A Facile Process to Synthesize Porous Adsorbents from Clays for CO₂ Capture

Anthropogenic CO2 emissions resulting from the combustion of fossil fuels contribute significantly to climate change. In this study, we examined the feasibility of employing low-cost, abundant, and environmentally friendly materials, such as kaolinite and bentonite clays, to synthesize solid adsorbents for CO2 capture. The synthesis method involved subjecting the clay to thermal treatment and then alkaline treatment via a hydrothermal method to enhance the textural properties such as porosity and specific surface area. This pretreated clay was then functionalized with 3-aminopropyltriethoxysilane (APTES) to prepare porous adsorbents suitable for CO2 capture. The synthesized adsorbents were characterized using field-emission scanning electron microscopy (FESEM), N2 physisorption, Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). FESEM revealed the fibrous morphology of the alkaline-treated adsorbents. The loading of APTES on the surface of the adsorbent was confirmed by FTIR. The porosity and specific surface area of both kaolinite and bentonite increased after the alkaline treatment. TGA analysis confirmed that the synthesized adsorbents exhibited thermal stability up to 150 °C. The CO2 adsorption capacity of these adsorbents under a simulated flue gas atmosphere, comprising of 15 vol % CO2 in N2, was determined using TGA. The CO2 adsorption capacities of both untreated kaolinite and bentonite were found to be negligible at 35 °C but increased to 6.11 and 10.56 mg/g, respectively, after alkaline treatment. With APTES functionalization, their CO2 adsorption capacities further increased to 20.69 and 25.96 mg/g at 35 °C, respectively. Moreover, the CO2 adsorption capacity of APTES functionalized adsorbents was found to increase with an increase in temperature (from 35 to 75 °C). The maximum CO2 adsorption capacities for kaolinite and bentonite-based adsorbents were found to be 30.36 and 38.72 mg/g, respectively, at the optimal temperature of 75 °C. Furthermore, TGA and a fixed-bed setup were employed to study CO2 adsorption–desorption over multiple cycles at 75 °C. The adsorption capacity of these adsorbents was found to be stable over 5 cycles, making them promising in practical applications.