In situ diffuse-reflectance infrared spectroscopic investigation of promoted sulfated zirconia catalysts during n-butane isomerization

Sulfated zirconia, unpromoted or promoted with either 2 wt% iron or manganese, was investigated by in situ diffuse reflectance IR spectroscopy during activation, n-butane isomerization (323-378 K, 1-5 kPa n-butane), and regeneration. During activation at 773 K in N2 or O2, more than 95% of the adsorbed water was removed, and comparison of the resulting pattern of sulfur-oxygen vibrations with DFT calculations (A. Hofmann and J. Sauer, J. Phys. Chem. B 108 (2004) 14652) suggests S2O72− as a major surface species. All catalysts underwent an induction period before maximum rates were measured, which at 1 kPa n-butane were 350 and 180 μmol g−1 h−1 for the Fe- or Mn-promoted catalyst (323 K) and 5 μmol g−1 h−1 for sulfated zirconia (358 K). During isomerization several bands increased in the range of 1700-1600 cm−1; the growth of a band at 1600 cm−1 was restricted to the period of increasing catalytic activity. The rate of isomerization was linearly correlated with the total area increase of the bands in this region, indicating that the absorbing species are either active surface intermediates or side products (possibly water) formed in an amount proportional to the active surface species. The rate per such species was much higher in the presence of Mn; that is, promoters facilitate reaction steps after the initial alkane activation. The promoted materials deactivated within hours under the applied conditions, but complete recovery of the spectral and catalytic properties of Mn-promoted sulfated zirconia was possible through regeneration in O2 at 773 K. Attempts to regenerate in N2 produced unsaturated surface species absorbing at 1532 and 1465 cm−1.