Condensation of Î±-Carbonyl Aldehydes Leads to the Formation of

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Condensation of α‑Carbonyl Aldehydes Leads to the Formation of Solid Humins during the Hydrothermal Degradation of Carbohydrates Ning Shi,*,† Qiying Liu,‡,§,∥ Rongmei Ju,† Xiong He,† Yulan Zhang,† Shiyun Tang,† and Longlong Ma‡,§,∥ †

School of Chemical Engineering, Guizhou Institute of Technology, Guiyang 550003, P. R. China Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, P. R. China § CAS Key Laboratory of Renewable Energy, Guangzhou 510640, P. R. China ∥ Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, P. R. China ACS Omega 2019.4:7330-7343. Downloaded from pubs.acs.org by 185.89.101.31 on 04/23/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: Catalytic hydrothermal conversion of carbohydrates could provide a series of versatile valuable platform chemicals, but the formation of solid humins greatly decreased the efficiency of the process. Herein, by studying the hydrothermal degradation behavior and analyzing the degradation paths of kinds of model compounds including carbohydrates, furan compounds, cyclic ketone derivatives, and some simple short carbon-chain oxy-organics, we demonstrate that α-carbonyl aldehydes and α-carbonyl acids are the key primary precursors for humin formation during the hydrothermal conversion process. Then, we analyzed the hydrothermal degradation paths of two simple α-carbonyl aldehydes including glyoxal and pyruvaldehyde and found that the α-carbonyl aldehydes could undergo aldol condensation followed by acetal cyclization and dehydration to form solid humins rich of furan ring structure or undergo Cannizaro route (hydration followed by 1,2-hydride shift) to form corresponding α-hydroxy acids. On the basis of the hydrothermal behavior of the αcarbonyl aldehydes, we mapped the hydrothermal degradation routes of carbohydrates (glucose, fructose, and xylose) and illuminated the formation details of α-carbonyl aldehydes, α-hydroxy acids, γ-lactones, furfural derivatives, and humins. Finally, we deduced the typical structure fragments of humins from three α-carbonyl aldehydes of pyruvaldehyde, 2,5-dioxo-6-hydroxyhexanal, and 3-deoxyglucosone, all of which could be formed during the hydrothermal degradation of hexose.

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INTRODUCTION

production via catalytic hydrothermal conversion of the abundant lignocellulosic biomass. Combination of the analysis methods of elemental analysis, Fourier transform infrared, 13C solid-state NMR, and pyrolysis−gas chromatography (GC)−mass spectrometry, multiple research studies have confirmed that humins are furan-rich polymer network containing different oxygen functional groups.15−19 On the basis of the structure of humins, several pathways of formation of humins from carbohydrates and furan compounds have also been proposed. Sumerskii et al. proposed that etherification/acetalization of HMF and polycondensation of furfural through electrophilic substitution all could lead to the formation of humins.17 Dee and Bell proposed that humins are formed through the acetalization of furfural/HMF with carbohydrates.20 Patil et al. proposed that aldol condensation of HMF with 2,5-dioxo-6hydroxy-hexanal, one kind of α-carbonyl aldehyde formed by

Polysaccharides including cellulose and hemicellulose are the main components of the abundant lignocellulosic biomass; thus, the conversion of these polysaccharides into valuable platform chemicals and liquid fuels plays a critical role for the biorefinery process. The catalytic hydrothermal conversion process is one efficient way to convert the abundant biomass into value-added chemicals and liquid fuels. During the catalytic hydrothermal conversion process, cellulose and hemicelluloses in biomass are hydrolyzed into monosaccharides, which could be further transformed into valuable platform chemicals such as 5-hydroxymethylfurfural (HMF),1,2 furfural, levulinic acid,3,4 dihydroxyacetone, glyceraldehydes, pyruvaldehyde, and lactic acid.5−7 Unfortunately, kinds of coal-like black solid residue, called humins or hydrothermal char,8−12 are formed during the hydrothermal conversion of carbohydrates, which leads to a considerable loss of the sugar feed and strongly decreases the efficiency of the biorefinery process.2,13,14 Therefore, studying the humin formation mechanism is of great significance for value-added platform chemical © 2019 American Chemical Society

Received: February 22, 2019 Accepted: April 3, 2019 Published: April 23, 2019 7330

DOI: 10.1021/acsomega.9b00508 ACS Omega 2019, 4, 7330−7343

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Table 1. Hydrothermal Degradation Behavior of Carbohydrates and Their Degradation Productsa

a

Reaction condition: 30 mL of H2O, 493 K, 5 h. b0.12 mol carbon atoms contained in raw material. cNot detected. dNot measured.

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RESULTS AND DISCUSSION Degradation Behavior of Carbohydrates and Their Derivatives. Because the reaction temperature, reaction time, and the concentration of feedstock all could greatly influence the formation of humins, we conducted all the hydrothermal degradation of all model compounds under temperature of 493 K for 5 h, employing 0.12 mol carbon atoms in the feedstock. Glucose, fructose, rhamnose, xylose, and dihydroxyacetone are five representative sugars, whereas HMF, 5-methylfurfural, furfural, and pyruvaldehyde are the dehydration products of glucose, rhamnose, xylose, and dihydroxyacetone, respectively;24−26 so, we first studied the hydrothermal degradation behavior of these nine chemicals. After hydrothermal treatment at high temperature of 493 K for 5 h, the conversions of all of these compounds have reached 100% except for 5-methylfurfural (62%) and furfural (74%), suggesting that these two compounds are relatively stable under hydrothermal condition compared to other studied compounds (Table 1, entries 1−9). Glucose, fructose, and HMF yielded over 50% of solid humins, suggesting that these three chemicals are more favor to generate humins. On the contrary, below 25% of humins is generated from 5-methylfurfural (12.1%) and furfural (23.4%), much less than the other compounds (over 35%), indicating that these two compounds are less favor to form humins. It should be noted that the humins generated from rhamnose

hydrolytic ring-opening of HMF, is the initial step for humin formation from glucose and HMF.21,22 van Zandvoort proposed that humins are formed by two main routes of nucleophilic attack of the furan ring on the carbonyl group of HMF and condensation reactions between HMF molecules.15 Recently, Cheng et al. proposed that aldol condensation of carbohydrates with furfural/HMF is the key initial step for humins formation.23 In conclusion, although advances have been achieved, the humins formation mechanism has not yet been unequivocally established and still needs to be deeply explored. Here, we studied the hydrothermal degradation of 37 model compounds including carbohydrates, furan derivatives, cyclic ketones, carboxylic acids, and simple short-chain oxy-organics containing 2−4 carbon atoms and found that solid humins could only be formed from these compounds which can generate α-carbonyl aldehydes or α-carbonyl acids during the hydrothermal degradation process. On the basis of the discovery, we proposed that the humins are formed by aldol condensation of the α-carbonyl aldehydes (acids), followed by acetalization and dehydration. At last, we analyzed the decomposition paths of glucose and confirmed that six kinds of α-carbonyl aldehydes could be easily formed from glucose, which is responsible for the high yield of humins formation during the hydrothermal degradation of glucose. 7331


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Scheme 1. Proposed Paths for Gluconic Acid To Form Aldehydes and Furfuryl Alcohol3,33−35

stable compared to the carbohydrates and furfural derivatives. No solid humins are formed from these carboxylic acids, suggesting that these carboxylic acids are not the key intermediates for humins formation. Hydrogenation of glucose can form sorbitol,29 whereas oxygenation of glucose can produce gluconic acid and glucolactone;30,31 so we further studied the hydrothermal behavior of sorbitol and glucolactone. Interestingly, no humins were formed from sorbitol, and only 17.4% of humins were formed from gluconic acid, indicating that these two compounds are disfavored to form humins compared with glucose (Table 1, entries 13,14). It is well known that sorbitol is lack of aldehyde group, and hydrothermal dehydration of sorbitol could hardly forms aldehydes or ketones;32 thus, we suspected that the aldehyde group in the glucose plays a critical role in the formation of humins for glucose. In contrast to sorbitol, gluconic acid can form unsaturated aldehydes and ketones under hydrothermal conditions.3 As has been shown in Scheme 1, because of the strong electronic absorption property of carboxylic group, gluconic acid can undergo β-elimination

(35.2%) and xylose (46.4%) were higher than that of 5methylfurfural (12.1%) and furfural (23.4%) (entries 3,4 and entries 7,8), suggesting that rhamnose and xylose may form humins through other routes in addition to 5-methylfurfural and furfural. Dihydroxyacetone and pyruvaldehyde, both of which are oxygenates with 3 carbon atoms in the molecular, all could yield around 45% of solid humins. This is the first time to report that compounds with carbon chain of below 5 could also generate humins under hydrothermal condition. Around 10−20% of soluble polymers and less than 20% of carboxylic acids are also formed during the hydrothermal process, except for pyruvaldehyde which formed 39.4% of carboxylic acid (lactic acid) and 6.7% of soluble polymers. Considering carboxylic acids such as formic acid, levulinic acid, and lactic acid all can be formed during the hydrothermal degradation of carbohydrates,5,27,28 we further studied the hydrothermal behavior of these three carboxylic acids (Table 1, entries 10−12). Results showed that the conversions of formic acid, levulinic acid, and lactic acid are 19.9, 2.9, and 7.8%, respectively, indicating that these carboxylic acids are relatively 7332


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Table 2. Hydrothermal Behavior of Furan Compounds To Produce Huminsa

a

Reaction condition: 30 mL of H2O, 493 K, 5 h. b0.12 mol carbon atoms contained in raw material. cNot detected.

and keto−enol tautomerism33−35 to form one α-carbonyl acid named 3-deoxyglucosonic acid. Decarboxylation of α-carbonyl acids to form aldehydes is one common reaction,36,37 so the formed 3-deoxyglucosonic acid could further undergo decarboxylation to generate 3,4,5-trihydroxy-pentanal (or named 2-desoxyribose), which could be further converted to 4,5-dihydroxypent-2-enal, furfuryl alcohol, and levulinic acid (Scheme 1, path 1). On the other hand, the 3-deoxyglucosonic acid could also undergo acetal cyclization and dehydration to generate 5-hydroxymethyl furoic acid and then further undergo decarboxylation to form furfuryl alcohol (Scheme 1, path 2). The formed 3-deoxyglucosonic acid, 5-hydroxymethyl furoic acid, 3,4,5-trihydroxy-pentanal, 4,5-dihydroxypent-2-enal, and furfuryl alcohol (marked in pink in Scheme 1) all could be the precursor of humins from gluconic acid. Degradation Behavior of Furan Compounds and Cyclic Ketones and the Degradation Path Analysis. Previous literatures confirmed that humins are kinds of furanrich polymers with aliphatic oxygenate linkages,4,15 indicating that compounds containing furan ring may play a critical role in the process of humins formation. In order to analyze the role of furan ring during the formation of humins, the hydrothermal degradation behaviors of furan derivatives including HMF, 5-methylfurfural, furfural, furfuryl alcohol, furoic acid, 2-acetyl furan, 2-methyl furan, and furan are compared (Table 2, entries 1−8). The results show that HMF, 5-methylfurfural, furfural, furfuryl alcohol, and furoic acid could produce 64.6, 12.1, 23.4, 29.2, and 11.8% of

humins, respectively, whereas no humins are formed from 2acetyl furan, 2-methyl furan, and furan. Obviously, the humin yields from HMF and furfuryl alcohol were higher than those from 5-methylfurfural, furfural, and furoic acid, which could be due to that the formation of intramolecular H-bond in HMF and furfuryl alcohol could aid the hydrolytic ring-opening reaction of these two compounds to form primary precursors for humins formation. The absence of humins from 2-acetyl furan, 2-methyl furan, and furan suggests that the unsaturated furan ring is not essential for humins formation. Above results showed that some furan derivatives could generate humins, whereas some could not generate humins under hydrothermal condition. We thought that these furan derivatives which could generate humins must have some special functional groups or special structures or could be converted to one kind of compounds containing some common special functional groups or special structures. Thus, we analyzed the degradation path of those compounds and tried to find out the common special functional groups. Before analyzing the degradation path of the furan derivatives, we should know which kinds of basic reactions could occur under the hydrothermal condition. As has been shown above (Scheme 1), hydrolytic degradation of gluconic acid involves the basic reactions of β-elimination, keto−enol tautomerism, decarboxylation of α-carbonyl aldehyde, acetal cyclization, and dehydration.3,35 It is also well known that the conversion of HMF into levulinic acid involves the basic steps of hydrolytic ring-opening, keto−enol tautomerism, 1,27333


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Scheme 2. Paths of HMF Degradation under Hydrothermal Conditions21,22,42−44

hydride shift, and dehydration21 (Scheme 2, path 1), whereas the conversion of pyruvaldehyde into lactic acid involves the basic reaction of hydration and 1,2-hydride shift.7 Besides, synthesis of cyclopentanone from furfural38−40 and synthesis of 1,2,4-benzenetriol from HMF41 all involve the basic reaction step of intramolecular aldol condensation. Assuming that all of these basic steps could occur under hydrothermal conditions, we then deduced the degradation paths of these compounds with the furan ring. Scheme 2 shows the proposed degradation paths of HMF under hydrothermal conditions. After hydrolytic ring-opening and keto−enol tautomerism, the HMF can yield one αcarbonyl aldehyde, named 2,5-dioxo-6-hydroxy-hexanal, as has been reported by Patil et al.21,22 Because of the strong electron absorption effect of the two adjacent carbonyl groups, the 2,5dioxo-6-hydroxy-hexanal is favored to undergo hydration accompanied with 1,2-hydride shift to form one corresponding α-hydroxy acid named 2,5,6-trihydroxyhex-3-enoic acid,42 which could be further converted to levulinic acid through the basic steps of hydrolysis, hydration, and dehydration (Scheme 2, path 1). Besides, as the 2,5-dioxo-6-hydroxyhexanal generated from HMF contains several active α-H atoms linked on 3-C atom, 4-C atom, and 6-C atom, it can also undergo intramolecular aldol condensation and keto−enol tautomerism to form 1,2,4-benzenetriol (Scheme 2, path 2).43 Also because of the strong electrophilic effect of the linked carbonyl groups and aldehyde group, the α-H atoms linked on the β-carbon atom of the 2,5-dioxo-6-hydroxy-hexanal are rather active, leading to the aldol condensation of the 2,5dioxo-6-hydroxy-hexanal with other aldehydes to generate polymers.21,22 On the basis of the above analysis, we deduced that 2,5-dioxo-6-hydroxy-hexanal is the key precursor during humins formation from HMF, as has been proposed by Patil et al.21,22 By analogy with HMF, we could also draw the degradation path of furfural, furfuryl alcohol, furoic acid, 2-acetyl furan, and 2-methyl furan (Schemes S1−S5). Through the basic steps of hydrolytic ring-opening and keto−enol tautomerism, both furfural and furfuryl alcohol could also yield corresponding α-

carbonyl aldehydes (2-oxopentanedial from furfural and 5hydroxy-2-oxopentanal from furfuryl alcohol) and cyclic ketones (cyclopent-3-ene-1,2-dione from furfural and cyclopentane-1,2-dione from furfuryl alcohol) (Schemes S1 and S2). Similar to 2,5-dioxo-6-hydroxy-hexanal formed from HMF, the 2-oxopentanedial formed from furfural can also undergo hydration followed by 1,2-hydride shift to yield corresponding α-hydroxy acid named 2-hydroxy-5-oxopentanoic acid or undergo aldol condensation followed by keto−enol tautomerism to form cyclopentanone derivate named cyclopent-3-ene1,2-dione. Selective synthesis of the cyclopentanone via hydrogenation of furfural has been reported.38,39 The path for conversion of furfuryl alcohol into levulinic acid is similar to that of HMF, except that furfuryl alcohol does not need to undergo the step of hydrolysis of α-hydroxy acid44 (Scheme S2, path 3). Different from HMF, furfural, and furfuryl alcohol, the hydrothermal degradation routes of furoic acid are relatively simple (Scheme S3). After hydrolytic ring-opening and keto− enol tautomerism, furoic acid can generate one α-carbonyl acid named 2,5-dioxopentanoic acid, which could be further converted into furan through decarboxylation followed by cyclo-dehydration. Therefore, 2,5-dioxopentanoic acid and furan are the two downstream compounds during the hydrothermal degradation of furoic acid. As hydrothermal degradation of furan could not generate humins (Table 2, entry 8), although hydrothermal degradation of furoic acid could generate humins, we suspect that the 2,5-dioxopentanoic acid (one α-carbonyl acid) should be the key precursor for humins formation from furoic acid. Both 2-acetyl furan and 2-methyl furan all could form neither α-carbonyl aldehydes nor α-carbonyl acids under hydrothermal condition (Schemes S4 and S5). Hydrolytic ringopening of 2-acetyl furan can form one α-carbonyl ketone, whereas hydrolytic ring-opening of methylfuran can form one γ-carbonyl aldehydes. The α-H atoms in the α-carbonyl ketone and γ-carbonyl aldehyde are relatively inactive compared to that in the α-carbonyl aldehydes and α-carbonyl acids because the two ketone groups in α-carbonyl ketone are linked to 7334


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Scheme 3. Path of Furan Compounds Generate α-Carbonyl Aldehydes and Humins42−44

relatively lower humins; and (c) both 2-acetyl furan and methylfuran could generate neither α-carbonyl aldehyde nor αcarbonyl acid, and they generate no humins during the hydrothermal degradation process. Thus, we suspected that the α-carbonyl aldehydes and α-carbonyl acids are the primary precursor of humins formation under hydrothermal conditions. In fact, because of the strong electrophilic effect of the aldehyde group (or carboxylic group) and the adjacent carbonyl groups, the α-H atoms linked on the β-C atom of the α-carbonyl aldehydes (acids) are quite active, which could cause the aldol condensation of the α-carbonyl aldehydes (acids) with other carbonyl compounds to generate solid humins under hydrothermal condition without any catalyst. Hydrothermal Degradation Behavior of C2−C4 Simple Chain Oxy-Organics. Glycol, acetaldehyde, glyoxal, and glycolic acid are four simple chain oxy-organics containing only two carbon atoms; 1,2-propylene glycol, glycerol, acetone, propanal, pyruvaldehyde, hydroxyacetone, and pyruvic acid are simple chain oxy-organics containing three carbon atoms, whereas n-butyl aldehyde, acetoin, and butanedione are compounds with four carbon atoms. We further studied the hydrothermal degradation behavior of these model compounds and hoped to obtain valuable information for humins formation. As shown in Table 3, among all of these simple chain oxyorganics, only glyoxal and pyruvaldehyde, both of which are αcarbonyl aldehyde, yielded 21.7 and 44.2% of solid humins, respectively (Table 3, entry 3 and entry 8), whereas the other oxy-organics all could not yield humins. It is well known that

electron-donating methyl group and methylene group, whereas the ketone group in the γ-carbonyl aldehydes is much too weak because of the distance between them. The low activity of α-H atoms in the α-carbonyl ketone and γ-carbonyl aldehyde could be responsible for the absence of humins formation during the hydrothermal conversion of 2-acetyl furan and 2-methyl furan. It should be noted that both carbonyl group and unsaturated furan ring are presented in the molecular of 2-acetyl furan, indicating that these two functional groups play minor role in humins formation. Above analysis also showed that all of these furan derivatives could form cyclic ketone under hydrothermal condition. Some literatures regard the cyclic ketone as the key precursor for humins formation.45,46 In order to clarify this viewpoint, we also studied the degradation behavior of several cyclic ketone compounds including 2-methyl-1,3-cyclopentanedione, 1,2cyclohexanedione, 1,3-cyclohexanedione, and 1,2,3-benzenetriol. No solid humins were formed from these cyclic ketones under the hydrothermal condition, although some nonvolatile chemicals are formed, suggesting that these cyclic ketone compounds are not the key processors of humins formation (Table 2, entries 9−12). According to the above results and analysis, we could find the following phenomena (Scheme 3): (a) HMF, furfural, and furfuryl alcohol can generate α-carbonyl aldehydes after the step of hydrolytic ring-opening, and they all formed relatively higher yield of solid humins during the hydrothermal degradation process; (b) furoic acid can generate α-carbonyl acid after hydrolytic ring-opening reaction, and it forms 7335


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Table 3. Hydrothermal Degradation Behavior of Compounds with 2−4 Carbon Atomsa

a

Reaction condition: 30 mL of H2O, 493 K, 5 h. b0.12 mol carbon atoms contained in raw material. cNot detected. dNot measured.

Scheme 4. Possible Paths of Degradation and Polymerization of Glyoxal

compared to those in the α-carbonyl aldehydes (acids), which could undergo irreversible aldol condensation to form solid humins without any catalyst. Hydrothermal degradation behavior of these simply chain oxy-organics further confirms the deduction that the αcarbonyl aldehyde (acid) compounds are key processors to form humins.

the carbonyl compounds such as acetaldehyde, propanal, acetone, hydroxyacetone, n-butyl aldehyde, acetoin, and butanedione all could undergo aldol condensation with other carbonyl compounds with the aid of catalyst. However, none of them could yield humins under the hydrothermal condition in this study. This could also be explained by the fact that the αH atoms in all of these carbonyl compounds are inactive 7336


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Scheme 5. Possible Paths of Degradation and Polymerization of Pyruvaldehyde

Scheme 6. Possible Path of Formation of α-Carbonyl Aldehydes, α-Hydroxy Acids, γ-Lactones, Furfural Derivatives, and Humins from Aldoses33,42,49−52

Degradation Paths of Glyoxal and Pyruvaldehyde. Glyoxal and pyruvaldehyde are the most simple α-carbonyl aldehydes. Therefore, we analyzed the hydrothermal conversion paths of these two compounds. As shown in Scheme 4, because of the strong electrophilic effect of the carbonyl group linked to aldehyde group, the aldehyde group of glyoxal could be attacked by water to form one hydrated intermediate, which could further undergo 1,2hydride shift reaction to form glycolic acid, one kind of αhydroxy acid (Scheme 4, path 1). This is the well-known Cannizaro route.47 In fact, glyoxal can generate about 76%

glycolic acid during the hydrothermal degradation process without any catalyst. On the other hand, also because of the strong electrophilic effect of the two carbonyl groups, the H atom linked on the β-C atom is quite active, making glyoxal be favored to undergo aldol condensation to form compounds with long carbon chain, which could further undergo intermolecular acetal cyclization and dehydration reactions to form polymers rich of furan rings (Scheme 4, path 2). The polymer formed by this route contains a large number of hydroxyl, carbonyl, and CC double bonds, which is in accord with the properties of solid humins reported in the 7337


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Scheme 7. Possible Paths of Hydrothermal Degradation of Glucose and Fructose7,33,42,49−52

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Scheme 8. Proposed Typical Structural Fragments of Humins from HMF

of the highly active α-H atoms linked on the β-C atoms of αcarbonyl aldehydes. Understanding the degradation path of α-carbonyl aldehydes is very helpful to analyze the degradation route of carbohydrates because α-carbonyl aldehydes could be easily formed through β-elimination of aldose (glucose and xylose, e.g.) followed by keto−enol tautomerism.33−35,42,48 As shown in Scheme 6, the formed α-carbonyl aldehydes from aldose could carry on Cannizaro route to form α-hydroxy acids, which could further undergo self-esterification to generate γ-lactones; the formed α-carbonyl aldehydes could also further undergo βelimination to generate β,γ-unsaturated α-carbonyl aldehydes and then carry on Cannizaro route to form β,γ-unsaturated αhydroxy acids.33,42,49−51 Besides, the α-carbonyl aldehydes can also undergo acetal cyclization and dehydration to generate furfural and related derivatives (HMF from glucose)52 or undergo aldol condensation to form solid humins. Intermediates and Byproducts Formed during the Degradation of Glucose. Above study has confirmed that the α-carbonyl aldehydes and α-carbonyl acids are the key primary precursors to form humins during the hydrothermal degradation of carbohydrates. Therefore, identifying the αcarbonyl aldehydes and α-carbonyl acids would be helpful to understand the humins formation routes from glucose and

literatures.15,16 Therefore, we proposed that the solid residue generated by glyoxal in the hydrothermal process is one polymer formed by aldol condensation reaction of glyoxal, followed by acetalization and dehydration reaction. Similar to glyoxal, pyruvaldehyde can also yield one αhydroxy acid (lactic acid) through the Cannizaro route6,7,27 (Scheme 5, path 1), or yield polymers rich of furan ring structure through the three steps of aldol condensation, acetal cyclization, and dehydration (Scheme 5, path 2). The methyl group in pyruvaldehyde can reduce its hydration activity, which should be responsible for the relatively lower yield of αhydroxy acid (40% of lactic acid) and higher humins (44.2%) from pyruvaldehyde than that from glyoxal (76% of glycolic acid and 21.7% of solid humins). According to the analysis of the conversion paths of glyoxal and pyruvaldehyde, we conclude that the α-carbonyl aldehydes can be digested by the following two paths under hydrothermal condition because of the electrophilic effect of the two linked carbonyl groups: (a) the α-carbonyl aldehydes could generate one corresponding α-hydroxy acid through two steps of hydration and 1,2-hydride shift (Cannizaro route) and (b) the α-carbonyl aldehydes could undergo the aldol condensation with other carbonyl compounds to form solid humins because 7339


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Scheme 9. Proposed Typical Structural Fragments of Humins Formed from 3-Deoxyglucosone

According to the above analysis, we could find that six kinds of α-carbonyl aldehydes including 3-deoxyglucosone, (S)-5,6dihydroxy-2-oxohex-3-enal, 2,5-dioxo-6-hydroxy-hexanal, 4hydroxy-2-oxobutanal, 2-oxobut-3-enal, and pyruvaldehyde all could be formed during the hydrothermal degradation of glucose (marked in pink in Scheme 7), and corresponding αhydroxy acids including 2,4,5,6-tetrahydroxyhexanoic acid, 2,5,6-trihydroxyhex-3-enoic acid, 2,4-dihydroxybutanoic acid, 2-hydroxybut-3-enoic acid, and lactic acid (marked in blue in Scheme 7) all could be formed from these α-carbonyl aldehydes through Cannizaro reaction. Because glucose could easily generate α-carbonyl aldehydes 3-deoxyglucosone through the simple steps of β-elimination and keto−enol tautomerism, whereas fructose can generate αcarbonyl aldehyde only by the formation of glucose or HMF as intermediates, we suspect that the formation of 3-deoxyglucosone is responsible for the higher by-products and lower HMF yield from glucose than from fructose. Typical Structural Fragments of Humins Formed from HMF and 3-Deoxyglucosone. Our above analysis has proved that six kinds of α-carbonyl aldehydes including 3deoxyglucosone, (S)-5,6-dihydroxy-2-oxohex-3-enal, 2,5-dioxo6-hydroxy-hexanal, 4-hydroxy-2-oxobutanal, 2-oxobut-3-enal, and pyruvaldehyde all could be generated during the hydrothermal decomposition of glucose, which all could lead to the formation of solid humins. Scheme 5 shows the typical structural fragments of humins formed from pyruvaldehyde. Other α-carbonyl aldehydes all could generate polymers containing similar structure through similar manner. Here,

fructose. Hence, we mapped the degradation paths of glucose and fructose based on the recent research progress and the hydrothermal behavior of the α-carbonyl aldehydes (Scheme 7). As one aldose, glucose can easily undergo β-elimination to form one α,β-unsaturated aldehyde, which could be further transformed to 3-deoxyglucosone through keto−enol tautomerism.33,35,48 The 3-deoxyglucosone is one kind of one αcarbonyl aldehyde, which could be digested via three paths: (1) the 3-deoxyglucosone can undergo Cannizaro route to yield one α-hydroxy acid, named 2,4,5,6-tetrahydroxyhexanoic acid,42 which then undergoes self-esterification to generate 3deoxy-γ-hexonolactone,33 or undergoes decarboxylation, acetal cyclization, dehydration, and rehydration to generate levulinic acid; (2) the 3-deoxyglucosone could undergo acetal cyclization and dehydration to form HMF;52 and (3) the 3deoxyglucosone can directly polymerize to humins through aldol condensation followed by acetal cyclization. The multiple by-products generated from 3-deoxyglucosone could explain the low selectivity to HMF from glucose.2,14 On the other hand, glucose could be transformed to fructose through 1,2hydride shift, which could be converted to HMF with the basic steps of acetal cyclization, dehydration, and keto−enol tautomerism. Besides, glucose and fructose may undergo retro-aldol reaction to generate dihydroxyacetone, glyceraldehyde, erythrose, and glycolaldehyde, which could be further converted corresponding α-hydroxy acids such as lactic acid,6,47 2,4-dihydroxybutanoic acid, and 2-hydroxybut-3enoic acid and corresponding γ-lactones.53 7340


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dihydroxyacetone (AR, 99%), HMF (98%), 5-methylfurfural (98%), furfural (99%), furfuryl alcohol (97%), furoic acid (98%), 2-acetyl furan (99%), 2-methyl furan (98%), furan (98%), levulinic acid (99%), formic acid (88%), lactic acid (85%), sorbitol (98%), glucolactone (99%), acetaldehyde (99%), glyoxal (40%), glycolic acid (98%), acetone (99.5%), propanal (97%), pyruvaldehyde (32%), hydroxyacetone (90%), pyruvic acid (70%), n-butyl aldehyde (98%), acetoin (97%), 2,3-butanedione (99%), 2-methyl-1,3-cyclopentanedione (97%), 1,2-cyclohexanedione (97%), 1,3-cyclohexanedione (97%), and 1,2,3-benzenetriol (99%) were all purchased from Aladdin Reagent Company. All of these chemicals were used without further treatment. Hydrothermal Decomposition of the Model Compounds. Model compounds containing 0.12 mol carbon atoms and 30 mL of deionized water were placed in a Teflonlined autoclave and kept at 493 K for 5 h to carry out the hydrothermal degradation process. After the hydrothermal degradation process, the solid products were separated by filtering, washed with distilled water and ethanol, and finally dried at 373 K and weighed, whereas the filtrates were evaporated and dried at 373 K to obtain the involatile products (403 K for conditions with water as reaction media). The carbon yields of the humins were calculated by dividing the total mass of carbon in solid residues (assuming that the carbon content in the solid residues be 65%) by the carbon in the feedstock, whereas the carbon yields of the involatile products were calculated by dividing the total mass of carbon in the involatile products (assuming that the carbon content in the involatile products to be 60%) by the total mass of carbon in the feedstock. Analysis of Concentration of Model Compounds in Solvents. The concentrations of carbohydrates and furfural derivatives were analyzed on an Agilent 1200 series highperformance liquid chromatography (Bio-Rad HPX-87H) with a refractive index (RI) and UV detector (210 nm), using a 5 mM aqueous sulfuric acid solution as the eluent at a flow rate of 0.5 mL/min. The column and RI detector temperature were set at 55 and 45 °C, respectively. The concentrations of other studied model compounds were analyzed by GC, which was performed using an Agilent 7890B GC system equipped. The column used was a HP-5ms Ultra Inert capillary column (30 m × 0.25 mm × 0.25 μm). The oven temperature was programmed to hold at 45 °C, ramp at 10 °C/min to 220 °C, and hold at 220 for 2 min. The flow rate of the He carrier gas was 1.2 mL/min. Experiments were performed in at least duplicate, and the results presented an average of two or three measurements.

we deduced the typical structural fragments of humins formed from 2,5-dioxo-6-hydroxy-hexanal and 3-deoxyglucosone. The proposed aldol condensation paths of three 2,5-dioxo-6hydroxy-hexanal molecules are shown in Scheme 8. The 3-C atom of 2,5-dioxo-6-hydroxy-hexanal could be linked to 1-C, 2C, and 5-C of another 2,5-dioxo-6-hydroxy-hexanal molecular by aldol condensation, leading to the formation of kinds of trimers. The structure fragments of trimer 1, trimer 3, and trimer 5 have been included in the structure model of humins proposed by van Zandvoort et al.,15,16 but the structure fragments of trimer 2 and trimer 4 containing parallel-ring structures are not included in their structural model. We speculate that the parallel-ring structure may be the kernel of humins because the parallel-ring structure is more stable. Scheme 9 shows the typical structural fragments of humins formed from condensation of three 3-deoxyglucosone molecules. The fragments of humins are also composed of a large number of furan rings, and these structures are also included in the humins structure model proposed by van Zandvoort et al.15,16 Explanation for High Yield of Humins from Rhamnose and Xylose Compared to the Furfurals. In the above study, we found that rhamnose and xylose could yield more humins than 5-methylfurfural and furfural (Table 1, entries 3,4 and entries 7,8). This result could be explained by regarding the α-carbonyl aldehydes as the primary precursors of humins. As has been shown in Schemes 6 and S6, both rhamnose and xylose could form corresponding α-carbonyl aldehydes by simple steps of β-elimination and keto−enol tautomerism. Therefore, rhamnose and xylose could form humins without generating 5-methylfurfural and furfural, leading to relatively higher humins yields from rhamnose and xylose than from 5methylfurfural and furfural.

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CONCLUSIONS

Through studying the hydrothermal degradation behavior of 37 model compounds and analyzing the degradation paths of these model compounds, we proved that α-carbonyl aldehydes and α-carbonyl acids are the key primary precursors for solid humins formation during hydrothermal degradation of carbohydrate. By analyzing the degradation path of glucose, we found that six kinds of α-carbonyl aldehydes including 3deoxyglucosone, (S)-5,6-dihydroxy-2-oxohex-3-enal, 2,5-dioxo6-hydroxy-hexanal, 4-hydroxy-2-oxobutanal, 2-oxobut-3-enal, and pyruvaldehyde all could be formed during the hydrothermal conversion of glucose, which are responsible for the complex routes of glucose degradation. Through two steps of β-elimination and keto−enol tautomerism, glucose could be easily converted to 3-deoxyglucosone, which could undergo Cannizaro reaction to generate corresponding α-hydroxy acid or undergo aldol condensation to form solid humins, thus leading to low selectivity to valuable HMF. Regarding aldol condensation, acetal cyclization, and dehydration as three steps for humins formation, we deduced the typical structural fragments of humins formed from pyruvaldehyde, 2,5-dioxo-6hydroxy-hexanal, and 3-deoxyglucosone. Further studies on the catalytic conversion of α-carbonyl aldehydes would be helpful to obtaining valuable chemicals from carbohydrates.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00508. Possible degradation paths of furfural; possible degradation paths of furfuryl alcohol; possible degradation path of furoic acid; possible degradation paths of 2-acetyl furan; possible degradation paths of 2-methyl furan; and possible paths of xylose conversion to α-hydroxy acids, γ-lactones, furfural, and humins (PDF)

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EXPERIMENTAL SECTION Experimental Materials. Glucose (AR, 99%), fructose (AR, 99%), rhamnose (AR, 99%), xylose (AR, 99%), 7341


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-851-88211104. Fax: +86-851-88210651. ORCID

Ning Shi: 0000-0003-1854-6354 Qiying Liu: 0000-0002-4957-7930 Shiyun Tang: 0000-0002-5337-4271 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research received support from the Guizhou province science and technology plan project ([2017]5789-08), the National Natural Science Foundation of China (51576199), and the Natural Science Foundation of Guangdong Province (2017A030308010).

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Condensation of Î±-Carbonyl Aldehydes Leads to the Formation of

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