CO₂ Adsorption Enhanced by Tuning the Layer Charge in a Clay Mineral

Due to the compact two-dimensional interlayer pore space and the high density of interlayer molecular adsorption sites, clay minerals are competitive adsorption materials for carbon dioxide capture. We demonstrate that with a decreasing interlayer surface charge in a clay mineral, the adsorption capacity for CO₂ increases, while the pressure threshold for adsorption and swelling in response to CO₂ decreases. Synthetic nickel-exchanged fluorohectorite was investigated with three different layer charges varying from 0.3 to 0.7 per formula unit of Si₄O₁₀F₂. We associate the mechanism for the higher CO₂ adsorption with more accessible space and adsorption sites for CO₂ within the interlayers. The low onset pressure for the lower-charge clay is attributed to weaker cohesion due to the attractive electrostatic forces between the layers. The excess adsorption capacity of the clay is measured to be 8.6, 6.5, and 4.5 wt % for the lowest, intermediate, and highest layer charges, respectively. Upon release of CO₂, the highest-layer charge clay retains significantly more CO₂. This pressure hysteresis is related to the same cohesion mechanism, where CO₂ is first released from the edges of the particles thereby closing exit paths and trapping the molecules in the center of the clay particles.