Density functional theory study on bond dissociation enthalpies for lignin dimer model compounds

Abstract

In order to enhance understanding of the mechanism of lignin pyrolysis, homolytic bond dissociation enthalpies (BDEs) in ten lignin dimer model compounds were calculated by using density functional theory methods at B3LYP/6-31G++(d,p) level and B3P86/6-31G++(d,p) level. The BDE values calculated at B3LYP/6-31G(d,p) level are about 20 kJ/mol lower than that at B3P86/6-31G(d,p) level, but the variation trends of BDE values at two levels are the same and the B3P86 functional was found to yield accurate BDEs. The calculation results show that the order of the BDE values for the β-O-4 type of linkage is as follows: Cβ-O < Cα-Cβ < C4-O < C1-Cα, and the substituents (methoxyl, carbonyl, and hydroxyl) on both the aromatic and alkyl groups in model compounds 1 have an important effect on the BDEs. It is found that the BDEs of Caromatic(1,4,5)-C(or O) in lignin model compounds are higher, but the BDEs of Calkyl(α, β)-Calkyl(α, β)(or O) are lower. Therefore, the initial step in pyrolysis of lignin is the homolytic cleavage of the Calkyl(α, β)-Calkyl(α, β)(or O) bond. The possible formation pathways of major products in the pyrolysis process of model compounds have been analysed.