Design, Synthesis And Gas Separation Of Molecular Sieve-type Metal-organic Framework Material

Both the separation of C4 hydrocarbons and the separation of ethanol and water are very important in industry,especially C4 hydrocarbons separation,which accounts for a significant portion of global energy consumption.Metal-organic frameworks（MOFs）,as a new type of functional crystalline materials,have shown significant potential in the field of gas adsorption separation and storage,by virtue of their tunable pore structure and easily modification.However,the gas separation of most MOF materials relies on the difference in interactions of gas molecules,which causes the co-adsorption phenomenon and low separation efficiency.In order to overcome this problem and obtain molecular-sieving MOFs with high separation selectivity,pore structures information of a large amount of MOFs were calculated in this work and MOFs with appropriate pore sizes were firstly selected for simple ethanol/water separation system.Then,MOFs with multi-point coordination were furtherly selected from them to reduce the changes of pore size caused by swing of ligands.At the same time,chemical stability,raw material cost,preparation process were further evaluated to screen the target MOFs,then,experimental preparation and performance studies were carried out.Furthermore,for the complex C4 separation systems,on the basis of the above method,the rational design of pore size of screened MOFs and the research on gas adsorption separation performance were performed to realize the molecular sieve separation of C4 hydrocarbons.Therefore,the molecular-sieving separations of ethanol/water and complex C4 components were achieved by screening and further designing of MOF materials.The main contents are given as follows:（1）According to multi-point coordination that proposed to be a strategy to determine the MOFs with frozen framework,together with computer-aided screening method,Cu-Tria MOF featuring molecular sieving separation for ethanol/water mixture was identified.The screened Cu-Tria MOF exhibited super hydrophilicity and typical Type-I adsorption isotherm for water adsorption while ethanol molecules were excluded.Benefiting from the strong coordination between N and Cu²⁺metal sites in triazole ligands,Cu-Tria MOF can maintain structural stability and integrity over a wide p H range from 1 to 14.In addition,its structure remained stable after immersion in water for 30 days.At the practical industrial temperature of 333 K,the efficient dynamic mixing separation of ethanol and water can still be achieved.Meanwhile,the cycle test showed that Cu-Tria MOF has good regeneration performance.These results show that the proposed multi-site coordination method and computer-aided MOFs screening is useful to identity optimal MOF for molecular sieving separation.（2）Finely-tuned pore window size and molecular sieving separation of n-C₄H₁₀ and iso-C₄H₁₀ can be realized in Zn-bzc MOF by precisely installing methyl groups at the bottleneck position.By introducing different numbers of methyl groups at the pore windows of Zn-bzc MOFs,Zn-bzc-CH₃ and Zn-bzc-2CH₃ were prepared.In particular,Zn-bzc-2CH₃ exhibited perfect molecular sieving effect on n-C₄H₁₀/iso-C₄H₁₀separation.Adsorption experiments showed that Zn-bzc and Zn-bzc-CH₃ exhibited obvious co-adsorption at 298 K.In contrast,the adsorption capacity of Zn-bzc-2CH₃for n-C₄H₁₀ is as high as 54.28 cm³/g,while it almost does not adsorb iso-C₄H₁₀.The uptake ratio for n-C₄H₁₀/iso-C₄H₁₀ at 1 bar is 83.4,which higher than that all of adsorbent materials reported.The DFT calculation results show that n-C₄H₁₀ can form multiple interaction sites with-CH₃ on surrounding ligands.The breakthrough experiments show that the dynamic separation of n-C₄H₁₀ and iso-C₄H₁₀ can be realized in Zn-bzc-2CH₃.In addition,the introduction of methyl groups greatly increases the water stability of the MOF and eliminating the effect of water vapor on the C4separation in humid environments.（3）The C₄H₆ separation and purification performance of Y-abtc MOF was studied.Single-component adsorption isotherm of Y-abtc MOF for C₄H₆ shows typical Type-I style and the saturated adsorption capacity is 49.5 cm³/g at 298 K.Meanwhile,it almost cannot adsorb n-C₄H₈,iso-C₄H₈,n-C₄H₁₀ and iso-C₄H₁₀ molecules,indicating that Y-abtc MOF has excellent molecular-sieving effect on C₄H₆ purification.The uptake ratios of C₄H₆/n-C₄H₈ and C₄H₆/iso-C₄H₈ are 40.9 and 63.1,respectively,which are higher than reported porous adsorbents.The breakthrough experiments showed that Y-abtc could separate C₄H₆/n-C₄H₈,C₄H₆/iso-C₄H₈,C₄H₆/n-C₄H₁₀ and C₄H₆/iso-C₄H₁₀mixtures and has good dynamic separation performance.（4）The gate-opening effect of flexible Mn-bpdc MOF on alkenes and the separation performance of C₄H₆ were investigated.The flexible nature of Mn-bpdc MOF was confirmed by VT XRD and guest molecule-induced structural transformation.The Type-S adsorption isotherms of C2 and C3 alkenes indicated that the gate-opening effects can be observed for alkenes adsorption process while alkanes cannot.It can be concluded that Mn-bpdc MOF can realize the gate-opening effect on C₄H₆but other C4cannot even at 1 bar.As expected,Mn-bpdc MOF exhibits unique gate-opening effect on C₄H₆with very low pressure（0.13 bar）,and almost cannot adsorb n-C₄H₈,iso-C₄H₈,n-C₄H₁₀ and iso-C₄H₁₀.In-situ XRD confirmed the structural transformation of Mn-bpdc MOF during C₄H₆adsorption,and in-situ FTIR and DFT calculations indicated that it has strong interactions with C₄H₆.The breakthrough experiments of C₄H₆/n-C₄H₈,C₄H₆/iso-C₄H₈,C₄H₆/n-C₄H₁₀,and C₄H₆/iso-C₄H₁₀ gas mixtures show that Mn-bpdc has good dynamic separation performances.In addition,after immersion in water for60 days,the structure of Mn-bpdc MOF remained intact and has excellent regeneration and cycling ability.