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)SiO<INF>2</INF> 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(MoO<INF>4</INF>)<INF>2</INF>. While Zr(x)SiO<INF>2</INF> supports stabilize surface molybdenum oxide species, silica is unable to spread molybdena, which segregates like bulk MoO<INF>3</INF>. 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.