Modified Preparation And Properties Of Metal-Organic Frameworks M₃（BTC）₂

Metal-organic frameworks have widely applied in the field of gas adsorption/separation and electrocatalysis,due to their advantageous and unique characteristics including extra-high surface area,structural tenability,porosity,modifiable channel and abundant components.MOFs with coordinatively unsaturated metal sites（CUS）is a kind of classical MOFs materials,CUS in MOFs can binding guest molecules based on the weak chemical interaction,which can be used for the modification of channel and for gas adsorption and separation.M₃（BTC）₂（also named M-HKUST-1）,is one of the most emblematic and part of an isostructural series that currently includes Cr,Fe,Ni,Cu,Zn,Mo,and Ru analogues.However,most of such MOFs are unable to expand their application fields well due to their simple structure or difficulty in preparation.In our paper,we firstly try to synthesize high purity MOFs via solvent assisted method,and to modify the channel by utilize the advantages of CUS which can act as interact node to coordinate functional atoms.And because open metal sites exhibit the most prominent impact on adsorption performance of MOFs for nonpolar ones such as CH₄,we synthesized M₃（BTC）₂ with strong metal active sites,and compared with Cu₃（BTC）₂ to study the interactions between the metal sites and CH₄.On the other hand,we prepared mixed-metal organic frameworks via postsynthesis and one pot method,respectively,and studied their gas adsorption and separation properties.Moreover,we also expanded the electrocatalysis properties for mixed-metal organic frameworks derived materials.The research of this thesis is mainly divided into the following parts:1.The low purity and yield of Ni₃（BTC）₂ has always been a difficult problem to solve,and there are few literatures were reported about the applications of Ni₃（BTC）₂.In this paper,we firstly synthesized the Ni₃（BTC）₂ with high purity（>90%）and yield（85%）via（CH₃）₂NH assisted method.During the synthesis process,（CH₃）₂NH will be coordinated on Ni metal sites to modify the channel.Subsequently,we test the CO₂/CH₄/N₂ adsorption separation properties for this materials.The results showed as follows:At 273 K and 1 atm,the selectivities for CO₂/CH₄ and CO₂/N₂ were 14.49 and 183.41,respectively,and the adsorption capacities for CO₂ and CH₄ were 223.87 cc/g and 67.89 cc/g,respectively.This is a rare demonstration of both high selectivity and adsorption capacity in the separation of CO₂,CH₄,and N₂.2.Based on the strong interaction energy between the coordinatively unsaturated metal sites（CUS）and the gas molecules,M₃（BTC）₂（M=Cr,Mo,Ni）,isostructural with Cu₃（BTC）₂（HKUST-1）,were successfully synthesized via the coordination reaction of the high activity divalent metal(Cr²⁺/Mo²⁺/Ni²⁺)and organic ligand（H₃BTC）,and then to gain insight into the impact of gas-metal interactions on the selective adsorption of CH₄ from N₂ by comparing with classical materials（Cu₃（BTC）₂）.The results show the good CH₄ selective adsorption properties of M₃（BTC）₂（M=Cr,Mo,Ni）with CUS.However,among these materials,Ni₃（BTC）₂ with unsaturated Ni²⁺sites presents the highest adsorption heat value for CH₄,leading to the excellent CH₄/N₂ adsorption separation properties;but the Cr₃（BTC）₂ with high Cr²⁺activity shows the lower ability for CH₄/N₂ separation than Cu₃（BTC）₂.Finally,combine the IAST calculation results,we verified the ability for the selective adsorption of CH₄molecules on Mo₃（BTC）₂（M=Cr,Mo,Ni）and Cu₃（BTC）₂followed as Ni₃（BTC）₂>Mo₃（BTC）₂>Cu₃（BTC）₂>Cr₃（BTC）₂.3.Cr₃（BTC）₂ is a classical oxygen adsorbent,but the difficulty of O₂desorption and unstable structure limited it’s industry application.The structure of Cu₃（BTC）₂ is stable,but the O₂ adsorption properties is poor,which is due to the low electronegativity of Cu²⁺.So,we chosen Cu₃（BTC）₂ as reaction substrate substituted part of Cu²⁺in MOFs to the Cr²⁺and Fe²⁺with higher electronegativity.The experiment data shows that the O₂ adsorption capacity and the isosteric heat of the M_xCu_3-x（BTC）₂（M=Cr,Fe）were higher than Cu₃（BTC）₂,and that of Cr_0.52Cu_2.48（BTC）₂ is highest,which was due to the highest electronegativity of Cr²⁺among these three metals.And the type of O₂ adsorption isotherms for Cr/Cu-BTC shows the physical adsorption type,which is easier to desorb O₂ from the MOFs.Subsequently,for M_xCu_3-x（BTC）₂（M=Cr,Fe）,there is almost no loss in absorbed quantity of O₂ after four cycles.More importantly,the structure and O₂adsorption capacity of M_xCu_3-x（BTC）₂（M=Cr,Fe）almost no change after placed these two MOFs in air for 1-3 days.4.MOF-derived catalysts exhibit homogeneous catalytically active metal sites,which are obtained from the regular placement of metal nodes within the original MOFs.Mixed-metal MOFs can offer an easier preparation method for catalysts doped with heteroatoms.In our research,we firstly synthesized the mixed-metal MOFs(Co_1.36Ni_1.64（BTC）₂)through one pot method,and then obtained catalyst Co doped Ni₃C/Ni uniformly dispersed in a graphitic carbon matrix by pyrolysis of Co/Ni-MOF under a flow of Ar/H₂ at 350℃,and Ni₃C/Ni@C was also prepared for comparison.The various characterization techniques confirmed the successful preparation of the heteroatom doped TMCs-based catalysts by pyrolysis of MOFs.Co doped Ni₃C/Ni@C exhibited superior electrocatalytic properties for OER.For example,Co-Ni₃C/Ni@C depicts a lower overpotential and smaller Tafel slope than Ni₃C/Ni@C and Ir O₂ during the OER in 1 M KOH solution,additionally,it shows a higher active surface area than Ni₃C/Ni@C.The outstanding electrocatalytic performance of Co-doped Ni₃C/Ni@C in the OER was mainly ascribed to the synergistic effect of the Co and Ni₃C/Ni active sites.