The effect of the thermal pretreatment on the performance of ZnO/Cr2O3 catalysts applied in high-temperature methanol synthesis

Abstract

Co-precipitated ZnO/Cr2O3 catalyst precursors were thermally pretreated in synthetic air (calcination), N2 (pyrolysis) and 10% H2 diluted in N2 (reduction) at temperatures in the range from 320°C to 600°C and then applied in methanol synthesis from CO and H2 at 260–300°C and 60bar. The X-ray diffraction patterns showed that increasing temperatures used in the thermal pretreatment lead to sintering of both the resulting non-stoichiometric Zn-Cr spinel and ZnO phases. However, reduced and pyrolyzed catalysts exhibit a much slower increase in particle size of the Zn-Cr spinel phase compared with the calcined catalyst. Correspondingly, at the same temperature of thermal pretreatment the specific surface areas of the catalyst follow the trend reduced catalyst>pyrolyzed catalyst>calcined catalyst. Chromate species were found to be present in the calcined catalyst, while their amount decreased with increasing calcination temperature. Infrared spectroscopy and temperature-programmed reduction confirmed the existence of carbonate species in the catalysts reduced or pyrolyzed below 500°C. NH3 temperature-programmed desorption showed that the calcined catalysts exhibit more acid sites due to the existence of chromate species. The reduced and pyrolyzed catalysts demonstrate significantly improved performance in high-temperature methanol synthesis compared with the calcined catalysts, and the maximum methanol productivity was achieved subsequent to reduction at 500°C. It is suggested that the thermal pretreatment in reducing or inert atmosphere favors the formation of oxygen vacancies, which are considered to be the active sites for the hydrogenation of CO.