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 ...