Role of Vanadium and Phosphorus in Substituted Keggin-Type Heteropolyoxo Molybdates Supported on Silica SBA-15 in Selective Propene Oxidation

Abstract

Vanadium-containing Keggin-type heteropolyoxo molybdate ([PV2Mo10O40]5−) was supported on silica SBA-15 (denoted as PV2Mo10-SBA-15). The structural evolution and catalytic activity of PV2Mo10-SBA-15 and a suitable reference V2Mo10Ox-SBA-15 were investigated under selective propene oxidation conditions by using in situ X-ray absorption spectroscopy. 31P MAS NMR measurements of supported PV2Mo10-SBA-15 and reference H3PO4-SBA-15 were performed after the catalytic reaction. PV2Mo10-SBA-15 formed a mixture of mainly tetrahedral [MoOx] and [VOx] units during thermal treatment under propene oxidation conditions. Changes in the average local structure around V centers coincided with the changes in the average local structure around Mo centers and the onset of catalytic activity. In addition, mainly tetrahedral [MoOx] and [VOx] units seemed to be in close proximity and interacted under catalytic conditions. Conversely, in the reference material V2Mo10Ox-SBA-15 synthesized with individual V and Mo source precursors, Mo and V centers appeared to be more separated from each other. The structural environment of P in PV2Mo10-SBA-15 under catalytic conditions corresponded to a mixture of various species. P was connected to both the support material SBA-15 via POSi bonds and [MoOx] or [VOx] units. Apparently, the proximity of V and Mo in Keggin precursors was a prerequisite for obtaining (Mo,V) oxide species on the support material.