ZIF-67 with precursor concentration-dependence morphology for aerobic oxidation of toluene

Article abstract of DOI:10.1016/j.jorganchem.2020.121597

Designing high-efficiency catalyst for the oxidation of toluene (TOL) into value-added products like benzyl alcohol, benzaldehyde and benzoic acid is of great significance in chemical industry. Herein, four type of ZIF-67 with different morphology (ZIF-67

Full text of DOI:10.1016/j.jorganchem.2020.121597

Journal of Organometallic Chemistry 930 (2020) 121597
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
ZIF-67 with precursor concentration-dependence morphology for
aerobic oxidation of toluene
Yepeng Xiaoa,b, Bingcheng Songa, Yaju Chena, Lihua Chenga,b,c, Qinggang Rena,∗
a College of Chemical Engineering, Guangdong University of Petrochemical Technology, Maoming, China
b Guangdong Provincial Engineering Technology Research Center of Petrochemical Corrosion and Safety, Guangdong University of Petrochemical Technology,
Maoming, China
c Key Laboratory of Inferior Crude Oil Processing of Guangdong Provincial Higher Education Institutes, Maoming, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Designing high-efficiency catalyst for the oxidation of toluene (TOL) into value-added products like benzyl
alcohol, benzaldehyde and benzoic acid is of great significance in chemical industry. Herein, four type of
ZIF-67 with different morphology (ZIF-67-6, ZIF-67-12, ZIF-67-18, and ZIF-67-24) are prepared by tuning
the concentration of precursor. About 90% of TOL conversion is obtained in the presence of ZIF-67-24
(0.1–0.2 μm, polyhedrons) by using O2 (>99.9%, 1 atm) as oxidant at 40 °C after 4 h of reaction. Excellent
performance of ZIF-67-24 could be attributed to two reasons: (I) The good crystallinity and smaller size
Received 1 August 2020
Revised 9 October 2020
Accepted 26 October 2020
Available online 27 October 2020
Keywords:
Toluene
with larger BET surface (2037 m2 −1). (II) The electron deficient Co in ZIF-67-24 may be responded to
g
Oxidation
the fast initiation of radical chain process of TOL oxidation. The research may provide a promising concept
for oxidation of C-H under mild condition.
ZIF-67
Morphological control
© 2020 Published by Elsevier B.V.
1. Introduction
electron orbitals, the superoxide intermediates accumulated dur-
ing chain growth can be rapidly decomposed by such catalytic sys-
Oxidation of toluene (TOL) is an important component of C-H
activation. Its products (benzyl alcohol, benzaldehyde and benzoic
acid) are in great demand in the chemical industry [1]. Although
the oxidation of TOL has been industrialized as early as the mid-
20th century, some problems exist in the catalytic system, such as
using corrosive fatty acids as the solvent, using bromide as the ini-
tiator, and over-oxidation to deep oxidation products (such as COx)
[2]. Hence, high-performance catalyst for oxidation of TOL is still
in an urgent need.
tems via the Haber-Weiss cycle [16,17]. However, the catalytic ac-
tivity are greatly influenced by their morphology and dispersion
[18–20]. Generally, well-dispersed active components with suitable
morphology can efficiently stimulate the lattice oxygen of metal-
lic oxide to participate in the reaction (mars-van krevelen redox
reaction mechanism) [18,21]. Thus the formation of reactive oxy-
gen species is accelerated, which can react with TOL by hydro-
gen extraction. As a result, the oxidation of TOL is facilitated. Re-
cently, an important progress was made by Pappo’ s group [22].
Over 90% of conversion with 90% of selectivity to benzaldehyde
was achieved in the oxidation of TOL using a cobalt acetate (cat-
alyst)/ N-hydroxyphthalimide (NHPI, initiator)/ molecular oxygen
(oxidant)/ 1,1,1,3,3,3-hexa-fluoropropan-2-ol (HFIP, solvent) system.
However, the separation of homogeneous catalyst from reaction
system is a obstacle for its further application in industry.
Past decades have witnessed a rapid development in oxidative
activation of C-H bonds, and the transition metals with half-full
outer electron orbitals such as V, Cr, Mn, Fe, Co, Ni, Cu, Ce were
one of the research hot spots. These transition metals and their
oxide were selected as the active components to form metallopor-
phyrin [3], layered bimetallic hydroxide [4], heteropoly acid [5] and
so on. Besides, supported catalysts (MnOx/SBA-15 [6], MnOx/ZSM-5
[7], FeOx/SBA-15 [8], NiOx/SBA-15 [9], CeO2/Al2O3 [10], CuO/ZSM-5
[11], VOHPO4/KIT-6 [12], CuCr2O4 [13], CoCuOx [14], and V-Mo-Fe-
O [15], etc) were widely reported, too. Due to the half-filled outer
Metal-organic frameworks (MOFs) are inorganic-organic com-
pounds with high surface areas and well-developed pores, which
can afford them excellent performance in gas adsorption and sep-
aration [23,24], new energy production [25], catalysis [26,27] and
so on. the properties of MOFs are significantly affected by a series
of important factors, including the composition, morphology, de-
fects and crystalline state [28]. However, one of the key doubts of
∗
Corresponding author.
E-mail address: 83842088@qq.com (Q. Ren).
https://doi.org/10.1016/j.jorganchem.2020.121597
0022-328X/© 2020 Published by Elsevier B.V.

Y. Xiao, B. Song, Y. Chen et al.
Journal of Organometallic Chemistry 930 (2020) 121597
utilizing MOFs in catalysis is their stability during the reaction pro-
cess. Zeolitic imidazolate frameworks (ZIFs), a new class of MOFs
that have exceptional chemical and thermal stability [29], are suc-
cessfully applied in many reactions including oxidation of benzylic
C-H bonds [30].
the head-space bottle before screw the lid. After that, O2 or air was
pumped into the bottle for 5 min with two hollow needles. Then
0.50 mmol of toluene (TOL), 0.5 mL of 1,1,1,3,3,3-hexafluoropropan-
2-ol (HFIP) with 0.025 mmol N-hydroxyphthalimide (NHPI) were
injected into the bottle, following by 1 min of ultrasonic treatment.
After the reaction was finished, the catalysts were separated by
centrifugation. The reaction mixture was analyzed by an Agilent-
7820A with a HP-5 capillary column and a flame ionization detec-
tor (FID), using ethylbenzene as an internal standard.
In this work, four type of ZIF-67 with different morphology are
prepared by tuning the concentration of precursor. The catalytic
activity of ZIF-67 (0.1–0.2 μm, polyhedrons) can be comparable
with a homogeneous catalyst in the oxidation of TOL. The possi-
ble mechanism involved in the oxidation is also discussed.
3. Results and discussion
2. Experimental
3.1. Morphological and structural characterization
2.1. Catalysts preparation
Fig. 1 shows that the morphology of ZIF-67 is depended by
precursor concentration. A gradual change from flower-like nanos-
tructure to polygon nanoparticles can be observed due to the di-
lution of Co(NO3)2·6H2O and Hmim solutions by methanol simul-
taneously. As shown in Fig. 1a, the size of flower-like ZIF-67–6 is
estimated about 1–2 μm, which is made up by smooth nanosheets
(SEM images are presented in Fig. S1, Electronic supplementary in-
formation). Irregular blocks (ca. 1–2 μm) with nanosheets (100 nm
in size) on the surface is presented from ZIF-67–12 (Fig. 1b). ZIF-
67–18 is consisted by ~2 μm polyhedrons (Fig. 1c). Furthermore,
Fig. 1d shows that the size of polyhedrons in ZIF-67–24 decreases
to only 0.1–0.2 μm with reducing precursor concentration.
The XRD patterns of the as-prepared ZIF-67 with different pre-
cursor concentration are shown in Fig. 2a, matching well with pre-
vious report [32]. Higher crystallinity could be observed as a result
of lower precursor concentration. As shown in Fig. 2b, photoelec-
tron peaks of C, N, and Co elements are detected. The peak of O
elements could be assigned to the absorbed water. The results of
TEM, XRD and XPS confirm the successful preparation of the ZIF-67
samples.
The synthesis of ZIF-67 was similar to the previous literature
with some modification [31]. Briefly, 3.75 mmol of Co(NO3)2·6H2O
and 7.5 mmol of 2-methylimidazole (Hmim) were dissolved in V
mL (V = 6, 12, 18, 24) methanol to form clear solution, respectively.
Then the two precursor solutions were mixed together under ul-
trasonic treatment for 5 min. After that, the mixture was main-
tained at room temperature for 24 h. The resulting samples were
isolated via suction filtration, washed thoroughly with methanol,
and finally dried in an oven at 70 °C overnight. The as-prepared
products were marked as ZIF-67-V.
2.2. Catalysts characterization
Transmission electron microscopy (TEM) images were obtained
by using a JEOL JEM-2010-TEM to determine the morphology of
samples. X-ray diffraction (XRD) patterns of the catalysts were
recorded with a Bruker D8 using Cu Kα radiation. X-ray photo-
electron spectroscopy (XPS) was performed by using an ULVAC PHI
Quantera microscope. BET surface areas and pore structure were
conducted with N2 adsorption-desorption on a Micromeritics ASAP
2010 micropore analysis system. Electron paramagnetic resonance
(EPR) spectra were collected by using a Bruker A300.
As shown in Fig. 3a and b, according to IUPAC classification,
typical IV type isotherm with a distinct hysteresis loop could be
seen in ZIF-67–6 and ZIF-67–12, indicating the existence of meso-
pore (inserts in Fig. 3a and b, which may be resulted by the
nanosheets structure). However, the curves in Fig. 3c and d are
more closer to type I nitrogen isotherm, suggesting the presence
of micropore structure (inserts in Fig. 3c and d). Moreover, other
obtained data, including BET surface areas, total pore volumes and
pore sizes, are summarized in Table 1.
2.3. General procedure for toluene oxidation
The reaction was carried out in a head-space bottle (10 mL)
with magnetic stirring at 25–40 °C under an O2 (>99.9%, 1 atm) or
air atmosphere. Firstly, 0.01 mmol catalysts were introduced into
Fig. 1. TEM images of ZIF-67-6 (a), ZIF-67-12 (b), ZIF-67-18 (c) and ZIF-67-30 (d).
2

Y. Xiao, B. Song, Y. Chen et al.
Journal of Organometallic Chemistry 930 (2020) 121597
Fig. 2. XRD pattern (a) and XPS survy spectra (b) of ZIF-67-6, ZIF-67-12, ZIF-67-18, and ZIF-67-24.
Fig. 3. N2 adsorption-desorption isotherms of ZIF-67-6 (a), ZIF-67-12 (b), ZIF-67-18 (c), and ZIF-67-30 (d).
Table 1
BET surface areas, total pore volumes, and pore sizes for the synthesized samples.
Sample
BET surface area (m² g−1
)
Total pore volume (cm3 g−1
)
Average pore diameter (nm)
ZIF-67-6
800
0.436
0.649
0.709
0.779
2.18
1.51
1.65
1.53
ZIF-67-12
ZIF-67-18
ZIF-67-24
1722
1719
2037
3.2. Catalytic performance
No apparent oxidative products is observed in a blank experiment,
or in the presence of Co(NO3)2·6H2O (Co(NO3)2·6H2O could not
be dissolved in HFIP) or Hmin, respectively. However, with ZIF-67–
24 as catalyst, high-performance in catalytic efficiency is observed.
87.9% and 63.1% of TOL conversion are detected by using O2 or air
as oxidant, respectively (entry 4 and 5), which can be compara-
The as-prepared ZIF-67 was employed as catalysts for the aer-
obic oxidation of TOL. The influence of various technological con-
ditions on TOL catalytic oxidation reaction was summarizes in Fig.
S2 (Electronic supplementary information). As shown in Table 2,
3

Y. Xiao, B. Song, Y. Chen et al.
Journal of Organometallic Chemistry 930 (2020) 121597
Table 2
Aerobic oxidation of TOL catalyzed by various catalysts[a]
.
Entry
Catalysts
Conv. (%)
Select. (%)
Benzyl alcohol
Benzaldehyde
Benzyl acid
1
Blank
trace
trace
trace
87.9
63.1
72.4
54.1
trace
91
–
–
–
2
Co(NO3)2·6H2O
Hmim
–
–
–
3
–
–
–
4
ZIF-67-24
ZIF-67-24
ZIF-67-24
ZIF-67-24
Co(II)TPP
7.8
20.4
11.3
18.1
–
66.9
70.7
68.7
64.2
–
25.3
8.9
20.0
17.7
–
5[b]
6[c]
7[d]
8[e]
9[f]
10[g]
Co(OAc)2·4H2O
CoOx/SiO2
9
90
1
91.2
8.4
68.8
20.3
[a]
Reaction conditions: TOL (0.5 mmol), NHPI (0.025 mmol), catalysts (0.01 mmol),
O2 (>99.9%, 1 atm), HFIP (0.5 mL), 40 °C, 4 h.
[b]
Use air as oxidant.
[c]
Reuse the catalysts from entry 4.
[d]
Reuse the catalysts from entry 5.
[e]
Use Co(II) meso-Tetraphenylporphine as catalyst.
[f]
Reference [21].
[g]
Reference [2].
tated by good crystallinity and smaller size with higher BET surface
(2037 m2 −1).
g
3.3. Study of probable mechanism
According to the earlier literature, the oxidation of TOL is in-
volved in a free radical reaction, and the central atom Co(II) in
ZIF-67 is proposed as the catalytic site. Fig. 5a shows the typical
characteristic peaks of PINO (a free radical generated from NHPI)
[33]. It is clear that the signal intensities have a good agreement
with the catalytic performance. Fig. 5b shows the XPS spectra of
Co 2p for ZIF-67-6, ZIF-67-12, ZIF-67-18 and ZIF-67-24. Compar-
ing to the binding energy of Co 2p in ZIF-67-6, positive shifts in
varying degrees are observed in ZIF-67-12 (ꢀE = 0.2 eV), ZIF-67-
18 (ꢀE = 0.4 eV) and ZIF-67-24 (ꢀE = 0.4 eV), respectively. This
phenomenon may be a result that less Co exist in the outer sur-
face of ZIF-67-18 and ZIF-67-24, so their crystal structure is more
complete (see Fig. 2a). As a result, the electron of Co may be dis-
perse by its connected ligand. This electron deficient Co in ZIF-67-
18 and ZIF-67-24 may have d-orbital vacancies, which will increase
the donation to O2 bonding orbital of the adsorbed O2 formed dur-
ing oxidation of TOL. Consequently, the bond between Co and O2
is reinforced, which will do good to the formation of Co(III) com-
plexes. As a result, the generation of PINO radical is facilitated by
the reaction between Co(III) complexes and NHPI, which will speed
up the initiation of radical chain process of TOL oxidation.
Fig. 4. Conversion efficiency of TOL oxidation catalyzed by ZIF-67-6, ZIF-67-12, ZIF-
67-18, and ZIF-67-24. Reaction conditions: TOL (0.5 mmol), NHPI (0.025 mmol), cat-
alysts (0.01 mmol), O2 (> 99.9%, 1 atm), HFIP (0.5 mL), 40 °C, 4 h.
ble with a homogeneous catalyst (Co(OAc)2·4H2O, entry 8) [22] or
a heterogeneous catalyst (CoOx/SiO2, entry 9) [2] in similar condi-
tions. Unfortunately, the catalytic stability of ZIF-67-24 is dissatis-
factory, nearly 40% of loss in conversion efficiency is observed af-
ter three consecutive runs. The poor stability may be caused by
the exposion to benzoic acid, which coud destroy the coordination
structure of ZIF-67. This phenomenon is also found in other acid
(such as acetic acid).
Based on the experimental results and the most accepted mech-
anism for the aerobic autoxidation reaction of methylarene by the
NHPI/Co(OAc)2 system proposed by Pappo’s group [22], we con-
clude a possible reaction mechanism for catalytic oxidation of TOL
over ZIF-67 (Fig. 6). This free radical reaction begins with the ox-
idation of Co(II) by molecular oxygen to afford Co(III) complexes
(Step 1), which further react with NHPI to generate PINO radical
(Step 2), so that the radical chain process is initiated. After that,
benzyl radical is generated from hydrogen abstraction by the PINO
radical (Step 3), readily reacts with molecular oxygen to eventually
form benzyl peroxy radical (Step 4). In the step of chain termi-
nation (Step 5), benzyl alcohol and benzaldehyde, which could be
Generally, the catalytic behavior is greatly influenced by the
morphology of catalyst. Hence, It is necessary to investigate the
morphology of ZIF-67 with more effective for this oxidation pro-
cess. As shown in Fig. 4, the conversions of TOL catalyzed by ZIF-
67-6, ZIF-67-12, ZIF-67-18 and ZIF-67-24 are 16.4%, 51.8%, 62.2%
and 87.9% after 4 h of reaction, respectively. Higher catalytic ac-
tivity of ZIF-67-24 is obtained compared to that of ZIF-67-6, ZIF-
67-12, and ZIF-67-18, implying that this oxidation process is facili-
4

Y. Xiao, B. Song, Y. Chen et al.
Journal of Organometallic Chemistry 930 (2020) 121597
Fig. 5. ESR signals of the PINO (a), and XPS spectra of Co 2p (b) for ZIF-67-6, ZIF-67-12, ZIF-67-18, and ZIF-67-24. Reaction condition: 0.5 mmol of TOL, 0.025 mmol of NHPI,
0.01 mmol of catalysts, 1 atm of O2, 0.5 mL of HFIP, 40 °C, 1 h.
Fig. 6. The proposed mechanism for the aerobic catalyzed NHPI/ZIF-67 system of TOL.
further oxidized to benzyl acid, are formed by the mutual destruc-
tion of two benzyl peroxy radical.
Acknowledgements
This work was financially supported by the Scientific Research
Foundation for Advanced Talents of Guangdong University of Petro-
chemical Technology [No. 2018rc52], Special Fund for Science
and Technology Innovation Strategy of Guangdong Province [No.
2018S00146], Natural Science Foundation of Guangdong Province
[No. 2018A030307004] and Maoming Public Service Platform for
Transformation Upgrading and Technological Innovation of Petro-
chemical Industry [No. 2016B020211001].
4. Conclusion
In summary, by adjusting the concentration of precursor, a se-
ries of ZIF-67 (ZIF-67-6, ZIF-67-12, ZIF-67-18, and ZIF-67-24) with
different morphology are prepared for TOL oxidation. Nearly 90%
conversion of TOL is obtained in the presence of ZIF-67-24 by using
O2 (>99.9%, 1 atm) as oxidant at 40 °C after 4 h of reaction. Ex-
cellent performance of ZIF-67-24 could be attributed to the good
crystallinity and smaller size with higher BET surface (2037 m2
Supplementary materials
g
−1), and more importantly, the initiation of radical chain process
of TOL oxidation may be accelerated by the electron deficient Co
in ZIF-67-24. The research may propose a promising concept for
oxidation of C-H under ambient condition.
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.2020.
121597.
Declaration of Competing Interest
None.
5

Y. Xiao, B. Song, Y. Chen et al.
Journal of Organometallic Chemistry 930 (2020) 121597
References
[17] Y. Xiao, J. Liu, H. Wang, C. Yang, H. Cheng, Y. Deng, Y. Fang, Photothermal oxi-
dation of cyclohexane by graphene oxide-based composites with high selectiv-
ity to KA oil, Mol. Catal. 493 (2020) 1–8.
[18] R. Peng, S. Li, X. Sun, Q. Ren, L. Chen, M. Fu, J. Wu, D. Ye, Size effect of Pt
nanoparticles on the catalytic oxidation of toluene over Pt/CeO2 catalysts, Appl.
Catal., B 220 (2018) 462–470.
[1] C. Deng, M. Xu, Z. Dong, L. Li, J. Yang, X. Guo, L. Peng, N. Xue, Y. Zhu, W. Ding,
Exclusively catalytic oxidation of toluene to benzaldehyde in an O/W emulsion
stabilized by hexadecylphosphate acid terminated mixed-oxide nanoparticles,
Chinese J. Catal. 41 (2020) 341–349.
[19] Y. Xiao, J. Liu, K. Xie, W. Wang, Y. Fang, Aerobic oxidation of cyclohexane cat-
alyzed by graphene oxide: effects of surface structure and functionalization,
Mol. Catal. 431 (2017) 1–8.
[2] G. Shi, S. Xu, Y. Bao, J. Xu, Y. Liang, Selective aerobic oxidation of toluene
to benzaldehyde on immobilized CoOx on SiO2 catalyst in the presence of
N-hydroxyphthalimide and hexafluoropropan-2-ol, Catal. Commun. 123 (2019)
73–78.
[20] Y. Xiao, J. Liu, Y. Lin, W. Lin, Y. Fang, Novel graphene oxide-silver nanorod com-
posites with enhanced photocatalytic performance under visible light irradia-
tion, J. Alloys Compd. 698 (2017) 170–177.
[21] R.K. Grasselli, Fundamental principles of selective heterogeneous oxidation
atalysis, Top. Catal. 21 (2002) 79–88.
[22] E. Gaster, S. Kozuch, D. Pappo, Selective aerobic oxidation of methylarenes
to benzaldehydes catalyzed by N-hydroxyphthalimide and cobalt(II) acetate in
hexafluoropropan-2-ol, Angew. Chem. Int. Ed. 56 (2017) 5912–5915.
[23] Z. Yan, Y. Gong, B. Chen, X. Wu, Q. Liu, L. Cui, S. Xiong, S. Peng, Methyl func-
tionalized Zr-Fum MOF with enhanced Xenon adsorption and separation, Sep.
Purif. Technol. 239 (2020).
[3] X.-L. Yang, M.-H. Xie, C. Zou, Y. He, B. Chen, M. O’Keeffe, C.-D. Wu,
Porous metalloporphyrinic frameworks constructed from metal 5,10,15,20-te-
trakis(3,5-biscarboxylphenyl)porphyrin for highly efficient and selective cat-
alytic oxidation of alkylbenzenes„ J. Am. Chem. Soc. 134 (2012) 10638–10645.
[4] X. Wang, G. Wu, F. Wang, H. Liu, T. Jin, Solvent-free selective oxidation of
toluene with O2 catalysed by anion modified mesoporous mixed oxides with
high thermal stability, Catal. Commun. 98 (2017) 107–111.
[5] B. Ma, Z. Zhang, W. Song, X. Xue, Y. Yu, Z. Zhao, Y. Ding, Solvent-free selec-
tive oxidation of C-H bonds of toluene and substituted toluene to aldehydes
by vanadium-substituted polyoxometalate catalyst, J. Mol. Catal. A Chem. 368
(2013) 152–158.
[6] W. Zhong, S.R. Kirk, D. Yin, Y. Li, R. Zou, L. Mao, G. Zou, Solvent-free selec-
tive oxidation of toluene by oxygen over MnOx/SBA-15 catalysts: relationship
between catalytic behavior and surface structure, Chem. Eng. J. 280 (2015)
737–747.
[7] Y. Meng, H.C. Genuino, C.-H. Kuo, H. Huang, S.-Y. Chen, L. Zhang, A. Rossi,
S.L. Suib, One-step hydrothermal synthesis of manganese-containing MFI-type
zeolite, Mn-ZSM-5, characterization, and catalytic oxidation of hydrocarbons, J.
Am. Chem. Soc. 135 (2013) 8594–8605.
[24] T. Rodenas, I. Luz, G. Prieto, B. Seoane, H. Miro, A. Corma, F. Kapteijn,
F.X. Llabres i Xamena, J. Gascon, Metal-organic framework nanosheets in poly-
mer composite materials for gas separation, Nature Mater. 14 (2015) 48–55.
[25] D. Sheberla, J.C. Bachman, J.S. Elias, C.-J. Sun, Y. Shao-Horn, M. Dinca, Con-
ductive MOF electrodes for stable supercapacitors with high areal capacitance,
Nature Mater. 16 (2017) 220–224.
[26] Y. Luo, Y. Zheng, J. Zuo, X. Feng, X. Wang, T. Zhang, K. Zhang, L. Jiang, Insights
into the high performance of Mn-Co oxides derived from metal organic frame-
works for total toluene oxidation, J. Hazard. Mater. 349 (2018) 119–127.
[27] Z. Sun, G. Li, Y. Zhang, H.-o. Liu, X. Gao, Ag-Cu-BTC prepared by postsynthetic
exchange as effective catalyst for selective oxidation of toluene to benzalde-
hyde, Catal. Commun. 59 (2015) 92–96.
[8] M. Lu, R. Huang, J. Wu, M. Fu, L. Chen, D. Ye, On the performance and mech-
anisms of toluene removal by FeOx/SBA-15-assisted non-thermal plasma at at-
mospheric pressure and room temperature, Catal. Today 242 (2015) 274–286.
[9] W. Xu, X. Xu, J. Wu, M. Fu, L. Chen, N. Wang, H. Xiao, X. Chen, D. Ye, Re-
moval of toluene in adsorption-discharge plasma systems over a nickel modi-
fied SBA-15 catalyst, Rsc Adv. 6 (2016) 104104–104111.
[28] J. Duan, Y. Li, Y. Pan, N. Behera, W. Jin, Metal-organic framework nanosheets:
an emerging family of multifunctional 2D materials, Coord. Chem. Rev. 395
(2019) 25–45.
[29] K. Zhou, B. Mousavi, Z. Luo, S. Phatanasri, S. Chaemchuen, F. Verpoort, Char-
acterization and properties of Zn/Co zeolitic imidazolate frameworks vs. ZIF-8
and ZIF-67, J. Mater. Chem. A 5 (2017) 952–957.
[10] S.M. Saqer, D.I. Kondarides, X.E. Verykios, Catalytic oxidation of toluene over
binary mixtures of copper, manganese and cerium oxides supported on
gamma-Al2O3, Appl. Catal., B 103 (2011) 275–286.
[30] C. Guo, Y. Zhang, X. Nan, C. Feng, Y. Guo, J. Wang, Efficient selective catalytic
oxidation of benzylic C-H bonds by ZIF-67 under eco-friendly conditions, Mol.
Catal. 440 (2017) 168–174.
[11] N. Viswanadham, S.K. Saxena, A.H. Al-Muhtaseb, Cu functionalized nano crys-
talline ZSM-5 as efficient catalyst for selective oxidation of toluene, Mater. To-
day Chem. 3 (2017) 37–48.
[31] H. Chen, K. Shen, Q. Mao, J. Chen, Y. Li, Nanoreactor of MOF-derived yolk-shell
Co@C-N: precisely controllable structure and enhanced catalytic activity, ACS
Catal. 8 (2018) 1417–1426.
[12] M. Rezaei, A. Najafi Chermahini, H.A. Dabbagh, Selective oxidation of toluene
to benzaldehyde by H2O2 with mesoporous silica KIT-6 supported VOHPO4
•
0
5H2O catalyst, J. Environ. Chem. Eng. 5 (2017) 3529–3539.
[32] G. Wei, Z. Zhou, X. Zhao, W. Zhang, C. An, Ultrathin metal-organic framework
nanosheet-derived ultrathin Co3O4 nanomeshes with robust oxygen-evolving
performance and asymmetric supercapacitors, ACS Appl. Mater. Inter. 10 (2018)
23721–23730.
[13] S.S. Acharyya, S. Ghosh, R. Tiwari, B. Sarkar, R.K. Singha, C. Pendem, T. Sasaki,
R. Bal, Preparation of the CuCr2O4 spinel nanoparticles catalyst for selective
oxidation of toluene to benzaldehyde, Green Chem. 16 (2014) 2500–2508.
[14] R. Xie, G. Fan, L. Yang, F. Li, Hierarchical flower-like Co-Cu mixed metal oxide
microspheres as highly efficient catalysts for selective oxidation of ethylben-
zene, Chem. Eng. J. 288 (2016) 169–178.
[33] L. Wang, Y. Zhang, R. Du, H. Yuan, Y. Wang, J. Yao, H. Li, Selective one-step
aerobic oxidation of cyclohexane to e-caprolactone mediated by N-hydroxyph-
thalimide (NHPI), ChemCatChem 11 (2019) 2260–2264.
[15] H. Xia, Z. Liu, Y. Xu, J. Zuo, Z. Qin, Highly efficient V-Mo-Fe-O catalysts for se-
lective oxidation of toluene to benzaldehyde, Catal. Commun. 86 (2016) 72–76.
[16] B.P.C. Hereijgers, B.M. Weckhuysen, Aerobic oxidation of cyclohexane by
gold-based catalysts: new mechanistic insight by thorough product analysis,
J. Catal. 270 (2010) 16–25.
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