Catalytic Processes from Biomass-Derived Hexoses and Pentoses: A Recent Literature Overview

catalysts Review Catalytic Processes from Biomass-Derived Hexoses and Pentoses: A Recent Literature Overview Jesús Esteban 1, Pedro Yustos 2and Miguel Ladero 2,* 1Max Planck Institute for Chemical Energy Conversion, Stiftstraße 34-36, 45740 Mülheim an der Ruhr, Germany; [email protected] 2Chemical Engineering and Materials Department, Chemical Sciences School, Complutense University, Ciudad Universitaria s/n, 28040 Madrid, Spain; [email protected] *Correspondence: [email protected]; Tel.: +34-91-394-4164 Received: 11 November 2018; Accepted: 3 December 2018; Published: 7 December 2018   Abstract: Biomass is a plentiful renewable source of energy, food, feed and chemicals. It fixes about 1–2% of the solar energy received by the Earth through photosynthesis in both terrestrial and aquatic plants like macroand microalgae. As fossil resources deplete, biomass appears a good complement and eventually a good substitute feedstock, but still needs the development of relatively new catalytic processes. For this purpose, catalytic transformations, whether alone or combined with thermal ones and separation operations, have been under study in recent years. Catalytic biorefineries are based on dehydration-hydrations, hydrogenations, oxidations, epimerizations, isomerizations, aldol condensations and other reactions to obtain a plethora of chemicals, including alcohols, ketones, furans and acids, as well as materials such as polycarbonates. Nevertheless, there is still a need for higher selectivity, stability, and regenerability of catalysts and of process intensification by a wise combination of operations, either in-series or combined (one-pot), to reach economic feasibility. Here we present a literature survey of the latest developments for obtaining value-added products using hexoses and pentoses derived from lignocellulosic material, as well as algae as a source of carbohydrates for subsequent transformations. Keywords: lignocellulose; algae; biorefinery; monosaccharide; platform chemical; catalysis; selectivity; stability; bio-based monomers; hexoses; pentoses; solvents 1. Introduction The advent of second generation biorefineries has opened the possibility to access a vast mass of monosaccharides, such as glucose, xylose, mannose, galactose and arabinose, apart from some of their derivatives such as galacturonic acid, to name the most abundant of the lignocellulosic biomass [ 1 ]. At the same time, the need for food and feed is directing research interests towards the exploitation of seaweeds, microalgae and aquatic plants, owing to their abundance. Their major components are alginic acid, agar or carrageenean, which could become sources of platform chemicals of the future [ 2 ]. Nowadays, seaweed production is in excess of 25 Mt/year. In first-generation biorefineries, now at a commercial scale, several problems are due to feedstock scarcity: free sugar from sugarcane and sugar beet, on one hand, and starch from corn, potatoes and other foods, on the other. Sucrose annual production amounts to approximately 170 Mt, while 88 Mt starch is obtained for several purposes (though the crop of potatoes alone in 2016 reached 377 Mt). Although high, this amount is still very far away from the 4700 Mt of oil and 7200 Mt of coal consumed in 2016 [ 3 ]. The food versus fuel debate cannot be maintained for long, even only considering this reason. From the mass balance perspective, second generation biorefinery processes, though less developed than first-generation ones, are more promising in the long term. CO 2 fixation Catalysts 2018,8, 637; doi:10.3390/catal8120637 www.mdpi.com/journal/catalysts Catalysts 2018,8, 637 2 of 39 via photosynthesis reaches up to 258,000 Mt each year, creating 447 Mt of terrestrial and aquatic biomass [4]. This type of biomass is not created to store energy and material resources by living beings, but for structural and reproductive purposes, so its use is much hindered by its structure, posing complex technological challenges to its transformation. To this end, thermochemical approaches focus on the production of carbon-rich solids, liquids and gases (biochar, biooil, and synthesis gas, a mixture of CO and H 2 -) [ 5 ]. Though the mixtures are complex, their nature is more similar to fossil resources, and present refinery and petrochemical technologies can be useful, but with a higher development of liquefaction, Fischer-Tropsch technologies and fast and slow pyrolysis processes, reducing byproduct formation in all cases and reducing energy consumption. One of the most promising strategies is pyrolysis and catalytic steam reforming to produce H 2 from biomass [ 6 ]. Further alternatives within thermocatalytic processing in recent years, hydrogenolysis at high H 2 pressures and catalytic transfer hydrogenolysis (in moderate conditions), are being developed to obtain allyl-alcohols from glycerol, furans and lactones from furfural and hydroxymethylfurfural (5-HMF), several acids and alcohols from levulinic acid and its esters and, most interestingly, several phenolic alcohols from lignin, the less reactive fraction of lignocellulosic biomass [7]. Less energy-intensive is the biochemical approach, based on biomass fractionation through the triad pretreatments-depolymerizations (chemical and/or enzymatic)-synthesis (biological and/or catalytic), although environmental concerns should be considered [ 8 ]. Even so, biological transformations of syngas are possible, mixing thermoand biochemical approaches for recalcitrant biomasses [ 9 ]. Although recalcitrant, biomass can be converted to chemicals directly by a number of catalytic routes, combining depolymerization, polymerization to humins, and reaction to several chemicals, with 5-HMF, furfural, levulinic acid and formic acid as the most prominent [ 10 ]. For example, from real biomass, 5-HMF can be obtained in 11–57% w/wyields using edible biomass and up to 60% w/wif lignocellulosic biomass was treated with acids in γ -valerolactone media [ 11 ]. Levulinic acid can be obtained from pure cellulose by using mesoporous catalysts resembling cellulase action [ 12 ], reaching yields up to 51% w/w. Similar results can be obtained with dicationic ionic liquids at 100 ◦ C for 3 h (yield = 55%) [ 13 ]. Hemicellulose is the second most abundant polymer in lignocellulosic biomass (15–30%) after cellulose, with furfural the main platform chemical that can be directly or indirectly produced in vast amounts [ 14 ]. Furfural can be obtained in one-pot processes from hemicelluloses rich in xylose, with yields up to 85% when using Brønsted acidic ionic liquids (BAILs) [ 15 ]. However, better yields, higher activities and less by-products can be reached when the constituent monosaccharides (glucose, xylose, fructose, etc.) are the raw materials to 5-HMF, furfural and levulinic acid [ 16 ]. From C5 and C6 monosaccharides, several acids (formic, acetic, oxalic, malonic, gluconic and glucaric) can be produced as well [ 17 ], while 5-HMF is the key to chemicals such as levulinic acid, adipic acid, 1–6 hexanediol, ε -caprolactam, and several furans and lactones [ 17 , 18 ]. Figure 1shows the main steps in the fractionation of biomass to obtain the monosaccharides and low molecular mass platform chemicals for further catalytic/biocatalytic processing. This review compiles the most relevant aspects of the latest research (from 2016 to the present day) on catalytic processes from C5 and C6 monosaccharides as platform chemicals obtained from lignocellulosic materials. In addition, reports on algal biomass as an interesting starting material for monosaccharides are also covered, with a slightly longer timespan given the novelty and emergence of the topic. Catalysts 2018,8, 637 3 of 39 Catalysts 2018, 8, x FOR PEER REVIEW 3 of 40 Figure 1. Chemical biorefinery (bio/catalyst routes) main process scheme. 2. Catalytic Studies for the Transformation of Glucose Carbohydrates constitute approximately 75% of the annual renewable biomass, among which cellulose is the most attractive owing to its wide availability and, most importantly, to the fact it does not compete with food products. Lignocellulosic material can undergo different types of pretreatment to release its components, including fractionation by thermochemical and physical methods, and biological or organosolv and other chemical procedures. Among such components are lignin, hemicellulose and cellulose, the latter being the major component, circa 45% [19–21]. Cellulose is undoubtedly the most abundant polymer in nature owing to it being the main constituent of plant cell walls. Therefore, owing to its massive availability, there is an opportunity to obtain sugars from its depolymerization by hydrolysis, which leads to obtaining soluble oligosaccharides and, especially, glucose. For such hydrolysis, cellulose can undergo enzymatic or chemical processes using mineral acids or, more recently, solid acid catalysts [22]. The present section will focus on works reporting the exploitation of glucose to obtain valueadded products and fuels of renewable origin through different catalytic procedures. For the valorization of this material, two types of transformations can be identified: (a) Routes through 5-HMF as a building block for further transformations to furan-based products. Such pathways require prior isomerization of glucose to fructose, from which 5-HMF can be produced. Figure 2 presents a scheme of a number of products and intermediates in some cases that can be synthesized via pathways from 5-HMF starting from glucose through isomer fructose. Additionally, this figure also shows the derivation of glucose from lignocellulosic material. (b) Reactions to products obtained by non-5-HMF related routes, thus avoiding isomerization and dehydration as the first steps of the corresponding conversion. These are schematized in Figure 3. 2.1. Isomerization to Fructose and Reactions to Products through 5-HMF Related Routes Table 1 presents a compilation of works in the literature that complement several of the routes presented in Figure 2, providing specific details about the products, side-products, catalysts employed and operating conditions, as well as the outcomes of their application. Figure 1. Chemical biorefinery (bio/catalyst routes) main process scheme. 2. Catalytic Studies for the Transformation of Glucose Carbohydrates constitute approximately 75% of the annual renewable biomass, among which cellulose is the most attractive owing to its wide availability and, most importantly, to the fact it does not compete with food products. Lignocellulosic material can undergo different types of pretreatment to release its components, including fractionation by thermochemical and physical methods, and biological or organosolv and other chemical procedures. Among such components are lignin, hemicellulose and cellulose, the latter being the major component, circa 45% [19–21]. Cellulose is undoubtedly the most abundant polymer in nature owing to it being the main constituent of plant cell walls. Therefore, owing to its massive availability, there is an opportunity to obtain sugars from its depolymerization by hydrolysis, which leads to obtaining soluble oligosaccharides and, especially, glucose. For such hydrolysis, cellulose can undergo enzymatic or chemical processes using mineral acids or, more recently, solid acid catalysts [22]. The present section will focus on works reporting the exploitation of glucose to obtain value-added products and fuels of renewable origin through different catalytic procedures. For the valorization of this material, two types of transformations can be identified: (a) Routes through 5-HMF as a building block for further transformations to furan-based products. Such pathways require prior isomerization of glucose to fructose, from which 5-HMF can be produced. Figure 2presents a scheme of a number of products and intermediates in some cases that can be synthesized via pathways from 5-HMF starting from glucose through isomer fructose. Additionally, this figure also shows the derivation of glucose from lignocellulosic material. (b) Reactions to products obtained by non-5-HMF related routes, thus avoiding isomerization and dehydration as the first steps of the corresponding conversion. These are schematized in Figure 3. 2.1. Isomerization to Fructose and Reactions to Products through 5-HMF Related Routes Table 1presents a compilation of works in the literature that complement several of the routes presented in Figure 2, providing specific details about the products, side-products, catalysts employed and operating conditions, as well as the outcomes of their application. Catalysts 2018,8, 637 4 of 39 Catalysts 2018, 8, x FOR PEER REVIEW 4 of 40 Figure 2. Exploitation of glucose and fructose derived from biomass to value-added chemicals via transformation of 5-hydroxymethylfurfural. Figure 2. Exploitation of glucose and fructose derived from biomass to value-added chemicals via transformation of 5-hydroxymethylfurfural. Catalysts 2018,8, 637 5 of 39 The first reaction before obtaining 5-HMF as a building block is the isomerization of glucose to fructose. LiBr has been reported to act as a catalyst obtaining yields to fructose of 30.3% in only 15 min, while the conversion amounted to 51.8%, with mannose being obtained mainly from a side epimerization reaction. This work reported mechanistic studies, where Li + catalyzed the isomerization through the intramolecular hydride shift mechanism from C2 to C1 and Br - through a proton transfer mechanism via an enediol intermediate [ 23 ]. Cu salts have also been used for this reaction, obtaining much lower conversions and yields, which were dependent on the pH of the reaction medium. At values of pH = 5.3–5.5, the detected species of the Cu salts was Cu(OH) + , identified as the Lewis acid active species, which triggers an intramolecular 1,2-hydride shift [24]. Further to sole isomerization to fructose, the synthesis of methyl fructosides has also been studied by etherification. A study combining different zeolites as suppliers of Lewis acidity and Dowex 50WX8-100 (Brønsted acidity) was made reaching yields of 72% in 1 h. In the mechanistic studies completed in this work it was proven that when catalysts with Brønsted acidity were employed, methyl glycosides prevailed, whereas when Lewis acidity was higher, isomerization was predominant [ 25 ]. In a different work, Leitner et al. used kaolin to conduct this reaction, whose advantage is its wide availability and inexpensive price. The study focuses on the screening of different kaolin samples, among which Strem-2008 is the most active, and, then, the optimization of conditions using rational experiment design in continuous flow [26]. As mentioned above, 5-HMF is the building block to ulterior reactions, and it has also been sought after as an end product. To generate 5-HMF, the reaction steps require the dehydration of fructose after prior isomerization of glucose. Glucose transformation was reported using organic catalysts featuring basic, Brønsted acidic or both types of functional groups. The basic groups have been found to isomerize glucose to fructose and Brønsted acidic groups catalyze the dehydration. Thus, sulfanilinic acid performed better than the other catalysts tested, reaching 44% yield to 5-HMF with 90% conversion of glucose. In addition, mechanistic studies disclosed that isomerization to fructose was found to be the rate-limiting step during the reaction [ 27 ]. Tin phosphates, whose active sites appear to be the tetracoordinated Sn 4+ centers, have also been employed for this reaction [Emim][Br] ionic liquid as solvent reaching yields up to 58.3%. The synergistic effect of this ionic liquid appears to have an influence in the high yields reported [ 28 ]. Cui et al. proposed the preparation of a catalyst based on chitosan nanoparticles doped with Cr ions, which were prepared from adsorption from aqueous solutions and can therefore have an application in the treatment of wastewater for removal of these species. The conversion reported was about 92% and the yields to 5-HMF reported as high as 64.7% [ 29 ]. Finally, making use of 5-sulfoisophthalic acid as ligand, a tin porous coordination polymer was synthesized on polydopamine-coated MnO 2 . This catalyst reached conversions of glucose above 90% and yields of 55.8%, showing good recycling capacity as no great activity loss was observed. Through 5-HMF, by direct one-pot conversion of glucose, 2,5-dimethylfuran (2,5-DMF) was obtained via hydrogenolysis, leaving other furan compounds as side products. For this reaction, catalysts based on Pd supported on a Zr-based metalorganic framework deposited on sulfonated graphene oxide was used, which obtained a yield to the desired product of 45.3% in 3 h [30]. 2,5-Diformylfuran (2,5-DFF or DFF) is a renewable platform chemical with multiple prospective applications in polymer, agrochemical and pharmaceutical industries. This compound is obtained with good yields from fructose (74% in a one-pot dehydration-aerobic oxidation process) and 5-HMF (94%) using 3D flower-like Ce − Mo micro/nano composite oxides with several ratios of Ce to Mo [ 31 ]. Likewise, in a one-pot process, fructose has been converted in DFF by using phosphomolibdic acid fixed in the metal–organic frameworks MIL-101, reaching a yield of 75.1% [32]. Methyl lactate can be synthesized through retro-aldol fragmentation of 5-HMF followed by dehydration, acetalization and isomerization. Yields of 47.7% were reported using ZnCl 2 after 3 h and a kinetic analysis of the overall transformation of glucose to methyl lactate with this catalyst revealed that the activation energy is 117.5 kJ mol −1 [ 33 ]. In a similar study, Sn-Beta zeolite was used, achieving slightly lower yields of 43% to product in a longer period giving fructosides as by-products. In fact, Catalysts 2018,8, 637 6 of 39 with this catalyst, the formation of methyl lactate shows two distinct kinetic regimes, where most of this product is formed in the slow reaction regime, which results from the accumulation of methyl fructoside as a masked form of glucose. Moreover, through methyl fructoside, the kinetics of the reaction can be accelerated in the presence of small amounts of water as long as they do not affect the catalyst stability [34]. Through dehydration of glucose to 5-HMF and further rehydration of this molecule, levulinic acid has been obtained with Cr-modified HZSM-5 zeolite. For this reaction, the kinetics were studied and, applying a simplified kinetic model, the calculated activation energies for glucose rehydration and 5-HMF rehydration were 69.1 and 54 kJ mol −1 , respectively. From the kinetic analysis, it was seen that the 5-HMF rehydration constant was higher than that of the dehydration of glucose, which implies that the latter is the controlling step [35]. Finally, furfural can be produced from isomerization followed by the cleavage of a C-C bond and dehydration with β -zeolites [ 36 ]. In addition, the solvent used in the transformation appears to play a role in the catalytic performance. Zhang et al. conducted the conversion of glucose to furfural with Snβ zeolite and found that using γ -valerolactone/water in the reaction medium enhanced yields to the product compared to water, DMSO or γ-butyrolactone [37]. Catalysts 2018,8, 637 7 of 39 Table 1. Summary of the details of work found in the literature dealing with the transformation of glucose (glu) to various products through hydroxymethylfurfural (5-HMF) related routes. Reaction Product Other Products Catalyst Reaction Conditions Main Results Reference Isomerization Fructose (Fru) Mannose Decomposition products LiBr T = 120 ◦C Cglu =10gL−1in water Ccat = 60% w/w t = 15 min Xglu = 51.8% Yfru = 30.3% TOF = 0.01 molfru molcat−1h−1 [23] Isomerization Fructose Mannose Cu(NO3)2and other Cu-containing catalysts. T = 110 ◦C Cglu = 1% w/win water Ccat = 60% w/w pH = 5.3 t = 90 min Xglu = 18% Yfru = 16% TOF = 0.45 molfru molcat−1h−1 [24] Isomerization -etherification Methyl fructoside (MF) Fructose Zeolites H-USY, H-Y, H-β (Lewis acidity) Dowex 50WX8-100 (Brønsted acidity) Si/Al ratio = 30 T = 120 ◦C Cglu = 3.13% w/win MeOH Ccat = 60% t = 60 min Xglu = 83% YMF = 72% TOF = 84.24 molMF gcat−1h−1 [25] Isomerization-etherification Methyl fructoside Fructose Kaolin: Strem-2008 and other kaolin samples SBET = 16 m2g−1 T = 120 ◦C Cglu = 3% w/win MeOH Ccat = 60% w/w t = 900 min Xglu = 93% YMF = 52% TOF = 1.15 molMF gcat−1h−1 [26] Isomerization Dehydration 5-HMF Fructose Sulfanilinic acid, aniline, PTSA, sulphamic acid T = 160 ◦C Cglu = 5% mol in H2O/DMSO/MIBK Ccat = 0.01 M t = 30 min Xglu = 90% YHMF = 44% TOF = 4.4 molHMF molcat−1h−1 [27] Isomerization Dehydration 5-HMF Fructose SnPO (from Sn3(PO4)4) SBET = 120.8 m2g−1 T = 120 ◦C Cglu = 20% w/w% in [Emim][Br] Ccat = 10% w/w t = 180 min Xglu = 94.1% YHMF = 58.3% TOF = 2.16 molHMF gcat−1h−1 [28] Isomerization Dehydration 5-HMF Fructose Chitosan nanoparticles doped with Cr(III) and Cr(VI) ions aided by H2SO4 SBET = 30.4 m2g−1 CCr(III) = 3% T = 180 ◦C Cglu = 3.13% w/win water/DMSO Ccat = 5% w/w t = 180 min Xglu = 92.3% YHMF = 64.7% TOF = 0.75 molHMF gcat−1h−1 [29] Isomerization Dehydration 5-HMF Fructose SnPCP@MnO2–PDA SBET = 240.6 m2g−1 T = 150 ◦C Cglu = 4% w/win DMSO Ccat = 1% w/w t = 300 min Xglu = 92.2% YHMF = 55.8% TOF = 2.47 molHMF gcat−1h−1 [38] Isomerization Dehydrations Hydrogenolysis 2,5-DMF Fructose, 5-HMF, 5-methylfurfural, 2,5-bis(hydroxymethyl)-furan; 5-methyl-furanmethanol 4.8Pd/UiO-66@SGO (Pd on a Zr-based MOF deposited on sulfonated graphene oxide) SBET = 715 m2g−1 CPd = 4.8% T = 160 ◦C PH2 = 1 MPa Cglu = 0.025 M in THF Ccat = 0.5% w/w t = 180 min Xglu = 87.3% YDMF = 45.3% TOF = 0.76 molDMF gcat−1h−1 [30] Catalysts 2018,8, 637 8 of 39 Table 1. Cont. Reaction Product Other Products Catalyst Reaction Conditions Main Results Reference Isomerization Retro-aldol fragmentation Dehydration Acetalization Isomerization Methyl lactate (MeLac) Fructose, glyceraldehyde, dihydroxyacetone, pyruvaldehyde, among many others reported ZnCl2and other Zn(II) salts T = 200 ◦C Cglu = 0.4% w/win EtOH and water Ccat = 0.004 M t = 180 min YMeLac = 47.7% TOF = 0.88 molMeLac molcat−1h−1 [33] Isomerization Retro-aldol fragmentation Dehydration Acetalization Isomerization Methyl lactate (MeLac) Fructose, fructofuranosides, fructopyranosides Sn-Beta zeolite SBET = 722 m2g−1 CSn = 0.977% T = 160 ◦C Cglu = 0.132 M in EtOH Ccat = 1% w/w t = 720 min YMeLac = 43% TOF = 0.47 molMeLac gcat−1h−1 [34] Dehydration Rehydration Levulinic acid (LevAc) 5-HMF, formic acid, humins Cr-HZSM-5 SBET = 308.9 m2g−1 CCr = 7.25% T = 180 ◦C Cglu = 2% w/win water Ccat = 0.75% w/w t = 180 min Xglu = 100% YLevAc = 64.4% TOF = 3.18 molLevAc gcat−1h−1 [35] Isomerization, C-C bond cleavage Dehydration Furfural Fructose, 2,5-HMF, lactic acid, arabinose H-βzeolite T = 150 ◦C Cglu = 5% w/win γ-valerolactone and water Ccat = 1% w/w PN2 = 2 MPa t = 60 min Xglu = 99.9% YFur = 56.5% TOF = 15.68 molFur gcat−1h−1 [36] Isomerization Dehydration Furfural 5-HMF Sn, Fe and Zr-βzeolite SBET = 539.9 m2g−1 CSn = 7.1 µmol g−1 T = 180 ◦C Cglu = 0.6% w/win γ-valerolactone and water Ccat = 2.4% w/w t = 33 min Xglu = 100% YFur = 69.2% TOF = 1.75 molFur gcat−1h−1 [37] Note: SBET stands for specific surface measured by BET isotherm; cat stands for catalyst. Catalysts 2018,8, 637 9 of 39 2.2. Other Routes from Glucose to Value-Added Products Some products obtained avoiding 5-HMF routes are included in Figure 3, whose details for the corresponding synthesis reactions are compiled in Table 2. Aldohexose mannose can be produced from the epimerization and hydrogenation of glucose, for which a continuous flow system has been implemented. The authors compared the performance of using a dual catalytic system consisting of Cs supported on heteropolyacid over carbon (Cs-HPA/C) and Ru supported on carbon simultaneously or in series, the latter option being optimal using first the Cs-HPA/C for epimerization and secondly a mixture of both catalysts to also perform hydrogenation [39]. Lactic acid has been obtained from glucose with a bifunctional Al(III)-Sn(II) catalyst via a series of tandem steps that include the isomerization to fructose and then the retro-aldol fragmentation into two C3 intermediates that eventually dehydrate and isomerize to lactic acid. Al(III) is accountable for isomerization via a 1,2-hydride shift mechanism and conversion of trioses to lactic acid; for its part, Sn(II) is liable for the fragmentation of fructose into two C3 intermediates (dihydroxyacetone and pyruvaldehyde) [40]. By hydrogenation and dehydration, D-isosorbide can be obtained using a one-pot synthesis employing a combined heterogeneous catalyst of Ru supported on a Dowex-H gel-type exchange resin. The yields reported were of up to 85% after 48 h of reaction in a pressurized autoclave [41]. The production of glycaldehyde, α -hydroxyγ -valerolactone and D-tetroses (D-(-)erythrose and D-(+)-erythrulose) was pursued in a different study, where they analyzed the profile of the products from the conversion of glucose with different catalysts. Ammonium tungstate gave the best conversion of 94.4%, obtaining mostly glycaldehyde as product, with a yield of 52.5% [42]. The oxidation to gluconic acid with exceptional high yields of 98% was performed photocatalytically, achieving practically total conversion of glucose. For this, Au nanoparticles were supported on TiO 2 and subjected to reaction under irradiation between 420 and 780 nm at very mild temperature conditions [ 43 ]. In another study, not only gluconic acid but also formates were obtained coupling dehydrogenation and (NH 4 ) 2 CO 3 hydrogenation by transfer leading to sorbitol as a side-product. In this case, Pd and Pt supported on activated carbonate were used simultaneously for 24 h obtaining conversions of 72.6% and yields to gluconic acid of almost 60% and formates in about 32% [ 44 ]. Finally, production of gluconic acid has also been attained by photooxidation of fructose, although xylitol has additionally been reported as a further decomposition product. For this reaction, TiO 2 catalysts were synthesized, for which the one prepared by the sol-gel method using polyethylene glycol as surfactant was most favorable to obtain gluconic acid, whereas using ultrasound and cetyltrimethylammonium bromide proved more efficient to produce xylitol [45]. Succinic acid can also be obtained as an oxidation product starting from glucose as substrate. In this way, García et al. have performed the oxidation and further cleavage using nitrogen-doped graphene as catalyst reaching yields as high as 68%, with the N loading playing a crucial role in the performance, reaching an optimum at 3.8% [ 46 ]. When carbon nanotubes doped with Fe and V were used, the major product of the oxidation reaction was oxalic acid (yield of 46%), with some succinic acid being observed as well, although to a much lower extent (only 7.8%) [47]. Last, a vinylized product that can be used as a monomer to interesting materials has also been prepared from fructose, as observed in Figure 3. The procedure to prepare it consisted of first protecting hydroxyl moieties by acetalization with acetone and further vinylation. For the former step an acidic catalyst like sulfuric acid was put to use, while for the latter, calcium carbide aided by basic compounds like KF and KOH. This process also depended on the polymerization of monomers produced from different sugars [48]. Catalysts 2018,8, 637 16 of 39 Table 3. Cont. Reaction Product Other Products Catalyst Reaction Conditions Main Results Reference Dehydration Hydrolysis Hydrogenation 1-hydroxy-2,5-hexanedione 5-HMF 2,5-bis-(hydroxymethyl) furan 1st step to 5-HMF: HCl 2nd step to 1-hydroxy-2,5-hexanedione: Cp*IrIII half-sandwich complexes with bipyridine ligands CIr = 3.64 mgL−1 1st step to 5-HMF: T = 130 ◦ C; C fru = 0.5 M in IPA/water CHCl = 0.05 M 2nd step to 1-hydroxy-2,5-hexanedione T = 130 ◦C C5-HMF = 0.517 M in aq. formate buffer solution (pH = 2.5) Ccat = 0.517 M t = 180 + 120 min Xfru = 71.9% YHDone = 99% TOF = 0.19 molHDone molcat−1h−1 [55] Dehydration, retro-condensation Isomerization Esterification Methyl lactate 5-HMF glyceraldehyde DHA fructosides Hierarchical Sn-βZeolite SBET = 719 m2g−1 Si/Al ratio = 12.5 CSn = 3.7% T = 160 ◦C; Cfructose = 0.15 M in methanol Ccat = 0.5% w/w PN2 = 1 MPa t = 1200 min Xfru = 100% YML = 86% TOF = 1.29 molMeLac gcat−1h−1 [57] Retro-aldol fragmentation Dehydration Acetalization isomerization Methyl lactate Glyceraldehyde, dihydroxyacetone, pyruvaldehyde, among many others reported ZnCl2 T = 200 ◦C; Cfru = 0.4% w/w in EtOH and water Ccat = 0.004 M t = 180 min YML = 52% TOF = 0.96 molMeLac molcat−1h−1 [33] Retro-aldol fragmentation Isomerization Esterification Methyl lactate Methyl levulinate and fructosides InCl3.4H2O/Bu2SnCl2and other In–Sn catalytic systems In/Sn ratio = 5 T = 160 ◦C Cfru = 0.125 M in methanol Ccat = 0.5% w/w PN2 = 0.5 MPa t = 600 min Xfru = 98% YMeLac = 72% TOF = 1.22 molMeLac molcat−1h−1 [58] Mechanism is not discussed Methyl lactate Not reported Sn-βzeolites SBET = 422 m2g−1 T = 160 ◦C Cfructose = 2.5% w/win methanol; Ccat = 1.6% w/w PN2 = 0.5 MPa t = 600 min YMeLac = 47% TOF = 0.39 molMeLac gcat−1h−1 [59] Hydrothermal Decomposition Levulinic acid 5-HMF [PrSO3HMIm] [Cl] and other ionic liquids T = 180 ◦C; Cfru = 2% w/w in water Ccat = 40% w/w t = 180 min Xfru = 100% YLevAc = 79% TOF = 0.31 molLevAc molcat−1h−1 [60] Dehydration Etherification Acetalization Hydration Ethyl levulinate 5-HMF, furfural, ethoxyfurfural Ti 0.75 TPA and other titanium exchanged heteropoly TPA T = 120 ◦C Cfru = 0.25 M in EtOH; Ccat = 2.25% w/w t = 360 min Xfru = 100% YEtLev = 63% TOF = 1.17 molEtLev gcat−1h−1 [61] Dimerization Oxidation Dicarboxylic acid monomer: 5,5’-[oxybis (methylene)]bis [2-furancarboxylic acid] 5,5’-[oxybis (methylene)]bis [2-furaldehyde] (OBFA) Dimerization: Dowex 50 W X8 Oxidation: 5% Pt/C Dimerization: T = 110 ◦C Cfru = 45% w/w in DMSO Ccat = 10% w/w. Oxidation: T = 23 ◦COBFA = 0.2 M in aq. NaOH (1.5 M) Ccat = 2% w/w; PO2 = 1 atm t = 1440 + 2880 min Xfru = 100% Ymonomer = 75% [62] C-C bond cleavage and dehydration Furfural Glucose 2,5-HMF Lactic acid Arabinose H-βzeolite Si/Al ratio = 25 Cacid = 0.366 mmol gcat−1 T = 150 ◦C Cfru = 5% w/win GBL/water Ccat = 1% w/w PN2 = 20 bar t = 60 min Xfru = 100% YFur = 64% TOF = 1.78 molFur gcat−1h−1 [36] Catalysts 2018,8, 637 17 of 39 Table 3. Cont. Reaction Product Other Products Catalyst Reaction Conditions Main Results Reference Dehydration Furfural 5-HMF Sn, Fe and Zr-Beta zeolite SBET = 539.9 m2g−1 CSn = 7.1 µmol g−1 T = 170 ◦C Cfru = 0.6% w/w in GBL/water Ccat = 2.4% w/w t = 30 min Xfru = 100% YFur = 69% TOF = 1.92 molFur gcat−1h−1 [37] Dehydration Etherification Ethoxymethyl furfural 5-HMF Ethyl levulinate Ar-SO3H-SBA-15 and other mesoporous silica SBET = 712 m2g−1 T = 116 ◦C, Cfru = 0.2 M in ethanol/DMSO (91.7:8.3 v/v) Ccat = 0.027 M t = 240 min Xfru = 100% YEMFur = 64% TOF = 1.19 molEMFur gcat−1h−1 [63] Dehydration Etherification Ethoxymethyl furfural HMF, lactic acid lignin-derived sulphated carbon SBET = 26 m2g−1 CS= 36 M T = 150 ◦C Cfru = 2% w/w in ethanol Ccat = 0.5% w/w PN2 = 20 bar t = 180 min Xfru = 100% YEMFur = 64% TOF = 3.03 molEMFur gcat−1h−1 [64] Dehydrations Hydrogenations Hydrogenolysis 2,5-DMF Fructose, 5-HMF, 5-MFA (5-methylfurfural), 2,5-BHMF (2,5-bis(hydroxymethyl)-furan; 5-MFM (5-methyl-furanmethanol) 4.8Pd/UiO-66@SGO (Pd on a Zr-based metalorganic framework deposited on sulfonated graphene oxide) SBET = 715 m2g−1 CPd = 4.8% T = 160 ◦C PH2 = 1 MPa Cfru = 0.5% mol in THF Ccat = 0.5% w/w t = 180 min Xfru = 92% YDMF = 71% TOF = 1.18 molDMF gcat−1h−1 [30] Dehydration Aldol condensation with methylisobutyl ketone (E)-1-(5-(hydroxymethyl) furan-2-yl)-5-methylhex -1-en-3-one 5-HMF Dehydration: KBr, H2SO4 Aldol condensation: NaOH Dehydration: T = 150 ◦C Cfru = 0.055 M in dioxane CKBr = 0.0375 M CH2SO4 = 0.125 M Aldol condensation: T = 55 ◦C CMIBK = 1 M in dioxane t = 1 + 180 min Xfru = 100% Yproduct = 73% TOF = 0.36 molproductr molcat−1h−1 [65] Catalysts 2018,8, 637 18 of 39 3.2. Additional Routes from Fructose to Further Products Very much like in the case of glucose, compounds of interest can be synthesized from fructose from reaction pathways not departing from dehydration, which are shown in Figure 4and detailed in Table 4. Sugar alcohol mannitol can be produced from the hydrogenation of fructose, for which copper-supported metallic nanoparticles were synthesized, where the best was a Cu catalyst supported on silica by precipitation-deposition method compared to preparation by incipient wetness method, which gave catalysts with lower activity and selectivity to mannitol [67]. Oxidation and cleavage within the molecule leading to oxalic and succinic acid have also been reported starting from fructose, for which iron supported on carbon nanotubes has been employed. Formic acid is obtained as a by-product of the reaction, also of use in many applications. These catalysts showed good reusability without much loss of efficiency [47]. The work by Rodygin et al. reported above also studied the production of a vinylized product as monomer to further polymerization, as observed in Figure 4for the case of fructose as substrate for the reaction. Using the same steps of the reaction and materials for the protection of hydroxyl moieties and vinylation, a yield of 92% was attained in the end [48]. Another example is the synthesis of lactic acid production, which is based on consecutive steps of retro-aldol fragmentation, dehydration and isomerization also using bifunctional Al(III)-Sn(II) as a catalyst, as described above for glucose valorization. When fructose was the substrate, somewhat higher yields to lactic acid of 90% were obtained compared to the 81% from glucose [40]. Finally, it is also worth briefly mentioning the synthetic approach to the production of 3-deoxy-L-fructose that Lu and Chang carried out, which consisted of a series of steps of benzoylation, bromination, dithioacetalization and final removal of the benzoyl and dithiane groups to obtain the desired product [68]. Catalysts 2018, 8, x FOR PEER REVIEW 18 of 40 3.2. Additional Routes from Fructose to Further Products Very much like in the case of glucose, compounds of interest can be synthesized from fructose from reaction pathways not departing from dehydration, which are shown in Figure 4 and detailed in Table 4. Sugar alcohol mannitol can be produced from the hydrogenation of fructose, for which coppersupported metallic nanoparticles were synthesized, where the best was a Cu catalyst supported on silica by precipitation-deposition method compared to preparation by incipient wetness method, which gave catalysts with lower activity and selectivity to mannitol [67]. Oxidation and cleavage within the molecule leading to oxalic and succinic acid have also been reported starting from fructose, for which iron supported on carbon nanotubes has been employed. Formic acid is obtained as a by-product of the reaction, also of use in many applications. These catalysts showed good reusability without much loss of efficiency [47]. The work by Rodygin et al. reported above also studied the production of a vinylized product as monomer to further polymerization, as observed in Figure 4 for the case of fructose as substrate for the reaction. Using the same steps of the reaction and materials for the protection of hydroxyl moieties and vinylation, a yield of 92% was attained in the end [48]. Another example is the synthesis of lactic acid production, which is based on consecutive steps of retro-aldol fragmentation, dehydration and isomerization also using bifunctional Al(III)-Sn(II) as a catalyst, as described above for glucose valorization. When fructose was the substrate, somewhat higher yields to lactic acid of 90% were obtained compared to the 81% from glucose [40]. Finally, it is also worth briefly mentioning the synthetic approach to the production of 3-deoxyL-fructose that Lu and Chang carried out, which consisted of a series of steps of benzoylation, bromination, dithioacetalization and final removal of the benzoyl and dithiane groups to obtain the desired product [68]. Figure 4. Reaction pathways using fructose as building block avoiding routes starting from 5-HMF. Figure 4. Reaction pathways using fructose as building block avoiding routes starting from 5-HMF. Catalysts 2018,8, 637 19 of 39 Table 4. Information in studies on the conversion of fructose to various products via alternative routes. Reaction Product Other Products Catalyst Reaction Conditions Main Results Reference Hydrogenation Mannitol Sorbitol Glucose Cu/SiO2-PD and other copper-supported metallic nanoparticles SBET = 225 m2g−1 CCu = 11.3% T = 200 ◦C PH2 = 40 bar Cfru = 0.055 M in EtOH/water Ccat = 0.5% w/w t = 360 min Xfru = 100% YMan = 78% TOF = 1.43 molmannitol gcat−1h−1 [67] Oxidation and cleavage Oxalic and Succinic acid Fructose, 2-formyl-5-furancarboxylic acid, formic acid Fe@CNT SBET = 78 m2g−1 T = 140 ◦C Cglu = 0.05 M in water Ccat = 2.5 g L−1 PO2 = 20 bar t=12h Xglu = 99% YOxAc = 46.8% YSucAc = 21% TOF = 0.003 molOxAc gcat−1h−1 [47] Acetalization (for protection) Vinylation Vinylized monomer Acetalized intermediate Acetalization: H2SO4 Vinylation: CaC2(KF, KOH) T = 130 ◦C Cfru = 0.33 M in DMSO/water CCaC2 = 1.2 M CKOH/KF = 0.22 M t = 180 min Ymonomer = 92% TOF = 1.29 molmonomer molcat−1h−1 [48] Retro-aldol fragmentation Dehydration Isomerization Lactic acid Glyceraldehyde Dihydroxyacetone Pyruvaldehyde Bifunctional Al(III)-Sn(II) catalysts T = 180 ◦C Cfru = 0.5% w/w in water pH = 2.8 Ccat = 0.005 M Al(III)/Sn(II) ratio = 1/1 t = 120 min Xfru = 100% YLA = 90% TOF = 2.50 molLacAc molcat−1h−1 [40] Catalysts 2018,8, 637 20 of 39 4. Catalytic Routes from Xylose Xylose, together with mannose, a pentose and a hexose, are the main components of hemicelluloses originating from wood. For example, in softwoods, the xylan share ranges between 3 and 8% w/wdry solid (DS), while mannans are present in a higher percentage—from 10 up to 13% w/wDS, with an overall content in hemicelluloses from 25 to 35% w/wDS [19]. Hardwoods are richer in hemicellulose fraction (24–40%), while grasses and leaves can contain as much as 85% w/wDS of hemicelluloses, with xylanand xylose-related polymers the main component of the hemicellulosic fraction in this case [ 69 ]. Hemicellulose is of amorphous nature, has a relatively low molecular weight (approx. 15 KDa) and its composition depends highly on the source, which can be very varied. They can be polymers with β -1,4-linked backbones with an equatorial configuration at C 1 and C 4 , which can be xylans, mannans and glucomannans, xyloglucans, and β -1,3;1,4-glucans, but also axially-directed β -1,4-galactans and arabinogalactans with a β -1,3-linked galactose backbone [ 69 ]. After being obtained and purified from hemicelluloses, xylose is a platform chemical that can be dehydrated to furfural, which is in turn an intermediate to a plethora of chemicals, whose exploitation can be conducted using several catalysts, as compiled in Tables 5and 6. This pentose can also be transformed directly to levulinic acid, furfural alcohol and more chemicals. All these pathways are summarized in Figure 5. Catalysts 2018, 8, x FOR PEER REVIEW 20 of 40 4. Catalytic Routes from Xylose Xylose, together with mannose, a pentose and a hexose, are the main components of hemicelluloses originating from wood. For example, in softwoods, the xylan share ranges between 3 and 8% w/w dry solid (DS), while mannans are present in a higher percentage—from 10 up to 13% w/w DS, with an overall content in hemicelluloses from 25 to 35% w/w DS [19]. Hardwoods are richer in hemicellulose fraction (24–40%), while grasses and leaves can contain as much as 85% w/w DS of hemicelluloses, with xylanand xylose-related polymers the main component of the hemicellulosic fraction in this case [69]. Hemicellulose is of amorphous nature, has a relatively low molecular weight (approx. 15 KDa) and its composition depends highly on the source, which can be very varied. They can be polymers with β-1,4-linked backbones with an equatorial configuration at C1 and C4, which can be xylans, mannans and glucomannans, xyloglucans, and β-1,3;1,4-glucans, but also axiallydirected β-1,4-galactans and arabinogalactans with a β-1,3-linked galactose backbone [69]. After being obtained and purified from hemicelluloses, xylose is a platform chemical that can be dehydrated to furfural, which is in turn an intermediate to a plethora of chemicals, whose exploitation can be conducted using several catalysts, as compiled in Tables 5 and 6. This pentose can also be transformed directly to levulinic acid, furfural alcohol and more chemicals. All these pathways are summarized in Figure 5. Figure 5. Reaction routes from xylose via furfural and direct routes from the pentose. Xylose Dehydratation Xylulose Furfural O OH OH OH OH O O O OH OH OH OH Anhydroxylose O OH OH O Dehydratation + reduction Furfuryl alcohol O OH Hydrogenation Retro aldol condensation OH OH OH OH OH Xylitol OH O CH 3 OH Esterification Lactic acid OH O CH 3 OCH 3 Methyl lactate Oxidation OH OH OH OH OH O Xylonic acid Oxidation OH OH OH OH OH O O Xylaric acid Dehydratation Dehydratation Reduction Hydrogenation Ring opening CH 3 OH O 1-hydroxyl-2-pentanone Hydrogenation CH 3 OH OH 1,2-pentanediol Hydrogenation Ring opening O OH OH O CH 3 trans-2,5-dihydroxy-3-pentenoic acid methyl ester (DPM) Transfer hydrogenation γ-valerolactone OO CH 3 Hydration Ring opening CH 3 OH O O Levulinic acid Transfer hydrogenation CH 3 O OH O R 1 4-hydroxypentanoates Figure 5. Reaction routes from xylose via furfural and direct routes from the pentose. Catalysts 2018,8, 637 21 of 39 Table 5. Information contained in papers on the dehydration of xylose to furfural in liquid-liquid systems (catalyst dissolved in one liquid phase) or liquid (homogeneous) systems. Reaction Product Other Products Catalyst Reaction Conditions Main Results Reference Dehydratation Resinification or self-polymerization Furfural polyfurfural Terephthalic acid (TPA) T = 190 ◦C Cxyl = 8.9% w/win H2O Vtoluene/VH2O = 2 Ccat = 0.5% w/v t = 180 min Xxyl = 92% YFur = 72% 100% stable for 5 cycles TOF = 0.05 molFur gcat−1h−1 [70] Dehydration Polymerization Furfural From xylan: Humins Monosaccharides From xylose: Not indicated (Low conc.) Ionic liquid [Choline-SO4H][CF3SO3] T = 120 ◦C Cxyl = 40% in 1,4-dioxane with 2% H2O (Cxylan identical) Ccat = 2% w/v From xylan t = 360 min Xxyl = 64% YFur = 62.4% TOF = 2.3·10−3molFur gcat−1h−1 From xylose t = 600 min Xxyl = 99.5% YFur = 91.5% TOF = 3.36 · 10 −3 mol Fur g cat−1 h −1 [71] Dehydration Furfural From xylose: xylulose CrPO4 T = 160 ◦C Cxyl = 10% w/vin H2O Vtoluene/VH2O = 3 CNaCl = 35% w/wH2O Ccat = 1.5% w/wH2O t = 60 min Xxyl = 98% YFur = 88% TOF = 0.094 molFur gcat−1h−1 [72] Dehydration Polymerization Resinification Furfural Humins Furfural polymers HCl T = 222 ◦C Cxyl = 10% w/vin H2O VGVL/VH2O = 4 Ccat = 5 mM Tubular reactor Liquid system t=96s Xxyl = 93% YFur = 82% TOF = 8.57 molFur mmolcat−1h−1 Maple: t = 30 min Xxyl = 93% YFur = 82% TOF = 0.084 molFur mmolcat−1h−1 [73] Catalysts 2018,8, 637 22 of 39 Table 6. Information contained in studies on the dehydration of xylose to furfural in solid-liquid systems (solid catalysts). Reaction Product Other Products Catalyst Reaction Conditions Main Results Reference Dehydratation Isomerization Furfural xylulose Cr-MOF with Sn phosphate nanoparticles SBET = 1000–2820 m2g−1 Cacid = 0.41–0.62 mmol gcat−1 T = 150 ◦C Cxyl = 10% w/win H2O Vtoluene/VH2O = 7/3 Ccat = 3% w/vH2O CNaCl = 70 ppt t = 180 min Xxyl = 97% YFur = 95% 100% stable up to 10 cycles TOF = 0.022 molFur gcat−1h−1 [74] Dehydratation Polymerization Furfural humins Amberlyts 70 M-20 ZSM-5-30 Cacid = 0.42–4.15 mmol gcat−1 T = 150 ◦C Cxyl = 10% w/win H2O PCO2 = 20 MPa QCO2 = 0.94 g min−1 Ccat = 10% w/vH2O Amberlyst 70 t=16h Xxyl = 91.4% YFur = 50.5% TOF = 1.24 ×10−3molFur gcat−1h−1 [75] Dehydration Polymerization Furfural Humins SO3H-KIT-6 SBET = 265 m2g−1 Cacid = 0.69–1.53 mmol gcat−1 T = 170 ◦C Cxyl = 4% w/vin H2O Ccat = 25% w/wH2O t = 120 min Xxyl = 97.5% YFur = 94.7% TOF = 1.99 ×10−4molFur gcat−1h−1 [76] Dehydration Polymerization Resinification Furfural Humins Anhydroxylose NbTiO-MNL SBET = 145 m2g−1 Cacid = 0.69–1.53 mmol gcat−1 T = 130 ◦C Cxyl = 20 mM VGVL/VH2O = 9 Wcat = 280 mg tresidence = 106 s Xxyl = 98% YFur = 29% TOF = 7.26 molFur gcat−1h−1 [77] Dehydration Polymerization Furfural Humins FDU and SBA mesoporous catalysts SBET = 500–900 m2g−1 Cacid = 0.07–0.53 mmol gcat−1 T = 160 ◦C PN2 = 2 MPa Cxyl = 5% w/vin H2O Vtoluene/VH2O = 2/1 Ccat = 2.5% w/vH2O FDU-5-7.5E-SO3H t = 240 min Xxyl = 96.81% YFur = 78.55% TOF = 5.13 ×10−3molFur gcat−1h−1 [78] Dehydration Furfural Not indicated Sulfonated graphitic carbon nitrides SBET = 10–35 m2g−1 Cacid = 5.47 mmol gcat−1 T = 100 ◦C Wxyl = 30 mg in H2O Wcat = 25 mg t = 30 min Xxyl = 100% YFur = 96% TOF = 5.36 ×10−3gFur gcat−1h−1 [52] Dehydration Furfural Not indicated Sulfonated active carbons (CA) SBET = 620–750 m2g−1 Cacid = 0.43–0.90 mmol gcat−1 T = 180 ◦C Cxyl = 1.4% w/vin H2O Ccat = 0.14% w/wH2O KOH-activated CA t = 180 min Xxyl = 95% YFur = 60% TOF = 0.0152 molFur gcat−1h−1 [79] Dehydration Esterification Reduction Ring opening Translocation Hydrogenation Furfural Xylose ethers Furfuryl alcohol Lactones GVL Levulinic acid Hydroxy-pentanoates Zr-USY zeolites with several Al/Zr ratios SBET = 308–418 m2g−1 Cacid = 0.137–0.650 mmol gcat−1 T = 170 ◦C Cxyl = 1 mol to 50 mol 2-propanol Ccat = 1% w/v Parent H-USY t = 180 min Xxyl = 100% YFur = 40% TOF = 3.48 ×10−3molFur gcat−1h−1 t = 60 min Xxyl = 80% YXylethers = 70% TOF = 8.21 ×10−3molXylethers gcat−1h−1 [80] Catalysts 2018,8, 637 23 of 39 4.1. Dehydration of Xylose to Furfural Furfural is a key building block to produce several fuels and chemicals (lactones, cetones, levulinic acid, etc.) that can derive from xylose making use of both homogeneous and heterogeneous catalysis. To overcome the shortcomings of homogeneous systems using classic mineral Brønsted acids, the use of heterogeneous catalytic systems (liquid-liquid and solid-liquid) has been deeply analyzed in recent decades [16] with the potential capability of more easily recycling the catalysts. When using liquid-liquid systems, furfural is usually recovered in the organic solvent, while the catalyst and the remaining xylose remains in the aqueous phase. Recently, terephthalic acid has been employed recently as a soluble catalyst for the production of furfural with high yields (up to 72–74%), using aqueous solutions of the xylose substrate suspended in up to two volumes of toluene. In this way, this acid that can be obtained from inexpensive PET, can be used up to five times with no reduction in activity (even a slight activation up to 5% is observed), as furfural is extracted mostly in the toluene phase during each cycle, adding more xylose to the aqueous phase for the next catalytic cycle [ 70 ]. In the work of Hui et al., several new SO 4 H-functionalized ionic liquids (SFILs) were synthetized and applied to the production of furfural from xylose, showing [Ch-SO 4 H][CF 3 SO 3 ] with the highest activity. This liquid catalyst can be easily separated from the aqueous phase simply by decantation, as it shows no miscibility with the xylose-containing phase. For recycling, the SFIL can undergo treatment at 80 ◦ C under vacuum to remove all volatile impurities and be subsequently used for the next catalytic cycle. Up to five cycles were performed to test the stability of the catalyst, resulting in a slight 5% decrease both in xylose conversion (reduced from 98 to 92%) and furfural yield (that dropped from 95 to 90%). The catalyst was also directly applied to xylan, with a reduction from 80 to 60% in the yield to furfural [ 71 ]. A third liquid-liquid catalytic system was designed to convert either xylose or wheat straw by the action of the Brønsted and Lewis acidity of chromium phosphate. Among several salts tested, this catalyst was able to yield 88% furfural with total conversion of xylose, at 160 ◦ C after 60 min, using toluene as the best hydrophobic solvent out of those tested. Again, worse results are achieved if biomass (wheat straw) is directly used: at 180 ◦ C during 90 min, the yield to furfural was 67% and to 5-HMF, 32%. Unlike with the previous catalysts, this one proved to be unstable: the activity towards xylose decreased 10% during 4 cycles, but the yield to furfural sharply decreased from 90 to 50%. A possible reason is the precipitation of CrPO4during each reaction cycle [72]. Further work in liquid systems at high temperatures is presented by Sener et al., where the production of furfural from concentrated aqueous solutions of xylose is approached. These authors used the classic catalyst HCl, although using γ -valerolactone as the main solvent in a mixture with water (80:20 v/v). In this case, temperatures used ranged from 200 to 220 ◦ C, where both solvents show miscibility and the concentration of the catalyst and the substrates were 1–10 mM HCl, 2–10% w/wxylose in a tubular reactor. In the best conditions, yields up to 85% in furfural and quantitative conversions of xylose are achieved in only 90–100 s residence time. Results were used to fit a power-law kinetic model comprised of the dehydration reaction xylose to furfural and side reactions from both chemicals to polymers, which are regarded as by-products. The approach is extended to a biomass rich in xylan (maple wood), reaching even better results using acid hydrolysates from this source treated at 222 ◦ C for 135 s (97% xylan oligomers conversion and 93% furfural yield). Therefore, the selection of the solvent and the temperature seem critical to reach high furfural yields at total xylose/xylan conversion [73]. The use of solid catalysts is investigated on a regular basis, comprising catalysts with Brønsted acidity as well as others with dual Brønsted-Lewis acidity. Of the latter type, Chartterjee et al. recently developed and tested metal organic frames (MOFs) with phosphate groups and Sn and Cr atoms, containing Brønsted and Lewis acidic sites, respectively. Starting with MIL-101, a MOF with Cr atoms, the addition of tin phosphate nanoparticles stabilized the catalyst (no change in activity in 10 cycles), while only 70 ppt of NaCl served to increase xylose conversion to 99% and furfural yield to 95% [ 74 ]. The presence of tin phosphate nanoparticles resulted in wider pores (mesopore structure) and avoided the formation of coke and humins deposits. Again, a hydrophobic phase, toluene, was used to extract Catalysts 2018,8, 637 24 of 39 furfural as it was obtained. Sato et al. have just published work on the application of supercritical CO 2 for such purpose. In their paper, a thermostable sulphonic acid resin (Amberlyst 70) and two zeolites (M-20 and ZSM-5-30) were tested, reaching xylose conversions up to 91% and furfural yields up to 51% [ 75 ]. The catalysts tested were also very active for polymerization due to their very strong acidity. The mesoporous silica KIT-6 has recently been functionalized with sulphonic groups to render a catalyst able to efficiently convert xylose into furfural. Several catalysts were synthetized by changing the molar ratio between 3-mercaptopropyl(methyl)dimethoxysilane (MPMDMS) and tetraethoxysilane (TEOS). At 170 ◦ C, the best catalyst (molar ratio TEOS-MPMDMS 8/2) could transform 97.5% of the pentose, achieving yields up to 94.6% furfural. However, while the capacity of the 0.2-SO 3 H-KIT-6 was maintained for four cycles, its selectivity to furfural dramatically decreased from 95 to 65%, with an increasing impact of coking side reactions [ 76 ]. Moreno-Marrodan et al. created several macroporous-mesoporous catalysts based on titania and niobia-titania, again with acid sites of the Brønsted and Lewis kind. These monolithic catalysts were prepared for and tested in continuous flow-through systems (fixed bed reactors), showing high stability on-stream for up to 30 h. However, selectivity to furfural was low (25%), while xylose conversion was stable and high (95–100%) when working at 130 ◦ C in a liquid-liquid-solid fixed bed reactor with γ -valerolactone and water mixed in a ratio 9/1 v/v[ 77 ]. To facilitate mass transfer within the porous structure and flow-through processing, another strategy followed was to create packed structures by liquid phase synthesis and evaporation-induced-self-assembly (EISA) methods. Hu et al. obtained by these means of several mesoporous sulfonated catalysts of the FDU and SBA type. Their best catalyst (FDU-5-7.5E-SO 3 H) was able to convert almost all xylose (97%), rendering up to 81% yield to furfural at 160 ◦ C. However, again, on successive reaction cycles, the yield to furfural decreased down to 60% after the fifth cycle [78]. By pyrolysis of urea, Verma et al. created several sulfonated graphitic carbon nitride (Sg-CN) catalysts, and tested them with xylose using several solvents and temperatures. Their best result was achieved using water, 100 ◦ C, and 30 min: a yield to furfural of 96% was reached. The same authors, using these catalysts, obtained up to 84% yield to benzimidazole derivatives (an important intermediate for drug synthesis) from xylose, 5-HMF from fructose (96%), and levulinic acid from glucose (41%) [ 52 ]. Lin et al. have also followed this strategy of using renewable materials to obtain catalyst by pyrolysis. In this case, polymeric by-products from the dehydration of xylose and hemicelluloses to furfural were pyrolyzed and activated by several means (non-activated, KOH, ZnCl 2 with further sulfonation in all cases). Activation with the hydroxide or the salt resulted in wider pores and a different distribution of carboxyl and sulfonic groups on the surface of the active carbons. All catalysts were active for the dehydration of xylose to furfural, reaching product yields of 60% and xylose conversions up to 95%. When used directly on hemicellulose, yields to xylooligosaccharides (XOS) were as high as 60%, while furfural yields could reach 40% [79]. Apart from the undesired side reactions to humins and furfural polymers, some catalysts can promote not only furfural formation but also its conversion into furfuryl alcohol, levulinic acid, lactones—including γ -valerolactone—and hydroxypentanoates to name a few relevant low molecular weight compounds. This is the case of the work by Lopez-Aguado et al., where the synthesis and test of several zeolytic catalysts of the type Zr-USY is reported. The ratios of Zr to Al are modified, influencing the distribution of products from xylose using 2-propanol as a solvent, with maximum yields to furfural of 40%, xylose ethers up to 60% and GVL ( γ -valerolactone) up to 5%. To increase yields to GVL, furfural was employed as the original reactant, reaching over 13% yield GVL [80]. 4.2. Direct Reactions of Xylose to Alcohols, Acids and Polymers Though furfural is a key intermediate to several products from xylose, some chemicals can be reached without the presence of furfural or, at least, without its build-up in the liquid phase, thus increasing the yields of such chemicals. Results are displayed in Table 7. In the case of furfuryl alcohol, which is the main monomer of furanic resins, the combination of sulfonic groups and Pt active phases on SBA-15 has allowed the direct production of the alcohol Catalysts 2018,8, 637 25 of 39 from xylose with selectivities ranging from 83 to 87%, although only at 20% xylose conversion. The unmodified SBA-Pt catalyst, however, mainly yielded xylitol (45% selectivity) in similar conditions. The main problem encountered by Canhaci and coworkers in this case was the extensive leaching of sulfonic groups during stability studies. This in turn resulted in an increased yield of xylitol and furfural and a very high reduction of furfuryl alcohol yield in just four cycles [81]. A dual catalyst was created and applied to also obtain 1,2-pentanediol, the building block for propiconazole (DMI)—a triazole fungicide—from xylose. In this work, Wang and coworkers used Ru/C as a hydrogenation catalyst and niobium phosphate as an acid catalyst. In the best conditions, 21.27% combined yield for 1,2-propanediol and 1-hydroxyl-2-pentanone was reached in a water/GVL/cyclohexane biphasic L-L system. Most importantly, the authors showed that a high Lewis acidity is critical to obtain a high selectivity to 1,2-propanediol in hydrogenating conditions, reducing the selectivity towards xylitol [82]. Xylitol is the main product from the hydrogenation of furfural and a main ingredient in the food industry, as a sweetening agent. Recently, Morales and coworkers synthesized highly dispersed NiO catalysts out of mixed oxide precursors (from Ce and Ni) with reduced leaching in aqueous medium. In all cases, the selectivity to xylitol was 50–60%, with xylulose, glycerol and ethylene glycol being the main by-products [83]. Its catalytic production has been deeply reviewed recently [84]. Lactic acid (LA) is a platform chemical usually obtained via anaerobic or microaerobic fermentation of glucose by Lactobacilli. To increase productivities, working with more concentrated feedstock, the catalytic strategy could be an alternative. Liu et al. have recently proved that pyrolytic sugars (levoglucosan, glucose and xylose) can be converted in acceptable yields to LA (75, 74 and 61% based on C content) using lanthanum triflate, a Lewis acid catalyst [ 85 ]. This approach can be successfully extended to cellulose (73% C yield). Further proof of this strategy is encountered in the work by Li et al., which focused on the conversion of hexoses (from cellulose) and pentoses (from hemicellulose) to methyl lactate (MeLac) in near-supercritical methanol and in the presence of several metal chloride catalysts [ 86 ]. In the case of glucose, the best yield to MLA (47%) is obtained with LaCl 3 , a catalyst that also renders good MLA yields from fructose (64%) and xylose (33%). Xylaric acid is an intermediate to glutaric acid and, thus, to PVC, resins, and synthetic rubbers, as well as several fine chemicals and agrochemicals. Therefore, xylaric acid has unquestionably been identified as one of the ten key platform chemicals from biomass. Very recently, Sabuda and Saha studied several commercial precious metals on carbon catalysts for the oxidation of xylose to xylaric acid in water, finding that Pt/C is the most active catalyst. High oxygen pressures and catalyst amounts facilitate deeper oxidation, hence reducing the presence of the intermediate xylonic acid and increasing the yield to xylaric acid, the end product [87]. Finally, like other pentoses, xylose can be employed as a monomer or as a source of monomers. Starting from xylose and using the catalytic activity of tin-containing silicates, Elliot et al. have created a new monomer for functional polyesters: trans-2,5-dihydroxy-3-pentenoic acid methyl ester (DPM). With Sn-Beta as the best catalyst and under optimized conditions, the authors reached a 33% yield to DPM [ 88 ]. Moreover, Lopez-Vidal et al. derived new xanthate (XAN) and thionocarbonate (TOC) cyclic monomers from 2-deoxy-D-ribose and D-xylose. A subsequent ring-opening polymerization (ROP) of these monomers rendered polycarbonates. To reach the monomers, CS 2 and CO 2 reacted with the sugars through a cyclocarbonation process, and further reaction with mesyl chloride with trimethylamine resulted in the cyclic xanthate [89]. Catalysts 2018,8, 637 32 of 39 Table 9. Summary of the details of works found in literature dealing with the transformation of micro and macroalgal biomass. Reaction and Biomass Source Product Other Products Catalyst Reaction Conditions Main Results Reference Acid hydrolysis Nanochloropsis salina Monosaccharides Formic acid Levulinic acid 5-HMF Furfural HCl H2SO4 T = 90 ◦C Cbiomass = 10% w/vin 10 mL H2O Ccat = 10% w/w t = 60 min Ymal = 90% TOF = 0.6 gmalt molcat−1h−1 [95] Acid hydrolysis Chlorella vulgaris Malodextrin H2SO4 T = 90 ◦C Ccarboh = 37.3% Ccat = 0.56 M t=5h Ymonosac = 243 mg/g TOF = 0.052 molsugar gcat−1h−1 [96] Acid hydrolysis Chlorella sp. and Nanochloropsis gaditana Levulinic ester H2SO4 T = 130 ◦C Cbiomass = 38 g/L alcohol Cglucose = 6.8–28.1% w/w Ccat = 15% w/v t=2h YLevulinate = 40% TOF = 0.0252 mollev molcat−1h−1 [97] Chemo-enzymatic hydrolysis Dunaliella tertiolecta Glucose H2SO4 α-amylase and α-glucosidase T = 50–90 ◦C Cpolyssac = 0.1–1 g/20–100 mL Ccat = 0.5–1.5% Acid catalyst T = 37 ◦C t=24h YMF = 90% TOF = 0.136 molgluc molcat−1h−1[98] Chemo-enzymatic hydrolysis Dunaliella tertiolecta Glucose Ethanol HCl and H2SO4 Saccharomyces cerevisiae T = 121 ◦C Cbiomass = 5% w/v Ccat = 0.05–1M Acid catalyst Cenzyme = 0.1–1.0 mL/g T = 35–55 ◦C pH = 3.5–6.5 t = 15 min Ysugar = 42.0% Csugar = 21 mg/mL Cethanol = 0.44 g/g glucose TOF = 0.467 molglu molcat−1h−1 [99] Chemo-enzymatic hydrolysis Gracilaria verrucosa Glucose Galactose 3,6-anhydro Galactose Levilinic acid 5-HMF Ethanol HCl H2SO4 Cellic Ctec2 T = 125 ◦C Cbiomass = 2% w/w Ccat = 0.01–1.5 N Acid catalyst Cenzyme = 150 FPU/mL T = 50 ◦C pH = 5 t = 60 min Xcarbohyd = 57.2% Ysugar = 21.3–37.4% TOF = 0.039 molmonosac molcat−1 h−1 [100] Chemo-enzymatic hydrolysis Scenedesmus sp. Monosaccharides Ethanol HCl Viscozyme L T = 121 ◦C Cbiomass = 2% w/v Ccat = 0.5 M Acid catalyst Cenzyme = 20 FBGU/g biomass T = 45 ◦C pH = 5.5 t = 45 min Ysugar = 37.9% (HCl) TOF = 0.098 mol monosac mol cat−1 h −1 t=72h Ysugar = 43.4% (Enz.) [101] Enzymatic hydrolysis Great Salt lake USU080 Lactic acid Lactobacillus casei 12A T = 37 ◦C Cbiomass = 15% w/v Cenzyme = 1% v/v 200–250 rpm t = 3–24 h Ylactic = 11.7g/L [102] Enzymatic hydrolysis Nanochloropsis oceanic 2,3-Butanediol Klebsiella oxytoca T = 37 ◦C Cbiomass = 15% w/v Csugar = 5 g/L 150 rpm t = 6–7 h Y2,3-BDO = 0.31 g/g sugars 0.0031 mol2,3-BDO L−1h−1 [103] Catalysts 2018,8, 637 33 of 39 Table 9. Cont. Reaction and Biomass Source Product Other Products Catalyst Reaction Conditions Main Results Reference Enzymatic hydrolysis Scenedesmus obliquus Monosaccharides (Glucose and xylose) Organic acids Celluclast1.5L Novozyme 188 Alkaline-peroxide pretreatment T = 50 ◦C Cbiomass = 6% w/w Cenzymes = 10FPU/g and 20CBU/g pH = 4.9 300 rpm t = 6–7 h Ysugrs = 0.098 g/g biomass [104] Catalytic-hydrothermal process Kappaphycus alvarezzi Glucose Galactose Levulinic acid 5-HMF Furfural H2SO4 T = 160–175 ◦C Cbiomass = 2 g/30 mL Ccat = 1% w/w t = 20 min Ymonosac = 14.5 g/L TOF = 0.364 molmonosac gcat−1h−1 [105] Catalytic-hydrothermal process Enteromorpha intestinalis Glucose Galactose Xylose Mannose Levulinic acid 5-HMF Furfural H2SO4T = 156 ◦C Ccat = 1.3% w/w t = 11 min Ymonosac = 28.6% [106] Catalytic-hydrothermal process Alginate from macroalgae and cellulose H+medium: Furfural, Mannuronic, Guluronic acids -OH medium: Lactic, Fumaric, Malic acids HCl NaOH T = 150 ◦C Calginate = 20 g/L pH = 1–13 t = 30 min Ymonomers = 43% in acid medium TOF = 0.955 molmonom molcat−1h−1 [107] Catalytic-hydrothermal liquefaction process Chlorella vulgaris Bi-oil Cyclic ketones Lactones Furans Phenols H2SO4 CH3CO2H T = 220–330 ◦C Cbiomass = 100 g/L in H2O Ccat = 0.01–0.1 mol/L t = 30 min Yproducts = 16% [108] Catalysts 2018,8, 637 34 of 39 7. Conclusions and Future Perspective Several catalytic strategies are presently being developed from biomass. A first approach involves pyrolysis or gasification to liquid and gases that are further separated and transformed into chemicals. Secondly, catalysis can be directly applied to plant or algal biomass or its main polymers, namely, starch, cellulose, lignin, and hemicelluloses. A final strategy is the development of catalytic routes from the monomers constituting these polymers, such as glucose, fructose, xylose, and mannose, among others. Concerning the two latter strategies, there has been an overwhelming amount of scientific and technical information reported in recent decades. Nevertheless, several hindrances remain, in particular the presence of a considerable variety and concentration of by-products, due to the presence of several side-reactions and the decomposition or further transformation of target products. In these regards, work on glucose or xylose to furans by dehydration using mixed Brønsted and Lewis catalysts is reaching very high yields to main products either directly from these monomers and, more recently, from cellulose and, most evidently, from xylose-rich hemicelluloses. It is worthwhile highlighting that the most adequate results are obtained when combining reaction with separation techniques, mainly extraction, to avoid further degradation of 5-HMF or furfural, typically in liquid-liquid or solid-liquid-liquid systems. For solid catalysis, epimerization, isomerization and hydrogenation combinations in in-series fixed bed reactors show the adequate way to combine transformations and reach higher yields to the products of interest. Therefore, challenges posed by selectivity (low yields) still remain for several products that need deep transformation of the C5 and C6 reactants, requiring more specific catalysts that reduce the impact of side-reactions. In the event that by-products were of interest in these cases, the combination of reaction and/or separation operations would be of interest for use here. In several routes, the stability and/or the less-studied regeneration capacity of the catalyst should be improved to gain economic feasibility in scale-up of processes. Finally, in recent years, researchers have turned their attention to other plentiful biomass feedstock: macroand microalgae. Though the application of catalysis to these is only starting, the knowledge gained with starchy and lignocellulosic biomass is being applied and several platform chemicals (galactose, glucose) and target products (lactic acid, ketones, lactones, and furans) are being produced. Author Contributions: J.E., P.Y., and M.L. contributed equally to this work. Funding: Funding from the Spanish Ministry of Science, Innovation and Universities (MISIU) through grant CTQ-2017-84963-C2-1-R is gratefully acknowledged. 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