Studies on the catalytic activity of zirconia promoted with sulfate, iron, and manganese

The catalytic properties of iron- and manganese-promoted sulfated zirconia (SFMZ) for the isomerization of n-butane to isobutane are investigated using various catalyst pretreatments and reaction conditions. The n-butane isomerization reactivity at 30 degrees C is effected by calcination of the catalyst at 650 degrees C in helium and vacuum treatment at room temperature indicating that superacidity is not likely to be responsible for activity. In addition, SFMZ samples exposed to dry air at over 450 degrees C are more active than those calcined in helium at a reaction temperature of 30 degrees C (n-butane conversions of 18.7% vs 0.4%) suggesting the presence of an active site involving a metal ''oxy'' species. The oxy species is capable of reacting CO to CO2 at room temperature and is present at a number density of 10-15 mu mol/ g. At a reaction temperature of 100 degrees C, SFMZ catalysts calcined in air then activated in helium show similar reactivities to those activated in air up to a preheating temperature of 450 degrees C; above 450 degrees C the metal oxy species is formed and provides additional activity (n-butane conversions of 37.1% in air vs 15.4% in He for calcinations at 650 degrees C). The nature of the active sites on SFMZ are investigated using temperature-programmed desorption of substituted benzenes. The liberation of CO2 and SO2 in the benzene TPD profile of SFMZ is attributed to the oxidation of benzene at the redox-active metal sites, resulting in the subsequent decomposition of the reduced iron (II) sulfate. Data from the TPD studies do not suggest the presence of superacidity on SFMZ that could contribute to the low-temperature n-butane isomerization activity. Instead, a bifunctional mechanism that involves a combination of a redox-active metal site and an acid site in close proximity is proposed. (C) 1996 Academic Press, Inc.