| Ethanol and other C2 oxygenates are not only the substitutes for oil fuel, but also the important chemical raw materials. The industrial production can be divided into two groups: fermentation and synthesis. Synthesis methods include direct and indirect hydration of ethylene, hydrogenation of acetaldehyde and CO. Compared to the fermentation process and the raw petrochemical materials, the production of C2 oxygenate from synthesis gas showed the potential for large-scale production, lower cost, wide material sources and etc.In 1975, the researcher from union carbide corporation found that the Rh-based catalyst showed great performances to produce C2 oxygenates such as ethanol, and the proper adsorption and dissociation ability of CO. Rh-based catalyst was considered to be the most promising. However, the problem of low catalytic activity, low ethanol selectivity, more side reactions, wider product distribution, which limited its industrialization. The supports, promoters and preparation methods showed great effects on the catalytic activity and selectivity, and the distribution altered with the nature of active ingredient, the precursor species, the promoters, the preparation methods and reaction conditions. The main difficulties lied on the development and improvement of catalyst. The researchers dedicated to minimize operating costs and maximize efficiency of precious metals, and improve the activity, stability, selectivity and sulfur resistance.By the development and modulation of the catalyst system, the combination of rare earth elements and transition metals, the adjustment of the loading, the choice of SiO2 with appropriate properties, the optimization of preparation methods and reaction conditions, which could improve the activity and selectivity. Combined with series of characterization methods, the interaction between the active components and promoters, and the reaction mechanism were discussed, and provided the effective methods to improve the activity and selectivity of Rh-based catalyst.The performances of Rh-Nd-V/SiO2 catalysts for the preparation of C2 oxygenates were studied by CO hydrogenation in fixed bed reactors. Combined with the results of XRD, TPR, TPD, TPSR, FT-IR, et al, the effects of supporters, promoters, preparation methods and reaction conditions, the catalytic of promoters and the interaction between the active components and promoters were investigated. It comes to the following conclusions:1. The addition of rare earth elements enhanced the CO conversion and C2 oxygenates selectivity greatly, especially to acetic acid selectivity. The activity and C2 oxygenates selectivity of Sm, and the atomic number of RE metals after Sm, was inversely related to magnetic moment of the RE ion.2. The addition of Nd improved the catalytic activity and the C2 oxygenates selectivity, under 563 K, 4.0MPa, 12,000h-1, the selectivity and space-time yield of C2 oxygenates on 1.0%Rh-0.5%Nd-1.0%V/SiO2 catalyst achieved 59.8% and 394.5g/(kg·h) respectively.3. The catalysts prepared by ethanol as solvent displayed 60.4% selectivity and 475.5g/(kg·h) space-time yield of C2 oxygenates. The impregnation sequences had significant impacts on the catalyst performance. Because of the medium interactions of Rh, Nd and V, the Rh-Nd-V/SiO2 showed moderate CO adsorption and dissociation ability, and higher catalytic activity.4. The pretreatment conditions of Rh-Nd-V/SiO2 catalyst were optimized. The results showed that the catalytic activity increased with the calcination temperature rised. When the temperature was below 623 K, the catalytic activity was low, and the interaction between Rh and V was weak. When the temperature increased more than 873 K, the surface area and pore volume educed, due to the high temperature made the excessive condensation of the carrier, and the collapse of pore. The catalytic activity reduced greatly, and the interaction was the strongest. At 673 K, the interaction was proper, and the catalytic activity was higher.5. The impact of the nature and structure of silica gel and Rh loading was investigated. The catalytic activity was affected by the diameter of SiO2, the content of impurity, et al. The carrier must be pretreated to obtain reproduced results. Macroporous silica gel suited with low Rh loading and the small pore silica was contrary. The appropriate Rh loading is 1.0-2.0%. The catalyst with low Rh loading showed higher activity, selectivity and stability.6. The reaction conditions were optimized. When the reaction temperature increased from 533 K to 573 K, the activity increased significantly, especially the alkanes selectivity such as methane, but the C2 oxygenates selectivity decreased. The high pressure was in favor of improving the catalytic activity and the space time yield of C2 oxygenates, but the promoting effect was not obvious after 4 MPa. The higher space velocity was better for the reaction products dropped from the catalyst surface, and the selectivity and space-time yields of C2 oxygenates were increased, but the CO conversion decreased and energy consumption increased greatly.7. The promoter of Fe showed strong effects on the performance of Rh-Nd-V/SiO2 catalyst. The interplay between Fe and Rh increased with the loading. When the Fe loading was 0.05%, the interaction was appropriate. The catalyst prepared by the first impregnation of Fe showed higher catalytic activity. The selectivity and space-time yield on 1.0%Rh-0.5%Nd-1.0%V/0.05%Fe/SiO2 catalyst reached 63.2% and 501.1g/(kg·h) under the condition of 563 K, 4.0MPa, 12,000h-1. Rh, Fe and V had strong interactions, and increased with the loading. TPR results showed the Fe promoter inhibited the reduction of Rh and V, and the first impregnation of Fe could easily lead to the interaction with Rh and V, exhibited the higher catalytic performance.8. The research on TPR, H2-TPD, CO and H2 chemical adsorption showed that: Nd can't be reduced under the experimental conditions, there was interaction between the active component and Nd, the Rh dispersion and the CO adsorption increased with the addition of Nd. The V particles suppressed the reduction of Rh, which were reduced to lower valence vanadium oxide, and covered the Rh surface.9. CO-TPD and TPSR experiments showed that only the strongest CO adsorption showed the dissociation and hydrogenation activity. The Nd promoter inhibited the dissociation and hydrogenation activity of CO, while the V promoter was the opposite. Rh-Nd-V/SiO2 catalyst exhibited the appropriate dissociation activity, and increased the activity of CO insertion relatively. The addition of Nd and V reduced the ratio of CO dissociation, so that the selectivity of the hydrocarbon gas such as methane was decreased, and increased the percentage of undissociated CO, which improved the catalytic activity and C2 oxygenates selectivity greatly.10. The CO adsorption ability, the desorption/conversion rate of CO adsorption influenced the CO hydrogenation performance greatly. The appropriate reaction conditions and additives doping could alter CO adsorption capacity, and adjust the desorption/conversion rate of adsorbed CO, change the desorption/conversion behavior of dicarbony adsorption CO(gdc), affect the conversion of CO(gdc) species to the H-Rh-CO species, and modulate the catalytic performance.When the Fe loading was low, the increasement of CO(gdc) desorption/ conversion rate played the dominant role, and the selectivity and the space time yield of C2 oxygenates increased. When the Fe loading was high, the decreasement of CO adsorption ability dominated, made the selectivity and the space time yield of C2 oxygenates decreased.11. By the kinetics study of Rh-Nd-V/SiO2 catalyst, the kinetic parameters of methane, methanol, ethanol, acetaldehyde, and C2 oxygenates were calculated. High temperature favored the methane formation, and the low temperature favored the formation of ethanol and other C2 oxygenates. The kinetic parameters of ethanol and acetaldehyde were different. Combined with the evaluation results of catalytic reaction, ethanol and acetaldehyde were from different active centers, and different intermediates. |
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