Introduction of metal-organic frameworks (MOFs)
1.1.1 Evolution of coordinated porous materials
The concept of combining metal ions with organic linkers, first introduced in 1959 with the coordination polymer {Cu(NO3)[NC(CH2)4CN]2}n by Kinoshita et al., paved the way for the development of various similar materials Subsequent notable compounds include {Cu[(NCN)2(CH3)2C6H2]2}n (1986) by Aumuller et al and [Cu(NO3)(4,4’-bipyridin)2]n (1995) by Yaghi et al These materials typically exhibit weak coordination between their organic and inorganic components, resulting in limited open spaces within their frameworks due to counter ion occupation To address this limitation, Yaghi et al developed a robust framework known as MOF-2 in 1998, utilizing benzenedicarboxylate linkers and Zn clusters This structure, characterized by four deprotonated carboxylic acid groups coordinating with two Zn ions to form a square planar cluster of [Zn2(CO2)4], demonstrated stability even after activation, as confirmed by 77K N2 adsorption isotherms The success of MOF-2 sparked a surge in research within porous materials science, leading to the discovery of thousands of MOF compounds By varying inorganic clusters, organic linker coordinates, and lengths, as well as employing techniques like linker mixing and functional group decoration, the potential for MOF production appears limitless.
1.1.2 Design and synthesis of metal-organic frameworks
Metal-organic frameworks (MOFs) are crystalline materials formed by the combination of inorganic metal clusters and organic linkers The inorganic component consists of metal ions, which can exist as single ions, clusters, or infinite arrangements, allowing for a diverse range of metal combinations with varying coordination properties These metal clusters are interconnected by multifunctional organic linkers, resulting in a wide variety of extended framework structures characteristic of MOFs.
Figure 1.1 The constructing of metal-organic framework yields by the combination of metal clusters and organic linkers – the construction of MOF-5
Research on the basic combinations and complex organic multi-topic linkers, along with diverse clusters of metals from the periodic table, has resulted in the creation and study of thousands of compounds annually, tailored to specific applications.
From 2000 to 2015, the publication status of Metal-Organic Frameworks (MOFs) has evolved significantly across various applications Research has focused on MOFs as luminescent materials, gas storage solutions, magnetic materials, drug delivery systems, and catalysts This period marks a diverse exploration of MOFs, highlighting their versatility and potential in multiple fields.
The careful selection of metal clusters and organic linkers enables the construction of materials with desirable properties, including high porosity, large pore apertures, and enhanced thermal and chemical stability These attributes make Metal-Organic Frameworks (MOFs) suitable for various applications such as gas adsorption, gas separation, catalysis, proton and electron conductivity, and energy conversion.
8 optimized structures can be the expending in linker size, incorporating chemical functional groups before or after MOFs formation (pre- or post-synthesis of MOFs), mixing different kinds of linkers
Figure 1.3 The crystal structures of MOF-5, UiO-66, MIL-101, and MIL-53 constructed by benzene dicarboxylate linker (BDC) with Zn4O(-CO2)6, Zr6O4(OH)4(-
CO2)12, Cr3O(-CO2)6, and [AlO(-CO2)2] clusters, respectively[6,7,18,39,88]
The combination of multi-topic linkers with various metal clusters can lead to a variety of structures with distinct topologies For instance, when using di-topic carboxylate linkers, the benzene dicarboxylate (BDC) linker interacts with a 4-coordinate Zn4O cluster to form MOF-5, which has a pcu network Additionally, it can also combine with a 6-coordinate Cr3O cluster.
12-coordinates-Zr6O4(OH)4 or Al-rod cluster give mtn (MIL-101), fcu (UiO-66), or sra (MIL-53) net, respectively (Figure 1.3).[6,7,18,39,88]
Modifying the shape of the multi-topic linkers
Figure 1.4 The crystal structures of Cu-BTC, HKUST-1, NU-110, and MOF-11 constructed by square planar Cu2(-CO2)4 cluster with BDC, BTC, ATC, and BHEHPI linkers, respectively[17,37,43a]
Using a chosen cluster, various metal-organic framework (MOF) structures are created by incorporating multiple types of linkers The resulting topological networks of these structures are influenced by both the number of linker types and the geometric shapes of the organic linkers used.
Ten combinations of four-coordinated paddle-wheel Cu2 clusters with different linkers—BDC (di-topic), BTC (tri-topic, regular triangular), ATC (tetra-topic, regular tetrahedron), and BHEHPI (hexa-topic, regular triple of di-topic)—yield various metal-organic frameworks (MOFs) such as MOF-2 (sql), HKUST-1 or MOF-199 (tbo), MOF-11 (pts), and NU-110 (ntt/rht net), as illustrated in Figure 1.4.
Figure 1.5 Crystal structures of MOF-74 DOT linker is joined by a metal oxide SBU to make the 3D MOF-74 with one-dimensional hexagonal channels (A) Atom colors:
M, blue polyhedra; C, grey; O, red; all H atoms are omitted for clarity Chemical structure of organic linkers used in the synthesis of a series of nine IRMOFs (B) [26]
The extension of organic linkers has led to the creation of new metal-organic frameworks (MOFs) with isoreticular structures, sharing the same topological net To enhance properties for applications like gas storage and catalysis, MOFs are designed by increasing the length of organic clusters, resulting in larger pore sizes with selected topologies However, this enlargement often leads to net penetration, where multiple frameworks are formed, decreasing pore size and altering the topology, as seen in the pcu net of MOF-5 or fcu net of UiO-66 To mitigate this issue, complex linkers with numerous uncoordinated groups have been employed, exemplified by the IRMOF-1 series Notably, certain MOF structures, such as the IRMOF-74 series with the etb net, demonstrate that net penetration can be avoided regardless of linker length.
The combination of various linkers, both in terms of length and number of topics, with metal clusters, or the use of multiple types of clusters with organic linkers, serves as an innovative approach to designing new Metal-Organic Frameworks (MOFs) An example is MOF-210, which is formed from a Zn4O cluster combined with tri-topic and di-topic linkers, recognized as one of the most porous materials until 2012.
Figure 1.6 Crystal structures of MOF-205 (A) and MOF-210 The yellow and orange spheres represent the empty spaces inside the pores Atom colors: Zn, blue polyhedra;
C, black; O, red; all H atoms are omitted for clarity [45]
Decorating the functional groups incorporated into the walls of frameworks
Functional groups are strategically incorporated into the walls of Metal-Organic Frameworks (MOFs) to optimize them for specific applications This process typically involves functionalizing organic linkers or attaching external ligands to uncoordinated sites on metal clusters For instance, to improve CO2 adsorption selectivity in MOF-5, various BDC linkers with distinct functional groups were utilized, resulting in a series of multivariate MOF-5 (MTV-MOF-5) materials This demonstrates the limitless potential for tailoring and designing MOF materials.
The decoration of Metal-Organic Frameworks (MOFs) can be accomplished either through pre-synthesis functionalization of reactants or post-synthesis modifications of the frameworks To improve CO2 capture efficiency, alkyl amine functional groups were incorporated during the pre-synthesis of the DH3PhDC linker, resulting in the creation of amine Mg-IRMOF-74-III This modification enhances CO2/N2 adsorption selectivity due to the formation of carbamate interactions between CO2 molecules and amine groups within the pore walls Additionally, a ditopic amine, N,N′-dimethylethylenediamine, was post-synthesis introduced to the open-metal sites of an expanded MOF-74, further increasing its CO2 adsorption capacity.
Methods to modify the structures and properties of metal-organic frameworks (MOFs) include ligand and metal exchange, achieved through post-synthesis modification This process involves immersing the MOF framework in a high-concentration solution of chosen organic ligands or metals.
Post-synthesis modification can be achieved through three main approaches: addition, exchange, and encapsulation In this process, metal clusters are depicted as yellow cubes, while organic linkers are represented by blue rods The addition of metal particles is illustrated by purple spheres, with only one organic linker undergoing transformation for clarity.
1.1.2.2 General synthetic procedure of MOFs
The synthesis of Metal-Organic Frameworks (MOFs) typically involves mixing metal salts with multi-topic organic linkers in polar solvents like alcohol or N,N-dimethylformamide (DMF) pH levels are adjusted using amines or organic acids, such as formic, acetic, or benzoic acid, which can act as modulators during MOF formation The reaction is commonly conducted using the solvothermal method, with temperatures ranging from 60 to 220 °C, although some MOFs can also be synthesized at room temperature.
Some application of metal-organic frameworks
Metal-Organic Frameworks (MOFs) exhibit diverse structures that make them ideal for various applications, including gas storage, separation, catalysis, conductivity, chemical sensing, drug delivery, and renewable energy Their high porosity and exceptional thermal and chemical stability set them apart from traditional porous materials like carbon, silica gel, and zeolites Additionally, MOFs can be engineered to modify their internal surface properties, enhancing their adaptability for a wide range of uses.
Table 1.1 Some physical properties of common adsorbents for adsorption systems
Properties ACs/CFs Silica gel Zeolite MOFs
Solid type amorphous amorphous crystal crystal
Structural design unavailable unavailable limited unlimited/desi gnable
Surface non-polar polar polar vary
Thermo/chemical stability stable stable stable vary
This article emphasizes the design of metal-organic frameworks (MOFs) with tailored properties for applications in adsorption-driven heat pump (ADHP) systems, toxic gas sensors, and methane storage These areas have not been extensively explored by Vietnamese researchers to date.
1.2.1 Metal-organic frameworks for ADHPs system
In 2016, the residential sector accounted for approximately 38% of total U.S electricity sales, with significant energy usage dedicated to space cooling (14%), heating (23%), refrigeration (6%), and hot water production (13%) As economic growth and quality of life improve, particularly in developing countries, residential energy consumption continues to rise, leading to increased total energy demand and reliance on fossil fuels like coal, oil, and natural gas for electricity generation Addressing primary energy consumption for heating and cooling is essential for reducing fossil fuel use and minimizing associated CO2 emissions.
Space cooling and refrigeration account for a significant portion of residential electricity consumption, primarily relying on conventional vapor-compression systems (VCCs) Despite their widespread use, VCCs contribute to high energy costs and environmental issues due to their reliance on refrigerants like chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), and hydrochlorofluorocarbons (HCFCs) The operation of these systems involves compressing low boiling-point gases into liquids, which requires substantial energy Furthermore, these refrigerants are known to be greenhouse gases and pose a risk of ozone depletion when released into the atmosphere.
Conventional vapor-compression systems (VCCs), which utilize CFC gases, operate through a series of processes depicted in Figure 1.8 (A) In contrast, adsorption-driven heat pumps (AHDPs) employ working fluids such as water or methanol to facilitate their functioning, as illustrated in Figure 1.8 (B).
Adsorption-driven heat pumps (ADHPs) are emerging as modern alternatives to traditional vapor compression cycles (VCCs), which are being phased out due to environmental concerns Unlike VCCs, ADHPs operate through two distinct cycles: the working cycle (charging) and the regeneration cycle During the working cycle, the working fluid evaporates at low pressure, absorbing energy (Qev) from the environment to provide cooling Vapor molecules, known as adsorbates, are then adsorbed by a dry sorption material (adsorbent), releasing adsorption heat (Qads) Once the adsorbent reaches saturation, the regeneration cycle commences, where adsorbates are desorbed using energy from a heat source (Qdes) The adsorbates then condense at medium temperatures, releasing condensation heat (Qcon) This system utilizes high latent heat of vaporization fluids such as water, methanol, ethanol, or ammonia, which have a significantly lower environmental impact compared to CFC refrigerants.
ADHPs operate at a low driving temperature of less than 100 °C during the regeneration process, allowing for the effective use of available solar energy and industrial waste heat as the primary heat source This capability suggests that the ADHPs system can function independently of electric energy, enhancing its sustainability and efficiency.
Table 1.2 Physical properties of some working fluids for cooling systems
Working Fluid Boiling point at 1 bar (C)
Saturated vapor pressure at 25 C (kPa)
Heat of vaporization (kJ/kg)
The performance of Adsorption Heat Pump Systems (ADHPs) is directly linked to the transferable energy during each cycle, which relies on the amount of heat transferred and the latent heat of the working fluid Promising working fluids for ADHPs include water, methanol, ethanol, and ammonia, noted for their high evaporation heat Among these, water stands out as the most effective choice due to its highest latent heat value, non-toxicity, environmental safety, and availability Consequently, water has been extensively researched in conjunction with various hydrophilic porous materials such as silica gel, zeolites, and Metal-Organic Frameworks (MOFs) However, a significant drawback of using water is its very low saturated pressure, approximately 3.16 kPa.
25 C) decreases transferred mass of working fluid by adsorption; and its relatively high frozen point (273.16 K) causes the application of water for the devices at below
0 C temperature is impossible [2,22,104] In additional, the strong hydrogen bonds formation with polar adsorbents led to the increasing of desorption temperature (>
Operating at 200 °C and extending the working cycle to several hours leads to a reduced coefficient of performance (COP), which is the ratio of heat generated to the energy supplied Although ammonia has a lower latent heat and is less thermodynamically efficient than water, its high vapor pressure allows for quicker mass transfer and shorter cycle times The metal-chloride/ammonia working pair is commonly utilized in industrial ice-making and cooling systems based on chemical adsorption, while ammonia is also paired with activated carbon in physical adsorption systems However, ammonia-based systems require an internal compressed pump to convert ammonia to its liquid phase, which increases energy input and lowers the COP Additionally, ammonia's high toxicity and corrosive nature pose challenges for adsorbent selection and system design In contrast, methanol and ethanol, which exhibit intermediate properties between water and ammonia, have gained significant attention in recent studies.
Methanol is favored over ethanol as a working fluid due to its smaller molecule size and higher heat of evaporation The combination of activated carbon and methanol shows significant potential for cooling systems, particularly those requiring a temperature lift greater than 30 degrees.
Activated carbon fiber (ACF) has gained significant attention for adsorption-driven heat pumps (ADHP) due to its high porosity, small pore diameter, and moderate interaction with methanol molecules, facilitating rapid adsorption and desorption processes Operating at a desorption temperature (Td) around 100 °C, ACF presents an efficient solution for enhancing the performance of ADHP systems.
20) showed the highest methanol adsorption capacity of 0.797 kg kg -1 among activated carbon materials [3] However, the amorphous nature of activated carbon
21 causes the difficulty in structural characterization That also limits the modification of their structure to extend the porosity and functionalize the internal wall of materials
Metal-organic frameworks (MOFs), known for their crystalline structure and high porosity, are being explored for use in advanced hydrogen production systems (ADHPs) with methanol Among these, MIL-101(Cr) stands out with a remarkable methanol adsorption capacity of up to 1.15 g g^-1, attributed to its high porosity of over 3000 m^2 g^-1 and polar structure featuring open metal sites Other hydrophilic MOFs, such as HKUST-1 and Zn(BDC)(TED)0.5, also demonstrate superior methanol adsorption capacities (0.60 g g^-1 and 0.50 g g^-1, respectively) compared to non-hydrophilic counterparts like ZIF-8 and MIL-53(Cr) However, research on methanol adsorption in MOFs remains limited, particularly lacking comprehensive adsorption/desorption cycling experiments to assess the efficiency of the MOF/methanol system.
This study presents a novel series of highly porous MOF-74 synthesized with new linkers featuring polar functional groups A thermodynamic assessment of these metal-organic frameworks (MOFs) is conducted to evaluate their working capacity and coefficient of performance (COP) Additionally, adsorption and desorption multi-cycling experiments are performed using MOFs and methanol working pairs to further assess their practical applications.
1.2.2 Metal-organic framework for gas sensors production
To detect the toxic, explosive or harmful gas such as chlorine, carbon monoxide, hydrogen, nitrogen dioxide, etc., n-type and p-type oxide semiconductors
The interaction between semiconductors and oxidizing or reducing gases involves the transfer of electrons from the surface layer of the semiconductor, leading to measurable changes in resistance When reducing gases such as H2 or H2S come into contact with n-type oxide semiconductors, the ionized oxygen anions on the surface oxidize the gas molecules, resulting in an increase of electrons in the core-shell structure and a decrease in resistance Conversely, exposure of p-type oxide semiconductors to reducing gases leads to electron injection, which reduces the hole-accumulation layer and increases resistance.
Until 2013, over 90% of research on oxide semiconductors for gas sensor production focused on n-type materials due to their superior response compared to p-type semiconductors Prominent n-type oxides like SnO2, ZnO, WO3, and Fe2O3 have been extensively studied for their chemi-resistive properties In contrast, p-type oxides such as NiO, CuO, Cr2O3, and Co3O4 received less attention, despite their unique advantages, including low humidity dependence and rapid recovery speed Recent approaches have involved integrating p-type particles or nanoclusters into n-type semiconductor components to improve sensor performance by reducing recovery time and humidity sensitivity while enhancing response and selectivity.
Figure 1.9 Core-shell structure of n-type (a) and p-type (b) oxide semiconductors [72]
Scope of The Dissertation
This dissertation presents the development of two new series of metal-organic frameworks (MOFs), designated as M-VNU-74-I and M-VNU-74-II, where M represents magnesium (Mg), nickel (Ni), or cobalt (Co) and VNU stands for Vietnam National University The study details the synthesis and comprehensive characterization of six novel compounds within these series.
M2(TDA) [M-VNU-74-I; M = Mg, Ni, or Co, H4TDA = 4,4′-[1,4-phenylenebis- (carbonylimino)]bis(2-hydroxybenzoic acid)], and M2(ODA) [M-VNU-74-II; M = Mg, Ni, or Co; H4ODA = 4,4′- oxalylbis(imino)]bis(2- hydroxybenzoic acid)]
The MOF design strategy focused on incorporating polar amide functionalities within pore walls to enhance methanol storage capacity Structural analysis revealed that both MVNU-74-I and -II share the same topology, with variations in pore metrics achieved by modifying the organic linker This facilitated an assessment of pore size, framework stability, and functionalization concerning methanol adsorption Methanol isotherm measurements indicated significant uptake in both series, but M-VNU-74-I exhibited structural instability during adsorption Consequently, the M-VNU-74-II series was prioritized due to its superior structural stability and smaller pore diameter, achieving the highest methanol uptake of over 0.75 g g -1 at low vapor pressure, along with a deliverable methanol amount of approximately 250 cm 3 cm -3, critical for thermal batteries Dynamic adsorption experiments on Mg-VNU-74-II under varying methanol vapor pressures confirmed consistent mass transfer capacity over three days, highlighting the material's regeneration ease.
29 between each cycle was accomplished under mild conditions (methanol/N2 flow at
Despite the promising potential of metal-organic frameworks (MOFs) for gas sensing, there is a lack of comprehensive studies on their sensing capabilities This research focuses on the preparation of Mg-VNU-74-I and Mg-VNU-74-II as resistance-based gas sensors, eliminating the need for metal oxide carrying agents The investigation systematically explores the gas sensing applications of these Mg-incorporated MOFs, which serve as the active sensing materials without any additional metal oxide layers The performance of gas sensors made from Mg-VNU-74 demonstrates their significant potential for environmental monitoring of toxic gases.
The innovative MOF-700, created by combining the H4ODA linker with zirconium ions, features amide moieties that allow for post-synthetic metalation By metalating MOF-700 with various Cu(II) salts, researchers effectively tailored the internal pore space, enhanced adsorption sites, and increased crystal density Notably, the metalation with Cu(NO3)2 significantly contributed to these improvements.
701) was found to achieve high volumetric methane storage working capacity of 211 cm 3 (STP) cm -3 at 298 K and 5.8-80 bar
Materials and General Procedures
4-Aminosalicylic acid (99% purity), terephthaloyl chloride (≥99% purity), magnesium nitrate hexahydrate (99% purity), nickel(II) nitrate hexahydrate, cobalt(II) nitrate hexahydrate (≥98.0% purity), N,N-dimethylformamide (DMF,
The chemicals used in the study include N-methyl-2-pyrrolidone (NMP, ≥99.5% purity) and oxalyl chloride (98% purity) sourced from Aldrich Chemical Company, as well as dichloromethane (99.8% extra dry grade) and tetrahydrofuran (99.9% extra dry grade) from Acros Organic Anhydrous methanol (99.8% extra dry) and ethanol (EMSURE Grade) were obtained from Merck Chemical Company, while acetic acid (≥98% purity) was acquired from Merck Millipore Additionally, potassium bromide (KBr) was supplied by Sigma Aldrich Corporation, and dimethyl sulfoxide-d6 was used as an NMR solvent.
DMSO-d6 (99.9% purity) was sourced from Cambridge Isotope, while diethyl ether (>99.5%, extra dry) was acquired from Fisher Scientific All other chemicals were obtained from commercial vendors and utilized without additional purification.
Powder X-ray diffraction (PXRD) data was obtained using a Bruker D8 Advance system with a LynxEye detector, utilizing reflectance Bragg-Brentano geometry and Ni-filtered Cu Kα radiation (1.54178 Å) at a power of 1600 W (40 kV, 40 mA) The measurement involved spreading an activated sample for analysis.
The structural refinement of M-VNU-74-I and -II was conducted using a 2θ range of 2-80° with a step size of 0.02° and a fixed counting time of 12.5 seconds per step In contrast, the structural refinement of MOF-700 and its metalated derivatives utilized a 2θ range of 3-70° with an exposure time of 6 seconds per step.
Thermogravimetric analysis (TGA) was conducted using a TA Q500 thermal analyzer, with activated samples placed in platinum pans and subjected to continuous airflow of 60 mL/min, while the balance pan received 40 mL/min The samples were systematically heated to a maximum temperature of 700 °C at a consistent rate of 5 °C/min throughout the experiments.
Low-pressure nitrogen (N2) adsorption experiments were conducted using an Autosorb iQ and a Micromeritics 3Flex volumetric gas sorption analyzer For all adsorption measurements, ultrapure nitrogen and helium (99.999% purity) were utilized A liquid nitrogen bath at 77 K was employed for all N2 isotherm measurements.
Scanning electron microscope (SEM) images were taken on a Hitachi S-
The 4800 scanning electron microscope operates at an accelerating voltage of 1 kV, allowing for detailed imaging of solvent-exchanged M-VNU-74-II Each sample was dispersed onto a sticky carbon surface fixed to a flat aluminum sample holder for optimal imaging results.
Electron paramagnetic resonance (EPR) spectroscopy was collected by CMS
Nuclear magnetic resonance (NMR) spectroscopy measurements for both 1H and 13C were conducted using a Bruker Advance II 500 MHz spectrometer Chemical shifts are expressed in parts per million (ppm), with tetramethylsilane serving as the reference point at 0 ppm, while coupling constants (J) are reported in hertz (Hz).
Electrospray-ionization mass spectrometry (ESI-MS) was conducted in negative ionization mode on an Agilent 1200 Series high-performance liquid chromatography coupled to a Bruker micrOTOF-Q II mass spectrometer detector
Fourier transform infrared (FT-IR) spectroscopy was measured on a Bruker Vertex 70 spectrometer using potassium bromide (KBr) pellets with output absorption bands described as: s, strong; m, medium; w, weak; and br, broad
The Raman measurements were collected using a Nicolet Almega XR Dispersive Raman spectrometer from Thermofisher A 532 nm solid state laser was used for excitation and the output power was reduced to 50%
Elemental microanalyses (EA) were conducted on activated or dried samples at the Microanalytical Laboratory of the College of Chemistry, UC Berkeley, utilizing a Perkin Elmer 2400 Series II CHNS elemental analyzer.
Synthesis of amide functional linkers
2.2.1 4,4'-[1,4-phenylenebis(carbonylimino)]bis(2-hydroxybenzoic acid) (H 4 TDA)
A solution of solid 4-aminosalicylic acid (1.28 g, 8.37 mmol) in NMP (15 mL) was prepared and cooled to 5 °C using an ice bath Terephthaloyl chloride (0.812 g, 4.00 mmol) was then added gradually over the course of 1 hour The reaction mixture was stirred at 5 °C for 2 hours, followed by continued stirring at room temperature for an additional 12 hours.
To synthesize the H4TDA linker, 20 mL of distilled water was added to the product, which was then filtered and washed successively with 20 mL of NMP, followed by five washes with 50 mL of distilled water and two washes with 20 mL of methanol The final product was dried at 65 °C for 12 hours, resulting in a white powder weighing 1.61 g, corresponding to 3.69 mmol and achieving a yield of 92%.
Scheme 2.1 Synthesis of 4,4'-[1,4-phenylenebis(carbonylimino)]bis(2- hydroxybenzoic acid) (H4TDA) from terephtaloyl chloride and 4-aminosalicylic acid; byproduct HCl was omitted
2.2.2 Synthesis of 4,4'-[oxalylbis(imino)]bis(2-hydroxybenzoic acid) (H 4 ODA)
In a controlled reaction, 4-Aminosalicylic acid (6.12 g, 40.0 mmol) was combined with diethyl ether (50 mL) and cooled to 5 °C using an ice bath Liquid oxalyl chloride (1.60 mL, 18.6 mmol) was gradually added over one hour The mixture was stirred at 5 °C for four hours and then allowed to stir at room temperature for an additional twenty hours After this period, distilled water (100 mL) was introduced to the mixture The resulting product underwent filtration and was washed with DMF (2 × 50 mL), distilled water (5 × 150 mL), and MeOH (2 × 100 mL) Finally, the product was dried at 65 °C for twelve hours to yield the final compound.
H4ODA linker as a light grey powder (4.75 g, 13.2 mmol, 71% yield)
Scheme 2.2 Synthesis of 4,4'-[oxalylbis(imino)]bis(2-hydroxybenzoic acid)
(H4ODA) from oxalyl chloride and 4-aminosalicylic acid; byproduct HCl was omitted
Synthesis of M-VNU-74-I and M-VNU-74-II series
Mg(NO3)2·6H2O (31.8 mg, 0.124 mmol) and H4TDA (18.5 mg, 0.0424 mmol) were dissolved in 3.75 mL of DMF in a sealed 10 mL glass vial, which was sonicated for 30 minutes until fully dissolved Ethanol (0.25 mL) and deionized water (0.25 mL) were then added to the solution, which was heated at 120 °C for two days, yielding colorless, needle-shaped crystals with a 58% yield based on the organic linker The samples underwent activation under reduced pressure (20 mTorr) at room temperature for 24 hours, followed by evacuation at 70 °C for 12 hours All analyses were conducted on the activated samples, with FT-IR results indicating significant peaks at 3195 (br), 1602 (m), and 1566 (s) cm-1, among others.
783 (m), 763 (m), 705 (m), 680 (m), 648 (m), 632 (w), 609 (s) EA (activated sample): Calcd for Mg2(C22H12N2O8)ã3H2O: C, 49.36; H, 3.40; N, 5.24% Found: C, 49.28;
Co(NO3)2·6H2O (43.3 mg, 0.149 mmol) and H4TDA (18.5 mg, 0.0424 mmol) were dissolved in 3.75 mL of DMF within a sealed 10 mL glass vial, which was then sonicated for 30 minutes until fully dissolved Following this, 0.25 mL of ethanol and 0.25 mL of deionized water were added to the solution The mixture was heated at 100 °C for 2 days in an isothermal oven, resulting in the formation of red needle-shaped crystals with a 70% yield based on the organic linker FT-IR analysis (KBr, 4000-400 cm-1) revealed peaks at 3319 (broad), 1654 (weak), 1598 (medium), 1564 (medium), and 1527.
1014 (w), 977 (w), 883 (w), 854 (w), 821 (w), 775 (s), 759 (s), 705 (s), 636 (s), 605 (s) EA (activated sample): Calcd for (Co2C22H12N2O8)ã2H2O: C, 45.05; H, 2.76; N, 4.78% Found: C, 44.98; H, 3.10; N, 4.55%
In a 10 mL glass vial, 43.3 mg of Ni(NO3)2·6H2O (0.149 mmol) and 18.5 mg of H4TDA (0.0424 mmol) were dissolved in 3.75 mL of DMF The mixture was sealed and sonicated for 30 minutes until fully dissolved Following this, 0.25 mL of ethanol and 0.25 mL of deionized water were added to the solution The resulting mixture was heated at 100 °C for 2 days in an isothermal oven, yielding green needle-shaped crystals with a 72% yield based on the organic linker FT-IR analysis revealed significant peaks at 3398 (br), 2923 (m), 2852 (w), 1608 (s), 1561 (s), 1420 (s), 1366 (s), 1289 (m), 1241 (m), 1185 (w), 1155 (w), 1120 (w), and 1016 (w) cm⁻¹.
983 (w), 862 (w), 785 (w), 710 (w), 595 (w) EA (activated sample): Calcd for (Ni2C22H12N2O8)ã3.6H2O: C, 42.97; H, 3.16; N, 4.56% Found: C, 42.94; H, 3.21; N, 4.19%
2.3.4 Synthesis of Mg-VNU-74-II
In a 10 mL glass vial, 43.5 mg of Mg(NO3)2·6H2O (0.170 mmol) and 15.3 mg of H4ODA (0.0425 mmol) were dissolved in 3.75 mL of DMF The vial was sealed and subjected to sonication for 20 minutes, followed by gentle heating until complete dissolution of the solid Subsequently, 0.25 mL of ethanol and 0.25 mL of deionized water were added to the solution This mixture was then heated at 120 °C for two days in an isothermal oven, yielding colorless needle-shaped crystals with a 65% yield.
36 the organic linker) FT-IR (KBr, 4000-400 cm -1 ): 3294 (br), 2929 (w), 1672 (w), 1600 (s), 1568 (s), 1500 (m), 1477 (m), 1415 (s), 1375 (s), 1280 (w), 1245 (m), 1159 (w),
1099 (w), 993 (w), 977 (w), 825 (w), 777 (m), 734 (s), 678 (s), 626 (s), 603 (s) EA (activated sample): Calcd for Mg2(C16H8N2O8)ã2.6H2O: C, 42.52; H, 2.95; N, 6.20% Found: C, 42.56; H, 3.46; N, 5.77%
2.3.5 Synthesis of Co-VNU-74-II
In a controlled experiment, 43.3 mg of Co(NO3)2·6H2O and 15.3 mg of H4ODA were dissolved in 3.75 mL of DMF within a sealed 10 mL vial The mixture was sonicated for 20 minutes and gently heated until fully dissolved Subsequently, 0.25 mL each of ethanol and deionized water were incorporated into the solution, which was then heated at 100 °C for two days in an isothermal oven, yielding dark red needle-shaped crystals with a 68% yield based on the organic linker FT-IR analysis revealed key absorption peaks at 3309 (br), 2916 (w), 1681 (w), 1595 (m), 1566 (s), 1492 (s), 1473 (s), 1427 (s), 1373 (s), 1317 (m), and 1236 (m) in the range of 4000-400 cm⁻¹.
607 (s) EA (activated sample): Calcd for (Co2C16H8N2O8)ã2.2H2O: C, 37.38, H, 2.44; N, 5.46% Found: C, 37.37; H, 2.45; N, 5.16%.513.6
2.3.6 Synthesis of Ni-VNU-74-II
In a 10 mL vial, 43.3 mg of Ni(NO3)2·6H2O (0.149 mmol) and 15.3 mg of H4ODA (0.0425 mmol) were dissolved in 3.75 mL of DMF After sealing the vial, the mixture was sonicated for 20 minutes and gently heated until the solid fully dissolved Subsequently, 0.25 mL of ethanol and 0.25 mL of deionized water were added to the solution, which was then heated at 100 °C for 2 days in an isothermal oven.
37 after which time green needle-shaped crystals were obtained (75% yield based on the organic linker) FT-IR (KBr, 4000-400 cm -1 ): 3418 (br), 2933 (w), 1655 (s), 1603 (s),
1569 (s), 1497 (m), 1435 (s), 1412 (s), 1371 (s), 1242 (m), 1159 (w), 1104 (w), 994 (w), 851 (w), 785 (w), 737 (w), 591 (w), 443 (w) EA (activated sample): Calcd for (Ni2C16H8N2O8)ã5.9H2O: C, 33.11; H, 2.95; N, 4.83% Found: C, 35.26; H, 3.29; N, 4.56%
2.3.7 Procedures of M-VNU-74-I and M-VNU-74-II activation
All as-synthesized M-VNU-74-I and -II were subsequently washed with DMF
Over a span of two days, six 5 mL methanol samples were exchanged with dry methanol (four 5 mL exchanges each day for three days) The samples underwent drying using a Tousimis Samdri-PVT-3D critical point dryer In this process, the methanol samples were placed in the dryer chamber and exchanged with liquid CO2 five times for five hours Subsequently, the sample chamber was heated to the supercritical point for one hour.
To produce guest-free activated porous materials, CO2 was gradually vented over a 12-hour period Prior to conducting gas and vapor adsorption experiments, all activated samples underwent a 24-hour evacuation at room temperature under a vacuum of 20 mTorr.
The M-VNU-74-I series was heated to 70 °C, while Mg-VNU-74-II underwent heating at 130 °C Additionally, Co-VNU-74-II and Ni-VNU-74-II were heated at 90 °C and maintained under a vacuum pressure of 20 mTorr for 12 hours.
Synthesis of MOF-700 and Post-Synthetic Metalation
A N,N-dimethylformamide (DMF) solution (120 mL, 1.55 mmol) containing zirconium(IV) chloride anhydrous (240 mg, 1.03 mmol) and acetic acid (7 mL, 0.12 mmol) was left undisturbed in a fume hood for 16 h before adding 4,4’-
The synthesis of [oxalylbis(imino)]bis(2-hydroxybenzoic acid) (H4ODA, 390 mg, 1.08 mmol) involved heating the mixture at 120 °C in an isothermal oven for three days After cooling to room temperature, the resulting MOF-700 was collected as a light brown microcrystalline powder through filtration To eliminate unreacted reagents, MOF-700 underwent a three-day wash with DMF, followed by a two-day exchange with anhydrous acetone (80 mL each day) Finally, supercritical CO2 was utilized to activate MOF-700, which was heated at 150 °C for 24 hours under low pressure.
The activated MOF-700 was synthesized with a yield of 27%, resulting in 230 mg of the compound based on zirconium(IV) chloride Subsequent analyses were conducted on the activated sample The elemental analysis (EA) for activated MOF-700, represented by the formula C48H32N6O28Zr3 4H2O, indicated calculated values of C at 36.99%, H at 3.10%, and N at 5.39% The experimental findings revealed C at 36.74%, H at 2.59%, and N at 5.01%.
As-synthesized MOF-700 was thoroughly washed with DMF for 2 days and then immersed in a 20 mL DMF solution containing 600 mg of copper(II) nitrate trihydrate (2.48 mmol) The mixture was stirred at 65 °C for 15 hours, resulting in a color change of the MOF.
700 clearly changed to green EA of activated MOF-701: Calcd for
C48Cu3H32N12O46Zr3 6H2O = Zr3O2(OH)2(C16H10N2O8)3[Cu(NO3)2]3 2H2O: C, 28.13;
The as-synthesized MOF-700, after being thoroughly washed with DMF for 2 days, was immersed in a 20 mL DMF solution containing 600 mg of copper(II) acetate (3.30 mmol) for 24 hours This treatment resulted in a noticeable color change of MOF-700 to dark green within minutes of exposure to the copper(II) solution.
Calcd For C72Cu6H68N6O52Zr3 4H2O Zr3O2(OH)2(C16H10N2O8)3[Cu(OAc)2]3 4H2O: C, 32.65; H, 3.20; N, 3.17% Found
The as-synthesized MOF-700 was thoroughly washed with DMF for two days and then immersed in a 20 mL DMF solution containing 600 mg of copper(II) chloride dihydrate (3.52 mmol) The mixture was stirred and heated at 125 °C for 24 hours, followed by stirring at 100 °C under open air for an additional 5 hours, resulting in a noticeable color change of MOF-703 to brown-green The elemental analysis (EA) of the activated MOF-703 indicated calculated values for C48Cl12Cu6H32N6O28Zr3·4H2O and Zr3O2(OH)2(C16H10N2O8)3 as C: 24.37%, H: 2.05%, and N: 3.55%, with the found values being C: 24.89% and H: 2.48%.
Figure 2.1 The images of MOF-700 and metalated MOF-700 which clearly demonstrated the color change
The powder of metalated MOF was subsequently collected and immersed into DMF and anhydrous acetone The activation procedure for MOF-701, MOF-702, and MOF-703 was similarly applied to MOF-700
Structural analysis of MOFs
2.5.1 Structure analysis of M-VNU-74-I and M-VNU-74-II
A structural model of M-VNU-74-I or -II was executed by using the Materials
Visualizer module of Materials Studio software (Materials Studio v 5.5.0.0, 2010,
The modelling method for the Mg-VNU-74-II structure, representative of both series, was based on the crystal structures of M-VNU-74-I and -II, which are expected to be isoreticular to M-MOF-74 due to the connectivity of TDA and ODA linkers Structural modelling and refinement of Mg-VNU-74-II utilized the single crystal structure of Zn-MOF-74, with the organic linker ODA constructed in GaussView for resonance geometry optimization This optimized ODA was then connected to the characteristic infinite-rod shaped Mg metal cluster of M-MOF-74, assuming an empty framework without guest molecules Following the structural model completion, energetic minimization was conducted using the universal force field in the Forcite module of Materials Studio, optimizing unit cell parameters until convergence was achieved at an energy criterion of 10^-4 kcal mol^-1, with a similar approach applied to TDA-containing structures.
Whole pattern profile fitting and integrated intensity extraction were performed using data from 2θ = 2.5° to 50°, as no peaks were detectable beyond 50°, excluding that region from further analysis Background correction was achieved using a 20-parameter Chebyshev polynomial function, while peak profiles were calculated and refined employing the Thomson-Cox-Hasting or Pseudo-Voigt methods.
A Rietveld refinement was conducted, achieving low residual values (R_p = 3.41%, R_wp = 4.46% for Mg-VNU-74-I and R_p = 4.58%, R_wp = 6.02% for Mg-VNU-74-II), indicating a strong correlation between the calculated PXRD pattern and the experimental data, thus confirming the accuracy of the cell parameters.
2.5.2 Structure analysis of MOF-700 and metalated MOF-700
MOF-700 was chosen to represent the crystallographic model Based on MOF-
700 model, a series of metalated MOF-700 was composed the structural modeling according to the chelation of copper(II) complex to amide linkage-based backbone
Unit cell determinations were carried out using Materials Studio software
(Materials Studio ver 6.0, Accelrys Software Inc.) A Face-centered Cubic lattice system (a = 32.9630(3) Å) with space group F23 (No 196) was selected for MOF-
700 Whole pattern profile fitting and extraction of the integrated intensities was carried out using data from 2θ = 3° – 45° A background correction was performed using a 20-parameter Chebyschev polynomial function
A structural model of MOF-700 was executed by using the Materials Visualizer module of Materials Studio software (Material Studio ver 6.0, Accelrys
The inorganic secondary building unit (SBU) composition and connectivity, Zr6O6(OH)4(CO2)12, were derived from the crystal structure of the UiO-66 framework The H4ODA linker was designed using the GaussView graphical interface for Gaussian to explore optimal resonance geometries Subsequently, the amide linker was integrated to connect 12-c Zr6O6(OH)4(CO2)12 clusters, resulting in the formation of a face-centered cubic (fcu) topological structure Following the structural model's completion, energetic minimization was conducted using the universal force field available in the software.
Forcite module of Materials Studio During this process, the unit cell parameters
42 were also optimized until proper convergence was achieved (energy convergence criteria were set at 10 -4 kcal mol -1 )
A comprehensive profile pattern fitting was conducted using the Pawley and Rietveld methods over the range of 2θ = 3° to 45°, revealing no discernible peaks beyond 45°, which were thus excluded from further analysis The resulting calculated PXRD pattern showed a strong correlation with the experimental data, as indicated by the satisfactory fitting results (R wp = 8.20%, R p = 6.18%), leading to the determination of final unit cell parameters of a = 33.1520(4) Å.
Gas and vapour adsorption properties
2.6.1 Determination of surface area by using B.E.T.equation
The surface area of solid materials is determined using the BET equation, which is derived from gas adsorption isotherm data at the boiling point of the adsorptive, typically nitrogen at 77 K or argon at 87 K The reliability of the BET equation is expressed in equation (2.1).
From the gas adsorption isotherm data, the plot of (P/Va(P0-P)) against (P/P0) is constructed which called BET plot
The BET plot should be a straight line which gives the intercept i = 1
V m c ; from these results the Vm and c value can be calculated by equation (2.2) and (2.3):
43 c = s i + 1 (2.3) The BET surface area (SBET, m 2 g -1 ) of solid adsorbent is obtained by equation (2.4)
22.4 N A σ gas 10 −18 (2.4) Here, NA is Avogadro number (6.02214 10 23 ); σ gas is occupying area of one gas molecular (σ N 2 = 0.162 nm 2 , at 77 K; σ Ar = 0.138 nm 2 , at 87 K)
• The linear relative pressure range (P/P 0) is usually in 0.05 – 0.35, but it can be varied for many adsorbents with different shapes of isotherms caused by their pore-size distribution
• In case of microporos materials with type I isotherm, the monolayer coverage volume Vm reaches to the total adsorption volume Va at very low pressure (P
Figure 2.2 The correlation of BET constant value and the shape of adsorption isotherms
2.6.2 Vapor adsorption measurements for ADHPs system
Volumetric methanol vapor adsorption measurements
Methanol vapor adsorption measurements were conducted using a BEL Japan, Inc Belsorp-aqua3 with ultrapure helium (99.999%) and anhydrous methanol (99.8% extra dry) To prevent methanol condensation, the manifold temperature was maintained at 50 °C The methanol isotherms were recorded at temperatures of 10, 15, and 25 °C for the M-VNU-74 sample.
At 25 °C, multi-cycle methanol isotherm data for Mg-VNU-74-II was obtained in a single measurement, where the sample underwent automatic evacuation for 3 hours between each cycle.
Principle of the isosteric enthalpy of adsorption (Q st ) and coefficient of performance (COP) calculation
The isosteric enthalpy of adsorption (𝑄 st) for the M-VNU-74-II series materials was determined using methanol adsorption isotherms at temperatures of 10, 15, and 25 °C This calculation was performed with the BEL Master software, utilizing the Clausius-Clapeyron equation.
Here, R is ideal gas constant
Utilizing the Polanyi adsorption potential, a characteristic curve was developed that relates the molar Gibbs free energy (A) to the volume adsorbed (W) The volume adsorbed is determined through the methanol isotherm using pressure and temperature (P, T), as outlined in equations (2.2) and (2.3).
Here, 𝑃 s (𝑇) is the saturated pressure at T, q(P, T) is the mass adsorbed, and liq wf (𝑇) is the liquid density of working fluid, here is methanol
In a refrigeration system designed for air conditioning, the condenser temperature is set at 303 K while the evaporator temperature is at 283 K, with the desorption temperature T_d being adjustable The maximum adsorption work, A_max, and the minimum adsorption work, A_min, are calculated using equations (2.4) and (2.5).
During operation, only equilibrium points (A, W) within the boundaries defined by A max and A min values are accessible These limits are solely influenced by the operational temperature (T con, T ev, and T d) and the working fluid pressure, which is temperature-dependent (P s(T)).
The volumes adsorbed, 𝑊 𝐴 min and 𝑊 𝐴 max, were calculated through interpolation of the characteristic curve corresponding to the adsorption potentials A min and A max The working capacity was derived using equation (2.6), and the volumetric working capacity was then adjusted based on the crystal density of each adsorbent.
∆𝑊 = 𝑊 𝐴 max − 𝑊 𝐴 min (2.6) The cooling energy is the energy taken up by the evaporator Q ev calculated by the equation (2.7)
𝑄 ev = − ∆ vap 𝐻(𝑇 ev ) liq wf 𝑚 sorbent ∆𝑊
Here, ∆ vap 𝐻(𝑇 ev ) is the enthalpy of evaporation of methanol at T ev , M W is molecule weight of working fluid (methanol).
The regeneration energy was obtained by the simplified equation (2.8) when the heat capacity of heat exchanger was ignored
Here, 𝑐 𝑃 sorbent is the heat capacity of adsorbent, which is assumed to be 1 J g -1 K -1 ,
∆𝑇 is the difference between the desorption and condensation temperature (∆𝑇 𝑇 d − 𝑇 con ), 〈∆ ads 𝐻〉 is the average value of isosteric enthalpy of adsorption or Q st The coefficient of performance becomes
The Q500 TGA was coupled with an HG-100 vapor generator from L&C Science Technology Co The TGA chamber was continuously flushed with dry N2
In the experiments, a balance flow of 10 mL min⁻¹ was maintained alongside a sample flow of a MeOH/N₂ mixture at 90 mL min⁻¹ The methanol pressure was systematically adjusted to 4.3, 8.5, 11.9, 15.3, and 16.9 kPa, creating relative methanol humidity levels of 22%, 45%, 63%, 81%, and 89% within the sample chamber at a constant temperature of 25 °C Approximately 15 mg of activated material was utilized for each experiment, with the furnace temperature consistently set at 25 °C during the adsorption phase.
The sample was heated to 50 °C for 50 minutes in a methanol-containing nitrogen flow, followed by an increase to 80 °C at a rate of 5 °C per minute, maintaining this temperature for 15 minutes under the same flow conditions Afterward, the furnace cooled naturally to 25 °C to initiate the next adsorption cycle A blank control measurement confirmed that no methanol condensation occurred in the system It is important to note that the sample mass was not adjusted for buoyancy effects, which could result in a slight underestimation of the methanol mass transfer capacity.
Gas sensors were fabricated using patterned-interdigital electrodes (PIEs) on SiO2-coated Si (100) substrates through a conventional photolithographic process Sequential deposition of bi-layers of titanium (Ti) and platinum (Pt) was achieved via DC magnetron sputtering, with thicknesses of 50 nm for Ti and 200 nm for Pt The Ti layer was critical for improving adhesion between the SiO2 and the Pt layer, while the Pt layer functioned as the electrode.
The gas sensor devices were prepared using a patterned interdigitated electrode (PIE) measuring 20 µm in width and 1.05 mm in length, consisting of 20 electrode pads spaced 10 µm apart To create the sensing material, 5 mg of synthesized magnesium metal-organic frameworks (Mg-MOFs) powder was ultrasonicated with 0.05 mL of 2-propanol for 15 minutes A 0.5 µL drop of this mixture was applied onto the PIEs using a micropipette After drying at 60 °C for 10 minutes, the Mg-MOFs established good contact with the electrodes, as illustrated in Figure 2.3, which depicts the preparation process for the gas sensor devices.
Figure 2.3 Schematic diagram of the procedure used to fabricate a sensor device with
Mg-MOFs (Mg-VNU-74-I and -II)
The gas sensing characteristics of the fabricated sensors were evaluated using a horizontal-quartz heating chamber, achieving target gas compositions by mixing air-balanced target gas with pure dry synthetic air at a flow rate of 500 standard cubic centimeters per minute (sccm) Dynamic resistance data were recorded in both air (Ra) and the target gas environment (Rg), with the gas sensor's response defined as R=Ra/Rg for NO2 and R=Rg/Ra for reducing gases The response time was measured as the duration for the sensor to reach 90% of its maximum response after target gas injection, while recovery time was the period taken to return to 90% of its initial resistance after air injection Further details on the gas sensing tests can be found in references [73,80].
2.6.4 High-Pressure Methane Adsorption Measurements
High-pressure CH4 adsorption isotherms were measured between 0-85 bar using a Hiden Isochema IMI-135 system An activated sample was carefully weighed and placed into a stainless-steel holder within an inert atmosphere glovebox Before attaching the holder to the high-pressure assembly's VCR fittings, a known quantity of glass wool (density = 2.06 g cm -3) was securely packed on top of the sample holder.
The fully assembled sample holder was transferred to Hiden Isochema IMI-
The sample was evacuated at 50 °C for 3 hours and then placed in a circulating dewar connected to a Grant TX150 isothermal bath filled with Silicate anti-freeze and Halfords coolant, maintaining a temperature stability of ± 0.02 °C Using the Hisorp v3.02.008 control software, the sample weight and glass wool volume were entered as sample information Helium expansion at 25 °C and 30 bar was conducted to determine the skeletal volume of the sample, and CH4 adsorption isotherms were measured at temperatures of 5, 15, and 25 °C.
Characterization of two amide functional linkers
H4TDA and H4ODA linkers were synthesized through N-acylation reactions of 4-aminosalicylic acid with terephthaloyl chloride and oxalyl chloride, respectively Each linker underwent a one-step synthesis under mild conditions, ranging from 5 °C to room temperature, without the need for reflux The resulting products achieved high purity and yields of 92% for H4TDA and 71% for H4ODA.
The H4TDA linker was synthesized using NMP solvent due to its non-proton property and the varying solubility of reactants and products; specifically, amino acids and chloride compounds are highly soluble in NMP, while H4TDA has low solubility, facilitating product collection through filtration The product underwent multiple washes with water to eliminate byproduct HCl, followed by methanol treatment to remove residual reactants and exchange NMP for a mild drying step Characterization of H4TDA's formula and structure was achieved through NMR, HRMS (ESI), and FT-IR analysis In the 1H-NMR spectrum, three signals at 7.77, 7.55, and 7.34 ppm indicated six aromatic protons from two salicylic parts, while a signal at 8.08 ppm corresponded to four protons of the central benzene ring, and a shape signal at 10.61 ppm identified two amide protons Additionally, a broad peak at 11.36 ppm represented acidic protons in DMSO-d6, and 13C-NMR confirmed the number of carbons and their chemical shifts Mass spectrometry revealed a mother peak for the ionization molecule (C22H15O8N2 +) with high accuracy.
Table 3.1 Characterization and structure analysis of H4TDA
= 11.36 (s, 2H), 10.61 (s, 2H), 8.08 (s, 4H), 7.77 (d, J = 8.5 Hz, 2H), 7.55 (d, J = 2 Hz, 2H), 7.34 ppm (dd, J = 8.5, 2 Hz, 2H)
HR-ESI-MS ([M-H] - ) Calculated for C22H15O8N2 +: m/z = 435.0823
FT-IR (KBr, 4000-400 cm -1 ) 3429 (br), 3100 (br), 1667 (s), 1637 (s), 1617 (s),
The acylation procedure was adapted to synthesize the shorter di-topic linker H4ODA by utilizing oxalyl chloride as the acylation reactant, resulting in a linker with one less benzene ring Diethyl ether was selected as the solvent due to its compatibility with the product's solubility, and a significant amount of distilled water was introduced to the reaction mixture to enhance product precipitation The formula and structure of H4ODA were characterized and confirmed using NMR, HRMS (ESI), and FT-IR spectroscopy, as detailed in Table 3.2.
Table 3.2 Characterization and structure analysis of H4ODA
= 11.36 (s, 2H), 11.06 (s, 2H), 7.76 (d, J = 8.5 Hz, 2H), 7.57 (d, J = 2 Hz, 2H), and 7.42 ppm (dd, J 8.5, 2 Hz, 2H)
107.6 ppm HR-ESI-MS ([M-H] - ) Calculated for C16H11O8N2 +: m/z = 359.0515
FT-IR (KBr, 4000-400 cm -1 ) 3432 (br), 3103 (br), 1699 (s), 1648 (s), 1620 (s),
Characterization of M-VNU-74 series
Figure 3.1 Synthesis of M-VNU-74 series and crystal images of MOFs under optical microscope
The synthesis of the M-VNU-74-I and -II series was successfully developed with minor modifications to the existing MOF-74 procedures All variants of the M-VNU-74-I and -II series yielded microcrystalline powders with an efficiency of 60-75% The materials' morphology was analyzed using optical microscopy and scanning electron microscopy (SEM).
54 revealed the micro-needle shape for all of MOFs series (Figure 3.2 and Appendix 5-
Figure 3.2 SEM images of (a)-(c) Mg-VNU-74-I, (d)-(f) Mg-VNU-74-II
The crystallinity of the synthesized and activated materials was validated through powder X-ray diffraction, with results compared to simulated PXRD models (Figure 3.3 and Appendix 9-12) The activation process was confirmed by the retention of main peaks and minimal noise across all patterns Both activated Mg-MOFs exhibited similar diffraction patterns with consistent peak positions; however, Mg-VNU-74-II displayed a slight shift towards larger diffraction angles compared to Mg-VNU-74-I, indicating that while both materials share a similar structure, they possess different unit cell sizes This phenomenon was also observed in the cases of Ni-VNU-74-I/-II and Co-VNU-74-I/-II (Appendix 9-12).
Figure 3.3 Powder X-ray diffraction patterns of as-synthesized (blue) and activated
(red) Mg-VNU-74-I (A) and Mg-VNU-74-II (B) The simulated pattern (black) generated from the structural model is provided as a reference
3.2.2 Structural analyses of M-VNU-74 series
Figure 3.4 PXRD analysis of activated Mg-VNU-74-I (A), and Mg-VNU-74-II (B)
The experimental pattern (blue), refined (red circles), and calculated pattern (black) The difference plot (green) and Bragg positions (pink) are provided for comparison
Utilizing the Zn-MOF-74 structure model alongside experimental X-ray diffraction data, we conducted full pattern profile fitting and Rietveld refinement This process achieved convergence with satisfactory residual values, specifically R_wp of 5.29% and R_p of 4.33%, indicating the reliability of the results.
The refinement results for Mg-VNU-74-I and Mg-VNU-74-II show a percentage of 5.22% (Figure 3.4), with additional data on other MOFs presented in Table 3.3 M-VNU-74-I and -II are isoreticular to MOF-74, and by substituting Mg with Ni and Co, Ni-VNU-74-I/-II and Co-VNU-74-I/-II structures were successfully refined using PXRD patterns (Appendix 13-16) These structures feature infinite, rod-shaped secondary building units connected by TDA 4- or ODA 4- linkers, resulting in an etb topology with octahedrally coordinated divalent metal atoms that form one-dimensional hexagonal channels lined with amide groups (Figure 3.5) The calculated pore apertures for Mg-VNU-74-I/-II, Ni-VNU-74-I/-II, and Co-VNU-74-I/-II are 26.4/22.5 Å, 27.3/23.6 Å, and 27.4/23.2 Å, respectively (Table 3.4), while the detailed refinement coordinates for each MOF can be found in Appendix 17-22 Comparatively, the pore apertures of M-VNU-74-I and -II are slightly larger than and similar to IRMOF-74-II (19.5 Å) and -III (27.3 Å), respectively.
Table 3.3 Crystal data and data refinement for M-VNU-74-I and -II
Mg-VNU-74-I Ni-VNU-74-I Co-VNU-74-I Mg-VNU-74-II Ni-VNU-74-II Co-VNU-74-II
Formula C 99 H 54 N 9 O 45 Mg 9 C 99 H 54 N 9 O 45 Ni 9 C 99 H 54 N 9 O 45 Co 9 C 72 H 36 N 9 O 45 Mg 9 C 72 H 36 N 9 O 45 Ni 9 C 72 H 36 N 9 O 45 Co 9
Crystal system Hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal
M-VNU-74-I and -II exhibit crystal structures similar to MOF-74, characterized by infinite, rod-shaped metal clusters, M3[(-O)3(-CO2)3], where M can be Mg, Ni, or Co These clusters are interconnected using either TDA 4- or ODA 4- linkers, contributing to the unique structural properties of M-VNU-74-I and -II.
C, grey; O, red; N, green, H, pink; metal atoms, blue H atoms of the coordinated water are omitted for clarity
3.2.3 Thermal gravimetric analyses of M-VNU-74 series
TGA measurements demonstrated that all members were thermally robust with decomposition occurring at temperatures >300 C (Figure 3.6 and Appendix 23-26); the
The initial weight loss in M-VNU-74 materials is primarily due to water elimination, followed by linker decomposition Notably, M-VNU-74-II, which features a smaller pore structure, exhibits greater thermal stability compared to M-VNU-74-I with larger pores, a finding that aligns with previous research Additionally, both Ni-VNU-74-II and Co-VNU-74-II demonstrate enhanced thermal stability compared to their counterparts, Ni-VNU-74-I and Co-VNU-74-I.
Figure 3.6 TGA trace of activated Mg-VNU-74-I (A) and Mg-VNU-74-II (B)
3.2.4 Gas adsorption analyses of M-VNU-74 series
To investigate porosity, N2 isotherms at 77 K were analyzed, revealing Type IV profiles characteristic of mesoporous materials Notably, the M-VNU-74-I isotherms exhibited a second gap at higher saturated pressures compared to the M-VNU-74-II curves, indicating larger pore sizes in the M-VNU-74-I series The calculated pore volumes at a saturation uptake of P/P0 = 0.9 were found to be 0.84/1.08, 1.10/1.27, and 1.42/1.68 cm³ g⁻¹ for Ni.
The VNU-74-I/-II, Co-VNU-74-I/-II, and Mg-VNU-74-I/-II series exhibit impressive high porosity, as evidenced by their Brunauer-Emmett-Teller (BET) surface areas, which range from 1820 to 3030 m²/g, with Mg-VNU-74-II exceeding 3000 m²/g This remarkable surface area positions Mg-VNU-74-II among the highest reported values for the isoreticular MOF-74 structure, surpassing the notable 3270 m²/g for Mg2(dobdc) with dobpdc being 4,4′-dioxido-3,3′-biphenyldicarboxylate.
Figure 3.7 N2 isotherm of Mg-VNU-74-I (blue) and -II (red) at 77 K Curves with filled and opened symbols represent the adsorption and desorption branches, respectively
The carbon dioxide and methane adsorption of Mg-VNU-74-II and Co-VNU-74-
The study focused on two metal-organic frameworks (MOFs), Mg-VNU-74-II and Co-VNU-74-II, which exhibit high porosity and small pore sizes, making them promising candidates for gas adsorption and separation Both MOFs demonstrated significant CO2 uptake capacities of 12.8% and 11.6% at 298 K and 1 bar, respectively, indicating their potential for effective gas storage comparable to other MOFs with similar porosity, such as IRMOF-74-III.
CH2NH2, SNU-50, MOF-5 and MIL-101-Cr with CO2 capacity is 14.4, 13.7, 4.5 and 4.2
At 298 K and 1 bar, Mg-VNU-74-II and Co-VNU-74-II exhibit significantly higher CO2 adsorption capacities, with values of 61 w% each, while their N2 and CH4 adsorption is notably low, showing CH4/N2 uptake ratios of 0.93/0.91 and 0.90/1.02, respectively These findings indicate that both metal-organic frameworks (MOFs) have promising potential for applications in CO2/N2 and CO2/CH4 separation.
Figure 3.8 illustrates the isotherms for CO2 (red), CH4 (orange), and N2 (blue) in Mg-VNU-74-II (A) and Co-VNU-74-II (B) at a temperature of 298 K, with filled symbols indicating the adsorption branches and open symbols representing the desorption branches.
3.2.5 Methanol adsorption capacity of M-VNU-74 series
Methanol adsorption isotherms at 25 °C were measured for the amide-containing M-VNU-74-I and -II series The M-VNU-74-I series exhibited convoluted isotherm profiles, attributed to structural collapse observed during subsequent nitrogen isotherm measurements at 77 K In contrast, the M-VNU-74-II series displayed fully reversible, Type-IV methanol isotherms, characterized by a steep uptake at P/P0 < 0.05, followed by an additional steep uptake step.
The M-VNU-74-II series demonstrated a negligible hysteresis across all members, indicating a lower regeneration energy requirement for subsequent re-use Additionally, the series exhibited a remarkably high maximum methanol capacity at P/P 0 = 0.7, achieving values of 1.04, 0.90, and 0.75 g g -1.
The maximum capacity values for Mg-VNU-74-II, Co-VNU-74-II, and Ni-VNU-74-II are 407, 435, and 367 cm³ cm⁻³, respectively, ranking among the highest reported for metal-organic frameworks (MOFs) An essential factor in evaluating adsorbent materials for methanol-based thermal batteries is the deliverable amount of methanol at relevant operating pressures, typically proposed within the range of P/P₀ = 0.1-0.3.
MIL-101(Cr) exhibits a high maximum gravimetric uptake capacity; however, its methanol isotherm profile indicates a lower deliverable amount of methanol compared to Mg-VNU-74-II, with values of 0.45 g g⁻¹ for MIL-101(Cr) versus 0.62 g g⁻¹ for Mg-VNU-74-II at P/P₀ = 0.1-0.3 This suggests that Mg-VNU-74-II offers superior methanol usability in both gravimetric and volumetric terms Additionally, N₂ isotherms of the Mg-VNU-74-II series were analyzed to verify that the stability and porosity of these metal-organic frameworks (MOFs) were maintained after a cycle of methanol adsorption and desorption.