Article pubs.acs.org/EF
Influence of Crop Residue Types on Microwave-Assisted Liquefaction Performance and Products Weihua Xiao,† Wenjuan Niu,† Fei Yi,‡ Xian Liu,† and Lujia Han*,† †
College of Engineering, China Agricultural University, Beijing 100083, People’s Republic of China Chinese Academy of Agricultural Mechanization Sciences, Beijing 100083, People’s Republic of China
‡
ABSTRACT: Microwave-assisted polyhydric liquefaction of crop residues was a useful method for the conversion of lignocelluloses into biopolyols. Five types of crop residues were selected to investigate the effect of feedstock varieties on the microwave-assisted liquefaction performance and products. The microwave-assisted liquefaction characteristic of different feedstocks in ethylene glycol (EG) was evaluated by residue component analysis and kinetics modeling. Cellulose was proven to be the main residual content, and hemicelluloses and lignin were liquefied rapidly. According to the first-order model, degradation of cellulose was indicated to be the rate-determining step in the microwave solvolysis liquefaction of crop residues. Among the five types of crop residues, rice straw was the material most difficult to be liquefied. Acid-insoluble ash (AIA) was found to be significantly negatively correlated with the liquefaction rate of crop residues and cellulose. The hydroxyl number and acid number of microwave-assisted liquefaction products (MLPs) from different feedstocks varied with different crop residues.
1. INTRODUCTION The use of lignocellulosic resources has received considerable attention in recent years for their high amount of organic constituents (i.e., cellulose, hemicelluloses, and lignin) and high energy content.1 However, large quantities of lignocellulosics, such as agricultural wastes from harvesting, are still incinerated or discarded in the field.2 Polyhydric sovolysis liquefaction is one effective technique for converting lignocelluloses into polyhydroxy compounds (biopolyols).3−9 The production of biopolyols from biomass by polyhydric liquefaction has long been of interest in the polyurethane (PU) industry.10 Microwave-assisted liquefaction is a novel technique used in lignocellulosic biomass conversion. The application of microwave technology to polyhydric liquefaction offers several advantages over conventional liquefaction with external heating, including saving processing time and improving the liquefaction yield as reported.11−14 Previous studies on liquefaction mainly focused on the effect of liquefying reagents, catalysts, reaction temperature, and reaction time on the liquefaction yield.3−13 Because agricultural crop residues are numerous in China, the components differ greatly according to biomass types. The liquefaction behavior and product property might vary with different feedstocks. Yamada et al.15 reported that the type of wood, i.e., softwood or hardwood, might affect the sovolysis liquefaction products with ethylene carbonate. Kurimoto et al.16 investigated the effect of wood species on the characteristics of liquefied wood. The mechanical properties of the PU films varied with the wood species significantly. The structural characterization of the hydroxyl groups and the condensed fractions of each liquefied product were assumed to be essential to the mechanical properties of PU films. The liquefaction rate was also compared in the conventional oil bath liquefaction; the liquefaction yield showed an obvious difference with varieties of crop residues.17 The reports mentioned above mainly studied influence of feedstock on the product property; our study aims to © 2013 American Chemical Society
investigate the influence of feedstock on microwave-assisted liquefaction behavior and to establish the relationship between the chemical components and liquefaction property. Experiments on five types of crop residues (corn stover, rice straw, wheat straw, cotton stalk, and corncob) were carried out. The liquefaction performance of different agricultural feedstocks was evaluated by chemical analysis and kinetic modeling. The liquefied products of different feedstocks were characterized by determination of the hydroxyl and acid numbers.
2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. Crop residues (corn stover, rice straw, wheat straw, cotton stalk, and corncob) used in the experiments were dried and ground with a blade-mill (FW135 medicine mill, China) and then sieved through a 40-mesh screen. The reagents used were of chemical grade (Beijing Chemical Plant, China). 2.2. Liquefaction Procedure. Liquefactions were carried out in a Milestone microwave labstation (Ethos Touch Control, Italy, with maximum output of 1000 W, ASM-400 magnetic stirrer) equipped with 100 mL sealed Teflon reaction vessels and an internal temperature sensor (ATC-400-CE automatic temperature control up to 300 °C with a fiber optic sensor). The frequency used by the microwave system was 2450 MHz. The system features both singleand multi-mode technologies in a single labstation. Samples were irradiated for less than 2 min under 600 W as starting microwave power. The sample temperature was controlled at 160 °C for preset duration from 1 to 30 min with the microwave power changed. The reaction mixture consisted of 5.0 g of crop residue, 25.0 g of liquefaction reagent ethylene glycol (EG), and 0.875 g of catalyst sulfuric acid. After liquefaction for a preset time, the vessels were allowed to cool at room temperature before opening. After cooling, the liquefaction product was diluted with 80% 1,4dioxane (dioxane/water = 80:20, v/v). The diluted resultant was Received: December 29, 2012 Revised: April 16, 2013 Published: April 30, 2013 3204
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Table 1. Chemical Composition of Five Different Crop Residues sample corn stover wheat straw rice straw cotton stalk corncob
cellulose (%) 37.34 41.42 39.59 46.40 35.94
± ± ± ± ±
hemicelluloses (%)
1.42 1.42 1.14 1.63 0.32
30.51 33.10 26.50 15.07 41.65
± ± ± ± ±
2.65 1.46 0.25 0.71 0.00
filtered, and the residue was dried at 105 °C for 12 h in an oven and then weighed. The liquefaction yield was calculated by the following equation:
lignin (%) 8.73 10.68 5.06 23.08 8.80
± ± ± ± ±
0.10 0.30 0.08 1.63 0.87
AIA (%) 1.18 3.14 10.43 0.18 0.47
± ± ± ± ±
0.02 0.12 0.12 0.01 0.00
The liquefaction yields as a function of the liquefaction time from 0 to 30 min were shown in Figure 1. About 71−82% of
liquefaction yield (%) = (1 − weight of residue/weight of crop material) × 100% 2.3. Composition Analysis. All samples were analyzed for neutral detergent fiber (NDF), acid detergent fiber (ADF), acid detergent lignin (ADL), and acid-insoluble ash (AIA) according to procedures described for use in an Ankom fiber apparatus (Ankom220, Fairport, NY). Cellulose, hemicelluloses, and lignin were calculated according to the method by Van Soest et al.18 as follows: hemicellulose = NDF − ADF; cellulose = ADF − ADL; and lignin = ADL − AIA. 2.4. Determination of the Acid and Hydroxyl Numbers. The acid numbers were determined as reported.16 A total of 8 g of microwave-assisted liquefaction product (MLP) was dissolved in a mixture of 80 mL of dioxane and 20 mL of water. The resulting solution was titrated with 0.1 M sodium hydroxide solution using an automatic titrator (Leici ZDJ-4A, Shanghai, China). Determination of the hydroxyl number was modified according to ASTM D4274-11. The phthalation reagent consisted of a mixture of 150 g of phthalic anhydride, 50 g of pyridine, and 900 g of dioxane. A mixture of 1 g of MLP and 25 mL of phthalation reagent were heated for 2 h at 98 °C. After that, 50 mL of 80% 1,4-dioxane was added and the mixture was titrated with 1 M sodium hydroxide solution to the equivalence point using an automatic titrator (Leici ZDJ-4A, Shanghai, China). In blank titration, exactly 25 mL of phthalation reagent and 50 mL of 80% 1,4-dioxane were added and the mixture was titrated. The hydroxyl number in milligrams of KOH per gram of sample was calculated by the following equation:
Figure 1. Microwave-assisted liquefaction yield of different feedstocks.
crop residues were degraded in the first 5 min, and the liquefaction yield increased slowly with time extending to 30 min. Over 95% of corn stover and corncob was liquefied at 20 min. The liquefaction yield of rice straw was significantly lower than those of other crop residues. 3.2. Chemical Analysis of the Microwave-Assisted Liquefaction Residue. The main components in residues of different feedstocks after microwave-assisted liquefaction, including cellulose, hemicelluloses, lignin contents, and AIA, were analyzed (Figures 2−4). The cellulose content in the liquefaction residue was shown in Figure 2. The content of hemicelluloses and lignin showed a sharp decline in the first 5 min and gradually decreased during the succeeding liquefaction period, except a slight increase in lignin at 30 min (Figures 3 and 4). Because a significant amount
hydroxyl number (mg of KOH/g) = [(B − A) × N × 56.1/W ] + acid number where A is the volume (mL) of sodium hydroxide solution after the phthalation reaction on the liquefied sample, B is the volume of the blank solution (mL), N is the normality of the sodium hydroxide solution, and W is the weight of the liquefied sample. 2.5. Statistical Analysis. All data collected were subject to analysis of variance (ANOVA) (p < 0.05) using SPSS. Correlation analysis was carried out to test for the association between the liquefaction rate and chemical components. Kinetic data were subject to analysis of linear regression using Excel. All of the analyses were carried out in duplicate.
3. RESULTS AND DISCUSSION 3.1. Microwave-Assisted Liquefaction Yield for Five Crop Residues. To compare the performance of different crop residues, corn stover, wheat straw, rice straw, cotton stalk, and corncob were selected for the microwave-assisted liquefaction. Chemical analysis of the five types of crop residues included was presented in Table 1. The chemical composition of corn stover and wheat straw was relatively similar. However, cotton stalk had a somewhat high lignin content and a low hemicelluloses content. The hemicelluloses content of corncob was almost 4 times higher than that of cotton stalk. Rice straw had the highest AIA content of 10.43% among the five crop residues.
Figure 2. Cellulose content of the liquefied residue with different crop residues. 3205
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According to the data in Figures 2−4, the liquefaction ratio of each organic component could be calculated. Cellulose accounted for 53−83% in various feedstock residues at 5 min, and only 62−67% of cellulose was liquefied at the initial stage, while 93−98% of hemicelluloses and 95−98% of lignin were liquefied at the same time. It could be concluded that hemicelluloses and lignin react more readily than cellulose in the microwave-assisted liquefaction of crop residues. A similar result was found in the conventional liquefaction of wood.20 It was deduced that access to cellulose limits the efficiency of solvolysis liquefaction. 3.3. Reaction Kinetics of Microwave-Assisted Liquefaction for Crop Residues. Because the initial concentration of liquefying reagent EG is much larger than the concentration of crop residues, the concentration of EG will not change appreciably during the course of the reaction. The concentration of the reactant in excess will remain almost constant. Thus, the dependence of the rate upon crop residues can be isolated, and the rate law could be written as a pseudo-firstorder reaction model
Figure 3. Hemicelluloses content of the liquefied residue with different crop residues.
R = A exp( −kt )
(1)
where k is the rate constant, R is the mass of unliquefied component, and A was a constant determined by the fitting data. Equation 1 can also be written as (2)
ln R = ln A − kt
The value of R at different time t could be obtained from the experimental curves (Figures 1 and 2). Plotting ln R against time gave five points corresponding to data mentioned in Figures 1 and 2. The five points created a straight line with slope k. The rate constant k was used to evaluate the liquefaction reaction rate of different crop residues in our study. The liquefaction rate constants of microwave-assisted liquefaction with various feedstocks were shown in Table 2.
Figure 4. Lignin content of the liquefied residue with different crop residues.
of hemicelluloses and lignin was liquefied during the initial liquefaction process, the relative cellulose content in the residue increased at 5 min (Figure 2). It then decreased with time extending to 20 min and increased slightly when the liquefaction time of corn stover and corncob was prolonged to 30 min. There are mainly two reactions in the entire lignocellulosic liquefaction, i.e., decomposition and polycondensation.19 The above results indicated polymerization of cellulose and lignin when the liquefaction time was increased to 30 min. The content of AIA showed an increased tendency at the late stage of liquefaction (20−30 min) in Figure 5 because it could not dissolve under the microwave-assisted liquefaction conditions.
Table 2. Fitting Results of Microwave-Assisted Liquefaction for Five Crop Residues residue
rate constant (k)
correlation coefficient (R2)
corn stover wheat straw rice straw cotton stalk corncob
0.085 0.069 0.045 0.077 0.095
0.689 0.650 0.639 0.805 0.723
All of the correlation coefficients (R2) of crop residues were above 0.65. In comparison to acid-catalyzed liquefaction in oil bathing,17 microwave heating provided a 4.5 times faster liquefaction rate with corn stover than the conventional method. The acceleration of liquefaction in corn stover indicated the non-thermal effect of microwave. The rate constant k increased in the following order: rice straw < wheat straw < cotton stalk < corn stover < corncob. The results suggested that the types of feedstock might affect the rate of microwave-assisted liquefaction. Rice straw was difficult to liquefy even if corn stover and corncob were easy to decompose with microwave heating in EG. Data in Figure 2 were fitted to a pseudo-first-order reaction model to study the reaction kinetics of cellulose similarly. The constant rates and correlation coefficients were listed in Table 3.
Figure 5. AIA content of the liquefied residue with different crop residues. 3206
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Table 3. Fitting Results for Microwave-Assisted Liquefaction of Cellulose in Crop Residues residue
rate constant (k)
correlation coefficient (R2)
corn stover wheat straw rice straw cotton stalk corncob
0.091 0.085 0.065 0.086 0.082
0.716 0.852 0.908 0.959 0.817
The fitting results showed that the liquefaction of cellulose fitted the pseudo-first-order reaction model well and the correlation coefficients R2 of cellulose varied from 0.716 to 0.959 with different crop residues. The rate constant of cellulose was comparable to that of the crop residue, although slightly higher than it. It indicated that the rate of microwaveassisted liquefaction depended upon the degradation of cellulose in the crop residue. The cellulose rate constant of corncob was less than that of the whole plant because of the considerable hemicelluloses content in it, which increased the liquefaction rate. To reveal the factors that affect microwave-assisted liquefaction of different feedstocks, the relationship between the chemical components and the liquefaction rate was analyzed (Table 4). The rate constant was equivalent to the
Figure 6. Acid numbers of MLPs with different crop residues.
components during liquefaction. Among the main components, hemicelluloses are more susceptible to oxidation than cellulose according to literature reports.22,23 Therefore, the highest hemicelluloses content in corncob resulted in the highest acid number among the MLPs and a rapid increase of the acid number from 18.79 to 23.46 mg of KOH/g. The abundance of acidic substances in corncob MLP and corn stover MLP accelerated esterification reactions, with the acid number decreasing as the liquefaction time increased from 20 to 30 min.24,25 Figure 7 showed variations in the hydroxyl numbers of MLPs as a function of the reaction time. The MLP hydroxyl numbers
Table 4. Pearson Correlation between Liquefaction Rates and Chemical Component chemical component (%)
a
liquefaction rate
AIA
cellulose
hemicelluloses
lignin
crop residues cellulose
−0.921a −0.905a
−0.293 0.08
0.396 0.034
0.265 0.467
Correlation is significant at the 0.05 level.
liquefaction rate in the analysis. It was evident that only AIA content was significantly negatively correlated with the liquefaction rate of crop residues and that of cellulose. That means that the higher the AIA content, the harder the liquefaction. The cell-wall environment is a highly complex matrix composed of cellulose, hemicelluloses, and lignin. The incorporation of silica within the plant cell wall has been well-documented by botanists and materials scientists.21 The sovolysis liquefaction of cellulose was required to cleave the numerous bonds within cell walls. The shield of hemicelluloses and lignin could be easily removed in the early stage of microwave-assisted liquefaction. On the contrary, AIA, primarily silica and silicates, acted as the main barrier for the liquefaction of crop residues and cellulose because of undissolving under acidic conditions. The result explicitly explained the reason why rice straw was most difficult to be liquefied. To improve the liquefaction rate, we should choose those residues containing less AIA content. 3.4. Characterization of MLPs from Different Crop Residues. The acid and hydroxyl numbers are important variables in the application of liquefied products as biopolyols. The acid and hydroxyl numbers of different crop residue MLPs were evaluated. The acid numbers of all samples increased as the liquefaction time increased (Figure 6). The highest acid number was obtained with corncob MLP. Acidic substances can be produced by the thermal oxidation of lignocellulosic
Figure 7. Hydroxyl numbers of MLPs with different crop residues.
of different crop residues ranged from about 304 to 441 mg of KOH/g (Figure 7), suggesting that MLPs contained a large number of active hydroxyl groups similar to conventional liquefaction products.26 The variations of hydroxyl numbers by reaction time appeared to be two distinct trends. The MLP hydroxyl number of corncob, wheat straw, and rice straw remained constant, while that of corn stover and cotton stalk decreased significantly with time extending to 20 min. The cellulose degradation mechanism in EG liquefaction4 indicated that the solvolysis reaction of the lignocellulosic component correlated to an increase in the hydroxyl number. On the contrary, various dehydration, oxidation, and/or condensation reactions might contribute to the decrease in the hydroxyl number of biopolyols.27,28 The decrease in the hydroxyl number was thought to be caused by polycondensation among liquefied products and polyhydric alcohols because there was a significant increase in the cellulose content with the corn stover residue (Figure 2) and an increase in the lignin content in the cotton stalk residue at 30 min (Figure 4). The hydroxyl 3207
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(9) Hassan, E. M.; Shukry, N. Ind. Crops Prod. 2008, 27, 33−38. (10) Xu, J. M.; Jiang, J. C.; Hse, C. Y.; Shupe, T. F. Green Chem. 2012, 14, 2821−2830. (11) Kržan, A.; Kunaver, M. J. Appl. Polym. Sci. 2006, 101, 1051− 1056. (12) Kržan, A.; Ž agar, E. Bioresour. Technol. 2009, 100, 3143−3146. (13) Pan, H.; Zheng, Z. F.; Hse, C. Y. Eur. J. Wood Wood Prod. 2011, 69, 1−10. (14) Xiao, W. H.; Han, L. J.; Zhao, Y. Y. Ind. Crops Prod. 2012, 34, 1602−1606. (15) Yamada, T.; Ono, H. Bioresour. Technol. 1999, 70, 61−67. (16) Kurimoto, Y.; Takeda, M.; Koizumi, A.; Yamauchi, S.; Doi, S.; Tamura, Y. Bioresour. Technol. 2000, 74, 151−157. (17) Liang, L.; Mao, Z.; Li, Y.; Wan, C.; Wang, T.; Zhang, L.; Zang, L. BioResources 2006, 1, 248−256. (18) Van Soest, P.; Robertson, J.; Lewis, B. J. Dairy Sci. 1991, 74, 3583−3597. (19) Niu, M.; Zhao, G. J.; Alma, M. H. For. Stud. China 2011, 13, 71−79. (20) Zhang, H. R.; Pang, H.; Shi, J. Z.; Fu, T. Z.; Liao, B. J. Appl. Polym. Sci. 2011, 123, 850−856. (21) Currie, H. A.; Perry, C. C. Ann. Bot. 2007, 100, 1383−1389. (22) Chatterjee, H.; Pal, K. B.; Sarkar, P. B. Text. Res. J. 1954, 24, 1− 12. (23) Gary, D.; McGlnnls, W. W.; Wllson, C. M. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 352−357. (24) Rezzoug, S. A.; Capart, R. Energy Convers. Manage. 2003, 44, 781−792. (25) Nasar, M.; Emam, A.; Sultan, M.; Abdel, H. A. A. Indian J. Sci. Technol. 2010, 3, 207−212. (26) Yu, F.; Liu, Y.; Pan, X.; Lin, X.; Liu, C.; Chen, P.; Ruan, R. Appl. Biochem. Biotechnol. 2006, 129−132, 574−585. (27) Yao, Y. G.; Yoshioka, M.; Shiraishi, N. J. Appl. Polym. Sci. 1996, 60, 1939−1949. (28) Lee, S. H.; Teramoto, Y.; Shiraishi, N. J. Appl. Polym. Sci. 2002, 83, 1482−1489. (29) Wang, T. P.; Yin, J.; Zheng, Z. M. Carbohydr. Polym. 2012, 87, 2638−2641.
number of liquefied corncob and liquefied wheat straw was as high as 411−441 mg of KOH/g because of the high hemicelluloses content in the feedstock. Similar results were observed in characterization on solvolysis liquefaction of various corn stalk factions.29 The lowest hydroxyl number was obtained with rice straw MLP (304−313 mg of KOH/g) for the high ash content, which resulted in less organic components available for solvolysis.
4. CONCLUSION Five types of crop residues, corn stover, rice straw, wheat straw, cotton stalk, and corncob, were included in our investigation. Over 95% of corn stover and corncob were liquefied in 20 min. Cellulose was the main residual content, while hemicelluloses and lignin were mostly decomposed at the initial stages of the reaction, which indicated that cellulose was the ratedetermining step in the microwave-assisted liquefaction of crop residues. Rice straw was the most difficult to be liquefied among the five crop residues. AIA was the main barrier for the liquefaction of crop residues and cellulose because of insolubility under liquefaction. Acid and hydroxyl numbers of MLPs varied significantly with different crop residues, indicating different application prospects.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: 86-10-62736313. Fax: 86-10-62736778. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Special Fund for Agro-scientific Research in the Public Interest (201003063), the National Natural Science Foundation of China (31201123), the Beijing Natural Science Foundation (6112014), and the Fundamental Research Funds for the Central Universities (2011JS017).
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NOMENCLATURE EG = ethylene glycol NDF = neutral detergent fiber ADF = acid detergent fiber ADL = acid detergent lignin AIA = acid-insoluble ash MLP = microwave-assisted liquefaction product
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REFERENCES
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