Magnetic nano-structured cobalt-cobalt oxide/nitrogen-doped carbon material as an efficient catalyst for aerobic oxidation of p-cresols

Article abstract of DOI:10.1016/j.mcat.2018.05.005

Efficient aerobic oxidation has been developed for the selective preparation of a sequence of valuable p-hydroxybenzaldehydes from corresponding p-cresols, using a new magnetically separable catalyst of nano-structured cobalt-cobalt oxide/nitrogen-doped carbon (CoO_x@CN) material. CoO_x@CN showed high activity for the 2-methoxy-4-cresol oxidation to vanillin, giving great yield (90%) and with good turnover number (210), as well as other p-cresols in good to great yields. The catalytic performance was investigated and related to the structural, chemical and magnetic properties which determined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FT-IR) and vibrating sample magnetometer (VSM). The effects of base to substrate molar ratio, catalyst concentration, temperature, and solvent on the conversion and selectivity patterns also have been studied. The investigation revealed that remarkable catalytic properties of CoO_x@CN could be ascribed to the active species cobalt oxide, doped nitrogen and porous carbon with large surface area. The size of the catalyst is a key factor for catalyst performance. The ferromagnetic property of catalyst enables to recycle easily by an external magnetic field and reuse six successive times without significant activity loss.

Full text of DOI:10.1016/j.mcat.2018.05.005

MolecularCatalysis453(2018)121–131
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Magnetic nano-structured cobalt–cobalt oxide/nitrogen-doped carbon
material as an efficient catalyst for aerobic oxidation of p-cresols
Cheng Lianga, Xuefeng Lib, Diefeng Sub, Qiyi Maa, Jianyong Maoc, Zhirong Chena, Yong Wangb,
⁎
Jia Yaob, Haoran Lia,b,
a
State Key Laboratory of Chemical Engineering, ZJU-NHU United R&D Center College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, 310027,
PR China
b
Department of Chemistry, Zhejiang University, Hangzhou, 310027, PR China
Zhejiang NHU Company Ltd., Xinchang, 312500, PR China
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
Efficient aerobic oxidation has been developed for the selective preparation of a sequence of valuable p-hy-
droxybenzaldehydes from corresponding p-cresols, using a new magnetically separable catalyst of nano-struc-
tured cobalt–cobalt oxide/nitrogen-doped carbon (CoOx@CN) material. CoOx@CN showed high activity for the
2-methoxy-4-cresol oxidation to vanillin, giving great yield (90%) and with good turnover number (210), as well
as other p-cresols in good to great yields. The catalytic performance was investigated and related to the struc-
tural, chemical and magnetic properties which determined by scanning electron microscopy (SEM), transmission
electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier-transform
infrared spectroscopy (FT-IR) and vibrating sample magnetometer (VSM). The effects of base to substrate molar
ratio, catalyst concentration, temperature, and solvent on the conversion and selectivity patterns also have been
studied. The investigation revealed that remarkable catalytic properties of CoOx@CN could be ascribed to the
active species cobalt oxide, doped nitrogen and porous carbon with large surface area. The size of the catalyst is
a key factor for catalyst performance. The ferromagnetic property of catalyst enables to recycle easily by an
external magnetic field and reuse six successive times without significant activity loss.
Aerobic oxidation
Heterogeneous catalyst
Magnetic
Cobalt
p-Hydroxybenzaldehydes
1. Introduction
and chloroform to give o-hydoxybenzaldehyde as a major product and
p-hydoxybenzaldehyde as a byproduct [11–13]; (ii) Glyoxylic acid
The selective direct oxidation of sp3 hybridized inert CeH bond
towards value-added oxygenates under mild conditions has been a
highly intriguing research topic not only in fundamental research but
also in industrial chemical synthesis [1–4]. P-cresols are one of typical
substituted phenols with three major CeH bonds that can be oxidized to
a series of oxygenated compounds such as p-hydroxy benzyl alcohols
[5], p-hydroxybenzaldehydes [6], and p-hydroxy benzoic acids [7],
which serve as versatile chemical intermediates. Among these, p-hy-
droxybenzaldehyde is the most important product in our daily life. The
versatile multifunctional p-hydroxybenzaldehydes such as vanillin [8],
3, 5-dimethyl-4-hydroxybenzaldehyde [9], and syringaldehyde [10]
have served as important feedstocks for pharmaceutical, perfumes,
dyes, and polymers.
process in which guaiacol and its homologues react with glyoxylic acid
to give vanillin and its homologues [14,15]; (iii) Classical method of
oxidation by using stoichiometric oxidants [16–18]. These processes
suffer severely from serious effluent disposal, multi-step procedures and
inefficiency due to lower yields. Hence, the use of molecular oxygen
with a suitable catalyst which involves the direct benzylic C(sp3)-H
oxyfunctionalization of non-substituted p-cresols such as p-cresol
[19–24], 2, 4, 6-trimethylphenol [25–28] and other substituted p-cre-
sols [29–35] is a preferred alternative for liquid–phase oxidations.
Many reports are available on direct oxidation of non-substituted p-
cresol using homogeneous as well as heterogeneous catalysts [19–24].
In almost all these reports, high pressure and large amount of catalyst
are needed. However, as for substituted p-cresols, this process has been
conventionally carried out by homogeneous complexes catalysts con-
taining metals like Co, Fe, Cu, and Mn [25–35]. Our group have
Traditionally, p-hydroxybenzaldehydes are synthesized by the fol-
lowing methods: (i) Reimer-Tiemann process from phenolic compound
⁎
Corresponding author at: State Key Laboratory of Chemical Engineering, ZJU-NHU United R&D Center College of Chemical and Biological Engineering, Zhejiang University,
Hangzhou 310027, PR China.
E-mail addresses: rebeccalc@zju.edu.cn (C. Liang), xuefeng_l@yeah.net (X. Li), ipersist@163.com (D. Su), maqy@zju.edu.cn (Q. Ma), chemmjy@zju.edu.cn (J. Mao),
chenzr@zju.edu.cn (Z. Chen), chemwy@zju.edu.cn (Y. Wang), yaojia@zju.edu.cn (J. Yao), lihr@zju.edu.cn (H. Li).
https://doi.org/10.1016/j.mcat.2018.05.005
Received 22 January 2018; Received in revised form 11 April 2018; Accepted 4 May 2018
2468-8231/©2018ElsevierB.V.Allrightsreserved.

C. Liang et al.
MolecularCatalysis453(2018)121–131
reported cobalt Schiff base with ionic ligand as a catalyst which can
oxidize 2-methoxy-4-cresol to vanillin efficiently [30]. However, the
complexity of the catalyst blocked its application in industry. Recently,
Ji and co-workers [32] reported Co(OAc)2/NaOH/O2 reaction system
for efficient oxygenation of a broad scope of electron withdrawing
Inspired by above results, we developed nano-structured co-
balt–cobalt oxide/nitrogen-doped carbon material (CoOx@CN) pre-
pared by a simple protocol as a novel magnetic heterogeneous catalyst.
It was found that CoOx@CN exhibits excellent qualities in selective
oxidations of varied p-cresols into correspond p-hydroxybenzaldehydes
with molecular oxygen under mild reaction conditions (353 K and
ambient pressure). On part of recycling, the catalyst can be easily se-
parated by an external magnet. Physicochemical properties obtained
from various characterizations are closely related with catalyst activity.
The effects of various reaction parameters such as base to substrate
molar ratio, temperature, catalyst concentration, and solvent on the
conversion of p-cresols and selectivity of p-hydroxybenzaldehydes were
also investigated. To our best knowledge, this is the first report de-
monstrating the utilization of CoOx@CN for magnetically recyclable
catalytic oxidation of p-cresols.
group-substituted
or
di-substituted
p-cresols
to
p-hydro-
xybenzaldehydes. It is a homogeneous catalyst system, besides, in-
separable tars (oligomers) generated from coupling side-reactions when
apply to mono-electron-donating group substituted p-cresols. While
homogeneously catalyzed liquid-phase oxidation using molecular
oxygen is a common practice in industry, these oxidation reactions have
drawbacks such as poor selectivity because of high active nature of free
radical intermediates [22], catalyst deactivation from metal μ-oxo
species [36], catalyst reusability and redundant recovery due to
homogeneous complex catalysts. Therefore, designing and constructing
of a proper heterogeneous catalyst, which has the strengths of ease of
recovery and amenability for continuous processing in p-cresols oxi-
dation is desired and significant [37].
2. Experimental
The nano-structured catalyst containing cobalt has wide application
as heterogeneous catalyst in the liquid-phase catalytic oxidation
[38–40] including liquid-phase oxidation of p-cresols [21,23,24].
Owing to the bad dispersion and agglomeration of nanoparticles (NPs),
diverse supports have been applied to immobilize them. In recent years,
nitrogen-doped porous carbon materials, as catalysts or catalyst sup-
ports, have aroused wide concern in heterogeneous hydrogenations and
oxidations [41–45]. On this point, the hybrid of cobalt NPs on nitrogen-
doped porous carbon offers a potential way to achieve high-perfor-
mance catalysts [46–48]. However, the inconvenient and inefficient
separation and recycling of supported NPs suffered from tedious cen-
trifugation or filtration processes impeded the economics and sustain-
ability of nano-structured catalyst [49]. In terms of practical applica-
tion, magnetically recyclable nanocatalyst development provides a
promising and effective option [50–53]. Magnetic nanocatalysts are
simply and efficiently recovered by an external magnetic field, sa-
tisfying the need of catalysts recovery and reusability. Various types of
magnetic nanocatalysts have been developed in recent years in cata-
lyzing oxidation reactions due to their high efficiency and recyclability.
2.1. Chemicals
All materials were of commercial reagent grade. D(+)-glucosamine
hydrochloride, melamine, glucose, cobalt (II) nitrate hexahydrate
(CoNO3·6H2O), Co3O4 and NaOH were obtained from Aladdin. 2-
Methoxy-4-cresol was purchased from Sahn Chemical Technology Co.,
Ltd. and other p-cresols were purchased from J&K Chemicals. The al-
dehydes and ether intermediate used as external standard in HPLC and
all the solvents ethylene glycol monomethyl ether (EGME), methanol
(MeOH), ethanol (EtOH), ethylene glycol (EG), ethylene glycol di-
methyl ether (EGDE) and ethyl acetate (EA) used of the highest com-
mercial quality supplied from J&K Chemicals and used without further
treatment. N2 was used for the catalyst preparation. O2 with purity of
≥99.99 mol% and air were used for the oxidation reaction.
2.2. Catalyst preparation
Nano-structured catalysts were prepared by thermal condensation
method reported earlier [54]. Accordingly,
a mixture of 1 g D
Fig. 1. Morphology analysis of CoOx@CN. (a) SEM image. (b) TEM image with an inset showing the particle-size distribution. (c)(d) HRTEM images.
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MolecularCatalysis453(2018)121–131
2.4. Oxidation of p-cresols
In a typical procedure, NaOH (5.4 g, 135 mmol) was first dissolved
with 20 g EGME in a four-necked round-bottom flask, then 50 mmol
substrate and 0.04 g catalyst (cobalt/substrate molar ratio, 0.4%) were
added to the above solution. The flask was equipped with a condenser,
a stirrer and an air-vent needle for continuous flow of 100 mL min−1
O2. The stirring speed was 600 rpm to minimize the effect of mass
transfer (Fig. S1). The reaction temperature is 80 °C and the reaction
time is 7 h. After the reaction, the CoOx@CN was separated by an ex-
ternal magnet for reuse and the decanted reaction solution was diluted
with water.
A high performance liquid chromatography (HPLC)
(Shimadzu LC-20AT) equipped with a UV detector connected to a C18
reversal pillar (size: 250 × 4.6 mm) was used for product analysis. A
mixture of methanol and water (volume ratio, 40:60) was used as
mobile phase at a flow rate of 1.0 mL min−1. The column temperature
was 40 °C and the UV detection wavelength was 230 nm. HPLC and
high performance liquid chromatography-mass spectrum (HPLC–MS)
were used to test the products and intermediates by comparison with
standard chemicals. The conversions (conv.) of p-cresols, yields (Yi) of
p-hydroxybenzaldehydes, selectivity (Si) of p-hydroxybenzaldehydes
and turnover numbers (TONs) of catalysts are defined as following
equations:
Fig. 2. XRD patterns of Co@CN, CoOx@M, CoOx@G and CoOx@CN.
(+)-glucosamine hydrochloride (GAH), 40 g melamine and 1.3 g cobalt
nitrate hexahydrate (CoNO3·6H2O) were dissolved in deionized water
and then stirred under 80 °C overnight to evaporate the water. The
dried solid was grinded into powder and directly calcined at 800 °C in
N2 atmosphere to give 0.92 g CoOx@CN. For comparisons, CoOx@G and
CoOx@M were made from mixtures of CoNO3·6H2O & GAH and
CoNO3·6H2O & melamine at 800 °C. In a similar way, CN and C were
moles of p-cresols reacted
moles of starting p-cresols
Conv.(%) =
× 100
moles of p-hydroxybenzaldehydes
moles of starting p-cresols
Y(%) =
× 100
× 100
i
moles of p-hydroxybenzaldehydes
moles of p-cresols reacted
made from mixtures of melamine
& GAH and glucose without
Si(%) =
CoNO3·6H2O at 800 °C. Furthermore, the Co@CN, which was made up
of metallic cobalt was acquired by acid treatment with 2 mol/L hy-
drochloric acid for 4 days at 50 °C. The cobalt oxide was removed out
which was confirmed by XRD (Fig. 2).
moles of product produced with catalyst
− moles of product produced without catalyst
TON =
moles of catalyst
2.3. Characterization of catalysts
3. Results and discussions
The Co content of CoOx@CN was 29.81% measured by inductively
coupled atomic emission spectrometer (ICP-AES, Thermo, iCAP6300).
The N content was measured by elemental analysis on Varo MACRO.
The N content of CoOx@CN was 1.66%. The O and C content of
CoOx@CN were 5.77% and 62.81%, respectively, measured by scan-
ning transmission electron microscopy-energy dispersive spectrometer
(STEM-EDS) elemental mapping. Scanning electron microscope (SEM,
Model 8100), transmission electron microscopy (TEM) (Model JEM-
1230), high resolution TEM (HRTEM, Tecnai G2 F30 S-Twin) was op-
erated to observe the catalyst morphology. Powder X-ray diffraction
(XRD) patterns were conducted on a D/tex-Ultima TV wide angle X-ray
diffractometer equipped with Cu Kα radiation (1.54 Å) at room tem-
perature. The Fourier-transform infrared spectroscopy (FT-IR) spectra
of CoOx@CN were acquired with KBr method in the range of
400–4000 cm−1 on a Nicolet FT-IR/Nexus 470. The X-ray photoelec-
tron spectra (XPS) were acquired with an ESCALAB MARK source. C 1s
line at 284.6 eV was used to correct all XPS spectra. The specific surface
area measured by using micromeritics ASAP 2020 HD88 based on the
Brunauer–Emmett-Teller (BET) method. Magnetization was measured
by vibrating sample magnetometer (VSM, J3426, Cryogenic Limited) at
300 K, while the applied magnetic field is from −30 kOe to 30 kOe.
After the oxidation of p-cresols, the amount of cobalt leached from
CoOx@CN was measured by atomic adsorption spectrophotometer
(AAS, AA800, PerkinElmer).
3.1. Catalyst characterization
The morphologies of CoOx@CN were examined by SEM and TEM
techniques (Fig. 1). It showed the flake-like shape (Fig. 1a) morphology
resulting from melamine forming sheets by thermal decomposition
[55,56]. For comparison, several different component materials
(CoOx@M and CoOx@G) were prepared and characterized in detail.
The TEM images of the CoOx@G and CoOx@M demonstrated agglom-
eration phenomena (Fig. S2), but uniformly dispersed nanoparticles in
CoOx@CN (Fig. 1b). As shown in the elemental mapping images (Fig.
S3), the elements (Co, O, C, and N) are distributed evenly across the
whole skeletal structure, which verifying the uniform dispersion of four
elements. Besides, BET analysis (Table S1) reaveals that all these three
hybrids appear to be porous. But the specific surface area of CoOx@CN
is larger than those of the other two materials. Such morphology of
CoOx@CN originates both from melamine soft template [57] and ni-
trogen-doped carbon matrix [54] which enhance the nanoparticles
dispersibility. The particle size distribution histogram presented in
Fig. 1b (inset) demonstrates that the cobalt-based NPs average diameter
is roughly 13 nm. The HRTEM images confirm the component of Co0
and cobalt oxide. As shown in Fig. 1c, lattice fringe spaces of 0.289 and
0.468 nm are accordance with the (220) and (111) plane of cubic Co3O4
spinel-phase, respectively, but can hardly find CoO. The Co0 is also
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C. Liang et al.
MolecularCatalysis453(2018)121–131
Fig. 3. FT-IR spectra of the CoOx@CN catalyst.
found, evidenced by the lattice fringes with a d-spacing of 0.204 nm
corresponding to the (111) plane in Fig. 1d. The SEM and TEM of used
catalyst (after 6 recycles) (Fig. S4) showed that the morphology and
particle size of the CoOx@CN was unchanged during the catalysis.
As showed in the XRD results (Fig. 2), there were different forms of
cobalt species in the obtained samples. The XRD patterns of the
CoOx@CN, CoOx@M and CoOx@G materials display peaks attributed to
Co3O4, CoO and Co, respectively. The XRD pattern of Co@CN only
exhibit the distinct peaks of Co0, indicating almost all cobalt oxide has
been washed out by HCl and the Co0 is reserved. The broad peaks in the
range of 20–30(2θ) can be interpreted as the ordered amorphous
carbon species [58]. In addition, the small particle size of the cobalt-
based NPs make contributions to these broad diffraction peaks [54].
The particle sizes of Co and Co3O4 in CoOx@CN were calculated to be
14 and 10 nm, respectively, according to the Scherrer equation. The
value is in line with the results of the particle size distribution.
To obtain more information of structure and interaction of major
constitutes, FT-IR spectra of CoOx@CN are demonstrated in Fig. 3. The
peak around 3400 cm−1 is assigned to the OeH and NeH stretching,
related from surface absorbed water and uncondensed amino from
melamine, respectively [59]. The peaks at 2918 and 2851 cm−1 were
ascribed to eCH3 symmetric and asymmetric stretching vibrations, re-
spectively. Peaks at 1581 and 1203 cm−1 were attributed to the C]N
and CeN vibrations [60], which indicated nitrogen was successful
doped into carbon material. Additionally, the Co3O4 spinal phase was
evidenced by fingerprint band at 665 and 570 cm−1 [61].
Fig. 5. Hysteresis loop of CoOx@CN catalyst.
confirmed the formation of metallic cobalt and cobalt oxide. The
comparison of the fresh and used CoOx@CN catalyst samples indicated
that there was a positive increase in binding energy of Co2p3/2 in the
used catalyst sample (Fig. S5). It could be interpreted as formation of
metal hydroxide species due to the strong electrostatic attraction be-
tween the large amount of OH− and positively charged Co2+ and Co3+
species atoms [63]. The spectra of O 1 s core level was deconvoluted to
two peaks (Fig. 4b). The peak at 530.1 eV could be attributed to the
CoeO species and the peak at 531.6 eV could be ascribed to low co-
ordinated oxygen defects or vacancies [64,65].
To prove the feasibility of catalyst recycling by an energy-and time-
saving magnetic process, the magnetic property of CoOx@CN was
measured by VSM at 300 K with a magnetic field up to 30 kOe. The
hysteresis loop (Fig. 5) demonstrated a ferromagnetic behavior with a
saturation magnetization of 26.22 emu/g and coercivity of 300 Oe. The
strong magnetization of CoOx@CN allows simple collection of catalyst
by a permanent magnet, as shown in Fig. 6. After separation, the so-
lution became transparent while remaining colored. The color was re-
sulted from EGME solution of NaOH (Fig. S6) other than dispersed NPs,
which was further confirmed by cobalt determination using AAS, which
showed the cobalt content was less than 0.5 ppm.
3.2. P-cresols oxidation over CoOx@CN
The XPS (Fig. 4) measurements were conducted to study the surface
properties and elemental valences. Fig. 4a demonstrates two primary
peaks located at around 780 and 796 eV corresponding to the binding
To study the impacts of the catalyst component and structure, the
model reaction of 2-methoxy-4-cresol (1a) to vanillin (2a) was pro-
ceeded at a moderate condition. The TONs were calculated and pre-
sented in Table 1. The results (Table 1, entry 1) revealed that 1a could
convert to 2a by applying CoOx@CN as catalyst and O2 as an oxidant.
energies of the Co
and Co
peaks, respectively. After decon-
2p3/2
2p1/2
volution, the peaks located at 779.8 and 795.2 eV are assigned to Co3O4
phase, while those around 778.3 eV corresponding to Co0 [62], which
Fig. 4. XPS core level spectra of CoOx@CN sample. (a) Co2p, (b) O1s.
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C. Liang et al.
MolecularCatalysis453(2018)121–131
Fig. 6. Images of (a) diluted solution after reaction; (b) catalyst separation by a permanent magnet.
materials (Co@CN, CoOx@M, CoOx@G, Co3O4, CN and C) were pre-
pared and their activities were tested for 2-methoxy-4-cresol oxidation
in detail. A blank experiment (Table 1, entry 4) was carried out without
the catalyst and it resulted with 50% conversion of 1a and 4% se-
lectivity of 2a. Fabricated CN without cobalt exhibited some activity
(Table 1, entry 5) comparing to blank experiment while fabricated C
did not show as good activity as CN (entry 6). It may related to large
surface area of CN and the adsorption and activation of molecular
oxygen over the N [69]. Both CoOx@G and CoOx@M were worse
(Table 1, entry 7, 8) than CoOx@CN. As can be seen from the TEM
images (Fig. S2), cobalt-based particles in CoOx@CN was much smaller
than those in CoOx@M and CoOx@G. Both the aforementioned analysis
and previous research revealed that the doped N is effective on pre-
venting aggregation of NPs, while particle size is a significant factor in
heterogeneous catalysis [70,71]. Therefore, the superior catalytic ac-
tivity of CoOx@CN to CoOx@G and CoOx@M may originate from
smaller particle which is owned to the introduction of nitrogen. Co@CN
mainly including Co0 presented much worse activity in oxidation
(Table 1, entry 9), revealing the Co0 is not the main active sites. Instead,
the CoOx plays an important role as active sites. On the contrary, the
commercial Co3O4 NPs exhibited a negligible yield and slight higher
yield was get when increase the amount of Co3O4 (Table 1, entries 10,
11). The dynamic light scattering (DLS) revealed that agglomeration
phenomena were observed during the reaction (Fig. S7). TEM char-
acterization demonstrated that the particle size of spent commercial
Co3O4 NPs after reaction was much larger than CoOx@CN (Fig. S8).
Table 1
Catalytic results of selective oxidation of 2-methoxy-4-cresol.a
Entry
Catalystb
Time [h]
Conv.[%]
Select. [%]
TON
TOF [h−1
]
1
CoOx@CN
CoOx@CN
CoOx@CN
No catalyst
CN
7
7
24
7
7
7
7
7
7
7
7
100
45
100
50
66
52
100
100
98
82
98
90
88
86
4
17
5
52
65
48
7
210
87
200
/
/
/
98
151
109
14
8
30
12.4
8.3
/
/
/
14
21.6
15.6
2
2c
3c
4
5d
6d
7
C
CoOx@G
CoOx@M
Co@CN
8
9
10
11
e
Co3O4
Co3O4
e
40
1.1
a
Reaction conditions: EGME, 20 g; temperature, 353.5 K; substrate,
50 mmol; [Co]/[substrate] = 0.4%; NaOH, 135 mmol; O2, 100 mL min−1
600 rpm.
;
b
The Co content of the samples is summarized in Table S3.
The reaction was conducted in air.
Catalyst, 0.04 g.
c
d
e
Commercial nano Co3O4 (Particle size = 50 nm) were purchased from J&K
Chemical.
3.3. Catalytic activity and efficiency of catalyst
The catalyst got 100% of 2-methoxy-4-cresol conversion with 90.0% of
selectivity of vanillin and led to the highest TON value of 210.
CoOx@CN shows superior performance compared with other reported
heterogeneous catalysts in air oxidation of 2-methoxy-4-cresol to va-
nillin, such as Co(x)Mn(y)M(z)O(1-x-y-z) [66] and Co-LDHs [67] (Table
S2). Inspired by the great performance of CoOx@CN, the reaction was
conducted in air instead of pure oxygen. The selectivity can reach 88%,
while the conversion was 45% after 7 h which could be explained by
lower concentration of oxygen in air (Table 1, entry 2). After extending
the reaction time to 24 h, 1a was consumed completely and the yield of
vanillin was 86% (Table 1, entry 5), close to the yield in pure oxygen.
These results are consistent with the kinetic model which demonstrated
first order dependence on oxygen [68]. The successful aerobic oxidation
makes the method potential for industry application.
By monitoring the reaction progress with time, we observed the
formation of intermediates 3a (vanillin alcohol glycol ether) and 4a
(vanillin alcohol), and trace by-product vanillic acid (Scheme 1). Fig. 7
CoOx@CN consists of cobalt oxide, metallic cobalt and nitrogen-
doped carbon. Which compound is the active specie or is poised to
dominate the reaction? For comparison, several different component
Scheme 1. Catalytic oxidation of 2-methoxy-4-cresol to vanillin.
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MolecularCatalysis453(2018)121–131
Fig. 7. Product distribution diagram for the optimized system. [■] 2-methoxy-
4-cresol; [ ] Vanillin; [ ] Vanillin alcohol glycol ether; [ ] Vanillin alcohol.
Reaction conditions: 2-methoxy-4-cresol,50 mmol; CoOx@CN, 0.04 g; NaOH,
135 mmol; temp., 353.5 K; EGME, 20 g; O2, 100 mL min−1; 600 rpm; time, 7 h.
Fig. 9. Effect of temperature on 2-methoxy-4-cresol oxidation. [■] 2-methoxy-
4-cresol; [ ] Vanillin; [ ] Vanillin alcohol glycol ether; Reaction conditions:
EGME, 20 g; NaOH, 135 mmol; substrate, 50 mmol; catalyst, 0.4mol% Co to
substrate; O2, 100 mL min−1.
Fig. 8. Effect of base to substrate molar ratio on 2-methoxy-4-cresol oxidation.
[■] 2-methoxy-4-cresol; [ ] Vanillin; Reaction conditions: EGME, 20 g; temp.,
353.5 K; substrate, 50 mmol; catalyst, 0.4mol% Co to substrate; O2,
100 mL min−1; 600 rpm; time, 7 h.
Fig. 10. Kinetic plot of ln[1/(1-xA)] against time. Reaction conditions: EGME,
20 g; NaOH, 135 mmol; 1a, 50 mmol; catalyst, 0.4mol% Co to substrate; O2,
100 mL min−1, 600 rpm.
shows the time course of product formation for 2-methoxy-4-cresol
oxidation over CoOx@CN. Many reaction parameters influenced this
type of oxidation. The molar ratio of alkali to substrate 1a had pro-
minent effect upon both the selectivity to the p-hydroxybenzaldehydes
and reaction rate (Fig. 8). The oxidation can’t start without alkali which
shows the importance of NaOH to activate the substrate [34]. The
conversion of substrate 1a remained unchanged in the range of alkali/
substrate ratios of 1:1 to 4:1 and then decreased remarkably at an al-
kali/substrate ratio of 6:1 because of the poor solubility and dispersity
of oxygen at these high alkali loadings [72]. The selectivity to vanillin
increased to the highest at a mole ratio of 2.7:1. It remained constant
although poor solubility and dispersity of oxygen at high mole ratio to
4:1 and decreased in alkali/substrate ratio of 6:1.The reaction rate was
accelerated from base free condition to increase the ration to 3:1 and
then decreased from 3:1 to 6:1.
resulting from the accelerated reaction rate. Meanwhile, an obvious
enhancement in consuming intermediate 3a followed by the improve-
ment in selectivity to 2a, was observed. The overall selectivity of va-
nillin and vanillin alcohol glycol ether increased from 78 to 90.0%
when increase temperature from 333 K to 353 K. Higher temperature is
benefit for the gas-liquid transfer. Futher increase of temperature was
unfavorable for the reaction due to the over-oxidation, producing more
vanillic acid and other side-products. These results showed that tem-
perature also plays a key role, while 353 K was an appropriate tem-
perature for the CoOx@CN catalyst system.
The apparent reaction order of 2-methoxy-4-cresol oxidation with
oxygen on substrate was assumed to be one according to the reference
[73] at first. In accordance with the plot of ln[1/(1 − XA)] against time
where XA is the 2-methoxy-4-cresol conversion, straight lines can be
fitted at different temperatures confirmed the first order kinetics
(Fig. 10). While the concentration of O2 was much lower than substrate
in reaction system, this reaction can be treated as a pseudo-first reac-
tion. The influence of temperature on the kinetics of the reaction was
also studied. The apparent activation energy was calculated with
The reactions were performed under different temperatures to de-
termine its influences (Fig. 9). Low conversion and selectivity were
obtained under 333 K due to the relatively low oxidation rate. The
conversion of 1a increased when increasing reaction temperature
126

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MolecularCatalysis453(2018)121–131
Table 2
Effect of solvent in oxidation of 1a.a
Entry
Solvent
Conv. [%]
Select. [%]
1
2
EGME
MeOH
EtOH
EG
EGDE
EA
100
100
100
100
62
90
86
83
88
12
0
3
4b
5
6
0
7
H2O
32
6
a
Reaction conditions: solvent, 20 g; 353 K; substrate, 50 mmol; CoOx@CN,
0.4mol% Co to substrate; NaOH, 135 mmol; O2, 100 mL min−1; 600 rpm; time,
7 h.
Fig. 11. Arrhenius plot for oxidation of 2-methoxy-4-cresol over CoOx@CN
catalyst.
b
Reaction time, 12 h.
various activity orders. The distinct differences between EGME and
other solvents essentially originate from their pKa values [34], solubi-
lities of oxygen [74] and solubilities of base [21]. When water was
applied as solvent, selectivity of 2a was only 6% with a lot of tars as by-
products (Table 2, entry 7). The low yield can be attributed to high
solubility of NaOH in water so that it was not available for protecting
the eOH group of 1a [21]. As for mass transfer, alcoholic solvents
promote the substrate to attach the catalyst surface and aldehyde to
transfer to the solvent. On the contrary, water obstructs the transfer of
substrate and product kinetically [20].
This methodology has broad suitability. Various p-cresols were
oxided with oxygen to the corresponding aldehydes in good to great
yields (Table 3). P-cresol and mono-substituted p-cresols were accord-
ingly oxidized to the corresponding aldehydes 2a∼f in excellent se-
lectivities of 81–92% otherwise 2e only gave trace aldehyde. To in-
vestigate whether the catalytic system extended to a more general
difference, di-substituted p-cresols were also applied. As shown in
Table 3, the catalytic system also successfully facilitated oxidations of 2,
6-dialkyl-, dialkoxyl-, and alkoxylalkyl-substituted p-cresols, with de-
sired products 2g∼l in good selectivities of 80–93%. Though, the se-
lectivity of 2, 6-dibromo-pcresol was bad. These above results convin-
cingly demonstrated that the CoOx@CN/NaOH catalyst system can be
applied in effectively transforming a sequence of substituted p-cresols,
particularly for 2-electron-donating group substituted p-cresols and 2,
6-di-electron-donating group substituted p-cresols into the corre-
sponding p-hydroxybenzaldehydes. For electron-withdraw substituted
p-cresols, either the reaction rate was slow or selectivity was un-
expected [75]. And, more remarkable, no quinones, benzoic acids and
salicylaldehydes (with regard to 2g, 2k and 2l) was observed, which
shows the catalyst exhibited favorable chemoselectivity.
Fig. 12. Effect of catalyst loading on 2-methoxy-4-cresol oxidation. [■] 2-
methoxy-4-cresol; [ ] Vanillin; Reaction conditions:353 K; EGME, 20 g; NaOH,
135 mmol; substrate, 50 mmol; temp., 353.5 K; O2, 100 mL min−1; 600 rpm;
7 h.
Arrhenius plot as 52.27 kJ/mol (Fig. 11).
Only small amount of vanillin was formed in the absence of
CoOx@CN. By increasing the catalyst loading to 0.04 g (0.4 mol% Co to
substrate), the conversion of 1a increased from 50% to 100% and the
selectivity of 2a increased from 4% to 90% (Fig. 12). The increase could
be explained by the increase of active sites. Further increase of the
catalyst loading from 0.04 to 0.1 g, the reaction was quicker, but had
almost no influence on the selectivity of 2a. When the catalyst loading
was increased further in the range of 0.1–0.4 g, the reaction was
quicker, but the selectivity to 2a decreased markedly due to the over-
oxidation.
3.4. Reaction mechanism
To study the solvent effect, the oxidations of 1a in different solvents
were compared. As displayed in Table 2, the oxidation proceeded
smoothly in methanol (MeOH) (Table 2, entry 2), ethanol (EtOH)
(Table 2, entry 3) and ethylene glycol (EG) (Table 2, entry 4) via a
similar reaction process with CoOx@CN. However, the oxidation was
inhibited in ethylene glycol dimethyl ether (EGDE) (Table 2, entry 5)
and ethyl acetate (EA) (Table 2, entry 6) as aprotic solvents. It may
relate to the difficulty in generating corresponding intermediates. More
specifically, various solvents produced different intermediates with
The oxidation system of CoOx@CN/NaOH/O2 showed great che-
moselectivity as trace overoxidation products aromatic acids were de-
tected during the oxidation of p-cresols [32–34]. For comparison, we
carried out control oxidations of p-cresol, toluene, p-nitrotoluene and p-
cresol methyl ether to investigate the role of phenolate. As displayed in
Scheme 2, compared to p-cresol, the methyl-substituted benzenes
without p-hydroxyl group were easily overoxidized to acids. Tradi-
tionally, hydrogen abstraction was considered the oxidation mechanism
of methyl-substituted benzenes [76–78] that explains the formation of
127

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MolecularCatalysis453(2018)121–131
Table 3
Chemoselective Oxidation of various 4-cresols 1 to aldehyde 2a.
aReaction conditions: EGME, 20 g; T = 353.5 K; substrate, 50 mmol; catalyst, 0.4 mol% Co to
substrate; NaOH, 135 mmol; O2, 100 mL min−1; 600 rpm; 7 h.
bPerformed with 20 h;
cPerformed with 185 mmol NaOH;
dPerformed with 33 h.
acids. With regard to oxidation of p-cresols, we inferred a different
mechanism that the p-hydroxyl group was oxidized to form quinone
methide other than oxidation of methyl group to form benzyl radical
[34].
magnet, as shown in Fig. 6, and the liquid was decanted. The catalyst
was thoroughly washed with water and methanol, and then it was dried
under vacuum at 60 °C overnight for the next reaction cycle under the
same condition. This procedure was followed for five subsequent runs
and these results are shown in Table 4. CoOx@CN showed the same
conversion with slight decrease in selectivity, indicating CoOx@CN was
stable and efficient. The small decline in selectivity from 90% to 88%
seems to originate from the handling loss of the catalysts. Furthermore,
the TEM, SEM (Fig. S4) and XPS(Fig. S5) spectra of fresh catalysts and
reused catalysts are very similar, demonstrating great stability of
CoOx@CN. Additionally, the reaction mixture was analysed by AAS
after separation of the catalyst and the cobalt concentration was less
than 0.5 ppm. The above results undoubtedly confirmed the nano-
structured CoOx@CN heterogeneous nature and it showed good
leaching resistance ability.
Based on all these results and the previous studied mechanisms
[27,33], a plausible mechanism was proposed (Scheme 3). As discussed
above, Co3O4 is the active site with Co3+ and Co2+. Between Co3+ and
Co2+ species, Co3+ is active for oxidation of p-cresols because of its
ability to be reduced to Co2+, thus forming a redox couple (Co3+
↔
Co2+). Phenolate of 1a would first attach to the positively charged
Co2+ and Co3+ species of Co3O4 surface and then active Co3+ species
react with phenolate, leading to the formation of a highly active specie-
quinone methide (5a, Scheme 2) by 2e− transfer, resulting in Co3+
reduction to Co2+. The next step involves in desorption of quinone
methide and nucleophilic addition of ethylene glycol monomethyl ether
to the quinone methide to form ether intermediate 3a (Scheme 2,
RO = CH3OC2H4O). The quinone methide 5a has been corroborated by
HPLC–MS. Subsequently, the phenolate of 3a, resembling the above-
mentioned process, is converted into the transitory enol ether 7a, which
goes through a rapid spontaneous aromatization hydrolysis sequence to
attain the desired 2a with releasing ROH [79–81].
4. Conclusions
In summary, we have prepared magnetic nano-structured cobalt-
based/nitrogen-doped carbon NPs as environmentally friendly, efficient
and chemoselective catalysts for aerobic oxidation of p-cresols. Among
the cobalt compound catalysts studied in this work, CoOx@CN with
average particle size of 13 nm showed the highest activity (100% con-
version, 90% selectivity, 210 TONs) for the selective liquid-phase oxi-
dation of 2-methoxy-4-cresol to vanillin and had a broad suitability for
other p-cresols. The Co3O4 was confirmed to be the active species. The
highest redox ability of CoOx@CN was due to the presence of Co3+ and
3.5. Catalyst reusability
To understand the stability of the nano-structured CoOx@CN in
reaction system, we carried out catalyst recycle experiments. The cat-
alyst after the first reaction cycle (7 h) was recovered by an external
128

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MolecularCatalysis453(2018)121–131
Scheme 2. Oxidations of p-cresol, toluene, p-nitrotoluene and p-cresol methyl ether under the standard conditions. Reaction conditions: EGME, 20 g; T = 353.5 K;
substrate, 50 mmol; catalyst, 0.4mol% Co to substrate; NaOH, 135 mmol; O2, 100 mL min−1; 600 rpm; 7 h [a]Conversions of substrates. [b]Yields of products.
Scheme 3. A plausible mechanism.
129

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MolecularCatalysis453(2018)121–131
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.mcat.2018.05.005.
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