Stepwise Enrichment of Sugars from the Heavy Fraction of Bio-oil

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Stepwise Enrichment of Sugars from the Heavy Fraction of Bio-oil Shurong Wang, Yurong Wang, Furong Leng, and Junhao Chen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00039 • Publication Date (Web): 15 Feb 2016 Downloaded from http://pubs.acs.org on February 16, 2016

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Stepwise Enrichment of Sugars from the Heavy Fraction of Bio-oil Shurong Wang*; Yurong Wang; Furong Leng; Junhao Chen

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027, China

ABSTRACT: Sugars are among the main compounds in bio-oil produced by biomass pyrolysis. However, they can easily form coke, resulting in fast deactivation of the catalyst and severe blockage of the reactor during catalytic upgrading of bio-oil. Molecular distillation may be performed to retain sugars and pyrolytic lignin in the heavy fraction and thus to provide a primary stage for the subsequent sugar separation and utilization. A new method involving extraction, heat treatment, and column chromatography was introduced to separate further the aqueous phase obtained from the bio-oil heavy fraction by methanol–water extraction. Thus, monophenols and sugars were coextracted through these combined separation technologies. A fraction rich in monophenols was obtained by solvent extraction. Other impurities were further removed by heat treatment and column chromatography. Alcohol sedimentation method further separated the combined sugar-rich fraction into an ethanol-soluble fraction (SF-1) and an ethanol-insoluble fraction (SF-2). Gas chromatography–mass spectrometry, Fourier transform infrared spectroscopy, nuclear magnetic resonance

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spectroscopy, and high-performance liquid chromatography, were performed to characterize sugars in the obtained fractions. SF-1 mainly contained monosaccharides such as levoglucosan, glucose, and xylose, whereas SF-2 still had a small amount of cellobiose besides monosaccharides. The ultimate recovery rates for several identified monosaccharides were in the range of 75‒86 wt%.

KEYWORDS: Bio-oil; Sugar; Separation; Identification; Molecular distillation.

1. INTRODUCTION

Fast pyrolysis is a promising technology that can convert solid biomass into liquid bio-oil.1 However, crude bio-oil is a typical low-grade fuel and therefore requires upgrading to allow its utilization as motor fuel.2,3 Sugars, which are among the main compounds in bio-oil, can easily form coke during direct catalytic upgrading of crude bio-oil, leading to catalyst deactivation and reactor blockage.4,5 Separation of sugars overcomes these problems, allowing a sugar-rich fraction to be obtained and to be used in the extraction of valuable levoglucosan (or 1,6-anhydro-β-D-glucopyranose, LG) or in the production of ethanol, 5-hydroxymethylfurfural, butanol, and other chemicals.6-8 Bio-oil has more than 300 compounds that have varied polarities.9 Since sugars usually have good hydrophilicity, water is a common solvent used in their separation. Through separation using a large amount of water, compounds in bio-oil that have low heating values, such as water, acetic acid, 5-hydroxymethylfurfural, and furfural, may be extracted along with sugars. This method thus results in good fuel properties of the water-insoluble fraction, namely, pyrolytic lignin. Meanwhile, 2


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pyrolytic lignin can be used to produce fuels or to synthesize chemicals via substitution of phenolic compounds.10,11 Bennett et al.12 investigated the influence of water-to-oil ratio, extraction temperature and contact time on the extraction of LG from Scots pine bio-oil. They found the optimal extraction conditions resulted in a LG concentration of 87 g/L. In a later study, Li et al.13 obtained a maximum LG concentration of 12.7 wt% under optimal conditions.

Most of the sugars in bio-oil may be enriched by using water. However, some small molecular compounds in the resulting aqueous phase, such as furfural, 5-hydroxymethylfurfural, acetic acid, and phenol, inhibit sugar fermentation.14 Therefore, removal of these inhibitors, especially lignin-derived compounds, is one of the key factors for successful sugar fermentation. Multistep separation of bio-oil may enable the primary enrichment of sugars and is suitable for further extraction of sugars and removal of aldehydes, monophenols, and other compounds. Rover et al.15,16 developed a five-stage condenser for bio-oil collection to enrich sugars in the first and second fractions. They performed extraction using water and organic solvent, ionic liquid, ion-exchange resin, sodium hydroxide, or lime to extract sugars and to remove inhibitors from these two sugar-rich fractions. Their results show that overliming treatment could remove most acidic compounds with high efficiency.

In multistep separation of bio-oil in traditional distillation, several experimental parameters usually have to be adjusted to realize optimal distribution of various compounds among the fractions. Atmospheric and vacuum distillation technologies

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were often used to separate bio-oil.17,18 Capunitan et al.19 employed atmospheric and vacuum distillation to fractionate the bio-oil at different temperature ranges. With atmospheric distillation, they collected 38.4 wt% of distillates at ambient temperature to 180 °C; under 0.5 bar, they obtained 32.5 wt% of distillates at ambient temperature to 160 °C. Elkasabi et al.20 also separated compounds from bio-oils at relatively high temperature with the combination of atmospheric and vacuum distillation, and recovered fractions rich in different chemical families. Although high-temperature stepwise distillation yields different fractions with distinct properties, condensation and polymerization reactions are inevitable because of the thermal sensitivity of bio-oil. An accelerated aging test of bio-oil carried out at 80 °C and subsequent analysis indicates that the physical properties and chemical distribution of aged bio-oil could vary.21 These findings reveal that high-temperature distillation is not suitable for bio-oil. In our previous study, we adopted molecular distillation to separate bio-oil.22,23 Through this approach, we could enrich small-molecule compounds such as water, acetic acid, acetol, furfural, and phenol in the distilled fraction while retaining sugars and pyrolytic lignin, which have relatively high molecular weights, in the heavy fraction. No coke formation was observed during the entire separation process. Suitable technology was employed to upgrade the fraction according to the property of each fraction with high efficiency.24,25 Since the heavy fraction has high viscosity and poor fluidity, it requires dilution with alcohol and subsequent emulsification with diesel to allow production of emulsion fuel.26 However, the emulsion fuel is unstable because it contains pyrolytic lignin. 4


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Meanwhile, valuable sugars enriched in the heavy fraction favor chemical extraction or fuel production. Consequently, the methanol–water method has been introduced to separate the bio-oil heavy fraction and to yield pyrolytic lignins with various reactivities.27 The residual aqueous phase (RAP) could be further treated to extract sugars and monophenols. In the present study, such treatment was performed with a new

multistep-separation

method.

Gas

chromatography–mass

spectrometry

(GC–MS), Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and high-performance liquid chromatography (HPLC), were employed to analyze and to identify the structures and amounts of sugars in the obtained fractions.

2. EXPERIMENTAL SECTION

2.1. Molecular Distillation of Bio-oil.

Bio-oil was produced from the fast pyrolysis of lauan sawdust in a 5 kg/h fluidized bed reactor designed by Zhejiang University. The pyrolysis temperature was 500−550 °C. Details of the operation could be found in the literature.28 Before experiments, the crude bio-oil (CB) was filtered to remove the solid particles. The separation pretreatment of bio-oil was carried out by using a KDL-5 molecular distillation apparatus (UIC Company). The operation parameters were as follows: 70 °C temperature, 120 Pa pressure, and 2 mL/min feeding rate. Detailed procedures of the operation are described in the literature.22 After pretreatment, three fractions were

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obtained, namely, the light fraction (LF), middle fraction (MF), and heavy fraction (HF).

2.2. Separation and Purification of Sugars.

Pyrolytic lignin in the bio-oil HF was removed by molecular distillation using methanol–water extraction method.27 Subsequently, multistep separation was applied to separate sugars from the RAP, as shown in Figure 1. Dichloromethane (15 × 3 mL) was used to extract 3 g of RAP for 30 min at room temperature with the aid of ultrasound. The dichloromethane-soluble phases were combined and condensed under vacuum at 25 °C to produce a phenolic fraction. Afterward, the raffinate was heated in a glass plunger tube at 120 °C for 5 h. After the removal of precipitate by filtration through a 0.45 µm organic membrane, the sample was loaded on the top of the column (25 × 140 mm), which was filled with neutral alumina (100‒200 mesh, upper layer) and silica gel (100‒200 mesh, bottom layer) at a volume ratio of 1:1. Water (150 mL) and 1:1 (v/v) ethanol–water solution (80 mL) were used in sequence to elute the sample. The water-eluted and ethanol–water-eluted fractions were evaporated under vacuum and 40 °C to remove the solvents. Finally, the two fractions were combined and dissolved in ethanol, and the resulting mixture was filtered through a 0.45 µm organic membrane to obtain an ethanol-insoluble solid. The solid was washed with ethanol, redissolved in water, and then filtered and re-evaporated under vacuum and 50 °C to obtain SF-2. The ethanol-soluble fraction was evaporated under

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vacuum and 25 °C to obtain a fraction named as SF-1. Both fractions were analyzed by FTIR spectroscopy, NMR spectroscopy, and HPLC.

2.3. Analysis of Bio-oil and Its Fractions.

The chemical distribution of each sample was determined by GC–MS (Trace DSQII, Thermo Fisher Company) using a DB-WAX poplar column (Agilent Company). The oven temperature was held at 40 °C for 1 min, increased to 240 °C at a heating rate of 8 °C/min, and then maintained for 20 min at this temperature. Quantification of the typical compounds in bio-oil and its fractions was carried out by GC (7890A, Agilent Company) using a HP-INNOwax capillary column, having the same temperature program with GC–MS. The standards were purchased from Alfa Aesar.

SF-1 and SF-2 were diluted by water, and were characterized by HPLC (U3000) using a Bio-Rad Aminex HPX-87H acidic sugar column (7.8 × 300 mm). The column temperature was kept at 60 °C, and 0.005 M H2SO4 solution (0.6 mL/min) was used as mobile phase. A RI2000 differential refraction detector was used at a temperature of 40 °C. The peaks in the chromatographs were attributed to the standard sugars (Aladdin), including glucose, xylose, mannose, levoglucosan, cellobiose, with the same retention times. Quantification of each sugar was based on its calibration curve calculated from the response values of different sugar concentrations.

SF-1 or SF-2 was ground with solid KBr, and the mixture was pressed into a thin film, which was dried under an infrared lamp and then scanned within the range of 4000‒400 cm−1. One-dimensional NMR spectroscopy and two-dimensional (2D) 7


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1

H‒13C heteronuclear single quantum coherence (HSQC) correlation NMR

spectroscopy of the sugar fractions were conducted on an Agilent 600MHz DD2 NMR apparatus. Appropriate samples were dissolved in 0.5 mL of D2O. The relaxation delay of the 1H dimension was 1 s, and that for the 13C dimension was 2 s. The total acquisition time lasted 10 h. The 1JC‒H value was 146 Hz.

3. RESULTS AND DISCUSSION

3.1. Chemical Composition of Bio-oil and Its Fractions.

Three bio-oil fractions with different physicochemical properties were obtained by molecular distillation. The recovery yields of LF, MF, and HF were 39.14, 8.95, and 49.58 wt%, respectively. The water content in the HF was as low as 6.50 wt%, while those in the LF and MF were 80.81 wt% and 11.51 wt%, respectively. GC–MS can detect about 40 wt% of bio-oil,9 and is used as a common identification technology which gives a broad insight into the composition of bio-oil. The relative amounts of compounds detected by GC–MS in bio-oil and its fractions were calculated by area normalization. Some typical compounds are quantified and listed in Table 1. Acetic acid and acetol occupied 3.62 and 1.92 wt% of crude bio-oil, respectively. After molecular distillation, their corresponding amounts increased to 6.24 and 2.33 wt% in the LF, respectively, whereas those in the HF decreased to < 1 wt%. Most furfural was enriched in the distilled fractions, whereas its content in the HF was as low as 0.004 wt%. Since LF had high water content and mainly contained small molecular compounds, it could be used as a feedstock for hydrogen production via steam 8


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reforming.24 The chemical distribution of MF was similar to that of bio-oil, but it almost had no large molecular polymers like pyrolytic lignin. Hence, MF could be upgraded through catalytic cracking or hydrotreatment. The large molecular compounds involving sugars and pyrolytic lignin would be retained in the HF, while most small molecular compounds were distilled out during this molecular distillation process. Since GC could only partially detect low-boiling compounds in HF, the typical compounds were only responsible for 5.76 wt% of HF. Besides that, the detectable compounds also included a high relative content of sugars and monophenols with various side chains.

3.2. Sugar Enrichment.

HF has high viscosity and occurs as a semisolid at room temperature. Therefore, effective removal of pyrolytic lignin using water extraction only is difficult. Methanol, a suitable diluent for HF, was first introduced to break the network structure covering the whole HF. After that, water was added to promote precipitation of pyrolytic lignins of various reactivities.27 The total yields of pyrolytic lignins based on the mass of CB were 9.86 wt% and 19.88 wt% with respect to HF. After the removal of pyrolytic lignins, the RAP contained 36.50 wt% of organics extracted from CB and was enriched with extractable monophenols (60.57%) and sugars (10.59%), as well as a small amount of ketones (9.52%). The main monophenols in the RAP had relatively good water solubilities and low molecular weights, including phenol, guaiacol, syringol, benzenediols, etc. In order to extract sugars of high purity

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from RAP, a new multistep-separation method was developed. Since dichloromethane has good selectivity toward phenolic compounds,29 we used it first to obtain a dichloromethane-soluble fraction, which contains 30.57 wt% of the total organics in RAP. The identification of dichloromethane-soluble fraction using GC–MS indicates that monophenols were the most abundant chemicals, and the total content of selected monophenols occupied 3.53 wt% of this fraction. This phenolic fraction could be used further to enrich phenolic compounds with high purity. Phenolic compounds mainly consisting of large molecules with various side chains, such as syringaldehyde, sinapaldehyde, and hydroquinone, were still present in the raffinate, but their total relative content decreased from 46.38% to 28.02%. Meanwhile, other small-molecule monophenols were removed effectively by dichloromethane. In addition, the raffinate also had a high relative content of ketones (17.18%) probably because of the poor extraction selectivity of dichloromethane.

After heat treatment and column chromatography of the raffinate, the water-eluted fraction was obtained (26.74 wt% yield with respect to the organics in the RAP). The chemical distribution of this fraction is displayed in Figure 2. Compounds that were abundant in this fraction are sugars (34.08%), ketones (24.64%), and phenols (20.21%). Ketones were mainly cyclic compounds such as cyclopentenones, furanones, and lactones. The most abundant phenols were vanillin (7.18%), hydroquinone (4.22%), and syringaldehyde (3.38%). As a typical furan derivative, 5-hydroxymethylfurfural still had a relative content of 1.86%. The above results indicate that these compounds are strongly retained during extraction, heat treatment, 10


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and column chromatography and are thus incompletely removed. Some researchers had tried to remove these inhibitors with different methods.14-16 However, their results also suggested that the complete removal of these impurities was difficult. During heat treatment, some large-molecule precipitates, that might be derived from reactions between double bond side chains or reactions with other functional groups (e.g., condensation and polymerization),21 were formed in the raffinate. It is worth noting that sugars can participate in these reactions at high temperature,30 resulting in depletion of sugars. The GC‒MS results show that the detectable sugars in this water-eluted fraction mainly include LG, 3,4-altrosan, 1,5-anhydro-D-mannitol, anhydro-D-mannosan, 1,4:3,6-dianhydro-β-D-glucopyranose, and glycolaldehyde (GA).

The water–ethanol-eluted fraction comprised only 2.91 wt% of the original organics in RAP. The GC‒MS results indicate that the main compounds in this fraction were sugars and that phenols were present in small amounts, implying that most of the compounds could be eluted simply by water. Nevertheless, some sugars were still adsorbed on the adsorbents probably because of the strong polarity of sugars. Finally, the recovered sugar-rich fractions comprised 29.65 wt% of the original organics in RAP. As is clear from the above analysis, some small molecular inhibitors, such as acetic acid and furfural, could be enriched in the distilled fractions during molecular distillation, whereas a small amount of benzenediols and 5-hydroxymethylfurfural were still retained in the final sugar fractions to impede the microbial fermentation process or sugar purification. Therefore, the removal of these impurities needs further 11


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investigation. GC‒MS can detect most of the compounds with low boiling points and high volatility but has limited capability for the detection of sugars. Hence, FTIR spectroscopy, NMR spectroscopy, and HPLC were used in order to detect and to quantify the sugars in the ethanol-soluble and ethanol-insoluble fractions (SF-1 and SF-2) obtained through alcohol sedimentation of the combined sugar-rich fraction. 3.3. Characterization of Sugar Fractions.

3.3.1.

FTIR Spectroscopy.

Since some of the combined sugar-rich fraction could be dissolved in ethanol, 24.82 wt% of SF-1 (based on organics in RAP) was obtained, while 4.83 wt% of SF-2 (based on organics in RAP) was precipitated. SF-1 was a bright yellow viscous liquid, while SF-2 was a brown, deliquescent solid. FTIR was used to determine the distribution of functional groups in SF-1 and SF-2, as shown in Figure 3. The broad band at 3570–3100 cm−1 is ascribed to sugar hydroxyls. Vibrations of their intramolecular and intermolecular hydrogen bonds occurred at 3570–3450 cm−1 and 3400–3200 cm−1, respectively.31 The hydroxyl peak of SF-2 is close to the band of intramolecular hydrogen bonds. The peak at 1730–1725 cm−1 is associated with carbonyl from esters,31 such as ethyl citrate and butyrates detected by GC‒MS, and that at around 1602 cm−1 is due to aromatic skeletal vibration and C=O stretching. Sugars produced characteristic vibration bands in the region of 1200–800 cm−1. The strong peak at around 1050 cm−1 represents C–O, C–C stretching, C–OH vibration, or 12


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glucosidic bonds.32,33 SF-1 contains a relatively high content of these groups because of the internal ether structure or the hydroxyl linked to the sugar ring from LG or other anhydrosugars. For SF-2, this peak is derived from anhydrosugars or glucosidic bonds in oligosaccharides. The vibration band due to the pyran ring at 930–925 cm−1 suggests the presence of pyran sugars such as LG and glucose. The peak at 895–892 cm−1 is indicative of stretching of C-anomeric groups, C1–H deformation, or ring stretching.31 A small sharp peak at 892 cm−1 is assigned to β-glucosidic linkages between neutral sugar side chains.34,35 This peak thus indicates the existence of neutral sugars in SF-1. The intensity of this peak, however, is low in the spectrum of SF-2.

3.3.2. NMR Spectroscopy. NMR spectroscopy is useful in the structural characterization of sugars and other compounds. The signal of the solvent (4.8 ppm) was used as a reference point for the internal chemical shift of 1H NMR. On the basis of the assignment of each region,36,37 the relative content of corresponding protons was calculated with respect to the total protons (water and ethanol signals were eliminated). The distribution of various protons is listed in Table 2. Characteristic protons of sugars mainly concentrated in the region of 3–6 ppm. 1H NMR spectra of SF-1 and SF-2 (Figure 4) are somewhat similar, suggesting the presence of similar compounds in these two fractions. Signals at 5.5–5.0 ppm mainly derive from acetals or vinyl, while signals for anomeric protons in sugars, such as that for C1 in LG (5.48 ppm), also occur in this region. Meanwhile, active aldehyde groups in bio-oil could form relatively stable acetal 13


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structures with alcohols. The vinyl signal may originate from vinyl groups in the side chain of phenolic compounds and from cyclic compounds containing double bonds, such as 4-allylsyringol and cyclopentenons. Signals appearing at 3.3–4.1 ppm correspond to protons attached to carbon atoms singly bonded to oxygen and thus imply the presence of alcohols, esters, ethers, or sugars. The proton intensity in this region is the strongest, occupying about 40% for both sugar fractions. Although the peak at 3.67 ppm is due to methylene in ethanol, some protons of sugars also produce signals around this peak, as clearly shown in the 2D HSQC NMR spectra.

Peaks at 6.6–7.0 ppm, which lie in the aromatic region, are due to protons in phenols and alkylbenzenes, and those at >7.7 ppm are attributed to protons in benzoic acid or aromatic esters. Figure 4 and Table 2 indicate that the relative content of aromatic protons of SF-1 is higher than that of aromatic protons of SF-2. This higher content is due to the good solubility of monophenols in ethanol; some small-molecule monophenols such as syringol and vanillin remained in the sugar fraction even though multiple separation steps had been employed. Nevertheless, the contents of aromatic acids and aromatic esters in these two fractions were low, resulting in weak signal intensities at high chemical shifts. Protons with relatively strong signal intensity at 8.48 ppm were connected to carbons at 173 ppm and could be observed in the HSQC NMR spectra (not shown in Figure 5). Carbons at δC 190–160 ppm derive from carboxylic acids37 and are consistent with the carbon signals from formic acid. However, formic acid is a low-boiling and reactive chemical and could thus react with alcohols or other compounds to form acetals or esters during the separation process. 14


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As a result, the signal at 8.48 ppm might be related to formates. Aldehyde groups at 9‒10 ppm indicate the existence of aldehydes in sugar fractions, albeit in low amounts, in accordance with the GC‒MS results. Therefore, the composition of SF-1 is more complex than that of SF-2 because it includes a large amount of sugars, as well as some phenolic compounds and ketones. Moreover, some side chains were also observed, such as aliphatic protons in extended alkyl chains (0.5‒1.9 ppm) and aliphatic protons attached to carbon atoms adjacent to a carbonyl or aromatic group (1.9‒3.2 ppm). Since the chemical shift window of the hydrogen spectra was narrow, we further identified the chemicals using 2D spectra.

2D HSQC NMR was used to determine the main compounds in the two sugar fractions. The main cross signals (δC/δH 110–50/5.6–3.0 ppm) for the sugar fractions (Figure 5) are assigned according to the nuclei resonance data for the sugars from the Biological Magnetic Resonance Data Bank.38 The main identified sugars are depicted in Figure 6. LG, glucose (G), xylose (X), and GA, were discovered in both fractions. GA is a linear, small molecule that forms during cellulose pyrolysis; some studies report that the formation of LG and GA have a competition- or inheritance-based relationship.39,40 Therefore, these two compounds are abundant in bio-oil. Some overlapping cross signals for xylose and glucose were observed and could be further identified by HPLC. In addition to signals for sugars, signals for other unknown compounds were also observed. As suggested by the GC–MS results, some anhydrosugars

such

as

1,4:3,6-dianhydro-β-D-glucopyranose

and

anhydro-D-mannosan might exist in the sugar fractions. Unfortunately, the lack of 15


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standard NMR spectra for these anhydrosugars resulted in uncertainties regarding the existence of these compounds in the sugar fractions. Cross signals for cellobiose in SF-2 overlap with those of glucose and xylose. Its content was low (as determined by HPLC), as evidenced by its low resonance intensity.

3.3.3. HPLC. HPLC was used to determine the mass content of each sugar in SF-1 and SF-2 (Table 3). The most abundant pyrolytic sugar was LG, comprising about 20 wt% of both sugar fractions. LG was typically produced by cellulose pyrolysis;40 its content in the CB was markedly higher than that of the other sugars. Nevertheless, LG only comprised 3.60 wt% of CB. Glucose may also be derived from cellulose pyrolysis, as cellulose is built from glucose units with β-1,4-glycosidic bonds. Its original content in CB was only 0.69 wt%, whereas it was around 4 wt% in the sugar fractions. Xylose was mainly from hemicellulose pyrolysis.41 After multiple separation steps including molecular distillation, methanol–water extraction, organic solvent extraction, heat treatment, and column chromatography, the recovery rates for glucose, xylose, and LG reached 85.79, 85.40, and 75.03 wt%, respectively. During molecular distillation, sugars were retained in the HF.22 Nevertheless, methanol–water extraction led to partial loss of LG accompanied by the precipitation of pyrolytic lignins.27 The subsequent multistep-separation process also led to some loss of sugars. Hu et al.42 found that LG can undergo hydrolysis under high temperature in the presence of phenolic compounds. This finding suggests that heat treatment can lead to LG hydrolysis. A small amount of cellobiose was also detected in SF-2 because of its 16


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poor solubility in ethanol. Since oligosaccharides are usually poorly soluble in ethanol and the identified sugars in SF-2 are in low amounts, some unknown oligosaccharides or anhydrosugars were perhaps present in SF-2. Other researchers also reported this possibility during the analysis of sugars obtained from bio-oil.7,12 However, HPLC was not able to identify such sugars. Moreover, the elution times of xylose and mannose are very similar; in contrast, NMR analysis indicates that xylose is present rather than mannose. The integrated results from these detection technologies confirm that the main monosaccharides in lauan bio-oil are LG, glucose, xylose, and GA, and that it contains a small amount of cellobiose.

4. CONCLUSIONS The present study adopted a new multistep-separation method involving extraction, heat treatment, and column chromatography to separate sugars from the aqueous phase of bio-oil HF after the removal of pyrolytic lignins. Our results show that dichloromethane could effectively extract a fraction rich in phenolic compounds. Subsequent heat treatment and column chromatography further removed impurities through chemical reactions and adsorption. Finally, the recovery rate for the combined sugar-rich fraction was 29.65 wt%. Alcohol sedimentation method could further separate the combined sugar-rich fraction into two fractions, both of which include some monosaccharides such as LG, glucose, xylose, and GA. The ethanol-soluble fraction, SF-1, contained more phenolic compounds and ketones compared with those in SF-2, but SF-2 had a small amount of cellobiose. The ultimate recovery rates for glucose, xylose, and LG (85.79, 85.40, and 75.03 wt%, 17


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respectively) suggest that our multistep-separation method is suitable for the separation of sugars from bio-oil.

AUTHOR INFORMATION Corresponding Author * Tel.: +86-571-87952801. Fax: +86-571-87951616. E-mail: [email protected].

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT The authors appreciate ﬁnancial support granted from the National Science and Technology Supporting Plan Through Contract (2015BAD15B06), the National Natural Science Foundation of China (51476142), and the National Basic Research Program of China (2013CB228101).

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Figure captions

Figure 1. Scheme of multistep separation of the bio-oil heavy fraction.

Figure 2. Total ion chromatogram of the water-eluted fraction.

Figure 3. FTIR spectra of the sugar fractions. Figure 4. 1H NMR spectra of the sugar fractions.

Figure 5. 2D HSQC NMR spectra of the sugar fractions.

Figure 6. Main sugars that were determined in the sugar fractions.

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Table 1. Some typical compounds in bio-oil and its fractions (wt%, based on aqueous product).

Compound

CB

LF

MF

HF

Acetic acid

3.62

6.24

3.26

0.82

Propionic acid

0.14

0.05

0.14

0.20

Butanoic acid

0.23

0.07

0.15

0.27

Acetol

1.92

2.33

3.00

0.52

2-Cyclopenten-1-one

0.09

0.07

0.19

0.06

2(5H)-Furanone

0.31

0.08

0.75

0.31

2-Furanmethanol

0.16

0.03

0.26

0.15

Furfural

0.23

0.48

0.33

0.004

Phenol

0.08

0.06

0.07

0.10

p-Cresol

0.14

0.03

0.17

0.22

Guaiacol

0.26

0.12

0.52

0.08

4-Methyl-guaiacol

0.18

0.05

0.38

0.09

Vanillin

0.15

/

0.12

0.25

Eugenol

0.12

0.02

0.17

0.18

iso-Eugenol

0.38

/

0.31

0.69

Syringol

0.55

/

0.43

1.00

Catechol

0.44

/

0.34

0.82

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Table 2. Peak assignments for the 1H NMR spectra of the sugar fractions.

Proton percentage (%) Range (ppm)

Assignment SF-1

SF-2

13.13

10.73

14.98

18.09

atoms

41.20

39.36

5.5‒5.0

Acetalic and vinylic H

5.44

6.13

8.3‒6.5

Aromatic H

1.39

0.73

10.0‒9.0

H in nonhydrated aldehydes

0.09

0.05

1.9‒0.5

Aliphatic H in extended alkyl chains

3.2‒1.9

H bound to aliphatic carbon atoms adjacent to unsaturated groups

4.1‒3.3

H bound to oxygenated aliphatic carbon

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Table 3. Content and recovery rate of each identified monosaccharide in the crude bio-oil and sugar fractions.

Content (wt%)

Recovery rate (%)

Sugar CB

SF-1

SF-2

SF-1

SF-2

Glucose

0.69

4.09

4.36

80.59

5.20

Xylose

1.20

7.20

5.81

81.42

3.98

Levoglucosan

3.60

18.59

21.14

70.19

4.84

Cellobiose

ND

ND

0.02

/

/

Recovery rate of a sugar = mass of the sugar in the fraction/mass of the sugar in crude bio-oil; ND = not detected.

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Figure 1.

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Figure 2.

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Figure 3.

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Figure 4.

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Figure 5.

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Figure 6.

33


Stepwise Enrichment of Sugars from the Heavy Fraction of Bio-oil

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