Controlled synthesis of Fe3O4/ZIF-8 nanoparticles for magnetically separable nanocatalysts

Article abstract of DOI:10.1002/chem.201405921

Fe₃O₄/ZIF-8 nanoparticles were synthesized through a room-temperature reaction between 2-methylimidazolate and zinc nitrate in the presence of Fe₃O₄ nanocrystals. The particle size, surface charge, and magnetic

Full text of DOI:10.1002/chem.201405921

DOI: 10.1002/chem.201405921
Full Paper
&
Heterogeneous Catalysis
Controlled Synthesis of Fe3O4/ZIF-8 Nanoparticles for Magnetically
Separable Nanocatalysts
Fei Pang, Mingyuan He, and Jianping Ge*[a]
Abstract: Fe3O4/ZIF-8 nanoparticles were synthesized
through a room-temperature reaction between 2-methylimi-
dazolate and zinc nitrate in the presence of Fe3O4 nanocrys-
tals. The particle size, surface charge, and magnetic loading
can be conveniently controlled by the dosage of Zn(NO3)2
and Fe3O4 nanocrystals. The as-prepared particles show both
good thermal stability (stable to 5508C) and large surface
area (1174 m2gÀ1). The nanoparticles also have a superpara-
magnetic response, so that they can strongly respond to an
external field during magnetic separation and disperse back
into the solution after withdrawal of the magnetic field. For
the Knoevenagel reaction, which is catalyzed by alkaline
active sites on external surface of catalyst, small Fe3O4/ZIF-8
nanoparticles show a higher catalytic activity. At the same
time, the nanocatalysts can be continuously used in multiple
catalytic reactions through magnetic separation, activation,
and redispersion with little loss of activity.
Introduction
systematic studies of their catalytic performance are seldom re-
ported in the literature.
In recent years, metal–organic frameworks (MOFs) have attract-
ed great attention in various research fields due to their poten-
tial applications in gas storage and separation,[1] optical devi-
ces,[2] sensors,[3] drug delivery,[4] and heterogeneous catalysis.[5]
Among all the MOF structures, zeolite imidazolate frameworks
(ZIFs) possess merits of conventional MOFs and zeolites be-
cause of their large specific surface area, high thermal stability,
and intersecting three-dimensional structure of the aluminosili-
cate zeolite.[6] Meanwhile, the ZIF compound is a bifunctional
substance with both acidic and basic active site on the surface,
which are induced by cations (Zn2+, Cu2+, Co2+, etc.) and imi-
dazole groups, respectively.[7] Therefore, the ZIF compounds
could be promising catalysts for many heterogeneous reac-
tions. Several successful demonstrations of using ZIFs as cata-
lysts include Friedel–Crafts alkylation,[8] Knoevenagel condensa-
tion,[9] cycloaddition reactions, the synthesis of styrene carbon-
ate from CO2 and styrene oxide,[7a,10] and so forth.[11] Some of
them may take advantage of the abundant surface area within
the nanopores, whereas for reactions such as Knoevenagel
condensation the active sites are generally located at the ex-
ternal surface of the ZIF particle,[8,12] so that it is necessary to
reduce the catalyst particle size as much as possible to effec-
tively increase the number of active sites on the surface. How-
ever, the syntheses of nanoscale, crystalline ZIF structures and
The nanocatalysts can be regarded as a quasi-homogeneous
catalyst, which combines the advantages of traditional homo-
and heterogeneous catalysts.[13] Similar to the homogenous
catalyst, the nanocatalyst is usually well dispersed in the reac-
tion system and fully accessible to the reactants, so that it
shows high reactivity in catalysis. It can also be separated from
the reaction for recycling usage similar to a heterogeneous cat-
alyst. Currently, a remaining problem for the generation of
promising nanocatalysts is that good dispersion and fast sepa-
ration are hard to achieve at the same time. It is difficult to
separate the nanoparticles from the reaction solution for redis-
persion in the next round of the reaction by using traditional
methods such as sedimentation and filtration.
A possible solution to this problem is the introduction of
magnetic materials to produce magnetically separable nano-
particles (MNPs).[14] Typically, the magnetic content should be
firmly anchored to the surface of the composite particle or em-
bedded inside that particle. At the same time, the composite
particle should have superparamagnetic characteristics, so that
the particles can not only strongly respond to the external
field during magnetic separation, but also disperse back into
the solution after withdrawal of the magnetic field.[15] Reported
magnetic nanocomposites include bimetallic MNPs,[16] core–
shell MNPs,[17] MNPs decorated on silica/polymer beads,[18]
MNPs in encapsulated mesoporous solids,[19] multinuclei core–
shell MNPs,[20] and MNP–silica nanocomposites.[14a] For instance,
bimetallic core–shell CoPt nanoparticles were synthesized by
reacting Co nanoparticles with a solution of [Pt(hfac)2] (hfac=
hexafluroacetylacetonate) in nonane and the formation of
CoPt alloy was explained by a redox transmetalation reaction
[a] F. Pang, Prof. M. Y. He, Prof. J. P. Ge
Shanghai Key Laboratory of Green Chemistry and Chemical Processes
School of Chemistry and Molecular Engineering
East China Normal University
Shanghai, 200062 (P.R. China)
E-mail: jpge@chem.ecnu.edu.cn
between Co0 and Pt2+ [16]
Lu and co-workers introduced ferro-
.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405921.
magnetic cobalt nanoparticles into ordered mesoporous car-
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bons to obtain a surface-grafted magnetic catalyst for octane
hydrogenation.[21] Tsang et al. utilized sequential spraying,
chemical precipitation, and controlled pyrolysis to prepare
carbon-encapsulated, iron-based magnetic nanoparticles (FeÀ
M; M=Ni, Ca, Mn, Zn, Cu, Co) in large quantities and these
particles were used as carriers for catalytically active species
such as platinum nanoparticles.[22] Kotani et al. synthesized the
magnetically separable heterogeneous catalyst Ru(OH)x/Fe3O4
for liquid-phase oxidation and reduction; most of the catalyst
could be recycled for the next round of the reaction.[23] Overall,
the involved magnetic content can be metals (Fe,[24] Co,[21]
Ni[25]), alloys (FeNi, FeCu, FeCo,[22] FePt[24]), metal oxides (FeO,
Fe2O3,[26] Fe3O4[27]), or ferrites (NiFe2O4,[28] CoFe2O4,[29]
MnFe2O4,[29b] ZnFe2O4[30]). Among them, magnetic iron oxides
have received most attention because of their chemical stabili-
ty in air and ease of synthesis.[13b]
Figure 1. Schematic illustration for the formation mechanism of Fe3O4/ZIF-8
particles. PAA=polyacrylic acid.
a negative surface charge (Figure 2b and Figure S1 in the Sup-
porting Information) are prepared in advance through a high-
temperature polyol reaction by using PAA as a surfactant.[15]
Because the Fe3O4 nanocrystals are grafted with highly charged
carboxylate groups, they are extremely stable against aggrega-
tion in an aqueous solution with a pH value higher than five,
and thereby, are also stable in an aqueous solution of MeIM.
When Zn(NO3)2 was injected into the solution of MeIM and
Fe3O4 nanocrystals, a composite nanostructure with magnetic
nanocrystals anchored on the surface of ZIF-8 or embedded
within the ZIF-8 nanocrystal could be produced (Figure 2c and
d).
Herein, the Fe3O4/ZIF-8 nanoparticles were synthesized
through the room-temperature reaction between 2-methylimi-
dazolate (MeIM) and zinc nitrate in the presence of Fe3O4
nanocrystals. Due to electrostatic attraction between negative-
ly charged Fe3O4 nanocrystals and positively charged ZIF-8 spe-
cies, Fe3O4/ZIF-8 composites were produced through the en-
capsulation of magnetic nanocrystals inside the ZIF-8 nanopar-
ticles. The particle size, surface charge, and magnetic loading
of Fe3O4/ZIF-8 nanoparticles were tuned by variations in the
synthetic parameters, and their thermal stability, specific sur-
face area, and magnetic properties were investigated. The
Knoevenagel reaction between benzaldehyde and cyanoaceta-
mide in methanol was used as a model reaction to evaluate
the catalytic performance and magnetic separation efficiency
of the as-prepared Fe3O4/ZIF-8 catalysts. It is believed that the
magnetic ZIF nanostructures will find potential applications in
practical catalytic processes and be convenient for recycling of
the nanocatalysts.
Results and Discussion
Synthesis and formation mechanism of Fe3O4/ZIF-8 nanopar-
ticles
The Fe3O4/ZIF-8 composite nanoparticles were synthesized
through a room-temperature, aqueous reaction between MeIM
and zinc nitrate in the presence of premade negatively
charged Fe3O4 nanocrystals (Figure 1). The synthesis was devel-
oped from a previously reported preparation of pure ZIF-8
nanoparticles, which utilized the injection of small amount of
Zn2+ into a concentrated solution of MeIM to initiate fast nu-
cleation and limited growth of ZIF nanocrystals.[31] In ZIF-8 crys-
tals, the Zn atoms are connected through the N atoms from
MeIM to form a MOF structure, which possesses nanosized
pores formed by four-, six-, eight-, and twelve-membered rings
of ZnN4 tetrahedral clusters.[6b]
Figure 2. TEM images of: a) ZIF-8, b) Fe3O4, and c,d) Fe3O4/ZIF-8 nanoparti-
cles.
The XRD patterns of Fe3O4/ZIF-8 nanoparticles were almost
same as those of pure ZIF-8 nanoparticles, except a peak was
observed at around 368, which indicated the inclusion of Fe3O4
content inside the hetero-nanostructures (Figure 3). Generally,
the XRD intensity of a MOF material is stronger than that of
nanocrystals by several orders of magnitude; thus the diffrac-
tion peaks attributed to inorganic nanocrystals can barely be
observed for hetero-nanostructures. It should be mentioned
that, although excess MeIM was used in the synthesis, after
separation of Fe3O4/ZIF-8 nanoparticles the reaction solution
can be used for the next round of the reaction by supplement-
ing with consumed MeIM, which makes the synthesis a green
and economic process.
As shown in Figure 2a, pure ZIF-8 nanoparticles with an
average diameter from 50 to 100 nm can be prepared by
using this method, and the XRD patterns of these particles are
consistent with reports in the literature.[31] To introduce mag-
netic content into the MOF structures, Fe3O4 nanocrystals with
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Figure 3. XRD patterns of ZIF-8, Fe3O4/ZIF-8, and Fe3O4 nanoparticles.
In synthesis, the particle size of Fe3O4/ZIF-8 composite nano-
structures can be finely tuned through continuous addition of
zinc ions into the reaction solution. It was not a surprise to
find that the average particle size increased from 58.3, 82.4, to
106 nm when 0.7, 1.4, and 2.1 mmol, respectively, of zinc ni-
trate was injected into the solution (Figure 4); this can be re-
garded as a typical seeding growth process. A common char-
acteristic for these Fe3O4/ZIF-8 particles is that all of the Fe3O4
nanocrystals have been combined with the ZIF-8 particles and
almost no isolated magnetic particles were observed. However,
the location of the magnetic content was different for the
three samples. For the smallest particle with a diameter of
58.3 nm, the Fe3O4 nanocrystals were attached to the surface
of ZIF-8 particles. Some small pits on the particle surface may
be caused by the detachment of Fe3O4 nanocrystals during
synthesis or post-treatment, which suggests that the combina-
tion may not be firm enough in this stage. The exposed mag-
netic nanocrystals can still be observed when the overall parti-
cle size is increased to 82.4 nm. As the particle size further in-
creases to 106 nm, almost all of the magnetic nanocrystals
have been embedded inside the surface layer of the particle,
producing stable and firmly encapsulated hetero-nanostruc-
tures. Apparently, there is an affinity between the ZIF-8 parti-
cles and Fe3O4 nanocrystals; however, no Fe3O4@ZIF-8 core–
shell structures were produced, which suggested that ZIF-8
particles originated from homogeneous nucleation at the be-
ginning of the reaction and the heterostructures were formed
when the ZIF-8 particles grew to a critical value. In addition to
tuning of the amount of Zn(NO3)2 in a single synthesis, the
composite particle size can be further increased by multiple
seeding growth. The molar amount of Zn(NO3)2 added in each
round equals the molar amount of zinc atoms in Fe3O4@ZIF-8
seeds.
Figure 4. TEM images and size distributions of Fe3O4/ZIF-8 nanoparticles
with average diameters of 58.3 (a–c), 82.4 (d–f), and 106 nm (g–i), when 0.7,
1.4, and 2.1 mmol Zn(NO3)2, respectively (samples 1, 2, and 3), were used in
the reaction. Scale bars: 200 (a, d, g) and 50 nm (b, e, h).
Figure 5. SEM images of Fe3O4/ZIF-8 particles with average diameters of
0.18, 0.38, 0.92, and 1.49 mm prepared by: a) 1, b) 2, c) 3, and d) 4 cycles, re-
spectively, of seeding growth with continuous addition of Zn(NO3)2 (sam-
ples 4, 5, 6, and 7).
nanocrystals are negatively charged because of surface carbox-
ylate groups, and the pure ZIF-8 nanoparticles are positively
charged due to imidazole acting as self-stabilizing ligands.
Taking the samples in Figure 5 as examples, one can find that
the zeta potential changes from negative to positive as the
particles grow larger (Figure 6a). For nanoparticles with aver-
age sizes of around 100 nm, the surface charge was largely
contributed to by the Fe3O4 nanocrystals in the surface layer,
and the whole heterostructure was negatively charged. As
more and more ZIF-8 grows into the Fe3O4/ZIF-8 particles in
subsequent cycles, Fe3O4 nanocrystals are deeply embedded
inside the composite, so that the whole particle behaves like
pure ZIF-8 with a positive surface charge. We found a similar
After separation by centrifugation, half of the as-made
Fe3O4/ZIF-8 particles could be redispersed in a solution of
MeIM for the next round of seeding growth (Figure 5). The
Fe3O4/ZIF-8 particles will gradually grow to submicron or 1–
2 mm sizes after 3 or 4 cycles; this proves that the current syn-
thesis is an effective method to produce magnetic ZIF-8 with
controllable sizes from the nano- to submicron range.
Upon changing the distribution of magnetic nanocrystals
within the Fe3O4/ZIF-8 particles, the surface charge of the
entire particle changed accordingly. It is known that the Fe3O4
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reversal of surface charge for samples with different magnetic
loadings (Figure 6b and Figure S2 in the Supporting Informa-
tion). Here, the production of ZIF-8 was fixed by adding the
same amount of Zn(NO3)2 (1.4 mmol) and the addition of Fe3O4
nanocrystals was changed from 0.017 to 0.667 mmol (Table 1).
According to the magnetic distribution in hetero-nanostruc-
tures and the surface charge of the particles, the growth of
Fe3O4/ZIF-8 composite particles can be attributed to a nano-
crystal encapsulation process driven by electrostatic attraction
between colloidal particles. At the beginning of the reaction,
small ZIF-8 crystal seeds quickly formed through homogene-
ous nucleation after injection of an aqueous solution of
Zn(NO3)2, even in the presence of Fe3O4 nanocrystals. As the
ZIF-8 nanocrystal grew larger, its positive surface charge was
gradually enhanced, so that the negatively charged Fe3O4
nanocrystals would be attracted to the surface of ZIF-8 due to
electrostatic interactions. The attachment of Fe3O4 nanocrystals
to the ZIF-8 particle and growth of the ZIF-8 particle may pro-
ceed simultaneously for some time, but the Fe3O4 nanocrystals
will eventually be encapsulated inside the ZIF-8 matrix when
excess Zn(NO3)2 is added and all magnetic nanocrystals are
consumed.
Table 1. Synthetic parameters for the preparation of Fe3O4/ZIF-8 nano-
particles.
Sample
n[Zn(NO3)2]
n[Fe3O4]
n[Zn(NO3)2]/n[Fe3O4]
[mmol]
[mmol]
1
2
3
4
5
6
7
8
0.7
1.4
2.1
2.8
5.6
11.2
22.4
1.4
1.4
1.4
0.167
0.167
0.167
0.167
0.167
0.167
0.167
0.017
0.067
0.167
0.667
4.19
8.38
12.6
16.7
33.5
67.1
134.1
82.4
20.9
8.38
2.01
The aforementioned reaction mechanism explained some
experimental phenomenon observed in our synthesis. For ex-
ample, no composite particles were obtained if the negatively
charged Fe3O4 nanocrystals were replaced by positively
charged ones (polyethylenimine (PEI)-grafted Fe3O4 nanocrys-
tals) because they repelled each other in solution. This proved
that electrostatic attraction rather than coordination between
imidazolate and Fe3O4 was the driving force to form composite
structures. Second, no Fe3O4@ZIF-8 core–shell structures were
produced during growth because attraction between the
Fe3O4 nanocrystals and small ZIF-8 seeds was weak and homo-
geneous growth of ZIF-8 was dominant at the beginning of
the reaction. Third, agglomeration of Fe3O4/ZIF-8 nanoparticles
was observed (Figure 5) when their sizes increased to several
hundred nanometers. Aggregation takes place because the
surface charge can be zero as it changes from negative to pos-
itive in the third cycle of addition of Zn2+, and the particle
cannot be stabilized in solution according to colloidal stability
theory.[32] However, the growth of ZIF-8 continues regardless of
whether ZIF-8 particles are well dispersed or not, so that the
aggregates are encapsulated inside to form submicron parti-
cles eventually (Figures 5 and 6a).
9
10
11
1.4
Thermostability, porosity, and magnetization of Fe3O4/ZIF-8
particles
ZIF-8 was perceived as a promising catalyst because it not only
has abundant ordered micropores similar to zeolite, but also
possesses better thermal stability than most of the other MOF
structures. The microporous structure of ZIF-8 can be main-
tained below 5008C in N2.[6b] Thermogravimetric analysis (TGA)
was performed on as-made Fe3O4/ZIF-8 (Figure 7). The TGA
curve shows a gradual weight loss of 3.4% when the tempera-
ture rises from 25 to 5008C, which corresponds to the loss of
guest species such as water and other gases adsorbed inside
the micropores. A larger weight loss is detected when the tem-
perature is raised above 5008C, which could be caused by col-
lapse of the MOF structures. Through a comparison of the TGA
curves of Fe3O4/ZIF-8 and ZIF-8, one can find that the thermal
stability is determined by the ZIF content, and the introduction
Figure 6. Zeta potentials of various Fe3O4/ZIF-8 particles prepared by: a) con-
tinuous seeding growth (samples 4, 5, 6, and 7), and b) with different mag-
netic loadings (samples 8, 9, 10, and 11).
The particle surface charge became negative upon increas-
ing the Fe3O4/ZIF-8 ratio because the magnetic nanocrystals
would inevitably attach to the surface layer of the composite
particles in this case (Figure S1 in the Supporting Information).
These two groups of parallel experiments showed that the
charge of the particle was determined by its surface constitu-
tion. In other words, one can estimate whether the magnetic
particles are embedded in the surface layer or deeply inside
the particle through zeta potential measurements.
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ior for micropores. The Langmuir and BET surface areas of ZIF-
8 are calculated to be 1932 and 1363 m2 gÀ1, respectively, ac-
cording to the adsorption branch with relative pressure (P/P0)
ranging from 0.05 to 0.28. As expected, the Langmuir and BET
surface areas for Fe3O4/ZIF-8 (1703 and 1174 m2gÀ1, respective-
ly) are smaller than those of pure ZIF-8 because the introduc-
tion of Fe3O4 nanocrystals makes no contribution to the micro-
porous structures and increasing surface area. On the other
hand, the retention of a large surface area suggests that the
addition of magnetic content does not interfere with the for-
mation of ordered micropores. A hysteresis loop appearing in
the sorption isotherm of Fe3O4/ZIF-8 with P/P0 from 0.4 to 0.75
shows the formation of large pores, which could be induced
by the uncompacted combination between Fe3O4 nanocrystals
and ZIF-8 during particle growth. Briefly, the introduction of
magnetic nanocrystals in the growth of ZIF-8 will generate
a highly porous, magnetically separable structure, which is
useful in recycling heterogeneous catalysis.
Figure 7. TGA results for Fe3O4/ZIF-8 and ZIF-8 particles.
of inorganic content has little contribution to the enhance-
ment of thermal stability. The long plateau in the temperature
range of 25–5008C proved the high thermal stability of the
Fe3O4/ZIF-8 composite particles; this makes them an excellent
heterogeneous catalyst for reactions at relatively high temper-
atures.
In addition to the porous characteristics inherited from ZIF-
8, the Fe3O4/ZIF-8 composite also possesses a superparamag-
netic response due to the encapsulation of Fe3O4 nanocrystals,
which is useful for fast separation of the catalyst in liquid-
phase reactions. Figure 9a shows the magnetic hysteresis
loops of Fe3O4/ZIF-8 particles at room temperature. The ab-
sence of remanence and coercivity indicates that the Fe3O4/
ZIF-8 particles have a typical superparamagnetic response. This
means that the particles can be readily separated from a liquid
suspension by using a magnetic field and they can be redis-
persed into the suspension without any agglomeration after
Fe3O4/ZIF-8 nanoparticles have rich, ordered micropores and
a large specific surface area. For ZIF-8 framework topologies,
the imidazolate unit bridges the zinc tetrahedra to make a Zn-
IM-Zn angle of around 1458, which is close to the Si-O-Si angle
in many zeolites. The expanded sod framework exhibits an in-
teresting structure that a large sod cage (11.6 ꢁ) is accessible
through a narrow six-ring pore (3.4 ꢁ).[6b] The porous structure
of ZIF-8 and Fe3O4/ZIF-8 was analyzed by a nitrogen adsorp-
tion–desorption isotherm swept at 77 K (Figure 8), which re-
vealed their microporous characteristics because both of the
isotherms showed typical type I adsorption–desorption behav-
Figure 9. a) Magnetic hysteresis loops, and b) separation efficiency of Fe3O4/
ZIF-8 nanoparticles with different Fe3O4 loadings. c) Separation of Fe3O4/ZIF-
8 nanoparticles by using a magnet.
Figure 8. Nitrogen adsorption–desorption isotherms of the as-synthesized:
a) ZIF-8 nanocrystals, and b) Fe3O4/ZIF-8 nanoparticles at 77 K.
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removal of that magnetic field. The magnetic loading of Fe3O4/
ZIF-8 particles can be controlled by the addition of Fe3O4 nano-
crystals in the synthesis. As the amount of Fe3O4 nanocrystals
increased from 0.017, 0.067, 0.167, to 0.667 mol, its saturated
magnetization was enhanced from 0.56, 1.98, 4.29, to
4.35 emugÀ1, accordingly. It should be noted that the magnet-
ic loading and response become saturated as the addition of
Fe3O4 nanocrystals is increased from 0.167 to 0.667 mol, proba-
bly because the positive ZIF-8 particles can barely absorb more
negative Fe3O4 nanocrystals during particle growth.
the Knoevenagel reaction between benzaldehyde and cyanoa-
cetamide was selected to evaluate the activity of Fe3O4/ZIF-8
catalysts and demonstrate the importance of catalyst synthesis
for high efficiency and reusable catalytic processes.
Some reaction conditions, including solvent, catalyst pre-
treatment, dosage, and size, were first optimized for a higher
conversion rate of substrate molecules. Typically, 2 mol% of
Fe3O4/ZIF-8 catalysts was used to catalyze the condensation
between benzaldehyde and cyanoacetamide in methanol at
room temperature (Figure 10). Aliquots were withdrawn from
the reaction mixture at different time intervals and the princi-
pal product, benzylidene cyanoacetamide, was analyzed by
GC; this gave kinetic data throughout the whole reaction. The
catalyst is highly active in protic solvent, such as methanol,
probably because the solvent facilitates the formation of a nu-
cleophilic carbanion and promotes condensation.[33]
To quantify the magnetic separation efficiency of Fe3O4/ZIF-8
particles, an in situ transmittance measurement system is es-
tablished for real-time evaluations. Typically, 2 mL of Fe3O4/ZIF-
8 solution (c=1.8 mgmLÀ1) was placed in a 1ꢂ1 cm optical
cuvette, and its transmission spectrum was measured by
means of a UV/Vis spectrometer. When the solution is diluted
from 1.0c, 0.8c, 0.6c, 0.4c, 0.2c, to 0c, it can be regarded as the
original solution with 0, 20, 40, 60, 80, and 100%, respectively,
of the particle separation. Therefore, a working curve can be
plotted by recording the transmittance of these diluted solu-
tions. In the formal test, a permanent magnet was placed
beside the cuvette, and the cuvette center could sense a mag-
netic field of 600 gauss. In the magnetic field, the Fe3O4/ZIF-8
particles gradually moved to the cuvette wall along the mag-
netic field gradient, and the transmittance of the solution, as
well as the magnetic separation efficiency, increased accord-
ingly. Herein, we recorded the separation time as every 20% of
Fe3O4/ZIF-8 particles separated from the solution, according to
the transmittance working curve. The experimental results are
consistent with magnetization hysteresis loops, and higher
magnetic loading leads to better separation efficiency. It takes
2.33, 3.75, 14.5, and 53.2 min for samples 7–10 to reach 80%
separation (Figure 9b). Magnetic separation under specific con-
ditions is seldom reported in the literature. The current results
are repeatable in different laboratories and comparable with
other magnetic particles. The short separation time indicates
that magnetic separation of the catalyst is feasible in practical
operations, even in a relatively weak magnetic field.
Figure 10. Knoevenagel reaction between benzaldehyde and cyanoaceta-
mide catalyzed by Fe3O4/ZIF-8 catalysts.
Various solvents, such as DMF, DMSO, acetonitrile, and meth-
anol, were investigated as potential solvents for the Knoevena-
gel reaction (Figure 11a), and methanol was found to be the
most effective solvent for condensation, which suggested that
the polarity had little influence upon the activity. The solvent
experiment was also consistent with previously reported Knoe-
venagel reactions, in which ethanol and methanol were select-
ed as most suitable solvents.[5c,34] Furthermore, a baking pre-
treatment of Fe3O4/ZIF-8 catalysts was essential for high cata-
lytic activity (Figure 11b). Thanks to the high thermal stability,
the catalysts could be baked at 3008C for 2 h under the pro-
tection of nitrogen, so that magnetite would not be oxidized
to hematite during calcination. It was observed that 72.3% of
benzaldehyde converted after 6 h with the baked catalyst,
whereas only 63.2% was converted with the unbaked catalyst.
The catalytic activity improved because the surface active sites
passivated by surfactant molecules were reactivated by anneal-
ing in nitrogen, and the crystallinity of ZIF-8 might also have
been improved after calcination.[35]
Nanocatalysts for Knoevenagel reaction
The Knoevenagel reaction is a nucleophilic aldol condensation
between aromatic aldehydes, aromatic ketones, or benzal bro-
mide and malonic ester, malonic acid, or cyanoacetic ester con-
taining an activated methylene group, and it is usually cata-
lyzed by a weak basic amine. Chizallet and co-workers investi-
gated the position of acidic/basic sites in ZIF-8 based on CO
adsorption monitored by FTIR spectroscopy and DFT calcula-
tions.[7b] Their study suggested that there were some Lewis
acid sites from zinc(II) species, some Brønsted acid sites from
NH groups, and some basic sites due to NÀ moieties and OH
groups on the surface of the catalyst. They also demonstrated
that all active sites were located at the external surface, but
not in the micropores of the material. Therefore, the Knoeve-
nagel reaction is an ideal model reaction to evaluate nanopar-
ticle-based catalysts, since their activities are closely related to
particle size, but not porosity. For the above considerations,
The optimal catalyst dosage was determined by comparing
the conversion of benzaldehyde in the corresponding reac-
tions. The catalyst concentration, in units of molar percentage
of the substrate benzaldehyde, changed from 0 to 5 mol% for
comparison (Figure 11c). It was observed that condensation
occurred even without the Fe3O4/ZIF-8 particles, but the con-
version was only 7.7% after 6 h and it increased slowly there-
after. When the catalyst dosage increased from 0.5 to
1.5 mol%, the conversion sharply increased from 25.1 to
65.2% for the same reaction over 6 h. However, as the catalyst
concentration was further raised to 2.0 or 5.0 mol%, the con-
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Figure 11. The influence of: a) solvent, b) baking treatment, c) catalyst con-
centration, and d) particle size on the conversion of benzaldehyde.
Figure 12. a) Kinetic curves for four continuous condensation reactions with
Fe3O4/ZIF-8 particles as catalysts, and b) conversion of benzaldehyde in 3 h
for each reaction.
version of substrate increased to 72.3 or 85.6%, respectively.
Therefore, the optimal catalyst dosage could be 1.5–2 mol%
because further addition of catalyst would lower the conver-
sion efficiency.
activation in vacuum at 2508C for 2 h, the Fe3O4/ZIF-8 catalysts
could be used again for the next round of the reaction. The
conversion of benzaldehyde after 3 h reaction reached 85% for
the first round of catalysis, and gradually decreased to about
75% for the fourth round of catalysis, which proved that the
Fe3O4/ZIF-8 catalysts could be recovered and reused without
significant loss in activity.
Because the Knoevenagel reaction is mostly catalyzed on
the external surface of ZIF-8, a smaller particle size can create
more contacts between reactants and active sites, leading to
a higher catalytic activity for Fe3O4/ZIF-8 particles. Here, cata-
lyst particles with average diameters of 58.6, 72.2, 100.3, and
128.6 nm were used to show the influence of particle size
upon activity (Figure 11d). Upon decreasing the particle size
from 128.6 to 58.6 nm, the conversion increased from 55.1 to
79.7%. This proves that the active sites on the external surface
of ZIF-8 were responsible for condensation, which was consis-
tent with the transesterification reaction reported by Chizallet
et al.[7b] Although the abundant active sites inside the micro-
pores may not be utilized in this reaction, the nanosized MOF
structures are still promising catalytic materials for other reac-
tions.
Conclusion
Fe3O4/ZIF-8 nanoparticles were synthesized through a room-
temperature reaction between MeIM and zinc nitrate in the
presence of Fe3O4 nanocrystals. Growth of the Fe3O4/ZIF-8
composite particles could be considered as a nanocrystal en-
capsulation process driven by electrostatic attraction between
positively charged ZIF-8 particles and negatively charged Fe3O4
nanocrystals. Through the adjustment of the amount of
Zn(NO3)2 and Fe3O4 nanocrystals, the particle size could be
conveniently controlled, along with the surface charge and
magnetic loading. The as-prepared Fe3O4/ZIF-8 nanoparticles
could be used as a magnetically separable nanocatalyst in het-
erogeneous catalysis due to their high thermal stability, large
surface area, and strong magnetic response. Herein, the Knoe-
venagel condensation reaction between benzaldehyde and cy-
anoacetamide was used to evaluate the catalytic performance
of Fe3O4/ZIF-8. The reaction solvent, catalyst pretreatment,
dosage, and size were optimized for the best conversion. Typi-
cally, the conversion reached 72.3% in 6 h when condensation
was catalyzed by 2 mol% of baked catalyst in methanol. Be-
cause condensation was catalyzed at the active sites on the ex-
ternal surface of catalyst, smaller Fe3O4/ZIF-8 particles usually
presented a higher catalytic activity. At the same time, the
Compared with most homogeneous catalysts for organic re-
actions, the heterogeneous catalysts have intrinsic advantages
in catalyst recycling and repeated usage. Because the nanoca-
talysts are usually well dispersed in solution as a quasi-homo-
geneous system, it is difficult to separate the catalyst from the
reaction medium by convenient methods. Therefore, the intro-
duction of Fe3O4 nanocrystals into ZIF-8 made the composite
structure magnetic, so that separation could be realized by ap-
plying a magnetic field. To demonstrate the convenience of
magnetic separation for continuous reactions, 10 mol% of
Fe3O4/ZIF-8 particles were used as catalysts for condensation at
room temperature. In each reaction, condensation proceeded
for 3 h and the corresponding conversions are recorded in
Figure 12. After separation by means of a magnetic field and
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and products in the Knoevenagel reaction were analyzed by using
an Agilent 7890A gas chromatograph equipped with an HP-5MS
capillary column and using N2 as the carrier gas. The magnetic
properties of the Fe3O4/ZIF-8 nanoparticles were measured by
using a Lakeshore 7312 vibration sample magnetometer.
nanocatalysts could be continuously used in multiple catalytic
reactions without loss of activity through magnetic separation,
activation, and redispersion into the reaction solution.
Experimental Section
Knoevenagel reaction and evaluation of catalytic activity
Chemicals
The Knoevenagel reaction between benzaldehyde and cyanoaceta-
mide could be catalyzed by the ZIF-8 nanoparticles. Typically, cya-
noacetamide (1.28 g, 15.2 mmol) and Fe3O4/ZIF-8 nanoparticles
(36 mg) were first dispersed in methanol (20 mL) to form a homo-
geneous suspension. Benzaldehyde (0.8 mL, 7.6 mmol) was added
to the above solution under stirring and the reaction was allowed
to proceed for 6 h at room temperature. It should be noted that
cyanoacetamide was used in excess to ensure complete conversion
of benzaldehyde, and the molar ratio between catalyst (Fe3O4/ZIF-
8) and substrate (benzaldehyde) was 2:100. A small amount of ani-
sole (0.4 mL) was added to the reaction solution as an internal
standard for chromatographic analysis. During the 6 h reaction,
samples (0.1 mL) were taken from the solution every 1 h, and then
directly injected into the GC instrument to obtain the conversion
of benzaldehyde. After the reaction, the Fe3O4/ZIF-8 nanocatalysts
were purified by washing with methanol and magnetic separation
(10 min) several times, and annealed under vacuum for 2 h at
2508C to regenerate the catalytic activity.
FeCl3 (97%), PAA (Mw =1800), and diethylene glycol (DEG, 99%)
were purchased from Sigma-Aldrich. Zn(NO3)2·6H2O (99%), MeIM
(98%), cyanoacetamide (98%), iron(III) acetylacetonate ([Fe(acac)3];
98%), and PEI (Mw =1800, 99%) were purchased form Aladdin Co.
Ltd. Benzaldehyde (98%) and anisole (99%) were obtained from
J&K Chemical Co. Ltd. All chemicals were used as received.
Synthesis of Fe3O4 nanocrystals
Negatively charged PAA-grafted Fe3O4 nanoparticles were prepared
according to a previously published procedure.[15] PAA (4 mmol)
and FeCl3 (2 mmol) were first dissolved in DEG (15 mL) to form
a transparent homogenous solution at 1208C. The mixture was
heated to 2208C, and a solution of NaOH (2.5 mmolmLÀ1, 4 mL) in
DEG was quickly injected to the above solution under vigorous
stirring. After reacting at 2208C for 2 h, the solution was cooled to
room temperature and the nanocrystals were separated by centri-
fugation, rinsed with deionized water and ethanol three times, and
finally dispersed in deionized water (10 mL) to form a brownish red
transparent solution. The positively charged PEI-grafted Fe3O4
nanocrystals were prepared in a similar way, in which a solution of
[Fe(acac)3] (2 mmol) and PEI (1 g) in DEG (15 mL) was directly
heated at 2208C for 2 h without the addition of NaOH.
Acknowledgements
J.G. thanks the Major State Basic Research Development Pro-
gram of China (2011CB932404), the National Science Founda-
tion of China (21222107, 21471058), the Shanghai Rising-Star
Program (13A1401400), and the Youth Talent Plan (Organiza-
tion Department of the Central Committee of the CPC) for sup-
port.
Synthesis of Fe3O4/ZIF-8 nanoparticles
In a typical process, an aqueous solution of MeIM (3.45 mol/L,
20 mL) was first mixed with the aforementioned aqueous solution
of Fe3O4 nanocrystals (2.5 mL) by sonication, and stirred at a speed
of 400 rpm at room temperature. Then the aqueous solution of
Zn(NO3)2 (0.35 molLÀ1, 4 mL) was loaded in a syringe and gradually
added to the above solution over 20 min by means of a syringe
pump to produce Fe3O4/ZIF-8 particles. After the addition of
Zn(NO3)2, the products were separated by centrifugation, washed
with deionized water twice, and dried at 3008C under the protec-
tion of N2 flow for 2 h. After complete separation of Fe3O4/ZIF-8
particles and replacement of consumed MeIM, the reaction solu-
tion could be recycled for the next synthetic cycle.
Keywords: heterogeneous catalysis · magnetic properties ·
metal–organic frameworks · nanoparticles · synthesis design
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FULL PAPER
Increasing attraction: Fe3O4/zeolite imi-
dazolate framework (ZIF-8) nanoparti-
cles were synthesized through a room-
temperature reaction between 2-meth-
ylimidazolate and zinc nitrate in the
presence of Fe3O4 nanocrystals (see
figure). The particles have a superpara-
magnetic response and can be separat-
ed by means of a magnetic field and re-
dispersed into the solution after with-
drawal of the field.
&
Heterogeneous Catalysis
F. Pang, M. Y. He, J. P. Ge*
&& – &&
Controlled Synthesis of Fe3O4/ZIF-8
Nanoparticles for Magnetically
Separable Nanocatalysts
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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