Heterogeneous catalysts based on Pd supported in mesoporous silica for selective hydrogenation: influence of the ionic liquid on the catalytic properties

In this work the ionic liquid 1-metyl-3-decyl-imidazolium bromide was used as structural template and functionalizer of mesoporous silica (180 m²g-1), that showed predominantly spherical morphology. This material was used as support for palladium nanoparticles, without removal of the IL from the structure. For the comparison, Pd nanoparticles were also supported on silica after removal of the IL. It was observed that the IL reduced the metallic clusters formation on support surface (Figure 1).After calcination, high surface area (901 m²g-1), mesopore volume (0.71cm³g-1) and presence of micropores were observed. However the Pd nanoparticles were less dispersed in this material. The catalytic results showed different performance due the presence of IL on the structure. The catalyst without IL reached high 1,5-cyclooctadiene (1,5COD)conversion rates, but with lower selectivity (60%). This was associated with cyclooctene (COE) consumption after total conversion of the substrate. On the other hand, the IL-containing catalyst showed high but slower conversion, due to the difficult access of 1,5-COD in the active sites, caused by the presence of IL (Figure 2). In addition, the IL molecules in the support showed a layer effect on the surface of the catalyst, similar to that obtained by the SCILL (Supported Catalyst with Ionic Liquid Layer) method. This avoided COE adsorption on the active sites because of its lower solubility in the IL, causing a repelling effect of their molecules for the solvent phase, leading to greater catalyst selectivity (93%).