Construction Of Caboxylic Acid-Based MOFs For Adsorption And Separation Of Rare Gases Xe/Kr

Non-radioactive xenon（Xe）devices,such as xenon lamps,lasers,and medical devices,are ubiquitous in daily lives.However,compared to other noble gases,Xe and krypton（Kr）have very low atmospheric concentrations.In industry,the separation of Xe/Kr gases is achieved through expensive cryogenic distillation methods.The existing separation processes based on the difference of single physical properties of molecules have disadvantages such as low adsorption capacity and poor selectivity,and there is an urgent need to develop more efficient adsorption materials and separation strategies.To address these problems,this paper systematically investigates the structural regulation and temperature response mechanism of carboxylic acid MOFs based on the pore size and pore environment tunability of such materials.On this basis,microporous MOFs with suitable pore size and pore environment were constructed in an oriented manner to achieve efficient separation of Xe/Kr gas mixture.A new mechanism for constructing Xe/Kr separated MOFs was further proposed.The mechanism of adsorption separation of Xe/Kr by these carboxylic acid MOFs was also explored by combining structural morphological characterization methods such as SCXRD,PXRD,SEM and XPS,giant regular system synthesis Monte Carlo（GCMC）simulation methods and penetration experiments simulating actual gas ratios,as follows:（1）Firstly,a new tetracarboxylic acid-based yttrium（Y）-based MOF（NU-1801）was constructed by changing the organic ligand length and changing the metal through the basic theory of reticular chemistry,using Zr-MOF（NPF-500）as the design source,and the performance of the material in Xe/Kr separation and its mechanism were investigated.NU-1801 retains the flu topology of NPF-500 structure with a reduced pore size of 8.0?,as well as a high stability in both air and water vapor.The capture of Xe at 298 K and 1 bar was 2.79 mmol/g,and the IAST selectivity of Xe/Kr（Xe/Kr=20:80,v/v）was 8.2 with an excellent Xe/Kr absorption ratio of up to 400%,while the penetration experiments further confirmed this excellent separation performance.GCMC simulations revealed that due to the richness of the pore channel in aromatic rings and Y-O sites,the separation is achieved by a high force on Xe with high polarizability,which provides theoretical guidance for the design of MOFs with preferential adsorption of Xe.（2）By further shortening the organic carboxylic acid ligand of NU-1801,two microporous indium（In）-based MOF（In-MOF-Cl and In-MOF-NO₃）with dia topology and possessing 4.7?were constructed by selecting1,3,5,7-adamantanetetracarboxylic acid as the organic unit and metal SBU.These two In-MOFs were found to be anionic MOFs containing free Cl^-and NO₃^-by XPS and EDX characterization.In-MOF-NO₃exhibited preferential Xe adsorption at 298 K and1 bar with an Xe adsorption amount of 1.79 mmol/g,which was larger than the Kr adsorption amount of 1.03 mmo/g.While at 0.27 bar and 273 K,preferential adsorption of Kr was found in In-MOF-Cl.Using ion exchange and heating,it was found that when In-MOF-Cl was treated at 200°C,its paradoxical adsorption behavior of Xe/Kr was also observed at room temperature conditions.This temperature-dependent Xe/Kr paradoxical adsorption provides a new research idea for the design of MOFs with preferential adsorption of Kr.