Stock material or miscellaneous articles – Structurally defined web or sheet – Honeycomb-like
Reexamination Certificate
1999-12-20
2002-04-02
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Honeycomb-like
C423S626000, C423S628000, C264S176100, C264S209100, C264S211000, C264S211200
Reexamination Certificate
active
06365259
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention addresses the need, for many applications, for an alumina honeycomb monolithic support that combines both high strength and a high B.E.T. surface area (over 100 m
2
/g). In the past, it has been found difficult to obtain both high strength and high surface area in extruded alumina honeycomb structures without the incorporation of binder materials that reduce the purity and/or chemical activity of the alumina for catalytic applications.
The petrochemical industries currently use a variety of pellet type structures formed of gamma alumina or other oxides, e.g., pellets, pills, beads, rings, trilobes, stars, and so forth, as catalysts or catalyst support media for catalytic reactions. These structures are typically formed by extrusion from batch mixtures of alumina or other selected oxides, followed by drying and calcining. The objective is to produce shapes which are crush- and attrition-resistant when packed into reactor beds; due to their thick cross-sections and compact geometric shapes, the tensile strengths of such extruded shapes is generally not of concern. U.S. Pat. Nos. 3,969,273, 3,917,808 and 4,132,669 provide examples of aqueous extrusion batches incorporating combinations of various acids for the preparation of extruded pellets or pills of hydrated alumina, calcined (e.g. gamma) alumina, and combinations of alumina with phosphorous or other oxides.
Although technologies for making durable, active pellet-type alumina supports or catalysts are well developed, such structures are not optimally configured for most catalytic reactor applications. Pellet beds tend to exhibit relatively high flow resistance in comparison with honeycomb supports, and also develop preferential flow paths which exhaust portions of the catalyst while leaving other portions relatively unused.
Ceramic honeycombs are used in many applications in which the ceramic substrate serves simply as a physical structural support for a chemically active, high-surface-area catalyst support coating. A typical coating for these applications is a high surface area washcoating of gamma-alumina deposited on the channel walls of the ceramic honeycomb. U.S. Pat. No. 4,965,243 describes coated honeycomb structures of this type useful for automotive catalytic converters.
However, for many applications porous washcoatings are inadequate and catalysts or catalyst supports made up mostly or entirely of active, high-surface-area material must instead be used. Such applications include chemical processes wherein the kinetics of the chemical reaction(s) on the catalyst are slow relative to the diffusion and mass transfer steps involved in the overall process. An example is the hydro-desulfurization of fossil fuels in the petrochemical industry to make low sulfur gasoline and diesel fuels. Since the reaction kinetics are the slow step in such processes, it is important to provide a relatively large accessible BET catalyst support surface (more catalyst sites in a given volume) in order to allow the most effective use of reactor volume. This in turn requires that the entire volume of the catalyst or catalyst support structure be made of active, high-surface-area material, and that the pore structure of the material be such that that the reactants can diffuse in and products diffuse out of the volume of the catalyst support effectively over relatively long distances. The potential advantages of honeycomb structures of appropriate porosity and surface area in such applications include better selectivity, higher yield, lower pressure drop, lower waste or emissions, and more compact reactor designs.
Even for applications such as automotive catalytic converters, where reaction kinetics are usually not rate limiting, thinner-walled, lower mass catalyst supports are being developed to decrease exhaust gas back-pressure and improve reactor efficiency. However, with decreasing wall thickness the thermal mass contribution of the ceramic substrate relative to the gamma alumina catalyst support coating becomes an increasingly important factor that restricts the light-off speed of the reactor. A honeycomb that incorporates only active support material while dispensing with inert supporting structure will offer substantial performance advantages, and eliminate the separate and costly alumina washcoating step as well.
In order to take advantage of the potential benefits of alumina honeycombs, however, both the strength and surface area of the honeycombs must be maintained or improved. Many potential honeycomb applications require a high B.E.T. surface area for effective catalyst function, i.e., at least about 50 m
2
/gram, and from 150-200 m
2
/gram or more for some applications. High strength and good resistance to flaking are needed to maintain the structural integrity of the support in a hostile reactor environment. Higher B.E.T. surface areas mean a more compact reactor, which could lead to significant cost reductions for the overall reactor system.
In contrast to the formulations employed to provide pelletized alumina or other active oxide supports, the extrusion of honeycomb structures from simple oxides or mixtures has involved the use of supplemental bonding agents. These are typically incorporated in the extrusion batch with the oxides and remain in the fired honeycombs as permanent binders, to achieve useful flexural strength levels in the fired structures. Relatively high tensile strength, as measured by flexural modulus of rupture tests of the fired oxides or mixtures, is required to impart useful strength and durability to the fired thin-walled structures. However, such strength must be attained at low to moderate firing temperatures in order preserve the high porosity and B.E.T. surface area of the oxide starting materials.
U.S. Pat. No. 4,631,267 teaches the manufacture of extruded honeycombs of alumina, silica and titania composition that incorporate precursors for permanent silica, alumina and titania binders in powdered alumina, titania or silica extrusion batches. The precursors for the permanent binders are generally liquid solutions or dispersions of oxide-yielding compounds such as titanium isopropoxide or silicone solutions or hydrated alumina slurries, these being converted to small crystallite bonding deposits of the respective oxides on firing. Alumina honeycombs produced by this method can exhibit B.E.T. surface areas in excess of 70 m
2
/g and MOR (flexural modulus of rupture) strengths above 2000 psi after firing at temperatures in the 500-1000° C. range.
Although the method of this patent provides alumina products of high surface area, it has not yet found extensive commercial application. Shortcomings of the disclosed method include the relatively high cost of the permanent binder materials, and the limited effectiveness of such binders in terms of the range of powders which can be successfully treated and the levels of fired product strength which may be obtained. Good results have been demonstrated for hydrated alumina batch powders, but the resulting batches are subject to high drying and firing shrinkages which create significant production and yield problems for fine cellular structures such as honeycombs.
What is therefore required is a method for producing alumina honeycombs that produces products of high strength and surface area, yet is still economic in terms of raw materials costs and processing yields.
SUMMARY OF THE INVENTION
The present invention is founded upon the discovery that the incorporation of relatively small amounts of inorganic or simple organic acids directly into powdered alumina extrusion batches can provide honeycombs exhibiting an excellent combination of high strength and high surface area. Moreover, the method can be used with powdered alumina batches that include substantial proportions of anhydrous high-surface-area (gamma) alumina powders, these offering a significant processing advantage in terms of reduced drying shrinkage, and therefore process yield. Strength increases of from 45-200% or more over the strengths typically
Brundage Kevin R.
Swaroop Srinivas H.
Boss Wendy
Corning Incorporated
Jones Deborah
Sterre Kees van der
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