Nanoreactor of MOF-Derived Yolk-Shell Co@C-N: Precisely Controllable Structure and Enhanced Catalytic Activity

Article abstract of DOI:10.1021/acscatal.7b03270

Hollow yolk-shell nanoreactors are of great interest in heterogeneous catalysis owing to their improved mass transfer ability and stability. Here, we report a facile and straight route to synthesize a highly efficient and recyclable yolk-shell Co@C-N nanoreactor with controllable properties by the direct thermolysis of a hollow Zn/Co-ZIF precursor. Based on systematical optimization of the pyrolysis temperature and the shell-thickness of Zn/Co-ZIFs, we could completely anchor and stabilize uniform Co nanoparticles (NPs) in the hollow yolk, accommodated by the Co-ZIF derived N-doped carbon nanosheets. This nanosheet-assembled yolk was further confined by a permeable and robust N-doped carbon (C-N) shell to protect the Co NPs against leaching and also enabled the reaction to take place in the hollow void. Consequently, the optimal yolk-shell Co@C-N nanoreactor showed a significantly enhanced catalytic activity for the aqueous oxidation of alcohols, yielding >99% conversion under atmospheric air and base-free conditions, which was much higher than that of the solid counterparts derived from pure ZIF-67 and solid core-shell ZIF-67@ZIF-8 precursors (with 14% and 59% conversion under the same reaction condition, respectively). The enhanced catalytic activity should be attributed to the yolk-shell structure that could facilitate the transport of reactant/product and the strong interaction between the Co NPs and N-doped carbon nanosheet to afford positive synergistic effects. Moreover, this catalyst also showed good recyclability, magnetically reusability, and general applicability for a broad substrate scope, further highlighting the structure superiority of our yolk-shell nanoreactor. This strategy might open an avenue to synthesize various hollow yolk-shell nanoreactors with controllable structures and enhanced catalytic performances.

Full text of DOI:10.1021/acscatal.7b03270

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Article
Nanoreactor of MOF-Derived Yolk-Shell Co@C-N: Precisely
Controllable Structure and Enhanced Catalytic Activity
Huirong Chen, Kui Shen, Qing Mao, Junying Chen, and Yingwei Li
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03270 • Publication Date (Web): 09 Jan 2018
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ACS Catalysis
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Nanoreactor of MOF-Derived Yolk-Shell Co@C-N: Precisely
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Huirong Chen, Kui Shen,* Qing Mao, Junying Chen, and Yingwei Li*
State Key Laboratory of Pulp and Paper Engineering, School of Chemistry and Chemical Engineering, South China
University of Technology, Guangzhou 510640, China.
ABSTRACT: Hollow yolk-shell nanoreactors are of great interest in heterogeneous catalysis owing to their improved mass
transfer ability and stability. Here, we report a facile and straight route to synthesize a highly efficient and recyclable
yolk-shell Co@C-N nanoreactor with controllable properties by the direct thermolysis of a hollow Zn/Co-ZIF precursor.
Based on systematical optimization of the pyrolysis temperature and the shell-thickness of Zn/Co-ZIFs, we could com-
pletely anchor and stabilize uniform Co nanoparticles (NPs) in the hollow yolk, accommodated by the Co-ZIF derived N-
doped carbon nanosheets. This nanosheet-assembled yolk was further confined by a permeable and robust N-doped car-
bon (C-N) shell to protect the Co NPs against leaching and also enabled the reaction to take place in the hollow void.
Consequently, the optimal yolk-shell Co@C-N nanoreactor showed a significantly enhanced catalytic activity for the
aqueous oxidation of alcohols, yielding >99% conversion under atmospheric air and base-free conditions, which was
much higher than that of the solid counterparts derived from pure ZIF-67 and solid core-shell ZIF-67@ZIF-8 precursors
(with 14% and 59% conversion under the same reaction condition, respectively). The enhanced catalytic activity should be
attributed to the yolk-shell structure that could facilitate the transport of reactant/product, and the strong interaction
between the Co NPs and N-doped carbon nanosheet to afford positive synergistic effects. Moreover, this catalyst also
showed good recyclability, magnetically reusability and general applicability for a broad substrate scope, further high-
lighting the structure superiority of our yolk-shell nanoreactor. This strategy might open an avenue to synthesize various
hollow yolk-shell nanoreactors with controllable structures and enhanced catalytic performances.
KEYWORDS:
of alcohols
n
anoreactor, hollow yolk-shell nanostructure, metal-organic frameworks, Co-based catalysts, oxidation
composed of a single-core nanocrystal and a porous
INTRODUCTION
nanoshell.6 For the catalytic applicability of a nanoreactor,
it is more desirable for the cavity to possess large num-
bers of tiny cores rather than a single core in large size.
Nevertheless, efficient synthetic methods for multi-
yolk@shell nanostructures have hardly been explored so
far.7 The reason is that the tiny nanocrystals, if left move-
able within the cavity, will undergo an inevitable sintering
process to form a single large particle in order to lower
their surface energies.8 As it is well known, the particle
size is critical for the use of nanoparticles (NPs) in heter-
ogeneous catalysis. A reduction in particle size usually
leads to a significant increase in reactivity due to the
higher surface-to-volume ratio of the active species.1a
Therefore, to obtain a highly efficient and stable hollow
yolk-shell nanostructure the key point lies in a balance
between the following two factors, i.e., embedding tiny
multi-core particles in the hollow cavity as much as pos-
sible and simultaneously protecting the small particles
from serious aggregation during the preparation and reac-
tion processes. Given these premises, we proposed that
introduction of nitrogen-doped carbon nanosheets into
the hollow cavity of the nanostructure would be a promis-
ing strategy to tackle this issue, because they cannot only
stabilize the tiny metal NPs in a highly dispersed fashion
In recent years, hollow yolk-shell nanostructures have
received increasing attention as they can be used as the
most ideal framework in nanoreactor for the catalytic
transformation of organic molecules.1 As compared to
their solid core-shell counterparts, the void space be-
tween the shell and core in the hollow system provides a
unique room for confined catalysis.2 Moreover, the per-
meable shell enables facile mass transportation for the
reactive species and protects the catalytically active sites
against leaching even under harsh reaction conditions,
leading to enhanced activity and stability for hollow yolk-
shell materials.3
For the synthesis of hollow nanoreactors, the most
important and challenging part would be selective func-
tionalization of the interior space in the hollow nanoshell,
which allows chemical reactions to take place within the
protected cavity.4 A few strategies have been developed
for the preparation of yolk-shell architectures during the
past few years. In these approaches, hard/soft-templating,
the Kirkendall effect, the Ostward ripening, ship-in-a-
bottle, post-impregnation, and selective etching are wide-
ly employed as the core technologies.5 However, most of
these approaches yielded yolk@shell nanostructures
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Figure 1. (a) Schematic illustration of the synthesis of hollow yolk-shell Co@C-N nanoreactors. (b) Schematic illustration
of the synthesis of solid Co-based nanocomposites by direct pyrolysis of ZIF-67 nanocrystals. (c) Schematic illustration of
the synthesis of solid core-shell Co-based nanocomposites by pyrolysis of core-shell ZIF-67@ZIF-8 precursors.
tions. In these regards, it would be a more reasonable
but also enhance the catalytic activity by providing a syn-
choice if we used MOFs with more sophisticated struc-
tures as precursors to fabricate a hollow core-shell nano-
composite.
ergistic effect to the metal active sites.
Metal-organic frameworks (MOFs) built with metal
ions and organic ligands have attracted continuously in-
creasing interest due to their high surface area, porosity,
Herein, we developed a facile strategy to prepare
and chemical tunability.9 More recently, a great deal of
multi-yolk@shell nanostructures by direct thermolysis of
a single hollow Zn/Co-ZIF precursor (Figure 1a). The ob-
research has been focused on preparation of new metal-
tained hollow Co@C-N materials feature a high surface
area, which consists of an N-doped carbon (C-N)
nanosheet anchored Co core and a porous C-N shell with
controllable structures. Accordingly, the porous C-N shell
would enable facile mass transfer and the C-N nanosheets
in the void space could play important roles in stabilizing
the tiny Co NPs and also providing electronic synergistic
effects between them. These characters endow this
unique hollow yolk-shell Co@C-N nanoreactor with supe-
rior catalytic activities in aerobic oxidation of alcohols
under neat water and base-free conditions using air as a
green oxidant, as compared to the Co-based materials
prepared by using ZIF-67 and ZIF-67@ZIF-8 as precursors
(Figure 1b and 1c). Furthermore, these highly efficient
hollow nanoreactors are easily separated and recovered
by a magnet due to the presence of magnetic Co NPs and
show no significant loss of catalytic activities after reuses.
carbon composites for advanced applications including
heterogeneous catalysis, electrochemistry, and gas ad-
sorption, by using MOFs as sacrificial templates.10 For
example, thermolysis of ZIF-67 has been frequently em-
ployed as a facile approach to synthesize novel nitrogen-
doped carbon supported Co materials.11 However, this
strategy would inevitably result in serious aggregation of
Co NPs at high pyrolysis temperatures (Figure 1b). There-
fore, we attempted to employ a core-shell structure ZIF-
67@ZIF-8 as sacrifice template (Figure 1c). Although this
method could effectively address the issue of Co aggrega-
tion, achieving highly dispersed Co NPs on N-doped car-
bon as the multi-core, the resultant core-shell material
also had a low surface area and solid structure together
with an unsatisfactory ability of mass transfer, which are
certainly unfavorable for heterogeneous catalytic reac-
2
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Characterization. Powder X-ray diffraction (XRD)
patterns of the samples were recorded with a Rigaku dif-
fractometer (D/MAX/IIIA, 3 kW) using Cu Kα radiation
(40 kV, 30 mA, λ=0.1543 nm). BET surface areas and pore
structure were measured with N2 adsorption/desorption
isotherms at 77 K on a Micromeritics ASAP 2020M in-
strument. Before the measurements, the samples were
degassed at 150 °C for 4 h. X-ray photoelectron spectros-
copy (XPS) was performed by using Kroatos Axis Ultra
DLD system with a base pressure of 10-9 Torr. The cobalt
contents in the samples were measured quantitatively by
atomic absorption spectroscopy (AAS) on a HITACHI Z-
2300 instrument. The surface morphology of the materials
was investigated by a high-resolution scanning electron
microscopy (SEM, MERLIN of ZEISS). The structure and
the element mapping were determined by a high resolu-
tion transmission electron microscope (TEM, JEOL, JEM-
2100F) with EDX analysis (Bruker Xflash 5030T) operated
at 200 kV.
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EXPERIMENTAL SECTION
Chemicals. All chemicals were of analytic grade and
purchased from commercial suppliers.
Preparation of hollow Zn/Co-ZIF. In a typical syn-
thesis, Co(NO3)2•6H2O (1.098 g), Zn(NO3)2•6H2O (1.116
g) and 2-methylimidazole (1.232 g) were dissolved in 15
mL methanol, 15 mL methanol, and 30 mL methanol, re-
spectively, to obtain three methanolic solutions. After
stirring for 10 min at room temperature, the methanolic
solution of Co(NO3)2 was slowly injected into the solu-
tion of 2-methylimidazole at room temperature. After
reacting for a certain time t, the methanolic solution of
Zn(NO3)2 was added into the above mixed solution. After
stirring for another 5 min, the resulting suspension was
transferred to a Teflon-lined stainless-steel autoclave and
hydrothermally treated at 120 ºC for 4 h. The formed
powder was collected by centrifugation, washed thor-
oughly with methanol, and dried overnight at 110 °C. By
adjusting the value of t (0.5, 1 and 5 min), we were able to
controllably synthesize three kinds of hollow Zn/Co-ZIF
with different shell thicknesses (denoted as Zn/Co-
ZIF(0.5), Zn/Co-ZIF(1) and Zn/Co-ZIF(5), respectively).
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RESULTS AND DISCUSSION
The preparation strategy for the hollow Co@C-N yolk-
shell nanoreactor is briefly illustrated in Figure 1a. The
synthesis started with the formation of a single ZIF-67
nanocrystal which was then coated with a ZIF-8 layer to
obtain a core-shell ZIF-67@ZIF-8 structure (Figure S1).
The as-prepared ZIF-67@ZIF-8 was then treated with Co2+
in methanol at 120 °C for 4 h to generate a hollow Zn/Co-
ZIF material (Figure S2).13 As shown in Figure S2 and S3,
both of pure ZIF-67 and ZIF-67 phase in ZIF-67@ZIF-8
core-shell structure can be transformed into a new Co-ZIF
phase by solvothermal reaction in a methanol solution
containing cobalt ions.14 Typically, in order to prepare
Zn/Co-ZIF with a relatively thin, medium or thick shell,
we choose three crystallization times of 0.5, 1, and 5
minutes, respectively, to regulate the growth process of
ZIF-67 (Figure S4). As a result, three samples with differ-
ent mean shell thicknesses of 138.8±16.5, 85.5±15.6 and
34.7±8.6 nm (Figure S5) (denoted as Zn/Co-ZIF(0.5),
Zn/Co-ZIF(1) and Zn/Co-ZIF(5), respectively) were suc-
cessfully prepared. The PXRD patterns of all the obtained
Zn/Co-ZIF materials matched well with the pristine ZIF-8
and ZIF-67, confirming their isostructural nature and
high crystallinity (Figure S6). From the TEM images (Fig-
ure S5), we confirmed that all the resultant particles pos-
sessed a dodecahedron-shaped hollow interior, together
with some Co-ZIF nanosheets confined by the ZIF-8 shell.
Preparation of ZIF-67@ZIF-8. The procedure was
the same as that for the synthesis of hollow Zn/Co-ZIF
but without the hydrothermal treatment.
Preparation of ZIF-67. ZIF-67 was prepared accord-
ing to the previous reports12 but with some modifications.
In a typical synthesis, Co(NO3)2•6H2O (1.091 g) was dis-
solved into methanol (30 mL) to form a solution. 2-
Methylimidazole (0.616 g) was dissolved in methanol (20
mL) to generate another clear solution. Then the two
methanolic solutions were mixed together under stirring
for 5 min. The mixture was kept on reacting at room tem-
perature for 12 h. The resulting bright purple powders
were collected by centrifugation, washed thoroughly with
methanol, and dried overnight at 110 °C.
Preparation of hollow yolk-shell Co@C-N nano-
reactor. In a typical synthesis, 0.5 g of hollow Zn/Co-ZIF
was placed in a tubular furnace and then heated at 800 °C
for 3 h with a heating rate of 1 °C/min from room temper-
ature under argon atmosphere. For comparison purposes,
another two control samples were also prepared by using
pure ZIF-67@ZIF-8 or ZIF-67 with similar crystal sizes as
single precursor under the same thermal conditions,
which were denoted as ZIF-67@ZIF-8-800 and ZIF-67-800,
respectively.
Finally, the Zn/Co-ZIF was subjected to high-
temperature pyrolysis at 800 ºC under Ar atmosphere to
yield the desired hollow Co@C-N nanoreactors. The as-
synthesized nanocomposites were denoted as Co@C-
N(0.5)-800, Co@C-N(1)-800 and Co@C-N(5)-800, respec-
tively, corresponding to the different Zn/Co-ZIF precur-
sors with various shell-thicknesses (i.e., Zn/Co-ZIF(0.5),
Zn/Co-ZIF(1) and Zn/Co-ZIF(5)).
Procedures for the aerobic oxidation of alcohols.
For a typical catalytic test, alcohol (0.1 mmol), H2O and a
certain amount of catalyst (10 mol% Co) were added in a
Schlenk tube. The reaction mixture was stirred at 110 ºC
under atmosphere air condition. After a given reaction
time, 100 µL of n-hexadecane as the internal standard was
added. Then the product was extracted with ethyl acetate
from the liquid mixture, and subsequently subjected to
GC-MS (Agilent, 7890B GC/5977A MS) equipped with a
HP-5 MS capillary column (0.25 mm × 30 m).
The morphologies and structures of the Zn/Co-ZIF
derived nanocomposites can be revealed by SEM and
TEM images. As shown in Figure 2, both of Co@C-N(0.5)
3
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