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

• Published in: Chemistry, an Asian journal an Asian journal, 2007-07, Vol.2 (7), p.837-848
• Main Authors: Takamizawa, Satoshi, Kachi-Terajima, Chihiro, Kohbara, Masa-aki, Akatsuka, Takamasa, Jin, Tetsuro
• Format: Article
• Language: English
• Subjects: adsorption; alcohols; crystal structures; host-guest systems; pervaporation
• ISSN: 1861-4728; 1861-471X
• DOI: 10.1002/asia.200600404

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. The long and short of it: The properties of alcohol‐inclusion compounds of the single‐crystal adsorbents [MII2‐(bza)4(pyz)]n (bza=benzoate, pyz= pyrazine, M=Rh (1), Cu (2)) depends on the length of the alcohol. Poly(dimethylsiloxane) membranes with microcrystals of 2 can separate methanol and ethanol from their aqueous mixtures.