Co-feeding water leads to a simultaneous attenuation of chain initiation and chain termination rates in HSSZ-13 catalyzed methanol-to-olefins (MTO) conversion. Density functional theory calculations and transient stoichiometric experiments support the plausibility of formaldehyde hydrolysis occurring over zeolitic Brønsted acid sites at MTO-relevant temperatures. A monotonic decrease in MTO chain initiation and termination rates, and a concurrent monotonic increase in total turnovers as a function of water co-feed partial pressure are consistent with the occurrence and mechanistic relevance of formaldehyde hydrolysis effected by co-fed water. Initiation/termination rates and total turnovers normalized by their corresponding values in the absence of water co-feeds at the same temperature show the expected trends as a function of reaction temperature, assuming equilibrium between formaldehyde and methanediol. These results underscore the implications of formaldehyde hydrolysis chemistry when assessing the mechanistic role of water in methanol-to-olefins conversion specifically, and deactivation mechanisms in zeolite-catalyzed hydrocarbon conversion processes more generally.
Bibliographical noteFunding Information:
We gratefully acknowledge financial support from Dow through the University Partnership Initiative, the National Science Foundation (CBET 1701534), and computational resources from the Minnesota Supercomputing Institute. We thank Dr. Andrew Hwang, Mr. Sukaran Arora, and Mr. Brandon Foley for helpful technical discussions.