Effects of steam explosion pretreatment on the chemical composition and fiber characteristics of cornstalks

Peer-Reviewed Journal of Bioresources and Bioproducts. 2017, 2(4): 153-157 ORIGINAL PAPER DOI: 10.21967/jbb.v2i4.100 Effects of steam explosion pretreatment on the chemical composition and fiber characteristics of cornstalks Jun Xua,b, Guoqiang Zhoua, Jun Lia* and Li-huan Moa a) b) State Key Laboratory of Pulp and Paper Engineer, South China University of Technology, Guangzhou 510640, China; Key Laboratory of Pulp and Paper Science & Technology of Ministry of Education of China, Qilu University of Technology, Jinan, Shandong, 250353 *Corresponding author: ppjunli@scut.edu.cn ABSTRACT This paper investigated the influence of steam explosion pretreatment with or without acid as the catalyst on the chemical composition and sugar contents of corn stalks. The fiber characteristics of the pretreated corn stalks were analyzed with a scanning electron microscope, a FS-300 automatic fiber analyzer and a fully automatic surface and pore analyzer. The results showed that the steam explosion pretreatment did not change the cellulose content of the corn stalks. However, hemicellulose was degraded and a portion of lignin was solubilized in the steam explosion pretreatment process. When acid was added in the steam explosion process, the fiber surface and cell wall structure of corn stalks were damaged, the specific surface area and pore size increased, and fiber length decreased, all of which were beneficial to subsequent enzymatic hydrolysis with cellulase. However, content of polysaccharides decreased after acid steam explosion pretreatment. Keywords: Corn stalk; Steam exploration pretreatment; Chemical composition; Specific surface area and pore size 1. INTRODUCTION With the rapid development of global economy, oil and other non-renewable resources have shrunk dramatically. Energy supply and environment protection have become increasingly important.1 Biomass resource is an important kind of renewable resources and ethanol fuel production from biomass meet the requirements of green and sustainable development, which has great prospects for our development. Biomass conversion has attracted increasing attention in the past decade.1,2 China s annual production capability is estimated at about 1.145 billion tons, including more than 700 million tons of straw as agricultural residue 3. At present, only a small portion of corn stalks is utilized for compost and most are burned, which causes air pollution and resource wasting. Thermal efficiency of corn stalks in direct combustion is only about 10%, but if they are transformed into gas or liquid fuel, thermal efficiency can reach 30% or even higher.3,4 Lignocellulose has a complex structure which consists of cellulose, hemicellulose and lignin that connect with each other by covalent bonds to form a network structure. The efficiency of direct enzymatic hydrolysis is low due to the compact crystalline structure of lignocellulose. Therefore a pretreatment is usually necessary before enzymatic hydrolysis2,5. Steam explosion is an effective pretreatment method which mainly uses steam with high temperature and high pressure to achieve the component separation and structure change of raw material through the process during which pressure is released instantly.6 Pure explosion pretreatment can effectively achieve the separation of www.bioresources-bioproducts.com chemical composition of lignocellulose. Steam explosion is considered as a green and efficient technology for biomass separation and conversion.7 However, hemicellulose degradation is severe under the conditions of steam explosion, which produces furfural and other chemicals which are toxic to the subsequent fermentation process.8 Acid-catalyzed steam explosion pretreatment may be effectively hydrolyzes hemicellulose into monosaccharaides and soluble oligomers, thus improving their transformation to mon sugars.8-10 However, little has been reported in the literature on the acid-catalyzed pretreatment of corn stalks and its effect on the chemical and physical properties of the biomass. In this paper, we investigated the influence of steam alone explosion pretreatment and acid-catalyzed steam explosion pretreatment on the structure, sugar contents and fiber structure characteristics of cornstalks. 2. EXPERIMENTAL 2.1 Materials Corn stalks from the northeast region in China were cut to 3~5 cm and air-dried to a moisture content of less than 10%. The dried samples were stored at room temperature. 2.2 Experimental equipment The equipment used in this study included a laboratory steam explosion set-up, a ZEISS Evo18 scanning electron 153

microscope, a ICS 3000 ion chromatograph, an Olympus BX51 biological microscope, a Metso FS-300 automatic fiber analyzer, and a Micromeritics automatic pore surface analyzer. 2.3 Explosion pretreatment Steam explosion pretreatment of the corn stalks was performed according to the conditions outlined in Table 1. After the pretreatment, the sample was centrifuged to separate the filtrate from the solid and analyzed for carbohydrate composition. The solid was stored in sealed plastic bag for moisture balancing, and then was used for yield analysis. A portion of the solid was air-dried and used for the analysis of chemical composition and fiber morphological. Table 1. Process conditions of steam explosion pretreatment No. Concentration Sulfuric acid Explosion pressure Time (%) (%) (MPa) (min) 1# 20 0 0.8-1.0 15 2# 20 2 0.8-1.0 15 2.4 Chemical composition analysis Pentosan content was tested according to GB/T2677.9, acid insoluble lignin according to GB/T2677.10, acid soluble lignin according to GB/T10337, holo-celluloses content according to GB/T2677.8, ash content according to GB/T2677.3, hot water extract according to GB/T2677.4, 1% NaOH extractive according to GB/T2677.5, benzyl alcohol extract according to GB/T2677.6, and pectin content according to GB/T10742. 2.5 Scanning Electron Microscopy (SEM) The samples were air-dried, and then glued on the sample holder and coated with carbon before SEM observation. 2.6 Specific surface area and pore size Specific surface area and pore size of samples were analyzed with a micromeritics automatic pore surface analyzer. Processing steps were carried out in accordance with the method described in the reference. 11 2.7 Fiber length and width measurement Samples were treated with 1:1 ice acetic acid and 30% H 2 O 2 solution at 60 to delignify the biomass until the fibers separated. The obtained fibers were then analyzed with a Metso FS-300 automatic fiber analyzer according to GB/T10336. 2.8 Sugar analysis of pretreatment liquor Contents of xylose, glucose, arabinose, galactose, mannose and rhamnose in solid and liquid sample were hydrolyzed with sulfuric acid and measured with an IC-5000 ion chromatograph system. 12 3. RESULTS AND DISCUSSION 3.1 Chemical composition of cornstalk biomass before and after steam explosion Table 2 and Fig.1 summarize the results on the main chemical components of cornstalks before and after steam explosion pretreatment with and without acid as the catalyst. The yield of the pretreatment was 83.84% and 72.56% for the steam alone explosion pretreatment (1# sample) and the acid-catalyzed steam explosion pretreatment (2# sample), respectively. Due to the action of acid, more organic substances were dissolved in the acid steam explosion process. From table 2, we can see that the pentosan content of cornstalks decreased after the steam explosion pretreatment. The pentosan content decreased from 19.94% to 17.02% in the steam alone explosion pretreatment, while it decreased to 16.13% in the acid-catalyzed steam explosion pretreatment. The added sulfuric acid provided more H + to accelerate the hydrolysis of hemicellulose. 13 The synergistic effect of acid-catalyzed steam explosion included the softening effect of dilute acid and the activation effect of steam explosion. 7 Dilute acid can remove most of the hemicellulose, and destroy the linkages with lignin so as to soften the fiber structure. Steam explosion can "freeze" the activated cellulose supra-molecular structure by reducing the pressure and temperature instantaneous, so that cellulase can have an easy access to the cellulose. Table 2. Chemical composition after steam explosion pretreatment Index Original 1# a 2# a 1# b 2# b (%) (%) (%) (%) (%) Oven dry 89.24 - - 83.84 72.56 Ash 3.86 5.35 4.79 4.49 3.48 Holo-cellulose 68.53 61.70 60.43 51.73 43.85 Pentosans 19.94 17.02 16.13 14.27 11.70 Acid insoluble lignin 14.50 9.18 7.42 7.70 5.38 Acid soluble lignin 3.00 5.95 4.91 4.99 3.56 Hot water extraction 20.07 13.60 14.79 11.40 10.73 1%NaOH extraction 54.05 45.15 48.79 37.85 35.40 Benzene-alcohol Extraction 3.12 6.47 9.67 5.42 7.02 Pectin 1.73 0.20 0.15 0.17 0.11 Tannin 1.58 0.68 1.58 0.57 1.15 a) Absolute content: based on the treated sample. b) Relative content based on the raw material. www.bioresources-bioproducts.com 154

% 70 60 50 40 30 20 10 Raw Material Pure steam explosion Dilute acid steam explosion were observed for the acid-catalyzed steam explosion pretreatment (Fig.2c). These changes to the fiber structure of cornstalks can improve the contact of enzyme molecules with the substrate in and thus increase the yield of reducing sugars in the subsequent fermentation process. 15 0 Holocellulose Pentosans Ligin : Fig. 1. Effects of steam explosion pretreatment on the chemical composition of cornstalks The total amount of lignin in the pretreated cornstalks decreased after the steam explosion treatment. For the steam alone explosion pretreatment, the lignin content decreased from 17.5% to 15.13%, while it decreased to 12.33% after the acid-catalyzed steam explosion pretreatment, indicating that the acid steam explosion pretreatment dissolved more lignin. However, the content of acid soluble lignin increased after the explosion pretreatment, which is particularly true for the steam alone explosion pretreatment. The steam alone explosion pretreatment probably converted more acid insoluble lignin into acid soluble lignin. Table 2 and Figure 1 show that the changes of holo-cellulose contents were the greatest, decreasing from 51.73% to 43.85%, which accounted for the major mass loss in the pretreatment. The lignin content (including acid insoluble lignin and acid soluble lignin) decreased from 12.69% to 8.94%. The contents of ash, hot water extract, 1% NaOH extractives and pectin also decreased to various extents. However, the contents of benzene-alcohol extractives and tannin increased after the steam explosion pretreatment, as the fats, wax and tannin did not dissolve under the acid conditions in the steam explosion pretreatment process. Similar to this finding, resin barrier has been reported in the literature for the acid steam explosion pretreatment. 14 3.2 Physical changes of the biomass in steam explosion pretreatment Fig.2 shows the SEM micrographs of the cornstalks before and after steam explosion pretreatment. The fibrous structure of the raw materials was relative compact and in order (Fig. 2a). After steam alone explosion pretreatment, broken fibers can be seen (Fig. 2b). More fiber damages a) Raw materials b) 1# samples c) 2# samples Fig. 2. SEM images of corn stalks before and after steam explosion treatment Table 3 shows specific surface area and mean pore size of corn stalks before and after acid steam explosion pretreatment. The specific surface area of increased by a factor of 3.5 after acid explosion pretreatment. This is highly desirable, as the increase in specific surface area will improve the effectiveness of enzymatic reactions. Table 3 also shows that the mean pore size of the biomass after the acid steam explosion pretreatment increased slightly from 18.7 nm to 19.4 nm. Table 3. Specific surface area and pore size of cornstalks before and after steam explosion pretreatment Samples Specific surface area(m 2 g -1 ) Mean pore size (nm) Raw material 0.083-1# (steam alone) 0.253 18.7 2# (acid steam) 0.288 19.4 Table 4. Fiber length, width and fines content of cornstalks before and after steam explosion pretreatment No. L(n) L(w) W (n) F (n) L/W (mm) (mm) (µm) (%) Raw material 0.29 1.37 13.78 100 56.6 1# (steam alone) 0.31 1.43 14.58 98 56.6 2# (acid steam) 0.37 1.53 14.70 104 47.6 Table 4 shows the fiber length, width and fines contents of cornstalks before and after steam explosion pretreatment. The weight-weighted average fiber length increased after steam explosion pretreatment probably due to dissolution of non-fiber cells in the pretreatment processes. Dissolution of non-fiber cells was more pronounced in the acid-catalyzed steam explosion pretreatment. 16 Fig.3 and Fig.4 shows fiber length distribution after steam explosion pretreatment. Compared with the sample www.bioresources-bioproducts.com 155

treated by the steam alone explosion, the sample treated by the acid-catalyzed steam explosion had more long fiber fractions and less short fiber fractions, indicating that more short fibers and non-fiber cells were dissolved in the acid-catalyzed steam explosion pretreatment process. Table 6. Carbohydrate analysis of the hydrolysate in acid catalyzed steam explosion pretreatment of cornstalks Xylose Arabinose Galactose Glucose Mannose Total Monosac C/g L -1 0.17 0.78 0.07 0.68-1.70 charide WP/% 0.07 0.31 0.03 0.27-0.68 Polysacc C/g L -1 1.48 0.49 0.53 1.22-3.72 haride WP/% 0.59 0.20 0.21 0.49-1.49 Total C/g L -1 1.65 1.27 0.60 1.90-5.42 sugar WP/% 0.66 0.51 0.24 0.76-2.17 Note: C: concentration; WP: weight percentage based on the oven dry raw material 12 a) b) Fig. 3. Fiber length frequency distribution (a) and weight weighted fiber length distribution of cornstalks after steam alone explosion pretreatment a) b) Fig. 4. Fiber length frequency distribution (a) and weight weighted fiber length distribution of cornstalks after acid-catalyzed steam explosion pretreatment 3.3 Carbohydrates in the steam explosion hydrolysate Table 5 shows that degraded carbohydrates in the hydrolysate were mainly polysaccharides in steam explosion pretreatment without acid, which accounted for 94.4% of the total carbohydrates in the hydrolysate. These polysaccharides were consisted with 69% glucose, 12.3% xylose, 13.0% arabinose and 5.6% galactose. Monosaccharides found in the hydrolysate were mainly arabinose and xylose. Some xylose degraded from hemicellulose could be further degraded to furfural at high temperature. 17 Table 5. Carbohydrate analysis of the hydrolysate in steam alone explosion pretreatment of cornstalks Xylos Arabino Galacto Glucos Mannos Tota e se se e e l Monosac C/g L -1 0.18 0.24 - - - 0.42 charide WP/% 0.07 0.10 - - - 0.17 Polysacc C/g L -1 0.87 0.92 0.39 4.85-7.03 haride WP/% 0.35 0.37 0.16 1.94-2.81 Total C/g L -1 1.05 1.16 0.39 4.85-7.45 sugar WP/% 0.42 0.46 0.16 1.94-2.98 Note: C: concentration; WP: weight percentage based on the oven dry raw material 12 Table 6 shows the composition of the carbohydrates found in the hydrolysate in the acid-catalyzed steam explosion pretreatment. About 68.6% of the dissolved carbohydrates were in polysaccharide form, which was much lower than the number for the steam alone explosion process. The monosaccharides found in the hydrolysate were mainly arabinose and glucose, while the polysaccharides were mainly comprised of xylose and glucose. Compared with the steam alone explosion process, the glucose content of the polysaccharides was lower in the acid-catalyzed steam explosion process, suggesting that the acid-catalyzed steam explosion process could transform part of the glucose polysaccharides to glucose monosaccharide. This notion is supported by the fact that a significant amount of glucose monosaccharide was found in the hydrolysate from the acid-catalyzed steam explosion process (Table 6). 4. CONCLUSIONS Steam explosion pretreatment changed the chemical composition and fiber characteristics of cornstalks. It was found that a substantial amount of hemicellulose and lignin were hydrolyzed and dissolved in the steam explosion processes. Damages to the fiber structure of cornstalks were evident in the steam explosion pretreatment, particularly in presence of acid. As a result, the specific surface area of the cornstalk biomass increased, which is desirable for the subsequent fermentation process. About 94.4% of total carbohydrates found in the hydrolysate were polysaccharides from the steam alone explosion process. In contrast, in the acid steam explosion process,only about 68.6% of the carbohydrates in the hydrolysate were polysaccharides. The acid conditions caused more polysaccharides dissolved in the process to hydrolyze into mono monosaccharides. ACKNOWLEDGMENTS The authors are grateful for the financial support from the Special Support Plan for High-Level Talent Cultivation of Guangdong Province (No. 2014TQ01N603), Key Laboratory of Pulp and Paper Science & Technology of Ministry of Education of China (No.KF201508) and Guangdong province science & technology plan projects (No.2015B020241001). www.bioresources-bioproducts.com 156

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