Cationic lignin as an efficient and sustainable homogenous catalyst for aqueous Knoevenagel condensation reactions

Knoevenagel condensation is a chemical reaction between aldehydes and active methylene-containing compounds in the presence of heterogeneous, basic homogenous organic or inorganic catalysts and solvent or neat systems. Herein, we introduced a new strategy for this synthesis by using the aqueous solution of cationic kraft lignin (CKL) as a catalyst. The CKL was synthesized through the reaction of kraft lignin (KL) with glycidyltrimethylammonium chloride (GTMAC) in a basic medium. The optimal reaction conditions for the Knoevenagel reaction were 5% catalyst load (weight of catalyst to the weight of benzaldehyde), water as the solvent, and at room temperature, which generated the products with a yield of 97%, illustrating that the CKL was an effective homogenous and green catalyst. The results confirmed that the increase in CKL charge density improved the product yield. The water-insoluble products were easily separated by filtration, and the filtrate containing the catalysts was reused effectively for 5 cycles without a significant decrease in the production yield, which would confirm the advantages of this catalyst for this reaction system. The CKL catalyst exhibited biodegradability comparable to KL. This paper discusses a novel method for Knoevenagel condensation reactions for different aldehydes in a green system utilizing a sustainable, biodegradable catalyst at room temperature and in an aqueous system.