Conversion of Lignin Models by Photoredox Catalysis

Visible light-driven ligand-to-metal charge transfer-mediated selective cleavage of β-O-4 lignin model compounds: a greener route to lignin valorization

Lignin is the most abundant renewable source of aromatics in nature. The β-O-4 bond is the most predominant linkage in lignin; therefore, methods for the selective cleavage of the β-O-4 bond are of great importance in order to break down lignin and produce value-added aromatic compounds. Herein, we report a visible light-driven, ligand-to-metal charge transfer (LMCT)-mediated, two-step approach for cleaving Cβ-O bonds in β-O-4 alcohol model compounds using titania (TiO2) as a photocatalyst. In the first step, the alcohol forms a visible light-absorbing LMCT complex on the surface of titania, which enables oxidation to the corresponding ketone under green light. The LMCT-mediated oxidation afforded high conversion of β-O-4 alcohol model compounds (79-97%) with high selectivity for β-O-4 ketones (>95%). Our studies reveal that the superoxide radical anion likely plays a key role in the oxidation. In the second step, the LMCT-assisted reductive cleavage of β-O-4 ketone is achieved by employing triethylammonium tetraphenylborate as a visible light sensitizer and proton donor. The LMCT-facilitated reductive cleavage of β-O-4 ketones exhibits high selectivity (up to 100%) for target fragmentation products under blue light. Experiments including EPR analysis suggest that in situ formed Ti3+ is responsible for the reductive cleavage of β-O-4 ketones. Moreover, a two-step, one-pot cleavage reaction was successfully carried out with good to high selectivity for Cβ-O bond cleavage products with a single catalyst. Our work offers a promising solution for the selective cleavage of β-O-4 bonds under mild conditions to promote lignin valorization. Furthermore, it provides potentially general strategies for enabling visible light-driven LMCT-mediated photocatalysis in related organic transformations.

Photochemical reactions of biomass derived platform chemicals

Platform chemicals obtained from biomass will play an important role in chemical industry. Already existing compounds or not yet established chemicals are produced from this renewable feedstock. Using photochemical reactions as sustainable method for the conversion of matter furthermore permits to develop processes that are interesting from the ecological and economical point of view. Furans or levoglucosenone are thus obtained from carbohydrate containing biomass. Photochemical rearrangements, photooxygenation reactions or photocatalytic radical reactions can be carried out with such compounds. Also, sugars such pentoses or hexoses can be more easily transformed into heterocyclic target compounds when such photochemical reactions are used. Lignin is an important source for aromatic compounds such as vanillin. Photocycloaddition of these compounds with alkenes or the use light supported multicomponent reactions yield interesting target molecules. Dyes, surfactants or compounds possessing a high degree of molecular diversity and complexity have been synthesized with photochemical key steps. Alkenes as platform chemicals are also produced by fermentation processes, for example, with cyanobacteria using biological photosynthesis. Such alkenes as well as terpenes may further be transformed in photochemical reactions yielding, for example, precursors of jet fuels.

Photoredox catalysis enabling decarboxylative radical cyclization of γ,γ-dimethylallyltryptophan (DMAT) derivatives: formal synthesis of 6,7-secoagroclavine

An unusual photoredox-catalyzed radical decarboxylative cyclization cascade reaction of γ,γ-dimethylallyltryptophan (DMAT) derivatives containing unactivated alkene moieties has been developed, providing green and efficient access to various six-, seven-, and eight-membered ring 3,4-fused tricyclic indoles. This type of cyclization, which was hitherto very difficult to comprehend in ergot biosynthesis and to accomplish by more conventional procedures, enables the synthesis of ergot alkaloid precursors. In addition, this work describes a mild, environmentally friendly method to activate, reductively and oxidatively, natural carboxylic acids for decarboxylative C-C bond formation by exploiting the same photocatalyst.

Photocatalytic Transformation of Biomass and Biomass Derived Compounds-Application to Organic Synthesis

Biomass and biomass-derived compounds have become an important alternative feedstock for chemical industry. They may replace fossil feedstocks such as mineral oil and related platform chemicals. These compounds may also be transformed conveniently into new innovative products for the medicinal or the agrochemical domain. The production of cosmetics or surfactants as well as materials for different applications are examples for other domains where new platform chemicals obtained from biomass can be used. Photochemical and especially photocatalytic reactions have recently been recognized as being important tools of organic chemistry as they make compounds or compound families available that cannot be or are difficultly synthesized with conventional methods of organic synthesis. The present review gives a short overview with selected examples on photocatalytic reactions of biopolymers, carbohydrates, fatty acids and some biomass-derived platform chemicals such as furans or levoglucosenone. In this article, the focus is on application to organic synthesis.

Lignocellulosic biomass valorization via bio-photo/electro hybrid catalytic systems

Lignocellulosic biomass valorization is regarded as a promising approach to alleviate energy crisis and achieve carbon neutrality. Bioactive enzymes have attracted great attention and been commonly applied for biomass valorization owing to their high selectivity and catalytic efficiency under environmentally benign reaction conditions. Same as biocatalysis, photo-/electro-catalysis also happens at mild conditions (i.e., near ambient temperature and pressure). Therefore, the combination of these different catalytic approaches to benefit from their resulting synergy is appealing. In such hybrid systems, harness of renewable energy from the photo-/electro-catalytic compartment can be combined with the unique selectivity of biocatalysts, therefore providing a more sustainable and greener approach to obtain fuels and value-added chemicals from biomass. In this review, we firstly introduce the pros/cons, classifications, and the applications of photo-/electro-enzyme coupled systems. Then we focus on the fundamentals and comprehensive applications of the most representative biomass-active enzymes including lytic polysaccharide monooxygenases (LPMOs), glucose oxidase (GOD)/dehydrogenase (GDH) and lignin peroxidase (LiP), together with other biomass-active enzymes in the photo-/electro- enzyme coupled systems. Finally, we propose current deficiencies and future perspectives of biomass-active enzymes to be applied in the hybrid catalytic systems for global biomass valorization.

Photocatalytic Reforming of Biomass: What Role Will the Technology Play in Future Energy Systems

Photocatalytic reforming of biomass has emerged as an area of significant interest within the last decade. The number of papers published in the literature has been steadily increasing with keywords such as 'hydrogen' and 'visible' becoming prominent research topics. There are likely two primary drivers behind this, the first of which is that biomass represents a more sustainable photocatalytic feedstock for reforming to value-added products and energy. The second is the transition towards achieving net zero emission targets, which has increased focus on the development of technologies that could play a role in future energy systems. Therefore, this review provides a perspective on not only the current state of the research but also a future outlook on the potential roadmap for photocatalytic reforming of biomass. Producing energy via photocatalytic biomass reforming is very desirable due to the ambient operating conditions and potential to utilise renewable energy (e.g., solar) with a wide variety of biomass resources. As both interest and development within this field continues to grow, however, there are challenges being identified that are paramount to further advancement. In reviewing both the literature and trajectory of the field, research priorities can be identified and utilised to facilitate fundamental research alongside whole systems evaluation. Moreover, this would underpin the enhancement of photocatalytic technology with a view towards improving the technology readiness level and promoting engagement between academia and industry.

Promising and efficient lignin degradation versatile strategy based on DFT calculations

The extraction of higher-value products from lignin degradations under mild conditions is a challenge. Previous research reported efficient two-step oxidation and reduction strategies for lignin degradation, which has great significance to lignin degradation. In this paper, the mechanism about the C-O bond cleavage of lignin with and without Cα oxidations has been studied systematically. Our calculation results show that the degradation of anionized lignin with Cα oxidations is kinetically and thermodynamically feasible. In addition, the calculations predict that the anionized lignin compounds without Cα oxidation also could be degraded under mild conditions. Moreover, we propose special lignin catalytic degradation systems containing the characteristic structure of "double hydrogen bonds." The double hydrogen bonds structure could further decrease the energy barriers of the C-O bond cleavage reaction. This provides a versatile strategy to design novel lignin degradation.

Cleavage via Selective Catalytic Oxidation of Lignin or Lignin Model Compounds into Functional Chemicals

Lignin, a complex aromatic polymer with different types of methoxylated phenylpropanoid connections, enables the sustainable supply of value-added chemicals and biofuels through its use as a feedstock. Despite the development of numerous methodologies that upgrade lignin to high-value chemicals such as drugs and organic synthesis intermediates, the variety of valuable products obtained from lignin is still very limited, mainly delivering hydrocarbons and oxygenates. Using selective oxidation and activation cleavage of lignin, we can obtain value-added aromatics, including phenols, aldehydes, ketones, and carboxylic acid. However, biorefineries will demand a broad spectrum of fine chemicals in the future, not just simple chemicals like aldehydes and ketones containing simple C = O groups. In particular, most n-containing aromatics, which have found important applications in materials science, agro-chemistry, and medicinal chemistry, such as amide, aniline, and nitrogen heterocyclic compounds, are obtained through n-containing reagents mediating the oxidation cleavage in lignin. This tutorial review provides updates on recent advances in different classes of chemicals from the catalytic oxidation system in lignin depolymerization, which also introduces those functionalized products through a conventional synthesis method. A comparison with traditional synthetic strategies reveals the feasibility of the lignin model and real lignin utilization. Promising applications of functionalized compounds in synthetic transformation, drugs, dyes, and textiles are also discussed.

Cleavage of Organosolv Lignin to Phenols Using Nitrogen Monoxide and Hydrazine

From the variety of methods known for the depolymerization of organosolv lignin, a broad range of diversely substituted aromatic compounds are available today. In the present work, a novel two-step reaction sequence is reported, which is focused on the formation of phenols. While the first step of the depolymerization strategy comprises the 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-catalyzed oxidation of organosolv lignin with nitrogen monoxide so that two waste materials are combined, cleavage to the phenolic target compounds is achieved in the second step employing hydrazine and potassium hydroxide under Wolff-Kishner-type conditions. Besides the fact that the novel strategy proceeds via an untypical form of oxidized organosolv lignin, the two-step sequence is further able to provide phenols as cleavage products, which bear no substituent at the 4-position.

Recent Advances in Photocatalytic Transformation of Carbohydrates Into Valuable Platform Chemicals

In response to the less accessible fossil resources and deteriorating environmental problems, catalytic conversion of the abundant and renewable lignocellulosic biomass to replace fossil resources for the production of value-added chemicals and fuels is of great importance. Depolymerization of carbohydrate and its derivatives can obtain a series of C5-C6 monosaccharides (e.g., glucose and xylose) and their derived platform compounds (e.g., HMF and furfural). Selective transformation of lignocellulose using sustainable solar energy via photocatalysis has attract broad interest from a growing scientific community. The unique photogenerated reactive species (e.g., h+, e−, •OH, •O2−, and 1O2), novel reaction pathways as well as the mild reaction conditions make photocatalysis a “dream reaction.” This review is aimed to provide an overview of the up-to-date contributions achieved in the selective photocatalytic transformation of carbohydrate and its derivatives. Photocatalytic methods, properties and merits of different catalytic systems are well summarized. We then put forward future perspective and challenges in this field.

Photocatalytic Conversion of Lignin into Chemicals and Fuels

Lignin, an underutilized component of lignocellulosic biomass, is regarded as a rich reservoir for the production of aromatic chemicals and fuels. Despite extensive research in recent years, lignin's potential is far from being fully unlocked. Photocatalysis that uses sustainable solar energy to drive lignin conversion under mild conditions has been identified as a promising strategy and received growing research interest. This review aims to present a critical introduction to the photocatalytic conversion of lignin, including a summary of lignin conversion pathways and mechanisms, as well as the latest cutting-edge innovations on photocatalyst design and reactor construction. Moreover, the screening of solvents and regulation of other key factors that are involved in photocatalytic lignin conversion are also elucidated and future perspectives and challenges for photocatalytic conversion of lignin into valuable products are discussed.

Photocatalytic transformations of lignocellulosic biomass into chemicals

This review highlights recent advances in photocatalytic transformations of lignocellulosic biomass (polysaccharides and lignin) into chemicals (in particular organic oxygenates).