Catalyst and process for preparing 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 a zinc (II) oxide wherein the proportion of the metal sulfide, based on the zinc (II) oxide, is between 15 wt% and 30 wt%, preferably between 15 wt% and 25 wt%, preferably between 15 wt% and 25 wt%. Particularly preferably between 15 wt% and 20 wt%.

Description

Technical Field

[0001] The present invention relates to a method for catalytically preparing methanol from carbon dioxide and hydrogen, in which carbon dioxide is reacted with hydrogen via a catalyst of zinc oxide and metal sulfide. In addition, the present invention relates to a catalyst for preparing methanol from carbon dioxide and hydrogen. Finally, the present invention relates to the use of a catalyst for preparing methanol from carbon dioxide and hydrogen. Background Art

[0002] Methanol is not only an important raw material for further processing into more complex chemical products, but also an energy carrier for direct combustion or conversion into fuel cells. The synthetic processes for extracting methanol usually provide a mixture of methanol and other alcohols, so selective processes for extracting methanol are very important.

[0003] According to the prior art, most of the industrial processes for preparing methanol are achieved by hydrogenation of carbon monoxide or carbon dioxide, in each case under high pressure and at a suitable catalyst. When hydrogenation reaction is carried out using syngas as raw material, two reactions will occur, but the yield of CO2 hydrogenation still needs to be improved.

[0004] Liu et al. described the preparation of higher alcohols by CO2 hydrogenation using a Mo-Co-K-sulfide catalyst in Journal of the Taiwan Institute of Chemical Engineers, 76 (2017), page 18, where the catalyst may mainly have MoS2.

[0005] Qi et al. described the CO hydrogenation by K / MoS2 in Catalysis Communication, 4 (2003), page 339. The addition of manganese to the catalyst was also described, and finally the Ni / Mn / K / MoS2 catalyst was described as a catalyst suitable for CO hydrogenation. The results showed a very high selectivity for alcohols, with a total yield of 81.7%. Among them, 45.8% was methanol and 53.3% was higher alcohols (C n alcohols, where n = 2, 3, 4 and 5).

[0006] Zeng et al. described the preparation of higher alcohols (C n alcohols, where n ≥ 3) containing at least three carbon atoms by CO hydrogenation via potassium-promoted MoS2 in Applied Catalysis B: Environmental, 246 (2019), page 232.

[0007] Ramadan A. Geioushy, Islam M. Hegazy, Said M. El-Sheikh, and Osama A. Fouad described a catalyst in "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 / j.jece.2022.107337). The catalyst consists of a 1:1 mixture of MoS2 and ZnO. The catalyst is used in a photochemical process in which CO2 is converted into methanol. Summary of the Invention

[0008] The selective processes known in the prior art for preparing methanol based on CO2 require expensive catalysts or complex process conditions and have low yields. Although the catalyst described by Geioushy et al. is much better in terms of specificity than other catalysts, the process management of Geioushy et al. is complex due to the need for ultraviolet irradiation and aqueous process conditions.

[0009] The object of the present invention is to provide a selective and cost-effective method and a highly selective catalyst for simply and selectively hydrogenating CO2 to methanol. The catalyst should also have sulfur resistance, i.e., be tolerant to trace sulfur compounds present in flue gas, which can be a raw material for methanol synthesis.

[0010] This object is achieved by a catalyst comprising a support and a catalytically active coating, wherein the catalytically active coating contains a metal sulfide, wherein the support is zinc (II) oxide, and wherein, based on the zinc (II) oxide, the proportion of the metal sulfide is between 15% by weight and 30% by weight, preferably between 15% by weight and 25% by weight, and particularly preferably between 15% by weight and 20% by weight.

[0011] The present invention is based on the following recognition: A catalyst comprising a ZnO support and a catalytically active coating containing a metal sulfide catalyzes the hydrogenation of CO2 with very high selectivity and high yield. It is crucial here that the catalyst not only comprises a mixture of ZnO and the metal sulfide, but also that the ZnO acts as a support. The ratio of ZnO to the metal sulfide is also important, because the higher the amount of the metal sulfide, the more disadvantageous the influence on the reaction conditions. With the correct support structure and catalytically active coating and the correct quantitative ratio between ZnO and the metal sulfide, the reaction of CO2 and H2 can proceed with very fast reaction kinetics without ultraviolet irradiation and in the gas phase.

[0012] In principle, the catalytic properties of metal sulfides are known in the preparation of mostly long-chain alcohols. However, the inventors have now found that the support plays a special role in the selectivity of the catalyst, where only ZnO leads to the selective formation of methanol. Other supports exhibit much less of this selectivity.

[0013] In this process, hardly any higher alcohols are formed, and the amount of by-products formed (such as CO or CH4) is also low. However, the inventors have now found that the support plays a special role in the selectivity of the catalyst, where ZnO significantly leads to the selective formation of methanol. Other supports do not exhibit this significance.

[0014] Molybdenum(IV) sulfide (MoS2) has proven to be particularly suitable as the metal sulfide.

[0015] MoS2 exists on the ZnO support in the form of rough layers. The particle size ranges from 100 nm to 1 μm. In the finished catalyst, Zn exists in the oxidized state +II and in the hexagonal ZnO phase.

[0016] The present invention also relates to a selective process for the preparation of methanol (CH3OH, MeOH) from carbon dioxide (CO2) and hydrogen (H2), in which CO2 is reacted with H2 in the gas phase via a catalyst, where the catalyst has a support and a catalytically active coating, where the catalytically active coating contains a metal sulfide, where the support is zinc(II) oxide (ZnO), and where the ratio of the metal sulfide, based on zinc(II) oxide, is between 15% by weight and 30% by weight, preferably between 15% by weight and 25% by weight, and particularly preferably between 15% by weight and 20% by weight.

[0017] The exact reaction mechanism is not yet clear, but in any case it cannot be a two-step reaction sequence first through the RWGS step (Reverse Water-GasShift Reaction) and then through the hydrogenation of CO to methanol, because this catalyst hardly shows the ability to convert CO and H2 into CH3OH. Therefore, the high yield and high selectivity are even more surprising.

[0018] Accordingly, the present invention 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, and wherein the support is zinc(II) oxide (ZnO), for the preparation of methanol from CO2 and H2.

[0019] More advantageous details of the catalyst and the method will be explained in more detail below, where two aspects of the present invention are coordinated with each other.

[0020] The metal sulfide molybdenum(IV) sulfide (MoS2) has proven to be a particularly suitable catalytically active material.

[0021] In this method, it has proven advantageous to use an elevated pressure compared to standard conditions as the reaction condition. Accordingly, it is preferably set such that the reaction is carried out at a pressure of ≥ 10 bar. For example, the pressure can be from 10 bar to 200 bar or from 10 bar to 100 bar. In one embodiment variant, the pressure is between 18 bar and 23 bar.

[0022] In principle, the reaction can be carried out in a wide temperature range. For example, suitable temperatures are between 160 °C and 260 °C, preferably between 180 °C and 220 °C.

[0023] It is preferably set such that the partial pressure ratio of CO2 to H2 is about 1 to 2.5 to 3.5, preferably up to about 3. This means that the partial pressure of hydrogen should be about 2.5 to 3.5 times the partial pressure of CO2.

[0024] In one embodiment variant, the CO2 can be from flue gas. In this case, the method according to the present invention is a selective method for the preparation of methanol from CO2 and H2, wherein the CO2 source is flue gas, wherein the CO2 is reacted with H2 via a catalyst, wherein the catalyst has a support and a catalytically active coating, wherein the catalytically active coating comprises a metal sulfide, and wherein the support is zinc(II) oxide (ZnO). This method is suitable for recycling flue gas.

[0025] Although metal sulfides are regarded as catalytically active materials, MoS2 is particularly effective.

[0026] One aspect of the present invention relates to a method for preparing a catalyst, wherein the catalyst has a support and a catalytically active coating, wherein the catalytically active coating contains a metal sulfide, wherein the support is zinc(II) oxide (ZnO), and the method comprises the following steps:

[0027] (i) forming a mixture of water, ammonium thio molybdate, in particular (NH4)2MoS4 or a hydrate of (NH4)2MoS4, and ZnO;

[0028] (ii) stirring this mixture; and

[0029] (iii) then evaporating water from the mixture.

[0030] The ratio (by weight) of the amount of ammonium thio molybdate, in particular (NH4)2MoS4 or a hydrate of (NH4)2MoS4, to ZnO is preferably 54 to 163 ((NH4)2MoS4) to 1 (ZnO). The amount of water is 200 to 1000 (by weight) based on the ZnO (1) used. Therefore, it is preferably set such that the ratio of ammonium thio molybdate, (NH4)2MoS4 or a hydrate of (NH4)2MoS4 to ZnO is between 54% by weight and 163% by weight.

[0031] Preferably, step (iii) is carried out at a temperature below 100 °C to prevent reaction with oxygen in the air. A temperature range from 60 °C to 80 °C is advantageous.

[0032] Therefore, it is beneficial that step (iii) is carried out at a pressure below 1 bar. A pressure less than 500 mbar is advantageous, and particularly preferably less than 250 mbar. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Further advantages and details of the present invention are shown in the accompanying drawings and are explained in more detail in the following description.

[0034] Figure 1 Shows the reaction yields of methanol, CH4 and CO from the reaction of CO2 with H2 via the catalyst according to the present invention as a function of temperature.

[0035] Figure 2 Shows a comparison of the yield of CO2 reacting with H2 via the catalyst according to the present invention with the yields of seven catalysts not according to the present invention.

[0036] Figure 3 Shows a comparison of the X-ray diffraction image (XRD) of the catalyst according to the present invention with MoS2 as the catalytically active layer and ZnO as the support with ZnO without a catalytic layer and MoS2 without a support.

[0037] Figure 4a Shows a scanning electron microscope image of ZnO.

[0038] Figure 4b Shows a scanning electron microscope image of the catalyst according to the present invention, where MoS2 is used as the catalytically active layer of ZnO. Detailed Description

[0039] In Figure 1 shows the reaction yields of methanol, CH4, and CO expressed as a function of temperature in the method according to the present invention. Here, one gram of the catalyst according to the present invention (composed of a ZnO support and a catalytically active coating containing 16.7 wt% MoS2) is reacted with CO2 and H2. First, the catalyst is pretreated for 4 hours at 400 °C and 100% H2, at a pressure of 21 bar and a total flow rate of 5 mlN.

[0040] Explanation: The total flow rate of the gas mixture through the catalyst is:

[0041]

[0042] In this formula, "mlN" represents milliliters under normal or standard conditions, i.e., at 273.15 K or 0 °C and 1 bar pressure. Normalizing to standard conditions is because at 21 bar, 1 ml may have a higher amount of substance than at 1 bar; therefore, the flow rate is converted and referenced to the volume flow rate under standard conditions.

[0043] The study of the reaction itself starts at 180 °C, in steps of 20 °C, where a gas mixture of CO2, H2, and He is applied at a pressure of 21 bar. The partial pressure of CO2 is 20%, the partial pressure of H2 is 60%, and the partial pressure of He is 20%.

[0044] It can be clearly seen that the methanol yield is highest at around 220 °C, and only a small amount of by-products are formed at this temperature. As the temperature increases, 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 approximately 180 °C to 230 °C.

[0045] In Figure 2 shows a comparison of eight different catalysts (V1 to V8) with the catalyst according to the present invention. Table 1 summarizes the composition of the individual catalysts.

[0046] Table 1: Figure 2 Catalyst compositions of the examples shown. V1 to V8 are comparative examples and E1 is an example according to the present invention :

[0047]

[0048] *Activated carbon

[0049] Figure 2 The reaction conditions in the illustrated examples were always chosen to be the same at the start of the reaction. The reaction yields of methanol, CH4, and CO are summarized in Figure 2 . In each case, one gram of catalyst (the catalytically active coating was always MoS2) was reacted with CO2 and H2. First, the catalyst was pretreated for 4 hours at 400 °C and 100% H2, at a pressure of 21 bar and a total flow rate of 5 mlN.

[0050] The study of the reaction itself started at 180 °C, where a gas mixture of CO2 with H2 and He was applied at a pressure of 21 bar. The partial pressure of CO2 was 20%, the partial pressure of H2 was 60%, and the partial pressure of He was 20%. Although the catalytically active coating was always MoS2, different reaction product distributions were observable depending on the carrier material. Here, it was observable that ZnO as the carrier material had the highest specificity for methanol, while the other materials provided large amounts of CH4 and / or CO.

[0051] Figure 3 Shows a comparison of the X-ray diffraction image (XRD) of a catalyst according to the invention with MoS2 as the catalytically active layer and ZnO as the carrier, with ZnO without a catalytic layer and MoS2 without a carrier. The XRD of MoS2 / ZnO indicates that the ZnO structure was retained. It is speculated that three-dimensional MoS2 islands are present on the ZnO carrier, and these islands have a lower crystallinity.

[0052] Figure 4a and Figure 4b Shows scanning electron microscope images of ZnO and MoS2 with ZnO as the carrier. One can see that Figure 4b the ZnO particles in the image are covered with MoS2.

Claims

1. A catalyst, the 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, Based on zinc(II) oxide, the proportion of the metal sulfide is between 15% by weight and 30% by weight, preferably between 15% by weight and 25% by weight, and particularly preferably between 15% by weight and 20% by weight.

2. The catalyst according to claim 1, wherein The metal sulfide comprises molybdenum(IV) sulfide (MoS2).

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

4. The method according to claim 3, characterized in that In the reaction, a pressure of ≥10 bar, preferably between 18 bar and 23 bar, is set.

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

6. The method according to one of claims 3 to 5, characterized in that The partial pressure ratio of CO2 to H2 is about 1 to 2.5 to 3.5, preferably about 1 to 3.

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

8. The method according to one of claims 3 to 7, characterized in that, The CO2 is derived from flue gas.

9. A method for preparing the catalyst according to claim 1 or claim 2, wherein the catalyst has a support and a catalytically active coating, wherein the catalytically active coating comprises a metal sulfide, and wherein the support is zinc(II) oxide (ZnO). The method comprises the following steps: (i) forming a mixture of water, ammonium thio molybdate, in particular (NH4)2MoS4 or a hydrate of (NH4)2MoS4, and ZnO; (ii) stirring the mixture; and (iii) then evaporating the water from the mixture, preferably at a temperature below 100 °C, particularly preferably at a temperature between 60 °C and 80 °C.

10. The 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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