Effects of ammonia/silica molar ratio on the synthesis and structure of bimodal mesopore silica xerogel

A surfactant-templated silica xerogel with high pore volume, high porosity and narrow bimodal mesopore size distribution (designated as BMS) was synthesized by a judiciously controlled two-phase sol-gel processing under basic conditions. The properties of BMS silica mesostructure and the effect of ammonia/silica molar ratio on the resultant silica structure and morphology were assessed using various analytical techniques such as XRD, ²⁹Si MAS NMR, SEM, TEM, TG-DTA and N₂ adsorption measurements. It is found that BMS silica xerogel consists of the packing of the micelle-encapsulated nanometer silica particles in the 20-40 nm range, depending upon the ammonia/silica molar ratio, and shows both an intraparticle framework mesopore resulting from the surfactant removal and a textural mesopore resulting from the interparticle voids, which can be adjusted in the 12.6-34.8 nm range by varying just the ammonia/silica molar ratio. The textural pore volumes for all BMS silica can be two or more times as large as the framework pore volumes, and the fill of a certain amount of surfactant between interparticle voids in as-synthesized BMS silica xerogel may play a crucial role for the formation, stability and ordering of textural mesoporosity. A fast hydrolysis and slow condensation process is beneficial to the formation of BMS silica mesostructure. Following a gradual increase of ammonia/silica molar ratio, thus an increase of condensation rate, the mesostructure evolution of the resultant silica products obtained from a bimodal mesopore via a disordered mesopore to a well-defined ordered hexagonal mesopore MCM-41 was observed.