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.