Ca‐Rich Ca–Al‐Oxide, High‐Temperature‐Stable Sorbents Prepared from Hydrotalcite Precursors: Synthesis, Characterization, and CO2 Capture Capacity

Abstract We present the design and synthesis of Ca‐rich Ca–Al–O oxides, with Ca 2+ /Al 3+ ratios of 1:1, 3:1, 5:1, and 7:1, which were prepared by hydrothermal decomposition of coprecipitated hydrotalcite‐like Ca–Al–CO 3 precursors, for high‐temperature CO 2 adsorption at 500–700 °C. In situ X‐ray diffraction measurements indicate that the coprecipitated, Ca‐rich, hydrotalcite‐like powders with Ca 2+ /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 Al 3+ 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 CO 2 adsorption capacity of approximately 51 wt. % can be obtained from Ca‐rich Ca–Al–O oxides prepared by calcination of 7:1 Ca–Al–CO 3 LDH phases at 600–700 °C. Furthermore, a relatively high CO 2 capture capability can be achieved, even with gas flows containing very low CO 2 concentrations (CO 2 /N 2 =10 %). Approximately 95.6 % of the initial CO 2 adsorption capacity of the adsorbent is retained after 30 cycles of carbonation–calcination. TEM analysis indicates that carbonation‐promoted CaCO 3 formation in the Ca–Al–O oxide matrix at 600 °C, but a subsequent desorption in N 2 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.