Synthesis, characterization, and catalytic activity of vanadium-incorporated, -grafted, and -immobilized mesoporous MCM-41 in the oxidation,of aromatics

Vanadium containing mesoporous molecular sieves synthesized by direct hydrothermal (V-MCM-41), grafting (V/MCM-41), and immobilization methods (V-NH2-MCM-41) has been studied in the one-step liquid-phase oxidation of naphthalene using aqueous H2O2 and TBHP as oxidants. The nature of vanadium species and its interaction with the support, MCM-41, were probed in detail by XRD, FTIR, BET surface area, N-2 sorption isotherms, UV-VIS, TPR, TG-DTG, C-13 CP MAS NMR, SEM, and TEM. XRD, FTIR, and N2 sorption results show that the characteristic structural features of MCM-41 are preserved after vanadia incorporation and surface modifications. Spectroscopic measurements reveal that vanadium exists mainly in tetrahedral positions for V-MCM-41 catalysts while with the grafting method the vanadium species exist as higher coordinated species and the immobilized catalyst shows that even slight changes in pretreatment conditions can change the oxidation state of vanadium ions. H-2-TPR measurements reveal that V-MCM-41 samples are reduced easily than the samples prepared by postsynthesis methods and thus point to the formation of nonreducible species in the grafted catalyst systems. Reaction data showed that the activity of the catalyst in oxidation reaction is greater at higher reaction temperatures and polar aprotic solvents promote the reaction drastically. The progressive activity of the V-MCM-41 than V/MCM-41 catalyst may be due to the presence of tetrahedral-coordinated vanadium ions in the framework positions compared to the V-O-V bond formed for V/MCM-41 and the observed higher catalytic behavior of the V-NH2-MCM-41 catalysts may result from active metal site isolations. The difference in the selectivity behavior of as-synthesized and calcined V/MCM-41 samples shows that apart from the active redox sites, the nature of hydrophilic-hydrophobic interactions also plays an important role in selective oxidation reactions. (C) 2004 Elsevier Inc. All rights reserved.