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The top image shows an MRI signal from thermally
polarized propylene, and the bottom image shows the signal
obtained with parahydrogen- polarized propylene. The
signal-to-noise ratio (SNR) of the bottom image is a factor of 300
larger than that of the thermally polarized propylene in the top
image.
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�This is the first time hyperpolarized gas has
been used to directly study catalytic reaction products on such a
small scale and without the use of tracer particles or gas,� says
Bouchard. �It opens the door for future studies of heterogeneous
catalysis in which all the unique benefits of MRI, such as velocimetry
and spatially dependent quantities, are available.�
Adds Burt, �Furthermore, our results indicate that our approach to
using parahydrogen can be extended to other chemical reactions beyond
hydrogenation, which significantly broadens the impact and potential
use of our technique.�
Pines, Bouchard and Burt are the co-authors of a paper published in
the January 25, 2008 edition of the journal Science, describing this
research. The paper is entitled: �NMR Imaging of Catalytic
Hydrogenation in Microreactors with the Use of para-Hydrogen.� Other
co-authors of this paper were Sabieh Anwar, who is a former member of
Pines� research group now at Lahore University, Pakistan, and Kirill
Kovtunov and Igor Koptyug, from the International Tomography Center,
Novosibirsk, Russia, who are experts in catalysis and the use of MRI
to study catalytic processes.
Commenting on the Science paper, Jeffrey Reimer, who chairs the UC
Berkeley Chemical Engineering Department, said, �The spatial and
temporal distribution of reactants and products in heterogeneous
systems has not been visited by researchers in recent years owing to
the lack of quantitative measures in situ. So while the sophistication
of mathematical modeling of such systems proceeds at the rate at which
computational power increases, the relevance of such models is dubious
since they cannot be compared with measurements other than bulk
properties of temperature, conversion, etc. The methods and data
presented in this paper portend a new era of measurement, modeling,
and design for more efficient reactors.�
Since nearly all manufacturing processes that involve chemistry start
with a catalytic reaction, there is a premium on the design of new and
better catalysts and catalytic reactors. This is especially true for
the growing field of microfluidic chip technology. MRI and nuclear
magnetic resonance (NMR), its sister technology, are among the most
powerful analytic tools known to science and could be immensely
valuable for characterizing catalytic reactors and reactions in
microfluidic devices. However, the low sensitivity of conventional
MRI/NMR techniques has limited their applicability to microscale
catalysis research.
For the results reported in their Science paper, Pines, Bouchard and
Burt were able to overcome the inherent low sensitivity of MRI/NMR
through the use of parahydrogen.
MRI/NMR signals are made possible by a property found in the atomic
nuclei of almost all molecules called �spin,� which gives rise to a
magnetic moment, meaning the nuclei act as if they were bar magnets
with a north and south pole. Obtaining an MRI/NMR signal depends upon
an excess of nuclei in a sample with spins pointing in one direction
or the other.
At standard temperature and pressure, hydrogen gas exists in one of
two molecular forms � ortho and para � with the former making up about
75-percent of the mixture. Both molecular forms are diatomic, but in
orthohydrogen, the spins of the two protons in the nuclei are pointed
in the same direction, whereas in parahydrogen, the spins of the two
protons point in opposite directions. By increasing the fraction of
parahydrogen in the gas mixture there is a net excess in the para spin
state even at room temperature and in the complete absence of a
magnetic field. Under the right conditions, this hyperpolarization can
be passed on to other nuclei and used to substantially boost the
strength of their MRI/NMR signals by several orders of magnitude.
Pines, Bouchard and Burt have found a way to use parahydrogen enhanced
gas in combination with propylene gas and a heterogenized catalyst to
achieve a strong MRI/NMR signal from samples in the gas-phase,
something that has only been done before using hyperpolarized noble
gases and expensive polarization equipment. A mixture of propylene and
parahydrogen enriched gas (about 40-percent parahydrogen) is flowed
through a reactor cell containing a catalyst (Wilkinson�s catalyst)
that�s been immobilized on a modified silica gel. As the parahydrogen
enhanced gas mixture passes over the catalyst, hydrogenation takes
place. This produces spin polarized propane gas that is transferred to
an MRI/NMR magnet. The catalyst-free hyperpolarized propane gas can
then be used to enhance MRI/NMR signals.
�The enhanced MRI/NMR sensitivity provided by parahydrogen induced
polarization allows us to overcome the inherent problem of low
sensitivity in thermally polarized gas-phase MRI/NMR,� says Bouchard.
�This is the reason we are able to get such high-spatial resolution
MRI images in the gas phase. Using conventional thermally-polarized
MRI/NMR signals, this would be an impossible task.�
It has been a persistent challenge for scientists and engineers who
study catalysts to correlate active catalytic regions with overall
morphology in heterogeneous catalyst beds. It has also been a
challenge to monitor the multistep reactions that take place within
the beds. This has hampered the design of catalytic reactors that give
optimal performances.
Says Burt, �Our MRI/NMR technique provides the ability to directly
measure the spatial dependence of conversion and allows one to do a
reality check on any simulations or assumptions used to design a
catalytic reactor. Design can therefore become an iterative process
that converges on an optimal performance.�
The costs of researching and developing new catalysts can be very
expensive, and the parahydrogen-enhanced MRI/NMR technique developed
by Pines, Bouchard, Burt and their collaborators has the potential to
significantly reduce these costs, as well as substantially speed up
the process.
Not only does it allow future studies of potential catalysts to be
carried out on a smaller and more economical scale, it is also
well-suited for �green chemistry,� the new approach that seeks to
maximize productivity and yield while minimizing costs, amounts of
reactants and waste products.
Pines, Bouchard and Burt say their technique is ready to be used in
the study of hydrogenation reactions now. In the future, they would
like to extend its applications beyond hydrogenation to study other
types of catalysts and chemical reactions.
Says Bouchard, �We also have new ideas on how to get high-resolution
temperature and pressure maps of the catalyst bed that will convey
information about the energetics of the chemical reaction and
mechanics of fluid transport during the reaction.
Says Burt, �This would be very exciting as there are few existing
techniques that provide such information apart from simulations. And
for microreactors, there is simply no competing method for studying
such gas-phase reactions at this level of detail and spatial
resolution.�
This work was supported by the Director, Office of Science, Office of
Basic Energy Sciences, Materials Sciences and Engineering Division of
the U.S. Department of Energy.
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