Partially hydrophobized silica supported Pd catalyst for hydrogenation reactions in aqueous media

The hydrophobic or hydrophilic nature of catalyst support materials may influence the reaction behaviour in three-phase catalytic oxidation or hydrogenation reactions in aqueous media. This may be attributed to the segregation or agglomeration behaviour of such support materials in water. We have systematically investigated the impact of surface hydrophobicity of silica supports during the catalytic hydrogenation of methyl acrylate in water. To this end, we deliberately varied the hydrophobicity of silica supported Pd catalysts by moderate silylation with dichlorodimethylsilane (DDMS). A range of materials was prepared in this way, with three-phase contact angles up to 90°, in order to allow the water to fill the catalyst pores. Hydrophobic silica supported catalysts were either prepared by silylation of hydrophilic mesoporous silica followed by incipient wetness impregnation of the support with H2PdCl4 solution (a), or by the reverse procedure (b). This way of rendering the support more hydrophobic leads to an increased three-phase contact angle, lower heat of immersion in water and lower heat of water vapour adsorption. These factors induce a more pronounced adhesion of particles to gas bubbles. During the hydrogenation of methyl acrylate in aqueous media, hydrophobic Pd/silica catalysts show higher activity than similar hydrophilic Pd/silica catalyst. The catalyst prepared by the route (a) shows the highest activity also as result of better Pd dispersion on the hydrophobic support.