Theoretical Studies On CH₄/CO₂ Adsorption And Separation In Zr-based Porphyrinic Metal-organic Frameworks PCN-224-Ms

Carbon dioxide?CO₂?produced from burning fossil fuels is one of the main greenhouse gases and also the significant factors affecting global warming.CO₂ is also considered as an impurity in natural gas.The presence of CO₂ reduces the energy content of natural gas,and leads to pipeline corrosion.Metal-organic frameworks?MOFs?are a class of porous materials composed of organic linkers and inorganic nodes and have attracted great attention in small gas?CH₄,N₂,CO₂,CO?and organic matter?alkanes,alkenes,alcohols?adsorption and separation applications.These have been revealed by a large number of experimental and computational studies.In recent years,studies have shown that MOFs with Zr₆O₈ inorganic node structure can significantly improve the stability characteristics of the material.A lot of researches have been investigated about MOFs with Zr₆ group after the first synthesis of Zr based MOFs material UIO-66,which becomes a hot research field of MOFs material.The appearance of MOFs with porphyrin ligand further increases the stability,and this aromatic heterocyclic compound with the conjugated macrocyclic structures is proved to have good catalytic properties.Indeed,the introduction of porphyrin ligand provides more plastic for the type of metal and chemical properties in MOFs,which will promote novel design and selection of MOFs.In this thesis,the grand canonical Monte Carlo?GCMC?molecular simulation method is performed to study the adsorption properties of single-component CO₂,CH₄ in PCN-224-Ni and ligand-metal modified PCN-224-M?M=Zr,Fe,Co,Mn,Mg?,as well as their adsorptive separation performances towards gas mixture CO₂/CH₄ and CO₂/CH₄/H₂O.At first,the crystral structures of ligand-metal modified PCN-224-M?M=Zr,Fe,Co,Mn,Mg?are constructed based on the crystal structure of PCN-224-Ni.The accuracies of the constructed srystral structures and the chosed force field parameters and calcution methods are vertified by comparing the results of molecular simulation in this thesis with the results from experiments and the simulation results from other literatures.On this basis,the single-component CO₂,CH₄ adsorption isotherms in PCN-224-M?M=Zr,Fe,Co,Ni,Mn,Mg?are obtained by molecular simulation.In addition,adsorption sites,binding energies,and radial distribution functions on the CO₂ adsorption are also performed to reveal the mechanism of ligand-modified impact.Furthermore,the adsorptive separation performances of the two-component gas mixture CO₂/CH₄ and three-component gas mixture CO₂/CH₄/H₂O in PCN-224-M?M=Zr,Fe,Co,Ni,Mn,Mg?are simulated.The effects of ligand modification on CO₂/CH₄ selectivity,electrostatic interaction,trace H₂O are discussed.For the single-component CO₂,CH₄ adsorption,the results show that the order of adsorption amount of CH₄ in PCN-224-Ni follows 273K>298K>323K,and the higher temperatures adverse to CH₄ adsorption.There is a little effect on CH₄ adsorption for modified PCN-224-Ms,but it can significantly improve the CO₂ adsorption properties.At low pressure,the order of adsorption amount of CO₂ follows PCN-224-Zr>PCN-224-Mn>PCN-224-Mg>PCN-224-Co>PCN-224-Ni>PCN-224-Fe,and the CO₂ adsorption capacity of PCN-224-Zr is 2.49 mol/kg,which is the largeset among the six PCN-224-Ms at 298 K and1 bar.The CO₂ adsorption sites suggest that the metal modified MTCPP ligand prefers to adsorb CO₂ molecules compared with the traditional inorganic metal node,possessing stronger CO₂ adsorption capacities.Therefore,ligand-metal modifications may significantly improve the CO₂ absorption capacity of the entire skeleton.In the process of CO₂ adsorption,there is 62.5%difference for the electrostatic interaction of CO₂/PCN-224-Ni and CO₂/PCN-224-Zr,and the other three interactions are generally on the same level.For the gas mixture CO₂/CH₄ and CO₂/CH₄/H₂O,the adsorption of CO₂ greatly exceeds the adsorption of CH_4,and ligand-metal modifications in PCN-224-Ms can significantly improve the selectivity of CO₂/CH₄,and the order of CO₂/CH₄ selectivity follows PCN-224-Zr>PCN-224-Mg>PCN-224-Mn>PCN-224-Co>PCN-224-Ni>PCN-224-Fe.In the process of CO₂/CH₄adsorption in Zr-based porphyrin PCN-224-Ms,a small amount of water has a negative effect on the selectivity of CO₂/CH_4,and this phenomenon is more obvious in 1%H₂O compared with 0.1%H₂O.After the introduction of 0.1%H₂O,its influence of CO₂ adsorption decreases in the sequence PCN-224-Zr?51%?>PCN-224-Mn?34%?>PCN-224-Ni?23%?against no H₂O.H₂O molecules prefer to be adsorpted on MOFs compared with CO₂ molecules.In the process of CO₂/CH₄/H₂O adsorption,there is serious competition of H₂O,which will prevent the adsorption of CO₂ and CH₄.