Estudo de MOFs zeolíticas de imidazolato com Zn(II) e Co(II) e emprego na condensação de Knoevenagel

Abstract

A new segment of porous materials has emerged last years: metal organic frameworks (MOFs). Zeolitic imidazolate frameworks (ZIFs), a subclass of MOFs, are formed by metallic cations and imidazolates presenting similar topologies to zeolites. The imidazolate linker have pyridinic nitrogen in its structure, which is part responsible for the basic character of these materials. In the present work, Zn(II) and Co(II) ZIFs with ZIF-8, ZIF-67 and ZIF-L structures were applied in Knoevenagel condensation model reaction. The method of synthesis of these materials stands out, where water was used as a solvent and synthesis was facilitated by a deprotonating agent. X-ray diffractometry, scanning electron microscopy, CHNS elemental analysis, thermogravimetry, N2, and CO2 physisorption and infrared spectroscopy with CDCl3 as probe molecule was performed. Zn-6TEA, with has ZIF-8 structure and was prepared with triethylamine as deprotonating agent, presented three bands in the region of weak basic sites from the infrared spectrogram with CDCl3. Co-6TEA, which has a ZIF-67 structure, showed bands at the same positions, demonstrating similar basic sites between these sodalite ZIFs. Zn-1FS, with a ZIF-8 structure, was synthesized using sodium formate, and due to the low basicity of this deprotonating agent, nucleation was slowed down, obtaining a high average particle size (~ 6 - 10 μm) when compared to the average particle size of Zn-6TEA (123 ± 25% nm). In Knoevenagel condensation, Zn 1FS showed 28% conversion with 30 min of reaction at 40 °C, while Zn-6TEA showed 64.6% under the same conditions. Thus, demonstrating that the synthesis method directly influences the properties and catalytic performance of ZIFs. It is suggested that, despite the flexibility of these structures, the reaction occurs on the surface and the smaller particle size and consequent larger external area favor the Knoevenagel reaction. The Zn-6TEA sample not only showed high conversion but was also stable in this reaction, maintaining 75% conversion after five reuses for 1 h at 40 °C.