An effective zeolite catalyst for partial methane oxidation

International research team establishes copper trimer as active intermediate

29 June 2015

An international research team with Dr Moniek Tromp of the Van 't Hoff Institute for Molecular Sciences is developing a new bio-inspired zeolite catalyst for the selective oxidation of methane to methanol. In Nature Communications the researchers reveal a copper trimer as active intermediate in the reaction mechanism. They expect the new zeolite to pave the way for a small scale 'gas-to-liquid' method for the conversion of natural gas to fuels and starting materials for the chemical industry.

In an era of depleting energy sources the production of natural gas is becoming ever more relevant, even though the gas is difficult to transport and not easily integrated in the existing industrial infrastructure. One of the solutions for this is to apply 'gas-to-liquid' technologies. These convert methane, the principal component of natural gas, to so-called synthesis gas from which subsequently methanol and hydrocarbons are produced. These liquids are then shipped to chemical plants or fuel companies all over the world.

This approach, however, is only feasible at very large scales. Currently there is no 'gas-to-liquid' chemistry available for the economical processing of methane from smaller sources at remote locations. This has spawned many research efforts regarding the chemistry of methane conversion.

Dr. M. (Moniek) Tromp

Dr Moniek Tromp

Of all the conceptually promising smaller scale processes for the direct conversion of methane, the partial oxidation to methanol seems the most viable since it allows for lower operating temperatures, making it more inherently safe and more energy efficient.

Bio-inspired

An international research team is currently focusing on a bio-inspired method enabling such partial methane oxidation. It combines the expertise of Moniek Tromp (UvA, Van 't Hoff Institute for Molecular Sciences), Evgeny Pidko and Emiel Hensen (Eindhoven University of Technology), Maricruz Sanchez-Sanchez (Technische Universität München) and Johannes Lercher (Technische Universität München and Pacific Northwest National Laboratory, USA) 

At the focus of the team is a modified zeolite, a highly structured porous material, developed at the Lercher research group. This so-called 'copper-exchanged zeolites with mordenite structure' mimics the reactivity of an enzyme known to efficiently and selectively oxidize methane to methanol.

In the recent publication in Nature Communications the researchers provide an unprecedented and detailed molecular insight in the way their zeolite mimic the active site of the enzyme particulate methane monooxygenase (MMO).

Highly selective

They show that the micropores of the zeolite provide a perfect confined environment for the highly selective stabilization of an intermediate copper-containing trimer molecule. This result follows from the combination of kinetic studies in Munich, advanced spectroscopic analysis in Amsterdam and theoretical modeling in Eindhoven. Trinuclear copper-oxo clusters were identified that exhibit a high reactivity towards activation of carbon–hydrogen bonds in methane and its subsequent transformation to methanol.

Trinuclear copper-oxo cluster

Structure and location of the trimere intermediate following from density functional theory (DFT) calculations. At left its position in the zeolite surroundings is shown. At right the trimere and its interaction with the zeolite is shown more in detail. Ilustration from the Nature Communications publication.

According to the researchers the developed zeolite is one of the few examples of a catalyst with well-defined active sites evenly distributed in the zeolite framework - a truly single-site heterogeneous catalyst. This allows for much higher efficiencies in conversion of methane to methanol than with zeolite catalysts previously reported.

Furthermore, it enables the unequivocal linking of the structure of the active sites with their catalytic activity. This renders the zeolite a "more than promising" material in achieving levels of catalytic activity and selectivity comparable to enzymatic systems.

Publication

Sebastian Grundner, Monica A.C. Markovits, Guanna Li, Moniek Tromp, Evgeny A. Pidko, Emiel J.M. Hensen, Andreas Jentys, Maricruz Sanchez-Sanchez & Johannes A. Lercher:  Single-site trinuclear copper oxygen clusters in mordenite for selective conversion of methane to methanol Nature Communications, published online 25 June 2015. DOI: 10.1038/ncomms8546

Published by  HIMS