Facile synthesis of vanillin from fractionated Kraft lignin

More than 90 % of global lignin production comes from Kraft pulp mills but due to shortage of economical viable methods to valorise lignin, it is commonly burned for energy. Therefore, finding new routes to utilize Kraft lignin (KL) as a renewable raw material for the chemical industry is of significant economic and environmental importance. Herein we report a novel, two-step procedure for facile synthesis of vanillin from technical KL by combining solvent fractionation and catalytic oxidation reaction. From the studied green solvents, 1-propanol was the most attractive for one-step, single solvent fractionation as it afforded uniform, low molecular weight lignin fractions (Mw=1300 g·mol−1, Mn=580 g·mol−1) with yield of 46 wt%. Using this homogeneous lignin as a raw material, CuSO4 catalyzed oxidation reaction proceeds smoothly, and under optimized conditions a high vanillin yield of 10.9 wt% was achieved. The method reported herein is promising as it facilitates straightforward and high yield vanillin synthesis from commercially available technical KL.