Hydrogen/Denterium exchange during n.butane conversion on H-ZSM-5

Steady-state isotope tracer studies and isotope transient response experiments ofn-butane conversion on H-ZSM-5 (Si/Al = 35) were carried out between 673 and 823 K. Among the three main reactions, the rate of H/D-exchange is at least one order of magnitude higher compared to the rates of cracking or dehydrogenation. Its apparent energy of activation is lower than that of the latter two processes. The rates of H/D-exchange are higher for larger molecules than for smaller ones and faster with olefins than with paraffins. Proton exchange proceeds stepwise, i.e., only one hydrogen (deuterium) of the substrate is exchanged with one deuterium (hydrogen) in a single catalytic turnover. A kinetic isotope effect was found for protolytic cracking, but not for dehydrogenation. Protonation of the feed (deprotonation of the zeolite) is concluded to be involved in the rate determining step of cracking.