The Preparation Of ZIFs-based Porous Carbon Materials And The Performance Of Xenon Selective Adsorption And Separation

The capture and separation of noble gas Xenon(Xe)is of great importance to the off gas purification of the used nuclear fuel(UNF)reprocessing in the nuclear industry,illegal nuclear tests monitor by Comprehensive Nuclear-Test-Ban Treaty(CTBT),nuclear reactor safety operation monitoring,environmental monitoring and industrial production of Xe.Methods to achive capture and separation of Xe reported uptodate mainly include cryogenic distillation,membrane separation,solvent absorption and physical adsorption based on porous materials,among which physical adsorption based on porous materials is recognized as the most promising one owing to its merits of simplicity,efficiency and low energy consumption.Porous materials are the key role of the physical adsorption method.Activated carbons,zeolitesand new developed materials such as MOFs are reported for the capture and separation of Xe.However,these materials have both advantages and disadvantages for the selective adsorption and separation of Xe,and there is still much room for development.Ideal materials for selective adsorption of Xe require high adsorption capacity,high selectivity and high stability.Theoretical studies show that pore structure is the main factor influencing the Xe selective adsorption properties of the porous materials and by regulating the pore structure,the selective adsorption ability of the porous materials to Xe can be effectively enhanced.Carbon materials have the advantages of excellent chemical stability and reliability,but selective adsorption performances to Xe are relatively low,and there is still much room for improvement.Adjusting the pore structure and properties provides a feasible way to achive high performaning porous materials for the capture and separation of Xe.However,traditional ways to adjust the pore structure of carbon materials like template methods and physical/chemical activated methods are limited by complexity and inefficiency.MOFs are characterized by high carbon content and order porosity,and with MOFs as sacrificed template and precursors,porous carbon materials prepared by calcination and carbonization of MOFs can inherit their pore structure,resulting in high specific surface area and porisity.Studies show that MOFs-derived porous carbons have excellent adsorption and separation performances for gases such as H2 and CO2,and are expected to be used for the selective adsorption and separation of noble gases,especially for Xe.Zeolite imidazolate frameworks(ZIFs),special MOFs with zeolite framework structures possessing high thermal stability,high carbon content and uniform pore size distribution,are ideal sacrifice templates and precursors for the preparation of porous carbons for Xe adsorption and separation.The introduction of the secondary carbon sources and the control of the calcination temperatures are also reported as important factors to the regulation of the pore structure of the MOFs derived porous carbons.In this work,two ZIFs Isomers,named ZIF-7 and ZIF-11,are specially selected as precursors,furfuryl alcohol(FA)as secondary carbon source,a series of ZIFs derived porous carbons are prepared under different calcination temperatures,which exhibited excellent Xe selective adsorption performances.Research content of this work is as follows:1)ZIF-7 and ZIF-11 Isomers of high purity have been successfully synthesized and used as precursors to prepare ZIF-7 derived and ZIF-11 derived porous carbons.With the furfuryl alcohol(FA)as a secondary carbon source,a series of ZIF-7 derived and ZIF-11 derived porous carbons were successfully prepared at 800℃ and 1000℃,respectively,under high-purity grade nitrogen atmosphere.2)The influences of the pore structure of ZIFs precursors,secondary carbon sources and the calcining temperature on the morphology,phase structure,chemical composition,graphitization degree,specific surface area and pore size distribution of the resulting porous carbons were studied in details.ZIF-7 derived and ZIF-11 derived porous carbons possess geometrically regular appearances similar to their ZIFs precursors and inherit the micro pore structure properties of the ZIFs precursors,which are clearly distinguished from the amorphous shape and the widely distributed pore size of the selected activated carbon.The specific surface areas and pore volumes of the ZIF-11 derived porous carbons are higher than those of their ZIF-7 derived counterparts,indicating the important influences of the ZIFs precursor on the pore structure of the resulting carbon materials.The introduction of the secondary carbon source FA has an obvious effect on increasing the porosity of the ZIF-11-based porous carbons,appearing significantly increase of the specific surface area and pore volume.Increasing the carbonization temperature is beneficial to improve the porosity of the carbon materials due to the in-situ pore-forming agent of Zn in ZIF-7 and ZIF-11 frameworks under calcining temperature as high as 1000℃.3)From the perspectives of static and dynamic adsorption,the selective adsorption properties of ZIF-7 derived and ZIF-11 derived porous carbons on Xe were investigated to reveal the relationships between pore structure of these ZIFs-derived porous carbons and their selective adsorption and separation performances to Xe.The ZIF-11 derived porous carbons has a higher adsorption capacity for Xe at room temperature and the same pressure than the ZIF-7 derived counterparts,owing to the higher porosity of the ZIF-11 derived porous carbons.The adsorptive capacity of Xe was further increased by the introducing FA as a secondary carbon source.Notably,ZIF-11 derived porous carbon material with introduction of FA named Z11CBF-1000 possess the adsorptive capacity at 298K and 100 kPa as high as 4.87 mmol/g,and for the Xe/Kr mixed gas of 20:80 ratio,the Xe/Kr IAST selectivity is calculated as high as 13,values comparable to those of the reported bench mark materials such as Ag@MOF-74.All the ZIF-7 derived and ZIF-11 derived porous carbons obsess higher Xe adsorption Henry’s coefficient,Xe/Kr Henry’s selectivity and zero adsorption heat of Xe compared to activated carbon at 298 K and 100 kPa.Remarkably,the adsorptive Henry’s coefficient of Xe on Z11CBF-1000 is as high as 80.0 mmol g-1 bar-1,and the Xe/Kr Henry selectivity is as high as 19.7,highest values reported up to date.The dynamic adsorption experiments showed that ZIF-7-deirved and ZIF-11derived porous carbons shows excellent Xe dynamic adsorption selectivity at 298 K and 100 kPa,among which the dynamic adsorption capacity to Xe of ZIF-7 derived porous carbon named Z7CB-1000 and ZIF-11 derived porous carbon named Z11CBF-1000 were as high as 17.6 mmol/kg and 20.6 mmol/kg,respectively.The Xe selective adsorption performances of ZIF-7 derived and ZIF-11 derived porous carbons are much better than those of the selected activated carbon,mainly owing to their high porosity and narrow pore size distribution,inherited from their ZIFs precursors.