Regiospecific oxyhalogenation of aromatics over SBA-15-supported nanoparticle group IV-VI metal oxides

Article abstract of DOI:10.1007/s10562-010-0350-z

TiO_x, VO_x, MoO_x and WO_x supported on SBA-15 exhibit efficient catalytic activity for oxyhalogenation of aromatics with the H₂O₂-halide ion system. Unlike the hitherto known solid catalysts, these reusable catalysts yield the para-halogenated product with 100% selectivity at 298 K and moderate acidic pH (3-5). The catalytic activity was enhanced by five orders of magnitude when supported on SBA-15. Springer Science+Business Media, LLC 2010.

Full text of DOI:10.1007/s10562-010-0350-z

Catal Lett (2010) 137:190–201
DOI 10.1007/s10562-010-0350-z
Regiospecific Oxyhalogenation of Aromatics Over SBA-15-
Supported Nanoparticle Group IV–VI Metal Oxides
•
•
•
L. Saikia M. Rajesh D. Srinivas
P. Ratnasamy
Received: 18 February 2010 / Accepted: 10 April 2010 / Published online: 4 May 2010
Ó Springer Science+Business Media, LLC 2010
Abstract TiOx, VOx, MoOx and WOx supported on SBA-
15 exhibit efficient catalytic activity for oxyhalogenation of
aromatics with the H2O2-halide ion system. Unlike the
hitherto known solid catalysts, these reusable catalysts
yield the para-halogenated product with 100% selectivity at
298 K and moderate acidic pH (3–5). The catalytic activity
was enhanced by five orders of magnitude when supported
on SBA-15.
they lack adequate process productivity [8, 9]. In contrast to
molecular bromine, stoichiometric brominating reagents
such as N-bromosuccinimide (NBS), N-bromoacetamide
(NBA) and bromodimethylsulfonium bromide do not pro-
duce HBr in bromination of organic molecules, but are
expensive and generate organic waste [10]. In this context,
oxyhalogenation, a bio-mimetic approach [11, 12], is safer
and greener since it avoids the hazardous X2 by replacing it
with a halide salt in the presence of an oxidizing agent under
acidic conditions (Eq. 1).
Keywords Oxyhalogenation of aromatics ꢀ
Haloperoxidase activity ꢀ Supported metal oxides ꢀ
SBA-15 ꢀ Ordered mesoporous silica
H2O2 þ Xꢁ þ RꢁH þ Hþ ! RꢁX þ 2H2O
ð1Þ
In the past, Raja and Ratnasamy reported [13] the appli-
cation of zeolite-encapsulated copper phthalocyanines as
chloroperoxidase mimics. There was an enhancement in
catalytic activity for substrate conversion when the copper
complexes were encapsulated in the cavities of zeolites X,
Y and L. There was, however, no regioselectivity among
the products. In the oxidation of anisole and toluene, for
example, both ortho (predominant) and para-halogenated
products were formed. Significant quantities of di- and tri-
halogenated products were also detected. Sels et al. [14,
15] had reported the application of transition metal ion
exchanged layered double hydroxides in oxyhalogenations.
We had reported that tungsten oxide supported on SBA-15
is highly efficient for oxyhalogenation of a range of aro-
matic compounds [16]. Aromatic compounds with electron
releasing groups (ca., aniline, anisole, phenol etc.) are
found to be the most reactive. Complete conversion of
those substrates to the corresponding halogenated (iodo,
bromo and chloro) compounds was achieved in just 4 h at
room temperature and moderate acidic conditions using
the H2O2-halide ion as the halogenating system Interest-
ingly, the para-isomer formed with 100% regioselectivity.
We now report here a more detailed study of the
1 Introduction
Halogen-containing compounds constitute an important
segment of fine and specialty chemicals, dyes, flame-retar-
dants, pharmaceuticals and agrochemicals [1–3]. Haloge-
nated arenes are extensively used as precursors in the
preparation of various bioactive molecules and pharma-
ceuticals [4, 5] and play a vital role in metal-catalyzed
coupling reactions [6]. Classical halogenations use molec-
ular halogen, despite the fact that it is a pollutant and a safety
and health hazard [7]. Perbromide (Br-3 )-exchanged poly-
mers are safer to handle and are commercially available.
Nevertheless, their preparation, still, involves direct contact
with Br2 and, like most supported stoichiometric reagents
L. Saikia ꢀ M. Rajesh ꢀ D. Srinivas (&) ꢀ P. Ratnasamy (&)
Catalysis Division, National Chemical Laboratory,
Pune 411 008, India
e-mail: d.srinivas@ncl.res.in
P. Ratnasamy
e-mail: p.ratnasamy@ncl.res.in
123

Regiospecific Oxyhalogenation of Aromatics
191
application of group IV–VI metal oxides (TiOx, VOx,
MoOx and WOx) supported on SBA-15 for this reaction.
Regiospecific oxybromination of phenol red is investigated
spectrophotometrically. The nature and dispersion of metal
oxide on catalytic activity is investigated. Some insight
into the mechanism of oxybromination is provided using
in situ UV-visible spectroscopic studies.
materials. For a 11 wt% (input) of V2O5 catalyst, 635 mg of
butylammonium decavanadate was added to a suspension of
1 g of propyl amine functionalized SBA-15 in 40 mL of
water. The contents were stirred for 12 h, filtered, washed
with water, and dried in air, yielding an orange powder,
which was then calcined at 823 K for 12 h.
2.1.2 TiOx-SBA-15
2 Experimental
In a typical synthesis, tetrabutyl orthotitanate (0.05 g) was
taken in 15 mL of glycerol and hydrolyzed with tetrapro-
pylammonium hydroxide (20 wt% TPAOH) while stirring
for 4 h. To that, calcined SBA-15 (200 mg) was added and
stirred at 373 K for another 72 h. The contents were fil-
tered under vacuum and the solid obtained was dried at
393 K and calcined at 773 K for 6 h.
2.1 Material Synthesis
SBA-15 was prepared using tetraethyl orthosilicate (TEOS,
Aldrich Co.) as a silica source, poly(ethylene glycol)-
block-poly(propylene glycol)-block-poly(ethylene glycol)
(EO20PO70EO20; average molecular weight = 5800;
Aldrich Co.) as a template and HCl as a pH controlling
agent [17].
2.1.3 MoOx-SBA-15
SBA-15 (2 g) was activated at 473 K prior to use and then
dispersed in 50 mL of aqueous solution of ammonium
heptamolybdate so that the molybdenum oxide loading was
20 wt%. The mixture was refluxed for 12 h, evaporated to
dryness and then calcined at 823 K to get a faint blue
colored supported molybdenum oxide catalyst.
2.1.1 VOx-SBA-15
This material was prepared in three steps. In the first-step,
propyl amine functionalized SBA-15 was prepared as
reported earlier [18]. Then, in the second-step, butylam-
monium decavanadate was prepared: 5 g of V2O5 and 2.42 g
of butyl amine were added to 10 mL of water taken in a
250 mL round-bottom flask equipped with a reflux con-
denser. The reaction mixture was heated until the solid was
completely dissolved. The pH of the solution was adjusted to
5 by a drop-wise addition of conc. HClO4. The deep orange
solution formed was refluxed overnight. The mixture was
filtered and kept for evaporation. Brilliant, orange colored
butylammonium decavanadate formed in several hours. The
powder was highly soluble in water and recrystallized from
acetone/water (10:1 v/v) mixture. In the third-step, SBA-15
supported vanadia was prepared using amine functionalized
SBA-15 and butylammonium decavanadate as starting
2.1.4 WOx-SBA-15
Tungsten oxide supported on SBA-15 was prepared in a
similar manner as above using activated SBA-15 (2 g) and
ammonium tungstate solution (50 mL). Ammonium tung-
state solutions were prepared in such a way that tungsten
oxide loading was 5, 10, 15 and 20 wt% of the total
catalyst.
The catalysts, thus, prepared were designated as
MOx(n)-SBA-15 (Table 1), where M represents Ti, V, Mo
or W and n stands for weight percent of metal oxide
loading.
Table 1 Structural and textural properties of SBA-15-supported metal oxide catalysts
Catalyst
XRD
N2 physisorption
Wall thickness
(nm)
EDAX—Si/metal
at.% (wt%)
d100
(nm)
Unit cell
parameter (nm)
Pore diameter
(nm)
SBET
(m2 g-1
Total pore
volume (cc g-1
)
)
WOx(5)-SBA-15
WOx(10-SBA-15
WOx(15)-SBA-15
WOx(20)-SBA-15
MoOx(20)-SBA-15
TiOx(6)-SBA-15
VOx(11)-SBA-15
SBA-15
11.0
11.0
11.1
11.0
11.3
11.4
11.7
9.5
12.7
12.7
12.8
12.7
13.1
13.2
13.5
11.0
7.2
7.2
7.1
6.5
6.6
6.5
6.0
6.5
432
419
391
322
452
452
349
692
0.77
0.76
0.70
0.68
0.75
1.08
0.52
1.13
5.5
5.5
5.7
6.2
6.5
6.7
7.5
4.5
47.2 (7.2)
23.1 (3.5)
18.2 (2.8)
10.1 (1.5)
10.9 (3.2)
11.2 (6.2)
8.8 (4.8)
–
123

192
L. Saikia et al.
2.2 Characterization Techniques
yields bromophenol blue (tetrabromophenolsulfonephthalein).
The conversion of phenol red to bromophenol blue is a
common test to monitor bromination activity under peroxi-
dative conditions. The widespread use of this dye stems from
its ability to undergo rapid and stoichiometric bromination
and from the ease with which this can be monitored using
UV-vis spectroscopy [14, 15].
X-ray diffractograms (XRD) were recorded on an X’Pert
Pro (Philips) diffractometer using Cu–Ka radiation and a
proportional counter as detector. A divergent slit of 1/32°
on the primary optics and an anti-scatter slit of 1/16° on
the secondary optics were used to measure the data in the
low-angle region. The XRD patterns were recorded in the
2h range of 0.5–108 at a scan rate of 18 min-1. Mea-
surements in higher 2h range of 10–808 (for detection of
crystalline metal oxides) were performed on a Rigaku
Geigerflex X-ray diffractometer with Ni-filtered Cu–Ka
radiation (40 kV, 30 mA). Crystallite size of the materials
was determined using the Scherrer equation: L = Kk/
bcosh, where h and k have their usual meanings; K is a
constant approximately taken as 0.9, and b is line width
on 2h scale in radians. Unit cell parameter for SBA-
15 was determined using the equation: a = d(111)
H(h2 ? k2 ? l2). Here d is inter-planar distance obtained
from the Bragg’s equation while considering the value of
n as unity. Morphological characteristics of samples were
determined using a high resolution transmission electron
microscope (HRTEM, JEOL, model 1200 EX operating at
100 kV).
Oxybromination of phenol red to bromo-phenol blue
was performed over nanoscopic tungsten oxide, molyb-
denum oxide, titanium oxide and vanadium oxide sup-
ported on SBA-15. In a typical experiment, 20 mL of
0.05 M phenol red solution (H2O:CH3OH = 4:1) was
taken in a double-necked round bottom flask. To it,
2 mmol of KBr and 2 mmol of H2O2 (30% aq.) were
added. After addition of 0.025 g of the catalyst, the pH of
the reaction mixture was adjusted to *5 by adding drop-
wise the buffer—HEPES (4-(2-hydroxyethyl)-1-piper-
azineethanesulfonic acid). The progress of the reaction
(298 K) was monitored by taking out small aliquots of the
samples at equal interval (15 min), separating the catalyst
by centrifugation and then by recording the UV-vis
spectrum which showed
a decrease in absorbance
at kmax = 430–450 nm corresponding to phenol red with
a concomitant increase in absorbance at kmax = 590–
600 nm due to bromophenol blue. The reaction was also
carried out with different loadings of tungsten oxide under
similar conditions. Effect of temperature on the conver-
sion of phenol red was studied over WOx(20)-SBA-15
catalyst.
The specific surface area (BET) of the samples was
determined using a NOVA 1200 Quanta Chrome instru-
ment. The data points of p/po in the range of about 0.05–
0.3 were used in the calculations. The micropore volume
was estimated from the t-plot and the pore diameter was
estimated using the Barret–Joyner–Halenda (BJH) model.
FTIR spectra (KBr pellets) were recorded on a Shima-
dzu 8201 PC spectrophotometer in the region 400–
4000 cm-1. Diffuse reflectance UV-visible (DRUV-vis)
measurements for powder samples were conducted on a
Shimadzu UV-2550 spectrophotometer equipped with an
integrating sphere attachment (ISR 2200). Spectral grade
BaSO4 was used as a reference material. FT-Raman spectra
were recorded on a Jobin-Yvon LabRam HR Laser Raman
spectrometer. Green light (514.5 nm) was used for exci-
tation. X-ray photoelectron spectra (XPS) of calcined
powder samples were acquired on a VG Microtech Mul-
tilab ESCA 3000 with Mg Ka radiation (ht = 1253.6 eV).
Base pressure in the analysis chamber was maintained at
3–6 9 10-10 mbar. Binding energies (BE) were calibrated
with respect to Au 4f7/2 core level (83.9 eV). The error in
BE values is 0.1 eV.
3 Results and Discussion
3.1 Structure and Spectroscopic Characterization
of Catalysts
3.1.1 XRD
The SBA-15 materials showed XRD peaks in the 2h range
of 18–2.3° attributable to 2D hexagonal p6 mm symmetry
[16, 18–24]. The well-resolved (110) and (200) reflections
revealed that incorporation of metal oxides did not alter the
long-range mesoporous ordering of the host SBA-15.
WOx(n)-SBA-15 showed distinct, additional peaks in the
2h range of 208–70°, typical of nanocrystalline WO3.
Intensity of these additional peaks increased with increas-
ing WOx loading (5–20 wt%). Similar peaks due to crys-
talline metal oxides were not detected in the case of TiOx,
2.3 Reaction Procedure: Oxybromination of Phenol
Red
VOx and MoOx supported on SBA-15. The d spacing (d100
)
Phenol red (phenolsulfonephthalein) is a conjugated system,
which shows a visible band at 430–450 nm in aqueous
solution. Bromination of phenol red at mild acidic conditions
and unit cell parameters, estimated from the position of the
low-angle (100) peak (Table 1) agree well with those
reported by others [18–24].
123

Regiospecific Oxyhalogenation of Aromatics
193
3.1.2 N2-Physisorption
3.1.3 HRTEM
All the materials exhibited type IV nitrogen adsorption–
desorption isotherms with a H1-type, broad, hysterisis loop
corresponding to a large pore mesoporous material with
one dimensional cylindrical channels. Capillary conden-
sation of nitrogen within mesopores occurred causing a
sudden step increase in nitrogen uptake in the characteristic
relative pressure (P/P0) range of 0.6–0.8, suggesting typical
mesoporous structure with uniform pore diameters [25].
The amount of nitrogen adsorbed decreased for SBA-15-
supported metal oxides compared to ‘‘bare’’ SBA-15. The
High resolution transmission electron micrographs
(HRTEM) of WOx(20)- and MoOx(20)-SBA-15 revealed
the highly ordered hexagonal arrays of mesopores charac-
teristic of the SBA-15 support (Fig. 1). No separate, bulk
metal oxide phase was detected. This observation reconfirms
the conclusions drawn from N2-physisorption measure-
ments that all the metal content is inside the pores of SBA-15
and not at the external surface. HRTEM images confirm the
conclusions drawn from XRD measurements that the mes-
oporosity of SBA-15 is intact even after modification with
metal oxides. The pore diameter calculated for WOx(20)-
SBA-15 from TEM matches well with that determined from
the nitrogen physisorption measurements.
decrease in BET surface area (from 692 to 322 m2 g-1
)
and pore volume (from 1.13 to 0.52 cc g-1) of the modi-
fied SBA-15 materials compared to ‘‘bare’’ SBA-15
(Table 1) is a clear indication of the location of the metal
oxides inside the pores of SBA-15. The increase in pore
diameter upon deposition of metal oxides is possibly due to
that a small part of the metal substitutes Si4? in the SBA-
15 lattice. It may be noted that the ionic radius of the metal
chosen is bigger than that of silicon and hence causes an
increase in the pore diameter of the lattice.
3.1.4 FTIR
SBA-15 showed FTIR peaks at 2900–3800, 1040–1260,
820 and 500 cm-1 due to O–H of silanols, adsorbed water
molecules and Si–O–Si stretching vibrations, respectively
[17]. SBA-15-supported metal oxides showed additional
Fig. 1 TEM images of
a MoOx(20)-SBA-15 and
b WOx(20)-SBA-15
123

194
L. Saikia et al.
peaks in the spectral range of 800–964 cm-1 which could
be attributed to M–O–Si and M–O–M vibrations [25, 26].
Intensity of these additional peaks increased with increase
in WOx loading indicating that they originate from the
supported metal oxide species.
pattern and the absence of 150 cm-1 band indicated that
titania is in a highly dispersed state in TiOx(6)-SBA-15.
Isolated vanadia centers in tetrahedral coordination
show Raman bands at 1042 cm-1. Cluster-type VOx spe-
cies show bands at 994, 697, 286 and 147 cm-1. Undis-
torted VO6 octahedra exhibit a strong and sharp band at
870–890 cm-1. The V2O5 phase exhibits typical spectral
bands at 994, 701, 526, 284, 144 cm-1 [28–30]. Absence
of these bands in VOx(11)-SBA-15 revealed a different
local environment for vanadium in VOx(11)-SBA-15.
Raman bands associated with V=O usually appear at 900–
1000 cm-1. This band for VOx(20)-SBA-15 appeared at
3.1.5 DRUV-vis
Diffuse reflectance UV-visible (DRUV-vis) spectroscopy
can differentiate the dispersed meal oxides from the bulk-
phase metal oxides. Bulk titania, vanadia, molybdenum
oxide and tungsten oxide show the O2 ? Mn? charge
-
transfer band above 380 nm [26]. In the case of dispersed
metal oxides this band shifts to higher energy side (blue
shift) [26]. In fact, TiOx(6)-SBA-15 and MoOx(20)-SBA-
15 showed an intense asymmetric band below 300 nm
indicative of isolated metal oxide species (Fig. 2).
VOx(11)-SBA-15 and WOx(20)-SBA-15 showed additional
bands at 385–400 nm due to bulk-like metal oxide species.
Intensity of this additional band increased with the metal
oxide content in WOx(n)-SBA-15. A blue shift of the
absorption edge in all these materials with respect to the
bulk-phase indicates formation of a spatially confined,
dispersed, nanoparticulate metal oxide species [26].
1034 cm-1
.
Intense, well-resolved Raman shifts were observed in
the case of MoOx(20)- and WOx(20)-SBA-15 (Fig. 3).
While the band at 995 cm-1 in MoOx(20)-SBA-15 corre-
sponds to the Mo=O group of two dimensional polymo-
lybdates (octahedral MoO6 species) that at 816 cm-1
corresponds to MoO5 type species [27]. The intense sharp
bands at 805, 717, 273, 137 cm-1 for WOx(20)-SBA-15
reveal the presence of octahedral MoO6 units as in the
MoO3 phase [27]. The Raman spectra, therefore, reveal
that the supported metal oxides are in a dispersed state and
the extent of dispersion differs in different SBA-15-sup-
ported metal oxides.
3.1.6 Laser Raman Spectroscopy
3.1.7 XPS
Raman spectroscopy of metal oxides is very sensitive to the
coordination environment of the metal [27]. Anatase
The binding energy peak corresponding to W 4f7/2 in
WOx(20)-SBA-15 appeared at 36.5 eV. This does not
match exactly the value for neat WO3 (35.8 eV), or WO2
(32.8 eV) [31]. The difference in binding energy found in
the present study from that with neat WO3 and WO2 can be
explained by the presence of SiO2 in the neighborhood of
the tungsten centers. The XPS study also reveals that all the
tungsten oxide is dispersed and located inside the pores of
SBA-15 and not present as a separate phase bulk oxide
outside or at the external surface of silica. The peaks
exhibits intense bands at 150, 395, 515 and 640 cm-1
.
TiOx(6)-SBA-15 shows a very weak Raman spectrum. No
trace of the 150 cm-1 band (a sensitive indicator of ana-
tase) was detected for TiOx(6)-SBA-15. On the other hand,
the sample showed bands at 955 and 1100 cm-1, corre-
sponding to a symmetric and antisymmetric Ti–O–Si
stretching vibratioins, respectively. The diffused spectral
MoOx(20)-SBA-15
WOx(20)-SBA-15
(d)
(c)
(b)
(a)
200
300
400
500
600
700
200 400 600 800 1000 1200
Raman shift (cm-1)
200 400 600 800 1000 1200
Raman shift (cm-1)
Wavelength (nm)
Fig. 2 Diffuse reflectance UV-vis spectra of a WOx(20)-SBA-15, b
TiOx(6)-SBA-15, c MoOx(20)-SBA-15 and d VOx(11)-SBA-15
Fig. 3 Laser Raman spectra of SBA-15-supported MoOx and WOx
123

Regiospecific Oxyhalogenation of Aromatics
195
arising from Mo 3d levels in MoOx(20)-SBA-15 appeared
at 235.8 and 233.1 eV. Supported vanadia showed a broad
peak at 515.6 eV corresponding to V 2p3/2. This peak for
extended VxOy aggregates such as in V2O5 appears at
517.5 [32–34]. The shift to lower energy is an indication
that vanadia is in dispersed and even in a partially reduced
?3 state. Based on XPS we assign that the oxidation state
of W and Mo in WOx(20)-SBA-15 and MoOx(20)-SBA-15,
respectively as ?6.
per mole of metal per min) of tungsten oxide was enhanced
by five orders of magnitude when dispersed and supported on
SBA-15 (Table 2, compare rows 2 and 9). Mesoporous SBA-
15 enable better dispersion of the metal oxides. The dis-
persed, nanoparticle metal oxides having a high exposed
surface area and Lewis acidity showed better performance
than the bulk metal oxides. Conversion of phenol red
increased with the bromide ion concentration up to 2 mmol
and beyond that it decreased with any further increase in the
2-
2-
NH3-TPD confirmed the presence of weak Lewis acidic
sites. The amount of NH3 desorbed at 373–573 K from
different supported catalysts decreased in the order:
WOx(20)-SBA-15 (0.35 mmol g-1) [ MoOx(20)-SBA-15
(0.224 mmol g-1) [ TiOx(6)-SBA-15 (0.201 mmol g-1).
concentration (Fig. 5). MgAl–WO4 and MgAl–MoO4
exhibited TOF values of about 0.3 min-1 [14, 15]. The TOF
values of 32 and 16 min-1 observed for WOx(20) and
MoOx(20) supported on SBA-15 indicate that nature of the
support influences the activity of the metal oxide species.
3.2 Catalytic Activity
3.2.2 Oxyhalogenation of Aromatics
3.2.1 Oxybromination of Phenol Red
3.2.2.1 Oxyiodination Iodination of arenes (Table 3) was
achieved at a moderate pH (*4). The reaction occurred at
room temperature (298 K) and atmospheric pressure. The
iodinating agent used was KI/H2O2; the pH of the medium
was adjusted with HNO3. Complete conversion of a range
of arenes into the corresponding iodo products was
achieved in just 4 h. Iodination of aniline and anisole can
occur at both ortho- and para-positions. However, only the
para-isomer formed selectively over WOx(20)-SBA-15
(Table 3, entry nos. 1 and 2). Oxyiodination of phenol
yielded both the para- and ortho-iodo products. However,
the former has higher selectivity than the latter isomer
(Entry no. 3). In the case of styrene and a-methyl styrene,
the olefinic bond is preferentially attacked forming diiodo
and diol compounds with former being more selective.
b-Methyl styrene yielded mainly the diol product (Table 3,
compare entry nos. 8–11). Iodination of aniline was per-
formed at different reaction temperatures. Increase in ani-
line conversion beyond 298 K was only marginal.
When phenol red was reacted with a mixture of aqueous
H2O2 and KBr in the presence of SBA-15-supported nano-
scopic metal oxide catalysts, the yellow color of the reaction
mixture changed rapidly to deep-blue. Electronic absorption
spectra recorded as a function of reaction time (Fig. 4),
provided clear evidence for the disappearance of phenol red
(kmax = 434 nm) and formation of a new compound—bro-
mophenol blue (kmax = 595 nm). The initial rate of bro-
mination over different supported oxide catalysts decreased
in the order: WOx(20)-SBA-15 (3.42 9 10-4) [ MoOx
(20)-SBA-15 (2.61 9 10-4) [ TiOx(6)-SBA-15 (2.24 9
10-4) [ VOx(11)-SBA-15 (1.65 9 10-4 mM min-1) indi-
cating that the supported tungsten oxide catalysts are supe-
rior to the rest of the metal oxides investigated in the present
study (Table 2). Controlled experiments revealed that oxy-
bromination doesn’t occur in the absence of any catalyst. The
supported metal oxides exhibited higher catalytic activity
than the corresponding bulk oxides. The catalytic activity
(turnover frequency (TOF)—moles of substrate converted
3.2.2.2 Oxybromination The reaction was carried out at
ambient temperature and pressure and under mild acidic
(pH = 4.4–5) conditions (Table 4). Among various sub-
strates investigated, substituted anilines, anisole, naphthol,
phenol, 1,3,5-trimethyl benzene were found to be the most
reactive (100% conversion). In case of aniline, anisole etc.,
the para-brominated isomer formed with 100% selectively
(Table 4, entry nos. 1 and 2). In oxidative brominations, a
mixture of acetonitrile and water was used as a solvent
system. When naphthol was brominated under similar
conditions, a-naphthol gave about 92% para-isomer (entry
no. 8) and b-naphthol gave 70% ortho-product along with
dibrominated product (entry no. 9). Non-activated aro-
matics like toluene are weakly active compared to acti-
vated aromatics. In the case of cyclohexane, a non-
activated cyclic alkane, a conversion of 63% was observed;
1.0
595 nm
90 min
WOx(20)-SBA-15
MoOx(20)-SBA-15
595 nm
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
434 nm
434 nm
15 min
350 400 450 500 550 600 650
Wavelength (nm)
350 400 450 500 550 600 650
Wavelength (nm)
Fig. 4 Reactivity of
toward oxybromination of phenol red
a WOx(20)-SBA-15, b MoOx(20)-SBA-15
123

196
L. Saikia et al.
Table 2 Oxybromination of phenol red over SBA-15-supported nanocrystalline metal oxides
Br
Br
O
OH
O
OH
Br
Br
SO3H
SO3H
Bromo phenol blue
Phenol red
Catalysts
Temperature (K)
Phenol red
conversion (mol.%)
TOF
(min-1
Initial Rate
(mM min-1
)
)
WOx(20)-SBA-15
293
298
303
313
323
298
298
298
298
298
49.8
62.6
63.0
70.1
82.0
44.0
42.0
12.0
4.0
32
40
40
45
52
16
10
2
3.15 9 10-4
3.42 9 10-4
4.11 9 10-4
4.43 9 10-4
7.86 9 10-4
2.61 9 10-4
2.24 9 10-4
1.65 9 10-4
MoOx(20)-SBA-15
TiOx(6)-SBA-15
VOx(11)-SBA-15
NH4WO3.2H2O
V2O5
2 9 10-4
3.2 9 10-4
11.6
Reaction conditions: phenol red = 0.05 mM, catalyst = 0.025 gm (SBA-15 supported metal oxides) and 0.1 mmol (V2O5 and NH4WO3ꢀ2H2O),
KBr = 2 mmol, H2O2 (30% aq.) = 2 mmol, solvent (water ? methanol (4:1)) = 20 mL, pH * 5 (adjusted with HEPES buffer), reaction
time = 90 min. Turnover frequency (TOF) = moles of phenol red converted per mole of metal per min
product of some herbicides and fungicides. p-Chloro anisole
0.045
0.040
0.035
0.030
0.025
0.020
0.015
is used with formaldehyde for preparation of condensation
polymers. The chlorophenols make up an important class of
industrial chemical compounds. They are used as either
intermediate in the synthesis of agrochemicals, dyestuffs,
and pharmaceuticals or directly in formulations [35, 36].
Chlorination of various organic compounds was carried
out at pH = 3. Activated aromatics like anisole, aniline
etc. gave very high conversion and chlorinated product
selectivity (Table 6). As in the case of bromination and
iodination, substrates like aniline and anisole gave para-
chlorinated product regioselectively.
0
1
2
3
4
[Br-] [mM]
Fig. 5 Influence of Br- concentration on oxybromination of phenol
red over WOx(20)-SBA-15
3.2.3 Catalyst Reusability Studies
The stability of the catalyst (WOx(20)-SBA-15) was
assessed by reusing it in several recycling experiments. In
each run, after completion of the reaction, the catalyst was
separated from the reaction mixture by filtration, washed
with a solvent mixture of dichloromethane and acetone and
dried in air followed by activation at 473 K for 2 h. The
catalyst was, then, used in the next recycle. The results
with the recycled catalyst invariably showed unaltered
activity in para-bromination of anisole (Fig. 6, left panel)
suggesting that WOx(20)-SBA-15 is a stable heterogeneous
catalyst. WOx(20)-SBA-15, in contrast to enzymes and
only one brominated product was formed. Out of all the
nanoscopic metal oxides investigated, WOx(20)-SBA-15
was the most active. Catalytic activity increased with
tungsten oxide loading up to 20 wt% (Table 5).
3.2.2.3 Oxychlorination Organic chlorinated compounds
play an important role in synthetic organic chemistry.
p-Chloroaniline is used as an intermediate in the manufac-
ture of dyes, pigments, agricultural chemicals and pharma-
ceuticals. It is a persistent environmental degradation
123

Regiospecific Oxyhalogenation of Aromatics
197
Table 3 Oxyiodination of organics over WOx(20)-SBA-15
Entry
No.
Substrate
Conversion
(mol %)
Product Selectivity (mol %)
Para-product
Others
–
H3CO
1
2
3
100
100
100
(100)
H3CO
H2N
I
H2N
(100)
(66)
–
I
I
HO
(34)
HO
HO
I
I
HO
4
100
–
(100)
HO
Cl
Cl
H3CO
I
5
6
H3CO
100
100
–
(60, 40)
H3CO
I
NH2
NH2
NH2
I
NH2
(100)
–
NH2
HO
HO
I
7
8
9
100
81.5
100
(100)
–
–
NO2
NO2
CH3
HO
H2N
HO
I
(100)
CH3
H2N
I
–
(60, 40)
(68, 32)
I
OH
I
OH
I
10
11
100
–
–
OH
I
H
OH
OH
17.8
(100)
(97)
OH
OH
12
13
100
(3)
OH
OH
I
I
17.8
–
(100)
OH
I
OH
Reaction conditions: Catalyst = 0.050 g, substrate = 2 mmol, H2O2 (30% aq.) = 5 mmol, KI = 5 mmol, solvent = a mixture of CH3CN and
H2O (5 ml each), pH was adjusted to*4 by adding required quantity of conc. HNO3 acid, temperature = 298 K, reaction time = 4 h
123

198
L. Saikia et al.
Table 4 Oxybromination of aromatics using WOx(20)-SBA-15
Entry
No.
Substrate
Conversion
(mol %)
Product Selectivity
Para-product
Others
–
H3CO
1
2
3
4
100
(100)
H3CO
H2N
Br
100 (69)a
100
(100, 57a)
(0, 39a)
Br
H2N
H2N
Br
Br
Br
OH
HO
(67)
(33)
HO
Br
O2N
O2N
Br
93.7
–
(100)
OCH3
OCH3
NH2
H2N
Br
O
NH2
OH
H3CO
5
6
100
100
(60)
(60, 40)
H2N
Br
HO
O
Br
NH2
HO
NH2
OH
(63)
(92)
(37)
HO
Br
Br
Br
Br
7
100
(8)
OH
Br
Br
OH
8
9
100
63
–
Br
(70) (30)
OH
OH
Br
CH3
CH3
H2N
(100)
(100)
–
–
H2N
H3C
Br
Br
10
14
H3C
Reaction conditions: Catalyst = 0.050 g, substrate = 2 mmol, H2O2 = 5 mmol, NaBr = 5 mmol, HNO3 = 0.180 g, solvent = a mixture of
acetonitrile and water (5 ml each), reaction temperature = 298 K, reaction time = 4 h
a
Values for neat tungsten oxide
Table 5 Oxybromination of anisole
homogeneous catalysts, is not sensitive to oxidative
Catalyst
Coversion
(mol.%)
Selectivity of para-
brominated product (%)
destruction and thereby, shows superior performance over
homogeneous catalyst systems. Reusability was also con-
firmed in the oxyiodination reaction. WOx(20)-SBA-15
was reused in nine recycles with negligible loss in activity/
product selectivity. To confirm that the observed catalytic
activity was not due to metal ions leached into the reaction
medium during the reaction, the catalyst was filtered and
separated at the end of 1 h and the reaction was continued
without the solid catalyst for another 3 h. The reaction did
not proceed any further in the absence of catalyst (Fig. 6;
right panel).
WOx(20)-SBA-15
WOx(15)-SBA-15
WOx(10)-SBA-15
MoOx(20)-SBA-15
TiOx(6)-SBA-15
VOx(11)-SBA-15
100
100
100
100
100
100
100
68.6
51.2
96.3
62.8
44.3
Reaction conditions: substrate = 2 mmol, KBr = 5 mmol, 30% aq.
H2O2 = 5 mmol, catalysts = 0.05 g, solvent (CH3CN ? H2O) =
10 mL, HNO3 = 2 mmol, time = 4 h
123

Regiospecific Oxyhalogenation of Aromatics
199
Table 6 Oxychlorination of organics over WOx(20)-SBA-15
Entry
No.
Substrate
Conversion
(mol %)
Product Selectivity (%)
Para-product
Others
–
H3CO
1
2
3
100
100
5.3
(100)
H3CO
H2N
Cl
H2N
(100)
–
Cl
Cl
(41.4)
OH
(58.6)
HO
HO
Cl
Cl
HO
4
5
33.3
100
–
(100)
HO
Cl
Cl
NO2
(100)
(100)
–
NO2
CH3
HO
HO
Cl
Cl
6
100
–
CH3
H2N
H3C
H2N
7a
8
8
–
(100)
CH2Cl
Cl
OH
54.3
–
(58.4, 41.6)
Cl
OH
Reaction conditions: Catalyst = 0.050 g, substrate = 2 mmol, H2O2 (30% aq.) = 5 mmol, HCl = 5 mmol, pH of the reaction was adjusted
to*3; solvent = a mixture of CH3CN and H2O of 5 ml each, reaction temperature = 298 K, reaction time = 4 h
a
Reaction conducted at 343 K
Anisole conversion
100
tungsten oxide loading in the order: WOx(20)-SBA-15
100
(100%) [ WOx(15)-SBA-15 (68.6%) [ WOx(10)-SBA-15
(51.2%) (Table 5). This variation in catalytic activity par-
80
80
60
40
20
0
allels the variation in Lewis acidity (NH3-TPD) of the
catalyst samples. The dispersed metal oxides showed
higher catalytic activity than the bulk oxides (Tables 2 and
5). Among several solvents CH3CN was found better. The
reaction medium contains both aqueous and non-aqueous
compounds. Perhaps, CH3CN acts as a medium promoting
the miscibility of these otherwise immiscible compounds.
N2-physisorption, TEM and XPS studies revealed that
the metal oxide is located mainly inside the pores of SBA-
15 and not on the external surface. Even if a small quantity
of such separate metal oxide phase is present that would
not have been the main contributor for the observed cata-
lytic activity. It may be noted (Table 2) that the specific
catalytic activity (TOF) of separate bulk oxides is five
orders of magnitude lower than that of the supported metal
oxides. Hence, the observed activity in the present study is
mainly due to the dispersed, supported, nanoparticle metal
60
40
20
0
1 h
0
50 100 150 200 250
Time (min)
1
2 3 4 5 6 7 8 9
No. of recycles
Fig. 6 Left: catalyst reusability: oxybromination of anisole over WOx
(20)-SBA-15. Right: catalyst leaching test: oxybromination of aniline
over WOx(20)-SBA-15
In general, catalytic activity decreased with halide ions
in the order: I- [ Br- [ Cl-. Conversion of anisole to
4-bromoanisole increased over different supported metal
oxides in the order: WOx-SBA-15 (100%) [ MoOx-SBA-
15 (96.3%) [ TiOx-SBA-15 (62.8%) [ VOx-SBA-15
(44.3%) (Table 5). Catalytic activity increased with
123

200
L. Saikia et al.
0.06
0.04
0.02
0.00
WOx(20)-SBA-15+H O2+Br-
VOx(11)-SBA-15+H O2+Br-
0.06
0.04
0.02
0.00
0.08
0.06
0.04
0.02
0.00
2
2
TiOx(20)-SBA-15+H O2+Br-
2
VOx(11)-SBA-15+H O2
2
WOx(20)-SBA-15+H O2
328
359
2
TiOx(20)-SBA-15+H O2
2
VOx(11)-SBA-15
WOx(20)-SBA-15
364
TiOx(6)-SBA-15
366
200
300
400
500
600
700
800
200
300
400
500
600
700
800
200
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Fig. 7 SBA-15-supported metal oxides and their interaction with aq. H2O2 and the H2O2 ? KBr system
oxides located inside the pores of SBA-15. The weak peaks
of tungsten oxide observed in XRD correspond to those
nanoparticle metal oxide species located inside the pores of
SBA-15 and not that from any separate bulk oxide phase.
catalytic activity with the Lewis acidity of the catalysts of
the present study.
4 Conclusions
3.3 Mechanistic Investigation: Spectroscopic Study
Nanoparticulate WOx, MoOx, TiOx and VOx supported on
SBA-15 showed efficient catalytic activity for oxyhalo-
genation of aromatic compounds. Oxybromination of
phenol red yielded exclusively bromophenol blue. The
catalytic activity decreased in the order: WOx-SBA-
15 [ MoOx-SBA-15 [ TiOx-SBA-15 [ VOx-SBA-15.
The catalytic activity of tungsten oxide was enhanced by
five orders of magnitude when dispersed on SBA-15. The
variation in catalytic activity followed the variation in
Lewis acidity of the catalysts. Diffuse reflectance UV-vis
spectroscopic studies revealed that oxyhalogenation
occurs, probably, via. peroxometal intermediates.
In situ diffuse reflectance UV-visible spectroscopy
(DRUV) was used to probe the interaction of SBA-15-
supported metal oxides with the H2O2/Br- system (Fig. 7).
Upon contact with H2O2, the white solid had changed rap-
idly to pale yellow in color. A new absorption band was
observed at 360–364 nm due to metal-peroxo species [14,
15]. The band at 260 nm corresponding to O2- ? W/Vn?
charge transfer was shifted to 220 nm. When the catalyst
was treated with H2O2 ? KBr solution, there was a relative
increase in overall intensity of the spectral bands. In addition
to the band at 360–364 nm due to metal-peroxo species, a
new band was observed at 267–274 nm. This band was
attributed in the past to Br3- species formed by the reaction
of Br- with molecular bromine (Br2 ? Br- $ Br3-) [13].
Br2 is produced by the oxidation of KBr by H2O2. The
liberation of Br2 from KBr and H2O2 in the presence of
homogeneous and enzyme catalysts is well known. So,
analogous to the active vanadium site in marine haloper-
oxidase enzymes, nanocrystalline metal oxides supported on
SBA-15 transform into their peroxo form upon exposure to
H2O2. This reactive peroxo intermediate in the presence of
bromide ions produces molecular bromine which, then,
participates in the halogenation reactions. Hence, haloge-
nation of arenes with H2O2-KX system over metal oxides
supported on SBA-15, probably, proceeds via a peroxidative
mechanism. Higher the acidity of the catalyst more facile
would be the formation of labile metal-peroxo species which
in turn produces halogen in situ by reaction with alkali
halide. This reasoning explains the parallel variation of
Acknowledgment L.S. acknowledges the Council of Scientific and
Industrial Research (CSIR), New Delhi for the award of senior
research fellowship.
References
1. Dawson JH (1988) Science 240:433
2. Littlechild J (1999) Curr Opn Chem Biol 3:28
3. Crans DC, Smee JJ, Gaidamauskas E, Yang L (2004) Chem Rev
104:849
4. Butler A, Carter-Franklin JN (2001) Nat Prod Rep 21:180
5. Nightingale PD, Malin G, Liss PS (1995) Limnol Oceanogr
40:680
6. Miyaura N, Suzuki A (1995) Chem Rev 95:2457
¨
¨
7. Ratnasamy P, Singh AP (2008) In: Ertl G, Knozinger H, Schuth F,
Weitkamp J (eds) Handbook of heterogeneous catalysis, vol 3, 2nd
ed. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, p 3575
8. Cacchi S, Caglioti L (1979) Synthesis 64
9. Bongini A, Cainelli G, Contento M, Manescalchi F (1980) Syn-
thesis 143
123

Regiospecific Oxyhalogenation of Aromatics
201
10. Choudary BM, Someshwar T, Venkat Reddy Ch, Lakshmi Kantam
M, Jeeva Ratnam K, Sivaji LV (2003) Appl Catal A: Gen 251:397
11. Coughlin P, Roberts S, Rush C, Willetts A (1993) Biotech Lett
15:907
12. Martinez JS, Carroll GJ, Tschirret-Guth RA, Altenhoff G, Daniel
Little R, Butler A (2001) J Am Chem Soc 123:3289
13. Raja R, Ratnasamy P (1997) J Catal 170:244
14. Sels BF, De Vos DE, Buntinx M, Jacobs PA (2003) J Catal
216:288
23. Das D, Lee JF, Cheng S (2001) Chem Commun 2178
24. Cano-Serrano E, Campos-Martin JM, Fierro JLG (2003) Chem
Commun 246
25. Xie S, Chen K, Bell AT, Iglesia E (2000) J Phys Chem 104:10059
26. Zhang Z, Suo J, Zhang X, Li S (1999) Appl Catal A: Gen 179:11
27. Ming X (2003) J Phys Chem 107:8972
28. Hardcastle FD, Wasch IE (1991) J Phys Chem 95:5031
29. Dai LX, Tabata K, Suzuki E, Tatsumi T (2001) Chem Mater
13:208
¨
30. Branda˜o P, Philippou A, Hanif N, Ribeiro-Claro P, Ferreira A,
15. Sels BF, De Vos DE, Jacobs PA (2001) J Am Chem Soc
123:8350
16. Srinivas D, Saikia L (2008) Catal Surv Asia 12:114
17. Saikia L, Satyarthi JK, Srinivas D, Ratnasamy P (2007) J Catal
252:148
18. Saikia L, Srinivas D, Ratnasamy P (2007) Micropor Mesopor
Mater 104:225
19. Feng F, Fryxell GE, Wang LQ, Kim AY, Kemner KM (1997)
Science 276:923
Anderson MW, Rocha J (2002) Chem Mater 14:1053
31. Scholz A, Schnyder B, Wokaum A (1999) J Mol Catal A: Chem
138:249
32. Liu W, Lai SY, Dai H, Wang S, Sun H, Au CT (2007) Catal Lett
113:147
33. Venkov TV, Hess C, Jentoff FC (2007) Langmuir 23:1768
34. Hesss C, Tzoliva-Muller G, Herbert R (2007) J Phys Chem C
111:9471
20. Mercier L, Pinnavaia TJ (1997) Adv Mater 9:500
21. Stein A, Melde BJ, Schroden RC (2000) Adv Mater 12:1403
22. Van Rhijn WM, De Vos DE, Sels BF, Bossaert WD, Jacobs PA
(1998) Chem Commun 317
35. Bailey JE, Bohnet M, Brinke J (eds) (1998) Ullmann’s encyclo-
pedia of industrial chemistry, 6th edn. Wiley-VCH, Weinheim
36. Menini L, Gusevskaya EV (2006) Appl Catal A: Gen 309:122
123