Exploring the reactivity of aliphatic and phenolic hydroxyl groups in lignin hydrogenolysis oil towards urethane bond formation

Lignin is the second most abundant natural polymer that exhibits a complex structure with various amounts of aliphatic and phenolic hydroxides. The use of lignin as a source of polyol to make lignin-based polyurethanes (PUs) has been the subject of intense studies in recent years. It is well known that there is a difference in reactivity between aliphatic and aromatic hydroxyl groups towards isocyanates. For simple model systems using conventional fossil-fuel based polyols the overall kinetics are well known. However, unveiling the complex kinetic behavior obtained when combining an isocyanate with lignin derived components exhibiting different types of hydroxyl groups is challenging. Therefore, an in-depth investigation using lignin model compounds and lignin hydrogenolysis oil to elucidate these differences of reactivity is presented and will be invaluable towards the development of lignin-based PUs. Understanding the kinetics involved will lead to optimized synthetic protocols and a higher valorization potential for lignin-based PUs. In this work, various monolignol model compounds are studied concerning their reactivity to aliphatic and aromatic isocyanates. Isothermal experiments revealed the kinetic rate constants and reaction orders while non-isothermal DSC was used to obtain the activation energies through the Friedman isoconversion model-free-kinetics (MFK) approach. These results were confirmed with FT-IR and 1H NMR. It was found that in all cases, aliphatic hydroxide groups react the fastest with aromatic isocyanates while aromatic hydroxide groups react the slowest with aromatic isocyanates. Reactions with aliphatic isocyanates, independent of the hydroxyl type, fall in between these extremes shown by the aromatic isocyanates. Apparent activation energies ranging from 55.2 kJ·mol-1 to 39.5 kJ·mol-1 were found with corresponding reaction rate constants ranging from 5.19·10-3 M1−n s-1 to 1.04·10-3 M1−n s-1. The presented work will be a capstone for the further valorization of lignin as a material for renewable PUs.