Preparation and Adsorption Properties of Novel Metal-organic Frameworks

Author:Zhou Lin

Supervisor:chen yun lin christian serre


Degree Year:2019





Metal-organic frameworks(MOFs)are crystalline porous coordination compounds with two-dimensional or three-dimensional frameworks,which are assembled by metal ions or clusters and organic ligands through coordination bonds.Compared with traditional porous materials such as zeolite,activated carbon and porous silicon,MOFs have many structural characteristics,such as diverse topology,large specific surface area and porosity,adjustable pore size and functions,and varies degree of flexibility.MOFs have been therefore extensively studied in the fields of adsorption,catalysis,luminescence,sensing and proton conduction.MOF materials were first used in the field of gas adsorption and separation,and the related individual applications have appeared in the commercial market.However,due to the complexity and wide range of practical problems to be solved in this field,it is still one of the hotspots,emphases and difficulties in the application of MOFs.On the one hand,it is necessary to select or synthesize appropriate MOFs according to the characteristics of the adsorbate molecules for adsorption separation.On the other hand,since a large number of MOFs have been experimentally synthesized,the key to expand their adsorptive applications is to design/find a proper separation system/process for each specific MOF,which has the potential as an adsorbent.On this basis,the low cost or green synthesis of MOFs is of great significance to promote the industrialization of MOF materials.Focusing on solving the above problems,we firstly improved the preparation methods of three MOF materials(UiO-66,A520 and hep UiO-66)and investigated the gas phase adsorption/separation properties.Secondly,two new MOF materials(Fe-PyC and Zr-Cu-PyC)were synthesized,and their gas-phase adsorptive separation and photocatalytic performance were tested,respectively.Thirdly,functionalized hep UiO-66 materials were synthesized and the optical properties were studied.Due to the structural differences among these MOFs,corresponding application systems were developed according to their structural characteristics.The research work of this thesis is mainly divided into the following parts:1.Preparation of UiO-66 based on Zr6 cluster and the adsorption behavior of dichloromethane(DCM).Firstly,a two-step synthesis method was proposed to solve the problem of poor crystallinity of the product obtained by one-step synthesis method.On this basis,the synthesis process was modulated by adding different amounts of acetic acid.The crystal size and microporous surface area of UiO-66 synthesized increased with the increase of acetic acid content.Thermogravimetric analysis(TGA)showed that the addition of acetic acid led to the loss of ligand in UiO-66,resulting in the formation of defects.The ratio of ligand loss was as high as 7/12.The vapor adsorption experiment showed that the maximum amount of dichloromethane adsorbed by UiO-66 samples was 530.2 mg/g,47.3%higher than that of the UiO-66 sample synthesized without acetic acid modulating.Besides,the samples kept good crystallinity after the adsorption was completed.2.Rapid preparation of A520 and its highly selective adsorption of dichloromethane(DCM)/trichloromethane(TCM).Firstly,the green rapid preparation of A520 nano-materials was realized by a common solution method with water as solvent.Secondly,the DCM and TCM adsorption isotherms on A520 for were measured.According to ideal adsorbed solution theory,A520 exhibited excellent selective adsorption of DCM over TCM.The adsorption selectivity at 298 K was up to 20.0,which is much higher than that of HKUST-1 and IRMOF-3(2.17 and 5.62,respectively).3.The preparation of hcp UiO-66 based on Zr12 cluster and its effective separation of benzene and cyclohexane.Firstly,hcp UiO-66 was synthesized by a solvothermal method using polyethylene terephthalate(PET)waste as ligand source.The BET surface area(995 m2/g)of the obtained sample was much higher thal its theoretical value(700 m2/g)due to the exist of missing linker defects.Secondly,the stability tests showed that the material had good thermal stability of at least 350 ℃ in air and excellent hydrothermal stability of at least 200 ℃.In addition,the gas phase adsorptive breakthrough experiments showed that the existing of a strong interaction between the material and benzene molecules,reasulting in the effective separation of benzene/cyclohexane.4.Synthesis of Fe-PyC and its separation of hexane isomers.Firstly,the first MOF constructed by the coordination of Fe(Ⅲ)with both carboxyl and nitrogen heterocycles,Fe-PyC,was synthesized by using a small size 4-pyrazolecarboxylic acid(H2PyC)ligand.The MOF has flu-e topology,and thus contains three kinds of cavity structures with different shapes and sizes.Secondly,based on the unique pore structure,the performance of Fe-PyC for separating hexane isomers was tested by gas breakthrough experiments.The results showed that five isomers of hexane can be completely separated by Fe-PyC at 100 ℃,which is of great significance for increasing the octane number of gasoline.5.The synthesis of Zr-Cu-PyC and the preliminary study of its photocatalytic activity.Firstly,a bimetallic mesoporous MOF material Zr-Cu-PyC with spn topology was designed and synthesized based on hard and soft acid-base theory.The MOF is prone to collapse after removal of solvent molecules in the pore,and thus is difficult to be used for gas adsorption.The photocatalytic cycloaddition of phenylacetylene with benzyl azide was investigated due to the existence of Cu redox active sites in the structure.It was found that the Zr-Cu-PyC had good photocatalytic activity under the irradiation of ultraviolet-visible(UV-Vis)light source,and the highest conversion of the reaction was 95%in tetrahydrofuran(THF)solvent.After the reaction,the crystal structure of Zr-Cu-PyC remained unchanged.6.Functionalization of hcp UiO-66 and the optical properties.Three kinds of functionalized hcp UiO-66 isomorphic MOFs were designed and synthesized:hcp UiO-66-CH3,hcp UiO-66-NH2 and hcp UiO-66-NDC.The optical bandgap and photoluminescence properties of the materials were studied respectively.It was found that introducing methyl,amino functional groups or using 1,4-naphthalic acid ligands could reduce the band gap of hcp UiO-66.Among them,amino and naphthalene rings could significantly change the band structure of the material and greatly reduce the band gap of MOF materials.Accordingly,the functionalization changed the luminescent properties of the MOFs.The luminescence wavenumber ranges of hcp UiO-66-NH2 and hcp UiO-66-NDC are extended to long wave direction compared with that of hcp UiO-66.