Synthesis of hierarchical MeAPO-5 molecular sieves - Catalysts for the oxidation of hydrocarbons with efficient mass transport

Hierarchical molecular sieves possess the characters of both microporous molecular sieves and mesoand/or macro-porous materials, and have potential application in adsorption and separation of macromolecules and diffusion limited catalytic reactions. In this work, hierarchical MeAPO-5 (MeAPO-5-meso, Me = Co, Mn, Fe, Mg and Ti) molecular sieves were synthesized directly using glucose as mesopore template. The synthesized MeAPO-5-meso molecular sieves were characterized by X-ray diffraction, X-ray fluorescence, N-2 physisorption, thermogravimetric analysis and scanning electron microscopy. It was proved that mesopores with the pore size distribution of 5-30 nm were introduced to MeAPO-5-meso. The improvement effect of the introduced mesopores on the catalytic performance of MeAPO-5-meso (Me = Mn, Fe and Co) was investigated in the oxidation of various hydrocarbons with different molecular dimensions including cyclohexene, ethylbenzene, indan, tetralin, diphenylenemethane and fluorene. For comparison, these oxidation reactions were also performed over the ordinary MeAPO-5. The results indicated that mesopores benefited the diffusion of the reactants and the products, so the conversion of the reactants was improved while the selectivity of the products was slightly improved or maintained at higher conversion. In addition, the promotion effect is dependent on the dimension of the substrates with respect to the micropore size of molecular sieves. (C) 2012 Elsevier Inc. All rights reserved.