Microwave assisted efficient protocol for the classic Ullmann homocoupling reaction using Cu-Mg-Al hydrotalcite catalysts

Article abstract of DOI:10.1016/j.molcata.2013.08.013

Four hydrotalcite catalysts containing different amounts of Cu and Mg cations, with varying atomic ratios of Cu:Mg:Al in the brucite-like layers of 0:2:1, 0.5:1.5:1, 1:1:1 and 2:0:1, were prepared by co-precipitation method. The synthesized catalysts were tested for classic Ullmann homocoupling of iodobenzene to produce biphenyl without using any stoichiometric amounts of base or reducing agent. The Mg-Al hydrotalcite catalyst without Cu ions is inactive, but the doping of Cu ions (Cu/Al = 0.5) into the Mg-Al hydrotalcite framework led to 63% conversion of iodobenzene under microwave irradiation. Gradual increase of Cu content and concomitant decrease of Mg content resulted in an increase in the conversion of iodobenzene and selectivity to biphenyl. The physico-chemical properties of hydrotalcite catalysts were studied by chemical analysis, XRD, FTIR, N₂-physisorption, SEM, XPS, ESR and TPR techniques to understand the state of Cu ions presented in the hydrotalcite framework. It appears that presence of easily reducible isolated Cu²⁺ ions is essential to obtain improved performance of the catalyst.

Full text of DOI:10.1016/j.molcata.2013.08.013

Accepted Manuscript
Title: Microwave assisted efficient protocol for the classic
Ullmann homocoupling reaction using Cu-Mg-Al hydrotalcite
catalysts
Author: Katabathini Narasimharao Ebtisam Al-Sabban Tamer
S Saleh Ainara Garcia Gallastegui Almudena Celaya Sanfiz
Sulaiman Basahel Shaeel Al-Thabaiti Abdulrahman Alyoubi
Abdullah Obaid Mohamed Mokhtar
PII:
S1381-1169(13)00296-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2013.08.013
MOLCAA 8834
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
13-6-2013
9-8-2013
12-8-2013
Please cite this article as: K. Narasimharao, E. Al-Sabban, T.S. Saleh, A.G. Gallastegui,
A.C. Sanfiz, S. Basahel, S. Al-Thabaiti, A. Alyoubi, A. Obaid, M. Mokhtar, Microwave
assisted efficient protocol for the classic Ullmann homocoupling reaction using Cu-
Mg-Al hydrotalcite catalysts, Journal of Molecular Catalysis A: Chemical (2013),
http://dx.doi.org/10.1016/j.molcata.2013.08.013
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Microwave assisted efficient protocol for the classic Ullmann
homocoupling reaction using Cu-Mg-Al hydrotalcite catalysts
a*
a
a
Katabathini Narasimharao , Ebtisam Al-Sabban , Tamer S Saleh , Ainara Garcia
b,c
b,c
a
a
Gallastegui , Almudena Celaya Sanfiz , Sulaiman Basahel , Shaeel Al-Thabaiti ,
a
a
a*
Abdulrahman Alyoubi , Abdullah Obaid , Mohamed Mokhtar
a
Department of Chemistry, Faculty of Science, King Abdulaziz University, P. O. Box
0203, Jeddah 21589, Kingdom of Saudi Arabia
8
b
Department of Chemistry, Imperial College London, London, SW7 2AZ , U.K.
c
Bio Nano Consulting, 338 Euston Road, London, NW1 3BT, U.K.
Abstract
Four hydrotalcite catalysts containing different amounts of Cu and Mg cations, with
varying atomic ratios of Cu:Mg:Al in the brucite-like layers of 0:2:1, 0.5:1.5:1, 1:1:1and
2:0:1, were prepared by co-precipitation method. The synthesized catalysts were tested for
classic Ullmann homocoupling of iodobenzene to produce biphenyl without using any
stoichiometric amounts of base or reducing agent. The Mg-Al hydrotalcite catalyst without Cu
ions is inactive, but the doping of Cu ions (Cu/Al=0.5) into the Mg-Al hydrotalcite framework
led to 63% conversion of iodobenzene under microwave irradiation. Gradual increase of Cu
content and concomitant decrease of Mg content resulted in an increase in the conversion of
iodobenzene and selectivity to biphenyl. The physico-chemical properties of hydrotalcite
2
catalysts were studied by chemical analysis, XRD, FTIR, N -physisorption, SEM, XPS, ESR
and TPR techniques to understand the state of Cu ions presented in the hydrotalcite
2
+
framework. It appears that presence of easily reducible isolated Cu ions is essential to obtain
improved performance of the catalyst.
Keywords: Ullmann homocoupling; Iodobenzene; Aryl halides; Cu-Mg-Al hydrotalcites;
Microwave irradiation
*
Corresponding Authors, Fax: +966-26952292; Tel: +966-500558045;
E-mail: mmokhtar2000@yahoo.com; katabathini@yahoo.com
1
Page 1 of 34

1. Introduction
Numerous attempts have been made to develop heterogeneous catalysts for C-C bond-
forming reactions because of their increasing importance in pharmaceutical, agrochemical and
fine chemical industries [1-4]. The synthesis and utilization of biaryls is interesting because
biaryls can be used as monomers for the synthesis of conductive polymers [5-7], natural
products such as, steganone [8] and receptrice macrocyclic molecules [9]. There are many
methods to synthesize symmetrical biaryls; one of the main routes is the classic Ullmann
coupling reaction. Although the Ullmann reaction is over 100 years old, it is still used in
industry today. Conventionally, the Ullmann coupling reaction (Scheme 1) is carried out at
high temperatures (125-300 °C) in the presence of base such as NaOt-Bu [10], Cs
2 3 ,
CO [11]
K
2
CO [12] and K PO [13]. In addition, these reactions often require the use of stoichiometric
3
3
4
amounts of copper reagents [copper powder or copper (I) salt and copper(I) thiophene-2-
carboxylate], which result in the production of undesirable chemical waste [4].
Scheme 1: The classic Ullmann reaction for coupling of aryl iodides by reaction with copper
metal at high temperature
The homogenous catalytic methodologies reported in the literature suffer from
disadvantages, such as disposal of waste and difficulty in recovering the catalyst from the
products. In the last decade, there were notable improvements in the development of
heterogeneous catalyst for Ullmann coupling reaction for synthesis of biaryl [14].
2
Page 2 of 34

Wan et al. [15] synthesized Pd deposited hybrid mesoporous periodic organosilicas
Ph-MCM-41) with phenylene moieties embedded inside the silica and used these materials as
(
a heterogeneous catalyst for the Ullmann coupling reaction in water. They observed 75.4%
iodobenzene conversion after 10 h at 100 °C with 98% selectivity to biphenyl. However, Pd
based catalysts are air sensitive, expensive and require an additional reducing agent, thus limit
their relevance for industrial applications [16].
Due to these disadvantages associated with Pd based catalysts, many researchers have
begun to reinvestigate Cu based catalysts; Miao and Wang [17] developed alternative
approaches to the classic Ullmann reaction of C-C formation using functionalized silica-gel
immobilized Cu (II) catalyst. However, irreversible binding of metal ions and lack of
selectivity are the main disadvantages of silica bound ligands [18]. Ponce and Klabunde [19]
synthesized nanocrystalline particles of copper and their catalytic reactivity have been tested
in the Ullman homocoupling. The authors observed maximum biphenyl yield of 90% at 150
o
C after 6 h for the Cu nanoparticle sample and toulene as a solvent. Samim et al. [20] studied
effect of size of Cu nanoparticles in the conversion and yield in the Ullmann coupling
reaction. They observed a drastic change in the yield of biphenyl from 43% to 95% when they
switched the catalyst from micro size Cu particles to nano size (8 nm) Cu particles.
Unfortunately, most of the Cu based catalysts need an extra base [21, 22] in sub-
stoichiometric amounts and/or require long reaction times [23]. More recently, Li et al. [24]
reported for the first time the application of gold cluster supported catalysts [Au25
2
(SR)18/CeO ] for an Ullmann homocoupling reaction of aryl iodides. The catalyst was found
to give a 99.8% yield, using conventional heating, 48 h reaction time with iodobenzene and
excellent recyclability, however this gold nano-cluster catalyst is expensive, needs extra base
3
Page 3 of 34

and also has low thermal stability. Hence, it is of great practical interest to develop an
efficient, inexpensive, solid heterogeneous catalyst that can catalyze the Ullmann coupling of
iodoaryls to biaryls in the absence of base, with short reaction times yielding a high amount
of product, which can be easily separated and reused in the reaction several times.
Mg-Al hydrotalcites (HT) and mixed oxides derived from hydrotalcite precursors are
widely used catalysts for various chemical reactions because they are relatively cheap, do not
contain any toxic metals and are stable at high temperatures in polar and non-polar reaction
conditions [25]. Leon et al. [26] reported that the basic properties of these materials are
dependent on chemical composition and synthesis procedure used. On the other hand,
properties such as presence of basic sites, the nature of basic sites, their density, type of
solvent, largely affects its performance in different reactions, since the chemical reactions
involve a complex sequence of elemental steps catalyzed by active sites. Auer et. al. [27]
used mixed-metal oxides, derived from Cu-Mg-Al hydroxy carbonates, for the synthesis of
methyl amines and observed maximum activity with a composite that possessed a
hydrotalcite like structure. Iwai et al. [28] successfully used an alkali-promoted Cu-Mg-Al
hydrotalcite catalyst for selective oxidation of 2,6-di-tert-butylphenol. The authors also
2+
claimed that presence of Cu ions is essential to obtain improved catalytic performance.
Motivated by the aforementioned findings, and in continuation of our research interest
in the use of hydrotalcite catalysts for organic transformations [29], “Microwave induced
Organic Reaction Enhancement” (MORE) chemistry has been utilized for rapid synthesis of
biaryl molecules. Here, we report a new, simple protocol that does not require any reducing
agent or additional base via recyclable solid Cu-Mg-Al and/or Cu-Al hydrotalcite catalyst for
synthesis of biaryl. To the best of our knowledge, the application of Cu-Mg-Al based
4
Page 4 of 34

hydrotalcite catalysts for the this reaction has not previously been reported. A detailed
characterization of catalysts has been carried out to understand how the Cu ions in
hydrotalcite structure influence the reaction mechanism.
2
2
.Experimental
.1 Preparation of catalysts
MgLDH (Cu:Mg:Al = 0:2:1), CuLDH-1 (Cu:Mg:Al = 0.5:1.5:1), CuLDH-2
(Cu:Mg:Al = 1:1:1) and CuLDH-3 (Cu:Mg:Al = 2:0:1) materials were prepared by a co-
precipitation method. Two solutions, namely, solution (A) containing the desired amount of
metal (Cu, Mg and Al) nitrates and solution (B) containing the precipitating agents (i.e.,
4 2 3
NH OH and Na CO ) were added slowly and simultaneously into a beaker containing
distilled water, while maintaining the pH constant at around 100.1 with vigorous stirring at
o
5
0 C to obtain the precipitate. The precipitate was filtered and washed with distilled water
o
five times. The precipitate was then dried in air at 100 C for 24 h.
2.2 Characterization of the catalysts
The chemical analyses of the prepared hydrotalcites were characterized by inductively
coupled plasma-atomic emission spectroscopy (ICP-AES) with Varian Vista-Pro Axial using
a simple solution calibration, where they were dissolved in 50 % HNO . C, H and N
3
concentrations were determined from 5-20 mg of solid samples on a CHN Elemental
Analyzer. Scanning Electron Microscopy- energy dispersive X-ray spectrometer) (SEM-
EDX) measurements were carried out using a JEOL JSM840A fitted with an Oxford
Instruments INCA energy dispersive analytical system. Coating of each of the powder was
attached to an aluminium block using double sided carbon tape. SEM images (20X) were
acquired of each of the powders and EDX analysis of 3 points of each sample was performed.
5
Page 5 of 34

X-ray Diffraction (XRD) patterns were obtained using Bruker D8 advance target
diffractometer. The patterns were run with copper Kα with secondly monochromator (λ=
1.5405 Å) at 40 kV and 40 mA. The crystallite size of hydrotalcite phases was calculated
using Scherrer equation: D = B λ/ β1/2 cos Θ; where D is the average crystallite size of the
phase under investigation, B is the Scherrer constant (0.89), λ is wavelength of the X-ray
beam used, β1/2 is the full width at half maximum (FWHM) of diffraction peak and Θ is the
diffraction angle.
Textural properties (BET-surface area, micropore area, pore size, total pore volume)
o
were determined from nitrogen adsorption/desorption isotherms at -196 C using a model
Tristar 3000 automated gas sorption system (Micromeritics, U.S.A). Continuous-wave (cw)
Electron spin resonance spectroscopy (ESR) experiments were performed at 30 K on a Bruker
EMXplus spectrometer operating at 9.4 GHz equipped with a 4122SHQE resonator and an
Oxford Instruments ESR900 cryostat. All measurements were carried out with 0.2 mW
microwave power, 100 kHz modulation frequency, 0.1 mT modulation amplitude and 10 ms
conversion time and time constant. FTIR spectra were recorded on a Perkin-Elmer Spectrum
100 FTIR spectrometer using KBr pellets (1 wt.% sample in KBr matrix). The X-ray
photoelectron spectroscopy (XPS) measurements were carried out by using a SPECS GmbH
X-ray photoelectron spectrometer. Prior to analysis, the samples were degassed under vacuum
inside the load lock for 16 h. The binding energy of the adventitious carbon (C 1s) line a
284.6 eV was used for calibration, and the positions of other peaks were corrected according
2 2
to the position of the C 1s signal. The H TPR, CO TPD analyses were performed using
CHEMBET 3000, Quantachrome, U.S.A.
2.3 Typical procedure for Ullmann condensation reaction
6
Page 6 of 34

2.3.1 Microwave irradiation
The reaction using microwave irradiation was carried out using CEM Discover
TM
TM
Labmate microwave apparatus (300 W with ChemDriver Software). The catalyst (0.5 g),
in absence or presence of KOH or K CO (2.0 mmol), iodobenzene (1) (2.0 mmol) and
2
3
DMSO (4.0 mL) were mixed in a glass process vial. The vial was capped tightly and it was
o
irradiated with microwaves with power of 300 W to reach a reaction temperature of 120 C
under auto generated pressure. The vial was exposed to microwaves for a required time to
complete the reaction. The progress of the reaction was monitored by thin layer
chromatography (TLC) for every 4 minutes (eluent; petroleum ether: chloroform). Upon
completion of the reaction, the catalyst was removed by filtration and washed with CHCl
solvent and the solvent was evaporated under reduced pressure to obtain the product.
3
2.3.2 Conventional electrical heating method
For comparison, the reactions were performed on the same scale as described above
and carried out using convential chemistry methodology. The reactants and catalyst were put
o
in a flask. The reaction mixture was heated at 120 C in a thermostatic oil-bath for required
time to complete the reaction as monitored by TLC. After the completion of the reaction, the
reaction mixture was cooled to room temperature and the products were obtained as described
in the previous method.
2.4 Analyses of the reaction products
All melting points were measured on a Gallenkamp melting point apparatus and are
uncorrected. The infrared spectra were recorded in KBr disks on Shimadzu FTIR 8101 PC
infrared spectrophotometers. The NMR spectra were recorded on a Varian Mercury VX-300
7
Page 7 of 34

1
3
NMR spectrometer. H spectra were run at 300 MHz in CDCl . Chemical shifts were related
to that of the solvent. GC-MS analysis was carried out using Shimadzu, GCMSQP2010S
equipped with a direct insertion probe (DIP) was used to acquire the mass spectra of extracted
volatile organic compounds and detected in the electron impact mode (EI-MS) by the
application of 70 eV as the ionization energy.
3
3
.Results and discussion
.1 Ullmann homocoupling of iodobenzene
All the samples have been tested as catalysts for the classic Ullmann homocoupling of
iodobenzene reaction (Scheme 2) without using any base via conventional means and
microwave irradiation.
Hydrotalcite Catalyst
4
I
+ 2
+ 2I2
Conventional
or
Microwave
Benzene, 3
Iodobenzene, 1
Biphenyl, 2
heating
Scheme 2: Ullmann homocoupling of iodobenzene
The catalytic activity data of the catalysts is tabulated in Table 1. The MgLDH sample
was virtually non-active under both conventional and microwave conditions. In contrast,
CuLDH-1 CuLDH-2 and CuLDH-3 catalysts showed substantial activity, the maximum
biphenyl yield for the reaction ca. 54%, 77% and 91% respectively under microwave
irradiation and 69%, 72% and 89% respectively under conventional heating. The
dehalogenation product benzene is the only detected byproduct under the studied reaction
conditons. The main advantage of microwave heating over the conventional heating is that it
8
Page 8 of 34

needed only 12-20 minutes while conventional heating required 10 hours to obtain considerly
good biphenyl yield.
It is clear from results cited in Table 1 that the best yields of the biphenyl using either
microwave or conventional conditions were achieved when the hydrotalcite catalyst
containing the highest content of Cu (CuLDH-3) was used. Furthermore, when the microwave
irraditaion was used, the CuLDH-3 catalyst (where the Cu replaced all the Mg ions
completely in the cationic sheet) showed very good efficiency over the Cu and Mg (CuLDH-1
and CuLDH-2) containing catalysts, since it gave the highest yield in the shortest reaction
time .
Table 1: Catalytic performance of hydrotalcite catalysts under microwave and conventional
heating methods
a
a
Catalyst
Microwave irradiation
Time Conversion Selectivity Yield
Conventional heating
Time Conversion Selectivity Yield
b
b
(
min.)
60
20
15
(%)
0
63
77
91
(%)
0
85
100
100
(%)
0
54
77
91
(h)
24
10
10
10
(%)
0
89
74
89
(%)
0
100
97
(%)
0
69
72
89
MgLDH
CuLDH-1
CuLDH-2
CuLDH-3
12
100
a
Iododbenzene (1) (2.00 mmol), hydrotalcite catalyst (1.25 g), DMSO (4mL), reaction temperature;
o
b
1
20 C, Isolated yields of biphenyl (2)
1
1
1
00
Conversion of iodobenzene
Selectivity to biphenyl
80
60
40
20
0
K CO
2
3
00
80
60
40
20
0
KOH
00
80
60
40
20
0
No base
0
.0
0.5
1.0
1.5
2.0
Cu/Al ratio
2 3
Fig. 1: Catalytic performance of hydrotalcite catalysts using K CO , KOH and no base
9
Page 9 of 34

To determine the effectiveness of hydrotalcite catalyst in the Ullmann homocoupling
reaction, we performed the reaction under microwave conditions, by adding KOH and K CO
2
3
as additional base along with the catalyst. Figure 1 shows the conversion of iodobenzene and
selectivity to biphenyl over all the hydrotalcite samples. All the Cu containing hydrotalcite
catalysts offered almost similar catalytic activity in presence or absence of base. These results
indicate that the Cu containing hydrotalcite is not only catalyzing the reaction, but also acting
as a base. It is also worth noting that the MgLDH sample did not provide any catalytic activity
even though it contained same interlayer anions as Cu containing hydrotalcites. This
observation suggests that Cu ions are necessary for the Ullmann homocoupling reaction.
Table 2: Ullmann homocoupling of aryl iodides using CuLDH-3 catalyst under microwave
irradiation
a
Entry
Substrate
Time Conversion Selectivity
(min.)
(%)
to biphenyl
derivative
(%)
10
85
95
1
2
Cl
Br
10
95
95
I
I
I
Me
3
4
5
10
10
10
95
95
75
98
98
90
OMe
NO
2
a
o
Substrate (1) (2.00 mmol), hydrotalcite catalyst (1.25 g), DMSO (4mL), reaction temperature; 120 C
1
0
Page 10 of 34

The scope and generality of this protocol was tested for bromobenzene, chlorobenzene
and other derivatives of iodo benzene as shown in the Scheme 2 using highly active CuLDH-3
catalyst under the same reaction conditions listed in experimental section and corresponding
biphenyl derivatives were obtained in good yield (Table 2).
As shown in Table 2, The electron-rich aryl iodides (entry 3 and entry 4) were
homocoupled at a much higher conversion (95%) than the electron-deficient 1-iodo-4-
nitrobenzene (75%, entry 5). In general, the reaction gave very good yields and was highly
successful for iodides and bromides. Activity has been lowered for chlorides.
In order to show the significance of this work, we compared the catalytic activity of
CuLDH-3 in Ullmann homocoupling reaction with reported heterogeneous catalysts in the
literature.
Table 3: Comparison of CuLDH-3 activity with heterogeneous catalysts reported in
the literature
Conv.
iodobenzene
(%)
Sel.
biphenyl
(%)
Reference
Catalytic
system
Reaction conditions
o
Wan et al.
Samim et al.
Li et al.
Pd/Ph-MCM-41
Cu nanoparticles(5nm
Au (SR) /CeO
H O,KOH, HCOONa,100 C,10h
75.5
95
98
2
o
DMSO,200 C,5h
100
100
100
100
o
DMF,K CO ,130 C,48h
99.8
89
2
5
18
2
2
3
o
Ponce & Klabunde
Present work
Cu nanoparticles
CuLDH-3
Toluene,150 C,6h
o
DMSO,120 C,microwaves,12min
91
Table 6 shows experimental conditions and results found in the literature for different
heterogeneous catalysts. Comparing our best catalyst, we can state that our catalyst is more
reactive, considering that the Cu-Al LDH is stable, cheaper, highly active and that the reaction
temperature is either lower or the reaction time shorter.
3.2 Chemical analysis
1
1
Page 11 of 34

Chemical analyses of the dried hydrotalcite catalysts were carried out using an ICP-
AES technique. The atomic percentages of Cu, Mg, Al, C, H, N elements and Cu/Al and
Mg/Al atomic ratios of the hydrotalcite compounds are shown in Table 4. Detailed
characterizations of the fresh catalysts were performed to understand how the structure of
hydrotalcite and Cu ions are playing the role in reaction Ullmann homocoupling reaction
mechanism.
Table 4: Chemical analysis of hydrotalcite samples
Catalyst
C
H
Cu Mg Al Cu/Al Mg/Al
MgLDH 3.26 4.07 0.0 20.7 10.3
CuLDH-1 2.85 3.29 5.4 15.5 10.1 0.53
0.0
2.00
1.53
CuLDH-2 2.88 3.86 10.9 10.4 10.7 1.01
CuLDH-3 1.80 2.75 23.1 0.0 12.0 1.92
0.97
0.0
The atomic composition results obtained from EDX analysis were similar and in
accordance with this data. The actual Cu, Mg and Al species contents analyzed in the solids
were very similar to nominal contents indicating the efficiency of the preparation method.
3.3 Powder X-ray diffraction (XRD)
The XRD patterns of the as-synthesized hydrotalcite samples are shown in Fig. 2. All
o
o
o
o
o
o
the samples showed diffraction peaks at 2θ = 11 , 24 , 35 , 38 , 46 , and 60 ascribed to
diffraction by basal planes (003), (006), (105), (108), (009) and (110) respectively. These are
the typical diffraction patterns of hydrotalcite-like materials [JCPDS 22-0700] having layered
structure with intercalated carbonate anions [30]. A hydrotalcite phase was only detected in
MgLDH and CuLDH-3 samples. A small amount of malachite phase [Cu
0-0399] appeared in the XRD patterns of CuLDH-1 and CuLDH-2 samples. Hence, it
appears that there is a limitation on the amount of Cu incorporation in the Mg-Al hydrotalcite
2 2 3
(OH) CO , JCPDS
1
1
2
Page 12 of 34

framework.
1
1
200
000
M=malachite
8
6
4
2
00
00
00
00
0
CuLDH-3
CuLDH-2
2
1
000
000
0
1
1
500
000
CuLDH-1
MgLDH
5
00
M
M
0
00
4
3
2
1
00
00
00
0
-
100
1
0
15 20 25 30 35 40 45 50 55 60 65 70 75
 (degrees)
2
Fig. 2: XRD patterns of as-synthesized hydrotalcite samples
This result indicates that under the adopted preparation conditions, less than 16.6%
atomic ratio of Mg ions can be isomorphously substituted by Cu in the Mg-Al brucite layer to
obtain pure and homogeneously dispersed Cu-Mg-Al HT sample. Jiang et al. [31] reported
that the Jahn-Teller distortion of Cu in octahedral coordination could be the reason for lower
stability of the Cu hydrotalcite phase and its subsequent formation of malachite. No
significant positional changes in the basal plane reflections were observed with the variation
in the Cu/Mg atomic ratio. This may be due to a small difference in the octahedral ionic radii
2
+
2+
of Cu (0.73 Å) and Mg (0.72 Å). However, the Cu content influenced the crystallinity of
1
3
Page 13 of 34

the Cu-Mg-Al hydrotalcite samples, which can be seen from both XRD patterns (Fig. 2) and
structural parameters (Table 5). MgLDH, CuLDH-1, CuLDH-2 samples are highly crystalline
and the crystallanity of the samples is increasing with the increase of Cu content. In contrast,
the CuLDH-3 showed much lower crystallanity than the Mg containing samples.
Table 5: Crystallographic data from XRD patterns and textural properties of hydrotalcite
2
samples from N physisorption
Catalyst Lattice parameters Crystallite SBET
V
p
D
p
2
3
(Ả)
size (nm) (m /g) (cm /g) (nm)
MgLDH a = 3.068, c= 22.830
CuLDH-1 a = 3.068, c= 22.830
CuLDH-2 a = 3.068, c= 22.830
CuLDH-3 a = 3.050, c= 23.120
38.00
30.73
29.59
16.07
90 0.7420
73 0.4068
75 0.2614
30 0.1001
20
10
15
45
SBET: specific BET surface area; V : total pore volume; D : average pore diameter
p
p
The average crystallite sizes in the a direction were calculated from (003) plane using
Debye-Scherrer equation and summarized in Table 5. The crystallite sizes of the HT samples
varied in the range 16 nm to 35 nm. The CuLDH-3 sample, with the largest Cu content in a
single hydrotalcite phase, exhibits the smallest crystallite size at about 16 nm. The lattice
parameters of all the MgLDH, CuLDH-1 and CuLDH-2 samples are similar. This confirms
2+
2+
the uniform substitution of Cu ions to Mg ions in the cationic sheets of CuLDH-1 and
CuLDH-2 samples. The lattice parameters of CuLDH-3 sample were smaller, which indicates
that the removal of Mg from the hydrotalcite structure decreases the lamellar spacing.
3.4 FTIR spectroscopy
The FTIR spectra of Cu incorporated hydrotalcites are presented in Fig. 3. All the
-1
samples exhibited characteristic broad absorption band centered at 3420 cm , due to presence
of interlayer H O molecules and structural hydroxyl groups in the brucite layer. The weak
2
-1
2
peak at 1640 cm is attributed to an interlayer H O bending vibration and a strong peak at
1
4
Page 14 of 34

-1
2-
1
350 cm results from the ν
3
(CO
3
) antisymmetric stretching mode [25]. Another small band
-1
at 875 cm can be observed in all the samples and is assigned to an out-of-plane deformation
4
in the ν mode of carbonate ion [32].
CuLDH-3
300
200
100
CuLDH-2
CuLDH-1
900 1200 1500 1800 2100 2400 2700 3000 3300 3600 3900
-1
Wave number (cm )
Fig. 3: FTIR spectra of CuMgAl hydrotalcites
It was reported [33] that in the FTIR spectrum of malachite each of the six non-
2
-
degenerate modes of the CO
3
ion, could split into two IR active components (particularly in
mode of the carbonate ion) under C2h
-1
the region of 1520-1390 cm that are attributed to the ν
3
symmetry, as a result of the correlation field splittings. However, the FTIR spectra of
CuLDH-1 and CuLDH-2 samples did not show any such peaks with splittings, even though
XRD results showed that these two samples contained small of amount of malachite phase.
A major spectral change was noticed in the FTIR spectrum of CuLDH-3 sample (Fig.
-1
-1
3
) showing a new band at 1045 cm . The peak at 1045 cm could be identified as an
2-
unperturbed CO
3
1
ν symmetric stretching vibration; this peak is not presented in either the
2
-
CuLDH-1 and CuLDH-2 spectra. This observation suggests that two different types of CO
3
1
5
Page 15 of 34

ions are presented in the CuLDH-3 sample. It is possible that these new carbonate species
might be isolated metal carbonates.
3.5 Nitrogen gas physisorption
o
Nitrogen gas adsorption-desorption isotherms at -196 C for all the samples are shown
in Fig. 4. All the samples show the type II pattern based on the Brunauer, Deming, Deming
and Teller (BDDT) classification [34] with a type B hysteresis loop, the area of which
increased with increasing Cu/Al ratio from 0 (MgLDH) to 0.5 (CuLDH-1). Increasing the
Cu/Al ratio further to 1 (CuLDH-2) and 2 (CuLDH-3), caused a decrease in the area under
hysteresis loop.
The hysteresis loops are at a relative pressure of 0.5-1.0 for MgLDH and CuLDH-1
samples indicating pore architecture of large mesoporous dimensions. CuLDH-2 and CuLDH-
3
samples have hysteresis loops at relative pressures of 0.6-1.0 revealing that the interlayer
space is inaccessible for nitrogen molecules. These results are further corroborated by the
pore size distribution of these samples, shown as an inset in Fig. 4.
2
1
1
1
1
1
00
80
60
40
20
00
1
1
1
80
60
40
0.8
0
.7
.6
0
0.5
0.4
0.3
0.2
0.1
0.0
120
100
80
60
40
20
0
0
10 20 30 40 50 60 70 80 90 100
Pore diameter (nm)
8
0
6
0
MgLDH
0.8
4
0
CuLDH-1
2
0
0
.0
0.2
0.4
0.6
1.0
0.0
0.2
0.4
0.6
0.8
1.0
o
o
Relative Pressure (P/P )
Relative pressure (P/P )
1
6
Page 16 of 34

1
00
0
.016
.014
.012
.010
.008
.006
.004
.002
.000
0
.12
.10
300
250
200
150
100
0
0
0
8
0
0
0.08
0
0.06
0.04
0.02
0
60
0
0
4
0
0
0.00
0
15 30 45 60 75 90 105 120 135 150
Pore diameter (nm)
0
15 30 45 60 75 90 105 120 135
Pore diameter (nm)
5
0
20
0
CuLDH-2
CuLDH-3
0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
o
o
Relative pressure (P/P )
Relative pressure (P/P )
Figure 4: Adsorption-desorption isotherm (inset: BJH pore size distribution) of hydrotalcite
samples
It is known that the distribution of pores in hydrotalcites is influenced by the
crystallite size and packing arrangement of crystallites [35]. A relatively narrow distribution
of pores was observed for MgLDH and CuLDH-1 samples, whereas a broader distribution of
pores can be observed for CuLDH-2 and CuLDH-3 samples. This is indeed reflected in the
BJH pore diameter of these samples (plotted from the desorption branch of the isotherm). The
textural properties of all the samples are tabulated in Table 5. The average pore diameter of
MgLDH sample is around 20 nm, in contrast the CuLDH-3 sample showed 45 nm, and with
the specific surface area of the former was twice that of the latter. The specific surface area,
total pore volume and the area of the hysteresis loop decreased with an increase in Cu content.
These changes probably originate from morphological differences in the crystallites arising
with a change in Cu and Mg concentration..
3.6 Scanning electron microscopy (SEM)
SEM was used to study the morphology of these samples to get more information
about the arrangement of crystallites in these samples. The SEM images of MgLDH, CuLDH-
1
7
Page 17 of 34

1, CuLDH-2 and CuLDH-3 are shown in Fig.5. The crystallites in all the samples are clumped
together, poorly dispersed and exhibiting spongy type of morphology. The crystallites are
ambiguous in appearance and lack distinct particle edges, particularly in the CuLDH-1 and
CuLDH-2 samples. The crystallite particles in MgLDH and CuLDH-3 samples were arranged
in semi-hexagonal shape to create inter-particle channels.
Fig. 5: SEM images of (a) MgLDH (b) CuLDH-1 (c) CuLDH-2 and (d) CuLDH-3
The extent of agglomeration appears to be decreasing with increase of Cu content. In
MgLDH sample, the crystallites agglomeration occurs strongly in forming such spongy
particles 30-40 nm in diameter, whereas CuLDH-3 consists of particles of 20-25 nm. This is
in accordance with XRD and pore size distribution results where contributions of larger pores
were formed in greater numbers through interparticles for CuLDH-3 sample.
3.7 Electron spin resonance spectroscopy (ESR)
To determine the electronic properties of Cu species in the samples. Fig. 6 shows the
1
8
Page 18 of 34

ESR spectra for CuLDH-1, CuLDH-2 and CuLDH-3 samples.
4
3
CuLDH-1
2
1
0
1
2
3
-
-
-
1500 2000 2500 3000 3500 4000 4500 5000 5500
4
3
2
1
0
1
2
3
4
CuLDH-2
-
-
-
-
1500 2000 2500 3000 3500 4000 4500 5000 5500
30
20
10
0
CuLDH-3
-10
-20
-30
-40
1500 2000 2500 3000 3500 4000 4500 5000 5500
Field [G]
Fig.6: ESR spectra of Cu incorporated samples
MgLDH sample is ESR inactive. The ESR signals of Cu incorporated samples are
2
+
⊥
anisotropic with clearly defined g|| and g regions as expected for Cu in a surrounding of
axial symmetry. The spectral parameters can be interpreted as arising from distorted
1
9
Page 19 of 34

2
+
octahedrally cooridanated Cu ions [36].
According to Bahranowski et al. [37], appearance of well-resolved ESR signals means
2
+
that Cu ions are uniformly dispersed in the LDH structure surface. The higher the Cu
concentration the broader the ESR signal. CuLDH-1, which has the lower Cu-content (5.1%)
shows the sharpest peak. CuLDH-2 and CuLDH-3, with a higher Cu-content (10.9% and
23.1% respectively) in comparison with CuLDH-3, shows broader ESR signal.
The broad line width with the increase of Cu-content can be attributed to dipolar
2
+
broadening effects caused by mutual interactions between paramagnetic Cu ions, indicating
2
+
that the corresponding ions were located in a Cu containing aggregated phase of clusters
[38]. The presence of Cu contained clusters in different concentration on three catalysts lead
to the difference of ESR signal. It was clear that more Cu contained clusters appeared on
catalyst CuLDH-3 than CuLDH-2 and CuLDH-1.
3.8 X-ray photoelectron microscopy (XPS)
To gain further insight into the location and nature of the Cu species present in these
samples, XPS investigation was undertaken. Deconvoluted Cu 2p3/2 X-ray photoelectron
spectra for CuLDH-1, CuLDH-2 and CuLDH-3 samples are shown in Fig. 7. It was reported
that binding energy in the range of 933.0-933.8 eV for the Cu2p3/2 peak and shake-up peaks
are characteristic for CuO, while binding energy of 932.2- 933.1 eV and absence of shake-up
peaks is characteristic for Cu
2
O [39]. All Cu incorporated samples exhibited principle Cu
2p3/2 broad profile consisted of two peaks in between 930 eV and 938 eV, which could be
2+
attributed to Cu species [40]. The peaks have been deconvoluted into two contributions
with binding energies centered at 932.8 eV and 934.9 eV respectively. The former peak can
2+
be assigned to isolated Cu species [41] and the latter maybe related to another state where
2
0
Page 20 of 34

Cu ions coordinate with Mg and Al in a spinnel like species. Quantification of Cu ions
presented in Cu LDH samples was performed (Table 6). It is interesting to note that the
2
+
CuLDH-3 sample contained the highest amount of isolated Cu species (65%).
Fig.7: Deconvoluted Cu 2p3/2 XPS spectra for CuLDH-1, CuLDH-2 and CuLDH-3 samples
Table 6: Quantification of Cu species presented in the samples by XPS analysis and H
2
consumption values from TPR measurements
2
1
Page 21 of 34

2
+
2+
H
2
consumed Cu isolated species Cu cluster species
Sample
during TPR Binding Percentage Binding Percentage
-1
(μmol.g )
energy
eV)
energy
(eV)
(
CuLDH-1
CuLDH-2
CuLDH-3
245
284
328
932.8
932.8
932.8
22
46
65
934.9
934.8
934.9
78
54
35
3.9 H
2
-Temperature Programmed Reduction (H
2
-TPR)
2
H -TPR was used to investigate the reducibility of the samples and to understand
contribution made by different copper species found in these materials. The H
of all the samples are shown in Fig. 8.
2
-TPR patterns
CuLDH-3
CuLDH-2
CuLDH-1
MgLDH
100 200 300 400 500 600 700
o
Temperature ( C)
Fig. 8: H
2
TPR patterns of hydrotalcite samples
o
The MgLDH sample showed three reduction peaks with the major peak at 550 C.
o
o
Two small peaks at 300 C and 425 C are assigned to the reduction of a small amount of
2
2
Page 22 of 34

o
impurities remaining in the solid from the raw materials. The major peak at 550 C may be
2+
due to the reduction of Mg ions. With the gradual replacement of Mg for Cu cations, new
peaks can be identified in the overall TPR patterns. Broad peaks in the temperature range of
o
2
00-400 C can be observed for the CuLDH-1 sample and these peaks can related to the
2+
reduction of Cu cations, which also include isolated copper ions, weak magnetic associates,
and small two- and three-dimensional clusters [42].
Nagaraja et. al.[43] observed two broad peaks at high temperature, ranging from 300
o
o
C to 560 C, they suggested that these peaks are possibly due to the reduction of Cu ions that
o
are strongly associated with hydrotalcite. The peak at 650 C can be ascribed to the reduction
of bulky CuO phases that include large clusters. Isolated Cu ions can react easily with other
components of the sample and form various surface Cu components of different reducibility
[44]. With the increase of Cu content, the reduction peaks shifted toward low temperatures
and multiple weak peaks were transformed into a single intense peak - as can be observed in
the CuLDH-2 sample. This observation could infer an enhancement of reducibility of the
catalyst, with rapid reduction of Cu ions. Agrell et. al. [45] stated that the shift in the
reduction temperature peak in the TPR patterns of Cu-Zn-Al oxide catalysts is due to the
presence of larger CuO crystallites.
-1
The hydrogen consumption value of CuLDH-3 (328 μmolg ) is higher than CuLDH-2
-1
-1
(
284 μmolg and CuLDH-1 (245 μmolg ). A higher amount of Cu cations (CuLDH-3) in the
hydrotalcite causes an increase in the portion of easily reducible copper particles, showing a
o
reduction peak at temperatures lower than 400 C. This trend is similar to that described in the
catalytic test, suggesting that the catalytic activiity is related to the amount of Cu ions that are
2 2 3
easily reducable. Very similar H -TPR profiles for CuO supported on Al O has been reported
2
3
Page 23 of 34

by other authors and they indicated that the redox properties of the catalysts were influenced
2+
2 3
by the interaction of Cu ions with Al O support and small Cu cluster and/or isolated Cu
ions are reduced at lower temperature than Cu larger/cluster partilces [46]. From these
observations, we can state that a low concentration of Cu and a high concentration of Mg in
LDH materials led to deterioration of the reducibility of the Cu(II) component in the solids
and as a consequence decrease of catalytic activity was noticed.
It is evident from characterization results that the CuLDH-3 showed its unique
characterstics from the CuLDH-1 and CuLDH-2 catalysts and CuLDH-3 was also more
active. This is a somewhat expected result and can be explained using the physicochemical
data. An increase in Cu content resulted in increased contribution from the isolated Cu
species. Reduction of pore volume, due to the filling of interlayer space with Cu species, was
observed with the increase of Cu content; however, the increase of pore diameter due to the
formation of semi-hexagonal interparticle channels made active phase accessible to the
reagents.
XPS analysis of Cu incorporated samples revealed that Cu species distribution is
different for each sample (Table 6). The area under the two 2p3/2 peaks has been quantified for
three samples and the corresponding percentages have been calculated as a function of the
toal area of the peaks.
2
+
In the Fig.9, the percentages of isolated Cu ions are drawn against the Cu/Al ratio
and conversion of iodobenzene. As we can observe, CuLDH-3 possesed highest contribution
2+
2+
of most reduced Cu species, with 65% of isolated Cu species appearing with binding
energy at 932.8 eV offering highest conversion of iodobenzene.
2
4
Page 24 of 34

100
Conversion of iodobenzene
Cu 2p3/2 (932.8 eV) peak area
8
0
0
0
0
6
4
2
0
0.5
1.0
1.5
2.0
Cu/Al ratio
Fig.9: Correlation of Cu/Al ratio with Cu 2p3/2 peak area percentage and conversion of
iodobenzene
2
H -TPR data revealed that higher amount of Cu cations in the hydrotalcite layers led
to an increase of the portion of easily reducible copper particles, which are active species for
C-C coupling reactions. Isolated Cu ions that might be diluted (separated) by the high Mg-
content, might improve the selectivity to desired product, avoiding the further reaction of the
desired product (i.e. avoiding the formation of by products).
4.0 Ullmann homocoupling reaction mechanism over Cu hydrotalcites
The mechanistic pathways of Ullmann homocoupling of iodobenzene over Cu-Mg-Al
hydrotalcite can be depicted as shown in Fig. 10. For this catalytic cycle, the nucleophile
-
species in the hydrotalcite framework (OH ) reacts with the Cu (II) species followed by an
oxidative addition of the iodobenzene molecule to Cu (II) to form the Cu (III) intermediate.
Subsequently, the iodide on Cu is exchanged for the nucleophile and the obtained
intermediate, via a reductive elimination step, releases the coupling product and the active Cu
(II) catalyst is regenerated.
2
5
Page 25 of 34

I
HI + H O
2
I
Cu
(
III)
Cu-Mg-Al hydrotalcite
I
I
Cu
Iodobenzene
biphenyl
Cu-Mg-Al hydrotalcite
Fig. 10: Plausible mechanism of Ullmann homocoupling of iodobenzene over Cu-Mg-Al
hydrotalcite
In a recent review, Sperotto et al.[47] mentioned that pinpointing an individual Cu
oxidation state involved in Ullmann coupling reaction is difficult, since Cu is likely to be
present at more than one oxidation level, proportions of which may alter during the reaction
as a result of redox processes. It is also interesting to note that the MgLDH sample without
any Cu in the framework is completely inactive for this reaction even though it possessed
stronger basic sites than Cu doped catalysts [48]. This indicates that easily reducible Cu
species are enhancing the redox cycle, which is crucial for achieving high conversion and
yields.
5.0 Reusability of Cu hydrotalcite catalyst
The reusability of the CuLDH-3 catalyst was determined over several reaction cycles
2
6
Page 26 of 34

under optimized reaction conditions. The reactivation of the catalyst was carried out by
°
heating the recovered solid in an oven for 2 h at 60 C.
1
00
8
6
4
2
0
0
0
0
0
1
2
3
4
5
Number of cycles
Fig. 11: Reusability of CuLDH-3 catalyst
As can be seen in Fig. 11, a constant activity of the catalyst was observed until the
thirdregeneration cycle. However, a decrease in the yield of biphenyl was observed in the
fourth cycle. In the fourth regeneration cycle, the yield fell from 91% to 65%. In the fifth
regeneration cycle, the yield was only of 35%. The pronounced decay in the catalytic activity
may be attributed to the possible pore blocking and the loss of catalyst during the reaction
and regeneration processes.
Conclusions
In summary, Cu-Mg-Al hydrotalcites with different Cu:Mg:Al atomic compositions
were successfully synthesized. The synthesized materials were applied as catalysts for an
Ullmann homocoupling reaction under microwave irradiation without using any base or
reducing agent. The coupling of both electron-rich and electron-poor aryl halides was
2
7
Page 27 of 34

facilitated in the protocol. Microwave irradiation was shown to be rapid and more efficient
than conventional heating methods to produce biphenyl derivatives. Activity for the Ullmann
homocoupling of aryl iodides reaction increased with increasing Cu concentration. Maximum
activity was found for the catalyst that contained an atomic ratio of Cu/Al=2, Mg/Al=0
(CuLDH-3). Variation in the catalytic activity was due to varied phase compositions
(influenced by the Cu/Mg atomic ratio, as indicated by XRD, FTIR). The better intrinsic
activity for the CuLDH-3 sample with a high copper content was caused by several properties,
including low crystallite size, high pore diameter and large amount of easily reducible
2+
isolated Cu species, and its facile redox behavior.. The Cu contanined hydrotalcite catalyst
can be reused at least for three times without loss of activity.
Acknowledgements
Authors thank Felicity Sartain, Milo S.P. Shaffer and Chris Kay for useful
discussions, and Zlatko Saracevic (Department of Chemical Engineering and Biotechnology,
University of Cambridge, UK) for BET measurements. This project was funded by the
Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah under Grant No.
D-5/432. The authors, therefore, acknowledge DSR with thanks for their technical and
financial support.
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