Physical and chemical characterizations of corn stover and poplar solids resulting from leading pretreatment technologies
Introduction
Biomass pretreatment is an essential processing step for producing biofuels with high yields via biological routes (Chandra et al., 2007, Grethlein, 1984, Yang and Wyman, 2008). Although several pretreatment methods render biomass digestible to sugars with enzymes (Gharpuray et al., 1983, Lynd et al., 1996, Morrison, 1988, Tanaka et al., 1990, Yu et al., 1998), thermochemical pretreatments are more prone to be utilized for commercial purposes over biological or mechanical options owing to shorter processing times, higher yields, limited chemical use, and lower energy requirements (Knappert et al., 1980, Mosier et al., 2005a, Mosier et al., 2005b, Puri and Pearce, 1986, Weimer et al., 1986, Wyman et al., 2005b). Over the last few decades, several thermochemical pretreatments have been shown to be promising for a variety of feedstocks. As summarized elsewhere (Mosier et al., 2005b, Sun and Cheng, 2002, Yang and Wyman, 2008), these pretreatments are known for their unique features and can be put in three categories: low pH, high pH, and neutral pH pretreatments. Typically low pH pretreatments (e.g., dilute acid, flowthrough with dilute acid, and uncatalyzed and catalyzed steam explosion with either acid or SO2 as a catalyst) remove most of the hemicellulose and a small portion of biomass lignin, Near neutral pH pretreatments such as controlled pH and flowthrough with just water remove much of the hemicellulose but leave most of the cellulose and lignin intact (Mosier et al., 2005b, Yang and Wyman, 2008). By contrast, high pH pretreatments (e.g., lime and ARP) remove a large fraction of lignin and some hemicelluloses, but AFEX is the exception that removes little of anything.
Among physiochemical changes, increased surface area, enhanced pore volume due to xylan removal, reduced cellulose degree of polymerization, increased biomass crystallinity, and melting and relocation of lignin are thought to be the most important features impacted by steam explosion and dilute acid pretreatments (Clark et al., 1989, Excoffier et al., 1991, Grethlein, 1984, Grous et al., 1986, Michalowicz et al., 1991, Saddler et al., 1982, Selig et al., 2007, Tucker et al., 1998, Wong et al., 1988). In addition, AFEX has been reported to decrease cellulose crystallinity and disrupt lignin-carbohydrates linkages (Chundawat et al., 2007, Laureano-Perez et al., 2005). Yet, such data is still lacking for other pretreatments, and to further tailor their processing conditions, it is vital to understand how these methods change substrate features and their impact on subsequent enzymatic hydrolysis. In addition, performance of these leading pretreatments may vary with feedstock type, and causes for this variability have not been researched well. Finally, these pretreatments have never been fully characterized using common sources of feedstock or enzymes.
In this study, we sought to understand physiochemical changes resulting from pretreatments by leading options of AFEX, ARP, controlled pH, dilute acid, flowthrough, lime, and SO2 with corn stover, an agriculture residue, and poplar, a woody biomass. Chemical characterizations were performed to determine glucan, xylan, lignin, and acetyl contents for untreated and pretreated corn stover and poplar solids resulting from the leading pretreatments. Physical characteristics including cellulose crystallinity, cellulose degree of polymerization (DP), copper number, cellulase adsorption capacity of biomass solids and enzymatically extracted lignin solids, and changes in surface elemental composition were also measured.
Section snippets
Materials
Pure cellulose, Avicel PH-101, was purchased from FMC Corporation, Philadelphia, PA (Cat 11365, Lot 1094627). Regenerated amorphous cellulose (RAC) was prepared from Avicel PH 101 according to a method reported by Zhang and Lynd (2005). Bacterial cellulose was a commercially available product called CHAOKOH® (coconut gel in syrup, Thep. Padung Porn Coconut Co. Ltd, Bangkok, Thailand) that was further purified according to a procedure reported elsewhere (Kipper et al., 2005, Väljamäe et al., 1999
Compositional analysis
Compositions, preparation conditions, and severity parameter values are reported in Table 1 for untreated and pretreated solids, and the amount of each component left after pretreatment for both corn stover and poplar are presented in Table 3. Although no direct relation was found between severity level (shown in Table 1) and xylan removal by pretreatment, low pH pretreatment removed a major portion of xylan, and xylan removal increased with severity for a given pretreatment. The high pH
Discussion
Table 10 summarizes the physical and chemical features characterized in this first-of-a-kind study of solids produced by application of leading pretreatments to shared sources of corn stover and poplar wood, the expected impacts on sugar release by enzymatic hydrolysis based on literature reports, and the impact of each pretreatment on these features. As noted, AFEX and lime pretreatments removed the least and most acetyl groups, respectively, from the xylan backbone for both substrates.
Acknowledgements
Support from the US Department of Energy Office of the Biomass Program (Contract DE-FG36-04GO14017) and the National Institute of Standards and Technology (award 60NANB1D0064) made this research possible. We are also grateful to the Center for Environmental Research and Technology of the Bourns College of Engineering at the University of California, Riverside and the Thayer School of Engineering at Dartmouth College for providing key equipment and facilities. We are thankful to Dr. Frank Fisher
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