Experimental and Kinetic Modeling Study on the Gas-Phase Pyrolysis of Hydroxycinnamaldehyde Model Compounds

Hydroxycinnamaldehyde monomers are major pyrolysis products
of
lignocellulose biomass and important intermediates in lignin biosynthesis.
In this work, for the first time, the gas-phase pyrolysis of two hydroxycinnamaldehyde
monomers, p-coumaraldehyde (PA) and coniferaldehyde
(CoA), was studied experimentally and theoretically at 873–1123
and 723–923 K, respectively. The experimental data were gathered
using a tandem pyrolysis reactor hyphenated with a GC × GC FID/TOF-MS
and a customized GC for on-line analysis. This allowed us to quantify
polyaromatic compounds with up to four aromatic rings. The potential
energy surface calculations at the CBS-QB3 level helped us to identify
a new decomposition path of primary phenoxy-type radicals in PA and
CoA pyrolysis, causing the decarbonylation of side-chain (−CHCHCHO)
via a combination of trans–cis isomerization and H-atom migration.
This new pathway is essential to predict the conversion of the reactants
and major product yields accurately. It is thus essential that similar
pathways should be explicitly accounted for in all future first principles-based
models that are developed for lignin pyrolysis and oxidation.