Kinetic analysis and reaction mechanism for anisole conversion over zirconia-supported molybdenum oxide

Abstract

Gas-phase catalytic conversion of anisole and its reaction intermediates was studied over a 10 wt% MoO3/ZrO2 catalyst at temperatures between 553 and 633 K and H2 partial pressures (PH2) ≤ 1 bar. Benzene, phenol, cresol and methyl anisole were identified as the primary products from the hydrodeoxygenation (HDO), hydrogenolysis, intra- and intermolecular alkylation of anisole, respectively. The anisole to benzene conversion featured a first-order dependence with respect to PH2, while the conversion of phenol to benzene and m-cresol to toluene, showed PH2 and Poxygenate reaction orders of 1/2 and zero, respectively. A kinetic model showed that although the secondary pathway of phenol HDO to benzene has a rate constant ∼3 times higher than that for the HDO of anisole to benzene, the anisole HDO pathway is dominant at low anisole conversions. Apparent orders of ∼½ with Poxygenate for anisole hydrogenolysis and alkylation to form phenol, cresol, and methyl anisole implied the existence of different active sites than those responsible for HDO. Co-feed studies with H2O, pyridine, and di-tert butyl pyridine (DTBP) indicated that the active-sites responsible for HDO have a Lewis acid character that is associated with oxygen vacancies and that is distinct from the nature of sites responsible for hydrogenolysis and alkylation. Accordingly, co-feeding CH3OH resulted in increased phenol alkylation rates to form alkylated cresols along with inhibition of phenol to benzene HDO rates. A three-site model was proposed to unify the HDO, hydrogenolysis, and alkylation reactivity data obtained from the kinetic and co-feed studies.