Physical and chemical characterizations of corn stover and poplar solids resulting from leading pretreatment technologies

Abstract

In order to investigate changes in substrate chemical and physical features after pretreatment, several characterizations were performed on untreated (UT) corn stover and poplar and their solids resulting pretreatments by ammonia fiber expansion (AFEX), ammonia recycled percolation (ARP), controlled pH, dilute acid, flowthrough, lime, and SO₂ technologies. In addition to measuring the chemical compositions including acetyl content, physical attributes determined were biomass crystallinity, cellulose degree of polymerization, cellulase adsorption capacity of pretreated solids and enzymatically extracted lignin, copper number, FT-IR responses, scanning electron microscopy (SEM) visualizations, and surface atomic composition by electron spectroscopy of chemical analysis (ESCA). Lime pretreatment removed the most acetyl groups from both corn stover and poplar, while AFEX removed the least. Low pH pretreatments depolymerized cellulose and enhanced biomass crystallinity much more than higher pH approaches. Lime pretreated corn stover solids and flowthrough pretreated poplar solids had the highest cellulase adsorption capacity, while dilute acid pretreated corn stover solids and controlled pH pretreated poplar solids had the least. Furthermore, enzymatically extracted AFEX lignin preparations for both corn stover and poplar had the lowest cellulase adsorption capacity. ESCA results showed that SO₂ pretreated solids had the highest surface O/C ratio for poplar, but for corn stover, the highest value was observed for dilute acid pretreatment with a Parr reactor. Although dependent on pretreatment and substrate, FT-IR data showed that along with changes in cross linking and chemical changes, pretreatments may also decrystallize cellulose and change the ratio of crystalline cellulose polymorphs (Iα/Iβ).

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.