X-ray Absorption Spectroscopy Investigation into the Origins of Heterogeneity in Silica-Supported Dioxomonomolybdates

Dispersed Mo oxides associated with an oxide support catalyze a wide variety of hydrocarbon transformations. The supported catalysts typically contain several types of molybdate species. Because a small fraction is catalytically active, characterization of the precise nature of these active sites and their activation mechanisms is challenging. In this work, monomolybdate sites with uniform first coordination spheres were synthesized by a direct solid–solid reaction between MoO2Cl2 and silica. The Mo(═O)2(OSi)2 sites were characterized by X-ray absorption spectroscopy, enabled by comparison to several well-defined model compounds. In molecular MoO2(OSiPh3)2, a prominent feature at ca. 3 Å in the Fourier-transform extended X-ray absorption fine structure arises due to constructive interference between single-, double-, and triple-scattering paths involving the near-linear silanolate ligands. Coordination of additional ligands causes the Mo–O–Si angle to decrease, weakening this feature. Even stronger attenuation is characteristic of Mo(═O)2(OSi)2 sites, which are inferred to have smaller Mo–O–Si angles and a range of Mo–Si distances imposed by the nature of their attachment to the silica surface as molybdasiloxane rings; smaller rings with lower Mo–O–Si angles contribute little intensity. While Mo(═O)2(OSi)2 sites may be useful in mechanistic studies involving supported molybdate catalysts, they are not single-site materials. The impact of the intrinsic heterogeneity of their attachment to the support on their reactivity will be important to consider in structure-property studies.