Synthesis Methods and Crystallization of MOFs

*Yitong Han, Hong Yang and Xinwen Guo*

### **Abstract**

Metal-organic frameworks (MOFs) are a class of porous crystalline materials constructed of metal centres with organic linkers, creating one-, two-, or threedimensional well-organized frameworks with very high surface areas. The study of MOFs has become one of the research hot spots in many fields, owing to the broad potential applications projected for these materials in various areas. It is well recognized that synthesis strategies dictate the structure and thus the properties and performance of the resulted MOFs. This chapter provides a comprehensive up-to-date overview on the modulated synthesis strategies for MOFs. The ability to control crystal morphology and size by a number of modulated synthesis methods is illustrated by the zirconium-terephthalate-based MOF, the UiO-66, and a number of other MOFs.

**Keywords:** metal-organic frameworks, synthesis method, crystallization, crystal morphology and size control

## **1. Introduction**

Porous materials are a class of solid compounds with an ordered and/or disordered pore structure, high pore volume, and large surface area. These combined with some of their unique chemistries make them a unique class of chemical and engineering materials. Over the past three decades, porous materials have become one of the research hot spots in the fields of chemistry, physics, and materials science and engineering. The diversity in the pore orientation and dimensionality, combined with the multiplicity in pore shapes and sizes, makes the porous materials highly interested and widely studied. The IUPAC defines the porous materials based on the pore dimensions and classifies them into the microporous materials with pore dimension less than 2 nm, mesoporous materials with pore dimension ranging from 2 to 50 nm, and macroporous materials with pores larger than 50 nm [1].

Porous materials have shown great application values in some traditional industries, such as the oil and gas processing, industrial catalysis, adsorption/separation, and fine chemical industries [2–5]. In more recent decade, they are also being recognized and explored in the research fields of water treatment, sustained release of drugs, and fuel cells, to name a few [6, 7].

Among all porous materials, zeolites are perhaps the most famous microporous materials. In the frameworks of the zeolites, different numbers of TO4 tetrahedral primary structure units connect together to form shared apexes, which enable the creation of the interconnected secondary structure units of various shapes and the formation of the microporous zeolite structure. The "T" in the TO4 tetrahedrons is usually silicon, aluminum, and phosphorus. Chemically, zeolites have adjustable acidity, in addition to

their excellent pore channel selectivity and hydrothermal stability. These support the high research value of zeolites in industrial applications such as exchange, adsorption/ separation, and petroleum processing. However, the inorganic nature of zeolites limits their adjustability in chemical properties and designability in pore structures and sizes, which restrict the further development and applications of zeolites [2].

For decades, there have been research efforts trying to integrate various metal centres and functional organic molecules into porous structures for the modulation of physical and chemical properties. This effort has resulted in the development of a novel type of hybrid porous crystalline materials formed by the coordination of inorganic metal centres and organic linkers in the 1990s. This new class of porous materials is most commonly known as metal–organic frameworks (MOFs), although they are also sometimes known as porous coordination polymers (PCPs) or inorganic-organic hybrid materials [8–10]. Ever since their discovery, MOFs have received significant attention from scientists, engineers, and technologists due to their versatilities in structures and chemistries. Since the synthesis of MOFs directly influences the crystallization of the MOF structure, thus dictates its properties and functional performance, extensive work and thousands of research papers have continually emerged focusing on the development of synthesis methods.

The aim of this chapter is to provide an up-to-date overview on the synthesis methods of MOFs reported so far, with the objective to provide empirical guidance for developing synthesis strategy for MOF materials targeting specific properties and functionalities. We draw on our own extensive experiences on modulated synthesis of the UiO-66 MOF material to demonstrate how the crystal morphology and size can be controlled by different modulated synthesis methods, mechanisms of crystallization, and effect of metal doping on MOF crystals during synthesis.

#### **2. Structure and applications of MOFs**

As stated in the introduction, MOFs are a unique class of hybrid porous crystalline materials and have been widely studied over the past two decades for their inherent structure design flexibility and potential applications [11].

The structures of MOFs are constructed by self-assembly between the "nodes" of metal-containing secondary building units (SBUs) and the "bridges" of organic linkers, creating one-, two-, or three-dimensional well-organized network structures with very high pore volumes and surface areas. The framework topologies and pore structures and sizes of MOFs can be designed via selecting various metal centres and organic linkers. Their chemical properties can be modified by chemical functionalization of linkers and post modifications [12–14].

MOFs have extended the chemistry of the porous materials from inorganic to inorganic-organic hybrid. This compositional diversity, combined with the structural diversity, gives them unique properties and functionalities. MOFs are thus a class of highly attractive porous materials for a broad range of applications, including in gas adsorption/separation [15–18], luminescence and sensing [19–22], catalysis [23–25], and others [26–29].

### **3. Synthesis methods and crystallization of MOFs**

#### **3.1 Overview of synthesis methods for MOFs**

At their discovery, the method for synthesis of MOFs is solvothermal. Typically, metal precursors and organic linkers are dissolved in solvent and placed in a closed

**61**

**Figure 1.**

*Synthesis Methods and Crystallization of MOFs DOI: http://dx.doi.org/10.5772/intechopen.90435*

of days.

hybrid MOF crystals.

reaction vessel for the formation and self-assembly of MOF crystals. The common solvents used include N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF), methanol, ethanol, and acetonitrile. The synthesis temperature is generally below 220°C, and the crystallization time varies from several hours to several tens

After more than two decades of research and development, great advances have been made in the synthesis of MOFs. New synthesis methods such as the electrochemical, microwave-assisted, mechanochemical synthesis, microfluidic synthesis method, etc. have all been reported [30]. **Figure 1** summarizes the development

These diverse methods have enabled the synthesis of hundreds of new MOF structures since its first discovery. On top of this, as clearly stated in the principle of "structure dictates function" [31], having the ability to control and tailor the morphology and size and their chemical functionalization of MOF crystals is vital in delivering targeted properties and performances of the resulted MOF materials. This demands the development of more sophisticated synthesis strategies based on the understanding of mechanisms of crystallization occurred during synthesis. The following provides an overview on the currently developed modulated synthesis methods for morphology and size control of MOF crystals and doping to create

The synthesis of MOFs involves the process of crystallization during which the nucleation and growth of crystals occur. The nucleation and growth of MOF crystals involve the self-assembly between metal-oxygen clusters and organic linkers. Understanding the influencing factors on the nucleation and growth of MOF crystals during their synthesis will enable accurate controlling of crystal morphology and size. The following discusses the morphology and size development of the

It is well documented that synthesis conditions, such as temperature, time, solvent type, and reactant concentrations, play important roles in the morphology

For example, an NH2-MIL-125(Ti) MOF material can be synthesized by a solvothermal method in a mixed solvent of DMF and methanol. **Figure 2** shows the SEM images of the NH2-MIL-125(Ti) crystals synthesized with different reactant concentrations, as indicated by the total solvent volumes. By changing the total volume of the solvent alone while maintaining constant ratio between the DMF and methanol and amount of reactants, the morphology of the NH2-MIL-125(Ti)

timeline for the most common synthesis approaches of MOFs [30].

**3.2 Morphology and size control of MOF crystals**

MOF crystals during different modulated syntheses.

*Timeline of the most common synthesis approaches for MOFs [30].*

*3.2.1 Deprotonation regulation synthesis*

and size of resulted MOF crystals.

#### *Synthesis Methods and Crystallization of MOFs DOI: http://dx.doi.org/10.5772/intechopen.90435*

*Synthesis Methods and Crystallization*

**2. Structure and applications of MOFs**

catalysis [23–25], and others [26–29].

their excellent pore channel selectivity and hydrothermal stability. These support the high research value of zeolites in industrial applications such as exchange, adsorption/ separation, and petroleum processing. However, the inorganic nature of zeolites limits their adjustability in chemical properties and designability in pore structures and sizes,

For decades, there have been research efforts trying to integrate various metal centres and functional organic molecules into porous structures for the modulation of physical and chemical properties. This effort has resulted in the development of a novel type of hybrid porous crystalline materials formed by the coordination of inorganic metal centres and organic linkers in the 1990s. This new class of porous materials is most commonly known as metal–organic frameworks (MOFs), although they are also sometimes known as porous coordination polymers (PCPs) or inorganic-organic hybrid materials [8–10]. Ever since their discovery, MOFs have received significant attention from scientists, engineers, and technologists due to their versatilities in structures and chemistries. Since the synthesis of MOFs directly influences the crystallization of the MOF structure, thus dictates its properties and functional performance, extensive work and thousands of research papers have continually emerged focusing on the development of synthesis methods.

The aim of this chapter is to provide an up-to-date overview on the synthesis methods of MOFs reported so far, with the objective to provide empirical guidance for developing synthesis strategy for MOF materials targeting specific properties and functionalities. We draw on our own extensive experiences on modulated synthesis of the UiO-66 MOF material to demonstrate how the crystal morphology and size can be controlled by different modulated synthesis methods, mechanisms of crystallization, and effect of metal doping on MOF crystals during synthesis.

As stated in the introduction, MOFs are a unique class of hybrid porous crystalline materials and have been widely studied over the past two decades for their

The structures of MOFs are constructed by self-assembly between the "nodes" of metal-containing secondary building units (SBUs) and the "bridges" of organic linkers, creating one-, two-, or three-dimensional well-organized network structures with very high pore volumes and surface areas. The framework topologies and pore structures and sizes of MOFs can be designed via selecting various metal centres and organic linkers. Their chemical properties can be modified by chemical

MOFs have extended the chemistry of the porous materials from inorganic to inorganic-organic hybrid. This compositional diversity, combined with the structural diversity, gives them unique properties and functionalities. MOFs are thus a class of highly attractive porous materials for a broad range of applications, including in gas adsorption/separation [15–18], luminescence and sensing [19–22],

At their discovery, the method for synthesis of MOFs is solvothermal. Typically, metal precursors and organic linkers are dissolved in solvent and placed in a closed

inherent structure design flexibility and potential applications [11].

functionalization of linkers and post modifications [12–14].

**3. Synthesis methods and crystallization of MOFs**

**3.1 Overview of synthesis methods for MOFs**

which restrict the further development and applications of zeolites [2].

**60**

reaction vessel for the formation and self-assembly of MOF crystals. The common solvents used include N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF), methanol, ethanol, and acetonitrile. The synthesis temperature is generally below 220°C, and the crystallization time varies from several hours to several tens of days.

After more than two decades of research and development, great advances have been made in the synthesis of MOFs. New synthesis methods such as the electrochemical, microwave-assisted, mechanochemical synthesis, microfluidic synthesis method, etc. have all been reported [30]. **Figure 1** summarizes the development timeline for the most common synthesis approaches of MOFs [30].

These diverse methods have enabled the synthesis of hundreds of new MOF structures since its first discovery. On top of this, as clearly stated in the principle of "structure dictates function" [31], having the ability to control and tailor the morphology and size and their chemical functionalization of MOF crystals is vital in delivering targeted properties and performances of the resulted MOF materials. This demands the development of more sophisticated synthesis strategies based on the understanding of mechanisms of crystallization occurred during synthesis. The following provides an overview on the currently developed modulated synthesis methods for morphology and size control of MOF crystals and doping to create hybrid MOF crystals.

## **3.2 Morphology and size control of MOF crystals**

The synthesis of MOFs involves the process of crystallization during which the nucleation and growth of crystals occur. The nucleation and growth of MOF crystals involve the self-assembly between metal-oxygen clusters and organic linkers. Understanding the influencing factors on the nucleation and growth of MOF crystals during their synthesis will enable accurate controlling of crystal morphology and size. The following discusses the morphology and size development of the MOF crystals during different modulated syntheses.

## *3.2.1 Deprotonation regulation synthesis*

It is well documented that synthesis conditions, such as temperature, time, solvent type, and reactant concentrations, play important roles in the morphology and size of resulted MOF crystals.

For example, an NH2-MIL-125(Ti) MOF material can be synthesized by a solvothermal method in a mixed solvent of DMF and methanol. **Figure 2** shows the SEM images of the NH2-MIL-125(Ti) crystals synthesized with different reactant concentrations, as indicated by the total solvent volumes. By changing the total volume of the solvent alone while maintaining constant ratio between the DMF and methanol and amount of reactants, the morphology of the NH2-MIL-125(Ti)

**Figure 1.** *Timeline of the most common synthesis approaches for MOFs [30].*

#### *Synthesis Methods and Crystallization*

crystals can be modified from circular plates to tetragons and octahedrons [32]. As the crystal morphology changes, the light absorption band of the NH2-MIL-125(Ti) can be tuned from 480 to 533 nm, making it advantageous in photocatalytic applications.

It has been found that the reactant concentration has a significant effect on the deprotonation rate of the organic linkers during the synthesis of NH2-MIL-125(Ti) crystals. The deprotonation rate plays a critical role in the nucleation and growth of the NH2-MIL-125(Ti) crystals. Modulating crystal morphology and size of MOFs by changing the rate of deprotonation is called the deprotonation regulation synthesis [33].
