electrochemical hydrodimerization of formaldehyde to ethylene glycol

7/27/2019 Electrochemical Hydrodimerization of Formaldehyde to Ethylene Glycol

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JOURNAL OF APPLIED ELECTROCHEMISTRY 21 (1991) 895-901

El e c tr oc he mi c a l hydr odi me r i z a t i on o f for mal de hydeto e thylene g lyco l*N. L. WEINBERG, D. J. MAZURThe Electrosynthesis Company, Inc., P.O. Box 430, East Amherst, N Y 14051, USAReceived 12 December 1990; revised 1 March 1991

T h e e l e c t r o h y d r o d i m e r i z a t i o n o f f o r m a l d e h y d e p r o c e e d s i n c l o se t o q u a n t i ta t i v e y i e l d a n d c u r r e n te f fi c ie n c y t o g i v e e t h y l e n e g ly c o l . T h e o p t i m u m p r o c e s s c o n d i t i o n s r e q u i r e a n a q u e o u s f o r m a l d e h y d es o l u t i o n o f h i g h c o n c e n tr ~ t fi o n , a p H r a n g e o f a b o u t 5 t o 8 , t h e p r e s e n c e o f a q u a t e r n a r y a m m o n i u ms a lt , a n d r e a c t i o n t e m p e r a t u r e s i n e x ce s s o f a b o u t 8 0 ~ C . R e m a r k a b l y , t h e p r o c e s s o n l y w o r k s w e l l a tg r a p h i t e , a t w h i c h t h e e n z y m e - l i k e sp e c if ic i ty is b e l i ev e d t o b e r e l a t e d t o t h e c a r b o n s u r fa c e o x i d es t r u c t u r e .

1 . I n t r o d u c t i o n

Presently all ethylene glycol (EG) is manufacturedcommerically via petroleum-based ethylene at the rateof about 9 billion kilograms annually worldwide. Inthis process, ethylene is converted to ethylene oxide inthe presence of oxygen and a silver-based catalyst witha selectivity of about 80% and production of CO andCO2 as byproducts. Hydrolysis of ethylene oxideresults in EG. The major uses of EG are in antifreezeand in manufacture of polyesters, such as polyethy-lene terephthalate films, fibres and bottles. Many"C-I" routes, starting from syngas, methanol andformaldehyde have been proposed, and a number ofthese have been piloted over the years [1], but they arenot practiced commercially because of unusually highpressures or catalyst problems. Should an electro-chemical route to EG be commercialized, the processcould be the third largest in volume after aluminumand chloralkali and could significantly lower depend-ency on petroleum.

An electrochemical route based on formaldehydewas first described by Tomilov et al. [2] in 1973. Theseworkers electrolyzed an aqueous solution of formal-dehyde at graphite electrodes at a pH of 2-5 usingpotassium dihydrogen phosphate as a supporting elec-trolyte and a mercury (II) catalyst. EG was reportedlyformed at a current efficiency of 24.9%. This electro-hydrodimerization process is analogous to the com-mercial electrochemical conversion of acrylonitr ile toadiponitrile [3, 4], and is described as follows:

2CH20 + 2H + + 2e ~ HOCH2CH2OH (1)Watanabe and Saito [5] and Saito and Tokuyama [6]reported much improved current efficiencies (83%)using an aqueous alkaline solution of formaldehyde

at graphite cathodes at 50 ~ Mercury salt was notrequired and an additional product, 1,2-propyleneglycol, was found in low current efficiency. Weinbergand Chum [7] repeated the above work and discoveredthat mercury salt acts as a poison. Moreover, thepresence of high methanol concentrations serves tosuppress the process, while high concentrations offormaldehyde favor EG formation. One initial pro-posal was that glycolaldehyde, formed by conden-sation of formaldehyde, was reduced cathodically toEG:

2C H2 0 ' HOCH2CHO (2)HCOH2CHO + 2H + + 2e , HOCH2CH2OH

(3)However, no evidence of glycolaldehyde could befound using gas chromatography. An alternativemechanism involving the intermediacy of formal-dehyde anion radicals could not explain the unusualrequirement for graphite cathodes (only mercuryhas been found 'thus far to provide EG, but at lowcurrent efficiency [8], whereas all other metals andceramic materials investigated are not electrocatalytic).Additional work [9] demonstrated the importance ofpH control, catalysts and poisons; a mechanism wasproposed based on the role of the electrode's carbon-oxide surface.

Subsequently, we studied the reaction variablesin much greater detail and have published variousaspects elsewhere [9-12]. This work demonstrated theimportance of the cathode material, high concen-tration of formaldehyde, a less acidic pH range,high temperature and the presence of quaternaryammonium salt. Furthermore, an unusual mechanismwas proposed in which the graphite cathode assumes

* This paper is dedicated to Professor Dr Fritz Beck on the occasion of his 60th birthday.0021-891X/91 $03.00 + .12 O 1991 Ch ap ma n & Hall 895


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896 N.L. WEINBERG AND D. J. MAZUR"enzyme-like" behaviour. This report presents thefirst full publication, including results which havenot previously been published, detailing the roles ofthe variables, the importance of poisons and otheradditives, current density, flowrate, etc. and providingfurther evidence in support of the original mechanism.2 . E x p e r i m e n t a l d e t a i l sReagent grade chemicals were employed "as received"including aqueous formalin (Eastman Kodak, 39.3%formaldehyde, 11% methanol, and 0.053% formicacid), low methanol grade formalin (DuPont, 39.0%formaldehyde, and 2.6% methanol), sodium formate(Fluka), quaternary ammonium salts (SouthwesternAnalytical), various inorganic salts and other chemi-cals. Carbon cathode materials were soaked in diluteaqueous acid (5% HC1 or 5% H2SO4) for severalhours, followed by washing in distilled water toremove metallic and other impurities.Glass cell experiments were conducted with an Elec-trosynthesis Company (ESC) Model 415 PotentiostaticController, Model 420X DC Power Unit and Model640 Digital Coulometer. Larger scale flow cell studiesused a Sorenson Model DCR60-45B power supply.An ESC Model 800 IR Measurement System was usedfor correction of electrode potentials. Single compart-ment and two compar tment (ESC Model C-600) glassH-cells were provided with magnetic stirring, saturatedcalomel reference electrode (SCE), cathode (Ultra-carbon ST-50 rod) and platinum flag (10 cm2) anode.H-cells employed DuPont Nation R 324 and 390 cationexchange membranes. Approximately 100 ml o f for-malin solution was used in each experiment. In H-celland flow cell studies the anolyte was 10-20% byweight aqueous sulphuric acid. Glass cells were main-tained at constant temperature by means of a waterbath. Where sodium formate was used, the pH of thecatholyte was maintained by addition of formic acidor sodium hydroxide as required.

Flow cell experiments were performed with an Elec-troCell AB (Sweden) MP cell, typically fitted withNation 324 membrane, Union Carbide ATJ graphitecathode and platinized titanium anode assembled in aplate-and-frame manner with polypropylene framesand DuPont Viton R gaskets. The MP cell has a pro-jected electrode area of 0.01 m 2, an interelectrode gapof 12ram and an electrolyte flow rate capability of1-15 dm 3 rain-' per cell. Figure 1 depicts the arrange-ment of the cell, pumps, flow meters, solution reser-voirs and other equipment. Each compartment of themodule was supplied with electrolyte solution from anelectrolyte reservoir via a March (Model TE-MDX-MT3) explosion-proof magnetic drive pump, connectedvia polypropylene tubing. A glass condenser served asa heat exchanger in the anolyte loop, The reservoirswere constructed of 4dm 3 heavy-wall, cylindrical,wide-mouth Nalgene R jars with screw cap lids. Thecatholyte reservoir lid had fittings for recirculatingcatholyte (about 2din 3) from the cell and static loopreturn, vent, thermometer, gas (H2) sampling, liquid

4

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Fig. 1. Arrangement of cell piping, reservoirs, pumps, etc.

sampling, or pH adjustment. The anolyte reservoirhad fittings for recirculating anolyte (about 2dm 3,20% H2804)via the glass heat exchanger and staticloop vent, thermometer, and gas outlet. A pH probewas inserted into the catholyte solution return linenear the reservoir lid to facilitate monitoring andpH adjustment. Two SCE reference electrodes wereinserted by means of fittings into the electrolyte inletsat the MP cell to monitor the cell voltage, electrodepotentials and IR drop. All flow cell experiments wereperformed with continuous recirculation of electrolytethrough the cell and reservoirs. Generally, electrolyseswere conducted to the extent of about 2000 to 15 000coulombs in glass cells and 1 to 3F in MP cell studies.A Hewlett-Packard 7610A gas chromatograph withFID detector and Poropak Q column (2m x 0.63 cm)served to analyze solutions for EG, 1,2-propyleneglycol, glycerol and methanol, using 1,3-propanediolas an internal standard. Formaldehyde was deter-mined according to the method described by Walker[13].3 . R e s u l t s a n d d i s c u s s io nThe roles of the variables, including pH, tempera-ture, concentrations of formaldehyde, methanol andEG, supporting electrolyte, current density, cathodematerials and stirring/flowrate are described in thefollowing.3.1 R ole o f pHWalker [13] has reviewed the role of pH on the chemi-cal stability of formaldehyde solutions. At higher pH,the Cannizzaro Reaction occurs in which hydroxideion causes the dispropor tionation of formaldehyde tomethanol and formate (Equation 4), while at lowerpH, acid catalyzed polymerization occurs (Equat ion 5).


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HYDRODIMERIZATION OF FORMALDEHYDE TO ETHYLENE GLYCOL 897

80

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40t -

2 0r>

I I I I1 2 5 4

t t " \! 9

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w

5I I I I I I6 7 8 9 10 11

C a t ho l y t e pH

Fig. 2. Cu rrent effic iency against ca tholy tepH. Condit ions: anolyte (18% H2SO4), ca tho-lyte 3 M N a-form ate in formalin; temp. 60 ~ C;no additives; current densi ty 100 m A cm -2.

Thus,2CH20 4- OH - ~ CH3OH q- HCO2 (4)

nCH20 H+ ~ polymer (5)The pH range for optimum storage stability of for-maldehyde solutions is about 5 to 7, as is the optimumpH range for depolymerization. Fortuitously, theoptimum pH range for electrohydrodimerization offormaldehyde lies within the range of optimum stab-ility of formaldehyde.Figure 2 plots current efficiency vs pH for a series ofextended electrolyses under constant current con-ditions of 100mAcm -2 in the MP flow cell at 60~with no additives present in the aqueous formalin/sodium formate solution. The optimum pH range isabout 5.5 to 6.5 and the current efficiency dropssharply at both lower and high pH's. We are unable toexplain the results ofTo milo v et al. [2] who studied theprocess at pH 2-5. We could not find EG under thereported conditions. In contrast, the procedures andresults described by the Japanese workers [5, 6] werereadily repeated; under the alkaline conditions, inwhich the pH was not controlled, the pH of theprepared solutions dropped rapidly from about 11to about 8, even before electrolysis. Significant for-mate production therefore occurred (according toEquatio n 4) which served to neutra lize the added base.Furthermore, we found that addition of quaternaryammonium salt and higher electrolysis temperaturescaused the current efficiency maximum to reach nearly100% in the optimum pH region.3.2. The role of temperatureHigher temperatures are found to be beneficial inincreasing the current efficiency for EG formationfrom less than 30% below 40 ~ C to almost 100%, withadded quaternary ammonium salt, at 80 to 90~Higher temperatures are known to increase the avail-

ability of free formaldehyde in solut ion [13] as well asproviding the important additional benefits describedbelow.3.3. Role o f concentra tions o f formaldehy de ,m e t h a n o l a n d E G.In earlier work [7] it was shown tha t the formaldehydeconcentra tion in solution should be greater than about20% by weight to achieve good current efficiencies ofEG. Moreover, methanol was found to inhibit EGproduction, especially above about 20% by weight.These results and the requirement for higher tempera-tures are consistent with the need for free formal-dehyde in unsolvated or hemiacetal form:

C H 2 0 -F ROH . " HOCH2OR (R = H, alkyl)( 6 )

The equilibrium constan t for this hydra tion is quotedas5,7 x 10 4a t3 0o Can d3 .0 x 10 3at64oc[13],indicating that formaldehyde is almost entirely in thehemiacetal form as methylene glycol. Alcohols likemethanol and EG can react with formaldehyde evenmore strongly than water. Therefore, the most efficientproduction of EG requires low methanol grades offormaldehyde, higher temperatures in order to maxi-mize free formaldehyde, and EG concentrations insolution of no greater than aboout 15 to 20% byweight. This is illustrated in Fig. 3, where the use of alow methanol, high concentration (52%) formalinsolution greatly extends the high current efficiencyrange to provide up to 23% EG product concentration.Some scatter was observed in the current efficiencyvalues towards the latter stages of the experiment(Curve A) and this has been attributed to the difficultyin maintaining good temperature control and avoid-ing some evaporation losses of the more volatile 52%formalin solution.


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898 N. L. WEINBERG AND D. J. MAZUR

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20

0 I r [ p ] _ _I0 20 30 40 50 60

O ( I 0 3 )Fig. 3 . Ethylene glycol current efficiency against charge for experi-men ts pe r fo rmed in g lass H-ce l ls w i th Na t io nR 3 24 m e m b r a n e a t80~ wi th 1 .0 M NaCO OH , 1% by we igh t (CH 3)4NOH ca tho ly te ,20% I-I2804ano ly te , Ul t raca rbo n ST-50 ca thode , P t anode . Curves :(a ) 300m Acro -2 52% CH 20 , 2% CH3OH; (b ) 200m Ac m -2 , 39%CH20, 10% CH3OH; and (c ) 750 mA cm -2 , 39% CH 20 10%CH3OH.

3.4. S u p p o r t in g e lec t ro ly teUsing a single compartment glass cell, fitted withgraphite rod cathodes (Ultracarbon ST-50) and aplatinum anode (10cm2), a number of supportingelectrolytes were screened in order to evaluate anyunusual behavior of anions or cations in the process.Aqueous solutions of formalin at 55~ were elec-trolyzed at a cathode potential of -1.9 to -2.2V/SCE with 1.0 M concentrations of (CH3)4NC1, NaC1,Na2SO4 (0.2M), sodium formate, lithium formate,ammonium formate, trimethylammonium formate,potassium formate, NaOH, KOAc and NazHPO4.The pH of these solutions was maintained at 6.5-7.0.Current efficiencies were highest with the quaternaryammonium salt and lowest with ammonium andtrimethylammonium formate. The others behavedalmost equally. Moreover, a study of supporting

Table 1. E lec trolys is of form alin solut ions containing addit ivesAdd it ive (conc) Current e f f ic iency( % ) E GED TA (10 000 p .p .m. ) 82Con trol (no additive) 80FeC12 (50 p.p.m.) 70Pb(OAc)2 (800 p.p.m.) 67M g S O 4 (50 p.p.m.) 65NH4O H (5200 p.p.m.) 64CaCI2 (50 p.p.m.) 56CuC1 (800 p.p.m.) 0 .0

electrolyte c9ncentration showed that the currentefficiency for EG~formation was almost independentof concentration for sodium formate and potassiumacetate. Sodium formate soon became the supportingelectrolyte of choice for larger scale studies because ofits low cost, availability, and good solubility and con-ductivity in formalin solutions.

Quaternary ammonium salts such as (CH3)4N+and (nBu)4N+ when present at about 0.5 to 2% byweight were found to increase the EG current efficiencysignificantly.3.5. P o i s o n sVarious chemicals were screened as potential poisons,including substances which could be present as a resultof manufacture of starting materials as well as cor-rosion products from metallic reagent containers andpiping. Table 1 presents results of electrolysis at 70-100 mAcm -2 for seven addit ives examined in glass Hcells with a catholyte of 100cm 3 of 1.0M sodiumformate in formalin at pH 6.5-7.0 and 55-60 ~ C. Theanolyte was 20% aqueous H2SO4.None of the additives had a significant positiveeffect on EG current efficiency, while the negativeeffect of Mg2+, Ca 2+, NH + and especially Cu 2+ a renoteworthy. Further examination o f Mg 2+, Ca2+ andCu2+ over a range of concentrations demonstratedthat these ions become deleterious at concentrationsas low as 0.1 M, 0.01M and 0.0001 M, respectively.After electrolysis in the presence of Cu2+, a distinctlayer of metallic copper could be seen on the cathode.The marked inhibition noted for Mg2+ and Ca 2+ ions,unprecedented to our knowledge in electroorganicsynthesis, is discussed in w5.

The slight increase in current efficiency seen withEDTA was further examined under flow cell con-ditions at 100mAcro 2, 60_65oc and at a pH of6.5-7.0. Using 1.0M sodium formate in formalin,a control experiment (no additive) gave a currentefficiency for EG of 84%, whereas 1% by weightEDTA gave 86% and 1% ascorbic acid gave 90%.These small increases in current efficiency may beattributed to complexation of polyvalent metal ionswhich are present as impurities in the formalin sol-ution, leached out of graphite electrodes, or result-ing from other sources of contamination. Efficientproduction of EG requires scrupulous attention toimpurity levels of polyvalent metal ions, ammonia andpossibly primary, secondary and tertiary amineswhich could be present as byproducts in quarternaryammonium salts.3.6. Ca th o d ic cu rren t d en s i tyOperation at high current density is a desirable objec-tive for commercial operation in order to minimizecapital costs. Using a single compartment glass cell,with a catholyte of 1.0 M sodium formate in formalincontaining 1.0% by weight tetramethylammoniumformate, at pH 6.5 and temperatures of 80 and 90 ~ C,


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H Y D R O D I M E R I Z A T IO N O F F O R M A L D E H Y D E T O E T H Y L E N E G L Y C O L 899Table 2. Role of cathodic current density

Current density Charge Temperature Current efficiency,( m A c m 2 ) ( C ) ( ~ E G ( % )I 0 0 6 0 5 0 8 0 9 91 0 0 6 7 8 9 9 0 9 02 0 0 1 2 0 4 1 9 0 1 0 0200 13 350 90 904 0 0 1 5 3 7 1 8 0 9 74 0 0 * 1 5 0 5 2 9 0 8 5500 16 625 80 96500 15 500 90 92

1 0 0 0 7 2 3 3 9 0 9 1* N o q u a t e r n a r y s a l t w a s a d d e d .

a s e r ie s o f e l e c t r o l y s e s w e r e c o n d u c t e d o v e r t h e r a n g eo f c u r r e n t d e n s i t i e s g i v e n i n T a b l e 2 . S i m i l a rl y , u s i n gt h e M P c e ll , e l e c t r o l y s is o f 1 .0 M s o d i u m f o r m a t e i nf o r m a l i n c o n t a i n i n g 0 . 6 % b y w e i g h t ( n B u ) 4 N O H ,p H 6 . 5 - 7 .0 , 4 0 0 m A c m - 2 a n d 8 2 ~ C , g a v e a 9 7 % c u r -r e n t e f fi c ie n c y f o r E G u p o n p a s s a g e o f 3 F o f c h a r g e .T h u s E G c a n b e p r o d u c e d i n c u r r e n t e f fi ci e nc i es o f 9 0t o 1 0 0 % e v e n a t r e l a t iv e l y h i g h c u r r e n t d e n s i t ie s .3.7. C a t h o d e m a t e r i a lN o o t h e r e l e c t ro d e m a t e ri a l b u t g r a p h i t e w a s f o u n d t og i v e E G i n h i g h c u r r e n t e f f ic i e n ci e s o f u p t o 1 0 0 % .M o s t o f t h e m e t a l l i c e l e c t r o d e s t e s t e d i n u n d i v i d e dc e ll s a t 5 5 - 6 0 ~ C i n f o r m a l i n s o l u t i o n s ( 1 . 0 M N a C O O Ho r 1 .0 M K O A c ) g a v e n o E G . A n e x c e p t i o n is m e r c u r yw h i c h M o n t e n e g r o et a l . [ 8 ] d i s c o v e r e d c o u l d g i v eE G i n a b o u t 2 5 % c u r r e n t e ff ic i en c y , p r o b a b l y v i a a no r g a n o m e r c u r i a l i n t e r m e d i a t e . T h e c a t h o d e m a t e r i a l sw h i c h g a v e n o E G i n c l u d e P b , C d , S n , Z n , A 1 , C u ,s t a in l e s s s t e e l, c a r b o n s t ee l , N i , V , A g , E b o n e x R( T i 4 0 7 ) , P t / I r o n T i , t u n g s t e n c a r b i d e , a n d s i l i c o nc a r b i d e . O t h e r c a r b o n s p r o v e d t o b e p o o r e r i n p e r -f o r m a n c e t h a n g r a p h i t e , i n c l u d i n g v i t r e o u s c a r b o n( 6 % ) , r e t i c u l a t e d v i t r e o u s c a r b o n ( 2 3 % ) , g r a p h i t ec l o t h ( 2 % ) , a n d a g r a p h i t e f i b r e / e p o x y re s in c o m p o s i t e( 3 2 % ) . E l e c t r o c h e m i c a l p r e o x i d a t i o n o f g r a p h i t e i n1 0 % a q u e o u s H 2 S O 4 g e n e r a l l y r a i s e d t h e c u r r e n te f fi c i en c y . E v e n v i t r e o u s c a r b o n g a v e E G i n 1 7 %c u r r e n t e f fi c ie n c y a f t e r p r e o x i d a t i o n . O t h e r e x p e r i-m e n t s h a v e s h o w n t h a t t h e c u r r e n t e f f ic i e n c y i s a l s oi m p r o v e d i f g r a p h i t e c a t h o d e s a r e f ir s t s o a k e d i n a c id( s u c h a s H C 1 o r H 2 S O 4 ) f o r s e v e r a l h o u r s t o r e m o v ei m p u r i ti e s , f o l l o w e d b y t h o r o u g h w a s h i n g i n d i st il le dw a t e r .3 .8 . Ro l e o f s t i r r ing / f l ow ra t eE x p e r i m e n t s w e r e c o n d u c t e d u s i n g t h e g l a s s H - c e l lc o n t a i n i n g a g r a p h i t e r o d c a t h o d e , P t ( 1 0 c m 2 ) a n o d e ,1 M s o d i u m f o r m a t e a n d 2 % b y w e i g ht ( C H 3 ) 4 N O H i nf o r m a l i n a t p H 6. 5. T h e s o l u t i o n w a s m a i n t a i n e d a t5 5 ~ C a n d t h e c a t h o l y t e w a s m a g n e t i c a l l y s t i r r e d . T h ec u r r e n t w a s m o n i t o r e d a t a s et c a t h o d e p o t e n t i a l w h i let h e m a g n e t i c s t i r r e r s p e e d w a s v a r i e d o v e r s e t t i n g sf r o m 0 t o i t s h i g h e s t s p e e d . A t e a c h f i x e d p o t e n t i a l o f

- 1 . 6 , - 1 . 8 a n d - 2 . 0 V / S C E ( I R c o r r e c t e d ) t h ec u r r e n t l e v e l r e m a i n e d f a i r l y c o n s t a n t a t ( 25 +_ 3 ) m A ,( 8 3 +_ 0 . 5 ) m A a n d (1 9 9 + 2 ) m A , re s p e c t i v e l y . F l o wc e l l s t u d i e s l ik e w i s e d e m o n s t r a t e d t h a t t h e r a t e o f t h er e a c t i o n i s i n d e p e n d e n t o f c a t h o l y t e f l o w - r a t e o v e r t h er a n g e o f 1 - 1 5 d m 3 m i n -~ a t c o n s t a n t c a t h o d e p o t e n -t ia l. H e n c e , e l e c t r o h y d r o d i m e r i z a t i o n o f f o r m a l d e h y d ei s n o t m a s s t r a n s p o r t c o n t r o l l e d b u t i s a k i n e t i c a l l yc o n t r o l l e d p r o c e s s u n d e r t h e se c o n d i t i o n s .4 . C u r r e n t - p o t e n t i a l s t u d i e sA t t e m p t s a t c y c li c v o l t a m m e t r i c i n v e s t ig a t i o n u s i n g ag r a p h i te r o d o r a c a r b o n f ib r e m i c r o e l e c tr o d e m o u n t e di n g la s s w e r e u n s u c c e s s f u l b e c a u s e o f t h e h i g h c o n -c e n t r a t io n o f fo r m a l d e h y d e r e q u i r ed f o r E G f o r m a t i o n .P o l a r i z a t i o n c u r v e s ( F i g . 4) o f g r a p h i t e r o d s ( U l t r a -c a r b o n S T- 50 ) a t 5 5 - 6 0 ~ a n d p H 6 . 5 s h o w t h a ta q u e o u s s o d i u m f o r m a t e i s d i s c h a r g e d a t a b o u t- 1 . 9 5 V a n d a q u e ou s f o rm a l in a t a b o u t - 1 . 8 5 V /S C E , i n d i c a t i n g a d i r e c t e l e c t r o n t r a n s f e r p r o c e s s f o rf o r m a l d e h y d e . N o l i m i t i n g c u r r e n t f o r f o r m a l d e h y d er e d u c t i o n w a s o b s e r v e d u p t o 1 A c m -2 . U n d e r t h e sec o n d i t i o n s a 7 0 - 8 0 % c u r r e n t e f fi c ie n c y f o r E G w a su s u a ll y o b t a i n e d o n e x t e n d e d e l e c t ro l y s is . T e t r a b u t y l -a m m o n i u m s a l t ( 2 % ) i n f o r m a l i n i n h i b i t s h y d r o g e ne v o l u t i o n a n d r a is e s t h e c u r r e n t e f f ic i e n c y f o r E G t oa b o u t 8 0 - 9 0 % a t t h e s e t e m p e r a t u r e s u n d e r e x t e n d e de l e c t ro l y s i s , r e s u l t i n g i n a p o l a r i z a t i o n c u r v e w h i c h l i e sm o r e n e g a t i v e i n p o t e n t i a l t o t h a t w i t h f o r m a l i na b o v e .

T a f e l p l o t s w e r e c o n s t r u c t e d , u s i n g s o l u t i o n s o ff o r m a l d e h y d e i n t h e r a n g e o f 0 . 0 1 2 t o 1 2 M a t 5 5 ~ C a tc u r r e n t d e n s i t i e s o f 0 .0 1 t o a b o u t 2 m A c m - 2 [ 12 ]. T h eT a f e l s lo p e s li e b e t w e e n 3 9 - 6 2 m V d e c - t i n t h e c o n -c e n t r a t i o n r a n g e 0 . 0 1 2 t o 0 . 2 0 M a n d c h a n g e t o s l o p eso f 1 1 0 - 1 2 0 m V d e c t i n t h e c o n c e n t r a t i o n r a n g e 1 .2 t o1 2 M . T h i s i s c o n s i s t e n t w i t h t h e o b s e r v a t i o n t h a t E Gi s f o r m e d i n r e l a t i v e l y p o o r c u r r e n t e f f ic i e n ci e s a t c o n -c e n t r a t i o n s b e l o w a b o u t 6 M , w h e r e h y d r o g e n e v o l -

450

4 0 0

55 0

E 30ru, ~ - " ~ C H z O H(A )

) ~ O- cH20H/ : / " O - C H z O H

( B )O- r~HzOH

Ol"~_CH20 H(B )~ + C H 2 0 + H +Hr 176

H 2 0 H- C H 2 0 H

tf , , , , , , , ' ~ 0 - C H0 H/ ~ C H ~ O H-

9 ~ " O-CH20H(E )

~ - c ~ " G r a p h i t e " 4- HOCH2CH?_OH

HI' ~ O T C - C H 2 0 H~ / (~)H + "Graphite'=" -~ +H -~ - I-) ~ O ~ _ C H z O H H O C H2 CH C H2 0H(D) iOHH~ H- C H 2 0 H[F }C HI lf " ' ~ f O - C H O H " G r a p h i t e '+

~ / / L ~ C H , C HC H 2 ~~ O- H20H IOH(G )F i g . 5 . M e c h a n i s m s f o r e l e c t r o s y n t h e s i s o f p o l y o l s .

Higher temperatures in excess of 80~ are ben-eficial in at least two respects to the kinetics: (i) morefree formaldehyde is available for reaction; and (ii)promotion of carbon-carbon bond formation occursin adducts like (B) and (C). Higher temperatures alsoserve to minimize hydrogen evolution and lower thecell voltage and hence the energy requirement.6. C onc lus ionAt the time of writing two pilot studies are underwayat two electrical utilities in Canada, Hydro Quebecand Ontario Hydro. If successful, they could demon-strate the commercial viability of the approach. Whathas been established thus far is that successful elec-troorganic processes need not be limited to highervalue-added fine chemicals, but that commoditychemicals are also worthy of pursuit, providing theroutes are technically sound, economically feasibleand also possess other advantages such as improvedenvironmental impact and ready availability of simpler,less expensive starting materials.A cknow ledgement sThis work was supported by National Science Foun-dation Phase I and Phase II Small Business InnovationResearch awards. We are also grateful to ProfessorRichard Alkire, University of Ilinois, Professor EliezerGileadi, University of Tel-Aviv and John E. Colman,John E. Colman Associates who served as consultants,and to M. E. Bolster, J. Burn, P. Fiorica, A. J. Poslin-


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s ki , E . P . Weinberg , H. R . Weinberg and A. N . Youngwho as s i s ted wi th ana ly t i ca l work.References

[1] Fo r examp le see J . L. Ehrler and B. Jura n Chem. Technol.13 (1983 ) 188; J . Falbr e (ed. ), 'Chem ical Feed stocksFro m C oa l ' , John W i le y & Sons , Ne w York (1982).[2 ] A . P . T o m i l o v , E . L . K l y u e v a n d V . D . N e c h e p o r n o i,J. Obschei Khim 43 (1973) 2792.[3] D . E . Da nly a nd C . R . Ca m pbe l l , in 'Te c hnique of E le c t ro-organic Synthesis ' , Par t I I I , (edi ted by N. L. Weinberga nd B . V . T i lak) , John Wi le y & Sons , Ne w York (1982)pp. 286-90.[4] F. Beck, 'Elektro organ ische Che mic ' , Verlag Chem ie , Wein -helm (1974).[5] H. Wa tanab e and M. Saito, Toy o Soda Ke nk y o , Hok o k u 24(1979) 93.[6] M. Sa i to and Y . Toku yum a , Ge r. Often. Pat . 3 0 1 8 8 4 4(1980).[7] N .L . We inbe rg a nd H . L . Chum , Abs t r. E le c troc he m ic a lSoc ie ty Me e t ing , Mon t re a l (M a y 1982) pp . 948-9 .

[8] M .I . Mo ntene gro, D. Ple tcher, E. A. Liol ios , D. J . Ma zura nd C . Za wodz inski , J. Applied Electrochem. 20 (1990)54 .[9] N . I . . W e inbe rg , M. Re ic her , J . A . R ipa a nd H . R . We in-berg, Abstr . Elec trochemical Socie ty Meeting Cincinnati(May 1984) pp. 432-3.[10] N .L . We inbe rg , M. E ipsz tajn , D . J . Ma z ur , M. Re ic he r ,H. R. Weinb erg and E. P. Weinberg, in 'Recen t Advancesin Elec troorg anic Synthesis ' , (edited by S. Tori i) , Kod an-sha L td , Tokyo (1987) pp . 441-4 .[11] N .L . We inbe rg , U .S . Pa te n t 447869 4 (1984); N . L . We in-berg, J . D. Gen ders and D. J . Maz ur , U.S. Paten t4950 368 (1990).[12] N .L . Weinberg , M. E. Bols ter , J . D. Gend ers , S. Maffuci ,D . J . Ma z ur , M. Re ic he r , J .A . R ipa , Final Report,National Science Foundation, No. CPE-8313565 (1987) .[13] J .F . Wa lke r, 'Form a ld e hyde ' , 3rd e d ., Robe r t E . Kr ie ge rPubl ic a t ions , Hunt ington , NY (1975) .[14] E. Gileadi , E. Kirow a-Eisn er and J . Penciner , ' Interrac ia lE le c t roc he m is t ry ' , Addison-We s le y , Re a ding , MA(1975).

electrochemical hydrodimerization of formaldehyde to ethylene glycol

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