U.S. patent application number 17/744870 was filed with the patent office on 2022-09-01 for porous bodies with enhanced pore architecture prepared without a high-temperature burnout material.
This patent application is currently assigned to Scientific Design Company, Inc.. The applicant listed for this patent is Scientific Design Company, Inc.. Invention is credited to Jean Adam, Michael Di Mare, Paul E. Ellis, JR., Wojciech L. Suchanek.
Application Number | 20220274093 17/744870 |
Document ID | / |
Family ID | 1000006336974 |
Filed Date | 2022-09-01 |
United States Patent
Application |
20220274093 |
Kind Code |
A1 |
Suchanek; Wojciech L. ; et
al. |
September 1, 2022 |
POROUS BODIES WITH ENHANCED PORE ARCHITECTURE PREPARED WITHOUT A
HIGH-TEMPERATURE BURNOUT MATERIAL
Abstract
A precursor mixture for producing a porous body, wherein the
precursor mixture comprises: (i) at least one milled alpha alumina
powder having a particle size of 0.1 to 6 microns, (ii)
non-silicate powder that functions as a binder of the alpha alumina
powders, and (iii) at least one burnout material having a particle
size of 1-10 microns and a decomposition temperature of less than
550.degree. C., with the proviso that a burnout material having a
decomposition temperature of 550.degree. C. or greater is excluded
from the precursor mixture.
Inventors: |
Suchanek; Wojciech L.;
(Wyckoff, NJ) ; Ellis, JR.; Paul E.; (West New
York, NJ) ; Di Mare; Michael; (Morristown, NJ)
; Adam; Jean; (Roselle, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Scientific Design Company, Inc. |
Little Ferry |
NJ |
US |
|
|
Assignee: |
Scientific Design Company,
Inc.
Little Ferry
NJ
|
Family ID: |
1000006336974 |
Appl. No.: |
17/744870 |
Filed: |
May 16, 2022 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15834365 |
Dec 7, 2017 |
11331652 |
|
|
17744870 |
|
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|
62506301 |
May 15, 2017 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C04B 2111/0081 20130101;
C04B 38/00 20130101; B01J 37/04 20130101; B01D 71/022 20130101;
B01J 35/002 20130101; C04B 35/624 20130101; B01J 35/0006 20130101;
B01J 35/04 20130101; C04B 2235/5436 20130101; B01J 37/082 20130101;
C04B 2235/3218 20130101; B01J 35/1009 20130101; B01D 53/22
20130101; C04B 2235/3217 20130101; B01J 37/0018 20130101; C04B
2111/00793 20130101; C04B 2235/5454 20130101; B01J 21/04 20130101;
B01J 37/0201 20130101; C07D 301/10 20130101; B01J 23/50
20130101 |
International
Class: |
B01J 23/50 20060101
B01J023/50; B01J 21/04 20060101 B01J021/04; B01J 35/00 20060101
B01J035/00; B01J 35/10 20060101 B01J035/10; B01J 37/00 20060101
B01J037/00; B01D 53/22 20060101 B01D053/22; B01J 37/08 20060101
B01J037/08; B01J 37/02 20060101 B01J037/02; C07D 301/10 20060101
C07D301/10; B01D 71/02 20060101 B01D071/02; B01J 37/04 20060101
B01J037/04; B01J 35/04 20060101 B01J035/04; C04B 35/624 20060101
C04B035/624; C04B 38/00 20060101 C04B038/00 |
Claims
1. A method for producing a porous body, the method comprising:
providing a precursor mixture comprising (i) milled alpha alumina
powder having a particle size of 0.1 to 6 microns, (ii)
non-silicate binder of the alpha alumina powders, and (iii) a
burnout material having a particle size of 1-10 microns and a
decomposition temperature of less than 550.degree. C., with the
proviso that a burnout material having a decomposition temperature
of 550.degree. C. or above is excluded; forming the precursor
mixture into a predetermined shape; and subjecting the shape to a
heat treatment step in which the shape is sintered to produce the
porous body.
2. The method of claim 12, further comprising unmilled alpha
alumina powder having a particle size of 10 to 100 microns in said
precursor mixture.
3. The method of claim 13, wherein the weight ratio of milled to
unmilled alpha alumina powder is in a range of 0.25:1 to about
5:1.
4. The method of claim 12, wherein unmilled alpha alumina powder is
excluded from the precursor mixture.
5. The precursor mixture of claim 12, wherein the non-silicate
binder is nano-sized boehmite
6. The method of claim 12, wherein the providing the precursor
mixture comprises: (i) dispersing said non-silicate binder into
water to produce a dispersion of said binder; (ii) adding said
milled alpha alumina powder having a particle size of 0.1 to 6
microns to the dispersion of the non-silicate binder, and mixing
until a first homogeneous mixture is obtained, wherein said
non-silicate binder functions as a binder of the alpha alumina
powder; and (iii) adding said burnout material having said particle
size of 1-10 microns and said decomposition temperature of less
than 550.degree. C., and mixing until a second homogeneous mixture
is obtained.
7. The method of claim 12, wherein said heat treatment step
comprises: subjecting the formed shape to a heat treatment step
within a temperature in a range of 35.degree. C.-550.degree. C. to
remove water and burnout the burnout material to produce a
pre-fired porous body; and subjecting the pre-fired porous body to
a sintering step at a temperature within a range of 900.degree.
C.-2000.degree. C. to produce said porous body.
8. The method of claim 12, wherein said porous body possesses a
porosity derived only from said burnout material having said
decomposition temperature of less than 550.degree. C.
9. The method of claim 12, wherein said porous body possesses at
least one of a water absorption of at least 30%, average crush
strength of at least 30 N, and a BET surface area of at least 0.3
m.sup.2/g.
10. The method of claim 12, wherein said porous body possesses a
pore architecture that provides at least one of a tortuosity of 7
or less, a constriction of 4 or less, and a permeability of 30
mdarcys or greater.
11. The method of claim 12, wherein said burnout material having
said decomposition temperature of less than 550.degree. C.
comprises a polyolefin powder.
12. The method of claim 17, wherein said step (ii) includes, either
simultaneous or subsequent to adding and mixing the milled alpha
alumina powder, adding unmilled alpha alumina powder having a
particle size in a range of 10-100 microns, and mixing until said
first homogeneous mixture is obtained.
13. The method of claim 23, wherein the weight ratio of milled to
unmilled alpha alumina powder is in a range of 0.25:1 to about
5:1.
14. The method of claim 12, wherein unmilled alpha alumina powder
is excluded from the method to produce the porous body.
15. The method of claim 12, wherein a silicon-containing substance
is substantially excluded from the method to produce the porous
body.
16. The method of claim 12, wherein a sodium-containing substance
is substantially excluded from the precursor mixture.
17. The method of claim 12, wherein, after said heat treatment step
to form a porous body, said method further comprises depositing
silver on and/or in said porous body.
18. The method of claim 1, wherein the non-silicate binder is
nanosized having a particle size of less than 50 nm.
19. The method of claim 1, wherein the non-silicate binder is
selected from the group consisting of an aluminum hydroxide, an
oxide-hydroxide, a transition alumina, and an organic or an
inorganic precursor that produces alpha-alumina upon firing.
20. The method of claim 1, wherein the burn material comprises a
first burnout material having a first decomposition temperature of
less than 550.degree. C. and a second burnout material having a
second decomposition of less than 550.degree. C. bur greater than
the first decomposition temperature, and wherein the first burnout
material is present in the precursor mixture in a greater amount
than the second burnout material.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. patent application
Ser. No. 15/834,365, filed Dec. 7, 2017, and also claims the
benefit of U.S. Provisional Patent Application No. 62/506,301 filed
May 15, 2017, the entire content and disclosure of each is
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to porous bodies and more
particularly to porous bodies with enhanced pore architecture that
can be prepared utilizing a precursor mixture that facilitates gas
transport/diffusion during heat treatment of the precursor
mixture.
BACKGROUND
[0003] In the chemical industry and the chemical engineering
industry, reliance is oftentimes made on using porous bodies,
including porous ceramic bodies that are capable of performing or
facilitating separations or reactions and/or providing areas for
such separations and reactions to take place. Examples of
separations or reactions include: filtration of gases and liquids,
adsorption, reverse osmosis, dialysis, ultrafiltration, or
heterogeneous catalysis. Although the desired physical and chemical
properties of such porous bodies vary depending on the particular
application, there are certain properties that are generally
desirable in such porous bodies regardless of the final application
in which they will be utilized.
[0004] For example, porous bodies may be substantially inert so
that the porous bodies themselves do not participate in the
separations or reactions taking place around, on or through them in
a way that is undesired, unintended, or detrimental. In
applications where it is desired to have the components that are
being reacted or separated pass through, or diffuse into, the
porous body, a low diffusion resistance (e.g., high effective
diffusivity) would be advantageous.
[0005] In some applications, the porous bodies are provided within
a reaction or separation space, and so they are desirably of high
pore volume and/or high surface area, in order to improve the
loading and dispersion of the desired reactants, and also to
provide enhanced surface area on which the reactions or separations
can take place. These applications also require sufficient
mechanical integrity to avoid being damaged, i.e., crushed, chipped
or cracked, during transport or placement. However, combination of
high mechanical strength with high pore volume in a porous body is
not easy to achieve because the strength decreases exponentially
with increasing porosity.
[0006] In view of the above, there is a need for providing porous
bodies that have a pore architecture that has enhanced fluid
transport properties, particularly gas diffusion properties and
high mechanical integrity. Also, there is a need for providing a
method of preparing porous bodies having such enhanced pore
architecture, while also facilitating gas transport of oxygen,
products of the burn-out oxidation, etc., during heat treatment of
a precursor mixture that is used in providing the porous
bodies.
SUMMARY
[0007] The present invention is directed to porous bodies that have
an enhanced pore architecture and a porosity that is derived
totally from a burnout material that decomposes at a temperature of
less than 550.degree. C. (i.e., a low-temperature burnout
material).
[0008] The porous bodies of the present invention can be prepared
by first providing a precursor mixture, wherein the precursor
mixture comprises: (i) milled alpha alumina powder having a
particle size of 0.1 to 6 microns, (ii) optionally, unmilled alpha
alumina powder having a particle size of 10 to 100 microns (iii)
non-silicate binder, preferably nanosized, wherein it functions as
a binder of the alpha alumina powders, (iv) a burnout material
having a particle size of 1-10 microns and a decomposition
temperature of less than 550.degree. C., and (v) optionally, other
additives, such as solvents and lubricants. The precursor mixture
is absent of any burnout material whose decomposition temperature
is 550.degree. C. or greater (i.e., high-temperature burnout
materials). All components of the porous body precursor mixture are
homogeneously mixed.
[0009] Another embodiment of the present invention is directed to
methods for fabricating a porous body in which the above-described
precursor mixture is formed into a shape, and the formed shape is
subjected to a heat treatment process to remove volatiles (e.g.,
water and burnout materials) and sinter the shape into a porous
body.
[0010] In other aspects, the instant invention is also directed to
the porous body produced by the above-described method, as well as
filters, membranes, catalyst supports, and the like, particularly
ethylene oxidation (i.e., epoxidation) catalysts comprising the
porous body (i.e., carrier) described above, along with a catalytic
amount of silver. In some embodiments, the resulting epoxidation
catalyst exhibits an increased catalyst activity and/or a
maintained or improved selectivity.
[0011] The instant invention is also directed to a method for the
vapor phase conversion of ethylene to ethylene oxide (EO) by use of
the above-described catalyst. The method includes reacting a
reaction mixture comprising ethylene and oxygen in the presence of
the ethylene epoxidation catalyst described above.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
[0012] FIG. 1 is a schematic (i.e., not exact) pictorial
representation showing limited gas diffusion in a precursor mixture
used for providing a porous body in which the precursor mixture
contains both low-temperature and high-temperature burnout
particles.
[0013] FIG. 2 is a schematic pictorial representation showing
enhanced gas diffusion in a precursor mixture used for providing a
porous body in which the precursor mixture contains low-temperature
burnout particles and is void of any high-temperature burnout
particles.
[0014] FIG. 3 shows a plot of the thermogravimetric analysis of a
precursor mixture containing a burnout mixture of granulated
polyethylene and graphite; the precursor mixture also contains
organic lubricant, which undergoes low-temperature oxidation as
well.
[0015] FIG. 4 shows a plot of the thermogravimetric analysis of a
precursor mixture containing granulated polyethylene as the sole
burnout material; the precursor mixture also contains organic
lubricant, which undergoes low-temperature oxidation as well.
DETAILED DESCRIPTION
[0016] The present invention will now be described in greater
detail by referring to the following discussion and drawings that
accompany the present invention. In the following description,
numerous specific details are set forth, such as particular
structures, components, materials, dimensions, processing steps and
techniques, in order to provide an understanding of the various
embodiments of the present invention. However, it will be
appreciated by one of ordinary skill in the art that the various
embodiments of the present invention may be practiced without these
specific details. As used throughout the present invention, the
term "about" generally indicates no more than .+-.10%, .+-.5%,
.+-.2%, .+-.1% or .+-.0.5% from a number.
[0017] Porous bodies with enhanced pore architecture and their
method of preparation are disclosed in U.S. Patent Application
Publication Nos. 2016/0354760A1 and 2016/0354759A1, the entire
content of each of the aforementioned publications is incorporated
herein by reference. In such materials, the pore architecture is
derived from the burnout particles and pores originally present in
alumina agglomerates/aggregates. When the precursor mixture of the
porous body contains small amounts of unmilled alumina powder
(porous agglomerates) or when the content of the burnout material
is substantial compared to the alumina content, the pore structure
in the formed precursor mixture before heat treatment (pre-firing
and firing) is essentially closed/plugged by the burnout particles.
Therefore, heat treatment (pre-firing and firing) of such precursor
mixtures requires higher temperatures and/or longer durations to
allow for sufficient gas diffusion through all pores of the porous
body.
[0018] The above mentioned diffusion problem can become more severe
when some of the burnout particles undergo oxidation at
high-temperatures, for example, at about 800.degree. C. for
graphite particles. With a mixture of low-temperature (i.e., less
than 550.degree. C.) and high-temperature (i.e., 550.degree. C. or
above) burnout particles in the precursor mixture, even the
oxidation of the low-temperature burnout particles can be slowed
down by the presence of a burnout material that oxidizes at high
temperature, and thus limits the gas transport of oxygen and
products of the burn-out oxidation in the material. FIG. 1
schematically illustrates such an instance in which a mixture of
low-temperature and high-temperature burnout particles is present
in a precursor mixture that is used to form the porous body. In
FIG. 1, gas diffusion (i.e., transport of O.sub.2, burn-out
oxidation products, etc.) is limited.
[0019] The present invention provides a solution to this problem by
eliminating the high-temperature burnout material from the
precursor mixture so as to "unplug" the pores and thus facilitate
gas transport during the oxidation stage to enhance the oxidation
kinetics of the burnout. FIG. 2 schematically illustrates the
present invention in which the precursor mixture used to provide
the porous body is free of high-temperature burnout particles. In
FIG. 2, gas diffusion (i.e., transport of O.sub.2, burn-out
oxidation products, etc.) is enhanced.
[0020] According to the present invention, precursor mixtures that
are used in providing porous bodies of enhanced pore architecture,
which have a large content of burnout material and/or a small
content of unmilled alumina powder, will benefit from eliminating a
high-temperature burnout material. Such change facilitates gas
transport during heat treatment (pre-firing and firing) thus
reducing time and/or temperature of the heat treatment (pre-firing
and firing). Moreover the elimination of high-temperature burnout
materials in the precursor mixture increases the throughput of the
heat treatment.
[0021] In one aspect, the present invention is directed to a method
for producing a porous body in which a specially crafted precursor
mixture is formed to a shape and subjected to a heat treatment step
to produce the porous body. In particular embodiments, the
precursor mixture includes at least: (i) milled alpha alumina
powder having a particle size of 0.1 to 6 microns, or more
typically, 0.25 to 4 microns, (ii) non-silicate powder that
functions as a binder of the alpha alumina powders, and (iii) a
burnout material having a particle size of 1-10 microns and a
decomposition temperature of less than 550.degree. C., with the
proviso that a burnout material having a decomposition temperature
of 550.degree. C. or greater is excluded in (and is thus absent
from) the precursor mixture of the present invention. Burnout
materials having a decomposition temperature of less than
550.degree. C. may be referred to herein as a low-temperature
burnout material, while burnout materials having a decomposition
temperature of 550.degree. C. or greater may be referred to herein
as a high-temperature burnout material. Since high-temperature
burnout materials are excluded, the precursor mixture of the
present invention may be referred to herein as a high-temperature
burnout free precursor mixture.
[0022] As mentioned above, only burnout materials that can
decompose at a temperature of less than 550.degree. C. (i.e.,
low-temperature burnout materials) are present in the precursor
mixture of the present invention. In one embodiment, the
low-temperature burnout material that can be used in the present
invention has a decomposition temperature from 200.degree. C. to
550.degree. C. Exemplary low-temperature burnout materials that can
be used in the present application include, but are not limited to,
granulated polyolefins (e.g., polyethylene and polypropylene),
Vaseline.RTM., petroleum jelly, waxes, polymers, plastics, oils,
and other natural or artificial organic compounds and materials. In
some embodiments, a single low-temperature burnout material such
as, for example, granulated polyethylene, is employed. In other
embodiments, a combination of at least two low-temperature burnout
materials such as, for example, granulated polyethylene and
polypropylene, can be employed. In embodiments in which a mixture
of low-temperature burnout materials is employed, it may be
preferred, in some instances, to use a greater amount of the lowest
low-temperature burnout material as compared to a higher
low-temperature burnout material. The amount of the low-temperature
burnout material that is present in the precursor mixture is
typically from about 5% to about 50% by weight, more specifically
between about 9% and about 39% by weight.
[0023] In some embodiments, the precursor mixture further includes
an unmilled alpha alumina powder having a particle size of 10 to
100 microns, while in other embodiments, the precursor mixture
excludes the unmilled alpha alumina. In embodiments in which the
unmilled alpha alumina is included, the weight ratio of milled to
unmilled alpha alumina powder is generally in a range of 0.25:1 to
about 5:1, preferably 0.5 to 4, and more preferably, 0.75 to 3. The
precursor mixture may also include one or more additives, such as a
solvent and/or lubricant. Generally, the binder is present in an
amount of at least 10% or 25% by weight of total alumina content.
In some embodiments, a silicon-containing substance is
substantially excluded from the precursor mixture.
[0024] The method for producing the porous body may also be
practiced by adding components in at least two steps prior to the
heat treatment step. For example, in some embodiments, a dispersion
of non-silicate binder is first produced, i.e., in step (i), by
dispersing non-silicate binder particles into water, which may be
neutral water or acidified water. As well known in the art,
boehmite, which could be used as the non-silicate binder, is an
aluminum oxide hydroxide material, generally recognized as
conforming with the formula .gamma.-AlO(OH). For purposes of the
invention, the binder particles, as produced in the dispersion, are
preferably nanosized, e.g., up to or less than 200 nm, preferably
<100 nm, and more preferably <50 nm. The acid employed in the
acidified water is typically a strong mineral acid, such as nitric
acid, hydrochloric acid, or sulfuric acid. The acid can be also
weak acid, such as, for example, acetic acid. The acid employed in
the acidified water can be added to neutral water or be dissolved
from solid particles, such as, for example, non-silicate
binder.
[0025] A milled particulate form of alpha-alumina is then added to
the dispersion of non-silicate binder in step (ii), wherein the
milled form of alpha-alumina is characterized by an average or
median particle size (e.g., D.sub.50, the particle size where half
of the particle population lies below the indicated value) in a
range of 0.1 to 6 microns, and preferably 0.25 to 4 microns. The
mixture of non-silicate binder and milled alpha-alumina is mixed
until a first homogeneous mixture is obtained. The term
"homogeneous," as used herein, indicates that individual
macroscopic regions of agglomerated particles (i.e., of at least
100 or 200 microns) of each substance in the mixture (e.g.,
non-silicate binder and alpha-alumina) are typically not detectable
or present in the homogeneous mixture, although individual
microscopic regions of agglomerated particles (e.g., less than 100
or 200 microns), may or may not be present. In the homogeneous
mixture, the non-silicate binder functions as a binder of the alpha
alumina particles. In some embodiments, the alpha-alumina has a
very high purity, i.e., about 95 or 98 wt % or more. In some
embodiments, the alpha-alumina is a low sodium alumina or a low
sodium reactive alumina. The term "reactive alumina" as used herein
generally indicates an alpha-alumina with good sinterability and
having a particle size that is very fine, i.e., generally, of 2
microns or less. Generally, a "low sodium alumina" material
contains 0.1% or less sodium content. Good sinterability is
generally derived from a 2 micron or less particle size.
[0026] The particle sizes given above can refer to a diameter for
the case where the particle is spherical or approximately
spherical. For cases where the particles substantially deviate from
a spherical shape, the particle sizes given above are based on the
equivalent diameter of the particles. As known in the art, the term
"equivalent diameter" is used to express the size of an
irregularly-shaped object by expressing the size of the object in
terms of the diameter of a sphere having the same volume as the
irregularly-shaped object.
[0027] In some embodiments, step (ii) can include, either
simultaneous or subsequent to adding and mixing the milled alpha
alumina powder, adding unmilled alpha-alumina powder having a
D.sub.50 particle size in a range of about 10-100 microns, and
mixing until the first homogeneous mixture is obtained. The term
"subsequent" indicates that the additional material (e.g., unmilled
alpha-alumina) can be included in the same step (ii) or in a
succeeding step before the forming and firing steps (iv) to (vi).
Typically, the unmilled alpha-alumina has a D.sub.50 particle size
in a range of 10 to 100 microns, and more preferably 25 to 80
microns.
[0028] When unmilled alpha-alumina powder is included, the
resulting first homogeneous mixture contains a homogeneous mixture
of non-silicate binder, milled alpha-alumina, and unmilled
alpha-alumina. In some embodiments, the weight percentage of milled
alpha-alumina is greater than the weight percentage of unmilled
alpha-alumina, by weight of total alumina. For example, the milled
and unmilled alpha aluminas can be present in a weight ratio (i.e.,
milled to unmilled alumina) of about, at least, or above 1.1:1,
1.5:1, 1.8:1, or 2:1 and to up to or less than 1.5:1, 1.8:1, 2:1,
or 2.5:1. In other embodiments, the weight percentage of unmilled
alpha-alumina is greater than the weight percentage of milled
alpha-alumina, by weight of total alumina. For example, the
unmilled and milled alpha aluminas can be present in a weight ratio
(i.e., unmilled to milled alumina) of at least or above 1.1:1 or
1.5:1 and to up to or less than 1.8:1, 2:1, or 2.5:1. In other
embodiments, the weight ratio of milled to unmilled alpha-alumina
is about or at least 0.25:1 or 0.5:1 and/or about, up to, or less
than 2.5:1 or 3:1. In some embodiments, the milled alpha alumina is
the only alumina used in step (ii) or the only alumina employed in
the method and incorporated into the porous body, i.e., unmilled
alpha alumina is excluded from the method. In other embodiments,
the combination of milled and unmilled alpha aluminas is the only
alumina used in step (ii) or the only alumina employed in the
method and incorporated into the porous body.
[0029] In some embodiments, the weight percentage of non-silicate
binder is about the same or less than the weight percentage of
total alumina. For example, the non-silicate binder may be present
in an amount of at least or above 5% or 10%. In some embodiments,
the weight percentage of non-silicate binder is about the same or
greater than the weight percentage of total alumina. For example,
the non-silicate binder may be present in an amount of at least or
above 25% by weight of total alumina content. The total alumina
used in the method in the porous body precursor is typically at
least or above 25% or 35% by weight of total weight of solid
components incorporated into the porous body.
[0030] After formation of the first homogeneous mixture containing
non-silicate binder and alpha-alumina in step (ii), a
low-temperature burnout material as defined above is added to and
mixed into the first homogeneous mixture until a second homogeneous
mixture is obtained, i.e., in step (iii); again no high-temperature
burnout material is used. The low-temperature burnout material may
have a particle size in a range of about, for example, 1-10
microns, preferably 1-9 microns, and more preferably 1.5-8 microns.
The second homogeneous mixture preferably consists of free-flowing
particles that can be subsequently formed to a shape and sintered.
The low-temperature burnout material, which may also be considered
a temporary binder, is primarily responsible for imparting porosity
to the porous body, and to ensure the preservation of a porous
structure during the green (i.e., unfired phase) in which the
mixture may be shaped into particles by molding or extrusion
processes. In the present application, the low-temperature burnout
materials are completely removed during firing to produce the
finished porous body.
[0031] If a mixture of low-temperature burnout materials is used,
the low-temperature burnout materials in the mixture can have the
same or different particle sizes, and they can be added
simultaneously or sequentially. For example, in some embodiments,
after a granulated polyethylene is added and mixed until a second
homogeneous mixture is obtained, granulated polypropylene may be
added subsequently, wherein the term "subsequently" or
"sequentially" indicates that the additional material can be
included in the same step (iii) or in a succeeding step before the
forming and firing steps (iv) to (vi).
[0032] In one embodiment, steps (i), (ii), and (iii) are separated
and conducted in succession, i.e., the dispersion of non-silicate
binder is produced in step (i), followed by production of the first
homogeneous mixture in step (ii), followed by production of the
second homogenous mixture in step (iii). Steps (i), (ii), and (iii)
can be also conducted in reverse or in random order. In another
embodiment, steps (i) and (ii) may be combined as a single step,
i.e., non-silicate binder and alumina are combined in the presence
of acidified water to form a dispersion of non-silicate binder and
alumina, which functions as the first homogeneous mixture. In yet
another embodiment, steps (ii) and (iii) may be combined as a
single step, i.e., alumina and low-temperature burnout material are
combined during production of the first homogeneous mixture, which
now also functions as the second homogeneous mixture. In a further
embodiment, steps (i), (ii), and (iii) may be combined as a single
step, i.e., non-silicate binder, alumina, and burnout material are
combined in the presence of acidified water to form a dispersion of
non-silicate binder, alumina, and low-temperature burnout material,
which functions as the second homogeneous mixture.
[0033] In some embodiments, the method further includes (in any
step prior to forming and firing the second homogeneous mixture) a
binder material in sufficient amount. Permanent binders include,
for example, inorganic clay-type materials, such as silica and an
alkali or alkali earth metal compound. A convenient binder material
which may be incorporated with the alumina particles comprises a
non-silicate compound, a stabilized silica sol, and optionally
alkali or alkali earth metal salt. Preferred non-silicate binders
can be selected from aluminum hydroxides, oxide-hydroxides,
transition aluminas, and any organic or inorganic precursor that
produces alpha-alumina upon firing. In some embodiments, a
silicon-containing substance is substantially or completely
excluded from the method for producing the porous body. In the case
of a silicon-containing substance being substantially excluded from
the porous body, a trace amount of silicon derived from impurities
in the raw materials used to prepare the porous body may still be
present in the porous body. Such trace amounts are generally no
more than 1%, 0.5%, or 0.1% by weight of the porous body.
[0034] The precursor mixture, or the second homogeneous mixture
formed in step (iii), is then formed into a desired shape by means
well known in the art. The forming process can be by extrusion,
pressing, pelletizing, molding, casting, etc.
[0035] After forming, the formed shape is subjected to a heat
treatment step in which it is sintered (i.e., fired) to produce the
porous body. The heat treatment may include a single heating step
in which removal of volatiles such as, for example, water, and the
low-temperature burnout material occurs at a temperature from about
35.degree. C. to about 550.degree. C., and sintering (i.e., firing)
occurs at a temperature of from about 900.degree. C. to about
2000.degree. C. In another embodiment, the heat treatment step
includes a pre-firing step followed by a separate sintering (i.e.,
firing) step. In this embodiment, the pre-firing step of the heat
treatment is conducted before the sintering step in order to remove
volatiles and the low-temperature burnout material. The pre-firing
step of the heat treatment is performed at a temperature of about
35.degree. C. to about 550.degree. C., while sintering is performed
at a temperature of from about 900.degree. C. to about 2000.degree.
C. Generally, a heating and/or cooling rate within a range of
0.5-100.degree. C./min, preferably 1-20.degree. C./min, or more
preferably 2-5.degree. C./min, is used during the heat treatment of
the present invention. The heat treatment step of the present
invention is performed in an oxygen-containing ambient such as, for
example, air or O.sub.2.
[0036] It is noted that since the precursor mixture of the present
invention only includes a low-temperature burnout material, but not
any high-temperature burnout material, gas transport of oxygen and
burn-out oxidation products during the heat treatment step is
enhanced. Also, the removal of the low-temperature burnout material
from the precursor mixture during heat treatment can be performed
at lower temperatures and/or shorter durations as compared to a
precursor mixture that includes a high-temperature burnout
material. These advantages are clearly seen in the sole example of
the present invention.
[0037] After performing the heat treatment step, porous bodies that
have an enhanced pore architecture (as defined below) and a
porosity that is derived totally from a burnout material that
decomposes at a temperature of less than 550.degree. C. (i.e., a
low-temperature burnout material) are provided.
[0038] In order to properly characterize porous bodies for
applications in filters, membranes, or catalyst carriers, pore
architecture and consequently fluid transport-related properties
must also be determined.
[0039] Among very important parameters in determining the diffusive
gas transport through a porous body are tortuosity and
constriction. Tortuosity is determined by the ratio of the real
length of flow path through a porous body to the shortest distance
across that porous body (see, for example, B. Ghanbarian et al.,
Soil Sci. Soc. Am. J., 77, 1461-1477 (2013)). Constriction is a
function of the area ratio of large pores to small pores. Thus,
lowering the values of tortuosity and/or constriction enhances the
diffusive transport through a porous material, i.e., increases the
effective diffusivity, which is very important for instance in
catalytic applications.
[0040] If there is a pressure drop across the porous body,
permeability becomes important. Permeability indicates ability of
fluids to flow through porous bodies and can be described by the
Darcy's law shown in Equation 1, where V is fluid flow velocity, k
is permeability, .mu. is dynamic viscosity of the fluid, .DELTA.P
is pressure difference across porous body with thickness of
.DELTA.x:
V = k .mu. .times. .DELTA. .times. P .DELTA. .times. x ( Eq . 1 )
##EQU00001##
[0041] Thus higher values of permeability will enhance the
pressure-driven fluid flow across a porous body, which is important
in such applications as sorption, filtration, or catalysis.
[0042] Surprisingly, the aforementioned fluid transport-determining
properties of porous bodies cannot be found in the literature to
characterize porous architectures, particularly as related to
catalyst carriers for epoxidation of olefins. Moreover, there has
been no indication in the literature of the necessary values of
tortuosity, constriction or permeability which provide a pore
architecture to a porous body that can achieve enhanced properties,
especially in regard to catalyst performance. The present invention
provides porous bodies that have a pore architecture that has
enhanced fluid transport properties and high mechanical
integrity.
[0043] Unless otherwise specified the following methodology of
measurements were employed in the present application:
[0044] In the present invention, water absorption of the porous
bodies was measured by placing a 10 g representative sample of a
porous body into a flask, which was then evacuated to about 0.1
torr for 5 min. Subsequently, deionized water was aspirated into
the evacuated flask to cover the porous bodies while maintaining
the pressure at about 0.1 torr. The vacuum was released after about
5 minutes to restore ambient pressure, hastening complete
penetration of water into the pores. Subsequently, the excess water
was drained from the impregnated sample. Water absorption was
calculated by dividing total water weight in the pores (i.e., wet
mass--dry mass of the sample) by the weight of the dry sample at
room temperature.
[0045] Cumulative intrusion curves and Log differential intrusion
curves may be acquired for representative samples of the porous
bodies by mercury (Hg) intrusion porosimetry, principles of which
are described in Lowell et al., Characterization of Porous Solids
and Powders: Surface Area, Pore Size and Density, Springer, 2006.
The Hg intrusion pressure may range between, for example, 1.5 and
60,000 psi, which corresponds to pore sizes between 140 microns and
3.6 nm. The following Hg parameters may be used for calculations:
surface tension of 480 dynes/cm, density of 13.53 g/mL, and contact
angle of 140.degree.. Pore volumes for the porous bodies may be
measured from the Hg intrusion data, which are consistent with the
water absorption measurements. Additional pore architecture
parameters of the porous bodies, such as tortuosity, constriction,
and permeability, may also be calculated from the Hg intrusion
data, as discussed below.
[0046] The tortuosity, .xi. was calculated from Equation 2, where
D.sub.avg is weighted average pore size, k is permeability, .rho.
is true materials density, and I.sub.tot is total specific
intrusion volume (see, for example, AutoPore V Operator Manual,
Micromeritics, 2014):
.xi. = D avg 2 4 24 .times. k .function. ( .rho. .times. I tot ) (
Eq . 2 ) ##EQU00002##
[0047] The constriction, .sigma., was calculated from Equation 3,
where .xi. is tortuosity and .tau. is tortuosity factor, calculated
from the Carnigilia equation (see, for example, AutoPore V Operator
Manual, Micromeritics, 2014):
.sigma. = .xi. .tau. ( Eq . 3 ) ##EQU00003##
[0048] The permeability, as defined by the Darcy's law (Eq. 1,
above) can be calculated by combining Darcy's and Poiseuille'd
equations (see, for example, Lowell et al., Characterization of
Porous Solids and Powders, Springer, 2006). For an arbitrary pore
shape factor, f, the permeability k is expressed by Equation 4,
where .tau. is tortuosity factor, P is materials porosity, and d is
pore diameter:
k = p 3 .times. d 2 16 .times. f .times. .tau. .times. ( 1 - P ) 2
( Eq . 4 ) ##EQU00004##
[0049] Once tortuosity and pore volumes have been measured,
effective diffusivity can be calculated from Equation 5, where P is
materials porosity, D is diffusivity, D.sub.eff is effective
diffusivity, and .xi. is tortuosity [D. W. Green, R. H. Perry,
Perry's Engineering Handbook, 8.sup.th Edition, McGraw-Hill,
2007]
D eff = PD .xi. ( Eq . 5 ) ##EQU00005##
[0050] In order to calculate absolute values of effective
diffusivity, D.sub.eff, in a porous solid, absolute values of gas
diffusivity, D, must be known per Eq. 5, in addition to the
material porosity and tortuosity. However, in order to compare
effective diffusivity properties of different porous solids, it is
possible to calculate relative numbers of effective diffusivity
normalized to a standard material. With the assumption that gas
diffusivity, D, is the same in all cases, it requires only
knowledge of porosity and tortuosity of the porous materials (see
Equation 6).
D eff , 1 D eff , 0 = P 1 .xi. 1 .times. .xi. 0 P 0 ( Eq . 6 )
##EQU00006##
[0051] Total porosity is defined as the void volume divided by the
total volume of the sample. It can be calculated from mercury
porosimetry or water absorption, using theoretical density of the
carrier material.
[0052] The porous body of the present invention typically has a
pore volume from 0.3 mL/g to 1.2 mL/g. More typically, the porous
body of the present invention has a pore volume from 0.35 mL/g to
0.9 mL/g. In some embodiments of the present invention, the porous
body of the present invention has a water absorption from 30
percent to 120 percent, with a range from 35 percent to 90 percent
being more typical.
[0053] The porous body of the present invention typically has a
B.E.T. surface area from 0.3 m.sup.2/g to 3.0 m.sup.2/g. In one
embodiment, the porous body of the present invention has a surface
area from 0.5 m.sup.2/g to 1.2 m.sup.2/g. In another embodiment
body of the present invention has a surface area above 1.2
m.sup.2/g up to, and including, 3.0 m.sup.2/g. The B.E.T. surface
area described herein can be measured by any suitable method, but
is more preferably obtained by the method described in Brunauer,
S., et al., J. Am. Chem. Soc., 60, 309-16 (1938).
[0054] The porous body of the present invention can be monomodal,
or multimodal, such as, for example, bimodal. The porous body of
the present invention has a pore size distribution with at least
one mode of pores in the range from 0.01 micrometers to 100
micrometers. In one embodiment of the present invention, at least
90 percent of the pore volume of the porous body is attributed to
pores having a pore size of 20 microns or less. In yet another
embodiment of the present invention, at least 85 percent of the
pore volume of the porous body is attributed to pores having a size
from 1 micron to 6 microns. In yet a further embodiment of the
present invention, less than 15, preferably less than 10, percent
of the pore volume of the porous body is attributed to pores having
a size of less than 1 micron. In still a further embodiment of the
present application at least 80 percent of the pore volume of the
porous body is attributed to pores having a size from 1 micron to
10 microns. In a particular aspect of the present invention, there
are essentially no pores smaller than 1 micron.
[0055] In the case of a multimodal pore size distribution, each
pore size distribution can be characterized by a single mean pore
size (mean pore diameter) value. Accordingly, a mean pore size
value given for a pore size distribution necessarily corresponds to
a range of pore sizes that results in the indicated mean pore size
value. Any of the exemplary pore sizes given above can
alternatively be understood to indicate a mean (i.e., average or
weighted average) pore size. Each peak pore size can be considered
to be within its own pore size distribution (mode), i.e., where the
pore size concentration on each side of the distribution falls to
approximately zero (in actuality or theoretically). The multimodal
pore size distribution can be, for example, bimodal, trimodal, or
of a higher modality. In one embodiment, different pore size
distributions, each having a peak pore size, are non-overlapping by
being separated by a concentration of pores of approximately zero
(i.e., at baseline). In another embodiment, different pore size
distributions, each having a peak pore size, are overlapping by not
being separated by a concentration of pores of approximately
zero.
[0056] In one embodiment, the porous body of the present invention
may be bimodal having a first set of pores from 0.01 microns to 1
micron and a second set of pores from greater than 1 micron to 10
microns. In such an embodiment, the first set of pores may
constitute less that 15 percent of the total pore volume of the
porous body, while the second set of pores may constitute more than
85 percent of the total pore volume of the porous body. In yet
another embodiment, the first set of pores may constitute less than
10 percent of the total pore volume of the porous body, while the
second set of pores may constitute more than 90 percent of the
total pore volume of the porous body.
[0057] The porous body of the present invention typically has a
total porosity that is from 55 percent to 83 percent. More
typically, the porous body of the present invention typically has a
total porosity that is from 58 percent to 78 percent.
[0058] The porous body of the present invention typically has an
average flat plate crush strength from 10 N to 150 N. More
typically, the porous body of the present invention typically has
an average flat plate crush strength of at least 30 N, with an
average crush strength from 40 N to 105 N being typically in some
embodiments of the present invention. The flat plate crush strength
of the porous bodies was measured using a standard test method for
single pellet crush strength of formed catalysts and catalyst
carriers, ASTM Standard ASTM D4179.
[0059] In some embodiments, the porous body of the present
invention can have an attrition value that is less than 40%,
preferably less than 25%. In some embodiments of the present
invention, the porous body can have attrition less that 10%.
Attrition measurements of the porous bodies were performed using a
standard test method for attrition and abrasion of catalysts and
catalyst carriers, ASTM Standard ASTM D4058.
[0060] In some embodiments of the present invention, the porous
body of the present invention has an initial low alkali metal
content. By "low alkali metal content" it is meant that the porous
body contains from 2000 ppm or less, typically from 30 ppm to 300
ppm, of alkali metal therein. Porous bodies containing low alkali
metal content can be obtained by adding substantially no alkali
metal during the porous body manufacturing process. By
"substantially no alkali metal" it is meant that only trace amounts
of alkali metal are used during the porous body manufacture process
as impurities from other constituents of the porous body. In
another embodiment, a porous body having a low alkali metal content
can be obtained by performing various washing steps to the porous
body precursor materials used in forming the porous body. The
washing steps can include washing in a base, water, or an acid.
[0061] In other embodiments of the present invention, the porous
body has an alkali metal content that is above the value mentioned
above for the porous body having substantially no alkali metal
content. In such an embodiment the porous body typically contains a
measurable level of sodium on the surface thereof. The
concentration of sodium at the surface of the carrier will vary
depending on the level of sodium within the different components of
the porous body as well as the details of its calcination. In one
embodiment of the present invention, the porous body has a surface
sodium content of from 2 ppm to 150 ppm, relative to the total mass
of the porous body. In another embodiment of the present invention,
the porous body has a surface sodium content of from 5 ppm to 70
ppm, relative to the total mass of the carrier. The sodium content
mentioned above represents that which is found at the surface of
the carrier and that which can be leached, i.e., removed, by, for
example, nitric acid (hereafter referred to as acid-leachable
sodium).
[0062] The quantity of acid leachable sodium present in the porous
bodies of the present invention can be extracted from the catalyst
or carrier with 10% nitric acid in deionized water at 100.degree.
C. The extraction method involves extracting a 10-gram sample of
the catalyst or carrier by boiling it with a 100 ml portion of 10%
w nitric acid for 30 minutes (1 atm., i.e., 101.3 kPa) and
determining in the combined extracts the relevant metals by using a
known method, for example atomic absorption spectroscopy (See, for
example, U.S. Pat. No. 5,801,259 and U.S. Patent Application
Publication No. 2014/0100379 A1).
[0063] In one embodiment of the present invention, the porous body
may have a silica content, as measured as SiO.sub.2, of less than
0.2, preferably less than 0.1, weight percent, and a sodium
content, as measured as Na.sub.2O, of less than 0.2 weight percent,
preferably less than 0.1 weight percent. In some embodiments, the
porous body of the present invention may have an acid leachable
sodium content of 40 ppm or less. In yet further embodiments of the
present invention, the porous body comprises alumina crystallites
having a platelet morphology in a content of less than 20 percent
by volume. In some embodiments, the alumina crystallites having a
platelet morphology in a content of less than 10 percent by volume
are present in the porous body of the present invention.
[0064] In addition to the above physical properties, the porous
body of the present invention has a pore architecture that provides
at least one of a tortuosity of 7 or less, a constriction of 4 or
less and a permeability of 30 mdarcys or greater. A porous body
that has the aforementioned pore architecture has enhanced fluid
transport properties and high mechanical integrity. In some
embodiments, and when used as a carrier for a silver-based
epoxidation catalyst, a porous body having the aforementioned pore
architecture can exhibit improved catalyst properties. Typically,
the pore architecture of the porous body of the present invention
has a tortuosity of 7 or less and/or a constriction of 4 or
less.
[0065] In one embodiment of the present invention, the porous body
has a pore architecture that provides a tortuosity of 7 or less. In
another embodiment, the porous body of the present invention has a
pore architecture that provides a tortuosity of 6 or less. In yet
another embodiment, the porous body of the present invention has a
pore architecture that provides a tortuosity of 5 or less. In a
further embodiment, the porous body of the present invention has a
pore architecture that provides a tortuosity of 3 or less. The
lower limit of the tortuosity of the porous body of the present
invention is 1 (theoretical limit). In some embodiments, the
tortuosity can be any number bounded between 1 and 7.
[0066] In one embodiment of the present invention, the porous body
has a pore architecture that provides a constriction of 4 or less.
In another embodiment, the porous body of the present invention has
a pore architecture that provides a constriction of 3 or less, or
even 2 or less. The lower limit of the constriction of the porous
body of the present invention is 1. In some embodiments, the
constriction can be any number bounded between 1 and 4.
[0067] In another embodiment of the present invention, the porous
body has 2-4 times improved effective gas diffusivity due to the
combination of low tortuosity and high porosity.
[0068] In one embodiment, the porous body of the present invention
has a pore architecture that provides a permeability of 30 mdarcys
or greater. In another embodiment, the porous body of the present
invention has a pore architecture that provides a permeability of
200 mdarcys or greater.
[0069] The porous body can be of any suitable shape or morphology.
For example, the carrier can be in the form of particles, chunks,
pellets, rings, spheres, multi-hole shapes, wagon wheels,
cross-partitioned hollow cylinders, and the like, of a size
preferably suitable for employment in fixed bed reactors.
[0070] In one embodiment, the porous body contains essentially only
alumina, or alumina and non-silicate binder components, in the
absence of other metals or chemical compounds, except that trace
quantities of other metals or compounds may be present. A trace
amount is an amount low enough that the trace species does not
observably affect functioning or ability of the catalyst.
[0071] In another embodiment, the porous body may be used as a
catalyst carrier (i.e., catalyst support), in which case it
typically contains one or more catalytically active species,
typically metals, disposed on or in the porous body. The one or
more catalytically active materials can catalyze a specific
reaction and are well known in the art. In some embodiments, the
catalytically active material includes one or more transition
metals from Groups 3-14 of the Periodic Table of Elements and/or
lanthanides. In such applications, one or more promoting species
(i.e., species that aide in a specific reaction) can be also
disposed on or in the porous body of the present invention. The one
or more promoting species may be, for example, alkali metals,
alkaline earth metals, transition metals, and/or an element from
Groups 15-17 of the Periodic Table of Elements.
[0072] In the particular case of the porous body being used as a
carrier for silver-based epoxidation catalysis, the carrier
includes silver on and/or in the porous body. Thus, in the method
described above, generally after the sintering step, the silver is
incorporating on or into the carrier by means well known in the
art, e.g., by impregnation of a silver salt followed by thermal
treatment, as well known in the art, as described in, for example,
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, all of
which are incorporated herein by reference. The concentration of
silver salt in the solution is typically in the range from about
0.1% by weight to the maximum permitted by the solubility of the
particular silver salt in the solubilizing agent employed. More
typically, the concentration of silver salt is from about 0.5% by
weight of silver to 45% by weight of silver, and even more
typically, from about 5% by weight of silver to 35% by weight of
silver by weight of the carrier. The foregoing amounts are
typically also the amounts by weight found in the catalyst after
thermal treatment. To be suitable as an ethylene epoxidation
catalyst, the amount of silver should be a catalytically effective
amount for ethylene epoxidation, which may be any of the amounts
provided above.
[0073] In addition to silver, the silver-based epoxidation catalyst
of the present invention may also include any one or more promoting
species in a promoting amount. The one or more promoting species
can be incorporated into the porous body described above either
prior to, coincidentally with, or subsequent to the deposition of
the silver. As used herein, a "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 the catalyst when compared to a catalyst
not containing said component.
[0074] For example, the silver-based epoxidation catalyst may
include a promoting amount of a Group I alkali metal or a mixture
of two or more Group 1 alkali metals. Suitable Group 1 alkali metal
promoters include, for example, lithium, sodium, potassium,
rubidium, cesium or combinations thereof. Cesium is often
preferred, with combinations of cesium with other alkali metals
also being preferred. The amount of alkali metal will typically
range from about 10 ppm to about 3000 ppm, more typically from
about 15 ppm to about 2000 ppm, more typically from about 20 ppm to
about 1500 ppm, and even more typically from about 50 ppm to about
1000 ppm by weight of the total catalyst, expressed in terms of the
alkali metal.
[0075] The silver-based epoxidation catalyst may also include a
promoting amount of a Group 2 alkaline earth metal or a mixture of
two or more Group 2 alkaline earth metals. Suitable alkaline earth
metal promoters include, for example, beryllium, magnesium,
calcium, strontium, and barium or combinations thereof. The amounts
of alkaline earth metal promoters are used in similar amounts as
the alkali metal promoters described above.
[0076] The silver-based epoxidation catalyst may also include a
promoting amount of a main group element or a mixture of two or
more main group elements. Suitable main group elements include any
of the elements in Groups 13 (boron group) to 17 (halogen group) of
the Periodic Table of the Elements. In one example, a promoting
amount of one or more sulfur compounds, one or more phosphorus
compounds, one or more boron compounds or combinations thereof can
be used.
[0077] The silver-based epoxidation catalyst may also include a
promoting amount of a transition metal or a mixture of two or more
transition metals. Suitable transition metals can include, for
example, the elements from Groups 3 (scandium group), 4 (titanium
group), 5 (vanadium group), 6 (chromium group), 7 (manganese
group), 8-10 (iron, cobalt, nickel groups), and 11 (copper group)
of the Periodic Table of the Elements, as well as combinations
thereof. More typically, the transition metal is an early
transition metal selected from Groups 3, 4, 5, 6, or 7 of the
Periodic Table of Elements, such as, for example, hafnium, yttrium,
molybdenum, tungsten, rhenium, chromium, titanium, zirconium,
vanadium, tantalum, niobium, or a combination thereof.
[0078] In one embodiment of the present invention, the silver-based
epoxidation catalyst includes silver, cesium, and rhenium. In
another embodiment of the present invention, the silver-based
epoxidation catalyst includes silver, cesium, rhenium and one or
more species selected from Li, K, W, Zn, Mo, Mn, and S.
[0079] The silver-based epoxidation catalyst may also include a
promoting amount of a rare earth metal or a mixture of two or more
rare earth metals. The rare earth metals include any of the
elements having an atomic number of 57-71, yttrium (Y) and scandium
(Sc). Some examples of these elements include lanthanum (La),
cerium (Ce), and samarium (Sm).
[0080] The transition metal or rare earth metal promoters are
typically present in an amount of from about 0.1 micromoles per
gram to about 10 micromoles per gram, more typically from about 0.2
micromoles per gram to about 5 micromoles per gram, and even more
typically from about 0.5 micromoles per gram to about 4 micromoles
per gram of total catalyst, expressed in terms of the metal. All of
the aforementioned promoters, aside from the alkali metals, can be
in any suitable form, including, for example, as zerovalent metals
or higher valent metal ions.
[0081] The silver-based epoxidation catalyst may also include an
amount of rhenium (Re), which is known as a particularly
efficacious promoter for ethylene epoxidation high selectivity
catalysts. The rhenium component in the catalyst can be in any
suitable form, but is more typically one or more rhenium-containing
compounds (e.g., a rhenium oxide) or complexes. The rhenium can be
present in an amount of, for example, about 0.001 wt. % to about 1
wt. %. More typically, the rhenium is present in amounts of, for
example, about 0.005 wt. % to about 0.5 wt. %, and even more
typically, from about 0.01 wt. % to about 0.05 wt. % based on the
weight of the total catalyst including the support, expressed as
rhenium metal. All of these promoters, aside from the alkali
metals, can be in any suitable form, including, for example, as
zerovalent metals or higher valent metal ions.
[0082] After impregnation with silver and any promoters, the
impregnated carrier is removed from the solution and calcined for a
time sufficient to reduce the silver component to metallic silver
and to remove volatile decomposition products from the
silver-containing support. The calcination is typically
accomplished by heating the impregnated carrier, preferably at a
gradual rate, to a temperature in a range of about 200.degree. C.
to about 600.degree. C., more typically from about 200.degree. C.
to about 500.degree. C., more typically from about 250.degree. C.
to about 500.degree. C., and more typically from about 200.degree.
C. or 300.degree. C. to about 450.degree. C., at a reaction
pressure in a range from about 0.5 to about 35 bar. In general, the
higher the temperature, the shorter the required calcination
period. A wide range of heating periods have been described in the
art for the thermal treatment of impregnated supports. See, for
example, U.S. Pat. No. 3,563,914, which indicates heating for less
than 300 seconds, and U.S. Pat. No. 3,702,259, which discloses
heating from 2 to 8 hours at a temperature of from 100.degree. C.
to 375.degree. C. to reduce the silver salt in the catalyst. A
continuous or step-wise heating program may be used for this
purpose. During calcination, the impregnated support is typically
exposed to a gas atmosphere comprising an inert gas, such as
nitrogen. The inert gas may also include a reducing agent.
[0083] In another embodiment, the porous body described above can
also be used as a filter in which liquid or gas molecules can
diffuse through the pores of the porous body described above. In
such an application, the porous body can be placed along any
portion of a liquid or gas stream flow. In yet another embodiment
of the present invention, the porous body described above can be
used as a membrane.
[0084] In another aspect, the invention is directed to a method for
the vapor phase production of ethylene oxide by conversion of
ethylene to ethylene oxide in the presence of oxygen by use of the
silver-based epoxidation catalyst described above. Generally, the
ethylene oxide production process is conducted by continuously
contacting an oxygen-containing gas with ethylene in the presence
of the catalyst at a temperature in the range from about
180.degree. C. to about 330.degree. C., more typically from about
200.degree. C. to about 325.degree. C., and more typically from
about 225.degree. C. to about 270.degree. C., at a pressure which
may vary from about atmospheric pressure to about 30 atmospheres
depending on the mass velocity and productivity desired. Pressures
in the range of from about atmospheric to about 500 psi are
generally employed. Higher pressures may, however, be employed
within the scope of the invention. Residence times in large-scale
reactors are generally on the order of about 0.1 to about 5
seconds. A typical process for the oxidation of ethylene to
ethylene oxide 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). In one embodiment,
the tubes are approximately 0.7 to 2.7 inches O.D. and 0.5 to 2.5
inches I.D. and 15-45 feet long filled with catalyst.
[0085] In some embodiments, the silver-based epoxidation catalyst
described above exhibits a high level of selectivity in the
oxidation of ethylene with molecular oxygen to ethylene oxide. For
example, a selectivity value of at least about 83 mol % up to about
93 mol % may be achieved. In some embodiments, the selectivity is
from about 87 mol % to about 93 mole %. The conditions for carrying
out such an oxidation reaction in the presence of the silver-based
epoxidation catalyst described above broadly comprise those
described in the prior art. This applies, for example, to suitable
temperatures, pressures, residence times, diluent materials (e.g.,
nitrogen, carbon dioxide, steam, argon, and methane), the presence
or absence of moderating agents to control the catalytic action
(e.g., 1, 2-dichloroethane, vinyl chloride or ethyl chloride), the
desirability of employing recycle operations or applying successive
conversion in different reactors to increase the yields of ethylene
oxide, and any other special conditions which may be selected in
processes for preparing ethylene oxide.
[0086] In the production of ethylene oxide, reactant feed mixtures
typically 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
nitrogen, carbon dioxide, methane, ethane, argon and the like. Only
a portion of the ethylene is typically reacted per pass over the
catalyst. After separation of the desired ethylene oxide product
and removal of an appropriate purge stream and carbon dioxide to
prevent uncontrolled build up of inert products and/or by-products,
unreacted materials are typically returned to the oxidation
reactor.
[0087] An example has been set forth below for the purpose of
further illustrating the invention. The scope of this invention is
not to be in any way limited by the examples set forth herein,
Example
[0088] In this example, a first precursor mixture and a second
precursor mixture were prepared. The first and second precursor
mixtures were prepared utilizing the same procedure which is
accordance with one of the embodiments of the present application.
The first and second precursor mixtures are identical in
composition except that the first precursor mixture included a
burnout mixture of granulated polyethylene and graphite, while the
second precursor mixture included granulated polyethylene as the
sole burnout material with a volume equal to the volume of
polyethylene and graphite in the first mixture. In both cases, the
precursor mixture contains the same organic lubricant, which also
undergoes low-temperature oxidation. Thermogravimetric analysis was
then performed on each of the first and second precursor mixtures
utilizing a SDT Q600 analyzer from TA Instruments. FIG. 3 shows the
results of the thermogravimetric analysis of the first precursor
mixture, while FIG. 4 shows the results of the thermogravimetric
analysis of the second precursor mixture. In the case of the first
precursor mixture shown in FIG. 3, there is a strong exothermic
peak centered around 800.degree. C., which is derived from the
high-temperature burnout particles (graphite). As is shown in FIG.
4, such a peak is absent in the second precursor mixture. In
addition, temperatures recorded during oxidation of the
low-temperature burnout polyethylene particles are 10-20.degree. C.
lower in second precursor mixture as compared to the first
precursor mixture. Both the first and second precursor mixtures
produced, after high-temperature firing, alumina porous bodies with
about the same physical and chemical properties.
[0089] While the present invention has been particularly shown and
described with respect to preferred embodiments thereof, it will be
understood by those skilled in the art that the foregoing and other
changes in forms and details may be made without departing from the
spirit and scope of the present invention. It is therefore intended
that the present invention not be limited to the exact forms and
details described and illustrated, but fall within the scope of the
appended claims.
* * * * *