Understanding light olefin selectivity during Fischer–Tropsch syntheses over zeolites: A DFT exploration of the key role of ketene intermediates

Syngas conversion to light olefins (C 2 = – C 4 = ) over bifunctional oxide-zeolite catalysts via Fischer–Tropsch syntheses (FTS) is presently attracting a lot of attention, but the mechanism of olefin production and factors controlling olefin selectivity remain unclear. With a view toward a better understanding of Fischer–Tropsch syntheses over zeolites, in particular the factors controlling product selectivity at the atomic level, we carried out first-principles density functional theory (DFT) calculations on Mordenite (MOR) zeolites with eight and twelve-membered rings (8MR, 12MR) and a microporous SAPO-34 zeolite (8MR only). The obtained free energy profiles demonstrate that ketene is the key intermediate in the conversion of syngas to light olefins, enabling product distributions consistent with recent experimental reports. Specifically, we show that the channel size and the interconnectivity between channels in zeolites significantly influence the ability of the ketene intermediate to undergo chain growth propagation during syngas conversion. Our results provide fundamental new insights into the factors controlling olefin selectivity during FTS over zeolites, guiding future improvements in FTS catalyst design. • Ketene prefers to adsorb on the 8MR of MOR zeolites, whilst methanol prefers to adsorb on the 12MR of MOR zeolites. • An SN 2 -like mechanism was proposed to account for the chain propagation of ketene over zeolites. • On the 8MR of MOR zeolites, ketene converts to ethylene with lower energy barriers compared to chain propagation to form longer olefins, while on 12MR, the energy barrier of carbon chain propagation is lower than ethylene production. • On SAPO-34, ketene is converted to light olefins. Not significant selectivity to ethylene is found, but the chain length of products is limited beyond C 4 products. • The unique selectivity of zeolites for ketene conversion to olefins is attributed to both zeolite ring size and the connectivity of internal channels.