Photocatalytic CO2 reutilisation to value-added products on metal-organic framework supported copper clusters

Atmospheric carbon dioxide (CO2) is one of the biggest contributors to climate change and as CO2 concentration in the atmosphere is constantly increasing, solutions to prevent CO2 emissions are urgently needed. Shifting to renewable, carbon neutral or negative technologies could prevent further growth of CO2 levels. However, these technologies are able to reduce only further CO2 emissions. Meanwhile, CO2 storage and conversion to value added- products are promising strategies to tackle the problem in the current situation. Nonetheless, conversion of CO2 to other chemicals instead of storing it would be needed to solve the issue.

Promising approaches for CO2 reutilisation to value-added products is the reduction to methanol or the addition reaction to another molecule to produce useful chemicals, such as amino or carboxylic acids. CO2 can be converted to value-added products by photoactivation. Photoactivation saves energy and resources as it can be performed in the presence of catalysts and by using light instead of thermal energy and therefore, it would be more plausible option in terms of sustainability. Homogenous photocatalytic CO2 reutilisation has been performed in synthesis of numerous products using metal cluster complexes; however, performing it in a heterogenous manner would have advantages in terms of catalyst separation, durability, and reusability. Copper is frequently used for CO2 reutilisation due to its' ability to activate CO2 molecules. Single atom catalysts (SACs) or clusters of few atoms could increase the reactivity of the metal centre anchored in heterogeneous supports, while improving the atom-economy of the processes. To the best of our knowledge, CO2 reutilisation for the synthesis of amino acids or carboxylic acids using heterogeneous photocatalysts based on SACs has not been extensively explored.

Proposed solution and methodology

This project aims to design of heterogenous photocatalysis for CO2 reutilisation. The focus is on the use of metal-organic frameworks (MOFs) as photoactive supports for Cu as catalytic centres.

MOFs are highly uniform and crystalline porous materials with an enormous surface area forming a framework structure between organic linkers and metal nodes. Consequently, MOFs offer a great platform for catalyst deposition and designing of tailored catalysts for various applications. Porphyrin and pyrene-based organic molecules have excellent light absorption properties in the visible light region and therefore they are applicable photosensitizers. These photosensitizers can be incorporated to MOF structures as the linkers allowing the photocatalytic processes to undergo in the heterogenous phase, facilitating catalysts recovery and extending their lifetime. Moreover, the photocatalytic properties of the MOFs could be further tuned by deposition of Cu on them by post-synthetic deposition techniques.

Cu is proven to activate CO2 molecules as well as it can be incorporated with photosensitizers allowing tuning of photocatalytic properties. By applying Cu SACs or clusters on the MOFs, it could be possible to enhance efficiency of the MOF-based photocatalysts. The rigid and stable structure and wide mesopores of MOFs allow multiple strategies for post-synthetic catalyst deposition, such as atomic layer deposition (ALD), wet impregnation and magnetron sputtering as a solvent-free technique. To obtain deeper understanding of structure-activity relationship in the catalysts, catalytic reactions will be explored in situ by X-ray photoelectron spectroscopy and ex situ by spectroscopic and microscopic techniques. Finally, computational studies will be carried out for understanding of the experimental results comparing with theoretical models.

This CDT will deliver impact aligned to the following agendas:

People
A2P will provide over 60 PhD graduates with the skill sets required to deliver innovative sustainable products and processes into the UK chemicals manufacturing industry. A2P will inspire and develop leaders who will:
- understand the needs of industrial end-users;
- embed sustainability across a range of sectors; and
- catalyse the transition to a more productive and resilient UK economy.

Economy
A2P will promote a step change towards a circular economy that embraces resilience and efficiency in terms of atoms and energy. The benefits of adopting more sustainable design principles and smarter production are clear. For example, the global production of active pharmaceutical ingredients (APIs) has been estimated at 65,000-100,000 tonnes per annum. The scale of associated waste is > 10 million tonnes per annum with a disposal cost of more than £15 billion. Consequently, even a modest efficiency increase by applying new, more sustainable chemical processes would deliver substantial economic savings and environmental wins. A2P will seek and deliver systematic gains across all sectors of the chemicals manufacturing industry. Our goals of providing cross-scale training in chemical sciences with economic and life- cycle awareness will drive uptake of sustainable best practice in UK industry, leading to improved economic competitiveness.

Knowledge
This CDT will deliver significant new knowledge in the development of more sustainable processes and products. It will integrate the philosophy of sustainability with catalysis, synthetic methodology, process engineering, and scale-up. Critical concepts such as energy/resource efficiency, life cycle analysis, recycling, and sustainability metrics will become seamlessly joined to what is considered a 'normal' approach to new molecular products. This knowledge and experience will be shared through publications, conferences and other engagement activities. A2P partners will provide efficient routes to market ensuring the efficient translation and transferal of new technologies is realised, ensuring impact is achieved.

Society
The chemistry-using industries manufacture a rich portfolio of products that are critical in maintaining a high quality of life in the UK. A2P will provide highly trained people and new knowledge to develop smarter, better products, whilst increasing the efficiency and sustainability of chemicals manufacture.
To amplify the impacts of our CDT, effective public engagement and technology transfer will become crucially important. As a general comment, 'sustainability' styled research is often regarded in a positive light by society, however, the science that underpins its effective implementation is often poorly appreciated. The University of Nottingham has developed an effective communication portfolio (with dedicated outreach staff) to tackle this issue. In addition to more traditional routes of scientific communication and dissemination, A2P will develop a portfolio of engagement and outreach activities including blogs, webpages, public outreach events, and contribution of material to our award-winning YouTube channel, www.periodicvideos.com.

A2P will build on our successful Sustainable Chemicals and Processes Industry Forum (SCIF), which will provide entry to networks with a wide range of chemical science end-users (spanning multinationals through to speciality SMEs), policy makers and regulators. We will share new scientific developments and best practice with leaders in these areas, to help realise the full impact of our CDT. Annual showcase events will provide a forum where knowledge may be disseminated to partners, we will broaden these events to include participants from thematically linked CDTs from across the UK, we will build on our track record of delivering hi-impact inter-CDT events with complementary centres hosted by the Universities of Bath and Bristol.