Macromolecular Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States.
J Am Chem Soc. 2011 Nov 16;133(45):18257-63. doi: 10.1021/ja2055935. Epub 2011 Oct 19.
There are several compounds for which there exists a disconnect between porosity as predicted by crystallography and porosity measured by gas sorption analysis. In this paper, the Zn-based analogue of Cu(3)(btc)(2) (HKUST-1), Zn(3)(btc)(2) (Zn-HKUST-1; btc = 1,3,5-benzenetricarboxylate) is investigated. Conventional analysis of Zn-HKUST-1 by powder X-ray diffraction and gas sorption indicates retention of crystalline structure but negligible nitrogen uptake at 77 K. By using positron annihilation lifetime spectroscopy, a densified surface layer preventing the entry of even small molecular species into the crystal framework is revealed. The material is shown to have inherent surface instability after solvent removal, rendering it impermeable to molecular guests irrespective of handling and processing methods. This previously unobserved surface instability may provide insight into the failure of other microporous coordination polymers to exhibit significant porosity despite crystal structures indicative of regular, interconnected, microporous networks.
有几种化合物存在着晶体学预测的孔隙率与气体吸附分析测量的孔隙率之间的不匹配。在本文中，研究了 Cu(3)(btc)(2)（HKUST-1）的 Zn 类似物 Zn(3)(btc)(2)（Zn-HKUST-1；btc = 1,3,5-苯三甲酸）。通过粉末 X 射线衍射和气体吸附对 Zn-HKUST-1 进行的常规分析表明，结晶结构得以保留，但在 77 K 时氮气吸收可忽略不计。通过使用正电子湮没寿命光谱，揭示了阻止甚至小的分子物种进入晶体骨架的致密表面层。该材料在去除溶剂后表现出固有的表面不稳定性，使其对分子客体不可渗透，无论处理和加工方法如何。这种以前未观察到的表面不稳定性可能为其他微孔配位聚合物尽管具有规则的、相互连接的微孔网络的晶体结构，但未能表现出显著的孔隙率提供了一些启示。
