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 (CHCl) 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 CHCl 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 ∼5 nm when CHCl is higher than 2 M. Using the catalysts prepared with a high CHCl (>2 M), an excellent hydrogenation degree (HD) of ∼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 CCl– from 0.1 to 2 M with the addition of NaCl while keeping CH+ at 0.1 M. This is because PdCl42– is the predominant existing form of precursor at high CCl–, 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 CH+, and the higher the CH+, 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.