Cation-Exchange Approach to Tuning the Flexibility of a Metal-Organic Framework for Gated Adsorption

Achieving tailorable gated adsorption by tuning the dynamic behavior of a host porous material is of great interest because of its practical application in gas adsorption and separation. Here we devise a unique cation-exchange approach to tune the dynamic behavior of a flexible anionic framework, [Zn₂(bptc)(datrz)]^- (denoted as MAC-6, where H₄bptc = [1,1'-biphenyl]-3,3',5,5'-tetracarboxylic acid and Hdatrz = 3,5-diamine-1H-1,2,4-triazole), so as to realize the tailorable gated adsorption. The CO₂ adsorption amount at 273 K can be enhanced by exchanging the counterion of protonated dimethylamine (HDMA⁺) with tetraethylammonium (TEA⁺), tetrabutylammonium (TBA⁺), and tetramethylammonium (TMA⁺), where the adsorption behavior is transferred from nongated to gated adsorption. Interestingly, the P_go for gate-opening adsorption can be further tuned from 442 to 331 mmHg by simply adjusting the ratio of HDMA⁺ and TMA⁺. The origin of this unique tunable property, as revealed by X-ray diffraction experiments and structure models, is rooted at the cation-responsive characteristic of this flexible framework.