Evaluation of Pd/ZSM-5 catalyst for simultaneous reaction of transesterification and partial catalytic transfer hydrogenation of soybean oil under supercritical methanol

Abstract

Biodiesel, termed as Fatty Acid Methyl Esters (FAMEs), has an essential role in reducing greenhouse gases by replacing fossil fuel and is needed to be partially hydrogenated to enhance its fuel properties. In this study, biodiesel was synthesized by the simultaneous reaction of transesterification and partial catalytic transfer hydrogenation (CTH) of soybean oil under supercritical methanol over Pd/ZSM-5 or commercial Pd/Al2O3. At 300 °C, 10 MPa, 45:1 of the molar ratio of methanol: oil with 0.5 mg Pd/g oil for 30 min, the FAMEs yield for Pd/ZSM-5 was 97.1%. On the other hand, the FAMEs yield for Pd/Al2O3 was 58.0% under the same condition. Pd/ZSM-5 was less active for CTH despite the higher dispersion of Pd nanoparticles (NPs) than Pd/Al2O3. The reactivity of Pd/ZSM-5 for CTH would be affected by the unique pore structure of ZSM-5 that seems to inhibit the hydrogenation of FAMEs on Pd NPs by confining the reactant diffusion. Consequently, biodiesel obtained by Pd/ZSM-5 had satisfactory fuel properties to the biodiesel standard specification at once. This work highlights that the highly efficient catalyst for producing the upgraded biodiesel could be developed by manipulating the reactivity of active centers via shape selectivity.