Thermodynamics Perspective on the Stepwise Conversion of Methane to Methanol over Cu-Exchanged SSZ-13

Abstract

Transition-metal exchanged zeolites are known to convert methane to methanol with high selectivity, in a stepwise process, involving exposure to oxidants, followed by exposure to methane, and finally by exposure to water vapor. However, a comprehensive theoretical study on the nature of the possible active sites and their respective changes during this stepwise process is still lacking. Here, we use a combination of density functional theory calculations in its generalized-gradient approximation (DFT-GGA) and post-DFT methods to identify the thermodynamically preferred sites in Cu-exchanged zeolite SSZ-13 during the stepwise conversion of methane to methanol. We develop a thermodynamic model for an extensive set of possible active sites, that is, Cu monomers, dimers, and trimers, which are anchored in different ring structures and supported by a series of different local Al distributions. Subsequently, phase diagrams are constructed and used to identify thermodynamically favored sites at each step during the stepwise conversion of methane to methanol. We find that during exposure to O2, hydroxylated dimers—Cu2O2H2 and, depending on the local Al configuration, Cu2OH—are preferred. Upon exposure to methane, site-bound methanol molecules are formed. With the subsequent increase in water vapor pressure, a thermodynamic preference for monoatomic Cu and the release of methanol are observed. Furthermore, we compare our predicted results to experimental measurements published in the literature and find close agreement in terms of Cu coordination number and bond distances for some of the sites considered. We expect that the insights obtained here can be used to improve our understanding of the reaction mechanism and to optimize the stepwise conversion of methane to methanol.