Structural Modification of Lignin and Characterization of Pretreated Wheat Straw by Ozonation

Abstract

Ozonolysis is potentially an effective method for pretreating lignocellulosic biomass to improve the production of fermentable sugars via enzymatic hydrolysis. Further understanding of the ozonolysis process and identifying specific lignin structural changes are crucial for improving the pretreatment process. Investigation into pretreatment of wheat straw using ozonolysisis is reported in this paper, with special emphasis on selective modification/degradation of lignin subunits. The ozonolysis was performed for 2 h with less than 60 mesh particles in order to achieve maximum lignin oxidation. The results showed that the lignin structure was significantly modified under these conditions, leading to higher sugar recovery of more than 50% which increased from 13.11% to 63.17% corresponding to the control and ozone treated samples, respectively. Moisture content was found to be an important parameter for improving sugar recovery. Ninety percent (w/w) moisture produced the highest sugar recovery. The concentration of acid soluble lignin in the ozone treated sample increased from 4% to 11% after 2 h treatment. NMR analysis revealed that the S2/6 and G2 lignin units in the wheat straw were most prone to oxidation by ozone as the concentration of aromatic units decreased while the carboxylic acids became more abundant. The experimental data suggest the degradation of β-O-4 moieties and aromatic ring opening in lignin subunits. The pyrolysis-gas chromatography/mass spectrometry results revealed that the rate of lignin unit degradation was in the following order: syringyl > guaiacyl > p-hydroxyphenyl. Long ozone exposure resulted in few condensed lignin structure formation. In addition, the formation of condensed units during this process increased the activation energy from ASTM-E, 259.74 kJ/mol; Friedman-E, 270.08 kJ/mol to ASTM-E, 509.29 kJ/mol; Friedman-E, 462.17 kJ/mol. The results provide new information in overcoming lignin barrier for lignocellulose utilization.

Relationships between Biomass Composition and Liquid Products Formed via Pyrolysis

“…Grass lignins also contain G-and S-units with slightly higher amount of H-units than woody dicots. Wheat straw, for example, has a G:S:H ratio of 64:30:6 (Bule et al, 2013). Grass lignin possesses high levels of p-coumarate esters (Hatfield et al, 2008) and can also be etherified by tricin and ferulic acid (Ralph et al, 1995;Lan et al, 2015), as discussed further below.…”

Ozonolysis

“…Various mechanisms have been proposed for this lignin degradation by ozone attack, including selective reaction with carbonÀcarbon double bonds, high reactivity toward aromatics and glycosidic bond cleavage [5].…”

“…In the presence of oxygen and ozone, superoxide is easily converted to the hydroxyl radical, and vice versa [42]. Moisture content interferes with the efficiency of ozone pretreatment, owing to the generation of reactive hydroxyl radicals [45] or the possible reaction of hydroxyl radicals in addition to ozone with lignin in the presence of water [5]. The hydroxyl radicals react with carbohydrates and possibly result in random cleavage of glycosidic bonds.…”

“…Chemical pretreatments are considered to be the most promising [4], and ozonolysis is potentially an effective oxidative lignocellulosic biomass pretreatment for improving the production of fermentable sugars via enzymatic hydrolysis [5]. Ozone is a powerful oxidant; it is soluble in water and is readily available, and has been widely used for pulp bleaching in the paper industry [6,7].…”

“…Unwashed samples could not be fermented, because of the inhibition effect of lignin degradation products (carboxylic acids and phenolic compounds). Bule et al [5] investigated lignin structural modification of wheat straw during ozone pretreatment, hypothesizing in that study that ozone targets lignin subunits either through structural modification or by opening the aromatic ring to reduce recalcitrance for enzymatic hydrolysis. Ozonolysis pretreatment increased sugar recovery from 13% (with untreated sample) to 63% (with 90% w/w moisture ozonated wheat straw particles).…”