Microporous silica prepared by organic templating: Relationship between the molecular template and pore structure

Microporous silica materials with a controlled pore size and a narrow pore size distribution have been prepared by sol-gel processing using an organic -templating approach. Microporous networks were formed by pyrolytic removal of organic ligands (methacryloxypropyl groups) from organic/inorganic hybr id materials synthesized by copolymerization of 3-methacryloxypropylsilane (MPS) and tetraethoxysilane (TEOS). Molecular simulations and experimental measurements were conducted to examine the relationship between the microst ructural characteristics of the porous silica (e.g., pore size, total pore volume, and pore connectivity) and the size and amount of organic template ligands added. Adsorption measurements suggest that the final porosity of t he microporous silica is due to both primary pores (those present in the hy brid material prior to pyrolysis) and secondary pores (those created by pyr olytic removal of organic templates). Primary pores were inaccessible to N- 2 at 77 K but accessible to CO2 at 195 K; secondary pores were accessible t o both N-2 (at 77 K) and CO2 (at 195 K) in adsorption measurements. Primary porosity decreases with the amount of organic ligands added because of the enhanced densification of MPS/TEOS hybrid materials as the mole fraction o f trifunctional MPS moieties increases. Pore volumes measured by nitrogen a dsorption experiments at 77 K suggest that the secondary (template-derived) porosity exhibits a percolation behavior as the template concentration is increased. Gas permeation experiments indicate that the secondary pores are approximately 5 Angstrom in diameter, consistent with predictions based on molecular simulations.