Abstract
This study investigated the effect of oxygen plasma treatment on the glass transition temperature of enzymatic hydrolysis lignin (EHL) derived from the production of bio-ethanol. Differential scanning calorimetry (DSC) was used to obtain the glass transition temperature (Tg) of EHL. The results showed that the Tg value of EHL under different heating rates ranged from 160 to 200 °C, and there was a strong linear correlation between heating rate and Tg. The Tg value of oxygen plasma treated EHL decreased when compared with the untreated samples. The apparent Tg of the untreated sample was 168.2 °C, while the value of the treated sample was 161.5 °C. Distinct chain scission and introduction of oxygen-based functional groups on the surface of EHL were detected by XPS analysis. These changes may occur mainly on the bulky side chain and thus enhance molecular mobility of EHL. This indicates that oxygen plasma treatment can modify the structure and improve the reactivity of EHL efficiently.
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Glass transition of oxygen plasma treated enzymatic hydrolysis lignin
Xiaoyan Zhou,a,b,* Fei Zheng,b Xueyuan Liu, b Lijuan Tang,b Gi Xue,a Guanben Du,c Qiang Yong,b Minzhi Chen, b and Lili Zhu a
This study investigated the effect of oxygen plasma treatment on the glass transition temperature of enzymatic hydrolysis lignin (EHL) derived from the production of bio-ethanol. Differential scanning calorimetry (DSC) was used to obtain the glass transition temperature (Tg) of EHL. The results showed that the Tg value of EHL under different heating rates ranged from 160 to 200 °C, and there was a strong linear correlation between heating rate and Tg. The Tg value of oxygen plasma treated EHL decreased when compared with the untreated samples. The apparent Tg of the untreated sample was 168.2 °C, while the value of the treated sample was 161.5 °C. Distinct chain scission and introduction of oxygen-based functional groups on the surface of EHL were detected by XPS analysis. These changes may occur mainly on the bulky side chain and thus enhance molecular mobility of EHL. This indicates that oxygen plasma treatment can modify the structure and improve the reactivity of EHL efficiently.
Keywords: Enzymatic hydrolysis lignin (EHL); Oxygen plasma treatment; Glass transition temperature; Differential scanning calorimetry (DSC)
Contact information: a: School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China; b: College of Wood Science and Technology, Nanjing Forestry University, Nanjing 210037, China; c: Southwest Forestry University, Kuming, 650224, China;
* Corresponding author: xyzhou@yahoo.cn
INTRODUCTION
Bio-ethanol, as a form of renewable energy that can be produced from lignocellulosic materials, has recently received considerable attention with increasing environmental and energy concerns. Both cellulose and hemicellulose in lignocellulosic materials can be converted to simple sugars by enzymatic hydrolysis. The hydrolysis products can be subsequently fermented to ethanol, while lignin is a barrier to enzymatic saccharification of carbohydrates. Therefore, enzymatic hydrolysis lignin (EHL) is considered as a waste product in cellulosic ethanol processes. It is expected that a future lignocellulosic ethanol industry will generate large quantities of EHL (Zhu and Pan 2010). Value-added utilization of EHL can help offset the cost of bio-ethanol production, boost the economic viability of the bio-ethanol industry, and also provide a source of renewable materials.
Lignin is a highly branched polymer which contains a variety of functional groups; it is capable of undergoing a large number of modification reactions. Tremendous efforts have been made during the last 50 years to develop high-value lignin products (Lora and Glasser 2002; Kumar et al. 2009). Since both hydroxyl and aldehyde groups
are abundant in lignin, it has been used as a natural binder for making bio-composite products (Anglès et al. 2001; Velásquez et al. 2003; Dam et al. 2004; Zhou et al. 2011). Compared with formaldehyde-based resins, its environmental advantage could lead to a bright future for manufacturing bio-composites with lignin as a natural binder. However, bulky substituents on the aromatic ring, such as methoxy, diminish the available positions for polymerization and lower the reactivity of lignin. Therefore, ways to improve the reactivity of lignin has been the focus of attention.
Various physical (ultrasonic processing and thermal treatment), chemical (acid or alkali treatment), and biological (enzymatic processes) methods, and their combinations for lignin modification have been reported (Ren and Fang 2005; Crestini et al. 2006; Qiu and Chen 2008; Brosse et al. 2010). A low temperature plasma treatment has been widely used for modifying polymeric materials over the past decades. Desired outcomes such as enhanced wettability, superior adhesion characteristics, and improved chemical reactivity can be achieved by plasma treatment. Few studies have investigated modifying lignin by plasma treatments (Toriz et al. 2004; Sahin 2009; Titova et al. 2010; Klarhoefer et al. 2010). It is believed that plasma treatment is an effective method to modify the structure of lignin and implant reactive functional groups to lignin.
The objective of this work was to investigate the effect of oxygen plasma treatment on the grass transition temperature of EHL using differential scanning calorimetry (DSC), so that the sufficient data could be applied to optimize pressing parameters in the manufacture of bio-composites with EHL as a natural binder. To better understand the effects of plasma treatment on the thermo characteristic of EHL, the chemical changes of treated EHL were also evaluated using X-ray photoelectron spectroscopy (XPS).
EXPERIMENTAL
Materials
EHL was extracted from corn stover residues, which was derived from the production of bio-ethanol in the pilot plant in Hei Longjiang Province, China, according to Liu and Cheng (2007) with 3-wt% NaOH solution. The reaction was conducted at 65 °C with gentle stirring for 1h, and the solution was kept at this temperature for an additional half an hour. Then the mixture was filtered, and the alkaline filtrate and wash water were collected and acidified to pH 3 by the dropwise addition of 2-wt% dilute H2SO4 solution. The acidified mixtures were then heated in a water bath at 70 °C for 30 minutes under continuous stirring. The precipitated lignin fractions were separated from the mixtures and centrifuged, then washed with water until neutral. The final products were dried in a vacuum oven at 60 °C for 24 h and screened to an average diameter of 20 μm. Typical physical and chemical characteristics of EHL are: free-flowing brown powder; solid content 96.5%; moisture content (MC) 3%; elemental content C 57.3%, H 5.9%, O 34.1%; OMe content 9.2%; and ash content 6.7%.
Oxygen Plasma Treatment of EHL
Plasma treatments of EHL were carried out in a plasma reactor (HD-1B, made in Jiangsu, P.R. China), as shown in Fig. 1, with a radio frequency (RF) of 13.56 MHz. Powdery lignin was dispersed in a glass container to maximize surface exposure of the samples for uniform treatments. The system was evacuated to a base pressure of 0.1 to 0.3 mTorr, and then oxygen was fed directly into the chamber. Such a cycle was repeated five times to remove volatile contaminants. By operating the gas feeding system valves, a pressure of 1.3 to 1.5 mTorr and a steady-state flow rate were maintained. A RF magnetron sputtering unit was used to produce oxygen plasma. The input power was set at 200 W and sustained for a period of 3 min. At the end of the reaction, the chamber was pressurized, and the samples were removed and stored under dry conditions for later analysis.
Fig. 1. The schematic diagram of RF plasma reactor
DSC Measurement
A differential scanning calorimeter (Mettler toledo DSC823e, Switzerland) with high purity nitrogen (flow rate 10 mL/min) as the carrier gas was used to evaluate the thermokinetic behaviors of the EHL. Sample mass was ca. 12 mg. In order to eliminate the thermal history of the sample, two step scans were conducted. Firstly, the samples were heated in DSC from 25 oC to 100 oC, held at this temperature for 5 min, and cooled down to 25 oC naturally. At the second scan, the samples were heated from 25 oC to 250 oC at five different heating rates (5 oC/min, 10 oC /min, 15 oC/min, 20 oC/min, and 25 oC/min). Because the thermal history of the samples was removed during the first heating run, the 2nd heating run was used for analysis of glass transition temperature (Tg). To verify the Tg values, the untreated samples were annealed at a glass transition temperature region for 4 h to investigate the enthalpy relaxation of EHL. For each group, five replicates of untreated and treated EHL samples were scanned.
XPS Analysis
X-ray photoelectron (XPS) spectroscopy was conducted to investigate the changes of elemental distributions and surface functionality of EHL after oxygen plasma treatment. Measurements were performed on a K-α using an Al Kα (1480 eV) X-ray source operated at 100 W under a vacuum of 5×10−7 mbar. To average out the heterogeneity of the sample, survey scans and high-resolution regional spectra were recorded from at least three measurement points in each sample.
RESULTS AND DISCUSSION
Glass Transition Temperature of EHL
Figure 2 shows a DSC thermogram of the untreated EHL sample. As a typical amorphous polymer, EHL has a distinct stage of glass transition starting around 150 oC. The Tg of untreated EHL samples, indicated by an arrow as the medial point of the step change, was observed around 170 oC at the heating rate of 10 oC/min. The glass transition is a characteristic of the viscoelastic behavior of amorphous polymers. At temperatures below the transition, lignin is stiff and brittle. When lignin is thermally processed in the transition region, the stiffness of lignin decreases, and it exhibits rubber-like elasticity as a result of chain entanglements. If the material is not cross-linked, and provided that thermal degradation does not occur, further increase in temperature will eventually result in rubbery flow as the entanglements begin to slip. If cross-linking is present, such flow cannot occur (Irvine 1985). With regard to lignin, both dehydration and non-reversible reactions will take place within the lignin matrix with further increase in temperature above its glass transition. The non-reversible reactions might be caused by the formation of new ether-type linkages between phenylpropane units of lignin (Guigo et al. 2009). If lignin is used as a natural binder for bio-composites, a higher hot-pressing temperature in the composites is necessary in order to reach temperatures above the glass transition point of lignin to cure and bond the fibers. It has been shown that improved properties of fiberboards with EHL as a natural binder were obtained when the hot-pressing temperature was above the Tg value of EHL (Zhou et al. 2011). Lower Tg values of EHL samples imply that the temperature during hot-pressing can be decreased and it can also be optimized based on the Tg value.
Glass transition temperature of various kinds of lignin has been investigated by different techniques over the past few decades (Hatakeyama and Hatakeyama 2010). The Tg values are found in a wide temperature range depending on plant species, isolation methods, and post treatments (Hatakeyama et al. 1982; Glasser and Jain 1993; Jain and Glasser 1993; Hatakeyama and Quinn 1999; Laborie et al. 2004; Li and Sarkanen 2005). As shown in Fig. 2, the Tg value ranged from 160 to 200 oC at the different heating rates. These values were much higher than that of industrial hydrolysis lignin from 75 to 90 oC, as reported by Hatakeyama et al. (2010), based on EHL obtained as a by-product from the production of bio-ethanol (Hatakeyama et al. 2010). This is probably due to the different plant species and the different hydrolysis conditions. Moreover, the molecular motion of isolated lignin was confirmed at a temperature higher than that of lignin in situ by the above result (Salmén 1984; Hatakeyama 1992), which was expected as molecular weight is related glass transition temperature (Blanchard 1974).
Since amorphous polymers show enthalpy relaxation when glassification slowly takes place, EHL samples were annealed at 165 oC for 4 h, cooled slowly, and then heated at the rate of 10 oC/min. By annealing, enthalpy relaxation is identified as an endothermic shoulder peak in DSC curves (Hatakeyama et al. 2010). The enthalpy relaxation of EHL is visible as indicated by an arrow in Fig. 2. It indicates that lignin is a typical amorphous polymer having a broad spectrum of molecular higher order structure which coaggregates by annealing (Hatakeyama et al. 2010).
Fig. 2. DSC heating curve of EHL and enthalpy relaxation of EHL annealed at 165 oC for 4h
Effect of Oxygen Plasma Treatment on Glass Transition Temperature of EHL
Figure 3 presents the DSC thermograms of one group of untreated and oxygen plasma treated EHL samples under different heating rates. It can be seen that there was a clear step change being detected and the size of the step change increased with increasing heating rate for all samples. The Tg values, indicated by an arrow, increased with increasing heating rate for all samples and ranged from 160 to 200 oC. Because of the lag in heat conduction and sample relaxation, the characteristic temperature (T) of thermal properties was delayed due to the high heating rate. Theoretically, if all experimental conditions are constant, such as the heat conduction of the pan and sample thickness, the heating rate can only change the kinetics of the thermal behavior. In that case, the relationship between T and heating rate should be linear (Liu et al. 2009). The Tg was plotted as a function of heating rate in Fig. 4 based on the measured data from Fig. 3. It shows a strong linear correlation between heating rate and Tg for both untreated and treated samples. The intercept was assumed as an apparent Tg of samples, which represents Tg when the heating rate is extremely slow and approaching 0 oC/min (Park and Wang 2005; Liu et al. 2009). The apparent Tg of the untreated sample was 168.2 oC, while the value of the treated one was 161.5 oC. From Fig. 4, it is obvious that Tg values of oxygen plasma treated samples were lower than that of untreated samples at the different heating time. Molecular motion of lignin is characterized by phenylpropane units in the molecular chain; bulky side chain and slight cross-linking establish intra- and intermolecular chains. Rigid groups in the main chain and cross-linking restrict molecular motion and have the effect of increasing the glass transition temperature. In contrast, bulky side chains enhance molecular mobility of lignin through the local mode relaxation (Hatakeyama et al. 2010). The above results indicate that the bulky side chains or cross-linking of EHL might be changed; even the ring might be opened during oxygen plasma treatment, since the Tg values of treated samples decreased. Therefore, to better understand the effects of plasma treatment on the thermokinetic behavior of EHL, X-ray photoelectron (XPS) analysis of EHL samples was conducted.
Fig. 3. DSC curves of untreated and oxygen plasma treated EHL at different heating rates
Fig. 4. The glass transition temperature (Tg) of samples as a function of heating rate
Effect of Oxygen Plasma Treatment on Chemical Change of EHL
From the low-resolution XPS spectra results, the O/C atomic ratio was found to be 0.25 of the untreated samples and was increased notably to 0.40 after oxygen plasma treatment. Moreover, the deconvoluted peak areas of the untreated and treated EHL surfaces in the C1s spectra were significantly different as well (Fig. 5). The peak areas of oxygen-related functional groups concerning the peaks C2 (corresponds to carbon atoms linked to single oxygen), C3 (carbon atoms linked two non-carbonyl oxygen, or to single carbonyl oxygen), and C4 (carbon atoms bonded to carbonyl and non-carbonyl oxygen) of the treated samples were increased by 126, 37, and 246%, respectively, compared with the untreated samples. On the other hand, a considerable decrease in C1, which corresponds to carbon atoms only linked to carbon and/or hydrogen, such as C-C and C-H groups of plasma-treated lignin, was identified. This indicates that a great number of oxygen-related functional groups are attached to the surface of EHL after treatment (Dorris and Gray 1978a,b). During the oxygen plasma treatment, oxygen-based polar species with high energy (0 to 20 eV) as so-called oxygen plasma will be generated, such as oxygen free radicals and atoms, ionized oxygen molecules, ozone, and a number of other oxygen metastable states, while the bond energies of the atoms of organic structures are variable and less than the energy range of such species (Denes et al. 2005). Therefore, plasma can lead to polymer chain scission and promote the formation of free radicals and ions. As a result, fragmentation, isomerization, cross-linking, and the introduction of functional groups to the surface of polymer materials occur (Li et al. 2007). Considering the lignin structure, it has C-H, C-C, C=C, C-O, and C=O functionalities with bond energies less than 20eV (Lide 1995). Hence, oxygen plasma is intense enough to dissociate almost all chemical bonds involved in recovering lignin structures and to create free radical species. Meanwhile, oxygen-based polar species react with these free radical species by the generation of hydroxyl, carbonyl, and carboxyl groups on the surface of EHL. The XPS analysis results imply that the chain scission and the introduction of functional groups occur mainly on the bulky side chain. These changes enhance molecular mobility of EHL, so that the Tg values decrease after oxygen plasma treatment.
Fig. 5. The high-resolution C1s XPS spectra of untreated and oxygen plasma treated EHL
CONCLUSIONS
The Tg values of isolated enzymatic hydrolysis lignin (EHL) under different heating rates ranged from 160 to 200 oC. They were much higher than that of industrial hydrolysis lignin, which was also obtained as a by-product from the production of bio-ethanol.
The glass transition temperature (Tg) of EHL was decreased after being treated by oxygen plasma due to efficiently modified chemical structure of EHL. Distinct chain scission and introduction of oxygen-based functional groups were detected by XPS analysis. These changes may occur mainly on the bulky side chain and thus enhance molecular mobility of EHL.
Lower Tg values of oxygen plasma treated EHL samples imply that the temperature of hot-pressing can be decreased and also be optimized based on the Tg value so that good cross-linking between EHL and lignocellulosic fibers will probably be formed when EHL is used as an natural thermosetting binder.
ACKNOWLEDGMENTS
The authors are indebted to financial support by projects from the State Key Program of National Natural Science of China (Grant No. 30930074), the National Natural Science Foundation of China (Grant No. 31070504), the Program for New Century Excellent Talents in University (NCET-10-0177), the Governmental Public Industry Research Special Funds (Grant No. 201004001), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Sincere thanks go to the research group of bio-ethanol from lignocellulosic materials from Nanjing Forestry University for providing lignin and Jiangsu Engineering Research Center of Fast-growing Trees and Agri-fiber Materials and Nanjing National Laboratory of Microstructures for providing equipment for this study.
REFERENCES CITED
Anglès, M. N., Ferrando, F., Farriol, X., and Salvadó, J. (2001). “Suitability of steam exploded residual softwood for the production of binderless panels. Effect of the pre-treatment severity and lignin addition,” Biomass Bioenergy 21, 211-224.
Blanchard, L. P., Hesse, J., and Malhotra, S. L. (1974). “Effect of molecular weight on glass transition by differential scanning calorimetry,” Can. J. Chem. 52, 3170-3175.
Brosse, N., Hage, R. E., Chaouch, M., Pétrissans, M., Dumarçay, S., and Gérardin, P. (2010). “Investigation of the chemical modifications of beech wood lignin during heat treatment,” Polym. Degrad. Stabil. 96, 1721-1726
Crestini, C., Caponi, M. C., Argyropoulos, D. S., and Saladino, R. (2006). “Immobilized methyltrioxo rhenium (MTO)/H2O2 systems for the oxidation of lignin and lignin model compounds,” Bioorgan. Med. Chem. 14, 5292-5302.
Denes, F. S., Cruz-Barba, E., and Manolache, S. (2005), “Plasma treatment of wood,” in: R. M. Rowell (ed.) Handbook of Wood Chemistry and Wood Composites, CRC press, New York.
Dam, J. E. G., Oever, M. J. A., Wouter, T., Keijsers, E. R. P., and and Peralta, A. G. (2004). “Process for production of high density/high performance binderless boards from whole coconut husk. Part 1: Lignin as intrinsic thermosetting binder resin,” Ind. Crop. Prod. 19, 207-216.
Dorris, G. M., and Gray, D. G. (1978a). “The surface analysis of paper and wood fibers by ESCA (Electron spectroscopy for chemical analysis). I. Application to cellulose and lignin,” Cell. Chem. Technol. 12, 9-23.
Dorris, G. M., and Gray, D. G. (1978b). “The surface analysis of paper and wood fibers by ESCA II. Surface composition of mechanical pulps,” Cell. Chem. Technol. 12, 721-734.
Glasser, W. G., and Jain, R. K. (1993). “Lignin derivatives. I. Alkanoates, ” Holzforschung 47, 225-233.
Guigo, N., Mija, A., Vincent, L., and Sbirrazzuoli, N. (2009). “Molecular mobility and relaxation process of isolated lignin studied by multifrequency calorimetric experiments,” Phys. Chem. Chem. Phys. 11, 1227-1236.
Hatakeyama, H. (1992). “Thermal analysis,” in: Lin S. Y., and Dence C. W. (eds.), Methods in Lignin Chemistry Springer, Berlin.
Hatakeyama, H., and Hatakeyama, T. (2010). “Lignin structure, properties, and applications,” Adv Polym Sci. 232, 1-63.
Hatakeyama, H., Tsujimoto, Y., Zarubin, M. J., Krutov, S. M., and Hatakeyama, T. (2010). “Thermal decomposition and glass transition of industrial hydrolysis lignin,” Journal of Thermal Analysis and Calorimetry 101, 289-295.
Hatakeyama, T., and Quinn, F. X. (1999). “Applications of thermal analysis,” in: Hatakeyama, T., and Quinn, F. X. Thermal Analysis, Fundamentals and Applications to Polymer Science, 2nd Ed., John Wiley & Sons Ltd, New York.
Hatakeyama, T., Nakamura, K., and Hatakeyama, H. (1982). “Studies on heat capacity of cellulose and lignin by differential scanning calorimetry,” Polymer 23, 1801-1804.
Irvine, G. M. (1985). “ The significance of the glass-transition of lignin in thermomechanical pulping,” Wood Sci. and Tech. 19, 139-49.
Jain, R. K., and Glasser, W. G. (1993). “Lignin derivatives. II. Functional ethers,” Holzforschung 47, 325-332.
Klarhoefer, L., Vioel, W., and Maus-Friedrichs, W. (2010). “Electron spectroscopy on plasma treated lignin and cellulose,” Holzforschung 64, 331-336.
Kumar, M. N. S., Mohanty, A. K., Erickson, L., and Misra, M. (2009). “Lignin and its applications with polymers,” J. Biobased Mater. Bio. 3, 1-24.
Lora, J. H., and Glasser, W. G. (2002). “Recent industrial applications of lignin: A sustainable alternative to nonrenewable materials,” J. Polymers Environ. 10, 39-48.
Laborie, M. P. G., Salmén, L., and Frazler, C. E. (2004). “Cooperativity analysis of the in situ lignin glass transition,” Holzforschung 58, 129-133.
Li, T. T., Wang, X. L., Wei, D. X., Wei Y. L., and Gu, F. (2007). “Mechanism and kinetic analysis of NO／SO2／N2／O2 non-thermal plasma dissociation reaction process,” J. Eng. Thermophy. 28, 531-533.
Li, Y., and Sarkanen, S. (2005). “Miscible blends of kraft lignin derivatives with low-Tg polymers,” Macromolecules 38, 2296-2306.
Lide, D. R. (1995). Handbook of Chemistry and Physics, 76th Ed., CRC press, New York.
Liu, P., Yu, L., Liu, H. S., Chen, L., and Li, L. (2009). “Glass transition temperature of starch studied by a high-speed DSC,” Carbohyd. Polym. 77, 250-253.
Liu, X. L., and Cheng, X. S. (2007). “Study on separation and structure of enzymatic hydrolysis lignin,” J. Cellul. Sci. Technol. 15, 41-46.
Nakamura, K., Hatakeyama, T., and Hatakeyama, H. (1992). “Thermal properties of solvolysis lignin-derived polyurethanes,” Polym Adv Technol. 3, 151-155.
Park, B. D., and Wang, X. M. (2005). “Thermokinetic behavior of powdered phenol-formaldehyde (PPF) resins,” Thermochim. Acta. 433, 88-92.
Qiu, W. H., and Chen, H. Z. (2008). “An alkali-stable enzyme with laccase activity from entophytic fungus and the enzymatic modification of alkali lignin,” Bioresour. Technol. 99, 5480-5484.
Ren, S. X., and Fang, G. Z. (2005). “Effects of ultrasonic processing on structure of alkali lignin of wheat-straw,” Chem. Ind. Forest Prod. 25, 82-86.
Sahin, H. T. (2009), “RF-O2 plasma surface modification of kraft lignin derived from wood pulping,” Wood Research 54, 103-112.
Salmén, L. (1984). “Viscoelastic properties of in situ lignin under water saturated conditions,” J. Mater Sci. 19, 3090-3096.
Titova, Y. V., Stokozenko, V. G., and Maksimov, A. I. (2010). “The influence of plasma-solution treatment on the properties of hemp fiber lignin,” Surf. Eng. Appl. Electrochem. 46, 127-130.
Toriz, G., Ramos, J., and Young, R. A. (2004). “Lignin-polypropylene composites. II. Plasma modification of kraft lignin and particulate polypropylene,” J. Appl. Polym. Sci. 91, 1920-1926.
Velásquez, J. A., Ferrando, F., and Salvadó, J. (2003). “Effects of kraft lignin addition in the production of binderless fiberboard from steam exploded Miscanthus sinensis,” Ind. Crop. Prod. 18, 17-23.
Zhou, X. Y., Tan, L. J., Zhang, W. D., Lv, C. L., Zheng, F., Zhang, R., Du, G. B., Tang, B. J., and Liu, X. Y. (2011). “Enzymatic hydrolysis lignin derived from corn stover as an intrinsic binder for Bio-composites manufacture: Effect of fiber moisture content and pressing temperature on boards’ properties,” BioResources 6, 253-264.
Zhu, J. Y., and Pan, X. J. (2010). “Woody biomass pretreatment for cellulosic ethanol production: Technology and energy consumption evaluation,” Bioresource Technol. 101, 4992-5002.
Article submitted: June 7, 2012; Peer review completed: July 31, 2012; Revised version received: August 1, 2012; Accepted: August 2, 2012; Published: August 13, 2012.