Cu3(BTC)2 metal organic framework as heterogeneous solid catalyst for the reduction of styrenes with silane as reducing agent

Article abstract of DOI:10.1016/j.ica.2019.119026

In this work, a well known metal organic framework, Cu₃(BTC)₂ (BTC: 1,3,5-benzenetricarboxylate) is reported as a heterogeneous solid catalyst for the reduction of styrene and its derivatives with silane as a reducing agent. Under these reaction conditions, a quantitative conversion of styrene is achieved with very high selectivity to ethylbenzene. A control experiment with pyridine as a catalyst poison revealed that Cu²⁺ located within the framework plays a crucial role in promoting this reduction. Further, hot-filtration test indicated the absence of metal leaching and Cu₃(BTC)₂ is used four times with no significant decay in its activity. In addition, the four times used Cu₃(BTC)₂ was compared with the fresh solid by powder X-ray diffraction, FT-IR, UV–Visible diffuse reflectance spectra, scanning electron microscope and electron paramagnetic resonance methods and observing no significant changes in its structural integrity, crystallinity and morphology. This process is extended for other styrene derivatives to their respective reduced products.

Full text of DOI:10.1016/j.ica.2019.119026

Accepted Manuscript
Research paper
Cu3(BTC)2 metal organic framework as heterogeneous solid catalyst for the re-
duction of styrenes with silane as reducing agent
Nagaraj Anbu, Amarajothi Dhakshinamoorthy
PII:
S0020-1693(19)30378-0
https://doi.org/10.1016/j.ica.2019.119026
119026
DOI:
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Reference:
ICA 119026
Inorganica Chimica Acta
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21 March 2019
11 July 2019
17 July 2019
Please cite this article as: N. Anbu, A. Dhakshinamoorthy, Cu3(BTC)2 metal organic framework as heterogeneous
solid catalyst for the reduction of styrenes with silane as reducing agent, Inorganica Chimica Acta (2019), doi:
https://doi.org/10.1016/j.ica.2019.119026
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Cu3(BTC)2 metal organic framework as heterogeneous solid catalyst for the reduction of
styrenes with silane as reducing agent
Nagaraj Anbu and Amarajothi Dhakshinamoorthy
School of Chemistry, Madurai Kamaraj University, Madurai-625 021, Tamil Nadu, India.
E-mail: admguru@gmail.com
In this work, a well known metal organic framework, Cu3(BTC)2 (BTC: 1,3,5-
benzenetricarboxylate) is reported as a heterogeneous solid catalyst for the reduction of styrene
and its derivatives with silane as a reducing agent. Under these reaction conditions, a quantitative
conversion of styrene is achieved to with very high selectivity to ethylbenzene. A control
experiment with pyridine as a catalyst poison revealed that Cu2+ located within the framework
plays a crucial role in promoting this reduction. Further, hot-filtration test indicated the absence
of metal leaching and Cu3(BTC)2 is used four times with no significant decay in its activity. In
addition, the four times used Cu3(BTC)2 was compared with the fresh solid by powder X-ray
diffraction, FT-IR, UV-Visible diffuse reflectance spectra, scanning electron microscope and
electron paramagnetic resonance methods and observing no significant changes in its structural
integrity, crystallinity and morphology. This process is extended for other styrene derivatives to
their respective reduced products.
1

1. Introduction
Metal organic frameworks (MOFs) are crystalline porous materials whose crystal
structure is defined as the coordination between the metal ions or clusters with rigid organic
linkers resulting in the formation one-, two- and three-dimensional solids.[1-3] Thus, the
coordination between metal ions and linkers not only forms rigid structure but also results in
porous networks possessing high surface area and pore volume.[4] In recent years, it has been
demonstrated that by appropriate selection of metal ions and linkers, one can easily obtain a
MOF with very high pore volume and surface area. These interesting properties of MOFs
encouraged researchers to employ these solids in many applications including gas storage,[5] gas
separation,[6, 7] sensors,[8] drug delivery,[9] proton conduction[10] and heterogeneous
catalysis.[11-13]
MOFs have been widely used as heterogeneous catalysts for a wide range of reactions
including oxidations,[14, 15] condensations,[16] C-C coupling reactions,[17] tandem
reactions[18] and photocatalytic applications.[11, 19, 20] In recent years, considerable attentions
have been devoted for the use of MOFs as solid catalysts for the oxidation of alkanes, alkenes,
amines and other heterocyclic compounds.[21-23] In contrast, MOFs have not been very often
used as catalysts for reduction reactions due to the possibility that the metal ions located within
the framework can undergo reduction, thus collapsing the stability of MOFs under reducible
reaction conditions. Therefore, one of the prerequisites of using MOFs as heterogeneous
catalysts is to appropriately select reagents, solvents and substrates which are inert to MOF
framework in order to maintain its structural framework under the reaction conditions. From the
environmental point of view, hydrogen is a preferred reducing agent, but however, it often
2

requires the use of noble metal nanoparticles as active sites for the reduction of alkenes and other
functional groups.
Besides conventional reducing agents, silanes have also been reported as reducing agents
either with homogeneous or heterogeneous catalysts under mild reducing conditions for various
functional groups.[24-33] For example, porous metal-organic aerogel containing dirhodium
paddle wheels was reported as an efficient and stable heterogeneous catalyst for the reduction of
aldehydes and ketones using phenylsilane as a reducing agent.[34] Hydrogenation of carbonyl
compounds were performed under metal-free conditions using polymethylhydrosiloxane as
reducing agent under mechanochemical conditions.[35] Furthermore, HKUST-1 was also
reported as heterogeneous catalyst for the reduction of acetophenone to 1-phenylethanol using
silane as reducing agent at room temperature.[36] In another precedent, cobalt(II)phthalocyanine
was shown as an efficient and highly chemoselective catalyst for the reductive amination of
aldehydes and ketones using ethanol as a green solvent.[37] Also, amine-bis(phenolate)Fe(III)
catalyst was employed as moisture stable catalyst for the reduction of ketones and aldehydes to
secondary and primary alcohols, respectively using silane as a reducing agent.[38] Besides the
reduction of carbonyl compounds, silanes were also reported as reducing agents for the reduction
of nitroarenes. For instance, a chemoselective reduction of nitroarenes was reported with a stable
o
iron(III) catalyst using silane as reducing agent at 80 C.[39] Aromatic and aliphatic nitro
compounds were reduced to their respective amines using HSiCl3 as reducing agent under metal
free conditions.[40] Furthermore, N-heterocyclic carbene functionalized MOF by post-synthetic
ligand exchange was reported as an efficient and selective hydrosilylation of CO2.[41]
MOFs have been frequently used as catalysts or supports for the metal nanoparticles to
reduce styrene to ethylbenzene.[42-45] For instance, Pd/MOF-5 was reported as an efficient
3

solid catalyst for the reduction of 1 compared to Pd/AC.[46] Ni/Mes-MOF was also reported as
an efficient heterogeneous solid catalyst for the reduction of 1.[47] Similarly, Pd/Tm-MOF
catalyst exhibited a high catalytic activity in the hydrogenation of 1.[48] Pd/UiO-67 was used as
heterogeneous solid catalyst for the reduction of 1 at room temperature using hydrogen as
reducing agent.[49] In another precedent, Ir-pincer modified NU-1000 solid MOF catalyst was
reported as an active catalyst for the liquid phase reduction of 1.[50] However, there was no
report in the literature for the reduction of olefins to the saturated hydrocarbons using silane as a
reducing agent with MOFs as heterogeneous catalysts.
The present work aims to use Cu3(BTC)2 as heterogeneous solid catalyst for the reduction
of styrene to ethylbenzene using dimethylphenylsilane (DMPS) as a reducing agent without
affecting the framework stability of MOF. Among the various silanes screened as reducing
agents, DMPS showed higher activity and provided better stability to MOF framework. Also, the
available catalytic data indicate that Cu3(BTC)2 exhibits higher activity in the reduction of
styrene to ethylbenzene without undergoing significant structural damage. Hence, it is believed
that this catalytic method would trigger to develop other solid catalysts for the reduction of
olefins.
2. Experimental Section
2.1 Materials
Cu3(BTC)2 commercially known as Basolite C300 MOF, dimethylphenylsilane,
diphenylsilane, phenylsilane and triethylsilane used in the present study were purchased from
Sigma Aldrich and used as received. Styrene and its derivatives, phenylacetylene, allylbenzene
and cis-cyclooctene were obtained from Sigma Aldrich and used as received.
4

2.2 Instrumentation
FT-IR spectra were recorded using a Bruker tensor 27 series FT-IR spectrometer in the
region of 400-4000 cm-1 with 2 cm-1 resolution and 16 scans. UV-Visible UV-DRS studies were
recorded by Shimadzu UV-2700; ISR-2600 plus instrument in the range from 200-900 nm and
the samples were dispersed on the surface of BaSO4. Powder XRD diffraction patterns were
measured in the refraction mode in a Philips X’Pert diffractometer using the CuK radiation ( =
1.54178 Å) as the incident beam, PW3050/60 (2 theta) as Goniometer, PW 1774 spinner as
sample stage, PW 3011 as detector, incident mask fixed with 10 mm. PW3123/10 for Cu was
used as a monochromator. The samples were scanned between 2o and 70o with the scan rate of
0.02o/s. SEM studies were performed using a Hitachi S-3000H scanning electron microscope.
EPR spectra were recorded by using a Bruker EMX spectrometer, with the typical settings
(frequency 9.80 GHz, sweep width 30.6 G, time constant 80 ms, modulation frequency 100 kHz,
modulation width 0.2 G, microwave power 200 mW). Conversion and selectivity were
determined by Agilent 7820 A gas chromatography (GC) using HP-5 column of 30 m length and
high pure nitrogen (99.999%) as carrier gas with flame ionization detector. The initial oven
o
o
temperature was kept at 50 C and reached the final temperature to 280 C at the ramp rate 10
oC/min. The observed products in the final reaction mixture were confirmed by 5890A model of
Agilent GC-MS instrument using HP-5 column of 30 m length and high pure helium gas as
carrier gas.
2.3 Reaction Procedure
In a typical reaction, 20 mg of Cu3(BTC)2 was added to the reaction flask containing (0.5
mmol) of styrene (1) followed by 0.5 mL of tetrahydrofuran, ethanol (1 mmol) and DMPS
(1 mmol). After completion of sequencial addition of these reagents, the reaction flask was
5

closed by silicon rubber septum then this reaction mixture was stirred at 70 oC for required time
as indicated in Tables 1 and 2. It was observed visually that the reaction mixture turns from blue
to brown and returned back to original blue colour after 30 min with the evolution of hydrogen
gas. The reaction progress was monitored by GC and after completion of the reaction, the
mixture was washed three times with acetone and filtered. Then, the product was analyzed by
GC for its purity and selectivity. Conversion was determined by Agilent 7820 A GC using
internal standard method. The identical procedure was followed for the reusability experiments
except the use of recovered solid catalyst with fresh reactants.
3. Results and Discussion
The crystal structure of Cu3(BTC)2 is constituted by the coordination between the clusters
of two copper ions with four carboxylate groups of BTC linker forming a paddle-wheel building
unit that behaves as a structural node held in place by the tripodal BTC linkers. This MOF
possesses a BET surface area around 1050 m2/g with 11, 16 and 6 Å pore dimensions.[21]
Further, it has been shown that Cu2+ in Cu3(BTC)2 can behave as a Lewis acid in many organic
transformations.[21] These structural features and the Lewis acid site in Cu3(BTC)2 prompted us
to employ this solid for the reduction reactions.
As commented earlier, one of the main objectives of this work is to demonstrate that
MOF can be used as heterogeneous catalyst for the reduction reactions. Hence, Cu3(BTC)2 is
identified as a solid heterogeneous catalyst and the reactions are optimized by selecting styrene
(1) as a model substrate. Further, the catalytic activity of Cu3(BTC)2 was examined in the
reduction of 1 using silane as reducing agent under mild reaction conditions. A blank control
experiment in the absence of catalyst with DMPS as the reducing agent in THF showed no
o
conversion of 1 at 70 C after 30 min. In contrast, the reduction reaction of 1 was completed
6

quantitatively using Cu3(BTC)2 as catalyst under identical reaction conditions. The conversion of
1 was significantly reduced upon decreasing the reaction temperature. For example, the
conversion of 1 was drastically reduced from 100 to 9% when the reaction temperature decreased
from 70 to 50 oC, respectively under similar reaction conditions. Further, the conversion of 1 was
o
negligible at 30 C under identical conditions. These results clearly suggest that the reaction
temperature plays an important role to activate DMPS in the presence of Cu3(BTC)2 to produce
molecular hydrogen. Next, the catalyst loading was optimized under similar experimental
conditions. The conversion of 1 was decreased to 96% with the catalyst loading of 15 mg and
further decrease of catalyst loading to 10 mg, the conversion of 1 lowered to 82% at 70 oC using
DMPS as reducing agent after 30 min. The activity of Cu3(BTC)2 was reduced nearly 30% when
the loading of 1 increased to two-fold under optimized reaction conditions. This decrease of
activity upon increasing 1 may be due to its difficulty to reach the active sites through diffusion
limitations due to the presence of silanes, competitive binding of 1 versus DMPS. Hence, the
optimum loading of 1 was 0.5 mmol compared to other loadings. Finally, the next parameter to
be optimized was the nature of solvent. Hence, various solvents were employed for the reduction
of 1 using Cu3(BTC)2 as catalyst with DMPS as reducing agent. As it can be seen in Table 1,
ethanol and methanol afforded 85 and 83% conversions of 1 under identical conditions whereas
water, acetonitrile and 1,2-dichloroethane afforded negligible conversion of 1 under similar
conditions. In contrary, the conversion of 1 was 94 and 66% in acetone and toluene, respectively,
while the use of n-hexane as solvent afforded quantitative conversion of 1 under the optimized
reaction conditions.
7

Table 1. Optimization of the reaction conditions for the reduction of 1 to 2 using Cu3(BTC)2 as
heterogeneous solid catalyst.a
Cu3(BTC)2
DMPS
2
1
Entry
Catalyst (mg)
T (oC)
Solvent
Conv. of 1b TONg
(%)
c
1
2
-
70
70
50
30
70
70
70
70
70
70
70
70
70
70
THF
THF
-
-
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2 (15)
Cu3(BTC)2 (10)
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
100
9
10
0.9
-
3
THF
4
THF
0
5
THF
96
82
85d
67e
9.7
8.3
8.6
6.8
-
6
THF
7
THF
8
THF
f
9
THF
-
10
11
12
13
14
Ethanol
Methanol
Water
85
83
2
8.6
8.4
0.2
-
Acetonitrile
1,2-
-
1
0.1
Dichloroethane
Acetone
Toluene
n-Hexane
15
16
17
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
70
70
70
94
66
99
9.5
6.7
9.9
aReaction conditions: 1 0.5 mmol unless otherwise indicated, DMPS (1 mmol), solvent (0.5
b
mL), ethanol (1 mmol) and Cu3(BTC)2 20 mg unless otherwise specified, 30 min; Conversion
was determined by GC; cBlank control experiment in the absence of catalyst; d0.75 mmol of 1; e1
g
mmol of 1; fWith 1 mmol of pyridine; TON is calculated as mmol of substrate converted/mmol
of Cu.
8

A control experiment was performed with pyridine to reveal the nature of active site in
the present catalytic system. Hence, the reduction of 1 was carried out with DMPS as a reducing
agent in the presence of Cu3(BTC)2 and pyridine. The observed catalytic data are given in entry
6, Table 1. These results indicate that the reduction of 1 was completely interrupted upon
addition of pyridine, thus suggesting the coordination of pyridine with Cu2+ in the MOF
structure. This coordination disfavors the interaction of DMPS with Cu2+ by preventing the
liberation of hydrogen. It is believed that pyridine is acting as a catalyst poison.
As commented earlier, silanes were frequently used as reducing agents for the reduction
of olefins to their respective saturated hydrocarbons. Hence, series of silanes were tested for the
reduction of 1 to 2 using Cu3(BTC)2 as heterogeneous solid catalyst under the optimized reaction
conditions. Among the various silanes such as DMPS, diphenylsilane (DPS), phenylsilane (PS)
and triethylsilane (TES) tested for the reduction of 1 under the present conditions, DMPS, DPS
and PS showed quantitative conversion of 1 while TES exhibited no reduction. This may be due
to the inability of TES to interact with Cu3(BTC)2 to produce hydrogen in situ to reduce 1 to 2.
Table 2. Reduction of 1 to 2 using different silanes in the presence of Cu3(BTC)2.a
Entry Silane
Conversion
(%)b
100
100
100
-
1
2
3
4
DMPS
DPS
PS
TES
aReaction conditions: 1 (0.5 mmol), silane (1 mmol), THF (0.5 mL), ethanol (1 mmol) and
Cu3(BTC)2 (20 mg), 70 oC, 30 min; bDetermined by GC.
Figure 1 shows the time conversion plot for the reduction of 1 to 2 using Cu3(BTC)2 as
o
catalyst in the presence of DMPS as the reducing agent at 70 C. Further, the data in Figure 1
9

also indicate that thermal activation does not improve the reaction rate compared to non-
activated Cu3(BTC)2 under identical conditions. In order to demonstrate the heterogeneity of the
reaction, hot-filtration experiment was performed under the present experimental conditions and
the observed results are given in Figure 1. This figure indicates that the catalyst was removed by
filtration from the reaction mixture while in hot condition and the resulting solution without
Cu3(BTC)2 was allowed to stir for remaining time. Samples were periodically taken under
identical intervals as in the case of time conversion plot and these kinetic data are plotted in
Figure 1. These catalytic results suggest that the reduction of 1 is completely inhibited upon
removal of Cu3(BTC)2 from the reaction mixture indicating the absence of active sites to
promote this reaction. Hence, these data suggest the heterogeneous nature of the reaction.
Further, these data rule out the possibility of Cu leaching to the solution under these
experimental conditions.
Figure 1. Time conversion plot for reduction of 1 to 2 (■) in the presence of Cu3(BTC)2, (●) hot
filtration experiment catalyst filtered after 10 min from the reaction mixture further solution
stirred continuously under identical conditions in the absence of Cu3(BTC)2 for remaining time
and (▲) catalyst activated at 150 oC for 3 h.
10

Reusability experiments are one of the proofs to validate the stability of solid catalyst in
heterogeneous catalysis. These experiments also provide sufficient information regarding the
deactivation mechanism of a catalyst under the experimental conditions. Hence, the reusability of
Cu3(BTC)2 was studied under identical conditions as shown in Table 1. At the end of the
reaction, the catalyst was washed with THF, dried and reused for the successive cycle with fresh
1 and DMPS under similar conditions. The observed results are given in Figure 2. As it can be
seen here, Cu3(BTC)2 is used for four cycles with no noticeable decay in its activity. These
results clearly indicate that structural integrity of Cu3(BTC)2 is maintained and the active sites
are preserved under the present experimental conditions. These data are also in agreement with
hot-filtration experiment that the active sites are not leached to the solution. Therefore,
reusability experiments in combination with hot-filtration experiment unambiguously infer that
Cu3(BTC)2 is an ideal catalyst for the reduction of 1 to 2 using DMPS as a reducing agent in
THF at 70 oC without undergoing any structural deterioration.
11

Figure 2. Reusability plot for reduction of 1 to 2 using Cu3(BTC)2 as solid catalyst with DMPS
as reducing agent.
One of the important issues to be addressed in heterogeneous catalysis is to characterize
the spent catalyst by adequate techniques and to compare with the fresh catalyst. For instance,
powder XRD of the four times used Cu3(BTC)2 catalyst showed equivalent powder diffraction
pattern with less crystallinity compared to the recovered and fresh solids. Further, the less
crystallinity of the XRD peaks in the reused solid was due to the lack of sufficient sample. These
results indicate that the structural integrity and the framework of Cu3(BTC)2 is mostly retained
during the reuse experiment in repeated cycles. These results are shown in Figure 3.
12

Figure 3. Powder XRD patterns of (a) fresh, (b) recovered after the reaction and (c) four times
used Cu3(BTC)2.
The UV-Visible diffuse reflectance spectra of the fresh and four times used Cu3(BTC)2
were measured and the observed results are given in Figure 4. The fresh Cu3(BTC)2 shows an
absorption band in the visible region and this band is attributed to d-d transition due to the
presence of Cu2+ in the framework of Cu3(BTC)2. On other hand, the recovered and four times
used samples also showed mostly unchanged absorption bands in the visible region suggesting
the presence of Cu2+. However, a minor decrease in the intensity of absorption band in four times
used solid is due to the presence of 1 and 2 around 2%. These results confirm that the framework
metal ions are not reduced during the reduction of 1 to 2 using DMPS as a reducing agent.
Further, these studies indicate that Cu3(BTC)2 shows higher stability for the reduction of 1
without undergoing any damage to the framework stability and no reduction of metal ions occur
under these reaction conditions. In contrast, it was reported that Cu3(BTC)2 undergoes significant
13

changes in its crystalline nature and framework stability during the aerobic oxidation of
thiols[51] and the synthesis of 2-phenylbenzimidazole[52] as evidenced by powder XRD and
UV-Visible DRS analyses.
Figure 4. UV-DRS spectra of fresh (blue line), recovered after the reaction (black line) and four
times used (red line) Cu3(BTC)2.
FT-IR spectroscopy is one of the tools to characterize the spent catalyst after a catalytic
reaction. Hence, FT-IR spectra of the fresh, recovered and four times used samples were
recorded and the observed results are shown in Figure 5. Comparison of the fresh Cu3(BTC)2
with the recovered and reused samples indicate no additional stretching frequencies are observed
after catalytic reaction. Further, the metal ion and the linker bond is not broken during the course
of the reaction and further these studies prove that Cu3(BTC)2 shows high structural stability.
14

Figure 5. FT-IR spectra of (a) fresh, (b) recovered after the reaction and (c) four times used
Cu3(BTC)2.
The structure and nature of the oxidation state of Cu was verified by comparing the EPR
spectra of the fresh and four times used solid catalysts. These results are given in Figure 6.
Although the intensity of the four times used catalyst was comparatively lower than the fresh
solid, the typical EPR pattern corresponding to Cu2+ within the framework of Cu3(BTC)2 is
clearly observed. These results indicate that Cu exists in the divalent state.
15

Figure 6. EPR spectra of fresh (black line) and four times used Cu3(BTC)2 (red line).
Figure 7 shows the SEM images of the fresh and four times used Cu3(BTC)2 catalysts.
The highly crystalline nature of the fresh Cu3(BTC)2 is maintained during the repeated cycles for
the reduction of 1 to 2 using DMPS as a reducing agent. These comparison studies are in
agreement with other characterization methods, thus, supporting the morphology and framework
structure is retained during the course of the reduction reaction.
16

Figure 7. SEM images of (a) fresh and (b and c) four times used Cu3(BTC)2.
Considering the experimental results observed for the reduction of 1 to 2 using
Cu3(BTC)2 as heterogeneous solid, a plausible general mechanism is proposed in Scheme 1.
Initially, Cu3(BTC)2 interacts with DMPS and produces molecular hydrogen and the
corresponding siloxane. The in-situ generated hydrogen is adsorbed over Cu sites in Cu3(BTC)2
by replacing the water ligands. Later, 1 interacts with Cu sites through C=C double bond which
further undergoes hydrogen transfer to give 2 as the final product. However, the reactivity of
other alkenes depends on their ability to reach the active sites without any diffusion barrier and
their interaction with Cu sites.
17

O
O
O
O
O
PhMe2SiH
Cu
H2O
OH2
Cu
O
H2
O
+
O
-H2O
PhMe2Si-O-SiMe2Ph
O
O
O
O
O
O
H
O
O
O
O
Cu
Cu
Cu
Cu
H
O
O
O
O
O
O
O
O
O
O
O
H
H
Cu
O
Cu
O
O
Scheme 1. A general plausible mechanism for the reduction of 1 to 2 using Cu3(BTC)2 as
heterogeneous solid catalyst with DMPS as reducing agent.
These preliminary results observed for the reduction of 1 using Cu3(BTC)2 as
heterogeneous catalyst with DMPS as a reducing agent prompted us to study the scope of
Cu3(BTC)2 with other styrene derivatives under similar experimental conditions. The observed
results are presented in Table 3. The reduction of 1 to 2 was quantitative using Cu3(BTC)2 as
catalyst with DMPS as reducing agent within 30 min. Similarly, the reduction of 4-methylstyrene
was also reduced in 93% conversion after 30 min. Further, 4-methyl-β-methylstyrene was
reduced to its corresponding product in 44% conversion after 45 min. Similarly, 4-methoxy and
4-ethoxystyrenes were reduced to their respective products in 78 and 40% conversions,
18

respectively under identical conditions. The lower conversions of these substrates compared to 1
may be due to the different dimensions of the substrates which may experience in reaching the
active sites. Particularly, the diffusion limitation encountered by 4-methyl-β-methylstyrene is
clearly evidenced by showing lower activity than other related substrates. On the other hand, the
reactivity of 4-chloro and 3-nitrostyrenes were 8 and 2% conversions, respectively under
identical conditions. This lower activity in these two cases may be due to the electron
withdrawing nature of nitro and chloro substituents and decreasing the electron density around
C=C double bond thus lowering the interaction of these two substrates with the active sites. A
moderate conversion of 33% was obtained with 2,4-dimethylstyrene to its corresponding reduced
product under identical conditions. Further, phenylacetylene was transformed to 1 with 18%
conversion using Cu3(BTC)2 as heterogeneous catalyst with DMPS as reducing agent after 120
min without the subsequent reduction of 1 to 2. This may be due to the lack of hydrogen in the
reaction medium as well as preferential interaction of phenylacetylene than 1. Also, alkenes with
bigger dimensions were also converted in lower conversion of 4% conversions under identical
conditions. It is believed that these lower conversions may be due to the diffusion limitations
experienced by these substrates within the pores of Cu3(BTC)2.
Table 3. Reduction of various alkenes to alkanes by DMPS using Cu3(BTC)2 as solid catalyst.a
Entry
1
Alkenes
Alkanes
Time Conversion TONc
(min)
30
(%)b
100
10
9.4
4.4
2
3
30
45
93
44
19

4
5
30
45
78
40
7.9
4
H3CO
H3CH2CO
Cl
H3CO
H3CH2CO
Cl
6
7
120
120
8
2
0.8
0.2
NH2
NO2
8
9
30
33
18
3.3
1.8
120
10
11
45
60
4
4
0.4
0.4
aReaction conditions: alkene (0.5 mmol), DMPS (1 mmol), THF (0.5 mL), ethanol (1 mmol)
and Cu3(BTC)2 (20 mg), 70 oC. bDetermined by GC; cTON is calculated as mmol of substrate
converted/mmol of Cu.
4. Conclusions
This work has shown that Cu3(BTC)2 can be used as a heterogeneous solid catalyst for the
reduction of 1 to 2 using silane as a reducing agent under mild reaction conditions. The catalytic
data indicated that a complete conversion of 1 is observed using Cu3(BTC)2 as catalyst with
o
DMPS as a reducing agent at 70 C in 30 min. Cu3(BTC)2 stability was ascertained by leaching
experiment and showing the absence of metal leaching. Further, the solid catalyst was used four
cycles with no noticeable decrease in its activity. A series of techniques including powder XRD,
FT-IR, UV-Visible DRS, SEM and EPR analyses were used to characterize four times used solid
20

and compared with the fresh solid and observing no major differences in its structural integrity.
This solid catalyst was also extended to other styrene derivatives and observing moderate to high
conversions.
Acknowledgements
AD thanks the University Grants Commission, New Delhi, for the award of an Assistant
Professorship under its Faculty Recharge Programme. AD also thanks the Department of Science
and Technology, India, for the financial support through Extra Mural Research Funding
(EMR/2016/006500).
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22

Graphical Abstract
23

Graphical Abstract
24

Highlights
 Cu3(BTC)2 is reported as heterogeneous solid catalyst for the reduction of styrene to
ethylbenzene.
 Dimethylphenylsilane is used as reducing agent.
 Cu3(BTC)2 is used for four cycles with no drop in its activity.
 No leaching of Cu is detected.
25