Alkali metals-promoted capacity of ZIF-8 and ZIF-90 for carbon capturing: A molecular simulation study

Huge emission of greenhouse gases to the atmosphere owing to extensive industrial activities of human societies causes irreparable damage to the environment. In this paper, to enhance the adsorption capacity of the most important greenhouse gas, carbon dioxide (CO2), in two types of the most significant and stable of zeolitic imidazolate framework (ZIF) materials, named as ZIF-8 and ZIF-90, a subset of MOF materials, ligand functionalization using various alkali metals is performed through Density functional theory (DFT) and Grand Canonical Monte Carlo (GCMC) simulations. This strategy (ligand functionalization) was employed for enhancing the host-guest interactions and subsequently higher CO2 adsorption. Therefore, at first step, ZIF-8 and ZIF-90 were synthesized via a convenient and environmentally-friendly method and characterized with different techniques including XRD, FT-IR and N-2 adsorption-desorption. Afterwards, CO2 adsoprtion isotherms over the as-prepared samples were experimentally measured and compared to the results from GCMC simulation to confirm the usability of the selected force field in our simulations. The obtained results revealed a good agreement between simulation and experimental data over both pristine ZIF-8 and ZIF-90. At the second step, the alkali-functionalized ZIF structures are designed using lithium (Li), potassium (K) and sodium (Na) cations by DFT calculations. Thereafter, CO2 adsorption capacity in the all alkali-functionalized ZIF structures is evaluated by GCMC simulation. Li indicated the highest impact on the adsorption uptake in both structures through enhancing electrostatic and dispersion interactions between adsorbate molecules and adsorbent atoms leading to higher binding energies. Our simulation results also revealed that at 1 bar and 298 K, the CO2 uptakes over the Li-functionalized-ZIF-8 and -90 are increased to 7 and 9 times and reach to surprising values of 6 and 9 mmol/g, respectively. Heat of adsorption was eventually calculated for CO2 over the Li-functionalized-ZIF structures and chemisorption was observed which is very important for regeneration of the adsorbents in practical situations. (C) 2020 Elsevier Ltd. All rights reserved.