Reviews in Applied Electrochemistry Number61
Towards paired and coupled electrode reactions for clean organic microreactor electrosyntheses
CHRISTOPHER A.PADDON1,MAHITO ATOBE2,TOSHIO FUCHIGAMI2,PING HE3,PAUL WATTS3, STEPHEN J.HASWELL3,GARETH J.PRITCHARD4,STEVEN D.BULL5and FRANK MARKEN5,*
1Physical and Theoretical Chemistry Laboratory,Oxford University,Oxford,OX13QZ,UK
2Department of Electronic Chemistry,Tokyo Institute of Technology,4259Nagatsuta,Midori-ku,Yokohama,
226-8502,Japan
3Department of Chemistry,University of Hull,Hull,HU67RX,UK
4Department of Chemistry,Loughborough University,Loughborough,Leicestershire,LE113TU,UK
5Department of Chemistry,University of Bath,Claverton Down,Bath,BA27AY,UK
(*author for correspondence E-mail:F.Marken@Bath.ac.uk)
Received24June2005;accepted in revised form13January2006
Key words:coupled processes,electrolysis,electrosythesis,microfluidics,microreactor,paired reactions
Abstract
Electrosynthesis offers a powerful tool for the formation of anion and cation radical intermediates and for driving clean synthetic reactions without the need for additional chemical reagents.Recent advances in microfluidic reactor technologies triggered an opportunity for new microflow electrolysis reactions to be developed for novel and clean electrosynthetic processes.Naturally,two electrodes,anode and cathode,are required in all electrochemical pro-cesses and combining the two electrode processes into one‘‘paired’’reaction allows waste to be minimised.By decreasing the inter-electrode gap‘‘paired’’reactions may be further‘‘coupled’’by overlapping diffusion layers.The concept of‘‘coupling’’electrode processes is new and in some cases coupled processes in micro-flow cells are possible even in the absence of intentionally added electrolyte.The charged intermediates in the inter-electrode gap act as electrolyte and processes become‘‘self-supported’’.Hardly any examples of‘‘coupled’’paired electrochemical processes are known to date and both‘‘paired’’and‘‘coupled’’processes are reviewed here.Coupled electrode processes remain a challenge.In future‘‘pairing’’and‘‘coupling’’electrode processes into more complex reaction sequences will be the key to novel and cleanflow-through microreactor processes and to novel chemistry.
1.Introduction
Electrosynthesis is based on adding or removing elec-trons with defined energy to/from a chemical process and can be employed to drive a wide range of clean chemical transformations[1,2].The formation of highly reactive intermediates as well as of electrogenerated acids or bases are possible[3,4].In several industrial processes large scale electrosynthetic in the nylon-6,6synthesis[5])have been successfully applied and specially designed reactor systems have been developed such asfilterpress systems,membrane reactors,fluidised bed reactors,or parallel-plateflow systems with down to G1mm inter-electrode gaps[5]. In organic synthesis,over the recent years microreac-tor systems have gained popularity[6,7]and they have been applied in syntheses where good reaction control, reactive intermediates,high yields,modularity,and small scale are desirable[8–10].As will be shown in this overview,micro-flow reactor systems are also ideal for electrosyntheses.New types of‘‘paired’’or‘‘cou-pled’’electrosynthetic processes could be developed in future to be conducted in micro-flow systems.However, a better understanding or how to beneficially‘‘pair’’or ‘‘couple’’electrode processes is prerequisite.The main focus of this review is on reaction concepts and examples rather than on cell design and process envi-ronments.
All electrochemical reactions occur in pairs(oxidation and reduction),thus obeying the law of electroneu
tral-ity:the number of electrons added at the cathode must simultaneously be removed at the anode.Often atten-tion is focused on only one of the reactions in a divided cell arrangement or the accompanying reaction at the counter electrode is chosen to be innocuous,not to interfere with starting materials,intermediates,or prod-ucts.In‘‘paired’’electrochemical syntheses both the anodic and cathodic reactions can be matched and contribute to the formation of thefinal product(s)as has been emphasised and defined by Baizer[11].Using
Journal of Applied Electrochemistry(2006)36:617–634ÓSpringer2006 DOI10.1007/s10800-006-9122-2
paired electrochemical processes to produce chemicals can result in a considerable reduction in energy con-sumption and by pairing electrode processes at a closely spaced anode and cathode,paired electrochemical processes can now be conducted without the need for an intentionally added supporting electrolyte[12].The product is obtained directly by solvent removal and without laborious separation steps,which is of consid-erable benefit in both laboratory scale and commercial processes.
In this review a range of‘‘paired’’electrode processes are discussed and different types of processes going from ‘‘uncoupled’’to‘‘strongly coupled’’are compared.The benefits and challenge of coupling elect
rode processes and possible applications of micro-flow reactor systems for electrosyntheses are highlighted.The future chal-lenge of identifying new‘‘paired’’and‘‘coupled’’pro-cesses and the promise of new chemical reactions under micro-flow electrosynthesis conditions are emphasised.
2.Pairing electrosynthetic processes
Early work on‘‘paired electrosynthesis methodology’’was reported in the late1980s particularly by Baizer and co-workers[13].Special cells withflow-through opera-tion and novel porous electrodes have been developed [14].There are well-studied examples of paired electro-syntheses such as the formation of ferrocene with a sacrificial iron electrode[15]or the anodic and cathodic transformation of glucose to give gluconic acid and sorbitol[16].This latter process may be classified as ‘‘divergent’’since two products are obtained from one starting material.In addition,there are many examples for‘‘parallel’’and‘‘convergent’’processes.A method-ology often employed to pair electrode processes is based on the cathodic generation of hydrogen peroxide, H2O2,(or similar peroxo intermediates)from oxygen. Hydrogen peroxide as an intermediate is able to act as an oxidant despite being formed at the cathode and therefore both oxidation and reduction result in the formation of the same product(vide infra).
In commercial electrosynthetic processes,pairing electrode processes has always been important.The most prominent example of a paired electrosynthesis is the production of adiponitrile by electrohydrodimerisa-tion of acrylonitrile,which is an industrially important intermediate used in the manufacture of nylon-6,6[17].The Monsanto process may be regarded as a‘‘paired’’process employing a two-phase reaction mixture and was commercialised by utilising an undivided cell consisting of a bipolar stack of carbon steel evaporated on one face with a thin layer of cadmium.Plastic spacers set the inter-electrode gap at2mm.From100–200 electrodes are placed in a single stack,and the assembly installed in a pressure vessel,designed to provide a uniform,two-phase re-circulating aqueous emulsion of acrylonitrile,adiponitrile,and a bisquaternary salt/ phosphate buffer system[18].The cell stack is operated at a current density of20A dm)2and3.8V per cell and the power usage at2.4kWh kg)1is less than half that for the corresponding divided cell process (6.0kWh kg)1).For the recovery of the adiponitrile product,a side stream of catholyte is withdrawn from each circulating system,cooled,and counter-currently contacted with excess acrylonitrile in a multistage extraction column.A portion of the aqueous electrolyte in this traditional macroreactor process is also purged from the electrolysis circulating system to remove the organic and metal ion impurities that can adversely affect the reaction selectivity and current efficiency. The use of the undivided cell system and two-phase conditions are the keys to high chemical and cost efficiency. The overall reaction scheme for thi
s process,which is based on two starting materials(acrylonitrile and hydrogen) reacting at cathode and anode to give adiponitrile as a coupling product,is summarised in Figure1[19].
From this reaction scheme the importance of coupling anode and cathode processes is immediately apparent. For processes to be developed it is important to take into consideration the overall reaction scheme including anode and cathode reaction.It is very interesting to ask ‘‘what are the effects of bringing the two electrodes, anode and cathode,even closer together’’?The recent development of new micro-reactor and micro-flow cell technology for synthetic processes[20]is now stimulat-ing further research into electrochemical reactions which proceed in undivided cell reactions and/or under condi-tions where the diffusion layer of anode and cathode are made to overlap.
Inventing paired syntheses,particularly if they are going to be carried out in undivided cells,presents many intriguing challenges.The raw materials before electrol-ysis must be compatible.The intermediates(or prod-ucts)formed at one electrode must not react irreversibly with the intermediates or products formed at the
other Fig.1.Schematic presentation of the Monsanto process[19].
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electrode.Above all,an intermediate must not,before reacting,return to the counter electrode and undo the electron transfer(s)that were previously accomplished.For paired electrode processes the selectivity and design of electrodes is important to avoid unwanted reactions and the development of new types of modified [21]or selective enzyme electrodes [22]will further help to improve this technology.The material used for the electrode often influences the reaction.The chemical interaction o
f an electroactive species generated from the substrate and the electrode is a particularly impor-tant factor in the selection of the material for the cathode or anode.Novel materials such as boron-doped diamond [23]or Ebonex [24]are now available to control electrochemical processes and many cases of specific electrode–substrate interaction exist.The inter-action of graphite with formaldehyde is,for example,a promoting factor in the reductive dimerisation of formaldehyde to ethylene glycol [25].A tin cathode has been found particularly effective for the intramo-lecular coupling of a carbonyl group with an aromatic ring [26].Porous Ebonex can be beneficially employed in processes where anodic solvent decomposition is re-quired (e.g.for the eletrogeneration of acids or bases).The type of electrode material(s)is therefore critical to the selectivity of a process.When considering a paired or coupled process,each electrode may be chosen to support a particular process.
The reactor design is crucial for efficient electrosyn-theses.Flow through systems have been proposed with channel geometry [27]or with flow through porous plug electrodes [28].Rather than employing a flow-through
system with anode and cathode placed vis-a
reactor pressure vessel-vis,elec-trode geometries such as the interdigitated electrode array can be highly effective [29](see Fi
gure 2a).An interdigitated electrode system (arrays of microscopic electrodes made from metal sputtered into patterns onto a substrate)system can now be produced routinely and in extremely small dimensions via advanced lithographic techniques [30].Each electrode has a band geometry with a bandwidth of typically 0.1–100l m and a gap between electrodes of similar size.For the interdigitated
band electrode of equally sized electrodes and gaps,the steady-state feedback current has been determined [31,32]and the approximate (steady state)diffusion layer thickness d can be equated in good approximation to the width of the inter-electrode gap between the electrodes,w (Equation 1).d ¼w
ð1Þ
For all common (uniformly accessible)hydrodynamic electrodes the (steady state)diffusion layer thickness can be expressed based on the Nernst diffusion layer model (Equation 2)[33].d ¼n F A D C Bulk =I lim
ð2Þ
In this expression,d is the diffusion layer thickness,n the number of electrons transferred per molecule
diffusing to the electrode,F the Faraday constant,D the diffusion coefficient,C Bulk the bulk concentration,and I lim the mass transport limited current.Forced convection will reduce the diffusion layer thickness and therefore increase the current.Typical values for the diffusion layer thickness at hydrodynamic electrodes are 10–500l m [34]and in more forceful agitated systems,values down to 1l m [35]and even <0.1l m [36]can be achieved.
With reference to Figure 2and by comparing the above two equations it can be seen that ‘‘coupling’’of paired processes will occur when the inter-electrode gap (width w for interdigitated electrodes or height 2h for channel electrodes)approaches approximately 2d .For non-uniformly accessible electrode systems such as the channel electrode (Figure 2b)coupling will be increasing towards the trailing edge of the electrodes (The dotted line is indicating the approximate extend of the diffusion layer).On the other hand,increasing the rate of flow through the channel cell will decrease the diffusion layer thickness and therefore decouple the processes at anode and cathode (see Figure 3b).Similarly,flow across an interdigitated array electrode will also affect the cou-pling between processes and essentially decouple process at high flow rates (see Figure 3a).
There are further types of electrode systems similar to interdigitated band arrays but these are based on porous insulator separated electrodes with flow on both or on only one side of the electrode system
s (see Figure 3c).These can be expected to follow similar principles except that convective flow has a smaller effect on the concen-tration profile within the porous insulator or membrane.Finally,in addition to the effect the diffusion layer thickness has on processes in micro flow systems,the magnitude of the reaction layer [37]provides a further important parameter for the interaction of the two electrodes and for the pathway of overall chemical processes triggered by the electrochemical process.A full understanding of ‘‘coupled’’electrochemical processes will require a detailed understanding of both diffusion layer and reaction layer composition,for example based on a detailed numerical
simulation.
Fig.2.Schematic drawing of (a)an interdigitated electrode system and (b)a rectangular duct micro-flow cell electrode system.
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3.Parallel,divergent,and convergent paired electro-organic reactions
3.1.Parallel paired electroorganic syntheses
The least complicated approach to a paired electro-chemical process is to use two starting materials which are converted into two products without any chemical interaction during the electrolysis process.There is one prominent example for this type of process which is the formation of phthalide and 4-(t -butyl)benzaldehyde dimethylacetal.
In 1999,BASF AG introduced a parallel commercial paired electrosynthesis for the simultaneous formation of two products.Methyl phthalate is cathodically reduced to phthalide while at the anode 4-(t -butyl)tol-uene is oxidised to give 4-(t -butyl)benzaldehyde dime-thylacetal (Figure 4)[38,39].In this process methanol is used both as a reagent and as a solvent.However,overall as much methanol is rel
eased from reduction of the diester as is consumed in making 4-(t -butyl)benzal-dehyde dimethylacetal.Protons are simultaneously gen-erated at the anode and consumed at the cathode.3.2.Divergent paired electroorganic syntheses
The use of a single starting material can be beneficial when two distinct products are formed at cathode and anode.In this ‘‘divergent’’process the overall balance of
charges is maintained and the two products have to be separated post-electrolysis.The paired oxidation of glucose to gluconate with the simultaneous reduction of glucose to sorbitol is an example of a divergent paired,industrial scale electroorganic process.The indirect oxidation by anodically generated bromine of glucose to gluconate is used commercially.The electro-chemical reduction to sorbitol (and mannitol)was for some years an industrial process until a catalytic process replaced it.It was postulated that if the two could be paired into a single undivided flow cell,the economics for each would be improved and full details of this procedure have since been published [40,41].The reaction at the cathode involves the electrochemical hydrogenation of glucose.An indirect oxidant (Br 2)is then electrochemically generated at the anode and oxidises the reactant in the bulk solution.Hydrogen evolution is a side reaction at the cathode (Figure 5).A continuous,electrochemical tubular –stirred tank reac-tor was employed (see Table 1).
3.3.Convergent paired electroorganic syntheses Processes in which two starting materials upon oxida-tion and reduction give the same product may be employed in a ‘‘convergent’’process.Although it is unusual for oxidation/reduction processes to yield the same product,there are several examples of this type of
process.
Fig.3.Schematic drawing of the concentration profile (dotted line)in the presence of strong or weak convective flow (see arrow)for (a)interdigitated electrodes,(b)channel electrodes,and (c)a semi-open ‘‘sandwich’’
electrode.
Fig.4.Schematic presentation of an industrially applied (BASF AG)paired electrosynthesis of phthalide at the cathode and 4-(t -butyl)benz-aldehyde dimethylacetal at the anode [38,39].
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3.3.1.Formation of glyoxalic acid
Through paired electrosynthesis,oxalic acid and glyoxal were separately reduced and oxidised yielding glyoxalic acid [42].In the oxidation process,both chlorine and oxygen gas could be used as an oxidising agent for glyoxal.It was found that the percentage of transfor-mation and selectivity was higher with chlorine gas than with oxygen gas (Figure 6).Overall,this process high-lights the advantages of a paired synthesis with high percentage yield,selectivity,and purity,low waste and a ‘simple’synthesis that is normally very difficult to perform by conventional chemical methods.
3.3.2.Formation of 2-alkoxy-tetrahydrofurans
Ishifune et al.[43]describe the electroreduction of aliphatic esters to form 2-alkoxytetrahydrofurans using a novel electrolysis system (see Table 1).The paired electrolysis system consists of a cathodic reduction of aliphatic esters and an anodic oxidation of THF solvent forming the tetrahydrofuranyl-protected form of a primary alcohol (Figure 7).Ultrasound was employed to continuously unblock the electrodes.An interesting effect of magnesium in the process was postulated.The authors suggest that magnesium ions promote the electroreduction of aliphatic esters which are hardly reduced under usual electroreductive conditions.Over-all,from a synthetic viewpoint,this is a novel example of a paired electrosynthesis yielding protected alcohols directly from aliphatic esters.
3.3.3.Formation of propylene oxide
Interdigitated coplanar band electrodes were made from platinum ink printed on alumina and their application to the epoxidation of propylene investigated in both a tank cell and flow cell [44],using a convergent paired process.Bromide solutions were saturated in propylene,then the bromide oxidised to bromine at the anode,which then reacts with water to form hypobromous acid.This in turn reacts with propylene to form propylene bromohydrin.The hydroxide ions produced at the cathode were then necessary to complete the overall reaction (Figure 8).
The performances of various platinum arrays were investigated.Many factors were found to influence their results such as the flow conditions,applied voltage,cell temperature,electrolyte concentration and the inter-electrode distance.Experiments were carried out with inter-electrode distances of the different arrays varying between 250l m and 1mm (see Table 1).
3.3.
4.Formation of benzyl –nitroalkyl adducts
Chiba and co-workers have reported [45]a benzylic nitroalkylation process,accomplished by the paired reaction of various benzyl phenyl sulfides and nitroalk-anes in the presence of lithium perchlorate.Paired electrolyses were performed in an undivided cell,with divided cells actually giving th
e desired products in very poor yields (Figures 9and 10).Similar reactions using a benzylphenylether gave no desired
nitromethylated
Fig.5.Schematic presentation of the paired production of gluconic acid and sorbitol from glucose [40,
41].
Fig.6.Schematic presentation of the convergent glyoxalic acid electrosynthesis [42].
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