Growth and Suppression of Polycyclic Aromatic Hydrocarbon Precursors in Lignin Pyrolysis: Molecular Beam Studies

Abstract

Laser ablation of lignin at the front end of supersonic gas expansions, producing molecular beams, affords a method of observing pyrolysis chemistry occurring on microsecond time scales; laser based resonance enhanced multiphoton ionization facilitates sensitive detection of aromatic pyrolysis species. Our ability to probe at different time delays within the gas pulses, along with the use of H₂ as a carrier gas, now has provided an opportunity to characterize successive chemical reactions that lead to formation of more extended aromatic structures, i.e., polycyclic aromatic hydrocarbons (PAHs) and their derivatives. Structure attributions for parent mass peaks observed in molecular beam mass spectra have been facilitated with the use of data from complementary experiments using traditional chromatographic and evolved gas analysis methods. As a result, we can describe the sequentially evolving product chemistry of extended aromatic structure species from earlier ones, and pathways that can lead to PAHs. The key precursors appear to be hydroxymethoxystilbenes; their methylation and dehydrogenation yield the corresponding phenanthrene structures, which undergo deoxygenation at higher temperatures and longer reaction times. The molecular beam experiments show that the presence of H atom donating chemicals dramatically suppresses the formation of species with more extended aromatic structures.