PET upcycling reveals fundamental catalysis stages

Earlier this year, Evgeny Pidko’s group at TU Delft published work on the practical and fundamental aspects of plastic recycling for aviation fuels (covered here). In an unplanned collaboration with the University of St Andrews, they have now aimed to upcycle PET. ‘Polymer chemists often say that it’s nearly impossible to upcycle in an economically viable way’, explains Pidko, Professor of Inorganic Systems Engineering. ‘But as we’ve been working with esters for many years, we had some experience in turning fatty esters into alcohols.’

As PET (polyethylene terephthalate) contains ester bonds, the researchers used hydrogenation to transform the plastic into alcohol. ‘The benefit of creating alcohols is that they cannot polymerise again’, Pidko continues. ‘However, this also means that an oxidation step would be required to recreate polymers, which would waste hydrogen.’ They therefore focused on increasing the turnover number (TON).

Suboptimal

However, after hydrogenation, they observed only partial conversion. Pidko explains, ‘The main constituent was a molecule called EHMB, which is half alcohol and half ester. My colleague, Dr. Kumar from St. Andrews, noticed that this is a precursor for agrochemicals and pharmaceuticals, specifically, an anticancer drug.’ This increases the value of the wate plastic instead of downgrading it for use as fuel. ‘A suboptimal catalyst has become an asset!’

The team performed a life cycle analysis, comparing their EHMB production process with the conventional process involving paratoluenic acid. Taking all environmental aspects into account, the new catalytic method comes out on top. ‘From a practical perspective, it’s a very nice illustration of how something that didn’t work well can inspire new, interesting avenues.’

Stages

As exciting as this discovery was, Pidko’s group found another equally fascinating fundamental aspect. ‘Homogeneous catalysis is often portrayed as precise: you know exactly what goes into the solution and what reacts’, says Pidko. ‘But more and more studies show that there is a cocktail of catalysts, resulting in fluctuating activity.’

Pidko recognises three stages that a catalyst goes through. ‘The initial activity is enormous but short-lived. Then the catalyst state changes, the activity drops and a slower process begins, during which most of the transformations occur. Ultimately, the reaction stops. So our question now is: which stage should you optimise? What influence do the different components have on the catalyst species?

To shed light on the reaction mechanism, the team used a special form of nuclear magnetic resonance (NMR): chemical exchange saturation transfer (CEST). ‘Instead of seeing static protons, you’re observing the reaction in the system’, explains Pidko. ‘It reveals the structures that are chemically active in the solution.’ CEST-NMR is usually used in biological systems, but here, the researchers used it to obtain information about the dynamically exchanging hydrogenated species. ‘Instead of sharp singlets and doublets, a fascinating forest of peaks emerges.’

Counterintuitive

According to Pidko, most people overlook the different catalytic phases because they assume that there is only one active site in a homogeneous system. ‘It is difficult to shift this mindset. Besides, if you don’t model the reaction correctly, you won’t see the phases. The data must be presented differently to reveal the various regimes and to understand the individual activity of the species formed in solution. It is the reaction conditions, rather than the design of the ligand-metal combination, that determine or change the regimes.’

Temperature seems to be an important factor in the kinetics. ‘All catalysts deactivate, but we found that increasing the temperature causes faster deactivation.’ This seems counterintuitive: the usual strategy to achieve higher activity and yields is to provide more energy to the catalytic system by raising the temperature. ‘At a small scale and with a high concentration of catalysts, this usually doesn’t manifest, but when you scale up and decrease the concentration of catalysts to a realistic level, the reaction often fails due to deactivation.’ Instead of yields, the focus should be on reaction kinetics and mechanism. ‘Catalysis is a marathon, not a sprint’, Pidko concludes.

Kulyabin, P.S., Luk, J. et al. (2025) Angew. Chem. Int. Ed. e21838, DOI: 10.1002/anie.202521838