Probing the Nature and Local Structure of Phosphonic Acid Groups Functionalized in Mesoporous Silica SBA-15

Well-ordered mesoporous silicas SBA-15 functionalized with variable contents of phosphonic acid groups (up to 25 mol % based on silica) have successfully synthesized via cocondensation of tetraethoxysilane (TEOS) and diethylphosphatoethyltriethoxysilane (PETES) using triblock copolymer Pluronic P123 as the structure-directing agent under acidic conditions. The status and local structures of the phosphonic functional groups are investigated by extensive multinuclear solid-state NMR studies. Solid-state 13C and 31P NMR results reveal that phosphonic ester moieties are obtained for the as-synthesized samples and for the samples subjected to template removal by concentrated H2SO4. The generation of phosphonic acid groups can be accomplished by dealkylation reaction via treating the template-extracted samples with concentrated HCl. Two distinct local environments for the phosphorus sites of phosphonic acid groups have been observed at 32 and 22 ppm in the 31P magic angle spinning (MAS) NMR spectra. The relativ...