Indian Journal of Chemical TechnologyVol. 4, January 1997,pp. \8-24

Technological use of propionitrile electrosynthesis

Daniel A Lowy'", Maria Jitaru'', Bogdan C Toma", loan A Silberg" & Liviu Oniciu"

"Department of Chemistry, University of Memphis, Campyus Box 526060, Memphis, TN 38152, USA"Department of Chemistry & Chemical Engineering, CDepartment of Organic Chemistry, dDepartment of Physical

Chemistry, Babes-.Bolyai University, Str Arany Janos No 11, RO-3400 Cluj-Napoca, Romania, Europe

Received 22 April 1996, accepted 10 July 1996

Propionitrile (PN) is manufactured by the non-dimerizing electroreduction of acrylonitrile. Ther-modynamic calculations, kinetic studies, laboratory-scale preparative syntheses, and technological con-siderations made it possible to apply the electrochemical PN manufacturing on the pilot plant scale. AsPN formation is kinetically favoured over adiponitrile, the yield of PN raises with increasing currentdensities. Endurance tests are reported for a continuous operation of the pilot plant over 7-days. Overthis time period the specific material consumptions is of 1.12 kg AN (kg PN)-I, while the power usageis 4.73 kWh kg-I. The annual productivityofthis type of plant is 6.53 x 103kg.

Propionitrile is a compound highly demanded byindustry'. It is used as a non-protic electrolyte forzinc-bromine batteries, as a solvent for various or-ganic syntheses, and as a reaction intermediate inthe manufacturing of propyl amines!". In a large-scale industrial procedure adiponitrile (AD) is ob-tained by the reductive dimerization of acryloni-trile (AN) via the Baizer reaction/'P. During thisprocess, AN also undergoes a complementaryreaction which yields propionitrile (PN). Surpri-singly, the latter process, has been almost ignored,regardless of its possible industrial applications I.

It was a demanding task for the electrochemistto find experimental conditions that ensure the se-lective electroreduction of the homogeneous doublebond of AN Eq, (11, and which avoid the Baizerreaction Eq. (2):

2FCH2 = CH - CN + H20 ~-+ CH3 - CH2 - CN + 102t

2F ... (1)2CH2 = CH - CN +H20 _ NC - (CH2)4 - CN

+!02t ... (2)

The first process (Eq. (1)) is called the non-di-merizing electroreduction of AN (henceforthNDE). while the second one (Eq. (2)) is the elec-trohydrodimerization of AN (henceforth EHD).Eqs (1) and (2) show that the formation of PNwith a moleculer weight of 55D requires the same

*Author to whom correspondence should be addressed.

charge consumption per mol (2 faraday) as thesynthesis of AD, with an almost twice the greatermolecular weight (108D). This means that, underidentical electrochemical conditions (same cell vol-tage, current density and current efficiency) thepower usage for PN is approximately twice asmuch as fOf AD. In order to make PN electrosyn-thesis economically as advantageous as the manu-facturing of AD, the cell voltage had to bere-duced significantly. This was achieved primarily byexploiting the electrocatalytic activity of the ca-thodic metal and/or of redox systems added tothe supporting electrolyte. Also, the most import-ant electrochemical, chemical, electrical and physi-cal parameters controlling PN production had tobe optimized by means of systematic studiesl•13-17•

The relevant literature data on the productionof PN as a by-product in EHD of AN were sum-marized elsewhere'. In Table 1 procedures of PNmanufacturing by the NDE of AN have been re-portedl.13.15-21.

Enthalpy changes were calculated from bondenergies, and entropy changes were estimatedform group contributions". These free energieswere adjusted with the dissociation energies of thewater molecules involved in the reaction. Inde-pendent calculations made by the AMI semiem-pirical calculation method+ yielded standard for-mation enthalpies of the same order of magni-tude", As a result, the reaction EMF for theNDE of AN was calculated: 1.17 V. Depending onthe electrocatalytic properties of the electrodes,

WWY et al: PROPIONITRIlE ELECIROSYNTIIESIS

Table I-Propionitrile formed as the main product in the non-dimerizing electroreduction of. acrylonitrile

Working parameters

Fe deposited on stainless steel vs Ni, und, 0.7 mol L-I NaOH, 271-273 K, 0.2 kAm - 2, AN flux: 50 mL h - 1

Cd vs Ni, und, 0.7 mol L" ' NaOH, 271-273 K,0.1-0.2 kAm-2

Pb ore with 6 wt. % Sb and 0.1 wt. % Ag vs Pb02, div, catol: 20 wt. % NMe4,anol: 1.0 mol L-I H2S04, 323 K, 1 kA m - 2

Cd, Cu orPb vs stainless steel (18 wt. % Cr, 8 wt. % Ni, 0.5 wt. % Mo), undpressf, 10 wt. % K2HPO. and K2HPOS' 298 ± 1 K,pH 7,0.1 kA m-2

Ni or Cu vs stainless steel (18 wt. % Cr, 8 wt. % Ni, 0.5 wt. % Mo), and pressf,phosphate buffer, ionic strength, 1.14-5.19 mol L-I,296-331 K, pH 7.0 ± 0.1,0.1-0.15 kA m "?

Method

Knunyants et aL

Knunyants et al:

Yomiyarna et al.

Oniciu et al.

Oniciu et al.

SPN.,% Ref.

80 18,19

95 18-20

52.6 21

60.9-91.7 1,3,15,17

97-99 1,14,17

SPN-the selectivity of PN formation (see ctdinition in text); div=divided cell; und-undivided cell; pressf+pressfilter type cell;anol-anolyte; catol-catolyte; Ref-references.

the cell was operated at overpotentials from 0.83to 2.73 V, while the IR drop was typically in therangefrom 0.41 to 0.57 V (ref. 17).

Electrocatalytic phenomena encountered in theNDE of AN are of major interest for designing aneffective PN synthesis procedure. Unlike literaturedata?", it has been demonstrated that the selectiv-ity of the C = C double bond electroreduction inAN depends essentially on the crystalline struc-ture of the cathodic metal, regardless of whether itbelongs to the main or transitional metalgroupl,16,25. Lead was found as a good cathodicmetal: its electrocatalytic activity favoured C = Cdouble bond reductionI5.16,25.27, and ensured agood selectivity for the PN formation'-':'. In addi-tion, due to its high overpotential, the use of alead cathode can prevent the discharge of hydrog-en. Thus, the selective electro reduction of AN toPN proceeded with current yields (rF) greater than95%, on lead cathodes+':'.

Other studies revealed the importance of thecomposition of the supporting electrolyte/V".Thus, in the absence of surfactants, the NDE pro-cess was favoured, yielding PN with high selectiv-ity, whereas in the presence of a quaternaryammonium salts, in concentrations ten times grea-ter than the critical micellar concentration, a pref-erential formation of AD was found (upto 94%).

Earlier kinetic studies of the competing reac-tions yielding PN and AD30 revealed a slight kin-etic preference of AN for the non-dimerizing elec-troreduction. These studies were performed onthe laboratory scale set-up. It is assumed that themechanism of the NDE of AN, involve the suc-cessive electrochemical and chemical steps (ECECsequencies), shown in (Eqs (3)-(6)Xref. 1, 16)

CH2 = CH - CN + e " ....•CH2 = CH - CN-(ads)... (3)

CH2 = CH - CN-(ads) + H20 ....•

[CH2-CH2N-CN'](ads)+HO- ... (4)[CH2-CH2 -CN'Kads)+e- ....•CH2=CH-CN-:

... (5)CH2 - CH2 - CN-: + H20 ....•CH3 - CH2 - CN

+HO- ... (6)

Given the strong electrophilic character of theradical anion and the neutral radical species, thatare formed in Eqs (3) and (4), respectively, it isbelieved that the first three steps (Eqs (3 )-(5)) pro-ceed in the adsorbed state. Once the carbanion(CH2 = CH - CN -:) is formed, it is rejected byelectrostatic repulsion from the cathode surface,so that the last step (Eq. (6)) takes place in thebulk phase. Based upon this mechanism, rate con-stants and activation energies were derived for theNDE of AN on the Pb vs Pb02 electrode couple.The overall activation energy was of 14.1 kJmol-1 (refs 15,31-33).

In order to avoid leakage of the hydraulic cir-cuits of the pilot plant, appropriate elastomers hadto be used in sealing the lines of the cells. Giventhat AN is an extremely efficient solvent, a syste-matic investigation of a large variety of elastomerswas needed in order to identify the types of rub-ber which are resistent in the presence of AN.Two testing methods were used'", i.e., (i) the timedependence of the swelling and solubilization ofelastomers subjected to AN was observed and (ii)the eventual drying of these elastomers was moni-tored.

In order to calculate the accurate material bal-

20 INDIAN J. CHEM. TECHNOL., JANUARY 1997

Fig. l-Schematic of the pilot plant used for the electrochemi-cal manufacturing of propionitrile (see explanations in text)

ance of PN electrosynthesis, the reciprocal solubi-lites in the AN-PN-aqueous electrolyte systemwere determined, and phase equilibria in theabove systems were examinedl '.

In this paper, the technological data on themanufacturing of propionitrile at the pilot plantscale have been reported. Dependence of the yieldof PN on the current density is shown and endur-ance tests for a continuous operation of the pilotplant over a 7-day period are discussed. The spe-cific material- and energy consumptions, and theannual productivity of the plant are calculated.

Experimental ProcedureThe electrochemical plant-The schematic of

the pilot plant is shown in Fig. 1. The supportingelectrolyte was prepared in vessel 101 by dissolv-ing potassium phosphates (Ph) in de-ionized water(DIW). Acrylonitrile (AN), the organic raw materi-al, was pumped (with pump PI) from container102 into the measuring vessel 103. Next, the su-spension of AN in aqueous supporting electrolytewas prepared in vessel 104. A hydraulic pump(P2) ensured the forced convection of the electro-lyte (previously cooled with the heat exchanger201) through the electrochemical reactor (ER).The upward linear velocity of the supporting elec-trolyte through the cell was of 1.0 ± 0.1 ms - 1. Ves-sel (104) is used also to degas the suspension, theanodic gases being exhausted through two heat ex-changers (202 and 203), cooled with water and

with brine, respectively. A separation vessel (105)continuously removed the organic phase, whichwas stored in the reservoir 106. Temperature(temperature indication,· T), flow rates (flow con-trol, F), liquid level (L), and pressure (P) werecontinuously monitored. A previously describedelectrochemical reactor (ER) was used26•35• It in-corporated seven undivided pressfilter type cellsthat were operated simultaneously. The electrodeshad a surface area of 2100 em 2 and were con-nected to a stabilized current source. This currentsource should deliver currents up to 1 kA at cellvoltages of 30-32 V. The cathode potential wascontrolled with respect to saturated calomel elec-trodes brought into the proximity of the workingelectrodes via Luggin capillaries. A high puritybulk lead cathode (99.99%) was used in conjuc-tion with a Pb02-coated lead anode. The anodewas obtained by the in situ formation of a 1-3 mmthick compact Pb02 layer at the lead surface3S-37.The latter procedure was based on the anodic oxi-dation of the lead in concentrated aqueousphosphate buffer (pH 7, ionic strength: 1.19)36,37.Neutral aqueous potassium phosphate buffer wasused as the supporting electrolyte. To this, 20 vol% of AN was added, and the suspension of AN inthe aqueous phase was electrochemically reducedunder potentiostatic and quasi-isothermal condi-tions (295 ± 2K).

Analytical control-The reduction productswere analyzed by gas-chromatography on a M9

LOWY et al.: PROPIONITRILE ELECTROSYNTHESIS 21

type instrument (Institute of Isotopic and Molecu-lar Technology, Cluj-Napoca, Romania) connectedwith an ENDINE 621.01 integrator (Germany).The gas-chromatographic separation was per-formed according to a reported method:" on aconventional column (3.0 m x 2.2 rom i.d.) filledwith 5% OV-17 silicon oil on Chromosorb GAWDMS (100/120 mesh). High purity argon(99.98%) was employed as the carrier gas (flowrate: 20 mL min - 1) and a FID detector was used.The temperature program was: T; = 80°C (3 min)to T, = 200°C (10 min), by a gradient of 8"Cmin - I. The composition of the evolved gas mix-ture (C02, O2 and H2) was monitored with an Or-sat apparatus with three absorbing columns (KOH,pyrogallol and colloid Pd solutions).

Results and DiscussionThe Pb02 coating layer of the Pb anode was

obtained by the electrolysis in phosphate buffer atcurrent densities of approximately 20 mA em - 2,after a previous chemical cleaning of the Pb sur-face. The cleaning procedure involved the treat-ment of the lead surface with aqueous acetic acidsolution (20 wt%). Based upon literature data'""one can assume that Pb02 is formed by the gradu-al oxidation of Pb at increasingly positive poten-tials (Eqs (7)-)9)).

... (7)

60~---L----L---~----~--~20 60 100 140 180 220

j , mA/cm2Fig. 2-Plot of the yield of propionitrile and the current effi-ciency (CE) of the process with respect to the current density,J. The correlation coefficient for the yield of propionitrile is

0.987

PbO+(m-l)H20- 2(m-l)e- -PbOm

+ 2(m-l)H+ ... (8)

PbOm + (2 - m)H20 - 2(2 - m)e - - Pb02+ 2(2 - m)H+ ... (9)

where, m = 1.3-1.6

According to Pavlov and co-workers'P'" the oxi-dation of the non-stoichiometric oxide PbO toPb02 (Eq. (3)) proceeds in the potential r~gefrom 1.27 to 1.33 V vs SCE, with the participa-tion of OH- ions (adsorbed at the electrode sur-face). The obtained coating is composed of a- and~-PbOi7-49.

The supporting electrolyte used in PN synthesiswas KH2PO 4 and K2HPO 4 in deionized water. Itensured a convenient ionic conductivity and alsoprotected the electrodes from corrosion.

Endurance studies-As shown in Fig. 2, theproduct yield for PN, raised with the increasingcurrent density. This raise in product yield is dueto the fact that the reduction of AN to PN has aslight kinetic preference over AD formation. Thevariation of the current efficiency (CE) with thecurrent density has also been shown. As seen fromFig. 2, at j> 60 mA em - 2 the CE improves withincreasing current density, and reaches a limitingplateau in the range of 90-130 mA em - 1. At cur-rent densities greater than 140 mA em - 2 CEdrops significantly, due to cathodic hydrogen evo-lution. On the other hand, below 80 mA cm - 2 theproductivity of the pilot plant becomes insuffi-cient.

Endurance tests were performed over a 7-day

100r--------------~130o - ~PN)• - j

96 126[] oOCOcQ:]O 0000000 []

~cJ:J [] OOOOODOO

~ 92 122e1118f .76 88 ••

•

•.•.•.•. .•.. .. •. •.

84 114

80~~-L~L-~-L~-~-L~-~~o 16 32 48 64 80 98 112128144 160176 110

t, hFig .. 3-Enduranc~ tests performed over a 7-day period ofcontinuous operation of the pilot plant. Plot of the selectivityof PN formation, S(PN), and of the current density (secondary

y axis) vs time

22 INDIAN 1. CHEM. TECHNOL., JANUARY 1997

Table 2-Material balance of propionitrile electrosynthesisperformed on the pilot plant scale

Aqueous phasesupporting electrolyteacrylonitrile"propionitrile"Organic phaseacrylonitrilepropionitrileadiponitrileside-products"waterGas phaseacrylonitrile oxidized to CO2O2 released at the anodeH2 from electrolyzed waterDIW supplied to balance lossesTotalSamples for analysis + Losses

Input, kg148.5138.7

2.57.3

29.429.4

Output, kg148.1137.6

3.57.0

29.68.6

18.20.91.1

0.87.00.2

6.7#0.1

7.2185.l 184.7

0.4

*the used recycled supporting electrolyte was already saturatedin both AN and PN**side products include: succinonitrile, u-methylglutaronitrile,and trimers/oligomers of acrylonitrile.# includes the oxygen from water electrolysis

period of continuous operation of the pilot plant.Both the current density' and the selectivity of PN(SPN) defined by Eq. (10) had steady values (Fig.(3)).

S = [PN] x 100 (0/0)PN" [PN]+[AD]

... (10)

where [PN] and [AD] are the concentrations of PNand AD in wt%.

After 100 h of functioning there was a gradualincrease of the CO2 content of the gas mixture ob-tained by the anode reaction. This increase in CO2ranged from the initial 0.2 vol % to the final 3.5vol%. Carbon dioxide was formed by the anodicoxidation of AN in the undivided cell, a processwhich was enhanced by the "aging" of the support-ing electrolyte. In order to avoid the undesiredCO2 formation, the supporting electrolyte shouldbe replaced by freshly prepared phosphate bufferafter each operation period of about 120 h.

Productivity and power usage-Given the aver-age cell voltage of Ecell= 3.5 V, one can assumetypical product yields of 'YJ = 90%, and a currentefficiency of CE = 80%. For the continuous opera-tion of the cell at an average current density of

100 mA em - 2 (I= 210 A) over a time period ofone year (t=250 days (yeartl), the productivity(Prod) of the pilot plant derived from Faraday'slaw is given by Eq. (11).

FWProd= n - It CE 'YJ ••• (11)zF

where, n is the number of cells operated simul-taneously, FW is the formula weight of PN (55.08kg kmol " I), z is number of electrons exchanged/mol of PN and E is the Faraday's constant (96487C equiv - 1). By using the listed values and n= 7for the number of cells one can calculate a pro-ductivity of 6.53 x 103 kg (year)" I. From the mate-rial balance, shown in Table 2, the specific materi-al consumption of 1.12 kg AM (kg PNt 1 was cal-culated. As the process is performed in an undi-vided cell, approximately 1.2 wt% of the electro-chemically converted AN is oxidized to carbon di-oxide. Due to reciprocal solubilities in this systemca. 2.6 wt% of water is dissolved by the organicphase (Table 2).

The power usage (Wel) in the electrolytic pro-cess is given by Eq. (12),

w = _z F_E-=c.::.::ell,---el FW'YJ CE

... (12)

Using the values listed above WI = 1.70 X 104 kJkg-I =4.73 kWh kg-I. Thus the power usage dur-ing the electrolysis (performed at ambient temper-atures) is less than the heats involved in a conven-tional catalytic reduction. Also, this power usage isless than that for AD production in the dividedcell process'".

ConclusionBased on several preliminary studies, the elec-

trochemical propionitrile synthesis was applied onthe pilot plant scale. Selective and energeticallyconvenient operating conditions were found. Thematerial balance of the process was calculated,specific matter and energy consumptions were re-ported, and the endurance of the electrodes wasstudied. The annual productivity of the pilot plantcan be significantly increased by increasing thecurrent density, e.g., at 200 mA em - 1 the produc-tivity becomes 1.3 x 104 kg (yeartl. However, athigher current densities problems related to thecorrosion of the electrodes become more signifi-cant. All the technological data reported here areuseful for the scale up of the propionitrile electro-synthesis from the pilot plant scale to commercialproduction.

LOWY et al: PROPIONITRILE ELECTROSYNTIffiSIS 23

Acknowl~gementFinancial support from the Research Institute

for Auxiliary Organic Products (ICPAO), Medias,Romania and Azomures Co., Ltd., Tg. Mures, Ro-mania' is gratefully acknowledged. Authors thankDr. James, J. Beyer from West Virginia Universityfor critical reading of the w,anuscript.

NomenclatureAD, [AD] = adiponitrile and AD concentrationAN =acrylonitrileCE =current efficiencyDIW =de-ionized waterEC•II =cell potentialEHD =electrohydrodimerizationF Faraday's constant (9648 7°C equiv - I)FW =formula weightj current densityn =number of cellsNDE non-dimerizing electroreductionPN, [PN] =propionitrile and PN concentrationProd =productivitySCE = saturated calomel electrodeSPN' S(PN) = selectivity of PN formationWe! =power usagez =number of electrons exchanged" =product yield

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