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

Furfural is a promising renewable platform compound derived from lignocellulosic biomass that can be further converted to biofuels and biochemicals. The highly functionalized molecular structure of furfural makes it a desired raw material for the sustainable production of value-added chemicals containing oxygen atoms. The conversion of furfural to C4 and C5 chemicals by various catalytic processes is reviewed. The C5 chemicals are mainly produced through sequential steps of selective hydrogenation and/or hydrogenolysis, while most of the C4 chemicals are synthesized with selective oxidation as the first step. This review divides the chemical products from furfural into several groups according to their carbon numbers and synthesis routes, with emphasis on the catalysts and reaction mechanisms. The applications of these chemicals and their traditional production from fossil feedstocks have also been added as background information. Additionally, recent advances in the development of heterogeneous catalysts...

Furfural: A Promising Platform Compound for Sustainable Production of C4 and C5 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 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

A new catalytic system was developed for the selective conversion of biomass-derived furfural to cyclopentanone in aqueous solution. CuZnAl catalysts with different Cu/Zn molar ratios (0.5, 1, 2, and 3) and calcination temperatures (350, 500, and 700 °C) were investigated, and the CuZnAl-500–0.5 catalyst (Cu/Zn = 0.5, calcined at 500 °C) showed a remarkable catalytic performance in the reaction. A 62% yield of cyclopentanone was obtained at the optimized conditions (150 °C, 4 MPa H2, 6 h), and the TOF was 9.4 h–1. The catalysts were characterized by nitrogen adsorption, XRD, TEM, N2O titration, ICP, XPS, and a carbon–sulfur analyzer. The factors that influenced the activity of catalysts were also investigated. Additionally, the CuZnAl-500–0.5 was recycled five times and maintained good activity and stability. Hence, the current work presents a new and efficient catalytic system for the conversion of furfural to cyclopentanone. The low-cost nature of the CuZnAl makes it a potential catalyst for the product...

Selective Conversion of Furfural to Cyclopentanone with CuZnAl Catalysts

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

A catalytic system over Ru-HAP catalyst is established to hydrodeoxygenate various oils to long-chain alkanes in water, for potential large-scale renewable diesel production, which has the following advantages. (i) This system is versatile to different oil sources, including Jatropha oil, palm oil, waste cooking oil, and cooking waste. (ii) Ru-HAP is highly efficient at achieving full conversion from stearic acid to alkanes at as low as 100 °C, and the isolated yield from Jatropha oil, palm oil, and waste cooking oil to long-chain alkanes reached up to 95, 96, and 87 mol % at 180 °C and 2 MPa H2 within 4–4.5 h, respectively. (iii) The catalyst showed high stability during five runs of recycling, ICP-OES analysis, and a hydrothermal treatment. The activity decreased less than 5% after the catalyst was treated in water at 200 °C for 24 h with a stirring speed of 1000 rpm due to the strong metal and hydrothermally stable support interaction. (iv) Ru-HAP is compatible with most impurities such as various salt...

Guangyue Xu

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 Conversion of Furfural to Cyclopentanone with CuZnAl Catalysts

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

Efficient Hydrogenation of Various Renewable Oils over Ru-HAP Catalyst in Water