Primary Thermal Decomposition Pathways of Hydroxycinnamaldehydes

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.