1. Field of the Invention:
The present invention relates to materials for gas separation, and, more particularly, it is concerned with materials for gas separation which are useful for separation of oxygen.
Oxygen is one of those gases which have been used in a wide range of utility and in a large quantity. The fields of its use, which have so far been known, are as follows: welding and cutting of iron and steel material; iron making such as blowing of fuel into blast furnaces, open-hearth furnaces, an converters; refining and smelting of various metals; production of various petrochemical products as the starting materials for the chemical industries; production of cement, refractories, glass, etc. in the ceramic industries; activated sludge treatment of sewage and industrial waste water in ganeral; medical purpose including use of oxygen-enriched air; and so forth. In major industrial countries such as Japan, the quantity of use of oxygen amounts to nine to ten billions cubic meters per annum, the major field of its use being centered on iron and steel manufacturing industry.
2. Description of the Prior Art
The industrialized production of oxygen has been done by the chilling separation method since as far back as the beginning of this century. In case a large amount of oxygen is to be produced in a large scaled industrial plant, this method is considered to be the most appropriate. On the other hand, however, the method requires an extremely large quantity of energy, and, moreover, when oxygen thus produced is to be used at a distant site, it should once be filled in a pressure container for transportation with the consequence that the production cost of oxygen becomes remarkably high. Also, as the method for production of oxygen in a relatively small or medium sized scale, there has recently emerged a method of separating oxygen from air with high concentration in utilization of a difference in the quantity of adsorption of nitrogen and oxygen onto an adsorbing agent such as zeolite, molecular sieve, carbon, and so forth, the method of which is being particularly made use of in various waste water treatment, blowing fuel into various furnaces, medical treatment, and others. This method is also disadvantageous in that it expends high electric power necessary for production of oxygen, hence high production cost. As the other special method, there has been made researches and studies on a method of using metal complexes. It has been known since old that cobalt salt of Schiff base combines with oxygen to form an oxygen complex. In this case, however, the oxygen complex itself becomes decomposed, while it is repeating adsorption and desorption of oxygen with the result that the reaction is disadvantageously difficult to be used as an economical system. Toward the end of 1960's, continued studies and researches on improvement in durability of the oxygen complex were conducted at U.S. Air Force laboratories and other research institutions, as the result of which there were found out relatively durable oxygen complexes such as fluorine substituent called "fluomine". However, the method of using this oxygen complex requires that absorption of oxygen be done at a room temperature or its vicinity, i.e., 27.degree. C. to 38.degree. C., and that its discharge be done at a high temperature, e.g., 82.degree. C., on account of which the temperature increase and decrease are necessary during the separating operation. In an Unexamined Japanese Patent Publication No. 12707/1984 (U.S. patent application No. 393,712; European Patent Application No. 98731), there is disclosed a method of selectively separating oxygen from air, wherein air is caused to pass through an oxygen-complex-containing-solution held on a porous membrane carrier. According to this method, it is possible to continuously separate oxygen in utilization of a pressure difference on both sides of the membrane, while maintaining the temperature constant. In this membrane separation method, a ratio of the permeation rate between oxygen and nitrogen is high, and the permeation rate of oxygen is required to be greater than that of nitrogen, for which the magnitude of the reaction rate between oxygen and the complex, the magnitude of the diffusion coefficient of the resulted oxygen complex, and so forth are considered to be the important factors. However, in spite of the fact that a great many oxygen complexes have so far been discovered, on which studies have been conducted, as seen in Chemical Reviews, Vol. 79, page 139 (1979); Canadian Journal of Chemistry, Vol. 54, page 3424 (1976); Journal of the American Chemical Society, Vol. 102, page 3285 (1980), and others, all these publications having been cited in the above-noted Unexamined Japanese Patent Publication No. 12707/1984, a bulky ligand is required for stable and reversible adsorption and desorption of oxygen with the consequence that molecules of the oxygen complex should eventually be large. With such large molecules of the oxygen complex, it is not possible to expect a sufficiently large diffusion coefficient. On the other hand, there have been conducted studies and researches on various cobalt complexes having the ligand of relatively small molecular weight, although no substance which is able to reversibly absorb and desorb oxygen has been discovered at all upto the present.