A note on micro- and mesopores in the walls of SBA-15 and hysteresis of adsorption isotherms

The hysteresis of adsorption/desorption of condensable vapors in porous materials is a highly debated area of research. The adsorption isotherms is useful for getting important structural parameters such as pore size distribution and surface area. Earlier, hysteresis was attributed to the networking of the pores. Since the discovery of cylindrical pore MCM-41 materials, it has been attributed mainly to the adsorption temperature, pore diameter and heterogeneity of surface. Recently, Esparza et al. [J.M. Esparza, M.L. Ojeda, A. Campero, G. Hernandez, C. Felipe, M. Asomoza, S. Cordero, I. Kornhauser, F. Rosa, J. Mol. Catal. A: Chem. 228 (2005) 97] presented extremely valuable experimental data related to characterization of SBA-15 and other micro- and mesoporous materials. They found that adsorption on a 7.6 nm pore diameter SBA-15 sample shows hysteresis and attributed it to the presence of bulges and necks along the pore channel due to the U-shaped pores or sinuosity. In our opinion, the results open up a debate regarding the accurate structure of SBA-15 as well as the reasons behind hysteresis. The t-plot analysis of the nitrogen isotherm of their SBA-15 found that it consists of micropores (8% of total volume). These micropores are absent in MCM-41 and although not related to hysteresis, it is an important characteristics of SBA-15 not addressed during the discussion. The experimental as well as theoretical investigations of adsorption hysteresis of MCM-41/SBA-15 or cylindrical nanotubes leads us to conclude that these ordered porous materials (MCM-41, SBA-15) will always show hysteresis for nitrogen adsorption at 77 K if the pore diameter is above similar to 4 nm. The hysteresis criticality (i.e. absence of hysteresis beyond a certain diameter or temperature) is a fundamental property of adsorbed fluids inside cylindrical pores and it may depend upon the temperature, pore diameter, shape of meniscus or surface heterogeneity. However, the presence of a bulge/neck (U-shaped pores/sinuosity) may not be responsible for hysteresis in SBA-15. The present note represents our arguments in an ongoing debate in the area of hysteresis of adsorption and the structure of SBA-15. (c) 2005 Published by Elsevier B.V.