Novel organic-inorganic hybrid mesoporous silica supported oxo-vanadium schiff base for selective oxidation of alcohols

Article abstract of DOI:10.1002/adsc.201100018

Novel organic-inorganic hybrid mesoporous materials containing diimine moieties inside the pore wall were used for the grafting of an oxo-vanadium Schiff base complex at the mesopore surface. The prepared catalyst was found to be highly active and selective for the oxidation of a variety of primary, secondary and α-hydroxy carbonyl compounds to the corresponding aldehydes, ketones and 1,2-dicarbonyl compounds using tert-butyl hydroperoxide as oxidant under mild reaction conditions. After completion of the reaction, the catalyst could easily be recovered by simple filtration and could be reused several times without significant change in the catalytic efficiency. Importantly, the catalyst did not show any leaching during the reaction, confirming that the prepared catalyst was truly heterogeneous in nature. Copyright

Full text of DOI:10.1002/adsc.201100018

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DOI: 10.1002/adsc.201100018
Novel Organic-Inorganic Hybrid Mesoporous Silica Supported
Oxo-Vanadium Schiff Base for Selective Oxidation of Alcohols
a
b,c
b
a,
Sanny Verma, Mahasweta Nandi, Arindam Modak, Suman L. Jain, *
b,
and Asim Bhaumik *
Chemical Sciences Division, Indian Institute of Petroleum, (Council of Scientific and Industrial Research), Dehradun
a
248 005, India
E-mail: suman@iip.res.in
b
c
Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
E-mail: msab@iacs.res.in
Integrated Science Education and Research Centre, Siksha Bhavana, Visva-Bharati, Santiniketan 731 235, India
Received: January 10, 2011; Revised: April 25, 2011; Published online: August 4, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100019.
Abstract: Novel organic-inorganic hybrid mesopo-
rous materials containing diimine moieties inside
the pore wall were used for the grafting of an oxo-
vanadium Schiff base complex at the mesopore sur-
face. The prepared catalyst was found to be highly
active and selective for the oxidation of a variety of
primary, secondary and a-hydroxy carbonyl com-
pounds to the corresponding aldehydes, ketones
and 1,2-dicarbonyl compounds using tert-butyl hy-
droperoxide as oxidant under mild reaction condi-
tions. After completion of the reaction, the catalyst
could easily be recovered by simple filtration and
could be reused several times without significant
change in the catalytic efficiency. Importantly, the
catalyst did not show any leaching during the reac-
tion, confirming that the prepared catalyst was truly
heterogeneous in nature.
is an area of tremendous importance as these catalysts
combine the advantages of both homogeneous (high
reactivity, selectivity) and heterogeneous catalysts
(easy product separation, facile recovery and recy-
[
3]
cling of the catalyst). However, a vast number of im-
mobilized metal catalysts has been developed by
[
4]
[5]
using different organic and inorganic supports, but
many of them suffer from the drawbacks such as high
cost, poor efficiency of the support, low catalyst load-
ing, leaching of active species and limited accessibility
of the supported materials. These drawbacks motivat-
ed us to develop new catalytic materials for develop-
ing new synthetic methodologies for organic reac-
[
6]
tions.
The synthesis of organic-inorganic hybrid mesopo-
rous silica-based materials, particularly periodic meso-
[
7]
porous organosilicas (PMOs) has been considered
to be a real advancement in the recent years of mate-
rials chemistry. These materials are particularly prom-
ising because they have organic fragments, which are
integrated into the silica matrix by covalent bonds,
and their properties can easily be modified by chang-
ing the nature of the bridging organic groups. The
unique structural features of these materials make
them potentially useful in a wide range of advanced
Keywords: alcohols; heterogeneous catalysts; or-
ganic-inorganic hybrids; oxidation; periodic meso-
porous organosilicas (PMOs); PMOs
The design and synthesis of novel materials or cata- applications, such as selective separation, sensing, cat-
[
8]
lysts for chemical reactions that are inherently envi- alysis, optoelectronics, and so on. However to the
ronmentally and ecologically benign is a challenging best of our knowledge there is no literature report on
[
1]
task in developing chemical processes and products.
the use of diimine-containing PMO material as sup-
The growing environmental concerns in recent de- port for the immobilization of vanadium complexes
cades are particularly promoting the discoveries of via covalent attachment. Furthermore, these materials
novel reactions, new synthetic schemes and recycling have more thermal, chemical and mechanical stability
[
9]
of the catalytic materials with favorable environmen- than organic polymers. Selective oxidation of alco-
[2]
tal impacts. Immobilization of homogeneous metal hols to carbonyl compounds is one of the fundamen-
[
10]
catalysts on polymeric organic or inorganic supports tal reactions in organic synthesis. Apart from a vari-
Adv. Synth. Catal. 2011, 353, 1897 – 1902
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1897

Sanny Verma et al.
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ety of conventional methods based on using stoichio- mine incorporated silica (LHMS-1) support, designat-
metric amounts of transition metals such as Ru, Cr, ed as 1 support has been described in the previous
[
14]
Mn which produce copious amounts of undesirable report. The TEM (transmission electron microsco-
waste, a number of effective catalytic methods using py) image of 1 is shown in Figure 1. As seen from this
transition metal-based catalysts has also been report- image, low electron density spots (pores) are present
[
11]
ed in the literature. In the recent years, the green throughout the specimen and these are arranged in a
[
7,8,14]
or sustainable development of oxidation methodolo- honeycomb-like hexagonal array.
The dimension
gies, is advancing research in the direction of develop- of pores is ca. 2.7 nm. This was further confirmed
ing more economical, environmentally benign, recy- from the hexagonal FFT pattern shown in the inset of
clable and efficient catalytic systems by using clean Figure 1. The loading of the Schiff base per gram of
À1
oxidants such as molecular oxygen, hydrogen perox- support was found to be 1.11 mmolg as determined
[12]
ide, etc. Furthermore, a variety of metal-free cata- from the estimated amount of nitrogen content
lytic systems has also been reported for these funda- (3.1%) through CHN elemental analysis.
[
13]
mentally important organic transformations. In the
The stirring of 1 with vanadyl sulfate in THF at
present paper we report for the first time an efficient room temperature resulted in the successful formation
and recyclable PMO (periodic mesoporous organosili- of the oxo-vanadium Schiff base, 2 as shown in
ca)-immobilized oxo-vanadium Schiff base for the se- Scheme 1.
lective oxidation of alcohols to carbonyl compounds
The incorporation of the vanadyl ions or successful
by using tert-butyl hydroperoxide (TBHP) as oxidant. synthesis of corresponding oxo-vanadium Schiff base
The detailed synthesis and characterization of the was confirmed by IR spectroscopy as it exhibits a
À1
required organic-inorganic hybrid mesoporous dii- sharp band at 962 cm characteristic of the V=O and
À1
a lower shift from 1647 to 1627 cm of the band cor-
responds to the C=N of the Schiff base (see Support-
ing Information, Figure S1 and Figure S2). Further
confirmation of the presence of the Schiff base
moiety in mesoporous PMO samples 1 and 2 is ob-
1
3
tained from the C CP MAS NMR spectra (see Sup-
porting Information, Figure S3). Both samples exhibit
strong signals at ca. 8.5, 20.5, 29.0, 42.4, 118.0, 127.5
and 168.0 ppm. These peaks could be assigned to the
different carbon atoms of 2,6-bis(propyliminomethyl)-
[
14]
4
-methylphenol moiety. The considerable downfield
13
chemical shift of the C NMR signal from ca.
4.0 ppm to 42.0 ppm for the V-loaded sample sug-
5
gested a VÀC=N covalent attachment at the surface
of the mesopores. The percentage of vanadium in the
prepared catalyst was estimated by thermogravimetric
Figure 1. HR-TEM image of Schiff base 1.
Scheme 1. Synthetic pathway to the oxo-vanadium Schiff base.
1898
asc.wiley-vch.de
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 1897 – 1902

Novel Organic-Inorganic Hybrid Mesoporous Silica Supported Oxo-Vanadium Schiff Base
À1
be 0.56 mmolg . The catalytic potential of the pre-
pared catalyst 2 was tested for the oxidation of alco-
hols using anhydrous TBHP as oxidant (Scheme 2).
The protocol developed for the oxidation of various
primary and secondary alcohols, and a-hydroxy ke-
tones consists of the heating of the mixture containing
substrate (1 mmol), catalyst 2 (2 mol%) and anhy-
drous TBHP (1.5 mmol) at 658C. The progress of the
reaction was monitored by thin layer chromatography
(
TLC). After completion of the reaction, the catalyst
Scheme 2. Oxidation of various alcohols over V-LHMS-1.
was recovered by simple filtration and the resulting
filtrate was subjected to usual work-up to obtain the
analysis (TGA) under an oxygen atmosphere. The corresponding carbonyl compound.
catalyst 2 started losing weight at ca. 75–1508C, prob-
The results of these experiments are summarized in
ably due to the loss of coordinated solvent molecules Table 1. Among the various substrates studied a-hy-
(
and other adsorbed species) followed by maximum droxy ketones (Table 1, entries 15 and 16) were found
weight loss between 275–5508C probably due to the to be most reactive substrates in comparison to the
decomposition of the ligand moiety. However, stabili- primary and secondary alcohols. Aromatic secondary
ty of the final residue at ca. 700–9008C revealed the alcohols (Table 1, entries 1 and 2) in general were
formation of V O at the end and by its weight we es- found to be more reactive than alicyclic ones
2
5
timated the approximate percentage of vanadium ions (Table 1, entries 3–6), however alcohols containing
to be 3.2%, which corresponds to the loading of the electron-donating groups were found to be more reac-
À1
catalyst 0.62 mmolg . We also calculated the loading tive than the substrates containing electron-withdraw-
of the vanadium by ICP-AES analysis and found it to ing groups. It is important to mention that primary al-
[a]
Table 1. Oxidation of alcohols, diols and a-hydroxy ketones.
[b]
Entry
Substrate
Product
Cat. (mol%)
Time [h]
Yield [%]
2
5
10
2
2
3.0
3.0
2.5
2.0
3.5
95
96
96[
1
c]
96
[
d]
92
2
3
2
2
3.0
2.5
90
93
4
5
2
2
2.5
5.0
84
90
6
7
2
2
2.5
3.0
95
90
8
2
3.0
92
Adv. Synth. Catal. 2011, 353, 1897 – 1902
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1899

Sanny Verma et al.
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Table 1. (Continued)
[b]
Entry
9
Substrate
Product
Cat. (mol%)
2
Time [h]
3.5
Yield [%]
85
10
2
4.0
75
[
[
e]
e]
1
1
1
2
2
2
2.5
2.5
93
92
[
[
e]
e]
1
1
1
1
3
4
5
6
2
2
2
2
3.0
2.5
2.5
3.0
95
94
94
90
[
a]
Reaction conditions: substrate (1 mmol), acetonitrile 1 mL and TBHP (5M solution in decane, 0.3 mL, 1.5 mmol), temper-
ature 658C.
Isolated yields.
[
[
[
[
b]
c]
d]
e]
By using homogeneous oxovanadium Schiff base 3 as catalyst.
By using non-Schiff base vanadium oxide on mesoporous silica as catalyst.
Using 1 mL (5 mmol) of TBHP.
cohols were selectively converted to the correspond- the heterogenized homogeneous catalysts over the ho-
ing aldehydes under similar reaction conditions mogeneous ones. Furthermore, to see the effect of
(
Table 1, entries 7–10). In the absence of catalyst, the Schiff base in catalyst 2, we prepared the silica immo-
[16]
reaction did not proceed at all even after prolonged bilized non-Schiff base vanadium oxide
and per-
reaction times. Similarly the use of mesoporous silica formed the oxidation of benzhydrol under similar re-
supported Schiff base 1 did not produce any oxidized action conditions. Both silica immobilized oxo-vanadi-
product under identical reaction conditions. These um Schiff base 2 and non-Schiff base vanadium oxide
findings clearly establish that vanadium is the active gave comparable results (Table 1, entry 1), but we ob-
species which is essentially required for these trans- served significant leaching in the case of non-Schiff
formations. We also compared the catalytic potential base vanadium oxide during the reaction. These re-
of the catalyst 2 with the homogeneous oxo-vanadium sults established the superiority of the immobilization
Schiff base 3 synthesized by the reaction of the pre- of oxo-vanadium Schiff base to functionalized support
cursor Schiff base I (Scheme 1) with vanadyl sulfate. via covalent attachment.
Whereas the oxidation of benzhydrol was found to
occur faster with this homogeneous catalyst
Most importantly, the prepared catalyst showed
moderate catalytic reactivity even in the lower con-
3
(
Table 1, entry 1), the facile recovery and recycling centrations (1 mol%, 2 mol% were found to be opti-
ability of the catalyst 2 establishes the superiority of mum) whereas, no marginal change could be observed
1900
asc.wiley-vch.de
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 1897 – 1902

Novel Organic-Inorganic Hybrid Mesoporous Silica Supported Oxo-Vanadium Schiff Base
[
a]
Table 2. Results of recycling experiments.
for the oxidation of different kinds of alcoholic com-
pounds to the corresponding carbonyl compounds by
using anhydrous TBHP as oxidant. The catalyst
[b]
Run
1
2
3
4
5
6
7
8
Time [h] [
Yield [%]
3.0 3.0 3.0 3.0 3.0 3.25 3.25 3.25 showed very good catalytic activity and afforded high
c]
95
95
96
96
95
94
94
95
to excellent product yields under mild reaction condi-
tions. After completion of the reaction, the catalyst
could readily be recovered by simple filtration and
could be reused for several runs without any signifi-
cant decrease in catalytic activity. No leaching of the
metal or ligand was observed during the course of the
reactions.
[
a]
Reaction conditions: benzhydrol (1 mmol), catalyst 2
(
(
0.02 mmol, 2 mol%), acetonitrile (1 mL) and TBHP
5M solution in decane, 0.3 mL, 1.5 mmol), temperature
6
58C.
[
[
b]
c]
Recovered catalyst was used in the subsequent runs.
Isolated yield.
in reaction rates while using higher amounts of cata- Experimental Section
lyst (5 mol% and 10 mol%) (Table 1, entry 1). The
oxidation of alcohols was found to be highly depen- The desired Schiff base bridging organosilane source (I,
Scheme 1) was prepared by following the literature
dent upon the temperature, at room temperature the
[14]
report. For the synthesis of LHMS-1, I and tetraethyl or-
thosilicate (TEOS, Aldrich) were used as diimine organosili-
ca and pure silica sources, respectively, at a molar ratio of
reaction was found to be very slow, 658C was found
to be optimum, however a further increase in temper-
ature made the reaction less selective and afforded
poor yields of the desired product.
Next we tested the recycling of the catalyst by
choosing the oxidation of benzhydrol as a representa-
1
:3 in the synthesis gel. Tartaric acid (TA, Loba Chemie)
was used to adjust the initial pH of the mixture. Cetyltrime-
thylammonium bromide (CTAB, Loba Chemie) was used as
[14]
template.
FT-IR spectra were recorded on a Perkin–
tive reaction. At the end of the reaction (as analyzed Elmer FT-IR X 1760 instrument. Elemental analyses were
by TLC), the catalyst was separated by filtration, carried out by using ASTM d-3828 (Kjeldahl method). X-
washed, dried and reused for subsequent runs (eight ray diffraction patterns of the powder samples were ob-
tained with a Bruker AXS D8 Advanced SWAX diffractom-
eter using Cu Ka (l=0.15406 nm) radiation. High resolu-
runs) under the described reaction conditions. The
yield of the oxidized products remained unchanged
tion transmission electron microscopic images were record-
during these experiments carried out for the same re-
ed on a JEOL 2010 TEM operated at 200 kV. For the solid
action times, establishing the efficient recycling and
1
3
state C CP MAS NMR studies a Chemagnetics 300 MHz
CMX 300 spectrometer was used. GC analyses were per-
formed on Varian CP 3800 Gas Chromatograph for the
identification of the reaction products.
reusability of the catalyst (Table 2). Furthermore, we
did additional experiments for checking the leaching
of the metal during the reaction. Initially, the mixture
of catalyst in acetonitrile was stirred at 658C for 24 h.
The immobilized catalyst 2 was then recovered by fil-
tration of the hot solution and the filtrate thus ob-
Preparation of Catalyst 2
tained was charged with benzhydrol and TBHP and The mesoporous silica immobilized Schiff base
1
À1
(
1.11 mmolg , 1 g) was stirred with vanadyl sulfate (0.24 g,
the reaction was continued under similar reaction
conditions. No oxidation occurred even after pro-
longed reaction time (5 h), confirming that there was
no leaching during the reaction. Also the selected fil-
trate samples during the recycling experiments were
analyzed by inductively coupled plasma atomic emis-
sion spectroscopy (ICP-AES) and no solvated vanadi-
1
.5 mmol) in acetonitrile for 48 h at room temperature. The
heterogeneous dark colored oxo-vanadium Schiff base 2 was
separated by filtration and washed thoroughly with acetoni-
trile, methanol and dried under vacuum prior to use. FT-IR:
À1
n=2924, 1627, 1540, 1531, 1128, 962, 802 cm . Loading of
À1
the catalyst 2 was found to be 0.62 mmolg , as determined
by TGA analysis. CHN elemental analysis of catalyst 2
um could be detected (0.1 ppm detection limit), prov- gave: C 20.65, H 2.62, N 3.11.
ing that there was no leaching and the reaction was
truly heterogeneous in nature.
General Procedure for the Oxidation of Alcohols
To the mixture containing substrate (1 mmol) and heteroge-
neous oxo-vanadium Schiff base 2 (0.02 mmol, 2 mol%,
Conclusions
0
.03 g) was added TBHP (5M solution in decane, 0.3 mL,
1
.5 mmol) by a syringe and the resulting mixture was stirred
We have demonstrated a successful synthesis of
highly efficient silica-immobilized oxo-vanadium
Schiff base over novel organic-inorganic hybrid peri-
odic mesoporous organosilica material containing dii-
at 658C. The reaction progress was monitored by TLC using
ethyl acetate/hexane mixture (6:4) as eluent. After comple-
tion, the catalyst was recovered by filtration and the residual
mixture was extracted with diethyl ether. The combined or-
mine functionality via covalent attachments. The cata- ganic layer was washed with water and dried over anhydrous
lytic efficiency of the developed catalyst was tested sodium sulfate. Evaporation of the organic solvent under re-
Adv. Synth. Catal. 2011, 353, 1897 – 1902
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1901

Sanny Verma et al.
COMMUNICATIONS
duced pressure yielded the corresponding oxidized product
selectively. The crude product was purified by flash chroma-
tography.
After the oxidation, we did not observe unreacted TBHP
in the reaction mixture as ascertained by GC-MS analysis.
The TBHP was converted into the tert-butyl alcohol during
the oxidation in the presence of transition metal-containing
catalyst.
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Acknowledgements
We thank the Director of IIP for his kind permission to pub-
lish these results. SLJ and AB kindly acknowledge DST, New
Delhi for funding. SV and AM acknowledge CSIR, New
Delhi for their fellowships.
3
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