Interaction between sulfated zirconia and alkanes: Prerequisites for active sites - Formation and stability of reaction intermediates

Two sulfated zirconia catalysts were prepared via sulfation and calcination of zirconium hydroxide at 873 K; the zirconium hydroxide had been aged at room temperature for 1 h (SZ-1) or aged at 373 K for 24 h (SZ-2). SZ-1 was active for n-butane isomerization at 373 K; SZ-2 reached a similar performance level only at 473 K. Both materials contained about 9 wt% sulfate and were tetragonal. Because of a lower BET surface area (105 vs. 148 m²/g) SZ-1 featured a higher sulfate density, and XRD and EXAFS analysis showed larger (ca. 10 nm) and more well ordered crystals than for SZ-2. n-Butane TPD on SZ-1 showed a butene desorption peak at low temperature, whereas no obvious butene desorption was observed with SZ-2, suggesting that SZ-1 has a higher oxidizing power at low temperature than SZ-2. The number of sites capable of dehydrogenation are less than 5 μmol/g, because the differential heats of n-butane adsorption as measured by microcalorimetry were 45-60 kJ/mol for higher coverages, indicating weak and reversible interaction. TAP experiments describe the adsorption and desorption behavior of n-butane at different activity states and are the basis for a simple adsorption model. Reactant pulses and purge experiments show that the active species, presumably formed in an oxidative dehydrogenation step, are stable at the surface under reaction conditions.