Formation of Pyrolytic Lignin During Fast Pyrolysis of Biomass

Bio-oil obtained from fast pyrolysis of lignocellulosic biomass usually contains up to 30% of pyrolytic lignin, which mainly consists of phenolic oligomers. Phenolic oligomer is undesired because it contributes to thermal instability of bio-oil and catalyst deactivation in bio-oil upgrading. Formation mechanisms of phenolic oligomers and how the high molecular weight compounds are recovered in bio-oil are not well understood. Whether it is lignin fragments that are thermally ejected as aerosols or repolymerization products of phenolic monomers is still under debate. In the present study, lignin is fast pyrolyzed in a micropyrolyzer under conditions that limit secondary reactions. The pyrolysis vapor is analyzed using GC/MS and solvent-recovered (SR) liquid from the micropyrolyzer is analyzed using high resolution Fourier Transform Ion Cyclone Resonance MS (FT-ICR-MS) and orbitrap-MS utilized by Atmosphere Pressure Photoionization (APPI) to obtain nearly complete information about pyrolysis products. The origin of the products recovered from the micropyrolyzer is further investigated by pyrolyzing several phenolic monomers and dimers at the same experimental condition.

The results show that the pyrolysis products of lignin under conditions that suppress secondary reactions consist of phenolic monomers and oligomers up to molecular weight of 504. It is also found that primary phenolic monomers of lignin pyrolysis rapidly repolymerize and thermally crack via side chain rearrangement. At temperatures up to 500ºC, repolymerization is stronger than thermal cracking. The original structural components of lignin, such as phenylcoumaran, stilbene and biphenol decompose during fast pyrolysis. However, the extent was incomplete and most of their derivatives are still dimers. Therefore, phenolic dimers representing fragments of the original lignin can be primary products of lignin depolymerization during fast pyrolysis. On the other hand, resinol structure is thermally unstable and does not survive fast pyrolysis. These results suggest that primary products of lignin pyrolysis are mainly monomers and dimers. Most fractions of phenolic oligomers found in bio-oil are the repolymerization products during the vapor-phase recovery and bio-oil storage.