Zinc–aluminum hydrotalcites as precursors of basic catalysts: Preparation, characterization and study of the activation of methanol

Monophasic ZnAl hydrotalcite-like layered hydroxycarbonate has been prepared by the urea precipitation method. This material has been characterized by Rietveld refinement of the XRPD pattern, DTA-TG and skeletal IR. ZnAl mixed oxides have been prepared by calcinations of this precursor. According to XRPD, after calcinations at 450 °C this material is constituted by poorly crystallized ZnO phase. IR spectroscopy shows the presence of additional spinel-like domains. The surface of this catalyst is dominated by poorly acidic Zn2+ cations and terminal and bridging hydroxy groups bonded to Zn ions. This catalyst adsorbs methanol dissociatively giving rise to two types of methoxy groups. Type A, likely bridging on Al and Zn ions, is more stable to desorption. Type B, likely terminally bonded to Zn2+ cations, is more labile with respect to decomposition into CO and H2. Type B methoxy groups appear to be very ionic and are, likely, those involved in transesterification and polyethoxylation reactions. ZnO and ZnAl2O4 particles progressively crystallize and sinter by increasing calcination temperature.