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Chemical Engineering Journal

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Sustainable bioplastics: Recent progress in the production of bio-buildingblocks for the bio-based next-generation polymer PEF

Kyung-Ran Hwanga, Wonjin Jeona, Soo Youn Leeb, Min-Sik Kimb, Young-Kwon Parkc,⁎

a Biomass and Waste to Energy Laboratory, Korea Institute of Energy Research, Daejeon 34129, Republic of KoreabGwangju Bio/Energy Research Center, Korea Institute of Energy Research, Gwangju 61003, Republic of Koreac School of Environmental Engineering, University of Seoul, Seoul 02504, Republic of Korea

H I G H L I G H T S

• Current progress in the catalytic conversion of bio-based HMF into FDCA are reviewed.

• Recent progress in the synthesis of HMF from biomass are discussed.

• Biological conversion route of biomass feedstock into EG are given.

• Future outlooks and challenges of the production of bio-building blocks are provided.

A R T I C L E I N F O

Keywords:BiowastesBioplasticsBio-building blocksConversion of biomass

A B S T R A C T

Today, bioplastics are recognised as sustainable alternatives and one of the solutions to the crisis of petro-basedplastics. The next-generation polymer poly(ethylene 2,5-furandicarboxlate) (PEF) has gained great popularitysince it is associated with significant reduction of energy use and GHG emissions, as well as with superior barrierproperties. This drop-in polymer is a promising 100% bio-based alternative to its petro-based counterpart poly(ethylene terephthalate) (PET). PEF can be effectively synthesised by polymerisation between 2,5-fur-andicarboxylic acid (FDCA) and ethylene glycol (EG), which represent biomass-derived building blocks. Thesebio-building blocks are an important commodity and platform chemicals that can be used for a variety of ap-plications, including the synthesis of PEF. This review covers recent progress in the production of novel bio-building blocks for the preparation of PEF. Among various synthesis methods, we reviewed the catalytic con-version of biomass-derived hydroxylmethylfurfural (HMF) into FDCA and the biological route from biomassfeedstock into EG. In addition, we covered recent progress in the synthesis of HMF from biomass since a reliablesupply of HMF is important for the synthesis of FDCA. Finally, research goals and challenges for future devel-opment of bio-building blocks production were proposed.

1. Introduction

Plastics are most ubiquitous materials, being used almost every-where at any time during our lives. Their low cost, low weight, dur-ability, and ease of manufacture (as the name itself implies: “pliableand easily shaped”), allow their widespread applicability from in toys toin medical devices. Plastics are synthetic polymers prepared via poly-merisation from primary chemicals that are mainly derived from pet-roleum oil. It was estimated that about 8,300 Mt of virgin plastics havebeen manufactured to date worldwide. Remarkably, around 79% ofthem have been discarded in landfills or natural environments, whileonly 12% have been incinerated [1]. Semi-uncontrollable

environmental issues are occurring as a result: for example, shrinkinglandfill capacity, the accumulation of non-degradable waste, secondarynano- and micro-plastics pollution in the ocean, and the emission oftoxic substances from incinerators. Fortunately, public awareness aboutthe environmental crises and hazards related to present disposalmethods has been growing thanks to newspapers and publications, andpeople seem to be ready to accept new solutions and alternatives topetro-based plastics. However, the global production of plastics hassteadily increased over time, reaching almost 360 Mt in 2018 [2].Furthermore, the production of primary plastics is estimated to exceed28,000 Mt by the end of 2050 [1], probably due to population growth,market expansion, and a significant growth in the demand of new

https://doi.org/10.1016/j.cej.2020.124636Received 1 January 2020; Received in revised form 26 February 2020; Accepted 2 March 2020

⁎ Corresponding author.E-mail address: catalica@uos.ac.kr (Y.-K. Park).

Chemical Engineering Journal 390 (2020) 124636

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applications. Notably, polyester fibres, polyamide fibres, and polyacrylfibres are not included in that estimation. Packaging, mostly food andbeverages packaging, represents the largest end-use market (around40%) by far [2], where package plastics are non-degradable and have ashort-lifetime, during which they are used just once and then discarded.

Researchers are making efforts to collect global information aboutplastics’ end-of-life fate [1] and to promote the utilisation and recyclingof different waste plastics [3,4]. Among the current plastic utilisationtechnologies, plastics pyrolysis leads to lower greenhouse gas (GHG)net emissions than other current plastic recycling technologies (e.g.,incineration and gasification) [3]. Even though some issues linked topetro-based plastics can be solved by thermochemically convertingthem into oils, bioplastics represent a better alternative in view of theshortage of fossil resources and global warming, since they promote theadvance of circular economy and environmental sustainability. A gen-eral interest in bioplastics started almost 50 years ago and was recentlyrenewed by observations on the huge amount of plastic produced, theprohibition of plastic waste export, and nano- and micro-plastics pol-lution.

1.1. Bioplastics

The definition of bioplastics can be simply divided into two terms,that have been extensively reviewed in previous articles [5]: (1) bio-degradable (or not) and (2) biomass-derived (or not). Our study focusedon bioplastics that fit in the category of biomass-derived polymers,biodegradable and not (durable). Bio-based thermoplastic polyesters(e.g., polylactide (PLA), polyhydroxyalkanoates (PHAs), and poly(bu-tylene succinate) (PBS)) that are obtained through the fermentation ofbio-based feedstocks, are biodegradable and available at commercialscales [6]. On the contrary, drop-in bioplastics (e.g., bio-polyethylene(bio-PE), bio-propylene (bio-PP) and bio-poly(ethylene terephthalate)(bio-PET)) are durable.

Bio-PET is an alternative to its petro-based counterpart poly(ethy-lene terephthalate) (PET). Partial bio-PET (Bio-PET 30) occupies a greatpart of the bioplastics market share (approximately 58%) as rigidpacking materials [7] and this market trend is expected to continue.Fig. 1 shows an overview of the bio-PET production from petro- andbio-based building blocks. Recently, many efforts have been made toobtain a 100% bio-PET by replacing petro-based terephtalic acid (TPA)with bio-based TPA. The amount of GHG emitted during the life cycle ofBio-PET 100 can be 25% – 58% lower than that from petro-based PET,depending on the biomass sources used for the production of the bio-TPA [8]. Other current bio-based alternatives to PET are poly(ethylene2,5-furandicarboxylate) (PEF) and poly(trimethylene furandicarbox-ylate), both of which are furan-based polymers. Bio-based PEF, whichhas gained great attention, is referred to as a bio-based polyester of thefuture not only for its properties, but also due to the fact that it leads toa significant reduction of energy use and GHG emissions. Energy andGHG balance analyses conducted during the production of PEF fromcorn-based fructose showed that the amount of energy used and GHGemissions can be reduced by approximately 40% – 45% and 45% –50%, respectively, compared with those during PET production.Moreover, these reductions in energy consumption and GHG emissionsare higher than those obtained in the case of other bio-based polymers(i.e., PLA and bio-PE) [5]. The material properties (i.e., barrier per-formance, mechanical, and thermal) of PEF are superior to those ofPET. In particular, gas barrier properties of PEF is extremely good. Ithas a 10 times lower O2 permeability and a 20 times lower CO2 per-meability than those of PET. This high gas barrier properties are veryadvantageous for the food and beverage packaging [9,10]; therefore,Corbion and Synvina are currently working for PEF.

1.2. Bio-building blocks for PEF production

The next-generation polyester PEF is a 100% bio-based alternative

to petro-based PET. PEF is effectively synthesised through a poly-merisation between 2,5-furandicarboxylic acid (FDCA) and ethyleneglycol (EG) (with a yield of over 90%) [11]. Fig. 2 presents an overviewof the process followed to produce PEF from biomass-derived buildingblocks (e.g., EG and FDCA).

The synthetic route of PEF is similar to that of PET, except for theuse of FDCA in place of TPA. Bio-based EG and FDCA can be both ob-tained from biomass via biological and chemical-catalytic routes. EG isan important commodity used in a variety of applications (e.g., anti-freeze, coolant, heat transfer fluids, as a precursor for polyester) [19].FDCA is an important platform chemical that was selected as one of thetwelve building blocks derived from biomass: it can be transformed intoa number of high-value bio-based chemicals, such as polyesters, poly-amides, plasticisers, and medicines [16,17,20] via various synthesismethods [21,22].

Although these building blocks can be produced in a wide variety ofways and routes (Fig. 2), this review covers only current progress in: (1)the catalytic conversion of bio-based hydroxymethylfurfural (HMF) intoFDCA (which is a viable route for commercialisation) and (2) the bio-logical conversion of biomass feedstock into EG (which has been rarelyreviewed). In addition, we cover recent progress in the synthesis ofHMF (a valuable platform chemical) from biomass: a reliable supply ofHMF is important for the synthesis of FDCA. Finally, we provide futureoutlooks on the production of bio-building blocks.

2. Production of FDCA from biomass feedstock

HMF, a heterocyclic furanic compound substituted in the 2 and 5positions with hydroxide and aldehyde functional groups, is one of themost valuable platform chemicals derived from lignocellulosic biomass.This important furanic compound has been widely used to produce bio-based fuels and chemicals [23]. Recently, it has also received greatattention as a raw material for the synthesis of FDCA (Fig. 2).

The catalytic conversion of biomass to HMF and FDCA has beeninvestigated extensively from the early 2000s (Fig. 3). At lab scale,many efforts have been done to discover new combinations of catalystsand solvents, so to improve the effectiveness of HMF production.Meanwhile, AVA Biochem (Switzerland) began to produce bio-basedHMF from sugars in 2013 (at the commercial scale of 20 tons per year).Their production capacity is expected to reach 5000 tons per year by2022. In 2004, FDCA was chosen as one of the top 12 value-added bio-based chemicals by the U.S. Department of Energy (DOE) [11]; this facttriggered intensive research on the catalytic conversion of HMF intoFDCA during the 2010s.

2.1. Catalytic conversion of biomass feedstock to HMF

Numerous studies on the catalytic conversion of biomass to HMFconcluded that the production of HMF is strongly influenced by the typeof biomass substrates, catalysts, and solvents employed [24–26].Therefore, an understanding of the experimental factors is very im-portant in order to establish an efficient HMF production at both laband commercial scales.

2.1.1. Biomass feedstockBiomass resources for the synthesis of HMF can be categorised into

edible and non-edible biomass. From these, it is possible to obtain rawmaterials (e.g., glucose and fructose) (Fig. 4). Sucrose, commonlyknown as sugar, is produced by refining sugarcane and sugar beet. Inthis substance, glucose and fructose are linked via a glycosidic bond.Compared with polymeric biomass, such as cellulose and starch, thisdisaccharide compound can be more effectively converted to HMF,because the cleavage of the single glycosidic bond existing betweenglucose and fructose molecules occurs under mild reaction conditions(below 120 °C) [27]. The use of the edible biomass for HMF productionis still controversial; however, the cheap sugar produced yearly (about

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Fig. 1. Production of PET from petro and bio-based building blocks; (a) petro-based PET, (b) Bio-PET30 and (c) Bio-PET100.

Fig. 2. Production routes of bio-based PET from biomass-derived building blocks including ethylene glycol (EG) [12–14] and 2,5-furandicarboxylic acid (FDCA)[15–18].

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180 million tons) has potential as a raw material for the synthesis ofHMF [28,29].

The Food and Agriculture Organization of the United Nations (FAO)reported that globally wasted food amount to about 1.3 billion tons peryear, and that this amount is expected to increase up to 2.1 billion tonsper year by 2030 [30]. Food waste can represent a good feedstock forthe production of HMF, since it usually contains plenty of sugars andcarbohydrate (e.g., in the form of starch) [31].

Lignocellulosic biomass is regarded as the most promising feedstockfor the synthesis of HMF: the use of expensive bio-based monomericsubstrates (e.g., glucose and fructose) is less practical. However, thedirect conversion of biomass into HMF is technically and economicallymore challenging compared with that of monomeric biomass substrates.Recently, the direct conversion of polymeric biomass into HMF hasbeen explored through sustainable and eco-friendly catalytic processes.These led to considerable HMF yields, comparable to those obtained byusing fructose and glucose [32,33]. However, the one-pot synthesis ofHMF from polysaccharides is still challenging, because cellulose ishardly soluble in water or general organic solvents [34]. This propertyleads to an insufficient interaction between the cellulose and catalysts,as well as to an increase of the temperature and time required for thereaction. Moreover, it is responsible for a low selectivity to HMF andthe production of by-products (e.g., humin, levulinic acid, and formicacid). In other words, the development of a cellulose-soluble solventsystem would guarantee an effective catalytic conversion of cellulose-based biomass to HMF.

2.1.2. Catalysts and solvents for the one-pot conversion of cellulose to HMFThe reaction pathway for the chemical conversion of cellulose to

HMF is described in Fig. 5. Cellulose is first hydrolysed to glucose; then,the produced glucose is isomerised to fructose, and fructose is dehy-drated to HMF. As aforementioned, the direct conversion of cellulose toHMF is quite challenging, due to the crystalline structure of the formercompound. Therefore, strong acid catalysts are required for the depo-lymerisation of cellulose. The table embedded in Fig. 5 summarises theroles of catalysts and solvents in the reaction. Brønsted acid catalyststhat donate protons (e.g., the hydrogen cation H+) into a reaction so-lution have been extensively used in HMF production, because activeprotons promote both the hydrolysis of cellulose and the dehydration offructose. Hydrochloric acid (HCl), one strong Brønsted acid catalyst,has a good catalytic performance in the hydrolysis of cellulose in water;however, it can also accelerate the conversion of HMF, resulting into alow HMF yield [35]. Contrary to homogeneous Brønsted acid catalysts,well-designed heterogeneous catalysts can control each reaction stepduring the hydrothermal conversion of cellulose to HMF. A recent studyshowed that the catalytic activity of niobic acid (Nb2O5·nH2O) can beremarkably improved by treating it with strong acids (e.g., H2SO4 andH3PO4) [36]; in fact, strong acids grafted on the catalyst surface tendedto increase Brønsted acidity, while the intrinsic Lewis acid sites on pureniobic acid catalysts promoted the isomerisation reaction, leading to adoubled yield of HMF. However, the maximum yield of HMF was ofonly 16% at about 80% of the cellulose conversion, and was accom-panied by the formation of by-products. This indicates that the Brønstedand Lewis acid sites simultaneously catalysed undesired reactions, suchas the rehydration and polymerisation of HMF and its derivatives. Inorder to suppress such undesired reactions during the hydrothermalconversion of cellulose using acid catalysts, polar aprotic solvents havebeen introduced into the reaction system.

Polar aprotic solvents, such as tetrahydrofuran (THF), methyl iso-butyl ketone (MIBK), and dimethyl sulfoxide (DMSO), are organic sol-vents characterised by moderate polarity and high dielectricity. Foralmost two decades, they have received great attention as promisingreaction solvents for the production of HMF from biomass: their mul-tifunctional catalytic activity promotes the hydrolysis and dehydrationof biomass substrates, while suppressing the rehydration and poly-merisation of products [37]. In particular, a biphasic system composedof polar aprotic solvent and water can have a great synergic effect onthe catalytic conversion of biomass to HMF [38]. The co-existence ofpolar protic and aprotic solvents induces a phase separation of the re-action medium, which results in the separation of sugars and HMF intopolar protic and aprotic phases, respectively. This transfer phenomenoncan protect the HMF produced from further rehydration in the aqueousphase, resulting in a high HMF production. In a biphasic solvent system,catalyst can show its best performance: the composition of the reactionmedium can control both desired and undesired chemical reactions[39]. For example, Li and co-workers recently reported the remarkableeffect of a THF-water biphasic system on the production of HMF fromcellulose. In their experiment, niobia/C composite catalysts having bothBrønsted and Lewis acid sites promoted the conversion of cellulose toHMF with a 78% C yield at 170 °C for 8 h (HMF: 53%, glucose: 25%).This high C yield indicates that further conversions of HMF were

Fig. 3. The number of article publications on the production of HMF and FDCAfrom biomass by year up to 2019, identified in the Scopus Database.

Fig. 4. Various biomass resources for the production of HMF.

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effectively suppressed over a relatively long reaction time while byusing the biphasic reaction medium [40]. Phosphated TiO2 and ChCl/HMOR zeolite catalysts with Brønsted and Lewis acid sites also ex-hibited good catalytic performances in the conversion of cellulose toHMF within THF- and MIBK-water solvent systems [41,42]. These re-sults indicate that a combination of bifunctional heterogeneous cata-lysts and biphasic solvents can be very effective for the selective pro-duction of HMF from cellulose. Homogeneous acid catalysts alsoshowed a remarkable catalytic activity under biphasic solvent condi-tions [43]; however, this type of catalytic system involves a difficultseparation of the ionised catalysts from the biphasic mixture.

Ionic liquids have also gained much attention as promising reactionsolvents for the one-pot conversion of cellulose to HMF, due to theirexcellent cellulose dissolving ability [44]. In addition, the thermalstability and negligible vapor pressure of ionic liquids favour such re-action. The use of these unique solvents allows the conversion of cel-lulose at relatively low reaction temperatures (T = 120 – 150 °C) andleads to considerable HMF yields (40 – 50%) [45–47]. Moreover, somefunctionalised ionic liquids (e.g., [HMIM]HSO4) can effectively catalysethe conversion of cellulose to HMF without the need for catalysts [48].In addition to catalysing cellulose hydrolysis, these unique solventsexhibit considerable catalytic activity in the isomerisation of glucoseand dehydration of fructose. Among various ionic liquids, 1-butyl-3-methylimidazolium chloride (i.e., [BMIM]Cl) has been the most ex-tensively used for cellulose conversion. Recent studies also employedthe ionic liquid as a reaction solvent in the production of HMF fromcellulose with catalysts. Yan’s research group fabricated unique

bifunctional catalysts with acid-base sites for the conversion of celluloseto HMF. For example, they synthesised rod-like hollow catalysts bytemplating polymer solids on halloysite nanotubes [45]. Those catalystsshowed a good catalytic activity in [BMIM]Cl, with HMF yields of over40%; in addition, MOF-based bifunctional catalysts were employed forthe conversion of cellulose to HMF in the ionic liquid, exhibiting also aremarkable catalytic performance [46]. All the hierarchical porouscatalysts were reused a few times in the ionic liquid with no deacti-vation. Cr-doped zeolite catalysts (e.g., Cr-USY and Cr-Beta) alsoshowed a good catalytic activity in the synthesis of HMF; however, Cr-USY was deactivated by the leaching of Cr in [BMIM]Cl [47].

Despite the beneficial properties of polar aprotic solvents and ionicliquids, the difficult separation of HMF from the reaction mixture andthe high cost of ionic liquids still hinder a sustainable production ofHMF [43,49]. An effective HMF separation would allow the reuse of thereaction solvents and of the spent homogeneous catalysts dissolved inthe solvents, making the process more economical and eco-friendlier.Since HMF is a thermally unstable compound, it can be easily trans-formed into undesired chemicals; therefore, conventional distillationmethods are unsuitable for the separation of HMF from high-boilingsolvents. The separation of HMF through vacuum distillation can ef-fectively reduce HMF loss; however, this technique is costly at com-mercial scale. A recent study [50] indicated a solid–liquid adsorptiveseparation technique as promising for HMF separation: the authorsreported that 94.4% of the HMF produced by acid-catalysed fructosedehydration was successfully recovered from the reaction mixture. Thiswas achieved through adsorption on hollow-structured porous aromatic

Fig. 5. Effects of catalysts and solvents on reaction routes in the production of HMF from biomass.

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polymers at room temperature. Moreover, the polymeric adsorbentcould be scaled-up and reused 3 times without any significant perfor-mance loss.

To achieve an economic and eco-friendly HMF production, it isnecessary to ensure an efficient recycling of spent catalysts and reactionsolutions. Many catalysts exhibiting remarkable catalytic activities inthe conversion of biomass to HMF can have harmful effects on humansand in the natural environment. For example, metal chlorides (e.g.,AlCl3 and CrCl3) are well known as active Lewis acid catalysts that caneffectively isomerise glucose to fructose [51]; however, AlCl3 can alsocause neurotoxic damages in animals and humans. Meanwhile, Cr(III)has relatively low toxicity; however, its high oxidation activity to Cr(IV)can cause serious damages to internal organs (e.g., kidney and liver).Moreover, toxic transition metals usually exist in the form of metalsolutions: the recovery of the metal ions from the reaction mixture iscomplicated and usually requires the adoption of costly separationprocesses. Therefore, bifunctional heterogeneous catalysts with highleaching resistance can be more advantageous for the synthesis of HMFfrom biomass feedstock. Up-to-date studies on the catalytic conversionof cellulosic biomass to HMF are summarized in Table 1.

2.2. Synthesis of FDCA from HMF

Compared to the catalytic conversion of biomass feedstock to HMF,FDCA can be more easily synthesised from HMF via catalytic oxidationpathways. Traditionally, the catalytic oxidation of HMF has exhibitedremarkable FDCA production, however, the use of costly catalysts and ahigh concentration of base makes the reaction process less sustainable.Recently, various studies have been carried out to synthesise FDCAfrom HMF in more sustainable ways.

2.2.1. Traditional catalytic systemsThe synthesis of FDCA from HMF can be accomplished via a few

chemical oxidation routes in aerobic conditions with the aid of a baseand a catalyst. Oxygen has been widely used as an oxidizing agent inthe reaction, due to the fact that it is cheap, environmentally friendly,and has a good oxidizing ability. A recent study suggested that the useof a stoichiometric oxidant (e.g., tert-butyl hydroperoxide (t-BuOOH))can lead to a time- and energy-efficient HMF oxidation [57]. Thecombination of a MnFe2O4 spinel catalyst and t-BuOOH resulted in aremarkable catalytic performance during the synthesis of FDCA ,with ayield of 71% at 100 °C within 5 h.

Heterogeneous catalysts doped with noble metals (e.g., Pd, Ru, Pt,and Au) have been extensively used for the synthesis of FDCA fromHMF. These catalysts showed very remarkable catalytic performances,leading to a FDCA yield of more than 90% [58]. The reaction me-chanism for the conversion of HMF to FDCA using noble metal-

supported catalysts is described in Fig. 6. There, a catalyst promotes theoxidation of a HMF formyl group, leading to the production of 5-hy-droxymethyl-2-furancarboxylic acid (HFCA) and the oxidation of aHFCA hydroxymethyl group to FDCA on Route A. On Route B, instead,the catalyst attacks another side of the HMF, leading to the formation of2,5-diformylfuran (DFF) via the oxidation of a HMF hydroxymethylgroup. The reaction mechanism involved in the oxidation of HMF toFDCA strongly depends on the kinds of catalysts and base additivesemployed [59,60].

The catalytic oxidation of HMF to FDCA has been extensively stu-died in recent decades: it generally leads to very high FDCA yields(about 100%). Traditional HMF oxidation processes employed toachieve a high FDCA production require noble metal-based catalystsand several equivalent amounts of base additives dissolved in the

Table 1Up-to-date studies on the catalytic conversion of cellulosic biomass to HMF.

Substrate Reaction condition Catalyst Solvent HMF yield Ref.

Cellulose T = 200 °C, t = 1 h, PH2 = 6 MPa Ni/C H2O 85% [32]Cellulose T = 220 °C, t = 0.5 h RuCl3 NaCl/BuOH/H2O 83% [33]Cellulose T = 170 °C, t = 1 h H3PO4-treated Niobic acid H2O 16% [36]Cellulose T = 170 °C, t = 8 h Nb/C NaCl/THF/H2O 53% [40]Cellulose T = 180 °C, t = 2 h ChCl/HMOR_20 NaCl/MIBK/H2O 39% [41]Cellulose T = 175 °C, t = 4 h 15P-TiO2 THF/water 33% [42]Cellulose T = 190 °C, t = 4 h HfO(PO4) THF/H2O 70% [43]Cellulose T = 180 °C, t = 2 h AlCl3-H3PO4 DMOE/water 49% [52]Waxy corn starch T = 195 °C, t = 1 h Choline chloride/boric acid THF/water 60 mol% [53]Cellulose T = 177 °C, t = 1 h HCl MIBK/H2O 34% [54]Cellulose T = 150 °C, t = 2 h Halloysite nanotube [BMIM]Cl 42% [45]Cellulose T = 130 °C, t = 0.5 h CNTs-SO3H-NH2-Cr(III) [BMIM]Cl 40% [55]Cellulose T = 120 °C, t = 1 h PHs-SO3H@UiO-66-NH2 [BMIM]Cl 50% [46]Cellulose T = 120 °C, t = 1.5 h HPSC [BMIM]Cl 46% [56]Cellulose T = 130 °C, t = 1 h Cr-Beta, Cr-USY [BMIM]Cl 34% [47]Whatman CF-1 T = 180 °C, t = 0.5 h – [HMIM]HSO4 68% [48]

Fig. 6. Reaction pathways for catalytic oxidation of HMF to FDCA in aqueousmedia over noble metal-based catalysts.

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correspondent reaction media [61]. The oxidation reaction of HMF inthe presence of expensive catalysts in the basic reaction media guar-antees a FDCA yield of almost 100%; however, the high cost of suchcatalysts and of the subsequent neutralisation steps for the basic mix-ture make the oxidation process less practical and less eco-friendly.Noble metal-based reaction systems are still used to study specific re-action mechanisms, the recyclability of catalysts, or continuous FDCAsynthesis [62–65]. Meanwhile, new approaches involving the use ofnon-noble metals and base-free solutions have been recently in-vestigated for a more sustainable FDCA production.

2.2.2. Base-free FDCA production over non-noble metal catalystsAs listed in Table 2, a base-free oxidation of HMF to FDCA has been

recently performed using noble metal-based catalysts (i.e., Ru/ZrO2, Pt/ZrO2, and Pt–Ni/AC), resulting in significantly high FDCA yields (ofover 90%) [58]. Some bases can promote the desorption of acid inter-mediates (e.g., HFCA and FFCA) from the surface of an active metal bydeprotonating the acid compounds [66]. However, very small Ru par-ticles impregnated on high-surface-area ZrO2 showed outstanding cat-alytic activity, with a maximum FDCA yield of 97%, in the absence ofbase additives [67]. Pt-based catalysts prepared through an atomiclayer deposition (ALD) method also effectively enhanced the oxidationof HMF to FDCA in the absence of any base additives [58]. That ALDtechnique allowed Pt metals to be highly dispersed on the catalystsupports (e.g., of ZrO2 and activated C (AC)), significantly improvingthe adsorption of C = O by the catalysts. Additionally, transition metal-based and cheap catalysts have been used in place of noble metal cat-alysts for the production of FDCA. Recently, it was found that Mn-basedtransition metal oxides (e.g., MnO2 and MnCo2O4) have a good catalyticactivity in the production of FDCA, leading to considerable FDCA yields(of over 70%) [61,68]. However, the high catalytic performance ofthese transition metal-based catalysts was achieved only in the presenceof base additives. When a base-free solvent was used, in fact, theMnCo2O4 spinel catalyst exhibited very poor catalytic activity in theoxidation of HMF to FDCA, leading to a FDCA yield of 11% [59]. Inorder to achieve an effective FDCA production under base-free condi-tions, ionic liquids (e.g., [BMIM]Cl) have been recently employed asreaction media in the catalytic oxidation of HMF to FDCA. Interestingly,ionic liquid systems showed remarkable reaction performances in thesynthesis of FDCA in the presence of non-noble metal catalysts underbase-free reaction conditions. Zhang’s research group recently reportedthe effect of [BMIM]Cl on the catalytic conversion of HMF to FDCA inthe presence of heteropoly acid (H3PMo12O40) and Ce0.5Fe0.15Zr0.35O2

catalysts. The strong Brønsted acid sites of H3PMo12O40 selectivelycatalysed the oxidation of a HMF C = O group, leading to a FDCA yieldof about 90% [69]. In addition, a non-noble metal-supported catalyst(i.e., Ce0.5Fe0.15Zr0.35O2) also showed an impressive catalytic perfor-mance and recyclability in [BMIM]Cl [70]. Another research groupconducted the one-pot conversion of fructose to FDCA, using Amber-lyst-15 and Fe0.6Zr0.4O2 catalysts in [BMIM]Cl [71]. In this work,

Amberlyst-15 was used to catalyse the dehydration of fructose to HMF;simultaneously, the Fe0.6Zr0.4O2 catalyst promoted the oxidation of theproduced HMF to FDCA. The FDCA yield was of only 46%; however, theone-pot process was able to avoid complex and costly separation steps.

3. Bio-based EG

This section introduces the biotechnological production of EG,which is one of the major components of PEF, from biomass or sugarthrough a novel approach. Up to present, the biological production ofEG has been usually considered in the following terms: i) direct pro-duction of EG from cellulosic or lignocellulosic biomass through che-mocatalytic conversion [72] and ii) production of EG by dehydration ofbio-based ethanol [73]. These approaches include a hydrogenolysis orpyrolysis step having high pressure and temperature requirements, aswell as a low reaction selectivity that complicates the downstreamprocesses for the separation of EG [19]. Herein, the biotechnologicalproduction of EG through synthetic metabolic pathways using threeindustrial microorganisms (i.e., Escherichia coli, Corynebacterium gluta-micum, and Saccharomyces cerevisiae) is reviewed before introducing aselective and environmentally friendly approach. To date, no naturalpathways have been used to produce EG from biomass derivatives [74].Consequently, various synthetic pathways have been established toinduce the production of EG from pentose (e.g., xylose and lig-nocellulose) or hexose (e.g., glucose) by the three microorganismsmentioned above.

3.1. Synthetic metabolic pathways for EG production in E. coli

Many people tried to produce EG using E. coli, the most well-studiedbacteria. Among the five possible biological EG producing-pathways(i.e., Dahms, D-xylulose-1-phosphate, L-ribulose-1-phosphate, L-xylu-lose-1-P, and serine), three of them (i.e., Dahms, D-xylulose-1-phos-phate, and serine) have been tested on E. coli. The most common ge-netic modification has been the construction of a synthetic pathwaybased on the partially known D-xylonic acid catabolism in E. coli (i.e.,the Dahms pathway) [74–76]. This pathway includes four steps: D-xy-lose (Fig. 7A) is initially oxidised to D-xylonic acid by NAD+-dependentxylose dehydrogenase (Xdh) that is originated from Caulobacter cres-centus. The conversion of D-xylonic acid to EG cascades from the en-dogenous enzyme reactions in E. coli (through the native D-xylonic acidmetabolism) [77]. The disruption of the D-xylose isomerase (XylA) andaldehyde dehydrogenase (AldA) genes alleviated the bypassing routes(i.e., the D-xylose → D-xylulose and glycoaldehyde → glycolic acid re-actions, respectively), improving the EG production [14,75]. Liu et al.overexpressed the D-xylose dehydrogenase of C. crescentus (Xdh) andthe aldehyde reductase of E. coli (YqhD) in order to produce EG;moreover, a xylose isomerase (XylA) gene was disrupted to prevent thecarbon flux from xylose to the pentose phosphate pathway. When thisstrain was cultivated in a lab-scale fermenter with 40 g/L D-xylose, a

Table 2Recent studies on the catalytic oxidation of HMF to FDCA.

Reaction condition Oxidant Catalyst Base HMF conversion (%) FDCA yield (%) Ref.

T = 100 °C, t = 5 h 9 mmol TBHP MnFe2O4 Acetonitrile (base-free) 100 85 [57]T = 120 °C, t = 16 h 10 bar O2 Ru/ZrO2 H2O (base-free) 100 97 [67]T = 100 °C, t = 12 h 4 bar O2 Pt/ZrO2 H2O (base-free) 100 97 [58]T = 130 °C, t = 2.5 h 5 bar O2 Au/CeO2 NaOH 100 87 [62]T = 110 °C, t = 6 h 7 bar O2 Au/GC NaOH 100 97 [63]T = 140 °C, WHSV = 1 h−1 30 bar O2 Ru/Al2O3 Na2CO3 100 98 [64]T = 70 °C, t = 4 h 10 bar O2 Au/Al2O3 NaOH 100 99 [65]T = 100 °C, t = 24 h 10 bar O2 β-MnO2 NaHCO3 >99 86 [68]T = 85 °C, t = 10 h 1 bar O2 Co4Mn1-L Na2CO3 100 95 [61]T = 140 °C, t = 6 h 10 bar O2 H3PMo12O40 [BMIM]Cl (base-free) > 99 89 [69]T = 160 °C, t = 24 h 20 bar O2 Ce0.5Fe0.15Zr0.35O2 [BMIM]Cl (base-free) > 99 44 [70]T = 160 °C, t = 4 h 20 bar O2 Amberlyst-15, Fe0.6Zr0.4O2 [BMIM]Cl (base-free) 100 (fructose) 46 [71]

K.-R. Hwang, et al. Chemical Engineering Journal 390 (2020) 124636

7

maximum EG concentration of 11,69 g/L and a yield of 0.29 g-EG/g-D-xylose were achieved; moreover, the average productivity was of0.24 g-EG/L/h [14].

The pathway optimisation of endogenous enzymes involved in na-tive D-xylonic acid metabolism can also enhance the target reaction. Forexample, the recruitment of a suitable aldehyde reductase (ALR) en-zyme, able to catalyse the conversion of glycoaldehyde to EG, might benecessary in order to improve the titer product. Chung and co-workersoverexpressed four ALRs-encoding genes (i.e., adhP, yahK, yjgB andyqhD) in the engineered E. coli W3100 (xylAB− strain) [75]. Their re-sults suggests that YjbB is superior to other ALSs: it had a higher cat-alytic activity (24.28 ± 1.25 U/mg) than YqhD (0.39 ± 0.18 U/mg)in the case of short-chain aldehydes. The expression of xdh was reducedwith a weak promoter in order to avoid D-xylonic acid accumulation;moreover, aldehyde dehydrogenase (aldA) was knocked out to blockthe production of glycolic acid from glycolaldehyde. The same authorsscreened and selected a more efficient aldehyde reductase (yjgB) andoverexpressed it. When the E. coli strain was cultured in a batch reactorwith 20 g/L D-xylose, the maximum titer and EG yield were 7.72 g/Land 0.39 g-EG/g-D-xylose, respectively [75].

The whole gene set of the Dahms pathway (i.e, xdh, xylC, yjhG, yjhH,fucO) was overexpressed in E. coli BL21 by Wang et al.. FucO, a NADH-dependent aldehyde reductase, was employed instead of yqhD (whichwas dependent on NADPH). This strategy might be beneficial for EGproduction, considering the prevalence of NADH over NADPH in bac-teria. In order to prevent the production of glycolic acid from glycolaldehyde AldA, aldehyde dehydrogenase, was disrupted. Additionally,the global regulator ArcA was removed to overcome acetate overflow.The cells were grown in a bioreactor with 20 g/L D-xylose; also, 60%(w/v) D-xylose was fed after about 15 g/L of the initial D-xylose wasconsumed. A maximum of 72 g/L of EG accumulated, resulting in ayield of 0.40 g-EG/g-D-xylose and a productivity of 1.38 g/L/h [76].

Recently, a synthetic biology tool based on synthetic small reg-ulatory RNA (sRNA) targeting was introduced to modulate the Dahm

pathway flux [77]. The metabolic fluxes through the Dahms and thenative D-xylose catabolic pathways were fine-tuned using syntheticsRNA, based on the results of an in-silico genome-scale metabolic si-mulation. Moreover, an sRNA targeting of D-xylulokinase genes from C.rescentus (xylBccs) was conducted. The production of EG in the anti-xylBccs sRNA-expressed E. coli strain EG03 (pTacxylBC-P1-anti-xylB)was 2.7- and 3.9-fold higher than in previous studies. By using thisengineered strain in a fed batch culture, the authors obtained a titer of108.2 g/L and a productivity of 2.25 g/L/h.

The D-Xylulose-1-phosphate (X1P) pathway is an alternative syn-thetic pathway reaction for the production of EG in E. coli (Fig. 7B). Inthe native pathway, D-xylose is first converted into D-xylulose-5-phos-phate (X5P) via subsequent isomerisation and phosphorylation troughthe catalytic reactions of XylA and XylB, respectively [78,79]. In thecorrespondent synthetic pathway, however, the EG yield of the en-gineered E. coli (xylB−, pEXT20-khk-c-aldoB) strain (characterised bythe simultaneous expression of xylose-1-kinase and X1P aldolase fromhuman ketohexokinase-C (KhkC) and human aldolase-B (AldoB), re-spectively) was drastically increased to 0.45 ± 0.02 mol/mol (wild-type, 0 yield) [79]. Walther and co-workers expressed human hex-okinase (Khk-C) and aldolase (Aldo-B) in an E. coli strain, in which D-xylose-5-P kinase, XylB, glycolaldehyde dehydrogenase, and AldA en-coding-genes were deleted to exclude any competitive pathways. Ad-ditionally, FucO (an NADH- dependent glycolaldehyde reductase) wasoverexpressed. When this strain of E. coli was cultured in a bioreactor, itshowed a productivity of 0.37 g/L/h and a yield of 0.91 g/g [78].

Although D-xylose is a significant constituent of lignocellulose, otherpentoses (e.g., D/L-arabinose and L-lyxose) were included.Stephanopoulos and co-workers reported multiple metabolic networksin E. coli that can enable the production EG from various pentoses [80].As shown in Fig. 8, these metabolic networks were connected with D-tagatose epimerase (DTE) recruited from Pseudomonas cichorii. DTE canbe used to enable D-xylose and L-arabinose to proceed into the D-ara-binose and L-lyxose degradation pathways, leading to the production of

Fig. 7. Overview of biosynthesis route of EG from D-xylose in E. coli. (A) Dahms pathway (red arrows). (B)D-Xylose-1-phosphate pathway (blue arrows). Dottedboxes indicates synthetic D-xylose assimilation path-ways. Xdh, D-xylose dehydrogenase (C. crescentus);YjhG/YagF, D-xylonic acid dehydrogenases (E. coli);YjhH/YagE, 2-keto-3-dexoy-D-pentonate aldolases (E.coli). YqhD, aldehyde reductase (E. coli); XylA, D-xy-lose isomerase (E. coli); XylB, D-xylulokinase (E. coli);KhkC, D-xylulose-1-kinase (H. sapiens); Aldo B, D-xy-lulose-1-phosphage aldolase (H. sapiens). AldA, al-dehyde dehydrogenase (E. coli) [14,75,78] (For in-terpretation of the references to colour in this figurelegend, the reader is referred to the web version ofthis article.)

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EG. A strain containing dte and genes capable of converting D-ribuloseto EG (fucA-fucO-fucK) was constructed. Competitive pathways wereexcluded by deleting xylulose kinase gene (xylB) and aldehyde dehy-drogenase (aldA) genes. The performance of this strain was tested in afed-batch bioreactor, resulting in a final titer of 40 g/L and an EGproductivity of 0.35 g/g. A similar approach was adopted for the pro-duction of EG from L-arabinose. Genes converting L-ribulose to EG (i.e.,rhaB, rhaD, fucO) and dte were expressed in a D-ribulose kinase gene(araB) and aldA mutant strain. When this strain was cultured in a batchbioreactor with 50 g/L L-arabinose, it lead to a titer of 20 g/L and ayield of 0.38 g/g. The possibility of utilizing both D-xylose and L-ara-binose in the same culture was tested by integrating all genes (i.e., dte,fucA, fucO, fucK, rhaB, rhaD) for both pathways in one strain. This en-gineered strain was cultured in a shake flask, obtaining a maximum EGtiter of 10.5 g/L and a yield of 0.35 g/g.

In the synthetic pathways adopted for EG production in E. coli,pentose D-xylose is transformed and cleaved into 2- and 3-C inter-mediates. The 2-C (i.e., glycoaldehyde) and 3-C intermediates (i.e.,pyruvate and glycolic acid) are converted into EG and the central me-tabolism, respectively. Another synthetic pathway, based on the re-presentative 3-C intermediate glucose, was designed for the biosynth-esis of EG [81]. As shown in Fig. 9, EG can be synthesised from glucosethrough the serine pathway in E. coli (Set 1). Serine biosynthesis genes(i.e., serA, serC, serB) were overexpressed in order to increase the pro-vision of serine (Set 2). The genes required for the conversion of serineinto EG (i.e., serine decarboxylase encoding-gene (sdc) from Arabidopsisthaliana, ethanolamine oxidase encoding-gene (aao) from Arthrobactersp., and aldehyde reductase encoding-gene (fucO) from E. coli) wereoverexpressed (Set 3). When this strain was grown in a shake flaskculture it resulted in an EG production of 4.1 g/L and a yield of 0.14 g-EG/g-glucose.

3.2. Synthetic metabolic pathways for EG production in C. Glutamicum

The production of EG in C. glutamicum via synthetic metabolicnetworks can also be attractive approach, since serine can accumulatein great quantities in metabolic engineered strains [82,83]. Chen andco-workers proposed EG production routes derived from a serine bio-synthesis pathway [82]. In the proposed pathway, serine can betransferred into glycoaldehyde via two synthetic routes: i) the deami-nation reaction of serine to hydroxypyruvate by aminotransferase(E.C.2.6.1-) or amino acid dehydrogenase (E.C.1.4.1-) and the dec-arboxylation of hydroxypyruvate to glycoaldehyde by α-decarboxylase

(E.C.4.1.1-); ii) the decarboxylation of serine to ethanolamine by serinedecarboxylase (E.C.4.1.1.-) and the oxidation of ethanolamine to gly-coaldehyde by ethanolamine oxidase (E.C.1.4.3.8). The correspondingenzymes were screened and optimised the production of the strain bycombinatorial optimisation and metabolic engineering. When the best

Fig. 8. Metabolic network for EG produc-tion from various pentose in E. coli. Redreaction indicates synthetic route of D-ta-gatose epimerase (DTE) recruited from P.cichorii. Gray reactions correspond to path-ways attenuated or deleted in specifiedstrains. Native enzymes involved in thepathway in E. coli were listed as follow:XylA, D-xylose isomerase; XylB, D-xyluloki-nase; FucI, L-fucose isomerase; FucK, L-fu-culokinase; FucA, L-fuculose phosphate al-dolase; FucO, lactaldehyde reductase; AldA,aldehyde dehydrogenase; RhaA, L-rhamnoseisomerase; RhaB, L-rhamnulokinase; RhaD,L-rhamnulose-1-phosphate aldolase; AraA,L-arabinose isomerase; AraB, L-ribulokinase[80]. (For interpretation of the references tocolour in this figure legend, the reader isreferred to the web version of this article.)

Fig. 9. Metabolic network of EG production from glucose via serine biosynth-esis pathway in E. coli. Bold characters indicate enzymes associated with thereaction. Red reaction indicates synthetic route of serine decarboxylase (SDC)from Arabidopsis thaliana and ethanolamine oxidase (AAO) from Arthrobactersp. Gray reactions correspond to pathways deleted in specified strains. Nativeenzymes involved in the pathway in E. coli were listed as follow: SerA, 3-phosphoglycerate dehydrogenase; SerC, phosphoserine aminotransferase; SerB,Phosphoserine phosphatase; FucO, lactaldehyde reductase; SdaA, L-serine dea-minase; EutBC, ethanolamine ammonia lyase; AldA, aldehyde dehydrogenase A[81]. (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

K.-R. Hwang, et al. Chemical Engineering Journal 390 (2020) 124636

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strain (i.e., Mdlc-YqhD –Sdc-ASAO-YqhD expressing strain) was cul-tured in a batch bioreactor, 3.5 g/L of EG were produced from glucoseat a yield of 0.09 g/g.

3.3. Synthetic metabolic pathways for EG production in S. Cerevisiae

S. cerevisiae is the most widely used industrial microorganism inbiotechnological processes, since it presents a particularly high toler-ance to relevant conditions (e.g., low pH and osmotic-stress); however,it cannot utilise pentose sugars (i.e., xylose and arabinose) as carbonsources [84]. The production of EG in S. cerevisiae through the Dahmspathway was reported by Salusjärvi and co-workers [85]. The genesrequired for the production of 2-keto-3deoxyxylonic acid from D-xylose(i.e., D-xylose dehydrogenase (XylB) and D-xylonate dehydratase (XylD)from C. crescentus, aldolase (YagE/YjhH) from E. coli, and aldehydedehydrogenase (AldA) from E. coli) were successfully expressed in awild-type strain; additionally, the reduction of glycolaldehyde to EGwas mediated by an endogenous aldo–keto reductase. A titer of 14 mg/L EG was obtained by flask culture in a synthetic media containing20 g/L D-xylose. More recently, EG was produced from D-xylose in S.cerevisiae through the heterologous expression of the D-ribulose-1-phosphate-dependent pathway [86]. In the synthetic route, S. cerevisiaepossessed a native catalytic activity for the biosynthesis of EG from D-xylose, independently for the heterologous D-ribulose-1-phosphate-de-pendent expression. Hence, genes from different bacterial species (i.e.,P. cichorii D-tagatose epimerase (PcDTE)) and E. coli genes (i.e., L-fu-culose kinase (FucK), L-fuculose 1-phosphate aldolase (FucA), and L-1,2-propanediol oxidoreductase (FucO)) were codon optimised to be ex-pressed in S. cerevisiae. Native S. cerevisiae genes (e.g., includingphosphofructokinase (PFK1/PFK2) and fructose-bisphosphate aldolase(FBA1)) were directly amplified from the genomic DNA of the xylose-utilizing strain F251. This engineered strain was cultured in a fed-batchbioreactor. The cells were first grown in 20 g/L of a glucose-containingmedia and concentrated to an OD600 of 20 to achieve a high-densityculture; then, they were resuspended in synthetic complete media with50 g/L D-xylose. In order to improve cell viability, 100 g/L glucose wascontinuously fed. The strain consumed about 60% of the provided xy-lose and produced 4.05 g/L of EG at a yield of 0.12 g/g.

The maximum EG production (yield of 0.4 g/g and titer of 108.2 g/L) among those mentioned above was achieved through the Dahmspathway in a E. coli strain [76,77]. Notably, none of the existingpathways for the production of EG titer reached a commercial pro-duction level [74].

4. Concluding remarks

The drop-in polymer PEF has gained great interest as a good solu-tion for the petro-based plastics crisis, in view of the shortage of fossilresources and global warming, since it promotes the advance of circulareconomy and environmental sustainability. This review covered recentadvances in the production of novel bio-building blocks (i.e., HMF,FDCA and EG) for the preparation of the next-generation polymer PEF.

The synthesis of FDCA from biomass has been accomplishedthrough two-step catalytic reaction stages. HMF can be produced frombiomass substrates (e.g., sugars and cellulose) and then converted toFDCA by oxidation reaction routes with the aid of catalysts and sol-vents. So far, numerous studies have shown that the yields of HMF andFDCA obtained from each stage reached almost 100%. However, theone-pot conversion of cellulose to FDCA is highly desirable: the pro-duction, separation, and storage of HMF is significantly costly, due to itsunstable properties. FDCA can be easily separated from a reactionmixture by crystallisation; however, a one-pot conversion process maylead to a much lower FDCA production or require the use of noble metalcatalysts and less green solvents (e.g., ionic liquids) in order to maintainan acceptable productivity. Ionic liquids are known as the most effec-tive solvents for the hydrolysis of cellulose; however, their high priceTa

ble3

Summaryof

selected

metab

olically

engine

ered

strainsforEG

prod

uction

.Dah

ms,

Dah

mspa

thway

;X1P

,D-Xylose-1-ph

osph

atepa

thway

;R1P

,D-ribulose-1-ph

osph

atepa

thway

;Serin,S

erin

pathway

.

Strain

(gen

etic

mod

ification

)Pa

thway

Prod

uctivity

(g/L

/h)

Yield

(g/g

)Yield

(mol/

mol)

Titer(g/L

)Med

ium

type

(Feeding

)Ref.

E.coliW31

10(D

E3)(Δ

xylA

xdhyq

hD)

Dah

ms

0.24

0.29

0.70

11.69

Com

plex

(D-xylose,

40g/

L)14

E.coliW31

10(Δ

xylABΔa

ldA

ΔyjgBpK

MXpT

rcHis2A

)Dah

ms

–0.39

0.95

7.72

Com

plex

( D-xylose,

20g/

L)75

E.coliBL

21(D

E3)(△

arcA

△aldA

pETD

uet1-yjhH-xdh

-xylC/p

ACYCDue

t1-fuc

O-yjhG)

Dah

ms

1.39

0.40

0.97

72.0

Com

plex

( D-xylose,

20g/

L;60

%xy

lose

forfeed

batch)

76E.

coliWL3

110(pTa

cxylBC

-P1-an

ti-xylB)

Dah

ms

2.25

0.36

0.87

108.2

MR( D-xylose,

20g/

L;D-xylose(20g/

L)forfedba

tch)

77E.

coliMG16

55(Δ

xylB

ΔaldA

khk-Caldo

BfucO

)X1P

0.37

0.38

0.91

20M9( D-xylose,

55g/

L)78

E.coliMG16

55(Δ

xylB

ΔaldA

dtefucA

fucK

fucO

)R1P

0.58

0.35

0.85

40MM2( D-xylose,

35g/

L)80

E.coliMG16

55(Δ

araA

ΔaldA

dterhaB

rhaD

fucO

)X1P

–0.38

–20

MM2( D-arabino

se,50

g/L)

80E.

coliMG16

55(Δ

araB

ΔaldA

ΔxylBdterhaB

rhaD

fucA

fucK

fucO

)X1P

/R1P

–0.35

–10

.5MM2( D-xylose,

15g/

L;arab

inose,

15g/

L)80

E.coliMG16

55(D

E3)(Δ

aldA

serA

:317

serB

serC

fucO

sdcaa

o)Se

rine

–0.14

0.41

4.1

MM

(Gluco

se,3

0g/

L)81

C.g

lutamicum

(Mdlcyq

hDSd

cASA

Oyq

hDyq

hD)

Serine

–0.09

0.25

3.5

MM2(cornsteepliq

uor,

2g/

L;gluc

ose,

40g/

L)82

S.cerevisiae

H40

99(M

ATα

ura3

–52HIS3leu2

–3/1

12TR

P1MAL2

–8cSU

C2gre3::x

ylB

fra2

::Hyg

Rxy

lD)

Dah

ms

––

–0.01

4Sy

nthe

ticco

mpletemed

ia( D-xylose,

20g/

L;gluc

ose,

10g/

L)85

S.cerevisiae

(PeX

YLA

)47(R

PE1RKI1

TKL1

PsTA

L1PF

K1PF

K2)

X1P

–0.12

–4.05

Synthe

ticco

mpletemed

ia(D-xylose,

50g/

L;gluc

ose,

10g/

L)86

K.-R. Hwang, et al. Chemical Engineering Journal 390 (2020) 124636

10

and toxicity are critical obstacles for the application of commercialprocesses. Meanwhile, a combination of transition metal-based bi-functional heterogeneous catalysts and polar aprotic solvents would besuitable for an economical and eco-friendly production of FDCA.

Due to environmental issues, there is an increasing demand for theproduction of platform chemical EG from renewable resources andthrough sustainable biotechnological processes. Here, we reviewedpotential metabolic routes that enable the utilisation of pentose (e.g.,xylose and lignocellulose) or hexose (e.g., glucose) for the production ofEG in microorganisms. Table 3 summarizes several synthetic pathwaysfor biotechnological EG production have been established by using ei-ther E. coli, C. glutamicum or S. cerevisiae as hosts. The best reported EGproduction results (yield of 0.4 g/g; titer of 108.2 g/L) achieved withthe Dahms pathway in E. coli strain. However, none of the existingpathways for the production of EG titer resulted in levels suitable forcommercial production. An optimisation of bioprocesses (leading toimproved pathway enzyme activities, balanced enzyme expression, andbiomass growth) might be of major importance for future improve-ments in this field of research. Moreover, the screening of strains highlytolerant to EG accumulation by natural collection or adaptive evolutiontechnologies would be fundamental to fulfil the industrial demand.

Declaration of Competing Interest

The authors declare that they have no known competing financialinterests or personal relationships that could have appeared to influ-ence the work reported in this paper.

Acknowledgement

This work was conducted under framework of the research anddevelopment program of the Korea Institute of Energy Research (C0-2462). Also this work was also supported by the C1 Gas RefineryProgram through the National Research Foundation of Korea (NRF)funded by the Ministry of Science and ICT (2015M3D3A1A01064899).

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