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Room-Temperature Synthesis of Metal–Organic Framework Isomers in the Tetragonal and Kagome Crystal Structure

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journal contribution
posted on 31.05.2019, 19:34 by Julian Hungerford, Krista S. Walton
Two metal–organic framework (MOF) isomers with the chemical formula Zn2(X)2(DABCO) [X = terephthalic acid (BDC), dimethyl terephthalic acid (DM), 2-aminoterephthalic acid (NH2), 2,3,5,6-tetramethyl terephthalic acid (TM), and anthracene dicarboxylic acid (ADC); DABCO = 1,4-diazabicyclo[2.2.2]­octane] have been synthesized via a fast, room-temperature synthesis procedure. The synthesis solvent was found to play a vital role in directing the formation of the Kagome lattice (ZnBD) versus tetragonal topology (DMOF-1). When N,N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) was used as the synthesis solvent, the reaction resulted in the formation of ZnBD, whereas methanol, ethanol, acetone, N,N-diethylformamide (DEF), and acetonitrile each produced DMOF-1. Water adsorption isotherms of ZnBD and DMOF-1 were collected, and the materials were found to have similar adsorption characteristics and stabilities. Both MOFs degraded upon exposure to water at a relative pressure (P/Po) of 0.5 at 25 °C, but both are hydrophobic below a P/Po of 0.4, displaying very little water adsorption. Additionally, CO2 adsorption isotherms of ZnBD were collected and compared to those previously reported for DMOF-1. ZnBD adsorbs less CO2 at low pressure compared to DMOF-1 but reaches a similar capacity at 20 bar. This adsorption behavior can be explained by the structural features of the materials, where ZnBD possesses large hexagonal pores (15 Å) compared to the smaller pore opening (7.5 Å) in DMOF-1. The heat of adsorption of CO2 on ZnBD was calculated to be ∼22 kJ/mol at zero coverage. Attempts to functionalize the Kagome lattice proved to be unsuccessful but instead resulted in a new method for producing functionalized DMOF-1 at room temperature. This was hypothesized to be a result of the steric effects imposed by the functional groups that prevent the formation of the Kagome lattice.