1.  Field of the Invention
This invention relates to a catalyst for epoxidation of an alkene, such as ethylene, to an alkene oxide, such as ethylene oxide.  Specifically, this invention relates to a catalyst on which silver has been deposited on alumina as a support which has been modified with certain weak base compounds, such as oxides of a Group 1A, Group 2A, Group 3A or the first transition series of the Periodic Table of Elements, and with a high temperature heat treatment or calcination. 
2.  Description of the Prior Art
The production of alkene oxides from alkenes is known and is practiced commercially.  Of particular interest is the production of ethylene oxide from ethylene with a catalyst of a refractory material, such as alpha-alumina, on which silver has been deposited.  Other materials may also be deposited on the support to promote the reaction. 
U.S. Pat.  No.  4,939,114 discloses production of ethylene oxide by oxidation of ethylene with a catalyst on which silver and at least one cationic component selected from sodium, potassium, rubidium and cesium have been deposited on an alpha-alumina which does not show acidity and basicity within a particular pKa range tested by color change.  The impregnated carrier is heat treated for 5-30 minutes with air at 130-300° C.
U.S. Pat.  No.  5,187,140 epoxidation of ethylene to ethylene oxide with catalyst having a high silver content on carriers having a high surface area and a high pore volume.  The surface acidity of the carrier is less than about 2, preferably less than about 1.5 and often between 0.05 to 1.0 micromoles per gram of carrier.  The impregnated carrier is heated to reduce the silver compound to metallic silver at a temperature of 100-900° C.
U.S. Pat.  No.  5,102,848 discloses epoxidation of ethylene with a catalyst of a silver-impregnated support which has an essential absence of fluoride anion but may have a fluoride anion present in an amount sufficient to reduce ethylene oxide burning.  The surface acidity of the carrier is less than about 2, preferably less than about 1.5 and often between 0.05 to 1.0 micromoles per gram of carrier.  The impregnated carrier was calcined at 500° C. for 2.5 minutes in Examples 1-7 and at 500° C. for 3 minutes in Examples 8 and 9. 
U.S. Pat.  No.  4,829,044 discloses a process for the preparation of a silver-containing catalyst for the oxidation of ethylene to ethylene oxide by impregnating a calcined alkali metal enriched alumina carrier with a silver compound and a potassium, rubidium or cesium compound as a promoter. 
U.S. Pat.  No.  4,874,739 discloses a process for the preparation of a silver-containing catalyst for the oxidation of ethylene to ethylene oxide by mixing alumina with a tin compound and an alkali metal compound, calcining the modified alumina, impregnating the carrier with a silver compound and with an alkali metal compound and a rhenium compound as promoters.  The alkali metal promoter may serve to neutralize “acid sites” on the alumina surface to influence formation of carbon dioxide from ethylene oxide. 
U.S. Pat.  No.  6,656,874 discloses a process for depositing metals, such as silver, on a carrier by impregnation with a solution which has pH lowered to above 11.2.  Since the typical impregnation solution for an epoxidation catalyst is quite basic, a strong base is used to farther lower the pH. Examples of strong bases include alkylammonium-hydroxides, such as tetraethylammonium hydroxide, and metal hydroxides, such as lithium hydroxide and cesium hydroxide.  While lowering the concentration of ionizable species on the surface of the carrier prior to deposition may improve performance, use of aggressive media, such as acid or bases, is not recommended since they extract material and generate acidic or basic sites in the pores. 
U.S. Pat.  No.  6,579,825 discloses a catalyst carrier of a refractory inorganic material on which the concentration of ionizable species has been lowered, particularly such that the carrier has a sodium solubilization rate no greater than 5 ppmw/5 minutes.  Acids will remove cations on a carrier but are fairly ineffectual in removing anions, such as silicates. 
It would be advantageous to modify the alpha-alumina support without depending on production control of the raw materials in making alpha-alumina and to modify not only the surface of the alpha-alumina but also the bulk properties at least in part.