The active site of low-temperature methane hydroxylation in iron-containing zeolites

Abstract

An efficient catalytic process for converting methane into methanol could have far-reaching economic implications. Iron-containing zeolites (microporous aluminosilicate minerals) are noteworthy in this regard, having an outstanding ability to hydroxylate methane rapidly at room temperature to form methanol. Reactivity occurs at an extra-lattice active site called α-Fe(ii), which is activated by nitrous oxide to form the reactive intermediate α-O; however, despite nearly three decades of research, the nature of the active site and the factors determining its exceptional reactivity are unclear. The main difficulty is that the reactive species-α-Fe(ii) and α-O-are challenging to probe spectroscopically: data from bulk techniques such as X-ray absorption spectroscopy and magnetic susceptibility are complicated by contributions from inactive 'spectator' iron. Here we show that a site-selective spectroscopic method regularly used in bioinorganic chemistry can overcome this problem. Magnetic circular dichroism reveals α-Fe(ii) to be a mononuclear, high-spin, square planar Fe(ii) site, while the reactive intermediate, α-O, is a mononuclear, high-spin Fe(iv)=O species, whose exceptional reactivity derives from a constrained coordination geometry enforced by the zeolite lattice. These findings illustrate the value of our approach to exploring active sites in heterogeneous systems. The results also suggest that using matrix constraints to activate metal sites for function-producing what is known in the context of metalloenzymes as an 'entatic' state-might be a useful way to tune the activity of heterogeneous catalysts.

The Nature and Catalytic Function of Cation Sites in Zeolites: a Computational Perspective

“…Schoonheydt, Sels, Solomon and co‐workers combined a wide range of characterization techniques such as UV‐Vis spectroscopy with site‐selective magnetic circular dichroism (MCD) spectroscopy complemented by DFT calculations to identify the nature of methane oxidation sites in Fe/ZSM‐5 catalyst ,. It was shows that reactive α‐Fe is a mononuclear Fe 2+ cation in high‐spin electronic configuration located in a six‐membered ring featured with a square‐planar geometry coordinated by four framework oxygen atoms.…”

“…Middle, energetics of the CH 4 homolytic C−H activation. Bottom, evolution of the lowest unoccupied molecular orbital along the reaction coordinate . Reprinted with permission from Nature , 2016, 536, 317.…”

A zeolitic vanadotungstate family with structural diversity and ultrahigh porosity for catalysis

“…Transition metal elements have superior properties including redox, photochemical, electrochemical, and magnetic properties due to partially occupied d -orbitals, which enable catalytic functions and applications that main group elements cannot achieve. With the combination of the unique properties of transition metals with ordered microporosity, major breakthroughs in applications have been achieved 6–8 . The classical methods to achieve this kind of combination is the direct modification of zeolites through ion-exchange 9 , modifying the framework disorderly 10 , and forming mixed tetrahedral and octahedral frameworks 11,12 .…”

Metalloprotein entatic control of ligand-metal bonds quantified by ultrafast x-ray spectroscopy

“…Iron-containing zeolites exhibit excellent reactivity in the hydroxylation of methane to methanol at the α-O site. This 5 Fe(IV)=O species, which is active toward hydrogen atom abstraction (HAA) from methane, is destabilized by ~5 kcal/mol because of lattice constraints, effectively lowering the activation barrier for HAA and greatly increasing reactivity (40). The experimental value presented here is consistent with these computational estimates.…”