Chelating Metal-Organic Frameworks and Their Polymer-Composites For Water Remediation

CHELATING METAL-ORGANIC FRAMEWORKS AND THEIR POLYMERCOMPOSITES FOR WATER REMEDIATION MARÍA CALLES GARCÍA Directores: Dr. Roberto Fernandez de Luis Dr. Viktor Petrenko 2025 (cc) 2025 María Calles García (cc by-nc-sa 4.0) CHELATING METAL-ORGANIC FRAMEWORKS AND THEIR POLYMERCOMPOSITES FOR WATER REMEDIATION MARÍA CALLES GARCÍA Supervisors: Dr. Roberto Fernandez de Luis Dr. Viktor Petrenko 2025 ©2025 MARÍA CALLES GARCÍA Table of Contents Abstract .................................................................................................. i Resumen ............................................................................................... v 1. Chapter 1: Introduction .................................................................... 1 1.1. Water and environmental remediation ......................................................... 1 1.2. Metal-Organic Frameworks Chemistry ....................................................... 10 1.2.1. Clusters ............................................................................................ 13 1.2.2. Linkers ............................................................................................. 16 1.3. Synthesis of MOFs ..................................................................................... 18 1.4. Functionalisation strategies of Zr(IV)MOFs for heavy metals absorption ... 22 1.5. Polymer and Polymer composites for water remediation ........................... 28 1.6. Polymer@MOF composite ......................................................................... 33 1.7. Objectives and structure of the thesis ........................................................ 39 1.8. References ................................................................................................. 41 2. Chapter 2: Matherials and Characterisation ................................. 61 2.1. Chemicals .................................................................................................. 61 2.2. Characterisation techniques....................................................................... 63 2.2.1. X-ray Scattering Techniques ............................................................ 63 2.2.2. Neutron Scattering Techniques ........................................................ 67 2.3. Thermogravimetric analysis (TGA) ............................................................ 69 2.4. Proton Nuclear Magnetic Resonance ........................................................ 71 2.5. Magnetoelastic resonance ......................................................................... 72 2.6. Gas Adsorption/Desorption Isotherms ....................................................... 73 2.7. Zeta Potential ............................................................................................. 75 2.8. Scanning Electron Microscopy (SEM) ........................................................ 77 2.9. Infrared Spectroscopy (IR) ......................................................................... 78 2.10. X-ray Photoelectron Spectroscopy (XPS) .................................................. 80 2.11. High Performance Liquid Chromatography (HPLC) coupled with UV-visible detection ............................................................................................................... 81 2.12. Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) 82 2.13. Adsorption Experiments and Models ...................................................... 83 2.13.1. Kinetic models ................................................................................ 83 2.13.2. Isotherm models ............................................................................ 85 2.14. References ............................................................................................. 88 Chapter 3: Synthesis of C4 Dicarboxylic Acids MOFs ...................... 93 3.1. Introduction ................................................................................................ 93 3.2. Synthesis and Characterisation ................................................................. 96 3.3. Application of C4MOFs ............................................................................. 118 3.3.1. Absorption of phenolic compounds ................................................ 118 3.3.2. Water harvesting ............................................................................ 121 3.3.3. Heavy metal adsorption screening ................................................. 123 3.4. Conclusions ............................................................................................. 132 3.5. References ............................................................................................... 135 Chapter 4: Hybrid Biopolymer@MOF 3D-Sponges for Removal of the “Big Five” Heavy Metals ........................................................... 139 4.1. Introduction .............................................................................................. 140 4.2. Experimental procedure ........................................................................... 143 4.2.1. Synthesis of BCM-1 ....................................................................... 143 4.2.2. Synthesis of SPICHI@BCM-1 sponges ......................................... 143 4.3. Results and discussion ............................................................................ 144 4.3.1. Characterization of BCM-1 and SPICHI@BCM-1 sponges ............ 144 4.3.2. Functional Characterisation ........................................................... 155 4.4. Conclusions ............................................................................................. 185 4.5. References ............................................................................................... 187 Chapter 5: PVDF@MOF Membrane for Heavy Metal Capture ........ 195 5.1. Introduction ..................................................................................... 195 5.2. Experimental procedure ..................................................................... 197 5.2.1. Synthesis of BCM-5 ....................................................................... 197 5.2.2. PVDF-HFP@BCM-5 membranes processing ................................ 198 5.3. Results and discussion ...................................................................... 199 5.3.1. Characterization of BCM-5 ............................................................. 199 5.3.2. Characterization of PVDF-HFP@BCM-5 membranes ................... 207 5.3.3. Functional Characterisation ........................................................... 217 5.4. Conclusions ..................................................................................... 237 5.5. References ...................................................................................... 239 Chapter 6: Conclusions and Future trends ..................................... 245 2.1. Conclusions ............................................................................................. 245 2.2. Future trends ............................................................................................ 247 A. Annex ............................................................................................ 251 A.1. Education .................................................................................................... 251 A.2. Publications ................................................................................................. 251 A.2.1. Part of the Thesis ........................................................................ 251 A.2.2. Other Publications ....................................................................... 252 A.3. Contribution to conferences......................................................................... 252 A.4. Training courses and other contributions ..................................................... 252 A.5. Research stays ............................................................................................ 253 Resumen vii cristalográficos conduce al colapso de la estructura cúbica hacia un empaquetamiento hexagonal. En conjunto, se demuestra que tanto la funcionalidad quelante del enlazador como su disposición espacial dentro de la estructura cristalina final influyen de manera crítica en las cinéticas de adsorción y en las capacidades hacia iones metálicos, contaminantes orgánicos y vapor de agua. En el marco de este capítulo se encuentra un artículo en preparación. Una vez sintetizados los C4-MOFs, estos se incorporaron en dos tipos de matrices poliméricas: una mezcla biobasada de β–quitosano y aislado proteico de soja (SPICHI) (Capítulo 4), y un polímero sintético fluorinado, PVDF-HFP (Capítulo 5). El Capítulo 4 detalla cómo se obtuvieron esponjas tridimensionales compuestas MOF/polímero mediante la integración de uno de los C4-MOFs descritos (BCM-1) en una matriz biopolimérica de proteína de soja y quitina. Esta esponja multifuncional presenta gran estabilidad mecánica, alta permeabilidad y una amplia afinidad química, capaces de eliminar los cinco metales pesados más relevantes. Mediante experimentos de imagen de neutrones y microscopía electrónica de barrido se evidenció la estructura porosa 3D interconectada desde escala macrométrica hasta micrométrica, mientras que los análisis SAXS confirmaron, a escala nanométrica, la integración homogénea de BCM-1 como nanopartículas casi monodispersas (~50 nm) en la red polimérica SPICHI. Los estudios de adsorción mostraron eficiencias superiores al 90% para la mayoría de los metales pesados objetivo, con efectos sinérgicos para Cd(II) y As(III), y capacidades de retención acrecentadas con la carga de MOF. Aun cuando se identificaron ligeras limitaciones frente a oxianiones (As(V), Cr(VI)) a baja concentración de MOF, un mayor contenido del mismo mejoró el rendimiento general del compuesto. Este trabajo, presentado en el Capítulo 4, refuerza el potencial de los compuestos biopolímero–MOF como plataformas versátiles tanto estructural como funcionalmente para la eliminación eficiente de diversos iones metálicos pesados (Calles, M.; Salazar, H.; Britto, S.; Tomchuk, O.; Martins, P. M.; Pradhan, A.; Cássio F.; Lanceros-Mendez, S.; de la Caba, K.; Guerrero, P.; Petrenko, V.; Fernández de Luis, R., Hybrid Biopolymer/Metal–Organic Framework 3D-Sponges Towards the Capture of the ‘Big Five’ Heavy Metals, Chem. Eng. J. 2025, 524, 169442. https://doi.org/10.1016/j.cej.2025.169442). Resumen viii El Capítulo 5 describe el diseño de un MOF de circonio funcionalizado con grupos tioles (BCM-5), sintetizado empleando ácido dimercaptosuccínico (DMSA), e integrado en una membrana jerárquicamente porosa de PVDF-HFP mediante una estrategia de lixiviación salina. Este enfoque conserva los sitios activos del MOF y mejora su accesibilidad, favoreciendo la coordinación fuerte y selectiva con metales blandos pesados como Hg(II), Pb(II) y Cd(II). Los análisis estructurales y espectroscópicos confirmaron la conservación de la cristalinidad y la accesibilidad de los grupos funcionales en la membrana. Las membranas compuestas PVDFHFP@BCM-5 mostraron un rendimiento de adsorción sobresaliente, con capacidades muy superiores a las de sus componentes individuales, evidenciando un efecto sinérgico derivado de la dispersión optimizada del MOF y de la interconectividad de poros en la matriz polimérica, que facilita la difusión de contaminantes. Los análisis post-adsorción (XRD, FTIR y XPS) confirmaron un mecanismo de quimiosorción basado en la coordinación de los metales pesados con grupos tiol y carboxilo. Este capítulo presenta una estrategia versátil para desarrollar membranas compuestas MOF@PVDF-HFP robustas y funcionales que no solo mantienen, sino que incluso mejoran, el rendimiento de BCM-5 en la captura de metales pesados en aguas contaminadas (Calles, M.; Rosales, M.; Martins, P. M.; Lanceros-Mendez, S.; Petrenko, V.; Fernández de Luis, R., Zirconium(IV)-Succimer Metal-Organic Framework Functionalized PVDF-HFP Membranes for Heavy-Metals Capture, 2025, Chemistry European Journal, referees). En conjunto, este trabajo presenta una estrategia detallada para el diseño de MOFs con funcionalidad quelante y sus compuestos polímero@MOF, orientada a mejorar la eficiencia global de eliminación de metales tóxicos de medios acuosos. Al integrar adsorbentes de alto rendimiento en matrices estables y funcionales, estos resultados contribuyen al desarrollo de materiales de nueva generación para la descontaminación del agua y la protección ambiental sostenible. Introduction 9 Chapter 1 Introduction 1 Chapter 1 Introduction 1.1. Water and environmental remediation The rapid industrialisation of various countries worldwide over the last century has led to significant environmental challenges, particularly regarding the quality and availability of freshwater resources.1 As a consequence of anthropogenic activities, a wide range of pollutants, including persistent organic pollutants (POPs) such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), dioxins, and organochlorine pesticides,2 along with pharmaceuticals,3 microplastics,4 and toxic inorganic species such as heavy metals and metalloids,5 have been and continue to be released into aquatic environments. Although water monitoring agencies have shifted the focus of water pollution to contaminants of emerging concern, also known as forever chemicals, heavy metals continue to represent one of the most significant vectors of contamination in many regions worldwide.6 Since ancient times, heavy metals have been known to be one of the most hazardous classes of contaminants.7 In fact, they are defined by regulatory bodies as priority contaminants due to their persistence, mobility and toxicity,8–10 along with their tendency to bioaccumulate through the food chain.11 Unlike organic pollutants, which undergo natural degradation, heavy metals such as mercury, lead, cadmium, chromium, and arsenic, among others, remain stable indefinitely in the environment. The toxicological profile of heavy metals presents significant public health concerns even at trace concentrations.12 These elements demonstrate the capacity to disrupt multiple physiological systems even after prolonged exposure to ppb concentrations. There are widely documented impacts of heavy metals on neurological functions, renal performance and immune system integrity.9,13 In addition, the ecological implications of heavy metals extend beyond the effects of their direct toxicity. Heavy metals can Chapter 1 2 alter the balance of an aquatic ecosystem by affecting primary producers, disrupting predator-prey relationships and reducing biodiversity. As species that serve as indicators of ecosystem health, fish populations demonstrate measurable bioaccumulation patterns that spread contamination effects throughout food chains, ultimately impacting human consumers through dietary exposure.14 Focusing on their sources, heavy metals are introduced into aquatic environments (Fig. 1.1) primarily through industrial activities such as mining, electroplating processes, the agricultural application of pesticides and fertilisers, the manufacture of electronics, and the production of batteries.15 All of them contribute to the release of conventional heavy metals, but also to the release of highly valuable elements, such as some of the ones listed as critical raw elements by the European Commission (e.g. Co, Ni or REE).16 Figure 2.1. Heavy metal pollution sources. In regions without adequate wastewater treatment infrastructure, these sources of metal pollution create direct exposure hazards for vulnerable populations. It is important to point out that, in addition to anthropogenic activities, heavy metals' entry into water bodies can also arise from the natural weathering of rocks containing specific mineralogic associations. The global arsenic contamination crisis is a clear Introduction 3 example of this scenario. In many regions worldwide, such as northern Chile, some areas of the United States, or India, the levels of arsenic in drinking water are consistently above the World Health Organisation's (WHO) safety guidelines, putting millions of people worldwide at serious risk.16,17 For instance, in northern Chile, more than 250,000 residents were exposed to arsenic concentrations of 860 µg/L between 1958 and 1970, leading to elevated incidences of bladder, lung, and skin cancers.18 In the United States, namely in California, residents have faced prolonged arsenic contamination in groundwater, resulting in increased cancer and cardiovascular risks.19 Similarly, in India, the state of Bihar is severely affected, with over 10 million people consuming water with arsenic levels exceeding the WHO limit of 10 µg/L, and concentrations in some districts reaching up to 724 µg/L, causing widespread health problems including skin lesions and various cancers.20 Table 1.1 summarises the principal anthropogenic and natural sources of heavy metals, together with the typical concentration ranges reported in contaminated natural and wastewaters.21–27 Table 2.1. Main sources and critical concentrations of heavy metals in contaminated Waters. Metal Main sources of contamination Typical concentrations in contaminated waters* Arsenic (As) Natural (weathering of arsenic-bearing minerals, volcanism); arsenical pesticides; metal smelting22 10–500 µg/L22 (WHO guideline: 10 µg/L)21 Cadmium (Cd) Electronics industry (batteries, semiconductors); pigments; phosphate fertilizers26 1–50 µg/L26 (WHO guideline: 3 µg/L)21 Lead (Pb) Mining and smelting; leaded fuel (historical); old water pipes26 10–100 µg/L26 (WHO guideline: 10 µg/L)21 Mercury (Hg) Artisanal and illegal gold mining (amalgams); chlor-alkali industry; coal combustion25 0.5–50 µg/L24 (WHO guideline: 6 µg/L)21 Chromium (Cr) Tanneries; electroplating; pigment and stainless-steel production24 10–300 µg/L27 (WHO guideline: 50 µg/L)21 Nickel (Ni) Metal refining; stainless steel production; batteries26 20–200 µg/L26 (WHO guideline: 70 µg/L)21 Copper (Cu) Mining; copper plumbing; pesticides and fungicides26 50–2000 µg/L26 (WHO guideline: 2000 µg/L)21 Zinc (Zn) Galvanization; fertilizers; metallurgical effluents26 100–5000 µg/L (no WHO guideline; ~3000 µg/L often used as reference)26 *Values represent typical ranges reported in polluted natural and wastewaters; they may vary depending on region and specific activity. Chapter 1 4 This overview highlights the strong link between specific human activities and heavy metal pollution, with concentrations exceeding the WHO guidelines. In most cases, current remediation technologies are based on the conventional physico-chemical (precipitation, coagulation–flocculation, ion exchange, or electrochemical methods), biological (microbial biosorption, bioaccumulation, or enzymatic degradation) and absorption/membrane technologies applied in the different stages of the waste or industrial water treatment plants. Although the combination of all these treatment steps has been revealed as highly functional under specific conditions, they face significant limitations that restrict their broader application.28 Conventional approaches, including ion exchange, chemical precipitation, membrane filtration, and reverse osmosis, often require substantial capital investment, operational costs and energy inputs. These factors limit their accessibility in places with limited resources or isolated areas lacking conventional wastewater treatment facilities.29 Furthermore, these technologies often produce secondary waste products or water streams that demand additional treatments. Brines generated by desalination and membrane-filtration processes, as well as metal-laden sludges from conventional precipitation and coagulation treatments, exemplify this challenge. The safe disposal of these concentrated metallic residues requires specialised methods to prevent re-contamination of environmental systems.30 Despite its critical importance for ensuring the overall sustainability of water treatment, the management of these secondary waste streams often receives insufficient attention in technology development. Among all the remediation technologies mentioned, adsorption stands out as a particularly attractive approach due to its inherent simplicity, cost-effectiveness, operational flexibility, and potential for adsorbent regeneration and reuse.31 Adsorption can operate under ambient conditions with minimal infrastructure requirements, making it accessible for implementation in resource-limited settings. Moreover, the design of adsorption systems, combined with their modular scalability, allows for adaptation to varying treatment capacities and contamination levels without substantial modifications to existing infrastructure.32 However, the effectiveness of adsorption technology is fundamentally dependent on the careful selection and design of adsorbent materials that can be tailored to match the specific physicochemical properties of target metal contaminants.33 The success of adsorption relies on Introduction 5 optimising the interaction mechanisms between the adsorbent surface and the metal species, which vary significantly depending on factors such as ionic radius, charge density, hydration energy, and complexation behaviour of different metals. For instance, soft metals like mercury and lead exhibit a strong affinity for sulphurcontaining functional groups, while hard metals such as chromium and aluminium preferentially interact with oxygen-donor sites.34 This specificity requirement has driven extensive research into the development of functionalized adsorbent materials, including modified activated carbons, biopolymer-based composites, various porous materials and especially metal-organic frameworks (MOFs), each designed to enhance selectivity and adsorption capacity for particular metal contaminants.35,36 From a chemical point of view, heavy metal contamination is highly complex due to the variety of metal speciation that is observed in natural waters, depending on their pH or eV conditions, just to mention two of the most important parameters. As illustrated in Figure 1.2 for the “big five” heavy metals, these metals can be found in different forms, from positive ions (Pb(II), Cd(II)) to neutral molecules (HgCl2, As(OH)3) and negative complexes (HCrO4-, H2AsO4-). The chemical form has a direct impact on environmental mobility, bioavailability, and interaction with potential remediation materials.37 Chapter 1 6 Figure 2.2. a) Hg(II), b) Pb(II), c) Cd(II), d) Cr(VI), e) As(III) and f) As(V) equilibrium. Furthermore, the information presented above highlights the need for effective treatment strategies capable of addressing the chemical diversity of contaminants. In real-world scenarios, contamination often involves multiple metal species simultaneously, a situation particularly common in industrial effluents and acid mine drainage. These multi-metal environments present several challenges in terms of their remediation due to potential synergistic effects between this cocktail of ions to block Introduction 13 frameworks has led to the remarkable development of more than 100.000 reported MOF structures, each with tailored properties for specific functional applications.88 1.2.1. Clusters A wide range of metal ions has been employed for MOF-assembly, including transition metals, lanthanides, and actinides. Each class of metal provides distinct coordination geometries, oxidation states, and specifically, bonding modes with the coordination groups of the organic linkers that define the resulting structural building unit. The connectivity and geometry of both the inorganic units and organic linkers determine the framework's topology, stability, and functionality.86 Transition metals such as Cu(II) and Zn(II) are widely used in MOF synthesis due to their flexible coordination modes and accessible oxidation states. Lanthanides, in turn, provide unique optical properties resulting from their high coordination numbers and shielded 4f orbitals. However, frameworks based on these metals often display limited stability under hydrothermal or chemically harsh conditions.87,88 These limitations have stimulated the pursuit of alternative metal nodes with greater robustness.91,92 Within this context, tetravalent metals—particularly Zr(IV), Hf(IV), and Ce(IV)—have emerged as especially important. Their high charge density and strong oxophilicity enable the formation of durable bonds with carboxylate linkers, yielding MOFs with exceptional thermal, chemical, and hydrolytic stability.89 The well-defined coordination environment of these M(IV) centres also facilitates the construction of highly connected SBUs, which underpin the synthesis of remarkably robust and porous architectures.90 Zirconium Clusters in Zr-MOFs Zirconium clusters exhibit considerable structural diversity, with hexanuclear (Zr6) and dodecanuclear (Zr12) units being the most reported secondary building units in the literature. Additionally, octanuclear (Zr8) clusters, though less common and often arising from structural disorder, constitute another notable cluster type.91,92 This thesis focuses specifically on Zr6 and Zr12 clusters, as these are the structural motifs found in Chapter 1 14 the MOF materials synthesised and characterised in this work. A summary of these three clusters is presented in Table 1.3. Table 2.3. Different types of Zr clusters. Cluster Type Geometry Connectivity Coordination Sites Representative MOFs Zr6 Octahedral 3-12 12 (edges) UiO-66, MOF-808 Zr12 Bioctahedral 12-24 24 (variable modes) Catalytic frameworks Zr8 Cubic 12 12 (edges) PCN-221, NPF-200 The Zr6(μ3-O)4(μ3-OH)4 cluster (Fig. 1.5) is the most common and well-studied secondary building unit (SBU) in zirconium-based metal–organic frameworks (ZrMOFs). This hexanuclear unit adopts an octahedral geometry, in which six Zr(IV) centres occupy the vertices of a regular octahedron. Each cluster provides up to twelve coordination sites, enabling the formation of frameworks with diverse connectivity, typically ranging from 3 to 12, depending on the linker configuration and overall framework symmetry.93 Within the cluster, four μ3-oxo and four μ3-hydroxo ligands alternately cap the octahedral faces, forming a highly stable inorganic core with remarkable resistance to hydrolysis. The twelve carboxylate coordination sites are located along the twelve edges of the octahedron, where each carboxylate ligand usually adopts a μ2-η¹:η¹ coordination mode. This arrangement allows the Zr6 node to act as a versatile inorganic connector capable of supporting a wide range of topologies.94 Several structural variations of this cluster have been reported.95 The standard form, Zr6O4(OH)4, features mixed oxo/hydroxo bridges, whereas the fully deprotonated Zr6O8 version exhibits enhanced charge density and increased chemical stability. In addition, defective Zr6 clusters— resulting from missing linkers— introduce additional porosity and accessible active sites, thereby improving the material’s chemical functionality and catalytic potential. Introduction 15 Figure 2.5. Representation of the Zr6 cluster. Beyond the Zr6 motif, larger zirconium clusters such as Zr12 (Fig. 1.6) have also been identified. These structures form through the dimerisation of two Zr6 octahedral units, which are linked via additional bridging ligands to produce a bi-octahedral geometry of reduced symmetry. The intercluster distance varies depending on the nature of the bridging ligands.96 Ligands within Zr12 clusters can adopt four distinct coordination modes: (i) chelating, involving bidentate coordination to a single Zr centre; (ii) belt bridging, in which μ₂linkers connect two Zr atoms within the same Zr6 unit; (iii) intercluster bridging, where linkers span across both octahedral subunits; and (iv) inner-face bridging, occurring at the interface between the two octahedra. This structural diversity allows Zr12 clusters to serve as versatile nodes in the design of frameworks with complex threedimensional topologies.97 Figure 2.6. Representation of the Zr12 cluster. Zr6O4(OH)4 [Zr6O4(OH)4]2OH6 Chapter 1 16 Another important motif is the Zr8O6 cluster, which exhibits a cubic geometry fundamentally different from the octahedral Zr6 structure. In this configuration, eight Zr atoms occupy the cube vertices, while six μ4-oxo groups cap the cube faces. Each edge of the cube hosts a carboxylate coordination site, resulting in an exclusive 12fold connectivity.98 This high-symmetry configuration favours the formation of frameworks with the rare (4,12)-connected ftw topology, characterised by large cavities and high porosity. Due to its cubic geometry, the Zr8 cluster is particularly compatible with planar porphyrinic linkers, facilitating the construction of highly ordered, mesoporous frameworks with large accessible volumes.99 1.2.2. Linkers The organic component of MOFs offers remarkable structural diversity, with linkers ranging from simple dicarboxylates to complex multitopic ligands. The choice of organic linker enables precise tailoring of pore size, shape, and functionality of the final MOF by modulating their length, geometry, and chemical functionalization.100. 109 The increasing complexity of organic linkers has enabled the discovery of countless crystal structure variants in Zr-MOFs, each characterised by diverse porosity metrics, surface areas, and defect chemistries. Among these, architectures assembled from C4-linkers have demonstrated outstanding chemical robustness, environmentally benign synthesis routes, and microporous environments, together with high chemical and topological diversity.101 The chemical structure of these C4 linkers typically comprises a rigid aromatic or aliphatic core with four carboxylate groups positioned symmetrically or asymmetrically, often complemented by lateral functional groups (Fig. 1.7). These side groups, combined with modulation of synthetic parameters such as temperature, pH, and modulator concentration, guide the crystallisation of Zr-MOF-C4 materials into distinct crystalline phases and topologies.102 Introduction 17 Figure 2.7. Examples of various C4 linkers. The linkers employed in this thesis are highlighted with a dashed line. Several C4 linkers have been extensively studied in the literature, leading to Zr-MOFs with well-characterised structures that illustrate this structural diversity. Frameworks based on aspartic, trans-aconitic, fumaric, and bromosuccinic acids typically crystallise in a Pn-3 cubic structure, characterised by a robust 3D network with small to moderate pore sizes and high stability. The symmetry and positioning of the carboxylate groups favour the formation of highly connected secondary building units with octahedral Zr6 nodes. In contrast, MOFs constructed from fumaric and malic acids often adopt an Immm orthorhombic structure, featuring elongated pores and anisotropic connectivity owing to the more flexible or less symmetric linker conformation. Zr-MOFs utilising thiomalic acid display polymorphism, crystallising in distinct structures such as an F23 cubic phase and a P63/mmm hexagonal phase, where the presence of sulphur atoms imparts unique binding modes and potential functionalities that influence both crystal packing and pore connectivity. Fumaric acid Succinic acid Malic acid Mesaconic acid Methylsuccinic acid Trans-aconitic acid Thiomalic acid Aspartic acid Dimercaptosuccinic acid Sulphosuccinic acidBromosuccinic acid Dibromosuccinic acid Chapter 1 18 Frameworks based on sulfosuccinic acid form in the I4/m tetragonal space group, where the sulfonate functional group contributes to altered coordination environments, inducing the formation of unique pore architectures with potential hydrophilic character. Each of these crystalline phases exhibits distinctive structural features: variations in pore size and shape, connectivity of nodes, and framework dimensionality. These differences profoundly impact the physical properties of the MOFs, including gas adsorption behaviour, chemical stability, and potential catalytic activity. Thus, by carefully selecting the C4 linker chemistry and optimising synthetic conditions, it is possible to tailor the crystalline structure and topological variety of Zr-MOFs. This versatility underpins their suitability in diverse applications requiring precise control of pore environments and framework robustness. 1.3. Synthesis of MOFs Thus, the final framework of a MOF, even when constructed from the same structural units, can in many cases be highly dependent on the specific synthesis conditions employed to crystallise it. In general terms, the synthesis process aims to promote nucleation and crystal growth while minimising the precipitation or formation of dense phases.103,104 Below, we will present different parameters which impact the synthesis of MOF samples. The most widely employed synthetic approach is solvothermal synthesis, where metal salts and organic ligands are heated in sealed vessels between 80 and 200°C, creating controlled conditions that promote the self-assembly of highly crystalline framework structures.105 The application of this method provides the thermal energy necessary to overcome kinetic barriers and promote the formation of thermodynamically stable crystalline phases.106 The selection of solvent is critical as it influences the solubility of reactants, the coordination environment of metal centres, and the templating effect on pore formation.107–109 Hydrothermal synthesis is a technique that employs water as the primary solvent and has proven effective for the preparation of many hydrolytically stable MOFs. The high dielectric constant of water enables the dissociation of metal salts and enhances ionic Introduction 19 interactions, while its hydrogen bonding network enables their cooperative assembly and stabilisation.110 Beyond these fundamental physicochemical advantages, waterbased synthesis routes align with the principles of green chemistry by eliminating toxic organic solvents, reducing energy consumption, and minimising hazardous waste generation.111 This approach has gained increasing attention as the field moves toward more sustainable and scalable production methods that are compatible with industrial implementation and environmental regulations. Moreover, aqueous media prove particularly compatible with the solubility requirements of most metal salts employed in MOF synthesis, including zirconium salts such as ZrCl4, which readily dissolve and hydrolyse in water to generate reactive metal species. Similarly, C4-dicarboxylic linkers, including aspartic acid, fumaric acid, malic acid and their derivatives, exhibit sufficient solubility in water, especially under controlled pH conditions where deprotonation of carboxylate groups enhances their dissolution. This dual solubility enables straightforward mixing of reagents without requiring complex solvent systems or solubilising agents, thereby simplifying reaction protocols and facilitating reproducibility. The compatibility of both metal precursor and organic linkers with aqueous environments has therefore been instrumental in enabling milder, more sustainable synthesis pathways for Zr-C4-MOFs, which were prepared and studied in this thesis. These characteristics have opened the room to the progressive softening of the synthesis and crystallisation of Zr-C4-MOFs when employing the proper reaction conditions. In fact, room-temperature synthesis offers advantages for the preparation of kinetically stable phases containing thermally labile functional groups.112,113 For reaching faster synthesis times, microwave-assisted methods have gained popularity by reducing reaction times from days to minutes while maintaining excellent product quality.114 Moreover, electrochemical synthesis provides precise control over metal ion release through electrode dissolution, while sonochemical methods employ ultrasonic agitation for accelerated nucleation and enable precise regulation of pore architecture and framework topology.115,116 Mechanochemical synthesis is a viable, environmentally friendly approach that reduces solvent consumption.117 It offers simplicity, high reproducibility, and mild reaction conditions, while enabling the preparation of frameworks that might be difficult to synthesise through traditional methods. Chapter 1 20 It is now well established that the synthesis conditions of MOFs exert a significant influence on their final properties, thereby offering opportunities for systematic optimisation of these materials. In general terms, the crystallisation of MOFs follows the general rules of crystallisation, being nucleation kinetics and crystal growth being the two key steps of the process. Low reagent concentrations favour limited nucleation and extended crystal growth, resulting in larger, well-ordered, and defect-free single crystals. In contrast, high concentrations promote rapid nucleation, yielding smaller crystals with a higher density of defects and altered coordination environments.118 The metal-to-ligand ratio, the nature of the solvent or solvent mixtures, and the anionic counterions of the initial metal salts can exert a strong influence on the final framework, the metal coordination environment, and the incorporation of solvent molecules or anions into the MOF structure 125119 Temperature plays a crucial role in nucleation and crystal growth rates, with higher temperatures generally promoting larger crystal sizes.120 However, elevated temperatures may lead to framework decomposition in thermally sensitive systems.121 The pH of the reaction medium also influences the deprotonation of organic linkers, metal speciation, and coordination geometry, which in turn impact the framework topology.122,123 Reaction time is a critical factor in the optimisation of crystallinity and phase purity, where shorter reaction times often result in the formation of smaller crystals with higher surface areas, while extended reaction times promote crystal growth and enhance structural ordering.124 Synthesis of Zr(IV) MOFs All the factors mentioned above generally influence the crystallisation of all types of MOF materials; however, the synthesis of Zr-MOFs introduces an additional factor that plays a crucial role in controlling their crystallisation process and the defectivity of the resulting crystals: the modulators. Modulators are competitive coordination agents that control Zr-MOF synthesis by temporarily competing with primary ligands for zirconium coordination sites, thereby regulating crystal growth, particle size, and defect formation and distribution within the framework.125 In Zr-MOFs, defects are not merely structural imperfections but functional features that can be rationally engineered through modulation strategies.126 Introduction 21 The controlled introduction of missing-linker or missing-cluster defects has emerged as a powerful tool for tailoring the properties of Zr-based frameworks, enabling enhanced porosity, improved adsorption performance, and tunable catalytic activity.127 The efficacy of modulators in directing Zr-MOF crystallisation and defect engineering depends primarily on two factors: the pKa of the acidic group and the structural characteristics of the substituent.128 Monocarboxylic acids with lower pKa values (strong acids) coordinate more weakly to Zr6 clusters due to their higher tendency to remain deprotonated in solution, resulting in faster ligand exchange kinetics and reduced interference with framework assembly. Conversely, modulators with higher pKa values (weaker acids) bind more strongly to metal nodes, slowing crystal growth and promoting the formation of larger, more ordered crystals with fewer defects. This competition between linker and modulator for coordination to the Zr SBUs is key to effective modulation, where a close match in pKa favours larger and more well-defined crystallites.126 The nature of the substituent also plays a crucial role: aliphatic substituents, such as those in acetic or formic acid, provide flexibility and minimal steric hindrance, while aromatic substituents, such as those in benzoic acid, introduce π-π interactions and enhanced compatibility with aromatic linkers, influencing crystal morphology and framework topology. Among these, acetic acid remains the most widely employed modulator for Zr-MOF synthesis due to its intermediate pKa (~4.76), which provides balanced coordination strength that slows crystallisation and improves crystal quality without excessively hindering framework formation. Formic acid provides stronger coordination for particle size control, while benzoic acid offers better compatibility with aromatic frameworks.129 However, higher concentrations typically yield larger crystals and higher defect densities, sometimes even altering the framework connectivity. The incorporation of missing-linker and missing-cluster defects can lead to structural reorganisation at the nanoscale: for instance, in UiO-66(Hf), high concentrations of formic acid modulator during synthesis promote the formation of correlated cluster vacancy nanoregions that locally adopt an eight-connected reo topology, dispersed within the parent twelveconnected fcu framework. This topological heterogeneity arises from the preferential inclusion of ordered cluster vacancies, which reduce the coordination number of the zirconium nodes and induce local structural modifications. Such phenomena underscore the profound influence of modulator concentration not only on crystal Chapter 1 22 quality and defect density but also on the local framework connectivity, highlighting the importance of careful synthetic control in Zr-MOF synthesis.130 In general terms for Zr-MOFs, the use of stronger acid modulators increases phase diversity in samples containing flexible ligands, whereas higher synthesis temperatures tend to reduce crystallite size and enhance phase selectivity for rigid ligands.131 Nevertheless, the synthesis parameter space requires meticulous exploration. A thorough understanding of these parameters and their effects on MOF formation mechanisms is essential for designing materials with predictable structures and properties. 1.4. Functionalisation strategies of Zr(IV)MOFs for heavy metals absorption Synthesis-induced defectivity in Zr-MOFs is not only a means of controlling the final topology of these materials but also provides an ideal platform of diverse sites for postsynthetic functionalization. In general, the chemical encoding of these frameworks can be achieved through two primary approaches that offer complementary advantages: pre-synthetic functionalization, where functional groups are incorporated into the organic linkers before framework assembly, and post-synthetic modification, where existing frameworks are chemically modified after synthesis.132,133 In general terms, the absorption of anionic species by Zr-MOFs is governed by two complementary mechanisms. First, anionic contaminants such as chromate (CrO42-) or arsenate (AsO43-) can coordinate directly to uncorrelated and correlated defective positions in the Zr-oxo clusters, where missing linkers or cluster defects expose unsaturated metal sites with high affinity for these species. This coordination-based capture is particularly effective due to the hard Lewis acid character of Zr(IV) centres. Second, electrostatic interactions play a crucial role when the framework incorporates cationic groups, most protonated amines (-NH3+), which are either present in the organic linkers or introduced through post-synthetic modification. These positively charged moieties create an electropositive environment within the pores that facilitates the adsorption of anionic pollutants, often achieving high selectivity even in complex multi-ionic solutions.134 Introduction 29 biopolymers—such as carboxyl (-COOH), hydroxyl (-OH), amine (-NH2), amide (-CO NH2), thiol (-SH), and phosphate (-PO43-) groups—renders these materials inherently functional for heavy metal.175,176 Notable examples include chitosan, which possesses abundant amine and hydroxyl groups capable of chelating hard and borderline metal ions such as Pb(II), Cd(II), Cu(II), and Ni(II) through coordination and electrostatic interactions.177 Alginate, rich in carboxyl groups, exhibits high affinity for divalent cations like Pb(II), Cu(II), and Zn(II) through the formation of stable metal-carboxylate complexes.178 Cellulose and its derivatives, bearing hydroxyl and carboxyl groups, have demonstrated effective adsorption of Cr(VI), Hg(II), and As(V).179 Furthermore, the surface chemistry of polymeric membranes and filters can be modified post-processing, enabling the anchoring of additional metal-chelating groups to the structure of the polymeric scaffold and thereby enhancing or tailoring their adsorption properties. Post-synthetic modification strategies include grafting of functional monomers via free-radical polymerisation, plasma treatment to introduce reactive groups, chemical activation followed by coupling reactions, and layer-by-layer assembly of functional coatings.180 For instance, chitosan membranes modified with thiol groups through reaction with thioglycolic acid have exhibited significantly enhanced Hg(II) removal compared to unmodified chitosan.181 Similarly, cellulosebased filters functionalized with ethylenediaminetetraacetic acid (EDTA) or diethylenetriaminepentaacetic acid (DTPA) have demonstrated improved chelation capacity for Pb(II), Cd(II), and Cu(II).182 Polyamide membranes modified with carboxyl, amine, or phosphonate groups through surface grafting have shown selective adsorption toward rare-earth elements and transition metals.183 These postmodification strategies allow fine-tuning of selectivity, capacity, and kinetics without compromising the mechanical integrity or permeability of the filtration system. All in all, the chemical diversity, processability, and modifiability of polymers have opened the room to the development of water filtering systems based on absorption for the removal of heavy metals, with outstanding chemical and mechanical properties, as well as good absorption performances. Examples of successful heavy metal removal include chitosan-based membranes achieving >95% removal of Pb(II), Cd(II), and Cu(II) from industrial wastewater,184 alginate beads demonstrating capacities of 200-400 mg/g for Pb(II) and Cu(II),185 cellulose nanofiber aerogels removing >99% of Cr(VI) and Hg²⁺ from contaminated water,186 and thiol-modified polyacrylonitrile fibres Chapter 1 30 capturing Hg2+ with adsorption capacities exceeding 1200 mg/g.187 These polymeric systems offer cost-effective, scalable, and environmentally friendly alternatives to conventional water treatment technologies, bridging the gap between laboratory research and practical implementation in real-world remediation scenarios. But there is a parallel way to modify the polymers’ characteristics in terms of their physicochemical properties and functionality: the design of polymer composites made of a mixture of two or more different components: polymer matrix (the continuous phase) and reinforcing materials (the dispersed phase). The polymer matrix, generally a plastic or resin, acts as a binding, holding the parts of the material together. The reinforcing materials, such as fibres, particles or other fillers, improve mechanical properties and expand their functionality.188 Combining the low weight and the functional properties of these materials allows for the creation of materials with tailored properties that significantly overcome those of their individual components. The development of polymer composites has enabled to modification of both the matrix and the reinforcement in terms of composition, morphology, dispersion and interfacial interactions.189 This flexibility enables the obtaining of a wide variety of composite architectures, including fibre-reinforced systems, particle-filled polymers and nanocomposites.190 These materials present synergistic properties that allow the simultaneous optimisation of mechanical, thermal, electrical and chemical properties, just to mention some examples. In our case study, three different polymers or biopolymers have been used: Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) is a thermoplastic and highly engineered fluorinated copolymer which represents an advanced version of polyvinylidene fluoride (PVDF). It offers a combination of chemical stability, mechanical flexibility and electroactive behaviour.191 The incorporation of hexafluoropropylene units (HFP) into the polyvinylidene fluoride (PVDF) main structure induces alterations in the crystalline structures, increasing chain mobility and enabling the development of polar β-phases.192 These phases improve surface polarity, ionic conductivity, hydrophobicity and solubility through stronger dipole and electrostatic interactions. This polymer serves as an exceptional polymeric matrix for hosting MOFs due to its mechanical strength, thermal stability and chemical compatibility, providing a stable Introduction 31 and protective environment for incorporated MOFs.193 The components can retain their functional properties over extended periods while the polymer itself remains lowcost, non-reactive, and easily processable, allowing the composite properties to be optimised to obtain materials with excellent functional activity. The material's structural versatility and compatibility with a wide range of functional fillers represent a significant advantage in the design of next-generation materials, as it can be modified to create water filters and membranes with engineered porous structures highly effective for water remediation and the removal of heavy metals.194–198 There are many processing techniques employed in the production of these composites, but in this thesis, these materials have been synthesised by salt leaching, which is a fundamental and widely employed technique for creating porous composite polymer structures with controlled morphology and interconnected pore networks. This method employs water-soluble particles that are uniformly distributed within a polymer matrix during processing and then removed through aqueous extraction, resulting in the formation of a three-dimensional porous architecture within the polymer matrix. Sodium chloride is the most commonly used salt due to its chemical inertness, controlled particle size availability, and complete water solubility.199–202 This technique offers exceptional control over pore size, porosity, and pore connectivity by modifying the salt particle size distribution, loading percentage, and leaching conditions. β-chitin is a semi-synthetic polymer. Chitin is the second most abundant natural polymer after cellulose and is a linear aminopolysaccharide composed of β(1-4)-linked N-acetylglucosamine units, presented in the exoskeletons of arthropods. The material's molecular configuration is characterised by a high number of hydrogen bonds, which contribute to its mechanical strength, chemical stability, and biological functionality.203–205 This polymer exists naturally in three polymorphic forms (α, β, and γ), with different crystallinity and mechanical properties and conferring a high degree of versatility on the polymer. Among these, α-chitin is the most abundant and mechanically robust, forming the structural basis of many biological composites.206 However, β-chitin has a distinct arrangement of its chains, with a parallel configuration where all polymer chains are oriented in the same direction, resulting in a crystalline network that is more accessible and flexible in comparison to α.207 This structural configuration confers Chapter 1 32 greater flexibility, water affinity, and chemical reactivity, making β-chitin particularly suitable as a matrix for functional composites. The presence of hydroxyl and acetamido groups along the polymer backbone provides intrinsic coordination sites for metal ions, while the porous architecture resulting from chitin's fibrillar morphology facilitates mass transport and accessibility to embedded functional fillers. The incorporation of these nanostructures enables functionalities such as antimicrobial activity and water resistance, which are crucial for biomedical applications and sustainable packaging. 208 The environmentally beneficial properties of chitin, together with its adaptability through nanoscale structural design and chemical modification, highlight its potential as a sustainable, high-performance material for functional composites. Soy protein, a natural polymer or biopolymer and is extracted from soybeans, is structurally rich with biomolecules composed of functional groups such as hydroxyl, carboxyl, amino, and sulfhydryl moieties.209 These reactive sites enable a variety of chemical modifications and cross-linking strategies, allowing the creation of threedimensional polymer networks with tailored mechanical properties.210 For the synthesis of soy protein composites, additives such as glycerol or sorbitol are often employed as plasticisers to improve flexibility and processability.211 And for the processing methods, soy protein composites generally use aqueous or alkaline environments, followed by casting, compression moulding, or extrusion.212 In this thesis, we have used the protein isolate (SPI). These composites represent a sustainable and renewable alternative to conventional polymers. Thanks to their natural film-forming ability, biodegradability, and abundance, soy proteins are being most used as the matrix in composite materials, showing positive results for new applications, such as producing porous scaffolds.213 This approach enables precise control over protein conformation and surface reactivity and modifies the material’s solubility, mechanical strength, water resistance, and reduces moisture sensitivity. The potential application areas of soy protein-based composites are extensive and multidisciplinary.214 Soy protein composites are a key player in the development of next-generation sustainable materials due to their ability to tune performance through structural and chemical modification, combined with their low environmental footprint. Introduction 33 These three polymers demonstrate the wide range of approaches available for developing high-performance composite materials, extending from synthetic fluoropolymers to natural biopolymers (Fig.1.9). Figure 2.9. Representation of PVDF-HFP, β-chitin and soy protein isolated polymers used in the thesis, with main advantages for each polymer. The complementary characteristics of these polymer matrices, ranging from different degrees of hydrophilicity, crystallinity, mechanical strength, and inherent metal-binding functionality, provide a versatile platform for integration with porous materials. 1.6. Polymer@MOF composite The development of MOF-based composites arises from the need to integrate MOFs into stable matrices without compromising their structure or function.215 MOFs are highly promising materials for the remediation of heavy metal-contaminated water due to their exceptional adsorption capabilities. However, their main limitation is their physical form, as they are obtained as fine powders that are difficult to handle for direct application in real-world environmental scenarios.216–218 To overcome this challenge, MOF composites have been designed to embed these materials within supportive matrices that preserve their intrinsic porosity and metalbinding properties. Beyond the mere function of providing support, these matrices, n cc F F F F H HH cc F F c HH cF F c F F F c F c - Synthetic - Hydrophobic - Semicristaline - Flexible - No interaction with metals - Mechanical support - Natural - Hydrophilic - Low cristalinity - Flexible - Interation with metals - Improve flexibility - Natural - Hydrophilic/Anphiphilic - Amorph - Flexible - Interation with metals - Improve metal adsorption PVDF-HFP β-quitin Soy protein isolated -(CH2CF2)x-(CF2-CF(CF3))y- -[C8H13O5N]n- -[NH-CH(R)-CO]n- Chapter 1 34 composed of functional polymers, can also contribute to the overall adsorption performance. In some cases, the resulting composites present synergistic effects, showing better adsorption capacity compared to the individual components alone.219 Building upon established insights into MOF functionalities and polymer matrix integration, Table 1.5 summarises some studied PVDF@MOF composites for heavy metal adsorption. This compilation reflects the current state of laboratory-scale research, highlights promising candidates with diverse adsorption profiles and regeneration potentials, and underscores the existing gap towards practical field applications.220–222 Table 2.5. Comparative Table of different PVDF@MOF composites. Material Adsorption Capacity (mg/g) Adsorption Kinetics (PSO) (min) TRC* Sorbent Cost S&R* TRL*3 PVDF@MOF-808@Cys Hg(II): 30 240 No Medium Good, Chemical 3-4 PVDF@β–CD-ZIF-8 Pb(II): 708 Cu(II): 651 120 No High Good, Chemical 4-5 PVDF@GuanidineGrafted-NH2-MIL101(Fe) Pb(II): 29 720 No High - 3-4 *TRC: Tested in Real Conditions, S&R: Stability and Regenerability, TRL: Technology Readiness Level Similarly, the integration of MOFs with biopolymers has gained considerable attention due to the inherent advantages of natural polymers, including biocompatibility, biodegradability, and cost-effectiveness. Table 1.6 summarises several studied biopolymer@MOF composites for heavy metal adsorption. The resulting bio-based composites offer a sustainable alternative to synthetic polymer matrices while maintaining efficient adsorption performance and facilitating recyclability.223–229 Table 2.6. Comparative Table of different Biopolymer@MOF composites. Material Adsorption Capacity (mg/g) Adsorption Kinetics (PSO) (min) TRC* Sorbent Cost S&R* TRL* Chitosan@ZIF-67 Pb(II): 5.5 - No Medium Good, Chemical 3-4 Chitosan@NH2MIL-125 Pb(II): 945 40 No Medium Good, Chemical 3-4 Chitosan@MOF808 Cr(VI): 320 20 No Medium Good, Chemical 3-4 CNC@Zn-BTC Pb(II): 559 30 No Low Good, Chemical 3-4 Introduction 35 CHIPEC@UiO-66NH2 As(II): 168 As(V): 335 - No Medium Good, Chemical 3-4 Co-AlLDH@CHN/Fe3O4 Pb(II): 559 Cr(VI): 711 - No Medium High, Chemical 4-5 Cellulose@ZIF-8 Pb(II): 307 Cd(II): 143 Co(II): 350 Cu(II): 354 Fe(III): 261 30 No Medium High, Chemical 3-4 *TRC: Tested in Real Conditions, S&R: Stability and Regenerability, TRL: Technology Readiness Level The synthesis of composite@MOF can be achieved through various methods, among them the most common are in-situ polymerisation, electrospinning, physical blending and solvent casting, each offering distinct advantages for tailoring material structure and functionality.230 In situ polymerisation is a process that involves the growth of polymer chains within the pores of MOF particles. This improves interfacial adhesion, enabling polymer chains to penetrate the MOF pores or form covalent bonds with surface functional groups. This creates a more uniform distribution throughout the polymer matrix.231,232 Electrospinning is a technique that involves a strong electric field to draw a polymer solution into ultra-fine fibres. As the solvent evaporates, fibres with a high surface-to-volume ratio, controllable porosity, and tunable morphology are obtained.233–235 Physical blending offers another effective strategy by physically combining MOFs with polymers to create multifunctional composites. It involves the incorporation of pre-synthesised MOFs into polymer solutions through mechanical mixing, offering scalable production with adequate thermal stability.236,237 Solvent casting remains the most straightforward approach, where MOFs are dispersed in a polymer solution followed by controlled solvent evaporation to yield homogeneous composite films, a method particularly favoured for its simplicity and reproducibility.238,239 In this thesis, two different polymer@MOF composites have been developed, using natural and synthetic polymers as a matrix and two different synthesis methods: PVDF-HFP@MOF composite a combination of solvent casting with salt leaching methods were used, where MOF, polymer and NaCl (as pore forming agent) were mixed in a solution, moulded and dried at room temperature (Fig. 1.10). This technique allows a well-controlled structure with a high surface area, enabling a uniform distribution of the MOF within the matrix while introducing interconnected pores upon salt removal, improving the accessibility to active sites. It also avoids the use of high Chapter 1 36 temperatures or complex equipment, making it scalable and compatible with structurally sensitive MOFs.240 Figure 2.10. Representation of PVDF-HFP@MOF composite prepared by solvent casting with salt leaching method. SPICHI@MOF composite (Soy Protein-Chitin@MOF): is prepared via aqueousphase blending and freeze-drying of soy protein isolate, chitin and the MOF at a high temperature, followed by pH adjustment and the addition of glycerol. The resulting mixture is poured into moulds, frozen and freeze-dried to obtain the final composite (Fig. 1.11). This method avoids the use of harsh solvents or very high temperatures, enabling homogeneous dispersion of the components. Freeze-drying also allows the formation of highly porous structures that are necessary for adsorption applications.241,242 Additionally, these biopolymers increase the metal-binding capacity with their functional groups and preserve the original MOF structure. Figure 2.11. Representation of SPICHI@MOF composite. Introduction 37 The final properties of composites are determined by the interactions between MOFs and polymers, such as hydrogen bonding, electrostatic forces, coordination or covalent bonding.243 These interactions interfere with the dispersion and adhesion of MOFs within the polymer matrix and influence the material's mechanical strength, porosity, and overall stability. The presence of strong bonds improves the structural integrity, prevents MOFs from aggregation and keeps their porosity.244,245 Therefore, a deep understanding of the nature and strength of MOF–polymer interactions is essential for designing composites with specific functionalities. The remarkable performance of MOF-polymer composites in metal adsorption can be attributed to several synergistic mechanisms that address the limitations of individual components. Polymers as a matrix help us to keep and handle MOF particles for the filtration, as well as protect MOF structures from water-induced degradation while maintaining access to active sites.246 This protective effect manifests differently depending on the polymer's nature. In PVDF-HFP, hydrophobic and hydrophilic domains create an optimal environment for metal adsorption. The PVDF backbone provides hydrophobic protection that shields MOF crystals from direct water contact, preventing MOF degradation, while the HFP co-monomer introduces controlled hydrophilicity that facilitates metal ion transport.247 Fluorine atoms in PVDF-HFP generate strong dipoles, which interact with metal cations through ion-dipole forces, bringing them into proximity to the MOF surface. Despite the absence of functional groups capable of coordinating metals, the dipolar nature and physical encapsulation of PVDF-HFP facilitate MOF dispersion and ion accessibility.248 Furthermore, its chemical inertness contributes to structural integrity and long-term performance in harsh aqueous environments, supporting stable and efficient metal adsorption throughout extended operational periods. On the other hand, biopolymers such as chitin and soy protein create hydrophilic environments that facilitate metal ion transport and offer additional binding sites. The amino groups in chitin and the diverse functional groups in soy protein (carboxyl, amino, hydroxyl, and sulfhydryl groups) work synergistically to create a multi-functional binding matrix that complements MOF adsorption sites. Primary adsorption occurs at MOF active sites through coordination bonds, ion exchange, and electrostatic interactions, while secondary adsorption takes place at polymer functional groups.249 Chapter 1 38 In chitin-soy protein composites, the polymer matrix contributes significant additional chelation capacity through multiple mechanisms: chitin's amino groups can form coordination complexes with transition metals, while soy protein's diverse amino acid residues provide sulphur and nitrogen donor atoms that exhibit strong affinity for heavy metals. The interactions between MOF particles and biopolymer composites are different from those with synthetic polymers. In chitin-soy protein-MOF systems, the formation of hydrogen bonds between polymer hydroxyl/amino groups and MOF organic linkers creates a stable region.250 Furthermore, the protein component exhibits conformational changes when binding to metal ions, which result in the exposure of previously hidden binding sites, improving the selectivity of the process since it has the capacity to adapt its structure to better accommodate specific metals. In addition, the chitin component provides structural support through its crystalline regions, while its amorphous domains add flexibility.251 Both polymer matrices are capable of maintaining the optimum particle dispersion and preventing the agglomeration, ensuring that MOF pores remain accessible to metal ions in the adsorption process.252 This is very useful for selective metal removal, as it keeps the size-selective properties of MOF pores while providing mechanical stability.253 The metal adsorption process in MOF–polymer composites displays a multitude of interconnected mechanisms. The process begins with the diffusion of metal ions from the solution towards the composite surface until metal ions move deeper into the composite and reach the MOF particles embedded within the polymer matrix. The capacity of ions to access the active sites of the MOF is significantly influenced by the nature of the polymer-MOF interface. In well-integrated composites, polymer chains can act as molecular pathways that guide metal ions directly towards the MOF pores.254 The first binding affinity with metal ions is with the unsaturated metal sites of the MOF, followed by the functional groups of the MOF’s organic linkers, and next in affinity with the functional groups or high-affinity sites on the polymer.255,256 The way metals interact with the composite involves two types of selectivity: thermodynamic and kinetic. Thermodynamic selectivity depends on the different metal ions and on their preferences for specific binding sites, following trends as the IrvingWilliams series for transition metals.257 Meanwhile, kinetic selectivity is influenced by Introduction 45 (63) Poorkhalil, A.; Tayefehseyfi, E.; Farrokhzad, H.; Mohsenzadeh, A. Natural and Synthetic Zeolites for Arsenic Removal from Water: A Comprehensive Review of Mechanisms, Performance, and Future Perspectives. 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It also summarises the main chemicals involved, as well as the fundamental principles, working conditions, and equipment related to the key experimental and characterisation techniques applied during this research. 2.1. Chemicals All chemicals used in this thesis were of reagent grade and employed as received from commercial suppliers, without further purification. Tables 2.1 to 2.4 compile the relevant information for each compound, including the chemical formula, molecular weight (MW), commercial supplier (CS), assay (AS), and the corresponding Chemical Abstracts Service (CAS) number. Table 2.1. Reagents and solvents for MOF synthesis. Name Formula Cs As MW (g/mol) CAS Zirconium (IV) chloride ZrCl4 Alfa Aesar 99% 233.3 10026– 11–6 Thiomalic acid C4H6O4S Alfa Aesar 98% 150.2 70–49–5 Trans-aconitic acid C6H6O6 Alfa Aesar 98% 174.1 4023-65-8 L-Aspartic acid C4H7NO4 Alfa Aesar 99% 133.1 56-84-8 (2R, 3S)-2,3Dimercaptosuccinic acid C4H6O4S2 Fluorochem 95% 188.2 304-55-2 Formic acid HCOOH Labbox 99% 46.0 64-18-6 Sodium hydroxide NaOH Fluorochem 98% 40.0 1310-73-2 Methanol CH3OH Alfa Aesar 99% 32.0 67-56-1 Chapter 2 62 Table 2.2. Reagents and solvents for polymer composite synthesis. Name Formula Cs As MW (g/mol) CAS Poly(vinylidene fluoridehexafluoropropylene) (C2H2F2)x(C3F6) Solvay 8% PVDF 12% HFP 600.0 9011-17-0 Sodium chloride NaCl Fisher 100% 58.4 7647-14-5 N, N – dimethylformamide HCON(CH3)2 SigmaAldrich 99% 73.1 68-12-2 Soy protein isolate, PROFAM 974 C13H10N2 ADM 90% - 9010-10-0 β-chitin (C8H13NO5)x Squid pens 100% - - Glycerol C₃H₈O₃ Panreac 99% 92,1 56-81-5 Table 2.3. Metal salts are used to obtain metal solutions for adsorption experiments. Name Formula Cs As MW (g/mol) CAS Mercury (II) chloride HgCl2 Sigma 99% 271.5 7487-94-7 Lead (II) nitrate Pb(NO3)2 Labkem 99% 331.2 10099-74-8 Cadmium (II) nitrate tetrahydrate Cd(NO3)2·4H2O Sigma 98% 308.5 10022-68-1 Copper (II) chloride dihydrate CuCl2·2H2O Acros Organics 99% 170.5 10125-13-0 Nickel (II) chloride NiCl2 Sigma 98% 129.6 7718-54-9 Cobalt(II) chloride hexahydrate CoCl2·6H2O Sigma 98% 237.9 7791-13-1 Lanthanum (III) nitrate hexahydrate La(NO3)3·6H2O Sigma 100% 433.0 10277-43-7 Yttrium (III) chloride YCl3 Sigma 99% 195.3 10361-92-9 Europium (III) nitrate Eu(NO3)3·6H2O Alfa Aesar 99% 446.1 10031-53-5 Sodium (meta)arsenite NaAsO2 Sigma 90% 129.9 7784–46–5 Sodium arsenate dibasic heptahydrate Na2HAsO4·7H2O Sigma 98 312.0 10048-95-0 Potassium dichromate K2Cr2O7 Acros Organics 99% 294.2 7778-50-9 Materials and Characterisation 63 Table 2.3. Reagents for wet oxidation catalysis assays. Name Formula Cs As MW (g/mol) CAS Dopamine C8H11NO2 Sigma 98% 153.2 51-61-6 4-Aminoantipyrine C11H13N3O Alfa aesar 97% 203.2 83-07-8 Hydrogen peroxide H2O2 Sigma 30% w/w in water 34.0 7722-84-1 2.2. Characterisation techniques X-ray Scattering Techniques X-ray scattering involves the interaction between X-ray radiation and matter to analyse the structural properties of materials at the atomic and molecular levels. The technique is based on the elastic scattering of X-rays by electrons in the sample material, where interference occurs according to Bragg's law1 (Eq. 2.1), enabling the determination of crystal lattice parameters and structural arrangements from X-ray diffraction data. 𝑛𝜆 = 2𝑑𝑠𝑖𝑛 𝜃 Equation 2.1. Bragg’s law. When X-rays, whose wavelength (λ) is on the same order of magnitude as the interatomic distances in crystalline solids, strike a sample, they are scattered by the valence electrons of the atoms in the lattice. The resulting scattered waves interfere with one another, and this interference can be either constructive or destructive depending on the angle of incidence (θ) and the interplanar spacing (d). Figure 2.1. Schematic representation of the X-ray diffraction. θ Incident beam Difracted beam d θ 2θ Chapter 2 64 This non-destructive technique provides quantitative information about crystalline structure, particle size, shape, and orientation by analysing the angular distribution of scattered X-rays.2 2.2.1.1. Powder X-ray Diffraction Powder X-ray diffraction (PXRD) is an analytical technique used for identifying crystalline phases and characterising the structure of materials. The method is based on directing monochromatic X-ray radiation onto a finely ground sample and recording the intensity of diffracted beams as a function of scattering angle. In powder diffraction, the sample consists of numerous randomly oriented crystallites. This random orientation ensures that for any set of lattice planes, enough crystallites will be oriented at the appropriate angle to satisfy the Bragg diffraction condition. The diffracted X-rays form characteristic patterns that are detected and recorded as intensity peaks plotted against the diffraction angle (2θ). Each crystalline phase exhibits a distinctive diffraction pattern that serves as a unique structural signature where its structural model can be determined by comparing the experimental pattern with reference patterns obtained, for example, from the Cambridge Structural Database (CSD).3,4 Peak positions are directly related to the unit cell parameters through Bragg's law, while peak intensities depend on the atomic scattering factors and the distribution of atoms within the unit cell. Peak profile analysis provides additional structural information that allows for to determination of average crystallite size and the presence of structural defects within the material. The sharpness and narrowness of diffraction peaks, reflected in a low full width at half maximum (FWHM), are generally associated with the presence of large, well-ordered crystallites. In contrast, peak broadening, evidenced by an increased FWHM, is typically correlated with reduced crystallite size and the presence of structural disorder within the crystal lattice. Consequently, every material exhibits a distinctive diffraction fingerprint, which can be theoretically derived from its structural model and the experimental conditions under which the measurement is performed. Contemporary diffractometers utilise various detector technologies to enhance data collection efficiency. In this thesis, all samples were measured at room temperature Materials and Characterisation 65 using a Panalytical X´pert CuKα diffractometer in the following conditions: 2θ range = 5–70°, step size = 0.05°, exposure time = 10 s per step. Panalytical X´pert is a polycrystalline sample diffractometer with theta-theta geometry, a programmable slit, secondary graphite monochromator adjusted to a copper radiation and a fast solid state PixCel detector adjusted to a 3.347º active length in 2θ(°). This diffractometer belongs to the General Research Services (SGIker) of the UPV/EHU. The obtained results were analysed using FullProf5,6 to confirm the formation of the target phase after synthesis, exclude the presence of secondary co-crystallised compounds, and assess the crystallinity and stability of the materials under the applied experimental conditions. Full peak profile fitting was systematically performed for all samples to verify the absence of impurities and to characterise their structural features. The crystal structures of the MOFs described in Chapters 3 and 4 were already reported; thus, their structural models were retrieved from databases for comparison. In Chapter 5, the crystalline structure of BCM-5 was examined by Rietveld refinement, a method that enables the determination of detailed structural parameters, including precise atomic positions, through a least-squares adjustment of the entire diffraction pattern to a profile calculated from a structural model. 2.2.1.2. Small-Angle X-ray Scattering (SAXS) Small-Angle Neutron Scattering (SANS) and (SAXS are well-known techniques to probe structural features on length scales ranging from approximately 1 to 100 nanometers7 This investigates larger-scale structural inhomogeneities, nanoparticle size distributions, macromolecular dimensions, pore sizes at the nanoscale, and characteristic distances in partially ordered materials (Fig. 2.2). Chapter 2 66 Figure 2.2. Small-angle scattering scheme. For scattering objects exceeding several nanometers to hundreds of nanometres, SAXS provides information on internal interfaces by analysing the elastic scattering of X-rays (λ = 0.06–0.3 nm, which corresponds to a photon energy range of approximately 4 to 25 keV) at small angles (0.1–10°). The technique is particularly powerful for characterising porous materials, as it can provide information about pore size distributions, specific surface areas, and pore connectivity. SAXS measurements were conducted on the I22 beamline at Diamond Light Source (Didcot, U.K.).8 Data were collected with a Pilatus P3-2 M detector. SAXS data were reduced and azimuthally averaged to obtain the isotropic scattering intensity as a function of scattering vector modulus q = 4π/λ sin(θ/2), where θ is the scattering angle and λ = 1 Å is the X-ray wavelength, using the DAWN software package.9,10 All scattering curves were fitted according to functions provided by SasView 6.0.1, including the Indirect Fourier Transformation analysis routine. qy qx Ø ~ π/R Materials and Characterisation 67 Neutron Scattering Techniques Neutron scattering techniques are based on the interaction of neutrons with atomic nuclei through the strong nuclear force, rather than electromagnetic interactions with electrons as in X-ray methods. This nuclear interaction mechanism provides unique analytical capabilities: deep penetration into dense materials, exceptional sensitivity to light elements (especially hydrogen), and the ability to distinguish isotopes of the same element.11 Neutrons exhibit wave-particle duality described by the de Broglie relation (Eq. 2.2), λ is the wavelength, m the mass of the neutron, ʋ the velocity of the neutron and ħ the reduced Planck constant. This equation reveals that thermal neutrons possess wavelengths comparable to those of X-rays (λ ≈ 0.1 nm) but with significantly lower energies in the range of 1 to 100 meV, which is particularly advantageous for studying dynamic processes, as it allows the detection of small energy changes. 𝜆 = ℎ 𝑚𝑣 Equation 2.2. Broglie equation. Neutron production facilities are classified as either reactor-based sources, which provide continuous neutron beams through nuclear fission, or spallation sources, which generate pulsed neutron beams by bombarding heavy metal targets with highenergy protons. Reactor sources typically employ monochromators for wavelength selection, while spallation sources utilise time-of-flight techniques to determine neutron energies and perform experiments simultaneously with different wavelengths of neutrons. Scattering events are categorised as elastic, where no energy exchange occurs between the neutron and sample, or inelastic, involving energy transfer that provides information about molecular dynamics and phonon spectra. Neutron Imaging Neutron imaging is a non-destructive characterisation technique that produces visual representations of the internal structure of materials by measuring how neutrons are absorbed or transmitted as they pass through a sample.12 The technique creates Chapter 2 68 contrast based on the different neutron absorption properties of various elements and compounds within the material (Fig. 2.3). The measurement process involves directing a beam of neutrons through the sample and detecting the intensity of neutrons that successfully transmit through the other side of the sample. Materials that strongly absorb neutrons will appear dark in the resulting image, while materials that allow neutrons to pass through easily will appear bright. This creates a visual map showing the distribution of different components within the sample. Neutron imaging provides unique advantages due to the specific way neutrons interact with matter. The technique shows exceptional sensitivity to hydrogen-containing materials such as water, making these substances clearly visible even when embedded within other materials.13 This technique can be performed in different configurations depending on the information required. Two-dimensional radiography produces cross-sectional images, while tomographic approaches reconstruct complete three-dimensional representations of the sample interior. Figure 2.3. Schematic view of a basic neutron imaging experimental set-up. In Chapter 4 of this thesis, the composite materials have been studied by neutron tomography measurements that were carried out at IMAT, the neutron imaging instrument at the ISIS Neutron and Muon Source (Didcot, U.K.). Samples were wrapped in aluminium foil and placed inside an aluminium tube, which was mounted Materials and Characterisation 69 on a rotation stage. Tomographic projections were acquired by rotating the sample over 360 degrees, with angular steps chosen to satisfy the Nyquist criterion. The instrument geometry was configured to a collimation ratio (L/D) of 260, where L is the distance from the pinhole to the detector and D is the pinhole aperture. Images were taken with the Andor Ikon-L 936 CCD camera coupled with a 105mm lens and an 80µm thick ZnS/6LiF scintillator providing a pixel size of 48µm. An exposure time of 60s was used for each projection. Flat-field images (recorded without the sample in the beam) and dark-field images (recorded with the beam off) were acquired for image normalisation. Tomographic reconstructions of the projection data were carried out using the Mantid Imaging14 software package. Porosity analysis was performed using Porespy.15 For the porosity profile analysis, an ROI that falls within the sample area was first selected for the analysis. Image segmentation was then done using Otsu thresholding before the calculation of percentage porosity. 2.3. Thermogravimetric analysis (TGA) TGA is a thermal analytical technique that measures the mass changes of a sample as a function of temperature or time under controlled atmospheric conditions.16 The technique operates by subjecting the sample to a programmed temperature profile while continuously monitoring mass variations using a precision microbalance. The resulting data is presented as a thermogram, which provides valuable information about thermal stability, decomposition processes, and material composition. The fundamental principle of TGA relies on detecting mass changes associated with thermal events such as dehydration, decomposition, oxidation, sublimation, or desorption processes. The measurement atmosphere can be precisely controlled, utilising inert gases (nitrogen, argon), oxidising environments (air, oxygen), or reducing conditions (forming gas mixtures), depending on the analytical requirements. The controlled heating rate and atmosphere selection enable the differentiation of various thermal processes and provide insights into material behaviour under specific conditions. Modern TGA instruments are often coupled with differential scanning calorimetry (DSC) capabilities, creating simultaneous thermal analysis systems.17 DSC measures the heat flow differences between a sample and an inert reference material as both Chapter 2 70 are subjected to identical temperature programs. This technique detects thermal transitions that may or may not involve mass changes, such as phase transitions, crystallisation, melting, or glass transitions. The DSC signal is obtained by monitoring the temperature difference between the sample and reference crucibles, where endothermic processes require additional heat input to maintain temperature equilibrium, while exothermic processes release heat. The combination of TGA and DSC provides comprehensive thermal characterisation by simultaneously monitoring mass changes and thermal events. This dual approach enables the correlation of mass loss processes with their corresponding enthalpy changes, facilitating the identification and understanding of complex thermal degradation mechanisms. The technique is particularly valuable for analysing multicomponent systems where overlapping thermal events may occur, as the complementary information from both measurements aids in process identification and quantification. TGA-DSC analysis in this thesis was carried out under synthetic air (80% N2 and 20% O2) with a 25 mL/min flux in a NETZSCH STA 449F3 DSC–TGA instrument, with a precise sample preparation, where 25 mg of material was placed in an alumina crucible, that was heated at 5 °C min–1 in the temperature range 30–700 °C. This technique was performed to evaluate thermal degradation processes and quantify the degree of linker defects per formula unit in the MOFs. The average number of linker-defect positions per formula unit was determined from the weight loss corresponding to the calcination of the organic linker. This approach was applied in Chapter 3 to investigate the defect structure of C4MOFs and in Chapter 5 to establish the chemical formula of BCM-5, where the defect degree of each sample was further confirmed through complementary 1H-NMR analysis. The defect degree of each sample was determined from the weight loss corresponding to the calcination of the organic linker at temperatures above ~300 °C.18 The TGA profile of these materials reveals three distinct weight-loss regions. The first, occurring between 30 and 100 °C, corresponds to the removal of solvent molecules confined within the porous framework. The second step, in the 100–300 °C range, is attributed to the dehydration and dehydroxylation of the Zr-hexanuclear clusters. The final weight-loss event, Materials and Characterisation 77 2.8. Scanning Electron Microscopy (SEM) SEM is an electron microscopy technique with a detailed visual image of a particle with high-quality and spatial resolution within the range of 5µm to 1mm. SEM is a multipurpose state-of-the-art instrument which is largely employed to observe the surface phenomena of the materials.31 This technique employs a focused beam of high-energy electrons generated by an electron gun, which is directed through a series of electromagnetic lenses under high-vacuum conditions. Upon interacting with the sample surface, these electrons provide detailed information about its morphological and structural features. Modern SEM instruments are equipped with multiple detectors to capture different types of signals simultaneously. Moreover, energy-dispersive X-ray (EDX) detectors are commonly integrated into SEM systems, allowing for elemental analysis and compositional mapping of the sample surface with 1-2 µm resolution.32 Both techniques provide information about particle size and shape, surface texture, crystal structure, and elemental distribution, making them an indispensable tool for materials characterisation. In this thesis, two different scanning electron microscopes have been used. For the study of particle morphology of C4MOF in Chapter 3 and BCM-5 in Chapter 5, a HITACHI S-4800 with Field Emission Gun (FEG) cold cathode (0.5-30 kV) from SGIker was used. The samples were dispersed in methanol under ultrasonication, and afterwards a drop was deposited in the copper sample holder and metallised with a gold conductive layer to prevent charge formation on non-conductive samples under the electron beam, which can impact the quality of the image and also destroy the sample. For the SEM images, including EDX mapping of C4MOF in Chapter 3 to study the empirical formula, a Carl-Zeiss EVO-40 Electronic Scanning Microscopy equipment from SGIker was used, whereby both images in high vacuum mode (with metallization of the samples) and in extended pressure or low vacuum and wet sample mode (without the need to metallise the samples) can be employed. The equipment features a secondary electron detector (SED) and a retro-scattered electron detector, together with an EDX elementary analyser, which allows performing both punctual and mapping analyses of the samples. Chapter 2 78 2.9. Infrared Spectroscopy (IR) IR represents a fundamental analytical technique extensively employed for the identification and characterisation of molecular structures through the detection of specific vibrational modes.33 This method relies on the interaction between infrared electromagnetic radiation and matter, specifically targeting the vibrational energy levels of chemical bonds within molecules. The infrared region of the electromagnetic spectrum, typically ranging from 4000 to 400 cm-1, provides sufficient energy to excite molecular vibrations without causing electronic transitions or bond dissociation. The fundamental principle underlying infrared spectroscopy is based on the selective absorption of infrared radiation by molecules that possess a permanent or induced dipole moment. When infrared radiation interacts with a molecule, energy transfer occurs only when the frequency of the incident radiation corresponds to the natural vibrational frequency of specific bonds or functional groups within the molecule. This resonance condition results in the excitation of vibrational modes, leading to measurable absorption of infrared energy at characteristic frequencies. Molecular vibrations can be categorised into two fundamental types: stretching and bending modes (Fig. 2.5). Stretching vibrations involve periodic variations in bond lengths along the internuclear axis, whereas bending vibrations encompass changes in bond angles between adjacent atomic bonds.34 These vibrational modes generate characteristic absorption patterns that serve as diagnostic fingerprints for specific functional groups and molecular structures. The intensity of absorption bands in infrared spectra depends on the magnitude of the change in dipole moment during the vibrational motion. Symmetric vibrations that do not result in dipole moment changes are infrared-inactive and therefore undetectable by this technique. Conversely, asymmetric vibrations that produce significant dipole moment variations yield intense absorption peaks. Materials and Characterisation 79 Figure 2.5. Schematic representation of molecular vibrations. FTIR spectrometers offer superior performance compared to conventional dispersive instruments. FTIR systems simultaneously measure all infrared frequencies, resulting in enhanced signal-to-noise ratios, improved spectral resolution, and reduced measurement times. The mathematical transformation of the interferogram through Fourier analysis yields the final absorption spectrum. Conversely, Attenuated Total Reflection (ATR) sampling is a technique that eliminates the complexity associated with traditional sample preparation procedures while maintaining high spectral quality and reproducibility. In ATR-FTIR measurements, the infrared beam undergoes total internal reflection within a high refractive index crystal, generating an evanescent wave that penetrates the sample in contact with the crystal surface. The depth of penetration, typically ranging from 0.5 to 2 micrometres, allows for surface-sensitive analysis of various sample types, including solids, liquids, and pastes. The resulting infrared spectrum provides a comprehensive molecular fingerprint that enables the identification of functional groups, the determination of molecular structure, and the monitoring of chemical transformations. Each absorption band corresponds to specific vibrational modes, allowing for detailed structural analysis and quantitative measurements when appropriate calibration standards are employed. In this thesis, the infrared spectra have been obtained in a spectrophotometer with a Fourier-transform Jasco FT/IR-6100 spectrometer in Attenuated Total Reflectance Stretching vibrations Bending vibrations AsymmtricalSymmtrical Twisting Wagging Rocking Scissoring Chapter 2 80 mode (FTIR-ATR). Each spectrum was recorded from 600 to 4000 cm-1 wavenumbers with a 1 cm-1 resolution. 64 scans were measured and averaged to obtain the final spectra. FTIR spectroscopy was employed to characterise the materials synthesised in this work, allowing the identification of functional groups incorporated into the MOFs during synthesis (Chapter 3). In Chapters 4 and 5, the technique was further applied to investigate possible chemical interactions between the MOFs and the polymeric matrix during membrane fabrication, and to analyse the MOFs after metal adsorption to evaluate structural modifications induced by metal exposure and to elucidate the binding mechanisms. 2.10. X-ray Photoelectron Spectroscopy (XPS) XPS is a surface-sensitive analytical technique based on the photoelectric effect, where high-energy X-ray photons interact with core-level electrons in atoms, causing the emission of photoelectrons with characteristic kinetic energies.35 In this process, the surface of a sample is irradiated with X-ray photons that interact with core electrons, where photoelectrons are emitted if the photon energy exceeds the binding energy of the electrons. XPS provides surface-sensitive analysis of materials, revealing chemical states, oxidation states, and bonding environments through characteristic core-electron binding energies. Different chemical states of the same element produce distinct peaks in the XPS spectrum, enabling the identification and quantification of various chemical species present on the surface. In this thesis, the materials described in Chapters 4 and 5 were studied after metal adsorption. Each sample was immersed in independent 100 ppm solutions of the target metals for 4 hours. Subsequently, the materials were recovered, extensively rinsed with distilled water, dried at 80 °C for 2 hours, and finally studied by XPS analysis. To this purpose, a Versaprobe III AD system (Physical Electronics, ULVAC) equipped with a monochromatic Al Kα radiation source (1486.7 eV) from SGIker was used. An initial survey scan was performed to identify the elements present (wide scan: step Materials and Characterisation 81 energy 0.2 eV, pass energy 224 eV), followed by high-resolution scans of the detected elements (detail scan: step energy 0.05 eV, pass energy 27 eV, time per step 20 ms) using an electron take-off angle of 45°. The spectrometer was previously calibrated using silver (Ag 3d5/2, 368.26 eV). The spectra were fitted using CasaXPS software version 2.3.26, which models the signal contributions after background subtraction using the Shirley method. 2.11. High Performance Liquid Chromatography (HPLC) coupled with UV-visible detection HPLC is a separation technique that enables the resolution of complex mixtures into individual components based on their differential interactions with stationary and mobile phases. The separation mechanism relies on the distinct affinities of analytes for the stationary phase packed within the chromatographic column and the mobile phase that flows through it. Components with different polarities, molecular sizes, or chemical properties migrate through the column at different rates, achieving temporal separation as they elute from the column. The chromatographic separation occurs as the mobile phase carries sample components through a column packed with stationary phase particles. The coupling of HPLC with UV-visible detection (HPLC-UV-Vis) monitors the absorption of light by eluting compounds as they pass through a flow cell, enabling immediate detection and quantification of separated components. Each compound exhibits characteristic absorption at specific wavelengths, allowing qualitative identification and quantitative determination based on Beer-Lambert law principles. HPLC-UV-Vis is useful for organic compounds with chromophore groups, offering excellent selectivity through chromatographic separation and sensitive detection through UV-Vis absorption. Quantitative analysis is achieved by comparing peak areas with calibration standards of known concentrations. The chromatographic analyses for Chapter 3 of this thesis were performed using an Agilent Infinity 1260 chromatographic system, equipped with a Diode Array Detector (DAD) set to monitor at a wavelength of 277 nm for phenol and catechol, 292 nm for hydroquinone, 280 nm for dopamine and 264 nm for paracetamol. The system Chapter 2 82 was equipped with an InfinityLab Poroshell 120 EC-C18 column (3.0 × 100 mm, 2.7 µm particle size) coupled with a guard column of the same material. Mobile phase A was 0.1% (v/v) formic acid in water, and mobile phase B was acetonitrile containing 0.1% (v/v) formic acid. The chromatographic gradient began with 0% of mobile phase B at 0.00 minutes. From 0.00 to 7.00 minutes, the percentage of mobile phase B increased linearly to 60%. At 7.01 minutes, a rapid step increase brought mobile phase B to 100%, which was maintained isocratically from 7.01 to 8.00 minutes. Finally, the gradient returned to 0% mobile phase B at 8.00 minutes and was held at this composition until 15.00 minutes to re-equilibrate the column. The mobile phase flow rate was maintained at 0.5 mL/min, and the column temperature was set to 40°C to ensure consistent retention times and peak resolution. Sample injection volumes were 5 µL. Calibration curves for all the molecules were obtained with solutions of 0.1, 1, 2.5, 5, 7.5, 10, 100 and 1000 ppm prepared from a parent solution at a concentration of 1000 ppm. The measurements were done in triplicate for each of the solutions 2.12. Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) ICP-AES is an analytical technique that employs high-temperature plasma to atomise and excite elements in a sample, causing them to emit characteristic electromagnetic radiation. The technique is based on the principle that atoms and ions, when excited to higher energy states, return to their ground states by emitting photons at wavelengths specific to each element. Liquid samples are introduced through nebulization, converting them into fine aerosols that are transported into the plasma by argon carrier gas. The extreme plasma temperatures ensure complete atomization and excitation of analyte elements, resulting in characteristic emission spectra.36 The emitted radiation is collected and dispersed by an optical system, enabling simultaneous or sequential detection of multiple wavelengths corresponding to different elements. The intensity of emission lines is directly proportional to elemental concentrations, allowing quantitative analysis through calibration with standard solutions. ICP-AES offers a wide linear dynamic range, multi-element capability, and low detection limits. The high plasma temperature minimises chemical interferences and matrix effects, enabling accurate analysis of complex samples.37 Materials and Characterisation 83 Throughout Chapters 3, 4, and 5 of this thesis, the concentration of the metal solutions was quantified using a Horiba Yobin Yvon Activa atomic emission spectrometer with inductively coupled plasma (ICP-AES) from SGIker. The system is equipped with a glass and Teflon nebuliser, allowing the analysis of samples obtained from acidic digestion using nitric acid. The instrument is controlled by the Activa Analyst 5.4 software, which enables sequential multi-element analysis and supports the accurate determination of both major and trace elements. For the adsorption assays, samples were diluted to the detection limits of the equipment, performing triplicate measurements for each solution. 2.13. Adsorption Experiments and Models In general, all adsorption experiments were conducted using 1 mg of sorbent per 1 mL of solution. The total volume varied depending on the requirements of each experiment. Different concentrations were tested, but all experiments were run for 4 h, since previous studies in this group have shown MOF degradation beyond that time. The pH was maintained within the range in which the MOF remains stable. It is important to note that the heavy-metal capture mechanism of the composite relies on a combination of absorption within the MOF’s pore structure and adsorption at both the biopolymer scaffold surface and the MOF/polymer interface. For simplicity, the term “adsorption” will be used throughout this thesis. However, depending on whether the MOF, the composite, or the composite@MOF components are employed, absorption, adsorption, or a combination of both processes may be responsible for metal immobilisation. In the following section, the isotherm and kinetic models applied to fit the experimental metal-adsorption data obtained in this thesis are described. Kinetic models Adsorption kinetic models describe how fast adsorbates are removed from solution and bind onto the adsorbent surface over time. These models help understand adsorption mechanisms and the steps that control the rate, which may include physical adsorption, chemical interactions, and diffusion processes.38 In this thesis, the Chapter 2 84 experimental kinetic data obtained from metal-ion adsorption experiments have been analysed using the pseudo-first-order (PFO) and pseudo-second-order (PSO) models. The PFO model assumes that the adsorption rate depends on the number of unoccupied sites,39 And it is often suitable for processes dominated by physical adsorption or diffusion-controlled steps. Its linearised form is (Eq. 2.5): 𝑙𝑛(𝑞𝑒− 𝑞𝑡)=𝑙𝑛𝑞𝑒− 𝑘1𝑡 Equation 2.4. Pseudo-first order equation. Where k1 is the pseudo-first order constant rate, qt is the amount of adsorbate adsorbed at time t, and qe is the equilibrium adsorption capacity. The PSO model assumes the adsorption rate relates to the square of the number of unoccupied sites, often interpreted as representing chemisorption processes.40 The linearised form of the pseudo-second-order equation is given as (Eq. 2.6): 𝑡 𝑞𝑡=1 𝑘2𝑞𝑒 2+𝑡 𝑞𝑒 Equation 2.5. Pseudo-first order equation. Where k2 is the pseudo-second order constant rate, qt is the amount of adsorbate adsorbed at time t and qe is the equilibrium adsorption capacity. While the PSO model often provides a better fit to experimental data, it is important to remember that these models are empirical. Good fitting alone cannot definitively prove whether the adsorption is physical or chemical. The models’ applicability depends on experimental conditions such as initial adsorbate concentration and adsorbent properties, and their parameters should be interpreted cautiously and complemented with additional mechanistic evidence. To illustrate the application of both models, Figure 2.6 shows the adsorption kinetics. The experimental data are presented as points, and the solid line corresponds to the model fit. In the pseudo-first order model, from the linearised plot of ln(qe−qt) versus t, the adsorption parameters qe and k1 can be obtained, and in the pseudo-second order model, from the linearised plot of t/qt versus t, the adsorption parameters qe and k2 can be determined. Comparing goodness-of-fit (R2) provides insight into ratecontrolling steps but is insufficient to conclusively determine mechanisms without further characterisation. Materials and Characterisation 85 Figure 2.6. Adsorption kinetics of a) pseudo-first order model and b) pseudo-second order model. Comparing goodness-of-fit (better R2) provides insight into rate-controlling steps but is insufficient to conclusively determine mechanisms without further characterisation. Isotherm models Adsorption isotherm models mathematically describe the equilibrium relationship between the amount of adsorbate bound to the adsorbent surface and its concentration in solution at a constant temperature.41 In this thesis, metal-ion adsorption isotherms were fitted using the Langmuir and the Freundlich models, which offer insights into adsorption mechanisms and surface characteristics. The Langmuir model assumes monolayer adsorption on a homogeneous surface with a finite number of identical sites. It postulates equal adsorption energies for all adsorbed molecules and no interaction between them. The Langmuir equation is expressed as (Eq. 2.7): 𝑞𝑒= 𝑄𝑚𝑎𝑥𝐾𝐿𝐶𝑒 1 + 𝐾𝐿𝐶𝑒 Equation 2.6. Langmuir equation. Where qe is the equilibrium adsorption capacity, Qmax is the maximum monolayer adsorption capacity, Ce is the equilibrium concentration, and KL is the Langmuir equilibrium constant. This model suits adsorption systems where monolayer coverage is predominant and provides parameters related to adsorption capacity and affinity.42 qe= 0.03 k1 = 11.8 R2 = 0.9998 qe= 0.001 k2 = 17.2 R2 = 0.9993 a) b) Chapter 2 86 The Freundlich model describes adsorption on heterogeneous surfaces with nonuniform sites and variable adsorption energies. This empirical model accounts for multilayer adsorption and is expressed as (Eq. 2.8): 𝑞𝑒= 𝐾𝐹𝐶𝑒1/𝑛 Equation 2.7. Freundlich equation. Where qe is the adsorption capacity at equilibrium conditions, Ce is the equilibrium concentration, and KF is the Freundlich constant related to adsorption capacity (the higher KF, the more adsorption capacity we have), and n is the heterogeneity factor that indicates the degree of non-linearity and adsorption intensity. Values of n between 1 and 10 generally indicate favourable adsorption conditions, with higher values suggesting more favourable adsorption processes.43 To illustrate the application of both models, Figure 2.7 shows the adsorption isotherms. The experimental data are presented as points, while the solid lines correspond to the Langmuir and the Freundlich model fits. In the Langmuir model, from the linearised plot of Ce/qe versus Ce, the adsorption parameters qmax and KL can be determined. In the Freundlich model, from the linearised plot of lnqe versus lnCe, the constants KF and 1/n are obtained. Figure 2.7. Adsorption isotherms of a) the Langmuir model and b) the Freundlich model. While these two models remain the most commonly applied due to their simplicity and interpretability, no single isotherm perfectly captures all adsorption behaviours. The Langmuir model assumes uniform sites and monolayer coverage, whereas the Freundlich model better describes heterogeneous surfaces and multilayer adsorption. Therefore, comparing fitting results can provide insights into whether adsorption occurs mainly as monolayer adsorption on homogeneous sites or involves multilayer a) b) qmax = 30.8 kL= 0.08 R2 = 0.987 KF = 5.43 1/n = 0.39 R2 = 0.917 Synthesis of C4 dicarboxylic acids MOFs 93 Chapter 3 Synthesis of C4 dicarboxylic acids MOFs This chapter presents the synthesis and characterisation of four zirconium-based metal-organic frameworks constructed using C4 dicarboxylic acids as organic linkers: aspartic acid (MIP-202), trans-aconitic acid (MIP-205), thiomalic acid (BCM-1), and meso-2,3-dimercaptosuccinic acid (BCM-5). A systematic exploration of water-based crystallisation conditions enabled the controlled transition of topology between the cubic "fcu", hexagonal "hcp" and mixed "fcu-reo" phases by modulating the concentrations of the precursor and formic acid. Multivariate combinations of thiol (- SH), amino (-NH2), and carboxyl (-COOH) functionalised linkers were synthesised in order to investigate the synergistic effects on the adsorption capacity of heavy metals. Comprehensive screening revealed exceptional selectivity towards Hg(II), Pb(II), Cu(II) and Cd(II). The results establish clear structure-composition relationships and demonstrate the tunability of C4 MOF-based materials for targeted heavy metal removal applications. 3.1. Introduction This chapter reports the synthesis of four different zirconium-based porous frameworks constructed using dicarboxylic acids based on four carbons (C4) as organic linkers. To introduce specific functional properties, four different dicarboxylic acids were employed: aspartic acid, trans-aconitic acid, thiomalic acid, and meso-2,3dimercaptosuccinic acid. These ligands led to the formation of the metal–organic frameworks (MOFs) MIP-202, MIP-205, BCM-1, and BCM-5, respectively. The main strategy for evaluating the performance of these MOFs is to create a multivariate Chapter 3 94 system by combining MIP-202, MIP-205 and BCM-1 in different proportions, in order to investigate their metal adsorption capacities. For the synthesis of the zirconium-based MOFs BCM-1, MIP-202, and MIP-205, an extensive screening of water-based crystallisation conditions was conducted. Controlling the addition of the formic acid modulator and the metal concentration has enabled modulating the particle size from the micrometric down to the nanometric range, resulting in materials with distinct topologies, referred to herein as C4MOF-SH, C4MOF-NH2 and C4MOF-COOH. Their multivariate combinations have been designated as C4MOF-SH/NH2, C4MOF-SH/COOH, and C4MOF-SH/COOH. The specific molar ratios of the linkers have been reported only for these multivariate systems, particularly when deviating from equimolar compositions. Each synthesised compound has been fully characterised before testing its adsorption capacity of Hg(II), Pb(II), Cd(II), La(III), Y(III), Eu(III), Cu(II), Ni(II) and Co(II). The structural versatility of highly ordered Metal-Organic Frameworks (MOFs) has opened the door to the design of a myriad of materials with different topologies and porosity metrics. More specifically, ZrIV, HfIV or CeIV based MOFs are one of the most promising groups of reticular materials due to their chemical and hydrolytic robustness arising from the high connectivity of their inorganic building units. Today, most of the members within MIV-MOFs are constructed from the hexameric [MIV6µ3-O4(µ3-OH)4]12+ clusters, which are assembled through organic linkers into three dimensional frameworks exhibiting permanent porosity. Depending on the connectivity of the inorganic and organic blocks, the topology, and hence the porosity metrics of the final framework can be controlled. Today, more than half of the possible topologies have already been synthesised for MIV-MOFs having [MIV6µ3-O4(µ3-OH)4]12+ clusters. Here, the design of new organic linkers has a prominent role to tune not only the topology of the final structure, but also the properties and functionality of the final MIV-MOF. Unfortunately, in most cases, the sophistication of the organic components in MOFs is linked to environmentally unfriendly synthesis paths, and energy, time and resourceconsuming processes. All of this hinders the applicability of MOF materials beyond the lab-scale. Here is where the design of MIV materials based on C4 organic linkers gains importance in terms of their potential applicability due to their low toxicity, limited price, Synthesis of C4 dicarboxylic acids MOFs 95 and chiral nature of the organic component. In addition, due to the high solubility of C4-linkers in water, MIV-MOFs can be obtained through high-yield green synthesis protocols at low to moderate temperatures, involving water as solvent, and formic acid as a modulator. Nonetheless, despite the chemical versatility of C4-linkers, to date only four variants of MIV-MOF materials have been synthesised based on fumarate (MOF801), aspartate (MIP-202), succinate and malate (MIP-203-S, MIP-203-M) and transaconitate (MIP-205) organic linkers. In comparison to the UiO-66 family, where countless variants and multivariate materials based on different compositions of the [MIVO4(OH)4]12+ units (i.e. ZrIV, HfIV, CeIV, REEIII, ZrIV-TiIV…) and organic linkers having different functions (i.e. -amino, -hydroxy, - nitro, -methyl, - alkyl, - thiol, -bromine…) have been synthesised, MIV-MOFs based on C4 linkers are still underexplored. In addition, as the research community working on MOFs gains control over the synthesis parameters controlling their crystallisation, the control of the crystallisation of variants arising from a local or long-range correlated disorder has been obtained in the same system. It is well known that missing linker (ML) local defectivity expands even more the richness of their chemical and physical properties of MIV-MOFs. Even more, when missing linker (ML) and cluster (MC) defects are correlated in a local to long-range order, the defectivity within the frameworks can give access to the design of materials beyond the archetypal “fcu” cubic variant usually obtained for MIV-MOFs. This is the case of ZrIV and HfIV UiO-66 and UiO-67 systems, where the linker and cluster defectivity guide the crystallisation of UiO-66 and UiO-67 materials towards five different topologies: 12-c “fcu”, 8-c “hex”, 8-c “reo”, 9-c “hcp” and 6-c “hxl”. More interestingly, the symmetry and topology within the structure of all these variants can be qualitatively interlinked by applying local or long-range correlated defectivity within the highly symmetric parent material. Thus, by systematically exploring the synthesis parameter space, it is possible to move from (I) “fcu” MIV-MOFs materials with different degrees of uncorrelated ML, to (II) “fcu + reo” and “reo” variants with correlated MC domains to finally, a collapse of the cubic symmetry to a hexagonal framework arising from the correlated ML defects into the same crystallographic plane. In this research we have duly investigated the synthetic parameters that control the local and long-range defectivity in C4and multivariate C4-MVI-MOFs based on thiomalic (THIO), aspartic (ASP), trans-aconitic (TRANS) and dimercaptosuccinic acid (DMSA), to guide the system from the ideal “fcu” cubic topology towards a local “reo” Chapter 3 96 defective framework, and finally to the “hcp” hexagonal variants where the ordered local ML defectivity generates the collapse of the ABCBC packing shown the “fcu-reo” framework into a ABABAB packing related with an hexagonal “hcp” variant showing Zr12 clusters as the nodes of the structure. We have explored how the dilution of the concentration of the initial reagents, and the addition of formic acid modulator to the media control defectiveness and guide the crystallisation towards the “hcp” variants, as well as how the multivariate combination of the linkers aids or suppresses the transition from a “fcu” to a “fcu-reo” and “hcp” system. Last but not least, once configured, the different variants of the materials, materials have been employed to study their adsorption capacity in a multimetal water environment. 3.2. Synthesis and Characterisation The design of metal-organic frameworks (MOFs) has evolved from basic synthesis methods to more complex approaches to target functional modifications and tune their properties.1,2. Their synthesis involves a complex interplay between various parameters that significantly affect crystallisation kinetics and final morphology. Modulators, solvents, pH, temperature, and other factors play crucial roles in controlling MOF formation. The use of chemical additives to adjust the properties of materials has become increasingly common in new approaches. Among these additives, modulators have emerged as powerful tools that allow control of the crystallisation process, offering a pathway to overcome the inherent challenges of controlling nucleation and growth in these complex materials. These molecular additives work through coordination modulation, where the modulator molecules compete with the linker molecules for metal coordination sites.3. This mechanism enables modulators to act as molecular switches for specific crystallisation pathways and thereby influence key material properties, including crystallinity, particle size, morphology, and defectivity.4. The acidity and concentration of modulators are important for achieving phase-pure crystalline frameworks.5 and thus require optimisation when working with robust frameworks such as Zr-based MOFs, where strong metal-ligand interactions can otherwise lead to poorly controlled crystallisation results. Synthesis of C4 dicarboxylic acids MOFs 97 Interestingly, while modulators typically decelerate MOF formation, unexpected accelerating effects can occur under certain conditions. For instance, the presence of water has been observed to accelerate Zr-fumarate MOF synthesis, highlighting the complex interplay between modulators, solvents, and other synthesis parameters.6 Linker design also plays a crucial role in tuning MOF structure and function, with adjustments to geometry, length, ratio, and functional groups allowing for tailored properties such as surface area, pore aperture, and molecular recognition.7. The interplay between modulators, solvents, and other synthesis parameters is complex and can significantly affect the kinetics of crystallisation. Understanding these mechanisms is crucial for designing MOFs with specific morphologies and properties.8 As a contribution to C4-based Zr-MOFs, in this work, we investigated the water-based synthesis and crystallisation of four different MOFs of zirconium, thiomalic (SH), aspartic (NH2) and trans-aconitic (COOH) microporous frameworks. To provide context for the synthesis conditions explored, a representative procedure is first outlined and illustrated in Figure 3.1. As a common procedure, ZrCl4 (2.33 g, 10 mmol) was dissolved in 10 mL of water under magnetic stirring in a 50 mL Pyrex® autoclave, then dicarboxylic acid linker (20 mmol) was added. Finally, 570 μL of formic acid (FA) was introduced as a modulator to the mixture. The Pyrex® reactor was closed and placed in a preheated oven at 120 °C for 24 h. The obtained solid was recovered by centrifugation (6000 rpm, 30 min) and afterwards washed three times for three consecutive days with water. Finally, the samples were dried at 80 °C for 24 h. Figure 3.1. Representation of the C4-based Zr-MOFs synthesis process. From these established conditions, we focused on two key parameters: the concentrations of the precursor (ZrCl4) and the modulator (FA). Varying these two parameters led to the formation of distinct crystalline phases. Once the optimal Chapter 3 98 conditions for progressively introducing defects into the framework and transitioning between crystallographic variants were identified, the synthesis protocol was applied to all C4-organic linkers studied in this work: aspartic acid (ASP), thiomalic acid (THIO) and trans-aconitic acid (TRANS) (Table 3.1). Table 3.1. Concentrations for C4-dicarboxylic acids and ZrCl4 for the crystallographic variants study. Name [ZrCl4] [FA] Geometry AspA1 0.1 1.5 hcp-t-c-fcu AspA2 0.1 5 hcp AspA3 0.1 10 hcp AspB1 0.5 1.5 c-fcu-monoclinic or tetragonal? AspB2 0.5 5 c-fcu-hcp AspB3 0.5 10 Hcp-monoclinic or tetragonal? AspC1 0.75 1.5 Monoclinic or tetragonal? AspC2 0.75 5 Monoclinic or tetragonal AspC3 0.75 10 monoclinic or tetragonal AspD1 1 1.5 c-fcu AspD2 1 5 c-fcu-monoclinic or tetragonal? AspD3 1 10 c-fcu-monoclinic or tetragonal? ThioA1 0.1 1.5 c-fcu ThioA2 0.1 5 c-fcu-hcp ThioA3 0.1 10 c-fcu-hcp ThioB1 0.5 1.5 c-fcu ThioB2 0.5 5 c-fcu ThioB3 0.5 10 t-fcu ThioC1 0.75 1.5 c-fcu ThioC2 0.75 5 c-fcu ThioC3 0.75 10 c-fcu ThioD1 1 1.5 c-fcu ThioD2 1 5 c-fcu ThioD3 1 10 c-fcu TransA1 0.1 1.5 c-fcu TransA2 0.1 10 c-fcu TransA3 0.1 20 c-fcu TransB1 0.5 1.5 c-fcu -reo TransB2 0.5 10 c-fcu-reo TransB3 0.5 20 c-fcu TransC1 0.75 1.5 c-fcu -reo TransC2 0.75 10 c-fcu -reo TransC3 0.75 20 c-fcu TransD1 1 1.5 c-fcu -reo TransD2 1 10 c-fcu -reo TransD3 1 20 c-fcu Synthesis of C4 dicarboxylic acids MOFs 99 Analysis of the diffraction patterns of the obtained ASP-type compounds reveals noticeable changes as the [ZrCl4] and [FA] concentration change. The diffraction pattern of AspD1, obtained using the typical procedure previously reported, shows strong similarities to that of MOF-801, which typically exhibits its most intense peaks at around 8° and 10° in 2θ. Additionally, the full profile matching analysis of the diffraction patterns confirms the formation of a cubic phase with the same structure as MOF-801 (Fig. 3.2). Figure 3.2. Pattern matching profile analysis of the aspartic acid (ASP) sample. Red points: experimental. Red line: calculated. Blue line: difference. Green vertical bars: Bragg planes. This symmetry reveals that, in general, increasing [FA] and decreasing [ZrCl4] produce noticeable changes in the diffraction peaks, leading to the formation of distinct structural geometries. As the concentrations of both [ZrCl4] and [FA] decrease from high to low, the diffraction patterns change significantly, with the most intense peaks shifting toward lower 2θ values. Comparison with known terephthalate-based ZrMOFs suggests that this pattern corresponds to a hexagonal structural variant. XRD analysis enabled the determination of the cubic and hexagonal phases in the different synthetic conditions explored, thus allowing the construction of crystallisation stability maps for each topology (Fig. 3.3a, 3.5a and 3.7a). Chapter 3 100 Figure 3.3. (a) Crystallisation phase diagram of the Zr-ASP system and (b) diffraction patterns of the screening synthesis. The initial screening was conducted using ZrCl4 and ASP at concentrations of 1.0 M and 1.5 M, respectively. Within this preliminary synthesis matrix, formic acid (FA) was added in volumes of 1.9 mL and 3.8 mL, corresponding to final FA concentrations of 5 M and 10 M. Under these conditions, a series of reactions was performed with varying ZrCl4 concentrations (0.75 M, 0.5 M, and 0.1 M), as summarised in the crystallisation stability map (Fig. 3.3a). By correlating the crystallisation phase diagram with the corresponding diffraction patterns, it becomes evident that higher concentrations of ZrCl4 favour the formation of the c-fcu phase, although with reduced crystallinity. As the ZrCl4 concentration decreases, a progressive morphological transition is observed: at 0.75 M, the product exhibits a monoclinic or tetragonal phase; at 0.5 M, a mixture of hexagonal and tetragonal (or monoclinic) phases emerges; and at 0.1 M, the material adopts a predominantly hexagonal structure. In these latter three cases, lower concentrations of formic acid are associated with a further decrease in crystallinity. The same screening strategy was applied to the THIO system. In this case, variation of the ZrCl4 and formic acid concentrations resulted in minimal changes in the diffraction patterns of the resulting THIO-based materials. Notably, the diffraction pattern of ThioD1 closely matches that of MOF-801. Full-profile pattern-matching analysis further confirms the formation of a cubic phase consistent with the structure of MOF-801 (Fig. 3.4). Synthesis of C4 dicarboxylic acids MOFs 101 Figure 3.4. Pattern matching profile analysis of the thiomalic acid sample. Red points: experimental. Black line: calculated. Blue line: difference. Green vertical bars: Bragg planes. A detailed correlation between the crystallisation phase diagram and the corresponding diffraction patterns shows that high concentrations of ZrCl4 favour the formation of a cubic phase but with a significant decrease in crystallinity (Fig. 3.5b). Reducing the ZrCl4 concentration to 0.1 M results in a progressive phase transformation towards a hexagonal topology. The concentration of formic acid further modulates this structural transition: at 0.5 M, a biphasic system consisting of cubic and tetragonal domains emerges, whereas at 0.1 M, the crystallisation pathway favours the formation of a predominantly hexagonal phase. Figure 3.5. (a) Crystallisation phase diagram of the Zr-THIO system and (b) diffraction patterns of the screening synthesis. A similar screening approach was applied to the TRANS system. In this case, analysis of the diffraction patterns of the resulting TRANS-type materials showed no significant changes as the concentrations of ZrCl4 and formic acid were varied. The diffraction pattern of TransD1 was also compared with that of MOF-801. Full-profile matching Chapter 3 102 analysis confirmed the formation of a cubic phase, displaying characteristic reo-type reflections consistent with the cubic structure of MOF-801 (Fig. 3.6). Figure 3.6. Pattern matching profile analysis of the trans-aconitic acid (TRANS) sample. Red points: experimental. Red line: calculated. Blue line: difference. Green vertical bars: Bragg planes. An initial screening was also carried out using ZrCl4 and TRANS at concentrations of 1.0 M and 1.5 M, respectively. In this synthesis matrix, formic acid (FA) was added in volumes of 3.8 mL and 7.6 mL, corresponding to final FA concentrations of 10 M and 20 M. A series of reactions was then conducted under these conditions, varying the ZrCl4 concentration to 0.75 M, 0.5 M, and 0.1 M. The outcomes of these experiments are summarised in the crystallisation stability map shown in Figure 3.7a. Figure 3.7. (a) Crystallisation phase diagram of the Zr-TRANS system and (b) diffraction patterns of the screening synthesis. Correlation of the crystallisation phase diagram with the corresponding diffraction patterns reveals that higher concentrations of ZrCl4 combined with lower concentrations of formic acid (FA) promote the formation of the reo-c-fcu phase. When Synthesis of C4 dicarboxylic acids MOFs 109 equimolar proportions, the material becomes predominantly amorphous (Fig. 3.14c). These findings clearly demonstrate that the introduction of the aspartic acid linker into the multivariate system promotes amorphisation of the framework. The PXRD pattern of C4MOF-NH₂ does not exhibit the superstructural reflections typically associated with reo-type nanodomains. As a result, the pattern cannot be accurately fitted using the higher-symmetry F23 model. Instead, lowering the symmetry to the Pn3 space group is required to obtain a better fit. This model successfully accounts for the superstructural reflection at 6.5° but still fails to reproduce the reflection consistently observed at 5.0°, indicating limitations in its ability to fully describe the structural features of the material. Figure 3.14. Representative XRD-patterns of C₄MOFs based on -NH₂, -COOH and -SH groups and of their equimolar on (a) -SH/-NH2, (b) -SH/-COOH, (c) -COOH/-NH2 and -COOH/-SH multivariate combinations and pattern matching profile analysis of the 1:1 multivariate compositions: (d) C4MOFSH/-NH2, (e) C4MOF-SH/-COOH and (f) C4MOF-COOH/-NH2. Red points: experimental. Red line: calculated. Blue line: difference. Green vertical bars: Bragg planes. Chapter 3 110 For the multivariate combinations between C4MOF-NH2 (Pn-3) and -COOH or -SH (F23/P23) variants, the transition between these endmembers of the sequence is disrupted within an intermediate compositional region. The integration of a secondary THIO or TRANS linker into the Pn-3 structure of the ASP variant initially induces the appearance of the superstructural reflections associated with cluster-defective reo nanodomains. This is followed by a sudden broadening of the diffraction maxima within a very narrow compositional window. Further integration of secondary linkers into the structure gives rise to the recovery of the crystallinity of the material, still the PRXD data show the presence of the superstructural reflections associated with -reo nanodomains. Fourier-transform infrared (FTIR) spectroscopy was employed to confirm the incorporation of the functional groups associated with the C4-MOFs (ASP, THIO, and TRANS) in both single-linker and multivariate Zr-based MOFs (Fig. 3.15–3.18). The spectra display the characteristic fingerprint regions associated with Zr-based C4MOFs, as well as the vibrational signatures of each functional linker. Notably, no significant shifts in vibrational frequencies are observed when comparing single-linker frameworks with their multivariate analogues, indicating that the incorporation of multiple ligands does not substantially alter the coordination environment at the molecular level (Fig. 3.15). Figure 3.15. FTIR spectra of compounds (a) C4MOF-(SH), (b) C4MOF-(NH2) and (c) C4MOF- (SH)/(NH2)/(COOH) and (c) C4MOF-(COOH). The most distinctive feature of the THIO ligand is the presence of its thiol group, which is clearly evidenced by a sharp S-H stretching band around 2550 cm-1 and a weaker Synthesis of C4 dicarboxylic acids MOFs 111 C-S vibration at approximately 635 cm-1. Although this latter signal partially overlaps with the Zr-O vibrations, it can still be seen in the spectra. Figure 3.16. FTIR spectra of compounds (a) C4MOF-(SH), (b) C4MOF-(SH)/(NH2) and (c) C4MOF- (SH). The spectra of all samples exhibit the typical bridging coordination of carboxylate groups, confirmed by the separation between the asymmetric (∼1575 cm-1) and symmetric (∼1420 cm-1) stretching bands of the carboxylate group. Figure 3.17. FTIR spectra of compounds (a) C4MOF-(SH), (b) C4MOF-(SH)/(COOH) and (c) C4MOF- (COOH). An additional absorption band at approximately 1210 cm⁻¹, associated with C–O stretching vibrations, is also detected and shows increased intensity in materials incorporating the TRANS ligand (Fig. 3.16 and 3.17). Furthermore, a weak absorption band centred around 1715 cm-1 is observed in most of the materials, which is attributed to the asymmetric stretching of non-coordinated Chapter 3 112 C=O bonds. This feature reflects the presence of free carboxylic acid groups in the framework. Figure 3.18. FTIR spectra of compounds (a) C4MOF-(NH2), (b) C4MOF-(COOH)/(NH2) and (c) C4MOF-(COOH). Thermogravimetric analysis (TGA) and 1H nuclear magnetic resonance (¹H NMR) spectroscopy provide complementary tools for evaluating the average of our materials (Fig. 3.19-3.24). As previously established, the weight loss observed in TGA, particularly that associated with cluster dehydration and organic linker decomposition, can be used to estimate the number of missing linkers per formula unit in M(IV)-based MOFs. To exemplify this approach, the thermogravimetric and differential scanning calorimetry (DSC) curves of the pure-linker frameworks were employed, as they represent the extreme behaviours within the series. The MTV materials exhibit thermal profiles that lie between those of their single-linker counterparts. In general, the TGA and DSC data reveal two main thermal events. The first weight loss, occurring between room temperature and ~125 °C, corresponds to the release of physically adsorbed or trapped solvent molecules within the pore system. The second weight loss process, initiated around 300 °C, is associated with the multistep decomposition of the organic linkers. Complete transformation into the corresponding M(IV) oxides occurs between 600 and 650 °C, depending on the chemical nature of the C4 linker employed in each material. To semi-quantitatively estimate the average linker defectivity in each MOF, the TGA weight loss recorded around 300 °C, just before the onset of intense exothermic Synthesis of C4 dicarboxylic acids MOFs 113 decomposition observed in the DSC trace, was taken as a reference. This value was normalised to 100% and used to recalculate the relative residue mass. The resulting experimental values were then correlated with the theoretical thermal decomposition described in Equation 3.1: Zr6O6+x(C4-L)6-x → (ZrO2)6 + CO2 + H2O Equation 3.1. Thermal decoposition of Zr compounds. Where x denotes the number of linker vacancies per formula unit. Theoretical weight loss values were calculated for different defect degrees (x) and plotted to generate a linear correlation, which was used to extract the experimental defectivity from the TGA data. It is important to highlight that the theoretical model behind Equation 3.1 assumes complete removal of formate modulator molecules, in addition to dehydration of the zirconium oxo-clusters, by 300 °C. In the current system, the onset of linker degradation may hinder full dehydration of the clusters. Nonetheless, at 300 °C, substantial formate removal and a significant degree of cluster dehydration are expected. Complementarily, 1H NMR was employed to quantify the amount of formate retained within the structure. The spectra were recorded after digesting the samples in 2 M NaOH solution in D2O, allowing accurate determination of the chemical formula of each material under ambient conditions. This analysis revealed that not all missing linker sites are compensated by formate anions; a significant fraction of the defects within the Zr6 clusters are instead occupied by terminal water molecules and hydroxyl groups. These findings support the hypothesis that defect compensation in these materials follows a mixed mechanism involving both modulator coordination and hydrolytic termination. To further clarify the composition of the MTV MOFs—particularly regarding the protonation state of the incorporated linkers—energy-dispersive X-ray spectroscopy (EDX) analysis was performed to support the determination of the empirical formula of the materials (Table 3.3). Based on the elemental data, it was concluded that the ASP linker is present in its protonated form as NH3Cl, while the remaining linkers (THIO and TRANS) are incorporated in their neutral forms. Chapter 3 114 Table 3.3. Quantification of atoms of MTV system components by EDX. Code C N O S Cl Zr C4MOF-NH2 35.4±3.61 2.98±0.16 19.4±1.91 - 6.86±0.46 20.6±1.43 C4MOF-SH/NH2 43.4±7.43 1.31±0.30 18.5±3.24 1.70±0.31 2.21±0.43 13.7±2.48 C4MOF-COOH/NH2 35.1±14.9 1.15±0.57 15.7±7.02 - 0.98±0.18 17.7±5.64 Considering these assumptions, the reaction conditions employed for the gram-scale synthesis of the MTV M(IV)-C4-MOFs yield average linker defectivities in the range of 0.2 to 2.3 vacancies per formula unit (Table 3.4). Figure 3.19. (a) 1H NMR and (b) TGA and DSC of C4MOF-SH. Figure 3.20. (a) 1H NMR and (b) TGA and DSC of C4MOF-(SH)0.52/(NH2)0.48. Synthesis of C4 dicarboxylic acids MOFs 115 Figure 3.21. (a) 1H NMR and (b) TGA and DSC of C4MOF-NH2. Figure 3.22. (a) 1H NMR and (b) TGA and DSC of C4MOF-(NH2)0.59/(COOH)0.41. Figure 3.23. (a) 1H NMR and (b) TGA and DSC of C4MOF-COOH. Chapter 3 116 Figure 3.24. (a) 1H NMR and (b) TGA and DSC of C4MOF-(COOH)0.41/(NH2)0.59. Table 3.4. Formula and defects of the MTV system calculated by NMR and TGA. Code Formula Defects C4MOF-SH Zr6O4(OH)4(C4SH4O4)4.9((H2O)(OH))2.2 1.1 C4MOF-SH/NH2 Zr6O4(OH)4(C4H3NH3ClO4)1.8(C4SH4O4)2.1(HCOO)0.07((H2O)(OH))4.1 2.1 C4MOF-NH2 Zr6O4(OH)4(C4H3NH3ClO4)5.6(HCOO)0.3((H2O)(OH))0,5 0.4 C4MOF-COOH/NH2 Zr6O4(OH)4(C4H3NH3ClO4)2.1(C6H3O6)1,6(HCOO)0.1((H2O)(OH))4.6 2.3 C4MOF-COOH Zr6O4(OH)4(C6H3O6)4,80((H2O)(OH))2.4 1.20 C4MOF-COOH/SH Zr6O4(OH)4(C6H3O6)2.4(C4SH4O4)3.4(HCOO)0.3((H2O)(OH))0,1 0.2 The porosity of the samples was evaluated through high-pressure CO2 adsorption isotherms (Fig. 3.25). All samples exhibit a type I isotherm profile, characteristic of microporous materials. In these curves, the majority of CO2 uptake occurs at low relative pressures, followed by a gradual increase at higher pressures due to intraparticle condensation. Figure 3.25. CO2 adsorption isotherms of the MTV system. Synthesis of C4 dicarboxylic acids MOFs 117 Surface area values were determined using the linear portion of the isotherms at low relative pressure, where adsorption behaviour follows the Brunauer–Emmett–Teller (BET) model. For both fcu and fcu/reo phase materials, regardless of the functional group present on the C4 linker, the calculated surface areas range from 215 to 390 m2/g (Table 3.5). These values are significantly lower than those predicted based on the crystallographic models (approximately 750-800 m2/g)9. This discrepancy is mainly attributed to the mild activation conditions employed, which are constrained by the thermal sensitivity of the materials and limit the complete removal of guest species. Table 3.5. Micropore surface areas of MTV materials by adsorption isotherms. Code Specific surface (m2/g) C4MOF-SH 259.44 C4MOF-SH/NH2 387.05 C4MOF-NH2 215.30 C4MOF-COOH/NH2 317.04 C4MOF-COOH 387.55 C4MOF-COOH/SH 307.53 Scanning electron microscopy (SEM) analysis (Fig. 3.26) offers valuable insight into the microstructure of the synthesised materials. Although the MOFs exhibit varying morphologies depending on their specific structural composition, all samples show a relatively uniform and monodisperse particle size distribution. Figure 3.26. SEM images for MTV samples. Zeta potential analysis provides insight into the surface electrical potential of particles by measuring the voltage difference between the particle surface and the surrounding Chapter 3 118 firmly attached counter-ions. This parameter is essential for understanding and controlling the electrostatic dispersion and colloidal stability of the particles.10 In this study, the zeta potential of the C4-MOFs was measured to evaluate their surface charge properties. As shown in Figure 3.27, measurements were conducted over a pH range of 3 to 9. Across this range, all materials exhibited a nearly consistently negative surface charge. Moreover, increasing the pH led to a progressive enhancement of the negative zeta potential, reaching values of approximately 40 mV at pH 9. These results indicate that the surface of the C4-MOF particles remains negatively charged under most conditions, which favours electrostatic interactions with positively charged species, such as heavy metal cations. Figure 3.27. Zeta potential measurements of MTV materials. 3.3. Application of C4MOFs 3.3.1. Absorption of phenolic compounds To evaluate the potential applications of the synthesised MOFs as adsorbent materials, a systematic adsorption study was conducted using five representative aromatic compounds: phenol, catechol, hydroquinone, dopamine, and paracetamol (Fig. 3.28). These molecules exhibit different functional groups and substitution patterns that allow for the analysis of selectivity and adsorption mechanisms of the synthesised materials. Synthesis of C4 dicarboxylic acids MOFs 125 Figure 3.31. Absorption capacity (mg/g) of MTV system materials over Co(II), Ni(III), La(III), Eu(III) and Y(III). Experiments were performed with an initial metal ion concentration of 10 ppm, using a 1 mg adsorbent/mL dosage. To facilitate a clearer interpretation of these trends, the results will be discussed separately for each group of metal ions (Fig. 3.32). As shown in Figure 3.32b, the ASP-based material exhibited negligible adsorption capacity for all metals except Hg(II), for which a moderate uptake was observed. In contrast, the THIO-based material demonstrated significant affinity for Hg(II), Pb(II), Cd(II), and Cu(II) (Fig. 3.32c), while the TRANS-based material stood out for its ability to adsorb rare-earth elements (RREEs), as shown in Figure 3.32a. When evaluating the multivariate materials, the TRANS/ASP combination (Fig. 3.32d) did not show any improvement in adsorption performance compared to the individual components. In the case of the THIO/ASP material (Fig. 3.32f), although no synergistic enhancement was evident, the individual adsorption capacities were largely retained. Notably, the THIO/TRANS material (Fig. 3.32e) exhibited a clear improvement, combining high adsorption capacities for Hg(II), Pb(II), and Cu(II) with moderate uptake for the remaining metal ions. Finally, the ternary TRANS/THIO/ASP material (Fig. 3.32g) displayed a slight enhancement in Hg(II) adsorption, while maintaining similar performance levels for the other metals compared to the binary systems. Chapter 3 126 Figure 3.32. Absorption capacity (mg/g) of each MTV system material over Hg(II), Pb(II), Cd(II), Cu(II), Co(II), Ni(III), La(III), Eu(III) and Y(III). Based on the initial results, two additional adsorption isotherms were performed using the THIO and THIO/TRANS materials, which had shown the most promising performance. These experiments aimed to further evaluate their adsorption capacities under varying metal concentrations. As in previous tests, all experiments were carried out in triplicate to ensure reproducibility and assess the reliability of the procedure. The MOF suspensions were stirred magnetically for 4 hours to allow equilibrium to be reached. Afterwards, the suspensions were filtered using hydrophilic 0.22 μm membranes. Adsorption capacity for each point of the isotherm curve was determined based on Equation 3.2. The experimental data obtained from the adsorption isotherms were fitted to the Langmuir and the Freundlich models to characterise the interaction between the MOF surface and the target metal ions. These models are widely applied in adsorption Synthesis of C4 dicarboxylic acids MOFs 127 studies involving porous materials, as they provide insight into the adsorption capacity, affinity, and surface heterogeneity of the materials. The Langmuir model assumes monolayer adsorption onto a homogeneous surface with a finite number of identical sites and is particularly useful for estimating the maximum adsorption capacity (Qmax) and the Langmuir constant (KL), which reflects the affinity between the adsorbent and the adsorbate. In contrast, the Freundlich model is empirical and assumes adsorption on a heterogeneous surface, allowing multilayer adsorption and variable adsorption energies. This model yields two key parameters: the Freundlich constant (KF), which is related to adsorption capacity, and the heterogeneity factor (n), which provides insight into the intensity of the adsorption process. Both the Langmuir and the Freundlich models were applied to the experimental data to evaluate the adsorption performance of the THIO and THIO/TRANS materials toward the selected metal ions (Figs. 3.33 and 3.37). The quality of the fitting, along with the extracted parameters, provides valuable insight into the adsorption mechanisms and surface characteristics of the materials. A summary of the fitted parameters is presented in Tables 3.7 to 3.10 and illustrated in Figs. 3.34, 3.35, 3.36, and 3.38. At lower metal concentrations (10 ppm), the THIO/TRANS material demonstrated slightly higher adsorption efficiencies for Hg(II), Pb(II), and Cd(II) compared to the single-linker THIO analogue. However, as the metal concentration increased, the isotherm profiles revealed that the THIO material exhibited superior adsorption capacities across the concentration range (Fig. 3.33). Figure 3.33. Absorption isotherms of THIO and THIO/TRANS over (a) Hg(II), (b) Pb(II) and (c) Cd(II). Chapter 3 128 This trend suggests that while the multivariate THIO/TRANS framework may present more accessible or higher-affinity binding sites at low metal concentrations—possibly due to synergistic effects between the ligands or improved surface dispersion—these advantages may become less significant at higher concentrations. Under such conditions, the THIO material likely benefits from a higher density of effective coordination sites or a more homogeneous surface chemistry, enabling greater uptake before reaching saturation. Additionally, the incorporation of two different linkers in the THIO/TRANS system might introduce a degree of structural heterogeneity or partial steric hindrance, reducing the number of available adsorption sites under high loading. This highlights the importance of optimising linker combinations not only for initial binding affinity but also for maintaining performance at elevated contaminant levels, which is critical for real-world applications in water remediation. Figure 3.34. Hg(II) isotherm curves for (a) THIO and (b) THIO/TRANS materials. Table 3.7. Hg(II) isotherms adjustments. Model Parameter THIO THIO/TRANS Langmuir Qmax (mg/g) 52.8±11.5 30.7±5.47 KL (L/mg) 15.0±6.50 0.25±0.11 R2 0.94 0.92 Freundlich KF (mg1-1/n·L1/n/g) 155±37.7 7.03±0.95 1/n 0.68±0.09 0.49±0.06 R2 0.93 0.95 Synthesis of C4 dicarboxylic acids MOFs 129 Figure 3.35. Pb(II) isotherm curves for (a) THIO and (b) THIO/TRANS materials. Table 3.8. Pb(II) isotherms adjustments. Model Parameter THIO THIO/TRANS Langmuir Qmax (mg/g) 151±17.10 40.24±5.28 KL (L/mg) 0.05±0.01 0.15±0.05 R2 1 0.96 Freundlich KF (mg1-1/n·L1/n/g) 7.92±0.73 6.28±0.42 1/n 0.77±0.04 0.54±0.03 R2 1 0.99 Figure 3.36. Pb(II) isotherm curves for (a) THIO and (b) THIO/TRANS materials. Chapter 3 130 Table 3.9. Cd(II) isotherms adjustments. Model Parameter THIO THIO/TRANS Langmuir Qmax (mg/g) 102±9.96 25.26±1.53 KL (L/mg) 0.01±0 0.03±3·10-3 R2 1 1 Freundlich KF (mg1-1/n·L1/n/g) 1.44±0.05 1.16±0.05 1/n 0.80±0.01 0.64±0.01 R2 1 1 Overall, the Langmuir model provided a superior fit to the experimental data for the THIO material, suggesting that the adsorption process predominantly follows a monolayer mechanism on a relatively homogeneous surface with energetically equivalent binding sites. This behaviour is consistent with a well-organised and uniform distribution of functional groups capable of coordinating the metal ions. In contrast, the THIO/TRANS material exhibited slightly better or comparable correlation with the Freundlich model, which accounts for adsorption on heterogeneous surfaces and allows for the possibility of multilayer formation. This implies that the introduction of the second linker (TRANS) may lead to a more complex surface environment, possibly introducing variations in the affinity and accessibility of adsorption sites. These results highlight the structural and chemical impact of multivariate linker combinations on adsorption behaviour, indicating that the balance between surface uniformity and functional diversity plays a key role in defining the dominant adsorption mechanism. Given that the THIO/TRANS material exhibited the most promising preliminary adsorption performance for the rare earth elements La(III), Y(III), and Eu(III), full isotherm studies were conducted to evaluate its capacity in greater detail. Among the three, Y(III) showed the highest uptake, followed by La(III) and then Eu(III), as illustrated in Figure 3.37. Synthesis of C4 dicarboxylic acids MOFs 131 Figure 3.37. Absorption isotherms of THIO/TRANS over (a) La(III), (b) Y(III) and (c) Eu(III). This trend may be attributed to differences in ionic radii, hydration energies, or coordination preferences among the rare earth elements, which can influence their interaction with the functional groups present in the MOF structure. The thiol and carboxylate functionalities from THIO and TRANS linkers likely play a synergistic role in facilitating the binding of trivalent metal cations, particularly Y(III), which appears to exhibit higher affinity for the available adsorption sites. Figure 3.38. THIO/TRANS isotherm curves for (a) La(III), (b) Y(III) and (c) Eu(III). Table 3.10. La(III), Y(III) and Eu(III) isotherms adjustments. Model Parameter THIO/TRANS La(III) THIO/TRANS Y(III) THIO/TRANS Eu(III) Langmuir Qmax (mg/g) 7.71±0.77 18.8±3.67 7.00±0.55 KL (L/mg) 0.11±0.03 0.02±0.01 0.22±0.06 R2 0.94 0.95 0.95 Freundlich KF (mg1-1/n·L1/n/g) 1.69±0.14 0.79±0.16 2.15±0.28 1/n 0.35±0.02 0.60±0.05 0.29±0.04 R2 0.98 0.97 0.93 The adsorption isotherm data for La(III), Y(III), and Eu(III) were better described by the Freundlich model, suggesting a heterogeneous distribution of binding sites on the surface of the THIO/TRANS material. This behaviour implies that the adsorption of Chapter 3 132 these trivalent cations does not occur through a uniform monolayer, but rather through a multilayer process influenced by variations in site affinity and accessibility. The improved fit to the Freundlich model may stem from the complex coordination behaviour of rare earth elements, which are known to interact with multiple functional groups and can adopt various coordination geometries. In the multivariate THIO/TRANS framework, the coexistence of thiol and carboxyl groups likely creates a diverse array of adsorption sites with different binding energies, favouring the stepwise adsorption characteristic of the Freundlich isotherm. Taken together, these results suggest that the adsorption mechanisms are strongly influenced by the interplay between surface functionality and structural organisation. While the THIO material tends to favour monolayer adsorption on more homogeneous surfaces—as evidenced by the better Langmuir fit—the introduction of TRANS linkers into the framework enhances surface heterogeneity, leading to multilayer or siteenergy-distributed adsorption, better described by the Freundlich model. Ultimately, this comparative analysis underscores the versatility and tunability of multivariate MOFs for targeting different classes of metal ions. It also highlights the importance of rational linker selection in optimising adsorption efficiency, selectivity, and mechanism, depending on the nature of the target contaminant. 3.4. Conclusions This chapter provides a thorough investigation into the synthesis, characterisation and application of C4 dicarboxylic acid-based zirconium metal-organic frameworks (MOFs), establishing a solid basis for the design of materials specifically developed for heavy metal adsorption applications. A systematic exploration of water-based crystallisation conditions successfully enabled the controlled synthesis of four different zirconium-based frameworks: MIP202, MIP-205, BCM-1 and the novel BCM-5. By systematically varying the concentrations of the ZrCl4 precursor and the FA modulator, it was possible to achieve precise control over the topology of the frameworks, enabling transitions between the cubic "fcu", hexagonal "hcp" and mixed "fcu-reo" phases. The developed crystallisation stability maps provide a valuable roadmap for targeting specific Synthesis of C4 dicarboxylic acids MOFs 133 structural variants and show that lower zirconium concentrations and higher formic acid concentrations favour the formation of hexagonal phases with better crystallinity. Introducing multivariate systems that combine thiomalic (THIO), aspartic (ASP), and trans-aconitic (TRANS) linkers revealed complex crystallisation behaviour that depends on the composition of the linkers. Notably, the incorporation of ASP consistently promotes framework amorphisation, while THIO-TRANS combinations facilitate the formation of reo-type nanodomains. These findings establish clear structure-composition relationships that are essential for the design of MOFs in a targeted way. Comprehensive characterisation confirmed the successful incorporation of the linker, while revealing significant defect densities ranging from 0.2 to 2.3 vacancies per formula unit. The materials exhibited microporous characteristics, with BET surface areas between 215 and 390 m²/g; however, values remained below theoretical predictions due to slight activation limitations imposed by thermal stability. Zeta potential measurements revealed consistently negative surface charges across pH 3– 9, with enhanced negativity at higher pH values. This indicates favourable electrostatic environments for cationic metal coordination. Initial screening using phenolic compounds demonstrated material selectivity, with catechol and dopamine exhibiting significant uptake, whereas phenol, hydroquinone and paracetamol showed negligible adsorption. Although TRANS had the highest BET surface area, THIO showed better catechol removal, suggesting that adsorption performance is more influenced by specific chemical interactions between functional groups than by pore size. Water vapour adsorption studies revealed strong correlations between functional group chemistry and hydrophilic behaviour. The THIO-ASP multivariate system demonstrated exceptional water uptake, outperforming its individual components, confirming the presence of synergistic effects between amino and thiol functionalities. These results demonstrate that the materials can interact effectively with aqueousphase species. Competitive multi-metal screening experiments revealed a clear pattern of selectivity, with all materials exhibiting an exceptional affinity for Hg(II), followed by Pb(II), Cu(II) and Cd(II). According to Langmuir modelling, the THIO-based framework performed Chapter 3 134 better for soft metal ions, while the THIO-TRANS multivariate system had a strong ability to remove rare earth elements. Isotherm analysis revealed different adsorption mechanisms. THIO and THIO/TRANS materials exhibited Langmuir behaviour, which is indicative of monolayer adsorption on homogeneous surfaces. In contrast, multivariate systems exhibited Freundlich characteristics, which suggest heterogeneous, multilayer processes. This diversity of mechanisms highlights the tunability that can be achieved through the strategic combination of linkers. Building upon the promising adsorption results obtained with the THIO material, the next stage of this thesis involves integrating the material into a biopolymeric membrane. The aim is to develop a stable, processable platform that can incorporate MOFs for practical applications in heavy metal removal. This approach not only addresses the limitations associated with powdered MOFs, such as recovery and reuse, but also opens the door to producing scalable, multifunctional materials suitable for real-world water remediation. At the same time, the potential of the DMSA material is also being explored. This is a structurally similar linker that contains an additional thiol group. Due to its enhanced functionalisation, it is expected that DMSA will exhibit improved adsorption affinity towards soft heavy metal ions. Consequently, a PVDFHFP-based composite membrane incorporating the MOF has been developed and evaluated for its metal uptake performance. In summary, the results presented in this chapter establish the basis for the design of MOF-based materials tailored for heavy metal adsorption, setting the stage for their integration into functional membrane systems explored in the following chapters. PVDF-HFP@MOF Membrane for Heavy Metal Capture 237 Figure 5.33. High resolution XPS-spectra of a) Hg(II), b) Pb(II) and c) Cd(II) after their immobilisation within BCM-5. The adsorption results suggest that the diffusion of metal ions into the internal regions of BCM-5 is restricted by mass transfer limitations due to its narrow pore dimensions. Incorporating the MOF into the membrane improves both particle dispersibility and the overall adsorption capacity, compared to the pristine MOF. Nevertheless, the membrane architecture can introduce additional diffusion barriers that limit the accessibility of certain metal ions to immobilised BCM-5 particles. While these kinetic limitations may slow the adsorption process, the improved dispersibility of BCM-5 within the PVDF matrix increases the equilibrium adsorption capacity. 5.4. Conclusions This study presents the development and comprehensive structural characterisation of a novel thiol-functionalized Zr(IV)-based metal-organic framework, denoted as CdSCdO c) PbSPbO α-HgSβ-HgS α-HgO a) b) Chapter 5 238 BCM-5. The framework of this material has been built employing commercially available succimer metal chelator as the organic linker. The unique combination of pore architecture consisting of thiol-rich and water/hydroxyl-abundant regions creates a main cavity structure containing three distinct hydrophilic, hydrophobic, and amphiphilic adsorption environments. These structural features can be linked with the selectivity trends of adsorption observed for Hg(II), Cd(II), and Pb(II) heavy metal species. Incorporating BCM-5 into porous PVDF-HFP membranes using a salt-leaching methodology significantly improves the dispersibility of the MOF particles, resulting in an improvement in adsorption capacity performance compared to pristine MOF material. Kinetic and equilibrium studies reveal the heterogeneous nature of the adsorption process, emphasising the importance of MOF loading concentration and particle distribution for optimal functionality. While the porous structure of the PVDFHFP membrane provides structural benefits, its inherent hydrophobic properties limit its ability to absorb heavy metals. These limitations arise from the PVDF-HFP matrix's fundamental properties and are also influenced by morphological changes that occur during solvent-induced phase separation in the presence of MOF particles. Spectroscopic and crystallographic analyses performed after adsorption reveal that the uptake of heavy metals is driven by the formation of stable metal–sulphur and metal–oxygen coordination bonds, involving both thiol and hydroxyl/water groups. These results show that the process is governed by a chemisorption mechanism. Overall, these findings demonstrate that integrating thiol-functionalised MOFs into polymeric membranes via salt-leaching yields a robust, scalable and versatile highperformance platform for selectively removing heavy metals from aqueous environments. 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Conclusions This doctoral thesis has explored the use of zirconium-based metal–organic frameworks and their incorporation into membrane systems for the adsorption of heavy metals for water remediation. Through a well-structured approach incorporating controlled MOF synthesis and integrating these frameworks into membranes, the study demonstrates the high efficiency and practical applicability of the materials. In light of the research findings, the following key conclusions can be drawn: Firstly, a robust protocol for the controlled synthesis of Zr-based MOFs under aqueous crystallisation conditions has been established. The development of crystallisation stability maps represents a significant methodological contribution, providing a precise roadmap for obtaining specific topologies (“fcu”, “hcp”, and “fcu–reo”) through the systematic control of ZrCl4 and formic acid modulator concentrations. This approach allowed the successful synthesis of four different frameworks: MIP-202, MIP-205, BCM-1, and the novel BCM-5. The analysis of multivariate systems has revealed complex but predictable structure-composition relationships. In particular, it was demonstrated that the incorporation of ASP linkers promotes framework amorphisation, whereas THIO–TRANS combinations facilitate the formation of reotype structures. These findings establish clear guidelines for the targeted engineering of MOFs with tailored properties. Extensive characterisation of the synthesised materials confirmed the successful incorporation of functionalised linkers, revealing significant defect densities that, it was found, increased adsorption performance. The BET surface areas obtained, although lower than theoretical predictions due to thermal activation limitations, were sufficient for effective adsorption applications. A key conclusion drawn from this work is that 246 adsorption performance is predominantly influenced by the nature of specific chemical interactions between functional groups, rather than by the accessible surface area. This is exemplified by the THIO-based materials, which, despite their lower BET surface areas, exhibit superior adsorption capacities compared to their TRANS counterparts. Zeta potential measurements revealed consistently negative surface charges (pH 3–9), creating favourable electrostatic environments for the coordination of cationic metals. Multi-metal screening experiments established a clear selectivity pattern: Hg(II) > Pb(II) > Cu(II) > Cd(II), with THIO-based frameworks exhibiting exceptional affinity towards soft metal ions. Isotherm analyses revealed the coexistence of different adsorption mechanisms: Langmuir-type (monolayer adsorption) behaviour for pure THIO frameworks, and Freundlich-type (heterogeneous, multilayer processes) for multivariate systems. The demonstrated ability of the THIO–TRANS system to remove rare earth elements extends the applicability of these materials beyond traditional heavy metal remediation, highlighting their potential for critical metal recovery. The development of two different membrane platforms (SPICHI@BCM-1 and PVDFHFP@BCM-5) demonstrated the possibility of incorporating MOFs into polymeric matrices while retaining their adsorption functionality. The SPICHI@BCM-1 system exhibited noteworthy adsorption capacities for Hg(II), Pb(II) and Cd(II), arising from interfacial interactions between the MOF and SPICHI components (thiol, amide, and carboxyl groups). These interactions enable the efficient simultaneous removal of multiple heavy metals from real surface and groundwater samples, along with the advantages of regeneration and tunable selectivity. The salt-leaching approach applied to PVDF-HFP@BCM-5 membranes resulted in better adsorption performance compared to the pristine MOF, thereby highlighting the critical role of controlled MOF particle dispersion within the polymer matrix. In addition, post-adsorption spectroscopic and crystallographic analyses confirmed that heavy metal uptake takes place via chemisorption, involving the formation of stable metal–sulphur and metal– oxygen coordination bonds. Taken together, the research of this thesis demonstrates that Zr-based chelating MOFs represent a potential solution for addressing the critical challenge of heavy metal contamination in water resources. The combination of controlled synthesis, Curriculum Vitae and Contributions 253 elements” M. Calles, L. Celaya. R. Fernández de Luis. European Open Day, May-2023, Brussels, Belgium. •3rd Internation School on Porous Materials. Como, Italy – Online (19th - 23rd of June, 2023). 30 hours •Dissemination Webinar: Raw Materials for sustainable development and the circular economy. Huelva, Spain - Online (12th of July, 2023). •Workshop: Materials for Environment and Beyond (INDESMOF-RISE-MSCA project). Braga, Portugal – Online (19th December, 2023). •ISIS Neutron and Muon Source workshop: Opportunities to build collaborations and impactful science. Leioa, Spain (7th – 8th of February, 2024) – Poster contribution. •Caracterización Químico-Física de la Superficie de Adsorbentes y Catalizadores II. Jarandilla de la Vera, Spain (11th – 14th of June, 2024). 15 hours. •Fortnightly Seminar: Development of New Metal-Chelator MOFs for metalsimmobilization. Leioa, Spain (10th of July, 2024) – Oral presentation. A.5. Research stays •Research stay at Instituto de Química y Metabolismo del Fármaco (IQUIMEFACONICET), Facultad de Farmacia y Bioquímica, Universidad de Buenos Aires, Argentina (1st of September – 1st of December, 2023). •Research stay at Wise4Automation company (W4A), Braga, Portugal (22nd of February – 27th of May, 2024). Annex 254