A Green Route to Produce Adipic Acid on TiO2–Fe2O3 Nanocomposites

Article abstract of DOI:10.1002/jccs.201700130

In this work, we study cyclohexene oxidation by molecular oxygen on doped-TiO₂. The improvement of the oxidizing capacity of titanium oxide by doping with iron oxide at different molar ratios is checked. All materials with different molar ratios (Ti/Fe = 9, 4, and 2) are prepared by the sol–gel method and fully characterized by ICP, XRD, SEM, DR/UV–vis, IR, and N₂ adsorption/desorption. The results show that iron is successfully incorporated into the titanium matrix but the incorporated amount is limited. In catalytic tests, improved activity is noticed while using TiO₂ in the presence of Fe₂O₃, which is due the improved oxidation. Conversion in the range of 21–42% depending on the presence of iron oxide was obtained with excellent yield of adipic acid (97% selectivity).

Full text of DOI:10.1002/jccs.201700130

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
A Green Route to Produce Adipic Acid on TiO2–Fe2O3 Nanocomposites
*
Nawal Ameur , Redouane Bachir, Sumeya Bedrane and Abderrahim Choukchou-Braham
Laboratory of Catalysis and Synthesis in Organic Chemistry (LCSCO), University of Tlemcen, Tlemcen, Algeria
(Received: April 10, 2017; Accepted: June 16, 2017; DOI: 10.1002/jccs.201700130)
In this work, we study cyclohexene oxidation by molecular oxygen on doped-TiO2. The improve-
ment of the oxidizing capacity of titanium oxide by doping with iron oxide at different molar
ratios is checked. All materials with different molar ratios (Ti/Fe = 9, 4, and 2) are prepared by
the sol–gel method and fully characterized by ICP, XRD, SEM, DR/UV–vis, IR, and N2 adsorp-
tion/desorption. The results show that iron is successfully incorporated into the titanium matrix
but the incorporated amount is limited. In catalytic tests, improved activity is noticed while using
TiO2 in the presence of Fe2O3, which is due the improved oxidation. Conversion in the range of
21–42% depending on the presence of iron oxide was obtained with excellent yield of adipic acid
(97% selectivity).
Keywords: Cyclohexene oxidation; Nanocomposite; Adipic acid; Iron; Titania.
INTRODUCTION
The oxidation of alkenes to value-added chemicals
TiO2 is largely employed to support metal nano-
particles and applied in several photocatalytic reactions
and to oxidize hydrocarbons.22–24 As nanomaterials,
TiO2 (nanoparticles, nanofibers, and nanotubes) demon-
strates an interesting behavior in cyclic olefin (1,5-
cyclooctadiene, indane, and cyclohexane) oxidation.
However, TiO2 shows conversion in the range of
58–66% depending on titania form, with a noticeable
production of unsaturated ketone (62–40%).25
TiZrCo catalysts have been studied for the oxida-
tion of cyclohexene by molecular oxygen (2 MPa,
120ꢀC for 12 h). For Ti60Zr10Co30, 92.2% cyclohexene
conversion is obtained at 57.6% selectivity for 2-cyclo-
hexen-1-one. The rest may consist of reaction inter-
mediates such as cyclohexene hydrogen peroxide and
highly oxidized products such as some ring-opening
acids. This result is explained as due to the presence of
active species such as CoO and Co3O4 on the surface.26
In this work, we study the improvement of the
oxidation capacity of TiO2 upon iron addition and the
application of these materials in the cyclohexene oxida-
tion reaction by O2. On one hand, different molar ratios
are selected to prepare TiO2–Fe2O3 nanocomposites by
the sol–gel method in order to introduce the maximum
of quantity iron particles into the titanium matrix. On
has recently received much attention.1–4 The resulting
products from this reaction attract a lot of interest
because of their use in the manufacture of perfumes,
pharmaceuticals, dyestuff, and agrochemicals.5–10
Different products can be obtained from cyclohex-
ene oxidation, but it is difficult to control the product
selectivity in this case. This is due to the existence of
two active bonds in this molecule. (1) The oxidation of
the C–H bond takes place at the allylic position, which
is responsible for the formation of cyclohexene hydro-
peroxide, 2-cyclohexene-1-ol, and 2-cyclohexene-1-one,
whereas (2) the oxidation of the C C bond leads to
cyclohexene oxide, cyclohexanol, cyclohexanone, and
cyclohexane dioladipic acid (Scheme 1).11–14
For cyclohexene oxidation, the traditional oxidants
are t-butyl hydroperoxide, H2O2, and molecular oxygen,
the last one being the greenest and the most
abundant.3,14–18 Cyclohexene oxidation with H2O2 is
another alternative to producing adipic acid (Scheme 2),
which is one of the most widely used dicarboxylic acids
in the chemical industry. It can be also used for the man-
ufacture of nylon 6,6 polyamide, polyurethanes, plastici-
zers, and other pharmaceutical chemicals.12,19–21
*Corresponding author. Email: ameurnawel@yahoo.fr
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Ameur et al.
Table 1. All the samples exhibit type IV isotherms typi-
cal of mesoporous structures with a sharp inflection at
a relative pressure of ~0.5–0.8, a narrow pore size distri-
bution of ~ 7–5 nm, and a H2-type hysteresis.
The X-ray diffraction (XRD) patterns of TiO2
and TiO2–Fe2O3 materials are shown in Figure 2. These
XRD patterns show for titania (a) the presence of peaks
at 2θ = 25.72ꢀ, 38.37ꢀ, 48.79ꢀ, 54.7ꢀ, 55.76ꢀ, 63.48ꢀ,
69.68ꢀ, and 71.23ꢀ. These peaks are attributed to the
faces (101), (004), (200), (105), (211), (204), (116), and
(220), respectively. These peaks confirm that the titania
is present only as the anatase phase (tetragonal, a = b =
3.775 Å, c = 9.4912 Å). In the doped materials, the
presence of the anatase diffraction peaks is observed,
but a small shift of the maximum of these peaks is
noticed in the presence of iron oxide.
Scheme 1. Cyclohexene oxidation products.
For instance, the peak at 25.41ꢀ in pure titania is
shifted to 25.47ꢀ, 25.52ꢀ, and 25.49ꢀ for TiFe-9, TiFe-4,
and TiFe-2, respectively. This phenomenon is observed
for all titania peaks and for all materials. This can be
related to the formation of a solid solution of iron and
titanium oxide. Moreover, the crystallites’ average size
also decreases with increasing amount of iron, which
confirms our previous conclusion.27
For TiFe-9 and TiFe-4, we observe a new peak at
30.96ꢀ related to the brookite phase. The intensity of this
peak increases with the iron amount, indicating that iron
oxide catalyzes the transformation of anatase into broo-
kite. Finally, iron particles are also present as the hema-
tite phase (α-Fe2O3) at 2θ = 69.36ꢀ in TiFe-9, 2θ = 75.4ꢀ
for TiFe-4, and 2θ = 69.5ꢀ, and 75.21ꢀ for TiFe-2.
The literature indicates that doping titania with
lower amounts of Fe2O3 is associated with the forma-
tion of the anatase phase without any remarkable influ-
ence on the crystallite size28 and that a low amount of
Fe2O3 is incorporated into the TiO2 lattice which is
attributed to the similarities of the ionic radii of Ti4+
and Fe3+.29
Scheme 2. Two routes for the synthesis of
adipic acid.
the other hand, the catalytic performance of the pre-
pared materials is studied in the cyclohexene oxidation
by molecular oxygen.
RESULTS AND DISCUSSION
The chemical composition and the surface proper-
ties of the prepared materials are summarized in
Table 1. We observe that the amount of iron incorpo-
rated into the materials depends on the initial Ti/Fe
ratio. Thus, we notice that, with the increase of the ini-
tial quantity of iron, the incorporated amount in TiO2
decreases, which may indicate that the maximum incor-
poration limit is reached.
The N2 adsorption–desorption isotherms and pore
size distribution are shown in Figure 1, and the corre-
sponding physical parameters are summarized in
Table 1. Chemical composition and textural properties of the prepared solids
Moles of Fe/g
Rate of Fe2O3
incorporation (%)
Crystallite
size (nm)a
Pore volume
(cm3/g)
SBET
Pore
diameter (nm)
Material
of oxide × 105
(m2/g)
TiO2
—
—
93
40
38
12.5
7.0
5.5
5.4
0.28
0.10
0.08
0.08
148
87
39
7
5
5
6
TiFe-9
TiFe-4
TiFe-2
a Estimated from XRD.
33.2
28.3
38.7
52
2
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Nanocomposites and Olefines Oxidation
Fig. 1. N2 adsorption–desorption isotherm of (a) TiO2, (b) TiFe-9, (c) TiFe-4, and (d) TiFe-2.
However, increasing the amount of iron in titania
accelerates the anatase–rutile transformation. This phe-
nomenon is related to the increasing number of Fe3+
ions occupying the substitutional positions in the tita-
nium crystal structure.27
We can conclude that iron particles were success-
fully incorporated in the titania matrix and that the
excess iron is present as hematite.
TiO2–Fe2O3 materials were also characterized by
scanning electron microscopy (SEM). Representative
micrographs are shown in Figure 3.
The micrographs confirm good crystallization of
the materials under study. We can conclude that the
presence of iron in these materials does not seem to
change the shape of the particles (Figure 3).
The deconvoluted diffuse reflection (DR)/UV–vis
spectra of all prepared samples are shown in
Figure 4. The spectrum of TiO2 showed absorption
bands in the range 200–330 nm. The original spec-
trum was fitted, which revealed three bands at
209, 272, and 326 nm. The first band is due to the
ligand–metal charge transfer between Ti4+ and ligand
(O–H, O–Ti, or H2O). The second band is related to
the Ti4+cations in octahedral environment,30,31 while
the third band due to the anatase phase appears at
326 nm.31,32
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Fig. 2. XRD diffractograms of (a) TiO2, (b) TiFe-9,
(c) TiFe-4, and (d) TiFe-2.
For doped materials, it is noticeable that the spec-
tral intensity increases in the visible region with increase
of the Fe content (Figure 4).
Thus, the fitted spectra show three absorption
zones:
1. Between 200 and 250 nm: these bands may be
related to the anatase phase of titania with compara-
ble intensities and peak positions for all materials.
Hence, in the same region, isolated Fe3+ cations
(charge transfer πd–πp between Fe and O) can
appear33,34
2. Between 250 and 400 nm: which is assigned to the
small (FeO)n33–35
3. Between 400 and 650 nm: related to larger iron
oxide nanoparticles or the polymeric species. A red
shift of these bands with the increase of iron content
indicates a quantum size effect in these species
(Figure 5).36
Figure 6 shows the Fourier transform infrared
(FTIR) spectra of all materials. The IR spectrum of
TiO2 nanoparticles reveals the presence of major
bands centered at 600 cm−1, which are attributed to
the stretching vibrations of the Ti–O–Ti group of the
anatase phase.27,37 It can be observed that there are
broad peaks at 3300 and 1210–1740 cm−1, assigned to
water stretches and bends, respectively.38,39 For the
a
new band at 1070 cm−1 is
Fig. 3. SEM micrographs of (a) TiFe-9, (b) TiFe-4,
and (c) TiFe-2.
doped materials,
observed, which corresponds to the vibration of
Ti–O–Fe.27
4
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Fig. 4. DR/UV–vis spectral (deconvoluted) bands (a) TiO2, (b) TiFe-9, (c) TiFe-4, and (d) TiFe-2.
Catalytic performance in the oxidation reaction
The catalytic performance of the TiO2–Fe2O3 cat-
alysts, in comparison with pure TiO2, is reported in
Table 2. Pure titania shows a cyclohexene conversion of
29%. This activity is improved by up to 42% by the
addition of iron oxide.
particles, which is associated with the changes in the
particle structure, size, and consequent oxidant behav-
ior, as described by Prieto-Centurion et al.36
In terms of selectivity, substantial production of
adipic acid is obtained for all catalysts (97–95%) inde-
pendent of the amount of iron oxide (Table 2). It may
be concluded that titanium oxide is very selective
toward adipic acid and that the presence of iron parti-
cles (TiFe-2) improves the overall conversion.
The catalytic performance of titanium oxide
appears to be modified on the incorporation of iron
According to the literature,11,40 cyclohexene oxi-
dation with molecular oxygen, and in the presence of
the catalyst, initially forms 2-cyclohexene-1-hydroper-
oxide. This product is not stable and can decompose
into 2-cyclohexene-1-ol and 2-cyclohexene-1-one (step
3), or 2-cyclohexene-1-one and water (step 2) or form
cyclohexane oxide and 2-cyclohexene-1-ol (step 4) in
presence of a second cyclohexene molecule. Adipic acid
Fig. 5. Iron species on titania surface.
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Ameur et al.
Fig. 6. Infrared spectra of (a) TiO2, (b) TiFe-9,
(c) TiFe-4, and (d) TiFe-2 materials.
is formed by a series of reactions (oxidation and hydrol-
ysis), as shown in Scheme 3.6
Although the mechanism is not established, we
propose that adipic acid is formed by a one-pot trans-
formation via a five-step scheme involving the hydroly-
sis of cyclohexane epoxide into cyclohexane-1,2-diol,
alcohol oxidation into 2-hydroxycyclohexanone,
Baeyer–Villiger oxidation into 7-hydroxyoxepan-2-one,
alcohol oxidation into oxepane-2,7-dione, and finally
hydrolysis to form adipic acid.5,13,41
The literature indicates that appropriate condi-
tions (medium or catalytic acid sites) facilitates the for-
mation of adipic acid and cyclohexene oxidation14 and
that the iron species present as Fe3+ improves the
Scheme 3. Proposed mechanism for the formation of
the different reaction products from cyclo-
hexene oxidation.
catalytic activity as they are able to activate the C–H
bond, which may also promote the O–O hemolytic
cleavage.36
In this case, we notice that the use of molecular
oxygen as oxidant is beneficial for this reaction, as oxy-
gen is a diradical facilitating a number of radical reac-
tions that can be useful for a range of low-temperature
oxidations.
Table 2. Comparison of the catalytic performance of TiO2
and TiO2–Fe2O3 on cyclohexene oxidation
C
TON
(mol/g)
SENONE
SENOL
SA
CONCLUSIONS
Catalysts
(%)a
(%)b
(%)c
(%)d
The oxidation process of cyclohexene with molec-
ular oxygen to produce adipic acid was studied on pure
and Fe-doped TiO2. First, the sol–gel method led to
binary oxides, where iron species were incorporated
into the titania matrix with an average size of 5–11 nm.
Excess iron was observed as the hematite phase.
Furthermore, the catalytic activity of the catalysts
showed excellent selectivity toward adipic acid
TiO2
29
28
21
42
0.2175
0.2100
0.1597
0.3150
02
02
06
02
01
01
02
01
97
97
92
97
TiFe-9
TiFe-4
TiFe-2
a Cyclohexene conversion.
b Cyclohexen-1,2-enone selectivity.
c Cyclohexen-1,2-enol selectivity.
d Adipic acid selectivity.
6
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formation (up to 97%) and improved cyclohexene con-
version in the presence of iron oxide in titania. This
may be related to the improvement of the oxidation
behavior of titania in the presence of the iron species.
Catalytic tests
The catalytic test of cyclohexene oxidation by
molecular oxygen was carried out in a Parr 5500 series
compact reactor (capacity = 0.14 L, maximum pressure
200 bar) as a function of the temperature (Parr control-
ler) and stirring. In a typical oxidation reaction, 4.5 mL
cyclohexene, 75 mL n-heptane, and 0.1 g of the catalyst
were placed in the reactor. The reactor was closed and
purged three times for air elimination. It was then
heated up to 353 K. When the desired temperature was
reached, molecular oxygen was introduced (P = 6 bar)
under stirring. The reaction was maintained under stir-
ring for 24 h. The reaction products were analyzed by
gas chromatography (GC).
EXPERIMENTAL
Material preparation
Different materials (TiO2–Fe2O3) were prepared
by the sol–gel method, as described by Kundu et al.42
We selected the following molar ration Ti/Fe = 9, 4,
and 2, and the corresponding materials were denoted
TiFe-9, TiFe-4, and TiFe-2, respectively. Typically, a
corresponding volume of titania precursor (titanium
isopropoxide (IV), (C12H28O4Ti)) was mixed with
40 mL of a mixture of acetic acid and ethanol
(v/v = 25:75), and this mixture was aged for 1 h.
Then, a corresponding amount of iron precursor was
dissolved in 10 mL of distilled water and aged for a
few minutes. Next, this lalter mixture was added drop-
wise to the first solution containing the titania precur-
sor. The obtained solution was aged for 72 h at
ambient temperature. The obtained sol was dried at
373 K for 7 days and finally calcined in air at 673 K
(10 K/min).
Cyclohexene conversion, selectivity, and turnover
frequency were calculated using the following
equations:
n
initial −nfinal
ninitial
Conversion ð%Þ =
Selectivity ð%Þ =
:100
moles of individual product
moles of total products
:100
Turnover frequency (TOF) of methanol was calcu-
lated by the following formula:
n
initial −nfinal
Characterization techniques
TOF =
TimeðsÞ:Weight of catalyst ðgÞ
The chemical composition of the samples was
determined by inductively coupled plasma optical emis-
sion spectroscopy (ICP ES) using a Varian ICP OES
instrument. The average crystallite sizes of all the sam-
ples were estimated by the Debye–Scherrer equation
d = λ/(βcosθ), where d is the TiO2–Fe2O3 crystallite
size, λ is the wavelength of X-rays (1.5406 Å), β is the
corrected full width at half-maximum, and θ is the
Bragg diffraction angle.43 The specific surface area was
determined from N2 adsorption/desorption isotherms at
77 K using a Quantachrom NOVA 1000 instrument.
Prior to the adsorption measurements, the sample was
outgassed at 523 K for 2.5 h.
where ninitial and nfinal are the number of moles of cyclo-
hexene at the beginning and end of the reaction, respec-
tively, and nproduct is the number of moles of the
product.
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