A novel preparation of microporous nanosilica derived from aluminum-extracted residue of coal fly ash for highly selective CO2 adsorption

Currently, the poor adsorption performance caused by the unsatisfactory pore size seriously restricts the further development of microporous nanosilica (MS). Herein, an excellent microporous nanosilica was constructed by preparing calcium silicate hydrate (CSH) support from the aluminum-extracted residue of coal fly ash and modifying it with different activating agents (H 2 SO 4 , HNO 3 , HCl, CH 3 COOH, KOH, NaOH and Na 2 CO 3 ). The results indicated that CH 3 COOH at 30 wt% was the optimal modifier, achieving a CO 2 uptake of 1.42 mmol/g at 298 K and 1.0 bar. Then the effects of synthesis parameters including mass fraction, liquid–solid ratio, temperature, and time on the adsorption capacity of CO 2 were further investigated. The obtained MSCp exhibited a high specific surface area (up to 505.71 m 2 /g), well-developed microporosity (0.085 cm 3 /g), and excellent CO 2 uptake (2.12 mmol/g at 273 K, 1.41 mmol/g at 298 K). Based on the study of kinetics, isosteric heat of adsorption (Qst), and situ diffuse reflectance infrared Fourier transformation spectroscopy (DRIFTS), the adsorption mechanism of MSCp was determined to be mainly physical adsorption. Besides, the excellent CO 2 /CH 4 selectivity of 20.91 (15/85, v/v) and 30.75 (50/50, v/v), CO 2 /N 2 selectivity of 53.79 (15/85, v/v) and 163.76 (50/50, v/v) were obtained from MSCp at 298 K and 1.0 bar. Finally, the binary gas breakthrough experiments and eight adsorption–desorption cycles confirmed the MSCp was suitable for selective adsorption of CO 2 in practical applications.