Upgrading lignin from lignin-containing residues through reactive extraction

A method for producing a functionalized lignin, the method comprising:

• exposing a biomass to a first fluid comprising water, thereby forming a first mixture comprising:
• a first liquid fraction;
• and
• a first solid fraction comprising a residue, wherein the residue comprises lignin and cellulose;
• wherein the first fluid has a first temperature of at least about 350° C. and a first pressure of at least about 180 bar;
• and
• subjecting the residue to a liquid comprising an organic compound, thereby forming a second mixture, wherein the subjecting is performed at a second temperature of about 100° C. or less and for a time of less than about 225 minutes, and wherein the second mixture comprises:
• a second liquid fraction comprising a solubilized functionalized lignin functionalized with the organic compound;
• and
• a second solid fraction comprising cellulose.