Effect of the Fast Pyrolysis Temperature on the Primary and Secondary Products of Lignin

This paper presents results on the primary pyrolysis products of organosolv lignin at temperatures between 360 and 700 °C. To study the primary products, a vacuum screen heater (heating rate of 8000 °C/s, deep vacuum of 0.7 mbar, and very fast cooling at the wall temperature of −100 °C) was used. The effect of the temperature on the primary and secondary lignin products was studied with a fluidized-bed pyrolysis reactor (Tᵣₑₐcₜₒᵣ between 330 and 580 °C) with pine wood. The results obtained with the screen heater show that the primary products of lignin were oligomers. Between 450 and 700 °C, the yield of these oligomers was very high, between 80 and 90%. After formation, the oligomers left the particle by evaporation or thermal ejection. Monophenols and other light compounds were formed by secondary reactions inside the particle or in the vapor phase. In the fluidized-bed reactor, significant quantities of lignin oligomers were formed along with monophenols, water, and other light compounds. The changes on the yield and composition of the lignin-derived oligomers as a function of the pyrolysis temperature are reported. The lignin oligomers were isolated by cold-water precipitation and analyzed with thermogravimetry (TG), Fourier transform infrared (FTIR) spectroscopy, pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS), electrospray ionization–mass spectrometry (ESI–MS), gel permeation chromatography (GPC), and proton nuclear magnetic resonance (¹H NMR). The yield of lignin-derived oligomers reached a maximum between 450 and 580 °C. The differential thermogravimetry (DTG) results show the existence of three major peaks, which where maximal between 450 and 480 °C, indicating that not only the yield but also the structure of the lignin oligomers changed at higher pyrolysis temperatures. Py–GC/MS and ¹H NMR results indicated that, as the temperature increased, the content of methoxylated phenols decreased and the content of alkylated phenols increased. FTIR spectroscopy revealed an increase in carbonyl groups produced by carbonylation reactions, from the hydroxyl groups or the cleavage of ether bonds. Both ESI–MS and GPC results show a negligible effect of the pyrolysis temperature on the molar weight distribution. This result differs with the increase in the molecular weight observed for organosolv lignin pyrolyzed in the screen heater and highlights the importance of secondary reactions on the outcome of fast pyrolysis reactors.