Catalyst-free activation of kraft lignin in air using hydrogen sulfate ionic liquids

In this research we use ionic liquids in combination with mild process conditions to provide a selective increase in the content of carbonyl groups in the kraft lignin structure. Such modification can improve the properties of the pristine biopolymer. In this study, aromatic substituted ionic liquids were synthesized using [C₄C₁Im][HSO₄] as a template structure. The substituents were intended to increase the affinity of the ionic liquid to the aromatic structure of kraft lignin, and to increase access to the oxidizing agent, which was atmospheric oxygen. [Benzyl(C₂OC₁)Im][HSO₄] and [benzylC₄Im][HSO₄] activate the surface of the biopolymer more effectively than [C₄C₁Im][HSO₄]. This was confirmed based on X-ray photoelectron spectroscopy, which showed the content of CO groups to be almost doubled compared with unmodified lignin. The analysis also revealed targeting of the hydroxyl groups of lignin to the carboxyl groups during activation in [C₄C₁Im][HSO₄].