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A direct, catalytic conversion of benzene to phenol would have wide-reaching economic impacts. Fe zeolites exhibit a remarkable combination of high activity and selectivity in this conversion, leading to their past implementation at the pilot plant level. There were, however, issues related to catalyst deactivation for this process. Mechanistic insight could resolve these issues, and also provide a blueprint for achieving high performance in selective oxidation catalysis. Recently, we demonstrated that the active site of selective hydrocarbon oxidation in Fe zeolites, named α-O, is an unusually reactive Fe(IV)=O species. Here, we apply advanced spectroscopic techniques to determine that the reaction of this Fe(IV)=O intermediate with benzene in fact regenerates the reduced Fe(II) active site, enabling catalytic turnover. At the same time, a small fraction of Fe(III)-phenolate poisoned active sites form, defining a mechanism for catalyst deactivation. Density-functional theory calculations provide further insight into the experimentally defined mechanism. The extreme reactivity of α-O significantly tunes down (eliminates) the rate-limiting barrier for aromatic hydroxylation, leading to a diffusion-limited reaction coordinate. This favors hydroxylation of the rapidly diffusing benzene substrate over the slowly diffusing (but more reactive) oxygenated product, thereby enhancing selectivity. This defines a mechanism to simultaneously attain high activity (conversion) and selectivity, enabling the efficient oxidative upgrading of inert hydrocarbon substrates. 

Abstract The Fe protein of nitrogenase plays multiple roles in substrate reduction and cluster maturation via its redox‐active [Fe₄S₄] cluster. Here we report the synthesis and characterization of a water‐soluble [Fe₄Se₄] cluster that is used to substitute the [Fe₄S₄] cluster of theAzotobacter vinelandiiFe protein (AvNifH). Biochemical, EPR and XAS/EXAFS analyses demonstrate the ability of the [Fe₄Se₄] cluster to adopt the super‐reduced, all‐ferrous state upon its incorporation intoAvNifH. Moreover, these studies reveal that the [Fe₄Se₄] cluster inAvNifH already assumes a partial all‐ferrous state ([Fe₄Se₄]⁰) in the presence of dithionite, where its [Fe₄S₄] counterpart inAvNifH exists solely in the reduced state ([Fe₄S₄]¹⁺). Such a discrepancy in the redox properties of theAvNifH‐associated [Fe₄Se₄] and [Fe₄S₄] clusters can be used to distinguish the differential redox requirements for the substrate reduction and cluster maturation of nitrogenase, pointing to the utility of chalcogen‐substituted FeS clusters in future mechanistic studies of nitrogenase catalysis and assembly.