Structure of Fe, Mn-promoted sulfated zirconia catalyst by X-ray and IR absorption spectroscopies

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Abstract
The rate of butane isomerization at low temperature is more than two orders of magnitude greater over sulfated zirconia with iron and manganese than over unpromoted sulfated zirconia. Therefore, the bulk and surface structures of the unpromoted and promoted catalysts have been investigated using X-ray absorption spectroscopy at the Zr and Fe K-edges and FTIR spectroscopy of adsorbed pyridine and CO. The Zr—O and Zr—Zr interatomic distances derived from EXAFS indicated that both zirconia catalysts were exclusively tetragonal regardless of composition. In addition, EXAFS revealed the Fe ions did not substitute in the tetragonal structure, but were likely to be present in small clusters or rafts supported on zirconia. The IR spectra, associated with surface sulfate groups, adsorbed pyridine and adsorbed CO, were minimally affected by the promoters indicating that Fe and Mn do not alter the surface properties of the oxides in an obvious manner. Thus, the dramatic effect of promoters on the reaction of butane is likely to be due to extremely low active site densities on the catalysts or the need for two surface sites in the reaction mechanism or both.