Ca-based layered double hydroxides as solids sorbents for CO₂ capture

Abstract

Calcium based layered double hydroxides (Ca LDHs, Ca₂M(OH)₆A·2H₂O) have emerged as promising materials for carbon capture due to their high density of alkaline adsorption sites and tunable physicochemical properties, which can be modified by altering the trivalent cation, M³⁺, and interlayer anion, A⁻. However, green synthesis of high-purity Ca-LDHs with different chemistries, together with assessment of their carbon capture performance, remains largely unexplored. This thesis investigates the effect of chemical tailoring and relative humidity on the CO₂ sorption performance of Ca LDHs at room temperature, providing new insight on how the molecular design of these sorbents affects their carbon capture potential.The first part focuses on understanding how Ca(OH)₂ thin films (known as portlandene) interact with CO₂ in the absence of water vapor, with results showing a complex adsorption mechanism dependent on film thickness. In thicker films, the interaction is dominated by strong chemisorption via CaCO₃ formation, whereas thinner films form a mixed metal oxide with the substrate that enables weaker chemisorption and even physisorption of CO₂. Next, the thesis examines the CO₂ sorption performance of two crystalline Ca LDHs: Ca₂Fe(OH)₆Cl·2H₂O and Ca₂Al(OH)₆Cl·2H₂O under humid conditions. Both Ca LDHs demonstrate higher CO₂ uptake and lower desorption temperatures than bulk portlandite, motivating further exploration of these Ca-LDHs. A novel green synthesis method is developed for high purity Ca LDHs of various chemistries, enabling comparative studies of four compounds, including Ca₂Fe(OH)₆Br·2H₂O, synthesized for the first time. In situ neutron and X ray scattering, along with density functional theory modeling, reveal the atomic structures of Ca LDHs and their transformation into layered double oxides (LDOs) at high temperatures, including the formation of Ca₂AlO₃Cl and Ca₂FeO₃Cl with perovskite like structures. Finally, a dual CO₂ adsorption mechanism is uncovered for Ca-LDHs: in dry conditions, CO₂ physisorbs within the interlayer space of Ca LDHs by anionic substitution, forming H bonds with the OH groups, whereas water vapor enables the chemisorption of CO₂ via CaCO₃ formation, leading to structural degradation. In practice, the physisorption kinetics can be controlled by selecting the appropriate M(III), while the rate of chemisorption can likely be minimized by drying the CO₂-rich gas before sorption.