111002pe

http://www.chemistrymag.org/cji/2009/111002pe.htm	Jan.1, 2009 Vol.11 No.1 P.2 Copyright

Kinetics and mechanism of ruthenium(III) catalyzed oxidation of isobutyl alcohol by cerium(IV) in acid medium

Qin Xinying¹, Lu Yunkai¹，Zhang Puzhe²
(¹College of Chemistry and Environmental Science, Hebei University, Key Laboratoryo f Analytical Science and Technology of Hebei Province, Baoding 071002, China; ²Hebei Handan foreign Language School，Handan 056000，China)

Received Oct. 29,2007.

Abstract The kinetics and mechanism of ruthenium(III) catalyzed oxidation of isobutyl alcohol(IBA) by cerium(IV) in sulfuric acid media has been investigated by titrimetric technique of redox in the temperature range of 298-313K. It is found that the reaction is first-order with respect to Ce(IV) and Ru(III), and a positive fractional order with respect to IBA. It is found that the pseudo first-order ([IBA]>>[Ce(IV)]>>[Ru(III)]) rate constant k_obsincreases with the increase of [H⁺] but decreases with the increase of [HSO₄^-]. Under the protection of nitrogen, the reaction system can not initiate polymerization of acrylonitrile, indicating generation of free radicals. On the basis of the experimental results, a suitable mechanism has been proposed. The rate constants of the rate determining step together with the activation parameters were evaluated.
Keywords Ruthenium(III) ion; Cerium(IV) ion; Isobutyl alcohol(IBA); Catalytic Oxidization; Kinetics and mechanism

1. INTRODUCTION
Transition metals in a higher oxidation state generally can be stabilized by chelation with suitable complex agent. Metal complexes such as silver (III)^[1], copper(III) ^[2] and Ce(IV) ions^[3] are good oxidants in a medium with an appropriate condition. However, our preliminary observations indicate that oxidation of some organic compounds by Ce(IV) in aqueous sulfuric acid is kinetically sluggish, the process can be efficiently catalyzed by various metal ions even at trace concentration. Different metal ion catalysts like chromium(III)^[4], ruthenium(III)^[5], iridium(III)^[6] etc. have been used in the oxidation by cerium(IV). Among the different metal ions, ruthenium(III) and iridium(III) are highly efficient. In the modern chemical industry, the higher requirement on catalyst selectivity has been advanced. However, the studies of active center structure by means of physical method can not calculate reaction pathways about complicated molecules. Reaction mechanism of various elementary reactions must be investigated to analyze the factors of affecting the selectivity^[7]. Therefore, the basic study of catalytic reaction will provide the scientific basis for improving catalyst selectivity and making high-efficiency catalyst. Now the lack of studying on the Ce(IV) oxidation of IBA stimulated us to explore the kinetic behavior of the title reaction in order to continue our studying on metal ion catalysis in cerium(IV) oxidation reactions.

2. EXPERIMENTAL
2.1 Materials and reagents
Ceric sulfate (Ce(SO₄)₂), ferrous ammonium sulfate (Fe(NH₄)₂(SO₄)₂), isobutyl alcohol(IBA) and ruthenium trichloride (RuCl₃) were of A.R. grade. Doubly distilled water was employed throughout the experiment. Ceric sulfate solution was prepared by dissolving it in sulfuric acid and standardized with ferrous ammonium sulfate using ferroin as an indicator. IBA was purified by distillation and its concentration was obtained from its density measurements. Stock solution of RuCl₃ was prepared by dissolving RuCl₃ in warm 1.0 mol·dm^-3H₂SO₄ solution and kept for several days at about 40^oC to attain the equilibrium. The concentration of Ru(III) in the stock solution was determined by spectrophotometry. The ionic strength (m) was maintained by adding NaNO₃ solution.
2.2 Procedure and kinetic measurements
Under the condition of [IBA]>>[Ce(IV)]>>[Ru(III)], 25mL of solution containing definite [Ce(IV)], [Ru(III)], [H₂SO₄] and [NaNO₃], and 25mL of IBA solution of appropriate concentration were transferred separately to the upper and lower branch tubes of l type two-cell reactor. After it was thermally equilibrated in a thermostat (about five minutes), the solutions were thoroughly mixed. The progress of the reaction was monitored by withdrawing aliquot of the reaction mixture at regular intervals, adding it into excess standard Fe(NH₄)₂(SO₄)₂ sulfate solution in sulfuric acid to quench the reaction, then back-titrating the unreacted iron(II) by standard cerium(IV) solution using ferroin as indicator. The pseudo first-order rate constants k_obs were evaluated (Figure 1) from the slope of the plots of ln[Ce(IV)]_t versus time (t) i.e. ln((V_∞-V_t) versus t where V_∞ and V_t denote the volume of standard cerium (IV) solution needed in back titration for the unconsumed iron(II) solution at infinity and time(t) respectively. To evaluate k_obs, generally 8-10 values at least up to 80% completion of the reaction were used. Average values of at least two independent determinations of k_obswere taken for analysis. The observed rate constants were reproducible within the experimental error±5%.

3. RESULTS AND DISCUSSION
3.1 Dependence on [Ce(IV)]
Under the condition [IBA]>>[Ce(IV)]>>[Ru(III)], the plot of ln (V_∞-V_t) versus t was linear(Fig.1), indicating that the reaction is first-order with respect to [Ce(IV)]. The pseudo first-order rate constants k_obs are independent upon the initial concentration of cerium (IV) in the 1.0~ 3.0×10^-3mol·dm^-3range. The dependence is given by:

Fig.1 Plots of ln (V_∞-V_t) vs. t at 298K
[Ce(IV)]=2.5×10^-3 mol·dm^-3, [IBA]=0.1 mol·dm^-3, μ=1.0 mol·dm^-3, [H₂SO₄]=1.0 mol·dm^-3, r=0.9995
(A) [Ru(III)]=0.0 mol·dm^-3, (B) [Ru(III)]=5.0×10^-7 mol·dm^-3

3.2 Dependence on [IBA]
At fixed [Ce(IV)], [Ru(III)], [H₂SO₄], ionic strength (m) and temperature (T), k_obsincreases with the increase of [IBA]. The plot of k_obsvs. [IBA] was a smooth curve, indicating fractional order in [IBA] ,observed reaction order n_ap= 0.61（r=0.9988）. The plot of 1/k_obs vs. 1/[IBA] exhibits excellent linearity (Fig.2) with a positive intercept and slope.

Fig.2 Plot of 1/k_obs vs. 1/[IBA] at 30^oC
[Ce(IV)]=2.5×10^-3 mol·dm^-3; [H₂SO₄]=1.0 mol·dm^-3, [Ru(III) ]= 1×10^-7 mol·dm^-3; I=1.0 mol·dm^-3

3.3 Dependence on [Ru(III)]
The fact that under the experimental conditions in the absence of Ru(III) ions the reaction practically does not take place is supported by an independent experiment (Fig.1 (a)).Addition of traces of Ru(III) enhances the rate significantly (Fig.1 (b)), k_obs vs. [Ir(III)] yielded good linear plots (Fig.3) nearly through the origin. This indicates that the reaction is of first-order with respect to [Ru(III)].Abtained from
k_obs=K[Ru(III)] ( n_ap=1).
[HSO₄^-] was varied in the range of 0.2~1.0 mol·dm^-3at fixed [H⁺] ([H⁺]=1.0 mol·dm^-3≈[HClO₄]+[H₂SO₄]), [IBA] and [Ru(III)]. [HSO₄^-] was calculated ignoring the dissociation of [HSO₄^-] in the presence of fairly high [HClO₄]. This leads to [HSO₄^-]≈[H₂SO₄]. From table 1, it can be seen that k_obs increases with the increase of [HSO₄^-]. Therefore HSO₄^- shows a rate retarding effect ( n_ap= 0.20).

Fig.3 Plots of k_obs vs. [Ru(III) ] at different temperatures
[Ce(IV)]=2.5×10^-3 mol·dm^-3; [IBA]=0.1mol·dm^-3; [H₂SO₄]=1.0 mol·dm^-3; I=1.0 mol·dm^-3

Table 1 Effect of varying [HSO₄^-] on k_obs at 298K

[HSO₄^-]/mol·dm^-3	[H⁺]/mol·dm^-3	10³k_obs/min^-1
0.1	1.0	44.94
0.5	1.0	63.04
1.0	1.0	71.46

[Ce(IV)]=2.5×10^-3 mol·dm^-3, [IBA]=0.1mol·dm^-3,·[Ru(III)]=1.0×10^-6 mol·dm^-3, m=2.0mol·dm^-3

3.5 Dependence on [H⁺]
[H⁺] was varied over the range 0.2~1.0 mol·dm^-3 at fixed [HSO₄^-] ([HSO₄^-] = 1.0 mol·dm^-3≈[H₂SO₄]+[NaHSO₄]), [IBA] and [Ru(III)], [H⁺] was calculated ignoring the dissociation of [HSO₄^-] and assuming [H⁺]≈[H₂SO₄](Table 2).

Table 2 Effect of varying [H⁺] on k_obs

[H⁺]/mol·dm^-3	0.2	0.4	0.6	0.8	1.0	a	b	r
[]/mol·dm^-3	1.0	1.0	1.0	1.0	1.0	1.193	0.330	0.993
10²k_obs/min^-1	1.989	2.368	2.731	3.130	3.329	1.193	0.330	0.993

[Ce(IV)]=2.5×10^-3mol·dm^-3; [Ru(III)]=1×10^-7mol·dm^-3; [IBA]=0.1mol·dm^-3; I=1.0mol·dm^-3; t=30^oC; a(intercept), b(slope) and r (correlation coefficient) for the linear regression of k_obs vs.H⁺.

3.6 Analysis of product
The completion of the reaction was marked by the complete fading of Ce(IV) color (yellow). One of the reaction products as isobutyl aldehyde was detected by spot test and estimated^[8], Another product Ce(III) was also detected by spot test^[9].
    Acrylonitrile solution(40%,V/V) was added to the reaction mixture under the protection of nitrogen gas, no white deposition could be found, indicating the reaction system can not initiate polymerization of acrylonitrile and proving free radicals is not formation in the reaction.
3.7 Mechanism of the reaction
The kinetic data (i.e. 1/k_obs vs. 1/[IBA]) fits well with Michaelis-Menten model^[10] , suggesting that 1:1 type complex of IBA and Ru(III) could be formed in the first preequilibrium step. Ce(SO₄)₂ has been found kinetically active in this study, free radicals can be generated in the reaction in acid media. Thus a mechanism consistent with the found kinetic characteristics is presented as follows:
(1)
(2)
Ru(III)－H Ru(III)+H⁺ (3)
(4)
[Ru(III)]_T=C+[Ru]_e
[Ru(III)]_e= (5)
    Subscripts T and e means total and equilibrium concentratration respectively.
    From step (2) and (5) the rate of [Ce(IV)]_T can be written as:
=2k_obs [Ce(IV)]_T =2k[C]=[Ce（IV）]_T (6)
Because of [IBA]>>[Ru(III)],[IBA]_T»[IBA]_e＋[C]» [IBA]_e ,
k_obs=k[C]= (7)
(8)
    Eq.(6) indicats that the reaction is first-order with respect to [Ce(IV)]_T, and Eq.(7) suggests that n_ap[Ru(III)]T=1.0, 0<n_ap[IBA]<1.0, which is consistent with the results of our experiments (confirmed by Fig.2). Eq.(8) suggests that a plot of 1/k_obs vs. 1/[IBA] at constant [Ru(III)] should be linear with positive intercept. The activation parameters were evaluated and listed in Table 3

Table 3. Rate Constants and Activation Parameters of the Rate-Determining Step

t /^oC	30	35	40	45	activation parameters (30^oC)
10⁴k/mol^-1.L. min^-1	7.57	10.50	16.45	26.34	Ea=54.5 KJ.mol^-1, △H^≠= 52 KJ.mol^-1 △S^≠=-149 J.K^-1.mol^-1, △G^≠=97 KJ.mol^-1

Acknowledgement
This research was supported by National Science Foundation of China (No. 20675024), Natural Science Foundation of Hebei Educational Committee (No. 2006407；No. 2008307), China Postdoctoral Science Foundation (No. 2005037629) and Science Foundation of Hebei University.

REFERENCES
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[2] SONG Wen-Yu，LIU Hong-Mei. Chinese Journal of Inorganic Chemistry, 2000, 16(4): 607-611.
[3] Lakshmi S, Renganathan. Int. J. Chem. Kinet., 1996, 28(10):713-720.
[4] SONG Wen-Yu, JIANG Qing-Mei. Acta Chimica Sinica, 2005, 63(2):109-113.
[5] Das A. K., Das M. K. Int. J. Chem. Kinet, 1995, 27(1): 7-16.
[6] Das A. K., DAS M. Indian J. Chem., 1995, 34A: 866-870.
[7] KONG Fan-zhi., TIAN Jian-hua., JIN Zi-lin.. Petrochemical Technology, 2002, 31(5): 387~392.
[8] Feigl F. Spot test in organic analysis. New York: Elsevier pulishing Co., 1956. P334.
[9] Department of Chemistry, Hangzhou University. Handbook of Analytical Chemistry. Beijing: Chemical Industry Press, 1982.
[10] Moore J. W., Pearson R. G. Kinetics and Mechanism. New York: John Willey and Sons, 1981. P379.

酸性介质中钌离子催化铈(IV)离子氧化异丁醇的反应动力学及机理
秦新英¹，吕运开¹，张普哲²
(河北大学化学与环境科学学院，河北省分析科学与技术重点实验室，保定，071002；2、邯郸市外国语学校，邯郸，056000)
摘要采用氧化还原滴定法，在298-313K温度范围内研究了在酸性介质中钌(III)离子催化铈(IV)离子氧化异丁醇的反应动力学及机理。通过实验发现，反应对钌(III)离子和铈(IV)离子的反应级数都是一级，对异丁醇为正分数级。准一级反应速率常数k_obs 随氢离子浓度的增大而增大，随硫酸根离子浓度的增大而减小。在氮气保护下，反应体系不能引发丙烯腈的聚合反应，表明反应过程中没有自由基生成。由此提出了适合此反应的反应机理并求出了相应的活化参数。
关键词 钌(III)离子；铈(IV)离子; 异丁醇; 催化氧化; 动力学及机理

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