ETHYLENE OXIDE CATALYST Background of the Invention Field of the Invention The present invention relates to a catalyst for the oxidation of ethylene to ethylene oxide consisting of silver, alkali metal such as cesium, a lanthanide component and sulfur deposited on a support such as alpha alumina and to the production of ethylene oxide using the catalyst; a fluorine component optionally can be included. The catalyst is essentially free of rhenium or transition metal components.

Description of the Prior Art Processes for the production of ethylene oxide involve the vapor phase oxidation of ethylene with molecular oxygen using a solid catalyst comprised of silver on a support such as alumina. There have been great efforts by many workers to improve the effectiveness and efficiency of the silver catalyst for producing ethylene oxide. U.S. Patent 5,051,395 provides a comprehensive analysis of these efforts of prior workers.

Among the many prior teachings in this area is that of U.S. Patent 4,007,135 (see also UK 1,491,447) which teaches variously silver catalysts for the production of ethylene and propylene oxides comprised of a promoting amount of copper, gold, magnesium, zinc, cadmium, mercury, strontium, calcium, niobium, tantalum, molybdenum, tungsten, chromium, vanadium, and/or preferably barium, in excess of any present in immobile form in the preformed support as impurities or cements (column 2, lines 1-15), silver catalysts for the production of propylene oxide comprising a promoting amount of at least one promoter selected from lithium, potassium, sodium, rubidium, cesium, copper, gold, magnesium, zinc, cadmium, strontium, calcium, niobium, tantalum, molybdenum, tungsten, chromium, vanadium and barium, in excess of any present in immobile form in the preformed support as impurities or cements (column 2, lines 16-34) , as well as silver catalysts for producing ethylene oxide or propylene oxide comprising (a) a promoting

amount of sodium, cesium, rubidium, and/or potassium, and (b) magnesium, strontium, calcium and/or preferably barium in a promoting amount (column 3, lines 5-8).

U.S. Patent 5,057,481, and related 4,908,343 are concerned with silver ethylene oxide catalysts comprised of cesium and an oxyanion of a group 3b to 7b element.

U.S. Patent 3,888,889 describes catalysts suitable for the oxidation of propylene to propylene oxide comprised of elemental silver modified by a compound of an element from Group 5b and 6b. Although the use of supports is mentioned, there are no examples. The use of cesium is not mentioned.

European Publication 0 266 015 deals with supported silver catalysts promoted with rhenium and a long list of possible copromoters.

U.S. Patent 5,102,848 deals with catalysts suitable for the production of ethylene oxide comprising a silver impregnated support also having thereon at least one cation promoter such as cesium, and a promoter comprising (i) sulfate anion, (ii) fluoride anion, and (iii) oxyanion of an element of Group 3b to 6b inclusive of the Periodic Table. Possibly for purposes of comparison since it is outside the scope of catalyst claimed, the patent shows at columns 21 and 22 a catalyst No. 6 comprised of Ag/Cs/S/F on a support, the Cs amount being 1096 ppm.

U.S. Patent 5,486,628 describes a silver catalyst promoted with alkali metal, rhenium and a rare earth or lanthanide component.

In the context of the bewildering and vast number of references, many of them contradictory, applicant has discovered a novel and improved catalyst for the production of ethylene oxide.

Brief Description of the Invention The present invention relates to an improved supported silver ethylene oxide catalyst containing a promoter combination consisting of a critical amount of an alkali metal component, preferably cesium, together with a sulfur component, and a rare earth or lanthanide component and to the

catalyst preparation and use; the catalyst is essentially free of rhenium and transition metal components and optionally can contain a fluorine component.

Detailed Description Preferred catalysts prepared in accordance with this invention contain up to about 30% by weight of silver, expressed as metal, deposited upon the surface and throughout the pores of a porous refractory support. Silver contents higher than 20% by weight of total catalyst are effective, but result in catalysts which are unnecessarily expensive. Silver contents, expressed as metal, of about 5-20% based on weight of total catalyst are preferred, while silver contents of 8- 15% are especially preferred.

In addition to silver, the catalyst of the invention also contains a critical promoter combination consisting of certain amounts of alkali metal, sulfur and rare earth or lanthanide.

The critical amount of alkali metal promoter component is not more than 2000 ppm expressed as alkali metal based on the catalyst weight; preferably the catalyst contains 400-1500 ppm, more preferably 500-1200 ppm alkali metal based on the catalyst weight. Preferably the alkali metal is cesium although lithium, sodium, potassium, rubidium and mixtures can also be used. Impregnation procedures such as are described in U.S. Patent 3,962,136 are advantageously employed for addition of the cesium component to the catalyst.

Necessary also for practice of the invention is the provision of sulfur as a promoting catalyst component. The sulfur component can be added to the catalyst support impregnating solution as sulfate, eg. cesium sulfate, ammonium sulfate, and the like. U.S. Patent 4,766,105 describes the use of sulfur promoting agents, for example at column 10, lines 53-60, and this disclosure is incorporated herein by reference. The use of sulfur (expressed as the element) in amount of 5-300 ppm by weight preferably 50-200 ppm by weight based on the weight of catalyst is essential in accordance with the invention.

The catalyst also contains a rare earth or lanthanide

promoter component. As used herein, the terms "lanthanide" or "rare earth" refer to the rare earth metals or elements having atomic numbers 57 through 71 in the Periodic Table of the Elements i.e., lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.

Preferred are praseodymium, neodymium, europium, terbium, dysprosium and holmium. Holmium is especially preferred.

Mixtures of the rare earth components can be used.

Promethium, being radioactive, is not preferred.

In the catalysts of the invention the rare earth or lanthanide component, expressed as the element, is used in amount of 10-500 ppm, preferably 50-300 ppm by weight based on the weight of the catalyst. The lanthanide component can be added to the catalyst support impregnating solution as a sulfate, chloride, acetate, nitrate or other salt.

The catalyst also optionally may contain a fluorine promoter in amount expressed as the element F of 10-300 ppm, preferably 50-200 ppm by weight based on the catalyst as an optional component. Ammonium fluoride, alkali metal fluoride, and the like can be used.

The catalysts are made with supports comprising alumina, silica, silica-alumina or combinations thereof. Preferred supports are those containing principally alpha-alumina, particularly those containing up to about 15 wt% silica.

Especially preferred supports have a porosity of about 0.1-1.0 cc/g and preferably about 0.2-0.7 cc/g. Preferred supports also have a relatively low surface area, i.e. about 0.2-2.0 m2/g, preferably 0.4-1.6 m2/g and most preferably 0.5-1.3 m2/g as determined by the BET method. See J. Am. Chem. Soc. 60, 3098-16 (1938). Porosities are determined by the mercury porosimeter method; see Drake and Ritter, "Ind. Eng. Chem. anal. Ed.," 17, 787 (1945). Pore and pore diameter distributions are determined from the surface area and apparent porosity measurements.

For use in commercial ethylene oxide production applications, the supports are desirably formed into regularly

shaped pellets, spheres, rings, etc. Desirably, the support particles may have "equivalent diameters" in the range from 3- 10 mm and preferably in the range of 4-8 mm, which are usually compatible with the internal diameter of the tubes in which the catalyst is placed. "Equivalent diameter" is the diameter of a sphere having the same external surface (i.e. neglecting surface within the pores of the particle) to volume ratio as the support particles being employed.

Preferably, the silver is added to the support by immersion of the support into a silver/amine impregnating solution or by the incipient wetness technique. The silver containing liquid penetrates by absorption, capillary action and/or vacuum into the pores of the support. A single impregnation or a series of impregnations, with or without intermediate drying, may be used, depending in part upon the concentration of the silver salt in the solution. To obtain catalyst having silver contents within the preferred range, suitable impregnating solutions will generally contain from 5- 50 wt% silver, expressed as metal. The exact concentrations employed, of course, will depend upon, among other factors, the desired silver content, the nature of the support, the viscosity of the liquid, and solubility of the silver compound.

Impregnation of the selected support is achieved in a conventional manner. The support material is placed in the silver solution until all of the solution is absorbed by the support. Preferably the quantity of the silver solution used to impregnate the porous support is no more than is necessary to fill the pore volume of the porous support.

The impregnating solution, as already indicated, is characterized as a silver/amine solution, preferably such as is fully described in U.S. Patent 3,702,259 the disclosure of which is incorporated herein by reference. The impregnation procedures described in U.S. Patent 3,962,136 are advantageously employed for the cesium component.

Known prior procedures of predeposition, co-deposition and postdeposition of the various promoters can be employed.

After impregnation, any excess impregnating solution is separated and the support impregnated with silver and the promoter or promoters is calcined or activated. In the most preferred practice of the invention, calcination is carried out as described in commonly assigned U.S. Patent 5,504,052 granted April 2, 1996 and copending application Serial No.

08/587,281 filed January 16, 1996, the disclosures of which are incorporated herein by reference. The calcination is accomplished by heating the impregnated support, preferably at a gradual rate, to a temperature in the range 200-500"C for a time sufficient to convert the contained silver to silver metal and to decompose the organic materials and remove the same as volatiles.

The impregnated support is maintained under an inert atmosphere while it is above 3000C during the entire procedure. While not wishing to be bound by theory, it is believed that at temperatures of 3000C and higher oxygen is absorbed in substantial quantities into the bulk of the silver where it has an adverse effect on the catalyst characteristics. Inert atmospheres as employed in the invention are those which are essentially free of oxygen.

An alternative method of calcination is to heat the catalyst in a stream of air at a temperature not exceeding 3000C, preferably not exceeding 2500C.

Catalysts prepared in accordance with the invention have improved performance, especially stability, for the production of ethylene oxide by the vapor phase oxidation of ethylene with molecular oxygen. These usually involve reaction temperatures of about 1500C to 4000C, usually about 2000C to 3000C, and reaction pressures in the range of from 0.5 to 35 bar. Reactant feed mixtures contain 0.5 to 20% ethylene and 3 to 15% oxygen, with the balance comprising comparatively inert materials including such substances as nitrogen, carbon dioxide, methane, ethane, argon and the like. Only a portion of the ethylene usually is reacted per pass over the catalyst and after separation of the desired ethylene oxide product and the removal of appropriate purge streams and carbon dioxide to

prevent uncontrolled build up of inerts and/or by-products, unreacted materials are returned to the oxidation reactor.

A disadvantage of the prior art rhenium promoted catalysts has been the instability associated with such catalysts. In accordance with the present invention, the rhenium free catalysts have advantageously high selectivity and high stability.

The following examples illustrate the invention.

Example 1 A silver solution was prepared using the following components (parts are by weight) Silver oxide - 834 parts Oxalic acid - 442 parts Deionized water - 2808 parts Ethylene Diamine - 415 parts Silver oxide was mixed with water, at room temperature, followed by the gradual addition of the oxalic acid. The mixture was stirred for 15 minutes and at that point the color of the black suspension of silver oxide had changed to the gray/brown color of silver oxalate. The mixture was filtered and the solids were washed with 3 liters of deionized water.

A container which contained the washed solids was placed in an ice bath and stirred while ethylene diamine and water (as a 72%/28% mixture) were added slowly in order to maintain the reaction temperature lower than 330C. After the addition of all the ethylene diamine water mixture the solution was filtered at room temperature. The clear filtrate was utilized as a silver/amine stock solution for the catalyst preparation.

The support used for the examples was obtained from Norton Company and was made primarily of alpha-alumina in the form of 5/16 inch cylinders. The support has a surface area of 0.65 m2/g, pore volume of .3 cc/g, and median pore diameter of 1.5 . For the examples, about 185 parts of the silver solution were mixed with varying amounts of: 1. CsOH solution, (8%Cs by weight in water),

2. ammonium fluoride, (3%F by weight in water) 3. ammonium hydrogen sulphate, (1%S by weight in water) and aqueous solution of the indicated lanthanide compound, the amounts of the promoter solutions being adjusted to give the promoter concentrations indicated in the tables. In certain comparative runs the use of various promoter solutions was omitted to give the indicated compositions. See Runs 1*, 2*, 3*, 4*, 6*, 7*, 8*, 9*, 10*, 11*, 12*, 13*, 14* and 15* in Table 2.

The mixture of silver stock solution and promoter solutions was stirred to assure homogeneity, then added to 400 parts of the support. The wet catalyst was mixed for ten minutes and then calcined.

Calcination, the deposition of silver compound, was induced by heating the catalyst up to the decomposition temperature of the silver salt. This was achieved via heating in a furnace that has several heating zones in a controlled atmosphere. The catalyst was loaded on a moving belt that entered the furnace at ambient temperature. The temperature was gradually increased as the catalyst passed from one zone to the next. It was increased, up to 4000C, as the catalyst passed through seven heating zones. After the heating zones the belt passed through a cooling zone that gradually cooled the catalyst to a temperature lower than 1000C. The total residence time in the furnace was 22 minutes. Atmosphere of the furnace was controlled through use of nitrogen flow in the different heating zones. In some instances, as indicated in the following table the calcination was carried out with air.

The catalysts were tested in a tube which was heated by a salt bath. A gas mixture containing 15% ethylene, 7% oxygen, and 78% inert, mainly nitrogen and carbon dioxide, was passed through the catalyst at 300 p.s.i.g., the temperature of the reaction was adjusted in order to obtain ethylene oxide productivity of 160 Kg per hour per m3 of catalyst and this temperature is given in the Table.

The results of the catalyst tests are summarized in Table 1.

Table 1 Run Cs F S Lanthanide Sel Temp metal ppm ppm ppm metal % CC cpd. ppm 1 565 75 85 150 La 83.3 236 La2 (S04)3 2 590 75 34 150 Ce 83.0 232 CeCl3 3 568 75 102 150 Ce 83.3 238 Ce (SO4)2 4 612 75 34 155 Pr 83.7 240 PrCl3 5 607 75 34 160 Nd 83.7 240 NdCl3 6 586 75 51 165 Sm 83.2 238 Sm2 (SO4)3 7 888 75 52 165 Eu 84.1 244 Eu2 (SO4)3 8 582 75 34 165 Eu 83.9 238 EuCl3 9 592 75 34 170 Gd 83.3 239 Gd(NO3)3 10 601 75 53 175 Tb 83.6 238 Tb2(SO4)3 11 616 75 34 175 Tb 83.6 242 Tb(NO3)3

Table 1 continued Run Cs F S Lanthanide Sel Temp metal ppm ppm ppm metal % OC cpd. ppm 12 598 75 34 175 Tb 83.8 237 TbCl3 13 638 75 52 180 Dy 83.7 236 Dy2 (SO4)3 14 598 75 34 180 Dy 83.9 240 DyCl3 15 633 75 34 180 Ho 84.4 240 HoCl3 16 628 75 34 180 Ho 83.8 240 Ho (NO3)3 17 597 75 53 185 Er 83.5 237 Er2 (SO4)3 18 621 75 34 185 Er 83.5 238 Er (NO3)3 19 611 75 34 185 Er 83.6 237 ErCl3 20 631 75 53 185 Tm 83.4 236 Tm2 (SO4)3 21 637 75 53 186 Yb 83.6 235 Yb2(SO4)3 22 618 75 34 189 Lu 83.5 237 Lu(NO3)3 From the results given above, it can be seen that the use of the lanthanide promoters in combination with cesium and sulfur components provides significantly improved catalyst performance.

The effect of fluoride can be seen in the results shown below in Table 2. The addition of 75 ppm F can, in some cases, result in a dramatic, cooperative increase in selectivity over that of the non-fluoride promoted catalysts.

In the table shown below, the best cases for non-fluoride and fluoride promoted catalysts are presented. In each instance, the lanthanide promoted catalysts outperformed the 82.8% selectivity achieved with similar levels of cesium, fluorine, and sulfur alone.

Table 2 Run Cs F S Lanthanide Sel Temp ppm ppm ppm metal % OC ppm *1 558 0 34 150 La 82.3 240 1 565 75 34 150 La 83.3 236 *2 607 0 34 150 Ce 81.9 234 3 568 75 102 150 Ce 83.3 238 *3 562 0 34 155 Pr 83.1 232 4 612 75 34 155 Pr 83.7 240 *4 578 0 34 160 Nd 82.9 234 5 607 75 34 160 Nd 83.7 240 *5 501 0 52 165 Eu 82.4 236 7 888 75 52 165 Eu 84.1 244 *6 465 0 52 170 Gd 82.7 231 9 611 75 52 170 Gd 83.3 242 *7 536 0 53 175 Tb 82.5 233 12 598 75 34 175 Tb 83.8 238 * Comparative

Table 2 continued Run Cs F S Lanthanide Sel Temp ppm ppm ppm metal % OC ppm *8 455 0 53 180 Dy 81.8 226 14 598 75 34 180 Dy 83.9 236 *9 500 0 34 180 Ho 82.2 231 15 633 75 34 180 Ho 84.4 240 *10 493 0 53 185 Er 82.5 230 18 611 75 34 185 Er 83.6 237 *11 606 0 53 185 Tm 83.3 235 20 631 75 53 185 Tm 83.4 236 *12 ; 625 0 53 186 Yb 83.1 235 21 637 75 53 186 Yb 83.6 235 *13 612 0 53 189 Lu 83.2 236 22 618 75 34 189 Lu 83.5 237 *14 611 0 34 0 82.4 237 *15 606 80 34 0 82.8 238 * Comparative