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Mn-promoted sulfated zirconia (0.5 or 2 wt% Mn) was investigated during activation (in inert or oxidizing atmosphere at 703–773 K), during n-butane isomerization (323–333 K, 1 kPa n-butane) and during regeneration (see activation conditions) using in situ UV–vis–NIR, diffuse reflectance IR, and X-ray absorption spectroscopies. While the activation atmosphere did not influence the degree of hydration and the state of the sulfate, manganese—which was always present as a mixture of Mn2+ and higher oxidation states—was slightly reduced in the absence of oxygen. Catalytic data did not give a clear picture as to whether activation in oxygen-containing atmosphere is advantageous or not. The catalysts deactivated within hours, but complete recovery in O2 was possible. Heating of a deactivated catalyst in N2 produced an inactive material and unsaturated compounds on the surface.

In situ DRIFT and DR-UV/Vis spectroscopies were performed during n-butane (1 or 5 kPa partial pressure, 358 – 453 K) and n-pentane (1 kPa, 298 – 323 K) isomerization in the presence of two different sulfated zirconia catalysts: a sulfate containing ordered mesoporous zirconia of the MCM-41 structure and a conventional sulfated zirconia catalyst with a tetragonal zirconia bulk structure. After a 100 min induction period, the mesoporous zirconia deactivates slowly during n-butane isomerization at 453 K while an absorption band at 285 nm grows, indicating the formation of unsaturated surface species. The conventional catalyst which is more active and produces the same maximum rate already at 378 K deactivates rapidly and a band grows at 310 nm, indicating the formation of allylic carbocations on the surface. During n-pentane isomerization, both catalysts deactivate rapidly while bands at 285 and 310-320 nm (mesoporous) and 335 nm (conventional) are formed. The spectra clearly show that the surface species on the two catalysts differ, although in principle the same gas phase products are observed, indicating an influence of the underlying wall or tetragonal bulk structure, respectively.

Two different sulfated zirconia catalysts were produced through precipitation from zirconyl nitrate solutions, followed by aging of the precipitate either at 298 K for 1 h (SZ-1) or 373 K for 24 h (SZ-2). After drying, the samples were sulfated with ammonium sulfate and calcined for 3 h at 873 K. SZ-1 had a smaller surface area (90 m2 g-1) than SZ-2 (140 m2 g-1) but displayed a one order of magnitude higher maximum n-butane isomerization rate (373–423 K, 1–5 kPa n-butane at 101.3 kPa total pressure). Both materials consisted predominantly of tetragonal ZrO2, contained 9 wt% of sulfate, and adsorbed about 0.5 mmol g-1 NH3. Measurements of adsorption isotherms and differential heats for propane and iso-butane at 313 K reveal a larger number of adsorption sites on SZ-1 than on SZ-2, extrapolated to 1 kPa, 42 vs. 20 µmol g-1 (propane) and 120 vs. 44 µmol g-1 (iso-butane). At coverages > 2 µmol g-1 the heats were similar for both samples with both probes and decreased from 60 to 40 kJ mol-1. Temporal analysis of products measurements indicated shorter residence times for n-butane than for iso-butane, and SZ-1 retained both of these molecules longer than SZ-2. The activation energy for n-butane desorption was 45 kJ mol-1 for both samples. Interaction with pulses of CO2 suggested that non-sulfated, basic ZrO2 surface is exposed on SZ-2, consistent with the larger surface area at the same sulfate content as SZ-1. The results suggest that only a fraction of the sulfate groups participates in adsorption and that product desorption may be of importance.