Circular Chemistry; the enabler to help solve global challenges

Rowena Fletcher-Wood  
Freelance
rowena.fletcherwood@gmail.com

Helena Rapp Wright 
Imperial College London
​h.rapp-wright@imperial.ac.uk

Laura Alcock 
Edwards Ltd
​laura.alcock@edwardsvacuum.com

ECG Bulletin January 2023

This event, jointly organised by the Environmental Chemistry Group and Applied Materials Chemistry Group (AMCG), and sponsored by the Department for Environment, Food, and Rural Affairs (DEFRA), attracted 30 delegates and ran as a hybrid event on Zoom and in Burlington House on 30th November. 

The meeting began with an introduction to the ECG, and Dr Rowena Fletcher-Wood chaired the opening session, introducing Dr Chris Slootweg (University of Amsterdam), with ‘CHNOPS’: from the origin of life to the origin of waste – the principles of circular chemistry. These key elements, needed in large quantities to make living organisms, also form our key waste streams: whilst the 12 Green Chemistry guiding principles (1) are important, they optimise linear processes, not circular. With two students
who gave the best responses to his challenge “define the principles for circular chemistry”, Dr Slootweg published new principles (2). His approach focuses on circularity, functionality, and safety, leading to a more sustainable future. Sodium borohydride is under investigation as a potential source of hydrogen storage. The solid is relatively easy to transport, and on (catalytic) hydrolysis releases four mole hydrogen per mole sodium borohydride. The problem of recycling the hydrolysis products is an obstacle at present for its application

Figure. credit: Dr Slootweg

Inspired by Hennig Brand, discoverer of phosphorus from urine, Dr Slootweg is also exploring natural wastestreams as sources for phosphate and ammonia via the mineral struvite – an alternative to mining. Struvite (NH₄MgPO₄·6H₂O) has shown promising results in removing and recovering metals such as Cu from wastewater effluents, reducing environmental risks.

Professor Matthew Jones (University of Bath) spoke next on Catalytic Upgrading of Polymers – is Chemical Recycling the Answer? Lightweight, durable, and flexible​ materials, plastics were originally hailed as an environmental solution – and they still can be, as a renewable feedstock. Professor Jones’ work concentrates on keeping carbon in its highest value form (most reactive) to enable chemical recycling of polymers. This can take place through depolymerisation (into original monomers) or degradation (into other useful monomers such as the breakdown of PLA to lactate esters, rather than lactic acid). Professor Jones identified polycarbonates as a key area of opportunity for metal-based, sustainable solutions and plastics diversification (3, 4). Using simple ligands, he demonstrated the possibility of compostable, renewable and biocompatible polymers made from coke bottles, processed through aminolysis, glycolysis, and methanolysis. However, the need for smarter and simpler design, including preclusion of additives like plasticisers, fillers, and pigments, was also highlighted. 

Inspired by geological timescales, Professor Colin Hills (University of Greenwich) next took us on a journey into Using CO₂ as a raw material: CO₂ used in the treatment of contaminated soils and waste to produce building aggregates. During the thermal maximum of the Cretaceous period, CO₂ was absorbed by the oceans, transformed into carbonate skeletons of microfauna and deposited as carbon ooze on the seabed, where it later underwent diagenesis to limestone. Professor Hills’ work now aims to establish industrially-managed pathways to make similar carbonates on anthropogenic timescales. Using worldwide samples of industrial waste that naturally contain high amounts of Ca and Mg oxides, hydroxides and silicates that react with CO₂ to form carbonates, Professor Hills used two routes, wet and semi-dry (or thin film) to create construction aggregates. These additionally reduce pH and stabilise potentially problematic heavy metals including Zn, Pb, and Ni. Professor Hills highlighted that public perception of waste safety, incentives such as landfill tax and legal obligations, were essential for uptake of this technology.

Session 2, chaired by Alan Armour (AMCG) began with Professor Alex Cowan (University of Liverpool), presenting Electrocatalytic reduction of carbon dioxide for a circular chemical economy. He described the UKRI Interdisciplinary Centre for Circular Chemical Economy’s aims to create a shift towards a fossil-independent climate positive and environmentally friendly circular chemistry economy, considering not only the technical, but also policy, society, and finance aspects of enabling technologies. In particular, he focussed on advancements in CO₂ electrolysis, using the main waste product from fossil fuel combustion, and changing operating conditions from high pH (unsuitable for carbonate regeneration) to low pH, using the catalysts Ni(cyclam)²⁺, Mn(bpy)(CO)₃Br and derivatives (5). Whilst the 50% efficiencies achieved may not seem impressive, this is novel under these conditions. Professor Cowan also solved the problem of manganese catalyst poisoning by its products, a two-step process with reversible first step, initiated by rapid cycling. However, he did identify these conditions need to be carefully controlled to prevent dominance of hydrogen generation over hydrocarbons. Next, he is exploring impurity tolerance.

Professor Leon Black (University of Leeds/UKCRIC), presented The use of industrial by-products (PFA and GGBS) in concrete. Professor Black began his talk by highlighting the fact that concrete is the second most widely used resource in the world (only after water), and cement production is responsible for 6-8% of global CO₂ emissions. During the production of a single tonne of the material, 880 kg CO₂e is released. There are several misconceptions around the hydration of cement, with few realising that a series of chemical reactions takes place as cement particles hydrate and dissolve. This is crucial because curing time, or prolonged hydration, affects microstructure. In his design of sustainable, low carbon concretes, and assessment of lifetime performance, Professor Black discovered that adding ground granulated blastfurnace slag (GGBS) to cement creates additional C-S-H bonds, improving durability. 35% calcined clay with 15% limestone, (both widely available), could reduce CO₂e by 250 million tonnes (whereas scale is limited by 350 million tonnes of worldwide available GGBS). Simultaneous thermal analysis showed that longer curing with ambient CO₂ provided more Portland-cement like material with intact microstructures. Construction is a conservative industry, and these findings highlighted the need for good site practice to ensure quality materials and achieve lower CO₂ emission targets. Working with Nigerian partners, Professor Black also identified the need to explore other curing temperatures in developing standards. At higher temperatures, high densification of hydrate was observed (greater porosity) – implying that standards of durability need to be local – British standards are not always relevant.

Professor Peter Edwards (University of Oxford) described two projects: converting CO₂ into aviation fuel and the catalytic deconstruction of plastics into hydrogen and carbon nanotubes. Current catalysts are still being refined and optimised for sustainable aviation fuel, along with the separation of high value by-products including light alkanes, light olefins, water, and other liquid hydrocarbons. Professor Edwards shared a graphic which showed the human development index of various nations correlated with oil consumption per capita.

The other project involved a change in paradigm: plastics have a high gravimetric density (weight %) of H₂. Using this, Professor Edwards proposed catalysed, microwave driven plastic waste destruction, producing hydrogen fuel, and demonstrated that effective microwave penetration of materials provided rapid extraction: 20 s is sufficient time to decompose 0.3 g of wax, crude oil, or plastics, and release hydrogen. They are still exploring the mechanism.

Session 3, chaired by Katie Hobson (DEFRA), began with an introduction to DEFRA, and then Sandra Averous Monnery (Knowledge and Risks Unit, UN Environment Programme, Switzerland) presented online, outlining the UNEP Framework Manual on Green and sustainable chemistry (6). This freely-available framework provides guidance in a variety of languages for a holistic, integrated approach to chemical innovation, including new molecules and compounds, but also opportunity for reducing resource use, conversion technologies, and some technical and policy aspects outlined in other talks today. The aim is to find a solution not just for pollution in general but to enhance the triple planetary crisis: climate action nature ​action, and chemicals and pollution action. “Business as usual” is not an option with chemicals and waste. Ms Averous-Monnery outlined 10 ways to promote innovation and unveil the full potential for chemistry to support the implementation of the 2030 Sustainable Development agenda. The unit also facilitates conversation between chemists and the sustainable building and construction sector, encouraging research. 

Aptly, our next speaker, Dr Katherine Adams (Alliance for Sustainable Building Products), discussed Circular economy in construction – what’s it got to do with chemistry? Her analysis of material opportunities for circularity in construction showed that although the UK performs well in terms of recycling, most material is downcycled, losing value. This is aggravated by a doubling global population – tripling materials extraction. Reuse could also be increased: in new building, embodied carbon content may be reduced using reused, recycled, and fewer materials, and replacing materials with lower carbon alternatives. New regulations have catalysed chemical analysis of materials by requiring demolition wastes to prove that they are not hazardous (otherwise they must be treated as hazardous), such as treated wood which will need to be incinerated. Other interesting findings have been the huge extent of microleakage from wear and tear of microplastic-loaded paints. Other factors, such as keeping buildings in use for as long as possible, since “the most efficient building is one that is already built”, and designing for deconstructing (e.g. via modularity) rather than demolishing at end of life, are crucial for adaptability. Dr Adams highlighted some materials that are particularly problematic as demolition wastes: structural insulated panels (SIPs), introduced as an energy-saving, easy construction material, cannot be recycled, are incredibly difficult to separate, and the insulation material is often a hazardous waste. Carpet underlay is equally challenging, providing an interesting insight into how front-end work seldom accommodates back-end recovery of materials

Our final speaker, Dr Richard Sheridan (University of Birmingham), talked on Recovery/Recycling of Critical Raw Materials. Source minerals contain a mixture of rare earth materials. Many of these are desirable with applications such as making magnets for media storage. As we get more creative, more of the mineral is usable, but some of the more common rare earth materials cannot be used – and are stockpiled. It was highlighted that the primary applications of these materials is in renewable energy resources and electric vehicle components – particularly within wind turbines and traction motors, which use highly powerful magnets to generate electricity, either for the local infrastructure or the vehicle in which they are installed. Meanwhile, consumer demands such as higher power and faster cars drives the need for bigger​ and stronger magnets, and the creation of products such as mobile phones lock up critical raw materials that are challenging to recycle. 

​One major sticking point is the presence of epoxy resins, which are found ubiquitously across devices containing critical raw materials. Dr Sheridan described his work using hydrogen for decrepitation and linking various stages of the processing route to reclaiming magnetic source materials. However, many challenges arise from rare earth magnets such as economy, and the small supply chain in the European Union. Overcoming corroded magnetic materials, and minimising the energy demands of the process are hurdles for scalability, and separation of magnets from traction motors and computer hard drives for recycling was particularly challenging. Dr Andrew Dunster (AMCG) provided the overview of the AMCG and closing remarks. 

References
1. Anastas, P. T., & Warner, J. C. Green Chemistry: Theory and Practice, Oxford University Press: New York, 1998.
2. Keijer, T., Bakker, V., & Slootweg, J. C. (2019). Circular chemistry to enable a circular economy. Nature Chemistry, 11(3), 190-195. https://doi.org/10.1038/ s41557-019-0226-9
3. Payne, J., & Jones, M. D. (2021). The chemical recycling of polyesters for a circular plastics economy: challenges and emerging opportunities. ChemSusChem, 14(19), 4041-4070. https://doi.org/10.1002/cssc.202100400  
4. Román-Ramírez, L. A. et al. (2018). Poly (lactic acid) degradation into methyl lactate catalyzed by a well defined Zn (II) complex. ACS Catalysis, 9(1), 409-416 . https://doi.org/10.1021/acscatal.8b04863
5. Siritanaratkul, B. et al. (2022). Zero-gap bipolar membrane electrolyzer for carbon dioxide reduction using acid-tolerant molecular electrocatalysts. Journal of the American Chemical Society, 144(17), 7551-7556. https://doi.org/10.1021/jacs.1c13024
6. United Nations Environment Programme (2021). Green and Sustainable Chemistry: Framework Manual. https://wedocs.unep.org/20.500.11822/34338