Synthesis Of Metal-organic Framework Materials Based On Calix[4]arene And Their Adsorption Properties For UO₂²⁺ And Pb²⁺ In Water

In recent years,due to rapid economic and social development,there has been an increasing amount of research and development into new materials.Metal-organic framework materials（MOFs）have received a lot of attention as a new type of material that not only contains a large specific surface area and regular pore channels,but also allows the modification of its pore channels using different functional groups and,secondly,the construction of different structures of MOFs by selecting different ligands.Functionalized MOFs with different functional groups can be obtained by pre-modification of the ligands to achieve specific adsorption and removal of certain pollutants.In this paper,two tetracarboxylic azobenzene calix[4]arene were obtained by selecting third-generation supramolecular calix[4]arene with a special cavity structure as ligands and modifying their upper ends functionally.Three different structural MOFs（Eu-MOF,Mn-MOF,and Co-MOF）were synthesized using these two tetracarboxylic calix[4]arene ligands with different metal ions(Eu³⁺,Mn²⁺,Co²⁺)under solvent heat conditions,and their structure,stability and detailed study of their adsorption properties on pollutants(UO₂²⁺and Pb²⁺)in water were characterized.The main results are as follows.（1）Synthesis of Eu-MOF with cage structure using self-assembly of para-substituted carboxy azobenzene calix[4]arene(abbreviated as H₄L_pair)with Eu（NO₃）₂·6H₂O at room temperature.The characterization revealed that Eu-MOF not only has the unique cavity structure of calix[4]arene but also has high water stability under acidic conditions（p H=2-5 range）,which provides a basis for uranium removal in acidic wastewater.The adsorption study showed that the maximum adsorption of uranium by Eu-MOF could reach 142.15 mg/g and was in accordance with the Langmuir adsorption isotherm model,and the adsorption of uranium was monolayer.The kinetic study of uranium adsorption found that the adsorption of uranium by the adsorbent can reach adsorption equilibrium within 180 minutes,which is by following the secondary kinetic adsorption model,and the adsorption process is chemisorption.By analyzing the mechanism of adsorption of uranyl ions by Eu-MOF,it is suggested that the reason why Eu-MOF shows such an excellent affinity for uranyl ions is due to the strong electrostatic attraction and the interaction between the COO^-functional group and the uranyl ion(UO₂²⁺).（2）The synthesis of Mn-MOF with a two-dimensional layered structure using the reaction of para-substituted carboxy azobenzene calix[4]arene(abbreviated as H₄L_pair)with Mn Cl₂·4H₂O under solvent heat conditions,which can be exfoliated into MOF nanosheets by dispersing them in the solvent under ultrasonic conditions.The prepared Mn-MOF nanosheets not only have a large number of unliganded carboxylic acid groups（-COOH）but also large cage-like cavities（21.236?×12.709?）.Kinetic studies have revealed that uranium can also be removed to 18.29 ppb（below the US Environmental Protection Agency standard:30 ppb）in a very short time（1 min）at an extremely low uranium concentration of 150 ppb,achieving efficient capture of uranyl ions(UO₂²⁺)and an ultra-fast adsorption rate.In addition,the adsorption of UO₂²⁺by the 2D-Mn-MOF nanosheets reached 343.22 mg/g,which is consistent with the Langmuir adsorption isotherm model,and the adsorption of uranium was monolayer.（3）The synthesis of one-dimensional Co-MOFs with cage structures using meso-substituted carboxy azobenzene calix[4]arene(referred to as H₄L_meso)and Co Cl₂·6H₂O under solvothermal conditions.This Co-MOF has a chain structure and contains unliganded carboxylic acid oxygen adsorption sites.By studying the effect of adsorbent on the p H of uranium adsorption,it was found that Co-MOF can have an excellent removal effect on Pb²⁺in the p H range of 4-7（removal rate of over 99%）.In contrast,when the kinetics of uranium adsorption was studied,the adsorption process was found to fit a secondary kinetic fitting model,indicating that the adsorption of Pb²⁺by Co-MOF is chemisorption.When different coexisting ions(K⁺,Na⁺,Ca²⁺,Ba²⁺,Ni²⁺,Mg²⁺,Sr²⁺)were added to a Pb²⁺solution at a concentration of 10 ppm,the coexisting ions were found to have little effect on the adsorption rate of Pb²⁺.In addition,the Co-MOF adsorbed 493.89 mg/g of Pb²⁺.This good removal performance of Pb²⁺can be attributed to the presence of a large number of accessible carboxylic acid oxygen adsorption sites in the Co-MOF framework,which can create coordination bonds with Pb²⁺and subsequently achieve good removal results.