Molecular sieve abstract
A small pore hydrophilic molecular sieve body and method of making
same involves forming a plasticized mixture of hydrophilic molecular
sieve powder having a pore size of no greater than about 5.0 angstroms,
temporary binder, silicone resin binder emulsion, and polar vehicle,
shaping the mixture into a green monolithic body, and drying and
heat-treating the green monolithic body to impart strength to the
green body and form the product molecular sieve monolith. The body
is made up of about 80% to 95% by weight molecular sieve with the
balance being silica binder.
Molecular sieve claims
What is claimed is:
1. A method of making a small-pore hydrophilic zeolite molecular
sieve body, the method comprising:
a) forming a plasticized mixture consisting of hydrophilic zeolite
powder selected from the group consisting of 3A zeolite, 4A zeolite,
and 5A zeolite, and having a pore size of no greater than about
5.0 angstroms, temporary binder, aqueous silicone resin binder emulsion,
and polar vehicle;
b) shaping the plasticized mixture into a green monolithic body;
c) drying the green body;
d) heat-treating the green monolithic body to impart strength to
the green body and form the product zeolite molecular sieve body.
2. A method of claim 1 wherein the zeolite is 4A.
3. A method of claim 1 wherein the temporary binder is a cellulose
ether binder selected from the group consisting of methylcellulose,
methylcellulose derivatives, and combinations thereof.
4. A hydrophilic zeolite molecular sieve produced according to
the method of claim 1 having a pore size of no greater than about
5.0 angstroms, and consisting essentially of in percent by weight
about 80% to 95% zeolite, with the balance being a silica binder.
5. A molecular sieve of claim 4 wherein the molecular sieve is
4A zeolite.
6. A molecular sieve of claim 4 wherein the molecular sieve is
in the shape of a honeycomb.
7. A molecular sieve of claim 4 wherein the molecular sieve is
a rod consisting essentially of in percent by weight about 85% 4A
zeolite with the balance being silica, and having an MOR of at least
about 2000 PSI.
8. A molecular sieve of claim 4 wherein the molecular sieve is
a honeycomb structure consisting of in weight percent by weight
about 83% to 87% 4A zeolite with the balance being silica, said
honeycomb structure having about 62 cells/cm.sup.2 and wall thickness
of about 0.17 mm, having a crush strength of about 770 psi.
Molecular sieve description
This invention relates to a method of making hydrophilic small
pore sizes molecular sieve bodies by shaping a plasticized mixture
of the molecular sieve and a silicone resin binder that is provided
as an emulsion. Use of the silicone resin as an emulsion results
in production of a stronger body for a small pore size hydrophilic
molecular sieve than if the silicone resin were dissolved in a solvent.
BACKGROUND OF THE INVENTION
Molecular sieve monoliths, such as zeolite monoliths find use in
catalytic applications such as catalysts, catalyst supports, or
adsorbing structures, where they must be strong and of uniform composition
throughout the body. Depending on their pore size, they are suitable
for adsorbing various molecular size contaminants or for catalyzing
various chemical reactions, or for moisture removal. They are made
by shaping e.g. extruding plasticized mixtures of the molecular
sieve and various binders and vehicle such as water. One effective
binder is silicone resin. Ordinarily the silicone resin is dissolved
in a solvent as described in U.S. Pat. Nos. 4631267 and 5633217.
For hydrophobic molecular sieves such as e.g. ZSM zeolites, this
poses no problem. However, with hydrophilic molecular sieves, repelling
of the silicone resin binder solution poses a problem. For example,
in forming a plasticized batch of a hydrophilic zeolite small pore
such as A4 zeolite with silicone resin dissolved in a solvent, the
batch becomes soft and gummy upon addition of the silicone resin
solution prior to addition of the water vehicle. Though rods and
ribbons can be extruded, the products are so gummy and soft that
it is impossible to maintain their shapes for further processing.
When the monoliths are honeycombs especially honeycombs with thin
walls, it is even more important that the plasticized mixture be
stiff and able to maintain its shape, otherwise the honeycomb walls
will collapse.
U.S. Pat. No. 5492883 relates to shaping zeolites using a silicone
resin emulsion as a binder. However, the zeolites are medium to
large pore size.
It would be desirable to have a method of shaping plasticized mixtures
of small pore molecular sieves wherein the shaped products would
have strength in both the green state and the final heat-treated
structure.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, there is provided
a method of making a small pore molecular sieve monolithic body
that involves forming a plasticized mixture of hydrophilic molecular
sieve powder having a pore size of no greater than about 5.0 angstroms,
temporary binder, silicone resin binder emulsion, and polar vehicle,
shaping the mixture into a green monolithic body, and drying and
heat-treating the green monolithic body to impart strength to the
green body and form the product molecular sieve monolith.
In accordance with another aspect of this invention, there is provided
a small pore hydrophilic molecular sieve monolithic body having
a pore size of no greater than about 5.0 angstroms, and made up
of about 80% to 95% by weight molecular sieve with the balance being
silica binder.
DETAILED DESCRIPTION OF THE INVENTION
This invention provides a method to shape small pore hydrophilic
molecular sieves into monolithic bodies of sufficient strength to
maintain their shape through handling, drying, and heat-treating
operations. The bodies are shaped from a plasticized mixture of
the molecular sieve, temporary binder, silicone resin binder emulsion,
and polar vehicle which is preferably water.
The Molecular Sieve
This invention is useful for hydrophilic molecular sieves of small
pore size. By hydrophilic is meant those having a SiO.sub.2 :Al.sub.2
O.sub.3 mole ratio of no greater than about 30:1 and preferably
between about 30:1 to 1:1 and more preferably about 10:1 to 1:1.
By small pore size according to this invention is meant pore sizes
no greater than about 5.0 angstroms. These molecular sieves find
use in specialized adsorption applications and/or where water absorbing
capability is desired.
Some suitable molecular sieves that fit this description are A3
A4 A5 ferrierite, erionite, and chabazite. Especially suited are
A3 A4 and A5 with A4 being especially preferred.
Temporary Binders
Temporary binders are so called because they are removed during
the final heat-treatment. The temporary binders are plasticizing
organic binder with optional additions of a co-binder.
The plasticizing Organic Binder
The organic binder contributes to the plasticity of the mixture
for shaping into a body. The plasticizing organic binder according
to the present invention refers to cellulose ether binders. Some
typical organic binders according to the present invention are methylcellulose,
ethylhydroxy ethylcellulose, hydroxybutyl methylcellulose, hydroxymethylcellulose,
hydroxypropyl methylcellulose, hydroxyethyl methylcellulose, hydroxybutylcellulose,
hydroxyethylcellulose, hydroxypropylcellulose, sodium carboxy methylcellulose,
and mixtures thereof. Methylcellulose and/or methylcellulose derivatives
are especially suited as organic binders in the practice of the
present invention with methylcellulose, hydroxypropyl methylcellulose,
or combinations of these being preferred. Preferred sources of cellulose
ethers are Methocel A4M, F4M, F240 and K75M from Dow Chemical Co.
Methocel A4M is a methylcellulose, while Methocel F4M, F240 and
K75M are hydroxypropyl methylcellulose.
The organic binder content is typically is about 2% to 12% and
preferably about 3 to 8 wt. %.
Co-binders
Co-binders can be used to enhance the plasticity of the mixture.
Some co-binders that are useful are low molecular weight water-soluble
binders such as for example, polyvinyl alcohols such as those available
from Air Products, Allentown Pa., under the designation Airvol,
e.g. Airvol 205 (molecular weight 31000-50000) and Airvol 350
(molecular weight 124000-186000). Other useful water soluble binders
include polyvinylpyrrolidones such as those available from GAF,
Linden, N.J., under the designation PVP K-30 (row 40000) and PVP
K-60 (row 160000). Airvol 205S is particularly useful. Polyvinyl
acetate is also suitable.
The silicone resin binder emulsion
Aqueous silicone resin emulsions such as phenylmethyl silicone
resin emulsions available from Dow Corning Corporation, Midland
Mich. and sold under the designations 1-0468 and 1-0469 for example
are particularly useful for the practice of the invention. These
silicone resin emulsions are characterized by about 60 weight percent
resin solids that sinter to yield about 52 weight percent silica
having a particle size of about 7000 Angstroms, and can contain
very small amounts (0.5 to 1 lb/gal of the emulsion) of an organic
aromatic solvent.
A homogeneous formable mixture is made of the raw material, permanent
binder and/or permanent binder precursors, temporary binder, and
vehicle. Although any vehicle can be used that is safe, feasible
and does not adversely affect the mixture, the preferred vehicle
in extrusion processing is water
The water content is typically about 12% to 50 wt. %, and preferably
about 28% to 45 wt. %.
The weight percents of the organic components and vehicle are calculated
as superadditions with respect to the non-organic solids by the
following formula: ##EQU1##
Normally, the molecular sieve and silicone resin binder emulsion
are provided in amounts sufficient to result in a weight ratio of
molecular sieve to silica in the product of about 80:20 to 95:5.
One advantageous mixture composition consists essentially of in
percent by weight based on the fired zeolite and silica, about 2%
to 12% temporary binder that is methylcellulose and/or methylcellulose
derivatives, about 12% to 50% water, sufficient zeolite to result
in about 80% to 95% zeolite on a fired basis, and sufficient silicone
resin emulsion to result in about 5% to 20% silica on a fired basis.
More advantageously the mixture composition consists essentially
of in percent by weight based on the fired zeolite and silica, about
3% to 8% temporary binder that is methylcellulose and/or methylcellulose
derivatives, about 28 to 45% water, sufficient zeolite to result
in about 83% to 92% 4A zeolite on a fired basis, and sufficient
silicone resin emulsion to result in about 17% to 8% silica on a
fired basis.
The mixture components are combined to form a homogeneous or substantially
homogeneous mixture. Normally the dry ingredients are first dry
blended, preferably in an intensive mixer, and then combined with
the wet ingredients It is highly desirable that the raw material
be well mixed into a plasticized batch with the silicone resin.
Conventional mixing equipment, e.g. mix-muller or high shear mixer
can be used. To effect further mixing, the batch can be first extruded
through a "noodling" die one or more times.
The mixture is formed into any desired shape depending on the application.
Some typical shapes can be, for example, pellets, rods, tubes, ribbons,
or disks, or multicellular structures such as honeycombs, etc.
The method of this invention is particularly well suited to the
preparation of structures in the shape of honeycombs.
Generally honeycomb cell densities range from 235 cells/cm.sup.2
(about 1500 cells/in.sup.2) to 1 cell/cm.sup.2 (about 6 cells/in.sup.2).
Examples of honeycombs produced by the process of the present invention,
although it is to be understood that the invention is not limited
to such, are those having about 94 cells/cm.sup.2 (about 600 cells/in.sup.2),
about 62 cells/cm.sup.2 (about 400 cells/in.sup.2), or about 47
cells/cm.sup.2 (about 300 cells/in.sup.2), those having about 31
cells/cm.sup.2 (about 200 cells/in.sup.2), or those having about
15 cells/cm.sup.2 (about 100 cells/in.sup.2).
Cell wall thicknesses range from about 0.1 to about 1.5 mm.
The resulting shaped green bodies are dried first to remove the
vehicle. This can be done by steam or controlled humidity drying,
dielectric drying of combinations of these techniques.
For example one suitable drying technique is use of a dielectric
dryer at low power or short drying times, followed by conventional
oven or air drying. Dielectric drying is known in the art. Those
skilled in the art can adjust the dielectric dryer in known ways
to obtain low power drying, such as adjusting plate height, adjusting
RF voltage, or adjusting drying time. This is especially suited
for honeycomb structures.
Another suitable drying technique is a relative humidity drying
technique. This is especially suitable for rods or bodies having
thicker cross sections than for example, honeycombs. One especially
preferred relative humidity drying technique is carried out according
to the following schedule:
Temp (.degree. C.) Relative Humidity (%) Time (hrs) 1. from 30
to 60 95 in 10 2. hold at 60 95 for 48 3. hold at 60 95-50 in 30
4. drop from 60 to 30 50 in 12 Total hours 108
The dried bodies are heated (fired) at a temperature of about 400.degree.
C. to 850.degree. C. to develop strength. The heating temperatures
depend on the particular support material. For example, for a zeolite
the temperatures are most advantageously about 500.degree. C. to
750.degree. C. The heating times depend on factors such as the type
of material, temperature, size and shape of the body, etc.
According to the present invention, stiff batches are obtained
for small pore hydrophilic molecular sieves enabling them to be
shaped such as by extrusion into cohesive green bodies that can
be handled without deformation and dried and heat-treated to produce
high strength bodies.
The hydrophilic small pore molecular sieve bodies of the present
invention consist essentially of in percent by weight about 80%
to 95% molecular sieve, with the balance being silica binder.
It has been found that rods made of about 83-87 wt. % zeolite with
the balance being silica, have an MOR of at least about 2000. An
example of such rods are those having about 85% 4A zeolite and the
balance silica.
It has been found that honeycombs made of about 83-87 wt. % 4A
zeolite with the balance being silica, having about 62 cells/cm.sup.2
(about 400 cells/in.sup.2) and 0.17 mm thick walls or webs, have
a crush strength of about 770 psi.
To more fully illustrate the invention, the following non-limiting
examples are presented. All parts, portions, and percentages are
on a weight basis unless otherwise stated.
COMPARATIVE EXAMPLE 1
The following example illustrates that a small pore hydrophilic
zeolite cannot be satisfactorily made using silicone resin dissolved
in a solvent as opposed to being in emulsion form.
A plasticized batch or mixture is made up to contain about 400
g of 4A zeolite from Union Carbide, about 50 g of Methocel A4M,
(this is about 10% based on fired zeolite/silica mass) about 290.7
g of silicone resin solution (Dow Corning 6-2230 dissolved in dibasic
ester solvent in a 3/1 ratio), to yield a composition of about 80%
zeolite/20% silica after firing. In most cases water is needed to
fully plasticize the Methocel binder for a proper extrusion. This
batch immediately became soft, mushy, and gummy upon addition of
the silicone resin solution prior to any water addition. Although
rods 0.79 cm (5/16") in diameter and ribbons about 0.96 mm
thick can be extruded, the products are so gummy and soft that it
is impossible to keep their shapes for further processing.
COMPARATIVE EXAMPLE 2
The presence of up to about 20% bentonite clay (Bentolite L from
Southern Clay Products) permits some minor amount of water (about
6%). However, fired rod samples for MOR are very weak. When bentonite
clay is present at a level of less than about 20% with decreasing
water levels, e.g. about 10% bentonite clay with 0% water, and about
15% bentonite clay with 2% water, rod samples are not only weaker
but also nonuniform in cross section. An attempt to extrude a honeycomb
from the 15% clay 2% water batch failed due to the product being
too soft for a cellular structure to be maintained.
INVENTIVE EXAMPLE 1
A batch was made up to contain about 360 g of 4A zeolite, about
24 g of A4 Methocel (6%), about 128.2 g of silicone resin emulsion
1-0469 to yield a composition of about 90% zeolite and about 10%
silica after firing. After the addition of about 42.8% water in
a muller, the well-plasticized batch was extruded into rods of about
0.79 cm (5/16") diameter, ribbons 3.2 cm (11/4") wide
and about 0.96-1.44 mm (40 and 60 mils respectively) thick, and
a honeycomb having about 62 cells/cm.sup.2 0.17 mm wall thickness,
2.54 cm diameter, (400 cells/in.sup.2 7 mil wall thickness, 1"
diameter). The ribbons are further cut to yield disks of about 2.06
cm (13/16") in diameter. After drying and firing at about 600.degree.
C. for about 10 hours in air at a heating rate of about 25.degree.
C./hr, rod samples had an MOR of about 986 psi. The products were
homogeneous and strong.
INVENTIVE EXAMPLE 2
A batch was made up to contain about 680 g A4 zeolite, about 48
g A4M Methocel (about 6%), and about 384.6 g of 1-0469 silicone
resin emulsion to yield a composition of about 85% zeolite and about
15% silica after firing. After the addition of about 31.7% water
in a muller, the well-plasticized batch yields uniform extruded
rods, ribbons/disks, and honeycombs as in Inventive Example 1. After
drying in a relative humidity controlled oven for about 4 days for
the rods (according to the relative humidity control drying technique
previously described), for about 95.degree. C./4 days for the ribbons/disks/honeycombs
or dielectric oven about 15 minutes for disks/honeycombs, the disks
can be fired perfectly at about 600.degree. C./10 hrs/air at a heating
rate of about 25.degree. C./hr. The rods can also be readily fired
under the same conditions with about 10-20.degree. C./hr heating
rates to yield MOR of about 2031 psi for the 10.degree. C./hr rate.
This is a very strong zeolite. For honeycombs, a more gentle firing
schedule is needed to avoid cracking: 600.degree. C./nitrogen/15
hr, followed by 500.degree. C./air/5 hr all at heating rate of about
10.degree. C./hr to yield a crush strength of about 770 psi. Scanning
electron microscope (SEM) and x-ray diffraction analyses show uniform
structure and distinct A4 zeolite characteristics.
It should be understood that while the present invention has been
described in detail with respect to certain illustrative and specific
embodiments thereof, it should not be considered limited to such
but may be used in other ways without departing from the spirit
of the invention and the scope of the appended claims. |