Bioresource Technology 173 (2014) 399–405
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Effects of metal ions on the hydrolysis of bamboo biomass in 1-butyl-3-methylimidazolium chloride with dilute acid as catalyst Nan Wang a, Jie Zhang a, Honghui Wang a, Qiang Li b, Sun’an Wei a, Dan Wang a,⇑ a b
Department of Chemical Engineering, School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, PR China National Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China
h i g h l i g h t s Metal ions was used in acidic hydrolysis of bamboo biomass in ILs. The mechanism of the hydrolysis system was investigated. The reaction time was significantly decreased along with the increased TRS yield. 2+
The most effective ion is Cu
a r t i c l e
ion and the maximal TRS yield of 67.1% was achieved.
i n f o
Article history: Received 29 June 2014 Received in revised form 23 September 2014 Accepted 24 September 2014 Available online 2 October 2014 Keywords: Bamboo biomass Ionic liquids Metal ions Hydrolysis Dilute hydrochloride acid
a b s t r a c t In this study, the effects of six metal ions including Na+, K+, Mg2+, Ca2+, Cu2+ and Fe3+ on hydrolysis of bamboo biomass by diluted hydrochloride acid (HCl) in ionic liquid [C4mim]Cl under mild conditions was investigated. These metal ions as co-catalysts exhibited significant effects on accelerating the hydrolysis process and improving the yield of total reducing sugar compared to single diluted hydrochloride acid hydrolysis in [C4mim]Cl at the same conditions. The most effective ion was Cu2+ and the total reducing sugar yield of 67.1% was achieved at 100 °C with CuCl2 as co-catalyst after 4-h reaction. The total reducing sugar yield was increased by about 7% and the reaction time was decreased by 3 h. The kinetic model was also investigated to give an insight into the mechanism of hydrolysis process. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Lignocellulosic biomass has shown promise as an efficient renewable clean energy resource that can be converted to fuels and high-valued chemicals. Extensive research and development programs have been initiated worldwide to transform the lignocellulosic biomass into various valuable products through biorefinery, in which liberation of reducing sugars from lignocellulose is the first key step (Ragauskas et al., 2006; Jäger and Büchs, 2012). Unfortunately, lignocellulosic biomass confers a notorious resistance to hydrolysis due to its complex hetero-matrix structure (Himmel et al., 2007). Up to now, hydrolysis of lignocellulose or cellulose to its component monomeric sugars has been widely investigated. ⇑ Corresponding author at: P. O. Box 94, 174 Shazheng Street, Shapingba District, Department of Chemical Engineering, School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, PR China. Tel./fax: +86 23 65111179. E-mail address: [email protected] (D. Wang). http://dx.doi.org/10.1016/j.biortech.2014.09.125 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
Among all the hydrolysis technologies, enzymes and acid catalysts are the most promising hydrolysis catalysts (Lenihan et al., 2010; Wang et al., 1996). However, these common hydrolysis methods exhibit certain disadvantages and are not cost-effective for large-scale applications. For example, enzymatic hydrolysis suffers a relatively low hydrolysis rate and enzymes are usually expensive and difficult to recycle (Mora-Pale et al., 2011). The traditional diluted acid hydrolysis process requires harsh conditions and results in the decomposition of monosaccharide (Mora-Pale et al., 2011). What is more, pretreatment, as a high cost process, is usually required to decompose the rugged construction of lignocellulose in that hydrolysis above. Concentrated acids such as sulfuric acid and hydrochloric acid have also been used to treat lignocellulose (Heinonen et al., 2012). Though they are powerful agents for cellulose hydrolysis, concentrated acids are toxic, hazardous and require corrosion-resistant reactors (Sun and Cheng, 2002). And the concentrated acid will become environmental pollutant without thorough recycling. Therefore, hydrolysis of
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lignocellulose via an economical and environmentally friendly approach remains a challenge. The very high solvating property of ionic liquids (ILs) was exploited in the dissolution of cellulose by Rogers et al. in 2002 (Swatloski et al., 2002). Since then, research on treatment of lignocellulose in ILs has gained much more attention. It has already been shown that ILs can dissolve a large number of biomass, such as corn stalk, wheat straw, rice straw and wood (da Costa Lopes et al., 2013; Weerachanchai and Lee, 2013; Sun et al., 2009). And ILs can present a high efficiency in the pretreatment of biomass (Viell et al., 2013; Bian et al., 2014). Furthermore, since Li and Zhao (2007) firstly found that mineral acids could facilitate the hydrolysis of cellulose in the ionic liquid 1-butyl-3-methylimidazolium chloride, much more attention has been paid on the hydrolysis of biomass in ILs with acid catalysts directly (Rinaldi et al., 2008; Sievers et al., 2009; Vanoye et al., 2009). Li et al. (2008) reported biomass hydrolysis in ILs with different inorganic acids as catalysts and a maximum 81% liberation of the total reducing sugars was achieved. Amarasekara and Owereh (2009) used Brønsted acid ILs, which act as both the solvent and catalyst, to dissolve and hydrolyze cellulose and obtained a total reducing sugar yield of 62%. Without enzyme hydrolysis subsequently, combined IL dissolution and acid catalysis may provide an economical approach to release reducing sugars from biomass. In recent years, addition of metal ions in traditional dilute acid pretreatment or hydrolysis has become a research highlight, since metal ions can increase the decomposition rate of cellulose and hemicellulose and reduce the severity of hydrolysis conditions (Kamireddy et al., 2013). Monavari et al. (2011) found that addition of small amount of ferrous sulfate in dilute-acid pretreatment resulted in a slightly increased overall glucose yield. Wei et al. (2011) reported that during dilute acid and ferrous ion co-catalyst pretreatment, solubilized sugars increased in the hydrolysate concomitantly with an increase of reducing sugars in the biomass residues. Yan et al. (1996) reported that a high yield of sugar could be obtained from hydrolysis of peat with ferric chloride and hydrogen chloride as catalysts. Zhao et al. (2013) investigated the acid hydrolysis of corn stover under microwave radiation by adding various metal salts individually as co-catalysts and found that FeSO4 had exhibited a better catalytic effect. Though some works have been executed to examine the effectiveness of the addition of acid as catalyst in ILs for lignocellulose hydrolysis and the supplementation of metal ions for the acidic hydrolysis respectively, there is seldom information on the acidic hydrolysis of lignocellulosic biomass in ILs with metal ions added as co-catalyst. The purpose of this study is to investigate the effects of metal ion additions on the dilute acid hydrolysis with ILs as reaction medium and provide an insight on the selectivity of metal ions that can yield high reducing sugars. In order to reduce the energy consumption and the products degradation, the hydrolysis were conducted at low severity conditions. In this work, diluted hydrochloride acid catalyzed hydrolysis of bamboo powder in 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) were performed with six kinds of metal chlorides (NaCl, KCl, MgCl2, CaCl2, CuCl2, FeCl3) added respectively as co-catalyst. The possible decomposition mechanism of lignocellulose in [C4mim]Cl with hydrochloride acid (HCl) and CuCl2 was proposed.
raw bamboo expressed in the weight percent of the dry matter is as follows: 40.1% glucan, 20.3% xylan, 22.3% lignin. The content of each composition was determined using a two-step acid hydrolysis method developed by the National Renewable Energy Laboratory (NRTL) (Sluiter et al., 2011). ILs used in the degradation, namely 1-butyl-3-methyl-imidazolium chloride ([C4mim]Cl), was prepared according to the method reported by Webb et al. (2003). 1-Butyl-3-methyl-imidazolium chloride exists in its liquid form at temperatures between 60 °C and 70 °C. 2.2. Hydrolysis of bamboo Experiments were carried out in a micro-glass round-bottom flask. 4 g of [C4mim]Cl was placed in the reactor and heated to 100 °C. 0.2 g bamboo was then added, followed by the addition of an appropriate amount of metal chlorides and different concentrations of HCl. The mixture was stirred at 20 rpm under atmospheric pressure for 7 h. At different time intervals, samples (0.1 mL) were withdrawn, quenched immediately with cold water and then weighed and recorded as MS. The samples were neutralized with an appropriate amount of 0.05 M NaOH, centrifuged at 10,000 rpm for 5 min and subjected to sugar analysis. The volume measured was recorded as V1. 2.3. Sugar analysis The total reducing sugars (TRS) was measured using DNS method (Miller, 1959). The color tests were made with 1.5 mL aliquots of DNS regent added to 1.5 mL aliquots of reaction sample. The mixtures were heated for 5 min in a boiling water bath and then cooled to ambient temperature. The color intensities were measured in a JASCO V-530 Model spectrophotometer at 540 nm with a slit width of 0.06 mm. The concentration of TRS in the reaction sample was calculated based on a standard curve obtained with glucose standard. The monomeric sugars were analyzed by high performance liquid chromatography (HPLC) using an Aminex HPX-87H ion-exchange column (Bio-Rad, USA) and Agilent 1200 chromatography working station system (Agilent Technologies, USA) equipped with UV absorbance detector (Agilent Technologies, G1315D) and refractive index detector (Agilent Technologies, G1362A). Samples were centrifuged at 10,000 rpm for 10 min. Each supernatant was diluted with 10 volumes of 5 mM H2SO4, and 20 lL of the diluted sample was injected. The column was eluted isocratically at a rate of 0.6 mL min1 with 5 mM H2SO4 under 55 °C. The mass of TRS and the yield of TRS were calculated as follows:
MT ¼ M1 ðM 0 =M S Þ
ð1Þ
M1 ¼ TRS concentration ðg=LÞ V 1
ð2Þ
TRS yield ¼ M T =ð200 A 1:1Þ 100%
ð3Þ
in which, MT is the mass of TRS, M1 is the mass of TRS in the reaction sample, M0 is the total mass of the reaction solution, MS is the mass of sample, 200 is the mass of bamboo biomass (mg), and A is the weight percentage of the polysaccharides contained in lignocellulosic materials. For bamboo, A = 40.1% + 20.3% = 60.4%.
2. Methods
2.4. XRD analysis
2.1. Materials
After the hydrolysis reaction was completed, the mixture was diluted with cold water, neutralized with 0.05 M NaOH, and centrifuged at 10,000 rpm for 5 min. The solid was collected, re-suspended in deionized water, and centrifuged. The purification process was repeated for 5 times. The recovered solid material
The feed stock material was obtained freely from a bamboo grove in Chongqing City, China. Bamboo was milled to pass through an 80 mesh sieve. The main chemical composition of
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was dried at 108 °C to a constant weight, then squashed on glass solid with a smooth surface, and submitted to XRD analysis. The X-ray powder diffraction pattern of the original and regenerated bamboo preparations was measured using an XRD-6000 instrument (Shimadzu, Japan) with a Cu Ka radiation source (k = 0.154 nm) at 40 kV and 30 mA. Samples were scanned from 2h = 5–50° at a speed of 2°/min. 2.5. FTIR spectroscopy Fourier transform infrared spectroscopy (FTIR) was conducted using a Nicolet iN10 FT-IR microscope (Thermo Nicolet Corporation, Madison, WI). The samples were pressed uniformly against the diamond surface with a spring-loaded anvil. The sample spectra was obtained in duplicates using an average of 64 scans over the range 4000–500 cm1 with a spectral resolution of 8 cm1 (Bian et al., 2014). 2.6. SEM analysis The morphology and porous features of bamboo were observed by JSM-6700F (JEOL, Japan) scanning electron microscopy (SEM) operated at 20 kV accelerating voltage. Prior to imaging, the samples were sputter-coated with gold to make the fibers conductive, avoiding degradation and building up the charge of the specimen. 2.7. Recovery of sugars and ILs After the hydrolysis reaction was completed, 1 mL of the mixture solution was placed into a 2 mL eppendorf tube and an appropriate amount of 50% (w/w) NaOH were added to give the final NaOH concentration of 20% (w/w). The mixture was agitated at 1400 rpm for 0.5 h and then centrifuged at 14,000 rpm for phase separate. The upper IL phase and lower NaOH phase was separated and collected. The mass and density of both phases was calculated and the sugar content was quantified (Sun et al., 2013). 3. Results and discussion 3.1. Effect of different metal ions on bamboo hydrolysis Table 1 summarized the TRS yields of bamboo biomass in [C4mim]Cl catalyzed by 0.45% (w/w) HCl with different metal ions as co-catalysts. As shown in Table 1, it took 7 h for the bamboo treatment without metal ion additions to achieve the maximum TRS yield of 60.82%. The maximum TRS yields were 64.80% and 64.00%, respectively, after hydrolysis for 7 h with Na+ and K+ ions (alkali metal ions) as co-catalysts. When Mg2+ and Ca2+ ions (alkali earth metal ions) were added into the hydrolysis system, the reaction time was reduced for 1 h with a similar TRS production, indicating that alkali earth metal ions are more effective than alkali metal ions on bamboo acidic hydrolysis. Hydrolysis experiment with transition metal ions (Cu2+ and Fe3+) were also performed. TRS yields were 64.82% and 63.04% after hydrolysis for 4 h,
respectively, with Cu2+ and Fe3+ as co-catalysts. The reaction time was decreased for 2 h compared to the cases with alkali earth ions as co-catalysts when the similar TRS production was obtained. These results demonstrated that a low content of metal ions as the co-catalysts was an efficient method for biomass acidic hydrolysis. Transition metal ions exhibited better effects than the alkali earth metal ions and alkali metal ions. Among all the six metal ions, Cu2+ ions showed the best effects both on the hydrolysis rate and TRS production. The hydrolysis experiment with gradually reduced acid loading was executed, and the results were summarized in Tables 2 and 3. Metal ions had little effect on the TRS yield except for Cu2+ ion when HCl concentration was 0.23% (w/w), demonstrating once again that Cu2+ can effectively improve the yield of TRS and accelerate the hydrolysis than any other metal ions. Further reducing the acid concentration to 0.05% (w/w) produced lower TRS yields after 7 h. So the optimal HCl concentration for bamboo biomass hydrolysis was chosen as 0.45% (w/w). Besides, the metal ions were found to have a slight influence on the glucose and xylose production. Experiments was conducted with 0.45% (w/w) HCl and 0.13% (w/w) metal ion as co-catalysts, and the results were shown in Table 4. The glucose yield was increased with metal ions addition while the xylose yield was decreased, which indicated that metal ions could promote the hydrolysis of cellulose, meanwhile accelerate the degradation of xylose to furfurals (Salmi et al., 2014). As reported by Liu and Wyman (2006), the inorganic salts KCl, NaCl, CaCl2, MgCl2, and FeCl3 significantly increased xylose monomer degradation at 180 °C in water. The condition in this system was more moderate, thus less degradation of xylose would occur during the hydrolysis. Table 5 illustrated the effects of water used as the solvent on the sugar yield and conversion with 0.45% (w/w) HCl and different metal ions as co-catalysts. It took about 7 h to achieve the TRS yield of 8.54% when the bamboo biomass was treated by HCl without metal ion additions. When metal ions were added into the hydrolysis system, the TRS production was increased by about 1.54–4.24% compared to the control. The result showed that metal ions also worked well on the acidic hydrolysis with water as solvent. However, the TRS yields were decreased with water as solvent under otherwise identical conditions, compared to the results obtained with IL as solvent in Table 5, suggesting that IL as solvent played an important role in this reaction system. The possible reason was that lignocelluloses and cellulose in the bamboo biomass could be dissolved in [C4mim]Cl as their complex, and the hydrolysis was conducted easily compared with the water system, in which hydrolysis just occurred at the surface of lignocelluloses and cellulose (Li and Zhao, 2007). 3.2. Effect of Cu2+ ions concentration on total reducing sugar production To further investigate the effect of Cu2+ ion on the acidic hydrolysis with [C4mim]Cl as solvent, different concentrations of Cu2+ ion were used in the hydrolysis process. The HCl concentration
Table 1 Total reducing sugar yields of the samples hydrolyzed by 0.45% (w/w) HCl without and with 0.13% (w/w) metal ion additions at different reaction time. Time (h)
1 2 3 4 5 6 7
TRS yield (%) Blank
Na+
K+
Mg2+
Ca2+
Cu2+
Fe3+
10.78 ± 1.89 26.89 ± 1.45 42.21 ± 2.26 51.13 ± 2.05 55.72 ± 1.78 58.87 ± 0.20 60.82 ± 0.17
14.88 ± 1.26 35.56 ± 1.06 53.02 ± 1.27 60.78 ± 1.58 63.89 ± 1.36 64.67 ± 0.87 64.80 ± 1.66
11.52 ± 1.45 28.89 ± 1.27 44.31 ± 1.96 56.91 ± 1.78 61.73 ± 1.56 63.44 ± 1.21 64.00 ± 1.12
12.50 ± 0.97 31.32 ± 1.16 46.64 ± 0.89 57.04 ± 1.36 61.32 ± 1.18 63.72 ± 1.54 66.62 ± 1.38
11.62 ± 1.58 31.31 ± 1.45 47.87 ± 1.06 58.22 ± 0.96 61.78 ± 1.15 63.72 ± 0.81 65.33 ± 1.27
15.14 ± 2.06 45.67 ± 1.67 62.23 ± 1.78 64.82 ± 1.16 65.78 ± 1.57 59.21 ± 0.96 53.84 ± 1.56
15.32 ± 1.59 46.42 ± 1.06 60.33 ± 0.89 63.04 ± 1.27 57.78 ± 1.21 53.82 ± 1.60 50.23 ± 1.45
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Table 2 Total reducing sugar yields of the samples hydrolyzed by 0.05% (w/w) HCl without and with 0.13% (w/w) metal ion additions at different reaction time. Time (h)
1.5 2 3 4 5 6 7
TRS yield (%) Blank
Na+
K+
Mg2+
Ca2+
Cu2+
Fe3+
5.13 ± 1.20 8.21 ± 0.98 14.46 ± 1.11 20.03 ± 0.83 21.89 ± 0.96 24.42 ± 0.87 27.67 ± 1.15
5.33 ± 0.90 9.64 ± 1.10 14.31 ± 0.97 17.78 ± 0.78 23.11 ± 0.62 23.82 ± 1.21 25.78 ± 1.10
4.23 ± 1.36 7.62 ± 1.17 10.84 ± 1.46 16.42 ± 1.26 21.53 ± 1.17 22.67 ± 1.03 24.32 ± 1.10
4.89 ± 1.18 8.34 ± 1.45 12.21 ± 0.72 16.62 ± 1.13 21.73 ± 1.02 23.04 ± 1.36 26.32 ± 1.58
4.56 ± 1.46 8.42 ± 0.97 13.10 ± 1.21 17.23 ± 1.56 21.87 ± 1.08 23.21 ± 0.78 25.89 ± 1.36
9.23 ± 0.72 14.03 ± 0.97 18.11 ± 0.78 23.52 ± 1.18 27.22 ± 0.93 29.89 ± 1.11 33.14 ± 0.96
2.52 ± 1.28 4.21 ± 1.07 7.52 ± 1.56 12.78 ± 1.18 19.21 ± 1.35 22.34 ± 1.46 25.72 ± 1.11
Table 3 Total reducing sugar yields of the samples hydrolyzed by 0.23% (w/w) HCl without and with 0.13% (w/w) metal ion additions at different reaction time. Time (h)
1.5 2 3 4 5 6 7
TRS yield (%) Blank
Na+
K+
Mg2+
Ca2+
Cu2+
Fe3+
11.13 ± 1.21 16.02 ± 1.52 26.08 ± 2.10 37.34 ± 1.61 47.38 ± 1.37 55.87 ± 1.81 59.03 ± 1.12
14.52 ± 0.83 22.13 ± 1.71 32.58 ± 1.24 45.23 ± 1.76 55.71 ± 1.85 58.34 ± 1.68 59.52 ± 1.46
13.64 ± 1.77 19.61 ± 2.26 30.28 ± 1.85 42.64 ± 1.55 53.17 ± 1.13 56.78 ± 1.51 59.22 ± 1.18
14.13 ± 1.36 20.24 ± 1.77 31.59 ± 1.26 43.88 ± 1.51 54.12 ± 1.57 57.10 ± 1.78 59.33 ± 1.86
13.88 ± 1.03 22.14 ± 1.36 31.82 ± 1.07 44.37 ± 1.56 55.31 ± 1.18 57.82 ± 1.86 59.11 ± 1.45
13.78 ± 0.72 19.67 ± 1.10 39.31 ± 0.87 56.74 ± 1.17 66.62 ± 1.09 66.21 ± 1.45 69.38 ± 1.01
9.11 ± 0.63 15.62 ± 0.89 30.34 ± 1.07 50.71 ± 0.82 58.03 ± 1.01 58.38 ± 0.91 59.83 ± 1.07
Table 4 Hydrolyis rate and monosaccharides yields of 0.2 g bamboo catalyzed by 0.45% (w/w) HCl with 0.13% (w/w) metal ions as co-catalysts in 4 g [C4mim]Cl at 100 °C. Entry
Metal ion
Hydrolysis time (h)
Hydrolysis rate (g L1 h1)
YieldTRS (%)
Yieldglucose (%)
Yieldxylose (%)
1 2 3 4 5 6 7
Blank Na+ K+ Mg2+ Ca2+ Cu2+ Fe3+
7 7 7 6 6 4 4
3.05 ± 0.01 3.25 ± 0.09 3.21 ± 0.05 3.73 ± 0.08 3.73 ± 0.04 5.69 ± 0.11 5.52 ± 0.12
60.82 ± 0.17 64.80 ± 1.66 64.00 ± 1.12 63.72 ± 1.54 63.72 ± 0.81 64.82 ± 1.16 63.04 ± 1.27
32.32 ± 1.03 37.61 ± 1.23 36.59 ± 0.93 37.05 ± 1.4 37.32 ± 1.25 38.56 ± 1.53 37.53 ± 1.41
28.5 ± 1.62 27.20 ± 1.54 27.41 ± 1.12 26.70 ± 1.0 26.40 ± 1.21 26.21 ± 0.81 25.43 ± 1.33
Table 5 0.2 g bamboo hydrolysis in 4 g water catalyzed by 0.45% (w/w) HCl with 0.13% (w/w) metal ions as co-catalysts at 100 °C. Entry
Metal ion
Hydrolysis time (h)
Hydrolysis rate (g L1 h1)
YieldTRS (%)
1 2 3 4 5 6 7
Blank Na+ K+ Mg2+ Ca2+ Cu2+ Fe3+
7 7 7 7 7 7 7
0.37 ± 0.02 0.45 ± 0.03 0.48 ± 0.02 0.51 ± 0.04 0.50 ± 0.03 0.56 ± 0.03 0.51 ± 0.02
8.54 ± 0.46 10.08 ± 0.60 10.83 ± 0.37 11.74 ± 0.53 11.52 ± 0.43 12.78 ± 0.46 11.62 ± 0.35
80 70
TRS yield (%)
60 50 40
0% 0.02% 0.05% 0.13% 0.23% 0.32%
30 20
was fixed as 0.45% (w/w) in this section. As shown in Fig. 1, the lignocellulose degradation is affected by the Cu2+ ion concentration significantly. When the ion additions concentration was 0.02% (w/w), the TRS yield increased with the reaction time. After 7 h, the TRS yield reached a peak value, which increased by approximately 6% compared to the sample treated without ion addition. With the increase of Cu2+ ion concentration, the releasing rate of reducing sugar improved obviously. After reacted for 4 h, the TRS yield reached 67.1% with Cu2+ ion concentration of 0.23% (w/w). Compared with the reaction without metal additions, the TRS yield increased by about 7%, and the reaction time of peak TRS yield was decreased for 3 h. Then as the hydrolysis reaction progressed, the TRS yield decreased. As can be seen in Fig. 1, the higher the Cu2+ ion concentration, the faster the total reducing sugar yield reached the peak value. This phenomenon could be explained as: the higher
10 0 0
1
2
3
4
5
6
7
8
Reaction time (h) Fig. 1. Total reducing sugar yields of the samples hydrolyzed by 0.45% (w/w) HCl with different Cu2+ concentrations.
the Cu2+ ion concentration, the lower the hydrolysis reaction activation energy required, thus the maximum TRS yield could be reached after a short reaction time. However, as the reactions proceed, the degradation of monosaccharide predominated. An analysis of the mass and composition of the products has been carried out when using 0.2 g of bamboo in 4 g of [C4mim]Cl
N. Wang et al. / Bioresource Technology 173 (2014) 399–405 Table 6 Recovery yields of sugars, ILs and metal ions. 0.2 g bamboo was hydrolyzed by 0.45% (w/w) HCl with 0.23% (w/w) Cu2+ ion as co-catalyst in [C4mim]Cl at 100 °C.
Glucose Xylose IL Cu2+ ions
Mass before reaction (g)
Mass after reaction (g)
Recovery yield (%)
0.088 ± 0.001 0.045 ± 0.001 4.000 ± 0.002 0.011 ± 0.002
0.022 ± 0.003 0.009 ± 0.002 3.326 ± 0.002 0.005 ± 0.001
25.33 ± 2.61 19.46 ± 3.34 83.15 ± 0.15 46.53 ± 1.13
with Cu2+ ion concentration of 0.23% (w/w) and HCl concentration of 0.45% (w/w) (Table 6 and Fig. 2). 0.022 g glucose and 0.009 g xylose were recovered in the alkali phase, accounting for a recovery of 25.33% and 19.46%, respectively. Besides, 83.15% of the IL present in the system and 46.53% of the Cu2+ ion was regenerated. When the recovery recycle was conducted for 3 times, a higher glucose yield could be reached as 32.53%. Compared to the maximum recovery yields of 53% glucose reported by Sun et al. (2013), the glucose recovery yield in our study was lower, which was attributed to the lower temperature and HCl concentration. 3.3. XRD spectra, FTIR spectra and SEM analysis To gain an insight into the possible mechanism revealing the enhancement of the hydrolysis by metal ions, the structural features of regenerated bamboo after 2 h treatment by HCl with/without Cu2+ ion in [C4mim]Cl at 100 °C were examined using XRD, FTIR and SED and compared to the corresponding untreated bamboo samples. Fig. 1 in the Supplementary materials showed the X-ray diffraction spectra of untreated and treated bamboo. The X-ray diffraction of untreated bamboo samples showed a main peak of crystalline cellulose at 22° with a broad shoulder of amorphous cellulose and hemicellulose and lignin at 15°. Bamboo treated with HCl presented a decrease of X-ray diffraction intensity compared with that of untreated bamboo. However, the significant decreases of intensity was derived from using HCl/Cu2+ ion for bamboo treatment, suggesting that Cu2+ ion was efficient to destroy the crystal structure of cellulose. In Fig. 2 in the Supplementary materials, several FTIR bands were used to monitor the chemical changes of lignin and carbohydrates. Compared to the untreated and HCl treated bamboo, the intensity of bands at 1510 cm1 (aromatic skeletal from lignin) and 1329 cm1 (syringyl and guaiacyl condensed lignin) decreased significantly for the HCl/Cu2+ ion treated bamboo, indicating the removal of lignin. The reason might be that metal ions act as electrophiles with an ability to attract electrons. Therefore, they would react with the p electrons of the benzene ring of lignin, resulting in a typical electrophilic aromatic substitution and further ring cleavage reactions (Xiang and Lee, 2000). Meanwhile, the a-aryl ethers bonds are attacked by the metal ions, giving rise to the cleavage of a-aryl ethers bonds, forming lignin fragments. In addition, a
403
decrease in the peak at 1098 cm1 (referring to the crystalline cellulose) were observed for the HCl/Cu2+ ion treated bamboo, exhibiting a transformation of the crystalline cellulose to amorphous cellulose after the combined treatment of bamboo. The phenomenon was consistent with the XRD pattern, which indicated a decrease in cellulose crystallinity (Zhu et al., 2012). Fig. 3 in the Supplementary materials was a collage of SEM images obtained from samples of HCl treated bamboo and HCl/ Cu2+ ion treated bamboo. HCl treated and HCl/Cu2+ ion treated bamboo samples both followed a similar structural modification in the vascular tissue. However, the inside and outside surfaces of the HCl treated samples appear smooth and littered with small to large droplets of lignin. In contrast, the surfaces of HCl/Cu2+ ion treated bamboo samples appear to have a large percentage of the matrixing material (hemicelluloses) removed, with the cellulose macrofibers intact but floating above the surface in a delicate, lacelike pattern with interspersed lignin droplets (Wei et al., 2011). From above experiments, a three-step mechanism to depolymerize bamboo into monosaccharides by metal ions might be deduced: firstly, the metal ions reacted with the lignin to break the intricate structure of biomass and release cellulose; secondly, the cellulose was dissolved into the ILs and the crystal structure of cellulose was destroyed by both ILs and metal ions; thirdly, the metal ions promoted the cleavage of glycosidic bond. The metal ions interacted with the oxygen of the C–O–C glycoside bond between the D-glucose units in cellulose, forming a complex intermediate and weakening the heterocyclic ether bond between the sugar monomers. As Wei et al. (2011) reported that, metal ions associated with cellulose in natural, untreated biomass. Then the weaken ether bond was broken by hydrogen ion, producing sugar monomers and oligomers. As reported by previous studies (Salmi et al., 2014), the attachment of the hydroxonium ion to the glycosidic bond promoted the cleavage of C–O bond, followed by further degradation of monosaccharide, which could be prominent under more severe conditions. In our hydrolysis system, the hemicellulose reaction mechanism might follow a similar way. The metal ions interacted with the oxygen of the glycoside bond in hemicellulose, forming a complex intermediate and weakening the C–O bond. Then the hydroxonium ion promoted the cleavage of the weaken bond, forming sugar monomers and oligomers. Due to the different ability to attract electrons, different metal ions exhibited different effects on the hydrolysis of bamboo. The order of ability to attract electrons for the six metal ions studied was: Fe3+ > Cu2+ > Mg2+ > Na+ > Ca2+ > K+. Theoretically, the Fe3+ ions should show the best effect on the hydrolysis, while the result was different in our study. This may be attributed to the largest ionic radius of Fe3+, resulting in the steric effects which hinder the interaction between Fe3+ ions and the components in bamboo. 3.4. Kinetic model Various kinetic studies on the acidic hydrolysis of cellulosic materials in ILs have been reported in the literature (Vanoye
Fig. 2. Representative mass balance of the whole process. 0.2 g bamboo was hydrolyzed with 0.45% (w/w) HCl and 0.23% (w/w) Cu2+ at 100 °C in 4 g [C4mim]Cl.
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Fundamental Research Funds for the Central Universities (project No. CQDXWL-2013-019).
TRS yield (%)
100
80
Appendix A. Supplementary data
60
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2014. 09.125.
40
References
20
0 0
1
2
3
4
5
6
7
8
Reaction time (h) Fig. 3. Experimental data and the simulated curve for bamboo hydrolysis. 0.2 g bamboo was hydrolyzed with 0.45% (w/w) HCl and 0.23% (w/w) Cu2+ at 100 °C in 4 g [C4mim]Cl.
et al., 2009; Zhang et al., 2012). In this study, a new model for bamboo hydrolyzed with metal ions, dilute acid in IL solvent was constructed as follows, based on a first-order random chain scission followed by a first-order reducing sugar degradation: k1
k2
bamboo ! reducing sugar ! decomposition products
ð4Þ
where k1 is the rate constant for bamboo hydrolysis and k2 is the rate constant for reducing sugar degradation. The time course of TRS yield from bamboo in [C4mim]Cl with Cu2+ ions as co-catalyst for 7 h was monitored in this study. Regression analysis of the experimental data by non-linear least squares curve was fitted using software Origin 7.0, as shown in Fig. 3, where R2 = 0.9913, k1 and k2, the rate constants for TRS formation and TRS degradation, were determined as 0.1844 h1 and 0.5143 h1, respectively. TRS yield increased remarkably with the increase of reaction time in the beginning, the yield of glucose reached 67.1% within 4 h. Thereafter, decreased TRS yield was obtained with the increased reaction time, indicating that reducing sugars were decomposed. The results was in good agreement with previous works (Zhuang et al., 2013), suggesting that this model gave reasonable levels of agreement with experimental data of biomass hydrolysis. 4. Conclusion This study has shown the effect of different types of metal ions on the acid catalyzed hydrolysis of Bamboo in [C4mim]Cl. The most effective ion was Cu2+ ion and the maximum reducing sugar yield of 67.1% was achieved at 100 °C after 4-h hydrolysis with a Cu2+ ion concentration of 0.23% (w/w). This hydrolysis method required no pretreatments in the process and effectively improved the efficiency of hydrolysis. The co-addition of copper ion and dilute hydrochloride acid in ILs could be a valuable method to facilitate cost-efficient conversion of biomass into biofuels or biobased products. Acknowledgements This research was supported by the National Natural Science Foundation of China (Grant 21106191, 21206175), Natural Science Foundation Project of CQ CSTC (Grant cstcjjA50002), and
Amarasekara, A.S., Owereh, O.S., 2009. Hydrolysis and decomposition of cellulose in bronsted acidic ionic liquids under mild conditions. Ind. Eng. Chem. Res. 48, 10152–10155. Bian, J., Peng, F., Peng, X.P., Xiao, X., Peng, P., Xu, F., Sun, R.C., 2014. Effect of [Emim]Ac pretreatment on the structure and enzymatic hydrolysis of sugarcane bagasse cellulose. Carbohydr. Polym. 100, 211–217. da Costa Lopes, André M., João, Karen G., Rubik, Djonatam F., Bogel-Łukasik, Ewa, Duarte, Luís C., Andreaus, Jürgen, Bogel-Łukasik, Rafał, 2013. Pre-treatment of lignocellulosic biomass using ionic liquids: wheat straw fractionation. Bioresour. Technol. 142, 198–208. Heinonen, J., Tamminen, A., Uusitalo, J., Sainio, T., 2012. Ethanol production from wood via concentrated acid hydrolysis, chromatographic separation, and fermentation. J. Chem. Technol. Biotechnol. 87, 689–696. Himmel, M.E., Ding, S.Y., Johnson, D.K., Adney, W.S., Nimlos, M.R., Brady, J.W., Foust, T.D., 2007. Biomass recalcitrance: engineering plants and enzymes for biofuels production. Science 315, 804–807. Jäger, G., Büchs, J., 2012. Biocatalytic conversion of lignocellulose to platform chemicals. Biotechnol. J. 7, 1122–1136. Kamireddy, S.R., Li, J.B., Tucker, M., Degenstein, J., Ji, Y., 2013. Effects and mechanism of metal chloride salts on pretreatment and enzymatic digestibility of corn stover. Ind. Eng. Chem. Res. 52, 1775–1782. Lenihan, P., Orozco, A., O’Neill, E., Ahmad, M.N.M., Rooney, D.W., Walker, G.M., 2010. Dilute acid hydrolysis of lignocellulosic biomass. Chem. Eng. J. 156, 395–403. Li, C.Z., Wang, Q., Zhao, Zongbao K., 2008. Acid in ionic liquid: an efficient system for hydrolysis of lignocellulose. Green Chem. 10, 177–182. Li, C.Z., Zhao, Zongbao K., 2007. Efficient acid-catalyzed hydrolysis of cellulose in ionic liquid. Adv. Synth. Catal. 349, 1847–1850. Liu, C.G., Wyman, C.E., 2006. The enhancement of xylose monomer and xylotriose degradation by inorganic salts in aqueous solutions at 180 °C. Carbohydr. Res. 341, 2550–2556. Miller, G.L., 1959. Use of dinitrosalicylic acid reagent for determination of reducing sugar. Anal. Chem. 31, 426–428. Monavari, S., Galbe, M., Zacchi, G., 2011. The influence of ferrous sulfate utilization on the sugar yields from dilute-acid pretreatment of softwood for bioethanol production. Bioresour. Technol. 102, 1103–1108. Mora-Pale, M., Meli, L., Doherty, T.V., Linhardt, R.J., Dordick, J.S., 2011. Room temperature ionic liquids as emerging solvents for the pretreatment of lignocellulosic biomass. Biotechnol. Bioeng. 108, 1229–1245. Ragauskas, A.J., Williams, C.K., Davison, B.H., Britovsek, G., Cairney, J., Eckert, C.A., Frederick Jr., W.J., Hallett, J.P., Leak, D.J., Liotta, C.L., Mielenz, J.R., Murphy, R., Templer, R., Tschaplinski, T., 2006. The path forward for biofuels and biomaterials. Science 311, 484–489. Rinaldi, R., Palkovits, R., Schüth, F., 2008. Depolymerization of cellulose using solid catalysts in ionic liquids. Angew. Chem. Int. Ed. 47, 8047–8050. Salmi, T., Murzin, D.Y., Maki-Arvela, P., Kusema, B., Holmbom, B., Willffor, S., Warna, J., 2014. Kinetic modeling of hemicellulose hydrolysis in the presence of homogeneous and heterogeneous catalysts. AIChE J. 60, 1066–1077. Sievers, C., Valenzuela-Olarte, M.B., Marzialetti, T., Musin, I., Agrawal, P.K., Jones, C.W., 2009. Ionic-liquid-phase hydrolysis of pine wood. Ind. Eng. Chem. Res. 48, 1277–1286. Sluiter, A., Hames, B., Ruiz, R., Scarlata, C., Sluiter, J., Templeten, D., Crocker, D., 2011. NREL-Laboratory Analytical Procedure (LAP). NREL/TP-510-42618. Sun, Y., Cheng, J.Y., 2002. Hydrolysis of lignocellulosic materials for ethanol production: a review. Bioresour. Technol. 83, 1–11. Sun, N., Liu, H.B., Sathitsuksanoh, N., Stavila, V., Sawant, M., Bonito, A., Tran, K., George, A., Sale, K.L., Singh, S., Simmons, B.A., Holmes, B.M., 2013. Production and extraction of sugars from switchgrass hydrolyzed in ionic liquids. Biotechnol. Biofuels 6, 39. Sun, N., Rahman, M., Qin, Y., Maxim, M.L., Rodriguez, H., Rogres, R.D., 2009. Complete dissolution and partial delignification of wood in the ionic liquid 1ethyl-3-methylimidazolium acetate. Green Chem. 11, 646–655. Swatloski, R.P., Spear, S.K., Holbrey, J.D., Rogres, R.D., 2002. Dissolution of cellulose with ionic liquids. J. Am. Chem. Soc. 124, 4974–4975. Vanoye, L., Fanselow, M., Holbrey, J.D., Atkins, M.P., Seddon, K.R., 2009. Kinetic model for the hydrolysis of lignocellulosic biomass in the ionic liquid, 1-ethyl3-methyl-imidazolium chloride. Green Chem. 11, 390–396. Viell, J., Wulfhorst, H., Schmidt, T., Commandeur, U., Fischer, R., Spiess, A., Marquardt, W., 2013. An efficient process for the saccharification of wood chips by combined ionic liquid pretreatment and enzymatic hydrolysis. Bioresour. Technol. 146, 144–151. Wang, W.J., Powell, A.D., Oates, C.G., 1996. Sago starch as a biomass source: raw sago starch hydrolysis by commercial enzymes. Bioresour. Technol. 55, 55–61.
N. Wang et al. / Bioresource Technology 173 (2014) 399–405 Webb, P.B., Sellin, M.F., Kunen, T.E., Williamson, S., Slawin, A.M.Z., Cole-Hamilton, D.J., 2003. Continuous flow hydroformylation of alkenes in supercritical fluidionic liquid biphasic systems. J. Am. Chem. Soc. 125, 15577–15588. Weerachanchai, P., Lee, J.M., 2013. Effect of organic solvent in ionic liquid on biomass pretreatment. ACS Sustain. Chem. Eng. 1, 894–902. Wei, H., Donohoe, B.S., Vinzant, T.B., Ciesielski, P.N., Wang, W., Gedvilas, L.M., Zeng, Y.N., Johnson, D.K., Ding, S.Y., Himmel, M.E., Tucker, M.P., 2011. Elucidating the role of ferrous ion cocatalyst in enhancing dilute acid pretreatment of lignocellulosic biomass. Biotechnol. Biofuels 4, 48. Xiang, Q., Lee, Y.Y., 2000. Oxidative cracking of precipitate hardwood lignin by hydrogen peroxide. Appl. Biochem. Biotechnol. 2000 (84–6), 153–162. Yan, Y.J., Li, T.C., Ren, Z.W., Li, G.Z., 1996. A study on catalytic hydrolysis of peat. Bioresour. Technol. 57, 269–273.
405
Zhang, Z.H., Wang, W.Q., Liu, X.Y., Wang, Q., Li, W.X., Xie, H.B., Zhao, Zongbao K., 2012. Kinetic study of acid-catalyzed cellulose hydrolysis in 1-butyl-3methylimidazolium chloride. Bioresour. Technol. 112, 151–155. Zhao, S.L., Li, P., Zhang, Q.K., Zhang, J.H., Kong, L.Z., 2013. Production of reducing sugar from corn stover by dilute acid hydrolysis co-catalyzed with metal salts under microwave radiation. Res. Chem. Intermed. 39, 3803–3812. Zhu, Z.S., Zhu, M.J., Wu, Z.Q., 2012. Pretreatment of sugarcane bagasse with NH4OHH2O2 and ionic liquid for efficient hydrolysis and bioethanol production. Bioresour. Technol. 119, 199–207. Zhuang, J.P., Lin, L., Pang, C.S., Liu, Y., 2013. Kinetics of bamboo fiber hydrolysis reaction in saturated formic acid. Key Eng. Mater. 531, 679–683.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Effects of metal ions on the hydrolysis of bamboo biomass in 1-butyl-3-methylimidazolium chloride with dilute acid as catalyst Nan Wang a, Jie Zhang a, Honghui Wang a, Qiang Li b, Sun’an Wei a, Dan Wang a,⇑ a b
Department of Chemical Engineering, School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, PR China National Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China
h i g h l i g h t s Metal ions was used in acidic hydrolysis of bamboo biomass in ILs. The mechanism of the hydrolysis system was investigated. The reaction time was significantly decreased along with the increased TRS yield. 2+
The most effective ion is Cu
a r t i c l e
ion and the maximal TRS yield of 67.1% was achieved.
i n f o
Article history: Received 29 June 2014 Received in revised form 23 September 2014 Accepted 24 September 2014 Available online 2 October 2014 Keywords: Bamboo biomass Ionic liquids Metal ions Hydrolysis Dilute hydrochloride acid
a b s t r a c t In this study, the effects of six metal ions including Na+, K+, Mg2+, Ca2+, Cu2+ and Fe3+ on hydrolysis of bamboo biomass by diluted hydrochloride acid (HCl) in ionic liquid [C4mim]Cl under mild conditions was investigated. These metal ions as co-catalysts exhibited significant effects on accelerating the hydrolysis process and improving the yield of total reducing sugar compared to single diluted hydrochloride acid hydrolysis in [C4mim]Cl at the same conditions. The most effective ion was Cu2+ and the total reducing sugar yield of 67.1% was achieved at 100 °C with CuCl2 as co-catalyst after 4-h reaction. The total reducing sugar yield was increased by about 7% and the reaction time was decreased by 3 h. The kinetic model was also investigated to give an insight into the mechanism of hydrolysis process. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Lignocellulosic biomass has shown promise as an efficient renewable clean energy resource that can be converted to fuels and high-valued chemicals. Extensive research and development programs have been initiated worldwide to transform the lignocellulosic biomass into various valuable products through biorefinery, in which liberation of reducing sugars from lignocellulose is the first key step (Ragauskas et al., 2006; Jäger and Büchs, 2012). Unfortunately, lignocellulosic biomass confers a notorious resistance to hydrolysis due to its complex hetero-matrix structure (Himmel et al., 2007). Up to now, hydrolysis of lignocellulose or cellulose to its component monomeric sugars has been widely investigated. ⇑ Corresponding author at: P. O. Box 94, 174 Shazheng Street, Shapingba District, Department of Chemical Engineering, School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, PR China. Tel./fax: +86 23 65111179. E-mail address: [email protected] (D. Wang). http://dx.doi.org/10.1016/j.biortech.2014.09.125 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
Among all the hydrolysis technologies, enzymes and acid catalysts are the most promising hydrolysis catalysts (Lenihan et al., 2010; Wang et al., 1996). However, these common hydrolysis methods exhibit certain disadvantages and are not cost-effective for large-scale applications. For example, enzymatic hydrolysis suffers a relatively low hydrolysis rate and enzymes are usually expensive and difficult to recycle (Mora-Pale et al., 2011). The traditional diluted acid hydrolysis process requires harsh conditions and results in the decomposition of monosaccharide (Mora-Pale et al., 2011). What is more, pretreatment, as a high cost process, is usually required to decompose the rugged construction of lignocellulose in that hydrolysis above. Concentrated acids such as sulfuric acid and hydrochloric acid have also been used to treat lignocellulose (Heinonen et al., 2012). Though they are powerful agents for cellulose hydrolysis, concentrated acids are toxic, hazardous and require corrosion-resistant reactors (Sun and Cheng, 2002). And the concentrated acid will become environmental pollutant without thorough recycling. Therefore, hydrolysis of
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lignocellulose via an economical and environmentally friendly approach remains a challenge. The very high solvating property of ionic liquids (ILs) was exploited in the dissolution of cellulose by Rogers et al. in 2002 (Swatloski et al., 2002). Since then, research on treatment of lignocellulose in ILs has gained much more attention. It has already been shown that ILs can dissolve a large number of biomass, such as corn stalk, wheat straw, rice straw and wood (da Costa Lopes et al., 2013; Weerachanchai and Lee, 2013; Sun et al., 2009). And ILs can present a high efficiency in the pretreatment of biomass (Viell et al., 2013; Bian et al., 2014). Furthermore, since Li and Zhao (2007) firstly found that mineral acids could facilitate the hydrolysis of cellulose in the ionic liquid 1-butyl-3-methylimidazolium chloride, much more attention has been paid on the hydrolysis of biomass in ILs with acid catalysts directly (Rinaldi et al., 2008; Sievers et al., 2009; Vanoye et al., 2009). Li et al. (2008) reported biomass hydrolysis in ILs with different inorganic acids as catalysts and a maximum 81% liberation of the total reducing sugars was achieved. Amarasekara and Owereh (2009) used Brønsted acid ILs, which act as both the solvent and catalyst, to dissolve and hydrolyze cellulose and obtained a total reducing sugar yield of 62%. Without enzyme hydrolysis subsequently, combined IL dissolution and acid catalysis may provide an economical approach to release reducing sugars from biomass. In recent years, addition of metal ions in traditional dilute acid pretreatment or hydrolysis has become a research highlight, since metal ions can increase the decomposition rate of cellulose and hemicellulose and reduce the severity of hydrolysis conditions (Kamireddy et al., 2013). Monavari et al. (2011) found that addition of small amount of ferrous sulfate in dilute-acid pretreatment resulted in a slightly increased overall glucose yield. Wei et al. (2011) reported that during dilute acid and ferrous ion co-catalyst pretreatment, solubilized sugars increased in the hydrolysate concomitantly with an increase of reducing sugars in the biomass residues. Yan et al. (1996) reported that a high yield of sugar could be obtained from hydrolysis of peat with ferric chloride and hydrogen chloride as catalysts. Zhao et al. (2013) investigated the acid hydrolysis of corn stover under microwave radiation by adding various metal salts individually as co-catalysts and found that FeSO4 had exhibited a better catalytic effect. Though some works have been executed to examine the effectiveness of the addition of acid as catalyst in ILs for lignocellulose hydrolysis and the supplementation of metal ions for the acidic hydrolysis respectively, there is seldom information on the acidic hydrolysis of lignocellulosic biomass in ILs with metal ions added as co-catalyst. The purpose of this study is to investigate the effects of metal ion additions on the dilute acid hydrolysis with ILs as reaction medium and provide an insight on the selectivity of metal ions that can yield high reducing sugars. In order to reduce the energy consumption and the products degradation, the hydrolysis were conducted at low severity conditions. In this work, diluted hydrochloride acid catalyzed hydrolysis of bamboo powder in 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) were performed with six kinds of metal chlorides (NaCl, KCl, MgCl2, CaCl2, CuCl2, FeCl3) added respectively as co-catalyst. The possible decomposition mechanism of lignocellulose in [C4mim]Cl with hydrochloride acid (HCl) and CuCl2 was proposed.
raw bamboo expressed in the weight percent of the dry matter is as follows: 40.1% glucan, 20.3% xylan, 22.3% lignin. The content of each composition was determined using a two-step acid hydrolysis method developed by the National Renewable Energy Laboratory (NRTL) (Sluiter et al., 2011). ILs used in the degradation, namely 1-butyl-3-methyl-imidazolium chloride ([C4mim]Cl), was prepared according to the method reported by Webb et al. (2003). 1-Butyl-3-methyl-imidazolium chloride exists in its liquid form at temperatures between 60 °C and 70 °C. 2.2. Hydrolysis of bamboo Experiments were carried out in a micro-glass round-bottom flask. 4 g of [C4mim]Cl was placed in the reactor and heated to 100 °C. 0.2 g bamboo was then added, followed by the addition of an appropriate amount of metal chlorides and different concentrations of HCl. The mixture was stirred at 20 rpm under atmospheric pressure for 7 h. At different time intervals, samples (0.1 mL) were withdrawn, quenched immediately with cold water and then weighed and recorded as MS. The samples were neutralized with an appropriate amount of 0.05 M NaOH, centrifuged at 10,000 rpm for 5 min and subjected to sugar analysis. The volume measured was recorded as V1. 2.3. Sugar analysis The total reducing sugars (TRS) was measured using DNS method (Miller, 1959). The color tests were made with 1.5 mL aliquots of DNS regent added to 1.5 mL aliquots of reaction sample. The mixtures were heated for 5 min in a boiling water bath and then cooled to ambient temperature. The color intensities were measured in a JASCO V-530 Model spectrophotometer at 540 nm with a slit width of 0.06 mm. The concentration of TRS in the reaction sample was calculated based on a standard curve obtained with glucose standard. The monomeric sugars were analyzed by high performance liquid chromatography (HPLC) using an Aminex HPX-87H ion-exchange column (Bio-Rad, USA) and Agilent 1200 chromatography working station system (Agilent Technologies, USA) equipped with UV absorbance detector (Agilent Technologies, G1315D) and refractive index detector (Agilent Technologies, G1362A). Samples were centrifuged at 10,000 rpm for 10 min. Each supernatant was diluted with 10 volumes of 5 mM H2SO4, and 20 lL of the diluted sample was injected. The column was eluted isocratically at a rate of 0.6 mL min1 with 5 mM H2SO4 under 55 °C. The mass of TRS and the yield of TRS were calculated as follows:
MT ¼ M1 ðM 0 =M S Þ
ð1Þ
M1 ¼ TRS concentration ðg=LÞ V 1
ð2Þ
TRS yield ¼ M T =ð200 A 1:1Þ 100%
ð3Þ
in which, MT is the mass of TRS, M1 is the mass of TRS in the reaction sample, M0 is the total mass of the reaction solution, MS is the mass of sample, 200 is the mass of bamboo biomass (mg), and A is the weight percentage of the polysaccharides contained in lignocellulosic materials. For bamboo, A = 40.1% + 20.3% = 60.4%.
2. Methods
2.4. XRD analysis
2.1. Materials
After the hydrolysis reaction was completed, the mixture was diluted with cold water, neutralized with 0.05 M NaOH, and centrifuged at 10,000 rpm for 5 min. The solid was collected, re-suspended in deionized water, and centrifuged. The purification process was repeated for 5 times. The recovered solid material
The feed stock material was obtained freely from a bamboo grove in Chongqing City, China. Bamboo was milled to pass through an 80 mesh sieve. The main chemical composition of
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was dried at 108 °C to a constant weight, then squashed on glass solid with a smooth surface, and submitted to XRD analysis. The X-ray powder diffraction pattern of the original and regenerated bamboo preparations was measured using an XRD-6000 instrument (Shimadzu, Japan) with a Cu Ka radiation source (k = 0.154 nm) at 40 kV and 30 mA. Samples were scanned from 2h = 5–50° at a speed of 2°/min. 2.5. FTIR spectroscopy Fourier transform infrared spectroscopy (FTIR) was conducted using a Nicolet iN10 FT-IR microscope (Thermo Nicolet Corporation, Madison, WI). The samples were pressed uniformly against the diamond surface with a spring-loaded anvil. The sample spectra was obtained in duplicates using an average of 64 scans over the range 4000–500 cm1 with a spectral resolution of 8 cm1 (Bian et al., 2014). 2.6. SEM analysis The morphology and porous features of bamboo were observed by JSM-6700F (JEOL, Japan) scanning electron microscopy (SEM) operated at 20 kV accelerating voltage. Prior to imaging, the samples were sputter-coated with gold to make the fibers conductive, avoiding degradation and building up the charge of the specimen. 2.7. Recovery of sugars and ILs After the hydrolysis reaction was completed, 1 mL of the mixture solution was placed into a 2 mL eppendorf tube and an appropriate amount of 50% (w/w) NaOH were added to give the final NaOH concentration of 20% (w/w). The mixture was agitated at 1400 rpm for 0.5 h and then centrifuged at 14,000 rpm for phase separate. The upper IL phase and lower NaOH phase was separated and collected. The mass and density of both phases was calculated and the sugar content was quantified (Sun et al., 2013). 3. Results and discussion 3.1. Effect of different metal ions on bamboo hydrolysis Table 1 summarized the TRS yields of bamboo biomass in [C4mim]Cl catalyzed by 0.45% (w/w) HCl with different metal ions as co-catalysts. As shown in Table 1, it took 7 h for the bamboo treatment without metal ion additions to achieve the maximum TRS yield of 60.82%. The maximum TRS yields were 64.80% and 64.00%, respectively, after hydrolysis for 7 h with Na+ and K+ ions (alkali metal ions) as co-catalysts. When Mg2+ and Ca2+ ions (alkali earth metal ions) were added into the hydrolysis system, the reaction time was reduced for 1 h with a similar TRS production, indicating that alkali earth metal ions are more effective than alkali metal ions on bamboo acidic hydrolysis. Hydrolysis experiment with transition metal ions (Cu2+ and Fe3+) were also performed. TRS yields were 64.82% and 63.04% after hydrolysis for 4 h,
respectively, with Cu2+ and Fe3+ as co-catalysts. The reaction time was decreased for 2 h compared to the cases with alkali earth ions as co-catalysts when the similar TRS production was obtained. These results demonstrated that a low content of metal ions as the co-catalysts was an efficient method for biomass acidic hydrolysis. Transition metal ions exhibited better effects than the alkali earth metal ions and alkali metal ions. Among all the six metal ions, Cu2+ ions showed the best effects both on the hydrolysis rate and TRS production. The hydrolysis experiment with gradually reduced acid loading was executed, and the results were summarized in Tables 2 and 3. Metal ions had little effect on the TRS yield except for Cu2+ ion when HCl concentration was 0.23% (w/w), demonstrating once again that Cu2+ can effectively improve the yield of TRS and accelerate the hydrolysis than any other metal ions. Further reducing the acid concentration to 0.05% (w/w) produced lower TRS yields after 7 h. So the optimal HCl concentration for bamboo biomass hydrolysis was chosen as 0.45% (w/w). Besides, the metal ions were found to have a slight influence on the glucose and xylose production. Experiments was conducted with 0.45% (w/w) HCl and 0.13% (w/w) metal ion as co-catalysts, and the results were shown in Table 4. The glucose yield was increased with metal ions addition while the xylose yield was decreased, which indicated that metal ions could promote the hydrolysis of cellulose, meanwhile accelerate the degradation of xylose to furfurals (Salmi et al., 2014). As reported by Liu and Wyman (2006), the inorganic salts KCl, NaCl, CaCl2, MgCl2, and FeCl3 significantly increased xylose monomer degradation at 180 °C in water. The condition in this system was more moderate, thus less degradation of xylose would occur during the hydrolysis. Table 5 illustrated the effects of water used as the solvent on the sugar yield and conversion with 0.45% (w/w) HCl and different metal ions as co-catalysts. It took about 7 h to achieve the TRS yield of 8.54% when the bamboo biomass was treated by HCl without metal ion additions. When metal ions were added into the hydrolysis system, the TRS production was increased by about 1.54–4.24% compared to the control. The result showed that metal ions also worked well on the acidic hydrolysis with water as solvent. However, the TRS yields were decreased with water as solvent under otherwise identical conditions, compared to the results obtained with IL as solvent in Table 5, suggesting that IL as solvent played an important role in this reaction system. The possible reason was that lignocelluloses and cellulose in the bamboo biomass could be dissolved in [C4mim]Cl as their complex, and the hydrolysis was conducted easily compared with the water system, in which hydrolysis just occurred at the surface of lignocelluloses and cellulose (Li and Zhao, 2007). 3.2. Effect of Cu2+ ions concentration on total reducing sugar production To further investigate the effect of Cu2+ ion on the acidic hydrolysis with [C4mim]Cl as solvent, different concentrations of Cu2+ ion were used in the hydrolysis process. The HCl concentration
Table 1 Total reducing sugar yields of the samples hydrolyzed by 0.45% (w/w) HCl without and with 0.13% (w/w) metal ion additions at different reaction time. Time (h)
1 2 3 4 5 6 7
TRS yield (%) Blank
Na+
K+
Mg2+
Ca2+
Cu2+
Fe3+
10.78 ± 1.89 26.89 ± 1.45 42.21 ± 2.26 51.13 ± 2.05 55.72 ± 1.78 58.87 ± 0.20 60.82 ± 0.17
14.88 ± 1.26 35.56 ± 1.06 53.02 ± 1.27 60.78 ± 1.58 63.89 ± 1.36 64.67 ± 0.87 64.80 ± 1.66
11.52 ± 1.45 28.89 ± 1.27 44.31 ± 1.96 56.91 ± 1.78 61.73 ± 1.56 63.44 ± 1.21 64.00 ± 1.12
12.50 ± 0.97 31.32 ± 1.16 46.64 ± 0.89 57.04 ± 1.36 61.32 ± 1.18 63.72 ± 1.54 66.62 ± 1.38
11.62 ± 1.58 31.31 ± 1.45 47.87 ± 1.06 58.22 ± 0.96 61.78 ± 1.15 63.72 ± 0.81 65.33 ± 1.27
15.14 ± 2.06 45.67 ± 1.67 62.23 ± 1.78 64.82 ± 1.16 65.78 ± 1.57 59.21 ± 0.96 53.84 ± 1.56
15.32 ± 1.59 46.42 ± 1.06 60.33 ± 0.89 63.04 ± 1.27 57.78 ± 1.21 53.82 ± 1.60 50.23 ± 1.45
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Table 2 Total reducing sugar yields of the samples hydrolyzed by 0.05% (w/w) HCl without and with 0.13% (w/w) metal ion additions at different reaction time. Time (h)
1.5 2 3 4 5 6 7
TRS yield (%) Blank
Na+
K+
Mg2+
Ca2+
Cu2+
Fe3+
5.13 ± 1.20 8.21 ± 0.98 14.46 ± 1.11 20.03 ± 0.83 21.89 ± 0.96 24.42 ± 0.87 27.67 ± 1.15
5.33 ± 0.90 9.64 ± 1.10 14.31 ± 0.97 17.78 ± 0.78 23.11 ± 0.62 23.82 ± 1.21 25.78 ± 1.10
4.23 ± 1.36 7.62 ± 1.17 10.84 ± 1.46 16.42 ± 1.26 21.53 ± 1.17 22.67 ± 1.03 24.32 ± 1.10
4.89 ± 1.18 8.34 ± 1.45 12.21 ± 0.72 16.62 ± 1.13 21.73 ± 1.02 23.04 ± 1.36 26.32 ± 1.58
4.56 ± 1.46 8.42 ± 0.97 13.10 ± 1.21 17.23 ± 1.56 21.87 ± 1.08 23.21 ± 0.78 25.89 ± 1.36
9.23 ± 0.72 14.03 ± 0.97 18.11 ± 0.78 23.52 ± 1.18 27.22 ± 0.93 29.89 ± 1.11 33.14 ± 0.96
2.52 ± 1.28 4.21 ± 1.07 7.52 ± 1.56 12.78 ± 1.18 19.21 ± 1.35 22.34 ± 1.46 25.72 ± 1.11
Table 3 Total reducing sugar yields of the samples hydrolyzed by 0.23% (w/w) HCl without and with 0.13% (w/w) metal ion additions at different reaction time. Time (h)
1.5 2 3 4 5 6 7
TRS yield (%) Blank
Na+
K+
Mg2+
Ca2+
Cu2+
Fe3+
11.13 ± 1.21 16.02 ± 1.52 26.08 ± 2.10 37.34 ± 1.61 47.38 ± 1.37 55.87 ± 1.81 59.03 ± 1.12
14.52 ± 0.83 22.13 ± 1.71 32.58 ± 1.24 45.23 ± 1.76 55.71 ± 1.85 58.34 ± 1.68 59.52 ± 1.46
13.64 ± 1.77 19.61 ± 2.26 30.28 ± 1.85 42.64 ± 1.55 53.17 ± 1.13 56.78 ± 1.51 59.22 ± 1.18
14.13 ± 1.36 20.24 ± 1.77 31.59 ± 1.26 43.88 ± 1.51 54.12 ± 1.57 57.10 ± 1.78 59.33 ± 1.86
13.88 ± 1.03 22.14 ± 1.36 31.82 ± 1.07 44.37 ± 1.56 55.31 ± 1.18 57.82 ± 1.86 59.11 ± 1.45
13.78 ± 0.72 19.67 ± 1.10 39.31 ± 0.87 56.74 ± 1.17 66.62 ± 1.09 66.21 ± 1.45 69.38 ± 1.01
9.11 ± 0.63 15.62 ± 0.89 30.34 ± 1.07 50.71 ± 0.82 58.03 ± 1.01 58.38 ± 0.91 59.83 ± 1.07
Table 4 Hydrolyis rate and monosaccharides yields of 0.2 g bamboo catalyzed by 0.45% (w/w) HCl with 0.13% (w/w) metal ions as co-catalysts in 4 g [C4mim]Cl at 100 °C. Entry
Metal ion
Hydrolysis time (h)
Hydrolysis rate (g L1 h1)
YieldTRS (%)
Yieldglucose (%)
Yieldxylose (%)
1 2 3 4 5 6 7
Blank Na+ K+ Mg2+ Ca2+ Cu2+ Fe3+
7 7 7 6 6 4 4
3.05 ± 0.01 3.25 ± 0.09 3.21 ± 0.05 3.73 ± 0.08 3.73 ± 0.04 5.69 ± 0.11 5.52 ± 0.12
60.82 ± 0.17 64.80 ± 1.66 64.00 ± 1.12 63.72 ± 1.54 63.72 ± 0.81 64.82 ± 1.16 63.04 ± 1.27
32.32 ± 1.03 37.61 ± 1.23 36.59 ± 0.93 37.05 ± 1.4 37.32 ± 1.25 38.56 ± 1.53 37.53 ± 1.41
28.5 ± 1.62 27.20 ± 1.54 27.41 ± 1.12 26.70 ± 1.0 26.40 ± 1.21 26.21 ± 0.81 25.43 ± 1.33
Table 5 0.2 g bamboo hydrolysis in 4 g water catalyzed by 0.45% (w/w) HCl with 0.13% (w/w) metal ions as co-catalysts at 100 °C. Entry
Metal ion
Hydrolysis time (h)
Hydrolysis rate (g L1 h1)
YieldTRS (%)
1 2 3 4 5 6 7
Blank Na+ K+ Mg2+ Ca2+ Cu2+ Fe3+
7 7 7 7 7 7 7
0.37 ± 0.02 0.45 ± 0.03 0.48 ± 0.02 0.51 ± 0.04 0.50 ± 0.03 0.56 ± 0.03 0.51 ± 0.02
8.54 ± 0.46 10.08 ± 0.60 10.83 ± 0.37 11.74 ± 0.53 11.52 ± 0.43 12.78 ± 0.46 11.62 ± 0.35
80 70
TRS yield (%)
60 50 40
0% 0.02% 0.05% 0.13% 0.23% 0.32%
30 20
was fixed as 0.45% (w/w) in this section. As shown in Fig. 1, the lignocellulose degradation is affected by the Cu2+ ion concentration significantly. When the ion additions concentration was 0.02% (w/w), the TRS yield increased with the reaction time. After 7 h, the TRS yield reached a peak value, which increased by approximately 6% compared to the sample treated without ion addition. With the increase of Cu2+ ion concentration, the releasing rate of reducing sugar improved obviously. After reacted for 4 h, the TRS yield reached 67.1% with Cu2+ ion concentration of 0.23% (w/w). Compared with the reaction without metal additions, the TRS yield increased by about 7%, and the reaction time of peak TRS yield was decreased for 3 h. Then as the hydrolysis reaction progressed, the TRS yield decreased. As can be seen in Fig. 1, the higher the Cu2+ ion concentration, the faster the total reducing sugar yield reached the peak value. This phenomenon could be explained as: the higher
10 0 0
1
2
3
4
5
6
7
8
Reaction time (h) Fig. 1. Total reducing sugar yields of the samples hydrolyzed by 0.45% (w/w) HCl with different Cu2+ concentrations.
the Cu2+ ion concentration, the lower the hydrolysis reaction activation energy required, thus the maximum TRS yield could be reached after a short reaction time. However, as the reactions proceed, the degradation of monosaccharide predominated. An analysis of the mass and composition of the products has been carried out when using 0.2 g of bamboo in 4 g of [C4mim]Cl
N. Wang et al. / Bioresource Technology 173 (2014) 399–405 Table 6 Recovery yields of sugars, ILs and metal ions. 0.2 g bamboo was hydrolyzed by 0.45% (w/w) HCl with 0.23% (w/w) Cu2+ ion as co-catalyst in [C4mim]Cl at 100 °C.
Glucose Xylose IL Cu2+ ions
Mass before reaction (g)
Mass after reaction (g)
Recovery yield (%)
0.088 ± 0.001 0.045 ± 0.001 4.000 ± 0.002 0.011 ± 0.002
0.022 ± 0.003 0.009 ± 0.002 3.326 ± 0.002 0.005 ± 0.001
25.33 ± 2.61 19.46 ± 3.34 83.15 ± 0.15 46.53 ± 1.13
with Cu2+ ion concentration of 0.23% (w/w) and HCl concentration of 0.45% (w/w) (Table 6 and Fig. 2). 0.022 g glucose and 0.009 g xylose were recovered in the alkali phase, accounting for a recovery of 25.33% and 19.46%, respectively. Besides, 83.15% of the IL present in the system and 46.53% of the Cu2+ ion was regenerated. When the recovery recycle was conducted for 3 times, a higher glucose yield could be reached as 32.53%. Compared to the maximum recovery yields of 53% glucose reported by Sun et al. (2013), the glucose recovery yield in our study was lower, which was attributed to the lower temperature and HCl concentration. 3.3. XRD spectra, FTIR spectra and SEM analysis To gain an insight into the possible mechanism revealing the enhancement of the hydrolysis by metal ions, the structural features of regenerated bamboo after 2 h treatment by HCl with/without Cu2+ ion in [C4mim]Cl at 100 °C were examined using XRD, FTIR and SED and compared to the corresponding untreated bamboo samples. Fig. 1 in the Supplementary materials showed the X-ray diffraction spectra of untreated and treated bamboo. The X-ray diffraction of untreated bamboo samples showed a main peak of crystalline cellulose at 22° with a broad shoulder of amorphous cellulose and hemicellulose and lignin at 15°. Bamboo treated with HCl presented a decrease of X-ray diffraction intensity compared with that of untreated bamboo. However, the significant decreases of intensity was derived from using HCl/Cu2+ ion for bamboo treatment, suggesting that Cu2+ ion was efficient to destroy the crystal structure of cellulose. In Fig. 2 in the Supplementary materials, several FTIR bands were used to monitor the chemical changes of lignin and carbohydrates. Compared to the untreated and HCl treated bamboo, the intensity of bands at 1510 cm1 (aromatic skeletal from lignin) and 1329 cm1 (syringyl and guaiacyl condensed lignin) decreased significantly for the HCl/Cu2+ ion treated bamboo, indicating the removal of lignin. The reason might be that metal ions act as electrophiles with an ability to attract electrons. Therefore, they would react with the p electrons of the benzene ring of lignin, resulting in a typical electrophilic aromatic substitution and further ring cleavage reactions (Xiang and Lee, 2000). Meanwhile, the a-aryl ethers bonds are attacked by the metal ions, giving rise to the cleavage of a-aryl ethers bonds, forming lignin fragments. In addition, a
403
decrease in the peak at 1098 cm1 (referring to the crystalline cellulose) were observed for the HCl/Cu2+ ion treated bamboo, exhibiting a transformation of the crystalline cellulose to amorphous cellulose after the combined treatment of bamboo. The phenomenon was consistent with the XRD pattern, which indicated a decrease in cellulose crystallinity (Zhu et al., 2012). Fig. 3 in the Supplementary materials was a collage of SEM images obtained from samples of HCl treated bamboo and HCl/ Cu2+ ion treated bamboo. HCl treated and HCl/Cu2+ ion treated bamboo samples both followed a similar structural modification in the vascular tissue. However, the inside and outside surfaces of the HCl treated samples appear smooth and littered with small to large droplets of lignin. In contrast, the surfaces of HCl/Cu2+ ion treated bamboo samples appear to have a large percentage of the matrixing material (hemicelluloses) removed, with the cellulose macrofibers intact but floating above the surface in a delicate, lacelike pattern with interspersed lignin droplets (Wei et al., 2011). From above experiments, a three-step mechanism to depolymerize bamboo into monosaccharides by metal ions might be deduced: firstly, the metal ions reacted with the lignin to break the intricate structure of biomass and release cellulose; secondly, the cellulose was dissolved into the ILs and the crystal structure of cellulose was destroyed by both ILs and metal ions; thirdly, the metal ions promoted the cleavage of glycosidic bond. The metal ions interacted with the oxygen of the C–O–C glycoside bond between the D-glucose units in cellulose, forming a complex intermediate and weakening the heterocyclic ether bond between the sugar monomers. As Wei et al. (2011) reported that, metal ions associated with cellulose in natural, untreated biomass. Then the weaken ether bond was broken by hydrogen ion, producing sugar monomers and oligomers. As reported by previous studies (Salmi et al., 2014), the attachment of the hydroxonium ion to the glycosidic bond promoted the cleavage of C–O bond, followed by further degradation of monosaccharide, which could be prominent under more severe conditions. In our hydrolysis system, the hemicellulose reaction mechanism might follow a similar way. The metal ions interacted with the oxygen of the glycoside bond in hemicellulose, forming a complex intermediate and weakening the C–O bond. Then the hydroxonium ion promoted the cleavage of the weaken bond, forming sugar monomers and oligomers. Due to the different ability to attract electrons, different metal ions exhibited different effects on the hydrolysis of bamboo. The order of ability to attract electrons for the six metal ions studied was: Fe3+ > Cu2+ > Mg2+ > Na+ > Ca2+ > K+. Theoretically, the Fe3+ ions should show the best effect on the hydrolysis, while the result was different in our study. This may be attributed to the largest ionic radius of Fe3+, resulting in the steric effects which hinder the interaction between Fe3+ ions and the components in bamboo. 3.4. Kinetic model Various kinetic studies on the acidic hydrolysis of cellulosic materials in ILs have been reported in the literature (Vanoye
Fig. 2. Representative mass balance of the whole process. 0.2 g bamboo was hydrolyzed with 0.45% (w/w) HCl and 0.23% (w/w) Cu2+ at 100 °C in 4 g [C4mim]Cl.
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N. Wang et al. / Bioresource Technology 173 (2014) 399–405
Fundamental Research Funds for the Central Universities (project No. CQDXWL-2013-019).
TRS yield (%)
100
80
Appendix A. Supplementary data
60
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2014. 09.125.
40
References
20
0 0
1
2
3
4
5
6
7
8
Reaction time (h) Fig. 3. Experimental data and the simulated curve for bamboo hydrolysis. 0.2 g bamboo was hydrolyzed with 0.45% (w/w) HCl and 0.23% (w/w) Cu2+ at 100 °C in 4 g [C4mim]Cl.
et al., 2009; Zhang et al., 2012). In this study, a new model for bamboo hydrolyzed with metal ions, dilute acid in IL solvent was constructed as follows, based on a first-order random chain scission followed by a first-order reducing sugar degradation: k1
k2
bamboo ! reducing sugar ! decomposition products
ð4Þ
where k1 is the rate constant for bamboo hydrolysis and k2 is the rate constant for reducing sugar degradation. The time course of TRS yield from bamboo in [C4mim]Cl with Cu2+ ions as co-catalyst for 7 h was monitored in this study. Regression analysis of the experimental data by non-linear least squares curve was fitted using software Origin 7.0, as shown in Fig. 3, where R2 = 0.9913, k1 and k2, the rate constants for TRS formation and TRS degradation, were determined as 0.1844 h1 and 0.5143 h1, respectively. TRS yield increased remarkably with the increase of reaction time in the beginning, the yield of glucose reached 67.1% within 4 h. Thereafter, decreased TRS yield was obtained with the increased reaction time, indicating that reducing sugars were decomposed. The results was in good agreement with previous works (Zhuang et al., 2013), suggesting that this model gave reasonable levels of agreement with experimental data of biomass hydrolysis. 4. Conclusion This study has shown the effect of different types of metal ions on the acid catalyzed hydrolysis of Bamboo in [C4mim]Cl. The most effective ion was Cu2+ ion and the maximum reducing sugar yield of 67.1% was achieved at 100 °C after 4-h hydrolysis with a Cu2+ ion concentration of 0.23% (w/w). This hydrolysis method required no pretreatments in the process and effectively improved the efficiency of hydrolysis. The co-addition of copper ion and dilute hydrochloride acid in ILs could be a valuable method to facilitate cost-efficient conversion of biomass into biofuels or biobased products. Acknowledgements This research was supported by the National Natural Science Foundation of China (Grant 21106191, 21206175), Natural Science Foundation Project of CQ CSTC (Grant cstcjjA50002), and
Amarasekara, A.S., Owereh, O.S., 2009. Hydrolysis and decomposition of cellulose in bronsted acidic ionic liquids under mild conditions. Ind. Eng. Chem. Res. 48, 10152–10155. Bian, J., Peng, F., Peng, X.P., Xiao, X., Peng, P., Xu, F., Sun, R.C., 2014. Effect of [Emim]Ac pretreatment on the structure and enzymatic hydrolysis of sugarcane bagasse cellulose. Carbohydr. Polym. 100, 211–217. da Costa Lopes, André M., João, Karen G., Rubik, Djonatam F., Bogel-Łukasik, Ewa, Duarte, Luís C., Andreaus, Jürgen, Bogel-Łukasik, Rafał, 2013. Pre-treatment of lignocellulosic biomass using ionic liquids: wheat straw fractionation. Bioresour. Technol. 142, 198–208. Heinonen, J., Tamminen, A., Uusitalo, J., Sainio, T., 2012. Ethanol production from wood via concentrated acid hydrolysis, chromatographic separation, and fermentation. J. Chem. Technol. Biotechnol. 87, 689–696. Himmel, M.E., Ding, S.Y., Johnson, D.K., Adney, W.S., Nimlos, M.R., Brady, J.W., Foust, T.D., 2007. Biomass recalcitrance: engineering plants and enzymes for biofuels production. Science 315, 804–807. Jäger, G., Büchs, J., 2012. Biocatalytic conversion of lignocellulose to platform chemicals. Biotechnol. J. 7, 1122–1136. Kamireddy, S.R., Li, J.B., Tucker, M., Degenstein, J., Ji, Y., 2013. Effects and mechanism of metal chloride salts on pretreatment and enzymatic digestibility of corn stover. Ind. Eng. Chem. Res. 52, 1775–1782. Lenihan, P., Orozco, A., O’Neill, E., Ahmad, M.N.M., Rooney, D.W., Walker, G.M., 2010. Dilute acid hydrolysis of lignocellulosic biomass. Chem. Eng. J. 156, 395–403. Li, C.Z., Wang, Q., Zhao, Zongbao K., 2008. Acid in ionic liquid: an efficient system for hydrolysis of lignocellulose. Green Chem. 10, 177–182. Li, C.Z., Zhao, Zongbao K., 2007. Efficient acid-catalyzed hydrolysis of cellulose in ionic liquid. Adv. Synth. Catal. 349, 1847–1850. Liu, C.G., Wyman, C.E., 2006. The enhancement of xylose monomer and xylotriose degradation by inorganic salts in aqueous solutions at 180 °C. Carbohydr. Res. 341, 2550–2556. Miller, G.L., 1959. Use of dinitrosalicylic acid reagent for determination of reducing sugar. Anal. Chem. 31, 426–428. Monavari, S., Galbe, M., Zacchi, G., 2011. The influence of ferrous sulfate utilization on the sugar yields from dilute-acid pretreatment of softwood for bioethanol production. Bioresour. Technol. 102, 1103–1108. Mora-Pale, M., Meli, L., Doherty, T.V., Linhardt, R.J., Dordick, J.S., 2011. Room temperature ionic liquids as emerging solvents for the pretreatment of lignocellulosic biomass. Biotechnol. Bioeng. 108, 1229–1245. Ragauskas, A.J., Williams, C.K., Davison, B.H., Britovsek, G., Cairney, J., Eckert, C.A., Frederick Jr., W.J., Hallett, J.P., Leak, D.J., Liotta, C.L., Mielenz, J.R., Murphy, R., Templer, R., Tschaplinski, T., 2006. The path forward for biofuels and biomaterials. Science 311, 484–489. Rinaldi, R., Palkovits, R., Schüth, F., 2008. Depolymerization of cellulose using solid catalysts in ionic liquids. Angew. Chem. Int. Ed. 47, 8047–8050. Salmi, T., Murzin, D.Y., Maki-Arvela, P., Kusema, B., Holmbom, B., Willffor, S., Warna, J., 2014. Kinetic modeling of hemicellulose hydrolysis in the presence of homogeneous and heterogeneous catalysts. AIChE J. 60, 1066–1077. Sievers, C., Valenzuela-Olarte, M.B., Marzialetti, T., Musin, I., Agrawal, P.K., Jones, C.W., 2009. Ionic-liquid-phase hydrolysis of pine wood. Ind. Eng. Chem. Res. 48, 1277–1286. Sluiter, A., Hames, B., Ruiz, R., Scarlata, C., Sluiter, J., Templeten, D., Crocker, D., 2011. NREL-Laboratory Analytical Procedure (LAP). NREL/TP-510-42618. Sun, Y., Cheng, J.Y., 2002. Hydrolysis of lignocellulosic materials for ethanol production: a review. Bioresour. Technol. 83, 1–11. Sun, N., Liu, H.B., Sathitsuksanoh, N., Stavila, V., Sawant, M., Bonito, A., Tran, K., George, A., Sale, K.L., Singh, S., Simmons, B.A., Holmes, B.M., 2013. Production and extraction of sugars from switchgrass hydrolyzed in ionic liquids. Biotechnol. Biofuels 6, 39. Sun, N., Rahman, M., Qin, Y., Maxim, M.L., Rodriguez, H., Rogres, R.D., 2009. Complete dissolution and partial delignification of wood in the ionic liquid 1ethyl-3-methylimidazolium acetate. Green Chem. 11, 646–655. Swatloski, R.P., Spear, S.K., Holbrey, J.D., Rogres, R.D., 2002. Dissolution of cellulose with ionic liquids. J. Am. Chem. Soc. 124, 4974–4975. Vanoye, L., Fanselow, M., Holbrey, J.D., Atkins, M.P., Seddon, K.R., 2009. Kinetic model for the hydrolysis of lignocellulosic biomass in the ionic liquid, 1-ethyl3-methyl-imidazolium chloride. Green Chem. 11, 390–396. Viell, J., Wulfhorst, H., Schmidt, T., Commandeur, U., Fischer, R., Spiess, A., Marquardt, W., 2013. An efficient process for the saccharification of wood chips by combined ionic liquid pretreatment and enzymatic hydrolysis. Bioresour. Technol. 146, 144–151. Wang, W.J., Powell, A.D., Oates, C.G., 1996. Sago starch as a biomass source: raw sago starch hydrolysis by commercial enzymes. Bioresour. Technol. 55, 55–61.
N. Wang et al. / Bioresource Technology 173 (2014) 399–405 Webb, P.B., Sellin, M.F., Kunen, T.E., Williamson, S., Slawin, A.M.Z., Cole-Hamilton, D.J., 2003. Continuous flow hydroformylation of alkenes in supercritical fluidionic liquid biphasic systems. J. Am. Chem. Soc. 125, 15577–15588. Weerachanchai, P., Lee, J.M., 2013. Effect of organic solvent in ionic liquid on biomass pretreatment. ACS Sustain. Chem. Eng. 1, 894–902. Wei, H., Donohoe, B.S., Vinzant, T.B., Ciesielski, P.N., Wang, W., Gedvilas, L.M., Zeng, Y.N., Johnson, D.K., Ding, S.Y., Himmel, M.E., Tucker, M.P., 2011. Elucidating the role of ferrous ion cocatalyst in enhancing dilute acid pretreatment of lignocellulosic biomass. Biotechnol. Biofuels 4, 48. Xiang, Q., Lee, Y.Y., 2000. Oxidative cracking of precipitate hardwood lignin by hydrogen peroxide. Appl. Biochem. Biotechnol. 2000 (84–6), 153–162. Yan, Y.J., Li, T.C., Ren, Z.W., Li, G.Z., 1996. A study on catalytic hydrolysis of peat. Bioresour. Technol. 57, 269–273.
405
Zhang, Z.H., Wang, W.Q., Liu, X.Y., Wang, Q., Li, W.X., Xie, H.B., Zhao, Zongbao K., 2012. Kinetic study of acid-catalyzed cellulose hydrolysis in 1-butyl-3methylimidazolium chloride. Bioresour. Technol. 112, 151–155. Zhao, S.L., Li, P., Zhang, Q.K., Zhang, J.H., Kong, L.Z., 2013. Production of reducing sugar from corn stover by dilute acid hydrolysis co-catalyzed with metal salts under microwave radiation. Res. Chem. Intermed. 39, 3803–3812. Zhu, Z.S., Zhu, M.J., Wu, Z.Q., 2012. Pretreatment of sugarcane bagasse with NH4OHH2O2 and ionic liquid for efficient hydrolysis and bioethanol production. Bioresour. Technol. 119, 199–207. Zhuang, J.P., Lin, L., Pang, C.S., Liu, Y., 2013. Kinetics of bamboo fiber hydrolysis reaction in saturated formic acid. Key Eng. Mater. 531, 679–683.
