Thermal behavior of 12-molybdophosphoric acid supported on zirconium-loaded silica

The thermal behavior of 12-molybdophosphoric acid (HPMo) supported on silica and Zr-loaded silica (Zr content from 1.6 to 17.0 wt %) was studied by X-ray diffraction, Fourier-transform infrared, in situ Raman, and X-ray photoelectron spectroscopic techniques. A silica support stabilizes the Keggin unit of HPMo up to 773 K, which is ca. 100 K higher than the unsupported one. For the Zr(1.6)SiO2 support at 773 K, the thermal stability of HPMo is similar to that of the silica-supported counterpart. For Zr contents above 5.6 wt %, decomposition of HPMo occurs at lower temperatures, and this is accompanied by the formation of structure analogues of Zr(MoO4)(2). While Zr(x)SiO2 supports stabilize surface molybdenum oxide species, silica is unable to spread molybdena, which segregates like bulk MoO3. The interactions of the Mo-Zr oxide species appear to be responsible for the activity enhancement in methanol oxidation. The marked decrease of the dimethyl ether/formaldehyde selectivity ratio with increasing zirconium content on the samples calcined at 723 K indicates that the redox function of the interaction of Mo-Zr species increases formaldehyde selectivity.