Mesoporous titania microspheres composed of exposed active faceted nanosheets and their catalytic activities for solvent-free synthesis of azoxybenzenes

Article abstract of DOI:10.1039/c3ce42252f

Mesoporous titania microspheres composed of nanosheets with exposed active facets were prepared by hydrothermal synthesis in the presence of hexafluorosilicic acid. They exhibited superior catalytic activity in the solvent-free synthesis of azoxybenzene by oxidation of aniline and could be used for 7 cycles with slight loss of activity. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3ce42252f

CrystEngComm
View Article Online
View Journal | View Issue
COMMUNICATION
Mesoporous titania microspheres composed
of exposed active faceted nanosheets and
their catalytic activities for solvent-free
synthesis of azoxybenzenes†
Cite this: CrystEngComm, 2014, 16,
1620
Received 5th November 2013,
Accepted 23rd December 2013
a
a
b
a
a
*
Liuliu Yang, Guodong Shi, Xuebin Ke, Ruwei Shen and Lixiong Zhang
DOI: 10.1039/c3ce42252f
www.rsc.org/crystengcomm
Mesoporous titania microspheres composed of nanosheets with
exposed active facets were prepared by hydrothermal synthesis in
the presence of hexafluorosilicic acid. They exhibited superior
catalytic activity in the solvent-free synthesis of azoxybenzene by
oxidation of aniline and could be used for 7 cycles with slight loss
of activity.
and dandelion-like18 hierarchical structures exhibits out-
standing performance in DSSCs, lithium ion batteries,
and photocatalysis.
As a versatile catalytic material, TiO2 is mostly used as a
photocatalyst and a support of both metal and oxide catalysts
in heterogeneous catalysis. A few studies used nano-sized TiO2
as heterogeneous catalysts mainly for organic synthesis, such
as Michael addition of indoles to α,β-unsaturated ketones,19
Mannich synthesis of β-aminocarbonyls,20 synthesis of
substituted 2-oxodihydropyrroles and bis(indolyl)methanes,21
etc. It is interesting that there are limited studies on using TiO2
as a catalyst for catalytic oxidation reactions, although titanium
containing materials, such as titanium-substituted molecular
sieves,22 are good catalysts for this kind of reaction. Oxidations
of amines with H2O2 to oximes and azoxybenzenes are the
main two reactions reported so far using the nano-sized TiO2
as the catalysts in which solvents such as acetone, CH3CN,
CH3OH, etc., are used to achieve good yields.23,24 It would be of
both academic and industrial interest to examine the catalytic
oxidation activity of these TiO2 with highly exposed crystal
facets or with hierarchical structures.
Aromatic azo compounds are high-value intermediates for
production of dyes, pigments, food additives, and drugs.25
Reduction of nitroaromatics and oxidation of anilines are the
main routes to prepare these compounds. Oxidation of
anilines can be carried out with oxygen at high pressures or
H2O2 under mild conditions. From a practical viewpoint, a
mild reaction is more favorable. As mentioned above, nano-
sized TiO2 exhibits excellent catalytic activity in oxidation of
anilines to azoxybenzenes; however, a solvent has to be used.
Synthesis of azoxybenzene by oxidation of aniline with H2O2
without a solvent provides a green and mild route and needs
to be developed.
Titanium dioxide (TiO2) has been intensively investigated in
the last couple of decades due to its relatively low cost and
wide application in many areas such as photocatalysis,
energy conversion and storage, and as sensors, catalysts, and
catalyst supports.1 It is generally synthesized as nanoparticles
and nano/microspheres. TiO2 with other morphologies, such
as disks,2 rods,3 wires4 and tubes,5 is also produced, as these
morphologies can endow unique properties to TiO2. Recently,
TiO2 with highly exposed crystal facets and these exposed facet
crystal-constructed hierarchical structures has attracted much
attention. Various techniques have been developed to prepare
these kinds of TiO2, mainly by hydrothermal or solvothermal
treatment of titanium precursor solutions synthesized in the
presence of a fluoric ion6,7 and capping reagents such as
ammonia,8 H2O2,9 alcohols,10 etc., or by hydrothermal treat-
ment of titania powder, nanowires or nanotubes in acidic/basic
solutions.11 Further results demonstrate that exposure of {001},
{110}, {010}, {111}, {102}, {106} and {103} facets can enhance
the reactivity of TiO2, especially in photocatalysis and dye-
sensitive solar cells (DSSCs).12 Similarly, TiO2 with exposed
facet crystal-constructed V-shaped channels,13 nanosheet or
nanorod assembly14,15 and core–shell,16 sphere-in-sphere,17
a State Key Laboratory of materials-oriented Chemical Engineering, College of
Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing,
China. E-mail: lixiongzhang@yahoo.com; Fax: +86 25 83172263;
Tel: +86 25 83172265
b School of Chemistry, Physics and Mechanism Engineering, Queensland University
of Technology, Brisbane, Qld 4001, Australia
Herein, we reported the preparation of mesoporous TiO2
microspheres (MTMs) with particle size of 2–15 μm com-
posed of TiO2 nanosheets (side length of 10–20 nm, thickness
of 4–6 nm) with exposed {101} facets (56%) by a simple
† Electronic supplementary information (ESI) available: Experimental details,
XRD pattern, XPS spectra. See DOI: 10.1039/c3ce42252f
1620 | CrystEngComm, 2014, 16, 1620–1624
This journal is © The Royal Society of Chemistry 2014

View Article Online
CrystEngComm
Communication
hydrothermal synthesis. The microspheres exhibited superior
catalytic activities in solvent-free oxidation of arylamines to
azoxybenzenes rather than azobenzenes. To the best of our
knowledge, no such application of exposed faceted crystal-
constructed hierarchical TiO2 structures in heterogeneous
catalysis has been reported.
The (−101), (101) and (002) atomic planes with lattice spac-
ings of 0.35, 0.35 and 0.48 nm indicate that the top/bottom
surface exposed by truncation is bound by a {001} facet.28,29
The inset of Fig. 1c shows that the angle labeled in the corre-
sponding fast-Fourier transform (FFT) image is 68.3 0.3°,
which is identical to the theoretical value for the angle
between the {101} and {001} facets.28 These facets prove that
the anatase single crystals exhibit flat facets of {101} and
{001}. On the basis of the above structural information, the
percentage of {001} facets in the TiO2 nanosheets shown
in Fig. 1c is estimated to be 44% according to eqn (1).30
Correspondingly, the percentage of {101} facets is 56%.
The MTMs were hydrothermally synthesized at 180 °C
for 48 h using a synthesis solution with a molar ratio of
1TBOT : 0.1H2SiF6 : 1.5H2O prepared by adding a H2SiF6 aque-
ous solution into TBOT under vigorous stirring (ESI†). Fig. 1a
shows XRD patterns of the as-synthesized sample, indicating
formation of pure anatase TiO2 (tetragonal, I41/amd, JCPDS
no. 21-1272). The diffraction peaks at 2θ of 25.3°, 37.8°, and
48.5° can be observed obviously, corresponding to (101),
(004) and (200) faces of TiO2. The peak intensity at 2θ = 25.3°
is much stronger than those at 37.8° and 48.5°, implying the
preferred growth along the (101) face.26 The primary crystal-
lite size calculated from the (101) peak of the XRD pattern
using the Scherrer equation is about 14.7 nm. XRD patterns
of the samples synthesized at 2–96 h (Fig. S1, ESI†) indicate
that the intensity of the (101) face becomes stronger with
increasing synthesis time and reaches the strongest at 48 h,
indicating the highest crystallinity of the sample after synthe-
sis for 48 h.
Fig. 1b shows an SEM picture of the MTMs. It exhibits
spherical morphology with particle size of 2–12 μm (inset in
Fig. 1b). Close observation reveals that the microsphere is
composed of compactly packed nanocrystals.
To elucidate the fine structure of nano-sized sheets, we
examined the sample by TEM after grinding and ultra-
sound dispersing the sample. Analysis of a representative
high-resolution TEM image of a single nanosheet (Fig. 1c)27
reveals that these nanosheets are well-faceted nanocrystals
with side length of 10–20 nm and thickness of ca. 4.6 nm.
a2
b2  a2
100%
(1)


 a2
cos
where, a is the side length of the {001} facet, b the bottom
edge of the {101} facet and θ the value for the angle between
the {101} and {001} facets.
The nitrogen adsorption–desorption isotherm was mea-
sured to determine the specific area and pore volume of the
MTM. The corresponding results are presented in Fig. 1d.
The pore size distribution, derived from desorption data and
calculated using the BJH model, indicated that the average
pore of such a sample is around 10.2 nm. The Brunauer–
Emmett–Teller (BET) specific surface area of the sample
calculated from N2 adsorption is 117 m2 g−1, which is larger
than that of conventionally prepared TiO2 microspheres.13,15
The much higher BET surface area of the MTM may provide
the possibility of contacting the active center more effectively.
To investigate the elemental compositions and the binding
states of the MTM, the sample was analyzed by XPS (Fig. S3,
ESI†). Sharp photoelectron peaks appear at binding energies
of 458 (Ti 2p) and 531 eV (O 1s). An F 1s peak at 684.5 eV is
observed due to surface fluorination. High-resolution XPS was
used to detect the surface F 1s, Ti 2p, and O 1s core levels. No
signals for F− in the lattice (BE = 688.5 eV) are found after the
MTM was calcined at 600 °C, implying that all F− are physically
adsorbed on the surface of the MTM and do not substitute
oxygen in the TiO2 lattice. The binding energy of Ti 2p3/2
is equal to 458.7 eV and the binding energy of Ti 2p1/2
is equal to 464.4 eV, which are identical to those in the
reported literature.31 These results confirm that titanium
exists as a Ti(IV) state. The O 1s peak at 530.2 eV indicates
oxygen in Ti–O–Ti. No Si peak (101.8 eV) is observed,
suggesting that Si in H2SiF6 does not enter or is not adsorbed
on the MTM.
The MTM shows quite high catalytic activity in the cata-
lytic oxidation of arylamines with H2O2 to azoxybenzenes. In
particular, no solvent is used in these reaction systems,
unlike the reported system for conversion of aniline using
H2O2 to azoxybenzene, in which acetone, CH3CN and CH3OH
are usually used as solvents,23,24 implying a green catalytic
synthesis route. Table 1 lists the reaction results of oxidation
of some arylamines with H2O2. It is observed that aniline
Fig. 1 XRD pattern (a), SEM image (b), and TEM image (c) of the TiO2
microspheres composed of nanosheets, recorded from an anatase
single nanocrystal with the corresponding fast-Fourier transform (FFT)
pattern in the inset, and the nitrogen adsorption–desorption isotherm
and the Barrett–Joyner–Halenda (BJH) pore size distribution plot of the
MTM (d).
This journal is © The Royal Society of Chemistry 2014
CrystEngComm, 2014, 16, 1620–1624 | 1621

View Article Online
Communication
CrystEngComm
Table 1 Oxidation of arylamines to azoxybenzenes in the presence of
a TiO2/H2O2 systema
Entry
1
Arylamine
Time (h)
0.75
Conversion (%)
97.8
Yield (%)
95.6
2
3
4
2
98.7
91.0
94.5
86.5
4.5
4.5
6.0
85.4
80.8
81.3
75
5
a
Catalyst = 25 mg; arylamines = 0.02 mol; arylamines/H2O2 molar
ratio = 1 : 1.7; temperature = 333 K.
Fig. 2 Effect of reaction time on aniline conversion and azoxybenzene
yield with MTM and P25 and without a catalyst (catalyst = 25 mg;
aniline = 1.86 g; aniline/H2O2 molar ratio = 1 : 1.7; temperature = 333 K).
results in a good yield of azoxybenzene within a shorter reac-
tion time when compared with substituted anilines, while tolu-
idines are more reactive than chloro or methoxy substituted
anilines. This may be attributed to the smaller size of toluidine
atoms, which would allow the reactant molecules to gain closer
proximity to the active centers of the titanium peroxo species
compared to the larger chlorine atoms or methoxy groups.
However, the differences in the reactivity of various substituted
anilines are dependent on the synergetic effect of the molecu-
lar parameters such as the mesomeric (+M), the inductive
(+I/−I) and the steric effect of the substituents.24 Furthermore,
it should be noted that no obvious difference was observed on
the conversion of aniline and the yield of azoxybenzene
when the reaction was conducted with or without sunlight
irradiation, clearly suggesting that the MTM does not act as
a photocatalyst in the present reaction.
To compare the catalytic property of the TiO2 micro-
spheres with TiO2 powder P25 (Degussa), we conducted time-
dependent kinetics of aniline with H2O2 using the MTM and
P25 as the catalysts. A blank test without a catalyst was also
conducted. The reaction results shown in Fig. 2 clearly dem-
onstrate that this reaction proceeds at a quite slow reaction
rate with low conversion of aniline without a catalyst.
Increasing the reaction time directly increases the aniline
conversion using the MTM and P25. The reaction rate of the
MTM is significantly greater than that of P25. The yield of
azoxybenzene increases smoothly with the reaction time for
30 min to 45 min. After 45 min, aniline conversion reaches a
maximum value of 97.8% and the yield of azoxybenzene is up
to 95.6%, which are much higher than those by P25.
Fig. 3 Cycling curve for aniline oxidation with the MTM as the catalyst.
facets were prepared by changing the synthesis time, and the
results of aniline oxidation to azoxybenzene on these samples
are listed in Table 2 (entries 1 and 2). By extending the syn-
thesis time to 72 and 96 h at 180 °C, the percentages of {101}
facets of the MTM are 47% to 37%, respectively. This is simi-
lar to the result reported by Yang, et al.6a The conversion of
Table 2 Oxidation of aniline to azoxybenzene using different catalysts
Hierarchical
Fig. 3 shows that after 6 recycling uses, the activity of the
MTM remains above 93.3% with selectivity of azoxybenzene
being 95.6% , which indicates that the exposed crystal facets
of TiO2 microspheres have high catalytic stability in the
oxidation of arylamine. A slight loss in the catalytic activity
was observed in the 7th recycling use (conversion: 91.2%,
selectivity: 94.3%).
To identify the contributions of the {101} facets and the
hierarchical structure of the MTM on aniline oxidation,
diverse samples with varying percentages of exposed {101}
Entry Catalyst Conversion (%) Yield (%) P(101) (%) structure
1
2
3
4
5
6
MTMa
MTMb
MTMc
ATNM
ATNHS 61.8
NS 87.3
94.5
93.6
91.4
22.6
90.9
89.6
89.3
21.0
58.4
85.4
47
37
56
94
10
20
Yes
Yes
No
Yes
Yes
No
a
The MTM sample synthesized at 180 °C for 72 h. b The MTM sample
c
synthesized at 180 °C for 96 h. The MTM sample synthesized at
180 °C for 48 h and ground into mostly nanosheets. The reaction time
is 0.75 h.
1622 | CrystEngComm, 2014, 16, 1620–1624
This journal is © The Royal Society of Chemistry 2014

View Article Online
CrystEngComm
Communication
aniline and the yield of azoxybenzene are slightly decreased,
suggesting possibly more active sites on the MTM with a
higher percentage of {101} facets. To examine the effect of
the hierarchical structure of the MTM on aniline oxidation,
we ground the MTM sample to form mostly nanosheets. Even
with an increase in the percentage of {101} facets (56%), the
conversion still decreases slightly (entry 3), possibly due to a
lack of hierarchical structure. To further elucidate the effect,
anatase TiO2 nanosheet-based microspheres (ATNM)15 and ana-
tase TiO2 nanoparticle-based hierarchical spheres (ATNHS),29 as
well as titania nanosheets (NS)7 were also prepared, with {101}
facet percentages of 94%, 10%, and 20%, respectively. The
results indicated that the samples with hierarchical structures
and both very high or low {101} facet percentages (entries 4
and 5) exhibit quite low conversions and yields, while the
sample with a very low {101} facet percentage shows slightly
low conversion and yield (entry 6), compared with those for
the MTM. These results suggested that both the {101} facet
percentage and the hierarchical structure of the MTM play
significant roles in the oxidation of aniline. To obtain much
higher catalytic activity, TiO2 catalysts with intermediate {101}
facet percentages as well as hierarchical structures have to be
used (entries 1 and 2 in Table 2).
In conclusion, we developed a new approach to synthesize
anatase mesoporous TiO2 microspheres composed of nano-
sheets with exposed {101} facets (56%) by a simple hydro-
thermal synthesis using H2SiF6. The microsphere shows high
conversion and yield in the oxidation of arylamines to
azoxybenzenes with H2O2 without a solvent under mild
conditions. Both the appropriate proportion of {101} and
{001} facets and the hierarchical structure of the MTM
contribute to the high reaction activity. The MTM provides
an environment-friendly and easily recyclable heterogeneous
catalytic route for the preparation of azoxybenzenes.
8 (a) C. T. Dinh, T. D. Nguyen, F. Kleitz and T. O. Do, ACS
Nano, 2009, 3, 3737; (b) X. Y. Ma, Z. G. Chen, S. B. Hartono,
H. B. Jiang, J. Zou, S. Z. Qiao and H. G. Yang, Chem.
Commun., 2010, 46, 6608; (c) B. H. Wu, C. Y. Guo, N. Zheng,
Z. X. Xie and G. D. Stucky, J. Am. Chem. Soc., 2008, 130,
17563; (d) X. K. Ding, H. C. Ruan, C. Zheng, J. Yang and
M. D. Wei, CrystEngComm, 2013, 15, 3040; (e) H. Xu,
P. Reunchan, S. X. Ouyang, H. Tong, N. Umezawa, T. Kako
and J. H. Ye, Chem. Mater., 2013, 25, 405.
9 (a) X. L. Wang, H. L. He, Y. Chen, J. Q. Zhao and
X. Y. Zhang, Appl. Surf. Sci., 2012, 258, 5863; (b) M. Liu,
L. Y. Piao, L. Zhao, S. T. Ju, Z. J. Yan, T. He, C. L. Zhou and
W. J. Wang, Chem. Commun., 2010, 46, 1664; (c) Y. G. Miao
and J. C. Gao, J. Solid State Chem., 2012, 196, 372.
10 (a) H. G. Yang, G. Liu, S. Z. Qiao, C. H. Sun, Y. G. Jin,
S. C. Smith, J. Zou, H. M. Cheng and G. Q. Lu, J. Am. Chem.
Soc., 2009, 131, 4078; (b) J. M. Du, J. S. Zhang and D. J. Kang,
CrystEngComm, 2011, 13, 4270; (c) J. Zhu, S. H. Wang,
Z. F. Bian, S. H. Xie, C. L. Cai, J. G. Wang, H. G. Yang and
H. X. Li, CrystEngComm, 2010, 12, 2219; (d) W. G. Yang,
Y. L. Wang and W. M. Shi, CrystEngComm, 2012, 14, 230.
11 (a) W. Li, Y. Bai, W. J. Liu, C. Liu, Z. H. Yang, X. Feng,
X. H. Lu and K. Y. Chan, J. Mater. Chem., 2011, 21, 6718; (b)
J. M. Li and D. S. Xu, Chem. Commun., 2010, 46, 2301; (c)
M. Miyauchi, J. Mater. Chem., 2008, 18, 1858.
12 P. H. Wen, Y. Ishikawa, H. Itoh and Q. Feng, J. Phys. Chem.
C, 2009, 113, 20275.
13 H. Pan, J. S. Qian, Y. M. Cui, H. X. Xie and X. F. Zhou,
J. Mater. Chem., 2012, 22, 6002.
14 (a) J. S. Chen, Y. L. Tan, C. M. Li, Y. L. Cheah, D. Luan,
S. Madhavi, F. Y. C. Boey, L. A. Archer and X. W. Lou, J. Am.
Chem. Soc., 2010, 132, 6124; (b) G. Liu, J. Pan, L. C. Yin,
J. T. Irvine, F. Li, J. Tan, P. Wormald and H. M. Cheng, Adv.
Funct. Mater., 2012, 22, 3233; (c) J. Zhu, J. G. Wang, F. J. Lv,
S. X. Xiao, C. Nuckolls and H. X. Li, J. Am. Chem. Soc., 2013,
135, 4719; (d) J. M. Li, K. Cao, Q. Li and D. S. Xu,
CrystEngComm, 2012, 14, 83.
Notes and references
1 (a) X. B. Chen and S. S. Mao, Chem. Rev., 2007, 107, 2891; (b)
U. Diebold, Surf. Sci. Rep., 2003, 48, 53; (c) D. V. Bavykin,
J. M. Friedrich and F. C. Walsh, Adv. Mater., 2006, 18, 2807;
(d) Z. Y. Zhou, N. Tian, J. T. Li, I. Broadwell and S. G. Sun,
Chem. Soc. Rev., 2011, 40, 4167.
15 W. G. Yang, J. M. Li, Y. L. Wang, F. Zhu, W. M. Shi,
F. R. Wan and D. S. Xu, Chem. Commun., 2011, 47, 1809.
16 (a) B. S. Liu, K. Nakata, M. Sakai, H. Saito, T. Ochiai,
T. Murakami, K. Takagi and A. Fujishima, Catal. Sci.
Technol., 2012, 2, 1933; (b) B. S. Liu, K. Nakata, M. Sakai,
H. Saito, T. Ochiai, T. Murakami, K. Takagi and
A. Fujishima, Langmuir, 2011, 27, 8500.
2 A. Y. Zhang, M. H. Zhou, L. Han and Q. X. Zhou, J. Hazard.
Mater., 2011, 186, 1374.
3 Y. W. Jun, M. F. Casula, J. H. Sim, S. Y. Kim, J. Cheon and
A. P. Alivisatos, J. Am. Chem. Soc., 2003, 125, 15981.
4 Y. B. Mao and S. S. Wong, J. Am. Chem. Soc., 2006, 128, 8217.
5 T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino and
K. Niihara, Langmuir, 1998, 14, 3160.
6 (a) H. G. Yang, C. H. Sun, S. Z. Qiao, J. Zou, G. Liu,
S. C. Smith, H. M. Cheng and G. Q. Lu, Nature, 2008, 453,
638; (b) D. Q. Zhang, G. S. Li, X. F. Yang and J. C. Yu, Chem.
Commun., 2009, 29, 4381; (c) J. Pan, G. Liu, G. Q. Lu and
H. M. Cheng, Angew. Chem., Int. Ed., 2011, 50, 2133.
7 X. G. Han, Q. Kuang, J. M. Shang, Z. X. Xie and L. S. Zheng,
J. Am. Chem. Soc., 2009, 131, 3152.
17 H. X. Li, Z. F. Bian, J. Zhu, D. Q. Zhang, G. S. Li, Y. N. Huo,
H. Li and Y. F. Lu, J. Am. Chem. Soc., 2007, 129, 8406.
18 H. B. Wu, J. S. Chen, X. W. Lou and H. H. Hng, Nanoscale,
2011, 3, 4082.
19 A. Fischer, P. Makowski, J. O. Muller, M. Antonietti,
A. Thomas and F. Goettmann, ChemSusChem, 2008, 1, 444.
20 M. Z. Kassaee, R. Mohammadi, H. Masrouri and
F. Movahedi, Chin. Chem. Lett., 2011, 22, 1203.
21 M. L. Kantam, S. Laha, J. Yadav, B. M. Choudary and
B. Sreedhar, Adv. Synth. Catal., 2006, 348, 867.
22 (a) A. Tuel, S. Gontier and R. Teissier, Chem. Commun., 1996,
651; (b) Y. S. S. Wan, K. L. Yeung and A. Gavruulidis, Appl.
This journal is © The Royal Society of Chemistry 2014
CrystEngComm, 2014, 16, 1620–1624 | 1623

View Article Online
Communication
Catal., A, 2005, 281, 285; (c) S. Gontier and A. Tuel, J. Catal.,
CrystEngComm
26 R. L. Penn and J. F. Banfield, Geochim. Cosmochim. Acta,
1999, 63, 1549.
27 For the low-resolution TEM images of MTM synthesized for
48 h, see Fig. S2†.
28 Y. Q. Dai, C. M. Cobley, J. Zeng, Y. M. Sun and Y. N. Xia,
Nano Lett., 2009, 9, 2455.
29 D. H. Wang, J. Liu, Q. S. Huo, Z. M. Nie, W. G. Lu,
R. E. Williford and Y. B. Jiang, J. Am. Chem. Soc., 2006,
128, 13670.
1995, 157, 124; (d) S. Gontier and A. Tuel, Appl. Catal., A,
1996, 143, 125; (e) H. R. Sonawane, A. V. Pol, P. P. Moghe,
S. S. Biswas and A. Sudalai, J. Chem. Soc., Chem. Commun.,
1994, 1215.
23 (a) W. K. Chan, C. M. Ho, M. K. Wong and C. M. Che, J. Am.
Chem. Soc., 2006, 128, 14796; (b) C. F. Chang and S. T. Liu,
J. Mol. Catal. A: Chem., 2009, 299, 121; (c) N. Jagtap and
V. Ramaswamy, Appl. Clay Sci., 2006, 33, 89.
24 H. Tumma, N. Nagaraju and K. V. Reddy, Appl. Catal., A,
2009, 353, 54.
25 (a) S. Sakuae, T. Tsubakino, Y. Nishiyama and Y. Ishii,
J. Org. Chem., 1993, 58, 3633; (b) A. B. E. Vix, P. M. Buschbaum,
W. Stocker, M. Stamm and J. P. Rabe, Langmuir, 2000, 16, 10456.
30 Y. B. Zhao, W. H. Ma, Y. Li, H. W. Ji, C. C. Chen, H. Y. Zhu
and J. C. Zhao, Angew. Chem., Int. Ed., 2012, 124, 3242.
31 (a) J. C. Yu, J. G. Yu, W. K. Ho, Z. T. Jiang and L. Z. Zhang,
Chem. Mater., 2002, 14, 3808; (b) D. Li, H. Haneda,
S. Hishita and N. Ohashi, Chem. Mater., 2005, 17, 2596.
1624 | CrystEngComm, 2014, 16, 1620–1624
This journal is © The Royal Society of Chemistry 2014