Claims
- 1. A polymerization process comprising contacting at least one alpha-monoolefin having 2 to 8 carbon atoms with an activated chromium-containing catalyst supported on a silica-containing base and recovering a polymer, wherein the catalyst is first activated by the process comprising:
- (a) a first activation step of heating catalyst in an oxidizing atmosphere at an oxidizing activation temperature within the range of 600.degree.-850.degree. C. until at least a part of the chromium present in the catalyst is converted to the hexavalent state;
- (b) a second activation step, immediately following step (a), of cooling the catalyst in an oxidizing atmosphere until it reaches a temperature, below the oxidizing activation temperature, comprised between 550.degree. and 750.degree. C.;
- (c) replacing the said oxidizing atmosphere by a non-oxidizing atmosphere at the temperature reached at the end of step (b) and maintaining the catalyst at the said temperature for a period of time between 15 minutes and 20 hours sufficient to achieve at least partial self-reduction of at least a portion of the chromium present in the catalyst to a lower valence state; and
- (d) cooling the catalyst in the said non-oxidizing atmosphere from the temperature of step (c) to ambient temperature.
- 2. The process recited in claim 1, wherein, in step (c), the catalyst is maintained in the non-oxidizing atmosphere at the temperature reached at the end of the second step (b) for a period of time of from 1 to 10 hours.
- 3. The process recited in claim 1, wherein, in step (c), the non-oxidizing atmosphere is an inert atmosphere.
- 4. The process recited in claim 3, wherein the inert atmosphere is selected from the group consisting of a vacuum, helium, argon and nitrogen.
- 5. A process according to claim 1 wherein said alpha-monoolefin comprises ethylene.
- 6. A process according to claim 1 wherein said alpha-monoolefin is a mixture of ethylene and of a comonomer selected from propylene, 1-butene, 1-hexene and 1-octene.
- 7. A polymerization process comprising contacting at least one alpha-monoolefin having 2 to 8 carbon atoms with an activated chromium-containing catalyst supported on a silica-containing base and recovering a polymer, wherein the catalyst is first activated by the process comprising:
- (a) a first activation step of heating catalyst in an oxidizing atmosphere at an oxidizing activation temperature within the range of 800.degree.-850.degree. C. until at least a part of the chromium present int he catalyst is converted to the hexavalent state;
- (b) a second activation step, immediately following step (a), of cooling the catalyst in an oxidizing atmosphere until it reaches a temperature below the oxidizing activation temperature, at a temperature between 600.degree. and 650.degree. C.;
- (c) replacing the said oxidizing atmosphere by a non-oxidizing atmosphere at the temperature reached at the end of step (b) and maintaining the catalyst at the said temperature for a period of time between 15 minutes and 20 hours sufficient to achieve at least partial self-reduction of at least a portion of the chromium present in the catalyst to a lower valence state; and
- (d) cooling the catalyst in the said non-oxidizing atmosphere from the temperature of step (c) to ambient temperature.
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a division of application No. 07/203,647 filed Jun. 3rd, 1988, now U.S. Pat. No. 5,093,300 which is a continuation of application No. 06/652,147 filed Sep. 18th, 1984, now abandoned, which is a continuation of application No. 06/481,877 filed Apr. 4th, 1983, now abandoned, the specifications of which are incorporated herein by reference.
US Referenced Citations (15)
Foreign Referenced Citations (1)
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1083178 |
Sep 1967 |
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Divisions (1)
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203647 |
Jun 1988 |
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Continuations (2)
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652147 |
Sep 1984 |
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481877 |
Apr 1983 |
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