Hydrocarbons (e.g., CH4, C2H6, etc.), carbon dioxide, and aqueous solutions can occupy the pores or fractures of numerous types of Earth materials. The properties of confined fluids vary as a function of density, pressure, and temperature, and may also be affected by physical and chemical interactions with pore walls. The objective of this study is to provide a fundamental description of methane gas confined within nanoporous matrices. Mesoporous silica (SiO2) having 200 nm particle size and 4 nm pore size was used as a model material for a systematic and controlled study of methane structure and dynamics in confined states at pressures up to 120 bar. We utilized a recently established magic angle spinning (MAS) high-pressure NMR method [1, 2]. For these studies, proton decoupled 13C MAS NMR spectra were acquired with the high-pressure MAS probe at 30 bar, 60 bar, and 120 bar. At each pressure, the temperature was varied from 34 oC to 73 oC.For methane alone, i.e., no silica present, we find that the 13C chemical shifts do not exhibit any change over the temperature range studied, but pressure induces shifts in the methane peak position. A mixture of methane and mesoporous silica yields resolved13C resonances arising from both �bulk like� and confined methane. In control experiments where the sample is not undergoing MAS, residual line broadening obfuscates the resonances and only one peak is observed. Grand canonical Monte Carlo, molecular dynamics, and quantum chemistry simulations of bulk and mesoporous silica-confined methane show 13C chemical shifts change by varying methane density.Acknowledgment. This work was supported by A. P. Sloan Foundation and Basic Energy Sciences of the U.S. Department of Energy.References. 1)Hoyt, D.W. et al. J. Magn. Reson. 2011  , 212, 378.2) Turcu, R.V.F. et al. J. Magn. Reson. 2013  , 226, 64.

High-pressure MAS NMR study of confined methane

247th National Meeting (2014)

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