A new method has been used to obtain pore size characteristics of MCM-41 catalyst supports and vanadiumsubstituted MCM-41 catalysts. The approach is based on the nonlocal density functional theory (NLDFT) model for nitrogen and argon adsorption in MCM-41, proposed recently. Samples with pore sizes varying from ca. 25 to 37 Å were prepared by hydrothermal synthesis. Two synthesis procedures employing different sources of V were used to prepare V/MCM-41 catalysts. The samples were characterized by X-ray diffraction (XRD). N 2 and Ar adsorption isotherms at 77 K were measured starting from the relative pressure P/P 0 ) 1 × 10 -5 . Analysis of adsorption isotherms was carried out in two stages. The first stage implies comparison of a given isotherm with a reference isotherm measured on a well-characterized sample of MCM-41 with uniform pores. From such a comparison, micropore volume, specific surface area of mesopores, and the point of the beginning of the capillary condensation are determined. In the second stage, pore size distributions are calculated from the NLDFT. Pore size distributions obtained from N 2 and Ar isotherms at 77 K were in perfect agreement. These results were compared with the traditional Barrett-Joyner-Halenda (BJH) method, and with the XRD data. It is shown that the BJH method underestimates an average pore size in MCM-41 materials by ca. 10 Å. Adsorption studies of V/MCM-41 catalysts revealed that the synthesis procedure with the direct addition of V 2 O 5 yields samples with a more uniform pore structure than the procedure with the use of VOSO 4 ‚3H 2 O solution.

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, 51 V 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 V 5+ centers is proposed.

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