In situ FTIR studies on the electrochemical hydrodechlorination of 3,4,5,6-tetrachloropicolinic acid on Ag cathode

Article abstract of DOI:10.1016/j.electacta.2009.12.086

The electrochemical hydrodechlorination reaction from starting material 3,4,5,6-tetrachloropicolinic acid (3,4,5,6-TCP) to the end product 3,6-dichloropicolinic acid (3,6-DCP) was investigated by cyclic voltammetry and in situ Fourier transform infrared spectroscopy (in situ FTIR). Compared with copper and glassy carbon, Ag cathode showed a high electrocatalytic activity for the irreversible reduction process of 3,4,5,6-TCP in NaOH aqueous solution. In situ FTIR results suggested that electrochemical hydrodechlorination took place in the 4- or 5-position of 3,4,5,6-TCP on Ag cathode after receiving an electron to get mixed trichloropicolinic acid free radical, which could receive another electron and give 3,5,6-trichloropicolinic acid (3,5,6-TCP) and 3,4,6-trichloropicolinic acid (3,4,6-TCP) at the potential more positive than -1000 mV afterwards. Finally, 3,5,6-TCP and 3,4,6-TCP were further dechlorinated to produce 3,6-dichloropicolinic acid (3,6-DCP) at the potential more negative than -1000 mV. Further studies of preparative electrolysis experiments by constant current electrolysis were carried out. The results were in good agreement with those from in situ FTIR investigations.

Full text of DOI:10.1016/j.electacta.2009.12.086

Electrochimica Acta 55 (2010) 3171–3174
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
In situ FTIR studies on the electrochemical hydrodechlorination of
3,4,5,6-tetrachloropicolinic acid on Ag cathode
Chun An Ma1, Mei Chao Li∗, Yan Na Liu, Ying Hua Xu
College of Chemical Engineering and Materials Science, State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology,
Zhejiang University of Technology, 18 Chaowang Road, Hangzhou 310032, Zhejiang, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 October 2009
Received in revised form 7 December 2009
Accepted 23 December 2009
Available online 14 January 2010
The electrochemical hydrodechlorination reaction from starting material 3,4,5,6-tetrachloropicolinic acid
(3,4,5,6-TCP) to the end product 3,6-dichloropicolinic acid (3,6-DCP) was investigated by cyclic voltam-
metry and in situ Fourier transform infrared spectroscopy (in situ FTIR). Compared with copper and glassy
carbon, Ag cathode showed a high electrocatalytic activity for the irreversible reduction process of 3,4,5,6-
TCP in NaOH aqueous solution. In situ FTIR results suggested that electrochemical hydrodechlorination
took place in the 4- or 5-position of 3,4,5,6-TCP on Ag cathode after receiving an electron to get mixed
trichloropicolinic acid free radical, which could receive another electron and give 3,5,6-trichloropicolinic
acid(3,5,6-TCP)and3,4,6-trichloropicolinicacid(3,4,6-TCP)atthe potential more positive than −1000 mV
afterwards. Finally, 3,5,6-TCP and 3,4,6-TCP were further dechlorinated to produce 3,6-dichloropicolinic
acid (3,6-DCP) at the potential more negative than −1000 mV. Further studies of preparative electrolysis
experiments by constant current electrolysis were carried out. The results were in good agreement with
those from in situ FTIR investigations.
Keywords:
3,4,5,6-Tetrachloropicolinic acid
Electrochemical hydrodechlorination
In situ FTIR
Ag
3,6-Dichloropicolinic acid
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
by electrochemical hydrodechlorination has attracted more and
more attentions recently [10–12]. However, to the best of our
With high biological activity and low toxicity, heteroaro-
matic compounds containing nitrogen atoms often play important
roles as the scaffolds of bioactive substances [1–3]. Pyridine-
monocarboxylic acids are one of the most popular intermediates for
the synthesis of pharmaceutical and agrochemical products [4,5].
Among them, 3,6-dichloropicolinic acid (3,6-DCP) is proved to be
a highly active plant growth regulator and selective herbicide. Not
only has the 3,6-DCP a low toxicity to mammals, fish and birds, but
it is relatively short lived in soil [6,7].
knowledge, there are only a few reports about the electrochemical
hydrodechlorination mechanism of 3,4,5,6-TCP to 3,6-DCP so far.
The present contribution aims to investigate the electrochem-
ical hydrodechlorination reaction mechanism of 3,4,5,6-TCP in
NaOH aqueous solution at the molecular level by means of a
powerful technique such as in situ FTIR spectroscopy [13,14]. In
addition, preparative electrolysis experiments using constant cur-
rent electrolysis have been carried out, and high performance liquid
chromatography method is applied to detect the intermediates
during electrochemical hydrodechlorination reaction.
The common methods to prepare 3,6-DCP are acid
hydrolysis
with
the
starting
material
3,6-dichloro-2-
(trichloromethyl)pyridine in sulfuric acid or nitric acid [8],
and chemical dechlorination reduction with starting material
3,5,6-trichloro-4-hydrazino picolinic acid by excess hydrazine
hydrate in alkaline aqueous solution [9]. However, the process
cost of the former is relatively high, and the latter shows low
yield and poor quality. Due to low cost, high yield, good quality
and low environmental contamination, the preparation method
from 3,4,5,6-tetrachloropicolinic acid (3,4,5,6-TCP) to 3,6-DCP
2. Experimental
All reagents were of analytical reagent grade and used as
received. All solutions were prepared using Millipore-Q water. The
experiments were carried out at room temperature.
Cyclic voltammetry measurements were performed using
EG&G potentiostat/galvanostat Model 263A. The electrochemical
hydrodechlorination reaction from 3,4,5,6-TCP to 3,6-DCP was car-
ried out in 0.1 M NaOH solution containing 0.05 M 3,4,5,6-TCP by
cyclic voltammetry between −0.6 and −1.4 V at a scan rate of
50 mV s−1. Ag plate, copper plate and glassy carbon (GC) with the
geometric area of 0.2 cm2 were used as the working electrodes
respectively. A saturated calomel electrode (SCE) was used as the
∗
Corresponding author. Tel.: +86 571 88320305; fax: +86 571 88320961.
E-mail address: limc@zjut.edu.cn (M.C. Li).
1
Member of ISE.
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.12.086

3172
C.A. Ma et al. / Electrochimica Acta 55 (2010) 3171–3174
reference electrode, and a platinum sheet (1 cm × 1 cm) as the
counter electrode.
In situ FTIR spectroscopic experiments were performed on a
Nicolet 670 FTIR spectrometer equipped with a MCT-A detector
cooled with liquid nitrogen. The spectroelectrochemical cell was
provided with CaF2 disk window (diameter 32 mm and thickness
2 mm). The interferograms were acquired with the working elec-
trode surface pressed against the CaF2 disk window. The resulting
spectra recorded were defined as the potential-difference spectra,
and were calculated by the following formula [14]:
ꢀR
R
R(Es) − R(ER)
R(ER)
=
In the formula, R(ER) represents single-beam spectrum collected
at the reference potential and R(ES) at the sample potential. Two
hundred interferograms were collected and co-added into a single-
beam spectrum. The spectral resolution was set at 8 cm−1
.
Fig. 2. In situ FTIR spectra collected during electrochemical hydrodechlorination
reaction of 3,4,5,6-TCP on Ag cathode. The sample potential was stepped from −500
to −1400 mV and the reference potential was 0 V. The spectral resolution was set at
Preparative electrolysis experiments were performed in
two-compartment cell, divided by Nafion membrane,
assembled with magnetic stirring bar. The cathode was
Ag plate (2 cm × 3 cm × 0.05 cm) and platinum sheet
a
a
8 cm−1
.
a
a
(2 cm × 3 cm × 0.05 cm) was used as the anode to provide a
harmless count reaction. The electrolysis experiments were per-
formed on Model 263A potentiostat/galvanostat in 0.5 M NaOH
solution containing 0.05 M 3,4,5,6-tetrachloropiclinic acid by
galvanostatic mode with the constant current 0.5 mA. The com-
position variation of the electrolytic processes was monitored by
Waters high performance liquid chromatography (HPLC) system
equipped with a symmetry column and a Waters 2996 Photodiode
Array Detector.
tion of 3,4,5,6-TCP took place on Ag cathode. However, under the
similar conditions, no corresponding current peaks for the cathodic
process of 3,4,5,6-TCP are observed on Cu or GC electrode, though
Cu and GC electrodes are common cathodes for electroreduction
reaction. The electrocatalytic activity of Ag for electrocatalytic
reduction of 3,4,5,6-TCP in NaOH solution is highest as compared
with Cu and GC electrodes, and thus is chose as the cathode for
electrochemical hydrodechlorination reaction of 3,4,5,6-TCP in the
following investigations.
3. Results and discussion
3.2. In situ FTIR spectroscopic study
3.1. Cyclic voltammetry behavior of 3,4,5,6-TCP
In alkaline solution, 3,4,5,6-TCP exists in the form of its corre-
sponding base salt and adsorbed on the surface of Ag cathode. Fig. 2
shows a set of in situ FTIR spectra obtained during electrochemical
hydrodechlorination reaction of 3,4,5,6-TCP on Ag cathode in alka-
line solution. Firstly the reference spectrum is acquired at 0 mV,
then the Ag cathode is polarized from −500 to −1400 mV and
the sample spectra are collected. The polarization time at each
potential is about 90 s. The spectra in Fig. 2 display three character-
istic positive-going bands at 1511, 1418 and 1322 cm−1, which are
assigned to the C C stretching vibration and C–N stretching vibra-
tion of 3,4,5,6-TCP [13,15,16]. Another important positive-going
band at about 1095 cm−1 is related to the C–Cl stretching vibration
of 3,4,5,6-TCP [17,18]. Table 1 summarizes the assignments of the
peak frequencies observed for all the positive-going and negative-
going bands. When the sample potentials are more positive than
Since the electrode material can affect electrochemical reac-
tions, the electrochemical hydrodechlorination reaction has been
carried out on three different electrodes. Cyclic voltammograms
for the electrochemical reduction of 3,4,5,6-TCP in NaOH solution
on Ag, Cu and GC electrodes respectively are shown in Fig. 1. On
Ag cathode, a characteristic irreversible reduction current peak for
3,4,5,6-TCP reduction at about −1.1 V (vs SCE) can be obviously
observed from Fig. 1, which shows that the electrochemical reduc-
Table 1
Assignments of all the positive-going and negative-going bands.
Band
Frequency/cm−1 Assignments
Positive-going
1511, 1418
1322
1215
C C stretching of 3,4,5,6-TCP
C–N stretching of 3,4,5,6-TCP
C–O stretching of 3,4,5,6-TCP
C–Cl stretching of 3,4,5,6-TCP
1095
Negative-going 1368
C–N stretching of 3,5,6-TCP or 3,4,6-TCP
C–Cl stretching of 3,5,6-TCP or 3,4,6-TCP
C–O stretching of 3,5,6-TCP or 3,4,6-TCP
C–H in plane bending of 3,5,6-TCP or
3,4,6-TCP
1079
1226
1164
1403
1045
C–N stretching of 3,6-DCP
C–Cl stretching of 3,6-DCP
1550, 1442
1230
1145
C C stretching of 3,6-DCP
C–O stretching of 3,6-DCP
C–H in plane bending of 3,6-DCP
Fig. 1. Cyclic voltammograms for the electrochemical hydrodechlorination reaction
of 0.05 M 3,4,5,6-TCP in 0.1 M NaOH at Ag, Cu and GC electrodes at the scan rate of
50 mV s−1
.

C.A. Ma et al. / Electrochimica Acta 55 (2010) 3171–3174
3173
−600 mV, no obvious changes can be observed on the FTIR spectra.
It indicates that the electrochemical hydrodechlorination of 3,4,5,6-
TCP on Ag cathode is quite slow. When it steps to more negative
potential, the intensities of all the bands increase, thus showing
the consume of 3,4,5,6-TCP and the progress of electrochemical
hydrodehalogenation with the applied potentials.
During the reduction of 3,4,5,6-TCP at potentials of −500 and
−1000 mV, two negative-going bands are observed. Bands at 1164
and 1079 cm−1 are related to C–H in plane bending and C–Cl
stretching of 3,4,6- or 3,5,6-TCP [18,19], and the peak intensities
increase with the potential shifting to more negative values. With
the dechlorination of picolinic acid, the wavenumber of C–O the
stretching vibration shifts slightly from 1215 to 1226 cm−1, and
thus a positive-going band at 1215 cm−1 and a negative-going band
at 1226 cm−1 are observed. Heterocyclic aromatic ring is easier
to receive electron to form radical-anion on the ring rather than
on the chlorine atom [20]. In addition, on the basis of the induc-
tive effect of the adjacent electronegative nitrogen atom, chloride
in 4- or 5-position of 3,4,5,6-TCP is more active and easier to be
reduced than those in 3- and 6-position [21]. Therefore, the chlorine
atom in 4- or 5-position on carbon atom drops out selectively and
is replaced with the hydrogen to give 3,5,6-trichloropicolinic acid
(3,5,6-TCP) or 3,4,6-trichloropicolinic acid (3,4,6-TCP). At the same
time, there are no other positive-going bands at 1615–1545 and
1420–1300 cm−1, which suggest that no decarboxylation reaction
occurs to 3,4,5,6-TCP [13,22].
Fig. 3. TR-FTIR spectra collected during electrochemical hydrodehalogenation reac-
tion of 3,4,5,6-TCP on Ag cathode at −700 mV. The reference potential was 0 V. The
spectral resolution was set at 8 cm−1
.
However, at potentials more negative than −1000 mV, the two
peak intensities at 1164 and 1079 cm−1 decrease and two negative-
going bands at about 1403 and 1230 cm−1 emerges, which are
assigned to the C–N and C–O stretching vibration of 3,6-DCP. In
addition, three characteristic negative-going bands at about 1550,
1145 and 1045 cm−1 can be observed, which are assigned to C
C
stretching vibration, C–H in plane bending and C–Cl stretching of
3,6-DCP [17]. Their intensities increase with the potential shifting
to more negative values. It shows that both the 4- and 5-position
of 3,4,5,6-TCP are replaced with a hydrogen to generate the end
product 3,6-DCP.
In order to observe the reaction more carefully, time-resolved
FTIR (TR-FTIR) spectra are collected during electroreduction of
3,4,5,6-TCP on Ag cathode. Fig. 3 shows plots of the intensities of
the various features observed at −700 mV as a function of time.
From the Fig. 3, one can see that the positive-going bands at 1151,
1418, 1322 and 1095 cm−1 related to 3,4,5,6-TCP increase with the
time. The negative-going bands at 1442, 1368, 1164 and 1079 cm−1
related to 3,5,6- or 3,4,6-trichloropicolinic acid increase with the
Fig. 4. TR-FTIR spectra collected during electrochemical hydrodehalogenation reac-
tion of 3,4,5,6-TCP on Ag cathode at −1100 mV. The reference potential was 0 V. The
spectral resolution was set at 8 cm−1
.
time too. It indicated that at −700 mV, the main electrochemical
hydrodehalogenation reaction is from 3,4,5,6-TCP to form 3,5,6- or
3,4,6-trichloropicolinic acid.
Fig. 4 displays time-resolved FTIR spectra collected during elec-
troreduction of 3,4,5,6-TCP on Ag cathode at −1100 mV. In Fig. 4,
the negative-going bands at 1550, 1403, 1342 and 1045 cm−1 can
Scheme 1. Electrochemical hydrodechlorination reaction mechanism of 3,4,5,6-TCP on Ag cathode.

3174
C.A. Ma et al. / Electrochimica Acta 55 (2010) 3171–3174
to obtain the direct spectroscopic information of the intermedi-
ates and monitor the species of intermediate or product generated
on the electrode surface. It was used to analyse the electro-
chemical hydrodechlorination reaction mechanism of 3,4,5,6-TCP
in 0.1 M NaOH aqueous solution on Ag cathode for the first
time.
Firstly electrochemical hydrodechlorination reaction took place
in the 4- or 5-position of 3,4,5,6-TCP on Ag cathode to get mixed
trichloropicolinic acid free radical. Then the mixed trichloropicol-
inic acid free radical received another electron and gave 3,5,6-TCP
and 3,4,6-TCP at the potential more positive than −1000 mV.
Finally, with the potential shifting to more negative values, 3,5,6-
TCP and 3,4,6-TCP took off another chlorine ion to achieve 3,6-DCP
as the end product. The results were in good agreement between
the in situ FTIR measurements and constant current electroly-
sis.
Fig. 5. Constant current electrolysis of 3,4,5,6-TCP on the silver electrode, elec-
trolyte: 300 mL 0.5 M NaOH solution containing 0.05 M 3,4,5,6-tetrachloropiclinic
acid, electrolyte temperature: 298 K, current efficiency: 0.83 A/m2.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (20876151) and Analysis and Measurement
Foundation of Zhejiang Province (2008F70029).
be seen clearly. Their intensities increase with time. It indicates that
3,4,5,6-TCP is electroreduced to 3,6-DCP at −1100 mV by electro-
chemical hydrodechlorination.
References
From the above integrating analysis, the possible electrore-
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Scheme 1. Firstly 3,4,5,6-TCP receives an electron to form 3,4,5,6-
TCP radical-anion, which can lose a chlorine ion in the 4- or
5-position respectively to form trichloropicolinic acid free radical.
Then the free radical receives a proton to give 3,5,6-TCP and 3,4,6-
TCP. Finally, with the potential shifting to more negative values,
3,5,6-TCP and 3,4,6-TCP were further dechlorinated to generate the
end product 3,6-DCP.
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4. Conclusions
The in situ FTIR characterization method allows to obtain vibra-
tional information from very small amounts of material, and then