The Catalytic Activity of MoOx/ZrO2 in the Hydrogenation and Metathesis of Propene

Abstract

The catalytic activity of MoO x /ZrO 2 for propene hydrogenation has been studied in a flow apparatus at 195–298 K. At or above 273 K, propene metathesis is observed in addition to hydrogenation. The catalysts, prepared by equilibrium adsorption at pH=1, 2, 3 or 8, have a Mo surface concentration from 0.5 to 7.7 atoms nm −2 . Before ESR, XPS, or catalysis measurements, the samples were heated in O 2 at 773 K and reduced thereafter with H 2 to a controlled extent, expressed by e/Mo (electrons acquired per Mo atom) in the range 448 K to 803 K. Both ESR and XPS analyses show that a large fraction of total Mo (30%) is already reduced to Mo V at 473 K (e/Mo≃0.3). The Mo V content remains constant with further reduction up to e/Mo≃3. The ESR spectrum originates from a mononuclear Mo V species in a square pyramidal configuration (ESR of 95 Mo V ). About 80% of Mo V is destroyed upon O 2 adsorption at 195 K and restored by evacuation at 773 K. The turnover frequency for the propene hydrogenation (N h /molecules s −1 Mo atom −1 ) is very small up to e/Mo≃1 and increases markedly with e/Mo. The N h value strongly increases with the Mo content up to about 5.6 atoms nm −2 , and decreases at higher Mo loading (aggregation of Mo oxide). The turnover frequency for metathesis, N m , is nearly constant in the range e/Mo≃1 to 3 and is almost independent of Mo content. The results show that the active sites for hydrogenation and metathesis are different. It is suggested that mononuclear Mo V are the active sites for metathesis, whereas higher nuclearity species involving molybdenum in lower oxidation state are active for hydrogenation.