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2D Metal-Organic Frameworks

Fengxian Cao1╬, Jian Chen1╬, Qixun Xia1* and Xinglai Zhang2┼

1 Henan Key Laboratory of Materials on Deep-Earth Engineering, School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo, China

2 Shenyang National Laboratory for Materials Science (SYNL), Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS), Shenyang, China

Abstract

The metal organic framework (MOF) is a crystalline porous material formed of an inorganic metal ion or cluster and an organic ligand. The invention has the characteristics of large pore volume, high specific surface area, variable structures, and multiple functions. It was widely applied in the fields of gas storage, separation, catalysis, sensing, and biomedicine. The emergence of this kind of material, to a large extent, has provided opportunities for the common development of other disciplines. In this chapter, the recent research and development of MOFs materials, including the synthesis methods (sol-gel method, hydrothermal solvothermal method, and microwave synthesis, etc.), the development status, the applications, i.e., hydrogen storage, energy storage, gas adsorption, catalytic reaction, sensors, biomedical applications, and so on, and the research hotspots of MOFs will be addressed.

Keywords: MOF, biomedicine, gas storage, sensors, catalysis

1.1 Introduction

Amidst the highly porous materials, metal organic frameworks (MOFs) exhibited incomparable tunable and structural diversity. Furthermore, MOFs synchronously demonstrate porosity and excellent electrical conductivity, which are a burgeoning group of materials and provide a wide range of applications, for instance, energy storages, electrocatalytic oxidation, gas adsorption, biomedical [1-6]. The atomic-level control over molecular and supramolecular structure provided by MOFs gives the chance for exploiting some new materials for a variety of applications [7].

As a new type of porous inorganic-organic hybrid crystal material, MOFs materials have attracted extensive attention in chemistry, material, physics, and other fields. It combines the characteristics of inorganic and organic materials. It has a wide range of potential values in gas storage and separation, luminous, sensing, catalysis, magnetism, and other fields. When MOFS was made into membrane, the application of MOFs material in gas phase field was expanded. The gas separation application of MOFs extends from adsorption separation to membrane separation. By using the adjustable or modified characteristics of pore size, shape, and surface chemical properties of MOFs, MOFs material is endowed with better membrane separation performance for some light gas molecules. In addition, MOFs film extends the detection range of MOFs to gas, which can realize humidity detection and fluorescence detection of other gases or vapors. In these cases, the MOFs will play an important role in the generation, transmission, adsorption, and storage.

The objective of this chapter is to summary recent literature describing the progress of MOFs. We first review the technology about how to grow MOFs thin films, including sol-gel method, hydrothermal solvothermal method, and microwave synthesis, etc. Whereafter, we summarized the structural feature and physicochemical properties description of MOFs. Subsequent sections discuss the MOF films in various applications, including hydrogen storage, energy storage, gas adsorption, catalytic reaction, sensors, biomedical applications, and the like. Finally, we discuss some limitations of MOFs in practical application.

1.2 Synthesis Approaches

The synthesis of two-dimensional (2D) MOFs compounds materials is generally carried out by cultivating single crystals. X-ray single crystal structure analysis is the most important method to determine the structure of metallic organic skeleton materials [8]. The accurate molecular structure of organometallic skeleton materials can be obtained by analysis. At present, the methods of synthesizing organometallic skeleton materials reported in the literature mainly include solution volatilization method, diffusion method, and hydrothermal/solvothermal synthesis route. These methods complement each other and sometimes use different synthesis methods or the same method and different conditions to obtain materials with different structures and functions [9]. With the development of collocation chemistry and material chemistry, ultrasonic synthesis, ion-liquid method, solid phase reaction method, sublimation method, microwave synthesis, method and two-phase synthesis method have also been applied to the synthesis of MOFs materials. Various synthesis ways have their own advantages and disadvantages. Therefore, the choice of synthesis methods is very important for the synthesis of MOFs, and even affects its structure and properties.

1.2.1 Selection of Synthetic Raw Materials

When the synthesis of MOFs is started, it is important to maintain the integrity of skeleton looseness in addition to geometric factors. Therefore, it is necessary to find sufficient mild conditions to maintain the function and structure of the organic ligand, while having sufficient reactivity to establish the coordination bond between the metal and the organic [10].

First of all, the metal components are mainly transition metal ions, and most of the valence states used by Zn2+, Cu2+, Ni2+, Pd2+, Pt2+, Ru, and Co2+. Secondly, organic ligands should contain at least one multi-dentate functional group, such as CO2H, CS2H, NO2, SO3H, and PO3H. CO2H was more commonly used in multi-dentate functional groups, such as erephthalic acid (BDC), tribenzoic acid (BTC), oxalic acid, succinic acid, etc. The selection of suitable organic ligands can not only form MOFs with novel structure, but also produce special physical properties. In addition, solvents can dissolve and protonize ligands in the process of synthesis. Metal salt and most ligands are solid as solvent is needed to dissolve it. Before metal ions and ligands are coordinated, ligands (such as carboxylic acids) need to be deprotonized, so alkaline solvents are often used. At present, many deprotonated alkaloids are used as organic amines, such as triethylamine (TEA), N, N2 dimethyl formamide (DMF), N, N2 diethylamide (DEF), N2 methyl pyrrolidone. At the same time, they are good solvents. In recent years, there are gradually examples of deprotonation with strong bases such as sodium hydroxide. Sometimes, solvents can also coordinate with metal ions as ligands or form weak interactions with other ligands, such as hydrogen bonds, which can be excluded by heating and vacuum. Finally, in order to make the synthesized organometallic skeleton have ideal pores, it is necessary to select the appropriate template reagent. Template reagents are sometimes separate substances, sometimes the solvents used.

1.2.2 Solvent Volatility Method

Solvent volatility method is suitable for the metal salt and ligands with good solubility and the obtained products that have a poor solubility in the used solvent. If the solubility of the ligands is poor, the dissolution of the ligands can be promoted by proper heating, and the coordination reaction can also be accelerated. The crystallization of the obtained coordination products is precipitated in the process of cooling [10, 11].

Solvent volatilization method is the most traditional method to synthesize MOFs materials and the principle of this method is that the crystal precipitates from saturated solution by solvent volatilization or decreasing temperature, and slowing down the volatilization rate or cooling is beneficial to the cultivation of perfect crystal form [12]. Specifically, by dissolving the selected organic ligands and metal salts in the appropriate solvent and placing them at rest, waiting for their slow self-assembly to form complex crystals.

1.2.3 Diffusion Method

Diffusion method means that the metal salt organic ligands and solvents are mixed into solution in a certain proportion, put into a small glass bottle that is placed in a large bottle with deproteinized solvent, seal the bottle mouth of the large bottle, and then the crystal can be formed after a period of static setting. Diffusion methods can be divided into gas phase diffusion, liquid layer diffusion, and gel diffusion.

1.2.3.1 Gas Phase Diffusion

The gas phase diffusion method is to dissolve the selected organic ligands and metal salt in the appropriate solvent, and then cause the lazy volatile solvent or volatile alkaline substance (for the carboxylic acid ligand containing hydrogen protons) to diffuse into the solution to reduce the solubility of the obtained complex product or speed up the coordination reaction, so that the complex precipitates in the form of crystallization. The volatilization rate of volatile solvents or alkaline substances in gas phase diffusion method will affect the nucleation speed of the complexes, and then affect the quality of precipitated crystals.

1.2.3.2 Liquid Phase Diffusion

The liquid phase diffusion method is to dissolve the selected organic ligands and metal salt in different solvents, and then put the seed solution on top of the other solution, or add another solvent to the interface of the two layers of solution that can slow down the diffusion rate. The reactant diffuses slowly and reacts in the solvent, and the reaction product precipitates in the form of crystal. The diffusion rate of reactants in liquid phase diffusion method will...