| Summary: | 博士 === 元智大學 === 化學工程與材料科學學系 === 95 === A commercial Cu/ZnO/Al2O3 catalyst was pre-treated as calcined, partially reduced and completely reduced. Methanol decomposition reaction was carried out over the various pre-treated catalysts and the activity was found to almost same at steady-state. The pre-treatment effect is found to be relatively insignificant in methanol decomposition reaction when the catalyst was completely reduced in the reaction conditions. The surface species are slightly changing over various pre-treated catalysts, as indicated by in-situ sTPD–DRIFTS analysis. The intensity of methoxy is decreases as well as the formate intensity is increases on the various pre-treated Cu/ZnO catalysts when the increase of desorption temperature from 393 to 433 or 473 K. The surface species detected from DRIFTS experiment indicated that methanol adsorbed on ZnO surface or Cu-ZnO interface, although the methanol adsorbed on Cu0, in the presence of surface ‘O’ cannot be excluded. The C-H vibration of methoxy and formate on the reduced catalyst showed a red shift compared to that on ZnO or the calcined catalysts. On the other hand, the OCO vibration of formate showed a blue shift. And the commercial Cu/Cr catalyst shows the similar reaction phenomena as the Cu/ZnO.
The methanol reaction was investigated at 423 to 523 K with various co-feed gases, such as He, H2, H2O and O2 over Cu/ZnO/Al2O3 catalyst. In the presence of H2, a sequential H-abstraction of methanol was observed and it is identified by the formation of formaldehyde at low methanol conversion and formation of CO, dominated at high methanol conversion. In the absence of H2, formaldehyde was not identified in the product stream. In the case of oxygen or water as a co-feed in the methanol reaction, the formation of CO2 and suppression of CO and methyl formate were observed. The proposed reaction scheme is involved in mainly H-abstract and/or O-addition steps.
The effect of alkali metal impregnation on Cu/ZnO catalyst was examined for the MeOH + H2 reaction. The selectivity of higher alcohols is not observed in this reaction. The synthesis of long-chain oxygenates from methanol was observed while using a Mg-Al hydrotalcite supported Cu-ZnO catalyst. The produced oxygenates include ketones, aldehydes, esters, ethers and alcohols from C2 to C9. Formaldehyde is very likely as the intermediate to form long-chain oxygenates. The synergistic effect between Cu and ZnO, and the basicity from hydrotalcite can be responsible for this novel synthesis route. No obvious evidence shows the specific structure, which result in the promotion of C2+. The moderate basic sites, over reduced catalysts at 473 or 523 K are very likely lead to the C2+ formation.
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