CA1282772C - Carbonate-supported 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 comprising 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 and a solid catalyst, the catalyst comprising a catalytically-effective amount of silver on a solid porous support and an efficiency-enhancing amount of at least one efficiency-enhancing salt of a redox-half reaction pair, the support comprising at least one carbonate salt selected from the group of carbonates of cations consisting of barium, strontium, calcium, magnesium, and mixtures thereof.

Description

~;æ~7~

IMPROVED CARBONATE-SUPPORTED
CATALYTIC SYSTEM FOR
EPOXIDATION OF ALRENES

Technical Field-The pre~ent lnvention i~ directed to an lmproved ~y~tem for the preparation of alkene oxide from al-kene and an oxygen-containing ga~ employing a BUp-ported sllver catalyst and to the catalysts used ln the ~ystem. More particularly, the present invention relates to the oxidation of alkene~ to the cor-re~ponding epoxides in which enhanced per~ormance i~attained by comblnation of a gaseou~ member of a redox-half reaction palr pre~ent in the gaseous ml~-ture of oxygen and alkene, a salt of a member of a redox-half reaction pair ln comblnation wlth the cataly~t and use of a ~tablllty-~nhanclng carbonate salt ~upport.

Background Art:

The productlon of alkene oxlde~, or epoxide~, partlcularly ethylene oxide by the dlrect oxldatlon of the correspondlng alkene ln the pre~ence of a sllver-containlng catalyst ha~ been known for many year~. For example, the basic process was described by ~efort ln U. S. Patent 1,998,878 ~nd by Van Peskl -Si~82772 ln U. S. Patent 2,040,782. The basic reaction pro-ceed~, a~ illu~trated for ethylene, according to the equation:

2 CH2-CH2 1 2 ~ 2 CH2 CH2 (I) and production of an unwanted by-product according to the reactlon:
CH2-CH2 + 3 2 - ~- 2 C02 ~ 2 H20 (II) or by further oxidation Oe the epo~lde.
ln the year~ between the Van Peskl patent and 15 the present inventions, re~earch efforts have been directed to lmproving both the activity and longevity or useful llfe of the catalyst and the efficiency of ; the overall catalytic reactlon. AB 1S indicated by ¦ reaction~ I and Il, the oxldatlon oE an alkene may ? 20 produce either the alkene oxide ~I) sought in the ! proce~s or the by-product~ C02 and ~2-Several tér~s are commonly ueed to descrlbe some of the parameters of the catalytlc system. For ln-~tance, ~conver~lon~ has been deflned as the percent-25 age of alkene fed to the reactor which undergoe~
reaction. The ~efflciencyR or, a~ lt is sometime~
called, the ~aelectivity~ of the o~erall proces~ i~
an indication o~ the proportlon, u~ually repre~ented by a percent~ge, of the converted materlal or product 30 which iB alkene oxide. The commercial ~uccefi~ of a reactlon ~ystem depend~ in large mea~ure on the effl-clency of the syQtem. At pre~ent, maximum eficlen-cles in commerciai production of ethylene oxlde by epoxidation are in the low 805, e.g., 80 or 81 per-35 cent. Even a very ~mall increase ln efflclency will ~2a~7'72 proviae &ub~tantial cost benefit~ ln large-~cale operatlon. For example, taking 100,000 metric ton~
as a typical yearly yield for a conventlonal ethylene oxl~e plant ~nd as~uming 80 percent conver~ion~ an increaqe ln eff iciency of from 80 to 84 percent, all other thing~ being equal, would re~ult ln ~ savings of 3790 metric tons of ethylene per year. In ~ddi-tion, the heat of reaction for reactlon II (formation of carbon dloxide) is much greater than that of reac-tion I (formation of ethylene oxide) BO heat-remov~l problem~ are more burdensome as the efficiency de-crea~e~. Furthermore, as the efficiency decrease~, there i~ the potential for a greater amount of impur-itie~ to be pre~ent ln the reactor effluent whlch can complicate ~eparation of the de~ired alkene oxide product. It would be desirable, therefore, to de-velop a proce~s for the epoxidatlon of alkene ln which the eef iclency ~B greater than that obtained in conventional commerclal proces~es, e.g., wlth ethy-lene, efficiencies of B4 percent or greater, whllemalntalnlng other performance characterlstlc~, partl-cularly the activlty, as descrlbed below, ln A ~atl~-factory range.
The product of the efflclency and the conversion i~ equal to the yleld, or the percentage o~ the al-kene fed that i~ converted lnto the corre3pondlng oxide.
The ~activity~ of the cataly~t 1B a term used to indic~te the amount of alkene oxlde contalned in the outlet ~tream of the reactor relative to that ln the lnlet ~tream. Actlvlty i~ generally expre~ed ln terms of poundfi of alkene oxlde produced per ~ubic foot of catalyst per hour at speclfied reactlon con-ditlons and rate of feed. The activlty may ~180 be stated ln terms of the amount of ethylene oxlde ln ~8~:77~

the outlet stream or the d~fference between the ethy-lene ox~de content of the lnlet and outlet ~treams.
If the activ1ty of a reaction system ls low, th~n, all other thlngs belng equal, the commerclal value of that system will be low. The lower the activity of a reactlon ~ystem, the le~ product pro-duced in a unit time for a given feed rate, reactor temperature/ cataly~t, surface area, etcetera. A low activity can render even a high efflciency process commercially impractical. ~or production of ethylene oxide, an activity below 4 pound~ of ethylene oxide per hour per cubic ~oot of catalyst i8 unacceptable for commercial practlce. The activity 1~ preferably greater than 8 pounds, and in ~ome lnstance~ an acti-vity greater than ll pound~ of alkene oxlde per hourper cubic foot of catalyst i~ deslred.
In some lnstances, activity is measured over a period of time ln terms of the amount of alkene oxlde produced at A ~peclfled constant temperature. Alter-natively, ~ctivity may be measured as a function ofthe temperature required to sustain production of a specified constant amount of alkene oxide. Plots of such mea~urements yleld ~aglng rates~ whlch reflect the stability or useful llfe of the cataly~t. The useful life of ~ reaction system ls the length of time that reactants can be passed through the reac-tion sy~tem during whlch acceptable activlty 1~ ob-~erved. The area under a plot of activity versus time iB equAl to the number of pounds of ~lkene o~lde produced durlng the u~eful llfe of the catalyst per ~ublc ~oot of catalyst. The greater the ~r~a under such a plot, the more valuable the proce~ lnce regeneratlon or replacement of the catalyst lnvolves a number of expense~, sometlme~ referred to a~ turn-around costs. The rate at whlch activlty decreases, i2827;~

l.e., the rate of deactivatlon at a glven polnt int~me, can be repre~ented by the 810pe of the actlvlty plot, l.e., the der~vatlve of activity wlth re~pect to t~me:
deac~ivatlon ~ dlactlvlty]/dt.

The average rate of deactivation over a perlod o~ time can be represented then by the change in act.iv~ty divided by the time periods average deactivation ~ a activity/ ~

At ~ome polnt, the act~vity decrease~ to an unacceptable level, for example, the temperature required to maintain the ~ctivity of the ~ystem be-comes unacceptably high or the rate of production become~ unacceptably low. At thls point, the cata-lyst mu~t either be regenerated or replaced. Some of these deflnltions may be represented as set ou~ be-low:

ConverYlon O moles alkene reacted x 100 mole~ alkene fed Efflclency ~ moles alkene oxide Produced x 100 moles alkene reacted Yleld , moles alkene oxlde Produced ~ 100 moles alkene fed Typically, ln commercial productlon, ~ince the outlet or effluent stream emanatlng from the reactor may contaln Jubstantial nmounts of unreacted alkene, the effluent ~tream 18 recycled and comblned wlth the ~2~n2 feedstream after removal of at lea~t a portion of ~he alkene oxide. Geneeally, a~ the activlty o~ a cata-lyst decreases with time, ln order to obtain the same ultimate yleld of epoxlde product, the effluent stream must elther be recycled 2 greater number of time~ or the ~emperature wlthln the reactor mu~t be raised to lncrease the activity of the catalyst. The former approach to increasing the yield of product require~ additional energy e~penditures and the lat-ter, which i~ most frequently used, causes faseercataly~t deterloration.
~ s used herein, an activity-reduclng comp~und refer~ to a co~pound which, when present in an acti-vity-reduc~ng amount, cau~es a reductlon ln actlvlty, gome or all of whlch actlvlty may subsequently be regained by returning to a situatlon ln whlch the concentration o~ the compound ls below the minlmum actlvlty-reducing amount. The mlnlmum actlvlty-reduclng amount varles dependlng on the partlcular sy~tem, the feedstream and the actlvity-reduclng compound.
Conversely, deactivatlon, as u3ed hereln, refers to a permanent 1085 of actlvlty, l.e., a decrea~e ln activity which cannot be recovered~ As noted above, act~vity can be lncreased b~ ealslng the temperAture, but the need to operate at a higher temperature to malntaln a particular actlvity i~ representative of deactlvatlon. Furthermoce, cataly~t~ tend to deactl-vate more rnpldly when reactlon i8 carrled out at higher temperatures.
In contrast to problems assoclated wlth low or decre~sing cat~lyAt activitle~, less than ~atisfac-tory efflclencies result in los~ of starting mater-lal, the alkene, as the unwanted product C02. Ultl mately, thls also lncreases product costs.

1~8;~2 To be considered 6ati~factory, a cataly~t must not only have ~ 5uf f i~ient activlty ~nd the catalytlc system provide an acceptable efficiencyr but the cataly~t mu~t also demonstrate a minimum u~eful llfe ¦ 5 or 6tability. When a cataly~t 1~ spent, typic~lly ! the reactor must be shut down and partlally di~man-¦ tled to remove the spent catalyst. This re~ults ln lo~ses in time and productivity. In addition, the catalyst must be replaced and the silvet salvaged or, where pos~ible, regenerated. Even when a cataly3t 1~
! capable of regeneration in situ, generally productlon must be halted for some perlod of tlme. At be~t, replacement or regener~tion of cataly~t requires additional lo~es ln tlme to treat the ~pent cataly~t and, at worst, requlre~ replacement of the cataly~t wlth the ~socl~ted costs.
Since even ~mall improvements in actlvity, ef-ficiency or u~eful life may have slgnificance ln large ~cale commercial productlon, such lmprovement have been the ob~ect of a great deal of research ln the direct epoxidation of alkenes. The focu~ of attempt~ to improve performance, such 8~ the ~ctlvity and useful life of the catalyst and the efficiency of the sy~tem, has lncluded such areas ~ feed~tream additlve~ or removal of components therefeomt method~
of prep~ration of the cataly~t~ depo~ltlon or impreg-natlon of ~ partlcul~e type or form of ~llvee~ com-po~ltlon, formation, phy~ic~l propertles nnd morpho-logy of the support~ additlves deposited on or lm-pregn~ted ln the support~ shape of ~upport aggreg~tesused ln the reactor~ and varlous types of re~ctors and bed designs, ~uch ~ ~tatlonary and fluldlzed beds.

;8-~B2~7~

Early work on the ~ilver-catalyzed direct oxlda-eion of alkene~ to alkene oxldesi ln many ln~tances resulted in lmprovement~ in activity and particularly thel~electively of the sy~tem, in many case~ the efficlency lncrea~ing by ~everal percent. Recent modiflcation~ in such ~y~tem~ have resulted ln only small incremental improvements in efficiency; how-ever, in term-Q of operating costs, even fractions of a percent improvement in efficlency can translate into large ~avlngs in production. Accordlngly, cur-rent reaearch i8 still being directed to improvements in the activity and u~eul life of the catalyst and ~electlvity of the system.
Although a vast number of elementfi and compounds are known to have effective catalytic propertles in variou~ ceaction~, many have at least one shortcom-ing, such as very high cost and/or llmited avail~bil-ity, thermal inqtability in the temperature range in which the reaction is to be conducted, low mechanlcal strength, small sueface area per unlt of volume, ~usceptibillty to poisoning, short useful lifetime, etcetera. Suc~ undesirable characterl~tics make ~uch substances of limited utility as cataly3ts. Some of these shottcomlngs, however, may be overcome and in ~ome instances the e~eectiveness o~ the c~tnly~t may Ibe improved by applying the sub~tance to ~ c~rrler or support.
New support materlals are continuously being tried. How~ver, many of those which were employed in i 30 the early development of the ~llver-bearlng cataly~t~
are, with some modifications, still being u~ed.
Material~ which have found mo~t wldespread use are typlcally inorganic and generally are of a mlneral nature. Such materials commonly include alumlna, flre brick, clay, bauxlte, bentonite, kleselguhr, ~32~72 carbon, ~lllcate~, ~ilic~, ~ilicon carbide, zlrconla, dlatomaceous earth, and pumice.
In addition to the physical strength of the ~upport materlals, other physical properties, ~uch as S ~urface area, pore volume, pore dlmensions, and cata-ly~t slze have drawn considerable attentlon. The~e propertles have been examined with great scrutiny when evldence indlcated that there was a correlation between the size of the catalyst and the efficiency of the ove~all ~ystem or useful life of the cata-lyst. Some materials are also preferred for their chemical properties, l.e., their ~inertne~s~ or ~pro-moting~ properties.
The support serves a number of functions in a heterogeneous catalytic system. Ease of handllng facilitated by a support which generally takes the form o di~crete particles or aggregates of varying ~hape or eize which, depending on usage, have a ma~or dimen~ion of about 1 millimeter to about 20 mlll~-meter~. Thus it 18 not necessary for the catalyst toform a permanent or ~emi-permanent part of the reac-tor.
The support, however, serves pr~marily to in-crease the surface area of the ~active~ component of the catalyst, ~llver, which 1~ important in that mo~t epoxldation occurs at the ~llver eurfnce-fluld lnter-face. Many of the ~ubstance~ commonly employed a~
catalyst ~upports not only have the u~ual external sur~ace, whlch provldes a varying surface area, de-pendlng on the shape of the support bodi~s and thepacklng of the bodles, but are al~o of a porous na-ture and, therefore, have a large lnternal ~urface ~hich ~ontrlbutes to the overall surface ~rea of the supported catalyst. Such ~upport materlal6 pro~ide a greater capacity for sorbing not only the catalyst ~77%

material during catalyst preparation (when the ~up-port i8 impregnated with a ~olution containlng the catalyst component~) in ~oluble form) but al~o a greater capac~ty for the flow of the flu~d reactants within the cataly~t during the reactlon or which the catalyst is intended. The ~upport al~o improve~
performance by lower~ng the pressure drop through the reactor and by facilitating heat and mass tran~fer.
Among the large variety of ~ub~tances employed in the past a~ ~upports for catalytic materials, alumina ha~ exhibited ~uperiority in many respects as a cataly~t ~upport material. In addition to the low cost of the materlal, alumina has good ther~al sta-billty and some forms have a relatively large ~urface area.
Alumlna, ln lt~ varlous`forms, partlcularly alpha-alumina, ha~ been preferred as a support mater-ial for silver-containing cataly~ts in the prepara-tion of alkene oxides. Numerous varlatlon~ of ~ur-face area, pore dimen~ions, pore volume and particle ~lze have been sugge~ted a6 providing the ideal physical propérty or combination of properties for improving e~iciency, activity or u~eful llfe of ~he catalyst.
Holler ~U. S~ ~atent 3,908,002) dl~clo~es an Dl-pha-alumlna, useful as a c~taly~t support for reac-tlons conducted at temperatures below 800 degrees C, such as oxidation reactions of hydrocarbon~ to oxyhy-drocarbons. The support, having a ~urface area re-ported to be at lea~t about 40 m2/g, 1~ produced by thermally decompo~ing a porou~ alumlnum ion chain-bridged, polymeric carboxylate. Indlcatlng that a lnrge ~urface area ~n a carrier may be detrimental to its ef~icient operation and catalyst activity, Belon (U. S. Patent 3,172,866~ describes a method of pro-~r ~282~

ducing a macroporous catalyst carrler which may be u6ed ~n the catalytlc production of ethylene oxlde hav~ng pore diameters of between O.l and 8.0 mlcrons and a ~pecific surfa~e area between a few equare meters and one square de~imeter per gram. The eup-port i~ prepared by heating a mixture of actlve and calcined aluminum oxide~ and a small amount of boron oxide at temperature~ of between about l,C00 and 1,800 degree~ C. Waterman (U. S. Patent 2,901,441) describes a process for preparing highly active and selective cataly~ts for the oxidatlon of olefins to olefIn oxldes on a support havlng an average porosity of at lea~t 35 percent. The method involve~ washing an alpha-alumina or ~lllcon carbide support havlng an average poro~ity of between 3~ and 65 percent with an aqueous ~olution of lactic acid, washlng wlth water untll neutrbl, and then lmpregnating the ~upport wlth an aqueou~ solution of silver lactate. The lmpreg-nated ~upport 1~ thereafter heat-treated to deposlt elemental silver. A silver-supported catalyst for the vapor pha~e oxldatlon of ethylene to ethylene oxide, exhiblting lmproved productlon of ethylene oxide snd catalyst longevlty, i8 described by Brown et al ~U. S. !Patent 3,725,307). The catalyst 1~
2~ disclo~ed as being ormed from support partlcles havlng an average pore diameter of at lea~t lO ml-cron~ up to, preferably, 70 mlcrQns and a ~urface area of lefls than about l m2/g. The ~electivltlea reported do not range ~bove about 73 percent. The support 1~ preferably compo~ed of slllca-alumlna.
silver-~upported catalyst whlch lncludes a ~upport of ulpha-alumlna, sillcon carblde, fused aluminum oxlde, or mixture~ of lumlna and sillca was a~serted by DeMaio (U. S. Patent 3,664,970) to elimlnate the need or halogenated lnhibitors in the oxidation o~ ethy-~ ' ~.zaz7~

lene to ethylene oxldeO The ~upport i~ compo~ed ofparticles having a minimum apparent porosity of about 30 percent ~nd whereln at lea~t 90 percent of the pores have diametee~ in the range o 1 to 30 micron~, the average of the diameters being ln the range of 4 to 10 mlcron~. Wattimena (U. S. Pa~ent 3,563,914) dieclo~es sllver cataly~ts using aluminum o~lde BUp-port~ having pore volumes between 0.1~ and 0.30 cc/g and BUrface area~ below about 10 m2/g.
Hayden et al (U. K. Patent Application 2,014,133t disclose a sllver catalyct employing a support having a specific ~urface area in the range of 0.05 to 10 m2/g, an apparent poro~ity of at least 20 percent, and mean pore diameter~ of 0.1 to 20 mlcron~, the pore size di~tributlon being bimodal, in which the smaller pore~ preferably account for at least 70 percent of the total pore volume. Alpha-alumina supports are described by Rashkin (U. ~.
Patent Application 2,122,913A) having a ~relatlvely low ~urface area~ of less than 30 m2/g. Mitsuhata et al ~Japane~e Published Patent Application 56-089~43) and Mit~uhata et al ~U. S. Patent 4,368,144) de~cribe supported sllver catalysts ln which the ~upport i8 formed from alpha-alumina having a specific eurface area of 0.5 to 5 m~/g. Watanabe et ~1 ~Japane~e Published Patent Appllcatlon 56-105750) employ a simllar catalyst ~upport havlng a surface area of 1 to 5 m2/g. Hayden et al ~U. S. Patent 4,007,135) de~crl~e sllver-contalning catalysts ln whlch the j 30 porous heat-re~isting ~upport ha6 a ~pecific surface ~rea ~n the range o~ 0.04 to 10 m2/g, an nppaeent poro~ity of at lea~t 20 percent, and a median pore dlameter of 0.3 to 15 microns. Mit~uha~a et al (U. S. Patent 4,24a,740) describe the use of hlgh ~lpha-alumlna content ~upport6 havlng a ~peclflc surface area of not more than 10 ~2/g, an ~pparent poro~ity of 40 to 60 percent by volume, and a pore volume of 0.1 to 0.5 cc/g. Armstrong et al (U. S.
Pa~ent ~,342,667) disclose a supported sllver ~at~-lyst, useful ln the oxldation o ethylene to ethyleneoxlde, ln which the ~upport has a surf~ce area of 0.02 to 2 m2/g, an aver~ge pore dlameter of 0.5 to 50 microns and an average pore volume of 0.2 to 0.5 cc/g.
There has also been some lnterest in the purity of supports employed, both as to compositlon and phase. E~amples o~ hlgh purity alumina include U. S.
Patent 2,901,441 whlch uses alpha-alumlna having a purity of about 99.5 percent as a support for cat~-ly~ts used to o~ldlze olefln~ to olefin oxldes. Anethylene oxldatlon catalyst i8 disclosed ~n German Patent Publlcation DE 2,933,950 which attains a long catalyst life wlthout a 1088 in actlvlty or sele~-tlvity by u~ing an alpha-alumina ~upport havlng less ~han 0.001 welght percent of alkall-soluble ~llicon compound~. The cataly~t iB prepared by boiling com-mercial quallty alpha-alumina with 1 welght percent sodium hydroxide solution and washing to a pH v~lue of B. If desired, the ellicon compound conc~ntration may be reduced below 1 p~rt p~r milllon ~ppm) by washing further wlth 1 weight percent ~F. U. ~.
Patent Appllcation 2,122,913A de~cribes eupported sllver cataly~ts in which the support ls composed of sllica, ~lumina or mixture~ thereof, one example of whlch 1B ~n alumlna having a purlty of 99.3 percent by ~elght. The silver-supported cataly~t de~crlbed in Japanese Publi~hed Patent ~pplication 56-OB9843 ~mploys an alpha-alumina carrler h~ving a ~odlum content of le~s than 0.07 welght percent. Japanese Published Patent Applic~tlon 56-105750 dessribes the ~n2 , ' u~e of an alpha-alumlna ~upport in con~unction wieh a j ~ilve~ cataly~t or producing ethylene oxide, which ! ~upport hafi a Rodium content le58 than 0.07 welght ¦ percent. A ~ilver cataly~t lncludlng an alpha-j S alumina carrier having a sodium content of not more than 0.07 percent 1~ de3cribed by Mitsuhata et al (U.
S. Patent 4,368,144). The ~upport al~o ha~ ~ ~urface area withln the range of O.S to 5 m2/g, an appDrent porosity of 25 to 60 percent, a specific pore volume of 0.2 to 0.5 cc/g, and a partlcle dlameter ~ithin the range oE 3 to 20 mm. An alpha-alumlna support havlng a purity of 98~ weight percent, for use wlth ~ilver in the catalytic oxidatlon of ethylene, iB
descrlbed by Warner et al in U. S. Patent 4,455,392. ~he patent additionally disclose~ that the carrier iB generally a conventlonal microporou~
~upport with surface areas of le~ than 10 m2/g, pore volume~ ranging from about 0.15 to 0.8 cc/g, and pore j dlameters of about 0.1 to 100 microns.
In addition to compositional purity, both pha~e purity Dnd morphology of the support have been areas in which 1mprovements in efflciency, selectlvlty or ~tabillty of the catalyst have been sought. Example~
lnclude U. S. Patent 2,901,441 ln which alumlnum oxide ie sub~tantially completely converted to the alpha form of alumlna by heatlng aluminum oxlde to a temperature of ~bout 1,~00 to 2,050 degree3 C. WeiB~
(U. S. Patent 2,209,908) and Carter ~V. S. Patent 2,294,3~3) de~crlbe the u~e o~ ~Tabular Corundum~ a5 ¦ 30 a cataly3t support for metalllc oxide~, ~uch as those oxlde~ of metal~ selected from the flfth and sixth group of the periodic system, for example, vanadlum, molybdenum, uranlum, etcetera, ln the o%idatlon of varlous orgAnic materlal~ to maleic acid and maleic anhydride and silver for the catalytic oxldatlon of -lS-ethyle~e to ethylene o~ide, respecelvely. ~el~
indicate~ that Tabular Corundum, ~h$ch i8 almost entirely aluminum o~ide and ha~ the alpha-corundum crystalline form of ~luminum oxlde, may be formed by mlxing aluminum oxlde with one or more of several compounds, ~uch a8 sodlum oxlde and chromic oxl~e, and heating the mixture to a temperature in the range of about ~00 to about 1,800 degrees C. Tabular Co-rundum i~ further descrlbed a~ havlng lmpurlties present in only ~mall quantitle~, the material also includes ~readily bonded surfaces and consi~ting es~entially cf interlocked corundum crystal~ ln tabu-lar form, having the contalned lmpurlties dls-semlnated ln mlnute globules throughout the crystal-line alumlna~. ~rengle et al ~U. S. Patent2,709,173) also employ Tabular Corundum a~ ~ support in one of their examples.
U. S. Patent~ 4,039,481 and 4,136,063 to Rimura et al di~clo~e a catalyst carrler and a ~ethod for maklng ~ame, the catalyst being the type used ln catalytic converters ln automoblle exhaust system~.
Specifically,'the catalysts have ~ sur~ace layer containing alpha-phase alumina and ~n lnner portlon consl~tlng e~entially of alumlna of a phase other than that of the alpha pha~e. The por~s in the ~1-pha-alumina surface layer are larger than tho~e in the lnner p~rtion of the catalyst body. A method of preparlng the phase gradient support partlcles is descrlbed whlch provlde~ for treating the surface of the alumlna to a depth of about 400 mlcrons ~lth a tran~ltion element, particularly lron, and thereafter flring the carrler particles.
Weber et al (U. 5. Patent 4,379,134) descrlbe high purlty alpha-alumina bodies, at least 85 percent of the pore volume of ~he hodie~ having pores with a ~2 ! diameter of from 10,000 to 200,000 Ang~tro~. The ¦ high purity alpha-alumina bodies are prepared by ! peptlzing boehmlte ln an acldic aqueous, fluorlde ~nion-containlng mixture. An extrudable ml~ture 1 formed thereby whlch 1~ extruded and ~haped lnto formed bodie~ which are thereafter drled at 100 to 3Q0 degrees C, calclned at a temperature of from ~00 to 700 degree~ C to convert the alumlna to the gamma pha~e, and 3ub~equently calclned further at a temper-ature of from 1,200 to 1,700 degrees C to convert thegamma pha~e to alpha-alumlna phase.
A method of produclng granulated porous corundum havlng a homogeneou6 porous structure wlth a total pore volume of 0.3 to 1.0 cm3/g and ~ predomln~nt pore slze of 5,000 to 30,000 A 1~ de~crlbed by Bores-kov et al (U. S. ~atent 3,950,507). The method of preparlng the alpha-alumlna lnclude~ treating active alumlna or alumlnum hydroxlde havlng a porous struc-- ture to a flrst heat treatment in whlch the tempera-i 20 ture 1~ increased from 20 to 700 degrees C, a ~econd he~t treatment in the range of from 700 to 1,000 degrees C, ana a thlrd treatment ln the cange of from 1,000 to 1,400 degree~ C. Each of the heat treat-ments i8 for a period of nt lea~t one-half hour, the flrst heat treatment belng conducted ln ~n atmo~phere ~ of hydrogen 1uorlde ln whlch the alumlna absorbs the ! hydrogen fluorlde and the second heat treatment de-sorbs the hydrogen fluorlde. The patent al~o de-scrlbe~ a slmllar procedure employlng statlonary i 30 thermal condltions ln whlch the granuleR of alumlna or alumlnum hydro~lde are lmpregnated wlth other fluorlne-contalning sub6tances prlor to the flr~t thermal treatment. The recommended starting mater-lals u~ed to form alpha-alumlna lnclude granulated ~5 pseudo-boehmlte, boehmlte or bayerite as the granu--~1;2E~2772 lated aluminum hydroxide and granulated alpha-, ota-, or theta-alumlna aB the active alu~lna.
Although alpha-alumina ha~ been con~idered by ~o~t to be the preferred alumlna BUppOrt materlal, Smlth et al (U. S. Patent 2,422,172) have ~ugge~ed th~t beta-alumlna~ are more de~lrable than the alpha pha~e as A support material for ~ataly~t~, partlcu-larly tho~e used ln catalytic converslon proces~es such as dehydrogenatlon and hydroformlng.
In eeeklng the ldeal support materlal, there has been 80me departure from the commonly employed sub-stance3. For example, some u~e has been made of alkall metal and alkallne earth metal ¢arbonates, both a~ ~he ~ole ~upport materlal ~nd in comblnatlon wlth other materiale as the carrler for proce~se~
such as dlrec~ oxidatlon of alkenes to epoxldes.
A number of ~upported sllver-containlng c~ta-ly~t~ have been employed for epoxldatlon of alkene~
ln which the carrler lnclude~, ~ometl~e~ labelled a~
a promoter, a carbonate of a metal, generally an alkall metal or alkallne earth metal. Some examples of the use of;one or more alkall and/or alkallne earth carbonate~ may be ~ound ln U. S. P~tents 2,424,084, 2,424,086, 2,615,900, 2,713,586, 25 3,121,~99, 3,258,433, 3,~63,913, 3,563,gl4, 3,5B5,217, 4,007,135, 4,033,903, 4,039,561, 4,066,575, ~,094,889, 4,123,385, 4,125,480, 4,168,247, 4,186,106, 4,226,7B2, 4,229,321, 4,324,699, ~uropean Patent Publlcatlons 0,003,642 ~nd 30 0,011,356, Japane~e Patent~ 41-11847 and 57-107242, U. g. Patents 590,479, 1,571,123 and 2,014,133A, and Murray, ~A ~tudy Of The Oxldatlon Of ~thylene To Et~ylene Oxlde On A Silver Cataly~t~, Au~trallan Journal of æclentlflc Research, Volume 3A, P~ges 433-35 449 (1950). In addltlon, U. 5. Patent 3,332,~87 to .

-~772 Endler employs zlnc and/or cadmlum carbonate~, Gelb-~teln, (DS 2,352,608) di3clo~e~ the use of th~ latter carbonate and European Patent Publicatlon 0,003,64~
~entions the use of ~olybdenum carbonate. ~oreskov et al (U. S. Patent 4,130,570) describe a method of produclng ethylene oxide u~lng a catalyst whlch ln-clude~ sllver, cadmium carbonate and/or cad~ium oxlde and fu~ed alumlna. Oprescu et al (French Publlshed Patent Applicatlon 2,005,9781 descrlbe a 011ver-based catalyst for u~e in the preparation of ethylene oxlde whlch i8 a coprec~pltate of sllver and other carbon-ates.
SeverAl patents have descrlbed the use of fluo-rlne-contalnlng ~ubstances to treat support mater-lal~, ln some cases to provide a composltlonally pure~upport, and in other ca~e~ as a fluxing agent to improve the pha~e purity of the support. Thus, U. ~.
- Publlshed Patent Speciflcatlon 590,479 and U. S.
Patent 2,424,086 lndicate that a more actlve catalyst 1~ formed lf the support materlal has undergone a prellminary treatment wlth a dilute ~olution o hy-drofluorlc acld prlor to lmpregnatlon wlth sllver.
V. S~ Patent 4,379,134 teaches the preparatlon o~
hlgh purlty alpha-alumina bodle~ by peptlzing boah-mlte alumlna ln an aqueous acldlc mlxture containlngfluorlde anlon~ and water. German Patent 2,933,950 teaches the reductlon of ~lllcon content by treatment wlth ~F. U. S. Patent 3,950,507 teache~ the prepara-tlon of granulated porous corundum by n multlple step heat treatment ln which initlal steps may be carrled out ln an atmosphere of hydrogen fluorlde. ~osoda et al (U. S. Patent 3,144,4161 sugge~t tbat ~ small amount of a halogen compound, sulfur co~pound, nltro-gen compound, or phosphorous compound may be ~dded elther to the reaction ga~ or the catalyst to lmprove the selectlvity of the catalyst.
The n~tute of the ~ilver ltself h~s al80 been examlned and msdified ln attempts to lmprove the efficlency and stabillty of the catalyst. Cavltt tU. S. Patent 4,229,321) teaches that a supporeed silver catalyst of lmproved aelectlvlty and nctlvlty ~ay be psepared by mechanlcally removlng t~e outer ~urface or skin of the cataly~t ~fter the impresnated catalyst has been heated to evaporate volatlle mater-~al and reductlon of the sllver salt to ~llver metal,thereby actlvating the cataly~t.
Since the early work on the dlrect catalytlc oxldatlon o ethylene to ethylene oxide, workers ln ~he fleld have ~uggested that the addltion of certain compound~ to the gaseou~ feed~tream or dlrect lncor-poratlon of metals or compounds in the cataly~t could enhance or promote the production of ethylene ox-ide. Such metal~ or compounds have been known vari-ou~ly a~ ~anti-cataly~ts~, ~promoters~ and ~lnhl-bitor~. These substances, which are not themselve~consldered catalysts, have been proposed by prior workers to contrlbute to the efficiency of the pro-ces~ by lnhibiting the form~tlon of carbon dloxlde or promotlng the productlon of othylene oxlde. The sclentlfic llterature 1~ rcplete ~lth e%ample~ of the use of alkall metals ~nd alkallne earth metal~ and thelr catlon~ to promote the efflclency of sllver cataly~ts u~ed in epoxldatlon reactlons. For ~%a~-ple, eodlum, potasslum and calclum ~ere ~lsclosed ae belng sultable promoters ln ~. S. P~tent 2,177,361.
Numerou3 examples may be found ln llterature of pref-erence for one or ~everal metal~ or cations and e~-clu~ion of one or more metal~ or catlons a~ pro~oter~
in silver catalysts.

.~ .

Among those anlon~ 2s~0ciated ~ith the catlon promoter~ used ln preparlng sllv~r-contalnlng c~ta-ly5t~ employed ln direct epoxldatlon reactlon~ that ha~e been suggested as belng ~ultable include cAr-~ 5 boxylates, for example, formate, acetate, malonate, I oxalate, l~ctate, tartrate, nnd/or cltrate, and ~nor-ganic salt~, ~uch as carbonates, bicarbonates, phos-phate~, nltrate~, and/or nltrltes, chlorides, lo-dides, bromate~, and 13Opropoxldes. ~owever, al-though many examples may be found ln the literature lndlcatlng that such compound~ are sultable, numerous patent~, such a~ U. S. Patents 3,962,136~ 4,012,425S
4,066,5~5~ 4,207,210; nnd 4,4~1,0~1, suggest that no unu~ual effectlvenes~, partlcularly wlth regard to catalytic actlvlty, 18 observed wlth any partlcular anlon of an alkall metal promotec. U. S. Patent~
4,007,135; 4,094,889; 4,125,480; 4,226,782~
4,235,757; 4,324,699; 4,342,667; 4,356,312;
j 4,36B,144; and ~,455,392 dlsclose that potas~lum 1 20 nltrate may be added to the catalyst as a su~table promoting materlal. Pota~lum nltrate may al~o be formed in sltu when a carr$er materlal 1~ tre~ted wlth certaln amlnes ln the pce~ence o~ potn~lum lon~
a8, for lnetance, when 011ver i~ lntroduced to a carrler materlal ~n a ~llver-impregnatlng ~olutlon contalnlng an amlne and pota~sium ion~, followed by roa~tlng.
A number of compound~ have been proposed in the llterature as nddltlves to the feedstream or re-1 30 actants to l~prove the efflciency of the dlrect, ~llver-catalyzed oxldation of alkenes to ~lkene ox-Ides. For example, Law and Chltwood (U. S. Patent 2,194,602) dlsclose the use of a ~repre~sAnt~, l.e., antl-cataly~t, such a~ ethylene dlchlorlde, chlorlne, sulfur chlorlde, sulur trloxlde, nltrogen dloxlde, ~28~72 or other halogen contalnlng or acld-formlng ~ter . Numerous additional anti-catalyst6 are pre-~ented by the same paten~ee~ ln U. 8. ~at~nt 2,279,469. The antl-catalysts, broadly ll~te~ ln S categorle~ euch aB halogen~ and compounds contalnlng halogen, hydrocarbons, compounds cont~lnlng c~rbon, hydrogen and oxygen, compounds contalnlng sulfur, an~
compound~ contalnlng nltrogen are repre~ented ~nd, ln addltlon to those compound3 already mentloned ~bove, adaitlonal representat~ve compounds ~nclude, as nl-trogen-contalning compounds, nitrlc oxlde, ~m~onla, amlnes such aB ethylenedlamlne, diphenyla~lne ~nd analine, nitro compound~ such as o-nltroanisole and o-nltrotoluene as organic oxygen-cont~lnlng organlc compounds, alcohols ~uch as methyl, ethyl ~nd l~o-propyl alcohols, ether~ ~uch a~ l~opropyl and dibutyl ethers~ ~8 well a~ glycol ethers, ketone~ ~uch BB
methyl ethyl ketone and acetone, a~ hydrocarbon~ ~uch a~ benzene, ~nd N-he~anes sulfur compounde ~uch as sulfur dloxlde, hydrogen ~ulfide and dlethyl~ulflde~
chlorine-cont~lnlng compounds ~uch as carbon tetr~-chlorlde, chlorobenzene and dlchlor~ethyl et~r.
~erl (U. S. Patent 2,270,780) ll~t~ a number of com-pounds a8 antl-detonatlng or ~ntl-knock m~terlal~ to control the o%ldatlon o~ ethyl~ne an~ propylene to their o~lde~. nlsclo~ure~ of other feedstream addl-tlves u~ed :In the productlon of alkene o%ldes, partl-cularly halogen compound~, may be found ln U. ~.
Patent~ 2,279,470l 2,799,687~ 3,144,416s ~,007,135 4,206,12~ and 4,368,144. In addltlon, EPO Patent 0,003,642 and U. ~. Patent Appllcatlon 2,014,133A
dlsclo~e processe~ for the productlon o ol~fln o~-ldes employlng ~lver-contalnlng cataly~t~ ln ~hlch a chlorine-contalnlng reactlon modlfler and ~ nltrlte or nltrlte-formlng substance are de~crlbed. Rumanl~n 1`

~.28Z77~

Patent 530l2, publiRhed Dece~ber 2, 1971, ~isclo~es dlrect, ~llver-caealyzed direct epoxldatlon procedure ~hlch employs oxldes of nltrogen ln the Eeed~t~eam.
U. ~. Patent 524,007 lnclude~ ethylene dlchlorlde or nitrogen dlo~lde in the feedstream of a sll~er-caea-lyzed epoxlaation procedure.
Although much of the art dl~cu~ed above h~
resulted ln lmprovements ln the efflclency, activity or ~tabllity of the catalytic ~yatem, many of the lmprovements have indlvidually been rather ~llght.
In some of the catalytlc system~, gain~ in one of these performance parameter~ have been frequently ofset by 106~es ln anotherS th~t i8, enhancement of one lndex of performance has been accompanled by a deleterlous effect on another of the lndlces. For example, lf a reaction ~y~tem 18 de~igned whlch has a very short useful llfe, the system may be commer-cially impractical even though the efflclency and initlal actlvlty of the catalyst are out~tanding.
Accordingly, a ey~tem that provlde~ ~n lncrease ln the efficlency of the overall catalytlc reactlon ~y~tem, whlle only mlnlmally decreaslng the actlviey and u~eful Llfe of the cataly3t, would be partlcular-ly beneflclal aq would a sy~tem ln whlch the u~eful llfe ls lmproved at llttle expen~e to th~ e~flclency or actlvlty.

Di~closure Of The Inventlon:

The pre~ent inventlon 1~ dlrected ~o catalytlc processe~ for the epoxldation o~ alkone ln the pre~-ence of an oxygen-cont~lnlng gas and to the catnlyst u~ed therefor. The process compri~e~ contactlng an ~lkene wlth an oxygen-contalnlng ga~ under epoxida-tlon condltlon~ ln ~he pre6ence of at least one 9~-, eou~ efficlency-enhanclng member of a redox-half reactlon palr and a ~olid catalyst. The catalyst comprlse~ a catalyeically effectlve amount of ~llver on a solld porous ~upport and an efflciency-enhanclng S amount of at lea~t one eficlency-enhanclng salt of a member of a redox-half reactlon palr. The support employed for the catalyst compri~e~ at least one carbonate salt of barium, ~trontium, calclum, mag-ne~lum, or mixture~ theceof. The porous ~upport may be either one whlch consi~ts essentially of a carbon-ate ~alt or one in which the carbonate ~lt 1~ as-~ociated wlth an inert sub~tructure, ~uch a5 alumlna.
The pre~ent invention i~ al50 directed to a cataly~t for u~e in epoxidatlon of alkene wlth an lS oxygen-contalning ga~ wh~ch comprise~ a catalytlcally effectlve amount of silver on a solld ~upport and at least one efficlency-enhancing ~alt of a member of a redox-half re~ction palr. The comblnatlon of B por-ou~ ~upport comprising a carbonate salt and the pees-ence of a ~alt of a member of a redox-half reaction pair produce~ a cataly~t of enhanced performance which 18 both highly actlve and capable of ~alntaln-ing ~uch activlty for extended perlod~ of tlme. In additlon to these lndlce~ of performance, a catalytic ~ygtem employing such a catalyst, partlculnrly when used ln the pre~ence of a ga~eou~ member of a redo~-half reactlon pair, also re~ults ln enhancement of efficlency. Thus, efflclencles on the order of about ~4 percent for the epoxldatlon of ethylene ~re the norm under Standard Test Condlt~on~ and of about 88-92 percent or greater are not uncommon. In a~dltlon, the catalyst~ of the present lnventlon are c-pable of maintalnlng hlgh actlvlty well as long term ~ta-bility.

Detailed ~escription Of The Inventlon-The pre~ent lnventlon ~s dlrected to a proce~s for the vapor pha~e oxidation of alkenes to alkene oxldes, l.e., an epox~datlon proces~, in the presence of an oxygen-contalnlng ga~ and to ehe ~llver cata-lyQt~ employed therein.
The proce~s and catalyst o~ the pre~ent lnven-tlon are useful ln the epoxldatlon of the alkene~
ethylene and propylene, the epoxide~ of whlch are ln great demand for uBe a8 intermedlates ln produclng such materials as polymer~, surfactants, ~ynthetlc flbers and antlfreeze. However, the present lnven-tlon 1B not llmlted to the~e compounds but may be used to o~ldlze cycllc and acycllc alkenes whlch are ~n the gaseous state or have ~lgnlflcant vapor pres-sures under epoxidatlon conditions. Typically these compound~ are characterlzed as having on the order of 12 carbon atoms or le~ whlch are gaseou~ under epox-ldation condltlons. In addltlon to ethylene andpropylene, e~amples of other alkenes whlch may be used ln the present lnventlon lnclude such compound~
a~ butene, dodecene, cyclohexene, 4-vlnylcyclohexene, Gtyrene and norbornene.

~E~

The aupport materlal used ln the present lnven-tlon ~ay be ~elected from one of several carbonate-contalnlng carrler materlals. In each embo~lment ofcarrler, thc carbonate employed le an ~norganlc car-bonate, preferably havlng a cation whlch ls an alka-llne earth metal lon, partlcularly calclum, stron-tlu~, magnesium or barium wlth calcium belng mo~t preferred. The carrlers of the pre~ent lnvention may ~28;~772.

e%ist ln varlou~ forms~ In one e~bodi~ent, the car rier 1~ one ln which the curbonate 1B the predo~lnant or, preferably, ~ubstantially ~he exclusive co~ponent of the ~upport. In other embodl~ents of the lnven-tlon, the lnorganic support materlal 1~ used ln con-~unction with a ~olid substrate, l.e., a ~ub~uppcrt or ~ub~tructure composed of a more conventlonal ~up-port materlal, such as alumina and preerably alpha-~lumina. Thl~ latter type of ~upport may employ the carbonate materlal coated on lndlvidual, relatlvely small partlcle~ of substructure or subsupport or on 2 larger unlt such a~ a three-dlmen~lonal framework havlng a honeycomb-type of 6tructure.
When used wlth a subsupport, the percentage of cnrbonate materlal present in the ~upport materlal 1~, by welght, frequently about 0.5 to about 15, ba~ed on the weight of the total support. Most often the carbonate 13 present ln an amount of about 2 to about ll percent, by welght.
A granular or cry~talline form of the carbonate ~upport materlal i~ preferred ln the pre~ent lnven-tlon, partlcularly when used a~ the excluslve or predomlnant component of the eupport~ Commerclally avallable c~lrbonate materlal~ suitable for use ln the pre~ent lnventlon may be obtalned ~B powders ~hlch can be convl~rted to the preCerred granulJr ~orm by formlng a paste and ~preadlng lt thlnly on a flat ~urf~ce, ~u~h as a large tray. After ~preadlng the materl~l on the surEace to a ~ultable depth, uaually appro%lmately one-quarter lnch, the carbonate ~ater-lal ~ay be dried and calclned at a tempera~ure of from about 2~ to about 75 degree~ C below th~ ~eltlng or ~ecomposltlon temperature of the ~aterlal. In one embodl~ent t~e pa~te of the carbonate ml~ture IB
drled and calclned by placlng lt lnto a ~urnace at ~2 about 120 degree~ C and heatlng to about 500 degree~
C over a flfteen m~nute perlod and holdlng ~t at th~t ~emperature for an additlonal flfteen minutes. The caacined carbonate 18 then broken into ~maller pleces ~nd 4creened to partlcles of the appropriate ~lze, on the order of one-eighth to three-elghths Inch, pref-erably ~bout one-quarter lnch in dlameter. As de-scclbed ln greater detall below, the carbonate BUp-port may then be lmpregnated, or coated, wlth a ~olu-tlon containlng a ~ilver compound and thereafterreduced to elemental silver.
Alternatlvely, as descrlbed below, the powdered carbonate materlal may be comblned wlth ~n approprl-ate sllver-contalnlng solution, ~uch ~8 that u~ed conventlonally to lmpregnate solld ~upportq to form a slurry or pa~te. Thl~ materlal may then be spre~d on a su~table surface and drled and roasted at ~n appro-prlate temperature, such as ~bout 500 degree~ C, on belt roaster. Thls result~ ln a carbonate support wlth ~llver belng supported thereon ln lts elemental state~ The cataly~t may then be lmpregnated ~ith ~olutlon of a salt of a member oE a redox-h~lf reactlon palr and thereafter drled. As an alterna-tlve, the 8alt of a member o~ a redox-hal reactlon palr m~y be ~alssolved ln the ~ame ~llver-cont~inlng impregnatlon ~olution used to form the aoating paste or ~lurry with the carbonate materlal.
The partlculate carbonate partlcles, ~hether or not prepared from a sllver-contalnlng pa~te, oan be 30 formed lnto shaped composite~ suItable for u~e ln alkylene oxlde manuf~cture. The composltes may be for~ed by any sultable technlque. For ln~tance, lt 1~ po3sIble to form the composltes by compre~slng the partlcle3 lnto a mold havlng a de~lred conf lgurn-tlon. Sul~able pre~ures may be at le~st 1,000 pslg, ~7Z , say ~bout 3,000 to ~bout 20,0Q0 p~lg. The ~l~e of the partlcle~ may be ~elected to be ~pproprlate or the formatlon of the compo~lte and are often In the ra~ge of about 0.301 to about 5 milllmeter~ ln ~a~or dlmenslon.
When coated cataly~t~, l.e., tho~e catalysts ln which the carbonate materlal iB coated on a ~ub~truc-ture, are employed, a ~lurry of the carbonate ~ater-lal, ln either powder or granular form, 1B mlxed wlth the partlcle~ of support ma~erlal or the honeycomb structure and thereafter drled in nn oven. A~ wlth the predomlnant or exclu~lve carbonate ~upport mater-lalA de~crlbed above, the coated cataly~t~ may also be prepared by u~lng n ~olutlon of ~ ~llver compound or the silver compound and a ~alt of a member of a redox-half reactlon palr to form the ~lurry, followed by suitable drying and roa~tlng.
The surface areas of the carbonate ~upport ma-terials generally r~nge from about 0.6 to about 14 m2/g, preferably from about 1.5 to about 10 m2/g.
The ~urface area is measured by the conventlonal B.
E. T. method u~lng nltrogen or krypton descrlbed by Brunauer, Emmet and Teller in J. Am. Chem. Soc. 60, 309-16 ~1938).
The carrler matetlals of the pre~ent lnventlon may generally be deecrlbed ~ porous or microporous.
As lndlcated above, when the carbonate ~upport materlals are used ln con~unctlon wlth a con~entlonal carrler a~ a substructure, the preferred support materlal 1~ alpha-alumlna. It 1B ~1BO preferr~d that the alpha-alumina be of high purlty, partlcularly oontalnlng a low sodlum content, and ~lffo be formed from ~afer or platelet-type partlcles, at lea~t 80~e of which form an ~nterpenetratlng crystalllne ~atrl%, as de~cribed ln commonly ~ssigned, copendlng appllca-tions to Notermann et al, entitled "Improved Catalytic System For Epoxidation Of Aklenes Employing Low Sodium Catalyst Support", having Canadian Application No. 515,864, filed August 13, 1986 and to Naumann et al, entitled "Improved Catalytic System For Epoxidation of Alkenes", having Canadian Application No. 515,816, filed August 13, 1986.
The carbonate supported particles of the present invention are generally used as individual particles of irregular shape and size. This is true both for the predominant or exclusive carbonate supports as well as the carbonate-coated supports.
However, in some instances the supports, particularly the carbonate-coated supports, may have a particular shape and size and this is especially true of the subsupports used with the carbonates. Typically the subsupports are formed into aggregates or "pills" of a size and configuration to be usable in co~nercially operated ethylene oxide tubular reactors. These pills may be formed by conventional extrusion and firing techniques, l~he pills generally range in size from about 2 mm to 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 appllications, sizes rangin~ from about 3 mm to about 10 mm have been found to be most suitable in the 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 as defined in U.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 ,,,l,../ji, . , ~282~72 one partitlon extending between walls), cyllnder~
having ga~ ~hannels from ~lde wall to ~1ae wall, cylinders hsvlng two or more gas channel~, an~ rlbbed or flnned ~tructure~. ~hlle the cyllnder~ are often clrcul~r, other cro~-sec~lon3, such a~ oval, hexa-qonal, quadrllateral, trilateral, etcetera, may be usef ul .

Catalvst3:

A~ lndlc~ted above, the catalyst~ of the present inventlon may lnclude, a~ a ~upport material, ~t lea~t one carbonate o~ or an alkallne earth metal catlon, preEerably Mg, Ca, ~ or Sr, mo~t pre~erably 5 C6. The support may be pre~ent clther ~8 pre-dominantly or exclu~lvely the carbonate, de~lgnated hereln as ~carbonate-~upport~. The corre~pondlng cataly~t which lncludes ~uch ~upport 1B de~ignated ~carbonate-~upported catalyst~. When the carbonate 1~ coated on or ln the pre~ence of a ~ub~trate or sub~upport, the support 1~ de~ignated ~carbonate-coated support~ and when the 3upport 18 u~ed ln a complete catalyst, the de~lgnatlon for the c~taly~t i8 a ~carbonate-coated cataly~t~. A~ u~ed hereln, 2S the term ~coated~ 1~ not intended to lmply that one ~ubstance neces~arlly form~ a layer on or en~elop~ a ~econd substance but merely refers to the procedure lnvolved in the preparatlon of such materlal.
The carbonate- and carbonate-coated 6upport~ may be prepared as lndicated above or, when a carbonate ~upport 1~ u~ed, obtained commerclally. The carbon-ate-supported cataly~t of the present lnvent~on may be prepared by any known method of lntrodu~lng ~ilver - and/or a salt, such as a sale of a redo~-half reactlon palr, ln solu~le form, to a ~upport. A

1~2~72 preferred method of introduclng sllver to the carbon-ate ~upport 1~ by an lmpregnatlon proce~s ln whlch a solu~ion of a soluble 3alt or complex of ~llver ln an amount suficlent to depo~lt the de~ired welght of ,! 5 ~llver upon the carrier 1~ dlssolved ln ~ sultable solvent or ~complexing/solubllizlng~ ~gent. The solutlon may be used to impregnate the support or carrler by immer~ing the carrier ln the ~llver-, contaln~ng lmpregnatlng ~olutlon and formlng ~ pa~ty 3 10 mlxture or slurry. The slurry 1~ then ~pread on flat lnert surface, fiuch aB h tr~y, to a sultable depth of about one-slxteenth to about one-quarter lnch, preferably a thickness of about one-elghth inch. Th~ carbonate/~llver compound mixture 1~ then 15 dried and roa~ted by placlng the mi~ture ln a furnace nt about 100 to ~bout 120 degree~ C ~nd heatlng the mixture to about 400 to about 600 degrees C over ~
I flfteen minute period, thereafter holdlng the ml~ture ¦ at a temperature wlthin thi~ range for an ~dditlonal ¦ 20 fifteen mlnute~ and removlng the calclned materlal from the fucnace. Thi~ procedure accomplishes drying of the carbonate/sll~er m~xture, remove~ volatile components ~nd reduce~ the silver present to lt~
element~l form.
The ~alt Oe a redox-hal~ re~ctlon palr may be ¦ lntroduced to the cataly~t D~ ~n lmpregnbtion ~olu-I tlon ln A separate impregnatlon step. Ag~ln, thl~
may be done by any known m~nner of lmpregnatlng Q
porous materlal. Convenlently, thls may be carrled ¦ 30 out by placlng the cataly~t materlal ln a contalner, I evacuatlng the contalner and thereafter lntroduclng I the solutlon of a salt of a member of a redo~-half eeaction p~lr. Alternativ01y, the support may be ~prayed or ~prlnkled wl~h the lmpregnatlng ~olu-35 tlon. The exces~ ~olutlon may then be allowed to ~2~ .

drain off or the ~olvent may be removed by ~vapor~-tion under reduced pres~ure at a sul~able te~pera-ture. The catalyst may then be drled at 120 degrees C ln an oven for two hours. Such a procedure 1~
known a8 a a~equentlal~ or ~con~ecutlve~ method of preparatlon. The carbonate-supported catalyst may ~180 be prepared by a ~slmultaneous~ or ~colnclden-tal~ ~ethod of preparatlon. Wlth thls method, the salt of a member of a redox-half reactlon palr 18 lncluded ln the sllver compound-containlng solution used to lmpregnate the carbonate ~upport.
The carbonate-coated catalysts are prepared by coatlng a sultable sub~tructure or 6ubsupport mater-lal, preferably alumlna, ~nd most preferably alpha-alumlna~ wlth a carbonate-containlng slu~ry, Thls may contaln only the carbonate, ln ~hlch case the carbonate-coated support i8 further treated as indl-cated above to produce a sllver or a sllver/salt of redox-half reactlon pair carbonate-coated catalyst.
Alternatlvely, a carbonate/sllver compound slurry or a carbonate/sllver compound/salt of a member of a redox-half reactlon palr slurry ln a sequentl~l or colncldental procedure. Thu~, ln a 8equentlal pro-cedure, partlcles or pills of a ~ultable aubsupport materlal, ~uch a~ ~lph~-~lumlna, are coated with a 61urry of ~ carbonate m~terlal and a ~oluble ~alt or complex of allver di~solved ~n a comple~lng/solu-billzing agent. The partlcles or plll8 are there-after dralned and calclned ln an oven at a tempera-ture of about 250 to about 600 degrees C for aboutthree mlnutes to ~bout four hours, the ~uratlon o~
heatlng belng lnversely proportlonal to the tempera-ture employed. The catalyst i~ then lmpregnated ln the manner descrlbed ~bove wl~h a solutlon of ~t least one salt of a member of a redox-half reactlon -32~
~2~Zm pair and then drl~a. The carbonate-coated supports may al~o be formed by a colncldental procedure ln whlch a carbonate/~ilver compound/~alt of a ~ember of redox-half react~on pair ~lurry 1~ u~ed to coat S partlcle~ or pills of a sultable ~ub~upport. After drainlng, the cataly~t l~ dried at a temperature and for a durat~on indicated above for those carbonate-j coated cataly~ts prepared by the sequentlal proce I dure. The partlcular silver salt or compound used to form the ~llver-containlng impregnatlng ~olutlon ln a solvent or a complexing/solublllzing agent l~ not particularly critical and any ~ilver salt or compound generally known to the art which iB both soluble ln and does not react wlth the solvent or comple~lng/-solubillzlng agent to form an unwanted product m~y beemployed. Thus, the ~llver may be introduced to the solvent or complexing/solubilizing agent a~ an oxlde or a salt, such as nitrate or carboxylate, for exam-ple, ~n ~cetate, propionate, butyrate, oxalate, ~ malonate, malate, maleate, lactate, cltrate, phth~-late, generally the sllver salt~ of hlgher fatty acids, and the like.
The chemical practitioner may choo~e frcm a large number of ~ultable eolvents or complexlng/~olu-billzing agents to form the ~llver-cont~inlng lmpreg-n~ting solutlon. Be~ide~ adequ~tely dis~olving the silver or converting lt to a solubl¢ form, a sultable solvent or comple%ing/solubllizing agent ~hould be capable of being readlly removed in sub~equent steps, elther by ~ washlng, volatilizing or oxl~atlon proce-dure, or the llke. The complexlng/solublllz1ng agent, preferably, should also permit solutlon to psovide sllver ln the finlshed catalyst to the extent of about 2 to about 60 percent silver or hlgher, based on the total weight of the cataly~t. It 18 9:;282~7;~

al~o generally preferred that the solvent~ or co~-plexlngJ~olublllzing agen~ be readlly ~l~clble wlth water ~lnce aqueou~ ~olutions may be ~onvenlently em-ployed~ Among the materiale found ~ultable a~ ~ol-vent~ or comple~lng/~olubllizing agents for the pre-paratlon the ellver-contAinlng ~olution~ are alco-hols, lncluding glycol~, such a~ ethylene glycol tV. S. Patent~ 2,B2S,701 to Endler et al and 3,~63,914 to Wattlmena), ammonla (U. S. Patent 2,463,228 to We~t et al), amines and aqueous ml~ture~
of a~lne~ (V. S. Patents 2,459,896 to Schwartz, 3,563,914 to Wattimena, 3,702,25g to Nlel~en, and 4,097,414 to Cavltt, and carboxyllc ac~d~, such a~
lactlc acid (U. S. Patents 2,4~7,435 to Arles ~nd 3,501,417 to DeMalo)~
Typlcally, a silver-containlng ~olutlon 18 pre-pared by dlssolvlng ~llver ln a ~ultable ~olvent or comple~lng/solublllzlng agent as, for e~ample, a mlxture of ~ater, ethylenedlamlne, oxallc acld, ~
ver o~lde, and monoethanolamlne. ~he solutlon 18 then mlxed wlth support partlcle~ and dralnea.
Thereafter the particles are sultably drled.
As lndicated above, after impregnatlon, the silver-lmpregnated carr~er partlcles are treated to convert the ~ er salt or complex to ~llv~r metal and thereby effect depo~ition o~ sllver on the sur-fnce of the support. A~ used hereln, the term ~sur-face~, as applled to the support, lncludes not only the externnl ~urface~ of the carrler but al~o the lnternal ~urfaces, that 1~, the surface~ deflnlng the pore~ or lnternal portlon of the ~upport partlcle~.
Thl~ may be done by trea~lng the lmpregnated partl-cles wlth a reduclng agent, ~uch aQ oxallc acld or alkanolamine and/or by roastlng, at an elevated tem-3~ perature to decompose the sllver compound and reduce the sll~er to lt~ free metalllc state.
The concentratlon of sllver ln the flnlshed cataly~t ~ay vaey from about 2 percent to 60 percen~, by weight, based on the total welght of ~he catalyst, S more preferably the sllver concenttation ranges from about 8 percent to about 50 percent, by weight. When a ~hlgh ~ilver~ content cataly~t i8 preferred, the sll~er range~ from about 30 ~o about 60 percent, by welght~ The preferred concentratlon for ~l~w ~llver~
content catalyst range~ from about 2 to about 20 welght percent. Lower ~llver concentratlons sre preferred from a capltal expen~e standpolnt. ~ow-ever, the optlmum sllver concentratlon for a partlcu-lar cAtalyst should al90 take lnto conslderatlon increased productlvlty resultlng from performance chal~Cter 18tlc~, such a~ ~atalyst actlvlty, 5y8tem efficiency and the rate of cat~lyst aging. In many lnstances higher concentratlons of sllver are prefer-red slnce t~ey demonstrate levels of enhanced per-formance, partlcularly catalyst stability, whlchcompen~ate~ ~or the greater capltal expendlture.

EfficlencY-Enhanclnq Compound:

A preferred aspect of the present inventlon lncludes an efflciency-enhancing amount of at least one efficlency-enhancing salt of a member o a redo~-half reactlon palr. The term ~redox-half rea~tlon~
18 deflned hereln to mean half-reactlons llke those found ln equatlon~ pre3ented ln table~ of etandard reauctlon or o~ldatlon potentlals, aleo known AB
~tandard or eingle electrode potentlals, of t~e type found in, for lnstance, ~Nandboo~ of Che~l~try~
A. L~nge, ~dltor, McGr~lw-Hlll Book Comp~ny, Inc., pages 1213-1218 (1961) or ~CRC Nandbook of Chemlstry ~2~!32772 ~nd Physlc~, 65th 2dltlon, CRC Pre~, Inc~, Boc~
~aton, Florlda, page~ D15S-162 (198~). The te~m ~redox-half reaction pair~ refer~ to the palrs of atoms, ~olecule~ or lons or mixtures thereof wh~ah are depicted a~ undergoing oxldatlon or reductlon ln such half-reactlon equa~lons. A member of a redox-half re~ction palr 18, therefore, one of the atoms, moleculeq or ions that appears ln a partlcular redox-half reactlon equatlon. Such term~ a~ redox-balf reactlon pairs or like terms ~re used hereln to ln-clude tho~e member6 of the clas~ of ~ub~tance whlch provlde the deslred performance enhancement, rather than a mechanl~m of the chemlstry occurrlng. Prefer-ably, such compounds, uhen as~oclatea ~lth the cata-lyst ~8 salts of members of a half-reactlon pnlr, are salts ln which the anions are oxyanions, preferably an oxyanion of a polyvalent atoms that 18, the ~tom of the anlon to whlch oxygen ~8 bonded i~ capable of existing, when bonded to a dl~lmllar ato~, in dlf-ferent v~lence states. Pota~slum 18 the preferredcatlon and the preferred anlons are nltrate, nltrlte and other anlons capable of undergolng dlsplacement or other che~lcal reactlon and forming nltrate Anlon~
under epoxldatlon or catalyst prepar~tlon condl-tlon~. Preferrod salt~ lnclude ~NO3 nnd ~NO2, ~lth~NO3 belng mo~t preferred.

Introduction Of Efficlency-~nhanclng Salt To The Carrler:
3~
The ef~lclency-enhanclng salt of a member of a redox-half reactlon p~ir may be lntroduced to the cataly~t ln any known manner. Thus, lmpregnat~on ~nd ~eposltion of sllver and an efficlency-enhanclng ~alt oP a member of a redox-half reactlon palr ~ay be X

effected coincldentally or sequentlally, a~ descrlbed above under the heading ~Catalyst~ hen more than one 8alt of a member of a redox-half re~ction pair 18 I employed, ~hey may be depo~ited together or sequentl-~lly. It is preferred, however, to introduce the ~alt~ to the support in a ~ingle solutlon, rather than use ~equential treatment~ u~ing more th~n one ~olution and a drylng step between impregnation steps, since the latter technigue may result in le~chlng o~ the fir3t introduced salt by the ~olution contalning the secona salt. Typical, and in m~ny ca~es preferred, of such method~ include concurrent, or coincidentnl, impregnation in whlch the solution which 18 used to impregnate the support wlth ~ilver also ~ontains at least one dl~olved efflclency-¦ enhancing Ralt member of a redox-half reaction ¦ pair. This procedure permits introductlon of both the ~ilver compound and the efflciency-enhancing ~alt ~imultaneou~ly to the support in a eingle ~tep and ~olutlon.
The otheF commonly employed method i8 the se-quential impregnation of the ~upport in wh~ch initlal introdu~tlon of the silver-contalning solution or ef1clency-enhancing ~alt ~olution ~d~pendlnq upon the sequen~e employed) iB followed by drylng o~ the ~ilver-contaln~ng support ~and heating ~nd/or chemi-cai reduction of the ~ilver if thi~ iB the flrst added eub~tance). This ~upport iB then impregn~ted with a solutlon of the second 3ubstance, that 18, the ~ficiency-enh~ncing salt ~lf the ~llver wa~ the first ~dd~d subst~nce).
In order to perform the former procedure, l.e., coincldental lmpregnation, the efflciency-enhancing ~alt must be eoluble ln the same solvent or complc~-lng/~olubillzing llquid u~ed with the sllver-lmpreg-~8Z~

natlng ~olutlon. ~ith the sequentlal procedure ln which ~he ~ilver 1~ added first, any solvent capable of di~solvlng the ~alt which wlll neither react ~lth the ~llver nor leach lt from the Bupport 18 BU~t-able. Aqueou~ solutlons are generally prefer~ed, butorganlc llquld~, ~uch ~ alcohol~, ~ay al~o be e~-ployed. Sultable procedures for effectlng introduc-tlon of the efflclency-enhancing salt to the solld ~upport may be found ln many of the patents ll~ted 10 ~,bove.
The ~alt of a member of a redox-half reactlon palr 18 added ln an amount ~ufficlent to enhance the efflclency of the epoxldatlon reactlon. The preclse amount ~111 vary dependlng upon ~uch varlables ~8 the gaseous efflclency-enhancing member of a redox-half reactlon palr used and concentratlon thereof employed ln the epoxldatlon procedure, the concentratlon of other components in the gas pha~e, the amount of sllver contained in the catalyst, the ~urface srea of the ~upport, the proce~s condltlons, e.g., space veloclty and temperature, ~nd morphology of 8Up-port. Generally, however, a sultable range of con-centration of the added efflc1ency-enhanclng ~Dlt, calculated a8 cation, 1B about 0.01 to about 5 por-2~ cent, preferably ~bout 0.02 to about 3 percent, byweight, based on the total welght of the cataly~t.
Most pre~erably the Balt 18 added in an amount of about 0.03 to about 2 welght percent.
It has been noted that when conventlonal ana-ly~e~ have been conducted wlth catalyste prepared by co-lmpregnatlon wlth sllver and efflclency-enhanclng ~alt, not all the anlon as~oclated wlth the catlon has been accounted for. For example, cataly~ts pre-pared by co-lmpregnatlon ~lth a pota~slum n~trate 3~ ~olutlon have been analyzed by conventlonal tech-niques and abo~t 3 ~oles of the nitrate anlon h~ve been observed for every 4 moles of the pota3sium catlon. This 18 believed to be due to lim~ta~lons ln the conven~lonal analytlcal technlqueQ and doe~ not S nece~arily mean that the unaccounted ~or anlons are not nitrate. For thl~ reason, the amount of the efflclency-enhancing ~alt ln the cataly3t la given, in some lnstances, in terms of the welght percentage of the catlon of the efflclency-enhanclng salt (based ~n the welght of the entlre cataly~t), wlth the un-derstandlng that the anlon associated wlth the cation ~8 al~o pre~ent in the cataly~t in an amount roughly proportional (on a molar basis) to the catlon.

Epoxldatlon Procedure:

AB in conventional proce~es of thls type, an alkene and an oxygen-contalnlng ga~ are brought to-~ether ln a reactor in the presence of a ~ultable epoxidation cataly~t under epoxldation condltlons.
Typlcal epoxldatlon conditions lnclude temperatures withln the reactlon zone of the reactor on the order of about 180 to 300 degrees C and pres~ure~ ~rom about 1 to about 30 atmospheres.
The gaseou~ eEflclency-enhanclng member of ~
redox-half reactlon palr may generally be supplied to the reaction zone wlthln the reactor by lntroducing the component to the feeastreAm contalning alkene and oxygen. Under commerclal epoxldatlon ~onditlons, such a8 those u~ed ln the pre~ent lnventlon, the feedstream al~o contalns a gas ph~se halogen com-pound, ~uch a~ an alkyl hallde, a hydrocArbon, and, ~hen the effluent stream from the reactor 1~ re-cycled, unreacted alkene. When recycle of the e~flu-ent stream 1~ used, carbon dloxlde may al~o be pres-ent. The presence and amount of carbon dlo%lde de-pend~ on, ~mong other thlng~, whether ~ ~crubblng devlce i~ used ln the process and, if ~o, to the extent lt ~emalns carbon dlo%lde.
The term~ ~gaseou~ member of a redo~-half reaction palr~, ~gaseous efflclency-enhanclng member of a redox~half reactlon palr~, or llke term~
referred to here~n have a meaning ~lmll~r to that for the ~salt of a member of a redox-half reactlon palr~
or.like teem~, deflned above. ThAt 1B~ the~e e~rms refer to members of half-reactions, repre~ented ln standa~d or slngle electrode potentlal tables ln ~tandard reference texts or h~ndbooks whlch ~re ln a ga~eous ~tate and are ~ubstances ~hlch, ln the reactlon equatlone repre~ented ln the text~, are elther o%idlzed or reduced. The preferred gaseous efficlency-enhancing materlals are compounds contain~ng an element capable of exl~tlng ln more than two valence ~tates, preferably nltrogen, ~n~
another element which 1B~ preerably, o~ygen.
~xamples of pteferred ga~eous e~ficiency-enhanclng members of a redo~-half reactlon palr lnclude at lea~t one o~!NO, NO2, N2O4~ N2O3 or any gaBeou3 substance capable of forming one of the aforementloned gases, particularly N0 and ~2~ ~nder epoxld~tion con~itions, and ml~tures Oe one o~ the foregolng, pa~tlcularly NO, with one or more o CO, ~H3, SO3 and SO2. NO 1B mo~t pre~erred as thc ga~-eou~ efflclency-enhanclng member of a redox-half reactlon palr.
Although ln ~ome ca~es lt i8 preferre~ to employ members of the same half-reactlon palr ln the reac-tion æy~tem, l.e., both the ef~iciency-enhanclng salt member a~sociated with the catalyst and the gaseous member ln the eed~tream, as, for example, ~i~h a .~

, .

- ~o -preferred comblnatlon of pota~slum nitrate and nltrlc oxlde, thl~ 1B not nece~ary ln all ca~e~ to achleve sati~factory re~ult~. Other preferred combinatlons, ~uch ~8 ~N03/N203, RN03/N02, RN03/N204, ~N02/~, ~NO2tNO2, and KNO3/a mixture of SO2 ~nd NO, may al~o be employed ln the same ~y~tem. In some in~tanc~s, the ~alt and gaseous members may be found in differ-ent half-reactions which repre~ent the flr~t and l~st reactlons ln a series of half-reaction equatlon~ of an overall reactlon.
The gaseou~ efficlency-enhanclng member of a redox-half react~on pair i~ also present ln an amount ~ufficlent to enhance the performance, such as the actlvlty of the catalyst, and, particularly, the efficiency of the epoxldat~on reaction. ~he preclse amount i6 determined, in part, by the particular efficiency-enhancing salt of a member of 8 redox-half reaction pair u~ed and the concentratlon ~hereof, the partlcular alkene undergolng oxidatlon, and by other factor~ noted above which influence the amount of efficiency-enhancing salt of a member of a redo~-half reaction pair. Typlcally, a suitable concentration of the ga~eou~ member oE a redo~-half reactlon pair for epoxldatLon of most alkenes, lncludlng propylene, i~ ~bout 0.1 to ~bout 2,000 ppm, by volume, ~hen N2 i~ ù~ed ~B bi~lla~t. When ~ preferred ga~eous member of ~ redox-half reactlon pair, such a8 NO, ls used in the epo~ldatlon of propylene, the preferred concentrat~on i9 about 5 to about 2,000 ppm, by volume, wlth an N2 balla~t. ~owever, when ethylene 1~ being oxidized, a ~uitable concentratlon 18 ln the range of from about 0.1 to ~bout 100 ppm, by volume, of the gaseous feed~trcam components. Prefer~bly, when ethylene 1~ being oxldlzed, the gaseous e~ficlency-enhancing member of ~282~

a redox-half reactlon palr i8 pre~ent ~n an amount of about l to about 80 ppm when about 3 percent, by volume, C02 19 pre3ent. When nitric ox~de i~
employed as the gaseous efflciency-enhancing member of a redox-half reaction pair ln ~n ethylene epo~ldatlon ~ystem, it 1B pre~ent ln an amount of about O.l to about 60 ppm, preferably about l to about 40 pp~, when C02 1~ pre~ent.
The ~oxygen-cont~lnlng gas~ employed ln the reactlon may be deflned as lncluding pure molecular oxygen, atomlc oxygen, any tr~n~lent radical ~pe~les derlved from atomlc or molecular oxygen capable of exl~tence under epoxldatlon condltlons, mlxture~ of another ga~eou~ sub~tance with at lea~t one o~ the oregoing, and ~ubstance~ capable of formlng one of the foregolng under epoxldatlon condltlon~. Such oxygen-containlng ga~ 1~ typlcally lntroduced to the reactor elther a~ air, commerclally pure oxygen or other aubstance whic~ under epoxldatlon condltlon~
both exl~t~ ln a gaseou~ state and form~ molecular oxygen.
The ga~eous component~ whlch are supplied to the reactlon zone~ or that reglon of the reactor where reactant~ and catalyst are brought togethee under epo%ldatlon condltlons, are gener~lly comblned beore belng lntroduced to the reactor. The reactor~ ln whlch the process and catalyst of the present lnven-tlon are employed may be o~ any type known to the art. A brlef descrlpt~on of ~everal of the r~actor parame~ers whlch may be used ln the present Inventlon are pre~ented below.
In additlon to an alkene, oxygen, and the ga~-eous cfflclency-enhanclng member of a redo~-half reactlon palr, the feed~tream also contaln~ ~ per-formance-enhanclng halogen-containlng sompound, pref-~Z8Z~;2 erably an organlc hallde, $ncludlng both saturated and unsa~urated halldes, such A~ e~hylene dichlorlde, ethyl chlorlde, vlnyl chlorlde, methyl chloride ~nd methylene chlorlde. Preferably, in commerclal pro-ductlon, ethylene dichlorlde i~ employed as ~he halo-~ gen-contalnlng compound~ l'he amount of hallde em-¦ ployed wlll vary depending upon a varlety of factors, j includlng the part~eular alkene belng o~idlzed and ¦ the concentration thereof, the particular efficiency-enhanclng salt and gaseous members of redox-half reaction pairs and the concentration~ thereof, ~9 well as other factors noted above as influenclng the amount of efflciency-enhanclng salt and gaseou~ com-pound. However, a sultable range of concentratlon for the halogen-contalnlng compound in the oxidatlon of mo~t alkenes, lncluding propylene, 18 typlcally abou~ 0.1 to about 2,000 ppm, by volume, of the gas-eous makeup feedstream. When ethylene 18 oxldlzed, the range of concentratlon for the halogen-contalnlng compound 18, however, about 0.1 to about 60 ppm, by volume. In addltlon, a hydrocarbon, ~uch a8 ethane, may be included ln the feedstream. ~he feedstream may also contaln ~ ballast or diluent, such a~ nltro-gen, or other inert ga8, particularly w~en alr 10 2~ used as the oxygen-containing ga5. Vaeylng amounts of carbon dloxlde and water vapor may also be pres-ent, dependlng upon whether means bave been provlded to remove such substances from the effluent ~tr~am prlor to comblnatlon of at lea~t a portlon of the effluent stream with the lnlet stream. Other than the gaseou~ efflclency-enhanclng member o ~ redox-half reactlon palr, the components are typ$cally pre~ent ln amounts shown ln the followlng tQbles for propylene ~nd ethylene.

~2 827~7~

" Volume ~ercent (or ppm) ComPonent for PropYlen~

prapylene about 2 to about 50 oxygen about 2 to about 10 alkyl hallde about 5 to about 2~000 ppm 10 hydrocarbon 0 to about 5 carbon dloxlde up to about 15 nltrogen or remalnder.
15 other ballast gas, e.g., ~ethane Volume Percent (or pp~
omPonent for EthYlene Oxldatlon ethylene at least about 2, often about 5 to ~bout 50 oxygen about 2 to about a alkyl hallde about 0.1 to about 60 ppm hydrocarbon 0 to about 5 30 carbon dlo~lde up to about 7 nltrogen or remalnder.
other ballast ga3, e.g., methane ~.Z8~

~ hen hlgher alkene~, such a~ those preYlouuly Aiscussed, are epoxldl~ed, condielon~ and concentrn-elons typlcally u~ed for the epox~dation of propylene ~ay be employed.

Standard Alkene Oxide Proce~ Te~t Condltlona:

The ~uccessful commerclal productlon of alkene oxlde~, partlcularly ethylene oxldeF by the ~llver-catalyzed oxldatlon of alkene, particularly ethylene,depends upon a varlety of factors. Many of the~e factors lnfluence, elther dlrectly or lndirectly, the e~flclency of the cataly~lc sy~tem, the actlvl~y or the aglng rate l.e., stablllty, of the cat~ly~t. The manner in whlch cataly~t~ and catalytlc ~ystems are evaluated ln the laboratory strongly lnfluence~ the value~ obtalned for the~e parameter~. Technlque~ and experlments deslgned to asses~ ~uch catalyst~ and catalytic ~ystem~ commonly employ mlcroreactors (i.e., tlny tubular reactor3 for testlng crushed catalyst partlcles) or back-mlxed autocl~ve~ of the Berty type (i.e., larger reactors whlch te~t full-~ized catalyst pellets and generally employ full ga~
recycle) as ~le~crlbed in Flgure 2 of the article by 2S J. M. ~erty, ~Reactor For Vapor Pha~e-Cat~lytlc Studles~, Chomlcal Englneerlng Proqre~, 70, Number 5, pages 78-84 (1974), and partlcularly Flgure 2.
Microreactor~ are capable o~ yieldlng, ln moat te~t ~ltuatlon3, the hlghe~t efflciency number~, typlcally approxlmately the same a~ or ~omewhat greater than tho~e obt~lnable ln commercl~l tubular reactor oper~-tlon~ employlng the ~ame catalyst~ ln non-cruahed condition. ~ack-mlxed autoclave~ commonly provide lower efficiency values because, al~hough condltlons c~n be varled, generally the entire cataly~t 1~ cx-.
. ~ ,.

'12~ 772 posed to the outlet gas ~hi~h ha~ the lowe3t concen-tratlon of re~ctant~ ~nd the hlghest concentratlon o product~. Values obtained u~ing one type of reactor are,seldom ldentlcal to those obtalned in ~noth~r reactor sy~tem. A~ a result, clalms of ~uperlor results or the deslrablllty of one catalyst over another are preferably based on tests conducted unaer controlled and comparable conditlons.

~5 . -46-lZ~Z~

Although the conditlon~ ~et forth ~pra may be e~ployed both for reactor~ employed in commercial production a~ well a~ those employed ln a laboratory, ~8 a ba~i~ of comparl~on, the cataly~t~ and oatalytlc syatem~ for epoxldation of ethylene employed ln the examples ~et forth below have been te~ted under ~om-parable conditlona known a~ Standard Alkene Oxide Proce~s Test Conditlons, or Standard Test Condltlons (referred to hereinafter as STC). The STC employed for testing and characterlzlng the catalyst~ ~nd the catslytlc sy~tem~ of the present lnventlon lnYolve the use of a standard back-mixed boteom-agitated ~Magnedrlve~ autocl~ve, or ~erty autoclave, a~ de-. scrlbed above.
In dlscu~slng the enhancement of effloiency provlded by the pre~ent lnvention, lt may be notedthat, when an effiolency-enhanclng amount o~ a salt of a member redox-half reaction pair 1B employed, an efflclency for the epoxldatlon of ethylene of at least about 84 percent 16 obtalned under Standard Test Condltlons. ~Standard Te~t Condltlons~ for ethylene may be deflned aB compri~lng ehe followlng:
by volume, 30 percent C2H4, 8 percent 2~ 5 ppm ethyl chlorlde, 5 ppm, by welght, NO, no added C2~6 or CO2, N2, ballast, 240 degree8 C, 275 p~lg, gae hourly ~pace veloci~.y ~GHSV) ~ 8,000 hr 1.
Although the pre~ent lnventlon can be used wlth any slze and type of alkene oxlde reactor, Includlng both flxed bed and fluldlzed bed reactors known to 3~ the ~rt, lt is contemplated that the pre~ent lnven-tlon wlll flnd ~08t wldespread applicatlon ln ~tan-~ard fl%ed bed, multl-tubular reactor~. The8e gener-~lly lnclude wall-oooled aB well a~ adlabatlo or non-wall-oooled reactors. Tube lengths may typlcally range from about 5 to about 60 feet but wlll fse-X

~8Z~

quently b~ in the range of from about 1~ to ~bout 40~eet~ The tube~ may have internal dlameter~ from ~bout 0.5 to about 2 lnche~ and are expectea to be typically from about 0.8 to about 1.5 lnches. G~SV
5 generally range from about 16~000 to about 1,000 br~l. ~yp~cally GHSV value~ range from about 2,000 to about 8,000 hours 1 at pres~ure~ from about 1 to about 30 atmo~phere~, commonly about 10 to about 25 atmospheres.
While the lnventlon 1~ susceptlble to varlous modiflcatlons and alternatlve forms, certaln spe~lfic embodl~ent~ thereof are descrlbed ln the examples ~et forth below. It should be understood, however, that these examples are not lntended to llmit the lnven-15 tlon to the partlcular forms dlsclosed but, on the contrary, the lnventlon 18 to cover all ~odlfl~a-tlons, equlvalent~ and alternatlve3 falllng ~lthln the ~plrlt and scope of the lnventlon.

20 Ex~mple 1 - Preparatlon Of A CaCO3-Supported CatalYst BY A Sequentlal Procedure:

Calclum carbonate ln granular form ~a~ prep~red by comblnlng ioo. o 9 of powdered cnlclum car~onate ~Baker Analyzed Reagent) and 250.0 9 ~lstllled water to form a thlck pa~te which wa~ ~pread to a thlcknes~
of approxlmate one-quarter lnch on a stalnless eteel tray and calclned at a temperature of sbout 800 de-grees C for slxteen hour~. The c~lclned product ~a8 30 then fractured lnto fragment~ whlch were Dcreenea to obtaln plece~ approxlmately one-guarter lnch ln ~la-~eter. The partlcle~ were lmpregnated wlth an ague-ou~ sllver ~mlne ~olution by allowlng the~ to ~tand ln ~ solutlon contalnlng 41.2 9 ~thylenedla~lne, 40.8 35 g dlstllled water, 41.2 9 oxallc ~cld, 72.2 9 sllver lZ82772 oxlde, and 14.4 9 monoethanola~lne. The ~lurry or paste formed from the nllver-aontainlng ~olutlon and c~lclum carbonate were spread on a ~t~lnle~ ~teel trpy to a th~ckne~s of approxlmately l/~ Inch. ~he S m~xture was c~lcined by placlng lt lnto ~ furnace at l20 degrees C and heating to a temperature of 500 degrees C over a ~lfteen minute period. The ml~eure waQ thereafter held at a temperature of 500 degrees C
for a fleteen mlnute perlod and then removed from the furnace~ Upon cooling, the calclum carbonate ~up-ported sllver catalyst was impregnated wlth ~N03 by immerslng 50.7 9 of the cAtalyst ln ~ 40lutlon for~e~
by dls~olving 2.03 9 KN03 ln lO0 mI of dl~tllled ~atee. The cataly~t partlcles were drled ~t 120 degrees C ln ~n oven for two hours. The catalyst contalned 40 percent Ag and 0.6 percent ~, a~ deter-~lned by ~nalysis.
After autoclave testlng for twelve days, the above catalyst was lmpregnated a second tlme to ln-crease the RN03 concentration. The catalyst pleces~47.7 g) were, lmmersed ln a ~olutlon prepared from 7.6 g RN03 d~olved ln lO0 ml water. The resulting materlal was'drled at 120 degrees C for a p~rlod of two hourY. The re~ultlng catalyst h~d a Ag concen~
tratlon of 39 percent and a X aoncentr~tlon of 1.7 percent, by welght, based on the total welght of catalyst a~ determlned by analysls.

-4g-~282m ~xample 2 - Preparation Of A Calclum CnrbDnate-Coated, Potassium Nitrate-Impregn~ted Silver Cataly~t By A Coincidental Procedure:

About 23.5 9 of low den~lty ~30 pore3/lnch), irregularly-shaped catalyst support partlcles o alpha-alumina formed from a honeycomb structure available from ~lgh Tech Ceramic~, Alfred, ~ew Yosk, having average dlameter~ of 5/8 lnch, were coated by thoroughly mlxlng the partlcles wlth a slurry formed from calclum carbonate ln ~n aqueous sllver a~ine/po-tassium n~trate solutlon for ~ perlod of flve mlnute~
~nd then drainlng. The sllver amine ~olutlon ~as prepared by ml%lng 41.1 9 ethylenediamlne, 40.B g dlstllled ~ater, 41.2 g oxalic acld, 71.2 g ~ilver oxlde, 14.4 g monoethanolamine, and 6.2 g potassium nltrate. After adding an ~dditional 15 ~1 dl~tilled water, 51.4 9 of Mallinckrodt ~pre wriptlon grade~
calclum carbonate was added with thorough mlxlng.
2~ The lrre~ularly-shaped catalyst support partlcles were then added to the sllver amlne/calclum carbonate slurry and t~oroughly mlxed. The exce~s slurry or llquld was dralned ~nd the coated partlcles ~ere calcined in an oven at 300 degree8 C for ~ perlod of - 25 three hours. The cataly~t ¢ontalnod 29 percent Ag, 0.7 percent R and 11 percent CaCO3, by ~elght, based on the tot~l welght of the catalyst.

~o-~12~nz ~xample~ 3-5 - Productlon Of Ethylene Oxide With Pota~lum Nitrate-Containing, Calcium Carbonate-SuPPorted CatalY~t~:
-The epoxidation re*ctlons descrlbed ln the e~amD
ples ~et forth below were conducted employlng cata-ly~tP of the type prepared in Example 1. The epoxi-datlon ~tud~e~ for whlch data are pre~ented below were conducted ln A contlnuously Rtlrred tank reac-tor, al~o known as a back-mixed autoclave, of the type describea above. The procedure involved charg-lng approximately 80 ml of catalyst to the auto-clave. The volume of catalyst was measured ln a one lnch ;.D. graduated cyllnder after tapplng the cylln-der several t~mes to thoroughly pack the catalyst.The back-mlxed autoclave was heated to about reaction temperature under a nltrogen atmosphere wlth the f~n of the autoclave operating at about 1,500 rpm. The nitrogen flow wa~ then dl~contlnued and the feed-stream was lntroduced to the reactor.
All ethylene epoxldation reactions, except where lndicated otherwise, were examlned under Standard Te~t Cond~tions comprl~lng, by volume, 30 percent C2H4, ~ percent 2~ 5 ppm ethyl chlorlde, 5 ppm nl-trlc oxlde, no ~dded C2H6 or C02, N2 balla~t, 240 degrees C, 27S pslg, at a 10w rate of 22.6 8CFH, ~nd GHSV ~ 8,000 hour 1.

~8~

T~ e I
epo~ld~lon Ot ~thyl-n~ In An Autool-vo ~ y~n~ C C ~-Su~eort~ C~t-ly-t~
~-opl- Perc-ntL9n An-ly~ed Perc~ntagff ~-~lnu~ P~rc~nt D-y-Ag ~lcentag~ C- ~YIcerl~ IS~tlcl~ncy Ob~ved 1~ IK/A9~ ~Ag/C-Co3) eo 0 ~A91ng R~te, eO/d~ y ) _ 39 1.7 25 ~0.62) 1.0 ~3 47 ~0.041~ ~.6 _3~
2~ 0.6 3n ~1).29~ 1.2 09 12 ~0.02~ 15.2 lo-2) 0.5 35 ~0.11) 0.9 ~9 3 (0.050) ~7 ~2~3Z77~

Example~ 6-19 - Production Of Ethylene Oxlde With Pota~31um Nitra~e Containing Cal¢ium CarbonateLAlumlna-Coated_Silver CatalystR

The epoxidation examples ~et forth below were conducted using cataly5t8 prepared in the manner de~cribed in Example 2, other hlkaline e~rth carbon-ates belng substituted for CaCO3 ln Example~ 1~ to l9~ A back-mlxed autoclave of the type discu~sed above was used for the epoxldatlon ~tudie~ for whlch dat~ are presented below. The procedure lnvolved charging approximately 80 ml of c~taly~t to the auto-clave. The volume of cataly~t wa8 mea ured ln a one inch I~D. graduated cyllnder nfter tapping the cylln-der several tlme~ to thoroughly pack the c~talyst.The back-mixed autoclave was he~ted to ~bout reactlon temperature ln a nltrogen flow of 11~3 SCF~ wieh the fan operatlng ~t about 1,500 rpm. The nltrogen flow wa~ then discontlnued and the feedstream ~as lntro-duced to the reactor.
An ~aging rate~, defined as the 810pe of a plotof activity or outlet ethylene oxide (EO) (i.e., -d(~EO)/dt, at constant temperature) v~. time, ha~
al~o been provlded in T~ble 2.

~%~7æ

TA~tlC 2 ~hrl4n- ~polld~tlon ~n A~ Autocl~v- ~Itb Pot~-~lu~ Nltr~t--C~n~htnlng Al~-lln- e-r~h C-~bonet~
5 On Alph -Alu~ln- C~t-lVJt~

b~pl~alln~ Perc-nt~q~ P~rcent~g- P-~ent-g~ u~ P~tc~nt e-rt~ Ag K l~/A9~ lln~ P-rc-nt ~ttlcl-ncy Cerbon~- E~rth EO
(Ag/ ~Aglnq Al~ R~t-, eerth ~ EO/d-y) C-rbon~t~
_ C-COl 29 ~ 7 11 1 1 88 0~6~~1,03~ ~1 3 1o~2~
~ '~ 20 1.2 1.2 1.0 ~9 lo.n~o~ ~6 67~
O ~ 13 0 3 e l.o 90 ~0.023~~0 ~5~ 11 3 lo-2~
~ 6 0 05 0 ~ 0 S ~6 ~0 008~ ~3 00~ ~1.
lo 1~
10 ~' 24 1 ~2 2 2 I.S 91 10 076~ ~.36~ ~6 S
10 ~
1~ " 2~ 0 46 2 2 I S ~n (0 0191 ~-,16~

~827 72 I1J~ P~ IIn- ~-rc1 nt-g~ Ps~c~ntn~ cont-ge tl~ ua ~-~c-nt ~rth Ag 11 l~t~Ay~ All~llno ~ nt ~ttloloncy C-tbon-t- ~rth 1~0 ~Ag~ IA41n9 A l ll - l l n~ tc e~ t~ ~ ~O~d~y~
C- r bon~l t- ~

1~ ''' 1~ 1.~ 1.5 1.~ ~0 0 ~o.n76~~2.12~ ~9.6 1~
10-~) 11 1~.~1 1.~ 0.9 1.5 91 ~0.076~~1.16~ 19.
10 ~
1~ ~I Cl 1~ 1 2.7 0.75 ~15 lo-n76~12.07~
lo-3~

16 1.~ 3 ~ 1.7 ~1 0.00~~2.~ 7.6 10'~
1~ 20 0. ~ ~.0 1.~ ~1 - 10.02~ 12.~7~ ~9.5 -lo-~) 17 D~CO~ 17 I.~ 5.6 1.2 90 0. 02~ 12. 1 1~ 16. 1 10'~
1~ 20 0.~ 6.~ 1.5 90 ~0.02~ ~2.1~ ~9.0 10'~
25 1~ ~IgC01 17 0.~ 1.0 1.00 ~7 1(~.1~2~ IS.0~ 1-.5 ~1 10-~

` i,~"
. . .

12~3Z~

~xamples 20-21 - Preparation Of Pres~ed Alkaline Rarth Carbonate-Supported Catalyst~
Bv A Sequential Procedure:

Example 20:

~ calcium carbonate catalyst was prepared by combining 30.0 9 CaC03 with 81.8 9 of a Bilver-lmpregnatlng ~olution prepared by comblnlng 101.7 9 ethylenediamine, 100 9 distilled water, 103.B g oxalic acid, 181.6 9 ~llver oxide, and 38.3 9 ~ono-ethanolamlne and dllutlng the ~olutlon to 500 ml with dl~tllled water. The slurey formed by combinatlon of the calclum carbonate and ~llver-lmpregnating ~olu-tlon was placed in A porcelain dish and roasted ln amuffle furnace at 300 degrees C for a perlod of about 16 hours. ~he dried catalyst w~ subsequently oooled, removed from the porcelain dl~h and ground to a powder ~lth a mortar and pestle. ~he powdered material was placed ln a steel die havlng a dlameter of 1 lnch and force of 10,000 pounds ~a~ ~pplled wlth a hydraullc E~res~ to fcrm the materlal lnto a 1 lnch dlameter w~fe!r havlng a thicknes~ of about 1/16 tc 1/8 lnch. 5everal wafer~ orme~ ln thls m~nner were then cru~hed and claa~l1~d to pa~s a 14/30 meeh screen.
The cataly~t was lmpregnated wi~h RN03 by lm-mer~lng 5 9 of the cataly~t ln a ~olutlon formed by dissolving 0.20 g ~N03 ln 90 drop~ of dl~tllled water. ~he lmpregnated materlal was then drled ln an oven ~t 110 degrees C foe a perlod of about 1 to about 2 hour~. The re~ultlng catalyst had a Ag con-centratlon of 40 percent and a RN03 concentraeion of 4 percent, by weight, ba~ed on ~he total welght of the catalyst.

~:8Z77';2 Example 21:

A ~N03-lmpregnated SrC03-supported cat~lyst wa~
prepared ~n the same manner aB in Example 20. The ~ame amount~ of SreO3 and Ag solutlon were used to prepare the ~ilver-containlng cataly~t, 5 9 of whl~h waq lmpregnated wlth a solution containlng 0.05 9 ~03 dlssolved in 55 drop~ of water. The resultlng catalyst had a Ag concentratlon of 40 percent and a K~03 concentr~tlon of l percent, by welght, ba~ed on the total weight of the catalyst.

Examples 22-23 - Productlon Of Ptopylene Oxlde With Pot~sium Nltrate-Containing, Alkallne Earth Carbonate-SuPPorted Catalyst~:

The examples of epo~ldatlon set forth below ~ere conducted uslng catalyst~ pcepared ln the manner described in Example 20 and 21. A tubular rea~tor or mlcroreactor of the type dlscu~sed above wa~ ueed for the epoxldation ~tudle~ for whlch data are presented below. For ejch oE the te~t~, a me~ured ~mount of ~atalyst was placed ln the tube of c etalnless ~teel tubular microreactor, the tube havlng ~ length of 10~2 centlmeters, an outslde dlameter of 9.52 mllll-meters, and an lnslde dlameter of 7.75 mllll~eters.
Prlor to lnitlatlng reactlon, ~he reactor was heated to the preferred temperature, wlth the cataly~t ln place, ln a 10~1ng nltrogen atmosphere.

128Z77æ

E~ample 22:

A mlxture of ga~ con~alning, by volume, 9.67 percent propylene, 9.15 percent oxygen, 200 ppm ethyl chlorlde, 200 ppm nltrlc oxlde, 1.43 percent methane, and nltrogen a~ a balla~t ga~ W~8 lntroduced lnto a heated tubular reactor at 245 degrees C. The tube of the tubular reactor contained 1 gram (0.9 ml) of 14/30 mesh cataly~t at a flow rate of 1,330 hr 1 at sl~ghtly greater than atmospheric pre~sure. The catalyst had a composltlon, by welght, of 4 percent KNO3 and 40 percent Ag on pressed calclum ca~bon-ate. After 155 hours, the outlet propylene oxlde concentratlon lactlvltY) was 1.6 percent ~lth a æe-lectlvlty of 47 percent.

ExamPle 23:

A ml~ture o~ gas contalnlng, by volume, 9.21 percent propylene, 9.06 percent oxygen, 200 ppm ethylchlorlde, 200 ppm nltric oxlde, 2.21 percent methGne, and nltrogen ga~ a~ a ball~st wa~ lntroduced to a heated tubuLar reactor malntained at a temperature of 245 degrees C. The tube of the tubular re~ctor con-talned 1 gr~m ~ ml) of 14/30 m~h cat~ly~t ~t Aflow ra~e of 1,500 hr~l at slightly grester than atmospheric pressure. The cataly~t employed in the mlcroreactor h~d a compo~ltion of, by weight, 1 per-cent KNO3 and 40 percent Ag on pre~sed strontlum carbon~te. After 35 hours, the outlet propylone oxl~e concentr~tlon (actlvlty) was 0.62 percent and the selectlvlty was S3 percent.

'~`.

Claims (23)

1. An improved process for the epoxidation of alkene selected from the group consisting of cyclic and acyclic alkenes containing up to about 12 carbon atoms to form the corresponding alkene oxide wherein said alkene is contacted with oxygen-containing gas in the presence of (i) a performance-enhancing gaseous organic halide compound, (ii) at least one gaseous efficiency-enhancing member of a redox-half reaction pair selected from compounds containing oxygen in combined form with a polyvalent element, (iii) a supported silver catalyst, comprising a catalytically effective amount of silver and an efficiency-enhancing amount of at least one efficiency-enhancing salt selected from the group consisting of salts of oxyanions of polyvalent elements on a support, said oxyanion of said efficiency-enhancing salt and said gaseous efficiency-enhancing member (ii) containing a common polyvalent element and either belonging to the same redox-reaction pair or belonging to different half reaction pairs in a series of chemically-related half reaction equations, and (iv) carbon dioxide, wherein the improvement comprises the support for said catalyst consisting essentially of a carbonate salt selected from the group consisting of the carbonates of barium, strontium, calcium, magnesium, and mixtures thereof.

2. The process of claim 1 wherein said support comprises particles of said carbonate associated with an inert substructure.

3. The process of claim 2 wherein said substructure comprises alumina.

4. The process of claim 3 wherein said substructure comprises alpha-alumina.

5. The process of claim 2 wherein the support has a surface area of about 0.6 to about 14 m2/g.

6. The process of claim 1 wherein the support consists essentially of a carbonate.

7. The process of claim 1 wherein said gaseous and said salt members of a redox-half reaction pair comprise members of the same redox-half reaction and said reaction is conducted in the presence of a per-formance-enhancing, halogen-containing compound.

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

9. The process of claim 8 wherein said gas capable of generating one of the aforementioned gases generates at least one of NO and NO2.

10. The process of claim 8 wherein said at least one efficiency-enhancing salt of a member of a redox-half reaction pair comprises potassium nitrate.

11. The process of claim 10 wherein the alkene comprises ethylene.

12. The process of claim 10 wherein the alkene comprises propylene.

13. The process of claim 1 wherein said at least one gaseous member comprises NO and said at least one salt comprises potassium nitrate.

14. The process of claim 2 wherein said carbon-ate is calcium carbonate.

15. A catalyst suitable for epoxidation of alkene in the presence of an oxygen-containing gas comprising a catalytically effective amount of silver on a solid porous support and an efficiency-enhancing salt of a member of a redox-half reaction pair, said solid support comprising at least one carbonate salt selected from the group of carbonates of cations consisting of barium, strontium, calcium, magnesium, and mixtures thereof.

16. The catalyst of claim 15 wherein said sup-port comprises particles of said carbonate associated with an inert substructure.

17. The catalyst of claim 16 wherein said sub-structure comprises alumina.

18. The catalyst of claim 17 wherein said sub-structure comprises alpha-alumina.

19. The catalyst of claim 16 wherein the sup-port has a surface area of about 0.6 to about 14 m2/g.

20. The catalyst of claim 15 wherein the sup-port consists essentially of a carbonate.

21. The catalyst of claim 20 wherein said car-bonate comprises calcium carbonate.

22. The catalyst of claim 15 wherein said at least one efficiency-enhancing salt of a member of a redox-half reaction pair comprises potassium nitrate.

23. The catalyst of claim 15 wherein said car-bonate comprises calcium carbonate.