Batch microreactor studies of lignin and lignin model compound depolymerization by bases in alcohol solvents

The depolymerization of Kraft- and organosolv-derived lignins by KOH in supercritical methanol or ethanol was studied in rapidly heated batch microreactors. High conversions were realized, with only 7% ether insoluble material remaining after treating organosolv lignin in KOH/ethanol at 290°C. The conversion was rapid, reaching the maximum value within 10–15 min. Further studies with KOH, NaOH, CsOH, LiOH, Ca(OH)2, and Na2CO3 indicate that strong bases give superior conversion, and that combinations of bases can give both positive and negative synergistic effects. An excess of base relative to lignin monomer units was required for maximum conversion. Model compound studies confirm the dominant depolymerization route is the solvolysis of ether linkages. Ethanol solvent is also partially converted to acetic acid, higher alcohols, and other products in addition to alkylating phenolic rings. The formation of acidic products may account for the excess base requirement.