Study On The Structure And Gas Adsorption Properties Of MOFs Constructed By Carboxylic Acid Ligands

The acquisition of high-purity raw materials such as acetylene,ethylene,ethane and carbon dioxide gas are crucial in energy,environmental protection and industrial production.However,due to the relatively similar properties of these gas molecules as well as many shortcomings of traditional separation methods（large equipment scale and high energy consumption）,it is difficult to completely separate their mixtures.Therefore,it is necessary to develop a new technology that can efficiently separate these gas mixtures（with the advantages of simple equipment and fast response time）.Currently,adsorption separation method is considered to be a potential technology to replace traditional separation methods.The principle is to utilize the differential adsorption between different gases to achieve the purpose of effective gas separation.Metal-Organic Frameworks（MOFs）,as a new type of porous crystal materials that have developed most rapidly in the past two decades,have become one of the most popular materials for gas adsorption and separation due to their structural diversity,high specific surface area,multiple binding sites,and porosity.In view of the current research on MOFs materials in improving gas adsorption capacity and effective separation,this paper has constructed six MOFs materials using simple and inexpensive organic ligands and metal salts through solvothermal methods:[Cu₄（3,5-dpa）₃（Cl O₂）₃（OH）₂]_n（1）,[Ca（2,4-dpa）（DMF）]_n（2）,[Ca（2,5-dpa）（DMF）]_n（3）,[Ca₄（3,5-dpa）₃（DMF）₄]_n（4）,[In₃（btc）_8/3（H₂O）₄]_n（5）,[In（bpdc）]_n（6）.These materials have been studied in detail using single crystal X-ray diffraction,elemental analysis,thermogravimetric analysis,infrared spectroscopy,powder X-ray diffraction and gas adsorption separation.The main purpose of the study is to explore the effective adsorption of gases by MOFs materials and the effective separation of binary mixtures of gases.The specific research content and results are summarized as follows:The complexes 1-4 have a three-dimensional porous network structure,among which complex 1 is a copper-based MOF containing a large number of polar oxygen atoms and electron withdrawing group chlorine atoms;Complexes 2-4 are calcium-based MOFs with novel topological structures.The stability of 1-4 was studied through PXRD and TGA analysis,and the results showed that 1-4 had high phase purity and thermal stability.The rich functional sites in the pores of complex 1provide a research basis for gas adsorption and separation.It exhibits good adsorption performance for C₂H₂gas,with an adsorption capacity of 136.5 cm³/g at 273 K and 110.7 cm³/g at 298 K.The calculated adsorption heat is 36.98 k J/mol.The high adsorption capacity and moderate adsorption heat indicate that there is a strong interaction force between complex 1 and C₂H₂molecules.Compared with the high adsorption capacity of complex 1 for C₂H₂gas,its adsorption force for CO₂gas is lower,with an adsorption capacity of 97.0 cm³/g at 273 K and80.5 cm³/g at 298 K.Based on the difference in adsorption capacity between C₂H₂and CO₂by complex 1,it is predicted that it can separate the C₂H₂/CO₂mixture.Using IAST（Ideal Adsorption Solution Theory）simulation,it was calculated that the separation ratio of complex 1 to the binary gas mixture C₂H₂/CO₂is 8.5.Compared with many reported MOFs materials that can effectively separate C₂H₂/CO₂,the separation performance of complex 1 is more prominent.This result indicates that it can effectively separate the mixture of C₂H₂/CO₂.Complex 5 and complex 6 are structurally novel In-group MOFs,through studying the gas adsorption properties of these two complexes,it is found that the adsorption capacities of complex 5 for CO₂,C₂H₆,C₂H₄,and C₂H₂are 32.3 cm³/g,35.6 cm³/g,38.2 cm³/g and 50.5 cm³/g,respectively.The adsorption capacities of complex 6 for CO₂,C₂H₆,C₂H₄and C₂H₂are 26.8 cm³/g,14.5 cm³/g,13.7 cm³/g and 27.4 cm³/g,respectively.Based on the differences in adsorption capacity between complex 5 and C₂H₂and CO₂,as well as between complex 6 and C₂H₂and C₂H₄,it is predicted that these two complexes can separate the mixture of C₂H₂/CO₂and C₂H₂/C₂H₄,respectively.Through IAST simulation,the separation ratio of complex 5 to C₂H₂/CO₂is 3.3,and that of complex 6 to C₂H₂/C₂H₄is 3.6,which are equivalent to some reported MOFs materials,this indicates that complex 5 pairs of C₂H₂/CO₂and complex 6 pairs of C₂H₂/C₂H₄can be effectively separated.