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

Abstract

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.