Kinetics of Isobutane Dehydrogenation and Cracking over HZSM-5 at Low Pressures

Isobutane cracking and dehydrogenation over HZSM-5 have been investigated under conditions where gas-phase collisions are minimized. The experimental vacuum system consists of a low-flow leak value, two interchangeable, thermally heated reactors (volumes=8700±5 mm3 and 2114±5 mm3), each with a small exit aperture (effective areas=0.16 mm2 and 0.37 mm2), and a quadrupole mass-spectrometer detector. The flow rate, reactor, and exit aperture dimensions are crucial to the kinetic analysis and have been accurately measured. For kinetic measurements the temperature of the reactor is heated at a constant rate from 200–500°C to minimize heat fluctuations, which can result from nonequilibrium adsorption at reaction temperatures, and to obviate the need for an internal standard. Cracking and dehydrogenation reaction channels dominate isobutane decomposition over the temperature range. Propene and methane formation are more prevalent than isobutene evolution. The rate constant for cracking is k2a(°C−1)=1017.3±1.1 exp(−170±6 kJ mol−1/RT), and for dehydrogenation it is k2b(°C−1)=1016.8±1.1 exp(−172±6 kJ mol−1/RT), Large preexponential factors indicate that the coverage of active sites is less than the coverage of exposed sites available for direct adsorption of isobutane. The similarity in the activation energies for the two channels agrees with theoretically determined proton affinities.