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

<jats:title>Abstract</jats:title><jats:p>The design, synthesis, and structural characterization of two new microporous metal‐organic framework (MMOF) structures is reported; Zn(BDC)(DMBPY)<jats:sub>0.5</jats:sub>·(DMF)<jats:sub>0.5</jats:sub>(H<jats:sub>2</jats:sub>O)<jats:sub>0.5</jats:sub> (1; H<jats:sub>2</jats:sub> BDC = 1,4‐benzenedicarboxylic acid; DMBPY=2,2′‐dimethyl‐4,4′‐bipyridine) and Zn(NDC)(DMBPY)<jats:sub>0.5</jats:sub>·(DMF)<jats:sub>2</jats:sub> (2; H<jats:sub>2</jats:sub>NDC = 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 Zn<jats:sub>2</jats:sub>(<jats:italic>L</jats:italic>)<jats:sub>2</jats:sub>(<jats:italic>P</jats:italic>) type and are made of a paddle‐wheel Zn<jats:sub>2</jats:sub>(COO)<jats:sub>4</jats:sub> secondary building unit (SBU), with the dicarboxylate and DMBPY as linker (<jats:italic>L</jats:italic>) and pillar (<jats:italic>P</jats:italic>) ligands, respectively. Comparisons are made to the parent structures Zn(BDC)(BPY)<jats:sub>0.5</jats:sub>·(DMF)<jats:sub>0.5</jats:sub>(H<jats:sub>2</jats:sub>O)<jats:sub>0.5</jats:sub> (3; BPY = 4,4′‐bipyridine) and Zn(NDC)(BPY)<jats:sub>0.5</jats:sub>·(DMF)<jats:sub>1.575</jats:sub> (4) to analyze and understand the effect of methyl functionalization. CO<jats:sub>2</jats:sub>‐adsorption studies indicate substantially enhanced isosteric heats of CO<jats:sub>2</jats:sub> adsorption (<jats:italic>Q</jats:italic><jats:sub>st</jats:sub>) for both compounds, as a result of adding methyl groups to the BPY ligand. The CO<jats:sub>2</jats:sub> uptake capacity, however, is affected by two opposing and competing factors: the enhancement due to increased MMOF–CO<jats:sub>2</jats:sub> interactions (higher <jats:italic>Q</jats:italic><jats:sub>st</jats:sub> 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 CO<jats:sub>2</jats:sub> 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 CO<jats:sub>2</jats:sub> 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 CH<jats:sub>4</jats:sub>, N<jats:sub>2</jats:sub>, and O<jats:sub>2</jats:sub>. The separation ratios of CO<jats:sub>2</jats:sub> to O<jats:sub>2</jats:sub> and N<jats:sub>2</jats:sub> (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 CO<jats:sub>2</jats:sub> interactions with 1′ and 2′ provides indirect support of the importance of the methyl groups in the interaction of CO<jats:sub>2</jats:sub> within these systems.</jats:p>