Optimization of Phenolic Monomer Production from Solvent Liquefaction of Lignin

Process conditions used for solvent liquefaction of technical lignin in mixtures of o-cresol and tetralin were explored for optimizing the yield of phenolic monomers (PM) and liquid products. The effects of solvent mixture, reaction temperature, solids loading, and residence time were evaluated using a central composite response surface statistical model. Six response variables were monitored to evaluate the influence of the four factors. Liquid and solid yield were tracked via mass balance. The yield of distillable products (distillate) was determined using a thermogravimetric analyzer (TGA). Gas chromatography (GC) was used to monitor the individual yields of phenol, guaiacol, and 2,6-xylenol. Tetralin, which served as a hydrogen donor, was most effective in enhancing liquid production, reducing solid products, and increasing selectivity toward PM when present at less than 30 wt % of the solvent mixture. The interaction of solids loading and solvent mixture further indicated o-cresol was more effective than tetralin at solubilizing lignin and stabilizing the liquid products. The hydrogen-donating capability of tetralin was most beneficial at temperatures near 340 degrees C. Residence time was not found to be a significant factor for experiments lasting up to 30 min. Distillate yields as high as 40 wt % from lignin on a dry, ash-free basis indicated the ability of this process to generate low-boiling-point PM suitable for recycle solvent. These results demonstrate this process to be robust and effective in converting technical lignin to valuable PM.