Process for isomerizing an ethylenically unsaturated alcohol

Abstract

A process for isomerizing a first ethylenically unsaturated alcohol by shifting of a double bond to obtain a second ethylenically unsaturated alcohol comprises contacting a stream of the first ethylenically unsaturated alcohol with at least one heterogeneous isomerization catalyst, characterized in that the stream of first ethylenically unsaturated alcohol which is contacted with the heterogeneous isomerization catalyst comprises less than 25 ppmw of organically bound nitrogen, relative to the weight of the first ethylenically unsaturated alcohol. The process allows for isomerizing ethylenically unsaturated alcohols with high selectivity.

Description

[0001] Process for Isomerizing an Ethylenically Unsaturated Alcohol

[0002] The invention relates to a process for isomerizing a first ethylenically unsaturated alcohol, in particular isoprenol, by shifting a double bond to obtain a second ethylenically unsaturated alcohol, in particular prenol. The invention moreover relates to a process for the preparation of citral, menthol and linalool from prenol.

[0003] Ethylenically unsaturated alcohols such as isoprenol (3-methyl-3-buten-1-ol) are important chemical intermediates, e.g. for the preparation of terpene-based fragrances, such as citral. Therefore, such ethylenically unsaturated alcohols are of great technical and economic importance.

[0004] WO 2008 / 037693 describes a process for the production of 3,7-di methy l-octa-2,6-dienal (citral), including a step of producing 3-methyl-2-butene-1-ol (prenol) from 3-methyl-3-butene-1-ol (isoprenol) by isomerization in the presence of hydrogen and a noble metal catalyst comprising palladium and selenium and / or tellurium. Specific reference is made therein to the isomerization process described in EP 0 841 090 A2, which relates to a noble metal catalyst comprising palladium and selenium and / or tellurium on a silicon dioxide support for producing 2-butene-1-ol compounds, in particular by isomerization of 3-butene-1-ol compounds.

[0005] WO 2009 / 106622 describes a method for isomerizing olefinically unsaturated alcohols on supported noble metal catalysts comprising a carbon-based support in an oxygen-containing atmosphere. Palladium and gold are preferred noble metals for the isomerization of isoprenol to prenol.

[0006] The known noble metal isomerization catalysts exhibit good alcohol conversion and excellent selectivity, and may exhibit, depending upon the exact nature of the catalyst, long lifetime. Over time, however, the selectivity deteriorates and the catalysts may lose some activity, and occasionally, may become sufficiently deactivated so as to render the catalyst impractical to use. At this stage of partial or full deactivation, the catalyst must be regenerated or replaced.

[0007] The present invention is based on the insight that trace amounts of organically bound nitrogen, for example, nitrogen in the form of amines, are attracted to the isomerization catalyst and cumulatively act to poison the isomerization catalyst. For example, isoprenol is typically obtained from the reaction of isobutene and formaldehyde. The reaction of isobutene and formaldehyde may be carried out in the presence of a catalyst such as an amine base, e.g., hexamethylenetetramine (urotropin), as described, e.g., in US 3,574,773. The amine base also intercepts the formic acid formed by disproportionation of the formaldehyde. As a result, the obtained isoprenol stream may contain organically bound nitrogen impurities in an amount of several ppmw.

[0008] DE 199 10 504 A1 describes a process for reducing the amine content, e.g., monomethylamine, of amine- contaminated N-substituted lactams, e.g., N-methyl-2-pyrrolidone, by treating the contaminated N- substituted lactams with an acidic macroporous cation exchanger. At the same time, metal cations contained as impurities in the N-substituted lactams may be depleted.

[0009] There remains a need for an improved process for isomerizing ethylenically unsaturated alcohols, in particular higher selectivity. In a first aspect, the present invention provides a process for isomerizing a first ethylenically unsaturated alcohol over at least one heterogeneous isomerization catalyst by shifting of a double bond to obtain a second ethylenically unsaturated alcohol, characterized in that the first ethylenically unsaturated alcohol comprises less than 25 ppmw of organically bound nitrogen.

[0010] In a second aspect, the present invention moreover provides a process for isomerizing a first ethylenically unsaturated alcohol by shifting of a double bond to obtain a second ethylenically unsaturated alcohol, comprising

[0011] (i) contacting a stream of the first ethylenically unsaturated alcohol with a solid adsorbent having a specific affinity for organically bound nitrogen compounds in preference to hydrocarbons, and which does not interfere with the double bond of the first ethylenically unsaturated alcohol, to obtain a treated stream, wherein the treated stream preferably comprises less than 25 ppmw of organically bound nitrogen, relative to the weight of the first ethylenically unsaturated alcohol; and

[0012] (ii) contacting the treated stream with at least one heterogeneous isomerization catalyst under isomerization conditions to obtain the second ethylenically unsaturated alcohol.

[0013] Herein, the term "organically bound nitrogen” is intended to denote any compound containing at least one nitrogen atom directly bound to one or more carbon atoms. For example, such compounds containing at least one nitrogen atom may be selected from amines, such as ethylamine, trimethylamine, aniline, pyridine or piperidine. An amine particularly significant in practice is hexamethylenetetramine (urotropin).

[0014] Ethylenically Unsaturated Alcohols

[0015] The first ethylenically unsaturated alcohol and the second ethylenically unsaturated alcohol each comprise at least one carbon-carbon double bond.

[0016] In the process of the invention, a double bond of the first ethylenically unsaturated alcohol is shifted to obtain the second ethylenically unsaturated alcohol. In a preferred embodiment, a terminal double bond of the first ethylenically unsaturated alcohol is shifted to obtain the second ethylenically unsaturated alcohol having an internal double bond. For example, the first ethylenically unsaturated alcohol stream may be a p,y-unsaturated alcohol.

[0017] The first ethylenically unsaturated alcohol is preferably selected from 3-buten-1-ol compounds of formula (I)

[0018] HR1C=CR2-CHR3-CR4R5-OH (I) wherein

[0019] R1, R2and R3are independently selected from hydrogen and Ci-Ci2-alkyl which may be substituted by OH, OR6where R6is Ci-Ci2-alkyl, COOH or halogen;

[0020] R4and R5are independently selected from hydrogen and Ci-Ci2-alkyl; wherein R2and R5together with the carbon atoms located between them may form an alicyclic ring. The second ethylenically unsaturated alcohol is preferably a 2-buten-1-ol compound of formula (II)

[0021] H2R1C-R2C=CR3-CR4R5-OH (II) wherein R1to R5are defined as in formula (I).

[0022] Each Ci-Ci2-alkyl is preferably independently selected from Ci-Ce-alkyl, in particular Ci-Cs-alkyl.

[0023] R1to R5are preferably independently selected from hydrogen and Ci-Ci2-alkyl, in particular from hydrogen and Ci-Ce-alkyl, such as from hydrogen and Ci-Cs-alkyl, most preferably from hydrogen and methyl.

[0024] In a preferred embodiment, the first ethylenically unsaturated alcohol is 3-methylbut-3-en-1-ol (isoprenol), and the second ethylenically unsaturated alcohol is 3-methylbut-2-en-1 -ol (prenol).

[0025] Removal of Organically Bound Nitrogen

[0026] The reliable purification of ethylenically unsaturated alcohols from trace amounts of organically bound nitrogen on an industrial scale is not a trivial task. Generally, such ethylenically unsaturated alcohols are reactive compounds. Said reactivity may lead to difficulties, e.g., when performing purification steps for removing impurities from such ethylenically unsaturated alcohols. For example, upon treatment with a solid adsorbent, isoprenol is suspected to form a tertiary carbocation that can give rise to undesired side reactions. In addition, the nitrogen compounds, which are only present in low concentrations, compete with the abundant alcohol for absorption sites.

[0027] In one embodiment, a stream of the first ethylenically unsaturated alcohol is contacted with a solid adsorbent having a specific affinity for organically bound nitrogen compounds in preference to hydrocarbons, and which does not interfere with the double bond of the first ethylenically unsaturated alcohol, to obtain a treated stream. The treated stream is subsequently contacted with at least one heterogeneous isomerization catalyst under isomerization conditions to obtain the second ethylenically unsaturated alcohol.

[0028] In a preferred embodiment, the first ethylenically unsaturated alcohol is contacted with an acidic solid adsorbent in the substantial absence of an oxidant. Generally, the alcohol stream is contacted with the weakly acidic solid adsorbent in the absence of a catalytically active metal catalyst.

[0029] The weakly acidic adsorbent may include an adsorbent material having sufficient acidity to adsorb the organically bound nitrogen from the stream.

[0030] In an embodiment, the solid adsorbent is a crosslinked resin having phosphonic functional groups.

[0031] Preferably, the resin polymer is a vinyl aromatic copolymer, preferably crosslinked polystyrene and more preferably a polystyrene divinylbenzene copolymer. Other polymers having a phosphonic functional group may also be used.

[0032] Preferably, the crosslinked resin having phosphonic functional groups is of the macroporous type. A preferred solid adsorbent is Purolite S956.

[0033] The resin is typically used in bead form and loaded into a column. The stream is passed through the column, contacting the resin beads. During contact, the organically bound nitrogen in the stream reacts with the functional group and an exchange occurs where a proton is transferred to the nitrogen and an ionic bond is formed to the anionic site of the resin. Contact is maintained until a threshold level is reached i.e. the breakthrough concentration. At this breakthrough point, the process reaches an equilibrium where additional organically bound nitrogen cannot be removed effectively. The flow is halted and the column is backwashed with water, preferably deionized or softened water. By flowing in reverse, the resin is fluidized and solids captured by the beads are loosened and removed.

[0034] In another embodiment, the solid adsorbent is a silica-alumina hydrate. Numerous silica-alumina catalyst compositions and processes for their preparation are described in the patent literature, see, e.g., US 4,499,197.

[0035] Preferably, the alumina content of the silica-alumina hydrate is from about 10 to about 90 wt.-% of AI2O3. The preferred range of alumina content is from about 30 to about 70 wt.-% of AI2O3.

[0036] The introduction of silicon dioxide into aluminum oxide leads to the introduction of acidic centers. The number of acidic centers can be controlled by the amount of introduced silicon dioxide. The number of acidic centers increases with the amount of introduced silicon dioxide up to a maximum number of acidic centers, and decreases again with a further increasing amount of silicon dioxide after having reached the maximum number of acidic centers.

[0037] Examples of commercially available silica-alumina hydrates are Siral® available from Sasol Germany Gmbh, Hamburg, Germany. Siral® is based on orthorhombic aluminum oxide hydroxide (boehmite; AIOOH) and doped with SIO2. Various Siral® grades having different ratios of AI2O3 to SIO2 are available: Siral 1 (AI2O3 / SiO2= 99 / 1), Siral 5 (AI2O3 / SiO2= 95 / 5), Siral 10 (AI2O3 / SiO2= 90 / 10), Siral 20 (AI2O3 / SiO2= 80 / 20), Siral 28M (AI2O3 / SiO2= 72 / 28), Siral 30 (AI2O3 / SiO2= 70 / 30), Siral 40 (AI2O3 / SiO2= 60 / 40). Siral 40 is especially preferred.

[0038] In a suitable determination method, the solid adsorbent is characterized by temperature programmed desorption of ammonia (TPAD) carried out on an apparatus constructed from Raczek analyzing technique GmbH, Hannover (Germany). For this purpose, the samples are conditioned at a temperature of 400 °C in helium flow. Afterwards, a mixture of 10% NHs / He is passed over the sample at 70 °C. The physisorbed ammonia is removed by flushing with helium at 120 °C for 2 h. The chemisorbed ammonia is removed by passing helium over the sample which was heated up to 400 °C with a linear heating rate of 15 °C / min. The integration values of the peaks in the amount of ammonia that desorbs from the solid adsorbent is reported as amount of acidic centers.

[0039] In an embodiment, the first ethylenically unsaturated alcohol is passed over a bed of the weakly acidic solid adsorbent.

[0040] Suitably, said step of "passing over a bed” denotes that a layer ("bed”) of the weakly acidic solid adsorbent is provided in a customary reaction vessel known to the skilled person which may preferably be equipped with a stirring device, e.g., in a stirred-tank reactor. The first ethylenically unsaturated alcohol is then introduced into the reaction vessel and guided through the same in a manner that it gets into contact with the weakly acidic solid adsorbent.

[0041] Alternatively, the weakly acidic solid adsorbent may be provided in a reaction tube, e.g., of a tubular reactor and the ethylenically unsaturated alcohol stream then continuously flows through said reaction tube(s) while getting into contact with the weakly acidic solid adsorbent.

[0042] The first ethylenically unsaturated alcohol comprises less than 25 ppmw of organically bound nitrogen. In order to obtain a first ethylenically unsaturated alcohol with less than 25 ppmw of organically bound nitrogen, the first ethylenically unsaturated alcohol may be contacted with an absorbent as described above.

[0043] In one embodiment, the first ethylenically unsaturated alcohol comprises less than 20 ppmw, such as less than 10 ppmw or less than 5 ppmw of organically bound nitrogen. For example, the first ethylenically unsaturated alcohol may comprise 0.1 to 25 ppmw, more preferably 0.2 to 20 ppmw, such as 0.3 to 100 ppmw or 0.5 to 5 ppmw of organically bound nitrogen. In an embodiment, the first ethylenically unsaturated alcohol comprises less than 2 ppmw of organically bound nitrogen. The term "ppmw” denotes wt.-ppm of compounds incorporating organically bound nitrogen, relative to the total weight of the first ethylenically unsaturated alcohol.

[0044] The content of organically bound nitrogen in the ethylenically unsaturated alcohol stream may be determined by Kjeldahl analysis. Alternatively, an oxidative combustion method with a chemiluminescence detector according to DIN 51444 may be used.

[0045] Isomerization

[0046] The isomerization of the first ethylenically unsaturated alcohol over at least one heterogeneous isomerization catalyst by shifting of a double bond to obtain a second ethylenically unsaturated alcohol may be carried out by any of the suitable processes known to the skilled person.

[0047] In one embodiment, contacting the stream of the first ethylenically unsaturated alcohol with the heterogeneous isomerization catalyst comprises passing the stream through a fixed bed of the catalyst.

[0048] For example, the isomerization may be carried out over a supported noble metal catalyst comprising a carbon-based support in an oxygen-containing atmosphere in accordance with WO 2009 / 106622 or WO 2017 / 157897. Palladium is preferred for the isomerization of isoprenol to prenol.

[0049] Preferably, however, the isomerization is carried out in the presence of hydrogen. Such a process may be referred to as hydroisomerization. In this case, the isomerization is preferably carried out over a supported noble metal catalyst. In one embodiment, the noble metal catalyst is a fixed-bed catalyst.

[0050] In a preferred embodiment, the supported noble metal catalyst comprises palladium. The catalyst may contain 0.1 to 2.0% by weight, preferably 0.2 to 0.8% by weight, in particular 0.4 to 0.6% by weight, of palladium, based on the total weight of the catalyst. Notably, when using pure palladium in the presence of hydrogen, significant hydrogenation of the double bond of the compounds may occur and a saturated product may be formed. In addition, low-boiling compounds such as hydrocarbons and aldehydes may be formed as by-products, for example by hydrogenation and isomerization. The hydrogenation of the double bond is undesired, in particular as separation of the hydrogenation by-product and the isomerized product by distillation is difficult.

[0051] In a preferred embodiment, the noble metal catalyst comprises palladium, and further comprises selenium, tellurium, or a mixture thereof.

[0052] In one embodiment, the noble metal catalyst comprises 0.01 to 0.2%, preferably 0.02 to 0.08% by weight, in particular 0.04 to 0.06% by weight by weight of selenium, tellurium or a mixture of thereof, based on the total weight of the catalyst.

[0053] The BET surface area of the noble metal catalyst may be in the range of 80 to 380 m2 / g, preferably 100 to 150 m2 / g, in particular 110 to 130 m2 / g. The BET surface area may be determined by nitrogen adsorption in accordance with DIN 66131.

[0054] In one embodiment, the pore volume of the noble metal catalyst may have a pore volume of 0.6 to 0.95 cm3 / g, preferably 0.8 to 0.9 cm3 / g, in particular from 0.8 to 0.85 cm3 / g, in the pore diameter range of 3 nm to 300 m, wherein 80 to 95%, preferably 85 to 93%, of this pore volume are in the pore diameter range of 10 to 100 nm. The pore volume may be determined by Hg porosimetry.

[0055] Apart from the active components mentioned, further metals may be present on the catalysts in small amounts. Preferably, only palladium, selenium and / or tellurium, in particular only palladium and selenium, are present on the support.

[0056] In one embodiment, the support of the noble metal catalyst is selected from refractory materials, such as silicon dioxide, aluminum oxide and mixtures thereof. In a particularly preferred embodiment, the support is a silicon dioxide support comprising at least 90 wt.-% of silicon dioxide, preferably at least 98 wt.-% of silicon dioxide.

[0057] Further details regarding suitable catalysts and their production are described, e.g., in EP 0 841 090 A2.

[0058] An isomerization is eventually arrived at, with the double-bond migrating within the molecule of the ethylenically unsaturated alcohol.

[0059] The isomerization may be carried out batchwise, for example in a stirred reactor using the suspension method, or continuously. However, since the double-bond isomerization of ethylenically unsaturated alcohols is an equilibrium reaction, complete conversions are not obtained but part of the starting material always remains and for further use has to be separated from by-products formed. To carry out the isomerization in a more economical way, the reaction should be carried out continuously.

[0060] The isomerization may be carried out in any suitable apparatus. In one embodiment, the isomerization is carried out in the up-flow mode in a tube reactor containing a fixed-bed catalyst as described above. The tube reactor preferably contains a gas distributor in the lower part, for example in the form of a filter plate, a static mixer or a nozzle. The gas distributor serves to feed in hydrogen which is preferably distributed uniformly across the reactor cross section. The first ethylenically unsaturated alcohol is introduced into the reactor from below and treated with hydrogen.

[0061] The space velocity over the catalyst is set such that a conversion of preferably from 45 to 65%, particularly preferably from 50 to 60%, is achieved at the reactor outlet. The introduction of hydrogen is set as a function of temperature and total pressure in such a way that a hydrogen partial pressure of from 0.5 to 5 bar, preferably from 0.5 to 2 bar, in particular from 0.6 to 1 bar, is maintained. The hydrogen which has passed through the reactor can be discharged as waste gas after condensing out low boilers or can be recirculated to the process.

[0062] The isomerization is carried out at a temperature of 50 to 150 °C, preferably 50 to 120 °C, preferably 80 to 100 °C. Depending on the starting compound used, space velocities of the catalyst of from 0.5 to 5 L / L(catalyst) x h, preferably from 0.5 to 1.5 L / L (catalyst) x h, are employed.

[0063] In one embodiment, contacting the stream of the first ethylenically unsaturated alcohol with the heterogeneous isomerization catalyst yields a product stream comprising the second ethylenically unsaturated alcohol and unconverted first ethylenically unsaturated alcohol, and the process comprises removing unconverted first ethylenically unsaturated alcohol from the product stream and recycling the unconverted first ethylenically unsaturated alcohol to the isomerization.

[0064] For this purpose, the reaction product mixture comprising the second ethylenically unsaturated alcohol obtained from the equilibrium reaction is preferably passed directly to work-up by distillation. The separation by distillation of the second ethylenically unsaturated alcohol from the first ethylenically unsaturated alcohol and the return of the first ethylenically unsaturated alcohol increase the economic viability of the isomerization process. The separation by distillation is preferably carried out continuously in suitable apparatuses.

[0065] In the first aspect of the invention, recycling the unconverted first ethylenically unsaturated alcohol to the isomerization involves combining the unconverted first ethylenically unsaturated alcohol with the stream of the first ethylenically unsaturated alcohol to obtain a combined stream, and contacting the combined stream with the heterogeneous isomerization catalyst.

[0066] In the second aspect of the invention, recycling the unconverted first ethylenically unsaturated alcohol to the isomerization involves combining the unconverted first ethylenically unsaturated alcohol with the treated stream to obtain a combined stream, and contacting the combined stream with the heterogeneous isomerization catalyst.

[0067] The process of the present invention can be carried out in the presence or absence of an inert organic solvent. Inert organic solvents which can be used are, for example, ethers such as diethyl ether, dioxane or tetrahydrofuran, alcohols such as ethanol or isobutanol, aromatic or aliphatic hydrocarbons such as heptane or benzene or mixtures thereof. Preference is given to carrying out the process without an inert organic solvent. Production of Isoprenol

[0068] In one embodiment, the first ethylenically unsaturated alcohol is isoprenol. Isoprenol may be obtained by introducing a formaldehyde source and isobutylene into a reactor through at least one nozzle and reacting the formaldehyde source and isobutylene under supercritical conditions. In order to achieve supercritical conditions, formaldehyde and isobutylene are preferably reacted at a temperature of at least 220 °C, for example in the range of 220 to 290 °CC, and an absolute pressure of at least 200 bara. The reaction of isobutene and formaldehyde may be carried out in the presence of a catalyst such as an amine base, e.g. hexamethylenetetramine (urotropin).

[0069] Formaldehyde may be provided as a liquid, for example as a solution of paraformaldehyde. Preferably, the formaldehyde source is an aqueous formaldehyde solution.

[0070] Further details regarding reacting a formaldehyde source and isobutylene to obtain isoprenol may be found in WO 2020 / 049111 A1.

[0071] Separation of the isoprenol from formaldehyde is complicated by the fact that monomeric formaldehyde (as well as polymeric formaldehyde) forms both hydrates with water and hemiformals with isoprenol. The hydrates and hemiformals of varying formaldehyde polymerization degree have intermingling boiling points. However, formaldehyde can be separated virtually completely from isoprenol via distillation at a temperature at which the hemiformal is cleaved to formaldehyde and isoprenol, so that the formaldehyde can be easily separated from the isoprenol.

[0072] Hence, crude isoprenol may be purified by subjecting a stream of crude isoprenol containing isoprenol, water and formaldehyde, or an isoprenol containing fraction thereof, to distillation in a low-boiler separation tower operated at a pressure of 2 bara or higher, preferably 2.5 bara or higher, to obtain a distillate stream containing aqueous formaldehyde and a bottoms stream containing isoprenol essentially free of formaldehyde.

[0073] A concentrated aqueous formaldehyde suitable for recycling into the isoprenol synthesis can be obtained in a distillation train involving a first distillation at a temperature at which the equilibrium is shifted towards the hemiformal of formaldehyde and isoprenol, so that essentially all formaldehyde remains in the bottoms of the distillation, and a second distillation at a temperature at which the hemiformal is cleaved to formaldehyde and isoprenol, so that the formaldehyde can be easily separated from the isoprenol.

[0074] In order to permit 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, two low-boiler separation towers operated at different pressures are envisioned.

[0075] In a more preferred embodiment, the purification process comprises

[0076] (I) directing the stream of crude isoprenol to a first low-boiler separation tower operated at a pressure of 1.5 bara or lower, to obtain a first bottoms stream containing isoprenol and formaldehyde, and a first distillate stream containing water and low-boilers; (ii) directing the first bottoms stream to a second low-boiler separation tower operated at a pressure of 2 bara or higher, to obtain a second distillate stream containing aqueous formaldehyde, and a second bottoms stream containing isoprenol; and

[0077] (ill) directing the second bottoms stream to a finishing tower to obtain pure isoprenol as a distillate stream, and a bottoms stream containing high-boilers.

[0078] The second distillate stream constitutes concentrated aqueous formaldehyde fit for recycle into the isoprenol synthesis.

[0079] Suitably, the second low-boiler separation tower is operated at a pressure of 2.5 bara or higher, preferably 2.8 bara or higher, most preferably 2.9 bara or higher. The bottoms temperature of the second low-boiler separation tower is preferably in the range of 160 to 200 °C, more preferably 170 to 185 °C, most preferably 175 to 180 °C. The temperature at the top of the second low-boiler separation tower is preferably in the range of 115 to 160 °C, more preferably 125 to 145 °C.

[0080] In a particularly preferred embodiment, the second low-boiler separation tower is operated at a pressure in the range of 2.9 to 3.5 bara, a bottoms temperature in the range of 175 to 180 °C and a temperature at the top in the range of 130 to 140 °C.

[0081] Further information on the process for recovering isoprenol essentially free of formaldehyde may be found in WO 2022 / 189652 A1.

[0082] Production of Citral

[0083] 3,7-Dimethyl-octa-2,6-dienal (citral) can be prepared by obtaining prenol in accordance with a process as described above, further comprising the steps of condensing the prenol with prenal to obtain diprenyl acetal of prenal; and subjecting the diprenyl acetal of prenal to cleaving conditions to obtain citral via prenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene.

[0084] In particular, 3,7-dimethyl-octa-2,6-dienal (citral) can be prepared by a process comprising the steps of: a) condensing prenol with prenal in the presence of at least one catalyst in a reaction column while withdrawing an acetal fraction comprising the diprenyl acetal of prenal from the reaction column; b) subjecting the acetal fraction in a cleaving column to cleaving conditions in the presence of at least one catalyst while withdrawing from the cleaving column a cleaving 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 c) reacting the cleaving fraction in a plug-flow type reactor to obtain citral. The overall reaction sequence is illustrated by the reaction scheme below. prenol prenal diprenyl acetal citral 2,4,4-trimethyl-3- prenyl (3-methyl- formyl-1,5-hexadiene butadienyl) ether

[0085] In step a), the unsaturated acetal 3-methyl-2-butenal-diprenyl acetal (herein referred to as "diprenyl acetal of prenal” or "diprenyl acetal”) is formed from prenol and prenal using a catalyst. For this purpose, prenal is reacted together with prenol in the presence of catalytic amounts of an acid and with separation of the water formed during the reaction in a reaction column. In step b), the resulting 3-methyl-2-butenal diprenyl acetal (diprenyl acetal) of step a) is cleaved in the presence of a catalyst in a cleaving column with elimination of 3-methyl-2-buten-1-ol (prenol) to give prenyl (3-methylbutadienyl) ether. Claisen rearrangement of the obtained prenyl (3-methylbutadienyl) ether yields 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene which subsequently undergoes Cope rearrangement yielding 3,7-dimethyl-2,6-octadienal (citral).

[0086] Step a) is carried out in the presence of a catalyst, preferably an acid. In an embodiment, the catalyst in step a) is nitric acid.

[0087] Preferably, in steb b), the acetal fraction is continuously subjected to cleaving conditions in a cleaving column. "Cleaving conditions” denotes reaction conditions selected such that the diprenyl acetal contained in the acetal fraction is cleaved to prenyl (3-methylbutadienyl) ether which may subsequently rearrange to 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene and citral.

[0088] The acetal fraction comprises diprenyl acetal as a main constituent. The acetal fraction does not necessarily need to consist of pure diprenyl acetal, but may also comprise prenol, prenal and citral building blocks.

[0089] Step b) is carried out in the presence of a catalyst, preferably an acid catalyst. Suitable acid catalysts are selected from non-volatile protic acids such as sulfuric acid, p-toluenesulfonic acid and phosphoric acid.

[0090] Suitably, the continuous cleaving in the cleaving column of step b) may be carried out in the lower part or the sump of the distillation column acting as cleaving column. Preferably, the acetal fraction and / or the catalyst(s) are introduced into the lower part of the distillation column, into the sump of the distillation column or into the evaporator of the distillation column.

[0091] If desired, a high-boiling inert compound can be introduced into the sump of the cleaving column in order to ensure a minimum filling level of the sump and the evaporator. Suitable high-boiling inert compounds are selected from liquid compounds which are inert under the reaction conditions and have a higher boiling point than citral and diprenyl acetal. For example, the high-boiling inert compounds may be selected from hydrocarbons such as tetradecane, pentadecane, hexadecane, octadecane, eicosane; or ethers such as diethylene glycol dibutyl ether; white oils; kerosene oils; or mixtures thereof.

[0092] Suitably, the distillation conditions are selected such that the diprenyl acetal is predominantly retained in the lower part or the sump of the distillation column. During the cleaving reaction, a cleaving fraction is continuously withdrawn from the cleaving column, the cleaving 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. For the 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 are intermediates on the reaction route to citral and can be converted into citral in the subsequent step c).

[0093] Additionally, the prenol formed during the cleaving reaction in step b) is continuously removed from the reaction mixture, generally at the top of the cleaving column.

[0094] The cleaving fraction together with the formed prenol may be withdrawn at the top of the distillation column.

[0095] Alternatively and preferably, it is also possible to withdraw the cleaving fraction in liquid or vaporous form at a side draw of the distillation column.

[0096] In step c), the cleaving fraction is reacted in a plug-flow type reactor to obtain citral. To this end, the cleaving fraction is guided through the plug-flow type reactor at a suitable temperature for carrying out the rearrangement reaction(s) yielding citral. By employing a combination of a highly back-mixed cleaving column and a plug-flow reactor, it is possible to increase the selectivity and the yield of the cleaving reaction. All of the catalyst(s) required for the cleaving reaction is preferably introduced into the cleaving column in step b) and preferably, no catalyst is introduced into the plug-flow reactor.

[0097] In an embodiment, prenol eliminated in step b) is recycled to step a). This allows for improved yields to be achieved in the process of the invention.

[0098] In one aspect, the invention hence relates to an improved process for the preparation of citral (3, 7-dimethy I- octa-2,6-dienal), comprising the steps of

[0099] A) reacting a formaldehyde source and isobutylene to obtain 3-methylbut-3-en-1-ol (isoprenol), and subjecting at least part of the obtained isoprenol to isomerization to obtain prenol in accordance with the present process;

[0100] B) providing prenal (3-methylbut-2-en-1 -al) by either B1 or B2:

[0101] B1) oxidative dehydrogenation of 3-methylbut-2-en-1 -ol (prenol);

[0102] B2) oxidative dehydrogenation of isoprenol to obtain 3-methylbut-3-en-1 -al (isoprenal), and subsequent isomerization of isoprenal; and

[0103] C) condensing the prenal with prenol to obtain diprenyl acetal of prenal; and subjecting the diprenyl acetal of prenal to cleaving conditions to obtain citral via prenyl (3-methyl-butadienyl) ether and 2,4,4- trimethyl-3-formyl-1 ,5-hexadiene. Step A can be performed as described above or by other methods known in the art, preferably via distillation at a temperature at which the hemiformal is cleaved to formaldehyde and isoprenol, so that the formaldehyde can be easily separated from the isoprenol, and more preferably by subjecting a stream of crude isoprenol containing isoprenol, water and formaldehyde, or an isoprenol containing fraction thereof, to distillation in a low-boiler separation tower operated at a pressure of 2 bara or higher, preferably 2.5 bara or higher, to obtain a distillate stream containing aqueous formaldehyde and a bottoms stream containing isoprenol essentially free of formaldehyde.

[0104] Step B comprises oxidative dehydrogenation of prenol and / or isoprenol and can be performed by any of the methods known in the art. The conversion of isoprenol with a catalytically active metal catalyst forms a reaction mixture of 3-methylbut-3-en-1 -al and 3-methylbut-2-en-1 -al. The former isomer may then isomerize under base catalysis to give the desired 3-methylbut-2-en-1 -al.

[0105] Step C can be performed as described above, for example via steps a) to c).

[0106] The thus obtained citral is a useful intermediate for, e.g., menthol or linalool.

[0107] Production of Menthol and Linalool

[0108] Menthol may be prepared from citral via a process comprising the steps of

[0109] - catalytic hydrogenation of citral to obtain citronellal;

[0110] - cyclization of citronellal to obtain isopulegol in the presence of an acidic catalyst; and

[0111] - catalytic hydrogenation of isopulegol to obtain menthol.

[0112] The overall reaction sequence is illustrated by the reaction scheme below.

[0113] The hydrogenation of citral to obtain citronellal may be achieved by hydrogenation in the presence of a rhodium-phosphine catalyst.

[0114] The cyclization of citronellal to isopulegol may be achieved by cyclization in the presence of a Lewis-acidic aluminum-containing catalyst, such as a bis(diarylphenoxy)aluminum compound, which may be used in the presence of an auxiliary, such as a carboxylic anhydride. The isopulegol may be recovered from the catalystcontaining reaction product by distillative separation to give an isopulegol-enriched top product and an isopulegol-depleted bottom product. From the bottom product, the catalyst may be regenerated. The isopulegol obtainable in this way by the cyclization of citronellal can be further purified by suitable separating and / or purification methods, in particular by crystallization, and be at least largely freed from undesired impurities or by-products.

[0115] The hydrogenation of isopulegol may be achieved by hydrogenation in the presence of a heterogeneous nickel-containing catalyst, preferably a heterogeneous nickel- and copper-containing catalyst.

[0116] Further details regarding the reaction sequence from citral to menthol may be found in US 2013 / 46118 A1 , which is incorporated by reference herein.

[0117] In one aspect, the invention thus relates to an improved process for the preparation of menthol by producing citral using the above processes and then producing menthol from the citral. Menthol may be prepared as described herein or by other methods known in the art.

[0118] Linalool may be prepared from citral via a process comprising catalytic hydrogenation of citral to obtain nerol and / or geraniol, and isomerization thereof.

[0119] The hydrogenation of citral to obtain nerol and / or geraniol may be achieved by hydrogenation in the presence of a supported ruthenium, rhodium, osmium, iridium or platinum catalyst, preferably a ruthenium catalyst supported on carbon black.

[0120] The isomerization of nerol and / or geraniol to obtain linalool may be achieved by isomerization in the presence of a tungsten catalyst, in particular a dioxotungsten (VI) complex. Further details regarding the isomerization of nerol and / or geraniol may be found in US 7,126,033 B2.

[0121] In one aspect, the invention thus relates to an improved process for the preparation of linalool by producing citral using the above processes and then producing linalool from the citral. Linalool may be prepared as described herein or by other methods known in the art.

[0122] The present invention is further illustrated on the basis of the following examples.

[0123] Examples

[0124] Example 1 - Removal of Urotropin from an Isoprenol Stream Using Siral 40

[0125] 100 mL of isoprenol having an initial content of 27 ppmw of urotropin was stirred with 2 g of Siral 40 for 6 h. The urotropin content of the treated isoprenol was determined as 3 ppmw.

[0126] Example 2 - Hydroisomerization of Isoprenol

[0127] Isoprenol subjected to removal of organically bound nitrogen and untreated isoprenol were hydroisomerized at 80 °C and 1.5 bar in the presence of an isomerization catalyst in a stirred-tank reactor. The results are shown in the table below.

[0128] It is evident that the selectivity for the isomerization of isoprenol to prenol is significantly higher when the content of organically bound nitrogen in the isoprenol stream is reduced.

Claims

Claims1 . A process for isomerizing a first ethylenically unsaturated alcohol by shifting of a double bond to obtain a second ethylenically unsaturated alcohol, comprising contacting a stream of the first ethylenically unsaturated alcohol with at least one heterogeneous isomerization catalyst, characterized in that the stream of first ethylenically unsaturated alcohol which is contacted with the heterogeneous isomerization catalyst comprises less than 25 ppmw of organically bound nitrogen, relative to the weight of the first ethylenically unsaturated alcohol.

2. A process for isomerizing a first ethylenically unsaturated alcohol by shifting of a double bond to obtain a second ethylenically unsaturated alcohol, comprising(I) contacting a stream of the first ethylenically unsaturated alcohol with a solid adsorbent having a specific affinity for organically bound nitrogen compounds in preference to hydrocarbons, and which does not interfere with the double bond of the first ethylenically unsaturated alcohol, to obtain a treated stream; and(II) contacting the treated stream with at least one heterogeneous isomerization catalyst under isomerization conditions to obtain the second ethylenically unsaturated alcohol.

3. The process according to claim 2, wherein the treated stream comprises less than 25 ppmw of organically bound nitrogen, relative to the weight of the first ethylenically unsaturated alcohol.

4. The process according to claim 2 or 3, wherein the solid adsorbent is a weakly acidic solid adsorbent.

5. The process according to any one of the preceding claims, wherein a terminal double bond of the first ethylenically unsaturated alcohol is shifted to obtain the second ethylenically unsaturated alcohol having an internal double bond.

6. The process according to any one of the preceding claims, wherein the isomerization is carried out in the presence of hydrogen.

7. The process according to any one of the preceding claims, wherein contacting the stream of the first ethylenically unsaturated alcohol with the heterogeneous isomerization catalyst comprises passing the stream through a fixed bed of the catalyst.

8. The process according to any one of the preceding claims, wherein the heterogeneous isomerization catalyst is a supported noble metal catalyst.

9. The process according to claim 8, wherein the noble metal comprises palladium.

10. The process according to claim 9, wherein the noble metal catalyst comprises selenium, tellurium, or a mixture thereof.11 . The process according to any one of claims 8 to 10, wherein the support is a refractory material, preferably silicon dioxide.

12. The process according to any one of the preceding claims, wherein contacting the stream of the first ethylenically unsaturated alcohol with the heterogeneous isomerization catalyst yields a product stream comprising the second ethylenically unsaturated alcohol and unconverted first ethylenically unsaturated alcohol, the process comprising removing unconverted first ethylenically unsaturated alcohol from the product stream and recycling the unconverted first ethylenically unsaturated alcohol to the isomerization.

13. The process according to any one of the preceding claims, wherein the first ethylenically unsaturated alcohol is 3-methylbut-3-en-1 -ol (isoprenol) and the second ethylenically unsaturated alcohol is 3-methylbut-2-en-1-ol (prenol).

14. A process for the preparation of 3,7-dimethyl-octa-2,6-dienal (citral), comprising obtaining prenol by the process according to claim 12, further comprising the steps of condensing the prenol with prenal to obtain diprenyl acetal of prenal; and subjecting the diprenyl acetal of prenal to cleaving conditions to obtain citral via prenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1 ,5- hexadiene.

15. A process for the preparation of menthol, comprising preparing citral by the process according to claim 14, and reacting citral to obtain menthol.

16. A process for the preparation of linalool, comprising preparing citral by the process according to claim 14, and reacting citral to obtain linalool.