Laser-Powered Homogeneous Pyrolysis (LPHP) of Lignin Dispersed into Gas Phase

The gas-phase delivery of lignin into the hot zone of cw-CO₂ laser-powered homogeneous pyrolysis (LPHP) reactor under “wall-less” conditions led to the breakdown of lignin macromolecules into neutral oligomers and paramagnetic fragments deposited onto the reactor cell walls. The formation of PAHs was observed during the defragmentation of lignin, accelerated with increased laser power. Remarkably, no phenolic compounds were detected among lignin fragments—intermediate radicals and neutral oligomers. It is concluded that the PAH and soot-like conjugated particulates are formed in the hot zone of the LPHP reactor, resembling the high-temperature combustion processes. The key role of the resonantly stabilized radicals in the formation of low-molecular-weight PAHs is outlined. An alternative pathway is proposed for the generation of PAH involving the formation of cyclopentadienyl radical precursors (CPDa) that are adsorbed onto or trapped within lignin macromolecules.