Electrochemical synthesis of niobium(V) ethylate

Article abstract of DOI:10.1134/S1070427206050090

The effect exerted by the supporting electrolyte and cathode material on the anodic dissolution of niobium in ethanol solutions was studied. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S1070427206050090

ISSN 1070-4272. Russian Journal of Applied Chemistry, 2006, Vol. 79, No. 5, pp. 741 745. Pleiades Publishing, Inc., 2006.
Original Russian Text
No. 5, pp. 752 756.
M.Yu. Berezkin, I.N. Chernykh, E.G. Polyakov, A.P. Tomilov, 2006, published in Zhurnal Prikladnoi Khimii, 2006, Vol. 79,
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
Electrochemical Synthesis of Niobium(V) Ethylate
M. Yu. Berezkin, I. N. Chernykh, E. G. Polyakov, and A. P. Tomilov
State Research Institute of Organic Chemistry and Technology, Federal State Unitary Enterprise,
Moscow, Russia
St. Petersburg State Polytechnic University, St. Petersburg, Russia
Received January 27, 2006
Abstract The effect exerted by the supporting electrolyte and cathode material on the anodic dissolution of
niobium in ethanol solutions was studied.
DOI: 10.1134/S1070427206050090
The goal of this study was to develop an electro-
chemical method for preparing Nb(V) alkoxide. The
latter can serve as the starting material for preparation
of ultrapure Nb(V) and Nb(V) oxide, which are wide-
ly used in various areas of industry.
Cathode:
+e
MHal2
+e
+2HOR
[MHal2]
[MHal2]2
H2 + 2Hal
(1)
+ M(OR)2.
The anodic dissolution of a metal in the corre-
sponding alcohol in the presence of a supporting elec-
trolyte is one of promising but insufficiently studied
methods of preparing transition metal alkoxides. At
the same time, in an extensive monograph [1] review-
ing in detail the properties and methods of preparation
of all the presently available metal alkoxides, the elec-
trochemical method is given the least attention. Al-
koxides prepared by the electrochemical method are
listed in a review [2], demonstrating the potential
of the method.
Based on the general views on the possible mech-
anism of the formation of transition metal alkoxides
by the anodic dissolution in alcohol electrolyte solu-
tions, Shreider et al. [4] suggested the following reac-
tions as the most probable:
Anode:
Hal +OR
M
e
M(OR)nHalm [+M(OR)pHalq ],
Hal
Cathode:
.
.
M(OR)nHalm + e
+ROH, 1/2H
M(OR)nHalm
M(OR)nHalm
As a rule, it is recommended to perform the syn-
thesis in a diaphragmless electrolyzer from electro-
lytes chosen empirically, with as high electrical con-
duction as possible. The influence of electrolysis con-
ditions on the yield of alkoxides virtually was not
studied. The existing views on the mechanism of the
alkoxide formation are controversial. According to
Lemkuhl et al. [3], who synthesized iron, cobalt, and
nickel alkoxides using alkali metal halides as electro-
lytes, the initial anodic process is the discharge of
halide ions. Then, the atomic halogen reacts with a
metal of the anode to form the corresponding halide,
which is reduced to the alkoxide on the cathode, i.e.,
the reaction pattern is as follows:
1
+e
...
2
.
M(OR)n + 1Halm
1
+ROH, 1/2H , Hal
2
M(OR)n + m
.
(2)
Contrary to Shreider et al. [4], who ruled out the
alcohol dissociation with the formation of RO and
a proton with its subsequent reduction on the cathode,
Bukhtiarov et al. [5] detected by cyclic voltammetry
intense hydrogen evolution during the electrolysis of
alcohol solutions in aprotic solvents on the cathode
made of transition metals. Based on these observa-
tions, Bukhtiarov et al. [5] suggested an alternative
mechanism of the alkoxide formation, involving ioni-
zation of a metal with the formation of the correspond-
ing cations on the anode and decomposition of an al-
Anode:
.
.
Hal
Hal + e, M + 2Hal
MHal2;
741

742
BEREZKIN et al.
cohol molecule accompanied by evolution of hydro-
gen and formation of the RO anion on the cathode.
In the bulk of a solution, alkoxide anions react with
the metal cations to form alkoxide. The reaction pat-
tern can be presented as follows:
magnetic stirrer. The electrolyzer was hermetically
closed with a lid, in which a thermometer, an outlet
pipe with a calcium chloride tube, and electrode cur-
rent leads were inserted. A flat niobium anode was
arranged between the two cathodes.
2
The niobium anode (working area 8 cm ) was made
Anode: M0
M2+ + 2e,
of an Nb-1 niobium sheet [GOST (State Standard)
16099 80], and the cathodes (working area 18 cm ),
2
Cathode: ROH + e
OR + 1/2H2,
of a 1-mm platinum wire. Prior to the electrolysis, the
electrodes were weighed after trimming and degreas-
ing with Vienna lime. After the experiment was com-
plete, the solution was removed for further treatment.
The weight of the dissolved metal was determined
from the weight loss of the anode.
In solution bulk: M2+ + 2RO
M(OR)2.
(3)
Thus, with rather scarce and controversial data
available on the electrochemical formation of transi-
tion metal alkoxides, we started a systematic study
of this process with niobium ethylate as example.
Shreider [4] obtained for the first time niobium ethyl-
ate by the electrochemical method involving electroly-
sis of an ethanol solution of triethylamine hydro-
bromide with a platinum cathode and a niobium
anode. Niobium pentaethoxide was isolated and iden-
tified, but the Shreider’s experiment is the only exam-
ple of niobium alkoxide electrosynthesis described in
detail in the literature.
The product was isolated by fractional distillation.
First alcohol was distilled off, and then niobium
pentaethoxide, a light-yellow liquid, was distilled in
a vacuum (boiling point 165 C/1.5 mm Hg).
The current voltage dependences were measured
on an IPC-2000 electronic potentiostat and processed
on a computer. For the measurements we used a
3
45 cm diaphragmless cell equipped with a tempera-
ture-control jacket. The electrodes and an outlet rube
were inserted into the cell lid. The electrode system
consisted of a stationary working electrode made
of the corresponding metal (working area 0.5 cm ),
a platinum auxiliary electrode, and a silver chloride
reference electrode.
This study is the first step of the investigation of
the electrochemical synthesis of niobium alkoxide; we
examined the anodic dissolution of niobium in an
ethanol solution of various electrolytes.
2
EXPERIMENTAL
To measure the anodic curve, the cell was charged
with an electrolyte solution of the required concentra-
tion and was purged with dry argon through the outlet
tube for 0.5 h. Then, the trimmed electrode was set
into the cell preheated to the required temperature and
was polarized by the prescribed program.
According to the published data, alkoxide synthesis
is a moisture-sensitive process. Therefore, particular
attention was given to the ethanol drying. After per-
forming a number of the experiments, we used in
the study the following method.
Water was preliminarily separated from ethanol by
distillation of a ternary azeotrope with benzene [6].
Then the solution was treated with sodium metal and
diethyl oxalate. After distillation, the ethanol fraction
(boiling point point 78.2 78.3 C) contained 0.03
To record the cathodic curves in the presence of
niobium ethoxide, the electrodes were withdrawn and
trimmed prior to each measurement, and niobium
ethoxide was added to the electrolyte just before the
measurement. The current voltage curves were proc-
essed on a computer.
0.05 wt % H O. The content of water in the alcohol
2
was determined by Fischer titration.
Niobium forms penta-, tetra-, and trialkoxy com-
pounds [1], i.e., it may exist in the penta-, tetra-, or
trivalent state. With lithium chloride, lithium p-tolu-
enesulfonate, triethylamine hydrohalides, or tetraethyl-
ammonium quaternary salts in ethanol as electrolytes,
the niobium anode dissolves with the formation of a
Nb(V) compound. This is confirmed by calculation of
the current efficiency and by the lack of solution
coloration typical for Nb(III) and Nb(IV) alkoxides
[1]. At the same time, it was shown experimentally
[1] that, at a cathodic current density from 0.01 to
As supporting electrolytes were used chemically
pure grade salts. Tetraethylammonium acetate was
prepared by mixing an aqueous solution of tetra-
ethylammonium hydroxide with an excess amount
of dilute acetic acid. After that, the solution was
evaporated to dryness and the residue was dried at
100 120 C/5 mm Hg.
The experiments were performed in a diaphragm-
3
less electrolyzer in the form of a 90 cm glass cylinder
equipped with a temperature-control jacket and a
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 5 2006

ELECTROCHEMICAL SYNTHESIS OF NIOBIUM(V) ETHYLATE
743
(a)
(b)
I, mA
I, mA
E, V
E, V
(c)
I, mA
E, V
Fig. 1. Current voltage curves measured in a 0.5 N solution of LiCl in ethanol (0.027% water) in the presence of Nb(OEt)
additions. T = 25 C, V = 0.2 V s ; the same for Fig. 2. (I) Current strength and (E) potential (vs. silver chloride electrode);
5
1
the same for Fig. 2. Cathode: (a) Pt, (b) Ni, and (c) stainless steel. Nb(OEt) concentration (M): (1) 0, (2) 0.001, (3) 0.01,
5
(4) 0.02, and (5) 0.04.
2
0.0025 A cm , niobium pentaalkoxide can be
reduced to the tetraalkoxide on the cathode, with a
colorless solution becoming brown in the course of
the electrolysis. After testing a number of cathodic
materials (lead, niobium, titanium, stainless steel,
molybdenum, nickel, and platinum), we chose metals
with the lowest hydrogen overvoltage, platinum and
nickel, as the working materials.
The cathodic reduction was also examined voltam-
metrically. The reduction of niobium pentaethoxide on
the cathodes made of different materials was deter-
mined by measuring the voltammograms on platinum,
nickel, and stainless steel at the cathodic polarization
in a 0.5 N solution of lithium chloride supporting
electrolyte at different concentrations of niobium
pentaethoxide (Figs. 1a 1c). The curves obtained
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 5 2006

744
BEREZKIN et al.
Table 1. Comparison of total currents and currents spent
for reduction of niobium pentaethoxide at its different
concentrations in solution and at different potentials
I, mA
Current, mA
Addi-
tion, M
Current spent for
reduction, %
background
total
Potential 2.0 V
0.001
0.01
0.02
10
10
10
10
12
17
0
16.7
41.2
E, V
Potential 2.5 V
Fig. 2. Current voltage curve obtained on Nb in a 0.5 N
solution of Et NCl in ethanol (0.027% water).
0.001
0.01
0.02
27
27
27
27
33
48
0
15.2
43.8
4
noticeably depend on the cathodic current density in
the examined range. Thus, the current voltage curves
obtained are in agreement with the results of the pre-
parative electrolyses.
Table 2. Effect of the anion on the dissolution of a niobi-
um anode (Swork = 8 cm2) at an anodic current density of
The effect of the electrolyte on the efficiency of the
anodic dissolution of Nb was studied in two series of
experiments. The fact that the solution remains color-
less indicates that the dissolution occurs to Nb(V).
In the first series of the experiments (Table 2) we
studied the effect of the anion, with tetraethylammo-
nium as cation. As seen from Table 2, niobium dis-
solves at a current efficiency exceeding 95% [as cal-
culated for Nb(V)]. Hence, the anion only slightly af-
fects the anodic dissolution of niobium. The effect of
the cation is also weak, at least in the case when nio-
bium is dissolved in chloride solutions (Table 3).
The further study showed that, whereas the supporting
electrolyte affects the dissolution of a niobium anode
only slightly, its influence on the formation of niobi-
um alkoxide is decisive. Table 2 shows that niobium
ethylate is best formed in chloride solutions, whereas
in bromide solutions it is not formed at all. Although
with tetraborate and acetate anions the alkoxide is
formed, its yield (based on Nb) is considerably lower
than in chloride solutions. We do not have sufficient
data yet to account for this trend, and the study in this
area is being continued.
2
0.03 A cm from a 0.1 M electrolyte with a Pt cathode
(Swork = 18 cm2) at 35 C
Current efficiency Nb(OEt)5 yield
Electrolyte
by Nb(V), %
based on Nb(V)
Et4NCl
Et4NAc
Et4NBF4
Et4NBr
LiCl
99.16
99.92
97.72
95.79
99.46
95.70
42.9
1.71
20.9
0
65.4
23.1
Li(CH3C6H4SO3)
Table 3. Effect of the cation on the current efficiency by
Nb(V) in electrolysis with Nb anode (Swork = 8 cm2) and
Pt cathode (Swork = 18 cm2) in a 0.1 M electrolyte at
2
an anodic current density of 0.03 A cm and 35 C
Electrolyte
LiCl
Et4NCl
Et3N HCl
Et4NBr
Current efficiency by Nb(V)
99.46
99.16
100.37
95.79
78.69
90.09
To study the anodic process in more detail, we
measured the current voltage curves. One of typical
cyclic voltammograms obtained at anodic polarization
of a niobium electrode in an ethanol solution of tetra-
ethylammonium is shown in Fig. 2. The voltammo-
gram has a clear hysteresis. At forward potential scan-
ning, the dissolution is not observed up to 1.2 V,
whereas at the reverse scanning the electrode dissolu-
tion continues down to 0.25 V. Presumably, the anod-
ic polarization breaks the passivation film on the elec-
Et3N HBr
Bu4NBr
show that niobium pentaethoxide is reduced only on
the steel electrode (Fig. 1c). The parameters charac-
terizing this process are listed in Table 1.
Table 1 shows that the current spent for the reduc-
tion of niobium pentaethoxide depends on the concen-
tration of the substance in a solution and does not
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 5 2006

ELECTROCHEMICAL SYNTHESIS OF NIOBIUM(V) ETHYLATE
745
Table 4. Potentials of the niobium transition from the
active to the passive state in solutions of various composi-
tions.* Solvent: ethanol containing 0.027% water
electrolyte anion. Probably, the anion is involved in
the synthesis reaction. Presumably, the intermediate
formation of alkoxy halides, suggested in [4], plays
a certain role in the alkoxide synthesis.
Ea
Ep
E
Electrolyte
CONCLUSIONS
V
(1) The anodic dissolution of niobium is accom-
panied by transition of niobium into the pentavalent
state. The supporting electrolyte affects only slightly
the current efficiency of the metal dissolution.
LiCl
Et4NCl
Et4NOOCCH3
0.65
1.20
1.60
0.30
0.25
0.40
0.35
0.95
1.20
*
E = Ea
Ep.
(2) The supporting electrolyte strongly affects
the yield of niobium alkoxide. The highest yield is
observed in chloride electrolytes.
trode surface, making the electrode active. The poten-
tial of the onset of the niobium dissolution at forward
scanning can be designated as the potential of transi-
tion to the active state, and the potential of the pas-
sivation at reverse scanning, as the passivation poten-
(3) To avoid the reduction of Nb(V) to Nb(IV), it
is advisable to use cathode materials with the lowest
hydrogen overvoltage.
tial E . The similar effect was also observed for solu-
p
REFERENCES
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lyte significantly affects the potential of transition to
the active state, but does not noticeably affect the pas-
sivation potential. From the viewpoint of easiness of
niobium anode activation, it is preferable to use lithi-
um chloride as electrolyte.
1. Turova, N.Ya., Turevskaya, E.R., Kessler, V.G., and
Yanovskaya, M.I., The Chemistry of Metal Alkoxides,
Boston: Kluwer, 2002.
2. Tomilov, A.P., Elektrokhimiya, 2000, vol. 36, no. 2,
pp. 115 132.
3. Lemkuhl, H., and Eisenbach Liebigs, W., Ann. Chem.,
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efficiency in dissolution of a niobium anode depends
only slightly on the supporting electrolyte and is close
to 100% [as calculated for Nb(V)] in all the media
studied. This result and the closeness of the passiva-
tion potentials for different electrolytes show that,
contrary to Lemkuhl’s suggestion, niobium ionization
rather than oxidation of the anion takes place in the
course of the anodic process. Another important ob-
servation consists in that the yield of niobium ethylate
based on Nb significantly depends on the supporting
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