Ligand Functionalization and Its Effect on CO2 Adsorption in Microporous Metal-Organic Frameworks

We report two new 3D structures, [Zn3(bpdc)3(2,2-dmbpy)] (DMF)x(H2O)y (1) and [Zn3(bpdc)3(3,3-dmbpy)](DMF)4(H2O)0.5 (2), by methyl functionalization of the pillar ligand in [Zn3(bpdc)3(bpy)] (DMF)4(H2O) (3) (bpdc=biphenyl-4,4-dicarboxylic acid; z,z-dmbpy=z,z-dimethyl-4,4-bipyridine; bpy=4,4-bipyridine). Single-crystal X-ray diffraction analysis indicates that 2 is isostructural to 3, and the power X-ray diffraction (PXRD) study shows a very similar framework of 1 to 2 and 3. Both 1 and 2 are 3D porous structures made of Zn3(COO)6 secondary building units (SBUs) and 2,2- or 3,3-dmbpy as pillar ligand. Thermogravimetric analysis (TGA) and PXRD studies reveal high thermal and water stability for both compounds. Gas-adsorption studies show that the reduction of surface area and pore volume by introducing a methyl group to the bpy ligand leads to a decrease in H2 uptake for both compounds. However, CO2 adsorption experiments with 1 (guest-free 1) indicate significant enhancement in CO2 uptake, whereas for 2 (guest-free 2) the adsorbed amount is decreased. These results suggest that there are two opposing and competitive effects brought on by methyl functionalization: the enhancement due to increased isosteric heats of CO2 adsorption (Qst), and the detraction due to the reduction of surface area and pore volume. For 1, the enhancement effect dominates, which leads to a significantly higher uptake of CO2 than its parent compound 3 (guest-free 3). For 2, the detraction effect predominates, thereby resulting in reduced CO2 uptake relative to its parent structure 3. IR and Raman spectroscopic studies also present evidence for strong interaction between CO2 and methyl-functionalized moieties. Furthermore, all compounds exhibit high separation capability for CO2 over other small gases including CH4, CO, N2, and O2.