Abstract:
New aerogels are disclosed which comprise silica and at least one organic polymer having polar functional groups. Also disclosed are methods for making such aerogels and for making them as substantially transparent objects. In addition, gas detectors, liquid and gas absorbing objects, and optical devices made using these aerogels are disclosed. The present invention concerns optical elements which are substantially transparent and exhibit low refractive indices. The invention further relates to detectors of reactive and absorbate gases based on the physical changes exhibited by the aerogels upon reacting with or absorbing such gases. The present invention further concerns printable objects comprising these aerogels, specifically when the print media are imaged by the absorption of liquid and the spatial localization of pigments or dyes. The invention further concerns reactive gas absorbers comprising these aerogels and the catalytic function the aerogels serve to catalyze subsequent chemical reaction. Furthermore, the present invention concerns the absorption of chemical compounds from condensed states including liquid, critical and supercritical fluids.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]    The present application claims priority from U.S. provisional application Serial No. 60/055,146, filed Aug. 8, 1997; U.S. provisional application Serial No. 60/085,469, filed May 14, 1998; and, U.S. application Ser. No. 09/130,943, filed Aug. 7, 1998. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to non-metal aerogels comprising silica and a particular class of organic polymers having polar functional groups. The invention also relates to various methods for producing the aerogels of this invention. In addition, the present invention pertains to numerous applications and devices utilizing the new aerogels.  
         BACKGROUND OF THE INVENTION  
         [0003]    An aerogel is a gel which has a lower density than the fully condensed form of the material comprising the gel. Aerogels typically are produced by replacing the liquid of a gel by air or another gas without allowing complete collapse of the structure. The seminal report on this was made by Kistler in 1931 (Nature, 127, 741(1931)), who described the goal of the research as being “to test the hypothesis that the liquid in a jelly can be replaced by a gas with little or no shrinkage”. This early work led to aerogels through the use of supercritical fluids to extract liquid, and it led to the hypothesis that the gel structure itself can be preserved in the supercritical drying process, as disclosed by Marshall in U.S. Pat. No. 285,449 (1942).  
           [0004]    There have been many successes in the aerogel field, as disclosed in the scientific and technical literature and in patents. Of relevance to the current invention is the area known in some contexts as Organically Modified Ceramics, referred to as ORMOCERS or called CERAMERS, which have been widely studied. A descriptive review of this area is that of R. C. Mehrotra (Present Status and Future Potential of the Sol-Gel Process, Chapter 1 in Chemistry,  Spectroscopy and Applications of Sol - Gel Glasses, Structure and Bonding Series  77, Eds. R Reisfeld and C. K. Jorgensen, Springer-Verlag, Berlin, 1992). This reference points to the distinction between composite materials that are mixed at the molecular level and those that have mechanically combined components. This reference also discusses work directed to organically modified gels in the form of aerogels and their subsequently dried, fused, oxidized and otherwise treated forms. Also of relevance are works concerning aerogels and their applications. The book Aerogels edited by J. Fricke (Springer Proceeding in Physics 6, Springer-Verlag, Berlin, 1985), the book  Sol - Gel Science, The Physics and Chemistry of Sol - Gel Processing  by C. J. Brinker and G. W. Scherer (Academic Press, Inc. Harcourt Brace Jovanovich, Publ., New York, 1990) and the book  Sol - Gel Technology for Thin Films, Fibers, Preforms, Electronics and Specialty Shapes , Ed. L. C. Klein (Noyes, Park Ridge, N.J., 1988) are of relevance and show the great importance attached to the formation of aerogels with specific properties and functions. U.S. Pat. No. 4,440,827 discloses in claim 13 the use of silica particles in media for ink jet recording and optical bar code printing, wherein one of the methods for preparing the synthetic silica that may be used is an aerogel process. The use of silica aerogel in Cerenkov detectors is described, for example , by M. Cantin, L. Koch, R. Jouan, P. Mestreau, A. Roussel, F. Bonnin, J. Moutel and S. J. Tiechner, in their paper in Nuclear Instruments and Methods, 118, 177 (1974).  
           [0005]    Examples of aerogels which contain or have added to them, metal ions or metal containing species are well known. Those known fall into several categories, including: (1) a silica aerogel that has been dipped into a solution or dispersion containing the metal ion source; (2) a polymer matrix aerogel, such as a polyacrylonitrile aerogel, that contains metal ions added to the aerogel or to the gel before formation of the aerogel (eg. L. M. Hair, L. Owens, T. Tillotson, M. Froba, J. Wong, G, J. Thomas, and D. L. Medlin, J. Non-Crystalline Solids, 186, 168 (1995), and S. Ye, A. K. Vijh, Z-Y Wang, and L. H. Dao, Can. J. Chem. 75, 1666 (1997)); or (3) a silica aerogel having metal ions (eg. M. A. Cauqui, J. J. Calvino, G. Cifredo, L. Esquivias, and J. M. Rodriguez-Izquierdo, J. Non-Crystalline Solids, 147&amp;148,758 (1992)) or small metal compounds bound in it (eg. Y. Yan, A. M. Buckley and M. Greenblatt, J. Non-Crystalline Solids, 180, 180 (1995)).  
           [0006]    The use of supported metal and metal ions for catalysis is widely known and practiced, and many reports exist in the scientific, technical, engineering, and patent literature. Relevant studies include “The Chemistry of Ruthenium in PSSA Ionomer: Reactions of Ru-PSSA with CO, H 2 and O 2  and Alcohols” (I. W. Shim, V. D. Mattera, Jr., and W. M. Risen, Jr., Journal of Catalysis, 94, 531 (1985)), which includes a report of a static Fischer-Tropsch reaction catalysis by supported ruthenium under mild conditions of 150° C. and 600 Torr total pressure, “A Kinetic Study of the Catalytic Oxidation of CO over Nafion-Supported Rhodium, Ruthenium, and Platinum”, by V. D. Mattera, Jr., D. M. Barnes, S. N. Chaudhuri, W. M. Risen, Jr., and R. D. Gonzalez, Journal of Physical Chemistry, 90, 4819 (1986), which shows this catalysis, and “Chemistry of Metals in Ionomers: Reactions of Rhodium-PSSA with CO, H 2 , and H 2 O”, Inorganic Chemistry, 23, 3597 (1984), which shows relationships between spectroscopic observations on supports that have been exposed to these gases and the compounds formed and the oxidation states of rhodium species.  
           [0007]    Despite the known aerogels and metal supports, a need still remains for improved aerogels and related materials having superior properties and applicability.  
         SUMMARY OF THE INVENTION  
         [0008]    In a first aspect, the present invention provides a new and unique aerogel comprising silica and at least one polymer having a polar functional group.  
           [0009]    In yet another aspect, the present invention provides a medium for printing that comprises an aerogel comprising silica and an organic polymer.  
           [0010]    In an additional aspect, the present invention provides a gas absorber that comprises an aerogel according to the present invention.  
           [0011]    In still a further aspect, the present invention provides a detector element comprising an aerogel in accordance with the invention.  
           [0012]    The present invention also provides, in yet another aspect, an optical element that comprises an aerogel material in accordance with the invention.  
           [0013]    In an additional aspect, the present invention provides a catalyst that comprises an aerogel material in accordance with the present invention.  
           [0014]    The present invention also provides a method for producing a transparent, or substantially so, aerogel.  
           [0015]    Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. It should, however, be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIGS. 1 a  to  1   f  illustrate the ultraviolet-visible range spectrum of various silica-chitosan-M aerogels in accordance with a preferred aspect of the present invention;  
         [0017]    [0017]FIGS. 2 a  to  2   e  illustrate the near infrared range spectra of various silica-chitosan-M aerogels in accordance with a preferred aspect of the present invention;  
         [0018]    [0018]FIG. 3 illustrates the infrared spectra of a silica-chitosan rhodium aerogel, presented as absorbance as a function of wave number, before and after exposure to CO; and  
         [0019]    [0019]FIG. 4 illustrates the infrared spectra of a silica-chitosan-ruthenium aerogel, presented as absorbance as a function of wave number, before and after exposure to NO.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    The present invention concerns new aerogels that comprise silica and at least one organic polymer having polar functional groups. The present invention also provides a process for producing some of these novel materials, in which the silica source, the organic polymer, and liquid form a clear gel. In accordance with this process, the gel may be placed in a liquid extractable form by replacing the liquid with an alcohol in the gel without removing the constituents completely, and then the alcohol and remaining liquid are extracted from the gel by supercritical extraction using carbon dioxide. This process can include an adjustment of the pH of the system at each stage to control the rate of gelation and the sol-gel morphology, or to control the solubility. The present invention process can be carried out in stages such that the silica source and the polar functional organic polymer form a clearsol which then forms a clear gel, which-may be treated to adjust the pH. The wet gel is then replaced with alcohol. A supercritical extraction operation is carried out to provide a monolithic aerogel.  
         [0021]    These monolithic aerogels in accordance with the present invention possess a number of properties that lead to novel applications. They have high surface areas and high void volumes related to their low densities. Monolith is a word used in its conventional sense, as “a single stone or block of stone” (Webster&#39;s New Collegiate Dictionary), in order to distinguish the materials of this invention (before any intentional fracturing, powdering or other mechanical processes) from the types of small particles or aggregate particles commonly obtained in the prior art. Typically, the monoliths obtained are limited only by the size of the supercritical extraction equipment and pieces as large as about 50 mm×50 mm have been prepared. Their thicknesses may typically vary from about 0.02 mm to about 4 mm. Greater or lesser thicknesses are contemplated. The monolithic structure permits applications of these materials as optical elements of low refractive index, optical filters, gas and liquid absorbent materials, gas filters, as well as those that are possible with the smaller particles.  
         [0022]    The term “polymer” as utilized herein generally refers to a macromolecular substance composed of repeating atomic groups, ie. monomers. The term encompasses all types of polymers such as homopolymers, copolymers, dimers, trimers, oligomers, for example.  
         [0023]    The aerogels of the present invention may comprise ions that can interact with reactive gases and unreactive but absorbable gases to yield a changed material whose changes can be probed to indicate the presence of the molecules. The aerogels may comprise ions that can interact with many dye molecules to immobilize them by a mordanting interaction. Moreover, the aerogels can be transparent to light in regions of interest, such that they may be used as optical filters, active optical elements, and detectors based on the optical, magnetic, and magneto-optic and related properties of the metals and their surroundings. The aerogels of the present invention have pore sizes in the range of about 2 to about 6 nanometers and very high surface areas.  
         [0024]    It has been discovered that certain organic polymers with polar functional groups are particularly useful constituents in such aerogels. These preferred organic polymers can be dissolved in the solution in which the silica source is provided, which form with the silica source, a clear sol and subsequent gel. For the case in which the silica source is an alkoxide, such as tetraethyl orthosilicate (TEOS), formation of a monolithic gel occurs preferably under acidic conditions with water and alcohol present. Note that TEOS can also be written as tetraethoxy silane, and it belongs to the class that includes tetraalkoxysilanes in general, trialkoxyalkyl silanes, dialkoxydialkyl silanes and other closely related compounds, which can hydrolyze to give 2,3, or 4 hydroxy groups with which to form part or all of a silica network, such as would be well known to one reasonably skilled in the art. Other alternate silica sources could be employed. Thus, the preferred organic polymer must have sufficient solubility to permit its incorporation in the sol and not precipitate to produce a visual cloudiness during gelation.  
         [0025]    A preferred organic polymer that has these characteristics is chitosan, which is the name given to materials derived from chitin by deacylation. These materials vary in degree of deacylation and molecular weight according to the source of the chitin and the deacylation process. Commercial chitosan typically is prepared from chitin from the skin or shell of anthropods and thus often is a recovered waste product of the fishing industry. Chitosans in the range of 50 to 100% deacylated (replacement of 50 to 100% of acylamine groups by amine groups) and molecular weights in the 35,000 to 3,000,000 Dalton can be used. More preferred are chitosans in the range of weight average molecular weights of 150,000 to 2,500,000 g/mol and degrees of deacylation from about 70 to about 100%. Most preferred are chitosans with such weights of 300,000 to 2,100,000 g/mol and degrees of deacylation from about 80 to about 100%.  
         [0026]    Chitosan is a copolymer containing both beta-(1-4)-2-acetamido-2-deoxy-D-glucose and beta-1(1-4)-2-amino-2-deoxy-D-glucose units. The amine group of the deacylated units can form coordinate covalent bonds to metal ions by complexation. The extent to which the amine group, which coordinates to metal ions, is present relative to its protonated form depends on the pH of the system. A characteristic of chitosan that is unusual for compounds with primary amine groups is that the pKa is about 6.3. This means that a significant fraction of the groups are in the amine form at pH less than 7. Furthermore, this also means that chitosan can coordinate effectively at pH less than 7 rather than at the higher pH values at which the silica network is subject to base hydrolysis and instability.  
         [0027]    Another characteristic of chitosan is that it has OH groups. Without being limited to any particular theory, it is contemplated that the OH groups assist in the interaction of the polymer with the silica as it is present in the various stages of preparation and in the final form of aerogel material. This characteristic as well as the interactions of other groups of the chitosan copolymer with the silica, and of the OH groups with the metal ions and with other chitosan units may be the reason that the polymer is not extracted to a deleterious extent when the wet gel is exposed to aqueous metal ion containing solutions and alcohols.  
         [0028]    Other organic polymers suitable for forming the aerogels of this invention include, but are not limited to, alginic acid, gelatin, pectic acid (from apples), carboxylate-modified poly(acrylamide), carboxylate modified chitosan, polyvinyl alcohol that is about 100% hydrolyzed and of weight average molecular weight in the 100,000 to 200,000 range, poly(acrylic acid). Other carbohydrates with polar groups, such as carrageenans, modified starches such as amylopectin, epichlorohydrin or alkyene oxide modified amylopecin and certain dextrins may also be suitable. Clearly polymers such as polyamines and others fulfilling the role of the polymer but whose polar groups are thiol, thioether, phosphate, also could be useful in the formation of these aerogels using the methods of the invention.  
         [0029]    It has been discovered that the aerogels of this invention can be friable so that they are easily broken into relatively small pieces for use as particles. The minimum particle size is about 25 nanometers. When this property is combined with the ability to absorb liquids containing dyes, the aerogel of this invention provides a novel particle for use on or as part of imaging surfaces such as those imaged with solvent borne inks.  
         [0030]    Another important feature of the aerogels of the present invention is their transparency to light in certain wavelength regions. This feature, particularly when taken on combination with the high surface areas and absorptive structure of the aerogels, has provided a wide array of novel uses for the aerogels as detection elements and optical elements. This transparency is immediately evident in certain regions of the visible spectrum, at which they are colored but relatively transparent, resembling pieces of colored but clear glass. Absorption of gases by the aerogels can cause a color change in the aerogel. Representative examples of gases that may be absorbed by the aerogels of the present invention, include but are not limited to, CO, H 2 , H 2 O, NH 3 , CO 2 , N 2 O 4 , NO, and NO x . Thus, gas absorption can be detected using the aerogel as the detection element and a suitable light source and detector. Absorption of gases which cause changes to the infrared or near infrared spectrum of the aerogels in the region where the aerogels have a spectral transmission of greater than about 10% provides an element for a detector for those gases. It has been discovered that detection of CO, NO, and N 2 O 4  with detection in the mid infrared region provides a detector element for these gases.  
         [0031]    The present invention is further illustrated by the following examples. It is to be understood that the present invention is not limited to the examples, and various changes and modifications can be made in the invention without departing from the spirit and scope thereof.  
       EXAMPLES  
     Example 1  
       [0032]    A sample of the SiO  2 —X aerogel based on 10:90 ratio, was touched with a drop from a micropipet of each of the following dyes and ink, selected to have a range of colors, solvent and molecular type: eriochrome black T in methanol, phenyl red in water, crystal violet in water, malachite green in water, methyl red in ethanol, and standard pen recorder blue (for Hewlett Packard pen recorder). For comparison, the same experiment was done with a piece of SiO 2 —X—Cu aerogel, SiO 2 -pectic acid- Yb aerogel, SiO 2 —X—Co aerogel, and SiO 2 —X—Ni aerogel, as well as a piece of aerogel made without either the organic polymer or the metal ion containing species, which will be labeled “SiO 2  aerogel prep”. The results were as follows. The ink and dyes were taken up very fast by the aerogels. The drops dried very fast in terms of spread of the image, such that the shape of the image was essentially that of the drop and its placement on the aerogel monolith. By using aerogel monoliths of about 0.3 mm thickness, it was possible to make the foregoing observations concerning lateral spread, but also about spread through the thickness and the effect on transparency. With small drop sizes relative to the thickness of the monoliths, the whole drop could be absorbed without noticeable spread laterally and without penetrating to the back side of the aerogel through its thickness. In addition, the interaction of the dye solutions and ink with the aerogels was found to be observable through the still transparent samples. The colors of the dyes as absorbed are given in Table 1.  
                                                                                 TABLE I                                                   methyl                   chloro   phenyl   eriochrome   crystal   red   blue pen   malachite           phenyl red   red (in   black T   violet   (in   recorder   green           (in H 2 O)   H 2 O)   (in CH 3 OH)   (in H 2 O)   EtOH)   ink   (in H 2 O)                                    SiO 2     brownish   yellow   burgundy   purple   red       green       aerogel   red   orange       blue       SiO 2 -   dark   red   black   purple   red       green       chitosan   brown           blue       aerogel       SiO 2 -   black   deep   black-violet   violet   pink-       blue       chitosan-       brick           orange       Co       red       SiO 2 -   brown   orange   black   violet   pink   deep   blue       chitosan-                       blue   green       Cu       SiO 2 -   black   red   violet   violet   pale red       blue       chitosan       Ni       Aerosil   brown   orange   brownish   purple   red       green       200       red   red   blue       Degussa       com&#39;cl       Lot No:       c/2/30c       TLC   brown   orange   brown   deep   red       green       plate       red       purple                  
 
       Example 2  
       [0033]    Samples of powder of the SiO 2  aerogel prep, SiO 2 —X aerogel, SiO 2  from a standard thin layer chromatography (tlc) plate, and SiO 2  aerosil (Degussa Aerosil 200, Lot c/2/30c), and of SiO 2 —X—Cu aerogel were obtained. The uptake of the solutions to apparently dry particles was instantaneous With the aerogels prepared with a chitosan to silica dry weight ratio of 10:90 as examples of this invention. It was observed to be much faster than the uptake by the Degussa Aerosil 200. The results also are included in Table 1.  
       Example 3  
       [0034]    Samples of the particles of Example 2 were dispersed in a solution of polyvinyl alcohol (100% hydrolyzed, molecular weight 115,000, Cat. #002 from Scientific Products), and cast onto microscope slides, and dried at about 70° C. for about 10 minutes. Then drops of the dye solutions were applied to these films. The aerogels of this invention took up the dyes with essentially the same colors as observed on the monoliths and did so faster and with greater control of lateral spread than either the silica gel from the tic plate or the Degussa aerosil.  
       Example 4  
       [0035]    A mixture of 12.5 mL of tetraethylorthosilicate (TEOS), 40 mL of an aqueous solution containing 1 wt % of 80% deacylated chitosan (supplied by Fluke, molecular weight of 2,000,000) and 1 wt % acetic acid, and 0.9 mL of glacial acetic acid was prepared, and stirred at room temperature (about 25° C.) for 6-9 hours. A clear, colorless and viscous sol formed, and a portion of it was poured into a polystyrene box. The box is approximately 2 in.×2 in. and about 1 in. high. The sol was poured into the box to a depth of about 1 to 2 mm., and the lid was placed on the box. This film was allowed to gel and then to age overnight. Then about 10 mL of a solution of absolute ethanol and ammonium hydroxide was added to the gel. The solution contained the amount of ammonium hydroxide required to adjust the pH of the solution, once in contact with the gel, to about 7.0. (Note that even though the solution added was alcohol, the solution present after solvent exchange and neutralization was water based, so the pH value is a valid parameter.) After this was accomplished, the supernatant solution was replaced with absolute alcohol, the lid was replaced, and the alcohol remained in contact with the wet gel for a few hours. This was repeated at least four times over several days to produce a wet gel in which the liquid was nearly all ethanol.  
       Example 5  
       [0036]    The wet gel produced in Example 4 was placed in the pressure vessel of a supercritical fluid extractor, and the solvent was extracted with carbon dioxide (CO 2 ) under supercritical conditions, which reached 1400 psi and 35° C. The result was a clear, hard, brittle monolithic aerogel, which can be labeled Si—X, for silica and chitosan containing material. Its density was found to be 0.25 g/cm 3 , and its BET surface area was found to be 673 m 2 /g.  
         [0037]    Aerogels of this type with the chitosan to silica dry weight ratios of 5:95, 10:90, and 20:80 were prepared. These aerogels utilized chitosan materials of molecular weights 2,000,000; 750,000; and 70,000.  
       Example 6  
       [0038]    A wet gel produced as in Example 4 was treated to one more step, in which the supernatant ethanol was replaced by a solution made by dissolving a compound of a transition metal in ethanol. The lid was placed on the box, and the interaction between the gel and solution was allowed to proceed for three days. Thus, one gel was treated with one solution. Then the wet gel was processed by supercritical fluid extraction, as described above, to produce a metal-ion-containing aerogel. In this example, the metal ion containing compound was 0.01 M RhCl 3 , and the final aerogel was a thin clear, yellow orange monolithic solid. Its density was found to be 0.27 g/cm 3 , and its BET surface area was found to be 555 m 2 /g. Its refractive index was measured at 632.8 nm using a Metricon 2010 Prism Coupling Instrument (Metricon Corporation) and found to be 1.17. This aerogel is labeled Si—X—RhCl 3 (or Si—X—RhCl 3  (0.01M).  
         [0039]    Aerogels containing other metals were prepared. They were prepared with treatment solution concentrations ranging from 0.001 to about 0.1 M, with a range of chitosan to silica dry weight ratios. The compounds used for treatment included the following: rhodium trichloride, ruthenium trichloride, molybdenum trichloride, cobalt (II) acetylacetonate (Co(acac) 2 ), Ni acetylacetonate (Ni(acac) 2 ), cobalt chloride, cobalt nitrate, silver nitrate, copper chloride, manganous chloride, chloro-1,5-cyclooctadiene iridium(I) dimer, and palladium chloride. Nickel nitrate and nickel chloride could also be used.  
         [0040]    These aerogels exhibited similar properties. For example, the density of the SiO 2 —X—RuCl 3  (0.02M) aerogel, prepared using a 0.02 M RuCl 3 , was found to be 0.27 g/cm 3 , its BET surface area was 494 m 2 /g., and it refractive index at 632.8 nm. was found to be 1.17. As another example of this type, SiO 2 —X—Co (0.01M) had a density of 0.26, a BET surface area of 500 m 2 /g, and a refractive index at 632.8 nm of 1.14. All of the aerogels could be obtained as clear, hard, brittle monolithic pieces. All BET surface areas obtained were in the 382 to 957 m 2 /g range, and densities in the 0.15 to 0.32 g/cm 3  range.  
       Example 7  
       [0041]    A mixture of 3.1 mL of tetraethylorthosilicate (TEOS), 10 mL of an aqueous solution containing 1 wt % of 80% deacylated chitosan and 1 wt % acetic acid, and 0.2 mL of glacial acetic acid was prepared, and stirred at room temperature (about 25° C.) for 6-9 hours. During the initial stage of stirring, 2.5 mL of an aqueous solution of potassium tetrachloroplatinate was added drop by drop very slowly. The amount of potassium tetrachloroplatinate added was one Pt per eight amine groups of chitosan. During the stirring for 7 hours, the viscosity increased and the color of the clear gel became a bright yellow. Once this sol was formed, a portion of it was poured into a polystyrene box. The box is approximately 2 in.×2 in. and about 1 in. high. The sol was poured into the box to a depth of about 1 to 2 mm., and the lid was placed on the box. This film was allowed to gel and then to age overnight. Then about 10 mL of a solution of absolute ethanol and ammonium hydroxide was added to the gel. The solution contained the amount of ammonium hydroxide required to adjust the pH of the solution, once in contact with the gel, to about 7.0. After this was accomplished, the supernatant solution was replaced with absolute alcohol, the lid was replaced, and the alcohol remained in contact with the wet gel for a few hours. This was repeated at least four times over several days to produce a wet gel in which the liquid was nearly all ethanol. The wet gel produced was placed in the pressure vessel of a supercritical fluid extractor, and the solvent was extracted with carbon dioxide (CO 2 ) under supercritical conditions.  
       Example 8  
       [0042]    A SiO 2 —X—Rh(III) aerogel sample was placed in a cell for measuring its infrared spectrum and controlling the surrounding atmosphere. The cell body was made of glass, and the windows were made of potassium bromide crystals. The air in the cell was pumped out and the infrared spectrum of the aerogel film was measured. Then, carbon monoxide (CO) was admitted to the cell at a pressure of 0.1 atmosphere. After about 30 minutes the CO was evacuated and the infrared spectrum of the exposed aerogel was measured. It showed the presence of three new infrared absorption bands, at 2032 and 2090, and at 2137 cm−1. The first two are assigned to a Rh(I)(CO) 2 , or rhodium(I) dicarbonyl species, as shown in the paper A Kinetic Study of the Catalytic Oxidation of CO over Nafion-Supported Rhodium, Ruthenium and Platinum, V. D. Mattera, Jr., D. M. Barnes, S. N. Chaudhuri, W. M. Risen, Jr. and R. D. Gonzalez,  J. Physical Chemistry,  90, 4819(1986). The third band is due to physical adsorption of the CO by the aerogel. These results are that the aerogel adsorbs CO and that it reacts with it in a manner that is detectable by infrared spectroscopy. In addition, it shows that some of the Rh(III) was reduced to Rh(I).  
       Example 9  
       [0043]    Example 8 was repeated but the CO gas was replaced with NO, and the result was the appearance of an infrared band at 1727 cm−1.  
       Example 10  
       [0044]    Example 8 was repeated but the aerogel was SiO 2  —X—RuCl 3 . The infrared spectrum showed new bands at 1968, 2018, 2080, and 2144 cm−1. The first is assigned to a Ru(II) carbonyl, the next two to a Ru(III) carbonyl and the last to physically absorbed CO, based on the paper.  
       Example 11  
       [0045]    Example 6 was repeated but the aerogel was SiO 2 —X—PdCl 2  and the interaction with NO gas produced new infrared bands at 1820 and 1858 cm−1.  
       Example 12  
       [0046]    In a typical preparation of an aerogel based on a carboxylic acid containing polymer, 40 mL of a 1 wt. % pectic acid aqueous solution at pH4 was combined with. 7.6 mL tetramethylorthosilicate (TMOS) at about 25° C. After about 10 min of stirring, the solution became clear and homogeneous. After being stirring for an additional 1.5 hours, the solution was placed in a polystyrene box, of the type described above. About 4 mL of solution was placed in each box, and the lid was placed on the box. The sol aged overnight to form a clear colorless gel. Then the reaction of these films with various metals was carried out by placing the solutions containing the metal ion in contact with the wet gel. Then the gel liquid was replaced in stages beginning with a solution that was 3:1, V/V water to ethanol and ending with alcohol. Finally, the wet gel containing ethanol as its primary liquid had the liquid extracted by supercritical fluid extraction with CO 2 . This method was used to prepare aerogels comprising Er(III), Yb(III), Sm(III), and Dy(III). In several cases, two or more lanthanide metal ion containing compounds were added together. For example, Yb(III) and Er(III) were so combined. Aerogel monoliths were obtained.  
       Example 13  
       [0047]    A powdered sample of SiO 2 —X—Co(II) aerogel was powdered and placed in a glass tube and studied by electron spin resonance spectroscopy. Similarly, the esr spectrum of the SiO 2 —X—Rh(III) and SiO 2 —X—Ru(III) materials were studied. In all cases, the esr spectra characteristic of the metal ions in their environment and oxidation state were observed.  
       Example 14  
       [0048]    A monolith of SiO 2 —X—Co(II) was placed in a glass cell consisting of a glass tube with glass windows attached to the ends and a stopcock attached to the side, and the cell was evacuated at 25° C. and the visible spectrum was measured in transmission. Then the sample was exposed to the vapors of water. The evident color change was recorded spectroscopically. This experiment was repeated with SiO 2 —X—Cu(II) aerogel and the dried material was exposed to water vapor. The evident color change was recorded spectroscopically. The result in the visible region of exposing SiO 2 —X—Ru(III) to CO is a color change from the red of the original material to a light yellow.  
       Example 15  
       [0049]    A piece of SiO 2 —X aerogel was placed in a 0.01M nickel nitrate solution in ethanol. It was removed after about 10 seconds and then it was allowed to dry in the air. Then it was rinsed by dipping it into ethanol for about 3 seconds. The light green color of the gel showed that it had absorbed nickel ions.  
       Example 16  
       [0050]    A piece of each of the following SiO 2 —X—M aerogels, those of Er(III), Yb(III), and Sm(III)measuring about 1 cm by 2 cm, was placed in a furnace in the air and heated to 800° C. The aerogels collapsed to form dense glassy materials.  
       Example 17  
       [0051]    A SiO 2 —X—Rh(III) aerogel was placed in a cell and dehydrated in vacuo at 95° C. for 10 hours, and then exposed to CO gas for about 1.5 hours at 0.5 atm. pressure. The infrared spectrum was measured. After subtraction of the spectrum of the background, the spectrum showed bands at 1821, 2025, 2097, and 2138, and a shoulder at about 2000 cm −1 , which are believed to be due to bridging CO on Rh(0) (1821), terminally bound CO on reduced (Rh(0)) (2000), terminally bound CO on Rh(I) (2025, 2097), and physically absorbed CO (2138).  
       Example 18  
       [0052]    A sample of SiO 2 —X—Rh(III) aerogel was dehydrated at 95° C. in vacuo and treated with H 2  at 100° C. and 0.5 atm. for about 2 hours. Then it was reacted with CO at 0.3 atm. for 1.5 hours. The infrared spectrum of the product, less that of the dehydrated material, exhibited bands at 1824, 2002, 2022, and 2082 cm −1 , which are believed to be characteristic of CO on Rh(0) and Rh(I). No bands were observed in the region 2100 to 2200, which is the region in which a band for CO absorbed on the original material containing Rh(III) was seen.  
       Example 19  
       [0053]    A SiO 2 —X—Rh(III) aerogel was analyzed to determine the Rh(III) loading. One piece of the aerogel was studied by thermogravimetric analysis, and it was found that 15% of the mass was lost on heating the material under inert gas flow from 25 to 300° C. Another piece was decomposed in strong base (NaOH). The basic solution was neutralized with HCl, taken to pH 3 with HCl, filtered and diluted quantitatively. Its spectrum was measured in the UV-visible region and compared to a calibration based on rhodium chloride solutions in HCl solution. These results were combined to yield a value of 3% (w/w) for Rh(III).  
       Discussion of the Examples  
       [0054]    Two processes for making monolithic aerogels comprising silica, a polar functional organic polymer, and, for comparison purposes, a metal-ion-containing species have been described. The compositions containing the polar functional organic polymer, such as an amine functional polymer, form aerogels that are clear, transparent, hard, brittle, and friable with transition metals. The aerogels comprising lanthanide ions were prepared using the carboxylate functional polymer. Additional preparations were made using polymers selected from poly(acrylic acid), alginic acid, gelatin, carboxylate modified poly(acrylamide).  
         [0055]    [0055]FIG. 1 shows the ultraviolet-visible range spectra, presented in absorbance as a function of wavelength in the 300-820 nm range, of 0.3 mm thick (T=0.3 mm) monolithic pieces of silica-chitosan-metal aerogels. FIG. 1 a  is a sample comprising rhodium ions with the source of Rh having been rhodium trichloride;  1   b  is a sample comprising nickel ions with the source of Ni having been nickel nitrate;  1   c  is a sample comprising ruthenium ions with the source of Ru having been ruthenium chloride;  1   d  is a sample comprising copper ions with the source of Cu having been copper chloride;  1   e  is a sample comprising cobalt ions with the source of Co having been cobalt nitrate; and  1   f  is a sample comprising cobalt ions with the source of Co having been cobalt acetate.  
         [0056]    [0056]FIG. 2 shows the near infrared range spectra, presented in transmittance (percent transmission) as a function of wavelength in the 700-3100 nm range, of 0.3 mm thick (T=0.3 mm) monolithic pieces of a silica-chitosan aerogel and silica-chitosan-metal aerogels. FIG. 2 a  is a sample comprising ruthenium ions with the source of Ru having been ruthernium trichloride;  2   b  is a sample of the silica-chitosan aerogel with no metal ions;  2   c  is a sample comprising nickel ions with the source of Ni having been nickel acetylacetonate;  2   d  is a sample comprising cobalt ions with the source of Co having been cobalt acetate; and  2   e  is a sample comprising rhodium ions with the source of Rh having been rhodium chloride.  
         [0057]    [0057]FIG. 3 shows the infrared spectra, presented as absorbance as a function of wave number in the 1450-2350 wave number (1/cm) region, of (a) a monolithic 0.2 mm thick piece of a preferred embodiment silica-chitosan-rhodium aerogel dehydrated at 95° C. and 0.001 Torr pressure for 10 hours, and (b) the same sample after exposure to CO at 380 Torr at 25° C. for 1.5 hours followed by evacuation of the sample chamber to 0.001 Torr for 1 hour. The absorbance scales are offset arbitrarily for display.  
         [0058]    [0058]FIG. 4 shows the infrared spectra, presented as absorbance as a function of wave number in the 1450-2350 wave number (1/cm) region, of (a) a monolithic 0.2 mm thick piece of a preferred embodiment silica-chitosan-ruthenium aerogel dehydrated at 95° C. and 0.001 Torr pressure for 10 hours, and (b) the same sample after exposure to NO at 76 Torr at 25° C. for 10 minutes followed by evacuation of the sample chamber to 0.001 Torr for 0.5 hour. The absorbance scales are offset arbitrarily for display.  
         [0059]    The absorption of gases from an atmosphere has been demonstrated for the case of CO and NO. Once absorbed, the gases can undergo further reactions, as demonstrated in the case of metal ion reduction for Rh(III) to Rh(I) and Ru (III) to Ru(II). These are typical reactions for catalysts that are useful in a range of reactions including hydroformylations. Also once absorbed, the spectrum of the aerogel is changed so that the aerogel can be used as a detector for the presence of the gases. This detection can be done by optical spectroscopy in the infrared, visible or ultraviolet regions, light scattering by Raman, or other dispersive or non-dispersive methods, by spin resonance spectroscopy using electron or nuclear spins, or by magnetic interactions. Thus, the changes that the aerogels undergo when they interact with gas molecules or other molecules can be detected by these means. In addition to changes in light transmission, the magnetic changes that result from the oxidation or reduction of the adsorbate or absorbent are observable magnetically. It is known that aerogels comprising silica are useful in some forms of particle detection, such as Chrenkov radiation detectors. The incorporation of the organic polymers and metal ions selected from a broad group of metals provides a new class of particle detectors whose advantage is in the metal specific interactions with the particles.  
         [0060]    The high absorbance of liquids by the aerogels tested and their apparent ability to mordant dyes contained in the liquids, has been shown to make the aerogels, either in monolithic or particulate form, suitable for their use in imaging systems. Thus, the use of these materials in the coating on the imaging layer of paper, could produce a rapid drying agent to absorb ink and analogous fluids from other imaging systems.  
         [0061]    Based on the behavior of the aerogels containing rodium, ruthenium, platinum, and palladium, and the field of catalysis using species of these elements on high surface area supports, it is contemplated that the materials of this invention will function as catalysts for many reactions beyond those demonstrated. The catalysts of the Fischer Tropsch reaction, the reduction of nitrogen oxides, and the oxidation of CO, and the oxidation of nitrogen oxides are particularly contemplated.  
         [0062]    The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon a reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations in so far as they come within the scope of the appended claims or the equivalents thereof.