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

-e catalytic activity of various Keggin polyoxometalate catalysts has been investigated in the gas-phase partial oxidation of toluene to produce benzyl alcohol and benzaldehyde. -e catalyst systems HPMo12O40, HPMo11VO40, FePMo12O40, and PMo11FeO39 were prepared and characterized by FT-IR, UV-visible, SEM, XRD, TGA, and cyclic voltammetry. -e 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 dependmainly on the catalyst composition and operating conditions. In addition to benzaldehyde, benzyl alcohol is obtained with a high selectivity on the PMo11FeO39 catalyst.-e kinetic data show that the reoxidation of the reduced catalyst is the rate-limiting step for the partial oxidation reaction of toluene.


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. is 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][5][6][7] and the conversion of alcohols to aldehydes or ketones [8][9][10][11].Supported vanadium oxide catalysts are also used in gas-phase catalytic partial oxidation of toluene [12][13][14].
e 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].e main reaction products were maleic anhydride, benzaldehyde, and carbon oxides [16].e 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 Al2O 3 , SiO 2 , TiO 2 , and transition metal oxides and favorable for the catalytic gas-phase toluene oxidation to benzaldehyde [17][18][19][20][21][22][23][24], but the toluene conversion is very poor at low temperature ( less than 10%). is catalyst system favors the formation of benzoic acid and degradation of carbon dioxide and benzaldehyde selectivity is not very satisfactory.
erefore, 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][25][26][27].Keggintype 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 O 2 (strong oxidant) leads to complete oxidation of hydrocarbons to COx.As an alternative to the strong oxidant O 2 , carbon dioxide, the main contributor to the greenhouse effect, was proposed as nontraditional mild oxidant for catalytic oxidation of light alkanes [31][32][33].CO 2 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]. is approach is expected to open new technology for CO 2 utilization.
In this paper, heteropolyacids H  Fe) 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.e 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.e reaction mechanism and kinetics are discussed.e kinetic study was carried over PMo 11 Fe catalyst at 200 °C.FeO 39 salt was prepared by the method described in the literature [36].e paramolybdate of ammonium was dissolved at 50 °C.e mixture of H 3 PO 4 , HNO 3 , and Fe(NO 3 ) 3 was introduced slowly in the aqueous solution of ammonium paramolybdate at 0 °C.Salt of ammonium was precipitated immediately by ammonium nitrates (NH 4 NO 3 ).

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).en, the UV-vis spectrum of the solution was measured at 25 °C.
e X-ray diffraction (XRD) patterns were obtained on a Siemens "D5000" diffractometer with Cu Kα radiation.
e diffractograms were registered at Bragg angle (2θ) � 5-45 °at a scan rate of 5 °/min.e 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.
e 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.e reaction products were quantified by gas chromatography, using an FID and TC detector.

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.e carbon dioxide stream (2 L/h) was saturated with toluene (50 Torr).e oxydehydrogenation (ODH) of toluene was carried out at a relatively lower temperature, 250 °C-350 °C, in which the carbon formation is thermodynamically unflavored.e 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.

FT-IR Analysis.
IR spectra of all compounds are given in Figure 1. e 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-O a , Mo�O t , interoctahedral Mo-O b -Mo, and intraoctahedral Mo-O c -Mo bands, respectively.e FT-IR spectra presented in Figure 1 showed splitting of P-O a band of value equal to 40 cm −1 for PMo 11 V and 24 cm −1 for PMo 11 Fe; the result showed clearly that vanadium and iron ions were introduced into octahedral position.ere is no apparent difference between the PMo 12 and FePMo 12 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.e absorption band around 1600 cm −1 is indicative of the presence of oxonium ions (H 3 O + ) or more ).e band at 1420 cm −1 was assigned to N-H stretching.

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).ese 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.

SEM Analysis.
e scanning electron micrographs of heteropolycompound catalysts are shown in Figure 4. ese images show that the morphology of the particles depends on the heteropolycompound composition and content.e principle structure of FePMo 12 and PMo 11 Fe was the same and consisted of rosette-shape plate-like crystals.e particles of HPMo 12 di er from that of HPMo 11 V in morphology.e samples of both catalysts contain particles with nonuniform size and shape.

Voltamperometric Analysis.
e 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 [PMo 12 O 40 ] 3-anion in acetonetrile/ water (ratio 1/1 by volume), using a glassy carbon rotating electrode.ree-step, one-electron redox wave was obtained with midpoint potentials, E mid of −0.75, −0.4, and −0.05 V assigned to the successive reduction molybdenum centers, where E mid (E pc −E pa )/2; E pc and E pa are the cathodic and the anodic peak potentials, respectively.e same result is observed with iron-substituted heteropolymolybdate (FePMo 12 ).
Cyclic Voltammetry studies of HPMo 11 V and PM 11 Fe were particularly informative.Figures 5(c 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 PMo 11 V and at −0.85, −1.68, and −1.76 for the PMo 11 Fe.e 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.

ermogravimetric Analysis.
e TGA profile for the thermal decomposition of the polyoxometalates is shown in Figure 6. e 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 HPMo 12 , HPMo 11 V, and FePMo 12 samples and three steps in that of PMo 11 Fe compounds.
e first loss at low temperatures (<120 °C) corresponds to the liberation of physisorbed water (12-13 H 2 O), 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].
eir end product of thermal decomposition is the mixture of MoO 3 , P2O 5 , and V 2 O 5 (Scheme (1)), confirmed by X-ray diffraction and IR spectroscopy.
PMo 11 Fe exhibits high stability (Figure 6).It is known that the decomposition of (NH 4 ) 4 PMo 11 FeO 39 begins even at 175 °C and involves three stages in the process of transformation of (NH 4 ) 4 PMo 11 FeO 39 into mixture oxides P 2 O 5 , MoO 3 , and Fe 2 O 3 (Scheme 2), located at approximately 150 °C, 280 °C, and 450 °C.e first peak is associated with removal of water from the polyoxometalate.e main decomposition peak at 250 °C corresponds to the loss of constitution water.
e third peak may be related to the decomposition of ammonium cations to ammonia and can be explained as shown.

Acidic Properties of Polyoxometalates.
e 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).
e results obtained from the isopropanol decomposition using POMs as catalysts as a function of reaction temperature are presented in Table 1.
It is clear that HPMo 11 V and HPMo 12 , are the most active catalysts.
e 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 (H 3 PMo 12 O 40 ) are partially exchanged by iron and ammonium cations, [(NH 4 ) 2.5 Fe 0.1 H 0.2 PMo 12 O 40 ] improves the catalytic activity (Table 1). is 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 (PMo 11 Fe), the initial activity decreased, resulting in conversion relatively lower than the iron in cationic position (FePMo 12 ). is decrease attributed to the decrease in the number and the strength of the acid sites of the catalyst.e presence of redox species Mo VI /Mo IV , V V /V IV , and Fe III /Fe II in the catalysts can contribute to dehydration and dehydrogenation in the decomposition of isopropanol.

Catalytic Activity.
e effect of reaction temperature for the partial oxidation of toluene over polyoxometalates was estimated at 250 °C, 300 °C, and 350 °C.
e toluene conversion and the product selectivity in a CO 2 atmosphere at time on stream of 8 h are shown in Table 2.
e 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).
In the reaction with the H 3 PMo 12 O 40 catalyst, the toluene conversion increased from 7 to 52.3% with an increase in the reaction temperature from 250 °C to 350 °C.
e 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, HPMo 11 V displayed very similar conversion than HPMo 12 but is more selective towards benzaldehyde (80-67% selectivity) benzyl alcohol was also observed as a minor product.ese 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 V 5+ and V 4+ species were likely the active sites in the selective oxidation of toluene to benzaldehyde [42].In the case of H 4 PMo 11 VO 40 , , the reduction of V 5+ to V 4+ is very quick leading to stability of benzaldehyde selectivity from the beginning of the reaction.is 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 V 4+ species act as reducing species towards Mo 6+ leading to Mo 5+ and V 5+ species.In the reaction with the FePMo 12 catalyst (Table 2), toluene conversion is similar to that of the HPMo 12 catalyst observed.
e 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 Journal of Chemistry reaction temperatures.e obtained results indicated that the selectivity of benzene increases with increasing temperature; thus, FePMo 12 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.e same products (benzyl alcohol, benzaldehyde, and benzene) are obtained with PMo 11 Fe, and the results are listed in Table 2. Generally, the conversion of toluene into oxidized products was much more e cient than for the catalysts listed in Table 2.With PMo 11 Fe, conversion of toluene is more important and becomes highly selective to the formation of benzyl alcohol at low temperatures.e catalyst also gave benzene but only as a minor product, benzaldehyde being the major one.is is suggested that iron in anionic position decreases the acidity and accelerates more the redox process.e results for PMo 11 Fe show that the conversion of toluene to oxygenate products (benzyl alcohol + benzaldehyde) is higher than the dealkylation process (benzene) indicating the redox of PMo 11 Fe surface.
e easy redox cycle between oxidized and reduced iron species (F 3+ /Fe 2+ ) generates a highly active catalyst.

Reaction Kinetics.
A kinetic study of toluene partial oxidation was performed at 200 °C over PMo 11 Fe, with a total ow rate of 50 ml/min and catalyst weight of 200 mg.e reaction rate of toluene oxidation by carbon dioxide was given by where α and β are the partial order of toluene and carbon dioxide.
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 CO 2 pressure was changed from 50 to 100 Torr in the second (Figure 8(b)).
e results indicated a reaction of zero order with respect to carbon dioxide and first order with respect to toluene.
ese 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.e 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 , and [S] 0 are, respectively, the concentrations of the oxidized, reduced, and total sites.e 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) P CO2 .
Hence, r k(red) P toluene k P toluene ,where k is the rate constant of the global process.e kinetics is indeed of the rst 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: 1 r e 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 −1 Torr −1 and k(ox) 0,50 mol•min −1 •g −1 Torr −1 .
is conclusion was in good agreement with the decomposition of isopropanol (Table 2) and cyclic voltammetry studies showing the redox properties of the PMo 11 Fe 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 C 6 H 5 -CH 3 OH/C 6 H 5 -CHO (γ) or C 6 H 5 -CHO/C 6 H 5 -CH 3 OH (1/γ) ratios tended towards zero or an in nite value, respectively (Figure 9).ese trends suggested that benzyl alcohol was an intermediate in benzaldehyde formation, as shown by the following reaction pathway: e same trend was observed during the study of the e ect 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.is result con rmed that benzyl alcohol was a precursor in benzaldehyde formation.
Activation energy of the catalytic oxidation of toluene to benzaldehyde, using CO 2 as oxidant, was determined from the plot of lnk against 1/T (Figure 11).e activation energy was calculated using the following equation: slope −Ea/R, where Ea is activation energy, R is the gas constant.e value of activation energy was found to be 62.639 KJ/mol.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 C 6 H 5 -CH 3 ⟶ C 6 H 5 -CH 2 OH ⟶ C 6 H 5 -CHO.

Mechanism and Pathway of Toluene Partial Oxidation by CO 2 . It has been well recognized that the selective
is 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 CO 2 , 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 CO 2 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.is 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). is is in agreement with the work of Busca et al. [46].ey studied by IR spectroscopy the interaction of light hydrocarbons over the MgCr 2 O 4 .It has been concluded that every hydrocarbon reacts at its weakest C-H bond giving rise to the corresponding alkoxide species.In the rst step of the mechanism, C 6 H 5 -CH 3 adsorbed on the M O centers to form an benzyloxide species (C 6 H 5 -CH 2 -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 C 6 H 5 -CHO; desorption of benzaldehyde (II) and dihydrogen lead to the nal products and a reduced catalyst surface (Step 3).e pathway for the oxidation of toluene over polyoxometalates is shown in the reaction scheme Figure 12.
Reoxidation of the catalyst by CO 2 restores the lattice oxygen atom of the M metal, then the M O centers are reestablished for the next turnover cycle.is sequence is represented by (Figure 13) Toluene, when passed over di erent 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.

Conclusion
Both IR spectroscopy and X-ray di raction indicate for FePMo 12 and PMo 11 Fe 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 H 3 PMo 12 O 40 and H 4 PMo 11 VO 40 .ese 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 NH 4 + and Fe 3+ cations or Mo 6+ by V 5+ and Fe 3+ induced important changes in the acid-base and oxidoreduction properties of phosphomolybdate heteropolyanions of Keggin structure.In this study, PMo 11 V and PMo 11 Fe catalyzed the toluene partial oxidation with carbon dioxide as an oxidant to the corresponding oxygenate compounds e ciently and selectively.
e partial oxidation of toluene selectively produced oxygenated compounds besides benzene.e reaction orders were 1 and 0 in toluene and carbon dioxide, respectively.
e 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

2
Journal of Chemistry likely to dioxonium ions (H 5 O 2 +

Figure 7 :
Figure 7: Isopropanol decomposition on acid and redox sites.

Figure 8 :Figure 9 :Figure 10 :
Figure 8: Reaction order of partial oxidation of toluene by carbon dioxide.

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

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