Characterization of WOx/ZrO2by Vibrational Spectroscopy andn-Pentane Isomerization Catalysis

Abstract

Tungstated zirconia catalysts containing WO3loadings ranging from 3.6 to 23.9 wt% were prepared by refluxing an aqueous suspension of hydrous zirconia containing appropriate amounts of metatungstate, drying, and calcinating at 923 K or 1098 K. The acidic properties of the samples were characterized by low-temperature infrared spectroscopy with adsorbed CO as a probe molecule. Both Lewis and Brønsted acid centers with enhanced acid strength were created as the WO3loading increased. The Lewis sites are coordinatively unsaturated surface Zr4+ions. Their density decreased significantly as the WO3loading approached saturation, and simultaneously the surface ZrOH groups disappeared. These observations are consistent with high (perhaps monolayer) dispersion of the surface tungstate. The Brønsted acid strength also increased with increasing WO3loading up to saturation and remained constant at higher loadings. These results show that large WO3domain or cluster sizes are required to create strong Brønsted acidity. Zr-heteropolytungstates containing charge-compensating protons are proposed as surface structures to account for the observations. The catalytic activities of these materials forn-pentane isomerization at 523 K in a flow reactor were found to parallel the measured Brønsted acid strengths. However, because an induction period of increasing catalytic activity was found, followed by catalyst deactivation, it is suggested that the detected Brønsted acidity may not simply account for the catalytic activity. Catalytic andin-situRaman spectroscopic evidence indicates the formation of carbonaceous surface deposits during the induction period. These may be converted into catalytic sites and ultimately into deposits that cause catalyst deactivation.