HYDROGEN DEUTERIUM EXCHANGE DURING N-BUTANE CONVERSION ON H-ZSM-5/

Steady-state isotope tracer studies and isotope transient response exp eriments of n-butane conversion on H-ZSM-5 (Si/Al = 35) were carried o ut between 673 and 823 K. Among the three main reactions, the rate of HID-exchange is at least one order of magnitude higher compared to the rates of cracking or dehydrogenation. Its apparent energy of activati on is lower than that of the latter two processes. The rates of HID-ex change are higher for larger molecules than for smaller ones and faste r with olefins than with paraffins. Proton exchange proceeds stepwise, i.e., only one hydrogen (deuterium) of the substrate is exchanged wit h one deuterium (hydrogen) in a single catalytic turnover. A kinetic i sotope effect was found for protolytic cracking, but not for dehydroge nation. Protonation of the feed (deprotonation of the zeolite) is conc luded to be involved in the rate determining step of cracking. (C) 199 6 Academic Press, Inc.