Structural Characterization and Comparison of Switchgrass Ball-milled Lignin Before and After Dilute Acid Pretreatment

To reduce the recalcitrance and enhance enzymatic activity, dilute H₂SO₄ pretreatment was carried out on Alamo switchgrass (Panicum virgatum). Ball-milled lignin was isolated from switchgrass before and after pretreatment. Its structure was characterized by ¹³C, HSQC, and ³¹P NMR spectroscopy. It was confirmed that ball-milled switchgrass lignin is of HGS type with a considerable amount of p-coumarate and felurate esters of lignin. The major ball-milled lignin interunit was the β-O-4 linkage, and a minor amount of phenylcoumarin, resinol, and spirodienone units were also present. As a result of the acid pretreatment, there was 36% decrease of β-O-4 linkage observed. In addition to these changes, the S/G ratio decreases from 0.80 to 0.53.