Force-activated organocatalysts for plastic recycling

Synthetic polymers are used in almost every aspect of modern life. For this reason, they also present an important environmental challenge. Although some polymers can be easily recycled (e.g.), required costly and/or challenging processes. This is especially true if the plastic is composed of mixed polymers and/or if the recovery of the monomer is targeted. This step could be facilitated if plastic where embedded with an additive that could accelerate their degradation at the end of their life. This goal could be achieved by implementing mechanoresponsive molecules, called mechanophores, that react upon application of a force to generate a useful chemical entity. For example, a force-activated catalyst could be used to facilitate depolymerisation. In this project we will produce new mechanocatalysts based on varied NHC-precursors, and investigate their mechanocatalytic properties in solution, using ultrasound and in the solid state. Ultimately, we aim to produce a mechanocatalyst able to increase the degradability/depolymerisation of challenging commodity polymer (e.g. PET, PU) in recycling/processing conditions such as : extrusion or batch hydrolysis/aminolysis.
Only a handful of mechanocatalysts have been reported but they are all metal-based, which make them expensive and difficult to process due to their low thermal and mechanical stability. The ideal mechanocatalyst is only activated when needed (and not for example during the production or the normal usage of the product). In other words, it must display a minimal sensitivity to a change in pH, temperature and humidity, but respond to a desired level of force. We have recently introduced a thermally-stable and environment-friendly alternative, based on a force-activated N-heterocyclic carbene (NHC) precursor (Nat. Chem. 2020, 12, 826; JACS 2021, 143, 3033).
Recent heterogeneous catalysis work in CEAS has demonstrated that a number of pure polyolefin feedstocks (polyethylene, polypropylene and polystyrene) and blends of these three can be successfully hydrocracked rapidly at much reduced temperatures yielding a predominantly C3 - C9 hydrocarbons (Ind. Eng. Chem. Res., 58 (45), 20601; EP Patent 2437886B1 -2019). Further work on the condensation polymer, polyethylene terephthalate (PET) using hydrolysis has led to conversions of 80% with around 80% selectivity to terephthalic acid at 220oC in 90 minutes. This project will extend this current work and will target the production of a mechanocatalyst able to increase the depolymerisation of challenging commodity polymer [e.g. PET, polyurethane (PUR)] in recycling/processing conditions such as, extrusion or batch hydrolysis/hydrocracking.
Objective 1: Catalyst design and optimisation of the mechanochemical and catalytic properties in solution
Objective 2: Exploration of the mechanochemical and catalytic properties in the solid state/melt
Objective 3: Investigation of various modes of activation in batch reactor (e.g. turbulent flow, vapor-induced elongational flow)
Objective 4: Optimisation of the mechanochemical and catalytic properties in hydrolysis/hydrocracking conditions for PET and PU.
With over 4.0 Mte of plastic waste being sent to landfill or incinerated annually in the UK (8 % is PET and a similar amount of PUR, PlasticsEurope, The Facts 2020), the development of integrated catalytic approaches for plastic recycling would allow us to make a significant and lasting contribution to this important area. The ability to control the lifecycle of polymers would have a dramatic impact on the flux of plastic waste (Nature 2016, 540, 363). Embedded molecular mechanisms act to prolong the life of high-performance polymers. The mechanocatalysts we propose to investigate could be used to implement self-degrading mechanisms in PET, PU, or polyamides to facilitate their degradation/depolymerisation at the end of their lifecycle.

iCAT will work with industry partners to create an holistic approach to the training of students in biocatalysis, chemocatalysis, and their process integration. Traditional graduate training typically focuses on one aspect of catalysis and this approach can severely restrict innovation and impact. Advances in technology and fundamental reaction discovery are rendering this silo-approach obsolete, and a new training modality is needed to produce the next generation of chemists and engineers who can operate across a far broader chemical continuum. iCAT will meet this challenge with a state-of-the-art CDT, equipping the next generation of scientists and engineers with the skills needed to develop future catalytic processes and create the functional molecules of tomorrow.

The UK has one of the world's top-performing chemical industries, achieving outstanding levels of growth, exports, productivity and international investment. The UK's chemical industry is a significant provider of jobs and creator of wealth, with a turnover in excess of £50 billion and a contribution of over £15 Billion of value to the UK economy [2015 figures]. iCAT will deliver highly skilled people to lead this industry across its various sectors, achieving impact through the following actions:

1. Equip the next generation of science and engineering leaders with the interdisciplinary skills and knowledge needed to work across the bio and chemo catalytic remit and build the functional molecules we need to structure society.

2. Provide a highly skilled workforce and research base, skilled in the latest methodologies, strategies and techniques of catalysis and engineering that is crucial for the UK's Chemical Industry.

3. Build the critical mass necessary to support effective cohort-based training in a world-class research environment.

4. Develop and disseminate new catalytic technologies and processes that will be taken up by industrial and academic teams around the world.

5. Encourage Industry to promote research challenges within the CDT that are of core relevance to their business.

6. Provide cohesion in the integration of biocatalysis, engineering and chemocatalysis to create a more unified voice for strategic dialogue with industry, funders and policy makers, and more generally outreach and public engagement.

7. Draw-in and bring together Industrial partners to facilitate future Industrial collaborations.

8. Benefit Industrial scientists through interactions with the CDT (e.g. training and supervisory experience, exposure to cutting-edge synthesis and catalysis etc).

9. Link with other activities in the landscape: bringing unique expertise in catalysis to, for example, externally-funded University-led initiatives, EPRSC Grand Challenge Networks, and the National Catalysis Hub.