Anchoring of Fe(III)salicylamide onto MCM-41 for catalytic hydroxylation of phenol in aqueous medium using hydrogen peroxide as oxidant

Article abstract of DOI:10.1007/s10562-010-0319-y

Fe(III)salicylamide immobilized on MCM-41 has been developed for hydroxylation of phenol in water medium using hydrogen peroxide as the oxidant. In the first step, the amine group containing 3-aminopropyltrimethoxy silane has been immobilized on the surface of MCM-41 via co-condensation. The amine group upon condensed with methyl salicylate afford a bidentate ligand in the mesoporous matrix for anchoring Fe(III) ions in the second step. Small angle XRD, N₂ adsorption-desorption isotherms, ¹³C and ²⁹Si cross-polarization magic angle spinning NMR spectroscopy, Fourier transform infrared spectroscopy (FT-IR) and diffuse reflectance UV-vis spectra were employed for the characterization of the catalysts. The heterogeneous iron salicylamide complex shows excellent catalytic activity in phenol hydroxylation using hydrogen peroxide as the oxidant and water as the solvent. Notably, phenol shows high conversion (64.6%) with very high selectivity for dihydroxybenzene compounds. The heterogeneous catalyst shows a successive slight decrease in catalytic activity when reused for three more times. Graphical Abstract: Fe(III)salicylamide-MCM-41 has been synthesized successfully through an easy co-condensation route. XRD and N₂ adsorption-desorption showed mesoporosity is retained even after complexation and FT-IR and diffuse reflectance UV-vis spectra confirmed the formation of complex. The heterogeneous catalyst exhibit excellent catalytic performance and stability in phenol hydroxylation reaction for the formation of dihydroxybenzene derivatives using H₂O₂.

Full text of DOI:10.1007/s10562-010-0319-y

Catal Lett (2010) 136:155–163
DOI 10.1007/s10562-010-0319-y
Anchoring of Fe(III)salicylamide onto MCM-41 for Catalytic
Hydroxylation of Phenol in Aqueous Medium Using Hydrogen
Peroxide as Oxidant
•
K. M. Parida Sudarshan Singha P. C. Sahoo
•
Received: 10 January 2010 / Accepted: 24 February 2010 / Published online: 10 March 2010
Ó Springer Science+Business Media, LLC 2010
Abstract Fe(III)salicylamide immobilized on MCM-41
has been developed for hydroxylation of phenol in water
medium using hydrogen peroxide as the oxidant. In the first
step, the amine group containing 3-aminopropyltrimethoxy
silane has been immobilized on the surface of MCM-41 via
co-condensation. The amine group upon condensed with
methyl salicylate afford a bidentate ligand in the meso-
porous matrix for anchoring Fe(III) ions in the second step.
Small angle XRD, N2 adsorption–desorption isotherms,
13C and 29Si cross-polarization magic angle spinning NMR
spectroscopy, Fourier transform infrared spectroscopy
(FT-IR) and diffuse reflectance UV–vis spectra were
employed for the characterization of the catalysts. The
heterogeneous iron salicylamide complex shows excellent
catalytic activity in phenol hydroxylation using hydrogen
peroxide as the oxidant and water as the solvent. Notably,
phenol shows high conversion (64.6%) with very high
selectivity for dihydroxybenzene compounds. The hetero-
geneous catalyst shows a successive slight decrease in
catalytic activity when reused for three more times.
need for finding clean oxidants for oxidation of organic
compounds. Hydrogen peroxide is used as a clean oxidant
because the oxidation affords only water as byproduct.
Catalytic hydroxylation of phenol to catechol and hydro-
quinone is an important industrial reaction [1]. It is most
desirable that the dihydroxybenzenes could be produced by
direct hydroxylation of phenol with environmentally
benign oxidants like hydrogen peroxide. Mineral acids,
simple metal ions and metal complexes are traditional
catalyst used for this reaction. Homogeneous processes for
production of dihydroxybenzenes are the Rhone-Poulenc
process [2], where the oxidation of phenol is carried out by
strong mineral acids or the Hamilton process [3] where
Fenton reagent is used as a catalyst. But these homoge-
neous catalysts are difficult to separate and recover from
the reaction mixture, which prevents their practical utili-
zation in phenol hydroxylation. The heterogenizations of
homogeneous catalyst have gained considerable interest
over the last few decades, because one of the most
attractive advantages is the easy separation of the products
from the catalyst without tedious experimental work-up
[4–6]. The first commercial application of TS-1 for phenol
hydroxylation was by Enichem in 1986 in Italy. An
increasing impetus of studies on various metalosilicates
such as TS-2, Ti-b, TAPO-5, TAPO-11, Ti-ZSM, VS-2 and
Cu-AIPO4-5 has been shown by many publications [7, 8].
Although some of these transition metal framework
substituted zeolites have exhibited attractive catalytic
conversion, selectivity and stability in phenol hydroxyl-
ation, their complicated synthesis, high cost, and low
reaction rate limit their commercial application.
Keywords Mesoporous silica ꢀ Salicylamide complex ꢀ
Immobilization ꢀ Phenol hydroxylation
1 Introduction
Environment benign technology is one of the major goals
of the present research in chemistry. There is an urgent
K. M. Parida (&) ꢀ S. Singha ꢀ P. C. Sahoo
Colloids and Materials Chemistry Department, Institute of
Minerals & Materials Technology, Bhubaneswar 751013,
Orissa, India
On the other hand, metal-modified mesoporous silica
(MCM-41, SBA-15, MCM-48) and complex immobilized
mesoporous silica have become target catalysts aimed at
accelerating the reaction rate of phenol hydroxylation
e-mail: paridakulamani@yahoo.com
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156
K. M. Parida et al.
[9, 10]. Mesoporous silica, especially MCM-41 is a good
choice because of its high surface area, high thermal and
chemical stability and easy preparation method. The silica
based hybrid materials, which offer pore sizes in the range
(CDH), FeCl3ꢀ6H2O (Merck), phenol (Aldrich), catechol,
hydroquinone, 1, 4-benzoquinone and acetic acid (Merck)
were used without further purification.
˚
20–100 A are suitable for liquid phase reactions, because
2.2 Synthesis Procedures
they allow easy diffusion of reactant to the active sites
[11, 12]. Their surface silanol groups can also be func-
tionalized by different organoalkoxy groups, hence allow-
ing subsequent attachment of the catalytic active species.
Wu and coworker [13] reported that immobilization of
[Fe(II)phen]2? onto MCM-41 induces enhancement of
hydroxylation activity by about two times (27.5–53.1%).
Here in, we report the satisfactory heterogenization of
Fe(III) onto organically modified MCM-41 by a two-step
process. The prepared catalyst is robust enough to achieve
high catalytic performance for liquid phase hydroxylation
of phenol using hydrogen peroxide as oxidant and water as
solvent. Moreover, the anchored catalyst can be recycled
three times without significant loss in catalytic activity.
The effect of reaction temperature, time, solvents and
phenol to oxidant ratio on conversion and product selec-
tivity were also examined for the optimization of the pro-
cess parameters.
2.2.1 Preparation of Hybrid Mesoporous Material:
Salicylamide Functionalized MCM-41 (I)
MCM-41 was prepared by a standard procedure [14]. Organic
amine modified mesoporous silica (Scheme 1) was prepared
according to literature [15] using C16H33N(CH3)3 Br (CTAB)
as template, tetraethylorthosilicate (TEOS) as silica precursor
and 3-aminopropyltrimethoxysilane (3-APTMS) as organ-
oalkoxysilane precursor. The mixture contained CTAB/
TEOS/APTMS/NaOH/H2O in the molar ratio 1.0:8.16:
1.05:2.55:4857. The mixture of CTAB (2.0 g, 5.49 mmol),
2.0 M of NaOH (aq) (7.0 mL, 14.0 mmol) and H2O (480 g,
26.67 mol) was heated at 80 °C for 30 min to reach pH 12.3.
To this clear solution, TEOS (9.34 g, 44.8 mmol) and APTMS
(1.03 g, 5.75 mmol) were added sequentially and rapidly via
injection. Following the injection, a white precipitation was
observed after 3 min of stirring. The reaction temperature was
maintained at 80 °C for 2 h. The product was isolated by hot
filtration, washed with copious amount of water and ethanol
and dried under vacuum. To this dried product, an acid
extraction process was performed in a methanol (100 mL)
mixture of concentrated hydrochloric acid (1.0 mL) at 60 °C
for 6 h. with continuous stirring. The resulting surfactant free
product was filtered and washed with water and ethanol and
then dried under vacuum. The solid was then refluxed with
methyl salicylate (5.75 mmol) in ethanol for 3 h at 60 °C. The
2 Experimental Methods
2.1 Materials
(3-Aminopropyl) trimethoxysilane (APTMS), tetraethyl
orthosilicate Si(OC2H4)4 (TEOS), cetyl trimethyl-
ammoniumbromide (CTAB) (Aldrich), methylsalicylate
Scheme 1 Preparation of
heterogeneous catalyst
Fe(III)salicylamide–MCM-41
(II). Conditions: (a)
Condensation in ethanol. (b)
FeCl3ꢀ6H2O in ethanol
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Anchoring of Fe(III)salicylamide onto MCM-41 for Catalytic Hydroxylation of Phenol
157
resulting product was dried in a desiccator and denoted as
MCM-41–salicylamide.
of the samples were examined by diffuse reflectance
UV–vis spectroscopy. The spectra were recorded on a
Varian-100 spectrophotometer in the wavelength range of
200–800 nm in BaSO4 phase. The Fe loading and leaching
of the reaction solution were determined by atomic
absorption spectroscopy (AAS) with a Perkin-Elmer
Analysis 300 using acetylene (C2H2) flame.
2.2.2 Preparation of Fe(III)salicylamide–MCM-41 (II)
The metal complex (II) (Scheme 1) was prepared by taking
FeCl3ꢀ6H2O (0.045 g, 0.17 mmol) in 10 mL of ethanol
along with 0.1 g of the hybrid organoalkoxy silane func-
tionalised MCM-41 and stirring the suspension for 12 h at
room temperature. Atomic absorption spectrometric anal-
yses showed an iron content of the sample is ca. 0.37%.
2.4 Catalytic Reaction
Catalytic tests of the prepared heterogeneous catalysts
(Scheme 2) were carried out in a 100 mL two necked
round bottom flask fitted with a reflux condenser. About
0.47 g (5 mmol) of phenol was dissolved in 10 mL of
deionized water and 0.05 g of the catalyst was added to it.
To this mixture 0.16 mL (5 mmol) of aqueous H2O2 (30%)
was added through the septum using syringe pump over
10 min at the beginning of the reaction. Reaction was
carried out for 2 h at 80 °C. The reaction products were
analyzed by off-line gas chromatography (Shimadzu GC-
17A) equipped with a capillary column (ZB-1, 30 m
length, 0.53 mm I.D. and 3.0 lm film thickness) using
flame ionization detector (FID). The self-decomposition of
H2O2 was measured by the amount of O2 released under
reaction conditions in controlled experiments. The con-
version and selectivity were calculated as follows.
2.2.3 Preparation of Fe(III)salicylamide
An equimolar amount of APTMS (5.75 mmol) in dichlo-
romethane (10 mL) was added to a stirred solution of
methyl salicylate (5.75 mmol) in dichloromethane
(10 mL). The resulting solution was stirred for 4 h at room
temperature. Removal of solvent under vacuum yields a
bright white solid. To this FeCl3ꢀ6H2O (0.17 mmol) was
added in 20 mL of ethanol and stirred for 24 h. The
resulting solid was filtered, washed with copious amounts
of ethanol and dried under vacuum. From the AAS analysis
the iron content was found to be 0.43%.
2.3 Physico-Chemical Characterizations
Moles of the phenol reacted
Conversion ð%Þ ¼
Selectivity ð%Þ ¼
ꢁ 100
ꢁ 100
Powder X-ray diffraction (XRD) patterns of the samples
were obtained on Rigaku D Max III VC diffractometer
with Cu Ka radiation at 40 kV and 40 mA in the range of
2h = 0–10°. The scanning rate was 2° min-1. Nitrogen
adsorption–desorption isotherms were measured at liquid
nitrogen temperature using ASAP 2020 (Micromeritics).
Samples were out gassed at 100 °C for 3 h to evacuate the
physically adsorbed moisture before the measurement.
Specific surface area was calculated using the BET method.
Solid-state 13C and 29Si cross-polarization magic-angle
spinning NMR spectra were recorded at 100.58 and
79.46 MHz, respectively using a Bruker Avance 400 MHz
spectrometer. The FTIR spectra of the samples were
recorded using Varian 800-FTIR in KBr matrix in the
range of 4000–400 cm-1. The co-ordination environments
Total moles of the phenol
Moles of the product formed
Moles of the phenol reacted
3 Results and Discussion
3.1 Characterizations
3.1.1 XRD Studies
The small angle X-ray diffraction patterns of MCM-41,
MCM-41–salicylamide and Fe(III)salicylamide–MCM-41
are shown in Fig. 1. In the low angle region (2h &
Scheme 2 Reaction pathway
for phenol hydroxylation
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158
K. M. Parida et al.
d110 and d200 reflections correspond well to the hexagonal
arranged pore structure of MCM-41. But XRD pattern of
Fe(III)salicylamide–MCM-41 shows slightly broadening of
(110) and (200) reflections compared to parent MCM-41
but are well resolved, indicates that immobilization of the
salicylamide complex has little effect on the long range
ordering of the parent support. The broadening of the
reflections may be mainly due to contrast matching
between the silicate framework and organic moieties,
which are located inside the framework of MCM-41 [16,
17]. All the peaks are well resolved, indicative of the good
quality of the material.
3.1.2 N2 Sorption Studies
N2 adsorption–desorption isotherms of MCM-41 and
Fe(III)salicylamide–MCM-41 are given in Fig. 2. These
samples exhibit type IV isotherms (defined by IUPAC)
with small hysteresis, which is characteristics for meso-
porous materials. A sharp increase in the adsorption of N2
between the relative pressure of 0.3 and 0.4 can be assigned
to the capillary condensation [18]. The nitrogen sorption
study showed that the BET surface area of MCM-41 is
Fig. 1 XRD spectra of (a) Fe(III)salicylamide–MCM-41, (b) MCM-
41–salicylamide, (c) MCM-41
1.5–5°), three characteristic Bragg’s reflections were
observed in all these materials. The diffraction lines can be
indexed by assuming a hexagonal symmetry of these
materials. The pronounced d100 reflection and less intense
Fig. 2 N2 adsorption–
desorption isotherms of
a MCM-41,
b Fe(III)salicylamide–MCM-41
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Anchoring of Fe(III)salicylamide onto MCM-41 for Catalytic Hydroxylation of Phenol
159
1350 m2 g-1 and the mesopore volume is 1.26 cm3 g-1
˚
.
The average pore diameter is calculated to be 21.9 A using
the BJH method. All calculated values are in agreement
with those reported for good quality mesoporous silica.
Fe(III)salicylamide–MCM-41 shows less N2 uptake (BET
surface area 880 m2 g-1) and pore volume 0.78 cm3 g-1. It
was reported that organic groups incorporated in MCM-41
are mainly located on internal surfaces close to the pore
windows. The decrease in pore volumes and size is also
possible proof that metal complex are successfully intro-
duced into the framework [19].
3.1.3 13C CP-MAS NMR Studies
The 13C CP-MAS NMR spectrum of MCM-41–salicyl-
amide is shown in Fig. 3a. The sharp peak at 10.4 ppm is
ascribed to the carbon atom bonded to silicon. The signal at
21.7 ppm corresponds to methylene carbon and the peak at
43 ppm can be attributed to the carbon atom attached to the
nitrogen atom. The existence of a peak at 164.7 ppm is due
to the carbon atom of the amide group. Resonance in the
range 120–148.6 ppm is evidence for the presence of aro-
matic ring.
3.1.4 29Si CP-MAS NMR Studies
Figure 3b shows the 29Si CP-MAS NMR spectra of MCM-
41–salicylamide. Three resonance peaks due to the Si
environments of Q4 [siloxane, (SiO)4Si] (d = -110 ppm),
Q3 [single silanol(SiO)3Si(OH)] (d = -102 ppm), and Q2
[geminal silanol, (SiO)2Si(OH)2] (d = -92 ppm) can be
seen. In addition to these three peaks, the sample displays
two more resonance peaks at d = -66 ppm, assigned to T3
[C–Si(OSi)3], and at -57 ppm, attributed to T2 [C–Si(O-
Si)2(OH)]. The existence of T3 confirms the modification of
MCM-41 by organic moieties [20, 21]. The appearance of
the Q3 signal indicates the presence of some residual
noncondensed OH groups attached to the silicon atom. The
29Si CP-MAS NMR provides direct evidence that the
hybrid MCM-41–salicylamide sample consists of a highly
condensed siloxane network with organic fragments
covalently bonded to the silica frame work. Figure 3c
shows the CP-MAS 29Si NMR spectral deconvolution plot
for T type silicon species of MCM-41–salicylamide. The
relative integral intensities of T type silicon species allow
the quantitative assessment of the degree of incorporation
of the organic moieties.
Fig. 3 13C CP-MAS NMR spectrum of a MCM-41–salicylamide,
29Si CP-MAS NMR spectrum of b MCM-41–salicylamide, c spectral
deconvolution curve of T typed silicon species of MCM-41–
salicylamide
vibrations of Si–O–Si bond in parent MCM-41 as well as in
all modified samples appear around 1050 cm-1 indicate
that silica framework was not affected after modifica-
tion, which is in accordance with the XRD results. A
characteristic peak around 3400 cm-1 (stretching) for
[Fe(III)salicylamide–MCM-41] indicates some free Si–OH
groups along with some unreacted phenolic –OH groups
are still present after formation of the complex. The peak at
3100 cm-1 is assigned to N–H stretching vibration and
3.1.5 IR Studies
Figure 4 displays the IR spectra of MCM-41(4(a)), MCM-
41–salicylamide (4(b)) and Fe(III)salicylamide–MCM-
41(4(c)) in the range 400–4000 cm-1. The stretching
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160
K. M. Parida et al.
Fe(III)salicylamide–MCM-41 and Fe(III)salicylamide but
with slightly less intense, which could be attributed to
attachment of metal to nitrogen and oxygen. The broad
band around 580 nm in Fe(III)salicylamide–MCM-41 may
be assigned to the d–d transition in metal as reported in the
literature [23–26].
3.2 Phenol Hydroxylation
3.2.1 Catalytic Activity
The results on catalytic activities for hydroxylation of
phenol over various catalysts are listed in Table 1.
Hydroxylation over pure MCM-41 is negligible indicating
that the host is inactive even in the presence of H2O2.
Fe(III)salicylamide–MCM-41 showed a very high activity
and resulted in a phenol conversion 64.6%. This shows that
the complex immobilized MCM-41 has an enormous effect
on the catalytic performance. The coordination environ-
ment around iron in Fe(III)salicylamide–MCM-41 is less
crowded in comparison to the bulkier metal-porphyrins or
metal-phthalocyanines anchored on MCM-41 matrix [27].
Probably, hydrogen peroxide has enough space to bind iron
in the intermediate stage of the catalytic reaction and
thereby facilitating the hydroxylation reaction. We have
also carried out the reaction using the corresponding
homogeneous analogue Fe(III)salicylamide under identical
reaction conditions, but conversion is only 32%. This may
be due to the formation of catalytically inactive l-oxo-
bridge dimer by the homogeneous complex which has been
speculated as the cause of catalytic deactivation. Among
other heterogeneous catalysts, titanium silicates (TS-1)
which is a titanium containing zeolite gives only 36%
conversion, when the reaction was carried out at 70 °C
taking acetonitrile as solvent [8]. J. S. Choi and co worker
reported phenol hydroxylation by using Fe-MCM-41 as
catalyst, which showed 60% conversion [28]. In compari-
son to the above two cases our catalyst is found to be more
efficient in terms of conversion using water as the solvent.
Further, it is worth mentioning here that the selectivity of
thermodynamically more stable product, i.e. catechol is
high compared to hydroquinone [29]. Ray et al. have
reported immobilization of salicyladimine complex onto
amorphous silica and MCM-41 for evaluation of their
efficacies towards hydroxylation of phenols. They found
MCM-41 supported catalyst showed a higher phenol con-
version than the amorphous silica, which is due to higher
surface area of MCM-41 [30].
Fig. 4 FT-IR spectra of (a) MCM-41, (b) MCM-41–salicylamide, (c)
Fe(III)salicylamide–MCM-41
2970 cm-1 is due to asymmetric stretching vibration of
–CH2 units. But in the case of metal complex immobilized
MCM-41 all peaks are slightly shifted to a lower frequency
due to coordination with iron. Sharp absorption peak
around 1600 cm-1 may be attributed to the presence of
amide group [22].
3.1.6 UV–Vis Spectral Studies
The immobilization of Fe(III) complex on MCM-41 is
further confirmed by diffuse reflectance UV–vis spectra.
The spectra of Fe(III)salicylamide (a), Fe(III)salicylamide–
MCM-41 (b) and MCM-41–salicylamide (c) are shown in
Fig. 5. A series of characteristic bands at 220, 260 and
380 nm appear due to intra ligand electronic transition of
the phenyl ring and lone pair present in nitrogen of MCM-
41–salicylamide. A similar type of results are also found in
The mechanism of phenol hydroxylation is known to
proceed via a redox mechanism involving a Fe(III)/Fe(II)
redox pair which is illustrated in Scheme 3. The electro-
philic attack of the hydroxyl radical is proposed to be the
reaction pathway for dihydroxybenzene over this catalyst.
Fig. 5 UV–vis spectra of (a) Fe(III)salicylamide, (b) Fe(III)salicyl-
amide–MCM-41, (c) MCM-41–salicylamide
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Anchoring of Fe(III)salicylamide onto MCM-41 for Catalytic Hydroxylation of Phenol
161
Table 1 Hydroxylation of phenol by H2O2 under the same reaction conditionsa
Samples
Phenol
conv. (%)
Product selectivity (%)
CAT/HQ
(mol/mol)
Hydrogen peroxide
TOFe
CAT
HQ
BQ
SDb (%)
Conv.c (%)
SEd (%)
MCM-41
–
–
–
–
–
Fe(III)salicylamide–MCM-41
Fe(III)salicylamide
64.6
32
67.4
63.4
30.2
31
2.4
5.6
2.23
2.04
37
63
64
68
68
33
494
149
CAT catachol, HQ hydroquinone, BQ 1,4-benzoquinone
a
Reaction temperature 80 °C, reaction time 2 h, phenol/H2O2 (molar ratio) = 1, amount of catalyst 0.05 g, water as solvent
Self-decomposition (SD) of H2O2 = volume of oxygen released in the reaction
b
c
d
e
Conversion of H2O2 = consumption of H2O2 (including self-decomposition)/initial amount of H2O2
Selectivity (SE) of H2O2 = H2O2 consumption for product formed/total consumption of H2O2
TOF = Moles of product formed per mole of Fe in the catalyst per hour
Scheme 3 Mechanistic
pathway for phenol
hydroxylation
3.2.2 Effect of Time
As OH radical is an electrophile, which behaves different
from that OH-. So OH radical tends to have an attack
preferably at ortho and para position of the phenol mole-
cule to produce catechol and hydroquinone. The self-
decomposition of H2O2 is obvious in the reaction system;
this may hinder the effective utilization of H2O2 (Table 1),
thus the formation of dihydroxybenzene derivatives. Gen-
erally, the catalytic activity and product selectivity in
phenol hydroxylation in presence of hydrogen peroxide are
highly influenced by the reaction time, temperature, types
of solvents, phenol to hydrogen peroxide ratio, which are
investigated systematically and described as follows.
Time dependence of activity in phenol hydroxylation is
summarized in Table 2. The phenol conversion and
selectivity for hydroxylation products are enhanced with
increase in time and the reaction acquires a steady state
after 2 h. It is also found that 1, 4-benzoquinone dominates
the reaction products at early stage of the reaction. But the
selectivity for 1, 4-benzoquinone decreases whereas those
for catechol and hydroquinone increases with increase in
reaction time. Similar types of results were found on TS-1
[8] and metal complex oxides [31]. It may presumably due
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162
K. M. Parida et al.
Table 2 Effect of reaction time on phenol hydroxylation
Table 4 Effect of solvents on phenol hydroxylation
Reaction medium Phenol conversion (%) Product selectivity (%)
Reaction
time (h)
Phenol
conversion (%)
Product selectivity (%)
CAT
HQ
BQ
CAT
HQ
BQ
0.5
1
7
53.0
53.3
62.3
67.4
20.7
25.8
26.4
30.2
26.3
20.9
11.3
2.4
Water
64.6
29.6
8.6
67.4
43.3
46.4
56.4
30.2
39.9
29.7
30.1
2.4
28
Acetonirtile
Ethanol
16.8
23.9
13.5
1.5
2
46
64.6
Dichloromethane
3.8
Reaction temperature 80 °C, phenol/H2O2 (molar ratio) = 1, amount
of catalyst 0.05 g, water as solvent
Reaction temperature 80 °C, reaction time 2 h, phenol/H2O2 (molar
ratio) = 1, amount of catalyst 0.05 g
to the fast oxidation of phenol in the reaction medium by a
large concentration of H2O2, which could cause the for-
mation of 1, 4-benzoquinone in relatively large amount.
The subsequent disappearance of 1, 4-benzoquinone can be
explained due to its deeper oxidation by H2O2 with the
formation of hydroquinone [32].
significant increase in phenol conversion. This may be
attributed that, both phenol and H2O2 dissolve easily in
water to form active hydroxyl radicals. The generation of
radical is more stable in polar solvents and polarity order is
water [ acetonitrile [ alcohol [ dichloromethane. Etha-
nol is a well-known scavenger for hydroxyl radical; hence
negligible conversion takes place [33].
3.2.3 Effect of Temperature
3.2.5 Effect of the Molar Ratio of Phenol to H2O2
Effect of temperature on phenol hydroxylation is depicted
in Table 3. It is observed that phenol conversion increases
as the temperature increases from 40 to 80 °C. At 40 °C,
phenol conversion is only 22.4%. But the 1, 4-benzoqoui-
none selectivity is comparatively high (10.8%). Conversion
goes on increasing up to 80 °C, above which a sudden
minimal yield of dihydroxybenzene products was obtained,
which is due to enhanced H2O2 decomposition. Similar
observation was reported earlier for Fe containing catalyst
[28].
The molar ratio of phenol to H2O2 has significant effect on
phenol hydroxylation activity. Reactions were performed
varying the amount of H2O2 to a fixed amount of phenol in
1:1 to 1:0.33 (phenol:H2O2) ratio. At the standard reaction
conditions (in 1:1 ratio) 64.6% conversion was obtained.
As the H2O2 amount reduced to 1/3 (phenol:H2O2 ratio
1:0.33), conversion decrease to 37%.
3.2.6 Catalytic Stability and Reusability
3.2.4 Effect of Solvent
Fe(III)salicylamide–MCM-41 catalyst was recovered from
reaction mixture by filtration after each reaction and
washed thoroughly with ethanol and dichloromethane. It
was reused successively three times under the same reac-
tion conditions. The recovered catalyst is found to exhibit
almost the same catalytic activity (slightly less in second
and third run) for phenol hydroxylation (shown in Table 5).
To test if iron is leaching out of the catalyst, the hot
reaction mixture was filtered after the reaction is over and
Solvent plays an important role in phenol hydroxylation. It
has a profound effect on phenol conversion, hydrogen
peroxide decomposition and the ratio of catechol to
hydroquinone formation. We have performed the reaction
in water, acetonitrile, ethanol and dichloromethane. The
conversion and product selectivity are illustrated in
Table 4. A change from organic solvent to water leads to a
Table 3 Effect of temperature on phenol hydroxylation
Temperature (°C)
Conversion
of phenol (%)
Product distribution (%)
CAT/HQ
(mol/mol)
CAT
HQ
BQ
40
50
60
70
80
22.4
36.9
50.3
61.3
64.6
62.3
65.7
65.8
67.3
67.4
26.9
27.1
29.5
29.5
30.2
10.8
7.2
4.7
3.2
2.4
2.31
2.42
2.23
2.28
2.23
Reaction time 2 h, phenol/H2O2 (molar ratio) = 1, amount of catalyst 0.05 g, water as solvent
123

Anchoring of Fe(III)salicylamide onto MCM-41 for Catalytic Hydroxylation of Phenol
163
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Table 5 Catalytic reusability test for hydroxylation of phenol
Run
Phenol
conversion (%)
Selectivity (%)
wt% of iron
from AAS
CAT
HQ
BQ
1
2
3
64.3
62.5
61.8
67.3
67.2
67.3
30.2
29.9
30.0
2.5
2.9
2.7
0.36
0.35
0.33
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Reaction temperature 80 °C, reaction time 2 h, phenol/H2O2 (molar
ratio) = 1, amount of catalyst 0.05 g, water as solvent
the filtrate was subjected to atomic absorption spectro-
scopic analysis. The analysis showed iron was absent in the
filtrate. Besides, filtrate mixture also did not show any
catalytic activity towards phenol hydroxylation reaction.
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and reusable.
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Fe(III)salicylamide complex was successfully anchored to
a mesoporous silica matrix via a covalent bond. XRD and
N2 adsorption–desorption confirm that the mesoporosity is
retained even after complexation. The 13C CP MAS NMR
spectra show all possible peaks corresponding to both the
aliphatic and aromatic carbon atoms. Solid-state 29Si CP
MAS NMR shows that the organic group is successfully
bonded to mesoporous silica. FT-IR and diffuse reflectance
UV–vis spectra confirm formation of the complex. The
catalysts exhibit excellent catalytic performance and sta-
bility in phenol hydroxylation reaction for the formation of
dihydroxybenzene derivatives using H2O2. One of the most
interesting thing for water as solvent is that it is safe, cheap,
environmental friendly and meets the needs of sustainable
development in chemical industries.
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Acknowledgements The authors are extremely thankful to Prof. B.
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his constant encouragement and permission to publish the paper. The
authors are also thankful to Department of Science and Technology,
New Delhi for financial support.
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