Molecular Tuning of Reactivity of Zeolite Protons in HZSM-5

In acidic HZSM-5 zeolite, the reactivity of a methanol molecule interacting with the zeolite proton is amenable to modification via coadsorbing a stochiometric amount of an electron density donor E to form the [(E)(CH₃OH)(HZ)] complex. The rate of the methanol in this complex undergoing dehydration to dimethyl ether was determined for a series of E with proton affinity (PA) ranging from 659 kJ mol^–1 for C₆F₆ to 825 kJ mol^–1 for C₄H₈O and was found to follow the expression: Ln(Rate) – Ln(Rate_N₂) = β(PA – PA_N₂)^γ, where E = N₂ is the reference and β and γ are constants. This trend is probably due to the increased stability of the solvated proton in the [(E)(CH₃OH)(HZ)] complex with increasing PA. Importantly, this is also observed in steady-state flow reactions when stoichiometric quantities of E are preadsorbed on the zeolite. As demonstrated with E being D₂O, the effect on methanol reactivity diminishes when E is present in excess of the [(E)(CH₃OH)(HZ)] complex. It is proposed that the methanol dehydration reaction involves [(E)(CH₃OH)(CH₃OH)(HZ)] as the transition state, which is supported by the isotopologue distribution of the initial dimethyl ether formed when a flow of CH₃OH was passed over ZSM-5 containing one CD₃OH per zeolite proton. Furthermore, the implication of this on the mechanism of catalytic methanol dehydration on HZSM-5 is discussed.. Authors: Chen, Yaxin [Northwestern University, Evanston, IL (United States)]; Ma, Xinyou [University of Chicago, IL (United States)]; Hack, John H. [University of Chicago, IL (United States)] (ORCID:0000000300420921); Zhang, Shuhao [Northwestern University, Evanston, IL (United States)]; Peng, Anyang [Northwestern University, Evanston, IL (United States)] (ORCID:0000000206302187). DOE Contract: FG02-03ER15457. Subjects: 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Alcohols; Dehydration; Infrared light; Molecules; Zeolites