Surface silanol groups of mesoporous silica FSM-16

Abstract

The surface structure of mesoporous silica crystals FSM-16 with a narrowly distributed pore size has been studied by FTIR, measurement of SiOH number and adsorption of N₂ and H₂O. IR spectra of FSM-16 outgassed at temperatures above 170 °C showed a sharp band at 3740 cm^–1 due to free SiOH and a broad shoulder at 3600–3500 cm^–1 due to hydrogen-bonded SiOH. The SiOH number, determined by a weight-loss method, amounted to 3.3 groups nm^–2, which is less than that of mesoporous silica gels. The amount of H₂O adsorbed on FSM-16 decreased with calcination above 800 °C, owing to the dehydroxylation of SiOH. On treating with alcohol, the number of free SiOH decreased, leading to a decrease in H₂O adsorption. The crystallinity of FSM-16 was lowered by pressing. With increasing pressure the specific surface area decreased and the SiOH number increased, up to 9.5 groups nm^–2.