The Synthesis Of Several Micro-Porous Metal-Organic Frameworks And Their Separation Performances Toward C₂H₄/C₂H₆ And C₃H₆/C₃H₈ Mixtures

The separation of light olefin/alkane mixtures is a very challenging separation process with great importance in the petrochemical industry.At present,such separations heavily rely on the energy-intensive cryogenic distillation in industry.In contrast,the adsorption separation technology could achieve such separation at room temperature and pressure,which has attracted extensive attention.The core of adsorption separation technology is high-performance adsorbents.Hence,this work mainly deals with the synthesis of several stable microporous metal-organic frameworks(MOFs)and the C₂H₄/C₂H₆and C₃H₆/C₃H₈ separation performance of these MOFs.Combined with molecular simulation,the mechanism of MOFs surface pore structure and chemical environment on the adsorption of guest molecules was studied,and efficient separation toward C₂H₄/C₂H₆ or C₃H₆/C₃H₈mixtures is achieved based on the synergistic effect of thermodynamic and kinetic separation effects.The research content of this paper mainly involves the intersecting fields of chemical engineering,material chemistry,and energy-related research fields,which have important scientific research value and practical significance.The apply of C₂H₆-selective adsorbent can largely simplify the C₂H₄/C₂H₆ separation process and reduce energy consumption.In this thesis,an indium-based C₂H₆-selective MOF,In-soc-MOF-1,was synthesized and its separation performance toward C₂H₆/C₂H₄ mixtures was investigated.At 298 K and 100 k Pa,the uptake of C₂H₆ and C₂H₄ were 4.04 and 3.72mmol/g,respectively,and the selectivity toward C₂H₄/C₂H₆(1/15)mixtures was 1.4.The difference in the intensity of van der Waals interaction between the skeleton and C₂H₆ and C₂H₄ molecules is the key factor for the C₂H₆-selective behavior of In-soc-MOF-1.A nickel-based C₂H₆-selective MOF,Ni-4Py C,was synthesized.At 298 K and 100 k Pa,the uptake of C₂H₆ and C₂H₄ were 3.84 and 3.55 mmol/g,respectively,and the selectivity toward C₂H₄/C₂H₆(1:15)mixtures was 1.7.Besides,five cycles of breakthrough experiments toward C₂H₄/C₂H₆(1:15)mixtures confirmed its good dynamic separation performance and cyclic stability.The molecular simulation showed that the framework contains two channels with different pore size(denoted as the channel I and channel II),in which channel II with a larger pore size is the key factor for the C₂H₆-selective adsorption behavior,due to the C₂H₆with stronger hydrogen-bonding interactions and more C-H···?interactions,especially at low-pressure.In this thesis,we proposed a new strategy to enhance the C₂H₆/C₂H₄ separation performance of the iron-based C₂H₆-selective MOFs,that is,the adsorption selectivity of iron-based MOFs to C₂H₆/C₂H₄ can be improved by increasing the positive partial charge of metal sites together with a decreased charge density of benzene ring(BR)sites from the organic ligands.To validate this strategy,a series of C₂H₆-selective MOFs,PCN-250,and its bimetal version PCN-250(Fe₂M,M=Ni,Co,Zn,Mn),were synthesized to investigate the influence of different metal compositions on their C₂H₆/C₂H₄ separation performance.At 298 K and 100k Pa,the C₂H₆uptakes of the five materials follows the order of:Fe₂Co(6.21 mmol/g)>Fe₂Ni(6.19 mmol/g)>Fe₃(6.00 mmol/g)>Fe₂Zn(5.95 mmol/g)>Fe₂Mn(5.53 mmol/g),and was higher than the most reported C₂H₆-selective MOFs.Besides,Fe₂Zn exhibited the highest IAST selectivity(1.70).The molecular simulation revealed that the difference in C₂H₆/C₂H₄separation performance of the five materials was due to the introduced metal ions have different influences on the atomic partial charge within the framework.Moreover,the introduction of Zn²⁺decreased the charge density of benzene rings and improved the electropositivity of metal sites.The increased positive partial charge enhanced the polarization induced effect on C₂H₆ molecules while the lower polarity channel is beneficial to strengthen the C-H···?weak interaction between the Fe₂Zn framework and C₂H₆ and weaken the?···?interactions toward C₂H₄ molecules.Therefore,PCN-250(Fe₂Zn)has the best C₂H₆/C₂H₄separation performance among the five materials.A cobalt-based MOF material,Co-aip-bpy,with the rare thermodynamic and kinetic dual separation mechanism was designed and synthesized.At 298 K and 100 k Pa,the uptake of C₃H₆ and C₃H₈ on Co-aip-bpy were 1.99 and 0.48 mmol/g,respectively,and the adsorption selectivity toward equal molar C₃H₆/C₃H₈mixture was 21,which has exceeded most reported MOFs adsorbents.The stronger supramolecular interaction(?···?stacking and hydrogen bonding)between the material and C₃H₆ molecules are the key factors for the thermodynamic separation performance toward C₃H₆/C₃H₈ mixtures.Besides,in its one-dimensional"zigzag"shaped channel,the diffusion resistance of C₃H₈ molecules with larger molecular size in the channel is greater than that of C₃H₆,and the kinetic separation selectivity of this material for C₃H₆/C₃H₈ is 29.7 at 303 K.The excellent dynamic separation performance and cyclic stability of Co-aip-bpy material were confirmed by the 5 cyclic breakthrough experiments toward C₃H₆/C₃H₈ mixtures.Besides,stability tests suggested that the material has high water and moisture stability.In this thesis,the influence of framework flexibility on the C₃H₆/C₃H₈ dynamic separation performance of MOFs was systematically studied.The C₃H₆/C₃H₈ separation performance of Ni-aip-bpy was systematically investigated and compared with the two isostructural materials Co-aip-bpy and Zn-aip-bpy.At 298 K and 100 k Pa,the uptake of C₃H₆and C₃H₈ were 1.94 and 0.45 mmol/g,respectively,and the adsorption selectivity toward equal molar C₃H₆/C₃H₈ mixtures was up to 34.4 which were higher than that in Co-aip-bpy(21)and Zn-aip-bpy(19).The CO₂ adsorption-desorption isotherms at 195 K of the three materials suggested that the order of their framework flexibility were:Zn-aip-bpy>Ni-aip-bpy>Co-aip-bpy.The molecular simulation combined with C₃H₆/C₃H₈ kinetic adsorption experiments showed that the moderate framework flexibility of Ni-aip-bpy material leads to appropriate deformation of the pore structure during the adsorption process,which significantly enhances the diffusion rate of smaller C₃H₆ molecules and restricts the diffusion of C₃H₈ in the pore channels.At 303 K,the C₃H₆/C₃H₈ kinetic separation selectivity of Ni-aip-bpy was high up to 93.4,which was higher than that of Co-aip-bpy(29.7)and Zn-aip-bpy(14.2).The excellent dynamic separation performances together with cyclic stability of Ni-aip-bpy material were confirmed by the 5 cyclic breakthrough experiments toward C₃H₆/C₃H₈mixtures.