Chromium-on-silica catalysts and methods of making the same
A composition, such as a catalyst precursor or a catalyst comprising a Cr coated silica support with particularly defined levels of Na and Al, such that the resulting Cr/Silica catalyst has an increased MI potential is disclosed. In an embodiment, the disclosed catalyst composition comprises a silica-containing substrate made using a base-set gel and comprising a catalytically active metal consisting of Cr, with Al impurities of less than 50 ppm and Na in an amount of less than 800 ppm of the catalyst composition. The disclosed composition has an increased MI potential over a catalyst having higher Al content, a lower Na:Al ratio, or both. Methods of making the disclosed composition, and methods of using it to prepare a polyethylene are also disclosed.
1. A method for polymerizing ethylene with a catalyst comprising: a silica-containing substrate derived from a base-set gel, the silica-containing substrate comprising: a catalytically active metal comprising Cr; Al in an amount of less than 50 ppm of said catalyst; and Na in an amount of less than 800 ppm of said catalyst, the method further comprising:
activating the catalyst with one or more thermal steps to form an activated catalyst;
cooling the activated catalyst;
charging the activated catalyst in a polymerization reactor; and
polymerizing said ethylene above room temperature.
2. The method of claim 1 , wherein the one or more thermal steps comprise heating the catalyst at a temperature ranging from 200 to 1200° C., for an activation period ranging from 30 minutes to 15 hours.
3. The method of claim 2 , wherein the one or more thermal steps is performed in a fluidized bed reactor in dry air.
4. The method of claim 1 , wherein polymerizing said ethylene results in the formation of polyethylene.
5. The method of claim 1 , further comprising copolymerizing a mixture of ethylene and one or more C3 to C8 α-alkenes.
6. The method of claim 5 , wherein said one or more C3 to C8 α-alkenes comprise 1-hexene.
7. The method of claim 1 , wherein one or more thermal steps is performed in a fluidized bed reactor at a temperature ranging from 750 to 850° C.
8. The method of claim 1 , wherein cooling the activated catalyst in an inert atmosphere comprises switching the atmosphere from air to nitrogen at 300° C.
9. The method of claim 1 , wherein Al is present in the catalyst in an amount ranging from 10-40 ppm of said catalyst.
10. The method of claim 1 , wherein Na is present in the catalyst in an amount ranging from 50 to 800 ppm of said catalyst.
11. The method of claim 1 , wherein the Na:Al molar ratio is greater than 5.
12. The method of claim 11 , wherein the Na:Al molar ratio ranges from 10-40.
13. The method of claim 1 , wherein the catalyst has a surface area ranging from 250 to 400 m 2 /g.
14. The method of claim 1 , wherein Cr is present in an amount ranging from 0.1-2.0% by weight.
15. The method of claim 1 , wherein Cr is added from at least one compound chosen from chromium acetate, nitrate, sulfate, acetylacetonate, chromium trioxide, ammonium chromate, and tert-butyl chromate.