Kinetic modeling of oleic acid esterification with UiO-66: from intrinsic experimental data to kinetics via elementary reaction steps

This work reports the experimental conversion of oleic acid (OA) into methyl oleate via UiO-66 MOF catalyst. Strong focus is on the kinetic modeling: 67 models, based on elementary reaction steps for Eley-Rideal, Langmuir-Hinshelwood and Hattori kinetic mechanisms, are proposed. Via the application of initial reaction rate theory and nonisothermal kinetic modeling, one model adequately describes the experimental data. It is a variant on the Eley-Rideal type of model, such that the surface reaction includes the oleic acid adsorbate, methanol reacts from the liquid phase and one additional active site is considered to make the ester and water as surface reaction products, so that overall two active sites are used in the surface reaction. An activation energy of 54.9 ± 1.8 kJ mol⁻¹ is reported and the desorption of the ester was found to be irreversible. In addition, NH₃ TPD revealed at least two types of acid sites (#) on the UiO-66 MOF, from which turnover rates were calculated in the range of 2.54 ± 0.33 to 8.82 ± 0.87 mmol_OA mol_#⁻¹ s⁻¹. Since the catalyst is recyclable and no additional acids have to be used, this system can be considered as a green alternative for the well-known homogeneously catalyzed esterification reaction.