Improved process for preparing 3,7-dimethyl-octa-2,6-dienal

Abstract

The process for preparing 3,7-dimethyl-octa-2,6-dienal (citral) comprises: a) continuously condensing prenol with prenal in the presence of at least one catalyst in a reaction column in which the water of condensation is distilled off as a prenal-water azeotrope as vapor, wherein the vapor is at least partially condensed, the condensate is separated into an aqueous phase and an organic phase, the organic phase is partially introduced into the reaction column as reflux, and the organic phase is partially discharged as a purge stream, while an acetal fraction containing the diprenyl acetal of prenal is continuously removed from the reaction column, the reaction temperature is less than 100°C, the catalyst is nitric acid, and the concentration of nitric acid is less than 500 ppm; and b) continuously condensing prenol with at least one catalyst in which the water of condensation is distilled off as a prenal-water azeotrope as vapor, the condensate is at least partially condensed, the organic phase is partially introduced into the reaction column as reflux, and the organic phase is partially discharged as a purge stream, while an acetal fraction containing the diprenyl acetal of prenal is continuously removed from the reaction column, the reaction temperature is less than 100°C, the catalyst is nitric acid, and the concentration of nitric acid is less than 500 ppm. The method includes the steps of: continuously subjecting the acetal fraction in the cracking column to cracking conditions in the presence of (a) and (b) continuously withdrawing a cracked fraction containing at least one of prenyl(3-methylbutadienyl)ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene, and optionally citral, from the cracking column; wherein the conversion rate of the diprenyl acetal of prenal in step b) is maintained at more than 90% and less than 100%, and the unreacted diprenyl acetal is at least partially contained in the withdrawn cracked fraction; c) reacting the cracked fraction in a plug flow reactor to obtain citral; and d) recycling a portion of the prenol obtained in step b) to step a). This process can suppress the formation of unwanted by-products during the production of citral.

Description

[Technical Field]

[0001] The present invention relates to an improved process for preparing citral (3,7-dimethyl-octa-2,6-dienal), which can provide citral in high yield. Citral is a mixture of the isomeric compounds neral and geranial. Citral is a valuable intermediate for the production of various odorants and fragrances, such as geraniol. Furthermore, citral is becoming increasingly important as a starting material for the production of vitamins, especially vitamin A. [Background technology]

[0002] German Patent Application Publication No. 198 46 056 describes a process for preparing citral, which comprises pyrolyzing 3-methyl-2-buten-1-al-diprenyl acetal, optionally in the presence of an acidic catalyst, to cis / trans-prenyl-(3-methyl-butadienyl) ether, with decomposition of 3-methyl-2-buten-1-ol (prenol), and then subjecting the butadienyl ether to Claisen rearrangement to 2,4,4-trimethyl-3-formyl-1,5-hexadiene, followed by Cope rearrangement to obtain citral. Prenol, intermediates, and citral are continuously distilled from the reaction mixture.

[0003] WO 2008 / 037693 discloses a method for producing citral, the method comprising: a) a process for producing 3-methyl-3-buten-1-ol (isoprenol) from isobutylene and formaldehyde; b) a process for producing 3-methyl-2-butenal (prenal) and 3-methyl-3-butenal (isoprenal) from 3-methyl-3-buten-1-ol (isoprenol) by oxidative dehydrogenation with an oxygen-containing gas over a silver-supported catalyst; c) producing additional 3-methyl-2-butenal (prenal) from a mixture containing 3-methyl-3-butenal (isoprenal) by isomerization; d) a process for producing 3-methyl-2-buten-1-ol (prenol) by isomerization from 3-methyl-3-buten-1-ol (isoprenol); e) a process for producing the unsaturated acetal 3-methyl-2-butenal-diprenyl acetal from 3-methyl-2-buten-1-ol (prenol) and 3-methyl-2-butenal (prenal) using an acidic catalyst; f) obtaining citral from 3-methyl-2-butenal-diprenyl acetal by decomposition and subsequent rearrangement; Includes.

[0004] This complex multi-step process is prone to undesirable side reactions that reduce the achievable citral yield. In other words, individual steps may exhibit less than 100% selectivity, resulting in the formation of greater amounts of by-products than desired. Such by-products reduce the desired selectivity of the conversion and generally must be removed from the citral product prior to further use. A significant amount of energy is required to separate the by-products from the citral, typically resulting in significant losses of citral. Such losses can make the use of an otherwise advantageous reaction sequence commercially unattractive.

[0005] To date, relatively little is known about the nature of unwanted by-products and the mechanisms of their formation.

[0006] Therefore, the present invention aims to suggest reaction conditions that effectively inhibit the formation of undesired by-products during the production of citral, and to remove the by-products that are inevitably formed without impairing the formation of citral and its components.

[0007] This problem is solved by the following process and its preferred embodiment. Summary of the Invention [Means for solving the problem]

[0008] The present invention provides a process for preparing 3,7-dimethyl-octa-2,6-dienal (citral), comprising: a) continuous condensation of prenol with prenal in the presence of at least one catalyst in a reaction column in which condensation water is distilled off as a prenal-water azeotrope as vapor, wherein the vapor is at least partially condensed, the condensate is separated into an aqueous phase and an organic phase, the organic phase is partially introduced into the reaction column as reflux, and the organic phase is partially discharged as a purge stream, while an acetal fraction containing the diprenyl acetal of prenal is continuously removed from the reaction column, the reaction temperature is less than 100°C, the catalyst is nitric acid, and the concentration of nitric acid is less than 500 ppm; b) continuously subjecting the acetal fraction in the cracking column to cracking conditions in the presence of at least one catalyst for removing prenol, and simultaneously continuously removing a cracked fraction from the cracking column comprising at least one of prenyl(3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene, and optionally citral, wherein the conversion of the diprenyl acetal of prenal in step b) is maintained at more than 90% and less than 100%, and the unreacted diprenyl acetal is at least partially contained in the removed cracked fraction; c) reacting the decomposition fraction in a plug flow reactor to obtain citral; d) recycling a portion of the prenol obtained in step b) to step a); The present invention relates to a process including: [Brief explanation of the drawings]

[0009] [Figure 1] The evolution of the residual concentration of by-product 5 versus diprenyl acetal at each of the temperatures and catalyst concentrations tested (see Tables 1 to 7) is shown in Figure 1. From Figure 1 it can be seen that for all temperatures and catalyst concentrations, the formation of by-product 5 increases sharply as the diprenyl acetal content approaches 0%, i.e., as the conversion approaches 100%. [Figure 2]Figure 2 shows the concentration of diprenyl acetal over time for each of the temperatures and catalyst concentrations tested (see Tables 1-7). It can be seen that the higher the temperature and catalyst concentration, the faster the reaction proceeds. [Figure 3] Figure 3 shows the concentration of Σ citral components (prenyl(3-methyl-butadienyl) ether, 2,4,4-trimethyl-3-formyl-1,5-hexadiene, and citral) over time for each of the temperatures and catalyst concentrations tested (see Tables 1-7). Figure 3 shows the initial increase in the concentration of Σ citral components. For runs at temperatures above 160°C, the content of Σ citral components reaches a maximum and then decreases. [Figure 4] Figure 4 shows the concentration of by-product 5 over time for each of the temperatures and catalyst concentrations tested (see Tables 1-7). From Figure 4, it can be seen that the formation of by-product 5 is initially slow, followed by a rapid increase in the concentration of by-product 5. From Figure 4, it can be seen that the formation of by-product 5 begins earlier and is steeper with increasing decomposition temperature and phosphoric acid concentration. DETAILED DESCRIPTION OF THE INVENTION

[0010] The overall reaction sequence is shown by the following reaction scheme: [ka]

[0011] In step a), a catalyst is used to form the unsaturated acetal 3-methyl-2-butenal-diprenyl acetal (hereinafter referred to as "diprenyl acetal of prenal" or "diprenyl acetal"). For this purpose, prenal is reacted with prenol in the presence of a catalytic amount of nitric acid, the nitric acid concentration is less than 500 ppm, the reaction temperature is less than 100°C, and the water produced during the reaction is separated in a reaction column.

[0012] In step b), the 3-methyl-2-butenal diprenyl acetal (diprenyl acetal) obtained in step a) is decomposed in a decomposition column in the presence of a catalyst to remove 3-methyl-2-buten-1-ol (prenol) and obtain prenyl (3-methylbutadienyl) ether. The Claisen rearrangement of the obtained prenyl (3-methylbutadienyl) ether gives 2,4,4-trimethyl-3-formyl-1,5-hexadiene, followed by Cope rearrangement to give 3,7-dimethyl-2,6-octadienal (citral).

[0013] To make the process economically viable, the prenol obtained in step b) is recycled to step a). However, it has now been discovered that unwanted by-products can accumulate in the recycle stream. The problem is that this may not be apparent until a large amount of by-products has accumulated in the recycle loop. Therefore, the present invention proposes purging unwanted by-products by discharging a portion of the organic entrainer liquid (i.e., the condensation distillate after removal of the aqueous phase) to the top of the reaction column where prenol condenses with prenal. The by-products are concentrated in this organic entrainer liquid, thereby minimizing the loss of valuables.

[0014] The structures of by-products that may accumulate in the recycle loop from step b) to step a) were elucidated and identified as by-products 1a and 1b shown below.

[0015] In a preferred embodiment, the rate of the purge flow is [ka] is maintained at a rate of less than 7% by weight, preferably between 2 and 7% by weight, in the feed to step a).

[0016] "Feed to step a)" means the sum of the feed of fresh prenol and prenal and recycled prenol from step b).

[0017] Under adverse reaction conditions, the following by-products 2, 3, 4, 5a and 5b formed in steps b) and c) have been identified: [ka]

[0018] Surprisingly, it has been found that when the conversion rate of the diprenyl acetal of prenal in step b) reaches complete conversion, the concentration of by-products increases sharply.According to the present invention, the conversion rate of the diprenyl acetal of prenal in step b) is maintained at more than 90% and less than 100%.Preferably, the conversion rate of the diprenyl acetal of prenal 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 the diprenyl acetal of prenal in step b) is maintained at more than 91%, for example 92% or more, or 93% or more, or 94% or more, or 95% or more.In a preferred embodiment, the conversion rate of the diprenyl acetal of prenal in step b) is more than 94% and less than 99%, for example 95% and less than 98%. If the conversion rate is low, the process becomes economically unprofitable, or otherwise requires the recovery and recycling of unreacted diprenyl acetal.However, complete conversion in step b) is undesirable, since it will result in a decrease in the yield of citral components and an increase in the formation of by-products.The conversion rate is affected by various parameters, including the decomposition temperature, nature and concentration of the catalyst in step b) and the residence time in step b), i.e., in the decomposition column.

[0019] It is also a feature of the present invention that the distillation conditions in the cracking column are effective to at least partially distill off unreacted diprenyl acetal.

[0020] The acetal fraction is continuously subjected to cracking conditions in a cracking column. By "cracking conditions" is meant reaction conditions selected such that the diprenyl acetal contained in the acetal fraction can be cracked to prenyl (3-methylbutadienyl) ether, which can then be rearranged to 2,4,4-trimethyl-3-formyl-1,5-hexadiene and citral.

[0021] The acetal fraction contains diprenyl acetal as the main component. The acetal fraction does not necessarily consist of pure diprenyl acetal, but may also contain prenol, prenal, and citral components. In this regard, it is desirable to maintain the diprenyl acetal content in the acetal fraction above a certain level, for example, to avoid excessive carryover of prenol and prenal to downstream reaction steps and subsequent separation of the latter. Furthermore, in this regard, ensuring that the diprenyl acetal content is appropriately high in the acetal fraction also means that prenol-derived by-products, such as by-product 1 exemplified below, are minimized. Thus, in a preferred embodiment, the acetal fraction contains at least 65 wt. % diprenyl acetal, preferably at least 75 wt. %, for example, 75-90 wt. %.

[0022] As the diprenyl acetal content in the acetal fraction increases beyond a certain point, a point of diminishing returns is reached. An economic balance must be struck between the improvement from increasing content and the cost of achieving such content. The temperature stress caused by additional separation steps and increased residence time can even lead to decomposition of the diprenyl acetal, possibly caused by trace amounts of the acetalization catalyst.

[0023] Because decomposition of diprenyl acetal can already occur to some extent under the conditions of the condensation reaction between prenal and prenol, attempting to complete the condensation can result in the premature formation of citral components (i.e., resulting from the reaction of the citral components with prenal and prenol) that can lead to the undesired formation of by-products 3 and 5.

[0024] Step b) is carried out in a cracking column. Preferably, the cracking column is a distillation column equipped with an evaporator and a condenser. Suitable internals for the cracking column are trays, packings, and in particular structured packings made of sheet metal or metal mesh. The number of theoretical plates in the cracking column can range from 5 to 100.

[0025] In one embodiment, the decomposition temperature in step b) is greater than 150°C and less than 200°C, preferably greater than 155°C and less than 180°C.

[0026] Step b) is carried out in the presence of a catalyst, preferably an acid catalyst. The catalyst can be a single catalyst type or a combination of two or more different catalyst types. 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 in step b) is 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. A higher concentration of the (acid) catalyst may result in a decrease in the yield of the citral component.

[0027] Preferably, the continuous cracking in the cracking column of step b) can be carried out in the lower part or sump of the distillation column that functions as the cracking column. Preferably, the acetal fraction and / or catalyst are introduced into the lower part of the distillation column, the sump of the distillation column, or the evaporator of the distillation column. If necessary, the volume of the sump of the cracking column can be increased by a vessel to provide a larger reaction volume.

[0028] Typically, the bottoms from the cracking column are a mixture of high boiling materials composed of diprenyl acetal and C5 oligomers resulting from the thermal instability of the citral building blocks.

[0029] If necessary, a high-boiling inert compound can be introduced into the sump of the cracking column to ensure a minimum filling level of the sump and evaporator.Suitable high-boiling inert compounds are selected from liquid compounds that are inert under reaction conditions and have a boiling point higher than that of 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 a mixture thereof.

[0030] A portion of the bottoms is continuously removed from the cracking column, which helps to avoid the accumulation of high boilers. Since it is an important feature of the present invention that the cracking of diprenyl acetal in step b) is carried out at less than complete conversion, the portion of the bottoms removed from the cracking column may also contain unreacted diprenyl acetal.

[0031] Preferably, the distillation conditions are selected so that the diprenyl acetal is primarily retained in the bottom or sump of the distillation column. During the cracking reaction, a cracking fraction is continuously removed from the cracking column, and this cracking 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 building blocks." This is because the former is an intermediate in the reaction pathway to citral and can be converted to citral in the subsequent step c).

[0032] In step b), the conversion rate of the diprenyl acetal of prenal is maintained below complete conversion, and the unreacted diprenyl acetal is at least partially contained in the decomposition fraction removed. The concentration of the unreacted diprenyl acetal in the decomposition fraction can be 1 to 6 wt.% relative to the citral component.

[0033] In addition, the prenol formed during the decomposition reaction of step b) is generally continuously removed from the reaction mixture at the top of the decomposition column.

[0034] The cracked fraction can be removed from the top of the distillation column together with the prenol formed.

[0035] Alternatively and preferably, the cracked fraction can be withdrawn in liquid or vapor form from a side draw of the distillation column. The side draw is preferably located in the middle or lower part of the cracking column, in particular 2 to 20 theoretical plates above the addition point of the acetal fraction. Preferably, the cracking column has 2 to 80 theoretical plates above the side draw.

[0036] The feed rate of the acetal fraction is adjusted to control the residence time of the diprenyl acetal in step b). In one embodiment, the residence time in step b) is greater than 5 minutes and less than 90 minutes, preferably greater than 15 minutes and less than 80 minutes. Residence times in this interval represent a good trade-off between the yield of the reaction product and minimizing the formation of by-products.

[0037] In step c), the decomposition fraction is 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 of 100 to 200°C, which is suitable for carrying out the rearrangement reaction to produce citral.

[0038] Applicants have found that the selectivity and yield of the cracking reaction can be increased by using a highly backmixed cracking column in combination with a plug flow reactor, with all of the catalyst required for the cracking reaction preferably being introduced into the cracking column in step b), and preferably no catalyst being introduced into the plug flow reactor.

[0039] The acetal fraction to be subjected to step b) is formed from prenol and prenal in step a). According to the present invention, in step a), prenol is continuously condensed with prenal in a reaction column in the presence of at least one catalyst, while the acetal fraction of prenal is continuously removed from the reaction column. Further optimization of this process relates to the conditions of step a).

[0040] In step a), the formation of the following by-products has been observed under adverse reaction conditions: [ka]

[0041] The preparation of unsaturated acetals by reacting an olefinically unsaturated aliphatic compound with allyl alcohol in a reaction column in the presence of a distillable acid is known per se. For this purpose, a mixture of at least 2 moles of prenol and 1 mole of prenal can be introduced into the reaction column, while the water formed during the reaction is distilled off overhead and removed by a phase separator. The diprenyl acetal can then be removed as a crude product from the bottom of the reaction column or from an evaporator. Due to the thermal instability of diprenyl acetal, extensive purification of the crude diprenyl acetal is generally undesirable. However, concentration of the crude diprenyl acetal, for example, in a short-path evaporator, can be advantageous. This removes unreacted prenal and prenol from the crude diprenyl acetal, which is then sent to a cracking column. Preferably, the crude diprenyl acetal sent to the cracking column is essentially free of aldehydes.

[0042] Preferably, the apparatus for preparing unsaturated acetals comprises a distillation column used as a reaction column. The vapor rising at the top of the reaction column is condensed in a condenser and sent to a phase separation vessel, where water separates as a lower phase. The upper phase mainly consists of organic compounds such as unreacted aldehyde (i.e., prenal), unreacted alcohol (i.e., prenol), and low-boiling secondary compounds (e.g., prenol formate). The majority of the organic phase is recycled to the top of the reaction column as reflux, and a smaller portion is discharged to remove minor components.

[0043] Suitably, the reflux amount per 1000 kg of freshly added aldehyde is in the range of 200 kg to 50,000 kg, preferably 1,000 kg to 20,000 kg. Depending on the purity of the feed, the amount of the discharged portion per 1000 kg of freshly added aldehyde is in the range of 1 kg to 400 kg, preferably 5 kg to 200 kg.

[0044] The reaction temperature in step a) is less than 100°C, preferably in the range of 70 to 80°C. In one embodiment, the reaction pressure in step a) is less than 150 mbar, preferably in the range of 90 to 110 mbar. Suitably, the residence time of the reaction mixture in the reactor is in the range of 0.1 seconds to 10 hours, preferably in the range of 60 seconds to 2 hours.

[0045] According to the present invention, step a) is carried out in the presence of a catalyst, which is nitric acid. The concentration of nitric acid is less than 500 ppm, more preferably in the range of 100 to 300 ppm, based on the total amount of the starting materials prenol and prenal. A lower amount of (acid) catalyst may result in a lower conversion rate in the reaction column. A higher amount of (acid) catalyst may disadvantageously result in increased formation of by-products and reduced selectivity.

[0046] Suitably, the catalyst is added to the reaction column, preferably at the bottom of the reaction column. In a preferred embodiment, the catalyst is added to the evaporator. It is also possible to add nitric acid at different locations, for example at two or more points within the reaction column.

[0047] The point of addition of fresh aldehyde and / or fresh alcohol is not critical. The aldehyde and alcohol may be added separately at different points in the reaction column. Preferably, the aldehyde and alcohol are mixed with the effluent from the condenser. The amount of freshly added alcohol is controlled so that the ratio of alcohol to aldehyde is in the range of 1 to 3, preferably 1.5 to 2.5.

[0048] In step d), the prenol removed in step b) is recycled to step a), thereby achieving an improved yield in the process of the present invention.

[0049] The applicants have found that it is important to control the concentration of contaminants contained in the prenol recycled to step a). Specifically, the concentration of 2,4,4-trimethyl-3-formyl-1,5-hexadiene in the prenol recycled from step b) to step a) is preferably controlled so that the concentration of 2,4,4-trimethyl-3-formyl-1,5-hexadiene in step a) is less than 1 wt %, preferably less than 0.5 wt %, based on the total weight of prenol and prenal. Similarly, the concentration of citral in the prenol recycled from step b) to step a) is preferably controlled so that the concentration of citral in step a) is less than 1 wt %, preferably less than 0.1 wt %, based on the total weight of prenol and prenal.

[0050] The inventors have found that it is advantageous to control the amount of 2,4,4-trimethyl-3-formyl-1,5-hexadiene and / or citral introduced into step a) together with the recycled prenol from step b), as higher concentrations of 2,4,4-trimethyl-3-formyl-1,5-hexadiene and / or citral as described above can lead to the formation of by-products.

[0051] Prenols useful as starting materials for the present invention can be obtained by reacting isobutylene with at least one formaldehyde source in a reactor, typically under elevated temperature and pressure, to produce 3-methylbut-3-en-1-ol (isoprenol), and isomerizing the resulting isoprenol.

[0052] In one embodiment, isoprenol is obtained by mixing at least one formaldehyde source with isobutylene, injecting the resulting mixture into a reactor through at least one nozzle, and reacting the at least one formaldehyde source with isobutylene under supercritical conditions. To achieve supercritical conditions, the formaldehyde and isobutylene are preferably reacted at a temperature of at least 220°C, e.g., in the range of 220-290°C, and at an absolute pressure of at least 200 bara. The reaction of isobutene with at least one formaldehyde can be carried out in the presence of a catalyst, such as an amine base, e.g., hexamethylenetetramine (urotropine).

[0053] In one embodiment, isoprenol is produced by mixing at least one formaldehyde source and isobutylene and injecting the resulting mixture into a reactor through multiple nozzles operated in parallel, where the reactor comprises a vertically disposed vessel, a sidewall, an upper portion, and a lower portion, the formaldehyde source and isobutylene are injected into a mixing chamber of the upper portion of the reactor, and a fluid comprising formaldehyde and / or isoprenol is released from the mixing chamber to a post-reaction reactor located in the lower portion. A draft tube is delivered to the chamber and disposed essentially concentrically below each of the nozzles in the mixing chamber, the draft tube providing a descending conduit inside the draft tube and an ascending conduit outside the draft tube, whereby the formaldehyde source and isobutylene injected through the nozzles travel generally downward in the descending conduit, and then the fluid containing formaldehyde and / or isobutylene and / or isoprenol is diverted to a generally upward path in the ascending conduit, whereby the fluid is backmixed with the injected formaldehyde source and isobutylene.

[0054] The formaldehyde may be provided as a liquid, for example as a solution of paraformaldehyde. Preferably, the at least one formaldehyde source comprises or is an aqueous formaldehyde solution.

[0055] Although initial rapid and vigorous mixing of the reactants is desirable, it may be advantageous to continue the reaction to completion under conditions where backmixing is limited. Thus, the reaction mixture can be sent to a post-reaction chamber located after or below the reactor, where backmixing is limited.

[0056] In one embodiment, the reactor includes an upper and lower portion, where reactants are injected and mixed in a mixing chamber of the reactor located in the upper portion, and a fluid containing formaldehyde and / or isobutylene and / or isoprenol is passed from the mixing chamber to a post-reaction chamber located in the lower portion.

[0057] Further details regarding reacting at least one formaldehyde source with isobutylene to obtain isoprenol can be found in WO 2020 / 049111 A1.

[0058] In one embodiment, reacting the at least one formaldehyde source with isobutylene comprises mixing the at least one formaldehyde source with isobutylene and injecting the isobutylene into an internal loop reactor through at least one nozzle and into a first conduit, the internal loop reactor comprising: a vertically disposed cylindrical container including a sidewall; - at least one draft tube vertically disposed within the vessel, the draft tube having a tube inlet end and a tube outlet end, the draft tube being concentrically disposed with the nozzle and having an inner surface and an outer surface, the draft tube providing a first conduit within the draft tube and a second conduit on the exterior of the draft tube and within a sidewall, the first conduit being in fluid communication with the second conduit; - reactor fluid outlet means; Including, the inner surface of the draft tube is convexly curved such that the first conduit exhibits an annular constriction in cross section between the tube inlet end and the tube outlet end, the constriction being located closer to the tube inlet end, and the convex curvature of the inner surface of the draft tube extends over at least 70%, preferably at least 80%, and most preferably at least 90% of the length of the draft tube; the outer surface of the draft tube is convexly curved such that the draft tube exhibits a circumferential protrusion between the tube inlet end and the tube outlet end, the circumferential protrusion preferably being located closer to the tube outlet end, and the convex curvature of the outer surface of the draft tube extends over at least 70%, preferably at least 80%, and most preferably at least 90% of the length of the draft tube; The end of the draft tube is rounded so that at least one formaldehyde source and isobutylene injected through a nozzle travel generally downwardly within a first conduit to obtain a reacted fluid, which is then diverted in the opposite direction to travel through a second conduit and then backmixed with the injected fluid.

[0059] This draft tube configuration allows for control of the boundary layer flowing at the end of the draft tube. When the angle of attack of the flow relative to the solid body reaches a certain limit, the adverse pressure gradient becomes too great for the flow to overcome. The flow then separates from the top surface of the solid body, a condition commonly referred to as stall. This configuration allows for reduced or delayed flow separation, respectively. Reduced flow separation reduces fluid friction, thereby reducing the pressure drop along the recirculating flow streamlines, which in turn results in an improved circulation rate for the configuration. The curved shape of the inner surface of the draft tube wall guides the fluid through the draft tube in an optimized manner, mimicking the fluid flow over an airfoil.

[0060] The inner surface of the draft tube is curved in the longitudinal direction of the draft tube, i.e., has a convex shape, so that the first conduit presents a minimum cross-sectional area between the tube inlet end and the tube outlet end, i.e., the cross-section of the first conduit decreases from the cross-section at the tube inlet end to the minimum cross-sectional area and increases from the minimum cross-sectional area to the cross-section at the tube outlet end.

[0061] The draft tube has a curved, generally conical section between the tube inlet end and the constriction, which is wider at the inlet end and narrower at the constriction. At least a portion of the fluid flowing downstream through the draft tube is deflected to flow along the inner surface of the draft tube until the draft tube terminates. Because the flow through the tube remains primarily attached, little pressure loss occurs. Near the constriction, the fluid flowing downstream through the draft tube accelerates. Between the constriction and the tube outlet end, the cross-sectional area of ​​the draft tube expands again. As a result, due to the change in area, and in conjunction with mass conservation, the velocity through a larger area is slower than the velocity through a smaller area, resulting in the conversion of dynamic pressure to static pressure. As the fluid flowing downstream through the draft tube accelerates near the constriction, a radial velocity component is added to the flow, increasing mixing between the recirculation flow and the inlet flow. By avoiding flow separation in this case, no significant pressure loss occurs.

[0062] 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 the injection velocities of these two jets are preferably different. In this embodiment, the isobutylene jet has a large shear plane toward both the central jet of the at least one formaldehyde source and the reaction mixture in the reactor, allowing for favorable high-speed mixing of the reactants.

[0063] In a preferred embodiment, the loop reactor comprises a deflection means arranged between the nozzle and the draft tube, the deflection means being suitable for deflecting the fluid traveling in the second conduit in the opposite direction.

[0064] The deflection means preferably comprises a surface that is concave relative to the end of the draft tube that defines the tube inlet end. In a preferred embodiment, the deflection means has a partial toroidal surface. It is particularly preferred that the deflection means is provided in the shape of the upper part of a ring torus bisected by a plane parallel to the toroidal direction. This shape allows for a particularly efficient deflection of the fluid moving in the second conduit. The deflection means may allow for stabilization of the injection 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 may lead to eccentricity of the injection fluid flow. If left unaddressed, such eccentricity may reduce the circulation rate.

[0065] When the first conduit is a descending conduit and the second conduit is an ascending conduit, the deflection means preferably has a shape such that the upper portion of the ring torus is bisected by a plane parallel to the toroidal direction, the ring torus being bisected at 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 deflection means has a shape such that the upper portion of the ring torus is bisected by a plane parallel to the toroidal direction, the ring torus being bisected at at most 85% of its height, for example at most 80% of its height. In these ranges, the inlet of the deflection means is angled in a way that is particularly suitable for deflecting the fluid.

[0066] The reaction of formaldehyde and isobutylene to produce isoprenol in high yields requires high temperatures. Effective heat removal is important for product quality and process safety. The heat removed from the isoprenol is used to raise the temperature of the isoprenol before it enters the reactor. The hot isoprenol stream contains sensible heat from the chemical reaction. Sensible heat is potentially renewable energy and can be reused.

[0067] Advantageously, reacting at least one formaldehyde source with isobutylene preferably comprises heat exchanging a hot isoprenol stream removed from the reactor with an isobutylene stream directed to the reactor, the heat exchange occurring in at least two shell-and-tube heat exchangers, each of the heat exchangers comprising a plurality of tubes and shell-side heat exchange passages, the hot isoprenol being conducted through the tubes of the heat exchanger and the isobutylene being conducted through the shell-side passages, and at least two of the heat exchangers being connected in series for both the shell-side and tube-side streams.

[0068] Such a configuration allows for longer operating intervals between maintenance breaks in such processes. The term "maintenance break" is intended to mean the periodic process shutdown required to clean heat exchanger tubes clogged with fouling. An indicator of the need for a maintenance break is typically when the isobutylene exiting the last heat exchanger is not sufficiently preheated, and even subsequent heaters are unable to add much additional external heat to the isobutylene to bring it to the required temperature before it enters the reactor. With the current configuration, the preheat of the isobutylene stream can be maintained at a sufficiently high level for a longer period of time so that the desired isobutylene temperature can be easily reached before it enters the reactor.

[0069] One particular area prone to fouling in conventional shell-and-tube heat exchangers is the tube region near the tube sheet, near the inlet where the tube-side fluid exits the individual tubes. Excessive fouling in this area can result in clogging of individual tubes and stagnation of fluid along the entire length of these tubes. Fluid stagnation generally results in reduced heat transfer performance.

[0070] A further consequence of the reduced heat transfer performance caused by fouling is the increased energy required in the heater to adjust the temperature of the preheated isobutylene stream to the desired reaction temperature, resulting in more additional external heat being required, which is detrimental in terms of energy demand and process economics, and often negatively impacts the carbon dioxide footprint of the product.

[0071] The use of two or more heat exchangers reduces the impact of fouling in individual tubes on the overall heat exchange capacity compared to configurations where only a single heat exchanger is used. As a result, the heat transfer rate is maintained at a desired level for a longer period of time, thereby increasing the operational interval between maintenance breaks and requiring less additional external heat to preheat the isobutylene stream compared to a plant with a single heat exchanger in a heavily fouled state.

[0072] Separating isoprenol from unreacted formaldehyde is not a simple task. This difficulty arises from the fact that monomeric formaldehyde (and polymeric formaldehyde) forms both hydrates with water and hemiformals with isoprenol. The hydrates and hemiformals, which have different degrees of polymerization of formaldehyde, have mixed boiling points.

[0073] However, it has been found that formaldehyde can be substantially completely separated from isoprenol by distillation at a temperature at which hemiformal decomposes into formaldehyde and isoprenol, and as a result, formaldehyde can be easily separated from isoprenol.

[0074] Thus, crude isoprenol can be purified by distilling a crude isoprenol stream comprising isoprenol, water, and formaldehyde, or an isoprenol-containing fraction thereof, in a light-boiling separation column operated at a pressure of at least 2 bara, preferably at least 2.5 bara, to obtain a distillation stream comprising aqueous formaldehyde and a bottoms stream comprising essentially formaldehyde-free isoprenol.

[0075] In particular, it has been found that formaldehyde can be substantially completely separated from isoprenol, and that a concentrated aqueous formaldehyde solution suitable for recycle to isoprenol synthesis can be obtained in a distillation train involving a first distillation at a temperature at which the equilibrium is shifted toward the hemiformal of formaldehyde and isoprenol, so that essentially all of the formaldehyde remains in the bottom of the distillation column, and a second distillation at a temperature at which the hemiformal is decomposed into formaldehyde and isoprenol, so that the formaldehyde can be easily separated from the isoprenol.

[0076] Two light-end separation columns are envisioned, operated at different pressures to allow a first distillation below the dissociation temperature of isoprenol-formaldehyde and a second distillation above the dissociation temperature of isoprenol-formaldehyde. Thus, at the relatively low pressure prevailing in the first light-end separation column, a first distillate containing water and low boilers essentially free of formaldehyde is obtained. At the relatively high pressure prevailing in the second light-end separation column, substantially all of the formaldehyde is separated from the isoprenol. This process allows for the production of isoprenol essentially free of formaldehyde.

[0077] Thus, in a more preferred embodiment, the purification process comprises: (i) passing said stream of crude isoprenol to a first light-end cut column operated at a pressure of 1.5 bara or less to obtain a first bottoms stream comprising isoprenol and formaldehyde, and a first distillation stream comprising water and light boilers; (ii) passing the first bottoms stream to a second light-end cut column operated at a pressure of at least 2 bara to obtain a second distillation stream comprising aqueous formaldehyde and a second bottoms stream comprising isoprenol; (iii) sending the second bottoms stream to a finishing column to obtain pure isoprenol as a distillate stream and a bottoms stream comprising high boilers; Includes.

[0078] The second distillation stream constitutes a concentrated aqueous formaldehyde solution suitable for recycle to isoprenol synthesis.

[0079] The second light-ends cut column is suitably operated at a pressure of 2.5 bara or more, preferably 2.8 bara or more, and most preferably 2.9 bara or more. The bottom temperature of the second light-ends cut 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 light-ends cut column is preferably in the range of 115 to 160°C, and more preferably 125 to 145°C.

[0080] In a particularly preferred embodiment, the second light end cut column is operated at a pressure in the range of 2.9 to 3.5 bara, a bottom temperature in the range of 175 to 180°C, and an overhead temperature in the range of 130 to 140°C.

[0081] Further information regarding processes for recovering essentially formaldehyde-free isoprenol can be found in WO 2022 / 189652 A1.

[0082] The resulting isoprenol can be subjected to catalytic isomerization by contacting the reactant stream containing isoprenol with at least one heterogeneous isomerization catalyst to obtain prenol.

[0083] The isomerization of isoprenol to 3-methyl-2-buten-1-ol (prenol) can be carried out over 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. 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 are described in WO 2008 / 037693.

[0084] When isoprenol is subjected to catalytic isomerization, it may be preferred to maintain the weight ratio of formaldehyde to isoprenol in the reactant stream at less than 0.04, preferably less than 0.03, especially less than 0.02, or less than 0.01. In even more preferred embodiments, the weight ratio of formaldehyde to isoprenol is maintained at less than 0.002, or less than 0.001.

[0085] It has been found that the presence of formaldehyde in the reactant stream is detrimental to the activity and selectivity of the process and can promote catalyst deactivation and / or poisoning.

[0086] Hydrogen is required to isomerize isoprenol to prenol. The mechanism of poisoning resulting from the presence of formaldehyde is thought to involve the dehydrogenation of formaldehyde to carbon monoxide, which is more strongly chemisorbed on the catalyst than hydrogen.

[0087] A further cause of catalyst deactivation that may occur in combination with the aforementioned causes of catalyst poisoning is the formation of paraformaldehyde or trioxane, which can deposit in solid form on the catalyst and shield the catalytically active surface from the isoprenol being treated, leading to the gradual deactivation of the catalyst.

[0088] By-products such as prenal or isoamyl alcohol that are typically included in isoprenol due to the manufacturing process are expected to have little or no effect on the performance of the isomerization reaction.

[0089] The weight ratio of formaldehyde to isoprenol in the reactant stream can be maintained below a certain level. However, if the weight ratio of formaldehyde to isoprenol in the reactant stream is reduced beyond a certain point, a point of rapid decline is reached. Formaldehyde removal requires additional equipment and operating costs. An economic balance must be struck between the improvements from reducing the ratio and the costs of maintaining such a ratio. Therefore, the weight ratio of formaldehyde to isoprenol is preferably 0.0005 or greater, or in some cases 0.005 or greater.

[0090] The presence of formaldehyde in the reactant stream comes from two main sources: Formaldehyde can be present in the fresh feed stream sent to the reactor as an impurity from the isoprenol production process. Any formaldehyde that cannot be separated in the post-isoprenol synthesis purification process ends up in the reactant stream.

[0091] In addition, formaldehyde is also produced in the system, and some of the isoprenol is decomposed back to isobutene and formaldehyde.

[0092] Because the double bond isomerization of isoprenol is an equilibrium reaction, conversion is necessarily incomplete. For economic operation of the process, unconverted isoprenol must be removed and recycled. If steps are not taken to purify the stream containing unreacted isoprenol, recycling of isoprenol can unintentionally (re)introduce formaldehyde into the isomerization step.

[0093] Reducing the weight ratio of formaldehyde to isoprenol in the reactant stream can be accomplished in several different ways: In one embodiment, formaldehyde is removed from the unreacted isoprenol stream before it is combined with the fresh feed stream.

[0094] In one embodiment, the unreacted isoprenol stream is mixed with a fresh feed stream and formaldehyde is removed from the combined stream.

[0095] Alternatively, a quantity of a sufficiently purified fresh feed stream can be mixed with the unreacted isoprenol stream to obtain the desired weight ratio of formaldehyde to isoprenol in the mixed stream.

[0096] Formaldehyde can be removed from the isoprenol stream by conventional separation methods such as distillation, selective adsorption and / or selective reaction, particularly the purification process involving pressure swing distillation described above.

[0097] Isoprenols, particularly those obtained above, can be converted to prenal by isomerization and oxidative dehydrogenation in any order. Thus, it is possible to first isomerize isoprenol to prenol, and then oxidize prenol to prenal; or it is possible to first oxidatively dehydrogenate isoprenol to isoprenal, and optionally isomerize at least a portion of the isoprenal to prenal.

[0098] Prenol, particularly prenol obtained as described above, can be oxidized to obtain prenal by contacting a reactant stream containing prenol with at least one oxidizing agent and at least one oxidation catalyst, preferably in the presence of a liquid phase.

[0099] Suitable oxidizing agents include hydrogen peroxide and oxygen, especially oxygen.

[0100] The oxidation is preferably carried out using oxygen as an oxidizing agent in the presence of a liquid phase. The liquid phase 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, based on the total weight of the liquid phase, measured at a temperature of 20° C. and a pressure of 1 bar. These conditions have been found to enable a simple and efficient process for preparing prenal from prenol.

[0101] The 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, the at least one oxidation catalyst comprises platinum. In a preferred embodiment, the at least one oxidation catalyst is a supported catalyst.

[0102] The oxidation is suitably carried out at a temperature of from 20° C. to 100° C., preferably from 20° C. to 70° C. The oxidation is suitably carried out under an oxygen partial pressure of from 0.2 to 8 bar.

[0103] The oxidative dehydrogenation of isoprenol typically involves contacting a reactant stream, particularly a gaseous reactant stream 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 an active layer of silver. Alternatively, a bulk (all-metallic) silver body may be used.

[0104] In one embodiment, the oxidative dehydrogenation is a shell-side heat exchange passage for circulating a heat transfer medium and a reaction passage including a plurality of reaction tubes; an inlet for introducing a reactant stream into the reaction passage; an outlet from the reaction passage for recovering the exhaust stream from the reaction tube; Including, The reaction tube is a reactant preheat zone adjacent the inlet; a reaction zone downstream of the reactant preheat zone, the reaction zone having a catalytically active wire matrix insert having silver on at least a portion of its surface; This is done by passing isoprenol through multiple reaction tubes in a shell-and-tube heat exchanger containing

[0105] The term "reactant preheat zone" refers to a section of the reactor tube, i.e., a section within the reactor tube where essentially no catalytic oxidative dehydrogenation reaction occurs and where the gas flow through the reactor tube exchanges heat with a circulating heat transfer medium through the tube walls. The preheat zone, upstream of the reaction zone, involves a net heat flow into the reactor tube and ensures that the reactant streams are sufficiently heated to a temperature close to or at the reaction temperature when they reach the reaction zone.

[0106] Upon contact with the catalyst surface, the oxidative dehydrogenation reaction is initiated immediately. Otherwise, if a "cold" reactant stream reaches the catalyst surface, which does not reach the reaction's initiation temperature, coke formation can occur. Less coke formation advantageously allows for longer reactor runs without the need to burn off the coke from the catalyst surface.

[0107] Preferably, the reactant preheat zone is adapted to allow laminar flow of the reactants within the reactant preheat zone, i.e., the reactant preheat zone does not have any obstructions to the reactant flow that would cause a transition from laminar to turbulent flow. Thus, the reactant preheat zone preferably has an essentially free cross section, i.e., the preheat zone is empty.

[0108] In the case of an "essentially free cross section," the reactant preheat zone can be empty. Alternatively, the reactant preheat zone can contain a fixture made of a material with zero or limited catalytic activity, the fixture having a negligible cross section in a plane perpendicular to the longitudinal axis of the reaction tube. Such a fixture can be attached to a catalytically active wire matrix present in the reaction zone, allowing such wire matrix insert to be easily placed in or removed from the reaction zone. For example, the optional attachment can be a stainless steel wire or rod.

[0109] This setup allows heating of only that portion of the total reactant stream that flows near the hot reactor tube walls. As a result, the portion of the reactant stream flowing through the center of the reactor tube is not heated to the reaction temperature, thus reducing or even avoiding blind reactions of unstable starting materials. A "blind reaction" is a non-selective oxidation reaction that occurs in the absence of a catalyst. Once the reactant stream reaches the reaction zone, an oxidative dehydrogenation reaction is initiated. Because this reaction is exothermic, energy is released, and the remainder of the reactant stream 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, thus improving selectivity.

[0110] Alternatively, the reactant preheat zone may have a wire matrix insert with zero or limited catalytic activity. The wire matrix insert can reduce or eliminate temperature gradients without creating any flow disturbances that would promote turbulent flow characteristics. A wire matrix insert is considered to have zero catalytic activity (or, in other words, "inactive") if it does not catalyze the gas-phase partial oxidation reaction of interest to any significant extent and does not significantly change the chemical composition of the flow passing through the wire matrix insert. Similarly, a matrix insert is considered to have limited catalytic activity if its catalytic activity is lower than the activity of the reaction zone. In one embodiment, a wire matrix insert with zero or limited catalytic activity is made of an inert material, preferably stainless steel.

[0111] As used herein, the term "reaction zone" refers to the region of a reactor tube where the catalytic gas-phase partial oxidation reaction occurs. The reaction zone contains a catalytically active wire matrix insert bearing a catalytically active noble metal on at least a portion of its surface. The wire matrix contained in the reaction zone has a more open structure than packing of individual elements, allowing a large portion of the heat of reaction to be radiated to the reactor tube wall and not dissipated by the reactant flow. The unique flow characteristics of the reactant flow through the reactor tube fitted with the wire matrix insert improve heat transfer through the tube wall. This avoids the formation of significant hot spots. This in turn avoids the deposition of organic components of the reactant flow on the surface of the active catalytic material and the associated pressure drop. Overall, this reduces the need for periodic maintenance in the form of catalyst regeneration and / or replacement. This allows for increased annual operating hours and maximized utilization of existing production capacity, reducing operating costs and increasing profits.

[0112] In contrast to discrete catalyst bodies, wire matrix inserts can be formed continuously or integrally, thus greatly facilitating placement and removal of wire matrix inserts from the catalyst containment region of the reactor tube.

[0113] A "reaction zone" can consist of a single, continuous reaction zone, or it can include alternating regions with catalytically active wire matrix inserts and regions with essentially free cross-sections or wire matrix inserts with no or limited catalytic activity.

[0114] A "wire matrix insert" is understood to be a free-standing skeletal structure made of coiled, curved, or crimped metal wires adapted to be inserted into the reactor tube of a shell-and-tube reactor. A wire matrix insert is a structure with a larger volume than the longitudinal wires.

[0115] A fixture, such as a stainless steel wire or rod, can be attached to the wire matrix insert, allowing the wire matrix insert to be easily placed in or removed from the reaction zone.

[0116] In one embodiment, the catalytically active wire matrix insert includes an elongated core having a plurality of wire loops extending therefrom, the wire loops being longitudinally arranged and helically shifted, i.e., adjacent wire loops having an angular offset. The loops may be formed by helically bending 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 twisted together to form a core wire winding, and the wire loops are received within the core wire winding.

[0117] The wire loop can be formed from a single wire or from one or more intertwined wires, preferably four intertwined wires.

[0118] The wire matrix insert contained in the reaction zone has catalytically active precious metal silver on at least a portion of its surface. The wire constituting the wire loop can be a block silver wire or a silver-coated wire. The core wire can be made of brass alloy or high-grade steel. The thickness of the silver coating layer superimposed on the core surface is, for example, 10 μm. However, block silver wire generally has a longer service life and is preferred. When the wire loop is formed from multiple entangled wires, at least one of the entangled wires can be made of block silver wire or a silver-coated wire, while the other entangled wires can be made of an inert material.

[0119] A silver wire having the same composition throughout its cross section and containing at least 92.5% Ag by weight is suitable. The silver wire is helically bent to form a wire loop and combined with at least two longitudinal core wire members, which are twisted together to form a core wire winding, and the wire loop is housed in the core wire winding. The longitudinal core wire members can also be silver wire or an inert metal wire.

[0120] In a preferred embodiment, the catalytically active wire matrix insert comprises an elongated core having a plurality of wire loops extending therefrom, the wire loops being longitudinally arranged and helically shifted, the wire loops comprising block silver wire.

[0121] Generally, the catalytically active wire matrix insert has a cylindrical envelope with a diameter that matches the inner diameter of the reactor tube. This includes situations where the diameter of the cylindrical envelope of the undeployed wire matrix insert is slightly larger than the inner diameter of the reactor tube. The resilience or elasticity of the wire matrix insert allows it to be inserted into the reactor tube with only slight back pressure so that the wire loops fit snugly against the inner wall of the reactor tube.

[0122] Suitable constructions of wire matrix inserts are known per se, see, for example, GB 2097910. Some inserts of this type are disclosed in GB 1570530. Other inserts, and their manufacturing process, are disclosed in GB 2097910A. Matrix inserts are commercially available from the company Cal Gavin Ltd., England, and are sold under the trade name HiTRAN®.

[0123] When isoprenol is subjected to an oxidative dehydrogenation reaction, it may be preferred to maintain the weight ratio of formaldehyde to isoprenol in the reactant stream at less than 0.04, preferably less than 0.03, especially less than 0.02, or less than 0.01. In even more preferred embodiments, the weight ratio of formaldehyde to isoprenol is maintained at less than 0.002, or less than 0.001.

[0124] The weight ratio of formaldehyde to isoprenol in the reactant stream can be maintained below a certain level. However, if the weight ratio of formaldehyde to isoprenol in the reactant stream is reduced beyond a certain point, a point of rapid decline is reached. Formaldehyde removal requires additional equipment and operating costs. An economic balance must be struck between the improvements from reducing the ratio and the costs of maintaining such a ratio. Therefore, the weight ratio of formaldehyde to isoprenol is preferably 0.0005 or greater, or in some cases 0.005 or greater.

[0125] Reactor plugging and increased pressure drop were found to be significantly affected by the presence of formaldehyde in the reactant stream. Catalyst fouling reactions by condensation and polymerization are believed to be the primary reactions responsible for the formation of carbon or coke on the catalyst. This carbon formation is believed to involve the thermal condensation of formaldehyde or formaldehyde with the olefinic hydrocarbons isoprenol and (iso)prenal. In the presence of the catalyst, the primary condensation products undergo dehydrogenation and polymerization-type reactions, deposit on the catalyst, and tend to undergo further dehydrogenation and decomposition until a carbonaceous deposit forms.

[0126] The presence of formaldehyde in the reactant stream comes from two main sources: Formaldehyde can be present in the fresh feed stream sent to the reactor as an impurity from the isoprenol production process. Any formaldehyde that cannot be separated in the post-isoprenol synthesis purification process ends up in the reactant stream.

[0127] In addition, formaldehyde is also produced in the process. A portion of the isoprenol is decomposed back to isobutene and formaldehyde. Because most continuous industrial processes operate with a single-pass conversion level of 50-60% and a recycle of unconverted isoprenol, formaldehyde may be present in the unconverted isoprenol recycle stream unless a purification step is taken to purify the stream containing unreacted isoprenol. It has now been discovered that the unconverted isoprenol recycle stream is the largest source of formaldehyde contamination in the reactant stream. The process is typically run at partial conversion, e.g., 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 to provide the reactant stream. The unreacted isoprenol stream contains isoprenol as the major component, but may also contain prenal, isoprenal, isoamyl alcohol, isovaleraldehyde, isovaleric acid, prenol, formaldehyde, etc. It may also contain trace amounts of other C3 and C2 aldehydes and acids.

[0128] Reducing the weight ratio of formaldehyde to isoprenol in the reactant stream can be accomplished in several different ways: In one embodiment, formaldehyde is removed from the unreacted isoprenol stream before it is combined with the fresh feed stream.

[0129] In one embodiment, the unreacted isoprenol stream is mixed with a fresh feed stream and formaldehyde is removed from the combined stream.

[0130] Alternatively, a quantity of a sufficiently purified fresh feed stream can be mixed with the unreacted isoprenol stream to obtain the desired weight ratio of formaldehyde to isoprenol in the mixed stream.

[0131] Formaldehyde can be removed from the isoprenol stream by conventional separation methods such as distillation, selective adsorption and / or selective reaction, particularly the purification process involving pressure swing distillation described above.

[0132] It may be advantageous to treat (iso)prenol to remove organically bound nitrogen 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, respectively. In other words, this process can remove organically bound nitrogen from (iso)prenol.

[0133] The term "organically bound nitrogen" is intended to refer 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. An amine of particular practical interest is hexamethylenetetramine (urotropine). (Iso)prenols can contain approximately 5 to 30 ppm of organically bound nitrogen.

[0134] We found that the weakly acidic solid adsorbent was capable of adsorbing organically bound nitrogen in the presence of abundant (iso)prenol without interfering with the reactive carbon-carbon double bond.

[0135] The weakly acidic adsorbent may comprise an adsorbent material having sufficient acidity to adsorb organically bound nitrogen from (iso)prenol. In one embodiment, the solid adsorbent is a cross-linked resin having phosphonic acid functionality. Preferably, the resin polymer is a vinyl aromatic copolymer, preferably cross-linked polystyrene, more preferably a polystyrene-divinylbenzene copolymer. Other polymers having phosphonic acid functionality may also be used. Preferably, the cross-linked resin having phosphonic acid functionality is macroporous. A preferred solid adsorbent is Purolite S956.

[0136] The resin is typically used in the form of beads and packed into a column. (Iso)prenol passes through the column while in contact with the resin beads. During contact, organically bound nitrogen in the (iso)prenol reacts with functional groups, resulting in a proton transfer to the nitrogen and an ionic bond formation at the anionic sites of the resin. Contact is maintained until a threshold level, i.e., breakthrough concentration, is reached. At this point, the process reaches equilibrium and additional organically bound nitrogen can no longer be effectively removed. The flow is stopped and the column is backwashed with water, preferably deionized or softened water. The backflow fluidizes the resin and loosens and removes solids captured by the beads.

[0137] In another embodiment, the solid adsorbent is silica-alumina hydrate. Numerous silica-alumina catalyst compositions and processes for their preparation are described in the patent literature, see, for example, U.S. Pat. No. 4,499,197.

[0138] Preferably, the alumina content of the silica-alumina hydrate is about 10 to about 90% by weight Al2O3, with a preferred range of alumina content being about 30 to about 70% by weight Al2O3.

[0139] When silicon dioxide is introduced into aluminum oxide, acidic centers are introduced. 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 a maximum number of acidic centers, and after reaching the maximum number of acidic centers, the number decreases again with further increase in the amount of silicon dioxide.

[0140] An example of a commercially available silica-alumina hydrate is Siral®, available from Sasol Germany GmbH, Hamburg, Germany. Siral® is based on orthorhombic aluminum oxide hydroxide (boehmite, AlOOH) and is doped with SiO2. Various grades of Siral® are available with different ratios of Al2O3 to SiO2: Siral 1 (Al2O3 / SiO2 = 99 / 1), Siral 5 (Al2O3 / SiO2 = 95 / 5), Siral 10 (Al2O3 / SiO2 = 90 / 10), Siral 20 (Al2O3 / SiO2 = 80 / 20), Siral 28M (Al2O3 / SiO2 = 72 / 28), Siral 30 (Al2O3 / SiO2 = 70 / 30), and Siral 40 (Al2O3 / SiO2 = 60 / 40). Siral 40 is particularly preferred.

[0141] In one embodiment, (iso)prenol is passed over a bed of a weakly acidic solid adsorbent. The aforementioned "passing over a bed" step preferably refers to providing a layer ("bed") of the weakly acidic solid adsorbent in a typical reaction vessel known to those skilled in the art, which may be equipped with a stirring device, such as a stirred-tank reactor. The (iso)prenol is then introduced into the reaction vessel and guided through the reaction vessel to contact the weakly acidic solid adsorbent.

[0142] Alternatively, the weakly acidic solid adsorbent can be provided, for example, in the reaction tube of a tubular reactor, and then (iso)prenol flows continuously through said reaction tube while contacting with the weakly acidic solid adsorbent.

[0143] In one embodiment, the (iso)prenol contains less than 2 ppm of organically bound nitrogen after contacting the alcohol stream with a weakly acidic solid adsorbent. As used herein, "ppm" refers to ppm by weight of compounds incorporating organically bound nitrogen relative to the total weight of the (iso)prenol.

[0144] Preferably, the content of organically bound nitrogen in (iso)prenols can be determined by Kjeldahl analysis. Alternatively, an oxidative combustion method with a chemiluminescence detector according to DIN 51444 can be used.

[0145] Citral is a useful intermediate for, for example, menthol or linalool.

[0146] Menthol is - catalytically hydrogenating citral to obtain citronellal; - cyclization of citronellal in the presence of at least one acidic catalyst to obtain isopulegol; - catalytic hydrogenation of isopulegol to obtain menthol.

[0147] The overall reaction sequence is shown by the following reaction scheme: [ka]

[0148] Hydrogenation of citral to give citronellal can be achieved by hydrogenation in the presence of a rhodium-phosphine catalyst.

[0149] 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, such as a carboxylic acid anhydride. Isopulegol can be recovered from the catalyst-containing reaction product by distillation, yielding an isopulegol-rich top product and an isopulegol-depleted bottom product. At least one catalyst can be regenerated from the bottom product. The isopulegol obtained by cyclizing citronellal in this way can be further purified by appropriate separation and / or purification methods, in particular by crystallization, to remove at least a large portion of undesired impurities or by-products.

[0150] The 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.

[0151] Further details regarding the reaction sequence from citral to menthol can be found in U.S. Patent Application Publication No. 2013 / 46118 A1, which is incorporated herein by reference.

[0152] Thus, in one aspect, the present invention relates to an improved process for the preparation of menthol by producing citral using the process described above and then producing menthol from the citral. The menthol can be prepared as described herein or by other processes known in the art.

[0153] Linalool can be prepared from citral via a process that involves catalytic hydrogenation of citral to give nerol and / or geraniol, and their isomers.

[0154] 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.

[0155] 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. 7,126,033 B2.

[0156] Therefore, in one aspect, the present invention relates to an improved process for preparing linalool by using the above-mentioned process to produce citral, and then producing linalool from citral.Linalool can be prepared as described herein or by other processes known in the art.

[0157] The present invention is further illustrated by the following examples. [Example]

[0158] In the examples, the term "by-product" is abbreviated as "BP." For GC analysis, all "%" are reported as "area %."

[0159] Example 1 This example shows the conversion and by-product formation in a model reaction that mimics the decomposition reaction of diprenyl acetal. The conversion and by-products are reported as a function of time for various combinations of decomposition temperature and catalyst concentration.

[0160] Diprenyl acetal (25 g, 73.8% purity, containing 1.6% 2,4,4-trimethyl-3-formyl-1,5-hexadiene, 5.4% prenyl(3-methylbutadienyl) ether, and 5.8% BP1a / b) was mixed with H3PO4 (85%) at 250 ppm, 530 ppm, or 750 ppm, respectively, and equally divided into six microwave vessels (10 mL capacity). The microwave vessels were sealed with caps and heated to temperatures of 150 °C, 160 °C, 170 °C, or 180 °C, respectively. Individual microwave vessels were heated for runtimes of 0, 5, 10, 15, 30, and 60 minutes. After subsequent cooling to room temperature, solid K2CO3 (100–200 mg) was added to each reaction mixture, and the reaction mixtures were filtered using a syringe filter. The neutralized reaction mixture was then analyzed by GC, and the results are reported in Tables 1-7.

[0161] [Table 1]

[0162] [Table 2]

[0163] [Table 3]

[0164] [Table 4]

[0165] [Table 5]

[0166] [Table 6]

[0167] [Table 7]

[0168] The evolution of the residual concentration of by-product 5 versus diprenyl acetal at each of the temperatures and catalyst concentrations tested (see Tables 1 to 7) is shown in Figure 1. From Figure 1 it can be seen that for all temperatures and catalyst concentrations, the formation of by-product 5 increases sharply as the diprenyl acetal content approaches 0%, i.e., as the conversion approaches 100%.

[0169] Figure 2 shows the concentration of diprenyl acetal over time for each of the temperatures and catalyst concentrations tested (see Tables 1-7). It can be seen that the higher the temperature and catalyst concentration, the faster the reaction proceeds.

[0170] Figure 3 shows the concentration of Σ citral components (prenyl(3-methyl-butadienyl) ether, 2,4,4-trimethyl-3-formyl-1,5-hexadiene, and citral) over time for each of the temperatures and catalyst concentrations tested (see Tables 1-7). Figure 3 shows the initial increase in the concentration of Σ citral components. For runs at temperatures above 160°C, the content of Σ citral components reaches a maximum and then decreases.

[0171] Figure 4 shows the concentration of by-product 5 over time for each of the temperatures and catalyst concentrations tested (see Tables 1-7). From Figure 4, it can be seen that the formation of by-product 5 is initially slow, followed by a rapid increase in the concentration of by-product 5. From Figure 4, it can be seen that the formation of by-product 5 begins earlier and is steeper with increasing decomposition temperature and phosphoric acid concentration.

[0172] Example 2 Example 2 illustrates the continuous reactive distillation of diprenyl acetal with continuous removal of decomposition fractions (prenyl(3-methyl-butadienyl) ether, 2,4,4-trimethyl-3-formyl-1,5-hexadiene and citral).

[0173] An external evaporator, a circulation pump with a holding power of 240 mL and an upper condenser, and an 800 mm high rectification column with structured packing (Montz A3-750) equipped with a reflux splitter were used.

[0174] The column was operated at a maximum pressure of 50 mbar. The diprenyl acetal feed stream (75% content) was mixed with the sump stream and introduced into the evaporator. The H3PO4 catalyst was diluted with prenol and introduced into the sump circulation stream as a 1% solution. For all experiments, a sufficient lead time of 5–6 h was selected to obtain steady-state conditions. In Runs 8-1–8-4, the distillation stream was sampled overhead. In Runs 8-5–8-6, a side draw and a head stream were sampled. Samples of the sump and distillation stream were taken and analyzed by GC. The exact H3PO4 concentration in the sump was determined by elemental P analysis. The reaction conditions (residence time, amount of H3PO4, and temperature) are shown in Tables 8a and 8b.

[0175] [Table 8]

[0176] [Table 9]

[0177] [Table 10]

[0178] [Table 11]

[0179] Tables 8a and 8b show higher yields of citral building blocks with less by-product formation compared to Example 1. Comparison of Runs 8-2, 8-3, 8-4, and 8-5 shows that a balance must be maintained between the concentration of H3PO4 and the reaction temperature. Too high a concentration of H3PO4 at high temperatures can be detrimental to the yield of the citral building block.

[0180] Example 3 Example 3 shows the formation of by-products BP1a / b, BP2, BP3, BP4, and BP5a / b when intermediate 2,4,4-trimethyl-3-formyl-1,5-hexadiene is carried over to step a) and subjected to the reaction conditions of step a). This example also elucidates the fate of 2,4,4-trimethyl-3-formyl-1,5-hexadiene in step b) and the products formed therefrom in step a).

[0181] In a 500 mL reactor equipped with a 15 cm Vigreux distillation column, a condenser, a distillation template, and a vacuum pump, 2,4,4-trimethyl-3-formyl-1,5-hexadiene (176 g, 92% purity, containing 6% citral) was dissolved in prenol (208 g, 2.4 mol) and cyclohexane (300 mL). After the addition of HNO (500 ppm), the reaction mixture was refluxed (80–95 °C, 200 mbar), and the water formed during the reaction was removed by azeotropic distillation using a Dean-Stark apparatus. After refluxing for 3 h, 13 g of water was removed from the reaction mixture. The reaction mixture was analyzed by GC. The results are shown in Table 9.

[0182] [Table 12]

[0183] The vacuum was gradually reduced to 1 mbar and low boiling compounds (HNO3, cyclohexane, prenol, prenal, BP1a / b, citral) were distilled from the reaction mixture. 103 g of a yellowish sump residue was obtained. The sump was analyzed via GC. The results are shown in Table 10.

[0184] [Table 13]

[0185] Catalyst H3PO4 (85%, 0.03 g) was added to the residue, and the temperature was gradually increased to 140-145 °C under a vacuum of 10 mbar. During the temperature increase, the formed product was distilled from the reaction mixture. After 2 h, 55 g of distillate was collected. The distillate was analyzed via GC. The results are shown in Table 11.

[0186] [Table 14]

[0187] Example 3 demonstrates that under the reaction conditions of step a), 2,4,4-trimethyl-3-formyl-1,5-hexadiene is largely converted to BP2 and BP3 (with prenol as a by-product), with only a small amount of citral. The sump fraction remaining after removal of low-boiling compounds consists primarily of BP2 and BP3 (see Table 10). The sump fraction is believed to mimic the contaminants carried over to step b) along with the crude diprenyl acetal. Under the reaction conditions of step b), most of the BP3 is formed from BP2, with prenol as a by-product. No further citral is obtained from the sump fraction under the reaction conditions of step b). Therefore, BP3 cannot be converted to citral and can be considered a dead end in the reaction pathway.

[0188] Example 4 Example 4 shows the effect of the reaction temperature and concentration of nitric acid in step a).

[0189] The starting materials, prenol (120.4 g, 1.4 mol) and prenal (58.8 g, 0.7 mol), were added to a 250 mL three-neck flask equipped with an oil bath, a magnetic stirrer, a 15 cm Vigreux distillation column, a condenser, a distillation template, and a vacuum pump and stirred at room temperature. After adding 100 ppm, 250 ppm, or 500 ppm HNO3 (65%), respectively, the reaction mixture was evacuated to 100 mbar and the temperature was increased to 80-82 °C or 90-91 °C, respectively, until the reaction mixture began to reflux. The resulting water was removed by azeotropic distillation, and the vacuum was gradually reduced to 70 mbar. After 1 h, the reaction was stopped. The sump and organic phase in the water separator were analyzed by GC. The results are shown in Table 12.

[0190] [Table 15]

[0191] From Table 12 it can be seen that an increase in the reaction temperature and the concentration of nitric acid in step a) leads to an increase in the formation of by-product 1 and high-boiling by-products (BP2 to BP5) and a decrease in the selectivity to prenol and prenal.

Claims

1. 1. A process for preparing 3,7-dimethyl-octa-2,6-dienal (citral), comprising: a) continuously condensing prenol with prenal in the presence of at least one catalyst in a reaction column in which condensation water is distilled off as a prenal-water azeotrope as vapor, at least partially condensing the vapor, separating the condensate into an aqueous phase and an organic phase, partially directing the organic phase to the reaction column as reflux, and partially discharging the organic phase as a purge stream; At the same time, an acetal fraction containing the diprenyl acetal of prenal is continuously removed from the reaction column; the reaction temperature is less than 100°C, the catalyst is nitric acid, and the concentration of the nitric acid is less than 500 ppm; b) continuously subjecting the acetal fraction in a cracking column to cracking conditions in the presence of at least one catalyst that removes prenol, and simultaneously continuously withdrawing a cracked fraction from the cracking column comprising at least one of prenyl(3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene, and optionally citral, wherein the conversion of the diprenyl acetal of prenal in step b) is maintained greater than 90% and less than 100%, and the unreacted diprenyl acetal is at least partially contained in the withdrawn cracked fraction; c) reacting the decomposition fraction in a plug flow reactor to obtain citral; d) recycling a portion of the prenol obtained in step b) to step a); A method comprising:

2. 2. The method of claim 1, wherein the decomposition temperature in step b) is greater than 150°C and less than 200°C.

3. 3. The method according to claim 1 or 2, wherein the catalyst in step b) is phosphoric acid.

4. 4. The method of claim 3, wherein the concentration of the phosphoric acid at the bottom of the decomposition column is greater than 100 ppm and less than 1500 ppm.

5. 5. The method according to any one of claims 1 to 4, wherein the residence time in step b) is more than 5 minutes and less than 90 minutes.

6. 6. The process according to claim 1, wherein the decomposition fraction is removed from the decomposition column as a side draw and the removed prenol is removed via the top of the decomposition column.

7. The velocity of the purge flow is such that by-products 1a and 1b 【Chemical 1】 7. The process according to claim 1, wherein the rate is such that the total steady-state concentration of is maintained below 7% by weight, preferably between 2 and 7% by weight, in the feed to step a).

8. 8. The process according to any one of claims 1 to 7, wherein the reaction pressure in step a) is less than 150 mbar.

9. The method according to any one of claims 1 to 8, wherein the concentration of 2,4,4-trimethyl-3-formyl-1,5-hexadiene in the prenol recycled from step b) to step a) is controlled so that the concentration of 2,4,4-trimethyl-3-formyl-1,5-hexadiene in step a) is less than 1 wt% based on the total weight of prenol and prenal.

10. 10. The method according to claim 1, wherein the concentration of citral in the prenol recycled from step b) to step a) is controlled so that the concentration of citral in step a) is less than 1% by weight based on the total weight of prenol and prenal.

11. 11. The process of any one of claims 1 to 10, wherein prenol is obtained by reacting at least one formaldehyde source with isobutylene to obtain 3-methylbut-3-en-1-ol (isoprenol) and subjecting at least a portion of the obtained isoprenol to isomerization by contacting a reactant stream comprising isoprenol with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen.

12. the isoprenol is obtained by reacting the at least one formaldehyde source with isobutylene in a reactor to obtain isoprenol; Reacting the at least one formaldehyde source with isobutylene preferably produces α, β, and γ: a) mixing the at least one formaldehyde source and isobutylene and injecting the resulting mixture into a reactor through a plurality of nozzles operated in parallel, and reacting the formaldehyde source and isobutylene under supercritical conditions, the reactor comprising a vertically disposed vessel, a side wall, an upper portion, and a lower portion, the formaldehyde source and isobutylene being injected into a mixing chamber of the reactor located in the upper portion, and a fluid comprising formaldehyde and / or isobutylene and / or isoprenol being sent from the mixing chamber to a post-reaction chamber located in the lower portion; providing a draft tube disposed essentially concentrically below each of the nozzles in the mixing chamber, the draft tube having a descending conduit inside the draft tube and an ascending conduit outside the draft tube, whereby the formaldehyde source and isobutylene injected through the nozzles travel generally downward within the descending conduit, and then a fluid comprising formaldehyde and / or isobutylene and / or isoprenol is diverted to a generally upward path within the ascending conduit, whereby the fluid is backmixed with the injected formaldehyde source and isobutylene; β) mixing said at least one formaldehyde source with isobutylene and injecting the mixture into an internal loop reactor through at least one nozzle and into a first conduit, said internal loop reactor comprising: a vertically disposed cylindrical container including a side wall; at least one draft tube disposed vertically within said vessel, said draft tube having a tube inlet end and a tube outlet end, said draft tube being disposed concentrically with said nozzle and having an inner surface and an outer surface, said draft tube providing said first conduit within said draft tube and a second conduit on the exterior of said draft tube and within said sidewall, said first conduit being in fluid communication with said second conduit; reactor fluid outlet means; Including, the inner surface of the draft tube is convexly curved such that the first conduit exhibits an annular narrowing in cross section between the tube inlet end and the tube outlet end, the narrowing being located closer to the tube inlet end, and the convex curvature of the inner surface of the draft tube extends over at least 70%, preferably at least 80%, and most preferably at least 90% of the length of the draft tube; the outer surface of the draft tube is convexly curved such that the draft tube exhibits a circumferential protrusion between the tube inlet end and the tube outlet end, the circumferential protrusion preferably being located closer to the tube outlet end, and the convex curvature of the outer surface of the draft tube extends over at least 70%, preferably at least 80%, and most preferably at least 90% of the length of the draft tube; the end of the draft tube is rounded so that the at least one formaldehyde source and isobutylene injected through the nozzle travel generally downwardly within the first conduit to obtain a reacted fluid, which is then diverted in the opposite direction to travel through the second conduit and thereafter backmixed with the injected fluid; γ) exchanging heat between a hot isoprenol stream removed from the reactor and an isobutylene stream fed to the reactor, the heat exchange occurs in at least two shell-and-tube heat exchangers, each of the heat exchangers including a plurality of tubes and shell-side heat exchange passages; the hot isoprenol is directed through the tubes of the heat exchangers and the isobutylene is directed through the shell-side passages, and at least two of the heat exchangers are connected in series with respect to both the shell-side flow and the tube-side flow. The method of claim 11 , comprising at least one of:

13. αα and ββ: αα) purifying the isoprenol by distilling a crude isoprenol stream comprising isoprenol, water and formaldehyde, or an isoprenol-containing fraction thereof, in a low boiler separation column operated at a pressure of at least 2 bara, preferably at least 2.5 bara, to obtain a distillation stream comprising aqueous formaldehyde and a bottoms stream comprising essentially formaldehyde-free isoprenol; ββ) maintaining a weight ratio of formaldehyde to isoprenol in said reactant stream of less than 0.

04.

13. The method of claim 11 or 12, further comprising at least one of:

14. Prenal is γ-i) and γ-ii): γ-i) subjecting isoprenol to oxidative dehydrogenation to obtain prenal and / or isoprenol by contacting a reactant stream comprising isoprenol with at least one heterogeneous oxidative dehydrogenation catalyst in the presence of molecular oxygen, and optionally isomerizing at least a portion of said isoprenol to prenal, γ-i), optionally characterized by maintaining a weight ratio of formaldehyde to isoprenol in said reactant stream of less than 0.04; γ-ii) oxidizing prenol to obtain prenal by contacting a reactant stream comprising prenol with at least one oxidizing agent and at least one oxidation catalyst, preferably in the presence of a liquid phase. The method according to any one of claims 1 to 13, wherein the method is provided by at least one of

15. The oxidative dehydrogenation in step γ-i) is carried out using a shell-and-tube heat exchanger, a shell-side heat exchange passage for circulating a heat transfer medium and a reaction passage including a plurality of reaction tubes; an inlet for introducing said reactant stream into said reaction passage; an outlet from said reaction channel for recovering the discharge stream from said reaction tube; Including, The reaction tube is a reactant preheat zone adjacent to the inlet; a reaction zone downstream of said reactant preheat zone, said reaction zone having a catalytically active wire matrix insert having silver on at least a portion of its surface; 15. The method of claim 14, wherein the isoprenol is passed through a plurality of reaction tubes of a shell-and-tube heat exchanger comprising:

16. 16. The method of claim 15, wherein the catalytically active wire matrix insert comprises an elongated core having a plurality of wire loops extending therefrom, the wire loops being longitudinally arranged and helically shifted, the wire loops comprising block silver wire.

17. 17. A method for preparing a citral-derived chemical, comprising preparing citral by the method of any one of claims 1 to 16 and ααα, βββ or (βββ + γγγ): ααα) converting the citral to obtain menthol; βββ) converting the citral into geraniol and / or nerol; γγγ) converting the geraniol and / or nerol to obtain linalool The method includes at least one of:

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