Spectroscopic evidence for a persistent benzenium cation in zeolite H-beta

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