Proton-conductive Metal-organic Frameworks Research Articles

A porous Cu(II)-MOF containing [PW12O40]3− and a large protonated water cluster: synthesis, structure, and proton conductivity

A proton-conducting metal–organic framework (MOF), {[Cu4(dpdo)12][H(H2O)27(CH3CN)12][PW12O40]3}n (where dpdo is 4,4′-bipyridine-N,N′-dioxide) (1), was synthesized by the reaction of CuHPW12O40·nH2O and dpdo at room temperature. Single-crystal X-ray diffraction analysis at 293 K revealed that 1 crystallized in the cubic space group Im-3 and presented a non-interwoven 3-D framework with cubic cavities and guest molecules. A large ionic water cluster H+(H2O)27, consisting of a water shell (H2O)26 and an encaged H+(H2O) as a center core, was trapped in the cubic cavity of the MOF {[Cu4(dpdo)12(PW12O40)3]−}∞. Thermogravimetric analysis suggests that 1 has high thermal stability, indicating that such a non-interwoven 3-D framework with cubic cavities is a suitable host for researching protonated water clusters. Its water vapor adsorption isotherm at room temperature and pressure shows that the water vapor adsorbed in it was 65.1 cm3 g−1 at the maximum allowable humidity. It exhibits good proton conductivities of 10−5–10−4 S cm−1 at 100 °C in the relative humidity range 35–98%.

PH-dependent proton conducting behavior in a metal-organic framework material.

A porous metal-organic framework (MOF), [Ni2(dobdc)(H2O)2]⋅6 H2O (Ni2(dobdc) or Ni-MOF-74; dobdc(4-)=2,5-dioxido-1,4-benzenedicarboxylate) with hexagonal channels was synthesized using a microwave-assisted solvothermal reaction. Soaking Ni2(dobdc) in sulfuric acid solutions at different pH values afforded new proton-conducting frameworks, H(+)@Ni2(dobdc). At pH 1.8, the acidified MOF shows proton conductivity of 2.2×10(-2) S cm(-1) at 80 °C and 95% relative humidity (RH), approaching the highest values reported for MOFs. Proton conduction occurs via the Grotthuss mechanism with a significantly low activation energy as compared to other proton-conducting MOFs. Protonated water clusters within the pores of H(+)@Ni2(dobdc) play an important role in the conduction process.

Control of crystalline proton-conducting pathways by water-induced transformations of hydrogen-bonding networks in a metal-organic framework.

Structure-defined metal-organic frameworks (MOFs) are of interest because rational design and construction allow us to develop good proton conductors or possibly control the proton conductivity in solids. We prepared a highly proton-conductive MOF (NH4)2(adp)[Zn2(ox)3]·nH2O (abbreviated to 1·nH2O, adp: adipic acid, ox: oxalate, n = 0, 2, 3) having definite crystal structures and showing reversible structural transformations among the anhydrate (1), dihydrate (1·2H2O), and trihydrate (1·3H2O) phases. The crystal structures of all of these phases were determined by X-ray crystallography. Hydrogen-bonding networks consisting of ammonium ions, water molecules, and carboxylic acid groups of the adipic acids were formed inside the two-dimensional interlayer space in hydrated 1·2H2O and 1·3H2O. The crystal system of 1 or 1·2H2O (P21/c, No. 14) was changed into that of 1·3H2O (P1̅, No. 2), depending on water content because of rearrangement of guests and acidic molecules. Water molecules play a key role in proton conduction as conducting media and serve as triggers to change the proton conductivity through reforming hydrogen-bonding networks by water adsorption/desorption processes. Proton conductivity was consecutively controlled in the range from ∼10(-12) S cm(-1) (1) to ∼10(-2) S cm(-1) (1·3H2O) by the humidity. The relationships among the structures of conducting pathways, adsorption behavior, and proton conductivity were investigated. To the best of our knowledge, this is the first example of the control of a crystalline proton-conducting pathway by guest adsorption/desorption to control proton conductivity using MOFs.

Immobilization of N-(3-aminopropyl)-imidazole through MOFs in proton conductive membrane for elevated temperature anhydrous applications

The development of anhydrous proton-conductive membranes operating at temperatures above 100°C is a challenge for practical application. To meet the requirements, the N-(3-aminopropyl)-imidazole (NAPI), a proton-carrier molecule, is encapsulated in the frameworks of the Fe-MIL-101-NH2 and then allowed to react with sulfonated poly (2,6-dimethyl-1,4-phenylene oxide) (SPPO) for preparing hybrid proton conductive membrane. Results showed that the dispersed proton conductive metal–organic frameworks (MOFs) in the membrane provide a convenient pathway for proton mobility under high temperature and anhydrous condition. Tuning of the host–guest interaction can generate a good proton-conducting pathway at temperatures above 150°C. It is also presumed that the proton transfer in the membrane is simultaneously contributed from the Grotthuss-type mechanism and the vehicle-type mechanism. This work suggests that a combination of guest molecules and microporous frameworks could afford highly mobile proton carriers in solids. Therefore, the new proton conductive membranes can be designed by incorporating the MOFs into functionalized polymers for applications at high temperature and anhydrous condition.

Enhancing Proton Conduction in a Metal–Organic Framework by Isomorphous Ligand Replacement

Using the concept of isomorphous replacement applied to entire ligands, a C(3)-symmetric trisulfonate ligand was substituted with a C(3)-symmetric tris(hydrogen phosphonate) ligand in a proton conducting metal-organic framework (MOF). The resulting material, PCMOF2½, has its proton conduction raised 1.5 orders of magnitude compared to the parent material, to 2.1 × 10(-2) S cm(-1) at 90% relative humidity and 85 °C, while maintaining the parent MOF structure.

Promotion of Low-Humidity Proton Conduction by Controlling Hydrophilicity in Layered Metal–Organic Frameworks

We controlled the hydrophilicity of metal-organic frameworks (MOFs) to achieve high proton conductivity and high adsorption of water under low humidity conditions, by employing novel class of MOFs, {NR(3)(CH(2)COOH)}[MCr(ox)(3)]·nH(2)O (abbreviated as R-MCr, where R = Me (methyl), Et (ethyl), or Bu (n-butyl), and M = Mn or Fe): Me-FeCr, Et-MnCr, Bu-MnCr, and Bu-FeCr. The cationic components have a carboxyl group that functions as the proton carrier. The hydrophilicity of the cationic ions was tuned by the NR(3) residue to decrease with increasing bulkiness of the residue: {NMe(3)(CH(2)COOH)}(+) > {NEt(3)(CH(2)COOH)}(+) > {NBu(3)(CH(2)COOH)}(+). The proton conduction of the MOFs increased with increasing hydrophilicity of the cationic ions. The most hydrophilic sample, Me-FeCr, adsorbed a large number of water molecules and showed a high proton conductivity of ~10(-4) S cm(-1), even at a low humidity of 65% relative humidity (RH), at ambient temperature. Notably, this is the highest conductivity among the previously reported proton-conducting MOFs that operate under low RH conditions.