Tracking Chain Populations and Branching Structure during Polyethylene Deconstruction Processes.

Catalytic deconstruction has emerged as a promising solution to valorize polyethylene (PE) waste into valuable products, such as oils, fuels, surfactants, and lubricants. Unfortunately, commercialization has been hampered by inadequate optimization of PE deconstruction due to an inability to either truly characterize the polymer transformations or adjust catalytic conditions to match the ever-evolving product distribution and associated property changes. To address these challenges, a detailed analysis of molar mass distributions and thermal characterization was developed herein and applied to low-density polyethylene (LDPE) deconstruction to enable more thorough identification of polymer chain characteristics within the solids (e.g., changes in molar mass or branching structure). For example, LDPE hydrocracking exhibited comparable rates of polymer chain isomerization and C-C bond scission, and the solids generated possessed a broadened molar mass distribution with a disappearance of a significant fraction of highly linear segments, indicating polymer-structure-dependent interactions with the catalyst. Solids analysis from pyrolysis yielded starkly different results, as the resulting solids were devoid of unreacted polymer chains and had a narrowed molar mass distribution even at short times (e.g., 0.2 h). By tracking the polymeric deconstruction behavior as a function of reaction type, time, and catalyst design, we mapped critical pathways toward PE valorization.

Structure-Property Relationships for Nickel Aluminate Catalysts in Polyethylene Hydrogenolysis with Low Methane Selectivity.

Earth-abundant metals have recently been demonstrated as cheap catalyst alternatives to scarce noble metals for polyethylene hydrogenolysis. However, high methane selectivities hinder industrial feasibility. Herein, we demonstrate that low-temperature ex-situ reduction (350 °C) of coprecipitated nickel aluminate catalysts yields a methane selectivity of <5% at moderate polymer deconstruction (25-45%). A reduction temperature up to 550 °C increases the methane selectivity nearly sevenfold. Catalyst characterization (XRD, XAS, 27Al MAS NMR, H2 TPR, XPS, and CO-IR) elucidates the complex process of Ni nanoparticle formation, and air-free XPS directly after reaction reveals tetrahedrally coordinated Ni2+ cations promote methane production. Metallic and the specific cationic Ni appear responsible for hydrogenolysis of internal and terminal C-C scissions, respectively. A structure-methane selectivity relationship is discovered to guide the design of Ni-based catalysts with low methane generation. It paves the way for discovering other structure-property relations in plastics hydrogenolysis. These catalysts are also effective for polypropylene hydrogenolysis.

Electronic modulation of metal-support interactions improves polypropylene hydrogenolysis over ruthenium catalysts.

Ruthenium (Ru) is the one of the most promising catalysts for polyolefin hydrogenolysis. Its performance varies widely with the support, but the reasons remain unknown. Here, we introduce a simple synthetic strategy (using ammonia as a modulator) to tune metal-support interactions and apply it to Ru deposited on titania (TiO2). We demonstrate that combining deuterium nuclear magnetic resonance spectroscopy with temperature variation and density functional theory can reveal the complex nature, binding strength, and H amount. H2 activation occurs heterolytically, leading to a hydride on Ru, an H+ on the nearest oxygen, and a partially positively charged Ru. This leads to partial reduction of TiO2 and high coverages of H for spillover, showcasing a threefold increase in hydrogenolysis rates. This result points to the key role of the surface hydrogen coverage in improving hydrogenolysis catalyst performance.

Plastic waste to fuels by hydrocracking at mild conditions.

Single-use plastics impose an enormous environmental threat, but their recycling, especially of polyolefins, has been proven challenging. We report a direct method to selectively convert polyolefins to branched, liquid fuels including diesel, jet, and gasoline-range hydrocarbons, with high yield up to 85% over Pt/WO3/ZrO2 and HY zeolite in hydrogen at temperatures as low as 225°C. The process proceeds via tandem catalysis with initial activation of the polymer primarily over Pt, with subsequent cracking over the acid sites of WO3/ZrO2 and HY zeolite, isomerization over WO3/ZrO2 sites, and hydrogenation of olefin intermediates over Pt. The process can be tuned to convert different common plastic wastes, including low- and high-density polyethylene, polypropylene, polystyrene, everyday polyethylene bottles and bags, and composite plastics to desirable fuels and light lubricants.

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.