Purification method of octafluorocyclobutane

Abstract

Provided is an octafluorocyclobutane purification method by which mixed-in fluorocarbons can be removed to obtain high purity octafluorocyclobutane. This octafluorocyclobutane purification method comprises: a decomposition step in which oxygen gas or air is mixed into crude octafluorocyclobutane containing fluorocarbons, which are impurities, and octafluorocyclobutane to form a mixed gas in which the concentration of oxygen gas or air is 1-90 vol.%, the mixed gas is caused to contact, in a temperature of 100-500ºC, an impurity-decomposing agent that contains alumina and an alkaline earth metal compound and that decomposes fluorocarbons, and the fluorocarbons are decomposed; and a concentrating step in which gas containing oxygen gas or air (but excluding octafluorocyclobutane) is removed from the mixed gas in which the fluorocarbons were decomposed in the decomposition step, and the concentration of the octafluorocyclobutane is increased.

Description

Technical Field

[0001] The present invention relates to a method for purifying octafluorocyclobutane.

Background Art

[0002] Octafluorocyclobutane (FC-C318) is a colorless gas with a boiling point of -5.8°C and can be suitably used as a semiconductor material gas such as an etching gas and a cleaning gas. In recent years, with the improvement of the performance of electronic devices, in order to form circuit patterns by etching or the like with higher precision, a high-purity etching gas with impurities removed as much as possible has been demanded. As a method for producing octafluorocyclobutane, for example, a method of purifying octafluorocyclobutane produced as a by-product when producing tetrafluoroethylene or hexafluoropropene can be mentioned. Octafluorocyclobutane before purification usually contains impurities such as oxygen gas, nitrogen gas, carbon dioxide, and water vapor, and most of these impurities can be removed by distillation.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] Before purification, octafluorocyclobutane contains impurities such as perfluorobutane (FC-3110, boiling point -2°C), decafluoroisobutane (FC-i3110, boiling point 0°C), perfluorocyclobutene (FC-C316, boiling point 5°C), hexafluorobutadiene (FC-316, boiling point 6°C), and other fluorocarbons. Since these fluorocarbons have boiling points close to that of octafluorocyclobutane, it was very difficult to obtain high-purity octafluorocyclobutane with the concentration of these fluorocarbons reduced to less than 1 mass ppm by distillation. In particular, perfluorobutane and decafluoroisobutane are difficult to separate even using molecular sieves or adsorbents because their physical properties and molecular diameters are close to those of octafluorocyclobutane. In the present invention, fluorocarbons mean those other than octafluorocyclobutane among organic compounds having carbon-fluorine bonds.

[0005] Patent Document 1 discloses a technique for decomposing fluorocarbons using a decomposition reagent. However, when the technique disclosed in Patent Document 1 is applied to the purification of octafluorocyclobutane described above, there is a risk that octafluorocyclobutane will also be decomposed together with the fluorocarbons. Further, Patent Document 2 discloses a technique for removing fluorocarbons from crude octafluorocyclobutane containing fluorocarbons as impurities by bringing the crude octafluorocyclobutane into contact with an impurity decomposing agent composed of iron oxide and an alkaline earth metal compound under heating to decompose the fluorocarbons. However, in the technique disclosed in Patent Document 2, there is a risk that perfluorobutane and decafluoroisobutane cannot be sufficiently removed. An object of the present invention is to provide a method for purifying octafluorocyclobutane capable of obtaining high-purity octafluorocyclobutane by removing the mixed fluorocarbons.

Means for Solving the Problems

[0006] To solve the above problems, one aspect of the present invention is as follows [1] to [7]. [1] A method for purifying octafluorocyclobutane for removing the fluorocarbon from crude octafluorocyclobutane containing octafluorocyclobutane and a fluorocarbon as an impurity, mixing oxygen gas or air with the crude octafluorocyclobutane to obtain a mixed gas in which the concentration of the oxygen gas or air is 1% by volume or more and 90% by volume or less, and contacting the mixed gas with an impurity decomposer containing alumina and an alkaline earth metal compound and decomposing the fluorocarbon at a temperature of 100°C or more and 500°C or less to decompose the fluorocarbon; a decomposition step, removing a gas containing the oxygen gas or air (excluding octafluorocyclobutane) from the mixed gas in which the fluorocarbon has been decomposed in the decomposition step to increase the concentration of the octafluorocyclobutane; a concentration step, and a method for purifying octafluorocyclobutane comprising the above steps.

[0007] [2] The method for purifying octafluorocyclobutane according to [1], wherein the alkaline earth metal compound is at least one carbonate of magnesium, calcium, strontium, and barium, and the mass ratio of the alumina to the alkaline earth metal compound is 1:9 to 1:1.

[0008] [3] The method for purifying octafluorocyclobutane according to [1] or [2], wherein the impurity decomposer further contains at least one oxide of copper, tin, nickel, cobalt, chromium, molybdenum, tungsten, and vanadium, and the mass ratio of the total amount of the oxide to the total amount of the alumina and the alkaline earth metal compound is 1:99 to 5:95. [4] The method for purifying octafluorocyclobutane according to any one of [1] to [3], wherein the concentration of the fluorocarbon in the crude octafluorocyclobutane is 1 mass ppm or more and 10,000 mass ppm or less.

[0009] [5] The purification method of octafluorocyclobutane according to any one of [1] to [4], wherein the fluorocarbon is at least one of perfluorobutane, decafluoroisobutane, perfluorocyclobutene, hexafluorobutadiene, tetrafluoromethane, hexafluoroethane, hexafluoropropene, octafluoropropane, octafluorocyclopentene, and fluoroform.

[0010] [6] The purification method of octafluorocyclobutane according to any one of [1] to [5], wherein the concentration of octafluorocyclobutane in the crude octafluorocyclobutane is 10% by volume or more. [7] In the concentration step, at least one of a distillation method and a membrane separation method is used to remove the gas containing oxygen gas or air (excluding octafluorocyclobutane). The purification method of octafluorocyclobutane according to any one of [1] to [6]. [Effect of the Invention]

[0011] According to the present invention, fluorocarbons in crude octafluorocyclobutane can be removed to obtain high-purity octafluorocyclobutane. [Brief Description of the Drawings]

[0012]

Figure 1

Figure 2

[0013] One embodiment of the present invention will be described below. It should be noted that this embodiment shows an example of the present invention, and the present invention is not limited to this embodiment. Also, various changes or improvements can be made to this embodiment, and forms with such changes or improvements can also be included in the present invention.

[0014] Octafluorocyclobutane (C4F8) having a cyclic structure has relatively high chemical stability. The inventors of the present invention have found that by utilizing the chemical stability of octafluorocyclobutane, it is possible to remove the mixed fluorocarbon to obtain high-purity octafluorocyclobutane, and thus have completed the present invention.

[0015] That is, a method for purifying octafluorocyclobutane according to an embodiment of the present invention is a method for purifying octafluorocyclobutane that removes fluorocarbon from crude octafluorocyclobutane containing octafluorocyclobutane and fluorocarbon as an impurity, the method comprising a decomposition step of decomposing the fluorocarbon and a concentration step of increasing the concentration of octafluorocyclobutane.

[0016] The decomposition step is a step of mixing oxygen gas (O2) or air with the crude octafluorocyclobutane to form a mixed gas in which the concentration of oxygen gas or air is 1% by volume or more and 90% by volume or less, and then bringing the mixed gas into contact with an impurity decomposing agent that decomposes the fluorocarbon at a temperature of 100°C or higher and 500°C or lower to decompose the fluorocarbon. This impurity decomposing agent contains alumina and an alkaline earth metal compound. The concentration step is a step of removing a gas containing oxygen gas or air (excluding octafluorocyclobutane) from the mixed gas in which the fluorocarbon has been decomposed in the decomposition step to increase the concentration of octafluorocyclobutane.

[0017] Although fluorocarbon may be mixed as an impurity in octafluorocyclobutane, if the crude octafluorocyclobutane containing octafluorocyclobutane and fluorocarbon is purified by the method for purifying octafluorocyclobutane according to this embodiment, it is possible to selectively decompose the fluorocarbon while suppressing the decomposition of octafluorocyclobutane by the impurity decomposing agent, so that high-purity octafluorocyclobutane (hereinafter, may also be referred to as "purified octafluorocyclobutane") can be obtained.

[0018] For example, while suppressing the decomposition rate of octafluorocyclobutane to 1% by mass or less, the concentration of fluorocarbon in purified octafluorocyclobutane can be reduced to less than 1 ppm by mass. That is, the purity of the purified octafluorocyclobutane can be made 99.999% by mass or more. In particular, perfluorobutane (C4F 10 ) and decafluoroisobutane (C4F 10 ), etc., which were difficult to sufficiently remove by conventional purification methods such as distillation and adsorption methods, can be effectively removed. The purity of octafluorocyclobutane can be analyzed by gas chromatography using a thermal conductivity detector (TCD) or a flame ionization detector (FID) as a detector.

[0019] The high-purity octafluorocyclobutane obtained by the purification method of octafluorocyclobutane according to this embodiment is useful, for example, as an etching gas, a cleaning gas, etc. used in semiconductor manufacturing processes. The purification method of octafluorocyclobutane according to this embodiment will be described in more detail below.

[0020] 〔Crude octafluorocyclobutane〕 The crude octafluorocyclobutane to which the purification method of octafluorocyclobutane according to this embodiment can be applied is not particularly limited as long as it contains fluorocarbon as an impurity, and the purification method of octafluorocyclobutane according to this embodiment can be applied to octafluorocyclobutane produced by a known method or commercially available octafluorocyclobutane.

[0021] However, the concentration of the fluorocarbon in the crude octafluorocyclobutane is preferably 1 mass ppm or more and 10,000 mass ppm or less, more preferably 1 mass ppm or more and 1,000 mass ppm or less, and even more preferably 1 mass ppm or more and 100 mass ppm or less. If the concentration of the fluorocarbon in the crude octafluorocyclobutane is within the above range, the fluorocarbon can be decomposed at a temperature at which decomposition of octafluorocyclobutane is less likely to occur.

[0022] Also, the concentration of octafluorocyclobutane in the crude octafluorocyclobutane is preferably 10% by volume or more, more preferably 20% by volume or more, and even more preferably 50% by volume or more. If the concentration of octafluorocyclobutane in the crude octafluorocyclobutane is within the above range, the effect of suppressing the heat of reaction is achieved. Among the components contained in the crude octafluorocyclobutane, examples of components other than octafluorocyclobutane and fluorocarbon include oxygen gas and nitrogen gas (N2).

[0023] 〔Fluorocarbon〕 The fluorocarbon contained as an impurity in the crude octafluorocyclobutane is not particularly limited as long as it is other than octafluorocyclobutane among organic compounds having a carbon-fluorine bond. However, the method for purifying octafluorocyclobutane according to the present embodiment is particularly suitable for removing the following fluorocarbons.

[0024] That is, perfluorobutane, decafluoroisobutane, perfluorocyclobutene (C4F6), hexafluorobutadiene (C4F6), tetrafluoromethane (FC-14, CF4), hexafluoroethane (FC-116, C2F6), hexafluoropropene (FC-216, C3F6), octafluoropropane (FC-218, C3F8), octafluorocyclopentene (FC-418, C5F8), and fluoroform (HFC-23, CHF3). The fluorocarbons contained in the crude octafluorocyclobutane may be one type or two or more types.

[0025] 〔Impurity decomposing agent〕 In the method for purifying octafluorocyclobutane according to this embodiment, the impurity decomposing agent used contains alumina and an alkaline earth metal compound. Since the impurity decomposing agent contains alumina and an alkaline earth metal compound, the decomposition of octafluorocyclobutane is suppressed while the fluorocarbons are selectively and efficiently decomposed.

[0026] The type of alumina is not particularly limited, but in order to increase the gas adsorption amount, those having a specific surface area of 50 m 2 / g or more are preferable, and those having a specific surface area of 100 m 2 / g or more and 300 m 2 / g or less are more preferable. Also, the alumina is preferably in powder form. In order to be uniformly mixed with other substances, the average particle size of the alumina particles is preferably more than 0.1 μm and 100 μm or less, more preferably 30 μm or less for the upper limit value, and even more preferably 5 μm or less. In the present invention, the average particle size is the d50 value in the volume distribution, which corresponds to the median value that bisects the area under the frequency curve and can be obtained by measuring the particle size by laser diffraction.

[0027] Furthermore, the content of alkali metals (such as sodium and potassium) contained as impurities in the alumina is preferably 0.1% by mass or less, more preferably 0.01% by mass or less, and even more preferably 0.001% by mass or less in order to ensure the reactivity with the fluorocarbons.

[0028] The type of the alkaline earth metal compound is not particularly limited, but it is preferably at least one carbonate of magnesium, calcium, strontium, and barium, and more preferably a carbonate of calcium. When the alkaline earth metal compound is calcium carbonate (CaCO3), the fluorine gas generated by the decomposition of the fluorocarbon reacts with the calcium carbonate and is fixed as calcium fluoride (CaF2), so the fluorination of alumina is suppressed. As a result, the performance (activity) of the alumina in decomposing the fluorocarbon is likely to be maintained.

[0029] Also, the alkaline earth metal compound is preferably in powder form. In order to be uniformly mixed with other substances, the average particle size of the alkaline earth metal compound particles is preferably 1 μm or more and 100 μm or less, more preferably 30 μm or less for the upper limit value, and even more preferably 5 μm or less.

[0030] Furthermore, the content of alkali metals (such as sodium and potassium) contained as impurities in the alkaline earth metal compound is preferably 0.1% by mass or less, more preferably 0.01% by mass or less, and even more preferably 0.001% by mass or less in order to ensure the reactivity with the fluorocarbon.

[0031] The amount of the alkaline earth metal compound in the impurity decomposer can be the same as or more than the amount of alumina. The mass ratio of alumina to the alkaline earth metal compound is preferably 1:9 to 1:1, and more preferably 1:4 to 2:3. If the ratio of the amount of alumina to the total amount of alumina and the alkaline earth metal compound is 10% by mass or more, the number of active sites of the catalyst increases, so the performance of decomposing the fluorocarbon becomes higher. Also, if the ratio of the amount of alumina to the total amount of alumina and the alkaline earth metal compound is 50% by mass or less, the amount of the alkaline earth metal compound is 50% by mass or more, so the effective utilization coefficient of the impurity decomposer becomes higher.

[0032] In the method for purifying octafluorocyclobutane according to this embodiment, the impurity decomposer used may further contain at least one oxide of copper, tin, nickel, cobalt, chromium, molybdenum, tungsten, and vanadium, if desired. Among these metal oxides, copper oxide (Cu2O, CuO), tin oxide (SnO, SnO2), and vanadium oxide (V2O5) are more preferable, and copper oxide and tin oxide are even more preferable.

[0033] If the impurity decomposer has the above oxide, since the above oxide functions as a promoter, the ability of the impurity decomposer to decompose fluorocarbon is improved. Further, since carbon monoxide generated by the decomposition of fluorocarbon is oxidized to carbon dioxide, the amount of carbon monoxide generated can be reduced.

[0034] Also, the above oxide is preferably in powder form. In order to be uniformly mixed with other substances, the average particle size of the oxide particles is preferably 1 μm or more and 100 μm or less, more preferably 30 μm or less for the upper limit value, and even more preferably 5 μm or less.

[0035] Furthermore, the content of alkali metals (such as sodium and potassium) contained as impurities in the above oxide is preferably 0.1% by mass or less, more preferably 0.01% by mass or less, and even more preferably 0.001% by mass or less in order to ensure reactivity with fluorocarbon.

[0036] When the impurity decomposer contains the above oxide, the total amount of the above oxide may be small compared to the total amount of alumina and alkaline earth metal compounds, and the mass ratio of the total amount of the above oxide contained in the impurity decomposer to the total amount of alumina and alkaline earth metal compounds is preferably 1:99 to 5:95.

[0037] If the ratio of the amount of the above oxide to the total amount of the above oxide, alumina, and alkaline earth metal compound is 1% by mass or more, the additive effect of the above oxide is sufficiently exhibited. Further, even if the ratio of the amount of the above oxide to the total amount of the above oxide, alumina, and alkaline earth metal compound exceeds 5% by mass, the additive effect of the above oxide is saturated. If it is 5% by mass or less, since the total amount of alumina and alkaline earth metal compound becomes 95% by mass or more, the effective utilization coefficient of the impurity decomposer becomes high.

[0038] The shape of the impurity decomposer is not particularly limited, but it is preferably in the form of powder, particles, pellets, spheres, etc. And the average particle size of the impurity decomposer having such a shape is preferably 0.5 mm or more and 10 mm or less, and more preferably 1 mm or more and 5 mm or less. If the average particle size is within the above range, the surface area involved in the adsorption and diffusion of the mixed gas becomes appropriately large, so that the diffusion rate of the mixed gas to be purified is likely to be suitable.

[0039] Next, a manufacturing method of the impurity decomposer will be described with an example. The manufacturing method of the impurity decomposer is not particularly limited, but it can be manufactured by mixing the powder of alumina and the powder of alkaline earth metal compound. When mixing the above two powders, by adding a binder, it becomes easy to granulate even if the average particle sizes of the two powders are different. The type of the binder is not particularly limited, but for example, fine powder alumina and alum are preferable.

[0040] If fine powder alumina is added as a binder, the uniform electrodeposition property of the powder of alumina and the powder of alkaline earth metal compound is improved, and the powder of alkaline earth metal compound is easily granulated. The average particle size of the fine powder alumina as a binder is preferably 0.1 μm or less, and more preferably 0.05 μm or less.

[0041] Further, the alum is preferably in powder form, and its average particle size is preferably 0.1 μm or less. Furthermore, the content of alkali metals (such as sodium and potassium) contained as impurities in the alum is preferably 0.1% by mass or less, more preferably 0.01% by mass or less, in order to suppress the decrease in the number of reactive sites on the surface of the alumina and the decrease in the ability to decompose the fluorocarbon of the impurity decomposer.

[0042] When producing the impurity decomposer by mixing the alumina powder and the alkaline earth metal compound powder, first, water is added to the above two powders and kneaded, then pulverized and granulated to obtain particulate matter. Next, for example, the particulate matter is dried at a temperature of 100°C or higher and 200°C or lower in an inert gas such as nitrogen or in air to remove water from the particulate matter. Then, the dried particulate matter is fired to obtain a particulate impurity decomposer.

[0043] Since the hardness is improved by firing, when performing operations such as loading the particulate impurity decomposer into the reactor, the particulate impurity decomposer is suppressed from being crushed or pulverized. The firing temperature is preferably 400°C or higher and 1000°C or lower, more preferably 400°C or higher and 700°C or lower, still more preferably 500°C or higher and 700°C or lower. By firing at such a temperature, the alumina becomes pseudo-boehmite alumina. Within this temperature range, the decomposition of the alkaline earth metal compound and the decrease in the activity of the impurity decomposer are less likely to occur.

[0044] 〔Decomposition step〕 In the decomposition step, first, oxygen gas or air is mixed with crude octafluorocyclobutane to obtain a mixed gas. At this time, the concentration of oxygen gas or air in the mixed gas is 1% by volume or more and 90% by volume or less, preferably 10% by volume or more and 90% by volume or less.

[0045] When the fluorocarbon contained as an impurity in crude octafluorocyclobutane is perfluorobutane and the alkaline earth metal compound in the impurity decomposer is calcium carbonate, the decomposition reaction formula is as follows. However, carbon monoxide may also be generated in addition to carbon dioxide. As described above, carbon monoxide is converted to carbon dioxide by the presence of the aforementioned oxide. The carbon dioxide gas generated in this reaction is removed in a subsequent concentration step, and calcium fluoride is removed as a solid precipitate. 5CaCO3+ C4F 10 + (3 / 2)O2→ 5CaF2+ 9CO2

[0046] When carbonates of magnesium, strontium, or barium other than calcium carbonate are used as the alkaline earth metal compound, or when a fluorocarbon other than perfluorobutane is decomposed, a reaction that produces metal fluoride and carbon dioxide also occurs in the same manner. When the fluorocarbon has a hydrogen atom, water vapor (H2O) is generated, and when it has a chlorine atom, a solid precipitate of metal chloride is generated. An amount of oxygen gas corresponding to this reaction is required. However, if the content of the impurity fluorocarbon is about 1000 mass ppm or less, the concentration of oxygen gas or air in the mixed gas may be 1% by volume or more.

[0047] If the concentration of oxygen gas or air in the mixed gas is 1% by volume or more, the heat of reaction when the fluorocarbon decomposes will not become excessively large, and the activity of the impurity decomposer will be sufficiently high, so that the fluorocarbon can be sufficiently decomposed. Also, if it is 90% by volume or less, the concentration of crude octafluorocyclobutane in the mixed gas will be high, so that the purification of crude octafluorocyclobutane can be efficiently performed. If the concentration of oxygen gas or air in the mixed gas exceeds 90% by volume, the decomposition of octafluorocyclobutane is likely to occur, which is not preferable.

[0048] Next, the mixed gas is brought into contact with the impurity decomposing agent at a temperature of 100°C or higher and 500°C or lower to decompose the fluorocarbon. If the temperature at which the mixed gas is brought into contact with the impurity decomposing agent is 100°C or higher, the activity of the impurity decomposing agent becomes sufficiently high, so that the fluorocarbon can be sufficiently decomposed. Further, if it is 500°C or lower, the decomposition of octafluorocyclobutane by the impurity decomposing agent is suppressed. The temperature at which the mixed gas is brought into contact with the impurity decomposing agent needs to be 100°C or higher and 500°C or lower, but it is preferably 250°C or higher and 500°C or lower.

[0049] The pressure conditions when the mixed gas is brought into contact with the impurity decomposing agent are not particularly limited, and any of atmospheric pressure conditions, pressurized conditions, and reduced pressure conditions may be used. However, it is preferably -0.1 MPaG or higher and 0.3 MPaG or lower, and more preferably 0 MPaG or higher and 0.2 MPaG or lower. If it is 0.3 MPaG or lower, the liquefaction of octafluorocyclobutane is less likely to occur.

[0050] The purification treatment rate of the crude octafluorocyclobutane can be represented by the space velocity of the mixed gas in contact with the impurity decomposing agent. The space velocity of the mixed gas in contact with the impurity decomposing agent is 1 h -1 or more and 300 h -1 or less, and preferably 10 h -1 or more and 150 h -1 or less. Within the above range, the fluorocarbon can be efficiently decomposed, and it becomes easier to reduce the concentration of the fluorocarbon in the purified octafluorocyclobutane to less than 1 mass ppm.

[0051] The method of bringing the mixed gas into contact with the impurity decomposing agent is not particularly limited, and examples include a method of supplying crude octafluorocyclobutane to a reactor filled with the impurity decomposing agent. For example, a method of continuously flowing the mixed gas through a fixed bed equipped with the impurity decomposing agent can be adopted.

[0052] 〔Concentration step〕 The concentration step is a step of removing a gas containing oxygen gas or air (excluding octafluorocyclobutane) from the mixed gas obtained by decomposing fluorocarbons in the decomposition step, thereby increasing the concentration of octafluorocyclobutane. The mixed gas obtained by decomposing fluorocarbons in the decomposition step contains nitrogen gas, oxygen gas, and carbon dioxide, and in some cases, also contains water vapor and the like. Therefore, by removing these in the concentration step and increasing the concentration of octafluorocyclobutane, purified octafluorocyclobutane with a purity of 99.999% by mass or more can be obtained, for example. The method for removing nitrogen gas, oxygen gas, carbon dioxide, etc. is not particularly limited, but a distillation method or a membrane separation method can be employed.

[0053] As an example of the distillation conditions in the distillation method, the temperature of the low-boiling cut distillation column is -80°C or higher and 70°C or lower, and the operating pressure is -0.1 MPaG or higher and 1.0 MPaG or lower. The type of separation membrane used in the membrane separation method is not particularly limited as long as it is a membrane through which nitrogen gas, oxygen gas, carbon dioxide, etc. easily permeate and octafluorocyclobutane hardly permeates (more preferably, octafluorocyclobutane hardly permeates at all). For example, membranes formed of aromatic polyimide, silicone hollow fiber, or porous carbon fiber are preferred.

[0054] The environmental temperature during separation in the membrane separation method is preferably 0°C or higher and 100°C or lower, and more preferably 20°C or higher and 80°C or lower. If it is 0°C or higher, since the vapor pressure of octafluorocyclobutane is high, the concentration rate of octafluorocyclobutane becomes high. Also, if it is 100°C or lower, deterioration of the separation membrane hardly occurs.

[0055] The pressure when supplying the mixed gas obtained by decomposing fluorocarbons in the decomposition step to the separation membrane is preferably 0 MPaG or higher and 1 MPaG or lower, and more preferably 0 MPaG or higher and 0.5 MPaG or lower. If it is 1 MPaG or lower, it is not necessary to set a high temperature to prevent liquefaction of octafluorocyclobutane, so deterioration of the separation membrane hardly occurs.

Example

[0056] The present invention will be described more specifically by showing examples and comparative examples below. 〔Example 1〕 Using the purification apparatus of FIG. 1 in which the concentration step is performed by a membrane separation method, purification of crude octafluorocyclobutane composed of octafluorocyclobutane and perfluorobutane as an impurity was carried out. The concentration of perfluorobutane in the crude octafluorocyclobutane is 100 mass ppm.

[0057] First, the impurity decomposer used in Example 1 will be described. 30 parts by mass of alumina (average particle size 5 μm), 70 parts by mass of calcium carbonate, 5 parts by mass of copper oxide, and 0.1 part by mass of fine powder alumina (average particle size 0.1 μm) were formulated to a total of 30 g and mixed with a Henschel mixer. 10 mL of water was added thereto, granulated, dried at 110 ° C. for 3 hours, and further sieved to obtain a particulate impurity decomposer 7 having a particle size of 0.85 to 2.8 mm.

[0058] Next, the configuration of the purification apparatus of FIG. 1 and the method for purifying crude octafluorocyclobutane using the purification apparatus of FIG. 1 will be described. The purification apparatus of FIG. 1 includes a crude octafluorocyclobutane container 1 filled with crude octafluorocyclobutane, an air container 2 filled with air, a tubular reactor 6 containing an impurity decomposer 7, a heating device 8 for heating the reactor 6, and a membrane separation device 11 having a separation membrane (not shown).

[0059] The reactor 6 is a quartz tube having an inner diameter of 38 mm and a length of 1000 mm, and contains 350 g of the impurity decomposer 7. The impurity decomposer 7 is fired in the reactor 6 before being used for the purification of crude octafluorocyclobutane. That is, while flowing nitrogen gas through the reactor 6, it was heated at 350 ° C. for 12 hours and then at 600 ° C. for 3 hours to bake the impurity decomposer 7. The nitrogen gas was introduced from the upper side of the reactor 6 and exhausted from the lower side (not shown).

[0060] The crude octafluorocyclobutane in the crude octafluorocyclobutane container 1 was supplied to the mixed gas supply pipe 5 through the pipe 3 for crude octafluorocyclobutane while controlling the flow rate by a mass flow controller or the like. Also, the air in the air container 2 was supplied to the mixed gas supply pipe 5 through the air pipe 4 while controlling the flow rate by a mass flow controller or the like. Thereby, the crude octafluorocyclobutane and air were mixed in the mixed gas supply pipe 5 to obtain a mixed gas (the concentration of air in the mixed gas is 50% by volume, and both the volume ratio and the concentration are shown in Table 1) in which the volume ratio of crude octafluorocyclobutane to air is 50 / 50.

[0061] This mixed gas was supplied to the reactor 6 through the mixed gas supply pipe 5 and brought into contact with the impurity decomposer 7 controlled at 400 °C by the heating device 8. The pressure in the reactor 6 was set at normal pressure (0 MPaG). Also, the space velocity of the mixed gas was 15 Hh -1 , the linear velocity was 0.074 m / min, and the residence time was 4 minutes. Thereby, the perfluorobutane contained in the crude octafluorocyclobutane was decomposed (decomposition step).

[0062] Next, the mixed gas in which perfluorobutane was decomposed was supplied to the membrane separation device 11 through the decomposition gas pipe 9 and passed through a tubular separation membrane (not shown) at room temperature. This separation membrane is made of aromatic polyimide and has a length of 1 m. When the permeate side of the separation membrane was depressurized to -0.1 MPaG by the vacuum pump 14, components other than octafluorocyclobutane (nitrogen gas, oxygen gas, etc.) permeated through the separation membrane and flowed out to the permeate side pipe 13.

[0063] Then, octafluorocyclobutane did not permeate through the separation membrane and flowed out into the non-permeate side pipe 12. When the concentration of perfluorobutane in the octafluorocyclobutane discharged from the non-permeate side pipe 12 was measured by gas chromatography, it was less than 0.1 mass ppm. Also, when the ratio (decomposition rate) of the octafluorocyclobutane decomposed during purification was measured by gas chromatography, it was less than 1 mass%. The decomposition rate of the octafluorocyclobutane decomposed during purification was determined by the following formula. Decomposition rate (%) = 100 × {(molar amount of octafluorocyclobutane before the purification process) - (molar amount of octafluorocyclobutane after the purification process)} / (molar amount of octafluorocyclobutane before the purification process)

[0064] [Table 1]

[0065] [Examples 2 to 8 and Comparative Examples 1 and 2] Purification of crude octafluorocyclobutane was carried out in the same manner as in Example 1, except that the mass ratio of alumina / calcium carbonate in the impurity decomposing agent, the volume ratio of crude octafluorocyclobutane to air, the concentration of perfluorobutane in the crude octafluorocyclobutane, and the decomposition temperature were as shown in Table 1. The results (the concentration of perfluorobutane in the purified octafluorocyclobutane and the decomposition rate of octafluorocyclobutane) are shown in Table 1.

[0066] [Example 9] Purification of crude octafluorocyclobutane was carried out in the same manner as in Example 1, except that oxygen gas was mixed with the crude octafluorocyclobutane to form a mixed gas. The results (the concentration of perfluorobutane in the purified octafluorocyclobutane and the decomposition rate of octafluorocyclobutane) are shown in Table 2.

[0067] [Example 10] Purification of crude octafluorocyclobutane was carried out in the same manner as in Example 4, except that oxygen gas was mixed with the crude octafluorocyclobutane instead of air to obtain a mixed gas. The results (the concentration of perfluorobutane in the purified octafluorocyclobutane and the decomposition rate of octafluorocyclobutane) are shown in Table 2.

[0068]

Table 2

[0069] 〔Example 11〕 Purification of crude octafluorocyclobutane was carried out in the same manner as in Example 1, except that the fluorocarbon, which is an impurity in the crude octafluorocyclobutane, was decafluoroisobutane. As a result, the concentration of decafluoroisobutane in the purified octafluorocyclobutane was less than 0.1 mass ppm, and the decomposition rate of octafluorocyclobutane was less than 1 mass%.

[0070] 〔Example 12〕 Purification of crude octafluorocyclobutane was carried out in the same manner as in Example 1, except that the fluorocarbon, which is an impurity in the crude octafluorocyclobutane, was perfluorocyclobutene. As a result, the concentration of perfluorocyclobutene in the purified octafluorocyclobutane was less than 0.1 mass ppm, and the decomposition rate of octafluorocyclobutane was less than 1 mass%.

[0071] 〔Example 13〕 Purification of crude octafluorocyclobutane was carried out in the same manner as in Example 1, except that the fluorocarbon, which is an impurity in the crude octafluorocyclobutane, was hexafluorobutadiene. As a result, the concentration of hexafluorobutadiene in the purified octafluorocyclobutane was less than 0.1 mass ppm, and the decomposition rate of octafluorocyclobutane was less than 1 mass%.

[0072] 〔Example 14〕 Purification of crude octafluorocyclobutane was carried out in the same manner as in Example 1, except that the fluorocarbon, which is an impurity in the crude octafluorocyclobutane, was tetrafluoromethane. As a result, the concentration of tetrafluoromethane in the purified octafluorocyclobutane was less than 0.1 mass ppm, and the decomposition rate of octafluorocyclobutane was less than 1 mass%.

[0073] [Example 15] Purification of crude octafluorocyclobutane was carried out in the same manner as in Example 1, except that the fluorocarbon, which is an impurity in the crude octafluorocyclobutane, was hexafluoroethane. As a result, the concentration of hexafluoroethane in the purified octafluorocyclobutane was less than 0.1 mass ppm, and the decomposition rate of octafluorocyclobutane was less than 1 mass%.

[0074] [Example 16] Purification of crude octafluorocyclobutane was carried out in the same manner as in Example 1, except that the fluorocarbon, which is an impurity in the crude octafluorocyclobutane, was hexafluoropropene. As a result, the concentration of hexafluoropropene in the purified octafluorocyclobutane was less than 0.1 mass ppm, and the decomposition rate of octafluorocyclobutane was less than 1 mass%.

[0075] [Example 17] Purification of crude octafluorocyclobutane was carried out in the same manner as in Example 1, except that the fluorocarbon, which is an impurity in the crude octafluorocyclobutane, was octafluoropropane. As a result, the concentration of octafluoropropane in the purified octafluorocyclobutane was less than 0.1 mass ppm, and the decomposition rate of octafluorocyclobutane was less than 1 mass%.

[0076] [Example 18] Purification of crude octafluorocyclobutane was carried out in the same manner as in Example 1, except that the fluorocarbon, which is an impurity in the crude octafluorocyclobutane, is octafluorocyclopentene. As a result, the concentration of octafluorocyclopentene in the purified octafluorocyclobutane was less than 0.1 mass ppm, and the decomposition rate of octafluorocyclobutane was less than 1 mass%.

[0077] [Example 19] Purification of crude octafluorocyclobutane was carried out in the same manner as in Example 1, except that the fluorocarbon, which is an impurity in the crude octafluorocyclobutane, is fluoroform. As a result, the concentration of fluoroform in the purified octafluorocyclobutane was less than 0.1 mass ppm, and the decomposition rate of octafluorocyclobutane was less than 1 mass%.

[0078] [Example 20] Purification of crude octafluorocyclobutane was carried out in the same manner as in Example 1, except that a concentration step by distillation was carried out instead of the concentration step by the membrane separation method. The results (purity of the purified octafluorocyclobutane) are shown in Table 3.

[0079] Purification of the crude octafluorocyclobutane of Example 20 was carried out using the purification apparatus of FIG. 2 for performing the concentration step by distillation. The configuration of the purification apparatus of FIG. 2 and the method for purifying the crude octafluorocyclobutane using the purification apparatus of FIG. 2 will be described below. Note that the configuration of the purification apparatus of FIG. 2 is the same as that of the purification apparatus of FIG. 1 for the parts other than the part where the concentration step is carried out, so the description of the same parts is omitted. Also, in FIG. 2, the same parts as those of the purification apparatus of FIG. 1 are denoted by the same reference numerals as in FIG. 1.

[0080] First, a decomposition step was carried out in the same manner as in Example 1 to decompose perfluorobutane contained in crude octafluorocyclobutane. Then, the mixed gas in which perfluorobutane was decomposed was supplied to a distillation column 21 through a decomposition gas pipe 9 for distillation. The diameter of the distillation column 21 was 100 mm and the number of stages was 10. Further, the packing filled in the distillation column 21 was Cascade Mini Ring (registered trademark) No. 0P manufactured by Matsui Machine Co., Ltd., and the packing height was 2 m. The distillation conditions were a packing temperature of 1 °C, an operating pressure of 0.05 MPaG, and a cut rate of 5 mass%.

[0081] When distillation was carried out under the above conditions, low-boiling components flowed out to a low-boiling side pipe 22 connected to the top of the column, and high-boiling components flowed out to a high-boiling side pipe 23 connected to the bottom of the column. The high-boiling components emerging from the high-boiling side pipe 23 were purified octafluorocyclobutane. When the concentration of perfluorobutane in the purified octafluorocyclobutane was measured by gas chromatography, it was less than 0.1 mass ppm. That is, the purity of the purified octafluorocyclobutane was 99.999 mass%.

[0082]

Table 3

[0083] 〔Comparative Example 3〕 Purification of crude octafluorocyclobutane was carried out in the same manner as in Example 1, except that the concentration step by the membrane separation method was not carried out. The results (purity of purified octafluorocyclobutane) are shown in Table 3.

Explanation of Signs

[0084] 1 ··· Crude octafluorocyclobutane container 2 ··· Air container 6 ··· Reactor 7 ··· Impurity decomposing agent 11 ··· Membrane separation device 21 ··· Distillation column

Claims

1. A method for purifying octafluorocyclobutane by removing fluorocarbon, which is an impurity, from crude octafluorocyclobutane containing octafluorocyclobutane and the fluorocarbon, comprising: mixing oxygen gas or air with the crude octafluorocyclobutane to obtain a mixed gas in which the concentration of the oxygen gas or air is 1% by volume or more and 90% by volume or less, and bringing the mixed gas into contact with an impurity decomposer containing alumina and an alkaline earth metal compound and decomposing the fluorocarbon at a temperature of 100°C or more and 500°C or less to decompose the fluorocarbon (decomposition step); removing a gas containing the oxygen gas or air (excluding octafluorocyclobutane) from the mixed gas in which the fluorocarbon has been decomposed in the decomposition step to increase the concentration of the octafluorocyclobutane (concentration step); characterized by comprising: the alkaline earth metal compound being at least one carbonate of magnesium, calcium, strontium, and barium; and the fluorocarbon being at least one of perfluorobutane, decafluoroisobutane, perfluorocyclobutene, hexafluorobutadiene, tetrafluoromethane, hexafluoroethane, hexafluoropropene, octafluoropropane, octafluorocyclopentene, and fluoroform. A method for purifying octafluorocyclobutane.

2. The method for purifying octafluorocyclobutane according to Claim 1, wherein the mass ratio of the alumina to the alkaline earth metal compound is 1:9 to 1:

1.

3. The method for purifying octafluorocyclobutane according to Claim 1 or Claim 2, wherein the impurity decomposer further contains at least one oxide of copper, tin, nickel, cobalt, chromium, molybdenum, tungsten, and vanadium, and the mass ratio of the total amount of the oxides to the total amount of the alumina and the alkaline earth metal compound is 1:99 to 5:

95.

4. The method for purifying octafluorocyclobutane according to any one of Claims 1 to 3, wherein the concentration of the fluorocarbon in the crude octafluorocyclobutane is 1 ppm by mass or more and 10,000 ppm by mass or less.

5. The purification method of octafluorocyclobutane according to any one of claims 1 to 4, wherein the concentration of octafluorocyclobutane in the crude octafluorocyclobutane is 10% by volume or more.

6. The purification method of octafluorocyclobutane according to any one of claims 1 to 5, wherein in the concentration step, the gas containing oxygen gas or air (excluding octafluorocyclobutane) is removed by at least one of a distillation method and a membrane separation method.

Images