Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
2001-08-30
2004-02-17
Nguyen, Ngoc-Yen (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C423S339000, C588S249000, C588S254000
Reexamination Certificate
active
06692715
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to new silica gel nanofibers and new silica glass nanofibers.
The invention also relates to methods for preparing such nanofibers from chrysotile asbestos.
BACKGROUND OF THE INVENTION
Nanometer scale fibers, filled or hollow, are of a great interest since the advent of carbon nanotubes.
Catalysis, separation, filling of plastic and optical communication are a few of the fields where the morphology of fibers plays an important role. These fields are also rich in applications where non-fibrous silica gel or silica glass is an active component. Silica nanofibers would therefore offer new possibilities of applications just like carbon nanotubes opened new avenues less than a decade ago.
It is well known that all silicates, whether natural or synthetic, react in water with acids, leading to the replacement of cations by hydrogen ions. The general formula of the end-product is SiO
2
.nH
2
O. Hydrolysis of many organic compounds containing silicon leads to the same end-product. Under diluted conditions silica acid (SiO
2
.2H
2
O or Si(OH)
4
can exist as a solute and a monomer in solution. However, under most conditions SiO
2
.nH
2
O is a solid known as silica gel, having a polymeric structure consisting of chains, sheets or three-dimensional networks. Firing of silica gel gives silica glass.
U.S. Pat. No. 5,980,849 discloses a method for preparing three-dimensional mesoporous material by incorporating a surface-active agent in the sheet structure of silica gel obtained from acid attack on natural silicates. This method provides specific surface area of 500 m
2
/g or less.
U.S. Pat. No. 6,169,135 discloses a method for preparing powder, beads or granules of silica having specific surfaces up to 240 m
2
/g, by acidifying silicates with strong or weak acids. Silica particulates with specific surface up to 300 m
2
/g and mean pore diameter ranging from 10 to 50 nm are the result of reactions between silicates and acids in water (see also U.S. Pat. No. 5,968,470).
U.S. Pat. Nos. 5,989,510 and 6,235,258 disclose methods for preparing silica solids having a surface area of up to about 800 m
2
/g by means of polymeric substances and acid neutralisation of silicates. These solids are amorphous, granular, spherical or of undefined morphology.
U.S. Pat. No. 6,221,326 discloses a method for preparing hollow silica particles, which consists in precipitating active silica on a core followed by its elimination, thus leaving a silica shell.
U.S. Pat. No. 4,838,914 discloses a method is also known to produce silica glass fibers from spinning a silica sol solution and sintering the gel fibers. The diameter of the so prepared fibers is of about 20 &mgr;m. Mesoporous silica fibers can also be made by a spinning process (see U.S. Pat. No. 5,923,299) with diameter of the order of 40 &mgr;m and high specific surface.
U.S. Pat. No. 5,573,983 discloses a method for preparing fine silica tubes from a reaction involving a synthetic silicon compound and an acid. The so prepared silica gel tubes and silica glass tubes have diameters of 50 to 2000 nm and lengths of up to 500 &mgr;m.
U.S. Pat. No. 5,958,098 discloses a method by which metal hydride particles are embedded in a silica network.
U.S. Pat. No. 6,136,736 discloses a method for preparing silica glass doped with many elements.
The large number of existing patents pertaining to silica products shows the importance of silica material having high surface area, chemical and thermal stability, and special morphology. The availability of silica nanofibers should therefore be welcome. If such nanofibers were also abundantly and economically produced, numerous applications could be developed.
Indeed, small diameter fibers are recognized to be more effective in applications such as strengthening and filtration. Silica gel and silica glass nanofibers would therefore expand the field of applications of granular silica gel and silica glass.
A natural silicon-based nanofiber is chrysotile asbestos. This mineral a fibrous silicate mineral, as are other asbestiform silicates like amosite, crocidolite and anthophyllite. The chemical composition of chrysotile is Mg
6
(OH)
8
.Si
4
O
10
.
The reactivity of chrysotile asbestos in the presence of acids, complexing agents and inorganic salts is well documented. For example, chrysotile is known to decompose in hydrochloric acid to magnesium ions and amorphous gel-like silica. In this connection, reference can be made to the following disclosure and Master theses available at Université Laval:
“Evaluation of chrysotile by chemical methods”, C. Barbeau, Short course in Mineralogical techniques of Asbestos determination, Mineralogical Association of Canada, 1979, 197-212;
“Étude de la réactivité du chrysotile”, L. Gendreau, Master thesis, Université Laval, 1985, 92 pages;
“Dissolution séquentielle des feuillets du chrysotile en milieu acide”, C. deBlois, Master thesis, Université Laval, 1987, 143 pages; and
“Adsorption de métaux de transition sur l'amiante chrysotile”, L. Dussault, Master thesis, Université Laval, 1990, 106 page).
Partial decomposition of chrysotile occurs in aqueous and weakly acidic solutions, thereby producing soluble silicic acid and magnesium ions. The remaining solid retains the original morphology and chemical composition, but the diameter of the fibers is usually reduced.
U.S. Pat. No. 5,516,973 discloses a method to destroy the crystal structure and the fibrous nature of the chrysotile asbestos, which consists in spraying a water solution of a weak acid onto asbestos-containing material.
U.S. Pat. No. 6,005,185 also discloses a method which makes use of a fluoro acid agent for converting chrysotile asbestos material to environmentally benign components. In the latter case, the tubular silicate structure is transformed to an open and unrolled silica product.
SUMMARY OF THE INVENTION
The present invention is based on the discovery that chrysotile asbestos can be converted to silica gel without loss of its tubular morphology. Such a discovery is of a great interest inasmuch as it permits to obtain fibers having a length of up to several millimeters and a diameter of less than 100 nanometers. Moreover, the so-obtained nanofibers of silica gel may thereafter be converted by firing into nanofibers of silica glass. Such new nanofibers can be produced at low cost and have numerous industrial applications due to their unique morphology.
More specifically, the invention is based on the discovery that by heating chrysotile in an aqueous solution containing the reactive combination of a controlled-proton-releasing agent and a cation-complexing agent, one may replace and dissolve the cations of the silicate by protons and thus obtain solid fibrous, amorphous hydrated silica also called “silica gel nanofibers”. The so-obtained silica gel nanofibers may thus be converted to silica glass nanofibers by deshydration at a temperature of 900 to 1200° C., preferably close to 1000° C.
Thus, a fist object of the invention is to provide a method for preparing silica gel nanofibers comprising the step of heating a chrysotile asbestos in an aqueous solution containing at least one controlled-proton-releasing agent and at least one cation-complexing agent, and subsequently recovery the silica gel nanofibers that have been prepared from the aqueous solution.
A second object of the invention is to provide silica gel nanofibers of improved structure. These fibers which may be obtained by the above mentioned method, have an outer diameter lower than 100 nm, a length up to 1 cm, a specific surface area of from 600 to 1000 m
2
/g and pore diameters of from 2 to 10 nm.
A third object of the invention is to provide a method for preparing silica glass nanofibers, comprising of the step of heating the above silica gel nanofibers at a temperature of 900° C. to 1200° C.
A fourth object of the invention is to provide silica glass nanofibers of improved structure. These fibers which can be obtained by the above-mentioned method, have an outer diameter and a length similar to that of the
Mine Jeffrey Inc.
Nguyen Ngoc-Yen
Robic
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