PROCESS FOR PREPARING METHANOL

A process for the production of methanol (CH3OH) from carbon dioxide (CO2) and hydrogen (H2), wherein CO2 is reacted with H2 over a manganese-promoted molybdenum(IV) sulfide catalyst; as well as a catalyst for such a process and a production process for the catalyst.

The present invention relates to a process for the catalytic production of methanol from carbon dioxide and hydrogen. Furthermore, the invention relates to a catalyst for the production of methanol from carbon dioxide and hydrogen. Finally, the invention relates to the use of a catalyst for the production of methanol from carbon dioxide and hydrogen.

BACKGROUND OF THE INVENTION

According to the prior art, various processes are available for the fully synthetic production of alcohols. In industrial terms, processes in which carbon monoxide (CO) or carbon dioxide (CO2) serve as starting materials are of particular importance. These processes are divided into processes for the production of methanol (CH3OH) on the one hand and processes for the production of higher alcohols, i.e., alcohols with more than one carbon atom, on the other hand. The selectivity and the yield of the desired alcohol are important criteria.

According to the prior art, the industrial production of methanol is effected, for example, via the hydrogenation of carbon monoxide or carbon dioxide, in each case at high pressures, using suitable catalysts. Both reactions occur during the hydrogenation starting from a synthesis gas, although the yield of CO2hydrogenation is in need of improvement.

Liu et al., Journal of the Taiwan Institute of Chemical Engineers, 76 (2017), page 18, describe the production of higher alcohols by means of CO2hydrogenation with a Mo-Co-K sulfide catalyst, wherein the catalyst may comprise MoS2, among other things.

Qi et al., Catalysis Communication, 4 (2003), page 339, describe CO hydrogenation by means of K/MoS2. The addition of manganese to the catalyst is described as well, with a Ni/Mn/K/MoS2catalyst finally being described as suitable for the hydrogenation of CO. The result shows a very high selectivity for alcohols with an overall yield of 81.7%. Therein, methanol accounts for 45.8% and higher alcohols account for 53.3% (Cn alcohols with n=2, 3, 4 and 5).

Zeng et al., Applied Catalysis B: Environmental, 246 (2019), page 232, describe the production of higher alcohols with at least three carbon atoms (Cnalcohols with n≥3) by means of CO hydrogenation over MoS2promoted with potassium.

DETAILED DESCRIPTION OF THE INVENTION

The selective processes known from the prior art for the production of methanol are often based on carbon monoxide as the starting material. Processes based on CO2either are not very selective or require expensive catalysts or, respectively, complex process conditions. If CO is to be used as a starting material, an additional reaction step is necessary for the production of CO, whereas CO2is available in practically unlimited quantities (for example, it accumulates as a waste product in the form of flue gas during the combustion of hydrocarbons).

It is the object of the present invention to provide a selective and inexpensive process and a catalyst for the selective hydrogenation of CO2to methanol. Furthermore, the catalyst should be sulfur tolerant, i.e., tolerant of trace amounts of sulfur compounds in the reaction gas.

This object is achieved by a selective process for the production of methanol (CH3OH, MeOH) from carbon dioxide (CO2) and hydrogen (H2), wherein CO2is reacted with H2over a manganese-promoted MoS2catalyst.

The invention is based on the finding that a manganese-promoted MoS2catalyst catalyzes the CO2hydrogenation in a highly selective manner. In particular, the very high yield of methanol and the high specificity of the formation of methanol are surprising in comparison to already known sulfur-tolerant catalysts. In this process, there is almost no formation of higher alcohols, and the amount of by-products formed, such as CO or CH4, is also small.

Although the exact reaction mechanism is not yet known to the inventors, a two-stage reaction sequence with an upstream RWGS step (Reverse Water-Gas Shift Reaction) and a subsequent CO hydrogenation to methanol

is rather improbable, since comparative tests on the MoS2catalyst promoted with manganese have shown hardly any conversion of CO with H2to form CH3OH. The high yield and selectivity is all the more surprising.

Therefore, in one aspect, the invention relates to the use of a manganese-promoted MoS2catalyst for the production of methanol from CO2and H2.

The object stated at the outset is further achieved by a catalyst comprising manganese-promoted molybdenum(IV) sulfide (MoS2), the manganese-promoted molybdenum(IV) sulfide having a layered structure which can have various disorders. The structure can be described by way of the borderline cases 2H-MoS2and 3R-MoS2. The proportion of manganese is such that the molar ratio of Mn to Mo is 0.1 to 0.5:1, preferably 0.2 to 0.4:1. Furthermore, XPS studies have shown that manganese can exist in the oxidation stages (II) and (III).

According to the invention, the manganese-promoted molybdenum(IV) sulfide (MoS2) can be a mixed crystal of manganese sulfide(s) and MoS2, with the basic structure being formed by the MoS2and manganese sulfide(s) being incorporated into this basic structure, wherein, optionally, manganese oxide(s), manganese hydroxide(s) and/or MnOOH are additionally incorporated into the basic structure according to the previous paragraph.

Additionally, the catalyst can be promoted with potassium. In this case, a phase of a K(I) salt, preferably K2CO3, is present on the surface of the manganese-promoted molybdenum(IV) sulfide. Such a catalyst is hereinafter referred to as manganese-promoted molybdenum(IV) sulfide with potassium.

The catalyst can additionally have a carrier on which the manganese-promoted molybdenum(IV) sulfide (optionally with potassium) is applied. The carrier can be a porous material. For example, the carrier can be an aluminium oxide or aluminium oxide hydroxide such as Al2O3or AlO(OH).

The catalyst preferably consists of Mn(0.1 to 0.50)MoS2, preferably Mn(0.2 to 0.4)MoS2, optionally with a carrier as mentioned above.

The catalyst described above has proved to be useful for the process. Therefore, in one aspect, the invention relates to such a catalyst.

Furthermore, reaction conditions in the process have proved to be advantageous which have a pressure that has been increased in comparison to standard conditions. Therefore, it is preferably intended for the reaction to take place at a pressure of ≥10 bar. For example, the pressure can be 10 bar to 200 bar or 10 bar to 100 bar. In one embodiment variant, the pressure was between 18 bar and 23 bar.

In principle, the reaction can proceed over a wide temperature range. Suitable temperatures are, for example, between 140° C. and 320° C. If a pure manganese-promoted MoS2catalyst is used, the ideal temperature range is preferably between 170° C. and 220° C.

If a manganese-promoted MoS2catalyst mixed with potassium is used, the ideal temperature range for the reaction is somewhat higher, namely preferably between 260° C. and 300° C.

It is preferably envisaged that the partial pressure ratio of CO2to H2is about 1:2.5 to 3.5, preferably approximately 3. This means that the partial pressure of hydrogen should be about 2.5 to 3.5 times higher than the partial pressure of CO2.

Furthermore, it has surprisingly been shown that the addition of an inert gas to the reaction mixture of CO2and H2, for example, of a noble gas (such as, e.g., helium) or of nitrogen, hardly impedes the reaction. The yield decreased only slightly. The partial pressure of inert gas can be about 1:0.5 to 1.5—based on CO2. The realization that inert gas does not interfere with the reaction means that flue gas, which contains mostly nitrogen, can also be used as a source of CO2.

In one embodiment variant, the CO2can therefore come from flue gas. In this case, the process according to the invention is a selective process for the production of methanol from CO2and H2, the source of CO2being flue gas, wherein CO2is reacted with H2over a manganese-promoted MoS2catalyst. Such a process is suitable for subjecting flue gas to recycling.

Although all manganese-promoted MoS2catalysts are suitable as catalysts, those presented according to the next-described process prove to be particularly efficient. Therefore, in one aspect, the invention relates to a process for the production of a manganese-promoted MoS2catalyst for the production of methanol from CO2and H2, comprising the steps of:(i) forming a mixture of water, ammonium molybdate (particularly (NH4)6Mo7O24.4H2O ), thiourea (CH4N2S) and a water-soluble manganese(II) salt;(ii) raising the temperature of this mixture in an autoclave to 150-250° C. and increasing the pressure to such a level that part of the water remains liquid, maintaining the temperature and pressure until the thiourea decomposes and a sulfide mixture comprising manganese sulfide and MoS2forms;(iii) washing the sulfide mixture obtained from step (ii);(iv) drying the washed sulfide mixture from step (iii);(v) calcining the dried and washed sulfide mixture from step (iv) under inert gas to obtain the manganese-promoted MoS2catalyst.

The sulfide mixture in step (ii) and in subsequent steps may comprise Mn(II) oxide, Mn(III) oxide, Mn(II) hydroxide, Mn(III) hydroxide or MnOOH. In addition, these compounds form, in particular, at the upper end of the temperature range.

The pressure in the autoclave preferably ranges from 5 to 40 bar, preferably it is about 15.5 bar.

Optionally, potassium can also be added to the washed sulfide mixture before it is calcined, but after it has been dried. The addition of potassium can take place in the form of an aqueous K(I) solution, e.g., a K2CO3solution, wherein a drying step is then provided before the calcination. The K(I) solution can be added via ultrasonic dispersion.

Furthermore, a carrier can be provided for the catalyst. In this case, the sulfide mixture is mixed with a carrier prior to the calcination step. The carrier can be a porous material. Aluminium oxides, e.g., AlO(OH) or AlO2O3, have proved to be suitable carriers.

Preferably, the carrier is precipitated, preferably from a precursor compound, while raising the temperature of the mixture in the autoclave to 150-250° C. and increasing the pressure to such a level that part of the water remains liquid. The precursor can be Al(NO3)3, for example, and initially it is present in a dissolved state. Subsequently, the dissolved precursor can be precipitated as Al(OH)3or AlO(OH).

The reaction conditions in the examples shown in the figures at the beginning of the reaction, the way how the gas mixture is passed over the catalyst, are summarized in Table 1.

The total flow of the gas mixture as it is passed over the catalyst is:

In this formula, “ml N” stands for millilitres under normal or standard conditions, i.e., at 273.15 K or 0° C. and 1 bar pressure. The normalization to normal conditions is carried out because, under 21 bar, 1 ml would have a higher molar number than under 1 bar; therefore, the flow is converted and related to the volume flow under normal conditions.

InFIG.1, the reaction yield of methanol as a function of temperature in a process according to the invention is shown, when CO2is allowed to react with H2over a simple molybdenum(IV) sulfide catalyst promoted with manganese. It can be seen very clearly that there is a maximum yield of methanol at around 200° C. to 210° C., while only a few by-products are formed at this temperature. With rising temperature, the formation of methane (CH4) increases, while the yield of methanol decreases. The amount of carbon monoxide (CO) formed also increases with rising temperature. The ideal temperature range is therefore around 180° C. to 220° C.

FIG.2shows, in comparison to the example ofFIG.1, that the reaction yield of methanol as a function of temperature is extremely low in a process in which CO is allowed to react with H2over a molybdenum(IV) sulfide catalyst promoted with manganese. As the temperature rises, the formation of CO2and CH4begins. CO should therefore be irrelevant during the formation of methanol on said catalyst.

InFIG.3, the reaction yields of the reaction CO2+2H2→CH3OH over a simple MoS2catalyst promoted with manganese (▪; see example ofFIG.1) and over a MoS2catalyst promoted with manganese further with potassium (●) as a function of temperature in the process according to the invention are compared. As already described inFIG.1, in case of simple manganese-promoted molybdenum(IV) sulfide, the reaction processes show a maximum yield at around 200 to 210° C. In case of the manganese-promoted MoS2catalyst with potassium, the maximum yield shifts to around 280° C. The addition of potassium therefore shifts the maximum yield towards higher temperatures, while a reduction in the yield (mol % based on the CO2used) from just under 0.7% to approx. 0.4% can be observed at the same time. However, the disadvantage resulting from the use of the manganese-promoted MoS2catalyst with potassium in the form of a lower yield combined with a higher ideal temperature range is accompanied by the advantage of a significant decrease in the formation of CH4, with CH4being an undesirable by-product. This correlation is also illustrated inFIG.4, where it is evident in this chart that, in case of simple (i.e., potassium-free) manganese-promoted molybdenum (IV) sulfide, the maximum yield of CH3OH is already associated with a significant increase in the yield of CH4at approx. 200 to 210° C. In case of the manganese-promoted MoS2catalyst with potassium, the methane yield is still low at the maximum yield for CH3OH at 280° C.

FIG.5shows the reaction yield of the reaction CO2+2H2→CH3OH as a function of temperature in a process over a MoS2catalyst promoted with cobalt. The maximum yield occurs at around 280° C. It is not difficult to see that, in comparison to MoS2catalysts promoted with manganese (with and without potassium), not only is the amount of CH4formed comparatively high, but especially also the amount of CO formed is so high that this catalyst is mostly unselective for the formation of methanol. The yield of CO is higher by orders of magnitude than that of methanol already at approx. 200° C., and the yield of CH4also increases significantly from around 280° C.

FIG.6shows, in comparison to the example ofFIG.5, the reaction yield of methanol as a function of temperature in a process in which CO reacts with H2over a cobalt-promoted MoS2catalyst with potassium. The yields of methanol and methane are slightly higher overall, but CO2is the main product even at low temperatures and from about 300° C. the CH4yield exceeds the amount of CH3OH formed.

FIG.7shows a comparison of the yield of methanol formed in the reaction of CO2with H2over various catalysts. A nickel-promoted MoS2catalyst with potassium (▴) provides the lowest yields. A MoS2catalyst promoted with cobalt shows only a slightly higher methanol yield (●). A MoS2catalyst with K (▪) shows significantly better yields, but the highest yields can be found in the process according to the invention with manganese-promoted MoS2with potassium (▾).

The chart ofFIG.8shows the comparison of the reaction yields of methanol (MeOH) or, respectively, CO and CH4from the reaction of CO2with H2in a process according to the invention over various manganese-promoted MoS2catalysts with different proportions of Mn and Mo. The abscissa shows the molar proportion of manganese in relation to molybdenum. The maximum methanol yield is from 0.2 to 0.4. (Reaction conditions: 21 bar, 180° C., 20% CO2, 60% H2, 20% He, 300 mlN/(gcatalyst*h)

The column chart ofFIG.9illustrates the reaction yields of methanol, CH4and CO from the reaction of CO2with H2over a Mn(0.30)MoS2catalyst in the presence and absence of helium as an inert gas. The yields of a mixture of 20% CO2, 60% H2and 20% He are illustrated in the left-hand chart, a mixture of 25% CO2and 75% H2is illustrated in the right-hand chart. It can be seen that the yield of methanol decreases only slightly in the presence of He, surprisingly, the yield of CO decreases at the same time by a significant amount. (The reaction conditions are in each case 21 bar, 180° C., 300 mlN/(gcatalyst*h)).

The column chart ofFIG.10illustrates the reaction yields of methanol, CH4and CO from the reaction of CO2with H2over three different catalysts. The left-hand chart shows the yields over a manganese-promoted MoS2catalyst (Mn(0.25)MoS2), the middle chart shows yields on a “simple” MoS2catalyst, and the right-hand chart shows those on a manganese-promoted MoS2catalyst (Mn(0.25)MoS2) applied to an AlO(OH) carrier. A significantly higher selectivity of the two manganese-promoted MoS2catalysts with regard to methanol can be seen, the significantly lower yields of the undesired by-product CH4are particularly striking (reaction conditions: in each case 21 bar, 180° C., 20% CO260% H220%, He 300 mlN/(gcatalyst*h))

FIG.11shows the comparison of the reaction yields of methanol of a manganese-promoted MoS2catalyst and a cobalt-promoted MoS2catalyst with potassium from the reaction of CO2with H2as a function of temperature. The reaction yields with the manganese-promoted MoS2catalyst are not only higher, but also shifted toward lower temperatures. (Reaction conditions: in each case 21 bar, 180° C., 20% CO260% H220% He, 300 mlN/(gcatalyst*h).

Furthermore,FIG.12shows the comparison of the reaction yields of methanol of a manganese-promoted MoS2catalyst and a cobalt-promoted MoS2catalyst with potassium from the reaction of CO with H2as a function of temperature. In this depiction, the selectivity of the manganese-promoted MoS2catalyst can be seen even more clearly. (Reaction conditions: in each case 21 bar, 180° C., 20% CO 60% H220% He, 300 mlN/((gcatalyst*h). Since flue gas can comprise residual amounts of CO, the high selectivity which occurs when flue gas is used as a source for CO2constitutes an advantage over other catalysts.

In contrast to the prior art of sulfur-insensitive catalysts, the selective formation of methanol by means of CO2hydrogenation on a manganese-promoted MoS2(with or without potassium) catalyst is therefore significantly greater.