Preparation Of Metal-Organic Frameworks And Their Adsorption Study For Precious Metal Ions

This research mainly describes the preparation process of ZIF-8,NH₂-ZIF-8,cyanoethyl-MOFs,2,5-TP and other metal organic framework materials,as well as their adsorption effects on Au???and Ag???in solution.The effects of pH value,adsorption time,initial concentration and temperature on adsorption were studied.At the same time,the selective and reusable properties of the materials were investigated.Various characterization methods were used to analyze the morphologies,structures,compositions and valence states of the metal organic framework materials?MOF?before and after the adsorption of metal ions,so as to explore the adsorption mechanism.The results of this paper are as follows:1.ZIF-8 has a large specific surface area and a large number of-NH and Zn-OH active sites,showing good adsorption performance for Au???and Ag???,and the maximum adsorption capacity for Au???and Ag???is 1192 mg/g and 477.6 mg/g,respectively.The experimental results showed that the acidic conditions were favorable for the adsorption of Au???,while the increase of pH value was more favorable for the adsorption of Ag???.Kinetic studies show that the adsorption of Au???and Ag???by ZIF-8 conforms to the pseudo-second order kinetic model,and adsorption isotherms show that both of them conform to the Freundlich model.Through the study of temperature,it is found that the adsorption of Au???by ZIF-8is an endothermic process,and the adsorption of Ag???is an exothermic process.According to the mechanism analysis,the adsorption of Au???by ZIF-8 mainly depends on the electrostatic attraction generated by the protonation of-NH and the coordination of Zn-OH.However,the adsorption equilibrium time of ZIF-8 for Au???and Ag???was relatively long,requiring 40 hours and 6 hours,respectively.2.In order to improve the adsorption rate,ethylenediamine was grafted onto ZIF-8 by post modification method to synthesize NH₂-ZIF-8.The adsorption time of Au???and Ag???was shortened to 4 hours and 1.5 hours respectively.The experimental results show that the adsorption trend of Au???and Ag???on NH₂-ZIF-8 by pH are consistent with those of ZIF-8,and the kinetic and adsorption isotherms are also the same as those of ZIF-8.Mechanism studies showed that-NH₂ and Zn-OH played a major role in the adsorption process.However,its adsorption capacity was significantly reduced compared with ZIF-8.3.In order to solve the problem of decreasing adsorption capacity,we synthesized cyanoethyl-MOF by using 1-?2-cyanoethyl?-methylimidazole as ligand and conducted adsorption studies on Au???and Ag???.The results showed that the maximum adsorption capacity of cyanoethyl-MOF for Au???was 674 mg/g,and that of Ag???was 598 mg/g.The optimal pH value of cyanoethyl-MOF adsorption of Au???was 5.5,while the change of pH had no effect on the adsorption of Ag???.The results of kinetic and adsorption isotherm fitting were consistent with those of ZIF-8.4.Based on the strong binding effect between S-Au,sulfur ligands were introduced to synthesize 2,5-TP materials,which were used for the adsorption study of Au???.The adsorption effect was greatly improved,with the adsorption capacity reaching 1238.5 mg/g,especially suitable for the concentration of 50 mg/L-300 mg/L,and the adsorption rate reaching more than 90%.The adsorption properties were investigated,and the results showed that the adsorption of Au???onto 2,5-TP was favorable when the aqueous solution was neutral.The kinetic data were in line with the pseudo-second-order kinetic model,and the adsorption isotherm was in line with the Freundlich model.Moreover,the adsorption belonged to the endothermic reaction,and the higher the temperature,the better the adsorption.Mechanism studies show that there are two main adsorption processes,one is the coordination between S element and Au???,and the other is the ion exchange between Zn-OH produced by unsaturated central ion Zn element and Au???.