Separation of aromatic monomers from oxidatively depolymerized products of lignin by combining Sephadex and silica gel column chromatography

The conversion of lignin into value-added aromatic monomers has attracted much attention as these monomers can be potentially used to prepare liquid fuel rather than heavy oil by further hydrodeoxygenation. A new process combining Sephadex and silica gel column chromatography was employed to obtain aromatic monomers with higher purity. It was found that pH and polarity of mobile phase were the most important factors affecting the separation effect for Sephadex G-10 and silica gel column chromatography, respectively. When the pH of mobile phase reached 10.5, the ionization of hydroxyl together with carboxyl groups resulted in electrostatic repulsion between the aromatic compounds and Sephadex G-10 gel, which counteracted the adsorption caused by hydrogen bond between aromatics and gel, causing the increase of the separation effect. Under the condition of pH above 10.5, Sephadex gel column chromatography gave 78.1% of purity of aromatic monomers (PAM) and 99.5% of recovery of aromatic monomers (RAM) due to the elimination of non-size exclusion effect. Silica gel column chromatography was further conducted to obtain 99.8% of PAM and 73.9% of RAM. The polar mobile phase was favorable for eluting aromatic monomers, while the non-polar mobile phase was beneficial to remove oligomers and increasing the PAM.