Jun 29, 1994 - converts into a peroxo-metal complex of brown, green, or gray color. A mechanism describing the ... 0 1995 John Wiley & Sons, Inc. Introduction.
Kinetics of Hydrogen Peroxide Decomposition with Fe(II1) and Cr(II1)-Ethanolamines Complexes Sorbed on Dowex-50W Resin IBRAHIM A. S K E M Chemistry Department, Faculty of Science, Tanta Uniuersity, Tanta, Egypt
Abstract The kinetics of the H202 decomposition in presence of Fe(II1)- and Cr(II1)-complexes of mono-, di-, and triethanolamine supported on Dowex-BOW resin have been investigated. The decomposition process proceeded with first-order kinetics for the substrate concentration. The rate of reaction increased with increasing number of the coordinated ligands in the metal complex as well as with increasing ligand basicity. The decomposition reaction involved the formation of an intermediate active species, which converts into a peroxo-metal complex of brown, green, or gray color. A mechanism describing the decomposition process is proposed. 0 1995 John Wiley & Sons, Inc.
Introduction Many efforts have been made to understand the catalytic decomposition of hydrogen peroxide in aqueous medium with metal complexes supported on cation exchange resins [l-71. Such investigations involved the use of ammonia [1,21, organic amines [4], ethanolamines [5-71, and Schiff-bases [3] as complexing ligands with various transitions metal ions. These complexes were highly stable and have an enzymelike activity with respect to the hydrogen peroxide [81. The peroxo-metal complex formed (on the resin) at the early stages of the reaction was stable even after the completion of the reaction. This peroxo-metal complex underwent self-decomposition with the evolution of 0 2 . With iron(II1)-aliphatic diamine complexes, the rate of H202 decomposition decreased with increasing the length of the methylene chain between the two amino-groups of the ligand [41. Also, the rate of the decomposition reaction decreased with the increase of either the %DVB or the particle size of the resin [l-31. The aim of this work is to investigate the kinetics of the heterogeneous decomposition of HzOz in presence of Fe(II1)- and Cr(II1)-complexes of mono-, di-, and triethanolamine supported on Dowex-50W resin. One of the objectives of this work is to study the effect of number of the coordinated ligands in the metal complex as well as the ligand basicity on the reaction rate.
Experimental Preparation of the Catalyst
Dowex-5OW resin (8% DVB, 20-50 mesh) has been previously described [41. Its moisture content and the total capacity were 24.6% and 2.39 meq/g dry resin, respectively [41. To a definite weight of the resin in the H+-form, a known excess International Journal of Chemical Kinetics, Vol. 27, 499-505 (1995) CCC 0538-80661951050499-07 0 1995 J o h n Wiley & Sons, Inc.
500
SALEM
amount of iron(II1) chloride or chromium(II1) chloride solution M) was added dropwise with stirring. After equilibration, the resin in the metal ion form was filtered and washed repeatedly with bidistilled water in order to remove any traces of excess metal chloride. The filtrate was then collected and the excess metal ion was determined [9], from which the amount of Fe3+ or Cr3+ ions supported on the resin was calculated. A known weight of the resin in the metal-ion form was immersed with bidistilled water whereby a stoichiometric amount of an aqueous solution (lop2 M) of the ligand was added dropwise to form 1: 1 complex. The same was also done for the preparation of 1: 2 complexes. In both cases the amount of the ligand adsorbed on the resin was determined using the same resin in the Hc-form. For 1:3 complexes, a known high excess amount of the ligand solution was added to a known weight of the resin in the metal-ion form. After equilibration, the resin in the metal complex form was then filtered, washed repeatedly with bidistilled HzO, and air dried. The filtrate was collected and the amount of the excess ligand was determined using a standard solution of HC1, from which the amount of the complexed ligand was calculated. The M : L ratio was also determined as described previously [5-71 and was found in agreement with the stoichiometry of the prepared complexes.
Chemicals and Reagents Reagent grade chemicals and bidistilled HzO were used throughout the work. An H202 solution (30% A.R. grade from Merck, Munch, Germany) was used. Standard H202 stock solution was prepared by dilution and its concentration was determined iodometrically using sodium thiosulphate solution. Monoethanolamine (meal, diethanolamine (dea), and triethanolamine (tea) were obtained from Aldrich Chemical Company, Inc., USA.
Measurements Kinetic measurements were carried out following the procedure previously described [l-41.The measurements were confined at 25-40°C temperature range in order to avoid the self-decomposition of H202 at high temperatures. The pH measurements were made by Crison digit 101 pH meter.
Results and Discussion Prior to the addition of H202 solution to the catalyst system, the pH of the medium (residmetal complex + water) showed noticeable changes, depending upon the metal complex (Table I). Upon the addition of H202, the pH decreased gradually with the progress of the reaction, reaching a constant value as the reaction ends. Such decrease in the pH did not displace the complex from the resin [41. Table I also shows the color of the different complexes before and after their reactions with hydrogen peroxide. The [metall/[ligandl ratio, the capacity of the resin and its water content were determined [5-71 at the end of the reaction and were found to be unchanged. Thus the resin and the complexes are not degraded during the decomposition of H202, indicating the stability of the resin under the present working conditions. The initial concentration of H202 was kept constant at 0.072 M whereas the weight of the air dry resin was 1g with iron(II1) complexes and 0.5 g with chromium(II1) complexes. The reaction was first-order with respect to [H2021 in all cases (Fig. 1).
HYDROGEN PEROXIDE DECOMPOSITION
501
TABLEI. The [metall/[ligandl ratio, the pH, and the color of the transition metal ethanolamine complexes associated with Dowex-50W (8% DVB, 20-50 mesh) resin before and after reaction with H2Oz. ~
~~
PH Complex Formula
Color
(n)
Before Reaction
After Reaction
Before Reaction
After Reaction
1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
6.38 7.15 7.52 6.95 7.42 7.98 7.25 7.73 8.12 3.8 4.12 9.19 4.36 4.65 9.36 4.62 4.91 9.65
5.73 6.65 7.31 6.48 7.25 7.66 6.95 7.45 7.85 3.7 4.01 9.02 4.26 4.49 9.02 4.71 4.65 9.20
faint brown brown brown faint brown brown brown faint brown brown brown faint green green gray faint green green €!ray faint green green gray
dark brown dark brown dark brown dark brown dark brown dark brown dark brown dark brown dark brown faint green dark green dark gray faint green dark green dark gray faint green dark green dark gray
~~
LFe(mea),13+
[Fe(dea),13+
[Fe(tea),13+
[Cr(mea), i3 +
LCr(dea),13+
[Cr(tea),13+
~~
L
Figure 1. First-order rate equation for H2Oz (0.072 M) decomposition in the presence of 1.0 g of the air-dried resin Dowex-SOW (8% DVB, 20-50 mesh) in the form of [Fe"'(mea)3I3+ a t different temperatures: (0):25°C; ( X ) : 30°C; (A): 35°C; and (m): 40°C.
502
SALEM
The rate constant k(per g dry resin) was obtained from the expression:
x ) = kWt where a is the initial concentration of H ~ 0 2x, the amount decomposed of HzOz at a ln(a/a -
(1)
given time t , and W the weight in g of dry resin. With Fe(II1) and Cr(II1)-complexes,independent on the stoichiometry of the complex, the rate constant k (per g dry resin) decreased in the following order, mea > dea > tea which is the same order of the basic strength of the ligands [lo] (Table 11). In general, with more basic ligands, a more stable complex is formed 1111. Such complexes, however, are more capable of attacking the HzOz molecule, and consequently higher reaction rate can usually be observed [4-71. Table I1 also demonstrates that, the rate of the decomposition reaction increased with increasing number of coordinated ligands to the metal ion. This can be attributed to the increased stability of the metal complexes with higher coordination number [lo]. In spite of the extremely low k values obtained with 1 : l and 1 : 2 complexes of Cr(III), the k value for 1 : 3 complex was very high and is much higher than the corresponding iron(II1) complex, (Fig. 2). The activation energies, Ea, were calculated from the Arrhenius plots and were in agreement with the k values obtained (Table 11). Lower Ea values are associated with higher k ones, i.e., smaller activation energy is necessary for HzOz decomposition. Since the value of AH' is a measure of the height of the energy barrier it must be overcome to obtain the transition state. A glance at Table I1 shows that values of AH' increases along with the decrease in the corresponding k values. This would suggest that the reaction rates are therefore only enthalpy controlled [121. The change in AG# values are within the range (96.1-102.41) k J mol-l, in agreement with those obtained for the Hz02 decomposition with iron(II1)-sal-o-phen complex supported on Dowex resin [31. The AS' values decreased in the following order; 1: 1 > 1: 2 > 1:3 as well as with increasing ligand basicity; tea > dea > mea which is the reverse order of the k values. TARLE11. Rate constant (per g of dry resin) and the activation parameters for the decomposition of Ha02 (0.072 M) in presence of Dowex-50W (8% DVB, 20-50 mesh) resin in the form of Fe(II1)- and Cr(II1)-complexes of ethanolamines.
K x 105 s-l Complex
M:L
25°C
30°C
35°C
40°C
Ea kJ/mol
AH# kJ/mol
AG# kJ/mol
Fe(II1)mea
1:l 1:2 1:3 1:1 1:2 1:3 1:l 1:2 1:3 1:3
12.86 16.3 22.28 3.65 5.04 7.32 0.847 1.43 2.68 148.3
19.36 24.8 30.1 5.91 7.82 10.97 1.54 2.14 4.18 186.5
29.5 34.35 41.07 9.94 12.37 16.03 2.72 3.37 5.72 234.8
43.6 49.6 55.4 14.81 18.83 22.08 4.36 5.55 8.63 283.8
63.4 56.9 47.24 73.33 68.5 57.34 85.2 70.15 61.54 33.25
60.86 54.36 44.7 70.79 65.96 54.8 82.65 67.61 58.96 30.75
96.1 95.62 95.12 99.03 98.36 97.64 102.41 101.58 100.17 90.6
1:3
33.95
42.17
68.77
85.41
44.15
41.6
93.96
-
1:3
14.4
20.66
28.92
40.30
52.9
50.4
96.1
-149.5
Fe(II1)dea Fe(II1)tea Cr(II1)mea Cr(II1)dea Cr(II1)tea
AS# Jdeg-' mol-' -115.3 135.0 - 165.0 -92.4 -106.0 140.0 -64.5 -111.2 - 134.9 -195.9 -
-
171.4
HYDROGEN PEROXIDE DECOMPOSITION
503
Figure 2. Dependence of the rate constant k (per g dry resin) on the stoichiometry of the Fe(II1)- and Cr(II1) complexes of triethanolamine a t 40°C.
Figure 3 represents two experiments carried out under the same experimental working conditions. In the first one, [Fe(dea)3I3+was used for Hz02 decomposition. After completion of this experiment, the resin in the peroxo-iron complex form (dark brown compound) was collected, washed with bidistilled HzO, and used in the second experiment to decompose Hz02. It is clear that the reaction rate with the peroxoiron complex was greater than that with the [Fe(dea)3I3+complex ion. This provides evidence for an intermediate (active species) formed at the early stages of the reaction and having an inhabiting effect on the reaction rate, i.e., the formation of the active species needs some time and accordingly the overall reaction rate decreased [l-41. This experiment showed that in neither case did the order change and also the peroxoiron complex was capable of oxidizing HzOz [4]. The same result was also found with chromium(II1) complexes. It was found that the rate of reaction in presence of scavengers (ethanol and ally1 acetate) is much slower than that in aqueous medium. Inhibition of rate by scavengers suggested the involvement of radicals or radical ions in the mechanism [4,13,14].Since the Ea values lie in the range of chemical reaction throughout the catalyst particles [ E l , an the peroxide anion, HOz- exists [161 in the pH range of the present study, the reaction mechanism can thus be described as: Hz02
HOzfast
(3)
(4)
[M1I1L,I3+ + HOz-
+ Hf
3 [M111L,(HOz)]2+ fast
[M111L,(H0z)]2+A [M"'L,(HOZ')]~+ slow
intermediate (active species)
-
HO~-
kz
perox-metal complex
SALEM
504
,*t
/ /
Dowex-5OW/~%OVS,W-50moshl
Figure 3. Decomposition-time curves for H202 (0.072 M) in the presence of 1.0 g air-dried resin Dowex-50W (8%DVB, 20-50 mesh) in the form of: (0):[Fe"'(dea)3I3+ and (B): peroxo-iron complex at 40°C.
The rate equation can be written as follows:
From eqs. (2) and (3):
dx dt
- --
d[H2021 dt
=
klK1K2[M111L,]3+[H202]/[H+]
Where k l is the rate constant of the rate determining step (eq. (411, n is the coordination number, K1 and Kz are the fast equilibrium constants of eqs. (2) and (31, respectively. According to eq. (6), the rate of reaction is inversely proportional to the acidity of the medium. This might explain the reason for the extremely low rate obtained with 1 : l and 1 : 2 Cr(II1)-complexes and the higher rate obtained with [CrL3I3+ complexes compared to the corresponding iron(II1)-complexes. The intermediate formed in eq. (4)may contain the free radical (HOZ'), i.e., the active species contains a divalent ferrous ion. Such a redox cycle Fe3+ Fez+ was found in the homogeneous H202 decomposition with o-phenanthroline-Fe(II1) complex and in the heterogeneous HzO2 decomposition with Fe(II1)-sal-o-phen [3] and Fe(II1)-diamine 141 complexes supported on Dowex-50W resin. Using the steady-state approximation for the calculation of the concentration of the intermediate, we get:
2
(7)
d[M1I1L,(H02')l2+/dt = kl[M111L,(H02)]2f
-
kz[M1*L,(H02')]2f[HOz-]
=
0
This equation agrees well with that found earlier for the decomposition of Hz02 in the presence of Dowex-50W resin in the form of some transition metal complexes [3-61. It proved impossible to carry out the decomposition reaction in the presence of an acid or buffer solution which regenerates the resin.
HYDROGEN PEROXIDE DECOMPOSITION
505
Bibliography 111 M. Y. El-sheikh, F. M. Ashmawy, I. A. Salem, and A. B. Zaki, 2. Phys. Chemie (Leipzig), 269, 126 (1988). [21 F. M. Ashmawy, M. Y. El-sheikh, I.A. Salem, and A. B. Zaki, Transition Met. Chem., 12, 51 (1987). [31 M.Y. El-sheikh, F.M. Ashmawy, I. A. Salem, A. B. Zaki, and U. Nickel, Transition Met. Chem., 16, 319 (1991). 141 I.A. Salem, J. Mole. Catal., 80, 11 (1993). [ 5 ] M.Y. El-Sheikh, A.M. Habib, F.M. Ashmawy, A.H. Gemeay, A.B. Zaki, and J. Bargon, Transition Met. Chem., 17, 299 (1992). [6] M.Y. El-Sheikh, A.M. Habib, F. M. Ashmawy, A. H. Gemeay, A. B. Zaki, and J. Bargon, J. C h i n . Phys., 89, 2057 (1992). [71 M.Y. El-Sheikh, A.M. Habib, F. M. Ashmawy, A. H. Gemeay, A. B. Zaki, and J. Bargon, J. Mol. Catal., 77, 15 (1992). [8] Y. Shibata and H. Kaneko, J. Chem. SOC.Jpn., 44, 166 (1923). [9] A. I. Vogel, Quantitative Inorganic Analysis, 4th ed., Longman, London, 1978, pp. 322, 361. [lo] A. E. Martell, Stability Constants of Metal-Zon Complexes, Part 11, Special Publication No. 25, Burlington House, London, W 1V OBN, 1971. [ll]B. Douglas, D. McDanial, and J. Alexander, Concepts and Models of Inorganic Chemistry, 2nd ed., Wiley, 1983, p. 539. [12] J. H. Espenson, Chemical Kinetics and Reaction Mechanism, McGraw-Hill, New York, 1981. [13] U.K. Gupta, Thermochemica Actu, 69, 389 (1983). [14] A.M. Ferreira and L. L. Duarte, J. Coord. Chem., 24, 339 (1991). [ E l U.S. Sharma and J. Schubert, J. Am. Chem. SOC.,91, 6291 (1969). [16] C.F. Wells and D. Mays, J. Chem. SOC.A, 2987 (1973).
Received June 29, 1994 Accepted October 7, 1994
