Kinetic Mechanism of Pyrolysis of Lilac Lignin Dimer Memes

Lilac-based lignin is an important lignin. Most of the natural lignins are connected through β-O-4 bonds to form a reticular structure. β-O-4-type lilac-based lignin dimeric modulators are closer to the actual lilac-based lignin structure, as multiple methoxy groups were added to it on the basis of the modulators studied by the previous researchers. In the Dmol³ module of the software Materials Studio 2019, the pyrolysis reaction paths of β-O-4 lilac-based lignin dimer modulators were simulated based on the density flooding theory using the B3LYP hybridization flooding at 875 K and 101 kPa. The enthalpy values of the reactants and products were calculated for each step of the reaction, and the frequency analysis was carried out by the Vibration Analysis module to confirm that there were only real frequencies and no imaginary frequencies; the enthalpy change of each step of the reaction was calculated and the total enthalpy change of the reaction paths was compared. The smaller the total enthalpy change was, the easier the paths were to take place thermodynamically, and then the more advantageous reaction paths were obtained to get the pyrolysis products of the corresponding paths finally. The results show that the initial pyrolysis of β-O-4 lilac-based lignin dimer modulators at 875 K and 101 kPa is more likely to involve the breakage of the C_α—C_β bond and the β-O-4 bond, among which the breakage of the β-O-4 bond is the most likely to occur. The more favorable reaction paths include R4 with a total enthalpy change of-59.65 kJ/mol, R10 with a total enthalpy change of-219.44 kJ/mol, R12 with a total enthalpy change of-14.93 kJ/mol, R21 with a total enthalpy change of-389.29 kJ/mol, R23 with a total enthalpy change of-466.24 kJ/mol, and R24 with a total enthalpy change of-276.72 kJ/mol, with the most favorable paths being R21, R23 and R24. The main products of pyrolysis are o-benzenetriol, 3,4,5-trihydroxybenzyl alcohol, 3,4,5-trihydroxybenzaldehyde and ethanol, among which o-benzenetriol, 3,4,5-trihydroxybenzyl alcohol and ethanol are the pyrolysis products of R21, R23 and R24. The simulation results obtained in this study can lay the foundation for further simulation calculations for generating biomass coke.