Characterization of micro- and mesoporosity in SBA-15 materials from adsorption data by the NLDFT method

SBA-15 siliceous materials templated using triblock copolymers have recently received considerable attention because of their potential applications in catalysis and separations of large molecules. SBA-15 materials possess a regular hexagonal array of cylindrical channels of 5-30 nm in diameter and, unless specially treated, an appreciable volume of intrawall pores which may constitute up to 30% of the total porosity. The intrawall pores include micropores within the silica matrix and also mesopores that are narrower than the main channels. We present a consistent method for evaluating the pore structure parameters of SBA-15 materials from the nitrogen adsorption isotherms. The proposed method is based on the nonlocal density functional theory (NLDFT) of adsorption and capillary condensation in cylindrical pores. The NLDFT method allows one to calculate the mesopore size distribution and to evaluate the pore wall thickness and the amount of intrawall porosity. The structural parameters obtained by the NLDFT method are in agreement with geometrical considerations and X-ray diffraction data. Interpretation of adsorption isotherms on SBA-15 by means of the NLDFT method resolves some of the recently published discrepancies between the pore sizes determined from different methods. The NLDFT method is applicable to other micro-mesoporous materials.