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Electron beam irradiation of cellulose
ARTICLE IN PRESS Radiation Physics and Chemistry 78 (2009) 539–542
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Electron beam irradiation of cellulose Mark Driscoll a,, Arthur Stipanovic a, William Winter a, Kun Cheng a, Mellony Manning a, Jessica Spiese a, Richard A. Galloway b, Marshall R. Cleland b a b
State University of New York, College of Environmental Science and Forestry, Syracuse, New York IBA Industrial, Inc., Edgewood, New York
a r t i c l e in fo
Keywords: Electron beam irradiation Microcrystalline cellulose Cellulose depolymerization Molecular weight reduction Crystallinity reduction Surface area increase
abstract Using a 90 kW, 3 MeV DynamitronTM, the molecular weight of microcrystalline cellulose (MCC) was reduced from 82,000 to 5000 Da with a dose of 100 kGy. The relative crystallinity of the MCC was reduced from 87% to 45% with a dose of 1000 kGy. The available surface area, an indication on how well cellulose will react with chemical agents, was increased from 274 m2/g for the control sample (0 kGy) to 318 m2/g at a dose 1000 kGy. & 2009 Elsevier Ltd. All rights reserved.
1. Introduction Cellulose is the major structural component of wood and plant fibers and is the most abundant polymer synthesized by nature. Despite this great abundance, cellulosic biomass has seen limited application outside of the paper industry. Its use as a feedstock for fuels and chemicals has been limited because of its highly crystalline structure, inaccessible morphology and limited solubility. Any economic use of lignocellulosic resources for the production of ethanol will require a ‘‘pretreatment’’ technology to enhance the accessibility of the biomass to enzymes and/or chemical reagents (Alen and Sjostrom, 1985; McMillan, 1992, 1994; NRC, 2000; Himmel et al., 2005; Mosier et al., 2005; Wyman et al., 2005a, b). Electron beams (EB), X-rays or gamma rays produce ions in a material which can then initiate chemical reactions and cleavage of chemical bonds (Charlesby, 1995; Charlesby and Davison, 1957; Chapiro, 1967; Phillips and Arthur, 1985; Cheng and Kerluke, 2003). Such ionizing radiation predominantly scissions and degrades or depolymerizes cellulose. Charlesby (1995) showed that radiation caused the molecular weight (MW) of cellulose to decrease with increasing radiation dose. Dose is defined as the amount of energy absorbed per unit mass. The dose of one kGy is the absorption of one Joule of energy per gram of material. Many researchers have since studied the relationship between radiation dose and cellulose fiber degradation (Charlesby, 1960; McLaren, 1978; Kumakura and Kaetsu 1978, 1979, 1983, 1984a,b; Han et al. 1981; Petryaev et al., 1988; Forsberg and Lepoutre, 1994; Taka´cs et al., 1999, 2000; Iller et al.,
Corresponding author. Tel.: +1 315 470 6848; fax: +1 315 470 6998.
E-mail address: [email protected] (M. Driscoll). 0969-806X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2009.03.080
2002; Borsa et al., 2003; To´th et al., 2003; Foldvary et al., 2003; Dubey et al., 2004; Kasprzyk et al., 2004; Bouchard et al., 2006; Stupinska et al., 2007; Yang et al., 2008). The degree of polymerization (DP) of cotton cellulose was reduced from about 1600 to 300 after 10 kGy of gamma radiation (Taka´cs et al., 1999). To´th et al. (2003) showed similar results on preswollen cotton cellulose, where the DP decreased from 1600 to 800 after 3 kGy and to 400 after 10 kGy. They also reported that the number of cleavages per 1000 monomer units was 6 and 16 for doses of 3 and 10 kGy, respectively. Forsberg and Lepoutre (1994) studied the degradation of thermomechanical (TMP) and kraft pulp by electron beams. While kraft pulp was degraded quickly, the TMP samples were degraded more slowly. Stupinska et al. (2007) showed that the DP of cellulose pulp was reduced as the electron beam dose is increased from 10 to 50 kGy. Iller (2002) studied the electron beam irradiation of various cellulose pulps, the DP was reduced for all pulps as the dose was increased from 10 to 50 kGy. The percent alpha cellulose, defined as the fraction of cellulose that does not dissolve in 17.5% aqueous NaOH, also decreased as the dose increased. This suggests that the amount of crystallinity was also reduced. Other studies also showed that radiation reduced the crystallinity of cellulose (Kasprzyk et al., 2004; Alberti et al., 2005; Khan et al., 1987). Kasprzyk et al. (2004) showed that gamma radiation could reduce the amount of crystallinity in cellulose fibers. The reduction was most evident at doses above 100 kGy. The crystallinity index of microcrystalline cellulose (MCC), flax, cotton and viscose was reduced up to 12% with a dose of 200 kGy (Alberti et al., 2005). The viscose (rayon textile) industry has evaluated electron beams as a means to reduce process cost and environmental problems. From a mechanistic perspective, the electron beam treatment disturbs the crystalline structure of the cellulose, thus
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M. Driscoll et al. / Radiation Physics and Chemistry 78 (2009) 539–542
allowing the penetration of chemicals into the crystalline regions (Horio et al., 1963; Fischer and Goldberg, 1987; Stepanik et al., 1998, 2000). This results in a reduction of up to 50% in carbon disulfide and 30% caustic usage, according to Stepanik et al. (1998). The DP of viscose pulp appears to remain relatively constant after a dose of 25 kGy (Stepanik et al., 2000). Waste newspaper (predominately cellulose and residual lignin) irradiated with an electron beam to 100 kGy produced 30% more glucose by acid hydrolysis than samples without electron beam treatment (Brenner et al., 1979). The best glucose yield was obtained by irradiating the paper to 100 kGy and using maleic acid at 3% for the hydrolysis. Wheat and rice straw, sawdust and chaff irradiated to 500 kGy showed very high glucose yields, when hydrolyzed with cellulase (Kumakura and Kaetsu, 1978, 1979). The sugar yield gradually increased with doses less than 100 kGy and then increased more rapidly from 100 to 500 kGy. Kumakura and Kaetsu (1983) studied both acid and enzymatic hydrolysis of electron beam-irradiated bagasse. Bagasse irradiated with 1000 kGy yielded two or four times as much glucose in enzymatic and acid hydrolysis, respectively, as compared to non-irradiated bagasse. The smaller increase in the enzymatic system could be possible due to the presence of radiolysis products and long-lived radicals (Phillips and Jett, 1985; Nakamura et al., 1985; Dubey et al., 2004; Polovka et al., 2006; Yang et al., 2008). Yang et al. (2008) showed that the % yield of reducing sugars by enzymatic hydrolysis increased as the storage time after gamma irradiation increase from 0 to 22 days. As far back as 1956 Glegg and Kertesz (1956) showed that the viscosity of purified wood cellulose continued to decrease for at least two weeks after irradiation. Similar results were obtained from cotton cellulose (Gong et al., 1998). The continuing reduction of the DP after irradiation is most likely due to the presence of long-lived radicals in the cellulose (Phillips and Arthur, 1985; Nakamura et al., 1985; Iller et al., 2002; Bayram and Delincee, 2004; Dubey et al., 2004; Polovka et al., 2006).
2. Experimental 2.1. Materials Microcrystalline cellulose Avicel PH101 was obtained from FMS Corporation. HPLC grade dimethyl sulfoxide (DMSO) was obtained from Burdick and Jackson, tetrabutylammonium fluoride trihydrate from Fluka. Congo red was purchased from Aldrich chemical company. Pullulan standards were obtained from Showa Denko K. K.
detector, and two Polymer Laboratories Polypore columns (330 7.5 mm2). MCC samples were dissolved in a solvent of 10% tetrabutylammonium fluoride trihydrate in DMSO that has been shown to dissolve high molecular weight cellulose (Chen, 2005). Eight pullulan standards in the molecular weight range from 5900 to 788,000 were used as standards. The relationship between pullulan molecular size and that of cellulose, at comparable molecular weights, has previously been discussed (Chen et al., 2007). The relative1 crystallinity was determined using a Rigaku DMAX-1000 X-ray diffractometer with Ni-filtered Cu Ka radiation (l ¼ 0.15418 nm). The diffraction data was collected at 50 kV and 120 mA. Each sample was scanned 9 times from 2y ¼ 51 to 501 (Chen et al., 2007). The percent crystallinity was calculated as described by Sottys et al. (1984). The available surface area of both irradiated and non-irradiated MCC samples were determined using the congo red dye adsorption method. The method was similar to those used by Inglesby and Zeronian (1996) and Goodrich and Winter (2007). Absorbance was measured at 492 nm using a Beckmann DU-600 spectrophotometer.
3. Results and discussion Fig. 1 shows the X-ray diffraction intensity of MCC as a function of diffraction angle, for the control and irradiated samples. Table 1 illustrates that the relative percent crystallinity of MCC decreases from 87% to 45%, when irradiated at doses of 0, 10, 100 and 1000 kGy in dry air calculated from the ratio of crystalline to amorphous cellulose in Fig. 1. The lower the crystallinity of the cellulose makes it more accessible to chemical reagents, and thus easier to hydrolyze to sugars (Stepanik et al., 1998, 2000; Kasprzyk et al., 2004; Mosier et al., 2005). Table 1 shows that as the dose is increased, the molecular weight decreases. The control sample’s (0 kGy) MW is 82,000 Da, but it is reduced to 2187 Da for the 1000 kGy sample. As can be seen, the majority of the MW reduction is achieved in the first 100 kGy. Other studies have shown a larger MW reduction at 10 kGy. This may be due to the relatively high crystallinity of the MCC that was used in this study. Radiation cleaves the amorphous region of the cellulose more easily than the crystalline region. Studies have shown that as the MW of cellulose is reduced, it becomes much more amenable to treatment with other agents
1000
0 kGy 10 kGy 100 kGy 1000 kGy
2.2. Irradiation of microcrystalline cellulose
2.3. Methods Molecular weight was determined using a Waters Breeze size exclusion chromatography system fitted with a Waters 1515 isocratic HPLC pump, a Waters 2414 refractive index detector, a Precision Detectors Enterprise PD2100 Series laser light scattering
750 Intensity
Microcrystalline cellulose was irradiated at doses of 10, 100 and 1000 kGy in dry air sealed in polyethylene bags. The irradiation was conducted at IBA Industrial, Edgewood, New York USA, using a 90 kW, 3 MeV DynamitronTM. The samples were less than 0.2 cm thick with a bulk density of o1.0 g/cc, thus giving equal in equal out dose. The dose was determined with cellulose triacetate films. The 1000 kGy samples were irradiated using ten 100 kGy passes to prevent overheating the samples.
500
250
0
10
15
20
25
30 35 2 Theta
40
45
50
Fig. 1. The X-ray diffraction intensity of MCC as a function of diffraction angle (2y degrees). From top to bottom: control MCC (0 kGy), 10 kGy electron beam dose, 100, 1000 kGy.
ARTICLE IN PRESS M. Driscoll et al. / Radiation Physics and Chemistry 78 (2009) 539–542
Table 1 Effect of electron beam irradiation on MCC. Dose (kGy)
Relative crystallinity (%)
Molecular weighta (Da)
Surface area (m2/g)
0 10 100 1000
87 76 65 45
82,000 80,800 5389 2187
274 281 285 318
a
Weight average molecular weight (MW).
(Kumakura and Kaetsu, 1978, 1979; Stepanik et al., 1998, 2000). This will allow electron beam-treated cellulose to be used as a raw material for the production of sugars, ethanol and other chemicals. It should be noted that MCC samples that were irradiated to a dose of 1000 kGy went into solution (10% tetrabutylammonium fluoride trihydrate in DMSO) much faster than the lower dose samples, seconds versus hours for the 0 and 10 kGy samples. This is most likely due to the larger reduction in the crystallinity of the 1000 kGy samples. Table 1 also shows that the available surface area of MCC increases with increasing dose from 274 m2/g in the control sample to 318 m2/g at 1000 kGy. While there was an increase in surface area with increasing dose, the increase was much less than expected, when compared to the large decrease in crystallinity and MW. Some researchers have shown a good correlation between the increase in available surface area and ease of chemical and enzymatic hydrolysis (Stone et al., 1969; Huang, 1975; Grethlein, 1985; Grous et al., 1986), however Fan et al. (1981) did not see any correlation.
4. Conclusion Treatment with high-power electron beams appears to be an energy efficient and environmentally benign method to reduce the MW and crystallinity of MCC cellulose and increase its available surface area. Using a 90 kW, 3 MeV DynamitronTM, the molecular weight of the cellulose (MCC) was reduced from 82,000 to 5000 Da with a dose of 100 kGy. Increasing the dose to 1000 kGy only reduced the MW to 2200 Da. The relative crystallinity of the MCC was reduced from 87% to 45%, with a dose of 1000 kGy. The available surface area increased from 274 m2/g for the control sample (0 kGy) to 318 m2/g at a dose 1000 kGy. These changes in the chemical and physical properties of the cellulose will make it more amenable to treatment with other chemical agents, thus allowing cellulose to be used as a raw material for the production of sugars, ethanol and other chemicals.
Acknowledgement The authors would like to thank the New York State Energy Research Development Agency and the United States Department of Agriculture for funding this research. References Alberti, A., Bertini, S., Gastaldi, G., Iannaccone, N., Macciantelli, D., Torri, G., Vismara, E., 2005. Electron beam-irradiated textile cellulose fibers. Eur. Polym. J. 41 (8), 1787–1797. Alen, R., Sjostrom, E., 1985. Degradative conversion of cellulose-containing materials into useful products. In: Nevell, T.P., Zeronian, S.H. (Eds.), Cellulose Chemistry and its Applications. Wiley, New York, pp. 531–542.
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Mosier, N., Wyman, C., Dale, B., Elander, R., Lee, Y.Y., Holtzapple, M., Ladish, M., 2005. Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresour. Technol. 96, 673–686. Nakamura, Y., Ogiwara, Y., Phillips, G.O., 1985. Free-radical formation and degradation of cellulose by ionizing radiations. Polym. Photochem. 6 (2), 135–159. NRC, 2000. Biobased Industrial Products—Priorities for Research and Commercialization, National Research Council. National Academy Press, Washington, DC. Petryaev, E.P., Boltromeyuk, V.V., Kovalenko, N.I., Shadyro, O.I., 1988. Mechanism of the radiation-initiated degradation of cellulose and its derivatives. Vysokomol. Soedin., Ser. A 30 (10), 2069–2075. Phillips, G.O., Arthur Jr., J.C., 1985. Effects of high-energy radiation on physical and chemical properties of purified fibrous cellulose. In: Nevell, T.P., Zeronian, S.H. (Eds.), Cellulose Chemistry and its Applications. Ellis Horwood Ltd., Chichester, UK, pp. 290–311. Phillips, G.O., Jett, A.C., 1985. Photochemistry and radiation chemistry of cellulose. In: Nevell, T.P., Zeronian, S.H. (Eds.), Cellulose Chemistry and its Applications. Wiley, New York, pp. 290–311. Polovka, M., Brezova, V., Stasko, A., Mazur, M., Suhaj, M., Simko, P., 2006. EPR investigations of gamma-irradiated ground black pepper. Radiat. Phys. Chem. 75 (2), 309–321. Sottys, J., Lisowski, Z., Knapczyky, J., 1984. X-ray diffraction study on the crystallinity index and structure of the microcrystalline cellulose. Acta Pharm. Technol. 30 (2), 174–180. Stepanik, T.M., Rajagopal, S., Whitehouse, R., Ewing, D., 1998. Electron processing technology: a promising application for the viscose industry. In: Proc. Tenth Int. Meet. Radiat. Process., 11–16 May, Anaheim, USA. Radiat. Phys. Chem., vol. 52, p. 505.
Stepanik, T.M., Ewing, D.E., Whitehouse, R., 2000. Electron treatment of wood pulp for the viscose process. In: Proc. Eleventh Int. Meet. Radiat. Process., 13–19 March 1999, Melbourne, Australia. Radiat. Phys. Chem., vol. 57, p. 377. Stone, J.E., Scallan, A.M., Donefer, E., Ahlgren, E., 1969. Digestibility as a simple function of a molecule of similar size to a cellulase enzyme. Adv. Chem. Ser. 95, 219–241. Stupinska, H., Iller, E., Zimek, Z., Wawro, D., Ciechanska, D., Kopania, E., Palenik, J., Milczarek, S., Steplewski, W., Krzyzanowska, G., 2007. An environment-friendly method to prepare microcrystalline cellulose. Fibres Text. East. Eur. 15 (5–6), 167–173. Taka´cs, E., Wojna´rovits, L., Borsa, J., Fo¨ldvry, Cs., Hargittai, P., Zo¨ld, O., 1999. Effect of g-irradiation on cotton-cellulose. Radiat. Phys. Chem. 55, 663–666. Taka´cs, E., Wojna´rovits, L., Fo¨ldva´ry, Cs., Hargittai, P., Borsa, J., Sajo´, I., 2000. Effect of combined gamma-irradiation and alkali treatment on cotton-cellulose. Radiat. Phys. Chem. 57, 399–403. To´th, T., Borsa, J., Taka´cs, E., 2003. Effect of preswelling on radiation degradation of cotton cellulose. Radiat. Phys. Chem. 67, 513–515. Wyman, C.E., Dale, B.E., Elander, R.T., Holtzapple, M., Ladisch, M.R., Lee, Y.Y., 2005a. Coordinated development of leading biomass pretreatment technologies. Bioresour. Technol. 96 (18), 1959–1966. Wyman, C.E., Dale, B.E., Elander, R.T., Holtzapple, M., Ladisch, M.R., Lee, Y.Y., 2005b. Comparative sugar recovery data from laboratory scale application of leading pretreatment technologies to corn stover. Bioresour. Technol. 96 (18), 2026–2032. Yang, C., Shen, Z., Yu, G., Wang, J., 2008. Effect and aftereffect of g radiation pretreatment on enzymatic hydrolysis of wheat straw. Bioresour. Technol. 99 (14), 6240–6245.
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Electron beam irradiation of cellulose Mark Driscoll a,, Arthur Stipanovic a, William Winter a, Kun Cheng a, Mellony Manning a, Jessica Spiese a, Richard A. Galloway b, Marshall R. Cleland b a b
State University of New York, College of Environmental Science and Forestry, Syracuse, New York IBA Industrial, Inc., Edgewood, New York
a r t i c l e in fo
Keywords: Electron beam irradiation Microcrystalline cellulose Cellulose depolymerization Molecular weight reduction Crystallinity reduction Surface area increase
abstract Using a 90 kW, 3 MeV DynamitronTM, the molecular weight of microcrystalline cellulose (MCC) was reduced from 82,000 to 5000 Da with a dose of 100 kGy. The relative crystallinity of the MCC was reduced from 87% to 45% with a dose of 1000 kGy. The available surface area, an indication on how well cellulose will react with chemical agents, was increased from 274 m2/g for the control sample (0 kGy) to 318 m2/g at a dose 1000 kGy. & 2009 Elsevier Ltd. All rights reserved.
1. Introduction Cellulose is the major structural component of wood and plant fibers and is the most abundant polymer synthesized by nature. Despite this great abundance, cellulosic biomass has seen limited application outside of the paper industry. Its use as a feedstock for fuels and chemicals has been limited because of its highly crystalline structure, inaccessible morphology and limited solubility. Any economic use of lignocellulosic resources for the production of ethanol will require a ‘‘pretreatment’’ technology to enhance the accessibility of the biomass to enzymes and/or chemical reagents (Alen and Sjostrom, 1985; McMillan, 1992, 1994; NRC, 2000; Himmel et al., 2005; Mosier et al., 2005; Wyman et al., 2005a, b). Electron beams (EB), X-rays or gamma rays produce ions in a material which can then initiate chemical reactions and cleavage of chemical bonds (Charlesby, 1995; Charlesby and Davison, 1957; Chapiro, 1967; Phillips and Arthur, 1985; Cheng and Kerluke, 2003). Such ionizing radiation predominantly scissions and degrades or depolymerizes cellulose. Charlesby (1995) showed that radiation caused the molecular weight (MW) of cellulose to decrease with increasing radiation dose. Dose is defined as the amount of energy absorbed per unit mass. The dose of one kGy is the absorption of one Joule of energy per gram of material. Many researchers have since studied the relationship between radiation dose and cellulose fiber degradation (Charlesby, 1960; McLaren, 1978; Kumakura and Kaetsu 1978, 1979, 1983, 1984a,b; Han et al. 1981; Petryaev et al., 1988; Forsberg and Lepoutre, 1994; Taka´cs et al., 1999, 2000; Iller et al.,
Corresponding author. Tel.: +1 315 470 6848; fax: +1 315 470 6998.
E-mail address: [email protected] (M. Driscoll). 0969-806X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2009.03.080
2002; Borsa et al., 2003; To´th et al., 2003; Foldvary et al., 2003; Dubey et al., 2004; Kasprzyk et al., 2004; Bouchard et al., 2006; Stupinska et al., 2007; Yang et al., 2008). The degree of polymerization (DP) of cotton cellulose was reduced from about 1600 to 300 after 10 kGy of gamma radiation (Taka´cs et al., 1999). To´th et al. (2003) showed similar results on preswollen cotton cellulose, where the DP decreased from 1600 to 800 after 3 kGy and to 400 after 10 kGy. They also reported that the number of cleavages per 1000 monomer units was 6 and 16 for doses of 3 and 10 kGy, respectively. Forsberg and Lepoutre (1994) studied the degradation of thermomechanical (TMP) and kraft pulp by electron beams. While kraft pulp was degraded quickly, the TMP samples were degraded more slowly. Stupinska et al. (2007) showed that the DP of cellulose pulp was reduced as the electron beam dose is increased from 10 to 50 kGy. Iller (2002) studied the electron beam irradiation of various cellulose pulps, the DP was reduced for all pulps as the dose was increased from 10 to 50 kGy. The percent alpha cellulose, defined as the fraction of cellulose that does not dissolve in 17.5% aqueous NaOH, also decreased as the dose increased. This suggests that the amount of crystallinity was also reduced. Other studies also showed that radiation reduced the crystallinity of cellulose (Kasprzyk et al., 2004; Alberti et al., 2005; Khan et al., 1987). Kasprzyk et al. (2004) showed that gamma radiation could reduce the amount of crystallinity in cellulose fibers. The reduction was most evident at doses above 100 kGy. The crystallinity index of microcrystalline cellulose (MCC), flax, cotton and viscose was reduced up to 12% with a dose of 200 kGy (Alberti et al., 2005). The viscose (rayon textile) industry has evaluated electron beams as a means to reduce process cost and environmental problems. From a mechanistic perspective, the electron beam treatment disturbs the crystalline structure of the cellulose, thus
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allowing the penetration of chemicals into the crystalline regions (Horio et al., 1963; Fischer and Goldberg, 1987; Stepanik et al., 1998, 2000). This results in a reduction of up to 50% in carbon disulfide and 30% caustic usage, according to Stepanik et al. (1998). The DP of viscose pulp appears to remain relatively constant after a dose of 25 kGy (Stepanik et al., 2000). Waste newspaper (predominately cellulose and residual lignin) irradiated with an electron beam to 100 kGy produced 30% more glucose by acid hydrolysis than samples without electron beam treatment (Brenner et al., 1979). The best glucose yield was obtained by irradiating the paper to 100 kGy and using maleic acid at 3% for the hydrolysis. Wheat and rice straw, sawdust and chaff irradiated to 500 kGy showed very high glucose yields, when hydrolyzed with cellulase (Kumakura and Kaetsu, 1978, 1979). The sugar yield gradually increased with doses less than 100 kGy and then increased more rapidly from 100 to 500 kGy. Kumakura and Kaetsu (1983) studied both acid and enzymatic hydrolysis of electron beam-irradiated bagasse. Bagasse irradiated with 1000 kGy yielded two or four times as much glucose in enzymatic and acid hydrolysis, respectively, as compared to non-irradiated bagasse. The smaller increase in the enzymatic system could be possible due to the presence of radiolysis products and long-lived radicals (Phillips and Jett, 1985; Nakamura et al., 1985; Dubey et al., 2004; Polovka et al., 2006; Yang et al., 2008). Yang et al. (2008) showed that the % yield of reducing sugars by enzymatic hydrolysis increased as the storage time after gamma irradiation increase from 0 to 22 days. As far back as 1956 Glegg and Kertesz (1956) showed that the viscosity of purified wood cellulose continued to decrease for at least two weeks after irradiation. Similar results were obtained from cotton cellulose (Gong et al., 1998). The continuing reduction of the DP after irradiation is most likely due to the presence of long-lived radicals in the cellulose (Phillips and Arthur, 1985; Nakamura et al., 1985; Iller et al., 2002; Bayram and Delincee, 2004; Dubey et al., 2004; Polovka et al., 2006).
2. Experimental 2.1. Materials Microcrystalline cellulose Avicel PH101 was obtained from FMS Corporation. HPLC grade dimethyl sulfoxide (DMSO) was obtained from Burdick and Jackson, tetrabutylammonium fluoride trihydrate from Fluka. Congo red was purchased from Aldrich chemical company. Pullulan standards were obtained from Showa Denko K. K.
detector, and two Polymer Laboratories Polypore columns (330 7.5 mm2). MCC samples were dissolved in a solvent of 10% tetrabutylammonium fluoride trihydrate in DMSO that has been shown to dissolve high molecular weight cellulose (Chen, 2005). Eight pullulan standards in the molecular weight range from 5900 to 788,000 were used as standards. The relationship between pullulan molecular size and that of cellulose, at comparable molecular weights, has previously been discussed (Chen et al., 2007). The relative1 crystallinity was determined using a Rigaku DMAX-1000 X-ray diffractometer with Ni-filtered Cu Ka radiation (l ¼ 0.15418 nm). The diffraction data was collected at 50 kV and 120 mA. Each sample was scanned 9 times from 2y ¼ 51 to 501 (Chen et al., 2007). The percent crystallinity was calculated as described by Sottys et al. (1984). The available surface area of both irradiated and non-irradiated MCC samples were determined using the congo red dye adsorption method. The method was similar to those used by Inglesby and Zeronian (1996) and Goodrich and Winter (2007). Absorbance was measured at 492 nm using a Beckmann DU-600 spectrophotometer.
3. Results and discussion Fig. 1 shows the X-ray diffraction intensity of MCC as a function of diffraction angle, for the control and irradiated samples. Table 1 illustrates that the relative percent crystallinity of MCC decreases from 87% to 45%, when irradiated at doses of 0, 10, 100 and 1000 kGy in dry air calculated from the ratio of crystalline to amorphous cellulose in Fig. 1. The lower the crystallinity of the cellulose makes it more accessible to chemical reagents, and thus easier to hydrolyze to sugars (Stepanik et al., 1998, 2000; Kasprzyk et al., 2004; Mosier et al., 2005). Table 1 shows that as the dose is increased, the molecular weight decreases. The control sample’s (0 kGy) MW is 82,000 Da, but it is reduced to 2187 Da for the 1000 kGy sample. As can be seen, the majority of the MW reduction is achieved in the first 100 kGy. Other studies have shown a larger MW reduction at 10 kGy. This may be due to the relatively high crystallinity of the MCC that was used in this study. Radiation cleaves the amorphous region of the cellulose more easily than the crystalline region. Studies have shown that as the MW of cellulose is reduced, it becomes much more amenable to treatment with other agents
1000
0 kGy 10 kGy 100 kGy 1000 kGy
2.2. Irradiation of microcrystalline cellulose
2.3. Methods Molecular weight was determined using a Waters Breeze size exclusion chromatography system fitted with a Waters 1515 isocratic HPLC pump, a Waters 2414 refractive index detector, a Precision Detectors Enterprise PD2100 Series laser light scattering
750 Intensity
Microcrystalline cellulose was irradiated at doses of 10, 100 and 1000 kGy in dry air sealed in polyethylene bags. The irradiation was conducted at IBA Industrial, Edgewood, New York USA, using a 90 kW, 3 MeV DynamitronTM. The samples were less than 0.2 cm thick with a bulk density of o1.0 g/cc, thus giving equal in equal out dose. The dose was determined with cellulose triacetate films. The 1000 kGy samples were irradiated using ten 100 kGy passes to prevent overheating the samples.
500
250
0
10
15
20
25
30 35 2 Theta
40
45
50
Fig. 1. The X-ray diffraction intensity of MCC as a function of diffraction angle (2y degrees). From top to bottom: control MCC (0 kGy), 10 kGy electron beam dose, 100, 1000 kGy.
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Table 1 Effect of electron beam irradiation on MCC. Dose (kGy)
Relative crystallinity (%)
Molecular weighta (Da)
Surface area (m2/g)
0 10 100 1000
87 76 65 45
82,000 80,800 5389 2187
274 281 285 318
a
Weight average molecular weight (MW).
(Kumakura and Kaetsu, 1978, 1979; Stepanik et al., 1998, 2000). This will allow electron beam-treated cellulose to be used as a raw material for the production of sugars, ethanol and other chemicals. It should be noted that MCC samples that were irradiated to a dose of 1000 kGy went into solution (10% tetrabutylammonium fluoride trihydrate in DMSO) much faster than the lower dose samples, seconds versus hours for the 0 and 10 kGy samples. This is most likely due to the larger reduction in the crystallinity of the 1000 kGy samples. Table 1 also shows that the available surface area of MCC increases with increasing dose from 274 m2/g in the control sample to 318 m2/g at 1000 kGy. While there was an increase in surface area with increasing dose, the increase was much less than expected, when compared to the large decrease in crystallinity and MW. Some researchers have shown a good correlation between the increase in available surface area and ease of chemical and enzymatic hydrolysis (Stone et al., 1969; Huang, 1975; Grethlein, 1985; Grous et al., 1986), however Fan et al. (1981) did not see any correlation.
4. Conclusion Treatment with high-power electron beams appears to be an energy efficient and environmentally benign method to reduce the MW and crystallinity of MCC cellulose and increase its available surface area. Using a 90 kW, 3 MeV DynamitronTM, the molecular weight of the cellulose (MCC) was reduced from 82,000 to 5000 Da with a dose of 100 kGy. Increasing the dose to 1000 kGy only reduced the MW to 2200 Da. The relative crystallinity of the MCC was reduced from 87% to 45%, with a dose of 1000 kGy. The available surface area increased from 274 m2/g for the control sample (0 kGy) to 318 m2/g at a dose 1000 kGy. These changes in the chemical and physical properties of the cellulose will make it more amenable to treatment with other chemical agents, thus allowing cellulose to be used as a raw material for the production of sugars, ethanol and other chemicals.
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