Low loadings of dispersed metal oxides on silica are amorphous at multiple scales. The support is amorphous and possesses a distribution of OH groups and siloxane rings, while the supported oxide can exist in a number of nuclearities and connectivities to the surface. This heterogeneity poses a significant challenge to understanding surface reactivity. Highlighting a few examples from group IV and V supported oxides, this talk will demonstrate how selective titration by phenylphosphonic acid can shed light onto kinetically-relevant structures. This technique is a counterpart to more spectroscopic probes, which may be ex situ, selectively sensitive, or qualitative. Several results are discussed that utilize this method. Non-aqueous photooxidation rates scale with the number of geometrically-accessible speices, which are probed by aggressive deposition of the phosphonic acid.[1] This reactivity is independent of surface structure, whereas epoxidation with hydrogen peroxide is strongly dependent on the surface structure.[2] The latter comes from in situ titration and extrapolation down to the total loss of activity. In this manner, it is also shown that bulky precursors lead to ~100% dispersion for several supported metal oxides,[3] as has been inferred but not quantified directly. In addition, when comparing titrations of Ti-silica to Nb-silica and Ta-silica epoxidation catalysis, the results appear to be consistent with the catalytic relevance of a monohydroxy in the former, and a dihydroxy in the latter.[4] As a counterpoint to the amorphous silica-supported oxides, it is shown that Nb supported on NU-1000, a zirconia-containing metal organic framework, gives epoxidation catalysts with near 100% dispersion even at high Nb surface densities and without particular attention to grafting method.[5] Finally, extensions to other reactions such as sulfoxidation, outstanding questions, and limitations of the technique will be discussed.

In situ site titration of silica-supported metal oxide catalysts

253rd National Meeting (2017)

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