QM/MM study of the stability of dimethyl ether in zeolites H-ZSM-5 and H-Y

The methanol-to-hydrocarbons (MTH) process transforms C1 carbon sources to higher hydrocarbons, but details of the mechanism that leads to the formation of the first carbon-carbon bond remain unclear. Here, we present a computational investigation of how a crucial intermediate, dimethyl ether (DME), interacts with different zeolite catalysts (H-ZSM-5, H-Y) to gain insight into the initial stages in the MTH process. We use QM/MM computational simulations to model the conversion of methanol to DME in H-ZSM-5, which is a well characterised and important reaction intermediate. We analyse and compare the stability of DME on several acid sites in H-ZSM-5 and H-Y, and show that the more acidic and open "intersection sites" in the H-ZSM-5 framework are able to bond strongest with DME, with complete deprotonation of the acid site occurring. The conversion of methanol to DME in H-ZSM-5 is calculated as requiring a higher activation energy than framework methoxylation, which indicates that a stepwise (indirect) mechanism, through a methoxy intermediate, is the most likely route to DME formation during the initiation of the MTH process.