Microwave-Assisted, Ni-Induced Fabrication of Hollow ZIF-8 Nanoframes for the Knoevenagel Reaction

Article abstract of DOI:10.1021/acs.cgd.8b00045

Morphology-controlled synthesis of metal-organic frameworks (MOFs) is of tremendous importance to understand the morphology-structure-property relationships. Although great emphasis has been placed on the controllable synthesis of MOFs, the preparation of hollow and nanoframe-like MOFs, especially using the microwave irradiation method, remains a great challenge. In the present work, we report microwave-assisted synthesis of hollow butterfly and ring zeolitic imidazolate framework-8 (ZIF-8) with the stimuli of Ni ions. Interestingly, the sphere, butterfly, and fiber ZIF-8 catalysts were also fabricated when adjusting the weight ratio of Zn and Ni resources. Specifically, sphere, butterfly, hollow butterfly, and ring particles have maintained the ZIF-8 structure. On the basis of the time sequential experiments, we inferred that the formation of hollow butterfly and ring particles is driven by the change of surface energy. The Knoevenagel reaction was carried out to explore the catalytic activity of these ZIF materials. The ring particles with ZIF-8 structure showed the highest catalytic activity. The open structure, thin walls, and the amount of basicity have been extensively discussed to shed light on the superior catalytic performance of ring particles.

Full text of DOI:10.1021/acs.cgd.8b00045

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Microwave-assisted, Ni-induced Fabrication of
Hollow ZIF-8 Nanoframes for Knoevenagel Reaction
Pengfang Zhang, Yun Xiao, Hui Sun, Xiaoping Dai, Xin Zhang, Haixia Su, Yuchen
Qin, Daowei Gao, Axiang Jin, Hai Wang, Xiubing Wang, and Shi-Gang Sun
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00045 • Publication Date (Web): 20 Apr 2018
Downloaded from http://pubs.acs.org on April 20, 2018
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Page 1 of 17
Crystal Growth & Design
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Microwaveꢀassisted, Niꢀinduced Fabrication of Hollow
ZIFꢀ8 Nanoframes for Knoevenagel Reaction
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Pengfang Zhang,a† Yun Xiao,a† Hui Suna, Xiaoping Dai,a Xin Zhang,*a Haixia Su,a Yuchen Qin,a Daowei
Gao,a Axiang Jin,b Hai Wang,b Xiubing Wang,a and Shigang Sunc
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a State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of
Petroleum Beijing 102249 (China)
b National Institute of Metrology, Beijing 100013, China
c College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China.
†
These authors contributed equally to this work and should be considered coꢀfirst authors.
ABSTRACT: Morphologyꢀcontrolled synthesis of metal organic frameworks (MOFs) is of tremendous
importance to understand the morphologyꢀstructureꢀproperty relationships. Although great emphasis has
been placed on the controllable synthesis of MOFs, the preparation of hollow and nanoframeꢀlike
MOFs, especially using microwave irradiation method, remains a great challenge. In the present work,
we report microwaveꢀassisted synthesis of hollow butterfly and ring ZIFꢀ8 with the stimuli of Ni ions.
Interestingly, the sphere, butterfly and fiber ZIFꢀ8 catalysts were also fabricated when adjusting the
weight ratio of Zn and Ni resources. Specifically, sphere, butterfly, hollow butterfly and ring particles
have maintained Zeolitic imidazolate framework ꢀ 8 (ZIFꢀ8) structure. Based on the time sequential
experiments we inferred that the formation of hollow butterfly and ring particles is driven by the change
of surface energy. Knoevenagel reaction was carried out to explore the catalytic activity of these ZIF
materials. The ring particles with ZIFꢀ8 structure showed the highest catalytic activity. The open
structure, thin walls and the amount of basicity have been extensively discussed to shed light on the
superior catalytic performance of ring particles.
characterized
crystalline
architectures,
controllable pore size, and suitable chemical
functionalities, the MOFs have been widely
applied in gas separation,6,7 sensors,8 catalysis,9
drug delivery,10 and energy storage.11 In recent
years, the synthesis of nanoframeꢀlike MOFs
attracted extensive interest because of unique
INTRODUCTION
MOFs, assembled by metal ions and organic
ligands through coordination bonds, are a class of
highly crystalline hybrid porous materials.1−5
Owing to their superior properties such as
exceptionally high specific surface area, wellꢀ
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Crystal Growth & Design
Page 2 of 17
structures, intriguing properties that differ from
their solid counterparts. Surfactants,12 metal
salts,13 lingands,14 solvents15,16 and even the
synthesized methods, such as solvothermal
microwave irradiation method assisted by two
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kinds of metal ions to synthesize different
morphologies of MOFs materials including
hollow and nanoframeꢀlike structures highly
efficient for Knoevenagel reaction. In the present
work, we report microwave irradiation method to
produce a series of wellꢀdefined morphologies
including sphere, butterfly, hollow butterfly, ring
and fiber using 2ꢀmethylimidazole (2ꢀMeIM) as
organic ligands, zinc and nickel ions as mixed
metallic nodes, poly (vinylpyrrolidone) (PVP) as
dispersive and protecting reagent and N,Nꢀ
method,17−19
microwave
irradiation,20
microemulsion21 and electrochemical deposition
methods22, have been tried to control the size and
morphology of hollow MOF materials. It has
been well established that mixed solvents, postꢀ
synthesis modification, sacrificing templates are
the most used methods to prepare hollow MOF
materials.23−26 For example, Wang and coꢀ
workers synthesized hollow MOFꢀ5 and MOFꢀ2
by adding two kinds of solvents including N,Nꢀ
dimethylformamide and ethanol.23,24 Lou et al
prepared nickel sulfide nanoframes using
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dimethylformamide
(DMF)
as
solvent.
Interestingly, the morphological evolution
process can be facilely controlled by just
changing weight ratio of Zn and Ni salts. The
structure of ZIFꢀ8 could be retained when zinc
ions are used as metallic nodes. However, the asꢀ
prepared fibers exhibits nonꢀMOF structure when
nickel ion was used as single metal resource.
The Knoevenagel condensation is an excellent
tool for the formation of C–C structure unit from
structureꢀinduced
anisotropic
chemical
etching/anion exchange methods.25 Li and coꢀ
workers produced hollow ZIFꢀ8 nanospheres
using silica as the sacrificing template.26 While
the reaction time of these methods was at least
six hours or even more.
The microwave irradiation method is on behalf
of a very energy efficient method of heating with
rapid heating rate, shorter reaction time and
higher selectivity and yield in contrast with
conventional solvothermal methods.27 The
synthesis of MOFs assisted by microwave
radiation technology has been extensively used
and emphasized on four aspects: (I) the
acceleration of crystallization, (II) the preparation
of nanoꢀranged particles, (III) the improvement
of product purity and (IV) the selective synthesis
of polymorphs.28 ZIF is a subclass of MOFs. ZIFꢀ
8 is one of the most representative ZIF materials
due to its robust chemical and thermal
stabilities.29,30 Although many efforts have been
made on the morphologyꢀcontrolled synthesis of
MOFs31ꢀ35, to our knowledge, it is barely reported
an active methylene compound and
a
carbonyl.41,42 This reaction could be used to the
synthesis of some drug intermediates such as
anticancer, antimicrobial and antituberculosis
agents. The catalyst containing basic group is the
most used catalyat for this reaction. To explore
the catalytic applications of asꢀsynthesized ZIFꢀ8
nanostructures, Knoevenagel reaction was carried
out. The ring particles exhibit the highest
catalytic activity among these five samples. The
open structure, thin walls and the amount of
basicity play crucial effects on the different
catalytic activities.
EXPERIMENTAL SECTION
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Page 3 of 17
Crystal Growth & Design
Materials and Reagents. Zinc nitrate
detector (SS420x), ultraviolet spectrophotometer,
and C18 chromatographic column (silica, 250
mm X 4.6 mm). The methanol (MeOH) and
acetonitrile were chosen as mixed solvents with
volume ratio of 6:4. The flow rate of mixed
solvent is 1 mL·min−1. The differential refractive
index detector was used to detect the different
materials. The retention time of malononitrile,
benzaldehyde, final product, and methylbenzene
are 3.2, 6.3, 8.9, 19.2 min, respectively. The
extent of conversion was calculated based on the
ratio of benzaldehyde and final product.
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hexahydrate (Zn(NO3)2·6H2O, 98%) was
purchased from Acros. Malononitrile was
purchased from aladdin. All other chemicals such
as Nickel (II) acetylacetonate (Ni(acac)2) 2ꢀ
MeIM (99%) PVP (M.W. 30000), benzaldehyde
toluene and ethanol were purchased from
Sinopharm Chemical Reagent Co. Ltd. Solvents
and all other chemicals were obtained from
commercial sources and used without further
purification.
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Synthesis of different morphologies of
particles from ZIF-8 sphere to non-MOF fiber.
A precursor solution was prepared by mixing
Zn(NO3)2·6H2O (300, 200, 175, 125 or 0 mg),
Ni(acac)2 (0, 100, 125, 175 or 300 mg), 2ꢀMeIM
(41 mg, 0.50 mmol) and 1092 mg PVP in 8 mL
of DMF. The mixed solution was stirred for 20
minutes and then transited to 100 mL Teflonꢀ
lined autoclave and microwave irradiated
(purchased from Michem Beijing Co. Ltd) for 5
min before it was cooled to room temperature
naturally. The products were separated via
centrifugation at 7000 rpm for 10 minutes and
further purified by ethanol three times. The yield
of the five products (Zn(NO3)2·6H2O=300, 200,
175, 125 or 0 mg) are 60%, 63.6%, 65.2%,65%
and 54% respectively.
Characterization. The size and morphology
of the particles were analyzed by JEM 2100
transmission electron microscope (TEM) at 200
kV. The samples were conducted by dropping
ethanol dispersion onto carbonꢀcoated copper
grids with a pipettor, and the solvent was allowed
to evaporate for TEM detection. The scanning
electron microscope (SEM) images were
determined by ULTRA 55 scanning electron
microscope at 200 K. The Xꢀray diffraction
(XRD) patterns of samples were recorded on a
Bruker D8ꢀadvance Xꢀray diffractometer
operated at a voltage of 40 kV and current of 40
mA with CuK radiation (λ = 1.5406 Å). The
elemental analysis was done using Rigaku
Nanohunter totalꢀreflection Xꢀray fluorescence
spectroscopy (TXRF) (Excitation line: MoꢀKa 50
kV, 0.60 mA; Peak: net; Angle (deg): 0.050; Time
(sec): 300; Dead rate (%): 0.2). A certain amount
of samples was first dissolved in aqua regia and
then was dropwise added on the silica wafer to
dry for TXRF detection. Thermogravimetric
analyses (TGA) were measured on a Mettler
Toledo TGA/SDTA 851 instrument in flowing air
(50 mL min−1) with a rate of 10 K min−1. The
Fourierꢀtransformed infrared resonance (FTIR)
The test of catalytic activity of Knoevenagel
reaction. To a 30 mL glassy bottle was
sequentially added benzaldehyde (63 mg, 0.6
mmol), malononitrile (198 mg, 3 mmol), toluene
(5 mL) and the asꢀprepared catalysts (10 mg).
The solution was stirred at room temperature (25
oC) for 2 hours. The reaction mixture was
injected
to
high
performance
liquid
chromatography.
High performance liquid chromatography is
equipped with a differential refractive index
spectra were obtained on
a
TENSORII
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