CO2 As an Expanded Solvent for the Fractionation of Lignin

Lignin is an abundant byproduct in both the nascent biofuel and pulp-and-paper industries. Because it is the only abundant, renewable biopolymer with aromaticity, it has been proposed as an alternative for petroleum-derived products for applications ranging from coatings to carbon fibers. However, preliminary work indicates that such applications will require the fractionation of lignin by molecular weight. In this work, CO₂ was added to acetic acid–water mixtures into which the lignin had been dissolved a priori to create expanded solvents. These systems proved to be highly effective in separating the lignin polymer into fractions with well-defined molecular weights. Precipitation of lignin fractions began at CO₂ pressures as low as 7 bar and continued up to the maximum pressure of 55 bar, where the system volume had approximately doubled. The resultant fractions were characterized by GPC, ¹H, ¹³C, and HSQC-NMR, and interesting trends in functionality were obtained, different from those currently being observed by more conventional separation techniques. Finally, the use of CO₂ as an expanded solvent facilitates the separation of the solvent system (acetic acid–water) from the anti-solvent (CO₂) by pressure reduction, eliminating the need for an energy-intensive separation step such as distillation.