Synthesis Of Metal-Organic Frameworks And Their Application In Gas Separation

Light hydrocarbons(C1-C4)are important industrial raw materials for the production of various polymer materials and organic intermediates.These light hydrocarbons are usually derived from petroleum cracking gas and alkanes reforming,etc.The resulting crude product is usually composed of a mixture of several light hydrocarbons and various impurities.When producing various chemical products,the purity of the raw materials is required to be high.For example,in the production of polyethylene,a trace amount of acetylene can poison the catalyst Therefore,in order to obtain a high-purity product which is advantageous for the next production,it is necessary to separate the crude product.Traditionally,cryogenic distillation and absorption method have been the two main separation methods,and these methods have high energy consumption and require large scale equipment.Therefore,it is necessary to develop new separation technologies to reduce energy consumption.The adsorption separation method based on selective adsorption for different gases to achieve separation,and is considered to be very promising to replace the traditional separation method.The most important of the adsorption separation method is the development of adsorbents.Metal-organic framework(MOFs)materials are a class of organic-inorganic hybrid porous materials with adjustable pore size and adjustable functional sites.These characteristics make it attractive in the field of gas adsorption separation and has been extensive researched.In general,the adsorption capacity and selectivity cannot be equilibrized,which limits the adsorption separation method is further studied.In response to this problem,the main work of this paper is to find materials with both selectivity and adsorption capacity,which will lay the foundation for further research.The main research contents as follows.(1)By designing and synthesizing two organic ligands,L1 and L2,four kinds of MOF-74 isoreticular with larger pore diameters were successfully prepared by solvothermal reaction with metal ions.The structure was determined by high-precision powder x-ray diffraction and DFT-D calculation,and characterized by XRD,BET and TG.Ni-L1 and Ni-L2 were selected for gas adsorption and desorption experiments.The adsorption and separation capacities of C2H2/CO2 and C2H2/C2H4 were studied.The calculations showed that the selectivity of Ni-L1 to both were 12 and 4.5,and the Ni-L2 to both were 6.2 and 1.7,respectively,indicating good separation performance.At the same time,the adsorption and separation ability of other gases was also studied,and the reasons for its adsorption separation ability were preliminarily analyzed.(2)NTU-9 is a Ti based MOF containing a large number of polar oxygen atoms.The polar oxygen atoms provide abundant functional sites for gas separation.NTU-9 was successfully prepared by solvothermal method and characterized by XRD,BET and TG.Subsequently,the gas adsorption desorption experiment was carried out to study its adsorption separation ability for C2H2/CO2 and C2H2/C2H4,and the theoretical selectivity was calculated to be 3.5 and 2.2,respectively.Compared with other MOFs which have similarly C2H2/CO2 selectivity,NTU-9 has a higher C2H2 adsorption capacity of 118 cm3/g and an adsorption heat of 30 kJ/mol.The high adsorption capacity and the low adsorption heat make NTU-9 has a good separation performance.Subsequently,the hydrothermal stability was studied,and the reasons for its adsorption separation ability were preliminarily analyzed.(3)Hofmann-type MOFs are a class of MOFs with one-dimensional pores and open metal sites.They have a pore size range of about 4 A,are in the ideal separation pore size range and are an ideal separation material.Three kinds of Hofmann-type MOFs of FeNi,FePt and FePd were prepared rapidly by stirring at room temperature.Their structures,specific surface area and pore structure were determined by XRD and BET.The separation performance of C2H2/CO2 was studied by gas adsorption desorption experiment.The separation ability was evaluated by its theoretical selectivity and adsorption heat.Among them,FeNi has the highest C2H2/CO2 separation selectivity of 24,exceeding most MOFs,and its adsorption capacity is 133 cm3/cm3,which also exceeds most MOFs,achieving equilibrium of adsorption capacity and selectivity,setting a new separation benchmark.Subsequently,DFT-D calculations and neutron powder diffraction experiments were carried out on FeNi.The results show that it is the synergistic effect of proper pore size and functional sites that FeNi has highest separation performance.