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Catalytic decomposition of hydrogen peroxide in the presence of. N,N'-bis(salicylidene)-o-phenylenediamineiron(III) sorbed on to. Dowex-50W resin. Mohamed ...
Transition Met. Chem., 16, 319-323 (1991) Decomposition of N,N'-bis(salicylidene)-o-phenylenediamine Fem

319

Catalytic decomposition of hydrogen peroxide in the presence of N,N'-bis(salicylidene)-o-phenylenediamineiron(III) sorbed on to Dowex-50W resin Mohamed Y. El-Sheikh, Fathy M. Ashmawy, Ibrahim A. Salem and Ahmed B. Zaki* Department of Chemistry, Faculty of Science, Tanta University, Tanta, A.R. Egypt Ulrich Nickel Universiti~t Erlangen, Erlangen-Niirnberg, D8520, Germany

Summary The tetradentate Schiff-base ligand N, N'-bis(salicylidene)o-phenylenediamine (salph) is very strongly sorbed by cation exchange materials with transition metal counter ions, forming stable complexes. The kinetics of catalytic decomposition of H202 in the presence of (salph)-Fe m sorbed on Dowex-50 W resin has been studied in aqueous medium. The reaction is first order with respect to [H202]. The rate constant, k (per g of dry resin) decreased with increasing degree of resin cross-linkage due to a salting out effect. The activation parameters were calculated and a reaction mechanism is proposed.

Introduction Ammonia and organic amines are very strongly sorbed by cation exchangers with transition metal counter ions to form stable complexes ") . We have recently studied the kinetics of catalysed hydrogen peroxide decomposition in the presence of Dowex-50W resin in the form of some transition metal-ammine and amine complexes (2-7). We have now extended our investigations to the tetradentate N a O z Schiff-base ligand N, N'-bis(salicylidene)-o-phenylenediamine (salph) complexed with transition metals bound to Dowex-50W resin. We report here on the kinetics of catalysed decomposition of hydrogen peroxide in the presence of Dowex-50W resin loaded with iron(III) (salph). A 1:1 complex ion has been found with iron(III) and salph (8), i.e. the complex ion is likely to have the formula [Fe(salph)(H20)2] +, since iron(III) complexes are generally octahedral.

Experimental Salph was prepared according to the standard procedure ~ and characterised by its m.p., i.r. spectrum and elemental analyses. Dowex-50W resin with varying degrees of cross-linkage [2,4, 8% divinylbenzene (DVB), 200-400 mesh and 2% DVB, 100-200 mesh] in the hydrogen form was used as a strongly acidic cation exchanger. The resin has been described previously('~). Its moisture content was determined by drying samples overnight at l l0~ at normal pressure (Table 1). The total capacity of the exchanger was determined by the batch method and was found to decrease with increasing resin cross-linkage (lm. Values of 5.16, 4.74 and 4.62meq/g dry H + form of the resin were obtained at 2, 4 and 8% DVB respectively. The resin was converted into the Fem form by equilibration with Fe(C104)3 solution (0.001 M) using the batch

process, then washed with doubly distilled H 2 0 until free from an excess of Fe m solution. The resin was washed with EtOH and added to an EtOH solution of salph (0.001 M) and the mixture stirred for ca. 2h. The resin was collected and washed with doubly distilled H 20 until the residual salph was undetectable by u.v. spectroscopy. The resin is yellowish brown with 2 and 4% DVB (200-400mesh) and deep brown with 2% DVB (100-200 mesh) and 8% DVB (200-400 mesh).

Hydrogen peroxide solution An H202 solution (30% A.R. grade from MerckSuchardt) was used. Four different initial concentrations of H 2 0 2 were chosen in the (1.344-3.672)x 10-2M range by dilution with doubly-distilled H20; the concentration of H20/; was checked iodometrically using Na2S203 solution.

Kinetic measurements A conical flask (100 cm 3) containing a weighed quantity of the resin in the [Fe(salph)(H20)2] + form together with doubly distilled H 2 0 (19cm 3) was placed in a water-shaker thermostat for 30rain. Standard HaO 2 stock solution (lcm 3) was added quickly via a micropipette and zero time was taken as the half addition point. After a measured time the reaction was quenched by quickly filtering the mixture through a G2 sintered glass. Aliquots of the aqueous phase (5cm 3) were withdrawn with a micropipette and the undecomposed H202 was determined iodometrically. The reaction temperature was varied between 25 and 40~ __+0.1 ~C. Before the addition of H202 the pH of the doubly distilled H 2 0 alone was 7.1 and in the presence of the resin-[Fe(salph)(H20)2] + it was 6.9. After the addition of H202 the pH decreased within the first minute of the reaction and reached 6.3 and then increased gradually till it reached a value 7.9 at the end of the reaction, A deep brown peroxo-iron complex with 2 and 4% DVB (200-400mesh) and a blackish brown one with 2% DVB (100-200mesh) and 8% DVB (200-400 mesh) formed at the beginning of the reaction. The former turned yellowish brown on standing for several hours. No pronounced change in colour with time was noticed with the latter. Attempts to carry out the decomposition in the presence of an acid or buffer solution led to regeneration of the resin and decomposition of both the Fem(salph) complex and the peroxo-iron complex.

* Author to whom all correspondenceshould be directed. 0340-4285/91$03.00+ .12

9 1991Chapman and Hall Ltd

320

M.Y. El-Sheikh et aI.

Transition Met. Chem., 16, 319-323 (1991)

Table 1. Rate constants (per g of dry resin) and activation parameters for the decomposition of H20 z (0.02413 M) in presence of the air-dried resin (0.2 2.0 g) Dowex-50W (200-400 mesh) in the form of (salph)-Fe m complex ions. DVB (%)

Moisture content(%)

2

20.50

2~

19.58

4

18.19

8

16.93

Temp (~

k x 105 (Mr-as -1 )

25 30 35 40

56,60 90.60 151.40 225,10

25 30 35 40

17.30 27.90 43.10 67.30

25 30 35 40

11.55 16.63 25.03 38.62

25 30 35 40

0.76 1.14 1.70 2.27

E (kJ mo1-1 )

AH * (kJ mo1-1 )

AG * (kJ mol -I )

AS~ (J deg- t mol =1)

71.3

68.8

92.1

- 76.3

69.8

67.3

95.1

- 91.0

62.7

60.2

96,4

- 118.5

54.6

52.1

103.4

- 167.9

~The air-dried resin Dowex-50W (100-200 mesh).

Results and discussion The total capacity and moisture content of the resin were determined at the end of the experiment and found to be unchanged. Thus the resin was not degraded during hydrogen peroxide decomposition. This resin is therefore more stable than the radioactive waste resin ~tt) (based on the same structure) which was partially decomposed by Fe-catalysed H 2 0 z.

from the relationship n = log(Vo/Vo)/log(a/a'), where Vo and V o are the initial reaction velocities for initial concentrations a and a' respectively. This order makes use of experimental data corresponding to a pure medium, i.e. to an ideal reaction. Conversely, the order obtained according to Equation 1 informs us about the

/

22 2.0

Rate constant and order of reaction The initial H 2 0 2 concentration was kept constant at 2.413 x 1 0 - 2 M whereas the mass of the resin and the degree of cross-linkage were varied in the 0.2-2.0 g range (air-dried resin) and 2 - 8 % DVB respectively. The reaction was first order with respect to [ H 2 0 2 ] (Figure 1). The rate constant, k (per g of dry resin) was obtained from the expression~t21: Log a/(a - x) = kwt/2.303

(1)

where a is the initial concentration of H202, x the amount of H 2 0 2 decomposed at time t and w is the mass in g of dry resin. The rate constant, k (per g of dry resin) decreased with increasing degree of resin cross-linkage (Table 1) due to a salting out effectu' 5,6~ Moreover, when the mesh size of the resin particles was changed from 200-400 mesh to 100-200 mesh the value of k decreased (Table 1). This can be attributed to the decrease of the effective surface area with the decrease of the mesh size of the resin particles. The order of the reaction (Equation i) was found to decrease from 1.8 to 0.5 upon increasing initial H2Oz concentration from 1.344 x 10 -2 to 3,672 x 10-2M; this is a sign of a stepwise mechanism (la). The reaction velocity at time tending to zero (Vo) was determined by extrapolation of the reaction rate time plots to zero time at constant mass (0.4 g) of the air-dried resin 4% DVB, 200-400 mesh). The order of the reaction n at time tending to zero was found to be unity

1.8

(

1.6 1.4 [ 1.2 1.0

/

0.8 0.6 0.4 /

0 =~a____L_.a_ 0

80

160

240

320 400 Time(rain)

480

560

640

720

Figure l. First-order integrated rate equation for H202 (2.413 • 10-2 M) decomposition in the presence of the air-dried Dowex-50W resin in the form of (salph)-Fe m complex ion at 40 ~C and at various masses, % DVB of the resin as well as at various mesh size of the resin particles: O: 0.2g (2% DVB, 200-400 mesh), x : 0.4g (2% DVB, 100-200 mesh), []: 0.4g (4% DVB, 200-400mesh) and A: 0.8 g (8% DVB, 200-400mesh).

, 9( . . . . Fem Transition Met. Chem., 16, 319-323 (1991) Decomposition of N,N-bls,sahcyhdene)-o-phenylenedlamme

321

2.25 [

10.0 r

2.00 I ~" 1.75 80 1.50 ~1.25 % 6.0 o9

x

% > 4.0

0

0

40

2.0

0,0

J

l

0,0

0.01

I

002 [H202] M

I

I

0.03

I

l

0.04

Figure 2. Variation of the initial reaction rate with initial [HzOa] for the decomposition of HzO2 in the presence of air-dried Dowex-50W (4% DVB, 200 400mesh) in the form of (salph)-Fe m complex ion: O: 0.4 g (25~C), x : 0.4 g (30~C), A: 0.4g (35~C), D: 0.4g (40~C) and O: 0.Sg (40~C).

actual reaction in all its complexity. The equality of the two orders means that the chemical process is unaffected by secondary reactions u4). When Vo is plotted against the initial concentration of H 2 0 2 (Figure 2), the curves give an intercept with the abscissa at a concentration of 0.325 x 10 -2 M, which represents 13.5% of the working concentration (2.413 x 1 0 - 2 M ) used in the present investigation. This means that the reaction does not start unless the initial H202 concentration is > 0.325 x 10-z M. In other words the formation of the active species requires a certain rcdox potential which is realised only above this H202 concentration. The relation between V0 and the initial H 2 0 2 concentration a in Figure 2 obeys the following equation: Vo = kow(a - c) where the constant c = 0 . 3 2 5 x 1 0 - 2 M and ko is the initial rate constant per g of dry resin (Table 2). Figure 3 illustrates two sets of experiments representing the % decomposition-time relationship observed for 2% DVB resin (200-400 mesh). Of the two curves obtained at 35 ~C, one was obtained with a resin in the form of [Fe(salph)(H20)2]+; after completing the experiment, the resin (in the form of the peroxo-iron complex) was collected, washed and then used to get the second curve. The reaction rate with the peroxo-iron complex was greater than that with [Fe(satph)(H20)2] + and the order was the same in both cases, indicating that the precursor of the active species in not [Fe(salph)(H20)z] + but a product of the latter with H 2 0 2. This experiment showed that the intermediate (active species) which was formed at the beginning of the reaction had an inhibiting effect on the reaction rate. It also showed that the peroxo-iron complex was capable of decomposing HzO2. The second set consists of two curves at 25~ with a resin in the

80

120 Time (rain)

160

200

Figure 3. Decomposition-time curves for H20 2 (2.413 x 10- 2 M) decomposition in the presence of 0.4 g air-dried resin Dowex-50W (270 DVB, 200-400mesh) in the form of: • (salph)-Fe m complex ion (35~ 9 peroxo-iron complex (35~C), [2: (salph)-Fe l" complex ion (25~C) and A: (salph)-Fe m complex ion (25~C) in presence of stream of argon. form of [Fe(salph)(HzO)23 +; one of them represents an experiment which was carried out in the presence of a stream of argon. The reaction rate in the presence of argon was obviously lower than that in the absence of it. This means that the argon gas expels the 0 2 formed during H 2 0 2 decomposition and consequently prevents the formation of the peroxo-iron complex. This experiment is of great importance because it proves the formation of the peroxo-iron complex during the reaction and shows its role in HzO 2 decomposition. Peroxo-metal complexes have been detected in homogeneous and heterogeneous decomposition reactions of H 2 0 2 with transition metal amine (and ammine) complexed z- 7,15-~ 7).

Activation parameters The k and ko values (per g of dry resin) are used in Arrhenius plots to yield the activation energies, E (Tables 1 and 2). E value decreased with increasing degree of resin cross-linkage (Table 1) due to dipole-dipole interactions (1) of the polar solvent molecules (H20) with the polar groups of the solute. The smaller the amount of the free water in the resin the less significant the influence of the dipole-dipole interactions and the smaller the activation energy. The E value at time tending to zero (Table 2) was smaller than that of the overall reaction (Table 1) with 4% DVB (200-40Omesh) resin. Thus the activated complex formed during the overall reaction is more stable than that formed at time tending to zero. The change in the enthalpy of activation, AH*, the change in the free energy of activation, AG* and the change in the entropy of activation, AS* were calculated ~2-~ (Tables 1 and 2). The values of AG$ are in good agreement with those found for H 2 0 2 decomposition in the presence of Dowex-50W resin in the form of transition metal amine complexes (5'6). The greater the degree of resin cross-linkage and the smaller the mesh size of the resin particles, the smaller the value of k (per g of dry resin) and the smaller the value of AS s (Table 1). Thus the probability of activated complex formation decreased with increasing degree of

2,

322

M.Y. El-Sheikh et al.

Transition Met. Chem., 16, 319-323 (1991)

2. Kinetic and activation parameters at time tending to zero for the decomposition of H2Oe in presence of the air-dried resin (0.4 g) Dowex-50W (4~ DVB, 200-400 mesh) in the form of (salph)-Fe m complex ions.

Table

Temp

(~ 25

30

35

40

[H202"] • 102 (M)

(Ms -x)

1.344 t.950 2.413 3.672

0.575 0.697 1.060 1.670

1.344 1.950 2.413 3.672

0.780 1.120 1.510 2.440

1.344 1.950 2.413 3.672

1.060 1.750 2.500 3.790

1.344 1.950 2.413 3.672

1.445 2.750 3.500 5.500

Vo • 106

Ko x 105 (Mien s -1)

E (kJ tool -1)

AH* (kJ tool i)

AG* (kj mol- t)

AS* (Jdeg - a tool- 1)

59.9

57.4

96

- 126.4

13.65

19.76

30.56

43.79

resin cross-linkage as well as with decreasing mesh size of the resin particles. The probability of activated complex formation at time tending to zero was smaller than that of the overall reaction with 4 ~ DVB (200400 mesh) resin (Tables 1 and 2).

[Fe(salph)(OH)(H20)] = K 2 [Fe(salph)(H20)2 ] +/[H + ] and [Fe(salph)(OH)(H02)] = K3 [Fe(salph)(OH)(H20)] [ H O 2 ] thus,

Reaction mechanism

dx/dt = - d[-H2Oz]/dt

Since the values of E (Table 1) are in the range of chemical reaction, the more likely mechanism is reaction throughout the catalyst particles (as) . Since the peroxide anion, HO~- exists (a9) in the pH range used in the present work the reaction mechanism is probably: K1

2H20 2 ,

' 2H + + 2HO~-

(2)

fast

[Fe(salph)(H20)2] + K2 ~ [Fe(salph)(OH)(H20)] + H+

(3)

fast

[Fe(salph)(OH)(H20)] + H O z

= kl ~ / - ~ KzK3 [Fe(salph)(H2 O)2 ] + [ H z O 2 ] / [ H + ]2 (8) where k a is the rate constant of the rate-determining step (Equation 5); Ka, K2 and K 3 are the equilibrium constants of Equations 2, 3 and 4 respectively. The involvement of solvent H 2 0 can not be defined since its concentration is not varied. From Equation 8 the reaction rate is proportional to [Fe(salph)(H20)2] +, [H2021 and [ H + ] -1. The intermediate in Equation 5 may contain the free radical (HO)), i.e. the active species contains a divalent ferrous ion. Such a redox cycle, +oFe 3+ . "Fe z+ was found in the homogeneous HzO 2 -e-

~:3

[Fe(salph)(OH)(HO2)]- + H 2 0

(4)

fast

[Fe ul(salph)(OH)(HOz)]-

k~ , [Fe"(salph)(OH)(HO~)]Intermediate (active species)

~ow

(5)

decomposition with o-phenanthroline-Fe(III) complex ion (2~ Using the steady-state approximation for the calculation of the concentration of the intermediate, we get: d[FelI(salph)(OH)(HOh)]-/dt = kl [Fe(salph)(OH)(HO2)] - k2 [Fen(salph)(OH)(HOh)]- [ H O f ] = 0

[Fen(salph)(OH)(HO~)]- + H O f

(9)

But from Equations 2, 3 and 4 we have,

Although the kinetic study can tell us nothing about structure, nor even (in this work) stoichiometry, the structure of the peroxo-iron complex may be assumed to be [Fe(salph)(OH)202]- (Equation 6). Such structure was found in the case of H 2 0 2 decomposition with the same resin in the form of Cu(II)-ethylamine and Cu(II)ethylenediamine (5-7). The peroxo-complex is self-decomposed slowly with the evolution of 0 2 as follows"6' ave:

[HO~-] = x / ~ [HzO/]/EH+],

[Fe(salph)(OH)2Oz]- ~ [Fe(salph)(OH)2]- + 0 2

k2 , [Fem(salph)(OH)202 ]_ + O H -

(6)

fast

The rate equation can be written as follows: dx/dt = - d [ H 2 0 2 ] / d t = ka [Fem(salph)(OH)(HO2)]-

(7)

(10)

Transition Met. Chem., 16, 319-323 (1991) Decomposition of N,N'-bis(salicylidene)-o-phenylenediamine Fe In

In the light of the proposed mechanism the hydroxy iron complex (Equation 10) changes slowly to the original [Fe(salph)(H20)2 ] + as follows: [Fe(salph)(OH)2]- + H 2 0 [Fe(salph)(OH)(H20)] + O H -

(11)

[Fe(salph)(OH)(H20)] + H + ,-~-[Fe(salph)(H20)2] + (12) We can reach the conclusion that the use of solid ion exchangers has many advantages over the homogeneous catalytic decomposition of H2Oz and the presence of the resin facilitates isolation of the reaction intermediates as well as the quenching of the reaction. The change in the pH value during the reaction is consistent with the formation of H + and O H - as seen above in the reaction mechanism. The rate-determining step (Equation 5) involves the formation of the active species during the reduction of Fem to FeII, whereas the decomposition reaction of H202 (Equation 6) involves the formation of the peroxo-iron complex during the oxidation of iron(II) to iron(III). This explains the requirement of a certain redox potential for the performance of the reaction (Figure 2). According to the experiment which was carried out in the presence of the argon gas (Figure 3) it is worthwhile to conclude that the peroxo-iron complex [Fem(salph) (OH)eO2]- loses its catalytic activity when it loses O2. This means that the compound [Fe(salph)(OH)2](Equation 10) is inactive.

References ~1)F. Helfferich, Ion Exchange, McGraw-Hill, New York, 1962, Ch. 5.

323

(2)F. M. Ashmawy, M.Y. El-Sheikh, I.A. Salem and A.B. Zaki, Transion Met. Chem., 12, 51 (1987). ~3)M. Y. El-Sheikh, F.M. Ashmawy, I.A. Salem and A. B. Zaki, Z. Phys. Chemic, Leipzig, 268, 595 (1987). (4)M. Y. El-Sheikh, F. M. Ashmawy, I.A. Salem and A.B. Zaki, Z. Phys. Chemic. Leipzig, 269, 126 (1988). (5)M. Y. El-Sheikh, A.M. Habib, F.M. Ashmawy, A.H. Gemeay and A. B. Zaki, Transition Met. Chem., 13, 96 (1988). (6)M.Y. El-Sheikh, A.M. Habib, F.M. Ashmawy, A.H. Gemeay and A. B. Zaki, Transition Met. Chem., 14, 95 (1989). (7)M. Y. El-Sheikh, A.M. Habib, F.M. Ashmawy, A.H. Gemeay and A. B. Zaki, J. Mol. Catal., 55, 396 (1989). ~8)M. S, Masoud, A. Akelah and S. S. Kandil, Inidian or. Chem., 24A, 855 (1985). ~9)H. D. Saw, J. Am. Chem. Soe., 101, 154 (1912). (l~ A. Soldano and G. E. Boyd, or. Am. Chem. Soc., 75, 6091 (1954). ~I)N. Hawkings, K.D. Horton and K.W. Snelling. Report (1980) AEEWR-1390, from INIS Atomindex, 12 (1981), Abstr. No. 605735, Chem. Abstr., 95, 208999b (1981). a2)C. W. Davies and G. G. Thomas, J. Chem. Soc., 1607 (1952). (~3)j. H. Espenson, Chemical Kinetics and Reaction Mechanism, McGraw-Hill, New York, 1981, p. 93. 114)G. Pannetier and P. Souchay, Chemical Kinetics, Elsevier, New York, 1967, Ch. 4. (15)T. Kaden and H. Sigel, Helvetica Chimica Acta, 51, 947 (1968). (16)K. Hayakawa and S. Nakamura, Chem. Soc. Jpn. Bull., 47, 1162 (1974). I1~)K. Hayakawa and S. Nakamura, Rep. Fac. Sci., Kagoshima Univ. (Math. Phys. Chem.), 7, 55 (1974). (~s)F. Helfferich,Ion Exchange, McGraw-Hill, New York, 1962, Ch. 11. ~9)V. S. Sharma and J. Schubert, J. Am. Chem. Sot., 91, 6291 (1969). (2o)G. Wada, T. Nakamura, K. Terauchi and T. Nakai, Shokubai (Catalyst), 5, 199 (1963). (Received 31 May 1990)

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