Journal of Molecular Catalysis, 84 (1993) 61-75 Elsevier Science Publishers B.V., Amsterdam

6’7

Ml82

Kinetics of heterogeneous decomposition of hydrogen peroxide with some transition metal complexes supported on silica-alumina

in aqueous medium

Ibrahim A. Salem*, Mohamed A. Salem and Ali H. Gemeay

Chemistry Department, Faculty of Science, Tanta University, Tanta (Egypt); tel. (+20- 40)346925, fax. (+20-40)322785

(Received January 5,1993; accepted April 14,1993)

Abstract

Silica-alumina in the form of diethylamine (deam ) -, dimethylamine (dmam ) -, ethylamine (earn)-, ammonia (amm)- and aniline (an) -copper (II) complexes as well as deam-cobalt (II) complex have been used as potentially active catalysts for HzOz decomposition in an aqueous medium. The reaction was first order with respect to the H202 concentration and the rate con- stants (per g of dry silica-alumina) were determined. A coloured compound, a peroxo-metal com- plex, which formed at the beginning of the reaction in each case, was found to contain the cata- lytically active species. The activation energy and the change in the entropy of activation increased with the basicity of the ligands in the following order: an < amm < earn < dmam < deam, which is also the probability sequence for the formation of the activated complex. A reaction mechanism was proposed.

Key words: heterogeneous decomposition; hydrogen peroxide; kinetics; silica-alumina

Introduction

Reactions of hydrogen peroxide with various chemical species have been the subject of extensive studies because of their intrinsic and biological signif- icance [ 1,2]. It has been reported that the decomposition of H,O, in the pres- ence of metal ions as homogeneous catalysts is slow but it is relatively fast when these ions are used along with small amounts of alumina or beryllium oxide [ 3,4]. The kinetics of heterogeneous decomposition of H202 with some metal complexes supported on alumina [ 51 or on cation exchanger [ 6-121 has been studied in an aqueous medium. The coloured peroxo-metal complex formed at the start of the reaction is stable even after completion of the exper- iment. However, over several days autodecomposition of the peroxo-complex occurs, with the evolution of dioxygen, and the original colour of the transition metal complex returns. As a continuation of our studies on the heterogeneous

*Corresponding author.

68 LA. Salem et al. /J. Mol. Catal. 84 (1993) 67-75

decomposition of HzOz, we have extended our investigation using silica-alu- mina in the form of some ammine and amine copper (II) and cobalt (II) com- plexes in an aqueous medium. One of the objectives of the present work is to treat quantitatively the effect of ligand basicity as well as the transition metal ion on the kinetics of the heterogeneous decomposition of H,O,.

Experimental

Silica-alumina in the form of transition metal complex Silica-alumina (25% A1203) was obtained from Crossfield Co. (England).

It was washed repeatedly with boiling doubly distilled HzO, dried and activated at 150°C for 24 h. The required amount of activated silica-alumina was kept in contact with the solution of copper sulphate or cobalt sulphate (1 M) for two hours with constant stirring. The silica-alumina in the Cu2+ or Co2+ form was then filtered and washed with doubly distilled H,O until free from any excess of metal sulphate solution. Ammonia or amine solution was added to the silica-alumina in the transition metal ion form to yield the corresponding stable ammine or amine complexes. Finally the silica-alumina in the complex form was washed with bidistilled H,O until the detection of the ligand gave a negative result [ 81. The colour of the different complexes before and after the reaction are shown in Table 1.

Determination of [metd]/[ligand’] ratio in the catalyst The amount of metal ion sorbed per g of catalyst was determined by adding

a known, excess amount of the metal sulphate solution to a definite amount of dry silica-alumina [ 51. Unadsorbed metal ion in the filtrate was determined

TABLE 1

The [ ligand] / [metal] ratio of transition metal complexes associated with air-dried silica-alumina

Complex Complex formula PH

before reaction

after reaction

Colour

before reaction

after reaction

Cu(II)-diethyl-

amine Cu(II)-dimethyl-

amine Cu(II)-ethyl-

amine Cu(II)-ammine Cu(II)-aniline

Co (II) -dimethyl- amine

]Cu(EtzNH),l*+ 8.01 8.82

[Cu(Me2NH)d]2+ 7.93 8.55

[Cu(EtNH2)J2+ 7.87 8.42

lCuWWJ2+ 7.82 8.15 [CuWNHA2+ 7.20 7.92 [Co(Me2NH)a12+ 7.96 9.07

blue

blue

blue

blue blue faint red

brown

brown

brown

brown brown dark red

Z.A. Salem et al. /J. Mol. Catal. 84 (1993) 67-75 69

using an atomic absorption spectrometer (Shimadxu, A 670). The amount of complexed ligand was determined as described previously [ 181. The total [M ] / [L] ratio was 1: 4 with copper (II) and 1: 6 with cobalt (II) complexes (Table 1).

Hydrogen peroxide solution An H,O, solution (30% A.R. grade from Merck, Schuchardt, Munich,

Germany) was used. Four different initial concentrations of HzOz were chosen in the (0.038-0.14) M range for the copper (II) complexes and in the (0.077- 0.28) M range for the cobalt (II) complex. The initial concentration of Hz02 was determined iodometrically using standard sodium thiosulphate solution.

Kinetic measurements A number of conical flasks (100 cm3) containing a weighed quantity of

catalyst together with doubly distilled H,O (19 cm3) were placed in a water- shaker thermostat for 30 min. For each conical flask, standard H202 stock solution (1 cm3) was added quickly by micropipitte and zero time was taken as the half addition point. After a measured time interval the reaction was quenched by quickly filtering the reaction mixture through G2 sintered glass. Aliquots of the aqueous phases (5 cm3) were withdrawn by micropipette and the undecomposed H202 was determined iodometrically. The reaction temper- ature was varied between 25 and 40’ C ? 0.1 ‘C.

Before the addition of H202 the pH of the doubly distilled H20 in the presence of silica-alumina metal complex was varied according to the type of the complex (Table 1). After addition of H202 the pH decreased within the first minute of reaction and then increased at the end of the reaction (Table 1). The increase in the concentration of protons within the first minute of the reaction does not lead to any displacement of the transition metal complex ions from silica-alumina.

Results and discussion

Ammonia and organic amines are very strongly sorbed by Dowex-50W resin in the form of Cu (II) and Co (II) to form stable complexes [ 6-8,12,14]. The same was also found in our case with silica-alumina (25% ) . The [metal] to [ ligand] ratio was unchanged before and after the decomposition reaction. This means that these complexes are not degraded during the decomposition of H202. With all ligands the reaction was first order with respect to [ H202], (Fig. 1). The rate constant, k (per g dry catalyst) was obtained from the expression [ 151

In a ( >

= kwt a-x

70 I.A. Salem et al. /J. Mol. Catal. 84 (1993) 67-75

where a is the initial concentration of HzOz, x the amount of H202 decomposed at time t, and w is the mass in g of dry catalyst. With hydrogen peroxide con- centration equal to 0.14 M, the rate constant k was found to increase with increasing basicity of the ligands [lo], in the following sequence, deam > dmam > earn > amm > an [ 161. The more basic the ligand the more sta- ble the transition metal complex [ 171. The greater the stability of the complex, the less its formation energy, the greater its affinity to react with Hz02 and the greater the value of k [ 181. With cobalt or copper dimethylamine complex, the rate constant k, was found to decrease with increasing H,O, concentration (Fig. 2 ) . Also, the rate constant k with Cu (II) -dmam complex was found to be greater than that with Co (II)-dmam complex at a concentration of H,O, equal to 0.14 M. Similar behaviour was found using Dowex-50W resin as a catalyst support [ 81.

The formation of the brown and a dark red compounds upon addition of H202 to the Cu (II) and Co (II) complexes respectively (peroxo compound) facilitated the isolation of the peroxo compound from the reaction medium. Figure 3 show the decomposition-time curves for two reactions having the

Fig. 1. First order rate equation for HzOz (0.14 M) decomposition in the presence of 0.3 g of dry silica-alumina in the form of copper (II) -dimethylamine complex at various temperatures: ( 0 ) 25”C, (x) 3O”C, (A) 35”Cand (Cl) 40°C.

I.A. Salem et al. /J. Mol. Catal. 84 (1993) 67-75 71

__ ^^ ^^ 0.1 0.1

IH2Oz I/M 0.2

Fig. 2. Variation of the rate constant k (per g dry catalyst) with the initial concentration of hydrogen peroxide in presence of silica-alumina in the form of copper(dimethylamine com- plexatdifferenttemperatures (0)25”C, (~)30”C, (A)35”Cand (0)4O”C.

same origin. The first reaction was carried out in the presence of the silica- alumina in the form of [Cu(dmam),] complex ions. After this reaction had been completed, the silica-alumina was collected, washed with doubly distilled Hz0 and used in the second reaction in the form of the peroxo-copper complex (brown compound). Under the same working conditions, the rate of decom- position of Hz02 in the presence of the peroxo-copper complex was greater than that in the presence of [ Cu (dmam),] complex ions. This is evidence for an intermediate (an active species), formed at the beginning of the reaction, which has an inhibiting effect on the rate of the reaction [6-8,10,12]. This experiment showed that in neither case did the order change and that the per- oxo-copper complex was able to decompose H,O,.

The 12 values (per g dry catalyst), were used in an Arrhenius plot to give the activation energy, E. Normally, more stable complexes should give higher E and lower K values. However, with copper (II) complexes, lower k values are found with lower E values with decreasing basicity of the ligand (Table 2). This suggests that the reaction rate is governed by the entropy of activation [ 7,8,19]. The change in the enthalpy of activation dH+ (Table 2) was smaller than the E values with the quantity RZ’, (M-I’ =E- RT). The change in the

LA. Salem et al. /J. Mol. Catal. 84 (1993) 67-75

,

50 100 150 Time lmin)

Fig. 3. Decomposition-time curves of H202 (0.077 M) in presence of 0.3 g of dry silica-alumina in the form of (0 ) copper (II) -dimethylamine complex and (m) peroxo-copper complex (brown compound) at 25°C.

TABLE 2

Kinetic and activation parameters or HzOz (0.14 M) decomposition in the presence of air-dried silica-alumina associated with various transition metal-ammine and amine complexes.

Complex kx 10’s_’ E AHfl AG* AS* kJ/mol kJ/mol kJ/mol Jmdeg-‘.mol-’

25°C 30°C 35°C 40°C

Cu(II)- 6.91 10.73 17.37 25.91 69.1 66.5 68.28 - 5.81 diethylamine Cu(II)- 5.07 7.47 10.63 14.61 56.1 52.25 69.39 -56.1 dimethylamine Cu(II)- 4.21 6.05 8.41 11.59 52.2 49.75 69.94 - 66.08 ethylamine Cu(II)-ammine 3.83 5.27 7.33 10.17 50.07 48.1 70.25 - 72.50 Cu(II)- 1.05 1.396 1.89 2.45 44.1 41.6 73.69 - 105.1 aniline Co(II)- 0.88 1.35 1.98 2.94 61.09 59.55 73.67 - 46.38 dimethylamine

LA. Salem et al. /J. Mol. Catal. 84 (1993) 67-75 73

free energy of activation, AG # was calculated (Table 2) from the Eyring equa- tion and lies in the range 44.1-69.1 kJ mol-’ range which is in agreement with that found in the decomposition of Hz02 with resin-transition metal-amine and -ammine complexes [6,10,12]. The change in the entropy of activation, AS + was calculated (Table 2 ) from the relationship AG + = AH # - TAS #. The AS+ values increased in the following sequence an < amm <earn < dmam < deam. This is also the sequence of the probability of the activated complex formation.

Since the values of E (Table 2) are in the range of a chemical reaction and since the peroxide anion, HO,, exists [ 201 in the pH range used in the present work, it is possible to illustrate the reaction mechanism with copper complexes as follows:

Kl

2H 0 2 2 M 2H++2HO; fast

KZ

[CuL,12++H0, = [CuL, (HO,) I+ fast

[CuL,(HO,)]+ ” - intermediate (active species)

(2)

(3)

+HOf

- [peroxo-copper complex] (4) fast

from this proposed mechanism the following rate equation can be deduced:

$=d[H,O,]/dt=k, [CuL,(HO,)]+ (5)

But from eqns. (2) and (3) we have:

W,l=& WzQMH+l

and

[CuLWO2) I+ =K2[CuL412+ [HO,]

thus

d.r/dt= -d[H,O,]/dt = h&Kz PL12+ P-J2021

W+l

where k, is the rate constant of the rate-determining step (eqn. 4); KI and K2 are the equilibrium constants (2) and (3) respectively. From eqn. (6) the re- action rate is proportional to [ CuL,12+, [H202] and [H+ ] -l; the source of

74 Z.A. Salem et al. /J. Mol. Catal. 84 (1993) 67-75

the inverse [H+ ] dependence is the protolytic equilibrium in eqn. (2) [ 6,7,11]. The intermediate in equation (4) may contain the free radical (HO,). In this case the active species may be [ CuL,(HO,) ] +, i.e., it contains a univalent cuprous ion [ 201. Thus the free radical (HO,) and HO, can be incorporated in the proposed mechanism and the rate determining step (eqn. 4) may be written as follows [ 7,11,12] :

[CuLWW I+ - k1 [Cu’LJHO;)] + &XV

kz

[Cu’L,(HO;)] + +HO, - peroxo-copper complex (7) fast

using the steady state approximation for the calculation of the concentration of the intermediate, [ CuL4 (HO;) ] + , we obtain:

d[CuIL,(HO;)]+/dt=k, [CuL4(HOz)]+-kz [Cu’L,(HO;)]+ [HO,] =0

The peroxo-copper complex may have the structure [CuL4(0H),02] which decomposes spontaneously according to eqn. (8) [ 7,8,12]

[CuL,(OH),O,] + [CuLJ’+ +20H- +O, (8)

On standing for several hours the sky blue colour of [ CuL412+ is attained. This phenomenon occurs in the case of the brown peroxo-copper complex contain- ing ammine [6] and amine ligands [ 7,8,10,12].

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