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

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Abstract
Hydroxyl‐ and amino‐ functionalized [Zn(BDC)(TED)0.5]⋅2DMF⋅0.2H2O leads to two new structures, [Zn(BDC‐OH)(TED)0.5]⋅1.5DMF⋅0.3H2O and [Zn(BDC‐NH2)(TED)0.5]⋅xDMF⋅yH2O (BDC=terephthalic acid, TED=triethylenediamine, BDC‐OH=2‐hydroxylterephthalic acid, BDC‐NH2=2‐aminoterephthalic acid). Single‐crystal X‐ray 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]⋅2DMF⋅0.2H2O can be considered a 3D porous structure with three interlacing 1D channels, whereas both [Zn(BDC‐OH)(TED)0.5]⋅1.5DMF⋅0.3H2O and [Zn(BDC‐NH2)(TED)0.5]⋅xDMF⋅yH2O contain only 1D open channels as a result of functionalization of the BDC ligand by the OH and NH2 groups. A notable decrease in surface area and pore size is thus observed in both compounds. Consequently, [Zn(BDC)(TED)0.5]⋅2DMF⋅0.2H2O takes up the highest amount of H2 at low temperatures. Interestingly, however, both [Zn (BDC‐OH)(TED)0.5]⋅1.5DMF⋅0.3H2O and [Zn(BDC‐NH2)(TED)0.5] ⋅xDMF⋅yH2O show significant enhancement in CO2 uptake at room temperature, suggesting that the strong interactions between CO2 and the functionalized ligands, indicating that surface chemistry, rather than porosity, plays a more important role in CO2 adsorption. A comparison of single‐component CO2, CH4, CO, N2, and O2 adsorption isotherms demonstrates that the adsorption selectivity of CO2 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 CO2 and H2 adsorption potentials.

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