Transition metal coordination polymers: Synthesis and catalytic study for hydroxylation of phenol and benzene

Article abstract of DOI:10.1016/j.apcata.2012.05.048

New coordination polymers of Ni(II) and Cu(II) of the polymeric salen-type Schiff base ligand derived from the condensation of 5,5′-methylene bis-(salicyaldehyde) with 1,2-diaminopropane yielded N,N′-1,2- propylenebis(5-methylenesalicylidenamine) abbrevia

Full text of DOI:10.1016/j.apcata.2012.05.048

Applied Catalysis A: General 435–436 (2012) 148–155
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Transition metal coordination polymers: Synthesis and catalytic study for
hydroxylation of phenol and benzene
∗
Hanna S. Abbo , Salam J.J. Titinchi
Department of Chemistry, University of the Western Cape, Private Bag X17, Bellville 7535, Cape Town, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
New coordination polymers of Ni(II) and Cu(II) of the polymeric salen-type Schiff base ligand
Received 18 March 2012
Received in revised form 27 May 2012
Accepted 29 May 2012
ꢀ
derived from the condensation of 5,5 -methylene bis-(salicyaldehyde) with 1,2-diaminopropane yielded
ꢀ
N,N -1,2-propylenebis(5-methylenesalicylidenamine) abbreviated [ CH2(H2sal-1,2-pn) ]n have been
synthesized. Both coordinated polymers with the general formula of [ CH2(ML·XDMF) ]n, where X = 0,
Available online 8 June 2012
ꢀ
ꢀ
M = Cu; N,N -1,2-propylenebis(5-methylenesalicylidenaminato)copper(II) and X = 2, M = Ni; N,N -1,2-
propylenebis(5-methylenesalicylidenaminato)nickel(II) have been characterized by elemental analysis,
magnetic susceptibility measurements, IR, electronic spectra and thermogravimetric studies. The ligand
behaves as a bis-bidentate molecule coordinating through the phenolic oxygen and azomethine nitrogen
atoms.
Keywords:
Coordination polymer
Phenol hydroxylation
Hydroxylation of benzene
Heterogeneous catalysts
These coordinated polymers have been assessed as catalysts for liquid phase hydroxylation of phenol
and benzene using H2O2 as an oxidant. The results show a high activity and selectivity of both catalysts
toward the formation of diphenols from phenol, and a low activity in the oxidation of benzene. The Cu-
based catalyst exhibited higher activity than Ni-based catalyst for hydroxylation of phenol and benzene.
The activity and efficiency of H2O2 depends on the reaction parameters viz., temperature, molar ratio
of the reactants and the solvent. Concentration of the oxidant and other reaction parameters has been
optimised for the maximum oxidation of these substrates. These catalysts can be recovered and reused
without notable loss of activity.
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
TS-1 was revealed to be novel and the most highly efficient het-
erogeneous catalyst (Enichem) in this area [12]. To date, many
researchers have been interested in the development of heteroge-
neous catalysts which are more active than TS-1 for hydroxylation
of phenol, but most of them suffer from leaching of the active metal
in the liquid phase and behave as homogeneous rather than het-
erogeneous catalysts [13].
Production of phenol and diphenols from the hydroxylation of
aromatic compounds is an important industrial application. There-
fore, much attention has been focused on their production through
catalytic routes, in particular direct hydroxylation of benzene and
phenol by H O [1]. Hydrogen peroxide which has high active
2
2
oxygen content has been widely explored as a green oxidant in
oxidation reactions, because it is readily available and the result-
ing by-products (water and molecular oxygen) are environmentally
friendly. Hydroxylation of phenol usually produces catechol and
hydroquinone which are used in industrial syntheses as antioxi-
dants, polymerization inhibitors, photographic chemicals and fine
chemical industries [2,3]. Mineral acids, simple metal ions and
transition metal-based complexes are well-known traditional cat-
alysts for the hydroxylation of phenol [4]. Metal ion-exchanged
or supported on zeolites, hydrotalcites, and resins [5–11] have
been increasingly studied in order to overcome the disadvantage
of homogeneous catalysts viz., recovery and reuse of the catalyst.
On the other hand, phenol is mainly manufactured by a three-
step cumene process, and has many disadvantages such as low
yield, high energy consumption and production of an equal amount
of acetone as the by-product [14]. Hence, finding alternative ways
to produce phenol that overcome the disadvantages of the cur-
rent cumene process has become one of the most challenging
tasks in oxidation catalysis from an economical and environmental
viewpoint. Direct hydroxylation of benzene to produce phenol is
performed both in the liquid and gas phase. Direct hydroxylation
of benzene is mainly focused on three preparation routes using
the oxidants nitrous oxide [15–18], hydrogen peroxide [6,19,20],
and molecular oxygen [21]. Moreover, various heterogeneous cat-
alysts have been studied for this reaction [22]. Currently, transition
metal coordination polymers have found increasing interest due
to their possible technological and industrial applications such
as separation, ion-exchange and catalysis. Coordination polymers,
∗ Corresponding author. Tel.: +27 21 959 2217; fax: +27 21 959 3055.
E-mail address: habbo@uwc.ac.za (H.S. Abbo).
0
926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.05.048

H.S. Abbo, S.J.J. Titinchi / Applied Catalysis A: General 435–436 (2012) 148–155
149
i.e. polymeric metal complexes derived from polymeric chelating
ligands showed remarkable thermal stability and since they are
insoluble in common solvents, acted as heterogeneous catalysts in
the oxidation reactions [23–26].
solution obtained in each case was digested on a water bath for
2 h. A blackish-green precipitate was formed in the case of the
Cu-complex, whereas no precipitate was formed for the nickel com-
plex. The Ni-complex was obtained as a brown solid by adding
◦
Continuing our research on the hydroxylation of phenol and
benzene through the catalytic route; we wish to report on the syn-
thesis of new coordination polymers derived from bis-bidentate,
methanol to the reaction mixture and kept overnight at 5 C. The
solids were filtered, washed with DMF followed by methanol. The
crude solids were purified by refluxing with methanol for 1 h and
ꢀ
◦
N,N -1,2-propylenebis(5-methylenesalicylidenamine) and the
dried at 120 C.
ꢀ
metal ions, Cu(II) and Ni(II). Their chemical structures are shown in
Scheme 1. The coordination polymers were characterized by var-
ious physico-chemical techniques. These coordination polymers
were screened as catalysts for the liquid-phase hydroxylation of
phenol and benzene with 30% H O . Optimum reaction conditions
N,N -1,2-propylenebis(5-methylenesalicylidenaminato)copper
◦
(II); [ CH {Cu(sal-1,2-pn)} ]n, yield: 58%, m.p. >300 C; Anal.
2
Found: C, 60.9; H, 4.5; N, 8.2; Cu, 17.6. C18H16N O Cu, requires C,
2
2
60.8; H, 4.5; N, 7.9, Cu; 17.9%. ꢀ = 1.9 BM
eff
ꢀ
N,N -1,2-propylenebis(5-methylenesalicylidenaminato)nickel
2
2
◦
were investigated and are discussed.
(II)·2DMF; [ CH {Ni(sal-1,2-pn)·2DMF}]n, yield: 42%, m.p. >300 C.
2
Anal. Found: C, 58.2; H, 6.1; N, 11.1; Ni, 11.5. C
H N O Ni,
30 4 4
2
4
2
. Experimental
requires C, 58.0; H, 6.0; N, 11.3, Ni; 11.8%, ꢀeff = 3.3 BM
2.1. Materials, physical methods and analyses
2.3. Catalytic activity test
All chemicals and solvents were of AR grade and used without
The catalytic hydroxylation of phenol and benzene using 30%
H2O2 solution as an oxidant was carried out in a two-necked round
bottom flask fitted with a water condenser and immersed in a
thermostated oil bath that was maintained at different preset tem-
peratures.
ꢀ
purification. 5,5 -Methylene bis(salicylaldehyde) [27] was pre-
pared following the literature procedures. Elemental analyses of
ligands and metal complexes were determined by Carlo Erba 1108.
The metal content was measured by using GBC Avanta atomic
absorption spectrophotometer. Magnetic susceptibility measure-
ments were made on Gouy’s balance at room temperature with
In a typical hydroxylation process, a mixture of substrate and
30% H2O2 in a chosen solvent was warmed to the desired temper-
ature. To this mixture the catalyst was added to start the reaction.
The reaction mixture was continuously stirred for the prescribed
time. Each experiment was reproduced at least 2 or 3 times. Sam-
ples were periodically withdrawn during reaction, filtered and
analyzed by gas chromatograph (2 ␮l diluted with 1 ml H2O and
extracted with chloroform). After the reactions, catalysts were fil-
tered off, regenerated by Soxhlet extraction with acetonitrile and
Hg[Co(SCN) ] as a calibrant and the diamagnetic corrections were
4
made using Pascal’s constant. Infrared spectra were recorded in KBr
on a PerkinElmer model 1600 FT-IR spectrophotometer. Electronic
spectra were recorded in Nujol on a PerkinElmer Lambda 35 UV/VIS
spectrophotometer by layering mull of the sample to the inside of
one of the cuvette while keeping the other cuvette layered with
1
pure nujol as reference. H NMR spectra were recorded in CDCl + 2
3
◦
drops DMSO-d using a Varian XR200 spectrometer. Sample signals
dried at 120 C for 6 h, and then reused.
6
are relative to the resonance of residual protons on carbons in the
solvent. A LEO 435 VP model was used for Scanning Electron Micro-
scope (SEM). Thermal studies were carried out on PerkinElmer Pyris
Diamond TG/DTA under atmospheric air. Reaction products were
analyzed using gas chromatograph HP 5890 fitted with FID detector
and 10 m × 0.53 (i.d.) and hp1 capillary column.
3. Results and discussion
3.1. Characterization
The polymeric ligand was synthesized by the condensation
ꢀ
of equimolar of 5,5 -methylene bis-(salicyaldehyde) with 1,2-
2
2
.2. Preparations
diamino-propane as shown in Scheme 1.
1
The H NMR spectral data (␦ in ppm) of the polymeric ligand
ꢀ
.2.1. Synthesis of 5,5 -methylene bis(salicylaldehyde)
Salicylaldehyde was converted to 5,5 -methylene bis-
[
CH2(H2sal-1,2-pn) ]n are as follow; broad band at 13.15 (br,
2H, OH), two sharp peaks at 8.24 (s, 1H, CH ) and at 8.28 (s,
1H, CH ) are assigned to non-equivalent azomethine protons
ꢀ
N
(
salicylaldehyde) by the classical methylation method as shown in
N
Scheme 1.
of different environment, 6.77–7.36 (m, 6H, aromatic). An unre-
solved multiplet band ranging from 3.49 to 4.04 (m, 3H, CH2
(isopropyl) and CH (isopropyl)) having a singlet at ␦ 3.68 (s, 2H,
CH2 (bridging methylene)) was also shown by the repeating unit.
A well resolved doublet with 1:1 intensity ranging from 1.37 to
1.39 (d, 3H, methyl isopropyl group) has been developed due to the
coupling of methine and methyl protons of the isopropyl group. In
view of observed peaks in 1H NMR spectrum of the repeating unit,
the structure of the repeating unit is confirmed.
2
.2.2. Synthesis of
ꢀ
N,N -1,2-propylenebis(5-methylenesalicylidenamine) polymeric
ligand, [ CH (H sal-1,2-pn) ]n
2
2
The polymeric ligand was prepared by adding dropwise equimo-
lar amount of a methanolic solution of 1,2-diaminopropane (0.74 g,
0
0
.01 mol) in 10 ml, to a hot methanolic solution of MBSal (2.56 g,
.01 mol) in 30 ml. The reaction mixture was refluxed for 2 h,
the separated solid was filtered and purified by refluxing with
methanol for 2 h. The yellow solid obtained was dried at ambient
temperature. Yield 72%, m.p. 180–184 d. Anal. Found: C, 73.2; H,
The metal complexes are colored and insoluble in most coordi-
nating and non-coordinating solvents that confirm its polymeric
nature. Elemental and metal analyses support the formation of
complexes and the presence of metal ions in each pocket of coor-
dinating atoms as proposed.
6
.0; N, 9.6. C18H18N O requires C, 73.4; H, 6.1; N, 9.5%.
2 2
2.2.3. Synthesis of coordinated metal complexes
The coordinated polymers were prepared by dissolving
3.1.1. IR spectral studies
(
1
0.002 mol equivalent of repeating unit) of the ligand [ CH (H sal-
,2-pn) ]n in 10 ml of dimethyl formamide (DMF) at 90 C. To
In accordance with structure of the ligand [ CH (H sal-1,2-
2
2
2
2
◦
pn) ]n, a broad band of medium intensity appeared in the
−
1
this solution, the respective metal acetate [M = Cu(II) and Ni(II)]
0.002 mol) dissolved in 10 ml of DMF were added. The colored
frequency range of 2400–2700 cm
due to hydrogen bonding
(
between phenolic hydrogen and azomethine nitrogen. This band

1
50
H.S. Abbo, S.J.J. Titinchi / Applied Catalysis A: General 435–436 (2012) 148–155
H N
2
CH
3
OH
HO
OH
CH COOH
3
H N
2
CHO
+
HCHO
Trioxane
2
H SO4
OHC
CHO
CH3
CH3
X
N
N
N
N
M
X
M(CH COO)
3
2
CH2
O
O
CH2
n
CH2
OH
HO
CH2
DMF
n
X = 0;
M = Cu; 1
X = DMF; M = Ni; 2
Scheme 1. Synthetic route for the coordination polymers.
disappeared on complexation with metal ions, which indicates
the coordination of phenolic oxygen to the metal ion after depro-
tonation. This is further supported by the fact that new bands
−1
appeared between 471 and 521 cm on complexation with metal
ions, which has been attributed to ␯(M O) vibrations.
−1
Multiple bands centred at 2960 cm
in the ligand spectra is
attributed to CH2 group. These bands are also found in all metal
complexes at around the same position, which indicated that
methylene group remained undisturbed after complexation with
−
1
metal ions. Sharp C N absorption band at 1635 cm was found
in the spectrum of the ligand, which on complexation with metal
−1
ions shifted to lower frequency by 10–13 cm . This shift in the
peak suggested the coordination of metal ions with the nitrogen of
the azomethine group, which is also confirmed by the appearance
−
1
of new bands within the frequency of ∼432 cm due to ␯(M N)
vibrations [28]. Thus, each unit of polymeric ligand behaves as diba-
sic tetradentate ONNO donor.
In the Ni complex an additional band at ∼1640 cm−1 corre-
sponding to ␯(C O) was observed due to coordination of the oxygen
of DMF with metal ions. The free DMF exhibits the ␯(C O) band
Fig. 1. Electronic spectra of the polymeric ligand and its metal complexes.
−
1
at 1680 cm , but on complexation with metal ions is lowered by
−
1
2
0–25 cm , which further provided evidence for the coordination
of DMF with metal ions through its oxygen [29].
(
P) ␯ transitions along with a band at 462 nm corresponding to
3
1
1
the A1g → T transition as shown by most of the metal com-
2g
3.1.2. Electronic spectral studies
plexes [30]. The electronic spectrum of [ CH2{Cu(sal-1,2-pn)} ]n
exhibits two bands at 395 and 561 nm. The latter band is assigned
to the d–d transition, while the former one is due to a symmetry
forbidden ligand → metal charge transfer (LMCT) transition simi-
lar to that observed in Cu(CH3COO)2(H2O) at 370 nm [31]. Other
bands appearing at 261 and 287 nm are assigned as intra-ligand
transitions.
The electronic spectra of the polymeric ligand [ CH (H sal-
2
2
*
1
,2-pn) ]n had two bands at 239 and 307 nm due to ꢁ → ꢁ and
*
n → ꢁ transitions, respectively (Fig. 1). These bands were shifted
to lower energy indicating the coordination between metal ions
and the azomethine nitrogen of the ligand. The Ni (II) complex
has one new band at 429 nm, which is assigned to the 3A2g → T
3
1g
Table 1
Thermogravimetric analysis data for polymeric ligand and its metal complexes.
Repeating unit ligand/complex
Temp. range
% wt. loss obsd.
(calcd.)
Group
decom-
posed
Residue of
ligand/complex
Final wt. residue
obsd. (calcd.)
◦
(
C)
[
[
[
CH2(H2sal-1,2-pn) ]n
130–170
70–530
5.0 (4.8)
95.0 (95.2)
C
L-C
[H2sal-1,2-pn]
–
1
–
CH2{Cu(sal-1,2-pn)} ]n
CH2{Ni(sal-1,2-pn)·2DMF} ]n
200–255
55–570
3.1 (3.4)
74.3 (74.2)
C
[Cu(sal-1,2-pn)]
CuO
22.60 (22.4)
14.85 (15.04)
2
L-(C + O)
180–260
60–540
31.74 (31.81)
53.41 (53.15)
C + 2DMF
L-(C + O)
[Ni(sal-1,2-pn)]
NiO
3

H.S. Abbo, S.J.J. Titinchi / Applied Catalysis A: General 435–436 (2012) 148–155
151
Table 3
3
.1.3. Thermal analysis
The TGA data of the polymeric metal complexes along with
a
Effect of reaction temperature on % phenol conversion , product selectivity and Turn
Over Frequency values.
their parent polymeric ligand for comparison are given in Table 1.
Thermogram of the Ni-complex indicates weight loss correspond-
ing to two DMF molecules per repeating unit of polymer metal
◦
−1
)
Reaction temp. ( C)
%PhOH conv.
% Selectivity
Cat
TOF (h
HQ
◦
complex. This weight loss between 150 and 290 C indicated the
5
6
0
5
11.0
33.0
48.0
100
82.6
63.1
–
17.4
36.9
33
98
142
presence of DMF molecules as being the labile ligand. The simulta-
neous decomposition of the methylene group and two molecules
of DMF are responsible for the higher weight loss in these com-
plexes. The observed weight loss theoretically corresponds to the
removal of two molecules of DMF and one carbon atom in each case
80
a
Reaction conditions: phenol (0.05 mol); phenol/H2O2 molar ratio = 1; solvent,
CH3CN (2 ml); catalyst [ CH2{Cu(sal-1,2-pn)} ]n, (10 mg); time = 6 h.
(
Table 1). The thermogravimetric analysis has been recorded under
and no other oxidation product was detected from the reaction
mixture with a mass balance of about 95%. The effect of catalyst
concentration, temperature, solvent and phenol/H2O2 mole ratios
on conversion profiles were examined in detail using the repre-
atmospheric conditions; hence carbon has been removed as carbon
dioxide [24]. The Ni(II) complex decomposed further on increasing
the temperature beyond 260 C. The last decomposed fragment cor-
responds to the weight loss of a complete ligand without the oxygen
atom because one atom of oxygen is retained by the metal ion in
the formation of the metal oxide at high temperature (>540 C).
◦
sentative catalyst [ CH {Cu(sal-1,2-pn)} ]n to optimize reaction
2
◦
conditions. Catalytic phenol hydroxylation does not occur in the
absence of catalyst.
The Cu-complex is anhydrous in nature and does not show any
◦
weight loss up to 200 C. However, a weight loss equivalent of one
carbon atom per repeating unit of polymeric complex was observed
above 200 C, which confirmed breaking of the chelating polymeric
3.2.1.1. Effect of catalyst amount. Table 2 and Fig. 2S represent the
◦
effect of catalyst amount on activity and selectivity. Three amounts
of catalyst viz., 5, 10 and 25 mg were used. Phenol conversion
increases from 29 to 48% with increasing the amount of catalyst
from 5 to 10 mg, which was achieved about 40% of total conversion
in initial 1.5 h. However, increasing the amount of catalyst to 25 mg
showed no significant effect on conversion (51%) and may be due to
the fast decomposition of H O in the reaction in the presence of an
repeating unit. The second step of the thermogram shows contin-
uous weight loss due to the decomposition of the remaining ligand
carbon skeleton of polymeric repeating units until the formation of
metal oxide. The residual weight after decomposition corresponds
to the calculated weight of copper oxide.
Thus, on the basis of the thermogravimetric behavior of these
complexes, an octahedral geometry has been proposed for the
monomeric unit of the Ni-complex and square planer geometry
for the Cu-complex, which is similar to other related complexes
confirmed by X-ray crystal structure analysis [32,33]
2
2
excess of catalyst. Therefore, the best molar ratio of phenol to the
catalyst used is 1708 ideal for obtaining higher phenol conversion.
3
.2.1.2. Effect of reaction temperature. The hydroxylation of phenol
◦
was carried out at three temperatures (50, 65 and 80 C) keeping
other parameters constant to optimized reaction temperature. It is
observed that phenol conversion increases with temperature from
The DTA curves of the complexes gave an indication of an
◦
exothermic reaction between 250 and 300 C, which may be due
to formation of carbon dioxide by decomposition of the methylene
bridge. The second exothermic peaks is the DTA curves were due to
formation of carbon dioxide and nitrogen oxides from the decom-
position of remaining ligand of the monomeric repeating units of
the complexes.
◦
◦
1
1 to 48% from 50 to 80 C (Table 3, Fig. 3S). The reaction at 80 C has
an added advantage of achieving maximum conversion of phenol
within 1.5 h of the reaction time. Selectivity towards catechol for-
mation however decreases with increasing temperature. Therefore,
◦
the suitable temperature for this catalytic reaction is 80 C.
3
.1.4. Scanning electron microscopic
The SEM micrograph of unmetalized polymeric ligand, Fig. 1S,
3.2.1.3. Effect of H2O2 concentration. The reaction was carried out
with varying amount of H2O2 to a fixed amount of phenol in (0.5:1,
1:1 and 2:1) H2O2: phenol molar ratios. As shown in Table 4 and Fig.
4S, maximum phenol conversion was obtained with molar ratios
2:1 and 1:1 (53.7 and 48.2%) respectively. It was more H2O2 efficient
to run the reaction at 1:1 ratio (41%) than 2:1 ratio (24%). In fact,
indicated rough and porous surface with irregular shapes. This lig-
and on complexation with metal ions became compact, smoother
and less porous due to coordination with metal ions which indi-
cated the formation of the coordinated polymers.
excess H O is undesirable due to competing H O decomposition,
3
3
.2. Catalytic activity
2
2
2
2
which keeps H O2 to phenol ratios low. When the H O2 is reduced
2
2
to a 0.5:1 molar ratio, conversion decreased significantly to 19.1%
.2.1. Hydroxylation of phenol
Liquid phase hydroxylation of phenol catalyzed by these com-
with 33% H O2 efficiency. The amount of p-benzoquinone formed
2
plexes using H O2 as an oxidant has been studied in MeCN. Two
2
major products, catechol and hydroquinone were identified by GC
Table 4
a
Effect of H2O2: Phenol molar ratio on % phenol conversion as a function of time
over [ CH2{Cu(sal-1,2-pn)} ]n.
Table 2
a
Effect of catalyst weight on phenol hydroxylation .
H2O2:PhOH
molar ratio
% PhOH
conv.
% H2O2
eff.
Product selectivity (%)
b
PhOH/catal.b
% PhOH conv.c (6 h)
−1
TOF (h )
Catal. wt (g)
Cat.
HQ
BQc
1.8
0
0
0
.005
.010
.025
3037
1708
683
29
48
51
172
142
60
0.5:1
1:1
19.1
48.2
53.7
33
41
24
65.9
63.1
59.6
34.1
36.9
38.6
2:1
a
Reaction conditions: phenol (0.05 mol); phenol/H2O2 molar ratio = 1; solvent,
◦
a
CH3CN (2 ml); catalyst [ CH2{Cu(sal-1,2-pn)} ]n; reaction temperature 80 C.
Reaction conditions: Phenol (0.05 mol); catalyst [ CH2{Cu(sal-1,2-pn)} ]n
b
◦
Molar ratio per repeating unit.
Phenol conversion (% mol).
(10 mg); solvent, CH3CN (2 ml); reaction time = 6 h; reaction temperature 80 C.
c
b
% H2O2 efficiency = (moles of H2O2 utilized in products formation/moles of H2O2
TOF h−1: Turn Over Frequency-moles of substrate converted per mole of metal (in
solid catalyst) per hour.
added) × 100, (based on GC analysis data).
c
p-Benzoquinone.

1
52
H.S. Abbo, S.J.J. Titinchi / Applied Catalysis A: General 435–436 (2012) 148–155
Fig. 2. Effect of phenol concentration on hydroxylation of phenol as a function of
time catalyzed by [ CH2{Cu(sal-1,2-pn)} ]n.
Fig. 3. Effect of volume of solvent (CH3CN) on the hydroxylation of phenol as a
function of time catalyzed by [ CH2{Cu(sal-1,2-pn)} ]n.
at a molar ratio of H O to phenol (2:1) was less than 2% due to the
2
2
it is clear from the data given in Table 5 that the percentage phenol
conversion was low, which ultimately retarded the reaction rate
since the reaction becomes viscosity controlled in nitrobenzene.
The viscosity was increased due to formation of polymeric mate-
rial. Similar observations were obtained with chloroform which is
more polar than acetonitrile. It is clear that the type of solvent
has a very significant role in controlling the overall % conversion
of phenol which is not exclusively dependant on solvent polarity
but hydrophilicity and solvent size may also explain the effect of
selected solvents.
further oxidation of hydroquinone. However, product selectivity is
similar in all cases. Taken collectively it is found that a 1:1 molar
ratio is the most efficient to obtain maximum % conversion with
reasonable H O efficiency and high TOF value.
2
2
3.2.1.4. Effect of the oxidant. The reaction was carried out
using oxidants other than H O , viz., (70% aqueous) tert-butyl-
2
2
hydroperoxide (TBHP) and sodium hypochlorite solution, NaOCl
(
available chlorine ≥4%). Phenol hydroxylation was by and large
absent (less than ca. 2%) using these oxidants and is possibly due to
the lack of generation of an active oxidant species and the solubility
problems associated with the reactant vis-à-vis the oxidant which
is in agreement with our results [25].
In terms of selectivity, using chloroform the reaction was found
to be more selective towards catechol (85.6%) than the other sol-
vents, but not as efficient as in acetonitrile.
3
.2.1.7. Effect of volume of solvent. In order to study the influence
3
.2.1.5. Effect of phenol concentration. The phenol concentration
of the volume of the reaction medium, the reaction was performed
in various volume of solvent (CH CN). From Fig. 3, it can be con-
was varied in the reaction mixture from 0.025 to 0.1 mol keep-
ing other parameters constant. It is clear from Fig. 2 that with
1
was obtained. When the phenol concentration to that of H O2 was
reduced to 1:0.5 molar ratio a maximum of 51% conversion was
3
cluded that the volume of acetonitrile used has a great role on the
% conversion. Using 2 ml of MeCN was found to be sufficient to per-
form the reaction to give best phenol conversion. Increasing the
amount of solvent to 4 ml and 8 ml, the catalytic reaction always
led to the poor performance. A decreased solvent amount (1 ml)
was found not sufficient to dissolve the substrate and formed a
non-homogeneous reaction mixture.
:1 H O :PhOH molar ratio, a maximum of 48% overall conversion
2 2
2
2
obtained. Increasing the amount of PhOH to twice of that of H O2
resulted in the overall conversion to be reduced to 21%.
3.2.1.6. Effect of type of solvent. Many factors can influence reac-
tion rates viz., polarity, solvation power, donor ability, electrophilic
behavior and molecule size of solvent. It is clear from Table 5 and
Fig. 5S that acetonitrile is the best solvent with 48.2% conversion
compared to other solvents used in this study. Although high cat-
alytic activity was expected in nitrobenzene due its high polarity,
3
.2.1.8. Effect of reaction time. The dependence of the reaction con-
version and selectivity on reaction time was studied for 24 h. In
general phenol conversion increases with longer reaction times and
attains a maximum conversion (39%) within 1.5 h (Fig. 4). The per-
centage selectivity for formation of catechol and hydroquinone also
varies with reaction time. Benzoquinone could not be detected at
any stage of the reaction. Selectivity for catechol formation in the
product is 100% at the beginning of the reaction. When reaction
time is prolonged beyond 3 h, phenol conversion increased slightly,
hydroquinone selectivity gradually increased while the catechol
selectivity decreased, exhibiting 32.0 and 76.8% at 6 h, respec-
tively. The generation of phenoxy radicals occurs on the catalyst
surface which leaves the activated ortho-sites lie close to the oxo-
complex and would be more amenable to attack by the generated
hydroxy radical. This could be the plausible reason that the ortho-
hydroxylation is the predominant pathway in this catalysis [10c].
After certain reaction time, the catalyst surface could be partially
covered by the products which would create partial hindrance for
the substrate in approaching the catalyst surface. Thus, there will
Table 5
a
Influence of various solvents on percentage phenol conversion , product selectivity,
TOF and H2O2 efficiency after 6 h.
TOF (h−1
Solvent
PhOH conv. (%)
H2O2 eff. (%)
)
Selectivity (%)
Cat
HQ
CH3CN
MEK
n-Heptane
PhNO2
CHCl3
48.2
36.5
30.4
21.9
15.9
41
31
26
19
14
142
108
90
65
47
63.1
68.4
70.7
72.3
85.6
36.9
31.6
29.3
27.7
14.4
a
Reaction conditions: Phenol (0.05 mol); phenol/H2O2 molar ratio = 1; catalyst
◦
[
CH2{Cu(sal-1,2-pn)} ]n (10 mg); solvent (2 ml); reaction time = 6 h; at 80 C;
MEK, Methylethylketone.

H.S. Abbo, S.J.J. Titinchi / Applied Catalysis A: General 435–436 (2012) 148–155
153
Table 6
Catalytic performance in phenol hydroxylation , percentage selectivity and turn over frequencies of over the catalysts.
a
TOF (h 1
−
)
Catalyst
% PhOH conv.
Product distribution (%)
Catb
HQc
Cat/HQ
[
[
CH2{Cu(sal-1,2-pn)} ]n
CH2{Ni(sal-1,2-pn)·2DMF} ]n
48.2
40.1
67.6
70.7
32.4
29.3
2.15
2.41
142
166
a
b
c
◦
Reaction conditions: Phenol (0.05 mol); phenol/H2O2 molar ratio = 1; catalyst (10 mg); solvent, CH3CN (2 ml); reaction time = 6 h; reaction temperature 80 C.
Cat = Catechol.
HQ = Hydroquinone.
−
Table 7
be sufficient time for the OH to then attack at the para-position.
Interestingly, recycled catalysts after regeneration exhibit almost
similar catalytic activity and selectivity towards the products. This
observation supports the above explanation for the variation of
product selectivity with time.
Comparison of Turnover Frequencies (TOF) between our catalysts and related sys-
tems for hydroxylation of phenol.
Catalyst
TOF (h−1)
Ref.
[
[
[
[
[
[
CH {Cu(sal-1,2-pn)} ]n
2
142
166
136.7
130.3
138.6
221.4
40
This work
This work
[24]
[24]
[24]
[24]
9a
9a
10e
10e
34b
[35]
9a
9a
[12]
[36]
The ratio of formation of catechol to hydroquinone reached to
maximum (∼2) after 5 h. Both the % conversion and % selectivity
were not affected with reaction time after acquiring a steady state.
On increasing the reaction time to 24 h, only minor increase in the
transformation of phenol as well as percentage selectivity for the
formation of catechol and hydroquinone were observed for both
types of catalysts.
CH2{Ni(sal-1,2-pn)·2DMF} ]n
CH2{Cu(salen)} ]n
CH2{Ni(salen)} ]n
CH2{Cu(salpn)} ]n
CH2{Ni(salpn)·2DMF} ]n
Cu(Salen)-Y
Cu(5-Cl-Salen)-Y
88
[
[
[
Cu(saldien)]-Y
Ni(saldien)]-Y
Ni(salpn)]-Y
25.0
24.6
28.7
74-8
21
31
5.5
25
CuPc-Y
3
.2.1.9. Comparison of catalysts performance. Table 6 and Fig. 4
Cu(Salen)
Cu(5-Cl-Salen)
TS-1
show the reaction profile for the performance of Cu and Ni-based
catalyst for phenol hydroxylation. It is clear that the Cu complex is a
better catalyst than the Ni complex as it does not show an induction
period and reaction proceeded steadily in a continuous manner.
With the former a maximum conversion of ∼40% is reached after
Meso TS-1
1
.5 h. When the reaction was prolonged for 6 h, the conversion
our catalysts show comparable activity with the salen polymeric
complexes and lower activity than 1,3-salpn polymeric complexes.
This can be attributed to the greater flexibility of the 1,3-salpn
ligand compared to the salen and 1,2-salpn ligands and also the
branching of the backbone in these catalyst may influence its per-
formance. However, the TOF values for our catalysts are similar
or better than that of the above mentioned catalyst analogues
(Table 7). Also the results show that Cu(II)- and Ni(II)-polymeric
complexes are better catalysts over the corresponding encapsu-
lated ones, the monomeric complexes and TS-1 (Table 7). The
neat complexes suffer from dimerization (formation of oxo dimer)
which hinders the substrate to reach the active centers of the cat-
alysts [37]. Although the zeolite encapsulated metal complexes do
increased to 48%. The latter gave only 4% conversion after 1.5 h
and reached a maximum conversion of 40% after 6 h. The dissim-
ilarities in the induction period for the catalysts can be explained
by either slow formation of active peroxo intermediate species or
taking a longer time to transfer oxygen to the substrate [1]. Once
the active species is generated, this is rapidly converted to prod-
ucts, but acquires a steady state at different times. Both catalysts
show effective conversion to catechol (67–71%) with a catechol to
hydroquinone ratio of 2:1 (Table 6)
On comparing the catalytic data found herein for the liquid
phase hydroxylation of phenol with our data reported earlier on
similar Cu(II)- and Ni(II)-salen and salpn coordination polymers,
Fig. 4. Catalytic performance in phenol hydroxylation, percentage selectivity and turn over frequencies over the catalysts.

1
54
H.S. Abbo, S.J.J. Titinchi / Applied Catalysis A: General 435–436 (2012) 148–155
catalysts [6,39–43]. Furthermore the selectivity of our catalysts
appears to be highly selective to produce phenol.
3.2.3. Stability and reusability of the catalysts
The stability and reusability of the catalysts were investigated.
At the end of the reaction, the catalyst was successfully recovered
by first diluting the reaction mixture with acetonitrile followed by
filtration. The catalyst was then regenerated by Soxhlet extraction
with acetonitrile to remove the organic materials remaining on the
◦
catalyst’s surface. The catalyst was dried at 120 C for 6 h. The regen-
erated catalysts were reused three times under the same reaction
conditions. After two successive cycles the activity of the catalysts
were found to be comparable with that for the fresh catalysts. Small
lowering in the activity (8–10%) occurs after the second cycle.
The experimental observations have indicated that during the
reaction there was no metal leaching into the reaction mixture. This
is also confirmed by isolating the catalyst from the reaction mixture
after 2 h reaction time and the reaction was continued for another
4
h. It was found that the % conversion remained unchanged with-
out any further increase which proved that the reaction is catalyzed
heterogeneously. Moreover, the ICP analyses for the reaction fil-
trate indicates the absence of metal ions which confirm indicate
that no leaching has occurred during the reaction.
◦
Fig. 5. Kinetic plot for benzene conversion at 70 C over Ni- and Cu-based catalysts.
Comparable IR spectral patterns of fresh and regenerated cata-
lyst indicate that the structure remains unchanged. This suggests
that these catalysts could be reused after recovery without signifi-
cant loss of activity.
not suffer from dimerization, however, fewer metal centres per unit
cell of zeolite are available that make their catalytic activity better
but still poorer than the polymeric complexes.
The coordination polymers currently being discussed possess
many active metal centres available in the chelating polymer pock-
ets this could play a positive role in enhancing the catalytic activity.
In addition to the thermal stability and insolubility in common
solvents could give them an advantage in heterogeneous catalysis.
4. Conclusion
Coordinated metal complexes of polymeric ligand [ CH (H sal-
2
2
1
,2-pn) ]n with Cu(II) and Ni(II) metal ions have been synthesized
and characterized. The catalytic activity for hydroxylation of phenol
using the Cu-based catalyst is higher than the Ni-based catalyst. On
the other hand, both catalysts show excellent selectivity towards
catechol (67–71%) under the optimum reaction conditions (phenol,
3
.2.2. Hydroxylation of benzene
Direct liquid phase hydroxylation of benzene using H O2 as
2
an oxidant catalyzed by the synthesized catalysts under the reac-
tion conditions (0.02 mol of benzene, 0.02 mol of H O and 0.025 g
of catalyst at 70 C) gave only one product, i.e. phenol. Traces of
2
2
0.05 mol; phenol/H O2 molar ratio = 1; catalyst (10 mg); solvent,
2
◦
◦
CH CN (2 ml) reaction temperature 80 C.
3
higher oxidation products (<2%) could be detected by using 2:1
These catalysts were also tested for direct hydroxylation of ben-
zene. Both catalysts show low activity but excellent selectivity
towards phenol formation of nearly 100% for the oxidation of ben-
(
H O :benzene) molar ratio with partial improvement in % benzene
2 2
conversion.
To evaluate the efficiency of the two catalysts viz., [ CH {Cu(sal-
zene under the optimum reaction conditions (0.02 mol benzene,
2
◦
1
,2-pn)} ]n and [CH {Ni(sal-1,2-pn)·2DMF} ]n, for the hydroxy-
0.02 mol of H O2 and 0.010 g of catalyst at 70 C).
2
2
lation of benzene using 30% H O as an oxidant, the percentage
The catalysts can be recovered and reused without significant
loss of activity. This study would be valuable for developing a new
environmentally benign route for oxidation of phenol and ben-
zene.
2
2
◦
of benzene conversion is plotted as a function of time at 70 C
as shown in Fig. 5. The results indicate that conversion of ben-
zene increased with reaction time for both catalysts and attained
a steady state after ∼3 h. In case of Ni-based catalyst, the benzene
conversion was initially low (1.5 h) but improved with time and
reached the steady state at ∼3 h, thereafter, only a very slow con-
version was observed up to 24 h which may be due to inefficient
product desorption from the catalyst surface rather than deacti-
vation of the catalyst [38]. The Cu-based catalyst shows better
performance (∼7%) than Ni-based catalyst (∼5%) after 6 h, under
the reaction conditions: benzene:H O (1:1 molar ratio), catalyst
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.apcata.2012.05.048.
2
2
References
◦
10 mg); solvent, CH CN (2 ml) at 70 C. The Cu complex does not
3
(
show an induction period which can be explained that once the
active species is generated; it is rapidly converted into products.
Both catalysts showed >99% selectivity for producing phenol with-
out any by-product being detected by GC. The selectivity towards
phenol formation remained unchanged over a period of 24 h.
Our search of the literature reveals that to date no study has
been done on the hydroxylation of benzene using polymeric chelat-
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other catalytic systems (homogenous and heterogeneous) for the
hydroxylation of benzene shows similar or lower activity to these
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