Characterization of the Acid Strength of SiO2-ZrO2 Mixed Oxides

Abstract

The preparation of SiO2-ZrO2 mixed oxides and the single oxides SiO2 and ZrO2 from H2SiF6, and/or H2ZrF6 is described. The catalysts are characterized using Hammett colour indicators, temperature programmed desorption of ammonia, XRD, XPS, and infrared spectroscopy. New IR bands and absence of XRD crystallinity show mixing of SiO2 and ZrO2 on an atomic scale in the amorphous coprecipitate. However, as could be concluded from the difference between surface concentrations of Zr and Si measured with XPS and the mean bulk concentrations, the ZrO2 precipitates somewhat more quickly than SiO2. The acid strength of the SiO2-ZrO2 mixed oxides corresponds to an H0 value between −11.4 and −13.8 (super-acidity). There is a fair correlation between the surface concentration of strong acid sites, determined from NH3 TPD, and the performance in the acid catalyzed dehydration of cyclohexanol to cyclohexene. In addition to the concentration of acid sites, their acid strength is an important factor determining the performance of the SiO2-ZrO2 catalysts in the dehydration of cyclohexanol. The acid strength needed for the dehydration of cyclohexanol corresponds to H0 ≤ +2.8. Chemically mixed SiO2-ZrO2 oxides contain strong acid sites as opposed to the single oxides SiO2 and ZrO2, which contain only weak acid sites. XPS shows an oxygen depletion on the surface of the mixed oxides, which indicates that the strong acid sites are of the Lewis type.