Lignin is one of three components of lignocellulosic materials. During fast pyrolysis, the poly-aromatic structure of lignin is depolymerized to smaller phenolic compounds. Phenolic compounds found in bio-oil are both monomers and oligomers. Since the water-insoluble fraction of bio-oil mainly consists of phenolic oligomers, it is also called “pyrolytic lignin”. While phenolic monomers are valuable platform chemicals, pyrolytic lignin is less preferred. Pyrolytic lignin is not only known to attribute to thermal instability and high viscosity of bio-oil, but it also causes deactivation of catalyst during catalytic upgrading. For these reasons, lignin-derived compounds would be less problematic if they were present as monomers instead of oligomers. However, yields of phenolic monomers in bio-oil are usually very low (<10 wt%), whereas pyrolytic lignin is the majority of lignin-derivatives, accounting for 25-30 wt% of bio-oil. Thus, understanding the formation mechanism of pyrolytic lignin during pyrolysis is very important in promoting lignin-depolymerization to phenolic monomers. However, the origin of pyrolytic lignin is not yet very clear. Two primary hypotheses have been discussed in literature: the thermal ejection as lignin fragments and the repolymerization of primary phenolic monomers in the vapor phase.
Our research goal is to understand the formation pathway of pyrolytic lignin. During experiments, lignin and phenolic model compounds were pyrolyzed in a micropyrolyzer system. The pyrolysis vapor was analyzed using online gas chromatography/ mass spectrometry (GC/MS). A modified micropyrolyzer system and a methodology that can greatly reduce the secondary reaction of primary products were employed to collect liquid products of pyrolysis. The liquid products were then analyzed using electrospray ionization mass spectrometry (ESI-MS) and gel permeation chromatography (GPC). The study revealed that the primary pyrolysis products of lignin are mostly phenolic monomers and dimers with molecular weight below 400 Daltons (Da). Primary phenolic monomers with unsaturated C=C bonds or methoxy groups further react in the vapor phase to form phenolic oligomers or phenolic monomers with higher molecular weights. Phenolic dimers with biphenol-, phenycoumaran- and diaryl- structures are also the primary pyrolysis products because these structures have relatively high thermal stability in conventional pyrolysis temperature. The study further indicated that these primary phenolic dimers with m/z< 400 Da are likely evaporated instead of thermally ejected.
Figure 1. Proposed scheme of thermal degradation behavior of lignin
Acknowledgements
Funding for this research was provided by the National Advanced Biofuels Consortium and the Phillips 66 Company.