Mild and controlled lignin methylation with trimethyl phosphate: towards a precise control of lignin functionality†
The methylation of lignin is a key reaction for many different purposes, such as increasing the thermal stability and controlling lignin functionality for polymer applications, and increasing the yield of aromatic hydrocarbons during lignin pyrolysis. Methylation most often requires the use of toxic reagents, such as dimethyl sulfate. We developed an alternative protocol based on trimethyl phosphate as a safer and milder methylating reagent. The reaction proceeds without a solvent for only one hour to ensure complete and selective methylation of phenolic hydroxyl groups. This specific protocol was successfully applied to several lignins from different botanical origins (softwood, hardwood, and annual plants) and fractionation processes (Kraft, soda, and organosolv). This approach further allows a precise control of methylation in order to prepare lignins with tunable contents of phenolic hydroxyl groups for a large range of potential applications.