U.S. patent application number 11/010488 was filed with the patent office on 2005-05-05 for catalyst carriers.
This patent application is currently assigned to Saint-Gobain Norpro Corporation. Invention is credited to Gerdes, William H., Lockemeyer, John R., Remus, Donald J., Szymanski, Thomas, Yeates, Randall Clayton.
Application Number | 20050096219 11/010488 |
Document ID | / |
Family ID | 27753171 |
Filed Date | 2005-05-05 |
United States Patent
Application |
20050096219 |
Kind Code |
A1 |
Szymanski, Thomas ; et
al. |
May 5, 2005 |
Catalyst carriers
Abstract
The selectivity and activity of a silver-based olefin
epoxidation catalyst is found to be a function of the pore size
distribution in the alumina carrier on which it is deposited.
Specifically it is found advantageous to provide a carrier which
has a minimum of very large pores, (greater than 10 micrometers)
and a water absorption of 35 to 55% and a surface area of at least
1.0 m.sup.2/g. A method of making such carriers is also
described.
Inventors: |
Szymanski, Thomas; (Hudson,
OH) ; Remus, Donald J.; (Stow, OH) ;
Lockemeyer, John R.; (Sugar Land, TX) ; Yeates,
Randall Clayton; (Sugar Land, TX) ; Gerdes, William
H.; (Hudson, OH) |
Correspondence
Address: |
Ann M. Skerry
FAY, SHARPE, FAGAN, MINNICH & McKEE, LLP
Seventh Floor
1100 Superior Avenue
Cleveland
OH
44114-2518
US
|
Assignee: |
Saint-Gobain Norpro
Corporation
|
Family ID: |
27753171 |
Appl. No.: |
11/010488 |
Filed: |
December 13, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11010488 |
Dec 13, 2004 |
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10082761 |
Feb 25, 2002 |
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6831037 |
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Current U.S.
Class: |
502/439 ;
502/263 |
Current CPC
Class: |
B01J 23/66 20130101;
C01P 2006/17 20130101; B01J 37/0018 20130101; B01J 35/1009
20130101; C01P 2006/14 20130101; C01F 7/02 20130101; C01P 2006/16
20130101; C01P 2006/90 20130101; C07D 301/10 20130101; C01P 2006/12
20130101; B01J 35/1038 20130101; B01J 35/1042 20130101; B01J 35/108
20130101; B01J 21/12 20130101; B01J 21/04 20130101; B01J 35/10
20130101 |
Class at
Publication: |
502/439 ;
502/263 |
International
Class: |
B01J 023/48; B01J
023/50; B01J 021/04; B01J 023/02; B01J 021/08; B01J 021/12; B01J
021/14 |
Claims
1. A carrier for a catalyst which comprises at least 95% alpha
alumina with a surface area of from 1.0 to 2.6 m.sup.2/g and a
water absorption of from 35 to 55%, wherein the pores are
distributed such that at least 80% of the pore volume is in the
form of pores having pore diameters from 0.2 to 10 micrometers and
pores with diameters less than 0.2 micrometers represent from 0 to
10% of the total pore volume of the carrier.
2. A carrier according to claim 1 in which pores with diameters
between 0.2 and 10 micrometers provide a pore volume of at least
0.27 mL/g of the carrier.
3. A carrier according to claim 1 in which the pore volume is up to
0.56 mL/g.
4. A carrier according to claim 1 wherein the surface area is from
1.6 to 2.2 m.sup.2/g.
5. A carrier according to claim 1 wherein the pores are distributed
such that pores with pore diameters greater than 10 micrometers
represent less than 20% of the total pore volume.
6. A carrier according to claim 1 which further comprises from 0.2
to 0.8% of an amorphous silica compound.
7-11. (canceled)
12. A catalyst carrier for which comprises at least 95% alpha
alumina with a surface area of from 1.0 to 2.6 m.sup.2/g and a
water absorption of from 35 to 55%, wherein the pores are
distributed such that pores having pore diameters from 0.2 to 10
micrometers provide a pore volume of 0.3 to 0.56 mL/g of the
carrier.
13. A catalyst carrier formed by a method comprising: forming a
mixture comprising: a) from 50 to 90% by weight of a first
particulate alpha alumina having an average particle size
(d.sub.50) of from 10 to 90 micrometers; b) from 10 to 50% by
weight, based on the total alpha alumina weight, of a second
particulate alpha alumina having an average particle size
(d.sub.50) of from 2 to 6 micrometers; c) from 2 to 5% by weight of
an alumina hydrate; d) from 0.2 to 0.8% of an amorphous silica
compound, measured as silica; and e) from 0.05 to 0.3% of an alkali
metal compound measured as the alkali metal oxide; all percentages
being based on the total alpha alumina content of the mixture;
forming the mixture into particles; and firing the particles at a
temperature of from 1250 to 1470.degree. C. to form the
carrier.
14. A carrier according to claim 13 wherein the alumina hydrate is
boehmite.
15. A carrier according to claim 13 wherein the mixture comprises
up to 20% by weight of pore formers in the form of organic burnout
material.
16. A carrier according to claim 13 in which the mixture is
compounded with from 10 to 25% based on the mixture weight of
extrusion aids and pore formers and sufficient water to render the
mixture extrudable, and then extruded to form pellets which are
then dried and fired to produce the carrier.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to ceramic catalyst carriers
and particularly to carriers for catalysts useful in the
epoxidation of olefins such as for example the oxidation of
ethylene to ethylene oxide, ("EO"). For the sake of simplicity the
invention will be described in the context of this reaction but it
is understood to have wider applicability.
[0002] Catalyst performance is assessed on the basis of selectivity
and reactor temperature. The selectivity is the percentage of the
olefin in the feed stream converted to the desired product under
standard flow conditions aimed at converting a fixed percentage of
the olefin in the feed stream and in the commercial production of
ethylene oxide this figure is usually in the 80's. The percentage
of olefin reacted normally decreases with time and to maintain the
constant level the temperature of the reaction is increased.
However this adversely affects the selectivity of the conversion to
the desired product. In addition, the equipment used can tolerate
temperatures only up to a certain level so that it is necessary to
terminate the reaction when the temperature reaches a level
inappropriate for the reactor. Thus the longer the selectivity can
be maintained at a high level and at an acceptably low temperature,
the longer the catalyst/carrier charge can be kept in the reactor
and the more product is obtained. Quite modest improvements in the
maintenance of selectivity over long periods yields huge dividends
in terms of process efficiency.
[0003] Epoxidation catalysts usually comprise a silver component,
usually with a modifier co-deposited therewith on a ceramic
carrier. It has been found that the nature of this carrier exerts a
very significant influence of the performance of the catalyst
carried thereon but that the reasons for that influence are not
completely clear.
[0004] Carriers are typically formed of a temperature resistant
ceramic oxide such as alpha alumina and in general higher purity
has been found to correlate with better performance. However it has
been found for example that the presence of minor amounts of
elemental impurities in the carrier such as alkali metals and some
forms of silica can have a beneficial effect.
[0005] Intuitively it might also be considered that the higher the
surface area of the carrier, the greater the area available for
deposition of the catalyst and therefore the more effective the
catalyst deposited thereon. This is however found not always to be
the case and in modern carrier/catalyst combinations the tendency
is to use a carrier with a surface area of less than 1.0 m.sup.2/g
since these maintain an acceptable activity and selectivity levels
while maintaining the necessary crush strength to withstand long
term service in a commercial reactor without losing their physical
integrity. In addition it has been found that carriers with high
surface areas often have high activity but inferior
selectivity.
[0006] It has now been found however that the picture with respect
to carrier surface area is significantly more complicated than was
at first appreciated since the nature of the porosity of the
carrier has been found to play a most significant role. This
discovery is the foundation for the present invention which has led
to the development of a catalyst/carrier combination with excellent
activity and unusually prolonged retention of a very high
selectivity level at modest temperatures.
SUMMARY OF THE INVENTION
[0007] The present invention provides a carrier for an olefin
epoxidation catalyst which comprises at least 95% alpha alumina
with a surface area of from 1.0 to 2.6 m.sup.2/g and preferably at
least 1.6 to 2.2 m.sup.2/g and a water absorption of from 35 to
55%, wherein the pores are distributed such that at least 70%, and
preferably at least 80% of the pore volume is provided by pores
that have pore diameters from 0.2 to 10 micrometers and provide a
pore volume of at least 0.27 mL/g of the carrier. In preferred
carriers according to the invention pores with diameters greater
than 10 micrometers represent from 0 to 20% and preferably from 0
to 15% of the total pore volume. More preferably still pores with
pore sizes less than 0.2 micrometer represent from 0 to 10% of the
total pore volume. The mercury pore volume is typically up to 0.56
mL/g and more commonly from 0.35 to 0.45 mL/g.
[0008] "Surface area" as the term is used herein is understood to
refer to the surface area as determined by the BET (Brunauer,
Emmett and Teller) method as described in Journal of the American
Chemical Society 60 (1938) pp309-316. While the surface area
correlates with the number and sizes of the pores and hence the
pore volume, it should be noted that as a practical matter the
carriers need to have a certain minimum crush strength which in
turn is related to the thickness of the walls surrounding the
pores. Reducing this thickness makes the walls more likely to
rupture under normal loading conditions such that there is a
practical limitation to the surface area of the commercially
interesting carriers, at least as designed for incorporation in
catalyst combinations using current technology.
[0009] The pore volume and the pore size distribution are measured
by a conventional mercury intrusion device in which liquid mercury
is forced into the pores of the carrier. Greater pressure is needed
to force the mercury into the smaller pores and the measurement of
pressure increments corresponds to volume increments in the pores
penetrated and hence to the size of the pores in the incremental
volume. The pore volume in the following description was determined
by mercury intrusion under pressures increased by degrees to a
pressure of 3.0.times.10.sup.8 Pa using a Micromeritics Autopore
9200 model (130.degree. contact angle and mercury with a surface
tension of 0.473 N/m).
[0010] While the pore volume of the carriers according to the
invention is at least 0.27 mL/g it is preferred that pores that
have pore diameters from 0.2 to 10 microns provide a pore volume
between 0.30 to 0.56 mL/g to ensure that the carriers have
commercially acceptable physical properties.
[0011] Water absorption is measured by measuring the weight of
water that can be absorbed into the pores of the carrier as a
percentage of the total weight of the carrier. As indicated above
this can be in the range 35 to 55% but preferred carriers have a
water absorption of 38 to 50% and most preferably from 40 to
45%.
[0012] The invention also comprises a method of making a carrier
for an olefin epoxidation catalyst which comprises forming a
mixture comprising:
[0013] a) from 50 to 90% by weight of a first particulate alpha
alumina having an average particle size (d.sub.50) of from 10 to
90, preferably from 10 to 60, and most preferably from 20 to 40
micrometers; and
[0014] b) from 10 to 50% by weight, based on the total alpha
alumina weight, of a second particulate alpha alumina having an
average particle size (d.sub.50) of from 2.0 to 6.0
micrometers;
[0015] c) from 2 to 5% by weight of an alumina hydrate;
[0016] d) from 0.2 to 0.8% of an amorphous silica compound,
measured as silica; and
[0017] e) from 0.05 to 0.3% of an alkali metal compound measured as
the alkali metal oxide;
[0018] all percentages being based on the total alpha alumina
content of the mixture, and then foaming the mixture into particles
and firing the particles at a temperature of from 1250 to
1470.degree. C. to form the carrier.
[0019] The carrier particles can be formed by any convenient
conventional means such as by extrusion or molding. Where finer
particles are desired these can be obtained for example by a spray
drying process.
[0020] Where the particles are formed by extrusion it may be
desirable to include conventional extrusion aids, optional burnout
material and water. The amounts of these components to be used are
to some extent interdependent and will depend on a number of
factors that relate to the equipment used. However these matters
are well within the general knowledge of a man skilled in the art
of extruding ceramic materials.
[0021] The average particle size, referred to herein as "d.sub.50",
is the value as measured by a Horiba (or similar) particle size
analyzer after five minutes of sonification and represents the
particle diameter at which there are equal volumes of particles
larger and smaller than the stated average particle size.
[0022] The method of the invention is well adapted to produce the
carriers of the invention in view of the careful matching of
particles sizes of the alumina components. Adjustments to the water
absorption can be achieved by incorporation of conventional burnout
materials which are typically finely divided organic compounds such
as granulated polyolefins, particularly polyethylene and
polypropylene, and walnut shell flour. However burnout material is
used primarily to ensure the preservation of a porous structure
during the green, (or unfired), phase in which the mixture may be
shaped into particles by molding or extrusion processes. It is
totally removed during the firing to produce the finished carrier.
In practice the above pore size limitations mean that the carriers
according to the invention do not have excessive numbers of large
pores, (that is pores larger than about 10 micrometers), and have
relatively few pores below 0.2 micrometer than is usually the
case.
[0023] The carriers of the invention are preferably made with the
inclusion of a bond material comprising silica with an alkali metal
compound in sufficient amount to substantially prevent the
formation of crystalline silica compounds. Typically the bond also
contains a hydrated alumina component such as boehmite or gibbsite.
The silica component can be a silica sol, a precipitated silica, an
amorphous silica or an amorphous alkali metal silicate or
aluminosilicate. The alkali metal compound can be for example a
salt such as a sodium or potassium salt. A convenient bond material
to be incorporated with the alumina particles used to form the
carrier is a mixture of boehmite, an ammonia stabilized silica sol
and a soluble sodium salt. The same effect can be achieved by
incorporation of conventional ceramic bonds formulated to contain
aluminosilicates and an alkali metal component. It is further found
that the performance of the carrier/catalyst combination is
significantly enhanced if the carrier is washed to remove soluble
residues before deposition of the catalyst.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0024] Alumina based carriers can be made in a number of different
ways, each of which may affect the pore size distribution.
Generally they are made by firing particulate mineral components at
an elevated temperature until the particles sinter together.
Porosity can be affected by the size of the particles sintered
together as well as the time of sintering. In a preferred
embodiment of the present invention alumina particles with two
different median particle size distributions are used: large
particles and small particles. These may be added as separate
components of the formulation from which the carrier is formed or
they may be generated in situ by milling friable agglomerates until
the blend of particle sizes obtains has the desired bimodal
distribution. Thus, in theory it is also possible to achieve the
carriers of the invention by starting with alumina particles with a
single distribution mode. It is intended that all such approaches
shall fall within the scope of the process claimed herein.
[0025] If sintering is continued until the particles are bonded
together, either by the formation of bond posts from any added bond
material or through sintering, but not to the point at which the
porosity resulting from the packing of the particles is
significantly reduced, larger particles will result in larger pores
and smaller will result in finer pores. As indicated above, water
absorption can also be affected by the use of burnout material
which allows more complete sintering without too great a reduction
in surface area of the carrier.
[0026] The use of a bond material reduces the length of sintering
time required to bond the particles together and since sintering is
commonly associated with reduction in pore volume, the use of such
a bond is a feature of this-invention. The selection of the bond
can also act to produce a more receptive carrier surface. As
indicated above, the bond materials include silica in some form
together with an alkali metal component which ensures that the
silica is in an amorphous form.
[0027] The preferred alumina hydrate is boehmite though gibbsite,
bayerite or diaspore could also be used. It is also preferred that
the carrier be prepared in the form of pellets, the size of which
is in general determined by the dimensions of the reactor in which
they are to be deposited. Generally however it is found very
convenient to use pellets in the form of hollow cylinders with
length and cross-sectional dimensions about the same and from 5 to
10 mm. The pellets can be formed from the mixture by any convenient
molding process but preferably they are formed by extrusion of the
mixture. To facilitate such extrusion the mixture is usually
compounded with up to about 25% and preferably from 10 to 20% by
weight based on the mixture weight of extrusion aids and burnouts
and thereafter enough water is added to make the mixture
extrudable. Extrusion aids are well known in the art and include
materials such as vaseline, polyolefin oxides and polyethylene
glycol. Likewise organic burnouts are well known in the art and
include materials such as granulated polyolefins, powdered walnut
shells and other fine organic particulates.
[0028] The extrusion aids are added in the amounts necessary to
facilitate extrusion of the specific formulation and this will be
influenced by the particle sizes, amount of bond material, (if any)
and water present and the design of the extruder. The actual amount
of extrusion aids to be used is not critical to the final product
and appropriate amounts will be readily apparent to the man of
skill in the art. They are removed completely upon firing. The
burnout materials are also added in amounts as desired to cause an
increase in the water absorption of the product prepared in
accordance with the invention. It is possible that either extrusion
aids or burnouts, in appropriate circumstances, could provide the
whole of the combined weight of such additives, (as indicated
above), that could be added to the formulation.
[0029] The shaped pellets are then dried and fired at a temperature
high enough to ensure that the alumina particles are joined
together by a sintering action or by the formation of bond posts
formed from a bond material incorporated in the mixture or by a
mixture of the two mechanisms. Generally firing takes place between
about 1250 and 1470.degree. C. and preferably about 1300 to
1440.degree. C. for a period of up to about 5 hours and preferably
for from 2 to 4 hours. The effect on pore size distribution of the
selection of materials and bonds is illustrated by comparison of
the Carriers of the invention, (INV-1, INV-2 and INV-3) with a
comparative carrier, (COMP-A). The following Table 1 shows the
various significant physical characteristics of the carriers
according to the invention and those of Comparative Carrier.
1TABLE 1 INV-1 INV-2 INV-3 COMP-A % PORES <0.2.mu. 5 9 3 0 %
0.2-10.mu. 92 72 95 64 % >10.mu. 3 19 2 36 Total Hg P.V. mL/g
0.41 0.42 0.56 0.40 Surface Area m.sup.2/g 2.04 2.11 2.51 0.73
Water Absorption % 42.4 48.9 55 40.2 0.2-10.mu. P.V. mL/g 0.37 0.30
0.53 0.26 P.V. refers to pore volume.
[0030] Preparation of Carriers
[0031] As indicated above the carriers of the invention can be
prepared in a number of ways that would be understood by the man of
skill in the art. In the production of a preferred carrier,
(INV-1), a mixture was made of the following ingredients, all
proportions being by weight as the components exist in the fired
carrier:
[0032] 1. 67.4% of an alpha alumina with an average particle size,
(d50), of 29 micrometers;
[0033] 2. 29% of an alpha alumina with an average particle size,
(d50), of 3 micrometers;
[0034] 3. 3% of boehrnite;
[0035] 4. 0.5% of silica, (in the form of an ammonia stabilized
silica sol); and
[0036] 5. 0.1% of sodium oxide, (in the form of sodium
acetate).
[0037] The silica and sodium acetate were used together with the
boehmite to provide a bond conferring green strength. To this
mixture were added 5% by weight of petroleum jelly, 9% of a mixture
of fine particulate organic burnouts and 0.1% of the mixture weight
of boric acid. Water was then added in an amount to make the
mixture extrudable and this mixture was then extruded to form
hollow cylinders that are about 8 mm in diameter and 8 mm long.
These were then dried and fired in a kiln at 1425.degree. C. to
produce the porous alumina carrier of the invention.
[0038] The INV-2 carrier was prepared in exactly the same way as
INV-1 except that the mixture contained 14% of the mixture of fine
particulate organic burnouts rather than 9%. The INV-3 carrier was
prepared with 14% of petroleum jelly and 8% fine organic
burnouts.
[0039] The COMP-A carrier was made according to the process
described in Example 1 of U.S. Pat. No. 5,100,859. The carriers
evaluated were all made from aluminas and the proportions and
average particle sizes, d.sub.50, of these components are shown in
the following Table 2. The balance of the proportions, to make 100%
was bond material.
2TABLE 2 Al.sub.2O.sub.3 COMP-A INV-1 INV-2 INV-3 1 98.8% 3.mu. 29%
3.mu. 29% 3.mu. 20% 3.mu. 2 67.4% 29.mu. 67.4% 29.mu. 3* 3% 3% 3% 4
76.4% 16.mu. *indicates boehmite with the amount calculated as
Al.sub.2O.sub.3
[0040] Evaluation of the Carriers
[0041] The comparative carrier was then evaluated against the INV-1
carrier of the present invention. The carriers were used to prepare
ethylene oxide catalysts using the method generally described in
U.S. Pat. No. 5,380,697. The performance of the carrier according
to the invention was then evaluated against the comparative
carriers under equivalent conditions.
[0042] The catalysts were used to produce ethylene oxide from
ethylene and oxygen. To do this, 1.5 to 2 g. of crushed catalyst
were loaded into a 6.35 mm. inside diameter stainless steel
U-shaped tube. The tube was immersed in a molten metal bath (heat
medium) and the ends were connected to a gas flow system. The
weight of catalyst used and the inlet gas flow rate were adjusted
to give a gas hourly space velocity of 6800 cc/cc of catalyst/hr.
The inlet gas pressure was 210 psig.
[0043] The gas mixture passed through the catalyst bed, in a
"once-through" operation, during the entire test run including the
start-up, consisted of 25% ethylene, 7% oxygen, 5% carbon dioxide,
63% nitrogen and 2.0 to 6.0 ppmv ethyl chloride.
[0044] The initial reactor temperature was 180.degree. C. and this
was ramped up at a rate of 10.degree. C. per hour to 225.degree. C.
and then adjusted so as to achieve a constant ethylene oxide
content of 1.5 vol % in the outlet gas stream at an ethyl chloride
concentration of 2.5 ppmv. Performance data at this conversion
level are usually obtained when the catalyst has been on stream for
a total of at least 1-2 days.
[0045] The initial performance values for selectivity and
temperature are reported in Table 3 below.
3 TABLE 3 CARRIER Selectivity (%) Temperature (.degree. C.)
INVENTION-1 82.5 224 INVENTION-2 81.9 232 COMP-A 81.9 240
[0046] The catalysts based on the Carrier COMP-A had a selectivity
value significantly below that based on the INV-1 carrier and
required much higher temperatures. The fact that the reaction
maintained the superior selectivity level at such a low
temperature, indicated strongly that the formulation based on the
Carrier of the invention would have much greater longevity than
formulations based on the comparative carrier, (A).
[0047] These improvements are highly valuable commercially since
the longer the reaction can be run at high levels of activity and
selectivity without changing the very expensive catalyst/carrier
charge, the more economical is the process.
* * * * *