Pretreatment of Lignocellulosic Materials for Efficient Bioethanol Production

Abstract

Second-generation bioethanol produced from various lignocellulosic materials, such as wood, agricultural or forest residues, has the potential to be a valuable substitute for, or a complement to, gasoline. One of the crucial steps in the ethanol production is the hydrolysis of the hemicellulose and cellulose to monomer sugars. The most promising method for hydrolysis of cellulose to glucose is by use of enzymes, i.e. cellulases. However, in order to make the raw material accessible to the enzymes some kind of pretreatment is necessary. During the last few years a large number of pretreatment methods have been developed, comprising methods working at low pH, i.e. acid based, medium pH (without addition of catalysts), and high pH, i.e. with a base as catalyst. Many methods have been shown to result in high sugar yields, above 90% of theoretical for agricultural residues, especially for corn stover. For more recalcitrant materials, e.g. softwood, acid hydrolysis and steam pretreatment with acid catalyst seem to be the methods that can be used to obtain high sugar and ethanol yields. However, for more accurate comparison of different pretreatment methods it is necessary to improve the assessment methods under real process conditions. The whole process must be considered when a performance evaluation is to be made, as the various pretreatment methods give different types of materials. (Hemicellulose sugars can be obtained either in the liquid as monomer or oligomer sugars, or in the solid material to various extents; lignin can be either in the liquid or remain in the solid part; the composition and amount/concentration of possible inhibitory compounds also vary.) This will affect how the enzymatic hydrolysis should be performed (e.g. with or without hemicellulases), how the lignin is recovered and also the use of the lignin co-product.

Ionic liquid‐mediated selective extraction of lignin from wood leading to enhanced enzymatic cellulose hydrolysis

“…Lignocellulosic biomass, such as agricultural residues, forestry wastes, waste paper, and energy crops, has long been recognized as a potential sustainable source of sugars for biotransformation into biofuels and value-added bio-based products Li et al, 2008). Conversion of lignocellulosic materials into biofuels, for example, typically includes three steps: (1) pretreatment of lignocellulose to enhance the enzymatic or microbial digestibility of polysaccharide components; (2) hydrolysis of cellulose and hemicellulose to fermentable reducing sugars; and (3) fermentation of the sugars to liquid fuels or other fermentative products (Galbe and Zacchi, 2007;Sun and Cheng, 2002;Zhang et al, 2007).…”

“…Examples include physical (e.g., limited pyrolysis and mechanical disruption/comminution, Mosier et al, 2005), physicochemical (e.g., steam explosion, ammonia fiber explosion, Grous et al, 1986;Mes-Hartree et al, 1988), chemical (e.g., acid hydrolysis, alkaline hydrolysis, high temperature organic solvent pretreatment, oxidative delignification, Chum et al, 1988;Gierer and Noren, 1982;Zhang et al, 2007), and biological (e.g., lignin degradation by white-and soft-rot fungi, Hatakka, 1983;Lee, 1997) methods. In all cases, upon sufficient removal of the lignin, there was also substantial degradation of lignin and in many cases there was substantial loss in fermentable sugar content of the residual polysaccharides (Galbe and Zacchi, 2007). The degradation of lignin is unfortunate, resulting in loss of a highly functional natural product that comprises 20-35% of the mass of lignocellulose.…”

Ionic liquid-tolerant microorganisms and microbial communities for lignocellulose conversion to bioproducts

“…These pretreatments include dilute acid (Lloyd and Wyman 2005;Saha et al 2005;Schell et al 2003), hot water (Liu and Wyman 2004;Ruiz et al 2013), ammonia fiber expansion (Hoover et al 2014;Lau et al 2008;Murnen et al 2007), steam explosion (Grous et al 1986;Kaar et al 1998), lime (Chang et al 1997;Kim and Holtzapple 2005), organic solvent (Zhang et al 2007;Zhao et al 2009b), and pyrolysis and mechanical disruption (Mosier et al 2005). In all these treatments, the substantial degradation of lignin is accompanied by considerable reduction in fermentable sugar content of the feedstock, resulting in a loss of 20-35% of the mass of lignocellulose (Galbe and Zacchi 2007;Lee et al 2009). In contrast, recent reports give ample evidence that pretreating biomass with ionic liquids can disrupt the interactions between plant cell wall polymers, resulting in significant improvements in enzymatic hydrolysis kinetics while preventing the loss of fermentable sugars and facilitating the fractionation of biomass (Cheng et al 2011;da Costa Lopes et al 2013;Li et al 2010;Li et al 2009;Mora-Pale et al 2011;Singh et al 2009;Zakrzewska et al 2010;Zavrel et al 2009).…”

Biofuel alternatives to ethanol: pumping the microbial well

“…The liberation of monomeric sugars from this biomass is the subject of intensive research efforts that might be the key to the sustainability of any biofuel process. This extremely important area of research has been reviewed elsewhere [6,7]. Here, we concentrate on another important factor for the cost-effective production of biofuels from lignocellulosic feedstocks -namely the conversion of biomass hydrolysates (monosaccharides) to target molecules.…”