Competitive sorption of CO₂ and H₂O in 2:1 layer phyllosilicates

Expandable clays such as montmorillonite have interlayer exchange sites whose hydration state can be systematically varied from near anhydrous to almost bulk-like water conditions. This phenomenon has new significance with the simultaneous implementation of geological sequestration and secondary utilization of CO₂ to both mitigate climate warming and enhance extraction of methane from hydrated clay-rich formations. In this study, the partitioning of CO₂ and H₂O between Na-, Ca-, and Mg-exchanged montmorillonite and variably hydrated supercritical CO₂ (scCO₂) was investigated using in situ X-ray diffraction (HXRD), infrared (IR) spectroscopic titrations, and quartz crystal microbalance (QCM) measurements. Density functional theory calculations provided mechanistic insights. Structural volumetric changes were correlated to quantified changes in sorbed H₂O and CO₂ concentrations as a function of percent H₂O saturation in scCO₂. Intercalation of CO₂ is inhibited when the clay is fully collapsed (dehydrated interlayer), peaks sharply with the introduction of some H₂O and partial expansion of the interlayer region, and then decreases systematically with further hydration of the clay. This behavior is discussed in the context of recent theoretical calculations of the montmorillonite H₂O-CO₂ system.