U.S. patent application number 11/845180 was filed with the patent office on 2009-03-05 for process for production of an olefin oxide.
This patent application is currently assigned to SD LIZENZVERWERTUNGSGESELLSCHAFT MBH & CO. KG. Invention is credited to Christian J. Gueckel.
Application Number | 20090062557 11/845180 |
Document ID | / |
Family ID | 40387701 |
Filed Date | 2009-03-05 |
United States Patent
Application |
20090062557 |
Kind Code |
A1 |
Gueckel; Christian J. |
March 5, 2009 |
PROCESS FOR PRODUCTION OF AN OLEFIN OXIDE
Abstract
The invention relates to a process for the epoxidation of an
olefin, wherein the concentration of the olefin oxide in the outlet
is greater than about 2.2% by volume. More particularly, the
invention relates to a process for the epoxidation of ethylene by
contacting a feed including at least ethylene and oxygen with an
improved epoxidation catalyst. The catalyst which has improved
selectivity in the epoxidation process at high productivities,
includes a solid support having a surface, which has a first mode
of pores that have a diameter ranging from about 0.01 .mu.m to
about 5 .mu.m and having a differential pore volume peak in the
range from about 0.01 .mu.m to about 5 .mu.m. The surface also has
a second mode of pores, which is different from the first mode of
pores, having a diameter ranging from about 1 .mu.m to about 20
.mu.m and have a differential pore volume peak in the range from
about 1 .mu.m to about 20 .mu.m. On the bimodal pore surface is a
catalytically effective amount of silver or a silver-containing
compound, a promoting amount of rhenium or a rhenium-containing
compound, and a promoting amount of one or more alkali metals or
alkali-metal-containing compounds.
Inventors: |
Gueckel; Christian J.;
(Paramus, NJ) |
Correspondence
Address: |
SCULLY SCOTT MURPHY & PRESSER, PC
400 GARDEN CITY PLAZA, SUITE 300
GARDEN CITY
NY
11530
US
|
Assignee: |
SD LIZENZVERWERTUNGSGESELLSCHAFT
MBH & CO. KG
Munchen
DE
|
Family ID: |
40387701 |
Appl. No.: |
11/845180 |
Filed: |
August 27, 2007 |
Current U.S.
Class: |
549/536 |
Current CPC
Class: |
B01J 37/0203 20130101;
C07D 301/10 20130101; B01J 21/04 20130101; B01J 35/1038 20130101;
B01J 23/688 20130101; B01J 35/109 20130101; B01J 35/1009
20130101 |
Class at
Publication: |
549/536 |
International
Class: |
C07D 301/10 20060101
C07D301/10; C07D 301/03 20060101 C07D301/03 |
Claims
1. A process for the epoxidation of an olefin to an olefin oxide
comprising: contacting a feed including at least oxygen and an
olefin in a reactor with a catalyst that includes a support having
a bimodal pore size distribution, with a first mode of pores having
a mean diameter ranging from about 0.01 .mu.m to about 5 .mu.m and
a second mode of pores having a mean diameter ranging from about 5
.mu.m to about 30 .mu.m, a catalytically effective amount of silver
or a silver-containing compound, a promoting amount of rhenium or a
rhenium-containing compound, and a promoting amount of one or more
alkali metals or alkali-metal-containing compounds, said reactor
including at least a reactor outlet and said olefin oxide produced
by said contacting has a concentration in the reactor outlet that
is greater than about 2.2 % by volume.
2. A process for the epoxidation of an olefin to an olefin oxide
comprising: contacting a feed including at least oxygen and an
olefin in a reactor wit a catalyst that includes a support having a
bimodal pore size distribution, with a, first mode of pores having
a mean diameter ranging from about 0.1 .mu.m to about 4 .mu.m and a
second mode of pores having a mean diameter ranging from about 5
.mu.m to about 20 .mu.m, a catalytically effective amount of silver
or a silver-containing compound, a promoting amount of rhenium or a
rhenium-containing compound, and a promoting amount of one or more
alkali metals or alkali-metal-containing compounds, said reactor
including at least a reactor outlet and said olefin oxide produced
by said contacting has a concentration in the reactor outlet that
is greater than about 2.2% by volume.
3. The process of claim 1 wherein the first mode of pores comprises
at most 50% of a total pore volume and the second mode of pores
comprises at least 50% of the total pore volume.
4. The process of claim 1 wherein the first mode of pores comprises
at most 40% of a total pore volume and the second mode comprises at
least 60% of the total pore volume.
5. The process of claim 1 wherein the support comprises alumina,
charcoal, pumice, magnesia, zirconia, titania, kieselguhr, fuller's
earth, silicon carbide, silica, silicon dioxide, magnesia, clays,
artificial zeolites, natural zeolites, ceramics or combinations
thereof.
6. The process of claim 1 wherein the support comprises
alumina.
7. The process of claim 1 wherein the support comprises alumina
with a surface area of less than about 1 m.sup.2/g.
8. The process of claim 1 wherein said catalyst further comprises a
promoting amount of one or more Group IIA metal-containing
compounds, one or more transition metal-containing compounds, one
or more sulfur-containing compounds, one or more
fluorine-containing compounds, one or more phosphorus-containing
compounds, one or more boron-containing compounds, or combinations
thereof.
9. The process of claim 8 wherein the Group IIA metal-containing
compound comprises beryllium, magnesium, calcium, strontium, barium
or combinations thereof.
10. The process of claim 8 wherein the transition metal-containing
compound comprises an element selected from Groups IVA, VA, VIA,
VIIA and VIIIA of the Periodic Table of the Elements, or
combinations thereof.
11. The process of claim 8 wherein the transition metal-containing
compound comprises molybdenum, tungsten, chromium, titanium,
hafnium, zirconium, vanadium, thorium, tantalum, niobium or
combinations thereof.
12. The process of claim 8 wherein the transition metal-containing
compound comprises molybdenum or tungsten or combinations
thereof.
13. The process of claim 1 wherein the alkali metal-containing
compound comprises lithium, sodium, potassium, rubidium, cesium or
combinations thereof.
14. The process of claim 1 wherein the alkali metal-containing
compound comprises cesium.
15. The process of claim 1 wherein said olefin is ethylene and said
olefin oxide is ethylene oxide.
16. The process of claim 1 wherein said olefin oxide concentration
is obtained by adjusting conversion of said olefin and oxygen.
17. The process of claim 16 wherein said adjusting is achieved by
increasing the reaction temperature of said epoxidation.
18. The process of claim 1 wherein said reactor is a fixed bed,
tubular reactor
19. The process of claim 1 wherein said contacting is conducting in
a vapor phase and said oxygen includes molecular oxygen.
20. A process for the epoxidation of an olefin to an olefin oxide
comprising: contacting a feed including at least oxygen and an
olefin in a reactor with a catalyst that includes a support having
a bimodal pore size distribution, with a first mode of pores having
a mean diameter ranging from about 0.01 .mu.m to about 5 .mu.m and
a second mode of pores having a mean diameter ranging from about 5
.mu.m to about 30 .mu.m, a catalytically effective amount of silver
or a silver-containing compound, and a promoting amount of rhenium
or a rhenium-containing compound, cesium, lithium, tungsten and
sulfur, said reactor including at least a reactor outlet and said
olefin oxide produced by said contacting has a concentration in the
reactor outlet that is greater than about 2.2% by volume.
Description
FIELD OF THE INVENTION
[0001] The present invention pertains to a process for the
epoxidation of an olefin to an olefin oxide by contacting a feed
gas comprising an olefin and oxygen with a catalyst comprising a
silver compound and a rhenium compound deposited on a carrier with
bimodal pore size distribution, wherein the concentration of the
olefin oxide in the reactor outlet is greater than about 2.2% by
volume. More particularly, the invention pertains to an improved
catalyst useful for the epoxidation of ethylene to ethylene oxide
at high catalyst work rates.
DESCRIPTION OF THE RELATED ART
[0002] Generally, commercially practiced olefin epoxidation is
carried out by continuously contacting a feed gas comprising oxygen
and an olefin with a catalyst under defined operating conditions.
The resulting product mixture of olefin oxide and typically
un-reacted oxygen and olefin as well as the total combustion
products undergoes a separation procedure wherein the un-reacted
feed gas components are separated from the products and undesired
by-products.
[0003] For economical purposes, it is preferred to operate
production plants for olefin oxide at maximum productivity and
highest selectivity. In order to maximize the productivity, the
catalyst work rate has to be increased which is usually achieved by
either raising the flow rate, i.e., Gas Hourly Space Velocity, at
fixed olefin oxide concentration at the reactor outlet and/or
changing the concentration of the olefin oxide in the reactor
outlet by adjusting the olefin and oxygen conversion.
[0004] Since all olefin oxide plants have limited capability for
increasing the flow rate because of the plant design, the most
common procedure to increase the productivity is the adjustment of
the olefin oxide concentration in the reactor outlet. In general,
the outlet concentration adjustments are achieved in the prior art
by increasing the catalyst temperature and thereby increasing the
olefin and oxygen conversion. However, by increasing the level of
olefin oxide in the reactor outlet, the selectivity of the process
decreases significantly which counteracts the desired productivity
increasing. Therefore, most plants run at low olefin oxide outlet
concentration in order to achieve high selectivites at moderate
productivity. By "low olefin oxide outlet concentration", it is
meant that the olefin oxide outlet concentration is typically 1.8%
v or less.
[0005] There is continuing interest in producing improved catalysts
for the epoxidation of olefins at higher productivities. In this
respect, and of particular interest, are catalysts for the highly
selective epoxidation of ethylene since these catalysts are known
to lose selectivity significantly at high productivities.
[0006] These catalysts typically comprise a porous refractory
support such as alpha alumina, which has on its surface a catalytic
amount of silver and at least one promoter that helps to increase
selectivity in the epoxidation process. The use of alkali metals
and transition metals as promoters for silver catalysts is well
known for the production of ethylene oxide by the partial oxidation
of ethylene in the vapor phase. The catalyst may comprise further
elements like alkali metals, as are described in U.S. Pat. Nos.
3,962,136 and 4,010,115. In particular, the '136 and the '115
patents disclose Ag/alkali metal catalysts without rhenium, Re.
[0007] Over the last two decades, rhenium was described as being
effective in improving the selectivity of alkaline metal promoted
silver-based catalyst supported by a refractory porous support.
Some references in the art are U.S. Pat. Nos. 4,761,394 and
4,833,261. The further improvement of silver-based catalysts
promoted with alkaline metals and rhenium by the use of one of
sulfur, Mo, W, and Cr was disclosed, for example, in U.S. Pat. Nos.
4,766,105, 4,820,675 and 4,808,738.
[0008] Other examples of catalysts are disclosed, for example, in
U.S. Pat. Nos. 4,010,155; 4,012,425; 4,123,385; 4,066,575;
4,039,561 and 4,350,616. Such highly selective catalysts contain,
in addition to silver, selectivity-enhancing promoters such as
rhenium, molybdenum, tungsten and/or nitrate- or nitrite-forming
compounds, as are discussed in U.S. Pat. Nos. 4,761,394 and
4,766,105.
[0009] U.S. Patent Application Publication No. 20060009647 A1
discloses a process for the epoxidation of an olefin with a
catalyst comprising a silver component deposited on a
fluoride-mineralized carrier, wherein the partial pressure of
olefin oxide in the product mix is greater than 60 kPa. In
addition, this printed publication discloses a similar process
utilizing a catalyst comprising a silver component and one or more
high-selectivity dopants, wherein the partial pressure of olefin
oxide in the product mix is greater than 20 kPa. However, the
disclosure of the '647 publication does not teach about the
influence of the pore size distribution on the catalyst performance
at high productivities.
[0010] Beside the chemical composition of a supported silver-based
epoxidation catalyst, the physical characteristics of the finished
catalyst as well the support have been an integral part of catalyst
development. Generally, the silver-based catalyst support shows a
characteristic pore volume and pore size distribution. Furthermore,
the surface area and the water absorption are well-known
characteristics for such catalyst supports. It has now been found
that the physical characteristics of the finished catalyst and the
impact of the characteristics on the catalyst performance are more
complicated than heretofore believed, especially if the catalyst is
promoted with rhenium. In addition to the surface area, the pore
volume and the pore size distribution, the pattern of the pore size
distribution, especially the number and the specific
characteristics of different modes, has now been found to have a
significant positive impact on the catalyst selectivity. In
particular, this effect is especially distinguished when the
catalyst is operated at very high work rates, i.e., high levels of
olefin oxide production.
[0011] In view of the above, there is a continued need for
providing new and improved Ag-based epoxidation catalysts that
exhibited increased performance at high productivites.
SUMMARY OF THE INVENTION
[0012] An increased productivity (expressed herein by the
concentration of ethylene oxide in the reactor outlet gas) catalyst
containing silver and rhenium supported by a carrier with a bimodal
pore size distribution is provided that shows improved performance.
The catalyst according to the invention shows a minor loss in
selectivity at higher productivities whereas conventional catalysts
with monomodal pore size distribution show a significant loss in
selectivity at higher productivities.
[0013] In particular, the invention provides a process for the
epoxidation of an olefin to an olefin oxide which comprises
contacting a feed including at least oxygen and an olefin in a
reactor with a catalyst that includes a support having a bimodal
pore size distribution, a catalytically effective amount of silver
or a silver-containing compound, a promoting amount of rhenium or a
rhenium-containing compound, and a promoting amount of one or more
alkali metals or alkali-metal-containing compounds, said reactor
including at least a reactor outlet and said olefin oxide produced
by said contacting has a concentration in the reactor outlet that
is greater than about 2.2% by volume.
[0014] The support having the bimodal pore size distribution that
is employed in the present invention includes a pore size
distribution with a first mode of pores which has a mean diameter
ranging from about 0.01 .mu.m to about 5 .mu.m, and a second mode
of pores which has a mean diameter ranging from about 5 .mu.m to
about 30 .mu.m.
[0015] In some embodiments of the present invention, the olefin
oxide concentration in the reactor outlet is greater than about
2.4% by volume. In yet other embodiments of the present invention,
the ethylene oxide concentration in the reactor outlet is greater
than about 2.6% by volume.
[0016] In the present invention, the olefin oxide concentration in
the reactor outlet is obtained by adjusting the olefin and oxygen
conversion. That is, the olefin oxide concentration in the reactor
is obtained by increasing the reaction temperature during the
epoxidation reaction. An increase of the reaction temperature has
always a negative effect on the catalyst selectivity. This decrease
in catalyst selectivity is economically undesired and reduces the
benefit of the higher productivity. The selectivity decrease can be
significant, i.e., more points of selectivity for less than one
point increase of the olefin oxide concentration in the reactor
outlet. It was found, that catalysts according to the invention
show only a minor selectivity decrease at higher productivities,
i.e., higher olefin and oxygen conversion, and, therefore, have a
significant economical benefit compared to state of the art
catalysts.
[0017] The invention also provides a process for the oxidation of
ethylene to ethylene oxide which comprises the vapor phase
oxidation of ethylene with molecular oxygen in a fixed bed, tubular
reactor, in the presence of the aforementioned catalyst. In this
aspect of the present invention, the ethylene oxide concentration
in the reactor outlet is greater than about 2.2% by volume.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The sole drawing of the present invention, shows a
comparison of catalyst performances for the inventive supported
Ag-based catalyst and a prior art Ag-based catalyst in applications
in which the ethylene oxide concentration at the reactor outlet was
equal or greater than about 2.2.
DETAILED DESCRIPTION OF THE INVENTION
[0019] As stated above, the present invention provides a process
for the epoxidation of an olefin, preferably ethylene, to an olefin
oxide, preferably ethylene oxide, which comprises contacting a feed
including at least oxygen and an olefin in a reactor with a
catalyst that includes a support having a bimodal pore size
distribution, a catalytically effective amount of silver or a
silver-containing compound, a promoting amount of rhenium or a
rhenium-containing compound, and a promoting amount of one or more
alkali metals or alkali-metal-containing compounds, said reactor
including at least a reactor outlet and said olefin oxide produced
by said contacting has a concentration in the reactor outlet that
is greater than about 2.2, preferably greater than about 2.4, more
preferably, greater than about 2.6, % by volume.
[0020] In the present invention, the olefin oxide concentration in
the reactor outlet is obtained by adjusting the olefin and oxygen
conversion. That is, the olefin oxide concentration in the reactor
is obtained by increasing the reaction temperature during the
epoxidation reaction. An increase of the reaction temperature has
always a negative effect on the catalyst selectivity. This decrease
in catalyst selectivity is economically undesired and reduces the
benefit of the higher productivity. The selectivity decrease can be
significant, i.e., more points of selectivity for less than one
point increase of the olefin oxide concentration in the reactor
outlet. It was found, that catalysts according to the invention
show only a minor selectivity decrease at higher productivities,
i.e., higher olefin and oxygen conversion, and, therefore, have a
significant economical benefit compared to state of the art
catalysts.
[0021] The support employed in this invention may be selected from
a large number of solid, refractory supports that may be porous and
may provide the preferred pore structure. Alumina is well known to
be useful as a catalyst support for the epoxidation of an olefin
and is the preferred support. The alumina support may also contain
various impurities and additives that may or may not influence the
catalytic epoxidation reaction. In the process of making the
preferred alumina support, high-purity aluminum oxide, preferably
alpha-alumina, is thoroughly mixed with temporary and permanent
binders. The temporary binders, known as burnout materials, are
thermally decomposable organic compounds of moderate to high
molecular weight which, on decomposition, alter the pore structure
of the support. The permanent binders are typically inorganic
clay-type materials having fusion temperatures below that of the
alumina and impart mechanical strength to the finished support.
After thorough dry-mixing, sufficient water and/or other suitable
liquid is added to help form the mass into a paste-like substance.
Catalyst support particles are formed from the paste by
conventional means such as extrusion. The particles are then dried
and are subsequently calcined at an elevated temperature.
[0022] The support may comprise materials such as alpha-alumina,
charcoal, pumice, magnesia, zirconia, titania, kieselguhr, fuller's
earth, silicon carbide, silica, silicon carbide, clays, artificial
zeolites, natural zeolites, silicon dioxide and/or titanium
dioxide, ceramics and combination thereof. The preferred support is
comprised of alpha-alumina having a very high purity; i.e., at
least 95 wt. % pure, or more preferably, at least 98 wt. %
alpha-alumina. The remaining components may include inorganic
oxides other than alpha-alumina, such as silica, alkali metal
oxides (e.g., sodium oxide) and trace amounts of other
metal-containing or non-metal-containing additives or
impurities.
[0023] The solid support employed in the present invention has a
bimodal pore size distribution. More particular, the solid support
employed in the present invention has a surface including a first
mode of pores which have a mean diameter ranging from about 0.01
.mu.m to about 5 .mu.m. Preferably, the first mode of pores has a
mean diameter ranging from about 0.1 .mu.m to about 4 .mu.m. The
surface of solid support employed in the present invention also has
a second mode of pores which is different from the first mode of
pores. In particular, the second mode of pores has a mean diameter
ranging from about 5 .mu.m to about 30 .mu.m. Preferably, the
second mode of pores has a mean diameter ranging from about 5 .mu.m
to about 20. Typically, but not necessarily always, the first mode
of pores comprises from about at most about 50% of the total pore
volume and the second mode provides at least about 50% of the total
pore volume. In another embodiment, the first mode of pores
comprises at most about 45% of the total pore volume and the second
mode provides at least about 55% of the total pore volume. It is
believed, without limiting the scope of the invention, that a
catalyst with the described bimodal pore size distribution provides
advantageous pore structure with reaction chambers separated by
diffusion channels.
[0024] The final support typically, but not necessarily always, has
a water absorption value ranging from about 0.2 cc/g to about 0.8
cc/g, preferably from about 0.25 cc/g to about 0.6 cc/g. The BET
surface area of the finished support is preferred to be in the
range from about 0.3 to about 4.0 m.sup.2/g, more preferably from
about 0.3 to about 1.5 m.sup.2/g, and most preferably from about
0.3 m.sup.2/g to about 1 m.sup.2/g. Suitable porosity volumes
measured by mercury intrusion techniques are generally in the range
from about 0.2 ml/g to about 0.8 ml/g, and preferably from about
0.25 ml/g to about 0.60 ml/g.
[0025] Regardless of the character of the support used, it is
usually shaped into particles, chunks, pieces, pellets, rings,
spheres, wagon wheels, cross-partitioned hollow cylinders, and the
like, of a size suitable for employment in a fixed-bed epoxidation
reactor. The type of reactor is not limited as long as it is
capable of producing an olefin oxide by the catalytic oxidation of
an olefin. Desirably, the support particles may have equivalent
diameters in the range from about 3 mm to about 12 mm, and
preferably in the range from about 5 mm to about 10 mm, which are
usually compatible with the internal diameter of the tubular
reactors in which the catalyst is placed. Equivalent diameter is
the diameter of a sphere having the same external surface (i.e.,
neglecting surface within the pores of the particle) to volume
ratio as the support particles being employed.
[0026] In general and as briefly mentioned above, a suitable
catalyst support of the present invention can be prepared by mixing
the refractory material, such as alumina, water or other suitable
liquid, a burnout material or suitable porosity-controlling agent,
and a binder. Burnout materials include cellulose, substituted
celluloses, e.g., methylcellulose, ethylcellulose, and
carboxyethylcellulose, stearates, such as organic stearate esters,
e.g., methyl or ethyl stearate, waxes, granulated polyolefins,
particularly polyethylene and polypropylene, walnut shell flour,
and the like which are decomposable at the firing temperatures used
in preparation of the support. The burnout material is used to
modify the porosity of the support and it is essentially totally
removed during the firing to produce the finished support. Supports
of the present invention are preferably made with the inclusion of
a bonding material such as silica with an alkali metal compound in
sufficient amount to substantially prevent the formation of
crystalline silica compounds. Appropriate binders include inorganic
clay-type materials. For instant, a particularly convenient binder
material is a mixture of boehmite, an ammonia stabilized silica
sol, and a soluble sodium salt.
[0027] A paste is formed by mixing the dry ingredients of the
support with water or another suitable liquid, and the paste is
usually extruded or molded into the desired shape, and then fired
or calcined at a temperature from about 1200.degree. C. to about
1600.degree. C. to form the support. When the particles are formed
by extrusion, it may be desirable to also include extrusion aids.
The amounts of extrusion aids required would depend on a number of
factors that relate to the equipment used. However these matters
are well within the general knowledge of a person skilled in the
art of extruding ceramic materials. After firing, the support is
preferably washed to remove soluble residues. Washing is most
commonly done with water, but washing with other solvents or
aqueous/non-aqueous solutions can also be beneficial.
[0028] Suitable supports having a bimodal pore size distribution
are available from Saint-Gobain Norpro Co., Sud Chemie AG, Noritake
Co., CeramTec AG, and Industrie Bitossi S.p.A.
[0029] In order to produce a catalyst for the oxidation of an
olefin to an olefin oxide, a support having the above
characteristics is then provided with a catalytically effective
amount of silver on its surface. The catalyst is prepared by
impregnating the support with a silver compound, complex or salt
dissolved in a suitable solvent sufficient to cause deposition of a
silver-precursor compound onto the support. Preferably, an aqueous
silver solution is used. After impregnation, the excess solution is
removed from the impregnated support, and the impregnated support
is heated to evaporate the solvent and to deposit the silver or
silver compound on the support as is known in the art.
[0030] Preferred catalysts prepared in accordance with this
invention contain up to about 45% by weight of silver, expressed as
metal, based on the total weight of the catalyst including the
support. The silver is deposited upon the surface and throughout
the pores of a porous refractory support. Silver contents,
expressed as metal, from about 1% to about 40% based on the total
weight of the catalyst are preferred, while silver contents from
about 8% to about 35% are more preferred. The amount of silver
deposited on the support or present on the support is that amount
which is a catalytically effective amount of silver, i.e., an
amount which economically catalyzes the reaction of ethylene and
oxygen to produce ethylene oxide. As used herein, the term
"catalytically effective amount of silver" refers to an amount of
silver that provides a measurable conversion of ethylene and oxygen
to ethylene oxide. Useful silver containing compounds which are
silver precursors non-exclusively include silver oxalate, silver
nitrate, silver oxide, silver carbonate, a silver carboxylate,
silver citrate, silver phthalate, silver lactate, silver
propionate, silver butyrate and higher fatty acid salts and
combinations thereof.
[0031] Also deposited on the support, either prior to,
coincidentally with, or subsequent to the deposition of the silver
is a promoting amount of a rhenium component, which may be a
rhenium-containing compound or a rhenium-containing complex. The
rhenium promoter may be present in an amount from about 0.001 wt. %
to about 1 wt. %, preferably from about 0.005 wt. % to about 0.5
wt. %, and more preferably from about 0.01 wt. % to about 0.1 wt. %
based on the weight of the total catalyst including the support,
expressed as the rhenium metal.
[0032] Also deposited on the support either prior to,
coincidentally with, or subsequent to the deposition of the silver
and rhenium are promoting amounts of an alkali metal or mixtures of
two or more alkali metals, as well as optional promoting amounts of
a Group IIA alkaline earth metal component or mixtures of two or
more Group IIA alkaline earth metal components, and/or a transition
metal component or mixtures of two or more transition metal
components, all of which may be in the form of metal ions, metal
compounds, metal complexes and/or metal salts dissolved in an
appropriate solvent. The support may be impregnated at the same
time or in separate steps with the various catalyst promoters. The
particular combination of support, silver, alkali metal
promoter(s), rhenium component, and optional additional promoter(s)
of the instant invention will provide an improvement in one or more
catalytic properties over the same combination of silver and
support and none, or only one of the promoters.
[0033] As used herein the term "promoting amount" of a certain
component of the catalyst refers to an amount of that component
that works effectively to improve the catalytic performance of the
catalyst when compared to a catalyst that does not contain that
component. The exact concentrations employed, of course, will
depend on, among other factors, the desired silver content, the
nature of the support, the viscosity of the liquid, and solubility
of the particular compound used to deliver the promoter into the
impregnating solution. Examples of catalytic properties include,
inter alia, operability (resistance to runaway), 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. In the epoxidation process, it may be
desirable to intentionally change the operating conditions to take
advantage of certain catalytic properties even at the expense of
other catalytic properties. The preferred operating conditions will
depend upon, among other factors, feedstock costs, energy costs,
by-product removal costs and the like.
[0034] Suitable alkali metal promoters may be selected from
lithium, sodium, potassium, rubidium, cesium or combinations
thereof, with cesium being preferred, and combinations of cesium
with other alkali metals being especially preferred. The amount of
alkali metal deposited or present on the support is to be a
promoting amount. Preferably, the amount ranges from about 10 ppm
to about 3000 ppm, more preferably from about 15 ppm to about 2000
ppm, and even more preferably from about 20 ppm to about 1500 ppm,
and as especially preferred from about 50 ppm to about 1000 ppm by
weight of the total catalyst, measured as the metal.
[0035] Suitable alkaline earth metal promoters comprise elements
from Group IIA of the Periodic Table of the Elements, which may be
beryllium, magnesium, calcium, strontium, and barium or
combinations thereof. Suitable transition metal promoters may
comprise elements from Groups IVA, VA, VIA, VIIA and VIIIA of the
Periodic Table of the Elements, and combinations thereof. Most
preferably the transition metal comprises an element selected from
Groups IVA, VA or VIA of the Periodic Table of the Elements.
Preferred transition metals that can be present include molybdenum,
tungsten, chromium, titanium, hafnium, zirconium, vanadium,
tantalum, niobium, or combinations thereof.
[0036] The amount of alkaline earth metal promoter(s) and/or
transition metal promoter(s) deposited on the support is a
promoting amount. The transition metal promoter may typically be
present in an amount from about 0.1 micromoles per gram to about 10
micromoles per gram, preferably from about 0.2 micromoles per gram
to about 5 micromoles per gram, and more preferably from about 0.5
micromoles per gram to about 4 micromoles per gram of total
catalyst, expressed as the metal. The catalyst may further comprise
a promoting amount of one or more sulfur compounds, one or more
phosphorus compounds, one or more boron compounds, one or more
halogen-containing compounds, or combinations thereof.
[0037] The silver solution used to impregnate the support may also
comprise an optional solvent or a complexing/solubilizing agent
such as are known in the art. A wide variety of solvents or
complexing/solubilizing agents may be employed to solubilize silver
to the desired concentration in the impregnating medium. Useful
complexing/solubilizing agents include amines, ammonia, oxalic
acid, lactic acid and combinations thereof. Amines include an
alkylene diamine having from 1 to 5 carbon atoms. In one preferred
embodiment, the solution comprises an aqueous solution of silver
oxalate and ethylene diamine. The complexing/solubilizing agent may
be present in the impregnating solution in an amount from about 0.1
to about 5.0 moles per mole of silver, preferably from about 0.2 to
about 4.0 moles, and more preferably from about 0.3 to about 3.0
moles for each mole of silver.
[0038] When a solvent is used, it may be an organic solvent or
water, and may be polar or substantially or totally non-polar. In
general, the solvent should have sufficient solvating power to
solubilize the solution components. At the same time, it is
preferred that the solvent be chosen to avoid having an undue
influence on or interaction with the solvated promoters. Examples
of organic solvents include, but are not limited to, alcohols, in
particular alkanols; glycols, in particular alkyl glycols; ketones;
aldehydes; amines; tetrahydrofuran; nitrobenzene; nitrotoluene;
glymes, in particular glyme, diglyme and tetraglyme; and the like.
Organic-based solvents which have 1 to about 8 carbon atoms per
molecule are preferred. Mixtures of several organic solvents or
mixtures of organic solvent(s) with water may be used, provided
that such mixed solvents function as desired herein.
[0039] The concentration of silver in the impregnating solution is
typically in the range from about 0.1% by weight up to the maximum
solubility afforded by the particular solvent/solubilizing agent
combination employed. It is generally very suitable to employ
solutions containing from 0.5% to about 45% by weight of silver,
with concentrations from 5 to 35% by weight of silver being
preferred.
[0040] Impregnation of the selected support is achieved using any
of the conventional methods; for example, excess solution
impregnation, incipient wetness impregnation, spray coating, etc.
Typically, the support material is placed in contact with the
silver-containing solution until a sufficient amount of the
solution is absorbed by the support. Preferably the quantity of the
silver-containing solution used to impregnate the porous support is
no more than is necessary to fill the pores of the support. A
single impregnation or a series of impregnations, with or without
intermediate drying, may be used, depending, in part, on the
concentration of the silver component in the solution. Impregnation
procedures are described, for example, in U.S. Pat. Nos. 4,761,394,
4,766,105, 4,908,343, 5,057,481, 5,187,140, 5,102,848, 5,011,807,
5,099,041 and 5,407,888. Known prior procedures of pre-deposition,
co-deposition and post-deposition of various the promoters can be
employed.
[0041] After impregnation of the support with the silver-containing
compound, i.e., a silver precursor, a rhenium component, an alkali
metal component, and the optional other promoters, the impregnated
support is calcined for a time sufficient to convert the silver
containing compound to an active silver species and to remove the
volatile components from the impregnated support to result in a
catalyst precursor. The calcination may be accomplished by heating
the impregnated support, preferably at a gradual rate, to a
temperature in the range from about 200.degree. C. to about
600.degree. C., preferably from about 200.degree. C. to about
500.degree. C., and more preferably from about 200.degree. C. to
about 450.degree. C., at a pressure in the range from about 0.5 to
about 35 bar. In general, the higher the temperature, the shorter
the required heating period. A wide range of heating periods have
been suggested in the art; e.g., U.S. Pat. No. 3,563,914 discloses
heating for less than 300 seconds, and U.S. Pat. No. 3,702,259
discloses heating from 2 to 8 hours at a temperature of from
100.degree. C. to 375.degree. C., usually for duration of from
about 0.5 to about 8 hours. However, it is only important that the
heating time be correlated with the temperature such that
substantially all of the contained silver is converted to the
active silver species. Continuous or step-wise heating may be used
for this purpose.
[0042] During calcination, the impregnated support may be exposed
to a gas atmosphere comprising an inert gas or a mixture of an
inert gas with from about 10 ppm to 21% by volume of an
oxygen-containing oxidizing component. For purposes of this
invention, an inert gas is defined as a gas that does not
substantially react with the catalyst or catalyst precursor under
the conditions chosen for the calcination. Non-limiting examples
include nitrogen, argon, krypton, helium, and combinations thereof,
with the preferred inert gas being nitrogen. Non-limiting examples
of the oxygen-containing oxidizing component include molecular
oxygen (O.sub.2), CO.sub.2, NO, NO.sub.2, N.sub.2O, N.sub.2O.sub.3,
N.sub.2O.sub.4, or N.sub.2O.sub.5, or a substance capable of
forming NO, NO.sub.2, N.sub.2O, N.sub.2O.sub.3, N.sub.2O.sub.4, or
N.sub.2O.sub.5 under the calcination conditions, or combinations
thereof, and optionally comprising SO.sub.3, SO.sub.2 or
combinations thereof. Of these, molecular oxygen is a useful
embodiment, and a combination of O.sub.2 with NO or NO.sub.2 is
another useful embodiment. In a useful embodiment, the atmosphere
comprises from about 10 ppm to about 1% by volume of an
oxygen-containing oxidizing component. In another useful
embodiment, the atmosphere comprises from about 50 ppm to about 500
ppm of an oxygen-containing oxidizing component.
[0043] In another embodiment, the impregnated support, which has
been calcined as disclosed above, may optionally thereafter be
contacted with an atmosphere comprising a combination of oxygen and
steam, which atmosphere is substantially absent of an olefin, and
preferably, completely absent of an olefin. The atmosphere usually
comprises from about 2% to about 15% steam by volume, preferably
from about 2% to about 10% steam by volume, and more preferably
from about 2% to about 8% steam by volume. The atmosphere usually
comprises from about 0.5% to about 30% oxygen by volume, preferably
from about 1% to about 21% oxygen by volume, and more preferably
from about 5% to about 21% oxygen by volume. The balance of the gas
atmosphere may be comprised of an inert gas. Non-limiting examples
of the inert gas include nitrogen, argon, krypton, helium, and
combinations thereof, with the preferred inert gas being nitrogen.
The contacting is usually conducted at a temperature from about
200.degree. C. or higher. In one embodiment the contacting is
conducted at a temperature from about 200.degree. C. to about
350.degree. C. In another embodiment the contacting is conducted at
a temperature from about 230.degree. C. to about 300.degree. C. In
another embodiment the contacting is conducted at a temperature
from about 250.degree. C. to about 280.degree. C. In another
embodiment the contacting is conducted at a temperature from about
260.degree. C. to about 280.degree. C. Usually the contacting is
conducted for from about 0.15 hour or more. In one embodiment, the
contacting is conducted for from about 0.5 hour to about 200 hours.
In another embodiment, the contacting is conducted for from about 3
hours to about 24 hours. In another embodiment, the contacting is
conducted for from about 5 hours to about 15 hours.
[0044] Olefin Oxide Production
[0045] The epoxidation process may be carried out by continuously
contacting an oxygen-containing gas with an olefin, which is
preferably ethylene, in the presence of the catalyst produced by
the invention. Oxygen may be supplied to the reaction in
substantially pure molecular form or in a mixture such as air.
Molecular oxygen employed as a reactant may be obtained from
conventional sources. By way of example, reactant feed mixtures may
contain from about 0.5% to about 45% ethylene and from about 3% to
about 15% oxygen, with the balance comprising comparatively inert
materials including such substances as carbon dioxide, water, inert
gases, other hydrocarbons, and one or more reaction modifiers such
as organic halides. Non-limiting examples of inert gases include
nitrogen, argon, helium and mixtures thereof. Non-limiting examples
of the other hydrocarbons include methane, ethane, propane and
mixtures thereof. Carbon dioxide and water are byproducts of the
epoxidation process as well as common contaminants in the feed
gases. Both have adverse effects on the catalyst, so the
concentrations of these components are usually kept at a minimum.
Non-limiting examples of reaction moderators include organic
halides such as C.sub.1 to C.sub.8 halohydrocarbons. Preferably,
the reaction moderator is methyl chloride, ethyl chloride, ethylene
dichloride, ethylene dibromide, vinyl chloride or mixtures thereof.
Most preferred reaction moderators are ethyl chloride and ethylene
dichloride. Usually such reaction moderators are employed in an
amount from about 0.3 to about 20 ppmv, and preferably from about 1
to about 15 ppmv of the total volume of the feed gas.
[0046] A usual method for the ethylene epoxidation process
comprises the vapor-phase oxidation of ethylene with molecular
oxygen, in the presence of the inventive catalyst, in a fixed-bed
tubular reactor. Conventional, commercial fixed-bed ethylene-oxide
reactors are typically in the form of a plurality of parallel
elongated tubes (in a suitable shell) approximately 0.7 to 2.7
inches O.D. and 0.5 to 2.5 inches I.D. and 15-53 feet long filled
with catalyst. Such reactors include a reactor outlet which allows
the olefin oxide, un-used reactant, and byproducts to exit the
reactor chamber.
[0047] Typical operating conditions for the ethylene epoxidation
process involve temperatures in the range from about 180.degree. C.
to about 330.degree. C., and preferably, from about 200.degree. C.
to about 325.degree. C., and more preferably from about 225.degree.
C. to about 280.degree. C. The operating pressure may vary from
about atmospheric pressure to about 30 atmospheres, depending on
the mass velocity and productivity desired. Higher pressures may be
employed within the scope of the invention. Residence times in
commercial-scale reactors are generally on the order of about 0.1
to about 5 seconds. The present catalysts are effective for this
process when operated within these ranges of conditions.
[0048] The resulting ethylene oxide, which exits the reactor
through the reactor outlet, is separated and recovered from the
reaction products using conventional methods. For this invention,
the ethylene epoxidation process may include a gas recycle wherein
substantially all of the reactor effluent is readmitted to a
reactor inlet after substantially or partially removing the
ethylene oxide product and the byproducts including carbon dioxide.
In the recycle mode, carbon dioxide concentrations in the gas inlet
to the reactor may be, for example, from about 0.3 to about 5
volume percent.
[0049] The inventive catalysts have been shown to be particularly
selective for oxidation of ethylene with molecular oxygen to
ethylene oxide especially at high ethylene and oxygen conversion
rates. The conditions for carrying out such an oxidation reaction
in the presence of the catalysts of the present invention broadly
comprise those described in the prior art. This applies to suitable
temperatures, pressures, residence times, diluent materials,
moderating agents, and recycle operations, or applying successive
conversions in different reactors to increase the yields of
ethylene oxide. The use of the present catalysts in ethylene
oxidation reactions is in no way limited to the use of specific
conditions among those which are known to be effective.
[0050] For purposes of illustration only, the following are
conditions that are often used in current commercial ethylene oxide
reactor units: a gas hourly space velocity (GHSV) of 1500-10,000
h.sup.-1, a reactor inlet pressure of 150-400 psig, a coolant
temperature of 180-315.degree. C., an oxygen conversion level of
10-60%, and an EO production rate (work rate) of 7-20 lbs.
EO/cu.ft. catalyst/hr. The feed composition at the reactor inlet
may typically comprises 1-40% ethylene, 3-12% O.sub.2, 0.3-40%
CO.sub.2, 0-3% ethane, 0.3-20 ppmv total concentration of organic
chloride moderators, and the balance of the feed being comprised of
argon, methane, nitrogen or mixtures thereof.
[0051] The following non-limiting examples serve to illustrate the
invention.
EXAMPLES
Aluminum Oxides
[0052] The following aluminas designed as A and B in Table 1 were
used for the preparation of the catalysts. The different types of
alumina supports are commercially available.
TABLE-US-00001 TABLE I Physical characteristics of supports Mode 1
Mode 2 Mean Mean Total pore Surface Pore Pore Pore Pore Water
absorption volume.sup.(a) area.sup.(b) Diameter volume Diameter
volume Support [cc/g] [cc/g] [m.sup.2/g] [.mu.m] [%]* [.mu.m] [%]*
A 0.45 0.41 0.6 0.7 25 15.8 75 B 0.40 0.39 1.0 monomodal
distribution .sup.(a)Mercury intrusion data to 44.500 psia using
Micrometrics AutoPore IV 9500 (140.degree. contact angle, 0.480 N/m
surface tension of mercury) .sup.(b)Determined according to the
Method of Brunauer, Emmet and Teller *Percentage of the total pore
volume of the catalyst
[0053] Catalyst Preparation
[0054] Silver Solution
[0055] An 834 g portion of silver oxide (Sigma Aldrich) was added
to a stirred solution of 442 g oxalic acid dehydrate (ACS Certified
Reagent, Fisher) in about 2,800 g deionized water. A precipitate of
hydrated silver oxalate salt formed on mixing. Stirring was
continued for 0.5 hours. The precipitate was then collected on a
filter and washed with deionized water. Analysis showed that the
precipitate contained 50.5 wt % silver. Next, 213.9 g of the silver
oxalate precipitate was dissolved in a mixture of 77.2 grams
ethylenediamine (99+%, Aldrich) and 60.3 g deionized water. The
temperature of the solution was kept below 40.degree. C. by
combining the reagents slowly, and by cooling the solution. After
filtration, the solution contained roughly 30 wt % silver, and had
a specific gravity of 1.52 g/mL.
Example 1
Catalyst A
[0056] A 150 g portion of alumina support A was placed in a flask
and evacuated to approximately 0.1 torr prior to impregnation. To
the above silver solution aqueous solutions of cesium hydroxide,
perrhenic acid, and ammonium sulfate were added in order to prepare
a catalyst composition according to examples 3-10 through 7-20 of
U.S. Pat. No. 4,766,105. After thorough mixing, the promoted silver
solution was aspirated into the evacuated flask to cover the
carrier while maintaining the pressure at approximately 0.1 torr.
The vacuum was released after about 5 minutes to restore ambient
pressure, hastening complete penetration of the solution into the
pores. Subsequently, the excess impregnation solution was drained
from the impregnated carrier. Calcination of the wet catalyst was
done on a moving belt calciner. In this unit, the wet catalyst is
transported on a stainless steel belt through a multi-zone furnace.
All zones of the furnace are continuously purged with pre-heated,
ultra-high purity nitrogen and the temperature is increased
gradually as the catalyst passes from one zone to the next. The
heat is radiated from the furnace walls and from the preheated
nitrogen.
[0057] In this example the wet catalyst entered the furnace at
ambient temperature. The temperature was then increased gradually
to a maximum of about 450.degree. C. as the catalyst passed through
the heated zones. In the last (cooling) zone, the temperature of
the now activated was immediately lowered to less than 100.degree.
C. before it emerged into ambient atmosphere. The total residence
time in the furnace was approximately 45 minutes.
Example 2
Catalyst B (Comparative Example)
[0058] Catalyst B was prepared with alumina support B following the
procedure of Catalyst A.
[0059] Testing of the Catalyst
[0060] For testing, the catalyst was charged into a fixed-bed
stainless steel tube reactor (1/4 inch approximate inner diameter),
which was embedded in a heated copper block. The catalyst charge
consisted of 12 g crushed catalyst (1.0-1.4 mm particle size) and
the inlet gas flow was adjusted to 0.75 Nl/min. The feed gas
composition by volume was 25% ethylene, 7% oxygen, 2% carbon
dioxide, 0.5-5 ppmv ethyl chloride, and nitrogen ballast. Reaction
pressure was maintained at 300 psig. The reactor effluent was
analyzed by mass spectrometry at roughly 1-hour intervals.
[0061] The feed gas was introduced at 200.degree. C. and increased
by 1.degree. C./h until the EO concentration in the reactor outlet
reached 3.8% by volume. The ethyl chloride concentration in the
inlet gas was adjusted until the maximum selectivity was achieved.
Subsequently, the EO concentration in the outlet was lowered to 3%
by volume and the ethyl chloride concentration re-adjusted until
peak selectivity was observed. Finally, this procedure was repeated
at an EO outlet concentration of 2.2% by volume.
TABLE-US-00002 TABLE II Test results EO Catalyst A Catalyst B*
concentration Selectivity Temperature Selectivity Temperature [% v]
[% mole] [.degree. C.] [% mole] [.degree. C.] 3.8 90.0 248 86.4 244
3 90.9 240 88.0 242 2.2 92.0 236 89.8 239 *For comparison
[0062] The sole drawing clearly shows that although the catalysts
have similar intrinsic selectivites, i.e., the selectivity at 0%
ethylene and oxygen conversion, the catalyst according to the
invention has a significant lower selectivity decrease towards
higher conversion rates expressed by the EO concentration at the
reactor outlet.
[0063] Porosities are determined by the mercury porosimeter method;
see Drake and Ritter, "Ind. Eng. Chem. anal. Ed.," 17, 787 (1945).
The pore size distribution is determined by plotting the pores
diameter (.mu.m or Angstrom) against the differential pore volume
(ml.sub.Hg/g/pore diameter).
[0064] The specific surface area is determined according to the BET
method: See J. Am. Chem. Soc. 60, 3098-16 (1938).
[0065] While the present invention has been demonstrated and
described with reference to preferred embodiments, it will be
readily appreciated by those of ordinary skill in the art that
various changes and modifications may be made without departing
from the spirit and scope of the invention. It is therefore
intended that the claims be interpreted to cover the disclosed
embodiment, those alternatives which have been discussed, and any
and all equivalents thereto.
* * * * *