Solid-State NMR Spectroscopic Studies of Propylphosphonic Acid Functionalized SBA-15 Mesoporous Silica: Characterization of Hydrogen-Bonding Interactions

This work aims to investigate the local structure of mesoporous silica of the type SBA-15 functionalized with propylphosphonic acid groups by means of an extensive solid-state NMR spectroscopic study. Here, the functionalized SBA-15 samples are obtained by a one-step synthetic method that allowed us better control over the loading and the distribution of the surface functions. In addition to NMR spectroscopy, the materials are characterized by X-ray diffraction, nitrogen adsorptiondesorption and transmission electron microscopy. The 29Si, 13C, 31P and 1H one-dimensional NMR spectroscopic results are discussed according to the nature of the functional groups in dry and wet materials. This paper proposes a rigorous approach that shows possible hydrogen bonding between adjacent phosphonic acid groups in dry SBA-15, from the comparative study with an alkylphosphonic acid material. This outcome was demonstrated by two-dimensional NMR spectroscopic solidstate experiments, including the use of 1H1H spin-diffusion exchange experiments, 31P31P double quanta and an original pulse sequence that corresponds to 31P1H1H31P correlation experiments.