Electrochemical Oxidation of Low Molecular Weight Alkanes to Liquid Fuels at Molecular Interfaces

The partial oxidation of low molecular weight alkanes (LMWA), to their corresponding alcohols, at low temperature and pressure is considered to be one of the grand challenges in the area of catalysis and energy. Large scale exploitation of methane involves energy intensive processing, such as steam and carbon dioxide reforming, as well as liquefaction for transport from the extraction field. Considering the vast amount of natural gas distributed around the world, including the UK, in addition to local methane produced by feedstock, there is a compelling case to develop low cost/energy catalytic conversion of methane to easily transportable liquid fuels (rather than liquefaction). In this context, electrochemical methods provide an extremely attractive approach to the partial oxidation of methane, not only in terms of scalability, but also due to the inherent low carbon footprint of such technology. In principle, the required overpotential for the interfacial process can be generated by (i) direct photoexcitation of the catalyst or by (ii) coupling electrochemical reactors to photovoltaic devices. However, to date, no viable electrochemical method has been designed for alkane oxidation.
This project departs from all conventional catalytic approaches, in order to combine elements of heterogeneous catalysis, photocatalysis and nanoscale electrocatalysis. The complex oxidation of methane, the key LMWA in this project, will be activated at the interface between an aqueous and an immiscible organic solution, at which the Galvani potential difference can be tuned externally. The rationale behind this approach is to maximise the reaction cross-section between high performing nanostructured electrocatalysts, LMWA accumulated in the organic phase and water as a source of OH radicals. The interfacial potential difference can play an important role, not only in changing the driving force for the oxidation, but also in the assembly of highly reactive catalytic centres supported on oxides and carbon based-nanostructures. Colloidal oxide supports such as TiO2 can play multiple roles in the process, including: promoting the interaction between surface OH groups and the active centres, generating highly active OH radicals upon UV-illumination and avoiding irreversible aggregation/coagulation of the metallic active centres. In the case of carbon nanotubes and graphene, these supports will also enhance the stability of the electrocatalytically active nanocentres, as well as extracting electrons accumulated at the nanostructures during methane oxidation. The dynamics of the interfacial processes will be monitored by electrochemical and photoelectrochemical techniques, under potentiostatic control of the liquid/liquid interface. Furthermore, the generation of products and intermediates will be investigated by a variety of in-situ and ex-situ techniques such as Raman spectroscopy and chromatographic methods. Methane is a key target due to its natural abundance, but is recognised to be a particular challenge due to its low reactivity. Consequently the approach will also be broadened to span the electro-oxidation of other LMWA such as ethane, propane and butane. The project will focus on two key goals:
i. Establishing the physical principles underlying the electrochemical / photoelectrochemical oxidation of methane and other LMWA to the corresponding alcohols at polarisable liquid/liquid junctions
ii. Novel approaches and catalysts for multi-electron transfer reactions of relevance to the energy sector at molecular interfaces.

The selective partial oxidation of low molecular weight alkanes (LMWA) to their corresponding alcohols at low temperatures is considered as one of the grand challenges in the field of catalysis. Electrochemical approaches provide a scalable technology which can be operated with significantly lower energy input, and lower costs, than current technologies based on natural gas reforming. The key aspect of this project is the oxygenation of volatile LMWA, without using oxygen gas as reactant, at polarisable interfaces between two immiscible electrolyte solutions. Water will act as source of oxygenated species, significantly reducing risks associated with current technologies based on conventional heterogeneous catalysis. Although the goal of the project is to rationalise the fundamental aspects associated with the partial oxidation of methane at liquid/liquid interfaces incorporating nanostructured catalysts, important indicators linked to scalability will be investigated throughout the programme. For instance, issues such as the effect of interfacial potential, composition of the electrocatalysts at the interface, selectivity and transformation rate (turnover) are crucial in evaluating the scalability of this approach. This fundamental knowledge lies at the core of catalysis for energy, an area designated as important for the UK industry in the recent EPSRC Shaping Capabilities review.
Specific benefits associated with this strategy include:
1- Low energy input for the conversion of LMWA to liquid fuels
2- Scalable to small and medium natural gas reserves, including bio-methane
3- Compatible with local energy generation systems such as low temperature fuel cell technology
The electrochemical conversion of LMWA can bring significant benefits to the energy sector as a whole as it provides a viable mean of natural gas exploitation regardless of the size of the reserve. This is particularly important in the exploitation of bio-methane, a key renewable gas source. In view of the current emphasis on micro-scale generation of power and security energy supply, there is a strong case for the development of low-energy intensive technologies for processing gas to liquid fuels.
We stress that the work is "blue skies", exploratory research, but the outputs it will spawn should bring significant benefits for the UK academic and energy sectors, as discussed in more detail in the "Pathways to Impact" section.