Synergistic Effect of NiLDH@YZ Hybrid and Mechanochemical Agitation on Glaser Homocoupling Reaction

Article abstract of DOI:10.1002/chem.202100018

Herein, we report the synthesis of nickel-layered double hydroxide amalgamated Y-zeolite (NiLDH@YZ) hybrids and the evaluation of the synergistic effect of various NiLDH@YZ catalysts and mechanochemical agitation on Glaser homocoupling reactions. Nitrogen

Full text of DOI:10.1002/chem.202100018

Accepted Article
Accepted Article
Title: Synergistic Effect of NiLDH@YZ Hybrid and Mechanochemical
Agitation on Glaser Homocoupling Reaction
Authors: Mohamed Mokhtar, Ghalia Alzhrani, Elham S. Aazam, Tamer
S. Saleh, Sulaiman Al-Faifi, Subir Panja, and Debabrata Maiti
This manuscript has been accepted after peer review and appears as an
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202100018
Link to VoR: https://doi.org/10.1002/chem.202100018
01/2020

10.1002/chem.202100018
Chemistry - A European Journal
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Synergistic Effect of NiLDH@YZ Hybrid and Mechanochemical
Agitation on Glaser Homocoupling Reaction
Mohamed Mokhtar,*[a] Ghalia Alzhrani,[a] Elham S. Aazam,[a] Tamer S. Saleh,[b] Sulaiman Al-Faifi,[a]
Subir Panja,[c] and Debabrata Maiti*[c]
[a]
Dr. G. Alzhrani, Dr. S. Al-Faifi, Prof. Dr. E. Aazam, Prof. Dr. M. Mokhtar
Chemistry Department, Faculty of Science
King Abdulaziz University
Jeddah 21589, Saudi Arabia
E-mail: mmoustafa@kau.edu.sa
[b]
[c]
Prof. Dr. T. S. Saleh
Chemistry Department, Faculty of Science
University of Jeddah
P.O. Box 80329, Jeddah 21589, Saudi Arabia
Dr. S. Panja, Prof. Dr. D. Maiti
Department of Chemistry
Indian Institute of Technology Bombay Powai
Mumbai 400076 - India
E-mail: dmaiti@iitb.ac.in
Supporting information for this article is given via a link at the end of the document. ((Please delete this text if not appropriate))
Abstract: Herein, we report the synthesis of nickel-layered double
hydroxide amalgamated to Y-zeolite (NiLDH@YZ) hybrids and
evaluation of the synergistic effect of various NiLDH@YZ catalysts
and mechanochemical agitation on Glaser homocoupling reactions.
Nitrogen adsorption-desorption experiments were carried out to
estimate the surface area and porosity of NiLDH@YZ hybrids. The
basicity and acidity of these hybrids were determined by CO2-TPD
and NH3-TPD experiments respectively and this portrayed good acid-
base bifunctional feature of the catalysts. The NiLDH@YZ catalyzed
mechanochemical Glaser coupling reaction achieved best yield of
83% for 0.5NiLDH@0.5YZ hybrid after 60 mins of agitation, which
revealed the highest acid-base bifunctional feature compared to all
the investigated catalysts. The developed catalyst has proven itself as
a robust and effective candidate that can be successfully employed
up to four catalytic cycles without significant loss in catalytic activity,
under optimized reaction conditions. This work demonstrated a new
strategy for C–C bond formation enabled by the synergy between
mechanochemistry and heterogeneous catalysis.
impressive catalysts, oxidants, ligands, and innovative synthetic
protocols. Although this reaction requires an organic solvent,
but the risk of using an organo-volatile solvent has been
[5]
[6]
minimized by exploiting supercritical carbon dioxide,
ionic
liquids, (near-critical) water[8] and polyethylene glycol (PEG)[9]
as solvents. In addition, some solvent-free protocols were also
identified[10] which include 1) utilizing CuCl2–pyridine complex as
the catalyst[10a], 2) microwave irradiation in presence of an excess
of copper salt[10b] and 3) utilization of copper salt in the presence
of morpholine. [10c]
[7]
Chemistry in ball mills has gained prominence in being efficient,
ecofriendly, and a greener alternative to replace some solvent-
based protocols. Nowadays, this is considered to be a road to
sustainable synthesis because it offers relatively milder reaction
conditions, increased surface area and surface energy with a
[11]
solution towards the numerous defects in catalytic materials.
The increased surface energy in mechanochemical mode is an
outcome of alteration in the structure, chemical composition and
chemical reactivity which is a resultant of milling. [11e] Schmidt et
al. reported a method for the ligand and solvent free Glaser
coupling of alkynes in a vibrating ball mill. [12]
Introduction
The catalytic activity of Cu catalysts is enhanced when it is
combined with solid supports like reduced graphene oxide (rGO),
N-doped carbon materials, polymers and zeolites, owing to the
distribution of Cu on larger surface area,[13] presence of nitrogen-
containing moieties and enhanced electron transfer.[14] For the
first time, Kuhn et al.[15] reported the use of USY-zeolite supported
Cu(I) catalyst for the synthesis of diynes by homocoupling of
phenylacetylene and revealed the role of zeolite acidic sites in this
reaction. Furthermore, they pointed out a significant influence of
the pore size and topology of zeolite as higher yields were
obtained with zeolites having large internal cages. Transition
metals are involved in a wide range of chemical transformations
namely dehydrogenative reactions,[16] cross couplings[17] and in
C–H activation processes.[18] Selection of a suitable metal salt is
quite challenging and consequently it is recommended to
generate M(0) with suitable ligand that could possibly minimize the
Dehydrogenative coupling is a sustainable chemical process
whereby a new chemical bond is formed either by hydrogen
evolution or by formal removal of hydrogen from the substrate. [1]
Many daunting obstacles have been faced while constructing a
desired C-C/C-X/X-X bond by selectively cleaving a C-H/X-H
[2]
bond among a plethora of similar bonds. The dimerization of
phenylacetylene is an example of dehydrogenative coupling
process taking place in the presence of copper salts and air, as
[3]
reported by Glaser.
Glaser coupling is a fascinating and
effective tool that can be harnessed to construct C–C bonds and
this has innumerable applications in materials science, molecular
electronics, polymer synthesis, supramolecular materials, and
drug development. [4] Extensive new strategies have emerged to
[2]
achieve dehydrogenative coupling
along with a range of
1
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10.1002/chem.202100018
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formation of undesired byproducts.[19] It has been found that
Ni(cod)2 is one of the most widely used catalysts employed in
many reactions, when dehydrogenative coupling is considered. Ni
catalyzed Glaser coupling reaction of two terminal alkynes is
already well developed using THF as solvent, in aerobic
conditions.[20]
In this context herein, we are interested for the development of
Ni-layered double hydroxides (NiLDH) catalysts[21] which are
prepared through the intercalation with cage framework zeolite.
This work is aimed to develop a sustainable version of Glaser
reaction by employing synergistic use of NiLDH@Y-zeolite (YZ)
hybrid as heterogeneous catalyst and mechanochemical agitation
to afford homocoupling of terminal alkynes (Glaser type reaction),
under solvent free conditions utilizing ball milling technique.
20000
15000
10000
5000
YZ
0.33NiLDH@0.67YZ
0.5NiLDH@0.5YZ
16000
12000
8000
4000
6000
Y
Y
4000
2000
Y
L
L
L
Y
L
0
Results and Discussion
Catalysts characterization
6000
0.67NiLDH@0.33YZ
4000
2000
0
In the present study a series of NiLDH, YZ , and amalgamated
NiLDH @YZ catalysts were synthesized with the aim of testing
their efficiency in the Glaser reaction. The selection of such
catalytic materials based on the expected acidic/basic bifunctional
characteristic that could be necessary for improving the catalytic
process.
6000
NiLDH
4000
2000
0
10
20
30
40
50
60
70
80
X-ray diffraction (XRD)
Pos. [°2Th.]
X-ray diffraction patterns of NiLDH, YZ and their hybrids are
presented in Figure 1. The characteristic peak of LDH is at low
value of 2θ ≈ 12° and is sharp with high intensity for NiLDH and
0.67NiLDH@0.33YZ. While relatively lower intensity peaks are
observed for 0.5NiLDH@0.5YZ and 0.33NiLDH@0.67YZ. The
highly intense patterns in a and b planes of NiLDH were gradually
suppressed in the hybrids. In contrary, the XRD pattern of the
0.5NiLDH@0.5YZ hybrid completely matches with that of the
corresponding synthesized YZ (JCPDS 76-110).[22] The diffraction
peaks of YZ were indexed as per previous report. [23] There were
no significant changes in the number of diffraction peaks with
respect to YZ except the peak at 2θ =12.3°, 23.4°, 35.3° and 61.9°,
which refer to NiLDH peaks, indicating that NiLDH could be
coated inside the YZ pores. [24]
Figure 1. XRD patterns of NiLDH, YZ and their hybrids.
Transmission electron microscopy (TEM)
The TEM image (Figure 3a) shows that YZ has a defined shape
with sharp edges. Nevertheless, despite the vague picture taken
for this particular sample, YZ’s instability under HRTEM power
was not omitted.[27] TEM image (Figure 3c) for 0.5NiLDH@0.5YZ
catalyst shows the presence of a significant degree of NiLDH
coverage on the surface of the YZ crystallites. In Figure 3d, the
HRTEM image of 0.5NiLDH@0.5YZ shows a non-uniform lattice
fringe compared to YZ in Figure 3b , which proves the existence
of NiLDH on the YZ surface. In addition, the width of the adjacent
lattice fringe (Figure 3d) is approximately 0.751 nm which is
similar to d003 spacing of the NiLDH phase (0.727 nm) [24a] and this
is further consistent with the above discussion. NiLDH sample
shows extra d-spacing at 0.185 nm, which is specific to the
layered material (Figure 3f).
Scanning electron microscopy (SEM-EDX)
The morphology of parent YZ, NiLDH and their hybrids were
investigated by scanning electron microscopy. The YZ sample
displays a very well-defined and polished shape (Figure 2a), while
the morphology of the layered NiLDH flakes was typical of layered
double hydroxides (Figure 2i).[21] The influence of the intercalation
of NiLDH on the Y-zeolite structure can be confirmed from SEM
images. In 0.33NiLDH@0.67YZ, irregular spots begin to deposit
on the outer surface of the zeolite material (Figure 2c). The YZ
still retained the original form in 0.5NiLDH@0.5YZ (Figure 2e)
with the outer surface roughened from widely dispersed NiLDH,
without an aggregation of NiLDH on YZ surface. The degree of
dispersion increases monotonically with the increase in NiLDH
content, where 0.67NiLDH@0.33YZ showed the maximum
roughness of the zeolite surface (Figure 2g).
N2 physisorption measurement
Nitrogen adsorption-desorption for a series of NiLDH@YZ hybrid
samples was examined in order to estimate their specific surface
area and porosity. The results of N2 adsorption-desorption
isotherms are displayed in Figure 4a. According to IUPAC
classification,[26] all samples show type IV isotherm except YZ that
exhibits typical type I isotherm. The H4-type hysteresis loop
indicates the presence of mesoporous slit like shape. [27]
2
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900
600
300
b
Textural properties for NiLDH, YZ and NiLDHs@YZ hybrids have
been summarized in Table 1. The pore size distribution analysis,
using DFT method, displayed that all the samples have bimodal
distribution curves in the microporous and mesoporous ranges
(Figure 4b). The pore size of YZ was decreased after coating
process from 2.8 to 1.1 nm in 0.5NiLDH@0.5YZ hybrid (Table 1).
Similar phenomenon was reported when different metals were
introduced on porous materials. [28] The decrease in pore size of
the 0.5NiLDH@0.5YZ catalyst compared to the original YZ could
be due to NiLDH layers blocking the pores during the process of
Y-zeolite surface coating. Both BET and micropore surface areas
decreased with NiLDH addition, while the opposite occurred in
mesoporous areas. YZ showed the highest BET surface area and
micropore area values and pure NiLDH sample showed the
opposite values. The micropore volume of YZ decreased while
the mesopore volume increased after addition of NiLDH in the
hybrid materials. The pronounced decrease in micropore volume
in YZ after the coating process may be due to the coating of pores
by NiLDH particles. [29] The calculated hierarchy factor recorded
0.12 for 0.5NiLDH@0.5YZ hybrid catalyst which is double to the
value of YZ (0.06) and NiLDH (0.06) samples individually. The
suitable hierarchy factor can lead to the creation of shorter
diffusion path lengths and lower diffusion resistance in this
particular catalyst. In addition, the sustainable number of
mesopores could resist the formation of coke. [30]
Si
Al
O
0
2
4
6
8
10
Energy (KeV)
900
Si
O
d
Al
600
300
C
Ni
Ni
0
2
4
6
8
10
Energy (KeV)
900
f
600
300
Si
Al
O
Ni
Ni
0
2
4
6
8
10
Energy (KeV)
900
h
O
600
300
Ni
Al
Si
C
Ni
Ni
0
2
4
Energy (KeV)
6
8
10
900
j
O
600
300
0
Ni
Al
C
Ni
Ni
2
4
6
8
10
Energy (KeV)
X-ray photoelectron microscopy (XPS)
Figure 2. SEM and EDX images of NiLDH, YZ and their hybrids.
The XPS spectrum allows an analysis of the surface structure and
provides information about the presence of Ni, Al, O and Si within
the surface of catalysts along with the oxidation state of the metals.
Figure 5a illustrates an envelope for the Al 2p band at 75.1 eV in
YZ sample and 0.5NiLDH@0.5YZ hybrid and this refers to the
presence of Al3+ ion that gets bonded to oxygen and silicon (Al-O-
Si) in zeolite structure [31]. The binding energy of Al 2p in pure
NiLDH sample resulted in a peak at 73.8 eV, while the peak that
[32]
appeared at 68.2 eV should contribute to Ni 3p.
The
deconvoluted Al 2p peaks at 73.8 eV indicates the presence of
Al3+ bonded to hydroxide layers (Al-OH). [33]
Figure 5b displays the deconvoluted XPS band of 1s of O in YZ,
0.5NiLDH@0.5YZ and NiLDH samples and that showed the
presence of isolated oxygen (O2-), the lattice oxygen (OH-) and
oxygen from the adsorbed water. Figure 5c shows that the peak
intensity of 2p of Si in YZ and 0.5NiLDH@0.5YZ, which is reduced
after coating with NiLDH indicating that the Si content on the
surface decreased significantly [24a]. In Figure 5d, NiLDH spectrum
shows the two obvious shakeup satellites at 856.0 and 873.7 eV
which can be identified as signals for 2p3/2 of Si and 2p1/2 of Ni2+,
respectively. These satellite lines are the fingerprints of Ni2+ ions
in an environment of oxide ions (O2−).[34]
Figure 3. TEM image of YZ (a, b), 0.5NiLDH@0.5YZ (c, d) and NiLDH (e, f).
3
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a)
b)
220
YZ
200
180
200
YZ
0.33NiLDH@0.67YZ
180
160
140
0.33NiLDH@0.67YZ
0.5NiLDH@0.5YZ
0.67NiLDH@0.33YZ
NiLDH
120
180
0.5NiLDH@0.5YZ
0.67NiLDH@0.33YZ
NiLDH
160
140
120
120
100
80
60
40
1520
120
90
60
30
0.0
0.2
0.4
0.6
0.8
1.0
2
4
6
8
10
Relative Pressure (P/Po)
Half pore width (nm)
Figure 4. (a) N2 adsorption-desorption isotherms; (b) pore size distribution curves of NiLDH, YZ and their hybrids
Table 1. The textural properties of YZ, NiLDH and their hybrids.
[a]
[b]
[c]
[d]
[e]
[f]
SBET
Smicro
Smeso
Vtotal
Vmicro
Vmeso
Pore Size Distribution
(nm)
HF [g]
(m2/g)
(m2/g)
(m2/g)
(cc/g)
(cc/g)
(cc/g)
YZ
564.4
424.2
379.3
164.0
124.7
517.7
347.4
297.2
60.9
39.8
71.6
0.34
0.29
0.28
0.19
0.18
0.27
0.18
0.16
0.03
0.01
0.06
0.11
0.12
0.15
0.17
2.8
1.9
1.1
3.0
4.3
0.06
0.10
0.12
0.11
0.06
0.33NiLDH@0.67YZ
0.5NiLDH@0.5YZ
0.67NiLDH@0.33YZ
NiLDH
79.7
105.2
126.4
12.8
[a] Surface area from BET isotherm ≈ Smeso + Smicro; [b] Micropore area (From t-plot method); [c] Mesopore area (From BJH method); [d] Total pore volume = Vmeso
Vmicro; [e] Micropore volume (From t-plot method); [f] Mesopore volume (From t-plot method); [g] Hierarchy factor = (Vmicro/Vtotal) × (Smeso/SBET
+
)
In XPS spectrum of 0.5NiLDH@0.5YZ hybrid (Figure 5d), the Ni
2p3/2 peak shifted to higher binding energy from 856.0 eV to 857.1
eV.
The position of Ni 2p peak is usually influenced by the local
chemical or physical environment around Ni species besides the
formal oxidation state. The Ni 2p peak shifts to higher binding
energy when the charge density around it decreases. The
existence of the acidic sites of YZ (Al and Si) around the Ni
species leads to a decrease in the charge density around Ni and
the slight increase in binding energy. [35]
to the temperatures at which the peaks were shown. Bicarbonate
and bidentate species decompose at low and moderate
temperatures (~100 - 420 °C) and can be traced to relevant peaks.
The peak that decomposed at higher temperature (≈ 540 °C) can
be attributed to the presence of monodentate species. [36] Figure
6 revealed that each of the hybrids have different types of basic
sites, whereas TPD-CO2 profile of YZ catalyst shows just one
peak at 340 °C. The TPD-CO2 of 0.67NiLDH@0.33YZ and NiLDH
have identical profiles that are distinct from 0.5NiLDH@0.5YZ,
which has a unique profile. The TPD-CO2 profile of all hybrids
(except 0.33NiLDH@0.67YZ) and NiLDH show peaks within the
range of 100-355 °C due to the desorption of weakly bond CO2
and the breakdown of carbonate ions existing in the interlayer of
LDH samples. In the TPD-CO2 profiles of 0.5NiLDH@0.5YZ and
0.33NiLDH@0.67YZ, the peaks extend to up to 520 °C and this
can be related to tightly bonded species of monodentate or
bidentate carbonate. In addition, there is a peak at about 420 °C
Temperature-programmed desorption (TPD-CO2)
The basicity of the different catalysts was determined by CO2-
TPD because the types of basic sites could be detected by CO2
adsorption. The chemisorbed peaks can be compared according
4
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Figure 5. Deconvoluted XPS for investigated catalysts
that is attributed to bicarbonate species bonded to weak or
[37]
medium basic strength sites.
The profiles of TPD-CO2 for all
Temperature-programmed desorption (TPD-NH3)
catalysts were integrated to measure the amount of CO2 formed
from the subtraction of the corresponding profiles. From Table 2,
it is clear that 0.5NiLDH@0.5YZ catalyst has higher total number
of basic sites (40.4 µmol/g), which reveals the stronger dispersion
of NiLDH in the YZ cavities.
The acidity of catalysts was studied by NH3-TPD profiles. The
overall acidity of the catalysts can be measured from the amount
of ammonia desorbed at different temperatures. The strength and
number of acidic surface centers can be estimated from the
5
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maximum temperature intensity and location, respectively.
Ammonia desorption profile of the catalysts could be divided into
three regions namely (1) weak (100-250 °C), (2) medium (250-
400 °C) and (3) strong (400-700 °C) acid strengths. [38] The TPD-
NH3 profiles of NiLDH, YZ and hybrid catalyst are shown in Figure
7. The NiLDH exhibited two peaks at 329 °C and 378 °C which
are under medium temperature range corresponding to moderate
acid sites.
6
4
2
YZ
0
6
0.33NiLDH@0.67YZ
4
2
YZ
0.33NiLDH@0.67YZ
0.5NiLDH@0.5YZ
4
2
0
6
0.5NiLDH@0.5YZ
0.67NiLDH@0.33YZ
NiLDH
4
2
0
4
0
6
2
4
2
0
4
0
6
2
4
2
0
0
4
0.67NiLDH@0.33YZ
100
200
300
400
500
600
700
2
Temperature (C)
0
4
Figure 7. TPD-NH3 profile for NiLDH, YZ and their hybrids.
NiLDH
The TPD-NH3 profile of 0.33NiLDH@0.67YZ also shows two
different peaks corresponding to poor and moderate acid sites at
higher temperatures (117 and 350 °C) relative to YZ. The TPD-
NH3 profile of 0.5NiLDH@0.5YZ has exhibited five peaks at
different temperatures such as 110 °C (weak acid site), 252 °C,
299 °C , 372 °C (moderate acid sites) and at 428 °C (strong acid
sites) as depicted in Figure 7. The acidic-basic features of various
catalyst derived from NH3-TPD and CO2-TPD profile analysis are
displayed in Figure 8. The 0.5NiLDH@0.5YZ hybrid reveals the
highest acid-base bifunctional feature relative to all the
investigated catalysts. Higher acid-base bifunctional feature of
catalysts not only provides the surface to interact with organic
molecules but also results in it being the site for C‒C coupling
reactions. [39]
2
0
100
200
300
400
500
600
700
Temperature ( C)
Figure 6. TPD-CO2 profile for NiLDH, YZ and their hybrids.
Table 2. Results of NH3-TPD and CO2-TPD profiles
Ni
Total CO2 uptake
(µmol/g)
Catalyst
Total acidity (µmol/g)
(% w/w) [a]
YZ
-
9.3
277.1
310.2
549.1
269.4
256.8
0.33NiLDH@0.67YZ
0.5NiLDH@0.5YZ
0.67NiLDH@0.33YZ
NiLDH
0.6
0.7
0.8
0.81
20.9
40.4
24.4
29.4
[a] From ICP analysis
The TPD-NH3 profile of 0.67NiLDH@0.33YZ catalyst is similar to
NiLDH but the maximum temperature and peak intensities are
lesser in 0.67NiLDH@0.33YZ than NiLDH which suggests lesser
acidity. YZ catalyst also has two different peaks at lower
maximum temperatures (107 °C and 274 °C) suggesting both
weak and moderate acid sites.
Figure 8. Acid-base bifunctional features of NiLDH, YZ and their hybrids
derived from NH3-TPD and CO2-TPD profiles.
6
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Catalytic activity of NiLDH@YZ hydrids on Glaser homo-
coupling reaction
Entry 3), in which the desired product 2a was isolated in 83 %
yield and no other product was noticed.
NiLDH catalyst alone was not adequate for activate the terminal
alkyne and obtain Glaser product with significant yield even
though it has acid-base properties as shown from TPD-CO2 and
TPD-NH3 results. The 2a yield enhanced for the
0.33NiLDH@0.67YZ catalyst by about 23%. This is actually
attributed to the acid-base active sites were distributed over the
larger surface area (424 m2/g) for 0.33NiLDH@0.67YZ. The
highest yield was obtained by 0.5NiLDH@0.5YZ hybrid catalyst
that has the equal ratio from NiLDH and YZ (83%). The superior
activity of the former hybrid catalyst was attributed to their unique
basic properties than other hybrid catalysts. Moreover, the acidity
of YZ and its characteristic topology that appeared from high
hierarchy factor value (0.12) engaged on giving the hybrid
catalysts the exceptional catalytic activity. However, the efficiency
of the hybrid catalyst was slightly suppressed when the amount
ratio of NiLDH was more than YZ (0.67NiLDH@0.33YZ, yield
61%) which could be explained by the aggregate of NiLDH
particles inside the pores of YZ.
Prepared catalysts have been used to develop a protocol for
heterogeneous nickel catalyzed Glaser homocoupling reaction
under mechanochemical agitation in a ball mill. The challenge is
to study the catalytic activities of the prepared NiLDH@YZ
catalysts that allowed the transformation of terminal alkyne 1 into
the corresponding 1,4- buta-1,3-diynes derivative 2, (Scheme 1)
in a time- and energy-efficient manner.
Scheme 1. Heterogeneous Ni catalyzed Glaser type reaction.
At
the
outset,
NiLDH,
YZ,
0.67NiLDH@0.33YZ,
0.5NiLDH@0.5YZ and 0.33NiLDH@0.67YZ catalysts were tested
for their catalytic activity in ball mill, under solvent free conditions
and results were cited in Table 3. These catalysts have the
advantages that they have a minimal impact to the environment
and can be easily recycled. Surprisingly, only trace product was
formed using NiLDH and YZ alone (Table 3, Entries 2 & 3).
Table 4. Optimization of reaction condition (time and ball mill frequency for
homocoupling of terminal alkynes.
Entry
Time (min.)
Yield (%)
Ball-mill frequency (Hz)
1
2
3
120
90
62
70
83
20
25
30
60
Table 3. Optimization of reaction conditions (catalyst and time) for Glaser
coupling of terminal alkynes.a
Reaction conditions: 0.5NiLDH@0.5YZ catalyst (400 mg), phenylacetylene (2
mmol) and pyrrolidine (1 mol %).
Entry
Catalyst
Time (min.)
Yield (%)
For the synthesis of symmetrical diyne molecules, the scope and
generality of this protocol were investigated with various types of
terminal alkyne derivatives including electron donating group and
electron withdrawing group using 0.5NiLDH@0.5YZ catalyst
under the optimized conditions (Table 5). And the reaction
proceeds efficiently in both cases without any compatibility.
1
2
Catalyst-free
NiLDH
360
180
180
130
60
0
10 (trace)
12 (trace)
61
3
YZ
4
0.67NiLDH@0.33YZ
0.5 NiLDH@0.5YZ
0.33NiLDH@0.67YZ
0.5 NiLDH@0.5YZ
5
83
6
180
60
23
7b
15
Table 5. Scope of Glaser coupling reaction under optimized conditions.
aReaction conditions: catalyst (400 mg), phenylacetylene (2 mmol),
pyrrolidine (1 mol %) and ball mill frequency (30 Hz). b Without pyrrolidine.
Notably, the catalytic materials used in this reaction act as
grinding auxiliaries in addition to its role as a catalyst where all the
reactants are liquid. It is clear from results depicted in (Table 3)
that even after utilizing a ball mill even after 6 h, no desired
product 2a was formed in absence of catalyst (Table 3, Entry 1).
In addition, the best yield of 83 % of the desired product 2 was
reached using 0.5NiLDH@0.5YZ catalyst in 60 min (Table 3,
Entry 5). Furthermore, a lower yield with longer period of time
(61 % at 130 min. and 23 % at 180 min.) was observed using
0.67NiLDH@0.33YZ and 0.33NiLDH@0.67YZ catalysts (Table 3,
Entry 4 & 6), respectively. Here pyrrolidine acts as a base and
without pyrrolidine the reaction gives lower yield (Table 3, Entry
7). Both the pyrrolidine and hybrid catalyst are required, however
only pyrrolidine does not give any significant effect. Further, effect
of time and frequency of ball mill on catalytic system were
examined (Table 4) with same amount of catalyst. The best
frequency for this reaction protocol was noted at 30 Hz (Table 4,
Reaction conditions: 0.5NiLDH@0.5YZ catalyst (400 mg), phenylacetylene (2
mmol) and pyrrolidine (1 mol %), ball mill frequency (30 Hz), time 1 h.
7
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We have performed a reaction with a combination of electronically
different alkynes, 1-ethynyl-4-methylbenzene and phenyl
acetylene for the synthesis of 1-methyl-4-(phenylbuta-1,3-diyn-1-
yl)benzene under the same reaction condition. We have obtained
poor yields of the unsymmetrical product, which indicates mixture
of symmetrical diyne formation is favored rather than
unsymmetrical ones (Table 5, entry 6).
original and regenerated catalysts (after first and last cycles) to
demonstrate the degree of deformation of layered structure,
which was found as 53.6, 16.5 and 5 nm, respectively but layered
structure was retained even after the last cycle. We conclude
herein that the slightly decay in catalytic activity of
0.5NiLDH@0.5YZ could be attributed to weight loss during the
workup process, deformation in regenerated catalysts and
surface defects.
The results cited in Table 5 revealed that this catalyst showed the
same reactivity whether the substrates were bearing electron
donating or withdrawing groups. Similar results were reported in
the literature using poly styrene supported copper catalyst. [40]
All the products were well characterized using NMR spectral data.
In the 1H NMR spectra, disappearance of singlet peak of
acetylenic hydrogen confirms the formation of desired coupled
products (2a-f). (Supporting Information).
Tentative Mechanism
Results obtained from utilizing both NiLDH and YZ as individual
catalysts for the Glaser reaction shows almost no catalytic activity.
On the other hand, a noticeable catalytic activity has been
observed in the case of using the hybrid catalyst that has dual
acid-base properties. Therefore, the proposed tentative
mechanism (Scheme 2) is based upon results of catalytic tests
and characterization of hybrid catalysts.
Reusability of 0.5NiLDH@0.5YZ catalyst
The transmetallation reaction between the hybrid catalyst and the
acetylide ion (catalyst-assisted generation of nucleophile) gives
the alkynyl complex (1), which then converts to a dialkynyl-Ni
complex intermediate (2). In the presence of oxygen, the latter
complex (2) gets readily converted to the final product 1,3-diyne
via reductive elimination and regenerates the catalyst. [20, 43]
The reusability of the 0.5NiLDH@0.5YZ catalyst was tested for
several reaction cycles for Scheme 1 under optimized reaction
conditions, the catalyst removed after filtration, washed with hot
ethyl acetate and dried under vacuum. The recovered catalyst
was used under the same reaction conditions four times. Figure 9
shows that the regenerated catalyst conducts the reaction
efficiently under the same conditions even after a four-fold use. A
slight decay in the catalytic activity of the 0.5NiLDH@0.5YZ
catalyst was observed in third and fourth cycles which was studied
by comparing XRD (Figure 10) of the original catalyst with the
corresponding regenerated catalyst after first and last cycle of
Glaser reaction. XRD analysis shows regenerated catalysts
losing crystallinity under mechanochemical force while
maintaining their layered structure. It is clear from Figure 10 that
the intensity of diffraction peaks that originated from Y-zeolite
decreased dramatically due to the breakup of Si-O-Si and Si-O-Al
bonds as reported by previous studies [41]. Although peaks due to
NiLDH showed to be distinctly resistant to mechanical force of ball
milling, the intensities of its distinguished XRD peaks in
regenerated catalysts are significantly decreased. [42] XRD pattern
of regenerated catalyst after the first cycle showed marked peak
Figure 10. XRD of used 0.5NiLDH@0.5YZ catalysts after first and fourth cycles.
Pyrrolidine facilitates hydrogen abstraction from the terminal
alkyne to form acetylide ion (Scheme 2). Notably, the last step is
facilitated by the Lewis acid sites of the hybrid catalyst. The
superiority of the 0.5NiLDH@0.5YZ catalyst among all the
investigated catalysts can be explained on the basis of its highest
acid-base bifunctional properties as witnessed by CO2-TPD and
NH3-TPD
Figure 9. Results of recyclability of 0.5NiLDH@0.5YZ catalyst under optimized
reaction conditions.
broadening that became wider after the last cycle, suggesting
reduction of crystallinity and deformation of particle size induced
by the milling action. The crystallite size was calculated for the
8
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every 15 minutes. The milling process was repeated until the reaction is
thoroughly completed. Then the milling Jar was opened and allowed to
stand in an air atmosphere for 60 minutes. After the post-reaction oxidation,
the crude products were extracted using ethyl acetate (3×10 mL) and
sonicated for 5 minutes to desorb all adsorbed liquid on the catalyst
surface followed by filtration. The filtrate was concentrated under reduced
pressure and the residual crude products were purified by column
chromatography using n-hexane/ethyl acetate mixtures (7:3).
Acknowledgements
Scheme 2. The proposed mechanism for Glaser homo-coupling
reaction.
This research work was founded by Institutional Fund Projects
under grant no. (IFPHI-184-130-2020). Therefore, authors
gratefully acknowledge technical and financial support from the
Ministry of Education and King Abdulaziz University, DSR,
Jeddah, Saudi Arabia.
Conclusion
In summary, NiLDH@YZ hybrid catalysts with different
percentage compositions were synthesized and successfully
harnessed to carry out Glaser coupling utilizing solvent-free ball
mill technique. The catalytic activities of the catalysts were
examined by their ability to transform terminal alkyne into the
corresponding 1,4- buta-1,3-diynes derivatives in a time- and
energy-efficient manner. The results of NH3-TPD and CO2-TPD
profiles of 0.5NiLDH@0.5YZ hybrid catalysts revealed the highest
acid-base bifunctional feature relative to all the investigated
catalysts. The unique acid-base behavior of this particular hybrid
was an outcome of good dispersion and interaction between
NiLDH and YZ as witnessed by SEM, and HRTEM. The effect of
the mechanochemical agitation can lead to the reduction of the
particle size and creation of active sites by generating fresh active
surfaces, which maintains the chemical activity of the recycled
materials.
Keywords: Acid-base bifunctional catalyst; Ball milling; Glaser
reaction; NiLDH@Y-zeolite hybrid catalyst; Synergistic effect
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Entry for the Table of Contents
In the present study NiLDH, YZ and amalgamated NiLDH@YZ catalysts were tested for the Glaser homocoupling reaction. The catalyst
with 0.5 NiLDH@0.5 YZ showed the superior catalytic efficiency under mechanochemical agitation. A robust catalyst used for four
times provide a new strategy in C-C coupling reactions using mechanochemistry.
Institute and/or researcher Twitter usernames: https://twitter.com/mmokhtar2000
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