論文の公開元へStudies on direct synthesis of alcohols from syngas over metal oxides catalysts
概要
Nowadays, the increasing demand on clean fuel and valuable chemicals ignited the research on catalytic converting carbon contained substances. Syngas, composed by hydrogen and CO, is an important platform material for synthesing gasoline, diesel oil and many valuable chemicals. Among all the downstream products of syngas, higher alcohols are considered to be valuable chemicals and fuel additives to offset the disadvantage of alcohols production from petrochemical and biological fermentation. Generally, alkali modified methanol catalysts (mainly composed by Cu based catalyst and ZnCr catalyst) are used to produce iso-butanol and methanol owing to the higher selectivity of alcohols and pre-longed catalyst life span.
However, there are still some important issues that need to be further studied in these catalysts system. For example, the exacting form and their effects of alkali metals on the activation of syngas and their influence on the mechanisms of carbon chain growth; The relatively high reaction temperature and pressure (400 °C, 10.0 MPa) versus the lower CO conversion (around 30 %) and higher selectivity of CO2 (around 30 %) for K-ZnCr isobutanol catalyst. In this thesis, the effects of K2O on the catalytic performance of Cu based catalyst was investigated by density functional theory (DFT) calculation. In addition, the performance of K-ZnCr catalyst was boosted by both controlling the co-precipitation temperature to increase the concentration of surface OH groups, and doping Ga3+ in the lattice of ZnCr2O4·
Through the above investigation, the following conclusions were obtained. That is ( i ) K2O presents a strong adsorption on the Cu (111)surface owing to the strong interaction between atom O and surface Cu atoms. The difference of the adsorption energies among all the possible adsorption sites is very small. Owing to the strong interaction, K7O denotes its electrons to copper which further promotes the formation of CHO groups through CO hydrogenation and inhibit the formation of CH3OII through OCH3 hydrogenation that usually considered as the rate control step of methanol formation. In addition, on K2O/Cu(111) surface, CH2O reacted with CH2 is the most favorable C2 oxygenate formation route which presents only a 1.10 eV activation energy, (ii) By controlling the co-precipitation temperature during K-ZnCr catalyst preparation, the selectivity of alcohol and isobutanol can be modulated. DRFTS of CO adsorption show that for the catalyst prepared at 60 °C, the relatively higher amount of surface OH groups was produced which facilitate the formation of COOH intermediates and further boost the performance of K-ZnCr isobutanol catalyst, (iii) Ga3+ can be used as an eflective promoter to improve the selectivity and space time yield of alcohols of K-ZnCr catalyst. The reason lies in the formation of Ga3+doped ZnCr204 compound which facilitate the formation of CHO and weakened the adsorption of CO2 and 〇. Owing to the formation of new catalyst structure and weakened adsorption of CO2 and H2O, water-gas-shift reaction is suppressed which promotes the eiTicicncy of iso-butanol synthesis. The analysis to the structure of K-GaxZnCr catalysts show that the morphology of the catalysts and the interactions between ZnO and ZnCr spinel were all changed owing to the formation of the diflerent compound. When the molar faction of Ga3+ is lower than 1.10 %, ZG is the newly formed compound which promotes the performance of K-ZnCr catalyst.
However, by further increasing the amount of Ga3\ ZnGa204 is formed which only modify the surface acidity and basicity of K-ZnCr catalyst and showed negative impact on the performance of the catalyst.
By means of the above work, the roles of alkali oxides on the activation of CO and C2 oxygenate formation and the goals that further improve the efficiency of K-ZnCr catalyst are all realized in this thesis. The research will throw light on further improving the performance of alkali modified methanol catalysts that used to directly synthesis of higher alcohols from syngas.
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