Acid sites in sulfated and metal-promoted zirconium dioxide catalysts

Unpromoted sulfated zirconia (SZ) and sulfated zirconia that was promoted with iron and manganese ions (FMSZ) have been tested as catalysts for the isomerization of n-butane to isobutane. FMSZ is the superior catalyst; its activity at 60°C is similar to that of SZ at 180°C. Both catalysts deactivate rapidly. FTIR analysis of adsorbed CO and acetonitrile reveals the presence of Lewis and Brønsted sites in both catalysts. Acetonitrile is strongly adsorbed on FMSZ. Subsequent desorption is destructive; CO2, SO2, and O2 are formed, while temperature-programmed oxidation shows that no carbon-containing products are left on the surface. CO-FTIR reveals equal acidity, within experimental error, for the Lewis sites on SZ and FMSZ. Changes in proton NMR and FTIR parameters caused by adsorption of acetonitrile show that the acid strength of the Brønsted sites of SZ and FMSZ is similar to that of the lower OH-frequency protons in HY, but weaker than that of the protons in HZSM-5. The results indicate that the remarkable activity of SZ and FMSZ is not caused by exceptionally strong acid sites, but by stabilization of the transition state complex at the surface