Effect of solvent nature on the reaction of metallic iron with benzoic acid in a bead mill

Article abstract of DOI:10.1134/S1070427208040058

Results of an experimental study of the influence exerted by the solvent nature on the reaction rate, achieved degree of acid expenditure, selectivity with respect to salts of iron(II) and (III), and phase state of the reaction mixture in a single-stage reaction of iron with benzoic acid and atmospheric oxygen in the presence of a stimulating iodine additive are presented.

Full text of DOI:10.1134/S1070427208040058

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2008, Vol. 81, No. 4, pp. 597 602.
Pleiades Publishing, Ltd., 2008.
Original Russian Text
A.M. Ivanov, E.A. Grechushnikov, 2008, published in Zhurnal Prikladnoi Khimii, 2008, Vol. 81, No. 4, pp. 559 564.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Effect of Solvent Nature on the Reaction of Metallic Iron
with Benzoic Acid in a Bead Mill
A. M. Ivanov and E. A. Grechushnikov
Kursk State Technical University, Kursk, Russia
Received February 22, 2007
Abstract Results of an experimental study of the influence exerted by the solvent nature on the reaction
rate, achieved degree of acid expenditure, selectivity with respect to salts of iron(II) and (III), and phase
state of the reaction mixture in a single-stage reaction of iron with benzoic acid and atmospheric oxygen in
the presence of a stimulating iodine additive are presented.
DOI: 10.1134/S1070427208040058
Iron(III) and (II) benzoates are rather widely used
in various industries [1 11].
been synthesized by this method [15 18]. The process
occurs in a single technological stage to give the tar-
get product mainly in the form of a suspension easily
separated by filtration. The filtrate containing dissolv-
ed products of conversion of the stimulating additive
and iron salts is recycled and thereby requires neither
separation, nor repeated utilization.
Commonly, these salts are obtained by one of
the following three methods: (i) exchange interaction
of iron salts and mineral acids with sodium benzoate
or by a direct reaction of an iron oxide with benzoic
acid in the temperature range 125 260 C in the pres-
ence of an organic dispersing agent [12]; (ii) reaction
of benzoic acid with the metal nitrate in the presence
of an inert organic solvent, followed by addition to
the reaction mixture of ammonia and(or) amine, and(or)
tetraalkylammonium hydroxide at a temperature of
The overall processes (1) and (2) are heterogeneous,
complex, and multistage. Their principal stages are
the following [19 22]:
Fe + I2
2FeI2 + 4HA + O2
and(or)
FeI2,
(3)
5
2
2
0 40 C and pressure of (1 50) 10 N m [13];
and (iii) direct electrosynthesis [14]. These methods
require certain auxiliary substances and rather severe
conditions and yield by-products that need utilization.
Moreover, some of these methods cannot be perform-
ed in a single stage.
2FeA2 + 2I2 + 2H O, (4)
2
4FeI + 12HA + 3O2
4FeA + 4I + 6H O, (5)
2
3
2
2
It was of interest to obtain iron benzoates in a sin-
gle-stage process in accordance with the stoichio-
metric overall equations (HA stands for carboxylic
acid)
and also [17, 18]
4FeA2 + O2 + 4HA
4FeA3 + 2H O,
(6)
(7)
2
2
Fe + 4HA + O2
2FeA2 + 2H O,
(1)
2
Fe + 2FeA3
3FeA2.
or
Together, stages (3) + (4), (3) + (5) or (3) + (4) + (5)
constitute cyclic processes of consumption and forma-
tion of iodine, and just this circumstance is the reason
why it is used as a catalyst.
4Fe + 12HA + 3O2
4FeA3 + 6H O,
2
(2)
i.e., with atmospheric oxygen as an oxidizing agent,
crushed iron or its alloys (including those of waste
origin) as a reducing agent, and also an organic sol-
vent and iodine (the latter as a stimulating additive).
By now, iron(II) and (III) formates and acetates have
The main purpose of the liquid phase is the trans-
port function. The place where the basic stages occur
is the iron surface onto which the reactants are de-
597

598
IVANOV, GRECHUSHNIKOV
the case and bottom of the bead mill were protected
with an inserted steel shell and a well-fit false bottom
with a foot bearing for the blade stirrer. As a grinding
agent served glass beads 0.8 3.1 mm in diameter, both
individually and together with powdered reduced iron,
sieved fractions of crushed cast iron, and fragmented
steel shavings with the maximum linear particle size
of 5 mm. The mass ratio between the glass beads and
ground iron and(or) its alloys was widely varied.
The bead mill was equipped with a reflux condens-
er and heat supply via a liquid bath that could be
moved along the longitudinal axis of the reactor.
The course of the process was monitored by sampling,
with the content of carboxylic acid and its iron(II)
and (III) salts determined. The monitoring results
were used to plot the corresponding kinetic curves.
Fig. 1. Kinetic curves of benzoic acid consumption for
formation of iron salts at varied nature of a solvent in
the liquid phase of the charge. Reactor: vertical bead mill
with a four-blade stirrer rotating at 1560 rpm; mass of
beads 200 g; 1 : 1 mass ratio of beads to the liquid phase
of the charge; charge: crushed cast iron 60 g, iodine
1
The reagents were charged in various orders. Most
frequently, glass beads, solvent, and carboxylic acid
were charged first and then agitation was switched
on and thereby a solution of carboxylic acid was pre-
pared. Occasionally, this solution was prepared pre-
liminarily and then batched into the reactor. After
that, ground iron or iron alloys and the catalyst were
charged and air was fed into the reactor for bubbling.
The bubbler was fabricated in an elastic version to
preclude its disintegration upon contact with moving
beads or heavy particles of iron or its alloys.
0
.03 mol kg
liquid phase; temperature 60 2 C.
(
(
X
) Concentration of benzoic acid and ( ) time. Solvent:
BA
1) dimethylformamide, (2) 2 : 3 (v/v) mixture of dimeth-
ylformamide and butanol, (3) n-butanol, (4) chloroben-
zene, (5) bromobenzene, (6) ethyl cellosolve, (7) toluene,
(
8) petroleum spirit, and (9) ethylbenzene.
livered by means of adsorption. The solid products
formed on the surface should be removed into the so-
lution bulk to preclude full passivation of the surface
by their deposits. Their desorption cannot play any
significant part in systems of this kind. Considerably
more important is their dissolution or mechanical re-
moval (scraping, scuffing, stripping). The role of
the solvent is also important. On the whole, the sol-
vent nature should largely determine the competitive
ability of reactions (3) (7) and, consequently, that of
It was found that, without iodine, the process be-
gins, but rapidly terminates at acid conversions not
exceeding 2 3%. Therefore, if the chosen temperature
mode was to be strictly maintained (in the range 15
9
0 C), the stimulating additive was introduced at
the instant when the system reached the working tem-
perature.
(
1) and (2), as well as the selectivity with respect to
the iron(II) and (III) salts and the heterophase state of
the final reaction mixture, which may either facilitate
or hinder its separation and product recovery. In this
study, an attempt was made to evaluate the effect of
the solvent nature on the reaction of iron with benzoic
acid in a bead mill.
Figure 1 shows kinetic curves of benzoic acid
consumption for the process performed in various
solvents. The role of the solvent is manifested not
only in different rates of acid consumption, but also
in the existence of the period of self-accelerated de-
velopment (e.g., in petroleum spirit), intermediate
self-deceleration (in ethylbenzene), self-termination
EXPERIMENTAL
(in toluene), and self-resumption of the process (in
ethylbenzene), in its selectivity with respect to iron(II)
and (III) salts, and in the manner in which the selec-
tivity varies in the course of the process.
The experiment was performed in a vertical bead
mill with a high-speed (1440 and 1560 rpm) blade
stirrer whose shaft was connected to that of a 0.5-kW
motor. The stirrer case was made of stainless steel and
had a wall thickness of about 10 mm. The blades and
the shaft of the stirrer were made of st.45 or stainless
steel. In both cases, their working life was rather short
In all of the solvents chosen, the amounts of iodine
used for stimulation were completely soluble. There-
fore, the maximum concentrations of iodine were de-
termined only by its close. At the same time, its
running concentrations were 10 20 times lower
than the maximum values. This indicated that, in
the course of the process, the main part of iodine
(
1 2 months), which was predetermined by their in-
volvement in the redox process under consideration
as a source of reacting iron. To preclude rapid wear,
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EFFECT OF SOLVENT NATURE ON THE REACTION OF METALLIC IRON
599
Solubility of benzoic acid and its iron salts and of iron(II) iodide in the solvents used at 17 1 C
Solubility, mol kg 1
Solvent of the liquid phase
benzoic acid
iron(III) benzoate
iron(II) benzoate
iron(II) iodide* [21]
Formamide
Dimethylformamide
n-Butanol
1.712
4.398
2.058
3.607
0.041
0.412
0.110
0.281
<0.0011
0.0375
0.0087
0.0158
0.57
0.24
2
: 3 (v/v) mixture of dimethyl-
formamide and n-butanol
Ethyl cellosolve
Petroleum spirit
Toluene
Ethylbenzene
Chlorobenzene
Bromobenzene
Water
2.724
0.028
0.817
0.559
0.581
0.623
0.145
0.183
0.008
0.083
0.062
0.082
0.029
0.013
0.0274
<0.0003
<0.0003
<0.0003
<0.0003
<0.0003
0.0007
0.41
0.0004
0.0009
0.0005
>2.08
*
At 29 1 C.
being introduced was in the form of FeI , which was
2
confirmed by direct analysis.
The acid must also be to some extent soluble in
the solvent used (see table), but this extent remains
unknown. The process could also be performed in
dimethylformamide, in which the solubility of benzoic
1
acid exceeds 4 mol kg , and in petroleum spirit, in
which the solubility is more than 100 times lower and
the main part of the acid charged into the reactor re-
mained in the solid phase in the course of the process.
The conclusion that the solubility of the acid is not
a key factor is also confirmed by the fact that the rate
and time characteristics of the process depend on
the initial content of the acid in the charge not only
in a rather complicated way, but also not too strongly
Fig. 2. Conversion of benzoic acid in a single-stage re-
action with fragmented iron in the presence of iodine vs.
the content X of n-butanol mixed with dimethylformamide
B
as a solvent of the liquid phase. Dosage: benzoic acid in ac-
1
cordance with its solubility at 21 C and iodine 0.125 mol kg ;
(
Figs. 2, 3). In addition, clear requirements to the role
4 : 1 mass ratio of the liquid phase and reduced iron; mass
of the liquid phase 200 g; temperature 60 1 C; rotation
rate of the blade stirrer of the vertical bead mill 1560 rpm.
played by the solubility of salt products in the liquid
phase could not be formulated. At a high solubility of
the metal iodide, there appears an important mass-ex-
change pathway to disintegration and removal of its
surface deposits from iron. In this case, reactions (4)
and (5) partly pass into solution, i.e., into homoge-
neous conditions. The FeA and FeA salts being
(
1) (
) Conversion at the selectivity with respect to
S
1
the iron(III) salt approaching 100% and remaining so till
the end of the process; (2, 3) ( ) time in which (2) 50%
and (3) 100% conversion of benzoic acid is reached.
2
3
accumulated are first present in solution and then,
beginning at a certain instant of time, form indepen-
dent solid phases. The solid phase of green FeA2
appears first, which correlates with its poorer solubil-
ity, compared with FeA . As a direct experiment de-
3
monstrated, transformations of dissolved and sus-
pended salt products into components of surface de-
posits on the metal are unlikely.
Fig. 3. Solubility of (1) benzoic acid (P ) and (2) iron(III)
BA
At the same time, at a poor solubility of iron(II)
iodide in the liquid phase used (e.g., in petroleum
spirit), reactions (4) and (5) will be predominantly
benzoate (P ) vs. the content X of n-butanol in a mixture
S
B
with dimethylformamide as a solvent. Temperature ( C):
(1) 21 and (2) 18.
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600
IVANOV, GRECHUSHNIKOV
The following circumstance appeared to be the most
important in this regard: the viscosity of the system
grows as the product is accumulated both in solution
and as a suspension, with the increase being self-ac-
celerated in deep stages. This leads to a synchronous
decrease in the operation efficiency of the bead mill,
to the point of complete loss of its working capacity.
The results obtained demonstrate that this occurs
when iron salts are accumulated in a total amount
1
of 1.2 1.5 mol kg . Consequently, the maximum
content of the acid in the charge, determined with ac-
count of the fact that its whole amount can be con-
1
verted into salts, must not exceed 3.6 4.5 mol kg .
However, this does not mean that just these initial
concentrations of the acid will be optimal (Figs. 2,
4
, 5), because the corresponding dependences are
Fig. 4. Time
in which a virtually quantitative con-
1
complicated. For example, the solubilities of ben-
zoic acid and iron(III) benzoate substantially fall as
the content of n-butanol in the mixed solvent in-
creases (Fig. 3), whereas the time in which a 50%
or >98% conversion of benzoic acid into iron salts is
reached passes through a minimum at a 60 65% con-
tent of n-butanol in the mixed solvent. Consequently,
it is better that the solubility of the final product in
the liquid phase used should not be too high, so that
the product would accumulate in the form of a sus-
pension. In this case, the viscosity grows more slowly
and the product itself can be easily isolated by simple
filtration.
version of benzoic acid mainly into the iron(III) salt is
reached vs. the initial concentration X
a 2 : 3 (v/v) mixture of dimethylformamide and butanol,
taken as the liquid phase for the reaction of 30 wt % (rel-
ative to the liquid phase) crushed cast iron with benzoic
acid at 60 C in the presence of 0.075 mol kg of a stim-
ulating iodine additive and bubbling with air. Reactor:
vertical bead mill with a four-blade stirrer rotating at
0
of the acid in
BA
1
1
550 rpm; 1 : 1 mass ratio of beads and the liquid phase
1
1
charge; air expenditure for bubbling 2.2 l min kg liquid
phase.
Comparison of the data in Figs. 4 and 5 shows that
the manner in which the process duration increases
with the initial content of benzoic acid is associated
not only with the pronounced differences in the vis-
cosities of the reaction mixtures in deep-conversion
stages, but with the fact that the initial rates pass
through a maximum as functions of the content of
the acid. At the beginning of the process, the viscosity
of the liquid phase will also grow with the content of
benzoic acid. However, this increase is incomparable
with the rise in the viscosity of the reaction mixture,
observed as the acid expenditure for salt formation
becomes quantitative.
Fig. 5. Initial rate W of the conversion of benzoic acid
0
into iron salts vs. the initial content X
of benzoic acid
BA
in the liquid phase of the system. Process conditions
the same as those for Fig. 4.
Experiments with different solvents revealed var-
iants in which the reaction mixture becomes, in the
course of the process, a glue-like viscous mass that
entraps glass beads and solid particles of the metal.
In these cases, the process is always doomed to self-
termination because of the complete loss of working
capacity by the bead mill. This is a rather rare phe-
nomenon for benzoic acid (e.g., in toluene; Fig. 1,
curve 7). By contrast, this situation is one of the most
frequently occurring for butyric acid.
heterogeneous, and FeA2 and FeA remaining on
3
the surface can well be transformed into important
components of the surface deposits on iron. Mechan-
ical methods for disintegration and removal of sur-
face deposits become the most important in this case.
Because both these variants have been implemented
in practice, with the process rates and durations being
comparable, there is no good reason to prefer one of
them.
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EFFECT OF SOLVENT NATURE ON THE REACTION OF METALLIC IRON
601
However, not any self-termination of the process is
due to a strong thickening of the reaction mixture or
to its transformation into a glue-like mass. As an ex-
ample can serve curve 9 in Fig. 1. Whose intermediate
portion is associated with blocking of the metal sur-
face by surface deposits of salt products, which be-
comes weaker in the course of time and self-resump-
tion of the process occurs.
intermediate between a poorly flowable paste and
a glue-like mass that does not release the solid phase
neither under rapid cooling, nor in the case of natural
or artificially decelerated cooling. A similar pattern
is observed in all the other cases when the content
of n-butanol in the mixed solvent is smaller than
20 vol %.
The iron(III) salt can be selectively obtained at
the instant of virtually quantitative consumption of
the acid with such solvents as n-butanol, petroleum
spirit, butyl acetate (not shown in Fig. 1) and the
above-mentioned mixtures of dimethylformamide and
n-butanol. The kinetic curve of iron(II) salt accumula-
tion in these solvents passes through a weak maxi-
mum lying close to the ordinate axis and further ap-
proaches the abscissa axis and virtually runs into it.
In such solvents as dimethylformamide, formamide
In some cases, self-termination of the process was
observed in systems in which products were most-
ly accumulated as suspended solid phases, and no
noticeable decrease in the operation efficiency of
the bead mill was observed in the course of their
accumulation. For example, self-termination occurred
in water (not shown in Fig. 1, the corresponding
kinetic curve is similar to curve 7) by the 85th
9
0th minute, with no self-resumption observed in
the following 150 min. This self-deceleration also
occurred because of the blocking of the iron surface
by a mixture of benzoates of iron(II) (green) and
iron(III) (brown). An inspection of unreacted particles
of cast iron revealed a greenish-gray deposit on their
surface. The filtered-off precipitate of the salt products
had the same coloration.
(
not shown in Fig. 1; the process is slower than that
in dimethylformamide, but goes to completion and
shows no self-termination), and water, iron(II) and (III)
salts are always accumulated in comparable amounts.
The nature of the solvent used strongly affects,
through the dynamics of variation of the phase state
of the reaction mixture, the operation efficiency of
the bead mill. This, in turn, is transformed to a certain
extent to the apparent efficiency of the stimulating
additive used and, consequently, affects its optimal
amount, which is far from being the same in different
solvents.
The nature of the solvent used strongly affects
the accumulation kinetics of iron(II) and (III) salts
and, consequently, the achievable selectivity with re-
spect to the target product. This can be illustrated by
the example of the mixed solvent used in this study,
with varied ratio of dimethylformamide and n-butanol
in the solvent (Fig. 2, curve 1). The conversion on
reaching and exceeding which the selectivity with re-
spect to iron(III) salt tends to unity passes through
a minimum. This minimum is observed at a content of
n-butanol within the range 50 70% and acid conver-
sions into the product of about 50%. It is this range
of alcohol contents in the mixed solvent that should
be regarded as the most interesting, although the sol-
ubility of benzoic acid in this region is substantially
CONCLUSIONS
(1) The liquid phase predetermines the type of
the kinetic curve of consumption of benzoic acid in
its reaction with metallic iron in a mechanically acti-
vated process: quantitative characteristics of its sep-
arate portions, duration of the process as a whole,
occurrence of autoacceleration and autodeceleration,
self-termination and self-resumption, and selectivity
with respect to iron(II) and (III) salts, and also the dy-
namics of their variation in the course of the process.
1
lower than 4 mol kg (at 21 C).
The solubility of both the acid and iron(III) salt in
dimethylformamide is considerably higher (Fig. 3). It
additionally increases as temperature is raised. In di-
methylformamide, the process can be brought virtually
to complete expenditure of the acid for salt formation,
(2) At the instant of virtually quantitative con-
sumption of benzoic acid, a selectivity for iron(III)
benzoate that tends to 100% is provided by use of
such solvents as white spirit, n-butanol, butyl acetate,
and some mixtures of n-butanol and dimethylformam-
ide, in which case a significant amount of the target
salt is accumulated in the solid phase in the form of
a suspension.
provided that its initial concentration does not exceed
1
5
.5 mol kg . However, the 100% selectivity with re-
spect to the iron(III) salt cannot be reached neither in
the end, nor in the course of the process. It was found
that it is impossible to obtain products in the solid
phase in this medium: at so large initial contents of
benzoic acid, the final reaction mixture is something
(3) The complete dissolution of the acid in the sol-
vent for the liquid phase, used in the study, is not
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 81 No. 4 2008

602
IVANOV, GRECHUSHNIKOV
a necessary condition for rapid and, a fortiori, quan-
titative and selective course of the redox process of
the given type.
11. GB Patent 1446114.
12. Canadian Patent 970389.
1
1
3. US Patent 5220045.
4. US Patent 5443698.
(4) The least favorable variant, as far as the rate
and extent of the process, its selectivity for salts,
and their isolation from the reaction mixture are con-
cerned, was obtained with a solvent composed of di-
methylformamide and its mixtures with up to 20%
n-butanol (by volume). In this case, non of the salts
is accumulated and the reaction mixture turns into
a viscous glue-like system.
15. RF Patent 2268874.
16. RF Patent 2269509.
17. RF Patent 2291855.
1
1
8. RF Patent 2291856.
9. Medvedeva, Zh.V., Oxidation of Iron and Ironcon-
taining Materials with Molecular Iodine in Organic
Dispersion Media, Cand. Sci. Dissertation, Kursk,
2
000.
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