Metal-Organic Framework Materials For Acetylene Separation And Purification

Metal-organic framework materials(MOFs)are a class of promising materials that have received widespread attention in the past two decades.MOFs are regarded as a sub-category of coordination polymers.They are formed by self-assembly of metal ions or metal clusters.These metal ions or metal clusters are connected together with organic ligands by coordination bonds.They have permanent porosity,high specific surface area and adjusted topology.MOFs materials can be used in different fields,such as gas adsorption and separation,catalysis,sensing,and drug delivery.This paper mainly focuses on the application of MOFs in acetylene adsorption and separationIn this paper,we mainly focus on the design and synthesis of organic ligands and the application of MOFs constructed from them in the selective adsorption and separation of acetylene.The effect of the positional isomerization of the coordination group and functional group of the tetracarboxylic acid ligands on the structure of MOFs and the selective adsorption performance of acetylene was explored;synthetic strategies were developed to improve the selective adsorption performance of MOFs for acetylene;the ligand conformation engineering strategy were employed to control the structure and gas adsorption performance of the resulting MOFs.Details are listed as follows:1.Two pairs of isomeric tetracarboxylic acid ligands were designed,including one couple of 2,3',4,5'-biphenyltetracarboxylic acid and 3,3',4,5'-biphenyl tetracarboxylic acid featuring positional isomerism of carboxylate coordination group,and the other couple of 5,5'-(4-methylbenzylamine-2,6-diyl)bis(isophthalic acid)and5,5'-(4-methylbenzene-3,5-diyl)bis(isophthalic acid)featuring positional isomerism of functional groups.Two pairs of copper-based MOFs were constructed by solvothermal method:ZJNU-9 and ZJNU-10,ZJNU-11 and ZJNU-12.The first pair of MOFs belongs to the homometallic but hetero SBU SBU-type MOFs,with different secondary structural units,ligand configurations and network topologies.Compared with ZJNU-10,ZJNU-9 does not adsorb N₂ at 77 K,but exhibits better adsorption capacities of C₂H₂,CO₂,CH₄ and higher C₂H₂/CO₂ and C₂H₂/CH₄ adsorption selectivity at room temperature.The latter pair of MOFs has the same topological structure,but the different arrangement of functional groups leads to different framework stability and thus gas adsorption performance.As far as C₂H₂,CO₂ and CH₄ are concerned,the adsorption performance of ZJNU-11 is better than that of ZJNU-12,which is mainly due to the fact that the former has better framework stability than the latter.The above work not only reported rare examples of homometallic hetero SBU MOFs for selective gas adsorption applications,but also demonstrated that the positional isomerism of coordination groups and functional groups has a significant impact on the co-assembly results and gas adsorption performance of the resulting MOFs.Reasonable arrangement of functional groups and coordination moieties is very important for designing MOFs with better performance.2.In order to improve the selective adsorption performance of acetylene,a strategy of introducing abundant hydrogen binding acceptors on the surface of the framework is proposed.A N-oxidation functionalized dicarboxylic acid ligand,3,5-dicarboxypyridine oxide,was designed and synthesized,and its copper-based MOF ZJNU-13 was constructed by solvothermal method.Single-crystal X-ray diffraction analyses showed that ZJNU-13 has no open metal sites in the three-dimensional network structure,but has a large number of carboxylic acid oxygen atoms and N-oxide atoms as acetylene recognition sites.The pure-component gas adsorption tests and IAST adsorption selectivity calculations showed that ZJNU-13 has excellent C₂H₂ separation and purification capabilities.The dynamic breakthrough experiments confirmed its separation potential.DFT calculations clarified the key role of carboxylic acid oxygen atoms and pyridine N-oxide oxygen atoms in the selective adsorption of acetylene.This work demonstrates that N-oxide is a functional group for effective separation and purification of C₂H₂,which can be used to design and construct new MOFs materials,and provides an inspiration for the development of porous MOFs for efficient gas separation.3.Inspired by the above work,two N-oxide-functionalized linear bis(isophthalic acid)ligands with different methyl positions were designed and synthesized,namely5'-(3-methylpyridine N-oxide-2,5-diyl)diisophthalic acid and 5,5'-(4-picoline N-oxide-2,5-diyl)diisophthalic acid to construct Nb O-type NOT-101 platform(ZJNU-18 and ZJNU-19)to study the effect of methyl position and N-oxidation functionalization on the selective adsorption of acetylene.Through single-crystal X-ray diffraction,it is found that regardless of the position of the methyl group,ZJNU-19 and ZJNU-20 have the same network structure as the prototype MOFs NOTT-101,and the space group is R3m.Both ZJNU-19 and ZJNU-20 are connected by Cu₂(COO)₄(H₂O)₂ double copper paddlewheel unit and bis(isophthalic acid)anion to form a non-interpenetrating Nb O-type network.Gas adsorption shows that the difference in the position of the methyl group has little effect on the adsorption performance,while the N-oxidation functionalization can significantly improve the adsorption performance of acetylene and carbon dioxide.In particular,the gravimetric C₂H₂ adsorption capacity of these two MOFs at 295 K and 1 atm is the highest record of Nb O-type MOFs analogues reported so far.Research on the relationship between structures and properties showed that N-oxide functional groups play a vital role in enhancing the adsorption of C₂H₂ and CO₂.This provides new ideas for the future design and synthesis of N-oxide framework materials for selective gas separation and purification.4.Ligand conformation engineering to control the structure and adsorption performance of MOFs:In order to control the conformation of the biphenyl-3,4',5-tricarboxylic acid ligand,two asymmetric tricarboxylic acid ligands were designed and synthesized,namely 2'-Methoxy-[1,1'-biphenyl]-3,4',5-tricarboxylic acid and 2'-nitro-[1,1'-biphenyl]-3,4',5-tricarboxylic acid Acid,and used to successfully construct two copper-based MOFs:ZJNU-109 and ZJNU-110.Single crystal X-ray diffraction shows that ZJNU-109 belongs to the monoclinic system,while ZJNU-110 belongs to the triclinic system.Due to the asymmetry of the ligand,two different copper clusters were formed,greatly expanding the libray of the secondary structural units of the copper-carboxylate clusters.In addition,gas adsorption studies showed the ability of the two MOFs to separate and purify C₂H₂.At 298 K and 1 atm,the C₂H₂/CH₄ adsorption selectivity of ZJNU-109 and ZJNU-110 is 21.6 and 15.3,respectively.This work shows that simple ligand modification can be used to design the structure and adjust the gas adsorption properties of MOFs,and it also demonstrates that ligand conformation engineering is an effective strategy to adjust the structure and properties of MOFs.