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Peculiarities of Pyrolysis of Hydrolytic Lignin in Dispersed Gas Phase and in Solid State

Barekati-Goudarzi, Mohamad, Boldor, Dorin, Marculescu, Cosmin, Khachatryan, Lavrent
Energy & Fuels 2017 v.31 no.11 pp. 12156-12167
acetone, depolymerization, desorption, droplets, evaporation, free radicals, fuels, gases, lignin, mass spectrometry, molecular weight, nitrogen, phenolic compounds, pyrolysis, quartz, temperature
The unique decomposition pathways of hydrolytic lignin (HL) dissolved in an acetone/water mixture (9:1) and dispersed by a droplet evaporation technique under nitrogen gas flow has been investigated in a conventional reactor at atmospheric condition, a temperature region of 400–550 °C, and a residence time of 0.12 s. The results validate the fact that dispersion of the lignin into the gas phase by decreasing the sample size (as well as “minimizing the char area to avoid catalytic contact” of molecular products/radicals with the surface) may open new perspectives in understanding the chemistry of the depolymerization of lignin. Using Laser Desorption Ionization-Time of Flight-Mass Spectrometry (LDI-TOF-MS) the intrinsic ion m/z = 550, as the major MS peak from fresh HL dissolved in an acetone/water mixture before pyrolysis, was detected. Surprisingly, the expected phenolic compounds after pyrolysis were in trace amounts at less than 15% conversion of lignin. Instead, oligomeric intermediate substances with low (<550 Da) and high molecular weight (>550 Da) containing lignin-substructures (trapped on quartz wool located at the end of the reactor at ∼300 °C) were detected as major products using LDI-TOF-MS. 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 the free-radical mechanism of depolymerization of solid lignin by formation of free intermediate radicals from initial lignin macromolecules as well as from inherent, low molecular weight oligomer molecules is developed based on the Low Temperature Matrix Isolation (LTMI) EPR technique.