Abstract
Despite their surprisingly thick walls, SBA-15 mesoporous silicas are not as stable as expected in water, even at room temperature. The microporosity of SBA-15, synthesized at temperature under 110°C, is lost during water treatment, leading to a strong decrease in specific surface area and to an increase in mesopore size. Only SBA-15s without microporosity, as the ones synthesized at 130°C, are stable under water treatment. Investigations by nitrogen adsorption isotherms and hyperpolarized 129Xe NMR spectroscopy have been performed to understand the stabilities of three SBA-15s synthesized at different temperature (60, 100 and 130°C) and treated in water at room temperature and at the boiling point. The different behaviour are explained by a mechanism of dissolution/redeposition of silica based on the local curvatures of silica in the different structures. Similar experiments on MCM-41 silica lead to a totally different dissolution/redeposition behaviour, due to the thinner walls and lower pore size, surface area and pore volume of the material.