Catalyst – solid sorbent – or support therefor: product or process – To be used as a melt
Reexamination Certificate
1998-07-29
2001-04-17
Wood, Elizabeth D. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
To be used as a melt
C502S250000, C502S262000, C502S303000, C502S304000, C502S313000, C502S328000, C502S325000, C502S329000, C502S332000, C502S334000, C502S339000, C502S527240
Reexamination Certificate
active
06218326
ABSTRACT:
BACKGROUND OF THE INVENTION
Heterogeneous catalysts are of central importance in the fossil fuel refining, and synthesis of alternative fuels, chemicals, and fine chemicals, as well as in environmental applications, e.g., automotive exhaust treatment, and resource and energy conservation (Campbell, I. M.,
Catalysis at Surfaces,
Chapman and Hall, 1988; National Research Council,
Catalysis Looks to the Future,
National Academy Press, Washington, D.C., 1992; Ribeiro, F. H., and Somorjai, G. A., “Heterogeneous Catalysis by Metals”, in King, R. B., ed.
Encyclopedia of Inorganic Chemistry,
Vol. 3, p. 1359, John Wiley, New York, 1994). It has been estimated that the value of goods processed through catalysis is roughly 1 trillion dollars, or about 20% of the gross national product. However, there still are a number of challenges that remain to be addressed in the design of more efficient and cheaper catalysts. The functions of a catalyst include facilitating reactions under milder conditions, increasing selectivity of the desired reactions, and providing a large interfacial area to reduce the precious metal usage and reactor volume. The conventional heterogeneous catalysts are based on microcrystallites of transition metals supported on porous supports and frequently involve precious metals such as Pt, Pd, Rh, Ir, Ru, etc. These transition metal catalysts presumably act by providing degenerate electronic states involving unfilled d-electron orbitals and can, thus, bind with a variety of species by readily donating or accepting electrons (Somorjai, G. A., “The Building of Catalysts. A Molecular Surface Science Approach”, in Hegedus, et al., eds.,
Catalyst Design. Progress and Perspectives,
John Wiley, New York, 1987; Somorjai, G. A.,
Introduction to Surface Chemistry and Catalysis,
John Wiley, New York, 1994; Masel, R. I.,
Principles of Adsorption and Reaction on Solid Surfaces,
John Wiley, New York, 1996). Further, the common reactant gases such as H
2
, O
2
, N
2
and CO are atomized by these metals for supply to the reacting species, the bond strength of H, C, N, and O to the metal providing the thermodynamic driving force for the atomization. The best catalysts are those that form bonds of intermediate strength with adsorbates. If the binding is too strong, further reaction is discouraged. On the other hand, if the binding is too weak, reaction rates are too low.
The catalysts also provide the surface geometry and configuration conducive to reaction. Thus, different reactions require site ensembles of different size and are classified as “structure sensitive” or “structure insensitive”. Thus, besides altering the electronic characteristics, alloying of a metal with another reduces the multiple atom ensembles, thus altering rates of structure sensitive reactions, and hence selectivity. Further, the supported microcrystallites of solid metals possess different crystal faces as well as steps and kinks on the surface, thus providing a variety of catalytic sites of diverse electronic properties and coordination. As a result, heterogeneous catalysts often have a limited selectivity to the desired product and resistance to poisoning and deactivation, resulting in a substantial waste of chemicals and energy.
On the other hand, homogeneous catalysts (liquid phase) are promising because of their higher catalyst activity and product selectivity, milder operating conditions and better control of the nature of catalytic species. The commercial importance of such catalysts has been established by the success of the Wacker process for acetaldehyde and the oxo process, as well as the so-called Monsanto process for acetic acid. It has been found that homogeneous catalysts consisting of transition metal complexes catalyze a variety of reactions with greater selectivity and activity than the conventional solid catalysts. However, the applications of homogeneously catalyzed reactions in industry are still quite limited due to some engineering problems associated with them. Separation of the homogeneous catalysts from the reaction mixture is often difficult, and high catalyst recovery must be achieved. Lack of effective catalyst utilization in conventional gas-sparged reactor, corrosion problems and catalyst contamination in the product are among the other disadvantages of homogeneous catalysis. Many of the disadvantages of heterogeneous catalysts can be eliminated, while retaining many of the advantageous features of homogeneous catalysts through the process of “heterogenizing” the catalysts by dispersing them in the pores of a solid support or by binding them to a polymer substrate.
In recent years, there have been numerous attempts to “heterogenize” some homogeneous catalyst so that the resulting catalysts would have the advantages of both homogeneous and heterogeneous catalysts. Some of these techniques include Polymer Bound Catalysts, Immobilized Enzyme System, Supported Liquid Phase Catalysis (SLPC), Supported Aqueous-Phase Catalysis (SAPC), Supported Aqueous-Phase Enzymatic Catalysis (SAPEC), and Supported Molten Salt Catalysis (SMSC). None of these, however, have so far met with great success. Supported molten metal catalyst technique is based on the concept of physically heterogenizing homogeneous catalyst.
Some of the drawbacks of traditional solid metal catalysts, such as sintering, limited selectivity, and coking may be addressed by the use of molten metal catalysts, which are functionally like heterogeneous solid catalysts in that catalysis proceeds on the surface of the molten metal through chemisorption and surface reaction but, unlike heterogeneous solid catalysts, have uniform and fluid surface sites. Molten metal catalysis also differs from traditional homogeneous liquid-phase catalysis, in which gaseous reactants dissolve in the liquid phase and undergo bulk liquid-phase reaction.
The catalytic properties of liquid metals and alloys have been studied by many investigators since the turn of this century. Ipatiew, W.,
Ber.,
34, 1047,1901 found that metallic zinc catalyzed the decomposition of alcohols above its melting point. His work thus showed that molten metals possess catalytic activity. Hartman, R. J., and O. W. Brown, Catalytic Activity of Cadmium,
J.Phys.Chem.,
34,2651,1930 found that molten lead, thallium and bismuth gave high yields of aniline from nitrobenzene. They also reported that cadmium could make an excellent catalyst for the reduction of nitrobenzene to aniline. They observed that the cadmium metal prepared from different cadmium salts had different catalytic activities. Hydrogenation of nitrobenzene showed a maximum at the melting point of Cd metal. They explained these results on the basis of Taylor's concept of active centers, which would presumably disappear upon melting of the metal. Steacie, E. W. R., and E. M. Elkin,
Proc.R.Soc.London,
A142, 457,1933 investigated the catalytic activity of pure zinc above and below its melting point for the thermal decomposition of methyl alcohol. The result showed no sharp decrease or break of the catalytic activity at the melting point of zinc. Adadurow, I. E. and P. D. Didenko,
J.Am.Chem.Soc. ,
57,2718,1935 conducted the catalytic oxidation of ammonia by fusion of metallic tin and silver. For the experiments with tin as catalyst, no abrupt change in the catalytic activity was observed at the melting point of tin (231.8° C.). However, it was theorized that the catalyzing effect was obtained not by metallic tin, but by tin oxide which was formed by the reaction of tin and air. Adadurow and Didenko (1935) also attempted the decomposition of methyl alcohol by zinc chips at 360° C. to 400° C. and found that the zinc chips were covered by zinc oxide layer on their surface. Schwab, G. M. and H. H. Martin,
Z.Elektrochem.,
43,610,1937 examined the role of active centers at the melting point of the catalyst metals. They argued against Steacie's result and showed that pure zinc liquid was inactive, and that only the zinc contaminated by oxygen was active for their reaction. In 1950, Weller and his co-wor
Datta Ravindra
Halasz Istvan
Serban Manuela
Singh Ajeet
University of Iowa Research Foundation
Wood Elizabeth D.
Zarley McKee Thomte Voorhees & Sease
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