(165e) Thermal Degradation Mechanisms of Lignin

Lignin is an important component of lignocellulosic material that contains 40% of biomass energy. During pyrolysis, lignin is mainly converted into phenolic monomers, and phenolic oligomers/polymers (collectively referred to as pyrolytic lignin). While the yields of phenolic monomers in bio-oil are relatively low, the yield of pyrolytic lignin can be up to 30%. Compared to phenolic monomers, pyrolytic lignin is less desirable since it contributes to the thermal instability and high viscosity of bio-oil.  There is an ongoing debate on the formation mechanisms of phenolic monomers and pyrolytic lignin. One proposed mechanism is that pyrolytic lignin is thermally ejected from the biomass. Another is that pyrolytic lignin is formed by repolymerization of phenolic monomers in vapor phase. In this work, the thermal degradation mechanism of lignin was investigated through the pyrolysis of organosolv lignin, phenolic monomers, and pyrolytic lignin extracted from bio-oil. Several different analytic techniques, including thermogravimetric analysis connected with mass spectrometry (TGA-MS), gel permeation chromatography (GPC) and pyrolyzer-gas chromatography/mass spectrometry (Py-GC/MS), were adopted to systematically investigate the formation mechanisms of phenolic monomers and pyrolytic lignin.