Copper(II)‐Doped ZIF‐8 as a Reusable and Size Selective Heterogeneous Catalyst for the Hydrogenation of Alkenes using Hydrazine Hydrate
Nagarathinam Nagarjun, Kannan Arthy, Amarajothi Dhakshinamoorthy
Abstract
Abstract In recent years, synthesis of mixed‐metal organic frameworks has received considerable attention due to their superior performance than with mono‐metallic metal organic frameworks (MOFs). In the present manuscript, Cu 2+ ions are doped within the framework of ZIF‐8 (ZIF: Zeolitic Imidazolate Frameworks) to obtain Cu@ZIF‐8 and is characterized by powder X‐ray diffraction (XRD), Fourier transform infrared (FT‐IR), UV‐Visible diffuse reflectance spectra (DRS), scanning electron microscope (SEM) and transmission electron microcope (TEM) studies. The reaction conditions are optimized with styrene as a model substrate using Cu@ZIF‐8 as a solid catalyst. Heterogeneity of the reaction is confirmed by leaching test and the solid is reusable for three recycles with no diminishing activity. Further, the structural integrity of Cu@ZIF‐8 is also retained after hydrogenation of styrene as evidenced by powder X‐ray diffraction. The size selective catalysis of Cu@ZIF‐8 is demonstrated by comparing the activity of Cu 2+ ions adsorbed over ZIF‐8 solid (Cu/ZIF‐8) in the hydrogenation of 1‐hexene, 1‐octene, cyclohexene, cyclooctene and t‐stilbene. The catalytic results indicate that Cu/ZIF‐8 shows superior activity than Cu@ZIF‐8 for all these olefins due to the lack of diffusion to access the active sites (Cu 2+ ). In contrast, Cu@ZIF‐8 exhibits higher activity for those olefins with lower molecular dimensions (1‐hexene, 1‐octene) than the pores of ZIF‐8 indicating the facile diffusion of these substrates inside the pores ZIF‐8 while poor activity is observed with t‐stilbene due to its larger molecular dimension than the pore apertures of ZIF‐8. These catalytic data clearly establish the size selective hydrogenation of Cu@ZIF‐8 due to the effective confinement provided by ZIF‐8 framework and the presence of the active sites within the framework. Furthermore, this is the first report showing the size selective hydrogenation of olefins promoted by Cu@ZIF‐8 (mixed‐metal MOFs) compared to other noble metal nanoparticles (NPs) embedded over MOFs as catalysts.