Synthesis, characterization and modification of monolithic ZSM-5 from geopolymer for CO2 capture: Experiments and DFT calculations

Zeolites exhibit the highest performance metrics among the second generation of post-combustion CO2 capture materials; however, conventional synthetic zeolite powders are relatively expensive and need to be shaped by binding agents for end use. In this study, cost-effective monolithic Na/ZSM-5 was successfully prepared for the first time using metakaolin and industrial silica fume waste as starting materials via polycondensation and seed-induced hydrothermal crystallization. The synthesised Na/ZSM-5 exhibited an adsorption capacity of 1.79 mmol/g CO2. To further enhance the CO2 adsorption capacity, the Ni/ZSM-5 was modified by ion-exchange of Na+ with Ni2+, revealing an improved adsorption capacity of 2.38 mmol/g CO2. Density functional theory (DFT) calculations were employed to explain the adsorption mechanism and the superior CO2 adsorption performance of Ni/ZSM-5 compared to Na/ZSM-5. The results show that the sorption of CO2 in both Na/ZSM-5 and Ni/ZSM-5 is physisorption by ion–dipole interaction, and the energy emission using Ni/ZSM-5 is higher than that using Na/ZSM-5 for the same CO2 capture efficiency. This research demonstrates a new method of preparing cost-effective shape-controllable zeolites with excellent CO2 capture capacity. Furthermore, it enables the high value-added utilization of the industrial waste of silica fume.