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(RateN2 ) = β­(PA – PAN2 )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. The implication of this on the mechanism of catalytic methanol dehydration on HZSM-5 is discussed.