A catalytic composite and a cooker having the same

Abstract: This invention provides a catalytic composite for decomposing fats and oils, and a heating cooker having the catalytic composite mounted therein. The catalytic composite comprises a porous sheet made of ceramic fibers and/or metal fibers, and a catalyst. The catalytic composite is capable of decomposing oil grease or stains deposited on the inner wall of a to heating cooker and cooking exhausts emitted in the process of cooking by oxidation, at relatively low temperatures.

Description: The present invention relates to a catalytic composite for decomposing fats and oils, which can be attached to the inner wall of a heating cooker. The present invention also relates to a heating cooker having the catalytic composite for decomposing and removing grease or stains formed from fats and oils that are present in the cooker at comparatively low temperatures. In general, the following two methods are available for the decomposition and removal of grease or stains which are formed and attached to the surface of the inner wall of a heating cooker when fish, meat, or the like is cooked in the heating cooker. In this specification, the heating cooker includes an apparatus with a heating means; for example an oven, an electric range, etc. The first method employs a cooker, the cooking chamber of which has an enamel coated inner wall. Grease or stains that adhere to the surface of the enamel coating are thermally decomposed by raising the temperature of the chamber so that the surface temperature of the enamel coating is about 450°C or more. The second method employs a cooker having a porous film that contains oxidation catalysts the film being attached to the inner wall of the cooking chamber. Such a porous film is formed from a mixture obtained by dispersing an oxide of transition metals such as an oxides of Mn, Cu, Fe, Co, Ni, or the like, or an oxide of alkaline earth metals, in a binder such as a metal salt of inorganic acids (e.g., a metal salt of phosphoric acid or silicic acid), or enamel. Grease attached to the inner wall of the chamber can be decomposed by oxidation at comparatively low temperatures by the catalytic action of the metal oxide. The document US-A-3,738,350 discloses fibrous catalytic structures for oven walls which consist of a paper-like fibrous support selected from the group consisting of glass wool, rock wool, fibrous ceramic alumina, quartz fibers, silica fibers etc. containing distributed therein a catalytically active material of i.a. certain transition metal or rare-earth metal oxides. However, the catalytically active compositions are formed into rigid structures which are taken as a material for mounting into the inner cavity of a cooking oven. The materials taught in said document are not apt to be adapted to the inner cavity of a heating cooker by folding or bending. The document GB-A-2,019,240 discloses a catalytically active mass for the heterogeneous contact catalysis, in particular the flameless combustion of hydrocarbons. Said catalytically active mass consists of silica-alumina fibers as a support and a catalytically active elemental metal selected from the metals of the groups III, IV, VIIIb, VIIIc and the rare-earth elements of the periodic table. The materials disclosed in said document are used in the industrial flameless combustion of hydrocarbons, i.e. in the oxidation of compounds which differ from fats and oils to be catalytically decomposed according to the present invention. Furthermore, such catalysts do not work in the oxidative degradation of fats and oils at low temperatures. However, the foregoing conventional techniques have the following disadvantages. An enamel coating is liable to crack due to a heat shock when the coating is heated to a temperature of 500°C or more, resulting in a separation from the surface of the iron substrate (i.e., the wall substrate). Thus, the iron substrate corrodes and the cooker cannot be used. Furthermore, in the above-mentioned process, high temperatures are required, thus, a large capacity heater is needed. Therefore, a safety problem arises. In the method of using the porous film, it is desired that the porous film has a high degree of porosity for the purpose of having an area large enough to perform an efficient contact between the grease or stains to be decomposed and the catalyst, and having an area large enough to allow oxygen to be diffused throughout the interior of the film. However, if the porosity of the film is increased, then the hardness of the film lowers; the film is apt to be injured, or the adhesiveness between the film and the inner wall of the cooking chamber is decreased. Furthermore, tar and ash formed by the decomposition of various materials permeates the porous film and settles in voids of the film, resulting in a gradual deterioration of the purifying performance of the film. The present inventors prepared several kinds of porous films containing the above-mentioned catalyst, and tested the porous film by using salad oil to see the decomposition ability. It was found that it was necessary to heat the porous film at a temperature of at least 400°C for about two hours so as to decompose it completely. It was also found that if the heating temperature was lower, the decomposition was incomplete, and decomposition residues progressively increased. The invention is, hence, directed to a catalytic composite for decomposing fats and oils by oxidation, consisting of: a porous sheet consisting of aluminum fibers, at least one surface of which is supported on an air permeable metal support; a film of silicon oxide and/or aluminum oxide which is formed on at least one surface of said porous sheet by anodic oxidation; and an oxidation catalyst carried on the surface of said film said oxidation catalyst being at least one selected from the group consisting of manganese oxide, iron oxide, cobalt oxide, nickel oxide, copper oxide, lanthanum oxide, and cerium oxide. The invention also concerns a layered catalytic composite for decomposing fats and oils by oxidation, comprising a first catalytic composite, a second catalytic composite disposed on said first catalytic composite, and an air permeable metal support disposed on at least one surface of the layered body which is composed of said first and second catalytic composites, wherein said first catalytic composite is a sheet obtained by pressure molding of a mixture containing ceramic fibers and an oxidation catalyst, said ceramic fibers being made mainly of at least one selected from the group consisting of silicon oxide, aluminum oxide, and zirconium oxide, and said oxidation catalyst being at least one selected from the group consisting of manganese oxide, iron oxide, cobalt oxide, nickel oxide, copper oxide, lanthanum oxide, and cerium oxide, and wherein said second catalytic composite consists of a porous sheet consisting of aluminum fibers, a film of silicon oxide and/or aluminum oxide, which is formed on said porous sheet, and an oxidation catalyst, said oxidation catalyst being at least one selected from the group consisting of manganese oxide, iron oxide, cobalt oxide, nickel oxide, copper oxide, lanthanum oxide, and cerium oxide, wherein said oxidation catalyst is carried on the surface of said film. The invention is also directed to a heating cooker having a layered catalytic composite as described above wherein said layered catalytic composite is attached to the inner wall of said cooker for the purpose of decomposing by oxidation; fats and oils or cooking exhausts which are formed or emitted in the process of cooking so that the first catalytic composite of the layered catalytic composite is faced with said inner wall. The catalytic composite for decomposing fats and oils by oxidation of this invention comprises: a porous sheet made of aluminum fibers, at least one surface of which is supported on an air permeable metal support; a film of silicon oxide and/or aluminum oxide which is formed on at least one surface of said porous sheet by anodic oxidation; and an oxidation catalyst carried on the surface of said film, said oxidation catalyst being at least one selected from the group consisting of manganese oxide, iron oxide, cobalt oxide, nickel oxide, copper oxide, lanthanum oxide, and cerium oxide. The layered catalytic composite for decomposing fats and oils by oxidation, comprises a first catalytic composite, a second catalytic composite disposed on said first catalytic composite, and an air permeable metal support disposed on at least one surface of the layered body which is composed of said first and second catalytic composites, wherein said first catalytic composite is a sheet obtained by pressure molding of a mixture containing ceramic fibers and an oxidation catalyst, said ceramic fibers being made mainly of at least one selected from the group consisting of silicon oxide, aluminum oxide, and zirconium oxide, and said oxidation catalyst being at least one selected from the group consisting of manganese oxide, iron oxide, cobalt oxide, nickel oxide, copper oxide lanthanum oxide, and cerium oxide, and wherein said second catalytic composite comprises a porous sheet made of aluminum fibers and an oxidation catalyst carried on the surface of said porous sheet, said oxidation catalyst being at least one selected from the group consisting of manganese oxide, iron oxide, cobalt oxide, nickel oxide, copper oxide, lanthanum oxide, and cerium oxide, and wherein said layered catalyst composite further comprises a film of silicon oxide and/or aluminum oxide which is formed on said porous sheet, wherein said oxidation catalyst is carried on the surface of said film. The heating cooker of this invention has the above-mentioned layered catalytic composite, and the layered catalytic composite is attached to the inner wall of said cooker for the purpose of decomposing, by oxidation, fats and oils or cooking exhausts formed or emitted in the process of cooking so that the first catalytic composite of the layered catalytic composite is faced with said inner wall. Thus, the invention described herein makes possible the objectives of: (1) providing a catalytic composite having sufficient catalytic ability for decomposing fats and oils by oxidation even at comparatively low temperatures of lower than 400°C; and (2) providing a heating cooker having the above-mentioned catalytic composite in the cooking chamber in order to decompose and remove grease or stains formed in the process of cooking. This invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows: Figures 1(a) and 1(b) are partially enlarged photographs showing the catalytic composite of the present invention; Figure 2 is a partial sectional view of the catalytic composite of the present invention; Figure 3(a) is an enlarged partial perspective view showing a layered catalytic composite of the present invention; Figure 3(b) is a cross-section taken along line V(b)-V(b) in Figure 3(a); Figure 4(a) is a schematic view showing a heating cooker having a catalytic composite of Figure 3 which is attached to the inner wall of the cooking chamber; and Figure 4(b) is an enlarged partially view showing a catalytic composite attached to the inner wall of the heating cooker shown in Figure 4(a). The catalytic composite of the present invention is a porous sheet made of aluminum fibers said sheet carrying an oxidation catalyst. At least one surface of the sheet is supported on an air permeable metal support. The metal material used for the air permeable support should be heat-stable and should have good workability. For example, aluminum expanded metal is preferred for the support. For the fiber component, an aluminum filament having, for example, a diameter of about 80 to 160 µm and a length of 130 to 150 mm is used. For the oxidation catalyst, the same kinds of catalysts as used in the first catalytic composite can be used, but cupric oxide is most preferred as will be described below. For preparing the catalytic composite, first, the above-mentioned metal fibers are press-molded into a sheet. At this time, for protecting the surface of the sheet as well as reinforcing the sheet, it is preferable to layer an air permeable metal support, for example, aluminum expanded metal, on one side of the fiber mass, then compress it into a sheet. In accordance with the invention an anti-oxidation coating, is formed on the surface of the resulting aluminum fiber sheet with the support. For example, the sheet is anodized in a silicate bath so that an anti-oxidation coating made of aluminum oxide and/or silicon oxide is formed on the surface of the support, and on the surface and the vicinity of the surface of the sheet. The anodic oxidation is carried out under alkaline conditions. A coating formed by an anodic oxidation under acidic conditions is hard and has poor flexibility. On the other hand, a coating formed by treatment under alkaline conditions has excellent flexibility, so that the film can comply with any bending of the sheet. It has been found that the resulting sheet with a film of silicon oxide and/or aluminum oxide formed by anodic oxidation has a high degree of corrosion resistance. When the sheet was sprayed with a saline solution for 120 hours (5 days) continuously, no particular charge was observed. Next, a porous film containing an oxidation catalyst is formed on the surface of the above-mentioned coating using an inorganic silicate as a binder. For example, a mixture of the above-mentioned binder and the oxidation catalyst is coated on the surface of the sheet, and the sheet is baked, resulting in a porous film containing the catalyst. The catalytic composite thus obtained is porous as shown in Figures 1(a) and 1(b); the composite has a larger inner surface area than the conventional porous film containing a catalyst, so that the composite has far greater catalytic activity than the conventional porous film. Next, catalysts and binders suitable for the composite were investigated. Aluminum fibers (mean diameter, 150 µm: mean length 100 mm) and an aluminum lath were pressed into a porous sheet having a thickness of 0.8 mm. A porous film was formed on the side of aluminum lath of the sheet according to the method mentioned above, using each of the catalysts and the binders shown in Table 1. The obtained catalyst composite was layered on an other catalytic composite, which consists of a sheet obtained by pressure molding of a mixture containing ceramic fibers and an oxidation catalyst, so that the side without the porous film was facing this other catalytic composite. Then, salad oil was dropped on the porous film of the layered composite, and the layered composite was allowed to stand in an electric oven at 300°C to 320°C and the state of decomposition of the salad oil was visually observed. The results are shown in Table 1. In Table 1, ○ indicates that the salad oil was completely decomposed, and the surface of the catalytic composite was the same as the original; ▵ indicates that the surface of the catalytic composite was partially colored; and x indicates that a tar-like material remained on the catalytic composite. Catalyst(s) Inorganic silicate Binder Polyboro-siloxane Silicone CuO ○ ○ or ▵ x Fe₂O₃, Mn₂O₃ and CuO ▵ ▵ x Co₂O₃ ▵ ▵ x NiO ▵ or x ▵ or x x Generally, as oxidation catalysts for use in decomposing grease or stains produced in the process of cooking, oxides of manganese or oxides of cobalt are known to be preferred. Accordingly, catalysts for decomposing fats and oils which are used for conventional porous films in a commercially available cooker are mainly of the manganese type. However, as is apparent from the table, for the catalyst used in the present composite, copper oxide is most preferred. This may be explained as follows. Conventional porous films are generally of low porosity because they are basically formed on a flat plate, therefore their permeability to oxygen, and liquid absorption and liquid diffusing ability are limited. Thus, if a higher degree of catalytic function is desired, a catalyst with higher catalytic ability must be used. Therefore, oxides of manganese or cobalt were preferably used rather than copper oxide. In contrast, the catalytic composite has a highly porous structure made essentially of fibers, and therefore, it has a great advantage over any conventional porous film in both permeability to oxygen and ability to absorb and diffuse liquid. Therefore, even if the activity of the catalyst used in the composite is not particularly high, the composite has high ability of decomposing fats and oils by oxidation. Generally, during the process of an oxidation reaction, a part of oxidation catalysts is considered to be in the reduced state as follows: A decomposition reaction of fats and oils by oxidation is an exothermic reaction. Therefore, When manganese oxide or cobalt oxide, which is a highly active catalyst, is used, the catalyst is exposed to high temperatures during the process of oxidation, and the catalyst itself is reduced. As a result, the porous film becomes colored. On the contrary, when cupric oxide (CuO) is used as a catalyst, the catalyst is once reduced to cuprous oxide (Cu₂O), which can readily be reoxidized under a temperature of about 300°C or more. Therefore, the coloration due to the color of reduced catalyst (i.e., cuprous oxide) does not occur. As mentioned above, the catalytic composite having such a catalyst and a binder is well suited for the purpose of decomposing fats and oils by oxidation. Figure 2 shows a cross-section of an example of the catalytic composite. This catalytic composite comprises a porous sheet 1 made of aluminum fibers, each surface of which is supported on an air permeable metal support 7, film 2 made of aluminum oxide and/or silicon oxide formed on each surface of the porous sheet 1 by anodic oxidation, and a porous film 3 formed on each surface of the film 2 which contains fine particles 4 of cupric oxide. In the layered catalytic composite the first catalytic composite is highly capable of decomposing fats and oils by oxidation, while the second catalytic composite has excellent mechanical strength, although the second composite is slightly poor in the ability of decomposing fats and oils by oxidation as compared with the first catalytic composite. Therefore, when a catalytic composite is attached to the inner wall of the chamber of a heating cooker, a layered composite shown in Figures 3(a) and (b) containing the first catalytic composite 6 and second catalytic composite 5 is preferably used. Figure 3(a) is an enlarged partial perspective view of the layered catalytic composite comprising a first catalytic composite 6, a second catalytic composite 5, and air permeable metal supports 7 disposed on both surfaces of the layered body composed of the first and second composite. Figure 3(b) is a cross-section of the layered composite. Such a layered composite is prepared according to the following procedures. First, the first catalytic composite 6 and a porous sheet made of aluminum fibers, the sheet being subjected to anodic oxidation as necessary, are press-molded into a layered sheet. At the press-molding, an air permeable metal support 7 as shown in Figures 3(a) and 3(b), for example, an aluminum lath can be layered on at least one surface of the layered body of first and second catalytic composite. For example, the metal support is layered on the surface of the first composite for preventing the removal of the first composite from the second composite. When the layered composite is attached to the inner wall of a cooker so as to face the surface of the first composite to the surface of the wall, the support is layered on the surface of the second composite for the protection of the surface of the second composite. Such supports can be layered on both sides. Subsequently, a silicon oxide and/or aluminum oxide film is formed on the surface of the aluminum fiber side by anodic oxidation as required, and then a porous film containing an oxidation catalyst such as CuO, is formed successively. The layered catalytic composite obtained in this way looks good in appearance, and can be easily provided in a heating cooker. Such a layered catalytic composite is attached to the inner wall of a heating cooker so that the first catalytic composite of the layered composite is faced to the inner wall of the cooker. As the surface of the second catalytic composite 5 is faced to the inner space of the cooker, the catalytic composite can withstand possible mechanical impact which occurs during the process of cooking. Figure 4(a) is a schematic view showing a cooker of the present invention. This cooker 8 comprises a cooking chamber 80 and a heating unit (not shown) provided in the chamber. On the inner wall of the cooking chamber 80 the layered catalytic composite of this invention is attached so that the surface of the first catalytic composite faces the inner wall. Figure 4(b) is an enlarged partial view of the catalytic composite attached to the inner wall of the cooking chamber. Fish meat (mackerel pike, saurel, etc.) as well as chicken meat was grilled in the heating cooker shown in Figure 4(a) and after grease or stains scattered and deposited on the chamber wall were observed, blank baking was carried out for about 30 minutes. The temperature was maintained 300°C or more for about 15 minutes. The grease or stains on the inner wall of the chamber decomposed completely. Judging by visual inspection, the surface of the inner wall was not particularly changed, as compared with the wall prior to cooking. When a cycle of cooking and blank baking was repeated for 100 times, some oil stain residue was found on the inner wall at the region where the maximum wall surface temperature was about 250°C or lower, but on the whole the degree of decomposition of oil stains was completely satisfactory. The remaining residue at the lower temperature region can be improved by maintaining the interior of the chamber at high temperatures through improvement of adiabatic properties and temperature profile of the cooker. On the other hand, in a conventional cooker, oil stains were insufficiently decomposed. Thus, the amount of grease or stains in the cooking chamber was gradually increased. When the layered catalytic composite is attached to the inner wall of a cooker so that the surface of the second catalytic composite is positioned at the inner side, liquid substances (oil stains or grease) such as oils permeate the first catalytic composite through the second catalytic composite. The liquid oil stains or grease are decomposed mainly in the first catalytic composite. Small amounts of the liquid stains or grease remain in the second catalytic composite, where decomposition can also be carried out effectively. Even if a large amount of oil was scattered, the oil can be effectively decomposed, because the oil is allowed to penetrate through the second catalytic composite into the first catalytic composite. The temperature required for decomposition is 300°C or more, preferably 320°C or more, as mentioned in the description of the second catalytic composite. As mentioned-above, the catalytic composite of the present invention for decomposing fats and oils by oxidation, is highly air permeable because it uses a highly porous sheet made of aluminum fiber, so that sufficient oxygen necessary for decomposition of fats and oils can be easily supplied sufficiently. Furthermore, the composite has good wettability with fats and oils. Therefore, upon contact of fats and oils with the composite the fats and oils are allowed to penetrate deep into the composite for effective contact with the catalyst. Therefore, fats and oils can be decomposed at comparatively low temperatures. In the cooker having the layered catalytic composite, it is possible to remove grease or stains that are deposited on the wall at temperatures of about 300°C in a short time. If the catalytic composite is not used, high decomposition temperatures of 450 to 500°C and a longer period of time e.g., up to 2 to 3 hrs are required. According to this invention, the high capacity of the heater is not required, resulting in a saving of energy.

Claims: A catalytic composite for decomposing fats and oils by oxidation, consisting of: a porous sheet consisting of aluminum fibers, at least one surface of which is supported on an air permeable metal support; a film of silicon oxide and/or aluminum oxide which is formed on at least one surface of said porous sheet by anodic oxidation; and an oxidation catalyst carried on the surface of said film said oxidation catalyst being at least one selected from the group consisting of manganese oxide, iron oxide, cobalt oxide, nickel oxide, copper oxide, lanthanum oxide, and cerium oxide. A layered catalytic composite for decomposing fats and oils by oxidation, comprising a first catalytic composite, a second catalytic composite disposed on said first catalytic composite, and an air permeable metal support disposed on at least one surface of the layered body which is composed of said first and second catalytic composites, wherein said first catalytic composite is a sheet obtained by pressure molding of a mixture containing ceramic fibers and an oxidation catalyst, said ceramic fibers being made mainly of at least one selected from the group consisting of silicon oxide, aluminum oxide, and zirconium oxide, and said oxidation catalyst being at least one selected from the group consisting of manganese oxide, iron oxide, cobalt oxide, nickel oxide, copper oxide, lanthanum oxide, and cerium oxide, and wherein said second catalytic composite consists of a porous sheet consisting of aluminum fibers, a film of silicon oxide and/or aluminum oxide, which is formed on said porous sheet, and an oxidation catalyst, said oxidation catalyst being at least one selected from the group consisting of manganese oxide, iron oxide, cobalt oxide, nickel oxide, copper oxide, lanthanum oxide, and cerium oxide, wherein said oxidation catalyst is carried on the surface of said film. A heating cooker having a layered catalytic composite of claim 2, wherein said layered catalytic composite is attached to the inner wall of said cooker for the purpose of decomposing by oxidation, fats and oils or cooking exhausts which are formed or emitted in the process of cooking so that the first catalytic composite of the layered catalytic composite is faced with said inner wall.

Inventors: FUKUDA AKIO, KANEKO YASUNORI, MAKI MASAO, WAKI MAKIKO, FUKUDA, AKIO, KANEKO, YASUNORI, MAKI, MASAO, WAKI, MAKIKO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD
Application year: 1990
Main classification: B01J35
IPC codes: B01J35, B01J37, F24C15