Exclusively catalytic oxidation of toluene to benzaldehyde in an O/W emulsion stabilized by hexadecylphosphate acid terminated mixed-oxide nanoparticles

Article abstract of DOI:10.1016/S1872-2067(19)63417-0

A series of hexadecylphosphate acid (HDPA) terminated mixed-oxide nanoparticles have been investigated to catalyze the oxidation of toluene exclusive to benzaldehyde under mild conditions in an emulsion of toluene/water with the catalysts as stabilizers.

Full text of DOI:10.1016/S1872-2067(19)63417-0

Chinese Journal of Catalysis 41 (2020) 341–349
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Exclusively catalytic oxidation of toluene to benzaldehyde in an O/W
emulsion stabilized by hexadecylphosphate acid terminated
mixed-oxide nanoparticles
Changshun Deng a, Mengxia Xu a, Zhen Dong a, Lei Li a,b, Jinyue Yang a, Xuefeng Guo a, Luming Peng a,
Nianhua Xue a, Yan Zhu a, Weiping Ding a,*
a Key Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, Jiangsu, China
b Luoyang R&D Center of Technology, Sinopec Engineering (Group) Co., Ltd., Luoyang 471003, Henan, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A series of hexadecylphosphate acid (HDPA) terminated mixed-oxide nanoparticles have been in-
vestigated to catalyze the oxidation of toluene exclusive to benzaldehyde under mild conditions in
an emulsion of toluene/water with the catalysts as stabilizers. With the HDPA-Fe2O3/Al2O3 as the
basic catalyst, a series of transition metals, such as Mn, Co, Ni, Cu, Cr, Mo, V, and Ti, was respectively
doped to the basic catalyst to modify the performance of the catalytic system, in expectation of in-
fluencing the mobility of the lattice oxygen species in the oxide catalysts. Under normally working
conditions of the catalytic system, the nanoparticles of catalysts located themselves at the interface
between the oil and water phases, constituting the Pickering emulsion. Both the doped iron oxide
and its surface adsorbed hexadecylphosphate molecules were essential to the catalytic system for
excellent performances with high toluene conversions as well as the exclusive selectivity to benzal-
dehyde. Under optimal conditions, ~83% of toluene conversion and >99% selectivity to benzalde-
hyde were obtained, using molecular oxygen as oxidant and HDPA-(Fe2O3-NiO)/Al2O3 as the cata-
lyst. This process is green and low cost to produce high quality benzaldehyde from O2 oxidation of
toluene.
Received 8 May 2019
Accepted 30 May 2019
Published 5 February 2020
Keywords:
Toluene oxidation
Benzaldehyde
Hexadecylphosphate acid
Molecular oxygen
Pickering
© 2020, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
decades, of which performance, however, is still far below the
demand [3,5]. The present industrial processes have serious
The oxidation of toluene to benzaldehyde by O2 has been
one of the reactions highly demanded in industry in the past
decades [1–4] and it should be the best route to produce ben-
zaldehyde, a highly useful chemical in many aspects. The reac-
tion seems simple, just an insertion of one oxygen into the C–H
bond of benzyl, but very difficult, for the heterogeneous cata-
lytic processes for the reaction have been studied for several
environmental and corrosion problems, as shown in Scheme 1,
and thus produced benzaldehyde inevitably contained a little
amount of halogens [6], which hindered high-end uses of the
compound such as in perfumes or foods. Liquid oxidations of
toluene by oxygen with noble metals based catalysts including
Pd, Au, Pt, Ag, Ru and their alloys have become popular re-
search topics in recent years, owing to their excellent catalytic
* Corresponding author. Tel: +86-25-89685077; Fax: +86-25-89687761; E-mail: dingwp@nju.edu.cn
This work was supported by the National Natural Science Foundation of China (91434101, 91745108), the Ministry of Science and Technology of the
People’s Republic of China (2017YFB0702900).
DOI: S1872-2067(19)63417-0 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 41, No. 2, February 2020

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Changshun Deng et al. / Chinese Journal of Catalysis 41 (2020) 341–349
Scheme 1. The main routes used in the current industry for the pro-
duction of benzaldehyde.
activities [7–14], under solvents or solvent-free conditions
[1,3,15–20]. Ag modified mixed-node MOF catalyst has been
found highly selective for the oxidation of toluene to benzalde-
hyde, but just at very low conversions [8]. Bimetallic PdAu cat-
alysts reported by Hutchings and co-workers for the oxidation
of toluene toward benzyl benzoate do get outstanding activity
at a relatively mild temperature (160 °C, 1 MPa O2) [4,21,22].
Generally, high selectivity to benzaldehyde at high toluene
conversion has been still not available with these catalytic sys-
tems.
Fig. 1. (a) Schematic show of the toluene/water Pickering emulsion
with HDPA-Fe2O3/Al2O3 nanoparticles located at the interface of tolu-
ene and water. (b) Diagram showing the transfer of toluene, benzalde-
hyde and oxygen at the interface. (c) Magnified show of the interface
between the Fe2O3 (110, as an example) surface and the HDPA layer
and the access of reactant molecules to the catalytic sites as well as the
seceding of product molecules along the lanes of the HDPA chains. (d)
The real laser confocal fluorescence photograph of the emulsion with
small amount of rhodamine B as fluorescence dye adsorbed on the
catalyst.
For the green oxidation of toluene by O2 exclusively to ben-
zaldehyde, a highly efficient catalytic system was explored in
our previous work using a biphasic system with HDPA-FeOx
nanoparticles locating themselves at the interface of toluene
and water to stabilize the O/W emulsion, i.e, Pickering emul-
sion [2]. Resasco et al. [23–25] have made the first use of Pick-
ering emulsions for the biphasic hydrodeoxygenation and con-
densation in biomass refining as well as other reactions. We
report here the further investigation on the doping effect of a
series of metal oxides such as Mn, Co, Ni, Cu, Cr, Mo, V, and Ti on
the catalyst HDPA-FeOx, anticipating the enhancement of the
mobility of lattice oxygen of the catalyst. In addition, we also
use a special kind of nano alumina as support for the easy
preparation of the modified catalysts and the high stability of
the catalysts in application. With the well-defined particle
structures, the catalysts show significantly improved and relia-
bly repeated catalytic performance for the title reaction and the
NiO co-loaded catalyst exhibits the best performance with
~83% conversion of toluene (a TOF of 0.027 nm–2·s–1) and
~100% selectivity to benzaldehyde under mild conditions. This
is a very important progress toward the practical use of the
catalytic process for production of high quality benzaldehyde.
Fig. 1 schematically shows the composition of the Pickering
emulsion with the catalyst nanoparticles positioned at the in-
terface of O/W. The long organic chains of HDPA molecules in a
suitable density protect the nanoparticles from gathering and
ensure the catalyst particles at the interface of the O/W emul-
sion, boosting the access of toluene to the nanoparticles. The
phosphate head in HDPA adsorbed on the surface is important
to tune the catalyst surface as well as to switch off unneeded
sites unselective for the toluene oxidation. Considering the
solubility of the reactants, the water has a significant effect on
the transportation of O2 from gas phase to the liquid phase as
well as the products from the surface of the catalyst to the liq-
uid phase (Figs. 1(a), 1(b), and 1(c)). Fig. 1(d) depicts the real
laser confocal fluorescence photograph of the reaction mixture
with small amount of rhodamine B as fluorescence dye ad-
sorbed on the catalyst, confirming the O/W Pickering emulsion
of the reactive system. Under such conditions, the sur-
face-bonded HDPAs boost the access of the nonpolar toluene as
well as the seceding of the relatively polar product, i.e., benzal-
dehyde, from the nonpolar surface to the polar environment,
constituting a complete catalytic cycle of high performance.
2. Experimental
2.1. Catalyst preparation
The nano γ-Al2O3 used as support in current investigation
was obtained according to our previous report [26].
Fe2O3/Al2O3 sample was prepared by incipient wet impregna-
tion method with corresponding aqueous solution of
Fe(NO3)3·9H2O. The 25 wt% loading of Fe2O3 was set as the
weight ratio of Fe2O3/(Fe2O3 + Al2O3). The impregnating mix-
ture was sealed and aged at room temperature for 24 h. The
resulting material was dried at 90 °C overnight and calcined in
a tube furnace at 450 °C for 3 h in flowing air. The sample was
labeled as 25Fe2O3/Al2O3. The figure before the metal oxides
meant the content in weight percent of the transition metal
oxide in the catalyst.
The (Fe2O3-MOx)/Al2O3 (M = Mn, Co, Ni, Cu, Cr, Mo, V, Ti)
samples were also prepared by incipient wet impregnation
method. The precursors of the doped oxides were Mn(NO3)2,
Co(NO3)2·6H2O,
Ni(NO3)2·6H2O,
Cu(NO3)2·3H2O,
Cr(NO3)3·9H2O, (NH4)2MoO4·4H2O, NH4VO3, and tetrabutyl
titanate, respectively. The mass ratio of Fe2O3 to MOx was 4 : 1,
the loadings of (Fe2O3-MOx) in all samples were shown as the
weight ratio of (Fe2O3-MOx)/(Fe2O3-MOx + Al2O3) and fixed at

Changshun Deng et al. / Chinese Journal of Catalysis 41 (2020) 341–349
343
tion. The names and compositions of all the catalysts investi-
gated are elucidated in the Table 1 for clarity.
2.2. Catalyst characterization
The X-ray diffraction (XRD) patterns were recorded using a
XRD-6000 X-ray diffractometer (Shimadzu, Japan) with Cu Kα
radiation (0.15418 nm) as X-ray source. The voltage and cur-
rent were set at 40 kV and 30 mA, respectively. The surface
area and textural characteristics of the catalysts were meas-
ured by N2 sorption using a Micrometrics TriStar II 3020 ana-
lyzer. Transmission electron microscopy (TEM) images of cat-
alysts were obtained by using a JEOL JEM-2100 at a voltage of
180 kV. Thermogravimetric analysis and derivative thermo-
gravimetry (TG-DTA) analysis was conducted on a STA 449C
apparatus (Netzsch) in air from room temperature to 800 °C
with a heating rate of 10 °C/min. Inductively coupled plas-
ma-atomic emission spectrometry (ICP-AES) was carried out to
analyze the leached metal from the catalysts in the solution
after the reaction using an instrument of Optima 5300 DV
(PerkinElmer).
In-situ Fourier transform infrared (FT-IR) spectra of toluene
adsorption on the catalyst H-Fe25 were recorded to analyze the
interaction of toluene with the catalyst. The sample was
pressed to a self-supported disk in the diameter of 0.5 inch and
then was mounted in a quartz cell for evacuation, heat treat-
ment and adsorption of toluene. The transmission FT-IR spec-
tra were recorded through the CaF2 windows of the quartz cell
using a NEXUS870 FT-IR spectrometer.
Fig. 2. The schematic show of the process of catalyst preparation.
25 wt%. The treatment of these mixtures was similar to
Fe2O3/Al2O3 sample. These sample names were abbreviated as
Fe20Mn5, Fe20Co5, Fe20Ni5, Fe20Cu5, Fe20Cr5, Fe20Mo5, Fe20V5
(oxalate was used to help the dissolution of NH4VO3.), and
Fe20Ti5 (HCl was used to inhibit the hydrolysis of tetrabutyl
titanate during preparation.), respectively. In addition, a series
of (Fe2O3-xNiO)/Al2O3 or (Fe2O3-xCuO)/Al2O3 catalysts was
prepared with different doping amount of Ni or Cu and the
samples were correspondingly named as Fe20Ni5, Fe15Ni10
Fe10Ni15, Ni25, Fe20Cu5, Fe15Cu10, Fe10Cu15, and Cu25
,
.
The decoration of HDPA was performed using a modified
hydrothermal method. Taking the preparation of
HDPA-25Fe2O3/Al2O3 as an example, certain amount of HDPA
(Sigma-Aldrich) were added into a 100 mL beaker containing
40 mL of deionized water with vigorous stirring at 80 °C until
HDPA completely dissolved and then corresponding amount of
25Fe2O3/Al2O3 powder was added into the above mixture. After
stirring for 2 h, the solution was transferred into a Teflon-lined
autoclave and heated at 120 °C for 24 h. After that, the mixture
was washed with deionized water and absolute ethanol several
times and filtered by suction. The achieved solid powders were
dried at 80 °C overnight. For convenience, the final catalysts
were hereafter denoted as H-Fe25, H-Fe20Mn5, H-Fe20Co5,
2.3. Catalytic tests
H-Fe20Ni5, H-Fe20Cu5, H-Fe20Cr5, H-Fe20Mo5, H-Fe20V5, and
H-Fe25Ti5, respectively. For all of the samples, the HDPA loading
in the catalysts was expressed as the weight ratio of
HDPA/((Fe2O3-MOx)/Al2O3) or HDPA/(25Fe2O3/Al2O3) and
fixed at ~5 wt% (the density of surface HPDA was about ~1
HDPA/nm2 of the sample). The process of catalyst preparation
is schematically shown as Fig. 2. The nano alumina used as
support brings about great convenience in catalyst preparation
as well as the stability in performance of the catalyst in reac-
The liquid oxidation of toluene was performed in a 40-mL of
Teflon autoclave with 0.6 mL of toluene and 0.06 g of catalyst in
25 mL of deionized water, which formed biphasic system for
catalyst dispersion and transport of reactants and products.
The pH of the system was adjusted by H2SO4 solution. After
purging with O2 three times, 2.0 MPa of O2 was fed into the
autoclave. Then the autoclave was heated to reaction tempera-
tures under stirring. Qualitative determination and quantitative
Table 1
The abbreviated name of different samples and their corresponding compositions.
Sample name
FeM
Fe25
Composition of the sample
(Fe2O3-MOx)/Al2O3
Sample name
H-FeM
Composition of the sample
HDPA-(Fe2O3-MOx)/Al2O3
HDPA-25Fe2O3/Al2O3
25Fe2O3/Al2O3
H-Fe25
Fe20Mn5
Fe20Co5
Fe20Ni5
Fe15Ni10
Fe10Ni15
Ni25
(20Fe2O3-5MnOx)/Al2O3
(20Fe2O3-5CoOx)/Al2O3
(20Fe2O3-5NiO)/Al2O3
(15Fe2O3-10NiO)/Al2O3
(10Fe2O3-15NiO)/Al2O3
25NiO/Al2O3
H-Fe20Mn5
H-Fe20Co5
H-Fe20Ni5
H-Fe15Ni10
H-Fe10Ni15
H-Ni25
HDPA-(20Fe2O3-5MnOx)/Al2O3
HDPA-(20Fe2O3-5CoOx)/Al2O3
HDPA-(20Fe2O3-5NiO)/Al2O3
HDPA-(15Fe2O3-10NiO)/Al2O3
HDPA-(10Fe2O3-15NiO)/Al2O3
HDPA-25NiO/Al2O3
FeNi
(Fe2O3-NiO)/Al2O3
H-FeNi
HDPA-(Fe2O3-NiO)/Al2O3
HDPA-(20Fe2O3-5CuO)/Al2O3
HDPA-(15Fe2O3-10CuO)/Al2O3
HDPA-(10Fe2O3-15CuO)/Al2O3
HDPA-25CuO/Al2O3
Fe20Cu5
Fe15Cu10
Fe10Cu15
Cu25
(20Fe2O3-5CuO)/Al2O3
(15Fe2O3-10CuO)/Al2O3
(10Fe2O3-15CuO)/Al2O3
25CuO/Al2O3
H-Fe20Cu5
H-Fe15Cu10
H-Fe10Cu15
H-Cu25
FeCu
(Fe2O3-CuO)/Al2O3
H-FeCu
HDPA-(Fe2O3-CuO)/Al2O3

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Changshun Deng et al. / Chinese Journal of Catalysis 41 (2020) 341–349
Fig. 3. TEM images ofγ-Al2O3 (a) and H-Fe25 (b) samples.
analysis were conducted by GC-9560 (Shanghai Huaai) gas
chromatography equipped with a SE-54 column and FID de-
tector. After reaction, ethylbenzene was added as internal
standard to calibrate the conversion of reactant and selectivity
of products.
3. Results and discussion
3.1. Characterization of H-FeM nanorods
Fig. 3 depicts the TEM images for observation on the mor-
phology and microstructure of the catalysts. The alumina sup-
port and the H-Fe25 show almost the same morphologies of
nanorods, implying that Fe2O3 mainly exists in a highly dis-
persed state on the γ-Al2O3 surface. According to the specific
surface area of the alumina used, 132.7 m2/g, the thickness of
iron oxide in amount of 25 wt% can be calculated as ~2–3
atomic layers, which is too thin to be clearly observed by the
common TEM. The morphological features of H-FeM (not
shown) are also very similar to that of H-Fe25 and the results
are consistent with the XRD measurements, which, as shown in
Fig. 4, indicate the alumina is γ-Al2O3 (JCPDS no.29–0063). Af-
ter loading with iron oxide, apart from the characteristic peaks
of γ-Al2O3, some other weak peaks ascribed to Fe2O3 are de-
tected. But the extra characteristic peaks ascribed to the other
metal oxides are not observed for H-FeM, suggesting that these
metal oxides are well dispersed on the corresponding catalysts.
For the samples H-Fe20Ni5, H-Fe15Ni10 and H-Fe10Ni15, the dif-
fraction peaks other than those ascribed to γ-Al2O3 cannot be
Fig. 5. The N2 adsorption-desorption isotherms and BJH pore size dis-
tribution curves (inset) of the typical samples.
observed, indicative of the highly dispersion of the mixture of
oxides of iron and nickel in the catalysts (Fig. 4(b)). For the
catalyst H-Fe10Cu15, however, the diffraction peaks ascribed to
CuO are evident, implying the not so good compatibility or
mixing of copper oxide with the iron oxide on the γ-Al2O3 sup-
port (Fig. 4(c)). For the catalysts H-FeCr, H-FeMo, and H-FeV,
further characterizations on their structures have not been
performed, because of their complete inertness for the reac-
tion.
The N2 sorption isotherms of typical samples (Fig. 5) exhibit
all the IV-type isotherms with evident H2-type hysteresis loops,
which is characteristic of irregular mesostructure and inter-
stice mesoporous structure formed by nanoparticle assembly
according to IUPAC [27,28]. After loading metal oxides on the
Fig. 4. XRD patterns of the as prepared samples.

Changshun Deng et al. / Chinese Journal of Catalysis 41 (2020) 341–349
345
Table 2
Textural parameters of the γ-Al2O3, Fe25, and FeM samples.
Pore volume
(cm3/g)
0.48
Average pore
diameter (nm)
12.4
Sample
S
BET (m2/g)
γ-Al2O3
Fe25
Fe20Mn5
Fe20Co5
Fe20Ni5
Fe20Cu5
132.7
101.9
107.9
109.6
114.7
107.4
0.40
0.32
0.38
0.40
14.8
10.6
13.0
12.7
0.38
13.1
γ-Al2O3 surface, the porous textures of the samples seem un-
changed much compared with the support alumina. The tex-
tural parameters of the samples are summarized in Table 2.
The results of FT-IR and TG measurements, as shown in Fig.
6, confirm the existence of HDPA on the surface of FeM nano-
rods. The two peaks located at 3000–2800 cm–1 are attributed
to the stretching vibration of –CH2– and they show up with the
samples decorated by HDPA adsorption [29]. By the results of
TG measurements (Fig. 6(b)), the weight losses from the HDPA
adsorbed samples are close to 5 wt%, similar to the amount of
hexadecyl in the HDPA added in preparation of the catalyst.
According to these results, the density of HDPA on the surface
of the catalyst nanoparticles is ~1 HDPA/nm2, which should be
optimal to the catalytic performance of the catalyst for toluene
oxidation and by tuning the active sites on the surface of FeM
nanorods and the access of the reactants to the reactive sites,
and the full coverage of HDPA on the surface of these samples is
about 15 wt% according to our previous works [2].
Fig. 6. FT-IR spectra (a) and TG results (b) of the typical samples.
3.2. Catalytic performance of the catalysts
As shown in Fig. 7(a), the conversion of toluene is ~71%
and benzaldehyde is the only product with H-Fe25 used as cata-
lyst. A control experiment with Fe25 as catalyst reports a con-
version of toluene below 8% with a lower selectivity of ben-
zaldehyde (~40.1%), other byproducts are benzyl alcohol
(20.3%) and benzoic acid (39.6%). This means that the modifi-
cation of HDPA greatly promotes the toluene conversion and
ensures an exclusive selectivity in this catalytic system. With
the modification of other metal oxides, the benzaldehyde is still
the only product and the doping of NiO or CuO to H-Fe25 gives
Fig. 7. (a–c) The catalytic performances of H-FeM nanorods for the toluene oxidation (toluene: 0.6 mL; catalyst: 0.06 g; water: 25 mL; pH: 2.5; pO2: 2
MPa; temperature: 180 °C; time: 4 h); (d) XRD patterns of the spent H-FeM catalysts; (e) The TEM image of the spent H-Fe25 catalyst. *The concentra-
tion of Fe3+ and Ni2+ equals respectively to that listed in Table 3 determined by ICP.

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Changshun Deng et al. / Chinese Journal of Catalysis 41 (2020) 341–349
promoted catalytic performance. Surprisingly, the catalysts are
entirely inactive for the reaction when high valence metal ox-
ides such as Cr, Mo, V, or Ti are used as dopants, even the dope
amount is only 1~5 wt%, implying the limitation of the dopants
on the mobility of the lattice oxygen of the catalysts.
In order to further investigate the dope effect of NiO or CuO
on the property of the catalyst for the reaction, the H-FeM na-
norods with different molar ratio of Ni (or Cu) to Fe were pre-
pared and tested for the reaction, as shown in Fig. 7(b) and
7(c). It can be seen from the figures that the catalyst with iron
oxide as major active constituent and minor dopant of nickel
oxide gives the best catalytic performance for the reaction and,
interestingly, the catalyst with nickel oxide or copper oxide as
major active constituent shows fairly good catalytic perfor-
mance for the reaction. But the stability of iron oxide catalyst is
better than that of the nickel oxide and copper oxide catalysts,
as discussed below.
After the use in reaction, the crystal structures (XRD) and
morphology (TEM) of the catalysts, as shown in Figs. 7(d) and
7(e) are similar to those before reaction (Fig. 4(a) and Fig.
3(b)), indicating that the inherent structure of this kind of
composite catalysts is rather stable. In addition, the concentra-
tion of leaching ions from the catalysts is always very low in the
aqueous solution after reaction, though the leaching amount to
the solution of Mn, Co, Ni, or Cu ions seems a little higher than
that of iron (Table 3). As to the functions for the toluene con-
version of the metal ions in the solution, control experiment
with only Fe3+ and Ni2+ ions in the solution as catalysts, in simi-
lar concentrations to those with H-Fe20Ni5 as catalyst, was car-
ried out and the conversion of toluene is only ~5% with a ben-
zaldehyde selectivity of ~32% (Fig. 7(b)), other byproducts are
benzyl alcohol (45.2%) and benzoic acid (22.8%). These facts
indicate that the catalytic activity is mainly produced by the
metal oxides composite nanoparticles and Fe2O3 is very im-
portant in this catalytic system. Therefore, it can be concluded
that the loading of Fe2O3 has an essential influence on toluene
conversion and the HDPA appears to ensure the exclusive se-
lectivity of benzaldehyde as well as to promote the conversion
of toluene. The moderately doped NiO is beneficial to increase
the catalytic performance of the catalyst.
Fig. 8. (a) Effect of pH on toluene oxidation (Other reaction conditions
are the same); (b) Effect of reaction temperature on toluene oxidation
(Other reaction conditions are the same).
The optimal pH value is ~2.5, neither lower nor higher pH val-
ue is detrimental to the catalytic activity. According to a previ-
ous results [30], the oxidation of toluene proceeded by hydride
(H–) transfer from toluene to the catalyst in water, neverthe-
less, hydride (H–) was replaced by hydrogen atom (H·) in tolu-
ene solution. Moreover, the catalyst whose surface is positively
charged will benefit to the transfer of hydride (H–) to the cata-
lyst and boost the oxidation of toluene. Therefore, relatively
lower pH value in the biphasic catalytic system will lead to the
surface of H-Fe20Ni5 positively charged. More protons in the
solution, however, will lead to an unstable surface of the cata-
lyst. Fig. 8(b) shows the influence of temperature on the reac-
tion and the optimal value of temperature is 180 °C with exclu-
sive selectivity. Higher temperatures used would result in the
homogeneous oxidation initiated by O2 diradicals and generate
byproducts. The turnover frequencies (TOF, in unit of nm–2 s–1)
based on the surface area of the catalysts are listed in Table 4.
3.4. In-situ FT-IR study to explore the surface species during the
toluene oxidation
In order to explore the interaction of toluene with the cata-
lysts, in-situ FT-IR spectra of the catalyst H-Fe25 were recorded
with the surface adsorbed toluene. The sample was pressed
into a self-supported disk of 0.5 inch in diameter and then was
mounted in a quartz cell for evacuation, heat treatment and
adsorption of toluene. The transmission FT-IR spectra were
recorded through the CaF2 windows of the quartz cell using a
NEXUS870 FT-IR spectrometer. The results are shown in Fig. 9.
The bands at ~2912 and ~2847 cm–1 are attributed to the vi-
3.3. Effects of pH value and reaction temperature
With the H-Fe20Ni5 is used as catalyst in the reaction, it is
found that, similar to HDPA-FeOx catalyst, the pH value has a
prominent effect on toluene conversion, as shown in Fig. 8(a).
Table 4
Table 3
The turnover frequency (TOF) of the catalysts for toluene conversion.
The concentrations of metallic ions in aqueous solution after reaction
determined by ICP.
TOF
(nm–2 s–1 × 10–2
2.34
TOF
(nm–2 s–1 × 10–2
1.88
Sample
Sample
)
)
Concentrations (ppm)
Sample
H-Fe25
H-Fe10Ni15
H-Ni25
H-Fe20Mn5
H-Fe20Co5
H-Fe20Ni5
H-Fe20 Cu5
H-Fe15Ni10
0.86
1.62
2.74
2.07
1.61
1.53
1.41
2.27
0.16s-1
Fe
7.2
6.7
7.1
9.3
6.4
Mn
–
Co
–
Ni
–
Cu
–
H-Fe15Cu10
H-Fe10Cu15
H-Cu25
H-Fe25
H-Fe20Mn5
H-Fe20Co5
H-Fe20Ni5
26.7
–
–
–
12.2
–
–
–
15.9
–
–
–
–
41.5
2.05
*Fe3+, Ni2+
* The concentrations of Fe3+ and Ni2+ equal to those listed in Table 3
H-Fe20Cu5
–
–
determined by ICP.

Changshun Deng et al. / Chinese Journal of Catalysis 41 (2020) 341–349
347
touching the catalyst surface. The phosphate head in HDPA
adsorbed on the surface may tune the catalyst surface to switch
off unneeded sites unselective for the toluene oxidation. The
hole dispersion system forms a Pickering emulsion under the
stirring conditions, preventing the aggregation of droplets [34].
In addition, the water is helpful for the transfers of O2 from
gaseous as well as the products from the surface of the catalyst
to the liquid phase (Figs. 1(b) and 1(c)), considering their rela-
tively better solubility in water. In the process, the HDPA plays
a role indispensable. As the consequence of synergistic interac-
tions in the catalytic system, high-quality benzaldehyde is pro-
duced in exclusive selectivity at high conversions of toluene
under mild conditions with gaseous O2 as oxidant.
4. Conclusions
Fig. 9. Spectra of the samples. (a) toluene; (b) benzaldehyde; (c) H-Fe25
evacuated at room temperature; (d) H-Fe25 evacuated and followed by
adsorption of gaseous toluene at room temperature for 30 min; (e)
H-Fe25 evacuated at room temperature and followed by toluene adsorp-
tion at 180 °C for 30 min. The spectrum was recorded at room temper-
ature after the sample disk cooling down to room temperature and the
gaseous toluene in the quartz cell was pumped out.
In summary, we have demonstrated a reliable strategy to
prepare hexadecylphosphate acid (HDPA)-functionalized
(Fe2O3-MOx)/Al2O3 catalysts for the oxidation of toluene to
benzaldehyde in toluene/water biphasic system with exclusive
selectivity of benzaldehyde under mild conditions. The use of
Al2O3 nanorods as support is greatly beneficial to the prepara-
tion and modification of the catalyst, with which the catalyst is
more stable and easily tuned by deposition of HDPA and the
dope of other metals. In the catalytic system, the
HDPA-functionalized (Fe2O3-MOx)/Al2O3 nanoparticles locate at
the interface between oil and water phases to stabilize the
Pickering emulsion. The loading of Fe2O3 and the HDPA are
essential to ensure the exclusive selectivity of benzaldehyde
and the catalytic activity for toluene conversion. Among the
metal oxides of Mn, Co, Ni, Cu, Cr, Mo, V and Ti used as modifi-
ers, the moderately doped nickel oxide enhances the catalytic
performance of the system. Under optimal conditions, ~83% of
toluene conversion and ~100% selectivity to benzaldehyde are
obtained with molecular oxygen used as oxidant. This work
displays an effective process, green and low cost, to produce
high-quality (halogen-free) benzaldehyde from toluene oxida-
tion using O2 as oxidant and it should also be useful for other
similar reactions to produce important chemicals in high qual-
ity.
brations of C–H bonds in the methyl or methylene groups of the
toluene (curve a) or the HDPA in H-Fe25 (curves c, d and e) [29,
31]. The absorptions at ~3079 and ~3027 cm–1 should be re-
ferred to the vibration of C–H bond connected to the aromatic
ring of toluene. It appears that the toluene has hardly adsorbed
at room temperature but strongly adsorbed on the catalyst at
180 °C (curve e), by the intensities of the peaks at ~3079 and
~3027 cm–1. More importantly, the absorption at ~1219 cm–1
shows up by the chemical adsorption of toluene at 180 °C
which can be assigned to the formation of R–C–O–Fe by the
insertion of the lattice oxygen of Fe2O3 into the C–H bond of
benzyl [32,33]. Coincidently, the shoulder peaks near 3079 and
3027 cm–1 (i.e., 3091 and 3044 cm–1) are also observed, which
are reasonably attributed to the vibrations of aromatic C–H
bonds in the adsorbed C6H5–C–O–Fe, which are also influenced
by the narrow HDPA lanes. Summing up the results, we specu-
late that the lattice oxygen species are involved in the conver-
sion of toluene to benzaldehyde with the assistance of HDPA at
the catalyst surface.
Notes
3.5. Possible mechanism for the oxidation of toluene
The authors declare no competing financial interest.
With the results presented above, the mechanism of the cat-
alytic system seems clear (Fig. 1). The HDPA-functionalized
oxides nanorods locate themselves at the interface of toluene
and water in the biphasic system as observed by laser confocal
fluorescence microscope (Fig. 1(d)), and their hydro-
philic/hydrophobic characteristics play an important role to
stabilize the O/W emulsion (Fig. 1(a)). The surface HDPA mol-
ecules in a suitable density protect the nanorods from gather-
ing and endow the catalyst amphipathic property. The HDPA in
a surface density of ~1 molecule per nm2 boosts the access of
toluene toward the catalyst surface and, more important, it
seems that HDPA molecules can align the toluene molecules
Acknowledgments
The authors thank the financial supports from the National
Natural Science Foundation NSF of China (91434101,
91745108) and the MOST of China (2017YFB0702900). The
financial support from the Shanghai Research Institute of Pet-
rochemical Technology of Sinopec is also appreciated.
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Changshun Deng et al. / Chinese Journal of Catalysis 41 (2020) 341–349
Graphical Abstract
Chin. J. Catal., 2020, 41: 341–349 doi: S1872-2067(19)63417-0
Exclusively catalytic oxidation of toluene to benzaldehyde in an O/W emulsion stabilized by hexadecylphosphate acid
terminated mixed-oxide nanoparticles
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十六烷基膦酸配合的复合氧化物纳米催化剂稳定的O/W乳液中
甲苯单一氧化为苯甲醛
邓长顺a, 许孟霞a, 董 珍a, 李 磊a,b, 杨金月a, 郭学峰a, 彭路明a, 薛念华a, 祝 艳a, 丁维平a,*
a南京大学化学化工学院介观化学教育部重点实验室, 江苏南京 210093
b中石化炼化工程(集团)股份有限公司洛阳技术研发中心, 河南洛阳 471003
摘要: 苯甲醛是一种用途广泛的重要化学品, 通过O2氧化甲苯制取苯甲醛是最佳生产途径, 也是近几十年来工业界迫切
需要的反应之一. 虽然该反应在苄基上结合一个氧再脱除两个氢即可, 对该反应的多相催化过程也已经研究了几十年,
但其性能仍远远低于工业要求. 当前的工业过程主要有甲苯氯化水解法和甲苯均相氧化法两种, 但都存在严重的环境污
染和腐蚀问题, 且产品中含有少量卤素, 阻碍了其在诸如香水或食品中的高端应用. 近年来, 以O2作为氧化剂及Pd, Au,
Pt, Ag, Ru等贵金属或它们间的合金为催化剂的甲苯液相氧化反应研究取得了一些很好的进展, 但仍然不能在高甲苯转化
率下高选择性地得到苯甲醛. 本课题组曾报道了一种高效的混相催化体系, 以O2作为氧化剂将甲苯专一地催化氧化为苯
甲醛, 其中十六烷基膦酸-氧化铁(HDPA-FeOx)纳米颗粒处在甲苯和水的界面上, 稳定了该O/W类皮克林乳液(Pickering).
为了进一步提高催化剂晶格氧的移动性以提升催化活性，本文采用Mn, Co, Ni, Cu, Cr, Mo, V和Ti等一系列金属氧化物对催
化剂HDPA-FeOx进行掺杂, 同时使用一种特殊的纳米Al2O3作为载体, 大大地增加了催化剂制备的便捷性和保证了催化剂
在实际应用中的稳定性.
TEM和XRD结果表明, Al2O3负载了金属氧化物后, 其形貌仍为纳米棒状结构, 并只能观察到Al2O3的晶相衍射峰, 表
明金属氧化物均匀地负载在其表面. BET结果表明, 负载后的催化剂的孔结构与载体Al2O3类似. FT-IR结果表明, HDPA很
好地吸附在了催化剂表面. TG结果表明, 催化剂中HDPA含量与加入量相符, 质量分数为~5%. 结合前期工作可知, HDPA
能够调整FeM纳米棒表面催化性质, 且以1 HDPA/nm2的密度为最佳, 此时, 甲苯液相氧化为苯甲醛的催化性能最佳.
催化性能测试结果表明, 催化剂吸附了HDPA后, 甲苯的转化率显著增加, 且只生成苯甲醛. 在所考察的第二种掺杂
–
–
1
2
金属中, 以Ni的效果为最好. 该催化剂在最佳反应条件下, 甲苯转化率为83% (TOF = 0.027 nm ·s ), 苯甲醛选择性为
~100%. 而Cr, Mo, V和Ti等高价金属则抑制了该反应, 这也说明通过掺杂第二种金属调变晶格氧的活动性可影响反应性
能. 经过优化后, 最佳反应条件为: pH值为2.5, 反应温度为180 °C. 原位FT-IR结果表明, 180 °C下, 甲苯在吸附有HDPA的
催化剂表面能够发生化学吸附, 苄基C–H键解离并与晶格氧产生结合, 形成了C6H5–CH2–O–Fe中间物种, 该物种脱附即得
苯甲醛. 该温度下, 表面HDPA对甲苯的化学吸附不可缺.
关键词: 甲苯氧化; 苯甲醛; 十六烷基膦酸; 分子氧; 皮克林
收稿日期: 2019-05-08. 接受日期: 2019-05-30. 出版日期: 2020-02-05.
*通讯联系人. 电话: (025)89685077; 传真: (025)89687761; 电子信箱: dingwp@nju.edu.cn
基金来源: 国家自然科学基金(91434101, 91745108); 国家重点研发计划(2017YFB0702900).
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).