In situ 13C NMR measurements on samples prepared using a pulse-quench catalytic reactor show
that the 1,3-dimethylcyclopentenyl carbenium ion (1) is an intermediate in the synthesis of toluene from ethylene
on zeolite catalyst HZSM-5. Cation 1 forms in less than 0.5 s when ethylene is pulsed onto the catalyst bed
at 623 K, and its presence obviates the kinetic induction period 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 bed. Density functional theory calculations (B3LYP/
6-311G**) on a cluster model of the zeolite confirm that 1 is stable in the zeolite as a free cation and suggest
why the alternative framework alkoxy is not observed. A π complex of the neutral cyclic diene is only 2.2
kcal/mol higher in energy than that of the ion pair. Theoretical (GIAO-MP2/tzp) 13C isotropic shifts of isolated
1 are in good agreement with the experimental spectra of the cation in the zeolite. To understand how organic
species entrained in the catalyst could promote MTO chemistry, we calculated a number of methylation reactions
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 aggregate experimental and theoretical results reveal the essential features
of a mechanism for MTO and methanol to gasoline (MTG) chemistry on a working catalyst.
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