(576e) Deciphering Mechanisms of Reaction and Deactivation over Isostructural Metal-Organic Frameworks for Small-Molecule Oxidation

Metal-organic frameworks (MOFs) are promising solid catalysts for chemical synthesis applications, including alkene oxidation by benign oxidants like hydrogen peroxide (H2O2) [1,2]. This work focuses on rigorous mechanistic studies to elucidate reaction kinetics and deactivation, which are often not well-defined for liquid phase MOF-catalyzed reactions, for styrene oxidation by H2O2 over MIL-101(Cr) and MIL-101(Fe). Batch reaction studies at 323 K assert that the intrinsic reactivity of MIL-101(Fe) is higher than MIL-101(Cr), as evidenced by normalized first-order styrene turnover rate constants with excess oxidant that are further confirmed to be kinetic in nature via calculated Weisz-Prater criteria. Oxygenate product distributions indicate that MIL-101(Cr) is more selective for styrene oxide whereas MIL-101(Fe) prefers benzaldehyde, particularly at low conversions (<10 %). Disparities in reactivity and oxygenate selectivity are rooted in metal abilities to generate and stabilize reactive H2O2-derived surface intermediates (oxo, hydroperoxo, hydroxyl) and their subsequent reactivity. Quantitative DFT-calculated oxophilicities [3] and Pauling electronegativities suggest lower energy barriers for oxo and hydroperoxo intermediate formations, respectively, on Cr, yielding epoxide formation. However, Fe(IV)-oxo has a lower energy barrier than Cr(IV)-oxo for C—H activation [4], consistent with the higher reactivity of MIL-101(Fe). Both catalysts facilitate secondary conversion to styrene glycol and benzoic acid via styrene oxide hydration and benzaldehyde oxidation by H2O2, respectively. Neither prefers benzoic acid production, but complete styrene oxide conversion to styrene glycol occurs over MIL-101(Fe) whereas styrene oxide is observed over MIL-101(Cr), which is consistent with higher Lewis/Brønsted acidic site density and strength for MIL-101(Fe). Spiking experiments suggest similar pathway interconnectivity for both analogs, though energy barriers for oxygenate formation differ. Finally, relatively constant oxygenate production rates on MIL-101(Fe) from recyclability experiments suggest that its active sites are more stable and regenerable than those in MIL-101(Cr), which suffers from metal leaching during reaction.

[1] https://doi.org/10.1021/cr9003924

[2] https://doi.org/10.1039/C4RA05402D

[3] https://doi.org/10.1021/acs.inorgchem.6b01702

[4] https://doi.org/10.1002/anie.202004458