How do the products in methane dehydroaromatization impact the distinct stages of the reaction?

Methane dehydroaromatization reaction at 700 °C over Mo/ZSM-5 involves numerous modifications of the molybdenum species from the catalyst preparation and throughout the catalyst lifetime, composed of 4 successive steps: calcination, activation, induction, and deactivation. A thorough kinetic study was undertaken with the aim to understand the transformation phenomena occurring on the catalyst during each stage of the reaction, using methane gas hourly space velocity per gram of catalyst (M-GHSV) from 1 to 29 L_CH4 h⁻¹ g_cat⁻¹. Here from, unexpected behaviors were observed, supported by molecular modeling results. MoO₃ firstly reacts stoichiometrically during the calcination (ΔrH =0.86 eV) with bridged hydroxyl pairs yielding [Mo₂O₅]²⁺ species (calcination). Thereafter, [Mo₂O₅]²⁺ slowly reduces by methane to form [Mo₂C₂]²⁺ (activation). The latter converts methane to ethylene (E_A= 1.49 eV), which dimerizes two times faster to butene through hydrocarbon pool catalysis rather than through Brønsted acid sites (induction). The catalyst deactivates through an inhibition effect of aromatics, which adsorb strongly onto [Mo₂C₂]²⁺ (ΔH_ads ~ 0.7 eV) (deactivation). The large amount of autogenous hydrogen produced at lower space velocity allows preventing the active species poisoning, leading to slower deactivation rate