CONVERSION OF METHYL HALIDES TO HYDROCARBONS ON BASIC ZEOLITES - A DISCOVERY BY INSITU NMR

It is shown that methyl halides (I, Br, Cl) react to form ethylene and other hydrocarbons on basic, alkali metal-exchanged zeolites at low t emperatures. For example, methyl iodide is converted to ethylene on Cs X zeolite at ca. 500 K. The order of reactivity of various catalyst/ad sorbate combinations is consistent with the predictions of elementary chemical principles. The order of reactivity of the methyl halides fol lows the expected leaving-group trend. The activity of the catalyst fr amework correlates with its basicity (or nucleophilicity). All reactio ns were performed in a batch mode in sealed magic angle spinning (MAS) rotors while the contents were continuously monitored by in situ C-13 NMR. Methyl iodide reacts on CsX below room temperature to form a fra mework-bound methoxy species in high yield. An analogous ethoxy specie s readily formed from ethyl iodide. These species were characterized i n detail. The ethoxy species was quantitatively converted to ethylene below 500 K. Cs-133 MAS NMR was used to characterize the interactions of methyl iodide and other adsorbates with the cation in zeolite CsZSM -5. Solvation of the alkali metal cation was reflected in large, loadi ng-dependent chemical shifts for Cs-133. Interactions between the cati on and adsorbates were also reflected in the C-13 shifts of the alkyl halides and ethylene. The cumulative evidence suggests a mechanism for carbon-carbon bond formation analogous to one proposed by Chang and c o-workers for methanol-to-gasoline chemistry on acidic zeolites (J. Ch em. Soc., Chem. Commun. 1987, 1320) that involves framework-bound meth oxy and ethoxy species. The mechanism for methyl halide conversion is proposed to include roles for the basicity of the zeolite framework as well as the Lewis acidity of the cation.