CA1289936C - Fluorine-containing catalytic system for epoxidation of alkenes - Google Patents

Abstract

Abstract:

A process is provided for epoxidation of an alkene in the presence of an oxygen-containing gas which comprises contacting the alkene and the oxygen-containing gas under epoxidation conditions in the presence of at least one gaseous efficiency-enhancing member of a redox-half reaction pair, a performance-enhancing amount of a fluorine-containing substance and a supported silver catalyst comprising a cata-lytically-effective amount of silver on a solid por-ous support and at least one efficiency-enhancing salt of a member of a redox-half reaction pair.

Description

lX~'3~3~

AN IMPROVED FLUORINE-CONTAINING
CATALYTIC SYSTEM FOR
EPOXIDATION OF ALKENES

Technical Field:

The present invention is directed to an improved system for the preparation of alkene oxide from al-kene and oxygen-containing gas employing a supported silver catalyst. More particularly, the present invention relates to a direct oxidation of alkenes to the corresponding epoxides in which the stability of the catalyst is enhanced by the presence of a fluorine-containing substance.

Background Art:

The production of alkene oxides, or epoxides, particularly ethylene oxide by the direct oxidation of the corresponding alkene in the presence of a silver-containing catalyst has been known for many years. For example, the basic process was described 30 by Lefort in U. S. Patent 1,998,878 and by Van Peski in U. S. Patent 2,040,782. The basic reaction pro-ceeds, as illustrated for ethylene, according to the equation:

.

128~3~36 \
~ CH2=CR2 + 2 ~ 2 CH2 C~2 (I) and production of an unwanted by-product according to the reaction:
CH2=CH2 + 3 2 2 CO2 + 2 H20 ~II) or by further oxidation of the epoxide.
In the years between the Van Peski patent and the present inventions, research efforts have been directed to improving both the activity and longevity or useful life of the catalyst and the efficiency of the overall catalytic reaction. As is indicated by reactions I and II, the oxidation of an alkene may produce either the alkene oxide (I) sought in the process or the by-products CO2 and H2O.
Several terms are commonly used to describe some of the parameters of the catalytic system. For in-stance, ~conversion" has been defined as the percent-age of alkene fed to the reactor which undergoes reaction. The ~efficiency~ or, as it is sometimes called, the ~selectivity~ of the overall process is an indication of the proportion, usually represented by a percentage, of the converted material or product which is alkene oxide. The commercial success of a reaction system depends in large measure on the effi-ciency of the system. At present, maximum efficien-cies in commerc~al production of ethylene oxide by epoxidation are in the low 80s, e.g., 80 or 81 per-cent. Even a very small increase in efficiency will provide substantial cost benefits in large-scale operation. For example, taking 100,000 metric tons as a typical yearly yield for a conventional ethylene oxide plant and assuming 80 percent conversion, an `` 1~,89''336 increase in efficiency of from 80 to 84 percent, all other things being equal, would result in a savings of 3790 metric tons of ethylene per year. In addi-tion, the heat of reaction for reaction II (formation of carbon dioxide) is much greater than that of reac-tion I (formation of ethylene oxide) so heat-removal problems are more burdensome as the efficiency de-creases. Furthermore, as the efficiency decreases, there is the potential for a greater amount of impur-ities to be present in the reactor effluent which cancomplicate separation of the desired alkene oxide product. It would be desirable, therefore, to de-velop a process for the epoxidation of alkene in which the efficiency is greater than that obtained in conventional commercial processes, e.g., with ethy-lene, efficiencies of 84 percent or greater, while maintaining other performance characteristics, parti-cularly the activity, as described below, in a satis-factory range.
The product of the efficiency and the conversion is equal to the yield, or the percentage of the al-kene fed that is converted into the corresponding oxide.
The ~activity~ of the catalyst is a term used to 25 indicate the amount of alkene oxide contained in the outlet stream of the reactor relative to that in the inlet stream. Activity is generally expressed in terms of pounds of alkene oxide produced per cubic foot of catalyst per hour at specified reaction con-30 ditions and rate of feed. The activity may also bestated in terms of the amount of ethylene oxide in the outlet stream or the difference between the ethy-lene oxide content of the inlet and outlet streams.
If the activity of a reaction system is low, 35 then, all other things being equal, the commercial 1?J8~9~j value of that system will be low. The lower the activity of a reaction system, the less product pro-duced in a unit time for a given feed rate, reactor temperature, catalyst, surface area, etcetera. A low activity can render even a high efficiency process commercially impractical. For production of ethylene oxide, an activity below 4 pounds of ethylene oxide per hour per cubic foot of catalyst is unacceptable for commercial practice. The activity is preferably greater than 8 pounds, and in some instances an acti-vity greater than 11 pounds of alkene oxide per hour per cubic foot of catalyst is desired.
In some instances, activity is measured over a period of time in terms of the amount of alkene oxide produced at a specified constant temperature. Alter-natively, activity may be measured as a function of the temperature required to sustain production of a specified constant amount of alkene oxide. Plots of such measurements yield ~aging rates~ which reflect the stability or useful life of the catalyst. The useful life of a reaction system is the length of time that reactant~ can be passed through the reac-tion system during which acceptable activity is ob-served. The area under a plot of activity versus time is equal to the number of pounds of alkene oxide produced during the useful life of the catalyst per cubic foot of catalyst. The greater the area under such a plot, the more valuable the process is since regeneration or replacement of the catalyst involves a number of expenses, sometimes referred to as turn-around costs. The rate at which activity decreases, i.e., the rate of deactivation at a given point in time, can be represented by the slope of the activity plot, i.e., the derivative of activity with respect to time:

' -i. ~ - .
'-~289g3~

deactivation = dlactivity]/dt.

The average rate of deactivation over a period S of time can be represented then by the change in activîty divided by the time period:

average deactivation = ~ activity/ ~ t.

At some point, the activity decreases to an unacceptable level, for example, the temperature required to maintain the activity of the system be-comes unacceptably high or the rate of production becomes unacceptably low. At this point, the cata-lyst must either be regenerated or replaced. Some of these definitions may be represented as set out be-low:

% Conversion = moles alkene reacted x 100 moles alkene fed % Efficiency = moles alkene oxide produced x 100 moles alkene reacted % Yield s moles alkene oxide Produced x 100 moles alkene fed Typically, in commercial production, since the outlet or effluent stream emanating from the reactor may contain substantial amounts of unreacted alkene, the effluent stream is recycled and combined with the feedstream after removal of at least a portion of the alkene oxide. Generally, as the activity of a cata-lyst decreases with time, in order to obtain the same ultimate yield of epoxide product, the effluent 8993~i stream mus~ either be recycled a greater number of times or the temperature wiehin the reactor must be raised to increase the activity of the catalyst. The former approach to increasing the yield of product requires additional energy expenditures and the lat-ter, which is most frequently used, causes faster catalyst deterioration.
As used herein, an activity-reducing compound refers to a compound which, when present in an ac-tivity-reducing amount, causes a reduction in acti-vity, some or all of which activity may subsequently be regained by returning to a situation in which the concentration of the compound is below the minimum activity-reducing amount. The minimum activity-reducing amount varies depending on the particularsystem, the feedstream and the activity-reducing compound.
Conversely, deactivation, as used herein, refers to a permanent loss of activity, i.e., a decrease in activity which cannot be recovered. As noted above, activity can be increased by raising the temperature, but the need to operate at a higher temperature to maintain a particular activity is representative of deactivation. Furthermore, catalysts tend to deacti-vate more rapidly when reaction is carried out athigher temperatures.
In contrast to problems associated with low or decreasing catalyst activities, less than satisfac-tory efficiencies result in loss of starting mater-ial, the alkene, as the unwanted product CO2. Ulti-mately, this also increases product costs.
To be considered satisfactory, a catalyst must not only have a sufficient activity and the catalytic system provide an acceptable efficiency, but the catalyst must also demonstrate a minimum useful life .

1'289~36 or stability. When a catalyst is spent, typically the reactor must be shut down and partially disman-tled to remove the spent catalyst. This results in losses in time and productivity. In addition, the catalyst must be replaced and the silver salvaged or, where possible, regenerated. Even when a catalyst is capable of régeneration in situ, generally production must be halted for some period of time. At best, replacement or regeneration of catalyst requires additional losses in time to treat the spent catalyst and, at worst, requires replacement of the catalyst with the associated costs.
Since even small improvements in activity, ef-ficiency or useful life may have significance in large scale commercial production, such improvements have been the object of a great deal of research in the direct epoxidation of alkenes. The focus of attempts to improve performance, such as the ac~ivity and useful life of the catalyst and the efficiency of the system, has included such areas as feedstream additives or removal of components therefrom; methods of preparation of the catalyst; deposition or impreg-nation of a particular type or form of silver; com-position, formation, physical properties and morpho-logy of the support; additives deposited on or im-pre~nated in the support; shape of support aggregates used in the reactor; and various types of reactors and bed designs, such as stationary and fluidized beds.
Early work on the silver-catalyzed direct oxida-tion of alkene~ to alkene oxides in many instances resulted in improvements in activity and particularly the selectively of the system, in many cases the efficiency increasing by several percent. However, recent modifications in such systems have resulted in 1'~89~36 only small incremental improvements in eff~ciency.
In terms o~ operating costs, even fractions o~ a percent improvement in èfficiency can translate into large savings in production. Accordingly, current research is still being directed to improvements in the activity and useful life of the catalyst and selectivity of the system.
Although a vast number of elements and compounds are known to have effective catalytic properties in various reactions, many have at least one shortcom-ing, such as very high cost and/or limited availabil-ity, thermal instability in the temperature range in which the reaction is to be conducted, low mechanical strength, small surface area per unit of volume, susceptibility to poisoning, short useful lifetime, etcetera. Such undesirable characteristics make such substances of limited utility as catalysts. Some of these shortcomings, however, may be overcome and in some instances the effectiveness of the catalyst may be improved by applying the substance to a carrier or support.
New support materials are continuously being tried. ~owever, many of those which were employed in the early development of the silver-bearing catalysts are, with some modifications, still being used.
Materials which have found most widespread use are typically inorganic and generally are of a mineral nature. Such materials commonly include alumina, fire brick, clay, bauxite, bentonite, kieselguhr, carbon, silicates, silica, silicon carbide, zirconia, diatomaceous earth, and pumice.
In addition to the physical strength of ~he support materials, other physical properties, such as surface area, pore volume, pore dimensions, and par-ticle size have drawn considerable attention. These :~
,-`:

17,89~33~;
_g properties have been examined with great scrutiny when evidence indicated that there was a correlation between the size of silver particles and the effici-ency of the overall system or useful life of the catalyst. Some materials are also preferred for their chemical properties, i.e., their ~inertness" or ~promoting~ properties.
~ ~he support serves a number of functions in a heterogeneous catalytic system. Ease of handling is facilitated by a support which generally takes the form of discrete particles or aggregates of varying - shape or size which, depending on usage, have a major dimension of about 1 millimeter to about 20 milli-meters. Thus it is not necessary for the catalyst to form a permanent or semi-permanent part of the reac-tor.
The support, however, serves primarily to in-crease the surface area of the ~active~ component of the catalyst, silver, which is important in that most epoxidation occurs at the silver surface-fluid inter-face. Many of the substances commonly employed as catalyse supports not only have the usual external surface, which provides a varying surface area, de-pending on the shape of the support bodies and the packing of the bodies, but are also of a porous na-ture and, therefore, have a large internal surface which contributes to the overall surface area of the supported catalyst. Such support materials provide a greater capacity for sorbing not only the catalyst material during catalyst preparation, when the sup-port is impregnated with a solution containing the catalyst component(s) in soluble form, but also a greater capacity for the flow of fluid reactants within the catalyst during the reaction for which the catalyst is intended. The support also improves 1~012 l~J89'~3~

performance by lowering the pressure drop through the reactor and by facilitating heat and mass transfer.
Among the large variety of substances employed in the past as supports for catalytic materials, alumina has exhibited superiority in many respects as a catalyst support material. In addition to the low cost of the material, alumina has good thermal sta-bility and some forms have a relatively large surface area.
Alumina, in its various forms, particularly alpha-alumina, has been preferred as a support mater-ial for silver-containing catalysts in the prepara-tion of alkene oxides. Numerous variations of sur-face area, pore dimensions, pore volume and particle size have been suggested as providing the ideal phys-ical property or combination of properties for im-proving efficiency, activity or useful life of the catalyst.
~oller (U. S. Patent 3,908,002) discloses an al-pha-alumina, useful as a catalyst support for reac-tions conducted at temperatures below 800 degrees C, such as oxidation reactions of hydrocarbons to oxyhy-drocarbons. The support, having a surface area re-ported to be at least about 40 m2/g, is produced by thermally decomposing a porous aluminum ion chain-bridged, polymeric carboxylate. Indicating that a large surface area in a carrier may be detrimental to its efficient operation and catalyst activity, Belon - (U. S. Patent 3,172,866) describes a method of pro-ducing a macroporous catalyst carrier which may be used in the catalytic production of ethylene oxide having pore diameters of between 0.1 and 8.0 microns and a specific surface area between a few square meters and one square decimeter per gram. The sup-port is prepared by heating a mixture of active and .
.

~`2899~;

calcined aluminum oxides and a small amount of boronoxide at temperatures of between about 1,600 and 1,800 degrees C. Waterman (U. S. Patent 2,901,441) describes a process for preparing highly active and selective catalysts for the oxidation of olefins to ole~in oxides on a Rupport having an average porosity of at least 35 percent. The method involves washing an alpha-alumina or silicon carbide support having an average porosity of between 35 and 65 percent with an aqueous solution of lactic acid, washing with water until neutral, and then impregnating the support with an a~ueous solution of silver lactate. The impreg-nated support is thereafter heat-treated to deposit elemental silver. A silver-supported catalyst for the vapor phase oxidation of ethylene to ethylene oxide, exhibiting improved production of ethylene oxide and catalyst longevity, is described by Brown et al (U. S. Patent 3,725,307). The catalyst is disclosed as being formed from support particles having an average pore diameter of at least 10 mi-crons up to, preferably, 70 microns and a surface area of less than about 1 m2/g. The selectivities reported do not range above about 73 percent. The support is preferably composed of silica-alumina. A
silver-supported catalyst which includes a support of alpha-alumina, silicon carbide, fused aluminum oxide, or mixtures of alumina and silica was asserted by DeMaio (U. S. Patent 3,664,970) to eliminate the need for halogenated inhibitors in the oxidation of ethy-lene to ethylene oxide. The support is composed ofparticles having a minimum apparent porosity of about 30 percent and wherein at least 90 percent of the pores have diameters in the range of 1 to 30 microns, the average of the diameters being in the range of 4 to 10 microns. Wattimena (U. S. Patent 3,563,914) .

~ ~ -~`28-192~36 discloses silver catalysts using aluminum oxide sup-ports having pore volumes between 15 and 30 ml/g and surface areas below about 10 m2/g.
~ayden et al (U. R. Patent Application 2,014,133) disclose a silver catalyst employing a support having a specific surface area in the range of 0.05 to 13 m2/9, an apparent porosity of at least 20 percent, and mean pore diameters of 0.1 to 20 microns, the pore size distribution being bimodal, in which the smaller pores preferably account for at least 70 percent of the total pore volume. Alpha-alu~na supports are described by Rashkin (U. R.
Patent Application 2,122,913A) having a ~relatively low surface area~ of less than 30 m2/g. Mitsuhata et al (Japanese Published Patent Application 56-089843) and Mitsuhata et al (U. S. eatent 4,368,144) describe supported silver catalysts in which the support is formed from alpha-alumina having a specific surface area of 0.5 to 5 m2/g. WatanaSe et al (Japanese Publlshed Patent Application 56-105750) emplay a similar catalyst support having a surface area of 1 to 5 m2/g. Hayden et al (U. S. Patent 4,007,135) describe silver-containing catalysts in which the porous heat-resisting support has d specific surface area in the range of 0.04 to 10 m2/g, an apparent porosity of at least 20 percent, and a median pore diameter of 0.3 to 15 microns. Mitsuhata et al (U. S. Patent 4,248,740) describe the use of high alpha-alumina content supports having a specific surface area of not more than 10 m2/g, an apparent porosity of 40 to 60 percent by volume, and a pore volume of 0.1 to 0.5 cc/g. Armstrong et al (~. S.
~; . Patent 4,342,667) disclose a supported silver cata-: lyst, useful in the oxidation of ethylene to ethylene oxide, in which the support has a surface area of :~

... ~, ~ ' : ~ ' '. ' .

3?~6 0.02 to 2 m2/g, an average pore diameter of 0.5 to 50 microns and an average pore volume of 0.2 to O.S
cc/g .
There has also been some interest in the purity of supports employed, both as to composition and phase. Examples of high purity alumina include U. S.
Patent 2,901,441 which uses alpha-alumina having a purity of about 99.5 percent as a support for cata-lysts used to oxidize olefins to olefin oxides. An ethylene oxidation catalyst is disclosed in German Patent Publication DE 2,933,950 which attains a long catalyst life without a loss in activity or selec-tivity by using an alpha-alumina support having less than 0.001 weight percent of alkali-soluble silicon compounds. The catalyst is prepared by boiling com-mercial quality alpha-alumina with 1 weight percent sodium hydroxide solution and washing to a p~ value of 8. If desired, the silicon compound concentration may be reduced below 1 part per million (ppm) by washing further with 1 weight percent ~F. U. R.
Patent Application 2,122,913A describes supported silver catalysts in which the support is composed of silica, alumina or mixtures thereof, one example of which is an alumina having a pur~ty of 99.3 percent by weight. The silver-supported catalyst described in Japanese Published Patent Application 56-089843 employs an alpha-alumina carrier having a sodium content of less than 0.07 weight percent. Japanese Published Patent Application 56-105750 describes the use of an alpha-alumina support in conjunction with a silver catalyst for producing ethylene oxide, which support has a sodium content less than 0.07 wei~ht percent. A silver catalyst including an alpha-alumina carrier having a sodium content of not more than 0.07 percent is described by Mitsuhata et al (U.

-~ 8 S. Patent 4,368,144). The support also has a surface area within the range of O.S to 5 m2/g, an apparent porosity of 25 to 60 percent, a specific pore volume of 0.2 to 0.5 cc/g, and a particle diameter within the range of 3 to 20 mm. An alpha-alumina support having a purity of 98+ weight percent, for use with silver in the catalytic oxidation of ethylene, is described by Warner et al in U. S. Patent 4,455,392. The patent additionally discloses that the carrier is generally a conventional microporous support with surface areas of less than 10 m2/g, pore volumes ranging from about O.lS to 0.8 cc/g, and pore diameters of about 0.1 to 100 microns.
In addition to compositional purity, both phase purity and morphology of the support have been areas in which improvements in efficiency, selectivity or stability of the catalyst have been sought. Examples include U. S. Patent 2,901,441 in which aluminum oxide is substantially completely converted to the alpha form of alumina by heating aluminum oxide to a temperature of about 1,500 to 2,050 degrees C. Weiss (U. S. ~atent 2,209,908) and Carter (U. S. Patent 2,294,383) describe the use of ~Tabular Corundum~ as a catalyst support for metallic oxides, such as those oxides of metals selected from the fifth and sixth group of the periodic system, for example, vanadium, molybdenum, uranium, etcetera, in the oxidation of various organic materials to maleic acid and maleic anhydride and silver for the catalytic oxidation of ethylene to ethylene oxide, respectively. Weiss indicates that Tabular Corundum, which is almost entirely aluminum oxide and has the alpha-corundum crystalline form of aluminum oxide, may be formed by mixing aluminum oxide with one or more of several compounds, such as sodium oxide and chromic oxide, 3~

and heating the mixture to a temperature ~n the range of about 800 to about 1,800 degrees C, Tabular Co-rundum is further described as having impurities present in only small quantities, the material also includes "readily bonded surfaces and consisting essentially of interlocked corundum crystals in tabu-lar form, having the contained impurities dis-seminated in minute globules throughout the crystal-line alumina~. Brengle et al (U. S. Patent 2,709,173) also employ Tabular Corundum as a support in one of their examples.
U. S. Patents 4,039,481 and 4,136,063 to Rimura et al di8close a catalyst carrier and a method for making same, the catalyst being the type used in catalytic converters in automobile exhaust systems.
Specifically, the catalysts have a surface layer containing alpha-phase alumina and an inr.er portion consisting essentially of alumina of a phase other than that of the alpha phase. The pores in the al-pha-alumina surface layer are larger than those in the inner portion of the catalyst body. A method of preparing the phase gradient support particles is described which provides for treating the surface of the alumina to a depth of about 400 microns with a transition element, particularly iron, and thereafter firing the carrier particles.
Weber et al (U. S. Patent 4,379,134) describe high purity alpha-alumina bodies, at least 85 percent of the pore volume of the bodies having pores with a diameter of from 10,000 to 200,000 Angstroms. The high purity alpha-alumina bodies are prepared by peptizing boehmite in an acidic aqueous, fluoride anion-containing mixture. An extrudable mixture is formed thereby which is extruded and shaped into formed bodies whicb are thereafter dried at 100 to ~89 ~3 300 degrees C, calcined at a temperature of from 400 to 700 degrees C to convert the alumina to the gamma phase, and subsequently calcined further at a temper-ature of ~rom 1,200 to 1,700 degrees C to convert the gamma phase to alpha-alumina phase.
A method of producing granulated porous corundum having a homogeneous porous structure with a total pore volume of 0.3 to 1.0 cm3/g and a predominant pore size of 5,000 to 30,000 A is described by Bores-kov et al (U. S. Patent 3,950,507~. The method ofpreparing the alpha-alumina includes treating active alumina or aluminum hydroxide having a porous struc-ture to a first heat treatment in which the tempera-ture is increased from 20 to 700 degrees C, a second heat treatment in the range of from 700 to 1,000 degrees C, and a third treatment in the range of from 1,000 to 1,400 degrees C. Each of the heat treat-ments is for a period of at least one-half hour, the first heat treatment being conducted in an atmosphere of hydrogen fluoride in which the alumina absorbs the hydrogen fluoride and the second heat treatment de-sorbs the hydrogen fluoride. The patent also de-scribes a similar procedure employing stationary thermal conditions in which the granules of alumina or aluminum hydroxide are impregnated with other fluorine-containing substances prior to the first thermal treatment. The recommended starting mater-ials used to form alpha-alumina include granulated pseudo-boehmite, boehmite or bayerite as the granu-lated aluminum hydroxide and granulated alpha-, eta-, or theta-alumina as the active alumina.
Although alpha-alumina has been considered by most to be the preferred alumina support material, Smith et al (U. S. Patent 2,422,172) have suggested that beta-aluminas are more desirable than the alpha .

89'33~;

phase aq a support mater~al for catalysts, particu-larly those used in catalytic conversion processes such as dehydrogenation and hydroforming.
In seeking the ideal support material, there has been some departure from the commonly employed sub-stances. For example, some use has been made of alkali metal and alkaline earth metal carbonates, both as the sole support material and in combination with other materials as the carrier for processes such as direct oxidation of alkenes to epoxides.
A number of supported silver-containing cata-lysts have been employed for epoxidation of alkenes in which the carrier includes, sometimes labelled as a promoter, a carbonate of a metal, generally an alkali metal or alkaline earth metal. Some examples of the use of one or more alkali and/or alkaline earth carbonates may be found in U. S. Patents 2,424,084, 2,424,086, 2,615,900, 2,713,586, 3,121,099, 3,258,433, 3,563,913, 3,563,914, 3,585,217, 4,007,135, 4,033,903, 4,039,561, 4,066,575, 4,094,889, 4,123,385, 4,125,480, 4,168,247, 4,186,106, 4,226,782, 4,229,321, 4,324,699, European Patent Publications 0,003,642 and 0,011,356, Japanese Patents 41-11847 and 57-107242, U. ~. Patents 590,479, 1,571,123 and 2,014,133A, and Murray, ~A Study Of The Oxidation Of Ethylene To Ethylene Oxide On A Silver Catalyst~, Australian Journal of Scientific Research, Volume 3A, Pages 433-449 (1950). In addition, U. S. Patent 3,332,887 employs zinc and/or cadmium carbonates, Gelbstein, (DS 2,352,608) discloses the use of the latter car-bonate and European Patent Publication 0,003,642 mentions the use of molybdenum carbonate.
Several patents have described the use of fluo-rine-containing sùbstances to-treat support mater-.
-:- : .-.
:~ .
.: .

~'~ 8~ 9 3 ials, in some cases to provide a compositionally pure support, and in other cases as a fluxing agent to improve the phase purity of the support. Thus, U. R.
Published Patent Specification 590,479 and U. S.
Patent 2,424,086 indicate that a more active catalyst is formed if the support material has undergone a preliminary treatment with a dilute solution of hy-drofluoric acid prior to impregnation with silver.
U. S. Patent 4,379,134 teaches the preparation of high purity alpha-alumina bodies by peptizing boeh-mite alumina in an aqueous acidic mixture containing fluoride anions and water. German Patent 2,933,950 teaches the reduction of silicon content by treatment with HF. U. S. Patent 3,950,507 teaches the prepara-tion of granulated porous corundum by a multiple stepheat treatment in which initial steps may be carried out in an atmosphere of hydrogen fluoride. ~osoda et al (U. S. Patent 3,144,416) suggest that a small amount of a halogen compound, sulfur compound, nitro-gen compound, or phosphorous compound may be addedeither to the reaction gas or the catalyst to improve the selectivity of the catalyst.
The nature of the silver itself has also been examined and modified in attempts to improve the efficiency and stability of the catalyst. Cavitt (U. S. Patent 4,229,321) teaches that a supported silver catalyst of improved selectivity and activity may be prepared by mechanically removing the outer surface ot skin of the catalyst after the impregnated catalyst has been heated to evaporate volatile mater-ial and reduction of the silver salt to silver metalt thereby activating the catalyst.
Since the early work on the direct catalytic oxidation of ethylene to ethylene oxide, workers in the field have suggested that the addition of certain .

39'33S

compounds to the gaseous feedstream or direct incor-poration of metals or compounds in the catalyst could enhance or promote the production of ethylene ox-ide. Such metals or compounds have been known vari-ously as "anti-catalysts", "promoters" and ~inhi-bitorsn. These substances~ which are not themselves considered catalysts, have been proposed by prior workers to contribute to the efficiency of the pro-cess by inhibiting the formation of carbon dioxide or promoting the production of ethylene oxide. The scientific literature is replete with examples of the use of alkali metals and alkaline earth metals and their cations to promote the efficiency of silver catalysts used in epoxidation reactions. For exam- ~
ple, sodium, potassium and calcium were disclosed as being suitable promoters in U. S. Patent ~,177,361.
Numerous examples may be found in literature of pref-erence for one or several metals or cations and ex-clusion of one or more metals or cations as promoters in silver catalysts.
Among those anions associated with the cation promoters used in preparing silver-containing cata-lysts employed in direct epoxidation reactions that have been suggested as being suitable include car-boxylates, for example, form~te, acetate, malonate,oxalate, lactate, tartrate, and/or citrate, and inor-ganic salts, such as carbonates, bicarbonates, phos-phates, nitrates, and/or nitrites, chlorides, io-dides, bromates, and isopropoxides. ~owever, al-though many examples may be found in the literatureindicating that such compounds are suitable, numerous patents, such as U. S. Patents 3,962,136; 4,012,425;
4,066,575; 4,207,210; and 4,471,071, suggest that no unusual effectiveness, particularly with regard to catalytic activity, is observed with any particular ~'~893~S

anion of an alkali metal promoter. U. S. Patents 4,007,1~5; 4,094,889; 4,125,480; 4,226,782;
4,235,757; 4,324,6g9; 4,342,667; 4,356,312;
4,368,144; and 4,455,392 disclose that potassium nitrate may be added to the catalyst as a suitable promoting material. Potassium nitrate may also be formed _ situ when a carr$er material is treated with certain amines in the presence of potassium ions as, for instance, when silver is introduced to a carrier material in a silver-impregnating solution containing an amine and potassium ions, followed by roasting.
A number of compounds have been proposed in the literature as additives to the feedstream or react-lS ants to improve the efficiency of the direct, silver-catalyzed oxidation of alkenes to alkene oxides. For example, Law and Chitwood ~U. S. Patent 2,194,602) disclose the use of a ~repressant~, i.e., anti-catalyst, such as ethylene dichloride, chlorine, sulfur chloride, sulfur trioxide, nitrogen dioxide, or other halogen-containing or acid-forming mater-ials. Numerous additional anti-catalysts are pre-sented by the same patentees in U. S. Patent 2,279,46y. The anti-catalysts, broadly listed in 25 categories such as halogens and compounds containing halogen, hydrocarbons, compounds containing carbon, hydrogen and oxygen, compounds containing sulfur, and compounds containing nitrogen are represented and, in addition to those compounds already mentioned above, 30 additional representative compounds include, as ni-trogen-containing compounds, nitric oxide, ammonia, amines such as ethylene diamine, diphenylamine and analine, nitro compounds such as o-nitroanisole and o-nitrotoluene as organ$c oxygen-containing 35 compounds, alcohols such as methyl, ethyl and isopro-:

pyl alcohols, ethers such as isopropyl and dibutyl ethers, as well as glycol ethers, ketones such as methyl ethyl ketone and acetone, as hydrocarbons such as benzene, and N-hexane, sulfur compounds such as sulfur dioxide, hydrogen sulfide and diethylsulfide;
chlorine-containing compounds such as carbon tetra-chloride, chlorobenzene and dichloroethyl ether.
Berl tU. S. Patent 2,270,780) lists a number of com-pounds as anti-detonating or anti-knock materials to control the oxidation of ethylene and propylene to their oxides. Disclosures of other feedstream addi-tives used in the production of alkene oxides, parti-cularly halogen compounds, may be found in U~ S.
Patents 2,279,470; 2,799,687; 3,144,416; 4,007,135;
4,206,128; and 4,368,144. In addition, EPO Patent 0,003,642 and U. R. Patent Application 2,014,133A
disclose processes for the production of olefin ox-ides employing silver-containing catalysts in which a chlorine-containing reaction modifier and a nitrite or nitrite-forming substance are described. Rumanian Patent 53,012, published December 2, 1971, discloses a silver-catalyzed direct epoxidation procedure which employs oxides of nitrogen in the feedstream. U. R.
Patent 524,007 includes ethylene dichloride or nitrogen dioxide in the feedstream of a silver-catalyzed epoxidation procedure.
Although much of the art discussed above has resulted in improvements in the efficiency, activity or stability of the catalytic system, many of the improvements have individually been rather slight.
In some of the catalytic systems, gains in one of these performance parameters have been frequently offset by losses in another; that is, enhancement of one index of performance has been accompanied by a 35 deleterious effect on another of the indices. For 1~28~3~33~;

example, if a reaction system is designed which has a very short useful life, the system may be commer-cially impractical even though the efficiency and initial activity of the catalyst are outs~anding.
S Accordingly, a system that provides an increase in the efficiency of the overall catalytic reaction system, while only minimally decreas~ng the activity and useful life of the catalyst, or perhaps lncreas-ing one of those performance indices, would be parti-cularly beneficial.

Disclosure Of The Invention:

The present invention is directed to a catalytic lS process for the epoxidation of alkene in the presence of an oxygen-containing gas and to ~he catalyst used therefor. The process comprises contacting an alkene with an oxygen-containing gas under epoxidation con-ditions in the presence of at least one gaseous effi-ciency-enhancing member of a redox-half reaction pair, a performance-enhancing amount, such as a sta-bility-enhancing amount, of a fluorine-containing substance, an efficiency-enhancing amount of at least one salt of a member of a redox-half reaction pair, preferably potassium nitrate, and a catalytically-effective amount of silver on a solid support. Con-tact between the alkene and the oxygen-containing gas may be carried out in the presence of a performance-enhancing gaseous halide, such as ethylene dichloride or ethyl chloride. The present invention also con-templates a catalyst comprising a catalytically-effective amount of silver, a salt of a member of a ~ redox-half reaction pair, preferably potassium ni-; trate, and a performance-enhancing amount of a fluo-rine-containing substance on a solid support.

`, .

3'3~

The present catalytic system for epoxidation of alkenes provides enhancement of performance, such as improvement in efficiency and also an improvement in the useful life, or stability, of the catalyst. This effect may be attributed to the presence of a fluo-rine-containing substance either in the gaseous feed-stream containing the alkene and oxygen or it may be combined with the catalyst itself. The enhance~ent of efficiency for the catalytic system is provided by the inclusion of a gaseous efficiency-enhancing com-pound in the gaseous feedstream. However, efficiency may be further improved by the inclusion of a second efficiency-enhancing compound associated with the catalyst itself.
Detailed DescriPtion Of The Invention:

The present invention is directed to a process for the vapor phase oxidation of alkenes to alkene oxides, i.e., an epoxidation process, in the presence of an oxygen-containing gas and to the silver cata-lysts employed therein.
The process and catalyst of the present inven-tion are useful in the epoxidation of the alkenes 25 ethylene and propylene, the epoxides of which are in great demand for use as intermediates in producing such materials as polymers, surfactants, synthetic fibers and antifreeze. ~owever, the present inven-tion is not limited to these compounds but may be 30 used to oxidize higher alkenes, particularly cyclic and acyclic alkenes which are in the gaseous state or have significant vapor pressures under expoidation conditions. Typically these compounds are character-ized as having on the order of 12 carbon atoms or 35 less which are gaseous under epoxidation condi-39~

tions. In addition to ethylene and propylene, exam-ples of alkenes which may be used in the present invention include such compounds as butene, dodecene, cyclohexene, 4-vinylcyclohexene, styrene, and norbor-nene.

SuP~ort:

The support material used in the present inven-tion may ~e selected from suitable solid, porous, refractory material which can withstand roasting tem-peratures, if that is the method employed to reduce the silver present to its free metallic state. Re-gardless of the method used to reduce the silver, the support should also be able to withstand the tempera-tures employed within the reactor under epoxidation process conditions. The support should not have any undue deleterious effect on the performance of the system. Examples of suitable materials include mag-nesia, zirconia, silica, silicon carbide, and alu-mina, preferably alpha-alumina. Suitable support materials also have a surface area of at least about 0.7 m2/g and preferably about 0.7 to about 16 m2/g, particularly preferred are those materials having surface areas in the range of about 0.7 to about 7 m2/g. The surface area is measured by the conven-tional ~. E. T. method using nitrogen, described by Brunauer, Emmet and Teller in J. Am. Chem. Soc., 60, 309-16 ll938).
The carrier materials of the present invention may generally be described as porous or microporous, having median pore diameters of about 0.01 to about 100 microns, preferably about 0.5 to about 50 mi-crons, and most preferably about 1 to about ~ mi-crons. Generally, they have pore volumes of about . .
: ', ' : --1'~89~36 0.6 eO about 1.4 cc/g, preferably about 0.8 to about 1.2 cc/g. Pore volumes may be measured by any con-ventional technique, such as conventional mercury porosity or water absorption technique.
~enerally improved results have been demon-strated when the support material is compositionally pure and also phase pure. By ~compositionally pure~
is meant a material which is substantially a single substance, such as alumina, with only trace impuri-ties being present. "Phase purity" or like terms refer to the homogeneity of the support with respect to its phase. In the present invention, alumina, having a high or exclusive alpha-phase purity (i.e., alpha-alumina), is preferred. Most preferred is a material composed of at least 98 percent, by weight, of alpha-alumina. Under some conditions, as when used in conjunction with the sait and gaseous members of redox-half reaction pairs, even small amounts of sodium can adversely affect the activity and useful life of the catalyst, i.e., have a deactivating ef-fect on the catalyst. Improved results have been observed when the support contains leachable sodium levels less than about 50 parts per million (ppm) by weight, preferably less than 40 ppm, based on the weight of the total support. Most preferred are supports having a leachable sodium content of less than about 20 ppm based on the weight of the total support. Suitable alpha-aluminas having concentra-tions of sodium below 50 ppm may be obtained commer-cially from suppliers, such as the Norton Company.
i Alternatively, suitable alpha-alumina support mater-ials may be prepared so as to obtain sodium concen-trations below 50 ppm by the method described by Weber et al in U. S. Patent 4,379,134.
i5012 1~89936 The use of low sodium supports in catalysts used for epoxidation of alkene is discussed by Notermann et al in copending application, entitled "Improved Catalytic System For Epoxidation of Alkenes Employing Low Sodium Catalyst Supports", filed August 13, 1986 and having Canadian Serial No. 515,866.
Introduction of Efficiency-Enhancing Compound And Performance-Enhancing Fluorine-Containing Substance To The Carrier:
The enhanced performance of the catalysts of the present invention appears to be related to the presence of a fluorine-containing substance in the catalytic system. The manner in which the fluorine-containing substance functions is not clearly understood at this point. However, such fluorine-containing substances appear to improve the performance, particularly the stability, of the catalyst independent of the manner in which they are introduced to the system. Thus, the fluorine-containing substance may achieve its effect by being combined or associated with the catalyst itself in a manner similar to that of the silver or an efficiency-enhancing compound or the fluorine-containing substance may be introduced to the reactor in the gaseous phase, such as by introduction with the alkene, oxygen or other gaseous component.
When the former method is employed, i.e., incorporating the stability-enhancing fluorine-containing substance in the support material, incorporation of the fluorine-containing substance may be achieved in A

.~ ... ~ ~. . ` . .
- . . . . . . .
.
-. ~ `` . ``. - -~7J89~3~i any known manner. ~owever, a suitable procedure involves treatment of a support material, particular-ly alpha-alumina, with an organic or inorganic fluo-rine-containing substance in any solvent capable of dissolving the fluorine-containing substance, prefer-ably in aqueous solution, and thereafter firing the treated support material at a suitable temperature and removing a portion of the fluorine-containing substance. In the present invention, the support material may either be extruded and formed into pel-lets after fluorine treatment and before firing or formed, i.e., extruded, pellets may be fluorine-treated and then fired.
The performance-enhancing fluorine-containing substance and compounds which are combined with the support, such as the efficiency-enhancing compound, may be introduced to the support in any known man-ner. Preferably, the fluorine-containing substance is introduced to the support material prior to im-pregnation with the silver-containing solution by initial impregnation of the support with a solution of the fluorine-containing substance. Suitable sol-vents depend upon the particular fluorine-containing substance employed but, where effective, aqueous solutions are preferably employed. Thereafter, the support is heated to an elevated temperature and a portion of the fluorine-containing substance is re-moved. When alpha-alumina is the support to be em-ployed for the catalyst in the epoxidation reaction, 30 ~ incorporation of fluorine may be effected by treating alpha or other form of alumina, such as boehmite, aluminum hydroxide, pseudo boehmite, or beta-alumina, preferably gamma- or impure alpha-alumina or mixtures of alpha- and another alumina, such as mixed alpha/theta-alumina, with an organic or inorganic .` ' "
-: :

1-~89~336 fluorine-containing substance capable of being incor-porated in the carrier. Such substances include ionic, covalent and polar covalent compounds as well as F2. When the alumina is a phase other than alpha phase, as with a "transitional" alumina, the fluorine-containing substance additionally assists the conversion or transition of the other alumina or impure alumina to alpha-alumina. The alpha-alumina formed according to the present invention has what appears to be a unique platelet morphology in which many of the individual particles are ~'interfused~ or "interpenetrated". By such terms is meant that these particles appear to be growing out of or passing through each other. In some cases the edges of some of the wafer-like particles form what appears to be an irregular "house of cards" arrangement. The morphology of the support is discussed by Notermann in copending application, entitled "Improved Catalytic System For Epoxidation of Alkenes", filed August 13, 1986 and having Canadian Serial No.
515,864.
The fluorine-containing substance should, preferably, be a somewhat volatile material or one which can be volatilized under firing conditions.
Examples of fluorine-containing materials which can be employed as recrystallizing agents which assist in the conversion to alpha-alumina and which supply at least a portion of the performance-enhancing fluorine-containing substance include aluminum trifluoride, ammonium fluoride, hydrofluoric acid, and dichloro-difluoromethane. It should be noted that the ulti-mate composition of the fluorine species present in the catalyst, i.e., after firing of the support, may '~ ~.

' , ` , . `

:

1~ 89~3~i be the same or different from that which is intro-duced to the support.
The fluorine-containing substance is used in an amount sufficient to achieve enhancement of per-S formance of the catalyst. Such enhancement istypically manifested as improvement in long-term stability. In some stances, however, the per-formance-enhancement is exhibited by an improvement in short-term stability or efficiency. The amount of fluorine-containing substance with which the support is treated and which remains in the support will vary with such factors as the process conditions under which the incorporation is effected, such as the firing temperature and heating rate, as well as the bed depth of the material to be treated, the amount of alumina being treated, the level of contamination of the alumina, and how well the firing chamber is sealed. Typically, the support material is treated with an amount of fluorine-containing substance or recrystallization agent of not more than about 3 percent, by weight, based on the weight of support material being treated. ~referably, the fluorine-containing substance is used in an amount of about 0.8 to about 2 percent, by weight. Use of about 0.8 to about 3 percent, by ~eight, of fluorine-containing substance to treat the carrier results in a carrier having about 0.1 to about 2.5 percent of a residual fluorine-containing material.
To effect incorporation of fluorine into the support and achieve the desired performance-enhancement, preferably stability-enhancement, the fluorine-treated alumina is fired at a temperature of less than about 1,200 degrees C, preferably from a temperature of about 750 to about 1,100 degrees C.
The properties of the resulting alpha-alumina depend 9 ~ 36 not only on the firing temperature but also, in part, on the rate of heating. With lower levels of fluo-rine, plate-like crystals having high crush strength and surface area can be obtained with rapid heat-ing. As used herein, "rapid heating~ refers to heat-ing from room temperature to the desired temperature in about l hour. However, with lower concentrations of fluorine recrystallizing agent, slower heating rates are generally preferred to achieve the same type of product. The "slow heating~ treatments gen-erally consist of heating from room temperature to about 750 degrees C in about O.S to l hour and from about 750 degrees C to the final temperature at a rate of about lO0 degrees C per hour. This procedure has collateral effects in that non-alpha-alumina is effectively converted to alpha-alumina by this proce-dure. In addition, the alpha-alumina which results from such treatment has a high surface area and a hexagonal platelet morphology, discussed above.
For most support materials, there is generally either a lack of correlation between crush strenqth and surface area or the crush strength decreases with lncreasing surface area or porosity. ~owever, it is interesting to note the direct correlation between crush strength and surface area of a preferred form of alpha-alumina supports of the present invention.
Alternatively, the performance-enhancing, pref-erably stability-enhancing, fluorine-containing sub-stance may be introduced to the support in the same manner as an efficiency-enhancing salt of a member of a redox-half reaction pair, discussed below. Thus, the performance-enhancing fluorine-containing sub-stance may be introduced separately or together with either an efficiency-enhancing compound or a silver-containing compound such that the impregnation may ~ 9 ~ 36 take place prior, during or after deposit of silver on the support. Typical of such methods include concurrent or coincidental impregnation in which the solution which is used to impregnate the support with S silver also contains the dissolved stability-enhancing compound andjor an efficiency-enhancing compound. This procedure permits introduction of both the silver compound and the stability-enhancing and/or efficiency-enhancing compound to the support in a single step and solution. Another commonly employed method, known as the ~silver-first" method, involves the sequential impregnation of the support with the silver-containing solution, subsequently drying the silver-containing support (and reduction of the silver), followed by impregnation of the sil-ver-containing support with a solution of the effi-ciency-enhancing salt. In order to perform the for-mer procedure, i.e., coincidental impregnation, the stability-enhancing compound and/or efficiency-enhancing compound must be soluble in the same sol-vent or complexing/solubilizing liquid used with the silver-impregnating solution. With the sequential procedure, any solvent capable of dissolving the stability-enhancing and/or efficiency-enhancing com-pounds which will neither react with the silver norleach it from the support is suitable. Leaching does not occur if the silver is reduced before being im-pregnated with the salt but may occur if reduction is carried out after impregnation with the salt solu-tion.
The support particles are preferably formed intoaggregates or ~pills~ of a size and configuration eo be usable in commercially operated ethylene dioxide tubular reactors. These pills may be formed by con-ventional extrusion and firing, as discussed above.

i;~8~3936 2 ~7 When alpha-alumina is the support material, treatment of the support with the fluorine-containing substance may be performed before or after extrusion. The pills generally range in size from about 2 mm to 5 about 15 mm, preferably about 3 mm to about 12 mm.
The size is chosen to be consistent with the type of reactor employed. In general, in fixed bed reactor appli~ations, sizes ranging from about 3 mm to about 10 mm have been found to be most suitable in the 10 typical tubular reactors used in commerce. The shapes of the carrier aggregates useful for purposes of the present invention can vary widely. Common shapes include spheres and cylinders, especially hollow cylinders. Other shapes include amphora (such 15 as defined in ~. S. Patents 3,848,033, 3,966,639 and 4,170,569), amorphous, Raschig rings, saddles, cross-partitioned hollow cylinders ~e.g., having at least one partition extending between walls), cylinders having gas channels from side wall to side wall, cylinders having two or more gas channels, and ribbed or finned structures. While the cylinders are often circular, other cross-sections, such as oval, hexa-gonal, quadrilateral, trilateral, etcetera, may be useful.

CatalYsts:

The catalysts of the present invention are con-ventional to the extent that silver is coated or ~30 deposited on and/or within a solid porous carrier.
Any known method of introducing the silver to the catalyst support may be employed. Numerous examples and procedures are given in the patents discussed above. ~n brief, in a coating or suspension process a slurry, preferably aqueous, of the active catlytic :' : `

'f~8993 material, such as silver or its oxide, is applied to the support to form a coherent silver layer on the support. In the preferred impregnation process, a solution of a soluble salt or complex of silver in an amount sufficient to deposit the desired weight of silver upon the carrier is dissolved in a suitable solvent or "complexing/solubilizing~ agent. This solution may be used to impregnate a porous catalyst support or carrier by immersing the carrier in the silver-containing impregnating solution. The impreg-nating solution is preferably introduced to the cata-lyst support in a container which has been previously evacuated. Alternatively, the support may be sprayed or sprinkled with the impregnating solution. The 1~ excess solution may then be allowed to drain off or the solvent may be removed by evaporation under re-duced pressure at a suitable temperature. The silver salt or compound used to form the silver-containing impregnating solution in a solvent or a complex-ing/solubilizing agent is not particularly criticaland any silver salt or compound generally known to the art which is both soluble in and does not react with the s~lvent or complexing/solubilizing agent to form an unwanted product may be employed. Thus, the silver may be introduced to the solvent or complex-ing/solubilizing agent as an oxide or a salt, such as nitrate or carboxylate, for example, an acetate, propionate, bu-tyrate, oxalate, malonate, malate, maleate, lactate, citrate, phthalate, generally the silver salts of higher fatty acids, and the like.
The chemical practitioner may choose from a large number of suitable solvents or complexing/solu-bilizing agents to form the silver-containing impreg-nat~ing solution. Besides adequately dissol~ing the silver or converting it to a soluble form, a suitable ~'~8~936 solvent or complexing/solubilizing agent should be capable of being readily removed in subsequent steps, either by a washing, volatilizing or oxidation proce-dure, or the like. ~he complexingtsolubilizing 5 agent, preferably, should also permit solution to provide silver in the finished catalyst to the extent of about 2 to about 60 percent silver or higher, based on the total weight of the catalyst. It is also generally preferred that the solvents or com-10 plexing/solubilizing agents be readily miscible withwater since aqueous solutions may conveniently be em-ployed. Among the materials found suitable as sol-vents or complexing/solubilizing agents for the pre-paration the silver-conta$ning solutions are alco-lS hols, including glycols, such as ethylene glycol(U. S. Patents 2,825,701 to Endler et al and 3,563,914 to Wattimena), ammonia (U. S. Patent 2,463,228 to West et al), amines and aqueous mixtures of amines ~U. S. Patents 2,459,896 to Schwartz, 3,563,914 to Wattimena, 3,702,259 to Nielsen, and 4,097,414 to Cavitt, and carboxylic acids, such as lactic acid (U. S. Patents 2,477,435 to Aries and 3,501,417 to DeMaio).
Typically, a silver-containing solution is pre-pared by dissolving silver oxide in a suitable sol-vent or complexing/solubilizing agent as, for exam-ple, a mixture of water, ethylenediamine, oxalic acid, and monoethanolamine. The solution is then mixed with support particles and drained. Thereafter the particles are suitably dried.
After impregnation, the silver-impregnated car-rier particles are treated to convert the silver salt or complex to silver metaL and thereby effect deposition of silver on the surface of the support.
As used herein, the term ~suracen, as applied to the , . !

~ _ . .

1~,89~36 -3s-support, lncludes not only the external surfaces of the carrier but also the ~nternal surfaces, that is, the surfaces defining the pores or internal portion of the support particles. This may be done by treat-5 ing the impregnated particles with a reducing agent,such as oxalic acid or alkanolamine and/or by roast-ing, at an elevated temperature on the order of about 100 to about 900 degrees C, preferably about 200 to about 650 degrees C to decompose the sllver compound 10 and reduce the silver to its free metallic state.
The duration of roasting is generally for a period of about 1 to about 10 minutes for temperatures above 200 degrees C, depending on the temperature used, and is commonly effectuated in a hot-air belt roaster.
The concentration of silver in the finished catalyst may vary from about 2 percent to 60 percent, by weight, based on the total weight of the catalyst, more preferably the silver concentration ranges from about 8 percent to about 50 percent, by weight. When a ~high silver~ content catalyst is preferred, the silver ranges from about 30 to about 60 percent, by weight. The preferred concentration for ~low silver~
content catalysts ranges from about 2 to about 20 weight percent. When a catalyst having a silver concentration in the preferred high silver range is prepared, the silver is preferably introduced in a series of at least two sequential impregnation and roasting cycles, as discussed in greater detail be-low. Lower silver concentrations are preferred from a capital expense standpoint. However, the optimum silver concentration for a particular catalyst should also take into consideration increased productivity resulting from performance characteristics, such as catalyst activity, system efficiency and the rate of 3S catalyst aging. In many instances higher concentra-89~36 tions of silver are preferred since they demonstrate levels of enhanced performance, particularly catalyst stability, which compensates for the greater capital expenditure.
s ~fficiencY-Enhancinq Compound:

A preferred aspect of the present invention includes an efficiency-enhancing amount of at least one efficiency-enhancing salt of a member of a redox-half reaction pair. The term "redox-half reaction"
is defined herein to mean half-reactions like those found in equations presented in tables of standard reduction or oxidation potentials, also known as standard or single electrode potentials, of the type found in, for instance, ~andbook of Chemistry~, N.
A. Lange, Editor, McGraw-Hill Book Company, Inc., pages 1213-1218 ~1961) or "CRC Handbook of Chemistry and Physics~, 65th Edition, CRC Press, Inc., Boca Raton, Florida, pages D155-162 (1984). The term ~redox-half reaction pair~ refers to the pairs of atoms, molecules or ions or mixtures tbereof which undergo oxidation or reduction in such half-reaction equations. A member of a redox-half reaction pair is, therefore, one of the atoms, molecules or ions that appears in a partlcular redox-half reaction equation. Such terms as redox-half reaction pairs are used herein to include those members of the class of substance which provide the desired performance enhancement, rather than a mechanism of the chemistry occurring. Preferably, such compounds, when associ-ated with the catalyst as salts of members of a half-reaction pair, are salts in which the anions are oxyanions, preferably an oxyanion of a polyvalent atom; that is, the atom of the anion to which oxygen .

.~, . . .

:

i~J891l336 is bonded is capable of existing, when bonded to a dissimilar atom, in different valence states. Potas-sium is the preferred cation and the preferred anions are nitrate, nitrite and other anions capable of 5 undergoing displacement or other chemical reaction and forming nitrate anions under epoxidatlon or cata-lyst preparation conditions. Preferred salts include RNO3 and RNO2, with KNO3 being most preferred.

10 Introduction Of Efficiency-Enhancinq Salt To The Carrier:

The e~ficiency-enhancing salt of a member of a redox-half reaction pair may be introduced to the catalyst in any known manner. Various sequences of impregnating or depositing silver and efficiency-enhancing salt on the surfaces of the carrier may be employed. Thus, impregnation and deposition of sil-ver and an efficiency-enhancing salt of a member of a redox-half reaction pair may be effected coinciden-tally or sequentially, i.e., the salt or salts may be deposited prior to, during or subsequent to silver addition to the carrier. When more than one salt of a member of a redox-half reaction pair is employed, they may be deposited together or sequentially. It is preferced, however, to introduce the salts to the support in a single solution, rather than use sequen-tial treatments using more than one solution and a drying step between impregnation steps, since the latter technique may result in leaching of the first introduced salt by the solution containing the second salt. Typical, and in many cases preferred, of such methods include concurrent, or coincidental, impreg-nation in which the solution which is used to impreg-nate the support with silver also contains at least 1~89~3~;

one dissolved efficiency-enhancing salt member of a redox-half reaction pair. This procedure permits introduction of both the silver compound and the efficiency-enhancing salt simultaneously to the sup-5 port in a single step and solution. The other com-monly employed method is the sequential impregnation of the support in which initial introduction of the silver-containing solution or efficiency-enhancing salt solution (depending upon the sequence employed) is followed by drying of the silver-containing sup-port (and heating and/or chemical reduction of the silver if this is the first added substance). This support is then impregnated with a solution of the second substance, that is, the efficiency-enhancing salt (if the silver was the first added substance).
In order to perform the former procedure, i.e., coin-cidental impregnation, the efficiency-enhancing salt must be soluble in the same solvent or complex-ing/solubilizing liquid used with the silver-impreg-nating solution. With the sequential procedure inwhich the silver is added first, any solvent capable of dissolving the salt which will neither react with the silver nor leach it from the support is suit-able. Agueous solutions are generally preferred, but organic liquids, such as alcohols, may also be em-ployed. Suitable procedures for effecting introduc-tion of the efficiency-enhancing salt to the solid support may be found in many of the patents listed above.
In some instances, the coincidental method of preparation of the catalyst is decidedly less prefer-red or may not be used as, for example, where ~a bari-um, calcium or magnesium salt is intended to be solu-bilized and the solution contains materials which may precipitate the cation as, for instance, a carboxylic '' .

,~.. . .
: , .

acid or dicarboxylic acid such as oxalic acid.
The salt of a member of a redox-half reaction pair is added in an amount suf f icient to enhance the efficiency of the epoxidation reaction. The precise 5 amount will vary depending upon such variables as the gaseous efficiency-enhancing member of a redox-half reaction and concentration thereof employed, the concentration of other components in the gas phase, the amount of silver contained in the catalyst, the 10 surface area of the support, the process conditions, e.g., space velocity and temperature, and morphology of support. Generally, however, a suitable range of concentration of the added efficiency-enhancing salt, calculated as cation, is about 0.01 to about 5 per-15 cent, preferably about 0.02 to about 3 percent, byweight, based on the total weight of the catalyst.
Most preferably the salt is added in an amount of about 0.03 to about 2 weight percent.
It has been noted that when conventional analyses have been conducted with catalysts prepared by co-impregnation with silver and efficiency-enhancing salt, not all the anion associated with the cation has been accounted for. For example, cata-lysts prepared by co-impregnation with a potassium nitrate solution have been analyzed by conventional techniques and about 3 moles of the nitrate anion have been observed for every 4 moles of the potassium catlon. ~his is believed to be due to limitations in the conventional analytical techniques and does not necessarily mean that the unaccounted for anions are not nitrate. For this reason, the amount of the efficiency-enhancing salt }n the catalyst is given, in some instances, in terms of the weight percentage of the cation of the efficiency-enhancing salt (based on the weight of the entire catalyst), with the un-- ' `,~' ' -1~899~6 derstanding that the anion associated with the cation is also present in the catalyst in an amount roughly proportional (on a molar basis) to the cation.
As indicated above, when the catalyst is in-5 tended to contain higher concentrations of silver,for example, equal to or above about 30 percent, by weight, it is generally preferred to use multiple cyclès of impregnation and reduction to elemental silver. This seems to result in a more uniform dis-tribution of silver throughout the catalyst pill. Aswith the catalyst containing lower amounts of silver introduced in a single impregnation, the introduction of the efficiency-enhancing salt of a member of a redox-hal~ reaction pair may also be introduced in a sequential or coincidental procedure. Regardless of the method employed, a first cycle includes impregna-tion only with silver (designated herein as a ~silver only~ impregnation) in an appropriate complex-ing/solubilizing agent followed by reduction to metallic silver, such as by roasting. If the sequen-tial procedure i~ to be followed, the first cycle of impregnation and reduction is repeated one or more times. The final step includes imprean~tion with a solution of the efficiency-enhancing salt of a member of a redox-half reaction pair only, followed by draining and drying, as discussed above.
When preparing a catalyst containing a high concentration of silver and the coincidental, or con-current, procedure is employed, the first cycle is the same as with the sequential procedure, i.e., an impregnation with silver and reduction (silver only). This is then followed either directly by a conventional coincidental impregnation (i.e., impreg-nation with a solution containing both silver in a soluble form and the efficiency-enhancing salt of a 1~899~6 member of a redox-half reaction pair, followed by a reduction of silver to its elemental form, as by roasting) or by one or more repetitions of the im-pregnation cycle with a silver only procedure inter-5 posed between the first si?ver only cycle and theco$ncidental impregnation and reduction cycle. Suit-able results have been obtained with both the sequen-tial and coincidental procedures with two silver impregnation cycles, although there are indications that greater amounts of silver with more uniform distribution of silver throughout the pill can be obtained by three or possibly more silver-impregna-tlon cycles. High silver-containing catalysts pre-pared by the coincidental impregnation technique lS provide somewhat better performance than those pre-pared by the sequential technique.

Epoxidation Procedure:

As in conventional processes of this type, an alkene and an oxygen-containing gas are brought to-gether in a reactor in the presence of a suitable epoxidation catalyst under epoxidation conditions.
Typical epoxidation conditions include temperatures within the reaction zone of the reactor on the order of about 180 to 300 degrees C and pressures from about 1 to about 30 atmospheres.
The gaseous efficiency-enhancing member of a redox-half reaction pair may generally be supplied to the reaction zone within the reactor by introducing the component to the feedstream containing alkene and oxygen. ~nder commercial epoxidation conditions, such as those used in the present invention, the feedstream also contains a gas phase halogen com-pound, sucb as an alkyl halide, a hydrocarbon, and, 1~9~6 when the effluent stream from the reactor i5 recy-cled, unreacted alkene. When recycle of the effluent stream is used, carbon dioxide may also be present.
The presence and amount of carbon dioxide depends on, 5 among other things, whether a scrubbing device is used.
The terms ~gaseous member of a redox-half reac-tion pair~, "gaseous effic~ency-enhancing member of a redox-half reaction pair~, or like terms referred to 10 herein have a meaning similar to that for the ~salt of a member of a redox-half reaction pair~ or like terms, defined above. That is, these terms refer to members of redox-half reaction pairs, represented in standard or single electrode potential tables in standard reference texts or handbooks which are in a gaseous state and are substances which, in the reac-tion equations represented in the texts, are either oxidized or reduced. The preferred gaseous efficien-cy-enhancing members of redox-half reaction pairs are compounds containing an element capable of existing in more than two valence states, preferably nitrogen and another element which is, preferably, oxygen.
Examples of preferred gaseous efficiency-enhancing members of redox-half reaction p~. s include at least one of NO~ NO2, N2O4~ N2O3 or any gaseous su~stance capable of forming one of the aforementioned gases, particularly NO and NO2, under epoxidation condi-tions, and mixtures of one of the foregoing, particu-larly NO, with one or more of CO, P~3, SO3, and SO2. NO is most preferred as the gaseous efficiency-enhancing compound.
Although in some cases it is preferred t~ employ - members of the same half-reaction pair in the reac-; tion system, i.e., both the efficiency-enhancing salt member associated with the catalyst and the gaseous ~, .

.
- ~

~89936 member in the feedstream, a~, for example, with a preferred combination of potassium nitrate and nitric oxide, this is not necessary in all cases to achieve satisfactory results. Other combinations, such as S RN03/N203, KN03/N02, E~N03/N204, RN03/S02, RN02/NO, KNO2/NO2, and RNO3/a mixture of SO2 and NO, may also be employed in the same system. In some instances, the salt and gaseous members may be found in differ-ent half-reactions which represent the first and last 10 reactions in a series of half-reaction equations of an overall reaction.
- The gaseous efficiency-enhancing member of a redox-half reaction pair iq also present in an amount sufficient to enhance the performance, such as the activity of the catalyst, and, particularly, the efficiency of the epoxidation reaction. The precise amount is determined, in part, by the particular alkene being oxidized, the efficiency-enhancing salt - of a member of a redox-half reaction pair used and the concentration thereof, the particular alkene undergoing oxidation, and by other factors noted above which influence the amount of efficiency-enhancing salt of a member of a redox-half reaction pair. Typically a suitable concentration of the gaseous member of a redox-half reaction pair for epoxidation of most alkenes, including propylene, is about 0.1 to about 2,000 ppm, by volume, when N2 is used as ballast. When a preferred gaseous member of a redox-half reaction pair, such as NO, is used in the epoxidation of propylene, the preferred concen-tration is about S to about 2,000 ppm, by volume, with an N2 ballast. However, when ethylene is.being oxidized, a suitable concentration in is the range of from about 0.1 to about 100 ppm, by volume, of the gaseous feedstream components. Preferably, when ethylene is being oxidized, the gaseous efficlency-enhancing member of a redox-half reaction pair is present in an amount of about 1 to about 80 ppm when about 3 percent, by volume, CO2 is present. When S nitric oxide is employed as the gaseous efficiency-enhancing compound in an ethylene epoxidation system, it is present in an amount of about 0.1 to about 60 ppm, preferably about 1 to about 40 ppm, when CO2 is present.
When the performance-enhancing fluorine-containing substance is introduced to the catalytic system by way of the feedstream, the substance should be either gaseous or sufficiently volatile to be in a gasous state under epoxidation conditions. Fluorine-containing substances which are suitable for use in the gaseous phase should not react adversely with the materials from which the internal surfaces of the reactor are formed. Suitable materials include or-ganic fluorine-containing substances, such as fluo-rinated hydrocarbons, exemplary of which are theFREONS.
The ~oxygen-containing gas" employed in the reaction may be defined as including pure molecular ` oxygen, atomic oxygen, any transient radical species derived from atomic or molecular oxygen capable of existence under epoxidation conditions, mixtures of another gaseous substance with at least one of the foregoing, and substances capable of forming one of - the foregoing under epoxidation conditions. Such oxygen-containing gas is typically introduced to the reactor either as air, commercially pure oxygen or other substance which under epoxidation conditions both exists in a gaseous state and forms molecular ~ oxygen. The gaseous components which are supplied to - 3S the reaction zone, or that region of the reactor ~, ' , - :
, 1'28993~

where reactants and catalyst are brought together under epoxidation conditions, are generally combined before being introduced to the reactor. The reactors in which the process and catalyst of the present invention are employed may be of any type known to - the art. A brief description of several of the reac-tor parameters which may be used in the present in-venti~on are presented below.
In addition to an alkene, oxygen, and the gas-eous efficiency-enhancing member of a redox-half reaction pair, the feedstream also contains a gaseous performance-enhancing halogen-containing compound, preferably an organic halide, including both satur-ated and unsaturated halides, such as ethylene di-chloride, ethyl chloride, vinyl chloride, methylchloride and methylene chloride as well as aromatic halides. Preferably, ethylene dichlorlde and/or ethyl chloride is employed as the halogen-containing compound. The amount of halide employed will vary depending upon a variety of factors, including the particular alkene being oxidized and the concentra-tion thereof, the particular efficiency-enhancing salt and gaseous members of redox-half reaction pairs and the concentrations thereof, as well as other factors noted above as influencing the amount of efficiency-enhancing salt and gaseous members of redox-half reaction pairs. ~owever, a suitable range of concentration ~or the halogen-containing compound ; in the oxidation of most alkenes, including propy-30- lene, i5 typically about 0.1 to about 2,000 ppm, by volume, of the gaseous makeup feedstream. When ethy-lene is oxidized, the range of concentration for the halogen-containing compound is, however, about 0.1 to about 60 ppm, by volume. In addition, a hydrocarbon, such as ethane, can be included in the ,: . ~
~; - , ' . .

- ': - `,, `

1~89~36 feedstrea~. The feedstream may also contain a ballast or diluent, such as nitrogen, or other inert gas, particularly when air is used as the oxygen-containing gas. Varying amounts of carbon dioxide S and water vapor may also be present, depending upon whether means have been provided to remove such sub-stances from the effluent stream prior to combination of at least a portion of the effluent stream with the inlet stream. Other than the gaseous efficiency-enhancing member of a redox-half reaction pair, the other components are typically present in amounts shown in the following tables for propylene and ethy-lene.

Volume Percent (or ppm) ComPonent for PropYlene Oxidation propylene about 2 to about 50 oxygen about 2 to about 10 20 alkyl halide about 5 to about 2,aoo ppm hydrocarbon 0 to about 5 carbon dioxide up to about 15 nitrogen remainder or other bal-25 last gas, e.g., methane ' , .
;

~89~'33S

Volume Percent ~or ppm) Component for Ethylene Oxidation ethylene at least about 2, often about 5 to about 50 oxygen about 2 to about 8 alkyl halide about 0.1 to about 60 ppm hydrocarbon 0 to about S
carbon dioxide up to about 7 10 nitrogen remainder or other bal-last gas, e.g., methane 15 When higher alkenes, such as those previously discussed, are epoxidized, conditions and concentra-tions typically used for the epoxidation of propylene may be employed.

Standard Alkene Oxide Process Test Conditions:

The successful commercial production of alkene oxides, particularly ethylene oxide, by the silver-catalyzed oxidation of alkene, particularly ethylene, depends upon a variety of factors. Many of these factors influence, either directly or indirectly, the efficiency of the catalytic system, the activity or the aging rate of the catalyst. The manner in which cataly~ts and catalytic systems are evaluated in the laboratory-strongly influences the values obtained for these parameters. Techniques and experiments designed to assess such catalysts and catalytic sys-tems commonly employ microreactors ~i.e., tiny tubu-lar reactors for testing crushed catalyst particles) or back-mixed autoclaves of the Berty type ~i.e., :`
' 1~89936 larger reactors which test full-sized catalyst pel-lets and generally employ full gas recycle) as de-scribed in Figure 2 of the article by J. M. Berty, ~Reactor For Vapor Phase-Catalytlc Studies~, Chemical 5 Enqineerinq Progress, 70, Number 5, pages 78-84 (1974), and particularly Figure 2. Microreactors are capable of yielding, in most test situations, the highest efficiency numbers, typically approximately the same as or somewhat greater than those obtainable in commercial tubular reactor operations employing the same catalysts in non-crushed condition. Back-mixed autoclaves commonly provide lower efficiency values because, although conditions can be varied, generally the entire catalyst is exposed to the out-let gas whlch has the lowest concentration of reac-tants and the highest concentration of products.
Values obtained using one type of reactor are seldom identical to those obtained in another reactor sys-tem. As a result, claims of superior results or the desirability of one catalyst over another are prefer-ably based on tests conducted under controlled and comparable conditions.
Although the conditions set forth suPra may be employed both for reactors employed in commercial production as well as those employed in a laboratory, as a basis of comparison, the catalyst and catalytic systems for epoxidation of ethylene employed in the examples set forth below have been tested under com-parable conditions known as Standard Alkene Oxide Process Test Condition~, or Standard Test Conditions (referred to hereinafter as STC). The STC employed for testing and characterizing the catalysts and the catalytic systems of the present invention involve the use of a standard back-mixed bottom-agitated ~Magnedrive~ autociave or Berty autoclave, as de-. . ~ .

89~33S

scribed above.
In discussing the enhancement of efficiencyprovided by the present invention, it may be noted that, when an efficiency-enhancing amount of a salt 5 of a member redox-half reaction pair is employed, an ef f iciency of at least about 84 percent is obtained under Standard Test Conditions. "Standard Test Con-ditiQns~ may be defined as comprising the following conditions for ethylene: by volume, 30 percent C2%4, 8 percent 2' 5 ppm ethyl chloride, 5 ppm, by weight, NO, no added C2H6 or CO2, N2, ballast, 240 deqrees C, 275 psig, gas hourly space velocity (GHSV) = 8,000 hr~l Although the present invention can be used with any size and type of alkene oxide reactor, including both fixed bed and fluidized bed reactors known to the art, it is contemplated that the present inven-tion will find most widespread application in standard fixed bed, multi-tubular reactors. These generally include wall-cooled as well as adiabatic or non-wall-cooled reactors. Tube lengths may typically range from about 5 to about 60 feet but will fre-quently be in the range of from about 15 to about 40 feet. The tubes may have internal diameters from about 0.5 to about 2 inches and are expected to be typically from about 0.8 to about 1.5 inches. G~SV
generally range from about 16,000 to about 1,000 hr~
1, Typically GHSV values range from about 2,000 to about 8,000 hr~I at pressures from about 1 to about 30 atmospheres, commonly about 10 to about 25 atmospheres.
While the invention is susceptible to various modifications and alternative forms, certain specific embodiments thereof are descr~bed in the examples set forth below. It should be understood, however, that -`

1289~36 these examples are not intended to limit the inven-tion to the particular forms disclosed but, oh the contrary, the invention is to cover all modifica-tions, equivalents and alternatives falling within 5 the spirit and scope of the inven~ion.

Example 1 - Preparation Of A
Fluorine-Containin~ Alpha-Alumina Support:

Approximately 1,200 grams of 1/4 inch hollow gamma-alumina pills or rings (obtained from Norton Corporation as Alumina 6573) were soaked for one-half hour in 1 molar NH4F with occasional shaking to li-berate bubbles that formed during impregnation.
Excess solution was drained from the rings, which were then dried overnight at 120 degrees C. The dried rings were distributed between four 3-inch diameter x 6 inch deep cylindrical alumina crucibles and were fired in a P. D. H. high-temperature fur-nace. Lids for the crucibles were placed askew sothat crucible openings were covered approximately 70 percent to maintain a fluoride atmosphere in the crucible but at the same time permit escape of excess fluoride. The firing schedule was one hour to raise the temperature to 700 degrees C, four hours to raise the temperature from 700 to 1,100 degrees C, and a hold temperature at 1,100 degrees C for one hour.
The product had a surface area of 1.56 m2/g and a sodium content of less than 50 ppm.

1~89~313 Example 2 - Sequential Preparation of A Potassium Nitrate-Containinq S~pported Silver Catalvst:

Formation of a potassium nitrate-containing 5 supported silver catalyst resulted from a multi-step procedure in which a support was initially impreg-nated with a silver-containing solution, the impreg-nated support was roasted, and the silver-inpregnated catalyst was thereafter impregnated with a potassium 10 nitrate solution and then dried.
The silver-containing solution usea in the first step was prepared by dissolving 51.49 grams ethylene-diamine in 51.03 grams of distilled water and stir-ring the mixture for a period of 10 minutes. To the stirred solution was slowly added 51.56 grams of oxalic acid dihydrate. The resulting solution was stirred for 10 minutes. ~o this solution was added, in portions, 90.34 grams silver oxide. The resulting silver-containing solution was thereafter stirred for an additional hour and 18.07 grams of monoethanol-amine was then added to the stirred silver-containing solution. Stirring was continued for an additional 10 minutes. This solution was then diluted to a total volume of 437 ml by addition of distilled 2S water.
In a catalyst-impregnation tube were placed 177.85 grams of an alpha-alumina carrier, similar to that prepared in Example 1. A catalyst-impregnation tube is an elongated tube, which in use is arranged vertically. The upper end of the tube is provided with an inlet to supply impregnation solution to the carrier contained in the tube and the lower end with an outlet from which solution may be drained. The tube is also provided with a means to connect the tube to a vacuum source. Prior to introduction of 1.~8~3~36 impregnation solution the tube was evacuated. The solution described immediately above was slowly poured into the tube to totally immerse the support. The carrier was allowed to remain in the impregnating solution for ! about 1 hour to achieve saturation of the support. The unabsorbed solution was thereaftér drained from the support with draining continuing for about 30 minutes. The wet, impregnated carrier was then roasted in a hot-air belt roaster for 2.5 minutes at 500 degrees C (66 SCF~ air flow) to produce a catalyst containing 11.39 percent silver.
Upon cooling, 44.57 grams of the roasted, im-pregnated carrier was placed in an impregnation tube and covered with a solution formed by dissolving 2.08 grams potassium nitrate in 100 ml distilled water.
After covering the roasted silver-containing carrier with the KNO3-containing solution and allowing to stand for 15 minutes, the catalyst pellets were drained on a funnel for 5 minutes. The resulting drained pellets were dried in an oven for 2 hours at 120 degrees C, producing a catalyst containing 0.46 percent R, calculated as 1.20 percent RNO3, by ~eight.

Example 3 - Preparation Of A Potassium Nitrate-Containing Silver Catalyst BY A Co-ImPreqnation Technique:

A co-impregnation solution was prepared by plac-ing 18.77 grams of ethylenediamine into a 400 ml beaker and mixing therewith 25 grams of distilled water to form a solution. To the stirred solution were slowly added 18.7 grams oxalic acid and, with continuous stirring, 35.2 gra~s of silver oxide was .

.

i~8993G

slowly added. When solution was complete, 6.55 grams of monoethanolamine were added directly to the solu-tion. To the silver-containing solution were added 7.14 grams of a potassium nitrate solution, 10 per-5 cent with respect to potassium (0.258 grams RNO3/gsolution). Sufficient water was added to the result-ing solution to dilute the solution to 100 ml.
To make 100 ml of finished catalyst, 53.5 grams of an alpha-alumina support material of the type 10 prepared in Example 1 but fired at a temperature of 1,000 degrees C in the form of ~uarter-inch ring extrudate and having a pore volume of 0.62 g H2O/g support were placed in an impregnation container. A
source of vacuum was attached to the container and evacuated and thereafter the silver/potassium-con-taining impregnating solution was added, completely covering the carrier. After about one hour the ex-cess solution was drained from the catalyst.
Roasting of the impregnated catalyst was con-ducted aa indicated above in Example 2. The result-ing catalyst included, by weight, 17.5 percent silver and, as calculated, about 1 percent potassium ni-trate.

Example 4 - Sequential Preparation Of A Potassium Nitrate-Containing SupPorted ~iqh Silver Concentration CatalYst:

A solution containing 17.5 g ethylenediamine, 17.3 g distilled water, 17.5 g oxalic acid, 30.~ g silver oxide, 6.1 g monoethanolamine, diluted to 125 ml, was prepared in the manner described in Ex~mple 2.
Into a catalyst impregnation tube were placed 43.9 g of an alpha-alumina carrier, similar to that - ~
.
-i~J8993~
-- 5~ -prepared in Example 1, and having a surface area of 1.12 m2/g, a pore volume of 0.8 cc/g, less than 50 ppm, by weight, leachable sodium, and a platelet morphology of the type disclosed in Canadian Serial No. 515,864 filed August 13, 1986. The support pellets were initially vacuum-impregnated at 28 inches mercury vacuum, releasing the vacuum when the pellets were completely covered with the aqueous silver amine solution, permitting them to stand in the amine solution for an additional hour at 1 atomsphere before draining. The impregnated pellets were then belt-roasted at 500 degrees C in an oven having an air flow of 66 SCFH for 2.5 minutes. The material contained 14.9 percent silver as determined by weight gain. This material was then impregnated a second and third time with fresh silver/amine solution having the composition indicated above. The impregnations and belt-roastings were conducted in the same manner as described above. The catalyst material was then impregnated with KNO3 by placing 67.4 g of the silver-impregnated pellets into a solution containing 2.1 g KNO3 dissolved in 100 ml distilled water. The KNO3-impregnated silver catalyst material was then dried at 120 degrees C for 1 hour to yield a catalyst containing, by weight, 34.7 percent silver and 0.33 percent K (calculated as 0.86 percent KNO3), as determined by weight gain.
Example 5 - Coincidental Preparation Of A Potassium Nitrate-Containing Supported Hiqh Silver _ nc ntration Catalyst A silver impregnation solution was prepared in a manner similar to that described in Example 2 by mixing 196.8 g ethylenediamine, 195.0 g distilled A~

~ ....

~8993S
--5s--water, 197.1 9 oxalic acid, 345.2 9 silver oxld~, 69.1 9 monoethanolamine and diluting to a total vol-ume of 780 ml with distilled water. After evacuation of an impregnation tube containing pellets of an alpha-alumina support material (293.1 9) similar to those described in Example 1 and having a B. E. T.
surface area of 1.16 m2/g by a Quantisorb apparatus, the pellets were impregnated by allowing them to stand in the silver impregnation solution for a per-iod of 1 hour and then draining. The impregnatedpellets were then belt-roasted at 500 degrees C in a 66 SCF~ alr flow for 2.5 minutes. As determined by gain in weight, the material contained 24.6 percent silver.
A co-impregnation solution was preparea in a manner similar to that described in Example 3 by mixing 37.5 9 ethylenediamine, 37.2 g distilled water, 37.6 g oxalic acid, 65.9 9 silver oxide, 13.2 g monoethanolamine, and 0.80 q potassium nitrate and diluting to 150 ml total solution with distilled water. The silver-impregnated catalyst pellets (77.8 g) were impregnated by immersing them in the sil-ver/potassium nitrate-impregnating solution for 1 hour and then draining. The resulti..~ pellets were then belt-roasted as described above. The pellets resulting from the two impregnations yielded a mater-ial, as determined by an analysis, containing, by :

1~,8993 weight, 39.8 percent silver and 0.098 percent R (cal-culated as 0.25 percent KNO3).

Examples 6 to 11 - Production Of 5 Ethylene Oxide With Potassium Nitrate-Containinq SuPPorted Silver Catalvst:
J
The examples set forth below were conducted employing catalyts using carriers of the type pre-pared in Example 1, the catalysts themselves beingprepared in the manner described in Examples 2 and 3 and identified as being prepared by either a sequen-tial (S) procedure or a coincidental impregnation (C) procedure. The epoxidation studies for which data are presented below were conducted in a continuously stirred tank reactor, also known as a back-mixed autoclave of the type described above. The procedure involved charging approximately 80 ml of catalyst to the autoclave. The volume of catalyst was measured in a one inch I. D. graduated cylinder after tapping the cylinder several times to thoroughly pack the catalyst. The weight of the catalyst was noted and is indicated in the table appearing below. The back-mixed autoclave was heated to about reaction tempera-ture in a nitrogen flow of 11.3 SCF~ with the fanoperating at about 1,500 rpm. The nitrogen flow was then discontinued and the feedstream was introduced to the reactor.

.

"

.

~899~6 All ethylene epoxidation reactions, except where indicated otherwise, were examined under Standard Test Conditions comprising, by volume, 30 percent C2H4, 8 percent 2' 5 ppm ethyl chloride, 5 ppm, by weight, NO, no added C~H6 or CO2, N2 ballast, 240 degrees C, 275 psig, GHSV = 8,000 hr~l. The data summarized beiow includes surface areas of the cata-lysts, Ag and RNO3 contents, maximum observed outlet ethylene oxide ~activity), maximum observed efficien-cy, and percent residual fluorine content, by weight,of the support.

TAB~E

15 Example Preparation S. A. % Ag Procedure (m2/9) 6 C 0.7 19.7 7 C 1.4 20.0 8 C 1.1 19.7 9 C 0.8 20.4 S 1.0 15.6 11 S 1.0 15.6 Maximum Maximum Example % BNO3 Activity Efficiency Residual (E. O.) % F

6 2.0 1.18 91.0 0.9 7 2.0 1.41 90.3 0.7 8 2.0 1.23 90.0 0.75 9 2.0 1.35 90.3 0.1~
1.0 1.12 86.5 0.21 11 2.0 1.18 88.5 0.21 128993~i Example 12 - Production of Propylene Oxide With Potassium Nitrate-Containin~Lsup~orted S_lver Catalyst:
Into a back-mixed autoclave, having a slightly smaller volume that the autoclave employed in the previous examples, were placed 5.0 grams (10.0 ml) of a catalyst having a support prepared as in Example 1. The support was an alpha-alumina having a surface area of 1.12 m /g, a pore volume of 0.8 cc/g, less than 50 ppm, by weight, of leachable sodium, a residual fluorine concentration of 0.7 percent, and a platelet morphology of the type disclosed in Canadian Serial No. 515,864, filed August 13, 1986. The catalyst included, by weight, a silver concentration of 17.7 percent and a potassium concentration of 0.38 percent (calculated as 1 percent potassium nitrate).
A mixture, by volume, of 9.9 percent propylene, 7.75 percent oxygen, 200 ppm ethyl chloride, 200 ppm nitric oxide, 2 percent methane, and the remainder being nitrogen ballast gas, was fed to the reactor at a temperature of 252 degrees C, a pressure of 25 psi and a gas hourly space velocity of 832 hr 1. After 22.5 hours, a selectivity of 47.3 percent was observed with an activity of 0.6 pounds of propylene oxide/cubic foo,t of catalyst/hour (corresponding to an outlet percent of propylene oxide of 0.465 percent).

A~

:

Claims (25)

1. A process for epoxidation of alkene in the presence of an oxygen-containing gas comprising con-tacting said alkene and said oxygen-containing gas under epoxidation conditions in the presence of at least one gaseous efficiency-enhancing member of a redox-half reaction pair, a performance-enhancing amount of a fluorine-containing substance, and a supported silver catalyst comprising a catalytically-effective amount of silver on a solid porous support and at least one efficiency-enhancing salt of a mem-ber of a redox-half reaction pair.

2. The process of claim 1 wherein the fluorine-containing substance is present in the catalyst.

3. The process of claim 2 wherein said fluo-rine-containing substance is present in the solid support as a residual fluorine-containing material.

4. The process of claim 3 wherein the support is comprised of an alpha-alumina converted from a non-alpha-alumina by heating in the presence of a fluorine-containing recrystallizing agent which pro-vides at least a portion of said fluorine-containing substance.

5. The process of claim 4 wherein said fluo-rine-containing recrystallizing agent and said fluo-rine-containing substance comprise the same sub-stance.

6. The process of claim 3 wherein said support contains about 0.1 to about 2.5 percent, by weight, based on the total weight of support of said residual fluorine-containing material.

7. The process of claim 6 wherein said support comprises alpha-alumina.

8. The process of claim 5 wherein said gaseous and said salt members of redox-half reaction pairs comprise members of the same redox-half reaction pair.

9. The process of claim 1 wherein said at least one gaseous member of a redox-half reaction pair comprises NO, NO2, N2O3, N2O4, or a gas capable of generating one of the aforementioned gases under epoxidation conditions.

10. The process of claim 9 wherein said gas capable of generating one of the aforementioned gases generates at least one of NO and NO2.

11. The process of claim 1 wherein said at least one efficiency-enhancing salt of a member of a redox-half reaction pair comprises potassium nitrate.

12. The process of claim 1 wherein said gas-eous efficiency-enhancing member of a redox-half reaction pair is NO.

13. The process of claim 1 wherein said at least one gaseous member of a redox-half reaction pair comprises NO and said at least one salt of a member of a redox-half reaction pair comprises potas-sium nitrate

14. The process of claim 1 wherein said support is alpha-alumina.

15. The process of claim 1 wherein said alkene comprises ethylene.

16. The process of claim 1 wherein said alkene comprises propylene.

17. The process of claim 1 wherein said alkene comprises ethylene, said at least one gaseous member of a redox-half reaction pair comprises NO, said at least one salt of a member of a redox-half reaction pair comprises potassium nitrate, and said solid support comprises alpha-alumina having a fluorine-containing substance physically associated therewith.

18. The process of claim 1 wherein the fluo-rine-containing substance is present in said oxygen-containing gas.

19. The process of claim 18 wherein said fluo-rine-containing substance comprises an organic com-pound.

20, A catalyst suitable for the epoxidation of alkene in the presence of an oxygen-containing gas comprising a catalytically-effective amount of sil-ver, a salt of a member of a redox-half reaction pair and a performance-enhancing amount of a fluorine-containing substance present on a solid porous sup-port.

21. The catalyst of claim 20 wherein said solid support is alpha-alumina.

22. The catalyst of claim 20 wherein said sup-port comprises an alpha-alumina converted from a non-alpha-alumina by a heat treatment in the presence of a fluorine-containing substance.

23. The catalyst of claim 22 wherein said sup-port contains about 0.1 to about 2.5 percent, by weight, based on the total weight of support of residual fluorine.

24. The catalyst of claim 23 wherein said sup-port comprises alpha-alumina.

25. The catalyst of claim 20 wherein said salt of a member of a redox-half reaction pair is present in an amount sufficient to provide a chemical effici-ency of at least about 84 percent under Standard Test Conditions.