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>

There is an urgent need for new technologies to enable circularity for synthetic polymers, spurred by the accumulation of waste plastics in landfills and the environment and the contributions of plastics manufacturing to climate change. Chemical recycling is a promising means to convert waste plastics into molecular intermediates that can be remanufactured into new products. Given the growing interest in the development of new chemical recycling approaches, it is critical to evaluate the economics, energy use, greenhouse gas emissions, and other life cycle inventory metrics for emerging processes, relative to the incumbent, linear manufacturing practices employed today. Here we offer specific definitions for classes of chemical recycling and upcycling and describe general process concepts for the chemical recycling of mixed plastics waste. We present a framework for techno-economic analysis and life cycle assessment for both closed- and open-loop chemical recycling. Rigorous application of these process analysis tools will be required to enable impactful solutions for the plastics waste problem. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering, Volume 13 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The development of technologies to recycle polyethylene (PE) and polypropylene (PP), globally the two most produced polymers, is critical to increase plastic circularity. Here, we show that 5 wt % cobalt supported on ZSM-5 zeolite catalyzes the solvent-free hydrogenolysis of PE and PP into propane with weight-based selectivity in the gas phase over 80 wt % after 20 h at 523 K and 40 bar H2. This catalyst significantly reduces the formation of undesired CH4 (≤5 wt %), a product which is favored when using bulk cobalt oxide or cobalt nanoparticles supported on other carriers (selectivity ≤95 wt %). The superior performance of Co/ZSM-5 is attributed to the stabilization of dispersed oxidic cobalt nanoparticles by the zeolite support, preventing further reduction to metallic species that appear to catalyze CH4 generation. While ZSM-5 is also active for propane formation at 523 K, the presence of Co promotes stability and selectivity. After optimizing the metal loading, it was demonstrated that 10 wt % Co/ZSM-5 can selectively catalyze the hydrogenolysis of low-density PE (LDPE), mixtures of LDPE and PP, as well as postconsumer PE, showcasing the effectiveness of this technology to upcycle realistic plastic waste. Cobalt supported on zeolites FAU, MOR, and BEA were also effective catalysts for C2–C4 hydrocarbon formation and revealed that the framework topology provides a handle to tune gas-phase selectivity.

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Isobutene is a specialty chemical used in the production of fuel additives, polymers, and other high-value products. While normally produced by steam cracking of petroleum naphtha, there is increasing interest in identifying routes to synthesizing isobutene from biomass-derived compounds, such as ethanol and acetone. Recent work has shown that zinc−zirconium mixed oxides are effective and selective catalysts for producing isobutene from ethanol. However, the reaction pathway, the roles of acidic and basic sites, and the role of water in promoting stability and selectivity are not yet clearly defined. In this study, a series of zinc− zirconium mixed oxides with tunable acid−base properties were synthesized and characterized with XRD, Raman spectroscopy, BET, CO 2 -TPD, NH 3 -TPD, and IR DRIFTS of adsorbed pyridine in order to probe the roles of acid and base sites for each step in the ethanol-to-isobutene reaction pathway. The observed reaction kinetics, supported by modeling of these kinetics, suggest that the reaction of ethanol to isobutene proceeds via a five-step sequence. Ethanol first undergoes dehydrogenation to acetaldehyde, which is then oxidized to acetic acid. This product undergoes ketonization to produce acetone, which dimerizes to form diacetone alcohol. The latter product either decomposes directly to isobutene and acetic acid or produces these products by dehydration to mesityl oxide and subsequent hydrolysis. The acetic acid formed undergoes ketonization to produce additional acetone. The dispersion of zinc oxide on zirconia was found to produce a balance between Lewis acidic and basic sites that prevent the loss of ethanol via dehydration to ethylene and promote the cascade reactions of ethanol and acetone to isobutene. Water, while inhibiting both isobutene and mesityl oxide formation, improves isobutene selectivity by suppressing side reactions such as unimolecular dehydration, acetone decomposition, and deactivation due to coke formation.