Catalyst – solid sorbent – or support therefor: product or process – Zeolite or clay – including gallium analogs – And additional al or si containing component
Reexamination Certificate
2002-05-09
2004-08-24
Nguyen, Cam N. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Zeolite or clay, including gallium analogs
And additional al or si containing component
C502S073000, C502S074000, C502S087000, C502S327000, C502S332000, C502S333000, C502S334000, C502S339000
Reexamination Certificate
active
06780805
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zeolite/alumina composite material exhibiting a high strength and a high surface area, specifically a surface area high enough to be suitable for catalyst impregnation.
This invention also relates to a process for the preparation of such composition, and to the use thereof, notably in the field of catalysis, whether as a catalyst and/or catalyst support for the conversion of vehicular exhaust gas.
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 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.
It is well known in the catalyst art that catalytic converters having smaller cell dimensions and cell wall thickness' exhibit increased catalyst geometric surface area resulting in increased diffusion of the pollutants into and in contact with the catalyst; i.e. an increased flux without significant increase in back pressure. Secondarily, honeycombs with high cell density have small cell dimensions, which increase the concentration gradients for pollutants to diffuse to the catalyst surface. This results in a high flux of pollutant gas flow to the surface of catalyst.
From the catalyst point of view, a minimum amount of catalyst is required in order to convert pollutant gases to environmentally benign gases. It follows then that ideally a high cell density honeycomb with a thin wall and a thin catalyst washcoating would be preferred in terms of pollutant gas diffusion; i.e., high or increased geometric surface. However, it is undesirable to washcoat a catalyst-containing slurry onto high cell density ceramic honeycombs utilizing a conventional washcoating process since the channel size of the honeycomb is reduced: In addition, the application of a relatively thick washcoat layer on thin wall, high cell density substrates, significantly increases the wall thickness. Increased washcoat thickness/wall thickness, in turn can increase the backpressure undesirably. Thus, catalytic converters comprising high cell density ceramic honeycombs that include washcoat are not the best means for increasing the conversion efficiency of the catalytic converter.
There is, accordingly, a clear need for, and thus an object of the present invention is to provide, a catalyst support material capable of being extruded into a catalyst support body exhibiting a high cell density, a thin wall thickness and sufficient strength for catalytic applications, and exhibiting a sufficiently high geometric surface area for precious metal catalyst impregnation without requiring a catalyst/high surface area material washcoating component.
SUMMARY OF THE INVENTION
The object of the present invention is to solve the above problems of the prior art and to provide a method for making a high surface area catalyst support capable of being extruded into a high cell density, thin wall thickness body and thereafter impregnated with precious metal catalyst material.
Specifically, the invention is directed at a zeolite/alumina composite for use as a catalyst substrate or catalyst carrier comprising a zeolite having a silica/alumina ratio of greater than 300 and a gamma alumina having a specific surface area of greater than 100 m
2
/g. The zeolite/alumina composite exhibits a modulus of rupture of at least 750 psi.
This invention also relates to a method for producing an extruded honeycomb monolith having a high surface area, comprising mixing into a substantially homogeneous body the following components:
(i) a zeolite having a silica/alumina ratio of at least 300 to 1 and a surface area of at least 250 m
2
/g;
(ii) a gamma alumina component having a specific surface area of greater than 100 m
2
/g, at least a portion of which is boehmite functioning as the permanent binder,
(iii) a temporary binder selected from the group consisting of methylcellulose, ethylcellulose, hydroxybutylcellulose, hydroxybutylmethylcellulose, hydroxyethylcellulose, hydroxymethylcellulose, hydroxypropylcellulose, hydroxypropylmethylcellulose, hydroxyethylmethylcellulose, and mixtures thereof; and,
(iv) water;
Following mixing of the raw materials, the method involves extruding the mixture to form a green honeycomb structure, and then heating the structure for a time in the range of 5 to 30 minutes to form a crack-free dry structure. Once the extruded body is dried, the method next involves heating the dried honeycomb structure to a temperature range of 500 to 1000° C. for a time period sufficient to form a sintered structure having a high surface area and high mechanical strength.
The invention is also directed at a three-way catalyst (TWC) system for use in the removal of hydrocarbons, carbon monoxide and oxides of nitrogen from waste gas, the TWC system comprising the following components:
(1) a zeolite/alumina composite catalyst support exhibiting a modulus of rupture of at least 750 psi and having a zeolite having a silica/zeolite ratio of at least 300 and the alumina comprising a gamma alumina having a specific surface area of greater than 100 m
2
/g; and,
(2) a noble metal catalyst impregnated on the catalyst support, the noble metal selected from the group consisting of platinum, rhodium, iridium and palladium.
DETAILED DESCRIPTION OF THE INVENTION
The product of the present invention is a zeolite/alumina composite for use as a catalyst substrate or catalyst carrier, specifically a composite wherein the zeolite exhibits a silica/alumina ratio of greater than 300 and a gamma alumina having a specific surface area of greater than 100 m
2
/g. The inventive composite exhibits a modulus of rupture of at least 750 psi, preferably at least 1000 psi. Expressed in parts by weight, the zeolite alumina composites, according to the invention, characteristically contain between about 30 to 70 parts by weight zeolite and between about 30 to 70 parts by weight gamma alumina.
Typically, zeolites comprise large particles on the order of several microns and exhibit a regular array of accessible micropores, a combination that provides th
Faber Margaret K.
Wu Shy-Hsien
Xie Yuming
Zaun Kenneth E.
Corning Incorporated
Gheorghiu Anca C.
Nguyen Cam N.
Sterre Kees van der
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