Journal of Chemistry
Volume 2019 (2019), Article ID 9521529, 11 pages
https://doi.org/10.1155/2019/9521529

Research Article

Direct Benzyl Alcohol and Benzaldehyde Synthesis from Toluene over Keggin-Type Polyoxometalates Catalysts: Kinetic and Mechanistic Studies

Allam Djaouida, Mansouri Sadia, and Hocine Smaïn

Laboratoire de Génie Chimique et Chimie Appliquée Hasnaoua I, Université de Mouloud Mammeri, BP. 17 RP 15000 Tizi-Ouzou, Algeria

Correspondence should be addressed to Hocine Smaïn; shocine_univ_to@yahoo.fr

Received 8 September 2018; Accepted 12 November 2018; Published 13 January 2019

Academic Editor: Fateme Razeai

Copyright © 2019 Allam Djaouida et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The catalytic activity of various Keggin polyoxometalate catalysts has been investigated in the gas-phase partial oxidation of toluene to produce benzyl alcohol and benzaldehyde. The catalyst systems HPMo₁₂O₄₀, HPMo₁₁VO₄₀, FePMo₁₂O₄₀, and PMo₁₁FeO₃₉ were prepared and characterized by FT-IR, UV-visible, SEM, XRD, TGA, and cyclic voltammetry. The acid/base properties were evaluated using the decomposition of isopropanol. Catalytic studies were carried under atmospheric pressure and over the temperature range 200°C–350°C, using carbon dioxide as a mild oxidant. Toluene conversion and product distribution depend mainly on the catalyst composition and operating conditions. In addition to benzaldehyde, benzyl alcohol is obtained with a high selectivity on the PMo₁₁FeO₃₉ catalyst. The kinetic data show that the reoxidation of the reduced catalyst is the rate-limiting step for the partial oxidation reaction of toluene.

1. Introduction

Aromatic aldehydes such as benzaldehyde are very valuable industrial organic intermediates, and their uses include the manufacture of flavors, fragrances, pharmaceutical precursors, and plastic additives. Industrially, benzaldehyde is produced exclusively by the liquid phase oxidation of toluene [1, 2]. Usually, benzaldehyde is made by a process in which toluene is treated with chlorine to form benzal chloride, followed by treatment of benzal chloride with water. This process suffers from several problems, such as the formation of undesirable products, equipment corrosion with chlorine, and complex products’ separation. An alternative is the gas-phase oxidation of toluene. However, toluene conversion and product distribution largely depended on catalyst composition and operating conditions. Vanadium oxide-based materials are the most commonly used catalysts in this process. Since the discovery of the effectiveness of vanadium pentoxide as catalyst in sulfur dioxide oxidation by Haen [3], in 1900, a considerable number of studies have been carried in homogeneous and heterogeneous catalysis, such as the conversion of alkanes to the corresponding alkenes [4–7] and the conversion of alcohols to aldehydes or ketones [8–11]. Supported vanadium oxide catalysts are also used in gas-phase catalytic partial oxidation of toluene [12–14]. The supported vanadium-based catalysts were reported to show high benzaldehyde selectivity, but the toluene conversion was below 10% at the low temperature (<400°C). However, at higher temperature (>400°C), the toluene conversion increased and the selectivity to benzaldehyde decreased and those to carbon oxides increased. Other workers have reported that pure and mixed molybdenum oxides are very efficient in gas-phase partial oxidation of toluene [15]. The main reaction products were maleic anhydride, benzaldehyde, and carbon oxides [16]. The investigation of the influence of various metal oxides on the behaviour of molybdenum oxide towards the oxidation of toluene with molecular oxygen concluded that toluene oxidation decreases in the following order: support oxides ≥ molybdenum oxide monolayer catalysts > molybdate salts > crystalline molybdenum oxide, but the benzaldehyde and benzoic acid selectivities follow the opposite trend [15]. Although there is a large number of catalysts described in the literature and in the patent literature composed of the vanadium oxides systems modified by various promoters, supported on carriers Al2O₃, SiO₂, TiO₂, and transition metal oxides and favorable for the catalytic gas-phase toluene oxidation to benzaldehyde [17–24], but the toluene conversion is very poor at low temperature ( less than 10%). This catalyst system favors the formation of benzoic acid and degradation of carbon dioxide and benzaldehyde selectivity is not very satisfactory. Therefore, a suitable catalyst for the selective production of benzaldehyde from toluene is strongly desired. More attention has been given to the application of the polyoxometalates having Keggin structure, as catalysts for the oxydehydrogenation of alkanes to alkenes [24–27]. Keggin-type polyoxometalates have been widely used as catalysts for one-step conversion of lower alkanes into value-added oxygenated products, such as partial oxidation of methane to methanol [28], oxidation of isobutane to methacrylic acid (MAA) [29], and partial oxidation of propane to acrylic acid (AA) [30]. Molecular oxygen was first proposed to be the oxidizing agent. However, it appeared that use of O₂ (strong oxidant) leads to complete oxidation of hydrocarbons to COx. As an alternative to the strong oxidant O₂, carbon dioxide, the main contributor to the greenhouse effect, was proposed as nontraditional mild oxidant for catalytic oxidation of light alkanes [31–33]. CO₂ utilization as a mild oxidant and an oxygen transfer agent is attracting considerable attention and considered as another route for increasing selectivity and to avoid total oxidation [34]. This approach is expected to open new technology for CO₂ utilization.

In this paper, heteropolyacids H₃PMo₁₂O₄₀ (HPMo₁₂) and H₄PMo₁₁VO₄₀ (HPMo₁₁V) and heteropolysalts (NH₄)2.5Fe0.1H0.2PMo₁₂O₄₀ (FePMo₁₂) and (NH₄)₄PMo₁₁(H₂O)FeO₃₉ (PMo₁₁Fe) were prepared, characterized by different methods (FT-IR, UV-visible, MEB, XRD, TGA, and cyclic voltammetry), and the acid/base properties were evaluated using the isopropanol decomposition. The catalytic performance of the catalysts was investigated in partial oxidation of toluene in the temperature range 200°C–350°C at atmospheric pressure using carbon dioxide as oxidant. The reaction mechanism and kinetics are discussed. The kinetic study was carried over PMo₁₁Fe catalyst at 200°C.

2. Experimental

2.1. Catalyst Preparation

Pure heteropolyacids H₃PMo₁₂O₄₀ and H₄PMo₁₁VO₄₀ samples were prepared in a classical way [35]. The ammonium-iron salt, (NH₄)2.5Fe0.1H0.2PMo₁₂O₄₀, was prepared by adding (NH₄)₂CO₃ solution to an aqueous solution of mixture of H₃PMo₁₂O₄₀ and Fe(NO₃)₃. The precipitate was dried at 50°C under vacuum for 5 h. (NH₄)₄PMo₁₁FeO₃₉ salt was prepared by the method described in the literature [36]. The paramolybdate of ammonium was dissolved at 50°C. The mixture of H₃PO₄, HNO₃, and Fe(NO₃)₃ was introduced slowly in the aqueous solution of ammonium paramolybdate at 0°C. Salt of ammonium was precipitated immediately by ammonium nitrates (NH₄NO₃).

2.2. Characterization

Fourier transform infrared spectra (FT-IR) of the heteropolycompounds were obtained in the 400–4000 cm⁻¹ wavenumber range using a FTIR-8400 Shimadzu spectrometer. UV-vis spectra were recorded on a UV-vis scanning spectrophotometer (Shimadzu UV-2100PC). A 2.5 mM reaction solution of each catalyst was sampled, and the solution was diluted with 400 ml acetonitrile (0.5 mM). Then, the UV-vis spectrum of the solution was measured at 25°C.

The X-ray diffraction (XRD) patterns were obtained on a Siemens “D5000” diffractometer with Cu Kα radiation. The diffractograms were registered at Bragg angle (2θ)  =  5–45° at a scan rate of 5°/min. The thermal decomposition of heteropolycomponds was studied using a Setaram TG-DTA 92. 20 mg of powder was placed in the sample holder, and the temperature was elevated at 5°C/min in flowing synthetic air.

Isopropanol decomposition and cyclic voltammetry are used to know the information about the redox properties of the hetepolyanions.

The isopropanol decomposition was carried out at atmospheric pressure, in a conventional flow fixed-bed reactor, using 0.2 g of catalyst. Nitrogen was the carrier gas with a flow rate of 50 mL/min. The reaction products were quantified by gas chromatography, using an FID and TC detector.

2.3. Catalytic Tests

Catalytic tests were carried out at atmospheric pressure in a continuous flow fixed-bed tubular glass reactor with 1 g of catalyst pretreated in nitrogen (2 L/h) for 1 h at the reaction temperature. The carbon dioxide stream (2 L/h) was saturated with toluene (50 Torr). The oxydehydrogenation (ODH) of toluene was carried out at a relatively lower temperature, 250°C–350°C, in which the carbon formation is thermodynamically unflavored. The reaction mixture (toluene, benzaldehyde, benzene, benzyl alcohol, methane, and CO) was analyzed by a gas chromatograph (DTC and FID CG, Shimadzu 14B) equipped with columns of Molecular Sieve 5A and Porapak QS.

3. Results and Discussion

3.1. Results of Characterization

3.1.1. FT-IR Analysis

IR spectra of all compounds are given in Figure 1. The characteristic peaks of Keggin unit were observed in the region 1100–500 cm⁻¹. According to literature data [37], the origin of the Keggin anion vibration bands, appearing at 1065, 961, 864, and 789 cm⁻¹, is attributed to P-Oa, Mo=Ot, interoctahedral Mo-Ob-Mo, and intraoctahedral Mo-Oc-Mo bands, respectively. The FT-IR spectra presented in Figure 1 showed splitting of P-Oa band of value equal to 40 cm⁻¹ for PMo₁₁V and 24 cm⁻¹ for PMo₁₁Fe; the result showed clearly that vanadium and iron ions were introduced into octahedral position. There is no apparent difference between the PMo₁₂ and FePMo₁₂ spectra; we can note from the results that the primary Keggin structure remains unaltered even when the protons form parental heteropolyacid are substituted by the ammonium and iron cations. The absorption band around 1600 cm⁻¹ is indicative of the presence of oxonium ions (H₃O⁺) or more likely to dioxonium ions (H₅O₂⁺). The band at 1420 cm⁻¹ was assigned to N–H stretching.

Figure 1: FTIR spectra of (a) HPMo₁₂; (b) FePMo₁₂; (c) HPMo₁₁V; (d) PMo₁₁Fe.

3.1.2. UV-Visible Analysis

In the near UV, the electronic spectra of Keggin-type polyoxometalates show two absorption bands around 220 nm and 300 nm (Figure 2). These bands attributed to metal oxygen charge transfer. According to the literature [37], these bands are assigned, respectively, to the vibrations of Mo-Ot bonds and Mo-Ob/Oc.

Figure 2: UV-visible absorption spectra of (a) PMo₁₁Fe; (b)FePMo₁₂; (c) HPMo₁₂; (d) HPMo₁₁V.

3.1.3. XRD Analysis

As shown in Figure 3, the XRD patterns of HPMo₁₂O₄₀ and HPMo₁₁VO₄₀ were substantially similar and have a Keggin-type structure. The acidic HPMo₁₂O₄₀ and HPMo₁₁VO₄₀ crystallize in the triclinic system, with a = 14.10 Å, b = 14.13 Å, c = 13.39 Å, a = 112.1°, b = 109.8°, = 60.73° and a =14.04 Å, b = 10.006 Å, c =13.55 Å and a = 112°, b = 109.58°, = 60.72°, respectively.

Figure 3: XRD patterns of (a) HPMo₁₂; (b) HPMo₁₁V; (c) PMo₁₁Fe; (d) FePMo₁₂.

The diffraction patterns of FePMo₁₂ shown in Figure 3 indicate for the compound the existence of a single crystallographic phase with patterns typical of a cubic phase (intense peak corresponding to [222] plane) (space group I m3) with a = b = c = 11.6812, Z = 2.

X-ray analysis of. (NH₄)₄PMo₁₁Fe(H₂O)O₃₉·xH₂O showed that it crystallizes in the monoclinic system, space group P 2₁/c, with, a = 20,07210 Å, b = 11,55938 Å, c = 18,77828 Å, β = 95,664 °, and Z = 2.

3.1.4. SEM Analysis

The scanning electron micrographs of heteropolycompound catalysts are shown in Figure 4. These images show that the morphology of the particles depends on the heteropolycompound composition and content. The principle structure of FePMo₁₂ and PMo₁₁Fe was the same and consisted of rosette‐shape plate‐like crystals. The particles of HPMo₁₂ differ from that of HPMo₁₁V in morphology. The samples of both catalysts contain particles with nonuniform size and shape.

(a)

(b)

(c)

(d)

Figure 4: Scanning electron micrographs of (a) HPMo₁₂; (b) FePMo₁₂; (c) HPMo₁₁V; (d) PMo₁₁Fe.

3.1.5. Voltamperometric Analysis

The redox properties of heteropolyanions have been studied by cyclic voltammetry. Keggin anions are able to accept reversibly up to six electrons on the vanadium, molybdenum, or tungsten atoms and thus are potential oxidants. Figure 5(a) shows a cyclic voltammogram of 0.50 mM [PMo₁₂O₄₀]³⁻ anion in acetonetrile/water (ratio 1/1 by volume), using a glassy carbon rotating electrode. Three-step, one-electron redox wave was obtained with midpoint potentials, Emid of −0.75, −0.4, and −0.05 V assigned to the successive reduction molybdenum centers, where Emid = (Epc−Epa)/2; Epc and Epa are the cathodic and the anodic peak potentials, respectively. The same result is observed with iron-substituted heteropolymolybdate (FePMo₁₂).

(a)

(b)

(c)

(d)

Figure 5: Voltamperograms of (a) HPMo₁₂; (b) FePMo₁₂; (c) HPMo₁₁V; (d) PMo₁₁Fe.

Cyclic Voltammetry studies of HPMo₁₁V and PM₁₁Fe were particularly informative. Figures 5(c) and 5(d) show typical cyclic voltammograms for the anions (HPMo₁₁VO₄₀)⁴⁻ and (PMo₁₁FeO₃₉)⁷⁻. The number of redox processes identified and the values of the peak potentials are quite similar. It consists of a reversible one-electron system at −0.11 and −0.7 V for the HPMo₁₁V and for PMo₁₁Fe, respectively, assigned to the vanadium (V/IV), iron (III/II), and molybdenum (VI/V, V/VI) couples, respectively, and followed by three reversible redox peaks at −0.90, −1.61, and −1.8 V for the PMo₁₁V and at −0.85, −1.68, and −1.76 for the PMo₁₁Fe. The redox properties of Keggin-type heteropolycompounds can be varied over a wide range by altering the chemical composition. In fact, the electron-accepting properties of Keggin anion are important for oxidative catalysis when electron transfer mechanisms are predominant. In other case, it is the ability of some oxygen atoms of the clusters or the possibility of being involved in complexation reactions that make them good catalysts for oxidation of hydrocarbons.

3.1.6. Thermogravimetric Analysis

The TGA profile for the thermal decomposition of the polyoxometalates is shown in Figure 6. The temperature at which their degradation starts depends on the protonation level and the composition of the polyoxometalates. It can be observed that the weight loss occurred in two steps in case of HPMo₁₂, HPMo₁₁V, and FePMo₁₂ samples and three steps in that of PMo₁₁Fe compounds. The first loss at low temperatures (<120°C) corresponds to the liberation of physisorbed water (12–13 H₂O), and the second weight loss, between 120°C and 380°C results from the elimination of the three or four acid protons associated with 1.5 or 2 oxygen atoms of the POMs, leading to formation of an anhydride [38].

Figure 6: The TGA profiles of (a) HPMo₁₂; (b) HPMo₁₁V; (c) FePMo₁₁; (d) PMo₁₁Fe.

Their end product of thermal decomposition is the mixture of MoO₃, P2O₅, and V₂O₅ (Scheme (1)), confirmed by X-ray diffraction and IR spectroscopy.

Scheme 1: Thermal decomposition process of HPMo₁₂ and HPMo₁₁V.

PMo₁₁Fe exhibits high stability (Figure 6). It is known that the decomposition of (NH₄)₄PMo₁₁FeO₃₉ begins even at 175°C and involves three stages in the process of transformation of (NH₄)₄PMo₁₁FeO₃₉ into mixture oxides P₂O₅, MoO₃, and Fe₂O₃ (Scheme 2), located at approximately 150°C, 280°C, and 450°C. The first peak is associated with removal of water from the polyoxometalate. The main decomposition peak at 250°C corresponds to the loss of constitution water. The third peak may be related to the decomposition of ammonium cations to ammonia and can be explained as shown.

Scheme 2: Thermal decomposition process of PMo₁₁Fe.

3.1.7. Acidic Properties of Polyoxometalates

The gas-phase decomposition of isopropanol is an important probe reaction, as its product selectivity depends on the surface concentration of redox, basic, and acidic sites [39]. Redox sites or strong basic sites produce propanone by dehydrogenation reaction (a), whereas acidic sites give rise to propene and di-isopropylether, respectively [40], by dehydration reaction (b) (Figure 7).

Figure 7: Isopropanol decomposition on acid and redox sites.

The results obtained from the isopropanol decomposition using POMs as catalysts as a function of reaction temperature are presented in Table 1.

Table 1: Catalytic activity of catalysts at different temperatures in isopropanol decomposition.

Catalysts
T (°C)
Conversion (%)
Products selectivities (%)

Propylene
Di-isopropylether
Acetone

H₃PMo₁₂O₄₀
100
17
60
30
10

150
60
80
13
7

200
90
92
4
4

H₄PMo₁₁VO₄₀
100
19
56
16
28

150
65
84
6
10

200
97
90
2
8

FePMo₁₂O₄₀
100
10
72
20
8

150
80
90
9
1

200
95
98
1
1

PMo₁₁FeO₃₉ 
100
5
15
5
80

150
32
35
3
62

200
45
69
1
30

It is clear that HPMo₁₁V and HPMo₁₂, are the most active catalysts. The high activity can be correlated with increasing the surface acidity associated with the presence of protons in Keggin structure. Propene is the major reaction product. However, it is also observed that, when the protons in the parent acids (H₃PMo₁₂O₄₀) are partially exchanged by iron and ammonium cations, [(NH₄)2.5Fe0.1H0.2PMo₁₂O₄₀] improves the catalytic activity (Table 1). This can be attributed to the partial hydrolysis of the polyanion during preparation, which formed weakly acidic H⁺. On the contrary, when the iron is in anionic position (PMo₁₁Fe), the initial activity decreased, resulting in conversion relatively lower than the iron in cationic position (FePMo₁₂). This decrease attributed to the decrease in the number and the strength of the acid sites of the catalyst. The presence of redox species MoVI/MoIV, VV/VIV, and FeIII/FeII in the catalysts can contribute to dehydration and dehydrogenation in the decomposition of isopropanol.

3.2. Catalytic Test

3.2.1. Catalytic Activity

The effect of reaction temperature for the partial oxidation of toluene over polyoxometalates was estimated at 250°C, 300°C, and 350°C. The toluene conversion and the product selectivity in a CO₂ atmosphere at time on stream of 8 h are shown in Table 2. The toluene conversion increased with increasing temperature for all catalysts. However, high temperatures also enhance an undesired parallel process, dealkylation reaction (benzene and methane) that reduces the selectivity to the desired product (benzyl alcohol or benzaldehyde).

Table 2: Effect of reaction temperature on the partial oxidation of toluene with CO₂ over polyoxometalates.

Catalysts
T (°C)
Conversion (%)
Products’ selectivity (%)

C₆H₆
C₆H₅–CH₂OH
C₆H₅–CHO

H₃PMo₁₂O₄₀
250
7
37
—
63

300
14.5
51
—
49

350
52.3
64
—
36

H₄PMo₁₁VO₄₀
250
6.1
12
8
80

300
16.4
18
10
72

350
54.7
31
2
67

PMo₁₁Fe
200
5
30
65
5

250
26
25
50
25

300
44.4
20
32
48

350
75.4
19
15
66

FePMo₁₂
250
14
39
21
40

300
31.0
51
19
30

350
55.1
61
18
21

In the reaction with the H₃PMo₁₂O₄₀ catalyst, the toluene conversion increased from 7 to 52.3% with an increase in the reaction temperature from 250°C to 350°C. The benzaldehyde selectivity decreased from 63% at 250°C to 36.0% at 350°C, that of benzene (dealkylation product) increased from 37% to 64%. Hence, higher temperatures cause higher dealkylation of toluene and formation of lower amounts of benzaldehyde. At all temperatures, HPMo₁₁V displayed very similar conversion than HPMo₁₂ but is more selective towards benzaldehyde (80–67% selectivity) benzyl alcohol was also observed as a minor product. These results are in accord with those obtained by other authors which have demonstrated that the use of vanadium-containing polyoxometalates improved catalytic performances (conversion and selectivity) as in the case of isobutane oxidation [29, 41]. On the other hand, it was reported that both V⁵⁺ and V⁴⁺ species were likely the active sites in the selective oxidation of toluene to benzaldehyde [42]. In the case of H₄PMo₁₁VO₄₀,, the reduction of V⁵⁺ to V⁴⁺ is very quick leading to stability of benzaldehyde selectivity from the beginning of the reaction. This agrees with the obtained results by Cavani et al., in oxidative isobutane [43]; the presence of vanadium will accelerate the reduction of compound suggesting that V⁴⁺ species act as reducing species towards Mo⁶⁺ leading to Mo⁵⁺ and V⁵⁺ species. In the reaction with the FePMo₁₂ catalyst (Table 2), toluene conversion is similar to that of the HPMo₁₂ catalyst observed. The benzaldehyde selectivity at the reaction temperature of 250°C is below 40%, which has been decreased to approximately 30% and 21% with an increase in the reaction temperature from 250°C to 300°C and 350°C, respectively. However, the benzyl alcohol selectivity remained constant in the range between 250°C and 350°C. Higher benzene selectivity is observed at all reaction temperatures. The obtained results indicated that the selectivity of benzene increases with increasing temperature; thus, FePMo₁₂ surface has strong acidic sites, which is evident from the higher dealkylation process than dehydrogenation; these results agree with those observed in dehydration of isopropanol to propene. The same products (benzyl alcohol, benzaldehyde, and benzene) are obtained with PMo₁₁Fe, and the results are listed in Table 2. Generally, the conversion of toluene into oxidized products was much more efficient than for the catalysts listed in Table 2. With PMo₁₁Fe, conversion of toluene is more important and becomes highly selective to the formation of benzyl alcohol at low temperatures. The catalyst also gave benzene but only as a minor product, benzaldehyde being the major one. This is suggested that iron in anionic position decreases the acidity and accelerates more the redox process. The results for PMo₁₁Fe show that the conversion of toluene to oxygenate products (benzyl alcohol + benzaldehyde) is higher than the dealkylation process (benzene) indicating the redox of PMo₁₁Fe surface. The easy redox cycle between oxidized and reduced iron species (F³⁺/Fe²⁺) generates a highly active catalyst.

3.2.2. Reaction Kinetics

A kinetic study of toluene partial oxidation was performed at 200°C over PMo₁₁Fe, with a total flow rate of 50 ml/min and catalyst weight of 200 mg. The reaction rate of toluene oxidation by carbon dioxide was given by

where and are the partial order of toluene and carbon dioxide.

The value of these orders can then be obtained by linearizing this equation and by plotting the corresponding logarithmic curves:

In most cases, a constant carbon dioxide pressure of 50 Torr was used in the first set of experiments as the pressure of the toluene was varied from 2.5 to 25 Torr (Figure 8(a)), and the Ph-CH3 pressure was fixed at 5 Torr while the CO₂ pressure was changed from 50 to 100 Torr in the second (Figure 8(b)).

(a)

(b)

Figure 8: Reaction order of partial oxidation of toluene by carbon dioxide.

The results indicated a reaction of zero order with respect to carbon dioxide and first order with respect to toluene.

These orders suggested the reaction was of the Mars and van Krevelen type, with the rate-determining step being the reduction of the catalyst surface by the hydrocarbon [44].

According to the Mars and van Krevelen model, the reaction of toluene partial oxidation occurs in two steps: the oxidation of toluene by the oxidized catalyst (3) and the reoxidation of reduced catalyst (4).

In these equations SO, S, k (red), and k (ox), respectively, represent the oxidized and reduced sites of the catalysts, the rate constants of the reduction, and oxidation stages of the catalyst. The rate of steps (3) and (4) would then be

In steady state, these rates are equal: r = r (red) = r (ox).

Moreover, the material balance corresponding to the reaction sites is [SO] + [S] = [S]₀; [SO], [S], and [S]₀ are, respectively, the concentrations of the oxidized, reduced, and total sites. The resolution of the system of equations above makes it possible to obtain the expression of the rate of reaction in steady state:

If [S]₀ = 1:

If the reduction step is limiting, it is possible to write k(ox) >>k(red) PCO2.

Hence, r ≅ k(red) Ptoluene = k Ptoluene,where k is the rate constant of the global process. The kinetics is indeed of the first order with respect to the toluene.

Moreover, the values of the rate constants of the reduction and reoxidation steps were determined graphically by linearizing the equation of the overall rate:

The plotting of the corresponding curves at constant pressure of toluene or carbon dioxide leads to the following values: k(red) = 4,1 10⁻⁴ mol·min⁻¹·g⁻¹Torr⁻¹ and k(ox) = 0,50 mol·min⁻¹·g⁻¹Torr⁻¹. This conclusion was in good agreement with the decomposition of isopropanol (Table 2) and cyclic voltammetry studies showing the redox properties of the PMo₁₁Fe catalyst. Since k(red) << k(ox), the limiting step of the reaction is the reoxidation of the catalyst as found by Van Der Wiele et al. [21]. In initial kinetic conditions, only benzyl alcohol and benzaldehyde as oxygenate products were detected. Besides, when the contact time tended to zero, the C₆H₅–CH₃OH/C₆H₅–CHO (γ) or C₆H₅–CHO/C₆H₅–CH₃OH (1/γ) ratios tended towards zero or an infinite value, respectively (Figure 9). These trends suggested that benzyl alcohol was an intermediate in benzaldehyde formation, as shown by the following reaction pathway:

Figure 9: Effect of the contact time on the relative selectivities in the partial oxidation of toluene at 200°C over PMo₁₁Fe.

The same trend was observed during the study of the effect of the reaction temperature on the product selectivities.

Figure 10 shows that selectivity of benzyl alcohol decreases as the temperature increases, while benzaldehyde selectivity increases with reaction temperature. This result confirmed that benzyl alcohol was a precursor in benzaldehyde formation.

Figure 10: Effect of the reaction temperature on the selectivities in the partial oxidation of toluene over PMo₁₁Fe.

Activation energy of the catalytic oxidation of toluene to benzaldehyde, using CO₂ as oxidant, was determined from the plot of lnk against 1/T (Figure 11). The activation energy was calculated using the following equation: slope = −Ea/R, where Ea is activation energy, R is the gas constant. The value of activation energy was found to be 62.639 KJ/mol.

Figure 11: Arrhenius plot for determination of activation energy.

3.2.3. Mechanism and Pathway of Toluene Partial Oxidation by CO₂

It has been well recognized that the selective oxidation of toluene proceeded via a van Krevelen redox mechanism [44], but the pathways and product distribution depended on the nature of catalysts and the reaction conditions. For toluene partial oxidation to benzyl alcohol and benzaldehyde using carbon dioxide as oxidant, the most strongly supported mechanism consists of a consecutive conversion scheme C₆H₅–CH₃ ⟶ C₆H₅–CH₂OH ⟶ C₆H₅–CHO. This indicates that benzyl alcohol can be formed as a precursor of benzaldehyde in the catalytic oxidation of toluene over a transition metallic oxide catalyst. According to the widely accepted mechanism of partial oxidation of toluene by CO₂, the breaking of the C–H bond in the toluene is the rate-determining step [45]. In the case of the catalytic partial oxidation of toluene by CO₂ over polyoxometalate catalysts, the formation of benzaldehyde and benzyl alcohol during the toluene adsorption suggests that toluene is activated by abstracting a hydrogen atom from the methyl group. This is followed by the bonding of the lattice oxygen nearby over the M=O centers with the primary carbon of benzyl species leading to the formation of benzyloxide species (a). This is in agreement with the work of Busca et al. [46]. They studied by IR spectroscopy the interaction of light hydrocarbons over the MgCr₂O₄. It has been concluded that every hydrocarbon reacts at its weakest C–H bond giving rise to the corresponding alkoxide species. In the first step of the mechanism, C₆H₅–CH₃ adsorbed on the M=O centers to form an benzyloxide species (C₆H₅–CH₂–O-M-H) (a) (Step 1). Benzyl alcohol (I) is then formed via the transfer of a hydrogen atom from the hybrid group to the benzyloxide (Step 2). Surface benzyloxide groups decompose to the adsorbed C₆H₅–CHO; desorption of benzaldehyde (II) and dihydrogen lead to the final products and a reduced catalyst surface (Step 3). The pathway for the oxidation of toluene over polyoxometalates is shown in the reaction scheme Figure 12.

Figure 12: Proposed mechanism for the partial oxidation of toluene on M=O centers.

Reoxidation of the catalyst by CO₂ restores the lattice oxygen atom of the M metal, then the M=O centers are reestablished for the next turnover cycle. This sequence is represented by (Figure 13)

Figure 13

Toluene, when passed over different heteropolycompound catalysts at 250°C–400°C, has been found to yield benzyl alcohol and benzaldehyde in addition to the hydrodealkylation [47] products benzene (III) and methane (IV). Here, the toluene reacts with the adsorbed hydrogen to produce benzene and methane as shown in Figure 14.

Figure 14: Toluene hydrodealkylation.

4. Conclusion

Both IR spectroscopy and X-ray diffraction indicate for FePMo₁₂ and PMo₁₁Fe salts, the presence of a single crystalline phase, and IR spectra typical of the Keggin anion; TGA shows that the salts containing iron are more stable than the pure acidic H₃PMo₁₂O₄₀ and H₄PMo₁₁VO₄₀. These results show also that the vanadium and iron play a very important role in benzaldehyde selectivity and indicate clearly that the substitution of protons by NH₄⁺ and Fe³⁺ cations or Mo⁶⁺ by V⁵⁺ and Fe³⁺ induced important changes in the acid-base and oxidoreduction properties of phosphomolybdate heteropolyanions of Keggin structure. In this study, PMo₁₁V and PMo₁₁Fe catalyzed the toluene partial oxidation with carbon dioxide as an oxidant to the corresponding oxygenate compounds efficiently and selectively.

The partial oxidation of toluene selectively produced oxygenated compounds besides benzene. The reaction orders were 1 and 0 in toluene and carbon dioxide, respectively. The overall process seemed to obey a redox mechanism of Mars and van Krevelen type, in which the reduction step would be rate determining. Benzyl alcohol appeared as the reaction intermediate in benzaldehyde formation.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Supplementary Materials

The supplementary materials for this work contain the detailed method of preparation of pure acids H₃PMo₁₂O₄₀ and H₄PMo₁₁VO₄₀ and heteropolysalts (NH₄)2.5Fe0.08H0.26PMo₁₂O₄₀ and (NH₄)₄PMo₁₁(H₂O)FeO₃₉ with iron outside and inside Keggin structure, respectively. The supplementary materials include the expressions of toluene conversion and products’ selectivities. (Supplementary Materials)

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