PROCESS FOR PREPARING ALKENES

Abstract

A process can be used to prepare alkenes by catalytic conversion of synthesis gas to a first mixture comprising alkenes and alcohols. The alcohols present in the first mixture are converted to the corresponding alkenes by dehydration in a subsequent step. At least one alkene having two to four carbon atoms is obtained as isolated product from a product mixture by processing thereof and/or separation steps. In the catalytic conversion, a catalyst is preferably used that comprises grains of non-graphitic carbon having cobalt nanoparticles dispersed therein. The cobalt nanoparticles have an average diameter d.sub.p of 1-20 nm. An average distance D between individual cobalt nanoparticles in the grains is 2-150 nm. A combined total mass fraction ω of metal in the grains is from 30%-70% by weight of a total mass of the grains such that 4.5 dp/ω>D≥0.25 dp/ω.

Claims

1.-18. (canceled)

19. A process for preparing alkenes by catalytic conversion of synthesis gas to give a first mixture comprising alkenes and alcohols, wherein alcohols present in the first mixture are converted to corresponding alkenes in at least one subsequent step by dehydration, wherein at least one alkene having two to four carbon atoms is obtained as an isolated product from a product mixture by processing thereof and/or separation steps, either before or after the dehydration of the alcohols.

20. The process of claim 19 wherein from the first mixture of alkenes, alcohols, and alkanes obtained after the catalytic conversion of synthesis gas, the alkanes and alkenes are separated from the alcohols first and then the separated alcohols are dehydrated.

21. The process of claim 20 wherein a mixture of separated alcohols is first separated into two or more fractions having a different number of carbon atoms and only then is at least one individual alcohol fraction dehydrated to obtain the corresponding alkene from the alcohol.

22. The process of claim 21 wherein the mixture of alcohols is separated at least into a C2 fraction, a C3 fraction, and a C4 fraction and at least one of ethene, propene, or butene is obtained from one or more of these fractions.

23. The process of claim 20 wherein after the alkenes and alkanes have been separated off, a mixture of predominantly C2-C4 alcohols comprising the alcohols is then dehydrated in the mixture to form the corresponding alkenes.

24. The process of claim 19 wherein after the dehydration, methanol is separated off from the alkenes and the alkenes are combined with a stream of alkenes and the alkanes separated off prior to the dehydration, wherein an alkene-alkane mixture is separated into individual compounds or compound groups that include fractions each having the same number of carbon atoms, including C2 or C3 or C4 hydrocarbons.

25. The process of claim 24 wherein the alkenes are each separated off from the alkanes from individual fractions, each having the same number of carbon atoms, so that ethene, propene, and butene are obtained.

26. The process of claim 19 wherein the dehydration is performed at temperatures in a range from 200° C. to 400° C. and/or at a pressure from 1 bar to 100 bar.

27. The process of claim 19 wherein the dehydration of the alcohols is performed with the first mixture of alkanes, alkenes, and alcohols without the alcohols having been separated off from the first mixture beforehand.

28. The process of claim 27 wherein after the dehydration, methanol is separated off from a product mixture obtained, after which a remaining mixture of alkanes and alkenes is separated into individual fractions each having the same number of carbon atoms, including C2, C3, or C4 fractions, and the alkene is separated off from the alkane from at least one of the individual fractions each having the same number of carbon atoms, so that at least one of ethene, propene, or butene is obtained.

29. The process of claim 24 wherein the methanol is removed at a lower temperature and a lower pressure than the dehydration.

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

31. The process of claim 30 wherein the gas phase is at least partially recycled to the catalytic conversion of the synthesis gas.

32. The process of claim 19 wherein after the catalytic conversion of the synthesis gas and after subsequent dehydration of the alcohols to corresponding alkenes, the process comprises separating a product mixture obtained in the catalytic conversion into a gas phase and a liquid phase, wherein methanol is then separated off from the liquid phase.

33. The process of claim 32 wherein the gas phase is at least partially recycled to the catalytic conversion of the synthesis gas.

34. The process of claim 32 wherein after the catalytic conversion of the synthesis gas the product mixture is processed by steps comprising: at least partially absorbing the alkenes and alcohols in a high-boiling hydrocarbon or hydrocarbon mixture as an absorption medium; separation of gases not absorbed into the absorption medium as a gas phase; separating an aqueous phase from an organic phase of the absorption medium; and desorption of the alkenes, the alkanes, and the alcohols from the absorption medium.

35. The process of claim 19 wherein using a catalyst in the catalytic conversion of the synthesis gas, wherein the catalyst 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 of 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 of 2 nm and 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 of 2 to 15.

Description

[0092] The present invention is described in more detail below on the basis of exemplary embodiments with reference to the accompanying drawings. In the figures:

[0093] FIG. 1 is a graphic representation of the product distribution after the catalytic conversion of synthesis gas to higher alcohols and subsequent dehydration of the product mixture at a temperature of 280° C. and a pressure of 60 bar, wherein the product distribution between the alcohols and the alkenes in thermodynamic equilibrium is shown under the assumption that an isomerization of the 1-alkenes to the 2-alkenes can take place.

[0094] FIG. 2 shows a graphic representation of the product distribution between the alcohols and the alkenes in thermodynamic equilibrium, assuming that no isomerization of the 1-alkenes to the 2-alkenes takes place.

[0095] For the working example shown in FIG. 1, the following equilibrium reactions were considered:

ethanol.Math.ethene+H.sub.2O

1-propanol.Math.propene+H.sub.2O

1-butanol.Math.1-butene+H.sub.2O

1-pentanol.Math.1-pentene+H.sub.2O

1-butene.Math.cis-2-butene

1-butene.Math.trans-2-butene

1-pentene.Math.cis-2-pentene

1-pentene.Math.trans-2-pentene

[0096] FIG. 1 shows the product distribution in thermodynamic equilibrium. According to this, after the first reaction step, the synthesis of the higher alcohols, the alcohols formed are predominantly ethanol and 1-butanol and the alkenes formed are predominantly 1-propene and 1-butene as well as some ethene and 1-pentene. After dehydration at 280° C., the main products in equilibrium are ethene and 1-propene, as well as the butenes trans-2-butene, cis-2-butene and 1-butene and some trans-2-pentene in decreasing proportions. This is due to the fact that in thermodynamic equilibrium, over a very long reaction time, trans- and cis-2-butene are formed from 1-butene, since these are thermodynamically more stable than 1-butene. However, if the residence time is shortened, it is possible to achieve that only, or at least predominantly, 1-butene is formed. Alcohols are practically no longer present, except for a small amount of methanol, which cannot be dehydrated and can be easily removed from the mixture.

[0097] Experiments and simulations on the catalytic synthesis of higher alcohols with subsequent dehydration according to the invention 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. 1). The synthesis of the higher alcohols was simulated on the basis of the experimental conversions and selectivities with the specific catalysts preferably used in this synthesis in the context of the present invention. The subsequent dehydration was calculated for an equilibrium reactor. The results clearly show that under the reaction conditions of the HA synthesis (280° C., 60 bar), a virtually complete conversion of the alcohols to the corresponding alkenes takes place. The results also show increasing formation of 2-butene and 2-pentene in thermodynamic equilibrium.

[0098] From a thermodynamic point of view, the dehydration of the alcohol mixture thus lends itself to the catalytic synthesis of higher alcohols at temperatures of ca. 280° C. The extent to which the dehydration actually proceeds under the reaction conditions may also depend on the respective catalysts used. It is also possible that other components of the product mixture (alkenes, alkanes, H.sub.2, CO, CO.sub.2) react under the conditions of the catalytic dehydration or affect the dehydration (e.g. also the C3+ alcohols) (see US 2009/0281362 A1). Among other things, these aspects depend on which of the aforementioned process variants is preferable in the individual case, for example, the in situ conversion of the alcohols to the corresponding alkenes may have advantages or disadvantages compared to a downstream dehydration, such as the dehydration of individual alcohols.

[0099] In the representation according to the diagram in FIG. 2, only the following equilibrium reactions are considered:

ethanol.Math.ethene+H.sub.2O

1-propanol.Math.propene+H.sub.2O

1-butanol.Math.1-butene+H.sub.2O

1-pentanol.Math.1-pentene+H.sub.2O

[0100] It is therefore assumed in FIG. 2 that no isomerization of the 1-alkenes to the 2-alkenes takes place. The diagram shows that dehydration of 1-butanol to 1-butene is possible. FIG. 2 thus represents the preferred product distribution in which no 2-alkenes are formed.

EXAMPLE 1

[0101] 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 selectivities of the catalytic conversion of the 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, CO.sub.2). The analysis of the CO conversion indicates that, in addition to the specified products detected, long-chain C6+ alcohols, C6+ alkenes and C6+ alkanes, and in some cases aldehydes, are also formed.

TABLE-US-00001 TABLE 1 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 0.0 1-Butanol 18.3 Alkanes (C2-C5) 12.2 Alkenes (C2-C5) 32.5 Higher alcohols 24.0

[0102] This example employed a pulverulent catalyst. The catalyst may alternatively also be pressed into tablets for example.

[0103] Table 1 above shows that the catalytic conversion of synthesis gas according to the invention affords a relatively high CO selectivity for the alcohols and for the alkenes. In comparison, the selectivity for the alkanes is lower. The higher alcohols (from C2) can be converted to further alkenes in the subsequent dehydration step, so that, including this dehydration step, in total the synthesis gas can be converted to alkenes with a CO selectivity of about 56%, for example, wherein the 1-alkenes are preferably obtained (see above) in the dehydration, so that 1-propene, 1-butene and some 1-pentene are formed in addition to ethene (see FIG. 2).

EXAMPLE 2

[0104] 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 is preferably used for process variants 1 and 2 and 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 variants 3 or 4, individual steps of this process can be adapted to the product mixture obtained after the conversion or omitted due to the previous conversion of the product mixture.

Inert Gas Removal

[0105] 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.

[0106] The subsequent gas-liquid separation, which serves in particular to separate the inert gases (for example nitrogen) and unreacted components of the synthesis gas (hydrogen, carbon monoxide, carbon dioxide and methane), is carried out by absorbing the product stream in a diesel oil (reference component dodecane) or alternatively in an alkane or a hydrocarbon mixture with a comparatively low viscosity of, for example, less than 10 mPas at room temperature and preferably with a comparatively high boiling point of, in particular, more than 200° C. The water is not absorbed in the process, but is largely condensed as the second liquid phase.

[0107] 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 fractions in the product stream of the catalytic conversion of synthesis gas, a condensation of the low-boiling components may alternatively also be considered.

Separation of Alcohols/Hydrocarbons

[0108] 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 be assisted by a solubility-based membrane.

Preparation of the Hydrocarbons

[0109] 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.

Dewatering of the Alcohol Fraction

[0110] The alcohol fraction may have a water content of about 10% for example. This water may be removed using a molecular sieve for example.

[0111] 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 proceed 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.

[0112] 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.

[0113] A further alternative method would be an azeotropic distillation, for example with butane or pentane as a selective additive.