Anionic oxyl radical formed on CrVI-oxo anchored on the defect site of the UiO-66 node facilitates methane to methanol conversion

The direct conversion of methane to methanol has attracted increasing interest due to abundant and low-cost natural gas resources. Herein, by anchoring Cr-oxo/-oxyhydroxides on UiO-66 metal-organic frameworks, we demonstrate that reactive anionic oxyl radicals can be formed by controlling the coordination environment based on the results of density functional theory calculations. The anionic oxyl radicals produced at the completely oxidized CrVI site acted as the active species for facile methane activation. The thermodynamically stable CrVI-oxo/-oxyhydroxides with the anionic oxyl radicals catalyze the activation of the methane C-H bond through a homolytic mechanism. An analysis of the results showed that the catalytic performance of the active oxyl species correlates with the reaction energy of methane activation and H adsorption energies. Following methanol formation, N2O can regenerate the active sites on the most stable CrVI oxyhydroxides, i.e., the Cr(O)4Hf species. The present study demonstrated that the anionic oxyl radicals formed on the anchored CrVI oxyhydroxides by tuning the coordination environment enabled facile methane activation and facilitated methanol production.

Modulating the Energetics of C-H Bond Activation in Methane by Utilizing Metalated Porphyrinic Metal-Organic Frameworks

In recent years, much effort has been directed toward utilizing metal-organic frameworks (MOFs) for activating C-H bonds of light alkanes. The energy demanding steps involved in the catalytic pathway are the formation of metal-oxo species and the subsequent cleavage of the C-H bonds of alkanes. With the intention of exploring the tunability of the activation barriers involved in the catalytic pathway of methane hydroxylation, we have employed density functional theory to model metalated porphyrinic MOFs (MOF-525(M)). We find that the heavier congeners down a particular group have high exothermic oxo-formation enthalpies ΔH_O and hence are associated with low N₂O activation barriers. Independent analyses of activation barriers and structure-activity relationship leads to the conclusion that MOF-525(Ru) and MOF-525(Ir) can act as an effective catalysts for methane hydroxylation. Hence, ΔH_O has been found to act as a guide, in the first place, in choosing the optimum catalyst for methane hydroxylation from a large set of available systems.

Exploring mechanistic routes for light alkane oxidation with an iron-triazolate metal-organic framework

In this work, we computationally explore the formation and subsequent reactivity of various iron-oxo species in the iron-triazolate framework Fe₂(μ-OH)₂(bbta) (H₂bbta = 1H,5H-benzo(1,2-d:4,5-d')bistriazole) for the catalytic activation of strong C-H bonds. With the direct conversion of methane to methanol as the probe reaction of interest, we use density functional theory (DFT) calculations to evaluate multiple mechanistic pathways in the presence of either N₂O or H₂O₂ oxidants. These calculations reveal that a wide range of transition metal-oxo sites - both terminal and bridging - are plausible in this family of metal-organic frameworks, making it a unique platform for comparing the electronic structure and reactivity of different proposed active site motifs. Based on the DFT calculations, we predict that Fe₂(μ-OH)₂(bbta) would exhibit a relatively low barrier for N₂O activation and energetically favorable formation of an [Fe(O)]²⁺ species that is capable of oxidizing C-H bonds. In contrast, the use of H₂O₂ as the oxidant is predicted to yield an assortment of bridging iron-oxo sites that are less reactive. We also find that abstracting oxo ligands can exhibit a complex mixture of both positive and negative spin density, which may have broader implications for relating the degree of radical character to catalytic activity. In general, we consider the coordinatively unsaturated iron sites to be promising for oxidation catalysis, and we provide several recommendations on how to further tune the catalytic properties of this family of metal-triazolate frameworks.