Computational Assessment of the Dominant Factors Governing the Mechanism of Methanol Dehydration over H-ZSM-5 with Heterogeneous Aluminum Distribution

Abstract

A van der Waals (vdW) corrected density functional theory (DFT) study of the methanol-to-DME reaction on H-ZSM-5 is conducted for both the associative and dissociative pathways. Calculations are performed for four different active site locations corresponding to Al sitings in sinusoidal and straight channels, and their intersections in the MFI zeolite framework. The Gibbs free energy landscape along the reaction paths computed for a typical set of conditions shows that the associative route is preferred, regardless of Al siting, but a transition in the mechanism from associative to dissociative is observed at higher temperatures. The crossover temperature, however, is not identical for the various active site locations, resulting in a temperature range over which both mechanisms are active. This observation may explain why methoxy, which is the key intermediate along the dissociative pathway, has been observed spectroscopically, whereas kinetic analysis points to dominant contributions of the associative pathway under similar conditions. Pore confinement effects largely contribute to transition state stabilization and have a significant impact on the reaction mechanism. The effect of acidity on kinetic performance is also tested by the substitution of three different heteroatom dopants (Al, Ga, In) at the active sites, but only a minor transition-state energy variation was observed. The fundamental information obtained in this study contributes to a better understanding of the complex interplay between pore confinement, acidity, and reaction conditions, and their effect on pathway selectivity. This knowledge can be utilized to either optimize DME production from methanol or facilitate the production of desired hydrocarbons in the methanol-to-hydrocarbon (MTH) process, which requires DME formation to initiate the conversion.