Demonstration of a Process for the Conversion of Kraft Lignin into Vanillin and Methyl Vanillate by Acidic Oxidation in Aqueous Methanol

In this work, a process for producing vanillin and methyl vanillate from an industrial kraft lignin by oxidation in aqueous methanol at acidic pH is proposed. A series of consecutive runs, comprising the steps of kraft lignin oxidation, monomer extraction, and filtration of residual lignin, were conducted. The depolymerization of kraft lignin (1 g per run) was Carried out in a stirred batch reactor with 80 vol % methanol/water as the solvent, with H3PMo12O40 as it homogeneous catalyst, and with 10 bar of oxygen initially present. The monomeric products were recovered from the process by extraction with chloroform and quantified by gas chromatography/mass spectrometry (GC/MS). Residual kraft lignin was recovered from the reaction medium by filtration. After extraction and filtration, the reaction medium, containing the homogeneous catalyst, was reused in the next run. The SLIM of all quantified products (gas phase, extract, and solids), as well as the amount of consumed oxygen, reached constant levels after five runs, and no indications of catalyst deactivation were found. Extracted products containing vanillin (3.5 wt %) and methyl vanillate (3.5 wt %) were recovered with a yield of 65 wt %. The extracted products were further analyzed by size exclusion chromatography (SEC) and Fourier transform infrared (FTIR) spectroscopy regarding their average molecular weight and functional group Contents.