U.S. patent application number 12/533322 was filed with the patent office on 2009-11-26 for catalysts having enhanced stability, efficiency and/or activity for alkylene oxide production.
This patent application is currently assigned to The Dow Chemical Company. Invention is credited to Albert C. Liu, Juliana G. Serafin, Seyed R. Seyedmonir, Hwaili Soo, Thomas Szymanski.
Application Number | 20090291847 12/533322 |
Document ID | / |
Family ID | 34520043 |
Filed Date | 2009-11-26 |
United States Patent
Application |
20090291847 |
Kind Code |
A1 |
Serafin; Juliana G. ; et
al. |
November 26, 2009 |
Catalysts Having Enhanced Stability, Efficiency and/or Activity For
Alkylene Oxide Production
Abstract
A catalyst for the manufacture of alkylene oxide, for example
ethylene oxide, by the vapor-phase epoxidation of alkene containing
impregnated silver and at least one efficiency-enhancing promoter
on an inert, refractory solid support, said support incorporating a
sufficient amount of zirconium component (present and remaining
substantially as zirconium silicate) as to enhance at least one of
catalyst activity, efficiency and stability as compared to a
similar catalyst which does not contain the zirconium
component.
Inventors: |
Serafin; Juliana G.;
(Charleston, WV) ; Liu; Albert C.; (Charleston,
WV) ; Seyedmonir; Seyed R.; (Charleston, WV) ;
Soo; Hwaili; (Charleston, WV) ; Szymanski;
Thomas; (Hudson, OH) |
Correspondence
Address: |
The Dow Chemical Company;Intellectual Property Section
P.O. Box 1967
Midland
MI
48641
US
|
Assignee: |
The Dow Chemical Company
Midland
MI
|
Family ID: |
34520043 |
Appl. No.: |
12/533322 |
Filed: |
July 31, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10573694 |
Jun 19, 2006 |
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PCT/US04/33219 |
Oct 7, 2004 |
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12533322 |
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60511975 |
Oct 16, 2003 |
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Current U.S.
Class: |
502/227 |
Current CPC
Class: |
B01J 37/0018 20130101;
B01J 23/686 20130101; B01J 37/0203 20130101; C07D 301/10 20130101;
B01J 35/1009 20130101; B01J 35/1076 20130101; B01J 23/50 20130101;
B01J 23/66 20130101; B01J 21/066 20130101; B01J 37/0201 20130101;
B01J 23/688 20130101; B01J 35/1042 20130101 |
Class at
Publication: |
502/227 |
International
Class: |
B01J 21/06 20060101
B01J021/06 |
Claims
1.-24. (canceled)
25. A process for preparing a catalyst for the epoxidation of an
olefin comprising: incorporating fluoride anions and zirconium
silicate into alumina; calcining the alumina at a temperature
between 1000.degree. C. and 1400.degree. C. to form a carrier
comprising alpha-alumina; and subsequently depositing a catalytic
species comprising silver onto the carrier.
26. A process for preparing a catalyst for the epoxidation of an
olefin comprising: incorporating fluoride anions and zirconium
silicate into alumina; calcining the alumina at a temperature
between 1000.degree. C. and 1400.degree. C. to form a carrier
comprising alpha-alumina; and subsequently depositing silver and
rhenium component onto the carrier.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Patent Application Ser. No. 60/511,975, filed Oct. 16, 2003.
FIELD OF INVENTION
[0002] This invention relates to catalysts for the epoxidation of
alkene, especially ethylene, to the corresponding alkylene oxide,
for example, ethylene oxide, which have enhanced stability,
efficiency and/or activity by incorporating sufficient amount of a
zirconium component substantially as zirconium silicate.
BACKGROUND OF THE INVENTION
[0003] The production of alkylene oxide, such as ethylene oxide, by
the reaction of oxygen or oxygen-containing gases with ethylene in
the presence of a silver-containing catalyst at elevated
temperature is an old and well-known art. For example, U.S. Pat.
No. 2,040,782, dated May 12, 1936, describes the manufacture of
ethylene oxide by the reaction of oxygen with ethylene in the
presence of silver catalysts which contain a class of
metal-containing promoters. In Reissue U.S. Pat. No. 20,370, dated
May 18, 1937, Leforte discloses that the formation of olefin oxides
may be effected by causing olefins to combine directly with
molecular oxygen in the presence of a silver catalyst. (An
excellent discussion on ethylene oxide, including a detailed
description of commonly used manufacturing process steps, is found
in Kirk-Othmer's Encyclopedia of Chemical Technology, 4.sup.th Ed.
(1994) Volume 9, pages 915 to 959).
[0004] The catalyst is the most important element in direct
oxidation of ethylene to produce ethylene oxide. There are several
well-known basic components of such catalyst: the active catalyst
metal (generally silver as described above); a suitable
support/carrier (for example alpha-alumina); and catalyst
promoters, all of which can play a role in improving catalyst
performance. Because of the importance of the catalyst in the
production of ethylene oxide, much effort has been expended to
improve catalyst's efficiency in producing ethylene oxide.
[0005] The use of zirconium and or silicon components as either
promoters in the ethylene oxide catalyst or as modifiers to
supports (that is carriers) used for such catalysts are also
known.
[0006] U.S. Pat. No. 5,703,001 describes a rhenium-free silver
catalyst promoted with an alkali metal component and a Group IVB
component wherein the Group IVB component is added as a compound
having a Group IVB cation. Soluble zirconium compounds where the
Group IVB component is a cation are preferred.
[0007] U.S. Pat. No. 5,145,824 describes a rhenium-promoted
ethylene oxide silver catalyst supported on a carrier comprising
alpha alumina, an added alkaline earth metal in the form of an
oxide, silicon in the form of an oxide, and from zero to about 10
percent (%) added zirconium in the form of the oxide. In U.S. Pat.
No. 5,145,824, the term "oxide" is used to refer to simple oxides
made up of only one metal as well as complex oxides made up of the
indicated metal and one or more of the other metals. The amount of
alkaline earth metal used in the carrier is from 0.05 to 4 weight
percent (wt. %), measured as the oxide. Similarly, U.S. Pat. No.
5,801,259 describes an ethylene oxide catalyst comprising silver
and promoters on a carrier prepared by mixing alpha alumina,
alkaline earth metal oxide, silicon oxide, and from zero to about
15% of zirconium in the form of the oxide. The particle sizes of
the ceramic components are chosen such that the packing density of
the dried carrier precursor is not greater than that of the fired
carrier; thereby eliminating the need for organic burnout agents.
In '824 and '259 patents, the carrier mixture is formed from a
starting mixture containing alpha-alumina, and requires the
addition of alkaline earth metal oxide. The addition of the
zirconium oxide component is optional.
[0008] There are several examples in the prior art of carriers used
for ethylene oxide catalysts which contain silicon-containing
compounds. U.S. Pat. No. 6,313,325 describes a method for the
production of ethylene oxide wherein the carrier of the catalyst is
obtained by adding an aluminum compound, a silicon compound and an
alkali metal compound to a low-alkali content alpha-alumina powder.
After calcination, this mixture is thought to provide a coating
layer of alkali metal-containing amorphous silica alumina on the
outer surface of the alpha-alumina carrier and the inner surface of
the pores thereof. Canadian patent 1,300,586 describes a catalyst
using a carrier composed mainly of alpha-alumina, silica, sodium,
which has measurable acidity and crystals of
Al.sub.6Si.sub.2O.sub.13 which are detectable by X-ray Diffraction
analysis (XRD).
[0009] Several terms are commonly used to describe some of the
parameters of catalytic systems for epoxidation of alkenes. For
instance, "conversion" is defined as the molar percentge of alkene
fed to the reactor which undergoes reaction. Of the total amount of
alkene which is converted to a different chemical entity in a
reaction process, the molar percentage which is converted to the
corresponding alkylene epoxide, that is alkylene oxide, is known as
the "efficiency" (which is synonymous with the "selectivity") of
that process. The product of the percent efficiency times the %
conversion (divided by 100% to convert from %.sup.2 to %) is the
percentage "yield", that is, the molar percentage of the alkene fed
that is converted into the corresponding epoxide.
[0010] The "activity" of a catalyst can be quantified in a number
of ways, one being the mole percent of alkylene epoxide contained
in the outlet stream of the reactor relative to that in the inlet
stream (the mole percent of alkylene epoxide in the inlet stream is
typically, but not necessarily, zero percent) while the reactor
temperature is maintained substantially constant, and another being
the temperature required to maintain a given rate of alkylene
epoxide production. That is, in many instances, activity is
measured over a period of time in terms of the molar percent of
alkylene epoxide produced at a specified constant temperature.
Alternatively, activity may be measured as a function of the
temperature required to sustain production of a specified constant
mole percent of alkylene epoxide. The useful life of a reaction
system is the length of time that reactants can be passed through
the reaction system during which results are obtained which are
considered by the operator to be acceptable in light of all
relevant factors.
[0011] Deactivation, as used herein, refers to a permanent loss of
activity and/or efficiency, that is, a decrease in activity and/or
efficiency which cannot be recovered. As noted above, production of
alkylene epoxide product can be increased by raising the
temperature, but the need to operate at a higher temperature to
maintain a particular rate of production is representative of
activity deactivation. Activity and/or efficiency deactivation
tends to proceed more rapidly when higher reactor temperatures are
employed. The "stability" of a catalyst is inversely proportional
to the rate of deactivation, that is, the rate of decrease of
efficiency and/or activity. Lower rates of decline of efficiency
and/or activity are generally desirable.
[0012] To be considered satisfactory, a catalyst must have
acceptable activity and efficiency, and the catalyst must also have
sufficient stability, so that it will have a sufficiently long
useful life. When the efficiency and/or activity of a catalyst has
declined to an unacceptably low level, typically the reactor must
be shut down and partially dismantled to remove the catalyst. This
results in losses in time, productivity and materials, for example
silver catalytic material and alumina carrier. In addition, the
catalyst must be replaced and the silver salvaged or, where
possible, regenerated. Even when a catalyst is capable of
regeneration in situ, generally production must be halted for some
period of time. At best, replacement or regeneration of catalyst
requires additional losses in production time to treat the catalyst
and, at worst, requires replacement of the catalyst with the
associated costs. It is therefore highly desirable to find ways to
lengthen the useful life of a catalyst.
SUMMARY OF THE INVENTION
[0013] One aspect of the present invention is a catalyst for the
manufacture of alkylene oxide by the vapor-phase epoxidation of
alkene, said catalyst containing impregnated silver and at least
one efficiency-enhancing promoter on a refractory solid support,
said support incorporating a sufficient amount of zirconium
component to enhance at least one of catalyst activity, efficiency
and stability as compared to a similar catalyst which does not
contain the zirconium component, said zirconium component being
present in the support substantially as zirconium silicate.
[0014] Another aspect of the present invention is the catalyst
described above wherein the refractory solid support is
alpha-alumina, particularly having a unique morphology consisting
of interlocking platelets.
[0015] Yet another aspect of the present invention is the process
for the manufacture of alkylene oxide, such as ethylene oxide or
propylene oxide, by the vapor-phase epoxidation of alkene using the
improved catalyst of this invention.
[0016] While the present invention should be understood as being
unconstrained by any particular theory, it is believed that the
zirconium silicate (commonly referred to as zircon), added as an
ingredient with other raw materials used to form the carrier
support, survives the rigors of the calcining process without being
oxidized or otherwise undergoing a substantial chemical change, and
thereby becomes an integral part of the modified carrier,
ultimately contributing to the favorable and unexpected
characteristics observed in catalysts of the present invention
employing such modified carriers.
[0017] A key distinguishing feature of the present invention is the
use of zirconium silicate with other raw materials to modify the
inert, refractory solid support (such as alpha-alumina) used as a
carrier in a manner described herein, prior to depositing silver
thereon with a well known promoter (and other optional additives)
to convert the carrier to a catalyst. Zirconium silicate is
employed in such a way and in sufficient amount that its presence
in the modified carrier ultimately enhances the activity,
efficiency and/or stability of the resultant catalyst of the
present invention. Zirconium silicate remains substantially the
same chemically throughout various preparation steps (including
multiple calcining or roasting steps involving relatively high
temperatures noted herein) for making the catalyst of the present
invention, from its initial introduction as a part of raw materials
for the modified carrier to the finished catalyst.
DETAILED DESCRIPTION OF THE INVENTION
[0018] Alkylene oxides made using the catalysts of this invention
are characterized by the structural formula
##STR00001##
wherein R.sup.1 and R.sup.2 are lower alkyl, for example, methyl or
ethyl or, preferably, hydrogen. Most preferably, the alkylene oxide
is ethylene oxide. The alkylene oxides are made from the
corresponding alkene, that is, R.sup.1HC.dbd.CHR.sup.2. The
following discussion is presented in terms of and with reference to
ethylene oxide and ethylene for the sake of simplicity and
illustration. However, the scope and range of the present invention
is generally applicable to catalysts for the epoxidation of
suitable alkenes.
[0019] In commercially useful catalysts for the production of
ethylene oxide, the carrier upon which the silver and promoters
reside must have a physical form and strength to allow proper flow
of gaseous reactants, products and ballast through the reactor
while maintaining physical integrity over catalyst life.
Significant catalyst breakage or abrasion is highly undesirable
because of the pressure drop and safety problems such degradation
can cause. The catalyst must also be able to withstand fairly large
temperature fluctuations within the reactor. The pore structure and
chemical inertness of the carrier are also important factors that
must be considered for optimum catalyst performance. Refractory
materials, particularly alpha-alumina, have been successfully used
as the carrier for ethylene oxide catalysts. Other porous
refractory carrier or materials may also be used as long as they
are relatively inert in the presence of the reactant feeds
introduced for epoxidation and the product epoxide, and are able to
withstand preparation conditions when converted into catalyst. For
example, carriers may be composed of alpha-alumina, silicon
carbide, silicon dioxide, zirconia, magnesia, various clays and
mixtures thereof.
[0020] The catalyst of the present invention which is useful for
the production of an alkylene oxide, such as ethylene oxide, from
alkene, such as ethylene, is supported on a zircon-modified
carrier. Zircon, a naturally occurring material which is also known
as zirconium silicate, has the chemical formula of ZrSiO.sub.4.
Zircon may also be prepared synthetically, following a number of
well-known procedures such as that given in R. Valero, B. Durand,
J-L. Guth, T. Chopin, "Hydrothermal Synthesis of Porous Zircon in
Basic Fluorinated Medium," Microporous and Mesoporous Materials,
Vol. 29 (1999) p. 311-318. In general, the carriers are made up of
an inert, refractory support, such as alpha-alumina, having a
porous structure and relatively high surface area, which has been
modified by the presence of zirconium silicate introduced with the
other raw materials used to produce the carrier. In preparing a
catalyst of the present invention, silver is deposited throughout
the pores of the carrier and reduced to silver metal. Promoters,
such as alkali salts, can be added with the soluble silver mixture
impregnated into the carrier or added in a separate step. These
promoters are generally associated with silver, although they may
also be present on the carrier. The promoters act to improve
catalyst efficiency, activity and/or stability.
[0021] The raw materials for the carrier must be of sufficient
purity so that there is limited reaction between any components
thereof and the zirconium silicate to be added during the
preparation of the carrier in accordance with the teachings of the
present invention. Limiting such reaction ensures that the added
zirconium silicate remains substantially unchanged chemically
throughout the processing of the carrier and the conversion of the
carrier into the catalyst. Even the partial decomposition of
zirconium silicate to zirconium oxide (ZrO.sub.2) is a particularly
undesirable reaction, which decreases significantly the benefits
from the addition of zirconium silicate to the carrier. At higher
zirconium silicate concentrations, the presence of zirconium
silicate may be easily ascertained by the use of X-ray diffraction
analysis of the fired carrier. At lower zirconium silicate
concentrations, zirconium silicate may not be detectable by the
same analysis. However, the presence of zirconium and silicon may
be detected using elemental analyses, such as X-ray fluorescence.
In any case, the beneficial effect on catalyst performance and life
are the primary indicator of the presence of zirconium silicate,
especially at lower zirconium silicate concentrations.
[0022] In addition, the zircon itself must be of sufficient purity
so that any impurities therein do not promote decomposition of
zircon to zirconia during the preparation of the carrier.
Impurities in zircon comprise primarily the inorganic compounds of
transition metals (excluding zirconium and halfnium, which
naturally occurs with zirconium), and are preferably limited to not
more than 1.5 wt. %. More common inorganic compounds of transition
metals occurring as impurities in zircon are oxides of transition
metals. Two of the common oxide impurities are titania and iron
oxides.
[0023] In the present invention, the zircon is mixed with the other
raw materials for the carrier prior to the final firing at high
temperature. The zircon may be incorporated in any number of ways,
including the adding of the zircon in the form of powder or flour
to the other dry raw materials, followed by mixing and adding of
liquid raw materials. The order of addition of the zircon to the
other raw materials is not critical.
[0024] Suitable shapes for the carrier of this invention include
any of the wide variety of shapes known for such catalyst supports,
including pills, chunks, tablets, pieces, pellets, rings, spheres,
wagon wheels, toroids having star shaped inner and/or outer
surfaces, and the like, of a size suitable for employment in fixed
bed reactors. Conventional commercial fixed bed ethylene oxide
reactors are typically in the form of a plurality of parallel
elongated tubes (in a suitable shell) about 1 to 3 inches O.D. and
15-45 feet long filled with catalyst. In such fixed bed reactors,
it is desirable to employ carrier formed into a rounded shape, such
as, for example, spheres, pellets, rings, tablets and the like,
having diameters from about 0.1 inch to about 0.8 inch.
[0025] There are many well-known methods of preparing carriers
suitable for use in ethylene oxide catalysts. Some of such methods
are described in, for example, U.S. Pat. Nos. 4,379,134; 4,806,518;
5,063,195; 5,384,302, U.S. Patent Application 20030162655 and the
like. As long as the carrier materials and method of preparation do
not substantially decompose zircon, these methods can be employed
to prepare the zircon modified carrier of the present invention.
For example, an alpha-alumina support of at least 95% purity
(exclusive of zirconium component) can be prepared by compounding
(mixing) the raw materials, extrusion, drying and a high
temperature calcination. In this case, the starting raw materials
usually include one or more alpha-alumina powder(s) with different
properties, a clay-type material which may be added as binder to
provide physical strength, and a burnout material (usually an
organic compound) used in the mix to provide desired porosity after
its removal during the calcination step. The levels of impurities
in the finished carrier are determined by the purity of the raw
materials used, and their degree of volatilization during the
calcination step. Common impurities may include silica, alkali and
alkaline earth metal oxides and trace amounts of metal and/or
non-metal-containing additives.
[0026] Another method for preparing a carrier of this invention
having particularly suitable properties for ethylene oxide catalyst
usage comprises mixing zirconium silicate with boehmite alumina
(AlOOH) and/or gamma-alumina, peptizing the aluminas with a mixture
containing an acidic component and halide anions (preferably
fluoride anions) to provide peptized halogenated alumina, forming
(for example, by extruding or pressing) the peptized halogenated
alumina to provide formed peptized halogenated alumina, drying the
formed peptized halogenated alumina to provide dried formed
alumina, and calcining the dried formed alumina to provide pills of
modified alpha-alumina carrier.
[0027] The modified alpha-alumina carrier prepared by the method
described above preferably has a specific surface area of at least
about 0.5 m.sup.2/g (more preferably from about 0.7 m.sup.2/g to
about 10 m.sup.2/g), a pore volume of at least about 0.5 cc/g (more
preferably from about 0.5 cc/g to about 2.0 cc/g), purity
(exclusive of zirconium component) of at least 99 weight percent
alpha-alumina, and median pore diameter from about 1 to about 50
microns. In this case, the modified alpha-alumina carrier comprises
particles each of which has at least one substantially flat major
surface having a lamellate or platelet morphology which
approximates the shape of a hexagonal plate (some particles having
two or more flat surfaces), at least 50% of which (by number) have
a major dimension of less than about 50 microns.
[0028] In the finished carrier of the present invention, including
those prepared by the two particular methods described above as a
way of illustration, zirconium silicate is present in an amount
which is preferably in the range of from about 0.01 to about 10.0%
by weight, more preferably from about 0.1 to about 5.0% by weight,
and most preferably from about 0.3 to about 3.0% based on the total
weight of the finished modified alumina carrier.
[0029] While the invention is not constrained by any particular
theory, the raw materials used to manufacture the carrier should
not contain large amounts of reactive calcium compounds in order to
minimize the reaction of these species with the added zirconium
silicate, resulting in the formation of less beneficial species,
particularly zirconia (ZrO.sub.2, also called zirconium oxide). The
cumulative concentration of calcium compounds in carrier raw
materials should be limited so that the fired carrier (excluding
zirconium component) contains less than 2000 ppmw calcium,
preferably less than 350 ppmw calcium.
[0030] In addition, certain other alkaline earth metal compounds
may also promote the decomposition of zirconium silicate to
zirconia. The cumulative concentration of alkaline earth metal
compounds in carrier raw materials should be limited so that the
fired carrier (excluding zirconium component) contains less than
500 ppmw alkaline earth metal (excluding calcium compounds),
measured as the alkaline earth metal oxide.
[0031] The calcination temperature (firing temperature) of the
carrier must also be controlled to limit the thermal decomposition
of zircon to zirconia which occurs in the pure state at
temperatures above 1540.degree. C.
[0032] Catalysts for the production of alkylene oxide, for example
ethylene oxide or propylene oxide, may be prepared on the modified
supports of the present invention by impregnating the carrier with
a solution of one or more silver compounds, as is well known in the
art. One or more promoters may be impregnated simultaneously with
the silver impregnation, before the silver impregnation and/or
after the silver impregnation. In making such a catalyst, the
carrier is impregnated (one or more times) with one or more silver
compound solutions sufficient to allow the silver to be supported
on the carrier in an amount which ranges from about 1 to about 70%,
more preferably from about 5 to about 50%, most preferably from
about 10 to about 40% of the weight of the catalyst.
[0033] Although silver particle size is important, the range is not
narrow. Suitable silver particle size can be in the range of from
about 100 to 10,000 angstroms.
[0034] There are a variety of known promoters, that is, materials
which, when present in combination with particular catalytic
materials, for example, silver, benefit one or more aspect of
catalyst performance or otherwise act to promote the catalyst's
ability to make a desired product, for example ethylene oxide or
propylene oxide. Such promoters in themselves are generally not
considered catalytic materials. The presence of such promoters in
the catalyst has been shown to contribute to one or more beneficial
effects on the catalyst performance, for example enhancing the rate
or amount of production of desired product, reducing the
temperature required to achieve a suitable rate of reaction,
reducing the rates or amounts of undesired reactions, etc.
Competing reactions occur simultaneously in the reactor, and a
critical factor in determining the effectiveness of the overall
process is the measure of control one has over these competing
reactions. A material which is termed a promoter of a desired
reaction can be an inhibitor of another reaction, for example a
combustion reaction. What is significant is that the effect of the
promoter on the overall reaction is favorable to the efficient
production of the desired product, for example ethylene oxide. The
concentration of the one or more promoters present in the catalyst
may vary over a wide range depending on the desired effect on
catalyst performance, the other components of a particular
catalyst, the physical and chemical characteristics of the carrier,
and the epoxidation reaction conditions.
[0035] There are at least two types of promoters--solid promoters
and gaseous promoters. A solid promoter is incorporated into the
catalyst prior to its use, either as a part of the carrier (that is
support) or as a part of the silver component applied thereto. When
a solid promoter is added during the preparation of the catalyst,
the promoter may be added to the carrier before the silver
component is deposited thereon, added simultaneously with the
silver component, or added sequentially following the deposition of
the silver component on the carrier. Examples of well-known solid
promoters for catalysts used to produce ethylene oxide include
compounds of potassium, rubidium, cesium, rhenium, sulfur,
manganese, molybdenum, and tungsten. During the reaction to make
ethylene oxide, the specific form of the promoter on the catalyst
may be unknown.
[0036] In contrast, the gaseous promoters are gas-phase compounds
and or mixtures thereof which are introduced to a reactor for the
production of alkylene oxide (for example ethylene oxide) with
vapor-phase reactants, such as ethylene and oxygen. Such promoters,
also called modifiers, inhibitors or enhancers, further enhance the
performance of a given catalyst, working in conjunction with or in
addition to the solid promoters. One or more chlorine-containing
components are typically employed as gaseous promoters, as is well
known in the art. Other halide-containing components may also be
used to produce a similar effect. Depending on the composition of
the solid catalyst being employed, one or more gaseous components
capable of generating at least one efficiency-enhancing member of a
redox half reaction pair may be employed as gaseous promoters, as
is well known in the art. The preferred gaseous component capable
of generating an efficiency-enhancing member of a redox half
reaction pair is preferably a nitrogen-containing component.
[0037] The solid promoters are generally added as chemical
compounds to the catalyst prior to its use. As used herein, the
term "compound" refers to the combination of a particular element
with one or more different elements by surface and/or chemical
bonding, such as ionic and/or covalent and/or coordinate bonding.
The term "ionic" or "ion" refers to an electrically charged
chemical moiety; "cationic" or "cation" being positive and
"anionic" or "anion" being negative. The term "oxyanionic" or
"oxyanion" refers to a negatively charged moiety containing at
least one oxygen atom in combination with another element. An
oxyanion is thus an oxygen-containing anion. It is understood that
ions do not exist in vacuo, but are found in combination with
charge-balancing counter ions when added as a compound to the
catalyst. Once in the catalyst, the form of the promoter is not
always known, and the promoter may be present without the
counterion added during the preparation of the catalyst. For
example, a catalyst made with cesium hydroxide may be analyzed to
contain cesium but not hydroxide in the finished catalyst.
Likewise, compounds such as alkali metal oxide, for example cesium
oxide, or transition metal oxides, for example MoO.sub.3, while not
being ionic, may convert to ionic compounds during catalyst
preparation or in use. For the sake of ease of understanding, the
solid promoters will be referred to in terms of cations and anions
regardless of their form in the catalyst under reaction
conditions.
[0038] It is desirable that the silver and optional one or more
solid promoters be relatively uniformly dispersed on the
zircon-modified carrier. A preferred procedure for depositing
silver catalytic material and one or more promoters comprises: (1)
impregnating a porous zircon-modified carrier according to the
present invention with a solution comprising a solvent or
solubilizing agent, silver complex and one or more promoters, and
(2) thereafter treating the impregnated carrier to convert the
silver salt to silver metal and effect deposition of silver and the
promoter(s) onto the exterior and interior pore surfaces of the
carrier. Silver and promoter depositions are generally accomplished
by heating the carrier at elevated temperatures to evaporate the
liquid within the carrier and effect deposition of the silver and
promoters onto the interior and exterior carrier surfaces.
Impregnation of the carrier is the preferred technique for silver
deposition because it utilizes silver more efficiently than coating
procedures, the latter being generally unable to effect substantial
silver deposition onto the interior surfaces of the carrier. In
addition, coated catalysts are more susceptible to silver loss by
mechanical abrasion.
[0039] The silver solution used to impregnate the carrier is
preferably comprised of a silver compound in a solvent or
complexing/solubilizing agent such as the silver solutions
disclosed in the art. The particular silver compound employed may
be chosen, for example, from among silver complexes, silver
nitrate, silver oxide or silver carboxylates, such as silver
acetate, oxalate, citrate, phthalate, lactate, propionate, butyrate
and higher fatty acid salts. Silver oxide complexed with amines is
a preferred form of silver for use in the present invention.
[0040] A wide variety of solvents or complexing/solubilizing agents
may be employed to solubilize silver to the desired concentration
in the impregnating medium. Among those disclosed as being suitable
for this purpose are lactic acid; ammonia; alcohols, such as
ethylene glycol; and amines and aqueous mixtures of amines.
[0041] For example, Ag.sub.2O can be dissolved in a solution of
oxalic acid and ethylenediamine to an extent of approximately 30%
by weight. Vacuum impregnation of such a solution onto a carrier of
approximately 0.7 cc/g porosity typically results in a catalyst
containing approximately 25% by weight of silver based on the
entire weight of the catalyst. Accordingly, if it is desired to
obtain a catalyst having a silver loading of greater than about 25
or 30%, and more, it would generally be necessary to subject the
carrier to at least two or more sequential impregnations of silver,
with or without promoters, until the desired amount of silver is
deposited on the carrier. In some instances, the concentration of
the silver salt is higher in the latter impregnation solutions than
in the first. In other instances, approximately equal amounts of
silver are deposited during each impregnation. Often, to effect
equal deposition in each impregnation, the silver concentration in
the subsequent impregnation solutions may need to be greater than
that in the initial impregnation solutions. In further instances, a
greater amount of silver is deposited on the carrier in the initial
impregnation than that deposited in subsequent impregnations. Each
of the impregnations may be followed by roasting or other
procedures to render the silver insoluble.
[0042] The catalyst prepared on the zircon-modified carrier may
contain alkali metal and/or alkaline earth metal as cation
promoters. Exemplary of the alkali metal and/or alkaline earth
metals are lithium, sodium, potassium, rubidium, cesium, beryllium,
magnesium, calcium, strontium and barium. Other cation promoters
include Group 3b metal ions including lanthanide series metals. In
some instances, the promoter comprises a mixture of cations, for
example cesium and at least one other alkali metal, to obtain a
synergistic efficiency enhancement as described in U.S. Pat. No.
4,916,243, herein incorporated by reference. Note that references
to the Periodic Table herein shall be to that as published by the
Chemical Rubber Company, Cleveland, Ohio, in CRC Handbook of
Chemistry and Physics, 46th Edition, inside back cover.
[0043] The concentration of the alkali metal promoters in the
finished catalyst is not narrow and may vary over a wide range. The
optimum alkali metal promoter concentration for a particular
catalyst will be dependent upon performance characteristics, such
as catalyst efficiency, rate of catalyst aging and reaction
temperature.
[0044] The concentration of alkali metal (based on the weight of
cation, for example cesium) in the finished catalyst may vary from
about 0.0005 to 1.0 wt. %, preferably from about 0.005 to 0.5 wt.
%. The preferred amount of cation promoter deposited on or present
on the surface of the carrier or catalyst generally lies between
about 10 and about 4000, preferably about 15 and about 3000, and
more preferably between about 20 and about 2500 ppm by weight of
cation calculated on the total carrier material. Amounts between
about 50 and about 2000 ppm are frequently most preferable. When
the alkali metal cesium is used in mixture with other cations, the
ratio of cesium to any other alkali metal and alkaline earth metal
salt(s), if used, to achieve desired performance is not narrow and
may vary over a wide range. The ratio of cesium to the other cation
promoters may vary from about 0.0001:1 to 10,000:1, preferably from
about 0.001:1 to 1,000:1. Preferably, cesium comprises at least
about 10, more preferably, about 20 to 100, percent (weight) of the
total added alkali metal and alkaline earth metal in finished
catalysts using cesium as a promoter.
[0045] Examples of some of the anion promoters which may be
employed with the present invention include the halides, for
example fluorides and chlorides, and the oxyanions of the elements
other than oxygen having an atomic number of 5 to 83 of Groups 3b
to 7b and 3a to 7a of the Periodic Table. One or more of the
oxyanions of nitrogen, sulfur, manganese, tantalum, molybdenum,
tungsten and rhenium may be preferred for some applications.
[0046] The types of anion promoters or modifiers suitable for use
in the catalysts of this invention comprise, by way of example
only, oxyanions such as sulfate, SO.sub.4.sup.-2, phosphates, for
example, PO.sub.4.sup.-3, titanates, e.g., TiO.sub.3.sup.-2,
tantalates, for example, Ta.sub.2O.sub.6.sup.-2, molybdates, for
example, MoO.sub.4.sup.-2, vanadates, for example,
V.sub.2O.sub.4.sup.-2, chromates, for example, CrO.sub.4.sup.-2,
zirconates, for example, ZrO.sub.3.sup.-2, polyphosphates,
manganates, nitrates, chlorates, bromates, borates, silicates,
carbonates, tungstates, thiosulfates, cerates and the like. The
halides may also be present, including fluoride, chloride, bromide
and iodide.
[0047] It is well recognized that many anions have complex
chemistries and may exist in one or more forms, for example,
orthovanadate and metavanadate; and the various molybdate oxyanions
such as MoO.sub.4.sup.2, and Mo.sub.7O.sub.24 and
Mo.sub.2O.sub.7.sup.-2. The oxyanions may also include mixed
metal-containing oxyanions including polyoxyanion structures. For
instance, manganese and molybdenum can form a mixed metal oxyanion.
Similarly, other metals, whether provided in anionic, cationic,
elemental or covalent form may enter into anionic structures.
[0048] While an oxyanion, or a precursor to an oxyanion, may be
used in solutions impregnating a carrier, it is possible that
during the conditions of preparation of the catalyst and/or during
use, the particular oxyanion or precursor initially present may be
converted to another form. Indeed, the element may be converted to
a cationic or covalent form. In many instances, analytical
techniques may not be sufficient to precisely identify the species
present. The invention is not intended to be limited by the exact
species that may ultimately exist on the catalyst during use.
[0049] When the promoter comprises rhenium, the rhenium component
can be provided in various forms, for example, as the metal, as a
covalent compound, as a cation or as an anion. The rhenium species
that provides the enhanced efficiency and/or activity is not
certain and may be the component added or that generated either
during preparation of the catalyst or during use as a catalyst.
Examples of rhenium compounds include the rhenium salts such as
rhenium halides, the rhenium oxyhalides, the rhenates, the
perrhenates, the oxides and the acids of rhenium. However, the
alkali metal perrhenates, ammonium perrhenate, alkaline earth metal
perrhenates, silver perrhenates, other perrhenates and rhenium
heptoxide can also be suitably utilized. Rhenium heptoxide,
Re.sub.2O.sub.7, when dissolved in water, hydrolyzes to perrhenic
acid, HReO.sub.4, or hydrogen perrhenate. Thus, for purposes of
this specification, rhenium heptoxide can be considered to be a
perrhenate, that is, ReO.sub.4. Similar chemistries can be
exhibited by other metals such as molybdenum and tungsten.
[0050] Another class of promoters, which may be employed with the
present invention, includes manganese components. In many
instances, manganese components can enhance the activity,
efficiency and/or stability of catalysts. The manganese species
that provides the enhanced activity, efficiency and/or stability is
not certain and may be the component added or that generated either
during catalyst preparation or during use as a catalyst. Manganese
components include, but are not limited to, manganese acetate,
manganese ammonium sulfate, manganese citrate, manganese
dithionate, manganese oxalate, manganous nitrate, manganous
sulfate, and manganate anion, for example permanganate anion, and
the like. To stabilize the manganese component in certain
impregnating solutions, it may be necessary to add a chelating
compound such as ethylenediaminetetraacetic acid (EDTA) or a
suitable salt thereof.
[0051] The amount of anion promoter may vary widely, for example,
from about 0.0005 to 2 wt. %, preferably from about 0.001 to 0.5
wt. % based on the total weight of the catalyst. When used, the
rhenium component is often provided in an amount of at least about
1, say, at least about 5, for example, about 10 to 2000, often
between 20 and 1000, ppmw calculated as the weight of rhenium based
on the total weight of the catalyst.
[0052] The promoters for catalyst employing the present invention
may also be of the type comprising at least one
efficiency-enhancing salt of a member of a redox-half reaction pair
which is employed in an epoxidation process in the presence of a
gaseous nitrogen-containing component capable of forming a gaseous
efficiency-enhancing member of a redox-half reaction pair under
reaction conditions. The term "redox-half reaction" is defined
herein to mean half-reactions like those found in equations
presented in tables of standard reduction or oxidation potentials,
also known as standard or single electrode potentials, of the type
found in, for instance, "Handbook of Chemistry", N. A. Lange,
Editor, McGraw-Hill Book Company, Inc., pages 1213-1218 (1961) or
"CRC Handbook of Chemistry and Physics", 65th Edition, CRC Press,
Inc., Boca Raton, Fla., pages D155-162 (1984). The term "redox-half
reaction pair" refers to the pairs of atoms, molecules or ions or
mixtures thereof which undergo oxidation or reduction in such
half-reaction equations. Such terms as redox-half reaction pairs
are used herein to include those members of the class of substance
which provide the desired performance enhancement, rather than a
mechanism of the chemistry occurring. Preferably, such compounds,
when associated with the catalyst as salts of members of a half
reaction pair, are salts in which the anions are oxyanions,
preferably an oxyanion of a polyvalent atom; that is, the atom of
the anion to which oxygen is bonded is capable of existing, when
bonded to a dissimilar atom, in different valence states. As used
herein, the term "salt" does not indicate that the anion and cation
components of the salt be associated or bonded in the solid
catalyst, but only that both components be present in some form in
the catalyst under reaction conditions. Potassium is the preferred
cation, although sodium, rubidium and cesium may also be operable,
and the preferred anions are nitrate, nitrite and other anions
capable of undergoing displacement or other chemical reaction and
forming nitrate anions under epoxidation conditions. Preferred
salts include KNO.sub.3 and KNO.sub.2, with KNO.sub.3 being most
preferred.
[0053] The salt of a member of a redox-half reaction pair is added
to the catalyst in an amount sufficient to enhance the efficiency
of the epoxidation reaction. The precise amount will vary depending
upon such variables as the gaseous efficiency-enhancing member of a
redox-half reaction used and concentration thereof, the
concentration of other components in the gas phase, the amount of
silver contained in the catalyst, the surface area of the support,
the process conditions, for example space velocity and temperature,
and morphology of support. Alternatively, a suitable precursor
compound may also be added such that the desired amount of the salt
of a member of a redox-half reaction pair is formed in the catalyst
under epoxidation conditions, especially through reaction with one
or more of the gas-phase reaction components. Generally, however, a
suitable range of concentration of the added efficiency-enhancing
salt, or precursor thereof, calculated as cation, is about 0.01 to
about 5%, preferably about 0.02 to about 3%, by weight, based on
the total weight of the catalyst. Most preferably the salt is added
in an amount of about 0.03 to about 2 wt. %.
[0054] The preferred gaseous efficiency-enhancing members of
redox-half reaction pairs are compounds containing an element
capable of existing in more than two valence states, preferably
nitrogen and another element which is, preferably, oxygen. The
gaseous component capable of producing a member of a redox-half
reaction pair under reaction conditions is a generally a
nitrogen-containing gas, such as for example nitric oxide, nitrogen
dioxide and/or dinitrogen tetroxide, hydrazine, hydroxylainine or
ammonia, nitroparaffins (for example, nitromethane), nitroaromatic
compounds (for example nitrobenzene), N-nitro compounds, and
nitrites (for example, acetonitrile). The amount of
nitrogen-containing gaseous promoter to be used in these catalysts
is that amount sufficient to enhance the performance, such as the
activity of the catalyst and particularly the efficiency of the
catalyst. The concentration of the nitrogen-containing gaseous
promoter is determined by the particular efficiency-enhancing salt
of a member of a redox-half reaction pair used and the
concentration thereof, the particular alkene undergoing oxidation,
and by other factors including the amount of carbon dioxide in the
inlet reaction gases. For example, U.S. Pat. No. 5,504,053
discloses that when the nitrogen-containing gaseous promoter is NO
(nitric oxide), a suitable concentration is from about 0.1 to about
100 ppm, by volume, of the gas stream.
[0055] Although in some cases it is preferred to employ members of
the same half-reaction pair in the reaction system, that is, both
the efficiency-enhancing salt promoter associated with the catalyst
and the gaseous promoter member in the feedstream, as, for example,
with a preferred combination of potassium nitrate and nitric oxide,
this is not necessary in all cases to achieve satisfactory results.
Other combinations, such as KNO.sub.2/N.sub.2O.sub.3,
KNO.sub.3/NO.sub.2, KNO.sub.3/N.sub.2O.sub.4, KNO.sub.2/NO,
KNO.sub.2/NO.sub.2 may also be employed in the same system. In some
instances, the salt and gaseous members may be found in different
half-reactions which represent the first and last reactions in a
series of half-reaction equations of an overall reaction.
[0056] In any event, the solid and/or gaseous promoters are
provided in a promoting amount. As used herein the term "promoting
amount" of a certain component of a catalyst refers to an amount of
that component that works effectively to provide an improvement in
one or more of the catalytic properties of that catalyst when
compared to a catalyst not containing said component. Examples of
catalytic properties include, inter alia, operability (resistance
to run-away), selectivity, activity, conversion, stability and
yield. It is understood by one skilled in the art that one or more
of the individual catalytic properties may be enhanced by the
"promoting amount" while other catalytic properties may or may not
be enhanced or may even be diminished. It is further understood
that different catalytic properties may be enhanced at different
operating conditions. For example, a catalyst having enhanced
selectivity at one set of operating conditions may be operated at a
different set of conditions wherein the improvement shows up in the
activity rather than the selectivity and an operator of an ethylene
oxide plant will intentionally change the operating conditions in
order to take advantage of certain catalytic properties even at the
expense of other catalytic properties in order to maximize profits
by taking into account feedstock costs, energy costs, by-product
removal costs and the like.
[0057] The promoting effect provided by the promoters can be
affected by a number of variables such as for example, reaction
conditions, catalyst preparative techniques, surface area and pore
structure and surface chemical properties of the support, the
silver and co-promoter content of the catalyst, the presence of
other cations and anions present on the catalyst. The presence of
other activators, stabilizers, promoters, enhancers or other
catalyst improvers can also affect the promoting effects.
Ethylene Epoxidation Process Conditions
[0058] A standard back-mixed autoclave with gas recycle is used for
catalyst testing. There is some variation in gas phase feed
concentrations depending on the process conditions used. Two cases
are illustrated: air process conditions, which simulate typical
conditions employed in commercial air-type ethylene epoxide
processes where air is used to supply molecular oxygen, and oxygen
process conditions, which simulate typical conditions in commercial
oxygen-type ethylene oxide processes where pure oxygen is added as
the oxygen source. Each case provides a different efficiency but it
is the rule for practically all cases that with air as the oxygen
feed, lower amounts of oxygen and ethylene are used which will
yield an efficiency to ethylene epoxide which is about 2 to 5
percentage points lower than that when pure oxygen is employed as
oxygen source. Well known, back-mixed, bottom-agitated "Magnedrive"
autoclaves described in FIG. 2 of the paper by J. M. Berty entitled
"Reactor for Vapor Phase-Catalytic Studies," in Chemical
Engineering Progress, Vol. 70, No. 5, pages 78-84, 1974, are used
as one of the reactors. The inlet conditions include the
following:
TABLE-US-00001 TABLE I Ethylene Epoxidation Inlet Process
Conditions Air Process Oxygen Process Oxygen Process Conditions-I
Conditions-I Conditions-II Component Mole % Mole % Mole % Ethylene
11.0 30.0 30.0 Oxygen 7.0 8.0 8.0 Ethane 0.00-0.24 0.5 0.0 Carbon
Dioxide 5.5 6.5 0.0 Nitrogen Balance of gas Balance of gas Balance
of gas Parts per million Optimum for Optimum for Optimum for Ethyl
Chloride Efficiency Efficiency Efficiency Parts per million None
None Optimum for Nitric Oxide Efficiency Type of Reactor CSTR CSTR
CSTR Amount of 80 cc 80 cc 40 cc Catalyst Total Inlet 22.6 SCFH
22.6 SCFH 11.3 SCFH Flow Rate
[0059] The pressure is maintained at about 275 psig (pounds per
square inch, gauge) and the total flow is maintained at about 11.3
or 22.6 SCFH (Standard Cubic Feet per Hour). SCFH refers to cubic
feet per hour at standard temperature and pressure, namely,
0.degree. C. and one atmosphere. Ethyl chloride concentration is
adjusted to achieve maximum efficiency. Temperature (.degree. C.)
and catalyst efficiency are obtained as the responses describing
the catalyst performance.
[0060] The catalyst test procedure used for autoclaves in the
Ethylene Epoxidation Process Conditions involves the following: 40
or 80 cc of catalyst is charged to the back-mixed autoclave and the
weight of the catalyst is noted. The back-mixed autoclave is heated
to about reaction temperature in a nitrogen flow of 10 or 20 SCFH
with the fan operating at 1500 rpm. The nitrogen flow is then
discontinued and the above-described feed stream is introduced into
the reactor. The total gas inlet flow is then adjusted to 11.3 SCFH
for 40 cc of catalyst or 22.6 SCFH for 80 cc of catalyst. The
temperature is adjusted over the next few hours to provide the
desired percent outlet ethylene oxide and the optimum efficiency is
obtained by adjusting ethyl chloride. The outlet epoxide
concentration is monitored to make certain that the catalyst has
reached its peak steady state performance. The ethyl chloride is
periodically adjusted, and the efficiency of the catalyst to
ethylene oxide and the rate of deactivation (temperature rise) is
thus obtained. In determining activity and efficiency, the process
and catalyst should be under steady state conditions.
[0061] The standard deviation of a single test result reporting
catalyst efficiency in accordance with the procedure described
above is about 0.3% efficiency units. The typical standard
deviation of a single test result reporting catalyst activity in
accordance with the procedure described above is about 1.2.degree.
C. The standard deviation, of course, will depend upon the quality
of the equipment and precision of the techniques used in conducting
the tests, and thus will vary. These standard deviations are
believed to apply to the test results reported herein.
[0062] The properties of the starting carrier materials and the
specifics of their modifications are detailed in Table II. Table
III sets forth the specifics of the catalyst preparations on the
carriers of Table II, including catalyst compositions.
Carrier Preparations
[0063] Carriers of the examples were prepared in the following
manner. Zirconium silicate (if used) was added with other solid raw
materials to obtain a dry mixture. In all cases where zircon was
used, it was introduced in a powder form with a median particle
size of about 130 microns. Liquids and additional dry raw materials
(optional) were then added. The amounts of such additives are
expressed as percentages by weight of the starting dry mixture.
Water was also added in an amount sufficient to obtain an
extrudable mixture. Such amount depends on a number of factors,
such as ambient humidity, hydration level of the raw materials,
etc. Unless otherwise noted in the following descriptions, the
mixture was extruded as cylinders with a single opening along the
axis, or as multi-partitioned cylinders. After drying, the extruded
greenware was fired to alpha-alumina under conditions chosen to
ensure complete conversion of the extrudates to alpha-alumina.
Firing temperatures between 1000.degree. C. and 1400.degree. C. and
firing times from 45 minutes to 5 hours were used. Outer diameter
dimensions of the fired greenware were 0.31-0.35 inches, cylinder
lengths 0.29-0.34 inches, and the wall thickness of the
multi-partitioned cylinders no greater than 0.075 inches. Physical
properties and the approximate weight percent of zircon in the
modified carriers and comparative carriers are given in Table II.
All percentages in the following descriptions are in weight
percent.
TABLE-US-00002 TABLE II Carrier Properties Carrier ID A B C D E F G
H I J K L M Surface Area 1.04 1.18 1.29 1.01 1.46 1.19 0.49 0.63
0.94 0.60 0.52 0.49 0.54 (m.sup.2/g) Packing 32.5 33.1 34.2 34.6
25.5 24.9 27.4 25.5 25.4 31.4 32.5 30.4 30.5 Density (lb/ft.sup.3)
Pore Volume 0.68 0.63 0.61 0.66 0.76 0.80 0.59 0.77 0.92 0.53 0.56
0.63 0.65 (cc/g) Zircon Target 2 0 1 0 2 0 3 0.3 0 2 0 2 0 (Wt.
%)
[0064] Carrier A was prepared from calcined alumina which
originally contained 0.06 wt. % CaO. The alumina was combined with
a 10% acetic acid solution and heated at 100.degree. C. for 15
minutes with stirring, then filtered and vacuum rinsed twice with
hot deionized water. The leached alumina was dried overnight at
100.degree. C. and was found to contain 0.03% CaO. A dry mixture
was prepared from 71.1% of the leached alumina, 22.8% organic
pore-forming burnout, 4.5% extrusion aids, <1% flux material and
1.4% granular zircon. 2.1 Percent additional extrusion aid and
<1% surfactant were added as aqueous slurries with sufficient
water to form an extrudable blend. This blend was extruded, dried
and fired to alpha alumina. The final sample contained about 2%
zircon.
[0065] Comparative Carrier B was prepared in a similar manner as
Carrier A except that no zircon was added.
[0066] Carrier C was prepared by blending a dry mixture of 79.2%
pseudoboehmite, 19.8% gamma-alumina and 1% granular zircon. 5.5%
formic acid and 4.6% ammonium bifluoride were added as aqueous
solutions with sufficient water to form an extrudable blend. After
mixing, the blend was extruded, dried and fired to alpha-alumina.
The final sample contained about 1% zircon.
[0067] Comparative Carrier D was prepared using the same procedure
as that given above for Carrier C except that no zircon was
added.
[0068] Carrier E was prepared by blending a dry mixture of 75.5%
gibbsite, 22.5% pseudoboehmite, and 2.0% granular zircon. To this
dry mixture was added 31.4% graphite with a particle size less than
600 microns. 1.7% Ammonium fluoride, 2.0% magnesium nitrate hydrate
and 1.2% nitric acid were then added as aqueous solutions with an
appropriate amount of water to form an extrudable blend. After
mixing, the blend was extruded, dried and fired to alpha-alumina.
The final sample contained about 2% zircon.
[0069] Comparative Carrier F was prepared using the same procedure
as that given above for Carrier E except that no zircon was
added.
[0070] Carrier G was prepared by blending a dry mixture of 74.8%
gibbsite, 22.3% pseudoboehmite and 2.9% granular zircon. To this
dry mixture was added 21.4% graphite with a particle size less than
600 microns. 4.6% Nitric acid, 1.9% magnesium nitrate hydrate and
1.6% ammonium fluoride were then added as aqueous solutions with an
appropriate amount of water to form an extrudable blend. This blend
was extruded, dried and fired to alpha alumina. The final sample
contained about 3% zircon.
[0071] Carrier H was prepared in an analogous manner to Carrier G
except that the amount of granular zircon added was sufficient to
give about 0.3% zircon by weight in the finished carrier.
[0072] Carrier I was prepared in an analogous manner to Carrier G
except that no zircon was added.
[0073] Carrier J was prepared by blending a dry mixture of 68.6%
pseudoboehmite, 29.4% gibbsite and 2.0% zircon. To this mixture was
added <1% hydroxypropyl methylcellulose. 5.4% Acetic acid and
4.0% hydrofluoric acid were then added as aqueous solutions with an
appropriate amount of water to form an extrudable blend. The
mixture was extruded, dried and fired to alpha-alumina. The final
sample contained about 2% zircon.
[0074] Comparative Carrier K was prepared in an analogous manner to
Carrier J except that no zircon was added.
[0075] Carrier L was prepared by blending a mixture of 98%
pseudoboehmite with 2% granular zircon, 5.4% formic acid and 2.1%
hydrofluoric acid were then added as aqueous solutions with an
appropriate amount of water to form an extrudable blend. The
mixture was extruded, dried and fired to alpha-alumina. The final
sample contained about 2% zircon.
[0076] Comparative Carrier M was prepared in an analogous manner to
Carrier except that no zircon was added.
Catalyst Preparations
[0077] The carriers were vacuum impregnated with a first
impregnation silver solution typically containing 30 wt. % silver
oxide, 18 wt. % oxalic acid, 17 weight percent ethylenediamine, 6
wt. % monoethanolamine, and 27 wt. % distilled water. The first
impregnation solution was typically prepared by (1) mixing 1.14
parts of ethylenediamine (high purity grade) with 1.75 parts of
distilled water; (2) slowly adding 1.16 parts of oxalic acid
dihydrate (reagent grade) to the aqueous ethylenediamine solution
such that the temperature of the solution does not exceed
40.degree. C., (3) slowly adding 1.98 parts of silver oxide, and
(4) adding 0.40 parts of monoethanolamine (Fe and Cl free).
[0078] The carrier was impregnated in an appropriately sized glass
or stainless steel cylindrical vessel which was equipped with
suitable stopcocks for impregnating the carrier under vacuum. A
suitable separatory funnel which was used for containing the
impregnating solution was inserted through a rubber stopper into
the top of the impregnating vessel. The impregnating vessel
containing the carrier was evacuated to approximately 1-2''mercury
absolute for 10 to 30 minutes, after which the impregnating
solution was slowly added to the carrier by opening the stopcock
between the separatory funnel and the impregnating vessel. After
all the solution emptied into the impregnating vessel (.about.15
seconds), the vacuum was released and the pressure returned to
atmospheric. Following addition of the solution, the carrier
remained immersed in the impregnating solution at ambient
conditions for 5 to 30 minutes, and was thereafter drained of
excess solution for 10 to 30 minutes.
[0079] The silver-impregnated carrier was then roasted as follows
to effect reduction of silver on the catalyst surface. The
impregnated carrier was spread out in a single layer on stainless
steel wire mesh trays then placed on a stainless steel belt (spiral
weave) and transported through a 2''.times.2'' square heating zone
for 2.5 minutes, or equivalent conditions were used for a larger
belt operation. The heating zone was maintained at 500.degree. C.
by passing hot air upward through the belt and about the catalyst
particles at the rate of 266 standard cubic feet per hour (SCFH).
After being roasted in the heating zone, the catalyst was cooled in
the open air to room temperature and weighed.
[0080] Next, the silver-impregnated carrier was vacuum impregnated
with a second silver impregnation solution containing both the
silver oxalate amine solution and the catalyst promoters. The
second impregnation solution was composed of all of the drained
solution from the first impregnation plus a fresh aliquot of the
first solution, or a new solution was used. The promoters, in
either aqueous solution or neat form, were added (in the ascending
numeric order listed in Table III) with stirring. In Catalysts 3
through 10, two equivalents of diammonium
ethylenediaminetetraacetic acid (EDTA) were added with the
manganese promoter in order to stabilize the manganese in the
impregnation solution. In Catalysts 11 and 12, one excess
equivalent of diammonium EDTA was added for the same purpose.
[0081] The impregnation, draining and roasting steps for this
second impregnation were carried out analogously to the first
impregnation.
[0082] The twice-impregnated carrier, that is, the finished
catalyst, was again weighed, and based upon the weight gain of the
carrier in the second impregnation, the weight percent of silver
and the concentration of the promoters were calculated (results
given in Table III). In some cases, the preparation of a catalyst
was carried out on a larger scale than that described here using
suitable scale-up of equipment and methods. The finished catalyst
was then employed in an ethylene epoxidation reaction, the results
of which are given in the Examples.
TABLE-US-00003 TABLE III Catalyst Preparations Part 1 Catalyst No.
1 2 3 4 Carrier No. A B C D Promoter 1 Na.sub.2SO.sub.4
Na.sub.2SO.sub.4 CsOH CsOH Promoter 2 Cs.sub.2SO.sub.4
Cs.sub.2SO.sub.4 Cs.sub.2SO.sub.4 Cs.sub.2SO.sub.4 Promoter 3
Mn(NO.sub.3).sub.2 Mn(NO.sub.3).sub.2 Chelating Agent
(NH.sub.4).sub.2H.sub.2(EDTA) (NH.sub.4).sub.2H.sub.2(EDTA) Total
Wt. % Silver 30.95 30.7 35.40 32.88 Promoter 1; ppm 340 Na 385 Na
575 Cs 552 Cs Promoter 2; ppm 653 Cs 742 Cs 162 SO4 150 SO4
Promoter 3; ppm 104 Mn 94 Mn Part 2 Catalyst No. 5 6 7 8 Carrier
No. E F G H Promoter 1 CsOH CsOH CsOH CsOH Promoter 2
Cs.sub.2SO.sub.4 Cs.sub.2SO.sub.4 Cs.sub.2SO.sub.4 Cs.sub.2SO.sub.4
Promoter 3 Cs.sub.2MoO.sub.4 Cs.sub.2MoO.sub.4 Mn(NO.sub.3).sub.2
Mn(NO.sub.3).sub.2 Promoter 4 Mn(NO.sub.3).sub.2 Mn(NO.sub.3).sub.2
Chelating Agent (NH.sub.4).sub.2H.sub.2(EDTA)
(NH.sub.4).sub.2H.sub.2(EDTA) (NH.sub.4).sub.2H.sub.2(EDTA)
(NH.sub.4).sub.2H.sub.2(EDTA) Total Wt. % Silver 34.8 35.6 37.0
38.8 Promoter 1; ppm 584 Cs 796 Cs 409 Cs 420 Cs Promoter 2; ppm
129 SO4 175 SO4 115 SO4 118 SO4 Promoter 3; ppm 15 Mo 20 Mo 74 Mn
77 Mn Promoter 4; ppm 53 Mn 71 Mn Part 3 Catalyst No. 9 10 11 12
Carrier No. I J K L Promoter 1 CsOH CsOH CsOH CsOH Promoter 2
Cs.sub.2SO.sub.4 Cs.sub.2SO.sub.4 Cs.sub.2SO.sub.4 Cs.sub.2SO.sub.4
Promoter 3 Mn(NO.sub.3).sub.2 Mn(NO.sub.3).sub.2 Mn(NO.sub.3).sub.2
Cs.sub.2MoO.sub.4 Promoter 4 Mn(NO.sub.3).sub.2 Chelating Agent
(NH.sub.4).sub.2H.sub.2(EDTA) (NH.sub.4).sub.2H.sub.2(EDTA)
(NH.sub.4).sub.2H.sub.2(EDTA) (NH.sub.4).sub.2H.sub.2(EDTA) Total
Wt. % Silver 39.5 28.4 27.4 32.2 Promoter 1; ppm 489 Cs 439 Cs 383
Cs 364 Cs Promoter 2; ppm 138 SO4 123 SO4 108 SO4 81 SO4 Promoter
3; ppm 89 Mn 79 Mn 70 Mn 9 Mo Promoter 4; ppm 32 Mn Part 4 Catalyst
No. 13 14 15 Carrier No. M J K Promoter 1 CsOH KNO.sub.3 KNO.sub.3
Promoter 2 Cs.sub.2SO.sub.4 K.sub.2Mn(EDTA) K.sub.2Mn(EDTA)
Promoter 3 Cs.sub.2MoO.sub.4 Promoter 4 Mn(NO.sub.3).sub.2
Chelating Agent (NH.sub.4).sub.2H.sub.2(EDTA)
(NH.sub.4).sub.2H.sub.2(EDTA) (NH.sub.4).sub.2H.sub.2(EDTA) Total
Wt. % Silver 32.9 31.5 32.7 Promoter 1; ppm 373 Cs 957 K 965 K
Promoter 2; ppm 83 SO4 144 Mn 144 Mn Promoter 3; ppm 9 Mo Promoter
4; ppm 33 Mn
[0083] In Tables IV through X "Mlb EO/CF" denotes units of 1000
pounds of ethylene oxide produced per cubic foot of catalyst.
Example 1
Catalyst 1 and Comparative Catalyst 2
[0084] Eighty cubic centimeters of each catalyst (61.4 g.) was
charged to an autoclave reactor and tested under Air Process
Conditions--I (Table I). Outlet ethylene oxide was set to 1.2 mole
percent until day 6 when it was increased to 1.4 mole percent.
Table IV compares the performance of the catalyst containing 2
weight percent zircon (Catalyst 1) with one containing no zircon
(Comparative Catalyst 2). The catalyst containing zircon has higher
initial efficiency and lower initial temperature (higher
activity).
TABLE-US-00004 TABLE IV Example 1 Catalyst Performance Efficiency %
Temp. .degree. C. 2 Mlb 5 Mlb 7 Mlb 2 Mlb 5 Mlb 7 Mlb EO/CF EO/CF
EO/CF EO/CF EO/CF EO/CF Catalyst 1 76.4 74.7 71.7 260 269 280
Catalyst 2 74.8 73.4 71.6 266 274 281 (comparative)
Example 2
Catalyst 3 and Comparative Catalyst 4
[0085] An equal weight (63.5 g.) of each catalyst was charged to an
autoclave reactor and tested under the Air Process Conditions--I
described in Table I. The outlet ethylene oxide was set to 1.4 mole
percent and temperature and efficiency monitored as the catalysts
aged.
[0086] Table V compares performance of the two catalysts as a
function of pounds of EO produced per cubic foot of catalyst.
Although the initial efficiency is lower than the comparative
catalyst, over time, the zircon-containing catalyst ages in
efficiency and temperature at a slower rate.
TABLE-US-00005 TABLE V Example 2 Catalyst Performance Efficiency %
Temp. .degree. C. 8 Mlb 20 Mlb 35 Mlb 8 Mlb 20 Mlb 35 Mlb EO/CF
EO/CF EO/CF EO/CF EO/CF EO/CF Catalyst 3 78.2 77.7 77.2 249 253 256
Catalyst 4 79.9 78.4 75.9 243 251 259 (comparative)
Example 3
Catalyst 5 and Comparative Catalyst 6
[0087] An equal weight (63.5 g.) of each catalyst was tested in an
autoclave under Oxygen Process Conditions--I (Table I). Outlet
ethylene oxide concentration was set to 1.0 mole percent. Catalyst
5 has higher efficiency and lower temperature compared to
Comparative Catalyst 6, which does not contain zircon.
TABLE-US-00006 TABLE VI Example 3 Catalyst Performance Efficiency
(%) Temp. (.degree. C.) 1 Mlb 2.5 Mlb 1 Mlb 2.5 Mlb EO/CF EO/CF
EO/CF EO/CF Catalyst 5 80.4 80.0 232 230 Catalyst 6 76.6 77.7 253
253 (comparative)
Example 4
Catalyst 7, Catalyst 8 and Comparative Catalyst 9
[0088] An equal weight (63.5 g.) of each catalyst was tested in an
autoclave under Oxygen Process Conditions--I (Table I). Outlet
ethylene oxide concentration was set to 1.2 mole percent. Catalyst
8, which contains .about.0.3% zircon has the highest efficiency and
lowest efficiency aging and temperature aging. Catalyst 7, which
contains .about.3% zircon is more active and efficient than
Comparative Catalyst 9 which contains no added zircon.
TABLE-US-00007 TABLE VII Example 4 Catalyst Performance Efficiency
% Temp. .degree. C. 1.5 Mlb 5.0 Mlb 7.0 Mlb 1.5 Mlb 5.0 Mlb 7.0 Mlb
EO/CF EO/CF EO/CF EO/CF EO/CF EO/CF Catalyst 7 79.8 78.2 76.0 244
250 256 Catalyst 8 80.2 79.6 78.3 246 244 246 Catalyst 9 79.5 76.0
-- 251 262 -- (comparative)
Example 5
Catalyst 10 and Comparative Catalyst 11
[0089] An equal weight (63.5 g.) of each catalyst was tested in an
autoclave under Oxygen Process Conditions--I. Outlet ethylene oxide
concentration was set to 1.0 mole percent. Catalyst 10 shows more
stable temperature than Comparative Catalyst 11 which does not
contain zircon.
TABLE-US-00008 TABLE VIII Example 5 Catalyst Performance Efficiency
% Temp. .degree. C. 1 Mlb 3 Mlb 5 Mlb 1 Mlb 3 Mlb 5 Mlb EO/CF EO/CF
EO/CF EO/CF EO/CF EO/CF Catalyst 10 80.9 81.1 80.1 231 230 230
Catalyst 11 80.8 81.1 80.0 233 233 235 (comparative)
Example 6
Catalyst 12 and Comparative Catalyst 13
[0090] An equal weight (63.5 g.) of each catalyst was tested in an
autoclave under Oxygen Process Conditions--I. Outlet ethylene oxide
concentration was set to 1.0 mole percent. Catalyst 12, prepared
with zircon, is initially less efficient and less active, but shows
significantly lower temperature aging than Comparative Catalyst 13,
prepared without zircon.
TABLE-US-00009 TABLE IX Example 6 Catalyst Performance Efficiency %
Temp. .degree. C. 2 Mlb 5 Mlb 7.5 Mlb 2 Mlb 5 Mlb 7.5 Mlb EO/CF
EO/CF EO/CF EO/CF EO/CF EO/CF Catalyst 12 80.7 79.8 79.2 242 242
244 Catalyst 13 81.1 80.5 79.6 241 244 248 (comparative)
Example 7
Catalyst 14 and Comparative Catalyst 15
[0091] Thirty cubic centimeters (26.8 g. for Comparative Catalyst
14 and 26.1 g. for Catalyst 15) of each catalyst was charged to an
autoclave reactor and tested under Oxygen Process Conditions--II
(Table I). After initial operation at temperatures between 220 and
255.degree. C., conditions were adjusted for a total flow of 21.3
SCFH, and temperature was controlled to maintain 1.2 mole percent
outlet ethylene oxide. Initial efficiency of Catalyst 14, which
contains zircon is higher than that of Comparative Catalyst 15, and
the efficiency decline rate is reduced.
TABLE-US-00010 TABLE X Example 7 Catalyst Performance Efficiency %
Temp. .degree. C. Day 12 Day 16 Day 21 Day 12 Day 16 Day 21
Catalyst 14 86.3 86.5 86.1 246 247 248 Catalyst 15 84.5 84.3 83.4
247 248 250 (comparative)
* * * * *