Designing isolated vanadium sites over mesoporous MCM-41: Characterization and Catalytic applications

Abstract

Vanadium-containing mesoporous molecular sieves were synthesized by three different routes viz. by the direct hydrothermal, grafting and immobilisation methods. The introduction of vanadium species as well as the nature of vanadium species formed over the MCM-41 support surface was probed by XRD, N2 sorption isotherms, UV-VIS and TPR techniques. XRD, N2 adsorption and TEM results show that the characteristic structural features of MCM-41 are preserved after surface modifications. Spectroscopic measurements reveal that vanadium exists mainly in tetrahedral positions for the hydrothermally synthesized catalyst while the grafting method tends the vanadium species to exist as higher coordinated species. Reaction data showed that the activity of the catalyst in the oxidation reaction of naphthalene is higher at high reaction temperatures and polar aprotic solvents like acetonitrile promotes the reaction drastically. The observed higher catalytic behaviour of the immobilized vanadium catalysts may result from the better active metal site isolation than the directly synthesized as well as grafted catalyst.