Abstract
We investigate the potential use of Fe(iv)oxo species supported on a metal-organic framework in the catalytic hydroxylation of methane to produce methanol. We use periodic density-functional theory calculations at the 6-31G**/B3LYP level of theory to study the electronic structure and chemical reactivity in the hydrogen abstraction reaction from methane in the presence of Fe(iv)O(oxo) supported on MOF-74. Our results indicate that the Fe(iv)O moiety in MOF-74 is characterised by a highly reactive (quintet) ground-state, with a distance between Fe(iv) and O(oxo) of 1.601 Å, consistent with other high-spin Fe(iv)O inorganic complexes in the gas phase and in aqueous solution. Similar to the latter systems, the highly electrophilic character (and thus the reactivity) of Fe(iv)O in MOF-74 is determined by the presence of a low-lying anti-bonding virtual orbital (3σ*), which acts as an electron acceptor in the early stages of the hydrogen atom abstraction from methane. We estimate an energy barrier for hydrogen abstraction of 50.77 kJ mol-1, which is comparable to the values estimated in other gas-phase and hydrated Fe(iv)O-based complexes with the ability to oxidise methane. Our findings therefore suggest that metal-organic frameworks can provide suitable supports to develop new solid-state catalysts for organic oxidation reactions.
Highlights
Natural gas has attracted attention in the last few decades for its potential use in the production of energy, as a clean and sustainable alternative to oil and coal and as a means to provide important feedstock for the chemical industry on a planetary scale
Natural gas is transported through high-pressure pipelines or in liquefied natural gas (LNG) carriers, which suffer from high compression and refrigeration costs
For the Densityfunctional theory (DFT) simulations with full symmetry we find that 82.28% of the initial spin is retained by the Fe atoms, whereas 12.69% is transferred to the O(oxo) atom
Summary
Natural gas has attracted attention in the last few decades for its potential use in the production of energy, as a clean and sustainable alternative to oil and coal and as a means to provide important feedstock for the chemical industry on a planetary scale. Global reserves of natural gas are estimated to be of the order of 1011 m3,1 the direct use of methane gas, its main component, is severely limited by transport costs from production sites to consumption areas. Natural gas is transported through high-pressure pipelines or in liquefied natural gas (LNG) carriers, which suffer from high compression and refrigeration costs. Methane can be oxidised to methanol using several multistep industrial processes.[3] Some of these reactions require catalysts, while others occur in the absence of a catalyst. In a two-step process methane is first decomposed into synthesis gas (CO + H2) via e.g. dry reforming
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