Selective Oxidation of Lignin into Aromatic Aldehydes Using Niobium Oxalate

Abstract

Abstract. There is a strong interest in using lignin as a precursor for synthesis of value-added chemicals such as aromatic aldehydes. Among the existing processes for converting lignin into aromatic aldehydes, catalytic oxidation appears to be most promising. In this research, we explored niobium oxalate as a selective oxidation catalyst and hydrogen peroxide as an oxidant for oxidizing lignin into vanillin and syringaldehyde. Research objectives were to (1) determine the effect of catalyst mass, lignin concentration, and H 2 O 2 concentration on synthesis of aromatic aldehydes; (2) study the effect of reaction temperature; and (3) optimize the reaction parameters. Batch experiments were performed via central composite design at 95°C using 4 to 12 g of lignin per 100 mL, 3 to 7 mL of H 2 O 2 per 100 mL, and 0.5 to 1.0 g of catalyst per 100 mL. Results indicated that under optimum conditions (3 mL of H 2 O 2 , 0.5 g of catalyst, and 7.26 g of lignin), 65.58 mg g -1 of vanillin and 23.12 mg g -1 of syringaldehyde were produced. In addition, production of aldehydes showed dependency on temperature in the range of 85°C to 100°C, with 100°C producing the highest product concentrations. It is theorized that perhydroxyl anion (HOO - ), being a strong nucleophile and the most active agent in alkaline hydrogen peroxide, breaks the I± and I²-aryl ether bonds of lignin to produce corresponding aldehydes. It is suggested that niobium is a potential catalyst for selective oxidation of lignin.