posted on 2021-07-27, 18:37authored byLiang Li, Ruben Van de Vijver, Andreas Eschenbacher, Onur Dogu, Kevin M. Van Geem
Hydroxycinnamaldehyde monomers are
important intermediates in the
lignin biosynthesis and can be incorporated into plants in large quantities
via genetic modification. They are also important products formed
during the pyrolysis or combustion of lignocellulose. In this work,
the decomposition of three hydroxycinnamaldehyde model compounds,
cinnamaldehyde, p-coumaraldehyde, and coniferaldehyde,
was studied in a micropyrolysis reactor equipped with an online GC × GC-FID/TOF-MS
coupled with a customized GC for the online analysis of all the pyrolysis
vapors including permanent gases. The vaporization or sublimation
process of these model compounds in the reactor was fitted well based
on the experimental time-resolved data. The dominating initial decomposition
pathways were elucidated from a first-principles based kinetic model
and usage of a rate of production analysis. For cinnamaldehyde (at
773–1123 K) and p-coumaraldehyde (at 873–1123
K), the concerted decarbonylation reactions in the condensed phase
determine the initial decomposition, with almost no contribution from
radical chemistry. For coniferaldehyde (at 773–1023 K), the
homolysis of the O-methyl (O–CH3) bond initiates
a radical chain mechanism at temperatures above 773 K, and the H atom
abstraction on the aldehyde group is the most dominant consumption
pathway at high temperatures. The presence of methoxy groups on the
aromatic rings accelerates the decomposition of hydroxycinnamaldehydes.
Models that do not account for these important structural differences
will have difficulties in the prediction of the decomposition of real
lignocellulosic biomass.