In pursuit of more sustainable and competitive biorefineries, the effective valorisation of lignin is key. An alluring opportunity is the exploitation of lignin as a resource for chemicals. Three technological biorefinery aspects will determine the realisation of a successful lignin-to-chemicals valorisation chain, namely (i) lignocellulose fractionation, (ii) lignin depolymerisation, and (iii) upgrading towards targeted chemicals. This review provides a summary and perspective of the extensive research that has been devoted to each of these three interconnected biorefinery aspects, ranging from industrially well-established techniques to the latest cutting edge innovations. To navigate the reader through the overwhelming collection of literature on each topic, distinct strategies/topics were delineated and summarised in comprehensive overview figures. Upon closer inspection, conceptual principles arise that rationalise the success of certain methodologies, and more importantly, can guide future research to further expand the portfolio of promising technologies. When targeting chemicals, a key objective during the fractionation and depolymerisation stage is to minimise lignin condensation (i.e. formation of resistive carbon–carbon linkages). During fractionation, this can be achieved by either (i) preserving the (native) lignin structure or (ii) by tolerating depolymerisation of the lignin polymer but preventing condensation through chemical quenching or physical removal of reactive intermediates. The latter strategy is also commonly applied in the lignin depolymerisation stage, while an alternative approach is to augment the relative rate of depolymerisation vs. condensation by enhancing the reactivity of the lignin structure towards depolymerisation. Finally, because depolymerised lignins often consist of a complex mixture of various compounds, upgrading of the raw product mixture through convergent transformations embodies a promising approach to decrease the complexity. This particular upgrading approach is termed funneling, and includes both chemocatalytic and biological strategies.

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Chemicals from lignin: an interplay of lignocellulose fractionation, depolymerisation, and upgrading

Lignin, a major component of lignocellulose, is the largest source of aromatic building blocks on the planet and harbors great potential to serve as starting material for the production of biobased products. Despite the initial challenges associated with the robust and irregular structure of lignin, the valorization of this intriguing aromatic biopolymer has come a long way: recently, many creative strategies emerged that deliver defined products via catalytic or biocatalytic depolymerization in good yields. The purpose of this review is to provide insight into these novel approaches and the potential application of such emerging new structures for the synthesis of biobased polymers or pharmacologically active molecules. Existing strategies for functionalization or defunctionalization of lignin-based compounds are also summarized. Following the whole value chain from raw lignocellulose through depolymerization to application whenever possible, specific lignin-based compounds emerge that could be in the fu...

Bright Side of Lignin Depolymerization: Toward New Platform Chemicals

Lignin is the most abundant aromatic polymer in nature. Due to its high amount of phenolic compounds storage, lignin is considered as an alternative source for various polymers and biomaterials production. Except for the native lignin in lignocellulose, a massive amount of technical lignin is being produced daily all over the world. However, the complex structure and low reactivity of lignin limit its further applications and currently, most of the lignin is burned for generating energy. Therefore, the depolymerization of lignin is considered one of the important challenges in lignin utilization. Methods for lignin depolymerization can be divided into thermochemical treatment, mechanical treatment, chemical catalysis, and biological treatment. Different methods for lignin depolymerization, their characteristics and products are extensively discussed in this review.

Lignin utilization: A review of lignin depolymerization from various aspects

From lignin to valuable products–strategies, challenges, and prospects

The profitability and sustainability of future biorefineries are dependent on efficient feedstock use. Therefore, it is essential to valorize lignin when using wood. We have developed an integrated biorefinery that converts 78 weight % (wt %) of birch into xylochemicals. Reductive catalytic fractionation of the wood produces a carbohydrate pulp amenable to bioethanol production and a lignin oil. After extraction of the lignin oil, the crude, unseparated mixture of phenolic monomers is catalytically funneled into 20 wt % of phenol and 9 wt % of propylene (on the basis of lignin weight) by gas-phase hydroprocessing and dealkylation; the residual phenolic oligomers (30 wt %) are used in printing ink as replacements for controversial para-nonylphenol. A techno-economic analysis predicts an economically competitive production process, and a life-cycle assessment estimates a lower carbon dioxide footprint relative to that of fossil-based production.

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A sustainable wood biorefinery for low–carbon footprint chemicals production

The hydrodeoxygenation (HDO) of lignin-derived phenols is important to produce the renewable biofuels. Herein, we reported a simple method to prepare magnetic nitrogen-doped carbon supported cobalt nitride catalysts (CoNx@NC) by copyrolysis of cellulose and cobalt nitrate under ammonia atmosphere. The catalysts were prepared at different temperatures and characterized by elemental analysis, atomic absorption spectroscopy (AAS), Brunauer–Emmett–Teller (BET) surface area analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and temperature-programmed reduction (TPR). The CoNx@NC-650 (pyrolyzed at 650 °C) exhibited the best HDO activity for eugenol conversion among a series of Co-based catalysts. The yield of propylcyclohexanol from eugenol was >99.9% under 2 MPa H2 at 200 °C for 2 h. Moreover, a high yield of propylcyclohexane (99.1%) could be achieved when the solid acid HZSM-5 was added to the reaction system. Other lignin-derived phenolic compo...

Selective Hydrodeoxygenation of Lignin-Derived Phenols to Cyclohexanols or Cyclohexanes over Magnetic CoNx@NC Catalysts under Mild Conditions

Depolymerization of lignin via a non-precious Ni–Fe alloy catalyst supported on activated carbon

Cyclohexanols are important feedstock for polymers, spices, and medicines production in industry. In this work, a series of cobalt-based catalysts with different supports were prepared and used to catalyze lignin-derived phenols to cyclohexanols. Among the catalysts, Co/TiO2 showed the best hydrodeoxygenation (HDO) activity. An equivalent of propylcyclohexanol (>99.9%) was achieved under 1 MPa H2, 200 °C for 2 h. According to the characterization results of transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) surface area analysis, powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR), hydrogen temperature-programmed desorption (H2-TPD) and NH3-TPD, the particle size and dispersion of Co could have important influence on catalytic activity. For Co/TiO2, the SMSI effect may significantly affect the catalytic activity. The influences of different temperature, H2 pressure and reaction time on the eugenol conversion by Co/TiO...

Selective Hydrodeoxygenation of Lignin-Derived Phenols to Cyclohexanols over Co-Based Catalysts

Tetrahydrofurfuryl alcohol (THFAL), as a green industrial solvent, can be obtained directly from biomass-derived furfural with 100% conversion and 100% yield over a hydroxyapatite-supported Pd catalyst (Pd-HAP) under relatively mild conditions (40 °C, 1 MPa H2, and 3 h) in 2-propanol. At room temperature and reacting for 8 h, the yield of THFAL was more than 99%. By capturing the intermediates, two pathways were proposed as follows: (1) Furfural was partially hydrogenated to furfuryl alcohol, and then, furfuryl alcohol was further hydrogenated to THFAL. (2) Furfural and 2-propanol first formed 2-(isopropoxymethyl)furan (2-IPMF) via etherification, and then, 2-IPMF was ultimately converted to THFAL. The Pd-HAP catalyst was characterized by various techniques including XRD, SEM, TEM, HAADF-STEM, XPS, and ICP-AAS. The results revealed that the outstanding catalytic performance of Pd-HAP was attributed to the quasicoordination effect between Pd and HAP, which not only contributed to highly dispersed and stabl...

Xiaohao Liu

Papers

Selective Hydrodeoxygenation of Lignin-Derived Phenols to Cyclohexanols or Cyclohexanes over Magnetic CoNx@NC Catalysts under Mild Conditions

Depolymerization of lignin via a non-precious Ni–Fe alloy catalyst supported on activated carbon

Selective Hydrodeoxygenation of Lignin-Derived Phenols to Cyclohexanols over Co-Based Catalysts

Hydrogenation of Biomass-Derived Furfural to Tetrahydrofurfuryl Alcohol over Hydroxyapatite-Supported Pd Catalyst under Mild Conditions

Hydrogenation of biomass-derived ethyl levulinate into γ-valerolactone by activated carbon supported bimetallic Ni and Fe catalysts