| Abstract
| - Experimental evidence of protonation of an aromatic ring by a zeolite is hereby presented forthe first time. The changes in vibrational properties and electronic transitions of the highest polymethylbenzene homologue, that is, hexamethylbenzene, were investigated after introducing the compound directlyinto a H-beta zeolite. Protonation of the aromatic ring, and thus the loss of symmetry, activated a vibrationalring mode at 1600 cm-1. Furthermore, an electronic transition around 26 000 cm-1, which was totally absentfor the neutral species, was an obvious consequence of protonation. A parallel study of hexamethylbenzeneadsorbed on a beta zeolite virtually free from protons did not show those distinctive spectral features. Onthe basis of the gas-phase proton affinity of hexamethylbenzene, a complete proton transfer from the zeoliteframework to the molecule is, according to conventional considerations, not expected. The hexamethylbenzenium ion is stable in the zeolite cavities at least up to 200 °C. The remarkable persistence of thiscarbenium ion may be attributed to spatial constraints imposed by the tight fit of the cation inside the zeolitechannels. Hexamethylbenzene is a relevant reaction intermediate in the methanol-to-hydrocarbons reactionand also plays a central role as a coke precursor in zeolite-catalyzed reactions that involve polymethylbenzenes.
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