Mg2−xCaxAl layered double hydroxide-derived mixed metal oxide porous hexagonal nanoplatelets for CO2 sorption

Abstract

Porous hexagonal nanoplatelets of mixed metal oxide (MMO) derived from the calcination of MgAl layered double hydroxide exhibits a CO₂ sorption capacity of 1.99 mmol g⁻¹ at 30 °C, with a retention of 87% sorption capacity over 10 carbonation–decarbonation cycles and a CO₂ sorption capacity of 1 mmol g⁻¹ at 200 °C with a 40% increase in capacity over 10 cycles. The high sorption capacity is attributed to the porous nanoplatelet structure of the MMO with a BET surface area of 115 m² g⁻¹, which enables increased CO₂ diffusion. Upon partially replacing magnesium with calcium (33, 50 and 66 mol%), the CO₂ sorption capacity of the MMO increases with an increase in temperature. MMO derived from LDH, in which 66% of magnesium is replaced by calcium (MgCaAl-66), delivers CO₂ sorption capacities of 1.38, 1.31, 2.50, 4.85 and 7.75 mmol g⁻¹ at 200, 300, 350, 400 and 600 °C, respectively, which is significant for application in the sorption-enhanced water gas shift (SEWGS) process. MgCaAl-66 MMO exhibits a sorption capacity of 1 mmol g⁻¹, which is stable over 10 cycles at 200 °C, and a sorption capacity of 3.68 mmol g⁻¹ at 400 °C with 85% capture efficiency retention over 10 cycles. While the incorporation of Ca²⁺ serves multiple purposes such as increasing basic defect sorption sites and improving stability to repress the sintering-induced limitation of MMO over sorption cycles, the porous nanoplatelets act as individual sorbent units resisting volume changes through carbonation–decarbonation cycles.