Kinetics of low olefins oxidation by iron(III) in Pd/ZrO₂/SO₄ precatalyst presence in system without chlorine ions

 

V.A. Matsura, V.V. Potekhin^*

 

Saint-Petersburg State Technological Institute (Technical University),

Saint-Petersburg, 198013, Moskovskiy pr. 26, Russia. This email address is being protected from spambots. You need JavaScript enabled to view it.

Tel. +7(812)2594787, fax. +7(812)1127791

 

Kinetics of ethylene, propene and 1-butene oxidation by iron(III) aquaions in Pd/ZrO₂/SO₄ precatalyst presence in water solution into corresponding  carbonyl compounds was studied under 40-80⁰C. Oxidation is promoted by dissolve palladium nanoparticles, which are formed from initial metal precatalyst under the influence of proton and acid-base properties of ZrO₂ support with SO₄^2- anion. Rate of olefins oxidation changes in C₂H₄<C₄H₈<C₃H₆ row and increases along with precatalyst mass and concentration both of perchloric acid and iron(III) and does not depend on olefin pressure. The rate of ethylene oxidation is described by similar Michaelis-Menten kinetics. In the case of 1-butene the positional isomerization of double bond with the formation of 2-buten (cis- and trans- isomer) takes place.

Keywords: palladium, iron(III) aquaions, catalysis, olefins oxidation.

 

Introduction

The liquid phase oxidation of unsaturated hydrocarbons into the corresponding carbonyl compound in palladium containing catalysts presence (Waker-processes) plays an important theoretical and practical role, that is why its investigation is very urgent nowadays [1-8].

 Earlier [9] we showed palladium nanoparticles formation (average size 5 nm.) and assumed their key role in the process of olefins oxidation by iron(III) in the presence of palladium(II) tetraaquacomplexes as well as of Pd/ZrO₂/SO₄ heterogeneous catalyst in water solution without chlorine system. Our work is devoted to kinetics of catalytic oxidation of ethylene, propene and 1-buten by iron(III) aquaions under the influence of Pd/ZrO₂/SO₄ precatalyst [reaction (1)].

 

Experimental section

 

Procedure of Pd/ZrO₂/SO₄ precatalyst preparation is described in details in work [9]. In all the cases the containing of palladium in the precatalyst was 1 wt%. The gases ethylene, propene, and 1-butene of >99% purity were purchased from Aldrich. Measured amount of Fe₂(SO₄)₃•9H₂O was dissolved in perchloric acid and then desired mass of Pd/ZrO₂/SO₄ was added. In all cases the volume of reaction solution were 10 ml. Catalytic reaction was performed under static conditions and atmosphere pressure. The reactions were carried out in a glass isothermal reactor installed in a shaker and connected to a gasometrical burette. The catalytic reaction is carried out providing the absence of diffusion limitations. The rate of reactions was calculated as relation of gaseous uptake to the time of reaction on the stage of constant rate.

The concentration of Pd(II) was determined by spectrophotometry after the addition of an excess of an SnCl₂ solution to the test sample [10]. Iron(III) was determined by spectrophotometry as a complex with sulfosalicylic acid [11].

The products of olefins oxidation were extracted from reaction solution by chlorobensol. The quantitative analysis of organic products was performed by GC/MC method (HP 5890 GCD) on 30m column HP-5 Crosslinked 5% PH Siloxane.

Runs with variations in the partial of olefins pressure were carried out using different gas mixtures of olefin and argon. In all cases the total pressure was 0.1MPa.

The products of positional isomerization of double bond in 1-buten were analysed by means of H¹ NMR method (Bruker DPX-300), by saturating a CDCl₃ solution with gas catalyst after the end of the reaction. Gas mixture of 2-buten, cis-buten-2 and trans-buten-2 was subjected to GC/MS quantitative analysis.

 

Result and discussion

 

Kinetics curves of ethylene, propene and 1-buten oxidation are shown on figure (1).

Fig. 1. The kinetics curves of olefins oxidation by Fe(III) in Pd/ZrO₂/SO₄ presence (10 mg), [Fe³⁺_aq]₀=0.03M, 65⁰C. 1 – C₂H₄, 2 – C₃H₆, 3 – 1-C₄H₈.

 

In all the cases of olefins oxidation induction period takes place. It happens due to the formation catalytic active palladium particles in reaction solution. These palladium particles were established and characterized in work [9]. Duration of induction period depends on temperature and initial ingredients concentration of reaction solution. When studying the kinetics of reaction (1) we took into account the maximum rate, which is fixed right after induction period end.

It may be seen from fig. (1) the maximum rate of olefins oxidation by iron(III) aquaions in Pd/ZrO₂/SO₄ presence increases in C₂H₄<C₄H₈<C₃H₆ row, and rates ratio is 1:1.4:3 at 65⁰C. The higher rate of propene oxidation compared to those of ethylene refers to the increase of nucleophilic properties among olefins from ethylene to propene. In relation to this we believe, that the reaction (1) proceeds with oxidated palladium species having the positive charge. In this case the rate of 1-buten oxidation among olefins should be the highest. However its not proved by experience. In case of 1-buten, oxidation proceeds along with double bond migration due to which 2-buten is formed. It was established with the help of H¹ NMR and GC/MS methods.

So, about 30% of 1-buten transforms into 2-buten, as mixture of cis- (10%) and trans- (20%) isomer,  during 70 minutes at 55⁰C, and only 3% of 1-buten is oxidized. It is known that oxidability of 2-buten is lowes then that of 1-buten [4, 5].

In our work it was established, that isomerization of 1-buten into 2-buten under the influence of sulfated zirconium oxide in acid water solution in iron(III) presence does not occur. Double bond migration takes place only in the presence of Pd/ZrO₂/SO₄ and iron(III). It is supposed, that palladium in intermediate oxidation level works as catalyst of double bond migration in olefin [6, 12]. The above said positional isomerization of C=C bond in 1-buten witnesses that reactions take place due to which palladium(0) transforms from metal catalyst into intermediate oxidated state, catalyzing the isomerization. The partial isomerization of 1-buten into 2-buten obviously explains the lower rate of oxidation in case of butene compared to those of propene.

The kinetics investigations were held mainly with ethylene.

            It may be seen from fig.2-4 that dependence of ethylene oxidation rate from precatalyst, iron(III) and perchloric acid is rather difficult. The rate of ethylene oxidation is described by similar Michaelis-Menten kinetics.

 

Fig.2 Dependence of ethylene oxidation rate in reaction (1) from precatalyst mass, [Fe³⁺_aq]₀=0.03M, [HClO₄]=0.4M, P(C₂H₄)=1 atm. 65⁰C.

                  

 

Fig.3 Dependence of ethylene oxidation rate in reaction (1) from Fe(III) concentration. 

precatalyst mass 40 mg, [HClO₄]=0.4M, P(C₂H₄)=1 atm. 65⁰C.

 

Fig. 4 Dependence of ethylene oxidation rate in reaction (1) from perchloric acid concentration. precatalyst mass 40 mg, [Fe³⁺_aq]=0.03M, P(C₂H₄)=1 atm. 65⁰C.

 

The dependence of oxidation rate on ethylene pressure was unpredictable. The rate of oxidation has zero order with respect to ethylene. It seems that ethylene oxidation step is preceded by slow step with the participation of Pd/ZrO₂/SO₄ metal palladium catalyst and iron (III).

When decreasing the initial ethylene pressure we may see evident slowing of the reaction (see fig. 5).

Fig. 5 Influence of ethylene pressure on oxidation rate. Precatalyst mass 10 mg, [Fe³⁺_aq]=0.03M, [HClO₄]=0.4M, 65⁰C. Р_ethylene, MPa. : 1 – 0.1, 2 – 0.05, 3 – 0.025, 4 – 0.015.

 

Thus, the kinetics equation of reaction (1) in the case of ethylene is as following:

where k=3.5∙10^-5 M^-1×c^-1, K^’=3.8×10^-4 M, K^’’=0.03M , K^’’’=0.1M at 65⁰С, [Pd] – the total amount of palladium mole (it corresponds to palladium mass in Pd/ZrO₂/SO₄ 1 wt%) in relation to solution volume.

Change of constant rate depending on temperature is rather difficult. The break appears on temperature curve of Arrenious dependence of ethylene oxidation. In the temperature range 40–55⁰C activation energy is (69±7) kJ/mol, and in the range 55–75⁰C – (141±14) kJ/mol. This fact can be explained by changes of the structure and energy state of active palladium centers in catalyst at temperature threshold value [13]. The higher temperature is, the more active centers appear, however their energy decreases.  In accordance with active centers theory it leads to increase of energy activation of catalytic reaction.

In case of propene and 1-butene oxidation at temperature range 55–75⁰C the observed activation energy makes (83±8) kJ/mol and (99±9) kJ/mol correspondingly.

In case there is no ethylene pressure influence on oxidation rate, it is impossible consider the reaction (1) as the heterogeneous one. Heterogeneous olefins oxidation includes adsorption of unsaturated hydrocarbon on catalyst surface and its latest oxidation by oxidant. It follows from oxidation kinetics of ethylene (see kinetics equation) that formation of carbonyl compound happens beyond the bounds of rate-determining stage. Obviously this stage turns out to be a transfer of palladium particles from the surface of solid palladium supported catalyst to solution with palladium nanoparticles formation and their latest oxidation by iron(III) aquaions.

A few articles are devoted to dissolution of metal from fine-dyspersated state in metal supported catalyst under the influence of acid protons [14-16]. The presence of hydroxonium in reaction solution due to perchloric acid provides the transfer of palladium from metal catalyst surface to solution. That corresponds to increase of oxidation rate when acid concentration increases as well (see fig. 4).

In water solution on sulfated zirconium oxide surface the acid centres of Broensted type are formed. The interaction between palladium metal particles and Broensted acid sites in sulfated zirconia forms metal–proton adduct of [Pd–H]^d+ type. In such an adduct electron transfers from the palladium particle to the proton. Therefore, whereas the metal becomes “less metallic”, palladium becomes partially positive .  To put it more simple it can be presented as following:

 

Earlier we established that palladium species in intermediate oxidation state could be oxidised by iron(III) aquaions [18, 19]. As known, the size of the particles determines its reductive-oxidative properties [20]. It seems that palladium nanoparticles show in adduct reductive properties sufficient for their oxidation by iron(III) aquaions. In work [21] it is presented some data concerning the partial oxidation of metal palladium atoms in palladium cluster by iron(III) aquaions with the formation of palladium particles in intermediate oxidation state. That is why in our case the interaction between palladium adduct and iron(III) aquaions  leads to the formation of palladium particles in oxidation state.

 

 

Further on palladium active species [Pd_(n-2)Pd₂²⁺]_aq fast olefins oxidation to corresponding carbonyl compound proceeds via the stage of formation of palladium-vinyl intermediate compound or palladium p-allylic complex in case of propene and 1-butene.  

 

After reaction (4) or (4.1) catalytic cycle repeats.

Oxidation of palladium nanoparticles by iron(III) aquaions depends on concentration of iron(II). Addition of iron(II) aquaions into start reaction (1) results in increase of induction period (see fig. 6) and decrease of olefins oxidation rate in general. It is necessary to mention that oxidation rate decreases directly proportional to initial iron(II) concentration.

Fig. 6. Iron(II) influence on ethylene oxidation in reaction (1). Precatalyst mass 40 mg, [Fe³⁺_aq]₀=0.03M, [HClO₄]=0.4M, 73⁰C.

[Fe²⁺_aq]₀, M: 1 – 0, 2 – 0.03, 3 – 0.06.

 

Obviously, iron(II) influence is determined by reversible reaction between palladium nanoparticles and iron(III) aquaions. The reversibility in reaction (2) is also proved by results of the influence of iron(III) initial concentration on ethylene oxidation rate. The slowing down of reaction (1) always takes place at a certain ratio of iron(II)/iron(II) concentration. Thus, at 65⁰C the ratio Fe(II)/Fe(III) is approximately 1:2 and does not depend on initial iron(III) concentration. 

Iron(II) aquaions which are formed during olefins oxidation act as a competitor agent for unsaturated hydrocarbon in interaction with active palladium species. It becomes evident when olefins pressure lows (see fig. 5). According to it the conversion degree of iron(III) usually does not achieve 100%. The higher reaction temperature is, the more iron(III) conversion degree increases. For example, during ethylene oxidation at 50⁰C the iron(III) conversion degree makes 47%, at 75⁰C – 90%.

It is established [22, 23] that formation of palladium p-allylic complexes at iron(III) aquaions presence dominates over oxidation of olefins into corresponding carbonyl compounds during the interaction between olefin and palladium(II) tetraaquacomplexes. That is why the formation of palladium p-allylic complexes on active palladium sites in the case of propene as well as 1-buten may cause the decrease of oxidation rate. As seen on fig. 1 in the case of propene and 1-butene sharp slowdown of reaction, compared to that of ethylene, is observed.

Basing on proposed scheme of olefins oxidation we evaluated total palladium concentration in solution during catalytic ethylene oxidation using kinetics method. For this purpose we considered ethylene oxidation by VO₂⁺ cation under the same conditions to compare with Fe(III) oxidation. Preliminary it was established that VO₂⁺ cation fully oxidised palladium(0) to palladium(II) under reaction with Pd/ZrO₂/SO₄ catalyst. That is why ethylene oxidation by VO₂⁺ cation in the presence of  Pd/ZrO₂/SO₄ can be treated as catalytic reaction with respect to palladium(II). Thus, when adding VO₂⁺ cation (0.06M) to Pd/ZrO₂/SO₄ 1 wt% catalyst (1 mg), which correlates with total palladium concentration 10^-5M, ethylene oxidation rate was 7.4×10^-6 M^-1×c^-1 at 65⁰C. The same rate of ethylene oxidation by Fe(III) can be achieved at catalyst mass Pd/ZrO₂/SO₄ 1 wt% 40 mg. and  initial Fe(III) concentration 0.06M. That means that approximately 2.5% of palladium is active in the process of ethylene oxidation by iron(III) aquaions in Pd/ZrO₂/SO₄ metal precatalyst presence.

Thus, formation of palladium nanoparticles from Pd/ZrO₂/SO₄ 1 wt% precatalyst under the influence of acid and acid-base property of sulfated zirconium oxide provides catalytic olefins oxidation by iron(III) aquaions similar to Waker-process in water solution, but without chlorine ions.

 

 

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