Elucidating the methanol conversion in H-SAPO-5 from first principles: Nature of hydrocarbon pool and scission style

Abstract

Silicoaluminophosphate (SAPO)-based zeolites have been demonstrated to be the potential active catalysts for the methanol conversion to produce hydrocarbons like light olefins. However the underlying reaction mechanism is yet to be fully understood. In this work, periodic density functional theory calculations were performed to address the hydrocarbon pool (HP) mechanism involving both aromatics and olefins as the hydrocarbon pool species in H-SAPO-5. We demonstrated that the olefins themselves rather than aromatics are likely to be the dominating HP species in H-SAPO-5. In the case of the olefin-based HP pathway, we proposed three different scission styles of cracking precursors to produce olefins, namely, concerted cracking with water, concerted cracking without water, and stepwise cracking without water. It was found that propene and butenes rather than ethene are the dominant products via the cracking of higher cracking precursors, like higher olefins and carbenium ions. The energy barriers of the olefin-based pathway are lower than 160 kJ/mol at 673 K, much lower than those in the aromatic-based pathway. The established linear scaling relations between the transition state energies and the number of carbon atoms in olefin-based cycle reveal that the van der Waals stabilization dominates the interaction between framework and organic moiety for olefin methylation and cracking in H-SAPO-5. This theoretical work further highlights the importance of olefin-based cycle and provides some implications to understand product distribution via different cracking style in zeolite catalyzed methanol conversion.