Probing Pore Size and Connectivity in Porous Silicas Using 13C MAS NMR Spectroscopy of Supercritical Methane

Measuring the pore size and pore-size distributions and exploring the fluid-exchange dynamics between different types of pores in porous materials remains a significant experimental challenge but is critical to understanding catalysis, chromatography, nutrient cycling, and a whole range of geochemical phenomena, including shale gas and tight gas extraction. Here, we present the results of 1D C-13 NMR and 2D exchange spectroscopy (EXSY) NMR investigations of a porous silica using supercritical methane (scCH(4)) as a direct probe of pore size and fluid exchange between pore types. The results show that the C-13 chemical shift of scCH(4) adsorbed in nanometer-scale silica pores becomes more negative with increasing pore diameter, in agreement with trends reported for gas hydrates, zeolites, MOFs, and clays and other microporous (<10 nm pores) geochemical materials. These C-13 chemical shifts follow a natural log-linear trend with pore size in the vacuum-dried porous silicas studied here, allowing one to predict pore size based on the C-13 chemical shift in dry silica nanopores. The EXSY and 1D C-13 NMR results both show that CH4 exchanges between pore and bulk fluid environments over rate scales from 0.01 to 1 kHz in vacuum-dried samples and that exchange dynamics must be considered when interpreting C-13 NMR of pore-adsorbed CH4. Introducing H2O to the system causes the C-13 chemical shift to become more negative with increasing H2O content as a result of H2O preferentially filling small pores, forcing CH4 to occupy larger pores that are better connected to the bulk environment. Likewise, the 1D C-13 data show a decreased distribution of exchange rates between pore-adsorbed and bulk CH4 in the presence of H2O.