Research On The Separation Of Light Hydrocarbons With Metal-Organic Frameworks

Energy resources are the most important material commodities available to human society,controlling economic development,social progress,and the lifeblood of modern civilization.Light hydrocarbons are important components of these energy resources and their production and application are key factors in the development and progress of today's world.Light hydrocarbons are important products and raw materials in industry,originating principally from the petroleum industry,coal chemical industry,and natural gas engineering.During production,these compounds are mostly present in a mixture,therefore light hydrocarbon separation/purification is a prerequisite for their use in the preparation of raw materials,in manufacturing,or in energy production.At present,the most commonly used industrial technology for light hydrocarbon separation is cryogenic distillation,which is highly energy-and cost-intensive.By contrast,the technique of adsorption separation has the advantages of low energy consumption,simple operation,and environmental friendliness–priorities which have risen to the forefront of global attention in recent years.The adsorbent is the core of adsorption technology,therefore the choice of low cost,high efficiency,and high stable the adsorbent is of paramount importance.In this work,separation of light hydrocarbons?C₂H₂/C₂H₄,C₃H₆/C₃H₈,CH₄/C₂H₆/C₃H₈,C₂H₄/C₃H₆?has been achieved using various metal-organic frameworks?MOFs?:by exploiting varying pore sizes,functionalized pore surface,or flexible structure,we expect to develop accurate separation strategies for separating the target gas mixtures and obtain methods for improving the separation performance.The principal observations and conclusions of this research are as follows:1.Metal-organic frameworks provide new possibilities for improving adsorption separation performance by virtue of their customizable and adjustable pore sizes.In this study,the low cost,easy synthesized and highly stable fcu-MOFs were prepared to explore their pore size and functionalization effects on the light hydrocarbons adsorption and separation.The MOF structures were characterized using powder x-ray diffraction?PXRD?,thermogravimetry?TG?,and scanning electron microscopy?SEM?,and their stability is also investigated under water or heat conditions.The adsorption and separation abilities of fcu-MOFs for CH₄,C₂H₂,C₂H₄,C₂H₆,C₃H₆,C₃H₈ and their mixtures are calculated and investigated through adsorption isotherms,isosteric heat of adsorption,and breakthrough experiments.In addition,cycling experiments were performed to test the regenerability of the fcu-MOFs.The results showed that:?1?the light hydrocarbons can be adsorbed by the studied fcu-MOFs and that there is a relation between the material pore size and the corresponding separation abilities.?2?C₂H₂/C₂H₄ separation can be achieved by Zr-FUM,indicating that the MOF with fcu topology has the potential to separate C₂H₂/C₂H₄ mixtures.Zr-FUM is effective in C₂H₂/C₂H₄ separation due to the obvious difference between the smaller pore size and the two gas molecules.?3?CH₄/C₂H₆/C₃H₈ and C₂H₄/C₃H₆mixtures can be separated by fcu-MOFs,and Zr-NDC with larger pore size has the best separation performance.?4?The fcu-MOFs have good stability and regeneration performance,showing potential useful for industrial application.2.In the previous chapter,the Zr-FUM material can separate C₂H₂/C₂H₄,but the smaller pore size limits the gas adsorption capacity.In this chapter,one of fcu-MOFs,Zr-BPDC,is selected to modify with the functional group/Lewis base?-SO₂/N?,targeting C₂H₂/C₂H₄ separation and overcomeing the trade-off effect between adsorption capacity and selectivity.The adsorption performance,heat of adsorption,Ideal Absorbed Solution Theory?IAST?calculation,Grand Canonical Monte Carlo?GCMC?simulation,and breakthrough experiments of these fcu-MOFs are studied in detail.The results show that:?1?Zr-DADTO modified by-SO₂ can effectively separate C₂H₂/C₂H₄ and showing a good regeneration performance,which can be used as an adsorbent for C₂H₂/C₂H₄ separation.The GCMC calculations show that the effective separation ability is attributable to the strong interaction between the-SO₂ group and C₂H₂ molecule,which provides some theoretical guidance for the design and synthesis of MOFs for C₂H₂/C₂H₄separation.Zr-BIPY functionalized by the Lewis base?N?has the similar adsorption and separation behavior with Zr-BPDC,indicating that the Lewis base?N?functionalization did not exhibit the expected effect in this MOF.?2?For the separation of CH₄/C₂H₆/C₃H₈ and C₂H₄/C₂H₆,all three materials show good adsorption capacity and separation effect.3.C₃H₆/C₃H₈ separation is a very important and challenging task among the separation of light hydrocarbons.In view of the gate-opening effect in flexible MOFs can lead to significant differences in adsorption of different gases,which is beneficial to increasing the separation performance of C₃H₆/C₃H₈.The flexible MOF NJU-Bai8 was prepared and investigated by adsorption,heat of adsorption,IAST selectivity analysis and breakthrough experiment.The single component gas adsorption curve reveals that NJU-Bai8 has a high C₃H₆/C₃H₈ adsorption selectivity from 298 to 348 K,which is attributed to the different interactions between flexible host structure and different guest molecules.The results show that the IAST selectivity of NJU-Bai8 to C₃H₆/C₃H₈?50%/50%?gas mixture is4.6 at 298 K and 20 kPa.Combined with the transient breakthrough simulation and the fixed bed breakthrough experiments,it was shown that the breakthrough time interval for C₃H₆ and C₃H₈ is more than 10 minutes.In addition,the cycling experiment show that the material has good reproducibility and stability.