Electrosynthesis
Electrosynthesis Technology Norman L. Weinberg The Electrosynthesis Go., Inc., P.O. Box 16, East Amherst, N Y 14051 Table 1. S o m e E l e c t r o c h e m i c a l Products An examination of introductory general chemistry textbooks leads me to believe that the majority of chemistry and Metals Al. Zn. Cu, Na. Mg, Ni, Mn, Ca, Cd, Cr. Li. Ag. Au, chemical eneineerine students eain an inadeouate awarerare earths ness of electksynth'sis princip~Hsand technology. Yet, the Gases CIS.Br2. F2, H2 suhiect matter is hiehlv" relevant to a fuller anoreciation of .. Inorganic Chemicals NaOH. KOH. NaOCI, CIOS-. CI04-. BrOz-. 1 0 ~ ~ such concepts as oxidation and reduction, solution properties, lo4-. S20aZ-, H,02, BOzZ-, MnOl. KMn04. heterogeneous reactions, etc., not to mention the many diverse We(CN), Oroanic Chemicals Adioonitrile: oroanofluarine comoounds: and important chemicals which are derived from electrotetraalkylead: dimethyl sebacate: ascorbic chemical processes. Tahle 1lists some of these urocesses 11-5) acid: anthraquinone: NaOCH3: (indirectly)includingthe aluminum and chloralkali indushies which will PVC, nylon be described more fully in separate reDorts. A further relevant point is that there are ever increasi& demands on industry where electrochemical routes produce unique products and for lower energy routes which are a t the same time, environdo so in reaction pathways which in themselves are unique. l~ n~em?Il\x c e p ~ m l ,m i l utilix. 111, re readily ~ w i l , ~mlc ~l le>. Over and over again, research workers have discovered that x p i 1t.1 ~ l ~ r ~ l cElt.ctt.cn kj. hc.miwl a y n f h e ~ (liieri i~ ,in simpler and less expensive starting materials may he used in ~ ~ c,l~jwtive . L ~ . ill th~s, errzctiw ~ltcrn,iri\.t.i n ,umz : I I > ~ AOur i s -elticm on t ~ ~ ~ ~ l r ~ ~ ~ ~electrochemical n l l i e ~ i ~conversions of organic chemicals (i.e., adiand Ill€ d h t r prex n l ~ l i . ~i l~l this ponitrile process, electrosynthesis of phenols). An appeal of is to provide an improved perspective of electrosynthesis electrochemical routes is that they can be controlled so well, technology for chemical educators and their students. by means of the electrode potential (and electrode material) Electrochemical processes by their nature are all heteroto give a high selectivity of desired product. Lastly, the same geneous reactions which occur a t electronic and ionic interreactor can sometimes produce useful products a t both the faces, namely electrodes and solutions. At the anode. oxidation occur;, in which electrons are removed from oxidizable anode and cathode, with considerable capital and energy cost benefits (i.e., chloralkali). Nevertheless, electrochemical solution comuonents and enter the electrode. Simultaneouslv. ". methods do not appear to compete favorably in some inthe cathode, in reduction processes, gives up electrons to reducible solution comuonents. stances. For instance, catalytic oxidations with 0 2 , or hydrogenations with Hz, are more economically done chemically Why should one do electrochemical reactions a t all, espethan by using analogous electrochemical routes. cially if chemical routes exist? Electrochemical reactions are To conduct an electrochemical svnthesis reaction. we need often much cleaner, with respect to possible pollutants, than to have an understanding of the reaction variahles and their chemical reactions. After all, electrons as a reagent, are ininterdependence (Table 2). In addition to the usual chemical herently pollution-free . . . a t least a t the point of use. Compare variables, due consideration must he given to the roles of the this with oxidations, for example, with CrOa or reductions with electrochemical variables. One might conclude from the list, Fe and HC1. In many cases, economic considerations favor the a t first sight, that chemical processes are complex enough and electrochemical route (i.e., Al, chloralkali) over any existing adding the further list of variahles would make electrosynchemical processes. But there are also a numher of examples
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268
Journal of Chemical Education
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Figure 1. Set-up for controlled-potentialeiectrolysis In a one-compartment cell (Beaker).
WORKING ELECTRODE rWXlLlARl ELECTRODE
thesis a nightmare to understand! Actually, this is far from true. The electrode potential offers us an unparalleled means of reaction control. The electrode potential is controllable bv m e u n s .,id .pwi.tl I ) ( ' l ~ ~ use~ rt p p l v~ I I L A \ I aia/~c.ie I u!Ld,tr ~ K xI t.. Tht- d w i o w i l l ~ ~ I I I I : I ~ ~I I h ~t x ~ t e n ~I J iI ;t h~ el I, ~ L I I L
Figure 2. Schematic drawing of electrolysis cell for lab use. The working electrode compartment is stirred well and the cell is thermostatedatthe desired temperature.
tentiostat is indispensable in the electrosynthesis laboratory but is impractical for industrial scale processes. Instead, the electrode potential is maintained reasonably constant by keeping the concentration of electrophore (i.e., the electroactive species) constant. The rate of an electrochemical process is expressed by the current density, that is the current flow per unit area. Practical current densities are greater than about 50 mA ~ m - depending ~, on the process. Lower current densities would signify very low rates of conversion of reactants. A laboratory scale electrosynthesis of a few grams is often conducted in a three-compartment cell, (Fig. 2) in which the anolvte. catholvte, and reference electrode compartment so-
We are normally concerned with these variables in chemical reaCtiMs
Table 2.
Variables to Consider in Electrochemical Reactions
Usual Variables
some preliminary electroanalytical studies (2-5). A controlled potential electrolysis is done using a potentiostat, and the product in the working electrode compartment is then analyzed and the yield calculated. Witha coulometer in the circuit (Fig. I),the current efficficlency may also be calculated. Theoretically, 96,487 coulombs are required to convert one mole of a substance in a one electron process. This is equivalent to passing 26.8 amperes through the cell for 1hour. In contrast, commercial cell installations may require currents of hundreds of thousands of amperes, operating continuously. The undereraduate laboratorv could easilv demonstrate manv electrosynthesis principles, with relatively inexpensive (under 52000) e a u i ~ m e n(6). t Several possible ex~erimentsinclude. 0
quinone. Suitable anode materials are fewer in number than cathode materials, because anodes most he stable to corrosion processes, as well as is often the case, possess chemical resistance to the intermediates and products produced. For example, Hg is too easily oxidized to be useful as an anode for most kinds of processes. Commonly used anode materials are P t , PbOz, carbon or graphite, and "ruthenized" titanium (DSA@anode). Somewhat less stable. excent under s~ecialconditions, are Ni. Au, and Ag. Fe and Ni are especially useful in aqueous alkaline solution, where they are reasonably stable to corrosion. Ni and Monel are excellent for electrofluorination reactions of organic com~ounds.Most, if not all anode materials. are covered with a conductive oxide layer, which is the actual electronic inter-
I
Concentrations 01: (1)Solventisupporting electrolyte (2) Substrate (3) Other components Temperature Presswe pH Time
Electrochemical Variables
An electrochemical reaction requires knowledge of both the usual and the electmchemical variables
I
Electrode potential Current density Electrode material Electric field Adsorption Cell design (i) compartmented? (2) membrane or porous separator? (3) static or flow cell? Solution Conductivity
face meeting the solution components. In the case of electrofluorinations, commonly carried out in anhydrous HF solution, the nickel electrode is believed to be covered with a high valent nickel fluoride species, namely NiF2-6, which, conveniently, is also the fluorinating agent. Useful cathode materials comprise a much broader list and include Hg, Pb, Al, Ag, Ni, Fe, Cu, Sn, Cd, carbon and graphite, Pt, Au, Pd, amalgams such as lead amalgam, and alloys. Besides chemical and electrochemical stability, electrodes are chosen for industrial use to be relatively inexpensive, highly conducting, and electrocatalytic. An electrocatalytic electrode is one which exhibits a low oueruoltage for a given process (i.e., the electrochemical reaction at that electrode occurs nearer to the theoretical potential for the oxidation or reduction). In addition. electrocatalvsis implies high selectivity, . product . especially important in the electrosynthesis of organic compounds. Figure 3 indicates the range of products possible from acetone when electrochemically reduced at various cathode materials. The ionic medium, in an electrochemical cell can be an aqueous solution (the chloralkali industry), a molten salt (the aluminum industry), an organic or inorganic non-aqueous solution (electrofluorination), or an aaueous emulsion (adi-
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Table 4. Potential Ranges of Some Non-Aqueous Solvent1 Supporting Electrolyte Systems on Pt versus SCE
I
Pt, Ni
Supporting Eiectrolvie
Solvent
+
I
isopropyl alcohol
1
Hg (strong acid)
- cT/
H-CHg--C-H
\
CH, CHS Some of the Electrogenerated Species Involved:
Acetic Acid Acetonitrile Ammonia (liq.) Oimethylformamide Dimethylsulfoxide Hydrogen fluoride (liq.) Methanol Methanol Methylene chloride Propylene carbonate Pyrldine Suifolane Sulfur dioxide (liq.) Tetrahydrofuran Trifluoroacetic acid Water
Fe3+/FeZ+ Fe(CN),-3/Fe(CN)6-4 Mn04-lMn04-2 Ni3+/NiF,-Z TI"lTI+ Co3+/CoZ' Sn4+/Sn2+ Ce2+/Ce3+ Cu2+/Cut VOS~1VO2+ HIOdH103 NaHgIHg NaOCiINaCl or NaOBrINaBr 0s04/[0s02/(OH)4]2BrdBri2/i-
--
nitro aromatic^ aniline% hydroquinone quinone Acrylonitrile polymerization Benzene oxidation Oxidation of aromatics Electrofiuorination I-Butene to methyl ethyl ketone Oxidation of aromatics Reduction of nitrocompounds Anthracene to anthraquinone Hydroxylation of aromatics Oxidation of aromatics Dialdehyde starch process Hydrodimeriration Propylene oxide from propylene. oxidation of sugars Olefins to glycols Aikoxylation of furans Halofun~tiona1,ization:Prevost reaction
om-., -a ,
+
Journal of
Chemical Education
--
- -+
+
+
+
-
-
+ COZ
Charge transfer occurs with some other species, which then reacts with the electrophore (see Table 3) with a species generated in the electrode surface (exampie: see electiofluorination) With a species generated on the electrode surface: (examples: C ,,I HOsada-. Hadr) With a redox species in solution: (example: Snd+/Sn2+ in reduction of nitroaromatics to anilinesl
Scope of Electrochemical Synthesis Reactions
1) Direct Electron Transfer to Form inorganic And Organic Compounds (a) Metal deposition (i.e.. Ai) (b) Oxidations and reductions of organics (i.e.. CH,OH to CH,O) 2) Direct Electron Transfer To Generate Reactive Species (a) inorganic Atoms And Radical Ion Species: (i.e., H., CI.. Na.. .OH. NOr, co2-.. 02--1 (b) Charged Organic Species: (i.e.. cation radicals (Rt), anion radicals (R-). carbonium ions (R.i R2+). carbanions (R-. R2-) 3) Indirect Electron Transfera (a) Via regenerative redox species in solution or on the electrode (ie.. Ti4+/Ti3+. Sn3+/Sn2+, solvated electron1 (b) Via displaceable electrogenerated haiogen CI, 2e (i.e., 2CICI, CH3CH=CH, 20Hpropylene oxide 2CI4) Generation 0 1 Acid At The Anode And Base At The Cathode (Direct) 5) Halogenation: 2XX2 2e (Direct) diholoethane) (i.e., Xz ethylene
+
270
+ -
+
+ + +
cationicarbonium ion formation: A A+ e (examples: ~ e ' + Fe3+ e: CH,C02 CH,+ 2 e) anionlcarbanion formation: A e A2e 21r; see acetone reduction) (examples: ,I
Table 6.
chloialkali indust&). What kinds of reactions can he carried electrochemicallv? There are two types of electrochemical synthesis processes (Tables 5 and 6). These may he described as direct or indrrect electron transfer reactions. In direct processes, electron transfer occurs directly with the electrophore. Indirect pro-
-
-
Electrochemical Conversion
and permit easy removal of the product. Often, special additives are present in these suluentlsupporting electrolyte systems to inhibit electrode corrosion, to adsorb on the electrode and thereby promote reactions of interest, while inhibiting other processes, or to act as "indirect" electrontransfer agents (Tahle 3). In spite of the many solvent possibilities, aqueous systems remain the most commonly employed (Table 4). In addition, electrochemical cells often contain microporous seuarators or ion-exchange membranes to senarate the anolvte solution from the catholyte solution. This separation prevents the ~ r o d u c t from s mixine. and uossiblvundereoine the reverse
-3.2 -0.6 -2.7iHal
Charge transfer is the primary act with electrophore A.+e; A+ e A. radical formation: AH.) (examples: Kolbe electrolysis: H+ e A+. e cation-radical formation: A (examples: oxidation of aromatic compounds) A-. e anion-radical formation: A (examples: reduction of aromatic compounds: O2 e
Indirect Electrolysis of Organic Compounds
Ti4+/Ti3+
1 . 0 -3.0 -2.8(Hg) -2.8 -3.4 -1.0 -1.0 -1.0 -1.7 -1.9 -2.2 -2.9
Examples of Direct And Indirect Electrochemical Processes
-
Figure 3. influence of electrode material on the electrochemical reduction of acetone.
Redox Couole
NaOAc LiCIOn (nBu)*NI (nBu)rNCIOn LiCIOI NaF KOH KCN (nBu),NCIO, EtdNClOn EtdNC104 EtnNClOn (~BU)~NBF~ LiCIOl Na02CCF3 (nBuLNCIOn
The useful potential region depends on the solvent. supporting electrolyte. elecbode material, and the impuri* level.
Table 5.
Table 3.
Approximate Useful Rangea Cathodic Anodic
-
+
-
+
+ +
-
+
-
'Electrochemical reactions may be mediated by regenepfable redax couples. which in effect act as "soluble electrodes."
cesses are those in which electron transfer occurs first with some other species present in or on the electrode surface, or in the solution. This oxidized or reduced species then reacts with the electrophore of interest. There are many thousands of electrochemical reactions known of organic and inorganic compounds. Only a few, as in the realm of chemical reactions, ever reach commercialization. Industry is reassessing the commercial viability of electrochemical processes to meet the demands of lower energy routes, environmental acceptability, and use of alternative feedstocks.
Literature Cited (11 'Yndwtrial Electrochemical Process? Kuhn. A. T.. IEddor). Elsevier Publishing Co., New York, 1971. 12) "Technique of Electraorganic Synthesis.(lPt. I, Weinberg, N. L., (Eddorl, 197*Pt. 11, Weinberp,N.L., (Editor], 1975;Pt.III. Weinberg, N.L.,andTilak.B.V.. IEdifars), 1982. John Wllevand Sons.New York.
( 5 ) Fry. A. .I.. "Synthetic Organic Eleetrochemistry.((Harperand Row, New York, 1972. (6) Weinhere, N . L., "Suggestions for Undergladuate Experiments in Eleetrosynthesis,(. TheElectrosynth~sisCa.,lnc..P.O. Boa 16,EAmherst. NY, 1982.
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Number 4
April 1983
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