GB2358195A

GB2358195A - Electrolytic synthesis of tetramethylammonium hydroxide

Info

Publication number: GB2358195A
Application number: GB0100628A
Authority: GB
Inventors: Francoise Andolfatto
Original assignee: Atofina SA
Current assignee: Arkema France SA
Priority date: 2000-01-13
Filing date: 2001-01-10
Publication date: 2001-07-18
Also published as: FR2803856A1; FR2803856B1; JP2001271193A; KR20010086305A; GB0100628D0; US20010025798A1; DE10101494A1; CN1312400A

Abstract

A process for the preparation of tetramethylammonium hydroxide by electrolysis of a tetramethylammonium salt in a cell comprising a cation-exchange membrane is carried out continuously under stationary conditions (concentrations of the solutions in the cell for a fixed current density) obtained by, on the one hand, introducing, into the anode electrolysis loop, a tetramethylammonium salt solution which is more concentrated than that present in the cell and introducing water into the cathode loop, and, on the other hand, withdrawing a portion of each of the solutions circulating in the anode and cathode loops. This process provides optimum operation of the membrane of the electrolysis cell and better performances which are retained over time. High, stable values of the current efficiencies, stable pH in the anode compartment and limited side reactions are obtained. The lifetime of the membrane is optimized, the quality of tetramethylammonium hydroxide synthesised is constant and energy consumption is minimised.

Description

2358195 J_ The present invention relates to tetramethylammonium hydroxide

and more particularly to the synthesis of tetramethylammonium hydroxide by a continuous electrolysis process under stationary conditions.

Tetramethylammonium hydroxide (TMAH) is one of the most widely 5 used products in the electronics industry (developing, etching, planarizing and photoresist stripping). In practice, TMAH is obtained in two stages, namely first the synthesis of a tetramethylammonium salt and subsequently the conversion of this salt to the hydroxide by electrolysis.

According to Japanese patent specification JP 57-155390 and US patent specifications US 4,634,509 and US 4,776,929, this electrolysis stage is carried out in an electrochemical cell comprising two compartments separated by a cation-exchange membrane and two electrodes, with an oxidation reaction on the anion of the tetramethylammonium salt at the anode, reduction of water at the cathode and transfer of the tetramethylammonium cation (TMA) through the membrane. The processes disclosed in the abovementioned patent specifications all proceed in the same way: filling the anode circuit with a concentrated solution of TMA' salt, filling the cathode circuit with deionized or demineralized water comprising from 0. 1 to I % of TMAH, in order to ensure minimum conductivity, and then beginning the electrolysis. The processes are inevitably discontinuous because the TMAH crystallizes at 50 weightO/o in the.form of a pentahydrate and because, after a certain time, which is dependent on the volume of the loop and on the current, there are'no longer any TMA ions in the anode circuit.

Such processes have several disadvantages:

1) The catholyte is initially not very conducting, resulting in a high ohmic drop.

During electrolysis, its conductivity increases but that of the anode compartment decreases in parallel. Overall, the ohmic drop of the system is therefore always very high. This is highly disadvantageous to the cell voltage, which fluctuates between 7 and 11 V for a current density of I kA/ml and between 15 and 23 V for a current density of 2 kA/m. The high ohmic drop can result in a significant increase in the temperature via the Joule effect.

2) The concentrations of TMAH and of TMA' salt (for example the chloride) are continually changing. The membrane therefore never operates under stationary conditions. This is harmful to the lifetime of the membrane and eventually leads to a fall in the yield and in the quality of the T synthesized.

The present invention provides a process for the synthesis of T by continuous electrolysis of a TMA' salt in a cell with a cation- exchange membrane under stationary conditions, that is to say the parameters of the electrolysis cell, in particular the concentrations of the various solutions, remain stable over time for a fixed current density.

The process according to the invention for the preparation of TMA-H by electrolysis of a tetramethylammonium salt in a cell with a cationexchange membrane is characterized in that the operation is carried out continuously under stationary conditions obtained, on the one hand, by the introduction, into the anode electrolysis loop, of a tetramethylammonium salt solution which is more concentrated than that present in the cell, and by an input of water into the cathode loop, and, on the other hand, by the withdrawal of a portion of each of the solutions circulating in the anode and cathode loops.

According to the present invention there is provided a process for the preparation of tetramethylammoniUM hydroxide by electrolysis of a tetramethylarnmonium salt in a cell comprising a cation-exchange membrane, which process is carried out continuously under. stationary conditions (concentrations of the solutions in the cell for a fixed current density) obtained by, on the one hand, introducing, into the anode electrolysis loop, a tetramethylammonium salt solution which is more concentrated than that present in the cell and introducing water into the cathode loop, and, on the other hand, withdrawing a portion of each of the solutions circulating in the anode and cathode loops.

The introductions and withdrawals according to the process of the present invention make it possible to obtain optimum operation of the membrane of the electrolysis cell and to obtain better performances which are retained over time. The cell voltage is also constant over time (for example, 7-11 V for a current density of 3 kA/ml) at a value far below those indicated in the abovementioned patent specifications. The membrane thus operates continuously under optimum conditions, resulting in high and stable values of the current efficiencies, stability of the pH in the anode compartment and limited side reactions. It is therefore possible to optimize the lifetime of the membranes, to obtain a constant quality for the TMAH synthesized. and to minimize the energy consumption, for example approximately 5 3000 kWh/t(TNiW continuously at 3 kA/m', compared with at best 4700 kWh/t(TNLkH) at 2 kA/m' in the prior art.

The process according to the invention is suitable for the synthesis of technical TMAH and electronic-grade TMAH. Preferred starting TMA' salts suitable for use in the process of the invention include tetramethylammonium chloride 10 (TMA-CI), tetramethylammonium hydrogencarbonate (TMA-HC03) and tetramethylammonium hydrogensulphate (TNIA-HSO,,). The anode reaction corresponding to each of these salts is given in the following table:

Nature of the salt Corresponding anode reaction TMA-Cl 1 - Cl- --)' /2C12 + e TNIA-HCO3 HC03- __ C02 +1/402 + 1/2H20 + e TMA-HS04 H20 ---).'/202 + 2H+ + 2e- Preferred anodes suitable for use in the process of the invention are based on platinum, or on a ruthenium, iridium or platinum oxide.

The production of hydroxide ions at the cathode can take place either by reduction of water to OR ions and to hydrogen or by reduction of oxygen with water to OR ions.

The invention is further illustrated by way of example with reference to the accompanying drawings in which:

Figure 1 shows a block diagram for electrolysis carried out with a cathode for the reduction of water and the release of hydrogen.

Figure 2 shows the electrolysis of TMA-Cl to TMAH using the device shown in Figure 1.

Figure 3 shows modification of the device shown in Figure 1 to render the cathode loop and storage of the TMAH produced inert.

Figure 4 shows a block diagram for electrolysis carried out with a cathode for the reduction of oxygen.

Figure 5 shows the electrolysis of TMA-Cl to TMAH using the device shown in Figure 4.

Figure 6 shows the device used in the Examples.

With reference to Figure 1 of the drawings, there is shown a block diagram for electrolysis carried out with a cathode for reduction of water and release of hydrogen, this cathode preferably comprising stainless steel or nickel. The device according to Figure 1 comprises:

- an electrolysis cell comprising an anode compartment (1) and a cathode compartment (2) separated by a cation-exchange membrane (m), - an anode degassing tank (3) for removing, via the pipe (9), the gas generated by the anode reaction, - a cathode degassing tank (4) for removing, via the pipe (10), the hydrogen generated by the cathode reaction, D - a tank (5) for storing the more concentrated solution of tetramethylammonium salt to be introduced into the anode loop via the pipe (5), - a tank (6) for storing the demineralized or deionized water to be introduced into the cathode loop via the pipe (6'), - a tank (7) for emptying, via the pipe (7), a portion of the tetramethylammonium salt solution exiting from the anode degassing tank (3), and - a tank (8) for storing the solution of synthesized TMAR which solution is drawn off from the cathode loop at the outlet of the cathode degassing tank (4) via the pipe (8').

With reference to Figure 2 of the drawings, there is shown the electrolysis of TMA-Cl to T, in which the electrolysis cell operates according to the following principles:

- oxidation of the chloride ion to chlorine at the anode according to the reaction:

2Cl- --) C12+ 2e - reduction of water to hydrogen and hydroxide ions at the cathode according to the reaction:

2H20 + 2e- --->. H2+2011- - transfer of the TMA' ion through the cation-exchange membrane, accompanied by a number of water molecules (number variable according to the nature of the membrane and the current density), and - separation by the cation-exchange membrane of the anolyte, of the catholyte and of the gases produced.

In one embodiment of the present invention the cathode loop and the storage of the T produced can be rendered inert with hydrogen, nitrogen or argon, or a mixture thereof. With reference to Figure 3 of the drawings, there is shown modification of the device with a cathode for reduction of water and release of hydrogen to render the cathode loop and storage of the TMAH produced inert. The region which has been rendered inert is represented in dotted lines, the inerting gas being introduced via the pipe (10) and the hydrogen generated by the cathode reaction being discharged via the pipe (11) exiting from the tank (8) for storage of the solution of synthesized TMAH.

With reference to Figure 4 of the drawings, there is shown a block diagram for electrolysis carried out with a cathode for the reduction of oxygen (preferably a cathode comprising platinized or silvered carbon). The cathode compartment (2) is fed with oxygen via the pipe (12) and where the pipe (10) acts as bleed for the oxygen.

With reference to Figure 5 of the drawings, there is shown the electrolysis of TMA-G to T, in which the electolysis cell operates according to the same principles as above, except that the reduction of water to hydrogen and hydroxide ions at the cathode is replaced by a reduction of oxygen with water to hydroxide ions according to the reaction:

02+ 2H20 + 4c---- 401r The optional inerting can in this case be carried out with oxygen, nitrogen or argon, or a mixture thereof The two electrodes can be pressed against the membrane (so-called "zero gap" arrangement) or the cathode can be placed a few millimetres from the membrane (so-called "finite gap" arrangement).

In the electrolysis cell, the membrane is an important component since it is this which ensures good separation of the two solutions. In order to obtain good current efficiencies and a high purity of the synthesized TMAH, the membrane must be permeable to the TMA' ions but impermeable to the anions of the starting salt (for example Cl- or HC03-) and to OH-. In order to separate an acidic medium, or a weakly basic medium, from a very basic medium (TMAH), the membrane must be chemically stable in both media. In addition, the membrane must be as conductive as possible in order to minimize the ohmic drop. To meet all these criteria, ionexchange membranes generally comprise at least two layers of polymers, these layers generally being colaminated together. The polymers can comprise perfluorosulphonated and/or perfluorocarboxylated chains. Such membranes are disclosed, for example, in US patent specifications US 4,401,711 and US 4,604,323, and European patent specifications EP 165,466, EP 253, 119 and EP 753,534. They are found commercially in particular under the names Nafion (RTM) N324, N902 and N966 from DuPont de Nemours, Flemion (RTM) 892 and 893 from Asahi Glass or Aciplex (RTM) 4203 from Asahi Chemicals.

In order to obtain an identical degree of swelling of the various polymers and thus to avoid a deterioration in the membrane (for example, by delamination of the layers), it is generally necessary to adjust the concentrations of the anolyte and of the catholyte. This is because deterioration in the membrane would result in a loss in performance of the cell, since the OIT ions can be oxidized at the anode to form oxygen, and in a decrease in the purity of the synthesized TMAFL since the delamination of the membrane makes possible a flow of the TMA' salt solution into the TNLA-H.

With reference to Figure 6 of the drawings, there is shown the device used in the Examples. The device according to Figure 6 comprises: - an electrolysis cell comprising an anode compartment (1) and a cathode compartment (2) separated by a cation-exchange membrane (m), - an anode degassing column (3) and a pipe (9) for removing the gas generated by the anode reaction, - a cathode degassing column (4), - a bottle (5) for storing the more concentrated solution of tetramethylammonium salt to be introduced into the anode loop via the pipe (5'), - a bottle (6) for storing the demineralized or deionized water to be introduced into the cathode loop via the pipe (6'), - a bottle (7) for emptying, via the pipe (7), a portion of the tetramethylammonium 5 salt solution exiting from the anode degassing column (3), - a bottle (8) for storing the solution of synthesized TMAH, which solution is drawn off from the cathode loop at the outlet of the cathode degassing column (4) via the pipe (8), - a pipe (10) for introducing inert gas, -a pipe (11) for removing the hydrogen generated by the cathode reaction, - a blow-off pipe (13) for the anode loop, and - a blow-off pipe (14) for the cathode loop.

The process according to the invention is advantageously carried out under the following conditions:

- current density of between I and 5 kA/m', preferably between 3 and 4 kA/m' - temperature of between room temperature and 80'C, preferably between 40 and 60'C - concentration of TMAH in the cathode loop of between 5 and 40% by weight, preferably between 10 and 25%, and - concentration of tetramethylammonium salt in the anode loop of between 15 and 40% by weight, preferably between 20 and 3 5 %.

The input of TMA' salt and of water via the concentrated solution of TMA' salt introduced into the anode loop is determined so as to compensate for the consumptions of TMA' salt and of water related to the electrochemical reaction at the anode and to the transfers through the membrane. Likewise, the input of water into the cathode loop must contribute, with the water transferred from the anode compartment to the cathode -compartment through the membrane, to compensating for the water consumed by the electrochemical reaction at the cathode and to providing the water required in order to obtain the desired final concentration of TMAH. These inputs and concentration of TNW salt depend in particular on the nature of the membrane used, on the current density chosen, on the area of the electrodes and on the desired concentration of TMAH.

EXAWLES The following examples, which illustrate the invention without limiting it, were produced by means of the experimental device represented in Figure 5 6.

The cell is composed of two independent circuits, one an anode circuit and the other a cathode circuit:

- The anode circuit is composed of a PTFE-base compartment which allows the anolyte to circulate in the electrochemical cell and which comprises the anode (made of titanium coated with Ru02-TiO2). This compartment is connected to an anolyte/gas degassing column in which the addition of concentrated TMA' salt solution and the discharge of the depleted anolyte are also carried out. During electrolysis, the gas generated at the anode is discharged via the back of the electrode and the circulation of the anolyte is carried out by "gas lift" (difference in relative density between the two-phase mixture and the solution). The temperature is adjusted using a heating tape surrounding the degassing column.

- The cathode circuit is symmetrical with the anode compartment and is based on the same operating principle. The cation-exchange membrane is placed between the anode and the cathode. The anode is pressed against the membrane and the cathode, composed either of a nickel grid or of a stainless steel plate pierced with small holes for the discharge of the gases, is placed 4 millimetres from the membrane ("finite gap" arrangement). The input of dernineralized water and the discharge of the synthesized TMAH take place in the degassing column.

The working area of the cell is 50 cmI. The material used for the electrochemical cell and the various pipes is of PTFE and, for the fittings (columns and tanks), of polypropylene.

The entire cathode loop is rendered inert with a mixture of hydrogen (generated at the cathode) and of argon (injected), in order to limit the dissolution of atmosphericC02 in the TMAH.

The circulation of the electrolytes takes place by a difference in relative density due to the gas releases (chlorine orC02at the anode, depending on the starting salt, and hydrogen at the cathode).

The water injected into the cathode circuit in order to maintain the concentration of TMAH is distilled water.

The chlorine produced during the tests from TMA-Cl is destroyed in a 5 scrubbing column (not represented) using sodium hydroxide and sodium sulphite.

The TNLkH storage bottle is sealed and equipped with a drawingoff/emptying valve in the bottom part. Two non-return bottles, one comprising water and the other empty, make it possible to prevent contamination by the outside atmosphere. Samples are taken under a controlled atmosphere (argon or nitrogen) in aglovebox.

The cell is started up according to the following protocol:

- filling the anode compartment with an aqueous solution of the TMA' salt at the operating concentration of the electrolysis, - filling the cathode compartment with an aqueous TMAH solution at the operating concentration of the electrolysis, - inerting the cathode circuit by purging with argon (if it is desired to limit the dissolution of atmosphericC02in the TMAH, in particular for a product for the electronics industry), and - switching on the heating tapes, in order to bring the device to the desired temperature, and gradual raising of the current density. Example 1 The electrochemical cell is equipped with an anode formed of Ru02T'02deposited on expanded titanium,. a cathode made of stainless steel (perforated plate) and a Naflon (RTM) N324 membrane preconditioned by immersion in a 10% TMAH solution for 24 hours. This cation-exchange membrane is a membrane with perfluorosulphonated chains sold by DuPont de Nemours.

The anode circuit is filled with 735 g of a 243 g/l aqueous TMA-HC03 solution. The cathode circuit is filled with 780 g of a 237 g/l aqueous TMAH solution. The entire assembly is heated with heating tapes and the cathode circuit is rendered inert by injection of argon. When the temperature of the fluids reaches 50'C, the electrical supply is switched on and the current is increased by I A every 3 minutes until 15 A is reached, i.e. 3 kA/ml.

The input of water is 100 g/h and the input of TMA-HC03 'S 125 g/h (588 g/1 solution).

After operating for 16 hours under these conditions, the following results were obtained:

a) The concentration of T is 244 g/1 in the storage tank and 23 5 g/1 in the degassing column of the cathode circuit. The current efficiency for the cathode reaction is 94%.

b) The concentration of TMA-HC03 is 247 g/1 in the emptying tank and 260 g/1 in the anode degassing column. The current efficiency for the anode reaction is 97%.

c) The voltage in the electrolysis cell remains stable at 3 kA/m' at a value of 10 V, 1. c. an energy consumption of 3 13 8 kWh/t(T).

Examples 2 to 6 Other examples according to the invention were produced by proceeding as in Example 1 but by using other materials (membrane, cathode) and/or by starting from another tetramethylammonium salt (TMA-X, X denoting the anion) and/or by varying the concentrations of TMA-X and of IMAR.

Naflon (RTM) N902 and N966 membranes are cation-exchange membranes sold by DuPont de Nemours.

The operating conditions and the results obtained after operating for 16 hours are summarized in the following table, in which:

E, cell denotes the cell voltage (V) and TI denotes the current efficiency, that is to say the ratio of the current portion which has actually been used to carry out the desired reaction to the total current used, TI, representing the current efficiency for the anode reaction (oxidation of chloride ions to Cl. or of hydrogencarbonate ions to C02) and il c representing the current efficiency for the cathode reaction (synthesis of hydroxide ions).

The energy consumption (W) of the electrolysis cell, expressed in kWh/tonne of TMAH, can be calculated from the data in the preceding table by the formula W = 295 x Ee11/TI, Example 1 2 3 4 5 6 Membrane N324 N966 N966 N324 N902 N324 Cathode Stainless Nickel Stainless Stainless Nickel Nickel steel steel steel 3C anion HC03- cl- Cl- cl- Cl- HC03 [TMA-Xl, 243 g/1 254 g/1 203 g/1 249 g/1 200 g/1 320 g/1 starting [TMA-Xl, 588 g/1 505 g/1 580 g/1 533 g/1 250 g/1 602 g/1 input [TMA-X], 247 g/1 243 g/1 191 g/1 23 5 g/1 176 g/1 332 g/1 emptying [TMAH], 237 g/1 106 g/1 107 g/1 247 g/1 100 g/1 244 g/1 starting [TMA1fl, 244 g/1 105 g/1 105 g/1 235 g/1 105 g/1 253 g/1 storage TI a 97% 97.5% 98% 94% 95.4% nd(a) TI c 94% 93% 92.5% 95% 91.8% 92% E, iov 9 v 8 v 11 v 7 V lO v 11 e (a) nd = not determined Comparative Examples 7 to 10 Four examples, produced by operating with the same device but under nonstationary conditions with regard to concentration at TNLA,-X (Example 10) or with regard to concentrations of TMA-X and TNLkH (Examples 7 to 9), are sumrn ari ed in the following table. Examination of the results obtained shows that the efficiencies are much lower than in Examples I to 6 in accordance with the invention.

Example 7 8 9 10 Membrane N902 N902 N902 N902 Cathode Nickel Stainless Stainless Nickel steel steel )C anion cl- cl- Cl- HC03 [TMA-X], 247 g/1 328 g/1 248 g/1 335 g/1 starting [TMA-X], 247 g/1 328 g/1 248 g/1 335 g/1 input [TMA-X], 144 g/1 270 g/1 160 g/1 222 g/1 final (b) [TMAH], 50 g/1 250 g/1 248 g/1 107 g/1 starting [TMAH], 203 g/1 3320 g/1 159 g/1 106 g/1 final (b) Duration of the 8 h 8 h 8 h 4 h test TI a 84% 59% 82% 86% TI C 80% 59% 78% 0/ 89/0 Ecell 7.5 V 7 V 7 V 1 is (b) As the operating conditions in these comparative examples are not stationary, the terms " [TNIA-X], final " and " [TMAH], final " are understood here to mean the concentrations of the solutions present in the degassing column at the end of the test. 25

Claims

1. Process for the preparation of tetramethylammonium hydroxide by electrolysis of a tetramethylammonium salt in a cell comprising a cation-exchange membrane, which process is carried out continuously under stationary conditions (concentrations of the solutions in the cell for a fixed current density) obtained by, on the one hand, introducing, into the anode electrolysis loop, a tetramethylammonium salt solution which is more concentrated than that present in the cell and introducing water into the cathode loop, and, on the other hand, withdrawing a portion of each of the solutions circulating in the anode and cathode loops. '

2. Process according to Claim 1, in which the tetramethylammonium salt is the chloride, hydrogencarbonate or hydrogensulphate.

3. Process according to Claim 1 or 2, which is carried out with a cathode for the evolution of hydrogen.

4. Process according to Claim 3, in which the cathode comprises stainless steel or nickel.

5. Process according to Claim 1 or 2, which is carried out with a cathode for the reduction of oxygen.

6. Process according to Claim 5, in which the cathode comprises platinized or silvered carbon.

7. Process according to any one of the preceding Claims, in which the anode is based on platinum, or on a ruthenium, iridium or platinum oxide.

8. Process according to any one of the preceding Claims, in which the current density is between 1 and 5 kA/ml.

9. Process according to Claim 8, in which the current density is between 3 and 4 kpJM2.

10. Process according to any one of the preceding Claims, which is carried out at a temperature of between room temperature and 8TC.

11. Process according to Claim 10, which is carried out at a temperature of between 40 and 6TC.

12. Process according to any one of the preceding Claims, in which the concentration of TNLAH in the cathode loop is between 5 and 40% by weight.

13. Process according to Claim 12, in which the concentration of 5 TMAH in the cathode loop is between 10 and 25% by weight.

14. Process according to any one of the preceding Claims, in which the concentration of tetramethylammonium salt in the anode loop is between 15 and 40% by weight.

15. Process according to Claim 14, in which the concentration of tetramethylammonium salt in the anode loop is between 20 and 35% by weight.

16. Process according to any one of the preceding Claims, in which the cation-exchange membrane comprises at least two layers of polymers comprising perfluorosulphonated and/or perfluorocarboxylated chains.

17. Process according to any one of the preceding Claims, in which the cathode loop and the storage of the tetramethylammonium hydroxide produced are rendered inert with hydrogen, nitrogen or argon, or a mixture thereof.

18. Process according to Claim 1, substantially as hereinbefore described.

19. Process for the preparation of tetramethylammonium.

hydroxide by electrolysis of a tetramethylammonium salt in a cell comprising a cation-exchange membrane, substantially as described in any one of Examples 1 to 6.

20. Tetramethylammonium hydroxide obtained by the process claimed in any one of claims I to 19.