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

Abstract

Nitrogen, neopentane, n-hexane, benzene and methanol adsorption isotherms were determined on five samples of silica grade MCM-41 with different pore sizes and a comparison of different methods for evaluating the pore size was carried out. With nitrogen we found a remarkably good agreement between the results obtained from the non-local density functional theory and geometric 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 radii 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 obtained with the four organic adsorptives was almost constant while the value obtained from nitrogen adsorption data was always higher. The results indicate that, for the nitrogen data, an average density of 0.84 g cm −3 can be used to calculate the pore volume and that in this case the surface area should be calculated by considering a value of 0.131 nm 2 for the cross-sectional area. In order to calculate hydraulic pore radii, either this pair of values 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.