Porous Nanomaterials For Adsorption And Separation Of Noble Gases On Molecular Simulation

With the continuous development of the chemical industry,the environmental problems have become increasingly severe.Every day,a large amount of radioactive noble gases are produced.In addition,the Radon present in building materials has become one of the most important pollutants in indoor air.They not only do harm to people's health but also pollute the environment seriously,if untreated.However,noble gases show many industrial applications due to their special physical properties,especially high-purity noble gases,including chemical industry,medicine,cutting-edge science,and daily life.For example,high-purity Xe has the exorbitant price exceeding $5000 kg-1,and many researchers have gradually paid significant attention to find cost-effective techniques to adsorb and collect high-purity Xe from noble gas mixtures.However,the commonly methods and the adsorption materials for separating noble gases expose many defects,so they cannot satisfy the needs for highly effective adsorption and separation of some specific gases.Furthermore,experiments on the adsorption of radioactive gas Rn in the laboratory have also been limited by practical experimental conditions.In this paper we mainly predict the adsorption and separation capabilities of porous nanomaterials for noble gases,which will guide the implementation of these experiments,and provide a preliminary guide for industrial applications.In the first work,we use the Monte Carlo(GCMC)simulation and adsorption integral equation combination method to theoretically characterize the pore size distribution of the experimentally synthesized nitrogen-doped porous carbon(Carbon-ZX)by fitting adsorption isotherms of Xe in a model adsorbent with different pore sizes.Then the grand canonical ensemble GCMC simulation method is used to predict the adsorption isotherms of the pure component gases Ar,Kr,Xe and Rn,and their Henry's constants and heat of adsorption values are calculated.It is found that they follow the sequence Rn>Xe>Kr>Ar,which means that there is a strong van der Waals interaction between Carbon-ZX and gas Rn and gas Xe,showing a great potential for adsorption.In addition,we further predict the selectivity of Carbon-ZX for Xe/Kr,Xe/Ar,and Rn/N2 mixtures.Results indicate that the selectivities of Carbon-ZX for Xe/Kr and Xe/Ar are apparently higher than that of other MOFs in the same conditions,which also is further confirmed by Henry's constant and isosteric adsorption heat.Whereafter,we calculate the mixtures of Rn/N2 with 7 different Rn molar contents.By observing the relationship between the molar concentration and the selectivity of Rn,it is found that they satisfy an inverted U-shaped curve relationship.Moreover,the Carbon-ZX for the Rn/N2 binary mixture shows the extremely high selectivity(about 900?1200)in the molar fraction XRn<0.001.In short,this work succeeds in reproducing the experimentally synthesized nanoporous materials by means of simulation and further explore their applications in adsorption and separation of noble gases.This material is found to have a great potential in the adsorption separation of gas Xe and gas Rn in the Xe/Kr,Xe/Ar mixed gas and Rn/N2 mixed gas.In the second work,we also use the Monte Carlo simulation method in the giant canonical ensemble to predict the adsorption capacities and selectivities of gas Rn in the mixing of Rn/N2 and Rn/O2 with 23 different kinds of porous metal framework materials(MOFs)under normal temperature and pressure.By analyzing the calculation results of the gas Rn,four kinds of excellent materials are selected those are ZIF-12,HKUST-1,IRMOF-62 and ZIF-11.Then we use these four MOFs to further explore the adsorption calculation of Rn/N2 and Rn/O2 mixed gases with different molar concentrations of Rn.It is found that when the molar concentration of Rn in the mixed gas is low,the four MOFs,especially the material ZIF-12,exhibit higher selectivity to the Rn,indicating that these materials have great potential for capturing low concentrations of Rn.When the molar concentration of gas Rn is 0.0001,the selectivities of Rn in the mixed gas of Rn/N2 and Rn/O2 are?2800 and?1750,respectively.We can see that the separation capability of the material ZIF-12 for gas Rn is much higher than that of the other three materials,indicating that ZIF-12 is a very promising material for purifying air at home and detecting the existence of indoor radioactive Rn.