ABSTRACT
Although polyethylene (PE) and polypropylene (PP) are by far the world's largest volume plastics, only a tiny fraction of these energy-rich polyolefins are currently recycled. Depolymerization of PE to its constituent monomer, ethylene, is highly endothermic and conventionally accessible only through unselective, high-temperature pyrolysis. Here, we provide experimental demonstrations of our recently proposed tandem catalysis strategy, which uses ethylene to convert PE to propylene, the commodity monomer used to make PP. The approach combines rapid olefin metathesis with rate-limiting isomerization. Monounsaturated PE is progressively disassembled at modest temperatures via many consecutive ethenolysis events, resulting selectively in propylene. Fully saturated PE can be converted to unsaturated PE starting with a single transfer dehydrogenation to ethylene, which produces a small amount of ethane (1 equiv per dehydrogenation event). These principles are demonstrated using both homogeneous and heterogeneous catalysts. While selectivity under batch conditions is limited at high conversion by the formation of an equilibrium mixture of olefins, high selectivity to propylene (≥94%) is achieved in a semicontinuous process due to the continuous removal of propylene from the reaction mixture.
Subject(s)
Polyethylene , Polypropylenes , Alkenes , Catalysis , Ethane , Ethylenes , PlasticsABSTRACT
The manuscript reports the synthesis, characterization, and analysis of electronic structure in a series of complexes of small perfluorocarbon ligands with the (PNP)Rh fragment (where PNP is a diarylamido/bis(phosphine) pincer ligand). Reactions of (PNP)Rh(TBE) as the source of (PNP)Rh with CHF3 and C2HF5 produced perfluoroalkylidene complexes (PNP)Rh[double bond, length as m-dash]CF2 and (PNP)Rh[double bond, length as m-dash]C(F)(CF3). (PNP)Rh[double bond, length as m-dash]CF2 could also be obtained via the reaction of (PNP)Rh(TBE) with Me3SiCF3/CsF, with an admixture of (PNP)Rh(C2F4), where TBE = tert-butylethylene. Abstraction of fluoride from these neutral (PNP)RhC x F y complexes was successful, although only abstraction from (PNP)Rh[double bond, length as m-dash]CF2 allowed unambiguous identification of the Rh product, [(PNP)Rh[triple bond, length as m-dash]CF]+. DFT computational studies allowed comparison of relative energies of (PNP)Rh(C2F4) and [(PNP)Rh(C2F3)]+ isomers as well as comparisons between the electronic structure of the [double bond, length as m-dash]CF2, C2F4, and [triple bond, length as m-dash]CF+ complexes and their hydrocarbon analogues.
ABSTRACT
The Pd(I)-Pd(I) dimer [((F)PNP)Pd-](2) reacts with O(2) upon exposure to light to produce either the superoxide ((F)PNP)PdO(2) or the peroxide [((F)PNP)PdO-](2), which exist in equilibrium with free O(2). Both complexes contain square-planar Pd(II) centers. The unpaired electron density in ((F)PNP)PdO(2) is localized on the superoxide ligand.
Subject(s)
Oxygen/chemistry , Palladium/chemistry , Peroxides/chemistry , Superoxides/chemistry , Dimerization , Models, MolecularABSTRACT
Desde 1902, con la comunicación, ahora clásica, de la secretina, la gastroenterología y la endocrinología han permanecido íntimamente relacionadas. El desarrollo a través de los años, de técnicas de investigación morfológica y funcional, ha permitido conocer el origen y las secreciones de las células endocrinas de aparato digestivo y del sistema neuro-entérico. Se describe de manera cronológica el desarrollo del conocimiento en este apasionante campo de la gastroenterología endocrina
Subject(s)
Gastrointestinal Hormones/history , Secretin/historyABSTRACT
Las hormonas gastrointestinales son químicamente aminas y polipéptidos, que de acuerdo a la secuencia de sus aminoácidos se agrupan en familias. Son producidas por una gran variedad de células distribuidas en todo el intestino y páncreas, así como en algunas porciones del sistema nervioso central. Se describen las principales acciones fisiológicas conocidas hasta ahora y las aplicaciones diagnósticas y usos terapéuticos de algunas de ellas
