Hydrogen–deuterium exchange reactions with isobutane over acid zeolites

Abstract

The kinetics of the hydrogen–deuterium (H–D) exchange reaction of isobutane over deuterated acid FAU- and MFI-type zeolites were studied using a batch recirculation reactor. On MFI-type zeolites, a fast exchange reaction takes place in the temperature range 140–200°C, while on FAU-type zeolites, higher temperatures (200–260°C) are required. On fully deuterated zeolites, the initial product is i-C 4D 9H. The degree of deuteration of the methyl hydrogen atoms is equal to that of the zeolite, while the methine hydrogen atom cannot be exchanged. On MFI-type zeolites, the exchange rate increases as the aluminium content [i.e., the Al/(Si+Al) ratio] increases, while on FAU a maximum exchange rate is found for a sample with Si:Al=5.5 [Al/(Si+Al)=0.15]. A good correlation was found between the amount of non-framework aluminium and the H–D exchange rate. The reaction is thought to proceed via carbenium ions that are formed during the induction period by hydride abstraction on Lewis acid non-framework aluminium species. Hydride abstraction takes place on the methine hydrogen atom, to form stabilised t-isobutyl carbenium ions that undergo a fast H–D exchange reaction. The carbenium ions leave the surface by hydride abstraction from a new isobutane molecule, thus not allowing for deuterium exchange of the methine hydrogen atom. The apparent activation energy for the H–D exchange reaction is 23–32 kJ mol −1. The kinetic isotope effect is ±1.4.