129Xe NMR of Mesoporous Silicas

The porosities of three mesoporous silica materials were characterized with 129Xe NMR spectroscopy. The materials were sythesized by a sol−gel process with r = 0, 25, and 70% methanol by weight in an aqueous cetyltrimethylammonium bromide solution. Temperature-dependent chemical shifts and spin−lattice relaxation times reveal that xenon does not penetrate the pores of the largely disordered (r = 70%) silica. For both r = 0 and 25%, temperature-dependent resonances corresponding to physisorbed xenon were observed. An additional resonance for the r = 25% sample was attributed to xenon between the disordered cylindrical pores. 2D NMR exchange experiments corroborate the spin−lattice relaxation data, which show that xenon is in rapid exchange between the adsorbed and the gas phase.