27 Search Results

Chloroaluminate ionic liquid catalyzes the disproportionation of alkanes, a reaction readily initiated by carbenium-ion precursors such as tert-butyl chloride, resulting in equimolar amounts of isobutane and methylpentanes. The carbenium ion-AlCl4- ion-pairs stabilized by the ionic liquid are the key intermediates in two distinctive kinetic phases, i.e., a transi-ent phase (0-5 minutes) and a steady-state phase (after 5 minutes). The transient phase constitutes the majority of iso-pentane conversion and is governed by the initial carbenium ion concentration. In the steady-state phase, disproportiona-tion occurs at a considerably lower rate, affected by the carbenium ion concentration, the concentration of the ionic liq-uid, andmore » the reaction temperature. The formation of olefins observed in the 1H NMR spectra of the reacting substrates, along with the DFT calculations, suggests that dehydrochlorination of active carbenium ion-pairs reduces their concentra-tion, decreasing, in turn, the reaction rate. Kinetic modeling indicates that the transient phase is significantly controlled by the hydride transfer (kHT) and the dehydrochlorination rate constants (kDC), while the steady-state phase is additional-ly influenced by the hydrochlorination rate constant (k–DC). The overall activation energy of the reaction at the steady state, expressed as Ea,steady-state = Ea,HT – Ea,DC + Ea,–DC, was 54 kJ/mol. The reaction mechanism and the kinetics highlight the potential of Lewis acid-catalyzed conversions of hydrocarbons under remarkably mild conditions.« less

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. Here, 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 amore » 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.« less

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. Crackingmore » 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.« less

In the presence of water, hydronium ions formed within the micropores of zeolite H-BEA significantly influence the sur-rounding environment and the reactivity of organic substrates. The positive charge of these ions, coupled with the zeolite's negatively charged framework, results in an ionic environment that causes strongly non-ideal solvation behavior of cyclohex-anol. This leads to a significantly higher excess chemical potential in the initial state and stabilizes at the same time the charged transition state in the dehydration of cyclohexanol. As a result, the free-energy barrier of the reaction is lowered, leading to a marked increase in reaction rates. Nonetheless, theremore » is a limit to the reaction rate enhancement by hydronium ion concentration. Experiments conducted with low concentrations of reactants show that beyond an optimal concentration, the required spatial rearrangement between hydronium ions and cyclohexanols inhibits further increases in the reaction rate, leading to a peak in the intrinsic activity of hydronium ions. The quantification of excess chemical potential in both initial and transition states for zeolites HBEA, along with findings from HMFI, provides a basis to generalize and predict rates for hydronium-ion catalyzed dehydration reactions in Brønsted zeolites.« less

Abstract 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 (AlCl ₃ ) and gallium chloride (GaCl ₃ ). Already at 60 °C, complete LDPE conversion was achieved, while maintaining the selectivity for gasoline‐range liquid alkanes over 70 %.more » AlCl ₃ 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.« less

Abstract 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 (AlCl ₃ ) and gallium chloride (GaCl ₃ ). Already at 60 °C, complete LDPE conversion was achieved, while maintaining the selectivity for gasoline‐range liquid alkanes over 70 %.more » AlCl ₃ 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.« less

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Brønsted-acid sites are introduced via –OSO₃H groups on the coordinatively unsaturated ZrO₂ nodes of UiO-66 metal organic frameworks (MOF). Such groups create strong Brønsted acidic sites that are active for cyclohexanol and ethanol dehydration (in liquid organic phase and gas phase, respectively). The intrinsic activity of Brønsted acid sites at nodes increased by increasing the concentration of sulfur, which is attributed to a shift from isolated µ₃-OSO₃H to two groups [(µ₃-OSO₃H)₂] interacting via hy-drogen bonding. For cyclohexanol dehydration, the relatively low activation enthalpies and negative transition entropies point to an E2 elimination mechanism, similar to dehydration with MFI zeolites inmore » organic solvents. Furthermore, our results, show that the catalytic activity can be manipulated via the functionalization of zirconia nodes of the MOF framework.« less

Ultraviolet (UV) photolysis of nitrite ions (NO₂^–) in aqueous solutions produces a suite of radicals, viz., NO^·, O^–, ^·OH, and ^·NO₂. The O^– and NO^· radicals are initially formed from the dissociation of photoexcited NO₂^–. The O^– radical undergoes reversible proton transfer with water to generate ^·OH. Both ^·OH and O^– oxidize the NO₂^– to ^·NO₂ radicals. The reactions of ^·OH occur at solution diffusion limits, which are influenced by the nature of the dissolved cations and anions. Here, we systematically varied the alkali metal cation, spanning the range from strongly to weakly hydrating ions, and measured the production ofmore » NO^·, ^·OH, and ^·NO₂ radicals during UV photolysis of alkaline nitrite solutions using electron paramagnetic resonance spectroscopy with nitromethane spin trapping. Comparing the data for the different alkali cations revealed that the nature of the cation had a significant effect on production of all three radical species. Radical production was inhibited in solutions with high charge density cations, e.g., lithium, and promoted in solutions containing low charge density cations, e.g., cesium. Through complementary investigations with multinuclear single pulse direct excitation nuclear magnetic resonance (NMR) spectroscopy and pulsed field gradient NMR diffusometry, cation-controlled solution structures and extent of NO₂^– solvation were determined to alter the initial yields of ^·NO and ^·OH radicals as well as alter the reactivity of NO₂^– toward ^·OH, impacting the production of ^·NO₂. The implications of these results for the retrieval and processing of low-water, highly alkaline solutions that comprise legacy radioactive waste are discussed.« less