Silica-coated alumina activator-supports for metallocene catalyst compositions

1. A catalyst composition comprising:

(a) at least one metallocene compound; and

(b) at least one activator-support;

wherein the at least one activator-support comprises at least one silica-coated alumina treated with at least one electron-withdrawing anion, wherein:

the at least one silica-coated alumina has a weight ratio of alumina to silica in a range from about 1:1 to about 100:1,

the at least one electron-withdrawing anion comprises fluoride, and

a catalyst activity of the catalyst composition is greater than that of a catalyst system containing the at least one metallocene compound and a silica-alumina treated with the at least one electron-withdrawing anion, under the same polymerization conditions.

2. The catalyst composition of claim 1 , wherein the weight ratio of alumina to silica is in a range from about 2:1 to about 20:1.

3. The catalyst composition of claim 1 , wherein the at least one activator-support comprises at least one silica-coated alumina treated with at least two electron-withdrawing anions, and wherein the at least two electron-withdrawing anions comprise fluoride and phosphate or fluoride and sulfate.

4. The catalyst composition of claim 1 , wherein the at least one silica-coated alumina has a surface area in a range from about 100 to about 1000 m 2 /g, a pore volume greater than about 0.5 mL/g, and a particle size in a range from about 5 microns to about 150 microns.

5. The catalyst composition of claim 1 , wherein the at least one silica-coated alumina is produced by a process comprising:

providing at least one alumina source, the at least one alumina source comprising an alumina, a hydrated alumina, aluminum hydroxide, boehmite, or a combination thereof;

contacting the at least one alumina source with a solution or suspension comprising at least one solvent and at least one silicon-containing compound capable of producing silica upon calcination;

depositing a coating of the at least one silicon-containing compound on at least a portion of the at least one alumina source; and

removing the solvent.

6. The catalyst composition of claim 1 , further comprising at least one organoaluminum compound having the formula:

Al(X 9 ) m (X 10 ) 3-m ;

wherein:

X 9 is a hydrocarbyl;

X 10 is an alkoxide or an aryloxide, a halide, or a hydride; and

m is from 1 to 3, inclusive.

7. The catalyst composition of claim 1 , further comprising at least one optional co-catalyst, wherein the at least one optional co-catalyst is at least one aluminoxane compound, at least one organoboron or organoborate compound, at least one ionizing ionic compound, or any combination thereof.

8. The catalyst composition of claim 1 , further comprising at least one optional activator-support, wherein the at least one optional activator-support is fluorided alumina, chlorided alumina, bromided alumina, sulfated alumina, fluorided silica-alumina, chlorided silica-alumina, bromided silica-alumina, sulfated silica-alumina, fluorided silica-zirconia, chlorided silica-zirconia, bromided silica-zirconia, sulfated silica-zirconia, fluorided silica-titania, or any combination thereof.

9. The catalyst composition of claim 1 , wherein a catalyst activity of the catalyst composition is greater than about 1000 grams of polyethylene per gram of activator-support per hour under slurry polymerization conditions, using isobutane as a diluent, with a polymerization temperature of 90° C. and a reactor pressure of 420 psig.

10. The catalyst composition of claim 1 , wherein a catalyst activity of the catalyst composition is greater than about 25,000 grams of polyethylene per gram of metallocene compound per hour under slurry polymerization conditions, using isobutane as a diluent, with a polymerization temperature of 90° C. and a reactor pressure of 420 psig.

11. The catalyst composition of claim 1 , wherein the at least one metallocene compound comprises an ansa-metallocene compound.

12. The catalyst composition of claim 1 , wherein the at least one metallocene compound comprises an unbridged metallocene compound.

13. A catalyst composition comprising:

(a) at least one metallocene compound;

(b) at least one activator-support; and

(c) at least one organoaluminum compound;

wherein the at least one organoaluminum compound comprises trimethylaluminum, triethylaluminum, tri-n-propylaluminum, tri-n-butylaluminum, triisobutylaluminum, tri-n-hexylaluminum, tri-n-octylaluminum, diisobutylaluminum hydride, diethylaluminum ethoxide, diethylaluminum chloride, or any combination thereof;

wherein the at least one activator-support comprises at least one silica-coated alumina treated with at least one electron-withdrawing anion, wherein:

the at least one silica-coated alumina has a weight ratio of alumina to silica in a range from about 1.5:1 to about 20:1,

the at least one electron-withdrawing anion comprises fluoride,

the at least one silica-coated alumina has a surface area in a range from about 250 to about 500 m 2 /g and a pore volume in a range from about 0.8 to about 1.7 mL/g, and

a catalyst activity of the catalyst composition is greater than that of a catalyst system containing the at least one metallocene compound, the at least one organoaluminum compound, and a silica-alumina treated with the at least one electron-withdrawing anion, under the same polymerization conditions.

14. The catalyst composition of claim 13 , wherein:

the weight ratio of alumina to silica is in a range from about 2:1 to about 12:1.

15. The catalyst composition of claim 14 , wherein the at least one metallocene compound comprises an ansa-metallocene compound.

16. The catalyst composition of claim 11 , further comprising at least one organoaluminum compound.

17. The catalyst composition of claim 16 , wherein the weight ratio of alumina to silica is in a range from about 1.2:1 to about 25:1.

18. The catalyst composition of claim 13 , wherein:

the at least one metallocene compound comprises an ansa-metallocene compound.