Synthesis and characterization of vanadium-substituted mesoporous molecular sieves

First-row transition metal containing silicalite zeolites are important oxidation catalysts. Special attention has been devoted to develop new systems with an uniform pore size distribution which would allow large molecules to reach the active sites inside the structure. In the present paper, a series of vanadium-containing mesoporous molecular sieves of the MCM-41 type have been synthesized, systematically varying the pore size and metal loading. The uniform mesoporous structure was confirmed by X-ray diffraction and physisorption. To obtain information on the local structure of incorporated vanadium, a series of vanadium model compounds with well-defined local symmetry were characterized, together with V-MCM-41, by UV Raman, DR UV-visible, V-51 solid-state NMR, and X-ray absorption spectroscopies. The UV-visible absorption edge energy may be correlated with the domain size (local symmetry) represented by the average bond length. On the basis of this principle and NMR and XANES results, it is deduced that the vanadium is incorporated into the framework of hydrated and dehydrated samples, mostly occupying isolated tetrahedral sites. A small portion of octahedral sites involving water coordination are observed in hydrated samples with large pore size. On the basis of these results, XAFS analysis provides quantitative information on localization of vanadium and agrees with the other results. A model of possible local structure of V5+ centers is proposed.