Microkinetic simulations of ketene conversion to olefins in H-SAPO-34 zeolite for bifunctional catalysis

Abstract

The characteristics of the bridging oxygenate (methanol vs. ketene) formed on an oxide and transformed inside a zeolite in bifunctional catalysts for the direct syngas conversion to light olefins remain ambiguous. Herein, the conversion of ketene to olefins in H-SAPO-34 zeolite via the olefin-based hydrocarbon pool mechanism was systematically studied by first-principles microkinetic simulations. The distribution of the retained olefinic species and their evolution preference using ketene as a methylating agent were revealed under different reaction conditions. The similarities and differences in activity and selectivity between the conversions of ketene and methanol were demonstrated. We clearly show that the conversion rate of ketene is lower than that of methanol, and the gap in activity reduces noticeably with increasing temperature. The higher energy barriers required for olefin methylation by ketene compared to methanol shift the distribution and interconversion of the retained olefinic species towards those with fewer C-atoms, improving the selectivity to ethene. The scaling relation can be established for the formation energies of transition states in the methylation steps between the conversion of ketene and methanol. This work thus provides a mechanistic understanding of the evolution of ketene in H-SAPO-34 and may benefit in lighting the role of both bridging oxygenates in bifunctional catalysis.