Nondegradable polyolefin plastics pose severe environmental threats and thus demand efficient upcycling technologies. In this work, we discovered that low-loading (≤0.25 wt %) Ru/CeO2 exhibits remarkable catalytic performance in the hydrogenolysis of polypropylene (PP), polyethylene (PE), and n-C16H34 that is superior to high-loading (≥0.5 wt %) Ru/CeO2. They possess high PP conversion efficiency (sevenfold increase over current literature reports), low selectivity toward undesired CH4, and good isomerization ability. In the low-loading range, the intrinsic activity of Ru in PP hydrogenolysis increases as the particle size decreases, opposite of the trend in the high-loading range. Detailed characterization revealed that the abrupt changes in catalytic behaviors coincide with Ru species transitioning from well-defined to highly disordered structures in the low-loading domain. The disordered Ru species were shown to be sub-nanometer in size and cationic. Mechanistically, the regioselectivity and the rate dependence on hydrogen pressure of C–C bond cleavage are different on low- and high-loading Ru/CeO2, both explained by the higher coverage of adsorbed hydrogen (*H) on low-loading Ru/CeO2. This work reveals the remarkable catalytic performance of highly disordered, sub-nanometer, cationic Ru species in polyolefin hydrogenolysis, opening immense opportunities to develop effective, selective, and versatile catalysts for plastic upcycling.
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.
We investigated the electrocatalytic hydrogenation (ECH) of model aldehydes and ketones over carbon-supported Pd in the aqueous phase. We propose reaction mechanisms based on kinetic measurements and on spectroscopic and electrochemical characterization of the working catalyst. The reaction rates of ECH and of the H 2 evolution reaction (HER) vary with the applied electric potential following trends that strongly depend on the organic substrate. The intrinsic rates of hydrogenation and H 2 evolution are influenced, in opposing ways, by the sorption of the reacting organic substrate. Strong interactions, that is, higher standard free energies of adsorption of the organic compound, induce high hydrogenation rates. The fast hydrogenation kinetics produces a hydrogen-depleted environment that kinetically hinders the HER and the bulk phase transition of Pd to a H-rich bulk Pd hydride, which is triggered by the applied potential in the absence of reacting organic compounds. As a consequence of strong organic−metal interactions, hydrogenation dominates at low overpotential. However, the coverages of organic substrates on the metal surface decrease, and the rates of H 2 evolution surpass those of hydrogenation with increasingly negative electric potential. We determined the range of electric potential favoring hydrogenation on Pd and quantitatively deconvoluted the effects of the sorption of the organic compound, and of the rates of proton-coupled electron transfers, on the kinetics of both ECH and HER. The results indicate that electrocatalysis offers hydrogenation pathways for polar molecules which are different and, in some cases, faster than those dominating in the absence of an external electric potential.
Hydrogenation on Mo and Wsulfides occurs at the edges of the sulfide slabs.T he rate of hydrogen addition is directly proportional to the concentration of sulfhydryl (SH) groups at the slab edge and the metal atom attached to it. Sulfhydryl groups vicinal to edge-incorporated Ni hydrogenate with muchh igher rates than SH close to Mo and W. Each subset of SH groups,however,exhibits nearly identical intrinsic activity and selectivity,independent of the sulfide composition. The higher activity of Ni-WS 2 compared to Ni-MoS 2 stems from ah igher concentration of SH groups on the former sulfide associated with ah igher tendency of its surface vacancies to react with H 2 .
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