Methyl halide reactions on multifunctional metal-exchanged zeolite catalysts

Article abstract of DOI:10.1021/ja00093a040

Proximal sites consisting of a divalent metal cation as a hydroxide and/or oxide and an anion site in the zeolite framework are very active for the conversion of methyl halides to ethylene. These studies further support the effects of zeolite framework and halide leaving-group stability reported previously (J. Am. Chem. Soc. 1993, 115, 4732-4741) and demonstrate multiple roles for divalent metal sites. Whereas the basic zeolite CsX converted methyl iodide to hydrocarbons at temperatures as low as 498 K, this reaction commenced at 248 K on the multifunctional catalyst ZnX. The orders of activity Zn²⁺ > Cd²⁺ and Mg²⁺ > Ba²⁺ suggest that Lewis acidity is one of these roles. Zeolite CuY is very selective for ethylene formation. These reactions were studied by in situ ¹³C solid-state NMR, by in situ FTIR spectroscopy with a flow cell, and with conventional flow reactors. The active sites of ZnZSM-5 were also probed with ¹H solid-state NMR. The first intermediate is a framework-bound methoxy group. The ¹³C NMR and FTIR properties of this species have been correlated, and it is shown that this species is very similar or identical to that observed during methanol to gasoline chemistry. A detailed mechanism is proposed for the reaction of methyl halides on Zn and Mg zeolites that includes several explicit roles for the metal.