Conversion of methanol to olefins or hydrocarbons over acidic catalysts, in particular H-ZSM-5 (a 3-D zeolite with MFI framework), is a feasible route for the production of commodity chemicals and liquid fuels, and has been a popular subject of catalytic studies. Although the overall process remains debated, the first step is accepted to be methanol dehydration to dimethyl ether (DME). The formation of DME from methanol may proceed through two competing pathways. The associative pathway is characterized by the co-adsorption of two methanol molecules followed by simultaneous formation of water and DME, while the dissociative pathway is initiated by water elimination and surface methoxy formation from a single methanol molecule followed by DME formation after a second methanol molecule adsorbs.
The active sites of H-ZSM-5 are Brønsted acid sites that can be found in 12 distinguishable locations, which may be found in the sinusoidal or straight channels, or at the channel intersections of the MFI framework. Our earlier studies have indicated non-identical structural and chemisorption properties of H-ZSM-5 Brønsted acid sites . Here, we use density functional theory (DFT) calculations with periodic boundary conditions to investigate the dominant reaction mechanism at industrially relevant conditions. The use of a dispersion-corrected functional (vdW-DF) enables our computational approach to account for the pore confinement effects at different active site locations. Through Gibbs free energy calculations we estimate the sensitivity of the reaction mechanism to temperature and pressure at which the reaction is conducted. Our results indicate that depending on temperature/pressure conditions the reaction may proceed along one of the two proposed pathways, or even both, as long as active sites are found at geometrically different locations. Since heterogeneous aluminum siting is possible depending on the zeolite synthesis conditions, the presence of dual mechanisms implies that experimental kinetic behavior compatible with the associative mechanism may be observed while infrared spectroscopy indicates the presence of surface methoxy groups characteristic of the dissociative pathway. Our results suggest that the neglect of dispersion forces in the stabilization of transition states can result in erroneous inference on the dominant reaction mechanism at practical conditions regardless of the active site location in question. We extend this investigation to the impact of acid site density by comparing kinetic parameters for several acid site pairs. Our studies have shown that the collaborative effect of two neighboring acid sites may result in considerable differences compared to the single site model, while no significant difference is observed when the two neighboring acid sites do not collaboratively interact with the transition state complex. In summary, the result of our van der Waals corrected DFT simulations can illuminate the underlying factors influencing the mechanism of the methanol-to-DME reaction over H-ZSM-5 and provide valuable insight into the design of improved catalysts and optimal reaction conditions.
 Arian Ghorbanpour, Jeffrey D. Rimer, Lars C. Grabow, Periodic, vdW-corrected density functional theory investigation of the effect of Al siting in H-ZSM-5 on chemisorption properties and site-specific acidity, Catalysis Communications 52 (2014) 98-102.