Catalytic Processes for Utilizing Carbohydrates Derived from Algal Biomass

catalysts

Review Catalytic Processes for Utilizing Carbohydrates Derived from Algal Biomass

Sho Yamaguchi 1,*, Ken Motokura 1, Kan Tanaka 2,3 and Sousuke Imamura 2,3

1 Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259-G1-14 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8502, Japan; [email protected] 2 Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, 4259-R1-30 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8502, Japan; [email protected] (K.T.); [email protected] (S.I.) 3 Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), Saitama 332-0012, Japan * Correspondence: [email protected]; Tel.: +81-45-924-5417

Academic Editor: Rafael Luque Received: 18 April 2017; Accepted: 15 May 2017; Published: 19 May 2017

Abstract: The high productivity of oil biosynthesized by microalgae has attracted increasing attention in recent years. Due to the application of such oils in jet fuels, the algal biosynthetic pathway toward oil components has been extensively researched. However, the utilization of the residue from algal cells after oil extraction has been overlooked. This residue is mainly composed of carbohydrates (starch), and so we herein describe the novel processes available for the production of useful chemicals from algal biomass-derived sugars. In particular, this review highlights our latest research in generating lactic acid and levulinic acid derivatives from polysaccharides and monosaccharides using homogeneous catalysts. Furthermore, based on previous reports, we discuss the potential of heterogeneous catalysts for application in such processes.

Keywords: algal biomass; carbohydrate; homogeneous catalyst; heterogeneous catalyst; lactic acid; levulinic acid

1. Introduction Many important chemicals and end products are produced from fossil fuels. For example, petroleum is widely used to produce gasoline, heating oil, and raw materials for plastics. Technologies have also been recently developed to obtain useful products from the hydrocarbons present in natural gas, such as methane and ethane. However, owing to the depletion of fossil fuels, new carbon resources are urgently required on a global scale. As such, various methods have been developed to allow the use of carbohydrates as alternative resources, and these processes could have a significant impact on reducing carbon dioxide emissions. Carbohydrates derived from both crop biomass (i.e., storage polysaccharides, such as starch) and woody biomass (i.e., cell wall polysaccharides, such as cellulose) have been considered for this purpose [1–8]. However, as the global population continues to increase, the use of limited farmland to grow crop biomass instead of food is clearly unrealistic. In addition, sufficient woody biomass can only be obtained through large-scale deforestation and subsequent environmental destruction. Thus, a more efficient and sustainable carbon resource supply system is required. In this context, microalgae have been investigated as alternatives to crop and woody biomass-based systems, as algae can be propagated industrially without using farmland [9], and its productivity per unit time and per unit area is extremely high [10,11]. The main characteristic

Catalysts 2017, 7, 163; doi:10.3390/catal7050163 www.mdpi.com/journal/catalysts Catalysts 2017, 7, 163 2 of 12 of algae is its high productivity of oil and carbohydrates (mainly starch) in cells under unfavorable growthCatalysts conditions, 2017, 7, 163 such as nutrient deﬁciency [9]. Indeed, the importance of algal-derived oil resources2 of 12 has recently been highlighted by applications of the obtained oil as biojet fuel and biodiesel [12–17]. However,conditions, unlike such cellulose, as nutrient the utilization deficiency of[9]. carbohydrates Indeed, the importance present inof algalalgal-derived cells after oil oilresources extraction has has been overlooked.recently been Therefore,highlighted it by is alsoapplications important of the to developobtained processesoil as biojet to fuel convert and biodiesel algal carbohydrates [12–17]. However, unlike cellulose, the utilization of carbohydrates present in algal cells after oil extraction into important chemicals. has been overlooked. Therefore, it is also important to develop processes to convert algal Table1 shows the typical carbohydrate compositions in green algae, including Chlorella vulgaris carbohydrates into important chemicals. (a health food)Table and1 showsChlamydomonas the typical carbohydrate reinhardtii compositions(often employed in green in algae, basic including research). Chlorella These vulgaris algae have a high(a productivity health food) and of both Chlamydomonas oil and carbohydrates, reinhardtii (often with employed the majority in basic of research). the latter These being algae in the have form a of starchhigh and productivity glucose. These of both carbohydrates oil and carbohydrates, may therefore with the be majority used as of new the latter carbon being resources in the form in light of of fossil fuelstarch depletion. and glucose. Although These carbohydrates a wide variety may of therefor microalgaee be used exist, as we new herein carbon focus resources on the in applicability light of of carbohydrate-richfossil fuel depletion. microalgae. Although a wide variety of microalgae exist, we herein focus on the applicability of carbohydrate-rich microalgae. Table 1. Carbohydrate contents of selected species of microalgae [18]. Table 1. Carbohydrate contents of selected species of microalgae [18].

TotalTotal Carbohydrate Carbohydrate Content of Carbohydrate SpeciesSpecies Content of Carbohydrate Components ReferenceReference ContentContent (wt %)(wt %) Components ChlamydomonasChlamydomonas reinhardtii reinhardtii 59.7%59.7% Starch Starch 43.6%; 43.6%; glucose glucose 44.7% 44.7% (w/w )[(w/w) 19][19] UTEXUTEX 90 90 Chlorococcum humicola 32.52% Starch 11.32%; glucose 15.22% (w/w)[20] Chlorococcum humicola 32.52% Starch 11.32%; glucose 15.22% (w/w) [20] Chlorococcum infusionum 32.52% Starch 11.32%; glucose 15.22% (w/w)[20] ChlorellaChlorococcum vulgaris FSP-E infusionum strain 50.39%32.52% Starch Starch 31.25%; 11.32%; glucose glucose 46.92% 15.22% (w/w )[(w/w) 21][20] ScenedesmusChlorella vulgaris obliquus FSP-ECNW-N strain 51.8% 50.39% Starch Glucose31.25%; 41.6%glucose 46.92% (w/w) [22][21] Scenedesmus obliquus CNW-N 51.8% Glucose 41.6% [22]

In thisIn review, this review, we examinewe examine the the chemical chemical transformation transformation of of algal algal carbohydrates carbohydrates into into alkyl alkyl lactate lactate and alkyland alkyl levulinate. levulinate. First, First, the the homogeneously homogeneously catalyzedcatalyzed conversion conversion of ofglucose glucose into into these these target target chemicals is discussed. Subsequently, we focus on the utilization of algal biomass, especially algal chemicals is discussed. Subsequently, we focus on the utilization of algal biomass, especially algal residue (which is commonly discarded after the abstraction of oil) to produce methyl lactate and residue (which is commonly discarded after the abstraction of oil) to produce methyl lactate and methyl levulinate. Finally, to put these results into a practical context, we discuss the development methylof levulinate.heterogeneous Finally, catalysts. to put these results into a practical context, we discuss the development of heterogeneous catalysts. 2. Homogeneously Catalyzed Transformation of Carbohydrates into Alkyl Lactate and Alkyl 2. HomogeneouslyLevulinate Catalyzed Transformation of Carbohydrates into Alkyl Lactate and Alkyl Levulinate Figure 1 shows that the molecular structures and applications of alkyl lactate and alkyl Figurelevulinate.1 shows Two that molecules the molecular of lactic structuresacid can be and dehydrated applications to yield of alkyl the lactone lactate lactide, and alkyl and levulinate. in the Two moleculespresence of ofcatalysts, lactic acidthis lactide can be can dehydrated polymerize toto give yield polylactide the lactone (PLA), lactide, a biodegradable and in the polyester presence of catalysts,[23]. this Indeed, lactide PLA can is an polymerize excellent ex toample give polylactideof a plastic that (PLA), is not a der biodegradableived from petrochemicals. polyester [23 Lactic]. Indeed, PLA isacid an excellentis also employed example in of pharmaceutical a plastic that istechnologies not derived to from produce petrochemicals. water-soluble Lacticlactates acid from is also employedotherwise-insoluble in pharmaceutical active technologies ingredients. In to addition, produce it water-soluble finds further lactatesuse in topical from preparations otherwise-insoluble and cosmetics as an acidity regulator and for its disinfectant and keratolytic properties. In contrast, active ingredients. In addition, it ﬁnds further use in topical preparations and cosmetics as an acidity levulinic acid is employed as a precursor for pharmaceuticals, plasticizers, and various other regulator and for its disinfectant and keratolytic properties. In contrast, levulinic acid is employed additives, and is recognized as a building block or starting material for a wide number of compounds. as a precursorIndeed, this for family pharmaceuticals, contributes to various plasticizers, large vo andlume various chemical other markets, additives, commonly and being is recognized applied as a buildingin biofuels block orsuch starting as γ-valerolactone, material for a2-methyl-tetrahydrofuran, wide number of compounds. and ethyl Indeed, levulinate this family as shown contributes in to variousReferences large volume[24–28]. chemical markets, commonly being applied in biofuels such as γ-valerolactone, 2-methyl-tetrahydrofuran, and ethyl levulinate as shown in References [24–28].

Figure 1. Molecular structures and applications of alkyl lactate and alkyl levulinate.

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Catalysts 2017, 7Figure, 163 1. Molecular structures and applications of alkyl lactate and alkyl levulinate. 3 of 12

The degradation pathway of polysaccharides such as cellulose, hemicellulose, amylose, and The degradation pathway of polysaccharides such as cellulose, hemicellulose, amylose, and amylopectin is shown in Scheme 1. Initially, cleavage of the glycosidic linkage in all polysaccharides amylopectin is shown in Scheme1. Initially, cleavage of the glycosidic linkage in all polysaccharides leads to the production of a common main component, namely the glucose monomer. Each molecule leads to the production of a common main component, namely the glucose monomer. Each molecule of glucose can then be decomposed to give a tetrose (C4) unit and a glycolaldehyde (C2) unit via a of glucose can then be decomposed to give a tetrose (C4) unit and a glycolaldehyde (C2) unit via retro-aldol reaction. Alternatively, the isomerization of glucose to yield fructose is a retro-aldol reaction. Alternatively, the isomerization of glucose to yield fructose is thermodynamically thermodynamically preferable, and the corresponding retro-aldol reaction of fructose affords trioses preferable, and the corresponding retro-aldol reaction of fructose affords trioses (C3), including (C3), including 1,3-dihydroxyacetone (DHA) and glyceraldehyde (GLA). A separate dehydration 1,3-dihydroxyacetone (DHA) and glyceraldehyde (GLA). A separate dehydration reaction of fructose reaction of fructose leads to the formation of furfural derivatives, which can react with alcohols to leads to the formation of furfural derivatives, which can react with alcohols to form alkyl levulinate. form alkyl levulinate. In general, the product selectivity is strongly dependent on the characteristics In general, the product selectivity is strongly dependent on the characteristics of the catalyst, i.e., of the catalyst, i.e., the balance between its Lewis and Brønsted acidities. the balance between its Lewis and Brønsted acidities.

Scheme 1. Degradation of carbohydrates and their transformation into several oxygenates. Scheme 1. Degradation of carbohydrates and their transformation into several oxygenates.

2.1. Production of Alkyl Lactate from Carbohydrates Using Homogeneous Catalysts 2.1. Production of Alkyl Lactate from Carbohydrates Using Homogeneous Catalysts In 2005, Hayashi et al. reported the one-pot synthesis of methyl lactate from DHA and GLA, In 2005, Hayashi et al. reported the one-pot synthesis of methyl lactate from DHA and GLA, which were obtained via fermentation and the retro-aldol reaction of glucose [29]. In this cascade which were obtained via fermentation and the retro-aldol reaction of glucose [29]. In this cascade transformation, sequential dehydration, a 1,2-hydride shift, and the addition of methanol take place transformation, sequential dehydration, a 1,2-hydride shift, and the addition of methanol take place to yield the desired product. The use of tin halides led to high product yields, while other Lewis to yield the desired product. The use of tin halides led to high product yields, while other Lewis acid acid catalysts (Cr, Zr, Al, etc.) exhibited low activities in this reaction system (Table2). To explain catalysts (Cr, Zr, Al, etc.) exhibited low activities in this reaction system (Table 2). To explain this this unique behavior of the homogeneous tin metal catalysts, we employed density functional theory unique behavior of the homogeneous tin metal catalysts, we employed density functional theory calculations to examine the coupling reaction between DHA and formaldehyde, in which tin catalysts calculations to examine the coupling reaction between DHA and formaldehyde, in which tin catalysts also accelerated the aldol condensation [30]. The calculation results indicated that the strong interaction also accelerated the aldol condensation [30]. The calculation results indicated that the strong between the tin catalyst and the reaction intermediates possessing carbonyl moieties can reduce the transition state energy, therefore accelerating the reaction.

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Catalystsinteraction 2017, between7, 163 the tin catalyst and the reaction intermediates possessing carbonyl moieties4 ofcan 12 reduce the transition state energy, therefore accelerating the reaction. interactionCatalystsIn 20172017, ,between7, Dusselier 163 the ettin al. catalyst reported and a the synthetic reaction proc intermediatesess to produce possessing methyl carbonyllactate from moieties cellulose4 ofcan 12 reduce(Scheme the 2) transition [31]. Following state energy, the initial therefore decomposition accelerating of the cellulose reaction. to give the glucose monomer, a In 2017, Dusselier et al. reported a synthetic process to produce methyl lactate from cellulose subsequentIn 2017, isomerization Dusselier et al.of reportedglucose followed a synthetic by process a retro-aldol to produce reaction methyl of fructose lactate from afforded cellulose the (Scheme 2) [31]. Following the initial decomposition of cellulose to give the glucose monomer, a (Schemedesired product.2)[ 31]. Interestingly, Following the the initial use of decomposition Sn(OTf)2 (Tf = triflate) of cellulose accelerated to give the the retro-aldol glucose monomer, reaction. subsequent isomerization of glucose followed by a retro-aldol reaction of fructose afforded the Ina subsequentaddition, the isomerization balance between of glucose Sn as followed a Lewis byacid a retro-aldoland TfOH reactionas a Brønsted of fructose acid has afforded a strong the desired product. Interestingly, the use of Sn(OTf)2 (Tf = triflate) accelerated the retro-aldol reaction. influencedesired product. on product Interestingly, selectivity, the therefore use of Sn(OTf) indicating(Tf that = triﬂate) tuning accelerated of the Lewis the and retro-aldol Brønsted reaction. acidic In addition, the balance between Sn as a Lewis 2acid and TfOH as a Brønsted acid has a strong ratioIn addition, is crucial the to balance achieving between the desired Sn as a degr Lewisadation acid and of the TfOH polysaccharides as a Brønsted acidand hasof glucose. a strong inﬂuence influence on product selectivity, therefore indicating that tuning of the Lewis and Brønsted acidic on product selectivity, therefore indicating that tuning of the Lewis and Brønsted acidic ratio is crucial ratio is crucial to achievingTable the 2. desired Catalytic degr conversionsadation ofof trioses the polysaccharides to methyl lactate. and1 of glucose. to achieving the desired degradation of the polysaccharides and of glucose. Table 2. Catalytic conversions of trioses to methyl lactate.1 Table 2. Catalytic conversions of trioses to methyl lactate.1

Yield of Methyl Entry Triose Catalyst Lactate (%) 2 Entry Triose Catalyst Yield of MethylYield Lactate of (%)Methyl2 Entry1 TrioseDHA CatalystSnCl2 89 2 1 DHA SnCl2 89 Lactate (%) 2 DHA SnBr2 83 21 DHADHA SnBr 2 SnCl2 83 89 3 DHA SnI2 71 3 DHA SnI2 71 2 DHA SnBr2 83 44 DHADHA SnCl 4-5H2OSnCl4-5H2O 82 82 3 DHA SnI2 71 55 DHADHA CrCl 3-6H2OCrCl3-6H2O 5 5 64 DHADHA ZrCl SnCl4-5H2O 13 82 6 DHA 4 ZrCl4 13 75 DHADHA AlCl 3-6H2OCrCl3-6H2O 62 5 7 DHA AlCl3-6H2O 62 8 GLA SnCl 85 6 DHA 2 ZrCl4 13 2 98 GLAGLA SnCl 4-5H2OSnCl 81 85 7 DHA AlCl3-6H2O 62 1 Reaction conditions:9 Triose (2.5 mmol),GLA catalyst (10 molSnCl %),4-5H methanol2O (4 mL), 90 ◦C,81 over 3 h. 2 Gas-liquid 8 GLA SnCl2 85 1chromatography Reaction conditions: (GLC) yield Triose (mol (2.5 %) mmol), were based catalyst on triose. (10 mol %), methanol (4 mL), 90 °C, over 3 h. 2 Gas- liquid chromatography9 (GLC) yieldGLA (mol %) were basedSnCl4 -5Hon triose.2O 81 1 Reaction conditions: Triose (2.5 mmol), catalyst (10 mol %), methanol (4 mL), 90 °C, over 3 h. 2 Gas- liquid chromatography (GLC) yield (mol %) were based on triose.

Scheme 2. Transformation of cellulose into methyl lactate. Scheme 2. Transformation of cellulose into methyl lactate. 2.2. Production of Alkyl Levulinate Scheme 2. from Transformation Carbohydrates of cellulose Using Homogeneous into methyl lactate. Catalysts

2.2. ProductionIn 2012, Wu of Alkylet al. Levulinatereported reported the thefrom synthesis synthesis Carbohydrates of of alkyl alkyl Using levulinate levulinate Homogeneous via via the the Catalystsdegradation degradation of of cellulose cellulose [32]. [32]. The use of supercritical methanol as the solvent an andd sulfuric acid as an acid catalyst provided the In 2012, Wu et al. reported the synthesis of alkyl levulinate via the degradation of cellulose [32]. desired product product in in a a high high yield. yield. This This result result indicate indicatess that that Brønsted Brønsted acids acids are areeffective effective for forcleaving cleaving the The use of supercritical methanol as the solvent and sulfuric acid as an acid catalyst provided the glycosidicthe glycosidic bonds bonds of cellulose of cellulose and for and the for sequential the sequential dehydration dehydration reactions reactions of glucose of glucose to generate to generate alkyl desired product in a high yield. This result indicates that Brønsted acids are effective for cleaving the levulinate.alkyl levulinate. glycosidic bonds of cellulose and for the sequential dehydration reactions of glucose to generate alkyl Based on these reports, we described how the ac acidicidic properties of the catalyst affect the levulinate. transformation of of polysaccharides polysaccharides into into alkyl alkyl lactate lactate and and alkyl alkyl levulinate. levulinate. As As shown shown in inScheme Scheme 3,3 a, Based on these reports, we described how the acidic properties of the catalyst affect the stronga strong acid acid such such as as sulfuric sulfuric acid acid or or TfOH TfOH is is re requiredquired to to degrade degrade the the polysaccharides polysaccharides (e.g., (e.g., cellulose) cellulose) transformation of polysaccharides into alkyl lactate and alkyl levulinate. As shown in Scheme 3, a into monosaccharides (e.g., glucose). However, both Lewis and Brønsted acids are required for the strong acid such as sulfuric acid or TfOH is required to degrade the polysaccharides (e.g., cellulose) isomerization of glucose into fructose followed by the retro-aldol reaction of fructose to afford alkyl

Catalysts 2017, 7, 163 5 of 12

Catalysts 2017, 7, 163 5 of 12 intoCatalysts monosaccharides 2017, 7, 163 (e.g., glucose). However, both Lewis and Brønsted acids are required5 offor 12 the isomerization of glucose into fructose followed by the retro-aldol reaction of fructose to afford alkyl into monosaccharides (e.g., glucose). However, both Lewis and Brønsted acids are required for the lactate.lactate. InIn contrast,contrast, aa strongstrong BrønstedBrønsted acidacid isis requiredrequired forfor thethe formationformation ofof furfuralfurfural derivativesderivatives andand isomerization of glucose into fructose followed by the retro-aldol reaction of fructose to afford alkyl alkylalkyl levulinatelevulinate viavia dehydration dehydration and and subsequent subsequent reaction reaction with with an an alcohol. alcohol. lactate. In contrast, a strong Brønsted acid is required for the formation of furfural derivatives and alkyl levulinate via dehydration and subsequent reaction with an alcohol.

Scheme 3. Effect of the catalyst acidic properties on the transformation of polysaccharides into alkyl Scheme 3. Effect of the catalyst acidic properties on the transformation of polysaccharides into alkyl lactate and alkyl levulinate. lactateScheme and 3. alkyl Effect levulinate. of the catalyst acidic properties on the transformation of polysaccharides into alkyl lactate and alkyl levulinate. 2.3.2.3. ChemicalChemical ProductionProduction UsingUsing AlgalAlgal ResidueResidue asas aa Carbon Carbon Source Source 2.3. Chemical Production Using Algal Residue as a Carbon Source InIn 2017,2017, wewe achieved achieved thethe successful successful transformationtransformation ofof algalalgal residueresidue intointomethyl methyllevulinate levulinate andand In 2017, we achieved the successful transformation of algal residue into methyl levulinate and methylmethyl lactatelactate [33[33].]. FollowingFollowing oiloil abstractionabstraction fromfrom CyanidioschyzonCyanidioschyzon merolaemerolae 10D,10D, thethe algalalgal residueresidue methyl lactate [33]. Following oil abstraction from Cyanidioschyzon merolae 10D, the algal residue containedcontained amyloseamylose andand amylopectinamylopectin asas starchstarch polysaccharides,polysaccharides, whichwhich werewere thenthen usedused toto produceproduce thethe contained amylose and amylopectin as starch polysaccharides, which were then used to produce the targettarget chemicals chemicals (Figure (Figure2 ).2). target chemicals (Figure 2).

Figure 2. A concept of the full utilization of algae.

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Catalysts 2017, 7, 163 Figure 2. A concept of the full utilization of algae. 6 of 12

Initially, we optimized the reaction conditions required for the conversion of authentic starch Initially, we optimized the reaction conditions required for the conversion of authentic starch into either methyl levulinate or methyl lactate (Table 3). According to a previous report [31] on the into either methyl levulinate or methyl lactate (Table3). According to a previous report [ 31] on the degradation of cellulose, the use of Sn(OTf)2 led to the production of methyl levulinate and methyl degradation of cellulose, the use of Sn(OTf) led to the production of methyl levulinate and methyl lactate in comparable yields. In addition, as 2a strong Brønsted acid is required for the synthesis of lactate in comparable yields. In addition, as a strong Brønsted acid is required for the synthesis of methyl levulinate (see Scheme 3), the use of Sn(OTf)2 was examined, and in this case, only methyl methyl levulinate (see Scheme3), the use of Sn(OTf) was examined, and in this case, only methyl levulinate was obtained in a high yield (entry 1). Furthermore,2 a high yield of methyl levulinate was levulinate was obtained in a high yield (entry 1). Furthermore, a high yield of methyl levulinate was achieved using only TfOH, indicating that TfOH is crucial both for the degradation of achieved using only TfOH, indicating that TfOH is crucial both for the degradation of polysaccharides polysaccharides and for the sequential dehydration reactions (entry 2). In contrast, to selectively and for the sequential dehydration reactions (entry 2). In contrast, to selectively synthesize methyl synthesize methyl lactate, a balance between the ratio of Lewis and Brønsted acids is important, as lactate, a balance between the ratio of Lewis and Brønsted acids is important, as mentioned above. mentioned above. Based on these considerations, we examined the effect of different tin halides on Based on these considerations, we examined the effect of different tin halides on the transformation, the transformation, including SnCl4·5H2O, SnBr4, and SnI4. Although the use of SnCl4·5H2O did not including SnCl4·5H2O, SnBr4, and SnI4. Although the use of SnCl4·5H2O did not result in the desired result in the desired product (entry 3), the use of SnBr4 and SnI4 gave methyl lactate in satisfactory product (entry 3), the use of SnBr and SnI gave methyl lactate in satisfactory yields (entries 4 and 5). yields (entries 4 and 5). Other metal4 halides4 were not active in this transformation. We also found Other metal halides were not active in this transformation. We also found that the product selectivity that the product selectivity was reversible in the presence of HBr, which confirms that the use of tin was reversible in the presence of HBr, which conﬁrms that the use of tin metal as a Lewis acid is metal as a Lewis acid is essential in the retro-aldol reaction of fructose. essential in the retro-aldol reaction of fructose.

TableTable 3. 3. ConversionConversion of authentic of authentic starch starch into methyl into methyl levulinate levulinate and methyl and methyllactate using lactate various using 1 catalysts.various catalysts. 1

Yield of Methyl Entry Catalyst Yield of Methyl Yield of Methyl Lactate (%) 5 Entry Catalyst Levulinate (%) 5 Yield of Methyl Lactate (%) 5 Levulinate (%) 5 1 Sn(OTf)2 48 2 1 Sn(OTf)2 48 2 2 2 TfOH 53 <1 2 2 3 TfOH 53 <1 3 SnCl4-5H2O 3 0 3 3 3 4 SnClSnBr4-5H4 2O 73 270 4 4 35 SnSnIBr44 37 2027 3 HBr 21 10 5 46 SnI4 3 20 1 Reaction6 3 conditions:HBr Starch (50 mg), methanol (5.021 mL), catalyst (0.024 mmol), naphthalene10 (0.156 mmol, as an internal standard), Ar (5 atm), 24 h, and 160 ◦C. 2 0.048 mmol catalyst. 3 0.24 mmol catalyst. 4 0.96 mmol 1 catalyst. Reaction5 Product conditions: yields S (moltarch %) (50 are mg), carbon-based. methanol The (5.0 produced mL), catalyst quantities (0.024 of each mmol), product naphthalene were determined (0.156 by mmol1H NMR, as analysis.an internal standard), Ar (5 atm), 24 h, and 160 °C. 2 0.048 mmol catalyst. 3 0.24 mmol catalyst. 4 0.96 mmol catalyst. 5 Product yields (mol %) are carbon-based. The produced quantities of each 1 producAccordingt were to determined previous by reports, H NMR SnCl analysis.4 is the most effective tin halide catalyst for the production of methyl lactate from 3- [29] or 6-carbon [34] sugars. This can be explained by examining the According to previous reports, SnCl4 is the most effective tin halide catalyst for the production ofelectronegativity methyl lactate (X fromp) of 3 the- [29 different] or 6-carbon halogen [34 atoms] sugars. (see TableThis 4 can) and be the explained pK a values by of examining the hydrogen the halides (see Table5). However, in our case, the use of SnCl failed to result in the desired methyl electronegativity (Χp) of the different halogen atoms (see Table4 4) and the pKa values of the hydrogen lactate (Table3, entry 3). Although the strong Lewis acidity of Sn in SnCl appears to accelerate the halides (see Table 5). However, in our case, the use of SnCl4 failed to result4 in the desired methyl retro-aldol reaction, the dehydration, and the 1,2-hydride shift, the glycosidic linkages are not cleaved lactate (Table 3, entry 3). Although the strong Lewis acidity of Sn in SnCl4 appears to accelerate the retrosmoothly-aldol due reaction, to the the low dehydration, pKa of HCl. Inand contrast, the 1,2- ashydride the strong shift, Brønsted the glycosidic acid HBr linkages and HI are accelerate not cleaved the cleavage of the glycosidic linkages, the use of SnBr and SnI produced methyl lactate in moderate smoothly due to the low pKa of HCl. In contrast, as 4the strong4 Brønsted acid HBr and HI accelerate yields (i.e., 27% and 20% yields, respectively, see Table3, entries 4 and 5). the cleavage of the glycosidic linkages, the use of SnBr4 and SnI4 produced methyl lactate in moderate yields (i.e., 27% and 20% yields, respectively, see Table 3, entries 4 and 5).

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Table 4. Xp values of the halogen atoms. Table 4. Χp values of the halogen atoms.

Halogen Atom Xp Halogen Atom Χp ClCl 3.16 3.16 Br 2.96 IBr 2.96 2.66 I 2.66

Table 5. pKa values of the hydrogen halides. Table 5. pKa values of the hydrogen halides. Hydrogen Halide pK Hydrogen Halide pKa a HClHCl −8 −8 HBrHBr −9 −9 HI −10 HI −10

Based onon thesethese results results obtaining obtaining for for the the authentic authen starchtic starch sample, sample, the corresponding the corresponding reactions reactions using algaeusing werealgae investigated. were investigated. The cultivation The cultivation conditions conditions and pretreatment and pretreatment method employed method employed are described are indescribed the literature in the [ 33literature] along with [33] detailsalong regardingwith details quantiﬁcation regarding quantification of the neutral of sugars. the neutral Preparation sugars. of thePreparation algae for of use the as algae a reaction for use substrate as a reaction was carried substrate out aswas indicated carried inout Figure as indicated3. After cultivationin Figure 3. of After the algae,cultivation the centrifugation, of the algae, freezing,the centrifugation, and dehydration freezing, steps and resulted dehydration in a green steps powder. resulted This powderin a green was thenpowder. suspended This powder in methanol was then and subjectedsuspended to sonicationin methan tool yieldand thesubjected reaction to substrate sonication containing to yield both the oilsreaction and carbohydrates.substrate containing However, both when oils the and reaction carbohydrates. was attempted However, using this when suspension, the reaction no traces was of theattempted desired using products this weresuspension, obtained. no We traces therefore of the hypothesized desired products that the were mineral obtained. ions andWe pigmentstherefore presenthypothesized in the that green the powder mineral reduced ions and the pigments catalytic present activity, in likely the green through powder catalyst reduced poisoning. the catalytic Thus, theactivity, sonicated likely powder through suspension catalyst poisoning. was ﬁltered, Thus washed, the sonicated with methanol, powder and suspension dried in vacuo was filtered, prior to applicationwashed with in methanol, the reaction. and dried in vacuo prior to application in the reaction.

Figure 3. Method for preparation of the algal residue. Figure 3. Method for preparation of the algal residue.

The resulting dried sample was then employedemployed to examine the degradation of the algal carbohydrates (Scheme 44).). Initially,Initially, Sn(OTf)Sn(OTf)2 was applied as the catalyst under ths same conditions employed for the authentic starch sampe, and the des desiredired products (i.e., methyl levulinate and methyl lactate) were obtained in 37%37% andand 9%9% yields,yields, respectively.respectively. In contrast,contrast, thethe useuse ofof SnBrSnBr44 produced respective yields of 6% and 37%, with methyl lactatelactate being the major product, as predicted. Hence, the introduction of an optimized pretreatment methodmethod to remove the mineral ions and pigments was successful inin retainingretaining thethe catalystcatalyst activity activity and and producting producting the the desired desired products. products. However, However, the the goal goal of thisof this study study was was to to produce produce chemicals chemicals of of interest interest using using the the algal algal residue residue followingfollowing abstractionabstraction of the oil components. Therefore, Therefore, the the sonication sonication st stepep was was carried carried out out in in a amixed mixed solvent solvent (CHCl (CHCl3/MeOH)3/MeOH) to toseparate separate the the oil oil components components from from the the algal algal residue. residue. As As indicated indicated in in Scheme Scheme 44,, thethe productproduct yieldsyields obtained from the algal residue were comparable to those obtained without the the oil oil extraction stage. stage. These results conﬁrmconfirm that the homogeneous catalystscatalysts could also be employed for reactions involving the algal residues.

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Scheme 4. Conversion of the algal residue into methyl levulinate and methyl lactate using Sn(OTf)2 Scheme 4. Conversion of the algal residue into methyl levulinate and methyl lactate using Sn(OTf)2 and SnBr4, respectively. and SnBr4, respectively.

3.3. Heterogeneously Heterogeneously Catalyzed Catalyzed Transformation Transformation of of Carbohydrates Carbohydrates into into Methyl Methyl Levulinate Levulinate and and Methyl Lactate Methyl Lactate The use of heterogeneous rather than homogeneous catalysts is necessary in the development The use of heterogeneous rather than homogeneous catalysts is necessary in the development of of a green chemical process for the transformation of algal biomass-derived polysaccharides into a green chemical process for the transformation of algal biomass-derived polysaccharides into chemicals of interest. This is due to the easy separation and recovery of homogeneous catalysts, chemicals of interest. This is due to the easy separation and recovery of homogeneous catalysts, and and the absence of inorganic salt wastes associated with catalyst degradation. As such, the use of the absence of inorganic salt wastes associated with catalyst degradation. As such, the use of heterogeneous catalysts could lead to a sustainable and environmentally friendly process. A summary heterogeneous catalysts could lead to a sustainable and environmentally friendly process. A of related studies is given below. summary of related studies is given below. 3.1. Production of Alkyl Lactate from Carbohydrates Using Heterogeneous Catalysts 3.1. Production of Alkyl Lactate from Carbohydrates Using Heterogeneous Catalysts In 2009, Taarning et al. reported the zeolite-catalyzed transformation of triose sugars into methylIn lactate, 2009, Taarning where Lewis-acidic et al. reported zeolites the zeolite-catalyzed such as Sn-Beta transformation exhibited surprisingly of triose sugars high into activities methyl andlactate, selectivities where inLewis-acidic the isomerization zeolit ofes triosessuch as to methylSn-Beta lactate, exhibited and insu bothrprisingly the 1,2-hydride high activities shift and and theselectivities dehydration in the reaction isomerization [35]. In addition,of trioses to in methyl 2010, the lactate, West and group in both reported the 1,2-hydride the transformation shift and ofthe triosesdehydration into methyl reaction lactate [35]. using In addition, H-USY-zeolite in 2010, [the36]. West According group toreported their study, the transformation a purely Lewis of acidic trioses zeoliteinto methyl is highly lactate active using for H-USY-zeolite the formation [36]. of methyl According lactate to fromtheir triosestudy, sugars, a purely which Lewis supports acidic zeolite the hypothesisis highly active that Brønsted for the formation and Lewis of acidicmethyl sites lactate catalyze from triose two different sugars, which reaction supports paths, withthe hypothesis only the Lewisthat Brønsted acidic sites and leading Lewis to acidic the formation sites catalyze of lactic two acid/methyl different reaction lactate paths, from DHA/GLA with only the in appreciableLewis acidic amounts.sites leading These to results the formation clearly indicate of lactic that acid/methyl Lewis acids lactate are effectivefrom DHA/GLA for the transformation in appreciable of amounts. trioses intoThese lactic results acid/methyl clearly indicate lactate (Tablethat Lewis4, entries acids 1–6). are effective for the transformation of trioses into lactic acid/methylFurthermore, lactate in (Table 2010, 4, Holm entries et 1–6). al. developed an efﬁcient method to transform mono- and disaccharidesFurthermore, into methyl in 2010, lactate Holm (Table et al.6, entriesdeveloped 7–11) an [ 34efficient]. They method used zeolites to transform possessing mono- Lewis and acidicdisaccharides sites with into a beta-typemethyl lactate zeolite (Table containing 6, entrie as 12-membered 7–11) [34]. They pore used structure zeolites (pore possessing dimensions: Lewis 6.6acidic× 7.6 sites Å) exhibitingwith a beta-type an especially zeolite containing high activity. a 12-membered This result pore clearly structure demonstrates (pore dimensions: that Lewis 6.6 × acidic7.6 Å) sites exhibiting are effective an especially in the retro-aldol high activity. reaction This of result fructose, clearly while demonstrates the presence that of Lewis Brønsted acidic acidic sites sitesare leadseffective to unwantedin the retro-aldol side reactions. reaction Thisof fructose is consistent, while withthe presence the case of of Brønsted homogeneous acidic catalysts.sites leads Furthermore,to unwanted this side accelerating reactions. This effect is agrees consistent with with the report the case ofthe of homogeneous Román-Leshkov catalysts. group, whichFurthermore, states thatthis cooperative accelerating catalysis effect agrees in the zeolitewith the pores report promotes of the the Román-Leshkov aldol condensation group, between which DHAstates and that formaldehyde,cooperative catalysis and can bein attributedthe zeolite to pores the presence promotes of boththe aldol a Lewis condensation acidic site (Sn) between and a BrønstedDHA and acidicformaldehyde, site (lattice and oxygen) can be [37 attributed]. to the presence of both a Lewis acidic site (Sn) and a Brønsted acidic site (lattice oxygen) [37].

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Table 6. Conversion of carbohydrates to methyl lactate in methanol. Table 6. Conversion of carbohydrates to methyl lactate in methanol.

EntryEntry Catalyst Catalyst Si/Metal Si/Metal Ratio Ratio Substrate Substrate YieldYield of of Methyl Methyl Lactate Lactate (%)(%)4 4 1 1 1 1 H-H-USYUSY 66 DHA DHA 71 71 2 1 2 1 H-H-USYUSY 30 30 DHA DHA 47 47 2 3 2 3 AlAl-Beta-Beta 65 65 DHA DHA 0 0 4 2 Zr-Beta 125 DHA 1 4 2 Zr-Beta 125 DHA 1 5 2 Ti-Beta 125 DHA 2 2 5 6 2 TiSn-Beta-Beta 125 125 DHA DHA >992 6 2 7 3 SnAl-Beta-Beta 125 125 DHA Glucose >99 0 7 3 8 3 AlZr-Beta-Beta 125 125 Glucose Glucose 0 33 3 8 3 9 ZrTi-Beta-Beta 125 125 Glucose Glucose 33 31 3 9 310 TiSn-Beta-Beta 125 125 Glucose Glucose 31 43 11 3 Sn-Beta 125 Sucrose 64 10 3 Sn-Beta 125 Glucose 43 1 Reaction3 conditions: Catalyst (80 mg), substrate (1.25 mmol calculated as monomer), methanol (4 g), reaction time 24 h,11 temperature Sn 115-Beta◦C. 2 Reaction conditions:125 DHA (1.25Sucrose mmol) in 4 g of methanol (8064◦C) or water (125 ◦C), 1catalyst Reaction (80 conditions: mg) added toCatalyst the mixture (80 mg), and substrate stirred for (1.25 24 h. mmol3 Reaction calculated conditions: as monomer), Substrate (225 methanol mg), catalyst (4 g), ◦ 4 reaction(160 mg), naphthalenetime 24 h, temperature (120 mg), and methanol115 °C. 2 (8.0 Reaction g) were stirredconditions: in an autoclaveDHA (1.25 at 160 mmol)C for in 20 h.4 g Yieldsof methanol (mol %) of each product are carbon-based. The quantity of each product produced was determined by high performance (80liquid °C) chromatography or water (125 (HPLC). °C), catalyst (80 mg) added to the mixture and stirred for 24 h. 3 Reaction conditions: Substrate (225 mg), catalyst (160 mg), naphthalene (120 mg), and methanol (8.0 g) were 4 stirredAs mentioned in an autoclave above, theat 160 transformatio °C for 20 h. of Yields monosaccharides (mol %) of each into product alkyl lactate are carbon using-based. heterogeneous The catalystsquantity has of been each productsuccessfully produced developed. was determined However, by high a few performance examples liquid of chromatographythe degradation of (HPLC). polysaccharides into alkyl lactate can also be found. This latter transformation involves the hydrolysis of celluloseAs mentioned to glucose, above, the isomerizationthe transform ofatio glucose of monosaccharides into fructose and intothen intoalkyl trioses, lactate and using the heterogeneousconversion of these catalysts trioses has to alkyl been lactate. successfully Hence, the developed overall reaction. However, is sensitive a few to examples the ratio of of Lewis the degradationand Brønsted of acid polysacch sites. arides into alkyl lactate can also be found. This latter transformation involves the hydrolysis of cellulose to glucose, the isomerization of glucose into fructose and then in3.2.to Productiontrioses, and of the Alkyl conversion Levulinates of fromthese Carbohydrates trioses to alkyl Using lactate. Heterogeneous Hence, the Catalysts overall reaction is sensitive to theAs ratio previously of Lewis discussed,and Brønsted Lewis acid acidic sites. sites promote the production of methyl lactate through acceleration of the retro-aldol reaction of glucose, while Brønsted acidic sites promote the subsequent 3.2.dehydration Production reactions. of Alkyl Levulinates In 2013, Saravanamuruganfrom Carbohydrates Using et al. Heterogeneous reported a selective Catalysts synthesis of methyl levulinateAs previously from monosaccharides discussed, Lewis using acidic several sites promote zeolites the [38]. production As shown of in methyl Table 7lactate, as the through Si/Al accelerationratio increased, of the the retro Brønsted-aldol reaction acidity of wasglucose, reduced, while Brønsted and the yieldacidic ofsite thes promote desired the product subsequent was dehydrationlowered. These reaction resultss. In therefore 2013, Saravanamuruga conﬁrm that an Brønsted et al. reported acidic site a selective is crucial synthesis for promoting of methyl the levulinatedehydration from pathway. monosaccharides using several zeolites [38]. As shown in Table 7, as the Si/Al ratio increased, the Brønsted acidity was reduced, and the yield of the desired product was lowered. These results therefore confirm that a Brønsted acidic site is crucial for promoting the dehydration pathway.

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Table 7. Conversion of glucose to methyl levulinate over zeolites 1. Table 7. Conversion of glucose to methyl levulinate over zeolites 1.

EntryEntry Catalyst Catalyst Si/AlSi/Al Ratio Ratio YieldYield of Methylof Methyl Levulinate Levulinate (%) 2 (%) 2 1 1H H-Y-Y 2.62.6 32 32 2 2H H-USY-USY 66 49 49 3 3H H-USY-USY 3030 37 37 4 H-Beta 12.5 44 4 5H H-Beta-Beta 1912.5 47 44 5 6H H-Beta-Beta 15019 10 47 6 7 H-ZSM-5H-Beta 11.5150 8 10 7 8H- NoneZSM-5 -11.5 <1 8 1 Reaction8 conditions: GlucoseNone (250 mg), catalyst (150- mg), methanol (10 mL), and Ar (20<1 bar). The autoclave was heated to 160 ◦C and the stirring (300 rpm) was started once the temperature reached 150 ◦C. 2 Yields (mol %) were 1 determined Reaction conditions: by gas chromatography Glucose (250 (GC) mg), and HPLC.catalyst (150 mg), methanol (10 mL), and Ar (20 bar). The autoclave was heated to 160 °C and the stirring (300 rpm) was started once the temperature reached 2 150It is °C. therefore Yields (mol apparent %) were that determined as with the by homogeneousgas chromatography catalysts, (GC) theand balanceHPLC. between Lewis and BrønstedIt is therefore acidic sites apparent is important that as in with the the heterogeneously homogeneous catalyzedcatalysts, preparationthe balance between of alkyl lactateLewis and Balkylrønsted levulinate. acidic sites However, is important the degradation in the heterogeneously of polysaccharides catalyzed into alkylpreparation lactate andof alkyl alkyl lactate levulinate and alkylhas also levulinate. been reported However, for heterogeneousthe degradation catalysts. of polysaccharides As such, the into development alkyl lactate of and processes alkyl levulinate involving hasthe hydrolysisalso been reported of polysaccharides for heterogeneous to monosachcarides catalysts. As and such, the the conversion development of monosaccharides of processes involving into the thedesired hydrolysis products of ispol challenging.ysaccharides to monosachcarides and the conversion of monosaccharides into the desired products is challenging. 4. Conclusions 4. ConclusionsMicroalgae have attracted attention as novel sources of biomass due to their high biomass productivitiesMicroalgae per ha unitve attracted time and attention area. In addition, as novel algae sources can of be biomass propagated due industrially to their high without biomass the productivitiesrequiring extensive per unit areas time of and farmland. area. In Underaddition unfavorable, algae can be growth propagated conditions, industrially such as without a nutrient the requirdeficiency,ing extensive algae produce areas large of farmland. quantities Under of oil unfavorableand carbohydrates growth (mainly conditions, starch) such intheir as a cells,nutrient and deficiency,recently this algae algae-based produce oil large has beenquantities employed of oil asand both carbohydrates biojet fuel and (mainly biodiesel. starch) However, in their few cells reports, and rexistecent intoly th theis algae use of-based algae-derived oil has been starch employed remaining as in both the algal biojet cells fuel following and biodiesel. oil extraction. However, few reportsIn exist this review,into the we use discussed of algae-derived the conversion starch remaining of this starch in the into algal alkyl cells lactate following (a precursor oil extraction. of the biodegradableIn this review, polyesters we discussed of atactic the or syndiotacticconversion of polylactides) this starch i andnto alkylalkyl levulinatelactate (a precursor (a precursor of andthe biodegradablebuilding block polyesters for pharmaceuticals, of atactic or plasticizers, syndiotactic and polylactide other additives).s) and alkyl Various levulinate homogeneous (a precursor catalysts and buildingwere examined block tofor optimize pharmaceuticals, the selective plasticizers, production and of either other methyl additives). levulinate Various or methyl homogeneous lactate, II catalystsand the homogeneous were examine Snd totriflate optimize led the to increased selective yieldsproduction of alkyl of levulinate,either methyl while levulinate SnBr4 allowed or methyl the lactateselective, and production the homogeneous of alkyl lactate. SnII triflate However, led the to development increased yield of heterogeneouss of alkyl levulinate, catalysts while is required SnBr4 allowedfor the implementation the selective production of these systems of alkyl in practical lactate. applications. However, the development of heterogeneous catalystsWe thereforeis required confirm for the thatimplementation algae can be of utilized these systems for oil productionin practical andapplications. as a carbon resource to potentiallyWe therefore replace confirm fossil fuels. that algae Future can studies be utilized should for then oil production focus on the and underlying as a carbon mechanism resource to of potentiallystarch production replace infossil algae, fuels. as suchFuture information studies should will be th usefulen focus for on the the design underlying and selection mechanism of algal of starchspecies production that can produce in algae high, as yieldssuch information of biomass. will be useful for the design and selection of algal species that can produce high yields of biomass. Acknowledgments: This study was supported by the JSPS Grant-in-Aid for Scientific Research on Innovative AcknowledgementAreas (Grant no. 16H01010s: This study and 26105003). was supported by the JSPS Grant-in-Aid for Scientific Research on Innovative AreasAuthor (Grant Contributions: no. 16H01010S.Y. and and 26105003). S.I. conceived the concept and directed the project. K.M. and K.T. discussed the experiments and results. Author Contributions: S.Y. and S.I. conceived the concept and directed the project. K.M. and K.T. discussed the experimentsConflicts of Interest:and results.The authors declare no conflict of interest.

Conflicts of Interest: The authors declare no conflict of interest.

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