Preparation and characterization of a RHA/TiO2 nanocomposite: Introduction of an efficient and reusable catalyst for chemoselective trimethylsilyl protection and deprotection of alcohols and phenols

Article abstract of DOI:10.1039/c5ra26607f

In this work, rice husk ash (RHA), as a natural source of amorphous silica, was used as a support for the synthesis of anatase-phase titania nanoparticles leading to the RHA/TiO₂ nanocomposite. This nanocomposite was used as an efficient catalyst for the chemoselective trimethylsilylation of various alcohols and phenols and deprotection of the obtained trimethylsilyl ethers. The procedure gave the products in excellent yields in very short reaction times. Also this catalyst can be reused at least six times without loss of its catalytic activity.

Full text of DOI:10.1039/c5ra26607f

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Preparation and characterization of a RHA/TiO2
nanocomposite: introduction of an efficient and
reusable catalyst for chemoselective trimethylsilyl
protection and deprotection of alcohols and
phenols
Cite this: RSC Adv., 2016, 6, 23564
Mohadeseh Seddighi, Farhad Shirini* and Omid Goli-Jolodar
In this work, rice husk ash (RHA), as a natural source of amorphous silica, was used as a support for the
synthesis of anatase-phase titania nanoparticles leading to the RHA/TiO
2
nanocomposite. This
Received 13th December 2015
Accepted 18th February 2016
nanocomposite was used as an efficient catalyst for the chemoselective trimethylsilylation of various
alcohols and phenols and deprotection of the obtained trimethylsilyl ethers. The procedure gave the
products in excellent yields in very short reaction times. Also this catalyst can be reused at least six times
without loss of its catalytic activity.
DOI: 10.1039/c5ra26607f
www.rsc.org/advances
reagents and non-recoverability of the catalyst. Additionally, only
some of them are useful for the protection as well as deprotection
Introduction
Protection and deprotection of functional groups is oen of the mentioned silyl ethers. Therefore, introducing of simple,
necessary during a multistep organic synthesis in order to efficient and mild procedures with easily separable and reusable
obtain chemoselectivity. Hydroxyl groups are oen present in solid catalysts to overcome these problems is still in demand.
organic molecules, so protection of them is very important and
modern natural product synthesis is inconceivable without found much attention due to their unusual catalytic properties
In recent years, synthesis of nanocrystalline metal oxides has
1
5
their protection. Among various procedures, the conversion of compared to bulk metals. Among various metal oxides, TiO
hydroxyl groups to their corresponding trimethylsilyl ethers is nanoparticles is one of the most utilized ones which have been
2
1
6,17
one of the popular methods used for this purpose. Silylation widely investigated as catalyst, pigment, food coloring, etc.
derivatives are generally less polar, more volatile and thermally Preparation of the unagglomerated TiO nanoparticles, with
2
more stable. They are also used in analytical chemistry to a narrow size distribution in anatase form, is very important to
prepare volatile derivatives of alcohols and phenols for GC and achieve higher catalytic activity and more readily handling than
1
18,19
GC-MS analyses. They can also be transformed directly into the commercial TiO
other functionalities. Therefore, they were used as important synthesis of TiO over a silica support was explored as a useful
intermediates for the synthesis of various natural products such way for preventing the agglomeration and formation of dispersed
.
On the basis of this subject, the
2
2
20
as (+)-spongistatin 1, phorboxazole A, amphidinolide H and G, TiO
monorhizopodin, (+)-cephalostatin 1 and brevenal.
2
aggregates. Siliceous materials are desirable because they
are chemically inert and have a high specic surface area and
2
Many different organosilyl protecting groups are known, but thermal stability.
,1,1,3,3,3-hexamethyldisilazane (HMDS), which is an inex-
Rice husk, the outer covering of rice grains, is one of the
1
pensive and commercially available reagent and gives ammonia main agricultural residues which obtained during the milling
as the only by-product, is one of the best candidates. However, process. Application of rice husk as an energy source for
the low silylating power of HMDS is a main drawback for its biomass power plants, rice mills and brick factories is
21
application. For the activation of this reagent a variety of cata- increasing due to its high caloric power. In this combustion,
3
–14
lysts have been reported.
an improvement in the synthesis of trimethylsilyl ethers, many able amount of amorphous silica up to 80% and small
Although these procedures provide rice husk ash (RHA) is produced which it is contains consider-
22
of them suffer from disadvantages such as long reaction times, proportion of impurities such as K
2 2 2 3
O, Na O and Fe O .
harsh reaction conditions, need to excess amounts of the Therefore, RHA possesses high silica content, so it has been
23,24
reagent, thermal instability, by-products formation, use of toxic employed to produce zeolites and silica powders.
In recent years, investigation on the catalytic application of
rice husk ash became an important part of our ongoing research
Department of Chemistry, College of Sciences, University of Guilan, Rasht, Iran, 41335.
E-mail: shirini@guilan.ac.ir; Fax: +98 131 3233262; Tel: +98 131 3233262
4,25–27
program.
In continuation of this study, the high silica
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Table 1 Chemical composition of RHA conducted by X-ray fluores-
cence analysis (wt%)
Composition
Percent (%)
Composition
Percent (%)
SiO
Al
Fe
SO
2
80.82
0.25
0.38
0.39
0.44
K
Na O
2
CaO
Cl
LOI
2
O
1.25
0.96
0.82
1.99
12.70
2
O
O
3
2
3
3
a
P
2
O
5
a
Loss on ignition.
Scheme 1 Preparation of RHA/TiO
2
nanocomposites.
content of RHA encouraged us to investigate the possibility of
the preparation of TiO over this reagent.
2
Experimental
Characterization techniques
The FT-IR spectra were run on a VERTEX 70 Bruker company
(
Germany). Scanning election microphotographs (SEM) were
Fig.
1
FT-IR spectra of RHA, TiO
2
and RHA/TiO
2
various
obtained on a SEM-Philips XL30. X-ray diffraction (XRD)
measurements were performed at room temperature on
a Siemens D-500 X-ray diffractometer (Germany), using Ni-
nanocomposites.

ltered Co-Ka radiation (l ¼ 0.15418 nm).
completely. The prepared material was separated by ltration,
ꢀ
washed with water, and dried at 80 C in oven. Finally, the ob-
Preparation and chemical composition of RHA
ꢀ
tained solid was calcinated at 500 C for 3 h to obtain RHA/TiO
2
ꢀ
The rice husk sample was burned at 600 C for 1 h to obtain nanocomposites (Scheme 1). Throughout the subsequent
RHA. The X-ray uorescence (XRF) analysis showed that a high discussion, the samples will be named by indicating the titania
value of silica content (up to 80%) and a very small proportion content and support, as RHA/TiO (20%), RHA/TiO (30%) and
2
2
of metallic elements such as Al O , Fe O , SO , P O , K O, Na O RHA/TiO (50%).
2
3
2
3
3
2
5
2
2
2
25
and CaO are present in RHA sample (Table 1).
General procedure for trimethylsilylation of alcohols and
phenols
Preparation of RHA/TiO nanocomposites
2
1
3
.0 g of rice husk ash in absolute ethanol (6 mL) was stirred for HMDS (0.75 mmol) was added to a mixture of substrate (1
0 min at room temperature. Then the required amount of mmol), RHA/TiO (30%) (10 mg) and CH CN (3 mL) and the
2
3
ꢀ
titanium tetraisopropoxide (TTIP) (volume ratio TTIP : EtOH of resulting mixture was stirred at 50 C. Aer completion of the
: 6) was added dropwise to the solution with stirring for 2 h. reaction (monitored by TLC; eluent: CCl ), the mixture was
Then, deionized water was slowly added to the resulting mixture ltered and the catalyst was separated by ltration. The solvent
TTIP : water with ratio of 1 : 60) to cause the hydrolysis of the was removed under reduced pressure to afford the desired
TTIP and the stirring was maintained for 2 h to hydrolyze TTIP product (Scheme 2).
1
4
(
2
Scheme 2 The protection and deprotection of alcohols and phenols catalyzed by RHA/TiO (30%).
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General procedure for deprotection of trimethylsilyl ethers
A mixture of the substrate (1 mmol) and RHA/TiO (30%) (10
2
mg) in CH
completion of the reaction (monitored by thin layer chroma-
tography (TLC); eluent: CH Cl), the catalyst was ltered off and
3
OH (3 mL) was stirred at room temperature. Aer
3
the solvent was evaporated under reduced pressure to afford the
desired product.
Results and discussions
Characterization of the catalyst
Fig. 1 shows the FT-IR spectra of nano-TiO
2
(anatase), RHA and
. In all cases, the peaks at 3400 and Fig. 4 TEM images of RHA/TiO
630 cm are attributed to the stretching and bending modes
2
0, 30 and 50 w% RHA/TiO
2
2
(30%).
ꢁ1
1
of the hydroxyl groups related to Si–OH or Ti–OH and the
28
ꢁ1
adsorbed water. The strong peak at 1100 cm and the peaks
Fig. 2 XRD patterns of RHA and various samples of RHA/TiO
2
nanocomposites.
Table 2 XRD data of RHA/TiO
2
nanocomposites
Peak width [FWHM]
0.40
Entry
Nanocomposite
2q
Size [nm]
Fig. 5 The N
distribution (b) of RHA and RHA/TiO
2
adsorption–desorption isotherm (a), and pore size
(30%).
1
2
3
RHA/TiO
RHA/TiO
RHA/TiO
2
2
2
(20%)
(30%)
(50%)
25.35
22.38
25.36
20
21
22
2
0.38
0.36
ꢁ
1
at 801 and 468 cm are assigned to the asymmetric stretching,
symmetric stretching and bending modes of Si–O–Si, respec-
tively. The vibration modes of Ti–O–Ti lie in the range of 500–
ꢁ
1
29
7
00 cm (Fig. 1, spectrum of TiO ), so the vibrations appeared
2
ꢁ1
at 500–700 cm in the prepared samples (Fig. 1, spectra of
RHA/TiO nanocomposites) can be attributed to these bands.
2
Increasing of the intensity of these bands by increasing of the
titanium content of the samples can be described on the basis
of this fact. Theoretically, the stretching vibration of Ti–O–Si is
ꢁ
1
appeared as a weak peak at 960 cm , whereas such peak was
2
Fig. 3 SEM images of RHA (a) and RHA/TiO (30%) (b).
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3
0
Table 3 Optimization of the best ratio of RHA/TiO nanocomposite in the silica of rice husk ash was mainly in the amorphous form.
2
the silylation of 4-chloro benzylalcohol
The peaks appeared around 2q ¼ 25.3, 37.8, 47.8, 54.1 and 62.3
in RHA/TiO nanocomposites are related to the (101), (004),
2
Entry
Catalyst
Time (min)
Conversion (%)
(
200), (105) and (211) reections which indicated only anatase
31,32
1
2
3
RHA/TiO
RHA/TiO
RHA/TiO
2
2
2
(20%)
(30%)
(50%)
2
<1
<1
90
100
100
phase is present in all prepared materials.
The intensity of
anatase peaks increased by increasing in the titania contents,
however the broadening of these peaks shows that the nano-
composite particles have very small size. These observations
strongly suggested that the RHA inhibited growth of crystal
not observed in the prepared samples. So, from the FT-IR
spectra could be resulted that titania is only embedded into
33
grain of titan.
The average crystalline sizes of nanocomposites were esti-
mated from XRD using Scherrer's equation by considering the
full width and half maximum (FWHM) value (Table 2). As it can
be seen, all of the synthesized nanocomposites have almost
same crystalline sizes.
18
the RHA matrix.
Fig. 2 represents the X-ray diffraction (XRD) patterns of the
RHA and RHA/TiO nanocomposites. The broad peak appeared
around 2q equal to 22 in the RHA pattern clearly indicated that
2
Table 4 Silylation of alcohols and phenols and deprotection of the corresponding trimethylsilyl ethers catalyzed by RHA/TiO
2
(30%)
Protection
Deprotection
a
b
a
b
Entry
Substrate
CH OH
Product
CH
Time (min)
Yield (%)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
C
6
H
5
2
C
6
H
5
2
OTMS
Immediately
Immediately
Immediately
Immediately
Immediately
Immediately
Immediately
Immediately
Immediately
Immediately
1
96
91
97
96
95
92
94
95
93
92
90
4
3
3
3
3
2
3
5
3
4
5
95
94
96
93
97
92
94
94
91
94
95
2-ClC
4-ClC
6
H
4
H
4
CH
CH
2
OH
OH
OH
OH
2-ClC
4-ClC
6
H
4
H
4
CH
CH
2
OTMS
OTMS
6
2
6
2
3-BrC
4-BrC
6
H
4
H
4
CH
CH
2
3-BrC
4-BrC
6
H
H
4
4
CH
CH
2
OTMS
2
OTMS
6
2
6
2-NO
3-NO
4-NO
2-CH
3-CH
4-CH
2
C
6
6
6
6
H
H
H
H
4
4
4
4
CH
CH
CH
CH
2
2
2
2
OH
OH
OH
OH
2-NO
3-NO
4-NO
2-CH
3-CH
4-CH
2
C
6
6
6
6
H
H
H
H
4
4
4
4
CH
CH
CH
CH
2
2
2
2
OTMS
OTMS
OTMS
OTMS
2
2
3
3
3
C
C
C
OC
OC
2
2
3
3
3
C
C
C
OC
OC
1
1
0
1
6
H
H
4
CH
CH
2
OH
OH
6
H
H
4
CH
CH
2
OTMS
OTMS
6
4
2
6
4
2
1
2
3
1
92
3
91
1
1
89
3
94
1
1
4
5
1
1
95
90
3
5
97
89
16
17
18
19
20
21
22
23
C
6
H
5
OH
2-ClC
4-CH
3,5-(CH
2-NH
C
6
H
5
OTMS
2-ClC OTMS
4-CH OTMS
3,5-(CH OTMS
2-NH OTMS
NHTMS
CH NHTMS
STMS
2
2
2
2
97
94
97
95
94
0
3
3
3
3
94
89
94
93
94
—
—
—
6
H
4
OH
6
H
4
3
C
6
H
4
OH
3
C
6
H
)
H
4
3
6
)
2
C
6
H
4
OH
3
2
C
6
H
4
2
C
H
4
OH
2
C
6
4
2
3
c
c
C
C
6
H
H
5
NH
CH
2
C
C
6
H
H
5
120
120
120
—
—
—
6
5
2
NH
SH
2
6
5
2
0
0
4-BrC
6
H
5
4-BrC
6
H
5
2
4
120
0
—
—
c
2
2
5
6
4-ClC
4-ClC
6
6
H
H
5
CH
CH
2
OH + 4-BrC
6
H
5
SH
NH
C
C
6
H
H
5
CH
CH
2
OTMS + 4-BrC
6
H
5
STMS
NHTMS
1
1
100 + 0
—
—
—
—
c
5
2
OH + C CH
6
H
5
2
2
6
5
2
OTMS + C CH
6
H
5
2
100 + 0
a
b
c
Immediately means <1 min. Isolated yields. Conversion.
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Scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) were analyzed for determining the
size distribution, particle shape and surface morphology,
shown in Fig. 3 and 4. It should be noted that these analysis
were done on RHA/TiO
the highest catalytic activity was determined in the silylation of
-chlorobenzyl alcohol). The SEM pictures show that RHA has
2 2
(30%) (the best ratio of RHA/TiO with
4
34
a porous and irregular shape, and the porosity were substan-
tially maintained over dispersion of TiO in RHA/TiO (30%). It
2
2
is clear from the TEM images that TiO nanoparticles, which
2
can be seen as dark particles, are spherical with average size less
than 10 nm in diameter, which have been dispersed in the
matrix of RHA, as shown in Fig. 4.
2
N adsorption–desorption measurements based on BET and
BJH methods, which have been a powerful tool for nano- or mes-
oporous material characterization, were performed to obtain more
information about the catalyst. The adsorption–desorption
Scheme 3 Proposed mechanism of the reaction.
isotherm of RHA and RHA/TiO (30%) nanocomposite is showed
2
out in the absence of the catalyst, that the reaction was not
proceeded, indicated the necessity of the catalyst to produce the
products.
Aer optimization of the reaction conditions, different types
of alcohols were subjected to the silylation using this method
in Fig. 5a. Both samples exhibited type IV isotherm with H3
hysteresis loop categories according to the literature, which indi-
cates that the samples are mesoporous materials with slit-like
35
pores. These results also prove that the textural properties of
RHA were substantially maintained over preparation of nano-
(Table 4). Silylation of various benzylic alcohols containing
composite. The specic BET surface areas for RHA and RHA/TiO
2
2
ꢁ1
electron-withdrawing and electron-donating substituents in the
ortho, meta, and para positions of the benzene ring proceeded
efficiently with high isolated yields (Table 4, entries 1–11).
Primary, secondary and tertiary aliphatic alcohols were also
efficiently converted to their corresponding silyl ethers in
almost quantitative yields at room temperature (Table 4, entries
(
30%) is 57 and 50 m g , respectively. Furthermore, the curves of
pore size distribution evaluated from desorption data by utilizing
the BJH model are also shown in Fig. 5b. These curves show that
there are some particles with average size less than 10 nm in
diameter in RHA/TiO (30%) which do not exist in RHA. These
2
2
particles can be attributed to the TiO nanoparticles.
12–15). The trimethylsilylation of phenol and its derivatives was
also investigated using this method and the desired products
were obtained in excellent yields (Table 4, entries 16–20).
Amines and thiols are resistant to this reagent and do not
react under the selected conditions (Table 4, entries 20–24).
Thus, this methodology shows selectivity and is suitable for the
chemoselective trimethylsilylation of benzylic, primary,
secondary and tertiary aliphatic alcohols and phenols in the
presence of amines and thiols (Table 4, entries 25 and 26).
Catalytic activity
On the basis of the information obtained from the studies on
RHA/TiO nanocomposite, expected to this reagent could be
2
used as a catalyst for the promotion of the organic reactions. So
this reagent was used in the promotion of the trimethylsilyla-
tion of hydroxyl groups in alcohols and phenols.
At the rst step we focused our attention toward the opti-
2
mization of the ratio of RHA to TiO for obtaining the highest
catalytic activity. For this purpose, silylation of 4-chlorobenzyl
alcohol was studied and was found that the best results can be
obtained using RHA/TiO (30%) (Table 3). Further increase in
2
this ratio did not improve the activity of the prepared
nanocomposite.
Then, for optimization of the reaction conditions, the reac-
tion between 4-chloro benzylalcohol and HMDS to its corre-
sponding trimethylsilyl ether was selected as a model reaction
and the various conditions including amount of the catalyst
(0, 5, 10, 20 and 30 mg), amount of HMDS (0.5, 0.7 and 1 mmol),
solvent (CH
2
Cl
2
, n-hexane, acetone, CH
3
CN, EtOH and H
2
O)
ꢀ
and temperature (30, 40, 50, 60 C) were examined. Finally the
best result was obtained using 10 mg of RHA/TiO
2
(30%) and
ꢀ
0
.7 mmol HMDS in CH CN at 50 C (Scheme 2). Any further
3
increase of the catalyst or temperature did not improve
the reaction time and yield. To illustrate the efficiency of the
catalyst in these reactions, the model reaction was also carried
Fig. 6 Reusability of the catalyst.
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Table 5 Comparison of the results of the silylprotection and deprotection of benzyl alcohol and phenol catalyzed by RHA/TiO
2
(30%) with those
obtained by some of the other reported catalysts
a
b
a
b
Entry
Catalyst (mol%)
Time (yield)
Time (yield)
Time (yield)
Time (yield)
Ref.
1
2
3
4
5
6
7
8
9
Montmorillonite K-10 (100 mg)
H-b zeolite (10% w)
21 (92)
80 (97)
510 (92)
30 (96)
80 (99)
5 (94)
3 (91)
7 (92)
5 (98)
2 (90)
5 (58)
10 (95)
5 (94)
3 (90)
<1 (96)
7 (98)
—
5 (98)
—
—
—
—
—
—
—
60 (95)
90 (90)
10 (78)
90 (95)
120 (90)
3 (93)
4 (95)
6 (93)
5 (99)
—
6 (98)
—
5 (89)
—
—
—
—
—
—
—
6
7
8
9
10
11
12
13
14
—
—
3
SiO
[bmim][BF
SiO -Pr-SO
Silica-bonded S-SO
Al –HClO (2.5 mol%)
Bi(OTf)
2
–H
2
SO
4
(15 mmol%)
] (2 mL)
H (3 mol%)
H (2 mol%)
4
2
3
3
2
O
3
4
3
Fe
AlCl
3
O
4
(10 mol%)
(10 mg)
10
11
10
11
12
13
3
CaCl
Nano TiO
2
$2H
2
O (10 mg)
(20 mg)
—
—
—
—
—
—
—
—
—
2
180 (85)
16 (93)
13 (86)
2 (97)
RiH (80 mg)
RHA (20 mg)
4
5
—
RHA/TiO
2
(30%) (10 mg)
4 (95)
3 (94)
a
b
Time (min) and yield (%) in the protection reaction. Time (min) and yield (%) in the deprotection reaction.
2 2
Furthermore application of RHA/TiO (30%) as the catalyst catalyzed by RHA/TiO (30%) nanocomposite with some of the
for deprotection of trimethylsilyl ethers to their corresponding other results reported in the literature (Table 5). It is clear that
hydroxyl groups was investigated. For this purpose, the silylated the present method is superior in terms of the reaction time and
products were treated with 10 mg of the catalyst in methanol at catalyst amount, especially compared with rice husk (RiH), rice
room temperature. Under the selected conditions, the corre- husk ash (RHA) and TiO2.
sponding alcohols and phenols were obtained in good to high
yields, as the results are shown in Table 4.
As mentioned, XRF analysis showed that a high value of
silica content (up to 80%) and other composition such as Al
, P and CaO are present in RHA. For this purpose, we
2 3
O ,
The mechanism of the reaction is not clear but according to Fe
2
O
3
2 5
O
our studies and observations in all reactions, fast evolution of used various catalysts in the reaction of trimethyl silylation of
ammonia gas was released (identied by odor and litmus 4-chlorobenzyl alcohol under optimal condition (10 mg cata-
paper). With these observations we have proposed a mechanism lyst). As it can be seen, CaCl $2H O, AlCl and Fe O can also
2 2 3 3 4
in which the generation of NH and the catalytic role of RHA/ catalyze this type of reactions (Table 5, entries 7–9). However,
3
TiO (30%) are explained as the most correctable mechanism the long time observed for the same reaction using RHA and
2
(
Scheme 3). On the basis of this mechanism, RHA/TiO (30%) RHA/TiO conrms the important role of the TiO in RHA/TiO
2 2 2 2
activated the HMDS (I). The nucleophilic attack of alcohol or nanocomposite (Table 5, entries 10 and 13). Furthermore, these
phenol generated the silyl ether product and the reactive sily- results can be also explained that not only the small size of nano
lating agent (II). The addition of second molecule of substrate TiO
guided to the fast evolution of NH and release of another silyl higher catalytic activity of RHA/TiO
to TiO catalyst.
2
in RHA/TiO
2
nanocomposite but also RHA caused to
3
2
nanocomposite compared
3,13
ether product (III).
2
To check the reusability of the catalyst, the reaction of
-chloro benzylalcohol and HMDS acid under the optimized
4
reaction conditions was studied again. When the reaction
completed, the catalyst was easily separated by ltration,
Conclusions
washed with CH
This process was carried out over six runs and all reactions led ding rice husk ash as a natural source of silica, and RHA/TiO
3
CN, dried and reused for the same reaction. In conclusion, anatase-phase titania was prepared by embed-
2
to the desired products immediately without signicant nanocomposite was obtained. The FT-IR spectra and TEM
changes in terms of yield which clearly demonstrates practical showed the embedding of titania into the RHA matrix. TEM and
recyclability of this catalyst (Fig. 6).
BET methods also indicated that anatase phase of titania with
Efficiency of the present method was compared by the result very small size was formed. This reagent was successfully used
of the silylation of benzyl alcohol and phenol with HMDS and as a catalyst for the simple, efficient and chemoselective sily-
deprotection of the corresponding trimethylsilyl ethers lation of various alcohols and phenols and deprotection of the
obtained silyl ethers. All reactions are carried out in very short
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Paper
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