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. 2020 Sep 1;5(36):23322–23333. doi: 10.1021/acsomega.0c03169

Characterization of the Soluble Products Formed during the Hydrothermal Conversion of Biomass-Derived Furanic Compounds by Using LC–MS/MS

Ning Shi †,‡,§,*, Qiying Liu ‡,§, Ying Liu †, Lijun Chen †, Hongyan Zhang †, Hongsheng Huang †, Longlong Ma ‡,§
PMCID: PMC7496007  PMID: 32954183

Abstract

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To reveal the hydrothermal conversion routes of the biomass-derived furanic compounds, the soluble products formed during the hydrothermal conversion of 5-hydroxymethylfurfural (HMF), furfural, and furfuryl alcohol were analyzed by liquid chromatography–mass spectrometry (LC–MS) and LC–MS/MS. Multiple carbocyclic compounds containing hydroxy group and carbonyl group were detected, with a molecular mass in the range of 154–272 Da and carbon chain of the length 8–15. The formation of these soluble carbocyclic compounds was proposed to involve hydrolytic ring-opening of the furanic ring, intermolecular aldol condensation, intramolecular aldol condensation, and C–C cleavage reaction. The C–C cleavage reaction was proposed to occur on the dicarbonyl structure of the formed primary polymers.

1. Introduction

With the depletion of fossil resources, the transformation of abundant and renewable lignocellulosic biomass into fuels and fine chemicals has attracted widespread attention. One attractive route for the production of fuels and chemicals from biomass is a selective conversion of biomass into platform chemicals, followed by upgrading the platform chemicals into downstream fuels and chemicals. 5-Hydroxymethylfurfural (HMF) and furfural are two important biomass-based furanic platform compounds formed by the dehydration of the carbohydrates in lignocellulosic biomass,1−4 and they could be used as the starting material for the production of commodity chemicals such as 2,5-hexanedione, 2,5-dimethylfuran, 2,5-dihydroxymethylfuran, 5-ethoxymethylfurfural, 2,5-diformylfuran, 2,5-furandicarboxylic acid, furfuryl alcohol, and cyclopentanone.5−9 Among these downstream compounds, 2,5-diformylfuran and 2,5-furandicarboxylic acid could be used as monomers for renewable polyesters, while 2,5-hexanedione, 2,5-dimethylfuran, cyclopentanone, and furfuryl alcohol all could be used to produce liquid fuels.10−14 Due to the versatility of these biomass-derived furanic compounds, great efforts are devoted to efficiently produce HMF and furfural from renewable biomass during the past decades.

Production of furfural from raw biomass has been commercialized for decades,15 but the industrial production of HMF from biomass is blocked by the high cost of HMF production process.16,17 Water is cheap and green, making it be an ideal solvent for the production of HMF from biomass. Unfortunately, HMF is quite unstable under the hydrothermal condition, so the production of HMF from biomass with water as reaction media could generate great amount of byproducts such as hydrochar (HC), levulinic acid, and some other soluble compounds, leading to low selectivity to HMF.18 Thus, revealing the formation mechanism of the byproducts formed during the conversion of biomass into furanic compounds could aid to improve the efficiency of the furanic compound production.

Great efforts have been devoted to revealing the formation mechanism of the byproducts formed during the hydrothermal conversion of biomass into furanic compounds. Hydrochar (also named as humins in some literatures19−22), one main solid byproduct formed during the catalytic conversion of carbohydrates into HMF and furfural, is proposed to be formed from the furanic compounds through hydrolytic ring-opening of the furanic compounds into chain carbonyl compounds, followed by aldol condensation, acetalization, and dehydration.22−27 Levulinic acid is another main product formed during the production of HMF, which is proposed to be formed by hydrolytic ring-opening of HMF into 2,5-dioxohex-3-enal, followed by hydrolytic C–C cleavage reaction and rearrangement.28,29 In addition to levulinic acid, a certain amount of other soluble compounds are also formed during the hydrothermal conversion of HMF and furfural. Obviously, the full characterization of those soluble products could help to reveal the hydrothermal conversion route of HMF and furfural. However, to the best of our knowledge, little structural information about these byproducts was reported, let alone their formation mechanism.

To deeply reveal the hydrothermal conversion route of the furanic compounds, here we characterized the soluble compounds formed during the hydrothermal conversion of biomass-derived furanic compounds including HMF, furfural, and furfuryl alcohol by liquid chromatography–mass spectrometry (LC–MS) and LC–MS/MS and found that multiple carbocyclic compounds with the molecular mass in the range of 154–272 Da could be formed. The formation routes of those carbocyclic oxyorganic compounds were proposed to involve hydrolytic furan ring-opening, intermolecular aldol condensation, intramolecular aldol condensation, dehydration, and hydrolytic C–C cleavage reaction.

2. Results and Discussion

2.1. General Description of Product Distribution

Table 1 shows the carbon yields of hydrochar, the nonvolatile compounds, and the volatile compounds formed during the hydrothermal conversion of HMF, furfural, and furfuryl alcohol in various solvents. Obviously, these furanic compounds all generated considerable hydrochar in water but generated no hydrochar in the organic solvent of ethyl acetate and tetrahydrofuran. The formation of hydrochar from these furanic compounds in water was ascribed to the fact that hydrolytic ring-opening of the furanic compounds could generate chain compounds rich in carbonyl group, which were in the favor of polymerization, as has been reported previously.19 The yields of hydrochar from HMF, furfural, and furfuryl alcohol within the water as a solvent were 64.6, 23.4, and 29.2%, respectively, indicating that HMF was more favorable to generating hydrochar than to furfural and furfuryl alcohol. The total yield of hydrochar and nonvolatile compounds formed from furfural was lower than that of HMF and furfuryl alcohol in water, which could be because the hydroxy group in HMF and furfuryl alcohol could aid the hydrolytic ring-opening of the furan ring.23

Table 1. Carbon Yields of Hydrochar and Soluble Products from Furanic Compounds in Various Solventsa.

      yield of products (%)
entry reactantb solvent hydrochar nonvolatiles volatiles
1 HMF water 64.6 14.5 20.9
2 furfural water 23.4 12.4 64.2
3 furfuryl alcohol water 29.2 30.2 40.6
4 HMF ethyl acetate 0 10.5 89.5
5 furfural ethyl acetate 0 1.2 98.8
6 furfuryl alcohol ethyl acetate 0 2.9 97.1
7 HMF tetrahydrofuran 0 5.6 94.4
8 furfural tetrahydrofuran 0 2.0 98.0
9 furfuryl alcohol tetrahydrofuran 0 2.7 97.3
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a

Reaction condition: 30 mL of solvent, 493 K, 5 h.

b

0.12 mol carbon atoms were contained in reactant.

According to the above results, we found that when water was used as a solvent, around 35% of soluble compounds were formed from HMF, and over 70% of soluble compounds could be formed from furfural and furfuryl alcohol. To further reveal the hydrothermal conversion routes of these furanic compounds, we analyzed the soluble compounds (including the volatile and nonvolatile fractions) using LC–MS and LC–MS/MS.

2.2. Characterization of the Soluble Compounds Formed during the Hydrothermal Conversion of the Furanic Compounds

2.2.1. Characterization of the Formed Soluble Compounds from HMF

The LC–MS chromatograms and the corresponding formulas of the detected soluble compounds (determined according to the MS data of these compounds shown in Table S1) formed from HMF are shown in Figure 1. The formulas of compounds detected under the positive ionization mode were C5H8O3 (12.98 min), C11H12O3 (18.09 min), C12H12O4 (19.93 min), C12H10O5 (21.52 min), C11H14O3 (22.56 min), C11H12O5 (24.91 min), and C12H12O3 (26.37 min), while the formulas of compounds detected under the negative ionization mode were C6H6O3 (10.20 min), C5H8O3 (12.79 min), C12H10O5 (15.48 min), C11H12O3 (18.11 min), C12H12O4 (19.96 min), C12H10O5 (21.50 min), C12H10O4 (23.68 min), C11H12O5 (24.89 min), C12H12O4 (25.60 min), and C12H12O3 (26.39 min). Obviously, the carbon atoms of these detected compounds were in the range of 5–12, while the oxygen atoms of these detected compounds were in the range of 2–5. Besides, only the compounds C5H8O3 and C6H6O3 were with a molecular mass below 150 Da, while the molecular mass of the other compounds was in the range of 166–234 Da.

Figure 1.

Figure 1

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LC–MS chromatogram of the samples obtained by hydrothermal conversion of HMF.

The MS2 data of the detected compounds could show the information of the core structure and the functional groups of these compounds. Because the HMF only contains C, H, and O atoms, so the formed compounds could only contain these three kinds of atoms. The core structures of the compounds containing these three kinds of atoms could be divided into two main species of noncyclic species and cyclic species, while the cyclic species could be further divided into two species of furanic species and carbocyclic species. The fragment ions at m/z 81.03, 95.01, 97.03, 107.01, 123.01, and 125.02 corresponded to the formulas of [C5H5O], [C5H3O2], [C5H5O2], [C6H3O2], [C6H3O3], and [C6H5O3], respectively. These fragment ions all contained no less than one oxygen atom and no more than six carbon atoms, while the double-bond equivalents (DBEs) of these fragment ions were no less than 3, suggesting that these fragment ions should have a furanic structure. Thus, the detected compounds containing those fragment ions in the MS2 spectra were proposed to contain furanic structure. The fragmental ions at m/z 79.05, 91.05, 93.07, 95.05, 105.07, 107.05, 133.06, and 147.08 correspond to the formulas [C6H7], [C7H7], [C7H9], [C6H7O], [C8H9], [C7H7O], [C9H9O], and [C10H11O], respectively. These fragment ions all contained no more than one oxygen atom and no less than six carbon atoms, and the double-bond equivalents (DBEs) of these fragment ions were no less than 3, suggesting that these fragment ions should contain carbocyclic structure such as cyclohexanone, cyclohexadiene, benzene, and phenol. Especially, the fragment ions with formulas [C9H9O] and [C10H11O] were proposed to have a bicarbocyclic structure. Thus, the detected compounds containing those fragment ions in the MS2 spectra were proposed to have a carbocyclic structure. The compounds have no furanic structures and carbocyclic structures were proposed for noncyclic compounds. On the other hand, the mass losses of 18.01, 27.99, and 43.99 between the ion peaks were ascribed to the losses of H2O, CO, and CO2, respectively, and the loss of these groups indicated the existence of hydroxy group, carbonyl group, and carboxylic group in the detected compounds.

Table 2 shows the core structure and the functional groups of these detected compounds according to the MS2 data of these compounds (shown in Table S1). Similar to those compounds formed during the hydrothermal conversion of glucose,30 these compounds could also be roughly divided into three species of chain compounds, carbocyclic compounds, and furanic compounds. Among all these compounds, only C5H8O3 (12.98 min) was a noncyclic compound. The compounds C6H6O3 (10.20 min), C12H10O5 (15.48 min), C12H10O5 (21.50 min), and C11H12O5 (24.89 min) all had a furanic structure. Obviously, the compound C6H6O3 (10.20 min) should be the unreacted HMF, while the compounds C12H10O5 (15.48 min) and C12H10O5 (21.42 min) should be the dimer of HMF formed through esterification/acetalization.25,30,31 On the contrary, the compounds C11H12O3 (18.09 min), C12H12O4 (19.93 min), C11H14O2 (22.56 min), C12H10O4 (23.68 min), C10H14O2 (24.16 min), C12H12O4 (25.60 min), and C12H12O3 (26.37 min) all had a carbocyclic structure. Most of these carbocyclic compounds were all proposed to contain a bicarbocyclic structure because the fragment ions with the formulas [C9H9O] and [C10H11O] were presented in the MS2 spectra of these compounds. Obviously, the species of carbocyclic compounds were more than that of furanic compounds, suggesting that the hydrothermal conversion of HMF was favorable to generating carbocyclic compounds. Most of these compounds were proposed to contain hydroxy group and carbonyl group due to the presence of a mass loss of 18.01 and 27.99 in the MS2 data of these compounds. The mass loss of 43.99 (corresponding to the loss of CO2 group) was presented in the MS2 spectra of C5H8O3 (12.98 min), C12H10O4 (23.68 min), and C12H12O4 (25.60 min), suggesting that these compounds contained a carboxyl group. The presence of the carbonyl group and carboxylic group in the compound C5H8O3 (12.98 min) confirmed that it should be levulinic acid formed by the rehydration of HMF.28

Table 2. Structure Character of the Soluble Compounds Formed by Hydrothermal Conversion of HMF.
no tR/min formula molecular mass core structure functional group DBE DBE/C (H-2O)/C
1 10.20 C6H6O3 126.03 furanic hydroxy, carbonyl 4 0.67 0
2 12.98 C5H8O3 116.05 noncyclic carbonyl, carboxyl 2 0.4 0.40
3 15.48 C12H10O5 234.05 furanic hydroxy, carbonyl 8 0.67 0
4 18.09 C11H12O3 192.08 carbocyclic hydroxy, carbonyl 6 0.55 0.55
5 19.93 C12H12O4 220.07 carbocyclic hydroxy, carbonyl 7 0.58 0.33
6 21.50 C12H10O5 234.05 furanic hydroxy, carbonyl 8 0.67 0
7 22.56 C11H14O3 194.09 carbocyclic hydroxy, carbonyl 5 0.45 0.73
8 23.68 C12H10O4 218.06 carbocyclic hydroxy, carboxyl 8 0.67 0.17
9 24.16 C10H14O2 166.10 carbocyclic hydroxy, carbonyl 4 0.40 1.0
10 24.89 C11H12O5 224.07 furanic hydroxy, carbonyl 6 0.55 0.18
11 25.60 C12H12O4 220.07 carbocyclic hydroxy, carbonyl, carboxyl 7 0.58 0.33
12 26.39 C12H12O3 204.08 carbocyclic hydroxy, carbonyl 7 0.58 0.50
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The double-bond equivalents (DBEs), DBE/C, and (H-2O)/2 of these detected compounds could be determined through the formulas of these compounds, and the values are also shown in Table 2. The DBE/C denoted the value of DBE per carbon atom of compounds. Under the hydrothermal condition, the water addition reactions could decrease the DBE/C values of the formed compounds from the reactant, while the water-removal reactions were reversed. The DBE/C values of these detected compounds were all lower than that of HMF (equal to 0.67), indicating that water addition reaction must occur during the formation of these compounds. This result confirmed that the hydrolytic ring-opening reaction played a critical role in the formation of these soluble compounds from HMF in water.

The (H-2O)/C indicates the neat H atom per carbon atom of the organic compounds, and this parameter could not change under the hydrothermal condition unless the redox reactions occurred. The water addition/removal reactions could not change the (H-2O)/C value of the product from the reactant because those reactions were not redox reactions. On the contrary, the hydrolytic C–C cleavage reaction is an intramolecular redox reaction of organic compounds, leading to the formation of one oxidation product with (H-2O)/C value lower than the reactant and one reduction product with (H-2O)/C value higher than the reactant. Thus, (H-2O)/C could show whether the hydrolytic C–C cleavage reaction occurred during the formation of those compounds. Among all these compounds, the (H-2O)/C values of C12H10O5 (15.48 min) and C12H10O5 (21.42 min) were the same with HMF (all equal to 0), suggesting that the formation of these two compounds could not involve the C–C cleavage reaction. On the contrary, the (H-2O)/C values of all these carbocyclic compounds were not equal to 0, indicating that the formation of these compounds all involved C–C cleavage reactions. Our previous study confirmed that the formula of hydrochar formed from HMF in water could be approximately expressed as (C3H1.8O)n, so the (H-2O)/C value of hydrochar was below 0.27 Here, the (H-2O)/C values of these carbocyclic compounds were all higher than 0, indicating that these compounds should be the byproducts released during the formation of hydrochar.30

According to the core structure and the functional group of these detected compounds, the possible structures of these detected compounds are shown in Figure 2. Most of those compounds have been reported to be generated by the hydrothermal conversion of glucose,30 indicating that HMF should be one key intermediate during the hydrothermal conversion of glucose.

Figure 2.

Figure 2

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Proposed structure of the soluble compounds formed during the hydrothermal conversion of HMF.

2.2.2. Characterization of the Soluble Compounds Formed from Furfural

The LC–MS chromatograms and the corresponding formula of the detected soluble compounds (determined by the MS data shown in Table S2) formed from furfural are shown in Figure 3. The formulas of the detected compounds under the positive ionization mode were C10H8O4 (20.94 min), C15H12O5 (21.52 min), C10H10O3 (22.21 min), C10H8O4 (24.15 min), C11H10O5 (25.57 min), and C15H12O5 (26.38 min), while the formulas of the detected compounds under the negative ionization mode were C5H8O4 (10.13 min), C4H6O4 (10.57 min), C8H10O3 (16.60 min), C10H8O4 (17.10 min), C12H14O4 (18.39 min), C9H12O3 (19.12 min), C9H12O4 (19.80 min), C10H8O4 (20.94 min), C15H12O5 (21.50 min), C10H8O4 (24.13 min), C10H8O4 (25.22 min), and C15H12O5 (26.28 min). Obviously, except the compounds C5H8O4 (10.13 min) and C4H6O4 (10.57 min), the carbon atoms of the detected compounds were in the range of 8–15, and the oxygen atoms of these detected compounds were in the range of 3–5.

Figure 3.

Figure 3

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LC–MS chromatogram of samples obtained by hydrothermal conversion of furfural.

Similarly, the core structure and the functional groups of the detected compounds could be determined by the MS2 data of these compounds (shown in Table S2). As shown in Table 3, among these detected compounds, only the compounds C5H8O4 (10.13 min) and C4H6O4 (10.57 min) were noncyclic compounds with a molecular mass below 140 Da, while the other detected compounds were all carbocyclic compounds with molecular masses in the range of 154–272 Da. Hydroxy group and carbonyl group were presented in all these detected compounds, but the carboxylic group only presented in the compounds C5H8O4 (10.13 min), C4H6O4 (10.57 min), and C15H12O5 (26.38 min). The proposed structures of these compounds are shown in Figure S1. Obviously, most of these detected compounds were bicarbocyclic compounds containing hydroxy group and carbonyl group.

Table 3. Structure Character of the Soluble Compounds Formed from Furfural.
no tR/min formula molecular mass core structure functional group DBE DBE/C (H-2O)/C
1 10.13 C5H8O4 132.04 noncyclic hydroxy, carbonyl, carboxyl 2 0.4 0
2 10.57 C4H6O4 118.02 noncyclic hydroxy, carbonyl, carboxyl 2 0.5 –0.5
3 16.60 C8H10O3 154.06 carbocyclic hydroxy, carbonyl 4 0.5 0.5
4 17.01 C10H8O4 192.04 carbocyclic hydroxy, carbonyl 7 0.7 0
5 18.39 C12H14O4 222.09 carbocyclic hydroxy, carbonyl 6 0.5 0.5
6 19.12 C9H12O3 168.07 carbocyclic hydroxy, carbonyl 4 0.44 0.67
7 19.80 C9H12O4 184.07 carbocyclic hydroxy, carbonyl 4 0.44 0.44
8 20.94 C10H8O4 192.04 carbocyclic hydroxy, carbonyl 7 0.7 0
9 21.50 C15H12O5 272.07 carbocyclic hydroxy, carbonyl 10 0.67 0.13
10 22.21 C10H10O3 178.06 carbocyclic hydroxy, carbonyl 6 0.6 0.4
11 23.26 C10H8O4 192.04 carbocyclic hydroxy, carbonyl 7 0.7 0
12 24.13 C10H8O4 192.04 carbocyclic hydroxy, carbonyl 7 0.7 0
13 25.22 C10H8O4 192.04 carbocyclic hydroxy, carbonyl 7 0.7 0
14 25.57 C11H10O5 222.05 carbocyclic hydroxy, carbonyl 7 0.64 0
15 26.28 C15H12O5 272.07 carbocyclic hydroxy, carbonyl, carboxyl 10 0.67 0.13
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The DBE/C values of these detected compounds were all below 0.8, while furfural was with DBE/C value equal to 0.8, confirming that the formation of these compounds from furfural all involved water addition reaction such as hydrolytic ring-opening. The (H-2O)/C of furfural was 0, but only C5H8O4, C10H8O4, and C11H10O5 had (H-2O)/C value be 0 among the detected compounds, suggesting that the formation of these compounds does not involve the C–C cleavage reaction, while the formation of the other carbocyclic compounds all involves C–C cleavage reaction.

2.2.3. Characterization of the Soluble Compounds Formed from Furfuryl Alcohol

Similar to HMF and furfural, the hydrothermal conversion of furfuryl alcohol also generated multiple carbocyclic compounds. The LC–MS chromatograms and the corresponding formula of the soluble compounds formed from furfural are shown in Figure 4. The formulas of the compounds detected under the positive ionization mode were C10H12O4 (17.18 min), C9H14O3 (18.71 min), C10H12O4 (19.40 min), C10H10O3 (19.88 min), C11H12O3 (21.42 min), C14H12O2 (22.87 min), C10H10O3 (26.37 min), C11H14O4 (27.29 min), and C14H16O3 (17.86 min), while the formulas of the compounds detected under the negative ionization mode were C4H6O4 (10.53 min), C5H8O3 (12.99 min), C5H6O2 (13.45 min), C8H10O3 (15.46 min), C10H12O4 (16.03 min), C10H12O4 (17.20 min), C10H12O4 (18.01 min), C10H10O3 (19.98 min), and C15H14O4 (21.12 min). Obviously, C5H8O3 should be the well-known levulinic acid formed by the rearrangement of furfuryl alcohol,28 and C5H6O2 should be the unreacted furfuryl alcohol. Except for C4H6O4 (10.53 min), levulinic acid (12.99 min), and furfuryl alcohol (13.45 min), all those detected compounds contained 8–15 carbon atoms and 2–4 oxygen atoms.

Figure 4.

Figure 4

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LC–MS chromatogram of samples obtained by hydrothermal conversion of furfuryl alcohol.

Similarly, the core structure and the functional group of these detected compounds (shown in Table 4) were also determined according to the MS2 data of these compounds (shown in Table S3). Among these compounds, only the compounds C4H6O4 (10.53 min), C5H8O3 (12.99 min), and C5H6O2 (13.45 min) were not carbocyclic compounds, while the others were all carbocyclic compounds with a molecular mass in the range of 154–258 Da. Besides, the hydroxy group and the carbonyl group were proved to present in all the carbocyclic compounds. The proposed structures of these detected compounds are shown in Figure S2. Similarly, most of these detected compounds were bicarbocyclic compounds.

Table 4. Structure Character of the Soluble Compounds Formed from Furfuryl Alcohol.
no tR/min formula molecular mass core structure functional group DBE DBE/C (H-2O)/C
1 10.53 C4H6O4 118.03 chain hydroxy, carbonyl, carboxyl 2 0.5 –0.5
2 12.99 C5H8O3 116.05 chain carbonyl, carboxyl 2 0.4 0.4
3 13.45 C5H6O2 98.04 furanic hydroxy 3 0.6 0.4
4 15.46 C8H10O3 154.06 carbocyclic hydroxy, carbonyl 4 0.5 0.5
5 16.03 C10H12O4 196.07 carbocyclic hydroxy, carbonyl, carboxyl 5 0.5 0.4
6 16.53 C8H10O3 154.06 carbocyclic hydroxy, carbonyl 4 0.5 0.5
7 17.20 C10H12O4 196.07 carbocyclic hydroxy, carbonyl, carboxyl 5 0.5 0.4
8 18.01 C10H12O4 196.07 carbocyclic hydroxy, carbonyl, carboxyl 5 0.5 0.4
9 18.71 C9H14O3 170.09 carbocyclic hydroxy, carbonyl 3 0.33 0.89
10 19.40 C10H12O4 196.07 carbocyclic hydroxy, carbonyl 5 0.5 0.4
11 19.98 C10H10O3 178.06 carbocyclic hydroxy, carbonyl 6 0.6 0.4
12 21.12 C15H14O4 258.09 carbocyclic hydroxy, carbonyl 9 0.6 0.4
13 21.42 C11H12O3 192.08 carbocyclic hydroxy, carbonyl, carboxyl 6 0.55 0.55
14 22.87 C14H12O2 212.08 carbocyclic hydroxy, carbonyl 9 0.64 0.57
15 26.37 C10H10O3 178.06 carbocyclic hydroxy, carbonyl 6 0.6 0.4
16 27.29 C11H14O4 210.09 carbocyclic hydroxy, carbonyl 5 0.45 0.55
17 27.86 C14H16O3 232.11 carbocyclic hydroxy, carbonyl 7 0.5 0.71
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The DBE/C value of furfuryl alcohol was 0.6, while most of the compounds had DBE/C value below 0.6, suggesting that the formation of these compounds also involved water addition reaction. The (H-2O)/C value of furfuryl alcohol was 0.4, but the those of the detected compounds were not equal to 0.4, indicating that hydrolytic C–C cleavage reaction occurred during the hydrothermal conversion of furfuryl alcohol.

2.3. Hydrothermal Conversion Routes of Furanic Compounds

According to the characterization of the soluble compounds, we found that multiple carbocyclic compounds containing hydroxy group and carbonyl group could be formed during the hydrothermal conversion of biomass-derived furanic compounds. The hydrothermal conversion route of the furanic compounds could be speculated according to the structure information of those detected compounds.

The DBE/C values of most of the formed carbocyclic compounds were lower than the furanic reactants, indicating that the formation of the carbocyclic compounds involved water addition reaction. This could explain the fact that the furanic compounds could only generate hydrochar and those carbocyclic compounds with water as the reaction media. Previous studies have shown that the furanic compounds could undergo hydrolytic ring-opening reaction to generate chain aldehydes rich in carbonyl group, which were the key intermediates for hydrochar formation.19,23,24,28 Because the formation of these detected carbocyclic compounds also involved water addition reaction, we proposed that these chain aldehydes rich in carbonyl group were also the key intermediates for the furanic reactants to generate those carbocyclic compounds.

On the other hand, the carbon chains of these carbocyclic compounds were all longer than those of the reactants, indicating that the formation of these carbocyclic compounds must involve C–C coupling reactions to extend the carbon chain. Because the chain aldehydes formed by hydrolytic ring-opening of these furanic compounds favor aldol condensation,23,24,32 we proposed that the aldol condensation should occur between the chain aldehydes. Besides, the (H-2O)/C values of most of the formed carbocyclic compounds were not equal to that of the furanic reactants, indicating that the formation of those carbocyclic compounds should involve the hydrolytic C–C cleavage reaction.

By regarding that the above reactions all could occur during the hydrothermal condition, we analyzed the possible routes of carbocyclic compounds formation from HMF, furfural, and furfuryl alcohol. As shown in Figure 5, the hydrolytic ring-opening of HMF could generate 2,5-dioxo-6-hydroxyhexanal (DHH), which is regarded as the key intermediate for hydrochar formation from HMF.24,27,32 Three carbonyl groups and one hydroxy group were present in this compound, so it could undergo aldol condensation to form initial chain polymers rich in carbonyl group and a hydroxy group. Then, the chain polymer could undergo intramolecular aldol condensation to form polymers containing carbocyclic structure rich in a hydroxy group and a carbonyl group. The formed carbocyclic polymers contained carbonyl groups in the carbon ring and side chain, thus these polymers could further undergo aldol condensation to generate bicarbocyclic compounds. Furfural and furfuryl alcohol could also generate carbocyclic compounds through a similar route (shown in Figures S3 and S4).

Figure 5.

Figure 5

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Route of formation of a carbocyclic compound from HMF.

According to the above analysis, we proposed that the formation of these carbocyclic compounds involved the following basic steps: (1) hydrolytic ring-opening of the furanic compounds to generate chain aldehydes rich in carbonyl group; (2) intermolecular aldol condensation of the chain compounds to generate primary chain polymer rich in carbonyl group; (3) intramolecular aldol condensation of the primary chain polymer to generate polymer containing carbocyclic structure; and (4) C–C cleavage of the carbocyclic polymer to release the carbocyclic compounds containing the hydroxy group and the carbonyl group.

2.4. Hydrolytic C–C Cleavage Reaction

The above results confirmed that a certain kind of C–C cleavage reaction occurs frequently under the hydrothermal condition. The C–C cleavage should be induced by water attacking because only water molecular was present in the reaction system. The retro-aldol condensation is a kind of hydrolytic C–C cleavage reaction, but it could not change the (H-2O)/C value of formed compounds from the reactant with (H-2O)/C value equal to 0. For example, retro-aldol condensation of glucose could form glycolic aldehyde, dihydroxyacetone, glyceraldehyde, and erythrose,33,34 all of which had the (H-2O)/C value equal to 0. Thus, we proposed that some hydrolytic C–C cleavage reactions different from retro-aldol condensation should occur frequently to release the soluble compounds.

On the other hand, the hydrolytic C–C cleavage of the compounds containing dicarbonyl structure to release carboxylic acids and ketone/aldehyde is one reaction that occurs frequently during the hydrothermal conversion of dicarbonyl compounds.35−37 As shown in Figure 6, both α-carbonyl compounds and β-carbonyl compounds could undergo hydrolytic C–C cleavage to release one carboxylic acid with (H-2O)/C lower than the reactant and one ketone/aldehyde with (H-2O)/C higher than the reactant.38−41 For example, the formation of levulinic acid and formic acid from HMF involved the hydrolytic C–C cleavage reaction of the α-dicarbonyl intermediate 2,5-dioxohex-3-enal,28,29 while the formation of acetic acid from glucose involved the hydrolytic β-dicarbonyl cleavage of the formed β-dicarbonyl intermediates.38−41 Besides, Davídek et al. reported that the hydrolytic C–C cleavage of the β-dicarbonyl compound 2,4-pentanedione could form acetic acid and acetone, while the hydrothermal oxidative C–C cleavage of the α-dicarbonyl compound 2,3-pentanedione could form acetic acid and propanoic acid.42,43 Considering that hydrolytic ring-opening of HMF and furfural could generate 2,5-dioxo-6-hydroxyhexanal and 2-oxopentanedial,24,27,28 both of which are α-carbonyl aldehydes containing the α-dicarbonyl structure, while the condensation of these chain compounds could generate polymers containing the α-dicarbonyl structure and β-dicarbonyl structure (Figures 5, S3, and S4), we proposed that the hydrolytic C–C cleavage reaction occurred on the polymers having a dicarbonyl structure.

Figure 6.

Figure 6

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Hydrolytic C–C cleavage of the compounds containing (a) a α-dicarbonyl structure and (b) a β-dicarbonyl structure.

As mentioned above, the chain α-carbonyl aldehydes formed during the hydrothermal conversion of biomass were dicarbonyl compounds. Previous studies have shown that they were the key intermediates in the hydrochar formation from carbohydrates,19,23,27 so understanding the conversion route of the α-carbonyl aldehydes could be helpful in developing methods to suppress the formation of hydrochar. To the best of our knowledge, the α-carbonyl aldehydes could be converted through the following three routes: (1) the α-carbonyl aldehydes could undergo Cannizaro reaction to form α-hydroxy acids;33,34 (2) the α-carbonyl aldehydes could undergo aldol condensation to generate hydrochar;24,27,28 and (3) the α-carbonyl aldehydes could undergo hydrolytic C–C cleavage reaction to generate carboxylic acids and aldehydes.42,43 Because Cannizaro reaction route and hydrolytic C–C cleavage reaction route of the α-carbonyl aldehydes all could generate soluble compounds, further exploring the catalytic Cannizaro reaction and hydrolytic C–C cleavage reactions of the α-carbonyl compounds could be helpful in suppressing the formation of hydrochar and improving the selectivity of soluble compounds during the hydrothermal conversion of biomass.

3. Conclusions

The soluble products formed during the hydrothermal conversion of HMF, furfural, and furfuryl alcohol were analyzed by LC–MS and LC–MS/MS to prove that multiple carbocyclic compounds containing hydroxy group and carbonyl group could be formed from these furanic compounds under hydrothermal condition. The formation of these carbocyclic compounds from furanic compounds involved hydrolytic furan ring-opening, intermolecular aldol condensation, intramolecular aldol condensation, and hydrolytic C–C cleavage reaction. These carbocyclic compounds were proposed to be released from the precursor of hydrochar through C–C cleavage reaction occurring in the compounds containing dicarbonyl structure. Further exploring the hydrolytic C–C cleavage of the dicarbonyl structure could aid the production of carbocyclic compounds from hydrothermal conversion of biomass.

4. Experimental Section

4.1. Process of Hydrothermal Conversion of Furanic Model Compounds

Thirty milliliters of solvent (deionized water, ethyl acetate, or tetrahydrofuran) and furanic model compounds (HMF, furfural, and furfuryl alcohol) containing 0.12 mol carbon atoms were placed in a Teflon-lined autoclave and kept at 493 K for 5 h to carry out the hydrothermal conversion process. After the hydrothermal conversion process, the solid product was separated by filtering, washed with distilled water and ethanol, and finally dried at 378 K and weighed to obtain the hydrochar (HC), while the filtrate was dried at 403 K to obtain nonvolatile products. The yield of volatile compounds was calculated by the difference between the two values.

4.2. Analysis of the Soluble Products by LC–MS and LC–MS/MS

The samples for LC–MS and LC–MS/MS analyses were prepared by concentrating the collected filtrates under 323 K for 30–60 min, followed by dilution with methanol 100 times and filtration by a 0.22 μm microporous membrane. Most of the formed soluble products (including the volatile and nonvolatile products) remained in the samples. Pure methanol was used as blank control.

LC–MS and LC–MS/MS analyses were performed on ultrahigh-performance liquid chromatography (UHPLC–Q) Instrument (Dinonex Ultimate 3000) equipped with a mass spectrometer (Thermo Scientific Q Exactive) within heated electrospray ionization (HESI) in both positive and negative ionization. Accurate mass spectra were recorded across the range from m/z 50 to 500 by mass spectrometry. The mass axis was calibrated in the m/z 50–3200 range. Collision-induced dissociation (CID) energies for MS2 were 4.0 and 3.2 kV for positive and negative modes, respectively.

The column was an Eclipse Plus C18 250 mm × 4.6 mm, with 5 μm particle size. The column temperature was maintained at 25 °C. Mobile phases A and B were water with 0.1% formic acid and pure methanol, respectively. The system was equilibrated with 95% of mobile phase A solution for 3 min, and then a linear gradient progressed from 95% A to 50% A in 17 min, after which the mobile phase composition was linearly changed to 5% A and 95% B in the following 5 min, and maintained at 5% A for 5 min. The flow rate was 0.6 mL/min, and 5.0 μL of the samples was injected.

4.3. Analysis Process of the LC/MS and LC/MS2 Data

The core structures and the functional groups of the detected compounds were determined through the process reported previously.30 In brief, the detected compounds with MS2 data containing the characteristic ion peaks at m/z 81.03, 95.01, 97.03, 107.01, 123.01, and 125.02 were proposed to contain furanic structure, while the detected compounds containing the ion peaks at m/z 79.05, 91.05, 93.07, 95.05, 105.07, 107.05, 133.06, and 147.08 were proposed to contain carbocyclic structure (such as cyclohexanone, cyclohexadiene, benzene, and phenol). The mass loss of 18.01, 27.99, and 43.99 between the ion peaks in MS2 data were ascribed to the loss of H2O, CO, and CO2 during the fragmentation of ions, respectively, which was used to determine the functional groups in the detected compounds.

Acknowledgments

This research received support from the Natural Science Foundation of Guizhou Province (QiankeheJichu [2020]1Y239), the High Level Scientific Research Funding Project of Guizhou Institute of Technology (XJGC20190636), the Academic New Seedling Plan Project of Guizhou Institute of Technology (QianKeHe [2017]5789-08), and the Natural Science Foundation of Guizhou Provincial Department of Education (Qianjiaohe KY Zi [2016]014).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c03169.

  • MS2 data of these detected compounds formed from HMF, furfural, and furfuryl alcohol; proposed structure of the detected compounds formed from furfural and furfuryl alcohol; and formation routes of carbocyclic compounds from furfural and furfuryl alcohol (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c03169_si_001.pdf (115.7KB, pdf)

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Supplementary Materials

ao0c03169_si_001.pdf (115.7KB, pdf)

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