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 Ca2+/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+/Al3+ 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/N2=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.