Catalytic Production of Value-Added Chemicals and Liquid Fuels from Lignocellulosic Biomass

Please cite this article in press as: Jing et al., Catalytic Production of Value-Added Chemicals and Liquid Fuels from Lignocellulosic Biomass, Chem (2019), https://doi.org/10.1016/j.chempr.2019.05.022

Review

Catalytic Production of Value-Added Chemicals and Liquid Fuels from Lignocellulosic Biomass Yaxuan Jing,1,3 Yong Guo,1,3 Qineng Xia,2,* Xiaohui Liu,1 and Yanqin Wang1,*

The efficient utilization of lignocellulosic biomass has tremendous potential to reduce the excessive dependence on fossil fuels. Here, we provide an overview on the recent achievements in the catalytic production of value-added chemicals and fuels. When targeting chemicals, a key objective is to maximize the product selectivity to favor the subsequent separation. This can be achieved through the design of catalysts and optimization of catalytic systems based on the deep understanding of the catalytic mechanism. For production of fuels, attention should be paid to the establishment of an energy-efficient process for high-quality fuels. This can be realized through the design of C–C coupling reactions and the development of multifunctional catalysts to minimize the reaction steps from lignocellulose to fuels. In addition, the full utilization of lignocellulose into biofuels and chemicals in a single process is separately introduced. Finally, several personal perspectives on the opportunities and challenges within this promising field are discussed.

INTRODUCTION The rising global concerns on energy and environment caused by the excessive reliance on fossil fuels have intensified the interest in the utilization of clean and renewable energies. Biomass is the only renewable source of organic carbon in nature, and its conversion to a variety of value-added chemicals, liquid fuels, and carbon-based functional materials has attracted increasing attention.1–4 Lignocellulosic biomass is primarily composed of three biopolymers (cellulose, hemicellulose, and lignin) and represents the most abundant form of biomass.1,2 Therefore, the conversion of lignocellulose is of paramount importance and worth being given considerable attention.3,4 However, the recalcitrance and complexity of lignocellulose have led to big challenges over its effective utilization and augmented the necessity to explore the efficient and selective catalytic systems and reaction processes. Over the decades, researchers have endeavored to explore the efficient production of high-value-added chemicals and transportation fuels from lignocellulosic biomass. The past decade, especially, has seen an enormous increase in the number of publications on lignocellulosic biomass valorization, presenting an extremely important field of research. In the meantime, a number of reviews have been published on the production of chemicals and liquid fuels from lignocellulosic biomass. Corma’s two biomass conversion reviews, published in 20061 and 2007,2 provide comprehensive overviews on the transformation of biomass into chemicals and transportation fuels in terms of chemistry, catalysts, and engineering. Recently, Zhang et al. reviewed the catalytic conversion of lignocellulose into chemicals and fuels in ionic liquids.3

The Bigger Picture One of the grand challenges in chemistry, driven by the growing concern about diminishing nonrenewable fossil resources and the pressing need for environmental pollution mitigation, is to develop novel technologies for producing chemicals and fuels from renewable resources. Among various sustainable resources (e.g., solar energy, biomass, and wind), biomass is the only source of organic carbon in nature, which makes it an ideal alternative for fossil-based chemicals and fuels. However, only a few processes for biomass utilization have been realized in industrial demonstration as a result of the recalcitrance and complexity of biomass. Thus, efficient and selective catalytic systems and reaction processes are required for realizing the production of biochemicals and fuels from renewable lignocellulose. This review presents the latest research on catalytic systems and reaction networks in the catalytic production of value-added chemicals and liquid fuels from lignocellulosic biomass.

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Cellulose and hemicellulose components in lignocellulose are carbohydrates that can be depolymerized by enzymes or acids to monosaccharides5 and then further converted to a wide range of chemicals, such as 5-hydroxymethylfurfural (HMF) and furfural.6,7 With HMF and furfural as pivotal platform compounds, a variety of value-added chemicals, such as levulinic acid (LA), g-valerolactone (GVL), 2,5-furandicarboxylic acid (FDCA), and diols, as well as transportation fuels, such as liquid alkanes and 2,5-dimethylfuran (DMF), have been selectively produced through state-of-the-art technologies.6,7 Moreover, carbohydrates also serve as feedstocks for the direct production of ethylene glycol,8 isosorbide, and light alkanes.9 Lignin as another component of lignocellulose accounts for 10–35 wt % of woody biomass and is the only large-volume source of renewable aromatics in nature. As such, a variety of catalytic systems and processes have been developed to convert lignin to chemicals, fuels, and functional materials.10,11 Specifically, because of their richness in aromatic rings and methoxy, phenolic hydroxy, alkyl, and fatty alcohol groups, some pure, value-added and commodity chemicals (such as phenol, terephthalic acid, alkylbenzenes, methyl p-hydroxycinnamate, acetic acid, N,N-dimethylanilines, and so on) can be produced by selective functionalization and defunctionalization.10,11 At the same time, because of the complexity of lignin structures, plenty of advanced analysis techniques have been employed to elucidate its structure at the molecular level.12,13 In this review, we give a summary of the recent achievements, primarily from our laboratory, in the catalytic transformation of lignocellulose to value-added chemicals and liquid fuels, namely sugars, HMF, furfural, LA, GVL, FDCA, diols, phenols, arenes, terephthalic acid, acetic acid, N,N-dimethylanilines, and liquid hydrocarbons (Scheme 1), by mainly focusing on the developments of efficient and selective catalytic systems and advanced reaction processes. Moreover, we separately introduce the full utilization of lignocellulosic biomass into biofuels and chemicals in a single process because it is atom economic with great practical potential and avoids energy-intensive pretreatment.

VALUE-ADDED CHEMICALS FROM LIGNOCELLULOSE Sugars The depolymerization of hemicellulose and cellulose into soluble intermediates, such as sugars, is of paramount importance for lignocellulosic biomass valorization as sugars can be further transformed into a wide range of value-added chemicals.14 However, the complex, robust, and heterogeneous structure of lignocellulose makes it resistant to chemical transformation. Both physical and chemical treatment processes are therefore required to achieve the efficient depolymerization of lignocellulose. Several reviews have involved the topic of glucose production from lignocellulosic biomass, particularly concentrated on the glucose production by enzymes, liquid acids, and solid acids.5,14 Herein, we mainly focus on the recent developments in this area. Enzymatic hydrolysis is a very common method for producing sugars from lignocellulose and is used in industry after proper physical or chemical pretreatment because of its high selectivity.14,15 Acid-catalyzed hydrolysis is another choice and has thus attracted increasing attention.14 However, the solid lignocellulose is hard to contact with catalytic acid sites. To overcome the inaccessibility of acid catalysts to cellulosic solids, ionic liquids have been employed as cellulose solvents by disrupting the hydrogen-bond network.16,17 As an alternative, the contact problem in acid-catalyzed hydrolysis of lignocellulosic biomass can be addressed with versatile organic

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1Key

Laboratory for Advanced Materials, Feringa Nobel Prize Scientist Joint Research Center, Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Shanghai 200237, P.R. China

2College

of Biological, Chemical Science and Engineering, Jiaxing University, Jiaxing, Zhejiang 314001, P.R. China

3These

authors contributed equally

*Correspondence: [email protected] (Q.X.), [email protected] (Y.W.) https://doi.org/10.1016/j.chempr.2019.05.022

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Scheme 1. Catalytic Production of Value-Added Chemicals and Liquid Fuels from Lignocellulosic Biomass

solvents through the complete solubilization of biomass. For example, biphasic systems consisting of biomass-derived GVL and water have been successfully employed for the efficient hydrolysis of lignocellulosic biomass (e.g., corn stover, softwood, and hardwood) into soluble carbohydrates in high yields (70%90%) (Figure 1A).15 The use of GVL as a solvent decreases the apparent activation energy and increases the reaction rates for monosaccharide production from lignocellulosic biomass (Figure 1B).18 Moreover, GVL can be used as a mild pretreatment agent of lignocellulose for concentrated sugar production.19 The above examples for sugar production use homogeneous acid catalysts, which have drawbacks such as difficult separation, corrosion hazards, and high

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Figure 1. The Production of Sugars from Hemicellulose and Cellulose (A) Overview of the aqueous-phase soluble sugar production using GVL as a solvent. Reproduced with permission from Luterbacher et al. 15 Copyright 2014 AAAS. (B) Cellobiose hydrolysis rate constants (-; left axis) and apparent activation energies (B; right axis) versus GVL content in GVL-H2 O solvent mixtures. Reproduced with permission from Mellmer et al.18 Copyright 2014 Royal Society of Chemistry. (C) Yields of water-soluble products versus ball-milling time for a-cellulose impregnated with different acids. Reproduced with permission from Meine et al. 21 Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA. (D) Kinetics of milling reactions using microcrystalline cellulose with either A niger (red) or T. reesei (blue) cellulase. Solid lines are reactions with milling and the broken line is a control reaction without milling. Reproduced with permission from Hammerer et al. 22 Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA.

environmental risks. As an alternative, solid-acid catalysts have been increasingly investigated for eco-friendly saccharification in recent years.14 Again, the contact between a solid acid and external surface of cellulose is believed to be the most essential factor for hydrolysis efficiency. To improve the direct solid-solid contact of the catalyst with the cellulose surface for hydrolysis, adjusting the density and strength of acid sites on the external surface through catalyst modification could be a feasible method. Taking carbon-based materials as an example, a high local density of weak-acid sites endows a more negative zeta potential to the catalyst, which enables a closer solid-solid contact with the cellulose surface for the hydrolysis reactions.20

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In addition to the reaction solvent and catalyst modification, contact problems existing in solid-state cellulose reactions can be minimized by the co-milling process of the cellulose with acid or enzyme.21,22 H2SO4- and HCl-impregnated cellulose can be completely converted to water-soluble products after only ball milling for 2 h (Figure 1C).21 These soluble oligosaccharides are more easily hydrolyzed than cellulose and give a much higher yield of glucose (91%) at a relatively low temperature (130 C). Similarly, enzymatic hydrolysis of cellulose by milling reactions produce glucose more than three times that obtained by control reactions without milling (Figure 1D).22 Milling is a relatively energy-intensive process, but it does not necessarily mean it is not feasible for large-scale processing. Actually, ball milling has been widely used in the cement industry on a multimillion-ton-per-year scale. In terms of ethanol production from cellulose, it is estimated that the electrical energy demand is only slightly more than 10% of the energy content of ethanol produced.21 The energy cost could be even lower if the milling is performed only at low-electricity-cost times or uses fluctuating regenerative electricity (e.g., wind power). Overall, the accessibility of enzymes or acid catalysts to cellulosic solids is the most important feature for sugar production. To address this problem, ionic liquids or versatile organic solvents (e.g., GVL and H2O) have been employed as cellulose solvents by disrupting the hydrogen-bond network or decreasing the apparent activation energy. Catalyst surface modification is another viable choice because it is possible to reach a direct solid-solid contact with the cellulose surface. Mechanochemistry assisted acid hydrolysis may be the best option for sugar production in the future because of its unprecedented activity and selectivity, especially if the energy consumption is addressed by garbage power. Furan Derivatives (HMF and Furfural) The production of furan derivatives, namely HMF and furfural, from dehydration of carbohydrates has been intensively reported over the past decade given that a large number of value-added chemicals and liquid fuels can be formed with HMF and furfural as intermediates. At the same time, plenty of published reviews have given comprehensive overviews of the synthesis and application aspects of HMF and furfural.6,7 Although the synthesis of HMF from sugar dehydration has long been documented, the global interest in research on HMF production and utilization started from the pioneering work published by Roman-Leshkov et al.23 in 2006. Subsequent investigations were mostly concentrated on improving the HMF yield and reducing feedstock and purification costs through the design of more efficient and versatile catalysts and/or the employment of biphasic media. HMF formation from dehydration of fructose is straightforward and typically displays high selectivity over Brønsted acid catalysts, especially in dimethylsulfoxide (DMSO) solvent.24 Theoretical investigation revealed that in the presence of Brønsted acid, H+ prefers to interact with DMSO to form the [DMSOH]+ active species, which exhibits catalytic activity toward the removal of three H2O molecules from fructose (Scheme 2).25 Although the formation of HMF is highly selective in DMSO medium, the separation and purification of HMF from the high-boiling point of DMSO remain a big challenge.26,27 To address this problem, biphasic solvents have been extensively employed for the simultaneous extraction of HMF into an organic phase after its formation in a reaction phase.26 A tetrahydrofuran (THF)/DMSO (7:3) mixture as a biphasic solvent showed high performance in fructose dehydration with a 98% yield of HMF.27 In addition, to enhance the yield of HMF, biphasic solvents also make the

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Scheme 2. Schematic Diagram for DMSO-Enhanced Fructose Dehydration into HMF in the Presence and Absence of Brønsted Acid Reproduced with permission from Ren et al. 25 Copyright 2017 American Chemical Society.

process more separation friendly given that high purity of HMF can be obtained by a simple extraction method.27 In comparison to fructose, HMF formation from glucose-based carbohydrates is more complex and difficult in that it requires selective in situ isomerization of glucose to fructose (Scheme 3).16,17,28,29 Metal chlorides (e.g., CrCl2, CrCl3, and SnCl4) in ionic liquids are active and selective catalytic systems for the formation of HMF from glucose-based carbohydrates.16,17 Cr or Sn cations are believed to be responsible for the isomerization of glucose into fructose via a five-membered-ring chelate complex of the Cr (or Sn) atom and glucose.16,17 Inspired by these pioneering works, Cr- or Sn-contained heterogeneous catalysts have been extensively used as bifunctional catalysts for the direct conversion of glucose-based carbohydrates to HMF.28,29 For example, Sn-Mont showed excellent catalytic performance such that 53.5% and 39.1% yields of HMF were directly achieved from glucose and cellulose, respectively, under their respective optimal conditions.28 The most recent research also showed that tin phosphate (SnPO) is capable of catalyzing a high concentration (20 wt %) of glucose to HMF with a 58.3% yield in ionic liquid.29 The excellent catalytic performance of Cr- or Sn-contained catalysts is generally attributed to the cooperative catalysis of Lewis acid (Cr3+ or Sn4+) and Brønsted acid sites, which play a key role in the isomerization of glucose and dehydration of in-situ-generated fructose to HMF, respectively. To achieve the cooperative catalysis of Lewis acid and Brønsted acid sites for conversion of glucose-based carbohydrates to HMF, suitable acid density, strength, and a Brønsted/Lewis acid (B/L) ratio are extremely important.30–33 Generally, Lewis acid sites are essential for the isomerization of glucose into fructose, but strong or excessive Lewis acid sites will lead to the formation of undesired humins and reduce the HMF selectivity, whereas Brønsted acid sites are active for dehydration of fructose into HMF, but excessive Brønsted acid sites can suppress the isomerization reaction.30–33 Deeper investigations revealed not only that the B/L ratio influences the

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Scheme 3. Pathway for the Transformation of Glucose to HMF as well as the By-products

catalytic performance in glucose conversion but also (and more importantly) that the ratio of weak to total Lewis acid (L*/L) plays a key role in the selective production of HMF.33 As can be seen in Figure 2, the HMF selectivity had a nearly linear relationship with the L*/L ratio. The B/L ratio also had an influence on HMF selectivity; at every similar L*/L ratio, volcano curves were obtained as the B/L ratio increased, but the influence was minor in comparison with that of the L*/L ratio.33 From the ‘‘green chemistry’’ point of view, water is an extremely important solvent that is natural, widely available, and environmentally benign. However, only a few solid acids offer both high activity and stability in the aqueous phase. The fabrication of water-tolerant solid acids for HMF formation is therefore of significant importance.6 Niobium-based materials (i.e., NbOPO4 and Nb2O5) are water-tolerant catalysts that also possess tunable surface acid density, strength, and type. For instance, NbOPO4 synthesized at different pH values will lead to totally different acid properties of the material. NbOPO4 synthesized at pH 7 showed the best performance for HMF formation from glucose in water as a result of the balanced Brønsted and Lewis acid amounts and water-tolerant property.30 Investigation on the correlation between the structure and acidity of Nb2O5$nH2O revealed that the Lewis acid sites originated from the lower coordinate NbO5 or NbO4 sites after the formation of oxygen vacancies in NbO6 networks.32 Moreover, for HMF production, Niobium-based catalyst (Nb2O5) was also reported to be efficient for the dehydration of xylose into furfural in aqueous phase given that it achieved 93% conversion of xylose and 48% selectivity to furfural.34 Apart from the catalyst design, the reaction solvent also plays a crucial role in the efficient synthesis and purification of furan derivatives from carbohydrates. As previously mentioned, the employment of biphasic solvents for simultaneous extraction of HMF into an organic phase after its formation can remarkably enhance the yield of HMF. In addition to the extraction effect, some organic solvents also act as catalysts to stabilize the key transition state by altering the reaction activation energies to an extent.25,35,36 It has been found that an aprotic organic solvent (e.g., GVL) influences the reaction kinetics by changing the stabilization of the acidic proton relative to the protonated transition state, leading to remarkable increases in reaction rates and product selectivity in acid-catalyzed reactions compared to water.35,36 In water-involved biphasic solvents, NaCl is usually added into the aqueous phase to increase the partition coefficient of HMF in biphasic systems. Although the majority of research has focused on the partition effect of NaCl, only a few works have mentioned the role of halide ligands on the formation of HMF.17,37,38 Earlier works have demonstrated that Cl atoms can interact with the hydrogen atoms in hydroxyl groups of glucose, as evidenced by 1H NMR spectra of glucose before and after the addition of SnCl4.17 Therefore, halide ligands (Cl and Br ) are believed to facilitate

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Figure 2. Effects of Lewis Acid on Glucose Conversion, the Ratios of L*(Weak Lewis Acid)/L and B/L to HMF Selectivity Reproduced with permission from Li et al. 33 Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA.

the selective formation of HMF in addition to serving as ligands for metal ions.37 Our recent work has demonstrated that chloride ions can not only promote the isomerization of glucose via a 1,2-hydride shift pathway but also improve cellulose hydrolysis and fructose dehydration (Figure 3A), as confirmed by a combination of control experiments, density functional theory (DFT) calculations, and D2-glucose isotopic labeling (Figure 3C).38 As a result, cellulose can be efficiently converted into HMF (48.6% yield) without any acid addition in a THF-seawater biphasic system (Figure 3B), which is also applicable for the conversion of raw lignocellulose to furfural and HMF without destroying the lignin fraction.39 LA and GVL LA and GVL are two of the most promising biomass-derived platform chemicals because they can be easily and cheaply produced and can be converted into a wide range of chemicals and fuels with diverse applications.40,41 For the production of LA from lignocellulose, the first step involves the hydrolysis of cellulose and hemicellulose into monosaccharides, such as glucose and xylose, and the second step involves isomerization and dehydration of monosaccharides into HMF and furfural. Subsequently, HMF is converted into LA and formic acid (as a co-product) by rehydration under acidic conditions in the aqueous phase (Scheme 4). The homogeneous acid (e.g., H2SO4 and HCl)-catalyzed production of LA from biomass has been intensively studied.40 Although the processes over homogeneous acids are effective, mineral acids are corrosive, are difficult to separate and recover from the reaction, and pose an environmental risk. To overcome these drawbacks, heterogeneous acid catalysts, such as Amberlyst 7042 and Al-modified NbOPO4,43,44 have been used for the production of LA or its derivatives from lignocellulosic biomass. Similar to the formation of HMF, the acid strength and types are important factors to yield LA or LA derivatives. Control experiments on the conversion of HMF to LA have demonstrated that strong Lewis acid sites play a vital role in the formation of LA in aqueous phase.43 However, as opposed to the case in water, strong Brønsted acidity is efficient for the formation of methyl levulinate (ML) (an LA ester) from alcoholysis of cellulose in methanol.44 Mechanism studies have revealed that the alcoholysis of cellulose proceeds via a pathway different from that in water with methyl glucoside as the intermediate (Scheme 4). Brønsted acid sites enhance the dehydration of methyl glucoside to 5-methoxymethyfurfural and the following hydration to ML.44 As a result, catalysts with strong Brønsted acidity facilitate the formation of ML from cellulose in methanol, whereas catalysts with strong Lewis acidity favor the production of LA from cellulose in water.43,44

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Figure 3. The Production of HMF from Cellulose in the THF-Seawater System (A) Schematic representation of the role of chloride ions in cellulose hydrolysis, glucose isomerization, and fructose dehydration. (B and C) Transformation of cellulose in the THF-seawater system to HMF (B) and the D2-glucose isotopic labeling study (C). Reproduced with permission from Li et al. 38 Copyright 2018 American Chemical Society.

The catalytic hydrogenation of LA to GVL is considered an essential reaction for biomass refinery. Supported Ru catalysts have been reported to be efficient for catalytic hydrogenation with conversion up to 99.7% and selectivity up to 100%.45 Bimetallic random alloys of Au-Pd/TiO2 and Ru-Pd/TiO2 have also been found to be active and selective for this reaction.46 Another method for upgrading LA to GVL is through the hydrogen transfer reaction with formic acid as an in situ hydrogen source.47 Formic acid is formed as a co-product in an equimolar amount with LA in the acidic rehydration of HMF. The use of in-situ-generated formic acid eliminates hydrogen consumption, reduces emission, and increases atom economy. In addition to formic acid, isopropanol48 or 2-butanol49 as the hydrogen donor for GVL formation has also been investigated. For instance, Bui et al.49 demonstrated a one-pot conversion of furfural into GVL with an overall yield of ca. 80% through sequential transfer hydrogenation and ring-opening steps over a combination of solid Lewis and Brønsted acids. This procedure is highly promising because there is no requirement for high-pressure molecular H2 or precious-metal catalysts.

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Scheme 4. Reaction Pathways for the Acid-Catalyzed Conversion of Cellulose to LA in Water and to Levulinate Esters in Alcohol Reproduced with permission from Ding et al. 42 Copyright 2015 Royal Society of Chemistry.

FDCA FDCA is a potential substitute for terephthalic acid in the production of polyesters because of its unique structure and renewability. The synthesis of FDCA from HMF, a biomass platform molecule produced from dehydration of carbohydrates, has attracted extensive attention. The transformation of HMF to FDCA can be achieved by enzymatic catalysis, electro-catalysis, and chemical catalysis. Here, we mainly concentrate on the chemical catalysis given that it covers the majority of the research. Immense attention has been paid to the development of heterogeneous catalysts, including supported noble-metal catalysts, such as Au, Pt, Pd, and Ru catalysts,50–53 and non-noble-metal oxide catalysts.54–56 Supported noble-metal catalysts have displayed excellent performance for the aerobic oxidation of HMF to FDCA because noble metals can efficiently activate oxygen. Generally, the oxidation of HMF to FDCA is conducted in alkaline aqueous phase given that hydroxide ions can promote the activation of the C–H bond. With the assistance of NaOH, the Au/CeO2 catalyst displayed an excellent performance with a 99% yield of FDCA.51 However, the excessive use of alkaline solutions makes the process less green and non-economical. Solid bases were therefore employed as supports in the aerobic oxidation of HMF to FDCA in water.52 Unfortunately, most of the solid bases are unstable in the aqueous phase or when exposed to air. Hence, the development of solid bases with high stability is of significant importance. Carbon-based materials have excellent stability in the aqueous phase and are usually employed as water-tolerant supports. After being modified by alkali functional groups, they would be desired catalysts for the aerobic oxidation of HMF after metal loading. For instance, by combining the basic property of MgO with the hydrothermal stability of carbon materials, a novel Pt/C-O-Mg catalyst was prepared for the aerobic oxidation of HMF to FDCA, and it reached a 97% yield of FDCA under optimal conditions.53 The catalyst can be reused ten times with no significant loss of activity because of the formation of new, strong, and stable basic C–O–Mg sites. By further scaling up the reaction by 20-fold, the isolated yield of purified FDCA reached up to 74.9% with a purity >99.5%. It is straightforward to separate the crude FDCA product from water because it would crystallize and precipitate after the reaction. In addition, the crude FDCA could be purified simply by recrystallization. The high efficiency and stability of the Pt/Mg–O–C catalyst demonstrates its great potential for the large-scale production of FDCA under base-free conditions, supposing that the Pt loading can decrease to an acceptable amount. Although noble-metal catalysts show excellent performance in FDCA synthesis from HMF aerobic oxidation, the high costs may limit their industrial application.54 Recently, non-noble-metal catalysts, such as Fe-, Co-, and Mn-based catalysts,

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Scheme 5. Reaction Mechanism for the HMF Oxidation over the MC-6 Catalyst Reproduced with permission from Han et al. 55 Copyright 2016 Royal Society of Chemistry.

have been found to be efficient for the production of FDCA from HMF oxidation.54 Among these, MnO2-based catalysts have attracted more attention because of their unique diverse crystal structures and oxidation states. In the mechanism of HMF oxidation over MnO2-based catalyst, one has clarified that the surface Mn4+ ions are the active sites.55 Hence, several strategies have been conducted to increase the concentration of surface Mn4+ ions. For instance, the doping of CeO2 in MnOx increased the amount of surface Mn4+ ions and promoted lattice oxygen transfer from Ce to Mn (Scheme 5).55 As a result, the catalytic activity of the CeO2-MnOx catalyst was greatly enhanced. Furthermore, the catalyst could be reused for five runs with no significant loss in activity. Moreover, another mechanism proposed that the lower formation energy of oxygen vacancy plays a key role.56 The crystal structure of MnO2 has a great effect on catalytic performance, and b-MnO2 with exceptionally low oxygen vacancy formation energy was proven to be the best one. No matter which mechanism is more reasonable, the performances shown by non-noble-metal oxide catalysts are promising and comparable to those of noble-metal catalysts. Although MnO2-based catalysts are promising in FDCA synthesis, high yields of FDCA can be obtained only in the presence of mineral bases, such as NaHCO3. The use of mineral bases produces waste water, which fails to meet the ‘‘green chemistry’’ concept. Therefore, non-noble-metal oxides with higher activity should be designed and developed. Importantly, efforts toward ‘‘green’’ reaction medium without base or acid should be the major focus, and the use of water is the most desirable. As mentioned above, the most important application of FDCA is as a substitute for terephthalic acid. Hence, other renewable strategies for terephthalic acid

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production should be proposed and developed in future studies, especially from lignocellulosic biomass or its derived compounds. Actually, tandem Diels-Alder and dehydration reactions of lignocellulose-derived 2, 5-dimethlyfuran with ethylene have great potential for the production of p-xylene, which can be used as a precursor for terephthalic acid.57 Besides this, terephthalic acid can also be produced from chemical funneling of lignin by selective functionalization and defunctionalization.58 Diols (Ethylene Glycol, Pentanediol, and Hexanediol) Diols are important raw materials in the modern chemical industry, especially in the manufacture of polymers. They are also widely used as additives and intermediates in the food and pharmaceutical industries. Diols that can be synthesized from biomass include ethylene glycol, propylene glycol, pentanediol, and hexanediol among others. Propylene glycol is not discussed here because propylene glycol is mainly obtained from non-lignocellulosic glycerol, which is beyond the scope of this review. In 2008, Zhang’s group discovered a non-petroleum route for the production of ethylene glycol from cellulose by using a nickel-promoted tungsten carbide catalyst.8 The one-pot process for ethylene glycol from renewable feedstock presented the prominent advantage over the petroleum-dependent multistep processes. After this pioneering work, an array of catalysts have been developed to increase the efficiency of ethylene-glycol production from cellulose. Besides the catalyst, the reaction medium also plays an important role on the product distributions. With methanol as the medium instead of water, 64% total yield of ethylene glycol and its derived ester can be achieved from cellulose over the Ru-Ni/NbOPO4 catalyst with a small amount of propylene glycol.59 The addition of Ni to the Ru/NbOPO4 catalyst limited the further hydrogenolysis of ethylene glycol and its derived ester to CO and alkanes. This process had excellent selectivity to ethylene glycol and offered potential for industrial applications. 1,5-pentanediol is usually used as plasticizer and polyester monome. It can be generated from the hydrogenolysis of furfural, furfuryl alcohol (FFA), or tetrahydrofurfuryl alcohol (THFA) in addition to the petroleum-based route. The pioneering work on 1,5-pentanediol production from THFA was reported by Tomishige et al.60 over the Rh-ReOx/SiO2 catalyst. It will be more energy efficient if 1,5-pentanediol can be directly produced from furfural but faces a big challenge because of the steric effect of the hydroxymethyl group at the C(2) position of the furan ring. CoOx was reported to be able to selective absorb and activate the C–O bonds of the furan ring. Consequently, a 34.9% yield of 1,5-pentanediol can be achieved directly from furfural over the Pt/Co2AlO4 catalyst under mild conditions.61 However, the selectivity to 1,5-pentanediol was lower than that from hydrogenolysis of THFA.60 Therefore, further research should focus on improving the selectivity from furfural by designing more efficient catalytic systems. 1,2-pentanediol is a key intermediate of low-toxicity microbicides and has been widely used in the cosmetic industry. In contrast to the production of 1,5-pentanediol from THFA, the selective formation of 1,2-pentanediol was basically realized by the direct conversion of furfural or FFA. The publications on this topic are fewer than those on 1,5-pentanediol. In a typical work, a Ru/MnOx catalytic system was established and obtained a maximum 1,2-pentanediol yield of 42% from furfural alcohol.62 A Pt/CeO2 catalyst was reported as efficient for the direct production of 1,2-pentanediol from furfural.63,64 The morphology of CeO2 has great influence

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Figure 4. The Selective Formation of 1,2-Pentanediol from Furfural Alcohol (A) Participation of H 2 O in the opening of furan ring during furfural alcohol hydrogenolysis. Reproduced with permission from Ma et al. 63 Copyright 2017 American Chemical Society. (B) The influence of CeO 2 facet-induced Pt-CeO 2 metal-support interaction on the direct hydrogenolysis of furfuryl alcohol. Reproduced from Tong et al. 64 Copyright 2018 Elsevier.

on the catalytic performance for the hydrogenolysis of FFA.64 CeO2 with different facets displays diverse concentrations of surface oxygen vacancies that tune the chemical states and particle size of Pt, leading to different selectivity to 1,2-pentanediol (Figure 4B). Reaction medium can also play a vital role on 1,2-pentanediol selectivity; for instance, a combination of H218O isotopic tracing experiments and DFT calculations revealed that water can participate in the ring-opening process of furfural alcohol, leading to the selective formation of 1,2-pentanediol.63 1,6-hexanediol is one of the most important diols and is extensively used for the industrial production of polyester and polyurethane. Currently, 1,6-hexanediol is produced industrially from the hydrogenation of adipic acid. With the development and utilization of lignocellulosic biomass, 1,6-hexanediol can be prepared through the hydrogenolysis of HMF, but the yields are rather low. Compared to furfural, HMF has an additional hydroxymethyl group and usually produces 1,2,6-hexanetriol after the opening of the furan ring, making the selective conversion to 1,6-hexanediol problematic. Efforts have been made to enhance the efficiency of 1,6-hexanediol production by one-step hydroprocessing,65 but the yield was lower than that of 1,5-pentanediol from furfural. In future studies, more attention should be paid to the development of novel catalysts with higher selectivity in the one-step process or the development of new multistep processes that provide a high conversion while maintaining selectivity. Value-Added Chemicals from Lignin Apart from value-added chemicals from the carbohydrate fractions of lignocellulose, high-value and commodity chemicals can be also produced from the selective conversion of lignin part. The selective valorization of lignin to valuable chemicals mainly includes two steps, i.e., depolymerization followed by upgrading. Generally, starting from lignin, a wide array of products can be produced, which makes the separation process difficult and energy consuming. The past few decades have seen an enormous increase in the number of publications on lignin valorization, and considerable progress has been made.11 For a more elaborate discussion, the reader is referred to a dedicated review on this topic.11,13 Despite these advances, selective valorization of lignin or its derived monomers into a single-component, value-added, critical commodity chemical is of key interest but remains challenging regarding the diversity

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Scheme 6. Catalytic Production of Single Value-Added Chemicals from Lignin

and complexity of lignin. This fascinating target, viz. lignin-to-single chemical, forms the subject of the following section (Scheme 6). On the basis of the structural characteristics of lignin, some interesting catalytic systems that could be used to produce single high-value chemicals, such as phenol, terephthalic acid, alkylbenzene (toluene, ethylbenzene or propylbenzene), methyl p-hydroxycinnamate, acetic acid, N,N-dimethylanilines, and so on, have been developed.58,66–74 Lignin is abundant in aromatic rings and methoxy-, phenolic hydroxy, alkyl, and fatty alcohol groups, and the successful conversion of lignin to single value-added chemicals from functional groups is mainly focused on two aspects now: (1) valorization of aromatic rings and (2) utilization of non-aromatic rings (mainly methoxy and alkyl groups). The strategies for aromatic ring valorization are focused on functionalization and defunctionalization, including selective cleavage of the inherent bonds and the formation of new bonds. An attractive pathway was developed to convert lignin-derived monomers into phenol via dealkylation and/or hydro-demethoxylation.66–68 An acidic ZSM-5 zeolite was proven to be highly efficient in the dealkylation of alkylphenols.66 A gold-nanoparticle-activated TiO2 catalyst is a strikingly active and selective catalyst for the conversion of guaiacol, a lignin model compound, to phenol.67 However, the above substrates, namely alkylphenols and guaiacol, are limited to single alkyl groups or methoxy groups. To remove both alkyl and methoxy groups together, a single-step strategy was proposed to convert 2-methoxy-4-propylphenol into phenol by using a physical mixture of Pt/C and H-ZSM-5 as the catalyst; the former was efficient for the hydro-demethoxylation, and the latter was responsible for the dealkylation.68 Even though the above studies worked with lignin model compounds for phenol production, selective conversion of real lignin into phenol in a single-step conversion still remains challenging. Starting from native lignin to synthesize pure phenol in a single-step conversion, a multifunctional catalyst should be designed to simultaneously promote lignin depolymerization and upgrading toward

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a single phenol. It is hard to strike a balance over single-metal-supported catalysts, so supported bimetallic catalysts may be a suitable alternative, and the type of bimetals must be carefully chosen. Apart from phenol production, selective production of alkylbenzene has attracted significant attention. Preserving the aromatic rings and the alkyl groups while cleaving all the C–O bonds, especially the C–O bonds in phenols, is highly challenging because the C–O bond energy in phenols is strong.69 A porous Ru/Nb2O5 catalyst has enabled the complete removal of the oxygen content in lignin and resulted in a high selectivity to alkylbenzenes (toluene, ethylbenzene, and propylbenzene).69 However, the selective production of single alkylbenzene, such as ethylbenzene from lignin, is still an important scientific challenge. The design of modified Nb-based catalysts is worth being taken into consideration. The production of pure methyl p-hydroxycinnamate, an important starting material for a variety of fine chemicals, from lignin was demonstrated by selective tailoring of the H unit in lignin over metal-based ionic liquid with the capacity to break the ester bonds rather than ether bonds.70 It will be essential to develop catalytically unique materials for the cleavage or activation of specific bonds in complex lignin to tailor the desired groups. In this respect, DFT calculation is an available technique for the investigation of bonding energy to reveal the catalytic mechanism.69,70 Besides the above, step-wise systems mainly involving the cleavage of inherent bonds in lignin and the formation of new bonds are fascinating to achieve a single and valueadded chemical. A three-step strategy including de-methoxylation, carbonylation, and oxidation steps was developed to produce terephthalic acid from corn stover ligninderived monomers.58 Especially in the carbonylation and oxidation process, the introduction of two –COOH groups directly contributed to this achievement. Studies utilizing non-aromatic rings are mainly focused on alkyl groups and methoxy groups. In general, alkyl group valorization is accompanied by other processes. For example, during the phenol production process from lignin-derived alkylphenols, the alkyl groups in polyphenols can be converted into ethylene and propylene.66 In addition, benzene as a catcher is added to react with the alkyl groups to generate alkylbenzenes (propylbenzene, cumene, and toluene), but the mixture of alkylbenzenes is unsuitable for direct use as a commodity chemical.71 For methoxy group utilization, a suitable acceptor is necessary to capture the methoxy groups because it contains only two atoms, namely carbon and oxygen, and is almost impossible to use alone. Water and CO as co-acceptors were found to react with the methoxy group of lignin to obtain acetic acid with RuCl3 as a catalyst and LiI and LiBF4 as co-promoters.72 Aniline compounds were also preferential substrates to react with the methoxy group of lignin for the synthesis of methyl-substituted amines over a catalyst comprising LiI and an ionic liquid.73 Because only the methoxy groups in lignin are selectively converted, the solid residual lignin left after the reaction is abundant in hydroxyl groups, and its use remains a big challenge. Compared with native lignin, the solid residual lignin can be used as an efficient catalyst for the cycloaddition reaction of epoxy propane with CO2.73 Because it has rich methoxy groups, the solid residual lignin may be endowed with other applications, but more systematic studies are needed.

LIQUID HYDROCARBONS FROM CELLULOSE AND HEMICELLULOSE One of the structural characteristics of biomass feedstocks is the high carbon content (over 40 mol %), which can meet the carbon demand for transportation fuels,

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including gasoline, diesel, and jet fuel.1,2 Therefore, the production of liquid fuels from biomass is considered one of the most promising options. C–O bond cleavage and C–C bond saturation of carbohydrates to liquid alkanes are of key interest because carbohydrates are abundant, and the reactions are straightforward. This section provides an overview of the key catalytic systems and reaction networks in the production of liquid alkanes from cellulose and hemicellulose with a focus on the production of long-chain alkanes. For a more elaborate discussion on the light gasoline production from cellulose and hemicellulose, the reader is referred to a dedicated review on the topic.75 Apart from the light gasoline, the production of long-chain diesel (C12–C22) and jet fuel (C8–C16) from biomass and biomass-derived chemicals is extremely attractive. Typically, the process of producing long-chain liquid alkanes from biomass and biomass-derived chemicals involves two steps, namely the preparation of high-carbon fuel precursors followed by hydrodeoxygenation. One main strategy for synthesizing fuel precursors with targeted carbon numbers relies on C–C coupling reactions, such as aldol condensation, hydroxyalkylation-alkylation (HAA) condensation, Michael addition, and Robinson Annulation, to increase carbon chain lengths because biomass-derived platform compounds are mainly low-carbon molecules (carbon number % 6).76,77 C–C Coupling via Aldol Condensation Aldol condensation involving addition or addition-elimination reactions of ketones and aldehydes, catalyzed by acid, base, or acid-based bifunctional catalysts, has attracted considerable interest because numerous carbonyl compounds can be produced by chemical conversion and biological fermentation from biomass.76,77 The Dumesic group presented pioneering work for the conversion of biomass-derived carbohydrates to longchain alkanes (C7–C15) by using the dehydration of carbohydrates, aldol condensation, pre-hydrogenation, and subsequent hydrodeoxygenation.78 This general strategy opens the synthesis of long-chain alkanes from biomass-derived compounds. After this work, a multifunctional Pd/NbOPO4 catalyst was designed to integrate the pre-hydrogenation and following hydrodeoxygenation (HDO) in one step and showed excellent performance under mild conditions.79 In addition to the synthesis of linear C7–C15 alkanes from biomass or its derivatives, which need isomerization before they are used as transportation fuels, the synthesis of branched and cyclic alkanes also received rapid development to improve fuel properties, such as density, volumetric heating values, and freezing point. In general, the chain structure of target alkanes is consistent with the structure of fuel precursors. Therefore, selecting appropriate substrates to aldol condensation is of significant importance for the controlled synthesis of branched and cyclic oxygenates. Scheme 7 shows how to obtain target structured oxygenates by selecting biomassderived substrates. Generally, aldehydes and straight ketones (acetone, 2-pentanone, and 2-heptanone) have been employed as substrates to obtain straight oxygenates (Scheme 7A).76–79 Therefore, for the production of branched and cyclic fuel precursors, aldehydes, and branched or cyclic ketones must first be selected to conduct cross-aldol condensation (Schemes 7B and 7C). The second feasible strategy is using chained or cyclic ketones, including acetone, methyl isobutyl ketone, cyclopentanone, and cyclohexanone, as the only substrate to carry out self-aldol condensation (Schemes 7D and 7E).76,77 The third method is employing chain ketones containing middle a-H as donors for aldol condensation. Accordingly, the target fuel precursors are a,b-unsaturated carbonyl compounds containing the branched chain (Scheme 7F).76,77,80

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Scheme 7. Aldol Condensation Reactions of Different Aldehydes and Ketones (A) Aldol condensation of aldehydes (a; furfural, HMF, and butanal) and straight ketones (b; acetone, 2-pentanone, and 2-heptanone). (B) Aldol condensation of aldehydes (a; furfural, HMF, and butanal) and branched ketone (c; methyl isobutyl ketone). (C) Aldol condensation of aldehydes (a; furfural, HMF, and butanal) and cyclic ketones (d; cyclopentanone, cyclohexanone, and isophorone). (D) Self-aldol condensation of cyclic ketones (d; cyclopentanone, cyclohexanone, and isophorone). (E) Self-aldol condensation of straight ketones (e; acetone and methyl isobutyl ketone). (F) Aldol condensation of aldehydes (a; furfural, HMF, and butanal) and chain ketones (f; 3-pentanone, angelica lactone, and alkyl levulinate) containing the middle a-H.

C–C Coupling via HAA Condensation Apart from aldol condensation, acid-catalyzed HAA condensation of 2-methylfuran (2-MF) with biomass-derived carbonyl compounds is an attractive C–C bond-formation strategy. The HAA condensation involves the selective condensation of 2-MF at its C5 position with biomass-derived carbonyl molecules, yielding biofuel intermediates. The main advantage of this process is that HAA adducts possess inherently branched chain structure, leading to branched alkanes. The HAA condensation of 2-MF with a series of biomass-derived carbonyl molecules was explored for the synthesis of branched fuel precursors.76,77,81–83 Among these carbonyl compounds, acetone and furfural may be preferential substrates because they can be currently produced at an industrial scale. In addition, HAA condensation using 2-MF as the sole substrate was also achieved as a result of the in situ ring opening of 2-MF to generate 4-oxopentanal in the presence of water.81 Generally, HAA condensation is catalyzed by Brønsted acids, including p-toluenesulfonic acid, acidic resin, acidic carbon materials, NbOPO4, and graphene oxide.77,81–83 Nafion-212 resin was proven to be more effective than other commercial resins (Nafion-1135, Nafion115, Amberlyst-36, and Amberlyst-1) for HAA condensation and have almost the same performance as that of homogeneous sulfuric acid.83 Weak Brønsted acidic phenol and acetic acid are inactive for HAA condensation.82 Hence, HAA

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condensation catalysts should be stimulated to the introduction of strong Brønsted acidic groups into cost-effective materials in future studies. Although branched fuel precursors can be synthesized through HAA condensation, the degree of branched formation is low because there is only one alkyl branch. In addition, 2-MF is an indispensable substrate for the HAA condensation, but it cannot be currently industrially produced. The development of efficient and cost-effective preparation methods for 2-MF production is necessary for promoting its industrialization. Other C–C Coupling Reactions Highly branched alkanes and alkylcyclohexanes can be obtained through aldol or HAA condensation but are highly dependent on the structure of the substrates. Therefore, exploring the potential C–C coupling reaction to produce highly branched and cyclic oxygenates from straight-chain molecules is of key interest. Michael addition of b-dicarbonyl compounds with a,b-unsaturated carbonyl compounds is possible if the attacking position of 1,4-addition lies within the central position of the a,b-unsaturated carbonyl compounds. However, a,b-unsaturated carbonyl compounds, as the characteristic products of aldol condensation, cannot be directly obtained from biomass. Therefore, an upgrading aldol condensation should be designed to synthesize desired a,b-unsaturated carbonyl compounds. Michael addition is a feasible approach for the production of highly branched alkane from biomass-derived straight-chain chemicals.80 (Scheme 8) The Michael addition product, a multi-carbonyl compound containing active a-H atoms, can be used to perform intramolecular aldol condensation to produce cyclic oxygenates. This tandem reaction of Michael addition and intramolecular aldol condensation is known as Robinson annulation, which is a crucial six-membered ring-like structure formation reaction with chained substrates (ketones containing active a-H atoms and a,b-unsaturated carbonyl molecules). 2,4-Pentanedione was proven to be a suitable co-substrate for aldol condensation and Robinson annulation for the realization of the one-pot cyclization process.84 (Scheme 8) The alkylcyclohexanes obtained via Robinson annulation are of high quality and can be directly blended with conventional fuels to improve the combustion performance. Apart from the above C–C coupling reactions, the scope of C–C coupling reactions can be extended by employing organic synthesis methodology. In addition, several biomass-derived compounds, such as furfural, acetone, cyclohexanone, methyl isobutyl ketone, and so on, can be currently produced at the industrial scale, which makes them promising substrates of C–C coupling for the production of fuel precursors. Future studies should be conducted to produce biomass-derived platform compounds at the industrial scale, especially those rich in functional groups that can be used for conducting C–C coupling reactions. HDO of Condensation Adducts The ultimate goal of producing liquid alkanes from biomass is the removal of oxygen through total hydrodeoxygenation. Generally, biomass-derived oxygenates contain C=C, C=O, C–OH, and C–O–C bonds; hence, the total HDO typically involves dehydration, hydrogenation, and hydrogenolysis reactions. HDO reactions are usually catalyzed by bifunctional catalysts containing metals and acid sites. Among the catalysts, niobium-based acid materials have attracted intensive attention because of their unique properties. Therefore, this section presents an overview of niobiumbased catalysts for the HDO of condensation adducts by demonstrating catalytic properties and mechanisms. NbOx-based solid-acid-supported catalysts show better activity for the HDO of furfuralacetone aldol adducts than Al2O3-, SiO2-, silica-alumina-, and H-ZSM-5-supported

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Scheme 8. Reaction Pathway for the Production of Highly Branched Chain Alkanes and Alkylcyclohexanes through Michael Addition and Robinson Annulation Reproduced with permission from Jing et al. 80,84 Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA.

catalysts, and the total HDO reaction can be realized under very mild conditions (170 C, 2–2.5 MPa).79,85–87 The combination of control experiments and DFT calculations reveals that NbOx species play a key role in C–O bond cleavage, especially on the cleavage of tetrahydrofuran ring.79 Moreover, the multifunctional NbOx-based solid-acid-supported catalyst has two other functions, namely the noble-metal sites for hydrogenation and the acid sites of NbOx-based solid acid for dehydration, making the total removal of oxygen in aldol adducts feasible under very mild conditions.79 In addition, different states of NbOx show distinct catalytic activity, that is, NbOx with lower coordination numbers are beneficial for the HDO of furanic compounds.86 On the basis of the crucial role of the NbOx species in the cleavage of C–O bonds, Pd-loaded Nb2O5/SiO2 and Nb-SBA15 catalysts were prepared for the HDO of furanic compounds under mild conditions.86,87 Apart from the furanic compounds, the HDO of other biomass-related compounds (e.g., palmitic acid, tristearin, and diphenyl ether) can also be efficiently catalyzed by Nb-based solid-acid-supported catalysts.87 Given that NbOx-based catalysts can efficiently catalyze the HDO of biomassderived oxygenates, a series of attractive routes were developed to produce highquality transportation fuels.77,79,80,84,87 Typically, carbon-growth strategies were designed to synthesize fuel precursors, and this was followed by total HDO over the metal-loaded NbOx-based catalysts. The major features of Nb-based catalysts regarding HDO include (1) the unique oxygen affinity to activate C–O bonds, (2) the mesoporous structures and large surface areas for metal loading, (3) the synergistic effect between metal and NbOx species for oxygen removal, (4) the excellent acid properties to promote acid-catalyzed reactions, and (5) water resistance and high stability.79,86,87 Unlike the above HDO route, Sutton and co-workers88 presented a different approach for the removal of oxygen from aldol adducts to alkanes. This approach involved three steps: hydrogenation of aldol adducts over Pd/C, hydrolytic ring

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opening of the intermediates over aqueous acetic acid to polyketones, and the final HDO of the polyketones to alkanes with La(OTf)3 and Pd/C catalysts in an acetic acid system. In the HDO process, C–C cleavage unavoidably takes place through the breaking or decarbonization of the branched carbon atom, leading to shorter-chain alkanes and carbon loss. Therefore, developing highly efficient catalysts to restrain the C–C cleavage is a key objective. Generally, a large amount of molecular H2 is indispensable for the current HDO process; however, the molecular H2 is currently obtained from non-renewable resources. Hydrogen transfer strategy is an additional option to reduce the consumption of molecular H2. In addition, water-tolerant catalysts are highly desired because water can be generated in the HDO process.

LIQUID HYDROCARBONS FROM LIGNIN Apart from synthesizing fuel precursors from carbohydrate fractions of lignocellulose, lignin or lignin oils can act as natural jet-fuel precursors to produce high-quality fuels because of their middle carbon numbers and aromatic functionality. Some studies have targeted the production of cycloalkanes through the total HDO of lignin or lignin model compounds, such as phenol, guaiacol, and syringol.11 However, the pathway of converting into cycloalkanes loses the inherent aromatic properties of lignin with the penalty of additional hydrogen consumption. In addition, arenes have higher densities and volumetric heating values than chain alkanes and enable the shrinkage of aged elastomer seals to prevent the leakage of fuel. Therefore, converting lignin into arenes through a selective HDO reaction is a promising option. The biggest challenge in the process of arene production from lignin, driven by the evident competition between the excessive hydrogenation of the aromatic rings and the cleavage of C–O bonds, is to design desired catalysts that can break the C–O bonds in lignin while preserving the aromatic rings. After obtaining lignin oils through a liquid phase reforming reaction, the aromatic compounds can be synthesized through the CoMo/Al2O3- or Mo2C/CNF-catalyzed HDO at 300 C under 5 MPa H2.89 However, the development of new catalysts that can promote the direct conversion of native lignin into arenes is still crucial to take into consideration. The Ru/Nb2O5 catalyst is successful in arene production from organosolv lignin, and a high arene selectivity of 71 wt % was achieved. The excellent performance originates from the selective adsorption and activation of the Caromatic–OH bonds on Nb2O5 combined with the synergistic effect of Ru.69 In addition, the catalytic activities of lignin hydrogenolysis and product distributions were strongly dependent on the size of the Ru nanoparticles.90 However, the arenes obtained mainly contained three aromatic compounds, namely toluene, ethylbenzene, and propylbenzene, among which ethylbenzene and propylbenzene accounted for more than 85%.69 As mentioned in the previous section, the selective production of single alkylbenzene from lignin is still an important scientific challenge. The selective production of ethylbenzene, as a bulk chemical, from lignin may stand a chance through dehydrogenation followed by the decarbonization of terminal propyl hydroxyl groups. Hence, designing a highly efficient and selective catalyst is the key for ethylbenzene production.

BIOFUELS AND CHEMICALS FROM TOTAL CONVERSION OF LIGNOCELLULOSE Lignocellulosic biomass has tremendous potential for the production of valuable biochemicals and biofuels. Generally, it is first separated to cellulose, hemicellulose, and lignin through pretreatment and then catalytically upgraded to value-added

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chemicals and fuels. However, this process is associated with several disadvantages, including complex separation, loss of biomass functionality, and high energy consumption. Therefore, the total or direct transformation of lignocellulosic biomass into valuable chemicals and fuels in a single process has drawn immense attention. Different catalytic systems reported for the total conversion of lignocellulose are summarized in Table 1, Scheme 9, and Scheme 10. Actually, the difficulty of multistep processes for the full utilization lies in the first pretreatment of lignocellulosic biomass. In general, some easy-to-use compounds, such as lignin-oil, carbohydrates, and so on, can be obtained after the pretreatment.11,12,39,91–93 For a more elaborate discussion on the pretreatment of lignocellulosic biomass, especially lignocellulose fractionation, the reader is referred to a dedicated review on the topic.11 In addition, the solid catalyst usually mixes with solid cellulose and hemicellulose after delignification from lignocellulose, so the separation of the solid residue remains a key challenge.39,92 Through the full utilization, some valuable chemicals, including arenes, furfural, HMF, liquid alkanes, sugars, polymer or pharma building blocks, alcohols, and phenols, can be obtained.11,12,39,91,93 As a new solution, a photocatalytic lignin-first strategy for converting lignocellulosic biomass into the functionalized aromatics xylose and glucose has been presented.93 Cadmium sulfide quantum dots can break the b–O–4 bond at room temperature under visible light, and an aggregation-colloidization strategy overcomes the separation problem of the unconverted solid cellulose and hemicellulose residue.93 The photocatalytic method may be a promising tool for the utilization of lignocellulosic biomass in the future. In general, multistep processes for the full utilization of lignocellulosic biomass require complex separations and thus are extremely energy intensive. One-pot catalytic transformation of lignocellulose is of key interest because it circumvents pretreatments and tedious separations (Scheme 10). Specifically, multifunctional catalysts need to be designed to meet various requirements in the one-pot system. Several multifunctional catalyst systems, such as carbon-supported Ni-W2C, copperdoped porous metal oxide, partially reduced Ru/C, the combination of LiTaMoO6 and Ru/C, and Pt/NbOPO4, have been developed for conducting one-pot catalytic transformation of lignocellulose.74,94–98 Apart from multifunctional catalyst systems, the reaction mediums also play an important role in the one-pot conversion. For example, using cyclohexane as the reaction medium can lead to liquid alkanes with high yield through the total HDO of raw woody biomass.98 Conversely, with water, supercritical methanol, and aqueous phosphoric as the reaction media, the products were mainly oxygenated alcohols and phenols.94–97 From the product-utilization point of view, the mixture of liquid alkanes can directly serve as an additive to transportation fuels without further separation. Therefore, producing liquid alkanes from lignocellulose could be a solution that avoids the energy-intensive separation in future. The C–C coupling of alcohols with cyclopentanone into fuel precursors may be a promising way for the full utilization of lignocellulosic biomass.91,98 In addition, various other alcohols obtained from full utilization systems can also be used for conducting the C–C coupling reaction with highly active ketones, especially those containing active a-hydrogen or low steric hindrance, such as acetone78 and 2,4Pentanedione,80,84 to produce fuel precursor mixtures.

CONCLUSIONS AND PERSPECTIVES In this review, recent advances on the catalytic production of value-added chemicals and liquid fuels from lignocellulosic biomass have been summarized with an

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Table 1. The Complete or Direct Transformation of Lignocellulosic Biomass Feedstock

Catalyst

Reaction Conditions

Products and Yields

Reference

Birch tree

Ru/C+ Yb(OTf)3 (step 1); Ru/Nb2O5 (step 2a); seawater (step 2b)

0.05 g/mL in methanol, 200 C, 2 MPa H2, 2 h (step 1); 0.05 g/mL in H2O/cyclohexane, 250 C, 20 h (step 2a); 0.03 g/mL in THF-seawater, 200 C, 0.5 MPa N2, 5 h (step 2b)

C7–C9 hydrocarbons (95.6%), HMF (18.9 wt %), and furfural (5.6 wt %)

Guo et al.39

Pine tree

Cu20-PMO

0.1 g/mL in methanol, 180 C, 4 MPa H2, 18 h (step 1); 0.08 g mL in methanol, 320 C, 6 h (step 2)

aromatic monomers (10%) and aliphatic alcohols (68%)

Sun et al.91

Ash tree

Ni-W2C/AC

0.01 g/mL in H2O, 235 C, 6 MPa H2, 4 h

monophenols (40.5%) and diols (75.6%)

Li et al.94

Pine tree

Cu-PMO

4 wt % in methanol, 320 C, 8 h

C2–C6 aliphatic alcohols and C9+-substituted cyclohexanols

Liu et al.95

Cornstalk

Ru/C

0.1 g/mL in H2O, 200 C, 3 MPa H2, 8 h

alkylcyclohexanes (97.2%) and polyols (52.7 %)

Van den Bosch et al.96

Cornstalk

LiTaMoO6 + Ru/C + H3PO4

0.025 g/mL in H3PO4 solution, 230 C, 6 MPa H2, 24 h

phenols (35.7%) and gasoline (82.4%)

Wu et al.97

Beech wood

HCl + formaldehyde (extraction); Ru/C (hydrogenolysis)

0.1 g/mL in 1,4-dioxane-HCl-water solution, 80 C, 5 h (extraction); 0.05 g/mL in THF, 250 C, 4 MPa H2, 24 h (hydrogenolysis)

lignin monomers (47%), glucose (80%), furfural (40%), and xylose (50%)

Shuai et al.12

Birch tree

Ru/C

0.05 g/mL in methanol, 250 C, 3 MPa H2, 6 h

phenolic monomers (50%), aromatic dimers (18%), and carbohydrate retention (81%)

Li et al.92

Birch tree

CdS quantum dots

0.8 g/mL in CH3CN, visible light, room temperature, N2

aromatic monomers (27 wt %), xylose (84%), and glucose (91%)

Matson et al.93

Birch tree

Pt/NbOPO4

3 wt % in cyclohexane, 190 C, 5 MPa H2, 20 h

pentanes (10.2 wt %), hexanes (13.1 wt %), and alkylcyclohexanes (4.8 wt %)

Xia et al.98

emphasis on several important chemicals and fuels—including sugars, HMF, furfural, LA, GVL, FDCA, polyols, phenols, arenes, terephthalic acid, acetic acid, N,N-dimethylanilines, and liquid hydrocarbons—and their diverse applications. The past decade has witnessed a rapid development of efficient catalysts, versatile processes, and new catalytic mechanisms for lignocellulose transformation, making the valorization of renewable biomass an achievable reality. Despite tremendous achievements in this area, there is still a long way to go for industrial applications. (1) For the production of value-added chemicals, a high purity of final product is required, but the recalcitrance and complexity of lignocellulose and the polyfunctionality of biomass platform molecules usually result in a mixture of products, making the subsequent separation and purification energy intensive and costly. (2) Although the catalytic conversion of lignocellulose can offer great benefits with higher selectivity and milder conditions than the thermochemical way, the majority of catalytic reactions and processes are conducted in batch mode with low concentrations because of the insolubility of lignocellulose, making these processes less attractive. (3) Water is a natural and environmentally friendly solvent that is usually involved in lignocellulose conversion. However, only a limited number of catalysts can endure the hydrothermal environment with a long lifetime for industrial practice. (4) The efficient catalysts currently used for catalytic biomass valorization are mostly prepared at laboratory scale with high costs, so the magnification of catalyst preparation without an obvious decrease in performance remains a big challenge. In the future, we believe that all the challenging and long-standing issues in the current stage of biomass valorization will be resolved with new powerful and stable catalytic systems and a better understanding of the reaction mechanism. Here, we would

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Scheme 9. Schematic Representation of the Direct Catalytic Transformation of Lignocellulosic Biomass

Chem 5, 1–27, October 10, 2019

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Scheme 10. Schematic Representation of the Direct Catalytic Transformation of Lignocellulosic Biomass in One-Pot System

like to emphasize some future directions that could lead to significant progress in lignocellulose conversion. (1) Highly selective catalysts or catalytic systems for bioplatform molecules can be designed and developed on the basis of the deep understanding of the reaction mechanism to increase product selectivity and reduce the subsequent separation and purification costs. (2) Transportation fuels are able to tolerate different components (hydrocarbons or mono-oxygenates), so strategies starting from lignocellulose would be more focused on producing liquid fuels. Meanwhile, designing integrated technologies with fewer steps or one-pot processes is of significant importance to improve energy efficiency and reduce separation costs. (3) Smart catalytic systems can be established to simultaneously convert cellulose, hemicellulose, and lignin components in lignocellulose into biofuels and chemicals in a single process, which would enable the full utilization of lignocellulosic biomass with improved atom economy and reduced pretreatment costs. (4) The synthesis of high-value chemicals, such as phenol, m-xylylenediamine, p-xylene, ethylbenzene, and p-phthalic acid, from lignocellulose is an important scientific challenge. It is crucial to consider the development and design of improved step-wise systems. (5) Many chemicals and pharmaceuticals contain heteroatoms, such as nitrogen, sulfur, or phosphorus. However, most current studies focus on the utilization of inherent atoms (mainly carbon, hydrogen, and oxygen) within lignocellulosic biomass.99

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Introducing heteroatoms into chemical structures is a promising strategy for exploring potential high-value chemicals or pharmaceuticals. (6) Similar to the production process of high-carbon biofuels, namely C–C coupling followed by the HDO process, an additional direction that combines C–C coupling reactions with the subsequent introduction of heteroatoms (nitrogen, sulfur, or phosphorus) may be a promising pathway for the production of high-carbon heteroatom-containing chemicals.

ACKNOWLEDGMENTS This work was supported financially by the National Natural Science Foundation of China (21832002, 21872050, and 21808063), the National Key R&D Program of China (2017YFB0306505), the Shanghai Municipal Science and Technology Major Project (2018SHZDZX03), the Fundamental Research Funds for the Central Universities (222201718003), and the ‘‘Zhang Jiangshu’’ excellent PhD scheme of the East China University of Science and Technology.

AUTHOR CONTRIBUTIONS Y.W. proposed the topic of the review. Y.J., Y.G., and Q.X. wrote the manuscript. Y.J., Y.G., Q.X., X.L., and Y.W. discussed and revised the manuscript.

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