Structural changes of corn stover lignin during acid pretreatment

Abstract

In this study, raw corn stover was subjected to dilute acid pretreatments over a range of severities under conditions similar to those identified by the National Renewable Energy Laboratory (NREL) in their techno-economic analysis of biochemical conversion of corn stover to ethanol. The pretreated corn stover then underwent enzymatic hydrolysis with yields above 70 % at moderate enzyme loading conditions. The enzyme exhausted lignin residues were characterized by ³¹P NMR spectroscopy and functional moieties quantified and correlated to enzymatic hydrolysis yields. Results from this study indicated that both xylan solubilization and lignin degradation are important for improving the enzyme accessibility and digestibility of dilute acid pretreated corn stover. At lower pretreatment temperatures, there is a good correlation between xylan solubilization and cellulose accessibility. At higher pretreatment temperatures, lignin degradation correlated better with cellulose accessibility, represented by the increase in phenolic groups. During acid pretreatment, the ratio of syringyl/guaiacyl functional groups also gradually changed from less than 1 to greater than 1 with the increase in pretreatment temperature. This implies that more syringyl units are released from lignin depolymerization of aryl ether linkages than guaiacyl units. The condensed phenolic units are also correlated with the increase in pretreatment temperature up to 180 °C, beyond which point condensation reactions may overtake the hydrolysis of aryl ether linkages as the dominant reactions of lignin, thus leading to decreased cellulose accessibility.

Process Design and Economics for the Conversion of Lignocellulosic Biomass to Hydrocarbons: Dilute-Acid and Enzymatic Deconstruction of Biomass to Sugars and Biological Conversion of Sugars to Hydrocarbons

“…For example, high severity dilute acid pretreatment is thought to catalyze aryl-ether (C-O) bond cleavage in lignin and promote recondensation of C-C linkages, likely rendering lignin more recalcitrant [135,136]. Thus, the nature of the upstream pretreatment chemistry intended to (in some cases) remove hemicellulose, redistribute lignin, and render biomass more digestible by enzymes should be considered in light of the intended downstream lignin conversion process.…”

Pyrolytic production of phenolic compounds from the lignin residues of bioethanol processes

“…The higher S/G ratios of the corn stover lignin and the rice straw lignin (see Table 4) also caused reactive thermolysis because of the additional chances from one more -OCH 3 group of a syringyl unit [20]. On the other hand, the ReLignin had a much higher temperature range for primary pyrolysis than even the Kraft lignin because acid pretreatment might release more syringyl units from lignin depolymerization and, thus, could induce more condensed phenolic units with stronger thermal stability [31].…”

“…The total number of phenolic hydroxyl groups in the lignin complexes would also be increased by the acid pretreatment [31]. However, autohydrolysis (hot water only pretreatment) of lignin with increasing temperatures resulted in a decrease in molecular weight [35].…”

Lignin Structural Alterations in Thermochemical Pretreatments with Limited Delignification

“…Evidence has also suggested that syringyl units were more readily degraded than guaiacyl units under dilute acid pretreatment conditions, resulting in a lower S/G ratio in lignin in pretreated biomass [9]. In addition, DAP results in the formation of new phenolic groups due in part to the cleavage of β-O-4 aryl ether bonds [85]. Other structural changes such as a decrease in methoxyl group content, hydrolysis of acetyl groups, and cinnamaldehyde units are also observed during dilute acid pretreatment [9].…”