Fractionation and Characterization of Three Main Components from Pennisetum sinese Roxb. (P. sinese) by Microwave-assisted H2O2-NaOH Extraction
Zhongjian Tian,a,b Jiachuan Chen,b,* Xingxiang Ji,b,* Gaojin Lyu,b Lucian A. Lucia,b,c and Guihua Yang b
Fractionation of lignocellulosic components is a prerequisite for maximizing valorization of plant biomass in an integrated biorefinery. In this study, microwave-assisted H2O2-NaOH extraction was developed for fractionation of Pennisetum sinese Roxb. (P. sinese), a highly productive energy crop. Different reaction conditions including solid-liquid ratio, NaOH concentration, H2O2 concentration, temperature, and time were tested for their effects on P. sinese fractionation. The cellulose, hemicellulose, and lignin obtained under optimal fractionation conditions were characterized by Fourier transform infrared spectrometry (FT-IR) and nuclear magnetic resonance (NMR). The optimal fractionation conditions were a solid-liquid ratio of 1:25 using 0.5% H2O2 and 4% NaOH at 75 °C for 4 h, which gave cellulose, hemicellulose, and lignin yields of 42.8%, 21.9%, and 15.2%, respectively. FT-IR and NMR analyses of the fractionated components clearly confirmed their structural integrity and representation. The work demonstrates the potential of the alternative approach for efficient fractionation of lignocellulosic biomass components for further valorization.
Keywords: Fractionation; Characterization; P. sinese; Hemicellulose; Lignin
Contact information: a: School of Light Industry and Food Engineering, Guangxi University, Nanning, Guangxi 530004, People’s Republic of China; b: Key Lab of Pulp & Paper Science and Technology of the Ministry of Education, Qilu University of Technology, Jinan, Shandong 250353, People’s Republic of China; c: North Carolina State University, Dept. of Forest Biomaterials, Raleigh, NC 29675-8005, USA;
* Corresponding authors: firstname.lastname@example.org; email@example.com
Lignocellulosic biomass is one of the most abundant renewable resources that can be transformed to fuels, chemicals, and materials to support sustainable development. The conversion of lignocellulosic biomass into liquid fuels and chemicals has received increasing public interest and some commercial implementations (Demirbas 2009; Naik et al. 2010). Future biorefineries may have a tremendous potential for producing chemicals and materials that are traditionally produced from petroleum.
The lignocellulosic biorefinery concept integrates renewable resources and necessary processes for their conversion to fuels, power, and value-added chemicals (Sheldon 2014). As biomass is a complex mixture of several different polymer constituents, it is difficult to use directly, and a transition from simple pretreatment processes to fractionation is needed. As part of an integrated lignocellulose biorefinery, fractionation of the raw material into its main lignocellulosic components, i.e. hemicellulose, cellulose, and lignin, maximizes the conversion of valuable feedstock streams to biofuels and chemicals (Grande et al. 2015; Imman et al. 2015).
Diverse techniques have been investigated for biomass fractionation including chemical, physicochemical, thermochemical, and biological methods (Lawther et al. 1995; Huijgen et al. 2010; Karp et al. 2014; Saito et al. 2014; Van den Bosch et al. 2015). The alkaline oxidative method is one of the most attractive strategies because it is versatile, low cost, nontoxic, and environmentally friendly (Azarpira et al. 2014). It effectively and selectively removes and depolymerizes lignin from wood pulp under relatively mild reaction conditions (Villar et al. 2001). Hydrogen peroxide (H2O2) is an environmentally benign oxidizing agent. It is generally accepted that H2O2 is unstable in alkaline conditions and readily decomposes to form hydroperoxide anion (HOO–), hydroxyl radicals (HO˙), and superoxide anion radicals (O2–˙). These radicals may participate in the cleavage of some inter-unit bonds of biomass, eventually resulting in the dissolution of lignin and hemicelluloses (Pan et al. 1998). In an earlier paper, Sun et al. (2000) has been reported that H2O2 in alkaline media could serve as a mild agent for solubilizing the macromolecular hemicelluloses besides its dual role in delignification and bleaching. Then, hemicelluloses from rice straw have been isolated and compared by alkali and hydrogen peroxide (NaOH-H2O2) treatments by Sun et al. (2000), and they proposed that the treatment by NaOH-H2O2 did not affect the overall structure of macromolecular hemicelluloses except for a noticeable degradation of cellulose and hemicelluloses when with more than 1.0% H2O2. Microwave-assisted heating applies an electromagnetic field to the molecular structure of the heated object to accelerate organic reactions (Tsubaki and Azuma 2013). It could increase specific surface area, decreases polymerization and crystallinity of cellulose, and causes lignin depolymerization (Peng et al. 2014). For this reason, it has been successfully applied to promote the dilute alkali and/or dilute acid pretreatment of biomass, causing swelling and fragmentation (Wu et al. 2010). Rossberg et al. (2015) explored the effects of different pulping procedures on lignin separation and composition, finding that microwave-assisted pulping allowed reduced pulping time while obtaining lignin with similar purity and composition. Diaz et al. (2015) proposed microwave-assisted pretreatment of lignocellulosic biomass immersed in alkaline glycerol and demonstrated that lignin removal from corn straw and rice husk were 30% and 12%, respectively. Recently, Jin et al. (2016) investigated microwave-assisted alkaline pretreatment of catalpa sawdust to remove hemicellulose and lignin and to enhance enzymatic saccharification. In summary, microwave-assisted alkaline treatment has been tested to remove and/or isolate one or two components in pulping or bioconversion, but there has not been much investigation on the fractionation of three main components as an integrated concept.
P. sinese, an energy crop, is a monocot C4 perennial grass that is a hybrid of Pennisetum purpureum and Pennisetum americanum (del Río et al. 2012). It is versatile and adaptable and can grow and thrive in a variety of harsh climates. Its high productivity (~ 40 t ha-1 y-1) allows 3 to 4 harvests per year. The high productivity and low inputs for cultivation allow it to serve as a potential lignocellulosic feedstock for pulp and paper industries, textiles, bioethanol, and biofuels. Although P. sinese is an attractive alternative feedstock for biorefinery, it is largely underexploited.
This study evaluated the efficiency of microwave-assisted H2O2-NaOH treatment on P. sinese fractionation and characterized the three main components obtained. The operating conditions of solid-liquid ratio, NaOH concentration, H2O2 concentration, temperature, and time were optimized. Fourier transform infrared spectra (FT-IR) and nuclear magnetic resonance (NMR) spectra were used to determine the structural characteristics of the fractionated cellulose, hemicellulose, and lignin.
P. sinese was obtained from Shandong Province, China, and air-dried at ambient temperature. The dried P. sinese was ground with a Wiley mill, and the fraction between 40- and 60-mesh was collected as feedstock. The initial composition of the P. sinese was determined based on Chinese National Standards GB/T2677.8-1994, GB/T10337-2008, GB/T10741-2008, and methods in reference (Shi and He 2003), and the results were as follows: 44.0% cellulose, 25.8% hemicellulose, 17.4% lignin, 4.3% ash, and 8.2% extractives as the components of unfractionated biomass
Sodium hydroxide, benzene, ethanol, hydrochloric acid, hydrogen peroxide (30 wt.%), and other chemicals were of analytical grade and used as received.
The overall process for fractionation of the three main components from P. sinese is summarized in Fig. 1. Briefly, P. sinese was first treated with benzene-alcohol mixture (2:1, v/v, 6 h) and hot water (85 °C, 3 h) to remove waxes and impurities. For each run, 10 g of dewaxed P. sinese and the necessary amount of liquid solution (with right concentration of NaOH and H2O2 in deionized water) were loaded into a 500 mL flat-bottom flask. After pretreatment in a water bath with stirring at 50 °C for 30 min, the reactor was transferred into a microwave generator (MCR-3, Gongyi, China). Extraction time was counted when the temperature in the reactor reached the target extraction temperature. After the extraction was completed, the reactor was cooled to room temperature, and the solids and liquids were separated by vacuum filtration with fluted filter paper. The solid fractions were washed with deionized water until the pH was neutral and then oven-dried at 60 °C to retrieve the crude cellulose. The filtrate was acidified with 6 M HCl until the pH dropped to 11, and it was concentrated to 1/3 of the original volume. The concentrated filtrate was added to 3 volumes of 95% ethanol for precipitation and centrifuged at 8000 rpm for 10 min.
Fig. 1. Scheme for fractionation of cellulose, hemicellulose, and lignin from P. sinese
The precipitated hemicellulose solid was washed 3 times with 85% (v/v) ethanol, separated by centrifugation, and freeze-dried. The supernatant after ethanol precipitation was adjusted to pH 9 to 10 with 6 M HCl and rotary evaporated to recover ethanol. The supernatant was further acidified to pH 2 to 3 (drop-wise addition) to precipitate the lignin fraction. The lignin fraction was washed, freeze-dried, and quantified by weighing.
In this study, varying extraction parameters: solid-liquid ratio (1:15 to 1:35), NaOH concentration (1 to 5 wt.%), H2O2 concentration (0 to 1 wt.%), extraction temperatures (55 to 95 °C), and extraction times (2 to 6 h) were investigated by single factor design method. At least two duplicate runs were performed for each condition to confirm the reproducibility of each test. The yields of crude cellulose (Ycell), hemicellulose (Yhemi), and lignin (Ylig) were calculated as shown below,
Ycell (%) = Mcell/Mi × 100 (1)
Yhemi (%) = Mhemi/Mi × 100 (2)
Ylig (%) = Mlig/Mi × 100 (3)
where Mcell, Mhemi, and Mlig were the mass of fractionated crude cellulose, hemicellulose, and lignin respectively, and Mi was the mass of the initial dewaxed P. sinese loaded.
The fractionated cellulose, hemicellulose, and lignin were characterized by FT-IR and NMR. FT-IR spectra of the three samples were recorded on a VERTEX70 FT-IR spectrophotometer (Bruker, Karlsruhe, Germany) over 4000 to 400 cm-1 using a KBr disc containing 1% finely ground samples.
The solid state CP/MAS 13C-NMR spectra of cellulose was recorded using a Bruker Avance III 400 MHz spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) at 25 °C and 62.9 MHz for carbons. The MAS rate was 3 kHz. Each spectrum was obtained with an accumulation of 5000 scans. The delay time was 60 s, the proton 90° pulse width was 9 μm, and the contact time for cross polarization was 2 ms.
The solution-state 1H-NMR and 13C-NMR of hemicellulose and lignin were obtained on a Bruker AV 400MHz spectrometer operating in the FT mode at 100 MHz. The samples (20 to 100 mg) was dissolved in 1 mL DMSO-d6 (99.8% D) before NMR analysis. The spectrum was recorded at 25 °C after 18,000 scans. The central DMSO solvent peaks were used as internal reference (δH/δC = 2.50/39.51 ppm).
RESULTS AND DISCUSSION
Effect of Reaction Conditions on Fractionation
The effect of reaction conditions including solid-liquid ratio, NaOH concentration, H2O2 concentration, temperature, and time on the yields of crude cellulose, hemicellulose, and lignin fractionated from P. sinese is shown in Fig. 2. The results in Fig. 2(a) indicated that the solid-liquid ratio was a factor affecting the yields of fractionated components. As the solid-liquid ratio increased, the yield of crude cellulose decreased and the yield of lignin increased, while the yield of hemicellulose increased first and then decreased. When the solid-liquid ratio was below 1:25, the yield of crude cellulose was more than 44.1%, while that of hemicellulose and lignin were no more than 20.0% and 13.7%, respectively, which indicated that insufficient loading of alkaline solution (low solid-liquid ratio) resulted in impurities in the fractionated cellulose, such as hemicellulose and lignin. When the solid-liquid ratio was 1:35, the yields of crude cellulose, hemicellulose, and lignin were 38.8%, 17.5%, and 15.6%, respectively, showing rapid declines in the yield of cellulose and hemicellulose. Alkali has the greatest effect on breaking ester and ether bonds between hemicellulose and lignin (Ralph et al. 1994; Buranov and Mazza 2008). Increasing alkaline dosage favored removal of hemicellulose and lignin from raw material. However, when the alkaline dosage was beyond a certain point (e.g., solid-liquid ratio > 1:25), the yield of hemicellulose was reduced.
Fig. 2. Effects of (a) solid-liquid ratio, (b) NaOH concentration, (c) H2O2 concentration, (d) extraction temperature, and (e) extraction time on the yields of crude cellulose, hemicellulose, and lignin from P. sinese fractionation. Reaction conditions: (a) 75 °C with 0.5% H2O2 and 4% NaOH for 3 h; (b) solid-liquid ratio of 1:25 with 0.5% H2O2 at 75 °C for 3 h; (c) solid-liquid ratio of 1:25 with 4% NaOH at 75 °C for 3 h; (d) solid-liquid ratio of 1:25 with 0.5% H2O2 and 4% NaOH for 3 h; (e) solid-liquid ratio of 1:25 with 0.5% H2O2 and 4% NaOH at 75 °C.
In addition, varied dosage of H2O2 may affect the lignocellulose components besides the alkalinity. Further peeling reaction and alkaline degradation resulting in the decline in the yield of cellulose. Figure 2(b) shows the effect of NaOH concentration on P. sinese fractionation. When the concentration of NaOH increased from 1% to 5%, the yield of crude cellulose decreased from 52.9% to 40.1%. The yield of crude hemicellulose and lignin increased dramatically when the concentration of NaOH increased from 1% to 2%, i.e., both of which increased more than 5%. When the concentration of NaOH was 4%, the yield of crude hemicellulose reached maximum values of 20.0%. However, further reduction was observed when the concentration was raised to 5%. The yield of crude lignin increased with an increase of the NaOH concentration (1 to 5%).
The yield of cellulose, hemicellulose, and lignin was remarkably influenced by the concentration of H2O2 (Fig. 2(c)). The yield of crude cellulose decreased while lignin increased slightly as H2O2 concentration increased. The yield of hemicellulose increased at first and then decreased, with the maximum values at 0.5% H2O2. Thus, H2O2 obviously improved the removal rate of lignin and hemicellulose in a certain concentration range.
As shown in Fig. 2(d), the yield of hemicellulose increased when the temperature was lower than 75 °C, but the yield was reduced at higher temperatures. For example, the hemicellulose yield decreased to 14.8% when the temperature was 100 °C. The lignin yield increased with increasing temperature, showing that a high temperature was beneficial to lignin removal. Below 100 °C, the yield of crude cellulose was decreased to 38.4%. The reduction in the yield of hemicellulose and cellulose was ascribed to peeling reaction and alkaline degradation, which was promoted at higher temperatures.
The effect of reaction time on the yield of three components is illustrated in Fig. 2(e). The yield of cellulose declined sharply as time increased, while the hemicellulose and lignin yields increased first and then decreased, with the maximum yields achieved at 4 h and 5 h respectively. Initially, the dissolution of hemicellulose and lignin were higher than the degradation, and thus, the yield increased. However, as the extracting time was increased to a certain value, the dissolution was far below the degradation, resulting in a downward trend. In addition, longer time led to further alkaline oxidative degradation of lignin to mono-aromatics and organic acids.
The above observations suggested that tuning reaction conditions was crucial to maintaining a high yield of the fractionated components while avoiding excessive carbohydrate degradation. The optimal fractionation conditions were obtained temporarily, i.e., solid-liquid ratio of 1:25, 0.5% H2O2, 4% NaOH, 75 °C, and 4 h. The resulting fractionated cellulose, hemicellulose, and lignin were used for structural characterization. The optical photographs of cellulose, hemicellulose, and lignin obtained under optimal conditions are shown in Fig. 3, and their FT-IR spectra are shown in Fig. 4. The IR signal was assigned based on previous publications (Lawther and Sun 1996; Sun et al. 1998).
In the cellulose IR spectra (Fig. 4(a)), the absorption bands located at 2899 cm-1, 1429 cm-1, 1372 cm-1, 1310 cm-1, 1059 cm-1, and 897 cm-1 are related to cellulose characteristics. The absorption at 2899 cm-1 is due to the C-H stretching. The band at 1429 cm– is associated with the CH2 symmetric bending. The absorbances appearing at 1372 and 1310 cm-1 originate from the O-H bending and C-H bending. The band at 1059 cm-1 represents the C-O-C pyranose ring skeletal vibration (Lan et al. 2011). In addition, the absorption band at 1720 cm-1 is attributed to carbonyl groups, which may occur due to the presence of residual hemicellulose (Lawther and Sun 1996).
Fig. 3. The optical photographs of (a) cellulose, (b) hemicellulose, and (c) lignin prepared from P. sinese by microwave-assisted H2O2-NaOH extraction. Reaction conditions: solid-liquid ratio of 1:25, 0.5% H2O2, 4% NaOH, 75 °C, and 4 h.
Fig. 4. FT-IR spectra of (a) cellulose, (b) hemicellulose, and (c) lignin from P. sinese by microwave-assisted H2O2-NaOH extraction. Reaction conditions: solid-liquid ratio of 1:25, 0.5% H2O2, 4% NaOH, 75 °C, and 4 h.
Figure 4(b) shows the FT-IR spectra of hemicellulose fractionated from P. sinese under optimal conditions. The absorption band between 1636 and 800 cm-1 originated from the typical absorption of hemicellulose. Specifically, the absorbance band at 1423 cm-1 is the characteristic absorption of alkyl and xylan structure. The strong sharp band at 1043 cm-1 originated from C-O and C-OH, which reflects the xylan structures. The absorption peak near 897 cm-1 is due to β-glycosidic linkages between sugar units of hemicellulose (Lawther et al. 1995; Sun et al. 1998). There are inevitably some lignin contaminants in isolated hemicellulose because of their intricate connections. The peaks at 1510 cm-1, 1328 cm-1, and 1215 cm-1, which corresponding to C-O stretching vibrations from aromatic, syringyl, and guaiacyl units of lignin, were inconspicuous. These suggested that the hemicellulose isolated by microwave-assisted H2O2-NaOH extraction has a good structural representation.
The FT-IR spectra (Fig. 4(c)) demonstrated that P. sinese has a typical GSH-type lignin. The absorbance peaks at 1600, 1510, and 1425 cm-1 indicate aromatic skeleton vibrations (Lawther and Sun 1996). The band at 3424 cm-1 was assigned to the OH stretching of hydroxyl functional groups. Moreover, the peaks at 1329 and 1125 cm-1 associated with the presence of dialkyl ethers represent syringyl (S) structures; the band at 1030 cm-1 identified with alkyl aryl ether bond as well as the absorption peak located at 1267 cm-1 represent the guaiacyl (G) structure. The small band found between 1124 and 1223 cm-1 indicates only minor amounts of p-hydroxyphenyl (H) unit in P. sinese lignin (Martínez and Gutiérrez 2012).
Fig. 5. CP/MAS 13C-NMR spectra of cellulose isolated from P. sinese. Reaction conditions: solid-liquid ratio of 1:25, 0.5% H2O2, 4% NaOH, 75 °C, and 4 h
To confirm their structural integrity, the three components isolated under optimal reaction conditions were characterized using NMR, and the spectra peaks were assigned based on published values (Marita et al. 2001; Yang et al. 2011; Mansfield et al. 2012).
The CP/MAS 13C-NMR spectra of cellulose is illustrated in Fig. 5. The signals at 60 to 70 ppm corresponded to the chemical shift of C-6. The signals between 70 and 81 ppm were attributed to C-2, C-3, and C-5 of glucose units. The signals at 86.6 and 103.1 ppm represented to C-4 and C-1, respectively. The lack of signals near 122 and 98 ppm indicated that the cellulose was relatively free from impurities such as lignin and hemicellulose, which is consistent with the FT-IR results (Fig. 4(a)).
The 1H-NMR and 13C-NMR spectra of hemicellulose are shown in Fig. 6. The four cross peaks at 4.25, 3.86, 3.47, and 3.14 ppm (Fig. 6(a)) were assigned to H-1, H-5/H-4, H-3, and H-2 of β-D-Xylp units, respectively. The small signal at 5.20 ppm was identified as alpha anomeric proton of arabinofuranose in hemicelluloses. The strong signals at 3.23 and 3.02 ppm were ascribed to 4-O-methyl-α-D-glucoronic acids. Further details can be seen from the 13C-NMR spectrum (Fig. 6(b)).
Fig. 6. 1H-NMR (a) and 13C-NMR spectra (b) of hemicellulose isolated from P. sinese. Reaction conditions: solid-liquid ratio of 1:25, 0.5% H2O2, 4% NaOH, 75°C, and 4 h
The main 1,4-linked β-D-Xylp units were characterized by five strong signals at 101.7, 75.4, 74.0, 72.6, and 63.2 ppm, which were attributed to C-1, C-4, C-3, C-2, and C-5 of the β-D-Xylp units, respectively. In addition, the signals at 107.2, 86.1, 80.3, 77.6, and 61.8 ppm were assigned to C-1, C-4, C-3, C-2, and C-5 of α-L-arabinofuranose units that linked to β-D-xylans. The signal at 55.9 ppm was ascribed to the carbonyl and the 4-O-methy group of glucuronic acid residue in xylans. Thus, the hemicelluloses isolated with H2O2-NaOH from P. sinese were mainly composed of 1, 4-linked β-D-xylopyranose as the main chain, with 4-O-methy glucuronic acid as the main side chain connected by α-glycoside bond. In addition, some α-L-arabinofuranose residues linked to main chain at C-2 and C-3 (Lan et al. 2011).
The 1H-NMR and 13C-NMR spectra of lignin are presented in Fig. 7. As shown in Fig. 7(a), the 1H-NMR of isolated lignin presented a wide spectrum of peaks and overlapped proton signals. This may be due to the complex three-dimensional structure of lignin as well as the steric hindrance of the lignin molecule.
Fig. 7. 1H-NMR (a) and 13C-NMR spectra (b) of lignin isolated from P. sinese. Reaction conditions: solid-liquid ratio of 1:25, 0.5% H2O2, 4% NaOH, 75 °C, and 4 h
The integral of all signals between 6.26 and 9.00 ppm is a typical indication of GSH lignin. Among them, the syringyl unit (S), guaiacyl unit (G), and p-hydroxyphenyl unit (H) from P. sinese lignin were characterized by three signals at 6.26 to 6.82, 6.82 to 7.49, and 7.49 to 9.00, respectively. In addition, peaks from 1.60 to 2.16 were attributed to aliphatic acetyl and diphenyl-o-acetyl protons, whereas the signals at 2.16 to 2.50 were due to the acetyl protons of aromatic ring. The strong peaks at 3.49-4.09 ppm originated from methoxyl protons (–OCH3), indicating that methoxyl is one of the most important functional groups in lignin.
The structural features explored with 13C-NMR are shown in Fig. 7(b). The syringyl residues were identified by signals at 152.2 (C-3/C-5 in syringyl units) and 104.2 ppm (C-2/C-6 in syringyl units). The signals at 149.4 (C-3 in G units), 144.5 and 144.1 (C-1 in G units), 115.8 (C-5 in G units), 115.2 and 111.4 ppm (C-2 in G units) originated from the guaiacyl units, while hydroxyphenyl propane unit produced a signal at 129.9 ppm (C-2/C-6 in H units). Signals corresponding to the C-γ, C-4, and C-1 of the p-coumarate unit were observed at 167.8, 159.7, and 125.1 ppm respectively. The C-2 and C-6 of the ferulate moieties were thought to be indicated by weak signals at 111.0 and 123.3 ppm (Sun et al. 2005; del Río et al. 2012). Furthermore, the signals showing the linkages between lignin phenylpropane units are also obvious in Fig. 7(b). The Cβ, Cα, and Cγ in β-O-4 linkages were identified by three signals at 82.8, 72.6, and 59.8 ppm, respectively. The condensed substructures such as β-β’ and β-5’ presented their typical signals near 71.8 and 62.6 ppm.
- Microwave-assisted H2O2-NaOH extraction was found to be an efficient way to fractionate three components from P. sinese. The yields of fractionated crude cellulose, hemicellulose, and lignin were dependent on the reaction conditions including solid-liquid ratio, NaOH concentration, H2O2 concentration, temperature, and time. Tuning reaction conditions produced high yields of fractionated components while avoiding excessive degradation of carbohydrates. The yields of cellulose, hemicellulose, and lignin obtained under optimal fractionation conditions, i.e., solid-liquid ratio of 1:25, 0.5% H2O2, 4% NaOH, 75 °C, and 4 h, were 42.8%, 21.9%, and 15.2%, respectively.
- FT-IR and NMR analyses of the fractionated components clearly confirmed their structural integrity and representation. The structural features of the isolated cellulose under optimal reaction conditions was free from contaminants. The P. sinese hemicellulose was mainly 4-O-methyl-D-glucuronoxylans with some α-L-arabinofuranose residue linked to main chain. Isolated lignin from P. sinese was characterized as a typical GSH-type lignin that consisted of three basic structural units: syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H). The fractionated components could be converted to valuable products by further chemical conversion and modifications.
The authors are grateful for ﬁnancial support from the National Science Foundation of China (Grant No. 31370580, 31670590, 31670595, 31400511, and 31400517) and the Taishan Scholar Project special funds.
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Article submitted: September 26, 2016; Peer review completed: November 12, 2016; Revised version received and accepted: February 12, 2017; Published: February 21, 2017.