Novel, facile, and scalable synthesis of magnesium based adsorbents via the freeze-drying technique for CO2 capture†

In this study, MgO/Mg(OH)₂ based adsorbents were prepared via freeze-drying and electrospinning techniques, and their CO₂ adsorption capacities were investigated. The synthesized adsorbents were characterized by XRD, N₂-Ads–Des, FESEM, XPS, and CO₂-TPD, while their CO₂ capture efficiency and mechanism were evaluated by TGA and FTIR spectroscopy, respectively. The adsorbent prepared via freeze-drying displayed nearly 6.2 wt% CO₂ adsorption at room temperature compared to only 5.4 wt% by the adsorbent prepared via electrospinning. This adsorbent's superior CO₂ capture capacity was attributed to the high basic strength of the active sites and the presence of a substantial amount of surface oxygen vacancies/defects. The adsorbent prepared via freeze-drying exhibited abundant surface basic sites, which led to enhanced CO₂ molecule interaction with the O²⁻ (strong sites), Mg–O pairs (medium sites), and OH group (weak sites) forming firmly fixed unidentate/monodentate, bidentate chelate and bidentate bridged carbonates, respectively. Although both physical and chemical adsorption coexisted in the process, the CO₂ adsorption was mainly presided over by the chemisorption sites. The high surface basicity of the adsorbents dominated BET surface area in governing the CO₂ capture capacity. For the first time in this research, the freeze-drying technique was applied to enlighten the facile, sustainable, and scalable synthesis of magnesium-based adsorbents for efficient CO₂ capture at room temperature.