JP2000210560A - Catalyst for decomposing compound containing fluorine and method for treating compound containing fluorine by decomposition - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable a high decomposition performance to be maintained for a long time under practical reaction conditions by forming a catalyst for decomposing a gaseous compound containing fluorine from a perfluorocompound and/or a chlorofluorocarbon compound by adding a specific ratio of silicon dioxide to 100 γ-alumina. SOLUTION: The catalyst for decomposing a gaseous compound containing fluorine from a perfluorocompound and/or a chlorofluorocarbon compound is formed by adding 1-10 pts.wt. silicone dioxide to 100 pts.wt. γ-alumina. When this catalyst is used, the catalyst obtained by adding 1-10 pts.wt. silicon dioxide to 100 pts.wt. γ-alumina heated at 300-1,000 deg.C is brought into contact with a compound containing fluorine. Thus it is possible to maintain the high decomposition performance originally possessed by the γ-alumina to the compound containing fluorine for a long time by controlling the transition of the γ phase to the α phase which occurs in the alumina following a chemical reaction. Especially it is desirable to use the γ-alumina having 100 ppm or less Na content in terms of a metal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a catalytic decomposition treatment of a fluorine compound, particularly a perfluoro compound or a fluorocarbon compound such as carbon fluoride, nitrogen fluoride or sulfur fluoride, which maintains a high decomposition activity for a long time. The present invention relates to a method and a catalyst used for the decomposition treatment. According to the method of the present invention, not only is it possible to decompose chlorofluorocarbon, which is a problem as a substance causing ozone layer depletion, but also various production sites despite concerns that it is a factor in global warming In particular, it is possible to reduce the amount of a perfluoro compound which is more difficult to decompose than chlorofluorocarbons discharged from a semiconductor manufacturing plant into the atmosphere.

[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 destruction of the ozone layer. Volatile fluorine-containing compounds include, in addition to this fluorocarbon, carbon fluoride, nitrogen fluoride, sulfur fluoride, and fluorocarbon (HF).
There is a compound called a perfluoro compound (hereinafter referred to as PFC), which is generically referred to as C). Unlike PFC, this PFC does not contain chlorine and is very stable, so it does not contribute to the ozone layer destruction. Since there is no emission control yet, the PFC is used in the etching and cleaning processes at semiconductor manufacturing sites. It is often used in. However, its global warming potential is as large as 1000 times or more that of carbon dioxide, and its emission into the atmosphere is a compound that is very likely to be regulated in the future, as with CFCs. 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 sources are widely distributed and whose emission control is extremely difficult, is completely prohibited. Although it may only be possible, unlike PFC, it is used in the manufacturing process, but is not included in the product and shipped, and the emission source is specified at the factory. Is relatively easy to use while taking advantage of its original characteristics. Of course,
It is needless to say that when the reference value of the emission is set, it is assumed that the emission reference value is suppressed below the reference value.

[0003] As a method of suppressing the above gas emission,
There are two possible methods, a recovery method and a decomposition treatment method. In the case of PFC, for example, in the case of PFC, the concentration of PFC contained in the exhaust gas is essentially low and the recovery device is complicated. For this reason, decomposition is the preferred method. However, PFCs, especially fluorocarbons, are chemically more stable than CFCs, so that they are difficult to treat by the usual decomposition method used to decompose CFCs, and require more severe processing conditions. . 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.

[0004] JP-A-7-116466 and JP-A-7-116466
Each publication of -132211 discloses a technique of decomposing carbon fluoride using silica or zeolite as a decomposer or a fluorine scavenger, but it is necessary to decompose fluorocarbon at a practical rate. Again, not only does it require a high temperature of 1000 ° C. or higher, but also a part of the decomposer is consumed as a fluorine scavenger, so it is necessary to carry out the reaction while supplying a powder decomposer, which makes the apparatus complicated. It has. On the other hand, as a method for continuously treating a fluorine-containing compound containing PFC under practical temperature conditions, a catalytic decomposition method is disclosed in Japanese Patent Application Laid-Open No. H10-192563. As a result of actually testing the method using alumina, silica, titania, zirconia, or the like as a catalyst shown here, the decomposition activity significantly decreased with time, as will be described later, which was a critical factor in putting the catalyst decomposition method into practical use. It has been found that the problem is included.

[0005]

SUMMARY OF THE INVENTION In the present invention, there is provided a truly practical catalyst for decomposing a fluorine-containing compound, which maintains a high decomposition performance under practical reaction conditions for a long time. An object of the present invention is to provide a method for continuously decomposing fluorine compounds.

[0006]

Means for Solving the Problems The inventors of the present invention have essentially developed a γ-type compound having a high decomposition activity with respect to a fluorine-containing compound.
As a result of examining in detail why the deactivation of the alumina catalyst is remarkable, as a result of finding out that the crystal transition from the γ phase to the α phase of alumina as the catalyst is the cause, and variously examining methods for suppressing the phase transition, The addition of silicon oxide, ie, silica,
The present invention has been found to have a great effect in suppressing the phase transition of alumina to α-alumina, and to exhibit a remarkable effect in maintaining the high decomposition activity inherent to γ-alumina catalysts against fluorine-containing compounds. Was completed. That is, the present invention provides γ
The present invention relates to a catalyst for decomposing a gaseous fluorine-containing compound comprising a perfluoro compound and / or a fluorocarbon compound, which is constituted by adding 1 to 10 parts by weight of silicon dioxide to 100 parts by weight of alumina. Further, the present invention relates to contacting with a catalyst which is heated to 300 to 1000 ° C. and which is obtained by adding 1 to 10 parts by weight of silicon dioxide to 100 parts by weight of γ-alumina in the presence of oxygen and water. The present invention relates to a method for decomposing a gaseous fluorine-containing compound comprising a perfluoro compound and / or a fluorocarbon compound. Hereinafter, the present invention will be described in detail.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Decomposition of a fluorine-containing compound on a γ-alumina catalyst includes adsorption of the fluorine-containing compound onto the catalyst,
It is thought to occur via a bond in the fluorine-containing compound accompanying the adsorption, for example, dissociation of a carbon-fluorine bond. Alumina is considered to exist not as pure Al ₂ O ₃ but as a so-called reaction intermediate, which has some kind of bond with the constituent element of the adsorbed fluorine-containing compound, for example, fluorine. The reaction intermediate is catalyst body is decomposed by water coexisting in the reaction system is in the process of returning to Al ₂ O _3, any change that is considered sufficient to occur in the crystal structure of Al ₂ O _3, actually,
It has been confirmed that various changes occur in the catalyst used in the reaction as compared to before the reaction.

For example, an actual measurement chart of X-ray diffraction is presented and described later. The diffraction peak intensity attributed to the γ phase, which was solely shown by the alumina catalyst before being subjected to the reaction, is such that the catalyst is maintained for about 50 hours. C decreased significantly in ₂ after contact with F _6, a large increase in diffraction peak intensity assigned to the α-phase in place was observed, due to catalyze C ₂ F ₆ decomposition reaction, the γ alumina The occurrence of a phase transition to α is confirmed. Usually, the phase transition temperature of γ-alumina to α phase is said to be 1200 to 1300 ° C. in air, and the phase transition to α phase in a very low temperature range of 400 to 900 ° C. observed here. Is presumed to be based on the interaction between alumina and the adsorbate described above.

Further, in a certain γ-alumina catalyst, the specific surface area of 204 m ² / g,
After contact with C ₂ F _{6 for} 0 hours, the value is 26 m ²
/ G has been confirmed. Further, the result of the thermal desorption with the catalyst on which pyridine was adsorbed is that the catalyst before the reaction (b) is 600
The desorption peak appearing at around ° C. has disappeared, and it has been confirmed that the acid sites giving the adsorbed pyridine species desorbed at this temperature have disappeared.

Whether the physical change of the phase transition from the γ phase to the α phase and the reduction of the specific surface area and the chemical change of the disappearance of the acid sites occur independently, or one of them causes another phenomenon. It is unknown at this time, but it is generally thought that each of these acts as a negative factor in the development of catalytic activity, so these additive effects significantly reduce catalytic activity. In order to suppress the deactivation of the γ-alumina catalyst, it is necessary to suppress one or more of these changes occurring in the catalyst upon contact with the reactant. Is considered indispensable.

In the present invention, the addition of silicon dioxide, ie, silica, to γ-alumina suppresses the above-mentioned transition of the γ phase occurring in the alumina due to the reaction to the α phase,
This makes it possible to maintain high decomposition of the fluorine-containing compound inherent in alumina for a long time.

Alumina, which is an essential component of the catalyst of the present invention, includes various types having different crystal phases. However, in the present invention, alumina contains the largest number of strong acid sites and, as a result, has a high activity for decomposing fluorine-containing compounds. Is used. γ-alumina is the most common alumina, and various types having different geometric shapes and characteristics are commercially available. Not only is it easy to obtain, but also various types of alumina precursors are used. And can be easily prepared. Commercially available products contain Fe, Na, and the like as main impurities. Of these, Na significantly poisons the acid sites of γ-alumina, which acts as active sites for the decomposition of fluorine-containing compounds,
Since the decomposition activity of the fluorine-containing compound is remarkably reduced, the smaller the amount of Na as an impurity is, the more suitable for the use of the present invention.
The use of less than 00 ppm of γ-alumina gives favorable results. Na content is 1000 ppm by weight in metal conversion
For the following γ-alumina, it is best to select a commercially available product that satisfies the specifications.
Γ-alumina containing more than 000 ppm of Na is also washed with water, or washed with an inorganic acid such as hydrochloric acid, nitric acid, sulfuric acid or an organic acid such as acetic acid to obtain Na.
The content can be reduced to 1000 ppm or less in terms of metal.

On the other hand, as a raw material of silica which is another component of the catalyst, there are alkoxide represented by tetraethyl orthosilicate, silica sol, and silica gel.
The appropriate one will be selected according to the geometry of the γ-alumina and the catalyst preparation method. For example, the simplest method for preparing a catalyst, a method of impregnating a molded alumina having a desired particle size with an alkoxide is performed by impregnating a molded γ-alumina with a silicon alkoxide dissolved in an alcohol and then removing the solvent. I can do it. In this case,
After evaporating and removing only the solvent, after drying at an appropriate temperature and time, for example, 100 ° C. for 24 hours, 300 to 7 in air.
The catalyst can be obtained by calcining at 00 ° C. for 1 to 10 hours. Instead of molded alumina, powdery γ-alumina may be impregnated with silicon alkoxide using a solvent, and after removing the solvent, may be molded to a desired size if necessary, and then fired under the same conditions. A so-called kneading method in which powdery γ-alumina and powdery silica are kneaded and then molded as necessary can be naturally employed. Further, a catalyst composed of γ-alumina and silica can be produced from an alkoxide of aluminum and an alkoxide of silicon using a sol-gel method. Of course, a series of calcination treatments may be performed in the reaction tube after filling the unfired catalyst.

The effect of suppressing the phase transition of γ-alumina to α is as follows.
It changes depending on the amount of silica added. If the amount of silica is small, the effect of suppressing the transfer of γ-alumina to the α phase is not sufficiently exhibited, and as a result, the substantial effect of extending the catalyst life is reduced. Further, as the amount of addition increases, the effect of suppressing the transition increases, and it is possible to completely suppress the phase transition.However, it is presumed that this is based on the decrease in the number of active points in γ-alumina that acts on the decomposition of the fluorine-containing compound. Although this can lead to a decrease in the activity of decomposing the fluorine-containing compound and can be solved by raising the reaction temperature, it results in an unfavorable result in terms of reaction conditions. In the present invention, the content of silica in the catalyst is γ-alumina 1
Desirable results can be obtained by using 1 to 10 parts by weight, preferably 1 to 5 parts by weight as oxides per 100 parts by weight.

The added silicon is presumed to be present in the form of an oxide in the catalyst, as judged from the kind of starting material and the preparation method. As far as observed by X-ray diffraction measurement, no crystal phase other than γ-alumina as the main component was observed.
The addition of silica suppresses the rearrangement of γ-alumina to the α phase upon contact with the fluorine-containing compound, so that the added silicon is capable of causing strong interaction with alumina,
That is, although it cannot be denied that it is incorporated into the γ-alumina crystal, its existence form and the effect of suppressing the phase transition from γ to α when γ-alumina containing silica is used as a fluorine-containing compound decomposition catalyst. The mechanism of the expression is unknown at this time.

The decomposition reaction of a fluorine-containing compound using a silica-containing γ-alumina as a catalyst is performed while supplying a mixed gas of a fluorine-containing compound, oxygen and water onto the catalyst in a range of 300 to 100.
The reaction is performed at a temperature of 0 ° C, preferably 400 to 900 ° C.
The supply rate of the mixed gas to the catalyst layer depends on the reaction temperature, but is not more than 50,000 / hour, preferably 100 to 10 / hour.
000 / 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, P in exhaust gas discharged from semiconductor manufacturing plants
The FC concentration is 1% by volume or less, which is not a problem. However, when the concentration is 3% by volume or more, it is diluted with air, nitrogen, etc.
It is preferable that the content be 3% by volume or less.

The reaction gas contains oxygen and water in addition to the fluorine-containing compound. The oxygen contained in the reaction gas is used to convert elements other than halogen and hydrogen in the fluorine-containing compound into oxides thereof. Is a component necessary for converting carbon of the carbon-containing fluorine-containing compound into CO ₂ and CO, and water discharges the halogen adsorbed on the catalyst during the decomposition reaction as hydrogen halide to the outside of the catalyst system. Al, which is not only a necessary component for the catalyst but also a constituent metal of the catalyst,
It also has the effect of suppressing the scattering and reduction of Zn as a halide.

The amount of oxygen contained in the reaction gas may be any element other than halogen and hydrogen in the fluorine-containing compound, for example, an amount sufficient to convert carbon of the carbon-containing fluorine-containing compound into CO ₂ and CO. Although there is no limitation, if the concentration of the fluorine-containing compound in the reaction gas is within the above range, not only air can be used, but also the most preferable oxygen source from the viewpoint of economy, safety and the like.

On the other hand, the amount of water contained in the reaction gas is equal to or more than 10 times the molar amount of the total halogen amount converted from the halogen atoms contained in the fluorine-containing compound in the reaction gas, ie, CF ₄ If present, 4 to 40 times mol, C ₂ F ₆
In this case, an effective result can be obtained if the molar ratio is 6 to 60 times. Water supply is generally used,
That is, a method in which a predetermined amount is sent in a liquid state using a liquid pump and gasified before reaching a catalyst layer in the reactor,
Alternatively, a method of using a saturator to entrain the reaction gas or oxygen (air) in a gaseous state can be used.

The catalytic cracking reaction of the fluorine-containing compound with a catalyst can be carried out in either 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 addition, in the case of a flow type, any of a fixed bed and a fluidized bed can be applied.However, since the catalyst activity can be maintained for a long time, the use of a fixed bed makes use of the characteristics of the present catalyst. How to use.

The exhaust gas containing hydrogen halide after leaving the reactor is subjected to a conventional method of passing through a wet scrubber or a dry adsorber filled with an adsorbent to remove the hydrogen halide. Released into the atmosphere.

[0023]

Now, the present invention will be described in further detail with reference to specific examples. (1) Catalyst Preparation Alumina has a Na content of 50 ppm by weight in terms of metal.
The following γ-alumina (Note: below the limit value in ICP analysis) (manufactured by JGC Universal Co., Ltd., trade name: NST-7,
(Average particle size: 1.6 mm) was used after dehydrating overnight in an oven at 100 ° C. Silica-containing alumina catalyst Here, commercially available molded γ-alumina was added to a silica source as tetraethylorthosilicate [Si (OC ₂ H ₅ ) ₄ ,
Hereinafter, referred to as TEOS], an example in which silica is impregnated. A predetermined amount of TEOS is 50 ml of ethanol
After dissolving the above, 30 ml of the above alumina beads were added,
Stirred at 60 ° C. for 1 hour. Next, after the solvent was removed by an evaporator, the resultant was dried in an oven at 100 ° C. overnight. The dried product was calcined at 900 ° C. for 2 hours in air using a muffle furnace to obtain a catalyst.

Titanium-containing alumina catalyst 10% by weight of Ti was prepared in the same manner as the silica-containing catalyst except that titanium isopropoxide was used instead of TEOS.
An alumina catalyst containing O ₂ was prepared.

(2) Decomposition reaction of C ₂ F _{6 by} a catalyst 10 ml of a catalyst was charged into a Hastelloy alloy reaction tube having an inner diameter of about 21 mm, and heated at a reaction temperature for 1 hour in a nitrogen stream. , Gas composition: 1% by volume C ₂ F ₆ −
The decomposition reaction was carried out under a reaction condition of nitrogen mixed gas / air / steam = 50/39/50, total gas flow rate: 139 ml / min. The amount of undecomposed C ₂ F ₆ gas in the gas at the outlet of the reaction tube was periodically analyzed by TCD gas chromatography, and the change over time in the catalytic activity was examined. In each case, carbon dioxide was obtained by gas chromatography and F
Confirmed by TIR. FIG. 1 shows the change over time of the C ₂ F ₆ decomposition activity with the reaction time. Incidentally, in FIG. 1, C ₂ F ₆ conversion [(C ₂ F ₆ amount in the feed gas - C ₂ F ₆ amount in the outlet gas) ÷
C ₂ F ₆ amount in source gas] × 100.

(3) Catalyst analysis For each catalyst, X-ray diffraction analysis was performed before and after use in the catalytic reaction shown in FIG. 1 and silica or titania was added. The effect of the reaction on the change in catalyst structure was investigated. The results are shown in FIG. For some catalysts before use, the pyridine adsorbed at 100 ° C. was subjected to a thermal desorption test (temperature increasing rate of 10 ° C./min), and 500 to 800 ° C.
The amount of pyridine desorbed in the temperature range of ° C. was measured and used as a scale representing the amount of catalytic acid that contributes to the decomposition reaction of the fluorine-containing compound.

FIG. 1 shows the change over time of the C ₂ F ₆ decomposition activity (hereinafter simply referred to as decomposition activity) of each catalyst at a reaction temperature of 700 ° C. The catalyst composed of alumina alone shows stable decomposition activity at the beginning of the reaction, but abruptly decreases in the decomposition activity from about 20 hours after the start of the reaction, and thereafter monotonously decreases with the reaction time. In the catalyst to which silica was added, except for the catalyst in which the amount of silica added was 1% by weight, the addition of silica showed a decrease in decomposition activity immediately after the reaction.
The degree of the degradation activity decreases with the amount of silica, but in any case, after a certain period of time, the tendency of the decrease in the degradation activity reaches a plateau, and thereafter the stable degradation activity is maintained. In particular, when the amount of silica added is 5% by weight or less, the decrease in the initial activity due to the addition of silica is small, and particularly preferable results are obtained.
The titania-added catalyst has a large reduction in the decomposition activity at the beginning of the reaction, and does not sufficiently exert the effect of suppressing the reduction of the decomposition activity. .

The amount of pyridine desorbed in the temperature range of 500 to 800 ° C., which is considered to be a scale representing the amount of catalytic acid that contributes to the decomposition reaction of the fluorine-containing compound, together with the amount of silica added,
As shown in Table 1, it decreases. It is presumed that the decrease in the activity observed immediately after the start of the reaction in the catalyst containing a large amount of silica is caused by the decrease in the acid amount.

[0029]

[Table 1]

FIG. 2 shows an X-ray diffraction diagram of a catalyst consisting of γ-alumina alone before being subjected to the reaction. Naturally, only diffraction peaks attributable to γ-alumina are observed. Up to 10% by weight of the catalyst to which silica and titania are added, the catalyst before being subjected to the reaction shows only the diffraction peak attributed to γ-alumina, which is exactly the same as that shown in FIG. FIG. 3 shows an X-ray diffraction diagram of the catalyst after the reaction result was used in the reaction shown in FIG. It can be seen that the phase transition of γ-alumina to the α phase is remarkably advanced in the alumina-only catalyst. On the other hand, in the case of the catalyst containing silica, the effect of suppressing the phase transition of alumina is observed even when 1% by weight is added, and the phase transition of alumina is completely suppressed by adding 2.8% by weight. Titania also exhibits an alumina phase transition inhibitory effect, but its inhibitory effect is very small compared to silica.

From FIGS. 1 to 3, the remarkable effect of adding silica to the γ-alumina catalyst was confirmed in terms of both the catalyst life and the catalyst structure. However, excessive addition of silica
As shown by the amount of desorbed pyridine in Table 1, a decrease in the amount of catalytic acid and a decrease in catalytic activity are caused. Therefore, the amount of addition must be 10% by weight or less based on γ-alumina.

[0032]

The catalyst of the present invention can be used not only for CFCs,
Even in the case of PFC decomposition, which is difficult to decompose compared to CFCs, the high decomposition activity inherent in the γ-alumina catalyst is maintained for a long time. The adoption of the method became possible. PFC
Is a substance that is certain to be regulated in the future since the Global Warming Prevention Conference held in December 1997 set a future reduction target based on the magnitude of its global warming potential. is there. Despite the fact that its use is indispensable in the manufacturing process of semiconductors, which is also called rice in industry, this book has enabled stable catalytic decomposition of PFC, whose emission will be regulated in the future due to environmental considerations. The technical and social significance of the invention is significant.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of adding silica on suppressing the catalyst activity from decreasing.

FIG. 2 is an X-ray diffraction diagram of a catalyst before use.

FIG. 3 is an X-ray diffraction diagram of a post-reaction catalyst showing an effect of suppressing alumina phase transition by silica.

Continued on the front page (72) Inventor Tokuo Matsuzaki 5 of 1978 Kogushi, Ube-shi, Ube-shi, Yamaguchi Ube Research & Development Co., Ltd. Ube Research Laboratory F-term (reference) 4D048 AA11 AB03 BA03X BA06X BA13X BB01 4G069 AA08 BA03A BA03B BA04A BA04B BA45A BC02A BC02 CA02 CA19 DA05 EA02Y EB18Y EC22X EC22Y FA01 FB06 FB30 FC08

Claims (4)

[Claims] 1 to 100 parts by weight of γ-alumina.
A catalyst for decomposing a gaseous fluorine-containing compound comprising a perfluoro compound and / or a fluorocarbon compound, which is constituted by adding 0 parts by weight of silicon dioxide. 2. A gas containing a perfluoro compound and / or a CFC compound according to claim 1, wherein the γ-alumina constituting the catalyst is γ-alumina having a Na content of 1000 ppm or less. Catalyst for fluorine compound decomposition. (3) in the coexistence of oxygen and water, 300 to 10
A perfluoro compound and / or a catalyst heated to 00 ° C., wherein the catalyst is obtained by adding 1 to 10 parts by weight of silicon dioxide to 100 parts by weight of γ-alumina.
Alternatively, a method for decomposing a gaseous fluorine-containing compound comprising a fluorocarbon compound. 4. A gas containing a perfluoro compound and / or a CFC compound according to claim 4, wherein the γ-alumina constituting the catalyst is γ-alumina having a Na content of 1000 ppm or less. A method for decomposing fluorine compounds.