Separation of lignin oxidation products by organic solvent nanofiltration

The emerging field of organic solvent nanofiltration (OSN) opened up novel opportunities in fractionation and purification of polydisperse mixtures during the last decade. In this work we report about the application of OSN for the valorization of lignin as a renewable source of chemicals. Oxidation of lignin yields high-value monomeric chemicals including vanillin and methyl vanillate which have to be separated from the remaining mixture of high molecular weight reaction products. Five membranes from the image, image and image series with a molecular weight cut-off (MWCO) in the range of 250–900 g mol−1 were characterized concerning solvent flux of ethyl acetate and rejection of four model compounds. Afterwards, four of the membranes were employed in the separation of the lignin product mixture which has a weight average molecular weight of approximately 1150 g mol−1. Retentate samples were analyzed concerning monomer content and remaining molecular weight distribution by gas chromatography/mass spectrometry (GC/MS) and size-exclusion chromatography (SEC). All four studied membranes proved to be very effective in separating monomeric products from the reaction mixture. The best results in terms of permeate flux and rejection of high molecular weight compounds were obtained with image S380, which therefore turned out to be the most appropriate membrane for the desired separation step.