Comparison of Pd-UiO-66 and Pd-UiO-66-NH2 catalysts performance for phenol hydrogenation in aqueous medium

Abstract

UiO-66, a kind of Metal-organic framework materials (MOFs) was used as a support for nano-palladium particles (i.e., Pd-UiO-66) which was used in the catalytic hydrogenation of phenol in aqueous media in this work. The performance of Pd-UiO-66 was compared with Pd-UiO-66-NH2, in which the UiO-66 support had been modified through the introduction of NH2 groups. Pd-UiO-66 and Pd-UiO-66-NH2 were characterized by BET, XRD, XPS and TEM. The results indicated that the Pd particles are embedded in Pd-UiO-66 and fixed on Pd-UiO-66-NH2. With NH2 groups, the crystalline structure of UiO-66-NH2 is somewhat damaged and the interaction between Zr and Pd in Pd-UiO-66-NH2 is rather weaker than in Pd-UiO-66. Pd-UiO-66 has high activity for aqueous phenol hydrogenation, with 100% conversion of phenol being achieved within 2h at 120°C, which is much higher than with the Pd-UiO-66-NH2 catalyst. Pd-UiO-66 has higher selectivity for cyclohexanol, and Pd-UiO-66-NH2 has higher selectivity for cyclohexanone under the same conditions. The study of the chemical kinetics portion revealed that the hydrogenation reaction rates with the Pd-UiO-66 catalyst are over 10 times higher than those with Pd-UiO-66-NH2 under the same conditions. The phenol hydrogenation catalyzed by Pd-UiO-66 is more sensitive to the reaction temperature than when catalyzed by the Pd-UiO-66-NH2 catalyst. Thus, it has been shown that modifications of an MOF support for a catalyst can result in structural changes that lead to different catalytic activities and selectivities in aqueous phenol hydrogenation.