Fabrication of novel hybrid nanoflowers from boron nitride nanosheets and metal-organic frameworks: A solid acid catalyst with enhanced catalytic performance

Article abstract of DOI:10.1039/c4ta04230a

A double solvent replacement method was employed for the synthesis of novel hybrid nanoflowers from boron nitride nanosheets (BNNSs) and the metal-organic framework (MOF) MIL-53 in aqueous solutions under hydrothermal treatments. The strong binding abilit

Full text of DOI:10.1039/c4ta04230a

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Materials Chemistry A
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Fabrication of novel hybrid nanoflowers from
boron nitride nanosheets and metal–organic
frameworks: a solid acid catalyst with enhanced
catalytic performance†
Cite this: J. Mater. Chem. A, 2014, 2,
18731
Received 15th August 2014
Accepted 25th September 2014
Peng Wang,a Hao Li,b Qiang Gao,a Pei-Zhou Li,a Xin Yao,a Linyi Bai,a
DOI: 10.1039/c4ta04230a
a
cd
ade
*
*
Kim Truc Nguyen, Ru-Qiang Zou
and Yanli Zhao
www.rsc.org/MaterialsA
A double solvent replacement method was employed for the synthesis nanosheets (BNNSs) show an isostructure to graphene, which is
of novel hybrid nanoflowers from boron nitride nanosheets (BNNSs) formed based on the B–N covalent bonds. The layers of BN can
and the metal–organic framework (MOF) MIL-53 in aqueous solutions undergo a variety of noncovalent interactions forming multiple
under hydrothermal treatments. The strong binding ability of layered structures, namely h-BN, which is an isostructure to
aluminum ions onto the surface of BNNSs determines the 3D flow- graphite.3
erlike architectures of the BNNSs/MOFs hybrid, and the BNNSs act as a
Aer h-BN and BNNSs are hybridized with other molecules
structure-directing template. The BNNSs/MOFs showed an enhanced or materials, the resulting hybrid materials oen exhibit
catalytic activity in the acetalization of benzaldehyde with methanol extraordinary properties and potential applications.4 For
owing to the facilitated diffusion process in the hierarchical example, h-BN has already been employed as catalyst supports,
architectures.
mainly because of its stability with a strong mechanical
strength, as well as high resistance against corrosion, high
temperature and oxidation. BNNSs have also been hybridized
with polymers including polymethyl methacrylate (PMMA) and
poly(vinyl alcohol) (PVA). However, the materials used to inte-
grate with h-BN or BNNSs are restricted to polymers and metal
nanoparticles. Herein, we present a novel type of catalytic
material by hybridizing BNNSs with metal–organic frameworks
(MOFs). To the best of our knowledge, this composite material
is the rst example employing BNNSs as the templates to direct
the MOF growth and to control the hybrid morphology.
Although the investigation was inspired by MOF-graphite oxide
(GO) hybrids,5 it is still of great importance in terms of both
fundamental studies and practical applications. Unlike the GO
layers, BN is relatively chemically inert without any decorated
functional groups, such as hydroxyl and carboxyl units, on the
surface making it more difficult to grow MOFs on the surface.6
The ability of BN to undergo hybridization relies on the lone
electron pairs on their nitrogen atoms, which act as Lewis bases
to coordinate with metal-based species.7 When multiple layered
h-BN is sonicated in polar solvents, such as N,N0-dime-
thylformamide (DMF), single or few-layered BNNSs with
retained crystallinity could be obtained. The formation and
stabilization of BNNSs are attributed to the noncovalent inter-
actions between the oxygen atoms in the carbonyl groups of
DMF and the boron atoms on the BNNSs, which act as a Lewis
base and acid, respectively.4c However, the trials using the
BNNSs obtained from DMF to template the MOF formation
were unsuccessful. This result could be explained by the fact
In the past decade, two-dimensional (2D) nanomaterials have
attracted signicant attention in the scientic community when
Novoselov, Geim and co-workers isolated graphene nanosheets
and discovered their extraordinary electronic properties.1 The
signicance of the development of graphene and their related
materials relies on not only the potential technological appli-
cations, but also the fundamental scientic interest, which
provide the inspiration for scientists to develop novel types of
2D materials, such as layered transition metal dichalcogenides
including MoS2 and WS2, as well as hexagonal boron nitride (h-
BN). These 2D materials are of importance because of their
signicant impact on a variety of applications in electronics, gas
storage and separation, catalysis.2 Among these novel materials,
BN is isoelectronic to carbon-based structures including carbon
nanotubes, graphite and diamond. In addition, boron nitride
aDivision of Chemistry and Biological Chemistry, School of Physical and Mathematical
Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371,
Singapore. E-mail: zhaoyanli@ntu.edu.sg
bDepartment of Chemistry, University of Texas at Austin, Austin, Texas, USA
cDepartment of Materials Science and Engineering, College of Engineering, Peking
University, Beijing 100871, China. E-mail: rzou@pku.edu.cn
dSingapore Peking University Research Centre for a Sustainable Low-Carbon Future, 1
Create Way, Singapore 138602, Singapore
eSchool of Materials Science and Engineering, Nanyang Technological University,
Singapore 639798, Singapore
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ta04230a
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that DMF could serve as a type of competitive coordination solution. We monitored the coordination process of Al3+ onto
ligand, which may also coordinate with metal ions. As a the BNNS surface tracing the zeta-potential of the latter.
consequence, the formation of MOFs occurs in a multi-phase Without the Al3+ graing, the zeta-potential of the original
manner in DMF solution instead of on the surface of the h-BN BNNS aqueous solution was determined to be +0.60 mV. Aer
(see Fig. S1 in the ESI†). In order to solve these problems, we the addition of Al3+, the zeta-potential increased to +16.52 mV.
employed an alternative solution, namely water, in the BNNS The increased zeta-potential value indicates that the positively-
exfoliation and MOF formation; given that water is a relatively charged Al3+ cation was successfully adsorbed onto the BNNS
weaker coordinative ligand to metal ions. Even though these surface. One equivalent BDC with regard to AlCl3$6H2O was
trials were also difficult to be realized on account of the fact that added to the reaction mixture containing the BNNS-Al3+ hybrid,
ꢁ
the BNNS exfoliation is oen assisted by the addition of followed by heating the solution at 150 C for 24 h. Few white
surfactants to enhance the Lewis acid-base interactions precipitates were observed and collected by centrifugation.
between water and BNNS layers,8 or accomplished through the Then, DMF was added into the obtained precipitates, and the
hydrolysis of multiple layered h-BN.9 Thus, the surfactants, suspension was reuxed at 150 ꢁC for 8 h, during which the
which are either acidic or basic, could coordinate with metal residual water and BDC molecules were removed from the
ions and disturb the formation of the MOF structures. On the cavities of the formed MOF. Two types of BNNS/MOF hybrids
other hand, the hydrolysis of multiple layered h-BN into BNNSs were obtained using different amounts of AlCl3$6H2O, namely
without the addition of surfactants could lead to the formation 24 mg and 48 mg, which are denoted as BNNSs/MOF-1 and
of ammonia, while the ammonia by-product could rapidly BNNSs/MOF-2, respectively. For comparison, two types of MIL-
deprotonate the carboxyl groups of the organic ligands used for 53 without BNNSs were also synthesized with different
the MOF synthesis, as a consequence of which, the MOF concentrations of the precursors, denoted as MIL-53-com and
formation occurs in an uncontrollable manner.10
MIL-53-bulk (see experimental section in the ESI†). Moreover, it
In order to address these problems, we employed a so-called should be noted that if excessive aluminum ions are used (e.g.
“double-solvent replacement method” to fabricate a BNNS/MOF 96 mg AlCl3$6H2O for 20 mL BNNS aqueous solution) they could
hybrid material. In this case, we employed DMF as the solvent result in aggregation or even precipitation of the BNNS/metal
for the BNNS exfoliation followed by the replacement of DMF ion hybrids.12 Thus, the amounts of Al3+ and BNNSs should be
with water, such that the MOF formation on the BNNSs could controlled at a relatively lower ratio.
take place in water. First, 1.0 g commercially available h-BN
Then, we examined the inuence of BNNSs on the crystalli-
powder was sonicated in 200 mL DMF for over 24 h, and the zation of MIL-53, as well as its morphology, given that BNNSs
dispersion was then centrifuged three times at 4000 rpm, for a could potentially act as a template during the formation of MIL-
period of 30 min each time, in order to remove the non-exfoli- 53. Field emission scanning electron microscopy (FE-SEM)
ated BN particles. Then, the centrifugation was performed at a shows that both BNNSs/MOF-1 and BNNSs/MOF-2 were
higher centrifuging speed (e.g., 10 000 rpm), during which the composed of separate ower-like hierarchical architectures
BNNSs were precipitated. Aer removing the DMF supernatant, (Fig. 1 and S3†). The magnied SEM image is shown in Fig. 1b,
the same volume of water was added into the centrifuge tube, which reveals that these hierarchical structures contain
forming an aqueous suspension of BNNSs. This aqueous
mixture was vigorously sonicated by a probe sonicator at 500 W
for 1 h, yielding a light blue homogeneous solution of BNNSs.
This aqueous BNNS solution was placed for over 24 h until some
precipitates appeared. Aer removing the precipitates, the
aqueous supernatant was directly used for the MOF growth
without further purication. Using inductively coupled plasma
mass spectrometry (ICP-MS) analysis, the concentration (0.037
mg Lꢀ1 with regard to boron) of BNNSs in aqueous solution was
determined. The absence of dissolved ammonia was conrmed
by the unchanged color of the BNNS aqueous suspension aer
adding a few drops of Nessler's reagent (a K2HgI4/KOH solution)
through a qualitative colorimetric test, indicating that the BNNS
formation in DMF and the re-dispersion in water are a physical
exfoliation process rather than a chemical hydrolysis.
The MOF MIL-53, composed of the corner-sharing MO4(OH)2
(M ¼ Cr3+, Al3+) octahedra and benzenedicarboxylate (BDC) as
the interconnection ligands, has been widely used for gas
adsorption and catalysis.11 Furthermore, it was demonstrated
that this MOF can be successfully synthesized in water with a
well-controlled size and morphology. We rst tested the possi-
Fig. 1 SEM and TEM images of BNNSs/MOFs and MIL-53: (a) SEM
bility to gra Al3+ onto the BNNS surface adding aluminum(III)
image of BNNSs/MOF-2; (b) magnified nanosheets of (a); (c) TEM
chloride hexahydrate (AlCl3$6H2O) into a 20 mL BNNS aqueous image of BNNSs/MOF-2; (d) SEM image of MIL-53-bulk.
18732 | J. Mater. Chem. A, 2014, 2, 18731–18735
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numerous nanoplates with a thickness from 20 to 30 nm.
Furthermore, the transmission electron microscopy (TEM)
image of BNNSs/MOF-2 reveals that the hybrid nanosheets
appeared as a coating of MOF on the BNNSs (Fig. 1c). It is
difficult to directly distinguish the BNNSs from MIL-53 under
TEM, as the mass thickness contrast of the B and N elements in
BNNSs is similar to that of the C atom in the MOF. As a control,
we also obtained the SEM images of both the pristine h-BN and
MIL-53 prepared without BNNSs as the template. These SEM
images show that h-BN, BNNSs/MOF-1, and BNNSs/MOF-2
shared a similar plate-like morphology (Fig. S2† and 1), while
MIL-53 exhibited a nanorod morphology with 200–600 nm in
width and several micrometers in length (Fig. S4† for MIL-53-
com). These observations further support our assumption that
the BNNS layers act as the template for the MOF formation. It
was reported that benzoic acid-functionalized graphene could
serve as a structure-directing agent in the MOF synthesis, and
the number of binding sites between graphene and metal ions
played a key role. The amount of carboxyl groups on modied
Fig. 2 Nitrogen adsorption/desorption isotherms for BN, MIL-53-
bulk, and BNNSs/MOFs at 77 K.
graphene can reach up to 17.8 mmol gꢀ1 5b BNNSs contain 40.3
.
the hierarchical porous structure in BNNSs/MOFs, i.e., micro-
pores, mesopores, and macropores coexist in the BNNSs/MOF
hybrids. The porosity of the hierarchical nanoowers could be
tuned by varying the contents of BN. As shown in Table S2,† the
volume of the mesopores and macropores in BNNSs/MOF-1 (0.8
wt% in boron) was 0.18 cm3 gꢀ1. When the boron content was
increased to 1.6 wt% in the case of BNNSs/MOF-2, the volume
increased to 0.25 cm3 gꢀ1. In contrast, the volume of the
micropores decreased upon increasing the boron content. For
example, the microporous volumes were 0.30 and 0.11 cm3 gꢀ1
for BNNSs/MOF-1 and BNNSs/MOF-2, respectively. These vari-
ations in the pore structures of the hybrids could be explained
as follows. The BN components are mesoporous and macro-
porous materials with low surface areas, while the MIL-53
components are microporous materials with high surface areas.
Upon increasing the contents of the BN components, the Bru-
nauer–Emmett–Teller (BET) surface area and microporous
volume decreased, while the volume of mesopores and macro-
pores increased. Thermogravimetric analysis (TGA) and the
FTIR spectra also demonstrate the integrated growth of MIL-53
onto the BNNSs and the successful formation of the hybrid
materials (Fig. S10 and S11†). BN exhibits a characteristic
Raman peak around 1366–1373 cmꢀ1 because of the E2g
phonon mode. In this case, BNNSs/MOF-2 shows a Raman peak
at 1368 cmꢀ1, which is about a 3 cmꢀ1 shi from the Raman
peak of bulk h-BN at 1365 cmꢀ1, indicating a possible reduction
in the thickness of the sheets (Fig. S12†).
mmol gꢀ1 nitrogen atoms, and the nitrogen atoms on the plane
act as the coordination sites. Thus, the interaction between the
BNNSs and MOF units was strong enough to enable the BNNSs
to be the structure-directing template. Thus, the BNNSs were
used to template and control the morphology of the MOF
structures in the BNNS/MOF hybrid synthesis. Recently, Rao
and co-workers reported a hybrid formation from ZIF-8 and few-
layered BN.13 Furthermore, an ordinary bulky MIL-53 powder
was also synthesized according to previous reports,11 and the
obtained material consisted of micrometer-sized crystals
(Fig. 1d for MIL-53-bulk as the contrastive material).
The phase structures of the BNNSs/MOF nanoowers and
related materials were also examined using powder X-ray
diffraction (XRD), where the MIL-53-bulk and BN powder were
used as the reference materials. Because MIL-53 is the major
content in the hybrid materials, the predominance of the main
diffraction peaks for MIL-53 was featured on the XRD pattern of
the hybrids (Fig. S5–S8†). As expected, the presence of the
BNNSs does not interrupt the coordination between the
aluminum ion and BDC to build the small units of MIL-53. The
XRD patterns combined with the zeta-potential results indicate
that the interaction between the BNNSs and aluminum ions is
strong enough to facilitate the dispersion of BNNSs in the
matrix of MIL-53.
Fig. 2 shows nitrogen adsorption/desorption isotherms of
the samples at 77 K. At a relatively low pressure (<0.4 atm), the
isotherms of both MIL-53-bulk and BNNSs/MOFs showed a
typical IUPAC type I behavior and no hysteresis was observed,
which are consistent with the previously reported physisorption
processes of microporous materials. This observation indicates
that BNNSs/MOFs possess microporous regions. At relatively
high pressures (>0.8 atm), the isotherms of both BNNSs/MOFs
and BN powders exhibited the presence of H3-like hysteresis
loops, indicating the coexistence of mesopores and macropores.
These large-sized pores may be resulted from slit-shaped
structures owing to the stacking of the nanoakes.14 The pore
size distribution curve (Fig. S9 in the ESI†) also indicates that
On the basis of the structural analysis, the growth mecha-
nism of BNNSs/MOF nanoowers was proposed, as shown in
Scheme 1. Aer the replacement of DMF with water, the
nitrogen atoms on BNNS layers are able to coordinate with Al3+
cations. These Al3+ cations undergo nucleation reactions,
forming aluminum oxide clusters on the BNNS layers. Then, the
MOF growth takes place on the surface of the obtained nano-
sheets graed with a number of aluminum oxide clusters,
leading to the formation of the nanoowers. This proposed
mechanism is similar to that for the formation of GO/MOF
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hybrids, which was previously reported to investigate the
structure-directing roles of graphene in the synthesis of MOF
nanowires.5
These ower-shaped hybrids contain a number of thin
nanosheets, as a consequence of which their active sites are
more accessible to the substrates in catalysis and absorption on
account of their relatively short diffusion pathway.15 In addi-
tion, the hierarchical porous structure of these materials is also
benecial in terms of the mass transport because of the fact that
the presence of macropores and mesopores can considerably
favor the diffusion of the guest molecules and make the active
sites more accessible.15 In order to support this assumption, we
employed a probe molecule to explore the relationship between
the morphology of the hybrids and their adsorption capability.
We used a relatively big-sized molecule, namely uorescein
isothiocyanate isomer I (FITC) as the probe molecule, in order
to rule out the impact of micro-porosity on the adsorption. The
adsorption experiments demonstrate that BNNSs/MOF-2 con-
taining nanoower structures could adsorb FITC faster than
that of bulky MIL-53 crystals under the same experimental
conditions. Within 16 min, BNNSs/MOF-2 can adsorb 34.8% of
FITC from solution, while MIL-53-bulk can only adsorb 21.8%
of the dye (Fig. S13 and S14†).
We also investigated the inuence of the morphology and
hierarchical structure of the BNNSs/MOF hybrids on their
ability as a catalyst. We select the acetalization of benzaldehyde
with methanol as the model reaction using the hybrid materials
as the catalyst at room temperature, given that MIL-53 was
reported to successfully catalyze this reaction.16 Under the same
experimental conditions, BNNSs/MOF-2 containing a nano-
ower-like structure led to 70.4% conversion of benzaldehyde
(Fig. 3). In comparison, the catalytic performance of MIL-53-
Fig. 3 Time-dependent conversion plots for the acetalization of
benzaldehyde with and without MIL-53-bulk or BNNSs/MOF-2.
Reaction conditions: benzaldehyde (100 mL), methanol (6 mL) and
catalyst (60 mg), room temperature.
bulk was signicantly lower. Because the reaction cannot occur
without the catalyst, these observations rmly support the
assumption that the nanosheet structure in the BNNSs/MOF
materials can signicantly enhance the catalytic capability by
providing more accessible catalytic centers.
In summary, a novel approach for the preparation of BNNSs/
MOF hybrid materials has been developed by double solvent
replacement method. To the best of our knowledge, this is the
rst example for the use of BNNSs as the template for the
formation of MOFs. The resulting hybrid materials combine the
advantageous features of both the BN nanosheets and the
porous MOFs, leading to nanoower structures, which provide
more accessible active sites to some reaction substrates or guest
molecules. As a consequence, the hybrid materials exhibited
enhanced catalytic activity. Thus, this synthetic approach of
double solvent replacement may open up an opportunity for
developing other hybrid materials using nanosheet materials
(such as MoS2 and WS2) as the platforms. The obtained hybrids
with a well-dened structure and morphology show a promising
application potential in the areas of heterogeneous catalysts,
drug/gene delivery, and biosensing.
Acknowledgements
This research is supported by the National Research Founda-
tion (NRF), Prime Minister's Office, Singapore under its NRF
Fellowship (NRF2009NRF-RF001-015) and Campus for Research
Excellence and Technological Enterprise (CREATE) Pro-
gramme–Singapore Peking University Research Centre for a
Sustainable Low-Carbon Future, and the NTU-A*Star Centre of
Scheme 1 Proposed mechanism for the construction of BNNSs/MOF
hybrids.
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