Smectite-rich Clays for CO₂ Adsorption and Separation

Increased atmospheric concentrations of greenhouse gases such as CO₂ and CH₄ is the primary driver of global warming and the adverse effects thereof. The strategies to reduce greenhouse gas emissions and atmospheric concentrations include the capture of carbon and its subsequent storage or use, and the reduction of CH₄ emissions across industries. Smectites are naturally abundant layered clay minerals that can adsorb molecules in their interlayer galleries under specific conditions. Therefore, smectites may show potential for gas separation processes and are relevant to carbon sequestration processes. One relevant separation is that of CO₂ from CH₄ in biogas upgrading to produce two valuable gas products while preventing greenhouse gas emissions.

This Thesis focuses primarily on the (selective) adsorption of these gases on smectites under near-ambient conditions. The main variables that are studied and affect the interlayer adsorption are (i) the smectite interlayer cation species – tightly linked to the interlayer gallery height – and (ii) the presence of co-sorbed H₂O. The selective adsorption of CO₂ over the larger CH₄ requires an interlayer gallery height that is comparable to the size of the CO₂ molecule, provided by (i) sufficiently large cations, or (ii) small but strongly hydrated cations in the absence of competitively adsorbed excess H₂O. Moreover, interlayer galleries then show fast CO₂ adsorption and desorption kinetics and desorption requires no external heat input.

Numerical results of the performance of the smectites in an actual vacuum pressure swing adsorption process show that some smectites can upgrade biogas to a product that is compatible with grid injection in The Netherlands at a comparatively low specific energy consumption. Together, the low sorbent cost, comparatively low specific energy consumption, and high CO₂/CH₄ selectivity make smectites an excellent alternative to conventional sorbent materials for biogas upgrading and beyond.