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-butene, 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 the 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 toluene with the internal OHs. Hydrogen bondings with alkanes are precursors for dehydrogenation with the formation of carbenium ions that go fast towards the tert-butyl-sec-butyl equilibrium, giving rise to skeletal isomerization 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 can contribute also to cracking to light olefins. H-bonded species are also thought to be intermediates for the formation of the so-called 'sigma-bonded' intermediate of electrophilic aromatic substitution, giving rise to toluene dealkylation and/or dismutation, obtained in the IR cell. (C) 2000 Elsevier Science B.V. All rights reserved.