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_N) = β(PA - PA_N)^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.