Kinetics of butene isomer methylation with dimethyl ether over zeolite catalysts

The systematic investigation of 1-butene, trans-2-butene, cis-2-butene, and isobutene methylation with dimethyl ether (DME) over acidic zeolites FER, MFI, MOR, and BEA at low conversions (<0.1%) and high DME:olefin ratios (>15:1) showed linear rate dependencies on butene pressure and no dependence on DME pressure. Such dependencies are consistent with the zeolite surface being predominantly covered by DME-derived species, which either directly reacts with butene species in the rate-determining step or through the formation and subsequent degradation of a coadsorbed complex. A comparison of rate constants for butene methylation across isomers over MFI and BEA shows that, in the absence of hydride shift, a 10-fold increase is observed for reactants capable of forming more substituted carbenium ion-like transition states, as predicted for carbocation mediated mechanisms. High cis-2-butene pressure experiments over BEA show linear dependencies of the butene methylation rate on butene pressure for olefin to DME ratios as high as a ∼1.5, indicating that surface-bound DME derived species react with butene in Eley-Rideal type kinetics at low temperatures. Titration with water after steady-state methylation of cis-2-butene over BEA results in DME-derived intermediates being removed as methanol in a 1:1 ratio with zeolite Al suggesting that surface methyl groups are involved in olefin methylation reactions.