The consumption of non-renewable resources and the accumulation and proliferation of waste are the two major challenges in transitioning the production and use of plastic-based materials towards a sustainable economy. An important application of epoxy polymers is found in fibre reinforced composites, which are key components of wind turbine blades, aircrafts, racing bicycles and similar constructs that require high durability and light weight. The flipside of the durability is the lack of disassembly technologies for such materials at their end-of-use. Mechanical recycling strategies cannot be applied to crosslinked thermoset polymers, such as epoxy resins, which makes reducing waste and recovering value especially challenging for these materials. The demand for clean and renewable energy is rising, and as such the market for wind energy is growing. In turn, from wind turbines blades alone 43 million metric tons of non-processable composite waste are predicted to accumulate by 2050.
With a background in organic synthesis and transition metal catalysis, our group set out in 2021 to explore whether selective catalytic transformations could depolymerise state-of-the-art epoxy composites. The aim was to recover both the fibres and polymer building blocks that in theory could be reused to make virgin-grade polymers. In 2023, we published our first paper on this topic, demonstrating that a triphos-Ru catalyst in toluene and isopropanol at elevated temperatures can indeed disassemble a piece of a wind turbine blade, recovering bisphenol A, a chemical produced on a scale of 10 million metric tons per year. Furthermore, the glass fibres could be recovered in high quality1. While this proof-of-principle discovery demonstrates the ability to take these highly engineered materials and perform complex catalytical transformations, the perspective for application at scale is hindered by long reaction times and high catalyst loadings.
Catalyst systems utlising triphos-Ru are well established for a magnitude of hydrogenation and dehydrogenation-based transformations. However, for the C–O bond hydrogenolysis at play in this particular case, the molecular mechanism was not known. Therefore, we set out to study the catalyst activation and catalytic cycle in detail. Utilising experimental, spectroscopical and theoretical studies in collaboration with Ainara Nova from the University of Oslo. We identified a dual role of the additive, isopropanol, which both initiates the activation of the precatalyst and acts as stochiometric reductant. Furthermore, we provided evidence for the presence of ruthenium hydrides and phenolates as key intermediates, and gauged the energy profiles of the catalyst activation and catalytic cycle.2
With this in-detail understanding of the reaction mechanism at hand, we are now ready to go forward with efforts to improve the original catalytic system with the goal of advancing the perspective of applicability of this disassembly process for epoxy resins and composites.
1 Nature 2023, 617, 730-737.
2 Nat. Comm. 2024, 15, 5656.