This application claims the benefit of U.S. Provisional Application
No. 60/173,592, filed 12/29/99, entitled "High Strength and High Surface Area Catalyst,
Catalyst Support or Adsorber Compositions", by Wu et al.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a improved zeolite/silica/alumina
material and a method for making such material. In particular it relates to a zeolite/silica/alumina
composite material exhibiting a high strength and a high surface area.
2. Background and Discussion of the Related Art
The Clean Air Act of 1970 requires that a catalytic converter be installed
on an automobile to purify the exhaust gas stream. The catalytic converter removes
unburned gasoline, carbon monoxide and nitrogen oxides simultaneously in the exhaust
stream. A conventional catalytic converter consists of a multi-channel ceramic honeycomb
and includes a high surface area material that is, along with the actual catalytic
material (e.g., three-way catalyst (TWC)), washcoated onto the ceramic material.
The monolithic ceramic honeycomb provides a strong substrate for the catalyst, in
addition to meeting mechanical and thermal requirements. However, acting as an inert
structure, the catalyst substrate does not participate in the chemical reactions
for removal of unburned hydrocarbons, carbon monoxide and nitrogen oxides.
US Patent No. Re. 34,804 discloses the formation of extruded zeolite
honeycomb bodies that include a permanent binder silicone resin component. An improved
method for making the zeolite body is disclosed in U.S. Pat. No 5,492,883 (Wu) wherein
the zeolite material is mixed with an aqueous silicone resin emulsion and, a temporary
binder, such as methylcellulose, and the mixture is extruded to form a green honeycomb
body, which is thereafter dried and sintered. Another improved method for making
a zeolite body is disclosed in U.S. Pat. No. 5,633,217 (Lynn), wherein it is discloses
the use of a dibasic ester as the solvent for the silicone resin and the use of
a methylcellulose temporary binder. Finally, US Patent No. 5,565,394 (Lachman et
al.) discloses improved zeolite bodies that include a thermal expansion control
component such as calcium silicate, permanent binder such as silica or alumina and
a temporary binder such as methylcellulose. Although the zeolites disclosed in the
Wu, Lynn and Lachman references are not inert and are capable of use as a catalyst
material, they each require the application of a precious metal washcoat in order
to function as a three-way catalyst capable of the conversion of hydrocarbons, nitrogen
oxides and carbon monoxide into their nontoxic gaseous counterparts.
For zeolite based materials to be used as monolithic honeycombs at
increased temperatures (>300°C) the zeolite material should exhibit the following
combination of properties, not currently possessed by conventional zeolite bodies:
high strength, high surface area, high thermal stability (i.e. high thermal shock
resistance) and a low coefficient of thermal expansion. There is, accordingly, a
clear need for, and thus an object of the present invention is to provide, a zeolite
material exhibiting the aforementioned requisite properties.
SUMMARY OF THE INVENTION
The object of the present invention is to solve the above problems
of the prior art and to provide high strength, high surface area zeolite material
exhibiting a high thermal stability and low thermal expansion.
Specifically, the invention is directed at a zeolite body comprising,
expressed in weight percent: 30-70% of a zeolite having a silica/alumina ratio of
at least 300 and a surface area of greater than about 250m2/g, 15-20%
silica binder, and 10-50% gamma alumina having a specific surface area of greater
than 100 m2/g. The properties this zeolite based material exhibits include
the following: (1) a modulus of rupture of at least 1406138 kg/m2 (2000
psi); (2) a surface area of at least 100m2/g; (3) a coefficient of thermal
expansion of less than about +/-10 ppm/°C; and, (4) a thermal shock resistance of
at least 850°C.
DETAILED DESCRIPTION OF THE INVENTION
The product of the present invention is a zeolite body for use as
an adsorber or catalyst carrier, specifically a zeolite-based material wherein the
zeolite exhibits a silica/alumina ratio of at least more than 300 and a surface
area of at least 250 m2/g. Expressed in parts by weight, the zeolite
based bodies, according to the invention, characteristically contain between about
30 to 70 parts, by weight, zeolite and at 15-20 parts, by weight, silica binder.
Typically, zeolites comprise large particles on the order of several
microns and exhibit a regular array of accessible micropores, a combination that
provides the high surface attribute of zeolites; a feature that is retained by zeolites
after sintering. Generally, such catalyst support and adsorber applications require
substantial overall surface areas of at least 20m2/g, preferably greater
than 100m2/g, and most preferably greater than 150-200 m2/g.
The inventive zeolite based body is capable of being extruded into a high cell density,
thin walled monolithic body, e.g., a honeycomb structure exhibiting at least 400
cells/in2, exhibiting surface areas of at least 200 m2/g,
with surface areas in excess of 250 m2/g being readily attainable.
As detailed above, the zeolite component is desirably a high silica-containing
zeolite exhibiting a SiO2/Al2O3 molar ratio of
at least 300. The presence of a zeolite having the requisite high silica/alumina
ratio provides for a zeolite-based material having both a thermal stability at those
high temperatures typically experienced in the exhaust environment, and the expected
ability to adsorb and desorb hydrocarbons. In other words, the high silica content
of the zeolite provides the composite with the ability to maintain its structure
at high temperatures. On the other hand, the presence of a low alumina content in
the zeolite ensures that the zeolite will not experience the type of moisture problems
typically associated with zeolites having high alumina content; high alumina zeolites
typically de-aluminate in the presence of moisture at high temperatures. Furthermore,
the zeolites crystalline silica phase is maintained at high temperatures and is
responsible for the negative CTE characteristic that compensates to reduce the overall
thermal expansion of the composite body. In sum, the inventive material provides
for a zeolite adsorber material that allows the automotive exhaust system designer
a certain degree of flexibility in exhaust system design; adsorber material having
increased thermal stability.
Suitable zeolites include any silica-based zeolite having the requisite
very high silica/alumina ratio. Useful high silica/alumina ratio-containing zeolites
for the practice of the invention can be found among the zeolites selected from
the following: mordenite, ultrastabilized Y (USY), ZSM-5, ZSM-8, ZSM-11, ZSM-12,
Hyper Y, beta-zeolites, H-ferrierite, H-offretite, faujasite, X zeolite, type L
zeolite, mazzite, EMC-2, and combinations of these, preferably silicalite, and any
of the natural zeolites including erionite, clinoptilolite, chanazite and phillipsite.
One commercially available zeolite having the requisite high silica property is
CBV 3002 available from the PQ Corporation.
In addition to a high surface area, other features of this zeolite
body that make it suitable for use as an adsorber material include its relatively
low thermal expansion and high thermal stability; less than 10ppm/°C, preferably
5 ppm/°C, and thermal stability up to at least 1000°C, respectively. Furthermore,
catalyst support applications and filter/adsorber applications preferably require
a flexural strength in excess of 1500 psi. The zeolite body of the instant invention
exhibits flexural and crushing strengths that exceed this value and are on the order
of greater than about 1406138 kg/m2 (2000 psi), with MOR's in excess
of 2460743 Kg/m2 (3500 psi) being attainable.
A second embodiment of the inventive zeolite body comprises the inclusion
of a third component, a gamma alumina having a surface area of greater than 100
m2/g. The gamma alumina component having the high surface area also contributes
to result in an overall zeolite-based body that is well within the surface area
requirements of many catalyst support applications. Expressed in parts by weight,
the zeolite/silica/alumina bodies, according to the invention, characteristically
contain between about 30-70 parts, by weight, zeolite and 15-20 parts, by weight,
silica binder, and 10 to 50, parts, by weight, gamma alumina.
Although the presence of silica prevents the incorporation of PGM
catalysts into the extruded zeolite substrate, due to silica's known PGM incompatibility,
the presence of alumina in this embodiment provides the zeolite-based composite
structure the support material function for non-PGM catalysts. Specifically the
gamma alumina provides the necessary sites to enable binding of transition metal
oxide catalyst to the structure, such that the composite will have enhanced catalytic
activity and lifetimes over zeolite-only structures, when used in the certain harsh
environments typically associated with high temperature, such as seen in chemical
processing applications. Additionally, the alumina, whereby the transition metal
oxides are typically sited is porous enough and exhibits a high enough surface area
porous structure so as to inhibit sintering of the metal oxides present and to provide
for the accessibility of the transition metal oxides to the reactant stream.
Gamma alumina suitable for use in the formation of this composite
include those aluminas that after calcining provide the requisite gamma alumina
phase and exhibit a sufficiently high surface area suitable for functioning as the
catalytic support material. A suitable commercially available gamma alumina having
the requisite high surface area characteristic is GL-25 supplied by LaRoche Industries.
In another embodiment, the zeolite body should include a stabilized
high surface area alumina. The stabilized alumina should include an amount of stabilizing
agent selected from the group consisting of lanthanum oxide (La2O3)
or it equivalents, including barium oxide, strontium oxide and yttrium oxide. These
stabilizing agents are known for stabilizing the specific surface area of the alumina,
which in its pure form is typically unstable at high temperatures. Specifically,
the stabilizing agents inhibit the phase transformation of alumina at high temperatures,
thereby increasing the high temperature stability of the alumina. The stabilizing
agents are typically included in the alumina as a pre-dopant prior to the batching
of the composite, and more preferably they are added to the composite after firing
via an impregnation process.
A preferred stabilizing agent for the alumina is lanthanum oxide (La2O3),
which is included by impregnation into the gamma alumina component of the composite.
Lanthanum impregnation is such that the composite includes lanthanum oxide in the
weight range of 0.5-20, parts, by weight, with respect to the alumina component
amount. If lanthanum is added in an amount less than such range, then the beneficial
effect of increase in activity due to the lanthanum addition is not observed.
For catalyst applications, porosity, as measured by total porosity,
of the zeolite/alumina composite should be sufficient to permit access to the transition
metal oxide catalyst through the walls. For adsorber applications, porosity, as
measured by average pore size, should be sufficient to allow the support to function
effectively as an adsorber. The range of choice, for total porosity and average
pore size, may be varied to accommodate the proposed catalyst or adsorber applications.
Porosity is dependent upon the raw materials and the firing temperature, the higher
the temperature the more dense the resulting structure. For catalyst and/or catalyst
support applications, the inventive zeolite structures may exhibit a total porosity
of about at least about 30%, along with sub micron average pore sizes.
In addition to its use as a simple adsorber structure, as detailed
above, the inventive high silica/alumina zeolite material can be used as a catalyst
substrate, specifically, as a replacement for cordierite. Alternatively, the inventive
zeolite can be used as a catalyst substrate in those applications where it performs
the additional function of adsorbing hydrocarbon during the cold-start stage (e.g.,
Bag I and II emissions).
In the first catalyst substrate embodiment, the zeolite honeycomb
substrate is washcoated with a conventional three-way catalyst, and catalyst system
performs in the same manner as a regular cordierite supported three-way catalyst
system. Suitable catalytic materials for supporting on the high silica/alumina zeolite
substrate include platinum, palladium, rhodium and iridium. The zeolite substrate
is comprised of a zeolite material that exhibits high thermal stability under automotive
exhaust conditions. Suitable zeolites include those selected from the following
materials: mordenite, ultrastabilized Y (USY), ZSM-5, ZSM-8, ZSM-11, ZSM-12, all
exhibiting a silica/alumina ratio of 300 and above.
In the second embodiment, the zeolite substrate is washcoated with
an oxidation catalyst; suitable oxidation catalysts including platinum, palladium
rhodium or iridium. The washcoated zeolite catalyst functions as a hydrocarbon trap
for reducing hydrocarbon emissions during the cold-start stage. Zeolite material
choices for the substrate include the following: beta-zeolite, USY and ZSM-5 and
mordenite, all exhibiting the requisite high silica/alumina ratio of 300 or above.
These large pore zeolites exhibit a large capacity for the adsorption of hydrocarbon,
while at the same time exhibiting requisite thermal stability to survive the harsh
environment in an automotive exhaust stream. Once the washcoated oxidation catalytic
material reaches a sufficient material it functions to destroy harmful hydrocarbon
molecules by oxidizing these molecules, with oxygen present in automotive exhaust
stream, to environmentally benign molecules such as water and carbon dioxide.
The general method of producing porous sintered substrates, as one
skilled in the art can appreciate, is by mixing batch materials, blending the mixture,
forming a green body, and subsequently sintering the green body to a hard porous
structure. In the manner of making the body various lubricants, such as zinc stearate
and sodium stearate, and organic binders are added to the batch during the mixing
step to provide viscosity control and strength prior to firing and porosity to the
after fired structure.
A particularly preferred method for producing the composite of the
invention described herein, an extruded honeycomb monolith having a high surface
area, comprises mixing into a substantially homogeneous body certain raw materials
capable of forming the aforementioned composite. Specifically, the raw materials
that will form a composite include a zeolite raw material that exhibits a silica/alumina
ratio of at least 300 to 1 and a surface area of at least 250 m2/g, a
silica binder exhibiting a resin/solvent ratio of between 2/1 to 4/1 and, optionally,
a gamma alumina component exhibits a specific surface area of greater than 100 m2/g.
As is standard in the formation of ceramic structures, the batch mixture should
include a temporary organic binder and water. The preferred method of forming the
body includes extruding the body to form a green honeycomb structure. Once formed
into a honeycomb body the extruded green body is then dried by heating the structure
for a time period sufficient to form a crack-free dry structure.
The drying step is accomplished in a number of different ways. One
embodiment involves placing the structure in an oven at a temperature in the range
of 50 to 100°C, preferably, at a temperature in the range of 90 to 100°C for periods
of up to 4 days. In another, slightly modified, embodiment the drying step is accomplished
by placing the green structure in a relative humidity controlled oven (e.g., 90%
relative humidity) for similar time periods and temperatures as for the aforementioned
standard oven-drying embodiment. In a third embodiment, the drying step is accomplished
by placing the green structure in a dielectric oven for a period of time sufficient
to form a crack-free, self-supporting structure, preferably, a period of no greater
than 60 minutes, more preferably for a period of 5 to 30 minutes.
Sintering of the dried honeycomb structure involves heating or sintering
the honeycomb for a time period sufficient to form a sintered structure having a
high surface area. Specifically the sintering comprises heating the honeycomb in
a nitrogen atmosphere, at a rate 10-25°C/hr, to a first temperature of at least
500°C, preferably this temperature is then maintained for period of up to 10 hours,
more preferably 4 hours. Following this nitrogen pretreatment firing, the honeycomb
is cooled to ambient temperature. Once ambient temperature is attained, the honeycomb
is heated, in air, to a second temperature of at least 850°C, preferably 1100°C.
This second air-heating step, to at least 850°C, may involve two distinct heating
steps: (1) a first heating step, at a rate of between 10-25°C/hr to 500°C, whereupon
the temperature is held for period of time; and, (2) and a second heating step from
500°C to at least 850°C, at a rate of about 50°C/hr, whereupon the temperature is
again held for a period of time. For both air-heating steps, the temperature-holds
are preferably maintained for period of up to 10 hours, and more preferably 4 hours.
The pre-treatment nitrogen heating step is thought to result in sintered
structures exhibiting an increased MOR over those bodies not subject to this pre-treatment
step. While not intending to be limited by theory it is thought that the nitrogen
pre-treatment produces a gradual slow burnout of the organic binder that does not
disrupt the microstructure of the zeolite bodies, thus allowing for easier densification
of the zeolite body at the later achieved higher firing temperatures.
The purpose of the organic binder is to provide plasticity during
forming, and some green strength after drying. Organic binder according to the present
invention refers to cellulose ether type binders and/or their derivatives, some
of which are thermally gellable. Some typical organic binders according to the present
invention are methylcellulose, hydroxybutylcellulose, hydrobutyl methylcellulose,
hydroxyethylcellulose, hydroxymethylcellulose, hydroxypropylcellulose, hydroxypropyl
methylcellulose, hydroxyethyl methylcellulose, sodium carboxy methylcellulose, and
mixtures thereof. Methylcellulose and/or methylcellulose derivatives are typically
used in the practice of the present invention. Methylcellulose, hydroxypropyl methylcellulose,
and combinations thereof are especially preferred. Preferred sources of cellulose
ethers and/or derivatives thereof, are Methocel A4M, F4M, and F240M from Dow Chemical
Co. Methocel A4M is a methylcellulose binder having a gel temperature of 50-55°C
and gel strength of 5000 g/cm2 (based on 2% solution at 65°C). Methocel
F4M and F240M are hydroxypropyl methylcellulose.
To further illustrate the principles of the present invention, there
will be described certain examples of zeolite and zeolite/alumina bodies formed
according to the invention, as well as a comparative body. However, it is to be
understood that the examples are given for illustrative purpose only, and the invention
is not limited thereto, but various modifications and changes may be made in the
invention, without departing from the spirit of the invention.
EXAMPLES 1 and 2
Both examples involved thoroughly mixing together in a Littleford
mixer a batch mixture as provided Table I. The zeolite raw material comprised a
ZSM-5 zeolite having an SiO2/Al2O3 ratio of 300 (CBV-3002
from PQ Corporation) and a silica binder comprising an amount of a concentrated
silicone resin (6-2230 resin from Dow Corning) dissolved in a dibasic ester solution
with the resin/solvent ratio reported in the Table; in each example the amount of
resin resulted in the amount of silica permanent binder reported in the fired composition.
The mixed batch was transferred to a muller and an amount of water, in as reported
in Table I, was added to the batch and the batch was uniformly plasticized; the
water listed as a superaddition weight percent based on the fired composition.
Honeycomb bodies with a wall thickness of about 8 mil and exhibiting
400 cells/in2 and rods suitable for testing MOR, exhibiting a diameter
of 0.3125 in. were formed by extrusion through a ram extruder. Each of the two examples,
green extruded honeycombs and the rods, were dried in the following manner: RT to
95°C in a humidity oven (95-100% relative humidity) for a period of 4 days. After
drying, the extruded honeycombs and rods were pre-fired in an N2 atmosphere
at a rate of between 15-25°C/hr. to 500°C, where temperature was held for a period
of 4 hours. The honeycombs were cooled to room temperature and then heated in air,
at a rate of between 15-25°C/hr, to a temperature of 500°C, where again the temperature
was held for a period of 4 hours. Following this hold, the firing involved heating
the honeycombs, at a rate of between 25-50°C/hr, to a final temperature of 1100°C,
where the honeycombs were held for a final period of 4 hours. The composition of
the fired body is reported in Table I.
The resultant rods were used for characterization of mechanical properties
such as MOR, CTE, and E-modulus. The thermal shock resistance was calculated according
to the following formulas: TSR= MOR/(E-mod X CTE). The porosity and mean pore size
data was measured for the honeycombs and was generated utilizing a conventional
mercury intrusion porosimetry technique. All of these physical properties for the
canes and honeycomb are detailed listed in TABLE I.
Each example involved thoroughly mixing together in a Littleford mixer
a batch mixture as provided Table I. The zeolite raw material comprised a ZSM-5
zeolite having an SiO2/Al2O3 of 300 (CBV-3002 from PQ Corporation),
the gamma alumina raw material comprised GL-25 supplied from LaRoche Industries
(surface area of 260 m2/g), and the methylcellulose temporary binder
comprised Methocel A4M from the Dow Chemical Co. One variation included utilizing
a La-stabilized gamma alumina in Example 7; doped with 4% La2O3
and having a surface area of 110 m2/g). The batch mixture additionally
comprised an amount of a concentrated silicone resin (6-2230 resin from Dow Coming)
dissolved in a dibasic ester solution having the resin/solvent ratio reported in
the Table; in each example the amount of resin resulted in the amount of silica
permanent binder reported in the fired composition. Following treatment with an
amount of oleic acid and/or acetic acid, as reported in the table, the mixed batch
then was transferred to a muller and amount of water, as reported in Table I, was
added to the batch and the batch was uniformly plasticized; note that each of the
weight percents listed in Table 1 for the water, oleic and acetic acids are superaddition
weight percents based on the final fired composition.
Honeycomb bodies with a wall thickness of about 8 mil and exhibiting
400 cells/in2 and rods suitable for testing MOR, exhibiting a diameter
of 0,793 cm (0,3125 in) were formed by extrusion through a ram extruder. The green
extruded honeycombs and rods of Examples 3-5 and 7 were dried in manner similar
to that for Examples 1 and 2. Example 6 was dried in a dielectric oven for a period
of 20 minutes. After drying, the extruded honeycomb and rod green bodies were fired
in a manner similar to that utilized for Examples 1 and 2 with the exception that
the temperature was not held at 500°C, following the second ramp-up, in air.
The final fired composition, physical and mechanical properties of
the extruded zeolite/silica/alumina bodies are shown in TABLE I.
* Comparison Example
Oleic Acid (%)
Surface Area (m2/g)
Mean Pore Size (µ)
Modulus Of Rupture
1068665 Kg/m2 (1520 psi)
2397467 Kg/m2 (3410 psi)
2642135 Kg/m2 (3758 psi)
2081085Kg/m2 (2960 psi) 1595967 Kg/m2
1595967 Kg/m2 (2270 psi)
2134519 Kg/m2 (3036 psi)
339582 Kg/m2 (483 psi)
1877195Kg/m2 (2670 psi)
23201029 Kg/m2 (3300 psi)
3240447Kg/m2 (4609 psi)
1779469 Kg/m2 (2531 psi)
984297 Kg/m2 (1400 psi)
2139440 Kg/m2 (3043 psi)
558237 Kg/m2 (794 psi)
Mean Coefficient Of
Shock Resistance (°C)
An examination of TABLE I, reveals that the, inventive samples 3-6,
containing a gamma alumina ranging from 10 to 50 parts, by weight, exhibit the requisite
combination of properties. Specifically, the inventive samples 3-6 exhibit the following
combination of properties: (1) a surface area of not less than 170 m2/g;
(2) an MOR of at least 2270 psi; (3) a CTE less than 2 ppm/°C; and, (4) a calculated
thermal shock resistance of at least 1510°C.
Referring to Example 7, zeolite/silica body comprising 10 parts, by
weight silica, TABLE I reveals that this sample has a less than desirable MOR of
339582 Kg/m2 (483 psi). This low strength is likely due low amount of
silica binder present, specifically an amount not sufficient to cover the vast surface
area of the 90 parts, by weight combined, of the zeolite and gamma alumina.
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, as numerous modifications
are possible without departing from the broad spirit and scope of the present invention
as defined in the appended claims.