Amorphous TiO2 coated into periodic mesoporous organosilicate channels as a new binary photocatalyst for regeneration of carbonyl compounds from oximes under sunlight irradiation

Article abstract of DOI:10.1039/c2ob26907d

A new binary photocatalyst was easily prepared based on incorporation of amorphous titania into the periodic mesoporous organosilicate framework bearing photoresponsive isocyanurate species. The catalyst was found to be highly active in photocatalytic deoximation reaction under sunlight irradiation. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2ob26907d

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Amorphous TiO2 coated into periodic mesoporous
organosilicate channels as a new binary photocatalyst
for regeneration of carbonyl compounds from oximes
under sunlight irradiation†
Cite this: Org. Biomol. Chem., 2013, 11,
416
Received 28th September 2012,
Accepted 12th November 2012
Sedigheh Abedi,a Babak Karimi,*a Foad Kazemi,a Mihnea Bostinab and
Hojatollah Valib
DOI: 10.1039/c2ob26907d
www.rsc.org/obc
A new binary photocatalyst was easily prepared based on incor- thought to be more suitable for the catalytic applications.6 It
poration of amorphous titania into the periodic mesoporous organo- seems that the chemists have largely ignored the intrinsic
silicate framework bearing photoresponsive isocyanurate species. advantages of amorphous TiO2-based photocatalytic systems
The catalyst was found to be highly active in photocatalytic deoxi- such as simple preparation process and relatively high surface
mation reaction under sunlight irradiation.
area leading to high adsorptivity. Nevertheless, to date there
have been only a few studies successfully employing amor-
phous TiO2 and TiO2-based composites in photochemical
transformations.7
In recent years, the search for environmentally benign chemi-
cal processes or methodologies has received increasing atten-
tion due to their role in conservation of the global ecosystem.1
In this regard, owing to the simplified recovery and reusability,
the use of heterogeneous catalysis is often favoured for many
chemical reactions within both academic and industrial set-
tings.2 However, in addition to reusability, one of the most
important desiderata in the field of sustainable chemistry is to
replace the toxic and expensive metal-based catalysts by safer
analogs accompanied with employing renewable and more
accessible energy sources whilst demonstrating similar or even
higher levels of activity. Along this line, heterogeneous photo-
catalysts comprising semiconductor metal oxides have been
recognized as promising tools to attain this goal in organic
transformations.3 Amongst them, TiO2 has gained consider-
able attention due to its low cost, high stability under
irradiation and environmentally friendliness.4 However, the
major limitation when using TiO2 as a photocatalyst is its very
low selectivity due to the large band gap, which often resulted
in a variety of undesired by-products in many applications.5
Furthermore, most of the studies have been focused on modi-
fying crystalline forms of TiO2 to obtain a new improved
photocatalyst system, as these materials are traditionally
Another interesting approach to improve the photocatalytic
performance/selectivity of TiO2 is the immobilization of a tita-
nium precursor onto a large surface area and inert solid
support with a primary objective of achieving more active sites
per unit area. In this regard, the use of ordered mesoporous
silicas such as MCM-418 and SBA-159 with large surface area
and regular pore size in the range of 2–30 nm is well esta-
blished in preparing various types of supported titania cata-
lysts.10 One of the impressive breakthroughs in the area of
mesoporous material was the discovery of periodic meso-
porous organosilicas (PMOs),11 which are built from the con-
densation of bridge organosilane [(R′O)3Si–R–Si(OR′)3]. PMOs
have not only better hydrothermal and mechanical stability
than ordered mesoporous materials, but also organic groups,
R, that are homogeneously distributed throughout the stable
silica network. Moreover, incorporation of bridging groups
provides a means of precise tuning the surface properties of
PMOs to achieve the desired functionality and/or selectivity.
However, despite the unique characteristic of PMOs, their
impact as the supports in photocatalytic systems has been
unexplored to date. Inspired by our successful experience in
designing several PMO-supported catalyst systems,12 we
hypothesized that PMOs bearing an appropriate photorespon-
sive and photoresistance organic group inside their framework
might be an attractive or even better matrix than non-functio-
nalized mesoporous silicas in immobilizing amorphous TiO2.
If this hypothesis could be validated in practice, the approach
would offer the possibility of designing an unprecedented
family of heterogeneous photocatalysts based on a synergism
between integrated organic functional groups located in close
aDepartment of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS),
Gava Zang, Zanjan 45137-66731, Iran. E-mail: karimi@iasbs.ac.ir;
Fax: +98-241-4214949; Tel: +98-241-4153225
bAnatomy and Cell Biology and Facility for Electron Microscopy Research,
McGill University, Montreal, Quebec, H3A 2A7 Canada
†Electronic supplementary information (ESI) available: The experimental details
for the preparation of catalysts, recycling results, N2 absorption–desorption and
BJH analyses, IR spectra, TGA, elemental analyses and TEM images of PICS-Ti
(45). See DOI: 10.1039/c2ob26907d
416 | Org. Biomol. Chem., 2013, 11, 416–419
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proximity of titanium sites on a porous surface. However, the
efficiency-selectivity couples and durability of the photocatalyst
systems could not only depend on the above-mentioned syner-
gism but they may also strongly rely on the photoresponsive
and photoresistance nature of the functionalized organic
moieties.
The interesting findings of Tetzlaff and Jenks13 and Kisch
and Mitoraj14 concerning the outstanding photoresistance and
photoresponsive behavior of cyanuric acid stimulated our
interest in investigating whether the combination of these
photochemical properties and photocatalytic activity of amor-
phous TiO2 in the pore structure of PMOs would lead to an
improved catalytic performance. To validate this hypothesis,
we have chosen a known isocyanurate PMO (PMO–ICS) which
can be easily prepared by hydrolysis co-condensation of tris-
[3-methoxysilylpropyl]isocyanurate (ICS) and tetraethoxysilane
(TEOS) in the presence of pluronic P123 as a structure
directing agent under acidic conditions.15 The surface of the
resulting PMO–ICS was then modified by amorphous TiO2
Fig. 1 Electron microscopy of PICS-Ti (45). (A) Across the 2D-hexagonal chan-
nels. (B) An image showing a longitudinal view of the 2D-hexagonal channels,
through a layer-by-layer coating procedure to give PICS-Ti (x).16 homogeneous presence of TiO2 in the material in dark mode. The wall of meso-
channels can be clearly seen in bright color. (C) EELS spectra of the image pres-
Four catalysts with varying percentage of loaded TiO2 (25, 35,
45, 55) have been prepared by employing Ti(Oi-Pr)4 (TTIP) as a
titanium precursor and the resulting samples were denoted as
ented in B showing the presence of the TiO2. (D) EFTEM image of the image in
B demonstrating the homogeneous presence of TiO2 in the material in bright
mode. TiO2 layers are clearly visible.
PICS-Ti (25), PICS-Ti (35), ICS-Ti (45), PICS-Ti (55), respec-
tively. All PICS-Ti samples were thoroughly characterized by N2
adsorption–desorption analysis, X-ray diffraction, transmission mesostructure of the parent PMO–ICS was retained intact after
electron microscopy (TEM) and 3D electron tomography the coating of titania (Fig. 1). Moreover, both EELS spectra and
(3D-ET). The structural parameters of the materials are col- EFTEM of the materials clearly demonstrate the homogeneous
lected in Table 1. As can be clearly seen both BET surface area distribution of Ti in the materials (Fig. 1C, 1D).
and average pore diameters decrease upon increasing the
To gain more insight into the ultrastructure of the TiO2-
amount of TTIP in the coating stage, highlighting the notion loaded materials, ET and 3D-reconstruction techniques were
that the TiO2 layer might be indeed well incorporated in the employed (Fig. S9 in ESI†). These images clearly demonstrate
interior of mesochannels of the pristine PMO–ICS.17 All that all ordered arrays of a homogeneous titania thin layer are
samples were also found to be amorphous according to high- well associated with the internal surface of mesopores of the
angle X-ray diffraction, indicating the absence of any crystal- PMO–ICS, and there is no separate TiO2 nano-domain in the
line TiO2 domains. On the other hand, TEM images of materials. Furthermore, 3D-reconstruction images also indi-
materials provided strong evidence that 2D-hexagonal cated that no channel blockage has occurred in the PICS-Ti
(45) (Fig. S9 in ESI†), which is a very promising feature for
application in catalysis. Since our initial goal of this research
was to employ mesoporous organosilica/amorphous TiO2
nanocomposites with varying degree of loaded titania inside
their mesochannels in catalysis, we were then very interested
to compare the catalytic performance of these nanocomposite
materials in photochemical deprotection of oximes as a probe
under sunlight irradiation. Despite the fact that lots of
methods have been developed for the generation of carbonyl
compounds from oximes,18 the number of energy efficient
systems reported in the literature involving TiO2 are very
limited.19 In the first step of our study, we immediately
realized that among the different kinds of TiO2-loaded PMO–
ICS materials, which had varying degree of TiO2 content, the
catalyst having 45% TiO2 inside the mesochannels (PICS-Ti
(45)) is the most active in the oxidative deprotection of aceto-
phenone oxime using molecular oxygen under sunlight
irradiation (Table 1), giving almost quantitative yields of
acetophenone.
Table 1 Structural parameters of PMO–ICS and PICS-Ti with varied Ti loading
SBET
DBJH
[nm]
Vt
Acetophenone
yielda [%]
Catalyst
[m2 g−1
]
[cm3 g−1
]
PMO-ICS
564
445
443
400
307
355
—
6.5
3.32
2.4
2.4
2.4
2.4
—
0.804
0.59
0.42
0.578
0.203
0.485
—
—
PICS-Ti (25)
PICS-Ti (35)
PICS-Ti (45)
PICS-Ti (55)
PICS-Ti (45B)
PICS-Ti (45T)
PMO-Ph-Ti (45)
14b
54b
100c
78b
99d
99e
70f
440
2.2
0.650
a GC yield using an internal standard (biphenyl) method unless
otherwise stated, reaction condition: oxime (20 mg), catalyst (25 mg),
12 ml dry CH3CN, O2 atmosphere (1 atm.), daily sunlight (10 am–4
pm). b Reaction time 10 h. c During 2 h. d The reaction was performed
using the recovered catalyst, PICS-Ti (45R), during 8 h. e Thermally
treated catalyst PICS-Ti (45T) at 400 °C was employed, reaction time
2 h. f The reaction was performed using PMO-Ph-Ti (45) having a 100%
phenylene group, reaction time 7 h.
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sunlight intensity ranging 80–100 × 103 Lux. To our knowledge
there has been no precedent example of using amorphous
TiO2 under such mild reaction as well as optical conditions in
the deprotection of oximes. In order to make better compari-
son, the photocatalytic aerobic oxidation of benzophenone
oxime was also tested using PMO-Ph-Ti (45) bearing a 100%
phenylene group instead of an isocyanuric group under the
same reaction conditions with the same TiO2 loading as
PMO-Ph-Ti (45), gave only 70% acetophenone within 7 hours
(Table 1). In a separate experiment the use of mesoporous
silica SBA-15 having essentially the same amorphous TiO2 was
shown to afford much inferior results in the deprotection of
acetophenone oxime under the described reaction conditions
(Table 2, entry 14). Notably, we found that the catalytic per-
formance of amorphous TiO2 prepared using a known pro-
cedure was quite negligible under the same reaction
conditions (Table 2, entry 15). These data imply that the high
performance of the present photocatalyst system might be
most likely originated from the isocyanuric groups embedding
inside the silica frameworks.
Table 2 Regeneration of carbonyl compounds from oximes under sunlight
irradiation using PICS-Ti (45)
Daily
temp.
range [°C]
Time
[h]
Yielda
[%]
Entry
R1
R2
R3
1b
2
3
Me
H
H
H
H
Cl
Me
H
H
H
H
H
H
H
H
H
H
Br
H
H
15
7
10
6
2
4
1
1
6
6
4
6
7
10
10
15
6
95
93
97
100
100
100
100
100
100
100
100
100
82
25–35
29–35
27–33
24–33
23–32
33–39
35–39
27–33
26–35
22–35
20–32
25–35
27–37
28–35
27–35
28–37
4
H
5
Me
Me
Me
Me
Me
Me
Et
6c
7
H
Me
OCH3
Cl
H
H
8
9
10
11
12
13
14d
15e
16f
Ph
H
2-Pyridyl acetophenone oxime
Me
Me
Me
H
H
H
H
H
H
7
2
On the basis of the above observations, we may conclude
that the excellent activity and selectivity of PICS-Ti (45) in the
described deoximation can be correlated to an unprecedently
reported and unique double synergetic effect of the presence
of isocyanuric groups in the pristine PMO–ICS as well as the
existing mesoporosity of the support where the titania photo-
active sites are located.
a GC yield using an internal standard (biphenyl) method unless
otherwise stated, reaction condition: oxime (20 mg), catalyst (25 mg),
12 ml dry CH3CN, O2 atmosphere (1 atm.), daily sunlight (10 am–
4 pm, sunlight intensity between 80–10 × 103 Lux). b In the dark. c In
the presence of 10 mg of the catalyst. d 25 mg SBA-15 having 45% TiO2
loading as photocatalyst. e With 25 mg of pure amorphous TiO2 as
photocatalyst. f Without catalyst.
We also investigated the recycling PICS-Ti (45) photocatalyst
from the reaction mixture of acetophenone oxime. After each
run, the catalyst was separated and washed with a mixture of
A survey of reaction media for this photocatalytic reaction CH3CN–MeOH (1 : 1), and dried at 75 °C under air. The catalyst
revealed that dry CH3CN provides the highest product yields was then successfully reused in five consecutive runs and
and selectivity. On the basis of this encouraging result, we showed a slight decrease in the catalytic activity after each
then speculate that this catalyst system might be also useful to recycling (Table S2 in ESI†). While the elemental analysis of
accomplish the same transformation for other types of struc- the catalyst after the first reaction cycle showed that the TiO2
turally diverse oximes. With optimized reaction conditions in content reduced to 36%, the total organic content remained
hand, we then proceeded to examine briefly the scope of this unchanged (Table S3 in ESI†). On the other hand, TEM images
photocatalytic deoximation reaction.
of the recovered catalyst also displayed a few inhomogeneity
As summarized in Table 2, various types of aromatic ald- (segregation) in the supported TiO2 domains (Fig. S8 in ESI†).
oximes and ketoximes were selectively oxidized to their corres- N2 sorption analysis of the recovered catalyst also confirms
ponding carbonyl compounds in excellent yields within that the material mostly retains its mesostructure, whereas
relatively short reaction times. The reaction of acetophenone both BET surface area and BJH pore size have been consider-
oxime in the presence of PICS-Ti (45) in the dark gave only 6% ably reduced. Consequently, the small decrease of catalytic
of acetophenone after 15 h, which confirmed that the TiO2 activity of PICS-Ti (45) seems to be a result of either leaching
layer in our catalyst is not acting as a Lewis acid catalyst of titanium species during the reaction or TiO2 segregation.
(Table 2, entry 1). It was also verified that in the absence of a
The authors acknowledge IASBS Research Councils for
catalyst no reactions occurred in sunlight under the oxygen support of this work.
atmosphere (Table 2, entry 16). The higher activity of ket-
oximes particularly electron-rich substrates in comparison with
aldoximes implies that the difference is possibly due to cat-
Notes and references
ionic character of intermediates in deoximation reaction.19b
However, variation in either electronic nature or steric hin-
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out using as low as 25 mg PICS-Ti (45), which contains just
11 mg of amorphous TiO2, in daily ambient temperature and
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