A QUANTUM CHEMISTRY STUDY OF LIGNIN STRUCTURE AND PYROLYSIS MECHANISM

摘要

In this paper, quantum chemical calculations have been carried out on structure and pyrolysis mechanism of five typical lignin model compounds: guaiacol, vanillin, coniferol, 5-5 dehydrovanillin and guaiacyl β-O-4 dimer. The quantum chemical methods density functional theory (DFT) B3LYP and B3P86 were employed to investigate the equilibrium geometries of the five typical lignin model compounds and their corresponding radicals. The DFT combined methods B3LYP/6-311G**//B3LYP/6-31G* and B3P86 /6-311G**//B3P86/6-31G* were employed to calculate the Mulliken population, bond dissociation energies (BDEs) as well as the thermodynamic parameters in the temperature range 298K-1298K of the five typical lignin model compounds. The results showed that both DFT methods B3LYP and B3P86 could get the exact equilibrium geometries. Theoretical calculations revealed that the Mulliken population is not suitable as a criterion of bond strength for the five typical lignin model compounds because there exists large difference when it is compared with the BDEs. Furthermore, B3P86 method is superior to B3LYP method in the calculation of the BDEs when it is compared with experimental values available. B3LYP method significantly underestimated the BDEs. It was found that pyrolysis temperature of the five typical lignin model compounds was in the range of 1200~1300K.