An efficient and sustainable catalytic reduction of carbon-carbon multiple bonds, aldehydes, and ketones using a Cu nanoparticle decorated metal organic framework

Article abstract of DOI:10.1039/c8nj01704b

Transition metal (Cu, Mn, Ni, Zr) substituted metal organic frameworks (MOFs) are prepared for the reduction of carbon-carbon multiple bonds with hydrazine hydrate in ethanol under mild reaction conditions. Among the MOFs investigated in this study, the Cu framework substituted MOF exhibited the best activity in this study. Further, Cu nanoparticles (CuNPs) are supported on the surface of a Cu framework substituted MOF to achieve excellent reduction activity. The catalyst exhibits efficient recyclability with no appreciable loss in the catalytic activity even after five recycles. In order to establish the reaction mechanism, reactions are performed under N₂ and Ar atmospheres. A reaction is also performed under an Ar atmosphere but in the presence of H₂O₂ to elucidate the mechanism. The catalyst exhibits excellent activity in the reduction of alkynes. Under the optimum reaction conditions, the catalyst is also successful in reducing a wide range of aldehydes and ketones. The present catalytic process demonstrates several key advantages such as mild and convenient reaction conditions, a low substrate to hydrazine ratio, reusability, and cost-effectiveness of the catalyst (Pt or Pd free catalyst).

Full text of DOI:10.1039/c8nj01704b

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The role of atropisomers on the photo-reactivity and fatigue of
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An efficient and sustainable catalytic reduction of carbon-carbon multiple bonds,
aldehydes, and ketones using Cu nanoparticles decorated metal organic framework
Ashish Kumar Kar and Rajendra Srivastava*
Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, Punjab, India
Corresponding Author Eꢀmail: rajendra@iitrpr.ac.in
Phone: +91ꢀ1881ꢀ242175; Fax: +91ꢀ1881ꢀ223395
1

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Table of contents
An efficient and sustainable catalytic reduction of carbon-carbon multiple bonds,
aldehydes, and ketones using Cu nanoparticles decorated metal organic framework
Ashish kumar Kar and Rajendra Srivastava*
Cu nanoparticles decorated CuꢀBTCꢀMOF is successfully demonstrated in the reduction of
alkenes, alkynes, aldehydes, and ketones. The low cost, reusability, sustainability of the
catalyst, the use of low amount of reducing agent, the ecoꢀfriendliness of the process, and the
involvement of a simple workꢀup & separation of the products from the reaction system are
some of the attractive key features.
2

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___________________________________________________________________________
Abstract
Transition metal (Cu, Mn, Ni, Zr) substituted metal organic frameworks (MOFs) are prepared
for the reduction of carbonꢀcarbon multiple bonds with hydrazine hydrate in ethanol under a
mild reaction condition. Among the MOFs investigated in this study, Cu framework
substituted MOF exhibits the best activity in this study. Further, Cu nanoparticles (CuNPs)
are supported on the surface of Cu framework substituted MOF to achieve the excellent
reduction activity. The catalyst exhibits efficient recyclability with no appreciable loss in the
catalytic activity even after five recycles. In order to establish the reaction mechanism,
reactions are performed in N2 and Ar atmosphere. Reaction is also performed in Ar
atmosphere but in the presence of H2O2 to elucidate the mechanism. The catalyst exhibits
excellent activity in the reduction of alkynes. Under the optimum reaction condition, catalyst
is also successful in reducing a wide range of aldehydes and ketones. The present catalytic
process demonstrates several key advantages such as a mild and convenient reaction
condition, low substrate to hydrazine ratio, reusability, and costꢀeffectiveness of the catalyst
(Pt or Pd free catalyst).
Keywords: Metal organic framework; Cu nanoparticles; reduction of carbonꢀcarbon multiple
bonds; reduction of aldehyde and ketone; heterogeneous catalyst.
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1. Introduction
Reduction of olefins, alkynes, and carbonyl compounds is one of the important synthetic
steps used in many catalytic and organic reactions.1,2 A wide range of hydrogen rich fuels and
chemicals can be produced from the reduction of carbonꢀcarbon multiple bonds.3 Further,
reduction of aldehydes/ketones to alcohols produces a wide range of key fine chemicals and
pharmaceuticals.4 Alcohols produced by the reduction of carbonyl compounds can act as
solvents and precursors for the production of a wide variety of esters used in the flavour,
fragrance, and cosmetic industries.5,6 Reduction of carbonꢀcarbon multiple bonds are
generally catalyzed by Pd, Ru, Rh, and Pt based costly catalysts and H2 as an economical
reducing agent.7,8 A wide range of metal complexes and metal nanoparticles based on costly
metals have been investigated for the reduction of alkyne to alkene.9,10 Further, only a few
reports are available for the complete reduction of alkyne to alkane.11ꢀ14 Similarly, a wide
range of homogenous and heterogeneous catalysts are known for the reduction of carbonyl
compounds to alcohols by conventional reduction or transfer hydrogenation protocols.15ꢀ23
Therefore, it is important for researchers to develop catalytic reduction methodology based
on economical transition metals when compared to those costly scarce metals that are
exclusively obtained as byproducts of major industrial metals.24ꢀ32 Furthermore, for the
reduction using transition metals based catalysts, different reducing agents such as, NaBH4,
hydrazine hydrate, formic acid etc can be used.33,34 Hydrazine is an economical and
environmentally non ecoꢀfriendly chemical. Therefore, its consumption as a reducing agent
will be advantageous and convenient for the hydrogenation in the production of fine
chemicals. This would be one of the important methods to use surplus of the hydrazine
produced worldwide. Efforts have been made to use hydrazine as a reducing agent with wide
range of homogenous and metal nanoparticles supported catalysts.27ꢀ32 The aim of this study
is to develop robust and economical heterogeneous catalyst for the reduction of olefins,
alkynes, and carbonyl compounds using hydrazine hydrate in base free condition.
A wide range of porous materials are known in the literature that includes zeolite,
silica, metal oxide, carbon materials, and metalꢀorganicꢀframeworks (MOFs). Metals and
metal nanoparticles can be incorporated in these porous materials to develop various robust
catalysts.35ꢀ39 Large external surface area, functional surface, and pore walls help to prevent
the agglomeration of the NPs, thus resulting in narrow particle size and size distribution.
Several strategies are known to prepare metal NPs, for example, (i) reduction of transition
metal salt in the presence of porous support, (ii) chemical vapour deposition, (iii)
photochemical reduction, (iv) ligand reduction and displacement from organometallics, and
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(iv) electrochemical reduction method. Among these methods, the chemical reduction method
is very straight forward and requires simple lab equipments to produce metal NPs. In this
method, the reduction of corresponding metal salts is carried out in the presence of suitable
reducing agent
.
Among the porous materials discussed above, metal oxides and MOFs are interesting
because metals are present in their framework structure. In these materials, support itself can
act as a good catalyst. In the recent time, efforts are being made to develop catalysts based on
MOF because they exhibit high surface area and porosity.40ꢀ42 MOFs are being developed as a
replacement of homogenous metal complexes but in the case of MOFs, only limited metal
centres are accessible for the catalytic reactions. However, suitable metals and functional
sites can be introduced in MOFs via inꢀsitu synthesis and postꢀsynthesis modification which
makes them versatile catalysts.43ꢀ45 Efforts have been made to develop MOFs in the reduction
of carbonꢀcarbon multiple bonds. However, these MOF catalysts exhibited low activity
(Significant amount of the catalyst was required for the reduction).27 In the recent time efforts
have also been made to develop metal NPs incorporated MOF (prepared using costly ligands
& metal) for the reduction of styrene that involved additional base for the reduction to take
place.46 Ionic liquids mediated, PdNPs incorporated MOF has also been investigated for the
selective reduction of alkynes.47
The objective of this study is to develop an economical transition metal based ecoꢀ
friendly and reusable catalyst for the reduction of carbonꢀcarbon multiple bonds and carbonyl
compounds. Another aim of this study is the base free and oxidant free reduction of carbonꢀ
carbon multiple bonds using hydrazine hydrate as an economical and userꢀfriendly reduction
source. In this study, four different transition metal (Cu, Mn, Ni, Zr) substituted metal
organic frameworks (MOFs) are investigated for the reduction of olefin. Among these MOFs,
Cu based MOFs (CuꢀBDCꢀMOF and CuꢀBTCꢀMOF) exhibit better activity. Among the two
Cu based MOF, CuꢀBTCꢀMOF exhibits marginally higher activity. Therefore, CuꢀBTCꢀMOF
is investigated in details. Moreover, CuꢀBTCꢀMOF is synthesized in ecoꢀfriendly
water/ethanol medium whereas CuꢀBDCꢀMOF needs to be synthesized in DMF medium. It is
difficult
to
synthesize
CuꢀBDCꢀMOF
in
aqueous/ethanol
medium.
Further, CuꢀBTCꢀMOF exhibits large surface area and pore volume which are some of the
desired criteria for the catalyst to exhibit high catalytic activity. Catalytic activity of the best
MOF found in this study is further improved by the incorporation of Cu nanoparticles
(CuNPs) on its surface. CuNPs supported MOF is investigated in the reduction of wide range
of olefins. Not only olefins but alkynes are also reduced using this protocol. The applicability
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of this protocol is also extended for the reduction of aliphatic and aromatic aldehydes and
ketones.
2. Materials and Methods
2.1. Catalyst preparation
Synthesis of Cu-BTC-MOF
CuꢀBTCꢀMOF was synthesized by a solvothermal process.48 First solution A was
prepared by dissolving Cu(NO3)2·3H2O (0.88 g) in 12 mL deionised water. Solution B was
prepared by dissolving H3BTC (benzeneꢀ1,3,5ꢀ tricarboxylate) (0.42 g) in 12 mL of ethanol.
Solution A was added to solution B dropꢀwise in 30 minutes. Resultant solution was stirred at
ambient temperature for 15 minutes. Finally, it was transferred into Teflonꢀlined stainless
steel autoclave and hydrothermally treated at 110 °C for 18 h. After hydrothermal treatment,
the autoclave was allowed to cool to ambient temperature. Solid precipitate was separated
using centrifuge machine and the crystalline solid product was dried in vacuum oven at 80 °C
for 10 h to get CuꢀBTCꢀMOF (Cu3(BTC)2, molecular weight 604.87)49.
Other MOFs such as CuꢀBDCꢀMOF (CuꢀBDC, molecular weight 300.76)49, ZrꢀBDCꢀ
MOF (Zr6O4(OH)4(BDC)6, molecular weight 1628.034)49, MnꢀBDCꢀMOF (Mn3(BDC)3,
molecular weight 663.69)49, and NiꢀBTCꢀMOF (Ni3(BTC)2, molecular weight 596.1)49 were
prepared by following the reported procedure.50ꢀ53 Details of their synthesis are provided in
Supporting information section.
Synthesis of Cu(10%)/Cu-BTC-MOF
First CuNPs was synthesized. For the synthesis of CuNPs, Cu(NO3)2.3H2O (0.19 g)
was added to 50 mL of ethanol and stirred at 2ꢀ3 °C for 10 minutes to get solution A. Sodium
dodecyl sulphate (0.31 g) and sodium borohydride (0.18 g) were dissolved in 35 mL of
ethanol and stirred at 2ꢀ3 °C for 10 minutes to get solution B. Then the solution A was slowly
added to solution B and the resultant solution was stirred at 2ꢀ3 °C for 30 minutes. Further,
the solution was kept at ambient condition for 45 minutes. The resultant black solution
confirmed the formation of CuNPs.
CuꢀBTCꢀMOF (0.5 g) was dispersed in 15 mL of ethanol. The dispersed CuꢀBTCꢀ
MOF was added to the colloidal CuNPs solution at ambient temperature. Resultant solution
was kept for 5 h under stirring condition at ambient temperature. The resulting product was
centrifuged, washed with ethanol, and dried in vacuum oven at 80 °C for 12 h.
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2.2. Procedure of catalytic reduction
Substrate (0.6 mmol) was treated with N2H4.H2O (1.2 mmol) in the presence of
catalyst (20 mg) in ethanol (2 ml) for stipulated time period. Progress of the reaction was
monitored using gas chromatograph (GC,Yonglin 6100; BPꢀ5; 30 m × 0.25 mm × 0.25 µm).
Product was confirmed using GCꢀMS (Schimadzu GCMSꢀQP 2010 Ultra Rtxꢀ5 Sil Ms; 30 m
× 0.25 mm × 0.25µm). Most of the reductions were initially carried out at ambient
temperature (25ꢀ27 °C. However, in some cases, reactions were carried out at elevated
temperature (80 °C or 120 °C) to achieve higher product yield. GC and MS spectra are
provided for the reduction of styrene, phenylacetylene, and benzaldehyde in the supporting
information section as examples after the GCꢀMS analysis (Fig. S1ꢀS3).
3. Results and discussion
3.1. Physico-chemical characterization
Powder XRD was elucidated to confirm the formation of MOF materials. XRD pattern of
CuꢀBTCꢀMOF matches well with the XRD pattern reported for HKUSTꢀ1 (Fig. 1).48 XRD
patterns of CuꢀBDCꢀMOF, ZrꢀBDCꢀMOF, MnꢀBDCꢀMOF, and NiꢀBTCꢀMOF match well
with the XRD patterns of CuꢀBDC, UiOꢀ66, MnꢀBDC, and NiꢀBTC, respectively, reported in
the literature (Fig. S4).50ꢀ53 Cu nanoparticles (CuNPs) were supported on the external surface
of CuꢀBTCꢀMOF. XRD pattern of Cu(10%)/CuꢀBTCꢀMOF is similar to that of parent Cuꢀ
BTCꢀMOF (Fig. 1). However, the peak intensity in the diffraction pattern of Cu(10%)/Cuꢀ
BTCꢀMOF is significantly reduced (almost five times) when compared to the parent Cuꢀ
BTCꢀMOF. Distinguished reflection associated with CuNPs is not observed in the diffraction
pattern of Cu(10%)/CuꢀBTCꢀMOF which suggests that the sizes of CuNPs present on the
external surface of CuꢀBTCꢀMOF can be very small. Reduction in the peak intensity suggests
that during the synthesis of CuNPs, partial destruction of framework structure has taken
place. Intensity can also be reduced due to the partial occupation of the CuNPs on the
exposed surface of CuꢀBTCꢀMOF. With increase in loading of CuNPs, the intensity of XRD
peaks decreases (Fig. S5). This result suggests that more concentration of NaBH4 in the
solution and more occupation of CuNPs on the exposed surface of CuꢀBTCꢀMOF influence
the XRD peak intensity.
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Fig. 1. Powder XRD patterns of CuꢀBTCꢀMOF and Cu(10%)/CuꢀBTCꢀMOF.
N2ꢀadsorption experiments were carried out to determine the textural properties of the
material. CuꢀBTCꢀMOF exhibits a type II adsorption isotherm with gradual increase in the
adsorbed volume in the range of 0.2 to 0.95 (P/P0) followed by the steep adsorption with P/P0
greater than 0.95 (Fig. 2). Changes in the isotherm behaviour are observed after the CuNPs
loading. A somewhat reduced adsorption volume is observed in Cu(10%)/CuꢀBTCꢀMOF.
Changes in the isotherm behaviour suggest that the partial loss of surface coverage and
porosity has taken place after the CuNPs loading. It is also possible that reducing
environment influences the textural characteristics of the material. Micropore size
distribution was determined from nonlocal density functional theory (NLDFT) and mesopore
size distribution was determined from BarretꢀJoynerꢀHalenda (BJH) method. Micropore size
distribution is similar for parent CuꢀBTCꢀMOF and Cu(10%)/CuꢀBTCꢀMOF materials (Fig.
2, inset b). It shows micropores in the range of 0.5 to 0.9 nm. A difference in the mesopore
size distribution is observed. CuꢀBTCꢀMOF shows mesopore size distribution in the range of
3ꢀ5 nm with peak maximum at 3.8 nm whereas Cu(10%)/CuꢀBTCꢀMOF shows biꢀmodal
mesopore size distribution in the range of 1.5ꢀ7 nm with peak maxima at 1.9 nm and 2.9 nm
(Fig. 2, inset a). Table 1 clearly shows that the loading of CuNPs reduces the surface area and
pore volume of the material. CuꢀBDCꢀMOF, MnꢀBDCꢀMOF, and NiꢀBTCꢀMOF exhibit type
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II isotherms similar to that of CuꢀBTCꢀMOF but much less adsorbed volume (Fig. S6a).
Multiꢀmodal pore size distribution is observed for these three samples (Fig. S6a, inset).
Surface area and pore volume of these samples are significantly lower than CuꢀBTCꢀMOF
(Table S1). ZrꢀBDCꢀMOF exhibits type IV isotherm with H2 hysteresis loop (Fig. S6b). A
gradual increase in the N2ꢀadsoption is observed in the range of 0.2 to 0.6 (P/P0) which is
followed by a steep adsorption above P/P0 of 0.7. Adsorption above P/P0 = 0.7 corresponds to
capillary condensation in the mesopores. Biꢀmodal mesopore size distribution for ZrꢀBDCꢀ
MOF is observed (Fig. S6b, inset). Surface area and pore volume of ZrꢀBDCꢀMOF are lower
than CuꢀBTCꢀMOF but significantly higher than other MOFs investigated in this study
(Table 1 and Table S1).
600
500
400
1.4
(b)
2.0
(a)
1.2
1.0
1.5
300
0.8
1.0
0.5
0.0
0.6
0.4
0.2
0.0
200
100
0
2
4
6
8
10
2
4
6
Pore size8(n1m0 )12
Pore size (nm)
Cu-BTC-MOF
Cu(10%)/Cu-BTC-MOF
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Relative Pressure (P/P0)
Fig. 2. N2ꢀadsorption desorption isotherms of CuꢀBTCꢀMOF and Cu(10%)/CuꢀBTCꢀMOF.
Inset shows (a) BJH pore size distribution and (b) NLDFT pore size distribution.
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Table 1. Physicoꢀchemical properties of various catalysts investigated in this study.
aSurface area External surface
Total pore
volume (cc/g)
1.01
Cu content
(Wt %)b
17.8
Entry
SBET (m2/g)
932
area (m2/g)
298
CuꢀBTCꢀMOF
Cu(10%)/CuꢀBTCꢀMOF
cCu(10%)/CuꢀBTCꢀMOF
758
240
0.84
26.2
749
232
0.82
26.0
aSBET calculated from the adsorption branch in the region of 0.05<P/P0≤0.3.bObtained from
MPꢀAES.cTextural properties of the recovered catalyst after the recycling process.
Fig. 3. SEM images of (aꢀc) CuꢀBTCꢀMOF and (dꢀf) Cu(10%)/CuꢀBTCꢀMOF.
Morphology of the materials was investigated using SEM (Fig. 3). Small
magnification SEM image shows that CuꢀBTCꢀMOF crystals are of varied morphology such
as octahedral, triangular, and distorted octahedral and so on (Fig. 3aꢀc). One high
magnification image of octahedral crystal is shown in Figure 3c. The surface of this crystal is
very smooth. Morphology of Cu(10%)/CuꢀBTCꢀMOF crystals is somewhat distorted when it
is compared with the morphology of the parent CuꢀBTCꢀMOF as shown in the small
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magnification SEM image (Fig. 3dꢀf). One such distorted octahedral image is presented here
(Fig. 3f). High magnification SEM image shows that surface of the crystal is rough which
suggests that the external surface was corroded when it was in the direct contact with the
solution containing NaBH4 and SDS (Fig. 3f). NaBH4 reducing environment seems to affect
the outer morphology of the parent material while CuNPs are supported on its surface. Cuꢀ
BDCꢀMOF and MnꢀBDCꢀMOF shows aggregated crystal morphology with different
dimensions (Fig. S7). ZrꢀBDCꢀMOF shows aggregated flake like crystal morphology of
various sizes (Fig. S7). NiꢀBTCꢀMOF shows a large beautiful spherical morphology (Fig.
S7). The details of microꢀnano structure of the highly active catalyst Cu(10%)/CuꢀBTCꢀMOF
were investigated using high resolution transmission electron microscope (HRTEM). Cuꢀ
BTCꢀMOF and CuNPs domains are clearly visible in the TEM images (Fig. 4a,b). HRTEM
images also reveal that CuNPs are present in the sample (Fig. 4c,d). HRꢀTEM image further
confirms the loading of 3ꢀ7 nm CuNPs on the surface of CuꢀBTCꢀMOF (Fig. 4e). Lattice
fringes with interlayer spacing of 1.8 nm confirm the formation of CuNPs (Fig. 4e). Selected
area diffraction pattern further confirms the crystalline nature of CuNPs (Fig. 4f).
Fig. 4. TEM images of Cu(10%)/CuꢀBTCꢀMOF.
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Cu-BTC-MOF
Cu(10%)/Cu-BTC-MOF
100
80
60
40
20
0
14.5
7.1
31
7.5
41.5
73.2
32
81.8
8.6
200 400 600 800 1000
Temperature (°C)
Fig. 5. Thermograms of CuꢀBTCꢀMOF and Cu(10%)/CuꢀBTCꢀMOF.
Thermogravimetric analysis shows different weight loss profiles for both these
materials (Fig. 5). CuꢀBTCꢀMOF shows the weight loss in the range of 30ꢀ135 C due to the
removal of physisorbed H2O molecules present in the porous structure. Somewhat stable
profile is obtained in the range of 135ꢀ340 C. Loss occurred in this temperate range is due to
the removal of ethanol molecules present in the micropores. A sharp decrease in the weight
loss at 350 C is due to the framework decomposition of CuꢀBTCꢀMOF. Only a marginal
decrease in the weight loss is observed in the range of 400ꢀ1000 C confirming the stability
of copper oxide formed after the decomposition of CuꢀBTCꢀMOF. TGA profile shows that
°
°
°
°
18.2% of CuO is remained after 1000
°C heating. Cu(10%)/CuꢀBTCꢀMOF shows the weight
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loss in the range of 30ꢀ135
matrix. Further, a stable profile is observed in the range of 135ꢀ300
temperate range is due to the removal of ethanol molecules present in the micropores. A
sloping decrease in the weight loss in the range of 300 – 600 C is due to the framework
decomposition of CuꢀBTCꢀMOF and the removal of any occluded SDS. Only a marginal
decrease in the weight loss is observed in the range of 600ꢀ1000 C confirming the stability
°C due to the removal of physisorbed H2O molecules present in
°C. Loss occurred in this
°
°
of copper oxide and Cu present in Cu(10%)/CuꢀBTCꢀMOF. TGA profile shows that 26.8
wt% of CuO and CuNPs in the sample. Based on these results, it can be concluded that 8.6%
of CuNPs is loaded in the sample. Thermograms of various other MOFs synthesized in this
study are provided in Figure S8. Different compositional behaviour and stability are clearly
observed from the thermograms of various MOFs investigated in this study.
The FTꢀIR spectrum of CuꢀBTCꢀMOF shows characteristic absorptions at 1645 cmꢀ1
and 1371 cmꢀ1 which are attributed to asymmetric stretching frequency and symmetric
stretching frequency of deprotonated carboxylic group. This suggests the coordination of
Cu+2 with carboxylate group of BTC ligand (Fig. S9a). The ringꢀoutꢀofꢀplane vibration of the
linker BTC molecules appeared at 729 cmꢀ1, which further suggests the effective coordination
of the Cu ions to the BTC ligands. The peak at 490 cmꢀ1 can be assigned to CuꢀO stretching
vibration. FTꢀIR spectrum of Cu(10%)/CuꢀBTCꢀMOF is very similar to that of CuꢀBTCꢀ
MOF, except some marginal shift in the peak position (Fig. S9a). The broadening of peak at
490 cmꢀ1 confirms the incorporation of different types of Cu species (CuꢀO and Cu metal) in
the sample. The characteristic bands of the carboxylate group located at 1660ꢀ1510 cmꢀ1
(asymmetric vibration) and 1470ꢀ1390 cmꢀ1 (symmetric vibration) are observed for various
MOFs investigated in this study (Fig. S9b). These bands suggest the coordination of metal
ions with the deprotonated carboxylate group of the linker (BTC/BDC). Less intense bands
located around 1100 cmꢀ1 and in the range of 860ꢀ710 cmꢀ1 indicate stretching vibrations of
CꢀC groups and the inꢀplane & the outꢀofꢀplane deformation vibrations of the CꢀH groups in
the benzene ring of BTC and BDC ligands. FTꢀIR peaks in the range of 400ꢀ650 cmꢀ1 confirm
the formation of MꢀO in the MOF structure (Fig. S9b).
DRUVꢀvisible spectroscopy provides indirect evidence for the incorporation of Cu in
the MOF framework structure and CuNPs on the external surface of MOF (Fig. S10). Charge
transfer bands are observed in the range of 250ꢀ450 nm whereas absorption above 600 nm is
associated with dꢀd transition.54 In addition, absorption in the range of 500ꢀ600 nm is
attributed to the plasmonic resonance of Cun nanoparticles. CuꢀBTCꢀMOF exhibits charge
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transfer absorption in the range of 200ꢀ310 nm and 350ꢀ450 nm. In addition, absorption in the
range of 500ꢀ1000 nm with a peak maximum of 700 is observed (Fig. S10a). Cu(10%)/Cuꢀ
BTCꢀMOF exhibits almost similar absorption in the range of 200ꢀ500 nm. However, some
change in the absorption band in the range of 500ꢀ1000 nm is observed, which confirms the
incorporation of CuNPs/CuO species in the matrix of CuꢀBTCꢀMOF. Shift in the absorption
spectrum in the range of 500ꢀ600 nm further suggests the formation of CuNPs/CuO species.
DRUV–vis spectra of ZrꢀBDCꢀMOF, MnꢀBDCꢀMOF, CuꢀBDCꢀMOF and NiꢀBTCꢀMOF
exhibit absorptions in the range of 250–450 nm which are attributed to the metal to ligand
and ligand to metal charge transfer bands (Fig. S10b). In addition, absorption in the range of
500ꢀ1000 nm corresponds to dꢀd transition.
+2
Cu 2p3/2
(943.4) (939.3)
Satellites Cu 2p3/2
+2
0/+1
Cu 2p3/2
(934.2)
(932.2)
950 945 940 935 930 925
Binding Energy (eV)
Fig. 6. High resolution XPS spectrum for Cu species present in Cu(10%)/CuꢀBTCꢀMOF.
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The details of various Cu species present in Cu(10%)/CuꢀBTCꢀMOF were obtained
from XPS investigation. High resolution XPS spectrum obtained for Cu species is presented
in Figure 6. Two spinꢀorbit peaks in the Cu 2p spectrum are observed which are consistent
with the Cu 2p3/2 and Cu 2p1/2 electronic configurations. Deconvolution of Cu 2p3/2 shows
that it has peaks at about 932.2 and 934.2 eV, for Cu0/Cu+ and Cu2+ species, respectively.55
Shake up satellite peak at 943.4 is observed due to Cu2+ species. XPS analysis results are
consistent with the results obtained from DRUVꢀvisible investigation. Bulk chemical
composition was determined using elemental analysis (Table 1). Analysis shows that 17.8
wt% of Cu is present in CuꢀBTCꢀMOF. After the CuNPs incorporation, Cu content is
increased to 26.2 wt %, which signifies that amount of CuNPs loading is 8.4 wt % (Table 1).
Amount of CuNPs loading in Cu(5%)/CuꢀBTCꢀMOF and Cu(15%)/CuꢀBTCꢀMOF were
estimated to be 4.6 wt% and 13.1 wt%, respectively.
3.2.
Catalytic activity
The objective of this study was to develop economical transition metal based robust
catalyst for the reduction of various compounds. Reaction condition was optimized using
highly active catalyst. First, it was important to find suitable active catalyst for this
transformation. Styrene was chosen as a model reactant to screen the catalytic activity of
various MOFs prepared in this study (Table 2). CuꢀBTCꢀMOF exhibited the highest activity
among various MOFs investigated in this study (Table 2). The activity of CuꢀBDCꢀMOF was
only marginally lower than CuꢀBTCꢀMOF (Table 2). CuNPs were supported on the CuꢀBTCꢀ
MOF. With increase in the loading of CuNPs from 5 wt% to 10 wt%, the ethylbenzene yield
was increased from 74.2 wt% to 96.4 wt% (Table 2). However, with further increase in the
CuNPs loading from 10 wt% to 15 wt%, ethylbenzene yield was decreased from 96.4 to 86.9
% (Table 2). These optimizations lead us to conclude that 10 wt% CuNPs loading was
optimum to achieve the highest yield. Based on the results obtained, it can be concluded that
an optimum loading of CuNPs (10 wt%) in the catalyst could provide highly dispersed
CuNPs sites which were accessible and reactive during the reduction of styrene.
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Table 2. Influence of various MOF catalysts investigated in the reduction of styrene.
Entry No.
MOF (Amount in mmol)
Ethyl
TON
benzene
yield (%)
1.
2.
ZrꢀBDCꢀMOF (0.0122)
MnꢀBDCꢀMOF (0.0301)
15.6
7.6
4.8
24.2
3.
4.
CuꢀBDCꢀMOF (0.0664)
CuꢀBTCꢀMOF (0.0330)
61.1
66.3
22.2
96.4
74.2
86.9
91.3
5.5
12
5.
NiꢀBTCꢀMOF (0.0335)
4
7.
Cu(10%)@CuꢀBTCꢀMOF (0.0358)
Cu(5%)@CuꢀBTCꢀMOF (0.0355)
Cu(15%)@CuꢀBTCꢀMOF (0.0360)
Cu(10%)@CuꢀBDCꢀMOF (0.0358)
16.2
12.6
14.5
15.3
8.
9.
a10.
Reaction condition: Styrene (0.6 mmol), N2H4.H2O (1.2 mmol), ethanol (2.0 mL),
temperature (RT, 25ꢀ27
C), time (6 h). aCatalytic activity data after 5th recycle.
°
Influence of catalyst amount was investigated by varying the catalyst amount in the
range of 5 mg to 25 mg (Table S2). 20 mg of the catalyst was required to achieve the better
activity when catalyst amount and product yield both were considered. Having found the best
catalyst for the selective reduction of styrene to ethylbenzene, influence of reaction medium
was investigated (Table S2). Various polar and nonpolar solvents were investigated. Reaction
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took place in various medium but ethylbenzene yield was significantly varied by varying the
reaction medium. Non polar solvents were less effective than polar solvents (Table S2).
Reaction proceeded well in polar protic solvent ethanol. However, it was not preceded well in
the water medium. Further, the reaction also took place in the neat condition but the yield of
ethylbenzene was very low (Table S2). Therefore, ethanol was chosen as an optimum
reaction medium for further investigation. Several reducing agents/transfer hydrogenation
reagents such as isopropanol, formaldehyde, formic acid, and NH3 were employed but all of
them were found ineffective in the reduction process (Table S2).
Cu(10%)/CuꢀBTCꢀMOF was easily recovered using centrifuge machine and reused
for five times. Recycling study showed that no significant decrease in the ethylbenzene yield
was observed (Fig. S11). Elemental analysis of the recovered catalyst showed that 0.13 wt%
of Cu was leached to the solution after five recycles. After the fifth cycle, the recovered
catalyst was subjected to XRD and other physicoꢀchemical characterization. XRD pattern
(Fig. S12), SEM image (Fig. S13), and FTꢀIR spectrum (Fig. S14) of the recovered catalyst
were similar to that of fresh catalyst. Surface area of the recovered catalyst was similar to that
of the fresh catalyst (Table 1). To confirm that the reaction took place due to the
heterogeneous catalyst and not due to the leached Cu in the solution, catalyst was removed
from the reaction mixture after 1.5 h and the reaction was continued further for 4.5 h. The
plot of ethylbenzene yield and reaction time confirmed that the reaction did not proceed
further after the removal of catalyst (Fig. S15). Analysis of the reaction mixture solution
showed that only 0.05 wt% Cu was leached to the reaction mixture that was unable to
catalyze the reaction under this experimental condition.
Reduction using hydrazine is believed to be mediated through diimide (the
hydrogenating source. Diimide are produced inꢀsitu in the presence of H2O2 or air.56ꢀ57 In this
study, Cu(10%)/CuꢀBTCꢀMOF produces diimide via dissolved O2 present in the reaction
mixture, and transfers hydrogen to styrene resulting in the formation of ethylbenzene. To
prove this hypothesis, three control experiments were performed. Reactions were carried out
in N2 and Ar atmosphere. Before the reaction, reaction flask was deꢀaerated and the flask was
maintained under N2/Ar atmosphere before adding hydrazine. After 6 h of reaction only 6.7
and 4.9 % ethylbenzene yields were obtained in N2 and Ar atmosphere, respectively. These
experiments support the assumption that diimide is acting as a reagent in the reduction of
styrene (Scheme 1). Similarly, one more reaction was conducted. In this reaction, the flask
was deꢀaerated and then it was put under Ar atmosphere. After that H2O2 (two times excess to
that of hydrazine) was added after the addition of hydrazine. In this case 68.9 % ethylbenzene
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yield was obtained. These control reactions confirmed that oxygen was responsible for the
diimide formation which plays key role in the reduction process. Based on the catalytic
results obtained using Cu(10%)/CuꢀBTCꢀMOF, and the model reactions, a reaction
mechanism is proposed (Scheme 1). Two catalytically active sites are present in
Cu(10%)/CuꢀBTCꢀMOF, one is the framework Cu and the other is the CuNPs. Both these
sites simultaneously activate the hydrazine and the inꢀsitu generated hydrazine adducts
produce diimide which reduce the olefin via the concerted or double coordination mode27 to
produce the alkane (Scheme 1). CuꢀBTCꢀMOF exhibited large surface area with average pore
diameter of 3.8 nm. Therefore, the active sites present on the internal surface and on the
external surface were easily accessible for the reactant molecules in the reduction process.
However, CuNPs sizes were in the range of 3ꢀ7 nm that could only be present on the external
surface of the CuꢀBTCꢀMOF. Therefore, in the case of Cu(10%)/CuꢀBTCꢀMOF the reduction
due to CuNPs took place on the external surface but the reduction due to Cu present in the
CuꢀBTCꢀMOF could take place on the internal surface as well as on the external surface. In
one of the early literature, it is reported that Cu ion is the best among the wide range of metal
ions investigated that include Sb, As, Bi, Cr, Ce, Co, Au, Fe, Pb, Mg, Hg, Mo, Ni, Os, Pd, Pt,
Ag, Te, Sn, and V for the activation of hydrazine in the reduction reaction.58 The catalytic
hydrogenation is mediated through the activation of hydrazine hydrate to diimide followed by
the H2 generation for the reduction reaction which is facilitated efficiently by the CuꢀBTCꢀ
MOF/CuꢀBDCꢀMOF investigated in this study.
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Scheme 1: Mechanism for the reduction of alkene, alkynes, aldehydes, and ketones using
CuNPs decorated CuꢀBTCꢀMOF using hydrazine hydrate as reducing agent.
A range of styrene derivatives were reduced to corresponding ethylbenzene
derivatives in excellent yield at the ambient temperature (Table 3). Substituted styrene, for
example, αꢀmethyl styrene was converted to cumene. However, the yield was somewhat
lower than ethylbenzene (in case of styrene) under identical condition (Table 3, compare
entries 1 and 5). Similarly, aliphatic olefin such as 1ꢀoctene was reduced to octane in good
yield at ambient temperature (Table 3, entry 6). However, reduction had to perform at the
°
elevated temperature (80 C) to obtain the good yield of cyclohexane in the reduction of
cyclohexene (Table 3, entry 7). Similarly, αꢀsubstituted styrene such as
°
cinnammylalcohol/cinnamic acid was reduced at the elevated temperature (80 C) to get the
moderate to excellent yield (Table 3, entries 8ꢀ9). Comparative catalytic activity data reported
in literature for the reduction of styrene to ethylbenzene over various heterogeneous catalysts
using hydrazine hydrate as reducing agent are presented in (Table S3).27, 59ꢀ61 Comparative
data shows that Cu(10%)/CuꢀBTCꢀMOF exhibits better or comparable activity with other
heterogeneous catalysts reported in the literature.
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Table 3. Reduction of olefins using Cu(10%)/CuꢀBTCꢀMOF.
Entry
Substrate
Temp. (oC)
Product yield (%)
TON
1
25
16.2
(96.4)
(92.1)
(82.5)
(90.6)
2
3
4
5
25
25
25
25
15.5
13.9
15.2
12.9
(76.6)
(78.4)
6
7
25
80
13.2
13.3
(79.2)
CH2OH
8
9
80
80
12.6
15.1
(75)
(90)
Reaction condition: Substrate (0.6 mmol), N2H4.H2O (1.2 mmol), ethanol (2 mL), catalyst
(0.0358 mmol, 20 mg), time (6 h). (RT = 25ꢀ27 °C)
Having accomplished the reduction of a wide range of olefins, the interest was to
extend the application of this catalyst in the complete reduction of alkynes. Under the
optimum condition, applicable for styrene reduction, phenylacetylene was observed to get
reduced only to a small extent, wherein styrene was obtained in high selectivity (Table 4,
entry 1). When the same reaction was conducted at a higher hydrazine to phenylacetylene
ratio (4:1) at ambient temperature, the conversion was not improved much but the selectivity
towards ethylbenzene was improved (Table 4, entry 2). The conversion of phenylacetylene
and selectivity for ethylbenzene were significantly improved when the reaction was carried
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out at elevated temperature of 80 °C and hydrazine to phenylacetylene ratio of (4:1) (Table
4, entry 3). These reactions provided evidence that the increase in reaction temperature and
phenylacetylene to hydrazine hydrate ratio (1:4), excellent conversion and ethylbenzene
selectivity were achieved. Substituted phenylacetylene derivatives also exhibited impressive
activity under the optimized reaction condition (80 °C, 6 h) (Table 4, entries 4 and 5).
However, diphenylacetylene was difficult to reduce. In this case, only Eꢀstilbene was
obtained as a selective product at 80 °C. When the temperature of the reaction was increased
to 120 °C, diphenylacetylene conversion slightly improved but Eꢀstilbene was obtained as a
selective product. Due to the strained ring and hindered rotation, diimide was difficult to bind
in the vicinity of diphenylacetylene for providing H2 to the substrate and therefore low
reactivity was obtained. Aliphatic compounds such as 1ꢀoctyne was converted to octane with
more than 90% selectivity, however it required longer duration (12 h) and higher reaction
temperature (120 °C) when compared to the phenylacetylene. Recycling study showed that
no significant decrease in the ethylbenzene yield was observed after five recycles during the
reduction of phenylacetylene (Table 4, entry 12). XRD pattern (Fig. S12) and FTꢀIR
spectrum (Fig. S14) of the recovered catalyst were similar to that of fresh catalyst.
Comparative data shows that Cu(10%)/CuꢀBTCꢀMOF exhibited better or comparable activity
with other heterogeneous catalysts reported in the literature (Table S4).11ꢀ14, 21
Table 4. Reduction of alkynes using Cu(10%)/CuꢀBTCꢀMOF.
S.N
1a
2
Alkyne
Temp.
(oC)
Time
(h)
Alkyne conv.
(%)
Product select. (%)
(TON)
14.4
25
25
6
6
6
(2.4)
(5.4)
(44.2)
(94.4)
(94.6)
(55.8)
(5.6)
18.6
(3.1)
3
96.8
80 °C
(16.3)
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4
6
93.5
80 °C
(15.7)
(92.6)
(93.5)
(7.4)
(6.5)
5
6
6
6
94.4
80 °C
(15.9)
25
6.2
(1.0)
(100)
7
8
6
6
26.6
(4.5)
80 °C
(100)
(100)
34.2
(5.7)
120 °C
9
25
6
12
12
6
4.5
(0.8)
55.4
(99.7)
(34.7)
(9.7)
(0.3)
(65.3)
(90.3)
10
11
12b
80 °C
120 °C
80 °C
(9.3)
88.4
(14.9)
92.3
(15.5)
(94.8)
(5.2)
Reaction condition: Alkyne (0.6 mmol), N2H4.H2O (2.4 mmol), ethanol (2 mL), catalyst
(0.0358 mmol, 20 mg). (RT, 25ꢀ27° C). a N2H4.H2O (1.2 mmol). b Catalytic activity data after
five cycle.
Having got success in the reduction of olefins and alkynes, our interest was to
examine whether the same protocol can be extended to the reduction of C=O also. Reduction
of aldehyde and ketone to corresponding alcohol is an important organic transformation.
Similar to styrene, benzaldehyde was also completely reduced to benzylalcohol at ambient
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temperature in 6 h under the same reaction condition (Table 5). Though substituted
benzaldeyde was also reduced at ambient temperature, but good yield was obtained at
°
elevated temperature (80 C). Various aliphatic and aromatic aldehydes were converted to
°
alcohols in good yields at 80 C (Table 5). Similarly, aliphatic and aromatic ketones were
°
converted to corresponding alcohols in moderate to good yields at 80 C (Table 5). The
reduction of aldehyde was more facile when compared to the ketone. Cinamaldehyde was
°
reduced under the same reaction condition but at 120 C. Complete conversion of
cinamaldehyde was observed but the selectivity towards cinamylalcohol was only 18 %.
Progress is underway to develop selective catalyst based on MOF which can selectively
reduce cinamaldehyde to cinamylalcohol in high yield. Recycling study showed that no
decrease in the benzylalcohol yield was observed after five recycles during the reduction of
benzaldehyde (Table 5, entry 13). XRD pattern (Fig. S12) and FTꢀIR spectrum (Fig. S14) of
the recovered catalyst were similar to that of fresh catalyst. Comparative data shows that
Cu(10%)/CuꢀBTCꢀMOF exhibited better or comparable activity with other heterogeneous
catalysts reported in the literature (Table S5).15, 17ꢀ23
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Table 5. Reduction of aldehydes and ketones using Cu(10%)/CuꢀBTCꢀMOF.
Entry
Substrate
Time
(h)
6
Temp.
(oC)
25
Product yield (%)
TON
1
16.8
(100)
(48)
2
3
6
6
25
80
8.1
14.9
(89)
(45)
(84)
4
5
6
6
6
25
80
7.6
14.1
3.0
12
120
(18)
(36)
(45)
7
8
9
6
6
25
80
6.0
10.9
6
25
80
7.6
(45)
10
6
13.1
(78)
(32)
11
12
6
6
6
25
80
25
5.4
11.3
16.8
(67)
13a
(100)
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Reaction condition: Substrate (0.6 mmol), N2H4.H2O (1.2 mmol), ethanol (2 mL), catalyst
(0.0358, 20 mg), (RT, 25ꢀ27 °C). a Catalytic activity data after five cycle.
4. Conclusions
Transition metal (Cu, Zr, Mn, Ni) containing metal organic frameworks were
investigated in the reduction of olefins using hydrazine hydrate as a reducing agent under the
mild reaction condition. Among the MOFs investigated in this study, CuꢀBTCꢀMOF
exhibited the highest activity in the reduction of styrene. Further CuNPs decorated CuꢀBTCꢀ
MOF was successfully prepared and thoroughly characterized. CuNPs(10%)/CuꢀBTCꢀMOF
exhibited better activity than the parent CuꢀBTCꢀMOF. Loading of CuNPs on the surface of
CuꢀBTCꢀMOF provided CuNPs and Cu2O isolated active sites that enabled higher activity
than the parent CuꢀBTCꢀMOF. The Cu (10%)/CuꢀBTCꢀMOF exhibited efficient recyclability
even after five recycles. Leaching experiments showed that extremely low quantity of Cu was
leached during the reaction and the Cu present in the catalyst catalyzed this reaction in the
heterogeneous manner. CuNPs(10%)/CuꢀBTCꢀMOF was unable to reduce the styrene in
N2/Ar environment in the presence of hydrazine hydrate. However, they were able to reduce
styrene and produced good yield of ethylbenzene in the presence of H2O2 and hydrazine
hydrate in Ar environment. These investigations confirmed that air or O2 are required to
catalyze this reduction process. Based on these investigations, catalytic data, and
physicochemical characterizations, reaction mechanism for the reduction using hydrazine
hydrate was proposed. The reduction protocol was successful in the reduction of a wide range
of aliphatic and aromatic olefins. The highly active catalyst exhibited excellent activity in the
reduction of a wide range of aliphatic and aromatic alkynes. Finally, the application of the
highly active CuNPs(10%)/CuꢀBTCꢀMOF is successfully demonstrated in the reduction of
aldehydes and ketones. Following are the key advantages of the present protocol: (i) the
reusability and sustainability of the catalyst, (ii) the use of low amount of reducing agent, (iii)
the ecoꢀfriendliness of the process (because N2 is the byꢀproduct of this reduction process),
(iv) the involvement of a simple workꢀup and separation of the products from the reaction
system. The progress is underway to develop a unique catalyst which can reduce difficult
alkene and alkynes.
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Associated content
Supplementary Information
The Supporting Information is available free of charge on the RSC Publications website at
DOI: . Supporting information includes details of materials, catalyst preparation, catalyst
characterization. Figures S1ꢀS15 include GCꢀMS analysis data, XRD patterns, N2ꢀadsorption
desorption isotherms, SEM images, thermograms, FTꢀIR spectra, DRUVꢀvisible spectra,
recyclability and leaching studies plots, XRD, FTꢀIR and SEM of recycled catalyst. Table S1ꢀ
S5 includes textural properties of MOFs, influence of catalyst amount, various solvents &
reducing agents, comparative catalytic activity data reported in the literature for the reduction
of styrene, phenylacetylene, and benzaldehyde.
Acknowledgements
Authors are thankful to DST, New Delhi for funding (EMR/2016/001408). Authors are
thankful to SAIF, IIT Bombay for HRTEM analysis and ACMS, IIT Kanpur for XPS
analysis.
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