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

The methanol-to-hydrocarbons (MTH) process transforms C-1 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.

Automated summary

This study investigates the conversion of methanol to dimethyl ether (DME) in H-ZSM-5 using computational simulations, revealing that the more acidic and open intersection sites in H-ZSM-5 have the strongest bonding capability with DME, leading to complete deprotonation of the acid site. The conversion of methanol to DME in H-ZSM-5 requires a higher activation energy than framework methoxylation, suggesting a stepwise mechanism through a methoxy intermediate for DME formation in the MTH process initiation.