Ozonocatalytic decomposition of glyoxal and glyoxylic and formic acids in the presence of iron(III) ions

Article abstract of DOI:10.1134/S003602441002010X

Rate constants of reactions of ozone with glyoxal, glyoxylic and formic acid in aqueous solutions at pH 1.5 were determined. It was shown that iron(III) in the form of ions accelerates oxidation of glyoxal and glyoxylic acid, but does not influence reaction between ozone and formic acid. It was established that the catalyst acts effectively if its concentration is comparable to the concentration of the oxidized substrate, the optimal stoichiometric ratio (Fe³+/substrate) being close to 1/3. The catalytic reaction mechanism was studied using a competitive chelate ligand, oxalic acid. We concluded that the catalytic activity of iron(III) in the investigated reaction was due to its ability to form chelate complexes in which the substrate was more easily oxidized by molecular ozone. Pleiades Publishing, Ltd., 2010.

Full text of DOI:10.1134/S003602441002010X

ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2010, Vol. 84, No. 2, pp. 215–220. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © E.M. Ben’ko, V.V. Lunin, 2010, published in Zhurnal Fizicheskoi Khimii, 2010, Vol. 84, No. 2, pp. 266–271.
CHEMICAL KINETICS
AND CATALYSIS
Ozonocatalytic Decomposition of Glyoxal and Glyoxylic
and Formic Acids in the Presence of Iron(III) Ions
E. M. Ben’ko and V. V. Lunin
Department of Chemistry, Moscow State University, Moscow, 119991 Russia
Received December 10, 2008
Abstract—Rate constants of reactions of ozone with glyoxal, glyoxylic and formic acid in aqueous solutions
at pH 1.5 were determined. It was shown that iron(III) in the form of ions accelerates oxidation of glyoxal and
glyoxylic acid, but does not influence reaction between ozone and formic acid. It was established that the catꢀ
alyst acts effectively if its concentration is comparable to the concentration of the oxidized substrate, the optiꢀ
3+
mal stoichiometric ratio (Fe /substrate) being close to 1/3. The catalytic reaction mechanism was studied
using a competitive chelate ligand, oxalic acid. We concluded that the catalytic activity of iron(III) in the
investigated reaction was due to its ability to form chelate complexes in which the substrate was more easily
oxidized by molecular ozone.
DOI: 10.1134/S003602441002010X
The results of numerous investigations have shown С –С1. Based on the experimental data, it was preꢀ
2
the great effectiveness of catalytic ozonization as an sumed that acceleration of the reaction was due to an
alternative method of removing hardꢀtoꢀoxidize increase in activity of the oxidized substrate during the
organic pollutants in water purification processes. Traꢀ formation of chelate complex with metal ions.
ditional homogeneous and heterogeneous catalysts
The present paper is a continuation of our earlier
based on the transition metals, which are able to
research and is devoted to investigating the kinetics
decompose ozone with the formation of more active
and mechanism of the ozonocatalytic decomposition
hydroxyl radicals, and to activate an oxidized substrate
of glyoxal, glyoxylic and formic acid in the presence of
due to complex formation [1], are applied in this field.
A special place among the objects of investigation
belongs to ketoacids and aldehydes, shortꢀchained
carbon acids that are produced during ozonization of
unsaturated and aromatic compounds and are notable
for high stability with respect to the impact of ozone.
3+
Fe ions.
EXPERIMENTAL
The following organic reagents, produced by
Fluka, were used in our work: 40% aqueous solution of
It has been established that in the presence of tranꢀ glyoxal, monohydrate of glyoxylic acid C H O · H O
,
2
2
3
2
2+
2+
2+
2+
3+
sition metals such as Mn , Co , Cu , Ni , Cr , formic acid, and oxalic acid. Chemically pure iron sulꢀ
2
+
3+
Fe , and Fe , the rate of oxidation of oxalic [2–5], fate Fe (SO ) · 9H O was used as the source of iron
2 4 3 2
glyoxalic [5–7], pyrotartaric [7–9] acids by ozone ions. Aqueous solutions of the reagents were acidified
rises. However, these catalysts are not active in the to pH 1.5 using concentrated Н SO4
.
2
ozonization of formic [5, 10] and acetic [10] acids. It
Ozonization was performed in a bubbling reactor (a
thermostated glass cylinder with a brazedꢀin porous
membrane that served as a gas flow splitter). Ozone
was obtained from the air in a laboratory glass ozoꢀ
nizer. The initial ozone concentration was ~60 mg/L,
the volume rate of the gas flow was ~10 L/h, the volꢀ
ume of the ozonizated solution was 20 to 40 mL, and
has been also shown that ions of transition metals do
not significantly influence the interaction of ozone
with maleic acid [5] and phenols [10, 11] but accelerꢀ
ate decomposition of the intermediate products of the
reaction (glyoxylic and oxalic acids), thereby reducing
considerably the total carbon content in the ozonizaꢀ
ted solution.
the temperature was 20°С
.
Similar results were obtained in our previous study,
Concentration of ozone in the gas phase was deterꢀ
which was dedicated to investigation of the catalytic mined spectrophotometrically using a Medozone 254/3
activity of iron(III) ions during the ozonolysis of pheꢀ ozone analyzer at the reactor inlet and outlet.
nols and a number of their substitutes (guaicol, 2,6ꢀ
dimethoxyphenol, vanillin) [12]. It was established
that the catalytic effect of Fe develops only during
3+
RESULTS AND DISCUSSION
ozonolysis of the intermediate products of phenol
Mathematical model. According to the data of [13–
decomposition, saturated carbon acids and aldehydes 17], presented in the table, ozone reactions with glyꢀ
215

216
BEN’KO, LUNIN
Rate constants (
acid medium (
k
, M–1 s–1) of ozonization of organic compounds in various types of reactions: with molecular ozone in
k
+
); with molecular ozone at pH 8 in presence of quenchers of OHꢀradicals (
k
–
); with OHꢀradꢀ
O (H )
O (OH )
3
3
icals (kOH• ); at pH 3–4 in aqueous solutions ( pH 3⎯4); at pH 1.5 in solutions of sulfuric acid (kpH 1.5)
k
kрН 1.5 (experiꢀ
mental)
k
+
[13]
k
–
[13]
Compound
kOH• [14]
kрН 3–4
pKa [13]
O (H )
O (OH )
3
3
Formic acid
formate anion)
Glyoxylic acid
glyoxylate anion)
Oxalic acid
oxalate anion)
Glyoxal
5
1.3
3.2
×
×
108
109
30 ± 10 [15]
5.7
3.8
(
100
0.18
10–2
20 [15], 33 [16]
35 [17]
1.9
3.2
(
1.9
<10–2
<4
×
1.4
7.7
×
×
106
106
1.0/3.9
(
1.8
oxal and glyoxylic and formic acids can be classified as one can determine the concentration of ozone in liqꢀ
soꢀcalled slow reactions. When such reactions are
made to occur in a bubbling reactor, kinetic and diffuꢀ
sion–kinetic modes of ozonization can in practice be
implemented.
uid volume c (l) . By inserting it into Eq. (2) and takꢀ
O
3
ing Eq. (1) into account, the equation for the reaction
rate can be obtained as follows:
The film model [18], according to which a film
forms on the border of the phases and ozone can penꢀ
etrate it by means of molecular diffusion only, was used where
v
=
keffKcO3[А],
(5)
k
eff is the effective rate constant in the reaction
between ozone and substance A:
to describe ozonization kinetics.
Equilibrium concentrations of ozone in gas cO3 and
in surface film cO3* are related by Henry’s law:
kk a
L
keff = ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ.
(6)
k[A] + k a
L
K
For low concentrations of substance A and a low
constant of reaction rate value [А] kLa, it follows
from (6) that ; i.e., the reaction takes place in
the kinetic mode.
O (g)
O ,
3*
3
k
k
Ӷ
(1)
c
= Kc ,
O
3
O3*
k = k
eff
where
K is the solubility constant, the value of which is
the reciprocal of the Henry constant.
On the other hand, the rate of ozone absorption
can be obtained from the material balance equation:
Concentration of ozone in volume of liquid c (l)
O
3
v
=
w
(c –
c
O3t ),
(7)
is determined by ozone diffusion rate
k a cO3* c (l)).
Here, kL is the mass transfer coefficient in the liquid
phase (m/s), and a is specific interfacial area (m ).
The rate of the reaction between the dissolved
ozone and substance A,
v
D:
O3o
v
=
(
–
O
3
(2) where
w is the specific rate of gas mixture delivery into
the solution (volume rate divided by solution volume
D
L
–1
s ), and cO3o , and cO3t are concentration of ozone in
–1
the gas phase at the inlet and the outlet of the reactor,
respectively.
Simultaneous solution of Eqs. (5) and (7) allow us
to find the effective rate constant keff of the chemical
reaction:
k
O (l) + A
P
3
is expressed as follows:
v
=
k
c (l) [А],
(3)
w(c – c )
O
O
O
3t
3
3o
keff = ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ ꢀꢀ .
(8)
Kc [A]
where
k
is the reaction rate constant and [A] is the
O
3
concentration of the oxidized substance (for the initial
part of the kinetic curve, where the fraction of conꢀ
verted substance is small, it may be considered equal to
initial concentration [А]0).
Concentration of ozone in the gas phase cO3 for a
semibatch reactor is calculated according to the folꢀ
lowing formula:
From equation
cO3t = (cO3o
–
cO3t )/ln(cO3o
/
cO3t ).
(9)
v
=
v
(4)
D
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OZONOCATALYTIC DECOMPOSITION OF GLYOXAL AND GLYOXYLIC AND FORMIC
217
1
1
×
105, mol/L
[
0
O ], mol/L
3t
keff, L mol– s–
4
v
.0012
.0008
.0004
4
1
2
3
4
5
1
2
3
6
3
2
1
3
2
0
0
1
0
0
0.1
0.2
0.3
[A]0, mol/L
0.4
0.5
0.6
0
200
400
600
800
, s
t
Fig. 1. Time dependences of ozone concentration at the
Fig. 2. Dependences of rate (solid line) and effective rate
reactor outlet during glyoxylic acid ozonization. Ozonizaꢀ
constant (dotted line) of ozone absorption on the concenꢀ
tion in the absence of catalyst: [A] = 0 (
1
), 0.005 (
2),
0
tration of the oxidized substrate: glyoxal (
), formic acid ( ).
1), glyoxylic acid
0.015 (
3
), 0.05 (
4
), 0.1 M (
).
5). Catalytic ozonization:
3
+
(
2
3
[
A] = [Fe ] = 0.015 M (
6
0
coordinates 1/keff–[A]0 . The obtained rate values k of
Once we have studied the dependence of the effecꢀ
tive reaction rate constant on the concentration of oxiꢀ bimolecular reactions of ozone with glyoxal and glyoxꢀ
dized substance, and once we have inserted the experꢀ ylic and formic acids are presented in the table. Comꢀ
imental data in the coordinates of the equation
parison with the literature data shows that they are in
agreement with the interval of known values, if we
(10) consider that a great many of the glyoxylic and formic
acid molecules are in a nondissociated form at pH 1.5
1
1
k
1
ꢀ
ꢀꢀꢀꢀ = ꢀ + ꢀꢀꢀꢀꢀ ꢀ[A]
k
k a
eff
L
•
and reactions in which ОН radicals participate are
hardly likely [1].
we can determine reaction rate constant
transfer volume coefficient km k a
k and mass
=
.
L
The obtained values for the ozonization rate conꢀ
stants for glyoxal and glyoxylic acid are almost equal.
Ozonization in the absence of catalyst. The experiꢀ
mental dependences of ozone concentration in the gas
phase at the reactor outlet on the time of ozonization
Catalytic ozonization in the presence of Fe3+ ions. In
were used to determine the kinetic parameters of the our experiments on catalytic ozonization, the initial
reaction. The experiments were carried out at constant concentrations of ozone and organic substrate were
–3
ozone concentration (~1
10 M), while the concenꢀ constant, while the concentration of iron ions was varꢀ
×
tration of the oxidized substances varied over the range ied. We established that iron(III) accelerates oxidation
–
3
–1
of glyoxal and glyoxylic acid but does not influence the
rate of reaction between ozone and formic acid.
Figure 4 shows the kinetic curves of ozone absorpꢀ
tion in reactions with glyoxal, obtained for various
concentrations of the catalyst. Specific consumption
of ozone (mole О3/mole A) was determined by graphꢀ
5
×
10 to 5 10 M. The kinetic curves obtained in
×
the process of glyoxal acid ozonization are presented
in Fig. 1 as an example.
Rate of ozone absorption
ing to Eq. (7), using values of cO3t in the region of staꢀ
v
was calculated accordꢀ
tionary ozone concentrations. Values of effective rate
constants of ozone absorption keff were determined
using Eq. (8). Figure 2 shows the resulting depenꢀ
dences of
and keff on the initial concentration of oxiꢀ was calculated using Eq. (8). The dependence of keff. cat
dized substance [A]0. As can be seen, all of the reacꢀ on the concentration of Fe ions is presented in
ical integration of experimental dependences cO3(
t).
The effective rate constant of ozone absorption keff. cat
v
3+
tions took place in the diffusion–kinetic mode. The Fig. 5.
region of concentrations below 0.02 M may tentatively
be considered kinetic.
According to our results, a noticeable catalytic
effect is observed only when the concentrations of iron
Results from treating the experimental data in ions and oxidized substrate are comparable. The effecꢀ
accordance with Eq. (9) are presented in Fig. 3 in tive reaction rate constant rises along with an increase
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BEN’KO, LUNIN
, molꢀeq
keff, mol s L–1
/
X
1
1
1
0
0
.0
5
8
4
6
4
3
.6
.2
2
1
2
y
y
= 15.739
x
x
+ 0.5563 (
1
)
)
= 22.696
+ 0.5143 (
2
3
y
= 17.059x + 0.1768 (3)
0
0.1
0.3
0.5
A]0, mol/L
0
0.5
1.0
1.5
−3
[
t × 10 , s
Fig. 3. Experimental data in the dependence of the effecꢀ
tive rate constant of ozone absorption on the concentraꢀ
tion of oxidation of the oxidized substrate in coordinates of
Fig. 4. Kinetic curves of ozone absorption (where
x is
ozone consumption) in reactions with glyoxal, catalyzed
3
–3
–2
Eq. (9): glyoxal (
1), glyoxylic acid (2), formic acid (
3
);
V
=
by Fe + ions; [O ] = 1.1
×
10 M, [A] = 1.5
×
10 M;
), 1 ( ).
3
0
0
3
[Fe3+] : [A] = 0 (
1); 0.03 (2), 0.08 (
3), 0.3 (
4), 2 (
5
6
20 (1, 3) and 40 mL (
2), km = 0.06 (1, 3) and 0.04 m /s (2).
0
in Fe3+ concentration (up to a threefold increase) and competing chelateꢀforming ligand (oxalic acid) into
3+
the system.
reaches limit value when the molar ratio [Fe ]/[A] is
0
close to 1/3. Assuming that chelate complexes can
This approach was based on oxalic acid ability to
form between iron ions and glyoxylic acid or glyoxal, form strong complexes with iron(III) ions, which
such a ratio corresponds to the filling limit of the comꢀ allowed us to consider it as a strong competing ligand.
3+
plex’s coordination sphere, since the coordination The stability constants of the Fe complex ions with
3
+
number of the Fe ion is 6, and glyoxylic acid and glyꢀ
3+
2– (3 – 2n)
4
n
oxalate [Fe (C O ) ]
Kn were K1 = 2.5
×
109,
2
oxal are bidental ligands.
1
6
1019, respectively [19].
K2 = 1.6
×
10 , and K3 = 4.0
×
Iron(III) does not exhibit catalytic activity in the
ozonization of formic acids, which are incapable of
chelate formation. Similar results were obtained by the
authors of [10] when they were studying photochemiꢀ
cal decomposition of various organic acids as a result
of O /Fe /UV impact. They found that iron was regard to ozone impact; as the Table shows, the rate
active only with regard to acids that can form strong constant in acid medium is
The rate constants of complex formation between
iron(III) and the various organic ligands were on the
5
8
–1 –1
order of 10 –10 М s [20].
At the same time, oxalic acid is relatively stable with
3+
3
–
2
–1 –1
≤
4
×
10
М
s
[13].
3+
We established that the rate of oxalic acid decomposiꢀ
tion rises considerably in the presence of Fe ions
12], but the absolute value of the catalytic reaction
rate is relatively low. Under experimental conditions in
which oxalic acid is ozonizated along with glyoxylic
acid and glyoxal, the oxalic acid reaction’s contribuꢀ
tion to total ozone consumption may be ignored.
chelate complexes with Fe ions (oxalic, glyoxylic,
and citric, but not formic or acetic).
3
+
[
Qualitative analysis of the decomposition products
of glyoxylic acid using high performance gas chromaꢀ
tography revealed that oxalic acid is produced in the
process of catalytic ozonization, just as happens in
case of simple ozonization. No chromatographic
peaks corresponding to formic acid were found.
For our qualitative experiments, a molar ratio of
Fe ] : [A] : [ С Н О4] = 1 : 3 : 3 was chosen for the
reagents. The concentrations of the competing ligands
were equal, and they were in amounts sufficient to
bind the metal ion in the complex, once the high value
of the stability constants and the rate of formation of
such complexes were taken into account.
3+
[
0 2 2
The stoichiometric reaction coefficient deterꢀ
mined from kinetic curves cO3(t) is 1 for both the cataꢀ
lytic and simple ozonization of oxalic acid (Fig. 4).
Ozonization in the presence of Fe3+ and additions of
oxalic acid. In our previous paper [12], in which we
studied the catalytic mechanism of the ozonization of
The obtained results are shown in Fig. 6. In the case
phenols in the presence of iron(III) ions, we used the of glyoxylic acid, the effective rate constant of the catꢀ
approach of inhibiting the reaction by introducing a alytic reaction falls from a maximum value of 3.8 to
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219
keff, L mol–1 s–1
keff, L mol–1 s–1
5
5
1
1
2
2
4
3
2
1
3
1
0
2
4
3+
[
Fe ]/[A] , molꢀeq
0
Fig. 5. Dependences of effective rate constant of ozone
absorption in reactions with glyoxal ( ) and glyoxylic acid
(2) on Fe concentration; [O ] = 1.1 × 10 M, [A] =
1
3+
–3
3 0 0
1.5 ×
10–2 M.
0
2
.9 М s–1, thus indicating that over half the soluble
–
1
I
II
III
iron was removed from the reaction with glyoxal, since
it was bound in oxalic complexes that were inactive
with respect to ozone.
Fig. 6. Influence of oxalic acid on catalytic activity of Fe3+
in reactions of ozone with glyoxal ( ) and glyoxylic acid
): Simple ozonization (I); catalytic ozonization, [Fe ] :
A] = 1 : 3 (II); catalytic ozonization in the presence of
1
3+
(
[
2
In the case of glyoxal, the addition of oxalic acid
completely suppressed the catalytic activity of iron.
Since oxalic acid (which contains carboxyl groups
with mobile hydrogen atoms) is a stronger ligand than
the corresponding aldehyde, glyoxal cannot compete
with it in the binding of iron ions.
0
oxalic acid, [Fe3+] : [A] : [C H O ] = 1 : 3 : 3 (III);
0
2 2 4
[
O ] = 1.1 × ×
10–3 M; [A] = 1.5 10–2 M.
3 0 0
According to [11], combining ozone with iron ions
leads to the creation of a system similar to Fenton’s
reagent, the effect of which is based on oxidative–
The results therefore point to complexation’s leadꢀ
3+
ing role in ozonization processes catalyzed by Fe ion.
2+
3+
Mechanism of catalytic ozonization. It is well known
that glyoxal ozonization produces glyoxylic acid,
which is in turn oxidized into oxalic acid, and oxalic
acid decomposes into carbon dioxide and water. It was
assumed in [16, 17] that all reactions in weakly acid
solutions (pH 3–5) proceed according to a similar
chain radical mechanism.
reductive transitions Fe /Fe , with the participation
of hydrogen peroxide Н О2 generated in the process of
2
ozonization. The forming ОН• and OОH• radicals
are the initiators of chain reactions of organic subꢀ
strate oxidation.
However, it was established in a number of cases
In the literature, two aspects are usually examined (e.g., ozonization of oxalic [21] and glyoxylic [6] acids
2
+
2+
when discussing the mechanism of ozonocatalytic
reaction: ozone activization due to the formation of
more active ОН radicals, and activation of the oxiꢀ
dized compound due to the formation of more easily
oxidized complexes with a catalyst [1].
in the presence of Co [21] and Mn [6] at pH
≈
5)
•
that ОН radicals do not play a significant role in ozoꢀ
nocatalytic reaction. The key stage of the process is the
formation of a complex between the metal ion and the
organic substrate, which stabilizes the high oxidation
•
The first type of scheme assumes the cyclic oxidaꢀ state of the metal and simplifies intramolecular transꢀ
tion–reduction of the transition metal: ozone oxidizes
fer of electrons onto the oxidized ligand.
•
the reduced form of the metal, generating ОН radiꢀ
At the same time, the data obtained in [4, 5, 21]
substrate, and the cycle repeats. Such a mechanism lead us to believe that the catalytic ozonization of aliꢀ
was proposed in particular for the catalytic ozonizaꢀ phatic carbonic acids can occur in acid media at
tion of glyoxylic acid in the presence of Cu ions [5]. pH < 2 without changes in the oxidation state of the
The authors of [5, 6] credit the participation of metal. The role of the metal is to form a complex in
ОH ꢀradicals in the process for the product of the catꢀ which the redox potential of the substrate (ligand) is
cals; the oxidized form of the metal then oxidizes the
2+
•
alytic reaction being not oxalic, but formic acid.
reduced due to the redistribution of electronic density.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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2
20
BEN’KO, LUNIN
6. R. Andreozzi, R. Marotta, and R. Sanchirico, J. Chem.
Such a mechanism seems to us to be more approꢀ
Technol. Biotechnol. 75 (1), 59 (2000).
7. Zuo Yuegana and J. Hoigne, Atmos. Environ. 28, 1231
priate for interpreting the results obtained in the
present study:
(
1994).
—
the stoichiometric ratio of the oxidized subꢀ
8
. R. Andreozzi, V. Caprio, A. Insola, et al., Water Res.
, 1492 (1998).
stance and the catalyst;
3
2
—
the inhibition of the catalytic reaction by comꢀ
9
. R. Andreozzi, V. Caprio, R. Marotta, and V. Tufano,
petitive ligand;
Water Res. 35, 109 (2001).
—
the stoichiometric coefficient of the catalytic
1
0. Abe Keiji and Tanaka Keiichi, Chemosphere 38, 2747
reaction, which is 1;
(1999).
—
the absence of formic acid among the products 11. X. F. Zhu and X. H. Xu, J. Zhejiang Univ. Sci. 12, 1543
of glyoxylic acid decomposition;
(2004).
1
2. E. M. Ben’ko, A. V. Mukovnya, and V. V. Lunin,
—
the absence of catalytic activity in reactions with
Zh. Fiz. Khim. 81, 815 (2007) [Russ. J. Phys. Chem. A
81, 701 (2007)].
formic acid, which is incapable of forming chelate
complexes.
1
3. J. Hoigne, Handbook of Ozone Technology and Applicaꢀ
tions, Ed. by R. G. Roce and A. Netger (Ann. Aiber Sci.
Publ., 1982), Vol. 1.
Based on the above, we may assume that the cataꢀ
lytic activity of iron(III) in reactions of ozone with
glyoxal and glyoxylic acid results from its ability to
activate the substrate due to their joint formation of
chelate complexes in which the substrate has lower
redox potential and is more easily oxidized by molecꢀ
ular ozone.
14. G. V. Buxton, C. L. Greenstock, W. P. Helman, and
A. B. Ross, J. Phys. Chem. Ref. Data 17, 513 (1988).
15. S. D. Razumovskii and G. E. Zaikov, Ozone and Its
Reactions with Organic Compounds (Nauka, Moscow,
1
974) [in Russian].
6. V. Caprio, A. Insola, and A. M. Silvestre, Ozone: Sci.
Eng. , 13 (1987).
1
1
1
1
9
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Vol. 84
No. 2
2010