Synthesis, Assembly And Properties Of Rare Earth Metal-Organic Frameworks

Since 1999, a open metal-organic framework material, MOF-5, reported by O. M. Yaghi research group. This new type of porous material becomes a hotspot in the field of chemistry and material. This is because their structure can be tailored, they have variety of topology and great application prospects in the areas of ion exchange, adsorption, molecular recognition, catalysis, light, electricity, magnetism and chiral separation etc. In the past decade, thousands of metal-organic framework materials with novel structures have been reported, some of them have steady structure and superior performance. Now synthesis approaches are mature increasingly, with the emerging of many new structures. People begin to pay attention to the design of structural with the purpose of functional, and development of performance.In structural design, development and modification of zeolite topologies are considered the most promising synthetic methods of metal-organic framework materials. The traditional zeolite materials have attractive pore structures and relatively high thermal stability, therefore similar structures could give metal-organic framework materials more superiority and greater applicability. According to this, a series of metal-organic framework materials RE(BTC)(DMF)₂·(H₂O) [RE = Tb (1), Dy (2), Ho (3), Er (4), Tm (5), Yb (6)] with zeolite-like topology structure have been synthesized. They have the same skeleton structure. In there frameworks, one metal ion is linked with four phenyl groups ,which could be considered as an inorganic four-connected node. Likewise, each BTC ligand is connected with four metal ions. So it could be regarded as an organic four-connected node.The vertex symbols of inorganic and organic node both are 43 62 8, which is very common in zeolite topologies. Therefore, it is not unreasonable to regard it as a zeolite-like topology, although it is not an isostructure with a reported zeolite topology.In the function, this thesis developed from the following aspects:(1) First, Considering the functionality of the metal centre, rare earth metal has been choiced in hopes that the there unique 4f electronic structure coluld lead to more optolite topology, Tb(BTC)(DMF)₂·(H₂O) shows the characteristics emission of Tb ion. Eu and Tb doped JUC-32-Y have also been synthesized. They have the same skeleton structure with JUC-32-Tb and JUC-32-Eu. by low-cost metal yttrium Y in the mixed area of micro and visible fluorescence emission of metal Tb and Eu, not only greatly reduce the cost of materials, but also reduces the concentration quenching of their fluorescence properties to optimize the JUC-32-Tb and JUC-32-Eu performance. Mixing fractional Tb and Eu into the low-cost metal yttrium, not only the cost is greatly reduced, but also the concentration quenching is reduces to optimize the fluorescence properties of JUC-32-Tb and JUC-32-Eu.(2) As a new type of porous material, a large number of metal-organic framework materials with high surface area and high porosity are synthesized. It makes that the gas storage and transportation applications of this kind of materials are in possible. Especially, today in the energy shortage, fuel gas (hydrogen, methane, etc.) storage performance of such materials receives many attentions. Our group had synthesized a metal-organic framework JUC-32 with high thermal stability. It has high hydrogen adsorption capacity. As a continuation, we change the metal centre to a much lighter element yttrium, which has very similar coordination chemistry properties. A new material JUC-32-Y with the same skeleton structure was synthesized. The changes of metal center improved the l gas storage capacity of JUC-32 greatly. At 77 K, the hydrogen adsorption capacity increase to 1.73 wt% from 1.32 wt%. The high-pressure methane storage capacity of JUC-32-Y has also been characterized. At 40 bar, the saturated methane storage capacity of JUC-32-Y can reach 135 v (STP) / v. Even at 35 bar, that is the security pressure setted by U. S. DOE, the methane storage capacity can reach 130 v (STP) / v. ical and magnetic properties. In the above series of metal-organic framework materials with ze(3) At present, there are two main methods for hydrogen storage, one is physical method using of adsorption properties of porous materials, and the other is chemical hydrogen storage method. The so-called chemical method, refers to that there is a chemical bond between materials and H, or the material itself contains a large number of H atoms, which can be taken off by heating or other methods to form H2 molecules. Over approximately a decade of exploration, the scope of candidate materials has expanded greatly, from traditional metal hydrides to complex hydrides. ammonia borane (AB), NH₃BH₃ is focusing many attentions, owing to its extremely high stoichiometric hydrogen content (19.6 wt %). But for practical applications, there are still some technical challenges that must be overcome: (1) reducing the dehydrogenation temperature of AB (< 85 oC set by DOE) (2) remarkable increase of hydrogen release rate; and (3) preventing the formation of volatile by-products, e.g. borazine, ammonia and diborane. The research of X. D. Yao's group indicated that there is a synergistic effect by combining nanoconfinement and metallic catalyst (Li+) on enhancing the hydrogen release kinetics and preventing ammonia formation. Accordingly, we imagine that metal-organic frameworks (MOFs) could be ideal frameworks with metallic sites and micropores. Herein we report a successful synthesis of AB/MOF confined system by infusion method. JUC-32-Y was selected as the framework due to its proper microporous size, high surface area, high thermally stability and containing open unsaturated metal sites. In contrast, the decomposition temperature of AB inside JUC-32-Y decreases greatly. The MS peak temperature of AB decomposition shifted to 84 oC, compared to 114 oC of neat AB. The hydrogen release rate of AB inside JUC-32-Y is remarkable increased. At 95 oC, AB inside JUC-32-Y could release 8.5 wt % hydrogen in only 3 minutes and reach 10.2 wt % hydrogen in only 10 minutes. At even a low temperature of 85oC, AB could release 8.0 wt % hydrogen within 10 minutes. Furthermore, no volatile products are found during the decomposition process. It should be emphasized that there is no ammonia formed. Superior performance demonstrates our vision that metal-organic framework materials is a new choice for improving the hydrogenation properties ammonia borane. So far, this is the first example of using MOF as host materials to improve the thermal hydrogen release of AB, and good results has achieved. This opens up a new application direction of metal-organic framework materials, also brings more hope for the application of ammonia borane.