The effect of ionic liquid cation and anion combinations on the macromolecular structure of lignins

Imidazolium based ionic liquids (ILs) composed of anions such as chloride, acetate and alkyl phosphate have come to be considered as effective non-derivatizing solvents for cellulose, lignin and lignocellulosic biomass. After dissolution and thermal treatment of three technical lignins (organosolv, alkali and alkali low sulphonate) with an array of ILs, it was shown that these solvents behave as either reactants or catalysts, significantly reducing the molecular mass of these macromolecules and altering their structure. The degree of lignin structural modification is shown to be primarily influenced by the anion. Lignin fragmentation mechanisms were defined and a fragmentation hierarchy of the lignin macromolecule as a function of the IL anion was established. It was determined that sulfates > lactate > acetate > chlorides > phosphates in terms of the relative impact on reducing lignin molecular weight, with evidence of different anions causing cleavage of different linkages within the lignin. Of the ILs studied, sulfate based ionic liquids most comprehensively broke down the largest lignin molecules, resulting in fragments >1000–3000 u (by polysaccharide calibration). The lactate anion, while appearing less capable of breaking down the largest lignin molecules, causes the formation of significant quantities of the smallest sized fragments observed (2000–500 u). The new lower molecular mass species formed from the organosolv lignin are shown to have a more highly conjugated structure than their parent molecules, while a reduction in conjugation was observed in the alkali lignins. Using size exclusion chromatography coupled with UV detection, at least 40% of the original large-lignin molecules, from each of the lignins studied, were observed to remained intact. We hypothesize that fragmentation is effected either via catalytic means or through nucleophilic attack of inter-lignin β-O-bonds.