Pore Anisotropy and Microporosity in Nanostructured Mesoporous Solids John Knowles, † Gerasimos Armatas, ‡ Michael Hudson,* ,† and Philippos Pomonis* ,‡ School of Chemistry, UniVersity of Reading, Box 224, Whiteknights, Reading, RG6 6BD, Great Britain, and Department of Chemistry, UniVersity of Ioannina, Ioannina, 45110, Greece ReceiVed July 12, 2005. In Final Form: October 12, 2005 In this study, we carried out an investigation related to the determination of the anisotropy (b) of pores as well as the extent of microporosity (mic%) in various groups of nanostructured mesoporous materials. The mesoporous materials examined were fifteen samples belonging to the following groups of solids: MCM-48s, SBA-15s, SBA-16s, and mesoporous TiO 2 anatases. The porosities of those materials were modified either during preparation or afterward by the addition of Cu(II) species and/or 3(5)-(2-pyridinyl) pyrazole (PyPzH) into the pores. The modification of porosity in each group took place to make possible the internal comparison of the b and mic% values within each group. The estimation of both the b and mic% parameters took place from the corresponding nitrogen adsorption- desorption isotherms. The new proposed method is able to detect a percentage of microporosity as low as a few percent, which is impossible by any of the methods used currently, without the use of any reference sample or standard isotherms. A meaningful inverse relationship is apparent between the b and mic% values, indicating that large values of b correspond to small values of mic%. 1. Introduction The mesoporous materials are defined according to IUPAC 1-3 as possessing pore openings/diameters D p in the range 2 < D p < 50 nm. By that it is usually meant that, if the majority of pores, or the maximum D max of the pore size distribution (PSD), falls within this region, the material is considered to be mesoporous. The majority of mesoporous materials, including the nanostruc- tured ones such as MCM-41, MCM-48, and SBA-1 4-6 possess some microporosity, which is due to the fact that the nanostructure only rarely is developed in perfect order across the entire mass of the prepared material. This microporosity becomes clear in observations by transmission electron microscopy (TEM) where often only small fractions of the optical field show order, but these small regions are usually chosen for publication. Evidence of mesoporosity can be obtained from X-ray diffraction (XRD) data, but, again, a considerable fraction of the material might be amorphous, disordered, or unstructured and thus not identified. A safer criterion for checking the existence of well-ordered mesoporosity is the shape of nitrogen adsorption-desorption isotherms and the appearance of the unmistakable sharp increase of adsorption at relative pressure (P/P 0 ) ) 0.2-0.3. Even in such cases, many researchers in the field, on the basis of indirect evidence, suspect that a considerable fraction of porosity is due to micropores, 7 and various attempts have been made to estimate the extent of microporosity. 8 The established method of a s -plots or t-plots for the estimation of microporosity 2,3 in such cases provides erroneous evidence for zero microporosity, which must be due to the fact that these methods were proposed before the invention of MCM materials and are based on assumptions that do not apply to such materials. 9 The a s -plot or t-plot methods necessitate either the use of an additional adsorption isotherm from a sample without porosity but chemically similar to the porous one for comparison, 2,3,10 or the use of the so-called standard isotherms, which correspond to a specific Value of the C parameter of the Brunauer-Emmett-Teller (BET) equation as proposed by Brunauer 11 and by Lecloux and Pirard. 12 Clearly, the first method has the drawback of additional experiments, while the second has the drawback of choosing a single specific value of C, which is not a constant. 13,14 So it would be advantageous if there was a direct method for estimating the percentages of micro- * Corresponding authors. Department of Chemistry, University of Ioannina, Greece; phone: +302651098350, fax: +302651098795, e-mail: ppomonis@cc.uoi.gr. (P.P.). Department of Chemistry, University of Reading, Box 224, Whiteknights, Reading RG6 6BD, GB; phone: +441183- 786717, fax: +441183786331, e-mail: m.j.hudson@reading.ac.uk (M.H.). † University of Reading. ‡ University of Ioannina. (1) (a) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Piertti, R. A.; Rouquerol, J.; Siemienniewska, T. Pure Appl. Chem. 1985, 57, 603; (b) Rouquerol, J.; Avnir, D.; Fairbridge, C. W.; Everett, D. H.; Haynes, J. M.; Pernicone, N.; Ramsay, D. J. F.; Sing, K. S. W.; Unger, K. K. Pure Appl. Chem. 1994, 66, 1739; (c) Sing, K. S. W.; Schuth, F. In Handbook of Porous Solids; Schuth, F., Sing, K. S. W., Weikamp, J., Eds.; Willey-VCH: Weinheim, Germany, 2002; Chapter 1.2. (2) Gregg, J. C.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London 1982. (3) Rouquerol, F.; Rouquerol, J.; Sing, K. 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