Understanding the stability in water of mesoporous SBA-15 and MCM-41

Surprisingly, SBA-15 mesoporous silicas are not as stable as expected in water, even at room temperature, despite their thick walls. The microporosity of SBA-15, synthesized at a temperature below 110 degrees C, is lost during water treatment, leading to a strong decrease in specific surface area and an increase in mesopore size. Only SBA-15s without microporosity, such as the ones synthesized at 130 degrees C, are stable under water treatment. Investigations by nitrogen adsorption isotherms and hyperpolarized Xe-129 NMR spectroscopy have been performed in an effort to understand the silica dissolution/redeposition processes occurring during water treatment at room temperature and at the boiling point for three SBA-15s synthesized at different temperature levels: 60, 100, and 130 degrees C. The differences between the local curvatures of silica in the different structures explain the difference of behavior in water with respect to silica dissolution/redeposition. Similar experiments on MCM-41 lead to a totally different dissolution/redeposition process because of its thinner walls: decrease of pore size, surface area and pore volume.