Note: Descriptions are shown in the official language in which they were submitted.
~ ~ - 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 --
~ .
_ 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
~;:
- 11 -
... 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
- 12 -
' :,.~:
~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
- 14 -
A;
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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
- 15 -
~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.
- 16 -
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~ ` 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- ' ~'
.
