Design of highly ordered mesoporous Nb2O5-based hybrid catalysts bifunctionalized by the Keggin-type heteropoly acid and phenyl-bridged organosilica moieties for the synthesis of methyl levulinate

摘要

Highly ordered mesoporous Nb2O5-based hybrid catalysts bifunctionalized by the Keggin-type heteropoly acid and phenyl-bridged organosilica moieties (PW12/Nb2O5-Si(Ph)Si) were prepared by a P123-directed one step sol-gel route combined with hydrothermal treatment. The main steps included co-condensation of bridged silsesquioxane with NbCl5 in the presence of H3PW12O40 and carefully adjusting Si-to-Nb molar ratio in the initial gel mixture. The pore morphologies and porosity properties, Bronsted and Lewis acid nature of the hybrid catalysts as well as the structure integrity of the starting H3PW12O40 and bridging phenyl moieties in the hybrid catalysts were well characterized. The heterogeneous acid catalytic activity of the PW12/Nb2O5-Si(Ph)Si was evaluated in esterification of levulinic acid (one of the important biomass-derived platform chemicals) with methanol to produce methyl levulinate. The obtained excellent esterification activity was explained by the combination of advantages of strong Bronsted and moderate Lewis acidity, excellent porosity properties and hydrophobic surface of the hybrid catalysts. Additionally, the catalysts can be reused for three times without significant activity loss. (C) 2016 Elsevier Inc. All rights reserved.