Plastics play irreplaceable roles in a wide range of industries and are produced on a large scale. Nonetheless, the low recycle rate and resistance towards degradation of the plastic waste pose severe threats to the environment. Therefore, technologies that effectively convert plastics, ideally that upcycle them into useful products are in urgent demand. Here, we present an efficient approach to produce short-chain alkanes from polypropylene (PP) and polyethylene (PE), the two most common plastic components, by catalytic hydrogenolysis. We discovered that Ru/C effectively catalyzes the reaction at relatively low temperature to pyrolysis (175 °C) without any solvent, showing superior activity to other carbon-supported metals. A mixture of C1 to C38 alkanes is produced at appropriate temperatures, with isomerization occurring to a minor extent. Systematic studies revealed that liquid alkane products do not go through secondary hydrogenolysis or isomerization before complete solid conversion, which is due to less favored adsorption and leads to the insensitivity of carbon distribution among liquid and gas products with reaction time. In contrast, the selectivity can be steered by the hydrogen pressure (pH2): higher pH2 shifts the regioselectivity of hydrogenolysis towards more substituted C−C bonds that have less dehydrogenated transition state for C−C cleavage. This suppresses undesired CH4 formation and branched alkane formation from PE. Higher pH2 also favors hydrogenolysis over isomerization. Guided by the understandings, a high carbon yield of &gt; 57% for preferred liquid alkanes was achieve under optimal conditions. Besides, Cl from PVC-containing feedstock were found to be detrimental to the hydrogenolysis activity of Ru/C. This work demonstrates the potential of hydrogenolysis for plastic upcycling with low operation temperature, broad substrate scope, and high selectivity to alkanes in the desired range. In addition, the intriguing trends revealed by this work are instructive to the catalyst development, selectivity enhancement, and mechanistic understanding for the reaction.

Upcycling polyolefin plastics to liquid alkane by catalytic hydrogenolysis

ACS Fall 2021

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