Chemistry of inorganic compounds – Oxygen or compound thereof – Peroxide
Reexamination Certificate
2001-10-04
2004-01-13
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Peroxide
C423S588000, C502S325000, C502S339000, C568S799000, C568S862000, C585S266000, C585S269000, C585S270000, C585S273000, C585S274000, C585S275000, C585S276000, C585S277000
Reexamination Certificate
active
06676919
ABSTRACT:
The present invention relates to a novel process for producing catalysts by electroless deposition of at least one platinum metal on a metallic support, the catalysts obtainable by this process, the use of the catalysts for the synthesis of hydrogen peroxide from the elements, for the hydrogenation of organic compounds and also a process for preparing hydrogen peroxide using these catalysts.
Catalysts comprising platinum metals as catalytically active substances are used in a wide variety of forms and are of great industrial importance, e.g. in the reduction or hydrogenation of organic compounds and in the catalytic purification of waste gases from industry and traffic.
For industrial applications, use is made wherever possible of supported platinum metal catalysts which contain only small amounts of the expensive noble metals on usually catalytically inactive support materials having a large surface area, e.g. carbon, aluminum oxide, silicon oxide, ceramic or other mineral supports.
Catalysts which are particularly easy to handle are ones in which the support can be used in the form of relatively large units, e.g. in the form of granules, beads or, in particular, woven meshes, gauzes or shaped bodies such as monoliths. Such supported catalysts are usually used as fixed-bed catalysts and make it possible to carry out catalytic processes in an economically advantageous continuous manner.
The application of the catalytically active metals to such porous supports is usually achieved by impregnating the support with solutions of salts or organometallic compounds of the catalytically active metal and subsequent immobilization by precipitation, hydrolysis, heat treatment, calcination and/or reduction. This usually necessitates repeated heating of the impregnated catalyst to from 200 to 1200° C. Thus, for example, DE-A-2 317 560 describes the production of a catalytic device by impregnation of a mineral monolith with a melt of trialkylaluminum at about 120° C., treatment with steam at 120° C./18 psi and subsequent firing at 400° C. The procedure is subsequently repeated using tetraalkylzirconium and the oxidic support obtained in this way is subsequently impregnated with a hexachloropalatinate, heated at 300° C. and activated.
Disadvantages of such porous catalysts are not only the complicated production method but also the low resistance toward strongly acidic reaction media. In addition, the use of such porous catalysts as fixed-bed catalysts usually leads to a severe, undesirable pressure drop in the reaction vessel.
In order to circumvent the abovementioned disadvantages, attempts have been made for some time to coat metallic supports with catalytically active metals, in particular platinum metals.
Metallic supports firstly have an increased stability and secondly metals can be worked to form thin sheets, wires, knitted meshes and gauzes which have a large surface area and favorable flow behavior. However, the application of the catalytically active metal to the metallic support is problematical. Thus, EP-A-0 198 435 discloses a process for producing catalysts in which the active components, e.g. noble metals, are vapor-deposited on the support. This process allows vapor deposition on metallic supports too, but the process is very complicated technically and requires expensive apparatuses. Furthermore, the process is not suitable for shaped catalyst bodies such as monoliths, wire mesh rings and helices.
Electrochemical plating processes or electroless plating processes, as are employed for finishing surfaces of materials, lead to smooth uniform coatings. The palladium-coated metals obtained are suitable, for example, as an inexpensive substitute for gold-coated metal parts in the electronics industry. However, substrates which have been coated in this way are unsuitable as catalysts.
Attempts have therefore been made to coat supports, including metallic supports, by impregnation with the catalytically active metal in soluble form. In order to obtain the porous surface necessary for impregnation, the surface of the metallic support is oxidized or, for example as described in EP-A-0 075 124, a porous oxide layer is applied to the metallic support and this is, as described above for porous nonmetallic supports, impregnated with the catalytically active metal. The catalysts obtained in this way display a good catalytic activity but are not suitable for many reactions in aggressive media because, particularly at low pH values, dissolution of the oxide layer and thus irreversible deactivation of the catalyst occurs.
A further method of producing supported noble metal catalysts on porous oxidic supports is the electroless deposition of noble metal salts from aqueous solutions by means of reducing agents in the presence of complexing agents such as ammonium chloride, EDTA or DTPA, as described in EP-A-0 878 235. In general, the porous support has to be activated by impregnation with sensitizers prior to the deposition process. Substances mentioned as suitable sensitizers are formaldehyde or aqueous solutions of silver nitrate, titanium salts or tin halides. The palladium catalysts produced by this method display good activity as hydrogenation: catalysts in the anthraquinone process for preparing H
2
O
2
in an organic phase, but are, like most catalysts based on oxidic supports or coatings, not suitable for wet chemical processes in the presence of aggressive chemicals.
DE-A-196 42 770 discloses a process for preparing hydrogen peroxide by continuous reaction of hydrogen and oxygen over palladium catalysts in an aqueous or alcoholic medium. The metal-supported catalysts used in the examples are obtained by electroless deposition of Pd salts in a strongly acidic medium.
In “Catalysis of organic Reactions” (Scarrows and Prunier, editors), Marcel Dekker Inc., New York, 1995, pp. 115-124, J. R. Kosak describes the production of metal-supported noble metal catalysts and their use for the direct synthesis of H
2
O
2
from hydrogen and oxygen. Here, the noble metal is applied to the metallic support by electroless plating using palladium chloride or palladium chloride and platinum chloride in the presence of sodium hypophosphite as reducing agent. The reduction of the noble metal takes place in strongly acidic solution in the presence of the support metal and leads to the formation of finely divided noble metal particles in the solution which becomes turbid and gray as a result. With increasing reaction time, the solution is decolorized to the extent that the noble metal deposits in the form of a black coating on the support.
In “Studies of Surface Science and Catalysis”, Volume. 118, pp. 63-72, J. P. Reymond describes the production of metal-supported palladium catalysts and their use for the hydrogenation of acetophenones. The catalyst is produced by a method based on the work of Kosak, and here too palladium chloride is deposited in strongly acidic solution (pH <2.2) using sodium hypophosphite as reducing agent. Reymond, too, observes that the aqueous reaction medium becomes turbid and dark before commencement of the deposition of palladium on the metallic support, which Reymond attributes to the formation of very fine palladium particles in the aqueous medium. Reymond states that the formation of palladium particles in the solution and the deposition on the metallic support are processes which proceed simultaneously, with the formation of the particles in the aqueous medium proceeding more quickly than the deposition on the metal support. The fact that the catalysts produced in this way have a good catalytic activity compared to catalysts produced by conventional plating methods by means of deposition of noble metals from homogeneous solution leads Reymond to conclude that only the deposition of noble metals from a solution which has become inhomogeneous due to precipitated noble metal particles leads to catalytically active deposits on the metal supports.
A disadvantage of the catalysts produced by the methods of Kosak and Reymond is that the catalytically active coating obtained in th
Fischer Martin
Hölzle Markus
Quaiser Stefan
Stammer Achim
BASF - Aktiengesellschaft
Langel Wayne A.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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