Structure of Fe(II) Sites Situated within the Nodes of Metal-Organic Framework Materials and Their Reactivity for Low Temperature Methane to Methanol Conversion

One strategy for methane activation is the design of catalysts that mimic the structure and function of metal centers in the active pockets of enzymes. We demonstrate the ability of a biomimetic, high-spin (S=2) Fe(II) site situated in trimeric iron-oxo based nodes of a series of Metal-Organic Framework materials to convert methane to methanol of low temperatures. The single-phase crystallinity and stability of the materials subject to oxidative reaction conditions (378-408 K CH₄+N₂O) are confirmed using pair distribution function (PDF) analysis, N₂ isotherms, and X-ray diffraction. Density Functional Theory (DFT) calculations predict the reaction occurs via a radical rebound mechanism involving the oxidation of Fe(II) species to Fe(III) via a high-spin Fe(IV)=O intermediate, supported by X-ray adsorption and Mössbauer spectroscopy. N₂ production and CH₄ consumption rates were measured using a recirculating batch reactor, and determined to both be first order in N₂O pressure and zero order in CH₄ pressure, concurrent with DFT calculations that predict the reaction of Fe(II) with N₂O to be rate-limiting (ΔH^‡_exp = 78±6 kJ mol^-1). The identity of the Fe(II) active site is confirmed and quantified using in-situ chemical titrations in conjunction with infra-red spectroscopy, revealing different concentrations of Fe(II) sites (5-30% total Fe content) across different materials. Accounting for differences in active site concentration yield comparable values for rate constants of reaction between N₂O and Fe(II) (1.2-0.8 ×10^-6 mol mol_Fe(II)^-1 kPa^-1 s^-1 at 378 K) across different materials, highlighting the consistent ability of trimeric Fe-oxo nodes to activate methane at low temperatures and opening a wide class of novel materials for this chemistry. We aim to compare this to the activation of CH₄ over trimeric nodes subject to substitution by secondary transition metals to develop structure-activity relationships for the design of improved catalytic materials for low temperature methane activation.