Ethylene oxide catalyst

A silver catalyst for ethylene oxidation to ethylene oxide is provided containing a promoter combination consisting of an alkali metal component, a sulfur component, and a lanthanide component, the catalyst being essentially free of rhenium and transition metal components; optionally the catalyst contains a fluorine component.

BACKGROUND OF THE INVENTION 
1. 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. 
2. 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. Pat. No. 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. Pat. No. 
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. Pat. Nos. 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. Pat. No. 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. Pat. No. 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. Pat. No. 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. Pat. No. 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. Pat. No. 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 m.sup.2 /g, 
preferably 0.4-1.6 m.sup.2 /g and most preferably 0.5-1.3 m.sup.2 /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. Pat. 
No. 3,702,259 the disclosure of which is incorporated herein by reference. 
The impregnation procedures described in U.S. Pat. No. 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. Pat. No. 5,504,052 
and U.S. Pat. No. 5,646,087, 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.degree. 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 300.degree. C. during the entire procedure. While not wishing to be 
bound by theory, it is believed that at temperatures of 300.degree. C. 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 300.degree. C., preferably not 
exceeding 250.degree. C. 
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 150.degree. C. to 400.degree. C., 
usually about 200.degree. C. to 300.degree. C., 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 33.degree. 
C. 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 m.sup.2 /g, pore volume of 0.3 cc/g, 
and median pore diameter of 1.5.mu.. 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 400.degree. C., 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 
100.degree. C. 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 m.sup.3 of catalyst and this 
temperature is given in the Table. 
The results of the catalyst tests are summarized in Table 1. 
TABLE 1 
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Lanthanide 
Cs F S metal Temp metal 
Run ppm ppm ppm ppm Sel % 
.degree. C. 
cpd. 
__________________________________________________________________________ 
1 565 75 85 150 La 83.3 236 La.sub.2 (SO.sub.4).sub.3 
2 590 75 34 150 Ce 83.0 232 CeCl.sub.3 
3 568 75 102 150 Ce 83.3 238 Ce(SO.sub.4).sub.2 
4 612 75 34 155 Pr 83.7 240 PrCl.sub.3 
5 607 75 34 160 Nd 83.7 240 NdCl.sub.3 
6 586 75 51 165 Sm 83.2 238 Sm.sub.2 (SO.sub.4).sub.3 
7 888 75 52 165 Eu 84.1 244 Eu.sub.2 (SO.sub.4).sub.3 
8 582 75 34 165 Eu 83.9 238 EuCl.sub.3 
9 592 75 34 170 Gd 83.3 239 Gd(NO.sub.3).sub.3 
10 601 75 53 175 Tb 83.6 238 Tb.sub.2 (SO.sub.4).sub.3 
11 616 75 34 175 Tb 83.6 242 Tb(NO.sub.3).sub.3 
12 598 75 34 175 Tb 83.8 237 TbCl.sub.3 
13 638 75 52 180 Dy 83.7 236 Dy.sub.2 (SO.sub.4).sub.3 
14 598 75 34 180 Dy 83.9 240 DyCl.sub.3 
15 633 75 34 180 Ho 84.4 240 HoCl.sub.3 
16 628 75 34 180 Ho 83.8 240 Ho(NO.sub.3).sub.3 
17 597 75 53 185 Er 83.5 237 Er.sub.2 (SO.sub.4).sub.3 
18 621 75 34 185 Er 83.5 238 Er(NO.sub.3).sub.3 
19 611 75 34 185 Er 83.6 237 ErCl.sub.3 
20 631 75 53 185 Tm 83.4 236 Tm.sub.2 (SO.sub.4).sub.3 
21 637 75 53 186 Yb 83.6 235 Yb.sub.2 (SO.sub.4).sub.3 
22 618 75 34 189 Lu 83.5 237 Lu(NO.sub.3).sub.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 
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Lanthanide 
CS F S metal 
Run ppm ppm ppm ppm Sel % Temp .degree. C. 
______________________________________ 
*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 
*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 
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*Comparative