Tuning Hydrogenation Catalysis via pH-Responsive Microenvironments in Hydrophobic ZeoliteEncapsulating Palladium

Aqueous pH modulates the transition-metal-catalyzed hydrogenation reaction, enhancing the rate in the presence of high concentrations of hydronium ions (H₃O⁺) to the metal. Herein, Pd particles encapsulated in hydrophobic silicalite-1 zeolite pores (Pd/S1-in) were synthesized and tested for benzaldehyde hydrogenation in water. Hydrophobic silicatlite-1 excludes the retention of both H₂O and H₃O⁺ within its pores, preventing the direct proximity of H₃O⁺ to Pd particles. However, the hydrogenation rate of benzaldehyde on Pd/S1-in showed a pronounced dependence on the aqueous pH, with the rate increased by one order of magnitude as the pH decreased from 9 to 4. We propose that silanol groups (SiOH) of silicatlite-1 dissociate in water, forming an acid/conjugate base pair of SiOH/SiO^-. This pair creates an acidic environment within the silicalite-1 pores that mirrors the pH of the bulk aqueous solution, enhancing the hydrogenation activity of the encapsulated Pd particles and demonstrating an indirect yet substantial influence of bulk solution pH on the catalytic performance of Pd/S1-in.