Iridium nanoparticles stabilized by metal organic frameworks (IrNPs@ZIF-8): Synthesis, structural properties and catalytic performance

Article abstract of DOI:10.1039/c2dt31779f

Today, the synthesis of well-defined metal nanoparticles stabilized by the metal-organic frameworks (MOFs), which provide high specific surface areas, tunable pore sizes, and guest interactable organic linkers, and the discovery of their unique properties are still challenging goals. The chemically robust zeolitic imidazole framework (ZIF) is a subclass of MOF. In this study, the microporous sodalite-like ZIF-8 (Zn(MeIM); MeIM = 2-methylimidazole) was selected as host matrix to stabilize guest iridium nanoparticles (IrNPs). The iridium loading was achieved via gas phase infiltration of Ir(COD)(MeCp) (methylcyclopentadienyl)(1,5-cyclooctadiene)iridium(i)) precursor followed by hydrogenolysis of the inclusion compound Ir(COD)(MeCp)@ZIF-8 to form the IrNPs@ZIF-8. The characterization of IrNPs@ZIF-8 by ICP-MS, P-XRD, XPS, ¹³C MAS NMR, TEM, HRTEM, STEM, STEM-EDX, HAADF-STEM, DR-UV-vis, EA analyses and N₂-adsorption-desorption techniques reveal the formation of well-dispersed iridium nanoparticles (3.3 ± 1.7 nm) within the framework of ZIF-8 (IrNPs@ZIF-8) by keeping the host framework intact. The catalytic application of IrNPs@ZIF-8 in terms of activity, selectivity, reusability and durability was demonstrated in the hydrogenation of cyclohexene and phenylacetylene under mild conditions, in which they were found to be highly active catalysts. Moreover, they show great durability against sintering and leaching throughout the catalytic runs that make them highly reusable catalysts. They retain their inherent catalytic activity even at the fifth catalytic run in the hydrogenation of cyclohexene and phenylacetylene.

Full text of DOI:10.1039/c2dt31779f

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PAPER
Iridium nanoparticles stabilized by metal organic frameworks
(IrNPs@ZIF-8): synthesis, structural properties and catalytic performance†
Mehmet Zahmakiran*
Received 4th June 2012, Accepted 21st August 2012
DOI: 10.1039/c2dt31779f
Today, the synthesis of well-defined metal nanoparticles stabilized by the metal–organic frameworks
(MOFs), which provide high specific surface areas, tunable pore sizes, and guest interactable organic
linkers, and the discovery of their unique properties are still challenging goals. The chemically robust
zeolitic imidazole framework (ZIF) is a subclass of MOF. In this study, the microporous sodalite-like
ZIF-8 (Zn(MeIM); MeIM = 2-methylimidazole) was selected as host matrix to stabilize guest iridium
nanoparticles (IrNPs). The iridium loading was achieved via gas phase infiltration of Ir(COD)(MeCp)
(methylcyclopentadienyl)(1,5-cyclooctadiene)iridium(I)) precursor followed by hydrogenolysis of the
inclusion compound Ir(COD)(MeCp)@ZIF-8 to form the IrNPs@ZIF-8. The characterization of
IrNPs@ZIF-8 by ICP-MS, P-XRD, XPS, 13C MAS NMR, TEM, HRTEM, STEM, STEM-EDX,
HAADF-STEM, DR-UV-vis, EA analyses and N2-adsorption–desorption techniques reveal the formation
of well-dispersed iridium nanoparticles (3.3 1.7 nm) within the framework of ZIF-8 (IrNPs@ZIF-8) by
keeping the host framework intact. The catalytic application of IrNPs@ZIF-8 in terms of activity,
selectivity, reusability and durability was demonstrated in the hydrogenation of cyclohexene and
phenylacetylene under mild conditions, in which they were found to be highly active catalysts. Moreover,
they show great durability against sintering and leaching throughout the catalytic runs that make them
highly reusable catalysts. They retain their inherent catalytic activity even at the fifth catalytic run in the
hydrogenation of cyclohexene and phenylacetylene.
confined void spaces such as the zeolites,6 metal oxides,7 poly-
Introduction
mers,8 carbonaceous materials,9 and minerals10 have already rep-
The synthesis of metal nanoparticles with controllable size and
size distribution is of great importance because of their potential
applications in many fields,1 including catalysis.2,3 However, the
aggregation of nanoparticles ultimately to the bulk metal despite
using the best stabilizing agents,4,5 in their synthesis route is still
the most important problem that should be overcome in their cat-
alytic application as their agglomeration into bulk metal leads to
a significant decrease in their catalytic activity. In this context,
generation of nanoparticles in matrices of solid materials with
resented an especially promising direction for preventing
aggregation. Additionally, their well-defined pores can act as a
scaffold to direct the formation of nanoparticles with tunable
size and uniform shape, which are important factors for the
nanocatalyst.2,3 Besides these aforementioned support materials,
more recent studies11 have also shown that metal-organic frame-
works, which are highly crystalline hybrid materials that
combine metal ions with rigid organic ligands and have emerged
as an important class of porous materials,12 are also to be con-
sidered as suitable host materials to stabilize guest ligand-free
metal nanoparticles.11 They allow more flexible and systematic
modification of the pore structure by the proper selection of the
structural subunits and their connected ways,11 these are the
important advantages of the employment of MOFs as host
materials for guest metal nanoparticles with respect to silica
based microporous and mesoporous materials. Moreover, the
stabilization of metal nanoparticles within the structure of MOFs
can help us in the kinetic controlling of the catalytic reactions.
This study has been focused on the synthesis, characterization
and the catalytic use of iridium nanoparticles stabilized by ZIF-8
framework. The zeolitic imidazole framework (ZIF-8, Zn-
(MeIM)2, MeIM = 2-methylimidazole) has been selected as a
representative microporous metal–organic framework (MMOFs)
support material, because of its intersecting three-dimensional
Department of Chemistry, Yüzüncü Yıl University, 65080 Van, Turkey.
E-mail: zmehmet@yyu.edu.tr
†Electronic supplementary information (ESI) available: Fig. ESI-1.
TEM images of (a) IrNPs@ZIF-8 and (b) plain ZIF-8 (scale bars equal
to 10 nm); Fig. ESI-2. (a) HAADF-STEM image of IrNPs@ZIF-8 and
its elemental mapping collected from the selected region given in (a) for
(b) Cu, (c) Zn, and (d) Ir; Fig. ESI-3. Plot of the concentration of cyclo-
hexene (M) versus time (h) for the hydrogenation of neat cyclohexene
(1.0 mL, 9.8 mmol) catalyzed by IrNPs@ZIF-8 (100 mg with iridium
loading of 1.5% wt corresponds to 7.8 μmol) at 25 0.1 °C with 3 bar
initial H2 pressure; Fig. ESI-4. TEM images of IrNPs@ZIF-8 containing
(a) 2.3 wt% (b) 3.8 wt% Ir; Fig. ESI-5. Initial TOF values vs. catalytic
runs determined for the IrNPs@ZIF-8-catalyzed hydrogenation of cyclo-
hexene and phenylacetylene; initial TOF values were determined at con-
version levels of ≤20%. Fig. ESI-6. TEM images and PXRD patterns of
IrNPs@ZIF-8 isolated from the fifth catalytic run in the hydrogenation
of cyclohexene and phenylacetylene. See DOI: 10.1039/c2dt31779f
This journal is © The Royal Society of Chemistry 2012
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pore structure, high thermal stability (over 500 °C) and large
surface area (∼1400 m2 g−1).13 To date it has already been
demonstrated that ZIF-8 can act as a suitable host material for
Au,14 Ag,15 and Ni16 nanoparticles. In deciding, the active metal
as guest to host ZIF-8 framework iridium was selected on
account of its remarkable activity in various organic
transformations.17
Teflon coated magnetic stir bars were cleaned with acetone, fol-
lowed by copious rinsing with distilled water before drying in an
oven at 150 °C.
Preparation of Ir(COD)(MeCp)@ZIF-8: gas phase infiltration of
Ir(COD)(MeCp) into ZIF-8 framework
Iridium nanoparticles stabilized by ZIF-8 framework
(IrNPs@ZIF-8) were prepared by following the procedure com-
prising of (i) gas phase infiltration of Ir(COD)(MeCp) ((methyl-
Prior to iridium precursor (Ir(COD)(MeCp)) loading, ZIF-8 was
immersed in methanol under ambient conditions for 48 h. Then,
ZIF-8 powders were isolated by suction filtration and the isolated
ZIF-8 sample was evacuated at room temperature for over 8 h,
then at 250 °C for 4 h. In a typical experiment, a sample of 1.0 g
of dry, activated ZIF-8 powder and 110 mg (0.29 mmol) of
Ir(COD)(MeCp) were placed in two separate glass vials in a
Schlenk tube. The tube was then evacuated for 30 min, sealed,
and kept at 40 °C for 18 h under a static vacuum. A light yellow
composite material denoted as Ir(COD)(MeCp)@ZIF-8 was
obtained and stored in the glovebox afterward. Elemental and
ICP-MS analyses for Ir(COD)(MeCp)@ZIF-8 revealed (wt%):
Ir, 1.5; C, 41.8; H, 4.5; N, 22.5.
cyclopentadienyl)(1,5-cyclooctadiene)iridium(I)), which is
a
well-known iridium precursor for metal–organic chemical vapor
deposition (MOCVD),18 into ZIF-8 framework to yield Ir(COD)-
(MeCp)@ZIF-8, (ii) followed by hydrogenolysis of Ir(COD)-
(MeCp)@ZIF-8 at elevated temperature to form IrNPs@ZIF-8.
The characterization of the resulting material by inductively
coupled plasma mass spectroscopy (ICP-MS), powder X-ray dif-
fraction (P-XRD), X-ray photoelectron spectroscopy (XPS),
transmission electron microscopy (TEM), high resolution-TEM
(HRTEM), scanning transmission electron microscopy (STEM),
STEM modulated energy dispersive X-ray spectroscopy
(STEM-EDX), high-angle annular dark-field STEM (HAADF-
STEM), diffuse-reflectance UV-visible spectroscopy (DR-UV-
vis), NMR spectroscopy, elemental analyses (EA) and N2-
adsorption–desorption technique is revealing the formation of
Preparation of IrNPs@ZIF-8: hydrogenolysis of the inclusion
compound Ir(COD)(MeCp)@ZIF-8 at elevated temperature
250 mg Ir(COD)(MeCp)@ZIF-8 was taken into the quartz boot
and put into the oven and heated to 300 °C with a heating rate of
2 °C min−1. The hydrogenolysis of Ir(COD)(MeCp)@ZIF-8 was
done by the treatment of Ir(COD)(MeCp)@ZIF-8 with H2 (10%
H2: 90% N2) at 300 °C for 3 h. The light yellow color of
Ir(COD)(MeCp)@ZIF-8 changed into gray at the end of the
hydrogenolysis. The resulting material was then immersed into
methanol for 24 h to dissolve out cyclooctane and methylcyclo-
pentane. Finally, the IrNPs@ZIF-8 sample was dried under
vacuum at room temperature.
well-dispersed 3.3
1.7 nm iridium nanoparticles within a
ZIF-8 matrix (IrNPs@ZIF-8) without distorting the host material
framework. The catalytic performance of these new iridium
nanoparticles in terms of activity, reusability and durability has
been demonstrated in the hydrogenation of cyclohexene in the
solvent-free system under mild conditions (25 0.1 °C and with
3 bar initial H2 pressure). They provide high activity (initial TOF
= 370 h−1) in the hydrogenation of neat cyclohexene to cyclo-
hexane at complete conversion. In addition to the catalytic
reduction of cyclohexene, the hydrogenation of phenylacetylene
in the presence of IrNPs@ZIF-8 was also performed to analyze
the selectivity of IrNPs@ZIF-8. It was found that IrNPs@ZIF-8
can catalyze the hydrogenation of phenylacetylene to styrene
with high selectivity (≥90%) at complete conversion (100%)
under mild conditions (40 0.1 °C and with 3 bar initial H2
pressure).
IrNPs@ZIF-8 catalyzed hydrogenation of cyclohexene
All reaction mixtures were prepared in a nitrogen-filled oxygen-
free glovebox (O2 < 5 ppm, H2O < 1 ppm) following the pro-
cedure described elsewhere.19 In a glovebox 100 mg IrNPs@
ZIF-8 catalyst (with an iridium loading of 1.5% wt
corresponding to 7.8 μmol of Ir) was weighed into a 40 mm ×
20 mm borosilicate culture tube containing a 5/16 in. × 5/8 in. A
Teflon coated magnetic stir bar, and then 1.0 mL cyclohexene
(9.8 mmol) was added with a gastight syringe. The culture tube
was then placed inside the Fischer–Porter (FP) pressure bottle.
The FP bottle was sealed, brought outside the drybox, placed
inside a constant temperature circulating water bath at 25
0.1 °C, and connected via Swagelock TFE-sealed quick-con-
nects to the hydrogenation line which had already been evacu-
ated for at least 30 min to remove any trace oxygen and water
present and then refilled with purified H2 at 3 bar. Then, the FP
bottle was purged 10 times (15 s per purge, with stirring at >600
rpm). The pressure in the FP bottle was then set to a constant
3 bar H2, and the reaction was started. The reaction was followed
by monitoring the pressure change in the FP bottle via an
RS-232 module and using a Lab View 8.0 program on a PC. The
pressure vs. time data were processed using Microsoft Office
More importantly, IrNPs@ZIF-8 were found to be highly
stable against to leaching and sintering throughout the catalytic
runs, which make them highly reusable catalysts so that they
retain their inherent activity even at the 5th catalytic run in the
hydrogenation of cyclohexene and phenylacetylene.
Experimental
Materials
ZIF-8 (Zn(MeIM)2(DMF)(H2O)3, MeIM = 2-methylimidazole,
DMF = dimethylformamide), cyclohexane, cyclohexene, and
d-chloroform (CDCl3), phenylacetylene and styrene were pur-
chased from Sigma-Aldrich. Ir(COD)(MeCp) ((MeCp = methyl-
cyclopentadienyl)(COD = 1,5-cyclooctadiene)iridium(I)) was
purchased from Strem Chemicals. Cyclohexane and cyclohexene
were distilled over sodium under argon atmosphere and stored in
the glovebox (O2 < 5 ppm, H2O < 1 ppm). All glassware and
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Excel 2003 and Origin 7.0. Reaction rates were determined from
the slope of the linear portion of the H2 uptake curve. The com-
plete hydrogenation of cyclohexene to cyclohexane was also
confirmed by H NMR and GC analysis of the reaction solution
taken at the end of the reaction.
(MeCp)@ZIF-8 at 270 nm can be assigned to the ligand to
metal charge transfer transition as previously reported for olefin-
Ir(I) complexes.21 Upon hydrogenolysis of Ir(COD)(MeCp)-
@ZIF-8, UV-vis spectrum of IrNPs@ZIF-8 became identical
with that of the host material ZIF-8 indicative of the complete
hydrogenolysis of iridium(I) precursor within the framework
of ZIF-8.
1
IrNPs@ZIF-8 catalyzed hydrogenation of phenylacetylene
In addition to UV-vis spectroscopy, the structural changes
throughout the inclusion of Ir(COD)(MeCp) and the hydrogeno-
lysis Ir(COD)(MeCp)@ZIF-8 were also investigated by 13C
MAS NMR spectroscopy. Fig. 2 shows 13C MAS NMR spectra
of ZIF-8, Ir(COD)(MeCp)@ZIF-8 and IrNPs@ZIF-8 samples.
Three signals are observed in ZIF-8; the signal at about 150 ppm
can be assigned to the carbon atom between the two nitrogen
atoms in the imidazole-ring. The other signals observed at
120 ppm and 10 ppm correspond to CH- and CH3-groups of the
2-methylimidazole, respectively.22 The 13C MAS NMR spec-
trum of the inclusion compound Ir(COD)(MeCp)@ZIF-8 given
in Fig. 2 (middle) shows the signals of ZIF-8 along with the
characteristic signals of Ir(COD)(MeCp) observed at 43 ppm
(vCH in COD), 32 ppm (–CH2 in COD) and 79 ppm (Cp). The
methyl signal of MeCp could not be observed, which is most
probably overlapped with the methyl signal of 2-methylimida-
zole. This result is in good accordance to the literature-known
13C resonances of the analogous Ir(COD)(Cp) complex,23 confi-
rming the unchanged nature of the Ir precursor. The 13C MAS
NMR spectrum of the sample taken after hydrogenolysis and
purification (methanol washing followed by drying to remove all
volatile by products) is also given in Fig. 2 (top) that reveals
only the characteristic resonances of the ZIF-8 and no hydrogen-
ated linker (2-methylimidazole) could be detected. The crystalli-
nity of the host ZIF-8 framework throughout the formation of
both inclusion compound Ir(COD)(MeCp)@ZIF-8 and IrNPs@
ZIF-8 was investigated by PXRD. The PXRD patterns of ZIF-8,
The catalytic hydrogenation of phenylacetylene (1.0 mL,
8.9 mmol) in methanol (10.0 mL) was performed by following
the aforementioned experimental protocol at 40 0.1 °C and
with 3 bar initial H2 pressure. Samples were removed at regular
intervals and diluted to a 1 : 100 ratio with methanol. The liquid
reactants and products were analyzed by gas chromatography
(GC). The catalytic selectivity was characterized by considering
the selectivity toward styrene.
Results and discussion
In the preparation of IrNPs@ZIF-8, Ir(COD)(MeCp), which has
successfully been used before as an iridium precursor not only in
metal–organic chemical vapor deposition (MOCVD) tech-
niques18 for iridium coatings but also in the synthesis of ligand-
stabilized iridium colloids,20 was selected as the iridium precur-
sor because of its high stability under air and low sublimation
temperature. The loading of Ir(COD)(MeCp) was performed
under a static vacuum at 40 °C for 18 h in order to ensure
adsorption equilibrium and a homogeneous distribution of the
iridium complex throughout the ZIF-8 framework. Upon loading
with Ir(COD)(MeCp), the color of ZIF-8 powder turned from
white to light yellow, which was then changed into to gray upon
treatment with H2 at 300 °C. This color change reflects the incor-
poration of Ir(COD)(MeCp) into ZIF-8 framework then its
reduction to form Ir(0). Monitoring the UV-vis spectra of the
solid samples taken at each step provides a convenient way to
follow this conversion.
The UV-vis absorption spectra of Ir(COD)(MeCp)@ZIF-8
and IrNPs@ZIF-8 are given in Fig. 1 along with that of the host
material ZIF-8. The notable strong band observed for Ir(COD)-
Fig. 1 UV-vis absorption spectra of ZIF-8, Ir(COD)(MeCp)@ZIF-8
Fig. 2 13C MAS NMR spectra of ZIF-8, Ir(COD)(MeCp)@ZIF-8 and
and IrNPs@ZIF-8.
IrNPs@ZIF-8. Asterisk denotes spinning side band.
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Ir(COD)(MeCp)@ZIF-8 and IrNPs@ZIF-8 are given in Fig. 3.
The PXRD pattern of Ir(COD)(MeCp)@ZIF-8 exhibits new
reflection at 14.9° (shown by asterisk), which can be assigned to
the (111) reflection of Ir(COD)(MeCp) supported on the ZIF-8
surface.24 The XRD pattern of IrNPs@ZIF-8 shows the charac-
teristic reflections of the host matrix ZIF-8, confirming the intact
structure of ZIF-8 after the precursor decomposition. The
decrease of the overall Bragg peaks intensity with respect to the
parent, empty ZIF-8 is a consequence of the inclusion of guest
Ir(COD)(MeCp) molecules and formation IrNPs in the framework
and has been well studied for zeolites and mesoporous silica
materials.25,26 In summary, the sum of PXRD results reveal that
neither the crystallinity nor the lattice of ZIF-8 is distorted by
formation of IrNPs within the framework of ZIF-8.
The morphology and composition of IrNPs@ZIF-8 were
investigated by HR-TEM, TEM, STEM, STEM-EDX, HAADF-
STEM and ICP-MS analysis. Fig. 4(a–c) shows bright field
TEM images of IrNPs@ZIF-8 (with an iridium loading of 1.5%
wt found by ICP-MS) taken at different magnifications. They
show the presence of iridium nanoparticles in the range of
2.2–6.5 nm with a mean diameter of 3.3 1.7 nm (Fig. 4(d)) as
measured from the TEM images given in Fig. 4(a–c) by using an
NIH image program,27 whereby >100 non-touching particles
were counted. Expectedly, these iridium nanoparticles were
found to be supported on the surface of the ZIF-8 framework as
the kinetic diameter of Ir(COD)(MeCp) (5.1 Å)24 is larger than
the largest window size of ZIF-8 (3.4 Å).13,28 HR-TEM image
of the IrNPs@ZIF-8 given in Fig. 4(e) is indicative of the highly
crystalline feature of the resulting iridium nanoparticles, the
magnified image given as an inset in Fig. 4(e) clearly shows an
interplanar spacing of ∼0.23 nm, conforming to the {111} plane
of face centered cubic (fcc) Ir.20,29 The STEM-EDX spectrum
(Fig. 4(g)) taken from the region given in Fig. 4(f) confirms the
presence of iridium with ZIF-8 framework elements and copper
(from TEM grid). Moreover, HAADF-STEM images
(Fig. ESI-2a–d†) of IrNPs@ZIF-8 mapped for Cu, Zn, and Ir
show that the iridium nanoparticles are well-dispersed within the
ZIF-8 matrix. The oxidation state of iridium and surface compo-
sition of the IrNPs@ZIF-8 were analyzed by X-ray photoelectron
spectroscopy (XPS). The survey scan XPS spectrum of the
IrNPs@ZIF-8 sample (Fig. 5(a)) shows the presence of iridium
in addition to the ZIF-8 framework elements (Zn, C, N). The
deconvolution of the high resolution Ir 4f spectrum of IrNPs@
ZIF-8 sample (Fig. 5(b)) gives two prominent peaks at 64.9 and
Fig. 3 PXRD patterns of ZIF-8, Ir(COD)(MeCp)@ZIF-8 and IrNPs@
ZIF-8 in the 2θ = 5–40° range.
Fig. 4 (a–c) Low resolution TEM images of IrNPs@ZIF-8 taken at different magnifications, (d) the size histogram of IrNPs@ZIF-8, (e) HRTEM
image of IrNPs@ZIF-8, (f) STEM-EDX spectrum of IrNPs@ZIF-8.
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Fig. 6 N2 adsorption–desorption isotherms of (1) ZIF-8 and (2)
IrNPs@ZIF-8.
(∼300 °C). Furthermore, no hysteresis loop was observed in the
N2 adsorption–desorption isotherm of IrNPs@ZIF-8, indicating
that the procedure for preparation of IrNPs@ZIF-8 does not
create any mesopores.
The catalytic performance of IrNPs@ZIF-8 was separately
tested in the hydrogenation of cyclohexene and phenylacetylene
in different conditions as given in Schemes 1 and 2. Before
testing the catalytic activity of IrNPs@ZIF-8 in these two cataly-
tic hydrogenation reactions, one has to check whether the host
material ZIF-8 can catalyze these reactions under the same con-
ditions. For this reason, the catalytic activity of ZIF-8 was inves-
tigated separately in the hydrogenation of cyclohexene and
phenylacetylene. The results of these experiments showed that
ZIF-8 is catalytically inactive in the hydrogenation of these two
substrates. The catalytic hydrogenation of cyclohexene was per-
formed as an illustrative reaction to determine the effect of
iridium loading on the catalytic activity of IrNPs@ZIF-8, in
which IrNPs@ZIF-8 samples with different iridium contents of
1.5, 2.3 and 3.8 wt% were used as catalysts. The initial turnover
frequency (TOF) values were found to be 370, 320 and 285 h−1
in the IrNPs@ZIF-8 catalyzed hydrogenation of cyclohexene at
complete conversion for the samples with 1.5, 2.3 and 3.8 wt%
Ir loadings, respectively. The variation in catalytic activity
Fig. 5 (a) Survey scan XPS spectrum of IrNPs@ZIF-8, (b) high reso-
lution Ir 4f XPS spectrum and its simulated peak fit for IrNPs@ZIF-8.
68.0 eV, readily assigned to Ir(0) 4f7/2 and Ir(0) 4f5/2, respect-
ively.30 The peaks observed at 65.9 and 68.7 eV show the pres-
ence of IrO2 (IrO2 4f7/2 and IrO2 4f5/2),31 which may originate
from the surface oxidation of iridium nanoparticles during the
XPS sampling procedure. The shifts (1.2–1.5 eV) observed in
the binding energies of IrNPs with respect to metallic iridium
might be attributed to both the quantum size effect,32 and
peculiar electronic properties of the MOF matrix.33 The inter-
action of iridium nanoparticles with the nitrogen atoms exist in
the linker 2-methylimidazole-ring of ZIF-8 is expected to induce
a positive charge on the surface metal which would increase the
binding energies of iridium nanoparticles. The similar effect was
observed for the zeolite encapsulated ruthenium,34 palladium35
and platinum36 nanoparticles, which interact with the oxygen
atoms of zeolite framework.
Nitrogen adsorption–desorption isotherms of ZIF-8 and
IrNPs@ZIF-8 are given in Fig. 6 and both of them show a type I
shape, a characteristic of microporous materials.37 The micropore
volume and area were determined for ZIF-8 and IrNPs@ZIF-8
by the t-plot method.38 On passing from ZIF-8 to IrNPs@ZIF-8,
both the micropore volume (0.647 to 0.608 cm3 g−1) and the
BET surface area (from 1631 to 1554 m2 g−1) are reduced,
which can be attributed to the encapsulation of iridium nanopar-
ticles into some extent within the cavities of ZIF-8 that is prob-
ably resulting from the migration of surface bound iridium
nanoparticles into the cavities during hydrogenolysis or N2
adsorption pre-treatment which require high temperature
Scheme 1 IrNPs@ZIF-8 catalyzed hydrogenation of neat cyclohexene
to cyclohexane under mild conditions at 25 0.1 °C and 3 bar initial H2
pressure.
Scheme 2 IrNPs@ZIF-8 catalyzed hydrogenation of phenylacetylene
in methanol at 40 0.1 °C and 3 bar initial H2 pressure.
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Moreover, the filtrate solutions collected at the end of the each
catalytic runs were analyzed by ICP-MS and in none of them Ir
was detected, which confirms the retention of iridium within the
ZIF-8 matrix (no iridium passes into the solution). A control
experiment was also performed to show that the hydrogenation
reactions are completely stopped by the removal of IrNPs@
ZIF-8 from the reaction solution.
Overall, these results are indicative of IrNPs@ZIF-8 as a
highly active and selective catalyst in the hydrogenation of these
two substrates. Moreover, they provide easy separation and high
reusability performance in these reactions by keeping their stab-
ility against sintering and leaching throughout the catalytic runs.
Conclusion
Fig. 7 Conversion (square) and selectivity (circle) graphs for the semi-
hydrogenation of phenylacetylene in MeOH using IrNPs@ZIF-8
(100 mg with iridium loading of 1.5% wt corresponds to 7.8 μmol) at
40 0.1 °C with 3 bar initial H2 pressure.
In summary, the main findings of this study plus their impli-
cations can be summarized as follows:
(i) IrNPs@ZIF-8 can be prepared by a procedure comprising
the gas phase loading of Ir(COD)(MeCp) into the activated
ZIF-8 by sublimation followed by the hydrogenolysis of
inclusion compound Ir(COD)(MeCp)@ZIF-8 at elevated
temperature.
(ii) The characterization of IrNPs@ZIF-8 by using advanced
analytical tools including ICP-MS, P-XRD, XPS, 13C MAS
NMR, TEM, HRTEM, STEM, STEM-EDX, HAADF-STEM,
DR-UV-vis, EA analysis and N2-adsorption–desorption tech-
niques to show that the formation of highly crystalline well-
reflects the accessibility of iridium nanoparticles in the ZIF-8
framework by the substrate. The highest catalytic activity is
obtained by using IrNPs@ZIF-8 containing 1.5 wt% Ir, mostly
on the surface and readily accessible (Fig. ESI-3†). The smallest
size of IrNPs@ZIF-8 containing 1.5 wt% Ir (3.3 1.7 nm) with
respect to that of 2.3 wt% (3.9 1.8 nm) and 3.8 wt% (4.3
1.9 nm) may be another reason for its high activity
(Fig. ESI-4†).39
In order to investigate the selectivity of IrNPs@ZIF-8, the cat-
alytic hydrogenation of phenylacetylene was performed at 40
0.1 °C and with 3 bar initial H2 pressure in methanol
(Scheme 2). As can be seen from Fig. 7, IrNPs@ZIF-8 show
very good activity (initial TOF = 195 h−1) and selectivity
(90.2%) throughout the course of the reaction. It is particularly
remarkable that the selectivity values remain very close to 100%
until conversion values exceed 60%, the final selectivity value
>90 for 100% conversion was observed.40
The isolability and reusability of IrNPs@ZIF-8, as two crucial
measures in heterogeneous catalysis, were also tested in the
hydrogenation of cyclohexene and phenylacetylene. After com-
plete hydrogenation, IrNPs@ZIF-8 was isolated as a gray
powder from the first catalytic run by suction filtration, washed
with methanol, and dried under static vacuum (10−3 Torr) at
room temperature. The dried gray sample of IrNPs@ZIF-8 can
be bottled and stored under inert atmosphere. Furthermore, when
IrNPs@ZIF-8 was reused it was found to still be an active cata-
lyst in the hydrogenation of these two substrates, retaining >98%
and >92% of the initial catalytic activity even at the fifth cataly-
tic run in the hydrogenation of cyclohexene and phenylacety-
lene, respectively (Fig. ESI-5†). Moreover, they retain their
inherent selectivity at the fifth catalytic run in the semihydro-
genation of phenylacetylene. These results indicate that IrNPs@
ZIF-8 is isolable, bottleable, redispersible and yet, catalytically,
an active/selective catalyst. The characterization of the isolated
samples from the fifth catalytic runs by using XRD and TEM
(Fig. ESI-6†) reveal that (i) the crystallinity of the host ZIF-8
framework is retained, and (ii) there is no bulk iridium formed
within the framework of ZIF-8, at the end of the fifth catalytic
run in the hydrogenation of cyclohexene and phenylacetylene.
dispersed iridium nanoparticles (3.3
ZIF-8 matrix by keeping the host framework intact.
1.7 nm) within the
(iii) Testing the catalytic performance of IrNPs@ZIF-8 in the
hydrogenation of cyclohexene and phenylacetylene under mild
conditions in terms of activity, reusability, selectivity and dura-
bility shows that IrNPs@ZIF-8 is a highly active, selective,
reusable and durable (no leaching and agglomeration) catalyst in
these reactions.
(iv) More importantly, IrNPs@ZIF-8 catalyzed solvent-free
hydrogenation of cyclohexene is found to be “relatively green”
in terms of its environmental impact as it fulfils the majority of
the “green chemistry”41 requirements including: (a) it is 100%
selective and does not generate byproducts; (b) it maximizes the
incorporation of reactant into the products; (c) it is a solvent-free
system (i.e., uses neat cyclohexene as the substrate/solvent); (d)
it requires mild conditions so it needs relatively low energy
(25 0.1 °C with 3 bar initial H2 pressure); (e) it is catalytic, not
stoichiometric; (f) it does not use any protecting/deprotecting
group; (g) real-time monitoring is easy by measuring the H2
uptake, 1H NMR or GC analysis, for example.
This study clearly proved that ZIF-8 can act as a good host
material for the guest iridium nanoparticle catalyst by preventing
their agglomeration and leaching in the organic phase mediated
hydrogenation reactions under mild conditions. Understanding
the existing synergistic effects between the metal and ZIF-8
support and the development of well-organized and defined
metal@ZIF-8 composite materials for various catalytic trans-
formations remain an active area of research in our group. High
activity, reusability and simple isolation procedures make
IrNPs@ZIF-8 a very attractive catalyst for performing selective
hydrogenation reactions.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2012

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MZ thanks Yuzuncu Yil University for allowing him to conduct
some parts of this research in the Middle East Technical
University.
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This journal is © The Royal Society of Chemistry 2012
Dalton Trans.