Synthesis And Catalytic Application Of Zeolitic Imidazolate Frameworks (ZIFs)

Zeolitic Imidazolate Frameworks (ZIFs) is a new type of MOFs materials. It is a kind of porous crystalline material of topology structure, which is formed through the complexation of N atom on the imidazole ring with divalent transition metal ions. Different ZIFs structures could be obtained through modulation of the interaction between metal ions and the ligands. ZIFs combine the advantages of zeolite and traditional MOFs materials with high thermal stability (up to 550℃), chemical stability, high porosity, large specific surface area, and adjustable functional structure. Its superior performance won international scientists attaches great importance to, and quickly developed into interdisciplinary research hot spot.ZIF-8, ZIF-67, and ZIF-9 materials were synthesized and characterized with the XRD, SEM, TEM, TG, FT-IR, UV-vis, N2 physical adsorption and so on. We employed ZIF-67 and ZIF-9 in the catalytic oxidation of ethylbenzene system.In acetonitril as solvent, ZIF-67/NHPI realized the high efficient oxidation of ethylbenzene under mild conditions. It achieved 95.2% conversion of ethylbenzene with 90.3% selectivity to acetophenone, and phenyl ethyl hydrogen peroxide has completely decomposed at 373 K under 0.3 MPa O2 for 9 h. Using NHPI alone, the conversion of ethylbenzene was 57.7%, and the selectivity of phenyl ethyl hydrogen peroxide was 33.8%. The results showed that the decomposition is of low efficiency. When ZIF-67 was used alone, PEHP can be decomposed efficiently, suggesting the tetrahedral coordinated Co (Ⅱ) in ZIF-67 has strong ability to decompose PEHP. Then, we compare ZIF-67 with Co(OAc)2, CoSO4 and CoCl2. Cobalt acetate/NHPI achieved a conversion close to that with ZIF-67/NHPI, but the selectivity of target product AcPO is comparably low (72.6%), and the PEHP selectivity is 13.1%, suggesting that ZIF-67 has stronger ability in decomposition of PEHP than Co(OAc)2 at low loading. The imidazol-based ligands may strongly affect its catalytic performance when combing with NHPI.In solvent-free system, ZIF-67 and ZIF-9 were soaked in different solvents for 24 h, and the crystal structure remain unchanged after soaking through XRD characterization. It illustrates that the ZIF-67 catalyst was organic solvent resistant. Then ZIF-67 materials after different solvents soaking were employed as catalysts for ethylbenzene oxidation. Compared with that over ZIF-67 directly synthesized in methanol, their selectivities to acetophenone were almost unchanged but with lower conversion of ethylbenzene. Especially for displacement ZIF-67 by DMF, ethylbenzene conversion reduced obviously. This may be because the solvents adsorbed in the ZIF-67 channel or interacted with cobalt centers. The recycle experiments showed that the catalyst could be recycled for 3 times without obvious change in catalytic porformance.