Catalytic disconnection of C-O bonds in epoxy resins and composites

Abstract

Fibre-reinforced epoxy composites are well established in regard to load-bearing applications in the aerospace, automotive and wind power industries, owing to their light weight and high durability. These composites are based on thermoset resins embedding glass or carbon fibres¹. In lieu of viable recycling strategies, end-of-use composite-based structures such as wind turbine blades are commonly landfilled^1-4. Because of the negative environmental impact of plastic waste^5,6, the need for circular economies of plastics has become more pressing^7,8. However, recycling thermoset plastics is no trivial matter^1-4. Here we report a transition-metal-catalysed protocol for recovery of the polymer building block bisphenol A and intact fibres from epoxy composites. A Ru-catalysed, dehydrogenation/bond, cleavage/reduction cascade disconnects the C(alkyl)-O bonds of the most common linkages of the polymer. We showcase the application of this methodology to relevant unmodified amine-cured epoxy resins as well as commercial composites, including the shell of a wind turbine blade. Our results demonstrate that chemical recycling approaches for thermoset epoxy resins and composites are achievable.

Fig. 1. Targeted C–O bonds in thermoset epoxy resins and catalytic deconstruction of related model compounds.

a, Schematic illustration of a crosslinked epoxy resin matrix and molecular structures of linkage motifs. Blue circles represent linkage sections while black lines represent linear polymer sections. The C–O bonds adjacent to BPA (red) are targeted to deconstruct the polymer matrix. b, Optimized reaction conditions applied to different model substrates, considering linkage motifs and building blocks. Yields are given (in parentheses) for products isolated via column chromatography.

Fig. 2. Mechanistic considerations regarding Ru-catalysed C–O bond disconnection.

a, Ru-catalysed acceptorless dehydrogenation. b, Proposed catalytic cycle for disconnection of C–O bonds. BDEs were calculated with DFT at the (U)M06-2X/6-311++G(d,p) level of theory. c, Detection of acetone as the disconnection product. Yields were determined by ¹H NMR spectroscopy using 1,3,5-trimethoxybenzene as the internal standard. d, Kinetic profile of deconstruction of model 1. Operando monitoring experiment for C–O bond disconnection on model 1. Molecular structure of triphos-Ru-H₂-CO in the crystal (CCDC 2219777).

Fig. 3. Catalytic deconstruction of epoxy resins.

Scope of epoxy resins deconstructed using catalytic conditions. Experiments were set up under an argon atmosphere. Yields were determined after isolation of products via column chromatography.

Fig. 4. Recovery of BPA and fibres from commercial epoxy composites using Ru catalysis.

a, Scope of the composite samples subjected to catalysis. Composite pieces 1, 2 and 3 were 1.0–1.5 cm in both length and width. b, Upscaling of deconstruction conditions on wind turbine blade.

Fig. 5. Characterization of commercial fibre-reinforced composites and glass fibres.

a, X-ray µ-CT with virtual slices through reconstructed image stacks showing fibre cross-sections. Scale bars, 100 µm. b, 3D renderings of reconstructed image stacks showing fibre organization; grey levels corresponding to air have been rendered transparent; for scale refer to the two-dimensional slices in a. c, Histograms of fibre diameter obtained by analysis of X-ray μ-CT data. d, XPS C 1s high-resolution spectra of neat and recovered fibres. e–h, SEM images of neat (e,f) and recovered fibres (g,h). Scale bars, 50 µm (e,g); 2 µm (f,h). AU, arbitrary units; BE, binding energy.