WO1997036680A1

WO1997036680A1 - Epoxidation oxide catalysts

Info

Publication number: WO1997036680A1
Application number: PCT/EP1997/001622
Authority: WO
Inventors: Marek Matusz
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 1996-03-29
Filing date: 1997-03-27
Publication date: 1997-10-09
Also published as: AU2507497A

Abstract

This invention relates to a catalyst suitable for the epoxidation of olefine having no allylic hydrogen, in particular for the preparation of ethylene oxide, which catalyst comprises a catalytically effective amount of silver, a promoting amount of alkalimetal, a promoting amount of a heteropolyoxometalate of tungsten and/or molybdenum and optionally, a promoting amount of rhenium and/or a promoting amount of a rhenium co-promoter selected from sulphur, molybdenum, tungsten, chromium, phosphorus, boron and mixtures thereof, supported on a porous, refractory support.

Description

EPOXIDATION OXIDE CATALYSTS

The invention relates to a process for the preparation of silver-containing catalysts suitable for the epoxidation of olefins having no allylic hydrogen, in particular for the preparation of ethylene oxide and to the use of the catalyst.

Catalysts for the production of ethylene oxide from ethylene and molecular oxygen are generally supported silver catalysts. Such catalysts are typically promoted with alkali metals. The use of small amounts of the alkali metals potassium, rubidium and cesium were noted as useful promoters m supported silver catalysts in U.S. Patent No. 3,962,136, issued June 8, 1976, and U.S. Patent No. 4,010,115, issued March 1, 1977. The use of other co-promoters, such as rhenium, or rhenium along with sulphur, molybdenum, tungsten and chromium is disclosed in U.S. Patent No. 4,766,105, issued August 23,

1988, and U.S. Patent No. 4,808,738, issued February 28,

1989. U.S. Patent No. 4,908,343, issued March 13, 1990, discloses a supported silver catalyst containing a mixture of a cesium salt and one or more alkali metal and alkaline earth metal salts.

It has been found that catalysts containing a promoting amount of a heteropolyoxometalate of tungsten and/or molybdenum have improved initial selectivities when compared with those obtained with catalysts which contain no heteropolyoxometalate of tungsten and/or molybdenum.

The present invention therefore relates to a catalyst suitable for the epoxidation of olefins having no allylic hydrogen, in particular for the vapour phase production of ethylene oxide from ethylene and oxygen, which comprises a catalytically effective amount of silver, a promoting amount of alkali metal, a promoting amount of a heteropolyoxometalate of molybdenum and/or tungsten and optionally a promoting amount of rhenium supported on a support having a surface area m the range of from 0.05 m²/g to 10 m²/g.

Generally, in the vapour phase reaction of ethylene with oxygen to produce ethylene oxide, the ethylene is present m at least a double amount (on a molar basis) compared with oxygen, but frequently is often much higher. Therefore, the conversion is calculated according to the mole percentage of oxygen which has been consumed in the reaction to form ethylene oxide and any oxygenated by-products. The oxygen conversion is dependent on the reaction temperature, and the reaction temperature is a measure of the activity of the catalyst employed. The value T]_ _ ₅ indicates the temperature T expressed °C, at a constant ethylene oxide production level of 1.5 percent. This value is strongly dependent on the employed catalyst and the reaction conditions. The selectivity (to ethylene oxide) indicates the molar amount of ethylene oxide in the reaction product compared with the total molar amount of ethylene converted. In this specification, the selectivity is indicated as S^ ^, which means the selectivity at a constant ethylene oxide production level of 1.5 percent.

In general, the catalysts of the present invention are prepared by impregnating porous refractory supports with silver ions or compound (s) , complex (es) and/or salt(s) dissolved in a suitable solvent sufficient to cause deposition on the support of from 1 to 40 percent by weight, preferably from 1 to 25 percent by weight, basis the weight of the total catalyst, of silver. The impregnated support is subsequently separated from the - solution and the deposited silver compound is reduced to metallic silver. Also deposited on the support either prior to, coincidentally with, or subsequent to the deposition of the silver will be suitable ions, or compound (s) and/or salt(s) of alkali metal dissolved m a suitable solvent . Also deposited on the support either prior to, coincidentally with, or subsequent to the deposition of the silver and/or alkali metal will be suitable ions, or compound (s) and/or salt(s) of a heteropolyoxometalate of molybdenum and/or tungsten dissolved in a suitable solvent. Optionally deposited on the support either prior to, coincidentally with, or subsequent to the deposition of the silver and/or alkali metal and/or heteropolyoxometalates of molybdenum and/or tungsten will be suitable rhenium ions or compound(s) , complex (es) and/or salt (s) dissolved in an appropriate solvent, and/or suitable ions or salt (s) , complex(es) and/or compound (s) of sulphur, molybdenum, tungsten, phosphorus, boron and/or chromium dissolved in an appropriate solvent . The carrier or support employed in these catalysts in its broadest aspects can be any of the large number of conventional, porous refractory catalyst carriers or support materials which are considered relatively inert in the presence of ethylene oxidation feeds, products and reaction conditions. Such conventional materials are known to those skilled in the art and may be of natural or synthetic origin and preferably are of a macroporous structure, i.e. , a structure having a surface area below 10 m2/g and preferably below 3 m²/g. Particularly suitable supports are those of aluminous composition.

Examples of supports which have been used as supports for different catalysts and which could, it is believed, be used as supports for ethylene oxide catalysts are the aluminium oxides (including the materials sold under the trade name "Alundum") , charcoal, pumice, magnesia, zirconia, kieselguhr, fuller's earth, silicon carbide, porous agglomerates comprising silica and/or silicon carbide, silica, magnesia, selected clays, artificial and natural zeolites and ceramics. Refractory supports especially useful in the preparation of catalysts m accordance with this invention comprise the aluminous materials, m particular those comprising alpha alumina. 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 0.03 to 10, preferably from 0.05 to 5, more preferably from 0.1 to 3 m²/g, and a water pore volume as measured by conventional water absorption techniques of from 0.1 to 0.75 ml/g by volume. The B.E.T. method for determining specific surface area is described in detail in Brunauer, S., Emmet, P. Y. and Teller, E., _,_ 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 B.E.T. specific surface areas of from 0.1 to 3, preferably from 0.1 to 2 m²/g, and water pore volumes of from 0.10 to 0.55 ml/g. Typical properties of some supports found particularly useful m the present invention are presented in Table I. Suitable manu acturers of carriers comparable to those m Table I include Norton Company and United Catalysts, Inc. (UCI) .

TABLE I

Carrier A B C D E F G H

B.E.T. Surface Area, m²/g<^a) 0.21 0.42 0.51 0.48 0.68 2.06 0.97 0.78

Water Pore Volume, ml/g 0.26 0.36 0.47 0.49 0.37 0.65 0.46 0.37

Crush Strength, FPCS, kg <^b) 9.1 6.8 6.4 6.4 12.3 No Data --

Total Pore Volume, Hg, ml/g (^c> 1.26 0.42 -- 0.46 0.40 0.65 0.59

Average Pore Diameter, Hg, nm '^c' 62 56 -- 55 -- 100

Median Pore Diameter, Hg, 3.7 2.7 — 3.4 1.7 2.5 — — micrometer ^(C/d⁾

Percent Pore Volume in Pores 90.0 '- 88.5= 89.1% 94.1%

Greater Than 35 nm

Percent Pore Volume in Pores 87. .0% 82.5% -- 82. .3% 61.0% --

Greater Than 1 Micrometer (^c'

% Wt . Alpha Alumina 99. ,5 98 >98 98. 5 >98 70-75 >98 >98

Water Leachable Na, ppmw 12 53 19 24 18 No Data NA NA

Acid-Leachable Na, ppmw 40 96 36 51 172 No Data 553 75

Water Leachable K, ppmw 5 22 35 22 119 No Data NA NA

Acid-Leachable K, ppmw 303 490

Acid-Leachable Fe, ppmw 2 5 1.8 1 No Data NA NA

Acid-Leachable Siθ2, ppmw 1433 1121

% Wt . Sι0₂ .5 2 0.81 15 0.17 25-30 0.78 0.78

% Wt . Tι0₉ -- -- -- - -- 0.4

^(a) Method of Brunauer, Emmet and Teller, loc cit ⁽k⁾ Flat Plate Crush Strength, single pellet ⁽°⁾ Determined by mercury intrusion using Micrometncs

Autopore 9200 or 9210 (130 Contact angle, 0 473 N/m surface tension of Hg) .

⁽d⁾ Median pore diameter represents the pore diameter wherein 50% of the total pore volume is found in pores having less than (or greater than) the median pore diameter.

Of the carriers listed in TABLE I, G and H are preferred because they provide catalysts which have high initial selectivities.

The support, irrespective of the character of the support or carrier used, is preferably shaped into particles, chunks, pieces, pellets, rings, spheres, wagon wheels, and the like of a size suitable for use in fixed bed reactors. Conventional commercial fixed bed reactors are typically m the form of a plurality of parallel elongated tubes (m a suitable shell) approximately 1.8 to 6.9 cm O.D. and 1.8 to 6.4 cm I.D. and 4.5-14 m long filled with catalyst. In such reactors, it is desirable to use a support formed into a rounded shape, such as, for example, spheres, pellets, rings, tablets and the like, having diameters from 0 2 to 2 cm. The catalysts of the present invention are prepared by a technique m which the alkali metal promoter (s) , the heteropolyoxometalate of tungsten and/or molybdenum promoter, the rhenium promoter, if present, and the rhenium co-promoter, if present, in the form of soluble salts and/or compounds are deposited on the catalyst and/or support prior to, simultaneously with, or subsequent to the deposition of the silver and each other The alkali metals may be deposited at one step of the process, and the heteropolyoxometalate of molybdenum and/or tungsten, the rhenium, if present, and/or the rhenium co-promoter, if present, at a different step or steps. The preferred method is to deposit silver, alkali metal, heteropolyoxometalate of molybdenum and/or tungsten, rhenium, if present, and rhenium co-promoter, if present, simultaneously on the support, that is, in a single impregnation step.

Promoting amounts of alkali metal or mixtures of alkali metal are deposited on a porous support by impregnation using a suitable solution. Although alkali metals exist in a pure metallic state, they are not suitable for use in that form. They are used as ions or compounds of alkali metals dissolved in a suitable solvent for impregnation purposes. The carrier is impregnated with a solution of alkali metal promoter ions, salt(s) and/or compound(s) before, during or after impregnation of the silver ions or salt(s) , complex(es) , and/or compound (s) has taken place. An alkali metal promoter may even be deposited on the carrier after reduction to metallic silver has taken place. The promoting amount of alkali metal utilized will depend on several variables, such as, for example, the surface area and pore structure and surface chemical properties of the carrier used, the silver content of the catalyst and the particular ions used in conjunction with the alkali metal cation, heteropolyoxometalate of molybdenum and/or tungsten, rhenium, if present, and rhenium co-promoter, if present, and the amounts of heteropolyoxometalate of molybdenum and/or tungsten, rhenium, if any, and rhenium co-promoter, if any, present. The amount of alkali metal promoter deposited upon the support or present on the catalyst generally lies between 10 and 3000, preferably between 15 and 2000, more preferably, between 20 and 1500, and most preferably, between 50 and 1000 parts per million by weight of the total catalyst. The alkali metal promoters are present on the catalysts in the form of cations (ions) or compounds of complexes or surface compounds or surface complexes rather than as the extremely active free alkali metals, although for convenience purposes in this specification and claims they are referred to as "alkali metal" or "alkali metal promoters" even though they are not present on the catalyst as metallic elements. It is believed that the alkali metal compounds are oxidic compounds. In a preferred embodiment, at least a major proportion (greater than 50%) of the alkali metals comprise the higher alkali metals, i.e. potassium, rubidium, cesium and mixtures thereof. A preferred alkali metal promoter is cesium. A particularly preferred alkali metal promoter is cesium plus at least one additional alkali metal. The additional alkali metal is preferably selected from sodium, lithium and mixtures thereof, with lithium being preferred.

Promoting amounts of molybdenum and/or tungsten heteropolyoxometalate compounds or mixtures thereof are also deposited on the carrier. The promoting amount of heteropolyoxometalate of molybdenum and/or tungsten utilized will depend on several variables, such as, for example, the surface area and pore structure and surface chemical properties of the carrier used, the silver content of the catalyst and the particular ions used in conjunction with the alkali metal cation, rhenium, if present, or rhenium co-promoter, if present. The amount of molybdenum and/or tungsten heteropolyoxometalate promoter deposited upon the support or present on the catalyst, expressed as the metal, generally lies between 10 and 2000, preferably between 15 and 1500 parts per million by weight of the total catalyst, and most preferably between 30 and 1000 parts per million by weight of the total catalyst. As used herein, the term "heteropolyoxometalate of molybdenum and/or tungsten" refers to complex tungsten or molybdenum oxides containing one or more heteroatoms .

(Michael Thor Pope. "Heteropoly and Isopoly Oxo etalates" . Springer Verlag. 1983) . Non-limiting examples of suitable heteroatoms include main group elements such as, for example, silicon, phosphorus and arsenic; transition elements such as, for example, silver, copper, germanium, cobalt, and chromium; and lanthanides such as, for example, cerium and neodymium. The molybdenum and/or tungsten heteropolyoxometalate promoter or promoters are presumably present on the catalyst in the form of surface compounds or surface complexes rather than as metals, although for purposes of convenience in this specification and claims, they are referred to as "heteropolyoxometalates of molybdenum and/or tungsten", "heteropolyoxometalate compound(s) " , and/or "heteropolyoxometalate promoter(s) " even though they are not present on the catalyst as metals . For purposes of convenience, the amount of heteropoly¬ oxometalate of molybdenum and/or tungsten deposited on the support or present on the catalyst is expressed as the element rather than in the compounds or complexes or surface compounds or surface complexes. The molybdenum heteropolyoxometalate compound is typically selected from the group consisting of olybdophosphates, molybdosilicates and the like, in which one or more molybdenum atoms has been replaced with a heteroatom. Specific non-limiting examples of molybdenum compounds include Na5PMo₁₂C>4o,

(NH₄) ₂ [H₆CeMo₁₂0₄₂] , H SiMo₁₂θ4₀,and the like. The tungsten heteropolyoxometalate compound is typically selected from the group consisting of tungstophosphates, tungstosilicates and the like, in which one or more tungsten atoms has been replaces with a heteroatom. Specific non-limiting examples of tungsten hetero¬ polyoxometalate compounds include H3P ₁₂°₄₀' ^H4SiW₁₂θ₄₀, ^K6^P2^W18°62' ^Na5 ^oW12°40' ⁽n-Bu₄N⁾ ₆Zn ₁₂θ4₀, and the like. It should be understood that heteropolyoxometalates exist 5 in a complex equilibrium mixture and that it is not always possible to determine the exact nature of the heteropolyoxometalate, and further that one type of heteropolyoxometalate can be converted to a different type during catalyst preparation. The heteropoly- 0 oxo etalates can be used in their free (acidic) form or as salts. Typical cations would include lithium, sodium, potassium, rubidium, ammonium, amine, etc., as well as other inorganic and organic salts which would provide solubility in the impregnating solution. 5 In one embodiment, the carrier is also impregnated with rhenium ions, salt (s) , compound(s) , and/or complex (es) . This may be done at the same time that the alkali metal promoter is added, or before or later; or at the same time that the silver is added, or before or 0 later; or at the same time that the heteropolyoxometalate of molybdenum and/or tungsten is added, or before or later; or at the same time that the rhenium co-promoter, if present, is added, or before or later. Preferably, rhenium, if present, alkali metal, heteropolyoxometalate 5 of molybdenum and/or tungsten, rhenium co-promoter, if present, and silver are in the same impregnating solution, although it is believed that their presence in different solutions will still provide suitable catalysts. When a rhenium promoter is utilized, the 0 preferred amount of rhenium, calculated as the metal, deposited on or present on the carrier or catalyst ranges from 0.1 to 10, more preferably from 0.2 to 5 micromoles per gram of total catalyst, or, alternatively stated, from 19 to 1860, preferably from 37 to 930 parts per 35. million by weight of total catalyst. For purposes of convenience, the amount of rhenium present on the catalyst is expressed as the metal, irrespective of the form m which it is present.

Suitable rhenium compounds for use m the preparation of the instant catalysts are rhenium compounds that can be solubilized in an appropriate solvent. Preferably, the solvent is a water-containing solvent. More preferably, the solvent is the same solvent used to deposit the silver and the alkali metal promoter. Examples of suitable rhenium compounds include the rhenium salts such as rhenium halides, the rhenium oxyhalides, the rhenates, the perrhenates, the oxides and the acids of rhenium. A preferred compound for use in the impregnation solution is the perrhenate, preferably ammonium perrhenate. However, the alkali metal perrhenates, alkaline earth metal perrhenates, silver perrhenates, other perrhenates and rhenium heptoxide can also be suitably utilized.

U.S. Patent No. 4,766,105, issued August 23, 1988, teaches that if a rhenium co-promoter is added to an alkali metal/rhenium doped supported silver catalyst, an improvement in initial selectivity is obtained. While suitable catalysts can be prepared in the absence of both rhenium and a rhenium co-promoter, it is preferable that if the catalyst contains rhenium, the catalyst also contains a rhenium co-promoter. When a co-promoter is utilized, the co-promoter is selected from the group consisting of sulphur, molybdenum, tungsten, chromium, phosphorus, boron and mixtures thereof, preferably a compound of sulphur, molybdenum, tungsten, chromium, phosphorus, boron and mixtures thereof. The exact form of the co-promoter on the catalyst is not known. The co-promoter, it is believed, is not present on the catalyst m the elemental form since the co-promoter is applied to catalyst in the form of ions, salts, compounds and/or complexes and the reducing conditions generally used to reduce the silver to metallic silver are not usually sufficient to reduce the sulphur, molybdenum, tungsten, chromium, phosphorus or boron to the elemental form. It is believed that the co-promoter deposited on the support or present on the catalyst is in the compound form, and probably in the form of an oxygen-containing or oxidic compound. In a presently preferred embodiment, the co-promoter is applied to the catalyst in the oxyanionic form, i.e. , in the form of an anion, or negative ion which contains oxygen. Examples of anions of sulphur that can be suitably applied include sulphate, sulphite, bisulphate, bisulphate, sulfonate, persulphate, thiosulphate, dithionate, etc. Preferred compounds to be applied are ammonium sulphate and the alkali metal sulphates. Examples of anions of molybdenum, tungsten and chromium that can be suitably applied include olybdate, dimolybdate, paramolybdate, other isopoly olybdates, etc.; tungstate, paratungstate, metatungstate, other isopolytungstates, etc.; and chromate, dichromate, chromite, halochromate, etc. Examples of anions of phosphorus that can be suitably applied include phosphate, hydrogen phosphate, dihydrogen phosphate, metaphosphate, fluorophosphate, pyrophosphate, hypophosphate, diphosphate, triphosphate, etc. Examples of anions of boron that can suitable be applied include borate, metaborate, tetraborate, tetrafluoroborate, etc. Preferred are sulphates, molybdates, tungstates, chromates, phosphates and borates. The anions can be supplied with various counter-ions . Preferred are ammonium, alkali metal, mixed alkali metal and hydrogen (i.e. acid form) . The anions can be prepared by the reactive dissolution of various non-anionic materials such as the oxides such as S0₂, S0₃ , M0O3 , WO3 , Cr θ3 , ^p2^5 ' ^B2^<-⁾3 ' ^etc • ' ^{as e}ll ^as other materials such as halides, oxyhalides, hydroxyhalides, hydroxides, sulfides, etc., of the metals.

When a co-promoter is used, the carrier is impregnated with rhenium co-promoter ions, salt(s) , compound(s) and/or complex(es) . This may be done at the same time that the other components are added, or before and/or later. Preferably, rhenium co-promoter, rhenium, alkali metal and silver are in the same impregnating solution, although it is believed that their presence in different solutions will still provide suitable catalysts .

The preferred amount of co-promoter compound present on or deposited on the support or catalyst ranges from 0.1 to 10, preferably from 0.2 to 5 micromoles, expressed as the element, per gram of total catalyst. For purposes of convenience the amount of co-promoter present on the catalyst is expressed as the element irrespective of the form in which it is present.

The co-promoter compounds, salts and/or complexes suitable for use in the preparation of the instant catalysts are compounds, salts and/or complexes which can be solubilized in an appropriate solvent. Preferably, the solvent is a water-containing solvent . More preferably, the solvent is the same solvent used to deposit the silver, alkali metal promoter and rhenium. Generally, the carrier is contacted with a silver salt, a silver compound, or a silver complex which has been dissolved in an aqueous solution, so that the carrier is impregnated with said aqueous solution; thereafter the impregnated carrier is separated form the aqueous solution, e.g., by centrifugation or filtration and then dried. It is understood that the other dopants such as alkali metal promoter, heteropolyoxometalate of molybdenum and/or tungsten promoter, rhenium promoter, if - present, and rhenium co-promoter, if present, can be added to the silver-containing impregnation solution, if desired The thus obtained impregnated carrier is heated to reduce the silver to metallic silver It is conveniently heated to a temperature m the range of from 50 °C to 600 °C, during a period sufficient to cause reduction of the silver salt, compound or complex to metallic silver and to form a layer of finely divided silver, which is bound to the surface of the carrier, both the exterior and pore surface. Air, or other oxidizing gas, reducing gas, an inert gas or mixtures thereof may be conducted over the carrier during this heating step.

There are several known methods to add the silver to the carrier or support . One method of preparing the silver containing catalyst can be found in U.S. Patent 3,702,259, issued November 7, 1972. Other methods for preparing the silver-containing catalysts which m addition contain higher alkali metal promoters can be found in U.S Patent 4,010,115, issued March 1, 1977; and U.S. Patent 4,356,312, issued October 26, 1982; U.S. Patent 3,962,136, issued June 8, 1976 and U.S. Patent 4,012,425, issued March 15, 1977. Methods for preparing silver-containing catalysts containing higher alkali metal and rhenium promoters can be found in U.S. Patent No. 4,761,394, issued August 2, 1988, and methods for silver-containing catalysts containing higher alkali metal and rhenium promoters and a rhenium co-promoters can be found in U.S. Patent No 4,766,105, issued August 2, 1988.

The concentration of the silver (expressed as the metal) m the silver-containing solution will range from 1 g/1 up to the solubility limit when a single impregnation is utilized. The concentration of the alkali metal (expressed as the metal) will range from 1 x 10^"3 to 12 and preferably, from 10 x 10^'3 to 12 g/1 when a single impregnation step is utilized. The concentration of the heteropolyoxometalate of molybdenum and/or tungsten (expressed as the element) will range from 1 x 10^"5 to 1 and preferably, from 5 x 10^~4 to 0.1 g/1 when a single pre-impregnation step is utilized. The concentration of the rhenium (expressed as the metal) , if present, will range from 5 x 10^~3 to 20 and preferably from 50 x 10^~3 g/1 to 20 g/1 when a single impregnation step is utilized. The concentration of rhenium co-promoter (expressed as the element) , if present, will range from 1 x 10^~3 to 20 and preferably from about 10 x 10^~3 to 20 g/1 when a single impregnation step is utilized. The amount of silver deposited on the support or present on the support is to be a catalytically effective amount of silver, i.e., an amount that catalyzes the reaction of ethylene and oxygen to produce ethylene oxide. Preferably this amount will range from 1 to 40, more preferably from 1 to 25, and even more preferably from 5 to 20 percent by weight of the total catalyst.

The catalysts according to the present invention have been shown to have improved selectivities for ethylene oxide production in the direct oxidation of ethylene with molecular oxygen to ethylene oxide. The conditions for carrying out such an oxidation reaction in the presence of the silver catalysts according to the present invention broadly comprise those already described in the prior art. This applies, for example, to suitable temperatures, pressures, residence times, diluent materials such as nitrogen, carbon dioxide, steam, argon, methane or other saturated hydrocarbons, to the presence of moderating agents to control the catalytic action, for example, 1-2-dichloroethane, vinyl chloride, ethyl chloride or chlorinated polyphenyl compounds, to the desirability of employing recycle operations or applying successive conversations in different reactors to increase the yields of ethylene oxide, and to any other special conditions which may be selected in processes for preparing ethylene oxide. Pressures in the range of from atmospheric to about 3500 kPa are generally employed. Higher pressures, however, are not excluded. Molecular oxygen employed as reactant can be obtained from conventional sources. The suitable oxygen charge may consist essentially or relatively pure oxygen, a concentrated oxygen stream comprising oxygen in major amount with lesser amounts of one or more diluents, such as nitrogen and argon, or another oxygen-containing stream, such as air. It is therefore evident that the use of the present silver catalysts in ethylene oxide reactions is in no way limited to the use of specific conditions among those which are known to be effective. For purposes of illustration only, the following table shows the range of conditions that are often used in current commercial ethylene oxide reactor units.

TABLE I I

*GHSV 1500-10,000

Inlet Pressure 1034-2756 kPa Tnlet Feed

Ethylene 1-40%

0₂ 3-12%

Ethane 0-3%

Argon and/or methane and/or Balance nitrogen diluent

Chlorohydrocarbon Moderator 0.3-50 ppmv total

Coolant temperature 180-315 °C

Catalyst temperature 180-325 °C

0₂ conversion level 10-60%

EO Production (Work Rate) 26-325 Kg/1 of catalyst/hr.

* Units of volume of gas at standard temperature and pressure passing over one unit of volume of packed catalyst per hour.

In a preferred application of the silver catalysts according to the present invention, ethylene oxide is produced when an oxygen-containing gas is contacted with ethylene in the presence of the present catalysts at a temperature in the range of from 180 °C to 330 °C and preferably 200 °C to 325 °C.

While the catalysts of the present invention are preferably used to convert ethylene and oxygen to ethylene oxide, other olefins having no allylic hydrogens can be oxidized using the silver catalysts of the present invention to produce a high selectivity of epoxide derivatives thereof by contacting the olefin feed with an oxygen-containing gas in the presence of an organic halide and the silver catalyst described above under defined oxidation conditions.

The process for the selective epoxidation of olefins having no allylic hydrogens comprises contacting the feed olefin, preferably an olefin having at least 4 carbon atoms, with a sufficient quantity of an oxygen-containing gas so as to maintain the molar ratio of olefin to oxygen in the range of 0.01 up to 20, m the presence of an organic halide and a silver catalyst at a reaction pressure in the range of 10 to 10000 kPa and a temperature in the range of 75° up to 325 °C for a reaction time sufficient to obtain olefin conversions per pass in the range of 0 1 up to 75 mole percent.

The process is carried out in the presence of sufficient 0.1 to 1000 parts per million (by volume of total feed) of organic halide. Preferred quantities of organic halide for use in the practice of the present invention fall within the range of 1 to 100 parts per million, by volume of total feed. Prior to use for oxidizing olefins having no allylic hydrogens, the silver catalysts (either before or after further treatment with promoter) , are optionally calcined in an oxygen-containing atmosphere (air or oxygen-supple¬ mented helium) at 350 °C for about 4 hours. Following calcination, the silver catalysts are typically subjected to an activation treatment at a temperature m the range of 300-350 °C in an atmosphere initially containing 2-5% hydrogen in an inert carrier such as helium or nitrogen The hydrogen content of the activating atmosphere is gradually increased up to a final hydrogen concentration of 20-25% at a controlled rate so that the activation temperature does not exceed 350 °C After the tempera¬ ture is maintained for about 1 hour at a hydrogen concentration m the range of 20-25%, catalyst is ready for use More detailed descriptions of the silver catalysts and their use in oxidizing olefins having no allylic hydrogens are found in U.S. Patent Nos. 4,897,498, issued January 30, 1990 and 5,081,096, issued January 14, 1992. The invention will be illustrated by the following

Examples . EXAMPLES

Part A: Preparation of stock silver oxalate/ethylene-diamme solution for use m catalyst preparation:

1) Dissolve 415 grams (g) of reagent-grade sodium hydroxide in 2340 millilitres (ml) deionized water. Adjust the temperature to 50 °C.

2) Dissolve 1699 g of "Spectropure" (high purity) silver nitrate in 2100 ml deionized water. Adjust the temperature to 50 °C.

3) Add sodium hydroxide solution slowly to silver nitrate solution with stirring while maintaining a temperature of 50 °C. Stir for 15 minutes after addition is complete, and then lower the temperature to 40 °C.

4) Insert clean filter wands and withdraw as much water as possible from the precipitate created m step (3) in order to remove sodium and nitrate ions. Measure the conductivity of the water removed and add back as much fresh deionized water as was removed by the filter wands. Stir for 15 minutes at 40 °C. Repeat this process until the conductivity of the water removed is less than 90 μmho/c . Then add back 1500 ml deionized water. 5) Add 630 g of high-purity oxalic acid dihydrate in approximately lOOg increments. Keep the temperature at 40 °C and stir to mix thoroughly. Add the last portion of oxalic acid dihydrate slowly and monitor pH to ensure that pH does not drop below 7.8 6) Remove as much water from the mixture as possible using clean filter wands in order to form a highly concentrated silver-containing slurry. Cool the silver oxalate slurry to 30 °C. 7) Add 699 g of 92 percent weight (%w) ethylene¬ diamine (8% deionized water) . Do not allow the temperature to exceed 30 °C during addition.

The above procedure yields a solution containing approximately 27-33%w silver. Part B: Preparation of impregnated catalysts:

Catalyst A

For preparing impregnated Catalyst A, into a 10 ml beaker is added 0.0796 grams of H4Si _]_ θ4_Q and approximately 4 grams of deionized water, and the mixture is allowed to dissolve with stirring. This dopant solution is then added to 177.5 grams of the above-prepared silver solution (specific gravity = 1.565 g/ml) . One-third of this solution is further mixed with 0.1234 grams of stock Cs solution containing 45.8% weight cesium. The final impregnating solution is then used to impregnate carrier in the manner described below.

Approximately 30 grams of the carrier H (from Table I) are placed under 3.33 kPa vacuum for 3 minutes at room temperature. Approximately 50 to 60 grams of the doped solution is then introduced to submerge the carrier, and the vacuum is maintained for an additional 3 minutes. At the end of this time, the vacuum is released, and excess impregnating solution is removed from the carrier by centrifugation for 2 minutes at 500 rpm. The impregnated carrier is cured by being continuously shaken in a 8500 1/hour air stream flowing across the cross sectional area of 77 cm² inches at 250 °C for 5 to 6 minutes. The cured catalyst (Catalyst A ) is then ready for testing. Catalyst B

For preparing impregnated Catalyst B, into a 10 ml beaker is added 0.1592 grams of H4Si ₁₂θ_4Q and approximately 4 grams of deionized water, and the mixture is allowed to dissolve with stirring. This dopant solution is then added to 177.5 grams of the above-prepared silver solution (specific gravity = 1.565 g/ml) . One-third of this solution is further mixed with 0.1357 grams of stock Cs solution containing 45.8% weight cesium. The final impregnating solution is then used to impregnate carrier H (from Table I) . Carrier impregnation and catalyst curing is the same as described in the preparation of Catalyst A. Comparative Catalyst H Comparative Catalyst H was prepared in the same manner as Catalyst A, except that the catalyst contain no tungsten or molybdenum heteropolyoxometalate promoter. Catalyst C

For preparing impregnated Catalyst C, into a 50 ml beaker is added 0.1387 grams of H₄Si ₁₂θ4o and approximately 10 grams of deionized water, and the mixture is allowed to dissolve with stirring. This dopant solution is then added to 154.6 grams of the above-prepared silver solution (specific gravity = 1.565 g/ml) . The solution is further diluted with 16.4 grams of water. One-third of this solution is further mixed with 0.1253 grams of stock Cs solution containing 45.3% weight cesium. The final impregnating solution is then used to impregnate carrier G (from Table I) . Carrier impregnation and catalyst curing is the same as described in the preparation of Catalyst A. Catalyst D

For preparing impregnated Catalyst D, into a 50 ml beaker is added 0.0693 grams of H SiW₁₂0_4Q and - approximately 10 grams of deionized water, and the mixture is allowed to dissolve with stirring This dopant solution is then added to 154.6 grams of the above-prepared silver solution (specific gravity =

1.565 g/ml) . The solution is further diluted with 16.4 grams of water. One-third of this solution is further mixed with 0.1253 grams of stock Cs solution containing 45.3% weight cesium. The final impregnating solution is then used to impregnate carrier G (from

Table I) . Carrier impregnation and catalyst curing is the same as described m the preparation of Catalyst A.

Catalyst E

For preparing impregnated Catalyst E, into a 50 ml beaker is added 0.0462 grams of H 4.SιW-1204.0_n and approximately 10 grams of deionized water, and the mixture is allowed to dissolve with stirring. This dopant solution is then added to 205.9 grams of the above-prepared silver solution (specific gravity = 1.565 g/ml) . The solution is further diluted with 15.1 grams of water. One-forth of this solution is further mixed with 0.1113 grams of stock Cs solution containing

45.3 % weight cesium. The final impregnating solution is then used to impregnate carrier G (from Table I) . Carrier impregnation and catalyst curing is the same as described in the preparation of Catalyst A. Catalyst F

For preparing impregnated Catalyst F, into a 50 ml beaker is added 0.1611 grams of H3PW₁₂O₄0 and approximately 10 grams of deionized water, and the mixture is allowed to dissolve with stirring. This dopant solution is then added to 154.6 grams of the above-prepared silver solution (specific gravity = 1.565 g/ml) The solution is further diluted with

16.4 grams of water One-third of this solution is further mixed with 0 1253 grams of stock Cs solution _^ containing 45.3 % weight cesium The final impregnating solution is then used to impregnate carrier G (from Table I) . Carrier impregnation and catalyst curing is the same as described in the preparation of Catalyst A.

For preparing impregnated Catalyst G, into a 50 ml beaker is added 0.064 grams of KgP W₁3θg₂ and approximately 10 grams of deionized water, and the mixture is allowed to dissolve with stirring. This dopant solution is then added to 154.6 grams of the above-prepared silver solution (specific gravity = 1.565 g/ml) . The solution is further diluted with 16.4 grams of water. One-third of this solution is further mixed with 0.1182 grams of stock Cs solution containing 45.3 % weight cesium. The final impregnating solution is then used to impregnate carrier G (from

Table I) . Carrier impregnation and catalyst curing is the same as described in the preparation of Catalyst A. Comparative Catalyst I

Comparative Catalyst I was prepared in the same manner as Catalyst C, except that the catalyst contain no tungsten or molybdenum heteropolyoxometalate promoter.

The procedures set forth above for Catalysts A and B, Comparative Catalyst H, Catalysts C, D, E, F, G and Comparative Catalyst I will yield catalysts (Table III below) which contain approximately 13.5%w-14.5%w Ag with the following approximate dopant levels (expressed in parts per million by weight basis the weight of the total catalyst, i.e., ppmw) and which are approximately optimum in cesium for the given silver levels and support with regard to initial selectivity under the test conditions described below. TABLE I I I

CATALYST COMPOSITION

Ag, Heteropolyoxometalate W, CS, % wt . ppmw ppmw

Catalyst A 14 H₄SιW₁₂O₄₀ 138 438 Catalyst B 14 H₄SιW₁₂O₄₀ 276 479 Comparative 13. .9 None 0 418 Catalyst H Catalyst C 14. .4 H₄SiW₁₂O₄₀ 276 526 Catalyst D 14. .3 H₄SιW₁₂O₄₀ 138 518 Catalyst E 14. .5 H₄SiW₁₂O₄₀ 70 470 Catalyst F 14, .4 H₃PW₁₂O₄₀ 276 511 Catalyst G 14 .5 K₆P₂W₁₈0₆₂ 138 500 Comparative 14 .3 None 0 517 Catalyst I

The actual silver content of the catalyst can be determined by any of a number of standard, published procedures. The actual level of rhenium on the catalysts prepared by the above process can be determined by extraction with water, followed by spectrophotometric determination of the rhenium in the extract. The actual level of heteropolyoxometalate of tungsten and/or molybdenum on the catalyst can be determined by extraction and inductively coupled plasma (ICP) analysis. The actual level of cesium on the catalyst can be determined by employing a stock cesium hydroxide solution, which has been labelled with a radioactive isotope of cesium, in catalyst preparation. The cesium content of the catalyst can then be determined by measuring the radioactivity of the catalyst.

Alternatively, the cesium content of the catalyst can be determined by leaching the catalyst with boiling deionized water. In this extraction process cesium, as well as other alkali metals, is measured by extraction from the catalyst by boiling 10 grams of whole catalyst in 20 millilitres of water for 5 minutes, repeating the above two more times, combining the above extractions and determining the amount of alkali metal present by comparison to standard solutions of reference alkali metals using atomic absorption spectroscopy (using Perkin Elmer Model 1100 B or equivalent) .

Part D: Standard Microreactor Catalyst Test Conditions/Procedure

1.5 to 2 grams of crushed catalyst (0.841-0.595 mm, i.e. 20-30 mesh) are loaded into a 6 mm diameter stainless steel U-shaped tube. The U tube is immersed in a molten metal bath (heat medium) and the ends are connected to a gas flow system. The weight of the catalyst used and the inlet gas flow rate are adjusted to achieve a gas hourly space velocity of 6800 ml of gas per ml of catalyst per hour. The inlet gas pressure is 1448 kPa. The gas mixture passed thorough the catalyst bed (in once-through operation) during the entire test run (including start-up) consists of 25% ethylene, 7% oxygen, 5% carbon dioxide, 63% nitrogen, and 1.0 to 6.0 ppmv ethyl chloride. The initial reactor (heat medium) temperature is

180 °C. The temperature is then ramped at 10 °C per hour for 3 hours, and then at 5 °C per hour for 3 hours. The temperature is then adjusted so as to achieve a constant ethylene oxide production level of 1.5%. Stable performance data are usually obtained when the catalyst has been on stream for a total of at least 1-2 days. Due to slight differences in feed gas composition, gas flow rates, and the calibration of analytical instruments used to determine the feed and product gas compositions, the - measured selectivity and activity of a given catalyst may vary slightly from one test run to the next . To allow meaningful comparison of the performance of catalysts tested at different times, all catalysts described in this illustrative embodiment were tested simultaneously with a reference catalyst. All selectivity data reported in this illustrative embodiment are corrected relative to the average initial performance of the reference catalyst (Si.5 = 81.7%) .

The results are presented below in Table IV. All selectivity values are expressed as % and all activity values are expressed as °C.

TABLE IV CATALYST PERFORMANCE

Carrier ^s1.5' Tl.5 % °C

Catalyst A H 84.0 238

Catalyst B H 84.7 247

Comparative H 83.1 228

Catalyst H

Catalyst C G 82.8 252

Catalyst D G 82.7 243

Catalyst E G 82.4 234

Catalyst F G 84.6 266

Catalyst G G 82.7 245

Comparative G 82.0 233 Catalyst I

As can be seen from Table IV, catalysts which are prepared on catalyst carrier H and contain hetero¬ polyoxometalate of tungsten promoters, Catalysts A and B, showed improved initial selectivities when compared to a catalyst which is prepared on catalyst carrier H and contains no heteropolyoxometalate of tungsten promoter (Comparative Catalyst H) . It can also be seen that catalysts which are prepared on catalyεt carrier G and contain heteropolyoxometalate of tungsten promoters, Catalysts C, D, E, F, and G, showed improved initial selectivities when compared to a catalyst which is prepared on catalyst carrier G and contains no heteropolyoxometalate of tungsten promoter (Comparative Catalyst I) .

Claims

C L A I M S

1. A catalyst suitable for the epoxidation of olefins having no allylic hydrogen, in particular for the vapour phase production of ethylene oxide from ethylene and oxygen, which comprises a catalytically effective amount of silver, a promoting amount of alkali metal, a promoting amount of a heteropolyoxometalate of molybdenum and/or tungsten and optionally a promoting amount of rhenium supported on a support having a surface area in the range of from 0.05 m²/g to 10 m /g.

2. The catalyst of claim 1 wherein the support comprises a porous refractory oxide.

3. The catalyst of claim 2 wherein the support comprises alpha alumina.

4. The catalyst of claim 1 wherein the alkali metal is found individually or in any mixture thereof on the catalyst, on the support or on both the catalyst and the support .

5. The catalyst of claim 1 wherein the alkali metal is selected from potassium, rubidium, cesium, and mixtures thereof .

6. The catalyst of claim 1 wherein said heteroatom is selected from the group consisting of boron, aluminium, silicon, phosphorus, sulphur, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, germanium, arsenic, selenium, yttrium, zirconium, indium, cerium, praseodymium, neodymium, samarium, europium, gadolinium, thorium, uranium and mixtures thereof.

7. The catalyst of claim 1 wherein the heteropoly- oxometalate of molybdenum is selected from the group consisting of molybdo- or tungsto-phosphates, silicates and mixtures thereof .

8. The catalyst of claim 1 wherein the amount of heteropolyoxometalate of molybdenum and/or tungsten promoter is in the range of from 10 to 2000 parts per million, expressed as the element, by weight of the total catalyst, the amount of silver is in the range of from 1 to 40 percent by weight of the total catalyst, and the amount of alkali metal promoter is in the range of from 10 to 1500 parts per million, expressed as the metal, by weight of the total catalyst, and the optional amount of rhenium promoter is in the range of from 0.1 to 10 micromoles of rhenium, expressed as the metal, per gram of total catalyst.

9. The catalyst of claim 1 wherein said catalyst additionally contains a rhenium co-promoter on said support .

10. The catalyst of claim 9 wherein the rhenium co-promoter comprises an oxidic compound or an oxyanion.

11. The catalyst of claim 9 wherein the rhenium co-promoter is selected from sulphate, sulphite, sulfonate, molybdate, tungstate, chromate, borate, phosphate and mixtures thereof.

12. The catalyst of claim 9 wherein the amount of rhenium co-promoter is in the range of from 0.1 to 10 micromoles of co-promoter, expressed as the element, per gram of total catalyst .