Core-shell Co-MOF-74@Mn-MOF-74 catalysts with Controllable shell thickness and their enhanced catalytic activity for toluene oxidation

Article abstract of DOI:10.1016/j.jssc.2020.121803

A series of core-shell Co-MOF-74@Mn-MOF-74 samples with different shell thicknesses were prepared by the seed growth method, which were characterized by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), inductively coupled plasma mass spectrometry (ICP-MS), X-ray photoelectron spectroscopy, nitrogen adsorption-desorption and scanning electron microscopy (SEM). The shell thickness of these MOF samples were successfully controlled by adjusting the mass ratio between MnCl₂·4H₂O and Co-MOF-74. On this basis, these MOFs were applied to the catalytic oxidation of toluene. The results showed that with the growth of Mn-MOF-74 in the outer layer of Co-MOF-74, the oxidative selectivity of the substrate to benzaldehyde was greatly improved for the synergy between the core layer and shell layer. The conversion of toluene was 22.4%, and the selectivity of benzaldehyde was 98.1%. In addition, the catalyst can transform various substituted toluene into the corresponding aldehydes in highly selectivity and still keep good stability after four catalytic cycles. The selectivity of the corresponding aldehyde is generally above 80%.

Full text of DOI:10.1016/j.jssc.2020.121803

Journal Pre-proof
Core-shell Co-MOF-74@Mn-MOF-74 catalysts with Controllable shell thickness and
their enhanced catalytic activity for toluene oxidation
Xiangyu Gu, Cheng Huang, Zengchuang Xu, Hao Wu, Ruilan Dong, Rui Liu, Jing
Chen, Hongjun Zhu
PII:
S0022-4596(20)30634-4
DOI:
https://doi.org/10.1016/j.jssc.2020.121803
YJSSC 121803
Reference:
To appear in: Journal of Solid State Chemistry
Received Date: 1 September 2020
Revised Date: 13 October 2020
Accepted Date: 19 October 2020
Please cite this article as: X. Gu, C. Huang, Z. Xu, H. Wu, R. Dong, R. Liu, J. Chen, H. Zhu, Core-
shell Co-MOF-74@Mn-MOF-74 catalysts with Controllable shell thickness and their enhanced catalytic
activity for toluene oxidation, Journal of Solid State Chemistry (2020), doi: https://doi.org/10.1016/
j.jssc.2020.121803.
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Author Statement
Xiangyu Gu: Conceptualization, Methodology, Roles/Writing - original draft; Cheng
Huang: Formal analysis, Visualization, Writing - review & editing; Zengchuang Xu:
Funding acquisition; Hao Wu: Data curation, Investigation; Ruilan Dong: Data
curation, Investigation;; Rui Liu: Supervision, Writing - review & editing; Jing Chen:
Supervision; Hongjun Zhu: Project administration, Resources.

Core-shell Co-MOF-74@Mn-MOF-74 Catalysts with Controllable
Shell Thickness and Their Enhanced Catalytic Activity for Toluene
Oxidation
Xiangyu Gu,a Cheng Huang b,*, Zengchuang Xuc, Hao Wu,a Ruilan Dong,a Rui Liua,**
Jing Chena,*** and Hongjun Zhu
,
aSchool of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing,
211816, China
bDepartment of Chemical and Pharmaceutical Engineering, Southeast University
ChengXian College, Nanjing, 210088, China
cShanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai,
200083, China
E-mail addresses: rui.liu@njtech.edu.cn (R. Liu), huangcheng@cxxy.seu.edu.cn
(Cheng. Huang), jchen@njtech.edu.cn (J. Chen).

Abstract: A series of core-shell Co-MOF-74@Mn-MOF-74 samples with different
shell thicknesses were prepared by the seed growth method, which were characterized
by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), inductively
coupled plasma mass spectrometry (ICP-MS), X-ray photoelectron spectroscopy,
nitrogen adsorption-desorption and scanning electron microscopy (SEM). The shell
thickness of these MOF samples were successfully controlled by adjusting the mass
ratio between MnCl2·4H2O and Co-MOF-74. On this basis, these MOFs were applied
to the catalytic oxidation of toluene. The results showed that with the growth of
Mn-MOF-74 in the outer layer of Co-MOF-74, the oxidative selectivity of the
substrate to benzaldehyde was greatly improved for the synergy between the core
layer and shell layer. The conversion of toluene was 22.4%, and the selectivity of
benzaldehyde was 98.1%. In addition, the catalyst can transform various substituted
toluene into the corresponding aldehydes in highly selectivity and still keeps good
stability after four catalytic cycles. The selectivity of the corresponding aldehyde is
generally above 80 %.
Keywords: Metal-organic frameworks; Core-shell; Selective oxidation; Synergistic
effect

1. Introduction
Aromatic alcohols, aromatic aldehydes and aromatic acids are a class of
important intermediates used in pesticide and medicine, which are produced by
oxidation of corresponding aromatic methyl derivatives.1-3 In industry, aromatic
methyl derivatives are often oxidized by KMnO4, H2SO4, H2O2, bromide, etc., which
could cause a lot of pollution and corrosion of equipments.2, 4 It is urgent to prepare
new green and efficient catalysts to solve the defects of traditional catalysts, such as
high pollution and strong corrosion. In the study of toluene oxidation, the researchers
found that TBHP (tert-butyl hydroperoxide) could generate free radical substances in
the condition of transition metal catalyst, which would lead to further oxidation of
toluene.5 In this process, the metal ion can crack the weak O-O bond on TBHP to
form tert-butyl peroxic radicals, which can react with the substrate in an oxygen
7
atmosphere to produce corresponding oxides.6,
However, conventional metal
catalysts are often homogeneous, which greatly limits the reuse of catalysts.
Meanwhile, heterogeneous transition metal catalysts can be reused many times, while
maintaining high catalytic oxidation activity.
Metal-organic Frameworks (MOFs) is a kind of porous crystal material
composed of metal clusters connected by organic ligands.8 Due to its ordered pore
structure, large specific surface area, easy surface modification and good
thermochemical stability, the potential applications of MOFs in gas storage and
separation,9, 10 heterogeneous catalysis,11-14 ion detection15 and biomedicine16, 17 have
attracted wide attention. Especially for the advantages of catalytic efficiency, easy

separation and reuse, MOFs have attracted wide attention in the field of
heterogeneous catalysis11, 18-21 Among these MOFs, M-MOF-74 is a kind of MOF
material with 2,5-dihydroxyterephthalic acid as organic ligand and different transition
metals as nodes. Among them, the Co-MOF-74 material has high stability and
abundant coordination unsaturated metal sites, which can be used as the active site of
catalytic reaction to improve its catalytic activity.22, 23 In addition, the introduction of
Mn2+ can enhance the adsorption capacity of the catalyst to toluene molecules and
increase the alkaline sites on the catalyst surface, thus improving the selectivity of
benzaldehyde.6 Furthermore, the hybrid structure of MOF was beneficial to the
catalytic activity of catalysts.12, 24
Core-shell MOF@MOF composites prepared by encapsulating one MOF within
another can retain the chemical, physical, and structural advantages of both MOFs.25
26, 27
Moreover, MOF@MOF structures may produce some unexpected synergistic
effects.28 As is known, the core–shell catalyst can not only maintain the excellent
performance of both core and shell materials, but also effectively overcome the
disadvantages of single materials, exhibiting properties that are different from those of
core or shell of MOFs.24, 25, 29, 30 Wang31 reported a novel UiO-67@Ni-MOF
composite-based electrochemical sensor for nonenzymatic glucose sensing. The
synergistic effect between good conductivity of UiO-67 and excellent electrocatalytic
activity of Ni-MOF toward glucose made the composite a novel electrode material for
glucose sensing. Zhang group25 prepared ZIF-67@Co-MOF-74 catalyst by coating
2,5-dihydroxyterephthalic acid molecule on the surface of ZIF-67 crystal via the

ligand exchange method for light-driven water oxidation reaction. The core–shell
MOFs show enhanced catalytic performance for the water oxidation reaction
compared to individual ZIF-67 or Co-MOF-74 because of the synergistic effects
between ZIF-67 and Co-MOF-74. Based on the significant advances of the core-shell
MOFs, it is clear that the catalytic properties of the core-shell materials can be greatly
improved by synergistic effects. Also, this feature will be beneficial to improve the
catalytic activity in the oxidation of aromatic methyl derivatives.
In this study, a series of Co-MOF-74@Mn-MOF-74 samples were prepared. The
shell thickness of Co-MOF-74@Mn-MOF-74 was successfully controlled by the mass
ratio of Co-MOF-74 and MnCl2·4H2O. The growth of Mn-MOF-74 out of the
Co-MOF-74 can expose more active sites to improve the catalytic activity. In addition,
the interface of core-shell MOFs could be more conducive for rapid electron transport,
which is helpful for the effective separation of electrons and holes to sufficiently
inhibit charge recombination, resulting in effective increase in the interfacial charge
transfer rate. Their structures and compositions were confirmed by powder X-ray
diffraction (PXRD), thermogravimetric analysis (TGA), inductively coupled plasma
mass spectrometry (ICP-MS), X-ray photoelectron spectroscopy, nitrogen
adsorption-desorption and scanning electron microscopy (SEM). These materials
were used as catalysts for catalytic oxidation of toluene, and the influences of catalyst
structure on the oxidation reaction were studied. Among these core-shell MOF@MOF
composites, Co-MOF-74@Mn-MOF-74-4 sample shows the best catalytic activity
with 22.4 % toluene conversion and 98.1 % benzaldehyde selectivity. Besides, the

substrates were expanded and the catalysts showed good catalytic activity to all these
substrates. In addition, the stability of these catalysts as well as the possible reaction
mechanism were also investigated and discussed.
2. Experimental section
2.1 Materials
2,5-dihydroxyterephthalic acid (H4DOBDC), cobalt nitrate hexahydrate
(Co(NO3)2·6H2O), polyvinylpyrrolidone (PVP) and tetrahydrate manganese chloride
(MnCl2·4H2O) were purchased from Energy Chemical Co., Ltd. TBHP (50 %),
N,N-dimethylformamide (DMF), acetonitrile (MeCN), ethanol (EtOH), deionized
water (H2O), toluene and methanol (MeOH) were obtained from Sinopharm
Chemical Co., Ltd without further purification.
2.2 Synthesis of Co-MOF-74
Co-MOF-74 were synthesized by modifying the synthetic method reported in the
previous literature.32 2,5-Dihydroxyterephthalic acid (H4DOBDC, 0.30 g, 1.51 mmol)
and Co(NO3)2·6H2O (0.73 g, 3.03 mmol) were added into a 60 mL mixture solution
of EtOH, H2O and DMF (1:1:1). Above mixture was stirred rapidly to obtain the
homogeneous solution. The solution was transferred into 100 mL autoclave, and then
heated at 150 °C for 24 h. After reaction, the solid was obtained by filtration and
washed by DMF and MeOH for three times. After that, the as-synthesized materials
were dried under vacuum at 150 °C for 12 h to obtain final Co-MOF-74.
2.3 Synthesis of Co-MOF-74@Mn-MOF-74

he catalyst by improving the seed growth method reported in the literature.28 0.10
g Co-MOF-74 seed, 0.50 g PVP and a certain amount of MnCl2·4H2O were added
into 60 mL DMF, and then ultrasonic dispersion for 30 min. After that, H4DOBDC
(0.10 g, 0.51 mmol) was added into the above solution and then stirred for several
minutes to obtain a homogeneous solution. The solution was transferred into 100 mL
autoclave, and then heated at 150 °C for 24 h. After reaction, the solid was obtained
by filtration and washed by DMF and MeOH for three times. After that, the
as-synthesized materials were dried under vacuum at 150 °C for 24 h to obtain final
Co-MOF-74@Mn-MOF-74-x. x represents the mass ratio between MnCl2·4H2O and
Co-MOF-74.
2.4 Catalyst characterization
PXRD analyses were carried out on a Rigaku MiniFlex 600 with Cu Kα radiation
from 3 to 50°. The specific surface area and distribution of pore size were determined
on a Micromeritics ASAP 2460 apparatus using the Brunauer-Emmett-Teller (BET)
method from N2 adsorption-desorption. Before BET measurements, all samples were
pretreated at 160 °C for 6 h under vacuum. The element content of cobalt was
measured on an Agilent 7700 ICP-MS. SEM images were obtained on a Hitachi
S-3400N II equipped with Energy dispersive X-ray (EX-250, Horiba) analysis
operating at 30 kV.
2.5 Catalytic oxidation of toluene
The catalytic oxidation test of toluene was carried out under atmospheric
pressure using oxygen as an oxidant. The following steps were taken: 15 mL Schlenk

tube was mixed with 30 mg as-prepared catalyst and additive 10 mol% TBHP, 28
mmol toluene and 3 mL MeCN, stirred at a certain temperature, and assembled with
oxygen balloon (0.1 MPa). Meanwhile, the conversion of toluene and the selectivity
of products were quantitatively analyzed by high performance liquid chromatography
(HPLC, P680A LPG-4).
3. Results and discussion
3.1 Catalyst structure characterization
The crystal structure of as-prepared MOFs was identified by PXRD. The PXRD
pattern was shown in Figure 1 a. All the samples showed sharp and clear diffraction
peaks, indicating that the highly crystallized samples were successfully synthesized.
The peak at 2 θ = 6.7° in the XRD pattern of the sample belongs to the (110) crystal
plane of the Co-MOF-74, while the peak at 2 θ = 11.7° belongs to the (300) crystal
plane, which is consistent with the characteristic diffraction peaks reported in the
literature.18, 33 Diffraction peaks of Co-MOF-74 and Co-MOF-74@MOF-74-x were
quite similar and there were no redundant diffraction peaks, indicating that
Co-MOF-74 is not destroyed in the synthesis process. There were no new diffraction
peaks for Co-MOF-74@Mn-MOF-74-x, indicating that the material is a single phase
crystal and no hybrid peaks were generated. Figure 1 b showed that after a layer of
Mn-MOF-74 grew on the outer layer of Co-MOF-74, the diffraction peak originally
located at 2 θ = 6.7° was shifted to the left due to the lattice of Mn-MOF-74 is larger
than that of Co-MOF-74. The lattice enlargement is mainly due to the difference of
radius of Mn (97.0 pm) and Co (88.5 pm). The Mn-MOF-74 growing in the outer

layer made the original lattice larger, so that the diffraction peak was shifted to a
34-36
small angle.
Moreover, as the thickness of Mn-MOF-74 increased, this trend
became more obvious. This indicated that the Co-MOF-74@Mn-MOF-74-x (x = 1, 2,
4) had been successfully synthesized.
(a)
Co-MOF-74@Mn-MOF-74-4
Co-MOF-74@Mn-MOF-74-2
Co-MOF-74@Mn-MOF-74-1
Mn-MOF-74
Simulated Mn-MOF-74
Co-MOF-74
Simulated Co-MOF-74
5
10
15
20
25
30
35
40
2 Theta (degree)
(b)
Co-MOF-74@Mn-MOF-74-4
Co-MOF-74@Mn-MOF-74-2
Co-MOF-74@Mn-MOF-74-1
Mn-MOF-74
Simulated Mn-MOF-74
Co-MOF-74
Simulated Co-MOF-74
5.0
5.5
6.0
6.5
7.0
7.5
8.0
2 Theta (degree)
Figure 1 Powder XRD patterns of different, (a) 2 Theta = 5 - 40°, (b) 2 Theta = 5 - 8°

Table 1 Co and Mn element composition of different samples
Samples
The content of Co (wt.%)
The content of Mn (wt.%)
Co-MOF-74
27.66
-
-
Mn-MOF-74
24.44
6.11
Co-MOF-74@Mn-MOF-74-1
Co-MOF-74@Mn-MOF-74-2
Co-MOF-74@Mn-MOF-74-4
21.17
16.87
10.96
10.72
17.72
The morphology and size of all the MOFs were characterized by SEM. Figure 2
showed the SEM images of Co-MOF-74. The results showed that Co-MOF-74 has
uniform rod-like six-prism structure and smooth surface. The length of the rod was
between 70 and 100 μm. The Co-MOF-74@Mn-MOF-74-x obtained by epitaxial
growth exhibit the same morphology, which indicated that the structure of
Co-MOF-74 was not damaged during epitaxial growth. This conclusion is also
confirmed by the PXRD. In addition, Figure 3 illustrated the EDS mapping of
Co-MOF-74@Mn-MOF-74-x with different thicknesses. The results showed that the
shell thickness of Mn-MOF-74 increased with the increase of MnCl2·4H2O mass,
indicating that Co-MOF-74@Mn-MOF-74-x were successfully synthesized and their
shell thickness could be regulated by changing the mass ratio between MnCl2·4H2O
and Co-MOF-74.
As is shown in Figure 3 (a-d), when the mass ratio was 1:1, there is a thin layer
of Mn-MOF-74 wrapped around the outer layer of Co-MOF-74. The thickness of
Mn-MOF-74 was less than 1 μm. When the mass ratio increased to 2:1, the thickness

of the Mn-MOF-74 layer gradually increased, as shown in Figure 3 (e-h). Furthermore,
when the mass ratio was 4:1, it can be seen from Figure 3 (i-l) that the thickness of
Mn-MOF-74 increased to about 2 μm.
The contents of Co and Mn in the catalysts were measured by ICP-MS (Table 1).
The element compositions of Co and Mn element composition were 27.66 % and
24.44 % for Co-MOF-74 and Mn-MOF-74, respectively. Similarly, with the increase
of introduced Mn ion, the content of Mn increased from 6.11 % to 17.72 %, while the
content of Co decreased from 21.17 % to 10.96 %, which reflected the increase of
shell thickness.
Figure 2 SEM of different Co-MOF-74

Figure 3 SEM and mapping images of different Co-MOF-74@Mn-MOF-74: (a-d)
Co-MOF-74@Mn-MOF-74-1; (e-h) Co-MOF-74@Mn-MOF-74-2; (i-l)
Co-MOF-74@Mn-MOF-74-4
The surface element composition and chemical states of the all samples were
measured by XPS, as displayed in Figure 4. The XPS spectra of Co-MOF-74,
Mn-MOF-74, and Co-MOF-74@Mn-MOF-74-4 were illustrated in Figure 4 (a),
where C 1s (284.08 eV) and O 1s (531.08 eV) were all found in three samples. After
epitaxial growth,
a
new peak at 641.8 eV was observed in
Co-MOF-74@Mn-MOF-74-4, which was attributed to the introduction of
Mn-MOF-74. The Co 2p signal from Co-MOF-74 and the Mn 2p signal from
Mn-MOF-74 were observed to coexist in Co-MOF-74@Mn-MOF-74-4, thereby
indicating the successful combination of Co-MOF-74 and Mn-MOF-74.

(a)
Co-MOF-74 Co 2p
O 1s
C 1s
Mn 2p
Mn-MOF-74
Co-MOF-74@Mn-MOF-74-4
1200 1000 800 600 400 200
0
Binding energy (eV)
(b)
2p1/2
641.8 eV
Mn 2p
2p1/2
Mn-MOF-74
Co-MOF-74@Mn-MOF-74-4
655
650
645
640
635
Binding energy (eV)

(c)
Co 2p
2p2/3
781.03 eV
2p1/2
Co-MOF-74
780.88 eV
Co-MOF-74-Mn-MOF-74-4
810 805 800 795 790 785 780 775
Binding energy (eV)
Figure 4 (a) XPS spectra of prepared Co-MOF-74, Mn-MOF-74, Co-MOF-74@Mn-MOF-74-4; (b)
Mn 2p signal deconvolution; (c) Co 2p signal deconvolution
In In order to evaluate the thermal stability of the catalysts, TGA was used under
flowing air, and the curve was shown in Figure 5. All the samples showed two kinds
of weightlessness in the range of room temperature to 500 °C. The weight loss in
range of 100-150 °C should mainly be caused by the evaporation of water and DMF
in the channel. The second weight loss in the range of 250-330 °C could be attributed
to the decomposition of the organic linkers and the corresponding metallic oxide
(such as Mn3O4 and Co3O4) remained as a residue. These results indicated that the
catalysts were still stable at about 300 °C, which can avoid deactivation of catalysts
due to decomposition in the reaction process.

100
90
80
70
60
50
40
30
20
10
0
Co-MOF-74@Mn-MOF-74-4
Co-MOF-74@Mn-MOF-74-2
Co-MOF-74@Mn-MOF-74-1
Co-MOF-74
Mn-MOF-74
100
200
300
400
500
o
Temperature ( C)
Figure 5 TG curves of different samples
The porosity and specific surface area of Co-MOF-74 and
Co-MOF-74@Mn-MOF-74-x catalysts were tested by nitrogen adsorption. The N2
adsorption/desorption isotherms of the catalysts were characterized at 77 K, and the
isotherms of the types of MOFs were type Ⅳ according to the IUPAC classification
of isotherm shapes (Figure 6 a).37 It can be found that the specific surface area of
Co-MOF-74 and Mn-MOF-74 were 427 m2/g and 1031 m2/g, respectively, and the
specific surface area of Co-MOF-74@Mn-MOF-74-x ranged from 597 to 941 m2/g.
With the increase of shell thickness, the specific surface area of core-shell MOF
increases gradually. The average adsorption pore diameter of Co-MOF-74 and
Mn-MOF-74 were 2.69 nm and 3.5 nm, respectively, while the pore diameter of
Co-MOF-74@Mn-MOF-74-x ranged from 2.98 to 4.49 nm. The increase in pore size
is mainly attributed to the growth of outer Mn-MOF-74. The increased pore size can
provide more reactive sites to improve the catalytic activity.

(a)
Co-MOF-74@Mn-MOF-74-4
Co-MOF-74@Mn-MOF-74-2
Co-MOF-74@Mn-MOF-74-1
Co-MOF-74
Mn-MOF-74
0.0
0.2
0.4
0.6
0.8
1.0
Relatuve pressure (P/P )
0
Co-MOF-74@Mn-MOF-74-4
Co-MOF-74@Mn-MOF-74-2
Co-MOF-74@Mn-MOF-74-1
Co-MOF-74
(b)
Mn-MOF-74
0
10
20
30
40
50
Pore width (nm)
Figure 6 a) Nitrogen adsorption-desorption isotherms, and b) pore size distributions of
Co-MOF-74, Mn-MOF-74 and Co-MOF-74@Mn-MOF-74-x
3.2 Catalytic activity of catalysts in oxidation of toluene.
On the basis of synthesizing Co-MOF-74@Mn-MOF-74-x with different shell
thicknesses, their catalytic activity in toluene oxidation were evaluated by using

oxygen as oxidant and TBHP as promoter at 80 °C for 6 h . As shown in Table 2, the
catalyst exhibited good catalytic activity for toluene, and the main products were
corresponding alcohols, aldehydes and acids.
For the blank experiment without catalyst, it can be seen that the conversion of
toluene is very low and the reaction can hardly occur, suggesting that the catalyst
plays a key role in the oxidation of toluene. Simultaneously, the control experiment
showed that TBHP was not added in the reaction, the conversion of toluene dropped
sharply to 3.2 % (Table 2, Entry 9), which indicated that TBHP was a key promoter in
the reaction. When Co(NO3)2 was used as catalyst, the conversion rate of toluene
reached 18.3 %, but after researching the selectivity of the product, it was found that
the product was mainly converted to benzoic acid, reaching 57.1 % (Table 2, Entry 2 ).
Although Co(NO3)2 has high catalytic conversion and benzoic acid selectivity, it
cannot be reused after the reaction, also resulting in pollution and waste.
When Co-MOF-74 was used as catalyst, the conversion of toluene reached
16.7 %. The main product of oxidation was benzaldehyde, and the selectivity of
benzaldehyde reached up to 60.0 %, which was quite different from that of Co(NO3)2.
This is mainly because the Co content in Co-MOF-74 is much lower than that in
Co(NO3)2, thus benzaldehyde cannot be further oxidized to benzoic acid.38 Compared
with Co-MOF-74 catalyst, the Mn-MOF-74 catalyst showed higher selectivity of
benzaldehyde, reaching 80.3 %. However, the conversion of toluene for Mn-MOF-74
was lower than that of Co-MOF-74, reaching just 13.2 % (Table 2, Entry 4). These
results indicated that Co-MOF-74 facilitated the catalytic transformation of toluene

oxidation and Mn-MOF-74 benefited the formation of benzaldehyde. Based on the
above results, we used Co-MOF-74 as the main constituent of the catalyst and
introduced the Mn-MOF-74 with the aim of achieving a high catalytic activity and a
high selectivity of benzaldehyde via the synergy between shell layer and core layer.
The catalytic activities of Co-MOF-74@Mn-MOF-74-x catalysts were explored
and the results were listed in Table 2. It was found that all the
Co-MOF-74@Mn-MOF-74-x catalysts obtain a good conversion of toluene while
maintaining a high selectivity of benzaldehyde. The conversion of toluene ranged
from 18.5 % to 22.4 % and the selectivity of benzaldehyde changed from 92.6 % to
98.1 % with the shell growth (Table 2, Entry 5-7), which were higher than single
Co-MOF-74 and Mn-MOF-74.
There are four main reasons for the increasing conversion of toluene. First of all,
introduced Mn-MOF-74 as a shell layer can enhance the interaction between toluene
and catalyst in the catalytic system, allowing more substrates to participate in the
reaction.6 Furthermore, with the growth of Mn-MOF-74 in outer of the Co-MOF-74,
the number of active sites (Mn sites) and the specific surface area are increased and
then enhance the conversion of substrates. Additionally, the surface of core-shell
catalyst is no longer smooth, which exposes more active sites of the metal and allows
the catalyst to come into full contact with the substrate molecule.25 Finally, the
interface of core–shell MOFs is more conducive for rapid electron transport, which is
helpful for the effective separation of electrons and holes to sufficiently inhibit charge
recombination,39 resulting in effective increase in the interfacial charge transfer rate.

When the Co-MOF-74@Mn-MOF-74-x was used as catalyst, the selectivity of
benzaldehyde ranged from 92.6 % to 98.1 %, which was better than single
Co-MOF-74 or Mn-MOF-74. The increased selectivity was mainly due to the
synergistic effect between Co-MOF-74 and Mn-MOF-74 in the process of reaction.
More specifically, when the substrate molecules and TBHP contacted with the nuclear
layer of Co-MOF-74 through the catalyst channel, the Co metal site of Co-MOF-74
can generate peroxy radical by cracking TBHP via Haber-Weiss route. Subsequently,
when the main oxidation products benzyl alcohol and benzaldehyde were desorbed
from the pore channels of the Co-MOF-74, the Mn metal site in Mn-MOF-74 could
rapidly further oxidize benzyl alcohol to benzaldehyde for the presence of base side of
Mn-MOF-74 and desorbed from the Mn-MOF-74 pore channels with the original
benzaldehyde. This explained that the Co-MOF-74@Mn-MOF-74-x catalysts exhibit
excellent selectivity of benzaldehyde, while still having a good conversion of toluene.
Among
the
Co-MOF-74@Mn-MOF-74-x
catalysts,
the
Co-MOF-74@Mn-MOF-74-4 catalyst showed the best catalytic activity compared
with Co-MOF-74@Mn-MOF-74-1 and Co-MOF-74@Mn-MOF-74-2, which obtained
a toluene conversion of 22.4 % and a benzaldehyde selectivity of 98.1 % (Table 2,
Entry 7). As we all know, the composition of catalysts will influence the activity of
catalytic reactions when the reaction conditions remain unchanged. In this system, the
difference of catalyst mainly lies in the thickness of Mn-MOF-74. In short, the
catalytic activity of a catalyst is affected by the thickness of shell. The presence of Mn
ions can enhance the adsorption capacity of the catalyst to toluene molecules, which

has been mentioned above. And with the increase of Mn content, the catalyst can
absorb more substrate molecules. ICP-MS showed the content of Mn element
increased with the growth of shell thickness. When the mass ratio between
MnCl2·4H2O and Co-MOF-74 is 4:1, the content of Mn element in the
Co-MOF-74@Mn-MOF-74-4 is 17.72 % which is higher than that of
Co-MOF-74@Mn-MOF-74-1 (6.11 %) and Co-MOF-74@Mn-MOF-74-2 (10.72 %).
Furthermore, the specific surface area of Co-MOF-74@Mn-MOF-74-4 is 941 m2/g,
which is higher than that of other core-shell catalysts from Figure 6. It is well known
that the greater the specific surface area of the catalyst, the more active sites it can
provide to enhance the catalytic activity. These explain the superior catalytic activity
of Co-MOF-74@Mn-MOF-74-4 compared with other core-shell catalysts.
Table 2 Oxidation of toluene catalyzed over different catalystsa
Selectivityb/%
Entry
Catalyst
Conversion/%
BAL
30.1
22.3
38.1
8.7
5.4
1.6
0.7
12.1
-
BA
60.3
12.1
60.0
80.3
92.6
94.9
98.1
78.3
51.0
-
BAC
-
1
2
-
1.9
18.3
16.7
13.2
18.5
20.2
22.4
19.3
3.2
Co(NO3)2
57.1
1.8
-
3
Co-MOF-74
4
Mn-MOF-74
5
Co-MOF-74@Mn-MOF-74-1
Co-MOF-74@Mn-MOF-74-2
Co-MOF-74@Mn-MOF-74-4
Co-MOF-74+Mn-MOF-74
Co-MOF-74@Mn-MOF-74-4
Co-MOF-74@Mn-MOF-74-4
1.4
1.2
0.9
7.4
33.7
-
6
7
8
9c
10d
-
-
aReaction condition: 30 mg of catalyst, 3 mL toluene (28 mmol), O2 balloon, 10 mol% TBHP, 3 mL CH3CN, 80
oC, 6 h, bBAC = selectivity of benzoic acid, BA = selectivity of benzaldehyde, BAL = selectivity of benzyl
c
alcohol, Without TBHP, 3 mL CH3CN, 80 oC, 6 h，d1 eq. BHT.

To prove the synergistic effect between Co-MOF-74 and Mn-MOF-74, mixture
of Co-MOF-74 and Mn-MOF-74 used as catalyst to oxidize toluene. The results were
shown in Table 2, Entry 8. The catalytic activity of mixed catalyst
(Co-MOF-74+Mn-MOF-74)
was
quite
different
from
that
of
Co-MOF-74@Mn-MOF-74-4. The conversion of toluene catalyzed over mixed
catalyst was 19.3 % and the selectivity of benzaldehyde was 78.3 %, which indicated
that the separate Mn-MOF-74 cannot improve the catalytic activity of Co-MOF-74
compared with the Co-MOF-74@Mn-MOF-74-4. In other words, mixed catalyst
composed of Co-MOF-74 and Mn-MOF-74 via physical mixing cannot play a
synergistic role in the catalytic system to improve catalytic activity.
In order to expand the application of this catalyst, a series of toluene derivatives
containing different substituents were selected and the result of oxidation to the
corresponding aldehydes were summarized in Table 3. For the derivatives containing
donor groups, the catalyst demonstrated excellent selectivity of aldehydes, but the
relatively low conversion rate. For example, the conversion rate of p-xylene was
15.7 %, while the selectivity of the corresponding aldehyde reached up to 98.2 %.
However, for some derivatives containing electron-absorbing groups, the catalyst also
showed relatively low selectivity of corresponding aldehyde and high conversion rate.
For example, the conversion of p-nitrotoluene and the selectivity of corresponding
aldehyde was 22.1 % and 87.2 %, respectively. This is mainly due to the influence of
different groups connected to toluene and different electron-supplying and adsorbing
groups, which changes the C-H bond length on the methyl group. In addition, the

adsorption of substrates with different substituents in the catalyst channel is also
different.
Table 3 Aerobic oxidation of various substituted toluenes over Co-MOF-74@Mn-MOF-74-4a
Conversion [%],
(Selectivity [%])
15.7 (98.2)
13.8 (93.6)
19.1 (91.5)
Cl
11.3 (88.7)
10.9 (90.7)
9.3 (90.6)
19.3 (95.2)
23.3 (83.2)
16.7 (84.9)
18.5 (94.1)
20.1 (81.2)
22.1 (87.2)
aReaction condition: 30 mg of catalyst, 3 mL toluene (28 mmol), O2 balloon, 10 mol% TBHP, 3 mL CH3CN, 80
oC, 6 h
Based on the existing experimental results and the reaction process reported by
predecessors40, 41, the experimental mechanism of catalytic oxidation of toluene by
Co-MOF-74@Mn-MOF-74 under the conditions of oxygen and TBHP was supposed.
Without TBHP in the catalytic system, the conversion of toluene was just 3.2 %
(Table 2, Entry 9). Then, when the free radical scavenger 2, 6-2-tert-butyl, 4-methyl
phenol (BHT) and TBHP were introduced together in the catalytic system (Table 2,
Entry 10), the reaction did not occur. These results indicate that the reaction occurred

via a radical pathway. Therefore, there may be a free radical reaction course in this
reaction.
Based on the above experiments, a possible reaction mechanism was proposed and
shown in Figure 7. Toluene molecules and TBHP were absorbed by the catalyst and
entered into the catalyst through the pore, and the Co site decomposed TBHP to
produce t-BuOO•.t-BuOO• formed benzyl radicals by taking a hydrogen atom from
the methyl group. This benzyl radical captured an activated oxygen molecule to form
benzylperoxy radical. After that, the benzylperoxy radical reacted with TBHP/toluene
to produce benzyl hydroperoxide. Benzyl hydroperoxide was rapidly oxidized by the
Co metal site to form the intermediate benzyl alcohol. Benzyl alcohol was then
desorbed from Co-MOF-74 to the shell, and Mn-MOF-74 can rapidly oxidize benzyl
alcohol to benzaldehyde due to its own base site.
Figure 7 Proposed mechanism for the catalytic reaction of toluene over
Co-MOF-74@Mn-MOF-74
The stability of the catalyst before and after 4 cycles was determined by PXRD
as shown in Figure 8. The diffraction peaks of used catalyst were the same as that of

fresh catalyst, and no new peaks were generated, which indicated that the overall
structure of the catalyst did not change after the 4 cycles. Figure 9 displays the
conversion of toluene and selectivity of benzaldehyde for subsequent oxidation
experiments. The catalyst shows good reusability, and after four runs, the conversion
rate of toluene is 15.4% and the selectivity of benzaldehyde is 89.9 %. The results
showed that the catalyst has good stability and can be used for the oxidation reaction.
Used
Fresh
10
20
30
40
50
2 Theta (degree)
Figure 8 PXRD patterns of the fresh and used for four times Co-MOF-74@Mn-MOF-74-4
catalyst in toluene oxidation
120
100
80
60
40
20
0
120
100
80
60
40
20
0
Conversion of toluene
Selectivity of benzaldehyde
1
2
3
4
Recycling time

Figure 9 Catalyst reusability study of Co-MOF-74@Mn-MOF-74-4 in toluene oxidation
Conclusions
Co-MOF-74@Mn-MOF-74-x catalysts with core-shell structure were
synthesized by epitaxial growth method, and their structure and composition were
determined by different characterization techniques. The shell thickness of
Co-MOF-74@Mn-MOF-74 was successfully controlled by adjusting the mass ratio
between MnCl2·4H2O and Co-MOF-74. The Co-MOF-74@Mn-MOF-74-x catalysts
were used in the oxidation of toluene, exhibited high selectivity of benzaldehyde and
favorable conversion of toluene compared to single Co-MOF-74 or Mn-MOF-74.
Among them, the Co-MOF-74@Mn-MOF-74-4 showed enhanced catalytic activity
(22.4 %) and high selectivity towards benzaldehyde (98.1 %). The interaction
between toluene and catalyst was also investigated, and the results indicate that the
synergistic effect between Co-MOF-74 and Mn-MOF-74 could promote the
conversion of toluene to benzaldehyde significantly. Furthermore, the catalyst can not
only convert the toluene derivative to corresponding aldehyde, but also maintain high
selectivity in the oxidation process. In addition, the catalyst can be reused for 4 times
and has great potential in oxidation of toluene. These results indicate that compared to
single MOF in oxidation toluene, the core-shell MOFs could achieve higher catalytic
activity via synergistic effect between core and shell, which could be potentially used
in the oxidation of aromatic methyl derivatives.
Acknowledgements
The authors greatly acknowledge the financial support in part by Natural Science

Foundation of Jiangsu Province-Outstanding Youth Foundation (BK20170104),
National Key Research and Development Program of China (2017YFB0307202),
Strategic Pioneer Program on Space Science, Chinese Academy of Sciences
(XDA15013100, XDA15013101) and Youth Foundation of Southeast University
ChengXian College (z0017).
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