Polyolefins represent nearly two-thirds of the plastic market with no short-term alternatives, making the development of chemical recycling strategies essential. Beyond pyrolysis, hydrogenolysis is an emerging approach to support circularity, enabling the production of valuable alkanes. However, the process remains limited by modest catalytic performance and lack of clarity in the target products required to achieve environmental and economic feasibility, summarized in the ‘Plastic-to-X’ question.^[1] Integrating catalyst design with systems engineering may clarify technological prospects and guide future studies.^[2]

Our first study^[3] identified ruthenium nanoparticles supported on titania as highly active catalysts under optimized reaction conditions, although low-value gaseous products were predominantly formed. The product distribution could be controlled by incorporating nickel as a modifier to the ruthenium phase, with the supported Ru-Ni catalysts producing up to 25% liquid (C₆–C₄₅) products from high density polyethylene and plastic caps (Fig. 1a). The optimal composition, namely 2.5 Ru wt% and 5 Ni wt% (2.5Ru5Ni), exhibited an alloyed nature, as indicated by high resolution transmission electron microscopy and extended X-ray absorption fine structure analysis (Fig. 1b). Mechanistic studies with surrogate compounds and density functional theory simulations revealed facilitated internal C–C bond cleavage over the Ru-Ni catalyst and terminal C–C bond scission over the Ru catalyst, rationalizing the observed differences in selectivity. At a systems level, techno-economic and life cycle analyses predicted economically viable processing of plastic caps –even when using green H₂– with reduced CO₂ emissions compared to the alternative fossil route, presenting the first example of its kind. To generalize this, we estimate a minimum average chain length of C₁₁ in the product distribution as an effective answer to the ‘Plastic-to-X’ endeavor (Fig. 1c). This work highlights a practical route for valorizing polyethylene waste while emphasizing the importance of bridging analyses across scales to uncover design guidelines for future catalyst developments.

Fig. 1 a Catalytic performance of 2.5Ru5Ni and 2.5Ru for hydrogenolysis of HDPE and plastic caps. b Structural characterization of used 2.5Ru5Ni and 2.5Ru catalysts. c ‘Plastic-to-X’ analysis of the avoided CO₂ emissions and margin for profit in hydrogenolysis towards different product distributions compared to the equivalent business-as-usual operation, showing the minimum average product chain length for environmental and economic gains.

[1] A.J. Martín, S.D. Jaydev, C. Mondelli, J. Pérez-Ramírez, Chem 2021, 7, 1487.
[2] S.M. Mitchell, A.J. Martín, J. Pérez-Ramírez, Nat. Chem. Eng. 2024, 1, 13.
[3] S.D. Jaydev, et al., Angew. Chem. Int. Ed. 2024, 63, e202317526.