True microporosity and surface area of mesoporous SBA-15 silicas as a function of synthesis temperature

Abstract

A model based on hexagonal mesopores connected by micropores allows evaluation of the porosity of SBA-15 silicas and elucidation of the literature problems on the subject. The presence of micropores in the walls between the mesopores renders unreliable the surface area determination by the BrunauerEmmett-Teller equation and the microporosity assessment by the t-plot method. A geometrical model allows calculation of the true surface area, as well as the real mesoporous and microporous pore volumes, the wall thickness and the wall density from the total pore volume, the pressure of pore-filling, and the cell parameter. The increase of synthesis temperature leads to a continuous increase of mesopore size and a continuous decrease of microporous volume until the disappearance of micropores for materials synthesized at nearly 130 °C. The results are discussed on the basis of the collective properties of nonionic surfactants.