Monday, November 23, 2020

Polyethylene upcycling to long-chain alkylaromatics by tandem hydrogenolysis/aromatization

Polyethylene upcycling to long-chain alkylaromatics by tandem hydrogenolysis/aromatization

A new future for polyethylene

Most current plastic recycling involves chopping up the waste and repurposing it in materials with less stringent engineering requirements than the original application. Chemical decomposition at the molecular level could, in principle, lead to higher-value products. However, the carbon-carbon bonds in polyethylene, the most common plastic, tend to resist such approaches without exposure to high-pressure hydrogen. F. Zhang et al. now report that a platinum/alumina catalyst can transform waste polyethylene directly into long-chain alkylbenzenes, a feedstock for detergent manufacture, with no need for external hydrogen (see the Perspective by Weckhuysen).

Science, this issue p. 437; see also p. 400


The current scale of plastics production and the accompanying waste disposal problems represent a largely untapped opportunity for chemical upcycling. Tandem catalytic conversion by platinum supported on γ-alumina converts various polyethylene grades in high yields (up to 80 weight percent) to low-molecular-weight liquid/wax products, in the absence of added solvent or molecular hydrogen, with little production of light gases. The major components are valuable long-chain alkylaromatics and alkylnaphthenes (average ~C30, dispersity Ð = 1.1). Coupling exothermic hydrogenolysis with endothermic aromatization renders the overall transformation thermodynamically accessible despite the moderate reaction temperature of 280°C. This approach demonstrates how waste polyolefins can be a viable feedstock for the generation of molecular hydrocarbon products.

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