Method for preparing citral and related products

Abstract

The present invention relates to a method for preparing 3-methyl-2-butenal (prenal), 3-methyl-2-buten-1-ol (prenol), diprenyl acetate of prenal, and 3,7-dimethyl-octa-2,6-dienal (citral), wherein the aldehyde content in one or more reaction flows is maintained below a threshold. Furthermore, the present invention relates to available product flows of prenal, prenol, diprenyl acetate of prenal, and citral.

Description

[Technical Field]

[0001] The present invention relates to a method for preparing 3-methyl-2-butenal (prenal), a method for preparing 3-methyl-2-buten-1-ol (prenol), a method for preparing diprenyl acetate of prenal, and a method for preparing 3,7-dimethyl-octa-2,6-dienal (citral), wherein the aldehyde content in one or more reaction flows is maintained below a threshold. Furthermore, the present invention relates to available product flows of prenal, prenol, diprenyl acetate of prenal, and citral. In particular, the present invention relates to such a method using isoprenol as the free agent. Accordingly, the present invention relates to an improved method for preparing prenol from isoprenol, a method for preparing prenal from isoprenol, a method for preparing diprenyl acetate of prenal from isoprenol, and an improved method for preparing citral from isoprenol.

[0002] According to the present invention, a method for preparing 3,7-dimethyl-octa-2,6-dienal (citral) comprises the step of reacting at least one formaldehyde source with isobutylene to obtain isoprenol (step a). The isoprenol obtained in step a) is isomerized to obtain prenol by contacting the reaction flow containing isoprenol with at least one heterogeneous isomerization catalyst (step b). Furthermore, prenal is provided by at least one of the following steps: ci) oxidative dehydrogenation of the isoprenol obtained in step a) to obtain prenol and / or isoprenol; and c-ii) oxidative dehydrogenation of the prenol obtained in step b) to obtain prenal. Furthermore, the prenol obtained in step b) is condensed with the prenol obtained in step c) to obtain a diprenyl acetal of prenal (step d). Furthermore, the diprenyl acetal of prenal obtained in step d) is subjected to decomposition conditions to obtain citral via prenyl(3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene (step e). Step ci) is characterized by maintaining the weight ratio of aldehyde to isoprenol at less than 0.04 in the reaction logistics. [Background technology]

[0003] Plenal, prenol, diprenyl acetate of plenal, and citral are chemical compounds that can be used as products or free agents for many further reactions. The aforementioned chemical compounds can be obtained from reactions using isoprenol as a free agent and / or intermediate product.

[0004] Prenols are sought after as intermediates for the synthesis of fragrances, vitamins, and carotenoids. Their preparation by isoprenol isomerization on suitable catalysts is known to those skilled in the art and is widely described in the literature.

[0005] Isoprenol can be isomerized to 3-methyl-2-buten-1-ol (prenol) in the presence of hydrogen and a catalyst, and the catalyst used is a fixed-bed catalyst containing palladium and selenium or tellurium, or a mixture of selenium and tellurium, on a silica support. From pages 1549-1550 of J.Am.Chem.Soc., 85 (1963), the isomerization of unsaturated alcohols with carbonyl compound catalysts of metals from Group VIII of the periodic table is well known. This method yields numerous by-products and derivatives, such as the corresponding aldehydes.

[0006] German Patent No. C-1901709 describes a method for preparing a buten-2-ol-4 compound by reacting a buten-1-ol-4 compound with palladium or a palladium compound and hydrogen. However, when pure palladium is used in the presence of hydrogen, the double bond of the compound is hydrogenated to a considerable extent, and a saturated product is formed.

[0007] In addition, low-boiling point compounds such as hydrocarbons and aldehydes are formed as by-products, for example, by hydrogenation and isomerization. For some butenols, hydrogenation of the double bond is undesirable because there is only a small difference in boiling point between the unreacted starting product and the hydrogenated product. For example, the boiling point of 3-methyl-3-buten-1-ol at 1020 mbar is 131.5°C, while the boiling point of its corresponding hydrogenated product, 3-methyl-1-butanol, is 130.9°C. This makes it difficult to separate the hydrogenated product from the starting product by distillation.

[0008] A method for isomerizing a 3-buten-1-ol compound to its corresponding 2-buten-1-ol compound is known from German Patent Application Publication No. A-2751766, where the isomerization is carried out in the presence of palladium, selenium, or tellurium as a catalyst and hydrogen. Palladium and selenium are used as catalysts on activated carbon. Other supports that can be used include barium sulfate, silica gel, alumina, and zeolite. The catalyst can also be used without a support. Relatively high proportions of light-boiling products such as isoprene and methylbutene are formed. Known catalytic isomerizations are carried out discontinuously in suspension mode, for example, in a stirred tank. However, since the double-bond isomerization of substituted butenols is an equilibrium reaction, a complete transformation of the substance is not obtained, and instead, some of the starting material always remains, which must be separated from the formed by-products for further use. To carry out the isomerization more economically, it should be possible to carry out the reaction continuously, which should yield the smallest proportion of hydrogenation products or low-boiling products.

[0009] European Patent Application Publication A841090 describes a continuous method for producing 2-buten-1-ol compounds by isomerization of 3-buten-1-ol compounds, with very small amounts of hydrogenation products and light-boiling point compounds. The catalyst used contains palladium and selenium or tellurium or a mixture of selenium and tellurium on a silica support, with a BET surface area of ​​80-380 m². 2 The values ​​are / g, with pore sizes ranging from 3 nm to 300 μm and pore volumes from 0.6 to 0.95 cm³. 3 It is a fixed-bed catalyst with a density of / g, where 80-95% of the pore volume is in the pore size range of 10-100 nm.

[0010] There is a continuous need for methods to maintain high conversion rates and selectivity over long periods while avoiding catalyst deactivation.

[0011] Prenol plays an important role in the production of citral.

[0012] Citral is a valuable intermediate for the production of various odorants and fragrances, including geraniol. Furthermore, citral is becoming increasingly important as a starting material for the production of vitamins, particularly vitamin A.

[0013] International Publication No. 2008 / 037693 discloses a method for producing citral. The above method is as follows: a) A process for producing 3-methyl-3-buten-1-ol (isoprennol) from isobutylene and formaldehyde, b) A process in which 3-methyl-2-butenal (prenal) and 3-methyl-3-butenal (isoprenal) are produced from 3-methyl-3-buten-1-ol (isoprenal) by oxidative dehydrogenation with an oxygen-containing gas on a silver support catalyst, c) A process in which 3-methyl-3-butenal (isoprenal) is further produced by isomerization from a mixture containing 3-methyl-3-butenal (isoprenal), d) The process by which 3-methyl-3-buten-1-ol (prenol) is produced from 3-methyl-3-buten-1-ol (isoprenol) by isomerization, e) A process in which an unsaturated acetal 3-methyl-2-butenal-diprenyl acetal is produced from 3-methyl-2-butenal-1-ol (prenol) and 3-methyl-2-butenal (prenal) using an acidic catalyst, f) a step of obtaining 3,7-dimethyl-octa-2,6-diene-al (citral) from 3-methyl-2-butenal-diprenyl acetal by decomposition and subsequent rearrangement.

[0014] This complex multi-step process is prone to undesirable side reactions that reduce the achievable citral yield. In other words, individual steps exhibit less than 100% selectivity, and the amount of by-products formed may be greater than desired. Such by-products reduce the desired selectivity of the conversion and generally must be removed from the citral product before subsequent use. A considerable amount of energy is required to separate the by-products from the citral, usually resulting in a significant loss of citral. Such losses can make the use of other more favorable reaction sequences commercially unattractive.

[0015] There remains an unmet need for methods to prepare plenal, prenol, diprenyl acetate of plenal, and citral with improved selectivity and / or improved yield. [Overview of the Initiative] [Means for solving the problem]

[0016] Surprisingly, methods for preparing plenal, prenol, diprenyl acetate of plenal, and citral can be improved by maintaining the aldehyde content in one or more reaction logistics, particularly in the case of oxidative dehydrogenation of isoprenol to plenal and / or isoprenal, and / or isomerization of isoprenol to prenol, below a considerably low threshold.

[0017] In the first aspect, the object of the present invention is to provide an improved method for preparing 3-methyl-2-buten-1-ol (prenol) from 3-methyl-3-buten-1-ol (isoprenol) by isomerization, which maintains a high conversion rate and selectivity over a long period of time and avoids catalyst deactivation.

[0018] Surprisingly, it has now been found that the presence of aldehydes, especially formaldehyde and / or prenal, in the reaction stream is detrimental to the activity and selectivity of the process and can promote the deactivation and / or poisoning of the catalyst during the isomerization of isoprenol to prenol.

[0019] Thus, in a first aspect, the present invention provides a process for preparing 3-methyl-2-buten-1-ol (prenol) by contacting a reaction stream comprising 3-methyl-3-buten-1-ol (isoprenol) with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen, to obtain a prenol-containing product stream, characterized in that the concentration of aldehyde in the reaction stream is maintained below 0.5% by weight, preferably below 0.4% by weight, particularly below 0.3% by weight, or below 0.25% by weight, based on the total weight of the reaction stream, and the concentration of aldehyde in the reaction stream is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, based on the total weight of the reaction stream.

[0020] In an even more preferred embodiment, the concentration of aldehyde is maintained below 0.2% by weight, based on the total weight of the reaction stream, and the concentration of aldehyde in the reaction stream is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, based on the total weight of the reaction stream.

[0021] In a second aspect, the object of the present invention is to provide advice on an improved process for preparing citral.

[0022] Generally speaking, the present invention provides a process for preparing 3,7-dimethyl-octa-2,6-dienal (citral): a) reacting at least one formaldehyde source with isobutylene to obtain isoprenol; b) isomerizing isoprenol to obtain prenol; c) converting isoprenol or prenol to prenol, optionally with isomerization and oxidative dehydrogenation, in any order; d) reacting prenol with prenol to obtain the diprenyl acetal of prenol; e) subjecting the diprenyl acetal of prenol to decomposition conditions to obtain citral via prenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene, and providing a method comprising the steps.

[0023] In particular, a second aspect of the present invention is a method for preparing 3,7-dimethyl-octa-2,6-dienal (citral): a) reacting at least one formaldehyde source with isobutylene to obtain isoprenol; b) isomerizing the isoprenol obtained in step a) by contacting a reaction stream containing isoprenol with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen, to obtain prenol; c) c-i) and c-ii) c-i) subjecting the isoprenol obtained in step a) to oxidative dehydrogenation by contacting a reaction stream containing isoprenol with at least one heterogeneous oxidative dehydrogenation catalyst in the presence of molecular oxygen to obtain prenal and / or isoprenal, and optionally isomerizing at least a part of the isoprenal to prenal; c-ii) oxidizing the prenol obtained in step b) by contacting a reaction stream containing prenol with at least one oxidizing agent and at least one oxidation catalyst, preferably in the presence of a liquid phase, to obtain prenal; providing prenal by at least one of the above steps; d) condensing the prenol obtained in step b) with the prenal obtained in step c) to obtain the diprenyl acetal of prenal; e) subjecting the diprenyl acetal of prenal obtained in step d) to decomposition conditions to obtain citral via prenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene, and providing a method comprising the steps.

[0024] The method satisfies the following condition 1), preferably the following condition 2), or the method satisfies at least one of the following conditions 1) and 2): 1) Step ci) is characterized by maintaining the weight ratio of aldehyde to isoprenol at less than 0.04 in the reaction logistics. 2) Step b) is characterized by maintaining an aldehyde concentration in the reaction logistics of less than 0.5% by weight, preferably less than 0.4% by weight, particularly less than 0.3% by weight, or less than 0.25% by weight, based on the total weight of the reaction logistics, and optionally the aldehyde concentration in the reaction logistics is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, based on the total weight of the reaction logistics.

[0025] In a further embodiment, an object of the present invention is to provide an improved method for preparing plenal.

[0026] Generally speaking, the present invention is a method for preparing plenal, comprising the steps of contacting a reaction logistics system containing isoprenol with at least one heterogeneous oxidative dehydrogenation catalyst in the presence of molecular oxygen to oxidative dehydrogenate isoprenol to obtain plenal and / or isoprenal, and optionally isomerizing at least a portion of the isoprenal to plenal, The objective is to provide a method that includes a step of maintaining the weight ratio of aldehyde to isoprenol at less than 0.04 during the reaction logistics.

[0027] Surprisingly, as experimentally discovered, such a method for preparing prenal is particularly beneficial. It can provide high selectivity for the reaction to prenal while maintaining high yield.

[0028] Further aspects of the present invention relate to an improved method for preparing diprenyl acetal of prenal.

[0029] Generally speaking, the present invention provides a method for preparing a diprenyl acetal of plenal, comprising the step of condensing plenal with prenol, wherein at least one of plenal and / or prenol is available (or obtainable) according to the method of the present invention.

[0030] As is evident from the experimental results, such a method for preparing diprenyl acetal of plenal is particularly beneficial. It can provide high selectivity for the reaction to diprenyl acetal while maintaining high yield.

[0031] Preferably, when preparing plenal, prenol, diprenyl acetal of plenal, or citral, isoprenol can be obtained by reacting at least one formaldehyde source with isobutylene to obtain isoprenol.

[0032] Further embodiments of the present invention refer to product streams of plenal, prenol, diprenyl acetal of plenal, and citral available (or obtainable) from the methods of the present invention. In a preferred embodiment, a product stream containing prenol is available (or obtainable) according to any one of claims 20-25. In a preferred embodiment, a product stream containing plenal is available (or obtainable) according to any one of claims 1, 2, and 13-16. In a preferred embodiment, a product stream containing diprenyl acetal of plenal is available (or obtainable) according to any one of claims 3-7, 10-17, and 26-30. In a preferred embodiment, a product stream containing citral is available (or obtainable) according to any one of claims 8-19, 31, and 32. As is evident experimentally, the product streams may contain each product with particularly high selectivity. Thus, the available (or obtainable) products may have special and technically beneficial characteristics.

[0033] definition As used herein and hereafter, the term “concentration of aldehydes in the reaction logistics” refers to the total concentration of aldehydes present in the reaction logistics. Aldehydes include those specific to the isoprenol preparation method, as well as those formed by oxidation and isomerization. Thus, aldehydes typically include formaldehyde and / or prenal. In one embodiment, the aldehyde whose concentration is determined is the sum of formaldehyde and prenal. Therefore, if formaldehyde and prenal are the only aldehydes present in the reaction logistics, the concentration of aldehydes in the reaction logistics is the sum of the respective concentrations of formaldehyde and prenal.

[0034] In this specification and hereafter, the term "ppm" means parts per million (ppm, 10%). -6 ) refers to.

[0035] As used herein, “formaldehyde source” refers to any source that contains formaldehyde or is capable of decomposing formaldehyde. Examples of formaldehyde sources include aqueous formaldehyde solutions and formaldehyde oligomers or polymers such as paraformaldehyde.

[0036] Throughout this specification, the terms “wt.-%”, “wt.%”, “wt%”, “weight percent”, and “weight%” are used as synonyms.

[0037] As used herein and hereafter, the term “reaction flow” refers to a stream containing one or more reactants consumed in the course of a chemical reaction. In this sense, reaction flow may further include solvents, catalysts, additives, and / or any other substances involved in the chemical reaction.

[0038] As used herein and hereafter, the term “unreacted isoprenol stream” refers to a stream derived from an isoprenol isomerization method, process (especially in the case of a manufacturing method) and containing unreacted isoprenol from an isoprenol isomerization process. In this sense, the unreacted isoprenol stream may further include solvents, catalysts, additives, and / or any other substances involved in the isoprenol isomerization method.

[0039] As used herein and hereafter, the term “crude isoprenol stream” refers to the production stream of an isoprenol production method from which unreacted isobutylene has been removed. Removal of aldehydes such as formaldehyde and / or prenal is carried out in a purification unit after isoprenol synthesis. A preferred method for recovering aldehydes from a crude isoprenol stream mixed with an unreacted isoprenol stream will be described in more detail below.

[0040] Detailed description of the invention The first and second embodiments of the present invention will be discussed in more detail below.

[0041] First aspect of the invention: Isomerization of isoprenol Degradation of catalyst properties has been found to be associated with the presence of aldehydes, particularly formaldehyde and / or plenal, in the reaction flow. Formaldehyde is generally considered to be the most important of these aldehydes. Fouling reactions of the catalyst by condensation and polymerization are considered to be the main reactions involved in the formation of carbon or coke on the catalyst. This carbon formation is thought to involve the thermal condensation of aldehydes, such as formaldehyde and / or plenal, or these aldehydes with the olefinic hydrocarbon isoprenol. In the presence of the catalyst, the primary condensation product tends to undergo dehydrogenation and polymerization type reactions, adhering to the catalyst and undergoing further dehydrogenation and decomposition until a carbonaceous deposit is formed.

[0042] One of the catalyst poisoning mechanisms is thought to involve catalytic or non-catalytic dehydrogenation of aldehydes, particularly formaldehyde and / or planal, with carbon monoxide, which are chemiadsorbed on the catalyst and block the active site.

[0043] A further cause of catalyst deactivation, which can occur in combination with the aforementioned causes of catalyst poisoning, is the formation of paraformaldehyde or trioxane, which can deposit on the catalyst in solid form and shield the catalytically active surface from the isoprenol being treated. This leads to the gradual deactivation of the catalyst.

[0044] Accordingly, according to the present invention, the concentration of aldehyde in the reaction material is maintained at a specific concentration or less, i.e., less than 0.5% by weight, preferably less than 0.4% by weight, particularly less than 0.3% by weight, or less than 0.25% by weight, based on the total weight of the reaction material, and the concentration of aldehyde in the reaction material is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, relative to the total weight of the reaction material.

[0045] In a more preferred embodiment, the aldehyde concentration is maintained at less than 0.2% by weight based on the total weight of the reaction material, and the aldehyde concentration in the reaction material is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, relative to the total weight of the reaction material.

[0046] Preferably, the aldehyde present in the reaction logistics includes formaldehyde. Also preferably, the aldehyde present in the reaction logistics includes plenal in addition to formaldehyde.

[0047] More preferably, the aldehyde present in the reaction logistics consists of prenal and formaldehyde. In certain cases, the aldehyde present in the reaction logistics consists of formaldehyde.

[0048] Preferably, the aldehyde concentration in the reaction material is less than 0.5% by weight, or less than 0.4% by weight, or less than 0.3% by weight, more preferably less than 0.25% by weight, or less than 0.2% by weight, even more preferably less than 0.15% by weight, even more preferably less than 0.1% by weight, 0.08% by weight or less, or less than 0.05% by weight, based on the total weight of the reaction material, but at least 10 ppm relative to the total weight of the reaction material. In another embodiment, the aldehyde concentration is less than 0.025% by weight, and even more preferably less than 0.02% by weight, based on the total weight of the reaction material. In one embodiment, the aldehyde concentration in the reaction material is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, relative to the total weight of the reaction material. Those skilled in the art will understand that any of the upper limits of the aldehyde concentration can be combined with any of the lower limits of the aldehyde concentration, and that in certain embodiments, the aldehyde is formaldehyde, prenal, or formaldehyde and prenal.

[0049] Preferably, the concentration of formaldehyde in the reaction logistics is less than 0.5% by weight, or less than 0.4% by weight, or less than 0.3% by weight, more preferably less than 0.25% by weight, or less than 0.2% by weight, even more preferably less than 0.15% by weight, even more preferably less than 0.1% by weight, 0.08% by weight or less, or less than 0.05% by weight, based on the total weight of the reaction logistics, but at least 10 ppm relative to the total weight of the reaction logistics. In another embodiment, the concentration of formaldehyde is less than 0.025% by weight, more preferably less than 0.02% by weight, based on the total weight of the reaction logistics, and the concentration of formaldehyde in the reaction logistics is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, relative to the total weight of the reaction logistics.

[0050] If the aldehyde present in the reaction logistics contains formaldehyde or consists of formaldehyde, the concentration of formaldehyde in the reaction logistics is preferably less than 0.5% by weight, or less than 0.4% by weight, or less than 0.3% by weight, more preferably less than 0.25% by weight, or less than 0.2% by weight, even more preferably less than 0.15% by weight, still more preferably less than 0.1% by weight, or less than 0.05% by weight, most preferably less than 0.025% by weight, or less than 0.02% by weight, based on the total weight of the reaction logistics, and the concentration of aldehyde in the reaction logistics is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, relative to the total weight of the reaction logistics.

[0051] Preferably, the concentration of prenal in the reaction logistics is less than 0.3% by weight, more preferably less than 0.2% by weight, even more preferably less than 0.15% by weight, and particularly less than 0.1% by weight, based on the total weight of the reaction logistics, but is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, relative to the total weight of the reaction logistics.

[0052] Therefore, in a preferred embodiment, the aldehyde in the reaction logistics consists of or contains formaldehyde, and the concentration of formaldehyde is less than 0.5% by weight, less than 0.4% by weight, less than 0.3% by weight, more preferably less than 0.25% by weight, less than 0.2% by weight, even more preferably less than 0.15% by weight, even more preferably less than 0.1% by weight, 0.08% by weight or less, or less than 0.05% by weight, based on the total weight of the reaction logistics, but at least 10 ppm relative to the total weight of the reaction logistics. In another embodiment, the concentration of formaldehyde is less than 0.025% by weight, more preferably less than 0.02% by weight, based on the total weight of the reaction logistics, but 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, relative to the total weight of the reaction logistics. In one embodiment, the formaldehyde concentration is 0.08% by weight or less based on the total weight of the reaction materials, but optionally, it is at least 10 ppm relative to the total weight of the reaction materials.

[0053] In one embodiment, the aldehydes present in the reaction logistics consist of prenal and formaldehyde. Therefore, the concentration of aldehydes in the reaction logistics corresponds to the sum of the concentrations of prenal and formaldehyde. The concentration of aldehydes in the reaction logistics, i.e., the sum of the concentrations of prenal and formaldehyde, is less than 0.5% by weight, preferably less than 0.4% by weight, particularly less than 0.3% by weight, or less than 0.2% by weight, based on the total weight of the reaction logistics. The concentration of aldehydes in the reaction logistics is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, relative to the total weight of the reaction logistics.

[0054] In a group of preferred embodiments, the weight ratio of aldehyde, preferably prenal and / or formaldehyde, to isoprenol in the reaction logistics is adjusted to a certain level or less, i.e., less than 0.04, preferably less than 0.03, particularly less than 0.02, or less than 0.01. In a more preferred embodiment, the weight ratio of aldehyde, preferably formaldehyde and / or prenal, to isoprenol is adjusted to less than 0.002, or less than 0.001. In one embodiment, this ratio is less than 0.0009.

[0055] With respect to the aldehyde concentration in the reaction logistics, the terms "maintained in the reaction logistics" and "adjusted in the reaction logistics" or "maintained in the reaction logistics" or "adjusted in the reaction logistics" are used interchangeably herein.

[0056] However, if the weight ratio of aldehydes, preferably formaldehyde and / or planal, to isoprenol in the reaction logistics is reduced beyond a certain point, it rapidly reaches a point of decrease. Removing aldehydes, particularly formaldehyde and / or planal, involves additional equipment and operating costs. An economic balance must be struck between the improvement achieved by reducing the ratio and the cost of maintaining such a ratio.

[0057] Therefore, the weight ratio of aldehyde, preferably formaldehyde and / or prenal, to isoprenol is preferably 0.0005 or higher, or in some cases 0.0007 or higher.

[0058] Since the double bond isomerization of isoprenol to prenol is an equilibrium reaction, the complete conversion of the substances is not achieved in a single pass. Instead, some isoprenol always remains, and this unreacted isoprenol can be suitably separated from the desired prenol. The unreacted isoprenol can be recycled into isomerization reactions or induced into other isoprenol consumption reactions.

[0059] Generally, the reaction logistics include or consist of a fresh isoprenol stream. The term "fresh isoprenol stream" refers to the isoprenol stream obtained directly from the purification unit after isoprenol synthesis, i.e., from the purification unit where the crude isoprenol stream from the reaction of isobutene and formaldehyde is purified. The reaction logistics may further include recycled, unreacted isoprenol and / or isoprenol from other sources.

[0060] Preferably, the reaction logistics include or consist of a fresh isoprenol stream. Also preferably, the reaction logistics include or consist of a mixture of an unreacted isoprenol stream and a fresh isoprenol stream.

[0061] In yet another embodiment, the reaction flow consists of a mixture of unreacted isoprenol and isoprenol from another source. The other source of isoprenol is a process other than the reaction of isobutene with formaldehyde in which isoprenol is obtained as a by-product or target product (or isoprenol from a commercial source).

[0062] The presence of aldehydes, particularly formaldehyde and / or planal, in the reaction logistics reduces both catalytic activity and selectivity, leading to increased pressure loss and reactor clogging. In addition to aldehydes, particularly formaldehyde and / or planal, other impurities that may be present in the reaction logistics can also reduce catalytic activity and selectivity. Preferably, apparatus or operations used to maintain a specific concentration of aldehydes, preferably formaldehyde and / or planal, in the reaction logistics, or a constant weight ratio of aldehydes, preferably formaldehyde or planal, to isoprenol, are also effective in removing most of these impurities. In preferred embodiments, the concentration of at least one of the following impurities in the reaction logistics, particularly all of the following impurities, is kept below the indicated limits.

[0063] [Table 1]

[0064] Adhering to these restrictions is especially important when the reaction logistics include isoprenol flows from other sources.

[0065] Reducing the concentration of aldehydes, preferably formaldehyde and / or prenal, in the reaction logistics essentially reduces the weight ratio of aldehydes, preferably formaldehyde and / or prenal, to isoprenol in the reaction logistics. Therefore, the following applies to reducing the concentration of aldehydes, preferably formaldehyde and / or prenal, in the reaction logistics and to reducing the weight ratio of aldehydes, preferably formaldehyde and / or prenal, to isoprenol in the reaction logistics.

[0066] The presence of formaldehyde in the reaction logistics stems from two main sources. Formaldehyde can be present in the isoprenol stream sent to the reactor, i.e., as an impurity originating from the isoprenol production process. In industrial practice, isoprenol is synthesized from isobutene and formaldehyde. Any formaldehyde that cannot be separated during the purification process after isoprenol synthesis ultimately enters the reaction logistics.

[0067] In addition, formaldehyde is also produced within the system. Some of the isoprenol is broken down back into isobutene and formaldehyde.

[0068] Most continuous industrial processes operate with a 50-60% single-pass conversion level, involving the recycling of unconverted isoprenol. Therefore, if a purification step for the stream containing unreacted isoprenol is not performed, formaldehyde may be present in the recycled stream of unconverted isoprenol. In this study, it was found that the recycled stream of unconverted isoprenol is typically the greatest source of formaldehyde contamination in the reaction logistics. This method is usually performed with partial conversion, for example, at a conversion rate of 30-70%, preferably 50-60%. The unreacted isoprenol stream is separated from the product stream. The unreacted isoprenol stream is recycled, i.e., mixed with a fresh feed stream containing isoprenol (crude isoprenol stream) to supply the reaction logistics. The unreacted isoprenol stream contains isoprenol as its main component, but may also contain plenal, isoprenal, isoamyl alcohol, isovaleraldehyde, isovaleric acid, prenol, and formaldehyde. Furthermore, this may also contain trace amounts of other C3 and C2 aldehydes, as well as acids.

[0069] Plenal may be present in the isoprenol stream sent to the reactor, i.e., as an impurity originating from the isoprenol production process. The isoprenol stream may further contain trace amounts of ammonia and / or other C5 oxygen additives besides formaldehyde and / or plenal. All plenal and / or other impurities that cannot be separated in the purification process after isoprenol synthesis ultimately enter the reaction stream.

[0070] Since the double bond isomerization of isoprenol is an equilibrium reaction, the conversion is inevitably incomplete. To operate the method economically, unconverted isoprenol must be removed and recycled. Therefore, if a purification step of the stream containing unreacted isoprenol is not performed, the recycling of isoprenol may unintentionally introduce formaldehyde into the isomerization process.

[0071] Reducing the concentration of aldehydes, preferably formaldehyde and / or prenal, in the reaction logistics, or reducing the weight ratio of aldehydes, preferably formaldehyde and / or prenal, to isoprenol in the reaction logistics, can be achieved in several different ways.

[0072] In a preferred embodiment, the method includes separating an unreacted isoprenol stream from a prenol-containing product stream, optionally removing at least some aldehydes, preferably some formaldehyde and / or prenal, from the unreacted isoprenol stream, and subsequently combining the unreacted isoprenol stream with a fresh isoprenol stream to form a reaction stream.

[0073] In another preferred embodiment, the method includes separating an unreacted isoprenol stream from a prenol-containing product stream, combining the unreacted isoprenol stream with a crude isoprenol stream containing isoprenol, water, and aldehydes, and removing aldehydes, preferably water and aldehydes, from the combined streams to form a reaction stream.

[0074] As described above, the crude isoprenol stream is generally the productive stream of an isoprenol production method from which unreacted isobutylene has been removed. This means that the removal of aldehydes (preferably formaldehyde and / or prenal, particularly formaldehyde) is achieved in a purification unit after isoprenol synthesis. A preferred method for recovering formaldehyde from the crude isoprenol stream mixed with the unreacted isoprenol stream will be described in more detail below.

[0075] Aldehydes, preferably formaldehyde and / or plenal, can be removed from the isoprenol stream by conventional separation methods such as distillation, selective adsorption, and / or selective reaction.

[0076] The removal of aldehydes, preferably formaldehyde and / or plenal, by distillation may involve the use of a single or series of distillation columns. The columns or columns used may be conventional distillation columns. Suitable types of distillation columns include packed columns, e.g., randomly packed or structured packed columns, plate columns (i.e., tray columns), and mixed columns containing both packing material and trays.

[0077] A suitable tray column may include an internal structure through which the liquid phase flows. Suitable internal structures include sieve trays, bubble cap trays, valve trays, tunnel trays, and Thormann® trays, particularly bubble cap trays, valve trays, tunnel trays, and Thormann® trays. Randomly packed columns can be filled with various molded bodies. Heat and mass transfer is improved by increasing the surface area with molded bodies, typically having sizes ranging from 25 to 80 mm. Suitable molded bodies include Raschig rings (hollow cylinders), Lessing rings, Pall rings, Hiflow rings, and Intalox saddles. The packing material can be supplied into the column in a regular or irregular manner (as bulk material, i.e., loosely packed). Suitable materials include glass, ceramics, metals, and plastics.

[0078] Structured fillers are an evolution of regular fillers and have a regular molded structure. This reduces pressure loss in the gas flow. Suitable types of structured fillers include fabric fillers and metal sheet fillers.

[0079] The removal of aldehydes, preferably formaldehyde and / or prenal, by selective adsorption involves contacting a flow with an adsorbent that exhibits selectivity for low molecular weight aldehydes, particularly formaldehyde and / or prenal. Useful adsorbent materials need to provide high selectivity and high adsorption capacity. An additional crucial requirement is that the adsorbent material must not catalyze or participate in chemical reactions that could reduce the recovery rate of (iso)prenal and / or deactivate the adsorbent. Examples of adsorbents include ion exchange resins, mesoporous solids, activated carbon, and zeolites.

[0080] Removal of aldehydes, preferably formaldehyde and / or plenal, by selective reaction involves placing the flow under reaction conditions that selectively react the aldehydes, preferably formaldehyde and / or plenal, with a product that is less likely to deactivate and clog the catalyst, or with a product that can be more easily separated from the flow than the aldehydes, preferably formaldehyde and / or plenal.

[0081] Preferably, the removal of aldehydes, preferably formaldehyde and / or plenal, from the isoprenol-containing stream is carried out by distillation, selective adsorption and / or selective reaction, particularly by purification methods including pressure swing distillation.

[0082] The above applies to reducing the concentration of aldehydes other than formaldehyde or prenal and / or other impurities in the reaction logistics, and to reducing the weight ratio of aldehydes other than formaldehyde or prenal to isoprenol.

[0083] Generally, isomerization of isoprenol to 3-methyl-2-buten-1-ol (prenol) can be carried out on a supported noble metal, preferably in the presence of hydrogen.

[0084] A preferred catalyst is a fixed-bed catalyst containing palladium and selenium or tellurium or a mixture of selenium and tellurium supported on silicon dioxide (also called silicon dioxide).

[0085] The catalyst contains 0.1 to 2.0 wt% palladium and 0.01 to 0.2 wt% selenium, tellurium, or a mixture of selenium and tellurium, based on the total weight of the catalyst.

[0086] The BET surface area is, for example, 100-150m². 2 The range of / g, especially 110-130m 2 The range is / g. The BET surface area is determined by N2 adsorption according to DIN66131.

[0087] For pores with a diameter in the range of 3 nm to 300 μm, the pore volume is preferably 0.8 to 0.9 cm³. 3 / g, especially 0.8~0.85cm 3 The ratio is / g. Therefore, 80-95%, preferably 85-93%, of this pore volume has a pore size in the range of 10-100 nm. The pore volume is determined by Hg porosimetry.

[0088] Preferably, the catalyst contains 0.2 to 0.8% by weight, particularly 0.4 to 0.6% by weight, of palladium. Preferably, the catalyst contains 0.02 to 0.08% by weight, particularly 0.04 to 0.06% by weight, of selenium, tellurium, or a mixture of selenium and tellurium, preferably selenium. In addition to the active ingredients mentioned, small amounts of other metals may be present on the catalyst. Preferably, only palladium, selenium, and / or tellurium, particularly only palladium and selenium, are present on the silica support.

[0089] The isomerization of isoprenol to prenol on a fixed-bed catalyst, as described, is also described in European Patent Application Publication A841090, as cited by reference.

[0090] Isomerization is carried out at a temperature in the range of 50 to 150°C, preferably 60 to 130°C, more preferably 70 to 120°C, to produce a reaction mixture of prenol and isoprenol. The isoprenol can be recycled. Further details are provided in International Publication No. 2008 / 037693.

[0091] Generally, a regeneration cycle is performed periodically to remove coke accumulated from the catalyst. The regeneration cycle can be initiated when the pressure loss increases beyond a threshold, or it can be initiated at any time interval (e.g., once a week). The regeneration cycle consists of supplying diluted air or air to the reactor for a predetermined time, for example, 6 to 24 hours, while increasing the temperature of the salt bath to, for example, 400 to 450°C to burn the coke.

[0092] Unreacted isoprenol from isoprenol isomerization methods can be used for isoprenol isomerization, i.e., recycled.

[0093] Selective recovery of aldehydes, particularly formaldehyde and / or plenal, from product streams containing aqueous alcohol solutions is extremely difficult. For example, in the case of formaldehyde, this difficulty stems from the fact that monomeric formaldehyde (as well as polymeric formaldehyde) forms both hydrates with water and hemiformals with alcohols such as isoprenol, which are reactants of isoprenol isomerization, and these unreacted reactants can still remain in the product stream. Hydrates and hemiformals of formaldehyde, with different degrees of polymerization, have coexisting boiling points. The stability of hydrates and hemiformals, and the equilibrium between them, is temperature-dependent. Formalals formed in the upper region of the distillation column may decompose at the higher temperatures at the bottom of the column, further complicating the separation process.

[0094] In recent years, it has been reported that formaldehyde can be substantially completely separated from isoprenol in a distillation column consisting of a first distillation at a low temperature where the equilibrium shifts to the hemiformal side of formaldehyde and isoprenol, resulting in essentially all of the formaldehyde remaining at the bottom of the column, and a second distillation at a high temperature where hemiformal is cleaved into formaldehyde and isoprenol, resulting in the easy separation of formaldehyde from isoprenol. See M. Dyga, A. Keller, and H. Hasse, Industrial & Engineering Chemistry Research 2021, 60, 11, 4471-4483.

[0095] In one embodiment, an unreacted isoprenol stream is combined with a crude isoprenol stream containing isoprenol, water, and an aldehyde, preferably formaldehyde and / or prenal; and from the combined stream, the aldehyde, for example formaldehyde and / or prenal, preferably water and an aldehyde, particularly water, and formaldehyde and / or prenal is removed. (i) The mixed flow is sent to a first low-boiling-point separation column operated at a pressure of 1.5 bara or less to obtain a first bottom flow containing isoprenol and aldehydes, preferably plenal and / or formaldehyde, and a first distillation flow containing water and low-boiling-point substances. (ii) A step of sending the first bottom flow to a second low-boiling point separation column operated at a pressure of 2 bara or more to obtain a second distillation flow containing aqueous aldehydes, preferably plenal and / or formaldehyde, and a second bottom flow containing isoprenol, (iii) A step of sending the second bottom flow to the finishing column to obtain a bottom flow containing high-boiling point substances and a reaction flow as a distillation flow. Includes.

[0096] To enable a first distillation at a temperature below the isoprenol-aldehyde dissociation temperature of each aldehyde present, for example, below the isoprenol-formaldehyde dissociation temperature in the case of formaldehyde, and a second distillation at a temperature above the isoprenol-aldehyde dissociation temperature, such as the isoprenol-formaldehyde dissociation temperature, the present invention envisions two low-boiling-point separation columns operated at different pressures. Thus, a first distillate is obtained at a relatively low pressure spreading through the first low-boiling-point separation column, containing water and low-boiling-point substances that are essentially free of aldehydes, preferably formaldehyde and / or plenal. At a relatively high pressure spreading through the second low-boiling-point separation column, substantially all aldehydes, preferably all formaldehyde and / or plenal, are separated from the isoprenol. Thus, the method of the present invention makes it possible to obtain isoprenol that is essentially free of aldehydes, preferably formaldehyde and / or plenal.

[0097] The phrase "essentially free of aldehydes, preferably formaldehyde and / or prenal" is understood to mean that there are no significant amounts of aldehydes, preferably formaldehyde and / or prenal, in the resulting isoprenol. Therefore, the resulting isoprenol preferably contains less than 0.2% by weight, particularly less than 0.15% by weight, or less than 0.1% by weight of aldehydes, preferably formaldehyde and / or prenal, based on the total weight of the resulting isoprenol.

[0098] Preferably, the crude isoprenol stream is a liquid stream. The liquid stream may be a single-phase or two-phase liquid stream.

[0099] Crude isoprenol is sent to a first low-boiling-point separation column operated at a pressure of 1.5 bara or less. Any high-pressure crude isoprenol stream is preferably released before being sent to the first low-boiling-point separation column. The crude isoprenol stream is preferably supplied to the first low-boiling-point separation column as a side stream, defining the rectification section above the supply position and the stripping section below the supply position.

[0100] In the first low-boiling-point separation column, a first bottom flow containing isoprenol and an aldehyde, preferably formaldehyde and / or plenal, and a first distillation flow containing water and the low-boiling-point substance are obtained. The term "low-boiling-point substance" is understood to refer to organic compounds (other than aldehydes, particularly formaldehyde and / or plenal) having a boiling point lower than the boiling point of isoprenol at atmospheric pressure, and therefore below about 130°C. The most common low-boiling-point substance is methanol and / or isoprenyl formate, which are formed as by-products during the process.

[0101] In a preferred embodiment, the first low-boiling-point separation column is operated at a pressure of 1.2 bara or less, preferably 0.5 bara or less. The bottom temperature of the first low-boiling-point separation column is preferably in the range of 80 to 135°C, more preferably 90 to 115°C, and most preferably 95 to 105°C. The top temperature of the first low-boiling-point separation column is preferably in the range of 45 to 105°C, and more preferably in the range of 55 to 80°C.

[0102] In a particularly preferred embodiment, the first low-boiling-point separation column is operated at a pressure in the range of 0.2 to 0.5 bara, a column bottom temperature in the range of 90 to 115°C, and a column top temperature in the range of 55 to 80°C.

[0103] The first low-boiling-point separation column preferably has a theoretical number of 15 to 65, more preferably 25 to 40. In particular, the stripping section of the first low-boiling-point separation column preferably has a theoretical number of 10 to 25. The rectification section of the first low-boiling-point separation column preferably has a theoretical number of 5 to 40.

[0104] The first bottom flow contains isoprenol in an amount of isoprenol of preferably 75-95% by weight, more preferably 80-90% by weight, based on the total weight of the first bottom flow.

[0105] The first distillate is typically taken out in gaseous form at the top of the first low-boiling-point separation column and condensed to obtain a liquid two-phase flow. The liquid two-phase flow is preferably separated in a separation vessel to obtain an aqueous phase and an organic phase. The aqueous phase is preferably passed through a wastewater stripping column, as described later. The organic phase is preferably partially returned as reflux to the top of the first low-boiling-point separation column. Another portion of the organic phase is preferably discarded from the method to avoid the accumulation of water-insoluble low-boiling-point substances in the first low-boiling-point separation column.

[0106] In a preferred embodiment, at least a portion of the first distillation stream is sent to a wastewater stripping column to separate the low-boiling substances and the accompanying isoprenol from the water. Preferably, the portion of the first distillation stream sent to the wastewater stripping column is the aqueous phase obtained by condensation and phase separation of the first distillation stream, as discussed above.

[0107] In the wastewater stripping column, low-boiling-point substances are obtained as a low-boiling-point distillation stream, and wastewater is obtained as a bottom-of-column stream. Both the low-boiling-point distillation stream and the wastewater bottom-of-column stream are removed from the process, and each stream can be sent for further treatment.

[0108] Furthermore, isoprenol is preferably obtained as a side flow in the wastewater stripping column. The isoprenol side flow is typically a two-phase flow and contains preferably 15-40% by weight, more preferably 25-35% by weight, of isoprenol based on the total weight of the isoprenol side flow. The isoprenol side flow is preferably recycled to a first low-boiling point separation column.

[0109] The low-boiling-point distillation stream preferably contains 75-95% by weight, more preferably 80-85% by weight, of low-boiling-point substances, based on the total weight of the low-boiling-point distillation stream. The wastewater tower bottom flow preferably contains less than 1.2% by weight, more preferably less than 0.6% by weight, of organic matter, based on the total weight of the wastewater tower bottom flow. The wastewater tower bottom flow typically contains aldehydes, preferably formaldehyde and / or plenal, at concentrations such as 0.05-1.5% by weight, for example, 0.3-0.9% by weight, based on the total weight of the wastewater tower bottom flow.

[0110] The wastewater stripping column is preferably operated at a pressure of 1.5 bara or less, more preferably 1.1 bara or less. The bottom temperature of the wastewater stripping column is preferably in the range of 95 to 110°C, more preferably in the range of 97 to 103°C. The top temperature of the wastewater stripping column is preferably in the range of 65 to 100°C, more preferably in the range of 75 to 85°C.

[0111] In a particularly preferred embodiment, the wastewater stripping column is operated at a pressure in the range of 0.95 to 1.1 bara, a column bottom temperature in the range of 97 to 103°C, and a column top temperature in the range of 75 to 85°C.

[0112] The wastewater stripping column preferably has 6 to 30 theoretical plates, more preferably 10 to 20 theoretical plates.

[0113] The first bottom flow obtained in the first low-boiling-point separation column is sent to a second low-boiling-point separation column operated at a pressure of 2 bara or more. The first bottom flow is preferably supplied to the second low-boiling-point separation column as a side flow, defining the rectification section above the supply position and the stripping section below the supply position.

[0114] In the second low-boiling-point separation column, a second distillation stream is obtained containing, or essentially consisting of, aqueous aldehydes, preferably formaldehyde and / or plenal, and a second bottom stream containing isoprenol. The second bottom stream further contains high-boiling-point substances. The term “high-boiling-point substances” is understood to refer to organic compounds having a boiling point higher than the boiling point of isoprenol at atmospheric pressure, i.e., higher than about 130°C.

[0115] In a preferred embodiment, the second low-boiling-point separation column is operated at a pressure of 2.5 bara or higher, preferably 2.8 bara or higher, and most preferably 2.9 bara or higher. The bottom temperature of the second low-boiling-point separation column is preferably in the range of 160 to 200°C, more preferably 170 to 185°C, and most preferably 175 to 180°C. The top temperature of the second low-boiling-point separation column is preferably in the range of 115 to 160°C, more preferably 125 to 145°C.

[0116] In a particularly preferred embodiment, the second low-boiling point separation column is operated at a pressure in the range of 2.9 to 3.5 bara, a column bottom temperature in the range of 175 to 180°C, and a column top temperature in the range of 130 to 140°C.

[0117] The second low-boiling-point separation column preferably has a theoretical number of 20 to 60, more preferably 35 to 60. In particular, the stripping section of the first low-boiling-point separation column preferably has a theoretical number of 25 to 45. The rectification section of the first low-boiling-point separation column preferably has a theoretical number of 7 to 20.

[0118] At the top of the second low-boiling point separation column, an off-gas is typically obtained. The off-gas is mainly nitrogen-based and may contain trace amounts of isoprenol, formic acid, water, aldehydes, preferably formaldehyde, plenal, and / or decomposition gases.

[0119] The second bottom flow preferably contains 82-96% by weight, more preferably 87-91% by weight, of isoprenol. The relatively high pressure of the second low-boiling point separation column allows for high-level separation of isoprenol from aldehydes, preferably formaldehyde and / or planal. Thus, the second bottom flow preferably contains up to 0.5% by weight, more preferably up to 0.1% by weight, and even more preferably up to 0.008% by weight, of aldehydes, preferably formaldehyde and / or planal, based on the total weight of the second bottom flow.

[0120] The second distillation stream is an aqueous stream which, based on the total weight of the second distillation stream, preferably contains 25-60% by weight, more preferably 40-50% by weight, and particularly 45-50% by weight of an aldehyde, preferably formaldehyde and / or plenal. The second distillation stream, based on the total weight of the second distillation stream, preferably contains up to 15% by weight of isoprenol, more preferably up to 5% by weight of isoprenol.

[0121] Because the vapor condensation curve appearing at the top of the second low-boiling-point separation column is broad, it is advantageous to use a condenser that recycles the liquid. Direct condensation by rapid cooling with liquid circulation is particularly advantageous. Therefore, in a preferred embodiment of this method, a rapid cooling section is provided downstream of the rectification section of the second low-boiling-point separation column in the direction of vapor flow. The term "direction of vapor flow" refers to the direction of flow of the gaseous components within the separation column, i.e., upward toward the top of the column. The rapid cooling section is preferably provided within the second low-boiling-point separation column above the rectification section.

[0122] Furthermore, direct condensation by rapid cooling reduces the risk of fouling caused by various condensation and polymerization mechanisms of aldehydes, such as formaldehyde, which may occur in areas with high localized aldehyde concentrations, such as localized formaldehyde concentrations. To avoid the risk of fouling in the off-gas of the second low-boiling-point separation column and downstream methods, particularly the second low-boiling-point separation column, the concentration of aldehydes, preferably formaldehyde and / or plenal, in the second distillate is preferably 60% by weight or less, more preferably 55% by weight or less, and particularly 50% by weight or less, based on the total weight of the second distillation stream.

[0123] At the lower end of the quenching section, the aqueous liquid is recovered. If the quenching section is located within a second low-boiling point separation column, the aqueous liquid can be recovered, for example, in recovery trays above the rectification section and below the quenching section.

[0124] The aqueous liquid is partially circulated through a circulation line to the quenching section and partially withdrawn as a second distillate. Preferably, a portion of the aqueous liquid circulated to the quenching section is circulated to the upper part of the quenching section. Circulation of the aqueous liquid is typically achieved by the use of a pump.

[0125] By circulating a portion of the aqueous liquid into the rapid cooling section, it becomes possible to cool the vapor rising through the rapid cooling section and to absorb aldehydes, preferably formaldehyde and / or planal, from the vapor into the aqueous liquid. Therefore, aldehydes, preferably formaldehyde and / or planal, can be rapidly cooled from the vapor rising through the rapid cooling section.

[0126] Furthermore, the aqueous liquid is partially returned to the rectification section of the second low-boiling point separation column as reflux. This can be achieved by a reflux line, or the aqueous liquid can be partially returned to the rectification section as an overflow from a recovery tray below the quenching section.

[0127] The mass flow rate ratio of the reflux flow to the second distillate is preferably in the range of 2:1 to 10:1, more preferably in the range of 3:1 to 7:1. In a preferred embodiment, the aqueous liquid is cooled before being circulated to the quenching section. Preferably, a portion of the aqueous liquid taken out as the second distillate is a partial flow of cooled aqueous liquid.

[0128] The temperature of the aqueous liquid recovered at the lower end of the rapid cooling section is preferably in the range of 80 to 140°C, more preferably 125 to 135°C. The temperature of the cooled aqueous liquid circulated through the rapid cooling section is preferably 10 to 80°C lower than the temperature of the aqueous liquid recovered at the lower end of the rapid cooling section. This enables an energetically advantageous method.

[0129] The hot aqueous liquid removed at the lower end of the quenching section is useful for thermal integration. In a preferred embodiment, it is heat-exchanged with the flow of crude isoprenol flowing into the first low-boiling point separation column before being circulated back into the quenching section.

[0130] In one embodiment, a scrubbing section is provided downstream of the quenching section in the direction of vapor flow, and water is introduced to the top of the scrubbing section. Preferably, the scrubbing section is provided in a second low-boiling point separation column above the quenching section. The scrubbing section makes it possible to maintain the aldehyde, preferably formaldehyde and / or planar concentration in the second distillate below the critical concentration, and thus deposits, such as those of paraformaldehyde, are avoided, for example, in off-gas lines.

[0131] The mass flow rate ratio between the water introduced at the top of the scrub section and the first bottom flow obtained in the first low-boiling point separation column is typically in the range of 0.01:1 to 0.06:1, and more preferably in the range of 0.015:1 to 0.03:1.

[0132] The second bottom flow is sent to the finish column, where pure isoprenol is obtained as a distillation stream. High-boiling substances are removed via the bottom flow. Since the second bottom flow is essentially free of aldehydes, preferably formaldehyde and / or planal, the separation operation in the finish column is not significantly more complicated than when the efficiency of formaldehyde separation in the low-boiling substance separation section is low.

[0133] The phrase "essentially free of aldehydes, preferably formaldehyde and / or prenal" is understood to indicate that there are no significant amounts of aldehydes, preferably formaldehyde and / or prenal, in the resulting isoprenol. Therefore, the resulting isoprenol preferably contains less than 0.05% by weight, preferably less than 0.01% by weight, of aldehydes, preferably formaldehyde and / or prenal, based on the total weight of the second column bottom flow.

[0134] The pure isoprenol distillation stream preferably contains at least 97.0% by weight, more preferably 98.0% by weight, for example 98.1 to 99.5% by weight of isoprenol, based on the total weight of the pure isoprenol distillation stream. The high-boiling-point bottom stream preferably contains 90 to 99.9% by weight, more preferably 99 to 99.8% by weight of high-boiling-point substances, based on the total weight of the high-boiling-point bottom stream. Preferably, the high-boiling-point bottom stream contains less than 0.2% by weight of aldehydes, preferably formaldehyde and / or planal, based on the total weight of the high-boiling-point bottom stream, for example less than 0.05% by weight of aldehydes, preferably formaldehyde and / or planal.

[0135] In a preferred embodiment, the finishing column is operated at a pressure of 0.5 bara or less, preferably 0.25 bara or less. The bottom temperature of the first low-boiling point separation column is preferably in the range of 130 to 190°C, and more preferably in the range of 150 to 170°C. The top temperature of the finishing column is preferably in the range of 60 to 90°C, and more preferably in the range of 65 to 85°C.

[0136] In a particularly preferred embodiment, the finishing column is operated at a pressure in the range of 0.05 to 0.2 bara, a column bottom temperature in the range of 150 to 170°C, and a column top temperature in the range of 65 to 85°C.

[0137] The finishing tower preferably has 6 to 40 theoretical stages, more preferably 10 to 20 theoretical stages.

[0138] Oxidation of prenol to prenal The prenol obtained as described above can be oxidized to plenal by contacting the reaction flow containing prenol with at least one oxidizing agent and at least one oxidation catalyst, preferably in the presence of a liquid phase.

[0139] Suitable oxidizing agents include hydrogen peroxide and oxygen, particularly oxygen.

[0140] Oxidation is preferably carried out in the presence of a liquid phase, using oxygen as the oxidizing agent. The liquid phase, measured based on the total weight of the liquid phase at a temperature of 20°C and a pressure of 1 bar, preferably contains at least 25% by weight of water, more preferably at least 50% by weight of water, or at least 70% by weight of water. These conditions have been found to enable a simple and efficient method for preparing prenal from prenol.

[0141] Oxidation is typically carried out in the presence of at least one oxidation catalyst selected from the group consisting of platinum, palladium, and gold. Preferably, at least one oxidation catalyst includes platinum. In a preferred embodiment, at least one oxidation catalyst is a supported catalyst.

[0142] Oxidation is preferably carried out at a temperature of 20°C to 100°C, more preferably 20°C to 70°C. Oxidation is preferably carried out under an oxygen partial pressure of 0.2 to 8 bar.

[0143] Further conversion to citral The prenol obtained as described above may also be useful in the preparation of citral. Citral is a mixture of its isomer compounds, neral and geranial.

[0144] 3,7-dimethyl-octa-2,6-dienal (citral) can be prepared by obtaining prenol by the method described above, which further comprises the steps of condensing prenol with prenal to obtain a diprenyl acetal of prenal, and subjecting the diprenyl acetal of prenal to decomposition conditions to obtain citral via prenyl(3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene.

[0145] The prenol obtained as described above, preferably a product stream containing prenol, can be used as a feed for the process of condensing prenol with planal to obtain a diprenyl acetal of planal.

[0146] Further details regarding citral preparations using prenol obtained by the above method are described below.

[0147] Recycling flow from isomerization of isoprenol to prenol Unreacted isoprenol from the isomerization of isoprenol to prenol, carried out according to the present invention, can be used as a feed for an oxidative dehydrogenation step of isoprenol to obtain a flow containing plenal and / or isoprenal.

[0148] The oxidative dehydrogenation of isoprenol typically involves contacting a reaction flow, particularly a gaseous reaction flow containing isoprenol, with at least one heterogeneous oxidative dehydrogenation catalyst, particularly at least one silver-containing heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen. The at least one heterogeneous catalyst may consist of an inert support having a smooth surface with a silver active layer, or a large amount of (all-metallic) silver body may be used.

[0149] Further details regarding the recycling of unreacted isoprenol derived from the isomerization of isoprenol to prenol as a feedstock for the oxidative dehydrogenation process of isoprenol are described below.

[0150] Second aspect of the invention: Method for preparing citral It will be understood that the definitions and embodiments defined with respect to the method for preparing prenol will apply to the method for preparing citral with necessary modifications.

[0151] Isoprenol is obtained by reacting a formaldehyde source with isobutylene. A prenol useful as a starting material for the present invention can generally be obtained by reacting at least one formaldehyde source with isobutylene in a reactor under high temperature and high pressure to obtain 3-methylbuta-3-en-1-ol (isoprenol), and then isomerizing the obtained isoprenol.

[0152] In one embodiment, isoprenol is obtained by introducing at least one formaldehyde source and isobutylene, preferably mixing them, and preferably injecting them into a reactor through at least one nozzle, and reacting the at least one formaldehyde source and isobutylene under supercritical conditions. To achieve supercritical conditions, formaldehyde and isobutylene are reacted at a temperature of at least 220°C, for example, in the range of 220-290°C, and at an absolute pressure of at least 200 bara. The reaction of isobutene with formaldehyde can be carried out without a catalyst and in the presence of at least one catalyst. The reaction of the isobutene source with formaldehyde can also be carried out in the presence of one or more auxiliary chemicals such as ammonia and / or hexamethylenetetramine (urotropin). Carrying out this reaction in the presence of such auxiliary chemicals, in particular ammonia and / or urotropin, is described, for example, in German Patent No. 1279014B.

[0153] For example, the presence of a Brønsted acid compound, such as a Brønsted acid catalyst, can induce the removal of water from isoprenol, yielding isoprene and water. Therefore, it is preferable that the method of the present invention be carried out without essentially containing compounds that induce further reactions of isoprenol, such as Brønsted acid compounds. Of course, this does not relate to the starting materials and products of the method.

[0154] Therefore, this method is preferably carried out in the presence of less than 0.5 mol%, preferably less than 0.1 mol%, more preferably less than 0.01 mol%, and most preferably less than 0.001 mol%, of a compound that can induce further reactions of isoprenol, particularly Brønsted acid compounds. The Brønsted acid compound is understood to be any compound having a pH value of less than 7 when measured at 25°C and an absolute pressure of 1 bar.

[0155] In one embodiment, the method is carried out in the presence of a Brønsted base compound. The Brønsted base compound is useful for neutralizing acids that may be present in the starting materials or formed by side reactions during the method.

[0156] Suitable examples of Brønsted base compounds include hydroxides, carbonates, and bicarbonates of alkali metals and alkaline earth metals, ammonia or organic amines, and salts of acids weaker than formic acid. It is particularly advantageous to use ammonia or organic amines such as ethylamine, trimethylamine, hexamethylenetetramine (urotropin), aniline, pyridine, or piperidine. Substances exhibiting buffering properties, such as hexamethylenetetramine (urotropin), are particularly preferred.

[0157] The Brønsted base compound is preferably present in an amount of 0.001 to 10% by weight, particularly 0.01 to 1% by weight, based on the total weight of the reaction mixture. To use a weak base and avoid secondary reactions that may occur in the strongly alkaline range, it is advantageous to ensure that the starting mixture of the reactants after the addition of the Brønsted base compound has a pH value of 7 to 11, preferably 7 to 10, particularly 7.5 to 9, when measured at 20°C and an absolute pressure of 1 bar. If anhydrous starting materials are used, the pH value of the starting mixture is determined after dilution with an equal volume of water.

[0158] The Brønsted base compound can be supplied to the reactor at one or more preferred locations. If the formaldehyde source is an aqueous formaldehyde solution, the Brønsted base compound is preferably supplied to the reactor and dissolved in the aqueous formaldehyde solution. In another embodiment, the Brønsted base compound is supplied to the reactor separately as an aqueous solution.

[0159] At least one formaldehyde source and isobutylene are preferably introduced into the reactor to enable mixing of the reactants to obtain a homogeneous mixture. Methods of introduction include injection into the reactor, splashing, stirring, and / or spraying. Preferably, at least one formaldehyde source and isobutylene are injected or sprayed into the reactor through at least one nozzle.

[0160] Formaldehyde can be provided as a liquid, for example, as a solution of paraformaldehyde in methanol. Preferably, at least one formaldehyde source contains or is an aqueous formaldehyde solution.

[0161] While rapid and vigorous initial mixing of the reactants is desirable, it may also be advantageous to continue and complete the reaction under conditions of limited backmixing. Therefore, the reaction mixture may be passed to a post-reaction chamber located after the reactor or at the bottom of the reactor. In the post-reaction chamber, backmixing is limited.

[0162] In one embodiment, the reactor includes an upper and a lower section. The introduction of reactants, particularly the injection and mixing of reactants, takes place in a mixing chamber of the reactor located in the upper section, and a fluid containing formaldehyde and / or isobutylene and / or isoprenol is sent from the mixing chamber to a post-reaction chamber located in the lower section.

[0163] In one embodiment, reacting at least one formaldehyde source with isobutylene includes the steps of introducing at least one formaldehyde source with isobutylene, preferably mixing them, injecting them into an internal loop reactor through at least one nozzle, and injecting them into a first conduit, the internal loop reactor being -A cylindrical container arranged vertically, including side walls, -At least one draft pipe, vertically positioned within a container, having a pipe inlet end and a pipe outlet end, wherein the draft pipe is concentric with a nozzle and has an inner and outer surface, and the draft pipe provides a first conduit inside the draft pipe and a second conduit outside the draft pipe and within the side wall, the first conduit being in fluid communication with the second conduit, - Includes reactor fluid outlet means, The inner surface of the draft pipe is convexly curved such that the first conduit exhibits an annular constriction in its cross-section between the pipe inlet end and the pipe outlet end, the constriction being located closer to the pipe inlet end, and the convex curve on the inner surface of the draft pipe extends over at least 70%, preferably at least 80%, and most preferably at least 90% of the length of the draft pipe. The outer surface of the draft pipe is convexly curved such that the draft pipe exhibits a circumferential projection between the pipe inlet end and the pipe outlet end, the circumferential projection preferably located closer to the pipe outlet end, and the convex curvature of the outer surface of the draft pipe extends over at least 70%, preferably at least 80%, and most preferably at least 90% of the length of the draft pipe. The ends of the draft pipe are rounded so that at least one formaldehyde source and isobutylene introduced from a nozzle move roughly downward through a first conduit to react and obtain a fluid, which is then diverted in the opposite direction to move through a second conduit and subsequently back-mixed with the introduced fluid.

[0164] In one embodiment, this configuration of the draft pipe allows for control of the boundary layer flowing along the end of the draft pipe. When the angle of attack of the flow relative to the solid body reaches a certain limit, the reverse pressure gradient may become too large for the flow to overcome. At this point, the flow may separate from the top surface of the solid body, a condition generally called stall. This configuration can allow for either a reduction or a delay in flow separation, respectively. A reduction in flow separation can reduce fluid friction, thus reducing the pressure drop along the streamlines of the recirculating flow, and consequently improving the circulation rate of the configuration. The curvature of the inner surface of the draft pipe wall can guide the fluid through the draft pipe in an optimized manner, similar to the fluid flow over an airfoil.

[0165] The inner surface of the draft pipe may be curved in the longitudinal direction of the draft pipe, that is, it may have a convex shape, and as a result, the first conduit may have its minimum cross-sectional area between the pipe inlet end and the pipe outlet end. This means that the cross-section of the first conduit may decrease from the cross-section at the pipe inlet end to the minimum cross-sectional area, and increase from the minimum cross-sectional area to the cross-section at the pipe outlet end.

[0166] A draft pipe may have a curved, substantially conical section between the pipe inlet end and the constricted section, which is wider at the inlet end and narrower at the constricted section. At least a portion of the fluid flowing downstream through the draft pipe may be deflected to flow along the inner surface of the draft pipe until it ends. Since the flow through the pipe may remain mostly attached, the resulting pressure loss is small. Near the constricted section, the fluid flowing downstream through the draft pipe may be accelerated. Between the constricted section and the pipe outlet end, the cross-sectional area of ​​the draft pipe may widen again. As a result, the change in area, along with the conservation of mass, may cause the velocity through the larger area to be slower than the velocity through the smaller area, accompanied by the conversion of dynamic pressure to static pressure. When the fluid flowing downstream through the draft pipe is accelerated near the constricted section, a radial velocity component may be added to the flow, increasing the mixing between the circulating flow and the injecting flow. By avoiding flow separation in this case, no large pressure loss occurs.

[0167] In a preferred embodiment, the nozzle is a two-component nozzle. In particular, the two-component nozzle is designed to provide an annular jet of isobutylene around a central jet of at least one formaldehyde source, and it is preferable that the introduction speeds of these two jets, such as injection speed or spray speed, are different. In this embodiment, the isobutylene jet has a large shear surface toward both the central jet of at least one formaldehyde source and the reaction mixture in the reactor, allowing for preferred high-speed mixing of the reactants.

[0168] In a preferred embodiment, the loop reactor includes a deflection means positioned between a nozzle and a fume hood, wherein the deflection means is suitable for deflecting a fluid moving through a second conduit in the opposite direction.

[0169] The deflection means preferably includes a surface that is concave with respect to the end of the draft pipe defining the pipe inlet end. In a preferred embodiment, the deflection means has a partial toroidal surface. The deflection means is particularly preferably provided in the shape of the upper portion of a ring torus bisected by a plane parallel to the toroidal direction. This shape allows for particularly efficient deflection of the fluid moving in the second conduit. The deflection means can enable stabilization of the introduction, e.g., injection or spray fluid flow. This is particularly appropriate when the flow rate of the fluid moving in the second conduit is not uniform across the cross-section of the reactor, which can lead to eccentricity of the introduction fluid flow. Such eccentricity, if left unaddressed, can reduce the circulation rate.

[0170] When the first conduit is a descending conduit and the second conduit is an ascending conduit, the shape of the deflection means constitutes the upper portion of a ring torus bisected by a plane parallel to the toroidal direction, and the ring torus is preferably bisected at least 50% of its height, for example, at least 55% or 65% of its height. Thus, the upper portion of the ring torus is the same size as, or smaller than, the lower portion of the ring torus. In another preferred embodiment, the shape of the deflection means constitutes the upper portion of a ring torus bisected by a plane parallel to the toroidal direction, and the ring torus is bisected at most 85% of its height, for example, 80% of its height. In these ranges, the inlet of the deflection means is set to an angle particularly suitable for deflecting the fluid.

[0171] Further details regarding the above-described embodiments relating to the loop reactor can be found in International Publication No. 2023 / 104863, which is incorporated herein by reference in its entirety.

[0172] High temperatures are required to obtain isoprenol in high yield from the reaction of formaldehyde and isobutylene. Effective heat removal is crucial for product quality and process safety. The heat removed from isoprenol is used to raise the temperature of isobutylene before it enters the reactor. The high-temperature isoprenol flow contains sensible heat from the chemical reaction. Sensible heat is a potentially renewable energy source that can be reused.

[0173] Advantageously, the reaction of at least one formaldehyde source with isobutylene preferably involves heat-exchanging a high-temperature isoprenol stream taken out of the reactor with an isobutylene stream directed into the reactor, in which case the heat exchange takes place in one or more shell-and-tube heat exchangers, each of which includes a plurality of tubes and a shell-side heat exchange passage, the high-temperature isoprenol is guided through the tubes of the heat exchanger and the isobutylene is guided through the shell-side passage, and if there are two or more heat exchangers, at least two of the heat exchangers are connected in series with respect to both the shell-side flow and the tube-side flow.

[0174] In a group of preferred embodiments, heat exchange is performed in a single shell-and-tube heat exchanger.

[0175] In another group of preferred embodiments, the heat exchange is carried out in at least one shell-and-tube heat exchanger, where the hot isoprenol is introduced through the tubes of the heat exchanger and the isobutylene is introduced through the shell-side passages, and in the case of at least two heat exchangers, these are connected in series with respect to both the shell-side flow and the tube-side flow.

[0176] Such a configuration allows for longer operating intervals between maintenance interruptions in this method. The term “maintenance interruption” is intended to mean the shutdown of the method that is periodically required to clean the heat exchanger tubes that have become clogged with fouling. An indicator of the need for a maintenance interruption is typically when the isobutylene discharged from the last heat exchanger is not sufficiently preheated, and even subsequent heaters are hardly able to add any additional external heat to the isobutylene to bring it to the required temperature before it enters the reactor. One aspect of the present invention is that the preheating of the isobutylene stream can be maintained at a sufficiently high level for a longer period of time so that the isobutylene can easily reach the desired temperature before it enters the reactor.

[0177] In conventional shell-and-tube heat exchangers, one specific area prone to fouling is the tubular region near the tube sheet, close to the inlet where the fluid exits individual tubes. Excessive fouling in this area can lead to clogging of individual tubes and fluid stagnation along the entire length of these tubes. Fluid stagnation generally results in reduced heat transfer performance.

[0178] As a further consequence of the reduced heat transfer performance caused by fouling, the energy required in the heater to adjust the temperature of the preheated isobutylene stream to the desired reaction temperature increases. Consequently, more additional external heat is needed, which is detrimental in terms of energy demand and method economics, and often negatively impacts the carbon dioxide footprint of the products.

[0179] By using two or more heat exchangers, the impact of fouling in individual tubes on the overall heat exchange capacity is reduced compared to a configuration using only a single heat exchanger. As a result, the heat transfer rate is maintained at the desired level for a longer period, thus allowing for longer operating intervals between maintenance interruptions and reducing the amount of additional external heat required to preheat the isobutylene flow compared to a plant with a single heat exchanger in a highly contaminated state.

[0180] Further details regarding the above embodiments relating to heat exchangers and energy saving and reduced maintenance intervals can be found in International Publication No. 2023 / 198714A1, which is incorporated herein by reference in its entirety.

[0181] Removal of unreacted formaldehyde from crude isoprenol. Separating isoprenol from unreacted formaldehyde is not a simple task. This difficulty arises because monomeric formaldehyde (and polymeric formaldehyde) forms both hydrates with water and hemiformal with isoprenol. Hydrates and hemiformal, which have different degrees of polymerization of formaldehyde, have coexisting boiling points.

[0182] However, it was found that by distilling hemiformal at a temperature at which it decomposes into formaldehyde and isoprenol, formaldehyde can be virtually completely separated from isoprenol, and as a result, formaldehyde can be easily separated from isoprenol. The separated formaldehyde can be recycled for isoprenol synthesis.

[0183] Therefore, crude isoprenol can be purified by distilling a crude isoprenol stream containing isoprenol, water, and formaldehyde, or the isoprenol-containing fraction thereof, in a low-boiling-point separation column operated at a pressure of 2 bara or more, preferably 2.5 bara or more, to obtain a distillation stream containing aqueous formaldehyde and a bottom stream containing isoprenol that is essentially formaldehyde-free.

[0184] The term "formaldehyde-free" is understood to mean that the resulting pure isoprenol does not contain a significant amount of formaldehyde. Therefore, the resulting pure isoprenol preferably contains less than 0.5% by weight, more preferably less than 0.1% by weight of formaldehyde. As used herein, the term "crude isoprenol" refers to a stream containing isoprenol, water, and formaldehyde. Preferably, a crude isoprenol stream contains 50-75% by weight, more preferably 60-65% by weight of isoprenol. Preferably, a crude isoprenol stream contains 15-40% by weight, more preferably 22-35% by weight of water. Preferably, a crude isoprenol stream contains 1-5% by weight, more preferably 2-3% by weight of formaldehyde.

[0185] In particular, it was found that formaldehyde can be separated substantially completely from isoprenol, and that a concentrated formaldehyde aqueous solution suitable for recycling into isoprenol synthesis can be obtained in a distillation column involving a first distillation at a temperature at which essentially all formaldehyde remains at the bottom of the column, and a second distillation at a temperature at which hemiformal is decomposed into formaldehyde and isoprenol, resulting in formaldehyde being easily separated from isoprenol.

[0186] Two low-boiling-point separation columns operating at different pressures are envisioned to enable a first distillation at a temperature below the isoprenol-formaldehyde dissociation temperature and a second distillation at a temperature above the isoprenol-formaldehyde dissociation temperature. Thus, a first distillate containing water and a low-boiling-point substance essentially free of formaldehyde is obtained at the relatively low pressure spreading through the first low-boiling-point separation column. At the relatively high pressure spreading through the second low-boiling-point separation column, virtually all of the formaldehyde is separated from the isoprenol. By this method, isoprenol essentially free of formaldehyde can be obtained.

[0187] Therefore, in a more preferred embodiment, the purification method is (i) A step of sending a stream of crude isoprenol to a first low-boiling point separation column operated at a pressure of 1.5 bara or less to obtain a first bottom stream containing isoprenol and formaldehyde, and a first distillation stream containing water and low-boiling point substances, (ii) A step of sending the first bottom flow to a second low-boiling point separation column operated at a pressure of 2 bara or more to obtain a second distillation flow containing aqueous formaldehyde and a second bottom flow containing isoprenol, (iii) The process includes sending a second bottom flow to a finish column to obtain a bottom flow containing pure isoprenol and high-boiling substances as a distillation stream.

[0188] The second distillation stream constitutes a concentrated aqueous formaldehyde solution suitable for recycling into isoprenol synthesis.

[0189] The second low-boiling-point separation column is preferably operated at a pressure of 2.5 bara or higher, preferably 2.8 bara or higher, and most preferably 2.9 bara or higher. The bottom temperature of the second low-boiling-point separation column is preferably in the range of 160 to 200°C, more preferably 170 to 185°C, and most preferably 175 to 180°C. The top temperature of the second low-boiling-point separation column is preferably in the range of 115 to 160°C, more preferably 125 to 145°C.

[0190] In a particularly preferred embodiment, the second low-boiling point separation column is operated at a pressure in the range of 2.9 to 3.5 bara, a column bottom temperature in the range of 175 to 180°C, and a column top temperature in the range of 130 to 140°C.

[0191] Detailed information on methods for recovering isoprenol containing low concentrations of formaldehyde or that is essentially formaldehyde-free can be found in International Publication No. 2022 / 189652A1.

[0192] Further details relating to the reaction of at least one formaldehyde source with isobutylene to obtain isoprenol can be found in the brochure International Publication No. 2020 / 049111A1, which is incorporated herein by reference in its entirety.

[0193] Isoprenol isomerized to obtain prenol. The resulting isoprenol can be subjected to catalytic isomerization by contacting the reaction logistics containing the isoprenol with at least one heterogeneous isomerization catalyst in order to obtain prenol.

[0194] The isomerization of isoprenol to 3-methyl-2-buten-1-ol (prenol) can be carried out on a supported noble metal, preferably in the presence of hydrogen. A preferred catalyst is a fixed-bed catalyst containing palladium and selenium or tellurium or a mixture of selenium and tellurium supported on silicon dioxide (also known as silicon dioxide). The isomerization is carried out at a temperature of 50 to 150°C to produce a reaction mixture of prenol and isoprenol. The isoprenol can be recycled. Further details of preferred embodiments are described in International Publication No. 2008 / 037693.

[0195] When isoprenol is subjected to catalytic isomerization, it may be preferable to maintain the weight ratio of formaldehyde to isoprenol in the reaction logistics to less than 0.04, preferably less than 0.03, particularly less than 0.02, or less than 0.01. In a more preferable embodiment, the weight ratio of formaldehyde to isoprenol is maintained to less than 0.002 or less than 0.001.

[0196] It was found that the presence of formaldehyde in the reaction logistics can accelerate catalyst deactivation and / or poisoning.

[0197] The weight ratio of formaldehyde to isoprenol in the reaction logistics can be maintained below a certain level. However, once the weight ratio of formaldehyde to isoprenol in the reaction logistics is reduced beyond a certain point, it rapidly reaches a point of decrease. Removing formaldehyde requires additional equipment and operating costs. An economic balance must be struck between the improvement achieved by lowering the ratio and the cost of maintaining such a ratio. Therefore, the weight ratio of formaldehyde to isoprenol is preferably 0.0005 or higher, or in some cases 0.005 or higher.

[0198] The presence of formaldehyde in the reaction logistics stems from two main sources. Formaldehyde can be present in the fresh feed stream sent to the reactor, i.e., as an impurity originating from the isoprenol production process. Any formaldehyde that cannot be separated during the purification process after isoprenol synthesis ultimately enters the reaction logistics.

[0199] In addition, formaldehyde is also produced within the system. Some of the isoprenol is broken down back into isobutene and formaldehyde.

[0200] Since the double bond isomerization of isoprenol is an equilibrium reaction, the conversion is inevitably incomplete. To operate the method economically, unconverted isoprenol must be removed and recycled. If a purification step is not performed for the stream containing unreacted isoprenol, the recycling of isoprenol may unintentionally introduce formaldehyde into the isomerization process.

[0201] Reducing the weight ratio of formaldehyde to isoprenol in the reaction flow can be achieved in several different ways. In one embodiment, formaldehyde is removed from the unreacted isoprenol flow before it is mixed with a fresh feed flow.

[0202] In one embodiment, an unreacted isoprenol stream is mixed with a fresh feed stream, and formaldehyde is removed from the mixed stream.

[0203] The present invention relates to a first aspect of the invention concerning the isomerization of isoprenol to 3-methyl-2-buten-1-ol (prenol). The isoprenol obtained as described above is isomerized by contacting the reaction logistics containing isoprenol with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen, by optionally maintaining or adjusting the aldehyde concentration in the reaction logistics to less than 0.5% by weight, preferably less than 0.4% by weight, particularly less than 0.3% by weight, or less than 0.25% by weight, based on the total weight of the reaction logistics, to obtain prenol, further optionally, the aldehyde concentration in the reaction logistics is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, based on the total weight of the reaction logistics.

[0204] Alternatively, the desired weight ratio of formaldehyde to isoprenol can be obtained in the mixed flow by mixing a certain amount of well-purified fresh feed stream with an unreacted isoprenol stream.

[0205] Aldehydes, preferably formaldehyde and / or prenal, can be removed from a stream containing isoprenol by conventional separation methods such as distillation, selective adsorption and / or selective reaction, particularly by the purification method involving the pressure swing distillation described above.

[0206] In one embodiment, the method for preparing prenol is a continuous method. In one embodiment, all steps for preparing prenol are carried out continuously, and in particular as a continuous process. This may include, for example, a step of preparing isoprenol (e.g., from formaldehyde and isobutene) and isomerization to prenol, and one or more optional purification steps.

[0207] Provided by Plenar The isoprenol obtained above can be converted to plenal through isomerization and oxidative dehydrogenation in any order. Therefore, it is possible to first isomerize isoprenol to prenol and then oxidize prenol to plenal; or to first oxidative dehydrogenate isoprenol to isoprenal and then selectively isomerize at least a portion of the isoprenal to plenal.

[0208] Oxidation of prenol to prenal The prenol obtained as described above can be oxidized to plenal by contacting the reaction flow containing prenol with at least one oxidizing agent and at least one oxidation catalyst, preferably in the presence of a liquid phase.

[0209] Suitable oxidizing agents include hydrogen peroxide and oxygen, particularly oxygen.

[0210] Oxidation is preferably carried out in the presence of a liquid phase, using oxygen as the oxidizing agent. The liquid phase, measured based on the total weight of the liquid phase at a temperature of 20°C and a pressure of 1 bar, preferably contains at least 25% by weight of water, more preferably at least 50% by weight of water, or at least 70% by weight of water. These conditions have been found to enable a simple and efficient method for preparing prenal from prenol.

[0211] Oxidation is typically carried out in the presence of at least one oxidation catalyst selected from the group consisting of platinum, palladium, and gold. Preferably, at least one oxidation catalyst includes platinum. In a preferred embodiment, at least one oxidation catalyst is a supported catalyst.

[0212] Oxidation is preferably carried out at a temperature of 20°C to 100°C, more preferably 25°C to 80°C, particularly 30°C to 70°C, and especially 35°C to 50°C. In another embodiment, oxidation is carried out at a temperature of 20°C to 70°C. Oxidation is preferably carried out under an oxygen partial pressure of 0.2 to 8 bar.

[0213] Further details of the oxidation reaction can be found in International Publication No. 2023 / 222895A1, which is incorporated herein by reference in its entirety.

[0214] Oxidative dehydrogenation of isoprenol The oxidative dehydrogenation of isoprenol typically involves contacting a reaction flow, particularly a gaseous reaction flow containing isoprenol, with at least one heterogeneous oxidative dehydrogenation catalyst, particularly at least one silver-containing heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen. The at least one heterogeneous catalyst may consist of an inert support having a smooth surface with a silver active layer, or a large amount of (all-metallic) silver body may be used.

[0215] In one embodiment, unreacted isoprenol from the isomerization of isoprenol to prenol may correspond to step b) and be used as a feed for the dehydrogenation step.

[0216] Therefore, in one embodiment, the method includes separating the unreacted isoprenol stream from the prenol-containing product stream that may be obtained in step b), and leading the unreacted isoprenol stream to at least partially oxidative dehydrogenation to obtain plenal and / or isoprenal (which may be in step c).

[0217] In one embodiment, oxidative dehydrogenation is performed. - A reaction passage including a shell-side heat exchange passage and multiple reaction tubes for circulating a heat transfer medium, - An inlet for introducing reaction logistics into the reaction passage, - This is done by passing isoprenol through multiple reaction tubes of a shell-and-tube heat exchanger, including an outlet from the reaction passage for recovering the discharge flow from the reaction tubes. In this case, the reaction tube is The reaction preheating zone adjacent to the injection port, The system includes a reaction zone downstream of the reactant preheating zone, which has a catalytically active wire matrix insert having silver on at least a portion of its surface.

[0218] The term "reactant preheating zone" refers to a section of the reaction tube, i.e., a section within the reaction tube where no catalytic oxidative dehydrogenation reaction occurs, and where the gas flow through the reaction tube exchanges heat with a circulating heat transfer medium via the tube walls. The preheating zone, located upstream of the reaction zone, provides a net heat flow to the reaction tube, ensuring that the reaction material is sufficiently heated to near or at the reaction temperature by the time it reaches the reaction zone.

[0219] Upon contact with the catalyst surface, the oxidative dehydrogenation reaction begins immediately. Otherwise, if "cold" reaction fluid that does not reach the reaction initiation temperature reaches the catalyst surface, coke formation may occur. Less coke formation is advantageous because it allows for longer reactor operation without the need to burn off the coke from the catalyst surface.

[0220] Preferably, the reactant preheating zone is adapted to allow laminar flow of the reactants within the preheating zone. That is, the reactant preheating zone has no obstructions to the reaction flow that would cause a transition from laminar to turbulent flow. Therefore, the reactant preheating zone preferably has an essentially free cross section, i.e., the preheating zone is empty.

[0221] In the case of an "essentially free cross-section," the reactant preheating zone may be empty. Alternatively, the reactant preheating zone may house a fixture made of a material with zero or limited catalytic activity, which has a negligible cross-section in a plane perpendicular to the longitudinal axis of the reaction tube. The aforementioned fixture can be attached to a catalytically active wire matrix present in the reaction zone, allowing the aforementioned wire matrix insert to be easily positioned in or removed from the reaction zone. For example, stainless steel wire or rod may be optional.

[0222] This setup allows only the portion of the reaction logistics flowing near the high-temperature walls of the reaction tube to be heated. As a result, the portion of the reaction logistics flowing through the center of the reaction tube is not heated to the reaction temperature, and therefore, blind reactions of unstable starting materials are reduced or even avoided. A "blind reaction" is a non-selective oxidation reaction that occurs in the absence of a catalyst. When the reaction logistics reach the reaction zone, the oxidative dehydrogenation reaction is initiated. Since this reaction is exothermic, energy is released, and the rest of the reaction logistics is rapidly heated to the reaction initiation temperature, allowing the reaction to proceed. This rapid heating of the majority of the reaction mixture reduces unwanted side reactions and thus improves selectivity.

[0223] Alternatively, the reactant preheating zone may have a wire matrix insert with zero or limited catalytic activity. The wire matrix insert can reduce or eliminate the temperature gradient without causing any flow obstruction that would promote turbulent characteristics. The wire matrix insert is considered to have zero catalytic activity (or in other words, "inert") if it does not catalyze the gas-phase partial oxidation reaction in question to a significant extent and the chemical composition of the flow passing through the wire matrix insert does not change significantly. Similarly, the matrix insert is considered to have limited catalytic activity if its catalytic activity is lower than that of the reaction zone. In one embodiment, the wire matrix insert with zero or limited catalytic activity is made of an inert material, preferably stainless steel.

[0224] In this specification, the term “reaction zone” refers to the region of the reaction tube where a catalytic gas-phase partial oxidation reaction occurs. The reaction zone includes a catalytically active wire matrix insert having a catalytically active precious metal on at least a portion of its surface. Because the wire matrix contained in the reaction zone has a more open structure compared to the packing of individual elements, most of the reaction heat is released to the walls of the reaction tube by radiation and does not need to be dissipated by the reaction flow. The specific flow characteristics of the reaction flow through the reaction tube to which the wire matrix insert is installed improve heat transfer through the tube walls. The formation of significant hot spots can be avoided. As a result, organic components of the reaction flow do not accumulate on the surface of the active catalyst material, which would cause a pressure drop. Overall, the need for periodic maintenance in the form of catalyst regeneration and / or replacement is reduced. Annual uptime can be increased and existing production capacity can be maximized, thus reducing operating costs and increasing profits.

[0225] In contrast to individually existing catalysts, wire matrix inserts can be formed continuously or as a single unit. Therefore, it becomes very easy to place and remove wire matrix inserts from the catalyst containment region of a reaction tube.

[0226] The "reaction zone" may consist of a single, continuous reaction zone. Alternatively, the reaction zone may include alternating regions having catalytically active wire matrix inserts, regions having essentially free cross-sections, or regions having wire matrix inserts with zero or limited catalytic activity.

[0227] A "wire matrix insert" is understood to be a self-supporting skeletal structure made of coiled, curved, or crimped metal wires, adapted for insertion into the reaction tubes of a shell-and-tube reactor. The wire matrix insert is a structure with a larger volume than the longitudinal wires.

[0228] Fixtures such as stainless steel wires or rods can be attached to the wire matrix insert, which allows the wire matrix insert to be easily placed in or removed from the reaction zone.

[0229] In one embodiment, the catalytically active wire matrix insert includes an elongated core having a plurality of wire loops extending from an elongated core, in which case the wire loops are arranged longitudinally and helically shifted, i.e., adjacent wire loops have an angular offset. The loops can be formed by helically bending a wire over the length of the wire matrix insert. For ease of manufacture, the elongated core preferably includes at least two longitudinal core wire members, these core wire members twisted together to form a winding of core wire, and the wire loops are housed in the winding of core wire.

[0230] A wire loop can be formed from one wire, or one or more intertwined wires, preferably four intertwined wires.

[0231] The wire matrix insert contained in the reaction zone has silver, a catalytically active precious metal, on at least a portion of its surface. The wires constituting the wire loop may be solid silver wire or silver-coated wire. The core wire can be made from a brass alloy or high-grade steel. The thickness of the silver coating layer superimposed on the core surface is, for example, 10 μm. However, solid silver wire is generally preferred because it has a longer service life. When the wire loop is formed from multiple entangled wires, at least one of the entangled wires may be made from solid silver wire or silver-coated wire, while the other entangled wires may be made from an inert material.

[0232] A silver wire having the same composition throughout its entire cross-section and containing at least 92.5% by weight of Ag can be appropriately used. The silver wire is bent into a spiral to form a wire loop and combined with at least two longitudinal core wire members, which are twisted together to form a winding of the core wire, and the wire loop is housed in the winding of the core wire. The longitudinal core wire members may also be silver wire or inert metal wire.

[0233] In a preferred embodiment, the catalytically active wire matrix insert includes an elongated core having a plurality of wire loops extending from an elongated core, in which case the wire loops are arranged longitudinally and helically shifted, and the wire loops include a block of silver wire.

[0234] Generally, catalytically active wire matrix inserts may have a cylindrical envelope with a diameter matching the inner diameter of the reaction tube. This may include cases where the diameter of the cylindrical envelope of an undeployed wire matrix insert is slightly larger than the inner diameter of the reaction tube. The elasticity or resilience of the wire matrix insert allows the wire loop to be inserted into the reaction tube with minimal back pressure, ensuring a secure fit against the inner wall of the reaction tube.

[0235] Preferred structures for wire matrix inserts are known in themselves; see, for example, UK Patent No. 2097910. Several inserts of this type are disclosed in UK Patent No. 1570530. Other inserts and methods for manufacturing them are disclosed in UK Patent Application Publication No. 2097910A. Matrix inserts are commercially available from Cal Gavin Ltd., England, and are sold under the trademark name HiTRAN®.

[0236] Further details of the oxidative dehydrogenation carried out by passing isoprenol through multiple reaction tubes of the shell-and-tube heat exchange reactor described above can be found in International Publication No. 2023 / 241952A1, which is incorporated herein by reference in its entirety.

[0237] When isoprenol is subjected to an oxidative dehydrogenation reaction, it may be preferable to maintain the weight ratio of aldehyde, preferably plenal and / or formaldehyde, to isoprenol in the reaction mixture at less than 0.04, preferably less than 0.03, particularly less than 0.02, or less than 0.01. In another embodiment, the weight ratio of aldehyde, preferably plenal and / or formaldehyde, to isoprenol is maintained at less than 0.002, less than 0.001, or optionally at least 100 ppm.

[0238] The weight ratio of aldehyde, preferably planal and / or formaldehyde, to isoprenol in the reaction logistics can be maintained below a certain level. However, if the weight ratio of aldehyde, preferably planal and / or formaldehyde, to isoprenol in the reaction logistics is reduced beyond a certain point, it rapidly reaches a point of decrease. Removal of aldehyde, preferably planal and / or formaldehyde, involves additional equipment and operating costs. An economic balance must be struck between the improvement achieved by reducing the ratio and the cost of maintaining such a ratio. Therefore, the weight ratio of aldehyde, preferably planal and / or formaldehyde, to isoprenol is preferably 0.0005 or higher. In alternative embodiments, the weight ratio is 0.005 or higher.

[0239] It was found that reactor clogging and increased pressure drop were significantly affected by the presence of aldehydes, preferably planal and / or formaldehyde, in the reaction fluid. Fouling reactions of the catalyst by condensation and polymerization are considered to be the main reactions involved in the formation of carbon or coke on the catalyst. This carbon formation is thought to involve thermal condensation of aldehydes, preferably planal and / or formaldehyde, or thermal condensation of aldehydes, preferably planal and / or formaldehyde, with the olefinic hydrocarbons isoprenol and (iso)prenal. In the presence of the catalyst, the primary condensation product tends to undergo dehydrogenation and polymerization-type reactions, adhering to the catalyst and undergoing further dehydrogenation and decomposition until carbonaceous deposits are formed.

[0240] The presence of formaldehyde in the reaction logistics is due to two main sources. Formaldehyde can be present in the fresh feed stream sent to the reactor, i.e., as an impurity originating from the isoprenol production process. Any formaldehyde that cannot be separated in the purification process after isoprenol synthesis may ultimately enter the reaction logistics.

[0241] In addition, formaldehyde is also generated within the system. Some of the isoprenol decomposes back into isobutene and formaldehyde. Most continuous industrial processes operate with a 50-60% single-pass conversion level, involving the recycling of unconverted isoprenol. Therefore, if a process to purify the stream containing unreacted isoprenol is not performed, formaldehyde may be present in the recycled stream of unconverted isoprenol. In this study, it was found that the recycled stream of unconverted isoprenol was the biggest source of formaldehyde contamination in the reaction logistics. The method is generally carried out with partial conversion, for example, with a conversion rate of 30-70%, preferably 50-60%. The unreacted isoprenol stream is separated from the product stream. The unreacted isoprenol stream is recycled, i.e., mixed with a fresh feed stream containing isoprenol (crude isoprenol stream) to supply the reaction logistics. The unreacted isoprenol stream contains isoprenol as its main component, but may also contain prenal, isoprenal, isoamyl alcohol, isovaleraldehyde, isovaleric acid, prenol, and formaldehyde. It may also contain trace amounts of other C3 and C2 aldehydes and acids.

[0242] Reducing the weight ratio of aldehydes, preferably planal and / or formaldehyde, to isoprenol in the reaction flow can be achieved in several different ways. In one embodiment, the aldehydes, preferably planal and / or formaldehyde, are removed from the unreacted isoprenol flow before it is mixed with the crude isoprenol flow.

[0243] In one embodiment, the unreacted isoprenol stream is mixed with the crude isoprenol stream and an aldehyde, preferably plenal and / or formaldehyde, and removed from the mixed stream.

[0244] Alternatively, the desired weight ratio of formaldehyde to isoprenol can be obtained in the mixed flow by mixing a certain amount of well-purified fresh feed stream with an unreacted isoprenol stream.

[0245] Aldehydes, preferably plenal and / or formaldehyde, can be removed from the isoprenol stream by conventional separation methods such as distillation, selective adsorption and / or selective reactions, particularly by the purification method involving the pressure swing distillation described above.

[0246] Pretreatment of (iso)prenol by oxidative dehydrogenation or removal of nitrogen before oxidation It may be advantageous to treat (iso)prenol to remove organically bound nitrogen from it by contacting it with a weakly acidic solid adsorbent before contacting it with at least one oxidative dehydrogenation catalyst or at least one oxidation catalyst. In other words, organically bound nitrogen can be removed from (iso)prenol by this method.

[0247] The term "organically bonded nitrogen" refers to any compound containing at least one nitrogen atom directly bonded to one or more carbon atoms. For example, such compounds containing at least one nitrogen atom can be selected from amines such as ethylamine, trimethylamine, aniline, pyridine, or piperidine. A particularly important amine is hexamethylenetetramine (urotropin). (Iso)prenol can contain approximately 5–30 ppm of organically bonded nitrogen.

[0248] It was found that weakly acidic solid adsorbents can adsorb organically bonded nitrogen in the presence of abundant (iso)prenol without interfering with reactive carbon-carbon double bonds.

[0249] A weakly acidic adsorbent may include an adsorbent material having sufficient acidity to adsorb organically bound nitrogen from (iso)prenol. In one embodiment, the solid adsorbent is a crosslinked resin having a phosphonic acid functional group. Preferably, the resin polymer is a vinyl aromatic copolymer, preferably crosslinked polystyrene, and more preferably polystyrene-divinylbenzene copolymer. Other polymers having a phosphonic acid functional group can also be used. Preferably, the crosslinked resin having a phosphonic acid functional group is of the macroporous type. A preferred solid adsorbent is Purolite S956.

[0250] The resin is typically used in the form of beads and packed into the column. (Iso)prenol passes through the column in contact with the resin beads. During contact, organically bound nitrogen in the (iso)prenol reacts with the functional groups, and an exchange occurs in which protons move to nitrogen and ionic bonds are formed with the anionic moieties of the resin. Contact is maintained until a threshold level, i.e., the breakthrough concentration, is reached. At this breakthrough point, the method reaches equilibrium, and additional organically bound nitrogen can no longer be effectively removed. The flow is maintained / stopped, and the column is backwashed with water, preferably deionized water or soft water. The backflow fluidizes the resin, and any solids trapped by the beads are loosened and removed.

[0251] In another embodiment, the solid adsorbent is silica-alumina hydrate. Numerous silica-alumina catalyst compositions and methods for preparing them are described in the patent literature; see, for example, U.S. Patent No. 4,499,197.

[0252] Preferably, the alumina content of the silica-alumina hydrate is about 10 to about 90% by weight of Al2O3. The preferred range for the alumina content is about 30 to about 70% by weight of Al2O3.

[0253] Introducing silicon dioxide into aluminum oxide introduces acidic centers. The number of acidic centers can be controlled by the amount of silicon dioxide introduced. The number of acidic centers increases with the amount of silicon dioxide introduced up to the maximum number of acidic centers, and after reaching the maximum number of acidic centers, it decreases again when the amount of silicon dioxide is further increased.

[0254] An example of a commercially available silica-alumina hydrate is Siral®, available from Sasol Germany GmbH, Hamburg, Germany. Siral® is based on orthorhombic aluminum hydroxide (boehmite, AlOOH) and doped with SiO2. Various grades of Siral® with different ratios of Al2O3 to SiO2 are available: Siral1 (Al2O3 / SiO2=99 / 1), Siral5 (Al2O3 / SiO2=95 / 5), Siral10 (Al2O3 / SiO2=90 / 10), Siral20 (Al2O3 / SiO2=80 / 20), Siral28M (Al2O3 / SiO2=72 / 28), Siral30 (Al2O3 / SiO2=70 / 30), and Siral40 (Al2O3 / SiO2=60 / 40). Siral40 is particularly preferred.

[0255] In one embodiment, (iso)prenol passes over a bed of weakly acidic solid adsorbent. Preferably, the “passing over a bed” step described above means providing a layer ("bed") of weakly acidic solid adsorbent in a typical reaction vessel known to those skilled in the art, which can be equipped with a stirring device, for example, a stirred tank reactor. The (iso)prenol is then introduced into the reaction vessel and guided through the vessel to come into contact with the weakly acidic solid adsorbent.

[0256] Alternatively, a weakly acidic solid adsorbent can be placed, for example, inside the reaction tube of a tubular reactor, and then (iso)prenol flows continuously through the reaction tube while in contact with the weakly acidic solid adsorbent.

[0257] In alternative embodiments, the (iso)prenol contains less than 2 ppm of organically bound nitrogen after contacting an alcohol stream with a weakly acidic solid adsorbent. In this specification, “ppm” refers to the weight ppm of the compound incorporating the organically bound nitrogen relative to the total weight of the (iso)prenol.

[0258] For example, the content of organically bound nitrogen in (iso)prenol can be determined by Kjeldahl analysis. Alternatively, an oxidative combustion method with a chemiluminescence detector according to DIN 51444 can be used.

[0259] In one embodiment, the method for preparing plenal is a continuous method. In one embodiment, all steps for preparing plenal are carried out continuously, and in particular as a continuous process. This may include, for example, a step for preparing isoprenol (e.g., from formaldehyde and isobutene), an oxidation step to plenal and / or isoprenal, an optional step for converting isoprenal to plenal, and an optional one or more purification steps.

[0260] Further conversion to citral The prenal produced in the process of subjecting diprenyl acetal to separation conditions to obtain citral (which may be step c)) may be useful in the preparation of citral. Citral is a mixture of its isomer compounds, neral and geranial.

[0261] 3,7-dimethyl-octa-2,6-dienal (citral) can be prepared by obtaining prenal by the method described above, which further comprises the steps of condensing prenal with prenol to obtain a diprenyl acetal of prenal, and subjecting the diprenyl acetal of prenal to decomposition conditions to obtain citral via prenyl(3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene.

[0262] In particular, 3,7-dimethyl-octa-2,6-dienal (citral) - A step of condensing prenal with prenol in the presence of at least one catalyst in a reaction column, and simultaneously removing the acetal fraction containing the diprenyl acetal of prenal from the reaction column, -A step of subjecting the acetal fraction in the decomposition column to decomposition conditions in the presence of at least one catalyst, and simultaneously removing from the decomposition column a decomposition fraction that optionally contains citral and includes at least one of prenyl(3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene, -It can be prepared by a method that includes the step of reacting the decomposed fraction in a plug-flow reactor to obtain citral.

[0263] The overall reaction sequence is shown by the following reaction scheme. [ka]

[0264] At least one catalyst is used to form an unsaturated acetal 3-methyl-2-butenal-diprenyl acetal (hereinafter referred to as "diprenyl acetal of prenal" or "diprenyl acetal") from prenol and prenal. For this purpose, prenal can react with prenol in the presence of a catalytic amount of at least one acid, and the water produced during the reaction is separated by a reaction column.

[0265] It has been found that when the conversion rate of diprenyl acetal of plenal reaches complete conversion, the concentration of by-products increases sharply. Therefore, it is preferable to maintain the conversion rate of diprenyl acetal of plenal at a level greater than 90% but less than 100%. Preferably, the conversion rate of diprenyl acetal of plenal (so that it can be converted in step b) is maintained at 99.5% or less, preferably 99% or less, for example 98% or less, or 97.5% or less, or 97% or less. Preferably, the conversion rate of diprenyl acetal of plenal is maintained at a level greater than 91%, for example 92% or more, or 93% or more, or 94% or more, or 95%. In a preferred embodiment, the conversion rate of diprenyl acetal of plenal is greater than 94% and 99% or less, for example 95% or more and 98% or less. If the conversion rate is low, the method becomes economically unprofitable or otherwise requires the recovery and recycling of unreacted diprenyl acetal. However, complete conversion is undesirable because it leads to a decrease in the yield of citral components and an increase in by-product formation. The conversion rate depends on various parameters, including the decomposition temperature, properties, and concentration of the catalyst, as well as the residence time in the decomposition column.

[0266] The obtained 3-methyl-2-butenal diprenyl acetal (diprenyl acetal) is decomposed by a decomposition column in the presence of at least one catalyst to remove 3-methyl-2-buten-1-ol (prenol) and obtain prenyl (3-methylbutadienyl) ether. Claisen rearrangement of the obtained prenyl (3-methylbutadienyl) ether yields 2,4,4-trimethyl-3-formyl-1,5-hexadiene, followed by Cope rearrangement to obtain 3,7-dimethyl-2,6-octadienal (citral).

[0267] Decomposition is carried out in the presence of at least one catalyst, preferably an acid catalyst. The catalyst may be a single catalyst species or a combination of two or more different catalyst species. Suitable acid catalysts are selected from non-volatile protonic acids such as sulfuric acid, p-toluenesulfonic acid, and phosphoric acid. In one embodiment, the catalyst includes phosphoric acid. In a preferred embodiment, the concentration of phosphoric acid at the bottom of the decomposition column is maintained at more than 100 ppm and less than 1500 ppm, preferably more than 200 ppm and less than 1000 ppm. Higher concentrations of the (acid) catalyst may result in a decrease in the yield of citral components.

[0268] The condensation of prenol with prenol is carried out in the presence of at least one catalyst, preferably an acid. The catalyst may be a single catalyst species or a combination of two or more different catalyst species. In one embodiment, the catalyst is nitric acid. Preferably, the concentration of nitric acid is less than 500 ppm, more preferably in the range of 100 to 300 ppm, relative to the total amount of the starting materials prenol and prenal. A smaller amount of (acid) catalyst may result in a lower conversion rate in the reaction column. A larger amount of (acid) catalyst may, unfavorably, lead to increased byproduct formation and decreased selectivity.

[0269] Preferably, the acetal fraction is subjected to decomposition conditions continuously using a decomposition column. "Decomposition conditions" refers to reaction conditions selected so that the diprenyl acetal contained in the acetal fraction is decomposed to prenyl(3-methylbutadienyl) ether, and then rearranged to 2,4,4-trimethyl-3-formyl-1,5-hexadiene and citral.

[0270] The acetal fraction contains diprenyl acetal as its main component. The acetal fraction does not necessarily have to consist of pure diprenyl acetal, but may also contain prenol, prenal, and citral components.

[0271] The decomposition is carried out in the presence of at least one catalyst, preferably at least one acid catalyst. Suitable acid catalysts are selected from non-volatile protonic acids such as sulfuric acid, p-toluenesulfonic acid, and phosphoric acid.

[0272] Preferably, the continuous decomposition in the decomposition column can be carried out at the bottom or the sump of a distillation column functioning as the decomposition column. Preferably, the acetal fraction and / or the catalyst are introduced into the bottom of the distillation column, the sump of the distillation column, or the evaporator of the distillation column.

[0273] If necessary, a high-boiling inert compound can be introduced into the sump of the decomposition column to ensure the minimum filling level of the sump and the evaporator. Suitable high-boiling inert compounds are selected from liquid compounds that are inert under the reaction conditions and have a higher boiling point than citral and diprenyl acetal. For example, the high-boiling inert compound can be selected from hydrocarbons such as tetradecane, pentadecane, hexadecane, octadecane, eicosane, or ethers such as diethylene glycol dibutyl ether, white oil, kerosene, or mixtures thereof.

[0274] Preferably, the distillation conditions are selected such that diprenyl acetal is mainly retained in the bottom or the sump of the distillation column. During the decomposition reaction, the decomposition fraction is continuously withdrawn from the decomposition column, and this decomposition fraction contains at least one of prenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene, and optionally citral. For ease of reference, prenyl (3-methyl-butadienyl) ether, 2,4,4-trimethyl-3-formyl-1,5-hexadiene, and citral are collectively referred to as "citral components". This is because the former is an intermediate in the reaction pathway to citral and can be converted to citral in a subsequent passage through a plug-flow reactor.

[0275] In addition, the prenol formed during the decomposition reaction can generally be continuously removed from the reaction mixture at the top of the decomposition column.

[0276] The decomposed fraction, along with the formed prenol, can be removed from the top of the distillation column.

[0277] Alternatively, and preferably, the decomposed fraction can be removed in liquid or vapor form from the side outlet of the distillation column.

[0278] The decomposition fraction can be reacted in a plug-flow reactor to obtain citral. For this purpose, the decomposition fraction is introduced into the plug-flow reactor at a temperature suitable for the rearrangement reaction that produces citral. By using a combination of a highly backmixed decomposition column and a plug-flow reactor, the selectivity and yield of the decomposition reaction can be increased. All catalysts required for the decomposition reaction are preferably introduced into the decomposition column, and preferably no catalysts are introduced into the plug-flow reactor.

[0279] In one embodiment, the prenol removed in the decomposition reaction is recycled into the condensation reaction. This improves the yield in the method of the present invention.

[0280] In particular, the method of the present invention may include recycling the prenol obtained in a step (which may be step d) of condensing prenol with plenal to obtain a diprenyl acetal of plenal, into a step (which may be step e) of subjecting the diprenyl acetal of plenal to separation conditions to obtain citral, and in particular recycling the prenol obtained in step e) into step d), The concentration of 2,4,4-trimethyl-3-formyl-1,5-hexadiene in the prenol recycled from step e) to step d) is controlled so that the concentration of 2,4,4-trimethyl-3-formyl-1,5-hexadiene in step d) is less than 1% by weight relative to the total weight of prenol and prenal. The concentration of citral in the prenol recycled from process e) to process d) is controlled such that the citral concentration in process d) is less than 1% by weight relative to the total weight of prenol and prenal.

[0281] The present invention relates to a process for preparing 3,7-dimethyl-octa-2,6-dienal (citral), comprising the steps of subjecting the diprenyl acetal of prenal to separation conditions to obtain citral via prenyl(3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene, wherein at least one of prenal and / or prenol is available from the method of the present invention. In a preferred embodiment, in this method for preparing citral, the diprenyl acetal of prenal is obtained from the method of the present invention.

[0282] In one embodiment, the method for preparing citral is a continuous method. In one embodiment, the steps for preparing citral are carried out continuously, and in particular as a continuous process. This may include, for example, a step for preparing plenal (which may be steps a) and c) and / or prenol (which may be steps a) and b) as described above; a step of condensing prenol with plenal to obtain a diprenyl acetal of plenal (which may be step d); a step of reorganizing citral (which may be step e); and one or more optional purification steps.

[0283] Further aspects of the present invention: Method for preparing plenary The definitions and embodiments defined with respect to the preparation methods of citral and / or prenol will be understood to apply to methods for preparing prenal with necessary modifications.

[0284] In embodiments of the present invention, prenol is obtained by isomerizing isoprenol by contacting a reaction logistics system containing isoprenol with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen, to obtain prenol.

[0285] In embodiments of the present invention, isoprenol is obtained by reacting at least one formaldehyde source with isobutylene to obtain isoprenol.

[0286] In embodiments of the present invention, the step of isomerizing isoprenol is characterized by maintaining an aldehyde concentration of less than 0.5% by weight, preferably less than 0.4% by weight, particularly less than 0.3% by weight, or less than 0.25% by weight in the reaction logistics, based on the total weight of the reaction logistics, and optionally the aldehyde concentration in the reaction logistics is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, based on the total weight of the reaction logistics.

[0287] Further aspects of the present invention: Method for preparing diprenyl acetal of planar The definitions and embodiments defined with respect to methods for preparing citral, prenol and / or prenal will be understood to apply to methods for preparing diprenyl acetal of prenal, with necessary modifications.

[0288] In embodiments of the present invention, the present invention provides a method for preparing a diprenyl acetal of plenal, comprising the step of condensing plenal with prenol, wherein at least one of plenal and / or prenol is available (or obtainable) according to the method of the present invention.

[0289] In embodiments of the present invention, a method for preparing plenal is: a) A step of reacting at least one formaldehyde source with isobutylene to obtain isoprenol, b) A step of isomerizing the isoprenol obtained in step a) by contacting a reaction logistics system containing isoprenol with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen, to obtain prenol. c) A step of providing planar according to the method of the present invention, d) Step of condensing the prenol obtained in step b) with the prenol obtained in step c) to obtain the diprenyl acetal of prenal, and, is included.

[0290] Further aspect of the present invention: Product stream containing prenol It will be understood that the definitions and embodiments defined with respect to the method of the present invention apply to the product stream containing prenol with the necessary modifications.

[0291] Due to the particularly high selectivity in the step of isomerizing isoprenol to prenol and / or the step of oxidative dehydrogenation to isoprenol in the method of the present invention, the available (obtained) product stream containing prenol contains particularly pure prenol in the product stream and has few undesirable by-products. It will be understood. The content range and by-products in the product stream can be changed due to limiting the content of aldehyde in the reaction stream.

[0292] Further aspect of the present invention: Product stream containing prenol It will be understood that the definitions and embodiments defined with respect to the method of the present invention apply to the product stream containing prenol with the necessary modifications.

[0293] Due to the particularly high selectivity in the step of isomerizing isoprenol to prenol in the method of the present invention, the available (obtained) product stream containing prenol contains particularly pure prenol in the product stream and has few undesirable by-products. It will be understood. The content range and by-products in the product stream can be changed due to limiting the content of aldehyde in the reaction stream.

[0294] Further aspect of the present invention: Product stream containing the diprenyl acetal of prenal It will be understood that the definitions and embodiments defined with respect to the method of the present invention apply to the product stream containing the diprenyl acetal of prenal with the necessary modifications.

[0295] Due to the particularly high selectivity of isoprenol to plenal and / or to isoprenal in the oxidative dehydrogenation step and / or to prenol in the method of the present invention, it will be understood that the available (obtained) product stream containing plenal and / or prenol will contain particularly pure plenal and / or prenol in the product stream. It will be understood that this can clearly result in a reduction of particularly pure diprenyl acetal of plenal and undesirable byproducts in the product stream. The content range and byproducts in the product stream may be modified by limiting the content of aldehydes in one or more reaction streams.

[0296] Further aspects of the present invention: citral-containing product flow It will be understood that the definitions and embodiments defined with respect to the method of the present invention will be applied to citral-containing product flows with necessary modifications.

[0297] Due to the particularly high selectivity of isoprenol to plenal and / or to isoprenal in the oxidative dehydrogenation step and / or the isomerization step of isoprenol to prenol in the method of the present invention, it will be understood that the available (obtained) product stream containing plenal and / or prenol will contain particularly pure plenal and / or prenol in the product stream. It will be understood that this can clearly result in a reduction of particularly pure citral and undesirable byproducts in the product stream. The content range and byproducts in the product stream may be modified by limiting the content of aldehydes in one or more reaction streams.

[0298] In one embodiment, the method for preparing prenal and / or prenol is a continuous method. In one embodiment, all steps for preparing citral are carried out continuously, and in particular as a continuous process.

[0299] Citral is produced as the (2Z)- and (2E)-isomers shown in formula (Vb-1): (2Z)-isomer neral, and as the (2E)-isomer geranyl shown in formula (Va-1).

[0300] The term citral, available (or obtained) according to the method of the present invention, may be any mixture of the two isomers, preferably a mixture having a mass ratio of neral:geranial of 40:60 to 60:40, particularly 45:55 to 55:45, 48:52 to 52:48, 49:51 to 51:49, or (approximately) 50:50.

[0301] As is known in the art, neral and geranial can be selectively separated from each other. For example, neral and geranial can be separated by distillation. This allows the mass ratio of neral to geranial to be adjusted to a desired degree.

[0302] The citral available (or obtained) according to the present invention may include geranial of formula (Va-1) and / or neral of formula (Vb-1). [ka]

[0303] Further aspects of the present invention relate to geranial of formula (Va-1), neral of formula (Vb-1), or mixtures thereof that are available (or obtainable) from the methods of the present invention.

[0304] The definitions and preferred embodiments described with respect to the preparation of (iso)prenal and the further steps aimed at the above citral will be understood to apply, with necessary modifications, to the preparation of genianal of formula (Va-1), neral of formula (Vb-1), or mixtures thereof, and the characterization of the products. It will be understood that genianal of formula (Va-1), neral of formula (Vb-1), and mixtures thereof have certain characteristics.

[0305] Further conversion to menthol or linalool The available (obtainable) citral is, for example, a useful intermediate for menthol or linalool. It will be understood that such products may also have particularly beneficial properties, such as purity, when citral is obtained by the method of the present invention.

[0306] Menthol (p-menthal-3-ol) is a naturally occurring active ingredient widely used in the pharmaceutical, cosmetic, and food industries. Menthol has a cooling effect when it comes into contact with mucous membranes, particularly the oral mucosa. In natural sources, such as peppermint oil, menthol exists in the form of four diastereomers and enantiomer pairs. The following formula represents (-)-menthol or L-menthol, the main component possessing the desired taste and other sensory properties. [ka] IUPAC name: 1R,2S,5R 2-isopropyl-5-methylcyclohexanol

[0307] Menthol is, - A process of catalytically hydrogenating citral to obtain citronellal, - A step of cyclizing citronellal in the presence of at least one acidic catalyst to obtain isopulegol, - It can be prepared from citral via a method comprising the step of catalytically hydrogenating isopulegol to obtain menthol.

[0308] The overall reaction sequence is shown by the following reaction scheme. [ka]

[0309] The hydrogenation of citral to obtain citronellal can be achieved by hydrogenation in the presence of a rhodium-phosphine catalyst.

[0310] A further aspect of the present invention is a method for preparing menthol, -According to the present invention, a step of catalytically hydrogenating citral, which is available (or obtained) according to any one of claims 8 to 19, 31 and 32, to obtain citronellal, - The citronellal prepared in this manner is cyclized in the presence of at least one acidic catalyst to obtain isopulegol, The present invention relates to a method comprising the step of catalytically hydrogenating the isopulegol prepared in this manner to obtain menthol.

[0311] A further aspect of the present invention relates to a method for preparing optically active menthol using citral obtained by the method according to the present invention.

[0312] A further aspect of the present invention is a method for preparing optically active menthol, preferably L-menthol, o) Optionally, a step of separating citral, which is available (or obtained) according to the present invention, preferably according to any one of claims 8 to 19, 31 and 32, into geranial of formula (Va-1) and neral of formula (Vb-1), i) A step of preparing optically active citronellal by asymmetric hydrogenation of citral, which is preferably available (or obtained) according to any one of claims 8 to 19, 31 and 32, to geranial of formula (Va-1) and neral of formula (Vb-1) by a method according to the present invention, ii) A step of cyclizing the optically active citronellal thus prepared in the presence of a suitable acid, preferably a Lewis acid, to obtain optically active isopulegol, iii) The present invention relates to a method comprising the step of hydrogenating the optically active isopulegol prepared in this manner to obtain optically active menthol.

[0313] Further aspects of the present invention relate to menthol which may optionally be optically active menthol, preferably L-menthol which is available (or obtained) by the method of the present invention.

[0314] The definitions and preferred embodiments described above with respect to the preparation of (iso)prenal and / or prenol, citral, and further steps and available (or obtained) products will be understood to apply to the preparation of menthol and the characterization of the product, with necessary modifications. The available (or obtained) menthol will be understood to have specific properties.

[0315] The cyclization of citronellal to isopulegol can be achieved by cyclization in the presence of at least one Lewis acidic aluminum-containing catalyst, such as a bis(diarylphenoxy)aluminum compound, which can be used in the presence of an auxiliary agent such as a carboxylic acid anhydride. The isopulegol can be recovered from the reaction product containing the catalyst by distillation separation, yielding an isopulegol-rich upper product and an isopulegol-depleted lower product. At least one catalyst can be regenerated from the lower product. The isopulegol obtained by cyclizing citronellal in this way can be further purified by suitable separation and / or purification methods, particularly by crystallization, to remove at least a large portion of undesirable impurities or by-products.

[0316] Hydrogenation of isopulegol can be achieved by hydrogenation in the presence of at least one heterogeneous nickel-containing catalyst, preferably at least one heterogeneous nickel and copper-containing catalyst.

[0317] Further details regarding the reaction sequence from citral to menthol can be found in U.S. Patent Application Publication No. 2013 / 46118A1, which is incorporated herein by reference.

[0318] Isopulegol (5-methyl-2-(1-methylethenyl)-cyclohexanol) has three chiral carbon atoms and therefore has four stereoisomers, each arising as an enantiomer pair. (1R,3R,4S)-(-)isopulegol is also known as L-isopulegol.

[0319] A further aspect of the present invention is a method for preparing isopulegol, preferably optically active isopulegol, preferably L-isopulegol, o) Optionally, a step of separating citral, which is available (or obtained) according to the present invention, preferably according to any one of claims 8 to 19, 31 and 32, into geranial of formula (Va-1) and neral of formula (Vb-1), i) A step of preparing optically active citronellal by asymmetric hydrogenation of citral, which is preferably available (or obtained) according to any one of claims 8 to 19, 31 and 32, to geranial of formula (Va-1) and neral of formula (Vb-1) by a method according to the present invention, ii) The present invention relates to a method comprising the step of cyclizing the optically active citronellal thus prepared in the presence of a suitable acid, preferably a Lewis acid, to obtain optically active isopulegol.

[0320] Further aspects of the present invention relate to isopulegol, which may optionally be optically active isopulegol, preferably L-isopulegol available (or obtainable) from the method of the present invention.

[0321] The definitions and preferred embodiments described above with respect to the preparation of (iso)prenal and / or prenol, citral, and further steps and available (or obtained) products will be understood to apply to the preparation of isopuregol and the characterization of the product, with necessary modifications. It will be understood that available (or obtained) isopuregol will have certain properties.

[0322] Accordingly, in one aspect, the present invention relates to an improved method for preparing menthol by generating citral using the above method, and then generating menthol from citral. Menthol can be prepared as described herein or by other methods known in the art.

[0323] Linalool can be prepared from citral by a method that includes catalytically hydrogenating citral to obtain nerol and / or geraniol and its isomers.

[0324] Hydrogenation of citral to obtain nerol and / or geraniol can be achieved by hydrogenation in the presence of at least one supported ruthenium, rhodium, osmium, iridium, or platinum catalyst, preferably at least one ruthenium catalyst supported on carbon black.

[0325] Isomerization of nerol and / or geraniol to obtain linalool can be achieved by isomerization in the presence of at least one tungsten catalyst, particularly a dioxotungsten(VI) complex. Further details regarding the isomerization of nerol and / or geraniol can be found in U.S. Patent No. 7,126,033B2.

[0326] Accordingly, in one aspect, the present invention relates to an improved method for preparing linalool by generating citral using the above method and then generating linalool from citral. Linalool can be prepared as described herein or by other methods known in the art.

[0327] Isopulegol (5-methyl-2-(1-methylethenyl)-cyclohexanol) has three chiral carbon atoms and therefore has four stereoisomers, each arising as an enantiomer pair. (1R,3R,4S)-(-)isopulegol is also known as L-isopulegol.

[0328] A further aspect of the present invention is a method for preparing isopulegol, preferably optically active isopulegol, preferably L-isopulegol, o) Optionally, a step of separating citral, which is available (or obtained) according to the present invention, preferably according to any one of claims 8 to 19, 31 and 32, into geranial of formula (Va-1) and neral of formula (Vb-1), i) A step of preparing optically active citronellal by asymmetric hydrogenation of citral, which is preferably available (or obtained) according to any one of claims 8 to 19, 31 and 32, to geranial of formula (Va-1) and neral of formula (Vb-1) by a method according to the present invention, ii) The present invention relates to a method comprising the step of cyclizing the optically active citronellal thus prepared in the presence of a suitable acid, preferably a Lewis acid, to obtain optically active isopulegol.

[0329] Further aspects of the present invention relate to isopulegol, which may optionally be optically active isopulegol, preferably L-isopulegol available (or obtainable) from the method of the present invention.

[0330] The definitions and preferred embodiments described above with respect to the preparation of (iso)prenal and / or prenol, citral, and further steps and available (or obtained) products will be understood to apply to the preparation of isopuregol and the characterization of the product, with necessary modifications. It will be understood that available (or obtained) isopuregol will have certain properties.

[0331] A further aspect of the present invention relates to linalool available (or obtainable) from the method of the present invention.

[0332] The definitions and preferred embodiments described above regarding the preparation of the above (iso)prenal and / or prenol, citral, further steps, and the obtainable (or obtained) products will be understood to apply, with the necessary modifications, to the preparation of linalool and the properties of the products. The obtainable (or obtained) linalool will be understood to have specific properties.

[0333] A further aspect of the invention is a method for preparing vitamin A acetate, comprising: - converting citral (VII), obtainable (or obtained) preferably according to any one of claims 8 to 19, 31 and 32 of the present invention, to pseudoionone (VIII); - reacting pseudoionone (VIII) to obtain β-ionone (IX); - converting β-ionone (IX) to β-vinyllinalool of formula (X); - phosphorylating β-vinyllinalool of formula (X) to obtain a C 15 salt; - reacting the C 15 salt of formula (XI) with the C5 acetate of formula (XII) to obtain vitamin A acetate of formula (XIII).

[0334] A further aspect of the invention relates to vitamin A obtainable (or obtained) from the method of the invention.

[0335] The definitions and preferred embodiments described above regarding the preparation of the above (iso)prenal and / or prenol, citral, further steps, and the obtainable (or obtained) products will be understood to apply, with the necessary modifications, to the preparation of vitamin A and the properties of the products. The obtainable (or obtained) vitamin A will be understood to have specific properties.

[0336] Generally, the invention further relates to a method for preparing 3,7-dimethyl-octa-2,6-dienal (citral), comprising: a) A step of reacting a formaldehyde source with isobutylene to obtain isoprenol, b) A step of isomerizing isoprenol to obtain prenol, c) A step of converting isoprenol to plenal, involving isomerization and oxidative dehydrogenation in any order, d) A step of condensing prenol with plenal to obtain a diprenyl acetal of plenal, The present invention relates to a method comprising the step of (e) subjecting the diprenyl acetal of prenal to decomposition conditions to obtain citral via prenyl(3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene.

[0337] The present invention is further illustrated by the following embodiments.

[0338] A method for preparing 1,3,7-dimethyl-octa-2,6-dienal (citral), which is: a) A step of reacting at least one formaldehyde source with isobutylene to obtain isoprenol, b) A step of isomerizing the isoprenol obtained in step a) by contacting a reaction logistics system containing isoprenol with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen, to obtain prenol. c)ci) and c-ii) ci) A step of subjecting the isoprenol obtained in step a) to oxidative dehydrogenation by contacting a reaction logistics system containing isoprenol with at least one heterogeneous oxidative dehydrogenation catalyst in the presence of molecular oxygen, thereby obtaining plenal and / or isoprenal, and optionally isomerizing at least a portion of the isoprenal to plenal. c-ii) A step of oxidizing the prenol obtained in step b) to obtain prenal by contacting a reaction flow containing prenol with at least one oxidizing agent and at least one oxidation catalyst, preferably in the presence of a liquid phase. A process of providing a planar by at least one of the following, d) A step of condensing the prenol obtained in step b) with the prenal obtained in step c) to obtain a diprenyl acetal of prenal, e) The diprenyl acetal of prenal obtained in step d) is subjected to decomposition conditions to obtain citral via prenyl(3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene, and the process includes the following steps: At least step ci) is characterized by maintaining the weight ratio of aldehyde, preferably prenal and / or formaldehyde, to isoprenol at less than 0.04 in the reaction logistics, Step b) is characterized by maintaining an aldehyde concentration in the reaction logistics of less than 0.5% by weight, preferably less than 0.4% by weight, particularly less than 0.3% by weight, or less than 0.25% by weight, based on the total weight of the reaction logistics, and the aldehyde concentration in the reaction logistics is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, based on the total weight of the reaction logistics.

[0339] 2. The method according to Embodiment 1, wherein step a) preferably includes introducing at least one formaldehyde source and isobutylene into a reactor through a plurality of nozzles optionally operating in parallel, by mixing and injection or by spraying, and reacting the formaldehyde source and isobutylene under supercritical conditions, wherein the reactor preferably includes a vertically positioned container, side walls, top and bottom.

[0340] 3. The method according to Embodiment 2, wherein a formaldehyde source and isobutylene are introduced, for example, by injection or spraying, into a mixing chamber of a reactor located at the top, and a fluid containing formaldehyde and / or isobutylene and / or isoprenol is sent from the mixing chamber to a post-reaction chamber located at the bottom.

[0341] 4. The method according to Embodiment 2 or 3, comprising the steps of providing draft tubes essentially concentrically arranged below each of the nozzles in a mixing chamber, the draft tubes providing a descending conduit inside the draft tube and an ascending conduit outside the draft tube, thereby introducing, e.g., a formaldehyde source and isobutylene introduced from the nozzles and injected or sprayed, moving generally downward through the descending conduit, and then a fluid containing formaldehyde and / or isobutylene and / or isoprenol being diverted to a generally upward path through the ascending conduit, and the fluid being back-mixed with the introduced, e.g., a formaldehyde source and isobutylene introduced or sprayed.

[0342] 5. Step a) includes mixing a formaldehyde source and isobutylene, and injecting or spraying the mixture into an internal loop reactor through at least one nozzle and into a first conduit, the internal loop reactor being -A cylindrical container arranged vertically, including side walls, -At least one draft pipe, vertically positioned within a container, having a pipe inlet end and a pipe outlet end, wherein the draft pipe is concentric with a nozzle and has an inner and outer surface, and the draft pipe provides a first conduit inside the draft pipe and a second conduit outside the draft pipe and within the side wall, the first conduit being in fluid communication with the second conduit, - Includes reactor fluid outlet means, The inner surface of the draft pipe is convexly curved such that the first conduit exhibits an annular constriction in its cross-section between the pipe inlet end and the pipe outlet end, the constriction being located closer to the pipe inlet end, and the convex curve on the inner surface of the draft pipe extends over at least 70%, preferably at least 80%, and most preferably at least 90% of the length of the draft pipe. The outer surface of the draft pipe is convexly curved such that the draft pipe exhibits a circumferential projection between the pipe inlet end and the pipe outlet end, the circumferential projection preferably located closer to the pipe outlet end, and the convex curvature of the outer surface of the draft pipe extends over at least 70%, preferably at least 80%, and most preferably at least 90% of the length of the draft pipe. The method according to any one of embodiments 1 to 4, wherein the end of the draft pipe is rounded so that a formaldehyde source introduced, e.g., injected or sprayed from a nozzle and isobutylene move generally downward through a first conduit to react and obtain a fluid, and then the reacted fluid is diverted in the opposite direction to move through a second conduit and subsequently back-mixed with the introduced, e.g., injected or sprayed fluid.

[0343] 6. Reacting a formaldehyde source with isobutylene involves heat exchange between a high-temperature isoprenol stream removed from the reactor and an isobutylene stream directed into the reactor. Heat exchange is performed in one or more shell-and-tube heat exchangers, each of which includes multiple tubes and shell-side heat exchange passages. The method according to any one of embodiments 1 to 5, wherein high-temperature isoprenol is introduced through the tubes of the heat exchanger, isobutylene is introduced through the shell-side passage, and in the case of two or more heat exchangers, at least two of the heat exchangers are connected in series with respect to both the shell-side flow and the tube-side flow.

[0344] 7. The method according to any one of Embodiments 1 to 6, wherein the isoprenol obtained in step a) is purified by distilling a crude isoprenol stream containing isoprenol, water, and formaldehyde, or the isoprenol-containing fraction thereof, in a low-boiling-point separation column operated at a pressure of 2 bara or more, preferably 2.5 bara or more, to obtain a distillation stream containing aqueous formaldehyde and a bottom stream containing isoprenol that is essentially formaldehyde-free.

[0345] 8. The method according to any one of Embodiments 1 to 7, wherein step c) comprises isomerization of isoprenol to prenol and oxidative dehydrogenation of prenol to prenal, wherein prior to oxidative dehydrogenation, prenol is treated to remove organically bound nitrogen from prenol by contacting it with a weakly acidic solid adsorbent.

[0346] 9. The method according to any one of Embodiments 1 to 8, wherein step c) comprises oxidative dehydrogenation of (iso)prenol by contacting a gaseous reaction flow containing (iso)prenol with a silver-containing heterogeneous catalyst in the presence of molecular oxygen.

[0347] 10. At least step ci) is characterized by maintaining the weight ratio of aldehyde, preferably prenal and / or formaldehyde, to isoprenol at less than 0.04 in the reaction logistics. The method according to Embodiment 9, characterized in that step b) maintains an aldehyde concentration in the reaction logistics of less than 0.5% by weight, preferably less than 0.4% by weight, particularly less than 0.3% by weight, or less than 0.25% by weight, based on the total weight of the reaction logistics, and optionally the aldehyde concentration in the reaction logistics is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, relative to the total weight of the reaction logistics.

[0348] 11. The oxidative dehydrogenation in step c) is - A reaction passage including a shell-side heat exchange passage and multiple reaction tubes for circulating a heat transfer medium, - An inlet for introducing reaction logistics into the reaction passage, - Includes an outlet from the reaction passage for recovering the discharge flow from the reaction tube, The reaction tube is The reaction preheating zone adjacent to the injection port, A reaction zone downstream of the reactant preheating zone, having a catalytically active wire matrix insert having silver on at least a portion of its surface, The method according to any one of embodiments 1 to 10, which is carried out by passing isoprenol through multiple reaction tubes of a shell-and-tube heat exchanger.

[0349] 12. The method according to Embodiment 11, wherein the catalytically active wire matrix insert includes an elongated core having a plurality of wire loops extending from an elongated core, in which case the wire loops are arranged longitudinally and helically shifted, and the wire loops include a block of silver wire.

[0350] 13. The method according to any one of Embodiments 1 to 12, wherein step d) comprises sequentially condensing prenol with plenal in the presence of at least one condensation catalyst in a reaction column, and simultaneously sequentially removing an acetal fraction containing the diprenyl acetal of plenal from the reaction column.

[0351] 14. The method according to any one of Embodiments 1 to 13, wherein step e) is to continuously subject the acetal fraction in a decomposition column to decomposition conditions in the presence of at least one decomposition catalyst, and simultaneously continuously withdraw from the decomposition column a decomposition fraction containing at least one of prenyl(3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene, and optionally containing citral, and optionally react the decomposition fraction in a plug-flow reactor to obtain citral.

[0352] 15. The method according to Embodiment 14, characterized in that the conversion rate of diprenyl acetal of prenal is maintained at more than 90% and less than 100%.

[0353] 16. The method according to Embodiment 14 or 15, wherein the decomposition catalyst is phosphoric acid, and the concentration of phosphoric acid at the bottom of the decomposition column is greater than 100 ppm and less than 1500 ppm.

[0354] 17. The method according to any one of Embodiments 14 to 16, wherein the condensation catalyst is nitric acid and the concentration of nitric acid is less than 500 ppm.

[0355] 18. This includes recycling the prenol obtained in step e) into step d), The concentration of 2,4,4-trimethyl-3-formyl-1,5-hexadiene in the prenol recycled from step e) to step d) is controlled so that the concentration of 2,4,4-trimethyl-3-formyl-1,5-hexadiene in step d) is less than 1% by weight relative to the total weight of prenol and prenal. The method according to any one of embodiments 14 to 17, wherein the concentration of citral in the prenol recycled from step e) to step d) is controlled such that the concentration of citral in step d) is less than 1% by weight relative to the total weight of prenol and prenal.

[0356] 19. The method according to any one of Embodiments 1 to 18, wherein the aldehyde in the reaction logistics consists of or contains formaldehyde, and the concentration of formaldehyde is less than 0.2% by weight, more preferably less than 0.15% by weight, particularly less than 0.1% by weight, or less than 0.05% by weight, more particularly less than 0.025% by weight, and even more particularly less than 0.02% by weight, based on the total weight of the reaction logistics, and optionally 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, based on the total weight of the reaction logistics.

[0357] 20. The method according to any one of Embodiments 1 to 19, comprising the steps of separating an unreacted isoprenol stream from a prenol-containing product stream, optionally removing at least some aldehydes, preferably some formaldehyde and / or prenal, from the unreacted isoprenol stream, and subsequently combining the unreacted isoprenol stream with a fresh isoprenol stream to form a reaction stream, wherein the concentration of aldehydes, preferably formaldehyde and / or prenal, in the reaction stream is maintained at less than 0.5% by weight, preferably less than 0.4% by weight, particularly less than 0.3% by weight, or less than 0.25% by weight, based on the total weight of the reaction stream, and the concentration of aldehydes, preferably formaldehyde and / or prenal, in the reaction stream is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, based on the total weight of the reaction stream.

[0358] 21. The method according to any one of Embodiments 1 to 19, comprising the steps of: separating an unreacted isoprenol stream from a prenol-containing product stream; combining the unreacted isoprenol stream with a crude isoprenol stream containing isoprenol, water, and an aldehyde; and removing water, preferably water and an aldehyde, from the combined stream to form a reaction stream.

[0359] 22. The step of removing aldehydes, preferably water and aldehydes, particularly water, formaldehyde and / or plenal from a combined flow, (i) The combined flow is sent to a first low-boiling-point separation column operated at a pressure of 1.5 bara or less to obtain a first bottom flow containing isoprenol and aldehydes, and a first distillation flow containing water and low-boiling-point substances, (ii) A step of sending the first bottom flow to a second low-boiling point separation column operated at a pressure of 2 bara or more to obtain a second distillation flow containing aqueous aldehydes and a second bottom flow containing isoprenol, (iii) A step of sending the second bottom flow to the finishing column to obtain a bottom flow containing high-boiling point substances and a reaction flow as a distillation flow. The method according to Embodiment 21, including the method described above.

[0360] 23. The method according to any one of Embodiments 1 to 22, wherein step a) is carried out in the presence of isoprenol, particularly a compound that can induce a further reaction of at least one Brønsted acid compound, in an amount of less than 0.5 mol%, preferably less than 0.1 mol%, more preferably less than 0.01 mol%, and most preferably less than 0.001 mol%.

[0361] 24. The method according to Embodiment 23, wherein the Brønsted acid compound is selected from the group consisting of alkali metals and alkaline earth metals, ammonia or organic amines, particularly ethylamine, trimethylamine, hexamethylenetetramine (urotropin), aniline, pyridine or piperidine hydroxides, carbonates and bicarbonates, and salts of acids weaker than formic acid.

[0362] 25. The method according to Embodiments 1 to 22, wherein in step a), the Brønsted base compound is present in an amount of 0.001 to 10% by weight, particularly 0.01 to 1% by weight, based on the total weight of the reaction mixture.

[0363] 26. The method according to Embodiments 1 to 25, wherein, in step a), the starting mixture of the reactants after the addition of the Brønsted base compound has a pH value of 7 to 11, preferably 7 to 10, and particularly 7.5 to 9, when measured at 20°C and an absolute pressure of 1 bar.

[0364] 27. The method according to Embodiments 1 to 26, wherein in step a), the formaldehyde source is an aqueous formaldehyde solution, and the Brønsted base compound is preferably supplied to the reactor and dissolved in the aqueous formaldehyde solution.

[0365] 28. The method according to Embodiments 1 to 26, wherein in step a), the formaldehyde source is an aqueous formaldehyde solution, and the Brønsted base compound is supplied separately to the reactor as an aqueous solution.

[0366] The present invention will be described by the accompanying drawings and the following embodiments. [Brief explanation of the drawing]

[0367] [Figure 1] Figure 1 shows the conversion rate and selectivity over time during the operation of the isoprenol oxidative dehydrogenation reaction. [Figure 2] Figure 2 schematically shows schemes A to D, which offer various options for handling the recycling flow of unreacted isoprenol. [Figure 3A] Figure 3A shows the relative isoprenol conversion rates during the first and second phases of Experiment A, as described below, based on online gas chromatography measurements. [Figure 3B] Figure 3B shows the relative isoprenol conversion rates during the first and second phases of Experiment B, described below, based on online gas chromatography measurements. [Figure 4]Figure 4 shows the prenol selectivity for isoprenol conversion in Experiment A below, including extrapolation of the selectivity observed in Phase 1 (data points with conversion rates of 52.5% to 55%) and Phase 2 (data points with conversion rates of 56% to 57%), as indicated by the dotted lines. [Modes for carrying out the invention]

[0368] According to the present invention, maintaining a constant weight ratio of formaldehyde to isoprenol in the reaction flow can be achieved in many ways, as shown in Figure 2. Scheme A shows how the recycled flow b is typically incorporated into prior art methods. Scheme B shows how the recycled flow b can be sent back through the isoprenol separation process of an existing isoprenol production unit. Scheme C shows the entire reactor feed flow through an additional impurity separation process, such as a distillation process. Scheme D shows only the recycled flow through an additional impurity separation process, such as a distillation process. [Examples]

[0369] Example 1 The oxidation of isoprenol is investigated in a miniplant reactor. The catalyst bed consists of 30 cm of silver-coated (5 wt%) steatite spheres (2 mm in diameter) packed inside a stainless steel tube (12 mm inner diameter). The temperature of the cooling jacket is maintained at 380°C. A 300 g / h isoprenol load is employed along with 30 g / h of water and 92 L / h of air. The reaction flow is quenched through a water cooler. The condensate is analyzed offline by gas chromatography. The uncondensed gas flow is analyzed online by gas chromatography. The conversion rate and selectivity of (iso)prenal are calculated using both analyses.

[0370] Experiments were conducted using isoprenol with formaldehyde concentrations increased to 0, 2, 3, and 4% by weight.

[0371] Figure 1 shows the conversion rate and selectivity over time during operation. The formaldehyde concentration increased to 2% by weight on day 5 and to 3% by weight on day 14. The regeneration cycle is indicated by a plus sign at the 50% conversion rate position. A decrease in selectivity is observed with increasing formaldehyde concentration. In the initial stages of operation, a decrease in selectivity of approximately 1.5 percentage points per 1% by weight of formaldehyde in the feedstream is observed. As the formaldehyde concentration increased, regeneration cycles became more frequent. At 4% by weight of formaldehyde, regeneration cycles were required even more frequently.

[0372] Example 2 Similar to Example 1, isoprenols with various concentrations of formaldehyde were subjected to isomerization. The isomerization reaction was carried out according to International Publication No. 2008 / 037693. It was also found that isomerization proceeded favorably with isoprenols with low / reduced formaldehyde content.

[0373] According to the present invention, maintaining specific concentrations of aldehydes, particularly formaldehyde, and / or prenal, particularly formaldehyde, in the reaction flow can be achieved in many ways, as shown in Figure 2. Scheme A shows how the recycled flow b is typically incorporated into prior art methods. Scheme B shows how the recycled flow b can be sent back through the isoprenol separation process of an existing isoprenol production unit. Scheme C shows the entire reactor feed flow through an additional impurity separation process, e.g., a distillation process. Scheme D shows only the recycled flow through an additional impurity separation process, e.g., a distillation process.

[0374] Experiments A and B Isoprenol batches containing aldehydes of various concentrations, namely plenal and formaldehyde, were subjected to isomerization. The composition of the isoprenol batches, as determined by gas chromatography (GC) analysis, is shown below, except for the amounts of plenal and formaldehyde in batch 1, which were provided by quality control from isoprenol production.

[0375] [Table 2]

[0376] Isomerization of isoprenol batches was performed as follows: A double-walled glass reactor was packed with 100 mL (48.8 g) of isomerization catalyst (a fixed-bed catalyst containing Pd / Se supported on SiO2). The top of the reactor was connected to a phase separator, and the bottom of the phase separator was connected to the bottom of the reactor via an external circulation pump. The reactor temperature was controlled via an external oil jacket. Before each experiment, the catalyst was dried at 120°C for 16 hours under a nitrogen stream, reduced at 120°C for 2 hours using a 1:1 (volume:volume) mixture of hydrogen and nitrogen, and then flashed with nitrogen at 70°C for 16 hours.

[0377] During startup, the feed from each isoprenol batch was pumped at a rate of 100 g / hour from the bottom to the top of the reactor in the same flow direction as the concurrently supplied hydrogen (6 Nl / hour, 1.2 bara) released through the bubble frit at the bottom of the reactor. The product flow from the top of the reactor was supplied to the phase separator.

[0378] After a 24-hour start-up phase, sufficient product accumulated in the phase separator, and recycling was initiated via a circulation pump. The feed rate was reduced to 70 g / hour, and the liquid phase of the product flow obtained in the phase separator was recycled to the bottom of the reactor at a rate of 150 g / hour. Simultaneously, the reactor pressure was increased to 1.5 bara and the temperature was raised to 80°C. These conditions were maintained for the remainder of the experiment.

[0379] During the experiment, liquid samples were taken from the product stream extracted from the top of the reactor and analyzed by offline GC. Furthermore, online GC measurements were continuously performed.

[0380] As shown below, two supply switching experiments were conducted: one batch was supplied to the reactor for approximately 6-10 days (Phase 1), then the system was switched to the other batch, and measurements were taken until a new steady state was reached (approximately 2-5 days, Phase 2). In this way, it was possible to directly compare the performance of the two batches.

[0381] [Table 3]

[0382] The conversion rate (%) of isoprenol was calculated as 100 × ([weight % of isoprenol in feed] - [weight % of isoprenol in product]) / [weight % of isoprenol in feed]. The selectivity (%) of prenol was calculated as 100 × ([weight % of prenol in product] - [weight % of prenol in feed]) / ([weight % of isoprenol in feed] - [weight % of isoprenol in product]). The prenol yield was calculated as (prenol selectivity × isoprenol conversion rate) / 100.

[0383] The average isoprenol conversion rates determined via offline GC are shown in the table below.

[0384] [Table 4]

[0385] When high-purity isoprenol from batch 2 was used as the feedstock, the catalyst was found to be more active, and the conversion rate of isoprenol was higher than when batch 1 was fed. Furthermore, the yield of isoprenol was higher. The higher conversion rate is also evident from the results of online GC measurements, as shown in Figures 3A and 3B. The above figures also indicate that the deactivation rate was greater when batch 1 was fed, which suggests that the catalyst lost its activity more rapidly in the presence of isoprenol from this batch.

[0386] Furthermore, as shown in Figure 4, it was found that the prenol selectivity when using high-purity isoprenol from batch 2, when extrapolated to the isoprenol conversion rate, was higher than that of isoprenol from batch 1.

[0387] Figure 4 shows prenol selectivity as a function of isoprenol conversion rate, as determined by offline GC data from Experiment A. Square dots represent isoprenol batch 1 (standard quality isoprenol), and circular dots represent isoprenol batch 2 (high-purity isoprenol). Offline GC analysis performed when supplying isoprenol batch 1 (red dots in Figure 4) can be well fitted to a linear trend line. This trend line shows that, under these conditions, prenol selectivity increased at a rate of 2.5% selectivity per 1% conversion rate as the isoprenol conversion rate decreased along with the long-term function of time on stream (TOS). Extrapolating this trend line to the higher conversion rate achieved with the high-purity feed reveals a predicted selectivity of approximately 86% for prenol. However, prenol selectivity of over 89% was recorded with isoprenol batch 2, the high-purity feed (circular dots in Figure 4), indicating that a more selective catalytic reaction was achieved after switching to the higher-purity feed.

[0388] Interestingly, using batch 2, which had lower aldehyde levels compared to batch 1 during the startup phase, resulted in a higher catalyst activity level from the start. Consequently, the conversion rates after approximately 24 hours, before the first phase, and during the first phase were higher when batch 2 was used from the start, compared to experiments where batch 1 was used from the start and through the first phase.

Claims

1. A method for preparing plenal, comprising the steps of contacting a reaction logistics system containing isoprenol with at least one heterogeneous oxidative dehydrogenation catalyst in the presence of molecular oxygen to oxidative dehydrogenate isoprenol to obtain plenal and / or isoprenal, and optionally isomerizing at least a portion of the isoprenal to plenal, A method comprising the step of optionally maintaining the weight ratio of aldehyde to isoprenol at less than 0.04 in the reaction logistics.

2. The method according to claim 1, wherein the isoprenol is obtained by reacting at least one formaldehyde source with isobutylene.

3. A method for preparing a diprenyl acetal of prenal, comprising the step of condensing prenal obtained according to claim 1 or 2 with prenol.

4. The method according to claim 3, wherein the prenol is obtained by isomerizing the isoprenol by contacting a reaction logistics system containing isoprenol with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen.

5. The method according to claim 4, wherein the isoprenol is obtained by reacting at least one formaldehyde source with isobutylene.

6. The method according to claim 4 or 5, wherein the step of isomerizing isoprenol is characterized by maintaining an aldehyde concentration of less than 0.5% by weight, preferably less than 0.4% by weight, particularly less than 0.3% by weight, or less than 0.25% by weight in the reaction logistics, based on the total weight of the reaction logistics, and optionally the aldehyde concentration in the reaction logistics is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, based on the total weight of the reaction logistics.

7. a) A step of reacting at least one formaldehyde source with isobutylene to obtain isoprenol, b) A step of isomerizing the isoprenol obtained in step a) by contacting a reaction logistics system containing isoprenol with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen, to obtain prenol. c) A step of supplying the planar according to claim 1 or 2, The method according to any one of claims 3 to 6, further comprising the step of d) condensing the prenol obtained in step b) with the prenol obtained in step c) to obtain a diprenyl acetal of prenal.

8. A method for preparing 3,7-dimethyl-octa-2,6-dienal (citral), comprising the step of subjecting a diprenyl acetal of prenal obtained according to any one of claims 3 to 7 to decomposition conditions to obtain citral via prenyl(3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene.

9. A method for preparing 3,7-dimethyl-octa-2,6-dienal (citral), comprising: a) A step of reacting at least one formaldehyde source with isobutylene to obtain isoprenol, b) A step of isomerizing the isoprenol obtained in step a) by contacting a reaction logistics system containing isoprenol with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen, to obtain prenol. c) c-i) and c-ii) c-i) A step of subjecting the isoprenol obtained in step a) to oxidative dehydrogenation by contacting a reaction logistics system containing isoprenol with at least one heterogeneous oxidative dehydrogenation catalyst in the presence of molecular oxygen, thereby obtaining plenal and / or isoprenal, and optionally isomerizing at least a portion of the isoprenol to plenal. c-ii) A step of oxidizing the prenol obtained in step b) to obtain prenal by contacting a reaction flow containing prenol with at least one oxidizing agent and at least one oxidation catalyst, preferably in the presence of a liquid phase. A process of providing a planar by at least one of the following, d) A step of condensing the prenol obtained in step b) with the prenal obtained in step c) to obtain a diprenyl acetal of prenal, e) The diprenyl acetal of prenal obtained in step d) is subjected to decomposition conditions to obtain citral via prenyl(3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene, and the process includes these steps. Step c-i) is characterized by maintaining the weight ratio of aldehyde to isoprenol at less than 0.04 in the reaction logistics, The method is characterized in that, optionally, step b) maintains an aldehyde concentration in the reaction logistics of less than 0.5% by weight, preferably less than 0.4% by weight, particularly less than 0.3% by weight, or less than 0.25% by weight, based on the total weight of the reaction logistics, and optionally, the aldehyde concentration in the reaction logistics is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, based on the total weight of the reaction logistics.

10. The method according to any one of claims 2 to 9, wherein the step of reacting at least one formaldehyde source with isobutylene to obtain isoprenol, preferably step a) according to any one of claims 7 to 9, comprises introducing the at least one formaldehyde source and isobutylene into a reactor and reacting the formaldehyde source and isobutylene under supercritical conditions.

11. A step of reacting at least one formaldehyde source with isobutylene to obtain isoprenol, preferably step a) according to any one of claims 7 to 10, includes introducing the formaldehyde source and isobutylene into an internal loop reactor through at least one nozzle and into a first conduit, wherein the internal loop reactor - A cylindrical container arranged vertically, including its side walls, - At least one draft pipe, vertically arranged within the container, having a pipe inlet end and a pipe outlet end, wherein the draft pipe is concentric with the nozzle and has an inner and outer surface, and the draft pipe provides the first conduit within the draft pipe and the second conduit outside the draft pipe and within the side wall, and the first conduit is in fluid communication with the second conduit, - Includes reactor fluid outlet means, The inner surface of the draft pipe is curved convexly such that the first conduit shows an annular narrowing in the cross-section between the pipe inlet end and the pipe outlet end, the narrowing is located closer to the pipe inlet end, and the convex curve of the inner surface of the draft pipe extends over at least 70%, preferably at least 80%, and most preferably at least 90% of the length of the draft pipe. The outer surface of the draft pipe is curved convexly such that the draft pipe exhibits a circumferential projection between the pipe inlet end and the pipe outlet end, the circumferential projection is preferably located closer to the pipe outlet end, and the convex curvature of the outer surface of the draft pipe extends over at least 70%, preferably at least 80%, and most preferably at least 90% of the length of the draft pipe. The method according to any one of claims 2 to 10, wherein the formaldehyde source and isobutylene introduced from the nozzle move generally downward through the first conduit to react and obtain a fluid, and the end of the draft pipe is rounded so that the reacted fluid is then diverted in the opposite direction to move through the second conduit and subsequently mixed with the introduced fluid.

12. The reaction of the formaldehyde source with isobutylene includes heat exchange between the high-temperature isoprenol stream removed from the reactor and the isobutylene stream directed towards the reactor. Heat exchange is performed in one or more shell-and-tube heat exchangers, each of which includes a plurality of tubes and shell-side heat exchange passages. The method according to any one of claims 2 to 11, wherein the high-temperature isoprenol is introduced through the tube of the heat exchanger, the isobutylene is introduced through the shell-side passage, and in the case of two or more heat exchangers, at least two of the heat exchangers are connected in series with respect to both the shell-side flow and the tube-side flow.

13. The method according to any one of claims 1 to 12, wherein the isoprenol obtained preferably according to claim 2, and more preferably in step a) according to any one of claims 7 to 12, is purified by distillation of a crude isoprenol stream containing isoprenol, water and formaldehyde, or the isoprenol-containing fraction thereof, in a low-boiling-point separation column operated at a pressure of 2 bara or more, preferably 2.5 bara or more, to obtain a distillation stream containing aqueous formaldehyde and a bottom stream containing isoprenol that is essentially formaldehyde-free.

14. The method according to any one of claims 1 to 13, wherein the reaction logistics comprising isoprenol, preferably the reaction logistics in step c-i) according to any one of claims 7 to 13, is a gas, and at least one heterogeneous oxidative dehydrogenation catalyst is a silver-containing heterogeneous oxidative dehydrogenation catalyst.

15. Preferably, the oxidative dehydrogenation in step c-i) described in any one of claims 7 to 14 is - A shell-side heat exchange passage for circulating a heat transfer medium and a reaction passage including multiple reaction tubes, - An inlet for introducing the reaction logistics into the reaction passage, - Includes an outlet from the reaction passage for recovering the discharge flow from the reaction tube, The reaction tube is A reactant preheating zone adjacent to the inlet, The method according to any one of claims 1 to 14, wherein the isoprenol is passed through a plurality of reaction tubes of a shell-and-tube heat exchanger, which includes a reaction zone downstream of the reactant preheating zone, and which has a catalytically active wire matrix insert having silver on at least a portion of its surface.

16. The method according to claim 15, wherein the catalytically active wire matrix insert includes an elongated core having a plurality of wire loops extending from the elongated core, wherein the wire loops are arranged longitudinally and helically shifted, and the wire loops include a block of silver wire.

17. Preferably, the step of condensing prenol with plenal according to step d) of any one of claims 7 to 16 comprises continuously condensing prenol with plenal in a reaction column in the presence of at least one condensation catalyst, and simultaneously continuously removing an acetal fraction containing the diprenyl acetal of plenal from the reaction column, according to any one of claims 3 to 16.

18. Preferably, the step of obtaining citral according to step e) of any one of claims 9 to 17 comprises: continuously subjecting the acetal fraction in a decomposition column to decomposition conditions in the presence of at least one decomposition catalyst, and simultaneously continuously withdrawing from the decomposition column a decomposition fraction containing citral and optionally containing at least one of prenyl(3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene; and optionally reacting the decomposition fraction in a plug-flow reactor to obtain citral, according to any one of claims 8 to 17.

19. Preferably, this includes recycling the prenol obtained in step e) and / or step d) according to any one of claims 7 to 18, The concentration of 2,4,4-trimethyl-3-formyl-1,5-hexadiene in the prenol recycled from step e) to step d) is controlled such that the concentration of 2,4,4-trimethyl-3-formyl-1,5-hexadiene in step d) is less than 1% by weight relative to the total weight of prenol and prenal. The method according to claim 18, wherein the concentration of citral in the prenol recycled from step e) to step d) is controlled such that the concentration of citral in step d) is less than 1% by weight relative to the total weight of prenol and prenal.

20. A method for preparing prenol, preferably in the presence of hydrogen, by contacting a reaction logistics pathway containing isoprenol with at least one heterogeneous isomerization catalyst to obtain a prenol-containing product pathway, characterized in that the concentration of aldehyde in the reaction logistics pathway is maintained at less than 0.5% by weight, preferably less than 0.4% by weight, particularly less than 0.3% by weight, or less than 0.25% by weight, based on the total weight of the reaction logistics pathway, wherein the concentration of aldehyde in the reaction logistics pathway is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, relative to the total weight of the reaction logistics pathway.

21. The method according to claim 20, wherein the isoprenol is obtained by reacting at least one formaldehyde source with isobutylene.

22. The method according to claim 21, wherein the aldehyde in the reaction logistics consists of or contains formaldehyde, and the concentration of the formaldehyde is less than 0.2% by weight, more preferably less than 0.15% by weight, particularly less than 0.1% by weight, or less than 0.05% by weight, more particularly less than 0.025% by weight, and even more particularly less than 0.02% by weight, based on the total weight of the reaction logistics, but is 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, based on the total weight of the reaction logistics.

23. The method according to any one of claims 20 to 22, comprising the steps of: separating an unreacted isoprenol stream from the prenol-containing product stream; optionally removing at least some aldehydes, preferably some formaldehyde and / or prenal, from the unreacted isoprenol stream; and subsequently combining the unreacted isoprenol stream with a fresh isoprenol stream to form the reaction stream.

24. The method according to claim 20 or 23, comprising the steps of: separating an unreacted isoprenol stream from the prenol-containing product stream; mixing the unreacted isoprenol stream with a crude isoprenol stream containing isoprenol, water, and an aldehyde; and removing water, preferably water and an aldehyde, from the mixed stream to form the reaction stream.

25. The step of removing aldehydes from the mixed flow is as follows: (i) A step of sending the mixed flow to a first low-boiling-point separation column operated at a pressure of 1.5 bar or less to obtain a first bottom flow containing isoprenol and aldehydes and a first distillation flow containing water and low-boiling-point substances, (ii) A step of sending the first bottom flow to a second low-boiling point separation column operated at a pressure of 2 bar or more to obtain a second distillation flow containing aqueous aldehydes and a second bottom flow containing isoprenol, (iii) A step of sending the second bottom flow to a finishing column to obtain a bottom flow containing high-boiling point substances and the reaction flow as a distillation flow. The method according to claim 24, including the method described in claim 24.

26. A method for preparing a diprenyl acetal of prenal, comprising the step of condensing prenol obtained according to any one of claims 20 to 25 with prenal.

27. The method according to claim 26, wherein the prenal is obtained by contacting a reaction logistics system containing isoprenol with at least one heterogeneous oxidative dehydrogenation catalyst in the presence of molecular oxygen to oxidative dehydrogenate isoprenol to obtain prenal and / or isoprenal, and optionally isomerizing at least a portion of the isoprenal to prenal.

28. The method according to claim 27, wherein the isoprenol is obtained by reacting at least one formaldehyde source with isobutylene.

29. The method according to any one of claims 26 to 28, characterized in that the reaction logistics containing isoprenol maintains a weight ratio of aldehyde to isoprenol of less than 0.04 in the reaction logistics.

30. a) A step of reacting at least one formaldehyde source with isobutylene to obtain isoprenol, b) A step of isomerizing the isoprenol obtained in step a) by contacting a reaction logistics system containing isoprenol with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen, to obtain prenol. c) c-i) and c-ii) c-i) A step of subjecting the isoprenol obtained in step a) to oxidation and dehydrogenation by contacting a reaction logistics system containing isoprenol with at least one heterogeneous oxidative dehydrogenation catalyst in the presence of molecular oxygen, thereby obtaining plenal and / or isoprenal, and optionally isomerizing at least a portion of the isoprenal to plenal. c-ii) A step of oxidizing the prenol obtained in step b) to obtain prenal by contacting a reaction flow containing prenol with at least one oxidizing agent and at least one oxidation catalyst, preferably in the presence of a liquid phase. A process of providing a planar by at least one of the following, The method according to any one of claims 20 to 29, comprising the step of d) condensing the prenol obtained in step b) with the prenol obtained in step c) to obtain a diprenyl acetal of prenal.

31. A method for preparing 3,7-dimethyl-octa-2,6-dienal (citral), comprising the step of subjecting a diprenyl acetal of prenal obtained according to any one of claims 26 to 30 to decomposition conditions to obtain citral via prenyl(3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene.

32. A method for preparing 3,7-dimethyl-octa-2,6-dienal (citral), comprising: a) A step of reacting at least one formaldehyde source with isobutylene to obtain isoprenol, b) A step of isomerizing the isoprenol obtained in step a) by contacting a reaction logistics system containing isoprenol with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen, to obtain prenol. c) c-i) and c-ii) c-i) A step of subjecting the isoprenol obtained in step a) to oxidation and dehydrogenation by contacting a reaction logistics system containing isoprenol with at least one heterogeneous oxidative dehydrogenation catalyst in the presence of molecular oxygen, thereby obtaining plenal and / or isoprenal, and optionally isomerizing at least a portion of the isoprenal to plenal. c-ii) A step of oxidizing the prenol obtained in step b) to obtain prenal by contacting a reaction flow containing prenol with at least one oxidizing agent and at least one oxidation catalyst, preferably in the presence of a liquid phase. A process of providing a planar by at least one of the following, d) A step of condensing the prenol obtained in step b) with the prenal obtained in step c) to obtain a diprenyl acetal of prenal, e) The diprenyl acetal of prenal obtained in step d) is subjected to decomposition conditions to obtain citral via prenyl(3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene, and the process includes these steps. Step b) is characterized by maintaining an aldehyde concentration in the reaction logistics of less than 0.5% by weight, preferably less than 0.4% by weight, particularly less than 0.3% by weight, or less than 0.25% by weight, based on the total weight of the reaction logistics, and optionally the aldehyde concentration in the reaction logistics of 10 ppm or more, preferably 25 ppm or more, particularly 50 ppm or more, or 100 ppm or more, based on the total weight of the reaction logistics. The method is characterized in that, optionally, step c-i) maintains the weight ratio of aldehyde to isoprenol at less than 0.04 in the reaction logistics.

33. The method according to claim 32, further characterized by any one of the steps described in any one of claims 10 to 19.

34. A product stream containing plenal, available according to any one of claims 1, 2, and 13-16.

35. A product stream containing prenol, available according to any one of claims 20 to 25.

36. A product stream containing a diprenyl acetal of prenal, available according to any one of claims 3-7, 10-17, and 26-30.

37. A citral-containing product stream available according to any one of claims 8 to 19, 31, and 32.

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