Primary Homogeneous Pathways of Lignin Depolymerization in Gas Phase

Abstract

The unique decomposition pathways of hydrolytic lignin (HL) dissolved in acetone/water mixture and dispersed into gas phase has been investigated using two unique reactors. The first one is a non-isothermal continuous wave-IR CO2 Laser Powered Homogeneous Pyrolysis (LPHP) and the second one is an isothermal Continuous Droplet Evaporation (CDE) reactor. Both of these reactors operated using atmospheric N2 carrier gas. A temperature region of 400 – 550°C, and various residence time of 0.12-2 sec were chosen for CDE reactor. The temperature distribution in the LPHP reactor was evaluated via thermocouple measurements and validated by the method of “chemical thermometer” and COMSOL Multiphysics simulations. The pyrolysis results validate the fact that dispersion of the lignin into gas phase, as a way to decrease the sample size which also minimizes the char area to avoid catalytic contact of molecular products/radicals with the char surface, may open new perspectives to understand the chemistry of depolymerization of lignin. Delivery of HL into gas phase and subsequent pyrolysis, in both reactors, at very low mass delivery rates and conversion (less than 20%) revealed the primary processes of depolymerization; MALDI-TOF-MS analysis confirmed break down of HL macromolecules into oligomer-fragments after pyrolysis in a Continuous Atomization reactor (CA) reactor, with negligible amounts of phenolics detected. Surprisingly, the expected phenolic compounds after pyrolysis were in trace amounts at less than 15% conversion of lignin. The hypothesis about a largely disputed key question on lignin pyrolysis, as to whether the phenolic compounds or oligomers (dimers, trimers, etc.) are the primary products is discussed. Additionally, a focus on free radical mechanism of depolymerization of solid lignin by formation of free intermediate radicals from initial lignin macromolecule as well as from inherent, low molecular weight oligomer molecules is developed based on Low Temperature Matrix Isolation (LTMI) EPR technique.