Journal of Molecular Catalysis A: Chemical 238 (2005) 135–137 A note on micro- and mesopores in the walls of SBA-15 and hysteresis of adsorption isotherms C.G. Sonwane * , Peter J. Ludovice SchoolofChemical&BiomolecularEngineering,GeorgiaInstituteofTechnology,Atlanta,GA30332,USA Received 20 March 2005; received in revised form 18 May 2005; accepted 19 May 2005 Available online 5 July 2005 Abstract 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 ∼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. © 2005 Published by Elsevier B.V. Keywords: Microporous; Mesoporous; Adsorption; Isotherm; MCM-41; SBA-15; Hysteresis; Nanoporous; Nitrogen 1. Micro-andmesoporesinthewallsofSBA-15 Stucky and co-workers [1] developed SBA-15 using tri- block co-polymer templates. These materials consist of par- allel cylindrical pores with axes arranged in a hexagonal unit cell. Several characterization techniques such as small angle X-ray scattering (SAXS), gas adsorption as well as high resolution transmission electron microscopy (HRTEM) have revealed that the walls of SBA-15 in between the main mesopores channels contain micropores 0.5–1.5 nm in diam- * Corresponding author. Tel.: +1 763 234 5738; fax: +1 859 406 3838. E-mailaddresses: shekar@nanoporous.com (C.G. Sonwane), pete.ludovice@chbe.gatech.edu (P.J. Ludovice). eter [2–4]. Using HRTEM, Liu et al. [2] suggested that walls of SBA-15 consist of spherical cages of ∼0.5 nm in diame- ter in the walls. Goltner and co-workers [3] used SAXS and gas adsorption and found that up to 63% of the specific sur- face area of SBA-15 is due to microporosity. Miyazawa and Inagaki [4] studied the systematic variation of microporosity in the walls of SBA-15 by varying the synthesis conditions. Jaroniec and co-workers [5] with the help of gas adsorption suggested that in addition to the main mesopore channels, SBA-15 consists of micropores as well as mesopores of size smaller than 3.4 nm in the walls. Galarneau et al. [6] in a sys- tematic investigation prepared SBA-15 with no micropores. They suggested that the micropores in the walls are replaced by mesopores because of high-temperature synthesis 1381-1169/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.molcata.2005.05.013