Structure and properties of PVMo11O40 heteropolyoxomolybdate supported on silica SBA-15 as selective oxidation catalyst

Investigations into structure and properties of heteropolyoxomolybdates (HPOM) supported on nanostructured silica SBA-15 for the selective oxidation of propene are presented. H4[PVMo11O40] Keggin ions supported on SBA-15 (PVMo11-SBA-15) were prepared by incipient wetness of silica SBA-15 with suitable precursor solutions. Structure and properties of the resulting material were studied ex situ by X-ray diffraction and nitrogen physisorption measurements and in situ by combined X-ray absorption spectroscopy (XAS) and mass spectrometry under various reaction conditions. The characteristic structure of PVMo11-SBA-15 and its evolution under reactive gas atmosphere are compared to that of bulk H4[PVMo11O40]⋅xH2O. Structural investigations of as-prepared PVMo11-SBA-15 have shown that the respective Keggin ions can be readily supported on silica SBA-15. In situ XAS measurements under reducing or oxidizing conditions revealed a pronounced support interaction effect. This effect resulted in a further decreased thermal stability of the supported Keggin ions compared to bulk H4[PVMo11O40]⋅xH2O. Apparently, no stable HPOM Keggin ions could be obtained on silica SBA-15 under reaction conditions. However, in spite of their low thermal stability, HPOM supported on SBA-15 remain suitable well-defined precursors for preparing mixed oxide model catalysts for selective oxidation reactions.