Direct benzyl alcohol and benzaldehyde synthesis from toluene over keggin-type polyoxometalates catalysts: Kinetic and mechanistic studies

Article abstract of DOI:10.1155/2019/9521529

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.

Full text of DOI:10.1155/2019/9521529

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Hindawi
Journal of Chemistry
Volume 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 Smaın
´
´
´
Laboratoire de Genie Chimique et Chimie Appliquee Hasnaoua I, Universite de Mouloud Mammeri,
BP. 17 RP 15000 Tizi-Ouzou, Algeria
¨
Correspondence should be addressed to Hocine Smaın; 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. -is 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.
-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
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. -e kinetic data show that the reoxidation of the reduced catalyst is the rate-limiting
step for the partial oxidation reaction of toluene.
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 vana-
dium oxide catalysts are also used in gas-phase catalytic
partial oxidation of toluene [12–14]. -e supported
vanadium-based catalysts were reported to show high
benzaldehyde selectivity, but the toluene conversion was
1. Introduction
Aromatic aldehydes such as benzaldehyde are very valuable
industrial organic intermediates, and their uses include the
manufacture of flavors, fragrances, pharmaceutical pre-
cursors, and plastic additives. Industrially, benzaldehyde is
produced exclusively by the liquid phase oxidation of tol-
uene [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 al-
ternative 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
below 10% at the low temperature (<400°C). However, at
higher temperature (>400°C), the toluene conversion in-
creased 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, benzal-
dehyde, 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 de-
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
creases in the following order: support oxides ≥ molyb-
denum oxide monolayer catalysts > molybdate salts >
crystalline molybdenum oxide, but the benzaldehyde and

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benzoic acid selectivities follow the opposite trend [15]. Al-
though there is a large number of catalysts described in the
literature and in the patent literature composed of the va-
nadium oxides systems modified by various promoters,
supported on carriers Al2O3, SiO2, TiO2, and transition metal
oxides and favorable for the catalytic gas-phase toluene ox-
idation to benzaldehyde [17–24], 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 [24–27]. Keggin-
type polyoxometalates have been widely used as catalysts for
one-step conversion of lower alkanes into value-added oxy-
genated 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]. -is approach is expected to open
new technology for CO2 utilization.
2.2. Characterization. Fourier transform infrared spectra
(FT-IR) of the heteropolycompounds were obtained in
−1
the 400–4000 cm 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 aceto-
nitrile (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 at-
mospheric 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.
2.3. Catalytic Tests. Catalytic tests were carried out at at-
mospheric 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 chro-
matograph (DTC and FID CG, Shimadzu 14B) equipped
with columns of Molecular Sieve 5A and Porapak QS.
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. -e catalytic
performance of the catalysts was investigated in partial oxi-
dation 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 PMo11Fe catalyst at 200°C.
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. -e characteristic peaks of Keggin unit were
−1
observed in the region 1100–500 cm . According to liter-
2. Experimental
ature data [37], the origin of the Keggin anion vibration
−1
2.1.
H3PMo12O40 and H4PMo11VO40 samples were prepared in
classical way [35]. -e ammonium-iron salt,
Catalyst
Preparation. Pure
heteropolyacids
bands, appearing at 1065, 961, 864, and 789 cm , is at-
tributed to P-Oa, Mo�Ot, interoctahedral Mo-Ob-Mo, and
intraoctahedral Mo-Oc-Mo bands, respectively. -e FT-IR
spectra presented in Figure 1 showed splitting of P-Oa band
a
(NH4)2.5Fe0.1H0.2PMo12O40, was prepared by adding
(NH4)2CO3 solution to an aqueous solution of mixture of
H3PMo12O40 and Fe(NO3)3. -e precipitate was dried at
50°C under vacuum for 5 h. (NH4)4PMo11FeO39 salt was
prepared by the method described in the literature [36]. -e
paramolybdate of ammonium was dissolved at 50°C. -e
mixture of H3PO4, HNO3, and Fe(NO3)3 was introduced
slowly in the aqueous solution of ammonium para-
molybdate at 0°C. Salt of ammonium was precipitated
immediately by ammonium nitrates (NH4NO3).
−1
−1
of value equal to 40 cm for PMo11V and 24 cm for
PMo11Fe; the result showed clearly that vanadium and iron
ions were introduced into octahedral position. -ere 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. -e absorption band around 1600 cm−1 is
indicative of the presence of oxonium ions (H3O+) or more

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Journal of Chemistry
3
(a)
(b)
Mo=Ot
(c)
(d)
Mo-O-Mo
d
2000 1800 1600 1400 1200 1000 800
Wavenumber (cm–1
600
400
200
)
c
Figure 1: FTIR spectra of (a) HPMo12; (b) FePMo12; (c) HPMo11V;
(d) PMo11Fe.
b
+
−1
a
likely to dioxonium ions (H5O2 ). ꢀe band at 1420 cm
was assigned to N–H stretching.
100
200
300
400
500
600
λ (nm)
3.1.2. UV-Visible Analysis. In the near UV, the electronic
spectra of Keggin-type polyoxometalates show two ab-
sorption 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.
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. ꢀe acidic HPMo12O40 and
HPMo11VO40 crystallize in the triclinic system, with a ꢀ
d
c
14.10 A, b ꢀ 14.13 A, c ꢀ 13.39 A, a ꢀ 112.1°, b ꢀ 109.8°, g ꢀ
˚
˚
˚
b
60.73° and a ꢀ14.04 A, b ꢀ 10.006 A, c ꢀ13.55 A and a ꢀ 112°,
˚
˚
˚
a
b ꢀ 109.58°, g ꢀ 60.72°, respectively.
0
5
10 15 20 25 30 35 40 45
ꢀe dišraction patterns of FePMo12 shown in Figure 3
indicate for the compound the existence of a single crys-
tallographic 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.
2θ°
Figure 3: XRD patterns of (a) HPMo12; (b) HPMo11V; (c)
PMo11Fe; (d) FePMo12.
X-ray analysis of. (NH4)4PMo11Fe(H2O)O39·xH2O
on the vanadium, molybdenum, or tungsten atoms and thus
are potential oxidants. Figure 5(a) shows a cyclic voltam-
mogram of 0.50 mM [PMo12O40]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, 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. ꢀe same result is
observed with iron-substituted heteropolymolybdate
(FePMo12).
showed that it crystallizes in the monoclinic system,
˚
˚
space group P 21/c, with, a ꢀ 20,07210 A, b ꢀ 11,55938 A,
˚
°
c ꢀ 18,77828 A, β ꢀ 95,664 , and Z ꢀ 2.
3.1.4. 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 FePMo12 and PMo11Fe was the same
and consisted of rosette-shape plate-like crystals. ꢀe par-
ticles of HPMo12 dišer from that of HPMo11V in mor-
phology. ꢀe samples of both catalysts contain particles with
nonuniform size and shape.
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−. ꢀe number of redox processes iden-
tižed 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
3.1.5. Voltamperometric Analysis. ꢀe redox properties of
heteropolyanions have been studied by cyclic voltammetry.
Keggin anions are able to accept reversibly up to six electrons

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(a)
(b)
(c)
(d)
Figure 4: Scanning electron micrographs of (a) HPMo12; (b) FePMo12; (c) HPMo11V; (d) PMo11Fe.
0.0001
0.0000
0.00
–0.05
–0.10
–0.15
–0.20
–0.25
–0.0001
–0.0002
–0.0003
–0.0004
–0.0005
–0.0006
–0.0007
–2.0
–1.5
–1.0
–0.5
E\ECS (V)
0.0
0.5
1.0
–1.2 –1.0 –0.8 –0.6 –0.4 –0.2 0.0 0.2 0.4 0.6 0.8
E/ECS (V)
(a)
(b)
0.00010
0.00
–0.05
–0.10
–0.15
–0.20
–0.25
–0.30
–0.35
0.00005
0.00000
–0.00005
–0.00010
–0.00015
–0.00020
–0.00025
–2.0 –1.5 –1.0 –0.5
0.0
0.5
1.0
–1.2 –1.0 –0.8 –0.6 –0.4 –0.2 0.0 0.2 0.4 0.6
E\ECS (V)
E/ECS (V)
(c)
(d)
Figure 5: Voltamperograms of (a) HPMo12; (b) FePMo12; (c) HPMo11V; (d) PMo11Fe.