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Cleaner technologies and electrochemical reactions
~
J. Cleaner Prod., Vol. 2, No. 2, pp. 83-89, 1994
U T T E R W O R T H I N E M A N N
Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0959-6526/94 $10.00 + 0.00
Cleaner technologies and electrochemical reactions* James Parkes The Faraday Centre, Green Road, Carlow, Republic of Ireland
Electrochemistry is one of the cleanest technologies known; its main competitor as a clean technology is catalytic oxidation or reduction. Its use may help make the chemical industry less polluting. With electrochemistry, industry can achieve direct oxidation or reduction of substrates and avoid the use of potentially toxic metal reagents, and recycle spent processing reagents so that they do not need to be discarded. Compounds may also be prepared by using electrochemically recycled redox reagents. The reactions can be carried out in a range of modern electrochemical cells which give better flow characteristics and lower power consumption. Examples are given of electrochemical processes developed by, among others, ICI, BASF and EA Technology.
Keywords: electrochemistry; chemical industry; clean technology
The EC defines 'clean technologies' as 'new industrial processes designed to avoid or significantly reduce particular types of pollution or waste'. Electrochemistry is one of the cleanest technologies known; its main competitor as a clean technology is catalytic oxidation or reduction. Its use may help make the chemical industry less polluting. With electrochemistry, industry can: • Achieve direct oxidation or reduction of substrates and avoid the use of potentially toxic metal reagents. • Recycle spent processing reagents so that they do not need to be discarded. Electrochemistry is the science, or art, of producing useful chemicals by passing electrical current through a solution of the starting material. It may be argued that, because some processes for producing electricity may cause pollution, electrochemistry is not inherently a clean technology. This is true if one examines only old technologies for production of electricity, for example, coal burning generating stations. There are many ways of producing electricity that cause very little pollution, such as hydro-electricity (for example the Irish Electricity Supply Hydroelectric Generating Station at Ardnacrusha, Republic of Ireland, generates 85 MW without any hazardous emissions), fuel cells ~
*Paper presented at the P R E P A R E workshop 'Prevention of Waste and Emissions in the Fine Chemicals/Pharmaceutical Industry', Cork, Republic of Ireland, 4-5 October 1993
and solar panels 2. Some comparative emission figures for fuel cells and coal are shown ~ in Table 1. One simple example of electrochemical synthesis is the manufacture of hydrogen and oxygen by passing electricity through an aqueous solution of potassium. Potassium hydroxide is needed as an electrolyte because pure water has very low electrical conductivity. The hydrogen thus produced can be used as a local supply of hydrogen without the need of gas cylinders, or can be used where high purity product is essential, for example, in food applications or where catalyst poisoning is a problem 3. It can also be used as the basis of the hydrogen economy, in which it has been suggested that hydrogen be used as a means of storing and transporting energy 3,4. To produce hydrogen, all that is needed are two electrodes immersed in the solution of potassium hydroxide, and a rectifier to give direct current. For some electrolytic reactions, where the anode or cathode products may react at the other electrode, a membrane or diaphragm is necessary to separate the reactions at Table 1 Comparative emissions for electricity from coal and from fuel cells (lbs/million k W h of electricity generated) Pollutant emitted
Coal
Fuel cell
NOx CO NMHC SO2
3840 200 2 5900
15 30 0 0
Total
9582
45
J. Cleaner Prod. Volume 2 Number 2
83
Cleaner technologies and electrochemical reactions: J. Parkes
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I Figure 1 Basic electrochemical cell each electrode (Figure 1). Oxidation occurs at the anode and reduction at the cathode. Electrolytic oxidations are sometimes called anodic oxidations or anodic reactions, and electrolytic reductions are called cathodic reductions or cathodic reactions. Chemicals have been made by electrochemistry in this way since Davy 5 first isolated metallic potassium in 1807, so why is electrochemical technology described as a new technology? What is new is that there is a growing range of applications in novel areas and new customdesigned equipment to carry out these applications. For example, hydrogen is now produced in a range of cells, including filter press type cells with very small inter-electrode spacings (Figure 2), and in solid polymer cells in which the electrolyte is a thin ion exchange membrane. These give much improved performance 3. The advantages of electrochemistry which make it a clean technology can best be seen by comparing a conventional industrial process with an electrochemical process to make the same chemical. Thus a process has been developed in which picolinic acid is produced by the direct electro-oxidation of methylpyridine in sulfuric acid at a lead dioxide anode on a 10 tonne/year scale 6. This compares favourably with conventional
t
II
chemical methods which would use potentially toxic oxidizing agents such as chromic acid (Figure 3). As well as being a clean technology, electrochemical synthesis can also be economically favourable. An economic study of electrochemical regeneration of chromic acid etchant solution shows that the payback period is approximately 10 months for a 5000 litre volume of etchant 7. In doing a full economic comparison of an electrochemical process, as well as the low cost of electricity compared with most chemical reagents, many other factors must be taken into account, including capital costs of each type of equipment, whether there are fewer steps in the electrochemical process, the cost of disposal of spent reagents in the conventional chemical process and other annual running costs 8. In direct electrochemical processes, the only reagent used to achieve the oxidation or reduction is electric current and the only inherent by-products are those from electrolysis of water, that is, hydrogen at the cathode or oxygen at the anode. As an illustration, 2.5 kWh of electrical power has the same oxidizing capacity as approximately 1 kg of sodium dichromate, a powerful oxidizing agent which is also highly toxic. Of course, if the electrochemical conversion is less than 100% efficient, by-products of the starting material will be formed as for any normal chemical reaction. Some processes, however, do require the use of external oxidizing agents. For example, chromic acid is the oxidizing agent of choice for the production of saccharin by the oxidation of toluene-2-sulfonamide. This produces a spent chromium(III) waste liquor which is toxic, though much less toxic than the starting chromium(VI). It can be disposed of by precipitation as the hydroxide, which can be landfilled. This always carries the risk of contaminating ground water. It can also be disposed of to the leather industry for tanning. This is not always feasible if there are organic impurities present in the chromium(III) waste liquor. This process can be made less polluting by electrochemically recycling the waste chromium liquor to active chromium(VI)
t
II N
Anode -)
Cathode
CH3
N
CO2H
Direct electrolytic oxidation of picoline to picolinic acid, an important precursor to many industrial pyridine chemicals
Membrane -~
Na2Cr207/H2SO~ ~ + N
t
II
t
Figure 2 Typicalflow through filter press type electrochemicalcell
84
J. Cleaner Prod. Volume 2 N u m b e r 2
CH3
N
Cr2(SO4)3 CO2H
Oxidation using highly toxic chromic acid; gives chromic sulphate as by-product Figure 3 Comparison of electrolytic oxidation and chromic acid oxidation
Cleaner technologies and electrochemical reactions: J. Parkes O
C H 3
2+ Cr207
--.p-
+ C r 3+
,
-SO2NH2
~,~-J
SO2
Chemical oxidation using chromic acid
Cr207-2
= Cr3+
+
spent reagent is then regenerated in the electrolytic cell. Table 2 shows some of the typical redox reagents that can be recycled in this way. The advantage of indirect electrosynthesis is that the electrochemical step is usually faster, since inorganic ions react faster than larger organic molecules. Also, the follow-on redox reaction is a homogeneous reaction as compared with heterogeneous reaction at
6e-
Table 2 Redox reagents for indirect electrolysis Electrochemical recycling of spent Cr (III) at anode
Typical metal ions used for oxidations; oxidized form Cr(VI) Ce(IV) Ag(II) Mn(III) Mn(IV) Un(VII)
Figure 4 Productionof saccharin with electrochemicalrecyclingof chromic acid which can be re-used 9 (Figure 4). Normally, a membrane is necessary when recycling chromium(VI) to prevent reduction at the cathode. However, by proper choice of conditions, chromic acid can also be electrolytically recycled in a membrane-free system m. Figures 5 and 6 compare the set-up for direct electrosynthesis, in which cystine is reduced to cysteine, and indirect electrosynthesis u. Indirect electrosynthesis can be thought of as a chemical redox reaction in which the redox reagent is electrochemically regenerated. The oxidizing or reducing agent is produced in the electrochemical cell. It is then reacted in an external chemical reactor to give the desired product. The Divided electrolytic cell
Typical anions used for oxidations; reduced form CI BrI
Typical metal ions used for reductions; reduced form Cr(II) Co(O) Ni(O) Ni(I) Sn(O)
Catholyte tank
nolyte
Product (cysteine) Feed (cystine)
/ Anolyte maintenance Membrane loop (sulfuric acid) Electrochemical stage Figure 5
Direct electrosynthesis
Divided electrolytic Anolyte M n ( I I I ) tank cell
Catholyte tan k
Product
separator
°xidant~ L - - ~ M n ( i i i ) ~ l atholyte
Anolyte ~
-
-
~
Feed
~
Product
I
Catholyte maintenance loop (sulfuric acid)
/ Membrane
Recycle spent oxidant, Mn (II) AL
Figure 6
Electrochemical stage
) 4 - ~ Chemical stage
Indirect electrosynthesis
J. Cleaner Prod. Volume 2 Number 2
85
Cleaner technologies and electrochemical reactions: J. Parkes an electrode. There is no net waste discharged, and oxidations and reductions can be achieved which might be difficult or too slow by direct electrolytic reactions. However, we will concentrate on direct electrosyntheses which can be thought of as cleaner technologies since no harmful reagents are used. Table 3 shows some of the reasons which have been suggested for choosing electrochemical routes. Another way of judging when to use an electrochemical route is as follows: electrochemical reactions are advantageous if the following conditions are true: • carried out in aqueous solution • require fewer steps than their chemical alternatives • avoid the generation of inorganic products requiring disposal • produce unique products from simpler starting materials One reason for the interest in electrochemistry in recent years has been the availability of off-the-shelf cells or electrolysers which can be easily scaled up, e.g. the Dished Electrode Membrane cell developed by the Electricity Research Council in the U K 11, The FM Cell developed by ICI in the UK 12, and a filter press cell with improved fluid distribution for organic electrosynthesis developed by Reilly Tar & Chemicals Corp., U S A 13. Similar cells have been developed and are available in Sweden, among other countries. It is interesting to note that the ICI cell was awarded the Queen's Award for Industry in the U K in 1991 for 'a technique for producing chemicals with minimal damage to the environment'. What these cells have in common is that a process developed on the laboratory scale of one of these cells can be easily scaled up to tonne lots. The flow pattern, current profile and other engineering requirements are the same in the larger industrial unit as in the smaller laboratory scale cell. Figure 7 shows some processes developed by ICI to a 1 tonne/year scale on their FM celP 4. Table 4 lists some electrolytic processes developed by E A Technology TM in the UK. These are available for licensing. In Germany, BASF have developed processes to give three commercial products 15 (Figure 8). In addition they have developed 15 five other pilot scale processes for a range of aromatic aldehydes: Table 3
Reasons for choosing electrochemistry
Electrons are environmentally friendly Possible higher energy efficiency compared to thermal processes Less aggressive process conditions, e.g. lower temperatures with less degradation Precise control of oxidation or reduction level by control of electrode potential It can use catalytic quantity of regenerable chemical redox couple Sometimes electrochemistry can give unique processes to establish market position/control
86
J. Cleaner Prod. Volume 2 Number 2
ArCHOHR
• ArCH2R
Electrolytic reduction of secondary alcohol
F
F
Electrolytic epoxidation of alkene group [--.-CO2H
r~ c02cH3 2(CH2)2n
Electrolytic coupling of di-acid
CH2N (CH3}2
1 "
F,,,~----~F
+ 2H++ 2e
CH3 ~ F ~ F + 2N (CH3) ~
F ~ F CH2N(CH3)2
F ~ F CH3
Electrolytic reductive deamination Figure 7 Some processes developed by ICI to the 1 tonne/year scale on their FM cell Table 4 Some laboratory scale processes developed by EA Technology, UK Product
Method
Benzoquinone Menadione Naphthoquinone Anisaldehyde 4-Nitrobenzoic acid Dimethoxydihydrofuran 2-Methylindoline Glyoxylic acid L-Cysteine Dicarboxydiphenyl sulfone Dicarboxyphenylether Tolualdehyde
Direct oxidation Indirect oxidation Indirect oxidation Indirect oxidation Indirect oxidation Anodic substitution Direct reduction Direct reduction Direct reduction Indirect oxidation, Cr(VI) Indirect oxidation, Cr(VI) Indirect oxidation, Mn(IIl)
• • • • •
4-tolualdehyde 2-tolualdehyde 4-ethoxytolualdehyde teraphthaldehyde phthalaldehyde
The aldehyde group in all examples is probably produced by anodic oxidation of the corresponding methyl group ~6. They also have a pilot scale process for production of 4-benzoquinonetetramethylketal (Figure 9), a compound which would be difficult to prepare by conventional chemistry. BASF has also developed 12 other processes on a laboratory scale for production of a wide range of compounds, many of which would be difficult to prepare using conventional chemistry:
Cleaner technologies and electrochemical reactions: J. Parkes
c
ocH 3
\c CH3
2-Methoxy ben zaldehyde
2,5-Dimethoxy-2,5dihydrofuran
4-Tert-butylbenzaldehyde
Figure $ Three commercial products from BASF using electrochemical process
CH30~OCH3 CH30
CONH2 OCH 3
4- Benzoquinone-tetramethyl-ketal Figure 9 Pilot scale product from BASF using electrochemical process • 4-(1,1-dimethylpropyl)-benzaldehyde • 4-phenoxybenzaldehyde • 2-tert-butyl-4-benzoquinonetetramethyketal • 2,2-dimethoxycyclohexanol • 1,1-dimethoxy-l-phenyl-2-butanol • 2,5-dihydro-2,5-dimethoxyfurfurylamine • 2,5-dimethoxy-2,5-dihydro-2-furanmethanol • 2-(dimethoxymethyl)-2,5-dihydro-2,5dimethoxyfuran • 1,1,6,6-tetramethoxyhexane • 1,1,8,8-tetramethoxyoctane • 5-methoxy-2-pyrrolidone • 5-methoxy-l-pyrrolidine-carboxaldehyde B A S F report that they have found product purity, selectivity, reliability and environmental compatibility to be the key features of electrochemical processes. The chemicals produced commercially or semicommercially by ICI and by B A S F using electrochemistry, as well as the processes developed by E A Technology, give some indication of the wide range of organic chemicals that it is possible to m a k e using electrochemistry. To give a b r o a d e r view, we will look at some typical reactions which can be achieved using electrochemical techniques. Electrochemical reactions involve the loss or gain of one or m o r e electrons at the anode or cathode, followed by subsequent rearrangements or reactions. As well as simple oxidations or reductions, these can include anodic substitutions and dimerizations. 'Simple' oxidation or reduction does not necessarily mean that the mechanism is simple. It means that the end transformation appears simple. Thus, in electrochemical systems, a reactive intermediate is produced at the electrode which then reacts further with itself or with other reactants in solution or by a combination of these. The following are some examples of typical reactions. A simple oxidation, where one aldehyde group is oxidized selectively to a carboxylic acid while avoiding oxidation of the other aldehyde 17 is as follows:
~
~
N
CH2NH2 N
In aqueous H2SO4 containing Ti (III) at a lead cathode: yield 75%
CONH 2 In aqueous
CH2OH
H 2 S O 4 at a lead cathode:
conversion 95%, yield 86%
Figure 10 Cathodic reduction: effect of conditions on products
OCH-HCO ~
O C H - C O O H in aqueous HCI at carbon anode
In this example of a controlled electrochemical oxidation of glyoxal to glyoxylic acid, conversion is 98% and selectivity is 82%. Figure 10 shows a reduction of a carboxamide group, in which different products are obtained depending on the reaction conditions '8. Figure 11 shows an anodic substitution, followed by hydrolysis to give 1-naphthol with good selectivity '9. 1-Naphthol is an important intermediate for dyes, insecticides and herbicides. This produces less waste and gives a purer product that the traditional method via the sulfonate: naphthalene + H2SO 4 ---> mixed naphthalene sulfonic acids ~ naphthol The electrochemical route is much more energy efficient, mainly beecause it gives a purer product.
OCOCH3
OH
Anodic oxidation of naphthalene followed by hydrolysis to l-naphthol Figure 11 Anodic substitution reaction
J. Cleaner Prod. Volume 2 Number 2
87
Cleaner technologies and electrochemical reactions: J. Parkes
CH(OCH3)2 ~.
O-t-Bu
¢
CH~
hydrolysis
O-t-Bu
the female sex-pheromone of the rove beetle. The preparation of cis-9-heneicosene is shown in Figure
13. In Figure 14, the electrochemical and chemical routes to 2-chlorobenzaldehyde are compared. It is used as metal plating brightener, tear gas intermediate and intermediate for Voranil, an appetite suppressant 22. Many multi-stage processes in the fine chemical area require synthetic steps such as reduction (or oxidation), which lead to disposal problems for the spent redox reagent. One example is the reduction of a secondary alcohol by zinc amalgam:
OH
Figure 12 Oxidation of an aromatic methyl group to the corresponding benzaldehyde
Figure 12 shows an oxidation of an aromatic methyl group, to give the corresponding benzaldehyde via a tert-butoxy protecting group; tert-butoxy is chosen as the protecting group as it is readily recycled. This is produced on a 1000 tonne/year scale at an undivided carbon disc anode 2°. In the Kolbe reaction, electrolysis of a carboxylic acid salt leads to loss of carbon dioxide and the formation of a free radical at the electrode surface which will give a coupled product. It makes possible the preparation of the coupled product R - R from R C O O - as well as the unsymmetrical coupled product, R - R ' , from a mixture of two acids, R C O O and R ' C O O , in addition to the two possible symmetrical ones, R - R and R - R ' . A particular preferred product can be selected by careful choice of the relative proportions of the starting materials. Kolbe coupling with suitable acid substrates can be used to make quite unusual products. The cis isomer is the preferred product because of steric effects at the electrode surface. For example, Kolbe coupling 21 by co-electrolysis of oleic with valeric acid and oleic with either propionic or caproic acids can give cis-9-nonadecene, cis-9-heneicosene and cis-9-docosene in yields from 50 to 63%. The products are used as sex-attractants for the housefly and as one of the components of
R R ' C H - O H + 2H + + Zn ~
+ Z n 2+
In use, this reaction produced considerable amounts of spent reagents that required disposal, and the addition of zinc powder meant that the process was difficult to control in a smooth reaction. An alternative electrochemical route used Cr(II) to achieve the reduction, and recycled the spent reducing agent by electrochemical reduction 23. The electrochemical process eliminated the use of the Zinc/Hg and gave a process that was cheaper to operate. At the cathode: 4Cr 3+ + 4e- ---->4Cr 2+ Reduction reaction: R R ' C H - O H + Cr 2÷ ---> R R ' C H 2 + Cr 3+
ht situ electrolytic preparation of hazardous reagents can lessen the hazards involved in using such reagents. Thus, electrolytic halogenations can be carried out by electrolysis in halide solutions and avoid the use of chlorine gas. This approach can also be used to achieve
H3C(H2C)7 (CH2)10CH3
H3C(H2C)7 (CH2)7CO2H ~'k + HO2C(CH2)3CH3 H H Oleicacid
.
H
H
Housefly sex-attractant
n-Valericacid
Figure 13 Mixed Kolbe coupling reaction
CH3
CHO + Ce(III) anodicallyrecycled
+ Ce(IV) Electrochemicalroute
CH3 [ ~
CI
CI2
CHCI2 CI [ ~
CHO hydrolysis
[~C'
Alternativechemicalroute Figure 14 Comparison of electrochemical and chemical routes to 2-chlorobenzaldehyde
88
J. Cleaner Prod. Volume 2 N u m b e r 2
R R ' C H 2 + 2H20
+ 2HCI
Cleaner technologies and electrochemical reactions: J. Parkes o t h e r reactions. F o r e x a m p l e , t w o - p h a s e electrothiocya n a t i o n of o r g a n i c m o l e c u l e s can replace c h l o r i n e a n d b r o m i n e used in chemical t h i o c y a n a t i o n . T h i o c y a n a t o c o m p o u n d s are used for p r e p a r i n g dyes, insecticides etc. In the t h i o c y a n a t i o n of 2 - m e t h y l p h e n o l as a m o d e l c o m p o u n d , c u r r e n t efficiencies of 50% were achieved. This r e a c t i o n has b e e n used with p h e n o l s a n d amines. H o w e v e r , it can be a p p l i e d to a n y o r g a n i c c o m p o u n d that reacts with t h i o c y a n o g e n 24. This review has tried to show the possibilities of e l e c t r o c h e m i c a l t e c h n o l o g y as a useful a l t e r n a t i v e clean technology. It can p r o d u c e a wide r a n g e of chemicals with less net waste p r o d u c t i o n a n d at lower e n e r g y c o n s u m p t i o n . It will n o t solve all p r o b l e m s , b u t will solve some p r o b l e m s m u c h b e t t e r t h a n a l t e r n a t i v e technologies. It should be r e g a r d e d as a useful synthetic tool to k e e p in o n e ' s a r m o u r y to be used w h e n r e q u i r e d . It is easily used as a t e c h n i q u e b e c a u s e the reactors, electrolysis cells, can be b o u g h t in from r e p u t a b l e c o m p a n i e s with good k n o w l e d g e of this area. A s well as good texts a n d reviews of the chemical a p p r o a c h 25-28, t h e r e are also good b o o k s o n the chemical engineering approach to electrochemistry29 32. W h a t is essential is a willingness to explore this t e c h n o l o g y , a n d a willingness to c h a n g e , w i t h o u t which p o l l u t i o n p r o b l e m s will n o t be solved.
11 12
13 14 15 16 17 18 19 20 21 22
23
References
10
Fuel Cell Fact Sheet, Gas Research Institute, Chicago, May 1993 Coghlan, A. New Scientist 1994, 143 (20 August), p. 21 Pletcher, D. and Walsh, F.C. 'Industrial Electrochemistry' 2nd edn, Chapman and Hall, London, 1990, pp. 256-269 Greenwood, N.N. and Earnshaw, A. 'Chemistry of the Elements', Pergamon Press, Oxford, 1993, pp. 45-47 Davy, Sir H. in Knickerbocker, W.S. 'Classics of Modern Science', Knopf, New York, 1927, pp. 183-189 Pletcher, D. and Walsh, F.C. 'Industrial Electrochemistry' 2nd edn, Chapman and Hall, London, 1990, p. 322 Sports, D.A. 'Economic Evaluation of a Method to Regenerate Waste Chromic Acid-Sulfuric Acid Etchants', Bureau of Mines Information Circular 8931, United States Department of the Interior, 1983 Pletcher, D. Paper presented at 'Second International Forum on Electrolysis in the Chemical Industry', Florida, November 1988 Jackson, C. and Kuhn, A.T. in 'Industrial Electrochemical Processes', Ed. A. Kuhn, Elsevier, Amsterdam, 1971, p. 515 Parkes, J. and Grimshaw, J. in 'Summary Reports of the R&D Programme, Recycling of Non-Ferrous Metals', Report
24 25 26 27 28 29 30 31 32
EUR 13646, Commission of the European Community, Luxemburg, 1992, pp. 123-142 Brochure on Dished Electrode Membrane Cell, Electrocatalytic Inc., Milltown Court, Union, NJ 07083, USA Robinson, D. in 'Electrosyntheses, from I,aboratory, to Pilot, to Production' (Eds J.D. Gender and D. Pletcher), The Electrosynthesis Company, Inc., New York, 1990, pp. 21%226 Toomey, J.E. Jr US Patent 4,589,968 (Reilly Tar & Chemicals Corp.), 1986 Tait, S.J.D. in 'Electrochemical Technology in Industry, A UK Status Report" (Ed. S.J.D. Tait), Society of Chemical Industry, London, 1991, pp. 36-37 BASF Electrochemical Intermediates, 'A new generation with new advantages', Intermediates Brochure from BASF, Ludwigshafen, Germany Degner, D. in 'Electrochemistry III' (Topics in Current Chemistry 148) (Ed. E. Steckhan), Springer-Verlag, Berlin, 1988, p. 19 Degner, D. in 'Electrochemistry III' (Topics in Current Chemistry 148) (Ed. E. Steckhan), Springer-Verlag, Berlin, 1988, p. 31 Degner, D. in 'Electrochemistry III' (Topics in Current Chemistry 148) (Ed. E. Steckhan), Springer-Verlag, Berlin, 1988, p. 65 Parkes, J. British Patent 1,465,892, 1977 Weinberg, N.L. in 'Electrosyntheses, from Laboratory, to Pilot, to Production' (Eds J.D. Gender and D. Pletcher), The Electrosynthesis Company, Inc., New York, 1990, p. 9 Singh, K.N. and Misra, R.A. Bull Electrochem. (India) 1990, 6(10), 832 Kreh, R.P. et al. in 'Electrosyntheses, from Laboratory, to Pilot, to Production' (Eds J.D. Gender and D. Pletcher), The Electrosynthesis Company, Inc., New York, 1990, p. 195 Couper, A.M. in 'Electrochemistry for a Cleaner Environment' (Eds D. Genders and N. Weinberg), Electrosynthesis Company, Inc., New York, 1992, p. 241 Krishnan, P. and Gurjar, V.G.J. Appl. Electrochem. 1993, 23(3), 268 Baizer, M.M. and Lurid, H. (Eds), 'Organic Electrochemistry', 3rd edn, Marcel Dekker, New York, 1990 Electrochemical Processing--A Quarterly Commentary, from ICI Electrochemical Technology, PO Box 14, Runcorn, Cheshire, UK Roberts, R., Ouellette, R.P. and Cheremisinoff, P.N. 'Industrial Applications of Electroorganic Synthesis', Ann Arbor Science, Ann Arbor, Michigan, 1982 Steckhan, E. in 'Electrochemistry I' (Topics in Current Chemistry 142) (Ed. E. Steckhan), Springer-Verlag, Berlin, 1987 Heitz, E. and Kreysa, G. 'Principles of Electrochemical Engineering, Extended Version of a DECHEMA Experimental Course', VCH, Weinheim, 1986 Pickett, D.J. 'Electrochemical Reactor Design' (Chemical Engineering Monographs 9), Elsevier, Amsterdam, 1979 Walsh, F. 'A First Course in Electrochemical Engineering', 1993, The Electrochemical Consultancy, Queens Close, Romsey, Hants, UK Fahidy, T.Z. 'Principles of Electrochemical Reactor Analysis' (Chemical Engineering Monographs 18), Elsevier, Amsterdam, 1985
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J. Cleaner Prod., Vol. 2, No. 2, pp. 83-89, 1994
U T T E R W O R T H I N E M A N N
Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0959-6526/94 $10.00 + 0.00
Cleaner technologies and electrochemical reactions* James Parkes The Faraday Centre, Green Road, Carlow, Republic of Ireland
Electrochemistry is one of the cleanest technologies known; its main competitor as a clean technology is catalytic oxidation or reduction. Its use may help make the chemical industry less polluting. With electrochemistry, industry can achieve direct oxidation or reduction of substrates and avoid the use of potentially toxic metal reagents, and recycle spent processing reagents so that they do not need to be discarded. Compounds may also be prepared by using electrochemically recycled redox reagents. The reactions can be carried out in a range of modern electrochemical cells which give better flow characteristics and lower power consumption. Examples are given of electrochemical processes developed by, among others, ICI, BASF and EA Technology.
Keywords: electrochemistry; chemical industry; clean technology
The EC defines 'clean technologies' as 'new industrial processes designed to avoid or significantly reduce particular types of pollution or waste'. Electrochemistry is one of the cleanest technologies known; its main competitor as a clean technology is catalytic oxidation or reduction. Its use may help make the chemical industry less polluting. With electrochemistry, industry can: • Achieve direct oxidation or reduction of substrates and avoid the use of potentially toxic metal reagents. • Recycle spent processing reagents so that they do not need to be discarded. Electrochemistry is the science, or art, of producing useful chemicals by passing electrical current through a solution of the starting material. It may be argued that, because some processes for producing electricity may cause pollution, electrochemistry is not inherently a clean technology. This is true if one examines only old technologies for production of electricity, for example, coal burning generating stations. There are many ways of producing electricity that cause very little pollution, such as hydro-electricity (for example the Irish Electricity Supply Hydroelectric Generating Station at Ardnacrusha, Republic of Ireland, generates 85 MW without any hazardous emissions), fuel cells ~
*Paper presented at the P R E P A R E workshop 'Prevention of Waste and Emissions in the Fine Chemicals/Pharmaceutical Industry', Cork, Republic of Ireland, 4-5 October 1993
and solar panels 2. Some comparative emission figures for fuel cells and coal are shown ~ in Table 1. One simple example of electrochemical synthesis is the manufacture of hydrogen and oxygen by passing electricity through an aqueous solution of potassium. Potassium hydroxide is needed as an electrolyte because pure water has very low electrical conductivity. The hydrogen thus produced can be used as a local supply of hydrogen without the need of gas cylinders, or can be used where high purity product is essential, for example, in food applications or where catalyst poisoning is a problem 3. It can also be used as the basis of the hydrogen economy, in which it has been suggested that hydrogen be used as a means of storing and transporting energy 3,4. To produce hydrogen, all that is needed are two electrodes immersed in the solution of potassium hydroxide, and a rectifier to give direct current. For some electrolytic reactions, where the anode or cathode products may react at the other electrode, a membrane or diaphragm is necessary to separate the reactions at Table 1 Comparative emissions for electricity from coal and from fuel cells (lbs/million k W h of electricity generated) Pollutant emitted
Coal
Fuel cell
NOx CO NMHC SO2
3840 200 2 5900
15 30 0 0
Total
9582
45
J. Cleaner Prod. Volume 2 Number 2
83
Cleaner technologies and electrochemical reactions: J. Parkes
© +
j
m
L___
Anode--~
.
~
.
.
.
_ ..
..
. .
. . .
..
..
. .
.
. .
..
..
~
.
Cathode-~
I Figure 1 Basic electrochemical cell each electrode (Figure 1). Oxidation occurs at the anode and reduction at the cathode. Electrolytic oxidations are sometimes called anodic oxidations or anodic reactions, and electrolytic reductions are called cathodic reductions or cathodic reactions. Chemicals have been made by electrochemistry in this way since Davy 5 first isolated metallic potassium in 1807, so why is electrochemical technology described as a new technology? What is new is that there is a growing range of applications in novel areas and new customdesigned equipment to carry out these applications. For example, hydrogen is now produced in a range of cells, including filter press type cells with very small inter-electrode spacings (Figure 2), and in solid polymer cells in which the electrolyte is a thin ion exchange membrane. These give much improved performance 3. The advantages of electrochemistry which make it a clean technology can best be seen by comparing a conventional industrial process with an electrochemical process to make the same chemical. Thus a process has been developed in which picolinic acid is produced by the direct electro-oxidation of methylpyridine in sulfuric acid at a lead dioxide anode on a 10 tonne/year scale 6. This compares favourably with conventional
t
II
chemical methods which would use potentially toxic oxidizing agents such as chromic acid (Figure 3). As well as being a clean technology, electrochemical synthesis can also be economically favourable. An economic study of electrochemical regeneration of chromic acid etchant solution shows that the payback period is approximately 10 months for a 5000 litre volume of etchant 7. In doing a full economic comparison of an electrochemical process, as well as the low cost of electricity compared with most chemical reagents, many other factors must be taken into account, including capital costs of each type of equipment, whether there are fewer steps in the electrochemical process, the cost of disposal of spent reagents in the conventional chemical process and other annual running costs 8. In direct electrochemical processes, the only reagent used to achieve the oxidation or reduction is electric current and the only inherent by-products are those from electrolysis of water, that is, hydrogen at the cathode or oxygen at the anode. As an illustration, 2.5 kWh of electrical power has the same oxidizing capacity as approximately 1 kg of sodium dichromate, a powerful oxidizing agent which is also highly toxic. Of course, if the electrochemical conversion is less than 100% efficient, by-products of the starting material will be formed as for any normal chemical reaction. Some processes, however, do require the use of external oxidizing agents. For example, chromic acid is the oxidizing agent of choice for the production of saccharin by the oxidation of toluene-2-sulfonamide. This produces a spent chromium(III) waste liquor which is toxic, though much less toxic than the starting chromium(VI). It can be disposed of by precipitation as the hydroxide, which can be landfilled. This always carries the risk of contaminating ground water. It can also be disposed of to the leather industry for tanning. This is not always feasible if there are organic impurities present in the chromium(III) waste liquor. This process can be made less polluting by electrochemically recycling the waste chromium liquor to active chromium(VI)
t
II N
Anode -)
Cathode
CH3
N
CO2H
Direct electrolytic oxidation of picoline to picolinic acid, an important precursor to many industrial pyridine chemicals
Membrane -~
Na2Cr207/H2SO~ ~ + N
t
II
t
Figure 2 Typicalflow through filter press type electrochemicalcell
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J. Cleaner Prod. Volume 2 N u m b e r 2
CH3
N
Cr2(SO4)3 CO2H
Oxidation using highly toxic chromic acid; gives chromic sulphate as by-product Figure 3 Comparison of electrolytic oxidation and chromic acid oxidation
Cleaner technologies and electrochemical reactions: J. Parkes O
C H 3
2+ Cr207
--.p-
+ C r 3+
,
-SO2NH2
~,~-J
SO2
Chemical oxidation using chromic acid
Cr207-2
= Cr3+
+
spent reagent is then regenerated in the electrolytic cell. Table 2 shows some of the typical redox reagents that can be recycled in this way. The advantage of indirect electrosynthesis is that the electrochemical step is usually faster, since inorganic ions react faster than larger organic molecules. Also, the follow-on redox reaction is a homogeneous reaction as compared with heterogeneous reaction at
6e-
Table 2 Redox reagents for indirect electrolysis Electrochemical recycling of spent Cr (III) at anode
Typical metal ions used for oxidations; oxidized form Cr(VI) Ce(IV) Ag(II) Mn(III) Mn(IV) Un(VII)
Figure 4 Productionof saccharin with electrochemicalrecyclingof chromic acid which can be re-used 9 (Figure 4). Normally, a membrane is necessary when recycling chromium(VI) to prevent reduction at the cathode. However, by proper choice of conditions, chromic acid can also be electrolytically recycled in a membrane-free system m. Figures 5 and 6 compare the set-up for direct electrosynthesis, in which cystine is reduced to cysteine, and indirect electrosynthesis u. Indirect electrosynthesis can be thought of as a chemical redox reaction in which the redox reagent is electrochemically regenerated. The oxidizing or reducing agent is produced in the electrochemical cell. It is then reacted in an external chemical reactor to give the desired product. The Divided electrolytic cell
Typical anions used for oxidations; reduced form CI BrI
Typical metal ions used for reductions; reduced form Cr(II) Co(O) Ni(O) Ni(I) Sn(O)
Catholyte tank
nolyte
Product (cysteine) Feed (cystine)
/ Anolyte maintenance Membrane loop (sulfuric acid) Electrochemical stage Figure 5
Direct electrosynthesis
Divided electrolytic Anolyte M n ( I I I ) tank cell
Catholyte tan k
Product
separator
°xidant~ L - - ~ M n ( i i i ) ~ l atholyte
Anolyte ~
-
-
~
Feed
~
Product
I
Catholyte maintenance loop (sulfuric acid)
/ Membrane
Recycle spent oxidant, Mn (II) AL
Figure 6
Electrochemical stage
) 4 - ~ Chemical stage
Indirect electrosynthesis
J. Cleaner Prod. Volume 2 Number 2
85
Cleaner technologies and electrochemical reactions: J. Parkes an electrode. There is no net waste discharged, and oxidations and reductions can be achieved which might be difficult or too slow by direct electrolytic reactions. However, we will concentrate on direct electrosyntheses which can be thought of as cleaner technologies since no harmful reagents are used. Table 3 shows some of the reasons which have been suggested for choosing electrochemical routes. Another way of judging when to use an electrochemical route is as follows: electrochemical reactions are advantageous if the following conditions are true: • carried out in aqueous solution • require fewer steps than their chemical alternatives • avoid the generation of inorganic products requiring disposal • produce unique products from simpler starting materials One reason for the interest in electrochemistry in recent years has been the availability of off-the-shelf cells or electrolysers which can be easily scaled up, e.g. the Dished Electrode Membrane cell developed by the Electricity Research Council in the U K 11, The FM Cell developed by ICI in the UK 12, and a filter press cell with improved fluid distribution for organic electrosynthesis developed by Reilly Tar & Chemicals Corp., U S A 13. Similar cells have been developed and are available in Sweden, among other countries. It is interesting to note that the ICI cell was awarded the Queen's Award for Industry in the U K in 1991 for 'a technique for producing chemicals with minimal damage to the environment'. What these cells have in common is that a process developed on the laboratory scale of one of these cells can be easily scaled up to tonne lots. The flow pattern, current profile and other engineering requirements are the same in the larger industrial unit as in the smaller laboratory scale cell. Figure 7 shows some processes developed by ICI to a 1 tonne/year scale on their FM celP 4. Table 4 lists some electrolytic processes developed by E A Technology TM in the UK. These are available for licensing. In Germany, BASF have developed processes to give three commercial products 15 (Figure 8). In addition they have developed 15 five other pilot scale processes for a range of aromatic aldehydes: Table 3
Reasons for choosing electrochemistry
Electrons are environmentally friendly Possible higher energy efficiency compared to thermal processes Less aggressive process conditions, e.g. lower temperatures with less degradation Precise control of oxidation or reduction level by control of electrode potential It can use catalytic quantity of regenerable chemical redox couple Sometimes electrochemistry can give unique processes to establish market position/control
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J. Cleaner Prod. Volume 2 Number 2
ArCHOHR
• ArCH2R
Electrolytic reduction of secondary alcohol
F
F
Electrolytic epoxidation of alkene group [--.-CO2H
r~ c02cH3 2(CH2)2n
Electrolytic coupling of di-acid
CH2N (CH3}2
1 "
F,,,~----~F
+ 2H++ 2e
CH3 ~ F ~ F + 2N (CH3) ~
F ~ F CH2N(CH3)2
F ~ F CH3
Electrolytic reductive deamination Figure 7 Some processes developed by ICI to the 1 tonne/year scale on their FM cell Table 4 Some laboratory scale processes developed by EA Technology, UK Product
Method
Benzoquinone Menadione Naphthoquinone Anisaldehyde 4-Nitrobenzoic acid Dimethoxydihydrofuran 2-Methylindoline Glyoxylic acid L-Cysteine Dicarboxydiphenyl sulfone Dicarboxyphenylether Tolualdehyde
Direct oxidation Indirect oxidation Indirect oxidation Indirect oxidation Indirect oxidation Anodic substitution Direct reduction Direct reduction Direct reduction Indirect oxidation, Cr(VI) Indirect oxidation, Cr(VI) Indirect oxidation, Mn(IIl)
• • • • •
4-tolualdehyde 2-tolualdehyde 4-ethoxytolualdehyde teraphthaldehyde phthalaldehyde
The aldehyde group in all examples is probably produced by anodic oxidation of the corresponding methyl group ~6. They also have a pilot scale process for production of 4-benzoquinonetetramethylketal (Figure 9), a compound which would be difficult to prepare by conventional chemistry. BASF has also developed 12 other processes on a laboratory scale for production of a wide range of compounds, many of which would be difficult to prepare using conventional chemistry:
Cleaner technologies and electrochemical reactions: J. Parkes
c
ocH 3
\c CH3
2-Methoxy ben zaldehyde
2,5-Dimethoxy-2,5dihydrofuran
4-Tert-butylbenzaldehyde
Figure $ Three commercial products from BASF using electrochemical process
CH30~OCH3 CH30
CONH2 OCH 3
4- Benzoquinone-tetramethyl-ketal Figure 9 Pilot scale product from BASF using electrochemical process • 4-(1,1-dimethylpropyl)-benzaldehyde • 4-phenoxybenzaldehyde • 2-tert-butyl-4-benzoquinonetetramethyketal • 2,2-dimethoxycyclohexanol • 1,1-dimethoxy-l-phenyl-2-butanol • 2,5-dihydro-2,5-dimethoxyfurfurylamine • 2,5-dimethoxy-2,5-dihydro-2-furanmethanol • 2-(dimethoxymethyl)-2,5-dihydro-2,5dimethoxyfuran • 1,1,6,6-tetramethoxyhexane • 1,1,8,8-tetramethoxyoctane • 5-methoxy-2-pyrrolidone • 5-methoxy-l-pyrrolidine-carboxaldehyde B A S F report that they have found product purity, selectivity, reliability and environmental compatibility to be the key features of electrochemical processes. The chemicals produced commercially or semicommercially by ICI and by B A S F using electrochemistry, as well as the processes developed by E A Technology, give some indication of the wide range of organic chemicals that it is possible to m a k e using electrochemistry. To give a b r o a d e r view, we will look at some typical reactions which can be achieved using electrochemical techniques. Electrochemical reactions involve the loss or gain of one or m o r e electrons at the anode or cathode, followed by subsequent rearrangements or reactions. As well as simple oxidations or reductions, these can include anodic substitutions and dimerizations. 'Simple' oxidation or reduction does not necessarily mean that the mechanism is simple. It means that the end transformation appears simple. Thus, in electrochemical systems, a reactive intermediate is produced at the electrode which then reacts further with itself or with other reactants in solution or by a combination of these. The following are some examples of typical reactions. A simple oxidation, where one aldehyde group is oxidized selectively to a carboxylic acid while avoiding oxidation of the other aldehyde 17 is as follows:
~
~
N
CH2NH2 N
In aqueous H2SO4 containing Ti (III) at a lead cathode: yield 75%
CONH 2 In aqueous
CH2OH
H 2 S O 4 at a lead cathode:
conversion 95%, yield 86%
Figure 10 Cathodic reduction: effect of conditions on products
OCH-HCO ~
O C H - C O O H in aqueous HCI at carbon anode
In this example of a controlled electrochemical oxidation of glyoxal to glyoxylic acid, conversion is 98% and selectivity is 82%. Figure 10 shows a reduction of a carboxamide group, in which different products are obtained depending on the reaction conditions '8. Figure 11 shows an anodic substitution, followed by hydrolysis to give 1-naphthol with good selectivity '9. 1-Naphthol is an important intermediate for dyes, insecticides and herbicides. This produces less waste and gives a purer product that the traditional method via the sulfonate: naphthalene + H2SO 4 ---> mixed naphthalene sulfonic acids ~ naphthol The electrochemical route is much more energy efficient, mainly beecause it gives a purer product.
OCOCH3
OH
Anodic oxidation of naphthalene followed by hydrolysis to l-naphthol Figure 11 Anodic substitution reaction
J. Cleaner Prod. Volume 2 Number 2
87
Cleaner technologies and electrochemical reactions: J. Parkes
CH(OCH3)2 ~.
O-t-Bu
¢
CH~
hydrolysis
O-t-Bu
the female sex-pheromone of the rove beetle. The preparation of cis-9-heneicosene is shown in Figure
13. In Figure 14, the electrochemical and chemical routes to 2-chlorobenzaldehyde are compared. It is used as metal plating brightener, tear gas intermediate and intermediate for Voranil, an appetite suppressant 22. Many multi-stage processes in the fine chemical area require synthetic steps such as reduction (or oxidation), which lead to disposal problems for the spent redox reagent. One example is the reduction of a secondary alcohol by zinc amalgam:
OH
Figure 12 Oxidation of an aromatic methyl group to the corresponding benzaldehyde
Figure 12 shows an oxidation of an aromatic methyl group, to give the corresponding benzaldehyde via a tert-butoxy protecting group; tert-butoxy is chosen as the protecting group as it is readily recycled. This is produced on a 1000 tonne/year scale at an undivided carbon disc anode 2°. In the Kolbe reaction, electrolysis of a carboxylic acid salt leads to loss of carbon dioxide and the formation of a free radical at the electrode surface which will give a coupled product. It makes possible the preparation of the coupled product R - R from R C O O - as well as the unsymmetrical coupled product, R - R ' , from a mixture of two acids, R C O O and R ' C O O , in addition to the two possible symmetrical ones, R - R and R - R ' . A particular preferred product can be selected by careful choice of the relative proportions of the starting materials. Kolbe coupling with suitable acid substrates can be used to make quite unusual products. The cis isomer is the preferred product because of steric effects at the electrode surface. For example, Kolbe coupling 21 by co-electrolysis of oleic with valeric acid and oleic with either propionic or caproic acids can give cis-9-nonadecene, cis-9-heneicosene and cis-9-docosene in yields from 50 to 63%. The products are used as sex-attractants for the housefly and as one of the components of
R R ' C H - O H + 2H + + Zn ~
+ Z n 2+
In use, this reaction produced considerable amounts of spent reagents that required disposal, and the addition of zinc powder meant that the process was difficult to control in a smooth reaction. An alternative electrochemical route used Cr(II) to achieve the reduction, and recycled the spent reducing agent by electrochemical reduction 23. The electrochemical process eliminated the use of the Zinc/Hg and gave a process that was cheaper to operate. At the cathode: 4Cr 3+ + 4e- ---->4Cr 2+ Reduction reaction: R R ' C H - O H + Cr 2÷ ---> R R ' C H 2 + Cr 3+
ht situ electrolytic preparation of hazardous reagents can lessen the hazards involved in using such reagents. Thus, electrolytic halogenations can be carried out by electrolysis in halide solutions and avoid the use of chlorine gas. This approach can also be used to achieve
H3C(H2C)7 (CH2)10CH3
H3C(H2C)7 (CH2)7CO2H ~'k + HO2C(CH2)3CH3 H H Oleicacid
.
H
H
Housefly sex-attractant
n-Valericacid
Figure 13 Mixed Kolbe coupling reaction
CH3
CHO + Ce(III) anodicallyrecycled
+ Ce(IV) Electrochemicalroute
CH3 [ ~
CI
CI2
CHCI2 CI [ ~
CHO hydrolysis
[~C'
Alternativechemicalroute Figure 14 Comparison of electrochemical and chemical routes to 2-chlorobenzaldehyde
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J. Cleaner Prod. Volume 2 N u m b e r 2
R R ' C H 2 + 2H20
+ 2HCI
Cleaner technologies and electrochemical reactions: J. Parkes o t h e r reactions. F o r e x a m p l e , t w o - p h a s e electrothiocya n a t i o n of o r g a n i c m o l e c u l e s can replace c h l o r i n e a n d b r o m i n e used in chemical t h i o c y a n a t i o n . T h i o c y a n a t o c o m p o u n d s are used for p r e p a r i n g dyes, insecticides etc. In the t h i o c y a n a t i o n of 2 - m e t h y l p h e n o l as a m o d e l c o m p o u n d , c u r r e n t efficiencies of 50% were achieved. This r e a c t i o n has b e e n used with p h e n o l s a n d amines. H o w e v e r , it can be a p p l i e d to a n y o r g a n i c c o m p o u n d that reacts with t h i o c y a n o g e n 24. This review has tried to show the possibilities of e l e c t r o c h e m i c a l t e c h n o l o g y as a useful a l t e r n a t i v e clean technology. It can p r o d u c e a wide r a n g e of chemicals with less net waste p r o d u c t i o n a n d at lower e n e r g y c o n s u m p t i o n . It will n o t solve all p r o b l e m s , b u t will solve some p r o b l e m s m u c h b e t t e r t h a n a l t e r n a t i v e technologies. It should be r e g a r d e d as a useful synthetic tool to k e e p in o n e ' s a r m o u r y to be used w h e n r e q u i r e d . It is easily used as a t e c h n i q u e b e c a u s e the reactors, electrolysis cells, can be b o u g h t in from r e p u t a b l e c o m p a n i e s with good k n o w l e d g e of this area. A s well as good texts a n d reviews of the chemical a p p r o a c h 25-28, t h e r e are also good b o o k s o n the chemical engineering approach to electrochemistry29 32. W h a t is essential is a willingness to explore this t e c h n o l o g y , a n d a willingness to c h a n g e , w i t h o u t which p o l l u t i o n p r o b l e m s will n o t be solved.
11 12
13 14 15 16 17 18 19 20 21 22
23
References
10
Fuel Cell Fact Sheet, Gas Research Institute, Chicago, May 1993 Coghlan, A. New Scientist 1994, 143 (20 August), p. 21 Pletcher, D. and Walsh, F.C. 'Industrial Electrochemistry' 2nd edn, Chapman and Hall, London, 1990, pp. 256-269 Greenwood, N.N. and Earnshaw, A. 'Chemistry of the Elements', Pergamon Press, Oxford, 1993, pp. 45-47 Davy, Sir H. in Knickerbocker, W.S. 'Classics of Modern Science', Knopf, New York, 1927, pp. 183-189 Pletcher, D. and Walsh, F.C. 'Industrial Electrochemistry' 2nd edn, Chapman and Hall, London, 1990, p. 322 Sports, D.A. 'Economic Evaluation of a Method to Regenerate Waste Chromic Acid-Sulfuric Acid Etchants', Bureau of Mines Information Circular 8931, United States Department of the Interior, 1983 Pletcher, D. Paper presented at 'Second International Forum on Electrolysis in the Chemical Industry', Florida, November 1988 Jackson, C. and Kuhn, A.T. in 'Industrial Electrochemical Processes', Ed. A. Kuhn, Elsevier, Amsterdam, 1971, p. 515 Parkes, J. and Grimshaw, J. in 'Summary Reports of the R&D Programme, Recycling of Non-Ferrous Metals', Report
24 25 26 27 28 29 30 31 32
EUR 13646, Commission of the European Community, Luxemburg, 1992, pp. 123-142 Brochure on Dished Electrode Membrane Cell, Electrocatalytic Inc., Milltown Court, Union, NJ 07083, USA Robinson, D. in 'Electrosyntheses, from I,aboratory, to Pilot, to Production' (Eds J.D. Gender and D. Pletcher), The Electrosynthesis Company, Inc., New York, 1990, pp. 21%226 Toomey, J.E. Jr US Patent 4,589,968 (Reilly Tar & Chemicals Corp.), 1986 Tait, S.J.D. in 'Electrochemical Technology in Industry, A UK Status Report" (Ed. S.J.D. Tait), Society of Chemical Industry, London, 1991, pp. 36-37 BASF Electrochemical Intermediates, 'A new generation with new advantages', Intermediates Brochure from BASF, Ludwigshafen, Germany Degner, D. in 'Electrochemistry III' (Topics in Current Chemistry 148) (Ed. E. Steckhan), Springer-Verlag, Berlin, 1988, p. 19 Degner, D. in 'Electrochemistry III' (Topics in Current Chemistry 148) (Ed. E. Steckhan), Springer-Verlag, Berlin, 1988, p. 31 Degner, D. in 'Electrochemistry III' (Topics in Current Chemistry 148) (Ed. E. Steckhan), Springer-Verlag, Berlin, 1988, p. 65 Parkes, J. British Patent 1,465,892, 1977 Weinberg, N.L. in 'Electrosyntheses, from Laboratory, to Pilot, to Production' (Eds J.D. Gender and D. Pletcher), The Electrosynthesis Company, Inc., New York, 1990, p. 9 Singh, K.N. and Misra, R.A. Bull Electrochem. (India) 1990, 6(10), 832 Kreh, R.P. et al. in 'Electrosyntheses, from Laboratory, to Pilot, to Production' (Eds J.D. Gender and D. Pletcher), The Electrosynthesis Company, Inc., New York, 1990, p. 195 Couper, A.M. in 'Electrochemistry for a Cleaner Environment' (Eds D. Genders and N. Weinberg), Electrosynthesis Company, Inc., New York, 1992, p. 241 Krishnan, P. and Gurjar, V.G.J. Appl. Electrochem. 1993, 23(3), 268 Baizer, M.M. and Lurid, H. (Eds), 'Organic Electrochemistry', 3rd edn, Marcel Dekker, New York, 1990 Electrochemical Processing--A Quarterly Commentary, from ICI Electrochemical Technology, PO Box 14, Runcorn, Cheshire, UK Roberts, R., Ouellette, R.P. and Cheremisinoff, P.N. 'Industrial Applications of Electroorganic Synthesis', Ann Arbor Science, Ann Arbor, Michigan, 1982 Steckhan, E. in 'Electrochemistry I' (Topics in Current Chemistry 142) (Ed. E. Steckhan), Springer-Verlag, Berlin, 1987 Heitz, E. and Kreysa, G. 'Principles of Electrochemical Engineering, Extended Version of a DECHEMA Experimental Course', VCH, Weinheim, 1986 Pickett, D.J. 'Electrochemical Reactor Design' (Chemical Engineering Monographs 9), Elsevier, Amsterdam, 1979 Walsh, F. 'A First Course in Electrochemical Engineering', 1993, The Electrochemical Consultancy, Queens Close, Romsey, Hants, UK Fahidy, T.Z. 'Principles of Electrochemical Reactor Analysis' (Chemical Engineering Monographs 18), Elsevier, Amsterdam, 1985
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