Hydrothermal preparation of CuO-ZnAl2O4 catalyst for phenol ortho-alkylation with methanol: effect of the calcination temperature on the catalytic performance

Abstract

The effect of heat-treatment on 10 wt% CuO-ZnAl2O4 catalytic activity in methylation of phenol and the degree of interaction of CuO active phase with support spinel phase were investigated. The CuO-ZnAl2O4 sample was subjected to heat-treatment up to 1000°C. The thermal products were characterized by X-ray diffraction (XRD) analysis, nitrogen adsorption-desorption at -196°C and temperature-programmed desorption (TPD-MS) of CO2. Additionally, the reducibility of copper phases was investigated by temperature-programmed reduction (TPR). XRD patterns of the fresh catalyst sample (calcined at 600°C) indicated the presence of a mixture of poorly crystallized CuO and ZnAl2O4 spinel phase. The presence of two reducible copper species has been found on fresh CuO-ZnAl2O4 catalyst by TPR analysis. After subsequent calcinations in air at elevated temperatures some CuO disappeared with appearance of CuAl2O4 phase. The catalytic results revealed that the CuO addition to ZnAl2O4 increases the activity in ortho-methylation of phenol. Subsequent heat-treatment up to 900°C causes partial deactivation of copper centers, which is the result of transformation of CuO to the inactive CuAl2O4 phase.