Mathias

This review discusses recent developments in the chemistry of photocatalytic biomass conversion into chemicals and fuels, emphasizing the role of experimental factors and materials design. A critical technological analysis to facilitate the scaling up of this technology is provided based on the reported research studies and industrial need.

Abstract

Photocatalytic biomass conversion into high-value chemicals and fuels is considered one of the hottest ongoing research and industrial topics toward sustainable development. In short, this process can cleave C_β−O/C_α−C_β bonds in lignin to aromatic platform chemicals, and further conversion of the polysaccharides to other platform chemicals and H₂. From the chemistry point of view, the optimization of the unique cooperative interplay of radical oxidation species (which are activated via molecular oxygen species, ROSs) and substrate-derived radical intermediates by appropriate control of their type and/or yield is key to the selective production of desired products. Technically, several challenges have been raised that face successful real-world applications. This review aims to discuss the recently reported mechanistic pathways toward selective biomass conversion through the optimization of ROSs behavior and materials/system design. On top of that, through a SWOT analysis, we critically discussed this technology from both chemistry and technological viewpoints to help the scientists and engineers bridge the gap between lab-scale and large-scale production.

Synlett
DOI: 10.1055/s-0042-1751443

Applications of photoexcited nitroarenes have been underdeveloped in organic synthesis. Since early reports on the direct excitation of nitroaromatics with harsh UV light, these synthetically useful reagents have not been tamed for use in modern synthetic chemistry. We have developed practical synthetic protocols for the anaerobic oxidation of hydrocarbon substrates using commercially available nitroarenes as photochemically activated oxidants under visible light. Using this approach, a wide variety of olefins are anaerobically cleaved to their corresponding carbonyls, and aliphatic C–H bonds are hydroxylated to give alcohols. The anaerobic reaction conditions enable oxidatively sensitive functional groups to be tolerated and the employment of visible light makes this method highly sustainable. Mechanistic studies support that the photoexcited nitroarene biradical intermediate is responsible for the oxygen atom transfer events.1 Introduction2 Alkene Cleavage Promoted by Photoexcited Nitroarenes3 Photoinduced Nitroarene-Mediated C–H Hydroxylation4 Conclusions
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

Article in Thieme eJournals:
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Cleave the bond: Zeolite depolymerized β-guaiacyl ether to produce monomers (≈87 %). Identification of various products reveals the crucial role of hydrogen, water, and acid sites in the heterolytic cleavage of the β-O-4 bond. Mechanistic analysis reveals that dehydration, decarbonylation, and hydrogenolysis were the main reactions taking place along with demethylation and hydrogenation.

Abstract

Efficient cleavage of β-O-4 bonds in lignin to high-yield aromatic compounds for the potential production of fuels and chemicals is vital for the economics of the modern biorefinery industry. This work is distinct in that a detailed mechanistic analysis of the reaction pathways of veratrylglycero-β-guaiacyl ether (VGE) catalyzed by transition-metal-free solid acid zeolite in aqueous conditions at high hydrogen pressure has been performed. VGE degradation produced high monomers yields (≈87 %), including guaiacol (48.2 %), 1-(3,4-dimethoxyphenyl)ethanol (10.3 %), 1-(3,4-dimethoxyphenyl)-2-propanol (6.1 %), 3,4-dimethoxyphenylpropanol (4.7 %), 3,4-dimethoxycinnamyl alcohol (4.1 %), and 1,2-dimethoxy-4-propylbenzene (2 %). The products were identified and confirmed by the in situ solid-state magic angle spinning (MAS) ¹³C NMR spectroscopy in real-time conditions and the two-dimensional gas chromatography (GC×GC). A variety of products reveal the crucial role of hydrogen, water, and acid sites for heterolytic cleavage of the β-O-4 bond in VGE. Decarbonylation, hydrogenolysis, hydrogenation, and dehydration reaction pathways are proposed and further validated using first-principles calculations.

Synlett
DOI: 10.1055/a-1918-4191

An account of our development of reactions to construct N-heterocycles by triggering cyclization–migration tandem reactions from aryl azides, nitroarenes, and aryl amines is described. The reactivity patterns of metal N-aryl nitrenes, nitrosoarenes, N-aryl nitrogen radical anions, and N-aryl nitrenoids are compared.1 Introduction2 Unlocking the Reactivity Embedded in Aryl Azides3 Exploiting the Reactivity of Nitrosoarenes Generated from Nitroarenes4 Radical Anion N-Aryl Nitrogen Reactive Intermediates from Nitroarenes5 Oxidation of Aryl Amines to Access Electrophilic N-Aryl Nitrenoids6 Conclusion
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

Article in Thieme eJournals:
Table of contents  |  Abstract  |  Full text