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Energy & Fuels 2007, 21, 2386-2389
Hydrolysis of Cotton Fiber Cellulose in Formic Acid Yong Sun,† Lu Lin,*,† Chunsheng Pang,† Haibo Deng,† Hong Peng,† Jiazhe Li,† Beihai He,† and Shijie Liu*,‡ State Key Laboratory of Pulp and Paper Engineering, South China UniVersity of Technology, Guangzhou, P.R. China 510641, and Department of Paper and Bioprocess Engineering, State UniVersity of New Yorks College of EnVironmental Science and Forestry, 1 Forestry DriVe, Syracuse, New York 13210 ReceiVed March 14, 2007. ReVised Manuscript ReceiVed May 27, 2007
The hydrolysis of degreased cotton in formic acid has been studied in this paper. Hydrochloric acid is added as a catalyst. The crystalline index of degreased cotton decreases gradually with the increase in treatment time. The effects of the reaction temperature (from 55 to 65 °C) and reaction time (from 2 to 9 h) on hydrolysis have been evaluated. The hydrolysis of cellulose and degradation of glucose increase with the increases in reaction time and temperature. The optimal conditions for the hydrolysis of degreased cotton is found to be at 65 °C for 5 h. Under the optimal conditions, 23% of the degreased cotton is turned into glucose. The maximum transformation ratio of degreased cotton to reducing sugars is 32%.
1. Introduction Owing to the depletion of the limited fossil deposit sources in the world, stepwise substitution of fossil energy use by renewable resources is necessary for the lasting development of the world’s economy. Biofuel from plant materials is a natural alternative. When biofuels are burnt for energy, liberated CO2 is returned back to the atmosphere from which it was captured. The balance between CO2 consumption and liberation is preserved, and the use of biofuels as a source of energy could help to alleviate global ecological problems such as the greenhouse gas effect and global warming.1-4 In the United States, Europe, and China, biofuels such as bioethanol are increasingly used as a substitute for fossil fuels in the transportation sector. With gradual cost reduction, biofuels will be more competitive in the future. Although sugarcane and corn are the dominant feedstocks for the production of biofuels today, future demands for biofuels will require other starchrich grains, food processing byproducts, and agricultural and forest biomass to be processed.5 Cellulosic biomass from forest and agricultural residues is the most abundant and renewable resource on earth. The complex structure of cellulosic biomass provides a primary protective barrier that prevents cell destruction by chemical or biological methods. In order to convert cellulose to biofuels, the complex structure of cellulosic materials must be broken down to water-soluble reducing sugars so that microorganisms can utilize them.6 The hydrolysis of cellulose is one of the prevailing steps in breaking down the complex structure of cellulose to recover monosaccharides. There are three major hydrolysis processes * Corresponding authors. E-mail:
[email protected] (L.L.),
[email protected] (S.L.). † South China University of Technology. ‡ State University of New York. (1) Kuemmel, B; Langer, V; Jagid, M. Biomass Bioenergy 1998, 15, 407-416. (2) Marbe, A; Harvey, S; Berntsson, T. Energy 2006, 31, 1614-1631. (3) Bohlin, F. Biomass Bioenergy 1998, 15, 283-291. (4) Liu, S.; Amidon, T. E.; Francis, R. C.; Ramarao, B. V.; Lai, Y.-Z; Scott, G. M. Ind. Biotechnol. 2006, 2 (2), 113-120. (5) Wheals, A. E.; Basso, L. C.; Alves, D. M. G.; Amorim, H. V. Trends Biotechnol. 1999, 17, 482-487. (6) Hakansson, H; Ahlgren, P. Cellulose 2005, 12, 177-183.
for cellulose from agricultural and forest biomass to produce monosaccharides capable of being transformed to bioethanol: dilute acid, concentrated acid, and enzymatic hydrolysis.7,8 The main advantage of the dilute acid in the hydrolysis process is that acid recovery may not be required, and there will be no significant losses of acid. However, in a dilute acid hydrolysis process, the reaction needs to be carried out at a high temperature and pressure. Also, a poor yield of glucose from cellulose is obtained in the dilute acid hydrolysis. Due to a nearquantitative yield of glucose from cellulose, the use of highconcentration acid in the hydrolysis process may yield higher quantities of monosaccharides, but strongly acid-resistant vessels and a good acid recovery process will be required. The dilute acid hydrolysis process uses high temperatures (up to 160 °C) and pressures (up to 10 atm).9 The acid concentration in the dilute acid hydrolysis process is in the range of 0.15%.8,10 The acid concentration used in the concentrated acid hydrolysis process is in the range of 10-30%.8 Enzymes produced by a variety of microorganisms are also capable of breaking down cellulose in lignocellulosic materials to sugars with a longer residence time.11 Enzymatic hydrolysis requires feedstock pretreatments, enzyme production, and enzyme recovery, which may make this option economically unfeasible. The hydrolysis process needs further improvements in order to reduce the operating cost for a fuel-producing plant that uses forest and agricultural biomass as feedstock. Therefore, the development of a feasible process for the hydrolysis of cellulose that could lead to high sugar yields and result in higher yields of monosaccharides is highly desirable. The objective of this work is to effectively hydrolyze cellulose by using formic acid with a low concentration of hydrochloric acid. Cotton cellulose possesses a high degree of polymerization and a high crystallinity index. Therefore, cotton cellulose is more (7) Sun, Y; Cheng, J. J. Bioresour. Technol. 2005, 96, 1599-1606. (8) Liao, W; Liu, Y; Liu, C; Wen, Z; Chen, S. Bioresour. Technol. 2005, 1-9. (9) Sanchez, G; Pilcher, L; Roslander, C; Modig, T; Galbe, M; Liden, G. Bioresour. Technol. 2004, 93, 249-256. (10) Lee, Y. Y; Iyer, P; Torget, R. W. Biotechnology 1999, 65, 93115. (11) Lloyd, T. A.; Wyman, C. E. Bioresour. Technol. 2005, 96, 19671977.
10.1021/ef070134z CCC: $37.00 © 2007 American Chemical Society Published on Web 06/30/2007
Hydrolysis of Cotton Fiber Cellulose
Energy & Fuels, Vol. 21, No. 4, 2007 2387 Table 1. Value of Characteristic X-Ray Diffraction Peaks and Crystalline Index Value of Degreased Cotton during Hydrolysis treatment time, h 1
2
3
4
5
Figure 1. X-ray diffraction images of degreased cotton. The curves from 1 to 8 h correspond to the samples that have been treated for 1-8 h at 65 °C, respectively, in formic liquor (78.22% formic acid, 17.78% water, and 4% hydrochloric acid).
6
7
peaks
location, deg
intensity, Cps
2θ ) 18° peak intensity, Cps
Cr I, %
101 101 002 040 101 101 002 040 101 101 002 040 101 101 002 040 101 101 002 040 101 101 002 040 101 101 002 040 101 101 002 040
14.76 16.52 22.84 34.62 14.5 16.54 22.56 33.54 14.46 16.80 22.78 34.48 14.76 16.54 22.68 34.36 14.86 16.60 22.70 34.62 14.74 16.34 22.68 34.70 14.84 16.50 22.74 34.54 14. 74 16.92 22.84 34.54
338 334 975 177 296 250 880 173 240 249 817 160 241 270 777 169 215 231 724 185 218 249 712 199 209 234 687 179 200 209 641 177
96
91.7
101
87.3
110
85.
124
84.0
133
81.6
141
80.2
149
78.3
155
75.8
difficult to break down than other celluloses.12,13 After the hydrolysis of cellulose, the solid hydrolyzates of soluble sugar oligomers can be favorably obtained. The glucose content is significant in the resultant hydrolyzates.14 The hydrolysis of cellulose in formic acid with hydrochloric acid is a reaction system in which formic acid is a catalyst and also a product. The reaction temperature is below 70 °C, and the corrosion of organic acid is lower than that of concentrated inorganic acid. Formic acid together with hydrochloric acid can be effectively recovered and reused.
atmospheric pressure. After reaction, formic acid and hydrochloric acid were extracted by a depressurization procedure. The final products were obtained for further analysis after washing, filtration, and desiccation.
2. Materials and Methods
3. Results and Discussions
2.1. Materials. The degreased cotton was provided by the Advanced Science Company in Guanzhou, China, the cellulose content of which is above 99 wt %. Formic acid was purchased from the Shanghai Lingfeng Chemical Company. Hydrochloric acid was purchased from the Guangdong Donghong Chemical Company. The pure water was provided by the Wetsons company in Guangzhou. All chemicals are analytical grade. 2.2. HPLC Analysis. The high-performance liquid chromatography (HPLC) system consisted of a Waters 600E system controller, a Waters 717 automatic sampler, a Waters 410 differential refractometer, and a Waters Sugar pak I column. The mobile phase was pure water and ran at a flow rate of 1.1 mL/min. The LC system was operated at 90 °C. The sample volume injection was 10 µL. Standard samples and hydrolyzate samples were filtrated by a 0.45 µm filter and analyzed in duplicate. 2.3. X-Ray Diffraction Analysis. Wide-angle X-ray diffraction was conducted with a D/MAX-III, an instrument with a 12°/min scan speed. Cellulose powder samples were laid on the glass sample holders (35 × 50 × 5 mm) and analyzed under plateau conditions. Cu radiation was generated at a voltage of 40 kV and a current of 30 mA. The scan scope was between 2° and 50°. 2.4. Hydrolysis of Cellulose. The hydrolysis of cotton cellulose was carried out by placing 1 g of fiber material in a flask which contained 24 g of a formic acid solution (78.22 wt % formic acid, 17.78 wt % water, and 4 wt % hydrochloric acid). The reaction was carried out at different temperatures and residence times at
3.1. Degreased Cotton Cellulose Structure Changes During Hydrolysis in Formic Acid with Hydrochloric Acid. The X-ray diffraction profile of degreased cotton is shown in Figure 1, with the representative peaks at 2θ ) 14.7° and 16.3° for the (101) plane, 2θ ) 22.5° for the (002) plane, and 2θ ) 34.5° for the (040) plane. In these diffraction profiles, the peak at 2θ ) 22.5° for the degreased cotton gradually reduces with the prolongation of treatment time. The height of the peak decreases from 975 to 641. The separation between peaks at 2θ ) 14.7° and 16.3° is visibly clear in the sample treated for 1 h but gradually becomes ambiguous with the extension of the treatment time. Moreover, the peak is also decreased. Segal et al.15-18 suggested estimation of the crystalline index by
(12) Cao, Y.; Tan, H. J. Mol. Struct. 2004, 705 (1-3), 189-193. (13) Cao, Y; Tan, H. Enzyme Microb. Technol. 2005, 36 (2-3), 314317. (14) Kamm, B; Kamm, M; Schmidt, M; Starke, I; Kleinpeter, E. Chemosphere 2006, 62, 97-105.
8
Cr I ) 100(Ihkl - Iam)/ Ihkl
(1)
where Cr I is the crystalline index, Iam is the amorphous zone diffraction intensity of 2θ equal to 18°, and Ihkl is the crystalline zone diffraction intensity on the 002 lattice plane. Table 1 shows the crystalline index calculated from the X-ray diffraction profiles. The Cr I of absorbent cotton decreases from 91.7% to 75.8% after a treatment period of 1 to 8 h at 65 °C. Table 1 (15) Segal, L; Greely, J. J.; Martin, A. E., Jr.; Conrad, C. M. Text. Res. J. 1959, 8, 786-794. (16) Cao, Y.; Tan, H. Enzyme Microb. Technol. 2005, 36, 314-317. (17) Yang, G.; Zhang, L.; Cao, X.; Liu, Y. J. Membr. Sci. 2002, 210, 379-387. (18) Mihranyan, A.; Llagostera, A. P.; Karmhagc, R. Int. J. Pharm. 2004, 269, 433-442.
2388 Energy & Fuels, Vol. 21, No. 4, 2007
Sun et al.
Figure 2. Scheme of the hydrogen bonds between the chain of cellulose.
also shows the intensity of diffraction peaks in the crystalline zone and the amorphous zone of degreased cotton. While the intensities in both zones decrease during the treatment, it is clearer in the crystalline zone than in the amorphous zone. One can infer that the effects of formic acid on the intensities of diffraction in the amorphous zone and in the crystalline zone are different. The effect in the crystalline zone is more distinctive than that in the amorphous zone. Formic acid is a flat organic molecule with the average length of the OdC-O-H bonds being 0.171 nm and the length of the OdC-C-H bonds being 0.224 nm.19,20 The bond length of hydrochloric acid is 0.128 nm. In the crystal lattice of cellulose, adjacent chains of cellulose microfibrils are linked by a zigzag of repeating O-H‚‚‚O-H bonds. The distance of the adjacent layer chains is about 0.450 nm, and that of the two neighboring O1 bonds in the long molecular axis amounts to 0.240 nm. The stagger of intramolecular hydrogen bonds occurring between O3 and the adjacent O5 of the next glucosidic residue is 0.331 nm. The length of intermolecular hydrogen bonds O6‚‚‚O2 is about 0.238 nm, and that of intramolecular bonds O3‚‚‚O5 ranges from 0.272 to 0.279 nm.21 Therefore, the col (Figure 2) with an area of 0.238 × 0.279 × 0.240 nm, which is enclosed by the O6‚‚‚O2 bonds of one chain (or molecule), O3‚‚‚O5 bonds of another chain (or molecule), and the distance of two neighboring O1 between adjacent layer chains (or molecules), is the limit for other molecules to enter into the inner space of the crystal lattice of cellulose. A molecule with a longest bond length of more than 0.238 nm cannot pass the col of bonds for passage into the inner space of crystal cellulose. Therefore, formic acid with 0.224 nm and hydrochloric acid with 0.128 nm of intramolecular bond length can get through the limit to enter the inner space of crystal cellulose. The formic acid is a carbonyl compound; hydrogen bonds are easily formed between the formic acid and cellulose molecules. Once the formic acid penetrates into the crystal lattice, the crystalline lattice of cellulose strongly swells. The hydrogen bonds between the chains of cellulose molecules are broken down, and the new hydrogen bonds between formic acid and cellulose molecules are formed. Eventually, the rigid framework of the crystalline lattice of cellulose is crushed. Because of the (19) Åkerholm, M.; Hinterstoisser, B.; Salme´n, L. Carbohydr. Res. 2004, 339, 569-578. (20) Roy, A. K.; Thakkar, A. J. Chem. Phys. Lett. 2004, 393, 347-354. (21) Zugenmair, P. Polym. Sci. 2001, 26, 1341-1417.
Figure 3. FTIR spectra of degreased cotton after formic acid treatment. The spectra from 1 to 8 h correspond to the samples that have been treated for 1-8 h at 65 °C, respectively, in formic liquor (78.22% formic acid, 17.78% water, and 4% hydrochloric acid).
catalytic action of hydrochloric acid, the degreased cotton is hydrolyzed to glucose and oligosugar. The Fourier transform infrared spectroscopy (FTIR) spectra of degreased cotton during hydrolysis are shown in Figure 3. The peak of -OH stretching near 3400 cm-1 and the peak of -CH2 stretching near 2900 cm-1 are the distinguished features of cellulose. During the treatment, degreased cotton adsorbs some materials containing -OH (such as formic acid), and this results in a peak at 1637 cm-1. The peaks at 1370 cm-1 and 1430 cm-1 are the bending vibrations of -CH and -OH bonds, respectively. The peak of -CH2 swaying is at 1317 cm-1, whereas -CH bending is at 1333 cm-1, and -CH2 bending is at 1372 cm-1. The peak at 1430 cm-1 is attributed to the bending of the -CH2 bond because of the influence of hydrogen bonds in cellulose. The peaks at 1063, 1058, 1112, 1030, and 896 cm-1 correspond to the stretching of C-O-C-O-C bonds in cellulose. Compared with the control sample, the absorption peak at 1721 cm-1 of the degreased cotton treated by formic acid is stronger, which is the consequence of intramolecular hydrogen-bond formation. Thus, a certain amount of chemical agents with -OH bonds is adsorbed into the cellulose crystal lattice. Clearly, HCOOH is the probable candidate, with novel single or series-chained bonds of OH‚‚‚O formed.22-24 The intensity of intermolecular hydrogen bonds is responsible for the shift of the -OH vibration in the FTIR spectra. The (22) Takacs, E.; Wojna´rovits, L.; Borsa J. Nucl. Instr. Methods Phys. Res. 2005, 236, 259-265. (23) Oh, S. Y.; Yoo, D. I.; Shin, Y. Carbohydr. Res. 2005, 340, 23762391. (24) Ruan, D.; Zhang, L.; Mao, Y. J. Membr. Sci. 2004, 241, 265-274.
Hydrolysis of Cotton Fiber Cellulose
Energy & Fuels, Vol. 21, No. 4, 2007 2389
Figure 4. Effect of residence time and temperature on the release of glucose by degreased cotton in a formic acid solution (78.22% formic acid, 17.78% water, and 4% hydrochloric acid).
peak shifts to a higher wave number if the intensity of the intermolecular hydrogen bonds is weak.25 From the FTIR spectra of degreased cotton, the peak of -OH has shifted to 3400 cm-1. It is probable that only intramolecular hydrogen bonds are formed when formic acid molecules penetrate into the crystalline lattice of cellulose. Thus, swelling of the rigid molecular framework of degreased cotton occurs. 3.2. Effect of Temperature and Residence Time on the Hydrolysis of Degreased Cotton. In the hydrolysis of degreased cotton, the residence time and temperature are important factors. In order to explore optimal reaction conditions, the effects of reaction temperature (55, 60, 65, and 70 °C) and residence time (from 2 to 9 h) on the glucose yield have been examined. At 55 and 60 °C, the yield of glucose first increases and then decreases after 5 h at 65 °C and 6 h at 70 °C (Figure 4). At 55 °C, the yield of glucose is lower than that at 60 °C. The yield of glucose is higher at 65 °C than that at 60 °C. These results indicate that the hydrolysis of cellulose is strongly affected by the temperature. A higher temperature can enhance the transformation ratio of cellulose. The glucose degradation in a mineral acid solution has been reported in numerous papers.26-28 The data in Figure 3 also show that the rate of glucose degradation is faster than the hydrolysis rate of cellulose to glucose over 65 °C after 5 h. The yield of glucose at 70 °C does not increase notably as compared to that at 65 °C; it decreases gradually with the prolongation of residence time. However, the glucose yield continuously increases with residence time at 55 and 60 °C. Thus, the degradation rate of glucose increases with the temperature. The optimal hydrolysis conditions of degreased cotton in this study are thus found to be 65 °C for 5 h. Under optimal conditions, the yield of glucose is 22.5%. The results of an HPLC analysis also show that the mass of glucose clearly increases with the prolongation of residence time at the optimal temperature, 65°C (Figure 5). The percentage of glucose in the total water-soluble sugar and oligosaccharides mixture gradually increases. The yield of reducing sugars measured by the DSN method is shown in Table 2. The yield of reducing sugars rises with the extension of residence time until 4 h have elapsed. Over 60 °C, the yield of reducing sugars first increases and then decreases with residence time. It clearly increases at 65 °C over that at 60 °C. Therefore, at high temperatures, the reducing sugars are (25) Oh, S. Y.; Yoo, D. I.; Shin, Y.; Seo, G. Carbohydr. Res. 2005, 340, 417-428. (26) Johansson, L.; Virkki, L.; Anttil, H.; Esselstrom, H.; Tuomainen, P.; Sontag-Strohm, T. Food Chem. 2006, 97, 71-79. (27) Lloyd, T. A.; Wyman, C. E. Bioresour. Technol. 2005, 96, 19671977. (28) Mosier, N. S.; Ladisch, C. M.; Ladisch, M. R. Biotechnol. Bioeng. 2002, 79, 610-618.
Figure 5. HPLC spectra of hydrolysates of degreased cotton in formic liquor (78.22% formic acid, 17.78% water, and 4% hydrochloric acid). The spectra from 1 to 6 h correspond to the samples that have been treated for 1-6 h, respectively. Table 2. Yield of Reducing Sugars from Hydrolyzate of Degreased Cotton as Measured by the DSN Method the yield of reducing sugars, % time, h
55 °C
60 °C
65 °C
70 °C
1 2 3 4 5 6 7 8 9
3.1 7.0 5.5 6.1 7.9 8.0 8.3 9.8 11.1
3.9 9.2 17.4 20.5 17.5 14.9 12.5 11.3 10.5
8.0 17.9 17.3 19.3 31.4 21.8 23.5 19.8 20.4
16.1 20.1 22.1 26.8 28.0 33.0 27.1 24.5 23.9
severely degraded, which is consistent with the degradation of glucose in formic acid with hydrochloric acid. The optimal conditions are 65 °C for 5 h, which is also consistent with the optimal hydrolysis conditions of cellulose to glucose. The transformation of cellulose to reducing sugars is 31.4%. The percentage of glucose in the reducing sugars is 71.9%. 4. Conclusion During the hydrolysis of degreased cotton in formic acid liquor, intermolecular hydrogen bonds are formed when formic acid molecules penetrate into the crystalline lattice of cellulose, and the crystalline lattice of cellulose strongly swells. Eventually, the rigid framework of the crystalline lattice of cellulose is crushed, leading to hydrolysis in both the crystalline zone and the amorphous zone. The hydrolysis ratio of the degreased cotton increases with the reaction temperature and time. However, the degradation of glucose also increases with the extension of reaction time and temperature. The optimal conditions for the hydrolysis of degreased cotton are found to be 65 °C for 5 h. The yield of glucose is 22.5%. The yield of reducing sugars is 31.4%. Acknowledgment. This research was funded by the Key Research Project (Grant No. 750548) and Innovative Research Group Project (Grant No. IRT0552) of the Chinese Ministry of Education. The authors gratefully acknowledge the Centre of Analysis and Measurement of South China University of Technology for its technical assistance. EF070134Z
