A spectroscopic insight of the porous structure of hydrophobic silica aerogels by hyperpolarized 129Xe NMR

Silica aerogels are mesoporous materials with high specific surface areas and broad pore size distributions. Pore structure characterization is generally performed through nitrogen sorption experiments; however, the fragile silica network and the broad pore size distributions represent important limitations in the analysis. The pore structure of hydrophobic acid and base-catalyzed sodium silicate-based aerogels was studied by using hyperpolarized (HP) 129Xe NMR and complemented with other spectroscopic techniques for aerogel characterization. Two main pore species within an overall broad pore size distribution are distinguished through HP 129Xe NMR and specific surface areas ranging from 300 up to 600 m2/g are reported. When studying synthesized base-catalyzed aerogels, condensation reactions yielded a coarser particle network and the presence of particle aggregates, which in turn affects the resultant pore structure leading to the lowest specific surface areas when increasing aging times, as opposite to synthesized acid-catalyzed aerogels.