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Tuning selectivity in the direct hydrogenolysis of PET plastic over Co catalysts through interfacial hydrogen spillover
Summary
Scientists found a clever way to break down PET plastic (the kind used in soda bottles) into valuable chemicals using a cheap, stable cobalt-based catalyst, and by simply adjusting the temperature and pressure, they can steer the process toward one of two useful products instead of a messy mixture. This matters because it's a cleaner, simpler way to recycle plastic waste—meaning less of it ends up polluting the environment and breaking down into microplastics that can enter our water, food, and bodies—while also producing fewer planet-warming emissions than making these chemicals from fossil fuels.
Polyethylene terephthalate (PET) plastic upcycling typically features two-step processes involving cascade depolymerization and functionalization to mixed products. Here, we realize the direct hydrogenolysis of PET over a Co@CoO heterogeneous catalyst, with high yield of either p-xylene (PX, >97 %) or 1,4-dimethylcyclohexane (1,4-DMC, >90 %). Pyrolysis of Co-MOF-71 yields carbon-supported Co nanoparticles partially encapsulated by CoO, whose metal/oxide interface facilitates hydrogen spillover. Experimental and computational investigations reveal Coδ+ atoms proximate to CoO particles selectively catalyze carboxylate C–O bond cleavage at 250 °C and 1 MPa H2 to yield PX. In contrast, hydrogen spillover from Co metal to CoO sites at 280 °C and 3 MPa H2 promotes selective ring hydrogenation to 1,4-DMC. Co/CoO-800 has excellent stability and efficacy for depolymerizing commercial PET plastics. Life cycle assessment indicates the PET-to-PX process offers negative CO2 emissions and outperforms fossil-fuel PX production. Direct hydrogenolysis over Earth-abundant catalysts offers a simple strategy for polyester waste upcycling. Turning PET plastic waste into useful chemicals usually takes several steps and makes mixtures. This study uses a cobalt catalyst to directly convert PET into high yields of two valuable products, with good stability and lower emissions.