Structure property relationships affecting the proton conductivity in imidazole loaded Al-MOFs

The structures of the imidazole loaded derivatives of Al-MIL-53 [Al(OH)(1,4-BDC-(CH₃)_x)] (x = 0, 1, 2) and CAU-11 ([Al(OH)(SDBA)]) (1,4-H₂BDC = terephthalic acid; H₂SDBA = 4,4'-sulfonyldibenzoic acid) were determined from powder X-ray diffraction data. Impedance spectroscopy measurements were carried out to evaluate their proton conductivities under anhydrous conditions at temperatures up to 110 °C. In Al-MIL-53-(CH₃)_x_HIm (x = 0, 1, 2) the formation of hydrogen bonds between the framework and the guest molecules results in a decrease in proton conductivity (x₀ = 1.7 × 10^-6, x₁ = 1.9 × 10^-8 and x₂ = 1.7 × 10^-9 S cm^-1 at 110 °C and E_act = 0.42, 0.41 and 0.46 eV, for 0, 1 and 2 CH₃-groups, respectively). The highest conductivity has been measured for CAU-11_HIm with 3.0 × 10^-4 S cm^-1 at 110 °C (E_act = 0.19 eV), where no host-guest hydrogen bonding interactions are observed.