Jul 09, 2020PRESS RELEASE
Development of a novel porous material with selective CO2 adsorption
Keyword:RESEARCH
OBJECTIVE.
A research group at the Mirai Theme Project Laboratory (Smart Molecules Laboratory) that included Rikkyo University Prof. Mao Minoura, Assistant Prof. Koh Sugamata, and Visiting Prof. Teruyuki Iihama of the Department of Chemistry, as part of efforts to create environmentally friendly molecules, developed a novel material that selectively adsorbs the greenhouse gas, CO2 (Figure 1). The laboratory is an endowed Rikkyo University research project* funded by Nippon Soda Co.** This material can also adsorb hydrogen molecules, which are known to be difficult to handle, and could thus be used as a kind of "Molecular Cylinder" for storing hydrogen in fuel-cell vehicles. This is a significant achievement of industry-academia collaboration between Nippon Soda and Rikkyo University that received the high honor of being published in the Royal Chemistry Society's journal "Dalton Transactions."
*) Endowed Rikkyo University research projects
Since 2017, Rikkyo University has been pursuing research projects endowed by companies and other entities with the goal of promoting the development and vitalization of research and education, and contributing to society through technological collaboration. The hope is that endowed research projects will generate the collaborative effects associated with industry-academia partnerships.
**) Nippon Soda Co.
A comprehensive chemical company that celebrated its 100th anniversary in 2020. Akira Ishii is its president. Under a new slogan—"Shining with chemistry"—the company manufactures and markets high value-added chemical products such as agricultural chemicals, pharmaceutical additives, and electronic materials.
Since 2017, Rikkyo University has been pursuing research projects endowed by companies and other entities with the goal of promoting the development and vitalization of research and education, and contributing to society through technological collaboration. The hope is that endowed research projects will generate the collaborative effects associated with industry-academia partnerships.
**) Nippon Soda Co.
A comprehensive chemical company that celebrated its 100th anniversary in 2020. Akira Ishii is its president. Under a new slogan—"Shining with chemistry"—the company manufactures and markets high value-added chemical products such as agricultural chemicals, pharmaceutical additives, and electronic materials.
Figure 1: Molecular structure illustration of the new metal-organic framework (MOF) developed in this study
1. Key points of the study's achievements
1) The research group developed a material that selectively adsorbs the greenhouse gas CO2 using a porous material called a metal-organic framework (MOF).
2) In addition to selective adsorption of CO2, the material can store hydrogen gas, which is a promising clean energy source.
3) The research group succeeded in using hydroxamate groups, which have rarely been used to construct MOF.
Metal-organic frameworks*1) possess extremely small pores called micropores*2) with specific surface area that far exceeds those of conventional porous materials such as activated carbon and zeolite*3). It is expected to be applied to gas adsorption and separation are anticipated. By using benzene-1,4-dicarbohydroxamic acid as an organic ligand and isonicotinic acid as an auxiliary ligand, and reacting with cobalt nitrate, the research group succeeded in developing an MOF that can selectively adsorb carbon dioxide and store large amounts of hydrogen (Figure 1).
In addition, a hydroxamate (RCONHO-), which has rarely been used in MOF synthesis, was used as the coordination site, which raises hopes for future applications of this novel coordination site in MOF.
*1) Metal-organic framework (MOF): A general term for coordination polymers composed of organic ligands and metal ions. Also known as organometallic structures.
*2) Micropores: Tiny pores in porous materials with a diameter of 2 nm or less. Pores that are 2-50 nm are called a mesopores, and those 50 nm or larger are called macropores.
*3) Zeolite: A generic term for crystalline aluminosilicates. Discovered as a natural mineral, they are now synthesized to have various pore sizes and shapes.
2) In addition to selective adsorption of CO2, the material can store hydrogen gas, which is a promising clean energy source.
3) The research group succeeded in using hydroxamate groups, which have rarely been used to construct MOF.
Metal-organic frameworks*1) possess extremely small pores called micropores*2) with specific surface area that far exceeds those of conventional porous materials such as activated carbon and zeolite*3). It is expected to be applied to gas adsorption and separation are anticipated. By using benzene-1,4-dicarbohydroxamic acid as an organic ligand and isonicotinic acid as an auxiliary ligand, and reacting with cobalt nitrate, the research group succeeded in developing an MOF that can selectively adsorb carbon dioxide and store large amounts of hydrogen (Figure 1).
In addition, a hydroxamate (RCONHO-), which has rarely been used in MOF synthesis, was used as the coordination site, which raises hopes for future applications of this novel coordination site in MOF.
*1) Metal-organic framework (MOF): A general term for coordination polymers composed of organic ligands and metal ions. Also known as organometallic structures.
*2) Micropores: Tiny pores in porous materials with a diameter of 2 nm or less. Pores that are 2-50 nm are called a mesopores, and those 50 nm or larger are called macropores.
*3) Zeolite: A generic term for crystalline aluminosilicates. Discovered as a natural mineral, they are now synthesized to have various pore sizes and shapes.
2. Background of the study
Carbon dioxide is the most influential greenhouse gas behind global warming. Technologies are needed to separate and recover CO2 from the exhaust gases emitted from factories and other sources. However, the separation membranes currently used in factories are based on chemical adsorption, and thus require large amounts of energy to recover CO2. Therefore, the world needs CO2 separation and recovery technologies that use physical adsorption, which requires almost no energy for recovery. Moreover, hydrogen has been enthusiastically researched as a clean energy source that does not emit any greenhouse gases. Safe methods for manufacturing, storing, and transporting hydrogen are needed. In particular, there is an urgent need to develop hydrogen storage materials that can be handled safely to create practical machinery and equipment for fueling fuel-cell vehicles and other things with hydrogen gas. Conventionally, hydrogen storage materials have been metal hydrides and metal amides that react violently with moisture, and their usage environments have been limited by the extreme conditions required for hydrogen gas generation.
Figure 2: Synthesis and structure of metal-organic frameworks
Recently, porous organometallic structures, or metal-organic frameworks (MOF), have attracted a great deal of attention as materials that are highly efficient at adsorbing gases (Figure 2). MOF possess the properties of a sponge in that they physically adsorb gases, which makes adsorption and desorption easy just only adjusting the pressure or temperature. Therefore, it is anticipated they will be applied in separation membranes or as safe gas storage materials.
3. Results of the study
Organic ligands often have carboxylates (RCOO-) such as terephthalic acid as their coordination sites. Hydroxamic acid (RCONHOH) is a bioisostere of carboxylic acid (RCOH), but despite their similar properties hydroxamate (RCONHO-) has rarely been used as the coordination site for MOF. This is because hydroxamic acid breaks down under the conditions for synthesizing MOF, turning into carboxylic acid. In this study, the two novel mixed-ligand metal organic frameworks Zn-MOF and Co-MOF generated from benzene-1,4-dihydroxamic acid and isonicotinic acid were successfully synthesized under solvothermal conditions. (Figure 3). Single crystal X-ray structural analysis confirmed that their hydroxamate sites were arranged as metals. Zn-MOF does not contain a solvent-accessible void, while Co-MOF contains a void space that occupies 42% of the unit-cell volume.
Figure 3: Zn-MOF (left) and Co-MOF (right) with hydroxamate sites
Figure 4: Nitrogen and hydrogen adsorption isotherm at 77 K
Gas adsorption measurements of these MOF showed that the Zn-MOF had no gas adsorption ability. Given the moderate, yet appreciable gas-adsorption performance of Co-MOF (502 m2 g-1), hydroxamic-acid-based ligands may serve as useful ligands for MOFs with custom-tailored functionality. Co-MOF displays an uptake capacity of ca. 1 wt% for H2 at 77 K and 1.0 bar, which is comparable to the 1.3 wt% of MOF currently expected to be used as hydrogen gas storage materials [1], which indicates this substance could be a useful hydrogen storage material (Figure 4).
Next, the research group investigated the selectivity of the material's gas adsorption. First, the amount of nitrogen and CO2 adsorption was measured at room temperature. To predict the selectivity for a CO2/N2 binary mixture, ideal adsorbed solution theory (IAST) calculations, coupled with a dual-site Langmuir-Freundlich simulation, were employed on the basis of single-component isotherms. Figure 5 shows the predicated selectivity for CO2/N2 as a function of the pressure when the gas phase mole fraction is 15/85, which is a typical feed composition of flue gas (Figure 5). This showed that the MOF selectively adsorbed 39 times more CO2 than nitrogen at room temperature (Figure 6). This ranks among the best ever reported for selectivity of an MOF, and raises hopes for future applications to CO2 separation membranes.
Next, the research group investigated the selectivity of the material's gas adsorption. First, the amount of nitrogen and CO2 adsorption was measured at room temperature. To predict the selectivity for a CO2/N2 binary mixture, ideal adsorbed solution theory (IAST) calculations, coupled with a dual-site Langmuir-Freundlich simulation, were employed on the basis of single-component isotherms. Figure 5 shows the predicated selectivity for CO2/N2 as a function of the pressure when the gas phase mole fraction is 15/85, which is a typical feed composition of flue gas (Figure 5). This showed that the MOF selectively adsorbed 39 times more CO2 than nitrogen at room temperature (Figure 6). This ranks among the best ever reported for selectivity of an MOF, and raises hopes for future applications to CO2 separation membranes.
Figure 5: Nitrogen and CO2 adsorption with IAST fitting at room temperature
Figure 6: CO2/N2 selectivity at room temperature
4. Social contributions and ripple effects
Substances that adsorb CO2 are hoped to be used in purifying exhaust from factories and automobiles. The material developed in the present study can be synthesized with a simple method using readily available materials, and because of its selective adsorption of CO2, it could be a useful material for cleaning the environment. In addition, this study successfully used hydroxamate groups as coordination sites, which had been difficult to do with MOF. This could have significant ripple effects on MOF synthesis going forward.
5. Future developments
This study developed an MOF by adding isonicotinic acid as an auxiliary ligand to the coordination sites of the metals and organic ligands that strongly influence the structure and properties of MOF. This allowed hydroxamate to be used, which is a rare achievement. Going forward, new MOF with various structures will likely be developed with various auxiliary ligands and by reacting with other metallic salts.
In this future-themed project, a single research group selected and synthesized a target molecule, synthesized an MOF, evaluated its gas adsorption abilities, and studied its molecular design by analyzing the MOF's crystal structure. This framework allowed the group's members to make rapid decisions in the course of the study. Further industry-academia partnerships are planned to create environmentally friendly molecules and apply them to the difficult problem of hydrogen storage, which will give back to society by contributing to the development of clean energy.
In this future-themed project, a single research group selected and synthesized a target molecule, synthesized an MOF, evaluated its gas adsorption abilities, and studied its molecular design by analyzing the MOF's crystal structure. This framework allowed the group's members to make rapid decisions in the course of the study. Further industry-academia partnerships are planned to create environmentally friendly molecules and apply them to the difficult problem of hydrogen storage, which will give back to society by contributing to the development of clean energy.
6. References
[1] J. L. C. Rowsell, A. R. Millward, K. S. Park and O. M. Yaghi, J. Am. Chem. Soc., 2004, 126, 5666–5667.
7. Paper information
Title
Structural Analysis of and Selective CO2 Adsorption in Mixed-Ligand Hydroxamate-based Metal-organic Frameworks
Authors
Koh Sugamata, Chikaze Takagi, Awano Keiko, Teruyuki Iihama, and Mao Minoura
Journal
Dalton Transactions (Royal Society of Chemistry) 2020, 49, 9948-9952.
Structural Analysis of and Selective CO2 Adsorption in Mixed-Ligand Hydroxamate-based Metal-organic Frameworks
Authors
Koh Sugamata, Chikaze Takagi, Awano Keiko, Teruyuki Iihama, and Mao Minoura
Journal
Dalton Transactions (Royal Society of Chemistry) 2020, 49, 9948-9952.