JP2012057255A - Method for electrolytic synthesis of fluorine-containing substance using anode for electrolysis - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an anode material for electrolysis in an electrolytic bath containing anhydrous hydrofluoric acid, which is free from the occurrence of an anode effect, does not remarkably produce sludge due to the dissolution of an electrode, is capable of suppressing the production of CFand capable of stably continuing electrolysis without causing the breakdown of the electrode.SOLUTION: An electrolytic bath containing anhydrous hydrofluoric acid or anhydrous hydrofluoric acid to which a material to be fluorinated is added is subjected to electrolysis using an electrode for electrolysis formed by coating at least part of a conductive material base including glassy carbon with a conductive diamond film, whereby fluorine or a fluorine-containing compound is electrolytically synthesized.

Description

In the present invention, in electrolysis in an electrolytic bath containing anhydrous hydrogen fluoride, the anode effect does not occur even when a high current density is applied, no significant sludge is generated due to electrode dissolution, and CF ₄ is generated. It is related with the electrolysis method using the anode material which can suppress electrolysis and can continue electrolysis stably without raise | generating electrode collapse.

  An electrolytic method for synthesizing an inorganic fluorine compound, an organic fluorine compound, or fluorine gas by electrolysis using a solution in which an inorganic or organic compound is dissolved in anhydrous hydrogen fluoride (anhydrous HF) as an electrolytic bath has been put into practical use industrially. .

  In addition, anhydrous HF has insufficient conductivity, so when a high current density operation is intended, an alkali metal fluoride such as potassium fluoride (KF) or an alkaline earth metal fluoride is often used as a conductive aid. Is added to the electrolytic bath.

Fluorine gas (F ₂ ), which is widely used as a fluorinating agent in resin synthesis, chemical synthesis, and pharmaceutical synthesis, is a KF / HF electrolytic bath in which potassium fluoride (KF) is added to anhydrous HF as a conductive aid. Nitrogen trifluoride gas (NF ₃ ), which is synthesized by electrolyzing and is widely used as a dry etchant or cleaning gas in the semiconductor field, etc., is NH ₄ F · HF in which ammonia is dissolved in anhydrous HF as a fluorinated product. It is synthesized by electrolyzing a system electrolytic bath.

  In addition, a method of synthesizing a perfluoro compound by using a solution in which an inorganic or organic compound is dissolved in anhydrous HF as an fluorinated substance as an electrolytic bath and performing electrolysis at a voltage lower than the generation of fluorine gas is known as the Simmons method. It has been.

  In any of these electrolysis methods, the material that can be used for the electrolytic cell and the electrode material is limited due to the remarkable corrosiveness of anhydrous HF, and in particular, the material that can be used as the anode material is limited to nickel or carbon.

  When nickel is used for the anode, its consumption is remarkably accelerated, so carbon is frequently used for the anode.

  The carbon anode has an advantage that the consumption of the electrode seen in the nickel anode is small. However, there is often a problem that it is difficult to continue the electrolysis due to the non-moving phenomenon of the electrode, the so-called anode effect.

  In the anodic reaction by the carbon anode, a graphite fluoride forming reaction proceeds together with a discharge reaction of fluoride ions which is a target reaction. On the other hand, the produced graphite fluoride is partially decomposed by thermal decomposition due to Joule heat generated by the electrode reaction or disproportionation reaction. Since covalently bonded graphite fluoride has low wettability with an electrolytic bath, when the generation rate of graphite fluoride is higher than the decomposition rate, the electrode surface is coated with fluorinated graphite, and an anodic effect is generated. Since the generation rate of graphite fluoride depends on the current density, the higher the current density, the easier the anode effect occurs.

  When water is present in the electrolytic bath, the decomposition reaction of water, which has a base potential, takes precedence over the discharge reaction of fluoride ions. At this time, graphite oxide is generated by the reaction of water and the carbon anode. Since this graphite oxide is chemically unstable, the substitution reaction with fluorine easily proceeds and graphite fluoride is produced. Therefore, the higher the water concentration in the electrolytic bath, the more the generation of graphite fluoride is promoted and the anode effect is more likely to occur.

  Therefore, in order to suppress the anode effect at the carbon anode, it is necessary to minimize the water concentration in the electrolytic bath and to perform electrolysis at a current density equal to or lower than the current density (critical current density) at which the anode effect occurs. is necessary. In actual industrial electrolysis, complicated operations such as dehydration electrolysis are performed for the former purpose, and the operating current density is limited for the latter purpose. As a result, the production rate of the target product is limited, and the profitability improvement of electrolytic synthesis is hindered.

  On the other hand, since anhydrous HF permeates into the carbon electrode and the electrode expands, the electrode often breaks or collapses due to this expansion. In order to prevent the penetration of HF into the carbon electrode, a method of coating the surface of the electrode with nickel by thermal spraying or plating has been put into practical use. However, since there is a problem with nickel as described later, an essential solution Has not been found. In addition, the vapor pressure of anhydrous HF can be reduced by increasing the concentration of the conductive assistant in the electrolytic bath such as KF, but the operating temperature is increased because the melting point of the electrolytic bath increases due to the increase of the conductive assistant concentration. There is a limit.

  Nickel is widely used as an anode in electrolysis in an electrolytic bath in which fluorinated substances such as ammonia, alcohol, and amine are added to anhydrous HF. Nickel anodes have the advantage that the anode effect seen with carbon anodes does not occur, but wear progresses during electrolysis.

  The consumed amount of the nickel anode reaches 3 to 5% of the energized amount, and the replacement cost of the consumed nickel anode is almost equal to the electrolytic power cost. Further, since nickel dissolves in the electrolytic bath, the viscosity of the electrolytic bath increases and it becomes difficult to control the temperature of the electrolytic bath, so that regular electrolytic bath replacement is also required. As described above, in the nickel anode, it is indispensable to replace the anode, replace the electrolytic bath, and stop the operation, which is a factor that hinders the improvement of electrolytic synthesis profitability.

  Patent Document 1 discloses an electrode in which the surface of a silicon substrate is covered with a boron-doped diamond film and an electrolytic fluorination method using the electrode. Patent Document 2 discloses an electrode in which the surface of a conductive carbon material substrate is coated with conductive diamond and a method for electrolytic synthesis of a fluorine-containing substance using the electrode. Patent Document 3 discloses a graphite plate, an electrode obtained by coating a conductive substrate obtained by coating glassy carbon on nickel or stainless steel with conductive diamond, and a method for electrolytic synthesis of a fluorine-containing substance using the electrode.

JP 2000-204492 A JP 2006-249557 A International Publication No. 2007/083740 (paragraphs 0065 and 0068, Examples 1 and 2)

  As a result of intensive studies, the inventors of the present invention have found that in the electrode in which the surface of the silicon substrate described in Patent Document 1 is coated with a boron-doped diamond film, anhydrous HF in an electrolytic bath permeating through pinholes inevitably generated in the boron-doped diamond film. It is difficult to maintain the electrode structure due to the corrosion of the silicon substrate, and the conductive carbon material surface described in Patent Document 2 is coated with conductive diamond, and the graphite plate described in Patent Document 3 is electrically conductive. When the anhydrous HF concentration in the electrolytic bath is high, particularly when the molar concentration of HF in the electrolytic bath is more than three times the molar concentration of the fluorinated substance or the conductive auxiliary, The subject that the electrode base body collapse | disintegrated by anhydrous HF osmose | permeating material and a graphite board was discovered.

  As described above, as an electrode for electrolysis in an electrolytic bath containing anhydrous HF, there is a demand for an electrode that does not cause the anode effect and collapse seen in the carbon electrode and does not progress the wear seen in the nickel anode. .

  In the present invention, an electrolysis material in which a conductive material made of glassy carbon is used as a base and at least a part of the base is covered with a conductive diamond film is used for electrolysis in which fluorinated material is added to anhydrous HF or anhydrous HF. A method for electrolytically synthesizing fluorine or a fluorine-containing compound by electrolyzing a bath is provided.

The present invention is described in detail below.
As a result of intensive studies, the present inventors have found that an electrode for electrolysis in which at least a part of a conductive substrate made of glassy carbon is coated with a conductive diamond film is electrolyzed in an electrolysis bath containing anhydrous HF. It was discovered that even when the anhydrous HF concentration in the bath was high, the anode effect, electrode consumption, and electrode collapse did not occur, and the electrode could continue electrolysis for a long time.

  Glassy carbon is a carbon material with a glass-like appearance that is produced by using a thermosetting resin such as cellulose, cellulose resin, or furan resin as a precursor and then solid-phase carbonizing the precursor after molding. These include high hardness, chemical stability, abrasion resistance, gas and liquid impermeability, and the like. The structure is homogeneous and amorphous with no crystal form, and there are many pores, but most of them are closed, so there are almost no open pores. In a conductive diamond electrode using glassy carbon having such characteristics as a conductive substrate, anhydrous HF hardly penetrates into the substrate even in an electrolytic bath having a high HF concentration, and electrode expansion and subsequent electrode collapse occur. Does not occur.

  In addition, by covering a part of the substrate surface with conductive diamond, the anode effect and electrode wear due to the generation of graphite fluoride do not occur.

For example, in order to produce perfluorotrimethylamine as a fluorine-containing compound, it can be efficiently synthesized by using an electrolytic bath having a composition of (CH ₃ ) ₄ NF · 5HF as anhydrous HF and a fluorinated product. is there. When using a nickel electrode, there is an inconvenience of immobilization, and it is necessary to add CsF · 2HF. However, even when CsF · 2HF is added, electrode wear proceeds. When carbon is used as the anode, the anode effect occurs and the electrode collapses due to the penetration of anhydrous HF into the substrate. When an electrode having a conductive carbon material substrate surface coated with conductive diamond is used, the electrode collapses due to permeation of anhydrous HF into the substrate.

  On the other hand, when a conductive material made of glassy carbon is used as a base and an electrode in which at least a part of the base is covered with a conductive diamond film is used, the anode effect, electrode wear, and electrode collapse do not occur. Long-term electrolysis can be continued.

  In the present invention, in the synthesis of inorganic fluorine compounds, organic fluorine compounds, and fluorine gas by electrolysis of an electrolytic bath containing anhydrous HF, a conductive material made of glassy carbon is used as a base, and at least a part of the surface is made conductive. A method of electrolytic synthesis of fluorine or a fluorine-containing compound using an electrode coated with a diamond film is proposed, and therefore, even in an electrolytic bath having a high concentration of anhydrous HF, the anode effect, electrode consumption, and electrode collapse are prevented. It does not occur and electrolysis can be continued for a long time, and the productivity of inorganic fluorine compounds, organic fluorine compounds, and fluorine gas is improved.

The details of the electrode for electrolysis proposed by the present invention will be described.
The conductive substrate of the electrode according to the present invention is made of glassy carbon, and its shape is not particularly limited, and a plate shape, a rod shape, a pipe shape, a spherical shape, or the like can be used. The gas permeability of the glassy carbon is preferably 10 ⁻⁷ cm ² / sec or less, more preferably 10 ⁻¹⁰ cm ² / sec or less.

  The coating method of the conductive diamond film for coating at least a part of the surface of the conductive substrate is not particularly limited, and any method can be used. As typical manufacturing methods, a hot filament CVD (chemical vapor deposition) method, a microwave plasma CVD method, a plasma arc jet method, a physical vapor deposition (PVD) method, and the like can be selected.

  When coating a conductive diamond film, any method uses a mixed gas of hydrogen gas and a carbon source which is a diamond raw material. In order to impart conductivity to diamond, elements having different valences (hereinafter referred to as dopants) are used. Add a small amount. The dopant is preferably boron, phosphorus or nitrogen, and the preferred content is 1 to 100,000 ppm, more preferably 100 to 10,000 ppm. Further, regardless of which diamond film coating method is used, the coated conductive diamond film is polycrystalline, and amorphous carbon and graphite components remain in the diamond film.

Stability amorphous carbon or graphite component in view of the diamond film is preferably lesser, in Raman spectroscopic analysis, a peak intensity existing near 1332 cm ^-1 attributable to the diamond (range 1312~1352cm ^-1) I (D) And the ratio I (D) / I (G) of the peak intensity I (G) near 1560 cm ⁻¹ (in the range of 1540 to 1580 cm ⁻¹ ) attributed to the G band of graphite is 1 or more, and the diamond content Is preferably greater than the graphite content.

A hot filament CVD method, which is a typical conductive diamond film coating method, will be described.
An organic compound such as methane, alcohol, and acetone as a carbon source and a dopant are supplied to the filament together with hydrogen gas. The filament is heated to a temperature of 1800-2800 ° C. at which hydrogen radicals and the like are generated, and the conductive substrate is disposed so as to be in a temperature region (750 to 950 ° C.) where diamond is deposited in this atmosphere. The supply rate of the mixed gas depends on the size of the reaction vessel, but the pressure is preferably 15 to 760 Torr.

  Polishing the surface of the conductive substrate is preferable because adhesion between the substrate and the diamond layer is improved, and an arithmetic average roughness Ra of 0.1 to 15 μm and a maximum height Rz of 1 to 100 μm are preferable. Also, nucleating diamond powder on the surface of the substrate is effective for uniform diamond film growth. A diamond fine particle layer having a particle size of usually 0.001 to 2 μm is deposited on the substrate. The thickness of the diamond film can be adjusted by the deposition time, but is preferably 1 to 10 μm from the viewpoint of economy.

As the material of the electrolytic cell, mild steel, nickel alloy, fluorine resin, or the like can be used from the viewpoint of corrosion resistance against anhydrous HF. In order to prevent mixing of F ₂ or fluorine compound synthesized at the anode and hydrogen gas generated at the cathode, it is preferable that the anode side and the cathode side are all or partly partitioned by a partition wall, a diaphragm and the like.

  A trace amount of anhydrous HF accompanying the inorganic, organic fluorine compound, or fluorine gas generated at the anode can be removed by passing it through a column filled with granular sodium fluoride. Also, by-products such as trace amounts of nitrogen, oxygen, and dinitrogen monoxide are generated. Of these, dinitrogen monoxide is removed by passing water and sodium thiosulfate, and oxygen is removed by activated carbon. Inorganic, organic fluorine compounds, or fluorine gas with little by-product contamination can be obtained.

  EXAMPLES The present invention will be specifically described below based on examples, but the present invention is not limited to these examples.

Example 1
A glassy carbon plate was used as the conductive substrate, and a conductive diamond electrode was prepared using a hot filament CVD apparatus under the following conditions.

  First, the surface of the substrate was polished using an abrasive made of diamond particles having a particle diameter of 1 μm. The surface roughness Ra was 0.2 μm, and the surface roughness Rz was 6 μm. Next, diamond particles having an average particle diameter of 4 nm were nucleated on the substrate surface, and then mounted on a hot filament CVD apparatus. While supplying a gas mixture of hydrogen gas with 1 vol% methane gas and 0.5 ppm trimethylboron gas at a rate of 5 liters / min, the internal pressure is maintained at 75 Torr and power is applied to the filament. The temperature was raised to 2400 ° C. At this time, the substrate temperature was 860 ° C.

The CVD operation was continued for 8 hours, and the substrate was analyzed after the completion of the CVD operation. It was confirmed that diamond is deposited by Raman spectroscopy and X-ray diffraction analysis, the peak intensity ratio of the peak intensity and 1560 cm ^-1 in 1332 cm ^-1 in the Raman spectroscopic analysis was 1: 0.4. Further, when a part of the substrate was broken and observed by SEM, the thickness was about 4 μm.

  The produced conductive diamond electrode was attached as an anode to an anhydrous HF bath maintained at 0 ° C., a nickel plate as a cathode and platinum as a reference electrode, and a current-potential curve was measured by constant current chronopotentiometry. .

Immediately after the start of the measurement, the anode potential when applying a current density of 5 mA / cm ² was 0.6V. Thereafter, the anode potential was measured while increasing the current density by 5 mA / cm ^{2, and the} anode potential when the current density was 200 mA / cm ² was 3.2 V. As an electrolytic product, fluorine gas was obtained.

  When the electrolysis was stopped and the anode was taken out and observed for appearance, electrode collapse and peeling of the conductive diamond film were not observed.

(Comparative Example 1)
The current-potential curve was measured in an anhydrous HF bath maintained at 0 ° C. under the same electrolysis conditions as in Example 1 except that a graphite plate was used as the anode.

Immediately after the start of the measurement, the anode potential when a current density of 5 mA / cm ^{2 was} applied was 0.7V. Thereafter, when the anode potential was measured while increasing the current density by 5 mA / cm ² , the anode potential suddenly increased when the current density was 70 mA / cm ^{2 and} almost no current flowed, making it difficult to continue electrolysis.

  When the electrolysis was stopped and the anode was taken out, the anode was shattered in the electrolytic cell.

(Comparative Example 2)
The current-potential curve was measured in an anhydrous HF bath maintained at 0 ° C. under the same electrolysis conditions as in Example 1 except that a nickel plate was used as the anode.

Immediately after the start of the measurement, the anode potential when applying a current density of 5 mA / cm ² was 0.6V. Thereafter, when the anode potential was measured while increasing the current density by 5 mA / cm ^{2, the} anode potential increased with time when the current density was applied at 50 mA / cm ² , and eventually the current almost did not flow, making it difficult to continue electrolysis. became.

When the electrolysis was stopped and the anode was taken out, no electrode collapse was observed. When the surface of the electrode was analyzed, Ni—F bonds were observed, and the formation of an insulating NiF ₂ coating was presumed on the electrode surface.

(Comparative Example 3)
A conductive diamond electrode was prepared in the same procedure as in Example 1 except that a silicon plate was used as the conductive substrate.

  The current-potential curve was measured in an anhydrous HF bath maintained at 0 ° C. under the same electrolysis conditions as in Example 1 except that the electrode was used as the anode.

Immediately after the start of the measurement, the anode potential when applying a current density of 5 mA / cm ² was 0.6V. Thereafter, the anode potential was measured while increasing the current density by 5 mA / cm ^{2, and the} anode potential when the current density was 200 mA / cm ² was 3.8 V.

  When the electrolysis was stopped and the anode was taken out and the appearance was observed, a portion of the diamond film immersed in the electrolytic bath was partially lost, and corrosion of the silicon substrate surface where the diamond film was lost was observed.

(Comparative Example 4)
A conductive diamond electrode was prepared in the same procedure as in Example 1 except that a graffiti plate was used as the conductive substrate.

  A current-potential curve was measured in an anhydrous HF bath maintained at 0 ° C. in the same manner as in Example 1 except that the electrode was used as an anode.

Immediately after the start of the measurement, the anode potential when applying a current density of 5 mA / cm ² was 0.6V. Thereafter, when the anode potential was measured while increasing the current density by 5 mA / cm ² , the anode potential suddenly increased when the current density was 70 mA / cm ^{2 and} almost no current flowed, making it difficult to continue electrolysis.

  When the electrolysis was stopped and the anode was taken out, the anode was shattered in the electrolytic cell.

(Example 2)
Using a glassy carbon plate as the conductive substrate, a conductive diamond electrode was prepared in the same manner as in Example 1 using a hot filament CVD apparatus.

This electrode is attached to an (CH ₃ ) ₄ NF · 5HF electrolytic bath immediately after the building bath as an electrolytic bath made of anhydrous HF and a fluorinated substance, using a nickel plate as a cathode and Cu / CuF ₂ as a reference electrode, Constant current electrolysis was performed at a current density of 100 mA / cm ² . When the anode potential immediately after the start of electrolysis was measured, it was 4.6 V, and the anode potential after electrolysis 200 hours was 4.8 V. Perfluorotrimethylamine (CF ₃ ) ₃ N was synthesized as an electrolytic product.

  When the electrolysis was stopped and the anode was taken out and observed for appearance, electrode collapse and peeling of the conductive diamond film were not observed. There was no occurrence of the anode effect until 200 hours after electrolysis.

(Comparative Example 5)
The electrolysis was performed in a (CH ₃ ) ₄ NF · 5HF electrolytic bath immediately after the building bath in the same manner as in Example 2 except that a graphite plate was used for the anode.

  Immediately after the start of electrolysis, the anode potential rose rapidly and almost no current flowed, making it difficult to continue electrolysis.

  Electrolysis was stopped, the anode was taken out, and the contact angle with water on the electrode surface was measured. As a result, it was 150 °, and so-called anodic effect was observed.

(Comparative Example 6)
The electrolysis was performed in the (CH ₃ ) ₄ NF · 5HF electrolytic bath immediately after the building bath in the same manner as in Example 2 except that a nickel plate was used for the anode.

  Immediately after the start of electrolysis, the anode potential gradually increased, and finally, almost no current flowed, making it difficult to continue electrolysis.

When the electrolysis was stopped and the anode was taken out and the electrode surface was analyzed, Ni—F bonds were observed, and the formation of an insulating NiF ₂ coating on the electrode surface was presumed.

(Comparative Example 7)
A conductive diamond electrode was prepared in the same procedure as in Example 1 except that a silicon plate was used as the conductive substrate.

The electrolysis was performed in a (CH ₃ ) ₄ NF · 5HF electrolytic bath immediately after the building bath in the same manner as in Example 2 except that the electrode was used as the anode.

  Although the anode potential immediately after the start of electrolysis was 4.6 V, the anode potential gradually increased after 14 hours from the start of electrolysis, and finally no current flowed, making it difficult to continue electrolysis.

  When the electrolysis was stopped and the anode was taken out and the appearance was observed, it was confirmed that the diamond film immersed in the electrolytic bath was almost lost, and the silicon substrate surface was corroded.

(Comparative Example 8)
A conductive diamond electrode was prepared in the same procedure as in Example 1 except that a graphite plate was used as the conductive substrate.

The electrolysis was performed in a (CH ₃ ) ₄ NF · 5HF electrolytic bath immediately after the building bath in the same manner as in Example 2 except that the electrode was used as the anode.

  The anode potential immediately after the start of electrolysis was 4.6 V. However, the anode potential gradually increased after 70 hours from the start of electrolysis, and finally no current flowed, making it difficult to continue electrolysis.

  When the electrolysis was stopped and the anode was taken out, the anode was shattered in the electrolytic cell.

Claims (1)

Electrolysis using an electrode for electrolysis in which a conductive material made of glassy carbon is used as a base and at least a part of the base is covered with a conductive diamond film, and fluorinated material is added to anhydrous hydrogen fluoride or anhydrous hydrogen fluoride. A method of electrolytically synthesizing fluorine or a fluorine-containing compound by electrolyzing a bath.