A computational study of the formation of surface methoxy species in H-SSZ-13 and H-SAPO-34 frameworks

The methanol-to-hydrocarbons (MTH) reaction on zeolites is vital for the production of higher-order hydrocarbons from sustainable C₁ feedstocks. The formation of the first C-C bond is a key initiation step in the MTH reaction, and surface methoxy species (SMS) are important in many of the proposed pathways to form C₂ species, but the reaction steps that form SMS in zeotype frameworks remain uncertain. Therefore, we have investigated the reaction energies and activation barriers for SMS formation pathways in zeotype frameworks using accurate ab initio simulations, considering isostructural aluminosilicate and aluminophospate frameworks to allow scrutiny of how catalyst composition affects reaction steps. The SMS precursors dimethyl ether (DME) and trimethyl oxonium (TMO) are found to form directly from methanol with relatively low barriers (62 kJ mol^-1 and 94 kJ mol^-1, respectively); and the protonated forms of DME and CH₃OH, as well as TMO, form SMS with low kinetic barriers (36-48 kJ mol^-1). The activation barriers for processes occurring on H-SAPO-34 are consistently 10-23 kJ mol^-1 higher than reactions on the isostructural H-SSZ-13 framework, indicating that only kinetic differences exist between aluminosilicates and aluminophosphates for SMS formation. Significant differences are identified in the activation energies for reactions that proceed through the front-side attack S_N2 when compared to the back-side attack S_N2 mechanisms, with reduced electron donation to the carbocation intermediate leading to instability of the front-side attack S_N2 intermediate. Overall, the direct framework methylation step via protonated methanol has the lowest kinetic barrier, which agrees with experimental observations of direct SMS formation, and this result provides a foundation for further mechanistic investigations.