The acidity of sulfated zirconia-titania preparations was studied using NH3, pyridine, and benzene as the probe molecule. Temperature-programmed desorption curves of ammonia and the IR spectra of adsorbed pyridine were determined to distinguish acid sites. The Brønsted acidic strength of the surface hydroxyl groups was correlated with the red shift of the νOH IR band induced by adsorption of benzene. The conversion of n-hexane was used as the catalytic test. The thermally effected textural and structural changes were followed by thermogravimetry, X-ray diffraction, energy-dispersive X-ray analysis, and surface area measurements. Sulfate and titania were found to hinder the aggregation of the metal oxide component. The weight-specific concentration of strong Lewis and Brønsted acidic sites were found higher in the zirconia-rich mixed oxides than in SO4-2/ZrO2, but, no new kind of acidic center was detected. The strongest acidic sites are Lewis centers. The sulfate treatment of the oxides resulted in the formation of new, strong Brønsted sites. Regardless, whether the basic probe was protonated (as pyridine) or not (as benzene) the negative charge on the conjugated base of the Brønsted acid increased upon adsorption. This is indicated by the induced red shift of the νS=O band of the surface sulfate group. This effect decreases the acid strength of the proton to an extent depending on the degree of electron donation and the volume available for delocalization of the electrons. The use of basic probes is not suggested for comparing the acid strength of catalysts much different in structure and composition.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry