Success in developing a novel environmentally friendly porous material capable of storing large amounts of hydrogen and carbon dioxide



The “Mirai Theme Project” laboratory, as part of a Rikkyo University research project* endowed by Nippon Soda Co., Ltd.**, has succeeded in developing Trp-MOF, materials capable of adsorbing large amounts of hydrogen and the greenhouse gas carbon dioxide (Figure 1). The research group, whose members include Rikkyo University Department of Chemistry Professor Mao Minoura, Specially Appointed Associate Professor Koh Sugamata, Visiting Professor Akihiro Shirai, and Visiting Professor Natsuki Amanokura, has been researching novel environmentally friendly molecules. This material could be used in small “molecular cylinders” potentially capable of replacing typical large gas cylinders. This discovery is the fruit of industry-academia collaboration between Nippon Soda and Rikkyo University and was published to wide acclaim in the academic journal “Chemistry - A European Journal,” with the molecule featured on the cover.

*) Rikkyo University Endowed Research Project
Rikkyo University established a system of endowed research projects with the hope of facilitating collaboration between industry and academia. Since 2017, the university has carried out projects endowed by corporations with the goal of contributing to society by advancing and revitalizing research and education and by collaborating on technological issues.

**) Nippon Soda Co., Ltd.
A general chemical company (headed by President Eiji Aga) that celebrated its 100th anniversary in 2020. Under its new slogan “Brilliance through chemistry,” Nippon Soda manufactures and markets high value-added chemical products for industries such as agriculture, pharmaceuticals, and electronics.

Figure 1. Overview of the study

1. Main findings

Efficiently storing gas in regularly arranged hexagonal “honeycombs”

1) Use of a porous metal-organic framework (MOF) enabled the team to develop materials capable of adsorbing large amounts of hydrogen, a clean energy source, and carbon dioxide, a greenhouse gas.
2) Trp-MOFs, characterized by their honeycomb-like structure, exhibit an extraordinary capacity for the efficient storage of carbon dioxide and hydrogen.
3) The material demonstrated impressive thermal stability, remaining undecomposed even at temperatures exceeding 400 °C.

Metal-organic frameworks (MOFs)*1) have extremely small pores called micropores*2) that have much higher specific surface area*3) than conventional porous materials such as activated carbon and zeolite*4). This feature raises the possibility of using MOFs for gas adsorption and separation. The research group synthesized triptycene-2,3,6,7,14,15-hexacarboxylic acid and its derivatives as organic ligands and reacted them with zinc nitrate to create an MOFs with excellent carbon dioxide and hydrogen storage capacity (Figure 1). The use of a rigid and thermally stable molecule called triptycene as the organic ligand endowed the MOFs with impressive thermal stability. Further, functionalization of the triptycene ligand with an alkyl group or halogen further improved gas adsorption.

2. Study background

Figure 2. Synthesis and structure of metal-organic frameworks

Carbon dioxide is considered the most important of the greenhouse gases blamed for global warming. The world needs technology capable of separating and recovering carbon dioxide from the gasses emitted by factories and other CO2 producers. However, the separation membranes currently used for chemical adsorption in factories require large amounts of energy for recovering carbon dioxide. On the other hand, using physical adsorption for carbon dioxide separation and recovery would require almost no energy. In addition, hydrogen is currently being investigated as a clean energy source that does not generate greenhouse gases, though difficult challenges remain regarding how to safely store and transport hydrogen. In particular, there is a great need for hydrogen storage materials that are inexpensive, lightweight, and safe enough to be integrated into machines and devices that run on hydrogen, such as fuel-cell vehicles. Conventionally, metal hydrides such as NaAlH4 and LiAlH4 and metal amides such as LiNH2 have been used for hydrogen storage. However, these substances are highly sensitive to moisture and require extreme conditions to be able to generate hydrogen gas, which limits the environments in which they can be used.

Porous metal-organic frameworks (MOFs) have attracted much attention recently for their potential role as highly efficient adsorbent materials for these gases (Figure 2). MOFs have sponge-like properties that allow them to physically adsorb gases, allowing them to easily adsorb or desorb gases simply by changing the pressure or temperature of the environment. Thus, it is hoped they could be used as separation membranes or safe gas storage materials.

3. Study findings

Figure 3. Structure of triptycene ligand and MOF

Triptycene possesses a rigid skeleton with high thermal stability and has long been widely used in the fields of organic and supramolecular materials. Because the free spaces created by the benzene rings that make up triptycene tend to form pores (Figure 3 left), MOFs that use triptycene as an organic ligand have also been reported to have good thermal and water stability, with high specific surface area. In addition, data has shown that the free spaces created by triptycene interact to some degree with hydrogen molecules, which suggests it may be possible to develop hydrogen storage materials using triptycene. Therefore, the research group synthesized triptycene ligands and used them to develop MOFs, then measured their hydrogen storage capacity. They synthesized organic ligands with 6 coordination sites on the periphery of triptycene, as well as a Trp ligands and extended PET ligands. These were reacted with zinc nitrate hexahydrate to create a Trp-MOFs and PET-MOFs (Figure 3).
Single-crystal X-ray diffraction revealed the MOFs had isomorphic structures with a difference in pore size of only 5 Å. Their structures consisted of alternating layers of honeycomb-like pores. In nitrogen adsorption experiments at 77 K, a 3-fold difference in adsorption was observed between the Trp-MOF and PET-MOF, but under the same conditions the hydrogen storage amounts were similar (Figure 3). The fact that hydrogen adsorption was about the same despite the 3-fold difference in nitrogen adsorption indicates that the relatively small pores created by Trp-MOF are effective at adsorbing hydrogen (Figure 4).

Figure 4. Nitrogen, hydrogen, carbon dioxide adsorption experiments

To investigate the effects of introducing functional groups on gas adsorption, bromo groups were introduced at the triptycene bridgehead position of each ligand, and the same methods were used to synthesize Trp-MOF-Br and PET- MOF-Br. Nitrogen adsorption experiments showed decreases for both MOFs, but while hydrogen storage was higher with Trp-MOF-Br compared to Trp-MOF, this was not observed with PET-MOF. They believe that the small pore size of Trp-MOF makes it more sensitive to the introduction of functional groups, which increased hydrogen storage capacity.

Carbon dioxide adsorption experiments at room temperature were also conducted on each of the MOFs. Trp-MOF-Br and Trp-MOF both exhibited excellent carbon dioxide adsorption, about 3 times higher than PET-MOF, which has larger pores. Further, because of the excellent thermal stability of triptycene, both Trp-MOF and PET-MOF had extremely high thermal stability, remaining undecomposed even at 400 °C, demonstrating that these MOFs could be used even under harsh conditions.

4. Social contribution, ripple effects

Carbon dioxide adsorbents can be used to purify exhaust gas from factories and automobiles. The materials developed in this study, which can be synthesized using simple methods and easily available materials, are capable of adsorbing large amounts of carbon dioxide. Thus, they may be capable of serving as "environmental cleaners". They also are excellent at storing hydrogen, which indicates they can be used as an inexpensive, safe, and lightweight storage material that will help in developing a hydrogen-based society.

5. Future developments

The researchers in this study succeeded in synthesizing MOFs that can store large amounts of carbon dioxide and hydrogen thanks to the free spaces created by triptycene. Gas adsorption requires pores that are the right size for the type of gas being adsorbed. This study showed that rigid triptycene molecules can be used to create pores of various sizes. Going forward, they plan to attempt to further increase the gas storage capacity by changing the metal varieties used and making other chemical modifications.

As part of the “Mirai Theme Research Project,” an industry-academia partnership under Rikkyo University's Faculty of Science, the research group selected and synthesized target molecules, synthesized MOFs, evaluated their gas adsorption abilities, and verified the MOFs molecular design through crystal structure analysis. The project structure allowed for rapid decision-making, implementation, verification, and feedback. The aim of this industry-academia partnership is to create environmentally friendly molecules, to use the power of chemistry to develop materials to resolve the difficult problem of storing substances such as hydrogen, and to put these findings into practice in society as part of the shift to clean energy.

6. Glossary

*1) Metal-Organic Framework (MOF): A general term for coordination polymers that are composed of organic ligands and metal ions.
*2) Micropores: A type of tiny pore that exists in porous materials with a diameter of 2 nm or less. Pores that are 2-50 nm are called mesopores, and those 50 nm or larger are called macropores.
*3) Specific surface area: The surface area (m2) per unit mass (g) of a substance.
*4) Zeolite: A general term for crystalline aluminosilicate. The substance was discovered in nature and has since been synthesized with various pore sizes and shapes.

7. Article information

Zn-based Metal-Organic Frameworks Using Triptycene Hexacarboxylate Ligands: Synthesis, Structure, and Gas-Sorption Properties
Koh Sugamata, Shoko Yamada, Daichi Yanagisawa, Natsuki Amanokura, Akihiro Shirai, and Mao Minoura
Chemistry - A European Journal

8. Reference

J. L. C. Rowsell, A. R. Millward, K. S. Park and O. M. Yaghi, J. Am. Chem. Soc., 2004, 126, 5666–5667.

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