Surface chemistry of multicomponent silicate gels

The surface chemistry of a calcium boroaluminosilicate gel was studied and compared with the corresponding behavior of a pure silica gel. The experiments were performed, in situ, by transmission FTIR utilizing high surface area, free-standing, monolithic xerogel foils. The foils could be vacuum-evacuated, heated, and/or exposed to water and organic vapors within the FTIR cell. The spectra of the dehydrated calcium boroaluminosilicate showed only isolated surface silanols (Si-OH) and boranols (B-OH). They were found to be inactive upon subsequent exposure to water vapor, but readily participated in H-bonding with basic organic adsorbates. The frequency shifts associated with the adsorption of organic vapors were lower on the multicomponent surface than on the silica surface. This reveals that the silanols are less interactive, or acidic, on the multicomponent surface. The adsorption of molecular water occurred faster and with greater tenacity on the calcium boroaluminosilicate. This is attributed to the prevalence of surface sites associated with Ca, B, and Al where the chemisorption of water may occur. Some of these sites were revealed by pyridine adsorption studies; i.e., pyridine was found to form coordinate bonds with unsaturated B^III and Al^III Lewis acid sites. Otherwise, the chemical behavior of Ca and Al on the surface was not directly evident in the FTIR spectra. Finally, the chemisorption of methoxytrimethylsilane was examined and found to occur preferentially at the boranol sites. However, the chemisorbed silanes on the boranols were found to be more easily hydrolyzed than those on the silanols. In general, this study illustrates several principles about the surface chemistry of calcium boroaluminosilicates, and offers an experimental approach for direct examination of multicomponent gel or glass surface chemistry in any multicomponent system that can be synthesized using the alkoxide method.