Chemical modification of metal oxide surfaces in supercritical CO2: In situ infrared studies of the adsorption and reaction of organosilanes on silica

Infrared spectroscopy was used to probe the reaction of organosilanes with a fumed silica immersed in supercritical fluid CO2 (SCF CO2). Venting of th e CO2 solvent eliminates the experimental difficulties associated with solv ent absorption of the infrared radiation and enables repeated surface treat ment cycles without disturbance to the amount of silica in the beam. This s tability is requisite for detecting infrared bands due to adsorbed species in the spectral region containing the strong metal oxide bulk modes. SCF CO 2 has been shown to extract water from silica, and we now exploit this feat ure for the silane treatment of silica particles. This utility of CO2 as a solvent for the reaction of organosilanes with silica is demonstrated with hexamethyldisilazane (HMDS) and octadecyltrichlorosilane (OTS). The HMDS re action in SCF CO2 proceeds according to the conventional gas-phase process even though the ammonia generated as a byproduct reacts with the CO2 to pro duce ammonium carbamate. The volatile ammonium carbamate is weakly physisor bed on the surface hydroxyl groups and is easily removed with evacuation or by purging. Moreover, carbamate formation can be completely avoided by per forming the reaction at relatively low CO2 pressures. Physisorption of OTS from SCF CO2 does occur via a weak interaction with the surface hydroxyl gr oups. Although a small amount of OTS is hydrolyzed by the residual water pr esent in the SCF CO2 and adsorbs on the silica. the amount hydrolyzed is mu ch lower than that found with the use of tradition nonaqueous solvents.