ZIF-8 crystal coatings on a polyimide substrate and their catalytic behaviours for the Knoevenagel reaction

Article abstract of DOI:10.1039/c2dt32161k

ZIF-8 crystal coatings were prepared by direct growth on different flexible polyimide substrates including a thin membrane and electrospun nanofiber mat. In SEM pictures, the ZIF-8 crystals exhibit a rhombic dodecahedron morphology. Owing to the flexible

Full text of DOI:10.1039/c2dt32161k

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ZIF-8 crystal coatings on a polyimide substrate and
their catalytic behaviours for the Knoevenagel
reaction†
Cite this: Dalton Trans., 2013, 42, 3936
Rizhe Jin,a Zheng Bian,*a Jizhen Li,b Mengxian Dinga and Lianxun Gao*a
ZIF-8 crystal coatings were prepared by direct growth on different flexible polyimide substrates including
a thin membrane and electrospun nanofiber mat. In SEM pictures, the ZIF-8 crystals exhibit a rhombic
dodecahedron morphology. Owing to the flexible polyimide substrate, the MOF–polymer film may be
moderately bent or easily tailored when not destroyed. The ZIF-8 coatings can be activated at high temp-
erature owing to the excellent thermal resistance and low coefficient of thermal expansion of the poly-
imide. Furthermore, this material may be used as an efficient heterogeneous catalyst for the Knoevenagel
reaction and can be easily recovered. The ZIF-8 coatings and 2-methylimidazole show similar catalytic
behaviour as a weak base.
Received 18th September 2012,
Accepted 11th December 2012
DOI: 10.1039/c2dt32161k
www.rsc.org/dalton
strength and good chemical adaptability.8 In general, they are
amorphous and have almost no Tg transition. The highest
Introduction
Metal–organic frameworks (MOFs) as multifunctional porous work temperatures may reach 400 °C but the coefficients of
materials have continually received much interest from che- thermal expansion are very low. In particular, a large number
mists and materialists.1 In particular, some MOF films on of polar imide groups exist on a PI surface to possibly favour
solid surfaces have been fabricated2 and show potential in the bonding of a MOF crystal. Therefore, we plan to use a PI as
applications such as highly efficient sensors,3 separation mem- the growth substrate of a MOF film.
branes4 and catalytically active coatings. Recently, Hatton et al.
As one of the most studied prototypical zeolite imidazolate
have reported that a MIL-47 film,5 consisting of vanadium(III) framework (ZIF) compounds, ZIF-8 with 2-methylimidazole as
oxide octahedra and terephthalate linkers, can directly grow a ligand possesses exceptionally high thermal and chemical
on
a
polyacrylonitrile (PAN) substrate using microwave stability and has been extensively applied to gas storage,2,9
irradiation. The MOF–polymer film presents a new category of catalysis,10,13 sensing,11 and gas separation.12 Considering
functionalized organic–inorganic hybrids. Owing to the flexi- that there could be strong N–H⋯OvC hydrogen-bond
ble organic polymer substrate, the MOF–polymer films could interactions between the CvO group of the imide group and
be moderately bent or easily tailored when not destroyed. the N–H group of the 2-methylimidazole, we postulate that
Notably, a mixed-matrix MOF–polymer membrane is obtained ZIF-8 would grow well on a PI substrate (Scheme 1). In this
by dispersing MOF nanocrystals in a polyimide (PI) substrate.6 paper, the growth of ZIF-8 on a PI membrane or electrospun PI
Very recently, Qiu et al. have prepared a pure MOF film by nanofiber mat is studied in detail using scanning electron
using a polymer material as the substrate.7
microscopy (SEM), X-ray diffraction (XRD) and Fourier trans-
However, the glass transition temperature (Tg) of PAN is form infrared spectroscopy (FT-IR). Furthermore, this material
only 85 °C. When the MOF coatings need to be activated at can be easily used as a recycled catalyst for the Knoevenagel
higher temperature, it could be destroyed owing to the large reaction.
stress deformation of the PAN substrate. It is well known that
PI materials have excellent thermal stability, high mechanical
Results and discussion
Preparation and characterization of the PI/ZIF-8 film
aState Key laboratory of Polymer Physics and Chemistry, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, China.
E-mail: bianzh@ciac.jl.cn, lxgao@ciac.jl.cn; Fax: +86 431 85697831;
We first chose the commercially available poly[4,4′-oxydipheny-
Tel: +86 85262265
lenepyromellitimide] (PMDA-ODA) PI membrane (Kapton®) as
the research objective. Referring to the method previously
reported by Caro et al.,12b the ZIF-8 coatings were prepared by
bCollege of Chemistry, Jilin University, Changchun, 130012, China
†Electronic supplementary information (ESI) available: SEM pictures. See DOI:
10.1039/c2dt32161k
3936 | Dalton Trans., 2013, 42, 3936–3940
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Fig. 2 FT-IR spectra of the ZIF-8 film on the PI membrane after eight growth
cycles.
Scheme 1 Schematic description of ZIF-8 growing on a PI surface. The hydro-
gen atoms are omitted except for those that can form hydrogen bonds with the
CvO group of the imide.
Fig. 3 The calculated (a) and measured XRD patterns of the ZIF-8 crystal films
on the Kapton PI membrane (b, one growth cycle; c and d, four growth cycles,
before and after activation at 300 °C).
on the surface to give amic acids.14 Comparative experiments
indicate that there is no evident difference between hydrolyzed
and unhydrolyzed PI membranes on the surface (Fig. S3, ESI†).
This suggests that the partial hydrolyzation on the PI surface
contribute little to the growth of ZIF-8.
Fig. 1 SEM images of the ZIF-8 crystals on the Kapton PI membrane (a and b,
one growth cycle; c and d, eight growth cycles, before and after activation at
300 °C, respectively). Scale bars: a, 2 μm; b, 5 μm, c–d, 10 μm.
The peak positions presented in the FT-IR spectra of the as-
immersing the PI membrane into a mixed methanol solution prepared film are in good agreement with those of ZIF-8 crys-
of 2-methylimidazole : Zn(NO3)2·6H2O (2 : 1, molar ratio) at tals previously reported (Fig. 2).9 In contrast, some character-
room temperature for 24 h. The effect of the concentration of istic peaks (3400–2200 cm−1, strong and broad, N–H⋯N
the ligand/metal on the film growth was investigated. When hydrogen bond; 1843 cm−1, strong, N–H stretching vibra-
the concentration of Zn(NO3)2 is 1.3 × 10−3 M, only very sparse tion13b) relative to the N–H bond in 2-methylimidazole dis-
nanocrystals (about 100–200 nm) on the PI membrane are appear. The powder XRD data of the MOF coatings are almost
observed by SEM (Fig. S1a, ESI†) and at a high concentration identical to those of the ZIF-8 crystal (Fig. 3). These obser-
of 4.0 × 10−2 M, no crystal is formed (Fig. S1b, ESI†). However, vations testify to the existence of ZIF-8 on the PI membrane.
good ZIF-8 coatings (about 0.5–2 μm) appear for a moderate
The ZIF-8 crystals presented in the SEM pictures exhibit the
concentration of 1.0 × 10−2 M (Fig. 1a, b). After two growth rhombic dodecahedron morphology previously described by
cycles, ZIF-8 crystals can completely cover the substrate surface Caro et al. (Fig. 1a).12b Moreover, most ZIF-8 crystals have a
and more growth cycles result in thicker MOF coatings plane which bonds to the PI surface (Fig. S4, ESI†). Because
(Fig. S2, ESI†). Furthermore, the partially hydrolyzed PI mem- ZIF-8 belongs to a highly symmetrical cubic crystal system,2,9
branes were also explored as a growth substrate. The imido every rhombic plane on the dodecahedron crystal is the (110)
groups undergo basic hydrolysis and subsequent acidification one and there are six pairs of parallel rhombic planes in a
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ZIF-8 crystal. In a single cell of ZIF-8, all the imidazole ligands
lie on the (110) planes and every (110) plane contains six
2-methylimidazole molecules with the imidazole ring vertical
to the plane. When a 2-methylimidazole molecule is absorbed
onto the PI surface by hydrogen bond interactions, the N–H
bond of the imidazole directs it to the PI surface and the N
donor extends outside the PI surface. Therefore, the ZIF-8 crys-
tals could be apt at growing along the direction vertical to the
(110) plane parallel to the PI surface at the beginning of the
crystal growth (Scheme 1). However, the XRD patterns of the
ZIF-8/PI film (Fig. 3b) after one growth cycle didn’t support
this assumption, possibly due to the uneven surface of the PI
membrane.
Fig. 5 The calculated (a) and measured XRD patterns of the ZIF-8 crystal films
on the electrospun nanofiber mat (b and c, four growth cycles, before and after
activation at 300 °C).
The MOF–polymer film undergoing four growth cycles was
treated for three hours at 300 °C to remove small solvent
molecules. The SEM pictures before and after the activation
show no evident difference (Fig. 1c, d). The XRD experiments
show that the ZIF-8 films remain highly crystalline after the
activation (Fig. 3c, d). These observations indicate the good
thermal resistance of the MOF–polymer films.
Furthermore, we explored the growth of ZIF-8 on the elec-
trospun nanofiber mat of PMDA-ODA15 using the previously
optimized method. As expected, the relatively small ZIF-8 crys-
tals (1–2 μm) can adhere effectively to the surface of the PI
nanofibers after the first growth cycle (Fig. 4). From the
second growth cycle, crystals with a large volume also appear
(2–3 μm, Fig. 4c).
After the activation at 300 °C, the morphology and crystalli-
nity of the ZIF-8 coating remained unchanged (Fig. 4d and 5).
Scheme 2 The Knoevenagel reaction with the ZIF-8/PI film or 2-methylimida-
Catalysis studies
zole as the catalyst. Isolation yield.
Chizallet et al. have described that ZIF-8 can catalyze the trans-
esterification of vegetable oil with significant activity and indi-
cated the catalytic role of ZIF-8 with Zn2+ species as acid sites
and N− moieties and OH groups as basic ones.13a Recently,
Phan et al. reported that ZIF-8 can catalyze the Knoevenagel
reaction.13b However, only malononitrile as a reaction sub-
strate was studied. Here, we explore in detail the catalytic pro-
perties of the ZIF-8 crystals for the Knoevenagel reaction via
ZIF-8 coatings on PI nanofibers (Scheme 2).
At first, acetylacetone (1), ethyl acetoacetate (2), ethyl cyano-
acetate (3), ethyl nitroacetate (4) and malonitrile (5) were
chosen to react with 4-bromobenzaldehyde (6), using the
ZIF-8/PI film or 2-methylimidazole as the catalyst in toluene at
room temperature. Interestingly, a reaction barely happened
for 1–4 but almost quantitative product was acquired for 5.
Notably, a malononitrile molecule has two strong electron-
withdrawing nitrile groups and it is the most reactive among
the five substrates for the Knoevenagel reaction. The ZIF-8/PI
film and 2-methylimidazole therefore exhibited the same cata-
lytic property as a weak base.
Furthermore, reactions of different aromatic aldehydes and
malononitrile were carried out. The same excellent results
were acquired (Scheme 2). The ZIF-8/PI film after the reactions
can be easily recovered and reused in the next cycle. The GC
conversions of benzaldehyde in four cycles of the reaction of
benzaldehyde and malononitrile are 98%, 97%, 84% and 67%,
respectively. However, a partial destruction of the ZIF-8 coating
happened in the latter cycles.
Fig. 4 SEM images of the ZIF-8 crystals on the electrospun nanofiber mat (a, PI
nanofibers; b, one growth cycle; c and d, four growth cycles, before and after
activation at 300 °C). Scale bars: a, 5 μm; b, 10 μm; c–d, 5 μm.
3938 | Dalton Trans., 2013, 42, 3936–3940
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yellow solid. 1H NMR (400 MHz, CDCl3), δ: 8.39 (2H, d, J =
8.0 Hz), 8.07 (2H, d, J = 8.0 Hz), 7. 87 (1H, s).
Experimental
General information
2-(3-Bromobenzylidene)malononitrile 7c (yield, 92% for
The common commercial reagents were purchased from ZIF-8 and 97% for 2-methylimidazole) was obtained as a white
1
Shanghai Reagent Plant and used directly. The PI Kapton film solid. H NMR (400 MHz, CDCl3), δ: 7.97 (1H, s), 7.90 (1H, d,
was purchased from Yangzhou Jiabao Electric Co. Ltd. The J = 8.0 Hz), 7.76 (1H, d, J = 8.0 Hz), 7.71 (1H, s); 7.43 (1H, t, J =
electrospun nanofiber mat of PMDA-ODA was provided by 8.0 Hz).
Jiangxi Xiancai Nanofibers Technology Co. Ltd.
2-(2-Bromobenzylidene)malononitrile 7d (yield, 96% for
The X-ray diffraction patterns of the films were performed ZIF-8 and 91% for 2-methylimidazole) was obtained as a white
1
on a Bruker D8 AXS ADVANCE X-ray diffractometer with a step solid. H NMR (400 MHz, CDCl3), δ: 8.22 (1H, s), 8.13 (1H, dd,
size of 0.05° and scan rate of 2° min−1, using a ZIF-8/PI film of J = 8.0, 1.6 Hz), 7.75 (1H, dd, J = 8.0, 1.6 Hz), 7.50 (1H, td, J =
about 10 mm × 10 mm. Scanning electron microscopy (SEM) 8.0, 1.6 Hz), 7.46 (1H, td, J = 8.0, 1.6 Hz).
images were taken with an FEI company XL30 ESEM FEG field
2-(4-Chlorobenzylidene)malononitrile 7e (yield, 92% for
emission environmental scanning electron microscope. ZIF-8 and 95% for 2-methylimidazole) was obtained as a white
1
Fourier transform infrared (FT-IR) spectra were determined solid. H NMR (400 MHz, CDCl3), δ: 7.85 (2H, d, J = 8.4 Hz),
with a Bio-Red Digilab Division FTS-80 spectrometer. Gas chro- 7.73 (1H, s), 7.52 (2H, d, J = 8.4 Hz).
matographic (GC) analyses were performed using a Shimadzu
2-(4-Bromobenzylidene)malononitrile 7f (yield, 92% for
GC 2010 equipped with a flame ionization detector (FID) and ZIF-8 and 96% for 2-methylimidazole) was obtained as a pale
an XE54 column (length = 30 m, inner diameter = 0.25 mm yellow solid. 1H NMR (400 MHz, CDCl3), δ: 7.77 (2H, d, J =
and film thickness = 0.25 μm). Samples were analyzed by the 8.4 Hz), 7.71 (1H, s), 7.69 (2H, d, J = 8.4 Hz).
following method: injector temperature 300 °C, helium flow
2-(4-Methylbenzylidene)malononitrile 7g (yield, 95% for
rate 1.0 mL min−1, oven temperature 100 °C, detector temp- ZIF-8 and 93% for 2-methylimidazole) was obtained as a white
1
erature 320 °C. n-Dodecane was used as an internal standard solid. H NMR (400 MHz, CDCl3), δ: 7.81 (2H, d, J = 8.0 Hz),
to calculate the reaction conversions. 1H NMR spectra 7.72 (1H, s), 7.34 (2H, d, J = 8.0 Hz), 2.46 (3H, s).
were recorded on a Varian Unity spectrometer at 400 Hz with
tetramethylsilane as an internal standard.
2-(4-Methoxybenzylidene)malononitrile 7h (yield, 91% for
ZIF-8 and 94% for 2-methylimidazole) was obtained as a white
1
solid. H NMR (400 MHz, CDCl3), δ: 7.91 (2H, d, J = 8.8 Hz),
Growth of ZIF-8 on the PI membrane
7.65 (1H, s), 7.01 (2H, d, J = 8.8 Hz), 3.92 (3H, s).
Typically, 2-methylimidazole (0.41 g, 5.0 mmol) was dissolved
2-(3-Methoxybenzylidene)malononitrile 7i (yield, 94% for
in a solution of Zn(NO3)2·6H2O (0.74 g, 2.5 mmol) in methanol ZIF-8 and 94% for 2-methylimidazole) was obtained as a white
1
(25 mL) and then the PI membrane (about 2.0 × 2.0 cm2) was solid. H NMR (400 MHz, CDCl3), δ: 7.75 (1H, s), 7.50 (1H, d,
immersed into the reaction solution for 24 h at room temp- J = 2.0 Hz), 7.45 (1H, t, J = 8.0 Hz), 7.41 (1H, td, J = 8.0, 2.0 Hz),
erature. Finally, the PI membrane was washed with methanol 7.17 (1H, dt, J = 8.0, 2.0, 2.0 Hz), 3.87 (3H, s).
several times and dried under air at room temperature.
2-(2-Methoxybenzylidene)malononitrile 7j (yield, 96% for
Thicker ZIF-8 films on a PI membrane can be obtained by ZIF-8 and 98% for 2-methylimidazole) was obtained as a white
1
simply repeating the above mentioned procedure.
solid. H NMR (400 MHz, CDCl3), δ: 8.31 (1H, s), 8.19 (1H, dd,
J = 8.0, 2.0 Hz), 7.58 (1H, td, J = 8.0, 2.0 Hz), 7.08 (1H, t, J =
8.0 Hz), 6.98 (1H, d, J = 8.0 Hz), 3.93 (3H, s).
General procedure for the Knoevenagel condensation
ZIF-8 crystals on electrospun PI nanofibers and 2-methylimida-
zole were used as the catalysts. The ZIF-8/PI film contained
about 65 wt% ZIF-8. To illustrate the general procedure for the
Knoevenagel condensation, the preparation of 2-benzylidene-
malononitrile 7a is given as an example.
Conclusions
In conclusion, ZIF-8 crystal films can grow well on a PI mem-
Benzaldehyde (0.0531 g, 0.50 mmol), malononitrile brane and electrospun nanofiber mat. The MOF–polymer
(0.1982 g, 3.0 mmol), the ZIF-8 catalyst (0.0092 g, 0.025 mmol) materials may be treated at high temperature to make them
and toluene (5.0 mL) were mixed and this reaction was carried useful in practical applications. Owing to the fact that the PI
out with bubbling of Ar gas through the reaction mixture for substrate itself can be functionalized, we reason that the
12 h. The reaction mixture was purified using silica gel hybrid MOF–PI film would combine the advantages of both
column chromatography (petroleum ether–ethyl ether (v/v), the PI and MOF to become more interesting and useful func-
5 : 1) and 0.0723 g (yield 94%) of 7a as a white solid was tional materials.
obtained. The yield of 7a was about 89% for 2-methylimidazole
1
as the catalyst. H NMR (400 MHz, CDCl3), δ: 7.91 (2H, d, J =
8.0 Hz), 7.78 (1H, s), 7.64 (1H, t, J = 8.0 Hz), 7.53 (2H, t, J =
8.0 Hz).
Acknowledgements
2-(4-Nitrobenzylidene)malononitrile 7b (yield, 91% for This work is supported by the National Natural Science Foun-
ZIF-8 and 88% for 2-methylimidazole) was obtained as a pale dation of China (20904052) and the State Key Laboratory of
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