Catalyst – solid sorbent – or support therefor: product or process – Zeolite or clay – including gallium analogs – And additional al or si containing component
Reexamination Certificate
2000-12-12
2003-04-15
Langel, Wayne A. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Zeolite or clay, including gallium analogs
And additional al or si containing component
C502S065000, C502S067000, C502S071000, C502S073000
Reexamination Certificate
active
06548439
ABSTRACT:
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.
U.S. Pat. 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, U.S. Pat. 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 based material for use as a catalyst substrate, adsorber or catalyst carrier comprising a zeolite having a silica/alumina ratio of greater at least 300 and a surface area of 250 m
2
/g and no greater than about 20 parts, by weight, of a silica binder. The properties this zeolite based material exhibits includes the following: (1) a modulus of rupture of at least 1500 psi; (2) a surface area of at least 100 m
2
/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.
In a separate embodiment the zeolite-based material includes a third component, specifically gamma alumina having a specific surface area of greater than 100 m
2
/g.
This invention also relates to a method for producing an extruded honeycomb monolith having the aforementioned high strength and surface area properties, comprising mixing into a substantially homogeneous body the following components: (1) a zeolite having a silica/alumina ratio of at least 300 to 1 and a surface area of at least 250 m
2
/g; (2) no greater than 20 parts, by weight, of a silica binder derived from using a resin/solvent mixture having a ratio ranging between from 2/1 to 4/1; (3) a temporary binder; and, (4) water. Following mixing of the raw materials, the method involves extruding the mixture to form a green honeycomb structure, and then drying the structure for a time sufficient to form a crack-free dry structure. Once the extruded body is dried, the method next involves heating the dried honeycomb structure in a nitrogen atmosphere, to a first temperature of at least 500° C., cooling to ambient, and thereafter, heating, in air, to a second temperature of at least 850° C. to form a sintered structure.
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 m
2
/g. Expressed in parts by weight, the zeolite based bodies, according to the invention, characteristically contain between about 30 to 90 parts, by weight, zeolite and not more than 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 20 m
2
/g, preferably greater than 100 m
2
/g, and most preferably greater than 150-200 m
2
/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/in
2
, exhibiting surface areas of at least 200 m
2
/g, with surface areas in excess of 250 m
2
/g being readily attainable.
As detailed above, the zeolite component is desirably a high silica-containing zeolite exhibiting a SiO
2
/Al
2
O
3
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, pref
Wu Shy-Hsien
Zaun Kenneth E.
Corning Incorporated
Langel Wayne A.
Schaeberle Timothy M.
Sterre Kees van der
Strickland Jonas N.
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