Copper(II)-Doped ZIF-8 as a Reusable and Size Selective Heterogeneous Catalyst for the Hydrogenation of Alkenes using Hydrazine Hydrate

Article abstract of DOI:10.1002/ejic.202100126

In recent years, synthesis of mixed-metal organic frameworks has received considerable attention due to their superior performance than with mono-metallic metal organic frameworks (MOFs). In the present manuscript, Cu²⁺ ions are doped within the framework of ZIF-8 (ZIF: Zeolitic Imidazolate Frameworks) to obtain Cu@ZIF-8 and is characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR), UV-Visible diffuse reflectance spectra (DRS), scanning electron microscope (SEM) and transmission electron microcope (TEM) studies. The reaction conditions are optimized with styrene as a model substrate using Cu@ZIF-8 as a solid catalyst. Heterogeneity of the reaction is confirmed by leaching test and the solid is reusable for three recycles with no diminishing activity. Further, the structural integrity of Cu@ZIF-8 is also retained after hydrogenation of styrene as evidenced by powder X-ray diffraction. The size selective catalysis of Cu@ZIF-8 is demonstrated by comparing the activity of Cu²⁺ ions adsorbed over ZIF-8 solid (Cu/ZIF-8) in the hydrogenation of 1-hexene, 1-octene, cyclohexene, cyclooctene and t-stilbene. The catalytic results indicate that Cu/ZIF-8 shows superior activity than Cu@ZIF-8 for all these olefins due to the lack of diffusion to access the active sites (Cu²⁺). In contrast, Cu@ZIF-8 exhibits higher activity for those olefins with lower molecular dimensions (1-hexene, 1-octene) than the pores of ZIF-8 indicating the facile diffusion of these substrates inside the pores ZIF-8 while poor activity is observed with t-stilbene due to its larger molecular dimension than the pore apertures of ZIF-8. These catalytic data clearly establish the size selective hydrogenation of Cu@ZIF-8 due to the effective confinement provided by ZIF-8 framework and the presence of the active sites within the framework. Furthermore, this is the first report showing the size selective hydrogenation of olefins promoted by Cu@ZIF-8 (mixed-metal MOFs) compared to other noble metal nanoparticles (NPs) embedded over MOFs as catalysts.

Full text of DOI:10.1002/ejic.202100126

European Journal of Inorganic Chemistry
Accepted Article
Title: Copper(II)-Doped ZIF-8 as a Reusable and Size Selective
Heterogeneous Catalyst for the Hydrogenation of Alkenes using
Hydrazine Hydrate
Authors: Nagarathinam Nagarjun, Kannan Arthy, and Amarajothi
Dhakshinamoorthy
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202100126
Link to VoR: https://doi.org/10.1002/ejic.202100126
01/2020

10.1002/ejic.202100126
European Journal of Inorganic Chemistry
Copper(II)-Doped ZIF-8 as a Reusable and Size Selective Heterogeneous Catalyst for the
Hydrogenation of Alkenes using Hydrazine Hydrate
Mr. Nagarathinam Nagarjun, Ms. Kannan Arthy, Prof. Amarajothi Dhakshinamoorthy*
School of Chemistry, Madurai Kamaraj University, Madurai 625021, Tamil Nadu, India.
E-mail: admguru@gmail.com; adm.chem@mkuniversity.org; M:+91 99764 73669
https://mkuniversity.ac.in/new/school/sc/dhakshinamoorthy.php
Twitter: Madurai Kamaraj University @mku_official
Abstract
In recent years, synthesis of mixed-metal organic frameworks has received considerable attention
due to their superior performance than metal organic frameworks (MOFs) with mono-metallic MOFs. In
the present manuscript, Cu2+ ions are doped within the framework of ZIF-8 to obtain Cu@ZIF-8 and is
characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR), UV-Visible diffuse
reflectance spectra (DRS), scanning electron microscope (SEM) and transmission electron microcope
(TEM) studies. The reaction conditions are optimized with styrene as a substrate using Cu@ZIF-8 as
solid catalyst. Heterogeneity of the reaction is confirmed by leaching test and the solid is reusable for
three cycles with no diminishing activity. Further, the structural integrity is also retained after
hydrogenation of styrene. The size selective catalysis of Cu@ZIF-8 is demonstrated by comparing the
activity of Cu2+ ions adsorbed over ZIF-8 solid (Cu/ZIF-8) in the hydrogenation of 1-hexene, 1-octene,
cyclohexene, cyclooctene and t-stilbene. The catalytic results indicate that Cu/ZIF-8 shows superior
activity than Cu@ZIF-8 for all these olefins due to the lack of diffusion to access the active sites (Cu2+). In
contrast, Cu@ZIF-8 exhibits higher activity for those olefins with lower molecular dimensions (1-hexene,
1-octene) than the pores of ZIF-8 to facilitate facile diffusion inside the pores ZIF-8 while poor activity is
observed with t-stilbene due to its larger molecular dimension than the pore apertures of ZIF-8. These
catalytic data clearly establish the size selective hydrogenation of Cu@ZIF-8 due to the effective
confinement provided by ZIF-8 framework and the presence of the active sites within the framework.
Furthermore, this is the first report showing the size selective hydrogenation of olefins promoted by
Cu@ZIF-8 (mixed-metal MOFs) compared to other noble metal nanoparticles (NPs) embedded over
MOFs as catalysts.
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10.1002/ejic.202100126
European Journal of Inorganic Chemistry
1. Introduction
MOFs are crystalline porous solids whose structure is composed between metal ions or clusters
with rigid organic linkers leading to the formation of one, two and three dimensional solids.[1-4] One of the
unique properties of MOFs compared to other analogous porous crystalline solids is their high surface
area and pore volume.[5-7] Another interesting feature of MOFs than to analogous solids is their tunable
pore volume or surface area.[8] Furthermore, surface area and pore size of a MOF can also be predicted
through the appropriate selection of organic linkers and metal ions.[9] Due to the presence of well defined
pores in the architecture of MOFs, they can be readily employed as a suitable host for the encapsulation
of guests like metal nanoparticles (NPs), metal oxide NPs and metal complexes and their activity was
tested in many fields including catalysis,[10-12] photocatalysis[13] and photodegradation of pollutants.[14]
Selective hydrogenation of olefins to their respective saturated hydrocarbons is an important
industrial process and it is frequently performed for the production of fine chemicals, petrochemistry and
drug intermediates.[15, 16] Very often, noble metal NPs like Pd, Pt, Au and Ru supported on porous solids
have been reported as heterogeneous catalysts for hydrogenation reactions.[17] The main purpose of
loading these metal NPs on a solid support is to prevent their growth of particle size by agglomeration
and thereby harvesting their catalytic performance in a confined space. In this aspect, zeolites were one
of the extensively employed porous solids to achieve size selective catalysis in broad ranges of organic
reactions due to the effective stabilization of metal NPs in their well defined channels and pores.[18-21]
Inspired by these research findings, many research groups have focused on the encapsulation of metal
NPs over MOFs and their activities are examined in the size selective catalysis.[22, 23]
One of the prerequisites of MOFs to behave as hosts for metal NPs is their stability during the
loading of metal NPs. In this aspect, metal NPs encapsulated over MOFs have been reported as
heterogeneous catalysts for the size selective hydrogenation of olefins using Pd nanocubes@ZIF-8 under
photocatalytic conditions,[24] Pd@ZIF-8,[25, 26] Pt@ZIF-8,[27] Pt@ZIF-8 with different particle size,[28] Pt/UiO-
66,[29] SIM-1@Pt/Al2O3 (SIM: Substituted Imidazolate),[30] Pt-Cu frame@HKUST-1[31] and
RhCoNi@ZIF‐67(Co)/RhCoNi@MOF-74(Ni).[32] On the other hand, very recently, various kinds of Cu-
based heterogeneous catalysts were reported as heterogeneous solid catalysts for hydrogenation
reaction.[33, 34]
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European Journal of Inorganic Chemistry
ZIF-8 (ZIF: Zeolitic Imidazolate Frameworks) is constituted from tetrahedral units, in which Zn2+
ions are coordinated to four imidazolate ligands resulting in the formation of a three dimensional
frameworks with sodalite topology [Zn(MeIm)2] (MeIm: 2-methylimidazole).[35, 36] The structural analysis of
ZIF-8 reveals the existence of sodalite cages with a pore diameter of 11.6 Å and the opening between
two cages is 3.4 Å. ZIF-based solids were employed for large numbers of applications including gas
storage,[37, 38] catalysis,[4, 39] sensing,[40] chemical separation[41] and drug delivery.[42] Some of the notable
features that make ZIFs as appropriate solids for these applications include high surface area, high
density of functional groups, pH responsive degradation, high loading of drug, high thermal and chemical
stabilities.[43, 44] ZIFs and their composites have also been reported as multifunctional solids with superior
performances.[45, 46]
In recent years, preparation of mixed metal MOFs has received considerable attentions mainly
due to their superior catalytic activities than the MOF containing single metal ions.[47, 48] The enhanced
activity may be due to the combinations of both metal ions or it can be due to a single metal ion with
uniform distribution while other metal ion stabilizes the MOF framework. Further, Schneider and co-
workers have reported the synthesis of Cu2+-doped ZIF-8 (Cu@ZIF-8) and its catalytic activity was tested
in the cycloaddition and condensation reactions.[49] Although considerable progress made in using ZIF-8
as scaffolds for the loading of metal NPs in size selective hydrogenation, no attempts were made to use
mixed-metal MOFs as heterogeneous catalysts involving the framework cations as active sites to promote
hydrogenation of olefins. Further, the facile doping of Cu2+ ions within the framework of ZIF-8 to obtain
mixed metal ZIF-8 solids prompted us to report the use of Cu@ZIF-8 as size selective heterogeneous
solid catalyst in the hydrogenation of olefins with different dimensions using hydrazine hydrate as
reducing agent at room temperature. Furthermore, a control catalyst was also prepared by adsorbing
Cu2+ over ZIF-8 (Cu2+/ZIF-8) to prove the size selective hydrogenation of Cu@ZIF-8. Furthermore, this
work nicely illustrates that the rate of hydrogenation of olefins is enhanced with Cu@ZIF-8 compared to
ZIF-8 under similar conditions. Besides, catalyst stability is also proved by performing leaching and
reusability experiments. In addition, Cu@ZIF-8 exhibits wide substrate scope in the hydrogenation of
substituted styrenes under the optimized reaction conditions.
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European Journal of Inorganic Chemistry
2. Results and Discussions
Cu@ZIF-8 solid was synthesized by adopting previously reported procedure.[49] The catalytic
performance of Cu@ZIF-8 was studied in the hydrogenation of olefins and its activity is compared with
Cu/ZIF-8 under similar experimental conditions to demonstrate the size selective catalysis of Cu@ZIF-8.
The visual color changes from ZIF-8 to Cu@ZIF-8 and Cu/ZIF-8 upon doping and adsorbing of Cu2+ ions,
respectively. (Chart S1, Supporting Information). These images clearly suggest the presence of Cu2+ ions
at different chemical environments in these two solids. Further, Cu@ZIF-8 was characterized by powder
XRD, FT-IR, UV-Visible DRS, SEM methods and the observed results are in line with the reported
results.[49, 50] Briefly, powder XRD patterns of ZIF-8, Cu@ZIF-8 and CU/ZIF-8 are shown in Figure 1 and
they clearly indicate that the structural integrity and crystallinity are not altered upon doping/adsorbing
with Cu2+ ions. The facile doping of Cu2+ ions without affecting crystallinity of ZIF-8 is mainly due to the
closer ionic radii between Cu2+ (0.71 Ǻ) and Zn2+ (0.74 Ǻ) ions. In contrast, Li and co-workers have
reported the introduction of Ni2+ in ZIF-8 framework by mechanochemical method and observing
additional peaks in the powder XRD pattern due to the formation of clusters between Ni2+ ion and 2-
methylimidazole that are confined in ZIF-8 cavities.[51, 52] Interestingly, the doping of Cu2+ ions over ZIF-8
did not show any extra peaks other than ZIF-8, thus confirming the existence of Cu2+ ions exclusively
within the framework of ZIF-8.
Figure 1. Powder XRD patterns of (a) ZIF-8, (b) Cu@ZIF-8 and (c) Cu/ZIF-8.
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European Journal of Inorganic Chemistry
Figure 2 shows FT-IR spectra of ZIF-8, Cu@ZIF-8 and Cu/ZIF-8 solids. A sharp peak at 423 cm-1
corresponds to the stretching vibration of Zn-N bond indicating the coordination of Zn2+ ions to nitrogen
atoms from 2-methylimidazole linker.[53] Further, the stretching vibrations from 1100 to 1300 cm-1 are
assigned to C-H vibrations, while the peak located at 1466 cm-1 is assigned to C=N stretching vibration. In
addition, the peak at 1229 cm-1 confirms the trembling of C-N bond in the imidazole ring. A characteristic
stretching frequency of Cu-N bond in Cu@ZIF-8 solid is located around 575 cm-1 which is in good
correlation with earlier report.[54] However, this band is not seen in ZIF-8 and Cu/ZIF-8, thus suggesting
the facile doping of Cu2+ ion within the framework of ZIF-8 to afford Cu@ZIF-8.
Figure 2. FT-IR spectra of (a) ZIF-8, (b) Cu@ZIF-8 and (c) Cu/ZIF-8 solids.
Similarly, the presence of Cu2+ ion in the ZIF-8 framework was further confirmed by measuring
UV-Vis DRS for ZIF-8, Cu@ZIF-8 and Cu/ZIF-8. The observed results are shown in Figure 3. The doping
of Cu2+ ion shifts the absorption edge of ZIF-8 solid from UV to visible region. Furthermore, Cu@ZIF-8
displayed well-defined two bands, one at 490 nm while the second broad band starting at 600 nm, which
is due to the d-d transition of Cu2+ ion. These results convincingly demonstrate the presence of Cu2+ ion in
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European Journal of Inorganic Chemistry
Cu@ZIF-8 solid. Further, Cu/ZIF-8 also shows similar bands compared to Cu@ZIF-8. In contrast, ZIF-8
showed no such bands due to the absence of Cu2+ ions.
Figure 3. UV-Vis DRS spectra of (a) ZIF-8, (b) Cu@ZIF-8 and (c) Cu/ZIF-8 solids.
After out-gassing the samples under vacuum at 150 °C for 10 h, permanent porosities of ZIF-8,
Cu@ZIF-8 and Cu/ZIF-8 materials were measured by nitrogen adsorption-desorption studies at 77 K
(Figures S23-S25). The observed data exhibit type I isotherms which are in agreement with earlier
report.[49] The BET surface area of Cu@ZIF-8 and Cu/ZIF-8 solids was 1590 and 1612 m2/g which are
slightly lower than ZIF-8 (1712 m2/g). The pore volume of ZIF-8, Cu@ZIF-8 and Cu/ZIF-8 was 0.571,
0.518 and 0.517 cm3/g, respectively. The pore size of ZIF-8, Cu@ZIF-8 and Cu/ZIF-8 was 1.29, 1.21 and
1.20 nm, respectively. These results are in agreement with recent results.[49]
On the other hand, the morphology of ZIF-8 and Cu@ZIF-8 was measured by SEM images and
the observed results are presented in Figure 4. ZIF-8 solid exhibited well-defined truncated rhombic
dodecahedron morphology and it is not altered upon doping with Cu2+ ions within the framework of ZIF-8.
These characterization data are in agreement that Zn2+ and Cu2+ ions are part of the framework leading to
the formation of a crystalline solid with uniform morphology. In addition, it was reported by us that
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10.1002/ejic.202100126
European Journal of Inorganic Chemistry
Cu@ZIF-8 showed a characteristic EPR signal and is due to the presence of Cu2+ ion while such band is
absent in ZIF-8.[49] Furthermore, elemental composition of Cu@ZIF-8 was measured by EDX analysis and
the weight percentage of copper is 2.46%. Also, the elemental mapping of Cu@ZIF-8 was shown in
Figure 5 and clearly indicating the existence of copper throughout the framework. Hence, the doping of
Cu2+ ions within the framework of ZIF-8 is indirectly confirmed by these results.
Figure 4. SEM images of (a) ZIF-8, (b) Cu@ZIF-8 and (c) after 3rd reuse ofCu@ZIF-8.
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10.1002/ejic.202100126
European Journal of Inorganic Chemistry
Figure 5. SEM-EDX analysis of Cu@ZIF-8.
TEM images were measured for Cu@-ZIF-8 and Cu/ZIF-8 to compare the morphology and the
location of Cu2+ ions in these solids (Figure 6). These images clearly indicate that both samples exhibit as
a well-defined truncated rhombic dodecahedron structure which is the ideal morphology reported for ZIF-
8. These results unambiguously prove that the surface morphology and the framework structure of ZIF-8
are not altered during the doping of Cu2+ ions and are in line with SEM results.[49] In contrast, TEM images
of Cu/ZIF-8 illustrate that the Cu2+ ions are adsorbed on the outer surface of the crystals while such
adsorption is not seen with Cu@ZIF-8, indicating the facile doping of Cu2+ ions. Further, the characteristic
lattice spacing of 0.284 nm was observed for Cu2+ ions adsorbed on (011) plane of ZIF-8 phase.[55, 56]
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European Journal of Inorganic Chemistry
(a)
(b)
(e)
(c)
(f)
(d)
(g)
Figure 6. TEM images of (a), (b), (c) of Cu@ZIF-8 and (d), (e), (f) of Cu/ZIF-8. The d-spacing of Cu2+ ions
is shown in (g).
As commented earlier, size selective hydrogenations have been reported with noble metal NPs
encapsulated over MOFs like ZIF-8.[23] Hence, one of the main objectives of this work is to demonstrate
the catalytic activity of Cu2+ ions in the size selective hydrogenation of olefins under mild reaction
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European Journal of Inorganic Chemistry
conditions without the requirement of noble metal NPs. Interestingly, the framework Cu2+ ions promote
the hydrogenation of olefins while ZIF-8 framework offers a confined environment to achieve size
selective catalysis. In order to prove this hypothesis, the reaction conditions were optimized in terms of
catalyst loading, N2H4.H2O concentration by selecting styrene as a model substrate using Cu@ZIF-8 as a
solid catalyst. The observed catalytic data are presented in Table 1. A blank control experiment in the
absence of catalyst showed 20% conversion of styrene with N2H4.H2O as reducing agent in ethanol at
room temperature after 14 h. On the other hand, ZIF-8 exhibited 36% conversion of styrene in ethanol
under identical conditions. Interestingly, Cu@ZIF-8 afforded quantitative conversion of styrene in ethanol
under similar experimental conditions. These catalytic results clearly prove the efficient performance of
Cu2+ ions in achieving complete conversion of styrene. Figure 7 shows the time conversion plot for the
transformation of styrene to ethylbenzene using Cu@ZIF-8 as solid catalyst. Although the conversion of
styrene was 80% in methanol while other solvents showed poor to moderate conversions using Cu@ZIF-
8 as catalyst under same conditions. Further, the conversion of styrene reached to 51% under neat
conditions after 14 h at room temperature. The conversion of styrene was unsuccessful with isopropanol
as a reducing agent under same conditions.
In another set of experiments, the conversion of styrene was significantly reduced upon lowering
the catalyst loadings and Figure 8 indicates their respective time conversion plots. Similarly, the influence
of N2H4.H2O concentration in the conversion of styrene was monitored and achieving the highest
conversion with the maximum dosage of N2H4.H2O under identical conditions (Figure 9). A control
experiment with pyridine as a catalyst poison was performed in the hydrogenation of styrene using
Cu@ZIF-8 in ethanol at room temperature for 14 h. Surprisingly, the conversion of styrene was
completely quenched in the presence of pyridine thus suggesting the strong coordination of pyridine with
Zn2+/Cu2+ ions. As a consequence, the interaction of these metal ions with N2H4.H2O is inhibited and
therefore, liberation of hydrogen is stopped,[57] thus showing no conversion of styrene. To support these
findings, two addition control experiments were performed with homogeneous salts with identical Cu
loadings present in Cu@ZIF-8. The observed catalytic data indicated that 80 and 67% conversions of
styrene are achieved with Cu(NO3)2.3H2O and CuI salts, respectively under similar conditions.
Additionally, the physical mixture of the metal precursors of Cu@ZIF-8 was also examined in the
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10.1002/ejic.202100126
European Journal of Inorganic Chemistry
hydrogenation of styrene and observing 89% conversion under similar conditions. These control
experiments unambiguously prove that the incorporated metal ions within the framework of ZIF-8 must be
readily available to coordinate with N2H4.H2O. Thus, Cu2+ ion plays a crucial role in achieving quantitative
conversion of styrene under these conditions. Furthermore, the homogeneous Cu(NO3)2.3H2O and CuI
salts were deactivated after 7 h while Cu@ZIF-8 solid shows complete conversion of styrene without
deactivation (Figure 10).
Table 1. Optimization of the reaction conditions for the hydrogenation of styrene.a
Entry
Catalyst
Solvent
Reducing agent
N2H4.H2O
Conversionb
(%)
20
36
>99
80
38
47
27
51
0
34
51
79
42
77
95
-
1
2
3
4
5
6
7
8
9
-
Ethanol
Ethanol
Ethanol
Methanol
DCM
ACN
Toluene
Neat
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
ZIF-8
N2H4.H2O
N2H4.H2O
N2H4.H2O
N2H4.H2O
N2H4.H2O
N2H4.H2O
N2H4.H2O
Isopropanol
N2H4.H2O
N2H4.H2O
N2H4.H2O
N2H4.H2O
N2H4.H2O
N2H4.H2O
N2H4.H2O
N2H4.H2O
N2H4.H2O
N2H4.H2O
Cu@ZIF-8
Cu@ZIF-8
Cu@ZIF-8
Cu@ZIF-8
Cu@ZIF-8
Cu@ZIF-8
Cu@ZIF-8
Cu@ZIF-8
Cu@ZIF-8
Cu@ZIF-8
Cu@ZIF-8
Cu@ZIF-8
Cu@ZIF-8
Cu@ZIF-8
Cu(NO3)2.3H2O
CuI
10c
11d
12e
13 f
14g
15h
16i
17
18
19
80
67
89
Cu(NO3)2.3H2O +
Zn(NO3)2
aReactions conditions: styrene (1 mmol), N2H4.H2O (3 mmol), catalyst (20 mg), room temperature, 14 h.
bDetermined by GC. Selectivity of ethylbenzene was always more than 99%.
c Cu@ZIF-8 (5 mg)
d Cu@ZIF-8 (10 mg)
e Cu@ZIF-8 (15 mg)
f N2H4.H2O (1 mmol)
g N2H4.H2O (2 mmol)
h after 3rd reuse
i pyridine (1mmol).
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The stability of Cu@ZIF-8 was assessed under the optimized reaction conditions by hot filtration
and reusability tests. The styrene hydrogenation was performed with Cu@ZIF-8 solid and it was removed
from the reaction mixture at around 30% conversion. The kinetics of the reaction was progressed in the
absence of solid. Comparison of styrene conversion with and without solid catalysts revealed that the
conversion rate is significantly affected upon removal of the solid. Hence, this experiment clearly rules out
the contribution of the leached Cu2+ ions under these conditions, thus proving the heterogeneity of the
process. Furthermore, reusability of Cu@ZIF-8 was also performed under the optimized reaction
conditions. The catalytic results have shown that the activity of Cu@ZIF-8 is retained for three reuses with
no significant decay in styrene conversion. ICP analysis of the three times reused solid showed 2.12 ppm
of Cu, which is in agreement with the leaching and reusability results. In addition, powder XRD of the
fresh and three times reused solids was compared and no significant changes are observed in the
crystallinity as well as structural integrity of the reused solid compared to the fresh material (Figure S22).
Further, morphology of the three times reused solid exhibited identical features to the fresh material
(Figure 4). These catalytic data in combination with analytical and microscopic results clearly prove the
robust nature of Cu@ZIF-8 under these conditions.
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European Journal of Inorganic Chemistry
Figure 7. Time conversion plot for the hydrogenation of styrene to ethylbenzene (a) blank control without
catalyst, (b) ZIF-8, (c) filtration of solid after 1 h and the reaction mixture stirred under identical reaction
conditions and (d) Cu@ZIF-8.
Figure 8. Hydrogenation of styrene using (a) 5 mg, (b) 10 mg, (c) 15 mg and (d) 20 mg Cu@ZIF-8
loadings..
Figure 9. Hydrogenation of styrene using N2H4.H2O with (a) 1 mmol, (b) 2 mmol and (c) 3 mmol.
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European Journal of Inorganic Chemistry
Figure 10. Hydrogenation of styrene using (a) Cu(NO3)2.3H2O, (b) Cu(NO3)2+Zn(NO3)2 and (c) Cu@ZIF-8.
With the optimized reaction conditions in hand, the promising activity and stability of Cu@ZIF-8 in
the hydrogenation of styrene under mild reaction conditions prompted us to screen for other substituted
styrene derivatives with electron donating and withdrawing substituents. The observed results are shown
in Table 2. 4-Methyl and 4-methoxystyrenes afforded 100 and 97% conversions using Cu@ZIF-8 as
catalyst in ethanol after 14 h. On the other hand, 2,4-dimethylstyrene was also converted to its respective
product in 97% conversion under same conditions. The conversion of β-methylstyrene was decreased to
74% under identical conditions. This decrease in the conversion may be due to the diffusion limitation
imposed by this branched olefin. 4-Fluoro, 4-chloro and 4-bromostyrenes were also quantitatively
converted to their respective hydrogenated products. A significant decrease in the conversion was
noticed for 3-nitrostyrene and is believed due to diffusion limitations. On other hand, 4-ethoxystyrene was
also converted quantitatively to its corresponding hydrogenated product. Finally, phenylacetylene was
hydrogenated at 61% conversion with the selectivity of ethylbenzene to 78% while styrene selectivity to
22%.
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Table 2. Hydrogenation of various alkenes using Cu@ZIF-8 as a solid catalyst.a
Entry Conversion (%)b
Substrate
Product
1
100
2
97
3
4
97
74
5
6
7
8
99
100
99
20
9
99
61
10c
aReaction conditions: Substrate (1 mmol), N2H4.H2O (3 mmol), Cu@ZIF-8 (20 mg), ethanol (1 mL), room
temperature, 14 h.
bConversion of substrate was determined by GC using internal standard method. The selectivity of the
reduced products was always more than 99 %.
cSelectivity of ethylbenzene was 78 % while styrene was 22 %.
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After demonstrating the stability and wide substrate scope of Cu@ZIF-8, size selective
hydrogenation of olefins within the constrained spaces of ZIF-8 was performed and the observed catalytic
data are discussed. For this study, 1-hexene, 1-octene, cyclohexene, cyclooctene and t-stilbene were
selected as model olefins with varying molecular dimensions (Table 3). Hydrogenation of 1-hexene to 1-
hexane was selectively achieved with Cu@ZIF-8 at 92% conversion due to its smaller dimension than the
pore aperture of Cu@ZIF-8 (3.4 Å). Similarly, 1-octene and cyclohexene exhibited 84 and 80%
conversions to their respective hydrogenated products, respectively. This slightly lower conversion of
these substrates is due to their larger dimensions than to 1-hexene. The conversion of cyclooctene was
further decreased due to its bigger size than the pore size of Cu@ZIF-8. Importantly, the conversion of
t-stilbene was significantly decreased to 12% due to its larger size than the rest of the olefins. These
catalytic data convincingly prove that the conversion of olefins is gradually decreased upon increasing
molecular dimension of olefins. Hence, these experimental data prove that the hydrogenation reaction
occurs within the confinement of Cu@ZIF-8 pores.
Table 3. Size selective hydrogenation of various alkenes between Cu@ZIF-8 and Cu/ZIF-8 as solid
catalysts.a
Entry
Substrate
Molecular
dimension (Å)
Conversion (%)
Cu@ZIF-8 Cu/ZIF-8
1 h
14 h
84
1 h
14 h
99
1
2
3
2.5
4
20
24
15
38
48
40
92
80
99
96
4.2
4
5
5.5
5.6
14
2
71
12
61
16
100
36
aReaction conditions: Substrate (1 mmol), N2H4.H2O (3 mmol), catalyst (20 mg), ethanol (1 mL), RT, 14 h.
bDetermined by GC.
Figure 11 provides the conversion of 1-hexene, 1-octene, cyclohexene, cyclooctene and t-
stilbene using Cu@ZIF-8 and Cu/ZIF-8 after 1 h (Figure S6 after 14 h) under the optimized reaction
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10.1002/ejic.202100126
European Journal of Inorganic Chemistry
conditions. These catalytic data suggest that the initial reaction rate is relatively much higher for all these
olefins irrespective of their molecular dimensions with Cu/ZIF-8 compared to Cu@ZIF-8 (Figures S1-S5).
This enhanced activity of Cu/ZIF-8 than to Cu@ZIF-8 is that Cu2+ ions are adsorbed on the exterior
surface of ZIF-8 that can be readily accessed by the reactants without diffusion limitations (Figure S26).
In contrast, the reaction rate was highly controlled by molecular dimensions of olefins using Cu@ZIF-8 as
catalyst and is due to the diffusion limitations encountered by larger size of olefin to access Cu2+ ions
within the framework. These notable differences in the activity between Cu@ZIF-8 and CU/ZIF-8 clearly
indicate the operation of size selective hydrogenation with Cu@ZIF-8 as heterogeneous catalyst under
these conditions.
Figure 11. Size selective hydrogenation of various alkenes using Cu@ZIF-8 versus Cu/ZIF-8 after 1 h.
Table S1 provides the comparison of the hydrogenation of styrene with Cu@ZIF-8 with other
reported solid catalysts in the literature. These results clearly indicate that most of the hydrogenation
reactions have been performed with noble metal NPs as the active sites. Some of the catalytic systems
have also been reported at elevated temperature (60 or 101 oC). On the other hand, the hydrogenation of
styrene to ethylbenzene using Cu3(BTC)2 with hydrazine hydrae resulted in 60% conversion after 24 h.
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10.1002/ejic.202100126
European Journal of Inorganic Chemistry
Interestingly, the present catalytic system with relatively lower Cu2+ loading in Cu@ZIF-8 in respect to
Cu3(BTC)2 afforded much superior activity at shorter reaction time. Furthermore, Table S2 shows the list
of catalysts that have been reported for the size selective MOF catalysts for the hydrogenation of olefins.
These results clearly indicate that no attempts were made to use framework cations as active sites for the
hydrogenation of olefins. Hence, this is the first report showing the catalytic performance of mixed-metal
MOFs, Cu@ZIF-8 as a size selective heterogeneous catalyst for the hydrogenation of olefins.
3. Conclusions
In summary, Cu@ZIF-8 and Cu/ZIF-8 solids were synthesized and the former solid is
characterized by suitable spectroscopic, microscopic techniques. Besides showing the activity of
Cu@ZIF-8 for the hydrogenation of styrene and its derivatives at higher conversions to their
corresponding saturated benzylic hydrocarbons with high selectivities, the size selective hydrogenation of
Cu@ZIF-8 is clearly proved by comparing its activity with Cu/ZIF-8 under similar experimental conditions.
Cu/ZIF-8 showed no discrimination among the various olefins tested but, in contrast Cu@ZIF-8 showed
hydrogenation activity to those olefins having lower dimensions than the pore aperture of ZIF-8. These
catalytic data convincingly demonstrate the occurrence of size selective hydrogenation of olefins with
Cu@ZIF-8. This work clearly illustrates the catalytic performance of the framework metal ions acting as
active sites to promote hydrogenation of olefins compared to other reports involving metal NPs embedded
on MOFs.
4. Experimental procedure
4.1 Materials
Cu(NO3)2.3H2O, Zn(NO3)2.6H2O, 2-methylimidazole, styrene and other alkenes derivatives were
purchased from Sigma Aldrich and used as received. Solvents were also received from Sigma Aldrich
and used as received without any further purification.
4.2 Instrumentation
Powder XRD diffraction patterns were recorded with Philips X’Pert diffractometer in the refraction
mode having CuKα radiation (λ=1.5417 Å) as the incident beam, PW3050/60 (2 theta) as Goniometer.
UV-Visible DRS spectra were measured by Shimadzu UV-2700; ISR-2600 plus instrument in the range
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10.1002/ejic.202100126
European Journal of Inorganic Chemistry
between 200-800 nm by dispersing the sample on BaSO4 surface. FT-IR spectra were measured in the
range of 400-4000 cm−1 using Bruker tensor 27 series FT-IR spectrometer. SEM images were measured
with Hitachi S-3000H scanning electron microscope. Gas chromatography (GC) Agilent 7820A model was
used for the determination of conversion and selectivity using high pure nitrogen as carrier gas with flame
ionization detector. GC coupled with mass spectrometry (GC-MS) was employed under identical
conditions for the confirmation of the products.
4.3 Synthesis of Cu@ZIF-8
Cu@ZIF-8 was prepared by following the procedure of Schneider and coworkers[49] with a slight
modification. Briefly, Cu(NO3)2.3H2O and Zn(NO3)2.6H2O (total 2 mmol) were dissolved in 22.6 mL
methanol and 2-methylimidazole (1320 mg, 16 mmol) was dissolved in 22.6 mL of methanol separately.
Then, these two solutions were mixed in drop wise by adding 2-methylimidazole solution to Zn2+ and Cu2+
solutions in a two-neck flask. The synthesis was conducted at room temperature under nitrogen flow with
stirring for 1 h and later, it was kept for 24 h at room temperature. Cu@ZIF-8 solids were separated
through centrifugation, followed by washing with methanol (3x 30 mL). The isolated Cu@ZIF-8 solid was
o
dried in air oven at 60 C for 6 h before analysis. The vials containing Cu@ ZIF-8 powders were tightly
capped and stored at room temperature for further catalytic experiments.
4.4 Synthesis of Cu/ZIF-8
A metal solution was prepared by dissolving Cu(NO3)2.3H2O (10 mg) in 22.6 mL methanol which
is identical to the Cu loading in Cu@ZIF-8. Then, in a two-neck flask, ZIF-8 (100 mg) was added to
Cu(NO3)2.3H2O solution under nitrogen flow at room temperature with stirring for 1h. Later, Cu/ZIF-8
solids were separated by centrifugation and washed with methanol (2× 30 mL). The material was dried in
air oven at 60oC for 6 h before catalytic experiments.
4.5 Experimental procedure
In a typical hydrogenation reaction, 20 mg of Cu@ZIF-8 was added to the high pressure tube
containing styrene (1 mmol), hydrazine hydrate (3 mmol) and ethanol (1 mL). This reaction tube was
sealed by silicon rubber septum and this reaction mixture was stirred at room temperature for the required
time as shown in tables. The reaction progress was monitored by GC and after completion of the
reaction, the mixture was washed three times with acetonitrile and filtered. Then, the product was
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10.1002/ejic.202100126
European Journal of Inorganic Chemistry
analyzed by GC to determine final conversion and selectivity. Conversion of olefin was determined by
Agilent 7820 A using internal standard method and the formed products were confirmed by GC-MS
(Figures S7-S21). The above described procedure was repeated for reusability tests with the recovered
solid catalyst. Control experiments were performed with Cu(NO3)2.3H2O (15 mg) and CuI (9 mg) as
homogeneous catalysts under identical conditions with similar copper loadings as in the case of Cu@ZIF-
8. The physical mixture of Cu(NO3)2.3H2O (15 mg) and Zn(NO3)2.6H2O (148 mg) was used under similar
conditions for the hydrogenation reaction.
Acknowledgements
A.D. extends his thanks to University Grants Commission (UGC), New Delhi, for awarding
Assistant Professorship through Faculty Recharge Programme.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: Hydrogenation of olefins · Heterogeneous catalysis · Metal-organic frameworks · Porous
solids · Size selectivity
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Table of Contents
Cu2+-doped ZIF-8 (Cu@ZIF-8) is reported as a reusable heterogeneous solid catalyst for the
hydrogenation of styrenes to their respective reduced products using hydrazine hydrate as a reducing
agent at room temperature. Further, Cu@ZIF-8 is also shown to exhibit size selective catalysis, thus
proving the occurrence of reduction within the pores of Cu@ZIF-8.
23
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