(407a) Could Benzyl Hydroxyl Shielding Promote the Radical Induced Pyrolysis of Lignin?

Due to the inherent complexity and changing chemical structure of lignin, a strategy, which has been gradually accepted in liquid depolymerization, is to shield the benzyl hydroxyl to inhibit the self-condensation reaction of lignin. This information on whether benzyl hydroxyl shielded in lignin structure affected the pyrolysis behaviors of lignin, especially explained by the radical theory, however, is scarce in the literature. Accordingly, the influence of the shielded benzyl hydroxyl in lignin structure on thermal behavior and radical evolution was investigated in this study. The benzyl hydroxyl in the lignin structure was first stabilized by propionaldehyde and prepoxidized by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) respectively, compared with the raw lignin as a control (ball-milling wood lignin, MWL) for subsequent investigation. Then, the raw lignin and pretreated lignin were pyrolyzed at different temperatures, and the corresponding products were determined to find the best condition for obtaining organic liquids. In addition, through the electron paramagnetic resonance (EPR) spectroscopy, the evolution of pyrolytic radicals had been comprehensively studied to reveal the connection of the products formation with the radical induced reaction by different method of pretreatment.

Materials and Methods

The maple wood was first prepared for subsequently extraction of different types of lignin. Milled wood lignin (MWL) were obtained from the maple wood via the ball-milling method. Propionaldehyde stabilized lignin (PAL) was obtained by extraction in dioxane: water solutions with HCl and the propionaldehyde. To prepare the DDQ oxidized lignin (DOL), the maple wood was firstly oxidized by the DDQ oxidation with O₂(g) as the co-oxidants. Then the DOL was obtained by the ball-milling method extracted from the oxidized wood. TGA, 2D HSQC NMR and Ultimate analysis of raw and pretreated lignin samples were conducted to investigate the characterization and thermal stability.

Analytical pyrolysis of MWL, DOL and PAL was conducted with pyrolysis gas chromatography/mass spectrometry (Py-GC/MS) at 350-650 °C. Lab-scale pyrolysis was performed on a fixed bed to harvest chars from lignin. The radical concentration of sample was detected on a Bruker EMX-10/12 spectrometer.

Results and Discussion

The results showed that the process of inhibiting the coking of lignin cannot be achieved by shielding the α-OH. The maximum total yields (11.10×10⁹ area/mg) of MWL lignin was obtained at 450 °C compared with the oxidized lignin (3.71×10⁹ area/mg of DOL, 5.02×10⁹ area/mg of PAL), which indicated that the yields of bio-oil was inhibited by oxidized α-OH of the lignin.

To understand the pyrolysis radical induction, the radical evolution of MWL was first detected by EPR from 350 to 650 °C. The pyrolysis radical concentration increased rapidly from 26.82×10¹⁷ spin/g to 245.04×10¹⁷ spin/g at 550 °C, which means the lignin undergo an explosive fission reaction. For PAL radical detected at 550 °C, the lower radical concentration (169.32×10¹⁷ spin/g) and higher char yields (37.10%)was obtained compared to the MWL. As for DOL radical, the liquid yield of the DOL was lowered than the MWL with the higher spin concentration (259.32×10¹⁷ spin/g for DOL). Therefore, the possible pathway of radical evolution leading to various products distribution from raw and pretreated lignin was given a new sight to understand the lignin pyrolysis reaction.

Conclusions

The shielded α-OH in lignin structure showed significant effect on the thermal stability and yield distribution during fast pyrolysis. It was proved that the radical reactions play an important role and process of inhibiting the coking of lignin cannot be achieved by shielding the benzyl hydroxyl. The bio-oil yield was decreased after shielding the benzyl hydroxyl, while the char yield had been increased. Facts have proved that in the pyrolysis process, due to the violent radical-induced reaction, the caped hydroxyl groups might be quickly converted into oxygen radicals, which further increases the probability of formation of benzoquinone and caused carbonization. Thus, the effect of capping pretreatment on the distribution of lignin pyrolysis products is clarified, and the failure of benzyl hydroxyl shielding pretreatment and the possible inducement of promoting the condensation of lignin to carbon were proved from the perspective of radical induction.