Hydrogen-deuterium exchange reactions with isobutane over acid zeolites

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 take s place in the temperature range 140-200 degrees C, while on FAU-type zeoli tes, higher temperatures (200-260 degrees C) are required. On fully deutera ted zeolites, the initial product is i-C4D9H. The degree of deuteration of the methyl hydrogen atoms-is equal to that of the zeolite, while the methin e hydrogen atom cannot be exchanged. On MFI-type zeolites, the exchange rat e increases as the aluminium content [i.e., the Al/(Si + Al) ratio] increas es, 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 thou ght to proceed via carbenium ions that are formed during the induction peri od by hydride abstraction on Lewis acid non-framework aluminium species. Hy dride abstraction takes place on the methine hydrogen atom,:to form stabili sed t-isobutyl carbenium ions that undergo a fast H-D exchange reaction. Th e carbenium ions leave the surface by hydride abstraction from a new isobut ane molecule, thus not allowing for deuterium exchange of the methine hydro gen atom. The apparent activation energy for the H-D exchange reaction is 2 3-32 kJ mol(-1). The kinetic isotope effect is +/- 1.4. (C) 2000 Elsevier S cience B.V. All rights reserved.