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Ind. Eng. Chem. Res. 2008, 47, 7552–7557
Production of Tetramethyl Ammonium Hydroxide Using Bipolar Membrane Electrodialysis Haozhe Feng, Chuanhui Huang, and Tongwen Xu* Laboratory of Functional Membranes, School of Chemistry and Materials Science, UniVersity of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China
To produce tetramethyl ammonium hydroxide (TMAH) at lower energy consumption and with less environmental pollution, a bipolar membrane electrodialysis (BMED) of A-BP configuration (A, anion exchange membrane; BP, bipolar membrane) was adopted to electro-alkalize tetramethyl ammonium chloride. The results for one-unit BMED stack indicate that current efficiency increases and energy consumption decreases as feed concentration increases or current density decreases. The highest current efficiency can reach 99.9%, and the lowest energy consumption is 1.43 kWh kg-1. On the basis of a 3-unit BMED, the process cost is estimated to be 0.33 $ kg-1 of TMAH. This method is not only environmentally friendly but also cost-effective. 1. Introduction
2. Experimental Details
As a strong quaternary ammonium alkali, tetramethyl ammonium hydroxide (TMAH, formula shown in Scheme 1) has unique properties: (I) complete decomposition at 135∼140 °C, which is lower than that of any other quaternary ammonium hydroxide; (II) complete ionization in water without metal introduction. Owing to these properties, TMAH has been used in chemical syntheses and semiconductor production.1,2 Its preparation methods include synthesis from acetylene and trimethyl amine, anion exchange of tetramethyl ammonium salts, metathesis of inorganic alkalis and tetramethyl ammonium salts, reaction of tetramethyl ammonium halide with silver oxide Ag2O, and electro-electrodialysis.3 The method using Ag2O has often been adopted for TMAH production in China, but it is restricted from wide application because of its shortcomings, such as the high price of Ag2O and pollution from silver and chloride ions. For improvements, some electro-electrodialysis operations have been investigated as an alternative ever since the 1960s,4-9 but they have not found any industrial application because of their high energy consumption. In such context, bipolar membrane electrodialysis (BMED) came up as a new method10 for TMAH production owing to its energy efficiency and environmental benignity.11 BMED is electrodialysis with bipolar membranes, which can split water into OH- and H+ at their intermediate layers under reverse bias.12 It has been widely used in several industries or investigated in experimental setups for producing organic/amino acids or electroacidifying soybean protein.13-19 Compared with organic acid production by BMED, study on electrobasification (i.e., production of organic base by BMED) is insufficient. Therefore, in this paper, new technology for producing TMAH from tetramethyl ammonium chloride (TMAC) by BMED was proposed as an example of electrobasification. The parameters affecting the process efficiency and energy consumption, such as TMAC concentration and current density, will be discussed. Furthermore, three configurations, such as A-BP, C-BP, and A-C-BP (A, anion-exchange membrane; C, cation-exchange membrane; BP, bipolar membrane) will be tested and used to screen for optimum configuration.
2.1. Material. Na2SO4 and TMAC were of analytical grade and both were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Distilled water was used throughout. 2.2. Principle and Setup. As shown in Figure 1a, a BMED stack of A-BP configuration (one repeating unit) was used to produce TMAH. The stack was divided into the anodal, salt, and cathodal chambers by an anion-exchange membrane (FTFAB from FuMA-Tech GmbH, Germany; Table 1) and a bipolar membrane (FT-BP from FuMA-Tech GmbH, Germany, Table 1) with the effective area of 7.07 cm2; the electrodes were made of titanium coated with ruthenium. The three chambers were connected with three immersible pumps (AP1000, Zhongshan Zhenghua Electronics Co. Ltd., China, with the maximal speed of 27 dm3 h-1), which were placed in three 1000 mL beakers. A Na2SO4 solution (0.3 mol dm-3, 500 mL) was used as rinse for each electrode chamber; a TMAC solution of given concentration was added into the salt chamber. The electrodes were connected with a direct current power supply (DF1731SD2A, Zhongce Electronics Co. Ltd., China), and the voltage drop across the stack was recorded by a digital meter (DT9205, Zhangzhou Mater Electronics Co., Ltd.). Before the current was applied, the solution of each compartment was circulated for half an hour, and all the visible gas bubbles were eliminated. For comparison, C-BP (one repeating unit, Figure 1b) and C-A-BP (one repeating unit, Figure 1c) configurations were chosen to produce TMAH under the same condition as A-BP (Table 2). As a scale-up experiment, a 3-unit BMED stack of A-BP configuration (Figure 1d) was used to estimate the process cost. In this experiment, TMAC was higher in concentration (2 mol dm-3); moreover, the BMED operation lasted for 1120 min so that more concentrated TMAH could be obtained. Scheme 1. Molecular Configuration of TMAH
* To whom correspondence should be addressed. E-mail: twxu@ ustc.edu.cn. 10.1021/ie800558m CCC: $40.75 2008 American Chemical Society Published on Web 09/18/2008
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2.3. Determination of TMAH Concentration. The concentration of TMAH was determined by titration using a standard hydrochloric acid solution with phenolphthalein as indicator. 2.4. Determinations of the Energy Consumption and Current Efficiency. The energy consumption E is calculated as E)
UtI dt
∫ C VM
(1)
t
Where Ut is the voltage drop across the stack at time t, I is current intensity, Ct is the TMAH concentration at time t, V is the volume of base cycle, and M is the molar molecular weight of TMAH. Current efficiency η is calculated as η)
(Ct - C0)zVF × 100% It
(2)
Where Ct and C0 are the TMAH concentrations at time t and 0, respectively; z is the ion chemical valence (z ) 1); V is the volume of base cycle; and F is the Faraday constant. 3. Results and Discussion 3.1. Voltage-Time Curves. As shown in Figure 2, the voltage drop across the BMED stack decreases as TMAC concentration increases from 0.1 to 0.6 mol dm-3. The reason can be ascribed to common sense: the higher the concentration of electrolyte, the lower the electrical resistance of the membrane and solution. On the other hand, for all the six V-t curves, the voltage drop leaps shortly after current is applied and levels off afterward. This has the same explanation as Wilhelm reported.20 Before water splits, the electrolyte is depleted in the intermediate layer of bipolar membrane, which makes the layer extremely high in electrical resistance; after water splits,
Figure 1. BMED stacks of (a) A-BP(one-unit), (b) C-BP(one-unit), (c) A-C-BP(one-unit), and (d) A-BP (3-unit): A, anion-exchange membrane; BP, bipolar membrane; C, cation-exchange membrane. Table 1. Properties of the Membranes Used in the Experiments membrane
ion-exchange feature
thickness (µm)
IEC (meq g-1)
area resistance (Ω · cm2)
selectivity (%)
FT-BP FT-FAB FT-FKB
bipolar anion-exchange cation-exchange
450 120 120
0.8 0.8
2-4 5-10
>98 >96
voltage drop (v)
efficiency (%)
92
7554 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 Table 2. Estimation of Process Cost A-BP repeating units current density (mA cm-2) experiment time (min) effective membrane area (cm2) Na2SO4 concentration (mol dm-3) fluid flow speed (dm3 h-1) TMAC concentration (mol dm-3) TMAH concentration (mol dm-3) current efficiency (%) energy consumption (kWh kg-1) process capacity (kg year-1) electricity charge ($ kWh-1) energy cost for the TMAH ($ kg-1) energy cost for the peripheral equipment ($ kg-1 × 10-2) total energy cost ($ kg-1) membrane lifetime and the amortization of the peripheral equipment (year) monopolar membrane price ($ m-2) bipolar membrane price ($ m-2) membrane cost ($) stack cost ($) peripheral equipment cost ($) total investment cost ($) amortization ($ year-1) interest ($ year-1) maintenance ($ year-1) total fixed cost ($ year-1) total fixed cost ($ kg-1) total process cost ($ kg-1)
C-BP
A-C-BP
3 50 1000 7.07 0.30 27 2 1.02 76 0.47 21.36 0.1 0.05 0.25
1 30 90 7.07 0.30 27 0.30 2.16 91 2.37 3.77 0.1 0.24 1.20
1 30 90 7.07 0.30 27 0.30 2.09 87 4.86 3.65 0.1 0.49 2.45
1 30 90 7.07 0.30 27 0.30 2.26 95 4.04 3.89 0.1 0.40 2.00
0.053 3
0.25 3
0.51 3
0.42 3
135 1350 3.15 4.73 7.09 11.82 3.94 0.94 1.18 6.06 0.28 0.33
135 1350 1.05 1.58 2.37 3.95 1.32 0.32 0.40 2.04 0.54 0.79
135 1350 1.05 1.58 2.37 3.95 1.32 0.32 0.40 2.04 0.56 1.07
135 1350 1.15 1.73 2.60 4.33 1.44 0.35 0.43 2.22 0.58 0.99
however, the produced H+ and OH- will carry the current and thus decrease the resistance. Figure 3 shows the effect of current density on the voltage drop. As current density increases from 10 to 60 mA cm-2, the voltage drop increases almost in direct proportion to the increase in current density. In such sense, the stack is similar to an electrical resistor and its voltage increases with current intensity. 3.2. Concentration-Time Curves. As shown in Figure 4, TMAH yield increases as TMAC concentration increases from 0.1 to 0.6 mol dm-3. Although TMAH is a strong base and OH- has higher mobility than Cl-, Cl- will have more predominance in quantity as TMAC concentration increases, so there is less loss of OH- for TMAH production and thus an increase in TMAH yield. Figure 5 shows the effect of current density on TMAH yield. Similar to the production of other strong acid or base using BMED, the yield of product increases almost in direct proportion to the increase in current density. 3.3. Current Efficiency and Energy Consumption. As shown in Figure 6a, current efficiency increases as TMAC
Figure 3. The voltage-time curves at the TMAC concentration of 0.3 mol dm-3.
Figure 4. The concentration-time curves at 30 mA cm-2.
Figure 5. The voltage-time curves at the TMAC concentration of 0.3 mol dm-3.
Figure 2. The voltage-time curves at 30 mA cm-2.
concentration increases. The reason is the same as that for the increase in TMAH yield. However, as current density increases, current efficiency has no significant increase but a slight decrease (Figure 7a). This is relevant to the basicity of TMAH. Since TMAH is a strong electrolyte, molecular diffusion is not a main factor affecting current efficiency, so it is understandable that current efficiency does not increase with current density in such
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Figure 6. (a) Current efficiency and (b) energy consumption at 30 mA cm-2.
Figure 7. (a) Current efficiency and (b) energy consumption at the TMAC concentration of 0.3 mol dm-3.
a case. On the other hand, Cl- decreases in quantity as more TMAH is produced, so OH- will carry more current, which leads to a decrease in current efficiency. Figure 6b shows the effect of TMAC concentration on energy consumption. As TMAC concentration increases, the electrical conductances of the solution and the membranes increase, so there is less energy wasted on electrical resistance. Figure 7b shows the effect of current density on energy consumption. The same as other reports, energy consumption increases as current density increases since more energy is consumed on electrical resistance to produce heat. 3.4. Comparison with Other BMED Configurations and the Process Economics. For comparison, two other configurations, C-BP and A-C-BP, were also tested for TMAH production. Whether it is TMAH concentration or current efficiency, both have the same order as follows: C-BP < A-BP < A-C-BP. For C-BP, the H+ produced through water decomposition (electrode reaction) or water splitting (bipolar membrane) competes with TMA+ to migrate through the cationexchange membrane and neutralize OH- in the base chamber. For A-C-BP, however, the migration of H+ is suppressed by the anion-exchange membrane and a feed compartment, so there
is less loss of product. From the viewpoint of TMAH yield and current efficiency, A-C-BP is the best configuration. Figure 8 shows that the voltage drop across the stack has the following order: A-BP < A-C-BP < C-BP. Considering the Stokes radius (TMA+ > Cl-) and component number in a repeating unit, the BMED of A-BP configuration achieve the lowest voltage drop. Surprisingly, the stack of C-BP configuration has the highest voltage drop though it has less membrane and chamber than that of A-C-BP configuration. The reason may be related to the anode rinse. Different from A-C-BP configuration, the anolyte in the stack of C-BP configuration is TMAC; moreover, TMAC is consumed continuously during BMED operation. With a decrease in the concentration of anolyte and an increase in the volume of accumulated Cl2 bubbles (due to electrode reactions), the apparent electrical resistance of anode rinse will increase very sharply. As for energy consumption, there exists the following order: A-BP < A-C-BP < C-BP (Table 2). This can be reasoned out by considering the voltage drop and TMAH yield. The process cost is calculated by following the procedure as reported in literature.21 For A-BP configuration, the total process
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the net yield of TMAH levels off and then decreases as OHloss overpasses OH- supply. The maximum TMAH concentration is near 1.02 mol dm-3 (t ) 1000 min). Table 2 also gives an estimation of the process cost based on the scale-up experiment. Obviously, the 3-unit BMED stack of A-BP configuration has the lowest process costs0.33 $ kg-1 of TMAH. This is mainly because the energy consumption corresponding to electrodes is equally distributed among the three units. The price of TMAH is 200 $ kg-1 and that of TMAC is 100 $ kg-1 in the current market in China; if this method is used for TMAH production, the cost can be greatly reduced. Considering the environmental benignity and economical advantage, it is promising for this method to be applied in the industry. 4. Conclusions Figure 8. The voltage-time curves for A-BP, C-BP, and A-C-BP configurations. TMAC concentration, 0.3 mol dm-3; current density, 30 mA cm-2.
cost is estimated to be 0.79 $ kg-1, more than 0.20 $ kg-1 lower than the other two configurations. 3.5. Scale-Up Experiment. For industrialization, the BMED unit can be repeated between a pair of electrodes in order to treat more feed solution. In these multiunit stacks, such electrode factors as hydrolysis and electrode electrical resistance have less influence on the whole stack. In this preliminary research, a 3-unit BMED stack of A-BP configuration is used as the setup for a scale-up experiment. Figure 9a illustrates the relationship between the voltage drop and time. As time elapses, the voltage drop keeps decreasing after the initial leap and then levels off. The reason for the sharp decrease in voltage is that the system’s electrical resistance decreases sharply when H+ and OH- turn into the main current carriers in bipolar membranes and TMAC is converted to TMAHsan electrolyte with higher conductivity than TMAC. As shown in Figure 9b, TMAH concentration increases initially, levels off afterward, and decreases finally. With an increase in the amount of TMAH (a strong electrolyte), more OH- will compete with Cl- for the current carrier across anion-exchange membranes; meanwhile, more OH- will diffuse into the anodal chamber because of Donnan dialysis. Therefore,
Electrodialysis with bipolar membranes (BMED) provides a convenient way to produce tetramethyl ammonium hydroxide (TMAH) from tetramethyl ammonium chloride (TMAC). In this kind of operation, both cell configurations such as A-BP, C-BP, and A-C-BP as well as operation conditions such as TMAC concentration and current density play an important role on the BMED characteristics. From the viewpoint of energy consumption, it is suggested that A-BP be a favorable cell configuration for the production of TMAH from the corresponding salt. In the case of such a configuration, current efficiency increases as current density decreases and TMAC concentration increases, and energy consumption increases as current density increases and TMAC concentration decreases. The highest current efficiency can reach 99.9%, and the lowest energy consumption is 1.43 kWh kg-1. The process cost is estimated to be 0.33 $ kg-1 of TMAH based on a 3-unit BMED stack. To have practical data, we hope this preliminary study can attract the interest of entrepreneurs and we can gain more support to further this research. Naturally, there is still a disadvantage for this configuration. Since TMAC and TMAH are circulated in the same compartment, the purity of TMAH cannot be high from the viewpoint of economy. Therefore, if high-purity TMAH is needed, some
Figure 9. (a) The voltage-time curve and (b) the concentration-time curve for the 3-unit stack of A-BP configuration. TMAC concentration, 2 mol dm-3; current density, 50 mA cm-2.
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other measures should be taken for subsequent purification, e.g., ion-exchange. For long-term practical application, membrane fouling should be controlled by removal of multivalent metal ions (such as Ca2+, Mg2+, and Fe3+) as pretreatment, routine cleansing, enhancement on hydraulic conditions, etc. Acknowledgment This research was supported by the National Science Foundation of China (Grant No. 20636050) and Key Foundation of Educational Committee of Anhui Province (Grant Nos. KJ2007A016 and KJ2008A69). Literature Cited (1) Papet, P.; Nichiporuk, O.; Kaminski, A.; Rozier, Y.; Kraiem, J.; Lelievre, J. F.; Chaumartin, A.; Fave, A.; Lemiti, M. Pyramidal texturing of silicon solar cell with TMAH chemical anisotropic etching. Sol. Energy Mater. Sol. Cells. 2006, 90, 2319. (2) Sundaram, K. B.; Vijayakumar, A.; Subramanian, G. Smooth etching of silicon using TMAH and isopropyl alcohol for MEMS applications. Microelectron. Eng. 2005, 77, 230. (3) Zhu, X. J.; Chen, Y. S.; Zhang, X. S. The production and purification of TMAH (review). Jiangsu. Chem. Ind. 2003, 31, 20 (in Chinese). (4) Yagi, O.; Shimizu, S. Synthesis of highly purified tetramethyl ammonium hydroxide by electrolysis of its formate. Nippon Kagaku Kaishi. 1993, 3, 291. (5) Yagi, O.; Shimizu, S. Synthesis of pure Tetramethyl ammonium hydroxide solution free from chloride ion by electrolysis of its hydrogen carbonate. Chem. Lett. 1993, 12, 2041. (6) Franco, B.; Giuseppe, B.; Bruno, N. Process for preparing quarternary ammonium hydroxides by electrolysis. US Pat. 4578161, 1986. (7) Gomez, J. R. O.; Estrada, M. T. Electrosynthesis of quaternary ammonium hydroxides. J. Appl. Electrochem. 1991, 21, 365. (8) Rijkhof, E. J.; Tholen, J. P. P.; Mass, H. J. H.; Boxhoorn, G. Preparation of quaternary ammonium hydroxides. U.S. Pat. 5089096, 1992. (9) Wade, R. C.; Guilbault, L. J. Electrolytic method for producing quaternary ammonium hydroxides. U.S. Pat. 4394226, 1981.
(10) Hulme, D. R.; Moulton, R.; Wilson, W. W.; Hellums, M. Process for recovering onium hydroxides from solutions containing onium compounds. U.S. Pat. 5968338, 1999. (11) Huang, C. H.; Xu, T. W. Electrodialysis with bipolar membranes for sustainable development. EnViron. Sci. Technol. 2006, 40, 5233. (12) Mafe, S.; Ramirez, P.; Alcaraz, A.; Aguilella, V. Ion transport and water splitting in bipolar membranes: theretical background. In Handbook on Bipolar Membrane Technology; Kemperman, A. J. B., Ed.; Twente University Press: Enschede, The Netherlands, 2000; p 47. (13) Huang, C. H.; Xu, T. W.; Zhang, Y. P.; Xue, Y. H.; Chen, G. W. Application of electrodialysis to the production of organic acids: state-ofthe-art and recent developments (review). J. Membr. Sci. 2007, 288, 1. (14) Bazinet, L.; Lamarche, F.; Ippersiel, D. Bipolar-membrane electrodialysis: Applications of electrodialysis in the food industry. Trends Food Sci. Technol. 1998, 9, 107. (15) Grib, H.; Bonnal, L.; Sandeaux, J.; Sandeaux, R.; Gavach, C.; Mameri, N. Extraction of amphoteric amino acids by an electromembrane process-pH and electrical state control by electrodialysis with bipolar membranes. J. Chem. Technol. Biotechnol. 1998, 73, 64. (16) Quoc, A. L.; Lamarche, F.; Makhlouf, J. Acceleration of pH variation in cloudy apple juice using electrodialysis with bipolar membranes. J. Agr. Food Chem. 2000, 48, 2160. (17) van der Ent, E. M.; Thielen, T. P. H.; Stuart, M. A. C.; van der Padt, A.; Keurentjes, J. T. F. Electrodialysis system for large-scale enantiomer separation. Ind. Eng. Chem. Res. 2001, 40, 6021. (18) Saxena, A.; Gohil, G. S.; Shahi, V. K. Electrochemical membrane reactor: Single-step separation and ion substitution for the recovery of lactic acid from lactate salts. Ind. Eng. Chem. Res. 2007, 46, 1270. (19) Wilhelm, F. G.; Punt, I.; van der Vegt, N. F. A.; Strathmann, H.; Wessling, M. A. Symmetric bipolar membranes in acid-base electrodialysis. Ind. Eng. Chem. Res. 2002, 41, 579. (20) Wilhelm, F. G.; van der Vegt, N. F. A.; Strathmann, H.; Wessling, M. Comparison of bipolar membrane by means of chronopotentiometry. J. Electroanal. Chem. 2002, 199, 177. (21) Strathmann, H.; Koops, G. H. Process economics of electrodialytic water dissociation for the production of acid and base. In Handbook on Bipolar Membrane Technology; Kemperman, A. J. B., Ed.; Twente University Press: Enschede, The Netherlands, 2000; pp 191-220.
ReceiVed for reView April 7, 2008 ReVised manuscript receiVed July 8, 2008 Accepted August 12, 2008 IE800558M
