Lignin Valorization through Thermochemical Conversion: Comparison of Hardwood, Softwood and Herbaceous Lignin

In the present study, milled wood lignin (MWL) and organosolv lignin isolated from red oak (hardwood), loblolly pine (softwood) and corn stover (herbaceous biomass) were characterized by TGA, elemental analyzer, GPC, FTIR, 2D-HSQC NMR, and then pyrolyzed in the absence and presence of a zeolite catalyst. For all three biomass species, organosolv lignins contained fewer volatiles in comparison to the corresponding MWLs. Red oak lignin was affected most by the organosolv process, evident by the greatest decrease in volatile content and increase in carbon content of the organosolv lignin. Compared to the corresponding MWLs, organosolv lignins produced more char and less phenolic oil upon pyrolysis. Organosolv lignins also convert to catalytic coke and light hydrocarbons in higher selectivity in comparison to the MWLs during catalytic pyrolysis. When pyrolyzed, corn stover MWL produced 16.26% of phenolic monomers, which is a significantly higher yield compared to 8.61% from red oak MWL and 9.51% from loblolly pine MWL. During catalytic pyrolysis, corn stover lignins also produced higher yields of aromatic hydrocarbons in comparison to red oak or loblolly pine derived lignins. Overall, corn stover lignin had the highest potential for volatilization because it retains highly branched polymer structure enriched in tricin, ferulate and coumarate groups.