The Cu0/Cu+ ratio is very important in many copper‐catalyzed reactions. A series of Cu−Pd/SiO2 catalysts were prepared by composite evaporation of ammonia with subsequent Pd impregnation. The Cu0/Cu+ ratio could be controlled by changing the Pd content in the catalysts during reduction. The catalysts were applied to the hydrogenation of dimethyl oxalate, and experimental results showed that a suitable amount of Pd doping remarkably improved the activity and prolonged the lifetime of the catalyst. The optimized Cu‐0.5 % Pd/SiO2 catalyst showed complete conversion with 96.0 % ethylene glycol selectivity over a lifetime of 300 h. Enhancements in the activity and stability of the Cu−Pd/SiO2 catalysts were ascribed to the stable generation of hydrogen spillover, which tended to adjust the Cu0/Cu+ ratio with changes in Pd content in the reduced catalysts. Hydrogen spillover also promoted the release of intermediates in contact with Cu+ to inhibit copper sintering.
Abstract:The influence of Sn 2+ doping on the structure and performance of silica supported copper catalyst was systematically investigated and characterised. Catalytic evaluation showed that the suitable content of Sn 2+ introduced into a Cu/SiO 2 catalyst evidently improved the catalytic activity and stability of ethylene glycol synthesis from dimethyl oxalate. X-ray diffraction and X-ray auger electron spectroscopy indicated that the Cu 0 /Cu + ratio gradually increased with increasing Sn 2+ content, and an appropriate proportion of Cu 0 /Cu + ratio played a very significant role in this reaction. Transmission electron microscopy revealed that the active copper particles in the Cu-xSn/SiO 2 catalyst were smaller than those of the Cu/SiO 2 catalyst. This result may be due to the introduction of Sn 2+ species transformed into SnO 2 . Furthermore, SnO 2 effectively segregated the active copper. These effects are beneficial in inhibiting the aggregation of copper in the catalysts, thereby improving the stability of the catalyst and prolonging the life span.
A series of Cu/ZrO 2 -SiO 2 catalysts with different ZrO 2 -SiO 2 contents were prepared using an ammonia-evaporation method. The catalysts were used for the hydrogenation of dimethyl oxalate to methyl glycolate. The catalytic activity and stability of the Cu/ZrO 2 -SiO 2 catalyst was greatly enhanced with an optimized ZrO 2 -SiO 2 content when compared with the Cu/SiO 2 and Cu/ZrO 2 catalyst. The catalysts were characterized by N 2 physisorption, X-ray diffraction, H 2 temperature-programmed reduction, and X-ray photoelectron spectroscopy. The results showed that adding an appropriate amount of zirconium to Cu/SiO 2 catalyst strengthened the interaction between the cupreous species and the support, diminished the copper crystallite size, improved the copper dispersion, and adjusted the surface [Cu + ] species
Cu/x-SiO 2 catalysts with 4, 10, and 20 nm silica sols as supports was produced by ammonia evaporation method and characterized. Different nano-sized silica sols as supports significantly affected the structure and catalytic properties of the copper catalysts for ethylene glycol synthesis from dimethyl oxalate. Compared with Cu/20-SiO 2 and Cu/4-SiO 2 catalysts, the catalytic performance and stability of Cu/10-SiO 2 catalyst were greatly enhanced. The Cu/10-SiO 2 catalyst showed 99.9% conversion with 94% EG selectivity and a lifetime of over 3080 h if it is calculated by industrial weight liquid hourly space velocity (WLHSV) of 0.5 h −1 . The Cu/10-SiO 2 catalyst has one of the longest lifetimes among the catalysts and is a good alternative catalyst for this reaction. Improvement in the catalytic performance and stability of Cu/10-SiO 2 is attributed to the proper S BET , D p and larger dispersion of copper. In addition, the supports of Cu/10-SiO 2 catalyst have smaller particles than that of Cu/20-SiO 2 ; thus, the migration and growth of copper species in catalysts are restrained during the reaction.
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