Impact of Alkyl-Functionalized BTC on Properties of Copper-Based Metal-Organic Frameworks

Highly porous metal organic frameworks (MOFs) [Cu-3(MBTC)(2)(H2O)(3)](n) (where MBTC is methyl-1,3,5-benzenetricarboxylate) and [Cu-3(EBTC)(2)(H2O)(3)](n) (where EBTC is ethyl-1,3,5-benzenetricarboxylate) have been solvothermally synthesized. The MOFs are isostructural to each other but do not form the same framework as HKUST-1 (CuBTC) due to the steric impact of the alkyl-functionalized ligands. The new MOFs are formed by two different [Cu-2(O2CR)(4)] units (where R is an aromatic ring), which create the three-dimensional framework with open-metal sites and high surface areas. Compared with HKUST-1, these MOFs adsorb comparable amounts of carbon dioxide and methane at pressures up to 5 bar. However, water adsorption is significantly lower compared to HKUST-1 due to the hydrophobic functional groups. This work indicates that the strong adsorption preference of water on open-metal sites can be limited with the proper choice of the hydrophobic functional groups, without losing the strong interaction with other gases.