A straightforward hydrogenation process can transform difficult-to-recycle rubber waste into valuable upcycled materials. Functioning independently, two teams from Denmark and the UK utilized the nitrogen functionality in nitrile-butadiene rubber (NBR) to alter the chemical-resistant polymer into an absorbent carbon capture material or flexible polyol chain. Initial studies on actual post-consumer plastics – including disposable gloves – resulted in promising yields of the upcycled polymers, and both teams are now aiming to enhance their processes.
Nitrile rubber is an essential component in any chemistry laboratory. A copolymer made up of acrylonitrile and butadiene, it is flexible, waterproof, and, most importantly, chemically resistant, making it the preferred material for disposable lab gloves – over 800,000 tonnes are manufactured each year. However, these same characteristics render them nearly impossible to recycle. Significant crosslinking between the polymer chains – referred to as vulcanisation – hinders mechanical recycling through melting and reshaping. Additionally, the inert nature of the polymer chain makes chemical degradation difficult, resulting in around 99% of nitrile rubber eventually being sent to landfills.
Nonetheless, slight alterations to the currently existing polymer structure can change these properties, and two research teams have now created complementary methods to valorise this waste plastic.
Activating latent chemistry
For Troels Skrydstrup and his group at Aarhus University, the ultimate objective was to retransform this troublesome polymer into a carbon capture material. Amines are a typical feature in solid CO₂ sorbents, trapping the gas through a straightforward nucleophilic reaction to produce a carbonate. ‘Nitrile-butadiene rubber contains nitrogen that we would describe as dormant,’ clarifies Simon Stampe Kildahl, a lead author on the project. ‘However, following a hydrogenation reaction – our activation process – this nitrogen becomes active, prepared to capture the CO₂.’
With this shift from nitrile to amine envisioned, the group examined hydrogenation conditions, heating dissolved rubber stock with a commercial ruthenium catalyst. The optimized reaction successfully converted nearly 90% of the acrylonitrile units into polyamine. But, since carbon capture efficacy relies on the amount of nitrogen present in the sorbent material, the team contemplated whether they could initially enhance the nitrogen content of their rubber substrates.
Employing nickel-catalyzed hydrocyanation conditions, they added nitrile groups to a portion of the butadiene units, subsequently repeating the reaction on another rubber substrate, styrene-butadiene-styrene (SBS).
After establishing a sequence, Skrydstrup’s team then applied the procedure to commercial products, including nitrile rubber gloves and SBS rubber shoe soles. Overall, the reaction proved tolerant of consumer plastics without issue. ‘It’s remarkable how effectively it works given that it’s a vulcanized material,’ comments Clemens Kaussler, another lead author.
With polyamine in hand, Skrydstrup’s group redirected their focus to the adsorbent characteristics of the material. The upcycled rubber exhibited a promising affinity for CO₂, and the polyamine’s adsorption capacity rose with temperature, reaching a peak at 90°C. This contrasts with traditional carbon dioxide sorbents such as the metal–organic framework CALF-20, which functions most effectively at ambient temperatures, potentially positioning the new material for high-temperature uses like flue gas filtration, according to Kaussler.
They speculate that this surprising effect arises from the polyamine’s distinctive storage mechanism. ‘We can only theorize, but we have some signs indicating it is a membrane-like mechanism,’ states Stampe Kildahl. ‘Once exceeding a specific temperature threshold, both adsorption and desorption can occur. The CO₂ travels like a bucket brigade from one amine to another.’
The potential for circularity inherent in this recycling approach particularly attracts polymer chemist Marta Ximenis from Polymat in Spain. ‘I view it as a highly elegant method, demonstrating that effective catalyst design can enable upcycling through modification,’ she remarks. ‘This is promising for applications where it is crucial to valorise or utilize the CO₂ rather than bury it, such as producing new materials like polycarbonates or even methanol.’
In the short run, the team is focused on establishing a scalable catalyst system and enhancing the reaction’s tolerance toward mixed waste streams but believes the material could eventually be configured for flue-gas capture applications.
Overcoming the crosslinks
Conversely, for Amit Kumar at the University of St Andrews, the emphasis was on investigating the potential of nitrile rubber as a feedstock. The group initially optimized hydrogenation conditions to convert nitriles to amines, achieving a quantitative yield using a commercial ruthenium catalyst. Incorporating water into the solvent mix and readjusting the reaction provided a similarly efficient pathway to the polyol, offering the team two viable products.