Alkylene Oxide Catalysts Having Enhanced Activity and/or Efficiency

This invention relates to silver containing, supported catalysts for the epoxidation of alkene, especially ethylene, to the corresponding alkylene oxide, e.g., ethylene oxide, which contain an efficiency and/or activity enhancing amount of a scandium-containing component.

Ethylene oxide is commercially produced by the epoxidation of ethylene over silver-containing catalyst at elevated temperature. Considerable research efforts have been devoted to providing catalysts that increase the efficiency, or selectivity, of the process to ethylene oxide.

The manufacture of ethylene oxide by the reaction of oxygen or oxygen-containing gases with ethylene in the presence of a silver catalyst is an old and developed art. For example, U.S. Pat. No. 2,040,782, patented May 12, 1936, describes the manufacture of ethylene oxide by the reaction of oxygen with ethylene in the presence of silver catalysts which contain a class of metal promoters. In Reissue U.S. Pat. No. 20,370, dated May 18, 1937, Leforte discloses that the formation of olefin oxides may be effected by causing olefins to combine directly with molecular oxygen in the presence of a silver catalyst. From that point on, the prior art has focused its efforts on improving the catalyst's efficiency in producing ethylene oxide.

In characterizing this invention, the terms "conversion", "selectivity", and "yield" are employed as defined in U.S. Pat. No. 3,420,784, patented Jan. 7, 1969, at column 3, lines 24-35 inclusive. This definition of "selectivity" is consistent with that disclosed in U.S. Pat. No. 2,766,261 at column 6, lines 5-22, and U.S. Pat. No. 3,144,916, lines 58-61. The definitions of "yield" and "conversion" have more varied meaning in the art and are not to be employed as defined, for example, in the aforementioned U.S. Pat. No. 2,766,261. The terms "efficiency" and "selectivity", as used throughout the specification and claims are intended to be synonymous.

Silver catalysts employed in the manufacture of ethylene oxide have undergone significant changes since their initial period of development. As reported by the art, silver particles were first deposited upon support materials with little attention being paid to support properties, such as surface area, pore volume and chemical inertness. As the art evolved, special technologies related to carriers or supports containing silver were developed for catalysts used in the reaction of ethylene with oxygen to produce ethylene oxide. Today, most supports for the silver catalysts are shaped particulate materials which can be loaded in the interior of a reactor wherein the reacting gases and the gaseous products of the reaction are capable of flowing in and about these particulate materials to pass through the reactor and be recovered. The size and shape of the support are variable factors and the particular size and shape selected are peculiar to the reactor employed, the gas flow required, and the pressure drop across the reactor, with other factors also being considered.

The carriers that have been employed are typically made of inorganic materials, generally of a mineral nature. In most cases, the preferred carrier is made of alpha-alumina, such as has been described in the patent literature: see for example, U.S. Pat. Nos. 2,294,383; 3,172,893; 3,332,887; 3,423,328; and 3,563,914.

The carriers which are employed for the manufacture of most, if not all, commercially employed ethylene oxide catalysts are produced by companies who do not produce such catalysts. As a rule, the methods of making such carriers are trade secrets of significant value to the carrier manufacturers. Consequently, the catalyst manufacturer cannot know how the carrier is made. Critical to making a carrier which proves uniquely desirable for the manufacture of a successful catalyst can be a number of factors, such as the purity and other physical/chemical properties of raw materials used to make the carrier and the method by which the carrier is made.

The silver that is deposited on these carriers is thought to be in the form of small particles because that is all that can be seen by current microscopic techniques. The patent literature indicates that the size of the silver is a factor in the effectiveness of the catalyst and in most cases fine particle silver is obtained utilizing the standard processes in the art; see, for example, U.S. Pat. Nos. 2,554,459; 2,831,870; 3,423,328 (specifies that silver particles of 150-400 Angstroms are employed): 3,702,259 (disclosed a preparation procedure for forming silver particles less than 1 micron in diameter) and 3,758,418 (discloses silver particles having a diameter less than 1000 Angstroms). Improvements in microscopic examinations of silver catalysts enable the observation that the particle size ranges to even smaller values.

The deposition of silver onto the carrier can be achieved by a number of techniques but the two techniques which are most frequently employed involve, in one case, the impregnation of the support with a silver solution followed by treatment of the impregnated support to effect deposition of the silver on the support and, in the other case, the coating of the silver on the support by the precipitation of silver or the preformation of silver into a slurry such that the silver particles are deposited on the support and adhere to the support surface when the carrier or support is heated to remove the liquids present. These various procedures are exemplified in various U.S. Pat. Nos. such as 2,773,844; 3,207,700; 3,501,407; 3,664,970 (see British Patent 754,593) and 3,172,893.

The surface area provided by the support has been the subject of considerable interest in the development of silver catalysts. Disclosures concerning the surface area of the catalyst carrier can be found in U.S. Pat. No. 2,766,261 (which discloses that a surface area of 0.002-10 m.sup.2 /gm is suitable); U.S. Pat. No. 3,172,893 which depicts a porosity of 35-65% and a pore diameter of 80-200 microns); U.S. Pat. No. 3,725,307 which depicts a surface area of less than 1 sq.m/gm and an average pore diameter of 10-15 microns): U.S. Pat. No. 3,664,970 (which utilizes a support having a minimum porosity of about 30%, at least 90% of the pores having diameters in the range of 1-30 microns, and the average of such diameters being in the range of 4-10 microns); and U.S. Pat. No. 3,563,914 which utilizes a catalyst support having a surface area of less than 1 sq. m/gm, a volume of 0.23 ml/gm and a particle size between 0.074 and 0.30 mm). Low surface area, inert alpha-alumina is favored by the prior art.

It has been known for a long time that impurities present in the catalyst and/or the gas phase can materially impact upon the reaction. In the early development of the art, there were no techniques available for identifying or measuring such impurities. Consequently, one could not isolate the role that such impurities played. However, even in the earliest periods of the development of the art, the use of alkali metals as promoters for the silver catalyzed production of ethylene oxide was extremely well known in the art. U.S. Pat. No. 2,177,361, issued October 1939, has a teaching of the use of alkali metals in silver catalysts. U.S. Pat. No. 2,238,471 discloses that lithium is very desirable as a promoter but that potassium and cesium are detrimental when used in amounts of essentially 10% by weight of potassium hydroxide or cesium hydroxide to the silver oxide employed in making the catalyst. Later, U.S. Pat. No. 2,404,438 states that sodium and lithium are effective promoters for this reaction. Essentially the same teaching can be found in U.S. Pat. No. 2,424,084. U.S. Pat. No. 2,424,086 generalizes about alkali metals as promoters and specifies sodium in particular. In U.S. Pat. No. 2,671,764 (the Sacken sulfate patent), the patentees believe that alkali metals in the form of their sulfates are effective as promoters for such silver catalysts. In particular, the patentees state that sodium, potassium, lithium, rubidium or cesium sulfates may be used as promoters. U.S. Pat. No. 2,765,283 describes the pretreatment of a support with a dilute solution of a chlorine-containing compound and indicates that such chlorine compounds should be inorganic. Particular illustrations cited of suitable inorganic chlorine compounds included sodium chloride, lithium chloride and potassium chlorate. This patent specifies that the amount of the inorganic chlorine-containing compound which is deposited on the catalyst support is from 0.0001% to 0.2% by weight based on the weight of the support, U.S. Pat. No. 2,615,900 to Sears describes the use of metal halide in the treatment of the supported catalyst and-specifies that such halides can be of alkali metals such as lithium, sodium, potassium and cesium. The metal halide is present in the range of 0.01% to 50% based upon the weight of metallic silver. The patent also specifies that mixtures of the individual metal halides generally classified in the patent may be used to advantage to enhance the break-in period of a new catalyst composition while at the same time maintaining a moderate but steady activity of the catalyst over an extended period of time during normal operation. Thus, one particular metal halide treated catalyst would provide a short-term high initial activity whereas another of the metal halides would provide a longer term moderate activity for the catalyst. This patent takes the position that the metal halides which are provided in the catalyst serve to inhibit the combustion of ethylene to carbon dioxide and thus classifies these materials as catalyst depressants or anticatalytic materials. U.S. Pat. No. 2,709,173 describes the use of a silver catalyst for making ethylene oxide in which there are provided simultaneously with the introduction of silver to the solid support, any of the alkali metal halides such as lithium, sodium, potassium, and rubidium compounds of chlorine, bromine and iodine, to enhance the overall production of ethylene oxide. The patent specifies small amounts "of less than about 0.5% are desirable." In particular, the patent emphasizes "proportions of alkali metal halide within the range of about 0.0001 to about 0.1%" are most preferred. The patent states that "although the preferred catalyst composition contains a separate promoter it is not always necessary since during preparation of the catalyst the alkali metal halide may be converted to some extent to the corresponding alkali metal oxide which acts as a promoter." U.S. Pat. No. 2,766,261 appears to draw from the teachings of U.S. Pat. No. 2,238,474 in that cesium and potassium are said to be detrimental in silver catalysts; sodium and lithium are suggested as useful promoters. However, U.S. Pat. No. 2,769,016 finds that sodium, potassium and lithium are promoters when used in the silver catalysts. This latter patent also recommends the pretreatment of the support with dilute solutions of sodium chloride, lithium chloride or potassium chlorate. U.S. Pat. No. 2,799,687 to Gould, et al., states that the addition of metal halides within the range described by Sears in U.S. Pat. No. 2,615,900 is not productive of optimum results. This is said to be especially true in the case of alkali metal halides, particularly the chloride and fluoride of sodium and potassium. The patentees recommend that the inorganic halide component of the catalyst be maintained within the range of 0.01-5 weight percent, preferably 0.01 to 0.1 weight percent, based on the weight of the "silver oxidative catalytic component," i.e., the silver salt transformed into elemental silver. U.S. Pat. 3,144,416 mentions a variety of metals as promoters and one of them is cesium. U.S. Pat. No. 3,258,433 indicates that sodium is an effective promoter. U.S. Pat. No. 3,563,913 recommends the use of alkali metals such as lithium compounds as promoters. The preferred amount of promoting material is said to be about 0.03 to 0.5%, by weight of metal oxide based on the weight of the support. U.S. Pat. No. 3,585,217 states that alkali metal chlorides "are known to counteract the formation of carbon dioxide" and "may be incorporated into the catalyst." U.S. Pat. No. 3,125,538 discloses a supported silver catalyst containing a coincidentally-deposited alkali metal selected from among potassium, rubidium and cesium in a specified gram atom ratio relative to silver. The weight of silver is preferably 2-5% by weight of the catalyst. The patentees characterize this catalyst as being especially suitable for the reaction of nitric oxide with propylene. This same catalyst is produced inherently by the processes of the examples of U.S. Pat. No. 3,702,259, as discussed previously, which patent promotes their use for making ethylene oxide. U.S. Pat. Nos. 3,962,136 and 4,012,425 also disclose that same catalyst as being useful for ethylene oxide production. U.S. Pat. No. 3,962,136 describes the coincidental deposition of alkali metal with the silver on the support, the alkali metals being present in their final form on the support in the form of an oxide in which the oxide consists of cesium, rubidium or mixtures of both, optionally combined with a minor amount of an oxide of potassium. The amount of such oxide is from about 4.0.times.10.sup.-5 gew/kg to about 8.0.times.10.sup.-3 gew/kg of total catalyst. However, U.S. Pat. No. 4,010,115, patented Mar. 1, 1977, purports to distinguish itself from the other patents by employing as the oxide of the alkali metal the oxide of potassium optionally combined with a minor amount of an oxide of rubidium or cesium. U.S. Pat. No. 4,356,312 describes the use of the same catalyst. U.S. patent application Ser. No. 317,349, filed Dec. 21, 1972, which is a parent to U.S. Pat. Nos. 3,962,136 and 4,010,115 and others, contains some interesting data deserving of comment. According to example 2 which contains some comparative experiments, there is described the manufacture of a catalyst which contains 310 parts per million by weight of coincidentally-added potassium and that catalyst when employed as an ethylene oxidation catalyst was found to be inactive for the production of ethylene oxide.

U.S. Pat. No. 4,207,210 (corres. Belgium Patent No. 821,439, based upon British Patent Specification 1,489,335) discloses that a catalyst can be made that is equivalent to that produced in the so called parent applications cited in U.S. Pat. Nos. 3,962,136, 4,012,425, and 4,010,115 by using a sequential procedure by which the alkali metal is supplied to the support. Thus, the criticality in the method of deposition of alkali metal in the catalyst appears doubtful in the face of that type of disclosure and the disclosure of U.S. Pat. Nos. 4,033,903 and 4,125,480 which describe subjecting used silver-containing catalysts to a post-addition of one or more of potassium, rubidium or cesium. Apparently, such treatment regenerates the catalyst's ability to enhance selectivity to ethylene oxide. Another patent which tends to indicate that a post-addition of alkali metal such as cesium gives results equivalent to either pre-addition or simultaneous addition is U.S. Pat. No. 4,066,575.

German Offenlegungsschrift 2,640,540 discloses in its examples a silver catalyst for ethylene oxide production containing sodium and either potassium, rubidium or cesium.

Japanese Application Publication Disclosure No. 95213/75 is directed to a process for producing ethylene oxide using a catalyst composition comprising silver, barium, potassium and cesium in specified atomic ratios. Table I of this disclosure summarizes the efficiencies achieved with the various catalyst compositions of the examples.

U.S. Pat. No. 4,039,561 discloses a catalyst for preparing ethylene oxide containing silver, tin, antimony, thallium, potassium, cesium and oxygen in specified atomic ratios.

Belgium Patent 854,904 discloses silver catalysts containing various mixtures of sodium and cesium. U.K. Patent Application 2,002,252 discloses, in Table 2, supported silver catalysts containing various mixtures of cesium and thallium, some of which additionally contain potassium or antimony. U.S. Pat. No. 4,007,135 broadly discloses (in column 2, lines 25-30) silver catalysts for alkylene oxide production containing silver "together with 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 . . . ". U.S. Pat. Nos. 3,844,981 and 3,962,285 disclose catalysts and processes for epoxidizing olefins in the presence of a multimetallic component. The catalyst in the 3,962,285 patent is said to comprise a minor amount of one or more of palladium, ruthenium, rhenium, iron and platinum with a major amount of silver. The 3,844,981 patent discloses the preparation of the catalyst from a decomposible salt of group 7b, 1b or the iron group of group 8 of the Periodic Table of the Elements. Preferably, the salt is selected from the group of gold, copper, rhenium, manganese and iron salts. While the patentee contemplates that these metals are in the metallic state, oxidation during epoxidation conditions may occur with one or more of these metals, e.g., rhenium, to form oxyanions containing the metal.

U.S. Pat. No. 2,605,239 discloses the use of beryllium oxide as a promoter. Other promoter metals such as copper, aluminum, manganese, cobalt, iron, magnesium, gold, thorium, nickel, cesium and zinc are suggested. These promoter metals are to be incorporated into the catalyst by mechanical mixture or coprecipitation.

European Patent Publication No. 0003642 discloses, in Table 3, silver-containing catalysts which include mixtures of potassium and cesium, and a catalyst containing sodium and cesium.

Belgium Patent 867,045 discloses supported silver catalysts containing what is referred to as an effective proportion of lithium and a substantially lesser amount of alkali metal selected from among cesium, rubidium and/or potassium.

Belgium Patent 867,185 discloses supported silver catalysts for ethylene oxide production containing a specified amount of potassium and at least one other alkali metal selected from rubidium and cesium.

United Kingdom Patent No. 2,043,481, commonly assigned, describes the use of a synergistic combination of cesium and at least one other alkali metal in combination with silver on an inert support to provide catalysts which were superior to those known to the art at that time. Such catalysts have been widely employed commercially. The alkali metal components are provided to the support by a variety of ways. The alkali metal can be supplied to the support as a salt and many salts of the alkali metals are described. Specific illustration is made of the use of alkali metal sulfates as one of many usable alkali metal compounds.

European Patent Application 85,237 describes an ethylene oxide catalyst wherein the applicants believe they "chemically absorbed" by alcohol wash, cesium and/or rubidium onto the catalyst support.

Japanese patent application Kokai 56/105,750 discloses, among other things, ethylene oxide catalysts containing cesium molybdate or cesium tungstate or cesium borate. The catalyst is stated to have an alumina carrier having a sodium content of less than 0.07 weight % and mainly consisting of alpha-alumina having a specific surface area of 1 to 5 sq. m./gm. The carrier is impregnated with decomposible silver salt solution containing alkali metal boron complex, alkali metal molybdenum complex and/or alkali metal tungsten complex. No examples of mixtures of anions are disclosed. Japanese patent application Kokai 57/21937 discloses thallium-containing catalysts in which the thallium may be borate or titanate salt.

European patent application 247,414, published Dec. 12, 1987, discloses catalysts containing alkali metal and/or barium which may be provided as salts. The salts include nitrates, sulfates, and halides. U.S. Pat. Nos. 4,761,394 at 4,766,105 disclose catalysts containing a rhenium component, e.g., rhenium oxide, rhenium cation or rhenate or perrhenate anion.

U.S. patent applications Ser. Nos. 18,808, filed Feb. 20, 1987, now abandoned; 640,269, filed Aug. 13, 1984, now abandoned; and 251,573, now U.S. Pat. No. 4,908,343, and 251,814, both filed Oct. 3, 1988, M. M. Bhasin, disclose silver-containing, supported catalysts for ethylene oxide production containing cesium salts of oxyanions having an atomic number of at least 15 to 83 and being from groups 3b through 7b and/or groups 3a through 7a of the Periodic Table of the Elements (as published by The Chemical Rubber Company, Cleveland, Ohio, in CRC Handbook of Chemistry and Physics, 46th Edition, inside back cover). The oxyanions include, by way of illustration, sulfate, phosphates, manganates, titanates, tantalates, molybdates, vanadates, chromates, zirconates, polyphosphates, tungstates, cerates, and the like.

While improved efficiencies of conversion to ethylene oxide are desirable, the concommitant increase in temperature (i.e., loss of activity) can be troublesome for a commercially-viable catalyst. Commercial ethylene oxide plants are typically operated to provide a desired balance of productivity and efficiency. Less active catalysts are thus operated at higher temperatures to achieve desired productivity. However, the upper temperature range of the catalyst is limited. Consequently, catalysts that have high initial temperatures for a given conversion rate may have shorter useful lives. Not only is catalyst a major expense to the ethylene oxide plant owner, but also, the plant must be shut down for substantial periods of time to discharge the old catalyst and charge new catalyst to the typical tubular, fixed bed ethylene oxide reactors. Hence, without a useful lifetime, e.g., two years or more, the benefit of any enhanced efficiency is quickly lost in catalyst replacement costs and plant shut down time.

Methods are sought to enhance the activity and/or efficiency, without adversely affecting stability, or useful life, of silver-containing, supported ethylene oxide catalysts which have been promoted to enhance efficiency, which while providing desirable efficiencies, are typically less active and must be operated at higher temperatures to be useful in commercial production facilities. These high temperatures can unduly shorten the catalyst life such that the catalysts are unattractive for commercial facilities.

U.S. Pat. No. 3,957,690 discloses heterogeneous catalysts for the oxidation of propylene into propylene oxide. The catalysts have a mineral acid-modified carrier with at least one active component supported thereon. The active component is an oxide of a metal selected from the group consisting of scandium, yttrium, indium, gallium, thallium and rare earth elements of the lanthanum group. The patentees disclose as carriers, silica gel, aluminogel and zeolite. Example 1 reports a silica gel carrier containing 30 percent scandium oxide calculated as metal with reference to carrier weight. Acid treatment is conducted with 1:1 ratio of 35 percent hydrochloric acid and 50 percent nitric acid. The patentees state:

"Moreover, the proposed heterogeneous catalyst can also contain the active component-silver, or oxide of scandium, molybdenum, tungsten, bismuth, manganese, or tantalum, or mixtures of the above active components. The preferable content of silver in the catalyst is from 0.1 to 5 per cent of the carrier weight." (Column 1, lines 45 to 50)

U.S. Pat. No. 4,046,784 discloses boride catalysts for the liquid phase epoxidation of olefinic compounds using organic hydroperoxides. The catalysts are characterized as having the formula M.sub.x B or M.sub.x B.sub.y R wherein M and R are elements selected from groups II-A, III-B, IV-B, V, VI-B, VII-B, VIII, III-A, IV-A and V-A of the Periodic Table, the rare earths, and the actinides.

U.S. Pat. No. 3,946,057 discloses the preparation of metal complexes of 1,3-diketones by reacting in the presence of an alkylene oxide, a metal halide or hydrates thereof, and an organic 1,3 diketone. Among the metals for the metal halides is included scandium.

U.S. Pat. Nos. 3,836,481; 3,899,445 and 4,257,967 disclose silver catalysts for the oxidation of ethylene to ethylene oxide which catalysts comprise a promoter of yttrium, lanthanum, praseodymium, neodymium, terbium and dysprosium.

By this invention silver-containing, supported alkylene oxide catalysts suitable for the epoxidation of alkene to alkylene oxide are provided that have enhanced activity and/or efficiency. The catalysts contain a sufficient amount of at least one scandium component to increase the activity and/or efficiency of the catalyst as compared to a similar catalyst but not containing the scandium component. The catalysts of this invention preferably contain an amount of efficiency-enhancing scandium component at least sufficient to increase the efficiency of the catalyst, as determined under Standard Ethylene Oxide Process Conditions (herein defined), by at least about 0.5 efficiency percentage point, preferably at least about 1 efficiency percentage point. Often, the scandium component is present in an amount of at least about 10, e.g., about 30 to 2000, preferably 100 to 1000, ppm (weight) calculated as the weight of scandium based on the total weight of the catalyst.

The scandium component is provided as one or more scandium containing compounds which have promoter activity, i.e., enhance at least one of activity and efficiency. In an aspect of this invention, the catalyst is prepared such that the scandium component is provided in the catalyst as a compound having promoter activity.

As used herein, the term "compound" refers to the combination of a particular element with one or more different elements by surface and/or chemical bonding, such as ionic and/or covalent and/or coordinate bonding. The term "ionic" or "ion" refers to an electrically chemical charged moiety; "cationic" or "cation" being positive and "anionic" or "anion" being negative. The term "oxyanionic" or "oxyanion" refers to a negatively charged moiety containing at least one oxygen atom in combination with another element. An oxyanion is thus an oxygen-containing anion. It is understood that ions do not exist in vacuo, but are found in combination with charge-balancing counter ions.

The catalyst preferably contains at least one other promoter in an amount sufficient to enhance the efficiency and/or activity of the catalyst as compared to a similar catalyst but not containing the promoter. Often, the promoter comprises a compound of an element other than scandium component which is selected from Groups 3b to 7b of the Periodic Table. (References to the Periodic Table herein shall be to that as published by the Chemical Rubber Company, Cleveland, Ohio, in CRC Handbook of Chemistry and Physics, 46th Edition, inside back cover.) The preferred promoters include the halides, e.g., fluorides and chlorides, and the oxyanions of the elements other than oxygen having an atomic number of 5 to 83 of Groups 3b to 7b and 3a to 7a of the Periodic Table. Most preferably, the promoters are one or more of the oxyanions of nitrogen, sulfur, manganese, tantalum, molybdenum, tungsten and rhenium.

An aspect of this invention relates to the use of the aforementioned catalysts in epoxidizing in the vapor phase alkene to alkylene oxide, especially ethylene to ethylene oxide.

Alkylene oxides made using the catalysts of this invention are characterized by the structural formula ##STR1## wherein R.sup.1 and R.sup.2 are lower alkyl, e.g., methyl or ethyl or, preferably, hydrogen. Most preferably the alkylene oxide is ethylene oxide. The alkylene oxides are made from the corresponding alkene, i.e., R.sup.1 HC=CHR.sup.2. For purposes of ease of understanding, the following discussion will be made with reference to ethylene oxide and ethylene.

The catalysts of this invention are characterized by containing a sufficient amount of at least one scandium component to enhance the activity and/or efficiency of the catalyst as compared to a similar catalyst which does not contain the scandium component. Although the catalysts can be used under widely varying process conditions, for purposes of determining whether sufficient scandium component has been incorporated into the catalyst, a standard set of process conditions can be used.

The STANDARD ETHYLENE OXIDE PROCESS CONDITIONS (ABBR. "CONDITIONS") for characterizing the catalysts of this invention involve the use of a standard backmixed autoclave with full gas recycle including carbon dioxide The CONDITIONS may be operated with some variation in ethylene, oxygen and gas phase inhibitor feed. Two cases are illustrated: air process conditions, which simulates in the backmixed reactor the typical conditions employed in commercial air-type ethylene oxide processes where air is used to supply the molecular oxygen and the oxygen process conditions, which simulates in the backmixed reactor the typical conditions in commercial oxygen-type ethylene oxide processes where molecular oxygen, as such, is employed. Each case provides a different efficiency but it is the rule for practically all cases that air as the oxygen feed, using lower amounts of oxygen and ethylene will yield an efficiency to ethylene oxide which is about 2 to 4 percentage points lower than that when molecular oxygen is employed as oxygen feed. The CONDITIONS employ the well known backmixed bottom agitated "Magnedrive" autoclaves described in FIG. 2 of the paper by J. M. Berty entitled "Reactor for Vapor Phase Catalytic Studies", in Chemical Engineering Progress, Vol. 70, No. 5, pages 78-84. 1974. The CONDITIONS employ 2.0 mole % ethylene oxide in the outlet gas of the reactor under the following standard inlet conditions:

The pressure is maintained constant at 275 psig and the total outlet flow is maintained at 11.3 SCFH. SCFH refers to cubic feet per hour at standard temperature and pressure, namely, 0.degree. C. and one atmosphere. The outlet ethylene oxide concentration is maintained at 2.0% by adjusting the reaction temperature. Thus temperature (.degree. C.) and catalyst efficiency are obtained as the responses describing the catalyst performance.

The catalyst test procedure used in the CONDITIONS involves the following steps:

1. 80 cc of catalyst is charged to the backmixed autoclave. The volume of catalyst is measured in a 1 inch I.D. graduated cylinder after tapping the cylinder several times to thoroughly pack the catalyst. The volume of catalyst is alternatively calculated from the packing density of the carrier and the amount of silver and additives. The weight of the catalyst is noted.

2. The backmixed autoclave is heated to about reaction temperature in a nitrogen flow of 20 SCFH with the fan operating at 1500 rpm. The nitrogen flow is then discontinued and the above-described feed stream is introduced into the reactor. The total gas outlet flow is adjusted to 11.3 SCFH. The temperature is adjusted over the next few hours so that the ethylene oxide concentration in the outlet gas is approximately 2.0%.

3. The outlet oxide concentration is monitored over the next 4-6 days to make certain that the catalyst has reached its peak steady state performance. The temperature is periodically adjusted to achieve 2% outlet oxide. The selectivity of the catalyst to ethylene oxide and the temperature are thus obtained.

The standard deviation of a single test result reporting catalyst efficiency in accordance with the procedure described above is about 0.7% efficiency units. The standard deviation of a single test result reporting catalyst activity in accordance with the procedure described above is about 1.2.degree. C. The standard deviation, of course, will depend upon the quality of the equipment and precision of the techniques used in conducting the tests, and thus will vary. The test results reported herein are believed to be within the standard deviation set forth above. The running of a multiplicity of tests will reduce the standard deviation by the square root of the number of tests.

The amount of scandium component is sufficient to provide an increase in efficiency under Standard Ethylene Oxide Process Conditions of at least about 0.5, preferably at least about 1.0, efficiency percentage point under Oxygen Process Conditions at 2.0% outlet ethylene oxide. In determining the increase in efficiency, the process and catalyst should be under steady state conditions, and can often be ascertained promptly upon steady state conditions being achieved.

The optimal amount of the scandium component may vary with silver content and particle size, the amounts and types of other promoters present and the chemical and physical properties of the carrier. However, the scandium component is often present in an amount of at least 10 ppmw (parts per million by weight) calculated as the weight of scandium as the metal. If too much scandium component is used, the catalyst performance, e.g., efficiency and/or activity, may suffer. Usually, the amount of scandium component falls within the range of about 30 to 2000, preferably, 100 to 1000, ppmw calculated as the weight of scandium as the metal.

The form of the scandium component in the catalyst is not certain. Scandium can be in a chemical form which is very deleterious to catalyst activity. While not wishing to be limited to theory, it is believed that scandium component having promoter activity is one or more compounds of scandium in which scandium is covalently bonded, e.g., as scandium oxide, or a cation as present in halide, sulfate, phosphate, etc., salt.

The scandium species that provides enhanced activity and/or efficiency may be the component added or that generated either during catalyst preparation or during use as a catalyst. The scandium component may be provided with the carrier or later provided on the catalyst. Scandium components that may find use in preparing the catalyst include, but are not limited to, scandium acetate, scandium ammonium sulfate, scandium citrate, scandium dithionate, scandium oxalate, scandium nitrate, scandium sulfate, scandium beta-diketone complexes (e.g., scandium acetylacetonate), scandium chloride, scandium bromide and scandium fluoride.

As with any catalyst for making ethylene oxide which provides optimum performance, a correlation exists among many factors. Factors frequently considered include:

(i) the nature of the support;

(ii) the amount of silver on or in the support;

(iii) the components and amounts thereof in or on the support;

(iv) the impurities or contaminants provided with the silver or other components;

(v) the procedure to make the catalyst; and

(vi) the conditions under which the catalyst is used to produce ethylene oxide.

However, in attempting to define any catalyst, there must be a base value from which other factors ar determined especially when the factors are variables, each dependent upon the base value for meaning. In the case of this invention, the base value can be the amount of silver or a combination of the amount of silver and the nature of the support. In most cases the latter combination will be the base value. Because at least two values will comprise the base value for catalyst performance, it is apparent that correlations between such combinations and other factors can be quite complex. There is no common thread of logic which integrates all of these combinations and/or factors. To that extent, practice of the invention requires experimental efforts to achieve all or essentially all of the benefits of this invention. Without departing from this script, one skilled in the art can readily achieve the optimum performances of the catalysts of this invention. It should be recognized that such script is commonly followed by the artisan in making any commercially-employable ethylene oxide catalyst. The elements of the script are dependent upon the technology employed in making the catalyst.

The concentration of silver in the finished catalyst may vary from about 2 to 45 or more, often about 2 to 40 or more, weight percent, a commercially preferred range being from about 6% to about 35% by weight of silver. Lower silver concentrations are preferred from an economic standpoint. However, the optimum silver concentration for any particular catalyst will be dependent upon economic factors as well as performance characteristics, such as catalyst efficiency, rate of catalyst aging and reaction temperature.

The support or carrier employed in these catalysts in its broadest aspects is selected from the large number of porous refractory catalyst carriers or support materials which are considered relatively inert in the presence of the ethylene epoxidation feeds, products and reaction conditions. Many such materials are known to persons skilled in the art and may be of natural or synthetic origin and preferably are of a macroporous structure.

The chemical composition of the carrier is not narrowly critical. Carriers may be composed, for example, of alpha-alumina, silicon carbide, silicon dioxide, zirconia, magnesia and various clays. The preferred carriers are alpha-alumina particles often bonded together by a bonding agent and have a very high purity, i.e., at least 98 wt. % alpha-alumina, any remaining components being silica, alkali metal oxides (e.g., sodium oxide) and trace amounts of other metal-containing and/or non-metal-containing additives or impurities; or they may be of lower purity, i.e., about 80 wt. % alpha-alumina, the balance being a mixture of silicon dioxide, various alkali oxides, alkaline earth oxides, iron oxides, and other metal and non-metal oxides. The carriers are formulated so as to be inert under catalyst preparation and reaction conditions. A wide variety of such carriers are commercially available. Alumina carriers are manufactured by United Catalysts, Inc., Louisville, Ky., and the Norton Company, Akron, Ohio.

In the case of alpha alumina-containing supports preference is given to those having a specific surface area as measured by the B.E.T. method of from about 0.03 m.sup.2 /g to about 10 m.sup.2 /g, preferably from about 0.05 to about 5, more preferably from about 0.1 to about 3 m.sup.2 /g, and a water pore volume as measured by conventional water absorption techniques of from about 0.1 to about 0.85 cc/g by volume. The B.E.T. method for determining specific surface area is described in detail in Brunauer, S., Emmet, P. and Teller, E. J. Am. Chem. Soc., 60, 309-16 (1938).

Certain types of alpha alumina-containing supports are particularly preferred. These alpha alumina supports have relatively uniform pore diameters and are more fully characterized by having (1) B.E.T. specific surface areas of from about 0.1 m.sup.2 /g to about 3.0 m.sup.2 /g, preferably about 0.1 m.sup.2 /g to about 2.0 m.sup.2 /g and (2) water pore volumes of from about 0.10 cc/g to about 0.85 cc/g, preferably from about 0.25 cc/g to about 0.75 cc/g. Median pore diameters for the above described carriers range from about 0.01 to 100 microns, a more preferred range being from about 0.5 to 50 microns. The carriers may have monomodal, bimodal or multimodal pore distributions.

Regardless of the character of the support or carrier used, it is preferably shaped into particles, chunks, pieces, pellets, rings, spheres, wagon wheels, and the like of a size suitable for employment in fixed bed reactors. Conventional commercial fixed bed ethylene oxide reactors are typically in the form of a plurality of parallel elongated tubes (in a suitable shell) approximately 0.7 to 2.7 inches O.D. and 0.5 to 2.5 inches I.D. and 15-45 feet long filled with catalyst. In such reactors, it is desirable to employ a support formed into a rounded shape, such as, for example, spheres, pellets, rings, tablets and the like, having diameters from about 0.1 inch to about 0.8 inch.

As with any supported catalyst, the optimal performance will depend upon optimizing the carrier in terms of its chemical composition/including impurities), surface area, porosity and pore volume.

The catalysts of this invention preferably contain, in addition to the scandium component, at least one other promoter or modifier to enhance the performance of the catalyst, e.g., to enhance efficiency or reduce the burning of ethylene oxide or affect activity. These promoters or modifiers are generally provided as chemical compounds.

For the sake of ease of understanding, the promoters will be referred to in terms of cation promoters, e.g., alkali and alkaline earth metals, and anion promoters. Compounds such as alkali metal oxide or MoO.sub.3, while not being ionic, may convert to ionic compounds, e.g., during catalyst preparation or in use. Whether or not such a conversion occurs, they will be referred to herein in terms of cation and anion species, e.g., alkali metal or molybdate.

Frequently, the catalyst contains alkali metal and/or alkaline earth metal as cationic promoter. Exemplary of the alkali metal and/or alkaline earth metals are lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium and barium. Other cationic promoters include Group 3b metal ions including lanthanide series metals. In some instances, the promoter comprises a mixture of cations, e.g., cesium and at least one other alkali metal, to obtain a synergistic efficiency enhancement as described in British Patent No. 2,043,481 discussed above.

Cesium salts alone or in combination with other salts are often used. Frequently, mixtures of cesium salts are used which comprise at least one halide and/or at least one cesium salt of an oxyanion of an element (other than oxygen) having an atomic number of 5 to 83 and being from groups 3b to 7b or groups 3a to 7a, inclusive, of the Periodic Table. In some instances, it has been found beneficial to add more anion than is required to associate with the total alkali metal and alkaline earth metal being provided to the catalyst. The reason why such additional anion is beneficial in these situations is not known. The additional anion may be added in the form of an acid, an ammonium salt, an amine salt, etc., or a portion of the alkali metal and/or alkaline earth metal may be added as an acid salt, e.g., cesium hydrogen sulfate.

The concentration of cesium salt and any other alkali metal and alkaline earth metal salts in the finished catalyst is not narrowly critical and may vary over a wide range. The optimum cesium salt and other salt concentration for a particular catalyst will be dependent upon performance characteristics, such as, catalyst efficiency, rate of catalyst aging and reaction temperature. The concentration of cesium salt in the finished catalyst may vary from about 0.0005 to 1.0 weight percent, preferably from about 0.005 to 0.1 weight percent. Cesium salts alone, or together with at least one other alkali or alkaline earth metal salt, can be employed in the finished catalyst. The ratio of cesium salt to any other alkali metal and alkaline earth metal salt(s), if used, to achieve desired performance is not narrowly critical and may vary over a wide range. The ratio of cesium salt to the other salt(s) may vary from about 0.0001:1 to 10,000:1, preferably from about 0.001:1 to 1,000:1. Preferably, cesium comprises at least about 10, more preferably, about 20 to 100, percent (weight) of the total added alkali metal and alkaline earth metal in the finished catalyst.

The preferred amount of cation promoter deposited on or present on the surface of the carrier or catalyst generally lies between about 10 and about 4000, preferably about 15 and about 3000 and more preferably between about 20 and about 2500 ppm by weight of cation calculated on the total carrier material. Amounts between about 50 and about 2000 ppm are frequently most preferable.

The types of anion promoters or modifiers suitable for use in the catalysts of this invention comprise, by way of example only, oxyanions such as sulfate, SO.sub.4.sup.-2, phosphates, e.g., PO.sub.4.sup.-3, titanates, e.g., TiO.sub.3.sup.-2, tantalates, e.g. Ta.sub.2 O.sup.-2, molybdates, e.g. MoO.sub.4.sup.-2, vanadates, e.g. V.sub.2 O.sub.4.sup.-2, chromates, e.g., CrO.sub.4.sup.-2, zirconates, e.g., ZrO.sub.3.sup.-2, polyphosphates, manganates, nitrates, chlorates, bromates, borates, silicates, carbonates, tungstates, thiosulfates, cerates and the like. The halide ions may also be present as anions and include fluoride, chloride, bromide and iodide. Other salts such as sulfides may find application. It is well recognized that many anions have complex chemistries and may exist in one or more forms, e.g., orthovanadate and metavanadate; and the various molybdate oxyanions such as MoO.sub.4.sup.-2, Mo.sub.7 O.sub.24.sup.-6 and Mo.sub.2 O.sub.7.sup.-2. The oxyanions may also include mixed metal containing oxyanions including polyoxyanion structures. For instance, manganese and molybdenum can form a mixed metal oxyanion. Similarly, other metals, whether provided in anionic, cationic, elemental or covalent form may enter into anionic structures.

While an oxyanion, or a precursor to an oxyanion, may be used in solutions impregnating a carrier, it is possible that during the conditions of preparation of the catalyst and/or during use, the particular oxyanion or precursor initially present may be converted to another form. Indeed, the element may be converted to a cationic or covalent form. Preferably, the element is associated with oxygen, i.e., is an oxyanion, a covalent oxide or has an oxygen containing anion. In many instances, analytical techniques may not be sufficient to precisely identify the species present. The invention is not intended to be limited by the exact species that may ultimately exist on the catalyst during use but rather reference herein to oxyanions is intended to provide guidance to understanding and practicing the invention.

A particularly preferred anion promoter includes the sulfates and oxyanions of rhenium, molybdenum, tungsten and/or chromium. Examples of anions of sulfur that can be suitably applied include sulfate, sulfite, bisulfite, bisulfate, sulfonate, persulfate, thiosulfate, dithionate, dithionite, halosulfate, e.g., fluosulfate, etc. Preferred compounds to be applied are ammonium sulfate and the alkali metal sulfates. Examples of anions of molybdenum, tungsten and chromium that can be suitably applied include molybdate, dimolybdate, paramolybdate, other iso- and heteropolymolybdates, etc.; tungstate, paratungstate, metatungstate, other iso- and hetero- polytungstates, etc.; and chromate, dichromate, chromite, halochromate, etc. Preferred are sulfates, molybdates, tungstates and chromates. Examples of rhenium compounds include the rhenium salts such as rhenium halides, the rhenium oxyhalides, the rhenates, the perrhenates, the oxides and the acids of rhenium. However, the alkali metal perrhenates, alkaline earth metal perrhenates, silver perrhenates, other perrhenates and rhenium heptoxide can also be suitably utilized. Rhenium heptoxide, Re.sub.2 O.sub.7, when dissolved in water, hydrolyzes to perrhenic acid, HReO.sub.4, or hydrogen perrhenate. Thus, for purposes of this specification, rhenium heptoxide can be considered to be a perrhenate, i.e., ReO.sub.4. Similar chemistries can be exhibited by other metals such as molybdenum and tungsten.

The amount of anion promoter may vary widely, e.g., from about 0.0005 to 2 weight percent, preferably from about 0.001 to 0.5 weight percent based on the total weight of the catalyst.

Another class of promoters includes manganese components. In many instances, manganese components can enhance the activity and/or stability of catalysts. The manganese species that provides the enhanced activity and/or stability is not certain and may be the component added or that generated either during catalyst preparation or during use as a catalyst. Manganese components include, but are not limited to, manganese acetate, manganese ammonium sulfate, manganese citrate, manganese dithionate, manganese oxalate, manganous nitrate, manganous sulfate, and manganate anion, e.g., permanganate anion, manganate anion, and the like.

In any event, the cation and/or anion promoters are provided in a promoting amount. As used herein the term "promoting amount" of a certain component of a catalyst refers to an amount of that component that works effectively to provide an improvement in one or more of the catalytic properties of that catalyst when compared to a catalyst not containing said component. Examples of catalytic properties include, inter alia, operability (resistance to run-away), selectivity, activity, conversion, stability and yield. It is understood by one skilled in the art that one or more of the individual catalytic properties may be enhanced by the "promoting amount" while other catalytic properties may or may not be enhanced or may even be diminished. It is further understood that different catalytic properties may be enhanced at different operating conditions. For example, a catalyst having enhanced selectivity at one set of operating conditions may be operated at a different set of conditions wherein the improvement shows up in the activity rather than the selectivity and an operator of an ethylene oxide plant will intentionally change the operating conditions in order to take advantage of certain catalytic properties even at the expense of other catalytic properties in order to maximize profits by taking into account feedstock costs, energy costs, by-product removal costs and the like.

The promoting effect provided by the promoters can be affected by a number of variables such as for example, reaction conditions, catalyst preparative techniques, surface area and pore structure and surface chemical properties of the support, the silver and co-promoter content of the catalyst, the presence of other cations and anions present on the catalyst. The presence of other activators, stabilizers, promoters, enhancers or other catalyst improvers can also affect the promoting effects.

A variety of procedures may be employed for preparing catalysts in accordance with the present invention. It is desirable that the silver, scandium component and other promoters, if any, are relatively uniformly dispersed on the catalyst. The preferred procedure comprises: (1) impregnating a porous catalyst carrier with a solution comprising a solvent or solubilizing agent, silver complex in an amount sufficient to deposit the desired weight of silver and the aforementioned anion and/or cation promoters upon the carrier, and (2) thereafter treating the impregnated support to convert the silver salt to silver metal and effect deposition of silver and the anion and/or cation promoters onto the exterior and interior surfaces of the support. For sake of repeatability, in the use and reuse of impregnating solutions the carrier should preferably not contain undue amounts of ions which are soluble in the impregnating solution and/or exchangeable with the promoter supplied to the catalyst, either in the preparation or use of the catalyst, so as to upset the amount of promoter which provides the desired catalyst enhancement. If the carrier contains such ions, the ions should generally be removed by standard chemical techniques such as leaching. Silver and promoter depositions are generally accomplished by heating the carrier at elevated temperatures to evaporate the liquid within the support and effect deposition of the silver and promoters onto the interior and exterior carrier surfaces. Impregnation of the carrier is the preferred technique for silver deposition because it utilizes silver more efficiently than coating procedures, the latter being generally unable to effect substantial silver deposition onto the interior surface of the carrier. In addition, coated catalysts are more susceptible to silver loss by mechanical abrasion.

The sequence of impregnating or depositing the surfaces of the carrier with silver and promoters is optional. Thus, impregnation and deposition of silver and salts may be effected coincidentally or sequentially, i.e., the promoters may be deposited prior to, during, or subsequent to silver addition to the carrier. The promoters may be deposited together or sequentially. For example, one or more of the salts may be deposited first followed by the coincidental or sequential deposition of silver and additional or other salts.

Impregnation of the catalyst carrier is effected using one or more solutions containing silver and promoters in accordance with well-known procedures for coincidental or sequential depositions. For coincidental deposition, following impregnation the impregnated carrier is heat or chemically treated to reduce the silver compound to silver metal and deposit the salts onto the catalyst surfaces.

For sequential deposition, the carrier is initially impregnated with silver or promoter (depending upon the sequence employed) and then heat or chemically treated as described above. This is followed by a second impregnation step and a corresponding heat or chemical treatment to produce the finished catalyst containing silver and promoters.

In making the catalysts of this invention, some promoters such as some alkali and alkaline earth metal salts have such high melting temperatures that when deposited on the support with silver compound, and subjected to heating to convert the silver compound to silver metal, the salts may remain essentially unchanged. Of course, it is realized that alkali metal and alkaline earth metal salts having an unstable oxidation state will change to a stable oxidation state or states, e.g., sulfites to sulfates. When, for instance, the alkali metal or alkaline earth metal is deposited as the hydroxide or carbonate, it may be transformed in the presence of amines, which may be used in the impregnation of the catalyst, to a different salt form (i.e., nitrate) during the heating (roasting) step depending on the roast conditions.

The process for preparing the catalyst should not be deleterious to the performance of the catalyst. In some instances, the use of elevated solution temperatures when impregnating catalysts with the scandium component apparently enables the scandium component to convert to a catalyst deactivating component. This occurrence has been observed in solutions containing cesium hydroxide and scandium oxalate. Accordingly, solution temperatures below about 40.degree. C., especially below about 30.degree. C., are frequently used. Prompt usage of fresh solutions containing scandium component is typically desired. The order of addition of components to make the catalyst may also affect performance. Care should be taken to avoid adding components in an order in which the scandium component may be adversely affected. Thus, in some instances, scandium component is added simultaneously or subsequently to the addition of other promoters in salt form.

The silver solution used to impregnate the carrier is comprised of a silver compound in a solvent or complexing/solubilizing agent such as the silver solutions disclosed in the art. The particular silver compound employed may be chosen, for example, from among silver complexes, nitrate, silver oxide or silver carboxylates, such as silver acetate, oxalate, citrate, phthalate, lactate, propionate, butyrate and higher fatty acid salts. Desirably, silver oxide complexed with amines is the preferred form of silver in the practice of the invention.

A wide variety of solvents or complexing/solubilizing agents may be employed to solubilize silver to the desired concentration in the impregnating medium. Among those disclosed in the art as being suitable for this purpose are lactic acid (U.S. Pat. Nos. 2,477,436 to Aries; and 3,501,417 to DeMaio); ammonia (U.S. Pat. No. 2,463,228 to West, et al.); alcohols, such as ethylene glycol (U.S. Pat. Nos. 2,825,701 to Endler, et al.,; and 3,563,914 to Wattimina); and amines and aqueous mixtures of amines (U.S. Pat. Nos. 2,459,896 to Schwarz; 3,563,914 to Wattimina; 3,215,750 to Benisi; 3,702,259 to Nielsen; and 4,097,414, 4,374,260 and 4,321,206 to Cavitt).

A particularly preferred process for making high silver content catalysts involves two or more sequential impregnations of silver, with or without promoters, each of which impregnations may be followed by roasting or other procedure to render the silver insoluble. Advantageously, the carrier has a high pore volume and surface area when using high silver loadings.

Following impregnation of the catalyst carrier with silver and promoter, the impregnated carrier particles are separated from any remaining non-absorbed solution. This is conveniently accomplished by draining the excess impregnating medium or, alternatively, by using separation techniques, such as filtration or centrifugation. The impregnated carrier is then generally heat treated (e.g., roasted) to effect decomposition and reduction of the silver metal compound (complexes in most cases) to metallic silver and the deposition of alkali metal and alkaline earth metal salts. Such roasting may be carried out at a temperature of from about 100.degree. C. to 900.degree. C., preferably from 200.degree. to 700.degree. C., for a period of time sufficient to convert substantially all of the silver salt to silver metal. In general, the higher the temperature, the shorter the required reduction period. For example, at a temperature of from about 400.degree. C. to 900.degree. C., reduction may be accomplished in about 1 to 5 minutes. Although a wide range of heating periods have been suggested in the art to thermally treat the impregnated support, (e.g., U.S. Pat. No. 3,563,914 suggests heating for less than 300 seconds to dry, but not roast to reduce, the catalyst; U.S. Pat. No. 3,702,259 discloses heating from 2 to 8 hours at a temperature of from 100.degree. C. to 375.degree. C. to reduce the silver salt in the catalyst; and U.S. Pat. No. 3,962,136 suggests 1/2 to 8 hours for the same temperature range), it is only important that the reduction time be correlated with temperature such that substantially complete reduction of the silver salt to metal is accomplished. A continuous or step-wise heating program is desirably used for this purpose. Continuous roasting of the catalyst for a short period of time, such as for not longer than 1/2 hour is preferred and can be effectively done in making the catalysts of this invention.

Heat treatment is preferably carried out in air, but a nitrogen or carbon dioxide atmosphere may also be employed. The equipment used for such heat treatment may use a static or flowing atmosphere of such gases to effect reduction, but a flowing atmosphere is much preferred.

An important consideration in making the catalyst of this invention is to avoid the use of strongly acidic or basic solutions which can attack the support and deposit impurities which can adversely affect the performance of the catalyst. The preferred impregnation procedure of U.K. Patent 2,043,481 coupled with the high roasting temperature, short residence time procedure which the patent also described is especially beneficial in minimizing such catalyst contamination. However, the use of the salts of this invention coupled with the high purity supports allows one to use lower temperatures though short residence times are preferred.

The particle size of silver metal deposited upon the carrier is asserted by a portion of the prior art to be a function of the catalyst preparation procedure employed. This may seem to be the case because of the limited ability of the art to effectively view the surface of the catalyst. Thus the space between the silver particles seen on the carrier has not been characterized sufficiently to say whether only such particles of silver represent the silver on the carrier. However, the particular choice of solvent and/or complexing agent, silver compound, heat treatment conditions and catalyst carrier may affect, to varying degrees, the range of the size of the resulting silver particles seen on the carrier. For carriers of general interest for the production of ethylene oxide, a distribution of silver particle sizes in the range of 0.005 to 2.0 microns is typically obtained.

The silver catalysts of the invention are particularly suitable for use in the production of ethylene oxide by the vapor phase oxidation of ethylene with molecular oxygen. The reaction conditions for carrying out the oxidation reaction are well known and extensively described in the prior art. This applies to reaction conditions, such as temperature, pressure, residence time, concentration of reactants, gas phase diluents (e.g., nitrogen, methane and CO.sub.2), gas phase inhibitors (e.g., ethylene chloride and ethylene dichloride), and the like. The gases fed to the reactor may contain modifiers or inhibitors or additives such as disclosed by Law, et al., in U.S. Pat. Nos. 2,279,469 and 2,279,470, such as nitrogen oxides and nitrogen oxide generating compounds. See also, European Patent No. 3642. In addition, the desirability of recycling unreacted feed, or employing a single-pass system, or using successive reactions to increase ethylene conversion by employing reactors in series arrangement can be readily determined by those skilled in the art. The particular mode of operation selected will usually be dictated by process economics.

Generally, the commercially-practiced processes are carried out by continuously introducing a feed stream containing ethylene and oxygen to a catalyst-containing reactor at a temperature of from about 200.degree. C. to 300.degree. C., and a pressure which may vary from about five atmospheres to about 30 atmospheres depending upon the mass velocity and productivity desired. Residence times in large-scale reactors are generally on the order of about 0.1-5 seconds. Oxygen may be supplied to the reaction in an oxygen-containing stream, such as air or as commercial oxygen. The resulting ethylene oxide is separated and recovered from the reaction products using conventional methods. However, for this invention, the ethylene oxide process envisions the normal gas recycle encompassing carbon dioxide recycle in the normal concentrations.

The specific STANDARD ETHYLENE OXIDE PROCESS CONDITIONS are used in the examples below unless indicated otherwise. In commercial processes, typical operating conditions can vary and the amounts of the ingredients employed can be adjusted to achieve the best efficiencies. In particular the amounts of ethane, carbon dioxide and organic chloride can be varied to optimize efficiency for the manufacture of ethylene oxide. Ethane is an impurity contained in varying amounts in ethylene raw material. Ethane can also be added to a commercial reactor to provide better control of the chloride's inhibitor action. Typically, the amount of ethane used in commercial processes can vary from about 0.001 to about 5 mole percent for achieving optimization under both air process conditions and oxygen process conditions. As the concentration of ethane increases in the reactor, the effective surface chloride concentration on the catalyst is believed to be decreased thereby decreasing the ability of chloride to promote/inhibit reactions that increase efficiency for the manufacture of ethylene oxide. The amount of chloride, e.g., ethyl chloride or ethylene dichloride, can be varied to provide the needed promoter/inhibitor action commensurate with the ethane levels encountered in a particular process and the type of promoters or modifiers used in the catalyst. The amount of organic chloride used in commercial processes can typically vary from about 0.1 ppm to about 100 ppm for achieving optimization under both air process conditions and oxygen process conditions. Often, the optimum chloride concentration is less with the catalysts of this invention containing scandium component than with catalysts not containing scandium but otherwise identical.

Carbon dioxide is generally considered an inhibitor, and the inhibitor effect of carbon dioxide on process efficiency may be variable with its concentration. With different types of promoters or modifiers used in preparation of the catalysts of this invention, different concentrations of carbon dioxide may be more desirable in certain commercial processes. Typically, the amount of carbon dioxide used in commercial processes can vary from about 2 to about 15 mole percent for achieving optimization under both air process conditions and oxygen process conditions. The amount of carbon dioxide is dependent on the size and type of carbon dioxide scrubbing system employed. The optimization of the amounts of ethane, carbon dioxide and organic chloride provides catalysts which are especially suitable for obtaining desired efficiencies in commercial ethylene oxide manufacture. Catalysts which have been subjected to process conditions for ethylene oxide manufacture such as STANDARD ETHYLENE OXIDE PROCESS CONDITIONS are considered an important aspect of this invention.

The following detailed procedures are provided as illustrative of methods and carriers which are useful for preparing catalysts according to the invention. These examples are by way of illustration only and are not to be construed as limiting the scope of the invention described herein.

The carrier, as indicated, is impregnated under vacuum as hereinafter described with a solution of silver complex and alkali metal and alkaline earth metal salts. The alkali metal and/or alkaline earth metal-containing components need not be introduced as the salts. For instance, cesium hydroxide may be used in conjunction with an ammonium salt (e.g., ammonium sulfate) or acid (e.g., sulfuric acid) or organic compound (e.g., ethylsulfonate) and under conditions of catalyst preparation or use, conversion is made to the desired species. The impregnating solution is prepared at a concentration such that the finished catalyst contained the desired amounts of silver and promoter or modifier. The required concentration of silver and promoter in solution for the given carrier is calculated from the packing density (grams/cc) and pore volume of the carrier which are either known or readily determined. The relationship can vary depending upon the nature of the carrier, e.g., pore volume may influence the amount of silver deposited from a given solution. The required concentration of promoter in solution is obtained by dividing the solution silver concentration by the ratio of silver to promoter desired in the finished catalyst.

In preparing the catalysts, generally a desired amount of ethylenediamine (high purity grade) is mixed with indicated amounts of distilled water. Then oxalic acid dihydrate (reagent grade) is then added slowly to the solution at ambient temperature (23.degree. C.) while continuously stirring. During this addition of oxalic acid, the solution temperature typically rises to about 40.degree. C. due to the reaction exotherm. Silver oxide powder (Metz Corporation) is then added to the diamine-oxalic acid salt-water solution while maintaining the solution temperature below about 40.degree. C. Finally, monoethanolamine, aqueous alkali metal salt solution(s) and distilled water are added to complete the solution. The specific gravity of the resulting solution is often about 1.3-1.4 g/ml.

Carrier can be impregnated in a 12 inches long by 2 inches I.D. glass cylindrical vessel equipped with a suitable stopcock for draining the carrier after impregnation, however, other suitable flask sizes and types can be used. A suitable size separatory funnel for containing the impregnating solution is inserted through a rubber stopper equipped with a metal tube for attaching a vacuum line into the top of the impregnating vessel. The impregnating vessel containing the carrier is evacuated to approximately 1 to 2 inches of mercury pressure for about 20 minutes after which the impregnating solution is slowly added to the carrier by opening the stopcock between the separatory funnel and the impregnating vessel until the carrier is completely immersed in solution, the pressure within the vessel being maintained at approximately 2 inches of mercury. Following addition of the solution, the vessel is opened to the atmosphere to attain atmospheric pressure. The carrier then remains immersed in the impregnating solution at ambient conditions for about 1 hour, and thereafter is drained of excess solution for about 30 minutes. The impregnated carrier is then heat treated as follows (unless stated otherwise) to effect reduction of silver salt and deposition of promoter on the surface. The impregnated carrier is spread out in a single layer on a 25/8 inches wide endless stainless steel belt (spiral weave) and transported through a 2 inches by 2 inches square heating zone for 2.5 minutes, the heating zone being maintained at 500.degree. C. by passing hot air upward through the belt and about the catalyst particles at the rate of 266 SCFH. The hot air is generated by passing it through a 5 ft. long by 2 inches I.D. stainless steel pipe which was externally heated by an electric, furnace (Lindberg.TM. tubular furnace: 21/2 inches I.D., 3 feet long heating zone) capable of delivering 5400 watts. The heated air in the pipe is discharged from a square 2 inches by 2 inches discharge port located immediately beneath the moving belt carrying the catalyst carrier. After being roasted in the heating zone, the finished catalyst is weighed, and based upon the weight gain of the carrier, and the known ratios of silver to promoter in the impregnating solution, it is calculated to contain the wt. % of silver, and wt. % promoter desired.

Silver and promoter concentrations for all catalysts described in the specification are calculated values as described above.

Carriers are nominally ring shape having dimensions of about 1/8.times.5/16.times.5/16 inch or about 1/8.times.1/4.times.1/4 inch. The following carrier is used:

The carrier is an alpha-alumina carrier prepared by calcining gamma-alumina (N 6573 available from the Norton Company, Akron, Ohio) impregnated with about 3.4 weight percent ammonium fluroide solution to a maximum temperature of about 1025.degree. C. The chemical and physical properties of the carrier are given below:

Attrition Loss and Crush Strength Average and Range are determined according to Test No. 45 and Test No. 6, respectively, as referred to in Catalyst Carriers Norton Company, Akron, Ohio Bulletin CC-11, 1974. 25 Ft. Drop Test is determined by dropping carrier pills through a tube for a vertical distance of 25 feet onto a steel plate and observing for breakage. Non-breakage of carrier pills indicates percent passing.

The identity and amounts of water leachable components of carriers can be determined by any convenient analytical technique. Generally, the carriers are heated in distilled water at a temperature of about 50.degree. to 95.degree. C., often 90.degree. C., for about 0.5 to 2, e.g., 1 hour. The liquid is then subjected to ion chromatography and Inductively Coupled Plasma Spectroscopy techniques.

These examples demonstrate various catalysts. Examples 1, 3, 8, 19 and 20 are comparative. Table I below summarizes the details about the catalyst and the efficiencies at CONDITIONS. About 2 ppmv ethyl chloride are contained in the feed. It should be appreciated that the catalyst performance characterized in these examples were not reflective of optimization of catalyst formulation.

The catalysts are prepared using the general procedures set forth below. Except as noted in Table I, all catalysts are prepared using Procedure A.

A standard seven liter solution is prepared based on the packing density and water adsorption of the support.

1. Ethylenediamine (high purity grade, 1441.17 grams) is mixed with 1400 ml of distilled water.

2. Oxalic acid (oxalic acid dihydrate, reagent grade, 1445.60 grams) is slowly added to the aqueous ethylenediamine solution at ambient conditions. An exothermic reaction occurs and the solution temperature rises to about 40.degree. C.

3. Silver Oxide (powder from Ames Goldsmith, 2509.72 grams) is then added slowly to the solution of step 2.

4. To the solution of step 3 above is added monoethanolamine (Fe and Cl free, 532.80 grams).

5. Distilled water is then added to adjust the solution volume to seven liters.

Each catalyst is prepared by taking 130 ml of the stirred stock solution made as described above and adding the promoting salts in the following order as is required: scandium as scandium oxalate hexahydrate, cesium as cesium hydroxide and cesium as cesium sulfate. This solution is used immediately in the impregnating step.

1. Ethylenediamine (high purity grade, 22.56 grams) is mixed with 25 ml of distilled water.

2. Oxalic acid (oxalic acid dihydrate, reagent grade, 22.63 grams) is added to the aqueous solution. An exothermic reaction takes place and the temperature rises to about 60.degree. C.

3. Silver oxide (powder from Ames-Goldsmith, 39.29 grams) is added to the solution of step 2 and the solution is heated to between 70-80.degree. C. to ensure rapid dissolution of the silver oxide.

4. To the solution of step 3 above is added monoethanolamine (Fe and Cl free, 8.34 grams).

5. The salts are then added in the following order: cesium as cesium sulfate or cesium as cesium hydroxide then scandium as scandium oxalate hexahydrate.

6. Distilled water is added to adjust the solution volume to 125 ml. The solution is stirred for 15 minutes before the impregnation step.

1. The carrier is evacuated at room temperature and the appropriate impregnation solution as described above is added to the carrier under vacuum.

2. The excess solution is drained off.

1. The impregnation carrier is roasted in hot air using a belt roaster at about 500.degree. C. for 2.5 minutes over a belt roaster. Air flow is 66 SCFH/in.sup.2.

The catalysts are tested at STANDARD ETHYLENE OXIDE PROCESS CONDITIONS under oxygen conditions.

A summary of Examples 1 to 20 is provided in Table I.