Alcohol-Vapor Inclusion in Single-Crystal Adsorbents [MII2(bza)4(pyz)]n (M=Rh, Cu): Structural Study and Application to Separation Membranes

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Abstract

The vapor absorbency of the series of alcohols methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol was characterized on the single-crystal adsorbents [MII2(bza)4(pyz)]n (bza=benzoate, pyz=pyrazine, M=Rh (1), Cu (2)). The crystal structures of all the alcohol inclusions were determined by single-crystal X-ray crystallography at 90 K. The crystal-phase transition induced by guest adsorption occurred in the inclusion crystals except for 1-propanol. A hydrogen-bonded dimer of adsorbed alcohol was found in the methanol- and ethanol-inclusion crystals, which is similar to a previous observation in 2⋅2EtOH (S. Takamizawa, T. Saito, T. Akatsuka, E. Nakata, Inorg. Chem.2005, 44, 1421–1424). In contrast, an isolated monomer was present in the channel for 1-propanol, 1-butanol, and 1-pentanol inclusions. All adsorbed alcohols were stabilized by hydrophilic and/or hydrophobic interactions between host and guest. From the combined results of microscopic determination (crystal structure) and macroscopic observation (gas-adsorption property), the observed transition induced by gas adsorption is explained by stepwise inclusion into the individual cavities, which is called the “step-loading effect.” Alcohol/water separation was attempted by a pervaporation technique with microcrystals of 2 dispersed in a poly(dimethylsiloxane) membrane. In the alcohol/water separation, the membrane showed effective separation ability and gave separation factors (alcohol/water) of 5.6 and 4.7 for methanol and ethanol at room temperature, respectively.

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