Bimetallic Ni-Zn/TiO2 catalysts for selective hydrogenation of alkyne and alkadiene impurities from alkenes stream

This study investigates the alternative of replacing noble metals by base metals as catalysts for selective hydrogenation of polyunsaturated hydrocarbons. Nickel catalysts are active for this type of reaction, but poorly selective. However, former calculations anticipated that Ni-Zn alloys could have higher selectivity to alkenes than monometallic Ni depending on the Ni/Zn atomic ratio in the alloy. In this contribution, Ni-Zn alloy nanoparticles supported on TiO2 are synthesized as catalysts for selective hydrogenation of acetylene and butadiene. The designed catalysts with 0.5 wt% Ni loading and various Ni/Zn nominal ratios are prepared by deposition-precipitation with urea (DPU). STEM-HAADF imaging coupled with EDS analysis reveals that Ni-Zn bimetallic particles are formed after reduction treatment at 450 degrees C. Alloying Ni with Zn leads to a slight increase in the average particle size and a broadening of the size distribution compared to monometallic Ni. However, the average Ni/Zn atomic ratio measured by EDS in the bimetallic particles is always higher than the nominal one, which could be due to Zn evaporation under the beam. The results of acetylene hydrogenation show that although the catalytic activity is slightly reduced after alloying Ni with Zn, the selectivity to ethylene is enhanced from 50% up to 85%, at the expense of the formation of oligomers (coupling products). However, the bimetallic Ni-Zn catalysts suffer from progressive deactivation in this reaction. During butadiene hydrogenation performed in the presence of an excess of propene, the bimetallic Ni-Zn/TiO2 catalysts are significantly more stable, with a high and constant selectivity to butenes (> 95%), compared with Ni/TiO2, which deactivates rapidly in the first hours. Some hypotheses concerning the observed differences in catalytic stability are discussed.