Preparation and characterization of bonded silica hydride intermediate from triethoxysilane and dimethylmethoxysilane using supercritical carbon dioxide and dioxane as reaction medium

This research examines bonding methodology, surface coverage and silanol conversion efficiencies on the preparation of silica hydride (SiH) intermediate from triethoxysilane (TES) and dimethylmethoxysilane (DMMS) using sc-CO2 and dioxane as reaction solvent. Under sc-CO2 reaction conditions (at temperature and pressure of 100 °C, 414 bar, respectively and 3 h reaction time), the surface coverages of SiH (evaluated from %C obtained from elemental analysis) prepared with DMMS (3.39 μmol/m2) and TES (4.46 μmol/m2) increased by 2- and 4-folds respectively, when compared to reaction performed in dioxane (2.66 μmol/m2, SiH, DMMS and 0.69 μmol/m2, SiH, TES). The relatively higher surface coverage of SiH from TES over DMMS generated in sc-CO2 is due to the inherent trialkoxy moiety of the TES that favours siloxane crosslinkage, forming polymeric surface attachments to yield a higher ligand density than the monomeric DMMS ligand. A conversion efficiency of ∼84.4% of SiH prepared from TES in sc-CO2 estimated from 29Si CP/MAS NMR analysis is comparable to TES silanization in dioxane or toluene. Moreover, silica hydride (SiH) conversion efficiency of ca. 42.4% achieved for the hydride intermediate prepared from DMMS in sc-CO2 is more superior to 33.3% efficiency obtained in dioxane. The differences in conversion efficiencies is attributed to the ability of sc-CO2 being able to access silica pores that are inaccessible in organic solvents. Bonded silica hydride from TES, DMMS prepared in sc-CO2 were characterized using elemental analysis, thermogravimetric analysis (TGA), BET surface area, Fourier transform infrared (FI-IR) and solid state NMR spectroscopy. Silica hydride technology/chemical functionalization of silica in sc-CO2 avoid extended purification steps (i.e. filtration and washing), generation of waste organic solvent and the need of costly or energy consuming drying processing with improved modification efficiency.