The Effect of Methyl Functionalization on Microporous Metal-Organic Frameworks' Capacity and Binding Energy for Carbon Dioxide Adsorption

The design, synthesis, and structural characterization of two new microporous metal-organic framework (MMOF) structures is reported; Zn(BDC)(DMBPY)0.5 center dot(DMF)0.5(H2O)0.5 (1; H2 BDC = 1,4-benzenedicarboxylic acid; DMBPY=2,2'-dimethyl-4,4'-bipyridine) and Zn(NDC)(DMBPY)0.5 center dot(DMF)2 (2; H2NDC = 2,6-naphthalenedicarboxylic acid, DMF=N,N,-dimethylformamide), which are obtained by functionalizing a pillar ligand with methyl groups. Both compounds are 3D porous structures of the Zn2(L)2(P) type and are made of a paddle-wheel Zn2(COO)4 secondary building unit (SBU), with the dicarboxylate and DMBPY as linker (L) and pillar (P) ligands, respectively. Comparisons are made to the parent structures Zn(BDC)(BPY)0.5 center dot(DMF)0.5(H2O)0.5 (3; BPY = 4,4'-bipyridine) and Zn(NDC)(BPY)0.5 center dot(DMF)1.575 (4) to analyze and understand the effect of methyl functionalization. CO2-adsorption studies indicate substantially enhanced isosteric heats of CO2 adsorption (Qst) for both compounds, as a result of adding methyl groups to the BPY ligand. The CO2 uptake capacity, however, is affected by two opposing and competing factors: the enhancement due to increased MMOFCO2 interactions (higher Qst values) and detraction due to the surface area and pore-volume reduction. For 1' (the guest-free form of 1), the positive effect dominates, which leads to a significantly higher uptake of CO2 than that of its parent structure 3' (the guest-free form of 3). In 2' (the guest-free form of 2), however, the negative effect rules, which results in a slightly lower CO2 uptake with respect to 4' (the guest-free form of 4). All four compounds exhibit a relatively high separation capability for carbon dioxide over other small gases, including CH4, N2, and O2. The separation ratios of CO2 to O2 and N2 (at 298 K and 1 atm) are 39.8 and 23.5 for compound 1', 57.7 and 40.2 for 2', 25.7 and 29.5 for 3', 89.7, and 20.3 for 4', respectively. IR and Raman spectroscopic characterization of CO2 interactions with 1' and 2' provides indirect support of the importance of the methyl groups in the interaction of CO2 within these systems.