Chemical upcycling of waste polyolefins via hydrogenolysis offers unique opportunities for selective depolymerization compared to high temperature thermal deconstruction. Here, we demonstrate the hydrogenolysis of polyethylene into liquid alkanes under mild conditions using ruthenium nanoparticles supported on carbon (Ru/C). Reactivity studies on a model n -octadecane substrate showed that Ru/C catalysts are highly active and selective for the hydrogenolysis of C(sp 3 )–C(sp 3 ) bonds at temperatures ranging from 200 to 250 °C. Under optimal conditions of 200 °C in 20 bar H 2 , polyethylene (average M w ∼ 4000 Da) was converted into liquid n -alkanes with yields of up to 45% by mass after 16 h using a 5 wt % Ru/C catalyst with the remaining products comprising light alkane gases (C 1 –C 6 ). At 250 °C, nearly stoichiometric yields of CH 4 were obtained from polyethylene over the catalyst. The hydrogenolysis of long chain, low-density polyethylene (LDPE) and a postconsumer LDPE plastic bottle to produce C 7 –C 45 alkanes was also achieved over Ru/C, demonstrating the feasibility of this reaction for the valorization of realistic postconsumer plastic waste. By identifying Ru-based catalysts as a class of active materials for the hydrogenolysis of polyethylene, this study elucidates promising avenues for the valorization of plastic waste under mild conditions.

Catalytic depolymerization of polyolefins is a promising chemical recycling strategy to create value-added products from waste plastics, which are accumulating in landfills and the natural environment at unsustainable rates. The cleavage of strong C−C bonds in polyolefins can be performed using a noble metal and hydrogen via a hydrogenolysis mechanism. Previously, we identified ruthenium nanoparticles supported on carbon (Ru/C) as a highly active heterogeneous catalyst for the conversion of polyethylene into liquid and gaseous n-alkanes under mild conditions. In the present study, we investigated the catalytic depolymerization of polypropylene (PP) under mild conditions (200−250 °C, 20−50 bar H 2 ). We demonstrate that Ru/C produces C 5 −C 32 iso-alkane yields above 68% in the absence of solvent and identify trade-offs between product yield and temperature, hydrogen pressure, and reaction time. We apply a rigorous analytical method to quantify all liquid and gaseous alkane products. The characterized catalyst was found to be recyclable after the complete depolymerization of high molecular weight PP (M w ∼ 340,000 Da) to liquid and gaseous hydrocarbons and after depolymerization of a postconsumer PP centrifuge tube. Further, the catalyst was shown to be effective in depolymerizing a mixture of high-density polyethylene and PP to produce a mixture of linear and branched liquid alkanes, demonstrating feasibility for the depolymerization of streams of mixed polyolefin waste.

ReviewsScheme5.Synthesis of furanyl ethers via direct and reductive etherification of biomass-derived platform molecules.References are noted in brackets.

<p>Chemical upcycling of waste polyolefins via hydrogenolysis offers unique opportunities for selective depolymerization compared to high temperature thermal deconstruction. Here, we demonstrate the hydrogenolysis of polyethylene into liquid alkanes under mild conditions using ruthenium nanoparticles sup-ported on carbon (Ru/C). Reactivity studies on a model <i>n</i>-octadecane substrate showed that Ru/C catalysts are highly active and se-lective for the hydrogenolysis of C(sp³)-C(sp³) bonds at temperatures ranging from 200-250°C. Under optimal conditions of 200°C in 20 bar H2, polyethylene (average Mw ~4,000) was converted into liquid <i>n</i>-alkanes with yields of up to 45% by mass after 16 h using a 5 wt% Ru/C catalyst, with the remaining products comprising light alkane gases (C1-C6). At 250°C, nearly stoichiometric yields of CH4 were obtained from polyethylene over the catalyst. The hy-drogenolysis of long chain, low-density polyethylene (LDPE) and a post-consumer LDPE plastic bottle to produce C7-C45 alkanes was also achieved over Ru/C, demonstrating the feasibility of this reac-tion for the valorization of realistic post-consumer plastic waste. By identifying Ru-based catalysts as a class of active materials for the hydrogenolysis of polyethene, this study opens new avenues for the valorization of plastic waste under mild conditions.<br></p>