Catalytic Pyrolysis of Sinapic Acid on Nanoceria: Surface Complexes, Valorization of Products, Experimental, and Atomistic Approaches

In this work, we investigated the pyrolysis of sinapic acid (SA) as a lignin S-units model compound on the nanoceria catalyst. We employed various techniques to unravel the pyrolysis mechanism, including temperature-programmed desorption mass spectrometry, thermogravimetric, and IR spectroscopic techniques, complemented with atomistic simulations. From spectroscopic data and atomistic models, we report that SA interacts with the catalyst via its carboxyl group and aromatic functional groups; the amounts of various surface complexes depend on the acid concentration. Conformational analysis revealed that parallel adsorption on ceria was preferred over the perpendicular one (ΔE₀ = -154 kJ mol^-1). The main pyrolysis products are associated with transformations of phenolate complexes, with the predominant formation of syringol and with decarboxylation of carboxylates, forming 4-vinyl syringol, well known as canolol, thanks to its exceptional antioxidant properties. Modeling the transition state between the SA and its vinyl analog, canolol, displayed an additional intramolecular decarboxylation pathway with an activation energy barrier of +189 kJ mol^-1. This is consistent with the activation energy E^≠ = 194 kJ mol^-1 calculated from experimental kinetic data, and complements other established decarboxylation pathways. Methyl-syringol, cresol, phenol, toluene, benzene, and other aromatics were found among the catalytic pyrolysis products of SA.