Adsorption of high-temperature CO₂ by Ca²⁺/Na⁺-doped lithium orthosilicate: characterization, kinetics, and recycle

High-temperature solid adsorbent Li₄SiO₄ has received broad attention due to its high theoretical adsorption capacity, high regeneration capacity, and wide range of raw materials for preparation. In this paper, a Li₄SiO₄ adsorbent was prepared by MCM-48 as the silica precursor and modified by doping with metal ions (Ca²⁺ and Na⁺) for high-temperature capture of low-concentration CO₂. The results showed that the surface of the Ca-doped (or Na-doped) Li₄SiO₄ adsorbent developed some particles that are primarily composed by Li₂CaSiO₄ (or Li₃NaSiO₄). Furthermore, the grains of the adsorbents became finer, effectively increasing the specific surface area and enhancing adsorption performance. Under 15 vol% CO₂, the maximum CO₂ adsorption was 25.63 wt% and 32.86 wt% when the Ca²⁺ doping amount was 0.06 and the Na⁺ doping amount was 0.12, respectively. These values were both higher than the adsorption capacity before the metal ion doping. After 10 adsorption/desorption cycles, the adsorption capacity of Na-doped Li₄SiO₄ increased by 9.68 wt%, while that of Ca-doped Li₄SiO₄ decreased by 7.98 wt%. This difference could be attributed to the easy sintering of the Ca-containing adsorbent. Furthermore, a biexponential model was used to fit the CO₂ adsorption curve of the adsorbent in order to study the adsorption kinetics. Compared to the conventional Li₄SiO₄, the Ca/Na-doped adsorbent offers several advantages, such as a high CO₂ adsorption capacity and stable cycling ability.