(198e) Catalytic Depolymerization of a Phenolic β-O-4 Model Compound in Ionic Liquids

Despite its relatively large abundance (approx. 30 wt. % of biomass), lignin has historically received less attention than cellulose as a renewable source of commodity chemicals. Due to its significant aromatic functionality, lignin is a potential feedstock for the production of aromatic monomers, such as phenol, benzene and toluene.  However, the predominant use of lignin recovered from alkaline pulping of wood is as an additive in various adhesives or as a low-value heating fuel.  The lack of environmentally benign and selective catalysts for cleavage of major linkages in lignin is one of the challenges inhibiting its utilization for renewable fuels and value-added chemicals.  Ionic liquids (ILs) are a potential green reaction media for lignin depolymerization because of their high solubility of milled wood, negligible vapor pressure and tunable solvation properties.

This work investigates commercially available guanidine bases for decomposition of a phenolic β-O-4 lignin model compound in imidazolium ILs from 100 to 150°C. Guanidines successfully cleave the β-O-4 linkage in the aromatic dimer, guaiacylglycerol-β-guaiacyl ether (GG) to varying degrees, though stoichiometric amounts of coniferyl alcohol and guaiacol were not observed during the conversion of GG.  Coniferyl alcohol is known to be unstable in aqueous alkaline environment and readily undergoes condensation reactions to produce higher molecular weight products. The stability of coniferyl alcohol in ionic liquids has not been studied extensively, but preliminary results indicate that it is unstable in ionic liquids as well.  LC-MS confirms major reaction products are guaiacol (~30% yield) and various higher molecular weight condensation products. This supports formation of a quinone methide intermediate that participates in multiple reaction pathways, such as β-O-4 cleavage and Michael addition, depending on the respective activation energy barrier. This work studies the effect of IL cation/anion pair on the activation energy and selectivity for guaiacol liberation from GG.  Future lignin depolymerization strategies should employ optimized base catalyst/ IL systems that favor β-O-4 cleavage, while suppressing the formation of thermodynamically stable condensation products.