Ca-rich Ca-Al-oxide, high-temperature-stable sorbents prepared from hydrotalcite precursors: Synthesis, characterization, and CO₂ capture capacity

We present the design and synthesis of Ca-rich Ca-Al-O oxides, with Ca ²⁺/Al³⁺ ratios of 1:1, 3:1, 5:1, and 7:1, which were prepared by hydrothermal decomposition of coprecipitated hydrotalcite-like Ca-Al-CO₃ precursors, for high-temperature CO₂ adsorption at 500-700 °C. In situ X-ray diffraction measurements indicate that the coprecipitated, Ca-rich, hydrotalcite-like powders with Ca²⁺/Al ³⁺ ratios of 5:1 and 7:1 contained Ca(OH)₂ and layered double hydroxide (LDH) phases. Upon annealing, LDH was first destroyed at approximately 200 °C to form an amorphous matrix, and then at 450-550 °C, the Ca(OH)₂ phase was converted into a CaO matrix with incorporated Al³⁺ to form a homogeneous solid solution without a disrupted lattice structure. CaO nanocrystals were grown by thermal treatment of the weakly crystalline Ca-Al-O oxide matrix. Thermogravimetric analysis indicates that a CO₂ adsorption capacity of approximately 51 wt. % can be obtained from Ca-rich Ca-Al-O oxides prepared by calcination of 7:1 Ca-Al-CO₃ LDH phases at 600-700 °C. Furthermore, a relatively high CO₂ capture capability can be achieved, even with gas flows containing very low CO₂ concentrations (CO₂/N ₂=10 %). Approximately 95.6 % of the initial CO₂ adsorption capacity of the adsorbent is retained after 30 cycles of carbonation-calcination. TEM analysis indicates that carbonation-promoted CaCO₃ formation in the Ca-Al-O oxide matrix at 600 °C, but a subsequent desorption in N₂ at 700 °C, caused the formation CaO nanocrystals of approximately 10 nm. The CaO nanocrystals are widely distributed in the weakly crystalline Ca-Al-O oxide matrix and are present during the carbonation-calcination cycles. This demonstrates that Ca-Al-O sorbents that developed through the synthesis and calcination of Ca-rich Ca-Al LDH phases are suitable for long-term cyclic operation in severe temperature environments.