Acid properties of iron oxide catalysts dispersed on silica–zirconia supports with different Zr content

Abstract

The acid properties of surfaces comprising dispersed iron oxide supported on pure silica and silica–zirconia supports with variable zirconia to silica ratio (ZrO 2/SiO 2 molar ratio from 0.03 to 0.4) have been studied. The surface acidity of the bare supports and relevant iron oxide covered supports were studied by thermal programmed desorption (TPD) of 2-phenylethylamine (PEA) probe, performed in a thermogravimetric analyzer (TGA). Results from N 1s XPS study of the surfaces saturated with PEA probe complemented the study, permitting the determination of the nature of the acid sites on the different samples. A regularly increasing amount of the number of acid sites and acid strength of the supports was observed increasing the zirconia content on the silica matrix. After the iron oxide support coverage, the number of acid sites of the Fe-samples changed, it increased or decreased compared with that determined on the relevant supports depending on the acidity of the bare support. The average acid strength of the Fe-samples was higher than that of the supports and turned towards a prominent Lewis acidity, in any case. The predominant Lewis nature of all the Fe-catalysts was also confirmed by the results of the reaction of α-pinene oxide (POX) isomerization that gave the corresponding aldehyde with good selectivity.