| The separation and purification of C2H2,CH4,CO2,CO,N2and other gases are of vital importance to energy,environmental protection,and the chemical industry.Traditional gas separation and purification technology mainly adopts cryogenic distillation with high energy consumption and high cost or absorption.In contrast,non-thermal adsorption separation is expected to replace the traditional separation technology,with the advantages of low energy consumption,high efficiency and environmental protection.The efficiency of adsorption separation mainly depends on the pore structure and surface properties of the adsorbent.Compared with traditional porous materials?such as zeolite,activated carbon,etc.?,metal-organic frameworks,as a new type of porous material,can achieve precise regulation and functionalization of the pore structure.That provides the possibility to design and synthesize novel adsorption materials for specific separation targets according to different gas properties.In this work,the adsorption and separation of C2H2,CH4,CO2,CO,N2and other gases are the application objectives.In order to explore the effects of organic ligands and their functional sites on the synthesis,structural stability,and adsorption separation performance of materials,15 complexes were constructed with metal salts such as zinc nitrate or cobalt nitrate under solvothermal conditions,using 6-?4-carboxyphenyl?nicotinic acid with Lewis basic pyridine sites or isophthalic acid with electricity-donating groups?methoxy,ethoxy?as the main ligand,and 4,4'-bipyridine or its derivatives containing hydrophobic groups as auxiliary ligand.The structure characterization and stability performance tests were performed on the complexes,and their gas adsorption separation performance was systematically studied.The details are as follow:1. Using 6-?4-carboxyphenyl?nicotinic acid?H2cpna?and zinc nitrate as raw material, a complex with a 3D porous structure,namely,{[?Me2NH2?2Zn3?cpna?4]×3DMF}n?1?has been synthesized.However,its structural stability is poor and the structure easily collapses when exposed to the air,which seriously affects the adsorption performance.For the above reasons,three auxiliary ligands,4,4'-bipyridine?bpy?,2,2'-dimethyl-4,4'-bipyridine?dmbpy?,and 3,3'-dimethyl-4,4'-bipyridine?tmbpy?,were introduced to construct the porous materials of{[Zn3?cpna?3?bpy?]×4DMF}n?2?,{[Zn3?cpna?3?dmbpy?]×4DMF}n?3?,and{[Zn3?cpna?3?tmbpy?]×4DMF}n?4?.The complexes 2?4 exhibit the consistent 3D porous structure with the same secondary building unit of trinuclear zinc metal cluster,which have good stability and the framework structure will not change after direct activation.Moisture stability results have shown that the hydrophobic methyl group on the bipyridine ligand can be helpful to improve the structural stability of the material.The results of adsorption and separation performance showed that the three complexes have better adsorption property of acetylene and carbon dioxide,but poorer adsorption property of methane,carbon monoxide,and nitrogen.In comparison,complex 3 has better acetylene adsorption performance.At 273 K and 100 k Pa,its adsorption capacity for acetylene and carbon dioxide reaches 103 cm3·g-1and 81.7 cm3·g-1,respectively.Complex 4 has better adsorption selectivity for C2H2/CH4and C2H2/CO2,reaching 38.5?22.4 and 9.1?3.9 at room temperature respectively.And complex 2 has better adsorption selectivity of CO2/N2,CO2/CO and CO2/CH4.The complexes 5?7 were obtained by doping magnesium ions in complexes 2?4 seperately.The results of PXRD showed that the doping of magnesium ions did not change the structure of the original complex,and it could improve the moisture stability of the material structure.Adsorption performance studies have shown that magnesium ions doping can effectively improve the complex's adsorption capacity for acetylene and other gases,especially for complex 2,but it will cause the selectivity of some mixed gases to decrease.Complexes 2?7 have good reusability and simple regeneration conditions.In short,strategies such as introducing hydrophobic methyl groups and doping magnesium ions into the structure can significantly improve the hydrophobic properties of the material.2. Three cobalt complexes and two zinc complexes were constructed,using5-ethoxyisophthalic acid?5-H2EIA?as the main ligand,and bpy,dmbpy,tmbpy,4'-?3''-pyridyl?-2,4':6',4''-terpyridine?ptp?as auxiliary nitrogen ligands,respectively.The complex{[Co?5-EIA??bpy?]·H2O}n?8?has a 2D network structure,and its adsorption performance is general.The complex{[Co6?5-EIA?5?tmbpy???3-OH?2?H2O?2]?4H2O}n?9?is a 3D porous structure with a hexanuclear cobalt metal cluster secondary structural unit,and the coordination water molecules or hydroxyl groups can be removed to form open metal sites.The Cobalt complex 10?nitrogen-containing ligand dmbpy?is isomorphic to complex 9,but the positions of methyl groups on the nitrogen-containing ligands of the two complexes are different,resulting in different pore structures and surface chemical environments.The pore size of complex 9 is small,so it has good C2H2/CH4,CO2/CO,CO2/CH4adsorption selectivity,while complex 10 has better adsorption capabilities of acetylene and carbon dioxide.The acetylene adsorption of complex 10 was segmented due to its two different pore distributions.In the synthesis of complex 9,the second metal zinc ions was added to synthesize the complex 9-Co-Zn,which is isomorphic to 9 and has the same adsorption performance.However,its color reversively changes with the adsorption and desorption of water molecules in the sample.Complexes[Zn2?5-EIA?2?dmbpy?0.5]n?11?and{[Zn2?5-EIA?2?ptp??H2O?]?3H2O}n?12?have 3D porous structures,but their adsorption performances of gas are not good,and the reproducibility of complex 12 is poor.Complexes 8?10 have excellent stability,especially complexes 9 and 10 which show the ability of reversible adsorption and desorption of water molecules.The adsorption performance of complex 10 exposed in air for 150 d was basically consistent with the newly synthesized sample.3. Three cobalt complexes were constructed by combining the main ligand of5-methoxyisophthalic acid?5-H2MIA?with the auxiliary nitrogen-containing ligands of bpy,tmbpy and dmbpy,respectively.The complex{[Co?5-MIA??bpy?]·H2O}n?13?is isomorphic with complex 8 and has a 2D network structure.The pore size of complex 13 is about 0.35 nm,leading to the screening effect on carbon dioxide,so it has very high selectivity to carbon dioxide in mixed gas such as CO2/CH4,CO2/N2,CO2/CO,and even shows very rare,reversed CO2/C2H2selectivity.In addition,the adsorption performance of complex 13 is better than that of complex 8.The complex{[Co6?5-MIA?5?tmbpy???3-OH?2?ethanol?2]·2ethanol}n?14?has a 3D network structure and is isomorphic with complex 9.After activation,the sample 14 have open metal sites,coupled with its smaller pore size,which makes it have better acetylene and carbon dioxide adsorption capacity,as well as higher adsorption selectivity of mixed gas.The adsorption performance of complex 14 for acetylene and carbon dioxide is higher than 9.Similar to complex 10,there are also two distinct pore size distributions in the structure of 15,so the adsorption of acetylene shows a segmentation phenomenon,and adsorption capacity of acetylene and carbon dioxide is better than that of 10.On the basis of complexes 13?15,the corresponding bimetallic complexes were synthesized by adding the second metal zinc ions.Among them,14-Co-Zn and 15-Co-Zn have the same structure and adsorption performance as their parent complexes,while the structure of 13-Co-Zn is different from 13.The adsorption capacity of 13-Co-Zn for acetylene and carbon dioxide is higher than 13,and it has good C2H2/CH4adsorption selectivity.Complexes 13 and 14 exhibit excellent water phase stability.After activation,the properties of 13 is not even affected by water vapor in the air.Complex 14 has the ability of reversible adsorption and desorption water molecules. |
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