Iron(II) phthalocyanine immobilized SBA-15 catalysts: Preparation, characterization and application for toluene selective aerobic oxidation

Article abstract of DOI:10.1016/j.ica.2017.08.026

Iron(II) tetracarboxyl phthalocyanine (tcFePc) grafted onto amino-functionalized SBA-15 catalysts for selective oxidation of toluene by oxygen and N-hydroxyphthalimide (NHPI) were studied under mild conditions. The catalysts were characterized by nitrogen adsorption measurements, X-ray diffraction, element analyses, transmission electron microscopy, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Their catalytic performance depended on the isolated amino group and tcFePc contents, which induced the surface composition and porosity of the catalysts. Less isolated amino groups (2.4 μmol/m²) and more grafted tcFePc (20 wt%) facilitate the oxidation efficiency of toluene to benzoic acid. Meanwhile, more isolated amino groups (3.8 μmol/m²) and less grafted tcFePc (11 wt%) weaken the oxidation activity of toluene and tend to form benzaldehyde. Reaction parameters, reaction scope and catalyst reusability were also investigated over 20 wt% tcFePc@NH₂-SBA-15-3. Moreover, a plausible pathway for this catalytic process was proposed to involve a radical mechanism.

Full text of DOI:10.1016/j.ica.2017.08.026

Accepted Manuscript
Research paper
Iron(II) Phthalocyanine Immobilized SBA-15 Catalysts: Preparation, Charac-
terization and Application for Toluene Selective Aerobic Oxidation
Cheng Huang, Rui Liu, Wenyu Yang, Caiting Zhang, Hongjun Zhu
PII:
S0020-1693(17)31160-X
http://dx.doi.org/10.1016/j.ica.2017.08.026
ICA 17820
DOI:
Reference:
Inorganica Chimica Acta
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25 July 2017
11 August 2017
12 August 2017
Please cite this article as: C. Huang, R. Liu, W. Yang, C. Zhang, H. Zhu, Iron(II) Phthalocyanine Immobilized
SBA-15 Catalysts: Preparation, Characterization and Application for Toluene Selective Aerobic Oxidation,
Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.08.026
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Iron(II) Phthalocyanine Immobilized SBA-15 Catalysts: Preparation,
Characterization and Application for Toluene Selective Aerobic Oxidation
Cheng Huang, Rui Liu*, Wenyu Yang, Caiting Zhang, Hongjun Zhu*
Department of Applied Chemistry, College of Chemistry and Molecular Engineering,
Nanjing Tech University, Nanjing 211816, China
*Corresponding author. Phone: +86-25-58139539. Fax: +86-25-58139539. E-mail:
rui.liu@njtech.edu.cn (R. Liu)
*Corresponding author. Phone: +86-25-83172358. Fax: +86-25-83587428. E-mail:
zhuhj@njtech.edu.cn (H. Zhu)

Abstract
Iron(II) tetracarboxyl phthalocyanine (tcFePc) grafted onto amino-functionalized
SBA-15 catalysts for selective oxidation of toluene by oxygen and
N-hydroxyphthalimide (NHPI) were studied under mild conditions. The catalysts
were characterized by nitrogen adsorption measurements, X-ray diffraction, element
analyses, transmission electron microscopy, X-ray photoelectron spectroscopy and
Fourier transform infrared spectroscopy. Their catalytic performance depended on the
isolated amino group and tcFePc contents, which induced the surface composition and
porosity of the catalysts. Less isolated amino groups (2.4 µmol/m2) and more grafted
tcFePc (20 wt%) facilitate the oxidation efficiency of toluene to benzoic acid.
Meanwhile, more isolated amino groups (3.8 µmol/m2) and less grafted tcFePc (11
wt%) weaken the oxidation activity of toluene and tend to form benzaldehyde.
Reaction parameters, reaction scope and catalyst reusability were also investigated
over 20 wt% tcFePc@NH2-SBA-15-3. Moreover, a plausible pathway for this
catalytic process was proposed to involve a radical mechanism.
Keywords
Iron Phthalocyanine, SBA-15, Aerobic Oxidation, Toluene

1. Introduction
Selective aerobic oxidation of toluene has great significance to the industrial
production, because the oxygenated products such as benzyl alcohol (BAL),
benzaldehyde (BA) and benzoic acid (BAC) are applied extensively in the field of
pharmaceuticals, pesticides, herbicides, dyes and pigments [1-3]. The selectivity of
oxygenated products mainly depends on catalysts and reaction parameters. Traditional
preparation of high value-added benzoic acid, for example, was conducted by using
cobalt carboxylate as catalyst, bromides as initiators and acetic acid as solvent, which
may cause serious pollution problems and corrosion of reactor vessels [4]. Currently,
molecular oxygen as an environmentally friendly and economical oxidant is selected
and utilized in highly selective oxidation of toluene. However, due to the low
oxidizing capability of oxygen, specific catalysts to activate the oxygen molecule in
the oxidation process are particularly demanded.
In the past decade, metal phthalocyanines [5, 6], metalloporphyrins [7, 8], Cu-Mn
oxides [9], Au-Pd nanoparticles [10], etc. as efficient catalysts have been used in
selective oxidation of toluene. Among these catalyst systems, metal phthalocyanines
(MPcs) are widely used for oxidative reactions, such as the reaction of alkanes,
alkenes, alcohols by oxygen [11]. As a class of well-defined transition metal
complexes with special physical and chemical properties, MPcs exhibit high catalytic
activities, which could be attributed to their excited ground-state molecular oxygen
(3O2) to highly active singlet oxygen (1O2) transformations under mild conditions [12].
Additionally, their oxidation catalytic activity can be modulated by attaching different
substituents such as alkyl, and carboxyl groups onto the aromatic rings [13, 14]. When
environmentally friendly oxygen was used as oxidant, substituted phthalocyanines
acted as homogeneous catalysts in oxidation of toluene [15], benzyl alcohol [16],
olefins [17] and nitrotoluene [18]. However, these conventional reactions usually
required high temperature and pressure, and faced difficult to separate catalysts.
Furthermore, in order to enhance the catalytic activity of MPcs catalysts under mild
conditions, some additives were also used in this aerobic oxidation system. Recently,
N-hydroxyphthalimide (NHPI) has been attracted continuous attention due to its

widespread application in oxidation of toluene with cobalt salt [19] and manganese
dioxide [20] under mild conditions. Meanwhile, NHPI also exhibits effective
co-catalytic performance in selective oxidation substituted toluene with MPcs [21,
22].
The catalytic activity of MPcs however, could decrease due to MPcs molecules
are prone to aggregation, which would greatly decrease the active sites for oxidation
reaction [23]. On the other hand, as catalysts, substituted MPcs are usually soluble in
organic solvents, causing the difficulty in separation, recovery and reuse. To solve
these problems, immobilization of the MPcs onto solid supports via covalent grafting
has been studied in recent years [21, 24-26]. The MPcs are desired solid support
materials for synthesizing, stabilizing the well dispersed active sites, facilitate the
recovery and reuse of catalysts, and to reduce effluent contamination. As we known,
SBA-15 is a kind of mesoporous silica material, which shows high internal surface
area (>1000 m2/g), large pores diameters (4-6 nm), good thermal and mechanical
stabilities [27-29]. Therefore, SBA-15 as an ideal solid support, which could be in
favor of improving MPcs dispersion and promote reactants diffuse in reactions [30,
31]. Zhong and coauthors reported MnOx/SBA-15 catalysts used in toluene selective
oxidation by oxygen. The catalytic activity of catalysts was affected by the amount of
manganese species on the surface of SBA-15. However, the catalytic activity has an
obvious drop in the third recycle due to the catalysts with loss of Mn active species
during the oxidation process [32]. Recently, (3-aminopropyl)triethoxysilane (APTES)
was incorporated into the mesoporous SBA-15 network to graft some metal
complexes such as tricopper(II) polyoxotungstates [33], manganese porphines [34]
and vanadyl(IV) acetylacetonate [35]. The covalent functionalization of SBA-15
by APTES could decrease catalysts active species leaching during catalytic
reactions.
In this work, the amino-functionalized SBA-15 (NH2-SBA-15) was conducted by
introducing APTES onto the surface of support, and then covalently with iron(II)
tetracarboxyl phthalocyanine (tcFePc) to obtain tcFePc@NH2-SBA-15, as show in
Scheme 1. The performances of these heterogeneous catalysts for aerobic oxidation of

toluene were explored, with NHPI under mild reaction conditions. For further build
the relationship between catalytic selectivity and catalyst composition, the grafting
amounts of APTES and tcFePc were studied. The effects of reaction time, temperature
catalysts content, NHPI amount and catalyst stability were also investigated. In
addition, to evaluate the pathway of the catalytic system, the possible reaction
mechanism was proposed by using tcFePc@NH2-SBA-15.
Scheme 1. Preparation of tcFePc@NH2-SBA-15.
2. Experimental
2.1. Chemical Materials
The reagents and solvents were obtained from Energy, J&K or Sinopharm
commercial source and used without any pretreatment. Mesoporous SBA-15 material
was purchased from Nanjing JCNANO Technology Co., Ltd. The tcFePc was
synthesized by a solid phase method as described in literatures [13, 36].
2.2. Catalysts characterization
Fourier transformation infrared (FT-IR) spectra were collected on a Thermo
Scientific Nicolet iS5 spectrophotometer using the standard KBr disk method in the
range of 400-4000 cm-1. Powder X-ray diffraction (PXRD) analyses were acquired on
an ARL X'TRA diffractometer with CuKa radiation (λ= 1.542 Å) from 0.6 to 10o for
low-angle XRD. Specific surface areas were determined on a Micromeritics ASAP
2460 apparatus with the BET method from N2 adsorption-desorption. Before BET
analysis, all samples were degassed at 120°C for 6 h under vacuum. Iron element
contents were computed from atomic absorption spectrometry (AAS) measured on a

Hitachi 180-80 instrument. Nitrogen element content was obtained with elemental
chemical analysis (EA) using a Heraeus CHN-O-Rapid. The X-ray photoelectron
spectroscopy (XPS) of the catalysts was acquired on an Imaging Photoelectron
Spectrometer (Escalab 250 Xi, Thermo Scientific) with Al Ka radiation (1486.7 eV).
Binding energies of Fe 2p and N 1s were referred to C 1s peak (BE = 284.8 eV).
Transmission electron microscopy (TEM) images were performed on a JEOL 2000
electron microscope operating at 80 kV. TEM samples were placed on a grid covered
with a copper grid and then moved into the apparatus.
2.3. Catalysts preparation
The pure SBA-15 material was modified by APTES using normal procedure [34].
In a typical synthetic step, SBA-15 (1.0 g) was dispersed in 50 mL toluene and the
solution was stirred for 2 h, then proposed amount APTES (3.80, 1.90, 1.00 or 0.25 g,
respectively) was added to the mixture and refluxed for 24 h. The resultant white
powder was filtered from the solution, and washed with ethanol for three times, and
then dried in air. The different immobilized APTES amount were denoted as
NH2-SBA-15-x (x = 1, 2, 3, 4).
Preparing of tcFePc@NH2-SBA-15 was performed according to the procedure in
previous report [34, 37]. Catalysts of tcFePc@NH2-SBA-15 with grafting tcFePc on
NH2-SBA-15 were synthesized by adding of 0.40 g NH2-SBA-15-x with
2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate (TBTU) (400
mg) and N,N’-diisopropylethylamine (DIPEA) (300 mg) in 25 mL dichloromethane
containing 0.05g tcFePc. Then the green powder was filtered and washed with CH2Cl2
until the filtrate became colorless. The resulting material was dried in vacuum (90°C)
for 12 h. The iron content of the grafted tcFePc on NH2-SBA-15 was determined by
AAS. The final catalysts are designated as y wt% tcFePc@NH2-SBA-15-x, where y
represents the content percent of tcFePc in the samples.
2.4. General procedure for catalytic oxidation
The catalytic oxidation activity tests were performed with molecular oxygen under
atmospheric pressure in accordance with following steps: a certain amount of
tcFePc@NH2-SBA-15 and co-catalyst NHPI, 28 mmol substrates, 3 mL acetonitrile

and then the mixture were added into a 15 mL Schlenk tube fitted with an O2 balloon
(1 atm), under magnetic stirring at certain temperature. The oxidation reaction was
timed sampling, and evaluated through high-performance liquid chromatography
(Dionex, USA) with nitrobenzene as the internal standard substance. Each product
was referred to authentic samples. The recyclability of catalysts was carried out at the
same reaction conditions. After each reaction the catalyst was isolated, thoroughly
washed with methanol and dichloromethane, dried at 90 °C for 24 h, and then reused
in the next cycle.
3. Results and Discussion
3.1. Physical properties of the as-prepared catalysts
The structural parameters of SBA-15 and functionalized catalysts are presented in
Table 1. As shown in Table 1, unmodified SBA-15 displays high surface area (719
m2/g), pore diameter and pore volume were 63 Å and 0.91 cm3/g, respectively. It was
found that APTES immobilized on SBA-15 lead to a clear decrease of surface area
and pore volume (NH2-SBA-15), and a further decrease of APTES content was also
observed with the increase of APTES content. Obviously, the decline of surface area
and pore volume is owing to the partial channel blocked by the introduced APTES.
Besides, tcFePc@NH2-SBA-15 shows lower surface area and pore volume than that
of NH2-SBA-15, which indicates that tcFePcs were introduced into the channel of
NH2-SBA-15 successfully. One noteworthy case in the surface area and pore volume
of NH2-SBA-15-2 (161 m2/g, 0.24 cm3/g) is lower than that of NH2-SBA-15-3 (319
m2/g, 0.48 cm3/g), which result in the different content of tcFePc (11 wt% and 20 wt%)
grafted on the support. This phenomenon indicates that high surface area and pore
volume could promote the tcFePc incorporated onto the surface in NH2-SBA-15 and
obtained more active sites. As illustrated in Fig. 1, the N2 adsorption–desorption
isotherms of these catalysts are quite similar and essentially type-IV isotherms with
the type-H1 hysteresis loop. These features suggest that they are mesoporous materials,
and still have well-ordered cylindrical channels [32, 38]. Additionally, as show in Fig.
1, the pore-size distribution curves of these catalysts indicated they have uniform

mesopore structures.
Table 1
Physical properties and chemical composition of the materials.
Element
content
(wt %)b
Structural parametersa
Density of
isolated amino
(µmol/m2)
Sample
Surface
area
Pore
Pore
diameter
(Å)
volume
(cm3/g)
N
Fe
(m2/g)
SBA-15
NH2-SBA-15-2
719
161
319
126
188
0.91
0.24
0.48
0.18
0.28
61
60
60
59
59
-
-
-
4.08
2.73
6.3
-
-
4.0
2.8
3.8
2.4
NH2-SBA-15-3
11 wt% tcFePc@NH2-SBA-15-2
0.83
20 wt% tcFePc@NH2-SBA-15-3
a From N2 adsorption experiment
5.82 1.54
b Nitrogen was estimated from the element analysis, Fe content from AAS.
a
0
25
50
75
100
125
150
b
c
Pore Radius (Å)
d
e
0.0
0.2
0.4
0.6
0.8
1.0
P/P
0
Fig.1. N2adsorption–desorption isotherms, the BJH pore-size distribution curves (inset) of (a)
SBA-15, (b) NH2-SBA-15-3, (c) 20 wt% tcFePc@NH2-SBA-15-3, (d) NH2-SBA-15-2,(e) 11wt%
tcFePc@NH2-SBA-15-2.
The
low-angle
PXRD
patterns
of
SBA-15,
NH2-SBA-15
and

tcFePc@NH2-SBA-15 are tested in the 2θ range of 0.6 -5.0°. As illustrated in Fig. 2,
SBA-15 exhibits three intense peaks of 0.92, 1.59 and 1.85° at 2θ, which could be
indexed to the (100), (110) and (200) diffraction peaks of the hexagonal arrays pore
structure. After immobilizing with APTES, the position of reflection peaks remains
the same, indicating that the supporting mesoporous structure was well retained.
However, the intensity of these diffraction peaks is declined with the increased
APTES content, which is due to the APTES loading on SBA-15 and the decrease of
long-range order of hexagonally arranged pore structures [39]. Moreover, compared
to NH2-SBA-15, the peak intensity of tcFePc@NH2-SBA-15 decreased dramatically
when incorporation of tcFePc into channels of the NH2-SBA-15 support. The decrease
of peak intensity could be attributed to the organic components introduced into the
porous catalysts pores [40].
(100)
(110)
(200)
a
b
c
d
e
1
2
3
4
5
2θ
(degree)
Fig.2. Small-angle XRD patterns of (a) SBA-15, (b) NH2-SBA-15-3, (c) 20 wt%
tcFePc@NH2-SBA-15-3, (d) NH2-SBA-15-2, (e) 11 wt% tcFePc@NH2-SBA-15-2.
Fig. 3 shows the TEM images of 11 wt% tcFePc@NH2-SBA-15-2 (Fig. 3a-c) and
20 wt% tcFePc@NH2-SBA-15-3 (Fig. 3d-f). The highly uniform mesoporous
structure of SBA-15 was preserved over wide scopes after functionalized, which was
consistent with the results received from N2 adsorption–desorption and XRD spectra.
As can be seen from Fig. 3, the hexagonal mesostructure and parallel cylindrical pore

channels of SBA-15 were not destroyed after grafting of tcFePc. Meanwhile, no
contrast can be observed for tcFePcs from these TEM images, indicating that the
tcFePcs almost exist in an amorphous state and are well-distributed on the pore
surface in mesoporous matrix [41].
Fig.3. TEM micrographs of (a-c) 11wt% tcFePc@NH2-SBA-15-2 and (d-f) 20wt%
tcFePc@NH2-SBA-15-3.
3.2. Surface composition
We have synthesized the catalysts with different activity in toluene oxidation by
modifying the tcFePc and amino group loading. The Fe content of the
tcFePc@NH2-SBA-15 catalysts was examined through AAS which revealed loadings
of 0.15 and 0.28 mmol/g of the catalyst for the 11 wt% (0.11 g of tcFePc per 1 g
catalyst) and 20 wt% (0.20 g of tcFePc per 1 g catalyst) catalysts, respectively. The N
content of NH2-SBA-15 and tcFePc@NH2-SBA-15 were determined from CHN
elemental analysis. The loading of amino group was 2.9 and 2.0 mmol/g for
NH2-SBA-15-2 and NH2-SBA-15-3, respectively. Based on the specific surface areas
(719 m2/g), the density of isolated amino group was calculated to 4.0 and 2.8
µ
mol/m2
for NH2-SBA-15-2 and NH2-SBA-15-3. After tcFePc grafted, the density of isolated

amino group decreased to 3.8 and 2.4 µmol/m2 for 11 wt% and 20 wt% catalysts
respectively, as summarized in Table 1.
XPS measurements were used to detect fine surface information, such as surface
compositions and element chemical states in solid catalysts. Fig. 4 illustrates the Fe
2p and N 1s binding energy regions of 11 wt% tcFePc@NH2-SBA-15-2 and 20 wt%
tcFePc@NH2-SBA-15-3. In both cases of Fe 2p spectra, two main peaks can be
divided, which corresponded to lower energy (Fe 2p3/2) and higher energy (Fe 2p1/2)
asymmetric bands produced by the spin orbital splitting. The relative intensity ratio of
two bands is 2:1. For the 20 wt% catalyst, two bands with binding energies of 710.8
and 722.7 eV. Compared to 11 wt% catalyst, the locations are at 710.7 and 723.4 eV,
respectively. Referring the band positions to standard Fe 2p3/2/Fe 2p1/2 signals for
metallic iron [42-44], it could be inferred that the ferrous state is dominant in both
catalysts. Fe(II) existing in the catalysts is as expected because that raw material
employed in the functionalizing process is Fe(II) phthalocyanine. With the content of
tcFePc grafting increased, the Fe 2p signal intensity of 20 wt% catalyst slightly
increased, indicating an increase amount of surface Fe species.
The spectral regions of N 1s are given in Fig. 4B, where the N signal can be
deconvoluted into four peaks (marked NI, NII, NIII, and NIV, respectively). For the 20
wt% catalyst, the N 1s band can be separated in four contributions (NI, NII, NIII, and
NIV) with binding energies of 398.6, 399.9, 400.8 and 402.3 eV, respectively. NI
should be related to the pyridinic-N of the phthalocyanine rings. NII band is attributed
to the pyrrolic-N of phthalocyanine rings or amide bonds [42, 45]. NIII band is
assigned to isolated amino group on the channels of tcFePc@NH2-SBA-15 [35].
Additionally, the NIV designates the presence of hydrogen bonded or protonated
amino species (-NH2/NH+3 ) [46]. Similar binding energies can be observed at 398.7,
399.9, 400.6 and 402.5 eV for 11 wt% catalyst. However, there is a notable fall of the
NII bands proportion and a raise of NIII in 11 wt% catalyst compared with 20 wt%
catalyst. The change of N proportion caused by APTES content increased in 11 wt%
tcFePc@NH2-SBA-15-2, which increased the isolated amino group (NIII proportion)
and decreased the pyrrolic-N of phthalocyanine rings or amide bonds (NII proportion).

The N 1s spectra show that N and tcFePc content in our catalysts can be adjusted
flexibly via relatively simple route, which could affect the catalytic activity.
(A)
Fe 2p
2p
3/2
2p
1/2
20 wt% tcFePc@SBA-15-3
11 wt% tcFePc@SBA-15-2
740 735 730 725 720 715 710 705 700
Binding Energy (eV)
(B)
N 1s
NI
NIII
N
II
NIV
20 wt% tcFePc@SBA-15-3
11 wt% tcFePc@SBA-15-2
406
404
402
400
398
396
Binding Energy (eV)
Fig.4. High-resolution XPS spectra of (A) Fe 2p and (B) N 1s for tcFePc@NH2-SBA-15 catalysts.
In order to further evaluate the surface composition of these catalysts, FT-IR is
used to give detailed information of SBA-15, NH2-SBA-15 and
tcFePc@NH2-SBA-15, respectively. As shown in Fig. 5, these materials show strong
peaks at 1083 cm-1, which could be ascribed to the stretching vibration of Si-O-Si [34].
However, Si-OH vibration characteristic peak at 960 cm-1 becomes weaker after
immobilizing APTES compared with the pure SBA-15, especially when the APTES
content increased (Fig. 5b, d). This decrease of Si-OH peak intense was due to the
substitution of hydroxyl group with APTES onto the surface of SBA-15. The Si-OH
peaks were almost unchanged when the tcFePc was introduced into the NH2-SBA-15
(Fig. 5c, e), which indicates that the grafting has no effect on the surface of the
support. The band at 804 cm-1 confirmed the presence of isolated amino groups (Fig.

5b, c, d and e). For the NH2-SBA-15-2 and NH2-SBA-15-3, it was noted that
NH2-SBA-15-2 shows stronger amino peak than the latter, which indicates that more
amino groups were immobilized onto the surface of NH2-SBA-15-2. When tcFePc
was grafted onto the NH2-SBA-15, the isolated amino peak further decreased due to
the formation of amidic bond. FT-IR results show that more isolated amino sites on
11 wt% catalyst, but a greater amount of tcFePc exists in 20 wt% catalyst, which is
consistent with the density of isolated amino group result.
a
b
c
d
e
4000 3500 3000 2500 2000 1500 1000 500
Wavelength (cm-1)
Fig.5. FT-IR spectra of (a) SBA-15, (b) NH2-SBA-15-3, (c) 20 wt% tcFePc@NH2-SBA-15-3, (d)
NH2-SBA-15-2 and (e) 11wt% tcFePc@NH2-SBA-15-2.
3.3. Catalytic activity of tcFePc@NH2-SBA-15 catalysts
The catalytic performance of the as-prepared tcFePc@NH2-SBA-15 was tested in
toluene oxidation at 80 °C for 18 h with molecular oxygen. Table 2 summarizes the
results of oxidation of toluene over different catalysts. The main products are
benzaldehyde and benzoic acid, while other by products may also produced include
benzyl alcohol and benzyl benzoate [10]. For the blank experiment of no catalysts
(Table 2, entry 1), there was negligible oxidation conversion of the substrate.
Therefore, it is clear that the catalysts were a crucial factor in the selective oxidation
of toluene. Furthermore, the effect of different metal phthalocyanines on the aerobic

toluene oxidation was studied and the results are listed in Table S1. Compared to
other metal phthalocyanines (tcCoPc, tcZnPc and tcCuPc), tcFePc (Table 2, entry 2)
shows the higher conversion of toluene (13.3 %) and selectivity of oxygenated
products (87.3 %). Therefore, tcFePc was selected when functionalizing SBA-15.
Control tests were also conducted by NH2-SBA-15-x and physical mixture of
NH2-SBA-15-3 and tcFePc. The toluene conversions were 11.2 to 26.1 %,
respectively. Thus, grafting tcFePc onto NH2-SBA-15 significantly improved the
toluene conversion.
When the content of tcFePc increased from 3 to 20 wt%, the toluene conversion
and selectivity of benzoic acid improved accordingly, but the selectivity of
benzaldehyde was decreased. By using 11 wt% tcFePc@NH2-SBA-15-2 as catalyst in
oxidation, the toluene conversion and benzaldehyde selectivity reached 23.7 % and
56.4 % respectively. When the content of tcFePc further increased to 20 wt%, the
catalyst displayed the best catalytic performance, which afforded toluene conversion
of 35.3 % and a benzoic acid selectivity of 71.8 %. However, further increasing the
content of tcFePc to 32 wt%, the toluene conversion decreased. Only 19.4 % toluene
conversion and 59.6 % benzoic acid (Table 2, entry 12) were gave, which indicated
that the dispersity of tcFePc on the catalyst significantly influenced the catalytic
activity. Meanwhile, it is noted that the conversion of toluene without NHPI
remarkably decreased to 12.6 %, suggesting NHPI was a key co-catalyst in the
catalytic oxidation of toluene.
By taking the porosity of catalysts and the obtained catalytic results into account,
it is clear that the catalysts with highly specific surface area and pore volume (i.e. 20
wt% tcFePc@NH2-SBA-15-3) exhibit the best performance. This suggests that the
highly surface area and pore volume facilitate substrates or intermediates diffusion.
Additionally, the tcFePcs were highly dispersed and grafted onto the surface of
NH2-SBA-15, which could be exposed more active sites of the catalyst. These
favorable factors make 20 wt% tcFePc@NH2-SBA-15-3 catalyst exhibits excellent
catalytic performance with higher toluene conversion and selectivity of benzoic acid.
However, surface area and pore volume decreased with the APTES content increased

(NH2-SBA-15-2), and hence hinder the tcFePc grafting and reaction substrates into
the pore channel of SBA-15. This might result in weak catalytic activity of 11 wt%
tcFePc@NH2-SBA-15-2, which converts toluene to benzaldehyde.
On the other hand, immobilizing of APTES leads to a decrease of adsorption sites
covered on the catalyst surface, and hinders substrates or intermediates adsorbed on
the catalyst surface. These -OH groups on the mesoporous SBA-15 surface have
adsorption capacity, it is believed that they could absorb substrate and then facilitate
transformation of toluene generating products [47]. In the reaction system, the
intermediate benzyl alcohol has higher molecular polarization than toluene and
benzaldehyde, which makes more easily absorbed on the catalyst surface and then
oxidation to benzoic acid [32]. However, the increased APTES content could be
further reduced the isolated -OH groups. Meanwhile, the -NH2 groups could form
basic sites on the surface, which induced the dehydrogenation of benzyl alcohol to
benzaldehyde [48, 49]. For example, the selectivity of benzaldehyde for 3 wt% and 11
wt% catalysts with less isolated -OH groups and more -NH2 groups was higher than
that of 20 wt% catalyst. Furthermore, the selectivity of benzoic acid for 20 wt%
catalyst was higher than that of 3 wt% and 11 wt% catalysts, which could be
attributed to more -OH groups and lower density of isolated amino group on the
surface.
Molecular oxygen activation and oxygen atom donation to substrate in oxidative
reaction of toluene over tcFePc@NH2-SBA-15 could be explained by the oxygen
activated ability of iron phthalocyanine. Iron phthalocyanines produce high active 1O2
singlet oxygen species in the oxidation. This singlet oxygen is formed during the
extinction mechanism of the excited triplet state [6, 50]. For the oxidation of toluene,
with the tcFePc increased from 3 to 20 wt%, the conversion rapidly increased from
13.5 to 35.3 %. These results suggest that the formation of oxygenated product during
the oxidation process has a close relationship with oxygen activation on
tcFePc@NH2-SBA-15 catalysts.
Due to 20 wt% tcFePc@NH2-SBA-15-3 catalyst shows excellent catalytic
performance with higher toluene conversion and selectivity of benzoic acid, the

reaction scope of this catalytic system to toluene derivatives bearing different
substituents was studied, and the results are listed in Table S2. It is clear that the 20
wt% tcFePc@NH2-SBA-15-3 could readily oxidize the methyl group on different
substituted positions of toluene to the main product carboxylic acids and side product
aldehydes. Furthermore, the reactivity of toluene derivative with electron-donating
substituent is demonstrated with moderate conversions and well selectivity for
corresponding carboxylic acids. Meanwhile, this catalyst system is also active for the
oxidation of toluene derivatives with electron-withdrawing substituents. The halogen
substituted toluene derivatives were oxidized smoothly with the conversion around
30 %, and the main products were their corresponding acid. However, decreased
reactivity of 4-nitrotoluene with fair conversion of 15.2 % (Table S2, entry 8)
suggests that the strong electron-withdrawing substituent -NO2 leads to the
deactivation of the oxidation process.
Table 2
Oxidation of toluene catalyzed over tcFePc@NH2-SBA-15a.
Selectivityb(%)
Conversion
Entry
Catalyst
Additive
(%)
BAL
23.5
11.5
26.6
5.55
4.98
5.11
6.26
BA
60.1
37.7
51.1
57.4
46.6
49.3
68.3
BAC
10.3
38.1
15.3
25.9
38.8
41.9
20.5
1
2
3
4
5
6
7
-
tcFePc
NHPI
NHPI
NHPI
NHPI
NHPI
NHPI
NHPI
3.30
13.3
11.2
13.7
15.4
12.0
13.5
NH2-SBA-15-1
NH2-SBA-15-2
NH2-SBA-15-3
NH2-SBA-15-4
3 wt% tcFePc@NH2-SBA-15-1
8
11 wt% tcFePc@NH2-SBA-15-2
NHPI
23.7
4.37
56.4
36.3
9
20 wt% tcFePc@NH2-SBA-15-3
20 wt% tcFePc@NH2-SBA-15-3
tcFePc/NH2-SBA-15-3
NHPI
-
35.3
12.6
26.1
19.4
1.99
7.80
10.5
7.75
20.1
22.4
22.7
25.3
71.8
62.7
55.4
59.6
10
11c
12
NHPI
NHPI
32 wt% tcFePc@NH2-SBA-15-4
aReaction conditions: toluene 0.028 mol, NHPI 2.0 mol%, catalyst 0.03 g, 80 oC, O2 balloon,
reaction time 18 h, acetonitrile 3 mL.
bBAL = selectivity of benzyl alcohol, BA = selectivity of benzaldehyde, BAC = selectivity of
benzoic acid.
ctcFePc and NH2-SBA-15-3 were physical mixing.

3.4. Effect of reaction conditions
Fig. 6A shows the change of toluene conversion as well as the distribution of the
oxygenated products catalyzed by 20 wt% tcFePc@NH2-SBA-15-3 at 80 °C in
different reaction time. The toluene conversion increased from 19.2 to 38.1 % with
the reaction time from 6 to 21 h. However, loss of benzyl alcohol and benzaldehyde
selectivity was happened in the reaction period. Simultaneously, the benzyl acid
selectivity increased and then decreased slightly when the reaction time extended to
21 h. This could be attributed to the toluene conversion attained its equilibrium and
benzyl benzoate was produced in reaction process [32]. Thus, giving consideration to
conversion and selectivity, the optimum reaction time was 18 h.
The influence of reaction temperature on the toluene oxidation over 20 wt%
tcFePc@NH2-SBA-15-3 from 50 to 90 °C is illustrated in Fig. 6B. When the reaction
was regulated temperature at 50 °C, the toluene exhibited low conversion. This
phenomenon could be mainly attribute to the NHPI was not initiated at this
temperature. With the reaction temperature increased from 60 to 90 °C, the conversion
of toluene rose from 16.1 to 47.0 %. Meanwhile, the selectivity of benzoic acid
increased from 36.1 to 71.8 % first, and then decreased to 70.3 %. In addition, the
selectivity of benzyl alcohol declined from 11.2 to 1.87 %, and the selectivity of
benzaldehyde decreased from 46.5 to 17.6 %, which suggested that the benzyl alcohol
and benzaldehyde was further oxidized to benzoic acid when the reaction temperature
further increased.
The effect of the mass of 20 wt% tcFePc@NH2-SBA-15-3 on the oxidation was
tested by various amount of catalyst between 0.01 g and 0.10 g, while other
experiment parameters remain unchanged (Fig. 6C). The conversion of toluene and
selectivity of benzoic acid increased with the increase of catalyst concentration,
whereas both their conversion and selectivity decreased with further increased
tcFePc@NH2-SBA-15 mass. Meanwhile, the selectivity of benzaldehyde and benzyl
alcohol was decreased first, but increased slightly at high catalyst concentrations. This
could be explained that transition metals complexes at high loadings might act as
inhibitors in the oxidation, which reduces the catalysis activity of

tcFePc@NH2-SBA-15-3.
The influence of NHPI concentration was also studied to optimize toluene
conversion and product selectivity. As shown in Fig. 6D with the concentration of
NHPI increase, the conversion of toluene increased and the selectivity of
benzaldehyde and benzyl alcohol decreased significantly. However, when NHPI
concentration reached 2.0 mol%, the selectivity of benzoic acid decreased obviously,
which is probably due to the more by-product were formed in the oxidation.
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
(A)
(B)
Conv
BAL
BA
Conv
BAL
BA
BAC
BAC
6
8
10
12
14
16
18
20
22
50 55 60 65 70 75 80 85 90
Time (h)
o
Temperature ( C)
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
(C)
(D)
Conv
BAL
BA
Conv
BAL
BA
BAC
BAC
0.00
0.02
0.04
0.06
0.08
0.10
0
1
2
3
4
Catalyst mass (g)
NHPI amount (mol%)
Fig.6. Effect of various reaction parameters on oxidation of toluene over 20 wt%
tcFePc@NH2-SBA-15-3: (A) reaction time, (B) temperature, (C) catalyst mass, (D) NHPI amount.
Reaction conditions: 0.028 mol (3 mL) of toluene, Conv = conversion of toluene, BA = selectivity
of benzaldehyde, BAL = selectivity of benzyl alcohol, BAC = selectivity of benzoic acid.
3.5. Catalyst reusability and reaction mechanism
A series of catalytic reactions were carried out to investigate the catalytic
reusability of catalyst 20 wt% tcFePc@NH2-SBA-15-3 under the identical conditions.
As shown in Fig. 7, it is found that the catalyst maintained good catalytic activity and
conversion of toluene and selectivity of product were barely changed after three
successive recycles. Meanwhile, the conversion of toluene dropped obviously in the
fourth recycle, and the selectivity of product both decreased. It might be caused by the

blocking of support channels, resulting in the catalyst deactivation [51].
100
80
60
40
20
0
100
80
60
40
20
0
Conv
BA + BAC
0
1
2
3
4
Recyling number
Fig.7. Catalyst reusability study of 20 wt% tcFePc@NH2-SBA-15-3, Conv = conversion of
toluene, BA + BAC = selectivity of benzaldehyde and benzoic acid.
According to the results obtained in this work and reported literatures [21, 52], a
possible mechanism of the catalytic oxidation of toluene by the tcFePc@NH2-SBA-15
catalyst was proposed, which is shown in Scheme 2. As we known, NHPI is an
effective co-catalyst for the oxidation of toluene, which could generates
phthalimide-N-oxy (PINO) with the metal-oxygen complex under ambient conditions.
It is generally known that high valence state iron oxo species in porphyryin or
phthalocyanine are convinced active components capable of catalyzing various
oxidation reactions [11]. Additionally, the composition on the surface was also related
to the enhancement of catalytic performance. As show in Table 2, the catalysts possess
a greater amount of immobilized APTES would obviously decrease tcFePc grafting,
leading to a lower toluene conversion and a higher benzaldehyde selectivity (11 wt%
tcFePc@NH2-SBA-15-2). On the other hand, less APTES would promote the content
of tcFePc increase, leading to a higher catalytic activity and benzoic acid selectivity
(20 wt% tcFePc@NH2-SBA-15-3).
In the catalytic system using tcFePc@NH2-SBA-15/NHPI, the methyl group of
absorbed toluene is first abstracted by PINO to lose a hydrogen and then produce a
benzyl radical. Meanwhile, the tcFePc could promote O2 adsorption and generate
dioxygen-iron complexes (Fe(III)PcOO·), and then transformed into metal

hydroperoxide (Fe(III)PcOOH). The benzyl radical reacts with Fe(III)PcOOH to
produce the first oxygenated product benzyl alcohol, which could be detected by
GC/MS. We also found that the benzyl alcohol in oxidation system was rapidly
lowering with the reaction time prolonged (Fig. 6A). According to previous literature
[53], the surface amino group enhances the dehydrogenation of benzyl alcohol.
Benzyl alcohol might reacted with Fe(III)PcO· on the surface to form benzaldehyde
and water, as shown in Scheme 2. Furthermore, the complex (Fe(III)PcOO·) could
captures a hydrogen from the aldehyde group of benzaldehyde and then forms the
benzaldehyde radical and Fe(III)PcOOH. Then the benzaldehyde radical rapidly
reacts with metal hydroperoxide (Fe(III)PcOOH) to obtain final product benzoic acid.
In this catalytic pathway, the adsorption ability (benzyl alcohol and benzaldehyde)
and the composition of catalysts surface could well explain the product distribution in
the oxidation.
Scheme 2. Plausible mechanism for the oxidation of toluene over tcFePc@NH2-SBA-15.
4. Conclusions
In this work, we present novel functionalized SBA-15 catalysts for selective
aerobic catalysis of toluene. The results showed that the catalytic activity of these
catalysts is significantly affected by their porosity and surface composition. The
characterizations of nitrogen adsorption measurements and element analyses indicate
that the catalyst 20 wt% tcFePc@NH2-SBA-15-3 qualifies large surface area (188

m2/g), high pore volume (0.28 cm3/g) and abundant active sites (tcFePc). When the
content of APTES increases, the surface area and pore volume of the catalyst decrease
obviously. Simultaneously, the number of grafted tcFePc also decreases. For the
toluene oxidation, a maximum conversion of 35.3 % conversion with 71.8 %
selectivity for benzoic acid is obtained by 20 wt% tcFePc@NH2-SBA-15-3 catalyst,
whereas the 11 wt% catalyst gives 23.7 % conversion with 56.4 % selectivity for
benzaldehyde. Meanwhile, this catalytic system is suitable for oxidizing both
electron-donated and electron-withdrawing substituted toluene derivatives.
Additionally, these catalysts maintain almost unchanged catalytic activity and
selectivity after several times without activation treatment. This work provides a
simple method to develop heterogeneous supported catalysts for selective toluene
oxidation and highlights the great potential of catalytic system in the development of
industrialization.
Acknowledgements
The authors greatly acknowledge the financial support in part by National Natural
Science Foundation of China (21602106), Natural Science Foundation of Jiangsu
Province-Outstanding Youth Foundation (BK20170104), Industry-Academy-Research
Prospective joint project of Jiangsu Province (BY2016005-06) and National Key
Research and Development Program of China (2016YFB0301703).
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Graphical abstract
Iron(II)
tetracarboxyl
phthalocyanine
(tcFePc)
immobilized
onto
amino-functionalized SBA-15 catalysts for selective oxidation of toluene by oxygen
and N-hydroxyphthalimide (NHPI) were studied under mild conditions. It is found
that the surface composition and porosity of these catalysts could tune the selectivity
of products in toluene aerobic oxidation.

Highlights
Novel Fe(II) phthalocyanine modified SBA-15 heterogeneous catalysts were
prepared.
Efficient catalytic performance for toluene aerobic oxidation.
The selectivity of product depends on the catalyst surface composition and porosity.
Catalyst recycled for three times without significant decrease in catalytic activity.