Synthesis of cobalt and iron based matal organic framworks and their applications

THE CHEMISTRY & APPLICATIONS OF METAL ORGANIC

Definition of Metal Organic Framework

Metal-organic frameworks (MOFs) are compounds made up of metal clusters linked by organic carboxylates, such as 1,4-benzenedicarboxylic acid (H2BDC) These structures can be constructed in two or three dimensions and often exhibit porous characteristics.

Fig 1 Structure of MOF-5 constructed from Zn4O(CO2)6 cluster and BDC 2- linker 6

Fig 2 Recent progress on synthesizing high surface area material 1

Over 20,000 different metal-organic frameworks (MOFs) have been reported recently, with the highest surface area reaching an impressive 7140 m²/g This surpasses the surface areas of traditional porous materials, including zeolites and activated carbons, highlighting the superior capabilities of MOFs in various applications.

Applications of Metal-organic Frameworks

Metal-Organic Frameworks (MOFs) are highly promising materials due to their high porosity and modular design, allowing for easy customization of their backbone components, such as inorganic and organic secondary building units (SBUs) This versatility enables a wide range of applications, including gas storage and separation, catalysis, proton conduction, sensing, light harvesting, drug delivery, and energy storage in batteries and supercapacitors.

1.2.1 Applications of Metal-organic Frameworks as Heterogeneous Catalysis

Catalysts are categorized into homogeneous and heterogeneous types Homogeneous catalysts are known for their rapid kinetics and high conversion rates in organic synthesis; however, they present challenges such as difficulties in separation for recycling and contamination of desired products by the catalyst or its decomposition products In contrast, heterogeneous catalysts offer a greener alternative for organic synthesis due to their ease of separation from the reaction mixture, facilitating recycling Despite their advantages, the use of heterogeneous catalysts often leads to lower conversion rates, prompting research efforts aimed at developing more efficient heterogeneous catalysts.

Traditional heterogeneous catalysts, such as metal oxides, polymer resins, silica gel, and zeolites, face limitations due to the low surface area of metal oxides and polymer resins, as well as the small pore sizes of zeolites, which hinder the accessibility of large organic substrates to catalytic centers In contrast, mesoporous silica gel offers a solution with its large pore structure and high surface area, facilitating the transformation of larger organic substrates effectively.

5 area, however, the structure and pore size of the materials are not uniform and the immobilization of active centers within its pore has maintained challenges

The oxidative transformation of large organic substrates necessitates the generation of active radicals or high oxidation states of metal centers, which are often unstable and decay rapidly This instability demands swift diffusion of the organic substrate to the catalytic active sites Recently, metal-organic frameworks (MOFs) have emerged as a promising platform for the catalytic synthesis of various organic compounds However, many published MOFs exhibit small pore apertures and low surface areas, typically less than 8 Å and 2000 m²/g.

Metal-Organic Frameworks (MOFs) are distinguished by their large internal surface areas and ultralow densities, which contribute to their unique properties The uniform pore sizes and well-defined coordination environments of the metal active centers in certain MOFs enable them to act as effective catalysts for the oxidative transformation of large organic substrates However, instances of these catalytic reactions remain limited in the current literature.

1.2.1.1 Metal-organic Frameworks as Scaffold for Oxidative Transformation of Organic Substrates

1.2.1.1.1 Cobalt-based Metal-organic Frameworks for Oxidative Transformation of Small Organic Substrates

Pyrazolate-based materials, specifically [Co II 4O(bdpb)3]n, were synthesized by Volkmer through the reaction of H2bdpb and CoCl2·6H2O The resulting structure of [Co II 4O(bdpb)3]n was determined to exhibit a pcu net topology, akin to that of MOF-5, which features octahedral enclosures.

The material {Co4O(dmpz)6} features a pore size of 18.1 Å, replacing {Zn4O(CO2)6} The compound [Co II 4O(bdpb)3]n exhibits permanent porosity, as demonstrated by argon gas sorption experiments Additionally, the BET surface area of [Co II 4O(bdpb)3]n was determined to be 1525 m²/g based on the adsorption data, indicating significant potential for applications in gas storage and separation.

The synthesis of [Co II (bdpb)]n involves the reaction of H2bdpb with Co(NO3)2·6H2O, resulting in a three-dimensional square grid framework This structure is constructed from cobalt rod secondary building units (SBUs) and bdpb 2- ligands Notably, [Co II (bdpb)]n features a one-dimensional channel with a diagonal length of 18.6 Å.

Scheme 1 Formation of the observed products through the reaction of tert-butyl peroxy radicals with cyclohexene (b) Mechanism for the formation of tert-butylperoxy radicals catalysed by the cobalt(II) centres in [Co II 4O(bdpb)3] Their further reaction with cyclohexene, forming the main product 16,17

The study focused on the liquid-phase oxidation of cyclohexene using TBHP as the oxidant with two catalysts: [Co II 4O(bdpb)3]n and [Co II (bdpb)]n The results showed that after 22 hours, the maximum cyclohexene conversion was 27.5% for [Co II 4O(bdpb)3] and 16% for [Co II (bdpb)] The primary reaction products from both catalysts included tert-butyl-2-cyclohexenyl-1-peroxide, along with 2-cyclohexen-1-one and cyclohexene oxide.

Recent advancements in cyclohexene oxidation reactions have highlighted the effectiveness of cobalt-based metal-organic frameworks (MOFs) A newly synthesized MOF, Co3(OH)2-(tpta)(H2O)4, featuring terphenyl-3, 2’’, 5’’, 3’-tetracarboxyate (tpta), demonstrates promising catalytic properties Characterization studies indicate that this material can be dehydrated through heating, resulting in the formation of dehydrated Co3(OH)2(tpta) Heterogeneous catalytic tests reveal that Co3(OH)2(tpta) exhibits significant activity in the allylic oxidation of cyclohexene, marking a noteworthy development in the field.

The study demonstrated that Co3(OH)2-(tpta)(H2O)4 exhibits a sixfold increase in catalytic activity, primarily due to the coordinatively unsaturated Co II sites in Co3(OH)2(tpta), which significantly enhance the oxidation of cyclohexene The highest conversion rate achieved in this system reached approximately 73.6% Following this, a similar oxidative transformation of cyclohexene was conducted using nickel-based catalysts.

Zhaohui Li et al investigated MOF-74, Co-MOF-74, and a mixed Co & Ni-MOF-74, revealing that the incorporation of active cobalt into the Ni-MOF-74 framework enhanced its catalytic activity for cyclohexene oxidation, achieving a maximum conversion rate of 54.7% Additionally, the mixed framework demonstrated superior catalytic performance compared to pure Co-MOF-74.

1.2.1.1.2 Metal-organic Frameworks for Oxidative Conversation of Large Organic Substrates

While metal-organic frameworks (MOFs) can facilitate the oxidative transformation of small organic substrates, instances of their effectiveness with large organic substrates are rare Notable exceptions include cases where MOFs with large pore sizes incorporate porphyrin active centers, demonstrating their potential in this area.

Fig 3 a) Crystal structure of PCN-222; b) Peroxidase-like oxidation reaction of pyrogallol catalyzed by PCN-222(Fe) 20

Zhou et al recently introduced a porphyrin-based metal-organic framework (MOF) called PCN-222, leveraging its large pore size and porphyrin active center for catalytic applications The self-assembly of tetrakis(4-carboxyphenyl)porphyrin and zirconium clusters resulted in a csq framework featuring one-dimensional channels with hexagonal and triangular shapes, measuring 36 Å and 8 Å in diameter, respectively.

SYNTHESIS OF THE NOVEL METAL-ORGANIC FRAMEWORKS

Introduction

2.1.1 The Modular Nature in Design and Synthesis of MOFs and The Quest to Design and Synthesize New MOFs

In the chemistry of Metal-Organic Frameworks (MOFs), the selection of metals and organic linkers plays a crucial role in determining the structure and properties of the resulting materials Commonly used linkers feature multifunctional chelating organic groups, including carboxylates, pyridine, imidazole, phosphonates, and sulfonates Furthermore, the diversity in functional groups, linker length, and bond angles significantly influences the framework structure and characteristics of the MOFs.

Over 20,000 different Metal-Organic Frameworks (MOFs) have been reported, many of which address modern challenges Despite advancements in the synthesis and applications of MOFs, there are ongoing challenges that require the development of novel MOFs with enhanced properties Key objectives include the synthesis of proton-conducting membranes that maintain high conductivity (>10^-2 S cm^-1) at medium temperatures (≥ 100 °C) and the creation of porous materials with larger pore apertures These materials can serve as scaffolds for doping active guest molecules and act as catalysts for transforming large organic substrates.

Table 3 Famous MOFs that was synthesized by commercial linkers

Over the past two decades, a significant number of metal-organic frameworks (MOFs) have been synthesized using inexpensive and commercially available linkers Our survey of the Cambridge Structural Database revealed approximately 5,092 structures, with terephthalic acid (H2BDC) identified as a crucial building block To facilitate large-scale production at a low cost, we aimed to utilize affordable commercial linkers alongside abundant earth metals like iron and cobalt to develop innovative metal-organic frameworks.

Subsequently, our newly discovered crystal structure were employed as standpoints for initially justifying the interesting properties of novel MOFs in order to be employed in relevant applications

Over the past twenty years, a significant number of metal-organic frameworks (MOFs) have been synthesized using common organic building blocks, such as 1,4-diazabicyclo[2.2.2]octane (DABCO), terephthalic acid (H2BDC), trimesic acid (H3BTC), aminoterephthalic acid (NH2-H2BDC), and 1,6-naphthalene dicarboxylic acid (H2NDC) However, many synthetic conditions remain unexplored, indicating the potential for further research by combining various organic building blocks with different metal sources to create new MOFs.

We utilized both single and mixed linker strategies, integrating cobalt and iron metal sources to develop innovative metal-organic frameworks These frameworks exhibit enhanced or novel properties, making them suitable for various applications.

Materials and Instrumentation

9,10-Anthraquinone was purchased from Merck Co Iron sulfate heptahydrate (FeSO4ã7H2O, 99% purity), copper chloride dihydrate (CuCl2ã2H2O, 99% purity),

The study utilized high-purity chemicals including cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 98%), 1,4-diazabicyclo[2.2.2]octane (DABCO, 98%), trimesic acid (H3BTC, 95%), aminoterephthalic acid (NH2-H2BDC, 99%), benzene-1,4-dicarboxylic acid (H2BDC, 98%), and 2,6-naphthalene dicarboxylic acid (H2NDC, 98%), all sourced from Sigma-Aldrich Additionally, anhydrous N,N-dimethylformamide (DMF, 99% extra dry), dichloromethane (DCM, 99% extra dry), acetic acid (CH3COOH, 99.8%), and hydrochloric acid (HCl, 35-37%) were procured from Sigma-Aldrich, Acros, and Merck, and were used without further purification.

All reagents use for catalysis study were obtained commercially from Sigma- Aldrich and Merck, and were used as received without any further purification unless otherwise noted

2.2.2 Single Crystal X-ray Diffraction (SC-XRD) and Powder X-ray Diffraction (PXRD) Data Collection

X-ray diffraction data for VNU-10, VNU-15, Fe-NH2BDC, and Fe-BTC were obtained using a Bruker D8 Venture diffractometer with a PHOTON-100 CMOS detector The analysis utilized monochromatic microfocus Cu Kα radiation (λ = 1.54178 Å) at 50 kV and 1.0 mA Before data collection, single crystals of VNU-10 and VNU-15 were cooled to 100 K using a chilled nitrogen flow managed by a Kryoflex system.

Unit cell determination for the II system was conducted using the Bruker SMART APEX II software suite, with data reduction and multi-scan spherical absorption correction applied through the SCALE interface Structures were solved using direct methods and refined via the full-matrix least-squares method in the SHELX-97 program package After identifying all framework atoms in the difference Fourier maps, the SQUEEZE routine in PLATON was utilized to eliminate scattering from highly disordered guest molecules within the pores Crystallographic data for VNU-10 and VNU-15 has been deposited in the Cambridge Crystallographic Data Centre (deposition nos 1063411 and 1443413), and CIF files are available for free on the Crystallographic Data Centre website Additionally, powder X-ray diffraction patterns (PXRD) were analyzed.

The study utilized a Bruker D8 Advance diffractometer, which was equipped with a Ni-filtered Cu Kα radiation source (λ 1.54178 Å) To enhance accuracy, the instrument included an anti-scattering shield that effectively blocked incident diffuse radiation from reaching the detector.

2.2.3 Instruments for Characterization of VNU-10, VNU-15, Fe-NH 2 BDC, Fe-BTC

Nitrogen physisorption measurements of VNU-10 were conducted using Autosorb

The IQ volumetric adsorption analyzer system was utilized to conduct physisorption measurements of VNU-15, with samples pretreated by heating under vacuum at 100 °C for 12 hours using the MasterPrep system The adsorption experiments employed ultrahigh-purity-grade N2 and He (99.999% purity) and high-grade CO2 (99.95% purity), with temperature control maintained via a water circulator Additionally, water uptake of VNU-15 was assessed using a BELSORP-aqua 3, while thermal gravimetric analysis (TGA) was performed using a TA Instruments Q.

The thermal gravimetric analysis of VNU-10 and VNU-15 was conducted using a 500 thermal gravimetric analyzer under a gas mixture of 20% O2 and 80% N2, with a temperature ramp of 5 °C min-1 Fourier transform infrared spectroscopy (FT-IR) was performed on the samples using a Bruker ALPHA FTIR spectrometer with potassium bromide pellets The cobalt content in VNU-10 was quantified through atomic absorption spectroscopy (AAS) utilizing an AA-6800 Shimadzu analyzer, while the copper content in VNU-15 was measured at the Vietnam Academy of Science and Technology using a Shimadzu AA 6200 analyzer Elemental analysis for both VNU-10 and VNU-15 was conducted at the Microanalytical Laboratory of UC Berkeley, employing a Perkin Elmer 2400 Series II combustion analyzer Additionally, crystal images of VNU-10 and VNU-15 were captured using a Nikon SMZ1000 microscope.

Material Synthesis, Single Crystal Structure Analysis and Characterization for VNU-10

Scheme 2 Synthetic scheme for crystallizing green, needle VNU-10

In a typical synthesis procedure, a mixture of 1,4-benzenedicarboxylic acid (H2BDC) (0.1 g, 0.60 mmol), 1,4-Diazabicyclo[2.2.2]octane (DABCO) (0.075 g, 0.67 mmol), and Co(NO3)2ã6H2O (0.1 g, 0.34 mmol) was dissolved in a solvent mixture of

N,N-dimethylformamide (DMF) (20 mL), CH3COOH (2 mL, 0.01 mmol), and HCl (20 μL, 0.24 μmol) The resulting solution was then dispensed equally to ten vials (10 mL) The vials were heated at 120 °C in an isothermal oven for 12 h After cooling the vials to room temperature, the solid product was removed by decanting the mother liquor and then washed with DMF (3 × 10 mL) to remove any unreacted species The DMF solvent was exchanged with dichloromethane (DCM) (3 × 10 mL) at room temperature The product was then dried at 120 °C for 4 h under vacuum, yielding green needle- shaped crystals of VNU-10 (76% based on Co(NO3)2ã6H2O) (Scheme 2) EA: Calcd for Co2C22H26O11N2 = [Co2(BDC)2(DABCO)]∙3H2O: C, 43.15; H, 4.28; N, 4.58% Found: C, 43.19; H, 4.35; N, 4.50% AAS indicated cobalt amount of 20.0%, which matched with calculated value of 20.9%

The combination of acetic acid (CH3COOH) and hydrochloric acid (HCl) with a solution containing H2BDC, DABCO, and cobalt nitrate hexahydrate (Co(NO3)2·6H2O) in dimethylformamide (DMF) results in the formation of a novel cobalt metal-organic framework (MOF) characterized by a kgm topology, designated as VNU-10.

The structure of VNU-10 features paddle wheel clusters linked by BDC 2 in two distinct ways, creating DABCO-connected kgm layers and a DABCO-connected sql layer of Co2(BDC)2(DABCO) In the representation, carbon is shown in black, oxygen in red, cobalt in light blue, nitrogen in blue, while hydrogen is omitted for clarity.

The crystal structure of VNU-10 is illustrated in DABCO-connected kgm layers, highlighting the vertexes and edges assigned to cobalt nodes and their linkages The structure features black BDC 2- components, blue DABCO elements, light blue paddle wheel cobalt nodes, and yellow linkages connecting the iron nodes.

The VNU-10 structure was elucidated through single crystal X-ray diffraction, revealing its architecture composed of cobalt paddlewheel units connected by four BDC 2- linkers in a slightly bent arrangement This unique cluster formation, along with the bent linkages, facilitates the creation of two-dimensional kagome sheets Furthermore, DABCO ligands at the apical sites of the cobalt clusters interconnect these kagome sheets, resulting in a three-dimensional framework characterized by 4.5 Å triangular and 15 Å hexagonal channels along the c axis, as well as a ~3.5 Å rectangular channel along the a and b axes.

Fig 15 Thermal ellipsoid plot of the asymmetric unit of VNU-10 with 30% probability C, black; O, red; Co, light blue; N, blue; H, white

Table 4 Crystal data and structure refinement for VNU-10

Identification code VNU-10 VNU-10 with SQUEEZE

Empirical formula C11H10NO11Co C11 H10 NO4Co

Crystal size (mm 3 ) 0.2 × 0.04 × 0.04 θ range for data collection 2.37 ° to 65.10 °

R 1 , wR 2 (all data) R 1 = 0.1010, wR 2 = 0.2533 R 1 = 0.0644, wR 2 = 0.1551

Largest diff peak and hole (eãÅ -3 ) 0.831 and -0.321 0.365 and -1.171

Although DABCO connected kgm layers has been observed for

Zn2(BDC)2(DABCO) and Ni2(BDC)2(DABCO) are notable isomers, alongside cobalt-based metal-organic frameworks (MOFs) constructed from the BDC 2- linker and cobalt paddle wheel secondary building units (SBUs) These frameworks feature DABCO-connected square-grid layers supported by DABCO pillars, which create a well-defined structural arrangement The DABCO-connected kgm layer structure, derived from BDC 2- and cobalt paddle wheel SBUs, represents the first reported Co-based version of this innovative design.

Fig 16 Green needle crystal of VNU-10 at forty zooming times

Phase purity of the bulk sample was confirmed by PXRD analysis For this analysis, PXRD patterns were calculated and generated based on structural models of

The study examines Co2(BDC)2(DABCO) for both the pillared kgm and sql topological isomers, comparing the findings with the experimental pattern of VNU-10 The results confirm that the structure of the synthesized VNU-10 aligns with the single crystal data for the pillared kgm Co2(BDC)2(DABCO) isomer, as illustrated in Figure 17.

Fig 17 The calculated PXRD pattern of VNU-10 from single crystal data (red) compared with the experimental patterns from the as-synthesized VNU-10 (orange) and Co2(BDC)2DABCOsql

2.3.3.3 FT-IR Analysis of activated VNU-10

Fig 18 FT-IR of activated VNU-10; inset: zooming with wavelength from 1450 to 1690 cm -1

FT-IR analysis of activated VNU-10 reveals the presence of the DABCO ligand and bonded carboxylate organic linkers The spectrum shows a partial overlapping peak at 1620 cm -1, attributed to the stretching of coordinated carboxylate, while sharp peaks at 1043 cm -1 correspond to the vibration of the C-N bond.

Fig 19 Thermogravimetric analysis of VNU-10 in air stream under 20% O2 and 80% N2

The framework thermal stability and architectural robustness of VNU-10 was assessed by thermal gravimetric analysis (TGA) Accordingly, as-synthesized VNU-

At low temperatures (T < 150 °C), the material exhibited a significant weight loss of 40%, primarily due to the removal of H2O and DMF solvent from its pores The activated VNU-10 showcased impressive thermal stability and structural integrity, maintaining stability up to approximately 350 °C Additionally, the Co3O4 residue observed at 500 °C was consistent with the theoretical predictions, showing a close match between the experimental (29.6 wt%) and theoretical (28.8 wt%) values derived from the structural formula.

Nitrogen isotherm measurements at 77 K confirmed the permanent porosity of VNU-10 with calculated Brunauer-Emmett-Teller and Langmuir surface areas of 2396 m 2 g -1 and 2604 m 2 g -1 , respectively (Figure 20)

Fig 20 N2 adsorption isotherm of VNU-10 at 77 K

Taking from high surface area of VNU-10, we sought to analyze the CO2, CH4 and

VNU-10 demonstrated significant N2 adsorption capabilities, showcasing high CO2 uptake at room temperature The selectivity ratios for CO2 over CH4 and N2 were determined to be 3.5 and 11.8 times, respectively, based on the initial slopes of the CO2 isotherm compared to those of CH4 and N2 under Henry’s law pressure (refer to Figures 21, 22 and Table 5).

Fig 21 CO2, CH4, N2 adsorption isotherm of VNU-10 at 273 K

Table 5 CO2, CH4, N2 uptake at 802 Torr and selectivity in adsorption of CO2 over CH4 and

Material Synthesis, Single Crystal Structure Analysis and Characterization for the

A mixture of H2BDC (60 mg, 0.36 mmol), H2NDC (60 mg, 0.27 mmol), 9,10- anthraquinone (30 mg, 0.25 mmol), FeSO4ã7H2O (60 mg, 0.143 mmol), and CuCl2ã2H2O (60 mg, 0.345 mmol) were dissolved in 10 mL DMF The solution was

The sample was sonicated for 10 minutes and divided into six borosilicate glass tubes, each containing 1.7 mL These tubes were then flame sealed under ambient conditions and placed in a preheated isothermal oven at 165 °C for four days, resulting in the formation of reddish-yellow crystals of VNU-15.

Scheme 3 Synthetic scheme for crystallizing reddish-yellow, octahedral VNU-15

The crystals underwent a thorough washing process with 10 mL of DMF six times and were then immersed in DMF for three days Following this, the solvent was exchanged with 10 mL of DCM over two days, completing the solvent exchange six times Finally, VNU-15 was activated at 100 °C, resulting in the production of 34 mg (0.051 mmol) of dried VNU-15, achieving a yield of 71.3% based on iron.

53 was formed in the absence of 9,10-anthraquinone to the reaction mixture 85 Furthermore, MIL-88 was formed without CuCl2ã2H2O added to the reaction mixture 90

EA of activated VNU-15: Calcd for Fe4C37.8H71.4N4.68O38.64S4 {[Fe4(NDC)(BDC)2DMA4.2(SO4)4]ã0.4DMF}ã10H2O: C, 29.38; H, 4.62; N, 4.25; S, 8.29% Found: C, 28.95; H, 4.64; N, 4.74; S, 8.13% Atomic absorption spectroscopy (AAS) of activated VNU-15: 0.036 wt% copper

The Assembly of H2BDC, H2NDC and anthraquinone, Fe(SO4)2ã7H2O and CuCl2ã3H2O in DMF leading to new iron based MOF with new fob topology, which was termed as VNU-15 89

The crystal structure of VNU-15 is formed by BDC 2- and NDC 2- linkers that create corrugated infinite rods of [Fe2(CO2)3(SO4)2(DMA)2]∞, extending along the a and b axes to establish a three-dimensional framework The structure is illustrated from the [110] and [001] orientations, with specific atom colors indicating their identities: Fe in orange and blue polyhedra, C in black, O in red, S in yellow, N in blue, and DMA cations in light blue, while all other hydrogen atoms are excluded for clarity.

Single crystal X-ray diffraction (SCXRD) analysis revealed that VNU-15 crystallized in the orthorhombic space group, Fddd (No 70), with unit cell parameters, a = 16.7581, b = 18.8268, and c = 38.9998 Å (Table 6 and Figure

The architecture of VNU-15 is characterized by two unique linkers, BDC 2- and NDC 2-, which connect corrugated iron infinite rod secondary building units (SBUs) These SBUs, represented as Fe2(CO2)3(SO4)2(DMA)2]∞, consist of alternating independent octahedral iron atoms Each iron atom's coordination environment features two equatorial corner-sharing vertices from μ2-O atoms of the carboxylate functionality in NDC 2-, with these μ2-O atoms promoting the structural arrangement of the infinite rod SBU.

38 corrugated fashion The coordination sphere of each iron is then completed through bridging axial sulphate ligands and bridging carboxylate functionalities from BDC 2- (Figure 23a)

The VNU-15 structure is illustrated using fob topology, showcasing the assignment of vertexes and edges for iron nodes and linkages In this representation, atom colors are designated as follows: iron (Fe) is depicted in orange and blue polyhedra, carbon (C) in black, oxygen (O) in red, sulfur (S) in yellow, nitrogen (N) in blue, and DMA cations in light blue, with all other hydrogen atoms omitted for clarity.

Two BDC 2- and one NDC 2- linkers, positioned closely with an aromatic π–π interaction distance of 3.4 Å, connect infinite rods in a perpendicular arrangement at an angle of 83.4° This configuration leads to the development of a three-dimensional architecture characterized by the fob topology.

The π–π interactions significantly contributed to the formation of the realized fob topological structure Additionally, DMA counterions were observed to align with the infinite rod SBUs, facilitated by hydrogen bonding with the axial bridging sulfate ligands.

HãããO-S distances of 1.90 - 1.96 Å) (Figure 23) Taken together, the resulting pore size of VNU-15, as calculated by PLATON, is 2.52 Å

Table 6 Crystal data and structure refinement for VNU-15

Crystal size (mm) 0.131 × 0.143 × 0.234 θ range (°) 3.6931 to 67.9142

Largest diff peak and hole (eãÅ -3 ) 0.651 and -0.401

Fig 25 Thermal ellipsoid plot of the asymmetric unit of VNU-15 with 50% probability C, black; O, red; Fe, orange; S, yellow; N, blue; H, white

Fig 26 Orange octahedral crystal of VNU-15 at forty zooming times

The phase purity of bulk VNU-15 was validated through PXRD analysis, where calculated patterns based on structural models were compared with experimental measurements The findings confirmed that both the as-synthesized and activated forms of VNU-15 aligned with single crystal data.

Fig 27 The calculated PXRD pattern of VNU-15 from single crystal data (black) compared with the experimental patterns from the as-synthesized sample (blue) and samples after activation at 100 °C (red)

2.4.3.3 FT-IR Analysis of activated VNU-15

FT-IR spectroscopy analysis revealed the presence of hydrogen-bonded DMA to sulfate ions bridging two iron atoms in the activated VNU-15 sample A broad peak between 3400-3500 cm -1 indicated N-H vibrations, while a sharp peak at 2781 cm -1 corresponded to C-H stretches of the DMA molecules Additionally, distinct vibration modes of the bridging sulfate ligands were observed, with sharp peaks at 983, 1037, 1110, and 1143 cm -1 A partial overlapping peak at 1606 cm -1 was identified as the stretching of coordinated carboxylate.

Fig 28 FT-IR spectra of activated VNU-15

Thermogravimetric analysis (TGA) was conducted to evaluate the thermal stability and structural integrity of VNU-15 in an air stream consisting of 20% O2 and 80% N2 The TGA curve provides insights into the framework's thermal stability and architectural robustness.

The sample showed a minimal weight percentage loss of less than 2 wt% when heated from room temperature to 200 °C Additionally, the residual metal oxide, primarily Fe3O4, closely matched the expected values based on its crystal structure, with experimental and theoretical weights of 24.3 wt% and 23.4 wt%, respectively.

2.4.3.5 Porosity and Gas Adsorption of VNU-15

According to PLATON, 88 the resulting pore size of VNU-15 was calculated to be 2.52 Å which is smaller than kinetic diameter of N2, hence N2 isotherm at 77 K for VNU-15 displayed no appreciable uptake

Research on the adsorption of CO2, CH4, and N2 in VNU-15 revealed a notable capacity for CO2 uptake at temperatures of 0 °C and 25 °C, while only minimal adsorption of N2 and CH4 was observed under the same conditions This discovery indicates a significantly high selective uptake for CO2.

The selectivity of CO2 over CH4 and N2 in the VNU-15 adsorbent was found to be exceptionally high, with CO2/CH4 and CO2/N2 selectivity ratios calculated at 120 and 251 times, respectively This remarkable selectivity can be attributed to the small pore structure of VNU-15, which enhances its ability to preferentially adsorb CO2.

44 pore size of the material which allowed the diffusion of CO2 while CH4 and N2 with larger kinetic diameter could not diffuse into the pore of VNU-15 (Figure 30, 31 and Table 7)

Fig 31 CO2, CH4, N2 adsorption isotherm of VNU-15at 273 K

Table 7 CO2, CH4, N2 uptake at 802 Torr and selectivity in adsorption of CO2 over CH4 and N2

2.4.3.6 Water Uptake, PXRD and FT-IR of Corresponding VNU-15 Sample

Fig 32 Water uptake of VNU-15 at 25 °C as a function of P/P 0 ranging from 8% to 80% Inset: Water uptake of VNU-15 at 25 °C with P/P 0 ranging from 8% to 62.58%

PXRD analysis of VNU-15 demonstrated that the long-range order of its structure was preserved after absorbing water up to 60% relative humidity at 25 °C The experimental diffraction pattern (red) closely matched the simulated diffraction pattern (black) derived from single crystal data for VNU-15.

Water isotherm of VNU-15 was measured by BELSORP-aqua3 VNU-15 exhibited high water uptake at medium humidity (0.08 < P/P 0 < 0.6), the water uptake of VNU-15 are 102, 110 and 128 cm 3 g -1 at P/P 0 = 0.50, 0.55 and 0.60, respectively (Figure 32)

Fig 34 FT-IR spectra of VNU-15, post H2O uptake at 60% RH, as compared with activated VNU-15

PXRD analysis of VNU-15 demonstrated that its long-range structural order was preserved after water uptake at 60% relative humidity and 25 °C Additionally, FT-IR analysis confirmed that the atomistic connectivity of VNU-15 remained intact under these conditions.

2.5.1 Synthesis of Fe-NH 2 BDC

A mixture of NH2-H2BDC (0.13 g, 0.73 mmol), FeSO4·7H2O (0.13 g, 0.49 mmol), CuCl2·2H2O (0.13 g, 0.76 mmol), and 9,10-anthraquinone (0.02 g, 0.095 mmol) was prepared in a bottle, followed by the addition of 15 ml of DMF The heterogeneous solution was sonicated for 10 minutes to achieve homogeneity and then transferred into three autoclaves The mixture was heated in an oven at 165 °C for 72 hours, resulting in the formation of yellow square plate crystals of Fe-NH2BDC, which were subsequently washed six times with DMF.

The compound Fe-NH2-BDC was synthesized by exchanging 47 with dry DCM for 24 hours, followed by heating under dynamic vacuum at room temperature for 12 hours, resulting in a yield of 0.102 g (56% based on Fe) Notably, the absence of CuCl2·2H2O or 9,10-anthraquinone led to the formation of MIL-88B, as illustrated in Scheme 4.

Scheme 4 Synthetic scheme for reddish-yellow, blocked shape crystal of Fe-NH 2 BDC

2.5.2 Crystal Structures of Fe-NH 2 BDC

The structure of Fe-NH2BDC is characterized by the connection of Fe2(CO2)4(SO4)2 clusters via NH2-BDC, forming a cohesive framework Additionally, the sql layers are interconnected through hydrogen bonds involving (CH3)2NH2+ and sulfate ligands The crystal structure of Fe-NH2BDC is depicted in these sql layers, showcasing the arrangement and interactions of the atoms, each represented by distinct colors.

C, black; O, red; Fe, orange polyhedra; S, yellow; N, blue; H of nitrogen, white; H atoms connected to carbon are omitted for clarity

The Assembly of NH2-H2BDC, anthraquinone, Fe(SO4)2ã7H2O and CuCl2ã3H2O in DMF leading to new iron MOF composed of sql layers, 89 which was termed Fe-

The Fe-NH2BDC structure was determined using single crystal X-ray diffraction (XRD), revealing a crystallization in the I4 1 /amd space group with unit cell parameters of a = 10.9546(8) Å, b = 10.9546(8) Å, and c = 39.533(3) Å This compound features iron paddle wheel secondary building units (SBUs) with two apical sites capped by bi-dentate sulfate (SO4 2-) groups The paddle wheel SBUs are interconnected by NH2-BDC linkers, forming sql layers, which are further stabilized by weak hydrogen bonds between the coordinated components.

The interaction between SO4 2- and the amino function of NH2-BDC occurs at N-HãããOS distances of 2.098 Å Additionally, two adjacent SO4 2- ions are connected by DMA, with N-HãããOS distances of 2.148 Å, resulting in the formation of a two-dimensional structure.

Fig 36 Thermal ellipsoid plot of the asymmetric unit of Fe-NH2BDC with 30% probability

C, black; O, red; Fe, orange; S, yellow; N, blue; H, white; Cu green

Table 8 Crystal data and structure refinement for Fe-NH2BDC

2.5.3 Characterization of Fe-NH 2 BDC

2.5.3.1 Microscope Image of Fe-NH 2 BDC

Fig 37 Orange blocked crystal of Fe-NH2BDC at eighty zooming times

2.5.3.2 PXRD Analysis of Fe-NH 2 BDC

Fig 38 The calculated PXRD pattern of Fe-NH2BDC from single crystal data (black) compared with the experimental patterns from the as-synthesized sample (red)

The phase purity of bulk Fe-NH2BDC was confirmed through PXRD analysis, where calculated patterns based on structural models were compared to the experimental PXRD pattern The alignment of diffraction peaks indicated that the synthesized Fe-NH2BDC structure is consistent with the single crystal model However, the structural flexibility inherent in iron-based metal-organic frameworks prevented the Fe-NH2BDC crystals from being ground into powder, as no diffraction peaks were observed in the ground sample.

51 utilizing the plate crystal for PXRD measurement leaded to mismatched intensity as a result from orientation diffraction

2.5.3.3 FT-IR Analysis of Activated Fe-NH 2 BDC

Fig 39 FT-IR of activated Fe-NH2BDC

FT-IR spectroscopy analysis of Fe-NH2BDC revealed hydrogen-bonded DMA interacting with sulfate ions, which connect two iron atoms in the activated Fe-NH2BDC sample Notably, a broad peak observed between 3400-3500 cm⁻¹, along with a distinct stretching peak, underscores these interactions.

The peaks at 2778 cm⁻¹ correspond to the N-H vibrations and C-H stretches of the DMA molecules Additionally, the vibration modes of the bridging sulfate ligands were distinctly identified, highlighting their significance in the analysis.

1617 cm -1 is assigned as stretching of coordinated carboxylate (Figure 39)

2.5.3.4 Thermogravimetric Analysis of Fe-NH 2 BDC

Activated Fe-NH2BDC exhibits thermal stability up to 200 °C in air, experiencing approximately 3% weight loss At 750 °C, the residue aligns closely with the expected amounts of oxidation derived from its structural formula, showing 27.36 wt% and 27 wt% in experimental results compared to the theoretical values of 20.5 wt% for Fe3O4 and 6.5 wt% for CuO.

Fig 40 Thermogravimetric analysis of activated Fe-NH2BDC in air stream with 20% O2 and 80% N2.

Scheme 5 Synthetic scheme for reddish-yellow, blocked shape crystal of Fe-BTC

A homogeneous solution was prepared by mixing H3BTC (0.106 g), FeSO4·7H2O (0.067 g), CuCl2·2H2O (0.06 g), and 9,10-Anthraquinone (0.02 g) in DMF (12 ml) and sonicated for 10 minutes This mixture was then heated in an oven at 165 °C for 72 hours, resulting in reddish-yellow cube crystals of Fe-BTC The crystals were exchanged with DMF overnight (6 times) and subsequently exchanged with dry DCM for 24 hours (6 times) Finally, the activated Fe-BTC was obtained by heating at 100 °C under dynamic vacuum for 12 hours, yielding 0.06 g (72% yield based on Fe).

2.6.2 Crystal Structures of Fe-BTC

The structure of Fe-BTC was identified by single crystal XRD, in which Fe-BTC crystallized in P2 1 2 1 2 space group (a = b = 19.29005, c = 12.67994 Å) (Figure 42 and

The Fe-BTC structure is composed of two distinct types of secondary building units (SBUs): a tetrahedral single iron atom SBU and an iron paddle wheel SBU These SBUs are interconnected by a triangular BTC linker, forming the mmm-a architecture, which features both small octahedron and large cuboctahedron cages that share triangular faces Sulphate ligands cap the apical sites of the iron paddle wheel SBUs, while their free chelate oxygens extend into the interior of the small octahedron cage Additionally, DMA molecules are integrated into the mmm-a network through hydrogen bonding with SO4²⁻ moieties, positioned at the shared face of the octahedron-cuboctahedron cage.

The crystal structure of Fe-BTC is comprised of BTC 3-linkers and two distinct secondary building units (SBUs): a tetrahedral single iron atom SBU and an iron paddle wheel SBU When viewed along the [001] plane, the mmm-a topology of Fe-BTC reveals its intricate arrangement In this structure, iron atoms are represented by blue polyhedra, carbon atoms by black, oxygen by red, sulfur by yellow, nitrogen by blue, and DMA cations by light green, with hydrogen atoms omitted for clarity.

Table 9 Crystal data and structure refinement for Fe-BTC

Fig 42 Thermal ellipsoid plot of the asymmetric unit of Fe-BTC with 30% probability C, black; O, red; Fe, orange; S, yellow; N, blue; H, white

2.6.3.1 PXRD Analysis of Fe-BTC

Fig 43 The calculated PXRD pattern of Fe-BTC from single crystal data (black) compared with experimental patterns from the as-synthesized sample (red)

The phase purity of bulk Fe-BTC was validated through PXRD analysis, where calculated patterns based on structural models of Fe-BTC were compared to the experimental PXRD pattern The alignment of diffraction peaks in the PXRD pattern confirmed the structural integrity of Fe-BTC.

The synthesized Fe-BTC exhibits a structure consistent with the single crystal model, as shown in Figure 43 The inherent structural flexibility of Fe-BTC, typical of iron-based metal-organic frameworks, prevents the crystal from being ground into powder, resulting in no diffraction peaks from the ground sample Consequently, using the cube crystal for powder X-ray diffraction (PXRD) leads to mismatched intensity due to the orientation diffraction of the crystals.

2.6.3.2 Thermogravimetric Analysis of Fe-BTC

Activated Fe-BTC demonstrated to be stable up to 200 °C in air (20% O2 and 80%

N2) with around 2.2% weight lost and residue at 800 °C matched with amount of Fe3O4 derived from structure formula (27.35 and 23.18 wt% for the experimental and theoretical, respectively) (Figure 44)

Fig 44 Thermogravimetric analysis of activated Fe-BTC in air stream with 20% O2 and 80% N2

APPLICATIONS OF VNU-10 AND VNU-15

NEW TOPOLOGICAL Co 2 (BDC) 2 (DABCO) AS HIGHLY ACTIVE

3.1.1 The Quest for Large Pore Window (above 15 Å) and High Surface Area (above 2600 m 2 g -1 ) MOFs as Catalyst for Large Substrate Conversions

Metal-Organic Frameworks (MOFs) are increasingly utilized as platforms for the catalytic conversion of diverse organic compounds Typically, metal clusters serve as the active catalytic sites within MOFs, while these sites can also be anchored to the MOF framework through linkers before self-assembly, modified post-synthesis, or immobilized as guest molecules during self-assembly Although small molecules can easily access the pores of MOFs for reactions, the relatively small pore sizes restrict the catalytic conversion of larger organic molecules, which are often sought after in the pharmaceutical industry, as substrates struggle to effectively reach the active sites located within the MOF structure.

The pore size of Metal-Organic Frameworks (MOFs) can be increased by using longer linkers to create larger pore structures with higher surface areas However, this approach has drawbacks, including the complexity of organic synthesis and the tendency for the resulting structures to interpenetrate, which significantly reduces pore size Additionally, larger pore sizes may lead to structural collapse under activated conditions.

Large pore metal-organic frameworks (MOFs) with a high surface area exceeding 14 Å and 2600 m²/g are uncommon, particularly those constructed from affordable and commercially available linkers like 1,4-benzenedicarboxylic acid (H2BDC) and 1,4-Diazabicyclo[2.2.2]octane (DABCO) Notably, only a select few MOFs meet these criteria, including examples such as MIL-101, MIL-68, and MIL-100.

Zn2(BDC)2DABCO kgm, 102and Ni2(BDC)2DABCO kgm , 103

Hence, the quest for designing and synthesizing large pore window and high surface area MOFs from cheap linker and clarifying the advantages of large pore size MOF

58 compared with small pore size or nonporous material on specific catalytic reactions for large substrates were raised

3.1.2 Direct Amination of Azoles under Mild Reaction Conditions

Molecules with aryl- and heteroarylamine structures are prevalent in biologically active natural products and functional materials Traditional methods for synthesizing these molecules often rely on nitrenoid chemistry, but recent advancements in metal-mediated C-N bond formation from C-H bonds have gained attention, particularly since the work of Buchwald and Hartwig Typically, nitrogen moieties are introduced using protected amines or hydroxylamine derivatives, while simple amine coupling in direct C-N bond formation remains underexplored Palladium and rhodium are commonly used as catalysts, primarily for intramolecular reactions The Yu group pioneered copper-catalyzed directed amination of arene C-H bonds, with subsequent studies demonstrating ortho amination of 2-phenylpyridine derivatives and deprotonative amination of thiazoles and oxazoles Additionally, silver-mediated benzoxazole amination has been reported, along with the catalytic activity of cobalt and manganese salts Recent work by Chang and colleagues has shown the oxidative amination of various azoles under acidic conditions with peroxide oxidants Despite advancements in homogeneous catalysts, there is still a need for more economically and environmentally efficient heterogeneous catalytic systems for these transformations.

Scheme 6 Plausible mechanism of direct amination of azoles 123

Cobalt-based metal-organic frameworks (MOFs) with large pore sizes and high surface areas serve as highly effective heterogeneous catalysts for the chemical transformation of large organic molecules, a process that smaller pore size MOFs cannot achieve.

Previously revealing in chapter 2, novel cobalt MOF, named

Co2(BDC)2(DABCO) kgm (VNU-10) has been successfully synthesized, featuring a large pore aperture and an impressive surface area of 2604 m²/g with a pore size of 14 Å This unique structure makes it an ideal candidate for the chemical transformation of large organic molecules.

We have synthesized Co2(BDC)2(DABCO) sql, characterized by an 8 Å pore size and a surface area of 1600 m²/g This material shares the same structural components—metal clusters, linkers, and coordinated bond types—as VNU-10, but features a distinct arrangement that results in a different structure with a smaller pore diameter.

Scheme 7 Amination of Benzoxazole through N-H/CH bonds activation using VNU-10 as catalyst

A comparative analysis of the catalytic performance of VNU-10, Co2(BDC)2(DABCO) sql isomer, other metal-organic frameworks (MOFs), zeolites, and oxides is essential for the transformation of large organic substrates, specifically in the direct amination of azoles via N-H/C-H bonds under mild conditions This study aims to highlight the significance of MOFs with large channel diameters (greater than 14 Å) as highly effective heterogeneous catalysts for such transformations, as smaller pore size MOFs have been found inadequate for facilitating the reaction.

Gas chromatographic analyses were conducted using a Shimadzu GC 2010-Plus, featuring a flame ionization detector and an SPB-5 column (30 m length, 0.25 mm inner diameter, and 0.25 μm film thickness) The temperature program for the GC analysis initially held samples at 120 °C for 1 minute before gradually increasing the temperature.

The experimental procedure involved heating samples to 180 °C at a rate of 50 °C/min, maintaining this temperature for 1 minute, followed by a ramp to 280 °C at the same rate and holding for 3 minutes The inlet and detector temperatures were consistently set at 280 °C To calculate reaction conversions, diphenyl ether served as the internal standard Gas chromatography-mass spectrometry (GC-MS) analyses were conducted using a Hewlett Packard GC system.

The analysis was conducted using an MS 5972 equipped with an RTX-5MS column, measuring 30 m in length, 0.25 mm in inner diameter, and 0.5 μm in film thickness The gas chromatography-mass spectrometry (GC-MS) temperature program ramped from 60 to 280 °C at a rate of 10 °C/min, maintaining 280 °C for 2 minutes, with the inlet temperature fixed at 280 °C Mass spectrometry (MS) data were compared against the NIST library spectra, while 1H and 13C NMR spectra were obtained in CDCl3 using TMS as a reference.

61 internal standard on a Bruker NMR spectrometer at 500 MHz and 125 MHz, respectively

Using data from the GC diagram, which includes peak position and peak area, the conversion of the reaction based on the product was calculated using a specific formula.

 C(t) : The conversion of reaction based on product at the time t

 S product : The peak’s area of product

 S internal standard : The peak’s area of internal standard

 t : The time of each aliquot was withdrawn

 tc : The time when conversion achieved 100 %

S(internal standard)(tc) ratio was figured out when the proceeding ratio of

In a 25 mL round bottom flask, a mixture of benzoxazole (0.101 mL, 1 mmol), acetic acid (0.114 mL, 2 mmol), and diphenyl ether (0.15 mL, 0.95 mmol) as an internal standard was prepared in acetonitrile (5 mL).

The Co2(BDC)2(DABCO) (VNU-10) catalyst, weighing 0.0143 g and constituting 5 mol%, was calculated based on the cobalt/benzoxazole molar ratio Initially, the catalyst was magnetically stirred for 3 minutes to ensure complete dispersion within the liquid phase Following this, piperidine and tert-butyl hydroperoxide (TBHP) were introduced to the mixture, which was continuously stirred at room temperature for 1 hour to facilitate the reaction.

62 conversion was monitored by withdrawing aliquots from the reaction mixture at different time intervals, followed by quenching with aqueous KOH solution (5% (w/w),

The organic components were extracted using 2 mL of ethyl acetate, dried with anhydrous Na2SO4, and analyzed via GC with diphenyl ether as a reference The identity of the product was confirmed through GC-MS, 1H-NMR, and 13C-NMR techniques In recycling studies, the catalyst was separated by centrifugation, washed, and heated with DMF at 100 °C for 2 hours The recovered VNU-10 catalyst was activated under vacuum at room temperature for 4 hours and reused in subsequent reactions under the same conditions For the leaching test, the catalytic reaction was paused after 5 minutes, analyzed by GC, and the solid catalyst was removed by centrifugation, followed by an additional 55 minutes of stirring to monitor any further reaction progress.

Ni2(BDC)2(DABCO) sql , Cu2(BDC)2(DABCO) sql , Co2(BDC)2(DABCO) sql ,Co-ZIF-

67 were synthesized based on previous reported procedure 23,124–126

3.1.6 Investigations on VNU-10 Catalytic Performance for Direct Oxidative Amination of Benzoxazole with Piperidine

3.1.6.1 Conditions Screening for Direct Oxidative Amination of Benzoxazole with Piperidine Using Heterogeneous VNU-10

HIGH PROTON CONDUCTIVITY AT LOW RELATIVE HUMIDITY IN AN

3.2.1 Introduction of Hydrogen Fuel Cell, Impedance and Nyquist Plot of Impedance

A hydrogen fuel cell is a device that transforms chemical potential energy into electrical energy, utilizing hydrogen gas (H2) and oxygen gas (O2) as fuel The reaction within a Proton Exchange Membrane (PEM) cell produces water, electricity, and heat as its byproducts.

Fig 63 Typical structure of Hydrogen fuel cell

Typically, the fuel cell were assembled by three parts which included anode, Proton conducting membrane and cathode (Figure 62)

The anode features a thin film of high surface area carbon combined with a catalyst that effectively splits H2 into hydrogen atoms These hydrogen atoms are then oxidized to produce H+ ions, releasing electrons that travel through the activated carbon layer to the external circuit, ultimately reaching the cathode to generate electric current.

A proton-conducting membrane is placed between the anode and cathode in hydrogen fuel cells, enabling the movement of H+ ions from the anode to the cathode while acting as an insulator for electrons This design compels electrons to travel through an external circuit to reach the cathode To maintain high proton conductivity, the membrane is typically kept hydrated.

The cathode is made from a catalyst applied to porous carbon, facilitating the distribution of O2 and the flow of electrons from an external circuit This process enables the combination of H+ ions to produce H2O as the final product.

A catalyst is a specialized material that enhances the reaction between oxygen and hydrogen, typically composed of platinum nanoparticles thinly layered on porous carbon Its rough and porous structure maximizes the surface area of platinum, allowing for optimal exposure to hydrogen or oxygen The platinum-coated side of the catalyst is oriented towards the proton exchange membrane (PEM).

Fuel cells convert chemical potential energy directly into electrical energy, bypassing the thermal bottleneck associated with the second law of thermodynamics This direct conversion makes fuel cells inherently more efficient than combustion engines, which first transform chemical energy into heat before generating mechanical work.

Fuel cell vehicles produce only water vapor and minimal heat as direct emissions, representing a significant advancement over the greenhouse gas emissions from internal combustion engines Additionally, the absence of moving parts in fuel cells enhances their reliability compared to conventional engines.

Hydrogen can be produced in an environmentally friendly manner, while oil extraction and refining is very damaging

3.3.1.2 Definition of Impedance and Nyquist Plot of Impedance

Electrical impedance (Z) refers to the resistance encountered by a circuit when subjected to alternating current (AC) voltage It is defined as the ratio of voltage to current for a specific complex exponential at a given frequency (ω) in Hertz (Hz).

Impedance is a complex number measured in ohms (Ω), similar to resistance It is defined by the equation Z = Z’(ω) – jZ’’(ω), where Z’ represents the real resistance and Z’’ denotes the imaginary resistance.

Fig 64 Typical Nyquist plot and an equivalent circuit used for fitting Schematic representations: R c /R m , resistor; W, Warburg diffusion element; C, capacitor

Nyquist plots is the plot of Z’ (Ox axis) and Z’’ (Ox axis) into Cartesian coordinate system at particular frequency ω (Hz)

The Equivalent Circuit is an electrical circuit utilized to generate Nyquist plots that align with experimental results This circuit is instrumental in determining the resistance of proton-conducting membranes, as illustrated in Figure 63.

3.2.2 The Quest of Proton Conducting Membrane that Maintain High Conductivity at High Temperature and Low Humidity

The advancement of new electrolyte materials for proton exchange membrane fuel cells is gaining significant interest due to the demand for alternative energy solutions Traditional materials like fully hydrated Nafion can achieve impressive proton conductivities of 1 × 10^-1 S/cm.

To achieve a proton conductivity of S cm -1 at 80 °C, materials must be maintained in a high humidity environment (98% relative humidity), which presents significant challenges, including high costs and the risk of flooding the cathode, ultimately affecting fuel cell performance Additionally, while high operating temperatures can reduce CO poisoning in Pt-based catalysts and enhance efficiency, they also lead to decreased conductivities due to electrolyte dehydration Consequently, there is a strong demand for the development of innovative electrolyte materials that can sustain ultrahigh proton conductivity at elevated temperatures and low relative humidity.

Metal-organic frameworks (MOFs) are being investigated as promising electrolyte materials due to their modular design and synthesis capabilities This allows for the customization of backbone components, such as inorganic and organic secondary building units, to meet specific application needs Research has focused on enhancing MOFs as proton conductors by integrating proton transfer agents within their pores, functionalizing metal sites, adjusting pore acidity with specific functional groups, and modifying defect sites These advancements have resulted in notable improvements in proton conductivities, reaching approximately 10^-2 S cm^-1, although they necessitate high relative humidity levels of 90% or more.

85 conductivity under anhydrous conditions (T ≥ 100 °C) in MOFs has reached ultrahigh levels (10 -2 S cm -1 ), albeit in a limited number of reports.66,76,77,79

Achieving high proton conductivity at elevated temperature (T ≥ 95 ºC) under low humidity (RH ≤ 60%)

As the synthesis and full characterization of a novel iron-based MOFs (VNU-

The compound VNU-15, formulated as Fe4(BDC)2(NDC)(SO4)4(Me2NH2)4, features a unique three-dimensional fob topology and introduces new iron rod-shaped secondary building units (SBUs), marking a significant advancement in metal-organic framework (MOF) chemistry.

VNU-15 demonstrates ultrahigh conductivity at low relative humidity (RH ≤ 60%) due to the dense occupation of sulfate ligands around iron secondary building units (SBUs) The ordered dimethylammonium (DMA) cations occupy the pore channels of VNU-15 through hydrogen bonding, creating a feasible conduction pathway.

3.2.5 Method for Proton Conductivity Measurement

3.2.5.1 Preparation of Pelletized VNU-15 and Proton Conductivity Measurement

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