DFT studies of single and multiple molecular adsorption of CH 4 , SF 6 and H 2 O in Zeolitic-Imidazolate Framework (ZIF-4 and ZIF-6)

Abstract

Methane and sulfur hexafluoride contribute significantly to the greenhouse effect. The aim of this research is to accomplish a theoretical study of the capture and the storage of these two gases and water in Zeolitic Imidazolate Frameworks (ZIF). Density functional theory (DFT) is employed to understand the behavior of the gases absorbed in ZIF-4 and ZIF-6 cages. The adsorption capacities of the pure gases and gas mixtures in the two different types of ZIFs are compared over a large range of pressures. Negative adsorption energies in ZIF-6 for SF

and CH

are obtained when they are treated individually and in mixtures with H

O. The result analysis gives a clue to what sites of the ZIFs are the favored for molecular adsorption being close to the imidazole ring, how many molecules may be attached to the surface of the pores, and how such structures look like. Adsorption energies range between −13 eV < E

< 11 eV. Mechanisms for adsorption and adsorption isotherms are described. The results confirm previous theoretical studies and available experimental data.