Combination of liquid hot water pretreatment and wet disk milling to improve the efficiency of the enzymatic hydrolysis of eucalyptus
Highlights
• Two environmentally friendly pretreatments were combined to enhance the sugar yield. • The concentration of fermentation inhibitors in the prehydrolyzate are discussed. • Compositions of water-insoluble solids resulting from pretreatment are discussed. • Find out the optimal pretreatment conditions basis on the total sugars yield. • A further analysis of insoluble solid resulting from enzymatic hydrolysis.
Introduction
Due to a lack of resource sustainability of fossil fuels as well as negative environmental effects from emissions, research is being done to find a liquid fuel for use as a gasoline replacement (Arthur et al., 2011). Lignocellulosic materials are the most abundant biomass available to the world (Buranov and Mazza, 2008). The polysaccharides present in the lignocellulosic materials can be depolymerized to monosaccharide and then converted to ethanol via appropriate processes (Shuai et al., 2010). However, the natural structures of lignocellulosic materials make it hard for microorganism to utilize this material to produce ethanol. Therefore, efficient pretreatment method is needed so that sequential enzymatic hydrolysis gives maximal sugar productivity and at the same time any loss of sugar is minimized (Jorgensen et al., 2007).
Pretreatment techniques to overcome this recalcitrance are essential for saccharification and bioethanol production from lignocelluloses (Hendriks and Zeeman, 2008) in future biorefineries (Himmel et al., 2007). Various pretreatments using sulfuric acid, hydrochloric acid, formic acid and alkali reagents have been attempted (Cara et al., 2008, Vanderghem et al., 2012, Zhu and Pan, 2010). However, pretreatments using these chemicals have some problems such as the formation of inhibiting compounds for saccharification and subsequent ethanol fermentation and the pH requirements for downstream processes (Sun and Cheng, 2002). Moreover, the use of strong acids such as sulfuric acid in pretreatments has significant environmental risks.
The development of environmentally friendly pretreatments that do not involve chemicals, such as ball milling (Endo et al., 2006), wet disk milling (Hideno et al., 2012) and liquid hot water pretreatment (Goh et al., 2010, Liu and Wyman, 2005), has been studied. However, they also have disadvantages. For example, ball milling has a high energy requirement and is not economically feasible in general (Inoue et al., 2008). Liquid hot water pretreatment, in which pressure is utilized to maintain water in the liquid state at elevated temperatures, has been reported to have the potential to remove most hemicelluloses, but the water-insoluble solid resulting from pretreatment are not very good for enzyme hydrolysis (Yu et al., 2010). To eliminate these disadvantages, Inoue et al. investigated ball milling combined with hot-compressed water treatment of eucalyptus, which can save energy for pretreatment and enzyme loading for enzymatic hydrolysis (Inoue et al., 2008). However, there have been very few reports about wet disk milling combined with liquid hot water pretreatment (LHWP), especially, for eucalyptus raw material.
The objective of this work is to enhance the sugar recovery yield both in prehydrolyzate and enzymatic hydrolyzate through combination of liquid hot water pretreatment (LHWP) and wet disk milling (WDM). In addition, the compositions of water-insoluble solids resulting from liquid hot water pretreatment and the concentration of fermentation inhibitors such as acetic acid, HMF, and furfural in the prehydrolyzate are also discussed. Finally, there is a further analysis of insoluble solid resulting from enzymatic hydrolysis, found that most of the acid insoluble lignin can be recycled in the enzymatic hydrolysis residues.
Section snippets
Raw material
The eucalyptus chips used in this study was provided by a local factory near LongYan, FuJian, China. The raw materials were air-dried and then milled to give a size less than 5 mm by a Rub silk machine. The chemical composition of the raw material (on a dry weight basis) was 42.6% glucose, 15.4% xylose, 28.3% acid-insoluble lignin, 2.4% acid-soluble lignin, 3.3% extractives and 8.0% others.
Liquid hot water pretreatment
Liquid hot water pretreatment was performed in a laboratory scale stirred autoclave. The reactor has a
Compositions of WIS resulting from LHWP
The total gravimetric recovery (solids remaining after liquid hot water pretreatment divided by original oven-dried weight) and the composition of water-insoluble solids resulting from liquid hot water pretreatment are summarized in Table 1. From the table, it can know that cellulose and acid-insoluble lignin is the major chemical composition of WIS, the content of hemicelluloses, ASL, and other material is relatively small. Part of the material was dissolved during liquid hot water
Conclusions
Wet disk milling (WDM) with liquid hot water pretreatment (LHWP) at 180 °C for 20 min produced maximum xylose and glucose yields of 91.62% and 88.12%, respectively, which are higher than that of dilute acid pretreatment or individual LHWP. Corresponding concentration of acetic acid, HMF, and furfural in the prehydrolyzate are about 0.98, 0.07 and 0.78 g/L, respectively, which indicated that the detoxification methods are not required in next fermentation step. 90.7% of acid-insoluble lignin can be
Acknowledgements
The authors would like to thank Genencor, Inc. (Jiangsu, China) for providing enzymes for this research. This research was supported financially by the Science and Technology Department of Guangdong Province, China (No. 2010y1-C071), the National Natural Science Foundation of China (No. 21176095), and the Major Research Projects of Guangdong Province, China (No. 2011A090200006).
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