DoubleBond Migration of an Olefin without Protonated Species on H(D) Form Zeolites

A novel reaction path was found for the double-bond migration (DBM) of 1-butene on very strong Brønsted acid sites (BAS) of zeolites, ZSM-5 and mordenite (MOR), in the absence of protonation below 230 K with the activation energies of 49 ± 4 kJ·mol-1 on D-ZSM-5 and 38 ± 8 kJ·mol-1 on D-MOR (DM20). By using deuterated zeolites, it was proved that the protonated intermediate is not essential to the conversion of the 1-butene to the hydrogen-bonded cis- and trans-2-butenes on the BAS of ZSM-5 and MOR. If the widely accepted protonated intermediate is assumed, the acidic OD groups should be converted to OH groups upon the DBM of 1-butene to 2-butenes. Nevertheless, the OD groups of zeolites forming 1:1 complex with 1-butene remained unchanged even after the double-bond migration. The isotope exchange reaction of OD groups due to the proton transfer occurred at higher temperature than the DBM. However, the DBM on a deuterated faujasite (FAU) zeolite (DY5.6 and DY4.8) proceeded accompanied by the H/D exchange reaction of the acidic OD groups, suggesting that the protonated intermediate is essential for the reaction on rather weak BAS. Thus, the existence of a new type of reaction catalyzed by very strong BAS is demonstrated, which takes place more facilely than the proton transfer.