Performance of ZrO2-supported Nb- and W-oxide in the gas-phase dehydration of glycerol to acrolein

Gas-phase dehydration of glycerol to acrolein is studied over tungsten and niobium oxide supported on monoclinic zirconia. Supported tungsten oxide is slightly better than supported niobia with initial yields to acrolein around 78% and 75%, respectively, at complete conversion of glycerol. No synergy is observed using mixed structures with W and Nb. The addition of oxygen to the feed has almost no effect on the yield to acrolein but reduces the deactivation rate. XPS and HRTEM imaging reveals that deactivation is due to formation of an amorphous layer on the surface, consisting of adsorbed high-boiling compounds and coke. Regeneration experiments show that the catalysts are effectively regenerated in flowing air. Characterization of the catalysts with Raman, FTIR, and pyridine adsorption reveals that the active surface structure is polymeric oxide exposing Wdouble bond; length as m-dashO and/or Nbdouble bond; length as m-dashO species and Brønsted acidic Wsingle bondOH and/or Nbsingle bondOH groups. The results show that Brønsted acidic sites are required for the catalyst to be active and selective to acrolein.