Experimental and Kinetic Study on the Gas-Phase Pyrolysis of Syringol

Syringyl (3,5-dimethoxy-4-hydroxyphenyl) units are abundant
in
hardwood lignin, which explains the large amount of syringol present
in bio-oils derived from these feeds. To improve understanding of
the decomposition chemistry of these syringyl units, the gas-phase
pyrolysis kinetics of syringol have been studied experimentally and
theoretically under typical fast pyrolysis conditions. The experiments
were performed at 673–923 K in a micropyrolyzer unit hyphenated
with a gas chromatography (GC) × gas chromatography-flame ionization
detection/time-of-flight mass spectrometry and a customized gas chromatograph,
which allows to detect and quantify products with boiling points up
to 823 K next to permanent gases and water. New obtained experimental
data show that 2,3-dihydroxybenzaldehyde is the most important phenolic
product, and a large amount of pyrolytic water was formed. The proposed
decomposition pathways of syringol to these two products show two
new features: (1) 2,3-dihydroxybenzaldehyde is formed from a unimolecular
decomposition of syringol without any stable intermediates and (2)
both pathways leading to 2,3-dihydroxybenzaldehyde and water involve
one more H-atom migration step to form the more stable radicals. Potential
energy surface calculations at the CBS-QB3 level of theory indicate
that the new features are kinetically favorable. Furthermore, a detailed
kinetic model for syringol pyrolysis including 361 reactions and 93
species was constructed. Thermodynamic data for important species
and kinetics for crucial reactions were determined with ab initio
calculations at the CBS-QB3 level of theory. The model predicts well
the conversion of syringol and the formation of major products. A
rate of production analysis shows that the new proposed pathway is
the major decomposition pathway of syringol. These findings shed new
light on the pyrolysis and oxidation chemistry of syringyl species.