CRACKING OF N-BUTANE CATALYZED BY IRON-PROMOTED AND MANGANESE-PROMOTED SULFATED ZIRCONIA

Fe- and Mn-promoted sulfated zirconia was used to catalyze the convers ion of n-butane at atmospheric pressure and n-butane partial pressures in the range of 0.0025-0.01 atm. At temperatures <225 degrees C, the significant reactions were isomerization and disproportionation; in th e range of 225-300 degrees C, these reactions were accompanied by crac king, and at temperatures >350 degrees C, cracking and isomerization o ccurred. Catalyst deactivation, resulting at least in part from coke f ormation, was rapid. The primary cracking products were methane, ethan e, ethylene, and propylene. The observation of these products along wi th an ethane/ethylene molar ratio of nearly 1 at 450 degrees C is cons istent with cracking occurring, at least in part, by the Haag-Dessau m echanism, whereby the strongly acidic catalyst protonates n-butane to give carbonium ions. The rate of methane formation from n-butane crack ing catalyzed by Fe- and Mn-promoted sulfated zirconia at 450 degrees C was about 3 x 10(-8) mol/(g of catalyst . s); for comparison, the ra te of cracking of n-butane catalyzed by HZSM-5 under these conditions was estimated to be 4 x 10(-9) mol/(g of catalyst . s) [as determined by extrapolation of the data of H. Krannila, W. O. Haag, and B. C. Gat es (J. Catal. 135, 115, 1992)]. This comparison suggests that the cata lytic activity of the promoted sulfated zirconia at 450 degrees C is a bout the same as that of the zeolite, although its activity for n-buta ne isomerization and disproportionation at temperatures <100 degrees C is orders of magnitude greater than those of zeolites. Thus the indic ation of superacidity of the promoted sulfated zirconia does not exten d to high temperatures. The results raise questions about the nature o f 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. (C) 1995 Academ ic Press, Inc.