Catalytic oxidation of cyclic ethers to lactones over various titanosilicates

Article abstract of DOI:10.1016/j.molcata.2011.02.003

Various crystalline microporous metallosilicates have been used in the liquid phase catalytic oxidation of different cyclic ethers into their corresponding lactones in the presence of dilute aqueous H₂O ₂ as oxidant. Among the various metallosilicates studied for the oxidation of tetrahydrofuran to γ-butyrolactone, titanosilicates exhibited the best activity than the other metallosilicates such as chromium silicalite-1 (CrS-1), chromium silicalite-2 (CrS-2) and vanadium silicalite-1 (VS-1). The intrinsic activity of TS-1 was found to be marginally higher than the other titanosilicates. Cyclic ethers undergo α_C-H oxidation to give the corresponding lactones; whereas open-chain ether produce carboxylic acids by initial α_C-H bond oxidation to give ester as an intermediate product, which further undergoes cleavage of -O- linkage to give the final carboxylic acids. The conversion of substituted tetrahydrofuran is decreased with number of -CH₃ groups at α- and/or β-position. The lactone formation is hindered when both the α-positions are substituted with methyl substituents. Mechanistically, titanium hydroperoxo complex formed in the titanosilicate/H₂O₂/H₂O system is believed to oxidize the α_C-H bond of ethers producing the respective α-hydroxylated product, which undergoes further oxidation to give the lactones (for cyclic ethers) or carboxylic acids (for open-chain ethers).

Full text of DOI:10.1016/j.molcata.2011.02.003

Journal of Molecular Catalysis A: Chemical 338 (2011) 105–110
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Catalytic oxidation of cyclic ethers to lactones over various titanosilicates
Manickam Sasidharana, Asim Bhaumikb,∗
a Department of Chemistry, Faculty of Science and Engineering, Saga University, 1 Honjo-machi, Saga 840-8502, Japan
b Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Various crystalline microporous metallosilicates have been used in the liquid phase catalytic oxidation
of different cyclic ethers into their corresponding lactones in the presence of dilute aqueous H2O2 as oxi-
dant. Among the various metallosilicates studied for the oxidation of tetrahydrofuran to ␥-butyrolactone,
titanosilicates exhibited the best activity than the other metallosilicates such as chromium silicalite-1
(CrS-1), chromium silicalite-2 (CrS-2) and vanadium silicalite-1 (VS-1). The intrinsic activity of TS-1 was
found to be marginally higher than the other titanosilicates. Cyclic ethers undergo ␣C–H oxidation to
give the corresponding lactones; whereas open-chain ether produce carboxylic acids by initial ␣C–H bond
oxidation to give ester as an intermediate product, which further undergoes cleavage of –O– linkage to
give the final carboxylic acids. The conversion of substituted tetrahydrofuran is decreased with number
of –CH3 groups at ␣- and/or ␤-position. The lactone formation is hindered when both the ␣-positions
are substituted with methyl substituents. Mechanistically, titanium hydroperoxo complex formed in the
titanosilicate/H2O2/H2O system is believed to oxidize the ␣C–H bond of ethers producing the respective
␣-hydroxylated product, which undergoes further oxidation to give the lactones (for cyclic ethers) or
carboxylic acids (for open-chain ethers).
Received 25 October 2010
Received in revised form 17 January 2011
Accepted 6 February 2011
Available online 12 February 2011
Keywords:
TS-1
Ether oxidation
Lactones
Tetrahydrofuran
Liquid phase oxidation catalysis
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
[5], Ti-MWW [6] as well as mesoporous Ti-MCM-41 [7], Ti-HPVS-
1 [8] and amorphous extra large pore Ti-DMS materials [9] have
Selective oxidation is one of the key reactions in organic
chemistry for the preparation of various intermediates and fine
chemicals. However, most of these reactions catalyzed by homo-
geneous catalysts are stoichiometric and non-recyclable. Thus, in
the fine chemical synthesis, there is a significant trend towards sto-
ichiometric inorganic oxidants such as chromium and manganese
compounds with a direct catalytic method using heterogeneous
and environment-friendly catalysts, which are non-stoichiometric
and recyclable. The discovery of TS-1 in 1983 [1] through iso-
morphous substitution of silicon by titanium in microporous MFI
framework and its unique catalytic properties especially in selec-
tive oxidation reactions lead to industrial resurgence such as in the
production of catechol and hydroquinone from phenol, cyclohex-
anone oxime from cyclohexanone using dilute aqueous H2O2 as
oxidant. The advantage of high activity coupled with reusability,
no byproduct formation and the ease of separation of the catalyst
from reaction mixture, justified the use of TS-1 as the potential
heterogeneous oxidation catalyst. In light of these developments, a
variety of new titanium containing microporous crystalline materi-
als such as Ti-ZSM-11 (TS-2) [2], Ti-ZSM-48 [3], Ti-␤ [4], Ti-MCM-22
been successfully synthesized and studied for various oxidative
transformations under liquid phase conditions. The advantage of
transition metal containing microporous and mesoporous mate-
rials over homogeneous catalysts have already been successfully
demonstrated for the epoxidation of alkenes, hydroxylation of phe-
nol, ammoximation of ketones, and oxidation of alcohols, amines,
sulfides, etc. using dilute H2O2 under eco-friendly liquid-phase
conditions [10,11]. The oxidation of C–H bond in alkanes and ben-
zene is an important area of research due to large scale industrial
production of fine chemicals from cheaply available hydrocarbon
feed stocks [12]. However, not much research has been carried out
on the oxidation of C–H bond in cyclic as well as open-chain ethers
using heterogeneous catalysts under liquid-phase conditions.
The selective oxidative functionalization at the ␣C–H bonds of
ethers is one of the most useful reactions in organic synthesis since
it serves for efficient preparation of esters or lactones in the case of
cyclic ethers [13]. Furthermore, the ␣-methylene-␥-butyrolactone
structural unit is present in a wide variety of sesquiterpenes and
other natural products, and has been suggested to be of cen-
tral importance for the biological activities of those compounds
[14]. These oxidation reactions are usually accomplished by the
use of either stoichiometric amounts of chromium trioxide, lead
tetraacetate, or ruthenium tetroxide as oxidant [15] or the catalytic
amounts of RuO4 in the presence of hypochlorite or periodate [16]
∗
Corresponding author.
E-mail address: msab@iacs.res.in (A. Bhaumik).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.02.003

106
M. Sasidharan, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 338 (2011) 105–110
Table 1
Physico-chemical properties of various titano- and metallo-silicates.
Entry
Catalysts
Si/M ratioa
Input gel
Particle sizeb
Micropore volume (mL g−1
)
Final product
1
2
3
4c
5
6
7
8
TS-1
TS-2
Ti-␤ (OH)
Ti-␤ (F)
Ti-MCM-22
VS-1
40
40
40
35.0
45
60
60
60
41.1
59.0
43.0
48.3
51.0
96.0
85.2
72.1
0.1–0.2 ␮m
2–3 ␮m
0.2–0.3 ␮m
3–4 ␮m
0.2–0.5 ␮m
0.4–0.6 ␮m
0.3–0.5 ␮m
1–2 ␮m
0.14
0.12
0.28
0.22
0.26
0.13
0.12
0.11
CrS-1
CrS-2
a
Si/M ratio was estimated from ICP analysis.
Particle size measurements were made from SEM analysis.
Ti-␤ was synthesized under the fluoride medium.
b
c
under homogeneous conditions. Recently, several new oxidation
systems have been described using transition metal complexes of
Co, Pd, etc. as catalysts for the transformation of ethers to esters
[17]. Although a number of different reagents have been discov-
ered for the selective oxidation of ethers, only few have assumed
any synthetic and/or industrial importance because of the expense
of the catalysts, high acidity of the media, and poor selectivity in
terms of product distribution. The use of heterogeneous catalysts
in the liquid-phase, on the other hand, offers several advantages
compared with their homogeneous counterparts [18]. In this con-
text Sn-beta/H2O2 [19] and Sn-MCM-41/H2O2 [20] systems are
successfully employed for the chemoselective Baeyer-Villiger oxi-
dation of cyclohexanone to lactone. We have reported earlier, the
facile catalytic oxidation of ethers with dilute H2O2 over TS-1 [21].
Herein, we report the detailed investigation of oxidation of cyclic
ethers over various titanosilicates such as TS-1, TS-2, Ti-MCM-22,
and Ti-␤. The activity of these titanosilicates has been compared
with other metallosilicates such as VS-1, CrS-1, and CrS-2 under
similar experimental conditions. The activity and product selec-
tivity has also been investigated with respect to substituent effect,
substrate/H2O2 molar ratio and reaction time. The reusability of dif-
ferent titanosilicates after reactivation has also been investigated.
ous, Aldrich) followed by the addition of Aerosil-200 silica and 3%
dealuminated seed crystals and the stirring was continued for an
extended period to obtain a homogeneous gel. The resulting gel
(chemical composition: SiO2:0.025 TiO2:0.337 H2O2:5 TEAOH:6.62
H2O) was transferred into a Teflon-lined autoclave, and the crys-
tallization was carried out at 413 K for 5 days. The as-synthesized
materials were dried at 373 K and calcined in O2 flow for 12 h
at 793 K. The Ti-␤(F) was synthesized by using HF (47% aque-
ous) as mineralizer in a gel composition of SiO2:0.029 TiO2:0.337
H2O2:0.55 TEAOH:0.55 HF:5.5 H2O [10]. The gel was crystallized
in a Teflon-lined autoclave at 413 K for 5 days. The as-synthesized
materials were dried and calcined in O2 slowly for 12 h at 793 K.
2.2. Synthesis of Ti-MCM-22
The Ti-MCM-22 was synthesized according to the reported
procedure [5] from fumed silica (Cab-o-sil M7D), tetrabutyl orthoti-
tanate (Ti(Obut)4), boric acid, and piperidine (TCI, 98%). Piperidine
was dissolved in deionized water and divided into two portions,
and the required amount of Ti(Obut)4 or boric acid was added to
each piperidine solution under vigorous stirring. Further, silica was
also divided into two equal parts and added to the solution con-
taining Ti and B, respectively. The gels were stirred for 1 h and then
mixed together. The combined gel was stirred for another 1.5 h to
obtain a gel with a molar gel composition of SiO2:0.02 TiO2:0.67
B2O3:1.4 piperidine:19 H2O. The crystallization of the resulting gel
was carried out in a stirring autoclave at 403 and 423 K each for 1
day and further at 443 K for 5 days. After crystallization, the solid
product was washed, dried, and the framework boron and extra-
framework titanium were removed by acid treatment with 2 M
HNO3 at 373 K for 20 h. The samples were calcined at 803 K to burn
off the remaining organic species.
2. Experimental
2.1. Synthesis of TS-1, TS-2 and Ti-ˇ
TS-1 was synthesized by modifying the standard literature pro-
cedure [22,23]. In a typical synthesis 54.2 g tetraethyl orthosilicate
(98%, TCI) was hydrolyzed with 90 g of tetrapropylammonium
hydroxide (20% aqueous, TCI) with vigorous stirring for 3 h. Then
2.12 g tetrabutyl orthotitanate (98%, Aldrich) dissolved in dry iso-
propanol was added drop wise to clear homogeneous solution and
the stirring was continued for an extended period of time at 323 K
in order to remove the ethanol formed from the hydrolysis of TEOS.
Finally, 32 mL of water was added (gel composition: SiO2:0.025
TiO2:35 TPAOH:H2O) and the mixture was allowed to crystallize
in a rotating autoclave at 443 K for 24 h. After crystallization, the
product was centrifuged and washed several times to remove the
occluded organic template. Then the solid was dried and calcined
at 773 K for 12 h in the presence of air. For the synthesis of the cata-
lyst TS-2 TPAOH was replaced by tetrabutylammonium hydroxide
(TBAOH, 40%, Aldrich). Synthesis gel was prepared keeping the gel
composition: SiO:0.025 TiO2:0.35 TBAOH:33.3 H2O, followed by
hydrothermal treatment under static condition at 443 K for 48 h
[2].
2.3. Synthesis of other metallosilicates
The hydrothermal synthesis of CrS-1 [24] was carried out using
the gel composition: SiO2:0.017 Cr2O3:0.35 TPAOH:33.3 H2O. In a
typical synthesis, required amount of TEOS was added to an ice-cold
solution of TPAOH and after complete hydrolysis Cr(NO3)3·9H2O
was added into the mixture. After evaporation of the ethanol pro-
duced through 4 h stirring at 323 K, the clear solution obtained was
crystallized at 443 K under tumbling for 24 h. Similarly, CrS-2 was
prepared using tetrabutylammonium hydroxide according to the
reported procedures [25–28]. V-MFI was synthesized according to
the procedure reported by Carati et al. [29]. VOSO4 × H2O (Aldrich,
99.9%) was used as the source of vanadium. In a typical synthe-
sis, 21.0 g of TEOS in 20 mL of water was hydrolyzed with 41.0 g
of tetrapropyl ammonium hydroxide (20% aqueous solution) for
3 h. Then, 0.45 of vanadyl sulfate was added, and the stirring was
continued for another 3 h under the flow of helium gas in order to
The Ti-␤ was synthesized in a basic medium by modifying
the reported procedure [4]. Titaniumperoxo complex was first
prepared by the addition of Ti(OBut)4 and H2O2 under vigorous
stirring. This complex was then mixed with TEAOH (35% aque-

M. Sasidharan, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 338 (2011) 105–110
107
6 and 7) exhibit poor activity for the oxidation of C–H bond in
tetrahydrofuran, even though it showed better activity for other
functional groups oxidation [30–32]. Similarly, VS-1 also realized
lower activity under identical conditions (entry 8).
H2O2
TS-1
O
O
O
TS-1 / H2O2
In order to compare the lactone selectivity at similar conver-
sion levels as that of Cr- and V-silicates, additional experiments
were performed by reducing the reaction time over the titanosil-
icates; thus TS-1, TS-2, Ti-MCM-22, and Ti-␤ exhibited 85.7, 83.8,
86.0 and 88% selectivities at the conversion of 10.5, 8.3, 7.2, and
9.0%, respectively. The above results suggest that selectivity for
␥-butyrolactone is almost similar for titanium and chromium sili-
cates; whereas VS-1 gives slightly more side-products. In the case
of Cr- and V-silicates, in order to obtain information on the low
reactivity as well as to find out leaching of metal atoms, the cata-
lyst was filtered off, organics were removed through evaporation,
and the residual solution was analyzed by using an AAS. Leach-
ing of chromium was found to be 28 and 36% for CrS-1 and CrS-2,
respectively; whereas that for vanadium in VS-1 showed 33% on
extraction. However, neither vanadium nor chromium exhibited
the oxidation of tetrahydrofuran under homogeneous conditions.
In addition, there is not much difference in selectivity of lactone at
similar conversion levels of tetrahydrofuran for different titanosili-
cates: In view of these above observations titanosilicates have been
chosen for further investigations.
CO2H
O
Scheme 1. Catalytic oxidation of cyclic and open chain ethers over titanosilicates.
suppress the oxidation of V4+ to V5+ at high pH. Finally, 30 mL of
water added and the clear homogeneous solution was subjected
to crystallization at 443 K for 48 h. The white solid was washed
with distilled water, dried at 373 K, and calcined at 773 K under the
flow of O2 through slow increase of the oven temperature. All the
metallosilicates were thoroughly characterized through XRD, FT-
IR, UV–Vis, SEM, sorption and surface area measurements and the
composition by ICP analysis. Table 1 shows the important physico-
chemical properties of different metallosilicates investigated under
present study.
2.4. General liquid phase catalytic oxidations
The liquid-phase catalytic reaction was performed in a two-
necked glass reactor fitted with a water condenser at 343 K under
vigorous stirring. A typical reaction involves 10 mmol of substrate,
10 mmol of H2O2, and catalyst TS-1 (Si/Ti = 41), 20 wt% with respect
to the substrate. The reaction was carried out in the absence of any
cosolvents. Cycloheptanonewas used as internal standardforquan-
tification of products. The progress of the reaction was monitored
by analyzing the products obtained at different intervals through
a capillary gas chromatograph (Shimadzu 14A, OV-1 columns with
flame ionization detectors). The products were identified by GC–MS
splitting pattern as well as from authentic samples and in some
cases the mixture is distilled off to obtain pure products and ana-
lyzed through GC–MS and 1H NMR.
3.2. Oxidation of various cyclic- and open-chain ethers over TS-1
and Ti-ˇ
The results of the oxidation of various cyclic and open-chain
ethers over both TS-1/H2O2 and Ti-␤/H2O2 catalytic systems are
presented in Table 3. Entries 1–4 demonstrate the oxidation of
tetrahydrofuran and substituted tetrahydrofurans to the corre-
sponding lactones. The intrinsic activity of TS-1 is slightly higher
than that of Ti-␤ (entry 1) for simple unsubstituted tetrahydrofu-
ran. However, the activity of TS-1 is influenced by the bulkiness
of tetrahydrofuran derivatives and it decreased drastically when
a methyl group is located on one of the ␣-position (entry 2). The
marginal difference in reactivity between TS-1 and Ti-␤ indicates
that, the effect of methyl substitutent at the ␣-position is com-
paratively more pronounced for the former due to geometrical
constrains to approach to the metalperoxide or reaction sites. Fur-
thermore, the presence of two methyl groups at ␣, ␣ꢀ-position of
tetrahydrofuran (entry 3) not only leads to lower reactivity but
also reduced the selectivity presumably due to steric hindrance
as well as difficulty associated with the cleavage of C–C bond to
produce the corresponding lactone. Thus the observed results over
the titanosilicates are unique in the sense that the reactivity of the
C–H bond at tertiary carbon is supposed to be higher than the sec-
ondary and/or primary one for homogeneously catalyzed reactions
[11]. Therefore, the decreased reactivity of ␣-methyl substituted
tetrahydrofuran may be due to the inductive effect and steric hin-
drance of these molecules exerted at the active-sites located in the
porous framework. The oxidation of tetrahydrofuran is negligible
for both TS-1 and Ti-␤ when all available ␣-positions are substi-
tuted with methyl groups (entry 4) and it is obvious that the lack
of ␣ C–H bond results in no lactone formation.
3. Results and discussion
3.1. Activity of various titano- and metallosilicates
Table 2 shows the activity of various metallosilicates in the
oxidation of tetrahydrofuran to ␥-butyrolactone (Scheme 1). The
medium-pore TS-1 exhibits (entry 1) comparatively higher activ-
ity over the large-pore Ti-␤. TS-2 and Ti-MCM-22 (entries 2 and
3) show moderately lower activities than that of TS-1. This could
be attributed to the higher Si/Ti ratio coupled with larger crystal-
size of the respective catalysts rather than diffusion limitation,
which is almost negligible for molecule with small dimension
such as tetrahydrofuran. Although both TS-1 (Si/Ti = 41.1) and Ti-
␤(OH) (Si/Ti = 43) have similar titanium content and crystal sizes of
0.1–0.2 ␮m and 0.2–0.3 ␮m, respectively, the former exhibits more
activity than the later (entry 4). On first glance it was assumed that,
unlike TS-1, the structure of Ti-␤ itself slightly hydrophilic in nature
even in the complete absence of aluminum, which leads to compet-
itive adsorption of both reactant as well as H2O inside the pores and
channels, and eventually decreased the activity of Ti-␤. However,
Ti-␤(F) synthesized in the fluoride medium (entry 5), supposed
to be the better hydrophobic catalyst, realized still lower activity
(40.3%) presumably due to its lower titanium content coupled with
larger particle-size. Moreover, Ti-␤(OH) exhibited higher TON than
the Ti-␤(F) and therefore, the hydrophobicity cannot be the major
determining factor of the activity between TS-1 and Ti-␤(OH).
Nonetheless, the turn over frequencies of different titanosilicate
changes within a range (4.3–5.2) and the intrinsic activity of TS-1
are comparatively higher than the other titanosilicates investi-
gated. Loading of chromium in silicalite-1 and silicalite-2 (entries
Entry
5 shows the oxidation of tetrahydropyran to ␦-
valerolactone, and again TS-1 is slightly more active than Ti-␤ as
observed in the case of unsubstituted tetrahydrofuran. For sub-
stituted tetrahydropyrans, the presence of methyl group at the
␤-position (entry 6) also decreases the reactivity. The oxidation of
dihydropyran (entry 7) leads to a mixture of ␦-valerolactone and
epoxy products through epoxidation of –C C– double bond. Thus in
the presence of –C C– double bond, the ␣C–H of ethers seems to be
less reactive and subsequently leads to decreased lactone selectiv-
ity. Therefore, the conversion of ethers to lactones lacks functional

108
M. Sasidharan, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 338 (2011) 105–110
Table 2
Activity of various metallosilicates in the oxidation of tetrahydrfuran.a
Entry
Catalyst
Conv. (mol%)
H2O2 sel.%
TOFb
Product selectivities, %
␥-Butyrolactone
␣-Hydroxy tetrahydrofuran
Othersc
1
2
3
4
5d
6
7
8
TS-1
TS-2
Ti-MCM-22
Ti-␤(OH)
Ti-␤(F)
CrS-1
49.0
32.3
37.5
45.2
40.3
8.0
99.0
87.0
93.0
97.0
85.9
68.0
62.0
7.0
5.2
4.3
4.5
4.7
4.6
1.8
1.3
1.6
91.0
90.1
89.0
90.4
91.3
89.0
87.3
78.0
3.6
4.0
4.8
4.9
4.8
2.8
3.9
4.9
5.4
5.9
6.2
4.7
3.9
8.1
8.8
17.0
CrS-2
VS-1
6.9
7.1
a
Reaction conditions: 10 mmol tetrahydrofuran, 10 mmol H2O2 (31 wt% aqueous), 20 wt% of catalyst with respect to the substrate, temperature 343 K, reaction time 12 h.
Turn over frequency = number of moles of reactant converted per mole of metal per hour.
Mixture of products from the oxidation of C–H bonds at the ␤-position.
b
c
d
Ti-␤ was synthesized under the fluoride medium.
group selectivity, unlike the recently reported Sn-beta/H2O2 cat-
age in open-chain esters undergoes cleavage after the oxidation of
␣ C–H bond to give carboxylic acids as the major product. Similarly,
the oxidation of unsymmetrical benzyl methyl ether (Table 3, entry
9) leads to benzoic acid as the major product and this result is in
contrast to the homogeneously catalyzed oxidation over ruthenium
tetroxide, where open-chain ethers containing at least one pri-
alytic system, which serves as an effective chemoselective catalyst
[19]. Furthermore, open-chain ethers produce carboxylic acids as
the major product. Entry 8 exhibits the oxidation of open-chain
symmetrical dibutylether to butyric acid, and TS-1 is more active
than Ti-␤. The observed results clearly indicate that the –O– link-
Table 3
Oxidation of various cyclic- and open-chain ethers over TS-1 and Ti-␤.a
Entry
1
Substrate
Catalyst
Conversion (mol%)
H2O2 conversion (mol %)
Lactone sel., %
91.0
Othersb
TS-1
Ti-␤
49.1
44.6
99.0
97.0
9.0
10.0
90.0
91.9
87.0
–
O
TS-1
Ti-␤
TS-1
Ti-␤
TS-1
Ti-␤
TS-1
Ti-␤
TS-1
Ti-␤
TS-1
Ti-␤
TS-1
Ti-␤
TS-1
Ti-␤
18.2
26.0
6.0
64.0
79.0
49.0
62.0
30.0
49.0
93.0
96.5
65.0
72.5
78.0
72.9
98.0
99.0
95.0
99.0
8.1
13.0
100
2
3
4
5
6
7
8
O
CH3
H3C
CH3
O
O
9.1
–
100
1.0
–
100
H3C
H3C
CH3
CH3
2.3
–
100
36.0
32.5
22.0
29.7
38.9
41.2
48.0
42.5
39.0
46.8
89.9
88.1
86.0
84.4
51.0
47.5
–
10.1
11.9
14.0
15.6
49c
O
CH3
O
O
52.5c
100d
100d
100d
100d
O
–
–
O
9
–
a
Reaction conditions: 10 mmol substrate, 10 mmol H2O2 (31 wt% aqueous), 20 wt% of catalyst (TS-1 or Ti-␤) with respect to the substrate, temperature 343 K, reaction
time 12 h.
b
Mixture of ␣-hydroxylated ethers and other C–H oxidation products.
Mostly epoxidation products.
Carboxylic acids.
c
d

M. Sasidharan, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 338 (2011) 105–110
109
100
90
80
70
60
50
40
30
20
10
0
Conversion
Lactone
α-hydroxy
tetrahydrofuran
Others
H2O2 selectivity
0
2
4
6
8
10
12
Reaction Time, h
Fig. 1. Profiles for the product distribution with time in the oxidation of tetrahy-
drofuran over TS-1/H2O2.
Fig. 2. The activity of various titanosilicates in three successive liquid phase catalytic
recycles for the oxidation of tetrahydrofuran.
mary alkyl group can be oxidized to the corresponding carboxylic
esters [15]. In the case of oxidation of bulky benzyl methyl ether
Ti-␤ exhibits better activity than TS-1, probably the diffusion of
reactants may play a dominant role in controlling the activity of
medium-pore TS-1. The observed results of various cyclic ethers
suggest that TS-1 is more active and selective catalyst for less bulky
substrates, whereas Ti-␤ shows better activity for bulky molecules
due to the availability of large void-space around the active Ti sites.
number of recycles and TS-1 showed best activity after three suc-
cessive reuses (91.3%). Other medium-pore titanosilicates, TS-2 as
well as Ti-MCM-22 also shows good activity of 89.0 and 86.0%,
respectively. However, the activity of large-pore Ti-␤ was reduced
by 31.5% on third reaction cycle under similar experimental condi-
tions. The carbon content of used Ti-␤ estimated by CHN analysis
was found to be 1.9%, which was also highest among the titanosil-
icates studied here. Furthermore, the chemical analysis of the
solution obtained after dissolution of the framework indicated the
presence of mainly lactone, ␣-hydroxy tetrahydrofuran and small
amount of higher molecular weight products in addition to Ti, Si
and O. The decrease in activity may be due to the partial leaching
of framework titanium by interaction of these molecules as well as
action of H2O2 on the TiIV active sites [33–36]. Observed Si/Ti ratios
of TS-1, TS-2, Ti-MCM-22, and Ti-␤ increased to 45.3, 64.0, 58.5, and
64.0, respectively, after three successive reuses.
3.3. Effect of substrate to H2O2 molar ratio and reaction time
The oxidation of tetrahydrofuran has been carried out over
TS-1 at different conversion levels by changing the amount of
oxidant in order to get more information on various products dis-
tribution. Table 4 shows the product distribution and as expected,
the conversion increases with the amount of H2O2 (entries 1–5).
The observed results suggest that at low substrate to H2O2 mole
ratio, the intermediate ␣-hydroxy tetrahydrofuran is formed in
appreciable amounts (8.1%, entry 1). However, the concentration
of ␣-hydroxy tetrahydrofuran decreases (due to further oxidation
to ␥-butyrolactone) to certain extent with increase in substrate to
H2O2 molar ratio. Moreover, the lactone selectivity is not changed
significantly for up to equimolar ratio of substrate/H2O2. However,
use of H2O2/substrate molar ratio of 2 (entry 5), conversion level
increased but the selectivity for lactone decreased to 82.5%. This
observation indicates that the titanosilicate/H2O2 system is some-
what less efficient to that of the Sn-beta/H2O2 catalytic system,
where the ketone is selectively converted to the corresponding lac-
tone with high selectivity in the range of 99% with >95% conversion.
Fig. 1 displays the effect of reaction time on conversion and
selectivity of the various products formed towards the oxidation of
tetrahydrofuran over TS-1/H2O2 system. The conversion increases
with time as expected, and the selectivity of lactone increases from
85.7 to 91.0% from 2 h to 12 h. The selectivity for ␥-butyrolactone
increases mainly at the expense of ␣-hydroxy tetrahydrofuran,
which is further oxidized to give more stable lactone as the main
product. In the beginning, ␣-hydroxy tetrahydrofuran is formed in
considerable amounts as observed in the case of low H2O2 con-
centration. However, the selectivity of side products also increases
during the course of reaction, accounting for the oxidation of C–H
bond at different positions on the tetrahydrofuran ring.
3.5. Reaction pathways
The product distribution for the oxidation of tetrahydro-
furan over titanosilicates with respect to reaction time and
substrate/H2O2 molar ratio suggests that ␥-bytyrolactone is prob-
ably formed via ␣-hydroxy tetrahydrofuran. Fig. 1 revealed the
existence of an appreciable amount of the intermediate ␣-hydroxy
ether, which decreased gradually as the reaction is progressed. In
Scheme 2 we have proposed possible reaction mechanism involved
in this oxidation process. Here, the reaction involves the initial
oxidation of activated C–H bonds of the –CH2 groups ␣ to –O–
group to give ␣-hydroxy ethers. This is followed by further attack of
titaniumhydroperoxo species (I) to the C–H bond of the ␣-carbon
atom of the ␣-hydroxy ethers leading to the oxidation to lac-
tones in the case of cyclic ether. Further oxidation of ␣-hydroxy
tetrahydrofuran (Scheme 2, II) is favored due to the presence of
more activated C–H bond compared to tetrahydrofuran leading
to more stable lactone, ␥-butyrolactone. On the other hand for
open-chain ethers like dibutylether, ␣-hydroxy ethers formed at
the initial stage undergoes further oxidation to ester in the pres-
ence of titanium hydroperoxo species. This would be followed by
hydrolysis and oxidation to yield butyric acid. Formation of ben-
zoic acid as the major product in the oxidation of benzyl methyl
ether (Table 3, entry 9) supports the initial oxidation of methylene
carbon and followed by oxidative cleavage mechanism as pro-
posed in Scheme 2. However, one cannot completely ignore the
possibility of radical oxidations with hydrogen peroxide, where a
metal-hydroperoxide [32–35] appears as an intermediate, which
easily dehydrates to carbonyl compound by heating during either
under the reaction conditions or purification processes such as dis-
tillation and/or estimation through GC. However, similar to other
3.4. Reusability of various titanosilicates
The activity of various used titanosilicates for the oxidation of
tetrahydrofuran is shown in Fig. 2. The titanosilicates were reac-
tivated at 623 K for 5 h under the air and reused again under
the similar experimental conditions using equimolar amounts of
substrate/H2O2. The activity of the catalysts decreases with the

110
M. Sasidharan, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 338 (2011) 105–110
Table 4
Effect of substrate to H2O2 molar ratio over TS-1.a
Entry
Substrate/H2O2 mole ratio
Conversion (mol%)
H2O2 conversion, %
Product selectivities (%)
␥-Butyrolactone
␣-Hydroxy tetrahydrofuran
Others
1
2
3
4
5
4
3.3
2
1
0.5
10.9
14.6
27.3
49.1
61.5
59.0
68.5
81.0
99.0
96.0
88.0
91.5
90.6
91.0
82.5
8.1
5.4
4.5
3.6
5.5
2.8
3.6
4.9
5.4
12.0
a
Reaction conditions: 10 mmol tetrahydrofuran, 20 wt% of catalyst (TS-1, Si/Ti = 42.1) with respect to the substrate, temperature 343 K, reaction time 12 h.
tetrahydrofurans, whereas tetra-␣, ␣, ␣ꢀ, ␣ꢀ-substituted compound
exhibited no activity due to lack of ␣ C–H bond. Dihydropyran con-
taining olefinic functions produced a large amount of side products
via epoxidation of –C C– bond in addition to ␣ C–H oxidation of
ether. The distribution of products with reaction time and H2O2
concentration revealed ␣-hydroxy ethers are the initial reaction
product, which is transformed to lactone as the final product.
The oxidation of both symmetrical and unsymmetrical open-chain
ethers produced carboxylic acids as the major product through
oxidative cleavage. Further, TS-1 showed excellent recycling effi-
ciency than the other microporous titanosilicates.
References
[1] M. Taramasso, G. Perego, B. Notari, U.S. Patent 4,410,501, 1983.
[2] J.S. Reddy, R. Kumar, P. Ratnasamy, Appl. Catal. 58 (1990) L1.
[3] A. Tuel, Y.B. Taarit, Zeolites 15 (1995) 164.
[4] T. Blasco, M.A. Camblor, A. Corma, P. Esteve, J.M. Guil, A. Martinez, J.A. Perdigon-
Melon, S. Valencia, J. Phys. Chem. B 102 (1998) 75.
[5] W. Peng, T. Tatsumi, T. Komatsu, T. Yashima, Chem. Lett. (2000) 774.
[6] W. Fan, Y. Kubota, T. Tatsumi, ChemSusChem 1 (2008) 175–178.
[7] P.T. Tenev, M. Chibwe, T.J. Pinnavaia, Nature 368 (1994) 321–324.
[8] D. Chandra, S.C. Laha, A. Bhaumik, Appl. Catal. A: Gen. 342 (2008) 29–34.
[9] A. Bhaumik, S. Samanta, N.K. Mal, Micropor. Mesopor. Mater. 68 (2004) 29–35.
[10] M. Sasidharan, A. Bhaumik, J. Mol. Catal. A: Chem. 328 (2010) 60–67.
[11] S.L. Jain, B.S. Rana, B. Singh, A.K. Sinha, A. Bhaumik, M. Nandi, B. Sain, Green
Chem. 12 (2010) 374–377.
[12] R.A. Sheldon, R.S. Downing, Appl. Catal. A: Gen. 189 (1999) 163.
[13] P. Muller, in: S. Patai (Ed.), The Chemistry of Ethers, Crown Ethers, Hydroxyl
Groups and Their Sulphur Analogues, Wiley, New York, 1980, Suppl. E, p. 469.
[14] S.M. Kupchan, M.A. Eakin, A.M. Thomas, J. Med. Chem. 14 (1971) 1147.
[15] M. Kaname, S. Yoshifuji, H. Sashida, Heterocycles 79 (2009) 647–658.
[16] P.H.J. Carlsen, T. Katsuki, V.S. Martin, K.B. Sharpless, J. Org. Chem. 46 (1981)
3936.
Scheme 2. Proposed mechanistic pathways for the oxidation of ethers over
titanosilicate/H2O2 system.
[17] J. Muzart, J. Mol. Catal. A: Chem. 319 (2010) 1–29.
[18] R.A. Sheldon, J. Dakka, Catal. Today 19 (1994) 215.
oxidation reactions catalyzed by titanium silicates, it is highly likely
that, the oxidation of ethers to lactones would also be facilitated by
titanium-hydroperoxo complexes formed by the interaction of TiIV
species with H2O2 [33]. Furthermore, since titanium-hydroperoxo
complexes being mild Brönsted acidic, could promote the cleavage
of –O– linkage to produce carboxylic acid in the case of open-chain
ethers. A similar mechanism would be applicable for the oxida-
tion over the other metallosilicates/H2O2 catalytic system under
investigation.
[19] A. Corma, L.T. Nemeth, M. Renz, S. Valencia, Nature 412 (2001) 423.
[20] M. Sasidharan, Y. Kiyozumi, N.K. Mal, M. Paul, P.R. Rajamohanan, A. Bhaumik,
Micropor. Mesopor. Mater. 126 (2009) 234–244.
[21] M. Sasidharan, S. Suresh, A. Sudalai, Tetrahedron Lett. 36 (1995) 9071.
[22] A. Thangaraj, R. Kumar, S.P. Mirajkar, P. Ratnasamy, J. Catal. 130 (1990) 1.
[23] G. Belllussi, M.S. Rigutto, Stud. Surf. Sci. Catal. 85 (1994) 177.
[24] J.S.T. Mambim, H.O. Pastore, C.V. Davanzo, E.J.S. Vichi, O. Nakamura, H. Vargas,
Chem. Mater. 5 (1993) 166.
[25] B. Jayachandran, M. Sasidharan, A. Sudalai, T. Ravindranathan, J. Chem. Soc.
Chem. Commun. (1995) 1523.
[26] J. Reni, M. Sasidharan, A. Sudalai, T. Ravindranathan, J. Chem. Soc. Chem. Com-
mun. (1995) 1341.
[27] N.V. Barhate, M. Sasidharan, A. Sudalai, R.D. Wakharkar, Tetrahedron Lett.
(1996) 707.
4. Conclusions
[28] M. Sasidharan, P. Wu, T. Tatsumi, J. Catal. 205 (2002) 332.
[29] A. Carati, C. Flego, E.P. Massara, R. Millini, L. Carluccio, W.O. Parker Jr., G. Bellussi,
Micropor. Mesopor. Mater. 130 (1999) 137.
[30] L.J. Davis, P. McMorn, D. Bethell, P.C. Bulman Page, F. King, F.E. Hancock, G.J.
Hutchings, J. Catal. 198 (2001) 319.
[31] L.J. Davis, P. McMorn, D. Bethell, P.C. Bulman Page, F. King, F.E. Hancock, G.J.
Hutchings, J. Mol. Catal. A 165 (2001) 243.
[32] G. Bellussi, A. Carati, M.G. Clerici, G. Maddinelli, R. Millini, J. Catal. 133 (1992)
220.
[33] A. Bhaumik, T. Tatsumi, J. Catal. 176 (1998) 305–309.
[34] G.N. Vayssilov, Catal. Rev. Sci. Eng. 39 (1997) 209–251.
[35] S. Mukherjee, M. Nandi, K. Sarkar, A. Bhaumik, J. Mol. Catal. A: Chem. 301 (2009)
114–117.
The titanosilicates can be used as heterogeneous catalyst for
the oxidation of ethers to lactones or carboxylic acids in mod-
erate to good yields in the presence of dilute aqueous H2O2 as
oxidant. The intrinsic activity of medium-pore TS-1 was found to be
higher over the TS-2, Ti-MCM-22, and Ti-␤. Tetrahydrofuran exhib-
ited the best reactivity than the other cyclic ethers investigated
under the present study. The activity of substituted tetrahydrofu-
rans decreased in the order: mono-␣- > di-␣, ␣ꢀ- > tetra-␣, ␣, ␣ꢀ,
␣ꢀ-substitution. Steric hindrance and geometrical constraints at
reaction sites influence the activity of mono- and di-substituted
[36] J. Iglesias, J.A. Melero, J. Sainz-Pardo, J. Mol. Catal. A: Chem. 291 (2008) 75–84.