MoOx-ZrO2 based catalysts were prepared by equilibrium adsorption in basic (pH 8) or in acid (pH 2) conditions
with molybdenum content up to 3 wt.% (pH 8) and up to 8 wt.% (pH 2) using hydrous zirconium oxide, designated
as ZrO2(383), as support. The samples were characterized by textural analysis (BET), X-ray diffraction (XRD), Raman
and X-ray photoelectron (XPS) spectroscopies. The catalytic behavior was analyzed for the selective oxidation of diphenylsulfide
(DPS) to diphenylsulfone (DPSO2) or diphenylsulfoxide (DPSO) using H2O2 as oxidizing agent. The results
show that the pH of the contacting solution affects the uptake of the Mo species and the molecular state of the adsorbed
species. Raman spectroscopy identified polymolybdate structures at pH 2 and molybdate aggregates at pH 8. XRD analysis
revealed that at increasing molybdenum concentration the interaction between the supported species and the zirconia
surface favored the tetragonal volume fraction of zirconia at the expense of the thermodynamically stable monoclinic
phase in all series of samples.
High conversion of DPS (88%) and selectivity to diphenylsulfone (DPSO2) (60%) was obtained for the pH 2 series of
catalysts. These results suggest that the acid environment was the most efficient synthesis parameter leading to the formation
of polymolybdates species which are considered the active phases in this reaction.