A robust ultra-microporous cationic aluminum-based metal-organic framework with a flexible tetra-carboxylate linker

Al-based cationic metal-organic frameworks (MOFs) are uncommon. Here, we report a cationic Al-MOF, MIP-213(Al) ([Al(18)(μ(2)-OH)(24)(OH(2))(12)(mdip)(6)]6Cl·6H(2)O) constructed from flexible tetra-carboxylate ligand (5,5'-Methylenediisophthalic acid; H(4)mdip). Its crystal structure was determined by the combination of three-dimensional electron diffraction (3DED) and high-resolution powder X-ray diffraction. The structure is built from infinite corner-sharing chains of AlO(4)(OH)(2) and AlO(2)(OH)(3)(H(2)O) octahedra forming an 18-membered rings honeycomb lattice, similar to that of MIL-96(Al), a scarce Al-polycarboxylate defective MOF. Despite sharing these structural similarities, MIP-213(Al), unlike MIL-96(Al), lacks the isolated μ(3)-oxo-bridged Al-clusters. This leads to an ordered defective cationic framework whose charge is balanced by Cl(-) sandwiched between two Al-trimers at the corner of the honeycomb, showing strong interaction with terminal H(2)O coordinated to the Al-trimers. The overall structure is endowed by a narrow quasi-1D channel of dimension ~4.7 Å. The Cl(-) in the framework restrains the accessibility of the channels, while the MOF selectively adsorbs CO(2) over N(2) and possesses high hydrolytic stability.