Catalytic cracking, dehydrogenation, and aromatization of isobutane over Ga/HZSM-5 and Zn/HZSM-5 at low pressures

Isobutane cracking, dehydrogenation, and aromatization over Ga/HZSM-5 and Zn/HZSM-5 has been investigated in a Knudsen cell reactor and the kinetics of the primary reaction steps for isobutene and propene formation have been accurately determined. Although cracking is the dominant reaction channel, with propene and methane being primary products, methane formation is significantly less than propene formation. This indicates that a proportion of the cracking proceeds via Lewis acid attack at C-C bonds, and not just via alkanium ion formation at Bronsted acid sites. This is particularly apparent over Zn/HZSM-5. Intrinsic rate constants for cracking, calculated from the rate of propene formation, are k(3a)(degreesC(-1)) = 10(18.3 +/- 1.1) exp(-148 +/- 6 kj mol(-1)/RT) Ga/HZSM-5 k(3a)(degreesC(-1)) = 10(14.8 +/- 1.1) exp(-114 +/- 6 kj mol(-1)/RT) Zn/HZSM-5 and for dehydrogenation, calculated from the rate of isobutene formation, are k(3b)(degreesC(-1)) = 10(18.0 +/- 1.1) exp(-148 +/- 6 kj mol(-1)/RT) Ga/HZSM-5 k(3b)(degreesC(-1)) = 10(13.5 +/- 1.1) exp(-101 +/- 6 kj mol(-1)/RT) Zn/HZSM-5 Large preexponential factors for cracking and dehydrogenation over Ga/HZSM-5 indicate that either the coverage of active sites is significantly less than the coverage of exposed sites or the intrinsic reaction step involves a large entropy change between reactant and transition state For Zn/HZSM-5 the small preexponential factors suggest either small entropy changes during activation, perhaps initiated by Lewis acid sites, or a steady-state distribution of active and exposed sites is rapidly reached Differences in intrinsic activation energies may reflect the ratio of Lewis and Bronsted acid sites on the respective catalyst surfaces Aromatization is more prolific over Ga/HZSM-5 than over Zn/HZSM-5 under the low-pressure conditions. (C) 2002 Wiley Periodicals.