Methane Activation in Gold Cation-Exchanged Zeolites: A DFT Study

Abstract

Activation of methane has attracted a great deal of interest in laboratory chemical synthesis and in large-scale industrial processes. We performed density functional theory studies to investigate the C–H bond breaking of methane on Au+ and Au2+ ions in vacuum and inside different types of zeolites. The density functional M06-L and the 6-31G(d,p) basis set were employed as this level of theory had already been shown to be reasonably accurate and affordable for transition metal systems. We investigated four industrially important catalysts, ZSM-5, FAU, FER, and MCM-22, each with a particular framework topology, with respect to their performance for methane activation. The bicoordinated character of the cationic site in the ZSM-5 structure provides a higher activity than the FAU structure with a 3-fold coordination of its cationic site. The activation energy of the reaction catalyzed by Au-ZSM-5 is lower than the one with the bare Au+ cation (13.2 vs 21.3 kcal/mol) because of the structural constraint imposed by the zeolite that leads to an earlier transition state with a high charge difference of the C–H atoms where the bond is broken. It is also found that the activity of Aun+ decreases already with n = 2, due to the shared positive charge. For the zeolites with large pores, Au-MCM-22 provides a higher activity due to the spacious framework of this particular type of zeolite is perfect for stabilizing the transition state structure but not the corresponding adsorption complex. The small and medium pore-sized zeolites, Au-FER and Au-ZSM-5 stabilize both the adsorption complex and the transition states, thus causing the activation energy to remain the same.