Design And Performance Study Of Semi-rigid MOFs For Gas Separation

At present,there are numerous problems in the storge and separation of gas fuels/feedstocks,especially for light hydrocarbon gases,which must be separated and purified for subsequent use.Normally,low-temperature distillation and solvent extraction are the main methods currently used for gas separation in the chemical industry,but these methods generally have the disadvantages such as high energy consumption,complex processes and high costs.To achieve efficient and energy-saving separation of gas fuels/feedstocks,researchers propose a new separation method using solid adsorbents.Compared with conventional solid adsorbents such as zeolites,activated carbon and molecular sieves,as a new type of porous material,metal-organic frameworks（MOFs）,also known as porous coordination polymers（PCPs）,are considered as ideal gas separation materials due to their advantages such as large specific surface area,high porosity,and tunability of pore size and surface.In this work,we synthesize a series of MOFs materials for gas separation under the solvothermal conditions and their crystal structure,thermal stability and gas adsorption and separation properties have been investigated as follow:1.The flexible tetracarboxylic acid ligand 2,3’,4,5’-tetracarboxylic acid diphenyl ether（L1）and the main group metal ion In（III）were used to synthesize compound 1 under solvothermal method and compound 1 crystallizes in the chiral P3₁21 space group and displayed a 3D porous structure with 2D interlaced pores with both the central metal ion In（III）and the L1 ligand are both 4 coordination numbers.In gas adsorption studies for C₂H₂,CO₂ and CH₄,compound 1exhibits good C₂H₂ and CO₂ adsorption capacity and extremely low adsorption capacity for CH₄,therefore the selectivity of C₂H₂/CH₄ and CO₂/CH₄ was calculated to be 65.5 and 10.5,respectively.Meanwhile,GCMC simulation and breakthrough experiments show that compound1 as a solid adsorbent can effectively separate CH₄.Besides,compound 2 and 3 were constructed using the transition group metal ions Cu（II）and Ni（II）with L1 under solvothermal method,respectively.The adsorption experiments of compound 2 and 3 on C₂H₂,C₂H₄,C₂H₆,CO₂ and CH₄ indicate that both frameworks have the lowest affinity for CH₄ and exhibited the lowest CH₄adsorption capacity and the selectivities of other gases for CH₄ calculated by IAST method further demonstrate that compound 2 and 3 have the potential in the separation and purification of CH₄.2.The flexible tetracarboxylic acid ligand 2,3’,5,5’-tetracarboxylic acid diphenyl ether（L2）and the main group metal ion In（III）were used to synthesize compound 4 under solvothermal method,where L2 and L1 are isomers with different carboxylate positions,so that compound 1and 4 have completely different structures.Compound 4 crystallizes in the orthogonal Pnma space group with both the central metal ion In（III）and the L2 ligand are both 4 coordination numbers.In gas adsorption studies for CO₂,the CO₂ adsorption isotherm of compound 4 shows a"gate-opening effect"at 273-288 K,as well as a rapid increase in CO₂ adsorption when the pores of the framework open at a specific pressure.Meanwhile,compound 5 was synthesized with Co（II）and L2 under solvothermal method,and gas adsorption studies of compound 5 for C₂H₂ and CO₂ shows that its adsorption capacity of CO₂ exceeds that of C₂H₂,a relatively rare occurrence among MOFs materials.Therefore,compound 5 can be seen as a CO₂-selective adsorbent for the reverse separation of CO₂ from CO₂/C₂H₂ gas mixtures.The adsorption experiments of compound 4 and 5 indicate that they are potential MOF materials in the separation and purification of CO₂.3.The flexible pentacarboxylic acid ligand（2,2’-（（5-carboxy-1,3-phenyl）bis（oxy））di-phenylene terephthalic acid）（L3）and the transition group metal ion Mn（II）were used to synthesize compound 6 under solvothermal method,and compound 6 crystallizes in the orthogonal Imm2 space group with Mn-O multicore clusters.In gas adsorption studies for C₃H₈,C₂H₆,CO₂ and CH₄,compound 6 shows high C₃H₈,C₂H₆ and CO₂ adsorption capacity and very low CH₄ adsorption capacity.And the selectivities of compound 6 for equimolar C₃H₈/CH₄,C₂H₆/CH₄and CO₂/CH₄ calculated by IAST method shows that compound 6 exhibits great CH₄separation performance.Meanwhile,GCMC simulation and breakthrough experiments show that compound 6 as a solid adsorbent can effectively separate CH₄ in mixed shale gas.