Role of Cations in CO2 Adsorption, Dynamics, and Hydration in Smectite Clays under in Situ Supercritical CO2 Conditions

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• 作者：Geoffrey M. BowersH. Todd Schaef ; John S. Loring ; David W. Hoyt ; Sarah D. Burton ; Eric D. Walter ; R. James Kirkpatrick
• 刊名：Journal of Physical Chemistry C
• 出版年：2017
• 出版时间：January 12, 2017
• 年：2017
• 卷：121
• 期：1
• 页码：577-592
• 全文大小：610K
• ISSN：1932-7455

文摘

This paper explores the molecular-scale interactions between CO₂ and the representative smectite mineral hectorite under supercritical conditions (90 bar, 50 °C) using novel in situ X-ray diffraction (XRD), infrared (IR) spectroscopy, and magic angle spinning (MAS) nuclear magnetic resonance (NMR) techniques. Particular emphasis is placed on understanding the roles of the smectite charge balancing cation (CBC) and H₂O in these interactions. The data show that supercritical CO₂ (scCO₂) can be adsorbed on external surfaces and in the confined interlayer spaces of hectorite at 50 °C and 90 bar, with the uptake of CO₂ into the interlayer favored at low H₂O content and when the basal spacing is similar to a monolayer hydrate of hectorite (1WL, ∼12.5 Å). These results are in agreement with published spectroscopic and molecular modeling data for the related smectite Na-montmorillonite. Charge balancing cations with small radii, large hydration energies, and low polarizabilities tend to scavenge H₂O from humid scCO₂ or retain the H₂O they held before scCO₂ exposure, swelling spontaneously to a bilayer hydrate (2WL) dominated state that largely prevents CO₂-ion interactions and influences the extent of CO₂ intercalation into the interlayer. In contrast, ions with large radii, low hydration energies, and large polarizabilities more readily form close associations with CO₂ with the energetics enabling coexistence of CO₂ and H₂O in the interlayer over a wide range of scCO₂ humidities. Integrating our results with those from molecular dynamics simulations of wet CO₂-bearing montmorillonites suggest that adsorbed CO₂ in 1WL-type interlayers is oriented with its long axis parallel to the clay sheets and experiences dynamics dominated by anisotropic rotation about the axis perpendicular to the CO₂ long axis at rates of at least ∼10⁵ Hz. If appreciable CO₂ is adsorbed in 2WL-type interlayers, it must experience a mean orientation and dynamic averaging affects that mimic the 1WL-type adsorption environment. External surface adsorbed CO₂ is dynamically similar to the 1WL case, but the CO₂ long axis samples a larger range of orientations with respect to the smectite surface and adopts a different mean angle between the long axis and the smectite surface. Our data also suggest that equilibrating hectorite with a large volume of scCO₂ at 50 °C and 90 bar leads to interlayer dehydration, with the extent of dehydration correlating with the hydrophilicity of the CBC.