Regioselectivity and Reaction Mechanism of Ru-Catalyzed Hydrogenolysis of Squalane and Model Alkanes

Advances and Challenges in Low-Temperature Upcycling of Waste Polyolefins via Tandem Catalysis

Polyolefin waste is the largest polymer waste stream that could potentially serve as an advantageous hydrocarbon feedstock. Upcycling polyolefins poses significant challenges due to their inherent kinetic and thermodynamic stability. Traditional methods, such as thermal and catalytic cracking, are straightforward but require temperatures exceeding 400 °C for complete conversion because of thermodynamic constraints. We summarize and critically compare recent advances in upgrading spent polyolefins and model reactants via kinetic (and thermodynamic) coupling of the endothermic C─C bond cleavage of polyolefins with exothermic reactions including hydrogenation, hydrogenolysis, metathesis, cyclization, oxidation, and alkylation. These approaches enable complete conversion to desired products at low temperatures (<300 °C). The goal is to identify challenges and possible pathways for catalytic conversions that minimize energy and carbon footprints.

Ru-Catalyzed Polyethylene Hydrogenolysis under Quasi-Supercritical Conditions

Ru/C-catalyzed polyethylene (PE) and hydrocarbon hydrogenolysis under quasi-supercritical fluid of isopentane was kinetically and mechanistically investigated. PE hydrogenolysis with C-C and C-H cleavage showed zeroth order, suggesting strong adsorption of hydrocarbons. PE yielded broad product distribution of heavy (C21-40) and diesel-range (C11-20) hydrocarbons in the primary step of hydrogenolysis due to stochastic C-C cleavage over Ru surface. Catalytic hydrogenolysis of n-hexadecane, squalane, and light hydrocarbons such as n-pentane, iso-pentane, and n-hexane further described C-C cleavage reactivity between primary and secondary carbons, i.e., 1C-2C and 2C-2C, which has an order of magnitude higher hydrogenolysis rate than that involving a tertiary carbon. The PE saturated Ru surface and lower C-C cleavage reactivity of tertiary carbon in iso-pentane, therefore, imited sovlent conversion during hydrogenolysis, whereas leading to selective PE conversion. Using hexadecane, we observed comparable hydrogenolysis rates between H2 and D2 (k H /k D ∼ 1), indicating the kinetically relevant step of C-C cleavage with facilitating C-H cleavage and rehydrogenation. However, the normal kinetic isotope effect between hexadecane and deuterated hexadecane (k C16H34 /k C16D34 ∼ 5) revealed that the dehydrogenation, i.e., C-H cleavage, can be kinetically involved in the hydrogenolysis kinetic. By considering the 8-fold lower H-D exchange rate with deuterated hexadecane compared to n-hexadecane, the lower rate for hydrogenolysis and H-D exchange with deuterated hexadecane can be attributed to the C-D bond dissociation energy being 3 kJ/mol higher than that of the C-H bond. Increasing H2 pressure favors internal C-C bond cleavage over terminal one. This minimizes the formation of lower hydrocarbons, particularly methane. However, the increase in H2 pressure increases the coverage of adsorbed hydrogen on the Ru particles due to competitive adsorption of H2 and polyethylene, which, in turn, reduces the polyethylene conversion rates.

Unraveling the role of water in mechanism changes for economically viable catalytic plastic upcycling

The surge in global plastic production, reaching 400.3 million tons in 2022, has exacerbated environmental pollution, with only 11% of plastic being recycled. Catalytic recycling, particularly through hydrogenolysis and hydrocracking, offers a promising avenue for upcycling polyolefin plastic, comprising 55% of global plastic waste. This study investigates the influence of water on polyolefin depolymerization using Ru catalysts, revealing a promotional effect only when both metal and acid sites, particularly Brønsted acid site, are present. Findings highlight the impact of Ru content, metal-acid balance, and their proximity on this interaction, as well as their role in modulating the isomerization process, affecting product selectivity. Additionally, the interaction facilitates the suppression of coke formation, ultimately enhancing catalyst stability. A comprehensive techno-economic and life cycle assessment underscores the viability and environmental benefits of the process, particularly in the presence of water. These insights advance understanding and offer strategies for optimizing polyolefin plastic recycling processes.

Catalytic Upcycling of Polyolefins

The large production volumes of commodity polyolefins (specifically, polyethylene, polypropylene, polystyrene, and poly(vinyl chloride)), in conjunction with their low unit values and multitude of short-term uses, have resulted in a significant and pressing waste management challenge. Only a small fraction of these polyolefins is currently mechanically recycled, with the rest being incinerated, accumulating in landfills, or leaking into the natural environment. Since polyolefins are energy-rich materials, there is considerable interest in recouping some of their chemical value while simultaneously motivating more responsible end-of-life management. An emerging strategy is catalytic depolymerization, in which a portion of the C-C bonds in the polyolefin backbone is broken with the assistance of a catalyst and, in some cases, additional small molecule reagents. When the products are small molecules or materials with higher value in their own right, or as chemical feedstocks, the process is called upcycling. This review summarizes recent progress for four major catalytic upcycling strategies: hydrogenolysis, (hydro)cracking, tandem processes involving metathesis, and selective oxidation. Key considerations include macromolecular reaction mechanisms relative to small molecule mechanisms, catalyst design for macromolecular transformations, and the effect of process conditions on product selectivity. Metrics for describing polyolefin upcycling are critically evaluated, and an outlook for future advances is described.

Chemical Upcycling of Polyolefin Plastics Using Structurally Well-defined Catalysts

Single-use polyolefins are widely used in our daily life and industrial production due to their light weight, low cost, superior stability, and durability. However, the rapid accumulation of plastic waste and low-profit recycling methods resulted in a global plastic crisis. Catalytic hydrogenolysis is regarded as a promising technique, which can effectively and selectively convert polyolefin plastic waste to value-added products. In this perspective, we focus on the design and synthesis of structurally well-defined hydrogenolysis catalysts across mesoscopic, nanoscopic, and atomic scales, accompanied by our insights into future directions in catalyst design for further enhancing catalytic performance. These design principles can also be applied to the depolymerization of other polymers and ultimately realize the chemical upcycling of waste plastics.

Hydrogen Bubbles: Harmonizing Local Hydrogen Transfer for Efficient Plastic Hydro-Depolymerization

Hydro-depolymerization presents a promising avenue for transforming plastic waste into high-value hydrocarbons, offering significant potential for value-added recycling. However, a major challenge in this method arises from kinetic limitations due to insufficient hydrogen concentration near the active sites, requiring optimal catalytic performance only at higher hydrogen pressures. In this study, we address this hurdle by developing "hydrogen bubble catalysts" featuring Ru nanoparticles within mesoporous SBA-15 channels (Ru/SBA). The distinctive feature of Ru/SBA catalysts lies in their capacity for physical hydrogen storage and chemically reversible hydrogen spillover, ensuring a timely and ample hydrogen supply. Under identical reaction conditions, the catalytic activity of Ru/SBA surpassed that of Ru/SiO2 (no hydrogen storage capacity) by over 4-fold. This substantial enhancement in catalytic performance provides significant opportunities for near atmospheric pressure hydro-depolymerization of plastic waste.

Layered Double Hydroxide Derivatives for Polyolefin Upcycling

While Ru-catalyzed hydrogenolysis holds significant promise in converting waste polyolefins into value-added alkane fuels, a major constraint is the high cost of noble metal catalysts. In this work, we propose, for the first time, that Co-based catalysts derived from CoAl-layered double hydroxide (LDH) are alternatives for efficient polyolefin hydrogenolysis. Leveraging the chemical flexibility of the LDH platform, we reveal that metallic Co species serve as highly efficient active sites for polyolefin hydrogenolysis. Furthermore, we introduced Ni into the Co framework to tackle the issue of restricted hydrogenation ability associated with contiguous Co-Co sites. In-situ analysis indicates that the integration of Ni induces electron transfer and facilitates hydrogen spillover. This dual effect synergistically enhances the hydrogenation/desorption of olefin intermediates, resulting in a significant reduction in the yield of low-value CH4 from 27.1 to 12.6%. Through leveraging the unique properties of LDH, we have developed efficient and cost-effective catalysts for the sustainable recycling and valorization of waste polyolefin materials.

Catalytic Strategies for the Upcycling of Polyolefin Plastic Waste

Chemical upgrading of waste plastics is currently one of the most important methods for addressing plastic pollution. In comparison to the current methods of incineration or landfill, chemical upgrading enables the utilization of carbon and hydrogen elements in waste plastics as resources. This process strongly relies on efficient catalysts and reaction systems. Through catalyst design, waste plastics can be converted into fuels or chemicals under the optimized reaction conditions, extending their life cycles. In this review, we systematically discuss various chemical conversion methods for polyolefin waste plastics, which account for a large proportion of waste plastics. We further explore the remaining challenges and future development trends in this field, including improving product value through product engineering and shifting research perspectives to exploring the tolerance of catalysts toward impurities in practical waste plastic waste rather than using pure plastic feedstock.

Boosting Polyethylene Hydrogenolysis Performance of Ru-CeO2 Catalysts by Finely Regulating the Ru Sizes

Hydrogenolysis is an effective method for converting polyolefins into high-value chemicals. For the supported catalysts commonly used, the size of active metals is of great importance. In this study, it is discovered that the activity of CeO2 -supported Ru single atom, nanocluster, and nanoparticle catalysts shows a volcanic trend in low-density polyethylene (LDPE) hydrogenolysis. Compared with CeO2 supported Ru single atoms and nanoparticles, CeO2 -supported Ru nanoclusters possess the highest conversion efficiency, as well as the best selectivity toward liquid alkanes. Through comprehensive investigations, the metal-support interactions (MSI) and hydrogen spillover effect are revealed as the two key factors in the reaction. On the one hand, the MSI is strongly related to the Ru surface states and the more electronegative Ru centers are beneficial to the activation of CH and CC bonds. On the other hand, the hydrogen spillover capability directly affects the affinity of catalysts and active H atoms, and increasing this affinity is advantageous to the hydrogenation of alkane species. Decreasing the Ru sizes can promote the MSI, but it can also reduce the hydrogen spillover effect. Therefore, only when the two effects achieve a balance, as is the case in CeO2 -supported Ru nanoclusters, can the hydrogenolysis activity be promoted to the optimal value.

Ab Initio Thermochemistry of Highly Flexible Molecules for Thermal Decomposition Analysis

Pyrolysis is a promising technology for chemical recycling of waste plastics, since it enables the generation of high-value chemicals with low capital and operating cost. The calculation of thermodynamic equilibrium composition using the Gibbs free energy minimization approach can determine pyrolysis operating conditions that produce desired products. However, the availability of thermochemical data can limit the application of equilibrium calculations. While density functional theory (DFT) calculations have been commonly used to produce accurate thermochemical data (e.g., enthalpies of formation) of small molecules, the accuracy and computational cost of these calculations are both challenging to handle for large, flexible molecules, exhibiting multiple conformations at elevated (i.e., pyrolysis) temperatures. In this work, we develop a computational framework to calculate accurate, temperature-dependent thermochemistry of large and flexible molecules by combining force field based conformational search, DFT calculations, thermochemical corrections, and Boltzmann statistics. Our framework produces accurately calculated thermochemistry that is used to predict equilibrium thermal decomposition profiles of octadecane, a model compound of polyethylene. Our thermochemistry results are compared against literature data demonstrating a great agreement, and the predicted decomposition profiles rationalize a series of pyrolysis experimental observations. Our work systematically addresses entropic contributions of large molecules and suggests paths for accurate and yet computationally feasible calculations of Gibbs free energies. The first-principles-based thermodynamic equilibrium analysis proposed in this work can be a significant step toward predicting temperature-dependent product distributions from plastic pyrolysis and guide experimentation on chemical plastic recycling.

Investigating the impact of TiO2 crystalline phases on catalytic properties of Ru/TiO2 for hydrogenolysis of polyethylene plastic waste

A series of TiO₂-supported Ru catalysts with different TiO₂ crystalline phases was synthesized and employed for the hydrogenolysis of polyethylene (PE). CO chemisorption, high-angle annular dark-field-scanning transmission electron microscopy, temperature-programmed reduction, and CO-Fourier transform infrared spectroscopy suggested that the degree of strong metal-support interactions (SMSIs) varied depending on the type of the TiO₂ phase and the reduction temperature, eventually influencing the catalysis of PE hydrogenolysis. Among the synthesized catalysts, Ru/TiO₂ with the rutile phase (Ru/TiO₂-R) exhibited the highest catalytic activity after high-temperature reduction at 500 °C, indicating that a certain degree of SMSI is necessary for ensuring high activity in PE hydrogenolysis. Ru/TiO₂-R could be successfully employed for the hydrogenolysis of post-consumer plastic wastes such as LDPE bottles to produce valuable chemicals (liquid fuel and wax) in high yields of 74.7%. This work demonstrates the possibility of harnessing the SMSIs in the design and synthesis of active catalysts for PE hydrogenolysis.

Converting Plastic Wastes to Naphtha for Closing the Plastic Loop

To solve the serious environmental problem and huge resource waste of plastic pollution, we report a tandem catalytic conversion of low-density polyethylene (LDPE) into naphtha, the key feedstock for renewable plastic production. Using β zeolite and silicalite-1-encapsulated Pt nanoparticles (Pt@S-1), a naphtha yield of 89.5% is obtained with 96.8% selectivity of C5-C9 hydrocarbons at 250 °C. The acid sites crack long-chain LDPE into olefin intermediates, which diffuse within the channels of Pt@S-1 to encounter Pt nanoparticles. The hydrogenation over confined metal matches cracking steps by selectively shipping the olefins with right size, and the rapid diffusion boosts the formation of narrow-distributed alkanes. A conceptual upgrading indicates it is suitable for closing the plastic loop, with a significant energy saving of 15% and 30% reduced greenhouse gas emissions.

Catalytic hydrogenolysis of plastic to liquid hydrocarbons over a nickel-based catalyst

The catalytic hydrogenolysis of a typical model compound of mulching film waste, polyethylene, was investigated as a potential way to improve economic efficiency of mulching film recycling. Nickel-based heterogeneous catalysts are proposed for polyethylene hydrogenolysis to produce liquid hydrocarbons. Among catalysts supported on various carriers, Ni/SiO₂ catalyst shows the highest activity which may due to the interactions between nickel and silica with the formation of nickel phyllosilicate. As high as 81.18% total gasoline and diesel range hydrocarbon was obtained from the polyethylene hydrogenolysis at relatively mild condition of 280 °C, and 3 MPa cold hydrogen pressure. The result is comparable to what have been reported in previous studies using noble metal catalysts. The gasoline and diesel range hydrocarbon are n-alkanes with a distribution at a range of C₄-C₂₂. The gas products are primarily CH₄ along with a small amount of C₂H₆ and C₃H₈. High yield of CH₄ as much as 9.68% was observed for the cleavage of molecule occurs along the alkane chain.

Mechanistic classification and benchmarking of polyolefin depolymerization over silica-alumina-based catalysts

Carbon-carbon bond cleavage mechanisms play a key role in the selective deconstruction of alkanes and polyolefins. Here, we show that the product distribution, which encompasses carbon range and formation of unsaturated and isomerization products, serves as a distinctive feature that allows the reaction pathways of different catalysts to be classified. Co, Ni, or Ru nanoparticles immobilized on amorphous silica-alumina, Zeo-Y and ZSM-5, were evaluated as catalysts in the deconstruction of n-hexadecane model substrate with hydrogen to delineate between different mechanisms, i.e., monofunctional- (acid site dominated) or bifunctional-hydrocracking (acid site & metal site) versus hydrogenolysis (metal site dominated), established from the product distributions. The ZSM-5-based catalysts were further studied in the depolymerization of polyethylene. Based on these studies, the catalysts are plotted on an activity-mechanism map that functions as an expandable basis to benchmark catalytic activity and to identify optimal catalysts that afford specific product distributions. The systematic approach reported here should facilitate the acceleration of catalyst discovery for polyolefin depolymerization.

Product distributions have been used to classify the depolymerization pathways of polyolefins catalyzed by silica-alumina-based catalysts to construct an activity-mechanism map as a benchmarking tool to facilitate catalyst discovery.

Upcycling Plastic Wastes into Value-Added Products by Heterogeneous Catalysis

Plastics are playing essential roles in the modern society. The majority of them enter environment through landfilling or discarding after turning into wastes, causing severe carbon loss and imposing high risk to ecosystem and human health. Currently, physical recycling serves as the primary method to reuse plastic waste, but this method is limited to thermoplastic recycling. The quality of recycled plastics gradually deteriorates because of the undesirable degradation in the recycling process. Under such background, catalytic upcycling, which can upgrade various plastic wastes into value-added products under mild conditions, has attracted recent attention as a promising strategy to treat plastic wastes. This Review highlights recent advances in the development of efficient heterogeneous catalysts and useful strategies for upcycling plastics into liquid hydrocarbons, arene compounds, carbon materials, hydrogen, and other value-added chemicals. The functions of catalysts and the reaction mechanisms are discussed. The key factors that influence the catalytic performance are also analyzed.

Effect of reaction conditions on the hydrogenolysis of polypropylene and polyethylene into gas and liquid alkanes

Hydrogenolysis of polypropylene and polyethylene provides a pathway to smaller hydrocarbons. We describe the impact of the polyolefin structure, reaction conditions, and presence of chlorine on the product distribution and branching degree.

Direct Conversion of Polypropylene into Liquid Hydrocarbons on Carbon-Supported Platinum Catalysts

Efforts to selectively convert polypropylene (≈30 % of all plastic waste) have not been particularly successful. Typical distributions span from gas to solid products, highlighting a challenging cleavage control. Here, carbon-supported platinum nanoparticles were designed for complete hydrocracking into liquid hydrocarbons (C₅ -C₄₅ ). The metal and carrier phases operated synergistically. The cleavage activity depended on platinum and its rate rose with decreasing particle size. The carbon carrier controlled selectivity via hydrocarbon binding strength, which depended on the chain length and on the surface oxygen concentration. An optimal binding provided by carbons with high oxygen content promoted both adsorption of long chains and desorption of short products. This strategy achieved an unprecedented 80 % selectivity toward motor oil (C₂₁ -C₄₅ ). Carbons exhibiting too strong binding (low oxygen content) hindered product desorption, while non-binding materials (e. g., silica or alumina) did not promote plastic-Pt contact, leading in both cases to low performance. This work pioneers design guidelines in a key process towards a sustainable plastic economy.

Polyethylene Hydrogenolysis at Mild Conditions over Ruthenium on Tungstated Zirconia

Plastics waste has become a major environmental threat, with polyethylene being one of the most produced and hardest to recycle plastics. Hydrogenolysis is potentially the most viable catalytic technology for recycling. Ruthenium (Ru) is one of the most active hydrogenolysis catalysts but yields too much methane. Here we introduce ruthenium supported on tungstated zirconia (Ru-WZr) for hydrogenolysis of low-density polyethylene (LDPE). We show that the Ru-WZr catalysts suppress methane formation and produce a product distribution in the diesel and wax/lubricant base-oil range unattainable by Ru-Zr and other Ru-supported catalysts. Importantly, the enhanced performance is showcased for real-world, single-use LDPE consumables. Reactivity studies combined with characterization and density functional theory calculations reveal that highly dispersed (WO x )n clusters store H as surface hydroxyls by spillover. We correlate this hydrogen storage mechanism with hydrogenation and desorption of long alkyl intermediates that would otherwise undergo further C-C scission to produce methane.

Conversion of Polyolefin Waste to Liquid Alkanes with Ru-Based Catalysts under Mild Conditions

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(sp³)-C(sp³) bonds at temperatures ranging from 200 to 250 °C. Under optimal conditions of 200 °C in 20 bar H₂, 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 (C₁-C₆). At 250 °C, nearly stoichiometric yields of CH₄ were obtained from polyethylene over the catalyst. The hydrogenolysis of long chain, low-density polyethylene (LDPE) and a postconsumer LDPE plastic bottle to produce C₇-C₄₅ 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.

Selective hydrogenation of amides to alcohols in water solvent over a heterogeneous CeO2-supported Ru catalyst

CeO₂-supported Ru (Ru/CeO₂) worked as an effective and reusable heterogeneous catalyst for the selective dissociation of the C–N bond in amides, particularly primary amides, with H₂ in water solvent at low reaction temperature of 333 K.