METHANOL CONVERSION TO HYDROCARBONS OVER ZEOLITE H-ZSM-5 - INVESTIGATION OF THE ROLE OF CO AND KETENE IN THE FORMATION OF THE INITIAL C-C BOND

Mechanistic studies concerning the formation of the initial carboncarbon bond in the methanol conversion reaction over zeolite H-ZSM-5 are described and discussed. In particular, the possible roles of CO as a reaction intermediate or as a catalyst, via the formation of ketene, are evaluated. Experiments using 13CH3OH/12CO reactant mixtures demonstrate that no CO is incorporated into ethene, the primary product of this reaction. In addition, CO is found to have no significant effect on the induction period for this reaction. Model experiments for the methylation of ketene by reaction with Me2SO4 and Me3O+SbCl−6 did not provide evidence for methylation of ketene on either of the C atoms, and from the products observed it is apparent that ketene acts only as an acylating agent in the normal way. Experiments using aluminium silica compounds as models for the active site in the zeolite are described and discussed. Hydrogen/deuterium exchange experiments indicate that a common intermediate exists that can lead to the eventual formation of CH3OCH3, CH4, and C2H4. Coreaction of CD3OCD3 and CH3OCH3 indicates that at least two intermediates are required for the formation of the ethene primary product. The results of this study indicate that CO and CH2CO play no significant mechanistic role in the formation of the primary products of methanol conversion. A mechanism consistent with the experimental evidence available involving a surface bonded methoxy group is proposed and discussed. © 1993 by Academic Press, Inc.