Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
1999-08-17
2002-04-09
Griffin, Steven P. (Department: 1754)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C428S312600, C428S218000, C428S307700
Reexamination Certificate
active
06368703
ABSTRACT:
FIELD OF THE INVENTION
The invention is in the field of porous materials having large surface areas for use in chemical catalysis, molecular separations, and the like.
BACKGROUND OF THE INVENTION
The mechanical strength of articles that have a porous structure depend, among a variety of factors, upon the nature of the structural material, the sizes of the pores, the “porosity” or void volume, and the nature or shape of the pores. As a general but by no means rigorous rule, for a given rigid or brittle material, mechanical strength decreases as porosity increases. The specific surface of a porous material increases with decreasing pore size for a constant void volume, and increases also with increasing void volume. The decrease of mechanical strength with increasing porosity appears to be intuitive in that the greater the porosity, the thinner must be the walls separating adjacent pores, and hence the weaker the walls must be.
A porous material having a substantial void volume is described in Kistler, U.S. Pat. No. 2,093,454. Here, a liquid is removed from a gel at a temperature above the critical temperature of the liquid to leave a dry porous gel structure. The gel is said to undergo little shrinkage as the solvent is removed. Kistler refers to his materials as “aerogels”, and it is taught that these gels can contain as little as one percent or less of solid material by volume. They are, as a result, often very fragile and hence difficult to use.
Another porous material with extremely small pores is taught in “
Ultrastable Mesostructured Silica Vesicles”,
S. S. Kim, W. Zhang, T. J. Pinnavaia,
Science
282, 1302-1305 (1998). This porous material is of silica, and the pores may have mean diameters ranging from about 2.7 to about 4.0 nm. The authors refer to the use of such materials in chemical catalysis and molecular separations, and the authors report having incorporated redox-active Ti(IV) and acidic Al(III) centers into the framework of their materials. Specific surfaces measured in the hundreds of m
2
/g are reported.
Extremely weak and fragile porous materials having pores that are to be contacted with a liquid or gas are difficult to work with because they are difficult to shape and difficult to support. They may be supported as a layer upon a supporting surface, for example. Although microporous materials themselves may have very high specific surfaces (that is, surface per unit mass), the materials themselves, when coated upon a support, provide only a limited macrosurface to come into contact with gases or liquids. Because the highly porous materials are so fragile, they cannot generally be formed into separate shapes that have greater surface areas, on a macro level, for contact with liquids or gases.
It would be desirable to provide a structure in which porous materials may be so fashioned as to render them highly available for contact to liquids and gases.
SUMMARY OF THE INVENTION
It has been found that porous materials having small pores, that is, not greater than about 100 nm and preferably in the range of about 5 to about 100 nm may themselves be supportively nested in the pores and supported on the pore walls of a relatively strong porous carrier having pore sizes at least one and preferably at least two orders of magnitude greater than the pore size of the smaller pore material. The pores of the small pore material open onto the pores of the porous carrier. By utilizing the pore walls of the porous carrier to support the smaller pore material, the available “macro” surface of the smaller pore material is greatly increased so that liquids or gases flowing through the volume of pores of the porous carrier may readily access the pores of the smaller pore material. Because the area of contact between the smaller pore material and the supporting pore walls of the porous carrier material is quite large, the structural support provided by the porous carrier to the smaller pore material is substantial.
Accordingly, the invention relates to a strong, porous article comprising a porous carrier having an outer surface defining a shape having a bulk volume and having interconnecting pores providing fluid flow openings extending throughout the bulk volume and opening through the outer surface. The porous carrier has a plurality of continuous strong supportive struts defining walls bounding the pores, the pores preferably having a mean size not greater than about 100 microns. A second porous material, the mean pore size of which is at least an order of magnitude less than the mean pore size of the porous carrier, is nested within and structurally supported by the pore walls of the porous carrier so that the second porous material maintains its porous configuration. The pores of the second porous material open onto the fluid flow openings of the porous carrier and are accessible to a fluid flowing through the pores of the carrier.
In a preferred embodiment, the porous carrier has an mean pore size in the range of about 0.3 to about 10 microns, and the second porous material has a pore size at least an order of magnitude smaller, preferably in the range of about 1 to about 100 nm.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
For use in describing the invention, “mesoporous” may be used to describe a porous material having a substantial specific surface and to which access is desired. Mesoporous materials commonly have pore sizes in the nanometer range, often ranging from about 1 to about 100 nm. These materials often are characterized by having extremely weak physical structures that often are only barely self-supporting, although some mesoporous materials can be somewhat stronger. “Microporous” may be used herein to refer to pore structures having pore sizes in the micron range, particularly in the range of about 0.3 to about 10 microns. These materials commonly are much stronger in mechanical strength than the mesoporous materials. Finally, “Macroporous” may be used to refer to pores that are generally measured in hundreds of microns, commonly in the 100 to 500 micron range. Porous materials of this type are shown in International Publication No. WO 99/16479.
The present invention is particularly valuable in supporting mesoporous structures within the pores of a micro or macroporous structure, but on a broad basis, the invention is applicable to any combination of two porous structures in which one structure, having larger pores, serves as a support for the material having smaller pores. The pore size of the smaller pores is at least one and preferably at least two orders of magnitude less than the pore size of the carrier material. The pores of the smaller pore material open onto and communicate with the passages defined by the pores of the porous carrier. The pores of the porous carrier serve to support the second porous material, which is particularly valuable when the second porous material has little mechanical strength, especially when the second porous material is so weak as to be non self supporting or cannot be mechanically worked from one shape to another.
The sizes of pores of the various materials referred to herein are mean sizes. Conceptually, it is convenient to think of a pore having a minimum diameter of, for example, 100 microns, as being capable of accommodating the passage through it of a “worm” having a round cross section and a transverse diameter of 100 microns. Put another way, a 100-micron opening should enable passage through it of a sphere having a 100-micron diameter. Although I am aware of no completely satisfactory way for measuring the sizes of pores, it is possible to examine a scanning electron micrograph or other photograph of enlarged cross section of a porous material, and view the pores as planar projections of the structure. Here, lines may be drawn across the micrograph and the size of the openings that intersect the lines is measured. Averaging and standard deviation techniques may be used to permit the mean size of the openings to be assessed.
In the manufacture of articles of the invention, it is generally preferred to first complete
Fredrikson & Byron , P.A.
Griffin Steven P.
Ildebrando Christina
Phillips Plastics Corporation
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