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We explored the proton conductivities of two 3D Co metal-organic frameworks (MOFs), {[Co(m-ClPhIDC)(HO)]·2HO} [1; m-ClPhHIDC = 2-(m-chlorophenyl)imidazole-4,5-dicarboxylic acid] and {[Co(p-ClPhHIDC)(HO)]·6HO} (2; p-ClPhHIDC = 2-(p-chlorophenyl)imidazole-4,5-dicarboxylic acid), under water and aqua-ammonia vapors, respectively. The experimental results revealed that the proton conductivities of 1 and 2 at aqua-ammonia vapor were 2.89 × 10 and 4.25 × 10 S/cm, respectively, and approximately 2 orders of magnitude greater than those at water vapor. On the basis of the activation energy, water and ammonia vapor absorption, and powder X-ray diffraction patterns, their proton-conduction mechanisms have been discussed. We believe that this is a novel approach to drastically improving the proton conductivity of MOFs.

Lately, the progressive study of metal−organic frameworks (MOFs) for the detection of ammonia and amines has made infusive achievements. Nevertheless, the investigation of proton-conductive MOFs used to detect the low concentrations of ammonia and amine gases at different relative humidities (RHs) at room temperature is relatively restricted. Herein, by solvothermal reaction of Cd(NO 3 ) 2 with 2-methyl-1H-imidazole-4,5-dicarboxylic acid (H 3 MIDC), a three-dimensional ionic MOF {Na[Cd(MIDC)]} n (1) bearing ordered one-dimensional channels was successfully synthesized. Our research indicates that the uncoordination carboxylate sites are beneficial to proton transfer and the recognition of ammonia and amine compounds. The optimized proton conductivity of 1 reaches a high value of 1.04 × 10 −3 S•cm −1 (100 °C, 98% RH). The room temperature sensing properties of ammonia and amine gases were explored under 68, 85, and 98% RHs, respectively. Satisfactorily, the detection limits of MOF 1 toward ammonia, methylamine, dimethylamine, trimethylamine, and ethylamine are 0.05, 0.1, 0.5, 1, and 4 ppm, respectively, which is one of the best room-temperature sensors for ammonia among previous sensors based on protonconductive MOFs. The proton conducting and sensing mechanisms were highlighted as well.

Our investigation on the proton conductivities of two water-stable isostructural 3D Co(II) MOFs, {[Co(DMPhIDC)(HO)]·2HO} (1) [DMPhHIDC = 2-(3,4-dimethylphenyl)-imidazole-4,5-dicarboxylic acid] and {[Co(m-BrPhIDC)(HO)]·2HO} (2) [m-BrPhHIDC = 2-(m-bromophenyl)-imidazole-4,5-dicarboxylic acid], under water or aqua-ammonia vapor shows that the optimized proton conductivities of both 1 and 2 under aqua-ammonia vapor are 4.41 × 10 S·cm and 5.07 × 10 S·cm (at aqua-ammonia vapor from 1.5 M NH·HO solution and 100 °C), respectively, which are approximately 1 order of magnitude greater than those maximum values (8.91 × 10 S·cm and 7.64 × 10 S·cm) under water vapor (at 98% RH and 100 °C). The plausible proton pathways and mechanisms of the MOFs have been proposed in terms of the structural analyses, activation energy calculations, water and NH vapor absorptions, and PXRD determinations.

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