Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2002-05-03
2004-05-04
Dentz, Bernard (Department: 1625)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S239000, C549S248000
Reexamination Certificate
active
06730631
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for producing monolithic oxidation catalysts and to their use in the gas-phase oxidation of hydrocarbons.
2. The Prior Art
Supported catalysts for the gas-phase oxidation of hydrocarbons to give the corresponding oxidation products such as carboxylic acids, carboxylic anhydrides or aldehydes, which catalysts have a catalytically active surface coating consisting essentially of titanium dioxide (TiO
2
) and divanadium pentoxide (V
2
O
5
), have been known for a long time. A typical example of the use of such catalysts is the preparation of phthalic anhydride, in which mixtures of o-xylene and air or naphthalene and air or o-xylene, naphthalene and air are passed over an appropriate catalyst in a shell-and-tube reactor. The heat generated in this strongly exothermic reaction is customarily removed (cooling, isothermal reaction conditions) by means of a salt melt which surrounds the reaction tubes.
The supported catalysts used here comprise an inert support body, for example having a ring shape or a spherical shape, on which the actual catalytically active composition is present. The active composition consists predominantly of the main components TiO
2
in the anatase form and V
2
O
5
. To improve the control of the activity and to improve the selectivity, further activating or deactivating additives, for example oxides of transition elements or alkali metal compounds, are frequently added in small amounts as dopants (promoters) to the catalytically active composition.
The supported catalysts are generally produced by spraying aqueous suspensions or aqueous solutions of TiO
2
and V
2
O
5
, frequently with addition of promoters and possibly binders for improving adhesion of the active components to the support, onto the support bodies.
As support bodies, use is made of regularly shaped, mechanically stable bodies such as spheres, rings, half rings, saddles, etc., particularly preferably rings or spheres. The size of the support bodies is determined predominantly by the dimensions of the reactor, especially by the internal diameter of the reaction tubes.
Support materials used are, for example, steatite, Duranit, earthenware, silicon dioxide, silicon carbide, aluminates, metals and metal alloys.
EP-A 744214 (U.S. Pat. No. 5,792,719) discloses a procedure for producing catalysts in which TiO
2
, V
2
O
5
, SiC and possibly dopants such as CsCO
3
and (NH
4
)
2
HPO
4
are stirred in aqueous suspension for a number of hours, and the suspension is subsequently admixed with an organic binder. This suspension is sprayed onto the support material and the supported catalyst is dried.
In industry, it is customary for each of the reaction tubes to be filled with various catalysts which have different catalytically active compositions. These can be arranged, for example, in two superposed zones, an upper zone and a lower zone. This measure makes it possible to match the activity of the catalyst system in the reactor appropriately to the course of the reaction.
During the reaction, the major part of hydrocarbon is reacted in the upper part of the reaction tube. As a result, the highest temperatures inevitably also occur there. In the lower part of the tube, only a sort of after-reaction takes place. There, remaining o-xylene
aphthalene and intermediates, for example o-tolualdehyde and phthalide, are converted into phthalic anhydride. Furthermore, by-products such as quinones are also oxidized further.
As a result of aging processes, all catalysts lose activity as the time for which they have been used increases. This occurs predominantly in the main reaction zone, since this is where the catalyst is subject to the highest temperatures. During the life of the catalyst, the main reaction zone migrates ever further into the catalyst bed. This steadily decreases the length of the remaining catalyst bed and adversely affects the after-reaction. As a consequence, intermediates and by-products can no longer be reacted completely and the product quality of the phthalic anhydride produced therefore deteriorates to an increasing extent. An aging process is particularly critical in the case of high feed loadings. Although the fall-off in the reaction and thus the deterioration in product quality can be countered by increasing the reaction temperature, for example by means of the salt bath temperature, but only to a temperature of about 400° C., this temperature increase is always associated with a loss in yield.
DE-A 1793267 (GB-A 1274471) describes a process for preparing phthalic anhydride, in which the overall oxidative reaction is divided in process engineering terms into two parts. The reaction is controlled so that the reaction conditions in the second part, known as the after-reaction, are significantly more aggressive than in the first part. This can be achieved, for example, by carrying out the after-reaction without cooling, i.e. adiabatically. This after-reaction can be carried out in a separate reactor having different tube dimensions or even in a downstream shaft oven.
DE-A 2005969 describes a process for preparing phthalic anhydride, in which from about 80 to 99% of the total feed is reacted isothermally, i.e. cooled, in the main reaction. Conversion of the remaining unreacted feed occurs in a downstream adiabatic reactor. In addition, in the reaction procedure described, the gas mixture leaving the isothermal reactor is cooled further before it enters the downstream adiabatic reactor. This process variant is likewise intended to enable the phthalic anhydride formed to be obtained largely free of by-products and without a loss in yield. Here too, a shaft oven is claimed as adiabatic reactor.
Owing to the laminar flow occurring in honeycomb catalysts, they have only a very low pressure drop even at very high gas velocities. However, a disadvantage is that, owing to the lack of turbulent flow resulting from the shape, heat and mass transfer in the honeycomb channels, and thus heat removal, are greatly reduced. This situation makes use of honeycomb catalysts as catalyst supports virtually impossible for strongly exothermic processes in conjunction with a selective oxidation. Honeycomb catalysts have therefore become established industrially only in waste gas purification or waste gas incineration where all the organic constituents undergo total oxidation to CO
2
.
Coating monolithic support material with a catalytically active composition comprising the main constituents TiO
2
, V
2
O
5
and possibly dopants by generally known methods, for example a dipping process, is found to be impractical. This is because coating suspensions based on commercially available TiO
2
have a very high viscosity even at solids concentrations of 30-35% by weight and thus make coating of the channels of a monolithic support material virtually impossible without blocking the channels.
In order to coat monolithic catalyst supports with the necessary amount of catalytically active composition, for example 50-150 g of active composition per liter of catalyst, the coating process would have to be carried out with such a low-concentration “active composition” suspension that the necessary layer thickness would be achieved only after repeating the coating process a number of times. However, this at the same time once again increases the problem of blocking of the channels in the catalyst support because of the multiple coating steps. Furthermore, this is associated with significantly more work and thus with increased costs and is therefore uneconomical.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a simple and preferably single-stage process for producing monolithic catalysts based on TiO
2
/metal oxides.
It has now surprisingly been found that the viscosity of highly concentrated TiO
2
coating suspensions having a high solids content can be greatly reduced by addition of surfactants
The invention provides a process for producing monolithic supported catalysts for gas-phase oxidation by coating t
Eberle Hans-Jürgen
Helmer Olaf
Stocksiefen Karl-Heinz
Trinkhaus Stefan
Wecker Ulrich
Collard & Roe P.C.
Consortium für Elektrochemische Industrie GmbH
Dentz Bernard
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