Synthesis of Fe3O4@P4VP@ZIF-8 core-shell microspheres and their application in a Knoevenagel condensation reaction

Article abstract of DOI:10.1016/j.jssc.2017.07.032

In this work, a core-shell magnetic composite Fe₃O₄@P4VP@ZIF-8 microspheres were successfully designed and synthesized. A polymerization approach on the surface of pre-made Fe₃O₄ microspheres was employed for th

Full text of DOI:10.1016/j.jssc.2017.07.032

Author’s Accepted Manuscript
Synthesis of Fe O @P4VP@ZIF-8 Core-shell
3
4
Microspheres and their Application in a
Knoevenagel Condensation Reaction
Zongcheng Miao, Fengxia Yang, Luan Yi, Xin
Shu, Daniele Ramella
www.elsevier.com/locate/yjssc
PII:
S0022-4596(17)30294-3
http://dx.doi.org/10.1016/j.jssc.2017.07.032
YJSSC19882
DOI:
Reference:
To appear in: Journal of Solid State Chemistry
Received date: 23 May 2017
Accepted date: 22 July 2017
Cite this article as: Zongcheng Miao, Fengxia Yang, Luan Yi, Xin Shu and
Daniele Ramella, Synthesis of Fe O @P4VP@ZIF-8 Core-shell Microsphere
3
4
and their Application in a Knoevenagel Condensation Reaction, Journal of Solid
State Chemistry, http://dx.doi.org/10.1016/j.jssc.2017.07.032
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Synthesis of Fe O @P4VP@ZIF-8 Core-shell Microspheres and their
3
4
Application in a Knoevenagel Condensation Reaction
Zongcheng Miao , Fengxia Yang , Yi, Luan , Xin Shu,c1, Daniele Ramella d
a
b
,b
a
Xijing University, Xi’an, Shaanxi Province, 710123, P.R. China
b
School of Materials Science and Engineering, University of Science and Technology
Beijing, Beijing 100083, P. R. China
c
College of Science, Beijing University of Chemical Technology, Beijing 100029, P.
R. China,
d
Temple University-Beury Hall, 1901, N. 13th Street, Philadelphia PA 19122, United
States
yiluan@ustb.edu.cn
shuxin@mail.buct.edu.cn
Abstract: In this work, a core-shell magnetic composite Fe O @P4VP@ZIF-8
3
4
microspheres were successfully designed and synthesized. A polymerization approach
on the surface of pre-made Fe O microspheres was employed for the synthesis of
3
4
Fe O @P4VP. The zinc-derived Zeolite Imidazolate Framework (ZIF) shell was
3
4
introduced through a layer-by-layer strategy. The obtained Fe O @P4VP@ZIF-8
3
4
core-shell structure was employed as an efficient Knoevenagel condensation catalyst
for a variety of aldehydes. Furthermore, the inner P4VP layer also served as a basic
additive in the condensation reaction process, while much less homogeneous basic
1
Tel: (0086)1064434899

additive was used . High catalytic reaction efficiency was achieved when the P4VP
layer was utilized in combination with a Lewis acidity bearing ZIF-8 layer. The
Fe O @P4VP@ZIF-8 catalyst was tested for recyclability and no drop in the catalytic
3
4
activity was observed after more than five cycles.
Keyboards:
Metal-organic Framework; Core-shell; Knoevenagel Reaction; Heterogeneous
Catalysis; Polymer
Introduction
The Knoevenagel condensation reaction is widely used in industrial synthetic
organic processes, as simple starting materials can be facilely transformed to densely
functionalized products.[1] No wastes or by-products are generated during the
condensation process. Various homogeneous bases, such as pyridine and piperidine,
have been commonly used as the catalyst for the Knoevenagel condensation
reaction.[2] However, homogeneous catalysts bear implicit limitations in their
recycling and can cause severe equipment corrosion.[3] As a result, heterogeneous
catalysts are preferable alternatives.[4]

Core-shell structured materials are considered ideal heterogeneous catalysts and
gained increased interests due to their unique physicochemical and multi-functional
properties.[5] Magnetic core-shell structures are of particular interest because of their
ease of recovery through the application of an external magnetic field.[6] Hence,
numerous magnetic core-shell nanocatalysts were developed for immobilization of the
transition metals.[7] However, there are still several drawbacks associated with
immobilized magnetic core-shell nanocatalysts.[8] The amount of the active sites is
limited due to the small surface area and metal leaching may occur during the
catalytic process.[9]
Recently, metal–organic frameworks (MOFs) attracted lots of attention as porous
materials with an extremely large surface area, numerous active sites and good
stability.[10] They have been often investigated as heterogeneous catalysts for their
high surface areas, unique topology and tunable properties.[11] The strong covalent
bonds in the MOFs structure ensures the stability of the active metal centers,
addressing the issue of metal leaching and deactivation.[12] The introduction of
metal-organic framework structural layer as the Lewis acidic shell promotes the
organic reaction efficiently and protects the inner material core, furnishing a
chemically stable heterogeneous catalyst.[13] For example, Fe O @MOF
3
4
2+
nanocomposites were synthesized as a magnetically recoverable catalyst[14] or UO2
adsorption reagent.[15]
In this work, we wish to report a novel core-shell-shell structured catalyst,
Fe O @P4VP(poly(4-vinylpyridine))@ZIF-8(Zeolitic Imidazolate Framework-8),
3
4
involving a coated MOFs layer (Scheme 1).[16] The zinc derived ZIF-8 was grafted
onto the surface of the Fe O @P4VP microspheres through layer-by-layer
3
4
assembly.[17] The inner shell of P4VP plays an important role in the rapid
coordination with zinc ion, forming a ZIF-8 layer for the protection of the magnetic
core structure. This core-shell-shell offers more functionality and higher stability than
Fe O @MOF nanocomposites. Furthermore, the P4VP shell can also serve as a basic
3
4
catalyst for the Knoevenagel condensation.[18] In the meanwhile, the coated MOF

layer functions as a Lewis acid co-catalyst to facilitate the reaction.[19] During this
study, the efficient Knoevenagel condensation reaction of a variety of aldehydes was
achieved using the Fe O @P4VP@ZIF-8 multifunctional catalyst.[20]
3
4
Scheme 1. Schematic illustration for the preparation of Fe O @P4VP@ZIF-8.
3
4
Experimental section
Synthesis of core-shell-shell Fe O @P4VP@ZIF-8 microspheres
3
4
The core-shell-shell Fe O @P4VP@ZIF-8 microspheres were synthesized by
3
4
step-by-step strategy. In general, 0.05 g of Fe O @P4VP were dispersed in 5 mL (10
3
4
mM) Zn(NO ) ·6H O methanol solution for 15 minutes. Then the microspheres were
3
2
2
dispersed in 2-methylimidazole (2-MeIM) methanol solution (4 mL, 10 mM) for 30
minutes. After twenty cycles, the Fe O @P4VP@ZIF-8 nanoparticles were separated,
3
4
o
washed with absolute ethanol, and dried under vacuum at 80 C for 6 h.
Typical catalytic procedure for Knoevenagel condensation reaction

In a typical reaction, to a 5-mL reaction vessel, 1.0 mmol of benzaldehyde and 1.5
mmol of malononitrile were dissolved in 2.0 mL of toluene. To the reaction 1.0 mol%
of Fe O @P4VP@ZIF-8 catalyst was then added. The heterogeneous solution was
3
4
stirred at room temperature (23 ºC) for 2 h. The final yield and selectivity of the
Knoevenagel condensation product were measured by GC-MS using n-dodecane as
the internal standard.
Results and Discussion
The Fe O @PAA microspheres were synthesized according to a literature reported
3
4
solvothermal method;[19] the microspheres were composed of Fe O nanocrystals of
3
4
diameter not greater than 10 nm (Fig. 1a). The Fe O nanoparticles were successfully
3
4
coated with P4VP to form core-shell composite microspheres, taking advantage of the
hydrogen bond between carboxyl groups of PAA chains and pyridine. The diameters
of the magnetic core (central black sphere) and polymer shell (outer gray shell) were
about 200 nm and 38 nm, respectively, as shown in Fig. 1b.

Fig. 1 HRTEM images of (a) Fe O , (b) Fe O @P4VP, (c) Fe O @P4VP@ZIF-8 and (d)
3
4
3
4
3
4
recycled Fe O @P4VP@ZIF-8.
3
4
The Fe O @P4VP@ZIF-8 core-shell-shell catalyst was prepared by layer-by-layer
3
4
2
+
method with 20 assembly cycles (Fig. 1c). The Zn ions were adsorbed on P4VP
initially through pyridine coordination, and then reacted with 2-methylimidazole to
complete the assemble procedure. This process was repeated twenty times to achieve
the core-shell-shell structure. The introduction of a zinc salt has no impact on the
sphere morphology of core-shell material according to HRTEM characterization. The
SEM image of Fe O @P4VP@ZIF-8 shows microspheres with a diameter of 400 nm.
3
4
Fig. 2 EDX mapping of Fe O @P4VP@ZIF-8.
3
4
The EDX elemental maps verified the successful synthesis of core-shell structured
Fe O @P4VP@ZIF-8. The EDX map in Fig. 2 clearly shows that zinc derived ZIF-8
3
4
was coated on the surface of Fe O @P4VP.
3
4

Fig. 3 XRD patterns of (a) Fe O , (b) Fe O @P4VP, (c) Fe O @P4VP@ZIF-8 and (d)
3
4
3
4
3
4
recycled Fe O @P4VP@ZIF-8
3
4
To further confirm the crystalline structures of Fe O , Fe O @P4VP and
3
4
3
4
Fe O @P4VP@ZIF-8 microspheres, powder XRD was performed and the patterns
3
4
are shown in Fig. 3. The diffraction peaks of Fe O (Fig. 3a) agree with standard
3
4
JCPDS 75-1609.39 indicating the face-centered cubic lattice. After P4VP coating, the
XRD pattern of core-shell Fe O @P4VP microspheres was almost identical to the
3
4
pattern of Fe O , as layer of polymeric shell is not crystalline (Fig. 3b). Several peaks
3
4
of ZIF-8 were observed on the XRD pattern for the Fe O @P4VP@ZIF-8
3
4
core-shell-shell structure. The new ZIF-8 peak is in agreement with previously
published literature. In addition, these peaks also match our simulated ZIF-8 XRD
peaks (Fig. S1).
Fig. 4 FTIR spectra of a) Fe O (PAA), b) Fe O @P4VP and c) Fe O @P4VP@ZIF-8.
3
4
3
4
3
4
The FTIR spectra of the synthesized microspheres are shown in Fig. 4. The high
-1
intensity of the band at 592 cm is the characteristic peak of the Fe-O vibrations. In
-1
addition, absorption bands at 1707, 1250, and 1165 cm correspond to the C=O
stretching of the carboxylic acid group and to in-plane deformations of C-O-H, and
-(C-O)H stretching. This observation suggested that -COOH functional groups
derived from PAA are located on the surface of Fe O particles. In the FTIR spectrum
3
4

of Fe O @P4VP microspheres, the characteristic absorptions at 1603, 1562, and 1417
3
4
-1
cm are attributed to the vibration of the pyridine ring (Fig. 4b). After the
-1
introduction of ZIF-8 layer, a major new peak appears at 1683 cm , indicating the
large amount of carboxylate groups (Fig. 4c).
Fig. 5 N adsorption/desorption isotherms of Fe O @P4VP@ZIF-8.
2
3
4
The surface area of our synthesized Fe O @P4VP@ZIF-8 was calculated by
3
4
measuring the N adsorption/desorption isotherms and the value was determined to be
2
2
-1
4
10.3 m g . The isotherms shown in Fig. 5 are a combination of type I and IV
isotherms, which suggested a mixture of microporous and mesoporous structures (Fig.
). It is a significant improvement when compared with its non-porous Fe O @P4VP
3
3
4
2
-1
precursor, which only has a BET surface area of 14.8 m g . The highly porous shell
strongly facilitates the catalytic process, which takes advantage of the newly
introduced MOF layer.

Fig. 6 TGA curves of a) Fe O (PAA), b) Fe O @P4VP and c) Fe O @P4VP@ZIF-8.
3
4
3
4
3
4
TGA analysis of Fe O @P4VP@ZIF-8 and its precursors were measured and
3
4
shown in Fig. 6. PAA coated Fe O microspheres are relatively stable with a weight
3
4
loss of 17 wt%, which we attributed to the desorption of solvent and to the
decomposition of PAA chains. The TGA curve of Fe O @P4VP microspheres
3
4
showed three steps of weight loss with 52 wt% remaining. The loss of water,
decomposition of oligomer, and the desorption of P4VP shell respectively account for
o
the three weight losses. As expected, there is less mass loss between 300 to 400 C for
Fe O @P4VP@ZIF-8 in comparison to its precursor, due to the zinc oxide formed by
3
4
calcination.
Fig. 7 Magnetic hysteresis loops of a) Fe O , b) Fe O @P4VP and c) Fe O @P4VP@ZIF-8.
3
4
3
4
3
4
Vibrating sample magnetometry (VSM) was utilized to measure the magnetic
properties of Fe O , Fe O @P4VP and Fe O @P4VP@ZIF-8 (Fig. 7). According to
3
4
3
4
3
4
the VSM results, the magnetization saturation values of Fe O , Fe O @P4VP and
3
4
3
4
-1
Fe O @P4VP@ZIF-8 were 74.2, 48.4 and 27.7 emu g , respectively; these values
3
4
indicate a material which is superparamegantic. The saturation magnetic momentum
of Fe O , Fe O @P4VP and Fe O @P4VP@ZIF-8 are lower than that of Fe O due
3
4
3
4
3
4
3
4
to the additional P4VP and MOF shells layers. Moreover, the saturation magnetic
momentum shows linear dependence on the content of magnetite.

a
Table 1. Reaction condition optimization.
b
Entry
Catalyst
-
Solvent Yield
PhCH3 <5%
PhCH3 91%
PhCH3 <5%
PhCH3 58%
PhCH3 27%
1
2
3
4
5
6
7
8
9
Pyridine (5 mol%)
Fe O
3
4
Fe O @P4VP
3
4
ZIF-8
Fe O @P4VP + ZIF-8 PhCH3 73%
3
4
Fe O @P4VP@ZIF-8 PhCH 99%
3
4
3
Fe O @P4VP@ZIF-8 CH CN 94%
3
4
3
Fe O @P4VP@ZIF-8 CH Cl2 87%
3
4
2
1
1
0
1
Fe O @P4VP@ZIF-8 EtOH
74%
66%
3
4
Fe O @P4VP@ZIF-8 THF
3
4
a
Reaction conditions: To 1.0 mmol benzaldehyde and 1.5 mmol malononitrile in 2
mL of solvent 1 mol% of catalyst (based on active catalytic site) was added. The

b
reaction solution was stirred at 23 ºC for 2 h. The yield was measured by GC-MS
using n-dodecane as the internal standard.
Several reaction factors were evaluated during the optimization of the Knoevenagel
condensation reaction (Table 1). Firstly, a control experiment was performed to study
the background rate of the reaction. The result indicated that there was no reactivity in
the absence of the catalyst (Table 1, entry 1). Pyridine functioned as a decent
homogeneous base catalyst and a yield of 91% was achieved (Table 1, entry 2). Fe O4
3
microspheres showed no catalytic activity under our reaction conditions in presence
of benzaldehyde and malononitrile (Table 1, entry 3). After coating with P4VP,
Fe O @P4VP provided a moderate yield of 2-benzylidenemalononitrile taking
3
4
advantage of basic functionality of P4VP layer (Table 1, entry 4). ZIF-8 can also
promote the Knoevenagel condensation reaction as a Lewis acid although in low
yields (Table 1, entry 5). The combination of Fe O @P4VP and ZIF-8 significantly
3
4
increased the yield in 2-benzylidenemalononitrile product (Table 1, entry 6). The
existence of both Lewis basicity and Lewis acidity strongly facilitates the
Knoevenagel condensation reaction of benzaldehyde. In light of this, the synthesized
Fe O @P4VP@ZIF-8 catalyst was tested and it gave its optimal yield at 1 mol%
3
4
catalyst loading (based on the number of zinc equivalents) when stirred at room
temperature for 2 h (Table 1, entry 7). The amount of Fe O @P4VP and ZIF-8 in
3
4
Fe O @P4VP@ZIF-8 catalyst were consistent with the one used in entry 6, for
3
4
parallel comparison. The outer MOF layer allows the free entry and exit of the
organic molecule and it also serves as a good Lewis acid for aldehyde activation. The
basic functionality of P4VP ensures the deprotonation of the adduct intermediate and
its dehydration, which leads to the benzylidenemalononitrile product. The synthesized
Fe O @P4VP@ZIF-8 catalyst showed significantly improved turnover number (TON)
3
4
and turnover frequency (TOF) than several other MOF promoted Knoevenagel
condensation reaction (Table S1).
Further evaluation indicated that toluene is the most suitable reaction solvent for
further Knoevenagel catalytic studies (Table 1, entries 7-11). Dichloromethane and

acetonitrile gave comparable, but slightly compromised yields of the desired product
using Fe O @P4VP@ZIF-8 catalyst (Table 2, entries 8 and 9). Oxygen atom
3
4
containing solvents, such as ethanol and THF, gave only decent yields of the desired
benzylidenemalononitrile product (Table 2, entries 10 and 11).
a
Table 2. Knoevenagel condensation reaction promoted by Fe O @P4VP@ZIF-8.
3
4

a
Reaction conditions: 1.0 mmol of benzaldehyde, 1.5 mmol of malononitrile and 1
mol% of Fe O @P4VP@ZIF-8 were dissolved in 2 mL of solvent. The reaction
3
4
solution was stirred at 23 ºC for 2 h. The yield was measured by GC-MS using
b
n-dodecane as the internal standard. 6 h of reaction time.
A variety of aldehydes were evaluated employing the Fe O @P4VP@ZIF-8
3
4
catalyst using malononitrile as the nucleophile (Table 2). Non-substituted and methyl
substituted aromatic aldehydes were compatible with the optimized reaction
conditions, which gave yields of 99% in the desired benzylidenemalononitrile product
(Table 2, entries 1 and 2). The tert-Butyl substituted benzaldehyde showed a slightly
compromised yield (Table 2, entry 3). It is postulated that the small pore size of the
ZIF-8 support limits the catalytic activity towards the Knoevenagel condensation of
large aromatic aldehydes. Electron-rich aldehyde, such as 4-methoxybenzaldehyde, is
compatible with Fe O @P4VP@ZIF-8 catalyst under the optimal reaction condition
3
4

(Table 2, entry 4). Aldehydes bearing a 4-fluoro electron-withdrawing group were
suitable substrates due to their electron-deficient nature (Table 2, entry 5).
Furthermore, the heteroaromatic aldehyde reacted smoothly with malononitrile in the
presence of Fe O @P4VP@ZIF-8 catalyst (Table 2, entry 6). Lastly, an alkyl
3
4
aldehyde such as cyclohexane carbaldehyde was also compatible with the
malononitrile nucleophile employing 1 mol% of catalyst. However, extended reaction
time was required to achieve an almost quantitative yield (Table 3, entry 7).
Fig. 8 Hot filtration test for the Knoevenagel condensation reaction.
A hot filtration test was conducted to confirm the heterogeneous nature of the
aerobic oxidation catalysis (Fig. 8). The Fe O @P4VP@ZIF-8 catalyst was removed
3
4
using an external magnet and isolated after 40 min and the aldehyde/malononitrile
mixture was stirred for additional 80 min at room temperature. After the separation of
the Fe O @P4VP@ZIF-8 catalyst, no further conversion of benzaldehyde was
3
4
observed; the yield in benzylidenemalononitrile was determined to be only 57%. This
+
result indicates that the carboxylate ligand bonded to the Zn species are highly stable
and strongly attached to the surface of Fe O @P4VP. The ICP-AAS of the reaction
3
4
solution showed that the concentration of Zn was 5.4 ppm, which suggests extremely
2
+
low leakage of Zn ion.

Fig. 9 Fe O @P4VP@ZIF-8 catalyst recycling test.
3
4
Finally, the recyclability of Fe O @P4VP@ZIF-8 was studied using benzaldehyde
3
4
as the model reaction at room temperature. The same batch of catalyst was reused six
times and the conversion and selectivity of the product remained at 99% levels (Fig.
9
). The high yield and selectivity of the product after six cycles indicated the high
stability of the core-shell heterogeneous catalyst and an extremely low leakage of zinc
ion. The 6-time reused Fe O @P4VP@ZIF-8 catalyst was characterized by SEM,
3
4
XRD and FTIR. showing no structure alternation on the ZIF-8 framework (Fig. 1d, 3d
and S2). As we expected, Fe O @P4VP@ZIF-8 is stable under the Knoevenagel
3
4
reaction conditions.
Conclusions
In conclusion, a novel magnetic core-shell Fe O @P4VP@ZIF-8 material was
3
4
developed as efficient catalyst for the Knoevenagel condensation reaction. The
microspherical nanoparticles were composed of a magnetic Fe O (PAA) core, of a
3
4
P4VP middle layer, as well as of a zinc derived ZIF-8 shell layer. The magnetic
composite microspheres were utilized as an efficient catalyst taking advantage of
basicity of both P4VP and ZIF-8. High reaction efficiency were achieved utilizing the
newly developed Fe O @P4VP@ZIF-8 catalyst. A variety of aldehyde substrates
3
4
were converted to the corresponding benzylidenemalononitrile products in good

yields under the optimized reaction conditions. Furthermore, the conversion and
selectivity of benzaldehyde using Fe O @P4VP@ZIF-8 as a catalyst was maintained
3
4
after at least six consecutive reaction cycles. A hot filtration test also suggested the
extremely low leakage of zinc ions during the catalytic reaction.
Acknowledgements
We thank the Beijing Municipal Natural Science Foundation (Grant No. 2172037)
and the National Natural Science Foundation of China (No. 51503016 and No.
5
1673157) for financial support. Zongcheng Miao and Xin Shu also thank the Science
Research Foundation of Xijing University (Grant No. XJ16T02) and the BUCT Fund
for Disciplines Construction and Development (No. XK1529) for financial support.
Appendix A. Supporting information
Supplementary data associated with this article can be found in the online version.
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Highlights
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. A newly designed Fe O @P4VP@ZIF-8 core-shell catalyst is reported.
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. Layer-by-layer strategy was utilized to achieve core-shell structure at 400 nm.
. The Fe O @P4VP@ZIF-8 core-shell material can be applied in an efficient
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Knoevenagel reaction.
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. Rapid catalyst recycling through external magnetic field.