Multidimensional Utilization of Kraft Lignin: Upgrading Glycerol to Sustainable Fuel Additives Using Lignin-Derived Benzaldehyde over Lignin-Based Acid Catalyst

This study aims to explore a lignin-based catalyst for the green synthesis of sustainable fuel additive using lignin-derived benzaldehyde and glycerol, byproducts from the paper industry and biodiesel preparation, respectively. This sustainable fuel additive can be refilled into biodiesel to improve the carbon atom economy, promoting resource efficiency and environmental sustainability. Kraft lignin was modified with phosphoric acid and sulfuric acid to prepare solid acid catalysts KL-HP-SO₃H. A series of characterization techniques showed that phosphorylation can markedly enhance the specific surface area, while sulfonation can notably increase the acid sites and acid strength. The synergistic physical–chemical effect of phosphorylation and sulfonation collectively enhances the activity of the KL-HP-SO₃H catalyst. In addition, ball milling and thermal catalysis were compared and analyzed in this study. The results show that the ball milling technology offers an advantage: it achieves a high product yield within only 15 min. The ball milling process consumes less energy than the hydrothermal reactor (21.3 vs 493.8 kWh·mol^–1). From a long-term investment perspective, the ball milling process is economically superior. This study contributes an efficient catalyst strategy to green chemical synthesis while providing multifunctional utilization of lignin.