Patent Publication Number: US-11034636-B2

Title: Method for producing tetrafluoromethane

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP2018/047653 filed Dec. 25, 2018, claiming priority based on Japanese Patent Application No. 2018-005823 filed Jan. 17, 2018. 
     TECHNICAL FIELD 
     The present invention relates to a method for producing tetrafluoromethane. 
     BACKGROUND ART 
     Known methods for producing tetrafluoromethane include a method of reacting solid carbon with fluorine gas, a method of reacting a gaseous hydrocarbon with fluorine gas, and a method of reacting a mixture of a carbon material and a metal, a metal fluoride, or a fused alumina with fluorine gas (see PLTs 1 and 2). 
     The method of reacting solid carbon with fluorine gas is a combustion reaction with flames and generates a great amount of reaction heat, and thus the material itself of an outlet of fluorine gas or a reaction container may be reacted with fluorine gas to corrode. A reaction without flames may generate insufficient reaction heat, giving tetrafluoromethane at a lower yield. 
     The method of reacting a gaseous hydrocarbon with fluorine gas is also a combustion reaction with flames and generates a great amount of reaction heat, and thus the material itself of an outlet of fluorine gas or a reaction container may be reacted with fluorine gas to corrode. For a reaction without flames, fluorine gas is diluted with an inert gas such as nitrogen gas to suppress the reaction heat, but such a measure needs a step of separating and purifying the resulting tetrafluoroethane from an inert gas, and this increases the production cost unfortunately. 
     The method of reacting a mixture of a carbon material and a metal, a metal fluoride, or a fused alumina with fluorine gas is a method of mildly reacting a carbon material with fluorine gas, is not performed in such a reaction condition as to cut carbon-carbon bonds, and is unsuitable for the synthesis of tetrafluoromethane. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP 6-298681 A 
     PTL 2: JP 11-180706 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     As described above, in conventional methods for producing tetrafluoromethane, such a vigorous reaction as to damage a reaction apparatus is performed, whereas a reaction in a mild condition suppresses damages on a reaction apparatus but is unlikely to give tetrafluoromethane as a main product. The present invention is intended to provide a method that is for producing tetrafluoromethane, is unlikely to damage a reaction apparatus, and can produce tetrafluoromethane safely, inexpensively, and stably. 
     Solution to Problem 
     To solve the problems, aspects of the present invention are the following [1] to [3]. 
     [1] A method for producing tetrafluoromethane, the method including 
     introducing fluorine gas to a raw material liquid containing a fluorinated hydrocarbon represented by chemical formula C p H q Cl r F s  (in the chemical formula, p is an integer of 3 or more and 18 or less, q is an integer of 0 or more and 3 or less, r is an integer of 0 or more and 9 or less, and s is an integer of 5 or more and 30 or less) and having no carbon-carbon unsaturated bond and concurrently introducing a reaction inducer in a gas state, 
     in which the reaction inducer is reacted with the fluorine gas to induce a reaction of forming tetrafluoromethane from the fluorinated hydrocarbon and the fluorine gas and is at least one reaction inducer selected from a hydrocarbon gaseous at normal temperature and pressure and hydrogen gas. 
     [2] The method for producing tetrafluoromethane according to the aspect [1], in which the reaction inducer is hydrogen gas. 
     [3] The method for producing tetrafluoromethane according to the aspect [1] or [2], in which the fluorinated hydrocarbon is at least one fluorine-containing substance selected from a perfluorocarbon, a fluorohydrocarbon, a chlorofluorocarbon, a chlorofluorohydrocarbon, a chlorotrifluoroethylene polymer, and a perfluoropolyether. 
     Advantageous Effects of Invention 
     A method according to the present invention enables safe, inexpensive, and stable production of tetrafluoromethane while being unlikely to damage a reaction apparatus. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The Figure illustrates an embodiment of a method for producing tetrafluoromethane pertaining to the present invention and is a schematic view illustrating a structure of a reaction apparatus for tetrafluoromethane. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention will now be described. The present embodiment is merely an example of the present invention, and the present invention is not limited to the present embodiment. Various modifications or improvements can be made in the present embodiment, and such various modifications and improvements can be encompassed by the present invention. 
     In a conventional tetrafluoromethane production method of reacting activated carbon with fluorine gas to give tetrafluoromethane, routes through which reaction heat is removed from a reaction field include a route through which heat is discharged outside via a gas heated by reaction heat in an atmosphere and a route through which heat is discharged outside via a reaction apparatus heated by reaction heat (for example, an outlet of fluorine gas or a reaction container). A gas, which unfortunately has a small heat capacity, discharges a small amount of heat, and most of the reaction heat is consumed in heating a reaction apparatus. As a result, a reaction apparatus has a high temperature, thus the reaction apparatus is reacted with fluorine gas, and the reaction apparatus is corroded and damaged. 
     The inventors of the present invention have carried out intensive studies and consequently have found that by performing a reaction of forming tetrafluoromethane from a fluorinated hydrocarbon and fluorine gas in a liquid phase to reduce the temperature of a reaction field and by further using, in the reaction field, a reaction inducer that induces the reaction of forming tetrafluoromethane from a fluorinated hydrocarbon and fluorine gas, a cleavage reaction of carbon-carbon bonds of a fluorinated hydrocarbon, which is caused only at an extremely high temperature, can be caused in a liquid phase at a low temperature. 
     In other words, the inventors of the present invention have found that a fluorinated hydrocarbon is difficult to react with fluorine gas even when fluorine gas is blown into a liquid fluorinated hydrocarbon, but when a reaction inducer is used together, the reaction inducer is reacted with fluorine gas, and this reaction induces the reaction between a fluorinated hydrocarbon and fluorine gas at a low temperature to give tetrafluoromethane. 
     The mechanism is thought as follows: When fluorine gas and a gaseous reaction inducer are blown through outlets into a liquid fluorinated hydrocarbon, bubbles containing the fluorine gas and the reaction inducer are formed on the periphery of the outlet of fluorine gas, and in the bubbles, the fluorine gas is reacted with the reaction inducer. By the reaction heat of the reaction, the temperature of the bubbles increases. The reaction inducer is used only at the initial stage of the reaction to increase the temperature of bubbles. 
     On a gas-liquid interface between the bubbles having a higher temperature and the peripheral liquid phase, the fluorine gas in the bubbles is reacted with the fluorinated hydrocarbon in the liquid phase before the bubbles leave the outlet of fluorine gas. By the reaction heat of the reaction, the fluorinated hydrocarbon around the bubbles vaporizes to react with the fluorine gas in the bubbles. Accordingly, a region having a temperature higher than the temperature of the liquid phase by about 20° C. or more is formed around the outlet of fluorine gas (hereinafter called “high temperature reaction region”). When the supply of fluorine gas is continued, the reaction between a fluorinated hydrocarbon and fluorine gas continues in the high temperature reaction region. The resulting reaction heat continuously volatilizes the peripheral liquid phase (i.e., the fluorinated hydrocarbon), and thus the temperature increase of the liquid phase is supposed to be suppressed. 
     A method for producing tetrafluoromethane pertaining to the present embodiment includes introducing fluorine gas to a raw material liquid containing a fluorinated hydrocarbon represented by chemical formula C p H q Cl r F s  and having no carbon-carbon unsaturated bond (in the present description, also simply called “fluorinated hydrocarbon”) and concurrently introducing a reaction inducer in a gas state. The reaction inducer is reacted with fluorine gas to induce a reaction of forming tetrafluoromethane from the fluorinated hydrocarbon and the fluorine gas and is at least one reaction inducer selected from a hydrocarbon gaseous at normal temperature and pressure and hydrogen gas. In the chemical formula, p is an integer of 3 or more and 18 or less, q is an integer of 0 or more and 3 or less, r is an integer of 0 or more and 9 or less, and s is an integer of 5 or more and 30 or less. 
     Even when a fluorinated hydrocarbon is difficult to react with fluorine gas, the above mechanism allows the reaction between a reaction inducer and fluorine gas to induce the reaction between a fluorinated hydrocarbon and fluorine gas even at a low temperature. Hence, the temperature is unlikely to abnormally increase in a reaction field, fluorine gas is unlikely to damage a reaction apparatus, and tetrafluoromethane can be safely, inexpensively, and stably produced at a high yield. 
     In addition, the reaction apparatus is not required to be made from an expensive material having corrosion resistance against fluorine gas (for example, a nickel alloy, Hastelloy (registered trademark), or Monel (registered trademark)), and a reaction apparatus can be made from a general steel such as stainless steel, giving an inexpensive reaction apparatus. 
     The resulting tetrafluoromethane is useful, for example, as an etching agent for substrates and a cleaning agent for chambers in the semiconductor production process. 
     Hereinafter, the method for producing tetrafluoromethane pertaining to the present embodiment will be described in further detail. 
     (1) Fluorinated Hydrocarbon 
     The fluorinated hydrocarbon is a saturated hydrocarbon represented by chemical formula C p H q Cl r F s  and having no carbon-carbon unsaturated bond. The fluorinated hydrocarbon may be any of a linear hydrocarbon, a branched hydrocarbon, and a cyclic hydrocarbon and may be a compound containing no hydrogen atom or no chlorine atom. Examples of the fluorinated hydrocarbon include at least one fluorine-containing substance selected from a perfluorocarbon, a fluorohydrocarbon, a chlorofluorocarbon, a chlorofluorohydrocarbon, a chlorotrifluoroethylene polymer, and a perfluoropolyether. 
     Specific examples of the chlorotrifluoroethylene polymer include difreon oil (registered trademark), and specific example of the perfluoropolyether include Fomblin oil (registered trademark). The difreon oil is a polychlorotrifluoroethylene having flowability at normal temperature (a pour point of 5 to 15° C.) and having a molecular weight of about 1,000 or less. 
     The fluorinated hydrocarbon may be any of a gas, a liquid, and a solid at normal temperature and pressure but is preferably a liquid. In the present invention, the normal temperature means 25° C., and the normal pressure means 101.325 kPa (1 atm). 
     When a fluorinated hydrocarbon is a liquid, the fluorinated hydrocarbon may be used as a raw material liquid, or the fluorinated hydrocarbon may be mixed with a solvent, and the resulting mixture of the fluorinated hydrocarbon and the solvent can be used as the raw material liquid. When a fluorinated hydrocarbon is a gas or a solid, a solvent is required to be used in the reaction, and the fluorinated hydrocarbon is required to be mixed with the solvent to give a raw material liquid. In this case, a solid fluorinated hydrocarbon may be dissolved in a raw material liquid or may be dispersed in a powder form. Alternatively, an aggregated, fluorinated hydrocarbon may be contained in a raw material liquid. A gaseous fluorinated hydrocarbon may be dissolved in a raw material liquid or may be dispersed in a foam. In other words, in the method for producing tetrafluoromethane pertaining to the present embodiment, the synthesis reaction of tetrafluoromethane may be performed without solvent or may be performed in a solvent. 
     The above fluorinated hydrocarbon is an organic compound that is difficult to react with fluorine gas even when 100% by volume fluorine gas is blown at 40° C. and 101.325 kPa. The reaction formula of a fluorinated hydrocarbon with fluorine gas is represented by the following formula.
 
C p H q Cl r F s +(4 p+q+r−s )/2F 2 -&gt; p CF 4   +r ClF+ q HF
 
     In consideration of the reaction formula, to efficiently use the supplied fluorine gas to form tetrafluoromethane, q and r in the chemical formula C p H q Cl r F s  are preferably small values. 
     When p in the chemical formula C p H q Cl r F s  is 3 or more, a fluorinated hydrocarbon is not a gas at normal temperature and pressure in many cases (a liquid or a solid in many cases), thus is not required to be cooled or pressurized to make a gas into a liquid, and is economical. When p is 18 or less, a fluorinated hydrocarbon is not a solid at normal temperature and pressure in many cases (a gas or a liquid in many cases), thus is not required to be warmed to make a solid into a liquid, and is economical. p is an integer of 3 or more and 18 or less, preferably an integer of 3 or more and 10 or less, more preferably an integer of 3 or more and 5 or less, and is economically as small as possible because a smaller amount of fluorine gas is needed for production of 1 mol of tetrafluoromethane. 
     When q in the chemical formula C p H q Cl r F s  is 3 or less, a hydrogen atom is reacted with fluorine gas to form hydrogen fluoride as a by-product at a smaller rate, and such a condition is economical because a smaller amount of fluorine gas is needed for production of 1 mol of tetrafluoromethane. q is an integer of 0 or more and 3 or less, preferably an integer of 0 or more and 2 or less, and more preferably 0 or 1. To increase the reaction selectivity of tetrafluoromethane, the fluorinated hydrocarbon is more preferably a perfluorocarbon or a chlorofluorocarbon where q is 0. 
     When r in the chemical formula C p H q Cl r F s  is 0 or more and 9 or less, a fluorinated hydrocarbon is not a solid at normal temperature and pressure in many cases (a gas or a liquid in many cases), thus is not required to be warmed to make a solid into a liquid, and is economical. In addition, a chlorine atom is reacted with fluorine gas to form fluorine chloride as a by-product at a smaller rate, and such a condition is economical because a smaller amount of fluorine gas is needed for production of 1 mol of tetrafluoromethane. r is an integer of 0 or more and 9 or less and preferably an integer of 0 or more and 4 or less. Moreover, the fluorinated hydrocarbon is more preferably a perfluorocarbon where q and r are 0. 
     (2) Reaction Inducer 
     The reaction inducer is a compound easily reacted with fluorine gas. The reaction inducer is reacted with fluorine gas to induce the reaction of forming tetrafluoromethane from a fluorinated hydrocarbon and fluorine gas and is at least one reaction inducer selected from a hydrocarbon gaseous at normal temperature and pressure and hydrogen gas. The reaction inducer is introduced in a gas state into a raw material liquid, and may be an inducer that is dissolved in a raw material liquid or an inducer that is dispersed in a foam. 
     Examples of the reaction inducer include a saturated hydrocarbon that has 1 or more and 10 or less carbon atoms and is a gas at normal temperature and pressure, such as methane, ethane, and ethylene, and hydrogen gas. When a hydrocarbon is used, fluorine gas is reacted with the reaction inducer at a higher rate to reduce economic efficiency, and thus hydrogen gas is more preferred. The reaction inducer can be reacted with fluorine gas to form tetrafluoromethane in some cases. 
     The reaction inducer may be introduced in any amount as long as the reaction of forming tetrafluoromethane from a fluorinated hydrocarbon and fluorine gas can be induced, and the amount is preferably 15% by volume or less of the amount of fluorine gas introduced. When once the reaction inducer induces the reaction of forming tetrafluoromethane from a fluorinated hydrocarbon and fluorine gas, the reaction between the fluorinated hydrocarbon and the fluorine gas continues even after the introduction of the reaction inducer is subsequently stopped. Hence, after the reaction inducer induces the reaction of forming tetrafluoromethane from a fluorinated hydrocarbon and fluorine gas, the introduction of the reaction inducer to the raw material liquid may be stopped. 
     An outlet through which a gaseous reaction inducer is introduced to a raw material liquid is preferably provided near an outlet through which fluorine gas is introduced to the raw material liquid. The pipes through which fluorine gas and a reaction inducer are introduced to a raw material liquid in the reaction container may have any shape. For example, the pipe through which a gas is introduced to a raw material liquid may be a double pipe, and one of the fluorine gas and the reaction inducer may be introduced through an inner pipe, whereas the other may be introduced through an outer pipe. Alternatively, a pipe through which fluorine gas is introduced to a raw material liquid and a pipe through which a reaction inducer is introduced to a raw material liquid are provided in a reaction container, and the respective outlets of the pipes may be located closely. 
     (3) Reaction apparatus 
     An example of a reaction apparatus in which the method for producing tetrafluoromethane pertaining to the present embodiment is performed to give tetrafluoromethane will be described with reference to the Figure. 
     A reaction apparatus in the Figure includes a metal reaction container  11  in which a reaction for forming tetrafluoromethane is performed, a raw material liquid introduction pipe  21  through which a raw material liquid  1  containing a fluorinated hydrocarbon represented by chemical formula C p H q Cl r ,F s , and having no carbon-carbon unsaturated bond is introduced to the reaction container  11 , a fluorine gas pipe  23  having, at an end, an outlet  23   a  through which fluorine gas is introduced to a raw material liquid  1  in the reaction container  11 , a reaction inducer pipe  27  having, at an end, an outlet  27   a  through which at least one reaction inducer selected from a hydrocarbon gaseous at normal temperature and pressure and hydrogen gas is introduced in a gas state to a raw material liquid  1  in the reaction container  11 , and a gas discharge pipe  25  through which a gas phase in the reaction container  11  is discharged outside. Examples of the metal forming the reaction container  11  include stainless steel. 
     The reaction apparatus illustrated in the Figure further includes a circulator that extracts a portion of a raw material liquid  1  in the reaction container  11  during reaction outside the reaction container  11  and returns the solution into the reaction container  11 . In particular, the respective ends of a circular circulation pipe  28  are connected to the reaction container  11 , and a liquid circulating pump  15  installed on the circulation pipe  28  sends a raw material liquid  1 . The raw material liquid  1  extracted from the reaction container  11  can be returned through the circulation pipe  28  into the reaction container  11 . 
     A heat exchanger  19  is installed at a point midway of the circulation pipe  28  and at a downstream side of the liquid circulating pump  15  and can cool the extracted raw material liquid  1 . The raw material liquid  1  cooled by the heat exchanger  19  is returned into the reaction container  11 . In other words, the reaction apparatus illustrated in the Figure is configured to perform reaction while a portion of a raw material liquid  1  in the reaction container  11  is extracted and cooled and the cooled raw material liquid  1  is returned to the reaction container  11 . 
     A produced gas containing tetrafluoromethane formed by reaction can be discharged through the gas discharge pipe  25  outside the reaction container  11 . At a downstream side of the gas discharge pipe  25 , a heat exchanger  17  is installed and can cool a produced gas discharged from the reaction container  11 . Even when a fluorinated hydrocarbon as a material vaporizes and is contained in a produced gas, the fluorinated hydrocarbon can be liquified by cooling the produced gas with the heat exchanger  17  and can be returned to the reaction container  11 . Hence, an unreacted fluorinated hydrocarbon can be prevented from escaping from the reaction container  11  to the outside and from being lost. 
     The outlet  23   a  of the fluorine gas pipe  23  may have any shape, and the outlet  23   a  can be a round through-hole formed on the fluorine gas pipe  23 . The through-hole can have a diameter of, for example, 0.5 mm or more and 5 mm or less. The fluorine gas pipe  23  may have one or a plurality of outlets  23   a . Near the outlet  23   a , a temperature measurement device such as a thermocouple may be installed to measure the temperature near the outlet  23   a . The same is applied to the outlet  27   a  of the reaction inducer pipe  27 . 
     Near the outlet  23   a  of fluorine gas, a high temperature reaction region is formed as described above, and it is preferable that the high temperature reaction region be not in contact with a member of the reaction apparatus, such as a chamber wall of the reaction container  11 , a thermocouple, a stirring blade, and a baffle plate. A portion in contact with the high temperature reaction region has a higher temperature, and thus a member of the reaction apparatus may corrode. 
     The range of a high temperature reaction region can be represented by equation ln(LV)=a ln(L/D) (hereinafter also called equation (1)), where D is the diameter (mm) of an outlet  23   a , LV is the blowing linear velocity (m/s) of fluorine gas as converted at a temperature of 0° C. and a pressure of 0 MPaG, and L is the length (mm) (the length in a fluorine gas ejecting direction) of a formed high temperature reaction region. In the equation, ln is natural logarithm, and a is a constant and can be a value of 1.2 or more and 1.4 or less. From the equation, the length of an expected high temperature reaction region can be calculated, and this enables a design such that a high temperature reaction region is not in contact with a member of the reaction apparatus. 
     The direction along the major axis of a high temperature reaction region (the axis along a fluorine gas ejecting direction) may be any direction, and fluorine gas is preferably ejected from the outlet  23   a  at an angle of 90° (horizontal direction) or more and 180° or less so as to stably maintain a high temperature reaction region to a maximum extent, where the vertically downward direction is 0°, and the vertically upward direction is 180°. 
     The reaction apparatus includes a temperature measurement device (not illustrated) for measuring the temperature of a raw material liquid  1  and includes the circulator having the heat exchanger  19 , and thus the reaction can be performed while a raw material liquid  1  is cooled to control the temperature of the raw material liquid  1 . Accordingly, an abnormal temperature increase of a reaction field or damage on the reaction apparatus can be suppressed. The temperature of a raw material liquid  1  can be set at 0° C. or more and 200° C. or less, for example. The reaction pressure can be set, for example, at 0.01 MPaA (absolute pressure) or more and 1.0 MPaA (absolute pressure) or less and preferably normal pressure or more and 0.9 MPaG or less. 
     The reaction apparatus may include a device for measuring the liquid level of a raw material liquid  1 . For example, a device of measuring the liquid level from the differential pressure between a liquid phase and a gas phase in the reaction container  11  or a device of measuring the liquid level by using a float can be used. 
     As the synthesis reaction of tetrafluoromethane proceeds, the liquid level of a raw material liquid  1  decreases. If the liquid level can be measured, a raw material liquid  1  can be supplied into the reaction container  11  while the liquid level is continuously or intermittently monitored, and thus tetrafluoromethane can be continuously synthesized. 
     The concentration of fluorine gas used in the reaction is not limited to particular values, and 100% fluorine gas may be used, or a fluorine gas diluted with an inert gas such as nitrogen gas and argon may be used. Similarly, the concentration of the gaseous reaction inducer is not limited to particular values, and 100% gaseous reaction inducer may be used, or a gaseous reaction inducer diluted with an inert gas such as nitrogen gas and argon may be used. 
     To uniformly react the blown fluorine gas with a raw material liquid  1 , the reaction container  11  may include a stirrer having stirring blades for stirring the raw material liquid  1 . 
     EXAMPLES 
     The present invention will next be described more specifically with reference to examples and comparative examples. 
     EXAMPLE 1 
     Tetrafluoromethane was synthesized by using a reaction apparatus substantially the same as the reaction apparatus in the Figure except that a heat exchanger  19 , a circulation pipe  28 , and a liquid circulating pump  15  were not included. In an SUS reaction container having a capacity of 1 L, 600 mL (1,030 g) of perfluoro-n-octane having a boiling point of 103° C. at normal pressure was placed as a raw material liquid. 
     From an outlet having a diameter of 1 mm and provided at an end of the fluorine gas pipe, fluorine gas was introduced to the raw material liquid. Concurrently with the introduction of fluorine gas, hydrogen gas was introduced to the raw material liquid from an outlet having a diameter of 1 mm and provided at an end of the reaction inducer pipe. The outlet of the reaction inducer pipe was located near (at a position 2 mm apart from) the outlet of the fluorine gas pipe. The blowing flow rate of fluorine gas was set at 400 mL/min as converted at a temperature of 0° C. and a pressure of 0 MPaG, and the blowing linear velocity was set at 2.1 m/s. The blowing flow rate of hydrogen gas was set at 20 mL/min as converted at a temperature of 0° C. and a pressure of 0 MPaG, and the blowing linear velocity was set at 0.1 m/s. In the reaction, the introduction amount of hydrogen gas was 5% by volume relative to the introduction amount of fluorine gas. 
     When the value a in equation (1) is 1.27, a high temperature reaction region having a length of 1.8 mm is expected to be formed for each outlet. Hence, in the range where the high temperature reaction regions were to be formed, any member of the reaction apparatus was not placed except a single thermocouple. 
     When the introduction of fluorine gas and hydrogen gas was started, the temperature of the fluorine gas outlet increased to 200° C., and thus the introduction of hydrogen gas was stopped. The reaction continued while the reaction container was cooled from the outside, and the reaction was performed while the temperature of the raw material liquid was maintained at 25° C., and the reaction pressure was maintained at normal pressure. As a result, the reaction was performed while the temperature of the fluorine gas outlet was maintained at 200° C. even after the introduction of hydrogen gas was stopped. 
     The produced gas was sampled and analyzed, and consequently the produced gas contained 95% by volume tetrafluoromethane and 5% by volume hexafluoroethane. Of the reacted perfluoro-n-octane, 95 by mole was converted into tetrafluoromethane, and the yield of tetrafluoromethane was 95%. No unreacted fluorine gas was detected in the produced gas. 
     After the completion of the reaction, the outlet of the fluorine gas pipe was observed. No corrosion or the like was observed, and the outlet maintained the same shape as the shape before the reaction. In addition, no corrosion or the like was observed on the thermocouple for measuring the temperature of the fluorine gas outlet or the raw material liquid and on the reaction container. 
     Comparative Example 1 
     Reaction was performed in the same manner as in Example 1 except that no reaction inducer (hydrogen gas) was introduced. Although introduction of fluorine gas was continued for 5 hours, the temperature of the fluorine gas outlet was not changed, and the entire amount of the introduced fluorine gas was discharged in an unreacted state from the gas discharge pipe for discharging a gas phase in the reaction container to the outside. In the discharged fluorine gas, no tetrafluoromethane was detected, and the yield of tetrafluoromethane was 0%. 
     EXAMPLE 2 
     Tetrafluoromethane was synthesized by using a reaction apparatus substantially the same as the reaction apparatus in the Figure except that a heat exchanger  19 , a circulation pipe  28 , and a liquid circulating pump  15  were not included. In an SUS reaction container having a capacity of 1 L, 600 mL (1,000 g) of a chlorofluorobutane mixture having the following formula was placed as a raw material liquid. In other words, the chlorofluorobutane mixture is a mixture containing 20% by mass trichloroheptafluorobutane, 5% by mass dichlorooctafluorobutane, 70% by mass pentachloropentafluorobutane, and 5% by mass tetrachloropentafluorobutane. The chlorofluorobutane mixture is a by-product formed when tetrachlorohexafluorobutane is synthesized by reaction of tetrachlorobutane with fluorine gas. 
     From an outlet having a diameter of 1 mm and provided at an end of the fluorine gas pipe, fluorine gas was introduced to the raw material liquid. Concurrently with the introduction of fluorine gas, hydrogen gas was introduced to the raw material liquid from an outlet having a diameter of 1 mm and provided at an end of the reaction inducer pipe. The outlet of the reaction inducer pipe was located near (at a position 2 mm apart from) the outlet of the fluorine gas pipe. The blowing flow rate of fluorine gas was set at 600 mL/min as converted at a temperature of 0° C. and a pressure of 0 MPaG, and the blowing linear velocity was set at 3.2 m/s. The blowing flow rate of hydrogen gas was set at 60 mL/min as converted at a temperature of 0° C. and a pressure of 0 MPaG, and the blowing linear velocity was 0.32 m/s. In the reaction, the introduction amount of hydrogen gas was 10% by volume relative to the introduction amount of fluorine gas. 
     When the introduction of fluorine gas and hydrogen gas was started, the temperature of the fluorine gas outlet increased to 300° C., and thus the introduction of hydrogen gas was stopped. The reaction continued while the reaction container was cooled from the outside, and the reaction was performed while the temperature of the raw material liquid was maintained at 60° C., and the reaction pressure was maintained at normal pressure. As a result, the reaction was performed while the temperature of the fluorine gas outlet was maintained at 300° C. even after the introduction of hydrogen gas was stopped. 
     The produced gas was sampled and analyzed, and consequently the produced gas contained 80% by volume tetrafluoromethane and 20% by volume chlorotrifluoromethane. No unreacted fluorine gas was detected in the produced gas, but chlorine fluoride and hydrogen fluoride were detected. 
     After the completion of the reaction, the outlet of the fluorine gas pipe was observed. No corrosion or the like was observed, and the outlet maintained the same shape as the shape before the reaction. In addition, no corrosion or the like was observed on the thermocouple for measuring the temperature of the fluorine gas outlet or the raw material liquid and on the reaction container. 
     Comparative Example 2 
     Reaction was performed in the same manner as in Example 2 except that no reaction inducer (hydrogen gas) was introduced. Although introduction of fluorine gas was continued for 5 hours, the temperature of the fluorine gas outlet was not changed, and the entire amount of the introduced fluorine gas was discharged in an unreacted state from the gas discharge pipe for discharging a gas phase in the reaction container to the outside. In the discharged fluorine gas, no tetrafluoromethane was detected, and the yield of tetrafluoromethane was 0%. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  raw material liquid 
               11  reaction container 
               23  fluorine gas pipe 
               23   a  outlet 
               27  reaction inducer pipe 
               27   a  outlet