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. An ethylene polymerization catalyst composition 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 composition; and
Na in an amount of less than 800 ppm of said catalyst composition.
2. The ethylene polymerization catalyst composition of claim 1 , wherein Al is present in an amount ranging from 10-40 ppm of said catalyst composition.
3. The ethylene polymerization catalyst composition of claim 1 , wherein Na is present in an amount ranging from 50 to 800 ppm of said catalyst composition.
4. The ethylene polymerization catalyst composition of claim 1 , wherein the Na: Al molar ratio is greater than 5.
5. The ethylene polymerization catalyst composition of claim 4 , wherein the Na: Al molar ratio ranges from 10-40.
6. The ethylene polymerization catalyst composition of claim 1 , wherein the catalyst has a surface area ranging from 250 to 400 m 2 /g.
7. The ethylene polymerization catalyst composition of claim 1 , wherein Cr is present in an amount ranging from 0.1-2.0% by weight.
8. The ethylene polymerization catalyst composition of claim 1 , wherein Cr is added from at least one compound chosen from chromium acetate, nitrate, sulfate, acetylacetonate, chromium trioxide, ammonium chromate, tert-butyl chromate, other water-soluble chromium compounds, and other organic solvent-soluble chromium compounds.
9. A method of preparing the ethylene polymerization catalyst composition of claim 1 , the method comprising:
performing an acid-base reaction by reacting a sodium silicate comprising Al as an impurity, with an acid to form a hydrosol, wherein the acid-base reaction is non-stoichiometric and comprises more base than acid;
allowing the hydrosol to form an Al-containing hydrogel precursor;
treating the hydrogel precursor to reduce the amount of Al;
aging the hydrogel precursor to form a hydrogel;
drying the hydrogel er alcogel to produce a dried gel; and
impregnating the support with a solution containing a chromium compound followed by drying to form a Cr on silica catalyst composition.
10. The method of claim 9 , further comprising at least one additional step selected from: crushing the hydrogel precursor prior to further treatment; washing the hydrogel with an organic solvent to remove water from the hydrogel to form an alcogel; milling the dried gel; and sieving/classifying the dried gel to produce a support of desired particle size distribution.
11. The method of claim 9 , wherein the hydrogel precursor is treated to reduce the amount of Al to less than 50 ppm of the composition.
12. The method of claim 9 , wherein the hydrogel precursor is treated to reduce the amount of Al to an amount ranging from 10-40 ppm of said composition.
13. The method of claim 9 , further comprising adjusting the Na content of the composition to achieve Na in an amount of less than 800 ppm of said composition.
14. The method of claim 13 , wherein the Na content of the composition is adjusted before or simultaneous with the dried gel being impregnated with the chromium compound.
15. The method of claim 9 , wherein the amounts of Na and Al are adjusted such that the resulting Na to Al molar ratio is greater than 5.
16. A method for polymerizing ethylene using a catalyst composition according to claim 1 , 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.
17. The method of claim 16 , 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.
18. The method of claim 17 , wherein the one or more thermal steps is performed in a fluidized bed reactor in dry air.
19. The method of claim 16 , wherein polymerizing said ethylene results in the formation of polyethylene.
20. The method of claim 16 , further comprising copolymerizing a mixture of ethylene and one or more C3 to C8 α-alkenes.
21. The method of claim 20 , wherein said one or more C3 to C8 α-alkenes comprise 1-hexene.
22. The method of claim 16 , wherein one or more thermal steps is performed in a fluidized bed reactor at a temperature ranging from 750 to 850° C.
23. The method of claim 16 , wherein cooling the activated catalyst in an inert atmosphere comprises switching the atmosphere from air to nitrogen at 300° C.