Evaluation of Pore Structure Parameters of MCM-41 Catalyst Supports and Catalysts by Means of Nitrogen and Argon Adsorption

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). N2 and Ar adsorption isotherms at 77 K were measured starting from the relative pressure P/P0 ) 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 N2 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 V2O5 yields samples with a more uniform pore structure than the procedure with the use of VOSO4‚3H2O solution.