Interaction of Zirconia Catalysts with Alkanes and CO: Investigation by Microcalorimetry and IR Spectroscopy

Sulfated zirconia (SZ) is active for the isomerization of n-butane to isobutane at room temperature [1]. The activity was first ascribed to strongly (super) acidic sites, however, it was not possible to identify such sites on the surface of SZ [2]. The addition of cations of Fe and/or Mn increases the activity of SZ for n-butane isomerization by 1-2 orders of magnitude [3,4]. Particularly with respect to Fe and Mn it was suggested that functionalities other than acidic sites contribute to the reactivity of sulfated zirconia materials [5]. The investigation of acidic sites on sulfated zirconia with standard probe molecules such as pyridine, ammonia, or substituted benzenes is problematic because the sulfate may be displaced or-upon heating for thermal desorption spectroscopy- the sulfate and/or the probe may decompose [6,7]. Large or strongly basic probes can thus be considered unsuitable for sulfated zirconia. We used propane and isobutane, i.e. molecules that are chemically closely related to the reactant n-butane, and CO, a weak base, as probe molecules. Zirconium hydroxide and sulfated zirconium hydroxide were obtained from MEL Chemicals. The promoters, Fe and Mn, were added in the form of solutions of their nitrates according to the incipient wetness method [4]. Unpromoted samples were calcined at 823 K, promoted samples at 923 K. The batch size for calcination which has been demonstrated to have a significant effect on the activity of the obtained catalysts [8] was varied in some cases (10-25 g). IR spectroscopy was performed with a PE System 2000 spectrometer in transmission using self-supporting wafers, and CO was adsorbed at 77 K. Differential heats of adsorption were measured with modified SETARAM Calvet calorimeters, and the alkanes were adsorbed at 313 K. Samples were activated at 723 K in vacuum for IR and microcalorimetry experiments