processes Review Mini-Review on the Synthesis of Furfural and Levulinic Acid from Lignocellulosic Biomass Zhiwei Jiang , Di Hu, Zhiyue Zhao, Zixiao Yi, Zuo Chen and Kai Yan * School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China; [email protected] (Z.J.); [email protected] (D.H.); [email protected] (Z.Z.); [email protected] (Z.Y.); [email protected] (Z.C.) * Correspondence: [email protected]; Tel.: +86-20-39277362 Abstract: Efficient conversion of renewable biomass into value-added chemicals and biofuels is regarded as an alternative route to reduce our high dependence on fossil resources and the associated environmental issues. In this context, biomass-based furfural and levulinic acid (LA) platform chemicals are frequently utilized to synthesize various valuable chemicals and biofuels. In this review, the reaction mechanism and catalytic system developed for the generation of furfural and levulinic acid are summarized and compared. Special efforts are focused on the different catalytic systems for the synthesis of furfural and levulinic acid. The corresponding challenges and outlooks are also observed. Keywords: biomass; catalytic conversion; furfural; levulinic acid; mechanism; synthesis Citation: Jiang, Z.; Hu, D.; Zhao, Z.; 1. Introduction Yi, Z.; Chen, Z.; Yan, K. Mini-Review Biomass, the largest carbon-neutral renewable source on Earth, is considered a promis- on the Synthesis of Furfural and ing alternative to our dependence on limited fossil fuel sources and the associated environ- Levulinic Acid from Lignocellulosic mental issues [1–6]. Combustion and anaerobic digestion are adopted extensively to treat Biomass. Processes 2021, 9, 1234. biomass; however, these techniques are inefficient and possess problems. Therefore, the val- https://doi.org/10.3390/pr9071234 orization of biomass into conventional fuels and chemicals has influenced a large number of research efforts [7,8]. The conversion of biomass to low-molecular weight compounds Academic Editors: Francesco Parrino requires an effective decomposition method. In addition, the various biomass-derived and Antonio D. Moreno chemicals can be converted into highly valuable products [9–11], which offers competitive Received: 22 May 2021 superiority against fossil-based products. A wide range of technologies (e.g., gasification, Accepted: 14 July 2021 pyrolysis, catalysis and aqueous phase processing) are successfully employed to convert Published: 16 July 2021 biomass into fine chemicals and biofuels [12,13]. As DOE/NREL reports, the top 10 valuable chemical candidates generated from Publisher’s Note: MDPI stays neutral biomass were identified, and an updated top 10 + 4 list was proposed by Bozell et al. [14]. with regard to jurisdictional claims in Among the various biomass-based chemicals, furfural and levulinic acid (LA) are consid- published maps and institutional affil- ered two significant platform chemicals in lignocellulosic biorefineries to produce a wide iations. variety of liquid fuels [15–18]. They can be directly obtained through the dehydration reaction of six- and five-carbon sugars (e.g., fructose and xylose). The production pathways of furfural and levulinic acid from alternative carbon sources are illustrated in Figure1. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Processes 2021, 9, 1234. https://doi.org/10.3390/pr9071234 https://www.mdpi.com/journal/processes ProcessesProcesses 2021, 92021, x FOR, 9, 1234 PEER REVIEW 2 of 162 of 16 FigureFigure 1. Biorefining 1. Biorefining scheme scheme for forthe the upgrading upgrading of of lignocellulosic lignocellulosic biomassbiomass into into value-added value-added platform platform chemicals chemicals and liquidand liquid fuels [19].fuels Reproduced [19]. Reproduced with with permission permission from from Ref. Ref. [19]. [19]. A Avast vast number number of of chemicals chemicals (e.g.,(e.g., furfuryl furfuryl alcohol, alcohol, furanones furanones et. al)et. canal) becan yielded be yielded fromfrom furfural furfural that that has has a a global productionproduction of of ~300 ~300 kton/y. kton/y. For For example, example, furfuryl furfuryl alcohol alcohol derivedderived from from furfural furfural accounts accounts for aboutabout 62% 62% of of the the global global furfural furfural market market and 75%and of75% the of the U.S.U.S. market. market. Moreover, Moreover, more more than 8080 value-addedvalue-added chemicals, chemicals, as fossilas fossil fuel fuel alternatives, alternatives, can be synthesized using furfural as a feedstock via different reactions for the aldehyde can be synthesized using furfural as a feedstock via different reactions for the aldehyde group and aromatic ring in furfural [20]. The valorization approach of furfural is a highly groupattractive and aromatic topic. ring in furfural [20]. The valorization approach of furfural is a highly attractiveLevulinic topic. acid is regarded as one of the top platform chemicals for the biorefinery. ToLevulinic be economically acid is competitive, regarded as LA one is generallyof the top acquired platform from chemicals multiple stepsfor the of biorefinery. the hy- To drolysisbe economically of biomass competitive, via 5-hydroxymethylfurfural LA is generally intermediate acquired from [17]. Conversionmultiple steps of LA of can the hy- drolysisaugment of biomass the categories via 5-hydr of chemicalsoxymethylfurfural from renewable intermediate biomass, which [17]. is part Conversion of an important of LA can augmentexpanded the field categories of biomass of chemicals conversion. from As are sustainablenewable biomass, platform chemical,which is LApart with of an a im- portantbifunctional expanded gamma-keto-carboxylic field of biomass acidconversion is exploited. As ina asustainable wide range ofplatform applications chemical, such LA as polymer precursors, pyrrolidinones, pharmaceutical intermediates and fuel additives. with a bifunctional gamma-keto-carboxylic acid is exploited in a wide range of applica- LA also reacts with phenol or aldehydes to produce various important chemicals [21]. tionsThe such above as discussionpolymer precursors, demonstrates pyrrolidino that furfuralnes, and pharmaceutical LA hold impressive intermediates market poten- and fuel additives.tial. However, LA also the reacts outcome with of phenol this fascinating or aldehydes goal is to dependent produce onvarious the large-scale important and chemi- calssustainable [21]. The above production discussion of furfural demonstrates and LA with that high furfural efficiency and andLA athold acceptable impressive prices. market potential.In this review,However, we concentratethe outcome on of the this mechanism fascinating and goal various is dependent well-studied on production the large-scale androutes sustainable of furfural production and LA from of furfural different and biomass LA resources.with high Some efficiency novel homogeneousand at acceptable prices.(deep In eutectic this review, solvents, we ions concentrate liquids) and on heterogeneousthe mechanism catalysts and various (carbon-based well-studied acids, clay, produc- tionoxides, routes zeolites, of furfural cation and exchange LA from resin, differ heteropolyent biomass acids, resources. metal-organic Some frameworks) novel homogene- in various systems for the production of furfural and LA are also discussed, especially from ous (deep eutectic solvents, ions liquids) and heterogeneous catalysts (carbon-based acids, native biomass (such as wheat straw, corn stover, sugarcane bagasse and so on). Challenges clay,for oxides, large-scale zeolites, applications cation andexchange areas that resin, need heteropoly improvement acids, are metal-organic also highlighted. frameworks) in various systems for the production of furfural and LA are also discussed, especially from2. Mechanismnative biomass for the (such Formation as wheat of Furfural straw, corn stover, sugarcane bagasse and so on). ChallengesThe preparationfor large-scale of furfural applications is often and studied areas through that need the degradation improvement of pure are xylose also high- lighted.in an acidic environment [22–27]. Recently, there is a growing trend to utilize a complex carbohydrate and a native biomass as the starting reactant. However, the exact mechanism 2. Mechanismis not mature. for Two the popular Formation mechanisms of Furfural are proposed. The first one is the production of furfural from C5-sugar (e.g., xylose and arabinose) through the 1,2-enediol intermediate (FigureThe preparation2). The rate-limited of furfural step of is this often mechanism studied is through the formation the degradation of 1,2-enediol of interme- pure xylose in andiate. acidic Some environment halide ions (e.g., [22–27]. Cl−, BrRecently,−) can tremendously there is a growing improve trend the rate to ofutilize enolization a complex carbohydrateand the following and a dehydration native biomass reaction as inthe an st acidicarting environment reactant. However, [28]. the exact mecha- nism is not mature. Two popular mechanisms are proposed. The first one is the produc- tion of furfural from C5-sugar (e.g., xylose and arabinose) through the 1,2-enediol inter- mediate (Figure 2). The rate-limited step of this mechanism is the formation of 1,2-enediol intermediate.