Freeze Drying Significantly Increases Permanent Porosity and Hydrogen Uptake in 4,4-Connected Metal-Organic Frameworks
Freeze Drying Significantly Increases Permanent Porosity and Hydrogen Uptake in 4,4-Connected Metal-Organic Frameworks
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DOI：
10.1002/anie.200904983
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发表时间：
2009-01-01
影响因子：
16.6
通讯作者：
Lin, Wenbin
中科院分区：
文献类型：
--
作者：
Ma, Liqing;Jin, Athena;Lin, Wenbin
金属有机框架（mof）的许多潜在应用都利用了这类新兴可调谐材料提供的巨大内表面积例如，大的永久孔隙度已被证明是气体储存、[2]化学传感、[3]催化、[2]和药物控释的关键虽然通常可以通过将高沸点溶剂（通常存在于合成mof的孔隙和通道中）与低沸点溶剂交换，然后在高温下排出，从而获得永久孔隙，但许多mof的表面积仅为基于其结构预测的一小部分。对于由精心设计的有机桥接配体构建而成的mof来说，这个问题变得更加严重我们最近开发了一些策略来提高mof的永久孔隙度，并取得了不同程度的成功，方法是通过不同寻常的联锁和互穿来固化框架，或者通过增加桥接配体的连通性在此，我们报告了一种简单的物理方法，通过冷冻干燥在新的4,4连接mof中实现高永久孔隙率。正如Hupp及其同事所展示的那样，通过超临界二氧化碳（s-CO2）处理，锌mof的表面积可以显著增加，可能是由于s-CO2的低表面张力，可以防止介孔坍塌，从而提高微孔可达性我们设想，如果将加入mof中的高沸点溶剂与可以通过冷冻干燥去除的溶剂（如苯）交换，可以产生类似的效果。冷冻溶剂可以在低于其冰点的温度下通过真空固气转变从mof中去除。绕过液相消除了表面张力在诱导介孔坍塌中的不利影响，从而提高了mof的永久孔隙率。
Many potential applications of metal–organic frameworks (MOFs) take advantage of the enormous internal surface areas offered by this emerging class of tunable materials.[1] For example, large permanent porosity has been shown to be key in gas storage,[2] chemical sensing,[3] catalysis,[4] and controlled release of drugs.[5] Although permanent porosity can often be attained by exchanging high boiling-point solvents that are typically in the pores and channels of as-synthesized MOFs with low boiling-point solvents followed by evacuation at elevated temperatures, many MOFs exhibit surface areas that are only a small fraction of what is predicted based on their structures. This problem becomes even more severe for MOFs that are built from elaborately designed organic bridging ligands to impart desirable functions.[6] We have recently developed strategies to enhance permanent porosity in MOFs with varying degrees of success by rigidification of the frameworks with unusual interlocking and interpenetration or by increasing the connectivity of the bridging ligands.[7] Herein we report a simple physical means of achieving high permanent porosity in new 4, 4-connected MOFs by freeze drying.As elegantly demonstrated by Hupp and co-workers, the surface areas of zinc MOFs can be significantly increased by supercritical carbon dioxide (s-CO2) processing, presumably by preventing the mesopore collapse and thus enhancing micropore accessibility as a result of the low surface tension of s-CO2.[8] We envisaged that a similar effect can be elicited if the high boiling-point solvents incorporated in the MOFs are exchanged with a solvent (such as benzene) that can be removed by freeze drying. The frozen solvent can be removed from the MOFs by solid–gas transition under vacuum at temperatures below its freezing point. Bypassing the liquid phase eliminates the detrimental effect of surface tension in inducing mesopore collapse, thereby enhancing the permanent porosity of MOFs.