Descriptor-based microkinetic modelling for methanol-to-DME in zeotypes

Övrigt konferensbidrag, 2018

A descriptor-based approach to study the methanol-to-DME reaction in zeotypes from first-principles is herein used to investigate the different reaction mechanism by means of a mean-field microkinetic model. In this theoretical study we focus on Brønsted acidic zeotype catalysts. We demonstrate that the difference of OH stretch frequency for the bare site and when CO is adsorbed on the site is a good descriptor for the acidic strength of the site. Different types of zeotypes are herein studied; CHA, BEA and MFI zeolite frameworks, where besides the conventional Al exchange we also consider B, Ga and Fe exchanged structures, along with SAPO-34, Mg-AlPO-34, Zn-AlPO-34, and Ti-AlPO-34. These span a wide range of strength of the acidic sites. The top panel in the figure to the right shows the free energy landscape for the two main reaction mechanisms for BEA and B-BEA (which have quite different acidity). The concerted mechanism, where two methanol molecules adsorb simultaneously to form DME and water, is shown in lighter colours, and the stepwise mechanism, where one methanol dissociates to a methyl group, producing water, after which a second methanol molecule is adsorbed to form hydrogenated DME, which then easily dehydrogenates, is shown in the darker shades. The bottom panel in the figure shows the calculated turn-over frequency (TOF) for the two mechanisms as a function of temperature. This shows that the concerted mechanism is dominating at lower temperatures while the stepwise dominates at higher temperatures for both systems.