(PNNL), Richland, WA (United States) Sponsoring Org.: USDOE Office of Science (SC), Basic Energy Sciences (BES). Publication Date: Research Org.: Pacific Northwest National Lab. Michigan State Univ., East Lansing, MI (United States).
Likewise, the 1D 13C data show a decreased distribution of exchange rates between pore-adsorbed and bulk CH 4 in the presence of H 2O. Introducing H 2O to the system causes the 13C chemical shift to become more negative with increasing H 2O content as a result of H 2O preferentially filling small pores, forcing CH 4 to occupy larger pores that are better connected to the bulk environment. The EXSY and 1D 13C NMR results both show that CH 4 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 more » 13C NMR of pore-adsorbed CH 4. These 13C 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 13C chemical shift in dry silica nanopores. The results show that the 13C 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. Here, we present the results of 1D 13C 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. 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.
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