Research Of ZIF-8-Based Surface Catalytic Performance And Carbon Dots Preparation

The use of metal–organic frameworks(MOFs)as heterogeneous catalysts for a wide range of chemical reactions has attracted intriguing attention due to their tunable compositions and structures for easy introduction of catalytic sites into pores.1 Exploiting their internal structures has been a conventional way to upgrade the catalytic performance of MOFs,but the effect of their external surface structures has been seldom discussed.Unsaturated coordination metal ions on external surfaces can serve as Lewis acid catalytic sites with higher activity compared to saturated coordination metal ions of internal pores.Therefore,understanding external surface catalysis well provides the possibility to design MOF catalysts with extremely high catalytic activity for some specific reactions.On this basis,well-defined MOF polyhedral shapes enclosed by different facets could be engineered for study of external surface catalysis.To achieve this purpose,MOFs should possess high chemical and thermal stability,as well as high symmetry crystal structures.To meet these needs,zeolitic imidazolate framework-8(ZIF-8)with cubic sodalite-relatedstructures can be employed as a candidate for elaborating facet-dependent catalytic activity.3 In this research,we chose ZIF-8 in shapes of rhombic dodecahedron(named as RD)with {110} facets and nanocube(named as NC)with {100} facets with different densities of Zn2+ions as Lewis acid catalysts for Knoevenagel condensation.The structures of carbon dots(CDs)are extremely sensitive to synthetic system,thereby identical optical properties of CDs greatly affected by their structures is difficultly achieved while utilizing different solvents as reaction media.The convenient applications of CDs with the optimizing performance,however,are generally beneficial from the CDs featuring identical structures prepared in different solvents.Here we developed a top-down method under mild condition to explore the feasibility of preparation of identical CDs from solid carbon matrices.Porous carbons were obtained from zeolitic imidazolate framework-8(ZIF-8)by pyrolysis firstly.To attenuate the solvent effect,the porous carbons needed to be oxidized by diluted nitric acid prior to solvothermal/hydrothermal treatment.The use of oxidized porous carbons,comprised of graphitic carbons embedded into the amorphous carbon domains,as precursors was favorable for preparation of CDs with identical cores by gentle solution-based heat treatment.Finally,although the resulting three CDs prepared in dimethylformamide,in water and in ethanol exhibit excitation wavelength-dependent emission,CDs-d possesses the different maximum emission at 515 nm from other CDs at 450 nm in water.Moreover,CDs-w without any treatment can be applied to cellular imaging.