CA1061793A - Catalyst for preparation of ethylene oxide - Google Patents
Catalyst for preparation of ethylene oxideInfo
- Publication number
- CA1061793A CA1061793A CA314,173A CA314173A CA1061793A CA 1061793 A CA1061793 A CA 1061793A CA 314173 A CA314173 A CA 314173A CA 1061793 A CA1061793 A CA 1061793A
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- Prior art keywords
- silver
- support
- catalyst
- polyacrylonitrile
- ethylene
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Abstract
A B S T R A C T
The present invention relates to a process for the preparation of supported silver catalysts to be used in the conversion of ethylene into ethylene oxide, by coating the surface of an inert support with an overlayer of polyacrylonitrile complexed with a silver salt, the coated support being heated at a temperature up to 600°C to pyrolyze the polyacrylonitrile and to convert the silver ions of the complexed silver salt into discrete particles of silver. Especially good results have been obtained when the overlayer of polyacrylonitrile complexed with silver nitrate is prepared in situ by polymerization of silver nitrate complexed acrylonitrile monomer.
The present invention relates to a process for the preparation of supported silver catalysts to be used in the conversion of ethylene into ethylene oxide, by coating the surface of an inert support with an overlayer of polyacrylonitrile complexed with a silver salt, the coated support being heated at a temperature up to 600°C to pyrolyze the polyacrylonitrile and to convert the silver ions of the complexed silver salt into discrete particles of silver. Especially good results have been obtained when the overlayer of polyacrylonitrile complexed with silver nitrate is prepared in situ by polymerization of silver nitrate complexed acrylonitrile monomer.
Description
~ ~ - 2 - l~G~7~3 f ~his lnvention relates to a process for the preparation of ~upported ~ilver catalysts, their activation and u~e in the conversion of ethylene to ethylene oxide.
A variety of methods have been employed to prepare supported silver catalysts to be used in the production of ethylene oxide wherein the silver i8 deposited as discrete, minute metal particles on the surfaces of a non-catalytic support. Among the most attractive of these methods in terms of laying down a silver deposit of ultrafine (less than 1000 nm) and uniform part cle size are those methods involving deoomposition of a silver^monomeric or polymeric carboxylic acid salt or complex on the surface of the support to form the ~ree metal particles. For example, ~nited States Patent Specif$oation No. 3,702,259 describes a process whereby a porous inert support i~ impre~nated with a solution contain-ing a complexed silver salt of a low molecular weight carboxylic acid, e.g., silver osalate, complexed and solubilized with certain organic amines, and the impregnated (coated) support is then heated to a sufficiently high temper-ature to deaompose the silver salt and deposi~ the elemental silver in the form of hemispherical particles having a diameter of less than 1000 nm on the surfaces of the support. ~ second method which involves metallic silver deposition by decompo-sition (decarboxylation) of a polymeric carboxylic acid-silver ion oomplex is disclosed in United States Pstent Specification No. 3,7~8,418. With that method, discrete silver partioles having a particle size less than 100 nm are uniformly deposited on the support surface by a multi-step process which comprises coating a support with a polymerized ethylenically unsaturated 3~0 acid, ~uch as polyacrylic acid, contacting the coated support wlth silver ions present as silver salts or complexes in non-aqueous media and then heating to a temperature sufficient to decarbo~ylate the polymerized acid and convert the silver ions into the discrete metallic silver particles. -Other possible methods for depositing metallic silver in the form of fine particles on a support surface are described a~ 3 in ~nited States Patent Specification ~Jo. 3,043,854 wherein the silver deposit is laid down on the support surface by adding a slurry of fine particles of ~ilver carbonate to the support and thermally decomposing the carbonate salt, and in United States Patent Specification No. 3,575,888 wherein the support i~ impregnated with an aqueous solution of silver nitrate, dried and the silver reduced to metallic silver particles with hydrogen or hydrazine.
q!he ability of the nitrile functional group to form a vsriety of metal oomplexes is well known as such. ~igh molecular weight species bearing a multiplicity of such llgand groups, such as polyacrylonitrile, also complex metals, but the~e complexes have not been extensively studied. ~ few poly-acrylonitrile complexes containing very small amounts of metals have been prepared, pyrolyzed and the products partially eharae-terized aecording to literature report~. Thus a polyacry] o-nitrile complex containing 0.14%w Ni was pyrolyzed and the electrical conductance properties of the metal containing char were evaluated by L.A. Lyatifova et al. in Doklad~ Akad ~auk ~Zh:R3. SS~ 20 31-33 (1964) and by M.A. Magrupov et al. in V:vsokomol Soedin 12 664 (1970). ~180, a similar study on pyrolyzed polyacrylonitrile containing copper, present as cupric chloride prior to pyrolysis, has been reported by l~.V. Topchiev et al. in Journal of Pol~nner Science ~. 1, 591 (1963). Lastly, at least one study ia reported wherein the catalytic properties of a pyrolyzed polyacrylonitrile compo-sition containing very small qllantities of copper (0.01~q6 Cu before pyrolysis) were examined in several reactions none of ~rhich were oxidation reactions. In that study by E.S. Doknkins et al. reported in Doklady Akad ~auk SSR 1'i7 893 (1961) the presence of the copper was said to have a negligible influence on the reactions attempted.
Ho~rever, while the pyrolysis of metal containing poly-aorylonitrile compositions has been studied in the limited sQnse described above, no disclosure is known of any attempt to prepare a finely divided metal or metsl oside cataly~t on ::, : . , , ~0617~;~
the surface of an inert support having practical utility such as those supported silver catalysts to be used in the production of ethylene oxide descrlbed above, via deposition of a metal (silver) containing polyacryl-onitrile on the surface of the support followed by pyrolysis of the polymer.
British patent specification no. 1,305,596 discloses Group VIII metal catalysts supported on a pyrolyzed polyacrylonitrile polymer carrier which apparently have practical utility as hydrogenation or dehydrogenation catalysts in other processes. However, in the technique taught for catalyst preparation in this patent the polyacrylonitrile carrier particles are pyrolyzed prior to addition of the ~roup VIII metal via treatment with an aqueous solution of a Group VIII metal salt or acid. Thus, it appears that the function of polyacrylonitrile in the cited British patent ls merely as a substitute for other conventional carbonaceous char carriers such as charcoal rather than being a vehicle for deposition of elemental metal particles on a support surface viaapplication of a metal ion-polyacrylon-ltrile complex to a support surface followed by decomposition of the complex-es.
It has now been found that supported silver catalysts active in the conversion of ethylene to ethylene oxide, the catalyst contain~ng from about 2 to about 20% by weight of metallic silver deposited evenly on the surface of a catalyst support in the form of a uniform dispersion of particles having diameters less than 150 nm, can be prepared by coating the surface of a catalyst support with an overlayer of polyacrylonitrile co~plexed with a silver salt and heating the coated support at a temperature up to 600C for a period of time sufficient to pyrollze the polyacryloni-trile and convert the sllver ions of the complexed silver salt into discrete partlcles of silver. Thus, the present invention provides a process for the production of ethylene oxide, ch2racterised in that ethylene is contacted in the vapor phase with an oxygen-containing gas at a temperature of from 210C
30 to 285C in the presence of a supported silver catalyst containing about 2~ -to 20~ by weight o~ metallicsilverpresent in particlllAte form on the support surface, the catalyst being prepared by coating the surface of a catalyst ~ - 4 -i17'~3 support with an overlayer of polyacrylonitrile camplexed with a silver salt and heating the coated support at a temperature up to 600C for a period of time sufficient to pyrolyze the polyacrylonitrile and convert the silver ions of the complexed silver salt into discrete particles of silver7 In the preparation of the silver catalysts used in the present invention, it is preferred that the overlayer of poly-- 4a -.~' . ,.
10~17~3 acrylonitrile complexed with a silver salt is prepared in situ by polymerization of a silver salt c~nplexed acrylonitrile monomer on the surface of the support. Silver catalysts prepared by the above described process, which contain metallic silver in the form of a uniform disperslon of particles having diameters less than about 150 nm on the support sur~ace, exhibit enhanced activity and selectivity in the conversion of ethylene to ethylene oxide when activated by treatment with a flowing stream of a gaseous mixture of ethylene and oxygen in combination with an inert gas or a flowing stream of air at elevated temperature under treatment conditions which facllitate remDval of the excess carbon residue but avoid sintering of the catalyst partlcles.
In the first step of the catalyst preparation process, a catalyst support is coated with an overlayer of polyacrylonitrlle complexed with a silver salt. m is overlayer of the polyacrylonitrile-silver salt complex can be conveniently laid down on the support surface by a variety of methods including those wherein the silver salt complex is formed on a support sur-face precoated with uncomplexed polyacrylonitrlle and those wherein the poly-acrylonitrile silver salt complex is applied directly to the support surface elther as the preformed polymer salt complex or as the monom~r salt conplex which is then polymerized onto the support surface.
In the procedures wherein the silver salt complex is formed after precoating the support surface with polyacrylonitrile, the poly-acrylonitrile precoat may be suitably applied to the support as a nomer and then polynerized on the support surface or it may be applied to the surface in an already polymerlzed form. If the uncomplexed acrylonitrile is a~plied in monomeric form, lt nay be polymerized in accord with ~ .
_ 5 ~ 3 well-known methoda of polymerization ~uch as the free radical polymerization technique. In this technique the acrylonitrile monomer is polymerized while in contact with the support, either in the presence or absence of a diluent, by the addition of a smsll quantity of a free radical polymerization initiator at a temperature which typically ranges from 0 to 200C.
Conventional free radical initiators which are suitable as polymerization catalysts in this reaction include benzoyl peroxide and other peroxide catalystc, azobisisobutyronitrile, and perbenzoic acid. If the uncomplexed polyacrylonitrile is applied to the support, a polymerization substantially a8 dQscribed above in the absence of the support may be carried out and the polyaorylonitrile 80 formed may be laid down on the support by any suitable method, for example by dipping the support in a solution of polyacrylonitrile in an inert solvent or by spraying the support with the solution. Suitable inert diluents or solvents for both the polymerization of the un-oomplexed acrylonitrile monomer, in the presence or absence of the support, and the application of the uncomplexed polyacrylo-nitrile polymer to the support surface typically include those polar organic compounds in which polyacrylonitrile is sub-stantially soluble. Examples of such solvents or diluents are gamma-butyrolactone, ethylene carbonate, dimethylformamide and dimethylsulphoxide. When solvents of this variety are employed it is desirable to remove the excess solvent, after applicatlon of the solution to the support, by means such as vacuum drying, ~bich avoid estraction of the polyacrylonitrile coating o~f the support. Alternatively, the polyacrylonitrile can be precipi-tated onto the support surface from the polymer solution by the addition of non-~olvents such as methanol or toluene to mixtures of the support and the polymer solution. The polyacrylonitrile overlayer can also be applied by polymerization of the acrylo-nitrile in the prQsence of the support; non-polar organic solvent~ such as benzene, toluene, cyclohexane and n-hexane e~hiblting solvQncy for the monomer can be employed as polymer-izstion solvents since the polymer will preCipitatQ on the .... .
- 7- 10~1'7~3 f ~upport as it i9 formed. When these ~olvents are employed it i~ again de~irable to r~move the excess solvent by filtration and/or VBCUUm drying.
The complexing of the silver salt with the polyacrylo-nitrile, precoated on the ~upport surface according to either of the above described techniques, is carried out by contact-ing the polyacrylonitrile coated support with a silver(I) ~alt or complex in a ~olvent. Suitable solvents for this step in which the silver salt complex of the polymer is formed are those polar organic solvent3 which will not appreciably extraot the polymer coating off the support surface, though some slight solvency for the polymer is desired to soften and pla~ticize the polymer surface and allow for diffusion of the metal ions into the polymer layer. Examples of solvents having acceptable solvency characteristics in this application oomprise acetone, methanol, acetonitrile, ethylene glyool and mixtures thereof. In special case~ aqueous mixtures are useful. Since this complex forming step is carried out in a medium of the type described, the silver(I) ion must be added in the form of salts or complexes which are at least partially soluble in the media. Examples of such salts and complexes are salts of carboxylic acids such as formate, trifluoro-aoetate, butyrate, 2-ethyl hexanoate, lactate and citrate, complexes such as those derived from pyridine and silver acetate or 1,5-cyclooctadiene and silver nitrate, and the soluble salts of mineral acids such as sil~er nitrats. Of this class of silver(I) compounds silver nitrate and its com-plexes are preferred. To form the silver sa;t complex over-layer of this invention the polyacrylonitrile coated ~upport is conta¢ted with the silver salt solution, preferably con-taining from 0.03 to 3 equivalents of silver ion per litre, at temperature~ ranging from ambient to 125C for a period long enough to permit the ~ilver ions to complex with the polyacrylonitrile. After passage of sufficient contacting time to allow complex formation on a substantial portion of the aooeasible li~and sites, time periods of from 10~. 7~3 0.5 to 24 hours ~enerally being sufficient, the coated ~upport containing the silver ~alt-polyacrylonitril~ complex overlayer $8 recovered frorl the excess silver salt solution by conventional methods such as sieving or decanting. ~lternatively, the silver-depleted solvent can be removed by vacuum distillation. This coated product as recovered from the excess silver salt typicslly contains only a minor amount of residual solvent and can suitably be employed directly in the thermal pyrolysis procedure dis¢ussed below to afford the particulate silver cata-lysts of the invention. ~owever, due to the hazards encountered when flammable organic solvents are exposed vo high temperatures it is generally preferred to remove the residual solvent by oonventional means such as vacuum drying at ambient and/or moderately elevated temperatures prior to pyrolysis.
As indicated above, the polyacrylonitrile-silver salt oomplex can also be applied to the support surface as the pre-formed polymer-salt complex or as the monomer-salt complex which is then polymerized onto the support surface. In procedures where the preformed polymer-salt complex i8 employed as the coating agent, the polymer-salt compleY may be suitably pre-pared ex-support by polymerizing the acrylonitrile monomer in solution according to the procedure described above, and adding the appropriate amount of silver salt to the polymer solution.
It is most desirable to employ the very best solvents for poly-acrylonitrile, such ae gamma-butyrolactone, dimethylformamide, dimethylacetamide, or dimethylsu~oxide, in this procedure a~
certain marginal solvents such as ethylene carbonate ~nd hexa-fluoroisopropanol precipitate the polyacrylonitrile-silver salt complex when the solution of silver nitrate or silver trifluoro- -acetate is added. ~his preformed polymer salt complex in solution may then be applied to the support surface by con-ventlonal means such as spraying or dipping or merely by making up a slurry of support particles with a polymer salt comple~
solution of desired dilution in the polymerization solvent and remo~ing the solvent by vacuum dryin~ techniques. In any case, . ~
10~7~3 where solution coating is involved it is preerred to remove the residual solvent prior to charging the coated support particles to the thermal pyrolysis phase of the process discussed below.
Preference is given to the procedure wherein the polymer salt-complex is applied directly onto the support surface by poly-merization of the monomer salt complex in the presence of the support.
This procedure is preferred because it allows advantage to be taken of the high solvency and complex forming ability which acrylonitrile exhibits for silver salts and the ease with which acrylonitrile solutions of silver salts polymerize. In this procedure acrylonitrile and the silver salt are combined directly at ambient or moderately elevated temperatures in the desired molar proportions to form a solution containing the complexed silver salt, preferably complexed silver nitrate. After addition of the silver salt to acrylonitrile this solution can be applied almost immedi-ately to the surface of the support and polymerizedO To ensure a polymerized layer of proper thickness on the support, it is advantageous to combine the support with the acrylonitrile solution, using conventional means such as pouring the liquid solution into a reaction vessel containlng the support, in proportions such that the solution thoroughly wets the support surface without any appreciably excess liquid phase. Once the support surface is wet the acrylonitrile-silver salt complex may be polymerized onto the support surface, in the presence or absence of a conventional free radical polymerization initiator by heating the reaction mixture at temperatures ranging from 50C to 150C for a time period of from 0.5 up to 12 hours.
Often it is convenient to slurry the monomer-complex coated support with a non-polar organic solvent in which the monomeric silver salt complex and the polymerized product`are essentially non-soluble to effect efficient heat transfer at the temperature selected for the polymerization. Suitable non-polar organic heat transfer solvènts for the polymerization reaction include straight c~ain, branched-c~ain and cyclic aliphatic hydrocarbons 3.0~ 3 such as n-pentane, n-hexane, 3-ethylhexane, octane, cyclopentane and cyclohexane and aromatic hydrocarbons such as benzene, toluene and xylene.
Although the polymerization can be carried out to yield a polyacrylonitrile of acceptable molecular weight without the aid of a conventional initiator, it is preferable to carry out the polymerization reaction in the presence of a free radical initiator, azobisisobutyronitrile and benzoyl peroxide being most preferred. It is efficacious to dissolve the free radical initiator in the acrylonitrile-silver salt sodium prior to the wetting of the support surface with this solution. Upon completion of the polymer-ization reaction period, the coated support is suitably separated from thepolymerization solvent by conventional means such as sieving or decanting or the like. This coated product may be charged directly to the thermal pyrolysis step for conversion to the particulate silver catalysts accord-ing to the present invention, however for reasons of safety discussed above, it is preferable to remove any residual solvent by means such as vacuum and/or heat prior to pyrolysis.
With any of the above described procedures for application of the polyacrylonitrile-silver salt complex to the support surface, poly-acrylonitrile compositions having an average molecular weight of from 103 up to above 106 can be employed. For those procedures in which the poly-acrylonitrile is formed prior to application to the support surface, poly-meric compositions having an average molecular weight of from 103 to 105 are preferred because of the lower viscosities of the polymer solutions which must be handled. In those procedures including the preferred procedure, wherein the acrylonitrile or acrylonitrile-silver salt complex is polymerized onto the support surface, best results are obtained when the polymerization is carried out to obtain a polyacrylonitrile having an -average molecular weight of from 103 up to above 106, this higher molecular weight limit not being critical.
- lQ -.. : ' . , . : -10~7~3 The quantities of polyacrylonitrile and silver salt applied to the support surface may vary within wide limits and depend primarily on the proportion of particulate silver desired in the catalyst products of this invention. While the Goordinati~n stoichiometry in the polyacryl-onitrile-silver salt complex has not been established with certainty for all possible silver salts complexes~ it appears that for salts having monovalent anions such assilver nitrate, stable complexes can be formed having up to at least 34% by wei~ht silver based on the total complex weight. me lower limit on the a~.ount of silver salt in the polyacryl-onitrile-sllver salt complex coating of this invention is, for all practical purposeR, dependent on the quantity of silver desired in the final catalyst product with amounts of silver salt, expressed as per cent by weight silver in the polymer complex, as low as about 1% being suitable. Preferably, the quantity of silver salt present in the polyacrylDnitrile complex over-layer of this invention ranges from about 5 to 34% by weight silver based on total complex weight. The quantity of polyacrylonitrile which may be re~lly applied to the support surface and pyrolized according to the procedure described below to yield the catalyst products according to the present invention is not critically limited and conveniently ranges of from 5% by weight to 45% by weight based on the total supported catalyst weight.
When a quantlty of silver salt within the preferred range given above is uittltzed ln the polymer complex of the invention, the quantity of silver applied to the supp~rt surface preferably ranges between 3% by weight and 15g by weight, based on total supported catalyst weight.
The supported catalyst products utilized in the present invention and ccntaining particulate deposits of metallic ~ilver are prepared by heating the polyacrylonitrile-sil~er salt complex coated supports to a temperature up to 600C, pre~erably in the range of fron 200C to 600C.
At te~peratures in this range other studies have suggested that poly-acrylonitrile pyrolizes with concomitant cyclization and loss of ~;:
... i 't,~.'i ' , 1~17~
hydrogen and some ammonia and hydrogen cyanide to yield a polypyridire-like structure. While this mechanism could account for the reductlon of the coordinated silver salts in the polyacrylonitrile matrix which has been found to occur in the instant process, the weight loss on pyrolysis of the complexed polyacrylonitrile considerably exceeds the 4% required for the idealized dehydrocyclization of polyacrylonitrile to the polypyridine structure. It has not yet been possible to assign with certainity any definite mechanism to the pyrolysis which occurs in the instant process, nor moreover, to ascertain the exact structure of the pyrolyzed char which remains. The pyrolysis can be carried out in an oxygen-containing enviror~nent, for example, in air; in an inert environment such as in nitrogen or argon; or in a vacuum; or in a reducing atrnosphere such as hydrogen. Due to the tendency of certain polyacrylonitrile-silver salt complexes and especially of silver nitrate complexes, to spontaneously ignite at the pyrolysis temperatures, it is preferred to carry out the pyro-lysls in stages in an inert atmosphere such as nitrogen or in a vacuum.
At the above indicated temperature range substantial pyrolysis of the polyacrylonitrile matrix with concomitant conversion of all or substantially all of the complexed silver ions into discrete ultrafine particles of metallic silver is obtained through employment of pyrolysis tines ranging from about 2 to 12 hours. From a procedural standpoint, the pyr~lysis may be suitably carrled out by slowly increasing the pyrolysis temperature with increasing residence time until the maximum pyrolysis temperature is reached - i.e. 600C or, preferably, about 500C - or until substantial pyrolysls ls obtalned. Preferably, the pyrolysis is ca~ried out in stepwise fashion wherein the pyrolysis is initiated at temperatures of about 200C and the pyrolysis temperature is incre~lentally increased . . . .
wlth hold~ng perlods at each intermediate temperature level until a max~mum temperature of about 450C is attained. In this preferred pyrolysis ' :,.~:
~0ti17~
procedure, the intermediate temperature levels m~y suitably di~fer by 50C
to 100C with the residual time at each ternperature level rar~n~ ~rom abou~
0~5 to 5 hours.
The products of the pyrolysis carried out as described above are supported silver materials which are catalytically active in the conversion of ethylene to ethylene oxide. These supported catalysts suitably contain of from 2% to 20% by weight of metallic silver deposited evenly on the interior (pore) and exterior surfaces of the support.
Preferably they contain of from 3% to 15% by weight of silver metal on the same basis. The metallic silver is present as tiny individual particles generally having diameters less than 150 nm with many particles having diameters less than 20 nm. Scanning electron micrographs of typical supported silver catalysts prepared by the process according to the present invention show an average silver particle size of about 100 nm whereas other analytical techniques such as X-ray diffraction indicate many of the silver particles to have particle diameters of less than 20 nm. Generally a minor amount of nitrogenous and carbonaceous residue remains on the catalyst support when the pyrolysis is carried out according to this invention. Typically, this residue from the pyrolyzed polyacrylonitrile ranges from about 2% to 30% by weigpt of total catalyst weight.
me support employed in these catalysts in its broadest aspects is selected from the la~ge number of conventional porous refractory catalyst carriers or support materials which are es æntially inert in the presence of the feedstock to be used in the oxidation of ethylene to the products and under the prevailing reaction conditions. Such con~entional m~terials nay be of natural or synthetic origin and preferably are of a ~acropor~us structure, tbat is, a structure having a surface area below 10 m2/g and preferably below 5 m /g. m ese support materials typically have an "appabent pQrosityl~ of greater than 20%. Very su~table supports comprise those of s~l~ceous and/or aluminous composition. Specific examples of suitable sup-ports ,~ :
~ . . .
-., .
. .
. . . . : . .
17~3 are the aluminium oxides (including the r~aterials sold under the ~rade name "Alundum"), charcoal, pumice, r~snesia, zirconia, kieselguhr, fuller's earth, silicon carbide, porous agglomerates comprising silicon and/or silicon carbide, magnesia, selected clays, artificial and natural zeolites, metal oxide gel-type materials comprising oxides of heavy metals such a~
molybdenum or tungsten, ceramics, etc. Refractory supports particuLarly useful in the preparation of catalysts in accordance with this invention comprise the aluminous materials, in particular those containing alpha alumina. In the case of alpha alumina-containing supports, préference is given to those having a specific surface area ss measured by the B.E.T.
method of from 0.03 m2/g to 2.0 m2/g and an apparent porosity as measured by conventional mercury or water absorption techniques of from 25% to 50%
bg volume. The B.E.T. method for determindng specific surface area is described in detail by S. Brunauer, P.H. Emmet, and E. Teller (J. Am. C~em.
Soc.~ 60 309-16 (1938)). The physical shape and size of the support is wholly conventional and includes small partlcles of regular or irregular shape suitable for use in fluidlzed bed applications or larger chunks, pellets and the like appropriate for use in fixed bed catalytic processes.
Preferably the support particles are in the form of tablets, rings, pellets cr the like of a size suitable for use in fixed bed operations.
While the catalyst products of the polyacrylonitrile pyrolysis procedure exhibit catalytic activity for, and may be used directly in, the partial oxidation of ethylene to ethylene oxide, the activity and selectlvity o~ these catalgsts can be considerably enhanced by activation in an atm~sphere and under conditions ~hich substantially remove the residual carbon but av~ld or substantially minimize sintering of the catalyst particles. While -~
this catalyst activation step ls not limited to any particular method or methods which acco~plish the desired result - i.e. substantial burn off or remDval of the residual carbon without cancomitant sintering of catalyst particles - at least two different catalyst activation or residual carbon removal pr~cedures appear to be A;
~
-1()ti~73 particularly applicable to the catalyst products. The first of these procedures involves treatment of the catalyst particles with a flowing gaseous stream comprising a mixture of ethylene and oxygen, pre~era~ly in combination with an inert gas such as nitrogen~ at temperatures in the range of from 190C to 230C for a time period of not less than about 40 hours. Ihe amounts of ethylene and oxygen employed in this procedure are suitably 20g - 40% molar (m) and 5% - 10% m, respectively, of the total composition of the treatment stream with the inert gas, preferably nitrogen, compr.lsing the balance of the stream. In carrying out this activation procedure, best results are obtalned when a treatment stream containing about 30% m ethylene and about 8~noxygen (balance nitrogen) is e~ployed in a procedure wherein the treatment stream is slowly heated from about 190, initlal, up to about 230C, final, over a time period of not less than about 50 hours. Ihis prefer.red activation procedure not only gives excellent results, but additiona ly, is quite convenient in that the proportlons of ethylene and oxygen employed correspond rather closely to the reactant concentrations normally employed in a conventional feed stream to an -;
oxyg~n based ethylene oxide production process. Accordingly, once the -catalyst has been activated by this ~referred procedure, which is quite ~0 suitably car.ried out on catalyst loaded.~in the ethylene oxide reaction zone~
the te~perature and other reactian conditions can be easily adJusted to the ethylene oxide manufacturing conditions thereby facilitating dlrect con- ~
versian fron catPlyst activation to ethylene oxide production with the `.
activated catalyst.
. Ihe second method of catalyst actlvation involves treatment of the pyrolyzed catalyst particles with a flowing stream of air at temperatu~es in the range of about 150 to 210C. With this procedure temperatures above about 210C are to be avolded since substantial sintering of the catalyst .
partlcles occurs at these higher temperatures. In fact, at least a minor amount of catalyst sintering has been found to occur ~1 .
... .. ",~
10~17'~3 ~t temperatures in the range wherein the activation procedure is operative.
Accordingly, it is preferred to carry out the catalyst activation in thiæ
procedure at the lower end of the operative range, most preferably at temperatures of about 160C in order to achieve maxi~um catalyst activity cambined with a minimum amount of sintering of catalyst particles. m e treatment times employed with this air activation procedure are substantially similar to those employed in the ethylene/oxygen activation procedure with times of not less than about 20 hours being suitable.
me silver catalysts, prepared and activated in the manner described have been shown to be particularly selective catalysts in the direct oxidation of ethylene with molecular oxygen to ethylene oxide. The condltions for carrying out such an oxidation reaction in the presence of the silver catalysts broadly camprise those described in the prior art.
This applies, for example, to suitable temperatures, pressures, residence times, diluent materials, such as nitrogen, carbon dioxide, steam, argon, methane or other saturated hydrocarbons, presence or absence of ncderating ~-agents to control the catalytic action, for example, 1,2-dichloroethane, vinyl chloride or chlorinated po}yphenyl compounds, the desirability of employing recycle operations or applying successive conversion in different reactors to increase the yields of ethylene oxide, and any other special cond-itions which may be selected in processes for preparing ethylene oxide.
Pressures in the range of fram atmospheric to 35 atm. are generally e~ployed. HiEher pressures may, however, be employed within the scope of the inv~ntiQn. Molecular oxygen employed as reactant may be obtained from convention~l sources. Ihe suitable oxygen charge may consist essentially of relatively pure oXYgen. A concentrated oxygen stream comprising oxygen ln nE~or amount with lesser amounts of one or more diluents such as nitrogen, argon, etc., or another oxygen-containing stream such as air. It is there- ~;
Pore evident that the use of the present novel silver catalysts in ethylene 3 oxidation reactians is in no way limited to the use of specific conditions among those which are kncwn to be effective.
.,~
~ ` 7 -- ~ 17 1~617g3 In a preferred application of the ~ilrer catalyst~
according to the present i~vention ethylene oxide is produced when an oxygen-containing gas separated from air and containing not less thsn 95~ oxygen is contacted with ethylene in the presence of a cataly~t according to the present invention at a temperature in the range of from 210C and preferab y 225C
to 270C.
EXAMPLE I
A solution of 3.20 g acrylonitrile, 2.05 g silver nitrate and 0.03 g aæobisisobutyronitrile was poured onto 10 g of 30-40 mesh commercial aluminium oxide (known as ~orton Company's "~lundum" grade L~-5556), having a surface area of about O.2 m /g in a reaction vessel. ~his solution just wet the aluminium o~ide particles without any appreciable liquid phase present.
Thirty millilitres of n-hexane were then added to the reaction vessel to provide the desired heat transfer and the mixture was heated to reflux under nitrogen atmosphere for 1.5 hours.
Observation of the reaction indicated that the acrylonitrile/
silver nitrate complex polymerized during the initial heating period. ~pon completion of the reaction period the solids were filtered off and vacuum dried for 1 hour at 60C to yield 13.5 g of a mustard yellow granular solid. Twelve grams of this yellow solid were pyrolyzed under a nitrogen atmosphere in a tube furnace as follows: 2 hours at 200 C, 1 hour at 250C, 1 hour at 300C and 2 hours at 400C. The product of this pyrolysis was a grey solid containing 12.0% by weight particulate silver (in the metallic form). X-ray diffraction indicated that many of the silver particles obtained had crystallite sizes in the range of 10 nm.
This grey solid was tested for catalytic activity in the partial oxidation of ethylene to ethylene oxide by pa~sing a mixture of air and ethylene over this catalyst packed in a 0~51 cm dia~eter by 12.7 cm long reaction tube in the presence of a chlorine-containing moderator. The reaction conditions ~ere approximately as follow~: pressure 15 atm. abs.; space velocity hours 1 = 30; 30% m ethylene in the charge;
~ r~ ~k .
: . - - , . ~ ~ . .... . ..
~ 8 - ~0~17~3 f ethylene/oxygen ratio = 3.75; moderator concentration equi-valent to 10 to 15 ppm chlorine. Under these conditions at a reaction tempersture of 205C and an oxygen conversion of 18~, a aelectivity of ethylene to ethylene oxide of approximatelv 65% waa obtained.
EXAMPLE II
Using the polymerization procedure described in Example I, thirty grams of 20-30 mesh aluminium oxide (known as Norton Company's "~lundum"~ grade L~-5556) were coated with a solution 10 of acrylonitrile (10.60 g) containing silver nitrate (3.40 g) and azobisisobutyronitrile (0.10 g). This coated product was then sub~eot to pyrolysis in a vacuum at the following oonditions: 1 hour at 210C; 2 hours at 250C; 1 hour at 300Ç; 2 hours at 350C and 1,5 hours at 400C. ~he pressures 15 employed during the pyrolyFis period ranged between about 0.5 and 1 mm ~g with the final pressure being 0.1 mm Hg. The product of the pyrolysis was a grey solid containing 7% by welght of metallic silver particles. ~he product also con-tained about 2% by weight csrbon.
This grey solid was also tested for catalytic activity in the conver~ion of ethylene to ethylene oxide under reaction conditions similar to those described in Example I. In this test the reaction feedstock composition was sbout 55~ ethylene, 10 ~ oxygen and 35 ~w nitrogen and the catalyst was conditioned 25 prior to use by passing 0.05 mole of air per hour over the cata-lyst at about 15 atm. abs. and 210C for 15 to 24 hours. No halogenated moderator was employed. ~t a reaction temperature of 240C and an oxygen conversion of 36~ the cataly~t gave as oonversion of ethylene to ethylene oxide of about 76%.
30 EX~MPLE III
~ catalyst prepared according to the procedure described in Example II was activated by passlng a gaseous stream con-taining a mixture of 30% m ethylene, 8~ m oxygen, and 62% m nitrogen over the catalyst particles in an isothermal tubular 35 reactor at a pressure of about 15 atm. abs. while the tempera-ture was slowly increased over an activation period of about r~ ~
10~ 3 50 hours from about 190 to 230C. During thls period the catalyst weight declined by a factor of about 10% indicating removal of the carbonaceous residue. After activation the catalyst was tested for catalytic activity in the conversion of ethylene to ethylene oxide using a procedure similar to that described in Example I. Here at reaction temperatures of 225C and 230C
and oxygen conversions of 40% and 52%, respectively, selectivities of 78.3 and 76.5 (ethylene to ethylene oxide) were obtained.
EXAMPLE IV
Catalyst particles prepared according to the process described in Example lI were activated by treatment with air in a non-isothermal tubular reactor. These catalysts were then tested for activity in the conversion of ethylene to e~thylene oxide. Table I below gives the activation conditions and the results of the evaluations for catalytic activity, T40 being the temperature at which 40% oxygen conversion was obtained and S40 being the selectivity of ethylene to ethylene oxide at the 40% oxygen conversion level.
TABLE
Temperature of O- T40 1 S40 -air ~reatment, CC
300 250 71 ~ ~
210 232 75.7 ~ -160 228 76.7 - - - -160 238 77.1 ~
~19- ' ~' .
A variety of methods have been employed to prepare supported silver catalysts to be used in the production of ethylene oxide wherein the silver i8 deposited as discrete, minute metal particles on the surfaces of a non-catalytic support. Among the most attractive of these methods in terms of laying down a silver deposit of ultrafine (less than 1000 nm) and uniform part cle size are those methods involving deoomposition of a silver^monomeric or polymeric carboxylic acid salt or complex on the surface of the support to form the ~ree metal particles. For example, ~nited States Patent Specif$oation No. 3,702,259 describes a process whereby a porous inert support i~ impre~nated with a solution contain-ing a complexed silver salt of a low molecular weight carboxylic acid, e.g., silver osalate, complexed and solubilized with certain organic amines, and the impregnated (coated) support is then heated to a sufficiently high temper-ature to deaompose the silver salt and deposi~ the elemental silver in the form of hemispherical particles having a diameter of less than 1000 nm on the surfaces of the support. ~ second method which involves metallic silver deposition by decompo-sition (decarboxylation) of a polymeric carboxylic acid-silver ion oomplex is disclosed in United States Pstent Specification No. 3,7~8,418. With that method, discrete silver partioles having a particle size less than 100 nm are uniformly deposited on the support surface by a multi-step process which comprises coating a support with a polymerized ethylenically unsaturated 3~0 acid, ~uch as polyacrylic acid, contacting the coated support wlth silver ions present as silver salts or complexes in non-aqueous media and then heating to a temperature sufficient to decarbo~ylate the polymerized acid and convert the silver ions into the discrete metallic silver particles. -Other possible methods for depositing metallic silver in the form of fine particles on a support surface are described a~ 3 in ~nited States Patent Specification ~Jo. 3,043,854 wherein the silver deposit is laid down on the support surface by adding a slurry of fine particles of ~ilver carbonate to the support and thermally decomposing the carbonate salt, and in United States Patent Specification No. 3,575,888 wherein the support i~ impregnated with an aqueous solution of silver nitrate, dried and the silver reduced to metallic silver particles with hydrogen or hydrazine.
q!he ability of the nitrile functional group to form a vsriety of metal oomplexes is well known as such. ~igh molecular weight species bearing a multiplicity of such llgand groups, such as polyacrylonitrile, also complex metals, but the~e complexes have not been extensively studied. ~ few poly-acrylonitrile complexes containing very small amounts of metals have been prepared, pyrolyzed and the products partially eharae-terized aecording to literature report~. Thus a polyacry] o-nitrile complex containing 0.14%w Ni was pyrolyzed and the electrical conductance properties of the metal containing char were evaluated by L.A. Lyatifova et al. in Doklad~ Akad ~auk ~Zh:R3. SS~ 20 31-33 (1964) and by M.A. Magrupov et al. in V:vsokomol Soedin 12 664 (1970). ~180, a similar study on pyrolyzed polyacrylonitrile containing copper, present as cupric chloride prior to pyrolysis, has been reported by l~.V. Topchiev et al. in Journal of Pol~nner Science ~. 1, 591 (1963). Lastly, at least one study ia reported wherein the catalytic properties of a pyrolyzed polyacrylonitrile compo-sition containing very small qllantities of copper (0.01~q6 Cu before pyrolysis) were examined in several reactions none of ~rhich were oxidation reactions. In that study by E.S. Doknkins et al. reported in Doklady Akad ~auk SSR 1'i7 893 (1961) the presence of the copper was said to have a negligible influence on the reactions attempted.
Ho~rever, while the pyrolysis of metal containing poly-aorylonitrile compositions has been studied in the limited sQnse described above, no disclosure is known of any attempt to prepare a finely divided metal or metsl oside cataly~t on ::, : . , , ~0617~;~
the surface of an inert support having practical utility such as those supported silver catalysts to be used in the production of ethylene oxide descrlbed above, via deposition of a metal (silver) containing polyacryl-onitrile on the surface of the support followed by pyrolysis of the polymer.
British patent specification no. 1,305,596 discloses Group VIII metal catalysts supported on a pyrolyzed polyacrylonitrile polymer carrier which apparently have practical utility as hydrogenation or dehydrogenation catalysts in other processes. However, in the technique taught for catalyst preparation in this patent the polyacrylonitrile carrier particles are pyrolyzed prior to addition of the ~roup VIII metal via treatment with an aqueous solution of a Group VIII metal salt or acid. Thus, it appears that the function of polyacrylonitrile in the cited British patent ls merely as a substitute for other conventional carbonaceous char carriers such as charcoal rather than being a vehicle for deposition of elemental metal particles on a support surface viaapplication of a metal ion-polyacrylon-ltrile complex to a support surface followed by decomposition of the complex-es.
It has now been found that supported silver catalysts active in the conversion of ethylene to ethylene oxide, the catalyst contain~ng from about 2 to about 20% by weight of metallic silver deposited evenly on the surface of a catalyst support in the form of a uniform dispersion of particles having diameters less than 150 nm, can be prepared by coating the surface of a catalyst support with an overlayer of polyacrylonitrile co~plexed with a silver salt and heating the coated support at a temperature up to 600C for a period of time sufficient to pyrollze the polyacryloni-trile and convert the sllver ions of the complexed silver salt into discrete partlcles of silver. Thus, the present invention provides a process for the production of ethylene oxide, ch2racterised in that ethylene is contacted in the vapor phase with an oxygen-containing gas at a temperature of from 210C
30 to 285C in the presence of a supported silver catalyst containing about 2~ -to 20~ by weight o~ metallicsilverpresent in particlllAte form on the support surface, the catalyst being prepared by coating the surface of a catalyst ~ - 4 -i17'~3 support with an overlayer of polyacrylonitrile camplexed with a silver salt and heating the coated support at a temperature up to 600C for a period of time sufficient to pyrolyze the polyacrylonitrile and convert the silver ions of the complexed silver salt into discrete particles of silver7 In the preparation of the silver catalysts used in the present invention, it is preferred that the overlayer of poly-- 4a -.~' . ,.
10~17~3 acrylonitrile complexed with a silver salt is prepared in situ by polymerization of a silver salt c~nplexed acrylonitrile monomer on the surface of the support. Silver catalysts prepared by the above described process, which contain metallic silver in the form of a uniform disperslon of particles having diameters less than about 150 nm on the support sur~ace, exhibit enhanced activity and selectivity in the conversion of ethylene to ethylene oxide when activated by treatment with a flowing stream of a gaseous mixture of ethylene and oxygen in combination with an inert gas or a flowing stream of air at elevated temperature under treatment conditions which facllitate remDval of the excess carbon residue but avoid sintering of the catalyst partlcles.
In the first step of the catalyst preparation process, a catalyst support is coated with an overlayer of polyacrylonitrlle complexed with a silver salt. m is overlayer of the polyacrylonitrile-silver salt complex can be conveniently laid down on the support surface by a variety of methods including those wherein the silver salt complex is formed on a support sur-face precoated with uncomplexed polyacrylonitrlle and those wherein the poly-acrylonitrile silver salt complex is applied directly to the support surface elther as the preformed polymer salt complex or as the monom~r salt conplex which is then polymerized onto the support surface.
In the procedures wherein the silver salt complex is formed after precoating the support surface with polyacrylonitrile, the poly-acrylonitrile precoat may be suitably applied to the support as a nomer and then polynerized on the support surface or it may be applied to the surface in an already polymerlzed form. If the uncomplexed acrylonitrile is a~plied in monomeric form, lt nay be polymerized in accord with ~ .
_ 5 ~ 3 well-known methoda of polymerization ~uch as the free radical polymerization technique. In this technique the acrylonitrile monomer is polymerized while in contact with the support, either in the presence or absence of a diluent, by the addition of a smsll quantity of a free radical polymerization initiator at a temperature which typically ranges from 0 to 200C.
Conventional free radical initiators which are suitable as polymerization catalysts in this reaction include benzoyl peroxide and other peroxide catalystc, azobisisobutyronitrile, and perbenzoic acid. If the uncomplexed polyacrylonitrile is applied to the support, a polymerization substantially a8 dQscribed above in the absence of the support may be carried out and the polyaorylonitrile 80 formed may be laid down on the support by any suitable method, for example by dipping the support in a solution of polyacrylonitrile in an inert solvent or by spraying the support with the solution. Suitable inert diluents or solvents for both the polymerization of the un-oomplexed acrylonitrile monomer, in the presence or absence of the support, and the application of the uncomplexed polyacrylo-nitrile polymer to the support surface typically include those polar organic compounds in which polyacrylonitrile is sub-stantially soluble. Examples of such solvents or diluents are gamma-butyrolactone, ethylene carbonate, dimethylformamide and dimethylsulphoxide. When solvents of this variety are employed it is desirable to remove the excess solvent, after applicatlon of the solution to the support, by means such as vacuum drying, ~bich avoid estraction of the polyacrylonitrile coating o~f the support. Alternatively, the polyacrylonitrile can be precipi-tated onto the support surface from the polymer solution by the addition of non-~olvents such as methanol or toluene to mixtures of the support and the polymer solution. The polyacrylonitrile overlayer can also be applied by polymerization of the acrylo-nitrile in the prQsence of the support; non-polar organic solvent~ such as benzene, toluene, cyclohexane and n-hexane e~hiblting solvQncy for the monomer can be employed as polymer-izstion solvents since the polymer will preCipitatQ on the .... .
- 7- 10~1'7~3 f ~upport as it i9 formed. When these ~olvents are employed it i~ again de~irable to r~move the excess solvent by filtration and/or VBCUUm drying.
The complexing of the silver salt with the polyacrylo-nitrile, precoated on the ~upport surface according to either of the above described techniques, is carried out by contact-ing the polyacrylonitrile coated support with a silver(I) ~alt or complex in a ~olvent. Suitable solvents for this step in which the silver salt complex of the polymer is formed are those polar organic solvent3 which will not appreciably extraot the polymer coating off the support surface, though some slight solvency for the polymer is desired to soften and pla~ticize the polymer surface and allow for diffusion of the metal ions into the polymer layer. Examples of solvents having acceptable solvency characteristics in this application oomprise acetone, methanol, acetonitrile, ethylene glyool and mixtures thereof. In special case~ aqueous mixtures are useful. Since this complex forming step is carried out in a medium of the type described, the silver(I) ion must be added in the form of salts or complexes which are at least partially soluble in the media. Examples of such salts and complexes are salts of carboxylic acids such as formate, trifluoro-aoetate, butyrate, 2-ethyl hexanoate, lactate and citrate, complexes such as those derived from pyridine and silver acetate or 1,5-cyclooctadiene and silver nitrate, and the soluble salts of mineral acids such as sil~er nitrats. Of this class of silver(I) compounds silver nitrate and its com-plexes are preferred. To form the silver sa;t complex over-layer of this invention the polyacrylonitrile coated ~upport is conta¢ted with the silver salt solution, preferably con-taining from 0.03 to 3 equivalents of silver ion per litre, at temperature~ ranging from ambient to 125C for a period long enough to permit the ~ilver ions to complex with the polyacrylonitrile. After passage of sufficient contacting time to allow complex formation on a substantial portion of the aooeasible li~and sites, time periods of from 10~. 7~3 0.5 to 24 hours ~enerally being sufficient, the coated ~upport containing the silver ~alt-polyacrylonitril~ complex overlayer $8 recovered frorl the excess silver salt solution by conventional methods such as sieving or decanting. ~lternatively, the silver-depleted solvent can be removed by vacuum distillation. This coated product as recovered from the excess silver salt typicslly contains only a minor amount of residual solvent and can suitably be employed directly in the thermal pyrolysis procedure dis¢ussed below to afford the particulate silver cata-lysts of the invention. ~owever, due to the hazards encountered when flammable organic solvents are exposed vo high temperatures it is generally preferred to remove the residual solvent by oonventional means such as vacuum drying at ambient and/or moderately elevated temperatures prior to pyrolysis.
As indicated above, the polyacrylonitrile-silver salt oomplex can also be applied to the support surface as the pre-formed polymer-salt complex or as the monomer-salt complex which is then polymerized onto the support surface. In procedures where the preformed polymer-salt complex i8 employed as the coating agent, the polymer-salt compleY may be suitably pre-pared ex-support by polymerizing the acrylonitrile monomer in solution according to the procedure described above, and adding the appropriate amount of silver salt to the polymer solution.
It is most desirable to employ the very best solvents for poly-acrylonitrile, such ae gamma-butyrolactone, dimethylformamide, dimethylacetamide, or dimethylsu~oxide, in this procedure a~
certain marginal solvents such as ethylene carbonate ~nd hexa-fluoroisopropanol precipitate the polyacrylonitrile-silver salt complex when the solution of silver nitrate or silver trifluoro- -acetate is added. ~his preformed polymer salt complex in solution may then be applied to the support surface by con-ventlonal means such as spraying or dipping or merely by making up a slurry of support particles with a polymer salt comple~
solution of desired dilution in the polymerization solvent and remo~ing the solvent by vacuum dryin~ techniques. In any case, . ~
10~7~3 where solution coating is involved it is preerred to remove the residual solvent prior to charging the coated support particles to the thermal pyrolysis phase of the process discussed below.
Preference is given to the procedure wherein the polymer salt-complex is applied directly onto the support surface by poly-merization of the monomer salt complex in the presence of the support.
This procedure is preferred because it allows advantage to be taken of the high solvency and complex forming ability which acrylonitrile exhibits for silver salts and the ease with which acrylonitrile solutions of silver salts polymerize. In this procedure acrylonitrile and the silver salt are combined directly at ambient or moderately elevated temperatures in the desired molar proportions to form a solution containing the complexed silver salt, preferably complexed silver nitrate. After addition of the silver salt to acrylonitrile this solution can be applied almost immedi-ately to the surface of the support and polymerizedO To ensure a polymerized layer of proper thickness on the support, it is advantageous to combine the support with the acrylonitrile solution, using conventional means such as pouring the liquid solution into a reaction vessel containlng the support, in proportions such that the solution thoroughly wets the support surface without any appreciably excess liquid phase. Once the support surface is wet the acrylonitrile-silver salt complex may be polymerized onto the support surface, in the presence or absence of a conventional free radical polymerization initiator by heating the reaction mixture at temperatures ranging from 50C to 150C for a time period of from 0.5 up to 12 hours.
Often it is convenient to slurry the monomer-complex coated support with a non-polar organic solvent in which the monomeric silver salt complex and the polymerized product`are essentially non-soluble to effect efficient heat transfer at the temperature selected for the polymerization. Suitable non-polar organic heat transfer solvènts for the polymerization reaction include straight c~ain, branched-c~ain and cyclic aliphatic hydrocarbons 3.0~ 3 such as n-pentane, n-hexane, 3-ethylhexane, octane, cyclopentane and cyclohexane and aromatic hydrocarbons such as benzene, toluene and xylene.
Although the polymerization can be carried out to yield a polyacrylonitrile of acceptable molecular weight without the aid of a conventional initiator, it is preferable to carry out the polymerization reaction in the presence of a free radical initiator, azobisisobutyronitrile and benzoyl peroxide being most preferred. It is efficacious to dissolve the free radical initiator in the acrylonitrile-silver salt sodium prior to the wetting of the support surface with this solution. Upon completion of the polymer-ization reaction period, the coated support is suitably separated from thepolymerization solvent by conventional means such as sieving or decanting or the like. This coated product may be charged directly to the thermal pyrolysis step for conversion to the particulate silver catalysts accord-ing to the present invention, however for reasons of safety discussed above, it is preferable to remove any residual solvent by means such as vacuum and/or heat prior to pyrolysis.
With any of the above described procedures for application of the polyacrylonitrile-silver salt complex to the support surface, poly-acrylonitrile compositions having an average molecular weight of from 103 up to above 106 can be employed. For those procedures in which the poly-acrylonitrile is formed prior to application to the support surface, poly-meric compositions having an average molecular weight of from 103 to 105 are preferred because of the lower viscosities of the polymer solutions which must be handled. In those procedures including the preferred procedure, wherein the acrylonitrile or acrylonitrile-silver salt complex is polymerized onto the support surface, best results are obtained when the polymerization is carried out to obtain a polyacrylonitrile having an -average molecular weight of from 103 up to above 106, this higher molecular weight limit not being critical.
- lQ -.. : ' . , . : -10~7~3 The quantities of polyacrylonitrile and silver salt applied to the support surface may vary within wide limits and depend primarily on the proportion of particulate silver desired in the catalyst products of this invention. While the Goordinati~n stoichiometry in the polyacryl-onitrile-silver salt complex has not been established with certainty for all possible silver salts complexes~ it appears that for salts having monovalent anions such assilver nitrate, stable complexes can be formed having up to at least 34% by wei~ht silver based on the total complex weight. me lower limit on the a~.ount of silver salt in the polyacryl-onitrile-sllver salt complex coating of this invention is, for all practical purposeR, dependent on the quantity of silver desired in the final catalyst product with amounts of silver salt, expressed as per cent by weight silver in the polymer complex, as low as about 1% being suitable. Preferably, the quantity of silver salt present in the polyacrylDnitrile complex over-layer of this invention ranges from about 5 to 34% by weight silver based on total complex weight. The quantity of polyacrylonitrile which may be re~lly applied to the support surface and pyrolized according to the procedure described below to yield the catalyst products according to the present invention is not critically limited and conveniently ranges of from 5% by weight to 45% by weight based on the total supported catalyst weight.
When a quantlty of silver salt within the preferred range given above is uittltzed ln the polymer complex of the invention, the quantity of silver applied to the supp~rt surface preferably ranges between 3% by weight and 15g by weight, based on total supported catalyst weight.
The supported catalyst products utilized in the present invention and ccntaining particulate deposits of metallic ~ilver are prepared by heating the polyacrylonitrile-sil~er salt complex coated supports to a temperature up to 600C, pre~erably in the range of fron 200C to 600C.
At te~peratures in this range other studies have suggested that poly-acrylonitrile pyrolizes with concomitant cyclization and loss of ~;:
... i 't,~.'i ' , 1~17~
hydrogen and some ammonia and hydrogen cyanide to yield a polypyridire-like structure. While this mechanism could account for the reductlon of the coordinated silver salts in the polyacrylonitrile matrix which has been found to occur in the instant process, the weight loss on pyrolysis of the complexed polyacrylonitrile considerably exceeds the 4% required for the idealized dehydrocyclization of polyacrylonitrile to the polypyridine structure. It has not yet been possible to assign with certainity any definite mechanism to the pyrolysis which occurs in the instant process, nor moreover, to ascertain the exact structure of the pyrolyzed char which remains. The pyrolysis can be carried out in an oxygen-containing enviror~nent, for example, in air; in an inert environment such as in nitrogen or argon; or in a vacuum; or in a reducing atrnosphere such as hydrogen. Due to the tendency of certain polyacrylonitrile-silver salt complexes and especially of silver nitrate complexes, to spontaneously ignite at the pyrolysis temperatures, it is preferred to carry out the pyro-lysls in stages in an inert atmosphere such as nitrogen or in a vacuum.
At the above indicated temperature range substantial pyrolysis of the polyacrylonitrile matrix with concomitant conversion of all or substantially all of the complexed silver ions into discrete ultrafine particles of metallic silver is obtained through employment of pyrolysis tines ranging from about 2 to 12 hours. From a procedural standpoint, the pyr~lysis may be suitably carrled out by slowly increasing the pyrolysis temperature with increasing residence time until the maximum pyrolysis temperature is reached - i.e. 600C or, preferably, about 500C - or until substantial pyrolysls ls obtalned. Preferably, the pyrolysis is ca~ried out in stepwise fashion wherein the pyrolysis is initiated at temperatures of about 200C and the pyrolysis temperature is incre~lentally increased . . . .
wlth hold~ng perlods at each intermediate temperature level until a max~mum temperature of about 450C is attained. In this preferred pyrolysis ' :,.~:
~0ti17~
procedure, the intermediate temperature levels m~y suitably di~fer by 50C
to 100C with the residual time at each ternperature level rar~n~ ~rom abou~
0~5 to 5 hours.
The products of the pyrolysis carried out as described above are supported silver materials which are catalytically active in the conversion of ethylene to ethylene oxide. These supported catalysts suitably contain of from 2% to 20% by weight of metallic silver deposited evenly on the interior (pore) and exterior surfaces of the support.
Preferably they contain of from 3% to 15% by weight of silver metal on the same basis. The metallic silver is present as tiny individual particles generally having diameters less than 150 nm with many particles having diameters less than 20 nm. Scanning electron micrographs of typical supported silver catalysts prepared by the process according to the present invention show an average silver particle size of about 100 nm whereas other analytical techniques such as X-ray diffraction indicate many of the silver particles to have particle diameters of less than 20 nm. Generally a minor amount of nitrogenous and carbonaceous residue remains on the catalyst support when the pyrolysis is carried out according to this invention. Typically, this residue from the pyrolyzed polyacrylonitrile ranges from about 2% to 30% by weigpt of total catalyst weight.
me support employed in these catalysts in its broadest aspects is selected from the la~ge number of conventional porous refractory catalyst carriers or support materials which are es æntially inert in the presence of the feedstock to be used in the oxidation of ethylene to the products and under the prevailing reaction conditions. Such con~entional m~terials nay be of natural or synthetic origin and preferably are of a ~acropor~us structure, tbat is, a structure having a surface area below 10 m2/g and preferably below 5 m /g. m ese support materials typically have an "appabent pQrosityl~ of greater than 20%. Very su~table supports comprise those of s~l~ceous and/or aluminous composition. Specific examples of suitable sup-ports ,~ :
~ . . .
-., .
. .
. . . . : . .
17~3 are the aluminium oxides (including the r~aterials sold under the ~rade name "Alundum"), charcoal, pumice, r~snesia, zirconia, kieselguhr, fuller's earth, silicon carbide, porous agglomerates comprising silicon and/or silicon carbide, magnesia, selected clays, artificial and natural zeolites, metal oxide gel-type materials comprising oxides of heavy metals such a~
molybdenum or tungsten, ceramics, etc. Refractory supports particuLarly useful in the preparation of catalysts in accordance with this invention comprise the aluminous materials, in particular those containing alpha alumina. In the case of alpha alumina-containing supports, préference is given to those having a specific surface area ss measured by the B.E.T.
method of from 0.03 m2/g to 2.0 m2/g and an apparent porosity as measured by conventional mercury or water absorption techniques of from 25% to 50%
bg volume. The B.E.T. method for determindng specific surface area is described in detail by S. Brunauer, P.H. Emmet, and E. Teller (J. Am. C~em.
Soc.~ 60 309-16 (1938)). The physical shape and size of the support is wholly conventional and includes small partlcles of regular or irregular shape suitable for use in fluidlzed bed applications or larger chunks, pellets and the like appropriate for use in fixed bed catalytic processes.
Preferably the support particles are in the form of tablets, rings, pellets cr the like of a size suitable for use in fixed bed operations.
While the catalyst products of the polyacrylonitrile pyrolysis procedure exhibit catalytic activity for, and may be used directly in, the partial oxidation of ethylene to ethylene oxide, the activity and selectlvity o~ these catalgsts can be considerably enhanced by activation in an atm~sphere and under conditions ~hich substantially remove the residual carbon but av~ld or substantially minimize sintering of the catalyst particles. While -~
this catalyst activation step ls not limited to any particular method or methods which acco~plish the desired result - i.e. substantial burn off or remDval of the residual carbon without cancomitant sintering of catalyst particles - at least two different catalyst activation or residual carbon removal pr~cedures appear to be A;
~
-1()ti~73 particularly applicable to the catalyst products. The first of these procedures involves treatment of the catalyst particles with a flowing gaseous stream comprising a mixture of ethylene and oxygen, pre~era~ly in combination with an inert gas such as nitrogen~ at temperatures in the range of from 190C to 230C for a time period of not less than about 40 hours. Ihe amounts of ethylene and oxygen employed in this procedure are suitably 20g - 40% molar (m) and 5% - 10% m, respectively, of the total composition of the treatment stream with the inert gas, preferably nitrogen, compr.lsing the balance of the stream. In carrying out this activation procedure, best results are obtalned when a treatment stream containing about 30% m ethylene and about 8~noxygen (balance nitrogen) is e~ployed in a procedure wherein the treatment stream is slowly heated from about 190, initlal, up to about 230C, final, over a time period of not less than about 50 hours. Ihis prefer.red activation procedure not only gives excellent results, but additiona ly, is quite convenient in that the proportlons of ethylene and oxygen employed correspond rather closely to the reactant concentrations normally employed in a conventional feed stream to an -;
oxyg~n based ethylene oxide production process. Accordingly, once the -catalyst has been activated by this ~referred procedure, which is quite ~0 suitably car.ried out on catalyst loaded.~in the ethylene oxide reaction zone~
the te~perature and other reactian conditions can be easily adJusted to the ethylene oxide manufacturing conditions thereby facilitating dlrect con- ~
versian fron catPlyst activation to ethylene oxide production with the `.
activated catalyst.
. Ihe second method of catalyst actlvation involves treatment of the pyrolyzed catalyst particles with a flowing stream of air at temperatu~es in the range of about 150 to 210C. With this procedure temperatures above about 210C are to be avolded since substantial sintering of the catalyst .
partlcles occurs at these higher temperatures. In fact, at least a minor amount of catalyst sintering has been found to occur ~1 .
... .. ",~
10~17'~3 ~t temperatures in the range wherein the activation procedure is operative.
Accordingly, it is preferred to carry out the catalyst activation in thiæ
procedure at the lower end of the operative range, most preferably at temperatures of about 160C in order to achieve maxi~um catalyst activity cambined with a minimum amount of sintering of catalyst particles. m e treatment times employed with this air activation procedure are substantially similar to those employed in the ethylene/oxygen activation procedure with times of not less than about 20 hours being suitable.
me silver catalysts, prepared and activated in the manner described have been shown to be particularly selective catalysts in the direct oxidation of ethylene with molecular oxygen to ethylene oxide. The condltions for carrying out such an oxidation reaction in the presence of the silver catalysts broadly camprise those described in the prior art.
This applies, for example, to suitable temperatures, pressures, residence times, diluent materials, such as nitrogen, carbon dioxide, steam, argon, methane or other saturated hydrocarbons, presence or absence of ncderating ~-agents to control the catalytic action, for example, 1,2-dichloroethane, vinyl chloride or chlorinated po}yphenyl compounds, the desirability of employing recycle operations or applying successive conversion in different reactors to increase the yields of ethylene oxide, and any other special cond-itions which may be selected in processes for preparing ethylene oxide.
Pressures in the range of fram atmospheric to 35 atm. are generally e~ployed. HiEher pressures may, however, be employed within the scope of the inv~ntiQn. Molecular oxygen employed as reactant may be obtained from convention~l sources. Ihe suitable oxygen charge may consist essentially of relatively pure oXYgen. A concentrated oxygen stream comprising oxygen ln nE~or amount with lesser amounts of one or more diluents such as nitrogen, argon, etc., or another oxygen-containing stream such as air. It is there- ~;
Pore evident that the use of the present novel silver catalysts in ethylene 3 oxidation reactians is in no way limited to the use of specific conditions among those which are kncwn to be effective.
.,~
~ ` 7 -- ~ 17 1~617g3 In a preferred application of the ~ilrer catalyst~
according to the present i~vention ethylene oxide is produced when an oxygen-containing gas separated from air and containing not less thsn 95~ oxygen is contacted with ethylene in the presence of a cataly~t according to the present invention at a temperature in the range of from 210C and preferab y 225C
to 270C.
EXAMPLE I
A solution of 3.20 g acrylonitrile, 2.05 g silver nitrate and 0.03 g aæobisisobutyronitrile was poured onto 10 g of 30-40 mesh commercial aluminium oxide (known as ~orton Company's "~lundum" grade L~-5556), having a surface area of about O.2 m /g in a reaction vessel. ~his solution just wet the aluminium o~ide particles without any appreciable liquid phase present.
Thirty millilitres of n-hexane were then added to the reaction vessel to provide the desired heat transfer and the mixture was heated to reflux under nitrogen atmosphere for 1.5 hours.
Observation of the reaction indicated that the acrylonitrile/
silver nitrate complex polymerized during the initial heating period. ~pon completion of the reaction period the solids were filtered off and vacuum dried for 1 hour at 60C to yield 13.5 g of a mustard yellow granular solid. Twelve grams of this yellow solid were pyrolyzed under a nitrogen atmosphere in a tube furnace as follows: 2 hours at 200 C, 1 hour at 250C, 1 hour at 300C and 2 hours at 400C. The product of this pyrolysis was a grey solid containing 12.0% by weight particulate silver (in the metallic form). X-ray diffraction indicated that many of the silver particles obtained had crystallite sizes in the range of 10 nm.
This grey solid was tested for catalytic activity in the partial oxidation of ethylene to ethylene oxide by pa~sing a mixture of air and ethylene over this catalyst packed in a 0~51 cm dia~eter by 12.7 cm long reaction tube in the presence of a chlorine-containing moderator. The reaction conditions ~ere approximately as follow~: pressure 15 atm. abs.; space velocity hours 1 = 30; 30% m ethylene in the charge;
~ r~ ~k .
: . - - , . ~ ~ . .... . ..
~ 8 - ~0~17~3 f ethylene/oxygen ratio = 3.75; moderator concentration equi-valent to 10 to 15 ppm chlorine. Under these conditions at a reaction tempersture of 205C and an oxygen conversion of 18~, a aelectivity of ethylene to ethylene oxide of approximatelv 65% waa obtained.
EXAMPLE II
Using the polymerization procedure described in Example I, thirty grams of 20-30 mesh aluminium oxide (known as Norton Company's "~lundum"~ grade L~-5556) were coated with a solution 10 of acrylonitrile (10.60 g) containing silver nitrate (3.40 g) and azobisisobutyronitrile (0.10 g). This coated product was then sub~eot to pyrolysis in a vacuum at the following oonditions: 1 hour at 210C; 2 hours at 250C; 1 hour at 300Ç; 2 hours at 350C and 1,5 hours at 400C. ~he pressures 15 employed during the pyrolyFis period ranged between about 0.5 and 1 mm ~g with the final pressure being 0.1 mm Hg. The product of the pyrolysis was a grey solid containing 7% by welght of metallic silver particles. ~he product also con-tained about 2% by weight csrbon.
This grey solid was also tested for catalytic activity in the conver~ion of ethylene to ethylene oxide under reaction conditions similar to those described in Example I. In this test the reaction feedstock composition was sbout 55~ ethylene, 10 ~ oxygen and 35 ~w nitrogen and the catalyst was conditioned 25 prior to use by passing 0.05 mole of air per hour over the cata-lyst at about 15 atm. abs. and 210C for 15 to 24 hours. No halogenated moderator was employed. ~t a reaction temperature of 240C and an oxygen conversion of 36~ the cataly~t gave as oonversion of ethylene to ethylene oxide of about 76%.
30 EX~MPLE III
~ catalyst prepared according to the procedure described in Example II was activated by passlng a gaseous stream con-taining a mixture of 30% m ethylene, 8~ m oxygen, and 62% m nitrogen over the catalyst particles in an isothermal tubular 35 reactor at a pressure of about 15 atm. abs. while the tempera-ture was slowly increased over an activation period of about r~ ~
10~ 3 50 hours from about 190 to 230C. During thls period the catalyst weight declined by a factor of about 10% indicating removal of the carbonaceous residue. After activation the catalyst was tested for catalytic activity in the conversion of ethylene to ethylene oxide using a procedure similar to that described in Example I. Here at reaction temperatures of 225C and 230C
and oxygen conversions of 40% and 52%, respectively, selectivities of 78.3 and 76.5 (ethylene to ethylene oxide) were obtained.
EXAMPLE IV
Catalyst particles prepared according to the process described in Example lI were activated by treatment with air in a non-isothermal tubular reactor. These catalysts were then tested for activity in the conversion of ethylene to e~thylene oxide. Table I below gives the activation conditions and the results of the evaluations for catalytic activity, T40 being the temperature at which 40% oxygen conversion was obtained and S40 being the selectivity of ethylene to ethylene oxide at the 40% oxygen conversion level.
TABLE
Temperature of O- T40 1 S40 -air ~reatment, CC
300 250 71 ~ ~
210 232 75.7 ~ -160 228 76.7 - - - -160 238 77.1 ~
~19- ' ~' .
Claims (9)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Process for the production of ethylene oxide, characterized in that ethylene is contacted in the vapour phase with an oxygen-contain-ing gas at a temperature of from 210°C to to 285°C in the presence of a supported silver catalyst containing about 2% to 20% by weight of metallic silver present in particulate form on the support surface, the catalyst being prepared by coating the surface of a catalyst support with an overlayer of polyacrylonitrile complexed with a silver salt and heating the coated support at a temperature up to 600°C for a period of time sufficient to pyrolyze the polyacrylonitrile and convert the silver ions of the complexed silver salt into discrete particles of silver.
2. Process as claimed in claim 1, characterized in that the supported silver product of the polyacrylonitrile pyrolysis is activated by removal or burn off of the excess of carbonaceous residue under condi-tions which avoid or minimize sintering of the catalyst particles.
3. Process as claimed in claim 1 or 2, characterized in that the catalyst activation is accomplished by treatment with a flowing stream of a gaseous mixture of ethylene and oxygen in combination with an inert gas or a flowing stream of air at elevated temperatures.
4. Process as claimed in claim 1 or 2, wherein the silver part-icles on the support surface have diameters of less than 150 nm.
5. Process according to claim 1 or 2, characterized in that the quantity of silver on the support surface ranges between 3% and 15% by weight, based on total supported catalyst weight.
6. Process as claimed in claim 1 or 2, wherein the support is a porous refractory alumina-based support.
7. Process as claimed in claim 1 or 2, wherein the support is a porous refractory alumina-based support .alpha.-alumina.
8. Process as claimed in claim 1 or 2, wherein the catalyst consists essentially of x-alumina carrying on its surface silver particles having diameters of less than 150 nm.
9. Process as claimed in claim 1 or 2, characterized in that the ethylene is contacted with an oxygen-containing gas containing not less than 95% oxygen, in the absence of air and at a temperature of from 225°C to 270°C.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA314,173A CA1061793A (en) | 1973-09-24 | 1978-10-25 | Catalyst for preparation of ethylene oxide |
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US399905A US3892679A (en) | 1973-09-24 | 1973-09-24 | Catalyst for preparation of ethylene oxide |
| CA207,077A CA1062691A (en) | 1973-09-24 | 1974-08-15 | Catalyst for preparation of ethylene oxide |
| CA314,173A CA1061793A (en) | 1973-09-24 | 1978-10-25 | Catalyst for preparation of ethylene oxide |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1061793A true CA1061793A (en) | 1979-09-04 |
Family
ID=27163582
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA314,173A Expired CA1061793A (en) | 1973-09-24 | 1978-10-25 | Catalyst for preparation of ethylene oxide |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1061793A (en) |
-
1978
- 1978-10-25 CA CA314,173A patent/CA1061793A/en not_active Expired
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