Molecular structure and formation mechanism of hydrochar from

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Molecular structure and formation mechanism of hydrochar from hydrothermal carbonization of carbohydrates Ning Shi, Qiying Liu, Xiong He, Gui Wang, Ni Chen, Jiayu Peng, and Longlong Ma Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b02174 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Graphic for manuscript

Aldol condensation of α-carbonyl aldehydes is the initial step for hydrochar formation from carbohydrates

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Molecular structure and formation mechanism of hydrochar from hydrothermal carbonization of carbohydrates Ning Shi a, b, c *, Qiying Liu b, c, Xiong He a, Gui Wang a, Ni Chen a, Jiayu Peng a, Longlong Ma b, c a

b

c

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

Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, P. R.

China

ABSTRACT: Hydrochars are solid byproducts formed during the liquid phase biorefinery process and could be used to generate functional carbonaceous materials, but the detailed molecular structure and the formation mechanism are still unclear. Herein, the formation of hydrochars from liquid phase carbonization of biomass derived compounds including glucose, fructose, xylose, ribose, dihydroxyacetone (DHA), 5-hydroxymethylfurfural (HMF), furfural (FF) and pyruvaldehyde (PRV) in water and inert polar organic solvents of ethyl acetate (EAC) and tetrahydrofuran (THF) was studied. The carbohydrates were found to generate hydrochars in both water and the organic solvents, while the HMF and FF could only generate hydrochars in water. The

α-carbonyl

aldehydes

including

pyruvaldehyde

(PRV),

3-deoxyglucosone

and

2,5-dioxo-6-hydroxyhexanal (DHH) formed during the decomposition of carbohydrates were proposed to be the key primary precursors for hydrochar formation. The molecular structure of the hydrochars were characterized by elemental analysis, FT-IR analysis and solid-state

13

C NMR

analysis to confirm that the molecular formula of the hydrochars all could be approximately expressed as (C3H2O)n, and the molecular structure of the hydrochars are all consisted of


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polyaromatic hydrocarbon fragment, phenolic fragment, furanic fragment, aliphatic fragment and small amount of carbonyl/carboxyl groups. The presence of polyaromatic hydrocarbon fragment and phenolic fragment in the hydrochars suggested that the aldol condensation played critical role for hydrochars formation. By regarding the aldol condensation of α-carbonyl aldehydes as the initial step for hydrochar formation, we deduced the polymerization routes of these α-carbonyl aldehydes, and found that the α-carbonyl aldehydes all could undergo aldol condensation followed by acetal cyclization and etherification to form polymers (C3H2O)n rich in furanic framework, or undergo aldol condensation followed by 1,2-hydride shift, intramolecular aldol condensation and dehydration to generate polymers (C3H2O)n rich in phenolic framework. One molecular structure containing polyaromatic hydrocarbon fragment, phenolic fragment, furanic fragment and aliphatic fragment was proposed for the hydrochars. Keywords: hydrochar; aldol condensation; biomass; α-carbonyl aldehydes; hydrothermal carbonization


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1. INTRODUCTION Biomass has been regarded as one sustainable carbon resource to generate fuels, materials and fine organic chemicals for a long time. Liquid phase catalytic biorefinery process is an effective way to upgrade the abundant and renewable biomass into liquid fuels1-3 and valuable platform chemicals such as 5-hydroxymethylfurfural (HMF), furfural (FF), levulinic acid, lactic acid4-6, but the formation of solid hydrochars (also called humins in some literatures7-9) during the process greatly decreased the carbon efficiency of the process7,

10, 11

. On the other hand,

hydrothermal carbonization process (HTC) is an efficient method to convert the biomass into hydrochars, which could be used to produce novel carbon-based materials with potential application in crucial fields such as CO2 sequestration, water purification, energy storage and catalysis12-17. Therefore, understanding the structure and the formation mechanism of the hydrochars are of great significance for both liquid phase biorefinery process and hydrothermal carbonization process. In order to reveal the fundamentals of the hydrothermal carbonization process, the chemical structure and the morphological structure of the hydrochars synthesized from diverse biomass and biomass derivatives (glucose, xylose, maltose, sucrose, starch, HMF and FF) were analyzed by multiple methods including SEM, pyrolysis GC–MS, solid-state 13C NMR analysis, FT-IR analysis and elemental analysis. Combination of the characterization of the hydrochars, multiple researchers have confirmed that the hydrochars are polymers rich in unsaturated polycyclic structure and containing different oxygen functional groups8, 18-22. Two quite different structure models are proposed for hydrochar (Figure 1). Sevilla et al. proposed that the hydrochars formed during the hydrothermal carbonization of carbohydrates are consists of small clusters of


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condensed polycyclic benzene rings and some reactive/hydrophilic oxygen functionalities (i.e. hydroxyl, carbonyl, carboxylic, ester)

23, 24

(Figure 1(a)). On the contrary, Van Zandvoort et al.

proposed that the hydrochars (called humins in their literatures) are furan-rich polymer network containing different oxygen functional groups8,

18

(Figure 1(b)). Both of these two molecular

structure models are widely accept by academics. However, the elemental composition of the molecular model shown in Figure 1(a) was not consistent with the hydrochars reported by multiple literatures25-30, while the molecular model shown in Figure 1(b) is lack of polyaromatic hydrocarbon fragments, also inconsistent with the results reported by multiple literatures25, 29, 31. Therefore, both of the proposed molecular models of hydrochars are far from satisfactory. Based on the characterization of the structure of hydrochars and the identification of the formed intermediates during hydrothermal degradation of carbohydrate, one formation mechanism of the hydrochar from biomass is widely accepted17, 24, 26, 32: 1) the cellulose and hemicellulose in the biomass are hydrolyzed into mono-carbohydrates such as glucose and xylose; 2) the mono-carbohydrates are dehydrated into furanic compounds such as HMF and FF; 3) these compounds formed by dehydration of mono-carbohydrates further undergo series of polymerization–polycondensation reactions leading to the formation of polyfuranic type compounds; 4) the formed polymers further undergo aromatization to form the hydrochars with polyaromatic hydrocarbon network. However, the information of the primary precursors for hydrochar formation and the details of the initial step for polymerization–polycondensation of the primary precursors are still unclear. Patil et al. proposed that aldol condensation of the chain aldehydes such as 2,5-dioxo-6-hydroxyhexanal (DHH) formed by hydrolytic ring-opening of FF and HMF are the initial step for the condensation of the carbohydrates into hydrochars9, 33.


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Sumerskii et al. proposed that intramolecular etherification/acetalization of FF and HMF could be the key steps for hydrochars formation19. Some other researchers proposed that aldol condensation or acetalization of HMF/FF with mono-carbohydrates should be the primary steps for hydrochar formation27, 34. By studying the hydrothermal degradation behavior of around 40 kinds of model compounds, our previous studies suggested that the α-carbonyl aldehydes such as PRV, 3-deoxyglucosone and DHH were the key primary precursors of hydrochars7. Here, we studied the producing of hydrochars from carbohydrates and related dehydration compounds in various solvents, and analyzed the elemental composition and molecular structure of these hydrochars by elemental analysis, FT-IR analysis and

13

C solid-state NMR. The results

confirmed that the α-carbonyl aldehydes could be the key primary precursors for char formation, while the aldol condensation should be the initial step for char formation. Based on the characterization of hydrochars, one molecular structure containing polyaromatic hydrocarbon fragment, phenolic fragment, furanic fragment and aliphatic fragment was proposed for the solid hydrochars.

2. EXPERIMENTAL SECTION 2.1. Experimental materials Glucose (AR, 99%), fructose (AR, 99%), xylose (AR, 99%), ribose (AR, 99%), DHA (AR, 99%), HMF (98%), FF (99%), PRV (32%), ethyl acetate (AR, 99%) and tetrahydrofuran (AR, 99%) were all purchased from Aladdin reagent company (Shanghai, China). All these chemicals were used without further treatment.

2.2. Preparation of the solid hydrochars The process of solid hydrochar preparation by hydrothermal carbonization of all feedstocks


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was reported previously7. Briefly, 30 mL of liquid solvent and model compounds containing 0.12 mol carbon atoms were placed in a Teflon-lined autoclave and kept at 493 K for 5 h. Deionized water, ethyl acetate (EAC) and tetrahydrofuran (THF) were employed as solvent for different runs. After the liquid phase degradation process, the solid products were separated by filtering, washed with distilled water and ethanol, and finally dried at 373 K and weighed.

2.3 Characterization of the hydrochars Elemental analyses of the solid hydrochars were performed on a Vario EL cube elemental analyzer with CHNS model under 0.22 MPa He and 0.25 MPa of O 2 at 1145 °C, with a TCD detector. The oxygen contents in the hydrochars were calculated on the basis of the difference between the weight of the matrix and the combined weight of the measured elements (C, H, N, S). The experimental error of the elemental analysis was below ±0.1% for carbon content and hydrogen content. FT-IR spectra of the hydrochars were recorded on an IR Prestige-21 spectrometer (Shimadzu) and scanned in the range 400−4000 cm−1 with a resolution of 2 cm−1. Samples for FT-IR analysis were prepared by mixing the sample powders with KBr and compressing into thin slices. Solid-state

13

C CP/MAS NMR spectra of the solid hydrochars were recorded on a Agilent

600 DD2 spectrometer (Agilent, USA, magnetic field strength 14.1 T) at resonance frequency of 150.72 MHz for

13

C using the cross-polarization (CP), magic-angle spinning (MAS), and a

high-power 1H decoupling. The powder samples were placed into a 4.0 mm Zirconia rotor. The spectra were obtained at a spinning speed of 10 kHz (4.2 μs 90°pulses), a 2 ms CP pulse, and a recycle delay of 3 s. The C signal of tetramethylsilane (TMS) at 0 ppm was used as the reference of 13C chemical shift.


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3. RESULTS AND DISCUSSION 3.1. Determination of the key primary precursors for hydrochar formation Determination of the key primary precursors is the crucial points to reveal the formation mechanism of hydrochars from carbohydrates. To determine the key primary precursors for hydrochar formation, we studied the formation of hydrochars from carbohydrates and relate compounds in various solvents. The yields of hydrochars formed from carbohydrates (glucose, fructose, xylose, ribose and DHA) and their dehydration products (FF, HMF and PRV) within water and inert polar organic solvents (EAC, THF) as solvents are shown in Table 1. When water was employed as reaction media, these feedstocks all could generate over 40% of solid hydrochars except FF, which generated only 23.4% of hydrochar. However, when the reaction was conducted in the inert organic solvents, the carbohydrates still generated over 30% of solid hydrochars, while the FF and HMF generated no hydrochars. The above results were helpful to determine the key primary precursors for hydrochars formation. Both the furanic compounds (HMF and FF)

8, 19

and the chain aldehydes such as

2,5-dioxo-6-hydroxyhexanal (DHH) formed by hydrolytic ring-opening of these furanic compounds

9, 33

are proposed to be the primary precursors for hydrochar formation by previous

literatures. However, the above results confirmed that HMF and FF could only generate hydrochars in water, indicating that water is necessary for these two compounds to generate hydrochars. Thus we proposed that the key primary precursors for hydrochar formation should be chain aldehydes formed by hydrolytic ring opening of the HMF and FF such as DHH instead of these furanic compounds. Some literatures also proposed carbohydrates to be the key primary


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precursors for hydrochar formation27, 34, however, the formation of hydrochar from these furanic compounds alone ruled this possibility. On the other hand, when organic solvents were employed as reaction media, the carbohydrates could still generate masses of hydrochars while the HMF and FF were reverse, indicating that the carbohydrates could generate hydrochars in the organic solvents through other intermediates without the formation of HMF and FF. Besides, the C3 compounds of DHA and PRV could hardly generate HMF and FF under hydrothermal condition but they all generated amount of hydrochars in water, suggesting that these two compounds should also generate hydrochars through other intermediates. We believed that the hydrochars are formed from one species of compounds containing some special functional group. Noticing that DHH and PRV are all α-carbonyl aldehydes containing the functional group of –CO-CHO, and the carbohydrates all could generate α-carbonyl aldehydes such as 3-deoxyglucosone and 3-deoxyxylosone through simple steps of β-elimination and keto-enol tautomerism35-37, we proposed that the α-carbonyl aldehydes formed during the degradation of carbohydrates are the primary precursors for hydrochars formation, as has been reported previously7. Here we tried to explain the hydrochar formation behavior from carbohydrates and furanic compounds in various solvents by regarding the α-carbonyl aldehydes as the primary precursors. As shown in Figure 2, C5 and C6 carbohydrates all could generate α-carbonyl aldehydes through several routes. On one hand, the C5 and C6 carbohydrates could undergo dehydration reaction to generate furanic compounds such as HMF and FF, which could further undergo hydrolytic ring-opening reaction to form α-carbonyl aldehydes such as DHH 9, 33, 38. On the other hand, the C5


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and C6 carbohydrates could also undergo β-elimination and keto-enol tautomerism to generate α-carbonyl aldehydes such as 3-deoxyglucosone and 3-deoxyxylosone35-37, or undergo retro-aldol condensation to generate C3 and C4 carbohydrates, which could further undergo β-elimination and keto-enol tautomerism to generate α-carbonyl aldehydes such as PRV5, 39. Because HMF and FF could only generate α-carbonyl aldehydes through hydrolytic ring-opening reaction, so they could only generate hydrochars within water as reaction media. On the contrary, the β-elimination reaction could occur in both water and organic solvents, so the carbohydrates could generate α-carbonyl aldehydes such as 3-deoxyglucosone and 3-deoxyxylosone in all solvents, which is responsible for the hydrochar formation from carbohydrates in both water and the inert organic solvents. Because the C5 and C6 carbohydrates all could generate large amount of hydrochars in the organic solvent, we proposed that the α-carbonyl aldehydes formed by β-elimination and keto-enol tautomerism of these carbohydrate, such as 3-deoxyglucosone and 3-deoxyxylosone, played even more crucial role for the carbohydrates to generate hydrochars. PRV should be the intermediate for DHA to generate hydrochar because DHA could be converted into PRV under hydrothermal condition10. In summary, the above results confirmed that the α-carbonyl aldehydes formed during the liquid phase decomposition of carbohydrates, such as 3-deoxyglucosone, PRV and DHH, should be the key primary precursors for hydrochars formation.

3.2 Characterization of the formed hydrochars Elemental Analysis of Hydrochars. The elemental compositions of all solid hydrochars are also shown in Table 1. The hydrochars all contained around 62.5-70.2% of carbon, 2.7-4.0% of hydrogen and 26.0-33.9% of oxygen, accord with the results reported in the previous literatures


8,

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23, 25-28

. The elemental compositions of the hydrochars formed in EAC and THF are quite similar,

indicating that these organic solvents were all inert and did not participate in the hydrochar formation process. Obvious difference could be found that the hydrochars formed in water contained less oxygen and more carbon than those formed in organic solvents, indicating that the oxygen was removed more thoroughly in hydrochars formed in water. According to the elemental analysis, the H/O and C/O atom ratio of the hydrochars could be calculated8, 25, 30. As shown in Table 1, the H/O atom ratio of all the hydrochar were between 1.4 and 2.4, while the C/O ratios of the hydrochar were between 2.6 and 3.6, suggesting that the hydrochars all contained around 3 n moles of carbon atom, 2 n moles of hydrogen atom and n moles of oxygen atom. Considering that every mole of glucose, fructose, HMF and 1,2,4-benzenetriol all contained 6 mol of carbon atoms, we proposed that the molecular formula of all hydrochars could be approximatively expressed as (C3H2O)n. Specifically, the molecular formula of the hydrochars formed from C5 and C6 carbohydrates and their dehydration derivatives in water could be expressed as (C3H2-(0.1~0.5)O1~1.1)n, while the hydrochars formed from these compounds with inert organic solvents as reaction media could be approximatively expressed as [C3(H2O)1.1~1.2]n. Obviously, the H/O ratios in the hydrochars formed from C5 and C6 compounds in organic solvents was quite close to 2:1, while the H/O ratios in the hydrochars formed from C5 and C6 compounds in water were all lower than 2:1. It is well-known that the H/O atom ratio in the feedstocks are all 2:1, and the water addition/removal reactions such as dehydration, hydration, acetalization, etherification, esterification, aldol condensation and retro-aldol condensation all could not change the H/O atom ratio in the products from carbohydrates. On the contrary, the reactions involving C-C cleavage could change the H/O atom


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ratio of the products from carbohydrates. Thus, we proposed that some C-C bond cleavage reactions must be occurred during the formation of hydrochars in water. As has been mentioned above, two different molecule structure models of hydrochar are proposed by different research teams 8, 23. The molecule formula of the molecular structure model of the hydrochar in Figure 1(a) could be expressed as (C3H2.25O0.58)n, which contained more hydrogen atoms and less oxygen atoms than the hydrochars formed here, suggesting that the molecular structure model of hydrochar in Figure 1(a) was too aromatized compared to the hydrochar formed here. On the contrary, the molecular formula of the structure model shown in Figure 1(b) could be expressed as (C3H2.49O1.11)n, much close to the elemental composition of the hydrochars formed here. FT-IR Analysis of Hydrochars. The FT-IR spectra of these hydrochars were shown in Figure 3. The FT-IR spectra of these hydrochars were quite similar, and were accord with the reported literatures8, 9, 11, 23, 40. The broad peak positioned at 3400 cm-1 was assigned to O–H stretching vibration, indicating that all hydrochars contained considerable hydroxy group11. The presence of peaks at 2991 cm-1 (aliphatic C-H stretching vibration), 1483 cm-1 and 1392 cm-1 (all assigned to aliphatic C–H deformation vibration) suggested that all these hydrochars contained aliphatic structure23. The peak positioned at 1633 cm-1 (assigned to C=O stretching vibration) indicated the presence of carbonyl/carboxyl groups in these hydrochars11, while the presence of peak at 1176 cm-1 (assigned to C-O stretching vibration in O=C-O group) confirmed the presence of carboxylic acid/ester groups in these hydrochars40. The peaks positioned at 1598 cm-1 (assigned to C=C stretching of aromatic/furanic rings), 1070 cm-1 and 1023 cm-1 (both are assigned to C-O stretching in aromatic or α-unsaturated groups), and 787 cm-1 (assigned to aromatic out-of-plane


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C–H deformation) suggested the presence of phenolic/furanic ring in the hydrochars8, 23. Different interpretations on the attribution of 700-800 peaks were proposed by academics. Van Zandvoort et al. proposed that the presence of peaks at around 700-800 cm-1 are the evidence for furanic fragment in the hydrochars8, while Sevilla et al. proposed that the peaks at around 700-800 cm-1 are the evidence for phenolic fragment in the hydrochars23. However, both furanic and phenolic fragments all could show peaks at the region, we proposed that the presence of peaks at around 700-800 cm-1 could not determine whether phenolic structure or furanic structure was presented in the hydrochars. Solid-state

13

C NMR Analysis of Hydrochars. The solid-state

13

C NMR spectrums of

hydrochars are shown in Figure 4. According to the previous literatures8, 18, 22, 25, 26, 29, 31, the solid-state

13

C NMR spectra could be divided into four distinct zones of δ=10–60 ppm

(corresponding to aliphatic sp3 carbon atoms), δ=60–100 ppm (corresponding to aliphatic sp3 carbon atoms in C-O groups), δ=100–160 ppm (corresponding to sp2 carbon atoms in aromatic rings) and δ=160–220 ppm (corresponding to sp2 carbon atoms in C=O from carbonyl/carboxyl groups). As has been shown in Figure 4, all these hydrochars had peaks in 0-60 ppm, 100-160 ppm and 160-220 ppm, indicating that all hydrochars contained saturated aliphatic fragment, aromatic fragment and carbonyl/carboxyl functional groups. In the region of δ=10–60 ppm, the hydrochars formed from glucose, fructose, xylose, ribose, HMF and FF within all solvents all showed strong peaks at 33-35 ppm, suggesting that these hydrochars contained plenty of secondary carbon atoms8. On the contrary, the hydrochars formed from PRV and DHA all have peaks at 42 ppm and 58 ppm, but showed weak peaks in region of δ=0–40 ppm, indicating the hydrochars mainly contained tertiary and quaternary carbon atoms8. It


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should be noted that if the β-C atom of PRV did not participate in the hydrochar formation process, great amount of methyl group should be presented in the hydrochar formed from PRV. However, only very small amount of methyl group were detected in the hydrochar formed from PRV, suggesting that the β-C atoms must be involved in the hydrochar formation process. This result suggested that the aldol condensation should play critical role for PRV to generate hydrochar during the hydrothermal carbonization process. The peaks in the region of 60-100 ppm are ascribed to the C-O bonds in alcohol groups or ether groups25. Obvious difference could be found between the hydrochars formed in water and EAC in this region. The hydrochars formed in EAC all showed obvious peaks in the region, suggesting that considerable aliphatic C-O bond presented in these hydrochars. On the contrary, the hydrochars formed in water were all lack of peaks in the region, indicating that these hydrochars were lack of C-O functional groups. Noticing that the above FT-IR analysis had confirmed that plenty of hydroxy group were presented in all hydrochars, while the solid-state 13C NMR analysis suggested that the hydrochars formed in water were lack of aliphatic C-O bond, we proposed that the hydroxy groups should be linked on the aromatic structure of the hydrochar. In other word, multiple phenolic fragments should exist in the hydrochars formed in water. All the hydrochars showed peaks in the region of δ=100–160 ppm, suggesting that all these hydrochars had aromatic structures. Obvious difference could be found that the hydrochars formed from PRV and DHA all showed a dominant peak at 124 ppm, while the hydrochars formed from C5 and C6 model compounds all showed main resonances at around 147, and 110 ppm. Previous literatures have reported that the two main peaks at around 147 and 110 ppm are relate to O-C=CH and O-C=CH carbons in the phenolic/furanic ring 25, 26, 31, while the resonance at around 124 ppm


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is the signatures of benzenic carbons18, 25, 29, 31. Thus, we proposed that the hydrochars formed from PRV and DHA were mainly composed of polyaromatic hydrocarbon fragment, while the hydrochars formed from C5 and C6 compounds were mainly composed of furanic/phenolic structure. Weak peak at 124 ppm was also appeared in the spectra of the hydrochars formed from C5 and C6 compounds, indicating that these hydrochars also contained small amount of polyaromatic hydrocarbons structure. Small difference could also be detected that the peaks at 124 ppm of the chars formed in water were slightly more pronounced than those hydrochars formed in EAC. Because the content of polyaromatic hydrocarbon fragment is an indicator of the degree of aromatization, we proposed that the hydrochars formed from PRV and DHA had higher degree of aromatization than those hydrochars formed from C5 and C6 compounds, while the hydrochars formed in EAC had the lowest degree of aromatization. In brief, the FT-IR analysis and solid-state 13C NMR analysis confirmed that all the formed hydrochars contained polyaromatic hydrocarbon fragment, phenolic fragment, furanic fragment, saturated aliphatic fragment and small amount of carbonyl/carboxyl groups. The hydrochars formed in water were lack of aliphatic C-O bond and rich in phenolic fragment, while the hydrochars formed in EAC were rich in both C-O bond and furanic fragment. The presence of polyaromatic hydrocarbon fragment, phenolic fragment and furanic fragment suggested that some carbon chain extending reaction must be involved during the char formation process.

3.3 The formation mechanism of hydrochar Determination of The Initial Step for Hydrochar Formation. Determination of the initial step for hydrochar formation is also one key point to reveal the formation mechanism of hydrochars. Generally, etherification19, acetalization34 and aldol condensation9, 27 are all proposed


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to be the initial step for hydrochar formation. Our above results had confirmed that the polyaromatic hydrocarbon fragment, phenolic fragment and furanic fragment were all presented in the hydrochars, indicating that some C-C bond formation reaction such as aldol condensation reaction must be involved during the hydrochar formation process. Considering that the above study suggested that α-carbonyl aldehydes are the primary precursors for hydrochar formation, which are prone to aldol condensation9, 33, we proposed that aldol condensation of the α-carbonyl aldehydes should be the initial step for hydrochar formation. Polymerization Routes of α-carbonyl aldehydes into Hydrochars. The above results indicated that hydrochars should be formed by aldol condensation of the α-carbonyl aldehydes formed during the degradation of carbohydrates. PRV, 3-deoxyglucosone and DHH are three kinds of α-carbonyl aldehydes could be easily generated during the degradation of glucose9, 33, 35-38, thus we analyzed the polymerization routes of these three α-carbonyl aldehydes by regarding the aldol condensation as the initial step. PRV is the simplest α-carbonyl aldehydes formed from glucose. As shown in Figure 5, PRV contained two carbonyl groups on the 1-C and 2-C site, so it could undergo 3-1 aldol condensation and 3-2 aldol condensation to generate two kinds of long chain polymers

7, 9, 33

. The long chain

polymer formed by 3-1 aldol condensation could undergo acetal cyclization and dehydration to generate polycyclic polymer rich in furanic fragment7, or undergo 1,2-hydride shift reaction, intramolecular aldol condensation and dehydration to generate polycyclic polymer rich in phenolic structure41. The molecular formulas of these two polycyclic polymer all could be expressed as (C3H2O)n, which were accord with the elemental composition of the hydrochars formed from PRV and DHA. These polymers are rich in aromatic carbon atoms and lack of C-O bond, also accord


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with the solid-state

13

C NMR results. On the contrary, the long chain polymer generated by 3-2

aldol condensation of PRV could only undergo acetal cyclization to generate polycyclic polymer (C3H4O2)n lack of aromatic carbon atom and rich in C-O bond, which is inconsistent with elemental composition and solid-state 13C NMR analysis of the hydrochars formed from PRV and DHA. Therefore, we proposed that the 3-1 aldol condensation of PRV is the main route to generate hydrochar from PRV, while the 3-2 aldol condensation of PRV played less important role for hydrochar formation. Figure 6 showed the polymerization routes of 3-deoxyglucosone. Similar with PRV, the 3-deoxyglucosone could also undergo 3-1 aldol condensation and 3-2 aldol condensation to form long chain polymers, which could further undergo acetal cyclization and dehydration to form polycyclic polymers (C6H6O3)n rich in furanic fragment and C-O functional groups. Then these formed polycyclic polymers (C6H6O3)n all could further undergo etherification to generate polymers (C3H2O)n rich in furanic fragment and aliphatic C-O bond, or undergo hydrolytic ring-opening, 1,2-hydride shift, aldol condensation and dehydration to form polycyclic polymers (C3H2O)n rich in phenolic fragment and lack of aliphatic C-O bond. As shown in Figure 6, hydrolytic ring-opening of furanic fragment was involved during the formation of phenolic fragment from 3-deoxyglucosone, which could explain that the hydrochars formed in water contained more phenolic fragments, while the hydrochars formed in EAC was rich in furanic fragment. Besides, comparison of the routes shown in Figure 5 and Figure 6 could found that the process of formation of phenolic polymers from 3-deoxyglucosone involved more steps than that from PRV, which could be responsible for that the hydrochars formed from PRV and DHA had higher degree of aromatization than those hydrochars formed from C5 and C6 carbohydrates.


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Different from PRV and 3-deoxyglucosone, DHH contained three kinds of carbonyl groups at 1-C, 2-C and 5-C atoms. Thus the 2,5-dioxo-6-hydroxyhexanal could undergo three kinds of aldol condensation routes of 3-1 aldol condensation, 3-2 aldol condensation, 3-5 aldol condensation to generate long chain polymers rich in hydroxy group (Figure 7). Similarly, the long chain polymers also could undergo acetal cyclization to generate polycyclic polymers (C6H6O3)n rich in furanic fragment and aliphatic C-O groups, which could further undergo etherification to generate polycyclic polymer (C3H2O)n rich in furanic fragment and aliphatic C-O bond, or undergo 1,2-hydride shift, aldol condensation and dehydration to generate polycyclic polymers (C3H2O)n rich in phenolic fragment and lack of aliphatic C-O bond. Similar with HMF and glucose, FF and xylose all could generate similar polymers rich in furanic fragment and phenolic fragment through the same reaction steps. The polycyclic polymers with formula of (C3H2O)n were all consisted with the elemental composition of hydrochars, indicating these polycyclic polymers all could be existed in the hydrochars as fragments.

3.4 The molecular structure of the hydrochar According to the characterization and the formation mechanism of hydrochars, one molecular structure model of the hydrochar was shown in Figure 8. The formula of the molecular structure model could be expressed as (C3H2O)n, which is accord with the elemental composition of the hydrochars, and the proposed molecular structure model of hydrochar contained polyaromatic hydrocarbon fragment, phenolic fragment, furanic fragment and aliphatic fragment, which is also accord with the 13C NMR analysis. The content of polyaromatic hydrocarbon, phenolic fragment, furanic fragment and aliphatic fragment in the hydrochars could change with the harshness of the hydrochar formation condition


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8, 23, 25, 31

. Under mild formation conditions, the furanic fragment and aliphatic fragment were

dominant in the hydrochars8, while under the harsh formation conditions, the phenolic fragment could become the dominant in the formed hydrochars23, 31. When the hydrochars were treated under high temperature, the furanic fragment and the phenolic fragments all could be transformed into the polyaromatic hydrocarbon fragment25.

3.5 Inhibition of hydrochar formation during the hydrothermal decomposition of carbohydrates Liquid phase catalytic converting the abundant and cheap lignocellulosic biomass into valuable chemicals such as HMF and levulinic acid has been pursued for decades, but the industrialization of this biorefinery process is blocked by the high yield of hydrochar10, 11. Our previous work7 and this study all suggested that the α-carbonyl aldehydes formed during the hydrothermal degradation of carbohydrates are the primary precursors of hydrochar. Therefore, suppressing the formation of α-carbonyl aldehydes, or decreasing the concentration of α-carbonyl aldehydes in the solvent, are proposed to be efficient ways to suppress the formation of hydrochar during the liquid phase biorefinery process. It is well-known that both hydrogenation and oxidation condition all could suppress the formation of α-carbonyl aldehydes, which could be responsible for little hydrochars formation during the hydrogenation42-44 and oxidation45, 46 of biomass under hydrothermal condition.

4. CONCLUSIONS The formation of hydrochars from carbohydrates and their dehydration compounds in water and inert organic solvents was studied, and the α-carbonyl aldehydes including PRV, 3-deoxyglucosone and DHH were proved to be the key intermediates for hydrochars formation.


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The structures of formed hydrochars were characterized by elemental analysis, FT-IR analysis and solid-state 13C NMR analysis. The elemental analysis confirmed that the molecular structure of these hydrochars all could be approximately expressed as (C3H2O)n, while the FT-IR analysis and solid-state

13

C NMR analysis confirmed that all the formed hydrochars contained

polyaromatic hydrocarbon fragment, phenolic fragment, furanic fragment, aliphatic fragment and small amount of carbonyl/carboxyl groups, indicating that aldol condensation played crucial role during the hydrochars formation process. By regarding the aldol condensation of α-carbonyl aldehydes as the initial step for hydrochar formation, the α-carbonyl aldehydes were found to generate polycyclic polymers (C3H2O)n rich in both furanic fragment and aliphatic C-O groups, or generate polycyclic polymers (C3H2O)n rich in phenolic fragment but lack of aliphatic C-O groups.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]. ORCID Ning Shi: 0000-0003-1854-6354 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research received support from the Academic New Seedling Plan Project of Guizhou Institute of Technology (Qian Ke He [2017]5789-08), the National Natural Science Foundation of


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China (51576199), the Natural Science Foundation of Guangdong Province (2017A030308010).

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Figures and Tables

Figure 1. Two molecular structures of hydrochars speculated by previous literatures8, 23


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Table 1. Yield and elemental analysis of the chars formed from carbohydrates and derivatives a

Name of hydrochars

Feedstock

Solvent

Elemental percentage (%)

Carbon yield (%) C

Atom ratio

Hypothetical molecular formula

H

O

H/O

C/O

3.9

26.0

2.4

3.6

(C3H2O0.8)n

70.2 Char-PRV-WT

PRV

H2O

44.2

Char-DHA-WT

DHA

H2O

47.8

68.9

3.7

27.4

2.1

3.3

(C3H1.9O0.9)n

Char-HMF-WT

HMF

H2O

64.6

66.0

3.3

30.7

1.7

2.9

(C3H1.8O1)n

Char-FF-WT

FF

H2O

23.4

67.5

3.2

29.4

1.7

3.1

(C3H1.7O1)n

Char-GLU-WT

Glucose

H2O

60.7

66.1

2.7

31.2

1.4

2.8

(C3H1.5O1.1)n

Char-FRU-WT

Fructose

H2O

54.1

66.5

3.5

30

1.9

3.0

(C3H1.9O1)n

Char-XYL-WT

Xylose

H2O

46.4

66.5

3.2

30.3

1.7

2.9

(C3H1.7O1)n

Char-RIB-WT

Ribose

H2O

41.0

65.8

3.2

31.1

1.6

2.8

(C3H1.7O1.1)n

Char-HMF-EAC

HMF

EAC

0

-

-

-

-

-

-

Char-FF-EAC

FF

EAC

0

-

-

-

-

-

-

Char-GLU-EAC

Glucose

EAC

73.1

63.2

4

32.8

1.9

2.6

(C3H2.3O1.2)n

Char-FRU- EAC

Fructose

EAC

33.1

64.5

3.6

31.9

1.8

2.7

(C3H2O1.1)n

Char-XYL- EAC

Xylose

EAC

56.6

64.3

4

31.8

2

2.7

(C3H2.2O1.1)n

Char-RIB- EAC

Ribose

EAC

54.9

64.6

3.9

31.5

2

2.7

(C3H2.2O1.1)n

Char-HMF-THF

HMF

THF

0

-

-

-

-

-

-

Char-FF-THF

FF

THF

0

-

-

-

-

-

-

Char-GLU-THF

Glucose

THF

52.6

65.2

3.9

30.9

2.0

2.8

(C3H2.1O1.1)n

Char-FRU- THF

Fructose

THF

27.4

64.4

3.8

31.7

1.9

2.7

(C3H2.1O1.1)n

Char-XYL-THF

Xylose

THF

38.7

62.5

3.6

33.9

1.7

2.5

(C3H2.1O1.2)n

Char-RIB- THF

Ribose

THF

40.4

64.6

3.9

31.4

2.0

2.7

(C3H2.2O1.1)n

a

±0.1

Reaction condition, 30 mL solvents, 493 K, 5 h, 0.12 mol carbon atoms contained in raw

material.


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Figure 2. Routes of α-carbonyl aldehydes formation from carbohydrates (R denoted H atom or CH2OH group)


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787

1633 1598 1483 1392 1354 1176 1070 1002

2991

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2025

Page 27 of 32

Char-GLU-WT

3440

Char-FRU-WT Char-XYL-WT Char-RIB-WT Char-GLU-EAC Char-FRU-EAC Char-XYL-EAC Char-RIB-EAC Char-HMF-WT Char-FF-WT Char-PRV-WT Char-DHA-WT

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

Figure 3. FT-IR spectra of the formed hydrochars


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

aromatic carbon

C=O bond

124

aliphatic C-O bond

Page 28 of 32

aliphatic carbon

110

147

Char-GLU-WT Char-FRU-WT Char-XYL-WT Char-RIB-WT 240

220

200

180

160

140

120

100

80

60

40

20

0

Chemical shift (ppm) 147

124 110

Char-GLU-EAC Char-FRU- EAC Char-XYL- EAC Char-RIB- EAC 240

220

200

180

160

140

120

100

80

60

40

20

0

Chemical shift (ppm)

147

124

110

Char-HMF-WT Char-FF-WT 240

220

200

180

160

140

120

100

80

60

40

20

0

Chemical shift (ppm) 147

124 110

Char-DHA-WT Char-PRV-WT 240

220

200

180

160

140

120

100

80

60

40

20

0

Chemical shift (ppm)

Figure 4. The solid-state 13C NMR spectra of hydrochars


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Figure 5. Possible paths of polymerization of PRV into hydrochar


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Figure 6. Possible paths of polymerization of 3-deoxyglucosone into hydrochar


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Figure 7. Possible paths of polymerization of DHH into hydrochar


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Figure 8. Proposed typical structure model of hydrochar fragment


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Molecular structure and formation mechanism of hydrochar from

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