Synthesis of Pd/SiO2 Catalysts in Various HCl Concentrations for Selective NBR Hydrogenation: Effects of H+ and Cl- Concentrations and Electrostatic Interactions

A series of silica supported Pd (Pd/SiO2) catalysts were prepared in various HCl concentrations (C-HCl) of the impregnation solution with different electrostatic interactions between Pd precursor and support, and their catalytic properties were evaluated by the selective hydrogenation of nitrile butadiene rubber (NBR). The results show that with the C-HCl increasing from 0.1 to 5 M, the particle size of Pd nanoparticles dramatically decreases from 24.2 to 5.1 nm and stabilizes at similar to 5 nm when C-HCl is higher than 2 M. Using the catalysts prepared with a high C-HCl (>2 M), an excellent hydrogenation degree (HD) of similar to 94% with 100% selectivity to C=C can be acquired under mild conditions. Interestingly, the HD could be remarkably increased from 65 to 92% by increasing only C-Cl(-) from 0.1 to 2 M with the addition of NaCl while keeping C-H(+) at 0.1 M. This is because PdCl42- is the predominant existing form of precursor at high C-Cl(-), which has a strong electrostatic attraction with the positively charged support favorable for the formation of small-sized Pd nanoparticles over silica. Notably, Pd leaching behavior during the hydrogenation reaction is closely related to C-H(+), and the higher the C-H(+), the less Pd residues are detected in the hydrogenated NBR. Our contribution is to provide a facile strategy to synthesize effective and stable Pd/SiO2 catalysts via adjusting the electrostatic interaction, which exhibits a high activity and selectivity for NBR hydrogenation.