Design And Synthesis Of Multi-site Metal-organic Framework And Its Studies On Gas Adsorption

The massive emissions of CO₂ and NH₃ have caused a series of environmental problems,and carrying out their capture work is the precondition for reducing environmental pollution and resource utilization.Metal-organic frameworks（MOFs）,which have large surface areas,high thermal stabilities,and adjustable pore structures,are widely used in gas absorption and capture,catalysis and drug delivery.In gas absorption,the number of adsorption sites in MOFs has a great effect on the absorption ability.Multi-site MOFs contain more adsorption sites,which can enhance the gas adsorption capacity.Although MOFs have great application prospects,the difficulty in upscale preparation is one of the main reason that limits their industrial application.In this context,seven multi-site MOFs were designed and synthesized and eighteen metal organic polymers were prepared by simple and green methods for the adsorption and selective separation of CO₂and NH₃ in this work.On the basis of NH₃ and CO₂ adsorption capacity,the stabilities and selective adsorption capacities of the materials,and a preliminary analysis of adsorption mechanism were investigated.These results provided a scientific basis for design and synthesis of multi-site MOFs and the selective adsorption performance.The main contents are as follows:1.Four multi-site Al-MOFs（MOF-303,MIL-160,Al-fum,CAU-10-dypc）were synthesized and used for CO₂ adsorption and the selective separation of CO₂/N₂ and CO₂/CH₄.Selecting MOF-303 as an example,the stabilities,cyclicalities,and thermal effect during the adsorption process were investigated.It was shown from the results that,the CO₂ adsorption capacity of the four materials were 5.1,3.5,2.6 and4.0 mmol/g,respectively,at room temperature.Compared with the reported MOFs,MOF-303 has higher CO₂ adsorption capacity.The structure and crystallinity of the MOFs didn’t change after CO₂ adsorption and their adsorption capacities didn’t decrease after 50 adsorption-desorption cycles,indicating that these materials have high cyclicality to CO₂;the adsorption heats of four materials were 26.2,29.2,22.2 and 27.0k J/mol,respectively,indicating that physical adsorption during the adsorption process.The theoretical selective adsorption coefficients of the materials for CO₂/N₂ were 9.6,21.6,12.4 and 23.2,respectively at353 K,which have good selectivity of CO₂ and were expected to effectively separate CO₂ from CO₂/N₂/CH₄ waste mixtures.2.Three multi-site organic carboxylic acid ligands were designed and used to prepare multi-site Zr-MOFs by solvothermal method with Zr Cl₄.The Zr-MOFs were characterized by XRD,TG,SEM and BET techniques.The NH₃ adsorption capacity,selective separation capacity of NH₃/CO₂ and stabilities of the materials during the adsorption process were investigated,and a preliminary analysis of the adsorption mechanism was proposed.The results shown that the multi-site Zr-MOFs have good thermal stabilities,and the NH₃ adsorption capacity of them was 6.3,5.5 and 7.2 mmol/g,respectively,which were 2-3 times higher than the unfunctionalized Uio-68.Their structures and crystallinity didn’t change during the adsorption process,and the adsorption capacities were not reduced after five adsorption-desorption cycles;the theoretical selective adsorption coefficients of the materials for NH₃/CO₂ were 12.6,7.9 and 12.0,respectively.It is expected to efficiently and selectively separate NH₃ from NH₃/CO₂ waste gases.The mechanism results shown that-NH,C2-H of imidazole,-CN and-OH could play important roles during NH₃ absorbtion and effectively capture of NH₃.The contribution of the adsorption sites were as follows:NH>-OH>C2-H of imidazole.The results could provide scientific information for the designing of multi-site functionalized MOFs.3.Combination of seven metal salts and three organic ligands,18 kinds of metal organic polymers were synthesized by a simple and green method,and the materials were characterized by XRD,TG,SEM and BET techniques.NH₃ adsorption capacities of the materials were evaluated.Taking Cu-Py and Ni-Py as examples to study the selective separation of NH₃/CO₂,NH₃/CH₄ and NH₃/N₂ and the stabilities during adsorption process.The results showed that the NH₃ adsorption capacities of Cu-Py,Ni-Py,Cu-Pz,Co-Pz,Zn-Pz and Zn-Im were 16.5,21.1,14.7,15.7,14.7 and 13.6 mmol/g,respectively,especially the NH₃uptakes of Ni-Py was 21.1 mmol/g,which is closed to the highest value reported in literature.At room temperature,the theoretical selective adsorption coefficients of Cu-Py for NH₃/CO₂,NH₃/CH₄,NH₃/N₂,CO₂/CH₄ and CO₂/N₂ of were 13,1031,423,78 and 32,respectively,and as for Ni-Py,the theoretical selective adsorption coefficients were 281,21110,811,75 and 26,respectively for NH₃/CO₂,NH₃/CH₄,NH₃/N₂,CO₂/CH₄ and N₂/CO₂.It was shown that Cu-Py and Ni-Py could effectively separate the mixed gas of NH₃,CO₂,N₂ and CH₄.At 353 K,the theoretical selective adsorption coefficients of Cu-Py and Ni-Py for NH₃/CO₂ were 39 and 520,which can be used for efficiently separate NH₃ from industrial waste gases.