Morphological and compositional control of a Zeolitic Imidazolate Framework for catalytic applications

Abstract

The structures of metal-organic frameworks (MOFs) are highly tunable, which allows their properties to be regulated by varying the types of metal ions and organic linkers. As one of the most widely studied MOFs, ZIF-8 (ZIF: zeolite imidazolate framework) has demonstrated fascinating properties for separation and catalysis applications. However, there is a lack of studies on the morphological and compositional control of ZIF-8 crystals. In this thesis, we systematically investigate the effect of solvents (water and methanol) and metal sources (nitrate and acetate) on the properties of ZIF-8 and the efficiency of doping additional metals into ZIF-8. We found that the product obtained from nitrate and water had a broad crystal size distribution. When using nitrate and methanol, the size of product was not uniform while when using acetate and methanol, the doping amount is low. Interestingly, uniform ZIF-8 crystal with high Ni doping (up to 1.96%) can be obtained by using the combination of water and acetate. In addition, the developed water-based synthesis is environmentally friendly, efficient, and easy to scale up. A gram-level product can be obtained at room temperature for just 1 h, which fits the principle of green chemistry. Besides, compared with pristine ZIF-8, Ni-ZIF-8 shows a significantly enhanced catalytic conversion rate in the Knowevenagel reaction. Finally, a polymetallic ZIF-8 was synthesized using the same method since all single metallic ions doped ZIF-8 contribute to improving the catalytic activity. However, because the crystal size reaches the micrometer level and there is a slight decrease in the content of Ni, the reaction activity did not outperform that of 25%Ni-ZIF-8.