Wei Zhang

Lewis acidic chloroaluminate ionic liquid diluted in dichloromethane catalyzes the disproportionation of alkanes, such as isopentane, via carbenium ions in a reaction readily initiated by carbenium ion precursors such as tert-butyl chloride (TBC). The carbenium ion-AlCl₄^- ion-pairs stabilized by the ionic liquid-dichloromethane solution are the key intermediates in two distinctive kinetic regimes, i.e., a transient regime (0-5 min) and a steady-state regime (after 5 min). The transient regime constitutes the majority of isopentane conversion and is governed by the initial carbenium ion concentration. In the steady-state regime, disproportionation occurs at a considerably lower rate, affected by the carbenium ion concentration, the concentration of the ionic liquid, and the reaction temperature. The formation of alkenes observed in the ¹H NMR spectra of the reacting substrates, along with the DFT calculations, suggests that deprotonation of carbenium ion-pairs reduces their concentration, decreasing, in turn, the reaction rate. Kinetic modeling indicates that the transient regime is significantly controlled by the hydride transfer (k_HT) and the deprotonation rate constants (k_DP), while the steady-state regime is additionally influenced by the alkene protonation rate constant (k_-DP). The overall activation energy of the reaction at the steady state, expressed as E_{a,steady-state regime} = E_a,HT - E_a,DP + E_a,-DP, was 54 kJ/mol. The reaction mechanism and the kinetics highlight the potential of Lewis acid-catalyzed conversions of hydrocarbons under remarkably mild conditions.

Selective chemical upcycling of waste polyethylene (PE) into high-value aromatics is conceptually very attractive. Yet, up to now the aromatic product selectivity typically remains below 50%, mainly limited by competing hydrogenolysis. We report here a cascade process that combines the selective aerobic oxidation of hydrogen, in situ generated from PE via cleavage of C(sp³)–H, with oxygen and the endothermic aromatization of polyethylene. Our tandem aerobic oxidation-aromatization approach boosts the catalytic activity at moderate temperatures of up to 280°C in ambient air. Concurrently, hydrogen removal shifts the chemical equilibrium favorably towards formation of aromatic products, achieving an aromatic selectivity of 78 mol%. Demonstrated on a near kilogram scale with upscaled catalysts under mild conditions, our approach offers a scalable, energy-efficient solution for the high-selectivity conversion of plastic waste into valuable chemical intermediates and solvents.

Polyolefins and their chlorinated derivatives such as polyvinyl chloride (PVC) are among the most prevalent plastics in global production and waste streams. Traditional waste-to-energy methods such as incineration and pyrolysis, as well as most chemical upcycling methods for PVC utilization, require thorough, high-temperature dechlorination to prevent the release of toxic chlorinated compounds. We present here a strategy for upgrading discarded PVC into chlorine-free fuel range hydrocarbons and hydrogen chloride in a single-stage process catalyzed by chloroaluminate ionic liquids. This approach offsets endothermic dechlorination and carbon-carbon bond cleavage with exothermic alkylation and hydrogen transfer by isobutane or isopentane in a low-temperature tandem process. The light isoalkanes are available from refinery processes and partly from recycling of the product stream. This process is suitable for handling real-world mixed and contaminated PVC and polyolefin waste streams.

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 (C_21-40) and diesel-range (C_11-20) 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.

Chloroaluminate ionic liquids selectively transform (waste) polyolefins into gasoline-range alkanes through tandem cracking-alkylation at temperatures below 100 °C. Further improvement of this process necessitates a deep understanding of the nature of the catalytically active species and the correlated performance in the catalyzing critical reactions for the tandem polyolefin deconstruction with isoalkanes at low temperatures. Here, we address this requirement by determining the nuclearity of the chloroaluminate ions and their interactions with reaction intermediates, combining in situ ²⁷Al magic-angle spinning nuclear magnetic resonance spectroscopy, in situ Raman spectroscopy, Al K-edge X-ray absorption near edge structure spectroscopy, and catalytic activity measurement. Cracking and alkylation are facilitated by carbenium ions initiated by AlCl₃-tert-butyl chloride (TBC) adducts, which are formed by the dissociation of Al₂Cl₇⁻ in the presence of TBC. The carbenium ions activate the alkane polymer strands and advance the alkylation cycle through multiple hydride transfer reactions. In situ ¹H NMR and operando infrared spectroscopy demonstrate that the cracking and alkylation processes occur synchronously; alkenes formed during cracking are rapidly incorporated into the carbenium ion-mediated alkylation cycle. The conclusions are further supported by ab initio molecular dynamics simulations coupled with an enhanced sampling method, and model experiments using n-hexadecane as a feed.

Polyolefins and polyvinyl chloride (PVC) stand out as the most ubiquitous plastic products.1,2 The physical separation and mechanical recycling of mixed waste face significant technical and economic challenges and is, hence, typically uncomplete.3,4 Traditional waste-to-energy methods like incineration and pyrolysis necessitate thorough PVC dechlorination to prevent the release of toxic chlorinated compounds such as dioxins and polychlorinated biphenyls.5 Moreover, PVC's flame-retardant properties hinder energy recovery efficiency,6 leading to a substantial portion of PVC ending up in landfills.7 Despite progress in chemical upcycling of PVC under mild conditions, effective dechlorination still demands temperatures ranging from 200 to 250 °C and a two-stage approach to prevent chlorine-derived compounds from deactivating metal active sites.8,9 We present here a strategy for upgrading discarded PVC into liquid iso-alkanes and HCl at room temperature in a single-stage process, catalyzed by (the cost-effective) anhydrous aluminum chloride (AlCl3) and AlCl3-based chloroaluminate ionic liquids. This approach effectively integrates the endothermic dechlorination and C-C bond cleavage with the exothermic alkylation of light isoparaffins (isobutane/isopentane) as co-reagent into a tandem process that ensures kinetic and thermodynamic efficiency. The iso-alkanes are available via refinery processes10 and can be partly recycled from the product stream. This approach presents a novel and cost-effective route to jointly upcycle PVC and isoparaffins, conserving the carbon atoms in both compounds in the final chlorine free isoalkanes. It is also applicable to real-world mixed and contaminated PVC and polyolefin waste streams, offering a superior option for decentralized conversion compatible with existing refinery infrastructure.

Transforming polyolefin waste into liquid alkanes through tandem cracking‐alkylation reactions catalyzed by Lewis‐acid chlorides offers an efficient route for single‐step plastic upcycling. Lewis acids in dichloromethane establish a polar environment that stabilizes carbenium ion intermediates and catalyzes hydride transfer, enabling breaking of polyethylene C−C bonds and forming C−C bonds in alkylation. Here, we show that efficient and selective deconstruction of low‐density polyethylene (LDPE) to liquid alkanes is achieved with anhydrous aluminum chloride (AlCl₃) and gallium chloride (GaCl₃). Already at 60 °C, complete LDPE conversion was achieved, while maintaining the selectivity for gasoline‐range liquid alkanes over 70 %. AlCl₃ showed an exceptional conversion rate of 5000 , surpassing other Lewis acid catalysts by two orders of magnitude. Through kinetic and mechanistic studies, we show that the rates of LDPE conversion do not correlate directly with the intrinsic strength of the Lewis acids or steric constraints that may limit the polymer to access the Lewis acid sites. Instead, the rates for the tandem processes of cracking and alkylation are primarily governed by the rates of initiation of carbenium ions and the subsequent intermolecular hydride transfer. Both jointly control the relative rates of cracking and alkylation, thereby determining the overall conversion and selectivity.

Transforming polyolefin waste into liquid alkanes through tandem cracking‐alkylation reactions catalyzed by Lewis‐acid chlorides offers an efficient route for single‐step plastic upcycling. Lewis acids in dichloromethane establish a polar environment that stabilizes carbenium ion intermediates and catalyzes hydride transfer, enabling breaking of polyethylene C−C bonds and forming C−C bonds in alkylation. Here, we show that efficient and selective deconstruction of low‐density polyethylene (LDPE) to liquid alkanes is achieved with anhydrous aluminum chloride (AlCl₃) and gallium chloride (GaCl₃). Already at 60 °C, complete LDPE conversion was achieved, while maintaining the selectivity for gasoline‐range liquid alkanes over 70 %. AlCl₃ showed an exceptional conversion rate of 5000 , surpassing other Lewis acid catalysts by two orders of magnitude. Through kinetic and mechanistic studies, we show that the rates of LDPE conversion do not correlate directly with the intrinsic strength of the Lewis acids or steric constraints that may limit the polymer to access the Lewis acid sites. Instead, the rates for the tandem processes of cracking and alkylation are primarily governed by the rates of initiation of carbenium ions and the subsequent intermolecular hydride transfer. Both jointly control the relative rates of cracking and alkylation, thereby determining the overall conversion and selectivity.

Selective upcycling of polyolefin waste has been hampered by the relatively high temperatures that are required to cleave the carbon-carbon (C–C) bonds at reasonably high rates. We present a distinctive approach that uses a highly ionic reaction environment to increase the polymer reactivity and lower the energy of ionic transition states. Combining endothermic cleavage of the polymer C–C bonds with exothermic alkylation reactions of the cracking products enables full conversion of polyethylene and polypropylene to liquid isoalkanes (C ₆ to C ₁₀ ) at temperatures below 100°C. Both reactions are catalyzed by a Lewis acidic species that is generated in a chloroaluminate ionic liquid. The alkylate product forms a separate phase and is easily separated from the reactant catalyst mixture. The process can convert unprocessed postconsumer items to high-quality liquid alkanes with high yields.

Heptazine‐based conjugated polymeric carbon nitrides (PCNs) are promising metal‐free photocatalysts, yet their synthesis is challenging due to the electron‐deficiency and insolubility of heptazine units. Indeed, heptazine‐containing polymers have only been prepared through nucleophilic substitution with amines by using toxic cyameluric chloride as the starting material. Herein, we report the novel and environmentally friendly method for preparing heptazine‐based mesoporous PCNs with hydrazone links formed through a simple Schiff base condensation of melem‐NH₂ and aldehydes. Unlike cyameluric chloride, melem‐NH₂ is non‐toxic, stable, and can be readily obtained from melem and hydrazine in solution. We demonstrate that the hydrazone linkages and the heptazine units synergistically enhance the photocatalytic activity of PCNs in visible‐light‐driven aerobic oxidation of benzyl alcohol to benzaldehyde. In particular, the polymer constructed from melem‐NH₂ and p‐phthalaldehyde shows 17 times more activity than graphitic carbon nitride (g‐C₃N₄).