Influence of Smectite Structure and Hydration on Supercritical Methane Binding and Dynamics in Smectite Pores
In situ high pressure infrared (IR), 13C Bloch decay magic angle spinning nuclear magnetic resonance (MAS NMR), and exchange correlation (EXSY) NMR spectroscopic data acquired at T = 323 K and Pfluid = 90 bars CH4 pressure show that CH4 occurs in the interlayer nano-pores of smectite clays, in pores between the clay particles (inter-particle pores), and in bulk fluid likely in the head space of the NMR rotors. All three CH4 environments interact with the clay surfaces and are in dynamic exchange on 100 to 104 Hz rates. The 13CH4 chemical shifts for these three environments are well resolved, with the 13C shift becoming more positive with decreasing physical space accessible by the CH4. This effect is most pronounced for interlayer adsorbed CH4, where the layer charge, size of the charge balancing cation, and relative humidity of the fluid all influence the interlayer pore size, 13CH4 chemical shift, and the amount of interlayer CH4 adsorbed. Layer charge exhibits the strongest influence. 2D EXSY NMR spectra of dry Cs-Laponite® show that CH4 is exchanging among all three environments, with interlayer CH4 exchanging with inter-particle/bulk CH4 at a broad range of frequencies between 104 to 100 Hz, while inter-particle/bulk CH4 exchange occurs at <10 Hz frequencies. With increasing H2O, the progressively more negative 13C chemical shifts of the inter-particle CH4 and peak area reductions for CH4 in interlayer environments suggest that CH4 is displaced from interlayers and smaller inter-particle pores by H2O causing a greater fraction of CH4 to spend significant time in bulk-like environments, consistent with earlier literature. However, the Cs-smectites show that the influence of H2O on the interlayer CH4 chemical shift is complicated, with increasing H2O leading to larger interlayer basal spacings and decreased lateral pore space that may lead to net larger or smaller pores depending on the relative affinity of the charge-balancing cation for the clay surfaces vs. hydrating H2O. Clay T-O-T layers with larger lateral dimensions (basal surface area) exhibit greater structural and/or dynamical heterogeneity than smaller layers, suggesting that dynamic basal spacing fluctuations with characteristic wavelengths between 10 nm and 1 micron may be important to fundamental understanding of molecular behavior in clay interlayer galleries.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 1578012
- Report Number(s):
- PNNL-SA-146425
- Journal Information:
- Journal of Physical Chemistry C, Vol. 123, Issue 48
- Country of Publication:
- United States
- Language:
- English
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