Organic groups influencing microporosity in organosilicas

Abstract

The micropore structure of a series of organosilica materials with various organic groups in bridging (methylene, ethylene, hexylene, octylene, p-phenylene) and terminal (methyl, n-propyl) positions was analyzed and compared to that of inorganic amorphous silica. Vapor thermogravimetry with water, methanol, 1-propanol and cyclohexane vapors was used to measure accessible pore volumes, pore entrance sizes and surface chemistries. Gas pycnometry with He, Ar and N2 was used to measure skeletal densities, semi-quantitative surface-to-volume ratios and surface areas, pore entrance sizes and semi-quantitative pore cavity sizes. Conventional adsorption isotherms were measured for N2 at −196 °C to check for mesoporosity and for CO2 at 0 °C to obtain Brunauer-Emmett-Teller surface areas for comparison. The known classification of 1) short or rigid organic bridges that open up the pore structure, 2) longer and more flexible bridges that cause pore filling and 3) terminal organic groups that reduce pore formation is further specified. The incorporation of any organic group in the silica network increased the dispersity in micropore entrance sizes as compared to inorganic silica in the probed size range. A critical discussion is given of the commonly accepted ‘spacing concept’ of organic bridges.