Characterization of the water-insoluble fraction from fast pyrolysis liquids (pyrolytic lignin). Part IV: Structure elucidation of oligomeric molecules

Various wet chemical and spectroscopic methods used in lignin chemistry as well as pyrolysis-gas chromatography/mass spectrometry were applied to elucidate the structure of pyrolytic lignins (PLs) with the help of previously obtained data on molecular weight distribution and laser desorption ionization-time of flight-mass spectrometry. PL from beech wood (Fagus sylvatica L.) was used as reference and in addition results from elemental analysis of carbon and hydrogen were used as basis for proposing five chemical structures, ranging from a tetramer to an octamer. The oligomers are typically characterized by biphenyl, phenyl coumaran, diphenyl ethers, stilbene and resinol structures. The results were used to speculate the formation of the pyrolytic lignin. It is concluded that pyrolytic lignin emerges as result of both recombination reactions and thermal ejection of intact lignin fragments.