Introduction of metal-organic frameworks (MOFs)
Structure of MOFs
Metal-Organic Frameworks (MOFs) are intricate structures formed by organic linkers and metal-containing units, where the linkers act as binding bridges and the metal units serve as joints These components are interconnected through covalent bonds, resulting in extensive, infinite network structures The vast diversity and abundance of both organic linkers and metal units have led to the synthesis and documentation of thousands of new MOF structures annually Over the past decade, the number of reported MOF structures has seen a remarkable increase, effectively doubling.
4 time for number of new MOFs (3D) is the highest among all reported Cambridge Structure Database (CSD) [30,124]
Figure 2.2 Number of MOF structures reported in the Cambridge Structural Database
Organic linkers are multidentate compounds with functional groups like carboxyl, sulfo, hydroxyl, and nitrogen derivatives that form complexes with metal ions Carboxylate-containing compounds are particularly favored as linkers in metal-organic frameworks (MOFs) due to their ability to chelate various metals and create stable, rigid structures Additionally, organic linkers can be extended to enhance their structural properties.
5 adding one or more benzene rings to linker structure in order to increase pore sizes and surface areas.[30,68,116,124]
Figure 2.3 Some organic linkers used to synthesize MOFs [30]
Metal-organic frameworks (MOFs) are composed of metal clusters, where metal ions are interconnected by carboxylate and oxo groups These frameworks can be formed using various metals, including both alkali and transition metals However, transition metals are predominantly used in MOF structures due to their strong affinity for oxygen atoms in organic linkers and their unique properties that make them suitable for specific applications.
Figure 2.4 Metal containing units used for construction of MOFs [116]
Yaghi and co-authors have employed the well-known ―secondary building unit (SBU)‖ strategy for the construction of rigid and porous frameworks The term
The term "secondary building unit" (SBU) was initially applied to tetrahedral frameworks in zeolite chemistry The concept has since been adapted for metal-organic frameworks (MOFs), redefining SBUs as distinct, low-molecular metal-organic coordination units characterized by well-defined and highly symmetric coordination geometries In the design and analysis of MOF structures, both organic linkers and metal clusters are classified as SBUs.
Metal clusters utilize carboxylate groups to chelate metal ions, creating rigid, directional metal–oxygen–carbon clusters The carboxylate carbon atoms serve as points of extension, defining geometrical shapes known as secondary building units (SBUs) The organic linkers exhibit polygonal or polyhedral forms, influenced by their three-dimensional structures and the number of carboxylate groups present.
Figure 2.5 SBUs presentation of metal containing units and organic linkers [116]
Synthesis of MOFs
The formation of Metal-Organic Frameworks (MOFs) involves a crystallization process that balances the formation and dissociation of bonds between metal ions and organic linkers, resulting in highly symmetrical structures MOF crystals grow through the development of new monolayers on their surfaces Research by Attfield and colleagues utilized atomic force microscopy (AFM) to study the real-time crystal growth of MOF-5, revealing that the two-dimensional nuclei responsible for layer growth emerge through a two-step process Initially, 1,4-benzenedicarboxylate units attach to the MOF-5 surface, followed by the addition of a layer of zinc species and connecting 1,4-benzenedicarboxylate units.
Figure 2.6 Possible pathways to formation of a new monolayer on the surface of
The synthesis of Metal-Organic Frameworks (MOFs) requires mild synthetic conditions to preserve the conformation of organic linkers while ensuring sufficient reactivity to form coordinative bonds Typically, this process involves the reaction of organic linkers and metal ions in polar solvents at temperatures up to 200 ºC and pressures reaching 100 atm Key factors influencing MOF synthesis include the solubility of organic linkers and metal salts, solvent polarity, temperature, and pressure Highly polar solvents like N,N-dimethylformamide, dimethylsulfoxide, and N-methyl-2-pyrrolidone are preferred as they effectively dissolve the necessary components and prevent competitive coordination for metal sites Additionally, mixed solvent systems are frequently utilized to adjust solution polarity, and in some instances, a volatile amine is incorporated to deprotonate the organic linker before coordination.
Various methods for synthesizing Metal-Organic Frameworks (MOFs) include solvothermal, ultrasonic, and microwave techniques The solvothermal method involves dissolving organic linkers and metal salts in suitable solvents within sealed vessels, such as Teflon-lined stainless steel bombs or glass tubes, and heating the mixture at high temperatures near the solvents' boiling point for several days Despite the lengthy reaction time, this approach promotes the growth of large MOF crystals In contrast, ultrasonic and microwave methods significantly shorten the reaction time, although they typically yield smaller crystals.
Structural analysis and characterization of MOFs
Single crystal X-ray diffraction (SCXRD) is the most effective technique for determining atomic structures and coordinates This analytical method yields crucial data, including unit cell dimensions, space group, atomic coordinates, bond lengths, and bond angles, making it essential for material characterization.
Metal-organic frameworks (MOFs) can be characterized using various techniques, including powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), elemental analysis (EA), and nitrogen sorption measurements based on Brunauer-Emmett-Teller (BET) theory to determine their surface areas Additional methods such as scanning electron microscopy (SEM), nuclear magnetic resonance (NMR) in liquid or solid-state, and UV-Vis measurements can also be employed to assess the properties and applications of MOFs.
Zr- and Hf-based metal-organic frameworks
Chemistry of Zr-MOFs
According to SciFinder, there are currently over 200 reported zirconium metal-organic frameworks (Zr-MOFs), with new discoveries continuing to emerge annually These frameworks are constructed using organic linkers that can be di-, tri-, or tetracarboxylate in nature The classification of Zr-MOF structures is determined by zirconium-containing units known as secondary building units (SBUs), specifically Zr6(μ3-O)x(μ3-OH)y(−CO2)n, which exhibit varying coordination numbers Among these, the Zr6(μ3-O)4(μ3-OH)4 core is widely recognized as a common structural feature, while the Zr6(μ3-O)8 core is found in a limited number of frameworks.
Figure 2.8 The year-by-year increase of reported Zr-MOFs in the last eight years (by
Table 2.1 Summary of some reported Zr-MOFs
Zr-MOF Zr core n BET surface areas (m 2 /g)
UiO: University of Oslo; PIZOF: Porous Interpenetrated Zirconium Organic Framework; NU: Northwestern University; PCN: Porous Coordination Network; DUT: Dresden University of Technology
Table 2.1 outlines five types of coordinated Zr6 clusters in Zr-MOFs, with coordination numbers ranging from 4 to 12 The most prevalent cluster, Zr6O4(OH)4(-CO2)12, features six Zr4+ cations arranged in a Zr6-octahedron, which is extended by 12 carboxylate groups from organic linkers The triangular faces of the octahedron are capped by oxo and hydroxyl groups (denoted as à3-O and à3-OH), while the edges connect to carboxylate groups, resulting in a cuboctahedral-shaped secondary building unit (SBU) In Zr6 clusters with lower coordination numbers, monocarboxylate compounds replace carboxylate linkers, significantly influencing the nucleation and crystal growth during Zr-MOF synthesis.
Figure 2.9 The Zr 6 O 4 (OH) 4 (-CO 2 ) 12 cluster is presented in stick-and-ball (a) and polyhedron (b) The SBU shape is cuboctahedron (c) [10]
Zr-Metal-Organic Frameworks (Zr-MOFs) are renowned for their high surface areas and diverse pore sizes, primarily attributed to the use of extended organic linkers Notably, NU-1103, which is built using the tetratopic linker PyPTP, currently holds the record for the highest surface area among Zr-MOFs, measuring 5646 m²/g according to BET theory These frameworks exhibit exceptional stability in aqueous solutions, largely due to the strong affinity between Zr(IV) ions and carboxylate O atoms, as explained by Pearson's hard/soft acid/base concept This interaction classifies Zr(IV) ions as hard acids and carboxylate ligands as hard bases, resulting in robust coordination bonds Consequently, most Zr-MOFs demonstrate remarkable stability in both organic solvents and water, even under acidic and basic conditions.
Chemistry of Hf-MOFs
Hafnium (Hf) exhibits chemical properties closely resembling those of zirconium (Zr), particularly in terms of ionic radius, with Zr 4+ at 0.79 Å and Hf 4+ at 0.78 Å This similarity extends to their coordination chemistry, leading to hafnium metal-organic frameworks (Hf-MOFs) and zirconium metal-organic frameworks (Zr-MOFs) sharing comparable structures and properties However, the development of organic linkers for Hf 6 clusters is limited, resulting in a lower number of reported Hf-MOFs compared to Zr-MOFs.
Hafnium-based metal-organic frameworks (Hf-MOFs), including Hf-UiO-66, Hf-DUT-67, Hf-DUT-69, and Hf-NU-1000, exhibit remarkable stability in aqueous solutions, similar to zirconium-based MOFs (Zr-MOFs) However, Hf-MOFs generally possess lower surface areas compared to Zr-MOFs due to the higher density of hafnium (13.31 g/cm³) in contrast to zirconium (6.52 g/cm³), resulting in a greater overall density for Hf-MOFs.
Table 2.2 Comparison of BET surface areas Zr-MOF and Hf-MOF in same structures
MOF Cluster BET surface areas (m 2 /g) Reference
Syntheses of Zr- and Hf-MOFs
The synthesis of Zr- and Hf-MOFs requires careful consideration of factors such as the solubility of organic linkers and metal salts, solvent polarity, temperature, and pressure N,N-dimethylformamide is frequently used as a solvent in these processes, while zirconium and hafnium salts, including ZrCl4, HfCl4, ZrOCl4·8H2O, and Zr(NO3)4·5H2O, serve as the metal precursors To enhance the solubility of organic linkers, ultrasound may be employed during the synthesis Typically, the synthesis occurs in capped glass vials at temperatures exceeding 100°C for durations extending beyond one day.
In the synthesis of Zr- and Hf-MOFs, modulators, such as formic, acetic, and benzoic acids, play a crucial role in regulating nucleation rates and crystal growth Due to the oxophilic nature and high charge density of Zr4+ and Hf4+ cations, these metal ions rapidly bond with carboxylate groups of organic linkers, often resulting in poorly crystalline materials To mitigate this, modulators with similar chemical functionalities to the organic ligands are employed to maintain coordination equilibrium Research by Behrens and colleagues highlights how the addition of acetic and benzoic acids influences the size and morphology of Zr-UiO series MOFs, allowing for precise control over crystallite and particle sizes in the nanometer range, particularly in Zr-UiO-67.
The powder X-ray diffraction (XRD) patterns of Zr-UiO-67 reveal the effects of varying benzoic acid amounts, expressed as equivalents relative to ZrCl4, on the synthesis process Scanning electron microscopy (SEM) images illustrate the morphological differences in Zr–BPDC samples synthesized with 0, 3, and 30 equivalents of benzoic acid, highlighting the impact of the modulator on the material's structure.
The type and amount of modulator significantly influence the crystal size, morphology, and porosity of Zr- and Hf-MOFs Utilizing various modulators during the synthesis of these metal-organic frameworks results in distinct porosity characteristics, as demonstrated in Table 2.3.
Table 2.3 Comparison of BET surface areas of Zr-MOFs synthesized under different conditions [94]
The molar ratio of metal salt to organic linker is crucial in the synthesis of Zr- and Hf-MOFs, significantly influencing the resulting structures For instance, in the synthesis of DUT-67 and DUT-69, the organic linker used is 2,5-thiophenedicarboxylic acid (H2TDC), but the molar ratios differ, with DUT-67 using a ratio of 1.5:1 and DUT-69 a 1:1 ratio Similarly, the synthesis of MOF-525 and MOF-545, which both utilize tetrakis(4-carboxyphenyl)porphyrin as the linker, results in distinct structures due to their molar ratios of 1:1 for MOF-525 and 3:1 for MOF-545.
Zr- and Hf-MOFs used as heterogeneous catalysts
Photocatalysis
Photocatalysis is a process where light energy is transformed into chemical energy in the presence of a catalyst This catalyst acts as a semiconductor, enabling electrons to absorb photon energy that meets or exceeds the material's band gap, resulting in the generation and separation of positive holes in the valence bands and electrons in the conduction bands.
Figure 2.11 Photoabsorption by transition of electrons in the valence band (VB) to the conduction band (CB) in a semiconductor [79]
Heterogeneous photocatalysts like TiO2 and ZnO are widely used; however, they have significant limitations for practical applications One major drawback is their large band gaps, approximately 3.2 eV, which restricts their ability to effectively absorb light.
Recent advancements in photocatalysts have highlighted the potential of Metal-Organic Frameworks (MOFs) due to their unique properties MOFs offer the advantage of rational design for specific applications, possess a large surface area and pore volume, and have a high metal content, making them effective for photocatalytic processes However, traditional photocatalysts often struggle to operate efficiently in visible light and face challenges in recycling due to their suspension in water.
In 2007, H Garcia and colleagues demonstrated the semiconductor properties of MOF-5, specifically Zn4O(BDC)3 They suggested that the linker facilitates charge transfer within the MOF-5 structure, as evidenced by UV-Vis spectral analysis.
Figure 2.12 Photophysical processes occur after the irradiation of the MOF-5 solid material [1]
The calculated band gap of MOF-5 is approximately 3.4 eV, which is higher than that of TiO2 at around 3.2 eV, indicating that MOF-5 primarily absorbs light in the UV region To address this limitation, B Chen and colleagues have proposed a solution.
22 used 2,6-naphthalenedicarboxylic acid as organic linker to construct MOF (termed UTSA-38), which is isostructure with MOF-5 [20]
UTSA-38, a semiconducting metal-organic framework (MOF) with a unique interpenetrated structure and a BET surface area of 1090 m²/g, demonstrates significant potential as a photocatalyst due to its band gap of 2.85 eV The photocatalytic activity of UTSA-38 was evaluated through the degradation of methyl orange (MO) in an aqueous solution, showcasing its effectiveness in environmental applications.
The characteristic absorption band of methyl orange (MO) at 464 nm was monitored, revealing that UTSA-38 serves as an effective photocatalyst for the complete degradation of MO within 120 minutes under UV-Vis irradiation.
In 2008, Gascon and colleagues discovered that a Zn-MOF utilizing a 2,6-naphthalenedicarboxylate linker exhibited significant photocatalytic activity for the aerobic epoxidation of propylene Additionally, recent studies have highlighted the photocatalytic effectiveness of MOFs containing metal ions such as Cd²⁺, Mn²⁺, Gd³⁺, Fe³⁺, and Cu²⁺ in the degradation of dyes.
While metal-organic frameworks (MOFs) demonstrate exceptional photocatalytic activity, their industrial application is hindered by poor stability in moisture and protic solvents To address this challenge, researchers are investigating various metal ions and organic linkers to develop more stable MOF materials Notably, MOFs constructed from Zr(IV) clusters have emerged as promising candidates due to their high chemical and water stability, making them potential photocatalysts for future applications.
Attracted by stability of UiO-66, Wu and co-authors have studied the structure– photoactivity relation with an isoreticular series of MOFs UiO-66-X (X = H, NH 2 ,
NO 2 , Br) and showed that the electronic effect of the linker substituents had a systematic and important impact on the photocatalytic activity of the MOFs [98]
Optical absorption of the UiO-66-X samples has been investigated using UV- Vis spectroscopy The main absorption bands of UiO-66 were located in the UV region
The optical properties of UiO-66-X (where X represents functional groups such as NH2, NO2, and Br) are significantly influenced by the presence of these groups, as indicated by absorption edges at approximately 450, 400, and 360 nm, respectively Notably, UiO-66-NH2 exhibits a red-shift in its UV-Vis spectrum, likely due to the amino group's interaction with the p*-orbitals of the aromatic benzene ring, which enhances electron density in the antibonding orbitals This interaction results in an elevated HOMO level, shifting absorption into the visible spectrum Consequently, these modifications to the linker structure are shown to effectively narrow the band gap of UiO-66, highlighting the impact of functional group substitutions on the material's optical characteristics.
Substitutions on the linker significantly affect the surface areas of metal-organic frameworks (MOFs), leading to a reduction in their photoactivity The BET surface areas for UiO-66-X (where X represents H, NH2, NO2, and Br) were measured at 1141.1, 732.2, 464.8, and 455.9 m²/g, respectively This decrease in surface area is primarily due to the reduced free space and the increased weight of the MOFs caused by the introduction of larger, heavier functional groups A larger specific surface area in photocatalysts is theoretically beneficial, as it can provide more active sites and enhance charge carrier transport, positively impacting reaction rates However, the observed BET surface area ranking was UiO-66-H > UiO-66-NH2 > UiO-66-NO2 > UiO-66-Br, while the reaction rate order was UiO-66-NH2 > UiO-66 > UiO-66-Br > UiO-66-.
NO 2 This could be explained that NH 2 group drastically enhance photoinduced property of UiO-66-NH 2 though the group lessen surface areas of the framework [98]
UiO-66-NH2, known for its excellent photoinduced properties and chemical stability, has gained significant attention in photocatalytic applications In 2012, Wang and colleagues identified UiO-66-NH2 as a highly efficient and selective photocatalyst for the aerobic oxygenation of various organic compounds, including alcohols, olefins, and cycloalkanes, under visible light irradiation The UV-Vis diffuse reflectance spectrum reveals that UiO-66-NH2 has an absorption band-edge at 450 nm, leading to a calculated optical band gap of 2.75 eV using the relation E g = 1240/λ.
Under 400 nm laser irradiation at room temperature, UiO-66-NH2 exhibited a broad photoluminescence peak at 477 nm, indicating effective generation and separation of electron-hole pairs upon photon absorption.
UiO-66-NH2 demonstrates high efficiency and selectivity for the aerobic oxidation of various organic compounds under visible light irradiation Specifically, it catalyzes the oxidation of styrenes, yielding over 10% conversion to benzaldehyde, while primarily producing epoxides from alkenes In the case of alcohols, UiO-66-NH2 achieves a complete conversion to aldehydes or ketones However, the conversion yield for cyclohexane remains low due to the strong C-H bonds present in its structure.
The authors propose that the mechanism of UiO-66-NH2 is initiated by photogenerated electron transfer during organic transformations When exposed to light, electrons from the HOMO, which consists of O, C, and N 2p orbitals, are excited to the LUMO and subsequently transferred to O2 molecules adsorbed on Zr3+ sites, resulting in the formation of O2 Meanwhile, photogenerated holes oxidize the organic substrates adsorbed on amine sites into carbonium ions These superoxide radicals then react with the carbocations, ultimately leading to the desired final products.
Table 2.4 Photocatalytic activity of UiO-66-NH 2 for organic transformations [67]
Main products are either benzaldehyde or epoxide Products are a benzaldehyde, bcyclohexanone, c hexanal and, d cyclohexanone
In 2013, Wu and co-authors have systematically studies of selective oxidation of primary alcohols to their corresponding aldehydes over photocatalysis of UiO-66-
Lewis acid catalysis of Zr- and Hf-MOFs
In 2010, De Vos and colleagues investigated the catalytic properties of UiO-66 and its amino-modified variant, UiO-66-NH2, in cross aldolization reactions Their study demonstrated that UiO-66-NH2 can be synthesized using the same method as UiO-66.
NH 2 with simple replacement of terephthalic acid by 2-aminoterephthalic acid The UiO-66 materials were evaluated as catalysts in the synthesis of jasminaldehyde by the reaction between heptanal and benzaldehyde [108]
Scheme 2.1 Cross aldol condensation of heptanal and benzaldehyde catalyzed by UiO-
Prior to catalytic reactions, UiO-66 and UiO-66-NH2 were subjected to high-temperature pretreatment The findings indicated that UiO-66-NH2 exhibited a 10% increase in both activity and selectivity compared to UiO-66 The authors attributed this enhanced performance in the cross-aldol reaction to the synergistic effect of acid and base sites; specifically, a Lewis acid site interacts with the carbonyl group of benzaldehyde, enhancing its polarization and facilitating the nucleophilic attack by heptanal, which is activated at a nearby base site.
Table 2.5 Conversion and selectivity of reaction, and jasminaldehyde yield at full heptanal conversion for the solventless reaction of benzadehyde and heptanal in the presence of 10% w/w catalysts [108]
UiO-66-NH 2 c 38 90 92 a Pretreated under air (423K) b Pretreated under deep vacuum at high temperatures (573 K) c Pretreated under deep vacuum at high temperatures
Van Speybroeck and De Vos investigated the cyclization of citronellal using various UiO-66 metal-organic frameworks (MOFs) modified with different benzene-1,4-dicarboxylates (BDC-X; where X represents H, NH2, CH3, OCH3, F, Cl, Br, NO2) Each catalyst was activated at 493 K prior to the reaction The study found that the presence of electron-withdrawing groups, such as F, Cl, Br, and NO2, significantly increased the reaction rate, with UiO-66-NO2 being the most effective catalyst, achieving complete conversion in just 6 hours.
Scheme 2.2 Cyclization of (+)-citronellal catalyzed by UiO-66-X [109]
De Vos recently introduced a novel synthesis method for UiO-66 by incorporating trifluoroacetic acid (TFA), which partially replaces BDC with trifluoroacetate This innovative Zr-MOF was utilized in the "ene"-type cyclization of citronellal to isopulegol Prior to the reactions, the material underwent activation under vacuum at 320 °C for 12 hours to achieve complete dehydroxylation of the inorganic cluster The findings indicated that even small quantities of TFA significantly enhanced the catalytic activity.
In 2014, Ahn and colleagues presented two innovative methods for incorporating sulfonic acid groups as Brønsted acid sites within the UiO-66 framework These methods include post-synthetic grafting onto the zirconium cluster and a direct solvothermal approach utilizing monosodium 2-sulfoterephthalate.
Scheme 2.3 p-Xylene acylation with benzoyl chloride [14]
The catalytic activity of sulfonated UiO-66 materials, featuring Lewis sites on zirconium clusters and Brønsted acid sites, was investigated in the Friedel-Crafts acylation of p-xylene with benzoyl chloride The findings revealed that sulfonated UiO-66 materials demonstrated a 30% higher conversion efficiency compared to both UiO-66 and other metal-organic frameworks (MOFs).
Table 2.6 p-Xylene acylation with benzoyl chloride using various acid catalysts [14]
In 2015, Truong and co-authors developed a Zr-MOF, known as Zr-AzoBDC, utilizing an azobenzene-based organic linker, azobenzene-4,4ʹ-dicarboxylate, to facilitate the direct amidation of benzoic acid This innovative material leverages the Lewis acid sites of the Zr6 cluster and features an extended organic linker that creates ample space for substrate reactions As a result, Zr-AzoBDC demonstrated significantly higher activity for amide formation under milder conditions compared to other MOFs.
Scheme 2.4 Amidation of benzoic acid catalyzed by Zr-MOFs [42]
Table 2.7 Comparing catalytic ability of MOFs in amidation of benzoic acid [42]
DABCO: 1,4-diazabicyclo[2.2.2]octane, ZIF: Zeolitic Imidazolate
2.3.2.5 Cycloaddition reaction of CO 2 with epoxide
In 2014, a novel hafnium-based metal-organic framework, known as Hf-Nu-1000, was developed using the tetratopic linker 1,3,6,8-tetrakis(p-benzoate)pyrene This framework demonstrated exceptional catalytic performance in the cycloaddition reaction of CO2 with epoxide, achieving a remarkable 100% yield, surpassing the efficiency of other metal-organic frameworks.
Scheme 2.5 Cycloaddition reaction of CO 2 with epoxide catalyzed by Hf-NU-1000 [5]
Table 2.8 Comparing catalytic ability of MOFs in cycloaddition reaction of CO 2 with epoxide [5]
Scope of this dissetation
Zr- and Hf-MOFs have garnered significant interest from researchers due to their diverse coordinated clusters, high chemical stability, and porosity, making them suitable for environmental treatment and photocatalysis applications However, there is limited research on Zr- and Hf-MOFs that utilize long, linear, ethynyl-containing linkers and 12-coordinated clusters, primarily due to the linker's low solubility in organic solvents and the low activity of saturated sites on metal clusters My thesis addresses these challenges by exploring innovative solutions to enhance the performance of these materials.
33 synthetic conditions and exploring new applications Besides, I believed that using ethynyl group can be an effectivestrategy for boosting molecule-accessible gravimetric surface areas of MOFs [24]
Researchers at Vietnam National University have successfully synthesized new Zr(IV)- and Hf(IV)-based metal-organic framework photocatalysts, designated as VNU-1 and VNU-2 Utilizing the linker 1,4-bis(2-[4-carboxyphenyl]ethynyl)benzene, the team employed a modulator technique to effectively control the crystal formation and size of these materials The study details the optimization of the synthesis process, along with comprehensive structural analysis, characterization, and evaluation of chemical stability.
The incorporation of a highly π-conjugated linker in VNU-1 and VNU-2 resulted in a significant red-shift of their optical absorption properties into the visible light spectrum This enhancement, combined with the materials' exceptional water stability, facilitated improved photocatalytic degradation of organic dye pollutants in wastewater under ultraviolet-visible light Additionally, the study explored the regeneration and recyclability of these materials.
Inspired by the strong Lewis acidity and stability of Zr 4+ and Hf 4+ clusters, microwave irradiation was employed to enhance the Lewis acid activity of VNU-1 and VNU-2, effectively reducing the reaction time in the Friedel–Crafts benzoylation of aromatic compounds This study includes a comparison between microwave irradiation and conventional heating, alongside an analysis of materials and other isostructural MOFs used in the reaction Additionally, recycling experiments and structural integrity tests were conducted to assess the performance of the materials.
Objectives
In our research, we explored the use of novel chemically stable metal-organic frameworks, VNU-1 and VNU-2, based on Zr(IV) and Hf(IV), for applications in heterogeneous catalysis.
Objectives 1: Syntheses of two novel stable Zr- and Hf-MOFs
Synthesis of long, slim ethynyl-containing linker, 1,4-bis(2-(4-carboxy- phenyl)ethynyl)benzene (H 2 CPEB)
Syntheses of microcrystal VNU-1 and VNU-2 powder (termed VNU-1-P and VNU-2-P, respectively; where P = powder)
Syntheses of single crystals of VNU-1 and VNU-2 (termed VNU-1-SC and VNU-2-SC, respectively; where SC = single crystal)
Objective 2: Structural analysis and characterization of VNU-1 and VNU-2
Characterization of by PXRD, FTIR, TGA, and measurement of nitrogen, carbon dioxide, and methane isotherm
Investigation of the chemical stability
Objective 3: VNU-1-P and VNU-2-P used as heterogeneous photocatalysts
Photocatalytic ability of the materials in degradation of organic dye pollutants, methylene blue (MB) and methyl orange (MO)
Comparing photocatalysis of the synthesized materials and other materials
Structural stability and recyclability of the materials after reaction
Objectives 4: VNU-1-P and VNU-2-P used as heterogeneous catalysts in Friedel–Crafts benzoylation
Friedel–Crafts benzoylation of aromatic compounds
Scheme 3.1 Friedel–Crafts benzoylation of aromatic compounds catalyzed by VNU-1 and VNU-2
Comparison of microwave irradiation and conventional heating
Comparing Lewis acid catalytic ability of the materials and others such as UiO-66, UiO-67, and zirconium and hafnium salts
Recyclability and structural stability of the materials after reaction
Syntheses of two novel stable Zr- and Hf-MOFs
Synthesis of long, slim ethynyl-containing linker, 1,4-bis(2-[4-carboxy- phenyl]ethynyl)benzene (H 2 CPEB)
Scheme 3.2 Synthesis of the ditopic linker, H 2 CPEB, was accomplished by Sonagashira coupling and hydrolysis reaction
The ditopic linear linker was synthesized through a Sonogashira coupling reaction followed by hydrolysis, as detailed in previous studies (Scheme 3.2) The Sonogashira coupling involved 1,4-diethynylbenzene and methyl 4-iodobenzoate, catalyzed by Pd(PPh3)2Cl2 and CuI under a nitrogen atmosphere at room temperature, yielding 1,4-bis(4-carbomethoxyphenylethynyl)benzene (Me2CPEB) Subsequently, the hydrolysis reaction utilized potassium hydroxide in a methanol, THF, and water mixture to produce the final product, 1,4-bis(2-(4-carboxyphenyl)ethynyl)benzene (H2CPEB), with a total yield of 72.3%.
37 on molar amount of 1,4-diethynylbenzene The formula and structure of H 2 CPEB were characterized and confirmed by NMR, HRMS (ESI), and FTIR (Table 3.1)
Table 3.1 Characterization and structure analysis of H 2 CPEB
HRMS (ESI) Calculated for C 24 H 14 O 4 : m/z = 366.09; Found m/z
FTIR (KBr, 4000-400 cm -1 ) 3413 (br), 3076 (br), 3039 (w), 3013(w), 2966 (w),
3.2.2 Syntheses of microcrystal of VNU-1 and VNU-2 (VNU-1-P and VNU-2-P, respectively)
VNU-1 and VNU-2 were synthesized through a solvothermal reaction using H2CPEB and ZrOCl2·8H2O or HfCl4 in N,N-dimethylformamide (DMF), with carboxylic acid as a modulator We investigated the influence of various factors on the crystal formation of VNU-1 and VNU-2, including the molar ratio of salts to linker, starting material concentration, temperature, reaction time, and modulator concentration Powder X-ray diffraction (PXRD) measurements were employed to assess the crystallinity of the samples.
The synthesis of VNU-1-P was initiated by selecting the molar ratio of ZrOCl2·8H2O salt to the linker H2CPEB, with benzoic acid acting as a modulator Various concentrations of Zr salt were combined with a mixture of 5x10^-3 mol/L H2CPEB and 0.1 mol/L benzoic acid in DMF Following 30 minutes of ultrasound irradiation to achieve a clear solution, the reaction mixtures were maintained at 120°C under static conditions for 24 hours After cooling to room temperature, the resulting suspensions were washed three times with DMF to eliminate residual precursors and isolated through centrifugation The collected solids were then dried at room temperature and analyzed using PXRD measurements.
Table 3.2 Effect of molar ratio of ZrOCl 2 ã8H 2 O salt to H 2 CPEB linker on VNU-1-P crystallinity
The PXRD patterns of samples with varying molar ratios of ZrOCl2·8H2O and linker H2CPEB revealed a broad peak at 2θ = 4°, indicating a potential large unit cell structure according to Bragg's law Notably, the samples with equal molar amounts of ZrOCl2·8H2O and H2CPEB exhibited the highest intensity at 2θ = 4° and introduced a new signal at 2θ = 6.5° Consequently, equal concentrations of ZrOCl2·8H2O and linker H2CPEB were selected for further investigations.
Figure 3.1 Comparing PXRD patterns of samples with different molar ratios of ZrOCl 2 ã8H 2 O to linker H 2 CPEB
We investigated the impact of varying concentrations of ZrOCl₂·8H₂O and the linker H₂CPEB on the synthesis of VNU-1-P, using optimal molar ratios The reactions involved mixing equal molar amounts of ZrOCl₂·8H₂O and H₂CPEB, along with 0.1 mol/L benzoic acid, dissolved in DMF and subjected to ultrasound irradiation for 30 minutes The mixtures were then heated in an isothermal oven at 120°C for 24 hours Following cooling to room temperature, the precipitates were washed with DMF and collected via centrifugation, with the crystallinity of the resulting solid products analyzed through PXRD.
Table 3.3 Effect of concentration of ZrOCl 2 ã8H 2 O and linker H 2 CPEB on VNU-1-P crystallinity
The PXRD patterns of samples with varying concentrations of ZrOCl2·8H2O and the linker H2CPEB indicated that the collected solids exhibited low crystallinity However, it was observed that increasing the concentration in the reaction solution resulted in higher crystallinity of the solids Notably, the PXRD pattern for the sample at a concentration of 8x10^-3 mol/L displayed strong intensity signals at 2θ values of 4° and 6.5°, along with a new signal appearing at 2θ 15.2° Consequently, we identified 8x10^-3 mol/L as the optimal concentration for ZrOCl2·8H2O and H2CPEB in the synthesis of VNU-1-P.
Figure 3.2 Comparing PXRD patterns of samples with different concentrations of ZrOCl 2 ã8H 2 O and linker H 2 CPEB
Following the optimization of the molar ratio of salt to linker and precursor concentration, we investigated the impact of temperature on the synthesis of VNU-1-P In our experiments, we dissolved mixtures of ZrOCl2·8H2O, the linker H2CPEB, and benzoic acid in DMF, heating them at temperatures ranging from 80 to 150°C (as shown in Table 3.4) The resulting samples were analyzed using PXRD to assess their crystallinity.
Table 3.4 Effect of reaction temperature on VNU-1 crystallinity
The PXRD analysis of VNU-1-P samples revealed that the optimal synthesis temperature is 120 °C, as indicated by the distinct patterns observed (Figure 3.3) At 85 °C, the sample resulted in an amorphous solid with no detectable PXRD signals Samples heated at 100 °C and 150 °C exhibited some low-angle signals, but they presented fewer peaks and lower intensity compared to those heated at the optimal 120 °C.
Figure 3.3 Comparing PXRD patterns of samples heated at various temperatures
Following the optimization of the temperature for VNU-1-P synthesis, we further investigated the impact of reaction time on the synthesis process In this experiment, a solution containing 8.0x10^-3 mol/L of ZrOCl2·8H2O, 8.0x10^-3 mol/L of the linker H2CPEB, and 0.1 mol/L of benzoic acid was prepared in DMF and subjected to ultrasound irradiation for 30 minutes before heating the samples.
120 o C in isothermal oven for different periods of time to get yellow powder (Table 3.5) After reaction, the growth of VNU-1-P crystals by the time was monitored by PXRD method
Table 3.5 Effect of reaction time on VNU-1-P crystallinity
The PXRD patterns indicate that crystal growth is influenced by the reaction time, with precipitates first appearing after 12 hours of heating, although they remained amorphous Continued heating led to the formation of a powder characterized by two broad signals at low angles in the PXRD patterns Notably, the crystallinity of the samples heated for 24 hours was found to be optimal, as subsequent heating did not alter the PXRD patterns Therefore, we opted to heat the samples for this duration.
Figure 3.4 Comparing PXRD patterns of samples heated for 12, 18, 24, 30, and 36 h
In our investigation of VNU-1-P synthesis, we examined the impact of modulator concentration in the reaction solution Various amounts of benzoic acid were incorporated into a DMF solution containing ZrOCl2·8H2O and the linker H2CPEB (as detailed in Table 3.6) Following a 24-hour heating period at 120°C, the resulting solid products were analyzed using PXRD to evaluate the effects of modulator concentration on crystallinity.
Table 3.6 Effect of modulator concentration on VNU-1-P crystallinity
The concentration of the modulator plays a crucial role in the synthesis of crystalline VNU-1-P As indicated by the PXRD patterns in Figure 3.5, increasing the concentration of benzoic acid leads to improved crystallinity of VNU-1-P.
The analysis of samples with benzoic acid concentrations of approximately 1.4x10^-4 and 1.6x10^-4 mol/L revealed higher intensity signals compared to those from lower concentrations Additionally, samples with benzoic acid concentrations exceeding 1.8x10^-4 mol/L demonstrated excellent crystallinity, characterized by highly diffracted signals in the range of 3 to 30 degrees Given the similarity in patterns between samples at 1.8x10^-4 and 2.0x10^-4 mol/L, we opted to use 1.8x10^-4 mol/L as the modulator concentration for further investigations into VNU-P synthesis.
Figure 3.5 Comparing PXRD patterns of samples in which benzoic acid was used as modulator was added with various concentrations
In the synthesis of Zr-MOFs, monocarboxylic acids such as benzoic acid, acetic acid, and formic acid serve as modulators, significantly influencing compound formation This study investigates the impact of these modulators on the synthesis of VNU-1-P Experimental vials were prepared by adding benzoic acid, acetic acid, and formic acid to a DMF solution containing ZrOCl2·8H2O and the linker H2CPEB The crystallinity of the resulting samples was analyzed using the PXRD method.
Table 3.7 Effect of modulators on VNU-1-P crystallinity
The PXRD patterns revealed that the addition of various monocarboxylic acids significantly influenced the crystallinity of VNU-1-P powder, with acetic acid and formic acid yielding higher crystallinity compared to benzoic acid Notably, the PXRD results for VNU-1-P powder modified with acetic and formic acids exhibited sharp peaks and high intensity signals within the 3 to 30° range However, despite achieving a highly crystalline powder, formic acid was deemed unsuitable for VNU-1-P synthesis due to its low boiling point.
120 o C (~100.8 o C) and is very easy to decompose at high temperature to generate CO 2 leading to explosion of glass reaction vials Correspondingly, acetic acid was used as modulator in further studies
Figure 3.6 Comparing PXRD patterns of samples with various modulators
Structural analyses and characterization of VNU-1 and VNU-2
Structural analyses of VNU-1-SC and VNU-2-SC
The single crystal data for octahedral shaped crystal of VNU-1-SC and VNU-2-
Single crystals of VNU-1-SC and VNU-2-SC were isolated from the mother liquor using a nylon loop and mounted for analysis with the SCXRD system The crystallographic data for both VNU-1 and VNU-2 were solved and refined using the SHELXL-97 program, as illustrated in Figures 3.8 and 3.9.
Figure 3.8 The asymmetric unit of VNU-1 showing Zr 6 cluster coordinated by twelve CPEB linkers The fragment unit drawn by ORTEP with thermal ellipsoids style at 50% probability
Figure 3.9 The asymmetric unit of VNU-1 showing Hf 6 cluster coordinated by twelve CPEB linkers The fragment unit drawn by ORTEP with thermal ellipsoids style at 50% probability
Table 3.11 Summary of crystallographic data for VNU-1-SC and VNU-2-SC
Single crystal X-ray diffraction analysis revealed that VNU-1-SC and VNU-2-
The SC crystallizes in the Fd-3m space group with lattice parameters a = 39.8961 Å and 39.7901 Å, as detailed in Table 3.11 These materials are isostructural to the PIZOF series, differing primarily in that the CPEB linker remains unfunctionalized and VNU-2-SC is derived from Hf(IV) instead of Zr(IV) Both structures feature a doubly interpenetrated fcu-c net characterized by the formula [M 6 O 4 (OH) 4 (CO 2 ) 12 ], where M represents Zr(IV) or Hf(IV).
Hf(IV) for VNU-1 or VNU-2, respectively) SBUs, in which the inner
The M6O4(OH)4(CO2)12 SBU cores are capped alternately by à3-O and à3-OH groups, forming triangular faces The interpenetration occurs due to the length of the linkers, which create triangular windows large enough for three additional linkers to pass through The SBUs are interconnected by twelve carboxylate groups from CPEB linker units, resulting in both tetrahedral and octahedral cages In the case of two-fold interpenetration, the SBU from one fcu net is positioned at the center of the tetrahedral cages belonging to the second net, leading to one large tetrahedral cage (~25 Å) and one smaller octahedral cage (9 Å) The total solvent-accessible volumes for VNU-1 and VNU-2, as determined by PLATON, are both 68%.
Figure 3.10 Crystal structure and topology of VNU-1 and VNU-2: (a) Cuboctahedral
The crystal structures of VNU-1 and VNU-2 feature a secondary building unit (SBU) represented by [M 6 O 4 (OH) 4 (CO 2 ) 12 ], where M denotes Zr(IV) for VNU-1 and Hf(IV) for VNU-2 These structures include large 25 Å tetrahedral cages and smaller cages formed by the interpenetration of a second net In the interpenetrated fcu-c net, the cuboctahedral SBU of one net is centrally located within the tetrahedral cage of the second net The SBU is characterized by cuboctahedron shapes connected by linear linkers, with atom colors indicating Zr or Hf in blue and orange polyhedra, oxygen in red, carbon in black, and hydrogen atoms omitted for clarity The framework also features free space represented by yellow and salmon colored balls.
Characterization of VNU-1 and VNU-2
3.3.2.1 PXRD analyses of VNU-1-P and VNU-2-P
To improve the yield of microcrystalline powders for VNU-1 and VNU-2, the amount of acetic acid modulator was reduced, resulting in higher production rates The purity of the bulk phases was confirmed through PXRD measurements, which showed that the experimental patterns for VNU-1-P and VNU-2-P closely matched the calculated patterns for their respective single crystal structures Additionally, scanning electron microscope images revealed a homogeneous crystal morphology for both VNU-1-P and VNU-2-P, further validating the purity of these samples Consequently, the microcrystalline powders VNU-1-P and VNU-2-P were utilized for all subsequent characterizations and applications.
Figure 3.11 PXRD analysis of VNU-1 The calculated pattern from single crystal data is compared to the experimental patterns from the as-synthesized powder sample and activated powder sample
Figure 3.12 PXRD analysis of VNU-2 The calculated pattern from single crystal data is compared to the experimental patterns from the as-synthesized powder sample and activated powder sample
Figure 3.13 Scanning electron microscope image demonstrating uniform octahedral crystal morphology of the bulk VNU-1-P sample
Figure 3.14 Scanning electron microscope image demonstrating uniform octahedral crystal morphology of the bulk VNU-2-P sample
3.3.2.2 Thermal gravimetric analyses (TGA) of VNU-1-P and VNU-1-P
Before further characterization, the as-synthesized VNU-1-P and VNU-2-P samples were extensively washed with DMF to eliminate unreacted materials Subsequently, they underwent a solvent exchange with chloroform over three days The activation of the chloroform-washed VNU-1-P and VNU-2-P samples was conducted under reduced pressure.
The activation procedure and architectural robustness of VNU-1-P and VNU-2-P were evaluated using thermogravimetric analysis (TGA) under airflow at 120 °C for 24 hours Both structures showed minimal weight loss before reaching decomposition temperatures of 430 °C for VNU-1-P and 460 °C for VNU-2-P Post-decomposition, the residue weight percentages were 26.3% for VNU-1-P and 37.5% for VNU-2-P, aligning closely with theoretical values of 25.3% and 36.9%, respectively Additionally, the weight percentages of the linker derived from the TGA curves were 72.6% for VNU-1-P and 61.8% for VNU-2-P, corroborating with elemental microanalysis results of 73.5% and 63.8%.
Figure 3.15 Thermal gravimetric analysis of activated VNU-1-P at heating rate of 5 oC/min under airflow
Figure 3.16 Thermal gravimetric analysis of activated VNU-2-P at heating rate of 5 oC/min under airflow
3.3.2.3 Fourier transform infrared (FTIR) spectroscopic measurements
FTIR spectroscopy was utilized to analyze the materials, revealing significant findings in the FTIR spectra of activated samples and H2CPEB The spectra for VNU-1-P and VNU-2-P displayed characteristic adsorption bands for C-O vibrations, notably two prominent bands between 1400 and 1700 cm−1 The band at 1656 cm−1 is attributed to the asymmetric stretching of the carboxylate group (COO), while the band at 1607 cm−1 corresponds to the symmetric stretching vibration of the same group Additionally, a strong band at 1413 cm−1 is associated with the C−C skeletal vibration of the aromatic ring.
These values are consistent with the presence of CO 2 − groups coordinating to zirconium or hafnium
Figure 3.17 FTIR spectra of VNU-1-P, VNU-2-P, and H 2 CPEB in dry KBr
3.3.2.4 Gas adsorption analyses of VNU-1-P and VNU-2-P
The porosity of both guest free samples was established by N 2 isotherms at 77
K, which afforded Type-IV behaviors corresponding to the presence of mesopores within the structures (Figures 3.18 and 3.19) The calculated BET surface areas of VNU-1-P and VNU-2-P were 2100 and 1700 m 2 /g, respectively Although the materials have the same structure, the surface area value of VNU-1 is higher than that of VNU-2 due to the difference of their density values (Table 3.11)
The N2 isotherm for activated VNU-1-P at 77 K is depicted in Figure 3.18, showcasing both adsorption and desorption branches indicated by closed and open circles, respectively Additionally, the inset illustrates the linear region of the N2 isotherm analyzed through the BET equation, with a connecting line serving as a visual guide.
The N2 isotherm for activated VNU-2-P at 77 K is illustrated in Figure 3.19, showcasing both the adsorption and desorption branches, indicated by closed and open circles, respectively The inset features a linear plot of the N2 isotherm analyzed using the BET equation, with a connecting line serving as a visual aid.
Building on the findings of the elevated surface areas of VNU-1-P, we conducted an investigation into its carbon dioxide and methane adsorption capabilities The adsorption isotherms for activated VNU-1-P were measured at a temperature of 298 K across a pressure range up to 800 Torr (Figure 3.20).
Figure 3.20 CO 2 and CH 4 isotherms for VNU-1-P at 298 K Closed and open circles represent the adsorption and desorption branches, respectively The connecting line is provided as a guide for the eyes
The increasing levels of atmospheric carbon dioxide due to human activities pose a significant environmental threat To address this issue, carbon capture and sequestration technologies that effectively capture CO2 from emission sources are essential Metal-organic frameworks (MOFs) offer a promising solution for CO2 capture, providing a more sustainable and efficient method that captures larger amounts of CO2 while using less energy for regeneration Notably, MOFs with open metal sites, such as Mg-MOF-74 and HKUST-1, demonstrate impressive CO2 uptake capacities of up to 27.5 at 298 K and 800 Torr, as detailed in Table 3.12.
74 and 18.4% w/w, respectively These results were contributed by their high interaction of CO 2 with exposed metal sites on the frameworks According to the CO 2 isotherm at
At a temperature of 298 K, the VNU-1-P material demonstrated a calculated CO2 capture capacity of only 5.2% w/w, likely due to the reduced interaction between saturated zirconium sites and CO2 compared to other metal sites Consequently, this suggests that VNU-1-P may not be an effective option for CO2 capture applications.
Table 3.12 Lower-pressure carbon dioxide adsorption capacities for metal-organic frameworks at 298 K
Formula Common name BET surface areas (m 2 /g)
Zr 6 O 4 (OH) 4 (CPEB) 6 VNU-1-P 2100 5.2 This work
DOBDC: 2,5-dihydroxyterephthalate, BTC: benzene-1,3,5-tricarboxylate, BDC: benzene-1,4-dicarboxylate, BTB: benzene-1,3,5-tribenzoate, BPyDC: 2,2′-bipyridine- 5,5′-dicarboxylic acid
The methane adsorption capacity of VNU-1-P at 298 K and 800 Torr is relatively low compared to other metal-organic frameworks (MOFs) This reduced capacity can be attributed to the nature of methane as a non-polar molecule, which relies on Van der Waals interactions for effective adsorption.
75 methane and the framework is very vital in methane adsorption; therefore, the mesoporous VNU-1-P has lower interaction with methane than the microporous materials
Table 3.13 Lower-pressure methane adsorption capacities for metal-organic frameworks at 298 K
Al 8 (OH) 8 (BTB) 4 (H 2 BTB) 4 MOF-519 2400 14.8 [33]
Al 8 (OH) 8 (OOCH) 4 (BTB) 4 MOF-520 3290 13.4 [33]
Zr 6 O 4 (OH) 4 (CPEB) 6 VNU-1-P 2100 0.6 This work
DMeIm: 4,5-dimethylimidazole, 4Me5AlIm: 4-methylimidazole-5-carbaldehyde, 4Cy5AmIm:4-aminoimidazole-5-carbonitrile.
Evaluation of Brứnsted acidity in VNU-1-P and VNU-2-P
In the M6O4(OH)4(CO2)6 cluster, where M represents Zr or Hf, the hydroxyl group bonded to Zr4+ or Hf4+ acts as a Brønsted acid A key aspect often discussed is the interplay between Lewis and Brønsted acidity in the catalysis of Zr- and Hf-MOFs To investigate this for the VNU-1-P and VNU-2-P cases, we employed potentiometric acid-base titration to assess their acid-base properties.
Brứnsted acidity of the materials The titration and pK a calculation were performed following a literature procedure (see Experimental)
Table 3.14 Calculated pK a values for Zr- and Hf-MOFs
Formula Common name pK a à 3 -OH
Zr 6 O 4 (OH) 4 (BDC) 6 Zr-UiO-66 3.52 [56]
Hf 6 O 4 (OH) 4 (BDC) 6 Hf-UiO-66 3.37 [56]
Zr 6 O 4 (OH) 4 (BPDC) 6 Zr-UiO-67 3.44 [56]
Hf 6 O 4 (OH) 4 (BPDC) 6 Hf-UiO-67 3.36 [56]
Zr 6 O 4 (OH) 4 (BTC) 2 (HCOO) 6 MOF-808 3.64 [56]
Zr 6 O 4 (OH) 4 (CPEB) 6 VNU-1-P 3.49 This work
Hf 6 O 4 (OH) 4 (CPEB) 6 VNU-2-P 3.42 This work
Figure 3.21 Acid–base titration curve of VNU-1-P and first derivative curve The connecting line is provided as a guide for the eyes
Figure 3.22 Acid–base titration curve of VNU-2-P and first derivative curve The connecting line is provided as a guide for the eyes
The acid-base titration curves indicate that the pK a values for the M-à 3 -OH protons in VNU-1-P and VNU-2-P are 3.49 and 3.42, respectively, suggesting that these materials act as weak Brønsted acids The acidic catalysis primarily arises from the Lewis acid sites of metal clusters Notably, Hf-MOFs exhibit stronger acidity in their M-à 3 -OH protons compared to Zr-MOFs, which may be attributed to the differing dissociation enthalpies of Hf−O and Zr−O bonds (802 vs .).
776 kJ/mol),Hf is more oxophilic (i.e., has stronger M−O bonds) than Zr and should thus function as a stronger Brứnsted acid
Investigation of the chemical stability
The Zr 6 cluster in the structures of VNU-1 and VNU-2 is fully occupied by 12 carboxylates from CPEB linkers, resulting in the highest connectivity among Zr-MOFs and indicating excellent chemical stability Following a comprehensive structural analysis and characterization of VNU-1 and VNU-2, we evaluated the chemical stability of VNU-1-P, noting that stability tends to decrease with longer linker lengths In our experiments, activated samples (10 mg) were immersed in 10 mL of aqueous solutions at varying pH levels (prepared using HCl and NaOH) for different durations The samples were then filtered, dried at room temperature, and analyzed using PXRD.
We conducted a stability test of VNU-1 in water at room temperature, revealing that the PXRD patterns maintained the characteristic signals of the VNU-1-P structure throughout the experiment The results indicate that VNU-1 demonstrates significant stability, remaining unchanged after 7 days in water at room temperature.
Figure 3.23 PXRD patterns of VNU-1-P after treatment with water at room temperature
Following the evaluation of the water-table capability of VNU-1-P at room temperature, we proceeded to assess the material's stability at elevated temperatures The samples were subjected to heating in an isostatic oven at 50°C and 100°C, after which structural changes were analyzed using the PXRD method.
Figure 3.24 PXRD patterns of VNU-1-P after treatment with water at 50 o C
Figure 3.25 PXRD patterns of VNU-1-P after treatment with water at 100 o C
The results presented in Figures 3.24 and 3.25 indicate that the PXRD patterns remain largely unchanged after high-temperature treatment for 7 days The peaks observed in the powder XRD patterns of the treated samples align closely with those simulated from single crystal data, confirming the structural stability of the material.
The structural resistance of VNU-1-P to acidic aqueous solutions was investigated by immersing the samples in a hydrochloric acid solution (pH = 1) at room temperature for varying durations After immersion, the samples were filtered, washed with deionized water, and dried at room temperature before undergoing analysis through powder X-ray diffraction (PXRD).
Figure 3.26 PXRD patterns of VNU-1-P after treatment with acidic solution (pH = 1)
PXRD analysis reveals that VNU-1-P maintains its crystallinity after being immersed in an aqueous solution with a pH of 1 for 7 days, indicating remarkable stability in acidic conditions Typically, metal-organic frameworks (MOFs) are prone to instability in protonic solutions due to the weak metal ion-carboxylate bonds However, the high stability of VNU-1-P in acidic environments is likely attributed to the presence of zirconium, a highly oxophilic metal that, due to its high oxidation state, contributes to the formation of robust frameworks capable of withstanding acidic solutions.
To investigate the stability of VNU-1-P in a basic aqueous solution, samples were immersed in a NaOH solution with a pH of 11 at room temperature Following this treatment, the samples underwent filtration, were washed with deionized water, and then dried at room temperature before being analyzed using PXRD.
Figure 3.27 PXRD patterns of VNU-1-P after treatment with basic solution (pH = 11)
The PXRD analysis revealed that VNU-1-P maintained its crystalline structure after 8 hours of treatment with an aqueous NaOH solution (pH 11) However, after this duration, the material's crystallinity began to decline, ultimately leading to complete structural decomposition after 12 hours in the basic solution Thus, VNU-1-P demonstrates stability in a basic environment for up to 8 hours.
Figure 3.28 PXRD patterns of VNU-2-P after different treatments
Stability tests of VNU-2-P, treated with various aqueous solutions, reveal that the compound maintains well-defined PXRD patterns, indicating its stability Notably, VNU-2-P remains stable in boiling water and hydrochloric acid (HCl) at pH 1 for 7 days, as well as in sodium hydroxide (NaOH) at pH 11 for 8 hours.
The materials VNU-1-P and VNU-2-P exhibit exceptional stability in aqueous solutions across a pH range of 1 to 11 and various temperatures, attributed to the interpenetration of their frameworks This structural robustness enables their effective use in harsh chemical environments.
VNU-1-P and VNU-2-P used as heterogeneous photocatalysts
Introduction
Industrial plants are generating more wastewater that often contains toxic organic dyes, leading to significant environmental issues These dyes are resistant to standard biological treatments and hinder the removal of other pollutants by absorbing sunlight, which is essential for algal-bacterial growth A promising solution for degrading organic dye pollution is the use of heterogeneous photocatalysts, which harness sunlight to convert harmful pollutants into biodegradable or less toxic substances While commonly used metal oxides like TiO2 and ZnO serve as effective photocatalysts, they present challenges for practical applications.
The large band gaps of these materials, approximately 3.2 eV, limit their light absorption to wavelengths below 380 nm, making them effective primarily in the UV region for photocatalytic and photodegradation applications Additionally, their common suspension in aqueous solutions results in inefficiencies regarding recycling for subsequent use Consequently, there is a strong demand for the development of new heterogeneous photocatalytic systems that can operate in the visible light spectrum and allow for effective recycling.
Zr-MOFs, which are metal-organic frameworks based on Zr(IV) clusters, exhibit exceptional chemical and water stability, making them highly sought after for environmental applications that other MOFs cannot effectively address Despite their impressive water stability, the photocatalytic properties of these frameworks are primarily relevant in specific contexts.
UV region due to the linkers absorbing light primarily at wavelengths < 300
87 nm.[35,36,67,103] Integration of auxochromic and bathochromic functionalities, such as –
NH 2 or –NO 2 groups, on the linkers of MIL-125(Ti)[21,27,41,44,55,75] and UiO-66 and -
Recent studies have demonstrated that specific modifications can effectively shift optical absorption from the UV to the visible light spectrum, significantly enhancing the photocatalytic properties of materials Among the proposed methods for improving optical absorption in water-stable Metal-Organic Frameworks (MOFs), computational research suggests that increasing the π-conjugation of the linker backbone by incorporating additional aromatic rings or alkyne units is a promising approach.
Indeed, this was recently realized by an isoreticular (having the same topology) UiO-66 structure constructed from a chromophoric anthracene-derived linker [86]
In this study, we demonstrate a strategy for further tailoring the optical absorption properties of water-stable VNU-1 and VNU-2 through high π-conjugation of the linker CPEB.
Photosorption analysis of VNU-1-P and VNU-2-P
For investigating photoactivity of VNU-1P and VNU-2-P, we performed UV-Vis diffuse reflectance spectroscopy (UV-DRS) measurements on the activated samples and calculated the band gap values
Figure 3.29 Ultraviolet-visible light diffuse reflectance spectra of activated VNU-1-P,
VNU-2-P, and H 2 CPEB Inset: Optical image highlighting the materials‘ color
The VNU-1-P spectrum shows a strong absorption band at 380 nm, extending to 540 nm, which correlates with the yellow color of the powder sample In contrast, the UV-DRS spectra for VNU-2-P and the original H2CPEB linker reveal absorption band-edges at 369 nm and 320 nm, respectively Notably, the absorbance band-edge of VNU-1-P is significantly red-shifted compared to most other water-stable and photoactive metal-organic frameworks (MOFs), particularly UiO-.
The optical band gaps of VNU-1-P and VNU-2-P were determined to be 2.88 eV and 3.36 eV, respectively, using the formula E g = 1240 / λ Additionally, the results indicate that the CPEB linker did not enhance the optical absorption properties of VNU-2-P.
Table 3.15 Summary of the photoabsorption properties and pertinent structural information of VNU-1, VNU-2, and other related MOF structures
MIL-125(Ti) Ti 8 O 8 (OH) 4 (BDC) 6 345 3.60 [41]
UiO-66-Br Zr 6 O 4 (OH) 4 (BDC-Br) 12 360 3.44 [98]
VNU-2-P Hf 6 O 4 (OH) 4 (CPEB) 12 369 3.36 This work
UiO-66-NO 2 Zr 6 O 4 (OH) 4 (BDC-NO 2 ) 12 400 3.10 [98]
VNU-1-P Zr 6 O 4 (OH) 4 (CPEB) 12 430 2.88 This work
UiO-66-NH 2 Zr 6 O 4 (OH) 4 (BDC-NH 2 ) 12 450 2.75 [67]
UiO-66(AN) Zr 6 O 4 (OH) 4 (ANDC) 12 502 2.47 [86]
BDC: 1,4-benzenedicarboxylate; BPDC: biphenyl-4,4‘-dicarboxylate; TPDC: [1,1‘:4‘,1‖-terphenyl]-4,4‖-dicarboxylate; ANDC: anthracene-9,10-dicarboxylate; n.d = No experimental data available; a Based on theorical calculation
Figure 3.30 Band gap calculations for VNU-1-P (red), VNU-2-P (blue), and H 2 CPEB (black).
Photocatalytic degradation of organic dye pollutants
To evaluate the photocatalytic capabilities of VNU-1-P and VNU-2-P, methylene blue (MB) and methyl orange (MO) were selected as model dye pollutants in aqueous solutions Prior to the photocatalytic reactions, the mixture was stirred in the dark at 25 °C for 30 minutes to achieve equilibrium and ensure proper dispersion of the photocatalysts The effectiveness of VNU-1-P and VNU-2-P under UV-Vis light was evidenced by a noticeable color change from dark to light within just 30 minutes The photocatalytic performance was assessed by monitoring the reduction in absorbance of MB and MO at their respective wavelengths of λ max = 661 nm and λ max = 464 nm.
91 respectively, which is related to structural changes of the dye molecules‘ chromophoric unit (Figures 3.31 and 3.32)
Figure 3.31 UV-Vis absorption spectra of the (a) MB and (b) MO solutions over the course of UV-Vis light irradiation in the presence of VNU-1-P
Figure 3.32 UV-Vis absorption spectra of the (a) MB and (b) MO solutions over the course of UV-Vis light irradiation in the presence of VNU-2-P
The degradation rate profiles of the VNU-1-P photocatalytic system demonstrate its superior activity compared to the commercially available Degussa P-25 TiO2 under the same conditions After 3 hours of exposure, VNU-1-P achieved a complete degradation of 100% for methylene blue (MB) and 83% for methyl orange (MO).
The VNU-1-P catalyst demonstrates significantly higher conversion rates compared to the Degussa P-25 TiO2, attributed to its absorbance band-edge of 430 nm extending to 540 nm, allowing for greater visible light absorption Additionally, VNU-1-P's larger accessible surface area enhances photocatalytic activity by providing more active sites and facilitating charge carrier transport A control experiment confirmed that without any catalyst, the photodegradation of MB and MO was ineffective under UV-Vis light alone.
The degradation profiles of methylene blue and methyl orange under UV-Vis light were analyzed using VNU-1-P and Degussa P-25 TiO2 as catalysts, alongside a control experiment without a catalyst Results indicated distinct degradation patterns, as illustrated by the varying shapes representing each method The lines connecting these shapes serve as visual aids for interpreting the data.
To evaluate the performance of VNU-1-P against isoreticular structures without UV-visible light responsivity, control reactions were conducted using UiO-66 and UiO-67 as photocatalysts The results demonstrated that UiO-66 and UiO-67 exhibited considerably lower photocatalytic activity for dye degradation, underscoring the superior advantage of the visible light-responsive VNU-1-P.
Figure 3.34 Degradation profiles of methylene blue (diamonds) and methyl orange (squares) under UV-Vis light by UiO-66 (a) and UiO-67 (b) The lines connecting the shapes are a guide for the eyes
The photocatalytic activity of VNU-2-P is significantly less effective compared to the highly active VNU-1-P and the well-known Degussa P-25 TiO2, as illustrated in Figure 3.35 After irradiation, the final concentrations of methylene blue (MB) and methyl orange (MO) treated with VNU-2-P were notably lower.
3 h was calculated to be 53 and 72% of the initial concentration, respectively, which was expected due to the larger measured band gap for this material
Figure 3.35 illustrates the degradation profiles of methylene blue (a) and methyl orange (b) when exposed to UV-Vis light, comparing the effectiveness of VNU-2-P (triangles), Degussa P-25 TiO2 (squares), and a control experiment without a catalyst (diamonds) The connecting lines serve as visual guides for interpretation.
Photoluminescence (PL) spectroscopy was utilized to investigate the mechanism of photocatalytic degradation, revealing broad PL peaks for H2 CPEB, VNU-1-P, and VNU-2-P at 432, 450, and 410 nm, respectively, under 365 nm laser irradiation The reduced intensity of PL peaks in VNU-1-P and VNU-2-P indicates charge transfer from CPEB to metal oxo clusters, with VNU-1-P showing a redshift and lower intensities compared to H2 CPEB, suggesting efficient generation and separation of electron-hole pairs These findings imply that the photocatalytic degradation of MB and MO by VNU-1-P and VNU-2-P occurs through electron-hole pair generation under UV-Vis irradiation, leading to the production of hydroxyl radicals via photoinduced energy transfer to adsorbed oxygen and water molecules, a mechanism commonly observed in various photocatalytic MOF systems.
The resulting hydroxyl radicals effectively react and decompose MB and MO accordingly.[20,32,45,51]
Figure 3.36 PL spectra of H 2 CPEB (black), VNU-1-P (red), and VNU-2-P (blue) at excitation wavelength 365 nm.
Structural stability and recyclability of VNU-1-P and VNU-2-P after the
After the photodegradation of MB and MO dyes, VNU-1-P and VNU-2-P were collected through centrifugation and washed three times with 20 mL of ethanol over a 24-hour period Subsequently, VNU-1-P and VNU-2-P were immersed in 20 mL of chloroform for one day, followed by filtration and regeneration under reduced pressure.
Analysis of the regenerated photocatalysts revealed that their crystallinity was preserved, demonstrating the exceptional stability of VNU-1-P and VNU-2-P in aqueous reaction conditions.
Figure 3.37 PXRD analysis of VNU-1-P before and after the photocatalytic degradation reaction in comparison to the calculated pattern from single crystal data
Figure 3.38 PXRD analysis of VNU-2-P before and after the photocatalytic degradation reaction in comparison to the calculated pattern from single crystal data
The recyclability of the heterogeneous photocatalysts VNU-1-P and VNU-2-P was successfully demonstrated through their dispersion in 100 ppm solutions of methylene blue (MB) and methyl orange (MO) As illustrated in Figures 3.39 and 3.40, both photocatalysts showed effective photodegradation of MO and MB across three consecutive cycles, with only a 10% decrease in the final relative concentrations of both dyes.
Figure 3.39 Degradation profiles of methylene blue (a) and methyl orange (b) by VNU-1-P over three consecutive cycles The lines connecting the shapes are guides for the eyes
Figure 3.40 Degradation profiles of methylene blue (a) and methyl orange (b) by VNU-2-P over three consecutive cycles The lines connecting the shapes are guides for the eyes