Advances in Green Synthesis and Photo-/Electrocatalytic Applications of Zirconium-Based MOFs: A Review

Abstract

Metal–organic frameworks (MOFs), particularly zirconium-based frameworks (Zr-MOFs), have gained significant attention in recent years due to their unique structural and functional properties. This review focuses on eco-friendly synthetic methods for producing Zr-MOFs, addressing the environmental impacts and costs associated with conventional synthesis, which often relies on hazardous reagents and harsh conditions. We explore various green synthesis strategies, including the selection of raw materials (such as using zirconium acetate), organic ligands (recycling waste materials for ligand synthesis), and synthesis methods (solvothermal, microwave-assisted, ultrasound-assisted, electrochemical, and mechanochemical approaches). Additionally, the application of Zr-MOFs in photocatalysis and electrocatalysis is examined, highlighting their potential for environmental purification and energy conversion. Despite the progress made in laboratory settings, challenges remain in achieving cost-effectiveness, material stability, and scalability for industrial applications. Future research should concentrate on enhancing synthesis efficiency, optimizing catalytic properties, investigating structure–property relationships, and expanding applications to novel catalytic reactions, thus ensuring Zr-MOFs can contribute to sustainable development in chemical science and technology.

1. Introduction

Metal–organic frameworks (MOFs), also known as Porous Coordination Polymers [1,2] (PCPs), have become a major focus of research due to their unique structural and functional properties. Since the pioneering work by Yaghi et al. [3] in the 1990s, MOFs have developed into a large family with over 80,000 different structures [4]. The diversity of MOFs stems from the infinite combinatorial possibilities of their building blocks, i.e., the infinite structural diversity of various metal ions or clusters with organic ligands, expressed through self-assembly reactions. MOFs are widely studied due to their tunable porosity, high surface area, and diverse applications in gas storage, catalysis, and drug delivery [5,6,7,8].

Among various metal–organic frameworks (MOFs), zirconium-based metal–organic frameworks (Zr-MOFs) stand out due to their unique chemical and physical properties. Zr-MOFs can be functionalized by incorporating acidic or basic functional groups, allowing for the tuning of their properties and demonstrating significant potential, particularly in catalytic applications. These frameworks combine both Lewis [9] and Brønsted [10] acidity and exhibit varying active sites, which contribute to their excellent activity and selectivity in catalytic reactions.

In addition to inheriting the common characteristics of MOF materials—such as high thermal and chemical stability and tunable pore structures—Zr-MOFs attract attention due to their distinctive zirconium active sites [11]. These properties enable Zr-MOFs to efficiently adsorb specific gas molecules, significantly enhancing the selectivity and efficiency of catalytic reactions. Furthermore, Zr-MOFs can serve as drug carriers for targeted delivery, showcasing great potential for applications in various advanced fields, including gas storage, separation, catalysis, sensing, biomedicine [12,13], etc.

In 2008, Cavka et al. [14] reported Zr-MOFs for the first time, and their study showed that Zr-MOFs exhibit excellent chemical and thermal stability due to their strong Zr-O bonding, as well as the presence of internal Zr₆ metal ion clusters and μ₃-OH groups. Zr-based MOFs catalysts, such as UiO-66, UiO-67, MOF-801, MOF-808, UiO-66-NH₂, etc., are widely used in catalysis due to their stability under harsh conditions, large specific surface area, small particle size, and abundant active centers. In addition, different synthesis techniques and strategies lead to differences in particle size, pore size distribution, and active sites, which greatly affect the ability of Zr-MOFs in adsorption and catalytic applications.

However, the conventional synthesis process of Zr-MOFs is often accompanied by a harsh environment and the use of a large number of chemical reagents, which not only increases the environmental burden but also raises the production cost. Therefore, the aim of this review is to explore the synthetic methods used for green synthesis of Zr-MOFs and the impact of structural modulation on their catalytic applications. This review will discuss the full range of green synthesis strategies in detail, ranging from raw material selection and solvent use to synthesis pathway optimization, which are important guidelines in the industrial application of Zr-MOFs. Through these studies, we expect to provide a scientific basis for realizing the green synthesis of Zr-MOFs and improving the efficiency of their application in photo-/electrocatalysis, which in turn will contribute to the improvement of environmental protection and energy efficiency.

In the field of zirconium metal–organic frameworks (Zr-MOFs), green synthesis methods face a series of significant challenges. Conventional synthesis processes often rely on organic solvents, which can contribute to environmental pollution; thus, developing harmless solvents or aqueous-phase synthesis methods has become a key challenge for green synthesis. While high-temperature and high-pressure synthesis methods can enhance the crystallinity and stability of Zr-MOFs, their considerable energy consumption and potential safety hazards cannot be overlooked. Consequently, synthesizing high-quality Zr-MOF materials under mild conditions has emerged as an important issue in the realm of green synthesis.

Additionally, traditional synthesis methods may generate harmful by-products that not only pose a threat to the environment but also jeopardize human health. Therefore, a core focus of current research is the development of green synthesis methods aimed at reducing or eliminating the production of harmful by-products. Although some progress has been made in green synthesis methods at the laboratory scale, achieving large-scale green synthesis of Zr-MOFs and successfully applying them in industrial photo- and electrocatalytic processes remains a significant obstacle to their commercial application.

In the field of catalysis, Zr-MOFs have particularly promising applications. They have not only demonstrated remarkable environmental purification and energy conversion capabilities in photocatalysis, but also excellent efficiency in electrocatalysis, especially in CO₂ reduction and the degradation of organic pollutants. The high specific surface area, porousness, and chemical and thermal stability of Zr-MOFs provide abundant active sites in catalytic reactions and allow for the customization of the materials for specific applications through precise synthesis strategies to customize the materials for specific applications. Therefore, Zr-MOFs are not only valued for their intrinsic physicochemical properties, but also as a promising research and application platform in catalytic science and technology due to their potential in catalytic performance optimization and novel composite material development. The progress of these studies not only promotes the application of Zr-MOFs in catalytic science and technology, but also provides new material options for solving environmental and energy problems and further broadens their application prospects in the field of sustainable development. As a result, scientific reports on Zr-MOFs have continued to grow in recent years; in particular, the related citation frequency is quite high (Figure 1).

In summary, the research and development of Zr-MOFs will continue to be an active and important research direction in the field of catalytic science and green chemistry. Future work needs to focus on the development of novel green synthesis methods, the optimization of catalytic properties, the in-depth study of structure–property relationships, the exploration of novel catalytic reactions, and development of composites for the further development of Zr-MOFs in catalytic science and technology. Through continuous research efforts and innovative strategies, it is expected that the industrial application of Zr-MOFs will be realized, contributing to the improvement of environmental protection and energy efficiency.

In existing studies, Fu and Wu [15] provided a comprehensive review of green synthesis methods for zirconium metal–organic frameworks (Zr-MOFs), discussing in detail the use of green raw materials, solvents, and synthesis techniques. However, their review primarily focused on laboratory-scale synthesis methods, with only a brief analysis of industrial-scale production and its environmental impacts. In contrast, our review further investigates the specific applications of Zr-MOFs in photocatalysis and electrocatalysis, particularly emphasizing the catalytic properties and application examples of various Zr-MOFs. Additionally, we offer an in-depth analysis of the challenges and prospects associated with green synthesis methods in industrial applications, providing a more holistic perspective.

Unlike previous reviews on Zr-MOFs, our review centers on the green synthesis methods and recent advancements in photoelectrocatalytic applications. It not only addresses improvements to traditional synthesis methods but also thoroughly explores a wide range of green strategies, from raw material selection to the optimization of synthesis pathways. This review highlights recent progress in Zr-MOFs synthesized under various conditions for photoelectrocatalytic applications and offers a critical analysis of these green synthesis methods, examining their environmental and economic benefits. By systematically comparing conventional synthesis with green strategies, this work fills the gap in the existing literature regarding the analysis of industrial-scale production and its environmental impact.

2. Zirconium-Based MOF Synthesis Method

Zr-MOFs are porous materials formed by self-assembly reactions of zirconium salts with a variety of carboxylate ligands, which have become the focus of research in the field of materials science and engineering due to their excellent thermo-chemical stability, diversified topologies, excellent catalytic properties, and a wide range of potential applications. Various synthesis strategies have been used for the synthesis of Zr-MOFs including solvent-heated synthesis, microwave-assisted synthesis, ultrasound-assisted synthesis, electrochemical synthesis, and mechanochemical synthesis [16,17,18], etc.

To clarify the characteristics of different synthesis methods,

Table 1. Advantages and disadvantages of different methods for synthesizing Zr MOFs.

Synthesis Method	Advantages	Disadvantages	Ref
Solvothermal synthesis	Efficient formation of Zr-MOFs with high crystallinity Simpler reaction conditions to control Adjustable porosity and structure	Requires high temperature and pressure Use of organic solvents, which may be harmful to the environment Longer reaction time	[19]
Microwave- or ultrasound-assisted synthesis	Rapid synthesis and short reaction time High energy efficiency and uniform temperature distribution Controllable particle size and shape	Problems with higher equipment costsComplex control of the reaction system	[20]
Electrochemical synthesis	Environmentally friendly, can be performed at room temperature No organic solvents, fewer by-products Can be used for mass production	Slower reaction rate Requires special electrochemical equipment Product purity may be limited	[21]
Mechanochemical synthesis	Environmentally friendly with no solvents or little solvent required Simple operation and easy control Can be carried out at room temperature and pressure, low energy consumption	Longer reaction time The crystallinity and purity of the product may be low Requires special ball milling equipment, higher equipment costs	[22]

summarizes the advantages and disadvantages of each approach. As shown in the table, each synthesis method has its own strengths and weaknesses. In order to achieve industrial-scale green synthesis of zirconium metal–organic frameworks (Zr-MOFs), factors such as pollution emissions, energy consumption, yield, and purity must be carefully considered. For traditional solvothermal methods, the experimental protocols need to be improved to minimize pollution emissions. In contrast, for novel synthesis methods such as microwave-assisted or ultrasound-assisted synthesis as well as electrochemical synthesis, identifying optimal synthesis parameters and precisely regulating the synthesis conditions are crucial for enabling large-scale industrial green synthesis of Zr-MOFs. Furthermore, selecting the appropriate synthesis method is vital for producing Zr-MOFs with specific structures and desirable photo- and electrocatalytic properties.

Different synthesis methods lead to the diversity of and performance differences in Zr-MOFs. Even if the same synthesis method is used, differences in parameters such as energy input, pressure, and reaction time during the synthesis process can significantly affect the types and properties of the final products. In the field of photoelectrocatalysis, the factors affecting the reactivity mainly include structural properties (e.g., porosity, specific surface area, etc.), active sites and functional groups, defects, and modifications [23,24]. For the same Zr-MOFs, the synthesis method is the biggest factor affecting their catalytic activity. The synthesis method mainly affects the structure and properties of Zr-MOFs through energy input. For example, UiO-66 is usually synthesized by a high-temperature solvothermal method, where the zirconium ions combine terephthalic acid with sufficient time and energy to form an ordered and stable complete ortho-octahedral structure due to sufficient energy. In contrast, when using low-temperature methods such as ultrasound or microwave, insufficient energy may lead to the formation of amorphous or less stable phases, and only fused or semi-fused structures can be synthesized. The synthesis methods mainly enhance catalytic performance by influencing the distribution of defects, functional groups, and exposed active sites. In addition, the stability of Zr-MOFS under different catalytic conditions affects their lifetime and effectiveness [25]. For example, LANG [26] and his colleagues improved the structural stability of unstable mesoporous Zr(IV)-MOF NKM-809 by hybrid molecular ligand synthesis and ligand mounting, thus realizing its efficient application in acidic sensors. Therefore, the rigorous selection of synthetic methods as well as the precise control of synthetic conditions are crucial for regulating the structure and function of Zr-MOFs. The common ligands and synthesis methods of Zr-MOFs are shown in Table 2. The types of ligands commonly used for synthesizing Zr-MOFs are shown in Figure 2.

Table 2. Ligands and synthesis of common Zr-MOFs.

The Common Zr-MOFs	Organic Ligand	Common Synthesis Methods	Ref
UiO-66	1,4-benzenedicarboxylic acid BDC	hydrothermal synthesis, microwave-assisted synthesis, mechanochemical synthesis	[27]
UiO66-NH2	2-Aminoterephthalic acid 2-NH2-BDC	hydrothermal synthesis, microwave-assisted synthesis	[28]
UiO-67	4,4′-Biphenyldicarboxylic acid BPDC	hydrothermal synthesis, microwave-assisted synthesis, mechanochemical synthesis	[29]
UiO-68	2,2′-Bipyridine-5,5′-dicarboxylic acid BPYDC	hydrothermal synthesis, mechanochemical synthesis	[30]
MOF-808	1,3,5-Benzenetricarboxylic acid BTC	hydrothermal synthesis, microwave-assisted synthesis	[31]
MOF-801	Fumaric acid fum	hydrothermal synthesis, microwave-assisted synthesis	[32]
MIP-202	L-Aspartic acid Asp	hydrothermal synthesis	[33]
NU-1000	1,3,6,8-Tetrakis(4-benzoic acid)pyrene TBAPy	hydrothermal synthesis	[34]
PCN-777	4,4′,4′′-(1,1′:4’,1″-Terphenyl-4,5,6-triyl)tribenzoic acid TPT	hydrothermal synthesis	[35]

Table 2. Ligands and synthesis of common Zr-MOFs.

The Common Zr-MOFs	Organic Ligand	Common Synthesis Methods	Ref
UiO-66	1,4-benzenedicarboxylic acid BDC	hydrothermal synthesis, microwave-assisted synthesis, mechanochemical synthesis	[27]
UiO66-NH2	2-Aminoterephthalic acid 2-NH2-BDC	hydrothermal synthesis, microwave-assisted synthesis	[28]
UiO-67	4,4′-Biphenyldicarboxylic acid BPDC	hydrothermal synthesis, microwave-assisted synthesis, mechanochemical synthesis	[29]
UiO-68	2,2′-Bipyridine-5,5′-dicarboxylic acid BPYDC	hydrothermal synthesis, mechanochemical synthesis	[30]
MOF-808	1,3,5-Benzenetricarboxylic acid BTC	hydrothermal synthesis, microwave-assisted synthesis	[31]
MOF-801	Fumaric acid fum	hydrothermal synthesis, microwave-assisted synthesis	[32]
MIP-202	L-Aspartic acid Asp	hydrothermal synthesis	[33]
NU-1000	1,3,6,8-Tetrakis(4-benzoic acid)pyrene TBAPy	hydrothermal synthesis	[34]
PCN-777	4,4′,4′′-(1,1′:4’,1″-Terphenyl-4,5,6-triyl)tribenzoic acid TPT	hydrothermal synthesis	[35]

Figure 2. Diagram of ligand types commonly used for synthesizing Zr-MOFs. (a) Demonstration diagram of conductive ligand in synthetic Zr-MOFs. Reprinted with permission from Ref. [36] Copyright 2019 John Wiley and Sons. (b) Demonstration diagram of active ligand in synthetic Zr-MOFs. Reprinted with permission from Ref. [37] Copyright 2018 MDPI. (c) Demonstration diagram of heteroligand in synthetic Zr-MOFs. Reprinted with permission from Ref. [38] Copyright 2018 Elsevier. (d) Demonstration diagram of chiral ligand in synthetic Zr-MOFs. Reprinted with permission from Ref. [39] Copyright 2018 Nature Communications.

Figure 2. Diagram of ligand types commonly used for synthesizing Zr-MOFs. (a) Demonstration diagram of conductive ligand in synthetic Zr-MOFs. Reprinted with permission from Ref. [36] Copyright 2019 John Wiley and Sons. (b) Demonstration diagram of active ligand in synthetic Zr-MOFs. Reprinted with permission from Ref. [37] Copyright 2018 MDPI. (c) Demonstration diagram of heteroligand in synthetic Zr-MOFs. Reprinted with permission from Ref. [38] Copyright 2018 Elsevier. (d) Demonstration diagram of chiral ligand in synthetic Zr-MOFs. Reprinted with permission from Ref. [39] Copyright 2018 Nature Communications.

There are various modification methods for Zr MOFs, aimed at improving their catalytic performance, stability, and adaptability to specific applications. Common modification strategies include organic ligand modification, metal ion introduction, and functionalized surface modification. Organic ligand modification can alter the chemical and physical properties of Zr-MOFs by introducing different functional groups [28,40,41], such as amino and carboxyl groups, which can enhance the performance of Zr-MOFs in photoelectrocatalysis and adsorption reactions. The method of introducing metal ions involves doping other metal ions or metal nanoparticles into Zr-MOFs to enhance their catalytic activity and selectivity. This exhibits significant advantages, particularly in electrochemical and photocatalytic reactions. In addition, surface functionalization is also a commonly used modification method, which can improve the stability and repeatability of Zr-MOFs in specific reactions by introducing different functionalized molecules or small molecules on the surface. These modification methods significantly enhance the performance of Zr-MOFs, demonstrating broader application prospects in fields such as environmental protection and energy conversion.

2.1. Solvothermal Synthesis

The solvothermal synthesis method mixes and dissolves a metal source with an organic ligand in a specific solvent, which is subsequently heated to a predetermined temperature in a closed vessel and held for a certain period of time in order to promote the coordination reaction between the metal ions and the ligand to form Zr-MOFs with a specific structure. This method is able to promote the coordination reaction between the metal ions and the organic ligand under high-temperature and high-pressure conditions to form metal–organic frameworks. This method allows for the optimization of the structure and functionality of Zr-MOFs through the fine tuning of the experimental parameters, enabling the precise control of material properties [42]. Rong et al. [43] prepared UiO-66 membranes via a three-time solvothermal method, which exhibited unprecedented CO₂/N₂ selectivity (31.3). Wang [44] prepared a series of inverse opal metal–organic frameworks with intrinsic microporous/medium pores (inverse opal metal–organic frameworks, abbreviated as IO MOFs) (Figure 3), including UiO-66, MOF-808, NU-1200, NU-1000, and PCN-777) with tunable macropores as well as high yield. Compared with pristine MOFs, IO MOFs showed significantly higher uptake of organophosphorus (OPs) aggregates and faster initial hydrolysis rates. Jannah [45] successfully synthesized UiO-66 hybrid linker materials with the addition of acetate modifier via the solvothermal method at 120 °C. This hybrid linker modification improved the stability of UiO-66 in water with the systematic and functional modulation of its pores. As the concentration of each linker increases, the linkers merge in one place and create cluster defects, opening the way for potential applications in adsorption and catalysis. Solvothermal methods occupy a pivotal role in the synthesis of MOFs with specific pore properties and application performances. The pore size and pore structure of Zr-MOFs can be effectively tuned by finely tuning the solvothermal synthesis conditions, such as reaction temperature, time, solvent type, and the addition of different structure-directing agents or modifiers. These precisely designed Zr-MOFs show great potential for applications in various fields such as gas separation, water treatment, and dye adsorption [46]. Therefore, the solvothermal method not only provides an efficient route for the synthesis of MOFs, but also provides a powerful experimental means to study the structure–property relationship [46].

2.2. Microwave or Ultrasound-Assisted Synthesis

Microwave-assisted synthesis and ultrasound-assisted synthesis are two widely recognized techniques in the field of materials synthesis. They significantly reduce the time required for the synthesis of Zr-MOFs by providing fast nucleation rates and assisting in nucleation. Kevat’s group [47] prepared MOF-808 by ultrasound in a constant ratio of Zr–BTC of 3:1, employing water and ethanol as solvents. Through a series of characterizations, this study demonstrated that sonication affects the morphology, phase purity, specific surface area, and thermal stability of MOF-808. Ogura [48] and his colleagues shortened the synthesis time of UiO-66 by performing a short sonication at the initial stage of its preparation. After a series of characterization experiments, it was demonstrated that the ultrasound-assisted synthesis method resulted in faster particle growth than the solvothermal method, and the average particle size synthesized by this method was smaller than that synthesized using the conventional solvothermal method. Ultrasound-assisted synthesis and microwave-assisted synthesis have become important ways to synthesize Zr-MOFs due to their significant advantages in accelerating nucleation and growth, optimizing material properties, enhancing adsorption performance, precisely controlling the particle size and morphology, and shortening the synthesis cycle. In addition, microwave-assisted synthesis can prepare Zr-MOFs with different morphologies as well as different active sites, conferring unique catalytic adsorption properties and broadening the application scope.

2.3. Electrochemical Synthesis

Electrochemical synthesis refers to the method of forming target products by the anodic dissolution or self-assembly of metal ions in solution and organic ligands in solution on the electrode surface under the action of an external electric field. Depending on the synthesis mechanism, it is mainly divided into anodic synthesis, cathodic synthesis, indirect synthesis, and electroplating replacement method, etc. The product usually synthesized without the addition of metal salts, at room temperature, and under normal pressure. In 2019, Wei’s group [49] prepared UiO-66-NH₂, which has high crystallinity, a homogeneous morphology, and high porosity, by the electrochemical method at room temperature and normal pressure. This method is expected to realize large-scale synthesis of nanoscale products at the gram scale, thus representing an economical form of synthesis. Naseri et al. [50] synthesized the precursor of Zr-UiO-66-PDC by chemical and electrochemical methods at ambient temperatures and pressures, and then prepared the zirconium-based mesoporous metal–organic framework [Zr-UiO-66-PDC-SO₃H]Cl by reacting the precursor of Zr-UiO-66-PDC with SO₃HCl. This UiO-66-PDC has a homogeneous cauliflower-like structure with an average pore size of 13.5 nm and a specific surface area of 1081.6 m²·g⁻¹. [Zr-UiO-66-PDC-SO₃H]Cl can be used as a catalyst for the polymerization pairing of dihydropyridine derivatives in the electrochemical synthesis, and after performing calculations on the model, its efficiency as a catalyst is found to be good. The electrochemical synthesis of Zr-MOFs is favored by researchers because the products can be synthesized at room temperature and pressure without considering the problem of the low solubility of zirconium salts in the traditional synthesis method, and it possesses the advantages of low energy consumption, a short reaction time, and simple and controllable equipment.

2.4. Mechanochemical Synthesis

The mechanochemical method is a solid-phase chemical reaction technique realized using mechanical force under solvent-free or solvent-reduced conditions [51]. The method is based on the application of mechanical energy during ball milling, which induces the formation of new crystal structures or chemical reaction products in reactant powders by friction and impact. UiO-66 MOFs can even be successfully prepared without additional heat treatment using mechanochemical methods [52,53]. Karadeniz et al. [54] used pre-assembled dodecanuclear zirconium acetate clusters [Zr₁₂O₈(OH)₈(CH₃COO)₂₄] and prepared them by water-assisted milling for 90 min, with a specific surface area of up to 1145 m²·g⁻¹ for UiO-66 and UiO-66 _-NH₂, which have high-crystallinity face-centered cubic structures and particle size less than 100 nm. Furthermore, the synthesis method can be easily achieved at a 10 g scale by planetary milling and also by the twin-screw extrusion (TSE) solid-state flow synthesis method, allowing us to obtain more than 100 g of catalytically active UiO-66-NH₂ materials in a continuous process at a rate of 1.4 kg·h⁻¹. This provides a viable new theory for large-scale commercial synthesis. Fidelli [55] and his colleagues mechanically synthesized UiO-67 with a surface area of up to 2250 m²·g⁻¹ using a dodecanuclear zirconium source and 4, 4′-H₂ bisphenol A as a linker. The use of a dodecanuclear precursor for the direct synthesis of Zr-MOFs does not require intermediates, allowing for the fast, clean synthesis of UiO-67. Gómez-López and colleagues [56] doped nickel oxide nanoparticles into the structure of Zr-MOFs by solvent-free mechanochemistry and modified UiO-66 with different Ni loadings, which was used in the methyl acetylpropionate hydrogenation reaction using 2-propanol as a hydrogen donor solvent. Under optimized conditions, 5% Ni/UiO-66 had the best catalytic performance among all samples. The mechanochemical synthesis of Zr-MOFs, as a solid-state synthesis technique that does not require the use of solvents or even heat treatment, is a method that is favored by researchers for its low energy consumption, rapid reaction, and easy operability.

3. Green Synthetic Zirconium-Based MOF

In the process of synthesizing Zr-MOFs, traditional methods are often accompanied by the use of large amounts of organic solvents and the generation of numerous by-products, which not only increase the environmental burden but also raise the production cost. Therefore, the design of green synthetic formulations and the selection of environmentally friendly synthesis methods, from the selection of raw materials and solvent use to the optimization of the synthetic pathway, have become key strategies to achieve the green synthesis of Zr-MOFs. This strategy aims to reduce the use of hazardous solvents and the generation of by-products, while improving the synthesis efficiency and material properties to promote the sustainability of the MOF synthesis process [57].

3.1. Selection of Zirconium Sources

The earliest Zr-MOFs were synthesized in 2008 by the Norwegian University of Science and Technology (NTNU), using ZrCl₄ as the metal source for UiO-66, which is widely recognized as the raw material of choice for the conventional synthesis of Zr-MOFs due to the better stability and higher solubility of ZrCl₄. However, the metal chlorides produce certain acidic by-products during the reaction, such as HCl, which can cause the corrosion of the metal-based reaction vessel. Similarly, zirconium nitrate [58] and zirconium perchlorate [59] also produce oxidative by-products that are difficult to treat and recycle and which are toxic to humans and the environment. Similarly, zirconium nitrate and zirconium perchlorate produce oxidizing by-products; this is not in line with the concept of green synthesis. Furthermore, it produces a large number of harmful by-products that are difficult to treat and recycle, and which are toxic to the human body and the environment, and therefore researchers are committed to finding zirconium sources that can be effectively replaced. Katz et al. [60] used zirconium acetate as a metal source for the first time in 2013; the by-products produced during the reaction were acetic acid, which is also acidic, but less so, and is neither oxidizing nor corrosive, making it an excellent green raw material. In addition, organic zirconium salts such as zirconium n-propanol [61,62] are gradually providing researchers with new options by virtue of their greater solubility and the generation of fewer corrosive, oxidizing, or toxic byproducts.

3.2. Selection of Organic Ligands

MOFs have great designability, and a large part of the reason for this comes from the diversity of their formulations and the designability of organic ligands. In terms of commercializing and industrializing Zr-MOFs and solving the pain points of large-scale production (high cost, safety and toxicity issues, complicated operation, etc.), the recycling of waste materials for secondary use and the selection of green organic ligands are among the most important ways to achieve green synthesis. The Dyosiba team [63] recycled waste polyethylene terephthalate (PET) for the first time, and extracted from PET p-phenylene dicarboxylic acid and used it as an organic ligand [64] for the synthesis of UiO-66 (Zr); the structure obtained was highly consistent with the commercial synthesis. The recycling of waste products and secondary utilization of Zr-MOFs were realized, which realized the green synthesis of Zr-MOFs and provided a new possible route for the large-scale production of Zr-MOFs at low cost.

3.3. Choice of Synthesis Method

Choosing a certain mild and green synthesis method is also an important way to realize green synthesis. Modulated hydrothermal method is one of the solvothermal methods, involving the use of water, a non-polluting solvent, to prepare Zr-MOFs by precisely controlling the synthesis conditions at low temperatures and atmospheric pressures. Bohigues et al. [65] prepared Hf-MOF-808 under mild reaction conditions without alkali and as a non-homogeneous catalyst that can be reused in at least four consecutive runs without loss of any activity. Peh et al. [66] added the controlled secondary building unit (SBU) to the cluster nuclei of the target Zr using a modulated hydrothermal method and attached different functional groups such as -NH₂, -Br, and -F₄ to the BDC, which opened the way for exploring the different functional groups to confer new and unique Zr-MOFs. In addition, the modulated hydrothermal method can produce not only ultra-high-water-stability UiO-66 [67], such as UiO-66-(F)₄ and UiO-66-(COOH)₄, but also UiO-808 [68] with higher structural integrity (Figure 3) than the conventional solvothermal method. There are some other methods with natural advantages in terms of green synthesis, such as the use of water as a non-toxic, neutral, mild, and non-corrosive solvent. Electrochemical methods use electrons to replace toxic or corrosive reagents as redox agents at room temperature. By precisely controlling the electrode potential and current density, the reaction rate and selectivity can be efficiently controlled, the utilization of raw materials can be improved, the generation of by-products can be reduced, and the generation of wastewater, waste gas, and waste residue in the process of the reaction can be drastically reduced. When scaling up and synthesizing MOF materials in significant flux, the use of traditional solvothermal methods often involves the use of a large amount of solvents, which are difficult to recycle and dispose of, but mechanochemical methods can be used to carry out the reaction at room temperature, using a small amount of solvents or even under solvent-free conditions, without the need to take into account the dissolution of the metal source. Through real-time in situ monitoring technology, it is possible to monitor the changes in the different topologies of the MOFs in the process of mechanochemical milling, and to detect the structural template effects. The synthesis process of the microwave-assisted method is green and water-based, reducing the dependence on organic solvents. In addition, the reaction system is heated directly by microwave radiation; microwaves can penetrate deep into the substance and rely on dielectric heating to generate heat, which makes the reaction medium rapidly and uniformly heated, and the whole heating reaction process is completed in a short time. The flow chemistry method can increase the process yield and minimize the use of solvents. In the synthesis of MOF-808 (Figure 4), the flow chemistry method significantly reduces the amount of solvents and conditioners used, while increasing productivity by two orders of magnitude compared to the batch synthesis method [69].

4. Advances in Photo/Electrocatalytic Applications of Zirconium-Based MOFs

4.1. Photocatalysis

Photocatalysis is a process that uses light energy to stimulate catalysts to produce catalytic activity, which has important applications in the fields of environmental purification, energy conversion, and organic synthesis. In this process, light energy is absorbed by the catalyst, which excites electrons to jump from the valence band to the conduction band, forming electronhole pairs. Zr-MOFs are often used as highly promising photocatalysts for various catalytic reactions, as shown in Table 3. These high-energy electrons and holes react with the surrounding molecules to produce redox capacity, thus catalyzing chemical reactions [70] (Figure 5). The activity and stability of photocatalysts are the key factors determining their application potential. They need to be able to absorb light energy and generate electron–hole pairs, have a high photo-quantum efficiency, good chemical and physical stability, and exhibit suitable surface properties to adsorb the reactants and facilitate the reaction. Among many photocatalysts, MOFs have attracted much attention due to their unique pore structure, tunable chemical composition, and high specific surface area, especially Zr-MOFs, such as the UiO-66 series, which show great potential for application in photocatalysis due to their excellent stability and multifunctionality. Li et al. [71] prepared zirconium-based MOF particles in poly(ethylene glycol) terephthalate (PEGT) fiber textile NH₂-UiO-66 film on polyethylene terephthalate fiber textiles. The NH₂-UiO-66 particles grew well on PET fiber textiles, and the film exhibited excellent photocatalytic activity and reusability superior to that of NH₂-UiO-66 powders. Ahmed’s group [72] decorated the water-soluble free radical porphyrin, decorated with three different linkage structures and functionalities of the Zr-MOFs by self-assembling (UiO66, UiO66-NH₂ and MIP-202). After a series of characterizations, the highest-complexity interaction between porphyrin and UiO-66-NH₂ was found. The self-assembled porphyrin UiO-66-NH₂ composites are a promising material for the degradation of dyes in contaminated wastewater. Chiu et al. [41] studied the interaction between electron/hole dynamics and the photo-physical properties of MOFs, as well as interactions between physical properties. Four zirconium-based MOFs (UiO-66 analogs) (Figure 6) were synthesized using different nitrogen functionalized ligands. After a series of characterization approaches, it was demonstrated that Pt/UiO-66-pz (Pt: platinum nanoparticles, pz: pyrazine) had the highest H₂ yield and was the most efficient bifunctional photocatalyst. Kondo et al. [73] varied the number of missing linker defects introduced into the UiO-66-NH₂ structure by varying the acetic acid concentration. The use of defective UiO-66-NH₂ under light irradiation produced a higher concentration of hydrogen peroxide compared to pristine UiO-66-NH₂. Catalytic technology has a wide range of applications in the fields of environmental purification, energy conversion, and organic synthesis, etc. MOFs, especially Zr-MOFs, are used as photocatalysts due to their unique structure and tunability. They have good chemical and physical stability, can remain active for a long period of time under reaction conditions, and have suitable surface properties to adsorb the reactants and promote the reaction.

Table 3. Performance of Zr-MOFs based photocatalytic applications.

Zr-MOFs Composites	Degree of Contamination	Reaction Type	Photocatalytic Performance	Ref
NH2-UIO66 (NU)	general	Reduction (Cr(VI)) to (Cr(III))	NU12-H removed Cr(VI) almost completely in 10 min. (100 mg L−1)	[74]
D-NUiO66	general	Reduction of CO2 to CO	The reaction rate was 38.6 µmol·g−1·h−1, and the catalyzer reusable	[75]
ZnIn2S4/MOF-808	general	Reduction of CO2 to CO	The CO yield was 8.21 μmol g−1·h−1	[76]
CdS@NU-1000	comparatively large	hydrogen production from aqueous media	CdS@NU-1000 and CdS@NU-1000/1%RGO exhibit 9.35 and 12.1 times higher photocatalytic activities than commercial CdS under visible light.	[77]
NaBiO3/UiO-67 heterojunction	general	Catalytic degradation of tetracycline	The degradation efficiency of NaBiO3/UIO-67 against tetracycline was 88.6% (100 mg·L−1)	[78]
TiO2/MOF-801(Zr)	comparatively small	Reduction (Cr(VI)) to (Cr(III))	At pH 1, the photocatalytic efficiency was as high as 98.1%.	[79]

Table 3. Performance of Zr-MOFs based photocatalytic applications.

Zr-MOFs Composites	Degree of Contamination	Reaction Type	Photocatalytic Performance	Ref
NH2-UIO66 (NU)	general	Reduction (Cr(VI)) to (Cr(III))	NU12-H removed Cr(VI) almost completely in 10 min. (100 mg L−1)	[74]
D-NUiO66	general	Reduction of CO2 to CO	The reaction rate was 38.6 µmol·g−1·h−1, and the catalyzer reusable	[75]
ZnIn2S4/MOF-808	general	Reduction of CO2 to CO	The CO yield was 8.21 μmol g−1·h−1	[76]
CdS@NU-1000	comparatively large	hydrogen production from aqueous media	CdS@NU-1000 and CdS@NU-1000/1%RGO exhibit 9.35 and 12.1 times higher photocatalytic activities than commercial CdS under visible light.	[77]
NaBiO3/UiO-67 heterojunction	general	Catalytic degradation of tetracycline	The degradation efficiency of NaBiO3/UIO-67 against tetracycline was 88.6% (100 mg·L−1)	[78]
TiO2/MOF-801(Zr)	comparatively small	Reduction (Cr(VI)) to (Cr(III))	At pH 1, the photocatalytic efficiency was as high as 98.1%.	[79]

Figure 5. Schematic photoexcitation in a solid followed by deexcitation events. Reprinted with permission from Ref. [70]. Copyright 2022, Wiley-VCH.

Figure 5. Schematic photoexcitation in a solid followed by deexcitation events. Reprinted with permission from Ref. [70]. Copyright 2022, Wiley-VCH.

Figure 6. (a) Coordination environment around the Cu(II) centers with a dinuclear core in 1. (b) Perspective view of 3D framework 1 viewed along the c axis. (c) Space-filling representation of 1. (d) 2-fold interpenetration with the space-filling representation in 1. (e) Topological view of 1 (color code: metal centers, blue; 4-TPOM ligands, purple; dicarboxylates, green). For clarity, H atoms have been omitted. Reprinted with permission from Ref. [80]. Copyright 2021 American Chemical Society.

Figure 6. (a) Coordination environment around the Cu(II) centers with a dinuclear core in 1. (b) Perspective view of 3D framework 1 viewed along the c axis. (c) Space-filling representation of 1. (d) 2-fold interpenetration with the space-filling representation in 1. (e) Topological view of 1 (color code: metal centers, blue; 4-TPOM ligands, purple; dicarboxylates, green). For clarity, H atoms have been omitted. Reprinted with permission from Ref. [80]. Copyright 2021 American Chemical Society.

Table 3 summarizes the performance of various zirconium metal–organic frameworks (Zr-MOFs) in photocatalytic applications, particularly their catalytic efficiency in the degradation of environmental pollutants and energy conversion. A diverse range of Zr-MOF composites demonstrated excellent reaction rates and turnover frequencies. For instance, NH₂-UiO66 (NU) nearly completely reduced Cr(VI) to Cr(III) within 10 min, while D-NUiO66 achieved a reaction rate of 38.6 µmol g⁻¹ h⁻¹ in CO₂ reduction. Additionally, CdS@NU-1000 exhibited photocatalytic activities under visible light irradiation that were 9.35 and 12.1 times higher than those of commercial CdS. These Zr-MOF composites displayed catalytic efficiencies superior to those of conventional photocatalysts, owing to their high specific surface areas, tunable chemical compositions, and excellent capabilities in terms of generating and transferring electron–hole pairs.

However, the stability and reusability of these catalysts remain challenges for their practical applications. Nevertheless, most of the Zr-MOF composites showed improved recyclability in photocatalytic processes for CO₂ reduction and hydrogen production [81]. Future research should concentrate on optimizing the synthesis methods for photocatalysts, particularly through green synthesis strategies, to minimize the use of hazardous chemicals and by-products. Additionally, enhancing the stability of catalytic materials under practical operating conditions will be essential for promoting their widespread industrial application.

4.2. Electrocatalysis

Electrocatalysis is a process that promotes or inhibits electron transfer reactions occurring in modifiers on the surface of an electrode or in the solution phase under the action of an electric field. This chemical reaction can significantly change the potential or rate of the reaction without changing the nature of the modifier itself, allowing the electrode not only to transfer electrons, but also to act as a facilitator and selector for the electrochemical reaction. In all types of catalytic reactions, the activity and lifetime of the catalyst as well as the precise regulation of the reaction are key factors. In the field of magnetocatalysis, it was investigated as to whether the precise control of the time of RAFT polymerization was achieved by the magnetocatalyst, which can precisely regulate the molecular weight of the polymer and its distribution. Similarly, in the electrocatalytic process, Zr-MOFs can also realize the efficient catalysis of the polymerization reaction by precisely regulating the electric field and the reaction conditions on the electrode surface. As shown in

Table 4. Performance of Zr-MOF-based electrocatalytic applications.

Zr-MOFs Composites	Degree of Contamination	Reaction Type	Overpotential(mV)	Tafel Slope (mV/dec)	Electrocatalytic Properties	Ref
PCN-222(Fe)/CNTs	comparatively large	CO2RR	494	137	After 10 h of electrocatalysis, an average conversion of 90% of CO. (−0.6 V, vs. RHE)	[88]
Fe@MOF-808-HPO−/HPO	general	Electrocatalytic nitrate to ammonia	/	/	the highest Faraday efficiency of NH3 is 90.8 ± 0.5% (−0.9 V, vs. RHE)	[89]
80Pt/C-MOF	general	HER	42.1	24.45	HER outperforms commercial Pt/C catalysts	[90]
NU-1000@NiMn-LDHS	general	HER/OER	93	/	The HER and OER overpotentials in 2M KOH at a current density of 10 mA·cm−2 were 93 mV and 129 mV, respectively.	[91]
UiO-67/B	general	HER	0.75	220	The current density of UiO-67/B increased from 3.2 mA·cm−2 to 7.0 mA·cm−2 (−0.6 V, vs. RHE) The optimized carrier density obtained for UiO-67/B increased significantly by a factor of 2.15 under light irradiation for 30 min.	[92]

, Zr-MOFs are often used as highly promising electrocatalysts for various catalytic reactions. Typical applications (Figure 7) of electrocatalysis include the Hydrogen Extraction Reaction [82] (HER), Oxygen Extraction Reaction [83] (OER), and Carbon Dioxide Reduction Reaction [84] (CO₂RR). These occur in the field of sustainable energy. MOFs have been shown to be highly promising electrocatalysts [85] due to their high structural designability, high porosity, large specific surface area, and ease of functionalization. These unique properties give Zr-MOFs a distinct advantage in the design and application of electrocatalytic systems. Liu [40] and others developed a porphyrin–zirconium-based MOF (HPCN-222) with a hollow structure as a carrier for single Pt atoms and prepared the single-atom catalysts (SAC) PtHPCN-222. The high specific surface area of HPCN-222 provides a large number of anchoring sites for immobilizing single Pt atoms, which maximizes the utilization of the Pt atoms. The strong interaction between HPCN-222 and Pt atoms leads to the stable dispersion of single Pt atoms and changes the electronic structure of the Pt atoms, thus achieving high catalytic efficiency and good electrochemical activity of the SAC PtHPCN-222. HPCN-222 is an excellent catalyst for the Pt atoms. Strong interactions with Pt atoms stabilize the dispersion of single Pt atoms and change the electronic structure of Pt atoms, resulting in high catalytic efficiency, good electrochemical activity, and stability of the SACs PtHPCN-222. Verma’s team [86] prepared an iron-metallized MOF-525 with a film that can reduce CO₂ to CO, which is important for CO₂ capture and utilization. Jiang [87] and his team deposited white UiO-66 on titanium plates to create a bridging layer with controllable size and thickness by peritectic thin-film deposition. Different thicknesses of UiO-66/Ti plates were obtained by varying the ratio of electrochemical deposition solution. Pd/UiO-66/Ti was further fabricated by impregnating Pd²⁺ solution onto the surface of UiO-66, followed by two hours of impregnation in 0.1% NaBH₄ solution. The cathodic electrode of Pd/UiO-66/Ti showed an 80% degradation of tetrabromobisphenol A (TBBPA) within 60 min. The adsorption of TBBPA by UiO-66 and the reaction with hydrogen at the Pd/UiO-66 interface greatly improved its electrocatalytic performance. The chemical and thermal stability of Zr-MOFs is a key advantage that enables their application in electrocatalysis. In particular, the stability of the UiO-66 series under various chemical environments significantly enhances its application prospects in the electrocatalytic field, enabling it to maintain its structural and functional integrity under harsh operating conditions. In addition, the composites formed by combining Zr-MOFs with metal nanoparticles are able to fully utilize the advantages of both and overcome the limitations of a single component, which provides a new research direction and choice of material in the field of electrocatalysis. The design of such composites not only improves electrocatalytic activity, but also enhances the stability and selectivity of the catalyst.

As shown in Table 4, the PCN-222(Fe)/CNTs composite exhibited an overpotential of 494 mV and a Tafel slope of 137 mV/dec in the CO₂ reduction reaction, indicating lower energy loss and higher catalytic efficiency for this catalyst in CO₂ conversion. In contrast, the NU-1000@NiMn-LDHS composite demonstrated an overpotential of 93 mV and a Tafel slope of 129 mV, highlighting its excellent electrocatalytic performance and faster reaction kinetics. Compared to conventional electrocatalysts, zirconium metal–organic frameworks (Zr-MOFs) have established a pivotal role in catalytic applications due to their exceptional versatility, high efficiency, and structural tunability.

These materials have shown remarkable capabilities in environmental purification and energy conversion within photocatalysis [93], as well as excellent performance in electrocatalysis, particularly in CO₂ reduction and the degradation of organic pollutants. The properties of Zr-MOFs, including high specific surface area, porosity, and chemical and thermal stability, provide abundant active sites for catalytic reactions and enable the tailoring of materials for specific applications through precise synthesis strategies. Therefore, Zr-MOFs are not only valued for their intrinsic physicochemical properties, but also as a promising research and application platform in catalytic science and technology due to their potential in catalytic performance optimization and novel composite material development. The progress of these studies not only promotes the application of Zr-MOFs in catalytic science and technology, but also provides new material options for solving environmental and energy problems and further broadens their application prospects in the field of sustainable development [94].

5. Conclusions and Future Perspectives

The green synthesis of Zr-MOFs is particularly important in the context of sustainable development. The implementation of green synthesis methods not only reduces the emission of harmful by-products but also decreases the reliance on environmentally harmful organic solvents, which is essential for realizing environmentally friendly chemical synthesis processes. This review highlights a full range of green synthesis strategies from raw material selection and solvent use to synthesis methods, which are important guidelines to promote the industrial application of Zr-MOFs.

Zr-MOFs show great potential for applications in catalysis, especially in photocatalysis and electrocatalysis. They display impressive performances in environmental purification, energy conversion, CO₂ reduction, organic pollutant degradation, etc. The unique structure and tunable properties of Zr-MOFs enable high efficiency and selectivity in catalytic reactions, which provide new material options for solving current environmental and energy problems.

Despite the remarkable progress made in laboratory-scale research on Zr-MOFs, their industrial application still faces many challenges. Continued research and development are necessary to address these challenges, including improving synthesis efficiency, reducing costs, enhancing material stability, and achieving scale-up. These efforts will drive Zr-MOFs from the laboratory to the marketplace and enable their commercial application in catalysis.

Currently, the main challenges in the synthesis and application of Zr-MOFs include cost-effectiveness, material stability, and scale-up production issues. Green synthesis methods, although environmentally friendly, are often accompanied by higher costs and lower yields. In addition, the stability and durability of Zr-MOFs in practical applications need to be further improved. Quality control and cost management in the process of large-scale production are also key to industrialization.

In future research, the development of green synthesis methods for Zr-MOFs will be a central direction, which involves the exploration of more economical, efficient, and environmentally friendly synthesis techniques aimed at reducing the cost and environmental impact of Zr-MOF production. Meanwhile, enhancing the performance of Zr-MOFs in specific catalytic reactions through structural optimization and functionalization modification will be another research focus, which not only enhances their catalytic efficiency, but also expands their role in industrial applications. In addition, an in-depth study of the relationship between the structure and performance of Zr-MOFs is of great significance for understanding their catalytic mechanism and guiding the design of novel and efficient catalysts, which will provide a solid theoretical foundation for catalytic science. Finally, exploring the application of Zr-MOFs in novel catalytic reactions will help to broaden the scope of their application in the industrial field and provide a new impetus for the sustainable development of the chemical industry. These research directions will not only promote the application of Zr-MOFs in catalytic science and technology, but also have a far-reaching impact on solving environmental and energy problems and promoting the process of green chemistry.

Zr-MOFs have remarkable potential and broad prospects for green synthetic pathways and their catalytic applications. Nevertheless, there are still a series of challenges at technical and economic levels in terms of realizing the industrial application of Zr-MOFs. Future work needs to focus on breaking through these barriers, which include, but are not limited to, improving synthesis efficiency, reducing costs, enhancing material properties, and scaling up production. Through sustained research efforts and innovative strategies, the industrial application of Zr-MOFs is achievable, which will not only provide new solutions for environmental protection, but also make a significant contribution to the improvement of energy efficiency. Therefore, the research and development of Zr-MOFs will continue to be an active and important research direction in the field of catalytic science and green chemistry.