Controlling ZIF-8 nano- and microcrystal formation and reactivity through zinc salt variations

Article abstract of DOI:10.1039/c3ce42485e

We report here a simple method for controlling the crystal size and morphology of zeolitic imidazolate framework-8 (ZIF-8) nanocrystals in methanol solution. ZIF-8 crystals were prepared by mixing 2-methylimidazole (Hmim) with various zinc salts for 1 h a

Full text of DOI:10.1039/c3ce42485e

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Controlling ZIF-8 nano- and microcrystal
formation and reactivity through zinc
salt variations†
Cite this: CrystEngComm, 2014, 16,
493
4
a
b
a
c
Aleksandra Schejn, Lavinia Balan, Véronique Falk, Lionel Aranda,
c
a
Ghouti Medjahdi and Raphaël Schneider*
We report here a simple method for controlling the crystal size and morphology of zeolitic imidazolate
framework-8 (ZIF-8) nanocrystals in methanol solution. ZIF-8 crystals were prepared by mixing
2-methylimidazole (Hmim) with various zinc salts for 1 h and using a Hmim/Zn(+2) salt molar ratio of 8/1.
All products prepared were assigned to a sodalite-type structure and both particle size and morphology
were found to be dependent on the reactivity of the Zn(+2) salt. Small ZIF-8 crystals with diameters varying
between ca. 50 and 200 nm were obtained with reactive zinc salts like Zn(acac)
Zn(ClO as demonstrated by SEM, TEM and DLS analyses. The use of ZnCl , Zn(OAc)
crystals with sizes varying between ca. 350 and 650 nm. Finally, the low reactive ZnBr
to generate microsized crystals. Taking ZIF-8 crystals prepared from Zn(NO ) , Zn(OAc) and ZnBr2 as
2
, Zn(NO
or ZnI
salt was found
3
)
2
, ZnSO
4
or
)
4 2
2
2
2
afforded
2
Received 5th December 2013,
Accepted 12th March 2014
3
2
2
representatives and through thermogravimetric analysis and BET measurements, we also demonstrated
that changes in particle size induced changes in stability and adsorption properties. The small sized ZIF-8
DOI: 10.1039/c3ce42485e
2
−1
crystals produced from Zn(NO
3 2
) were found to exhibit the highest surface area (1700 m g ) and the best
www.rsc.org/crystengcomm
catalytic activity in Knoevenagel and Friedländer reactions.
surface area and pore volume of classical MOFs but also
the high chemical and thermal stability of conventional
Introduction
1
0,13
Metal–organic frameworks (MOFs) are new microporous inor-
ganic–organic hybrid materials that exhibit tunable struc-
tures, low density and ultrahigh surface areas. These
materials may therefore have various potential applications
zeolites.
for gas storage and separation.
ZIF-8 (Zn(mim) , mim = 2-methylimidazole) is the most
Due to these features, ZIFs show great potential
14–18
2
representative ZIF material with
a sodalite zeolite-type
1
–3
4–6
7,8
in catalysis,
Among MOFs, the so-called zeolitic imidazolate frameworks
ZIFs) have recently attracted considerable attention. In these
hydrogen storage,
and CO2 adsorption.
topology, in which ZnN4 tetrahedra are connected through
imidazolate linkers forming cages 11.6 Å in diameter which
1
3
(
are accessible through a narrow six-ring pore (3.4 Å). ZIF-8
2
+
materials, metal atoms such as Zn are linked through N
has a high thermal stability (550 °C in N ), large surface area
2
−
2
−1 13
atoms by a ditopic imidazolate (im ) ligand to form neutral
(BET: 1630–1700 m g ), and a high chemical resistance to
9
–12
2+
−
19,20
frameworks.
Each Zn atom connects four im and the
various solvents.
All of these properties are outstanding
2
+
−
{
Zn (im ) } units are analogous to the {SiO } tetrahedra in
among MOFs, making ZIF-8 an ideal candidate for numerous
industrial applications. Nevertheless, to open the way for
applications such as sensors, catalysts or as materials for gas
separation, a real effort has to be made on shaping the ZIF-8
hybrid frameworks. Variation of morphology and size of
ZIF-8 crystals is of great importance because these para-
meters can markedly affect the functional properties and
performance of these nanomaterials. Moreover, controlling
the shape and size of ZIF nanoparticles may not only provide
a fundamental understanding of the crystallisation mecha-
nisms but could also improve structural and morphological
control in the synthesis of these novel porous materials.
However, these important issues have only scarcely been
addressed in the field of ZIF-8 nanomaterials.
2
2
zeolites and the Zn–im–Zn bond angle is similar to the Si–O–Si
bond angle (145°). Guest-free ZIFs have not only the large
a
Université de Lorraine, Laboratoire Réactions et Génie des Procédés (LRGP),
UMR 7274, CNRS, 1 rue Grandville, BP 20451, 54001 Nancy Cedex, France.
E-mail: raphael.schneider@univ-lorraine.fr; Tel: +33 3 83 17 50 53
b
Institut de Science des Matériaux de Mulhouse (IS2M), UMR 7361,
1
5 rue Jean Starcky, 68093 Mulhouse, France
c
Université de Lorraine, Institut Jean Lamour (IJL), UMR 7198, CNRS,
Vandoeuvre-lès-Nancy Cedex, BP 70239, 54506, France
†
Electronic supplementary information (ESI) available: Size distributions of
ZIF-8 crystals and XRD patterns of ZnO crystals produced by thermal decompo-
sition of ZIF-8. See DOI: 10.1039/c3ce42485e
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3
3
In recent years, different strategies have been adopted to
and for a duration of 1 h. As recently described, the
structural evolution as a function of time for ZIF-8 nano-
particles can be divided into three stages: (i) fast nucleation
(t < 10 min), (ii) growth stage (10 < t < 60 min) during
which crystallinity increases with time, and (iii) stationary
phase (t > 60 min) during which the crystallisation remains
constant but ZIF-8 nanoparticles transform into larger ones
through Ostwald ripening (a thermodynamically driven process
in which small particles disappear at the expense of growing
manipulate the morphology and size of ZIF-8 crystals. Varia-
tion of synthetic parameters including solvent, concentration
and molar ratio of reactants, temperature and duration can
2
1,22
markedly affect the size of the crystals.
An excess of the
bridging ligand relative to the zinc salt is generally employed
to increase the nucleation rate. Addition of sodium formate
that acts as a base in the deprotonation equilibrium of mim
in the reaction medium was found to influence the crystal
2
3,24
34
shape and size of ZIF-8 nanoparticles.
Surfactants such as
larger ones, which are energetically favored). Therefore, one
2
5
poly(diallyldimethylammonium chloride) or cetyltrimethyl-
may assume that the ZIF-8 structure is fully developed after a
reaction period of 1 h and the synthesis was then stopped.
Products were then collected by centrifugation and washed
with methanol several times. Syntheses were conducted with
Zn(NO ) , Zn(acac) , Zn(ClO ) , ZnSO , Zn(OAc) , ZnCl , ZnBr
3 2 2 4 2 4 2 2 2
and ZnI in order to gain insight into the reactivity and role
2
6
27
ammonium bromide
or the gelatin biopolymer
have
also been used as growth inhibitors to limit the ZIF-8 crystal
size or to tune the crystal morphology. Finally, the use of
auxiliary ligands such as amines or carboxylates acting as
competitors to the mim bridging ligand can also modulate
the crystallisation process and thus the particle size and
2
of the zinc counterion in changing the morphology and size
of ZIF-8 crystals.
2
1
morphology.
Variation of zinc salts is largely unexplored for ZIF nano-
crystals despite the advantages this offers in terms of simplicity
Using ZnSO4 or Zn(acac) , addition of a few droplets of
Hmim caused the immediate precipitation of ZIF-8. With
2
and processability. For (Zn
influence of the zinc precursor on the morphology and size of
nanocrystals produced was demonstrated by Biemmi et al.
Very recently, Torad et al. showed that replacing Zn(NO
4
O[(OOC)
2
C
6
H
4
]
3
) (MOF-5), the
Zn(ClO
of Hmim and precipitates appeared over time. With Zn(NO3)2
and ZnCl , the solution became cloudy within 2 min but
white precipitates were much dense with Zn(NO . With
4 2
) , the solution became turbid just after the addition
2
8
2
)
3 2
3 2
)
with Zn(OAc) markedly slowed down the generation speed of
ZIF-8 crystals, thus leading to large-sized nanoparticles.
these reactive salts, the large amount of deprotonated Hmim
2
2
9
−
(mim ) accelerates the formation of ZIF-8 via a coordination
−
The importance of the Zn(+2) precursor was also identified
by Zhu et al. who discovered that ZnO nanoneedles could
be produced during the synthesis of ZIF-8 crystals using
reaction of the Zn cations and mim . Using Zn(OAc)
2
or
ZnI , a slower formation rate of ZIF-8 was observed and the
2
2
colloidal solution formed within 2.5 min. Finally, with ZnBr ,
30
zinc carbonate as the zinc source. However, these studies
have not yielded more detailed information concerning the
influence of the zinc precursor on the size and morphology
of ZIF nanocrystals produced.
the solution remained transparent at ambient temperature for
at least 30 min and then precipitates appeared. These first
observations clearly indicate that the Zn precursors may signifi-
cantly accelerate or slow down the particle formation rate.
Each synthesis yielded pure-phase ZIF-8 crystals as
demonstrated by X-ray diffraction (XRD) patterns (Fig. 1).
Patterns generated by the ordered porous structure of the
ZIF-8 particles between 2θ values of 5 and 40° can be observed
Here we report a systematic investigation of the influence
of the zinc source on the morphology and size of ZIF-8
crystals. All reactions were conducted in methanol at room
temperature. Our results demonstrate that the reactivity of
the zinc precursor is able to direct the assembly of metal–
organic systems such as ZIF-8 nanocrystals and thus their
catalytic activity. The surface areas and the thermal stabilities
of the networks, determined using the Brunauer–Emmett–
Teller (BET) method and thermogravimetric analysis (TGA)
respectively, are also reported.
Results and discussion
Methanol is the most commonly used organic solvent for the
production of ZIF-8 nanocrystals and was therefore used in
2
1,31
the experiments.
Moreover, methanol only weakly inter-
acts with the ZIF-8 framework and can thus be removed
3
2
much easier than more polar solvents like DMF. The proce-
dure employs an excess of the bridging Hmim ligand with
respect to the Zn(+2) source to increase the nucleation rate
and works well at room temperature without the need of any
activation. All syntheses were conducted with a molar ratio of
Fig. 1 XRD patterns of ZIF-8 nanocrystals prepared with different zinc
precursors.
3
3
Zn/Hmim/MeOH of 1/8/559 in analogy to a previous report
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and the peak broadening observed indicates the formation of
nanosized crystals. The relative intensities and prominent
peak positions, including 011, 002, 112, 022, 013, and 222 are
1
3,31
in good agreement with previous reports,
confirming the
sodalite structure, which is the typical structure of ZIF-8,
and the well-defined peaks revealed high crystallinity. The
interplanar spacings calculated using Bragg's law from the
reflection at different Bragg's angles correspond to a body
centered cubic structure with a unit cell parameter of 17 Å
and are in accordance with those reported in the literature.
The correlation between the growth rate, size and
morphology was next evaluated using Scanning Electron
Microscopy (SEM) and Transmission Electron Microscopy
(TEM) (Fig. 2, 3 and S1, ESI†). The growth of ZIF-8 crystals is
known to evolve with time from cubes exposing 6 {100} faces
to intermediate shapes, and finally to rhombic dodecahedra
23
exposing 12 {110} faces, the latter being most likely the stable
equilibrium morphology of ZIF-8 (Fig. 4). With Zn(acac) , an
2
average size of 45 nm was estimated from SEM and TEM
images. Some isolated particles could be observed but the
sample mainly constitutes aggregates containing particles
with poorly resolved shapes (Fig. 2a and 3a). With Zn(NO ) ,
3
2
Fig. 3 TEM images of ZIF-8 crystals obtained after 1 h reaction using a
Hmim/Zn ratio of 8/1 and starting from (a) Zn(acac)
2 3 2
, (b) Zn(NO ) ,
(
c) ZnSO , (d) Zn(ClO , (e) Zn(OAc) , (f) ZnCl , (g) ZnI , and (h) ZnBr .
4
4
)
2
2
2
2
2
Fig. 4 Illustration of the crystal morphology evolution with time:
a) cube, (b) cube with truncated edges, (c) rhombic dodecahedron
(
with truncated corners, and (d) rhombic dodecahedron.
well-defined truncated rhombic dodecahedral crystals with
an average size of 141 ± 48 nm, which is the typical ZIF-8
morphology, were obtained (Fig. 2b and 3b). Small sized
(
211 ± 60 nm) truncated nanoparticles were obtained from
ZnSO . However, all of the crystals exhibit holes indicating
4
that they are in the stage of dissolution and that Ostwald
ripening has likely taken place (Fig. 2c and 3c). Uniformly
sized nanoparticles with an average diameter of 224 nm were
obtained using Zn(ClO ) . The crystals were rather spherical
4
2
and had bumpy surfaces but some angles could be observed
on some of them (Fig. 2d and 3d). Using ZnCl , larger crystals
of ca. 300 nm together with many considerably smaller ones
(ca. 150 nm) can be seen. Both the larger and smaller
2
Fig. 2 SEM images of ZIF-8 crystals obtained after 1 h reaction using a
Hmim/Zn ratio of 8/1 and starting from (a) Zn(acac) , (b) Zn(NO
c) ZnSO , (d) Zn(ClO , (e) Zn(OAc) , (f) ZnCl , (g) ZnI , and (h) ZnBr
2
3 2
) ,
(
4
4
)
2
2
2
2
2
.
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crystals exhibit cubic and rhombic shapes (Fig. 2f and 3f).
(Fig. 5). From 20 to 250 °C, the TGA curves exhibited only a
very weak weight loss of less than 0.5%, corresponding to the
removal of guest molecules (methanol or Hmim) and/or CO
2
from the cavities as indicated by MS analyses. A long plateau
was then observed after the formation of the guest-free phase
2
+
With these five reactive zinc precursors (soft Zn
acid
−
−
associated with hard bases such as NO
3
or ClO
4
), a huge
2
+
amount of Zn ions is available to coordinate with Hmim
and the number of nuclei generated by the complex forma-
tion is probably large resulting in a decrease in crystal size
and crystal anisotropy. This is further supported by the
maximum size of the crystals produced in these syntheses.
The maximum size decreases because a larger number of
nuclei can only grow to a smaller individual crystal size and
the growth is stopped due to a rapid decrease in supersatura-
Zn(mim)
2
until 350 °C, indicating good thermal stability of
the three-dimensional network for all samples, which is
1
3
comparable to that in the literature. Since nearly no weight
loss is seen by TGA until 350 °C before the onset of the exo-
thermic decomposition of the mim bridging ligand, it is
clear that the solvent or unreacted species have left the intra-
crystalline cavities during workup and that the synthetic
protocol used affords highly pure nanocrystals that do not
need solvent exchange and extensive drying for further use.
The most significant feature observed in the TGA results is
the size-dependent thermal stability up to 300 °C in the case
of the smaller 141 nm sized nanocrystals prepared from
tion. With Zn(OAc)
rhombic dodecahedron shape and their mean size is ca.
00 nm. The images show also that the particles have good
uniformity, well-defined facets and sharp edges and corners
Fig. 2e, 3e and Fig. 2g, 3g). It is also worth mentioning that
the hydration degree of Zn(OAc)2 (anhydrous Zn(OAc)2 vs.
Zn(OAc) ·2H O) has no effect on the size and morphology of
ZIF-8 crystals produced (data not shown). The largest nano-
crystals, ca. 1050 nm in size, were observed with ZnBr
Fig. 2h and 3h). Separated large grains with a rhombic
2 2
and ZnI , the particles had the typical
5
(
2
2
3 2
Zn(NO ) , and up to 390 °C in the case of the larger 1050 nm
sized crystals prepared from ZnBr . A sharp weight loss step
2
2
of ca. 63–64% was observed for all nanocrystals upon further
increasing the temperature from 400 to 500 °C, indicating
thermal decomposition of ZIF-8 nanocrystals in that tempera-
ture range. This weight loss is in good agreement with the
theoretical weight loss of 64% and indicates that zinc oxide
[ZnO, Fw = 81] was formed as the final calcination product
(
dodecahedron shape were obtained together with cubes. All
of the particles have well-defined facets and sharp edges
and corners. For this zinc precursor with lower reactivity
2+
−
(
soft Zn acid associated with the soft Br base), the density
of nuclei is reduced and nanoparticles grow through further
direct addition of single monomeric mim and solvated Zn
ion species until the framework is formed.
The particle size was further studied by dynamic light
scattering (DLS). The DLS-determined mean particle size values
of 85, 192, 247, 312, 480, 388, 589 and 1160 nm obtained for
2
of ZIF-8 nanocrystals [Zn(mim) ; Fw = 229] (see Fig. S3 in
−
2+
the ESI† for the XRD pattern of the product obtained, which
is in good accordance with the ZnO reference pattern, JCPDS
card no 79-2205).
The N
2
sorption properties of ZIF-8 samples prepared with
Zn(NO ) , Zn(OAc) , and ZnBr were next investigated (Fig. 6).
3
2
2
2
ZIF-8 crystals prepared from Zn(acac)
Zn(ClO , Zn(OAc) , ZnCl , ZnI and ZnBr
2
, Zn(NO
3
)
2
, ZnSO
4
,
It was found that they exhibited typical type-I adsorption
isotherms with a high level of N adsorption, indicating the
)
4 2
2
2
2
2
, respectively, are in
2
relatively good agreement with the TEM and SEM determined
values (Fig. S2, ESI†).
microporous nature of the crystals. This was confirmed by
the increase in the volume adsorbed at low relative pressures
Thermogravimetric analyses coupled to mass spectrometry
0
(P/P < 0.08). The specific surface area and micropore
(
TGA/MS) were conducted on three representative samples
prepared from Zn(NO , Zn(OAc) and ZnBr to gain infor-
mation about the thermal stability of ZIF-8 nanocrystals
volume of the ZIF-8 crystals prepared from Zn(NO ) were
1700 ± 30 m g and 0.66 cm g , values in perfect accor-
3
2
2
−1
3
−1
3
)
2
2
2
2
−1
dance with those in the literature (SBET = 1700 m
g
and
3
−1 13
Vmicro = 0.64 cm g ). Similar results were obtained with
2
−1
crystals prepared from ZnBr
V
2
(SBET = 1713 ± 45 m
micro = 0.63 cm g ). The BET surface areas of nanocrystals
and pore volumes prepared from Zn(OAc) appear lower than
g
and
3
−1
2
those prepared from Zn(NO
3
3
)
2
and ZnBr
2
(SBET = 1477 ±
6 m g and Vmicro = 0.55 cm g ). The adsorption–desorption
hysteresis loop of N2 near P/P = 1, which originates from
2
−1
3 −1
0
3
5
interparticle mesopores, is consistent with the interparticle
voids observed by SEM and demonstrates the dual micro-
and mesoporosity of ZIF-8 nanoparticles. It can further be
observed that the adsorbed N
increased for ZIF-8 crystals prepared with Zn(NO )
2
amount near P/P
0
= 1
and
3 2
Zn(OAc)2 compared to the sample prepared from ZnBr2.
These results suggest that the interparticle porosity between
ZIF-8 crystals produced from Zn(NO ) and Zn(OAc) increases
3 2 2
due to the decrease in particle size and confirms the SEM
and TEM results (vide supra).
Fig. 5 TGA curves of ZIF-8 nanocrystals prepared from Zn(NO
black line), Zn(OAc) (red line), and ZnBr (blue line).
3 2
)
(
2
2
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Fig. 7 Catalytic activities of ZIF-8 crystals prepared from different zinc
sources in the Knoevenagel reaction.
Fig. 8 Time-dependent conversion plots for the Knoevenagel reaction
between 4-bromobenzaldehyde and malononitrile catalyzed by ZIF-8
crystals (reactions were conducted in toluene at room temperature).
3 2
catalyzed by ZIF-8 crystals produced from Zn(NO ) . With
ZIF-8 crystals prepared from Zn(OAc)2 and ZnBr , the reac-
2
tion proceeded with more difficulty, though conversions of
82 and 79% were observed after 30 min, thus indicating the
importance of the ZIF-8 crystal size in the reaction. It should
also be noted that significantly longer reaction times (5 h)
were required for the reaction using the latter catalysts.
The catalytic performance of the different-sized ZIF-8
crystals was next evaluated in the Friedländer reaction
between 2-aminobenzophenone and acetylacetonate. The
Friedländer reaction is an important synthetic approach to
Fig. 6 Nitrogen sorption isotherms measured at 77 K on the powder
ZIF-8 crystals obtained from (a) Zn(NO ) , (b) Zn(OAc) , and (c) ZnBr .
3 2 2 2
3
9,40
Black and red data correspond to the adsorption and desorption
branches, respectively.
prepare quinoline derivatives,
heterocycles possessing a
41
wide range of pharmaceutical and biological activities.
Cu(BDC) and Cu(BTC) MOFs (BDC = 1,4-benzenedicarboxylic
acid, BTC = 1,3,5-benzenetricarboxylic acid) have already
been demonstrated to be efficient catalysts for the Friedländer
In the last set of experiments, we evaluated the cata-
lytic properties of ZIF-8 crystals produced from Zn(NO ) ,
4
2–45
reaction.
As can be seen in Fig. 9, the 2-quinolone
3
2
Zn(OAc)
2
,
and ZnBr
2
.
A
Knoevenagel reaction between
derivative can be obtained in moderate to excellent yields using
ZIF-8 crystals as catalysts and the highest catalytic activity
4-bromobenzaldehyde and malononitrile, which is well-
3
6–38
established for ZIF-8 crystals, was first studied.
Conden-
sation was conducted for 5 h at room temperature and
isolated yields of 2-(4-bromobenzylidene)malononitrile are
reported in Fig. 7. As can be seen, the reaction worked effec-
tively with the three catalysts but was only complete with
3 2
ZIF-8 crystals produced from Zn(NO ) .
Fig. 8 shows the conversion of 4-bromobenzaldehyde
when reacting with malononitrile in the presence of the
different-sized ZIF-8 crystals as a function of time. As can be
seen, condensation proceeds very quickly at room tempera-
ture and is almost quantitative (96%) after 30 min when
Fig. 9 Catalytic activities of ZIF-8 crystals prepared from different zinc
sources in the Friedländer reaction.
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was as previously observed for the smallest ZIF-8 crystals
prepared from Zn(NO ) .
In both catalytic experiments, the conversion of the starting
materials increased over ZIF-8 crystals in the following
Scanning electron microscopy (SEM) pictures were prepared
using a JEOL scanning electron microscope JSM-6490 LV. The
ZIF-8 material prepared from zinc acetylacetonate was viewed
on an Isopore™ membrane (Merck Millipore) with 0.2 μm
pore size. The X-ray powder diffraction (XRD) diagrams of all
samples were measured using a PANalytical X'Pert Pro MPD
diffractometer using Cu Kα radiation. The X-ray powder
diffraction data were collected using an X'Pert MPD diffracto-
meter (PANalytical AXS) with a goniometer radius of 240 mm, a
fixed divergence slit module (1/2° divergence slit, 0.04 rad
Soller slits) and an X'Celerator as a detector. The powder
samples were placed on a silicon zero-background sample
holder and the XRD patterns were recorded at room temper-
3
2
sequence: ZIF-8 from Zn(NO3)2 > ZIF-8 from Zn(OAc)2
>
ZIF-8 from ZnBr . The reduced catalytic activity with the
2
decrease in the external surface area of the ZIF-8 crystals is
probably linked to the increase in adsorption rates of nano-
crystals compared to microcrystals, as recently observed with
4
6
metal–organic framework nanofibers, and to the increasing
concentration of accessible Lewis acid and basic sites at the
surface of the smallest nanoparticles.
Conclusions
ature using Cu K radiation (λ = 0.15418 nm). DLS measure-
α
ments were performed on a ZEN 3600 Zetasizer Nano ZS.
The solid materials were redispersed in methanol by sonica-
tion and transferred to quartz cuvettes through a syringe.
The textural properties of the materials were investigated
using a Micromeritics ASAP 2420 instrument using liquid
nitrogen (−196 °C). Prior to analyses, the samples were out-
gassed overnight in vacuum at 40 °C on the degassing port
followed by 4 h of out-gassing on the analysis port. The
resulting isotherms were analysed using the BET (Brunauer–
In conclusion, we demonstrated that phase pure ZIF-8 crystals
can be produced from Hmim and various zinc sources in
methanol at room temperature. The reactivity of the zinc salt
in the growth solution was found to markedly affect the size
and morphology of ZIF-8 particles produced after 1 h reaction
and at the same concentration of the zinc salt. Small ZIF-8
nanocrystals with diameters varying between ca. 50 and
2
00 nm were obtained with reactive zinc salts like Zn(acac)
Zn(NO , ZnSO or Zn(ClO . The use of ZnCl , Zn(OAc) or
ZnI afforded crystals with sizes varying between ca. 350 and
2
,
3
)
2
4
4
)
2
2
2
Emmett–Teller) method while the micropore volume (Vmicro
)
2
was determined using the Horvath–Kawazoe (HK) equation.
Thermogravimetric measurements were performed on a
SETARAM Setsys Evolution thermoanalyzer coupled with an
OmniStar GSD301C-Pfeiffer Vacuum mass spectrometer.
Samples were filled into platinum crucibles and heated in
6
2
50 nm. Finally, the use of the low reactive ZnBr was found
to generate microsized crystals. These significant changes in
particle size induced distinctive changes in the adsorption
properties as demonstrated by BET measurements but also in
the catalytic performance of ZIF-8 crystals in Knoevenagel
and Friedländer condensation reactions used as models.
−1
flowing air with a ramp of 5 °C min from room tempera-
1
ture up to 800 °C. H NMR spectra were recorded in CDCl
3
3 2
The small sized crystals produced from Zn(NO ) exhibited
using a 300 MHz spectrometer (Avance 300, Bruker, Bremen,
Germany). Chemical shift values were reported in ppm
relative to the residual peak of the solvent.
the highest surface area and the best catalytic activity.
Our results should open new opportunities to control the
reactivity of ZIF-8 crystals in applications such as films and
membranes.
Synthesis of ZIF-8 crystals
2
+
Solutions of the Zn salt (1 mmol) and Hmim (660 mg,
mmol) in 11.3 mL of methanol were prepared separately.
Then, in a three-neck-flask, the two solutions were mixed
by dropwise addition of the Hmim solution to the Zn salt
solution. Synthesis was conducted under nitrogen flow at
room temperature with stirring for 1 h. The ZIF-8 crystals
were separated by centrifugation (4000 rpm, 15 min) and
washed with methanol (3 × 30 mL). The nanocrystals were
dried in air overnight before analysis.
Experimental
Materials
8
2
+
The chemicals used in the experiments were zinc acetate
dihydrate (98+%, Aldrich), anhydrous zinc acetate (99.99%,
Aldrich), zinc bromide (98%, Riedel de Haën), zinc chloride
(
98%, Aldrich), zinc nitrate hexahydrate (98%, Aldrich), zinc
perchlorate hexahydrate (Aldrich), zinc sulphate heptahydrate
99%, Merck), 2-methylimidazole (Hmim, 99%, Aldrich),
-bromobenzaldehyde (99%, Aldrich), malononitrile (99%,
Aldrich), ethyl acetoacetate (99%, Aldrich), 2-aminobenzophenone
(
4
Catalytic experiments
(
(
98%, Aldrich), toluene (99%, Prolabo), and methanol
≥99.9%, Aldrich).
The ZIF-8 samples were activated by treatment of the pow-
ders at 200 °C in a programmable oven for 6 h, and then
cooled to room temperature naturally. The vials containing
the ZIF-8 powders were tightly capped and stored at room
temperature before use.
Knoevenagel condensation. 4-Bromobenzaldehyde (0.0925 g,
0.5 mmol) was dissolved in 5 mL of toluene, the ZIF-8 catalyst
(9.2 mg, 0.04 mmol) was added and the mixture was stirred for
General information
Transmission electron microscopy (TEM) images were taken
by placing a drop of the ZIF-8 particles in methanol onto a
carbon film-supported copper grid. Samples were studied
using a Philips CM20 instrument operating at 200 kV.
4498 | CrystEngComm, 2014, 16, 4493–4500
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5
min. Malononitrile was then injected (0.189 mL, 3 mmol)
13 K. S. Park, Z. Ni, A. P. Côté, J. Y. Choi, R. D. Huang,
F. J. Uribe-Romo, H. K. Chae, M. O'Keeffe and O. M. Yaghi,
Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 10186.
14 H. Wu, W. Zhou and T. Yildirim, J. Am. Chem. Soc., 2009,
131, 4995.
15 J. Pérez-Pellitero, H. Amrouche, F. R. Siperstein,
G. Pirngruber, C. Nieto-Draghi, G. Chaplais, A. Simon-Masseron,
D. Bazer-Bachi, D. Peralta and N. Bats, Chem. – Eur. J., 2010,
16, 1560.
and the reaction was conducted with bubbling of Ar gas
through the mixture for 5 h at room temperature. After
centrifugation and concentration of the toluene phase, the
crude reaction mixture was purified using silica gel column
chromatography (petroleum ether–ethyl acetate (v/v), 95 : 5)
1
1
and the product was analyzed by
H
NMR.
H NMR
(CDCl , 300 MHz): δ 7.74 (d, J = 8.7 Hz, 2H), 7.73 (s, 1H),
3
7.67 (d, J = 8.7 Hz, 2H).
Friedländer condensation. 2-Aminobenzophenone (82.1 mg,
.42 mmol) and ethyl acetoacetate (81.3 mg, 0.62 mmol) were
16 S. R. Venna and M. A. Carreon, J. Am. Chem. Soc., 2010,
132, 76.
0
added to ZIF-8 crystals (16 mg, 0.069 mmol) dispersed in 2 mL
of toluene. The mixture was heated at 90 °C and the progress
of the reaction was monitored by TLC. After cooling, the
reaction mixture was centrifuged and the toluene phase was
concentrated. The crude product was purified using silica
gel chromatography (petroleum ether–ethyl acetate (v/v),
17 R. Krishna and J. M. van Baten, J. Membr. Sci., 2010,
360, 323.
18 A. S. Huang, H. Bux, F. Steinbach and J. Caro, Angew. Chem.,
Int. Ed., 2010, 49, 4958.
19 P. Küsgens, M. Rose, I. Senkovska, H. Fröde, A. Henschel,
S. Siegle and S. Kaskel, Microporous Mesoporous Mater.,
2009, 120, 325.
1
1
9
5 : 5) and the product was analyzed by H NMR. H NMR
(
(
CDCl , 300 MHz): δ 12.05 (s, NH), 7.58–7.13 (m, 9H), 2.31
s, 3H).
20 K. A. Cychosz and A. J. Matzger, Langmuir, 2010, 26, 17198.
21 J. Cravillon, R. Nayuk, S. Springer, A. Feldhoff, K. Huber and
M. Wiebcke, Chem. Mater., 2011, 23, 2130.
3
2
2 J. Cravillon, C. A. Schröder, R. Nayuk, J. Gummel, K. Huber
Acknowledgements
and M. Wiebcke, Angew. Chem., 2011, 123, 8217.
This work was supported by the ICCEL and MICA Carnot
Institutes. We thank Jean-François Remy (LRGP, CNRS) for
SEM analyses and Anne Sapin-Minet (EA 3452, Cithéfor,
Université de Lorraine) for DLS measurements.
23 J. Cravillon, C. A. Schröder, H. Bux, A. Rothkirch, J. Caro and
M. Wiebcke, CrystEngComm, 2012, 14, 492.
24 M. Shah, H. T. Kwon, V. Tran, S. Sachdeva and H.-K. Jeong,
Microporous Mesoporous Mater., 2013, 165, 63.
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5 S. K. Nune, P. K. Thallapally, A. Dohnalkova, C. Wang, J. Liu
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