Process for isomerizing an ethylenically unsaturated alcohol in a cascade of reactors

Abstract

A process for isomerizing a feed comprising an ethylenically unsaturated alcohol by shifting a double bond to obtain an isomerized ethylenically unsaturated alcohol comprises providing a cascade of n reactors, wherein n is an integer of at least 2. The process further comprises combining an ith makeup stream with an ith recycle stream to obtain an ith reactor inlet stream; feeding the ith reactor inlet stream to an ith reactor, and isomerizing the ith reactor inlet stream over a heterogeneous isomerization catalyst to yield an ith effluent stream; and dividing the ith effluent stream into the ith recycle stream and an ith forward stream. In the process, i runs from 1 to n; wherein for i = 1, the feed constitutes the first makeup stream, and for i = 2 to n, the (i-1)th forward stream constitutes the (i)th makeup stream; and the nth forward stream is a product stream. The process allows for isomerizing ethylenically unsaturated alcohols at high conversion and with improved selectivity.

Description

[0001] Process for Isomerizing an Ethy lenically Unsaturated Alcohol in a Cascade of Reactors

[0002] The invention relates to a process for isomerizing an ethylenically unsaturated alcohol, in particular isoprenol.

[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-dimethyl-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.

[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] Known isomerization processes are typically carried out with recirculation of a partly isomerized product mixture. Recirculation increases the volume flow of liquid through the reactor and decreases the concentration of the starting ethylenically unsaturated alcohol. Thus, the conversion per single-pass and the adiabatic temperature increase can be limited. Excessive recirculation rates, however, give rise to formation of by-products. There exists a trade-off between having an acceptably high conversion and having a suitably high selectivity towards the desired isomerized product.

[0007] Certain side-products such as isoamyl alcohol cannot be easily separated from unconverted isoprenol due to the proximity of their boiling points. In order to avoid accumulation of isoamyl alcohol when unconverted isoprenol is recycled to the isomerization, a purge stream must be provided. This, however, is associated with the loss of valuables.

[0008] In view of the above, there remains a need for a process for isomerizing ethylenically unsaturated alcohols at high conversion and with improved selectivity.

[0009] The present invention provides a process for isomerizing a feed comprising an ethylenically unsaturated alcohol by shifting a double bond to obtain an isomerized ethylenically unsaturated alcohol, comprising providing a cascade of n reactors, wherein n is an integer of at least 2; combining an Ithmakeup stream with an Ithrecycle stream to obtain an Ithreactor inlet stream; feeding the Ithreactor inlet stream to an Ithreactor, and isomerizing the Ithreactor inlet stream over a heterogeneous isomerization catalyst to yield an Itheffluent stream; dividing the Itheffluent stream into the Ithrecycle stream and an Ithforward stream; and wherein I runs from 1 to n; wherein for I = 1, the feed constitutes the first makeup stream, and for I = 2 to n, the (i-1)thforward stream constitutes the (i)thmakeup stream; and wherein the nthforward stream is a product stream. In preferred embodiments, n is 2, 3 or 4.

[0010] Thus, in one embodiment, the invention relates to a process for isomerizing a feed comprising an ethy lenically unsaturated alcohol by shifting a double bond to obtain an isomerized ethy lenically unsaturated alcohol, comprising providing a cascade of reactors comprising a first reactor and a second reactor; combining the feed with a first recycle stream to obtain a first reactor inlet stream; feeding the first reactor inlet stream to the first reactor, and isomerizing the first reactor inlet stream over a heterogeneous isomerization catalyst to yield a first effluent stream; dividing the first effluent stream into the first recycle stream and a first forward stream; combining the first forward stream with a second recycle stream to obtain a second reactor inlet stream; feeding the second reactor inlet stream to the second reactor, and isomerizing the second reactor inlet stream over a heterogeneous isomerization catalyst to yield a second effluent stream; and dividing the second effluent stream into the second recycle stream and a product stream.

[0011] In another embodiment, the invention relates to a process for isomerizing a feed comprising an ethy lenically unsaturated alcohol by shifting a double bond to obtain an isomerized ethylenically unsaturated alcohol, comprising providing a cascade of reactors comprising a first reactor, a second reactor and a third reactor; combining the feed with a first recycle stream to obtain a first reactor inlet stream; feeding the first reactor inlet stream to the first reactor, and isomerizing the first reactor inlet stream over a heterogeneous isomerization catalyst to yield a first effluent stream; dividing the first effluent stream into the first recycle stream and a first forward stream; combining the first forward stream with a second recycle stream to obtain a second reactor inlet stream; feeding the second reactor inlet stream to the second reactor, and isomerizing the second reactor inlet stream over a heterogeneous isomerization catalyst to yield a second effluent stream; dividing the second effluent stream into the second recycle stream and a second forward stream; combining the second forward stream with a third recycle stream to obtain a third reactor inlet stream; feeding the third reactor inlet stream to the third reactor, and isomerizing the third reactor inlet stream over a heterogeneous isomerization catalyst to yield a third effluent stream; and dividing the third effluent stream into the third recycle stream and a product stream.

[0012] In yet another embodiment, the invention relates to a process for isomerizing a feed comprising an ethylenically unsaturated alcohol by shifting a double bond to obtain an isomerized ethylenically unsaturated alcohol, comprising providing a cascade of reactors comprising a first reactor, a second reactor, a third reactor and a fourth reactor; combining the feed with a first recycle stream to obtain a first reactor inlet stream; feeding the first reactor inlet stream to the first reactor, and isomerizing the first reactor inlet stream over a heterogeneous isomerization catalyst to yield a first effluent stream; dividing the first effluent stream into the first recycle stream and a first forward stream; combining the first forward stream with a second recycle stream to obtain a second reactor inlet stream; feeding the second reactor inlet stream to the second reactor, and isomerizing the second reactor inlet stream over a heterogeneous isomerization catalyst to yield a second effluent stream; dividing the second effluent stream into the second recycle stream and a second forward stream; combining the second forward stream with a third recycle stream to obtain a third reactor inlet stream; feeding the third reactor inlet stream to the third reactor, and isomerizing the third reactor inlet stream over a heterogeneous isomerization catalyst to yield a third effluent stream; and dividing the third effluent stream into the third recycle stream and a third forward stream; combining the third forward stream with a fourth recycle stream to obtain a fourth reactor inlet stream; feeding the fourth reactor inlet stream to the fourth reactor, and isomerizing the fourth reactor inlet stream over a heterogeneous isomerization catalyst to yield a fourth effluent stream; and dividing the fourth effluent stream into the fourth recycle stream and a product stream.

[0013] The double-bond isomerization of ethylenically unsaturated alcohols is an equilibrium reaction, and complete conversion is thus not obtained, even at prolonged residence time of the reaction mixture in contact with an isomerization catalyst. Moreover, the formation of undesired by-products is observed, in particular with increased residence time of the reaction mixture. For example, the selectivity in the hydroisomerization of isoprenol to prenol is typically in the range of about 91 to 94%, wherein the by-products include isoamyl alcohol, isovaleraldehyde and methylbutenes.

[0014] While the conversion may be increased by prolonging the residence time of the reaction mixture in the reactor, this typically increases the amount of undesired by-products and leads to a decreased selectivity of the process.

[0015] Notably, the double-bond isomerization of ethylenically unsaturated alcohols is generally a moderately exothermic reaction which is, however, accompanied by highly exothermic side-reactions. The reactor temperature is typically controlled by cooling a recycle stream which is fed to the reactor. The ratio between the recycle stream and the feed stream to the reactor, known as the recycle ratio, is adjusted such that the adiabatic temperature increase is not too large. In order to efficiently control the reactor temperature, the proportion of the recycle stream in the reactor inlet stream has to be sufficiently high.

[0016] In equilibrium reactions, the farther away a system is from equilibrium, the faster the reaction proceeds. The "backmixing” resulting from the recirculation of a partly isomerized product mixture that is close to equilibrium therefore inevitably slows down the reaction. It was found that by carrying out the isomerization in multiple reactors, improved reaction rates associated with improved selectivity can be obtained at desirably high conversion. When using multiple reactors, the deleterious effects of backmixing by the individual recycle streams are less pronounced, which in turn allows for increased selectivity. In one embodiment, a weight ratio of the ithrecycle stream to the ithmakeup stream defines an ithrecycle ratio, and the first recycle ratio is greater than each of the subsequent recycle ratios. Preferably, each ithrecycle stream is cooled, preferably by indirect heat exchange, before combining the ithrecycle stream with the ithmakeup stream. Since the concentration of available ethylenically unsaturated alcohol is highest in the first reactor inlet stream, the first reactor generally requires a greater degree of cooling and thus a higher recycle ratio than the subsequent reactors.

[0017] Ethylenically Unsaturated Alcohol

[0018] The ethylenically unsaturated alcohol and the isomerized ethylenically unsaturated alcohol each comprise at least one carbon-carbon double bond.

[0019] In the process of the invention, a double bond of the ethylenically unsaturated alcohol is shifted to obtain the isomerized ethylenically unsaturated alcohol. In a preferred embodiment, a terminal double bond of the ethylenically unsaturated alcohol is shifted to obtain the isomerized ethylenically unsaturated alcohol having an internal double bond.

[0020] For example, the ethylenically unsaturated alcohol stream may be a p,y-unsaturated alcohol. The ethylenically unsaturated alcohol is preferably selected from 3-buten-1-ol compounds of formula (I)

[0021] HR1C=CR2-CHR3-CR4R5-OH (I) wherein

[0022] 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;

[0023] 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.

[0024] The isomerized ethylenically unsaturated alcohol is preferably a 2-buten-1-ol compound of formula (II)

[0025] H2R1C-R2C=CR3-CR4R5-OH (II) wherein R1to R5are defined as in formula (I).

[0026] Each Ci-Ci2-alkyl is preferably independently selected from C i-Ce-alkyl, in particular Ci-Ca-alkyl.

[0027] 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-Ca-alkyl, most preferably from hydrogen and methyl.

[0028] In a preferred embodiment, the ethylenically unsaturated alcohol is 3-methylbut-3-en-1-ol (isoprenol), and the isomerized ethylenically unsaturated alcohol is 3-methylbut-2-en-1 -ol (prenol). Isomerization

[0029] The isomerization of the reactor inlet streams over at least one heterogeneous isomerization catalyst in each of the reactors may be carried out by any of the suitable processes known to the skilled person.

[0030] In one embodiment, isomerizing the Ithreactor inlet stream over a heterogeneous isomerization catalyst comprises passing the stream through a fixed bed of the catalyst.

[0031] 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.

[0032] 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 noble metal catalyst, in particular a supported noble metal catalyst. In one embodiment, the noble metal catalyst is a fixed-bed catalyst.

[0033] In a preferred embodiment, the noble metal 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.

[0034] 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.

[0035] In a preferred embodiment, the noble metal catalyst comprises palladium, and further comprises selenium, tellurium, or a mixture thereof.

[0036] 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.

[0037] 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.

[0038] 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. 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.

[0039] In one embodiment, the noble metal catalyst is a supported noble metal catalyst, and the support is preferably 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.

[0040] Further details regarding suitable catalysts and their production are described, e.g., in EP 0 841 090 A2.

[0041] An isomerization is eventually arrived at, with the double-bond migrating within the molecule of the ethylenically unsaturated alcohol.

[0042] The reactors of the present process are not particularly limited. In one embodiment, each 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 reactor inlet streams are introduced into the reactors from below. Hydrogen may be injected into the reactor inlet streams or introduced into the reactors in the bottoms or lower regions of the reactors.

[0043] The conversion level progressively increases from reactor to reactor. The term "conversion level” is intended to mean the (cumulative) conversion reached after the Ithreactor, relative to the feed (which is deemed to have a conversion level of 0%). The final conversion level attained in the effluent stream from nthreactor (and hence, in the product stream) is preferably in the range of 30 to 70%, more preferably 34 to 68%, in particular from 36 to 65%, relative to the feed.

[0044] Generally, the contributions of the individual reactors to the final conversion level are not equal, and it is preferred that the first reactor makes the largest contribution to the conversion level.

[0045] In a cascade of two reactors, the conversion level attained after the first reactor is suitably in the range of 20 to 38%, and the conversion level attained after the second reactor is suitably in the range of 34 to 60%.

[0046] In a cascade of three reactors, the conversion level attained after the first reactor is suitably in the range of 16 to 31 %, the conversion level attained after the second reactor is suitably in the range of 29 to 52%, and the conversion level attained after the third reactor is suitably in the range of 38 to 65%.

[0047] In a cascade of four reactors, the conversion level attained after the first reactor is suitably in the range of 14 to 27%, the conversion level attained after the second reactor is suitably in the range of 25 to 47%, the conversion level attained after the third reactor is suitably in the range of 35 to 61 %, and the conversion level attained after the fourth reactor is suitably in the range of 42 to 70%.

[0048] 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.

[0049] 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.

[0050] The isomerization 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.

[0051] The product stream obtained from nthreactor comprises the isomerized ethylenically unsaturated alcohol and unconverted ethylenically unsaturated alcohol. In one embodiment, the process comprises comprising separating from the product stream a stream of isomerized ethylenically unsaturated alcohol and a stream of unconverted ethylenically unsaturated alcohol.

[0052] For this purpose, the reaction product mixture comprising the isomerized 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, in particular in a dividing wall column.

[0053] In one embodiment, the process comprises recycling the stream of unconverted ethylenically unsaturated alcohol at least partially to the feed.

[0054] In another embodiment, the process comprises subjecting the stream of unconverted ethylenically unsaturated alcohol at least partially to oxidative dehydrogenation oxidation to yield an ethylenically unsaturated aldehyde. In particular, unconverted isoprenol may be at least partially subjected to oxidation to yield prenal. Since in particular isoamyl alcohol, which is undesired by-products of isoprenol isomerization, can only be separated from isoprenol with great difficulty due to the similar boiling points, subjecting the unconverted isoprenol

[0055] Oxidative dehydrogenation of unconverted ethylenically unsaturated alcohol, in particular isoprenol, typically comprises bringing a reactant stream, in particular a gaseous reactant stream, comprising the unconverted ethylenically unsaturated alcohol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in particular 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 having an active layer of silver. Alternatively, massive (full-metal) silver bodies may be used.

[0056] The present invention is further illustrated on the basis of the following figures and examples.

[0057] Fig. 1 shows a cascade of two reactors suitable for carrying out the present invention. According to Fig. 1 , the reactor cascade comprises a first reactor 101 and a second reactor 102.

[0058] A feed 103 comprising an ethylenically unsaturated alcohol is combined with a first recycle stream 104 to obtain a first reactor inlet stream 105. The first reactor inlet stream 105 and a hydrogen stream 106 are fed to the first reactor 101, wherein the first reactor inlet stream 105 is isomerized over a fixed bed of a heterogeneous isomerization catalyst to yield a first effluent stream 107, which is withdrawn as a side draw. A gaseous effluent stream 108 is withdrawn via the top of the first reactor 101. The first effluent stream 107 is divided into the first recycle stream 104 and a first forward stream 109.

[0059] The first forward stream 109 is combined with a second recycle stream 110 to obtain a second reactor inlet stream 111. The second reactor inlet stream 111 and a hydrogen stream 112 are fed to the second reactor 102, wherein the second reactor inlet stream 111 is isomerized over a fixed bed of a heterogeneous isomerization catalyst to yield a second effluent stream 113, which is withdrawn as a side draw. A gaseous effluent stream 114 is withdrawn via the top of the second reactor 102. The second effluent stream 113 is divided into the second recycle stream 100 and a product stream 115.

[0060] Example 1

[0061] The influence of the number of reactors in a cascade on prenol selectivity in the isomerization of isoprenol is examined using the software CHEMASIM. The set of parameters is based on operational data and equilibrium measurements for plants with Pd / Se on SiC>2 as isomerization catalyst in the presence of hydrogen.

[0062] Setups utilizing one, two and three reactors, respectively, are examined. Every reactor is provided with a recycle line, through which a substream of the effluent stream is recycled to the reactor inlet. The recycle ratios are indicated in the table below, along with the reactor temperature and adiabatic temperature increase.

[0063] Each reactor comprises a fixed bed of the isomerization catalyst. The total isoprenol conversion is set to 50%, while the isoprenol feed flow rate is 6.8 t / h and the prenol flow rate after final distillation is 3.1 t / h.

[0064] * temperature difference between Ithreactor inlet stream and Itheffluent stream

[0065] ** weight ratio of the Ithrecycle stream to the Ithmakeup stream

[0066] It is evident that a reactor cascade with multiple reactors exhibits increased selectivity at identical conversion.

Claims

Claims1 . A process for isomerizing a feed comprising an ethy lenically unsaturated alcohol by shifting a double bond to obtain an isomerized ethy lenically unsaturated alcohol, comprising providing a cascade of n reactors, wherein n is an integer of at least 2; combining an Ithmakeup stream with an Ithrecycle stream to obtain an Ithreactor inlet stream; feeding the Ithreactor inlet stream to an Ithreactor, and isomerizing the Ithreactor inlet stream over a heterogeneous isomerization catalyst to yield an Itheffluent stream; and dividing the Itheffluent stream into the Ithrecycle stream and an Ithforward stream; wherein I runs from 1 to n; wherein for I = 1, the feed constitutes the first makeup stream, and for I = 2 to n, the (i-1)thforward stream constitutes the (i)thmakeup stream; and wherein the nthforward stream is a product stream.

2. The process according to claim 1 , wherein a weight ratio of the Ithrecycle stream to the Ithmakeup stream defines an Ithrecycle ratio, and the first recycle ratio is greater than each of the subsequent recycle ratios.

3. The process according to claim 1 or 2, comprising cooling each Ithrecycle stream, preferably by indirect heat exchange, before combining the Ithrecycle stream with the Ithmakeup stream.

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

5. The process according to any one of the preceding claims, wherein the reactor inlet streams are liquid.

6. The process according to any one of the preceding claims, wherein the heterogeneous isomerization catalyst comprises a noble metal.

7. The process according to claim 6, wherein the noble metal comprises palladium.

8. The process according to claim 7, wherein the heterogeneous isomerization catalyst comprises selenium, tellurium, or a mixture thereof.

9. The process according to any one of claims 6 to 8, wherein the noble metal is supported on a support, which is preferably a refractory material, in particular silicon dioxide.

10. The process according to any one of the preceding claims, comprising separating from the product stream a stream of isomerized ethy lenically unsaturated alcohol and a stream of unconverted ethylenically unsaturated alcohol.11 . The process according to claim 10, comprising recycling the stream of unconverted ethylenically unsaturated alcohol at least partially to the feed.

12. The process according to claim 10, comprising subjecting the stream of unconverted ethylenically unsaturated alcohol at least partially to oxidative dehydrogenation to yield an ethylenically unsaturated aldehyde.

13. The process according to any one of claims 10 to 12, wherein separating from the product stream a stream of isomerized ethylenically unsaturated alcohol and a stream of unconverted ethylenically unsaturated alcohol comprises distilling the product stream, preferably distilling in a dividing wall column.

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