Three-Dimensional Metal-Organic Framework with Highly Polar Pore Surface: H2 and CO2 Storage Characteristics

A three-dimensional (3D) pillared-layer metal organic framework, [Cd(bipy)(0.5)(Himdc)](DMF)](n) (1), (bipy =4,4'-bipyridine and Himdc = 4,5-imidazoledicarboxylate) has been synthesized and structurally characterized. The highly rigid and stable framework contains a 3D channel structure with highly polar pore surfaces decorated with pendant oxygen atoms of the Himdc linkers. The desolvated framework [Cd(bipy)(0.5)(Himdc)(n), (I') is found to exhibit permanent porosity with high H-2 and CO2 storage capacities. Two H-2 molecules occluded per unit formula of 1' and the corresponding heat of H-2 adsorption (Delta H-H2) is about similar to 9.0 kJ/mol. The high value of Delta H-H2 stems from the preferential electrostatic interaction of H-2 with the pendent oxygen atoms of Himdc and aromatic bipy linkers as determined from first-principles density functional theory (DFT) based calculations. Similarly, DFT studies indicate CO2 to preferentially interact electrostatically (C delta+center dot center dot center dot O delta-) with the uncoordinated pendent oxygen of Himdc. It also interacts with bipy through C-H center dot center dot center dot O bonding, thus rationalizing the high heat (Delta H-CO2 similar to 35.4 kJ/mol) of CO2 uptake. Our work unveiled that better H-2 or CO2 storage materials can be developed through the immobilization of reactive hetero atoms (O, N) at the pore surfaces in a metal-organic framework.