Macromolecular Science and Engineering, University of Michigan, 2300 Hayward Avenue, Ann Arbor, Michigan 48109-2136, USA.
Langmuir. 2013 Jun 25;29(25):8146-53. doi: 10.1021/la401323t. Epub 2013 Jun 14.
Microporous coordination polymers (MCPs) have emerged as strong contenders for adsorption-based fuel storage and delivery in large part because of their high specific surface areas. The strategy of increasing surface area by increasing organic linker length has shown only sporadic success; as demonstrated by many members of the iconic Zn4O-based IRMOF series, for example, accessible porosity is often limited by interpenetration or pore collapse upon guest removal. In this work, we focus on Zn4O(ndc)3 (IRMOF-8, ndc = 2,6-naphthalene dicarboxylate), which exhibits typical surface areas of only 1000-2000 m(2)/g even though a surface area of more than 4000 m(2)/g is expected from geometric analysis of the originally reported crystal structure. We recently showed that a high surface area could be produced with zinc and ndc by room-temperature synthesis followed by activation with flowing supercritical CO2. In this work, we investigate in detail the porosity of both the low- and high-surface-area materials. Positron annihilation lifetime spectroscopy (PALS) is used to show that the low-surface-area material suffers from near-complete interpenetration, explaining why traditional synthetic routes have failed to yield materials with the expected porosity. Furthermore, the high-pressure hydrogen and methane sorption properties of noninterpenetrated Zn4O(ndc)3 are examined, and PALS is used to show that pore filling is not operative during room-temperature CH4 sorption even at pressures approaching 100 bar. These results provide insight into how gas adsorbs in high-surface-area materials at high pressure and reinforce previous contentions that increasing surface area alone is not sufficient for the simultaneous optimization of deliverable gravimetric and volumetric gas uptake in MCPs.
微孔配位聚合物（MCPs）在很大程度上因其具有高比表面积而成为基于吸附的燃料储存和输送的有力竞争者。通过增加有机连接体长度来增加表面积的策略仅取得了零星的成功；例如，许多标志性的基于 Zn4O 的 IRMOF 系列成员表明，可及孔隙率通常受到互穿或客体去除时孔塌陷的限制。在这项工作中，我们专注于 Zn4O(ndc)3（IRMOF-8，ndc = 2,6-萘二甲酸酯），尽管从最初报道的晶体结构的几何分析来看，预计其表面积超过 4000 m2/g，但它仅表现出 1000-2000 m2/g 的典型表面积。我们最近表明，通过室温合成并用流动的超临界 CO2 进行活化，可以产生具有高表面积的锌和 ndc。在这项工作中，我们详细研究了低表面积和高表面积材料的孔隙率。正电子湮没寿命谱（PALS）用于表明低表面积材料受到近乎完全的互穿，这解释了为什么传统的合成路线未能产生具有预期孔隙率的材料。此外，还研究了非互穿的 Zn4O(ndc)3 的高压氢气和甲烷吸附性能，并使用 PALS 表明，即使在接近 100 巴的压力下，室温 CH4 吸附时也不会发生孔填充。这些结果提供了对高压下高表面积材料中气体吸附的深入了解，并强化了先前的论点，即增加表面积本身不足以同时优化 MCPs 中可输送的重量和体积气体吸收。
