Roles for cyclopentenyl cations in the synthesis of hydrocarbons from methanol on zeolite catalyst HZSM-5

In situ C-13 NMR measurements on samples prepared using a pulse-quench cata lytic reactor show that the 1,3-dimethylcyclopentenyl carbenium ion (1) is an intermediate in the synthesis of toluene from ethylene on zeolite cataly st HZSM-5. Cation 1 forms in less than 0.5 s when ethylene is pulsed onto t he catalyst bed at 623 K, and its presence obviates the kinetic induction p eriod for conversion of a subsequent pulse of dimethyl ether, or methanol, into olefins (MTO chemistry). The kinetic induction period returns when the interval between pulses is many times the half-life of 1 in the catalyst b ed. Density functional theory calculations (B3LYP/ 6-311G**) on a cluster m odel of the zeolite confirm that I is stable in the zeolite as a free catio n and suggest why the alternative framework alkoxy is not observed. A pi co mplex of the neutral cyclic diene is only 2.2 kcal/mol higher in energy tha n that of the ion pair. Theoretical (GIAO-MP2/tzp) C-13 isotropic shifts of isolated 1 are in good agreement with the experimental spectra of the cati on in the zeolite. To understand how organic species entrained in the catal yst could promote MTO chemistry, we calculated a number of methylation reac tions in the gas phase. We found that the diene formed by deprotonation of 1 is far more easily methylated than ethylene, propene, or toluene. The agg regate experimental and theoretical results reveal the essential features o f a mechanism for MTO and methanol to gasoline (MTG) chemistry on a working catalyst.