Kimoon Kim talks with
ScienceWatch.com and answers a few questions about this month's
Emerging Research Front Paper in the field of Chemistry. The
author has also sent along images of their work.
Article: Rigid and flexible: A highly porous
metal-organic framework with unusual guest-dependent dynamic
behavior
Authors: Dybtsev, DN;Chun, H;Kim, K
Journal: ANGEW CHEM INT ED, 43 (38): 5033-5036 2004
Pohang Univ Sci & Technol, Natl Creat Res Initiat Ctr Smart
Supramol, San 31 Hyojadong, Pohang 790784, South Korea.
Pohang Univ Sci & Technol, Natl Creat Res Initiat Ctr Smart
Supramol, Pohang 790784, South Korea.
Pohang Univ Sci & Technol, Dept Chem, Div Mol & Life
Sci, Pohang 790784, South Korea.
Russian Acad Sci, Inst Inorgan Chem, Novosibirsk 630090,
Russia.
Why do you think your paper is highly
cited?
Metal-organic frameworks (MOFs) are an emerging field with a rapid growth
of the number of publications about this discipline in recent years. MOFs
have drawn special attention because of their potential in many areas
including separation, catalysis, and gas storage, particularly hydrogen for
their applications in fuel cells. In our paper "Rigid and flexible: A
highly porous metal-organic framework with unusual guest-dependent dynamic
behavior," as published in Angewandte Chemie in 2004, we reported
on the rigid nanoporous framework, Zn2(bdc)2(dabco)
(Figure 1), with the highest hydrogen storage capacity at that time.
Another striking feature of the reported framework is its unusual
guest-induced structural changes—the framework expands upon guest
release and shrinks upon guest uptake, a rare phenomenon in this class of
materials. The unique combination of such conflicting properties such as
rigidity and flexibility in the framework makes our results remarkable.
Therefore, researchers working on either gas storage or flexible frameworks
often cite our paper as one of the first and representative milestones in
this area.
Does it describe a new discovery, methodology, or
synthesis of knowledge?
In our paper, we described the facile synthesis of
Zn2(bdc)2(dabco), which has a rigid framework and
permanent porosity, from low-cost chemicals in a one-pot reaction. What
surprised us at that time was that the material exhibited exceptionally
high surface area and hydrogen storage capacity compared with conventional
porous materials such as zeolites and activated carbons.
Crystal structure of
Zn2(bdc)2(dabco) with a
rigid and flexible framework.
First homochiral
MOF, POST-1, demonstrating
enantioselective sorption and
catalysis.
Another surprising discovery was the unusual flexibility of the framework
during guest exchange. In particular, unlike conventional porous materials,
the framework of this material shrinks upon an uptake of guest molecules.
These structural changes depend on the nature of guest molecules, which
suggested its potential applications in sensor.
Would you summarize the significance of your paper in
layman's terms?
In our daily lives, everyone uses porous materials—for example, a
sponge to remove moisture from a kitchen table. In our paper we described
the synthesis of a nanoporous material, where the pores are so small that
it can absorb even small single molecules in a similar way. Therefore, with
the help of such a "nanosponge," one can do cleaning on a molecular level!
Another unique property of such nanoporous materials is their ability to
absorb a large amount of volatile gases. We all recognize the importance of
hydrogen as a green fuel which should replace conventional fossil fuels in
the not-too-distant future. One of the biggest obstacles to charge cars
with hydrogen today is the very low capacity of hydrogen fuel tanks. For
example, a regular 60 liter car tank can store about 500 grams of
compressed hydrogen, which is enough only for a 50 km drive.
The same fuel tank made of our nanoporous material, however, is able to
store several times more hydrogen under the same conditions. Although
today’s hydrogen storage capacity of nanoporous materials still does
not meet the US DOE's guideline, the use of such "nanosponges" is one of
the promising ways to achieve hydrogen-powered cars in the future.
How did you become involved in this research and were
any particular problems encountered along the way?
Design, synthesis, and applications of metal-organic porous materials have
been one of the main research interests of our group at POSTECH. We
successfully synthesized the first homochiral MOF, named as POST-1 (Figure
2), and demonstrated its enantioselective sorption and catalytic properties
as reported in Nature in 2000.
Although the potential of MOFs in gas storage, selective sorption,
separation, catalysis, etc., had been well demonstrated, one of the
challenging issues in MOFs at that time was the synthesis of stable
frameworks with permanent porosity upon removal of solvent molecules
trapped in the pores.
To tackle this problem we used simple rigid organic linkers such as
terephthalate, dabco, and formate, which allow us to synthesize a number of
stable porous frameworks. Interestingly, these stable frameworks
synthesized from rigid organic building units showed extraordinary gas
sorption properties compared with conventional porous materials. These
exciting results from our lab have been published in premier journals such
as Nature, Angewandte Chemie, Journal of American Chemical
Society, and Chemical Communications over the past 10 years.
Where do you see your research leading in the
future?
Shortly after our publication, other researchers took advantage of the
extraordinary guest sorption properties and facile synthesis of
Zn2(bdc)2(dabco). For example, Professor Susumu
Kitagawa (University of Kyoto, Japan), used this material to synthesize
polymers inside channels with a better control of molecular weight. Such
host-assisted polymers were shown to have a more regular length and
structure. As mentioned above, this work demonstrated the potential of MOFs
as a hydrogen storage material, which prompted many researchers to seek
MOFs with higher hydrogen sorption capacity.
Although MOFs have not met the guideline set by DOE in terms of hydrogen
sorption capacity, our efforts in searching for MOFs with higher hydrogen
sorption capacity still continue. Another application of MOFs being
explored is CO2 capture and storage to solve an important
environmental issue. We are also actively working on other challenging
projects such as the synthesis of well-defined metal clusters inside the
pores of MOFs and growth control of MOFs on surfaces for memory devices.
Do you foresee any social or political implications for
your research?
The field of MOFs has undergone explosive developments over the past
decade. With emerging properties and functions, MOFs have already shown
great promise in many applications including gas storage, separation and
catalysis. The successful applications of MOFs in hydrogen storage and
carbon dioxide capture would contribute greatly to a green and sustainable
society. Also, MOFs' usefulness for separation and catalysis would make a
great impact on the fine chemical and pharmaceutical industries. These are
only a few examples. Other applications of MOFs are yet to be explored and
their social and/or economical implications may be beyond our present
imagination.
Kimoon Kim, Ph.D.
National Creative Research Initiative Center for Smart Supramolecules
(CSS)
Department of Chemistry and Division of Advanced Materials Science
Pohang University of Science and Technology (POSTECH)
Pohang, Republic of Korea Web
Why do you think your paper is highly
cited?
