Selective Adsorption And Separation Of Inert Gas Xe/Kr By Functionalized Metal-Organic Framework Materials

The noble gases Xenon?Xe?and Krypton?Kr?are very important due to the wide industrial application foreground and environmental influence by the radioactive isotopes of the noble gases Xe and Kr.The efficient separation of Xe/Kr is attracting much more attention than before.Currently,the dominant separating method is cryogenic distillation,which can extract the noble gases Xe and Kr from air and separate each other.But it's an energy and capital intensive process.Thus finding an effective method to separate the noble gases Xe/Kr is significantly important.The technology-adsorptive separation,which can separate Xe and Kr using the solid adsorbents by the selective adsorption at room temperature is an alternative method that can curtail the cost and consume low energy.The adsorption capacity of Xe and Kr and separating selectivity are the key factors of this method.Metal-organic frameworks?MOFs?are novel porous materials,which possess large specific surface area,diverse channel structures,uniform pore architectures and possibility of pore modification compared to traditional adsorbents such as activated carbons and zeolites.Nowadays,MOFs are being developed as a kind of novel noble gases separating materials with extreme potential and excellent application foreground.Existing reports showed that the separation performance of Xe/Kr by MOFs have surpassed zeolites and activated carbons.Relevant researches showed that pore size,pore structures,and pore polarizing environment directly affect the performance of Xe/Kr separation by MOFs.Computational simulation showed that MOFs which have uniform tubular channel and the channel can accommodate no more than one Xe atom may be the best candidate to separate the noble gases.Thus,utilizing the modifiable properties of MOFs,adjusting the pore morphology and size and enhancing the polarizing environment by simple modification,could improve the performance of MOFs to separate noble gases in a simple and efficient way.The modification of MOFs could provide a new approach for developing novel noble gases separating materials of high performance.Based on this,this paper carried out the following work in two aspects:1)For the first time,we introduce electronegative ionic framework?CPM-5 and CPM-6?into the noble gases adsorptive separation.Compared to most neutral frameworks,MOFs with anionic frameworks have electric field inside the channel and possess more polarizing environment which could enhance the interaction between noble gas Xe and frameworks.And the guest cation inside the channels could be replaced by other cations via simple ion exchange process to tune the pore size and polarizing environment.Therefore,we choose an In-MOF materials with anionic frameworks-CPM-5 and CPM-6.We carried out the ion exchange process on CPM-6 and obtained the Co²⁺ exchanged analogue Co²⁺-CPM-6.The results showed that compared to CPM-5 and CPM-6 which own the organic cations inside the channels,the noble gases Xe/Kr separation performance of Co²⁺ exchanged Co²⁺-CPM-6 had been significantly improved,exhibiting high Xe adsorption capacity?3.2mmol/g at 1bar,298K?and Xe/Kr selectivity?9.3 at 298K,1bar?.And dynamic adsorptive breakthrough experiment also showed that under practical conditions,Co²⁺-CPM-6 had better Xe/Kr separating performance.The improved adsorptive performance of Xe/Kr for Co²⁺-CPM-6 could be due to the increased pore size or accessible micropore volume and enhanced electric field within pore space via the introduce of Co²⁺.On the one hand,the enhanced electric field environment improved the interaction between frameworks and Xe atom,on the other hand,it reduced the affinity with Kr.Thus it elevated the separating performance of Xe/Kr for Co²⁺-CPM-6.2)we utilized a solution immersion method to successfully introduce organic porous cage-like molecule cucurbit[6]?CB[6]?into the MOF-MIL-101,which owns stable properties and large pores.We partitioned the large cage-like pores?2-3nm?of MIL-101 by introducing CB[6]into the pores.And we improved the adsorptive strength of Xe by utilizing the exposed polarized microporous cage-like pores?5.7A?of CB[6]after CB[6]were dispersed in MIL-101.The element analysis and powder X-ray diffraction results indicated that CB[6]was successfully loaded on MIL-101,and kept the crystalline integrity.After loading,the adsorptive performance of Xe at low pressure has been significantly improved,the capacity of Xe at 1kPa increased by 4 times,and the Henry coefficient of Xe adsorption and Xe/Kr selectivity had enormous improvement,which reached 50.02 mmol-1g-1 and 26.6,respectively.It was much higher than that of MIL-101.The improved separation performance of Xe/Kr for CB[6]@MIL-101 could be attributed to the microporous polarizing cage and the more suitable pore size for Xe adsorption which was partitioned by CB[6].These two factors work synergistically to make CB[6]@MIL-101 have strong affinity with Xe,and make CB[6]@MIL-101 have excellent performance of Xe/Kr separation.