Lignin with Tunable and Predictable Yield and Molecular Properties

To control and predict lignin properties remains very challenging due to the complexity of chemical structures and recovery methods of lignin. Recently, an acidcatalyzed one-pot liquefaction technique was developed to produce Kraft lignin with improved molecular uniformity directly from black liquor. Herein, we investigated the effects of the liquefaction parameters (pH, reaction temperature, and reaction time) on the yield, molecular weights, polydispersity, and quantities of different types of hydroxyl groups of the Kraft lignin using the Box-Behnken response surface methodology (RSM). Computational models were generated and refined to establish the relationships between the liquefaction parameters and the Kraft lignin properties. The results showed that pH was the most influential factor followed by the reaction temperature affecting the properties of the Kraft lignin. The yield, molecular weight, and polydispersity were found to be more predictable (R-(pred)(2) values of 87.5-91.5%) than the type and quantity of hydroxyl groups (R-(pred)(2) values of 0) of the Kraft lignin. Additionally, the weight average molecular weight (Mw) could be used as a reliable predictor for both the number average molecular weight (M-n) and the polydispersity of the Kraft lignin, which was confirmed by both the experimental and the computational approaches. Such tunable and predictable molecular properties of the lignin may be associated with the combination of acetic acid, subcritical methanol, and one-pot method. This study provided insights into understanding, predicting, and even customizing the properties of the lignin products.