An FT-IR study of the reactivity of hydrocarbons on the acid sites of HZSM5 zeolite

The interaction of HZSM5 zeolite with n-butane, iso-butane, n-heptane, 1-bu tene, cis-2-butene, trans-2-butene, iso-butene, benzene and toluene in the temperature range 100-773 K has been investigated using FT-IR spectroscopy Low temperature experiments allowed to characterise hydrogen-bonded species of all adsorbates both with the internal bridging OHs of HZSM5 and with th e external terminal OHs. H-bonds apparently involve C-C sigma-type orbitals of alkanes and C=C pi-type orbitals of alkenes and aromatics. Evidence is provided for a partial steric hindrance in the interaction of benzene and t oluene with the internal OHs. Hydrogen bondings with alkanes are precursors for dehydrogenation with the formation of carbenium ions that go fast towa rds the tert-butyl-sec-butyl equilibrium, giving rise to skeletal isomeriza tion and cracking. Poly-isobutene like species are formed at the surface by high-temperature interaction with both alkanes and alkenes. Isobutene is a likely precursor for such species, that are intermediates of coking and ca n contribute also to cracking to light olefins. H-bonded species are also t hought to be intermediates for the formation of the so-called 'sigma-bonded ' intermediate of electrophilic aromatic substitution, giving rise to tolue ne dealkylation and/or dismutation, obtained in the IR cell. (C) 2000 Elsev ier Science B.V. All rights reserved.