Mechanistic insights into hydroxynaphthoic acid-based suppression of lignin repolymerization

As a renewable biomass resource, lignocellulose holds significant potential for the production of chemicals and fuels, due to its vast availability and low cost. However, its complex and chemically stable structure poses a major challenge to its efficient utilization. Acidic pretreatment methods (including autohydrolysis) are widely employed to improve the conversion efficiency of lignocellulose into valuable chemicals. While the acidic environment promotes the breakdown of the lignocellulose structure, facilitating its (bio)chemical conversion, it also induces lignin repolymerization reactions, thereby hindering the pretreatment and further valorisation of cellulose and lignin. The use of carbocation scavengers to cap reactive sites (carbocations) formed during the pretreatment process in lignin, has recently proven to be an effective method for mitigating these issues. Therefore, the discovery and evaluation of effective scavengers has emerged as a key research focus in lignocellulose valorisation. In this study, we investigate the performance of three hydroxynaphthoic acid isomers - 3-hydroxy-2-naphthoic acid, 6-hydroxy-1-naphthoic acid, and 6-hydroxy-2-naphthoic acid - as potential scavengers for suppressing lignin repolymerization. Fukui function calculations were employed to determine the most favourable nucleophilic sites within each compound. In addition, the acid-catalysed dehydration and cation scavenging processes were simulated using a representative lignin cluster model. Our results demonstrate that the binding interactions between the scavengers and the lignin-derived carbocations at favorable active sites play a critical role in suppressing repolymerization. Among the hydroxynaphthoic acid tested, 3-hydroxy-2-naphthoic acid exhibited the strongest binding affinity with the lignin carbocation (-16.8 kJ mol^-1) and the most significant scavenging effect. This result was further validated experimentally, by studying the consumption of the three hydroxynaphthoic acids and their effect on the lignocellulose structure in the autohydrolysis pretreatment of spruce wood. Among the investigated scavengers, 3-hydroxy-2-naphthoic acid showed the lowest recovery (2%) after pretreatment at 210 °C, indicating its high reactivity. Moreover, evaluation of the scavenger effect on lignocellulose enzymatic digestibility revealed that 3-hydroxy-2-naphthoic resulted in the highest glucose yield.