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

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 A). 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 sc...