Silver catalysts with improved size and distribution density of silver particles

1. A silver-based ethylene epoxidation catalyst comprising:

greater than about 20% by weight of silver disposed on an alpha-alumina carrier; and

a promoting amount of one or more promoters disposed on said alpha-alumina carrier, wherein said silver is present on said alpha-alumina carrier as silver particles, said silver particles having a diameter of greater than about 150 nm and having a distribution density of about 20 particles per 1 square micron or less.

2. The silver-based ethylene epoxidation catalyst of claim 1 , wherein said one or more promoters comprise Group IIA metal promoters, one or more transition metals, one or more alkali metals or any combination thereof.

3. The silver-based ethylene epoxidation catalyst of claim 2 , wherein said one or more transition metals are selected from the group consisting of Groups 4-10 of the Periodic Table of the Elements.

4. The silver-based ethylene epoxidation catalyst of claim 3 , wherein said one or more transition metals are selected from the group consisting of molybdenum, rhenium, tungsten, chromium, titanium, hafnium, zirconium, vanadium, thorium, tantalum, and niobium.

5. The silver-based ethylene epoxidation catalyst of claim 2 , wherein said one or more transition metals comprise rhenium, molybdenum, tungsten, or any combination thereof.

6. The silver-based ethylene epoxidation catalyst of claim 2 , wherein said one or more alkali metals are selected from the group consisting of cesium, lithium, sodium, potassium, and rubidium.

7. The silver-based ethylene epoxidation catalyst of claim 6 , wherein said one or more alkali metals comprise lithium and cesium.

8. The silver-based ethylene epoxidation catalyst of claim 1 , wherein said diameter of said silver particles is from about 150 nm to about 300 nm.

9. The silver-based ethylene epoxidation catalyst of claim 1 , wherein said distribution density of said silver particles is from about 10 particles per 1 square micron to about 20 particles per 1 square micron.

10. The silver-based ethylene epoxidation catalyst of claim 1 , wherein said alpha-alumina carrier has a pore volume from 0.40 mL/gm to 0.70 mL/gm, a surface area from 0.7 m 2 /gm to 1.2 m 2 /gm, an Al 2 O 3 content of greater than 80% and has a pore size from 0.01 micrometers to 1.0 micrometer of less than 25%, pore size from 1.0 micrometer to 10 micrometers of greater than 50%, and pore size from 10 micrometers to 100 micrometers of less than 25%.

11. The silver-based ethylene epoxidation catalyst of claim 1 , wherein said alpha-alumina carrier has a pore volume from 0.33 mL/gm to 0.55 mL/gm, a surface area from 0.5 m 2 /gm to 1.0 m 2 /gm, an Al 2 O 3 content of greater than 85% and has a pore size from 0.01 micrometers to 1.0 micrometer of less than 35%, pore size from 1.0 micrometer to 10 micrometers of greater than 30%, and pore size from 10 micrometers to 100 micrometers of less than 25%.

12. The silver-based ethylene epoxidation catalyst of claim 1 , wherein said one or more promoters comprise a promoting combination of rhenium, cesium and lithium.

13. The silver-based ethylene epoxidation catalyst of claim 1 , wherein from about 20 to about 45% by weight of silver is disposed on said alpha-alumina carrier.

14. A process for the vapor phase conversion of ethylene to ethylene oxide in the presence of oxygen, said process comprising:

providing a silver-based ethylene epoxidation catalyst to an ethylene oxide reactor, wherein said silver-based ethylene epoxidation catalyst comprises:

greater than about 20% by weight of silver disposed on an alpha-alumina carrier; and

a promoting amount of one or more promoters disposed on said alpha-alumina carrier, wherein said silver is present on said alpha-alumina carrier as silver particles, said silver particles having a diameter of greater than about 150 nm and having a distribution density of about 20 particles per 1 square micron or less; and

reacting a reaction mixture comprising ethylene and oxygen in the present of said ethylene epoxidation catalyst.

15. A silver-based ethylene epoxidation catalyst comprising:

from about 20% to about 50% by weight of silver disposed on an alpha-alumina carrier; and

a promoting amount of one or more promoters disposed on said alpha-alumina carrier, wherein said silver is present on said alpha-alumina carrier as silver particles, said silver particles having a diameter of greater than about 150 nm and up to about 300 nm, and having a distribution density of from about 1 particle per 1 square micron to about 20 particles per 1 square micron.