Structures, Sorption Characteristics, and Nonlinear Optical Properties of a New Series of Highly Stable Aluminum MOFs

Employing high-throughput methods, the synthesis conditions for a series of six new MOFs based on aluminum ions and the V-shaped linker molecule 1,3-benzene dicarboxylic acid, denoted as CAU-10-X (CAU = Christian-Albrechts-University) with the sum formula [Al­(OH)­(C8H3O4X)]·solvent, were established (X = functional group in 5-position of the aromatic ring; X = H (1), CH3 (2), OCH3 (3) NO2 (4), NH2 (5), or OH (6)). Because of the absence of macroscopic crystals, the obtained compounds were structurally characterized employing XRPD-methods. The crystal structures of 1, 2, and 3 were refined using Rietveld methods. Although the described MOFs are isoreticular, they crystallize in several, sometimes noncentrosymmetric space groups (1, 4, 6), because of slight structural changes induced by the functionalization. These space groups were confirmed with second-harmonic generation measurements. All compounds are highly stable as confirmed by temperature-dependent XRPD and IR experiments and decompose at temperatures above 350 °C. The stabilities of all compounds in aqueous solutions of varying pH were confirmed by XRPD measurements and their sorption properties toward nitrogen, hydrogen, carbon dioxide, and water vapor at low pressures are reported. A drastic influence of the functional group on affinity, capacity, and accessibility of the pores for these gases is observed. These properties depend on the polarity and size of the functional group as well as on subtle structural differences between the CAU-10-X compounds.本研究采用高通量合成方法，建立了基于铝离子与V形连接配体1,3-苯二甲酸的6种新型金属有机框架（Metal-Organic Frameworks, MOFs）的合成条件，该系列材料命名为CAU-10-X（CAU=基尔大学Christian-Albrechts-University），其通式为[Al(OH)(C8H3O4X)]·溶剂，其中X为芳环5位取代官能团，具体包括：X=H（1）、CH3（2）、OCH3（3）、NO2（4）、NH2（5）与OH（6）。 由于未获得宏观单晶，所合成的化合物通过X射线粉末衍射（X-ray Powder Diffraction, XRPD）方法进行结构表征。其中，化合物1、2与3的晶体结构采用里特维尔德（Rietveld）精修方法完成精修。 尽管该系列MOFs具有同拓扑骨架，但是由于官能化修饰引发的细微结构变化，它们分别结晶于多种空间群，其中部分为非中心对称空间群（1、4、6号）。上述空间群通过二阶谐波生成（second-harmonic generation, SHG）测试得以验证。 所有化合物均具有优异的热稳定性，该结论通过变温XRPD与红外（Infrared, IR）实验得到证实，其热分解温度均高于350℃。通过XRPD测试证实了所有化合物在不同pH水溶液中的稳定性；本文同时报道了该系列材料在低压条件下对氮气、氢气、二氧化碳与水蒸气的吸附性能。 研究发现，官能团对上述气体的吸附亲和力、吸附容量与孔道可及性具有显著调控作用；该类吸附性能取决于官能团的极性与尺寸，以及CAU-10-X系列化合物之间的细微结构差异。