Kinetics of formation of peroxyformic acid

Article abstract of DOI:10.1135/cccc19961457

Oxidation kinetics of formic acid with aqueous hydrogen peroxide (30-70%) has been studied at 45°C with 0-0.1 M H₂SO₄ as a catalyst. A kinetic model has been suggested which satisfactorily describes the oxidation process of formic acid to peroxyformic acid.

Full text of DOI:10.1135/cccc19961457

Short Communication
1457
KINETICS OF FORMATION OF PEROXYFORMIC ACID
1
Vladimir MOSOVSKY, Zuzana CVENGROSOVA, Alexander KASZONYI ,
2
Milan KRALIK and Milan HRONEC
Department of Organic Technology, Slovak Technical University, 812 37 Bratislava,
Slovak Republic; e-mail: 1 kaszonyi@checdek.chtf.stuba.sk, 2 kralik@checdek.chtf.stuba.sk
Received January 19, 1996
Accepted April 24, 1996
Oxidation kinetics of formic acid with aqueous hydrogen peroxide (30–70%) has been studied at 45 °C
with 0–0.1 M H2SO4 as a catalyst. A kinetic model has been suggested which satisfactorily describes
the oxidation process of formic acid to peroxyformic acid.
Key words: Peroxyformic acid; Hydrogen peroxide; Kinetics; Oxidation.
Aliphatic peroxyacids are obtained by oxidation of the corresponding carboxylic acids
with aqueous hydrogen peroxide. The reaction takes place at 20–70 °C in the presence
of mineral acids such as H2SO4, H3PO4 or sulfonated styrene–divinylbenzene co-
polymers1–4
.
H+
RCOOH
+
H2O2
RCOOOH
+
H2O
(A)
It is presumed5 that the rate-limiting step consists in a nucleophilic reaction of the
RC+(OH)2 or RC+(OH)(OOH) species with the H2O2 or H2O reagents, respectively.
The processes usually are continuous and the peroxyacid formed is either separated
from the reaction mixture by extraction with organic solvents6–8 or used directly as an
oxidant in the form of the peroxyacid–acid mixture.
The aim of the present work was to study the oxidation kinetics and hence suggest a
model describing the oxidation of formic acid into peroxyformic acid.
EXPERIMENTAL
Reagents
The solutions of hydrogen peroxide with concentrations from 32 to 70 wt.% were prepared by evapo-
rating water from the commercial H2O2 (32 wt.%) from a water bath. The commercial formic acid
(99%) was used without purification.
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Mosovsky, Cvengrosova, Kaszonyi, Kralik, Hronec:
Apparatus
The kinetics of formation of peroxyformic acid was studied in a glass through-flow stirred reactor of
40 cm3 volume. Hydrogen peroxide and a mixture of HCOOH and H2SO4 were dosed into the reactor
by means of peristaltic pumps. The inlets of H2O2 and HCOOH–H2SO4 mixture were placed near the
stirrer axis. The reactor was located in a thermostat with water bath. The reaction mixture left the
reactor by overflow. All the components of the apparatus in direct contact with the reaction mixture
were made of glass and Teflon. After reaching the required temperature and putting the whole ap-
paratus into operation, the reactants started to be dosed into the reactor. After reaching the steady
state (ca 3–4 h), samples of the reaction mixture were taken for analyses.
Analysis
The samples of reaction mixture were analyzed for H2O2 (cerimetry) and HCO3H (iodometry)9. The
values obtained represent the stationary concentrations of hydrogen peroxide and peroxyformic acid
in the reaction mixture.
RESULTS AND DISCUSSION
The kinetics of reaction of hydrogen peroxide and formic acid giving peroxyformic
acid and water was studied in a through-flow stirred reactor using the 1 : 2.67 molar
ratio of the above-mentioned reactants without any solvent at the temperature of 45 °C.
The formic acid introduced into the reactor was concentrated (99%, i.e. 26.24 mol dm–3).
Using various retention times, we introduced the hydrogen peroxide of the concentra-
tions varying in the interval from 32 to 70 wt.% (10.5 to 29.8 mol dm–3), the catalytic
sulfuric acid having the concentrations from 0 to 0.1 mol dm–3 in the reaction mixture.
The highest outlet concentrations of peroxyformic acid were obtained with the use of
70 wt.% hydrogen peroxide and at the sulfuric acid concentration of 0.06 mol dm–3 in
the reaction mixture, the inlet flow-rate of reactants being 0.0228 dm3 h–1. The results
depicted in Figs 1–5 show that the formation of peroxyformic acid is favourably af-
6
c
1
4
FIG. 1
Dependence of concentration
c
(mol dm–3) of hydrogen peroxide
(1) and peroxyformic acid (2) on
volume rate F/V (h–1). Conditions:
45 °C, inlet concentrations of com-
ponents: [H2O2] = 7.2 mol dm–3,
[HCOOH] = 19 mol dm–3, [H2SO4] = 0
mol dm–3. Solid line – calculated
values (MODEL 1)
2
2
0
0.2
0.4
0.6
0.8
F/V
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fected by increasing concentration of both the inlet formic acid and the inlet hydrogen
peroxide. At such conditions, minimum amounts of water (water suppresses the forma-
tion of peroxyformic acid) are introduced into the system. The results indicate that a
lower dosing rate of inlet reactants (i.e. their longer retention time in the reactor) causes
increasing decomposition of hydrogen peroxide. The lowering of hydrogen peroxide
concentration due to its increased decomposition in the reaction mixture affects unfa-
vourably the formation of peroxyformic acid, hence a lower concentration of peroxy-
formic acid is observed at the outlet of the reactor. At higher dosing rates of reactants,
the reduced reaction time affects the conversion of formic acid but slightly, which is
connected with the establishing of equilibrium of formation of peroxyformic acid at the
experimental conditions.
4
1
c
3
FIG. 2
Dependence of concentration
c
(mol dm–3) of hydrogen peroxide
(1) and peroxyformic acid (2) on
volume rate F/V (h–1). Conditions:
45 °C, inlet concentrations of com-
ponents: [H2O2] = 4.89 mol dm–3,
2
1
0
2
[HCOOH]
=
13.1 mol dm–3,
[H2SO4] = 0 mol dm–3. Solid line –
calculated values (MODEL 1)
0.2
0.4
0.6
0.8
F/V
4
3
2
1
0
c
1
FIG. 3
Dependence of concentration
c
(mol dm–3) of hydrogen peroxide
(1) and peroxyformic acid (2) on
volume rate F/V (h–1). Conditions:
45 °C, inlet concentrations of com-
ponents: [H2O2] = 4.89 mol dm–3,
2
[HCOOH]
=
13.1 mol dm–3,
[H2SO4] = 0.06 mol dm–3. Solid line
– calculated values (MODEL 1)
0.4
0.6
0.8
F/V
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Mosovsky, Cvengrosova, Kaszonyi, Kralik, Hronec:
The presence of sulfuric acid in the reaction mixture increases the overall concentra-
tion of hydrogen ions, which in particular affect the decomposition of hydrogen perox-
ide and peroxyformic acid. As it can be seen from the results the increasing sulfuric
acid concentration in the reaction mixture accelerates the decomposition of hydrogen
peroxide. The dependence of formation of peroxyformic acid on the sulfuric acid con-
centration shows an indistinct maximum at the sulfuric acid concentrations from 0.05 to
0.06 mol dm–3.
For kinetic treatment of the experimental data, various models were suggested de-
scribing the processes of formation and decomposition of peroxyformic acid and that of
decomposition of hydrogen peroxide. All the models presume complete dissociation of
sulfuric acid, the total concentration of hydrogen ions involving also the protons com-
6
c
1
4
FIG. 4
Dependence of concentration
c
(mol dm–3) of hydrogen peroxide
(1) and peroxyformic acid (2) on
sulfuric acid concentration [H2SO4]
(mol dm–3) in the reaction mixture.
Conditions: 45 °C, inlet concentra-
tions of components: [H2O2] = 7.19
mol dm–3, [HCOOH] = 19.2 mol
dm–3, (F/V) = 0.57 h–1. Solid line –
calculated values (MODEL 1)
2
2
0
2
4
6
8
10
2
[H SO ] . 10
2
4
4
3
c
1
FIG. 5
Dependence of concentration
c
(mol dm–3) of hydrogen peroxide
(1) and peroxyformic acid (2) on
sulfuric acid concentration [H2SO4]
(mol dm–3) in the reaction mixture.
Conditions: 45 °C, inlet concentra-
tions of components: [H2O2] = 4.9
mol dm–3, [HCOOH] = 13.1 mol dm–3,
(F/V) = 0.66 h–1. Solid line – calcu-
lated values (MODEL 1)
2
1
0
2
2
4
6
8
10
2
[H SO ] . 10
2
4
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ing from the dissociation of formic and peroxyformic acids. The individual models
differ in the reaction orders with respect to formic and peroxyformic acids and in the
extent of catalytic effect of H+ ions on the formation and decomposition of peroxyfor-
mic acid and hydrogen peroxide. The best agreement between the measured and calcu-
lated data was achieved by adopting the following chemical and kinetic equations.
k1
H2O2
+
HCOOH
HCOOOH
+
H2O
(B)
(C)
(D)
k2
H+
k3
HCOOOH
H2O2
CO2
+ H2O
H+
k4
H2O
+
O2
The rate equations:
r1 = (k1 c(H2O2) c(HCOOH) – k2 c(H2O) c(HCOOOH)) c(H+)
r2 = k3 c(HCOOOH) c(H+)
(1)
(2)
(3)
r3 = k4 c(H2O2) c(H2O2) c(H+) .
The starting data for the calculation of rate constants involved the following:
– the reactor volume
– the dissociation constants of HCO2H and HCO3H (1.68 . 10–4 for both)
– the reaction mixture flow rate
– the inlet and outlet concentrations of components.
The presumptions involved a constant volume flow rate of reaction mixture and a reac-
tor with perfect stirring. The rate constants ki, i = 1, 2, 3, 4, were determined by mini-
mizing the sum of squares of deviations of measured and calculated concentrations of
hydrogen peroxide and peroxyformic acid at the reactor outlet. The procedure accord-
ing to ref.10 was applied to find the minimum of objective function and to evaluate the
error of assessment of parameters and the correlation of parameters. All the experimen-
tal points shown in Figs 1–5 (38 measurements altogether) were treated simultaneously.
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Mosovsky, Cvengrosova, Kaszonyi, Kralik, Hronec:
The calculated rate constants (MODEL 1) are as follows:
k1 = 1.00 ± 0.49 dm6 mol–2 h–1
k2 = 2.68 ± 1.57 dm6 mol–2 h–1
k3 = 0
k4 = 0.29 ± 0.04 dm6 mol–2 h–1
.
The indicated error in determination of the parameters applies to the confidence inter-
val of 95%. From Figs 1–5 it can be seen that the kinetic model suggested leads to a
good agreement between the experimental and calculated values.
The relatively large error in the estimation of parameters k1 and k2 in MODEL 1
results from large correlation of these parameters (the coefficient of mutual correlation
is 0.993). Therefore, we adopted a kinetic model taking the reactions (B) and/or (A) as
equilibrium ones (MODEL 2) and we tried to find the value of the respective equili-
brium constant K(A) and rate constant of reaction (D) with rate equation (3) – the
kinetic constant k4. The following values were found for MODEL 2:
K(A) = k1/k2 = 0.32 ± 0.03
k3 = 0
k4 = 0.29 ± 0.04 dm6 mol–2 h–1
.
Like in the case of application of MODEL 1, in this case too the value of rate constant
of decomposition of peroxyformic acid turned out to be statistically insignificant, i.e.
zero. The coefficient of mutual correlation of the parameters K(A) and k4 was 0.14,
which positively affected also the accuracy increase of determination of parameters of
MODEL 2 as compared with those of MODEL 1. MODEL 2 represents an only slight
worsening of description of the experiments as compared with MODEL 1: the sum of
residues increased from 2.59 to 3.21.
In conclusion it can be stated that in the through-flow reaction system with the reten-
tion times between 1 and 4 h at 45 °C and with the concentrations of hydrogen peroxide
and formic acid equal to 2–5 and 12–18 mol dm–3, respectively, an equilibrium concen-
tration of peroxyformic acid is reached (in all the cases below 2.5 mol dm–3), the de-
composition of this acid being practically negligible at the given conditions. The
decomposition of hydrogen peroxide is catalyzed by acids and is relatively intense,
which distinctly lowers the selectivity of formation of peroxyformic acid with regard to
hydrogen peroxide (50% in average). The kinetic model described was applied success-
fully to oxidation of olefins at conditions analogous to those used for the formation of
peroxyformic acid.
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SYMBOLS
ci
kj
concentration of the i-th component, mol dm–3
rate constant, dm6 mol–2 h–1
K(A)
ri
equilibrium constant of formation of peroxyformic acid
rate of the i-th reaction, mol dm–3 h–1
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