Synthesis and characterisation of bonded mercaptopropyl silica intermediate stationary phases prepared using multifunctional alkoxysilanes in supercritical carbon dioxide as a reaction solvent

This research employed 29Si and 13C Cross-Polarisation/Magic Angle Spinning (CP/MAS) NMR spectroscopy to characterise the nature and amount of surface species of di-and trifunctional mercaptopropylsilane (MPS) bonded silica using supercritical carbon dioxide (sc-CO2) as a reaction solvent without additives (co-solvent) or catalysts. The MPS stationary phases were prepared within 1 h at a temperature and pressure of 70 °C and 414 bar, respectively. Complementary analysis including elemental analysis, thermogravimetric analysis (TGA), DRIFT spectroscopy and BET surface area measurements were employed to characterise the bonded MPS intermediate stationary phases in support of data obtained from solid-state NMR analysis. The results revealed that modification of silica with a trimethoxymercaptopropylsilane (MPTMS) results in ligand surface coverage that is larger than when dimethoxymethylmercaptopropysilane (MPDMMS) is employed as a silanisation reagent. This observation is attributed to greater reactivity and cross-linkage of trifunctional silane. Reaction in sc-CO2 in comparison to reflux in organic solvents, is rapid, reducing product recovery procedures.