Conversion of Methyl Mercaptan to Hydrocarbons over H-ZSM-5 Zeolite: DFT/BOMD Study

Methyl mercaptan-a harmful impurity in natural gas-may be selectively converted into H₂S and hydrocarbons [methyl mercaptan to hydrocarbon (M2TH) process], using zeolite catalysts. When M2TH is compared with the well-known MTH (methanol to hydrocarbons) process, significant differences emerge, essentially regarding the formation and distribution of products. Density functional theory (DFT) and Born-Oppenheimer molecular dynamics (BOMD) were employed to reveal possible origins for the experimentally observed differences. We established a close similarity between DFT intrinsic (electronic) reaction profiles in the stepwise mechanism of methanol and mercaptan dehydration, although no variance in reactivity was revealed. BOMD simulations at the experimental temperature of 823 K reveal rapid hydrogen abstraction from the methyl group in mercaptan, adsorbed in the zeolite cavity in the presence of the methoxy intermediate. The formation of •CH₂SH radical is 10 times faster than that of •CH₂OH at the same temperature. The varied reactivity of methanol and mercaptan in MTH and M2TH processes, respectively, can therefore first be attributed to very rapid hydrogen abstraction in mercaptan, which occurs in the zeolite cavity, following the formation of surface methoxy.