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Upload organization:Kyoto University Department of Synthetic Chemistry and Biological Chemistry  Upload date:2012/08/22

Pioneering studies on new porous materials

Susumu Kitagawa
Department of Synthetic Chemistry and Biological Chemistry
Kyoto University


Materials with nanosized spaces, which are well known as porous materials, are abundant in everyday modern life; they are used for gas storage, separation, and catalysis. One of the earliest historical records of a porous material is noted on papyrus; the record indicates activated carbon was used in medical treatments in ancient Egypt (BC.1550). About 3,000 years later in 1753, a new porous material, zeolite, was discovered in natural ores. Zeolite was successfully synthesized in the first half of the 20th century, and has remarkably contributed to industries such as the petroleum industry. The discovery of novel materials whose functions are superior to activated carbon and zeolite would drastically change human life. However, the synthesis of new porous materials remained stagnant until the early 1990s when Prof. Susumu Kitagawa became interested in the field. Based on a revolutionary concept, he successfully developed novel porous materials, called “porous coordination polymers” (PCPs) or “metal–organic frameworks” (MOFs), which are comprised of organic and inorganic materials. In this article, his scientific achievements are described. In recent years, porous materials have been used in diverse applications, including water purification, and are indispensable.

1. Pioneering studies on “chemistry of coordination space”

 Wellness and prosperity are due in part to progress in material and life sciences in which atoms and molecules play central roles. A major theme in 20th century chemistry was the synthesis of molecules from atoms. This work led to the development of supramolecular chemistry, the predecessor to nanoscience. In addition to microscopic building entities, nanoscience contains the unknown world of spaces surrounded and partitioned by atom. However, for nanoscience research to advance, a new group of porous materials along with a simple synthesis had to be discovered.
In the early 1990s, Prof. Susumu Kitagawa noticed that besides the framework entity, the space surrounded and partitioned by atoms and molecules could constitute another scientific world; he defined this as coordination space. Coordination space is where coordination bonds play important roles in the formation of spatial structures and where various physical and chemical properties are exhibited. Through the rational synthesis of porous crystalline metal organic materials, which involve assembling organic molecules (linkers) with coordination bonds (glue), he developed the field of “Chemistry of Coordination Space”.
  The hallmark of Prof. Kitagawa’s work is pioneering studies on the development of new porous materials called “porous coordination polymers” (PCPs) or “metal–organic frameworks” (MOFs), which are hereafter referred to as MOFs. MOFs are applicable to gas storage, separation, ion transport, and heterogeneous catalysis (Fig. 1). Moreover, these materials have produced an extensive class of highly stable crystalline materials with tunable metrics, organic functionality, and porosity as well as added a new category of MOF materials to the conventional classification of inorganic (e.g., zeolites and clays) and carbon materials. Now, the properties of the metal ions and functional organic ligands in MOFs, can be combined almost at will, making MOFs serious alternatives to conventional zeolites and activated carbons.

 Figure 1. Porous frameworks are designed and synthesized from
 metal ions and organic linkers, providing various functions.

 Looking back on the advent of synthetic materials, the development of new materials faced a steep path. Aristotle in the 4th century BC observed “Natura abhorret a vacuo”. Consequently, preserving porosity has been a difficult task even in the nanodomain. Until 1997, it was believed that crystalline compounds with a regular porous structure based on organic molecules were unlike inorganic zeolites and collapsed without guest filling. Kitagawa challenged this notion, and successfully synthesized robust crystalline porous materials from organic molecules; he became the first person in the world to synthesize a novel MOF, which is sufficiently robust to store gases such as carbon dioxide (CO2) and methane (CH4). This was epoch in materials science and technology; he discovered self-assemblies of metal ions, which act as coordination centers, link with various organic bridging ligands under mild conditions to yield tailored nanoporous host materials. These host materials are robust solids with high thermal and mechanical stabilities. Thanks to Prof. Kitagawa’s achievements, many researchers have been drawn to this new field. Today several hundred MOFs have been synthesized, and these materials exhibit sorption properties ranging from energetically and environmentally important gases such as H2, O2, NO, CO2, CH4, to other organic and inorganic compounds. Presently, more than 2,000 articles are published annually on this class of materials (Fig. 2).


 Figure 2.
 Number of published materials for coordination polymers
 and metal organic frameworks in eacn year in the world.

Highlights of Prof. Susumu Kitagawa’s outstanding achievements

(1) First to discover robust MOFs
Early in this field, Prof. Kitagawa noticed the importance of the chemical and physical properties of these architectures and their applications through the porous properties of the frameworks. He used these observations to discover new knowledge about the chemistry and physics of and in the micropores of MOFs. Additionally, he was the first person to synthesize a novel MOF capable of storing supercritical gases such as CH4 at ambient temperature (Angew. Chem. Int. Ed. (1997), cited 530 times). He then proceeded to expand its functions not only for gas storage, but also for separation and catalysis with a higher capacity than conventional materials. He established effective, large-scale synthetic routes to build desired nanosized spaces using self-assembly processes where coordination bonds play key roles in new synthetic technologies. During his studies to control pore surfaces of MOFs, he discovered exciting phenomena based on molecular coagulation, molecular stress, and activation of molecules (Review articles: Angew. Chem. Int. Ed. (2004), cited 3813 times; Chem. Soc. Rev. (2005), cited 590 times).

(2) Design and creation of new conceptual porous materials
Prof. Kitagawa’s research focused on novel porous materials based on the new concept of cooperative integration of “softness” and “regularity.” Proteins and zeolites, which have nanosized cavities, are representatives of extreme softness and regularity, respectively. Prior to Prof. Kitagawa, no one imagined the existence of porous materials simultaneously possessing both of these properties because they seemed contradictory. However, the concept that materials can be both crystalline and plastic or rubber-like is at the heart of a new field of materials science.
In 1998, Prof. Kitagawa classified MOFs into three categories—first, second, and third generations—predicting the presence and importance of flexible porous frameworks (Bull. Chem. Soc. Jpn. (1998), cited 483 times). First-generation materials have frameworks whose porosity collapses irreversibly after guest removal, i.e., porosity is not permanent. Second-generation materials have stable and robust frameworks, which maintain the original porous structures before and after guest sorption. Kitagawa pioneered second-generation MOF materials, which are analogous to zeolites. Third-generation materials are unlike other solid matter (e.g., zeolites, carbons, mesoporous silicates, and metal oxides), and Prof. Kitagawa was the first to notice this new property; he predicted the existence of flexible or dynamic porous frameworks, which reversibly respond to external physical and chemical stimuli. He discovered many flexible crystalline porous frameworks with a specific gate-opening pressure for guest gases (Angew. Chem. Int. Ed. (2002 and 2003) and Chem. Soc. Rev. (2005), cited 312, 495, and 574 times, respectively). In particular, he reported flexible frameworks for supercritical gases such as N2, O2, CH4, and CO2 at ambient temperature. His findings will or have led to applications in gas separation and sensors for air, flue, and biogas. Moreover, he established a new field of porous materials by introducing third-generation materials and developing their rational design and synthesis; his work demonstrates an in-depth understanding of sorption phenomena (a new category of porous materials, “soft porous crystal” is described in a review article, Nat. Chem. (2009)).

(3)Groundbreaking results emanating from the achievements of (1) and (2)
(a) Stable storage of explosive acetylene
Low molecular weight molecules, such as CO2, CH4, C2H2, and alkanes/alkenes/alkynes (C2–C4) are important gases for human life, because they are associated with the global energy and environmental issues. Acetylene (C2H2) is used as a starting material for many chemical and electric functional materials. To obtain highly pure C2H2 for chemical synthesis, C2H2 must be separated from a gaseous mixture containing CO2 as an impurity without a large expenditure of energy. Furthermore, C2H2 is a highly reactive molecule; compression above 0.2 MPa causes it to explode at room temperature even without oxygen. Therefore, more feasible and safer materials for C2H2 separation and storage are required. Prof. Kitagawa compared C2H2 with a very similar molecule, CO2, and attained extremely high levels of selective sorption of C2H2 molecules onto a MOF functionalized surface. His efforts led to stable storage of C2H2 at a density 200 times higher than the safe compression limit of free C2H2 at room temperature (Nature (2005), cited 562 times). The ACS Chemical & Engineering News highlighted this result as “outstanding chemistry in 2005,” with a cover story and picture. Thanks to his discovery, technology is now at the stage of on-demand synthesis of functional pores by tuning the size and shape as well as chemical properties.
(b) Low-dimensional molecular arrays
Due to keen interest in the characteristic magnetic and photo-physical properties of low dimensionality, chemists and physicists have been fascinated with a one-dimensional (1D) regular assembly of dioxygen (O2) molecules, which cannot be realized under normal conditions. One approach to form a 1D specific assembly of O2 molecules is to use a uniform nanosized channel in a microporous compound. Prof. Kitagawa was the first to observe detached O2 molecules in a solid arranged in a 1D ladder structure aligned to the host channel. His work improved the understanding of adsorption phenomena in nanochannels and led to novel nanotechnology (Science (2002), cited 318 times; Nature (2006), News & Views). To date, molecular arrays for other gases, N2, H2, Ar, and CH4, have also been determined.
(c) Opening the door to applications of porous materials in polymerization reactions (Review articles, Chem. Asian J. (2006), cited 57 times; Chem, Soc. Rev. (2009), cited 37 times)
The polymerization of monomers encapsulated within confined and designed nanospaces based on MOFs can yield polymeric materials with desirable structures. Therefore, this can be regarded as a tailor-made polymerization system.
  Since Prof. Kitagawa’s groundbreaking efforts on porous coordination frameworks in the porous materials field, MOFs have been extensively researched in both academia and industry. To date, materials with a much greater porous surface areas (exceeding 5000 m2, which is the size of a football field per gram), which are dramatically larger storage capacities than are available using zeolite or activated carbon. Industrial syntheses are rapidly advancing (currently they are at the barrel-size pilot scale). Unlike many other novel materials (e.g., mesoporous silicas, carbon polymorphs, fullerenes, buckyballs, and carbon nanotubes), the preparation and fabrication of MOF materials do not necessarily require additional capital investments for totally new synthesis technologies. To summarize the significance of Prof. Kitagawa, he is a global leader in the field, and the research that he personally conducted and inspired may lead to rapidly advancing, prosperous, and widespread innovations in materials science from both academic and industrial viewpoints.
  In addition to his original papers, Prof. Kitagawa has published numerous reviews on MOFs, some of which are the most cited reviews in this field to date (Fig. 3).


Figure 3. Number of times Kitagawa’s publications have been cited in the last 20
years. Total Articles in Publication List: 365, Articles With Citation Data: 364,
Sum of the Times Cited: 21046, Average Citations per Article: 57.8, h-index: 66,
Last Updated: March 7, 2012 (Web of Science)

  His accomplishments and contributions have been recognized. He has received various prestigious awards: the Chemical Society of Japan Award (2009), the Humboldt Research Award, Germany (2008), and the Japan Society of Coordination Chemistry Award (2007). Very recently, he was awarded the “2010 Thomson Reuter Citation Laureate Award” for the design and development of porous MOFs and their applications including gas storage, gas purification, and gas separation. He was also awarded the Medal with Purple Ribbon from the Japanese Government.

  In the past decade, Prof. Kitagawa has given over 100 presentations as an invited speaker at international symposia and conferences, most of which were plenary or keynote lectures during the last five years. He was the first to host an international symposium on MOFs (1st International Symposium on Chemistry of Coordination Space (ISCCS 2005, Okazaki, November 14–15, 2005), which has been followed by the 2nd ISCCS2006 (Hakata) and 3rd ISCCS2007 (Awaji), and Pacifichem 2005 (Honolulu, Hawaii; Symposium No.120 on Chemistry and Applications of Metal-Organic Frameworks). These symposia have been succeeded by the MOF conference (MOF2010, Augsburg and MOF2012, Marseille). In addition, Prof. Kitagawa serves as the Chair of the International Organizing Committee of the Eurasia Conference on Chemical Sciences (EuAsC2S 2008 and 2012), enthusiastically promoting advanced scientific research throughout Eurasia. 

  Porous materials developed by Prof. Kitagawa have great potential in applications for our imminent environment as well as a wide variety of fields, such as the global environment, resources, outer space, life, and energy, suggesting that porous materials are extremely valuable both scientifically and industrially. Moreover, he has opened a new scientific discipline, chemistry of coordination space, which involves many academics and industrial professionals. Moreover, Prof. Kitagawa is a highly acclaimed world-leading chemist.


Figure 4. Low molecular weight molecules, such as carbon dioxide (CO2), hydrogen (H2),
oxygen (O2), methane (CH4), acetylene (C2H2), and alkanes (C2–C3), are important
gases for human life because they are associated with the global issues of energy,
natural resources, the environment, and living systems. PCPs and MOFs can
contribute to gas science and technology for the capture, storage, separation, and
conversion of low molecular weight molecules into useful materials without a large
expenditure of energy.