-
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.
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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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24 INDIAN J. CHEM. TECHNOL., JANUARY 1997
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