Imidazolium-type ionic liquid-assisted formation of the MFI zeolite loaded with metal nanoparticles for hydrogenation reactions

Abstract

Zeolites loaded with metal nanoparticles (NPs) stand out for their distinct activity and selectivity as heterogeneous catalysts. However, the poor stability of metal species limits their applications. Herein, we report a novel synthetic strategy for stabilizing metal nanoparticles inside the MFI zeolite. For the first time, alkoxysilane functional imidazolium-type ionic liquids (ImILs) were used as assistant agents to protect metal precursors (Pt and Pd) against precipitation during the hydrothermal synthesis of the MFI zeolite. The positively charged imidazolium groups in ImILs interact with the negatively charged metal precursors (PtCl42-, PdCl42- etc.), while the alkoxysilane groups participate in zeolite crystallization. Scanning transmission electron microscopy images indicate that most of the Pt and Pd nanoparticles (average diameters of approximately 1.0 nm and 1.7 nm, respectively) are confined in channels or intersections of the MFI zeolite. The shape-selectivity effect on nitroarene hydrogenation over Pt@MFI_ImILs confirmed the successful encapsulation of metal NPs into the MFI matrix. The conversion of the small molecule 4-nitrotoluene is >99%, whereas the bulky 1,3-dimethyl-5-nitrobenzene shows 5.8% conversion. Simultaneously, the Pd@MFI_ImILs catalyst affords a higher reaction rate (25 mmol/s/gMe) than the impregnated Pd@MFI_Imp catalyst (9 mmol/s/gMe) in propene hydrogenation, which is ascribed to the uniform distribution of Pd nanoparticles. The ImIL-assisted synthesis strategy can be therefore successfully used to confine small metal nanoparticles in the MFI zeolite while maintaining its high catalytic activity and shape-selectivity.