Structure and properties of tungstated zirconia catalysts for alkane conversion

Abstract

Promoted tungstated zirconia (WZ) catalysts are active and selective for isomerization of light alkanes, offering good prospects for industrial application. This account is an abbreviated summary of what these catalysts are and how they work. WZ containing approximately a monolayer of tungstate covering the zirconia support was prepared by impregnation of zirconia, tested in a flow reactor, and characterized with a variety of spectroscopic methods. The catalytic activity is associated with interconnecting polyoxotungstate clusters on the surface of tetragonal zirconia. The polyoxotungstate species increase the acid strength of the catalyst relative to that of unmodified zirconia, but the acid strength is still less than that of zeolites and far below the superacidic range. Redox properties of WZ characterized by EPR spectroscopy suggest that alkane activation proceeds via homolytic CH bond cleavage, leading to the formation of W5+ centers and organic radicals, which can be converted to alkenes, initiating catalysis. Unpromoted WZ has a low activity; reaction intermediates remain adsorbed on the surface, where they undergo polymerization and cracking, leading to fast deactivation and poor selectivity. The addition of platinum to the catalyst and H2 to the feed drastically improves the catalytic activity, selectivity, and stability. The platinum enhances the desorption of reaction intermediates and minimizes the condensation reactions, so that monomolecular isomerization predominates. An additional promotion with iron compounds leads to further improvement in catalytic activity and selectivity.