complex reacts with a hydrogen peroxide molecule to form the peroxo-metal .... formation energy; the greater its affinity to react with Hz02 and, therefore, the.
Journal of Molecular Catalysis, 80 (1993) 11-19 Elsevier Science Publishers B.V., Amsterdam
11
MO42
Role of aliphatic diamine ligands in hydrogen peroxide decomposition with Dowex-50W resin as transition metal complex ions Ibrahim A. Salem Chemistry Department,
Faculty of Science, Tanta University,
Tanta (Egypt)
(Received June 28,1992; accepted October 22,1992)
Abstract Dowex-50W resin, as the aliphatic diamine-transition metal ion (Fe”‘, Cu”, Con) complexes, has been used as a potentially active catalyst for H,Oz decomposition in an aqueous medium. The rate constant, k (per g of dry resin) was evaluated over the temperature range 25-40°C. The reaction was first order with respect to [H202] in all cases. With 1,6_hexamethylenediamine as a ligand, the rate constant k decreased in the following order; Cu”> Con > Fem. Also, with Fe”‘complexes the value of k decreased with increasing the length of the methylene chain between the two amino groups of the ligand. The activation parameters were calculated and a reaction mechanism is proposed. Key words: cobalt; copper; diamine ligands; Dowex-50W resin; hydrogen peroxide; iron
Introduction The catalytic decomposition of H202 into 0, and H,O has been the subject of extensive investigations both with regard to the kinetics and to the mechanism of the reaction [ 1,2]. The catalytic effect of some transition metal ammine and amine complexes on H,O, decomposition has been studied in the presence of the strongly acidic cation exchanger, Dowex-50W resin in aqueous medium [ 3,6]. Recently we have used the tetradentate Nz02 Shiff-base ligand N,N’ -bis (salicylidine )-o-phenylenediamine (Sal-o-phen ) complexed with iron (III) bound to Dowex-50W resin [ 71. We have now extended our investigations to the aliphatic diamine ligands, NH,(CH,),NH,, n=2-7, and 9 with the Fe’““, Cu’“’ and Co’“’ bound to Dowex-50W x 8 resin. These complexes are very stable even after the decomposition reaction is complete. At the start of the reaction, the transition-metal complex reacts with a hydrogen peroxide molecule to form the peroxo-metal complex and the latter then reacts with another molecule of H,O, to yield the product [ 3-71. The peroxo-metal complex undergoes self-decomposition with the evolution of 02, and on standing for several days, the original colour of the transition metal diamine complex is attained [ 3,4,8,9].
0304-5102/93/$06.00
0 1993 - Elsevier Science Publishers B.V. All rights reserved.
LA. Salem/J.
Mol. Catal. 80 (1993) 11-l 9
Experimental
Dowex-50 W resin associated with transition metal-diamine complexes Dowex-50W resin (8% DVB, 20-50 mesh) in the hydrogen form was the strongly acid cation-exchanger used. It is produced as spherical beads of sulphonated styrene divinylbenzene copolymers in the hydrogen form by the Chemical Company, Midland, MI, USA. The resin was regenerated with 2 M HCl, then thoroughly washed and air-dried. Its moisture content (24.6% ) was determined by drying a sample overnight at 110°C under atmospheric pressure. The total weight capacity of the exchanger was determined using the batch method and was equal to 2.39 mequiv. per g dry of the H+ form of the resin. The resin was converted into the Fe(“‘), Cu”” and Co(“) forms by equilibrating it (by using the batch process) with the corresponding 1 M transition metal ion solution. In each case the resin was collected and washed with doubly distilled H,O until it was free from any excess transition metal solution. The diamine solution (2 M in doubly distilled HzO, except for 1,gdiaminonenane which dissolves in ethanol) was added to the resin in the transition metal ion form to yield the corresponding stable diamine complexes. Finally, the resin was washed with doubly distilled Hz0 (or ethanol for the 1,9diamine) until it was free from any excess diamine solution. Determination of the [ligand]/[metal] ratio The [ ligand]/ [ metal] ratio was determined by the addition of a known excess of standard diamine ligand (0.032 equiv. dnP3) to a definite amount of the air-dried resin as the transition metal ion. After equilibration, the excess of diamine was determined by titration against standard HCl (0.0087 equiv. dmw3). After correction for the amount of diamine sorbed by the ion exchanger TABLE
1
The ligand/ [metal] ratio of transition metal-diamine complexes associated with air-dried Dowex-50W (8% DVB, 20-50 mesh) resin Complex
Fe”‘-1,2-diaminoethane Fe”‘-1,3_diaminopropane Fe”‘-1,4-diaminohutane Fe”‘-1,5-diaminopentane Fe”‘-1,6-diaminohexane Fe”‘-1,7_diaminoheptane Fe”‘-1 9-diaminonenane Copped-1,6-diaminohexane Cobalt-1,6-diaminohexane
formula
[Fe(l,2-diamine)z]3+ [Fe(l,3-diamine),13+ [Fe(l,4-diamine),13+ [Fe(l,5-diamine)J3+ [Fe(l,G-diamine),13+ [Fe(l,7-diamine),13+ [Fe(l,9-diamine)2]3+ [Cu(l,G-diamine),]‘+ [Co(1,6diamine),]*+
Colour
PH Before reaction
After reaction
Before reaction
After reaction
9.93 9.86 9.72 9.61 8.5 7.62 6.9 6.97 7.2
9.69 9.42 9.33 9.10 8.32 7.25 6.5 7.2 7.6
reddish brown reddish brown reddish brown reddish brown reddish brown dark brown dark brown blue faint red
dark brown dark brown dark brown dark brown dark brown dark brown dark brown brown dark brown
LA. Salem/J. Mol. Catal. 80 (1993) II-19
13
(which was determined in a separate experiment using the ion-exchanger in the H+ form) the amount of diamine which had reacted could be determined. Knowing the capacity of the resin, and allowing one Fe3+ to replace 3H+ and one Cu2+ or one Co’+ to replace 2H+, the content of the transition metal ion in the resin could be determined [ 6,101. The [ ligand] / [metal] ratios of the various transition metal diamine complexes under investigation are listed in Table 1. Hydrogen peroxide solution A H,O, solution (30% A.R. grade from Merck) was used and its initial concentration (0.062 M) was obtained by mixing the standard Hz02 stock solution (2 ml, freshly prepared) with doubly distilled water (18 ml). The initial concentration of H,O, was determined iodometrically using standard sodium thiosulphate solution. Kinetic measurements .Kinetic measurements and the iodometric determination of undecomposed H,O, have been described elsewhere [ 3 1. The reaction temperature was in the range 25-40 -+0.1 oC. Before the addition of the Hz02, the pH of the doubly distilled water in the presence of the diamine-metal complex ion in the resin was varied depending on the type of complex (Table 1) . After the addition of H202, the pH decreased within the first minute of reaction and then increased to reach a constant value which is lower than before the H,O, was added in the case of the iron complexes and greater in the case of copper and cobalt complexes. The increase in proton concentration within the first minute of the reaction does not lead to any displacement of the transition metal ion from the resin. Results and discussion The total capacity and moisture content of the resin as well as the [ligand] / [metal] ratio were determined before and after the decomposition reaction and were found to be unchanged. Thus the resin was not degraded during the decomposition of H202. This resin is therefore more stable than the radioactive waste resin [ 111 (based on the same structure) which is partially decomposed by Fe-catalysed H,O, decomposition. The decomposition of H,O, was carried out at a constant Hz02 concentration (0.061 M) and at a constant weight (0.2 g) of the air-dried Dowex-50W (8% DVB, 20-50 mesh) resin. The reaction was found to be first order with respect to [ H,O,] (Fig. 1) . The rate constant, k (per g of dry resin) was obtained from the expression [12]: ln(a/a-x)
=kwt
(1)
14
LA. Salem/J. Mol. Catal. 80 (1993) 11-19
WlIIi-1.3.dtomne
b!!?, , IH202l=o.06iM
o 25’C
b 35.C
x
.
30’.
USC
/ n ”
0
LO
80
120
160
Time(min)
Fig. 1. First-order rate equation for H,O, (0.061 M) decomposition in the presence of 0.2 g airdried resin Dowex-50W (8% DVB, 20-50 mesh) as [Fe”‘( 1,3-diamine)z J3+ at different temperatures: (0) 25°C; (x) 30°C; (A) 35°C and (m) 40°C.
where a is the initial concentration of Hz02, x is the amount of H,O, decomposed at time t and w is the mass in g of dry resin. With the ferric complexes under investigation, the rate constant k (per g dry resin) (Table 2) was found to decrease with increase in the number of methylene groups between the two amino groups in the diamine ligand (Fig. 2). This can be attributed to the decreased stability of the complex formed with increased length of the methylene chain between the two coordinating groups as a result of the increased steric hindrance [ 131. The greater the stability of the complex, the less its formation energy; the greater its affinity to react with Hz02 and, therefore, the greater the value of k (per g dry resin) (Table 2). Also, with 1,6-hexamethylenediamine as a ligand, the rate constant k (Table 2) was found to decrease in the order Cu”> Con > Fem. The activation energy, E, calculated from the Arrhenius plot was found to decrease with increase in the number of methylene groups between the two amino groups (Table 2). Also, with the transition metal ions, the value of E decreased in the sequence Cu” > Con > Fe”’ (Table 2 ). Normally, higher E values were found with lower k values. However, the results depicted in Table 2
I.A. Salem/J.
Mol. Catal. 80 (1993) 11-19
15
TABLE 2 Kinetic and activation parameters of H,O, (0.061M) decomposition in the presence of air-dried Dowex50W (8% DVEI, 20-50 mesh) resin associated with various transition metal-diamine complexes Complex
kx104s-’
Fe”‘-1,2-diamine Fe”‘- 1 3 d&nine Fe”‘-114 diamine Fe’“- 1 5 diamine Fe”‘-1:6- diamine Fe”‘-1,7-diamine Fe”‘-1 9-diamine Cu”-1’6-diamine Con- 1:6-diamine
2
25°C
30°C
35°C
40°C
13.26 9.68 6.77 4.7 3.19 1.93 0.419 10.01 6.83
17.64 12.67 8.97 6.01 4.04 2.35 0.483 13.76 8.86
23.27 16.31 10.82 7.46 4.9 2.84 0.565 18.70 11.34
30.305 20.70 14.09 9.14 6.05 3.43 0.66 25.84 14.19
& no of methylene
6
E
AH+
AC+
AS+
(kJ/moI)
(kJ/mol)
(kJ/mol)
(J degg’ mol-‘)
42.8 39.33 37.05 34.4 32.81 29.73 23.58
40.26 36.79 34.51 31.9 30.27 27.2 21.04 46.4 35.36
90.7 91.6 92.5 93.5 94.5 95.91 99.96 91.25 92.48
-170 -180 - 190 -210 - 220 - 230 - 260 - 150 - 190
8
groupsln)
Fig. 2. Variation of the rate constant k (per g dry resin) with the number of methylene groups between the two amino groups in the presence of the air-dried resin Dowex-50W (8% DVB, 2050 mesh) as Fe”‘-diamine complex ions. (0) 25°C; (X ) 3O”C, (A ) 35°C and (m) 40°C.
indicate that higher k values (per g dry resin) are found with higher E values. This suggests that the reaction rate is governed by the entropy of activation [141. The changes in the free energy of activation, dG#, quoted in Table 2 were calculated from Eyring’s equation and lie in the range 90.6-99.9 kJ mol-l, in
I.A. Salem/J. Mol. Catal. 80 (1993) 11-19
16
agreement with that found for H,O, decomposition with resin-transition metal ammine [ 3,4], amine [ 56,101, and Fe”’ Sal-o-phen [ 71 complexes. With 1,6-diamine, the entropy of activation dS # (Table 2) decreases in the sequence Cu” > Co” > Fe”‘. Also, with ferric complexes (Table 2 ) , the LIS# values increased with decreasing number of methylene groups in the diamine ligand. The greater the value of dS #, the greater the value of k (per g dry resin ) and the greater the probability of the activated complex formation. Therefore, the probability of activated complex formation with 1,9-diamine complexes is lower than with 1,2diamine complexes as a result of the steric effect of the nine methylene groups. Figure 3 represents an experiment in which the resin in the [Fe”‘(1,5diamine ) 2] 3+ form was used in the H202 decomposition. After completion of the experiment the resin in the peroxo-iron complex form (dark brown compound) was collected, washed with doubly distilled water and used to decompose H,O,. It is clear that the reaction rate with the peroxo-iron complex was greater than that with the [Fe111(l,5-diamine)2]3+ complex ion. This is evidence for an intermediate (active species) formed at the beginning of the reaction [3-lo] and having an inhibiting effect on the rate of the reaction, i.e., the active species needs some time to be formed and accordingly the overall rate of the reaction decreases. This experiment showed that in neither case did the order change and that the peroxo-iron complex was capable of oxidizing I-&O, 161.
80
o FelIID-1,5dia,7,n= .
pToxo-compo”nd t=35.c
0
100
200
300
Timetmin)
Fig. 3. Decomposition-time
curves of H,O,
resin Dowex-50W (8% DVB, 20-50 complex at 35 oC.
(0.061
M) in the presence of 0.2 g of the air-dried
mesh) as (o ) [Fe”‘( 1,5-diamine)J3+
and (m) peroxo-iron
17
LA. Salem/J. Mol. Catal. 80 (1993) 11-l 9
Before proposing a mechanism for the reactions under study, it is important to point out here that the rate was greatly decreased by the addition of ally1 acetate to the reaction mixture. Inhibition of the rate by ally1 acetate suggests the involvement of radicals or radical ions in the mechanism [ 151, which is also supported by the low values of activation energy observed in the present studies. Since the values of E (Table 2)) lie in the range of chemical reaction throughout the catalyst particles [ 161, and the peroxide anion, HO;, exists [ 171 in the pH range in the present work the reaction mechanism is probably:
KI
2H 20 ,~===-2H0,+2H+
(2)
FL%St,
[Fe”‘(diamine),(H,O),]
‘+s
[Fe111(diamine),(OH)H,0]2++H+
Fe111(diamine)2(0H)HaO]2++HO;
3
[Felll(diamine)z(OH)
(3) (HO,)]’
+H20 [FeI”(diamine),(OH)
(HO,)]+-
(4)
k1 [Fe’I(diamine),(OH)
(HO;)]’
SLOW
intermediate (active species) [Fe11(diamine)2(0H)
(HO;)]++HO;-
(5)
Iz2 [Fe111(diamine)2(OH)20,]+ fast
+OH-
(6)
The rate equation can be written as follows: &/dt=
-d[H202]/dt=k1
[Fe”‘(diamine),(OH)
(HO,)]’
(7)
But from eqns. (2), (3) and (4) we have,
W,l=JK,W,W/[H+l [Fe111(diamine)z(OH)H20]2’
=K2 [Fe111(diamine)2(H20)2]3’/[H+]
and [Fe”‘(diamine),(OH)
(HO,)]‘=K,
[Fe111(diamine)2(OH)H2012+ [HO,]
thus
dx/dt=k,&K,K,
[Fe111(diamine)2(H20)213+[H~021/[H~12
(3)
18
LA. Salem/J. Mol. Catal. 80 (1993) 11-l 9
where $ is the rate constant of the rate determining step (eqn. (5) ); K,, & and K3 are the equilibrium constants of eqns. (2)) (3) and (4) respectively. The intermediate in eqn. (5 ) may contain the free radical (HO; ) i.e. the active species contains a divalent ferrous ion. Such a redox cycle, Fe3++Fe2+, was found in the homogeneous H,O, decomposition with o-phenanthroline-Fe(m) complex ion [ 181 and in heterogeneous H,O, decomposition with Fe”n’Sal-ophen complex ion [ 71. Using the steady state approximation for the calculation of the concentration of the intermediate, we get; d[Fe”(diamine),(OH)
(HO;)]‘/dt=lz,
[Fe’“‘(diamine),(OH)
-Iz, [Fe”(diamine),(OH)HOH)]
+ [HO;]
(HO,)]’ =0
(9)
The structure of the peroxo-iron complex may be assumed to be [Fem(diamine),(OH)202] + (eqn. (6). Such a structure has been proposed previously [ 3-101. The peroxo-complex is self-decomposed slowly with the evolution of 0, as follows [ 8.91 [Fem(diamine),(OH)202]+
--+[Fe”‘(diamine),(OH),]++O,
(19)
In the light of the proposed mechanism the hydroxy iron complex (eqn. ( 10) ) changes slowly to the original [Fe”’ ( diamine)z (H,O ), ] 3+ as follows: [Fe’“(diamine),(OH),]++H,O=
[Fe”‘(diamine)z(OH)
(H,O)]‘+
+OH[Fe”‘(diamine),(OH)
(11) (H,O)]‘++H+=
[Fe”‘(diamine)z(Hz0)2]3+
(12)
This phenomenon is also observed in the case of the brown peroxo-copper complex [Cu”(diamine) (OH),O,] and the dark-brown peroxo-complex [Co”(diamine),(OH),O,].
References P. Jones and A. Suggett, Biochem, J., 110 (1968) 621. M.L. Kremer, J. Chem. Sot., Faraday Trans. I, 79 (1983 ) 2125. F.M. Ashmawy, M.Y. El-She&h, I.A. Salem andA.B. Zaki, Transition Met. Chem., 12 (1987) 51. M.Y. El-Sheikh, F.M. Ashmawy, I.A. Salem and A.B. Zaki, Z. Phys. Chem. (Leipzig), 268 (1987) 595. M.Y. El-Sheikh, Colloids Surf. 60 (1991) 97. M.Y. El-Sheikh, A.M. Habib, F.M. Ashmawy, A.H. Gemeay and A.B. Zaki, Transition Met. Chem., 14 (1989) 95. M.Y. El-Sheikh, F.M. Ashmawy, I.A. Salem, A.B. Zaki and U. Nickel, Transition Met. Chem., 16 (1991) 319.
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K. Hayakawa and S. Nakamura, Bull. Chem. Sot. Jpn., 47 (1974) 55. K. Hayakawa and S. Nakamura, Bull. Chem. Sot. Jpn., 47 (1974) 1162. M.Y. El-Sheikh, A.M. Habib, F.M. Ashmawy, A.H. Gemeay and A.B. Zaki, J. Mol. Catal., 55 (1989) 396. N. Hawkings, K.D. Horton and K.W. Snelling, Report (1980) AEEW-R-1390, ZNZS Atomindex, 12 (1981) Abstr. No. 605735; Chem. Abstr., 95 (1981) 208999b. C.M. Davis and G.G. Thomas, J. Chem. Sot., (1952) 1607. J. Burgess, Ions in Solution, Ellis Horwood, Chichester, ch. 6,1988, p. 82. R.G. Wilkins, The Study of the Kinetics and Mechanisms of Reactions of Transition Metal Complexes, Allyn and Bacon, Boston, MA, 1974, p. 101. V.K. Gupta, Thermochim. Acta, 69 (1983) 389. F. Hellferich, Zon Exchange, McGraw-Hill, New York, 1969, p. 1962. V.S. Sharma and J. Schubert, J. Am. Chem. Sot., 91 (1969) 6291. G. Wada, T. Nakamura, K. Terauchi and T. Nakai, Shokubai (Catalyst), 5 (1963) 199.
