Propene adsorption and reaction on zeolites and pillared clays

Comparative IR and UV-Vis spectroscopic studies of propene adsorption and reaction on H-mordenite, dealuminated H-mordenite, dealuminated mazzite, montmorillonite and Al₁₃-pillared montmorillonite have been carried out. On all systems propene is first transformed into polymeric species (C_nH_2n+1 ⁺). On HMOR (both as such and dealuminated), allylic carbocations are successively produced by loss of H₂, the monoenic species (C_nH_2n?1 ⁺) being formed at room temperature and the dienic (C_nH_2n?3 ⁺) and trienic (C_nH_2n?5 ⁺) species at higher temperatures. These species are not observed on the other systems, although they are presumably formed as unstable intermediates. In fact, on all the zeolites studied here two cyclic penta-atomic and hexa-atomic allylic carbocations have been observed for the first time. On all systems, the final products of reaction are polyaromatic species which, on the basis of their reaction with NH₃ still exhibit unsaturated carbocation behaviour. The activity of the various samples depends on their pore dimensions and on the nature of acidic sites involved: the larger the available pore space, the more branched is the polymer and the more difficult it is to observe allylic carbocations. Evidence is provided for a Brønsted-induced mechanism.