Enhancement of CO2 Adsorption Performance on Hydrotalcites Impregnated with Alkali Metal Nitrate Salts and Carbonate Salts

Hydrotalcite is considered to be the potential high-temperature CO2 sorbent adaptable to operate in 473–673 K. However, pristine hydrotalcite has a lower CO2 adsorption capacity, which cannot satisfy the industrial requirements. In this work, commercial hydrotalcites (MG60 with a Mg/Al molar ratio of 1.91) were modified through impregnation with two kinds of alkali metal salts (K2CO3 and Na/KNO3) to improve the CO2 adsorption performance. Because Na/KNO3 was a mixed salt with a lower melting point of 493 K, the impregnated Na/KNO3 molten salt in micropores of hydrotalcites would accelerate CO2 diffusion to enhance CO2 adsorption and desorption kinetics when it captured CO2 from the process gas at more than 493 K. By measuring CO2 adsorption capacity, adsorption and desorption rates, and cyclic adsorption/desorption stability by a thermogravimetric analyzer, the enhancement of CO2 adsorption performance of impregnated hydrotalcites was evaluated. Furthermore, Raman spectra were used first to explore the CO2 adsorption enhancement mechanism on impregnated hydrotalcites in addition to IR spectra analysis. By comparison, it was found that the CO2 adsorption enhancement mechanism was different with two kinds of salts impregnation: Na/KNO3 promoted MG60 with the alkalinity increase of active sites and having the maximum CO2 adsorption capacity at 473 K and K2CO3 promoted MG60 with the formation of K2Mg(CO3)2 on adsorption active sites and having the maximum CO2 adsorption capacity at 623 K. The maximum CO2 adsorption capacities for both impregnated hydrotalcites were up to 1.0 mmol g–1 under 1 atm CO2 gas flow, which is more than that of pristine hydrotalcite with less than 0.4 mmol g–1 at 473–673 K.