Molecular Thermodynamic Evaluation of Various Cu-Doped ZIF-8 Ratios for SF6 Catalysis

Abstract

SF6 is a greenhouse gas with strong greenhouse effect. How to rationally deal with waste SF6 has become an extensive research topic for scholars at home and abroad. Studies have shown that ZIF-8 has certain superiority as a catalyst, while metal doping in ZIF-8 may promote its catalytic ability. In this article, the undoped ZIF-8 model and the Cu-doped ZIF-8 model (Zn:Cu =1:5; Zn:Cu =3:3) were constructed, and the most stable adsorption structures of SF6 at three typical sites were obtained based on density-functional theory (DFT), with the 6-membered ring (6MR) window as the optimal adsorption site. The stable adsorption energy was the largest in the Zn:Cu =5:1 doped model (−98.352 kcal/mol), and during the adsorption process, the SF6 molecular bond lengths changed most drastically, and even the bond breaking phenomenon appeared. The catalytic ability of the Zn:Cu =3:3 doping model was in between that of the ZIF-8 and Zn:Cu =1:5 doping models. The results of the Mulliken charge transfer and the HOMO-LUMO energy level showed that in the Zn:Cu =5:1 doping model, the Cu atom can both transfer a large number of electrons (−0.606 e) to SF6 and enhance the ability of Zn atoms to lose electrons, effectively weakening the S-F bond strength of SF6 gas molecules and thus promoting SF6 gas molecule catalysis. The highest occupied molecular orbital (HOMO) wave function in the system is mainly distributed around the doped Cu atoms, the energy gap is significantly reduced, and the catalyst reactivity is enhanced.