Catalyst and process for producing methanol

Abstract

A catalyst comprising a support and a catalytically active coating, wherein the catalytically active coating comprises a metal sulfide, wherein the support is zinc(II) oxide, wherein the proportion of metal sulfide is between 15% and 30% by weight, preferably between 15% and 25% by weight and particularly preferably between 15% and 20% by weight, based on zinc(Il) oxide.

Description

[0001] CATALYST AND PROCESS FOR THE PRODUCTION OF METHANOL

[0002] The present invention relates to a process for the catalytic production of methanol from carbon dioxide and hydrogen, wherein carbon dioxide is reacted with hydrogen over a catalyst composed of zinc oxide and metal sulfide. Furthermore, the invention relates to a catalyst for producing methanol from carbon dioxide and hydrogen. Finally, the invention relates to the use of a catalyst for producing methanol from carbon dioxide and hydrogen.

[0003] BACKGROUND OF THE INVENTION

[0004] Methanol is not only an important raw material for further processing into more complex chemical products, it also represents an energy source for direct combustion or conversion in fuel cells. Synthetic processes for the production of methanol often yield mixtures of methanol and other alcohols, making selective processes for the production of methanol of great importance.

[0005] State-of-the-art industrial processes for methanol production largely involve the hydrogenation of carbon monoxide or carbon dioxide, both at high pressures, over suitable catalysts. Hydrogenation from synthesis gas involves both reactions, but the yield of CCU hydrogenation still needs improvement.

[0006] Liu et al., Journal of the Taiwan Institute of Chemical Engineers, 76 (2017), page 18, describe the production of higher alcohols by CCb hydrogenation with a Mo-Co-K sulfide catalyst, where the catalyst can contain, among others, M0S2.

[0007] Qi et al., Catalysis Communication, 4 (2003), page 339, describe the hydrogenation of CO using K / MoS2. The addition of manganese to the catalyst is also described, with a Ni / Mn / K / MoS2 catalyst ultimately 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%. Of this, 45.8% is methanol and 53.3% is higher alcohols (C n - Alcohols with n = 2, 3, 4 and 5). eng et al., Applied Catalysis B: Environmental, 246 (2019), page 232, describe the preparation of higher alcohols with at least three carbon atoms (C n-alcohols with n > 3) by CO hydrogenation over potassium promoted M0S2.

[0008] Ramadan A. Geioushy, Islam M. Hegazy, Said M. El-Sheikh, Osama A. Fouad, “Construction of 2D MoS2@ZnO heterojunction as superior photocatalyst for highly efficient and selective CO2 conversion into liquid fuel” Journal of Environmental Chemical Engineering, Volume 10, Issue 2, 2022, 107337 (https: / / doi.org / 10.1016 / jjece.2022.107337) describe a catalyst consisting of a 1:1 mixture of MoS2 and ZnO. The catalyst is used in a photochemical process in which CO2 is converted to methanol.

[0009] BRIEF DESCRIPTION OF THE INVENTION

[0010] The prior art selective processes for producing methanol from CO2 require expensive catalysts or complex process conditions and deliver low yields. Although the catalyst described in Geioushy et al. is considerably better than other catalysts in terms of specificity, the process described in Geioushy et al. is complex due to the need for UV irradiation and the aqueous process conditions.

[0011] The object of the present invention is to provide a selective and cost-effective process and a catalyst for the simple, selective hydrogenation of CO2 to methanol with high selectivity. The catalyst should also be sulfur-tolerant, i.e., tolerant to traces of sulfur compounds present, for example, in flue gas, which can be a starting material for methanol synthesis.

[0012] This object is achieved by a catalyst comprising a support and a catalytically active coating, wherein the catalytically active coating comprises a metal sulfide, wherein the support is zinc(II) oxide, wherein the proportion of metal sulfide is between 15 and 30 wt.%, preferably between 15 and 25 wt.%, particularly preferably between 15 and 20 wt.%, based on zinc(II) oxide.

[0013] The invention is based on the discovery that catalysts comprising a ZnO support and a catalytically active coating with a metal sulfide catalyze CO2 hydrogenation with very high selectivity and high yield. It is crucial that the catalyst not only comprises a mixture of ZnO and metal sulfide, but that ZnO acts as the support. The ratio of ZnO to metal sulfide is also crucial, as higher amounts of metal sulfide have a negative impact on the reaction conditions. With the correct structure of support and catalytically active coating, as well as the correct ratio of ZnO to metal sulfide, the reaction of CO2 and H2 can be carried out without UV irradiation and in the gas phase with very rapid reaction kinetics.

[0014] The catalytic properties of metal sulfides in the production of mostly long-chain alcohols are generally known. However, the inventors have now discovered that the support plays a special role in the selectivity of the catalyst, with only ZnO leading to the selective formation of methanol. Other supports exhibit this selectivity to a much lesser extent.

[0015] This process results in almost no formation of higher alcohols, and the amount of byproducts such as CO or CH4 is also low. However, the inventors have now discovered that the support plays a special role in the selectivity of the catalyst, with ZnO leading to significantly more selective methanol formation. Other supports do not exhibit this clarity.

[0016] Molybdenum (IV) sulfide (MoS2) has proven to be particularly suitable as a metal sulfide.

[0017] MoS2 exists as a rough layer on the ZnO support. The particle size ranges from 100 nm to 1 pm. In the finished catalyst, Zn is in the +II oxidation state and exists as a hexagonal ZnO phase.

[0018] The invention also relates to a selective process for the production of methanol (CH3OH, MeOH) from carbon dioxide (CO2) and hydrogen (H2), wherein CO2 is reacted with H2 in the gas phase over a catalyst, the catalyst comprising a support and a catalytically active coating, the catalytically active coating comprising a metal sulfide, the support being zinc(II) oxide (ZnO), the proportion of metal sulfide being between 15 and 30 wt. %, preferably between 15 and 25 wt. %, particularly preferably between 15 and 20 wt. %, based on zinc(II) oxide. The exact reaction mechanism is still unknown, but the two-step reaction sequence with a preceding RWGS step (reverse water-gas shift reaction) and subsequent CO hydrogenation to methanol is unlikely, since the catalyst hardly showed any conversion of CO with H2 to CH3OH. Therefore, the high yield and selectivity are all the more astonishing.

[0019] The invention therefore also relates to the use of a catalyst, wherein the catalyst has a support and a catalytically active coating, wherein the catalytically active coating comprises a metal sulfide, wherein the support is zinc (II) oxide (ZnO), for the production of methanol from CO2 and H2.

[0020] Further advantageous details of the catalyst and the process are explained in more detail below, whereby both aspects of the invention are coordinated with each other.

[0021] The metal sulfide molybdenum (IV) sulfide (M0S2) has proven to be a particularly suitable catalytically active material.

[0022] Reaction conditions in the process with a pressure higher than standard conditions have proven advantageous. Therefore, it is preferred that the reaction 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, the pressure was between 18 bar and 23 bar.

[0023] The reaction can, in principle, proceed over a wide temperature range. Suitable temperatures are, for example, between 160 °C and 260 °C, preferably between 180 and 220 °C.

[0024] Preferably, the partial pressure ratio of CO2 to H2 is approximately 1 to 2.5 to 3.5, preferably approximately 3. This means that the partial pressure of hydrogen should be approximately 2.5 to 3.5 times higher than the partial pressure of CO2.

[0025] In one embodiment, the CO2 can originate from flue gas. In this case, the process according to the invention is a selective process for producing methanol from CO2 and H2, wherein the CO2 source is flue gas, wherein CO2 is reacted with H2 over a catalyst, wherein the catalyst comprises a support and a catalytically active coating, wherein the catalytically active coating comprises a metal sulfide, wherein the support is zinc(II) oxide (ZnO). Such a process is suitable for recycling flue gas.

[0026] Although metal sulfides can be considered as catalytically active materials, M0S2 is particularly efficient.

[0027] The invention relates in one aspect to a process for producing a catalyst, wherein the catalyst comprises a support and a catalytically active coating, wherein the catalytically active coating comprises a metal sulfide, wherein the support is zinc (II) oxide (ZnO), comprising the steps:

[0028] (i) forming a mixture of water, ammonium thiomolybdate, in particular (NH4)2MOS4 or a hydrate of (NH4)2MoS4, and ZnO;

[0029] (ii) stirring this mixture; and

[0030] (iii) then evaporating the water from the mixture.

[0031] The ratios (by weight) of ammonium thiomolybdate, in particular (NH4)2MoS4 or a hydrate of (NH4)2MoS4, to ZnO are preferably from 54 to 163 (NH4)2MoS4) to 1 (ZnO). The amount of water is from 200 to 1000 (by weight) based on the ZnO used (1). Therefore, it is preferably provided that the ratio of ammonium thiomolybdate, (NH4)2MoS4 or a hydrate of (NH4)2MoS4, to ZnO is between 54 and 163 wt. %.

[0032] Step (iii) is preferably carried out at temperatures below 100°C to prevent reaction with atmospheric oxygen. A temperature range of 60°C to 80°C is advantageous.

[0033] Therefore, it is advantageous if step (iii) is carried out at a pressure below 1 bar. A pressure of below 500 mbar is advantageous, particularly preferably below 250 mbar.

[0034] DETAILED DESCRIPTION OF THE INVENTION

[0035] Further advantages and details of the invention are shown in the accompanying figures and explained in more detail in the following description. Figure 1 shows the reaction yield of methanol, CFU, and CO from the reaction of CO2 with H2 over a catalyst according to the invention as a function of temperature in a process according to the invention.

[0036] Fig. 2 shows a comparison of the yields of the reaction of CO2 with H2 over a catalyst according to the invention compared to seven catalysts not according to the invention.

[0037] Fig. 3 shows X-ray diffraction images (XRD) of a catalyst according to the invention with M0S2 as catalytically active layer and ZnO as support in comparison to ZnO without catalytic layer and M0S2 without support.

[0038] Fig. 4a shows scanning electron microscope images of ZnO.

[0039] Fig. 4b shows scanning electron microscope images of a catalyst according to the invention with M0S2 as catalytically active layer with ZnO.

[0040] Figure 1 shows the reaction yields of methanol, CH4, and CO as a function of temperature in a process according to the invention. One gram of the inventive catalyst (consisting of a ZnO support with a catalytically active coating of 16.7 wt% MOS2) was reacted with CO2 and H2. The catalyst was initially pretreated at 400°C and 100% H2 for 4 h at a pressure of 21 bar and a total flow of 5 mlN.

[0041] For explanation: The total flow of the gas mixture as it is passed over the catalyst is:

[0042] 5 mlN gkat * min

[0043] In this formula, "mlN" stands for milliliters under normal or standard conditions, i.e., at 273.15 K or 0 °C and 1 bar pressure. Normalization to standard conditions occurs because at 21 bar, 1 ml would have a higher molar mass than at 1 bar; therefore, the flow is converted and related to the volumetric flow rate under standard conditions.

[0044] The reaction itself was investigated starting at 180 °C in 20 °C increments, using a gas mixture of CO2, FE, and He at 21 bar. The partial pressure of CO2 was 20%, the partial pressure of H2 was 60%, and the partial pressure of He was 20%. It is clearly evident that the maximum methanol yield is around 220 °C, while only a few by-products are formed at this temperature. With increasing temperature, the formation of methane (CH4) increases, while the methanol yield decreases. The amount of carbon monoxide (CO) formed also increases with increasing temperature. The ideal temperature range is around 180 °C to 230 °C.

[0045] Figure 2 shows a comparison of eight different catalysts (VI to V8) with a catalyst according to the invention. The composition of the individual catalysts is summarized in Table 1.

[0046] Table 1: Composition of the catalysts for the examples according to Fig. 2. VI to V8 represent comparative examples, El is according to the invention:

[0047] "Activated carbon

[0048] The reaction conditions in the examples shown in Fig. 2 were always identical at the start of the reaction. The reaction yields of methanol, CEU, and CO are summarized in Fig. 2. One gram of catalyst (catalytically active coating always M0S2) was reacted with CO2 and FE. The catalyst was initially pretreated at 400°C and 100% FE for 4 hours, at a pressure of 21 bar, and a total flow of 5 mlN.

[0049] The reaction itself was investigated at 180 °C, using a gas mixture of CO2, Fe, and He at 21 bar. The partial pressure of CO2 was 20%, the partial pressure of H2 was 60%, and the partial pressure of He was 20%. While the catalytically active coating was always M0S2, a different distribution of reaction products was observed depending on the support material. The highest specificity for methanol was observed with ZnO as the support material, while the other materials produced high amounts of CH4 and / or CO.

[0050] Figure 3 shows X-ray diffraction (XRD) images of a catalyst according to the invention with MoS2 as the catalytically active layer and ZnO as the support, compared to ZnO without a catalytic layer and MoS2 without a support. The XRD for MoS2 / ZnO shows that the ZnO structure is retained. Three-dimensional MoS2 islands with low crystallinity are suspected on the ZnO support.

[0051] Fig. 4a and Fig. 4b show scanning electron microscope images of ZnO and M0S2 with ZnO as the support. It can be seen that the ZnO particles in Fig. 4b are covered by M0S2.

Claims

REQUIREMENTS 1. Catalyst comprising a support and a catalytically active coating, wherein the catalytically active coating comprises a metal sulfide, wherein the support is zinc(II) oxide, wherein the proportion of metal sulfide is between 15 and 30 wt.%, preferably between 15 and 25 wt.%, particularly preferably between 15 and 20 wt.%, based on zinc(II) oxide.

2. Catalyst according to claim 1, characterized in that the metal sulfide comprises molybdenum (IV) sulfide (M0S2).

3. A process for producing methanol (CH3OH) from carbon dioxide (CO2) and hydrogen (H2), wherein CO2 is reacted with H2 in the gas phase over a catalyst according to claim 1 or claim 2.

4. Method according to claim 3, characterized in that a pressure of > 10 bar, preferably between 18 bar and 23 bar, is set during the reaction.

5. Method according to claim 3 or claim 4, characterized in that a temperature of between 160 °C and 260 °C, preferably 180 °C to 220 °C, is set during the reaction.

6. Method according to one of claims 3 to 5, characterized in that the partial pressure ratio of CO2 to H2 is approximately 1 to 2.5 to 3.5, preferably approximately 3.

7. Method according to one of claims 3 to 6, characterized in that the reaction is carried out in the absence of UV light.

8. Method according to one of claims 3 to 7, characterized in that the CO2 originates from flue gas.

9. A method for producing a catalyst according to claim 1 or claim 2, wherein the catalyst comprises a support and a catalytically active coating, wherein the catalytically active coating comprises a metal sulfide, wherein the support is zinc (Il) oxide (ZnO), comprehensively the steps: (i) Forming a mixture of water, ammonium thiomolybdate, in particular (NH4)2MOS4 or a hydrate of (NH4)2MoS4, and ZnO; (ii) Stirring this mixture; and (iii) subsequently evaporating the water from the mixture, preferably at temperatures below 100 °C, particularly preferably at 60 °C to 80 °C.

10. Method according to claim 9, characterized in that step (iii) is carried out at a pressure below 1 bar, preferably below 500 mbar.

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