Transition metal catalyzed oxidation of Alcelllignin, soda lignin, and lignin model compounds in ionic liquids

Abstract

Lignin is a component of lignocellulosic biomass from which important aromatic compounds can potentially be obtained. In the present work, Alcell and soda lignin were dissolved in the ionic liquid 1-ethyl-3-methylimidazolium diethylphosphate (EMIM DEP) and subsequently oxidized using several transition metal catalysts and molecular oxygen under mild conditions. CoCl₂·6H₂O in EMIM DEP proved particularly effective for the oxidation. The catalyst rapidly oxidized benzyl and other alcohol functionalities in lignin, but left phenolic functionality and 5–5′, β-O-4 and phenylcoumaran linkages intact, as determined by analysis of various lignin model compounds and ATR-IR spectroscopy. The catalyst system oxidized the alcohol functionality contained in cinnamyl alcohol to form cinnamaldehyde or cinnamic acid or disrupted the double bond to form benzoic acid or an epoxide. The benzyl functionality in veratryl alcohol, a simple non-phenolic lignin model compound, was selectively oxidized to form veratraldehyde at a maximum turnover frequency of 1440 h⁻¹, compared to 10–15 h⁻¹ reported for earlier systems. Phenolic functional groups contained in guaiacol, syringol, and vanillyl alcohol remained intact, although the benzyl alcohol group in the latter was oxidized to form vanillin. Incorporation of strongly bound tetradentate ligands to the catalyst yielded reduced activity relative to those derived from simple metal salts in EMIM DEP. The influence of reaction conditions, such as temperature, oxygen pressure and NaOH loading, were also investigated. The system represents a potential method in a biorefinery scheme to increase the oxygen functionality in lignin prior to depolymerization or additional functionalization of already depolymerized lignin.