Methanol-to-hydrocarbon initiation reactions over a zeolite catalyst: reactive molecular dynamics simulations

Abstract

Environmental concerns and energy security drive the need for renewable alternatives to fossil fuels. Methanol is an ideal candidate as it enables the synthesis of olefins and fuels from biomass using zeolite-catalyzed processes, which inspired the concept of “Methanol Economy” since it ideally implies an oil-independent scenario. Although MTH industrial units already exist, many fundamental aspects remain unknown. Therefore, in this contribution, we developed a ReaxFF reactive force field to study the dynamic features of the equilibration stage of the MTH process in an H-ZSM-5 zeolite. Simulations were run from 600 to 1200 K during 1000 ps, using a constant number of molecules, pressure, and temperature (NPT ensemble). Methanol conversion increases from 800 to 1000 K to form water and the crucial intermediate surface methoxy species (SMS), whose production diminishes at 1200 K because of the prevalence of undesired methane. Generally, temperatures above 1200 K can lead to questionable reactions due to entropy effects. Humidity at 800 K modifies the nature of the zeolite acidity from static to dynamic, embodied in hydronium ions, which enhances methanol conversion via hydrogen transfer reactions and framework activation, namely, water protonation leaves a negatively charged framework that eventually facilitates the dissociation of protonated methanol in water and an SMS. Cation diffusion was pervasive, and it is hypothesized that this relieves the entropic penalties of several relevant reactions. This phenomenon showcases a critical manifestation of dynamic effects that complements experimental and theoretical research mainly conducted by static density functional theory methods. Overall, dynamic effects involving diffusing cations and modification of the zeolite interior by water are complex and call for more extensive investigations.