(7a) Catalytic Cycles and Deactivation Mechanisms for Styrene Oxidation over First-Row Transition Metal Carboxylate MOFs

Metal-organic frameworks (MOFs) are modular solid catalysts that have demonstrated notable activity for liquid-phase oxidations; however, their underlying mechanisms are not well-understood.^1,2 Here, styrene oxidation by hydrogen peroxide (H₂O₂) is utilized as a probe reaction over Fe and Cr carboxylate MOFs to quantify the role of key material properties in observed oxidation rates and deactivation behaviors. Comparison of isoreticular MIL-101(Fe,Cr) shows higher oxygenate production rates over MIL-101(Fe) than over MIL-101(Cr) at 323 K in acetonitrile; Fe(II) generation during reaction and MIL-101(Fe)’s higher acid site density and strength compared to MIL-101(Cr) are significant contributors.^3,4 Replacing acetonitrile with methanol to quench solution-phase H­₂O_{2­­­­}-derived radicals (hydroperoxo, hydroxyl) does not result in cumulative (2 h) oxidation rate depression for either MIL-101(Fe) or MIL-101(Cr), indicating entirely surface-mediated reactions at low deactivation extents. Further, methanol does not change oxygenate product distributions by 2 h for MIL-101(Fe) (benzaldehyde > benzoic acid > styrene glycol) but shifts MIL-101(Cr)’s product profile towards higher epoxide fractions by >2x compared to acetonitrile; this is likely rooted in effective reactant concentration differences and in differential stabilization of epoxide metallocycle transition states through H-bonding. Both MIL-101(Fe) and MIL-101(Cr), however, suffer deactivation via metal leaching, though leached Fe species perturb neither rates nor oxygenate product distributions.⁴ Temporal metal leaching is spectroscopically quantified via UV-Vis (Cr) and colorimetric analysis (Fe), yielding a 1^st-order exponential functional form with a higher deactivation rate constant k_d for MIL-101(Cr) (280±8×10^-6 s^-1) than for MIL-101(Fe) (21±3×10^-6 s^-1). Finally, Cr leaching is the predominant deactivation mechanism in MIL-101(Cr), but additional factors are responsible for activity loss in MIL-101(Fe). Overall, this work provides fundamental insight into transition metal MOF kinetic and deactivation mechanisms that are imperative for rational design of reaction systems and next-generation oxidation catalysts.

References

1. https://doi.org/10.1021/cr9003924
2. https://doi.org/10.1039/C1CY00068C
3. https://doi.org/10.1021/jacs.9b08686
4. https://doi.org/10.1039/D1CY00567G