| Generally, commercial methanol catalyst (CZA) is comprised of Cu, ZnO and Al2O3, heterogeneously catalyzing methanol production from syngas (mainly CO and H2). However, CO2also can be a perfect carbon source to produce methanol. One promising process for this conversion is based on the reaction between renewable hydrogen and carbon dioxide to produce methanol. The development of efficient processes for the synthesis of chemicals and fuels from carbon dioxide would provide modern society with both an alternative to increasingly expensive petrochemical feed-stocks and a means to recycling a problematic pollutant-CO2.Although the catalysts developed for the conversion of syngas can be used for the conversion of CO2, it has been postulated that improvements can be made to the process by developing new catalysts specifically for CO2. The reson is that the synthesis of methanol from CO2or syngas differs in several respects. More specifically, for the synthesis of methanol from CO2, the catalysts are exposed to higher water concentrations and different gas compositions.Carbon has long been recognised as a good catalyst support for some applications, primarily because of its low reactivity under a wide range of conditions, but also because of the variety of structural forms available and its ability to support metal and metal oxide nanoparticles. Among these materials, ordered mesoporous carbons, with well-defined pore size distributions and high internal surface areas, appear to be particularly promising candidates. While activated carbons are relatively inexpensive, with high internal surface areas and pore volumes, also can be promising candidates.In this context, mesoporous carbon FDU-15(MC) and coconut activated carbon (AC) were used as catalyst supports. In-situ method, precipitation method, impregnation method, impregnation with ammonia treatment method and sequential impregnation method were used to prepare the catalyst. These catalysts were characterized and analysised by XRD, XPS, N2physisorption, Cu surface area, TG-DTG, SEM and TEM. Catalytic performances of the catalysts were tested and relative mechinism were explored based on the characteriziton and catalytic performances.In the control experiment and supports’catalytic performance test, CO2convertion and methanol production were beyond of the test limitation. While CO2convertion, methanol selectivity and methanol formation rate of commercial catalyst were25.2%,56%and5.6mmol· g-1· h-1, respectively.Mesoporous carbon (MC) as methanol synthesis catalyst support:1. In-situ method:the higher carbonized temperature (700℃) led to a catalyst with higher BET surface area and pore volume. The higher ramping rate (20℃/min) catalyst has well dispersed and similar particle sized metal oxides throughout the carbon support. None of these catalysts showed any catalytic activity.2. Precipitation method (Cu Zn precipitate; Cu precipitate; Zn precipitate; co-precipitation):the addition of Cu Zn precipitate in the carbon precursor and co-precipitation on MC support led to the form of Cu/ZnO aggregates which showed some catalytic activity. Among these catalysts, the catalyst prepared by co-precipitation method had the best catalytic performance with a CO2conversion of7.0%, methanol selectivity of28%and methanol formation rate of0.8mmol· g-1· h-1. By carefully investigating these catalysts, we concluded that the active cites of the methanol formation was the Cu/ZnO interaction.3. Impregnation with ammonia treatment method:the catalyst prepared by this method contained well dispersed mixture of CuO, Cu2O and ZnO which were well confined by the carbon channel. Ammonia treatment was beneficial for maintaining the carbon structure, while the carbon support therefore confined the growth of the metal oxides. Both contributing to the highly dispersed, small sized metal oxides in the resultied catalyst. Compared with the sample without ammonia treatment, MC-CZ-NH3-700showed some activity while catalyst MC-CZ-700showed no activity at all. Among these catalysts, MC-CZ-NH3-700had the highest activity with a CO2conversion of5.5%, methanol selectivity of27%and methanol formation rate of0.6mmol· g-1· h-1.Activated carbon (AC) as methanol synthesis catalyst support:1. Co-precipitation method:the catalysts prepared by this method had the metal oxides aggregating on the carbon surface. The catalyst with25%loading of CuO and ZnO had relatively better distribution. When the loading amount increasing up to50%, the metal oxides strated to sinter and the size of metal oxides increased significantly. The catalysts prepared by precipitation method with Cu or Zn only all showed some activity. The proper loading amount for co-precipitation method was25%. The catalyst AC-25CZ with a CO2conversion of12.7%, methanol selectivity of31%and methanol formation rate of1.5mmol· g-1· h-1which is the best catalyst prepared by precipitation method.2. Impregnation method and impregnation with ammonia treatment method: in genenal, the distribution of CuO/ZnO of these catalysts prepared by impregnation method and impregnation with ammonia treatment method were better than that of the catalysts prepared by co-precipitation method. Impregnation method:CuO/ZnO was randomly located in the catalyst AC-CZ, while CuO/ZnO was much better dispered in catalysts AC-APS-CZ and AC-HNO3-CZ with the particles size of around10nm. All these three catalysts contained the mixture of Cu, Cu2O, CuO and ZnO. When only loaded Cu or Zn on the AC-APS support, appearently, under the same condition, catalyst with Cu only had larger crystall size than the catalyst with Zn only. Cu was crystalline while ZnO was amorphous, none of these catalysts showed any activity. Impregnation with ammonia treatment method:the catalyst prepared by this method had the metal oxides well dispersed in the carbon support with the size less than10nm. Among these catalysts, AC-CZ has the best catalytic performance with a CO2conversion of15.3%, methanol selectivity of56%and methanol formation rate of3.1mmol· g-1· h-1.3. Sequential impregnation method:the catalyst prepared by the sequential loading of Cu and Zn on AC had similar activity with catalyst AC-CZ, but a much longer lifetime. The catalyst prepared by the sequential loading of Cu and Zn on AC-APS led to3.5times higher catalytic performance that of AC-APS-CZ. Among these catalysts, catalyst AC-C-Z had the best catalytic performance with a CO2conversion of17.0%, methanol selectivity of51%and methanol formation rate of3.2mmol· g-1· h-1. After tested in the reactor for160h, this catalyst showed only8%deactivation. Our finding suggests that catalyst AC-C-Z may potentially be better catalysts for methanol production. |
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