Reaction kinetics of vanillin hydrodeoxygenation in acidic and nonacidic environments using bimetallic PdRh/Al₂O₃ catalyst

The kinetics of hydrodeoxygenation (HDO) reaction in literature are mostly reported for single model compounds in bio-oil. However, these kinetic models may become invalid in the real bio-oil environment where other model compounds are present. This study investigates the effect of acetic acid, which is a major compound in bio-oil, on the liquid-phase HDO reaction kinetics of vanillin (VL). A synthesized bimetallic catalyst (PdRh/Al₂O₃) was utilized in a batch reactor at 308–328 K, 1–4 MPa H₂ gas partial pressure (P_H), 263–526 mM initial VL concentration (C_VL0), and 1.9–4.6 kg/m³ catalyst loading with ethyl acetate as the reaction solvent. N₂ adsorption–desorption, scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray fluorescence (XRF), CO chemisorption, and mercury porosimetry methods were used to determine the physicochemical properties of the catalyst. Transport limitations in the system were ruled out via the Madon–Boudart test, Weisz–Prater criterion, agitation, and particle size test. In vanillin-only and vanillin–acetic acid environments, nonfirst-order dependence on P_H and C_VL0 was found. Notably, lower reaction rates were observed in the presence of acetic acid compared to vanillin-only environment. Kinetic data for the vanillin-only and vanillin–acetic acid environments were successfully modeled using derived expressions from Langmuir–Hinshelwood–Hougen–Watson approach under the assumption of competitive dissociative hydrogen adsorption. The estimated activation energy for VL HDO reaction was 24.1 kJ/mol in vanillin-only environment and 51.0 kJ/mol in the presence of acetic acid.