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

Laser\nablation of lignin at the front end of supersonic gas expansions,\nproducing molecular beams, affords a method of observing pyrolysis\nchemistry occurring on microsecond time scales; laser based resonance\nenhanced multiphoton ionization facilitates sensitive detection of\naromatic pyrolysis species. Our ability to probe at different time\ndelays within the gas pulses, along with the use of H<sub>2</sub> as\na carrier gas, now has provided an opportunity to characterize successive\nchemical reactions that lead to formation of more extended aromatic\nstructures, i.e., polycyclic aromatic hydrocarbons (PAHs) and their\nderivatives. Structure attributions for parent mass peaks observed\nin molecular beam mass spectra have been facilitated with the use\nof data from complementary experiments using traditional chromatographic\nand evolved gas analysis methods. As a result, we can describe the\nsequentially evolving product chemistry of extended aromatic structure\nspecies from earlier ones, and pathways that can lead to PAHs. The\nkey precursors appear to be hydroxymethoxystilbenes; their methylation\nand dehydrogenation yield the corresponding phenanthrene structures,\nwhich undergo deoxygenation at higher temperatures and longer reaction\ntimes. The molecular beam experiments show that the presence of H\natom donating chemicals dramatically suppresses the formation of species\nwith more extended aromatic structures.