Rate parameters from low-pressure steady-state protolytic cracking and dehydrogenation of isobutane over zeolite catalysts

An improved low-pressure steady-state technique has been used to accurately determine kinetic parameters for isobutane protolytic cracking and dehydrogenation over HZSM-5 (SiO2/Al2O3 = 33.0), HZSM-5 (94.2), ferrierite (55.2) and mordenite (20.0). Apparent activation energies for cracking are 104 ± 5 kJ mol−1 over HZSM-5 (33.0) and 101 ± 10 kJ mol−1 over mordenite. For isobutane dehydrogenation, energy barriers are 136 ± 6 kJ mol−1 over HZSM-5 (33.0), HZSM-5 (94.2) and ferrierite, while over mordenite the dehydrogenation energy barrier is 95 ± 5 kJ mol−1. Differences in rate-determining steps, in desorption and in intrinsic reaction are proposed to explain the similarities and differences in these apparent energies. The magnitudes of these energies agree with most literature values and so validate the technique. Associated intrinsic preexponential factors are not as sensitive to the zeolite structure or the reaction pathway, but magnitudes agree with the assumption that rate-determining steps are first order. An exponent m for the coverage is required in the kinetic analyses because, under low-pressure conditions, a temperature-dependent or non-steady-state effect such as diffusion causes apparent non-linear Arrhenius plots.