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donglijie银虫 (初入文坛)
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Conventional and Unconventional MetalOrganic Frameworks Based
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The class of materials known as metalorganic frameworks, or MOFs, is the subject of an enormous amount of current research. The other reviews in this issue serve as testament to this fact. The defining character of these compounds is their porosity, which can be tuned by changing the geometry and size of the carboxylic acid ligands. In the early years of this research, carboxylate-based MOFs were designed to have ever-greater surface areas, resulting in the astonishing increase from 600 to >5000 m2/g.16 In addition, interest was keyed toward very large uptake of certain gases such as hydrogen, methane, and carbon dioxide and the possibility of a range of potential gas separations. The number of reported MOFs has skyrocketed as researchers have explored the vast number of possible combinations of ligand design, transition metals, coligands, and structuredirecting agents. Many investigators were fascinated by the idea that one could use a set of principles to define structures that would be porous. However, although many papers claim to have created porous products, the structural data is insufficient. One must have a sorption isotherm to prove the extent of porosity. Carboxylate-based MOFs have been shown to have remarkably high surface area and uniform pore size distribution, but their lack of stability in air and water poses a significant problem if they are to be used in industrial or commercial applications. Divalent metal carboxylates are subject to hydrolysis and usuallyquite soluble in acid solution. It should be pointed out that other types of MOFs, such as those based on pyrazolate ligands, can be much more robust.7 Phosphonates form stronger bonds than carboxylates do with metal atoms, so it is natural that investigators should try to prepare phosphonate-based MOFs. From our own experience, monovalent metal phosphonates are highly soluble, even though they may crystallize as supramolecular structures. As the valence of the metal increases, the solubility decreases. Divalent metal phosphonates are soluble enough that single crystals can be obtained by hydrothermal or solvothermal techniques, whereas trivalent and tetravalent metal phosphonates are rarely crystalline as they are highly insoluble and tend to precipitate as poorly ordered layered materials. The tetravalent metal phosphonates are insoluble even in strong acid solution. Phosphonates have three oxygen atoms capable of bonding to metals. They can also coordinate metals when they are in any state of protonation. This results in many possible modes of cocoordination and myriad different arrangements that the structures can take, many of which are not porous and therefore not MOFs. The majority of metal phosphonate compounds are not MOFs, and although there are a number of instances where phosphonate- based MOFs have been prepared, they were generally not by design. The interested reader is directed to a recently released book for more information. |
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hookhans
铁杆木虫 (著名写手)
Farmer
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phu_grassman: 金币+5, thanks 2013-03-11 16:23:05
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| 金属有机骨架化合物，即MOFs，是目前投入大量研究的一类材料。 在这方面的（大量)其它综述可以作为这一结论的佐证。这些化合物的确定特征是它们具有的孔性，可以通过改羧酸基配体的几何结构和尺寸来调变。在早些年的研究中，羧基的MOFs研究主要朝向设计前所没有的表面积，结果其表面积得到惊人的提高，从600 to >5000 m2/g. 此外， 研究兴趣还集中在其大量吸收一些气体上，比如， 氢气、甲烷、二氧化碳，以及其用于气体分离的潜在可能性。 随着研究人员发掘出大量配体设计、过度金属、共配体、结构导向剂之间的可行性组合方法，有关MFOs的报道数量急剧飙升。很多研究者满足于能够用一系列的既定方法来确定结构的孔道性。 然而，尽管很多文章声明他们构建了孔性材料，但是结构方面的数据还是欠缺。孔性必须用吸附曲线来证明。羧酸基的MOFs 已经显示其具有惊人的高表面积和均一的孔尺寸分布，但是由于它们在空气或水中缺乏稳定性，因此在工业或商业化应用中遇到很大的问题。二价金属的羧酸通常在酸性溶液中水解，并且很稳定。 应该指出的是， 其它类型的 MOFs, 比如那些基于吡唑特基配体的MOFs, 可以更稳定存在。因为磷酸基比羧酸基能与金属成更强的键，因此研究者们尝试制备磷酸基的MOFs是自然而然的事情。 从我们的实验来看， 单价的金属磷酸具有高溶解性，尽管它们会以超分子结构的形式结晶。随着金属价态的增加，溶解性降低。二价金属磷酸具有足够的溶解性，能够通过水热或溶剂热的技术得到单晶；而三价和四价的金属磷酸很少结晶，因为它们溶解性很差，通常趋向于形成无序的层状结构材料。三价和四价的金属磷酸甚至不溶于强酸。磷酸具有三个可以和金属成键的氧原子， 它们甚至在任何质子化状态下仍能和金属配位。这就导致材料结构可以采取很多的可能配位模型和大量的不同的组合，而其中的很多结构不是孔性的，因此不是MOFs。 大多数金属磷酸化合物不适MOFs， 尽管有一些例子报道说制备了磷酸基的MOFs， 但是它们都不是通过设计获得的。 更多信息，感兴趣的读者可以参考最近发布的一本书。 |
2楼2013-03-11 11:35:36