Synthesis And Properties Of Zeolitic Imidazolate Frameworks

Zeolitic imidazolate frameworks (ZIFs) are a new subclass of porous metal-organic frameworks (MOFs), which frequently possess expanded zeolite topologies. Because of these combined and desirable features, ZIFs possess tunable structure and functional properties of classical MOFs; at the same time, they exhibit the high chemical and thermal stability of conventional zeolites. Thus, ZIFs show great potential for many applications of porous materials, and the study of ZIFs will be expanded to other application areas. So far, many groups at home and abroad have dedicated to the synthesis of ZIFs with new topology. Meanwhile, the bulk of investigations on ZIFs have involved their application in gas adsorption and separation.We have focused our study on the synthesis conditions and rules of ZIFs and explored new properties of ZIFs. On the one hand, various synthetic factors are studied, such as the choice of the solvent, metal salts and imidazole derivatives as well as crystallization time and temperature, on the influence of the crystallization of ZIFs. These works will supply useful information for synthesis of ZIFs. On the other hand, according to the characteristics of ZIFs, the tribological behavior as additives in liquid lubricants and the liquid phase adsorption performance of ZIFs have been examined.1. The porous ZIFs have been synthesized by replacing the organic solvent, such as DMF, with H2O through a steam-assisted conversion. By steam-assisted conversion and separation of the solid and liquid phase, the solid phase (usually Zn(OAC)2or Co(OAC)2and mlm) was transformed into ZIF-8or ZIF-67upon contact with H2O vapor. ZIF-8and ZIF-67are the most representative ZIFs with a zeolite sod topology. When reactant mixtures could dissolve easily in the H2O used for hydrothermal synthesis, dense and nonporous dia(Zn) and dia(Co) was generated. Then, based on the mechanical properties and lipophilic characters of the ZIFs, tribological properties of ZIF-8and ZIF-67as solid lubricant additives have been examined. After1.0wt%ZIF-8and ZIF-67were added in base oil, the PB values were improved by16.8%, the WSD decreased from0.58mm to0.32and0.30mm, respectively.2. Applying steam-assisted conversion, various synthetic factors are studied, such as the Zn sourve, crystallization temperature and time on the influence of the crystallization of ZIF-8. Compared with the zinc nitrate, zinc acetate have relatively strong alkalinity, which will easily make the ligand2-methylimidazole lose proton. Thus, the use of zinc acetate is conducive to the formation of ZIF-8under the same experimental conditions. Then, ultrahigh field67Zn solid-state NMR (21.1T/900MHz) was used to investigate the gel transformation process. The intermediate metastable phase and the primary polymerization of Zn were firstly discovered before emergence of ZIF-8. Moreover,67Zn NMR can be used to determine the distribution of the guest molecules in ZIF-8. The results based on line-shape of signals suggested the water molecules are randomly distributed within the cages and the channels. Instead, the organic molecules are symmetric distributed within the cages.3. ZIFs show exceptional stability and resistance to boiling organic solvents and water due to their intrinsically hydrophobic nature and strong bonds. Due to their above-mentioned stability, we have described the adsorption of dye Acid Blue40(AB40) from aqueous solution onto isotypic ZIFs such as ZIF-8(Zn(mlm)2) and ZIF-67(Co(mlm)2). The results showed that the adsorption capacity for ZIF-67is nearly three times higher than that of ZIF-8. The results obtained suggested that the charged sites at the external surface of ZIFs play a key role in dye adsorption, involving electrostatic interactions between metal charged sites (Zn and Co) of ZIFs and negatively charged groups (-SO3-) of AB40. And Co in ZIF-67has higher affinity with the-SO3-groups of AB40.4. We discussed the substituted position of functional groups in the imidazole ligand (2position,4position,4,5position and2,4position) on the influence of the crystallization of ZIFs. Using Imidazole-2-carboxaldehyde (2position) as ligand, ZIF-90and the unknown phase (with high crystallinity) were obtained; using Imidazole-4-carboxaldehyde (4position) as ligand, only low crystallinity phase was obtained. Then, using5-Methyl-imidazole-4-carbaldehyde (4,5position) as ligand, ZIF-93and SIM-1were obtained; using2-Methyl-imidazole-4-carbaldehyde (2,4position) as ligand, only amorphous material was obtained. Therefore, when using imidazole derivatives with the symmetrical substituted (2or4,5positions) as ligands, ZIFs materials were easily obtained. Instead, imidazole derivatives asymmetrically substituted (4or2,4positions) are not easily coordinated with metal because it destroy the conjugated N balance of imidazole, which was not conducive to the generation of ZIFs materials.5. The three samples which have the same GIS topology are synthesized through the selection of complementary ligands:small-sized imidazole ligand (imidazole,2-methylimidazole,2-ethylimidazole) and one large-sized ligand (5,6-dimethylbenzimidazole). They are denoted as GIS-1:Zn(Im)0.73(dmbIm)1.27, GIS-2:Zn(mIm)o.64(dmbIm)1.36, GIS-3:Zn(eIm)0.65(dmbIm)1.35. GIS-1exhibited sphere architectures which are assembled by hundreds of sub-microcrystals. GIS-2exhibited well-defined nanosheets. GIS-3exhibited hierarchical hollow persimmon-like structures which are constructed by many nanosheets. It was found that the addition of NH3H2O was conducive to the formation of uniform hierarchical structure and primary particles, and also could control the diameter and aggregate mode of the primary particle.