Study On The Structure Regulation Of Metal Organic Framework Materials And Their Adsorption And Separation Properties Of Low Carbon Hydrocarbons

The separation and purification of low-carbon hydrocarbon mixtures is one of the most important separation processes in petroleum industry.The current distillation method has the disadvantages of high energy consumption and large equipment investment.In contrast,adsorption separation technology has advantages,and its key issue is to develop advanced adsorbent materials with low cost,good effect and high stability.In recent years,metal-organic frameworks（MOFs）have attracted extensive interest due to their structural and functional designability,high porosity and specific surface area.Since the pore size and pore environment of MOFs have a great influence on the adsorption performance of adsorbent materials,the precise structural regulation of MOF materials has become the key to the preparation of high-performance adsorbents.However,there are still some challenges for the development of high-performance adsorbents.First,most of the traditional adsorbents rely on the differenct affity of target molecules,which results in co-adsorption and unsatisfactory separation efficiency.Second,the presence of water vapor in the separation system normally significantly reduces the separation efficiency of adsorbents.In view of the above two challenges,this paper aims to the separation and purification of low-carbon hydrocarbons such as CO₂/C₂H₂ and C4 isomers.Through the fine-tuning of pore environment and pore size,a series of MOF adsorbents were developed to realize the purification of C₂H₂ and the molecular-sieving separation of C4 hydrocarbons under humidity conditions.The main research contents include:（1）Molecular sieve separation of n-C₄H_8/iso-C₄H₈ and n-C₄H₁₀/iso-C₄H₁₀ in C4isomers was achieved by changing the pore size of the anion column in the pillared layered MOFs structure.In order to accurately design molecular sieve materials to achieve efficient separation of C4 isomers,the adsorption and separation performances of a class of pillared layered isostructral MOFs（KAUST-7,KAUST-8,KAUST-9）were systematically studied.Single-component static and dynamic breakthrogh experiments show that KAUST-8 not only can efficiently adsorb n-C₄H₈ and n-C₄H₁₀,but also significantly exclude iso-C₄H₁₀ and iso-C₄H₈ by the size-exclusion effect.The results of breakthrough experiments show that the mixture of n-C₄H₈/iso-C₄H₈ and n-C₄H₁₀/iso-C₄H₁₀ could be efficient separated by KAUST-8.In addition,KAUST-8 also has excellent water stability and recycling performance,impying that it is a potential adsorbent for C4 isomer separation;（2）Weak-polar microporous MOFs（Ni-MA-BPY and Co-MA-BPY）with suitable pore size were prepared,which realized the efficient separation of n-C₄H₁₀/iso-C₄H₁₀ isomers under humid conditions.The pore sizes of Ni-MA-BPY and Co-MA-BPY prepared by hydrothermal method locate between the kinetic diameters of n-C₄H₁₀and iso-C₄H₁₀,which makes them be able to effectively adsorb n-C₄H₁₀ and inhibit the adsorption of iso-C₄H₁₀,and achive efficiently separation of n-C₄H₁₀/iso-C₄H₁₀ mixture in breakthrough experiments.In addition,due to the absence of open metal sites and polar groups in its channels,the interaction between water and framework is weak,which inhibits the competitive adsorption of water vapor and n-C₄H₁₀,and thus the breakthrough curves of n-C₄H₁₀/iso-C₄H₁₀ mixture is not affected by humidity.In addition,the two MOFs also have the advantages of low ligand cost,good stability,and mild desorption process,showing application prospects in C4 separation;（3）A strategy of three-dimensional restricted sieving mechanism induced by twisted pores of MOF was proposed and the molecular-sieving separation of CO₂ and C₂H₂ mixture was achieved.Traditional molecular sieve separation relies on the difference in the two-dimensional size of gas molecules to achieve gas separation,but CO₂ and C₂H₂ have the same molecular dynamics diameter and extremely close two-dimensional cross-sectional area,so the molecular sieve separation of CO₂ and C₂H₂ is considered to be difficult to achieve.Considering that the length of C₂H₂ molecule is significantly larger than that of CO₂,a three-dimensional molecular sieve separation mechanism is proposed in this paper.The separation of CO₂/C₂H₂ is achieved by the strong repulsion of C₂H₂ by the twisted channels in Cu-Trz MOF.Adsorption isotherms and adsorption kinetics experiments show that the one-dimensional twisted channels in Cu-Trz MOF can repel longer C₂H₂ molecules into the pores,and can effectively adsorb shorter CO₂ molecules,thereby achieving molecular sieve separation of CO₂/C₂H₂.Penetration experiments confirmed that the material can achieve efficient separation of CO₂/C₂H₂ mixture and can directly obtain high purity C₂H₂（>99.9%）.In addition,Cu-Trz has excellent chemical stability and adsorption cycle regeneration performance,ultra-low cost,and has important application prospects in CO₂/C₂H₂ separation;（4）A L-arginine（ARG）functionalized MOF（MOF-808-ARG）was developed to achieve water-promoted CO₂/C₂H₂ separation.MOF-808-ARG was prepared by grafting natrual ARG molecules on the Zr₆ clusters of MOF-808 via a simple water-assisted ligand exchange method.The experimental results showed that the static adsorption capacity of CO₂ on the wet MOF-808-ARG is 137%(71 cm³ g^-1,1 bar)of the value of dry sample,while the adsorption capacity of C₂H₂ decreased from 41.3 cm³g^-1 to 7.8 cm³ g^-1.At the same time,the wet MOF-808-ARG showed higher CO₂/C₂H₂selectivity（71）at 1 bar,which is 20 times higher than that under dry conditions.Breakthough experiments show that CO₂/C₂H₂ can be efficiently separated.More importantly,in situ IR and DFT calculations revealed that its hygroscopic promotion of CO₂ adsorption was due to the change of the adsorption pathway.The presence of water promotes the formation of bicarbonate product,which is both thermodynamically and kinetically preferred,compared with the carbamate product in dry conditions.Meanwhile,MOF-808-ARG also has excellent stability,good recycling ability,which makes it extremely promising in C₂H₂ purification;...