Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2001-10-24
2002-12-24
Padmanabhan, Sreeni (Department: 1621)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S243000, C502S348000, C549S534000, C549S536000
Reexamination Certificate
active
06498122
ABSTRACT:
The present invention relates to a silver catalyst useful for the production of ethylene oxide by a gas phase oxidation of ethylene with oxygen, and a process for the preparation of the catalyst. Ethylene oxide is co-polymerized with an active hydrogen compound for a non-ionic surfactant, or it is converted into ethylene glycol or polyethylene glycol by an addition of water which is used as a starting material for a polyester or polyurethane type polymer or as an antifreezing agent for engines.
A silver catalyst has been used as a catalyst which is useful for industrially producing ethylene oxide by a gas phase oxidation of ethylene with oxygen. In order to efficiently produce ethylene oxide, a demand for improvement of such a silver catalyst has been strong, and it is desired to develop a catalyst having higher activity, higher selectivity and longer life. Accordingly, various proposals have been made for improvements in e.g. a method of supporting silver, and alkali metal or other additive components serving as a reaction accelerator of ethylene oxide production, and a carrier development.
For supporting silver on a carrier, it is considered preferable to use silver in the form of a complex solution by means of a complex-forming compound. As such a method, the following has been proposed. For example, there is a method of using an aqueous solution of a silver complex having monoethanolamine bonded to silver nitrate (JP-B-46-19606), a method of using an aqueous silver lactate solution (JP-B-47-20079), a method of using an ethanol solution of a silver carbonate/acetylacetone complex (JP-B-49-26603), a method of using an aqueous solution of a silver oxalate/ethylenediamine, monoethanolamine complex (JP-A-47-11467), a method of using a silver oxalate/ethylenediamine, 1,3-diaminopropane complex (JP-A-61-54242), or a method of using a toluene solution of silver neodecanoate (JP-A-60-244338).
In order to have silver supported on a carrier by means of such a complex-forming compound, it is common that a carrier is impregnated with a solution of a complex of silver, and then the impregnated carrier is heat-treated in a gas to decompose the silver complex. For such heat treatment, a method is employed which comprises depositing a layer of the impregnated carrier and permitting a heated gas to pass through the deposited layer. This method can be carried out by a batch system i.e. by a fixed bed system. However, for the production on an industrial scale, a method is commonly employed wherein the impregnated carrier is put on e.g. a belt and continuously passed through a heating apparatus.
As the gas for heating, an inert gas such as nitrogen, helium, argon or superheated steam, or a mixture of such an inert gas with air or oxygen, may be employed.
As an apparatus to be used for such heat treatment, a gas flow band dryer as disclosed, for example, in “Chemical Engineering Handbook (5th edition)” 1988, compiled by Society of Chemical Engineers, Japan, published by Maruzen Co., Ltd. on Mar. 18, 1988, p. 674-675, may be employed, and the impregnated carrier is heated by circulating a heated gas.
However, when a complex-forming compound is used to prepare a silver complex for silver impregnation on a carrier, the gas which is circulated in the heat treatment apparatus, will contain the complex-forming compound formed by the decomposition of the silver complex, or its decomposed products, and the solvent used to dissolve the complex. Accordingly, if the circulation of the gas is continued, they will be accumulated in a large amount in the gas, and the complex-forming agent or its decomposed products are likely to deposit on the catalyst, or the catalyst tends to be hardly completely dried.
Accordingly, it is necessary to discharge the gas passed through the deposited layer of the impregnated carrier out of the system in a certain proportion. However, if the amount of such discharge increases, the amount of a heated gas to be introduced afresh to supplement the discharged amount, will have to be increased, such being uneconomical. Accordingly, it is known to heat the impregnated carrier while discharging about 10 vol % of the gas passed through the deposited layer, taking into consideration the catalytic performance of the obtained catalyst.
In the catalyst obtained by such a method, organic substances which are considered to be the complex-forming compound and its decomposed products, remain in an amount of from 0.1 to 0.3 wt %. However, in the production of ethylene oxide by using the catalyst immediately after the preparation of the catalyst, they present no adverse effect, and ethylene oxide can be produced at a high selectivity.
The amount of the catalyst to be used in an installation for producing ethylene oxide on an industrial scale, is usually as much as a few tens tons. Accordingly, it is usual that the production of the catalyst is started a few months ahead of using the catalyst, and it takes at least a few months to produce the necessary amount of the catalyst, although it depends also on the capacity for the production of the catalyst. Further, in a case where the installation for the production of the catalyst is located far from the installation for the production of ethylene oxide, it may take a few months until the catalyst is delivered to the installation for the production of ethylene oxide. For such reasons, it is common that the catalyst is stored for about one year before it is actually used for the production of ethylene oxide.
However, there is a problem that if a catalyst which is capable of producing ethylene oxide at high selectivity when it is used within one year after the production, is used after being stored for a long period of time, the selectivity for ethylene oxide decreases by a few percent. Such a decrease in the selectivity gives a serious adverse effect from the viewpoint of the production cost of ethylene oxide on an industrial scale.
Accordingly, the object of the present invention is to provide a catalyst useful for the production of ethylene oxide, of which the performance will not decrease even when stored for a long period of time, and a process for the preparation of the catalyst. Thus, the catalyst invented shows a stable performance against storage.
The present inventor has conducted an extensive study to solve the problems of the storage degradation, and as a result, have found that the above problems are somehow attributable to an organic substance which results from a complex-forming compound used for supporting silver on the carrier and its decomposition product remaining in a trace amount in the catalyst in the conventional process for producing a catalyst for the production of ethylene oxide. On the basis of this discovery, the present invention has been accomplished.
Namely, according to the present invention, it is possible to produce a catalyst which maintains as high performance after long storage as that on its preparation, by impregnating a carrier with a solution having silver dissolved in the form of a complex with an organic compound and heating this impregnated carrier by a heated gas at a temperature of from 120 to 500° C., until the content of the organic compound will be less than 0.1 wt %, in the production of an olefin oxidation catalyst having at least silver and an alkali metal supported on a carrier, particularly a catalyst for producing ethylene oxide by ethylene oxidation.
Now, the present invention will be described in detail.
An oxidation catalyst for olefin, particularly ethylene, of the present invention is a catalyst having at least silver and an alkali metal supported on a carrier. As the alkali metal, cesium, or a combination of cesium and another alkali metal, such as lithium, is preferred.
As the carrier, a refractory such as alumina, silicon carbide, titania, zirconia or magnesia, may be mentioned. Among them, a refractory comprising &agr;-alumina as a main component is preferred from the viewpoint of the performance of the final catalyst.
The carrier may be used, as it is, for supporting silver. However, it
Mitsubishi Chemical Corporation
Padmanabhan Sreeni
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