Structure and thermal property of alkaline hemicelluloses from steam exploded Phyllostachys pubescens
Introduction
Lignocellulosic biomass has complex structure and is mainly comprised of three components: cellulose, hemicelluloses, and lignin. This resource can serve as a source of carbon-neutral or carbon-negative feedback for the production of biofuels, chemicals, and polymers, generally referred to as the biorefinery (Ragauskas et al., 2006). The integration of biomass and biorefinery manufacturing technologies offers the potential for the development of sustainable biopower and biomaterials that will lead to a new manufacturing paradigm (Ragauskas et al., 2006).
As we known, the most promising strategy is to integrate biofuels production into a biorefinery scheme in which the major components of the lignocellulosic biomass can be converted into value-added products to offset the costs of the whole process (Pan et al., 2005). Most biorefinery processes currently focus on cellulose as the main active participant, while less attention has been paid to hemicelluloses and lignin. Consistent with the economic requirements of a biorefinery for biomass, the recovered hemicelluloses and lignin have physiochemical characteristics suitable for the development of valuable co-products (Pan et al., 2005). In this work, we attempt to develop a biorefinery process to effectively separate cellulose, hemicelluloses, and lignin.
Lignocellulosic biomass is resistant to microbial and enzymatic deconstruction, known as biomass recalcitrance (Himmel et al., 2007). Several natural factors are believed to contribute to the recalcitrance of biomass to chemicals and enzymes, including the degree of lignification, the structural heterogeneity and complexity of cell-wall constituents, the crystalline structure of cellulose, and the inhibitors to subsequent fermentations that exist naturally in cell walls or are generated during conversion processes (Himmel et al., 2007). Therefore, biomass recalcitrance must be overcome for high-value utilization of lignocellulosic biomass. Indeed, pretreatment is one of the key steps in biorefinery process (Stark, 2011). The objective of pretreatment is to effectively fractionate hemicelluloses and lignin, and is conducive to subsequent enzymatic digestibility of cellulose. A multitude of pretreatments have been developed, and can be classified into physical, chemical, physicochemical, and biological methods (FitzPatrick et al., 2010, Galbe and Zacchi, 2007). Among various pretreatments, steam explosion is one of the most common physical methods. The advantages of steam explosion include a significantly lower environmental impact, lower capital investment, and less hazardous process chemicals (Li, Lennholm, Henriksson, & Gellerstedt, 2001). This pretreatment has been shown to be effective in opening up biomass structure, increasing the susceptibility of cellulose fibres to enzymatic hydrolysis for the production of fermentable sugars (Ramos, Breuil, Kushner, & Saddler, 1992), and assisting the fractionation of biomass components (Heitz et al., 1991, Montané et al., 1997). However, steam explosion has some limitations, such as only partial destruction of hemicelluloses and incomplete disruption of the lignin-carbohydrate matrix (Sun & Cheng, 2002). Based on the above reasons, a two-stage approach by using steam explosion pretreatment followed by sequential alkali and alkali/ethanol extractions was developed to treat biomass in this study. In this approach, the main components (cellulose, hemicelluloses, and lignin) of bamboo (Phyllostachys pubescens) can be effectively fractionated and then used for subsequent high-valued conversion.
For comprehensive understanding of this fractionation process, the chemical structures and thermal properties of the hemicelluloses obtained were studied in detail, which will be very helpful for the further optimization of this process and the utilization of hemicelluloses.
Section snippets
Materials
P. pubescens, which is a kind of popular bamboo in Jiangxi province (China), was air-dried and cut into an average size of 100 mm × 15 mm × 15 mm. The component analysis of the dewaxed P. pubescens has been determined by our group (Wen, Sun, Xue, & Sun, 2013). All chemicals purchased were of analytical or reagent grade and used without further purification.
Steam explosion pretreatment and fractionation process
Before the steam explosion pretreatment, bamboo chips were separated into three batches (300 g for each batch). To evaluate the effect of water
Effects of steam explosion pretreatment on chemical components and yields of hemicelluloses
The component analysis of the raw material (without steam explosion) has been determined by our group (Wen et al., 2013). Meanwhile, the chemical compositions of the dry material (sample 1, without water impregnation) treated at 2.0 MPa and two water-immersed samples treated at 1.8 MPa (sample 2) and 2.0 MPa (sample 3) for 5 min, are given in Table 1. As compared with the raw material, the contents of hemicelluloses and lignin in steam-exploded samples decreased, which is due to the fact that steam
Conclusion
In this study, P. pubescens chips with or without water impregnation were pretreated by steam explosion followed with alkali and alkali/ethanol extractions to fractionate hemicelluloses. Hemicelluloses fractionated from the water-immersed materials were obtained in high yields and exhibited similar compositions. The alkali-extracted hemicelluloses had relatively higher molecular weights and xylose/arabinose ratios than the alkali/ethanol-extracted hemicelluloses. This study suggests that the
Acknowledgments
The authors are extremely grateful to financial support from the Fundamental Research Funds for the Central Universities (BLYJ201315), National Natural Science Foundation of China (31110103902), Major State Basic Research Projects of China (973-2010CB732204, 2012CB215302), and State Forestry Administration (201204803).
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