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Rapid atom-efficient polyolefin plastics hydrogenolysis mediated by a well-defined single-site electrophilic/cationic organo-zirconium catalyst
Summary
Researchers developed a highly efficient catalyst that breaks down polyethylene and polypropylene plastics into small hydrocarbon molecules using hydrogen gas at relatively low temperatures, converting common single-use plastics to fuel-like chemicals within less than an hour. This chemical recycling approach could offer a practical route to deconstruct the plastic waste that mechanical recycling cannot handle.
Polyolefins comprise a major fraction of single-use plastics, yet their catalytic deconstruction/recycling has proven challenging due to their inert saturated hydrocarbon connectivities. Here a very electrophilic, formally cationic earth-abundant single-site organozirconium catalyst chemisorbed on a highly Brønsted acidic sulfated alumina support and characterized by a broad array of experimental and theoretical techniques, is shown to mediate the rapid hydrogenolytic cleavage of molecular and macromolecular saturated hydrocarbons under mild conditions, with catalytic onset as low as 90 °C/0.5 atm H<sub>2</sub> with 0.02 mol% catalyst loading. For polyethylene, quantitative hydrogenolysis to light hydrocarbons proceeds within 48 min with an activity of > 4000 mol(CH<sub>2</sub> units)·mol(Zr)<sup>-1</sup>·h<sup>-1</sup> at 200 °C/2 atm H<sub>2</sub> pressure. Under similar solventless conditions, polyethylene-co-1-octene, isotactic polypropylene, and a post-consumer food container cap are rapidly hydrogenolyzed to low molecular mass hydrocarbons. Regarding mechanism, theory and experiment identify a turnover-limiting C-C scission pathway involving ß-alkyl transfer rather than the more common σ-bond metathesis.
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