PET Upcycling Reveals Fundamental Catalysis Stages

In early 2025, the group of Evgeny Pidko at TU Delft published an article on the practical and fundamental aspects of plastic recycling for aviation fuels (see here). In an unplanned collaboration with the University of St Andrews, they have now turned their attention to the upcycling of PET. ‘Polymer chemists often say that it is almost impossible to upcycle in an economically viable way,’ explains Pidko, Professor of Inorganic Systems Engineering. ‘But because we have been working with esters for many years, we had some experience in converting fatty acids into alcohols.’

Since PET (polyethylene terephthalate) contains ester bonds, the researchers used hydrogenation to convert the plastic into alcohol. ‘The advantage of making alcohols is that they cannot repolymerise,’ Pidko continues. ‘But that also means that to recreate the polymers you would have to carry out an oxidation step, which would waste hydrogen.’ They therefore focused on increasing the turnover number (TON).

Suboptimal

After hydrogenation, however, they observed only partial conversion. Pidko explains: ‘The principal component was a molecule called EHMB, one half of which contains an alcohol group and the other an ester. My colleague Dr Kumar from St Andrews noted that this is a precursor for agrochemicals and pharmaceutical products, in particular an anti-cancer drug.’ Rather than degrading it for use as a fuel, this actually increases the value of the waste plastic. ‘A suboptimal catalyst has become an asset!’

The team conducted a life cycle analysis, comparing their EHMB production process with the conventional process starting from para-toluic acid. When all environmental aspects are taken into account, the new catalytic method comes out on top. ‘From a practical standpoint, it is a fine illustration of how something that did not work well can lead to new and interesting avenues.’

Phases

Exciting as this discovery was, Pidko’s group found yet 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 a growing number of studies show that you are dealing with a cocktail of catalysts, resulting in fluctuating activity.’

Pidko distinguishes three phases through which a catalyst passes. ‘The initial activity is enormous, but short-lived. The catalyst then changes state, activity decreases, and a slower process begins, during which most of the transformations take place. Eventually, the reaction stops. Our question now is therefore: which phase should you optimise? What influence do the various components have on the catalyst species?’

To shed light on the reaction mechanism, the team employed a special form of NMR: chemical exchange saturation transfer (CEST). ‘Rather than observing static protons, you observe the reaction within the system,’ explains Pidko. ‘It reveals the structures that are chemically active in solution.’ CEST-NMR is most commonly used in biological systems, but here the researchers applied 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 there is only one active site in a homogeneous system. ‘It is difficult to change this way of thinking. Moreover, if you do not model the reaction correctly, you will not see the phases. The data must be presented differently to reveal the distinct regimes and to understand the individual activity of the species formed in solution. It is the reaction conditions, and not the design of the ligand–metal combination, that determine or alter the regimes.’

Temperature appears to be an important factor in the kinetics. ‘All catalysts are deactivated, but we have found that a higher temperature leads to faster deactivation.’ This seems counterintuitive: the usual strategy for achieving higher activity and yield is to add more energy to the catalytic system by raising the temperature. ‘On a small scale and at high catalyst concentrations this is generally not noticeable, but when you scale up and reduce the catalyst concentration to a realistic level, the reaction frequently fails as a result of deactivation.’ The focus should be on reaction kinetics and the reaction mechanism rather than on yields. ‘Catalysis is a marathon, not a sprint,’ concludes Pidko.

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