Abstract:

Selective hydrogenation of dienes and alkynes to monoenes is an important topic of research in the fields of pharmacology and organic synthesis. Catalyst design plays a key role in this process, where a general principle involves controlling the steric diene adsorption by modifying the surface of the metal nanoparticles. For example, upon introducing Bi species into Rh nanoparticles, the resulting RhBi/SiO₂ showed 90% selectivity to 2-hexene, with 95% conversion of 1, 4-hexadiene under ambient conditions, because of the suppressed adsorption of the internal C＝C bond. However, the catalyst activity decreased remarkably; that is, the activity of the unmodified Rh/SiO₂ was about 27 times higher than that of RhBi/SiO₂. Controlled steric adsorption of the diene molecules could also be achieved by the constructing porous channels around the metal nanoparticles. For example, metal-organic framework (ZIF-8) or mesoporous silica (MCM-41) encapsulated noble metals showed high selectivity for the hydrogenation of terminal C＝C bonds. However, these catalysts had poor durability under the thermal/hydrothermal reaction/regeneration conditions. In contrast, zeolites have superior durability under harsh reaction conditions, but they are rarely used in semi-hydrogenation reactions. We recently found that metal nanoparticles fixed within zeolite crystals (e.g., ZSM-5 and Beta) efficiently catalyze the selective hydrogenation of molecules bearing multiple reducible groups. Thus inspired, we developed a catalyst by fixing Rh nanoparticles within zeolite crystals via an inter-zeolite transformation method. The Rh@CHA catalyst was synthesized by introducing Rh species into the parent Y zeolite (Rh@Y) and transformation of the Y zeolite to chabazite (CHA zeolite) under hydrothermal conditions. X-ray diffraction patterns, N₂ sorption isotherms, scanning/transmission electron microscopy images, and model reactions (hydrogenation of probe molecules) confirmed the successful fixation of the Rh nanoparticles inside the CHA zeolite crystals. As expected, the Rh@CHA catalyst was highly selective for the hydrogenation of dienes. For example, Rh@CHA showed a 2-hexene selectivity of 86.7%, with 91.2% conversion of 1, 4-hexadiene. In contrast, the generally supported Rh nanoparticle catalyst (Rh/CHA) showed a low 2-hexene selectivity of 37.2% under identical reaction conditions. Considering that Rh@CHA and Rh/CHA comprise the same CHA zeolite crystals and have similar Rh nanoparticle sizes, the remarkably high selectivity of Rh@CHA is assigned to the steric adsorption of dienes on the Rh surface controlled by the micropores of the CHA zeolite. This work demonstrates that a zeolite-fixed metal core-shell structure is a powerful tool for developing efficient catalysts to be used in diene hydrogenation.

Key words: Heterogeneous catalysis, Diene hydrogenation, Rh nanoparticle, CHA zeolite, Core-shell structure, Inter-zeolite transformation