PROCESS OF PREPARING ALCOHOLS

Abstract

A process can produce alcohols having at least two carbon atoms by catalytic conversion of synthesis gas into a mixture containing alkanes, alkenes, and alcohols. Alkenes are converted into corresponding alcohols in a subsequent step by hydration of the alkanes. Before the hydration and after the catalytic conversion, gas and liquid phases may be separated. Specific catalysts can be employed that have a markedly higher selectivity for alkenes than for alkanes. These catalysts comprise grains of non-graphitic carbon having cobalt nanoparticles dispersed therein. The cobalt nanoparticles have an average diameter d.sub.p from 1 to 20 nm, and an average distance D between nanoparticles is from 2 to 150 nm. The combined total mass fraction of metal ω in the grains ranges from 30% to 70% by weight of the total mass of the grains of non-graphitic carbon, wherein 4.5 dp/ω>D≥0.25 dp/ω.

Claims

1.-18. (canceled)

19. A process for producing alcohols having at least two carbon atoms by catalytic conversion of synthesis gas into a first mixture containing alkenes, alcohols, and alkanes, wherein the alkenes present in the first mixture are subsequently converted into alcohols by hydration of the alkenes.

20. The process of claim 19 comprising: separating the alkanes and the alkenes in the first mixture from the alcohols to form a second mixture; and only subsequently hydrating the alkanes in the second mixture.

21. The process of claim 20 comprising initially separating the second mixture of the alkanes and the alkenes into two or more fractions having different numbers of carbon atoms and only subsequently hydrating at least one of the individual fractions to obtain a corresponding alcohol from the alkene in the fraction.

22. The process of claim 21 comprising separating the second mixture of the alkanes and the alkenes at least into a C2 fraction, a C3 fraction, and a C4 fraction.

23. The process of claim 20 wherein the second mixture contains a mixture of C2-C4 alkenes or C2-C5 alkenes that is hydrated to corresponding alcohols as a mixture.

24. The process of claim 19 comprising hydrating the alkenes with the first mixture of alkanes, alkenes, and alcohols without separating the alcohols from the first mixture beforehand.

25. The process of claim 20 comprising selecting conditions for hydration of the second mixture in terms of choice of catalyst and reaction conditions including temperature and pressure at which a hydration reaction is performed such that hydration of propene and/or 1-butene is favored over hydration of ethene.

26. The process of claim 19 wherein hydration occurs at temperatures above 80° C. and/or at a pressure of 5 bar to 150 bar.

27. The process of claim 26 comprising separating the alkanes from a product mixture that is obtained from the hydration.

28. The process of claim 19 wherein before hydration of the alkenes to corresponding alcohols and after the catalytic conversion of the synthesis gas, the process comprises separating a product mixture obtained into a gas phase and a liquid phase, wherein the liquid phase is used for subsequent hydration of alkenes to alcohols.

29. The process of claim 28 wherein the gas phase is at least partially recycled to the catalytic conversion of the synthesis gas.

30. The process of claim 19 wherein after the catalytic conversion of the synthesis gas and after subsequent hydration of alkenes to alcohols, the process comprises separating a product mixture that is obtained into a gas phase and a liquid phase, wherein the liquid phase contains at least the alcohols.

31. The process of claim 19 comprising at least partially converting at least one of alkenes or primary alcohols into secondary alcohols in the hydration.

32. The process of claim 30 wherein the gas phase obtained in the separation is at least partially recycled to the catalytic conversion of the synthesis gas.

33. The process of claim 19 wherein after the catalytic conversion of the synthesis gas, the process comprises processing a product mixture as follows: at least partially absorbing the alkenes and alcohols in a high-boiling hydrocarbon or a hydrocarbon mixture as an absorption medium; separating gases not absorbed into the absorption medium as a gas phase; separating an aqueous phase from an organic phase of the absorption medium; and desorbing the alkenes, the alkanes, and the alcohols from the absorption medium.

34. The process of claim 19 wherein before or after hydration of the alkenes, the separation of the alcohols from the alkanes comprises: separating the alcohols in at least one distillation column at an elevated pressure of at least 10 bar; and removing water from an alcohol fraction by extractive distillation, by pervaporation, or by azeotropic distillation.

35. The process of claim 19 wherein the catalytic conversion of the synthesis gas employs a catalyst that comprises grains of non-graphitic carbon having cobalt nanoparticles dispersed therein, wherein the cobalt nanoparticles have an average diameter d.sub.p in a range from 1 nm to 20 nm, wherein an average distance D between individual cobalt nanoparticles in the grains of non-graphitic carbon is in a range from 2 nm to 150 nm, wherein a combined total mass fraction ω of metal in the grains of non-graphitic carbon is in a range from 30% by weight to 70% by weight of a total mass of the grains of non-graphitic carbon, wherein 4.5 d.sub.p/ω>D≥0.25 d.sub.p/ω.

36. The process of claim 35 wherein a material of the catalyst is doped with a metal selected from Mn, Cu, or a mixture thereof, wherein the grains of non-graphitic carbon have a molar ratio of cobalt to doped metal in a range from 2 to 15.

Description

[0100] The present invention is described in more detail below on the basis of exemplary embodiments with reference to the accompanying drawings. In the figures:

[0101] FIG. 1 shows a graphical representation of the temperature dependence of the equilibrium of the hydration of ethene to ethanol at a pressure of 60 bar;

[0102] FIG. 2 shows a graphical representation of the temperature dependence of the equilibrium of the hydration of propene to propanol at a pressure of 60 bar;

[0103] FIG. 3 shows a graphical representation of the temperature dependence of the equilibrium of the hydration of butene to butanol at a pressure of 60 bar;

[0104] FIG. 4 shows a graphical representation of an exemplary product distribution after catalytic conversion of synthesis gas to higher alcohols and subsequent hydration of the product mixture consisting of the alcohols, alkenes and alkanes at a temperature of 150° C. and a pressure of 60 bar;

[0105] FIG. 5 shows a graphical representation of an exemplary product distribution after catalytic conversion of synthesis gas to higher alcohols and subsequent hydration of the product mixture consisting of the alcohols, alkenes, alkanes and synthesis gas at a temperature of 50° C. and a pressure of 60 bar;

[0106] FIG. 6 shows a graphical representation of an exemplary product distribution after catalytic conversion of synthesis gas to higher alcohols and subsequent hydration of the product mixture consisting of the alcohols, alkenes, alkanes and synthesis gas at a temperature of 130° C. and a pressure of 60 bar;

[0107] FIG. 7 shows a graphical representation of an exemplary product distribution after catalytic conversion of synthesis gas to higher alcohols and subsequent dehydration of the product mixture consisting of the alcohols, alkenes, alkanes and synthesis gas at a temperature of 280° C. and a pressure of 60 bar.

[0108] In the following, reference is first made to FIGS. 1 to 3 and the temperature dependence of the thermodynamic equilibrium is more particularly elucidated with reference to these representations. FIG. 1 shows graphically the temperature dependence of the equilibrium of ethene and ethanol at a pressure of 60 bar, FIG. 2 the temperature dependence of the equilibrium of propene propanol at a pressure of 60 bar and FIG. 3 the temperature dependence of the equilibrium of butene and butanol at a pressure of 60 bar. FIGS. 1 to 3 show that at the reaction conditions of 150° C. and 60 bar the thermodynamic equilibrium is on the side of the alcohols for all three reactions. The unconverted alkenes may together with the alkanes be converted for example into synthesis gas and recycled into the process. Indirect hydration of the alkanes may be preferable on account of the alkene/alkane mixture.

[0109] FIG. 2 further shows that it is 2-propanol that is practically exclusively formed, while the amount of 1-propanol is negligibly small.

[0110] It is apparent from FIG. 3 that at thermodynamic equilibrium at temperatures up to about 150° C. predominantly 2-butanol is formed, while at higher temperatures predominantly 2-butene is formed. The amount of 1-butene formed at higher temperatures is substantially lower compared to 2-butene, but continuously increases somewhat at still higher temperatures of 200° C. to 500° C. 1-Butanol is formed only in negligibly small amounts independently of temperatures.

[0111] In the following, reference is made to FIG. 4. In the embodiment of FIG. 4 the hydration of the reaction mixture consisting of alcohols, alkenes and alkanes was performed at a temperature of 150° C. As can be demonstrated using simulations and calculations of thermodynamic equilibrium the mixture of alkanes and primary alcohols is under these reaction conditions virtually completely converted into secondary alcohols. It is thought that the isomerization of the primary alcohols to the secondary alcohols proceeds via formation of the alkenes as intermediates. The hydration of the product mixture of the synthesis of higher alcohols from alcohols and alkenes thus makes it possible to shift the product spectrum in the direction of the secondary alcohols. The industrial hydration of propene and 1-butene proceeds at reaction temperatures of 120 to 150° C.

[0112] In the following, reference is made to FIG. 5. These two diagrams are used to elucidate for example the respective product distribution after the catalytic synthesis of higher alcohols according to the invention and the immediately subsequent step of hydration of the alkenes, wherein the hydration was performed at different temperatures in the two exemplary embodiments.

[0113] In the exemplary embodiment of FIG. 5 the hydration was simulated at a temperature of 50° C. This temperature is thermodynamically preferred as can be demonstrated using simulations and calculations. However, it must be noted in this purely thermodynamic view that the industrial processes for hydration are generally carried out at reaction temperatures of 130-260° C. It is therefore to be expected that at 50° C. the reaction proceeds at a markedly reduced reaction rate.

[0114] The product distribution in FIG. 5 shows that after the first reaction step, the synthesis of the higher alcohols, the alcohols formed are predominantly ethanol and 1-butanol and the alkanes formed are predominantly 1-propene and 1-butene as well as some ethene and 1-pentene. After the hydration at 50° C. the main products present are ethanol, 2-propanol and 2-butanol, while alkenes only remain in relatively small amounts, primarily butene and some pentene.

[0115] The hydration of the product mixture of the synthesis of higher alcohols from alcohols and alkenes thus makes it possible in principle to shift the product spectrum in the direction of the secondary alcohols. The industrial hydration of propene and 1-butene proceeds at reaction temperatures of 120 to 150° C.

[0116] FIG. 6 therefore shows in a similar diagram to FIG. 5 the respective product distribution after the synthesis of higher alcohols and also after the subsequent hydration, but in the present case at a higher temperature of 130° C. during the hydration. The product distribution of the alcohols and alkenes after the first synthesis step is the same as in FIG. 5.

[0117] The simulation of the thermodynamic equilibrium at 130° C. shows that propene and pentene are partially converted into the corresponding secondary alcohols. By contrast, ethanol and 1-butanol are dehydrated to afford the respective alkenes. 1-Propanol and 1-butanol are also partially isomerized to 2-propanol and 2-butanol. The isomerization of the linear alcohols to the secondary alcohols proceeds via formation of the alkenes as intermediates.

[0118] This simulation thus shows that according to the reaction conditions, product composition and reaction conditions this variant may be advantageous for the hydration of individual alkenes, thus making it possible to increase the yield of these alcohols. By contrast, for other alcohols this variant may have the result that the alcohol yield is reduced and the alcohols are preferentially converted into alkenes. This process variant may ultimately also achieve a shift in the product spectrum. Supplemental to the description of process variant 4 the product mixture thus obtained may be converted into alcohols in a further hydration reaction, for example by combination of process variant 4 with one of the process variants 1, 2 or 3.

[0119] In the following, reference is made to FIG. 7. The reaction conditions of the hydration of ethene and of propene are similar to the synthesis of the higher alcohols so that in an alternative variant of the invention it may optionally be advantageous to perform the hydration of the alkenes directly in a reactor arranged downstream of the catalytic synthesis of higher alcohols and without preceding separation of the product mixture. Here it is advantageous that the reaction mixture is already at a similar temperature and pressure level in the alcohol synthesis as is required for the conversion in the hydration. The reaction mixture need not be cooled and compressed to a low temperature and low pressure (for example 30° C., 1 bar) but rather may be converted directly.

[0120] However, simulations show that under the reaction conditions of the catalytic synthesis of higher alcohols the dehydration of the alcohols to the alkenes is thermodynamically preferred (see FIG. 7). The synthesis of the higher alcohols was simulated using the current experimental conversions and selectivities from catalyst development and testing. The subsequent dehydration/hydration was calculated with an equilibrium reactor. The results clearly show that under the reaction conditions of the catalytic synthesis of higher alcohols (280° C., 60 bar) an almost complete conversion of the alcohols into the corresponding alkenes is possible.

[0121] According to observation of the thermodynamic alcohol/alkene equilibrium the direct hydration of the product mixture of the catalytic synthesis of higher alcohols does not lead to an increase in alcohol yield. The formation of alkanes instead of the desired alcohols is preferred in some cases.

[0122] Conclusion: The consecutive hydration of the alkenes formed as byproducts in the catalytic synthesis of higher alcohols makes it possible to increase the alcohol yield with suitable reaction management. This equilibrium reaction further makes it possible in principle to convert the complex reaction mixture of primary alcohols and alkenes into secondary alcohols (with the exception of ethanol) by means of dehydration and hydration steps. The reduction of the products results in a small number of purification steps of the individual products and facilitates the marketing of the products of the higher alcohols synthesis.

Example 1

[0123] Example 1 which follows specifies an exemplary product composition obtained in the catalytic conversion of synthesis gas by the process according to the invention using a catalyst which comprises grains of non-graphitic carbon having cobalt nanoparticles dispersed therein, wherein the cobalt nanoparticles have an average diameter d.sub.p in the range from 1 nm to 20 nm and the average distance D between individual cobalt nanoparticles in the grains of non-graphitic carbon is in the range from 2 nm to 150 nm and ω, the combined total mass fraction of metal in the grains of non-graphitic carbon is in the range from 30% by weight to 70% by weight of the total mass of the grains of non-graphitic carbon, wherein d.sub.p, D and ω satisfy the following relationship: 4.5 dp/ω>D≥0.25 dp/ω. The catalyst used showed a high C2-C4 selectivity and alcohols, alkenes, and alkanes were formed. The CO selectivity in respect of the conversion to alcohols is about 28%, the CO selectivity in respect of the conversion to alkenes is about 32%. The precise CO selectivities of the catalytic conversion of synthesis gas are apparent from table 1 which follows. The selectivities reported in table 1 were normalized to the products detected in the catalytic tests (C1-C5 alcohols, C1-C5 alkenes and C1-C5 alkanes, CO2). The analysis of the CO conversion indicates that, in addition to the recited products detected, long-chain C6+ alcohols, C6+ alkenes and C6+ alkanes, and in some cases aldehydes, are also formed.

TABLE-US-00001 TABLE 1 Selectivity [CO selectivity] CO.sub.2 9.8 Methane 17.9 Ethane 4.6 Propane 4.3 Butane 3.0 Pentane 0.3 Ethene 6.0 1-Propene 15.1 1-Butene 7.2 Pentene 4.2 Methanol 3.7 Ethanol 4.6 1-Propanol 1.1 2-Propanol 0.00 1-Butanol 18.3 Alkanes (C2-C5) 12.2 Alkenes (C2-C5) 32.5 Higher alcohols 24.0

[0124] This example employed a pulverulent catalyst. The catalyst may alternatively also be pressed into tablets for example.

[0125] Table 1 above shows that the catalytic conversion of synthesis gas according to the invention affords a relatively high proportion of alcohols in addition to the alkenes. The proportion of alkanes in the product mixture is lower in comparison. In the hydration step which follows the alkenes may likewise be converted to alcohols so that inclusive of the subsequent hydration step the synthesis gas may be converted into alcohols with an overall CO selectivity of virtually 60%, wherein primary alcohols (methanol, ethanol, 1-propanol and 1-butanol) are obtained from the alcohol synthesis and ethanol and secondary alcohols (2-propanol, 2-butanol and optionally 2-pentanol) are obtained from the hydration step and wherein the methanol content is relatively low. Such an alcohol mixture is suitable for example as a fuel additive for blending with gasoline. Separation into the individual alcohols is alternatively possible.

Example 2

[0126] A possible process for separating the product mixture obtained in the catalytic conversion of synthesis gas is described below by way of example. The exemplary separation process described below describes the separation of the mixture of alcohols, alkenes and alkanes obtained by the reaction of the synthesis gas from the gas phase and its subsequent separation into a mixture of alcohols and a mixture of hydrocarbons. When using the different process variants and converting the product mixture obtained, the individual steps of this process for separating the product mixture can be varied and adapted to the product mixture obtained after the conversion.

[0127] Inert Gas Removal

[0128] Catalytic conversion of a synthesis gas stream under the conditions of the process according to the invention affords a product stream at a temperature of 280° C. and a pressure of 60 bar. This is initially decompressed to a pressure of 5 to 20 bar, preferably to about 10 bar, in a turbine to generate electrical energy which may be used for the power requirements of the process.

[0129] The subsequent gas-liquid separation, which serves in particular to separate the inert gases (nitrogen) and unreacted components of the synthesis gas (hydrogen, carbon monoxide, carbon dioxide and methane), is carried out by absorption of the product stream in a diesel oil (reference component dodecane) or alternatively in an alkane or a hydrocarbon mixture having a relatively low viscosity of less than 10 mPas at room temperature for example and preferably having a relatively high boiling point of more than 200° C. in particular. The water is not absorbed but rather is largely condensed as the second liquid phase.

[0130] The two liquid phases (organic phase and aqueous phase) can then be separated in a decanter, the hydrocarbons barely, but the alcohols partially, passing into the aqueous phase. The alcohols may be distilled out of the water again as azeotropes by means of a first column for example. Alcohols and hydrocarbons are then desorbed from the diesel oil, which may be done in a column. The diesel oil may be recycled into the absorption process after desorption. At relatively low inert gas factions in the product stream of the catalytic conversion of synthesis gas a condensation of the low-boiling components may alternatively also be contemplated.

[0131] Separation of Alcohols/Hydrocarbons

[0132] The subsequent separation of alcohols and hydrocarbons is carried out by distillation in a second column, preferably at a high pressure of 10 bar to 40 bar for example, in order that the C3 fractions remain condensable even in the presence of any residues of inert gas. This separation is preferably carried out such that the hydrocarbons are practically completely removed from the alcohol fraction at the column bottom, while smaller alcohol contents (in particular methanol) in the hydrocarbons may be tolerated. This process may optionally also be assisted by a solubility-based membrane.

[0133] Preparation of the Hydrocarbons

[0134] In a third distillation column the hydrocarbons are obtained overhead at elevated pressure of for example 5 bar to 20 bar while the remaining water and the alcohols dissolved therein are obtained in the bottoms and separated. This stream can be recycled to the first distillation column to recover the alcohols. The condenser of the column may be a partial condenser for example. The outputs of the column are a gas phase of hydrocarbons and inerts, a liquid phase of hydrocarbons and an aqueous phase which may be returned to the column as reflux.

[0135] Dewatering of the Alcohol Fraction

[0136] The alcohol fraction may have a water content of about 10% for example. This water may be removed using a molecular sieve for example.

[0137] A contemplated alternative method for removing the water from the alcohol fraction is extractive distillation for example with ethylene glycol, though this requires a further separation step since the water is pulled into the bottoms by the ethylene glycol while the alcohols methanol and ethanol are obtained overhead practically free from water. About half of the propanol and all of the butanol remain in the bottoms and these C3-C4 alcohols must likewise be removed from the ethylene glycol overhead in a subsequent column.

[0138] A third suitable alternative is pervaporation. Water passes selectively through a membrane and is withdrawn in vaporous form as permeate. Energy consumption is even lower than for a molecular sieve.

[0139] A further alternative method would be an azeotropic distillation, for example with butane or pentane as a selective additive.