Preparation And Characterization Of Cu-based Catalyst For Low-temperature Methanol Synthesis In Liquid-phase

In recent two decades, low-temperature methanol synthesis in liquid-phase has been well studied. Compareing with gas-phase reaction, reaction temperature was easy to be controled and one-pass conversions of feed gas were high in liquid-phase reaction. Methol synthesis in alcohol soltution by Cu-based catalyst was well studied recently. The reaction included two steps—carbonylation of alcohol and hydrogenolysis of intermediate ester. It was reported that carbonylation was a rate-determining step. To increase the yeild of methanol, it was a key point to improve carbonylation activity. The cognition of liquid-phase reaction mechanism was more ambiguous than gas-phase reaction.In this dissertation, low-temperature methanol synthesis was carried out in ethanol solution on Cu/SiO2catalyst prepared by adsorbed-layer reactor technique. Several analytical methods were used to characterize the catalysts:TEM used to analyze particles’ size and morphology, XRD used to analyze crytal form and calculate size of crystallite, SEM-EDAX and ICP-MS used to analyze the load factor of Cu, XPS used to analyze the valence state of Cu, CO-TPD used to analyze the adsorption of CO on Cu-based catalysts. The structure of catalysts was regulated by changing preparation condition and preparation process was also recognized by catalysis feature.For catalysts preparation, it was confirmed that Cu particles were synthesized in adsorbed-layer. Cu particels with1～2nm in diameter were well dispersed on the surface of SiO2. As Cu loading was low, the increment of Cu tended to form new Cu particles. While as Cu loading exceeded9.5wt%, the increment of Cu tended to cover pre-existing Cu particles, which made Cu partcles bigger and Cu dispersion worse.The results of catalysis indicated that Cu/SiO2performed much higher activity for carbonylation than CuO/ZnO/Al2O3prepared by coprecipitation. It was interesting that Cu/SiO2did not show any activity for hydrogenolysis. According to this feature, carbonylation was detailed studied, then zetetic study about hydrogenolysis was carried out and the point about active sites was proposed. For the carbonylation, the impacts of Cu loading, alcohol heat treatment, addition of H2and modified components on Cu/SiO2were studied. The results indicated:(1) The activity of unit mass Cu kept contant as Cu loading was low, while it begun to decrease when Cu loading was high. The optimal Cu loading was9.5wt%.The Cu particles with small size and great dispersion were in favor of carbonylation.(2) Alcohol heat treatment could change status of Cu particles and cause crosslinking of SiO2. Through appropriate alcohol heat treatment, the activity of low Cu loading catalysts was improved obviously.(3) During reaction, H2might induce adsorption of CO on Cu sites thus the acvitity was improved.(4) The addition of modified components decreased the activity, because Cu particles were covered by modified components. Alcohol heat treatment of catalysts could increase activity and decrease Cu size after reaction.For the hydrogenolysis, the activity was high as peak intensity of Cu and ZnO was close after Cu/ZnO/Al2O3reduction. We modified the adsorbed-layer reactor technique and successfully prepared a Cu-based catalyst with Cu and ZnO crystallization speaks. The carbonylation activity of the catalyst was lower than Cu/SiO2, but the catalyst showed a certain hydrogenolysis activity. The changing of two steps activities indicated that the requirement of catalyst’structure was different for two steps.Thus we inferred that the active sites for carbonylation and hydrogenolysis were different. The active site for carbonylation was Cu sites on the support surface and the active site for hydrogenolysis was Cu-ZnO.