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Efficient acid-catalyzed hydrolysis of cellulose in organic electrolyte solutions
Polymer Degradation and Stability 97 (2012) 573e577
Contents lists available at SciVerse ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Efficient acid-catalyzed hydrolysis of cellulose in organic electrolyte solutions Zehui Zhang a, b, *, Bing Liu a, Zongbao (Kent) Zhao b, c a
Key Laboratory of Catalysis and Materials Sciences of the State Ethnic Affairs Commission & Ministry of Education, College of Chemistry and Material Science, South-Central University for Nationalities, Wuhan 430074, China b Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China c Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian 116023, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 December 2011 Received in revised form 6 January 2012 Accepted 14 January 2012 Available online 21 January 2012
A novel method for cellulose hydrolysis catalyzed by mineral acids have been developed in organic electrolyte solutions at 70 C under atmosphere pressure without pretreatment. Several reaction parameters including reaction time, temperature, catalyst loading and the ratio of ionic liquids to organic solvents have been evaluated and optimized. Under optimal conditions, the maximum total reducing sugars (TRS) yield in 68.8% and glucose yield in 39.2% were obtained in 1.0 g NMP/3.0 g [Bmim]Cl system at 70 C. Due to the low temperature of cellulose hydrolysis, this method shows a promising potential as an energy-efficient and cost-effective approach for the biorefinery of lignocellulosic biomass. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Cellulose Hydrolysis Acid catalyst Organic electrolyte solutions
1. Introduction Concerns about global warming and energy security have led to the exploration of alternatives for fossil resources to supply chemicals and energy [1,2]. Biomass is the most abundant renewable resource with an estimated global production of around 1.0 1011 tons per year, and it can be converted to biofuels and biobased chemicals through biorefinery [3e5]. Cellulose is a major component of biomass, which is readily available and does not compete with the food supply. Therefore, there is tremendous interest to develop new processes not only for fuels but also as a starting material for our chemical industry from cellulose. Hydrolysis of cellulose is considered to be an entry point of biorefinery schemes, as the resulting glucose can be further converted into various chemicals and biofuels [6]. However, the poor solubility of cellulose limited this process. Cellulose is a linear polysaccharide chain consisting of hundreds to thousands of D-anhydroglucopyranose linked together b-glycosidic bonds. Due to the extensive network of inter- and intra-molecule hydrogen bonds and van der Waals force, cellulose is insoluble in water and most conventional organic solvents, leading to notoriously recalcitrant to process [7e9].
* Corresponding author. Key Laboratory of Catalysis and Materials Sciences of the State Ethnic Affairs Commission & Ministry of Education, College of Chemistry and Material Science, South-Central University for Nationalities, Minyuan Road 708, Wuhan 430074, China. Tel./fax: þ86 27 67842752. E-mail address: [email protected] (Z. Zhang). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2012.01.010
Much effort has been devoted to depolymerization of cellulose. These included acidic hydrolysis [10e12], enzymatic hydrolysis [13,14] and hydrolysis in supercritical water [15e17]. Acidic hydrolysis approach is commonly applied in the hydrolysis of cellulose to attain glucose. The major drawbacks are the high operating cost of acid recovery and the use of expensive construction material for both the hydrolyzer and the acid recovery system. Enzymatic process takes place under mild conditions, but the system is sensitive to contaminants originating from other biomass components. Furthermore, pretreatment of cellulose is usually required to increase the accessible area of cellulose to improve the hydrolysis rate. Supercritical water has recently been used as a medium for hydrolysis of cellulose, but hash conditions such as high temperature and high pressure are required. Therefore, it is pivotal to develop more efficient and environmentfriendly method for cellulose hydrolysis. Recently, ionic liquids (ILs) have attracted much attention as greener alternatives to conventional organic solvents [18e20]. Swatloski et al. [21] firstly reported that ILs could act as a new and powerful non-derivatizing solvent for cellulose with a high solubility up to 25 wt.% in 1-n-butyl-3-methylimidazolium chloride ([Bmim]Cl) under microwave irradiation. The good solubility of cellulose in ILs has paved a new way for its applications [22]. In our previous work, it was demonstrated that cellulose could be readily hydrolyzed in [Bmim]Cl in the presence of mineral acids at 100 C [23]. Even though good results was obtained, it is also desirable to carry out cellulose hydrolysis at a relative low temperature as one of the key goals among the 12 principles of
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Green Chemistry, which would be more energy-saving. Due to the high melting point and viscosity of ILs, hydrolysis of cellulose at a low temperature (e.g. at 70 C) seems difficult. Recently, Rinaldi [24] discovered that organic electrolyte solutions showed a superior solubility for cellulose. Organic electrolyte solutions are composed of ILs and polar organic solvents. This discovery would provide a more economical and convenient way to the utilization of cellulose. Therefore, in this work, we would like to present our results of acidic hydrolysis of cellulose in organic electrolyte solutions. 2. Experimental 2.1. Materials and instrument 1-Butyl-3-methylimidazole chloride ([Bmim]Cl) was synthesized according to a known procedure [25,26]. Spruce cellulose (Cat. No. 22182) was purchased from Sigma (St. Louis, USA), and was dried under vacuum at 100 C for 24 h before use. N-methylimidazole (99%) was obtained from Zhejiang Kaile Chemicals Co. Ltd. (Taizhou, China). 1-Chlorobutane (98%) was purchased from Guangfu Fine Chemical Research Institute (Tianjin, China), and freshly distilled before use. N-Methylpyrrolidinone (NMP) was purchased from Aladdin reagent Co. (Shanghai, China). All other chemicals were supplied by local suppliers and used without further purification. 2.2. Typical procedure for cellulose hydrolysis Cellulose (320 mg) was added into a mixture of [Bmim]Cl (3.0 g) and NMP (1.0 g) at 70 C. The mixture was stirred until a yellowish solution formed. Then 63 mg H2O (1.75 mol equiv to glucose unit) and 0.148 g 98 wt.% H2SO4 were quickly added into the cellulose solution. The reaction mixture was heated at 50, 70, or 100 C for 10 h. A portion of the mixture (about 60 mL) were withdrawn, weighed (recorded as M1) diluted with water with 2 mL, centrifuged at 10,000 rpm for 5 min, obtaining an aqueous solution with the volume to be V1. This aqueous solution was called sample, which was then subjected to TRS analysis using a DNS method in section 2.4 [27] and glucose analysis with a glucose analyzer in section 2.5. 2.3. Preparation of DNS reagent 182 g of potassium sodium tartrate, 6.3 g of 3, 5-dinitrosalicylic acid (DNS) and 262 mL of 2 M NaOH were added to500 mL of hot deionized water (50 C) in sequence, after dissolved, 5 g of phenol and 5 g of sodium sulfite were added in the solution, stirring until homogeneous solution was formed, then cooled to room temperature and diluted with deionized water to 1000 mL to give the DNS reagent. 2.4. TRS analysis according to DNS method A mixture of 0.5 mL of DNS reagent and 0.5 mL of sample was heated in a boiling water bath for 5 min (Note: The sample was derived from the prepared aqueous solution section 2.2, and we only used 0.5 mL for TRS analysis), and 1 mL mixture after the reaction of 0.5 mL of DNS reagent with 0.5 mL of sample was remained. Then the 1 mL mixture was cooled to room temperature, and diluted with 4 mL of deionized water, finally forming 5 mL solution. The color intensity of the final 5 mL solution was measured in a JASCO V-530 Model spectrophotometer at 540 nm with a slit width of 0.06 mm. The concentration of total reducing
sugars was calculated based on a standard curve obtained with glucose, and the yield of TRS were calculated as follows,
MT ðmgÞ ¼ TRS concentrationðmg=mLÞ 5ðmLÞ ðV1 =0:5Þ ðM0 =M1 Þ TRS Yield ¼ MT =½ðM2 =162Þ 180 100% ¼ MT 0:9=M2 100% As cellulose is composed of glucose unit (with the mole mass of unit to be 160), Therefore, the glucose mass after hydrolysis should be 180*(M2/162) theoretically. In which, MT is the mass of TRS, and M0 is the total mass of the reaction solution. M1 is the mass of a portion of the mixture withdrawn from the reaction mixture under magnetic stirring, keeping the whole reaction mixture to be homogenous. V1 is the volume of the aqueous solution in section 2.2, “5 (mL)” is the total volume of 1 mL mixture (0.5 mL sample and 0.5 mL DNS reagent) after reaction in boiling water, and 4 mL water, which was used to dilute the 1 mL mixture after reaction. “0.5” means we only used 0.5 mL of the aqueous solution, although the volume was as V1. M2 is the mass of cellulose initially loaded in the reaction, respectively. 2.5. Glucose analysis Glucose concentration was determined using a SBA-50B glucose analyzer (Shandong Academy of Sciences, Jinan, China). 3. Results and discussion 3.1. Effect of reaction temperature on cellulose hydrolysis in [Bmim] Cl/NMP system In our previous work, an efficient method has been developed for cellulose hydrolysis in [Bmim]Cl at 100 C [23]. Due to the high melting point and high viscosity of [Bmim]Cl, it is difficult for the hydrolysis of cellulose in [Bmim]Cl at low temperature (e.g. at 70 C). It was reported that cellulose solubility was improved extremely and the viscosity decreased remarkably in organic electrolyte solutions (the mixture of ILs and polar organic solvents) [24]. In our experiment, it only required 2 h for 320 mg cellulose dissolving in 1.0 g NMP/3.0 g [Bmim]Cl to form homogenous solution at 70 C, whereas, that was 12 h in [Bmim]Cl at 100 C. In addition, the cellulose solution was stirred smoothly at 70 C. Therefore, it inspired us to carry out cellulose hydrolysis in 1.0 g NMP/3.0 g [Bmim]Cl at a relative low temperature of 70 C. Unfortunately, the hydrolysis rate was slow. TRS yields increased gradually from 8.2% at 1 h to 63.7% at 10 h (Fig. 1), and the corresponding glucose yields were 2.7% at 1 h and 29.6% at 10 h (Fig. 2), respectively. Even though a long reaction time was required to obtain high TRS and glucose yields at 70 C, to the best of our knowledge it is the lowest reaction temperature for cellulose hydrolysis. In order to increase the reaction rate, a higher hydrolysis temperature of 100 C was evaluated. As expected, the reaction rate increased sharply, and the maximum yields of TRS in 53.9% (Fig. 1) and glucose in 32.4% (Fig. 2) were obtained at 1 h, respectively. However, the TRS and glucose yields sharply declined with the continual extension of reaction time. It indicated that the products were not stable at high temperature and degraded into other byproducts. Additionally, cellulose hydrolysis was also carried out at 50 C, and it was noted that the hydrolysis rate was rather slow, with TRS yield in 9.9% (Fig. 1) and glucose yield in 2.5% (Fig. 2) at
Z. Zhang et al. / Polymer Degradation and Stability 97 (2012) 573e577
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Fig. 1. TRS yield of cellulose hydrolysis catalyzed by H2SO4 at different temperatures in organic electrolyte solutions. Reaction condition: 0.320 g spruce cellulose dissolved in 1.0 g NMP and 3.0 g [BMIM]Cl, then 0.063 g H2O and 0.148 g 98 wt.% H2SO4 were added, the reaction was performed at a set temperature.
Fig. 3. TRS yield of cellulose hydrolysis catalyzed by different acids in organic electrolyte solutions. Reaction condition: 0.320 g spruce cellulose dissolved in 1.0 g NMP/3.0 g [BMIM]Cl, then 0.063 g H2O and 0.148 g 98 wt.% H2SO4 were added (or 0.300 g 36.5 wt.% HCl), the reaction was performed at 70 C.
10 h, respectively. The results indicated that temperature had a remarkable effect on cellulose hydrolysis. Considering the reaction rate and the stability of the products, we set the hydrolysis temperature as 70 C in the following experiments.
acid in aqueous solution corresponds to a high acidity in organic electrolyte solutions.
3.2. Effect of mineral acids on cellulose hydrolysis in [Bmim]Cl/NMP system Secondly, different mineral acids including HCl, H2SO4 and H3PO4 were tested for hydrolysis of cellulose in organic electrolyte solutions. As shown in Fig. 3 and Fig. 4, the strong mineral acids such as HCl and H2SO4 afforded satisfied results. They almost had the same reaction rate and sugar yields, with the same amount of protons. However, when the weak mineral acid H3PO4 was used, the hydrolysis was inferior. TRS and glucose yields were only 2.2% and 0.6% for 10 h at 70 C, respectively. These results suggested that the strength of acids played an important role in cellulose hydrolysis in organic electrolyte solutions. The high acidity of mineral
The effect of catalyst loading on cellulose hydrolysis was further investigated. The results were shown in Fig. 5 and Fig. 6. It was apparent that more acids loading led to higher sugar yields especially at the initial reaction stage. For example, the yields of TRS and glucose were only 13.2% and 2.3% at 2 h with 0.077 g H2SO4, whereas, those were reached up to 45.6% and 15.5% with 0.222 g H2SO4 at 2 h, respectively. The increase sugar yields with an increase in catalyst loading can be attributed to an increase in the availability and number of catalytically active site for cellulose hydrolysis. It only required 8 h to achieve a maximum TRS yield in 68.8% and glucose yield in 39.2% with 0.222 g H2SO4 at 70 C. After 8 h, TRS and glucose yields decreased gradually, due to the acidpromoted degradation of glucose.
Fig. 2. Glucose yield of cellulose hydrolysis catalyzed by H2SO4 at different temperatures in organic electrolyte solutions. Reaction conditions were the same as described in Fig. 1.
Fig. 4. Glucose yield of cellulose hydrolysis catalyzed by different acids in organic electrolyte solutions. Reaction conditions were the same as described in Fig. 3.
3.3. Effect of acids loading on cellulose hydrolysis in [Bmim]Cl/NMP system
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Fig. 5. TRS yield of cellulose hydrolysis catalyzed by different amount of H2SO4 in organic electrolyte solutions. Reaction condition: 0.320 g spruce cellulose dissolved in 1.0 g NMP/3.0 g [Bmim]Cl, then 0.063 g H2O and a set amount of 98 wt.% H2SO4 were added, the reaction was performed at 70 C.
Fig. 7. TRS yield of cellulose hydrolysis catalyzed by H2SO4 in different organic electrolyte solutions. Reaction condition: 0.320 g spruce cellulose dissolved in 4 g solvents, then 0.063 g H2O and 0.148 g 98 wt.% H2SO4 were added, the reaction was performed at 70 C.
Our method produced equal or higher glucose and TRS yields in comparison with those reaction procedures at high temperature and/or using concentrated H2SO4 (65 wt% or higher) [28,29]. When cellulose was completely dissolved in the electrolyte solutions and formed a homogeneous solution, it made the Hþ more accessible to the b-glycosidic bonds. This is likely the reason that the hydrolysis rate was much higher within the electrolyte solutions as the solvent when compared with those systems where hydrolysis occurred at the surface of cellulose. Therefore, a physical barrier for hydrolysis was overcome through formation of a solution. As a control experiment, cellulose hydrolysis was also carried out in water (4.0 g) catalyzed by 0.222 g of sulfuric acid at 70 C for 8 h. Glucose and TRS yield were 1.5% and 9.6%, respectively. The results clearly indicated that the solubility of cellulose in 3 g of [Bmim]Cl and 1 g of NMP played a key role in cellulose hydrolysis, in which the accessibility of acid to cellulose could be enhanced.
3.4. Effect of the ratio of NMP with [Bmim]Cl on cellulose hydrolysis
Fig. 6. Glucose yield of cellulose hydrolysis catalyzed by different amount of H2SO4 in organic electrolyte solutions. Reaction conditions were the same as described in Fig. 5.
Fig. 8. Glucose yield of cellulose hydrolysis catalyzed by H2SO4 in different organic electrolyte solutions. Reaction conditions were the same as described in Fig. 7.
It was reported that the content of [Bmim]Cl had an important effect on cellulose solubility. Cellulose dissolved much more easily with the increase concentration of [Bmim]Cl24. Herein, the solvent effect on cellulose hydrolysis was investigated. The results are shown in Fig. 7 and Fig. 8. It can be seen that the rate of cellulose hydrolysis became fast with the increase amount of [Bmim]Cl. It should be noted that the yields of TRS and glucose were extremely low in 3.0 g NMP/1.0 g [Bmim]Cl system. It was acceptable because cellulose was not dissolved in this system, which resulted in a heterogeneous hydrolysis. On the other hand, cellulose was dissolved easily in other systems, but the hydrolysis was hindered with the increase amount of NMP. It was reported that H2SO4 can react with NMP to form a new acidic ionic liquids [NMP]þ[HSO4] [30]. Therefore, the actual proton used for catalyzing the cellulose hydrolysis became less with the increase amount of NMP.
Z. Zhang et al. / Polymer Degradation and Stability 97 (2012) 573e577
4. Conclusion In this study, hydrolysis of cellulose was realized at a relative low temperature at 70 C in NMP/[Bmim]Cl system catalyzed by mineral acid. Under optimal reaction conditions, a maximum of TRS and glucose yields were obtained in 68.8% and 39.2% at 70 C, respectively. To the best of our knowledge, it is the lowest temperature for cellulose hydrolysis till yet. The application of this novel organic electrolyte solution for cellulose hydrolysis is more economical and energy-saving. This reaction system opens up a new opportunity for facilitating cost-efficient utilization of lignocellulosic biomass in biorefinery. Acknowledgments The Project was Supported by the Special Fund for Basic Scientific Research of Central Colleges, South-Central University for Nationalities (CZQ11024). References [1] Saidur R, Abdelaziz EA, Demirbas A, Hossain MS, Mekhilef S. A review on biomass as a fuel for boilers. Renew Sust Energ Rev 2011;15:2262e89. [2] Martin M, Eklund M. Improving the environmental performance of biofuels with industrial symbiosis. Biomass Bioenerg 2011;35:1747e55. [3] Corma A, Iborra S, Velty A. Chemical routes for the transformation of biomass into chemicals. Chem Rev 2007;107:2411e502. [4] Zhang ZH, Wang Q, Xie HB, Liu WJ, Zhao ZK. Catalytic conversion of carbohydrates into 5-hydroxymethylfurfural by Germanium(IV) chloride in ionic liquids. ChemSusChem 2011;4:131e8. [5] Gog A, Roman M, Tosa M, Paizs C, Irimie FD. Biodiesel production using enzymatic transesterification e current state and perspectives. Renew Energ 2012;39:10e6. [6] Zhang ZH, Zhao ZK. Production of 5-hydroxymethylfurfural from glucose catalyzed by hydroxyapatite supported chromium chloride. Bioresour Technol 2011;102:3970e2. [7] Jarvis M. Cellulose stacks up. Nature 2003;426:611e2. [8] Nishiyama Y, Sugiyama J, Chanzy H, Langan P. Crystal structure and hydrogen bonding system in cellulose Ia from synchrotron X-ray and neutron fiber diffraction. J Am Chem Soc 2003;125:14300e6. [9] Poletto M, Pistor V, Zeni M, Zattera AJ. Crystalline properties and decomposition kinetics of cellulose fibers in wood pulp obtained by two pulping processes. Polym Degrad Stab 2011;96:679e85. [10] Antonoplis RA, Blanch HW, Freitas RP, Sciamanna AF, Wilke CR. Production of sugars from wood using high-pressure hydrogen-chloride. Biotechnol Bioeng 1983;25:2757e73.
577
[11] Karimi K, Kheradmandinia S, Taherzadeh MJ. Conversion of rice straw to sugars by dilute-acid hydrolysis. Biomass Bioenerg 2006;30:247e53. [12] Torget RW, Kim JS, Lee YY. Fundamental aspects of dilute acid hydrolysis/ fractionation kinectics of hardwood carbohydrates. 1. Cellulose hydrolysis. Ind Eng Chem Res 2000;39:2817e25. [13] Ha SH, Mai NL, An G, Koo YM. Microwave-assisted pretreatment of cellulose in ionic liquid for accelerated enzymatic hydrolysis. Bioresour Technol 2011; 102:1214e9. [14] Shaikh HM, Adsul MG, Gokhale DV, Varma AJ. Enhanced enzymatic hydrolysis of cellulose by partial modification of its chemical structure. Carbohydr Polym 2011;2:962e8. [15] Mok Jr WSL, Antal MJ. Uncatalyzed solvolysis of whole biomass hemicelluloses by hot compressed liquid water. Ind Eng Chem Res 1992;31: 1157e61. [16] Kumar S, Gupta RB. Hydrolysis of microcrystalline cellulose in subcritical and supercritical water in a continuous flow reactor. Ind Eng Chem Res 2008;47: 9321e9. [17] Sasaki M, Kabyemela B, Malaluan R, Hirose S, Takeda N, Adschiri T. Cellulose hydrolysis in subcritical and supercritical water. J Supercrit Fluids 1998;13: 261e8. [18] Miao W, Chan TH. Ionic-liquid-supported synthesis: a novel liquid-phase strategy for organic synthesis. Acc Chem Res 2006;39:897e908. [19] Olivier-Bourbigou H, Magna1 L, Morvan D. Ionic liquids and catalysis: recent progress from knowledge to applications. Appl Catal A Gen 2010; 373:1e56. [20] Rahman M, Brazel CS. Ionic liquids: new generation stable plasticizers for poly(vinyl chloride). Polym Degrad Stab 2006;91:3371e82. [21] Swatloski RP, Spear SK, Holbrey JD, Rogers RD. Dissolution of cellulose with ionic liquids. J Am Chem Soc 2002;124:4974e5. [22] Li WY, Jin AX, Liu CF, Sun RC, Zhang AP, Kennedy JF. Homogeneous modification of cellulose with succinic anhydride in ionic liquid using 4-dimethylaminopyridine as a catalyst. Carbohydr Polym 2009;78:389e95. [23] Li CZ, Zhao ZK. Efficient acid-catalyzed hydrolysis of cellulose in ionic liquid. Adv Synth Catal 2007;349:1847e50. [24] Rinaldi R. Instantaneous of cellulose in organic electrolyte solutions. Chem Commun 2011;47:511e3. [25] Burrell AK, Del Sesto RE, Baker SN, McCleskey TM, Baker GA. The large scale synthesis of pure imidazolium and pyrrolidinium ionic liquids. Green Chem 2007;9:449e54. [26] Webb PB, Sellin MF, Kunen TE, Williamson S, Slawin AMZ, Cole-Hamilton DJ. Continuous flow hydroformylation of alkenes in supercritical fluid-ionic liquid biphasic systems. J Am Chem Soc 2003;125:15577e88. [27] Miller GL. Use of dinitrosalicylic acid reagent for determination of reducing sugar. Anal Chem 1959;31:426e8. [28] Roman M, Winter WT. Effect of sulfate groups from sulfuric acid hydrolysis on the thermal degradation behavior of bacterial cellulose. Biomacromolecules 2004;5:1671e7. [29] Xiang Q, Lee YY, Pettersson PO, Torget RW. Heterogeneous aspects of acid hydrolysis of a-cellulose. Appl Biochem Biotechnol 2003;105e108:505e14. [30] Tong X, Li Y. Efficient and selective dehydration of fructose to 5-hydroxymethylfurfural catalyzed by brønsted-acidic Ionic liquids. ChemSusChem 2010;3:350e5.
Contents lists available at SciVerse ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Efficient acid-catalyzed hydrolysis of cellulose in organic electrolyte solutions Zehui Zhang a, b, *, Bing Liu a, Zongbao (Kent) Zhao b, c a
Key Laboratory of Catalysis and Materials Sciences of the State Ethnic Affairs Commission & Ministry of Education, College of Chemistry and Material Science, South-Central University for Nationalities, Wuhan 430074, China b Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China c Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian 116023, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 December 2011 Received in revised form 6 January 2012 Accepted 14 January 2012 Available online 21 January 2012
A novel method for cellulose hydrolysis catalyzed by mineral acids have been developed in organic electrolyte solutions at 70 C under atmosphere pressure without pretreatment. Several reaction parameters including reaction time, temperature, catalyst loading and the ratio of ionic liquids to organic solvents have been evaluated and optimized. Under optimal conditions, the maximum total reducing sugars (TRS) yield in 68.8% and glucose yield in 39.2% were obtained in 1.0 g NMP/3.0 g [Bmim]Cl system at 70 C. Due to the low temperature of cellulose hydrolysis, this method shows a promising potential as an energy-efficient and cost-effective approach for the biorefinery of lignocellulosic biomass. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Cellulose Hydrolysis Acid catalyst Organic electrolyte solutions
1. Introduction Concerns about global warming and energy security have led to the exploration of alternatives for fossil resources to supply chemicals and energy [1,2]. Biomass is the most abundant renewable resource with an estimated global production of around 1.0 1011 tons per year, and it can be converted to biofuels and biobased chemicals through biorefinery [3e5]. Cellulose is a major component of biomass, which is readily available and does not compete with the food supply. Therefore, there is tremendous interest to develop new processes not only for fuels but also as a starting material for our chemical industry from cellulose. Hydrolysis of cellulose is considered to be an entry point of biorefinery schemes, as the resulting glucose can be further converted into various chemicals and biofuels [6]. However, the poor solubility of cellulose limited this process. Cellulose is a linear polysaccharide chain consisting of hundreds to thousands of D-anhydroglucopyranose linked together b-glycosidic bonds. Due to the extensive network of inter- and intra-molecule hydrogen bonds and van der Waals force, cellulose is insoluble in water and most conventional organic solvents, leading to notoriously recalcitrant to process [7e9].
* Corresponding author. Key Laboratory of Catalysis and Materials Sciences of the State Ethnic Affairs Commission & Ministry of Education, College of Chemistry and Material Science, South-Central University for Nationalities, Minyuan Road 708, Wuhan 430074, China. Tel./fax: þ86 27 67842752. E-mail address: [email protected] (Z. Zhang). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2012.01.010
Much effort has been devoted to depolymerization of cellulose. These included acidic hydrolysis [10e12], enzymatic hydrolysis [13,14] and hydrolysis in supercritical water [15e17]. Acidic hydrolysis approach is commonly applied in the hydrolysis of cellulose to attain glucose. The major drawbacks are the high operating cost of acid recovery and the use of expensive construction material for both the hydrolyzer and the acid recovery system. Enzymatic process takes place under mild conditions, but the system is sensitive to contaminants originating from other biomass components. Furthermore, pretreatment of cellulose is usually required to increase the accessible area of cellulose to improve the hydrolysis rate. Supercritical water has recently been used as a medium for hydrolysis of cellulose, but hash conditions such as high temperature and high pressure are required. Therefore, it is pivotal to develop more efficient and environmentfriendly method for cellulose hydrolysis. Recently, ionic liquids (ILs) have attracted much attention as greener alternatives to conventional organic solvents [18e20]. Swatloski et al. [21] firstly reported that ILs could act as a new and powerful non-derivatizing solvent for cellulose with a high solubility up to 25 wt.% in 1-n-butyl-3-methylimidazolium chloride ([Bmim]Cl) under microwave irradiation. The good solubility of cellulose in ILs has paved a new way for its applications [22]. In our previous work, it was demonstrated that cellulose could be readily hydrolyzed in [Bmim]Cl in the presence of mineral acids at 100 C [23]. Even though good results was obtained, it is also desirable to carry out cellulose hydrolysis at a relative low temperature as one of the key goals among the 12 principles of
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Z. Zhang et al. / Polymer Degradation and Stability 97 (2012) 573e577
Green Chemistry, which would be more energy-saving. Due to the high melting point and viscosity of ILs, hydrolysis of cellulose at a low temperature (e.g. at 70 C) seems difficult. Recently, Rinaldi [24] discovered that organic electrolyte solutions showed a superior solubility for cellulose. Organic electrolyte solutions are composed of ILs and polar organic solvents. This discovery would provide a more economical and convenient way to the utilization of cellulose. Therefore, in this work, we would like to present our results of acidic hydrolysis of cellulose in organic electrolyte solutions. 2. Experimental 2.1. Materials and instrument 1-Butyl-3-methylimidazole chloride ([Bmim]Cl) was synthesized according to a known procedure [25,26]. Spruce cellulose (Cat. No. 22182) was purchased from Sigma (St. Louis, USA), and was dried under vacuum at 100 C for 24 h before use. N-methylimidazole (99%) was obtained from Zhejiang Kaile Chemicals Co. Ltd. (Taizhou, China). 1-Chlorobutane (98%) was purchased from Guangfu Fine Chemical Research Institute (Tianjin, China), and freshly distilled before use. N-Methylpyrrolidinone (NMP) was purchased from Aladdin reagent Co. (Shanghai, China). All other chemicals were supplied by local suppliers and used without further purification. 2.2. Typical procedure for cellulose hydrolysis Cellulose (320 mg) was added into a mixture of [Bmim]Cl (3.0 g) and NMP (1.0 g) at 70 C. The mixture was stirred until a yellowish solution formed. Then 63 mg H2O (1.75 mol equiv to glucose unit) and 0.148 g 98 wt.% H2SO4 were quickly added into the cellulose solution. The reaction mixture was heated at 50, 70, or 100 C for 10 h. A portion of the mixture (about 60 mL) were withdrawn, weighed (recorded as M1) diluted with water with 2 mL, centrifuged at 10,000 rpm for 5 min, obtaining an aqueous solution with the volume to be V1. This aqueous solution was called sample, which was then subjected to TRS analysis using a DNS method in section 2.4 [27] and glucose analysis with a glucose analyzer in section 2.5. 2.3. Preparation of DNS reagent 182 g of potassium sodium tartrate, 6.3 g of 3, 5-dinitrosalicylic acid (DNS) and 262 mL of 2 M NaOH were added to500 mL of hot deionized water (50 C) in sequence, after dissolved, 5 g of phenol and 5 g of sodium sulfite were added in the solution, stirring until homogeneous solution was formed, then cooled to room temperature and diluted with deionized water to 1000 mL to give the DNS reagent. 2.4. TRS analysis according to DNS method A mixture of 0.5 mL of DNS reagent and 0.5 mL of sample was heated in a boiling water bath for 5 min (Note: The sample was derived from the prepared aqueous solution section 2.2, and we only used 0.5 mL for TRS analysis), and 1 mL mixture after the reaction of 0.5 mL of DNS reagent with 0.5 mL of sample was remained. Then the 1 mL mixture was cooled to room temperature, and diluted with 4 mL of deionized water, finally forming 5 mL solution. The color intensity of the final 5 mL solution was measured in a JASCO V-530 Model spectrophotometer at 540 nm with a slit width of 0.06 mm. The concentration of total reducing
sugars was calculated based on a standard curve obtained with glucose, and the yield of TRS were calculated as follows,
MT ðmgÞ ¼ TRS concentrationðmg=mLÞ 5ðmLÞ ðV1 =0:5Þ ðM0 =M1 Þ TRS Yield ¼ MT =½ðM2 =162Þ 180 100% ¼ MT 0:9=M2 100% As cellulose is composed of glucose unit (with the mole mass of unit to be 160), Therefore, the glucose mass after hydrolysis should be 180*(M2/162) theoretically. In which, MT is the mass of TRS, and M0 is the total mass of the reaction solution. M1 is the mass of a portion of the mixture withdrawn from the reaction mixture under magnetic stirring, keeping the whole reaction mixture to be homogenous. V1 is the volume of the aqueous solution in section 2.2, “5 (mL)” is the total volume of 1 mL mixture (0.5 mL sample and 0.5 mL DNS reagent) after reaction in boiling water, and 4 mL water, which was used to dilute the 1 mL mixture after reaction. “0.5” means we only used 0.5 mL of the aqueous solution, although the volume was as V1. M2 is the mass of cellulose initially loaded in the reaction, respectively. 2.5. Glucose analysis Glucose concentration was determined using a SBA-50B glucose analyzer (Shandong Academy of Sciences, Jinan, China). 3. Results and discussion 3.1. Effect of reaction temperature on cellulose hydrolysis in [Bmim] Cl/NMP system In our previous work, an efficient method has been developed for cellulose hydrolysis in [Bmim]Cl at 100 C [23]. Due to the high melting point and high viscosity of [Bmim]Cl, it is difficult for the hydrolysis of cellulose in [Bmim]Cl at low temperature (e.g. at 70 C). It was reported that cellulose solubility was improved extremely and the viscosity decreased remarkably in organic electrolyte solutions (the mixture of ILs and polar organic solvents) [24]. In our experiment, it only required 2 h for 320 mg cellulose dissolving in 1.0 g NMP/3.0 g [Bmim]Cl to form homogenous solution at 70 C, whereas, that was 12 h in [Bmim]Cl at 100 C. In addition, the cellulose solution was stirred smoothly at 70 C. Therefore, it inspired us to carry out cellulose hydrolysis in 1.0 g NMP/3.0 g [Bmim]Cl at a relative low temperature of 70 C. Unfortunately, the hydrolysis rate was slow. TRS yields increased gradually from 8.2% at 1 h to 63.7% at 10 h (Fig. 1), and the corresponding glucose yields were 2.7% at 1 h and 29.6% at 10 h (Fig. 2), respectively. Even though a long reaction time was required to obtain high TRS and glucose yields at 70 C, to the best of our knowledge it is the lowest reaction temperature for cellulose hydrolysis. In order to increase the reaction rate, a higher hydrolysis temperature of 100 C was evaluated. As expected, the reaction rate increased sharply, and the maximum yields of TRS in 53.9% (Fig. 1) and glucose in 32.4% (Fig. 2) were obtained at 1 h, respectively. However, the TRS and glucose yields sharply declined with the continual extension of reaction time. It indicated that the products were not stable at high temperature and degraded into other byproducts. Additionally, cellulose hydrolysis was also carried out at 50 C, and it was noted that the hydrolysis rate was rather slow, with TRS yield in 9.9% (Fig. 1) and glucose yield in 2.5% (Fig. 2) at
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Fig. 1. TRS yield of cellulose hydrolysis catalyzed by H2SO4 at different temperatures in organic electrolyte solutions. Reaction condition: 0.320 g spruce cellulose dissolved in 1.0 g NMP and 3.0 g [BMIM]Cl, then 0.063 g H2O and 0.148 g 98 wt.% H2SO4 were added, the reaction was performed at a set temperature.
Fig. 3. TRS yield of cellulose hydrolysis catalyzed by different acids in organic electrolyte solutions. Reaction condition: 0.320 g spruce cellulose dissolved in 1.0 g NMP/3.0 g [BMIM]Cl, then 0.063 g H2O and 0.148 g 98 wt.% H2SO4 were added (or 0.300 g 36.5 wt.% HCl), the reaction was performed at 70 C.
10 h, respectively. The results indicated that temperature had a remarkable effect on cellulose hydrolysis. Considering the reaction rate and the stability of the products, we set the hydrolysis temperature as 70 C in the following experiments.
acid in aqueous solution corresponds to a high acidity in organic electrolyte solutions.
3.2. Effect of mineral acids on cellulose hydrolysis in [Bmim]Cl/NMP system Secondly, different mineral acids including HCl, H2SO4 and H3PO4 were tested for hydrolysis of cellulose in organic electrolyte solutions. As shown in Fig. 3 and Fig. 4, the strong mineral acids such as HCl and H2SO4 afforded satisfied results. They almost had the same reaction rate and sugar yields, with the same amount of protons. However, when the weak mineral acid H3PO4 was used, the hydrolysis was inferior. TRS and glucose yields were only 2.2% and 0.6% for 10 h at 70 C, respectively. These results suggested that the strength of acids played an important role in cellulose hydrolysis in organic electrolyte solutions. The high acidity of mineral
The effect of catalyst loading on cellulose hydrolysis was further investigated. The results were shown in Fig. 5 and Fig. 6. It was apparent that more acids loading led to higher sugar yields especially at the initial reaction stage. For example, the yields of TRS and glucose were only 13.2% and 2.3% at 2 h with 0.077 g H2SO4, whereas, those were reached up to 45.6% and 15.5% with 0.222 g H2SO4 at 2 h, respectively. The increase sugar yields with an increase in catalyst loading can be attributed to an increase in the availability and number of catalytically active site for cellulose hydrolysis. It only required 8 h to achieve a maximum TRS yield in 68.8% and glucose yield in 39.2% with 0.222 g H2SO4 at 70 C. After 8 h, TRS and glucose yields decreased gradually, due to the acidpromoted degradation of glucose.
Fig. 2. Glucose yield of cellulose hydrolysis catalyzed by H2SO4 at different temperatures in organic electrolyte solutions. Reaction conditions were the same as described in Fig. 1.
Fig. 4. Glucose yield of cellulose hydrolysis catalyzed by different acids in organic electrolyte solutions. Reaction conditions were the same as described in Fig. 3.
3.3. Effect of acids loading on cellulose hydrolysis in [Bmim]Cl/NMP system
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Fig. 5. TRS yield of cellulose hydrolysis catalyzed by different amount of H2SO4 in organic electrolyte solutions. Reaction condition: 0.320 g spruce cellulose dissolved in 1.0 g NMP/3.0 g [Bmim]Cl, then 0.063 g H2O and a set amount of 98 wt.% H2SO4 were added, the reaction was performed at 70 C.
Fig. 7. TRS yield of cellulose hydrolysis catalyzed by H2SO4 in different organic electrolyte solutions. Reaction condition: 0.320 g spruce cellulose dissolved in 4 g solvents, then 0.063 g H2O and 0.148 g 98 wt.% H2SO4 were added, the reaction was performed at 70 C.
Our method produced equal or higher glucose and TRS yields in comparison with those reaction procedures at high temperature and/or using concentrated H2SO4 (65 wt% or higher) [28,29]. When cellulose was completely dissolved in the electrolyte solutions and formed a homogeneous solution, it made the Hþ more accessible to the b-glycosidic bonds. This is likely the reason that the hydrolysis rate was much higher within the electrolyte solutions as the solvent when compared with those systems where hydrolysis occurred at the surface of cellulose. Therefore, a physical barrier for hydrolysis was overcome through formation of a solution. As a control experiment, cellulose hydrolysis was also carried out in water (4.0 g) catalyzed by 0.222 g of sulfuric acid at 70 C for 8 h. Glucose and TRS yield were 1.5% and 9.6%, respectively. The results clearly indicated that the solubility of cellulose in 3 g of [Bmim]Cl and 1 g of NMP played a key role in cellulose hydrolysis, in which the accessibility of acid to cellulose could be enhanced.
3.4. Effect of the ratio of NMP with [Bmim]Cl on cellulose hydrolysis
Fig. 6. Glucose yield of cellulose hydrolysis catalyzed by different amount of H2SO4 in organic electrolyte solutions. Reaction conditions were the same as described in Fig. 5.
Fig. 8. Glucose yield of cellulose hydrolysis catalyzed by H2SO4 in different organic electrolyte solutions. Reaction conditions were the same as described in Fig. 7.
It was reported that the content of [Bmim]Cl had an important effect on cellulose solubility. Cellulose dissolved much more easily with the increase concentration of [Bmim]Cl24. Herein, the solvent effect on cellulose hydrolysis was investigated. The results are shown in Fig. 7 and Fig. 8. It can be seen that the rate of cellulose hydrolysis became fast with the increase amount of [Bmim]Cl. It should be noted that the yields of TRS and glucose were extremely low in 3.0 g NMP/1.0 g [Bmim]Cl system. It was acceptable because cellulose was not dissolved in this system, which resulted in a heterogeneous hydrolysis. On the other hand, cellulose was dissolved easily in other systems, but the hydrolysis was hindered with the increase amount of NMP. It was reported that H2SO4 can react with NMP to form a new acidic ionic liquids [NMP]þ[HSO4] [30]. Therefore, the actual proton used for catalyzing the cellulose hydrolysis became less with the increase amount of NMP.
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4. Conclusion In this study, hydrolysis of cellulose was realized at a relative low temperature at 70 C in NMP/[Bmim]Cl system catalyzed by mineral acid. Under optimal reaction conditions, a maximum of TRS and glucose yields were obtained in 68.8% and 39.2% at 70 C, respectively. To the best of our knowledge, it is the lowest temperature for cellulose hydrolysis till yet. The application of this novel organic electrolyte solution for cellulose hydrolysis is more economical and energy-saving. This reaction system opens up a new opportunity for facilitating cost-efficient utilization of lignocellulosic biomass in biorefinery. Acknowledgments The Project was Supported by the Special Fund for Basic Scientific Research of Central Colleges, South-Central University for Nationalities (CZQ11024). References [1] Saidur R, Abdelaziz EA, Demirbas A, Hossain MS, Mekhilef S. A review on biomass as a fuel for boilers. Renew Sust Energ Rev 2011;15:2262e89. [2] Martin M, Eklund M. Improving the environmental performance of biofuels with industrial symbiosis. Biomass Bioenerg 2011;35:1747e55. [3] Corma A, Iborra S, Velty A. Chemical routes for the transformation of biomass into chemicals. Chem Rev 2007;107:2411e502. [4] Zhang ZH, Wang Q, Xie HB, Liu WJ, Zhao ZK. Catalytic conversion of carbohydrates into 5-hydroxymethylfurfural by Germanium(IV) chloride in ionic liquids. ChemSusChem 2011;4:131e8. [5] Gog A, Roman M, Tosa M, Paizs C, Irimie FD. Biodiesel production using enzymatic transesterification e current state and perspectives. Renew Energ 2012;39:10e6. [6] Zhang ZH, Zhao ZK. Production of 5-hydroxymethylfurfural from glucose catalyzed by hydroxyapatite supported chromium chloride. Bioresour Technol 2011;102:3970e2. [7] Jarvis M. Cellulose stacks up. Nature 2003;426:611e2. [8] Nishiyama Y, Sugiyama J, Chanzy H, Langan P. Crystal structure and hydrogen bonding system in cellulose Ia from synchrotron X-ray and neutron fiber diffraction. J Am Chem Soc 2003;125:14300e6. [9] Poletto M, Pistor V, Zeni M, Zattera AJ. Crystalline properties and decomposition kinetics of cellulose fibers in wood pulp obtained by two pulping processes. Polym Degrad Stab 2011;96:679e85. [10] Antonoplis RA, Blanch HW, Freitas RP, Sciamanna AF, Wilke CR. Production of sugars from wood using high-pressure hydrogen-chloride. Biotechnol Bioeng 1983;25:2757e73.
577
[11] Karimi K, Kheradmandinia S, Taherzadeh MJ. Conversion of rice straw to sugars by dilute-acid hydrolysis. Biomass Bioenerg 2006;30:247e53. [12] Torget RW, Kim JS, Lee YY. Fundamental aspects of dilute acid hydrolysis/ fractionation kinectics of hardwood carbohydrates. 1. Cellulose hydrolysis. Ind Eng Chem Res 2000;39:2817e25. [13] Ha SH, Mai NL, An G, Koo YM. Microwave-assisted pretreatment of cellulose in ionic liquid for accelerated enzymatic hydrolysis. Bioresour Technol 2011; 102:1214e9. [14] Shaikh HM, Adsul MG, Gokhale DV, Varma AJ. Enhanced enzymatic hydrolysis of cellulose by partial modification of its chemical structure. Carbohydr Polym 2011;2:962e8. [15] Mok Jr WSL, Antal MJ. Uncatalyzed solvolysis of whole biomass hemicelluloses by hot compressed liquid water. Ind Eng Chem Res 1992;31: 1157e61. [16] Kumar S, Gupta RB. Hydrolysis of microcrystalline cellulose in subcritical and supercritical water in a continuous flow reactor. Ind Eng Chem Res 2008;47: 9321e9. [17] Sasaki M, Kabyemela B, Malaluan R, Hirose S, Takeda N, Adschiri T. Cellulose hydrolysis in subcritical and supercritical water. J Supercrit Fluids 1998;13: 261e8. [18] Miao W, Chan TH. Ionic-liquid-supported synthesis: a novel liquid-phase strategy for organic synthesis. Acc Chem Res 2006;39:897e908. [19] Olivier-Bourbigou H, Magna1 L, Morvan D. Ionic liquids and catalysis: recent progress from knowledge to applications. Appl Catal A Gen 2010; 373:1e56. [20] Rahman M, Brazel CS. Ionic liquids: new generation stable plasticizers for poly(vinyl chloride). Polym Degrad Stab 2006;91:3371e82. [21] Swatloski RP, Spear SK, Holbrey JD, Rogers RD. Dissolution of cellulose with ionic liquids. J Am Chem Soc 2002;124:4974e5. [22] Li WY, Jin AX, Liu CF, Sun RC, Zhang AP, Kennedy JF. Homogeneous modification of cellulose with succinic anhydride in ionic liquid using 4-dimethylaminopyridine as a catalyst. Carbohydr Polym 2009;78:389e95. [23] Li CZ, Zhao ZK. Efficient acid-catalyzed hydrolysis of cellulose in ionic liquid. Adv Synth Catal 2007;349:1847e50. [24] Rinaldi R. Instantaneous of cellulose in organic electrolyte solutions. Chem Commun 2011;47:511e3. [25] Burrell AK, Del Sesto RE, Baker SN, McCleskey TM, Baker GA. The large scale synthesis of pure imidazolium and pyrrolidinium ionic liquids. Green Chem 2007;9:449e54. [26] Webb PB, Sellin MF, Kunen TE, Williamson S, Slawin AMZ, Cole-Hamilton DJ. Continuous flow hydroformylation of alkenes in supercritical fluid-ionic liquid biphasic systems. J Am Chem Soc 2003;125:15577e88. [27] Miller GL. Use of dinitrosalicylic acid reagent for determination of reducing sugar. Anal Chem 1959;31:426e8. [28] Roman M, Winter WT. Effect of sulfate groups from sulfuric acid hydrolysis on the thermal degradation behavior of bacterial cellulose. Biomacromolecules 2004;5:1671e7. [29] Xiang Q, Lee YY, Pettersson PO, Torget RW. Heterogeneous aspects of acid hydrolysis of a-cellulose. Appl Biochem Biotechnol 2003;105e108:505e14. [30] Tong X, Li Y. Efficient and selective dehydration of fructose to 5-hydroxymethylfurfural catalyzed by brønsted-acidic Ionic liquids. ChemSusChem 2010;3:350e5.