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ScienceWeek
CHEMISTRY: ON MULTIPHASIC CATALYTIC REACTIONS
The following points are made by J. Kobayashi et al (Science 2004 304:1305):
1) Multiphase catalytic reactions play important roles not only in the research laboratory but also in the chemical and pharmaceutical industries (1). They are classified according to the phases involved, such as gas-liquid, gas-liquid-liquid, or gas-liquid-solid reactions. Although numerous multiphase catalytic reactions are known and many are used in industry, these reactions are still difficult to conduct when compared to homogeneous reactions, because the efficiency of interaction and mass transfer between different phases is extremely low, and thus in most cases the reaction rates are slow. In general, to accelerate multiphase catalytic reactions, some treatment producing high interfacial area between the two or three reacting phases, such as vigorous stirring or additional equipment, is needed, and the development of more effective, simple devices that can produce such a high interfacial area between different phases is a much-sought-after goal.
2) To achieve efficient multiphase catalytic reactions, the authors focused on a new device, which has a very small channel (nanometer- to micrometer-sized in width and depth and centimeter- to meter-sized in length) in a glass plate (2-5). A similar device, the so-called "microchannel reactor", is used mainly in the field of analytical chemistry. The device has a very large specific interfacial area per unit volume. In concrete figures, this area rises to 10,000 ~ 50,000 m^(2)/m^(3), as opposed to only 100 m^(2)/m^(3) for conventional reactors used in chemical processes. The idea is to immobilize a solid catalyst on the wall of the microchannel and then to flow liquid and gas materials into the channel. Provided that the flow is well controlled, it should be possible to pass the gas through the center of the channel and the liquid along the inner surface of the channel at a particular gas pressure. In this system, efficient gas-liquid-solid reactions might occur, since effective interaction of the three phases is expected because of the extremely large interfacial areas and the short path required for molecular diffusion in the very narrow channel space.
3) In summary: The authors report they have developed an efficient system for triphase reactions using a microchannel reactor. Using this system, the authors conducted hydrogenation reactions that proceeded smoothly to afford the desired products quantitatively within 2 minutes for a variety of substrates. The system could also be applied to deprotection reactions. The authors could achieve an effective interaction between hydrogen, substrates and a palladium catalyst using extremely large interfacial areas and the short path required for molecular diffusion in the very narrow channel space. The authors suggest this concept could be extended to other multiphase reactions that use gas-phase reagents such as oxygen and carbon dioxide.
References (abridged):
1. P. L Mills, R. V. Chaudhari, Catal. Today 37, 367 (1997)
2. P. D. I. Fletcher et al., Tetrahedron 58, 4735 (2002)
3. K. Joehnisch, V. Hessel, H. L÷we, M. Baerns, Angew. Chem. Int. Ed. Engl. 43, 406 (2004)
4. C. Bellefon, N. Tanchoux, S. Caravieilhes, P. Grenouillet, V. Hessel, Angew. Chem. Int. Ed. Engl. 39, 3442 (2000)
5. H. Hisamoto, T. Saito, M. Tokeshi, A. Hibara, T. Kitamori, Chem. Commun. 2001, 2662 (2001)
Science http://www.sciencemag.org
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Related Material:
ON HETEROGENEOUS CATALYSIS AND SURFACE SCIENCE
The following points are made by G. Ertl and H-J. Freund (Physics Today 1999 January):
1) The economic significance of heterogeneous catalysis is reflected in the fact that the world market for solid catalysts in the automotive, petroleum, and other industries is of the order of US$100 billion per year and growing rapidly.
2) In heterogeneous catalysis, the chemical transformation typically occurs in a flow reactor through which the reacting species pass. Atoms in the surface of the catalyst may form chemical bonds with atoms in impinging molecules, a phenomenon known as "chemisorption". If existing bonds in the impinging molecule break, the process is known as "dissociative chemisorption". The chemisorbed species are mobile on the surface and may bond to other particles, thus leading to new molecules, which eventually leave the surface (desorb) as the desired reaction products.
3) Detailed identification and characterization of these elementary processes of heterogeneous catalysis are hampered by several fundamental problems: a) The reacting systems exist merely as 2-dimensional phases for which most of the usual methods of investigation are not well suited. b) The surfaces of real catalysts are typically inhomogeneous as a result of methods to increase catalytic efficiency. For example, because in heterogeneous catalysis efficiency, in general, increases with total surface area of the solid catalyst, finely divided particles are usually applied to a support material which is only relatively inert. Also, catalytic activity is often further enhanced by the addition of compounds called "promoters". At the present time, analysis of the fundamentals of heterogeneous catalysis is largely dependent on the use of surface science models, real but simple systems such as single crystal surfaces whose structure may be varied by choosing different surface orientations.
Physics Today http://www.physicstoday.org
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Related Material:
ON CATALYSIS AND WILHELM OSTWALD
In the final quarter of the 19th century, Germany was leading the world in the study of the physical changes associated with chemical reactions. The outstanding worker in this field of physical chemistry was the Russian-German chemist Friedrich Wilhelm Ostwald (1853-1932). It was thanks to him, more than to any other individual, that physical chemistry came to be recognized as a discipline in its own right. By 1887, he had written the first textbook on the subject and founded the first journal to be devoted exclusively to the field.
Fittingly enough, Ostwald was among the first Europeans to discover and appreciate the work of Josiah Willard Gibbs (1839-1903). Ostwald translated Gibbs's papers on chemical thermodynamics into German in 1892. Ostwald proceeded to put Gibbs's theories to use almost at once in connection with the phenomenon of catalysis.
Catalysis (a word suggested in 1835 by J.J. Berzelius [1779-1848]) is a process whereby the rate of a particular chemical reaction is hastened, sometimes enormously so, by the presence of small quantities of a substance which does not itself seem to take part in the reaction. Thus, powdered platinum will catalyze the addition of hydrogen to oxygen and to a variety of organic compounds, as Humphry Davy (1778-1829) (the isolator of sodium and potassium) discovered in 1816. Again, acid will catalyze the breakdown to simpler units of a number of organic compounds, as G.S. Kirchhoff (1824-1887) first showed in 1812. At the conclusion of the reaction, the platinum or the acid is still present in its original quantity.
Ostwald prepared, in 1894, a summary of someone else's paper on the heat of combustion of foods, this summary to appear in his own journal. He disagreed strongly with the conclusions of the writer, and to buttress his disagreement discussed catalysis. He pointed out that the theories of Gibbs made it necessary to assume that catalysts hastened reactions without altering the energy relationships of the substances involved. The catalyst, he maintained, must combine with the reacting substance to form an intermediate that breaks up to give the final products. The breakup of the intermediate released the catalyst, which thus resumed its original form.
Without the presence of this catalyst-combined intermediate, the reaction would proceed much more slowly, sometimes so slowly as to be imperceptible. Hence, the effect of the catalyst was to hasten the reaction without itself being consumed. Furthermore, since a molecule of catalyst was used over and over, a small quantity of catalyst was sufficient to hasten a great deal of reaction.
This view of catalysis is still held today. It has helped to explain the activity of the protein catalysts ("enzymes") which control the chemical reactions in living tissue.
Adapted from: Isaac Azimov: A Short History of Chemistry. Doubleday 1965, p.155.
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