Experimental and molecular simulation study of CO2 adsorption in ZIF-8: Atomic heat contributions and mechanism

We successfully synthesised ZIF-8 using the solvothermal method at room temperature to study CO2 adsorption storage at 273 and 298 K up to 35 bar. Characterisation methods such as BET, SEM-EDS, XRD, and TGA were used to measure the physical and composition properties of ZIF-8. Grand canonical Monte Carlo (GCMC) simulation was conducted to compare with experimental data and get inside of the CO2 adsorption mechanism by calculating the isosteric heat and its fluid-fluid and solid-fluid contributions. The second was also split into fluid-solid atom contributions to understand in detail the interaction between CO2 and ZIF-8. The analyses revealed that there are three main stages during the CO2 adsorption gas-solid atom contributions, developing, pore-filling and densification. During the developing and pore-filling stages the largest fluid-solid atom contribution to the isosteric heat is CO2-C2 interactions, indicating that the CO2 is adsorbed close to the hexagonal windows of the ZIF-8 structure, while during the densification stage the largest contribution is CO2-N interactions. Where C2 and N refers to C-atom and N-atom, respectively in NCH group of the solid framework. This is because CO2 changes its orientation to be able to accommodate more molecules in the pore cavity. This work provides a better understanding of the adsorption mechanism of CO2 on ZIF-8 and shows how molecular simulation can be used to improve the understanding gas adsorption storage on metal-organic frameworks.