Nitrogen Coordination To Dramatically Enhance the Stability of In-MOF for Selectively Capturing CO2 from a CO2/N2 Mixture

Two In-based metal–organic frameworks (MOFs) have been designed and synthesized by the reaction of 3-phenylpyridine polycarboxylic ligands and In3+ cations: In3L14·(CH3NH2CH3)3 (H3L1 = 5-(3′,5′-dicarboxylphenyl)nicotic acid; 1) and In2L22(CH3COO)2·(CH3)2NH2·(CH3)3NH·H2O (H3L2 = 3-(2,4-dicarboxylphenyl)-6-carboxylpyridine; 2). Single-crystal X-ray diffraction analysis indicates that the In3+ centers in 1 are all coordinated by eight oxygen atoms from four carboxylate groups, resulting in a 3D framework with two different channels A and B with sizes of 14.4 × 8.75 and 7.24 × 6.7 Å2, respectively. In 2, each In3+ center was coordinated by six carboxylate oxygen atoms and a pyridine N atom, forming a 2D layerlike structure. These 2D layers are further fused together via electrostatic interactions into a 3D supramolecular framework, showing two kinds of channels A′ and B′ with pore sizes of 5.25 × 9.69 and 4.29 × 8.66 Å2, respectively. In this work, the coordination of N to the In3+ center in 2 was first achieved by regulating carboxyl sites in 3-phenylpyridine, thus increasing the stability of the In-based MOF. A detailed structural study reveals that the formation of In–N bonds promotes the conversion of the 3D framework of 1 into the supramolecular structure of 2. However, the supramolecular structure in 2 is more stable in air and methanol in comparison to the 3D framework of 1, confirming the important role of In–N bonds in enhancing the framework stability. Thus, 2 could be used as a stable adsorbent to selectively capture CO2 from a CO2/N2 mixture, while 1 broke down sharply in air and in a methanol solution.