Cracking of n-Butane Catalyzed by Iron- and Manganese-Promoted Sulfated Zirconia

Abstract

Fe- and Mn-promoted sulfated zirconia was used to catalyze the conversion of n-butane at atmospheric pressure and n-butane partial pressures in the range of 0.0025-0.01 atm. At temperatures <225°C, the significant reactions were isomerization and disproportionation; in the range of 225-300°C, these reactions were accompanied by cracking, and at temperatures >350°C, cracking and isomerization occurred. Catalyst deactivation, resulting at least in part from coke formation, was rapid. The primary cracking products were methane, ethane, ethylene, and propylene. The observation of these products along with an ethane/ethylene molar ratio of nearly 1 at 450°C is consistent with cracking occurring, at least in part, by the Haag-Dessau mechanism, whereby the strongly acidic catalyst protonates n-butane to give carbonium ions. The rate of methane formation from n-butane cracking catalyzed by Fe- and Mn-promoted sulfated zirconia at 450°C was about 3 × 10−8 mol/(g of catalyst · s); for comparison, the rate of cracking of n-butane catalyzed by HZSM-5 under these conditions was estimated to be 4 × 10−9) mol/(g of catalyst · s) [as determined by extrapolation of the data of H. Krannila, W. O. Haag, and B. C. Gates (J. Catal. 135, 115, 1992)]. This comparison suggests that the catalytic activity of the promoted sulfated zirconia at 450°C is about the same as that of the zeolite, although its activity for n-butane isomerization and disproportionation at temperatures <100°C is orders of magnitude greater than those of zeolites. Thus the indication of superacidity of the promoted sulfated zirconia does not extend to high temperatures. The results raise questions about the nature of the presumed superacidity: perhaps the low-temperature reactions may involve catalyst functions other than the acidic function responsible for high-temperature cracking reactions or perhaps superacidic sites may be very rapidly poisoned at cracking temperatures.