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 )(CH 3 OH)(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 C 6 F 6 to 825 kJ mol -1 for C 4 H 8 O and was found to follow the expression: Ln(Rate) - Ln(Rate N 2 ) = β(PA - PA N 2 ) γ , where E = N 2 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 )(CH 3 OH)(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 D 2 O, the effect on methanol reactivity diminishes when E is present in excess of the [( E )(CH 3 OH)(HZ)] complex. It is proposed that the methanol dehydration reaction involves [( E )(CH 3 OH)(CH 3 OH)(HZ)] as the transition state, which is supported by the isotopologue distribution of the initial dimethyl ether formed when a flow of CH 3 OH was passed over ZSM-5 containing one CD 3 OH per zeolite proton. The implication of this on the mechanism of catalytic methanol dehydration on HZSM-5 is discussed.