Impact of Pseudolignin versus Dilute Acid-Pretreated Lignin on Enzymatic Hydrolysis of Cellulose

To evaluate the inhibition effects of pseudolignin to enzymatic hydrolysis of cellulose in comparison to lignin, enzymatic mild acidolysis lignin (EMAL) was isolated from poplar after an 8 min pretreatment at 170 °C using 0.5% H₂SO₄. Fourier transform infrared (FT-IR) and ¹³C NMR characterization revealed that the poplar lignin was partially degraded during the pretreatment and did not contain detectable amounts of pseudolignin. Holocellulose was treated with varying amounts of pseudolignin and/or EMAL dissolved in p-dioxane and then dried. The treated and control holocellulose was then treated to a standard cellulase treatment, and the results from enzymatic hydrolysis of these samples showed that the dilute acid-pretreated lignin inhibited hydrolysis in the initial stage but had a negligible impact on the overall cellulose-to-glucose conversion yield. In contrast, pseudolignin significantly reduced the overall enzymatic conversion yield of cellulose to glucose. This study suggests that pseudolignin formation needs to be avoided because it is more detrimental to enzymatic hydrolysis of cellulose than dilute acid-pretreated lignin.