Reaction-probe infrared investigation on drastic change in reactivity of mesoporous silica for acetalization of cyclohexanone with methanol; pore-size dependence

Small-pored (<2.5 nm) mesoporous silica showed remarkably higher catalytic activity for cyclohexanone acetalization with methanol than large-pored ones (≥2.5 nm). For the clarification of the pore-size effect of mesoporous silicas (MCM-41 and SBA-15) on the reactivity, the reaction probe infrared (IR) method was applied. No evident differences were observed by conventional IR characterization methods using CO, pyridine and acetonitrile adsorption. IR spectra of methanol, one of the reactants, on small-pored (MCM-41) and large-pored (SBA-15) samples appeared the same. On the other hand, adsorption of cyclohexanone resulted in different IR spectra: a C[dbnd]O stretching band appeared at lower frequency side on MCM-41 at a small amount of adsorption in addition to that observed on SBA-15. This result indicates the presence of the stronger interaction of cyclohexanone with MCM-41 than that with SBA-15. When the time course of the reaction was observed by in-situ IR observation, the consumption of activated carbonyl groups and the generation of 1,1-dimethoxycyclohexane, the reaction product, were confirmed. The presence of the activated cyclohexanone was most probably due to the adsorption on two (or more) OH groups in vicinity, while a non-activated species occupied a single OH group. The reaction probe IR method is thus, a direct and effective approach to extract the small differences in catalysis.