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