Hydroxylation of phenol with hydrogen peroxide catalyzed by Ti-SBA-12 and Ti-SBA-16

Article abstract of DOI:10.1016/j.molcata.2012.11.026

We report here, for the first time, the application of ordered, three-dimensional, mesoporous titanosilicates, Ti-SBA-12 and Ti-SBA-16, as reusable solid catalysts, for hydroxylation of phenol, an industrially important organic transformation. The reactio

Full text of DOI:10.1016/j.molcata.2012.11.026

Journal of Molecular Catalysis A: Chemical 368–369 (2013) 112–118
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Hydroxylation of phenol with hydrogen peroxide catalyzed by Ti-SBA-12 and
Ti-SBA-16
Anuj Kumar, Darbha Srinivas∗
Catalysis Division, National Chemical Laboratory, Pune-411 008, India
a r t i c l e i n f o
a b s t r a c t
Article history:
We report here, for the first time, the application of ordered, three-dimensional, mesoporous titanosil-
icates, Ti-SBA-12 and Ti-SBA-16, as reusable solid catalysts, for hydroxylation of phenol, an industrially
important organic transformation. The reactions were conducted using 30% aqueous H2O2 as oxidant.
The catalysts of this work are more efficient than the hitherto known mesoporous Ti-silicates. They
are more para-product selective than TS-1 and other titanosilicates. Ti-SBA-12 exhibited higher activity
and hydroquinone/catechol selectivity (by nearly two times) than Ti-SBA-16. H2O2 efficiency of ∼90 mol%
was obtained. Our study reveals that framework substituted Ti with pseudo-tetrahedral geometry, three-
dimensional mesoporosity as well as surface structure are the unique features responsible for the high
catalytic activity and selectivity of these titanosilicate catalysts.
Received 18 October 2012
Received in revised form
26 November 2012
Accepted 30 November 2012
Available online xxx
Keywords:
Ordered mesoporous titanosilicates
Ti-SBA-12
© 2012 Elsevier B.V. All rights reserved.
Ti-SBA-16
Molecular sieves
Catalytic liquid-phase hydroxylation of
phenol
Oxidation with hydrogen peroxide
1. Introduction
Titanosilicates have been widely investigated due to their
remarkable catalytic activity for oxidation reactions at mild con-
Hydroxylation of phenol is an industrially important reaction as
the product diphenols, viz., catechol and hydroquinone (Scheme 1),
are widely used as photography chemicals, antioxidants, poly-
merization inhibitors, flavoring agents and drug intermediates [1].
Since 1970, phenol hydroxylation has been widely investigated
using various homogeneous and heterogeneous catalysts. In the
Brichima process (now Enichem, Italy) [2,3], the salts of Fe2+ and
Co2+ are used as catalysts for phenol hydroxylation by using 60%
aqueous H2O2 as oxidizing agent. In the Rhône-Poulenc process
[4,5], strong mineral acids such as phosphoric acid and catalytic
amount of perchloric acid are employed with 70% aqueous H2O2.
In the Hamilton process [6], Fenton reagent is used as a cata-
lyst. In the Ube-Rhone-Poulenc process [7], this reaction is carried
out with ketone peroxides (␣-hydroxy hydroperoxides) formed
in situ from a ketone and 60% aqueous H2O2 in the presence of
an acid catalyst (sulphuric or sulphonic acid). But these homoge-
neous catalysts are difficult to recover from the reaction mixture.
Heterogeneous catalysts are particularly attractive for hydroxyl-
ation of phenol due to their engineering and catalyst reusability
advantages.
ditions using aqueous hydrogen peroxide as oxidant [8–10].
Hydroxylation of phenol catalyzed by microporous titanosilicates
(TS-1 or TS-2) created an environmentally benign route for the
production of catechol and hydroquinone [11–15]. The process
has been commercialized by Enichem, Italy in 1986. However,
the application of TS-1 and TS-2 catalysts is limited to molecules
with smaller dimensions due to inherent constraints on pore size
(0.54 nm). Mesoporous M41S-type titanosilicates are less stable
and show lower intrinsic catalytic activity and selectivity toward
the use of H2O2 in phenol hydroxylation [10]. Ti-MCM-68 showed
superior activity to TS-1 but a decrease in product yield after
some duration of the reaction due to tar formation was noted [16].
Hence, there is a need to develop more efficient and stable, ordered
mesoporous silica materials with Ti substituted in their framework
for transformation of bulky organic molecules of pharmaceutical
interest.
Among SBA-type mesoporous silica, SBA-12 and SBA-16 attract
much attention due to their outstanding stability and three-
dimensional mesopore architecture [17,18]. We have recently
reported a method for direct synthesis of framework-substituted
Ti in Ti-SBA-12 and Ti-SBA-16 [19–21]. These materials exhibited
high catalytic activities for epoxidation of cyclic olefins and for
the synthesis of ␤-amino alcohols through ring-opening of epox-
ides with amines [19–21]. We, now, extend their application, for
the first time, for the liquid-phase hydroxylation of phenol with
∗
Corresponding author. Tel.: +91 20 2590 2018; fax: +91 20 2590 2633.
E-mail address: d.srinivas@ncl.res.in (D. Srinivas).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.11.026

A. Kumar, D. Srinivas / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 112–118
113
Scheme 1. Hydroxylation of phenol.
30% aqueous H2O2. These catalysts showed remarkably high cat-
alytic activity than the hitherto known mesoporous titanosilicate
catalysts.
2. Experimental
Ti-SBA-12 and Ti-SBA-16 with Si/Ti input molar ratios of 80, 50,
40, 30 and 20 were prepared and characterized as described by us
earlier (Supplementary information, S1) [19–21]. The Si/Ti output
molar ratios of the catalysts are listed in Table 1.
Liquid-phase hydroxylation of phenol was carried out in a 50 ml
glass round-bottom flask, fitted with a water-cooled condenser and
placed in a temperature-controlled oil bath. In a typical reaction,
10 mmol of phenol (0.94 g), 10 ml of distilled water and 0.1 g of cat-
alyst were taken in the glass reactor. To it, a known quantity of
30% aqueous H2O2 was added in one lot. Temperature of the reac-
tion mixture was raised to a specific value and the reaction was
conducted while stirring for a desired period of time. The progress
of the reaction was monitored by gas chromatographic technique
(GC – Varian 3400; CP-SIL8CB column; 30 m-long and 0.53 mm-
i.d.). Prior to GC analysis, solid catalyst, present if any in the
aliquot was removed by centrifugation followed by filtration. Sol-
vent (water) was separated by treating the liquid portion of the
reaction mixture with diethyl ether. Unreacted phenol and the
oxidation products which remained in the ether layer were then
separated. Ether was distilled out under reduced pressure using a
rotavapor. The reaction mixture thus recovered was diluted with
a known quantity of methanol and analyzed by GC. Identification
of the products was done by GC–MS (Varian CP-3800; 30 m-long,
0.25 mm-i.d., and 0.25 ␮m-thick CP-Sil8CB capillary column) and
by comparing with the standard samples. A typical mass balance
greater than 98% was obtained.
3. Results and discussion
3.1. Structural and spectroscopic properties
We present here, in brevity, the salient features of Ti-SBA-12
and Ti-SBA-16. Detailed characterization of these catalysts could be
found elsewhere [19–21]. Fig. 1 shows the X-ray powder diffraction
(XRD) patterns of the catalysts in the low-angle region (2ꢀ = 0.5–5◦).
SBA-12 shows an intense main peak at 1.68◦ corresponding to
(0 0 2) reflection and two well-resolved, weak peaks at 2.90◦ and
3.28◦ attributable to (1 1 2) and (3 0 0) reflections, respectively
(Fig. 1(a)). A weak shoulder (to the main peak) at 1.47◦ due to (1 0 0)
reflection is also observed. These characteristic features of SBA-
12 are consistent with the ordered, three-dimensional, hexagonal
mesoporous structure with a space group of p63/mmc [17]. SBA-16
shows a strong peak at 0.92◦ attributable to (1 1 0) reflection and
two poorly-resolved, weak peaks at 1.2◦ and 1.9◦ due to (2 0 0) and
(2 1 1) reflections, respectively (Fig. 1(b)). The diffraction patterns
of SBA-16 can be indexed to an ordered, three-dimensional, meso-
¯
porous, cubic structure with a space group of Im3m [17,22–26]. In
the case of Ti containing samples (Ti-SBA-12 and Ti-SBA-16), these
peaks shifted to lower 2ꢀ values evidencing an increase in the unit
cell parameters (a and c for the hexagonal Ti-SBA-12 and a for the

114
A. Kumar, D. Srinivas / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 112–118
0.84º (110)
(a) Ti-SBA-12
1.62º (002)
1.64º
(b) Ti-SBA-16
Si/Ti = 20
0.88º
0.90º
Si/Ti = 20
Si/Ti = 30
1.65º
Si/Ti = 30
Si/Ti = 40
0.91º
1.66º
1.66º
Si/Ti = 40
0.92º
0.92º
Si/Ti = 50
Si/Ti = 80
Si/Ti = 50
Si/Ti = 80
1.68º
1.47º (100)
2.90º (112)
1.90º (211)
3.28º (300)
SBA-16
1.20º (200)
SBA-12
0.5 1.0 1.5 2.0 2.5
3.0 3.5 4.0 4.5 5.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
2θ (degree)
2θ (degree)
Fig. 1. XRD profiles of Ti-SBA-12 and Ti-SBA-16 with Si/Ti molar ratios of 20–80.
450
375
300
225
150
75
450
(a) Ti-SBA-12 (Si/Ti = 20)
(b) Ti-SBA-16 (Si/Ti = 30)
375
300
225
150
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure, P/P0
Relative pressure (P/P0)
Fig. 2. Representative nitrogen adsorption–desorption isotherms of Ti-SBA-12 and Ti-SBA-16. Pore size distribution curves are shown in inset.
cubic Ti-SBA-16) and substitution of Ti4+ for Si4+ in the framework
location of mesoporous silica (Fig. 1 and Table 1). This increase in
the value of unit cell parameters and unit cell volume was found
linear up to a Ti content of 4.76 mol% [19,20].
Ti-SBA-12 shows typical type-IV N2 adsorption–desorption
isotherms with a H1-hysteresis loop characteristic of a hexago-
nal mesoporous material (Fig. 2(a); Supplementary information
S2) [19]. The N2 physisorption behavior of Ti-SBA-16 is typical of
Fig. 3. HRTEM images: (a) Ti-SBA-12 (Si/Ti = 20), (b) Ti-SBA-12 (Si/Ti = 30), (c) Ti-SBA-12 (Si/Ti = 50), (d) Ti-SBA-16 (Si/Ti = 20), (e) Ti-SBA-16 (Si/Ti = 30) and (f) Ti-SBA-16
(Si/Ti = 50).

A. Kumar, D. Srinivas / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 112–118
115
cage-like uniform mesopores interconnected by relatively narrow
aperture (Fig. 2(b); Supplementary information S2) [20,21,26–28].
BJH measurements showed that all these materials have a narrow
pore size distribution with an average pore size of 3.8–5.6 nm for
SBA-12 and 3.4 to 4.0 nm for SBA-16 materials.
Ti-SBA-12 has a spherical morphology with an average particle
size in the range of 4–6 ␮m. Ti-SBA-16 formed as planar sheets or
worm-like morphologies with particle size in the range of 7–15 ␮m.
Ti-content had little effect on the size and shape of titanosili-
cate particles (Supplementary information S3). High resolution
transmission electron micrographs (HRTEM) (Fig. 3) confirmed
the long-range, three-dimensional, mesoporous ordering in these
materials. While the HRTEM images of Ti-SBA-12 are consistent
with hexagonally arranged pores (space group: p63/mmc), those
of Ti-SBA-16 are typical of cubic cage-like interconnected pore
¯
arrangement (space group: Im3m). No separate bulk-like TiO2 par-
ticles were detected in the HRTEM images indicating that Ti is
isolated.
The catalyst samples showed two characteristic Fourier trans-
form infrared bands at 948 and 969 cm−1 (Supplementary
information S4). While the former is attributed to Si
tions of framework substituted Ti, the latter band is assigned
to Si Si vibrations. Intensity of Si Ti band increased with
O Ti vibra-
O
O
increasing Ti content in the sample [13,29–31]. The samples
showed an optical absorption band with maximum at 210 nm and a
shoulder band at 217–220 nm (Fig. 4) due to ligand → metal charge
transfer transition (O2− → Ti4+) which could be attributed to mono-
atomically dispersed Ti4+ ions in tetra-coordinated geometry [29].
While the band at 210 nm is corresponded to tetrapodal titanium
with Ti(OSi)4 structure that at 217–220 nm is attributed to tripo-
dal titanium with Ti(OH)(OSi)3 structure [29]. At higher Ti contents
(Si/Ti ≤ 50), an additional weak absorption at 315 nm correspond-
ing to agglomerated titania was observed in Ti-SBA-12 samples.
This anatase phase was absent in Ti-SBA-16 (Supplementary
information S3).
All these structural, textural and spectral features reveal that Ti
is isolated, possesses a pseudo-tetrahedral geometry and is substi-
tuted in the lattice of mesoporous silica. The Ti in Ti-SBA-12 and
Ti-SBA-16 has all the characteristic features as that of TS-1 for it to
be active in oxidation reactions with hydrogen peroxide as oxidant.
3.2. Catalytic activity
The liquid-phase hydroxylation of phenol was carried out in the
presence of Ti-SBA-12 and Ti-SBA-16 catalysts using 30% aqueous
H2O2 as oxidant (Tables 2–4). Catechol (CAT) and hydroquinone
(HQ) were the only products (Scheme 1). Surprisingly, benzo-
quinones, the over oxidation products, were not detected. The
reaction occurred in the presence of water as solvent. Blank con-
trolled experiments confirmed that no reaction takes place without
a catalyst at our experimental conditions. Also, no reaction was
detected when all-silica SBA-12 and SBA-16 or bulk TiO2 (Ti in octa-
hedral geometry) was used as catalyst in place of Ti-SBA-12 and
Ti-SBA-16. All these observations reconfirm [10–15] that isolated
Ti with pseudo tetrahedral geometry present in the framework
positions of Ti-SBA-12 and Ti-SBA-16 are the active sites for the
hydroxylation reaction to occur.
In general, the catalytic activity (phenol conversion) increased
with increasing Ti content (i.e., decreasing Si/Ti molar ratio) in the
catalyst (Table 2). Except for Si/Ti = 80, the intrinsic catalytic activ-
ity (phenol conversion or TOF) of Ti-SBA-12 was double that of
Ti-SBA-16. Based on stoichiometry, one expects a HQ/CAT product
molar ratio of about 0.5 in this reaction. In fact, that was the case
with Ti-SBA-16. Ti-SBA-12 was found to be a para-selective cata-
lyst. HQ/CAT molar ratio for Ti-SBA-12 was in the range 0.67–1.14.
The higher amount of para-product formation suggests that the

116
A. Kumar, D. Srinivas / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 112–118
Table 3
Influence of catalyst and oxidant amounts on the hydroxylation of phenol using Ti-SBA-12.
a
c
Catalyst/phenol (g mol−1
)
Phenol conv. (mol%)
Product selectivity (mol%)
TOF (h−1
)
CAT
HQ
HQ/CAT
5
10
15
20
4.8
8.6
10.2
11.8
57.5
52.3
51.0
50.4
42.5
47.7
49.0
49.6
0.7
0.9
1.0
1.0
4.4
3.9
3.1
2.7
Phenol/H2O2 (molar ratio)b
4
3
2
1
15.1
24.1
31.2
55.0
44.1
47.1
46.5
57.0
55.9
53.9
53.5
43.0
1.3
1.1
1.2
0.8
1.7
2.8
3.6
6.3
a
Reaction conditions: catalyst = Ti-SBA-12 (Si/Ti = 20, 2 M; 0.1 g), phenol = 0.94 g (10 mmol), oxidant = 30 wt% aqueous H2O2, phenol/H2O2 (molar ratio) = 3, solvent = water
(10 ml), reaction temperature = 353 K, reaction time = 6 h, CAT = Catechol, HQ = hydroquinone.
b
Reaction conditions: same as those of a except for phenol/H2O2 (molar ratio) = 1 to 4 and reaction time = 24 h.
Turnover frequency (TOF) = moles of phenol converted per mole of Ti (output) in the catalyst per hour.
c
Table 4
Comparative catalytic activity data of Ti-SBA-12 and TS-1 for the hydroxylation of phenol.a
Reaction time (h)
Ti-SBA-12
TS-1
b
b
Phenol conversion (mol%)
Productselectivity(mol%)
TOF (h−1
)
Phenol conversion (mol%)
Productselectivity(mol%)
TOF (h−1
)
CAT
HQ
HQ/CAT
CAT
HQ
HQ/CAT
1
3
6
12
18
24
–
4.8
8.6
15.0
20.2
24.1
–
–
–
–
6.0
21.7
26.7
31.0
31.4
32.0
70.5
60.0
56.7
52.3
52.4
48.0
29.5
40.0
43.3
47.3
48.6
52.0
0.4
0.7
0.8
0.9
0.9
1.1
11.9
14.3
8.8
5.1
3.5
55.0
52.3
51.5
50.6
47.1
45.0
47.7
48.5
49.4
53.9
0.8
0.9
0.9
1.0
1.1
4.4
3.9
3.4
3.1
2.8
2.6
a
Reaction conditions: catalyst, Ti-SBA-12 (Si/Ti = 20, 2 M) and TS-1 (Si/Ti = 30) = 0.1 g, phenol = 0.94 g (10 mmol), oxidant = 30 wt% aqueous H2O2 (0.37 g), phenol/H2O2 (molar
ratio) = 3, solvent = water (10 ml), reaction temperature = 353 K, CAT = catechol, HQ = hydroquinone.
b
Turnover frequency (TOF) = moles of phenol converted per mole of Ti (output) in the catalyst per hour.
extent of reaction is occurring mainly in the mesopores of Ti-SBA-
12 than on the external particle surface. This conclusion on shape
selectivity is made in line with the similarities observed in the
case of para-selective TS-1 catalyst for the hydroxylation of phenol
[10–15,32,33]. H2O2 selectivity was nearly the same (∼90 mol%)
irrespective of Ti content in the catalyst. This is because the Ti ions
were isolated even at higher concentrations. The small amount of
anatase-like TiO2 present in Ti-SBA-12 at Si/Ti ≤ 50 hardly influ-
enced the decomposition of H2O2. Irrespective of Si/Ti ratio, the
TOF values were nearly the same.
(Table 3). H2O2 efficiency was lower at higher amount of oxidant
concentrations as a part of the oxidant decomposed into water and
molecular oxygen. For high H2O2 efficiencies it is recommended
to conduct the reactions at higher phenol/H2O2 molar ratios (for
example, at 3 or 4). Under such conditions a maximum H2O2 effi-
ciency of ∼90% (Table 2) was observed.
A comparison of catalytic activity of mesoporous Ti-SBA-12 and
microporous TS-1 is reported in Table 4. The reaction was con-
ducted with phenol/H2O2 molar ratio of 3. Under these conditions,
for 100% H2O2 consumption, one should get 33.3 mol% of phenol
conversion. As seen from Table 4, a phenol conversion of 31 mol%
was obtained in 12 h over TS-1 while Ti-SBA-12 required more
than 24 h for such conversion. In other words, the initial rate of
reaction and intrinsic catalytic activity (TOF) of TS-1 is higher than
Phenol conversion and HQ/CAT selectivity increased with
increasing reaction temperature (Supplementary information S5)
and catalyst amount (Table 3). With increasing H2O2, the conver-
sion of phenol increased while HQ/CAT molar ratio had decreased
206 nm
206 nm
(b) Ti-SBA-16 (Si/Ti = 30)
(a) Ti-SBA-12 (Si/Ti = 50)
217 nm
315 nm
200
300
400
500
600
700
800
200
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 4. Diffuse reflectance UV–vis spectra of representative (a) Ti-SBA-12 and (b) Ti-SBA-16 samples.

A. Kumar, D. Srinivas / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 112–118
117
1.2
1.0
0.8
0.6
0.4
1.2
Si/Ti = 20
Ti-SBA-12
Si/Ti = 30
Ti-SBA-12
1.0
0.8
0.6
0.4
Si/Ti = 40
Si/Ti = 50
Si/Ti = 80
Si/Ti = 30
Si/Ti = 20
Ti-SBA-16
TS-1
Si/Ti = 50
Si/Ti = 40. 80
6
9
12
15
18
21
24
0
4
8
12 16 20 24 28 32
Phenol conversion (mol%)
Phenol conversion (mol%)
Fig. 5. Variation of HQ/CAT molar ratio as a function of phenol conversion: (left) for Ti-SBA-12 and Ti-SBA-16; data and reaction conditions taken from Table 2; (right) for
Ti-SBA-12 and TS-1; data and reaction conditions taken from Table 3.
that of Ti-SBA-12. A similar observation was also found with other
mesoporous titanosilicates like Ti-MCM-41 [10]. Clerici et al. [34]
proposed that the intrinsic activity of titanium sites declines pro-
gressively with increasing pore diameter. Thus, Ti-SBA-12 having
higher pore diameter (5.0–5.6 nm) showed lower intrinsic activity
than TS-1 having lower pore diameter (∼0.5 nm). Moreover, the
higher hydrophobic nature of TS-1 than Ti-SBA-12 and Ti-SBA-16 is
another possible cause for its higher catalytic activity. It is interest-
ing to note that at similar conversions, HQ/CAT selectivity is higher
over Ti-SBA-12 than over TS-1 (Fig. 5; right panel). A systematic
increase of HQ/CAT selectivity with increasing phenol conversion
was observed on both TS-1 and Ti-SBA-12. Such variation was not
found in the case of Ti-SBA-16 (Fig. 5; left panel). To demonstrate
the importance of three-dimensional pore architecture, we have
carried out the reactions in the presence of Ti-SBA-15 (Si/Ti = 30
and 20), a two-dimensional mesoporous titanosilicate catalyst at
reaction conditions of catalyst = 0.1 g, phenol = 10 mmol, H2O2 (30%
aqueous) = 0.37 g, phenol/H2O2 (molar ratio) = 3, reaction tempera-
ture = 353 K, reaction time = 24 h and H2O = 10 ml. Catalytic activity
of Ti-SBA-15 was found inferior to Ti-SBA-12 confirming that the
three-dimensional pore network of Ti-SBA-12 facilitates diffusion
of the reactant and product molecules and accessibility of active
sites. Unlike that with Ti-SBA-12, over Ti-SBA-15 catechol formed
in higher amounts than hydroquinone (Ti-SBA-15 (Si/Ti = 30):
phenol conversion = 6.6 mol%, CAT = 72 mol% and HQ = 28 mol%,
spent catalyst showed activity similar to that of a fresh catalyst. The
active Ti sites are not buried or obscured by the adsorbed product
molecules. It is interesting to note that with microporous TS-1 cata-
lyst, about 30% weight gain due to adsorption of product molecules
was observed at the end of one reuse itself. However, the weight
gain in the case of mesoporous titanosilicate catalyst of the present
study at the end of one recycle was lower by nearly four times
to that of TS-1 catalyst. Although the intrinsic catalytic activity is
lower, mesoporosity of Ti-SBA-12 enables prolonged usage of the
catalyst. Thus, the three-dimensional pore structure benefits in the
catalytic applications by reducing the pore blockage and prolonging
the catalyst life.
Tuel and Ben Taarit [35] proposed that steric hindrance of tran-
sition complex favors the formation of para-isomer, HQ which can
further undergo isomerization into the ortho-isomer, CAT on the
external surface of TS-1. Kerton et al. [36] stated that phenol oxi-
dation occurs on the exterior crystallite surface or within the pores
close to the surface of the crystallites. In order to find whether
any such isomerization is happening in the case of catalysts of the
present study, we have taken CAT or HQ as substrate and carried
out the reaction under similar conditions in liquid phase with and
70
Phenol conversion
HQ/CAT = 0.39;
Ti-SBA-15
(Si/Ti = 20):
phenol
conver-
Catechol selectivity
60
sion = 7.8 mol%, CAT = 73 mol% and HQ = 27 mol%, HQ/CAT = 0.37).
Reusability of Ti-SBA-12 (Si/Ti = 20) was studied in five recycling
experiments. At the end of each run, the catalyst was separated by
centrifugation, dried at 373 K for 4 h and then reused in a subse-
quent recycle experiment. As seen from Fig. 6, Ti-SBA-12 is quite
stable even after the 5th recycle. No loss in phenol conversion and
variation in product selectivity was detected.
The spent Ti-SBA-12 catalyst (after 5th recycle study) was char-
acterized by ICP-OES, XRD, thermal analysis and FTIR. No loss in
Ti content in the reused catalyst was detected (ICP-OES). While
the XRD pattern of the reused catalyst is nearly the same as that
of a fresh catalyst, a decrease in intensity of the XRD peaks with
a back ground signal below 1.5◦ was observed (Supplementary
information S6). In thermal analysis, the spent catalyst (at the
end of 5th recycle) showed an additional weight loss of 40% in
the temperature range of 450–900 K. FTIR spectrum of the reused
catalyst showed additional bands at around 1100 and 1600 cm−1
and a broad band at 3300–3450 cm−1 corresponding to the charac-
teristic peaks of the product dihydroxybenzenes (Supplementary
information S6). In spite of adherence of product molecules, the
Hydroquinone selectivity
50
40
30
20
10
0
0
1
2
3
4
5
Number of recycles
Fig. 6. Catalyst recyclability study. Reactions conditions: catalyst = Ti-SBA-12
(Si/Ti = 20, 0.1 g), phenol = 10 mmol, oxidant = 30 wt% aqueous H2O2 (0.37 g),
phenol/H2O2 (molar ratio) = 3, water = 10 ml, reaction temperature = 353 K, and reac-
tion time = 12 h.

118
A. Kumar, D. Srinivas / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 112–118
without H2O2. We did not observe any isomerization in this case.
We, therefore, propose that the differences in HQ/CAT molar ratio
can arise due to the following reasons: (1) differences in the binding
of CAT and HQ molecules on the catalyst surface and (2) differences
in the rates of conversion of ortho- and para-dihydroxy compounds
to corresponding quinones and their polymerized products. Non-
detection of benzoquinone products at our reaction conditions
suggests that the latter cannot be the cause for higher values
of HQ/CAT in the case of Ti-SBA-12. Hence, differences in the
desorption rates of HQ and CAT from the surface of Ti-SBA is
the possible cause for variation in HQ/CAT selectivity. Earlier, we
found [20,21], based on thermal and NMR studies, that TiSBA-
12 is relatively more hydrophilic than Ti-SBA-16. Ortho-hydroxy
compound (CAT) must be more strongly adsorbed than the para-
hydroxy compound (HQ) on hydrophilic Ti-SBA-12 surface than on
hydrophobic Ti-SBA-16. Hence, the HQ/CAT ratio in the product is
higher in the case of Ti-SBA-12 than with Ti-SBA-16. The increase in
HQ/CAT selectivity ratio with increase in phenol conversion is also
because of these differences in the adsorption characteristics. Tsai
et al. [37] have also observed such differences in diphenol prod-
uct selectivities over post-treated TS-1 catalysts. They too have
attributed these changes in product selectivity as due to changes
in textural properties and pore structure of the TS-1 catalyst
samples.
Acknowledgment
Anuj Kumar acknowledges the Council of Scientific and Indus-
trial Research (CSIR), New Delhi for the Research Fellowship.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.molcata.2012.11.026.
References
[1] H. Fiegel, H.-W. Voges, T. Hamamoto, S. Umemura, T. Iwata, H. Miki, Y. Fujita,
H.-J. Buysch, D. Garbe, W. Paulus, Phenol Derivatives in Ullmann’s Encyclopedia
of Industrial Chemistry, vol. 26, Wiley-VCH, New York, 2002, p. 521.
[2] P. Maggioni, US Patent 3,914,323 (1972).
[3] P. Maggioni, F. Minisci, Chim. Ind. (Milan) 59 (1977) 239–242.
[4] F. Bourdin, M., Costantini, M., Jouffret, G. Lartigan, German Patent DE 064,497
(1971).
[5] J. Varagnat, Ind. Eng. Chem. Prod. Res. Dev 15 (1) (1976) 212–215.
[6] G.A. Hamilton, J.P. Friedman, P.M. Campbell, J. Am. Chem. Soc. 88 (1966)
5266–5268.
[7] S. Umemura, N., Takamitsu, T., Hamamoto, N. Kuroda, U.S. Patent 4,078,006
(1978).
[8] M. Taramasso, G., Perego, B. Notari, U.S. Patent 4410501 (1983).
[9] J.M. Thomas, G. Sankar, Acc. Chem. Res. 34 (2001) 571–581.
[10] A. Corma, Chem. Rev. 97 (1997) 2373–2420.
The possible reason for the higher activity of Ti-SBA-12 is
due to its larger pore size than that of Ti-SBA-16. Differences
in pore arrangement in these two titanosilicate molecular sieves
(Ti-SBA-12 has a hexagonal mesopore structure while Ti-SBA-
16 has a cage-like cubic interconnected pore structure) could be
the other reason. Under similar experimental (reaction tempera-
ture = 353 K, reaction time = 24 h, phenol/H2O2 = 3; Si/Ti = 20) while
Ti-MCM-41 and Ti-SBA-15 showed a phenol conversion of 1.5
and 7.8 mol%, respectively, Ti-SBA-12 exhibited a conversion of
24.1 mol%. Thus the three-dimensional, mesoporous titanosilicates
(Ti-SBA-12 and Ti-SBA-16) of the present study are more active than
Ti-MCM-41 and other related mesoporous titanosilicate catalysts
[10,38].
[11] A. Thangaraj, R. Kumar, P. Ratnasamy, J. Catal. 131 (1991) 294–297.
[12] A. Thangaraj, R. Kumar, P. Ratnasamy, Appl. Catal. 57 (1990) L1–L3.
[13] A. Thangaraj, R. Kumar, S.P. Mirajkar, P. Ratnasamy, J. Catal. 130 (1991) 1–8.
[14] J.S. Reddy, R. Kumar, Zeolites 12 (1992) 95–100.
[15] J.S. Reddy, S. Sivasankar, P. Ratnasamy, J. Mol. Catal. 71 (1992) 373–381.
[16] Y. Kubota, Y. Koyama, T. Yamada, S. Inagaki, T. Tatsumi, Chem. Commun. (2008)
6224–6226.
[17] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998)
6024–6036.
[18] Y. Sakamoto, I. Diaz, O. Terasaki, D. Zhao, J.P. Pariente, J.M. Kim, G.D. Stucky, J.
Phys. Chem. B 106 (2002) 3118–3123.
[19] A. Kumar, D. Srinivas, P. Ratnasamy, Chem. Commun. (2009) 6484–6486.
[20] A. Kumar, D. Srinivas, Catal. Today 198 (2012) 59–68.
[21] A. Kumar, D. Srinivas, J. Catal. 293 (2012) 126–140.
[22] C.F. Cheng, Y.C. Lin, H.H. Cheng, S.M. Liu, H.S. Sheu, Chem. Lett. 33 (2004)
262–263.
[23] Y. Sakamoto, M. Kaneda, O. Terasaki, D.Y. Zhao, J.M. Kim, G.D. Stucky, H.J. Shin,
R. Ryoo, Nature 408 (2000) 449–453.
[24] C. Yu, H. Chu, T. Wan, D. Zhao, J. Mater. Chem. 20 (2010) 4705–4714.
[25] S. Shen, Y. Deng, G. Zhu, D. Mao, Y. Wang, G. Wu, J. Li, X.Z. Liu, G. Lu, D. Zhao, J.
Mater. Sci. 42 (2007) 7057–7061.
[26] K. Morishige, N. Tateishi, S. Fukuma, J. Phys. Chem. B 107 (2003) 5177–5181.
[27] T.-W. Kim, R. Ryoo, M. Kruk, K.P. Gierszal, M. Jaroniee, S. Kamiya, O. Terasaki, J.
Phys. Chem. B 108 (2004) 11480–11489.
4. Conclusions
Hydroxylation of phenol with 30% aqueous H2O2 was investi-
gated in the presence of three-dimensional, mesoporous titanosil-
icates, Ti-SBA-12 and Ti-SBA-16, as catalysts. The catalytic activity
data of these materials were compared with those of microporous
TS-1 and mesoporous Ti-MCM-41 and Ti-SBA-15. Although both
Ti-SBA-12 and Ti-SBA-16 are highly active, their intrinsic catalytic
activityis lower than that of TS-1. But they are relativelymore active
than Ti-MCM-41 and Ti-SBA-15. Catechol and hydroquinone are the
only products of phenol hydroxylation. The para-selective hydrox-
ylation was preferred more on Ti-SBA-12 to TS-1 and Ti-SBA-16.
The mesoporous Ti catalysts of this work are reusable with
little loss in activity and product selectivity even after the
fifth recycle. Isolated framework Ti with tetrahedral geom-
etry and three-dimensional, ordered mesoporosity are the
unique characteristic features responsible for the highly effi-
cient phenol hydroxylation activity of Ti-SBA-12 and Ti-SBA-16
catalysts.
[28] K. Chaudhari, R. Bal, T. Kr Das, A. Chandwadkar, D. Srinivas, S. Sivasanker, J.
Phys. Chem. B 104 (2000) 11066–11074.
[29] P. Ratnasamy, D. Srinivas, H. Knözinger, Adv. Catal. 48 (2004) 1–169.
[30] R. Millini, M.E. Previde, G. Perego, G. Bellussi, J. Catal. 137 (1992) 497–503.
[31] M.A. Camblor, A. Corma, J.P. Periante, Zeolites 13 (1993) 82–87.
[32] U. Wilkenhöner, G. Langhendries, F.V. Laar, G.V. Baron, D.W. Gammon, P.A.
Jacobs, E.V. Steen, J. Catal. 203 (2001) 201–212.
[33] A. Tuel, S. Moussa-Khouzami, Y. Ben Taarit, C. Naccache, J. Mol. Catal. 68 (1991)
45–52.
[34] M.G. Clerici, R.A. Sheldon, H. van Bekkum (Eds.), Fine Chemicals Through Het-
erogeneous Catalysis, Wiley-VCH, Weinheim, 2001, p. 538.
[35] A. Tuel, Y. Ben Taarit, Appl. Catal. 102 (1993) 69–77.
[36] O.J. Kerton, P. McMorn, D. Bethell, F. King, F. Hancock, A. Burrows, C.J. Kiely, S.
Ellwood, G. Hutchings, Phys. Chem. Chem. Phys. 7 (2005) 2671–2678.
[37] S.-T. Tsai, P.-Y. Chao, T.-C. Tsai, I. Wang, X. Liu, X.-W. Guo, Catal. Today 148
(2009) 174–178.
[38] S. Song, W. Zhao, L. Wang, J. Chu, J. Qu, S. Li, L. Wang, T., T. Qi, J. Colloid Interface
Sci. 354 (2001) 686–690.