Overview
Introduction of Metal-organic frameworks
In the past decade, metal-organic frameworks (MOFs) have gained significant attention due to their rapid development and characterization These materials are formed by linking metal-containing secondary building units (SBUs) with organic linkers, resulting in stable crystalline structures with permanent porosity The diversity of metal SBUs and organic linkers allows for the creation of thousands of new compounds annually.
Figure 1.1 The quantity of MOFs reported in the Cambridge Structural Database
Metal-Organic Frameworks (MOFs) exhibit remarkable porosity, making them highly valuable for various applications, including gas storage, separations, and catalysis Their significant role in energy technologies, such as fuel cells, super-capacitors, and catalytic conversions, has led to extensive research and industrial-scale production.
Organic linkers are multi-dentate compounds featuring functional groups like carboxyl, sulfonic, hydroxyl, or nitrogen derivatives, which can effectively form complexes with metal ions Carboxylate-containing compounds are particularly favored as linkers in metal-organic frameworks (MOFs) due to their ability to chelate various metals, thereby creating a rigid structure Additionally, the linker structure can be enhanced by incorporating one or more benzene rings, which helps to increase both pore sizes and surface area.
Figure 1.2 The linkers were employed for synthetic process of MOFs [43]
Metal clusters are formed by the combination of metal ions with carboxylate and/or oxo groups While most metal-organic frameworks (MOFs) are synthesized using alkali and transition metals, transition metals are favored due to their stronger affinity for oxygen atoms in organic linkers, leading to the common construction of metal clusters in MOFs.
Metal-Organic Frameworks (MOFs) feature a robust and porous structure, constructed using the strategy of secondary building units (SBUs) In the design and analysis of MOF structures, both organic linkers and metal clusters are categorized as SBUs based on their geometrical shapes Organic linkers can be represented as polygons or polyhedra, depending on their three-dimensional structures and the number of carboxylate groups present.
Figure 1.3 The metal clusters in MOFs structure [83]
Most metal-organic frameworks (MOFs) are synthesized through liquid-phase methods, where metal salt and ligand solutions are combined or a solvent is added to a solid mixture in a reaction vial The choice of solvent is crucial, influenced by factors like reactivity, solubility, and stability, impacting the thermodynamics and activation energy of the reaction While solid-phase synthesis of MOFs offers a quicker alternative, it often struggles to produce single crystals necessary for structural determination, a task more straightforward in liquid-phase methods The slow evaporation technique has been a common crystallization approach for decades Although solvo-thermal methods dominate MOF synthesis, alternative techniques such as microwave-assisted, electro-chemical, mechano-chemical, and sono-chemical methods have also been explored.
Zr-based metal-organic frameworks
Zinc (Zn) and copper (Cu)-based metal-organic frameworks (MOFs) have been extensively researched due to their high surface areas and structures formed from metal units and long organic linkers However, a significant limitation of these Zn-MOFs and Cu-MOFs is their poor stability in moist environments and protonic solvents, attributed to the lability of Zn(II)-carboxylate and Cu(II)-carboxylate bonds To address this issue, researchers have focused on developing more chemically stable MOFs.
Figure 1.4 Structure of Zn-MOFs (MOF-5) and Cu-MOFs (HKUST-1) [15]
The stability of metal-organic frameworks (MOFs) is significantly influenced by the inorganic units and the strength of chemical bonding between these units and organic components Group four transition metals, known for their strong interactions with oxygen, are ideal candidates for stable inorganic units when paired with oxygen-containing linkers Recent advancements have led to the synthesis of robust and porous zirconium-based metal-organic frameworks (Zr-MOFs), which feature highly stable secondary building units (SBUs) that exhibit remarkable resistance to humid conditions and protonic solvents.
Zr-MOFs are constructed using organic linkers such as di-, tri-, or tetracarboxylates, with their structures defined by Zr-containing secondary building units (SBUs) like Zr6(à3-O)x(à3-OH)y(-CO2)n, which vary in coordination numbers The Zr6(à3-O)4(à3-OH)4 core is commonly found in Zr-MOFs, while the Zr6(à3-O)8 core appears in only a few structures.
Table 1.1 Summary of several reported Zr-MOFs
Zr-MOFs Zr core BET surface areas (m 2 /g) Ref
The Zr 4+ cation in Zr-MOFs is capped by oxo and hydroxyl groups on the triangular faces of the octahedron, forming a cuboctahedral-shaped secondary building unit (SBU) In Zr6 clusters with lower coordination numbers, monocarboxylate compounds replace carboxylate linkers, significantly influencing the nucleation and crystal growth during the synthesis of Zr-MOFs.
Figure 1.5 The Zr6O4(OH)4(-CO2)12 cluster is presented in stick-and-ball (a) and polyhedron (b) The SBU shape is cuboctahedron (c) [9]
Zr-MOFs are known for their high surface areas and diverse pore sizes, attributed to the use of extended organic linkers Notably, NU-1103, which is built using the tetratopic linker PyPTP, stands out as the Zr-MOF with the highest surface area, measuring an impressive 5646 m²/g.
Current research on BET theory highlights the exceptional stability of Zr-MOFs in aqueous solutions The strong affinity between Zr(IV) ions and carboxylate O atoms, attributed to high charge density and bond polarization, is a key characteristic of many carboxylate-based Zr-MOFs.
Proton conduction in Zr-MOFs
Proton exchange membrane fuel cells (PEMFCs) are crucial for sustainable energy technologies, with the U.S Department of Energy (DOE) setting a target for their performance by 2020, requiring proton conductivity to reach approximately 0.4 S cm -1 at 80 °C Furthermore, the DOE emphasizes the need for electrolyte materials to exhibit chemical and mechanical stability for over 500 hours or 20,000 cycles As a renewable energy source, fuel cells convert chemical energy into electrical energy by combining hydrogen fuel with oxygen from the air, resulting in the production of water and heat.
A fuel cell consists of an anode (negative side), a cathode (positive side), and an electrolyte that facilitates charge movement between the two sides Among various fuel cell systems, polymer electrolyte membrane fuel cells (PEMFCs) are significant In PEMFCs, protons move from the anode to the cathode through a proton-exchange membrane, while electrons generated at the anode travel through an external circuit to the cathode, producing electricity.
Figure 1.6 Model representation of proton exchange membrane fuel cell [23]
Researchers are increasingly focused on finding materials that can efficiently transfer protons, given the significant advantages of fuel cells Among the most extensively studied proton-conducting materials are polymers, particularly Nafion, which demonstrates impressive conductivity values of up to 10 −1 S cm −1 at 80 °C and 98% relative humidity.
Figure 1.7 Chemical structure of Nafion [21]
At high temperatures, the mechanical strength of Nafion diminishes, leading to increased hydrogen and oxygen content over prolonged use Maintaining the necessary humidity (98% RH) to achieve optimal proton conductivity incurs high costs and can flood the cathode, further reducing conductivity Additionally, powering humidifiers limits Nafion's operational temperature range, posing risks such as CO poisoning of Pt catalysts below 100°C and sluggish reaction kinetics To meet the DOE's 2020 targets, there is a pressing need for the development of new, robust, non-swelling electrolyte materials that can operate at lower relative humidity levels Research is focusing on cheaper and higher-performing polymeric materials, including sulfonated polyether ketone, oxo acids, oxides, hydroxides, and inorganic-organic hybrids for proton conduction Recent studies have also explored the potential of porous solids like mesoporous silica, coordination polymers (CPs), metal-organic frameworks (MOFs), and organic porous materials, with Zr-MOFs emerging as promising candidates due to their permanent porosity and thermal stability for developing proton-conducting membranes.
Table 1.2 The proton conductivity of commercial materials, MOFs and Zr-MOFs
No Material Proton conductivity Condition Ref
9 H + @Ni2(dobdc) pH=1.8 2.20 × 10 −2 S cm −1 80 o C, 95% RH [55]
5 UiO-66(Zr)-(CO2H)2 2.30 × 10 −3 S cm −1 90 o C, 95% RH [6]
The study presents a conductivity measurement of 12 Zr6O4(OH)6(BDC)5 at 6.90 × 10 −3 S cm −1 under conditions of 65 °C and 95% relative humidity Various materials were analyzed, including a polyethylene-tetra-fluoro matrix with sulfonyls and a tetra-fluoroethylene grafted poly(styrene sulfonic acid) Additionally, sulfonated polyetheretherketone (SPEEK) was examined However, the synthesis of metal-organic frameworks (MOFs) involved multiple steps and revealed a lack of stable structural evidence following impedance analysis.
To enhance the conductivity of proton conductors, it is essential to focus on increasing the number of mobile charge carriers (n) and their mobility (μ), since the charge of the ion (q) remains constant at 1.602 × 10^-19 C for H+ The charge carrier density can be improved by adding more proton donors, such as Brønsted acids, amines, and alcohols, either by integrating them into the framework to line pore walls or incorporating them as guest molecules These proton donors can be introduced during the synthesis of metal-organic frameworks (MOFs), through post-synthetic modifications, or by managing guest exchanges within the host framework However, designing MOFs for proton conduction poses challenges, as the incorporation of proton donors often leads to deprotonation and coordination with metal linkers, resulting in fewer free acidic protons While guest molecules can be more easily introduced, their susceptibility to removal may compromise the material's longevity.
The Grotthuss mechanism involves a series of steps where a proton initially hops to a central oxygen atom, followed by a 180° reorientation that facilitates another proton transfer.
Adjusting proton mobility is a complex challenge due to the difficulty of achieving Ångstrüm-level control over structural elements For effective transport, a free proton must not only be present but also have access to an energetically favorable site for hopping, typically within 1-2 Å and with a clear pathway to minimize activation energy barriers This process, known as the Grotthuss mechanism, involves protons moving through a continuous hydrogen-bonding network, facilitated by a series of oxygen atoms acting as proton acceptor sites As illustrated, protons can bond to neighboring oxygen atoms and hop to the next site, although structural reorientation is necessary for continued movement Proton acceptor sites can include not just surface functional groups but also solvent or guest molecules within the pore While achieving precise control over the distance and availability of these sites, as well as the reorientation process, is challenging and often reliant on chance, integrating these principles into the design of Porous Coordination Metal-Organic Frameworks (PCMOFs) is feasible.
In larger pore metal-organic frameworks (MOFs), proton transfer occurs through an additional mechanism known as the vehicle mechanism, where protons are carried by species such as H3O+ In this process, water serves as a vehicle for proton transport, moving through solvent-filled channels in the framework As occupied vehicles travel in one direction, unoccupied ones diffuse back, facilitating proton conduction via diffusion The diffusion rate is influenced by factors such as pore size, vehicle size, its solvation sphere, and the diffusion coefficient.
Proton conduction in metal-organic frameworks (MOFs) involves a combination of the Grotthuss and vehicle mechanisms The activation energy (Ea) of the proton conduction process is crucial for determining the dominant mechanism: a low Ea, typically below 0.40 eV, indicates a Grotthuss-like mechanism, while a higher Ea, such as 0.8 eV for H3O+, suggests the prevalence of the vehicle mechanism.
Figure 1.9 Mechanism of proton conduction [67]
1.3.2 Challenges of proton conductivity in MOFs
Activation energies derived from AC impedance data are crucial for distinguishing between two conduction processes: the Grotthuss mechanism, characterized by lower activation energies (Ea) below 0.4 eV due to the cleavage of hydrogen bonds, and the vehicle mechanism, which involves larger ions like H3O+, requiring higher energies typically above 0.4 eV Solid-state ionic conduction occurs through the efficient movement of protons along low-energy barrier channels, facilitated by a protonation and de-protonation mechanism that demands accessible donor and acceptor sites Currently, two primary objectives for metal-organic frameworks (MOFs) in proton exchange membrane fuel cell (PEMFC) applications are to develop improved proton-conducting materials that operate efficiently under humid conditions (below 100 °C) and are more cost-effective than traditional materials like Nafion, as well as to create efficient anhydrous proton conductors capable of functioning at elevated temperatures (above 100 °C) and in dry conditions.
1.3.3 Measuring solid state proton conduction in MOFs
Solid state ion conductors are materials which can transport cations (e.g H + ,
Ionic materials, such as Li+ cations and OH- or O2- anions, function similarly to semiconductors, requiring an external potential for conduction Charge carriers migrate by jumping between occupied and unoccupied sites, driven by the potential difference that creates lower-energy vacant sites in the direction of the applied potential This energy gradient leads to a net movement of charge carriers, generating a current and resulting in observable ion conductivity (σ) The ion conductivity can be mathematically expressed as σ = nqμ, where n represents the carrier concentration, q the charge, and μ the mobility, forming the theoretical foundation for enhancing ion conductivity in materials.
Figure 1.10 Energy diagram for proton transport in a solid state [10]
To determine the activation energy (Ea) of a solid-state proton conductor, a theoretical model focusing on the proton hopping mechanism is utilized In the absence of an electric field, protons occupy energetically equivalent sites separated by energy barriers of magnitude Ea When an electric field is applied, the energy landscape shifts, lowering the energy of proton acceptor sites in the field's direction by qaV, where 'a' represents the distance between sites and 'V' is the electric field strength This alteration decreases the activation energy by 0.5qaV in the direction of the field and increases it by the same amount against the field, creating a favorable condition for protons to move toward lower energy sites Consequently, the reduced energy barrier enhances the likelihood of proton jumps, which can occur in either direction if sufficient thermal energy is present, as described in Equations (1.2) and (1.3).
The net proton jump rate, denoted as FNet, can be calculated by subtracting F- from F+ This relationship is expressed in terms of the vibration frequency of the proton (v), the Boltzmann constant (k), and the system's temperature in Kelvin (T), as outlined in equations (1.3), (1.4), and (1.5).
Equation (1.5) can be simplified by factoring out common elements, resulting in Equation (1.6) By applying the approximation in Equation (1.7), which is applicable for field strengths up to approximately 10^8 V/m, we arrive at Equation (1.8).
In order to determine conductivity, the velocity (v) of the protons, which is the product of the jump rate and the jump distance, must be known to give Equation (1.9) v = 𝑣𝑞𝑎 2 𝑉
𝑘𝑇 𝑒 −𝐸𝑎 𝑘𝑇 (1.9) Since proton mobility is defined as the proton velocity when V is 1 V/m, Equation (1.9) can then be substituted into Equation (1.1) to give Equation (1.10)
𝑘𝑇 𝑒 −𝐸𝑎 𝑘𝑇 (1.10) Since n, υ, q 2 , a 2 and k -1 are all constants, they can be combined into a single constant and can be rewritten to give Equation (1.11) ln(𝜎𝑇) = −𝐸 𝑎
𝑘𝑇 + ln(𝜎 𝑜 ) (1.11) Thus, plotting a series of conductivity measurements taken at different temperatures as ln(σT) against T -1 should give a straight line with a slope of –Ea/k (k is Botlzmann constant) [32]
The methods of controlling and enhancing proton conductivity in MOFs
Researchers are increasingly focusing on Metal-Organic Frameworks (MOFs) as promising proton conductors due to their advantageous properties Firstly, the ability to easily obtain and structurally characterize single crystals of MOFs through X-ray crystallography enhances the understanding of proton conduction mechanisms Secondly, the flexible control over MOF structures, particularly their pores, by selecting different metal ions or modifying multifunctional organic ligands, allows for improved proton conductivity Consequently, the pursuit of MOFs with high proton conductivity has emerged as a significant goal, with studies demonstrating enhanced conductivity in MOFs constructed with ligands containing -COOH, -SO3H, or -PO3H2 groups, as well as those incorporating protonic guest molecules like imidazole, triazole, and histamine.
1.4.1 Structure guest molecules and ions in MOFs
Bu et al investigated the enhancement of proton conduction in anionic metal-organic frameworks (MOFs) by introducing various metal ions They constructed anionic frameworks with different counter ions, specifically {[M2Cl2(BTC)4/3].(Me2NH2)2 +.4/3H2O}n, where M represents Co and Mn Despite being iso-structural, these two MOFs demonstrated significantly different proton conductivities Notably, the introduction of different metal cations resulted in a two-fold increase in proton conductivity for the Mn-MOFs at 19°C and 65% relative humidity, achieving a conductivity of 2.60 × 10^-4 S/cm.
S cm -1 ; Co-MOFs, 5.93 × 10 -4 S cm -1 ), indicative of the important role of metal ion substitution
Figure 1.19 The structure of Co(Mn)-MOFs [39]
Both sulfonate and phosphoric groups are widely used for chemical modification of proton conducting materials of proton exchange membrane fuel cells
Metal-organic frameworks (MOFs) often incorporate groups like sulfonate and carboxylate, which enhance unique properties such as proton conductivity A notable example is a tetranuclear copper cluster-based MOF, [Cu4(L)2(OH)2(DMF)2, L=5-Sulfoiso-phthalic acid monosodium salt], which demonstrates a remarkable proton conductivity of 7.4 × 10 -4 S cm -1.
95 o C and 95% RH [47] This MOFs possesses 1D irregular channels of approximately 7.0 Å in diameter, lined with sulfonate, carboxylate, and DMF molecules, among which the hydrogen bonds are formed (Figure 1.20)
Figure 1.20 (a) The asymmetric unit of the tetranuclear copper cluster-based MOFs;
(b) the 3D structure viewed along the b-axis; (c) Ball-and-stick and polyhedral representations of [Cu4(OH)2(CO2)4(SO3)2] cluster, respectively; (d) the (3,6)- connected 3D non-interpenetrating network [47]
Ramaswamy et al reported a 2D layered Mg-based MOFs (PCMOF10), which shows extremely high proton conductivity of 3.55 × 10 -2 S cm -1 at 70 o C and 95%
RH The linkers (2,5-Dicarboxy-1,4-benzenediphosphonic acid) form a robust backbone, and the hydrogen phosphonate groups and lattice water form a very efficient pathway for proton transportation [61]
1.4.2 Brứnsted acidity and functionalized group
The Brứnsted acidity of MOFs is highly correlated with their proton conductivity [31] The tuning of proton conduction with functionalized groups has been extensively investigated
Yang et al [87] found that highly acidic and strongly hydrophilic functional groups, such as -SO3H and -COOH, significantly enhance proton conductivity, while groups like -NH2, -H, and -Br result in lower conductivities under similar conditions The proton conductivities of UiO-66-SO3H and UiO-66-2COOH are measured at 0.34 × 10^-2 S cm^-1 and 0.10 × 10^-2 S cm^-1, respectively, at 30 °C and 97% relative humidity, with activation energy (Ea) values of 0.27 eV and 0.18 eV.
Figure 1.21 The structure of UiO-66-X and the proton conductivity of UiO-66-X with different %RH [87]
The exceptional proton conductivities of functionalized UiO-66 result from a synergistic effect of their strong hydrophilicity and elevated acidity, facilitating the development of dense hydrogen bond networks.
Phang et al demonstrated that the proton conductivity of acidified metal-organic frameworks (MOFs), specifically H + @Ni2(dobdc) with hexagonal channels, can be effectively enhanced by adjusting the pH value After immersion in sulfuric acid solutions at varying pH levels, the MOFs exhibited a remarkable proton conductivity of 2.2 × 10 -2 S cm -1 at 80 °C and 95% relative humidity (pH = 1.8) Notably, experimental results indicated the absence of elemental sulfur in the acidified solid, along with the leaching of Ni 2+ ions from the framework This suggests that protons may be linked to the release of anionic dobdc linkers following metal leaching.
Figure 1.22 The structure of [Ni2(dobdc)(H2O)2] (a) and proton pathway of [Ni2(dobdc)(H2O)2](b) [56]
In 2018, Rought et al successfully synthesized new barium-based metal-organic frameworks (MOFs) MFM-510, MFM-511, and MFM-512, which demonstrated excellent stability to water vapor MFM-510 and MFM-511 exhibited proton conductivities of 2.1 × 10^-5 S cm^-1 and 5.1 × 10^-5 S cm^-1, respectively, at 99% relative humidity (RH) and 25°C, attributed to limited free protons and hindered proton diffusion In contrast, MFM-512, which contains a pendant carboxylic acid group, showed a significant enhancement in proton conductivity to 2.9 × 10^-3 S cm^-1 Additionally, MOF-801, developed by Zhang et al., utilized Brønsted acid sites of μ-OH within its structure, achieving a remarkable proton conductivity of 1.88 × 10^-3 S cm^-1 at 25°C under 98% RH, along with enhanced stability against hydrochloric acid, diluted sodium hydroxide solutions, and boiling water.
1.4.3 Loading proton carriers within MOFs structure
The proton carriers such as Imidazole, Triazole, Histamine, etc molecules were frequently incorporated into porous materials to improve their proton conductivity
In 2009, Bureekaew et al introduced a novel approach by encapsulating the proton-carrier molecule Imidazole within Al-MOFs to develop a hybrid proton conductor suitable for anhydrous environments By optimizing the host–guest interactions, they established an effective proton-conducting pathway at temperatures exceeding 100 °C, achieving a proton conductivity of 2.2 × 10 −5 S cm −1 at 120 °C This research highlights the potential of combining guest molecules with various microporous frameworks to create highly mobile proton carriers in solid-state applications, paving the way for innovative proton conductors designed for high-temperature and anhydrous conditions.
Umeyama et al constructed a composite of aluminum-based metal-organic frameworks (MOFs) and Histamine, achieving a conductivity exceeding 10^-3 S cm^-1 at 150 °C in a completely anhydrous environment This conductivity is nearly 100 times greater than that of Imidazole in Al-MOFs at 120 °C The study utilized [Al(OH)(ndc)]n (where ndc = 1,4-naphthalenedicarboxylate), known for its high thermo/chemical stability, as the support for the composite The compound features one-dimensional channels with a pore diameter of 7.7 × 7.7 Ų Histamine serves as a proton-donating/accepting molecule for hybridization, with a melting point of 83 °C, lower than Imidazole's 89 °C Notably, Histamine does not undergo sublimation like Imidazole, allowing its introduction into MOFs through an immersion process.
Figure 1.23 a) Crystal structure of [Al(OH)(ndc)]n (1), b) Schematic view of Histamine with three proton-hopping sites [77]
Hurd et al introduced β-PCMOF2, a metal-organic framework (MOF) characterized by its ability to conduct protons through one-dimensional pores lined with sulfonate groups The proton conduction in β-PCMOF2 was enhanced by the precise loading of 1H-1,2,4-triazole (Tz) guests within these pores, achieving a remarkable conductivity of 5 ×.
Additionally, 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-
Homburg et al conducted studies on H2BDC (Terephthalic acid) and H2SDBA (4,4′-Sulfonyldibenzoic acid), as illustrated in Figure 1.24 In the Al-MIL-53-(CH3)xHIm framework, where x = 0, 1, and 2, the formation of hydrogen bonds between the framework and guest molecules leads to a reduction in proton conductivity, with values measured at x0 = 1.7 × 10^-6, x1 = 1.9 × 10^-8, and x2 = 1.7 × 10^-8.
At 110 °C, the conductivity values measured were 9 S cm -1 for samples with varying numbers of CH3-groups, showing activation energies (Ea) of 0.42, 0.41, and 0.46 eV for 0, 1, and 2 CH3-groups, respectively The highest conductivity recorded was for CAU-11-HIm, reaching 3.0 × 10 -4 S cm -1 at the same temperature, with an activation energy of 0.19 eV, indicating the absence of host-guest hydrogen bonding interactions.
Figure 1.24 The crystal structure of Al-MIL-53-CH3-HIm AlO6 polyhedra are shown in dark blue, carbon atoms in black and nitrogen atoms in light blue [26]
In 2017, Zhang et al explored the impact of different Imidazole arrangements in Metal-Organic Frameworks (MOFs) on proton conduction They studied a control Fe-MOF, an Imidazole@Fe-MOF (Im@Fe-MOFs) with physically adsorbed Imidazole, and an Imidazole-Fe-MOF (Im-Fe-MOFs) featuring chemically coordinated Imidazole molecules The parent Fe-MOFs were synthesized by exchanging carboxylates in preformed [Fe3(μ3-O)](carboxylate)6 clusters with multi-topic carboxylate ligands Im@Fe-MOFs were created by encapsulating free Imidazole molecules within the Fe-MOF pores, while Im-Fe-MOFs were synthesized in situ, allowing Imidazole ligands to coordinate with the metal nodes Proton conductivity tests indicated that Im-Fe-MOFs exhibited a proton conductivity approximately two orders of magnitude higher than both Fe-MOFs and Im@Fe-MOFs, achieving a remarkable conductivity of 1.21 × 10 −2 S cm −1 at 60 °C and 98% relative humidity.
Figure 1.25 (a) Nyquist diagram of Im-Fe-MOFs, (b) Impedance of Im-Fe-MOFs
The study examines the effects of varying relative humidity (RH) levels, specifically at 98% RH, on the Arrhenius plots and impedance spectra of different iron-based metal-organic frameworks (Fe-MOFs) The analysis includes Fe-MOFs, Im@Fe-MOFs, and Im-Fe-MOFs, highlighting their performance under conditions of 60 °C and 98% RH.
Improving proton mobility under low relative humidity and high temperatures is difficult due to the water affinity of the structure One approach to address this issue involves using metal-organic frameworks (MOFs) with proton transfer agents, although results have been inconsistent In anhydrous conditions at temperatures reaching 130 °C, these proton transfer agents tend to be released from the composite materials Furthermore, there is limited research on their performance in hydrous conditions, where the incorporation of such agents can lead to a loss of crystallinity in the MOFs.
Scope of this dissertation
Zr-MOFs have gained significant attention from researchers due to their diverse coordinated clusters, high chemical stability, and porosity, making them suitable for various applications in environmental treatment and proton exchange membranes These membranes incorporate linkers with proton and proton carrier agents within the MOF structure However, there is limited research exploring the acid-base interactions between SO3H and proton carrier agents such as Imidazole and Histamine in Zr-MOFs.
In my thesis, I addressed the limitations of materials that become ineffective after activation by employing an anchoring strategy with proton carriers like Imidazole and Histamine to stabilize the structure of Metal-Organic Frameworks (MOFs) I also proposed that incorporating proton transfer agents could enhance the proton density within MOFs and improve their affinity for water.
The novel Zr(IV)-based metal-organic frameworks, VNU-17 and VNU-23, developed at Vietnam National University, were synthesized using the linkers 4-Sulfonaphthalene-2,6-dicarboxylic acid (H3SNDC) and 4,8-Disulfonaphthalene-2,6-dicarboxylic acid (H4SNDC) A modulator technique was employed to effectively control the formation and size of the crystal materials This study highlights the optimization of synthesis, along with detailed structural analysis, characterization, and assessments of thermal and chemical stability.
Utilizing linkers with SO3H groups, akin to the Nafion structure, is anticipated to increase the density of protons in metal-organic frameworks (MOFs) This interaction with proton carriers significantly enhances proton conduction, achieving remarkable values in performance.
10 -2 S cm -1 at 95 o C and 85% RH The stability and proton conductivity remaining of the materials were also investigated after impedance measurements.
Experimental
Research content
As reported reviews, we took advantage of new chemically stable Zr(IV)- based metal-organic frameworks, termed VNU-17 and VNU-23, for proton conduction in our research
Content 1: Synthesis of two novel stable Zr-MOFs
Synthesis of linkers 4-Sulfonaphthalene-2,6-Dicarboxylic acid (H3SNDC) and 4,8-Disulfonaphthalene-2,6-Dicarboxylic acid (H4SNDC)
Synthesis of single crystals of VNU-17 and VNU-23
Synthesis of microcrystal VNU-17 and VNU-23 powder
Content 2: Structural analysis and characterization of VNU-17 and VNU-23
Structural analysis of single crystals by SCXRD
Characterization by EA, PXRD, FT-IR, TGA, measurement of nitrogen isotherm and water uptake
Content 3: Loading Imidazole and Histamine within VNU-17 and VNU-23, respectively
Loading Imidazole (HIm) within VNU-17 with different concentrations as 1M and 5M, respectively
Loading Imidazole (HIm, 5M) and Histamine (His, 0.5M) within VNU-23
Characterization by EA, PXRD, FT-IR, TGA, 1 H-NMR (digestion of sample for calculating the number of proton carriers inside MOFs) and measurement of water uptake
VNU-17, VNU-23, HImxVNU-17, HImyVNU-23, and HiszVNU-23 are utilized as exchange proton membranes, where 'x' denotes the number of Imidazole molecules in VNU-17, 'y' represents the number of Imidazole molecules in VNU-23, and 'z' indicates the number of Histamine molecules in VNU-23.
Measuring proton conductivity of VNU-17, HIm9VNU-17 and HIm11VNU-17 at different temperatures and relative humidity (RH)
Measuring proton conductivity of VNU-23, HIm12VNU-23 and
His8.2VNU-23 at different temperatures and RH
Comparing proton conductivity of the materials with other materials
Investigating structural and operated time stability after impedance measurement.
Chemicals
The following chemicals were sourced from Acros Organics: 2,6-Napthalenedicarboxylic acid (H2NDC, 95%), Zirconium Chloride Octahydrate (ZrOCl2·8H2O, 98%), Imidazole (HIm, 99%), Histamine (His, 99%), N,N-Dimethylformamide (DMF, 99% extra dry grade), Sulfuric acid (H2SO4, 98%), Oleum (SO3, 50%), Hydrochloric acid (HCl, 37%), Formic acid (HCOOH, 95%), Anhydrous Methanol (MeOH, 99%), and Dichloromethane (CH2Cl2, 99%).
General methods
The X-ray diffraction data were collected on a Bruker D8 Venture diffractometer outfitted with a PHOTON-100 CMOS detector using monochromatic microfocus Cu Kα radiation (λ = 1.54178 Å) that was operated at 50 kW and 1.0 mA The materials data were collected at 100 K by chilled nitrogen flow controlled by a Kryoflex II system before data collection Unit cell determination was performed in the Bruker SMART APEX II software suite The data sets were reduced and a multi- scan spherical absorption correction was implemented in the SCALE interface The structures were solved with direct methods and refined by the full-matrix least- squares method in the SHELXL-97 program package Once the framework atoms were located in the difference Fourier maps, the SQUEEZE routine in PLATON was performed to remove scattering from disordered guest molecules residing in the pores
Powder X-ray diffraction (PXRD) patterns were recorded using a Bruker D8 Advance operated at 40 kV and 40 mA with a Ni filtered Cu Kα radiation (λ = 1.54178 Å) source The diffractometer was also outfitted with an anti-scattering shield that prevented incident diffuse radiation from hitting the detector Sample preparation included placing samples on a zero background holder and flattening them with a spatula The 2θ range was 3-50 ° with a step size of 0.02 ° and a fixed counting time of 0.3 s/step
Thermal gravimetric analysis (TGA) was conducted using a TA Q500 thermal analysis system under an airflow environment Additionally, Fourier transform infrared spectroscopy (FT-IR) measurements were performed with a Bruker Vertex 70 spectrometer, utilizing the Attenuated Total Reflectance (ATR) sampling technique.
Solution 1 H nuclear magnetic resonance (NMR) spectra were acquired on a Bruker Advance II-500 MHz NMR spectrometer, typically, The samples (15 mg) were digested in 500 μL of DMSO-d6 solution containing HF (10 μL) The mixture was then sonicated for 10 minutes before 1 H-NMR measurement
Elemental analysis was conducted using a LECO CHNS-932 analyzer, while nitrogen adsorption isotherms at 77 K were obtained with a Quantachrome 3Flex, utilizing helium (99.999% purity) for dead space estimation and ultra-high purity N2 (99.999% purity) throughout the adsorption experiments The temperature during measurements was regulated with a water circulator Additionally, water uptake measurements were performed using a BELSORP-aqua3, with temperature control also managed via a water circulator.
Impedance analysis of pelletized samples, measuring 13 mm in diameter and pressed at 3.76 ton cm^-2, was conducted using a Gamry potentiostat (Interface 1000) with the two-probe method Humidity control was achieved with an Espec humidity chamber (model SH-222), while the measuring frequency spanned from 1 MHz to 10 Hz and the applied voltage ranged from 1 mV to 80 mV based on open circuit voltage The pellet thickness, typically between 0.4 to 0.5 mm, was measured using a Nikon SMZ1000 microscope Measurements were performed at temperatures from 25 to 95 °C and various relative humidities, with proton conductivity calculated using equation 1.14.
To obtain the pure impedance of the pelletized sample, the experimental impedance of the electric wire without the pelletized sample was recorded to account for the inductive effect This was achieved by subtracting the impedance of the pelletized sample from that of the electric wire The impedance of the pelletized samples was then determined using various fitting models.
Figure 2.1 An equivalent circuit used for fitting Schematic representations: W1, Warburg diffusion element; Q1/Q2/Q3, imperfect capacitor; R1, Contact resistor; R2, bulk resistor; R3, grain boundary resistor
Figure 2.2 Nyquist plot derived from equivalent circuit (black line) and experimental
Nyquist plot (red circles) of the pelletized sample Frequency ranged from 1 MHz to
Synthesis of H 3 SNDC and H 4 SNDC linker
Nyquist plot (red circles) of the pelletized sample Frequency ranged from 1 MHz to
2.4 Synthesis of H 3 SNDC and H 4 SNDC linker
A mixture of H2NDC (4 g, 18 mmol) and 20 mL of Oleum (SO3 in H2SO4, 20 wt%) was stirred continuously at 130 °C for two days in a 100 mL flask After cooling to room temperature, the product was dissolved in 200 mL of deionized water, filtered, and precipitated using 100 mL of concentrated HCl (37 wt%) The resulting beige mixture was filtered and thoroughly washed with concentrated HCl and water The final product, pure H3SNDC linker, was obtained by drying under vacuum at 120 °C, yielding 4.5 g (82%, 0.015 mol) The 1H-NMR analysis in DMSO-d6 at 500 Hz showed chemical shifts at δ = 9.56 (s, 1H), 8.6 (s, 1H), 8.47 (s, 1H), 8.17 (d, 1H), and 8.00 ppm (d, 1H).
Scheme 2.1 Synthetic process of H3SNDC linker
A mixture of 1 g (4.63 mmol) H2NDC and 5 ml of Oleum (50 wt% SO3 in H2SO4) was placed in a 50 mL flask and stirred continuously at 150 °C for 24 hours Subsequently, an additional 3 ml of fresh Oleum was added and the mixture was stirred further.
170 °C for 2 days Finally, fresh Oleum (2 mL) was added and stirred at 190 °C for
The reaction was allowed to cool to room temperature and the resulting product was dissolved in 100 mL of deionized water After filtration, the product was precipitated using 50 mL of concentrated HCl (37 wt%) The precipitate was filtered and thoroughly washed with 100 mL of concentrated HCl and water, then dried under vacuum at 150 °C, yielding pure H4SNDC linker at 70% (1.2 g, 3.2 mmol) 1H-NMR analysis in DMSO-d6 (500 Hz) showed peaks at δ = 9.59 (s, 2H) and 8.49 (s, 2H).
Scheme 2.2 Synthetic process of H4SNDC linker
Synthesis of VNU-17 and loading Imidazole within VNU-17
2.5.1 Synthesis of single crystals, VNU-17
A mixture of ZrOCl2·8H2O (1 mL, 0.08 M) and H3SNDC (1 mL, 0.08 M) was dissolved in 2 mL of DMF and combined with 1 mL of formic acid as a modulator in a 10 mL capped vial The solution was heated in an isothermal oven at 120 °C for five days, resulting in the formation of colorless crystals After cooling to room temperature, the crystalline product was isolated from the mother liquor through centrifugation, achieving a 40% yield based on Zr4+ ions The obtained single crystals were then utilized for single-crystal X-ray diffraction analysis.
Scheme 2.3 Synthetic process of VNU-17 single crystals
2.5.2 Synthesis of microcrystalline powder, H + VNU-17
A mixture of H3SNDC linker (0.108 g, 0.365 mmol), ZrOCl2·8H2O (0.104 g, 0.324 mmol), and HCOOH (2.6 mL) in DMF (16 mL) was heated at 120 °C for two days, yielding a white microcrystalline powder The product was washed with DMF (5 × 2 mL daily for 3 days), exchanged with deionized water (8 × 2 mL daily for 3 days), and immersed in a 0.3 M H2SO4 aqueous solution (2 × 5 mL daily for 2 days) After centrifugation, the sample was thoroughly washed with water (5 × 7.5 mL) until a pH of 5 was achieved, followed by washing with MeOH (5 × 3 mL) and activation under dynamic vacuum.
The fully acidified H + VNU-17 was synthesized at room temperature under a vacuum of 10 -3 Torr, achieving an 81% yield based on Zr(IV) metal The elemental analysis of the activated sample (EA) indicated the following composition for Zr6C48H70O59S4 [Zr6O8(H2O)8(HSNDC)4]ã15H2O: carbon (C) at 25.72%, hydrogen (H) at 2.76%, nitrogen (N) at 0.03%, and sulfur (S) at 6.02%, closely aligning with the calculated values of C, 25.42%; H, 3.09%; N, 0%; and S, 5.65% The FT-IR spectrum displayed significant peaks at 1696 (weak), 1606 (medium), 1552 (strong), and various other frequencies, confirming the compound's structural characteristics.
Scheme 2.4 Synthetic process of H + VNU-17 powder
VNU-17 (100 mg) was immersed in a methanol solution containing 1 M imidazole for 24 hours, followed by centrifugation and washing with dichloromethane to remove surface-adsorbed imidazole The sample was then activated overnight under dynamic vacuum at room temperature, resulting in the formation of HIm9VNU-17 The elemental analysis of the activated sample (EA) indicated the following calculated and found values: C, 33.93; H, 3.05; N, 9.50; and S, 4.83% The FT-IR spectrum revealed peaks at 3148 (w), 1606 (m), 1570 (s), 1544 (s), 1494 (w), 1410 (s), 1352 (s), 1321 (m), 1192 (s), 1098 (w), and 1065 (m).
Scheme 2.5 Synthetic process of HIm9VNU-17
VNU-17 (100 mg) was immersed in a methanol solution containing HIm (5 M) for two days, followed by centrifugation and washing with dichloromethane to remove surface-adsorbed HIm The sample was then activated overnight under dynamic vacuum at room temperature, resulting in the formation of HIm11VNU-17 The elemental analysis of the activated sample (EA) indicated the following calculated values for Zr6C81H95.2N22O49.6S4: C, 34.17; H, 3.35; N, 10.83; and S, 4.50% The found values were C, 34.26; H, 3.08; N, 10.5; and S, 4.13% FT-IR analysis revealed peaks at 3137 (weak) and 1606 (medium).
Scheme 2.6 Synthetic process of HIm11VNU-17
Synthesis of VNU-23 and loading Histamine within VNU-23
2.6.1 Synthesis of single crystals, VNU-23
A mixture of ZrOCl2·8H2O (1 mL, 0.08 M) and H4SNDC (1 mL, 0.088 M) was dissolved in 2 mL of DMF and combined with 1 mL of formic acid as a modulator in a 10 mL capped vial The solution was heated at 120 °C for four days in an isothermal oven, resulting in colorless crystals After cooling to room temperature, the crystalline product was separated from the mother liquor through centrifugation, achieving a 30% yield based on Zr4+ ions The collected single crystals were subsequently analyzed using single-crystal X-ray diffraction.
Scheme 2.7 Synthetic process of VNU-23 single crystals
2.6.2 Synthesis of pristine VNU-23 powder
A mixture of H4SNDC (0.033 g, 0.088 mmol) and ZrOCl2·8H2O (0.026 g, 0.081 mmol) was dissolved in 4 mL of DMF, followed by the addition of 1 mL of formic acid The solution was heated at 120 °C for three days to produce colorless microcrystals, which were then exchanged with DMF for two days (3 × 5 mL) This material underwent further exchange with CH2Cl2 (3 × 5 mL), dried, and activated under vacuum (10^-3 Torr) at room temperature, resulting in the pristine VNU-23 sample with an 80% yield based on Zr4+ The elemental analysis of the activated sample, Zr6C64H141.9O78.93S8N8 [Zr6O8(H2O)8(SNDC)4]·(DMA)8·22.93H2O (where DMA represents dimethylammonium), revealed a carbon content of 24.83%.
H, 4.59; N, 3.62; and S, 8.27% Found: C, 25.26; H, 4.01; N, 3.25; and S, 7.78% FT- IR: 3098 (w); 2808 (w); 1652 (s); 1582 (s); 1466 (m); 1416 (s); 1363 (s); 1227 (m);
Scheme 2.8 Synthetic process of pristine VNU-23 powder
The Pristine VNU-23 sample underwent a two-day exchange process using a MeOH/H2O solution with 0.3 M H2SO4, at a volume ratio of 4:1 (5 × 5 mL) Following the exchange, the sample was centrifuged and thoroughly washed with a significant volume of MeOH/H2O (5 × 5 mL) to ensure purity.
= 4/1 mL mL -1 ) until the decanted liquor reached pH = 5 Subsequently, this material was washed with MeOH (5 × 5 mL), dried and activated under dynamic vacuum (10 -
3 Torr) at room temperature to acquire H + VNU-23 1 H-NMR (Digested VNU-23, DMSO-d 6 , 500 Hz): δ = 9.59 (s, 2H) and 8.49 (s, 2H) EA: Calcd For
Zr6C49.96H95.88O81S8N0.98 = [Zr6O8(H2O)8(H2SNDC)3(HSNDC)]ã(DMA)0.98ã25H2O:
C, 21.30; H, 3.44; N, 0.49; and S, 9.10% Found: C, 21.31; H, 3.08; N, 1.00; and S, 9.45% FT-IR: 1696 (w); 1607 (m); 1557 (m); 1495 (w); 1414(s); 1364 (m);
Scheme 2.9 Synthetic process of H + VNU-23 powder
H + VNU-23 (100 mg) was soaked in a MeOH solution with 5 M Imidazole (HIm) for three days, followed by centrifugation and washing with CH2Cl2 to eliminate surface-adsorbed HIm The sample was then activated overnight under dynamic vacuum (10 -3 Torr) at room temperature, resulting in HIm12VNU-23 Elemental analysis of the activated sample (EA) showed calculated values for Zr6C84H142N24O79S8 [Zr6O8(H2O)8(H2SNDC)4]ã12HImã23H2O as follows: C, 28.33; H, 3.99; N, 9.44; and S, 7.20%, with found values of C, 28.12; H, 3.85; N, 9.49; and S, 7.28% FT-IR analysis revealed peaks at 3134 (w), 1607 (m), 1565 (s), 1551 (s), 1488 (w), 1410 (s), 1350 (s), 1318 (m), 1190 (s), 1092 (w), and 1065 (m).
Scheme 2.10 Synthetic process of HIm12VNU-23
120 mg of VNU-23 was added to 10 mL of MeOH solution of Histamine (His,
5 M) The mixture was shaken at 45 °C for 3 days, cooled to room temperature, then centrifuged and washed with mixture of MeOH and CH2Cl2 (3 × 2 mL; vMeOH/vCH2Cl2
To eliminate histamine from the intercrystalline regions, a solution of 1/19 mL was utilized Subsequently, the sample underwent activation overnight at room temperature under a dynamic vacuum of 10^-3 Torr, resulting in the formation of His8.2⊗VNU-23 The activated sample (EA) was then analyzed for its composition.
Zr6C89H165.4N24.6O81.8S8 = [Zr6O8(H2O)8(H2SNDC)4]ã8.2Hisã25.8H2O: C, 28.91; H, 4.48; N, 9.32; and S, 6.93% Found: C, 28.70; H, 3.96; N, 9.68; and S, 6.43% FT- IR: 3127 (m); 1607 (m); 1573 (s); 1466 (w); 1408 (s); 1358 (s); 1222 (m); 1178 (s);
Scheme 2.11 Synthetic process of His8.2VNU-23
It was noted that the aforementioned processes were surveyed at the different temperature and time to obtain the optimal condition.
Digestion of samples for 1 H-NMR analysis
Metal-organic frameworks (MOFs) were digested in a DMSO-d6 solution with hydrofluoric acid (HF) to facilitate their decomposition A total of 15 mg of MOFs was combined with 500 μL of DMSO-d6 and 10 μL of HF, followed by sonication for 10 minutes prior to conducting 1H-NMR measurements.
Sample preparation for impedance analysis
The activated samples, weighing 90 mg each, were compressed into pellets under a force of 4 tons The thickness of these pellets was measured using a microscope, and they were subsequently assembled into cell systems utilizing the two-probe method.
Table 2.1 Summary of synthesized materials and full characteristics
1 Single crystals of VNU-17 SCXRD
2 Powder of H + VNU-17 PXRD, FT-IR, TGA, EA, water uptake
PXRD, FT-IR, TGA, EA, water uptake,
4 HIm11VNU-17 PXRD, FT-IR, TGA, EA, water uptake,
5 Single crystals of VNU-23 SCXRD
6 Powder of pristine VNU-23 PXRD, FT-IR, TGA, EA, water uptake
7 Powder of H + VNU-23 PXRD, FT-IR, TGA, EA, water uptake
8 HIm12VNU-23 PXRD, FT-IR, TGA, EA, water uptake,
9 His8.2VNU-23 PXRD, FT-IR, TGA, EA, water uptake,
Results and Discussion
Synthesis of two novel Zr-MOFs as VNU-17 and VNU- 23
3.1.1 Synthesis and characterization of VNU-17
To gain insight into a proton-conducting MOFs platform, we supposed several design elements:
(1) The generation of densely integrating Brứnsted acid functional groups (i.e sulfonic acids) inside MOFs structure
(2) Appropriately sized pore channels, which permit diffusion, full loading, and immobilization of a proton transfer agent as Imidazole
(3) High chemical stability to retain conditions relevant to PEMFC applications
We explored the stability of Zr-based metal-organic frameworks (MOFs) as a potential platform material While conventional fcu structures, such as UiO-66 and DUT-52, feature narrow triangular pores, increasing pore size in UiO-67 limits the proximity of Brønsted acid functional groups However, we identified that synthesizing a bcu structure using shorter linkers could create an architecture that meets the desired criteria for optimal functionality.
As a consequence, a mixture of H3SNDC linker and ZrOCl2ã8H2O was dissolved in DMF in the presence of HCOOH modulator and isothermally reacted at
120 °C for 48 h to yield colorless, crystals of VNU-17 (Figure 3.1)
Figure 3.1 Optical microscopy picture of pristine VNU-17 crystals
Single crystal X-ray diffraction analysis showed that VNU-17 crystallized in the tetragonal space group, I4/m (No 87), with unit cell parameters, a = b = 17.75 and c = 22.49 Å (Table 3.1)
Table 3.1 Crystal data and structure refinement for VNU-17
The VNU-17 framework consists of 8-connected Zr6O8(H2O)8(-CO2)8 clusters linked by ditopic SNDC 3- linkers, resulting in a three-dimensional structure with bcu topology The rigid nature of the SNDC 3- linker effectively stabilizes the sulfonate groups along the [1 1 0] direction.
1 1 0] Miller indices of the bcu structure, thus affording VNU-17 with 6 Å pore channels running down the [0 0 1] plane (Figure 3.2c,d)
Figure 3.2 The crystal structure of VNU-17 is constructed from 8-connected, cubic
The Zr6O8(H2O)8(COO)8 clusters are interconnected by linear, ditopic HSNDC 2- linkers, resulting in a structure that exhibits the bcu topology, as viewed from the [001] plane The architecture of VNU-17, represented as Zr6O8(H2O)8(HSNDC)4, is illustrated, with atom colors indicating Zr as blue polyhedra, C as black, O as red, and S as yellow.
The phase purity of the synthesized VNU-17 was verified through powder X-ray diffraction (PXRD) analysis, which showed that the diffraction pattern of the sample closely matched the simulated pattern derived from its single crystal structure.
Figure 3.3 Powder X-ray diffraction analysis of as-synthesized VNU-17, activated
HIm9VNU-17, and activated HIm11VNU-17 patterns in comparison to the pattern simulated from the single crystal structure of VNU-17
The synthesized VNU-17 was thoroughly washed with DMF and water to eliminate any unreacted starting materials During the synthesis process, it was observed that the sulfonic groups predominantly converted to sulfonate due to the dimethylamine produced from DMF decomposition Consequently, VNU-17 was immersed in a 0.3 M aqueous solution for further treatment.
To protonate the sulfonate functionalities, H2SO4 was utilized, followed by washing with distilled water until the decanted liquid achieved a pH of 4.5 Finally, VNU-17 underwent solvent exchange with anhydrous MeOH to produce the fully acidified form of H + VNU-17.
Before conducting detailed structural characterization, H + VNU-17 was activated under a dynamic vacuum of 10 -3 Torr, resulting in a loss of some long-range order, as indicated by PXRD analysis This phenomenon has been observed in previous studies However, after re-solvation in water, the material's crystallinity was completely restored, as shown in Figure 3.4.
Figure 3.4 Powder X-ray diffraction patterns of VNU-17 after multistep-steps preparation
The experimental formula of VNU-17, [Zr6O8(H2O)8(HSNDC)4]ã15H2O, was determined through elemental microanalysis, confirming the predicted formula based on crystal structure calculations (Calcd: C, 25.42; H, 3.09; N, 0; S, 5.65%; Found: C: 25.72; H: 2.76; N: 0.03; S: 6.02%) The elemental analysis results indicated that the sulfonic acid moieties were fully protonated, as shown by the minimal nitrogen content ( 70 o C) without any Histamine release
The presence of three proton-donor/acceptor sites in the imidazole ring and the amine group of histamine molecules enhances acid-base interactions and increases water affinity within the pore.
Imidazole was incorporated into VNU-23, resulting in the formation of HIm12VNU-23, which exhibited a proton conductivity of 9.65 × 10^-3 S cm^-1 at 70 °C and 85% relative humidity However, this conductivity diminished during impedance analysis at 95 °C due to the release of Imidazole molecules at elevated temperatures.
Figure 3.36 Nyquist plot of pelletized HIm12VNU-23 at 70 °C under 85% relative humidity
Figure 3.37 Nyquist plot of pelletized HIm12VNU-23 at 95 °C under 85% relative humidity
This is to say that loading strategy of Histamine is extremely suitable, as expected, His8.2VNU-23 revealed the proton conduction value reached 1.79 × 10 -2
S cm -1 (95 o C and 85% RH) and without any appreciable loss of performance for at least 120 h
The H + VNU-23 sample underwent an anchoring process with Histamine by being immersed in a 5 M MeOH solution for three days After filtering, it was washed with a mixture of organic solvents to eliminate any Histamine trapped in the intercrystalline regions Finally, the product was activated under dynamic vacuum to yield the dried HiszVNU-23, where 'z' indicates the quantity of immobilized Histamine per formula unit of VNU-23.
Figure 3.38 Procedure for the proposed anchoring of Histamine on the internal surface of VNU-23 (a) and the relative position of anchored Histamine within VNU-
To investigate the successful loading of Histamine into the pore of H + ⊂ VNU-23, we noted that H + ⊂ VNU-23 lost its structural order upon activation Therefore, we conducted PXRD analysis as an initial assessment to determine if Histamine was effectively loaded, which would indicate that the structure retains its order upon activation.
The PXRD pattern indicates that HiszVNU-23 maintains its crystalline structure after activation, confirming the presence of anchored histamine within the pores of H + VNU-23 Additionally, HiszVNU-23 preserves its structural integrity post-pelletization, as verified by the PXRD analysis (Figure 3.39).
Figure 3.39 Powder X-ray diffraction patterns of simulated VNU-23 (1), activated
His8.2VNU-23 (2) and pelletized His8.2VNU-23 (3) pressing at 3.76 tons cm -2
Systematic investigations using FT-IR spectroscopy revealed distinct peaks at 3127 cm⁻¹ in the control Histamine spectrum, which were not present in the VNU spectrum.
23 These were assigned to the vN-H stretching modes of terminal amine groups (Figure 3.15) On the other hand, the absence of an absorption band centered at 1650 cm -1 (assigned to the N-H bending in the ring of Histamine molecules) in HiszVNU-
23 samples confirmed the absence of Histamine on the surface or within the intercrystalline regions (Figure 3.15)
The quantity of Histamine in H + VNU-23 was quantitatively assessed using 1 H-NMR and EA techniques To perform the 1 H-NMR analysis, HiszVNU-23 was digested in acidic DMSO-d6 The resulting spectra revealed a protonated Histamine/H4SNDC ratio of 1.98, indicating that there are 7.92 Histamine molecules incorporated per formula unit of VNU-23.
Figure 3.40 1 H-NMR analysis of digested His8.2VNU-23 (2) in contrast with 1 H- NMR from pure Histamine (1)
The 1 H-NMR peaks of the protons in protonated Histamine was observed to center at higher chemical shift in comparison with pure Histamine This phenomenon was assigned to effect, caused by the protonation of Histamine This value was further confirmed by EA analysis, in which, the chemical formulas were determined to be [Zr6O8(H2O)8(SNDC)4]ã8.2Hisã25.8H2O (Calcd: C, 28.91; H, 4.48; N, 9.32; and
S, 6.93% Found: C, 28.70; H, 3.96; N, 9.68; and S, 6.43%) All two quantitative
(2) measurements were consistent in determining the loading of z = 8.2 Histamine molecules, respectively
Single crystal X-ray diffraction studies of His8.2VNU-23 reveal the anchoring mechanism of Histamine on the internal surface of VNU-23 The analysis indicates that Histamine is deposited within VNU-23, characterized by a close distance (2.7 < d < 3.2 Å) between the nitrogen atoms of Histamine, sulfonate groups, and coordination water molecules.
Zr6O8(H2O)8(-CO2)8 clusters was observed This confirmed the formation of the hydrogen bonds between the protonated Histamine and the docking centers in
H + VNU-23 Furthermore, it noticed that the low site occupation of Histamine inside
The single crystal X-ray diffraction method revealed that H + VNU-23 (15%) exhibited a naturally large crystal size This size, combined with the dense occupation of Histamine near the surface, inhibited the diffusion of additional Histamine molecules into the crystal Notably, we highlighted that the presence of several large single crystals is rare, as they were predominantly collected from the walls of reaction vials, while the microcrystal product of H + VNU-23 was more commonly observed.
3.3.2 Proton conduction of H + VNU-23 and His z VNU-23
Before measuring the proton conductivity of His8.2VNU-23, water sorption experiments were conducted at 25 °C The results indicated that the water uptake of His8.2VNU-23 was 191 cm³/g at P/P₀ = 0.7, 248 cm³/g at P/P₀ = 0.8, and 339 cm³/g at P/P₀ = 0.85 Notably, water condensation was observed at P/P₀ = 0.87.
P/P 0 = 0.85 was chosen as cut off point for proton conductivity measurements in order to minimize the effect of condensed water during the measurement (Figure 3.41)
Figure 3.41 Water uptake of His8.2VNU-23 at 25 °C as a function of P/P 0 Closed and open circles represent the adsorption and desorption branches, respectively
We aimed to assess the proton conductivity of the His8.2VNU-23 composite, which is characterized by a high water uptake and a dense arrangement of sulfonate groups conjugated with protonated histamine.
In addition, His8.2VNU-23 was then tested the structural stability at wide temperature range from 70 - 120 o C As expected, the structure still remained during heating process (Figure 3.42)
Figure 3.42 Temperature-dependent PXRD analysis of activated His8.2VNU-23 from 70 to 120 °C
3.3.2.1 Proton conductivity of VNU-23 and His 8.2 VNU-23 under different relative humidities
Systematic investigations were conducted on the ac impedance spectroscopy measurements of pelletized pristine VNU-23, H + VNU-23, and His8.2VNU-23 at varying relative humidity levels (50-85%) and an elevated temperature of 95 °C This temperature was chosen as it demonstrated the highest proton conductivity for all materials tested.
Table 3.5 Conductivity of pelletized pristine VNU-23, H + VNU-23 and
His8.2VNU-23 at 95 °C from 50% RH to 85% RH
Figure 3.43 Nyquist plot of pelletized pristine VNU-23 at 95 °C under relative humidity varied from 50% to 85%
Pristine VNU-23 H + VNU-23 His8.2VNU-23
Figure 3.44 Nyquist plot of pelletized H + VNU-23 at 95 °C under relative humidity varied from 50% to 85%
As a consequence from Table 3.5, proton conductivity of pristine VNU-23 and
H + VNU-23 respectively resulted the conductivity value of 1.30 × 10 -4 and 9.97 ×
10 -6 S cm -1 under 85% RH and 95 °C, respectively It is noted that this value is at least
10 2 times lower than the conductivity value of His8.2VNU-23 under 85% RH and
95 °C (1.79 × 10 -2 S cm -1 ), thus the importance of immobilzed Histamine inside VNU-
The Nyquist plots of His8.2VNU-23 reveal a significant correlation with relative humidity (RH) levels, showing that conductivity increases from 1.3 × 10 -6 S cm -1 at 50% RH and 95 °C to 1.79 × 10 -2 S cm -1 at 85% RH and 95 °C.
Figure 3.45 Nyquist plot of pelletized His8.2VNU-23 at 95 °C under relative humidity varied from 50% to 85%
Figure 3.46 Dependence of proton conductivity in His8.2VNU-23 as a function of relative humidity at 95 °C, inset: Nyquist plot of His8.2VNU-23 under 85% RH and
3.3.2.2 Proton conductivity of VNU-23 and His 8.2 VNU-23 at different temperature
Comparing the proton conductivity of VNU-17, HIm x VNU-17, VNU-23,
and His z VNU-23 with other MOFs
We note that the proton conductivity of His8.2VNU-23 under 85% RH and
95 °C exceeds the values several of the highest performing MOFs materials reported conditions (Table 3.7)
Table 3.7 Proton conductivity values of VNU-17, VNU-23, HImVNU-17,
His8.2VNU-23 in comparison with high-performing water-mediated proton conducting MOFs
70 °C, 85% RH Im-Fe-MOF 1.21 × 10 -2 60 o C, 98% RH [90] Fe-CAT-5 7.94 × 10 -3 25 °C, 85% RH [53]
SA@Zr6O4(OH)L4ãxH2O 5.62 ì 10 -3 65 °C, 95% RH [74]
SA = Sulfoacetate; L = 2-sulfoterephthalate; BDC = Terephthalate a,b,c The proton conductivity is decreased after 2 h d Note the higher RH needed for MOFs.
The research findings indicate that the hybridization materials, specifically Imidazole and Histamine combined with Zr-MOF, exhibit significantly higher proton conductivity compared to other materials.
VNU-17 and VNU-23 feature a high density of protons due to the presence of SO3H groups within their structures, enabling effective anchoring of proton carriers in metal-organic frameworks (MOFs) through acid-base interactions The abundant distribution of polar molecules like Imidazole and Histamine fully occupies the pores, facilitating efficient proton transport pathways.
At elevated temperatures, many metal-organic frameworks (MOFs) exhibit reduced proton conductivity due to decreased water affinity However, HImxVNU-17 and HiszVNU-23 maintain high proton conductivity over extended operation times, thanks to their internal proton agents that effectively retain water molecules through strong polarity.
Sulfonic acid groups are generally more effective at supporting H+ conduction compared to imidazolium and histamine ions However, we observed an improvement in proton conduction in HImxVNU-17 and HiszVNU-23 compared to their parent compounds, VNU-17 and VNU-23 This enhancement is due to the complete hydration of the entire pore structure and grain boundaries.
Histamine molecules do not surpass the sublimation process, as demonstrated by His8.2VNU-23, which maintained proton conductivity for over 120 hours at 95 °C In contrast, HImxVNU-17 and other metal-organic frameworks (MOFs) release imidazole at the same temperature.
The structural integrity of hybridization materials HImxVNU-17 and HiszVNU-23 was evaluated post-impedance measurements using FT-IR and PXRD techniques, highlighting their stability In contrast, other metal-organic frameworks (MOFs) lack similar structural stability data following analysis.
1) For the first time, the two novel Zr-MOFs, namely VNU-17, VNU-23 and hybrid materials have been successfully synthesized via anchoring proton carriers into MOFs The use of SCXRD combined with the advanced characteristic methods such as PXRD, FT-IR, TGA, BET, 1 H-NMR has revealed that the materials adopted bcu topology and the structure was highlighted by densely packed sulfonic acid groups lining 6 Å channels They exhibited a high thermal, chemical stability and water resistance and suitable for proton exchange membrane of fuel cells The amount of Imidazole and Histamine molecules within MOFs was determined through 1 H- NMR, EA and TGA
2) By acid-base interaction of dense SO3H groups with Imidazole and Histamine, the new hybrid materials, HIm9VNU-17, HIm11VNU-17, HIm12VNU-23 and His8.2VNU-23, were successfully synthesized The number of Imidazole within VNU-17 is 9 and 11 molecules respectively, in addition, the Histamine molecules anchored within VNU-23 is 8.2 The results showed that the modification of VNU-17 and VNU-23 plays an important role in generation of new proton conductive materials High content of proton carriers in MOFs, Imidazole and Histamine, supported the fully hydration process in the pore and grain boundaries of MOFs This led to enhance proton conductivity at high temperature and low relative humidity (RH < 90%) The proton conductivity of HIm11VNU-17 (5.93 × 10 -3 S cm -1 under 85% RH at 70 °C) is 900 times higher than that of H + VNU-17 (5.03 ×
10 -7 S cm -1 at 70 °C but need 98% RH) Similarly, the proton conductivity of His8.2VNU-23 under 85% RH at 95 o C is higher than pristine VNU-23 and
3) The use of materials containing Histamine within MOFs instead of Imidazole affords to increase the operating temperature of the proton exchange membrane from 70 o C (for Imidazole) to 95 o C (for Histamine) As expected, at 85%
RH, His8.2VNU-23 showed a proton conductivity of 1.00 × 10 -2 S cm -1 at 70 o C and
1.79 × 10 -2 S cm -1 at 95 ° C These values are better when compared to HIm9VNU-
The study highlights the impressive conductivity of various metal-organic frameworks (MOFs) under 85% relative humidity, with HIm11VNU-17 showing a conductivity of 5.93 × 10 -3 S cm -1, while His8.2VNU-23 maintained high proton conductivity at 95 °C for over 120 hours without structural changes This stability is crucial for reducing CO poisoning in platinum catalysts at the electrode Consequently, this research marks the first development of novel hybrid materials that exhibit both high conductivity and structural stability under practical fuel cell conditions.
4) The activation energy and mechanism of proton conduction of the materials were also determined The activated energy of HIm11VNU-17 (Ea = 0.27 eV) is lower than H + VNU-17 (Ea = 0.46 eV) and HIm9VNU-17 (Ea = 0.44 eV) Thus, proton conduction of HIm11VNU-17 was supposed by Grotthuss mechanism (Ea < 0.4 eV) Subsequently, for His8.2VNU-23 there are two proton conduction zones
In the low temperature range of 30 to 50 °C, the activation energy for proton conduction is 0.81 eV, indicating that the process follows a vehicle mechanism, as the activation energy exceeds 0.4 eV Conversely, in the high temperature range of 60 to 95 °C, the activation energy decreases to 0.27 eV, and proton conduction occurs via the Grotthuss mechanism.
List of published works in thesis
[1] Lo, T H N * ; Nguyen, M V * ; Tu, T N (2017), An anchoring strategy leads to enhanced proton conductivity in a new Metal-organic frameworks, Inorganic Chemistry Frontiers, 4(9), pp 1509-1516 (ISI-Q1, IF = 5.934)
[2] Nguyen, M V.; Lo, T H N.; Luu, L C.; Nguyen, H T.T.; Tu, T N (2018),
Enhancing proton conductivity in a Metal-organic frameworks at T > 80 °C by anchoring strategy, Journal of Materials Chemistry A, 6(4), pp 1816-1821 (ISI-Q1,
[3] Tu, T N.; Nguyen, M V.; Nguyen, H L.; Yuliarto, B.; Cordova, K.; Demir, S
(2018), Designing bipyridine-functionalized zirconium metal-organic frameworks as a platform for clean energy and other emerging applications, Coordination Chemistry
[4] Nguyen, H T T.; Tu, T N.; Nguyen, M V.; Lo, T H N.; Furukawa, H.; Nguyen,
N N.; Nguyen, D M (2018), Combining linker design and linker exchange strategies for the synthesis of a stable large pore Zr-based Metal-organic framework, ACS Applied Materials & Interfaces, 10(41), pp 35462-35468 (ISI-Q1, IF = 8.456)
[5] Tu, T N.; Nguyen, H T T.; Nguyen, H D T.; Nguyen, M V.; Nguyen, D T.; Tran, N T.; Lim K T (2019), A new iron-based metal–organic framework with enhancing catalysis activity for benzene hydroxylation, RSC Advances, 2019(9), pp 16784-16789 (ISI-Q1, IF = 3.049)
[1] Agmon, N (1995), The Grotthuss mechanism, Chemical Physics Letter, 244(5), pp 456-462
[2] Alberti, G.; Casciola, M Annu., (2003), Composite membranes for medium- temperature pem fuel cells, Annual Review of Materials Research, 33(1), pp 129-
[3] Bai, Y.; Dou, Y.; Xie, L H.; Rutledge, W.; Li, J.-R.; Zhou, H C (2016), Zr-based metal-organic frameworks: design, synthesis, structure, and applications, Chemical Society Reviews, pp 2327-2367
[4] Bon, V.; Senkovska, I.; Weiss, M S.; Kaskel, S (2013), Tailoring of network dimensionality and porosity adjustment in Zr- and Hf-based MOFs, CrystEngComm, 15(45), pp 9572-9577
[5] Bon, V.; Senkovskyy, V.; Senkovska, I.; Kaskel, S (2012), Zr(IV) and Hf(IV) based metal-organic frameworks with reo-topology, Chemical Communications,
[6] Borges, D D.; Devautour-Vinot, S.; Jobic, H.; Ollivier, J.; Nouar, F.; Semino, R.; Devic, T.; Serre, C.; Paesani, F.; Maurin, G (2016), Proton transport in a highly conductive porous zirconium-based metal-organic framework: molecular insight,
Angewandte Chemie International Edition, 55(12), pp 3919-3924
[7] Bose, S.; Kuila, T.; Nguyen, T X H.; Kim, N H.; Lau, K.; Lee, J H (2011), Polymer membranes for high temperature proton exchange membrane fuel cell: Recent advances and challenges, Progress in Polymer Science, 36(6), pp 813-843
[8] Bureekaew, S.; Horike, S.; Higuchi, M.; Mizuno, M.; Kawamura, T.; Tanaka, D.; Yanai, N.; Kitagawa, S (2009), One-dimensional imidazole aggregate in aluminium porous coordination polymers with high proton conductivity, Nature Materials,
[9] Cavka, J H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti, C.; Bordiga, S.; Lillerud, K P (2008), A new zirconium inorganic building brick forming metal organic frameworks with exceptional stability, Journal of the American Chemical Society, 130(42), pp 13850-13851
[10] Chen, W.; Xu, H.; Zhuang, G.; Long, L.; Huang, R.; Zheng, L (2011), Temperature-dependent conductivity of Emim + (Emim + = 1-ethyl-3-methyl imidazolium) confined in channels of a metal–organic framework, Chemical Communications, 47(43), pp 11933-11935
[11] Chui, S S.-Y.; Lo, S M.-F.; Charmant, J P H.; Orpen, A G.; Williams, I D
(1999), A chemically functionalizable nanoporous material [Cu3(TMA)2(H2O)3]n,
A study by Costantino, Donnadio, and Casciola (2012) investigates the phase transitions of a flexible and robust Zr phosphonate coordination polymer, highlighting their impact on ion-exchange and proton-conduction properties Published in Inorganic Chemistry, this research provides insights into how these transitions influence the material's functionality, offering valuable information for applications in various fields.
[13] Crabtree, G W.; Dresselhaus, M S (2008), The hydrogen fuel alternative,
[14] Deng H.; Doonan C J.; Furukawa H.; Ferreira R B.; Towne J.; Knobler C B.; Wang B.; Yaghi O M (2010), Multiple functional groups of varying ratios in Metal- organic frameworks, Science, 327(5967), pp 846-850
[15] Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O'Keeffe, M.; Yaghi,
O M (2002), Systematic design of pore size and functionality in isoreticular MOFs and their application in methane storage, Science, 295 (5554), pp 469-472
[16] Feng, D.; Gu, Z.-Y.; Chen, Y.-P.; Park, J.; Wei, Z.; Sun, Y.; Bosch, M.; Yuan, S.; Zhou, H.-C (2014), A highly stable porphyrinic zirconium metal–organic framework with shp-a topology, Journal of the American Chemical Society, 136(51), pp 17714-17717
[17] Furukawa, H.; Cordova, K E.; O’Keeffe, M.; Yaghi, O M (2013), The chemistry and applications of metal-organic frameworks, Science, 341(6149), pp
[18] Furukawa, H.; Gándara, F.; Zhang, Y.-B.; Jiang, J.; Queen, W L.; Hudson, M
R.; Yaghi, O M (2014), Water adsorption in porous metal–organic frameworks and related materials, Journal of the American Chemical Society, 136(11), pp 4369-4381
[19] Gagnon, K.; Perry, H.; Clearfield, A (2012), Conventional and unconventional Metal-organic frameworks based on phosphonate ligands: MOFs and UMOFs,
[20] Gomes, S C.; Luz, I.; Llabrés i Xamena, F X.; Corma, A.; García, H (2010), Water stable Zr–benzenedicarboxylate metal–organic frameworks as photocatalysts for hydrogen generation, Chemistry – A European Journal, 16(36), pp 11133-11138
[21] Grot, W (1978), Use of Nafion perfluorosulfonic acid products as seperators in electrolytic cells, Chemical Inorganic Technology, 50(4), pp 299-301
A study by Gutov et al (2014) explores a water-stable zirconium-based metal-organic framework (MOF) known for its high surface area and impressive gas storage capabilities This innovative material demonstrates significant potential for applications in gas capture and storage, highlighting advancements in the field of MOFs and their role in addressing environmental challenges.
[23] Hamrock, S.; Yandrasits, M (2006), Proton exchange membranes for fuel cell applications, Journal of Macromolecular Science - Part C, 46(3), pp 219-244
[24] Haile, S M.; Boysen, D A.; Chisholm, C R I.; Merle, R B (2001), Solid acids as fuel cell electrolytes, Nature, 410 (6831), pp 910-913
[25] Hogarth, W H J.; Diniz da Costa, J C.; Lu, G Q (2005), Solid acid membranes for high temperature (140 o C) proton exchange membrane fuel cells, Journal of Power Sources, 142(1-2), pp 223-237
[26] Homburg, T.; Hartwig, C.; Reinsch, H.; Wark, M.; Stock, N (2016), Structure property relationships affecting the proton conductivity in imidazole loaded Al- MOFs, Dalton Transactions, 45(38), pp 15041-15047
[27] Horcajada, P.; Gref, R.; Baati, T.; Allan, P K.; Maurin, G.; Couvreur, P.; Férey, G.; Morris, R E.; Serre, C (2012), Metal–organic frameworks in biomedicine,
[28] Horike, S.; Umeyama, D.; Inukai,M.; Itakura, T.; Kitagawa, S (2012),
Coordination-network-based ionic plastic crystal for anhydrous proton conductivity,
Journal of the American Chemical Society, 134(18), 7612-7615
[29] Howarth, A J.; Liu, Y.; Li, P.; Li, Z.; Wang, T C.; Hupp, J T.; Farha, O K
(2016), Chemical, thermal and mechanical stabilities of metal–organic frameworks,
[30] Hurd, J.; Vaidhyanathan, R.; Thangadurai, V.; Ratcliffe, C.; Moudrakovski, I.; Shimizu, G (2009), Anhydrous proton conduction at 150 °C in a crystalline metal- organic framework, Nature Chemistry, 1(9), pp 705-710
[31] Jiang, J.; Yaghi, O M (2015), Brứnsted acidity in metal–organic frameworks,
[32] Jorcin, B.; Orazem, M.; Pébère, N.; Tribollet, B (2006), CPE analysis by local electrochemical impedance spectroscopy, Electrochimica Acta, 51(8), pp 1473-
[33] Kim, M.; Cohen, S M (2012), Discovery, development, and functionalization of Zr(IV)-based metal-organic frameworks, CrystEngComm., 14(12), pp 4096-4104
[34] Kreuer, K-D (1996), Proton Conductivity: Materials and Applications,
[35] Li, H.; Eddaoudi, M.; O'Keeffe, M.; Yaghi, O M (1999), Design and synthesis of an exceptionally stable and highly porous metal-organic framework, Nature,
[36] Liu, J.; Chen, L.; Cui, H.; Zhang, J.; Zhang, L.; Su, C.-Y (2014), Applications of metal-organic frameworks in heterogeneous supramolecular catalysis, Chemical Society Reviews, 43(16), pp 6011-6061
[37] Li, Q.; He, R.; Jensen, J O.; Bjerrum, N (2003), Approaches and recent development of polymer electrolyte membranes for fuel cells operating above 100 °C, Chemistry of Materials, 15(26), pp 4896-4915
[38] Liu, Q Y.; Wang, W F.; Wang, Y L.; Shan, Z M.; Wang, M S.; Tang, J
(2012), Diversity of lanthanide(iii)–organic extended frameworks with a 4,8- disulfonyl-2,6-naphthalenedicarboxylic acid ligand: syntheses, structures, and magnetic and luminescent properties, Inorganic Chemistry, 51(4), pp 2381-2392
[39] Liu, S.; Cao, C.; Yang, F.; Yu, M.; Yao, S.; Zheng, T.; He, W.; Zhao, H.; Hu, T.; Bu, X (2016), High proton conduction in two CoII and MnII anionic Metal- organic frameworks derived from 1,3,5-Benzenetricarboxylic acid, Crystal Growth
[40] Liu, S.; Yue, Z.; Liu, Y (2015), Incorporation of imidazole within the metal– organic framework UiO-67 for enhanced anhydrous proton conductivity, Dalton Transactions, 44(29), pp 12976-12980
[41] Liu, Y.; Yang, X.; Miao, J.; Tang, Q.; Liu, S.; Shi, Z.; Liu, S (2014), Polyoxometalate-functionalized metal–organic frameworks with improved water retention and uniform proton-conducting pathways in three orthogonal directions,
[42] Luo, H-B.; Wang, M.; Liu, S-X.; Xue, C.; Tian, Z-F.; Zou, Y.; Ren, X-M (2017), Proton conductance of a superior water-stable metal–organic framework and its composite membrane with poly(vinylidene fluoride), Inorganic Chemistry, 56(7), pp 4169-4175
[43] Ma, L.; Abney, C.; Lin, W (2009), Enantioselective catalysis with homochiral metal-organic frameworks, Chemical Society Reviews, 38(5), pp 1248-1256
[44] Maier, J (2009), In solid state electrochemistry i: fundamentals, materials and their applications, Kharton, V V., Ed., Wiley-VCH: Hoboken, NJ, pp 1-13
[45] Mallant, R K A M (2003), PEMFC systems: the need for high temperature polymers as a consequence of PEMFC water and heat management, Journal of Power
[46] Maréchal, M.; Souquet, J.-L.; Guindet, J.; Sanchez, J.-Y (2007), Solvation of sulphonic acid groups in Nafion ® membranes from accurate conductivity measurements Electrochemistry Communications, 9(5), pp 1023-1028
[47] Meng, X.; Song, S.; Song, X.; Zhu, M.; Zhao, S.; Wu, L.; Zhang, H (2015), A tetranuclear copper cluster-based MOF with sulfonate–carboxylate ligands exhibiting high proton conduction properties, Chemical Communication, 51(38), pp 8150-8152
[48] Meng, X.; Wang, H.; Song, S.; Zhang, H (2017), Proton-conducting crystalline porous materials , Chemical Society Reviews, 46(2), pp 464-480
[49] Moghadam, P Z.; Li, A.; Wiggin, S B.; Tao, A.; Maloney, A G P.; Wood, P A.; Ward, S C.; Fairen-Jimenez, D (2017), Development of a cambridge structural database subset: A collection of Metal-organic frameworks for past, present, and future, Chemistry of Materials, 29(7), pp 2618-2625
In a groundbreaking study published in the Journal of the American Chemical Society, Mondloch et al (2013) explored vapor-phase metalation through atomic layer deposition within a metal-organic framework This innovative approach demonstrates the potential for enhanced functionality in metal-organic frameworks, paving the way for advancements in various applications The findings emphasize the significance of precise metal incorporation techniques in improving the properties of these frameworks, which could lead to developments in fields such as catalysis and gas storage.
[51] Morris, W.; Volosskiy, B.; Demir, S.; Gándara, F.; McGrier, P L.; Furukawa, H.; Cascio, D.; Stoddart, J F.; Yaghi, O M (2012), Synthesis, structure, and metalation of two new highly porous zirconium metal-organic frameworks, Inorganic
[52] Nagarkar, S.; Unni, S.; Sharma, A.; Kurungot, S.; Ghosh, S (2014), Two-in-one: inherent anhydrous and water-assisted high proton conduction in a 3D metal-organic framework, Angewandte Chemie International Edition, 53(10), pp 2638-2642
[53] Nguyen, T T N; Furukawa, H.; Gándara, F.; Trickett, C A.; Jeong, H M.; Cordova, K E.; Yaghi, O M (2015), Three-dimensional metal-catecholate frameworks and their ultrahigh proton conductivity, Journal of the American Chemical Society, 137(49), pp 15394-15397
[54] O’Keeffe, M.; Peskov, M A.; Ramsden, S J.; Yaghi, O M (2008), The reticular chemistry structure resource (rcsr) database of, and symbols for, crystal nets,
Accounts of Chemical Research, 41(12), pp 1782-1789
[55] Oisaki, K.; Li, Q.; Furukawa, H.; Czaja, A U.; Yaghi, O M (2010), A metal- organic framework with covalently bound organometallic complexes, Journal of the
[56] Phang, W.; Lee, W.; Yoo, K.; Ryu, D.; Kim, B.; Hong, C (2014), pH-dependent proton conducting behavior in a metal-organic framework material, Angewandte Chemie International Edition, 53(32), pp 8383-8387
[57] Phang, W.; Jo, H.; Lee, W.; Song, J.; Yoo, K.; Kim, B.; Hong, C (2015), Superprotonic conductivity of a UiO-66 framework functionalized with sulfonic acid groups by facile postsynthetic oxidation, Angewandte Chemie International Edition, 54(17), pp 5142-5146
[58] Papadaki, I.; Malliakas, C D.; Bakas, T.; Trikalitis, P N (2009), Molecular supertetrahedron decorated with exposed sulfonate groups built from mixed-valence tetranuclear Fe3 3+Fe 2+ (μ3-O)(μ3-SO4)3(CO2)3 clusters, Inorganic Chemistry, 48(21), pp 9968-9970
[59] Ponomareva, V.; Kovalenko, K.; Chupakhin, A.; Dybtsev, D.; Shutova, E.; Fedin, V (2012), Imparting high proton conductivity to a metal–organic framework material by controlled acid impregnation, Journal of the American Chemical Society, 134(38), pp 15640-15643
[60]Qing, S B.; Huang, W.; Yan, D Y (2006), Synthesis and properties of soluble sulfonated polybenzimidazoles, Reactive & Functional Polymers, 66(2), pp 219-227
[61] Ramaswamy, P.; Wong, N.; Gelfand, B.; Shimizu, G (2015), A water stable magnesium MOF that conducts protons over 10 –2 S cm –1 , Journal of the American