Role of Lewis-acid sites in the transformation of alkylbenzenes on H-mordenite

Abstract

Adsorption of ethyl, n-propyl-, n-butyl-, isopropyl- and tert-butyl-benzenes on H-mordenite has been investigated by EPR in the temperature range – 120 to 170 °C. The influence of the alkyl group on the adsorption state of these molecules is very important as revealed by the results obtained at room temperature. At room temperature ethylbenzene loses an α-hydrogen to form a neutral radical, while dealkylation occurs after the adsorption of n-propyl-, n-butyl- and isopropyl-benzenes. Isopropylbenzene undergoes the fastest cracking of the three molecules, which is in agreement with what could be expected by a classical ionic process of aromatic ring protonation followed by dealkylation. However, the results obtained from the adsorption of tert-butylbenzene contradict the occurrence the ionic process at room temperature: tert-butylbenzene loses an electron from the aromatic ring to form a cation upon adsorption and no dealkylation was observed. This indicates that a different process should be envisaged to account for the room-temperature dealkylation of these alkylbenzenes. Two possible mechanisms involving Lewis-acid sites of the zeolite and α-hydrogen of the alkylbenzenes are discussed. The classical ionic pathway of dealkylation may predominate only when the temperature of reaction is higher than ca. 60 °C.