Reaction Mechanisms in Pyrolysis of Hardwood, Softwood, and Kraft Lignin Revealed by ReaxFF MD Simulations

Abstract

Reactions of linkages and monomer rings in hardwood, softwood, and kraft lignin pyrolysis were investigated using reactive force field (ReaxFF) molecular dynamics (MD) simulations. Four large lignin models with diverse linkages and oxygen-containing groups were constructed on the basis of the two-dimensional lignin models proposed by Adler and Freudenberg for softwood lignin, by Nimz for hardwood lignin, and by Marton for kraft lignin. Large-scale ReaxFF MD simulations were performed using the GMD-Reax code and analyzed with the VARxMD code. The breaking of Cα/Cβ–O bonds in α/β–O–4 linkages is the dominant pathway to convert the source lignin molecules into smaller fragments. Differences in pyrolyzate evolution and linkage behavior among hardwood, softwood, and kraft lignin can be attributed to different reactions of linkages and their linked monomers induced by their varied oxygen-containing substituents. The consumption evolution of β–O–4 linkages can be promoted by their active substituents (Cα═O and Cα–O–4). The conversion of β–β_γ–O−α linkages and their linked monomer rings can be accelerated by the Cα═O substituent on the β–β_γ–O−α linkages and by the substituents of Cα–O–4 and OCH3 on the monomer ring. The different evolution profiles of the softwood, hardwood, and kraft lignin models are caused by the differences in the inherent structures (mainly linkage and oxygen-containing functional groups) of lignin from different tree species or isolation methods. Despite distinct differences in the pyrolyzate evolutions and linkage behaviors, the ring substructure evolutions of five- to seven-membered rings are similar for the four lignin models. This work demonstrates a useful alternative approach for investigating the pyrolysis reaction mechanism of lignin from varied sources.