Abstract: The adsorption of CO₂ on MgAl layered double hydroxides (MgAl-LDHs) based adsorbents has been an effective way to capture CO₂, however the adsorption capacity was hampered due to the pore structure and the dispersibility of adsorption active sites. To address the problem, we investigate the effect of intercalated anion and alkaline etching time on the structure, morphology and CO₂ uptake performances of MgAl-LDHs. MgAl-LDHs are synthesized by the one-pot hydrothermal method, followed by alkaline etching of NaOH, and characterized by x-ray diffraction, N₂ adsorption, scanning electron microscopy and Fourier transform infrared spectroscopy. The CO₂ adsorption tests of the samples are performed on a thermogravimetric analyzer, and the adsorption data are fitted by the first-order, pseudo-second-order and Elovich models, respectively. The results demonstrate that among the three intercalated samples, MgAl(Cl) using chloride salts as precursors possesses the highest adsorption capacity of CO₂, owing to high crystallinity and porous structure, while MgAl(Ac) employing acetate salts as precursors displays the lowest CO₂ uptake because of poor crystallinity, disorderly stacked structure and unsatisfactory pore structure. With regard to alkaline etching, the surface of the treated MgAl(Cl) is partly corroded, thus the specific surface area and pore volume increase, which is conducive to the exposure of adsorption active sites. Correspondingly, the adsorption performance of the alkaline-etched adsorbents is significantly improved, and MgAl(Cl)-6 has the highest CO₂ uptake. With the alkaline etching time further increasing, the CO₂ adsorption capacity of MgAl(Cl)-9 sharply decreases, mainly due to the collapse of pore structure and the fragmentized sheet-structure. Hence, the CO₂ adsorption performance is greatly influenced by alkaline etching time, and appropriate alkaline etching time can facilitate the contact between CO₂ molecules and the adsorbent.

Key words: CO₂ adsorption, MgAl-LDHs, one-pot hydrothermal method, intercalated anion, alkaline etching