Reaction and Deactivation Pathways in Xylene Isomerization on Zirconia Modified by Tungsten Oxide

Abstract

The effect of H2 on isomerization pathways and on the acid site density on WOx-ZrO2 catalysts was explored using kinetic measurements of acid-catalyzed o-xylene isomerization reactions. Initial o-xylene isomerization rates on WOx-ZrO2 at 523 K are proportional to the H2 concentration even though H atoms are not involved directly in reaction turnovers, as shown by the low deuterium content in the isomers formed from D2/o–C8H10 mixtures. H2 chemisorption uptakes on WOx-ZrO2 and isomerization rates show a similar dependence on WOx surface density and structure, suggesting that Brønsted acid sites form via reductive processes that require H atoms from H2. This proposal is consistent with UV-visible spectra, which show the formation of reduced centers by H2; these centers correspond to the formation of acidic Hδ+ species with charge compensation by WO3δ− domains. The transients induced by the addition and removal of H2 (or O2) during xylene isomerization show that the formation of acid sites by H2 is reversible at reaction conditions and that H2 is involved in maintaining a steady-state density of acid sites. H2 also inhibits catalyst deactivation, apparently by reversing C–H bond activation steps leading to the formation of strongly adsorbed unsaturated hydrocarbons. A sequence of elementary steps including isomerization, deactivation, and acid site formation pathways was found to describe accurately the observed effects of H2 and o-xylene on deactivation, reaction rates, and the results of o-13C8H10/o-C8H10 and D2/o-C8H10 exchange experiments.