Resolving the Relations between Methanol and Ketene/Acetyl within HSAPO-34 in the Bifunctional Syngas-to-Olefin Process

Abstract

The syngas-to-olefin process represents a significant area of contemporary research; however, the identification of key intermediates remains a subject of ongoing debate. In this study, we employed ab initio molecular dynamics simulations, which revealed that two critical intermediates, i.e., methanol and ketene/acetyl ions, can interconvert in the presence of CO when utilizing HSAPO-34 as a catalyst. The simulation results indicated that, in the pathway from ketene/acetyl ions to ethylene, the rate-determining step is the formation of surface methoxy species. The introduction of methanol was found to effectively lower the free energy barrier, thereby enhancing the rate of initial olefin formation. Further analysis of potential chain growth processes for the production of C₃ species, specifically ethylene methylation and acrylketone methylation, indicates that the formation of C₃ species in HSAPO-34 is likely attributed to the latter process. Additionally, our simulation results for HMOR and HSAPO-34 zeolites demonstrated that variations in product selectivity can be attributed to differences in the acidic strength of Brønsted acid sites and their capacity to stabilize acyl ions.