Samples of bamboo material were collected from a local farm in Sichuan Province, China. It was ground to pass through a 40-mesh sieve to obtain the flour and then dried in an oven at 105 ℃ overnight. The dried ground samples were stored in polyethylene bags. Elemental composition and ultimate analysis of the bamboo are shown in Table 1. All other chemicals were purchased from Shanghai Aladdin Bio-chem Technology Co., Ltd., Shanghai, China.
Chemical composition Percent (wt%) Cellulosea 41.8 Holocelluloseb 59.8 Ligninc 29.3 Ashd 1.5 Moisturee 6.1 Extractivesf 3.3 Notes:a, b, c, d, e, and f were determined ac- cording to ASTM D 1103–60, ASTM D 1106– 96, ASTM D 1102–84, ASTM D4442–15 and ASTM D 1107–96, respectively.
Table 1. Chemical composition of bamboo biomass.
The solvothermal liquefaction of the bamboo was carried out in a 250 mL cylindrical autoclave pressurized reactor equipped with a magnetic stirrer. In each run, 7.0 g of dried bamboo powder and 3 wt% of sulfuric acid (based on dry material) was introduced into the reactor with 105.0 g ethanol. High purity N2 (99.9999%) was used to remove the inside air. The reactor was then heated to the desired temperature and held temperature for a certain period. After completing the reaction, the reactor was rapidly cooled down to room temperature. After releasing the gaseous products, the liquid/solid mixture was filtered through a pre-weighed filter paper under reduced pressure. The solid residue (SR) was dried in an oven overnight at 105 ℃, and then cooled to room temperature. All experiments were conducted in duplicate and reproduced with a standard deviation lower than 5%.
The filtrate was neutralized with 20% NaOH solution and then heated to 50 ℃ with evaporation to remove and recycle ethanol. When added deionized water into the neutralization solution (the ratio of water to neutralization solution was 1:5), the high molecular weight fraction from depolymerized lignin (fraction 1#) which is insoluble in ethanol-water was precipitated after centrifuging (4000 r/min, 20 min). The liquid mixture was then distilled under vacuum to remove and recycle ethanol, lower molecular weight fraction from depolymerized lignin (fraction 2#) was obtained with a centrifuge process. Furthermore, dichloromethane was added to the liquid (the weight ratio of dichloromethane to mixture liquid is 1:1) after removal of the ethanol completely, two fractions were then formed, a water-soluble phase and a dichloromethane soluble phase. Since the liquid fraction from depolymerized lignin (fraction 3#) was obtained after evaporating the respective solvent. Fig. 1 shows the photographs of raw sample, fractions 1#, 2#, 3# and water soluble phase.
Figure 1. Photos of (a) raw sample (b) fraction 1# (c) fraction 2# (d) fraction 3# (e) water soluble fraction.
The procedure used for the separation of products is shown in Fig. 2. These three fractions were vacuum dried overnight at room temperature and weighed. Lignin conversion, cellulose conversion and holocellulose conversion all expressed in wt%, were calculated based on the content before and after liquefaction relative to the composition contained in biomass. Bamboo conversion and yield of each fraction were calculated with Eqs. (1) and (5), respectively.
The aromatic compounds were dissolved in THF for characterization using gas chromatography/mass spectrometry (GC–MS, Agi-lent 7890 N/5975 N, America) equipped with an HP-5 column (30 m × 0.05 μm × 0.32 nm). The GC oven temperature was programmed from 60 ℃ to 90 ℃ (3 min) at a temperature ramp of 5 ℃ /min and then to 210 ℃ (10 min) at a temperature ramp of 10 ℃ /min; the detector temperature was 250 ℃. Carrier gas was helium (99.999%) at 1.6 mL/min with a split ratio of 1:20. The injection size was 0.2 μL. Compounds were identified by comparisons with the NIST08 library of mass spectra. The relative area of individual compounds is defined as the percentage of the chromatographic area of the compound out of the total area. The compounds with a high molecular weight and non-volatile nature could not be determined using GC–MS.
The molecular weight of the aromatic compounds was analyzed on a gel permeation chromatography (GPC, Waters-1515 series, America) equipped with a Shodex Refrative Index (RI) detector. The THF (chromatographically pure) was used as the solvent for the sample solution and eluent at a flow rate of 1.0 mL/min at 30 ℃. Polystyrenes with molecular weight ranging from 162 to 6000 g/mol were used as the calibration standards.
Two-dimensional heteronuclear single-quantum coherence (2D HSQC NMR) spectra were determined on a Bruker Avance 500 MHz spectrometer (Germany). For aromatic products, about 80 mg of samples were placed into a 5 mm NMR tube and dissolved in 0.5 mL of dimethyl sulfoxide (DMSO). Data processing was carried out with the standard Bruker Topspin-NMR software.
The surface morphology of the raw samples and the residues was observed by scanning electron microscope (SEM) (S-3400 N, Japan) at an acceleration voltage of 10 kV. The X-ray diffraction (XRD) profiles of bamboo fibers before and after reaction were collected, and the samples were in powder form and analyzed by using a D8 Advanced Instrument (Bruker AXS, Germany) with Cu K radiation and 2 from 10° to 70℃ypress crystallinity, as expressed by crystallinity index (CrI), was calculated according to the literature (Xu et al., 2012).
2.2. Solvothermal liquefaction and preparation of aromatic products
2.3. Product analysis
Temperature is an important influence in controlling the thermochemical reactions. The conversion of bamboo lignin, as well as cellulose was conducted at various temperatures in ethanol using H2SO4 as the acid catalyst. The conversion eﬃciency of the lignin increased markedly from 65.4% to 94.5% with increasing temperature from 140 ℃ to 200 ℃ (Fig. S1), while the conversion eﬃciency of cellulose increased from 55.3% to 83.6%. As reaction temperature increased to 220 ℃, the amount of the SR increased significantly. A similar trend was found in lignin conversion where the conversion rate decreased with increase of temperature (Singh et al., 2014). This was attributed to the repolymerization of lignin-degraded intermediates. It implies that an optimal reaction temperature for bamboo biomass depolymerization is 200 ℃.
It is obvious that the yield of fractions containing aromatic products can be dramatically increased with the increase of reaction temperature. When the reaction temperature was 140 ℃, the yield of fractions was 12.3% (Fig. 3). However, when the reaction tem-perature increased from 160 ℃ to 200 ℃, a significant increase of the yield of fractions containing aromatic product was observed. Our results are in good agreement with these previous reports (Ma et al., 2014; Custodis et al., 2015). It suggests that higher tem-perature benefits the cleavage of the lignin bond into smaller soluble fragments (Tymchyshyn and Xu, 2010). In this study, the yield of fractions containing aromatic products was higher than the lignin content in bamboo material. This significant increase could be attributed to the condensation and re-polymerization of the intermediates and liquefied products to form high molecular weight by-products, which precipitated with the aromatic compounds in the process of stepwise fractionation.
Figure 3. Effect of temperature on yield of products (bamboo biomass: 7.0 g; ethanol: 105.0 g; sulfuric acid: 0.21 g; reaction time: 30 min; stirring speed: 400 r/min).
The effect of temperature on the yield of water soluble phase is quite different, by comparing with the aromatic compounds. The content of water soluble phase increased dramatically from 12.7% to 41.8% as reaction temperature increased from 140 ℃ to 180 ℃. However, with an increase of temperature to 200 ℃, a decline of the yield of 38.3% was observed. A further increase of temperature to 220 ℃ would result in a lower yield of 20.5%. This decline of yield may be attributed to the side reaction, i.e., the re-polymerization of liquefied products into heavier compounds (Ye et al., 2012).
As shown in Table 2, the effect of the residence time from 10 min to 60 min on product yield was investigated at 200 ℃. With an increase of time from 10 min to 30 min, the yield of fractions containing aromatic products increased from 22.4% to 26.7%, and was plateaued if further increased the reaction time to 60 min. As with reaction time increased, the water soluble yield increased from 28.7% to 38.3% and then decreased to 23.4%, suggesting that the reaction time indeed affects the product distribution pattern. It demonstrates that: 1) high-molecular-weight compounds could be formed via the re-polymerization reactions with increased time; and 2) the prolonged time could highly increase the probability of secondary and tertiary reactions of intermediates, and then precipitated with aromatic compounds due to their mutual solubility and similar physio-chemical properties in the organic solvent (Xu et al., 2016).
Time (min) Bamboo conversion (wt%) Product yield (wt%) Aromatic product Water soluble phase 1# 2# 3# 10 59.4 ± 1.4 1.8 ± 0.5 17.5± 1.4 3.1± 0.5 28.7 ± 1.8 20 64.3 ± 1.5 2.0 ± 0.3 17.9± 1.2 2.6± 0.J 32.7 ± 1.7 30 78.4 ± 1.6 2.2 ± 0.4 19.5± 1.5 4.9± 0.8 38.3 ± 1.5 40 74.3 ± 1.9 2.0 ± 0.3 19.7± 1.8 4.2± 0.6 35.0 ± 2.4 50 76.6 ± 1.6 1.9 ± 0.6 20.0± 1.7 4.2± 0.5 31.7 ± 1.2 60 73.5 ± 1.3 1.7 ± 0.4 19.9± 1.2 5.4± 0.7 23.4 ± 2.1 Notes: Reaction conditions—bamboo biomass, 7.0 g; ethanol, 105.0 g; sulfuric acid, 0.21 g; reaction temperature, 200 ℃; stirring speed, 400 r/min.
Table 2. Effect of residue time on yield of products.
The GC/MS analysis of the fractions containing aromatic products was performed to identify the major monomeric phe-nols derived from lignin. The relative contents of phenolic compounds take up about 80% of the total area for each fraction. As expected (Fig. 4), the major components in three fractions were phenolic compounds such as 4-ethylphenol, vanillin, ethyl 4-hydroxy-3-methoxybenzoate, 2-(4-hydroxy-3, 5-dimethoxyphenyl) acetic acid, ethyl (E)-3-(4-hydroxyphenyl) acrylate, and ethyl (E)-3-(4-hydroxy-3-methoxyphenyl) acrylate (Table S1), and these high value compounds may be applied directly in phenolic resin synthesis and medicines (Effendi et al., 2008; Kim, 2015). The oxygen-containing components, such as ethyl levulinate in three aro-matic compounds fractions was identified to be 10.65%, 8.09%, and 6.71%, respectively. These results indicated that most of the hydrophobic aromatic compounds were well separated from the reaction mixtures by stepwise separation process. During the lique-faction of bamboo biomass, not only cellulose and hemicellulose were decomposed, but high molecular polymeric lignin was also decomposed into low molecular phenolic compounds, which reacted intermediately with C==C, —OH, and —C==O groups.
Figure 4. The GC–MS chromatograms of three aromatic compounds fractions (Reaction conditions for bamboo biomass liquefaction were 200 ℃ at 30 min.).
Table 3 presents the average molecular weight and polydispersity of the aromatic products from stepwise fractionation at different temperatures. The results demonstrated that the molecular fragments were released from the lignin plant cell corners, resulting in the formation of much lower molecular weight fractions reached 100% solubility in the THF. The aromatic products presented several potential industrial utilizations for fine chemicals, polyurethane foams, pharmaceuticals, and epoxy resins for printed circuit boards (Thakur et al., 2014).
Temperature (℃) and Time (min) Aromatic product Mw (g/mol) Mn (g/mol) Polydispersity (Mw/Mn) Solubility in THF (%) 180 ℃, 30 min 1# 1734 1000 1.73 100 2# 1124 739 1.52 3# 432 265 1.63 200 ℃, 30 min 1# 867 581 1.49 100 2# 573 383 1.83 3# 323 231 1.51 200 ℃, 60 min 1# 798 467 1.71 100 2# 552 324 1.70 3# 184 245 1.32
Table 3. Weight-average (Mw) and number-average (Mn) molecular weights and polydispersity (Mw/Mn) of aromatic products.
Both weight-average (Mw) and number-average (Mn) generally decreased with the increase of reaction temperature and holding time. The Mw and Mn of fraction 3# were lower than those of fraction 1# and 2# in most cases, and these results were perfectly consistent with the GPC curves (Fig. 5). As shown in Fig. 5, fraction 1# had the highest molecular weight. Fraction 2# showed a bi-model distribution of molecular weight, and the GPC curves of fraction 3# showed three peaks. These results demonstrated that the fraction 3# mainly consists of phenol and propylsyringol monomers and their dimmers or timers, which is smaller enough to be soluble in an extraction solvent, i.e., dichloromethane in this study.
Figure 5. The GPC analysis of three fractions of aromatic platforms (Reaction conditions for bamboo biomass liquefaction were 200 ℃ at 30 min.).
The aromatic products were further characterized by 2D HSQC NMR analysis, which provides important structural features and allows for the resolution of otherwise overlapping resonances observed in either 1H or 13C NMR spectra (Kim and Ralph, 2010; Yuan et al., 2011). As shown in Fig. S2, overall HSQC spectra of the aromatic product from liquefaction can be divided into side chain units (aliphatic region) and aromatic region. The side chain region was further divided into aliphatic C—O region (δC/δH 50–80/3.0–5.0) and aliphatic C—C region (δC/δH 15–50/1.0–3.0).
The HSQC correlation signals from the fractions were assigned according to the literature data (Thomas et al., 2017; Huang et al., 2019; Wang et al., 2019). The main cross-signals from syringyl (S)- and guaiacyl (G)-lignin units, as well as others lignin substructures were observed, which can be concluded that a large amount of lignin intermediates was present in three fractions of aromatics (Fig. 6). The signals at δC/δH 106.5/6.50 are attributed to C2, 6—H2, 6 correlations of syringyl (S) units, whereas the C2, 6—H2, 6 correlations in Cα-oxidized S units (S') were found at δC/δH 107.5/7.25. The G lignin units showed two different correlations at δC/δH 115.0/6.35 and δC/δH 115.2/6.67 for C5—H5 and C6—H6. Likewise, signals of p-hydroxybenzoate substructures (PB) and p-hydroxycinnamyl alcohol end groups (Ⅰ) were also detected. In the aromatic region of the HSQC spectrum of the fraction 1# and fraction 2#, a minor signal for the Cα—Hα correlations of substructures Ⅰ was observed at δC/δH 126.0/6.61. Slight shifts for three aromatic fractions from previously reported signals in literatures (Mansfield et al., 2012; Feng et al., 2017) to our observed signals might be due to interactions of other functional groups that were produced from the liquefaction process, i.e., on the altered side chain on the S, S', G, and PB aromatic rings.
Figure 6. Aromatic regions of HSQC spectra from (a) fraction 1#; (b) fraction 2#; and (c) fraction 3#.
The signal intensity of aliphatic C—C regions of the HSQC spectra is lower than that in the C—O region, as shown in Fig. S3. The aliphatic —O side-chain regions are shown in Fig. 7. Signals from methoxyl groups (δC/δH 55.7/3.42) in three aromatic fractions are the most prominent in this region. Lignin is a complex macromolecule composed of randomly branched units of bearing p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units. The phenylpropenyl building blocks are connected by several types of C—C or C—O—C linkages, including β-5', 5–5', β-β', β-O-4', and 4-O-5', forming biphenyl, resinol, phenylcoumaran, alkyl-aryl ether, and biphenyl ether substructures (Pandey and Kim, 2011). The -ether (β-O-4) units are the most frequent coupling linkage in lignin. The Cβ—Hβ and Cγ—Hγ correlations in β-aryl ether units were discovered at 55.0/3.42 and 63.0/3.62, respectively, whereas the Cγ—Hγ correlations were overlapped with other signals in fraction 3#. Besides, small signals corresponding to the Cγ—Hγ correlations in p-hydroxycinnamyl alcohol end groups (substructures Ⅰ) were also observed at δC/ δH 63.0/3.62 in the spectra of fraction 1#. These results suggest that the C—O linkages in the three fractions are different and -O-4-linkages in -ethers can break into small molecules during thermochemical conversion process.
The water soluble products from the directional liquefaction of bamboo at 200 ℃ were analyzed by using GC–MS to identify the composition (Table 4). From the results of the GC–MS analysis, the chemical compositions of this liquid phase consist of six carbon sugars such as ethyl-d-glucopyranoside, ethyl-d-galactopyranoside, and five carbon sugars like ethyl-d-riboside and ethyl-d-ribopyranoside, and the result are in agreement with previous reports (Xu et al., 2016; Feng et al., 2017). The presence of oxygenated components such as ethyl levulinate, levoglucosenone and 5-ethoxymethyl furfural were due to the decomposition of hemicellulose and cellulose. The total relative content of sugar derivatives (five carbon sugars and six carbon sugars) in the water soluble fraction was 86.22%.
No. Time (min) Compound name Area (%) 1 9.906 Ethyl levulinate 5.19 2 11.463 Levoglucosenone 0.3 3 15.417 5-Ethoxymethyl furfural 1.09 4 18.238 Ethyl-beta-d-riboside 22.68 5 22.008 Beta-d-Glucopyranose 2.6 6 25.253 Ethyl -alpha-d-glucopyranoside 27.03 7 25.533 Ethyl-beta-d-glucopyranoside 20.82 8 25.693 Ethyl-beta-d-ribopyranoside 4.82 9 25.899 Ethyl-beta-d-galactopyranoside 5.91 10 27.261 4-O-methyl-beta-d-xylopyranoside 2.36 Total sugar derivatives 86.22
Table 4. Main components in water soluble phase fraction.
Fig. 8 presents the mass balance of the product distribution of bamboo biomass at 200 ℃ for 30 min. Approximately 80% of the starting material was converted into liquid products within 30 min. Using 7.0 g of dry bamboo biomass, 1.88 g aromatic products were produced, including 0.16 g of fraction 1# (high molecular weight fraction), 1.36 g of fraction 2# (lower molecular weight fraction), and 0.34 g of fraction 3# (the lowest molecular weight fraction). Over 90% of the ethanol used was recycled under most conditions due to the lower boiling point. Mass loss of 10% was calculated by mass of bamboo (7.0 g) in the overall mass balances of the solid and liquid fractions. It is noticed that several factors caused the 10% loss: 1) some solids adhered to funnel wall and stirrer; 2) evaporation loss; and 3) water-soluble compounds that could not be extracted.