JPH11165071A - Fluorine-containing compound-decomposing catalyst and fluorine-containing compound-decomposing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To continuously decompose a fluorine-containing compound containing also perfluoro-compound, which is difficult to decompose, by carrying a metal selected from group VIA, group VIII and group IIIB and an inorganic acid selected from sulfuric acid, phosphoric acid and boric acid on alumina. SOLUTION: The metal carried on the alumina is at least one or more kinds selected from group VIA, group VIII and group IIIB and Cr, Fe, Co, Ni, Pd and Pt, which are the transition metal elements, and B and Ga, which are the typical metal elements, among them produces good result. A preferable result is attained by controlling the quantity of the metal to be carried on the alumina carrier to 0.01-20 pts.wt. expressed in terms of metal per 100 pts.wt. carrier. The inorganic acid is limited to sulfuric acid, phosphoric acid and boric acid from the view point of the reaction condition and sulfuric acid having strong acidity and low splash property is most preferably used among them. The suitable result is attained by controlling the content of the inorganic acid in the catalyst to 0.1-20 pts.wt. per 100 pts.wt. alumina.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a catalyst for catalytically decomposing a fluorine-containing compound, particularly a perfluoro compound such as carbon fluoride, nitrogen fluoride and sulfur fluoride, and a method for decomposing a fluorine-containing compound using the catalyst. About. By using the catalyst of the present invention, not only is it possible to decompose chlorofluorocarbon, which is a problem as a substance causing ozone layer depletion, but it is also discharged into the atmosphere from plants that use alternative chlorofluorocarbons, especially semiconductor manufacturing plants. It is also possible to reduce the amount of perfluoro compounds which are more difficult to decompose than chlorofluorocarbons.

[0002]

2. Description of the Related Art Among volatile fluorine-containing compounds, Freon containing chlorine and fluorine has been determined to be severely regulated in the future as the cause of ozone layer destruction. In addition to this fluorocarbon, the volatile fluorine-containing compound includes a volatile fluorine-containing compound called a perfluoro compound (hereinafter, referred to as PFC) composed of carbon fluoride, nitrogen fluoride, sulfur fluoride, and fluorocarbon. Unlike PFC, this PFC does not contain chlorine and does not contribute to ozone layer destruction because it is very stable. Since there is no emission control yet, it is often used for cleaning in semiconductor manufacturing sites. Is what is being done. However, the global warming potential is 10
It is as large as 00 times and its emission to the atmosphere is
It is a compound that is very likely to be regulated in the future. Among these fluorine-containing compounds, the production and use of specific fluorocarbon gases, which are contained in products such as air conditioners themselves and whose emission is very difficult to control due to their wide distribution, are completely prohibited. You may only have a hand, but PFC
As mentioned above, although it is used in the manufacturing process, it is not included in the product and is not shipped, and since the source is specified by the factory, for those that are relatively easy to control at the exit, It is advisable to use it while taking advantage of its original characteristics. Of course, it is needless to say that when the reference value is set, the emission is suppressed to a value lower than the reference value.

[0003] As a method of suppressing the above gas emission,
There are two methods, a recovery method and a decomposition method, but the recovery method is preferably a decomposition treatment because the concentration of PFC contained in the exhaust gas is essentially low and the recovery device is complicated. Is the way. However, PFC,
Above all, fluorocarbon is chemically stable as compared with chlorofluorocarbons, so that it is difficult to treat it with a normal decomposition method used for decomposition of chlorofluorocarbons, and further severe processing conditions are required. For example, the temperature required for a simple combustion process is 800 to 900 ° C. in the case of chlorofluorocarbon, but not only 1000 ° C. or more in the case of fluorocarbon, but also a combustion furnace or a nozzle of a combustion device. There is also a problem of corrosion at a portion that comes into contact with a corrosive gas at a high temperature, and there is still a problem to be solved for practical use. Also, Japanese Patent Application Laid-Open No.
Japanese Patent Application Laid-Open No. 7-132211, which discloses a technique for decomposing carbon fluoride by using silica or zeolite as a decomposing agent or a fluorine scavenger, decomposes fluorocarbon at a practical rate. Not only requires a high temperature of 1000 ° C. or more, but also requires a part of the decomposing agent to be consumed as a fluorine scavenger, so that it is necessary to carry out the reaction while supplying the decomposing agent in the form of a powder. It has the following disadvantages. For continuous treatment of a fluorine-containing compound containing PFC, a catalytic cracking method using a catalyst is considered to be the most effective, but a catalyst system showing a sufficient activity has not been found yet.

[0004]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a P which is difficult to decompose in the prior art under practical processing conditions.
It is an object of the present invention to provide a catalyst system capable of continuously decomposing a fluorine-containing compound, including FC, and a method for continuously decomposing a fluorine-containing compound using the catalyst.

[0005]

The present inventors have found that a catalyst in which alumina carries a certain metal and an inorganic acid is a catalyst system that solves the above-mentioned problems, and that the above-mentioned problems can be solved by using the catalyst properly. The present invention was found to be a method for continuously decomposing the obtained fluorine-containing compound, and the present invention was completed. That is, the present invention relates to a fluorine-containing compound decomposition treatment catalyst in which alumina carries at least one or more metals selected from 6A, 8 and 3B and at least one or more inorganic acids selected from sulfuric acid, phosphoric acid and boric acid. . Also, the present invention
In the coexistence of oxygen and water, group 6A, 8
The present invention relates to a method for decomposing a gaseous fluorine-containing compound, which is carried out by contacting a catalyst supporting at least one kind of metal selected from Group III and Group 3B and an inorganic acid.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION As a catalyst carrier in the present invention, alumina is used because it provides a catalyst having a very high activity of decomposing a fluorine-containing compound. Alumina, which is generally commercially available for use as a catalyst carrier, can be used as it is, but for the shape, granules, powders, honeycombs, etc. should be selected according to the reactor and reaction method. become.

The metal supported on alumina is Group 6A,
It is at least one or more metals selected from Group VIII and 3B, and among them, transition metal elements such as Cr, Fe and C
o, Ni, Pd, Pt and typical metal elements B, G
a gives favorable results. In carrying metal,
After dissolving various salts or oxides of these metals, such as chlorides, nitrates, sulfates, and phosphates, in an appropriate solvent, if the carrier is a molded article, a generally used impregnation method or It can be easily supported by an evaporation-to-drying method, or in the case of a powder, by a mechanical kneading method or an evaporation-to-dryness method. It can also be prepared by a coprecipitation method using alumina sol. If a metal sulfate and / or phosphate is used as a starting material, a sulfate group and / or a phosphate group are introduced simultaneously with the metal, so that the metal component and the inorganic acid can be introduced into the catalyst at the same time. It can be adopted as a preparation method. It is to be noted that the catalyst activity is further increased by introducing an excessive amount of sulfate groups and / or phosphate groups rather than introducing metal sulfates and / or phosphates in equal amounts of sulfate groups and / or phosphate groups. Can be.

In the present invention, the amount of metal supported on the alumina carrier depends on the type of metal, but a preferable result can be obtained by adjusting the amount of metal to 0.01 to 20 parts by weight per 100 parts by weight of the carrier. If the amount of the metal is too small, a sufficient effect of addition may not be exhibited.If the amount is too large, not only is the waste of resources, but a uniformly dispersed catalyst cannot be obtained, and the catalyst may have poor reproducibility. . Incidentally, since the form of the metal present in the catalyst, particularly under the reaction conditions, is unknown at this time, it is expressed as "support of metal" in the present invention, but this does not mean only a zero-valent metal. Absent.

The present invention is characterized in that a catalyst having a high activity of decomposing a fluorine-containing compound is obtained by supporting a metal species and an inorganic acid on alumina. Although the function of the inorganic acid is unknown at this time, it is coordinated around the Al ion of the carrier, and the F, which has a high electronegativity, is bound and the C, N, and S atoms in the reactant molecule polarized to δ + are adsorbed. It is thought to increase reactivity. Incidentally, the inorganic acid used in the present invention,
From the reaction conditions, it is limited to sulfuric acid, phosphoric acid and boric acid. Among them, the use of sulfuric acid which is strongly acidic and has low scattering property is most preferable.

[0010] The support of the inorganic acid is not only free acid,
Precursors that can be decomposed by use conditions such as ammonium salts such as ammonium sulfate or esters or by a pretreatment performed prior to use to give essentially the same support as when a free acid is used as a starting material can also be used. These free acids or their precursors are dissolved in water or other suitable solvent, as in the case of supporting the metal component, and then, when the carrier is a molded body, a commonly used impregnation method or The support can be easily carried out by an evaporation-drying method, or in the case of a powder, by a mechanical kneading method or an evaporation-drying method. When the amount of the supported inorganic acid is large, a high-concentration solution may be supported at once, or a low-concentration solution may be supported several times. Therefore, it is preferable to carry the low-concentration solution in two to four portions, since the solution is easily obtained. The order of addition of the metal and the acid on the carrier is not particularly limited, and either one may be carried first or the other may be carried simultaneously.

The content of the inorganic acid in the catalyst varies depending on the kind of the acid to be used. If the amount is too small, the effect of addition is not sufficiently exhibited. If the amount is too large, the content of the metal depends on the type of metal supported. The effect may be suppressed. In the present invention, 0.1 parts by weight per 100 parts by weight of the carrier γ-alumina is used.
A suitable result can be obtained by using 1 to 20 parts by weight.

When the catalytic activity is reduced due to the decrease in the amount of acid in the catalyst, the catalyst can be reactivated by adding an acid. The method of adding the acid is a method of supplying these inorganic acids to the reaction system together with the reaction gas in the form of an aqueous solution when supplying water which is an essential component to the reaction system, or a method of adding a volatile precursor such as an inorganic acid ester. It is possible to adopt a method of entraining the reaction gas in a gaseous state. In this case, the acid may be added either intermittently or constantly.
If it is carried out regularly, it is possible to prevent a decrease in catalytic activity due to a decrease in the amount of acid.

The catalyst precursor prepared by supporting the necessary components on alumina is dried at about 100 ° C., and then subjected to a heating pretreatment in a stream of air or nitrogen to obtain a catalyst. This activation treatment may be performed using a muffle furnace or the like, or after filling a necessary amount of the dried product in a reactor for decomposition reaction of a fluorine-containing compound, and circulating air or an inert gas prior to use. It may be activated by heating. The heating temperature depends on the type of the metal to be supported and the type of the precursor used for the support. However, the treatment temperature may cause a decrease in the surface area of the alumina carrier or the supported metal, which is too high. Without
In the early stage of the reaction, a change in the catalytic activity with the lapse of time is caused, which leads to undesirable results. Therefore, in the present invention, 200 to
The heat treatment is performed at 1000 ° C, preferably in the range of 400 to 800 ° C.

The decomposition reaction of the fluorine-containing compound is performed by supplying a mixed gas of the fluorine-containing compound, oxygen and water onto the catalyst.
It is carried out in a temperature range of 300 to 1000 ° C, preferably 400 to 900 ° C. The supply rate of the mixed gas is 50,000 /
Or less, preferably 100 to 100000 / hour.

The concentration of the fluorine-containing compound contained in the reaction gas of the present invention is preferably 3% by volume or less. If the concentration of the fluorine-containing compound contained in the reaction gas is too high, the life of the catalyst may be adversely affected. Generally, the PFC concentration in the exhaust gas discharged from a semiconductor manufacturing plant is 1
This is not a problem because it is not more than 3% by volume, but when it is contained at 3% by volume or more, it is preferable to add a diluent gas such as air or nitrogen so that the concentration becomes 3% by volume or less. The reaction gas contains oxygen and water in addition to PFC. Among them, oxygen is a component necessary for converting carbon of PFC into CO ₂ and CO, and water is generated by a decomposition reaction. In addition to being a component necessary for discharging halogen as HF out of the catalyst system, it also has a function of preventing Al in the carrier from escaping out of the catalyst system as aluminum fluoride.

The amount of oxygen contained in the reaction gas is not particularly limited as long as it is an amount sufficient to convert PFC carbon into CO ₂ and CO, but the PFC concentration in the reaction gas is within the above range. If not only air is available,
It is the most preferred oxygen source. On the other hand, the amount of water contained in the reaction gas is equal to or more than 10 times the amount of halogen contained in the reaction gas, that is, ₄ to 40 mole times for CF ₄ and ₆ to 40 times for C ₂ F ₆ . If it is 60 mole times, a favorable result can be obtained. The problem with water supply is that it is a commonly used method, that is, a method of supplying a liquid state to a reactor using a liquid pump, or a method of entraining a gaseous state with a reaction gas using a saturator. Applicable without.

The catalytic decomposition reaction of the fluorine-containing compound can be carried out by a flow system or a batch system, but the flow system is preferred in view of the simplicity of the apparatus and the high processing capacity.
In the case of a flow type, any of a fixed bed and a fluidized bed can be applied.

Exhaust gas after leaving the reactor is passed through a scrubber filled with an alkaline aqueous solution or through an adsorber filled with solid alkali to remove HF produced by the decomposition reaction. After being released into the atmosphere.

[0019]

Now, the present invention will be described in further detail with reference to specific examples. Example 1 Commercially available granular γ-alumina (particle size: 1 to 1.5 mm)
0 g, NiSO ₄ .6H ₂ O 0.5 g and distilled water 30 g
Was dried at 60 ° C. under reduced pressure using a rotary evaporator, and then dried at 100 ° C. overnight. Drying cooled to room temperature is laid flat on a glass dish,
After 10 ml of a 1 N aqueous sulfuric acid solution was evenly dropped with a pipette, the solution was dried again at 100 ° C. overnight. The sulfuric acid supporting step was repeated three times, and approximately 1.5 g of γ-alumina was added to 10.0 g.
A catalyst supporting × 10 ^-2 mol of sulfuric acid was prepared. The obtained catalyst (5.0 ml) was made of quartz having an inner diameter of 1.0 cm.
And baked at 700 ° C. for 1 hour in a nitrogen stream. Then, at the same temperature, 20 ml each of a 1% by volume C ₂ F ₆ / air mixed gas and water (NTT) was used.
P. ) / Min and the reaction was carried out at a rate of 0.36 g / hour, and the concentration of C ₂ F ₆ in the outlet gas at 2 hours after the start of the reaction was analyzed by gas chromatography to evaluate the catalytic activity. Was performed. As a result, the conversion of C ₂ F ₆ represented by the following formula was 90%. As a product, carbon dioxide was subjected to gas chromatography and FT.
Confirmed by IR. Conversion rate = [(C ₂ F ₆ amount in raw material gas−C in outlet gas)
₂ F ₆ amount) ÷ C ₂ F ₆ amount in raw material gas] × 100

Reference Example 1 A catalyst supporting only Ni was prepared in the same manner as in Example 1 except that sulfuric acid was not supported, and a C ₂ F ₆ decomposition reaction was performed under the same reaction conditions as in Example 1. . As a result, the C ₂ F ₆ conversion two hours after the start of the reaction was 68%.

[0021] Other omitting Reference Example ₂ NiSO ₄ · 6H 2 O in supporting step Example 1
To prepare a catalyst supporting only sulfuric acid,
A C ₂ F ₆ decomposition reaction was performed under the same reaction conditions as in Example 1. As a result, the C ₂ F ₆ conversion two hours after the start of the reaction was 81%.

A catalyst in which a metal and an inorganic acid are supported on γ-alumina has a higher C ₂ F ₆ decomposition activity than a catalyst in which only a metal or an inorganic acid is supported on γ-alumina. I understand.

[0023]

According to the catalyst of the present invention, not only chlorofluorocarbon but also PFC which is difficult to decompose as compared with chlorofluorocarbon can be decomposed at a reaction temperature of 1000 ° C. or less. Despite the fact that its use is indispensable in the semiconductor manufacturing process, which is also referred to as rice in the industry, it enables the decomposition of PFC, whose emission to the environment will be regulated in the future due to its large global warming potential. It has great technical and social significance.

Claims (7)

[Claims] 1. A fluorine-containing compound decomposition treatment catalyst comprising alumina carrying at least one metal selected from the group 6A, 8 and 3B and at least one inorganic acid selected from sulfuric acid, phosphoric acid and boric acid. 2. The method according to claim 1, wherein the metal species is Cr, Fe, Co, Ni, P.
The fluorine-containing compound decomposition treatment catalyst according to claim 1, wherein the catalyst is at least one metal selected from d, Pt, B, and Ga. 3. The catalyst according to claim 1, wherein the metal content in the catalyst is 0.01 to 20 parts by weight in terms of metal per 100 parts by weight of alumina. 4. The catalyst according to claim 1, wherein the content of the inorganic acid in the catalyst is 100% alumina.
The catalyst for treating a fluorine-containing compound according to any one of claims 1 to 3, wherein the amount is 0.1 to 20 parts by weight per part by weight. 5. The decomposition of a gaseous fluorine-containing compound according to any one of claims 1 to 4, wherein the fluorine-containing compound to be reacted is one or a mixture selected from a perfluoro compound and a fluorocarbon. Processing catalyst. 6. A method for decomposing a fluorine-containing compound, which is carried out by bringing the catalyst into contact with the catalyst for decomposing a fluorine-containing compound according to any one of claims 1 to 4 in the presence of oxygen and water. 7. The method for decomposing a gaseous fluorine-containing compound according to claim 6, wherein the fluorine-containing compound to be reacted is one or a mixture selected from a perfluoro compound and a fluorocarbon.