C-H Activation via Direct Oxidative Routes over Molecular Metal-oxo Species Situated in Metal-Organic Frameworks

Abstract

Metal Organic Frameworks (MOFs), crystalline materials composed of inorganic nodes connected by organic linker molecules, afford an opportunity to synthesize new biomimetic catalysts engendering the oxidative activation of light alkanes, opening new pathways for the enhancement of underutilized chemical feedstocks. We aim to demonstrate in this dissertation the ability of Fe(II) centers, bearing a similar geometric and electronic structure to sites found in non-heme enzymes, situated in the trimeric iron-oxo nodes of a family of MOFs to activate light alkanes at near ambient temperatures. The identity and quantity of the active site was determined using in situ X-ray Absorption and ex situ Mössbauer Spectroscopy, in concert with in situ chemical titrations. Reaction kinetics, measured by varying reactant concentration and temperature using a recirculating batch reactor, are consistent with the rate limiting reaction of this Fe(II) site with the oxidant, N2O, to form a highly reactive Fe(IV)=O species (k = 1.2-0.8 x10-6 mol molFe(II)-1 kPaN2O-1 s-1 at 378 K) capable of activating C-H bonds homolytically, in agreement with Density Functional Theory calculations that predict subsequent radical-mediated pathways for upgrading propane and methane. A major challenge in this chemistry is resisting the over-oxidation of desired products formed by these pathways, which is often both kinetically and thermodynamically favored. We evince, using in situ Infrared Spectroscopy, that methanol, the desired product of the oxidation of methane, is stabilized as methoxy groups on the MOF through reactions with surface hydroxyl species. Pursuant to this, we added a zeolite (H-ZSM-5, Si/Al = 11.5) in inter- and intra- pellet mixtures with the MOF, observing monotonic increases in methanol selectivity with increasing ratio and proximity of zeolitic H+ to MOF-based Fe(II) sites, signaling increased amounts of methanol being dehydrated and protected within the zeolite. This work demonstrates (i) the radical-rebound mechanism commonly invoked in this chemistry is insufficient to explain the reactivity of these systems, (ii) the selectivity controlling steps involve both chemical and physical rate phenomena, and (iii) offers a strategy to mitigate over-oxidation in these and other similar systems.