Enhancing Gas Adsorption and Separation Capacity through Ligand Functionalization of Microporous Metal-Organic Framework Structures

Hydroxyl-and amino-functionalized [Zn(BDC)(TED)(0.5)]center dot 2DMF center dot 0.2H(2)O leads to two new structures, [Zn(BDC-OH)(TED)(0.5)]center dot 1.5DMF center dot 0.3H(2)O and [Zn(BDC-NH(2))(TED)(0.5)]center dot xDMF center dot yH(2)O (BDC = terephthalic acid, TED = triethylenediamine, BDC-OH = 2-hydroxylterephthalic acid, BDC-NH(2) = 2-aminoterephthalic acid). Single-crystal Xray diffraction and powder X-ray diffraction studies confirmed that the structures of both functionalized compounds are very similar to that of their parent structure. Compound [Zn(BDC)(TED)(0.5)]center dot 2DMF center dot 0.2H(2)O can be considered a 3D porous structure with three interlacing 1D channels, whereas both [Zn(BDC-OH)-(TED)(0.5)]center dot 1.5DMF center dot 0.3H(2)O and [Zn(BDC-NH(2))(TED)(0.5)]center dot xDMF center dot yH(2)O contain only 1D open channels as a result of functionalization of the BDC ligand by the OH and NH(2) groups. A notable decrease in surface area and pore size is thus observed in both compounds. Consequently, [Zn(BDC)(TED)(0.5)]center dot 2DMF center dot 0.2H(2)O takes up the highest amount of H(2) at low temperatures. Interestingly, however, both [Zn(BDC-OH)(TED)(0.5)]center dot 1.5DMF center dot 0.3H(2)O and [Zn(BDC-NH(2))(TED)(0.5)]center dot xDMF center dot yH(2)O show significant enhancement in CO(2) uptake at room temperature, suggesting that the strong interactions between CO(2) and the functionalized ligands, indicating that surface chemistry, rather than porosity, plays a more important role in CO(2) adsorption. A comparison of single-component CO(2), CH(4), CO, N(2), and O(2) adsorption isotherms demonstrates that the adsorption selectivity of CO(2) over other small gases is considerably enhanced through functionalization of the frameworks. Infrared absorption spectroscopic measurements and theoretical calculations are also carried out to assess the effect of functional groups on CO(2) and H(2) adsorption potentials.