Journal of Molecular Catalysis, 87 (1994). 25-32. Elsevier Science B.V., Amsterdam. 25. M255. The catalytic effect of some transition metal.
Journal of Molecular Catalysis, 87 (1994) 25-32 Elsevier Science B.V., Amsterdam
25
M255
The catalytic effect of some transition metal hexamethylenetetramine complexes in hydrogen peroxide decomposition Ibrahim A. Salem* Chemistry Department, Faculty of Science, Tanta University, Tanta (Egypt) (Received March 11,1993; accepted July 6,1993)
Abstract The kinetics of the catalytic decomposition of Hz02 with Wofatit KPS resin (4% DVB, 4069 F) in the form of 1:l copper(H)-, manganese(H)-, cobalt(II)- and nickel(II)-hexamine complexes was studied in an aqueous medium. The decomposition reaction was first order with respect to [ H202] and the rate constant, k (per g of dry resin) increased in the following sequence: Mn (II) > Co (II) > Cu (II) > Ni (II). The active species, formed as an intermediate at the beginning of the reaction, had an inhibiting effect on the reaction rate. An anionic surfactant, sodium dodecyl sulphate (SDS), considerably inhibited the reaction rate. A probable mechanism for the decomposition process has been suggested, which is consistent with the results obtained. Key words: cobalt; copper; hexamethylenetetramine manganese; nickel
complexes; hydrogen peroxide decomposition;
Introduction The catalytic effect of some transition metal amine complexes on the decomposition of HzOz has been studied in the presence of Dowex-50W resin in an aqueous medium [l-6]. A coloured compound (peroxo-metal complex), which formed at the beginning of the reaction, was found to contain the catalytically active species. It was stable even after the completion of the decomposition reaction. The peroxo-metal complex undergoes auto-decomposition with the evolution of 0, to give the original complex [l-6]. With di- and triethylamine, the rate of H202 decomposition with cobalt(I1) was found to be greater than that with copper(I1) [3]. Also, with aliphatic diamine ligands, the rate of the decomposition reactions decreased in the following order Cu (II) > Co (II) > Fe (III) as well as with increasing length of the methylene chain between the two nitrogen atoms in the ligand [ 61. With copper (II) and nickel (II) ethanolamine complexes, the rate of HzOz decomposition was in the following order; monoethanolamine > diethanolamine > triethanolamine [ 4,5]. *Corresponding author.
0304-5102/94/$07.00 0 1994- Elsevier Science B.V. All rights reserved. SSDfi0304-5102(93)E0188-M
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I.A. Salem /J. Mol. Catal. 87 (1994) 25-32
As a continuation of our studies on the catalytic activity of resin-transition metal amine complexes in hydrogen peroxide decomposition we have extended our investigation to the use of hexamethylenetetramine (hexamine ) as a ligand. This ligand is strongly sorbed by Wofatit KPS resin (4% DVB, 4080 pm) in the form of Mn(II), Co(II), Cu(I1) and Ni(I1) to form the corresponding transition metal-hexamine complexes.
Experimental
Wofatit KPS resin associated with metal-hexumine complexes Wofatit KPS resin (4% DVB, 40-80 pm) in the hydrogen form was used as a strongly acid cation-exchanger. This resin is produced as sulphonated styrene divinylbenzene copolymer in the H+-form by EKP-Wolfen (Germany). Its moisture content (30.38%) was determined by drying a sample overnight at 110°C under atmospheric pressure [6-S]. The total weight capacity of the exchanger was determined using the batch method and was equal to 4.77 mequiv/g dry H+ form of the resin. This amount is equal to 2.38 mequiv/gdry Mn(II), Co(II), Cu(I1) and Ni(I1) form. The required amount of the resin in the hydrogen form was kept in contact with the metal sulphate solution (0.1 M) for 2 h with constant stirring. The resin in the metal-ion form was then filtered and washed with bidistilled Hz0 until free of any excess of the metal sulphate solution. An equivalent amount of the ligand solution (0.1 M) was added dropwise with constant stirring to the resin in the metal-ion form to form a 1: 1 metal-hexamine complex. A small excess amount of the ligand was added in order to confirm the formation of the 1: 1 complex. Finally the resin in the complex form was washed repeatedly with b&tilled H,O until free of any excess ligand. The colour of the different complexes before and after the reaction with HzOz are shown in Table 1.
Chemicals and reagents Reagent grade chemicals and bidistilled Hz0 were used throughout. An H202 solution (30% A.R. grade from Merck, Munich, Germany) was used. Standard HzOz stock solution was prepared by dilution and its concentration was determined iodometrically using standard sodium thiosulphate solution. Hexamethylenetetramine ligand was obtained from BDH Chemicals Ltd., Poole, England and used without further purification. Sodium dodecyl sulphate (SDS) was obtained from Aldrich Chemical Company, Inc., USA, and used without further purification.
Kinetic measurements Kinetic measurements and the iodometric determination of the undecomposed Hz02 have been described elsewhere [ 6,8]. The reaction temperature was in the range 25-40 5 0.1 ‘C.
LA. Salem /J. Mol. Catal. 87 (1994) 25-32
27
TABLE 1 The [ hexamine] / [metal] ratio of transition metal-hexamine Wofatit KPS (4% DVB, 40-80 pm) resin Complex
Mn(II)-hexamine Co(II)-hexamine Cu(II)-hexamine Ni(II)-hexamine
Complex formula
[Mn(hexamine)(H20)2]2+ [Co(hexamine)(H,0)2]2+ [Cu(hexamine)12+ [Ni(hexamine)(H20),12+
complexes associated with air-dried
Colour
PH
before after before reaction reaction reaction
after reaction
8.21 8.16 8.12 8.05
pale brown brown brown dark green
9.07 8.93 8.82 8.75
yellow-red red greenish blue green
The pH of the bidistilled H,O in the presence of the supported catalyst as well as the colour of the different complexes before and after the addition of Hz02 were varied according to the type of the complex (Table 1) .
Results and discussion Hexamethylenetetramine ligand is very strongly sorbed by the Wofatit KPS resin (4% DVB, 40-80 pm) in the form of Mn(II), Co(II), Cu(I1) and Ni (II) to form stable complexes. The [metal] : [ ligand] ratio was 1. This ratio was determined after the reaction of these complexes with HzOz [ 1,4] and was found to be unchanged. Also, the capacity and the moisture content of the resin were determined at the end of the reaction and were found to be unchanged. This means that the resin and the complexes are not degraded during the decomposition of H202. Therefore, the Wofatit KPS resin (4% DVB, 40-80 w) under the present experimental conditions is more stable than the radioactive waste resin [ 91 (based on the same structure) which was partially decomposed by Fe-catalysed HzOz. The decomposition reaction was carried out at constant H202 concentration (0.09 M) with 0.4 g of the air-dried resin (0.278 g dry resin). The reaction was first order with respect to [ H,OP ] (Fig. 1). The rate constant, k (per g dry resin) was obtained as described previously [ 2,3,6,8]. The rate constant K (per g dry resin) increased in the following order (Table 2) Mn (II) >C~(II)>CU(II)>N~(II). The activation energy, E, calculated from the Arrhenius plot, decreased in the following order (Table 2); Mn(I1) >Co(II) >Cu(II)>Ni(II). Normally, higher E values were found with lower k values. However, the results depicted in Table 2 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 [ 1,6,10].The E values are in agreement with those found
LA. Salem /J. Mol. Catal. 87 (1994) 25-32
50
I
I
100
150
llmelmin) Fig. 1. First order rate equation for the decomposition of HzOz (0.09 M) in the presence of 0.4 g of air-dried Wofatit KPS resin (4% DVB, 40-80 ,um) in the form of [Mn(hexamine) ] +’ at different temperatures: 0: 25”C, X: 3O”C, A: 35 and n : 40°C.
previously with some transition metal complexes supported on Dowex resin [4,5,8,11-131. Table 2 also demonstrates the change in the activation enthalpy, AH”, where AH* = E - RT. The changes in the free energy of activation, dG#, quoted in Table 2 were calculated from Eyring’s equation and lie in the range 91.7195.3 kJ/mol, in agreement with the value found for the decomposition of HzOz with resin-transition metal ammine and amine complexes [4,6,11,12]. The changes in the entropy of activation, AS* (Table 2) were calculated from the relationship AG#= AH”- TAs#. The AS* value increased in the following sequence; Ni (II) > Cu (II) > Co (II ) > Mn (II). The greater the value of AS”, the greater the value of k and the greater the probability of activated complex formation [ 1,6]. The formation of coloured compounds (peroxo-compounds) upon the ad-
I.A. Salem/J.
29
Mol. Catal. 87 (1994) 25-32
TABLE 2 Kinetic and activation parameters of HzOz (0.09 M) decomposition in the presence of air-dried Wofatit KPS (4% DVB, 40-80 pm) resin associated with various transition metal-hexamine complexes Complex
kxlo’s-’ 25°C 30°C
Mn(II)-hexamine Co(II)-hexamine Cu (II) -hexamine Ni(II)-hexamine
6.08 3.71 2.44 2.19
E (kJ/mol) 35°C
*
t
9
$/moI)
$/mol)
$deg-lmol-‘)
77.54 59.48 50.59 41.24
91.71 93.42 94.72 95.3
- 46.38 -111 - 144 - 176.9
40°C
10.68 16.97 29.10 80.08 5.76 8.09 12.55 62.02 3.45 5.01 6.74 53.13 2.73 3.77 5.04 43.79
I
I
80 -
60 -
50
Time lminl
Fig. 2. Decomposition-time curves of HzOz (0.09 M) in the presence of 0.4 g air-dried Wofatit KPS resin (4% DVB, 40-80 pm) in the form of 0: [Mn(hexamine)]‘+ and A: peroxo-Mn complex at 30 ’ C.
dition of H,O, to the resin-transition metal complexes allowed the isolation of these compounds from the reaction medium. Fig. 2 shows the decomposition-time curves for two reactions having the same origin. The first reaction was carried out in the presence of the resin in the form of [ Mn (hexamine) ] ‘+ complex ions. The resin (peroxo-compound) was collected after this reaction was completed, washed with bidistilled Hz0 and used in the second reaction. Under the same working conditions, the rate of decomposition of HzO, in the
)
30
I.A. Salem j J. Mol. Catal. 87 (1994) 25-32
Fig. 3. Effect of SDS concentration on the rate constant k (per g of dry resin) for decomposition of HtOP (0.09 M) in the presence of 0.4 g of the air-dried Wofatit KPS resin (4% DVJ3, 40-80 ,um) in the form of [Mn(hexamine)12+ at 25°C.
presence of the peroxo-manganese complex was greater than that in the presence of [ Mn (hexamine) ] 2+ complex ions. This is evidence for an intermediate (active species) formed at the beginning of the reaction which had an inhibiting effect on the rate of the reaction [l-8,11-13]. This experiment showed that in neither case did the order change and also that the peroxo-copper complex was able to decompose H,O,. Since the supported catalyst possesses positive sites, an anionic surfactant, such as sodium dodecyl sulphate (SDS), becomes adsorbed on the positive centres via electrostatic forces [ 141. The effect of SDS concentration was investigated in order to elucidate the role of SDS on the rate of the reaction. Different concentrations of SDS below and above the critical micelle concentration (6 x 10s3 M at 25 ‘C, determined experimentally under the same conditions) were employed. It was found that a decrease in the reaction rate was observed, even at low SDS concentration, Fig. 3. This can be explained by assuming that the positive active centres on the catalyst surface undergo blocking with SDS aggregates and this blocking increases with increasing SDS concentration. Under these conditions the blocked sites are no longer able to interact with substrate molecules. Before proposing a mechanism for the reactions under study, it is important to point out that the rate was greatly decreased by the addition of ally1 acetate to the reaction mixture. Inhibition of the rate by ally1acetate suggested the involvement of radicals or radical ions in the mechanism [ 6,151. Since the E values lie in the range of chemical reaction, the more likely
31
I.A. Salem /J. Mol. Catal. 87 (1994) 25-32
mechanism is one including reaction through the catalyst particles [ 16 ]. Since the peroxide anion HO, exists in the pH range used in the present work [ 171, the reaction mechanism is probably; K1 2H02 2ti2H0,
+2H+
(1)
fast
A .--
[M(hexamine)12++HO;
[M(hexamine) (HO,)]’
(2)
fast K2
[M(hexamine)
(HO,)] + -
intermediate (active species) &XV
1
+HO,,
fast
(3)
peroxo-metal complex The following rate equation can be deduced from this proposed mechanism: Rate= -d[H202]dt=k1
[M(hexamine)(HO:,)]+
(4)
From Eqns. 1 and 2 we get;
(5)
W,l=@&MM/W+l and [M(hexamine)
=K2[M(hexamine)12+[
(H02)]’
(HO;)]
(6)
Substituting Eqns. 5 and 6 into Eqn. 4; Rate= -d[H,O,]/dt=
kl&K2
[M(hexamine)12+
[ (H202]
[H+l
(7)
where k1 is the rate constant of the rate-determining step (Eqn. 3)) ICI and K2 are the equilibrium constants for Eqns. 1 and 2, respectively. Eqn. 7 shows that the reaction rate is proportional to [M(hexamine) 12+, [H,O,] and l/ [H+ ] i.e. the reaction rate decreases with increasing acidity of the reaction medium. It proved impossible to carry out the decomposition reaction in the presence of an acid or buffer solution which regenerates the resin. The structure of the peroxo-metal complex in Eqn. 3 may be assumed to be [M (hexamine) (OH),O, 1. This compound undergoes auto-decomposition with the evolution of oxygen as follows [8,18,19]; [M(hexamine)
(HO),O,]+
[M(hexamine)12++20H-+02
On standing for several days the resin- [ M (hexamine) ] 2+ entity is formed. This phenomenon is evident for many complexes supported on the cation exchange resin [l-8,11-13].
LA. Salem 1 J. Mol. Catal. 87 (1994) 25-32
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