Pore Environment Regulation Of Adsorbent To Enhance Adsorptive Separation Efficiency Of Xe/Kr

Currently,fossil fuels are the primary source of energy for human production and life.However,the limited reserves and non-renewable nature of fossil fuels make it difficult to meet the growing energy demand,prompting us to seek for cleaner alternative energy sources.Nuclear energy,with its efficient and economical implementation,has been well developed and widely used nowadays and is considered one of the most promising future energy sources for humans.However,the nuclear waste and reprocessing off-gas contain radioactive xenon(¹²⁷Xe,～400 ppm)and krypton(⁸⁵Kr,～40 ppm),which can cause air pollution and serious health risks.Therefore,the development of economic and effective technologies to capture trace amounts of radioactive ¹²⁷Xe and ¹²⁷Kr in nuclear waste is highly desirable.Xe and Kr are also gaseous fine chemical products,and high-purity Xe and Kr are very valuable commodities,known as"gold gases".Xe and Kr mainly come from air separation byproducts（Xe/Kr,20/80 v/v）,and further separation and purification processes are needed to obtain high-purity Xe and Kr.Currently,cryogenic distillation is the most mature technology for Xe/Kr separation,but suffers from disadvantages of high energy consumption,low economy,and tedious processes.Therefore,it is urgent to develop a new and economically efficient separation method to replace traditional thermally-driven,high-energy consumption separation methods.In recent years,adsorptive separation technology has been recognized as a promising separation method because of low energy consumption,high separation efficiency,and simple equipment.which has attracted great attention in gas separation and purification.The key to this technology is the development of porous adsorbents.Given that Xe and Kr atoms possess zero dipole moment,the development of highly selective porous adsorbents for Xe/Kr separation remains a significant challenge.A promising strategy for achieving efficient separation of Xe/Kr mixtures is to finely control the pore structure and size of the porous adsorbent and introduce functional groups to regulate the pore channel surface polarity and increase active sites based on the differences in properties such as the kinetic diameter（Xe is 4.1?,and Kr is 3.6?）and polarizability(Xe is40.4×10^-25 cm^-3,and Kr is 24.8×10^-25 cm^-3)of Xe and Kr.Therefore,in this thesis,metal-organic frameworks（MOFs）and porous carbon materials were chosen as porous adsorbents.Several high-performance adsorbents were constructed using strategies such as ligand functional group regulation and optimization of pore-forming methods.The adsorptive separation performance and mechanism of these adsorbents for Xe and Kr were studied,with specific details as follows:（1）Three isostructural metal-organic frameworks（MOFs）were prepared by self-assembly from different bidentate carboxylic acids and Ni.The pore size and pore environment of isostructural MOFs were finely regulated by changing the number of aromatic rings in the ligands.The results indicated that with an increasing number of aromatic rings,the pore size and pore environment are more conducive to generating stronger Xe-MOF host-guest interactions.The results showed that Ni（NDC）（TED）_0.5has a suitable pore size（5.3?）and a high BET surface area(954.8 m² g^-1),exhibiting an ultra-high Xe adsorption capacity of 5.56 mmol g^-1 and a Xe/Kr selectivity of 12.9at 298 K and 1.0 bar,providing a new benchmark adsorbent for Xe/Kr adsorption separation.Further narrowing of the pore size resulted in higher selectivity（24.7）,but a significant decrease in BET surface area and pore volume of Ni（ADC）（TED）_0.5 led to a lower Xe adsorption capacity of 3.25 mmol g^-1.In contrast,Ni（BDC）（TED）_0.5exhibited a larger pore size（6.7?）,which could not effectively restrict Xe atoms,resulting in a lower Xe/Kr selectivity of only 3.5,making it difficult to achieve efficient separation of Xe and Kr.DFT-D and GCMC simulations demonstrated that a narrower pore size and abundant C-H groups on the channel wall are more favorable for the separation of Xe/Kr gas mixtures.Dynamic breakthrough experiments further verified the high penetration capacity and cyclic stability of Ni（NDC）（TED）_0.5 in the practical separation of Xe/Kr gas mixtures.（2）The inherent drawbacks of carbons,such as lack of surface polar functional groups and wide pore size distribution usually lead to poor gas separation performance.In this thesis,the abundant and renewable bamboo was selected as the precursor for the preparation of carbon granules rich in oxygen groups（GBC-T）.The optimized GBC-900 showed a narrow pore size distribution centered at 6.3?,exhibiting high Xe adsorption capacity of 4.71 mmol g^-1 and exceptional Xe/Kr selectivity of 18.3 at 298K and 1.0 bar,breaking the trade-off effect between adsorption capacity and selectivity.Additionally,GBC-900 achieved the highest Xe adsorption capacity(2.78 mmol g^-1)among porous carbon materials at low pressure（0.2 bar）.DFT calculations showed that the oxygen groups had a stronger affinity for Xe atoms,leading to efficient Xe/Kr separation.Dynamic breakthrough experiments under dry and wet conditions for Xe/Kr mixtures（20/80 v/v）and UNF off-gas（400 ppm Xe,40 ppm Kr,and air as balance gas）confirmed the practical Xe/Kr separation capability of GBC-900,which also demonstrated high stability and excellent cyclic performance,making it a potential excellent material for industrial Xe/Kr separation.