Metal–organic frameworks (MOFs) are three-dimensional highly porous solids composed of metal ions connecting short and mostly rigid organic linkers to form a crystalline network. The resulting porosity and high surface area of MOFs make them prime candidates for gas separation and sequestration. Seth Cohen and co-workers at the University of California, San Diego and the University of Florida have now added new flexibility to the design of MOFs.
Challenging common wisdom in MOFs architecture, the researchers managed to use linear, floppy polymers instead of rigid ligands and show that the final material — a polymer-MOF hybrid — is indeed crystalline. The polymer ligands are made of a polyether unit with methylene spacers of various lengths and an aromatic dicarboxylic acid strut. The carboxylic acid coordinates with zinc(II) ions to form a crosslinked porous framework. The porosity, surface area and hydrophobicity of the poly-MOFs can be controlled by varying the molecular weight of the polymer and the temperature of the reaction with zinc. From a polymer chemistry perspective, poly-MOFs demonstrate that linear polymers, usually associated with amorphous and non-porous arrangements, can form porous and crystalline solids through a coordination chemistry strategy.
Crystallographic data show that there are optimal conditions to prepare poly-MOFs, and although the fine mechanistic details still need to be worked out, the two best behaving poly-MOFs have pores of 7 Å and 9 Å — sizes that are favourable for CO2 uptake.