Mesoporous silica synthesis: Energetics of interaction between framework and structure directing agent

The thermodynamics of mesoporous silicas (MCM-41, MCM-48, SBA-15, and SBA-16) were studied by solution calorimetry at 323 K in 25% aqueous HF. The enthalpies of formation were determined for calcined mesoporous silica (MS) and organic structure-directing agent (SDA) occluded samples (SDA: n-hexadeciltrimethylammonium bromide or CTAB, Pluronic P123, and Pluronic F127). The following are the measured interaction enthalpies between the MS and SDA: MCM-41/CTAB, -6.1 kJ/mol SiO₂; MCM-48/CTAB, -12.3 kJ/mol SiO ₂; SBA-15/P123, -19.7 kJ/mol SiO₂; SBA-16/F127, -19.9 kJ/mol SiO₂. Per unit surface area, these interactions are -0.08, -0.15, -0.43, and -0.40 J/m², respectively. Though these SDA-framework interaction energies are still small in magnitude, they are somewhat more exothermic than those in silica zeolite formation, reflecting the greater metastability of the MS materials and the role of the long chain SDA in stabilizing and space-filling the large pores. The cubic MS (SBA) show stronger SDA interactions than the hexagonal (MCM). The interaction energies confirm a complex landscape of many competing structures of similar energy; with the role of SDA kinetic in selecting a specific structure rather than energetic in strongly stabilizing a given state, as has already been noted for zeolites. The enthalpies of the calcined MS relative to quartz determined by HF solution calorimetry in this study are in excellent agreement with those determined previously by high temperature oxide melt solution calorimetry.