Synthesis and CO2 adsorption of SAPO-20 molecular sieve by coal gangue and aluminum ash

Silicoaluminophosphate-20 (SAPO-20) molecular sieve was synthesized from solid waste coal gangue (CG) and aluminum ash (Al-ash), and its adsorption capacity for CO 2 was evaluated. The content of Si and Al in raw materials was increased by hydrochloric acid pretreatment, and the synthesis conditions (NaOH/CG ratio, calcination temperature, hydrothermal reaction temperature and time, liquid–solid ratio) were optimized by single factor analysis. The synthesized products were characterized by XRD, SEM, BET, FTIR and TEM. The results showed that the SAPO-20 crystal form prepared under the optimal conditions had high purity, BET surface area of 175 m 2 /g, and pore structure with multistage pore distribution, which was conducive to gas diffusion and adsorption. Thermogravimetric analysis revealed that under the conditions of 25 °C and 1 bar, compared with the CO 2 adsorption capacity of 5.63 mg/g for CG, the CO 2 adsorption capacity of SAPO-20 was 27.92 mg/g. The adsorption process was in line with the Langmuir monolayer model and had exothermic characteristics (ΔH = –152 kJ/mol). The adsorption kinetics was fitted by Pseudo-First-Order Kinetic Model (R 2  = 0.740–0.995), and the adsorption rate increased with the increase of temperature. In addition, compared with the traditional synthesis method, the synthesis cost of this study was reduced by more than 70 %, and the carbon emission was reduced by about 75 %, realizing economic and environmental viability.