Electrochemical synthesis of sodium hexamethoxyniobate

Article abstract of DOI:10.1134/S1070427208040125

Anodic dissolution of niobium in anhydrous methanol in the presence of sodium methylate was studied. A method for recovery of niobium methylate from sodium hexamethoxyniobate by the reaction with ammonium chloride was developed.

Full text of DOI:10.1134/S1070427208040125

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2008, Vol. 81, No. 4, pp. 635 638.
Pleiades Publishing, Ltd., 2008.
Original Russian Text M.Yu. Berezkin, E.G. Polyakov, V.V. Turygin, A.P. Tomilov, 2008, published in Zhurnal Prikladnoi Khimii, 2008, Vol. 81, No. 4,
pp. 598 601.
PHYSICOCHEMICAL STUDIES
OF SYSTEMS AND PROCESSES
Electrochemical Synthesis of Sodium
Hexamethoxyniobate
M. Yu. Berezkin, E. G. Polyakov, V. V. Turygin, 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 February 13, 2008
Abstract Anodic dissolution of niobium in anhydrous methanol in the presence of sodium methylate was
studied. A method for recovery of niobium methylate from sodium hexamethoxyniobate by the reaction with
ammonium chloride was developed.
DOI: 10.1134/S1070427208040125
The permanent need of the electronic industry in
high-purity materials and, in particular, special-purity
niobium used in manufacture of niobium slug capac-
itors enforces a search for new methods for its produc-
tion and purification. Of particular interest are purifi-
cation of niobium to remove admixtures of other me-
tals and recovery of pure niobium from the raw metal
and alloys. The most promising for this purpose is
the electrochemical way to obtain metal alkoxides,
followed by their separation. The electrochemical
synthesis of niobium alkoxides has been considered
previously [1, 2]. A similar method can be used
to produce not only simple alkoxides, but also com-
plex compounds, e.g., sodium hexamethoxyniobate
ical method was considered and its properties and
formation mechanism were analyzed.
EXPERIMENTAL
Because, according to [5], the synthesis of alkox-
ides is sensitive to presence of moisture, methanol of
chemically pure grade used in the study was subjected
to additional drying by boiling with magnesium shav-
ings [6]. The final content of water was less than
0.01%. LiCl used in the study was also of chemically
pure grade. Lithium chloride was dried by calcination
at 350 C for 5 h. Sodium methylate was obtained by
a direct reaction of metallic sodium (chemically pure)
with methanol. The anodes were made of niobium of
Nb-1 brand [GOST (State Standard) 16099 80].
NaNb(OMe) , whose specific properties can serve as
6
a basis for development of a new method for recovery
of pure niobium from niobium-containing alloys.
Experiments were performed in a diaphragmless
3
The available published data for this compound are
rather scarce. This compound was first obtained by
a chemical reaction of sodium and niobium meth-
ylates [3]. The principal properties of the substance
were described and the electrical properties of its so-
lutions were reported. The existence of [Nb(OMe)6]
ions was also demonstrated in [4] by conductometric
titration. No evidence about the electrochemical meth-
od for synthesis of complexes of this kind could be
found in the literature.
electrolyzer having the form of a 90-cm glass cylinder
equipped with a thermostating jacket and a cock at
the bottom for discharging the electrolyzate. The elec-
trolyzer was hermetically closed with a cover into
which a thermometer, gas-discharge pipe with a cal-
cium chloride packing, and current leads to the elec-
trodes were inserted. The niobium anode was mounted
below the cathode at a distance of 0.5 cm.
2
The working area of the anode was 8 cm . The cath-
odes were made of a platinum wire 1 mm in diameter
2
In this study, the possibility of obtaining the so-
dium-niobium methoxy complex by the electrochem-
and had a working area of 24 cm . Prior to each elec-
trolysis, the electrodes were trimmed and degreased
635

6
36
BEREZKIN et al.
For measuring the anodic polarization curve,
an electrolyte solution of a required concentration was
poured into the cell and bubbled with dried argon via
the gas-discharge pipe for 0.5 h. Then the required
temperature was set and a trimmed electrode was
placed in the cell and polarized in accordance with
the preset program.
NaNb(OMe) was synthesized as follows. Prior to
6
electrolysis, the niobium anode was thoroughly de-
greased and weighed. The electrolyzer was charged
with an electrolyte solution [1.1 g of NaOCH in
3
90 ml of methanol (0.54 M solution)]. The content of
moisture in the solution was less than 0.05 wt %.
The electrolysis was performed at a constant current
strength of 1 A and temperature of 18 20 C. In
the course of electrolysis, the solution remained trans-
parent, with the electrolyzer voltage equal to 10 V
at the beginning of a run and increasing to 11 V by its
end. In 7 h 16 min, the current was switched off and
the niobium anode was weighed. The loss of mass by
the anode was 4.14 g, which corresponds to a 89%
current efficiency by pentavalent niobium. Methanol
was evaporated from the electrolyzate under a reduced
pressure (15 mm Hg). A white crystalline substance
Fig. 1. Voltammetric curves obtained on Nb in 1 M solu-
tions of (1) LiCl and (2) NaOCH in methanol. Tempera-
3
1
ture 25 C, potential sweep rate 0.2 V s ; the same for
Figs. 2, 3. (I) Current and (E) potential; the same for
Figs. 2, 3.
with Vienna lime, washed with water, and weighed.
After an experiment was complete, the electrolyzate
was discharged into a leak-proof flask for further
processing. The amount of the dissolved metal was
found from the loss of mass by the anode.
identified as NaNb(OMe) remained in the still bot-
6
To isolate niobium pentamethoxide (methylate),
the alcohol was evaporated from the electrolyzate and
then the product was extracted from the still bottoms
with hexane. After the extractive agent was evap-
orated, a light yellow liquid that crystallized upon
cooling remained in the flask. The resulting yellowish
crystals had mp 61 C.
toms.
A known method for obtaining alkoxides of the ali-
phatic series consists in anodic dissolution of a metal
in an appropriate alcohol containing a supporting elec-
trolyte that makes the system electrically conducting
[
1]. Previously, a method for synthesizing niobium
pentamethoxide by dissolution of a niobium anode in
methanol in the presence of lithium chloride has been
developed. With certain conditions satisfied, niobium
pentamethoxide can be obtained with a current effi-
ciency exceeding 95% [2].
Sodium hexamethoxyniobate was isolated by evap-
oration of the alcohol from the electrolyzate and wash-
ing of the remaining white needle-like crystals with
hexane.
A study of the influence exerted by the nature of
a supporting electrolyte on the niobium dissolution
potential in methanol solutions by cyclic voltammetry
The content of sodium methylate in the still bot-
toms was found by their dissolution in water and
titration of the resulting solution with an alkali.
The content of niobium in the still bottoms was found
by their dissolution in water, addition of an acid, and
gravimetric determination of the precipitated niobium
pentaoxide.
(CVA) revealed an anomalously high dissolution rate
of niobium in the presence of sodium methylate. Fig-
ure 1 shows CVA curves for niobium dissolution in
the presence of lithium chloride and sodium meth-
ylate. It can be seen from the CVA curve that, in
Voltammetric measurements were made with an IPC
computer-controlled electronic potentiostat in a 45-
the presence of NaOCH , the onset of niobium dis-
3
solution is shifted to the region of negative potentials
by approximately 750 mV. Because of this circum-
stance, the current of anodic dissolution of niobium
3
cm cylindrical diaphragmless glass cell equipped
with a thermostating jacket. Electrodes and a gas-
discharge pipe were inserted into the cell cover.
The electrode system comprised a niobium stationary
at a potential of 1 V in the presence of NaOCH ex-
3
ceeds that in the presence of LiCl by more than a fac-
tor of 2. The so pronounced shift of the niobium dis-
solution potential may indicate that niobium ions
formed at the anode are bound into a stable complex.
2
working electrode with an area of 0.5 cm , platinum
auxiliary electrode, and silver chloride reference elec-
trode.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 81 No. 4 2008

ELECTROCHEMICAL SYNTHESIS OF SODIUM HEXAMETHOXYNIOBATE
637
Results of a preparative synthesis of sodium hexamethoxyniobate (NaOCH concentration 0.5 M, voltage 6 11 V,
3
2
current density 0.125 A cm , temperature 25 30 C)
Current efficiency
by Nb(V)
Yield of
NaNb(OMe)6
Fraction of niobium bound
into sodium hexamethoxyniobate
Yield* of
Nb(OMe)5
Na : Nb
molar ratio
Run no.
%
1
2
3
1 : 1
1 : 2.5
1 : 5
99.9
98.8
97.2
99
97
98
99
41.6
18.9
0
55.3
79.09
*
Isolated by extraction with hexane after the evaporation of methanol.
The occurrence of two anodic processes is the most
clearly reflected by the voltammetric curve obtained
in the presence of lithium chloride with a minor ad-
dition of lithium methylate (Fig. 2). It can be seen
that, in this case, the dissolution of niobium begins
at a more negative potential, a region of limiting cur-
rents appears, and the onset of the second reaction is
observed when the niobium dissolution potential in
the presence of lithium chloride is reached. It follows
from the voltammetric curve obtained in the presence
of the methoxy complex (Fig. 3) that the niobium dis-
solution potential lies in the intermediate region be-
tween the dissolution potentials in the presence of
NaOCH3 and LiCl. However, the process rate is
lower.
Sodium hexamethoxyniobate is stable in a meth-
anol solution; however, a similar compound with
the ammonium cation does not exist. On the basis of
this property, the compound obtained in this study
was used to isolate niobium methoxide by the ex-
change reaction
NaNb(OCH3)6 + NH Cl
Nb(OCH3)5 + NaCl
4
+
CH OH + NH3
.
3
The reaction was performed in a methanol solution
of the complex to give niobium pentamethoxide in
8
7% yield.
The data obtained suggest that, in anodic polariza-
tion of the niobium electrode, the dissolution of
the metal is substantially facilitated in complexation
with methylate ions, whereas the reaction with chlo-
ride ions is more hindered, which leads to an increase
in the potential. In the absence of ions reacting with
niobium, the dissolution rate is lower.
In contrast to the process of electrosynthesis of
niobium pentamethoxide in the presence of lithium
chloride, in which both anodic and cathodic reac-
tion products are involved and niobium methylate is
formed in the solution bulk upon their mixing [2],
the hexamethoxy complex is probably formed directly
at the anode in the course of the anodic reaction
Fig. 2. Voltammetric curve obtained on Nb in a 1 M solu-
Nb + 6OMe
[Nb(OMe) ] .
6
5
e
tion of LiCl in methanol containing 0.1 M LiOCH .
3
The preparative electrolyses performed in the study
with 0.1 M NaOMe as a supporting electrolyte de-
monstrated that niobium is converted to the penta-
valent state in a nearly quantitative yield, but no
free niobium pentamethoxide is formed until the con-
centration of Nb(V) in solution reaches a value of
0
.1 M. Niobium pentamethoxide starts to appear only
after the Nb : Na molar ratio in solution exceeds 1 : 1
see table).
Fig. 3. Voltammetric curve obtained on Nb in a 1 M solu-
(
tion of NaNb(OCH ) in methanol.
3
6
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 81 No. 4 2008

6
38
BEREZKIN et al.
Sodium hexamethoxyniobate is formed as white
(2) It was demonstrated that the anodic dissolution
of niobium is substantially facilitated in the presence
of sodium methylate as a supporting electrolyte, com-
pared with solutions containing lithium chloride, be-
cause of the formation of complexes with methylate
ions directly in the anodic process.
thin needle-like crystals. The substance is well soluble
in methanol and insoluble in benzene and hexane.
The methanol solution is electrically conducting,
which indicates that this compound dissociates by
the equation
(
3) It was shown that pure niobium methoxide
NaNb(OMe)6
Na+ + [Nb(OMe) ] .
6
Nb(OMe)5 can be obtained in the reaction of
the NaNb(OMe) formed with ammonium chloride.
6
An elemental analysis and calculation gave the fol-
lowing:
(4) It was demonstrated that a water-soluble hy-
droxo complex NaNb(OH) is formed in hydrolysis
of NaNb(OMe)6.
6
Found, %: Na 7.4, Nb 30.6.
NaNb(OMe)6.
REFERENCES
Calculated, %: Na 7.6, Nb 30.79.
1
2
. Berezkin, M.Yu., Chernykh, I.N, Polyakov, E.G., and
Tomilov, A.P., Zh. Prikl. Khim., 2006, vol. 79, no. 5,
pp. 752 756.
Crystalline sodium hexamethoxyniobate is com-
paratively well soluble in water to give a weakly
opalescent solution that is stable at room temperature.
Evaporation of water yields coarse cubic crystals
containing no carbon. Presumably, such a behavior
is accounted for by instantaneous hydrolysis to give
. Berezkin, M.Yu., Polyakov, E.G., Turygin, V.V., and
Tomilov, A.P., Elektrokhimiya, 2007, vol. 43, no. 10,
pp. 1265 1267.
a hydroxo complex NaNb(OH) , which is well soluble
in water and insoluble in a water methanol solution.
3. Mehrotra, R.C., Agrawal, M.M., and Kapoor, P.N.,
6
J. Chem. Soc. (A), 1968, pp. 2673 2677.
4
. Cut, R., Helv. Chim. Acta, 1964, vol. 47, pp. 2262 2270.
The possibility of obtaining a compound stable in
aqueous solution may be of interest for separation of
niobium from admixtures of other metals.
5. Turova, N.Ya., Turevskaya, E.R., Kessler, V.G., and
Yanovskaya, M.I., The Chemistry of Metal Alkoxides,
Kluwer, 2002.
CONCLUSIONS
6. Weissberger, A. and Proskauer, E.S., Organic Solvents.
Physical properties and methods of purification, Rid-
dig, J.A. and Toops, E.E., Eds., New York: Inter-
science, 1955.
(
1) Sodium hexamethoxyniobate NaNb(OMe) was
6
electrochemically synthesized for the first time.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 81 No. 4 2008