Factors influencing the activity of SiO₂ supported bimetal Pd-Ni catalyst for hydrogenation of Α-angelica lactone: Oxidation state, particle size, and solvents

We have previously reported the superior catalytic activity of Pd-NiO/SiO₂ to Pd-Ni/SiO₂ in hydrogenation of α-angelica lactone (α-AL) to gamma-valerolactone (GVL) in a batch reactor. To gain deeper insight into the structure-activity relationship of Pd-Ni bimetal catalyst, a series of NiO/SiO₂ with different NiO particle sizes (about 3, 9, and 20 nm in average) were prepared by impregnation method in the presence or absence of ethylene glycol. These materials were used as the support of Pd catalysts for the liquid-phase hydrogenation of α-AL in a fixed bed reactor. The corresponding reduced catalysts, namely Pd-Ni(3, 9, 20)/SiO₂ with different Ni particle sizes were obtained by in situ reduction before reaction and then their catalytic activities were investigated. By comparing the catalytic performances of Pd-NiO/SiO₂ and Pd-Ni/SiO₂ in water or tetrahydrofuran (THF), the effects of oxidation state and particle size of nickel species on the hydrogenation of α-AL in terms of activity and stability were systematically studied. The bimetal catalysts were characterized by BET, XRD, TEM and in situ XAS. The catalytic tests suggested that there was no clear correlation between the initial activity and the particle size of NiO or Ni species, whereas the catalyst durability was particle size dependent and solvent dependent as well. The deactivation mechanism is likely associated with a strong adsorption of organic species on the catalyst surface according to FTIR study on the adsorption behavior of α-AL and GVL on various materials, as well as the C1s XPS spectra of the spent catalysts. The beneficial effect of water in hydrogenation is thereby explained by the less coke tendency due to the relative stronger interaction between water and catalyst surface than that between THF and surface.