CO2 Sorbents with Scaffold-like CaAl Layered Double Hydroxides as Precursors for CO2 Capture at High Temperatures

A highly stable high-temperature CO2 sorbent consisting of scaffold-like Ca-rich oxides (CaAlO) with rapid absorption kinetics and a high capacity is described. The Ca-rich oxides were prepared by annealing CaAlNO3 layered double hydroxide (LDH) precursors through a sol–gel process with Al(OiP)3 and Ca(NO3)2 with Ca2+/Al3+ ratios of 1:1, 2:1, 4:1, and 7:1. XRD indicated that only LDH powders were formed for Ca2+/Al3+ ratios of 2:1. However, both LDH and Ca(OH)2 phases were produced at higher ratios. Both TEM and SEM observations indicated that the CaAlNO3 LDHs displayed a scaffold-like porous structure morphology rather than platelet-like particles. Upon annealing at 600 °C, a highly stable porous network structure of the CaO-based CaAlO mixed oxide (CAMO), composed of CaO and Ca12Al14O33, was still present. The CAMO exhibited high specific surface areas (up to 191 m2 g−1) and a pore size distribution of 3–6 nm, which allowed rapid diffusion of CO2 into the interior of the material, inducing fast carbonation/calcination and enhancing the sintering-resistant nature over multiple carbonation/calcination cycles for CO2 absorption at 700 °C. Thermogravimetric analysis results indicated that a CO2 capture capacity of approximately 49 wt % could be obtained with rapid absorption from the porous 7:1 CAMO sorbents by carbonation at 700 °C for 5 min. Also, 94–98 % of the initial CO2 capture capability was retained after 50 cycles of multiple carbonation/calcination tests. Therefore, the CAMO framework is a good isolator for preventing the aggregation of CaO particles, and it is suitable for long-term cyclic operation in high-temperature environments.