Recent progress in aerobic oxidation of hydrocarbons by N-hydroxyimides

Synthesis and Deconstruction of Polyethylene-type Materials

Polyethylene deconstruction to reusable smaller molecules is hindered by the chemical inertness of its hydrocarbon chains. Pyrolysis and related approaches commonly require high temperatures, are energy-intensive, and yield mixtures of multiple classes of compounds. Selective cleavage reactions under mild conditions ( Collapse

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Grants

• H2020 European Research Council

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Affiliation(s)

Simon T. Schwab Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, Universitätsstraße 10, 78464 Konstanz, Germany
Maximilian Baur Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, Universitätsstraße 10, 78464 Konstanz, Germany
Taylor F. Nelson Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, Universitätsstraße 10, 78464 Konstanz, Germany
Stefan Mecking Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, Universitätsstraße 10, 78464 Konstanz, Germany

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Low-Temperature Upcycling of Polypropylene Waste into H2 Fuel via a Novel Tandem Hydrothermal Process

Plastic waste is a promising and abundant resource for H2 production. However, upcycling plastic waste into H2 fuel via conventional thermochemical routes requires relatively considerable energy input and severe reaction conditions, particularly for polyolefin waste. Here, we report a tandem strategy for the selective upcycling of polypropylene (PP) waste into H2 fuel in a mild and clean manner. PP waste was first oxidized into small-molecule organic acids using pure O2 as oxidant at 190 °C, followed by the catalytic reforming of oxidation aqueous products over ZnO-modified Ru/NiAl2 O4 catalysts to produce H2 at 300 °C. A high H2 yield of 44.5 mol/kgPP and a H2 mole fraction of 60.5 % were obtained from this tandem process. The entire process operated with almost no solid residue remaining and equipment contamination, ensuring relative stability and cleanliness of the reaction system. This strategy provides a new route for low-temperature transforming PP and improving the sustainability of plastic waste disposal processes.

Degradable and Recyclable Polyesters from Multiple Chain Length Bio- and Waste-Sourceable Monomers

Monomers sourced from waste or biomass are often mixtures of different chain lengths; e.g. catalytic oxidation of polyethylene waste yields mixtures of dicarboxylic acids (DCAs). Yet, polyesters synthesized from such monomer mixtures have rarely been studied. We report polyesters based on multiple linear aliphatic DCAs, present in chain length distributions that vary in their centers and ranges. We demonstrate that these materials can adopt high-density polyethylene-like solid state structures, and are ductile (e.g. Et 610 MPa), allowing for injection molding, or film and fiber extrusion. Melting and crystallization points of the polyesters show no odd-even effects as dipoles cannot favorably align in the crystal, similar to traditional odd carbon numbered, long-chain DCA polyesters. Biodegradation studies of 13 C-labelled polyesters in soil reveal rapid mineralization, and depolymerization by methanolysis indicates suitability for closed-loop recycling.

The design of PINO-like hydrogen-atom-transfer catalysts

Phthalimide-N-oxyl (PINO) is a valuable hydrogen-atom-transfer (HAT) catalyst for selective C-H functionalization. To advance and optimize PINO-catalysed HAT reactions, researchers have been focused on modifying the phthalimide core structure. Despite much effort and some notable advances, the modifications to date have centred on optimization of a single parameter of the catalyst, such as reactivity, solubility or stability. Unfortunately, the optimization with respect to one parameter is often associated with a worsening of the others. The derivation of a single catalyst structure with optimal performance across multiple parameters has therefore remained elusive. Here we present an analysis of the structure-activity relationships of PINO and its derivatives as HAT catalysts, which we hope will stimulate further development of PINO-catalysed HAT reactions and, ultimately, lead to much improved catalysts for real-world applications.

Selective Oxidation of Cyclohexanone to Adipic Acid Using Molecular Oxygen in the Presence of Alkyl Nitrites and Transition Metals as Catalysts

This paper presents a not previously reported catalytic system consisting of transition metals Co2+ and Mn2+ and alkyl nitrites R-ONO for the oxidation of cyclohexanone with oxygen to adipic acid. The influence of type and amount of catalyst, temperature, time, and type of raw material on conversion and product composition were determined. In addition, the oxidation of selected cyclic ketones such as cyclopentanone, cyclohexanone, cyclooctanone, cyclododecanone, 2-methylcyclohexanone, 3-methylcyclohexanone, and 4-methylcyclohexanone in acetic acid as solvent was performed. The results showed that R-ONO systems, under established reaction conditions, form NO·radicals, which oxidize to NO2 under a strong oxidization reaction environment. The Co2+/Mn2+/NO2 system was shown to be highly active in the oxidation of cyclic ketones with oxygen.

Mild air oxidation of boron nitride nanotubes. Application as nanofillers for thermally conductive polycarbonate nanocomposites

Boron nitride nanotubes (BNNTs) have experienced considerable growth in recent years due to their unique intrinsic properties, in particular for the fabrication of polymer nanocomposites. Dispersion of pure BNNTs in nanocomposites is often difficult due to their poor compatibility with most polymer matrices. An approach involving the creation of hydroxyl groups on their surface could improve their dispersion. While some harsh oxidation processes have been reported so far, a mild oxidation of BNNTs using air as the oxidant is reported here. This new catalytic reaction leads to slightly oxidized BNNTs, which were characterized by scanning electron microscope, x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and thermogravimetric analysis. Polycarbonate nanocomposites were then fabricated using pristine and oxidized BNNTs as nanofillers. The measured thermal conductivity increased linearly with the mildly oxidized BNNTs content. It reached a five-fold increase up to 1.19 W m.K^-1at 15% vol. content which is significantly improved over nanocomposites fabricated with severely oxidized BNNTs, while the electrically insulating character remained unchanged.

Mixed plastics waste valorization through tandem chemical oxidation and biological funneling

Mixed plastics waste represents an abundant and largely untapped feedstock for the production of valuable products. The chemical diversity and complexity of these materials, however, present major barriers to realizing this opportunity. In this work, we show that metal-catalyzed autoxidation depolymerizes comingled polymers into a mixture of oxygenated small molecules that are advantaged substrates for biological conversion. We engineer a robust soil bacterium, Pseudomonas putida, to funnel these oxygenated compounds into a single exemplary chemical product, either β-ketoadipate or polyhydroxyalkanoates. This hybrid process establishes a strategy for the selective conversion of mixed plastics waste into useful chemical products.

Selective Aerobic Oxidation of P-Methoxytoluene by Co(II)-Promoted NHPI Incorporated into Cross-Linked Copolymer Structure

A wide series of copolymer materials with various contents of 4-vinyl-diisopropyl-phtalate ester (10–90 mol%), divinylbenzene (1–11 mol%) and styrene, as monomers, were obtained by radical copolymerization. In the last steps of the synthesis, diisopropyl ester functionalities were converted into the form of N-hydroxyphthalimide (NHPI) rings. The obtained materials with the NHPI groups immobilized in the copolymer structure were studied by various physicochemical techniques, including FT-IR, UV-Vis-DR, XPS, elemental analysis, and tested as catalysts in aerobic oxidation of p-methoxytoluene in the presence of Co(II) acetate co-catalyst. Conversion of the aromatic substrate was correlated with the NHPI content and cross-linking degree. The best catalytic performance (conversions higher than 23%) was achieved for the copolymer catalysts containing 60% and 30% of 4-vinyl-diisopropyl-phtalate ester. At too high concentrations of NHPI and DVB, some of the NHPI groups were transformed into inactive (C=O)-N=O species or not available due to embedding inside the copolymer structure. The mechanism of the process involving both NHPI centers, forming phthalimide N-oxyl (PINO) radicals, and Co(II) cations was discussed. Stability of the developed catalysts was also tested. The opening of imide rings took place during the catalytic process, resulting in the formation of carboxyl groups and the release of hydroxylamine molecules. The deactivated catalyst could be easily regenerated by repeating two last steps of closing imide ring.

Covalent anchoring of N-hydroxyphthalimide on silica via robust imide bonds as a reusable catalyst for the selective aerobic oxidation of ethylbenzene to acetophenone

N-Hydroxyphthalimide is anchored on commercial silica by robust imide bonds, and the synthesized N-oxyl catalysts exhibit excellent activity, selectivity and reusability for the aerobic oxidation of ethylbenzene to acetophenone.

N-Hydroxybenzimidazole as a structurally modifiable platform for N-oxyl radicals for direct C-H functionalization reactions

A novel class of N-oxy radicals based on flexibly modifiable N-hydroxybenzimidazole skeleton was designed and applied to C–H functionalization reactions.

Methods for direct functionalization of C–H bonds mediated by N-oxyl radicals constitute a powerful tool in modern organic synthesis. While several N-oxyl radicals have been developed to date, the lack of structural diversity for these species has hampered further progress in this field. Here we designed a novel class of N-oxyl radicals based on N-hydroxybenzimidazole, and applied them to the direct C–H functionalization reactions. The flexibly modifiable features of these structures enabled facile tuning of their catalytic performance. Moreover, with these organoradicals, we have developed a metal-free approach for the synthesis of acyl fluorides via direct C–H fluorination of aldehydes under mild conditions.

Metal complex catalysis in the chemistry of lower diamondoids

The review presents the first survey of published data on the use of compounds, complexes and nanoparticles of transition metals (Fe, Co, Ni, Mn, V, Mo, Cu, Pd, Pt, Rh, Ru, Os, Au, Re and Th) in the catalytic transformations of lower diamondoids — adamantane, diamantane and their derivatives. Catalytic halogenation, oxidation, alkylation and cross-coupling reactions are considered, and the formation pathways of C–N, C–S and C–Se bonds in the series of adamantanoids are discussed. Reaction conditions, appropriate catalytic systems and the structures of products are presented.

The bibliography includes 242 references.

Catalytic Aerobic Oxidation of C(sp3 )-H Bonds

Oxidation reactions are a key technology to transform hydrocarbons from petroleum feedstock into chemicals of a higher oxidation state, allowing further chemical transformations. These bulk-scale oxidation processes usually employ molecular oxygen as the terminal oxidant as at this scale it is typically the only economically viable oxidant. The produced commodity chemicals possess limited functionality and usually show a high degree of symmetry thereby avoiding selectivity issues. In sharp contrast, in the production of fine chemicals preference is still given to classical oxidants. Considering the strive for greener production processes, the use of O₂ , the most abundant and greenest oxidant, is a logical choice. Given the rich functionality and complexity of fine chemicals, achieving regio/chemoselectivity is a major challenge. This review presents an overview of the most important catalytic systems recently described for aerobic oxidation, and the current insight in their reaction mechanism.

Heterogeneous Catalytic Oxidation of Amides to Imides by Manganese Oxides

Herein, we report a one-step peroxide mediated heterogeneous catalytic oxidation of amides to imides utilizing a series of manganese oxides. Among them, Cs/Mn2O3 was found to be the most active catalyst for the selective partial oxidation of N-benzylbenzamide to diphenyl imide. We have been able to apply an optimized oxidation method to other aromatic substrates. The feasibility of using air as an oxidant, the heterogeneous nature, inexpensive catalytic materials, respectable turnover numbers, and chemoselectivity to imides make this methodology an attractive choice for functional group transformations of amides to imides.

Distinguishing ionic and radical mechanisms of hydroxylamine mediated electrocatalytic alcohol oxidation using NO–H bond dissociation energies

An idea is proposed to sort N-oxyl radicals with respect to their mechanisms of electrocatalytic alcohol oxidation by knowing the NO–H bond dissociation energies of their precursors.

A dendritic TiO2–Co(ii) nanocomposite based on the melamine catalyzed one-pot aerobic photocatalytic synthesis of benzimidazoles

The nanocrystalline TiO₂ surface modified with cobalt melamine dendrimer amine complex under ultrasonic agitation produced a novel low band-gap photocatalyst (Co(ii)–TD@TiO₂). This catalyst system involves photooxidation of alcohols to aldehydes followed by cyclocondensation with 1,2-phenylenediamine to afford benzimidazoles in one-pot operation with good to excellent yield.

Oxidation of p-toluic acid to terephthalic acid via a bromine-free process using nano manganese and manganese–copper mixed oxides

Challenges and technological opportunities for nano-mixed oxide (Cu–Mn) allowing a green route to valued terephthalic acid production from p-xylene.

A cobalt Schiff base complex on TiO2nanoparticles as an effective synergistic nanocatalyst for aerobic C–H oxidation

A cobalt Schiff base complex on TiO₂nanoparticles possess excellent photocatalytic properties under visible light for the aerobic benzylic C–H oxidation.

Aerobic benzylic C–H oxidation catalyzed by a titania-based organic–inorganic nanohybrid

The visible light photocatalytic properties of TiO₂/AA/Co organic–inorganic nanohybrid in the aerobic benzylic C–H oxidation was exploited.