BACKGROUND OF THE INVENTION (a) Field of the Invention The present invention relates to an oxygen enriching membrane which selectively increases oxygen content in the air, and a method for preparing the same, and more particularly to an oxygen enriching membrane having superior oxygen permeability and selectivity while maintaining superior mechanical strength, and a method for preparing the same. (b) Description of the Related Art The air we breathe consists of about 20.9% of oxygen and about 78.1 % of nitrogen. A membrane that is selectively more permeable to oxygen than nitrogen in the air is called the oxygen enriching membrane. Air passing through the oxygen enriching membrane has a relatively higher oxygen content. Accordingly, the oxygen enriching membrane can improve fuel efficiency of internal combustion engines of vehicles, contributing to energy saving. Also, it can be widely used in the fields of medicine, oxidation processing, and environmental engineering which do not require high-level oxygen content. Also, since enriched nitrogen is simultaneously obtained accompanied with oxygen enrichment, the oxygen enriching membrane can be utilized in the fields requiring nitrogen-rich environment, e.g., fermentation processes. Using the oxygen enriching membrane, it is possible to separate gas with a simple apparatus. Thus, it is not limited in place or mobility. Since the 1980s, gas separation using membranes has undergone a lot of developments. Although membrane separation is inadequate for obtaining massive oxygen at a high concentration, compared with the freezing method it has drawn a lot of attention recently since it consumes less energy during the separation process. Since Kammermeyer first used silicone rubber for separation of gas in 1957 (K. Kammermeyer, Ind. Eng. Chem., 49, 1685, 1957), methyl-based polysiloxanes have been known to be superior to any other polymers in oxygen permeation properties until the advent of polytrimethylsilylpropyne (R. L. Riley et al., USP 4,243,701 ). However, if the membrane was made thin to improve oxygen permeability, it was torn easily. There was a need to solve this problem. The first commercially successful oxygen enriching membrane was one having improved membrane strength through copolymerization of siloxane and carbonate (see W. J. Ward III, J. Membrane Sci., 1 , 99, 1976). In this regard, research was performed using a graft copolymer of styrene and dimethylsiloxane, introducing an amide into the main chain using tetramethyldisiloxane and aliphatic dicarboxylate, substituting a variety of siloxanes at the para position of polystyrene, and so forth (see Y. Kawakami, J. Polym. Sci., Part A, 25, 1591 , 1987). Fluorine-substituted derivatives were also used as oxygen enriching membranes. This was because the higher the content of fluorine atoms, the higher the solubility of gaseous oxygen. Yoshio et al. synthesized a poly(phenylacetylene) derivative having a trifluoromethyl in the benzene ring to prepare an oxygen enriching membrane having an oxygen selectivity over nitrogen of 2.1 (see Y. Hayakawa, M. Nishida, T. Aoki, and H. Muramatsu, J. Polym. Sci., Part A, 30, 873, 1992). Paul et al. synthesized tetra-halogenated polycarbonates in which four hydrogen atoms of bisphenol A were substituted with halogen atoms, such as chlorine and bromine, to prepare oxygen enriching membranes having an oxygen selectivity over nitrogen of 6.37 and 7.47 (see D. R. Paul, J. Membrane Sci., 34, 185, 1987). However, in spite of these efforts, oxygen permeability tends to decrease as oxygen selectivity over nitrogen increases. Oxygen permeability can be improved by increasing a pressure difference across the membrane, but then a lot of energy is required to separate the oxygen. Korean Patent Publication No. 2001-0018930 disclosed a cellulose-based oxygen enriching membrane which is not torn easily, sacrificing neither oxygen selectivity nor oxygen permeability. However, it is limited in oxygen selectivity and permeability because of the structural limit of cellulose. In addition, it is inadequate for mass production, since an activation process is necessary to loosen the cellulose structure. SUMMARY OF THE INVENTION It is an object of the present invention to provide a metallosiloxane-metallic oxide compound for improving oxygen permeability and selectivity of an oxygen enriching membrane. It is another object of the invention to provide an oxygen enriching membrane having superior oxygen permeability and selectivity, and that is not torn easily and has good strength. It is still another object of the invention to provide a method for preparing the metallosiloxane-metallic oxide compound. It is further another object of the invention to provide a method for preparing the oxygen enriching membrane. To attain the objects, the present invention provides a metallosiloxane-metallic oxide compound represented by Formula 1 below. The invention also provides an oxygen enriching membrane comprising the metallosiloxane-metallic oxide compound, and a metallosiloxane compound represented by Formula 2 below or a mixture thereof. The invention further provides a method for preparing a metallosiloxane-metallic oxide compound comprising the steps of treating the metallosiloxane compound represented by Formula 2 with heat and acid or base to dissociate it into the metallosiloxane ion represented by Formula 3 below and adsorbing the metallosiloxane ion on the surface of the metal oxide represented by Formula 1. The invention further provides a method for preparing an oxygen enriching membrane comprising the steps of coating the metallosiloxane-metallic oxide compound powder represented by Formula 1 on woven or non-woven fabric and laminating another woven or non-woven

fabric on the metallosiloxane-metallic oxide compound layer.

The invention further provides a method for preparing an oxygen

enriching membrane comprising the steps of uniformly distributing the

metallosiloxane represented by Formula 2 in a polymerization monomer,

polymerizing the polymerization monomer, and coating the resultant

polymer on woven or non-woven fabric.

The invention further provides a method for preparing a membrane

comprising the steps of uniformly distributing the metallosiloxane

represented by Formula 2 and an air-permeable foaming agent in polymer

resin, foaming the air-permeable foaming agent, and processing the

polymer resin into a membrane,

(3)

where each of m and n is an integer of 4 to 1000; M1 is a metal selected from the group consisting of aluminum, boron, manganese, chromium, lead, titanium, tin, and germanium; M2O is an oxide of a metal selected from the group consisting of aluminum, arsenic, gold, boron, barium, beryllium, bismuth, calcium, niobium, cadmium, cerium, cobalt, chromium, cesium, copper, iron, gallium, germanium, mercury, indium, potassium, lanthanum, lithium, magnesium, manganese, molybdenum, iridium, sodium, nickel, osmium, lead, palladium, platinum, rubidium, rhodium, ruthenium, antimony, silicon, tin, strontium, tantalum, tellurium, thorium, titanium, thallium, uranium, vanadium, tungsten, zinc, and zirconium; and Rs -Si FT each of R1, R2, R3, and R4 is hydrogen, halogen, R7 unsubstituted or substrituted C1-C6 alkyl, unsubstituted or substrituted C2-C6 alkenyl, or unsubstituted or substrituted phenyl, wherein the substituted Ci-C6 alkyl, C2-C6 alkenyl, and phenyl are substituted by at least one substituent selected from the group consisting of

halogen and where each of R5, R6, and R7 is hydrogen, halogen, unsubstituted or halogen-substituted Ci-C6 alkyl, unsubstituted or halogen-substituted C2-C6 alkenyl, or unsubstituted or halogen-substituted phenyl. The definition of the substituents applies to all the formulas and schemes in this description. Preferably, M1 is aluminum or boron, because they form covalent and coordinate bonds better than other metal elements. Preferably, the polymerization monomer used in preparing the oxygen enriching membrane of the invention is at least one addition polymerization monomer selected from the group consisting of ethylene, propylene, methyl methacrylate, and styrene, or at least one condensation polymerization monomer selected from the group consisting of terephthalate/ethylene glycol, bisphenol-A/diphenyl carbonate, phenol/aldehyde, and urea/aldehyde. And, preferably, the air-permeable foaming agent used in preparing the oxygen enriching membrane of the invention is at least one selected from the group consisting of azobicarbonamide, p-toluenesulfonylhydrazide, benzenesulfonylhydrazide, and dinitrosopentamethylenetetramine. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of metallosiloxane compounds adhered on the surface of metal oxide particles. FIG. 2 is a schematic view of the pendant model showing motion of the metallosiloxane unit caused by relative pressure difference across the membrane. FIG. 3 is a schematic view of an apparatus for testing oxygen enriching performance of the oxygen enriching membrane according to an embodiment of the present invention. <Symbols used in the drawings> 1 : oxygen enriching membrane 2: pressure gauge 3: flow meter

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereunder is given a more detailed description of the present invention. The metallosiloxane represented by Formula 2 can be prepared from any compound having a siloxyl group. Because the distance from the central metal to oxygen atoms is longer than usual covalent bonds, the binding force between them is relatively weak. Accordingly, the bond between the metal and oxygen is relatively easily dissociated by heat, and an acid or base, to give the metallosiloxane ion repressed by Formula 3 according to Scheme 1 below. [Scheme 1]

The metallosiloxane ion readily reacts with a metal oxide particle to give the metallosiloxane-metallic oxide compound represented by Formula 1 according to Scheme 2 below. [Scheme 2]

The metallic oxide (M2O) is present in the form of particles. The process of Scheme 2 occurs at the surface of a metallic oxide (M2O) particle and the resultant metallosiloxane-metallic oxide compound has a structure shown in FIG. 1. Since the metallosiloxane-metallic oxide compound of the invention is present in the form of particles and is used in a powder form, a means to fix the particles or powders is required. In the present invention, woven or non-woven fabric may be used to fix the metallosiloxane-metallic oxide compound. The metallosiloxane-metallic oxide compound powder is coated on woven or non-woven fabric to a predetermined thickness and another woven or non-woven fabric is laminated thereon. If this process is repeated at least once, a lamination membrane in which metallosiloxane-metallic oxide compound powder layer(s) is formed between the woven or non-woven fabric is obtained. The lamination membrane itself can be used as an oxygen enriching membrane. Also, it can be laminated along with other lamination membranes to be described later as oxygen enriching membranes. In general, separation of a gaseous mixture through a porous membrane can be explained by the following four principles. First, Knudsen diffusion occurs when the pore size is smaller than the mean free path of a specific gas molecule. Second, surface diffusion occurs by interaction with the pore surface. Third, capillary diffusion occurs by condensation of a specific liquid. Fourth, in a molecular sieve, the pore acts as a sieve that passes only the molecules smaller than the pore size. In non-porous membranes, it can be explained by dissolution of gas at the membrane surface and penetration by diffusion. Here, solubility of oxygen to the membrane and degree of diffusion are important factors. Adding to these, the pendant model was proposed to explain the peculiar characteristics of the oxygen enriching membrane (Jaejin Hong, Research on gas separation of siloxylcellulose membrane, Dankook University, Thesis for doctorate, 1998). FIG. 2 is a schematic diagram for illustrating the pendant model. According to the pendant model, a plurality of metallosiloxane units protruding inside the pore move freely like cilia. When the pressure applied to the oxygen enriching membrane is relatively low, the metallosiloxane units align almost vertically to the direction of the pore, as seen in FIG. 2 (a). If the pressure is relatively high, the metallosiloxane units align almost parallel to the direction of the pore, as seen in FIG. 2 (b). The oxygen enriching membrane of the present invention not only offers superior oxygen selectivity and permeability even with a low pressure difference, but also has good strength and is not easily torn thanks to the lamination membrane structure. Also, since it is prepared by fixing the metallosiloxaiie-metallic oxide compound in the powder form on woven or non-woven fabric and laminating them repeatedly, membranes with uniform pore size can be produced in large amounts. Also, differently from cellulose, the metal losiloxane unit moves freely, so that superior permeability and selectivity are obtained even at low pressure. Since the compound having 4 to 1000 siloxane units, the m and n values in Formula 1 , is suspended and fixed to the metallic oxide (M2O), it offers good oxygen enriching performance even with a low pressure difference by the pendant model. Besides laminating the metallosiloxane-metallic oxide compound powder, a method of dissolving the metallosiloxane compound in a polymerization monomer and polymerizing the monomer to obtain a membrane is practicable. The metallosiloxane represented by Formula 2 dissolves well in most monomers of matrix polymer resins. After dissolving the metallosiloxane in a monomer of a matrix polymer resin to an adequate concentration, it is stirred sufficiently and the monomer is polymerized. Then, a polymer is obtained in which the metallosiloxane is uniformly distributed in the matrix. The resultant matrix polymer is coated on woven or non-woven fabric to obtain a lamination membrane. The resultant lamination membrane can act as an oxygen enriching membrane. Also, it is possible to prepare a membrane using an air-permeable foaming agent. That is, the metallosiloxane is added to a polymer which has not yet solidified just after polymerization. The polymer is stirred so that the metallosiloxane is dispersed uniformly. Then, an air-permeable foaming agent is added to obtain a polymer foam having numerous micropores. If the polymer foam is processed into a membrane, it can act as an oxygen enriching membrane. Hereinafter, the present invention is described in further detail through examples. However, the following examples are only for the understanding of the invention and the invention is not limited to or by the following examples.

EXAMPLES Sample A: Preparation of aluminosiloxane-metallic oxide compound 2.7 kg of silica (Siθ2) sieved to 80 mesh was put in a kneader. 300 g of aluminosiloxane (150 g of Sc.101 and Sc.011 each, Silochem LTD.) was mixed with the silica. Kneading was performed for 60 minutes while heating to 200-250 0C and rotating at 60-120 rpm, to obtain a metallosiloxane-silica compound.

Sample B-1 : Preparation of aluminosiloxane emulsion for fabric coating 100 mL of MMA (methyl methacrylate), 100 mL of EMA (ethyl methacrylate), and 20 g of aluminosiloxane were put in a beaker. The mixture was stirred at room temperature to completely dissolve it (a monomer solution was obtained). A mixture of 1000 mL of distilled water, 10 g of sorbitan monolaurate polyethylene oxide (Tween 20), a surfactant, and 0.1 g of APS (ammonium persulfate), an initiator, was added to the reactor. The mixture was heated to 40 °C while stirring (a surfactant solution was obtained). The monomer solution was added to the surfactant solution in a dropwise fashion. Reaction was performed at 80 °C for 4 hours to obtain an aluminosiloxane emulsion for fabric coating.

Sample B-2: Preparation of aluminosiloxane emulsion for fabric foam coating 0.3 wt% per 100 wt% of the polymer of a foaming agent was added to the aluminosiloxane emulsion (sample B-1 ), and dispersed uniformly.

Sample B-3: Preparation of polymer foaming master batch comprising aluminosiloxane 200 g of aluminosiloxane and 0.3 wt% per 100 wt% of the polymer of an air-permeable foaming agent were mixed with 800 g of PP to prepare a master batch.

Preparation Example 1 : Aluminosiloxane-metallic oxide compound coated fabric Fabric with a thickness of 1 mm was cut to a size of 30 cm * 30 cm. After placing the fabric on a vibrator, the aluminosiioxane-metallic oxide compound powder (sample A) was put on the fabric and the vibrator was turned on, so that the powder penetrated the fabric. For the aluminosiloxane metallic oxide powder, a 1 :1 mixture of those having a particle size of 60 mesh and 120 mesh or one having a particle size of 80 mesh was used.

Preparation Example 2: Aluminosiloxane emulsion coated fabric Fabric with a thickness of 1 mm was cut to a size of 30 cm x 30 cm. The fabric was immersed in the emulsion of sample B-1 , dried in the air, and heated in an oven at 180 0C for 30 minutes for crosslinking.

Preparation Example 3: Aluminosiloxane emulsion foamed fabric Fabric with a thickness of 1 mm was cut to a size of 30 cm * 30 cm. The fabric was immersed in the emulsion of sample B-2, dried in the air, and heated to 165 0C for foaming.

Preparation Example 4: Preparation of polymer foam membrane comprising aluminosiloxane The master batch of sample B-3 was put in a mold and heated to the foaming temperature to obtain a polymer foam membrane comprising aluminosiloxane.

Preparation Example 5: Napped fabric comprising aluminosiloxane Fabric comprising aluminosiloxane was napped for use as a support of the masks used in the examples.

The fabrics and foam membranes of Preparation Example 1 to Preparation Example 5 were laminated as in Table 1 below to prepare oxygen enriching membranes of Example 1 to Example 9. For each oxygen enriching membrane, oxygen enriching performance was measured as follows. The results as compared with the oxygen concentration in air (Comparative Example 1 ) are given in Table 1. A 1 :4 (in molar ratio) mixture gas of oxygen and nitrogen was used for oxygen enriching performance measurement. The test apparatus is shown in FIG. 3. Oxygen concentration was determined by gas chromatography. The analysis conditions were as follows. Column: 3" * 1/8" Filler: 6D/6D molecular sieve 5 A Oven temperature: 60 °C Detector: TCD Carrier gas: Helium (20 mL/min) [Table 1]

In Table 1 "LS" stands for "Lamination sequence", "Ex" stands for

"Example", "CE" stands for "Comparative Example", "PD" stands for

"Pressure difference", "OC" stands for "Oxygen concentration", "P" stands

for "Preparation Example", and the numbers below P 1 and so forth refer to

the particle size of the metallosiloxane-metallic oxide compounds. For

example, "80" means a particle of 80 mesh and "120/60" means a 1:1

mixture of particles of 120 mesh and 60 mesh.

An oxygen enriching membrane prepared by laminating a

metallosiloxane-metallic oxide compound and dissolving metallosiloxane in

a monomer or foaming metallosiloxane in a polymer resin by adding an

air-permeable foaming agent has superior oxygen permeability and

selectivity, and good strength. In addition, mass production becomes possible and high quality is attained.