Initial
reaction mechanisms of lignin pyrolysis were studied by
large-scale ReaxFF molecular dynamics simulations (ReaxFF MD) facilitated
by the first GPU-enabled code (GMD-Reax) and the unique reaction analysis
tool (VARxMD). Simulations were performed over wide temperature ranges
both for heat up at 300–2100 K and for NVT at 500–2100 K with a large lignin model, which contained
15920 atoms and was constructed based on Adler’s softwood lignin
model. By utilizing the relatively continuous observation for pyrolysate
evolution in slow heat up simulations, three stages for lignin pyrolysis
are proposed by pyrolysate fractions. The underlying mechanisms for
the three stages are revealed by analyzing the species structure evolution
and the reactions of linkages, aryl units, propyl chains, and methoxy
substituents. Stage I is characterized with the complete decomposition
of source lignin molecules at low temperatures dominated by breaking
of α-O-4 and β-O-4 linkages. The temperature in stage
II is relatively high where cracking of all the linkages occurs, accompanied
by conversion of propyl chains and methoxy substituents. Stage III
mapping to high temperature shows the formation of heavy pyrolysates
by recombination reactions of five-, six-, or seven-membered aliphatic
rings. The heterocyclic oxygen-containing rings are revealed as important
intermediates for the aryl monomer ring opening into aliphatic rings
of five-membered, seven-membered, or even larger. The pathways for
small molecule formation observed in this work are broadly in agreement
with the literature. This work demonstrates a new methodology for
investigating the overall behaviors and the underlying complex mechanisms
of lignin pyrolysis.
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