Colloid systems and wetting agents; subcombinations thereof; pro – Continuous or semicontinuous solid phase – The solid phase contains metal silicate or clay
Reexamination Certificate
2001-10-26
2004-11-02
Metzmaier, Daniel S. (Department: 1712)
Colloid systems and wetting agents; subcombinations thereof; pro
Continuous or semicontinuous solid phase
The solid phase contains metal silicate or clay
C516S098000, C516S111000, C516S112000, C423S335000
Reexamination Certificate
active
06812259
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
BACKGROUND OF THE INVENTION
Aerogels are low-density, high surface area solid materials, typically ceramic oxides, which have been expanded using an explosive release of pressure, typically in a supercritical fluid (SCF) or by flash evaporation of a solvent from a sol-gel precursor solution. One of the more common aerogels is composed of silicon dioxide (or “silica”), which is presently available from a variety of commercial vendors. Aerogels commonly display remarkably high surface areas, achieved at minimal cost due to the simplicity of the method used for their synthesis. For example, silica aerogels exhibiting surface areas of approximately 1,250 m
2
/g, are commercially available. No time-consuming and expensive templating process is necessary for the manufacture of aerogels, as both the flash evaporation and SCF routes for their synthesis are readily amenable to large-scale production.
The high surface area exhibited by aerogels suggests their use in a variety of scientific and industrial applications. However, various limitations have curtailed the utility of aerogels in industrial applications, and aerogels have not found widespread use in applications where materials having a high surface area would be expected to present significant advantages.
For example, aerogels commonly exhibit a random pore structure which typically includes “bottlenecks”, or regions within the aerogel wherein the pore sizes fall well below the average pore size for the material. This structure limits their use in applications where a consistent pore size is required. Also, aerogels are typically very fragile structures, rendering them unsuitable in applications where a high surface area material is only useful if it is able to withstand an applied force, even as slight a force as the capillary force of a liquid. Further, in many applications, a material having both a high surface area and exhibiting specific chemical properties is desired. In many instances, the aerogels will fail to provide the specific chemical properties necessary for a given application. To overcome both of these drawbacks, many having skill in the art have attempted to provide coatings for aerogels. The ability to chemically modify the internal surfaces of an aerogel would provide direct access to inexpensive, high-surface area materials useful in a variety of uses, including, without limitation, as sorbents, catalysts and sensor materials. In this manner, it has been proposed that the aerogels could be made to exhibit enhanced strength and/or that aerogels could be made to exhibit chemical properties desired for a particular application by coating the internal and external surfaces of the aerogels with materials bonded on one end to the aerogel, and having a molecule with desired chemical or “functional” properties at the other end.
Unfortunately, attempts to provide coatings on aerogels have so far met with little success. Traditional synthetic coating methods utilizing liquid carriers and the like have been unable to effectively coat the broad expansive surface area of aerogels for a variety of reasons. The random structure of the aerogel has a significant number of constrictions and/or blockages that hinder mass transport into the complex pore structure. Further, due to the high temperature nature of the synthetic protocol typically used to make aerogels, there is very little adsorbed water within the aerogel. Thus, in silica aerogels for example, the surface population of hydroxyl groups is quite low. This severely limits the amount of other species that can be bound by this surface. Also, as noted above, the ceramic oxide wall structure of the aerogels is extremely thin. Combined with the convoluted morphology of the aerogels, the presence of restrictive bottlenecks, and the hydrophobic nature of the material, it is difficult to form hydroxyl groups on aerogels at ambient pressure using standard solution phase methodologies. As a condensed liquid phase enters the pore structure, the capillary forces brought about by liquid column in the tiny pores can overcome the fragile strength of the aerogel wall, thereby crushing the internal structure of the aerogels simply by filling it with liquid.
Thus, there exists a need for methods and techniques whereby the hydroxyl groups may be formed on the surfaces of aerogels. There is a further need for methods and techniques which allow the pore distribution of the internal volume of aerogels to be narrowed, and the bottlenecks limiting transport into and out of the internal volume may be removed, thereby facilitating the deposition of other materials, such as strength enhancing monolayers and functionalized monolayers, on the surface of the aerogels without destroying their high surface area.
BRIEF SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method for forming aerogel having a high density of hydroxyl groups on the surface of the aergel. As used herein, a “high density” of hydroxyl groups on the aerogels refers to the aerogels exhibiting greater than 1 hydroxyl group per square nanometer of the surface of the aerogel, and preferably greater than 3 hydroxyl groups per square nanometer of the surface of the aerogel and more preferably greater than 5 hydroxyl groups per square nanometer of the surface of the aerogel.
It is a further object of the present invention to provide a method for altering the surface area of an aerogel in a manner which reduces the incidence of bottlenecks while preserving the high surface area exhibited by the aerogels. Preferably, the pore sizes of these aerogels are between about 150 Å and 250 Å and the bottlenecks of these aerogels are between about 110 Å and 150 Å. It is also preferable that these aerogels have a pore size distribution of less than 50% of the mean pore diameter. More preferably, these aerogels have a pore size distribution that is less than 20% of the mean pore diameter, and more preferably still is a pore size distribution is less than 10% of the mean pore diameter.
These and other objects of the present invention are accomplished and enabled by the surprising discovery that aerogels, when exposed to a mixture of water and a near critical or supercritical fluid, will resolve into structures having increased surface areas, with a lessened incidence of bottlenecks. The process of exposing an aerogel to a mixture of a supercritical fluid and water is referred to herein as “hydroetching.” An additional advantage of the hydroetching process is the formation of a high a density of hydroxyl groups on the aerogel's surfaces.
The method of the present invention provides aerogels that are amenable to the formation of mono-layers, which can be applied to render the aerogels into functionalized aerogels. The formation of such monolayers and functionalized aerogels is described in co-pending U.S. patent application Ser. No. 10/045,948, filed concurrently herewith, the entire contents of which are incorporated herein by this reference.
As mentioned above, the process of the present invention may also serve to modify the pore size distribution of the aerogel. Prior to processing, aerogel materials typically demonstrate a broad range of pore sizes and narrow bottlenecks, which impede the transport of material into and out of the internal void volume. BET analysis (Brunauer, Emmett, Teller) of aerogels produced by the method of the present invention has demonstrated that the process can narrow the pore size distribution (typically to approximately 200 Å+/−~50 Å) and remove the bottlenecks (typically to approximately 130 Å+/−~20 Å), thereby enhancing the transport of materials to and from the aerogel interior. As used herein, the “pore size distribution” is defined as that revealed by the adsorption isotherm of the BET experiment and bottlenecks are defined as that reveled by the desorption isotherm of the BET experiment.
Materials which have been formed into aerogels are gene
Fryxell Glen
Zemanian Thomas S.
Battelle (Memorial Institute)
McKinley, Jr. Douglas E.
Metzmaier Daniel S.
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