MCM-41-supported oxo-vanadium(IV) complex: A highly selective heterogeneous catalyst for the bromination of hydroxy aromatic compounds in water

Article abstract of DOI:10.1021/la202094p

An ecofriendly solid catalyst has been synthesized by anchoring vanadium(IV) into organically modified MCM-41. First, the surface of Si-MCM-41 was modified with 3-aminopropyl-triethoxysilane (3-APTES), the amine group of which upon condensation with ortho-hydroxy-acetophenone affords a N ₂O₂-type Schiff base moiety in the mesoporous matrix. The Schiff base moieties were used to anchor oxo-vanadium(IV) ions. The prepared catalyst has been characterized by UV-vis, IR spectroscopy, small-angle X-ray diffraction (SAX), nitrogen sorption, and transmission electron microscopy (TEM) studies. It is observed that the mesostructure has not been destroyed in the multistep synthesis procedure, as evidenced by SAX and TEM measurements. The catalyst has shown unprecedented high conversion as well as para selectivity toward the bromination of hydroxy aromatic compounds using aqueous 30% H ₂O₂/KBr in water. The reaction proceeds according to the stoichiometric ratio, and the monobrominated product was obtained as the major product using a stoichiometric amount of the bromine source. The immobilized complex does not leach or decompose during the catalytic reactions, showing practical advantages over the free metal complex.

Full text of DOI:10.1021/la202094p

ARTICLE
pubs.acs.org/Langmuir
MCM-41-Supported Oxo-vanadium(IV) Complex: A Highly Selective
Heterogeneous Catalyst for the Bromination of Hydroxy Aromatic
Compounds in Water
Susmita Bhunia, Debraj Saha, and Subratanath Koner*
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
S Supporting Information
b
ABSTRACT: An ecofriendly solid catalyst has been synthesized by
anchoring vanadium(IV) into organically modified MCM-41. First, the
surface of Si-MCM-41 was modified with 3-aminopropyl-triethoxysilane
(3-APTES), the amine group of which upon condensation with ortho-
hydroxy-acetophenone affords a N2O2-type Schiff base moiety in the
mesoporous matrix. The Schiff base moieties were used to anchor oxo-
vanadium(IV) ions. The prepared catalyst has been characterized by
UVꢀvis, IR spectroscopy, small-angle X-ray diffraction (SAX), nitrogen
sorption, and transmission electron microscopy (TEM) studies. It is
observed that the mesostructure has not been destroyed in the multistep
synthesis procedure, as evidenced by SAX and TEM measurements. The
catalyst has shown unprecedented high conversion as well as para
selectivity toward the bromination of hydroxy aromatic compounds
using aqueous 30% H2O2/KBr in water. The reaction proceeds according to the stoichiometric ratio, and the monobrominated
product was obtained as the major product using a stoichiometric amount of the bromine source. The immobilized complex does
not leach or decompose during the catalytic reactions, showing practical advantages over the free metal complex.
there has been an upsurge in the design of bromination protocols
based on bromide salt oxidation with hydrogen peroxide similar
to the haloperoxidase enzyme. For a metal halideꢀH2O2 system,
it requires a stoichiometric amount of mineral acids,14 and it is
believed that the bromination reaction proceeds via the forma-
tion of hypobromous acid, which is more unstable because of its
ionic nature and thus more reactive toward the aromatic nucleus.
Many vanadium complexes are employed in the oxidative
halogenation reaction15ꢀ18 in the homogeneous phase, but
there is one serious problem in the recovery of products from
the reaction mixture as they undergo decomposition during the
catalytic reactions. To avoid this problem, considerable attention
has been paid to the heterogeneous catalytic process because it
allows the production and ready separation of a large quantity of
the desired product with the use of a small amount of recyclable
catalyst. Different approaches such as encapsulation or the
immobilization of a catalytically active metal complex in a solid
support such as a zeolite19 and the covalent grafting of such an
active complex onto reactive polymer surfaces20 or inorganic
porous matrices21 have been used to develop the efficient
heterogeneous catalyst. Ordered mesoporous silica, for example,
MCM-4122 with a high surface area, high thermal stability, and
1. INTRODUCTION
Over the years, the peroxidative bromination reaction has
been an ongoing area of research for chemists because bromoaro-
matics are versatile intermediates in the manufacture of pharma-
ceuticals, agrochemicals, and other specialty chemical products.
Flame retardants, disinfectants, and antibacterial and antiviral
drugs also involve bromination.1 Bromophenols, in particular,
are important to the synthesis of flavor compounds.2,3 A number
of methods of bromination of aromatic compounds have been
reported in previous studies.4ꢀ11 The traditional procedure for
direct bromination is the use of elemental bromine, a pollutant
and health hazard for which half of the bromine ends up as
hydrogen bromide waste (eq 1).
ArH þ Br2 f ArBr þ HBr
ð1Þ
Therefore, for large-scale operations it creates an enormous
environmental and economical problem. The reoxidation of HBr
(e.g., with H2O2) can partially solve this problem because highly
toxic, corrosive HBr increases the reactor costs that exceed the
cost of purchasing more Br2. Again, the transportation and
storage of large quantities of molecular bromine or HBr are
extremely hazardous.
Marine enzyme haloperoxidase catalyzes the two-electron
oxidation of X (X = Cl, Br, or I) in the presence of aqueous
hydrogen peroxide under seawater conditions.12 However, a
large-scale enzymatic reaction is problematic.13 In view of this,
Received: June 4, 2011
Revised:
October 24, 2011
Published: October 25, 2011
r
2011 American Chemical Society
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Scheme 1. (a) Modification of the Si-MCM-41 Channel Wall with Aptes/Chloroform, (b) Condensation with ortho-Hydroxy-
Acetophenone in Methanol, and (c) Metal Complex Formation of VOSO4/WaterꢀMethanol
attractive pore structure, is the natural choice to use as the matrix.
Vanadium species could be functionalized into the mesoporous
matrix either by direct hydrothermal synthesis or by an impreg-
nation method.23ꢀ25
In this study, we report the simple, low cost, highly para
selective MCM-41 supported oxo-vanadium(IV) complex-cata-
lyzed bromination of hydroxyaromatic compounds in water at
room temperature. Herein bromides ions are used as halogenat-
ing agents, and aqueous 30% H2O2 is used as the oxidant; thus
this method may be considered to be a functional model for
enzyme vanadate-dependent haloperoxidase (V-HalPO)17 and
offers a green bromination option compared to the classical
electrophilic substitutions.
Postsynthesis organic modification of the mesoporous material was
performed by stirring 0.1 g of Si-MCM-41 with 0.18 g (0.81 mmol) of
3-APTES in 10 mL of dry chloroform for 12 h under a nitrogen
atmosphere. The resulting material, MCM-41-(SiCH2CH2CH2NH2)x,
was vacuum-filtered, washed with dry chloroform, and dried under
vacuum. The white solid was then refluxed with ortho-hydroxy-acet-
ophenone (8.18 mmol, 1 g) dissolved in 10 mL of methanol for 5 h at
60 °C. The resulting yellowish solid was filtered, washed with methanol,
and dried in a desiccator. Finally, catalyst V-MCM-41 was prepared by
refluxing the above-mentioned yellowish solid in 10 mL of methanol
with VOSO4 5H2O (0.405 mmol, 0.10 g) dissolved in 2 mL of water at
3
60 °C for 12 h. The resulting light-greenish solid was recovered by
vacuum filtration, washed with methanol using Soxhlet for 12 h to
remove any unreacted vanadyl species, and dried under vacuum
(Scheme 1). The atomic absorption spectrometric result showed the
vanadium content of the catalyst to be ca. 0.65 wt %. The elemental
analysis yields molar ratios of N/V ≈ 2.4 and for C/V ≈ 24.3,
representing the fact that oxo-vanadium(IV) ions are in a N2O3 ligand
environment as shown in Scheme 1.
2. EXPERIMENTAL SECTION
2.1. Materials. Cationic surfactant cetyltrimethylammonium
bromide (CTAB, 98%), tetraethyl ortho-silicate (TEOS, 98%), oxo-
2.3. Catalyst Characterization. Fourier transform infrared
(FTIR) spectra of KBr pellets were obtained on a Perkin-Elmer RX1
FTIR spectrometer. Electronic spectra were measured on a Shimadzu
CP-3101 UVꢀvis spectrophotometer. The vanadium content of the
sample was estimated on a Perkin-Elmer A-Analyst 200 atomic absorp-
tion spectrometer. Elemental analysis, for carbon, hydrogen, and nitro-
gen (CHN), was undertaken on a Perkin-Elmer 240C elemental
analyzer. The powder X-ray diffraction (XRD) patterns of the samples
were recorded with a Scintag XDS-2000 diffractometer using Cu Kα
radiation. A surface area measurement was carried out by nitrogen
adsorption at 77 K using a Quantachrome Autosorb iQ surface area
analyzer. Prior to sorption experiments, samples were outgassed at 80 °C
under vacuum until a final pressure of 10ꢀ2 Torr was reached.
Transmission electron microscopy (TEM) images were recorded with
a transmission electron microscope (FEI, model STWIN) operating at
an accelerating voltage of 200 kV. The samples were ground and
sonicated in isopropanol and dispersed over carbon-coated copper grids.
The products of the catalytic reactions were identified and quantified by
vanadium(IV) sulfate (VOSO4 5H2O), phenol red, salicylaldehyde,
3
ortho-hydroxy-acetophenone, phenol, para-chloro phenol, toluene, ben-
zaldehyde, and hydrogen peroxide (30% aqueous) were purchased
either from Sigma-Aldrich or Spectrochem (India) and were used as
received without further purification. The solvents were purchased from
Merck (India) and were distilled and dried before use.
2.2. Catalyst Preparation. Mesoporous Si-MCM-41 was synthe-
sized according to the reported procedure26 by the hydrolysis of
structure-directing agent CTAB (cetyltrimethylammonium bromide)
and TEOS as the silica source in basic solution with a reactant molar
composition of 1.0:7.5:1.8:500 CTAB/TEOS/NaOH/H2O. The gel
mixture was then hydrothermally held at 110 °C for 60 h in a Teflon-
lined autoclave. After being cooled to room temperature, the resultant
solid was recovered by filtration, washed with deionized water, and dried
in air. The collected product was calcined at 550 °C for 12 h to remove
the occluded polymeric surfactants. This mesoporous material is desig-
nated as Si-MCM-41.
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Figure 2. IR spectra of (a) Si-MCM-41, (b) MCM-41-(SiCH2CH2-
CH2NH2)x, and (c) V-MCM-41.
Figure 1. Small-angle XRD patterns of (a) Si-MCM-41, (b) MCM-
41-(SiCH2CH2CH2NH2)x, and (c) V-MCM-41.
phenyl-modified mesoporous sieves29 and by Lim et al. for
directly synthesized thiol-MCM-41.30 After postsynthesis graft-
ing, an overall decrease in the intensity of the diffraction lines was
noticed. This result could be attributed to the lowering of local
order, such as variations in the wall thickness, or might be due to
the reduction of scattering contrast between the channel wall of
the silicate framework and the vanadium complex present in
V-MCM-41, as previously mentioned by Lim et al.30 Marler et al.
have reported that the intensity of the diffraction lines decreases
systematically on increases in the concentration of organic
sorbates in boron-containing MCM-41.31 Therefore, the shift
in the diffraction lines to the higher angle and the decrease in the
intensity of the peaks upon organic modification of Si-MCM-41
as well as complex formation with vanadium(IV) are not
inconsistent.
using a Varian CP-3800 gas chromatograph equipped with an FID
detector. 1H NMR spectra were obtained on a Bruker Avance DPX 300
NMR spectrometer using TMS as the internal standard. Other instru-
ments used in this study were the same as reported earlier.27,28
2.4. General Procedure for the Bromination of Hydroxy
Aromatic Compounds. We have followed this general procedure for
all catalytic reactions. Catalytic reactions were carried out in a 50 mL
reaction flask. In a typical reaction, 5 mmol of the substrate was added to
5 mL of H2O containing 5 mmol of KBr. After the mixture was stirred for
1 min at room temperature, 30 mmol of 30% H2O2 was added dropwise
using a pressure-equalizing dropping funnel under stirring. Catalyst,
V-MCM-41 (25 mg) followed by 70% HClO4 (2 mmol) was added to
the above mixture when the reaction began. The progress of the reaction
was monitored by thin layer chromatography (TLC). After the disap-
pearance or no change in the starting material on TLC plates, the catalyst
was filtered and the reaction contents were subjected to multiple ether
extractions. The combined filtrates were washed with saturated sodium
bicarbonate solution. The organic extract was dried over anhydrous
sodium sulfate, and the solvent was evaporated under reduced pressure.
The crude product was then purified by crystallization or by column
chromatography over silica gel (60ꢀ120 mesh). The purities of the
3.2. Spectroscopic Measurements. The FTIR spectra of Si-
MCM-41, MCM-41-(SiCH2CH2CH2NH2)x, and catalyst V-MCM-
41 are shown in Figure 2. The stretching vibrational band for SiꢀOꢀ
Si of Si-MCM-41, MCM-41-(SiCH2CH2CH2NH2)x, and V-MCM-
41 samples appeared at around 1050 cmꢀ1, indicating that the silica
framework remain unaffected upon modification. The IR spectrum of
MCM-41-(SiCH2CH2CH2NH2)x exhibits two bands at 2919 and
1466 cmꢀ1 that are due to both the asymmetrical stretching vibration
of the CꢀH bond in the CH2 unit and in methylene and the CH2
bending vibration, respectively. The appearance of bands in the range
of 3200ꢀ3400 cmꢀ1 can be attributed to the NꢀH stretching
frequency of the primary amine. The appearance of these bands in the
IR spectra of MCM-41-(SiCH2CH2CH2NH2)x indicates the attach-
ment of amino-propyl groups to the solid surface that are absent in Si-
MCM-41. In catalyst V-MCM-41, the characteristic band of the
azomethine group of the vanadium(IV) complex moiety appears
at ca. 1616 cmꢀ1 in the IR spectra of V-MCM-41. The IR band of
the free azomethine group appears at 1643 cmꢀ1 whereas in
V-MCM-41 this band is shifted to a lower frequency, indicating
Schiff base formation between ortho-hydroxy-acetophenone and
APTES-modified MCM-41 as well as the coordination of azo-
methine nitrogen with vanadium(IV). The characteristic peak of
oxo-vanadium (VdO) that appears at 956 cmꢀ1 was obscured by
the strong SiꢀOꢀSi vibrational band of V-MCM-41.32
1
products were confirmed by H NMR spectra and quantified by gas
chromatography.
3. RESULTS AND DISCUSSION
3.1. XRD Studies. The small-angle X-ray diffraction patterns
of Si-MCM-41, MCM-41-(SiCH2CH2CH2NH2)x, and catalyst
V-MCM-41 are shown in Figure 1. Pristine Si-MCM-41 shows a
typical three-peak pattern,22a,b a very strong reflection at 2θ ≈
2.3° with d100 = 38.36 Å, and additional peaks with low intensities
at 2θ ≈ 4.1 and 4.75° for d110 and d200, respectively, for the quasi-
regular arrangement of mesopores with hexagonal symmetry. A
comparison of the X-ray powder diffraction patterns of Si-MCM-
41, MCM-41-(SiCH2CH2CH2NH2)x, and V-MCM-41 shows
that the typical three-peak pattern of MCM-41 has been retained
after organofunctionalization as well as metal complex formation
in Si-MCM-41. However, diffraction lines were shifted to higher
angles upon organic modification of MCM-41 and subsequently
vanadium incorporation into modified mesoporous silica MCM-41.
A similar type of behavior was observed by Burkett et al. in
The solid-state diffuse reflectance UVꢀvis spectrum of V-MCM-
41 is shown in Figure 3. The catalyst exhibits one broad band bet-
ween 301 and 352 nm that may be assigned either to a ligand-to-metal
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Figure 3. UVꢀvis spectra of the V-MCM-41 catalyst.
Figure 5. Transmission electron micrographs of (a) Si-MCM-41 and
(b) V-MCM-41.
at 234ꢀ286 and 201ꢀ233 nm can be attributed to intraligand
transitions. A relatively weak, broad band appeared in the range of
460ꢀ615 nm and had an absorption maximum at 515 nm that was
due to characteristic dꢀd transitions of (VO)2+ in the visible
region.34 The CT and dꢀd transitions confirm the presence of
anchored vanadium in the mesoporous matrix.
3.3. N2 Sorption Studies. The nitrogen sorption isotherms of
Si-MCM-41, MCM-41-(SiCH2CH2CH2NH2)x, and the V-MCM-
41 catalyst are shown in Figure 4. A gradual decrease in the BET
surface area, pore volume, and pore diameter could be observed at
each stage of modification as expected because the organic frag-
ments or metal complexes entered the channels. However, all of the
materials had type IV isotherms (according to the IUPAC
nomenclature) with hysteresis loops, indicating that the mesopor-
ous structure remained. The nitrogen sorption study exhibits that
the BET surface area of Si-MCM-41 is 1178 m2 gꢀ1 and the
mesopore volume is 0.67 cm3 gꢀ1. The average pore diameter is
calculated to be 33.1 Å using the NLDFT method. All calculated
values are in agreement with those reported for good-quality
mesoporous silica. MCM-41-(SiCH2CH2CH2NH2)x shows smal-
ler N2 uptake (BET surface area 835 m2gꢀ1), pore volume
(0.49 cm3gꢀ1), and pore diameter (29.4 Å) whereas the
V-MCM-41 catalyst shows even smaller N2 uptake (BET surface
Figure 4. (a) N2 adsorption/desorption isotherms of (a) Si-MCM-41,
(b) MCM-41-(SiCH2CH2CH2NH2)x, and (c) V-MCM-41. Adsorption
points are marked by filled circles, and desorption points are marked by
open circles. (b) Pore size distributions of (a) Si-MCM-41, (b) MCM-
41-(SiCH2CH2CH2NH2)x and (c) V-MCM-41.
charge-transfer band or to a Π f Π* transition originating
mainly in the azomethine chromophore.33 Other bands appearing
area 634 m2gꢀ1), and pore volume (0.31 cm3 ꢀ1) and
g
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exhausted. The reaction mixture contains 10 mL of a 0.5 M
KBr solution, 10 mL of 30% H2O2, 10 mL of 0.1 mM phenol red,
and 20 mL of distilled water. The redox activity was tested by
adding 50 mg of the V-MCM-41 catalyst to it and by monitoring
the spectral changes at regular intervals. From the spectral data
(shown in Figure 6), it is evident that as the reaction proceeds,
the peak at about λmax ≈ 432 nm corresponds to the phenol red
decreases whereas the peak at about λmax ≈ 592 nm corresponds
to the bromophenol blue increase, thus indicating the progress of
the reaction. After ∼1 h, no further changes in the absorption
peaks were observed, confirming the completion of the reaction.
3.6. Catalytic Activity Studies. Oxo-vanadium compounds
are able to mimic the vanadium-dependent haloperoxidase
enzyme by catalyzing the bromination of organic substrates in
the presence of H2O2 and bromide.37 During oxidation, vana-
dium coordinates with 1 or 2 equiv of H2O2, forming the
monoperoxo {VO(O2)+} or bis(peroxo) {VO(O2)2ꢀ} species
that oxidize bromide, possibly via a hydroperoxo intermediate.
Oxidized bromine species Br2, Br3ꢀ, or most likely HOBr
ultimately brominates the organic substrates. In this investiga-
tion, the catalytic efficacy of V-MCM-41 in the bromination
reaction was studied using KBr and H2O2 as the bromide source
and oxidizing agent, respectively. The catalyst shows high
selectivity toward the monobromination of hydroxy aromatic
compounds. It is noteworthy that the catalyst is highly para
selective. However, in the substrate in which the para position is
blocked, bromination takes place at the ortho position (entry 4,
Table 1). To achieve optimum reaction conditions, oxidative
bromination reactions were studied by varying parameters such
as the nature of the solvent and the volumes of acid and H2O2 at
room temperature. It was observed that for a fixed amount of
substrate (5 mmol), a catalyst (25 mg), KBr (5 mmol), and 30%
H2O2 (30 mmol) in 5 mL of H2O in the presence of HClO4 (2
mmol) are found to be the best conditions under which to
achieve 100% conversion. The addition of HClO4 at a time may
cause the slow decomposition of the catalyst. To stop the
decomposition of the catalyst, HClO4 was successively added
to the reaction mixture. The catalytic results are shown in Table 1.
The bromination of salicylaldehyde undergoes 100% conversion
with a 99% monobrominated product, 5-bromo salicylaldehyde,
in a short time under heterogeneous conditions. In the recent
past, the bromination of salicylaldehyde over vanadium-based
catalysts under heterogeneous conditions has been studied (Table 2).
Maurya et al. studied the oxidative bromination of salicylaldehyde
mediated by zeolite-Y-encapsulated dioxo-vanadium(V) complexes,
which shows a maximum of 39% conversion with 34% 5-bromo-
salicylaldehyde.38 When oxo-vanadium(IV)-based coordination poly-
mers having a bridging methylene group are employed in the
oxidative bromination of salicylaldehyde, the conversion in-
creases to 92% but the selectivity of 5-bromosalicylaldehyde
remains low (28%) and 3,5-dibromosalicylaldehyde was formed
as a major product (65%).39 Chloromethylated polystyrene-
supported oxo-vanadium(IV) complexes have also been used
as catalysts in the oxidative bromination of salicylaldehyde, which
gives a maximum 73% conversion with ∼81% product (5-
bromosalicylaldehyde) selectivity, and a dibrominated product
was also formed in this case.40 Recently, Maurya et al. have
studied this reaction over polymer-anchored dioxo-vanadium(V)
complexes, which shows a maximum of ∼85% conversion with
∼90% product (5-brompsalicylaldehyde) selectivity.41
Figure 6. Oxidative bromination of phenol red catalyzed by V-MCM-41.
pore diameter (27.2 Å). As the pores become bulkier, hysteresis
occurs at lower P/P0 with a concomitant decrease in pore size.23b
This indicates that the immobilization of the oxo-vanadium(IV)
complex has taken place substantially inside the pore channels of Si-
MCM-41. The average density of the attached APTES moiety in
MCM-41-(SiCH2CH2CH2NH2)x was ∼1.45 molecules/nm2.
3.4. TEM Studies. Figure 5A,B shows TEM micrographs of Si-
MCM-41 and the V-MCM-41 catalyst. Both Si-MCM-41 and
V-MCM-41 feature an open-ended lamellar-type arrangement of
hexagonal porous tubules. When the electron beam falls on the
catalyst perpendicular to the pore axis, the pores are seen to be
arranged in patches composed of regular rows, as has been
interpreted by Chenite et al.35 TEM micrograph of Si-MCM-41
viewed along the pore axis reveals a hexagonal array having a
channel dimension of ∼3.9 nm, which is consistent with the
XRD results. V-MCM-41 also exhibits a similar type of array of
regular rows in TEM micrographs. Therefore, it can be con-
cluded that both Si-MCM-41 and V-MCM-41 have a similar
type of external morphology. In our case, the morphology of
the tubules in Si-MCM-41 and in V-MCM-41 is rectilinear,
and the tubules are 1200ꢀ1700 Å long. The TEM image
provides strong evidence that the long-range ordering of the
support framework is retained even after the immobilization
of the oxo-vanadium(IV) Schiff base complex in the mesopor-
ous silica matrix.
3.5. Bromination Activity Test for Catalyst V-MCM-41. To
test the activity of the V-MCM-41 catalyst, the peroxidative
bromination of phenolsulfonaphthalein (phenol red) to tetra-
bromo-phenolsulfonaphthalein (bromophenol blue) was used.
This is a facile method and easy to monitor by a UVꢀvis
spectroscopic technique.36 λmax values of pure phenol red and
bromophenol blue were first determined from the UVꢀvis
spectra of the substrates. Phenol red traps the active bromine
species without influencing the rate of reaction until it is
To extend the scope of this reaction, similar reaction condi-
tions were applied to other aromatic compounds. Phenol and
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Table 1. Bromination of Hydroxyaromatic Compounds Using V-MCM-41 as a Catalysta
a Reaction conditions: Substrate, 5 mmol; KBr, 5 mmol; solvent, 5 mL; catalyst, 25 mg; H2O2, 30 mmol; and HClO4, 2 mmol. b Acetonitrile (1 mL) was
added to 4 mL of H2O to dissolve the substrate. c Conversion of the reactant is determined by GC. d Isolated yields were calculated from the mass of the
product after separation by column chromatography. All of the isolated products showed more than 99% GC purity. e TOF (turnover frequency) =
moles converted/(moles of active site ꢁ time).
Table 2. Heterogeneous Oxybromination of Salicylaldehyde Catalyzed by Other Vanadium-Based Catalysts
catalyst
conversion (wt %)
% selectivity of 5-bromo salicylaldehyde
TOFe
ref
NH4[VO2(sal-inh)]ꢀYa
39.3
92.0
34.0
28.0
81.36
90.4
99.0
52.0
33.8
38
39
40
41
[ꢀCH2{VO(sal-1,3-pn)}ꢀ]nb
polymer-supported-[VO(fsal-ohyba) DMF]c
polymer-supported K[VO2(sal-inh)(im)]d
73.0
100.0
775.0
448.0
3
85.2
V-MCM-41
100.0
this study
a H2 sal-inh is N-isonicotinamidosalicylaldimine. b Oxovanadium(IV) complex of the polymeric Schiff base derived from 5,5-methylenebis-
(salicylaldehyde) [CH2(Hsal)2] and 1,3-diaminopropane (1,3-pn). c H2fsal-ohyba is the Schiff base derived from 3-formylsalicylic acid and o-
hydroxybenzylamine. d H2 sal-inh is the Schiff base derived from salicylaldehyde and isonicotinolhydrazide. e TOF (turnover frequency) = moles
converted/(moles of active site ꢁ time).
para-chloro-phenol give monobrominated products with high
yields in a short time. The bromination of ortho-hydroxy-
acetophenone proceeds efficiently and furnishes the respec-
tive monobrominated product using a stoichiometric amount of
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Table 3. Effect of Solvents on the Oxidative Bromonation of ortho-Hydroxy-acetophenone Catalyzed by V-MCM-41a
a Reaction conditions: Substrate, 5 mmol; KBr, 5 mmol; solvent, 5 mL; catalyst, 25 mg; H2O2, 30 mmol; and HClO4, 2 mmol. b Conversion of the
reactant is determined by GC. c Isolated yields were calculated from the mass of the product after separation by column chromatography. All of the
isolated products showed more than 99% GC purity. d Without the addition of HClO4.
determine if vanadium was leaching out of the catalyst, the
reaction mixture was filtered out after the reaction and was
subjected to atomic absorption spectroscopic analysis. The
analysis showed the absence of vanadium in the filtrate.
Besides that, the filtrate mixture did not show any catalytic
activity toward the bromination reaction. These results in-
dicate that the catalyst is stable and heterogeneous in nature,
which has advantages over the homogeneous counterpart.
Figure 7. Chart showing the recyclability of the V-MCM-41 catalyst.
4. CONCLUSIONS
We have successfully immobilized the vanadium(IV)
Schiff base complex into the Si-MCM-41 matrix via covalent
bonds. The mesoporosity of the material is retained after the
immobilization of the vanadium complex. The catalyst
efficiently and selectively catalyzes the bromination of hy-
droxy aromatic compounds using KBr as the bromide source
and 30% H2O2 as the oxidizing agent in water at room
temperature. This methodology of bromination offers sim-
ple reaction conditions, commercial availability of the re-
agents, excellent product yield, no evolution of hydrogen
bromide, and an environmentally more benign alternative in
comparison to the hazardous classical bromination proto-
cols, thus affording an opportunity for easy access to a variety
of bromo-organics.
potassium bromide as the bromine source. We have also examined
thebehavior of benzaldehyde and toluene toward the oxidative
bromination reaction under similar reaction conditions using
V-MCM-41 as the catalyst, but they remained completely
inactive. Therefore, it can be concluded that the V-MCM-41
catalyst is highly selective toward hydroxyaromatic com-
pounds. Recently, Sharma et al. have studied the oxidative
bromination reaction of various aromatic compounds using
Cu2+-perfluorophthalocyanine-immobilized silica gel as the
catalyst. Although the yield is good, the reactions occur only in
an acetic acid medium at 60 °C.42 At variance with the above-
mentioned catalytic systems, V-MCM-41 showed the best
catalytic activity at room temperature and, even more im-
portantly, in an ecofriendly water medium.
Solvent plays an important role in the catalytic bromination
reaction. It was observed that among different solvents the
maximum conversion takes place in a water medium. The
effect of different solvents on the oxidative bromination of
ortho-hydroxy-acetophenone is shown in Table 3.
The catalyst can be easily recovered after the reaction by
simple filtration. After recovery, the catalyst was thoroughly
washed with ether and then methanol and dried at room
temperature. The recovered catalyst showed almost the same
catalytic activity in successive runs shown in Figure 7. To
’ ASSOCIATED CONTENT
Supporting Information. 1H NMR spectra of the com-
S
b
pounds used in this study. This material is available free of charge
via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: snkoner@chemistry.jdvu.ac.in.
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ARTICLE
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