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KARGEN Tech Blog | How MOF Materials Achieve Structural Regulation

2024-09-09 15:47:16

Introduction

When discussing Metal-Organic Frameworks (MOFs), we often highlight their "highly tunable structures," which is one of the advantages MOFs have over other advanced porous materials such as Covalent Organic Frameworks (COFs) and Porous Organic Polymers (POPs). The structure of a substance determines its properties, and the ability to orientally regulate the structure of MOFs to meet specific application needs allows for customized functionalities. This aspect is crucial in MOF applications. This paper first introduces the adjustable parameters of MOFs and their effects on MOF properties. It then explores how macroscopic methods can be used to regulate the microscopic structure of MOFs through various stages of chemical reactions. Finally, it briefly presents relevant application examples from KARGEN New Materials. The aim is to help readers concretize the abstract concept of "MOF structural tunability" and deepen their understanding of MOFs.

Categorical Topics: MOFs, Metal-Organic Frameworks, Pore Size, Porosity, Structural Regulation, Performance Customization, MOFs Synthesis, MOFs Modification

01. The Importance of Achieving Structural Regulation in MOFs

MOFs are porous crystalline materials formed by the self-assembly of metal centers (metal ions or metal clusters) and organic ligands. They are characterized by high porosity. Due to the vast variety of metal salts and ligands, numerous new MOFs have been synthesized, totaling in the thousands. However, not all synthesized MOFs are directly applicable in practical production. Many exhibit notable shortcomings such as poor water/thermal stability, inadequate gas adsorption/storage selectivity, or low yield. Therefore, it is crucial to adjust properties such as size, functional groups, internal porosity, and pore size distribution to meet specific application requirements.

The size of MOFs primarily refers to their particle diameter, which is usually below 1000 nm. Unlike bulk materials, the functional characteristics of nanocrystals are highly dependent on size. Variations in particle size affect the diffusion of guest molecules within the porous material, directly impacting catalytic reactions or molecular adsorption and separation. Hence, uniformly sized nanoscale MOFs often exhibit superior performance in catalysis, sensing, and drug delivery.

In addition to synthesizing original MOFs, incorporating specific functional groups onto the ligands can further enhance certain aspects of MOF performance while retaining their inherent advantages. For instance, in heavy metal wastewater treatment, introducing groups such as -NH3 and -COOH into MOFs can promote interactions with heavy metal ions and improve ion removal rates.

The internal structure of MOFs is a critical factor determining their performance. Pores are the sites of interaction between external molecules and MOFs. Increasing the internal porosity of MOFs effectively enlarges their specific surface area, enhances gas adsorption or storage capacity, and accelerates mass transfer rates. Additionally, mesopores and macropores can improve thermal conductivity, and adjusting pore size according to target gas molecules can enhance MOF selectivity for specific gases.

02. How to Adjust the Microscopic Structure of MOFs through Macroscopic Methods

2.1 Regulation through Reactants

2.1.1 Metal Connectors

In general, the strength of the coordination bonds between metals and ligands in MOFs affects their thermodynamic properties, which in turn determines the stability of MOFs. The charge density refers to the concentration of positive charge distributed in space. Choosing metal ions with high charge densities (such as Zr4+, Cr3+, Al3+, Fe3+, etc.) for interactions with ligands can form strong coordination bonds, resulting in MOFs with good stability. Additionally, the regulation of MOF structures can also be achieved by designing secondary building units (SBUs). The concept of SBUs, introduced by the "father of MOFs," Yaghi, in 1999, refers to structural units with specific geometrical shapes formed by the connection of metal ions and functional groups. Typically, SBUs are composed of metal-oxygen or metal-nitrogen bonds. In simple terms, the design of SBUs involves purposefully selecting metal ions based on their coordination patterns, predicting the resulting geometric structure, and then reacting with organic ligands to directionally control the resulting MOF structure. Figure 1 illustrates some classic MOFs constructed from SBUs and ligands.

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MOFs Constructed from SBUs and Ligands

Furthermore, strategies have been developed to incorporate two or more metal ions into MOF metal clusters. The synergistic effects between different metals and the varying activities of metal centers result in some mixed-metal MOFs exhibiting superior chemical stability and performance compared to single-metal MOFs. For example, mixing Co2+ with Cu2+ solutions can create new active centers with Co2+, resulting in HKUST-1, which shows stronger water affinity compared to single-metal products. Consequently, the obtained product not only has a larger porosity and specific surface area but also improved adsorption capacity for water vapor.

2.1.2 Ligands

Besides metal connectors, most organic ligands remain stable during synthesis. There is a wide variety of organic ligands used in MOF preparation, differing in geometry and length. Therefore, MOF structural regulation can also be achieved by selecting different organic ligands. As shown in Figure 2, using organic ligands with similar structures but varying sizes and chemical functionalities can effectively adjust the porosity of MOFs.

Figure 2. Carboxylic Ligands with Different Geometries, Lengths, and Functional Groups211n
Carboxylic Ligands with Different Geometries, Lengths, and Functional Groups

In addition, the design of mixed organic ligands also plays a crucial role in adjusting MOF structures and functionalities. The combination of ligands with different sizes, shapes, and coordination characteristics can lead to the creation of entirely new MOF structures, allowing for the modulation of pore size, shape, and functionality. Figure 3 illustrates how Yaghi's team demonstrated that up to eight different terephthalic acid-based ligands with various side functionalities could be incorporated into MOF-5, resulting in a 400% increase in selectivity for CO2/CO gas mixtures.

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Simplified Structure of MOF-5 with Five Different Ligands, as Demonstrated by Yaghi's Team

2.2 Regulation through Synthesis Methods

The same reactant mixture can yield MOFs with different structures depending on the synthesis method used, resulting in variations in the final product's morphology, particle size, and size distribution. Figure 4 provides an overview of MOF synthesis methods, reaction temperatures, and final reaction products. Details of each method are described in the first article of the tech blog and will not be reiterated here.

Traditional solvothermal synthesis is conducted under high temperature and pressure in a closed system, where high solubility of reactants leads to thorough reactions and high-quality large crystals. Electrochemical methods allow for the synthesis of MOFs under milder conditions, producing uniform films or coatings directly. Microwave-assisted and sonochemical methods can produce crystals smaller than 100 nm, enhancing the MOFs' loading capacity. Mechanochemical methods, which can be used in solvent-free systems, do not apply to all types of MOFs. While this method reduces reaction time, the porous materials synthesized by mechanochemistry differ in pore size distribution from those produced by solvothermal methods.

Figure 4. Overview of Synthesis Methods, Reaction Temperatures, and Final Reaction Products4utv
Overview of Synthesis Methods, Reaction Temperatures, and Final Reaction Products

2.3 Regulation through Reaction Control

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Factors Affecting MOF Synthesis

Figure 5 shows the factors affecting MOF synthesis, divided into reaction system parameters (such as solvent, pH, molar ratio of starting materials, concentration) and reaction condition parameters (such as pressure, time, temperature). Traditional solvothermal reactions are essentially Lewis acid-base reactions, where metal ions act as Lewis acids and deprotonated ligands serve as Lewis bases. If the reaction proceeds too quickly, it can produce a large amount of precipitate rather than crystalline growth. Therefore, adjusting the reaction conditions aims to regulate the rate of ligand deprotonation and coordination bond formation, leading to the formation of products with different structures.

It is important to note that there are no universal reaction conditions suitable for all ligands and metal ions; specific ligand and metal ion combinations require different conditions. Success often relies on a combination of experience and serendipity.

2.3.1 Reaction System

Common solvents used in MOF synthesis include dimethylformamide (DMF), diethylformamide (DEF), and dimethylacetamide (DMA). These solvents can hydrolyze at temperatures between 60°C and 85°C, releasing amines that deprotonate carboxylic ligands and promote the reaction. Additionally, solvents can act as structure-directing agents or mediums for crystal growth. Therefore, selecting appropriate solvents and determining the optimal solvent ratios are crucial steps in MOF synthesis.

The acidity and alkalinity of the reaction system significantly affect MOF crystal growth. The pH value of the reaction medium determines the deprotonation degree of the organic ligands. In reaction systems containing carboxylic ligands, the appropriate pH environment can also promote the formation of ligand-OH, facilitating the self-assembly of ligands with metal ions. In practical production, inorganic acids or bases (such as acetic acid, hydrochloric acid, sodium hydroxide) are often used to adjust the pH, affecting the particle size, specific surface area, and appearance of the product.

The molar ratio of reactants is another important factor affecting the structure and yield of MOF products. The coordination mode of MOFs is influenced by the stoichiometry of reactants. Under constant conditions, altering the molar ratio of reactants can induce changes in the product's framework structure from one-dimensional to three-dimensional.

2.3.2 Reaction Conditions

In addition to adjusting the reaction system, process parameters such as temperature, pressure, and time are also critical in influencing MOF structure. For instance, hydrothermal synthesis is typically conducted under high temperature and pressure conditions, leading to higher solubility and crystallinity of the reactants. Some ligands may form different spatial conformations at varying temperatures or reaction times, impacting the coordination mode and the coordination number of central metal ions. Therefore, adjusting reaction conditions can purposefully yield multi-dimensional structures.

2.4 Post-Synthesis Modification

When the stability or solubility of reactants limits the adjustment of reaction conditions, post-synthesis modification methods can be employed to achieve specific functional structures. At a microscopic level, post-synthesis modification involves replacing or chemically modifying the metal nodes and organic ligands of MOFs. A commonly used method is the solution-assisted exchange method, where the parent MOF is exposed to a solution containing target ligands or metals, with stirring or heating. The concentration of the solution is related to the MOF type. This method can introduce new chemical functions to the MOF, control the pore size and shape of the crystals, while maintaining the structure and stability of the parent framework.

3. Application Examples

Guangdong Advanced Carbon Materials Co., Ltd., as the first domestic high-tech enterprise to achieve ton-scale production of MOFs, offers a range of MOF products with mature preparation processes and excellent performance. With years of technological accumulation in MOF synthesis, KARGEN has not only achieved significant production capacity improvements but also adjusted MOFs based on actual customer needs.

For example, KAR-F39 is one of KARGEN's top-performing MOF products for dehumidification, with a maximum water vapor adsorption capacity of 1.0 g/g. By adjusting reaction conditions and adding reaction auxiliaries, the particle size was successfully reduced from ~900 nm to 100–200 nm, enhancing the material's specific surface area and pore volume, and increasing the water vapor adsorption capacity to 1.2–1.5 g/g. Figure 6 shows scanning electron microscopy images of samples with different particle sizes.

Figure 6. KAR-F39 with Different Particle Sizes6tnn
KAR-F39 with Different Particle Sizes

4. Conclusion

The structural tunability of Metal-Organic Frameworks (MOFs) is a key feature for their applications in scientific research and industry. By precisely controlling the structure of MOFs, materials can be customized to meet specific needs, significantly impacting areas such as gas separation and adsorption, environmental protection, and drug delivery. The synthesis of MOFs involves various factors like reactants, reaction conditions, and post-synthesis treatments, which all influence the MOF structure. While universal guidelines for reaction systems are still developing, the wealth of accumulated experience provides endless possibilities for structural tuning. Guangdong Advanced Carbon Materials Co., Ltd. is the first domestic technology-driven enterprise to achieve ton-scale MOF production, producing hundreds of functional MOFs. Our team has extensive expertise in MOF synthesis and applications. For more information on MOF applications and custom functionalities, please contact the KARGEN expert team.

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