Kinetics and mechanism of iridium(III) catalyzed oxidation of methanol by cerium(IV) in sulfuric acid media

Song Wenyu, Zhao Ronghui, Li Yanwei, Xu Junran
(College of Chemistry and Environmental Science, Hebei University, Baoding 071002, China)

Abstract  The kinetics and mechanism of iridium(III) catalyzed oxidation of methanol by cerium(IV) in sulfuric acid media has been investigated by titrimetric technique in the temperature range of 25 - 40ºC. The reaction order of [Ce(IV)] was found to be unity, and that in both methanol and iridium(III) to be positive fractional. It was found that the pseudo first order ([CH₃OH]＞＞[Ce(IV)]) rate constant k_obs increases with the increase of [H⁺], but decreases with the increase of []. Under the protection of nitrogen, the reaction system can not induce polymerization of acrylonitrile indicating no free radicals generated. On the basis of the experimental results, a suitable mechanism has been proposed. From the hydrogensulfate dependence, Ce(SO₄)₂ has been found as the kinetically active species. The preequilibrium constants, composite rate constants together with the parameters were evaluated.
Keywords   Iridium(III) ion, cerium(IV) ion, methanol, catalyst, 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 diperiodatocuprate(III)^[1], dihydroxydiperiodatonikelate(IV)^[2] and Ce(IV) ions^[3] are good oxidants in a medium with an appropriate conditions. 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 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 only can not calculate reaction channel about complicated molecules. Reaction mechanism of various elementary reactions must be investigated to analyze the effect on selectivity^[7]. Therefore, the basic study of catalytic reaction will prove the scientific basis for improving catalyst selectivity and making high-efficiency catalyst. Now the lack of any study on the Ce(IV) oxidation of methanol prompted us to explore the kinetic behavior of the title reaction on metal ion catalysis in cerium(IV) oxidation reactions.

2 EXPERIMENTAL
2.1 Materials and reagents
Ceric sulfate, ferrous ammonium sulfate, methanol and iridium trichloride were of A.R. grade. Twice distilled water was employed throughout the experiment. Ceric sulfate solution was prepared by warming it in sulfuric acid and standardized with ferrous ammonium sulfate using ferroin as an indicator. Methanol was purified by distillation and its concentration was obtained from its density measurements. Stock solution of iridium trichloride was prepared by dissolving in warm 1.0 mol·dm^-3 HCl solution and kept for several days at about 40ºC to attain the equilibrium. The concentration of iridium(III) in the stock solution was determined^[8] spectrophotometrically. The ionic strength was maintained by adding KNO₃ solution.
2.2 Procedure and kinetic measurements                   
Under the condition of [CH₃OH]>>[Ce(IV)]>>[Ir(III)], 25ml of solution containing definite [Ce(IV)], [Ir(III)], [H₂SO₄] and [KNO₃], and 25ml of methanol 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, the solutions were thoroughly mixed. The progress of the reaction was monitored by withdrawing aliquots of the reaction mixture at regular intervals, quenching the reaction by adding to excess standard ammonium.iron(II) sulfate solution in sulfuric acid and back-titrating the unreacted iron(II) against standard cerium(IV) 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 devote 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_obs were taken for analysis. The observed rate constants were reproducible within the experimental error 5%.

Fig.1 Plots of ln (V_∞-V_t) vs. t at 30ºC
[Ce(IV)] = 2.0×10^-3mol·dm^-3, [CH₃OH] = 0.05mol·dm^-3, [H₂SO₄] = 0.5mol·dm^-3, m= 1.0mol·dm^-3.
A ([Ir(III)] = 0 mol·dm^-3); B ([Ir(III)] = 4.0×10^-7 mol·dm^-3).

3 RESULTS AND DISCUSSIONS
3.1 Dependence on [Ce(IV)]
Under the condition [CH₃OH]>>[Ce(IV)]>>[Ir(III)], the plot of ln (V_∞-V_t) versus time t was linear(Figure.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 2 - 4.5 ×10^-3mol·dm^-3 range. The dependence is given by:

3.2 Dependence on [CH₃OH]                
At fixed [Ce(IV)], [Ir(III)], [H₂SO₄], ionic strength (m) and temperature, k_obs increases with the increase of [CH₃OH]. The plot of k_obs vs.[CH₃OH] was a smooth curve passing through the origin indicating fractional order in [CH₃OH] (observed reaction order n_ap = 0.2 ). The plot of 1/k_obs vs. 1/[CH₃OH] exhibits excellent linearity (Figure.2) with a positive intercept and slop.

Fig.2 Plot of 1/k_obs vs. 1/[CH₃OH] at 30ºC
[Ce(IV)] = 2.0×10^-3mol^·dm^-3，[Ir(III)] = 4.0×10^-7mol^·dm^-3,
[H₂SO₄] = 0.5mol^·dm^-3, m= 1.0mol^·dm^-3.

3.3 Dependence on [Ir(III)]                  
The fact that under the experimental conditions in the absence of Ir(III) ions the reaction practically does not take place is supported by an independent experiment ( Figure.1. A). Addition of traces of Ir(III) enhances the rate significantly (Figure 1. B), k_obs shows the fractional order in [Ir(III)] (n_{a p} = 0.56-0.69), 1/k_obs vs. 1/ [Ir(III)] yielded good linear plots (Figure 3) with a positive intercepts and slops at different temperatures. The kinetic data also fit well with Michaelis-Menten Plot.

Fig.3 Plots of 1/k_obs vs. 1/[Ir(III)] at different temperatures
[Ce(IV)] = 2.0×10^-3mol·dm^-3, [CH₃OH] = 0.05mol·dm^-3,
[H₂SO₄] = 0.5mol·dm^-3, m = 1.0mol·dm^-3.

3.4 Dependence on []              
[] was varied over the range (0.2-1.0 ) mol·dm^-3 at fixed [H⁺] ([H⁺] = 1.0 mol·dm^-3≈[HClO₄]+[H₂SO₄]), [CH₃OH] and [Ir(III)], [] was calculated ignoring the dissociation of in the presence of fairly high [HClO₄]. This leads to []≈[H₂SO₄]. Here [] shows a rate retarding effect ( n_ap = -0.70 ) and the plot of 1/ k_obs against [] was found to be linear (regression coefficient r = 0.997) with positive intercept and slope (Figure 4). Thus the hydrogensulfate dependence can be represented as follows:

                               (1)
where a, b and c are constants under the experiment conditions.


Fig. 4 Plot of 1/k_obsvs. [] at 30ºC
[Ce(IV)] = 2.0×10^-3mol·dm^-3, [CH₃OH] = 0.05mol·dm^-3,
[Ir(III)] = 4.0×10^-7mol·dm^-3, m= 1.0mol·dm^-3.

3.5 Dependence on [H⁺]
[H⁺] was varied over the range (0.2-1.0) mol·dm^-3 at fixed []([] = 1.0 mol·dm^-3 ≈[H₂SO₄]+[NaHSO₄]), [CH₃OH] and [Ir(III)], [H⁺] was calculated ignoring the dissociation of and assuming [H⁺]≈[H₂SO₄]. k_obs increases with the increase of [H⁺](TABLE 1).

Table 1 Dependence of k_obs on [H⁺]

[H+]/mol·dm-3     0.2        0.4       0.6       0.8       1.0

103 /min-1       4.51     5.62      6.84     7.83     8.76

[Ce(IV)]= 2.0×10^-3 mol·dm^-3, [Ir(III)]= 4.0×10^-7 mol·dm^-3,
[CH₃OH]= 0.05mol·dm^-3 , m= 1.0 mol·dm^-3, t = 30ºC

3.6 Product analysis and stoichiometry
The completion of the reaction was marked by the complete fading of Ce(IV) color (yellow). The reaction product as formaldehyde was detected by spot test^[9] and estimated^[10] gravimetrically as its 2, 4, dinitrophenylhydrazone derivative. It was found that the reaction is of 1:1 type. Another product Ce(III) was also detected by spot test^[11].
3.7 Acrylonitrile polymerization test
Acrylonitrile solution(40%,V/V) was added to the reaction mixture under the protection of nitrogen gas the reaction system can not initiate any polymerization of acrylonitrile indicating no free radical generated in the reaction.
3.8 Mechanism of the reaction
The kinetic data (i.e.1/k_obs vs.1/[CH₃OH]) fit well with Michaelis-Menten type model suggests that CH₃OH is involved in the reversible formation of a complex between the oxidant and the substrate, and the kinetic data (i.e.1/k_obs vs.1/ [Ir(III)]) also fit well with Michaelis-Menten type model indicates a probable way of association of Ir(III) and the complex formed in the first preequilibrium step. Ce(SO₄)₂ has been found kinetically active in this study. Thus a mechanism consistent with the found kinetic characteristics is presented as follows:
(2)
(3)
(4)
    From the mass balance relationship we have:
                  (5)
[II]_e                          (6)
    Subscripts T and e stand for total and equilibrium concentration respectively , f gives the fraction of the total cerium(IV) kinetically active.
    From step (4) of the above mechanism the rate of disappearance of [Ce (IV)]_T can be written as:
                             (7)

                               (8)

    The other two forms of Eq.(8) are as follows:
                       (9)

                                           (10)
    Eq.(8) can explain well positive fractional order dependence on both [substrate] and [catalyst]. Eq.(9) suggests that a plot of 1/k_obs vs.1/[CH₃OH] at constant [Ir(III)] and [H₂SO₄] should be linear, and was indeed evidenced in Figure 2. Eq.(10) shows that the plot of 1/k_obs vs.1/[Ir(III)] at fixed [CH₃OH] and [H₂SO₄] should also be linear at different temperatures, and the data in Figure 3 proved to be true (r ≥ 0.996)
    k_n (k_n=fk), K₁ and K₂ from the intercepts and slops of lines in Figure 3 and Figure 2 were evaluated. k_n at different temperatures and activation parameters are listed in Table 2.

Table 2 Rate constants and activation parameters

t /ºC	25	30	35	40	activation parameters (25ºC)
102 kn/ min-1	2.38	4.27	5.63	8.19	Ea*=62KJ·mol-1, DH≠=59KJ·mol-1 DS≠= -76J·K-1·>mol-1,DG≠=86KJ·mol-1

* r = 0.99, B = -7449, A = 21.32 for the linear regression of ln k_n vs.1/T

    The second equilibrium probably involves the outersphere association of the species(I) and catalyst followed by the electron transfer leading to the species(II) which may be Ce(III)·(CH₃OH)·Ir(IV), subsequently electron transfer occurs within the complex to give Ir(III) and the free radical which is rapidly oxidized by Ce(IV) at a fast step. Thus the oxidation of methanol occurs through the Ir(III)/Ir(IV) catalytic cycle.

3.9 Kinetically active Ce(IV) species and [] dependence
Under the experimental conditions in aqueous sulfuric acid medium, the important cerium(IV)-sulfato complexes are Ce(SO₄)²⁺ , Ce(SO₄)₂ and HCe(SO₄) and the relevant equilibrium^[12,13] are:
Ce⁴⁺+                                      (11)
+ Ce(SO₄)₂ + H⁺                          (12)
Ce(SO₄)₂ + HCe(SO₄)                   b₃ = 3.4                 (13)

    Among the different sulfato species, the kinetically active species can be inferred on the basis of kinetic data, not inferring it according to the magnitude of concentration. From the relation between hydrogensulfate and k_obs, Ce(SO₄)₂ has been found as the kinetically active species in the present study. The concentration of Ce(SO₄)₂ can be approximately obtained. The following Eqs. can be derived from Eqs.(11)-(13):
               (14)
    Substituting above Eqs. into Eq.(14) we get:
     (15)
    By considering the relative magnitudes of the successive formation equilibrium constants which are in the order ₁>>₂>>₃, the value of are much less than two terms later. Therefore, we get Eq.(16) from Eq.(15).

                  (16)

                                 (17)

    Substituting Eq.(17）into Eq.(8）we get:

        (18)
where  
    Eq.(19) can be derived from Eq.(18) after rearrangement:
                                  （19）
or                                            (20)
   Eq.(19) is the same as Eq.(1) and can explain well negative number order (n_{a p}= - 0.70) dependence on []. Eq.(20) suggests that 1/ k_obs vs.[] should be linear, and was evidenced in Figure 4. b₃( 6.66 ) from the ratio of slop to intercept of line in Figure 4 was estimated and is in good agreement with the previously reported value^[12,13]. All above show that Ce(SO₄)₂ is the kinetically active species indeed.
   Because of the existence of so many protondependent equilibria^[12] among the reactants, the quantitative interpretation of [H⁺] dependence is very much complicated and uncertain. Because of this complexity in the present system, no attempt was made to explain the observed [H⁺] dependence from the proposed reaction mechanism. However, the qualitative observation is in agreement with the fact that cerium(IV) oxidation reaction in aqueous sulfuric acid media are acid catalyzed. On protonation, positive charge on the cerium(IV) sulfato species increases and it facilitates the electron transfer towards cerium(IV) center.

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铱(III)离子催化铈(IV)离子氧化甲醇的反应动力学及机理
宋文玉 赵荣慧 李彦维 徐俊然
（河北大学化学与环境科学学院，保定 071002）
摘要 在酸性介质中用氧化还原滴定法研究了铈 (IV) 离子在痕量铱 (III) 离子催化作用下，于25-40℃区间氧化甲醇的反应动力学. 结果表明反应对铈 (IV) 离子为一级，对甲醇和铱 (III) 离子的表观反应级数均为正分数. 准一级速率常数（[CH₃OH]＞＞[Ce(IV)]）k_obs随 [H⁺] 增加而增大，而随 [] 增加而减小. 在氮气保护下，反应不能引发丙烯腈聚合，说明在反应中没有自由基产生.通过k_obs与 [] 的依赖关系，找到本反应体系的动力学活性物种是Ce(SO₄)₂，并计算出平衡常数，速控步骤的速率常数及相应的活化参数.
关键词 铱 (III) 离子， 铈 (IV) 离子， 甲醇， 催化剂， 动力学及机理