Hierarchical Microporous Anion Pillared Metal-Organic Frameworks (APMOFs) for Gas Separation

As an emerging class of porous materials, metal-organic frameworks (MOFs) have attracted considerable interest for their designable structures and tailorable functionality. Among them, anion-pillared MOFs (APMOFs) represent an important subclass that has gained widespread attention in gas adsorption and separation. The introduction of inorganic anions polarizes the pore surface of APMOFs, thereby enhancing their ability to discriminate between similar gas molecules. Typically, APMOFs are composed of cationic metal nodes, neutral organic linkers, and inorganic anions. Among these three components, modulation of the organic ligands is more significant for creating diverse APMOF structures. In this Account, we summarize our efforts to engineer hierarchical APMOFs for gas separation through modulation of the organic ligands. Employing a special class of polydentate pyridine ligands, we constructed a new series of APMOFs possessing hierarchical micropores ranging from ultramicropores (<7 Å) to supermicropores (7-20 Å). Based on our fabrication research, we found that the key to successfully synthesizing stable hierarchical frameworks lies in constructing an ultramicroporous tetrahedral cage composed of four dipyridyl amine units, four metal nodes, and two anions. Therefore, based on the dipyridyl amine motif, we have designed and synthesized four different ligands: N-methyl-N,N-di(pyridin-4-yl)amine (MDPA), tri(pyridin-4-yl)amine (TPA), N1,N1,N4,N4-tetra(pyridin-4-yl)benzene-1,4-diamine (p-TPBDA) and N1,N1,N4,N4-tetra(pyridin-4-yl)benzene-1,3-diamine (m-TPBDA). These ligands, when combined with various anions and Cu2+ cation, afford different APMOFs. All of them have double or triple-level hierarchical microporous structures featuring ultramicroporous tetrahedral cages. This pore architecture differs from the conventional one-dimensional straight channels in most APMOFs with pcu topology. Ultramicropores enhance separation selectivity through confinement effects, whereas supermicropores improve storage capacity and provide the rapid diffusion pathways. By combining both advantages, these hierarchical microporous APMOFs show enhanced performance in CO2 capture, propadiene/propyne separation, C3H4/C3H6 separation, C2H2/CO2 separation, C2H2/C2H4 separation, Xe/Kr separation, C3H8/C2H6/CH4 and one-step acquisition of C2H4 from ternary C2H2/CO2/C2H4 mixtures. In summary, our work provides new strategies for the design of APMOFs and offers fresh insights for further structural optimization to advance their gas adsorption and separation performance.