Adsorption of nitrogen, neopentane, n-hexane, benzene and methanol for theevaluation of pore sizes in silica grades of MCM-41

Nitrogen, neopentane, n-hexane, benzene and methanol adsorption isotherms w ere determined on five samples of silica grade MCM-41 with different pore s izes and a comparison of different methods for evaluating the pore size was carried out. With nitrogen we found a remarkably good agreement between th e results obtained from the non-local density functional theory and geometr ic methods, with corresponding values obtained by the two methods differing by less than 0.05 nm. On the other hand, the results confirm findings seen before by other workers that, in order to obtain reliable values of pore r adii by the hydraulic method, it is necessary to use a cross-sectional area of nitrogen in the monolayer smaller than the normally assumed value of 0. 162 nm(2). In addition, it was found that the effective pore volume obtaine d with the four organic adsorptives was almost constant while the value obt ained from nitrogen adsorption data was always higher, The results indicate that, for the nitrogen data, an average density of 0.84 gcm(-3) can be use d to calculate the pore volume and that in this case the surface area shoul d be calculated by considering a value of 0.131 nm(2) for the cross-section al area. In order to calculate hydraulic pore radii, either this pair of va lues can be used or if, on the other hand, the usual value of 0.808 g cm(-3 ) is considered for the nitrogen density, the value of 0.137 nm(2) must be used for the cross-sectional area. (C) 2001 Elsevier Science B.V. All right s reserved.