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

Abstract

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 relative ratio between these two species is not sensitive to the loading of phosphonic acid groups incorporated, but it strongly depends on the moisture present in the materials. The PO₃H₂ groups forming the hydrogen bonds with the nearby Q³ Si–OH are the major species responsible for the 22 ppm peak based on the results of ¹H → ³¹P → ²⁹Si double cross-polarization NMR experiments and density functional theory calculations (DFT). Of particular interest is that ²⁹Si{³¹P} rotational echo double resonance (REDOR) NMR experiments are utilized to measure ³¹P–²⁹Si distances between the phosphorus site in the functional groups and the silicon sites in the silica framework. A ²⁹Si–³¹P distance of 5.0 Å is obtained for the phosphorus site in the functional groups to the silicon site of the Q³ species for the as-synthesized sample. A reasonable fitting to the REDOR data for the acidified sample can also be achievable by assuming the presence of different structural units, whose ³¹P–²⁹Si distance information is referred from the DFT results. The combination of REDOR and ¹H → ³¹P → ²⁹Si double cross-polarization NMR measurements and the DFT calculations allow one to gain deeper insights into the local environments of the organic groups functionalized in mesoporous silica materials.