Preparation Of Metal Organic Frameworks And Application For Magnesium Adsorption And CH₄/N₂ Separation

As a new and highly ordered multifunctional material,metal-organic frameworks have become a new research hotspot in chemistry and materials science,which is mainly attributed to their exceptionally high surface area,tunable pores,as well as intriguing functionalities.Through nearly 20 years of development,scientists have studied MOFs from the early design of new materials gradually developed to functional modification,performance applications and so on.MOFs show potential applications in many areas,such as energy gas storage and separation,fluorescent probes,catalysis,transfer of drug molecules,molecular magnetic.With the intensification of the energy problem,MOFs as a new energy materials provide a new idea in order to solve the energy problem.In this thesis,we mainly study the application of MOFs in the magnesium adsorption and CH₄/N₂ separation.MOFs are firstly used in the extraction of magnesium from seawater where there are few reports.We focus on eight kinds of MOFs,the structures and properties of which are characterized by X-ray single crystal diffraction,thermogravimetric analysis,infrared analysis and X-ray powder diffraction.The main contents include the following points:（1）MIL-101-NH₂（Al）,MIL-53-NH₂（Al）,MIL-101-NH₂（Cr）and MIL-53-NH₂（Cr）were synthesized by hydrothermal method.Through modifying the-NH₂ groups in the frameworks,cationic MOFs were achieved,which exhibit selectively adsorbing Mg²⁺through ion exchange.The ion exchangeability was studied by Mg²⁺adsorption isotherm,kinetics,selectivity and cyclicality measurements.The results showed that Mg²⁺could be almost adsorbed completely just in a short time when c（MgCl₂）is 0.001 mol/L.Furthermore,protonated MOFs have a remarkable magnesium ion-sieve property with equilibrium distribution coefficients（K_d）of MIL-101-NH₂（Al）and MIL-53-NH₂（Al）in the order of Mg²⁺>>Li⁺>Na⁺>K⁺>Ca²⁺and Mg²⁺>>Li⁺>Na⁺>Ca²⁺>K⁺(Mg²⁺monolayer saturation amount of 0.70 mmol/g),and MIL-101-NH₂（Cr）and MIL-53-NH₂（Cr）in the order of Mg²⁺>>Ca²⁺>Na⁺>Li⁺>K⁺and Mg²⁺>Ca²⁺>Na⁺>K⁺>Li⁺(Mg²⁺monolayer saturation amount of 0.64 mmol/g)respectively.After two adsorption/desorption cycles,the adsorption capacity for these two materials only decreased by about 3%and 5%,10.51%and 9.46%respectively,which could be regenerated many times without obviously decrease.So MIL-101-NH₂（Al）,MIL-53-NH₂（Al）,MIL-101-NH₂（Cr）and MIL-53-NH₂（Cr）could selectively adsorb magnesium ions.In addition,MIL-101-NH₂（Cr）and MIL-53-NH₂（Cr）did not adsorb lithium ions,so it provided a good solution for the problems of Mg²⁺and Li⁺separation.（2）Using ligands contain methyl and carboxylic acid H₄L self-assembly with Zr⁴⁺got the rod-like crystals Zr-MOF and studied the effect of CH₄/N₂ separation compared with HKUST-1,MIL-101-NH₂（Cr）and Ni-MOF-74.The results showed that Zr-MOFs had good adsorption effect on methane which was about 8.2cm³/g at 273K and 1atm,and the adsorption effect on nitrogen was poor and hardly adsorbed.The adsorption heat of CH₄was about 20kJ/mol which was small,closed to HKUST-1,MIL-101-NH₂（Cr）and Ni-MOF-74.And the selectivity factor of CH₄/N₂ was about 6 that was the same as the better performance HKUST-1,and was higher than MIL-101-NH₂（Cr）and Ni-MOF-74,even the traditional adsorbents.Therefore,crystal Zr-MOFs had a good separation effect on CH₄ and N₂.