Catalyst – solid sorbent – or support therefor: product or process – Miscellaneous
Reexamination Certificate
1999-06-16
2002-08-20
Griffin, Steven P. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Miscellaneous
C502S325000, C502S328000, C502S330000, C502S339000
Reexamination Certificate
active
06436873
ABSTRACT:
The present invention relates to an impregnating process for the application of active composition to structured supports or monoliths. Furthermore it relates to catalytically active structured supports and monoliths which are produced by means of this process and to processes in which they are used as catalysts.
By structured supports we mean, within the scope of this invention, supports having a regular two-dimensional or three-dimensional structure, which thus differ from particulate supports used as a bulk bed. Examples of structured supports are supports composed of threads or wires, mostly in the form of supporting webs, such as woven fabrics, single-thread or multi-thread knit fabrics, or felts, or metal plates, which may exhibit recesses, such as perforated plates or expanded metal. Such substantially two-dimensional, structured supports can be converted to three-dimensional structures, so-called monoliths, having a shape appropriate to their end use, which structures can then be used as, say, catalyst packs or column packings. Packings may be composed of more than one monolith. Alternatively, monoliths need not be composed of two-dimensional supporting webs, but can be made directly, without inter-mediate stages, such as is the case with ceramic monoliths having flow channels, these being known to the person skilled in the art.
Active compositions can be applied to structured supports or monoliths so as to impart catalytic activity to the structured supports or monoliths. By active compositions we mean catalytically active substances or mixtures of substances containing at least one catalytically active ingredient. They can in addition contain further catalytically active ingredients, auxiliary substances and/or promoters for influencing the catalytic properties such as selectivity, activity and/or on-stream time. Catalytically active structured supports or catalytically active, monoliths are used eg in processes in which the use of a bulk catalyst bed is disadvantageous for example for fluid dynamic or reaction-kinetic reasons. Typical fields of application are their use for exhaust gas purification, primarily in automobiles, their use as catalyst packs in vapor-phase or liquid phase reactors or their use as column packings in reactive distillations.
Various techniques are known for application of active compositions to structured supports and for the production of catalytically active monoliths composed thereof, or for application of active compositions to monoliths.
Thus EP-A 198,435 discloses the vapor deposition of catalytically active substances and promoters onto net-like or woven supports. Catalyst packs for the detoxication of exhaust gas are prepared from these vacuum-metallized support materials. EP-A 564,830 teaches the production of catalyst packs, known as monoliths, by tempering a metallic support mate-rial in air, cooling, coating the support material with active components and promoters by vapor deposition in vacuo followed by cutting, shaping and processing to form monoliths. EP-A 412,415 teaches the vapor deposition of palladium and a metallic inhibitor to a structured support and also the use of the thus prepared catalysts for the hydrogenation of triple bonds to double bonds, eg hydrodehydrolinalool to hydrolinalool.
By the use of vapor deposition it is possible to apply the active composition to the support materials, very uniformly and normally without the formation of agglomerates of undesirable magnitude, in the form of firmly adhering fine particles, which method is, however, relatively time-consuming and technically elaborate, particularly when relatively large amounts of active composition are to be applied. The vapor deposition must be carried out in vacuo, which requires a batch process involving comparatively long set-up times, or high vacuum pump capacities in continuous plants. There has thus been no lack of attempts to prepare the active composition by the impregnating process well known to the person skilled in the art from the manufacture of catalysts on bulk supports in particulate form.
For this purpose a so-called “wash coat” is frequently applied to a pre-formed monolith, this being a coating composed of a finely divided oxidic catalyst-supporting powder, which is in turn coated with catalytically active substances. This mostly takes place by impregnation of the monolith with a suspension which contains the oxidic powders already provided with active composition, or the oxidic powders and precursors of the active composition, drying and, where applicable, conversion of the precursors to the active composition. Conventional processes for the application of wash coats are disclosed eg in DE-A 4,135,055 and DE-A 3,915,685. U.S. Pat. No. 4,746,537 discloses a process for coating a ceramic monolith with &ggr;-aluminum oxide precipitated in situ from the vapor phase. The monolith thus coated with finely divided alumina is then impregnated with a solution of salts of catalytically active metals, dried and calcined.
Wash-coated monoliths however show unsatisfactory properties, usually due to the fact that when the coating is applied by impregnation under active flow conditions precipitation preferentially takes place at the upstream end of the monolith or when the monolith is lifted out of a non-flowing impregnation bath precipitation preferentially takes place at that end of the monolith which is last in contact with the liquor, and in addition, there is poor adhesion of such a coating. As a result, comparatively large amounts of catalytically active material can become detached and, eg in the case of an automobile, be removed through the exhaust or, in the case of a chemical plant, be deposited at some other site. Such deposits can consititute a considerable safety risk, particularly when they catalyze exothermal reactions at an uncooled site.
Thus the attempt has also been made to apply active material or a pulverulent support material for active material to structured support materials, using an impregnating process, prior to shaping of the latter to form monoliths.
EP-A 68,862 discloses the preparation of units comprising on the one hand hydrophilic woven, knitted or felted fabrics of textile material, and on the other hand, open-mesh hydrophobic material coated with catalytically active substances, the catalytically active substances—ie platinum crystallites on a finely divided carbon support—being applied as aqueous suspension to the hydrophobic material and fixed by drying and sintering at elevated temperature. EP-A 56,435 teaches a continuous process in which metallic webs are passed through a dip bath, in which they are coated with an adhesive resistant to high temperature, eg a ceramic adhesive, and are then coated with a granular support material for the catalytically active substances. The substances deposited in the dip bath are then dried and the metallic web is fabricated to form monoliths
These impregnating processes suffer from the drawback, however, that the applied substances are not deposited on the structured support uniformly and or in uniform particle size. The particles always agglomerate to form comparatively large lumps. Such agglomeration is accompanied by a loss of active surface, and the activity of such catalysts is therefore always unsatisfactory. Furthermore, such agglomerated material as occurs in wash coats readily detaches itself from the support, particularly when the structured supports provided with active composition are being processed to form the monoliths, but also during industrial use of the monoliths, which likewise leads to the drawbacks described above with respect to wash coats.
Thus the problem to be solved is to find an industrially simple process for the application of catalytically active substances to structured supports, which avoids the drawbacks of the known processes.
Accordingly, we have found an impregnating process for the application of catalytically active substances to structured supports or monoliths, which is characterized in that an impregnating medium having a surface
Bröcker Franz Josef
Schwab Ekkehard
BASF - Aktiengesellschaft
Griffin Steven P.
Ildebrando Christina
Keil & Weinkauf
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