Direct syntheses of sulfonated mesoporous SiO2-TiO 2-SO3H materials as solid acid catalysts

Article abstract of DOI:10.1039/c0jm01174f

A facile one-step approach for a mesoporous solid acid catalyst SiO ₂-TiO₂-SO₃H network was developed by the simple chelation of a benzene disulfonate compound to the Ti ion. The resulting catalyst showed excellent catalytic performance for esterification.

Full text of DOI:10.1039/c0jm01174f

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www.rsc.org/materials | Journal of Materials Chemistry
Direct syntheses of sulfonated mesoporous SiO -TiO -SO H materials as
2
2
3
solid acid catalysts†
a
a
a
ab
Hyang Im Ryoo, Lan Young Hong, Sang Hee Jung and Dong-Pyo Kim*
Received 23rd April 2010, Accepted 29th June 2010
DOI: 10.1039/c0jm01174f
A facile one-step approach for a mesoporous solid acid catalyst
2 2 3
SiO -TiO -SO H network was developed by the simple chelation of
a benzene disulfonate compound to the Ti ion. The resulting catalyst
showed excellent catalytic performance for esterification.
There is an increasing need for eco-friendly and efficient catalysts due
to global legislation for environmental protection. Solid acids have
attracted interest as environmentally benign catalysts in most chem-
ical processes because of their advantages over homogeneous acids,
such as non-corrosive, easy separation by filtration, and less waste
production through recycling. Various insoluble solid acids have been
reported to increase catalytic activity, and zeolites are considered to
1
be good alternative acid catalysts owing to their large surface area.
ꢀ
However, the small pore size in the 20 A range makes them unsuit-
Scheme 1 (a) A synthetic diagram of sulfonated mesoporous network
and (b) sulfonation chemistry of the SiO
chelation.
2 2
-TiO structure via catecholic
able for reactions involving bulky substrates. More desirable is a large
regular pore structure of mesoporous solids with stronger Brønsted
and/or Lewis acidity. The sulfonic acid-functionalized mesoporous
silica provides a way of reinforcing the Brønsted acidity of solid acid,
and has been used for a variety of chemical reactions, such as ester-
2 2 3
Pure inorganic SiO -TiO -SO H mesoporous materials, labeled as
SP series, were prepared by binary sol–gel reactions, involving
separate pre-hydrolysis of tetraethylorthosilicate (TEOS) with a tri-
block copolymer surfactant (Pluronic P123) for the silica sol and that
ification. The SO H moieties on the surface of the ordered meso-
3
porous MCM-41, SBA-15, and Ph-SO H are mostly incorporated by
3
2
4
of TiCl for the titania sol, mixing of the two sols at Ti : Si mole ratios
a two-step process : co-condensation or anchoring of -SH containing
alkoxide precursors and subsequent oxidation in the presence of
of 0.1 and 0.2 for further reaction, and incorporating disodium
ꢀ
3
H O or a post-modification grafting route, which are somewhat
4,5-dihydroxy-1,3-benzene disulfonate (Tiron ) into the binary SiO
-
2
2
2
TiO2 network. Inorganic–organic hybrid mesoporous materials,
labeled HP series, were also synthesized by mixing the aforemen-
complicated and often incomplete. Therefore, a simpler synthesis
technique that produces the mesostructure and the sulfonic acid sites
simultaneously is needed.
2 2 3
tioned SiO -TiO -SO H composite sols with 3-(methacryloyloxy)-
propyl trimethoxysilane (MPTMS) and polyethylene glycol
dimethacrylate (PEG-DMA) to improve the mechanical reliability.
Here, we propose a facile one-step approach for the incorporation
2 2
of benzene disulfonate into a mesoporous SiO -TiO surface, thereby
The elemental analysis results in Table 1 show that the SO
ratios of the samples are stoichiometrically consistent with the initial
compositions of the precursors. These binary SiO -TiO mesoporous
3
H/Ti
opening a path to aromatic acid-functionalization with multiple acid
sites. The key synthetic approach is the chelation of a catecholic
2
2
2 6 2 3 2
compound including two sulfonate groups, (OH) C H (SO Na) , on
structures were characterized by small-angle X-ray diffraction, as
shown in Fig. 1. The unhybridized 1SP specimen with a Ti-to-Si ratio
of 0.1 showed typical diffraction patterns of a SBA15-like hexagonal
robust mesoporous networks, as illustrated in Scheme 1. This one-pot
aromatic acid-functionalization of mesoporous oxide support is
illustrated with esterification reactions under a variety of conditions.
In particular, it is expected that the incorporation of an aromatic
sulfonic acid with a more electron withdrawing phenyl group would
increase the acidic strength of the resulting mesoporous material
significantly, compared with the traditional aliphatic sulfonic acid-
Table 1 Experimental S/Ti elemental ratio, BET surface area, acid
density, catalytic activity and turn over frequency (TOF) of various
SiO -TiO -SO H mesoporous materials
2
2
3
4
functionalized samples.
Acid
density
2
BET (m /g)
Sample S/
code
Yield Time
(h)
3
+
Ti ratio [pore vol. (cm /g)] (meq H /g) (%)
TOF
a
Department of Fine Chemical Engineering and Chemistry, Chungnam
National University, Yuseong Gu, Daejeon, 305-764, Korea. E-mail:
1
1
2
1
SP-4T 2.00
404 [0.40]
313 [0.32]
4 [0.01]
58 [0.05]
120 [0.15]
1.08
0.30
0.21
0.14
0.08
97
97
97
90
73
1
5
10
12
18
150
107
77
89
86
dpkim@cnu.ac.kr; Fax: +82-42-823-6665; Tel: +82-42-821-6695
b
Graduate School of Analytical Science and Technology, Chungnam
National University, Yuseong-Gu, Daejeon, 305-764, Korea
SP-1T 0.49
SP-1T 0.25
HP-4T 1.83
† Electronic supplementary information (ESI) available: Further
experimental details. See DOI: 10.1039/c0jm01174f
1HP-1T 0.47
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ꢂ1
tint as well as a peak at ꢁ940 cm corresponding to the Si-O-Ti
8
stretching vibration, as measured by FT-IR spectroscopy (see ESI†
Fig. S4). This confirms that there is no phase separation of the
inorganic and organic moieties. This homogeneity also indirectly
supports the stoichiometric coordination of catechols to Ti atoms. It
should be noted that a benzene disulfonated mesoporous network
was formed directly for the first time using a facile one-step sol–gel
reaction. It is most likely that the surface of the binary mesoporous
2 2
SiO -TiO network structure was coincidentally functionalized by
forming a catechol-Ti complex, as illustrated in Scheme 1. Further-
more, the cured films of sulfonated resins on the glass substrates
exhibited improved hydrophilic behavior as expected. For example,
Fig. 1 (a) Small-angle XRD patterns of the sulfonated mesoporous
samples with different chemical compositions, and (b) TEM image of
mesoporous 1SP-1T sample.
ꢀ
1SP-1T film showed a lower contact angle of 37 against water
ꢀ
droplet, compared with the contact angle of 46 for unfunctionalized
SiO
2 2
-TiO .
ꢀ
structure, which shows a high intensity (100) peak at 0.88 and
ꢀ
2 2 3
The acid densities of the SiO -TiO -SO H specimens prepared at
additional higher order (110) peaks at 1.72 . This d(100) spacing of
various compositions were determined with 0.01 M NaOH (aq), as
9
.8 nm indicates the formation of highly ordered mesoporous SiO
2
-
summarized in Table 1. The acid density after the simple exchange of
+
the Na ion of the SO
TiO binary networks with pores, approximately 10 nm in diameter,
2
3
Na moiety with a proton was approximately
which is relatively larger than the 2–7 nm reported for mesoporous
2
silica. The transmission electron microscopy (TEM) image of
proportional to the extent of sulfonation, as shown in the elemental
analysis (see ESI† Table S2). This may provide a protocol for
introducing strong Brønsted acid sites to the surface. Alternatively,
the acid strength was also measured using the Hammett indicators,
anthraquinone and 2,4,6-trinitroaniline. The Hammett constants of
1SP-1T (see ESI† for notation) in Fig. 1(b) also supports the presence
of a highly ordered structure with no segregated SiO and TiO
2
2
phases. However, the incorporation of an organic moiety such as
MPTMS and PEG-DMA caused microstructural changes with peak
broadening as well as a slight shift in the (100) peak to a lower
diffraction angle, while the processability and toughness of the hybrid
resin were enhanced. The change in this peak suggests that the pore
size was made larger with slight disordering while maintaining the
binary SiO -TiO
2
2
-SO
3
H samples (1SP-4T) were in the range of ꢂ8.2
9
to ꢂ10.10, which were as acidic as nearly 90% sulfuric acid solution.
However, the acid density of the hybrid 1HP-1T and 1HP-4T
samples (see ESI† for notation) decreased drastically to 0.08 and 0.14
+
meq H /g, respectively, presumably due to a blocking effect of the
hexagonal pore structure. Interestingly the mesoporous SiO
skeleton was preserved even with high organic content
ca. 50 mol%), whereas previous work reported a restriction on the
2
-TiO
2
organic content on the active sites.
a
2 2 3
The solid acid catalytic performance of the SiO -TiO -SO H
(
mesoporous materials for esterification was evaluated with the
ꢀ
organic content due to the collapse of the mesoporous structure that
5
occurs for contents larger than 20 mol%.
condensation of acetic acid and ethanol at 80 C using the powdery
solid acids prepared in this study (Fig. 2).
On the other hand, the peak intensities were reduced significantly
when the titania portion was increased to a Ti : Si ratio of 0.2, as
shown in Fig. 1(a), presumably due to lack of homogeneity, i.e. dis-
ordering of the network structure. Table 1 lists the surface areas from
BET adsorption measurements. 1SP-4T and 1SP-1T show BET
The catalytic conversion was impressively high when compared to
unfunctionalized SiO -TiO and the background reaction with no
2 2
catalyst. In particular, the initial reaction rates were considerably
faster in nearly reaching completion in 60 min. In general, the
conversion rate increased with increasing acid density, i.e., the cat-
echolic sulfonic acid content. The 1SP-4T with the highest acid
density gave 73% and 99% yields in 10 min and 6 h, respectively. This
2
surface areas of 404 and 313 m /g, respectively, which are relatively
2
smaller than those of pure SiO mesoporous materials with a similar
5
pore size (see ESI† Fig. S1). This suggests that an increase in the Ti
content or the incorporation of organic components caused a signif-
icant decrease in surface area, which is consistent with the partial
collapse or decreased structural order. In addition, the TGA-DTA
thermograms showed high thermal stability of the 1SP sample with
ꢀ
only 4 wt% weight loss below 300 C due to the adsorbed moisture
and volatile organic portion (see ESI† Fig. S2).
A catecholic compound (dihydroxy benzene) can be readily
6
chelated to Ti ions. In this study, chelation chemistry was introduced
for the direct synthesis of a SiO -TiO
2
2
-SO
3
H network by coordi-
2
(SO Na) ,
nating 4,5-dihydroxy-1,3-benzene disulfonate, (OH)
2
C
6
H
2
3
as a catecholic compound containing a sulfonate group to a Ti atom.
Under the acidic conditions of the sol–gel reaction, chelation was
facilitated due to the conversion of the catecholic salt to a phenolic
7
form as reported previously. This chelation turned the solution to
ꢀ
Fig. 2 Esterification of acetic acid and ethanol at 80 C catalyzed by
a red-orange color due to Ti–catechol charge transfer, which was
confirmed by the intense absorption at 440 nm (see ESI† Fig. S3). On
the other hand, all samples showed optical transparency with a yellow
various SiO
mmol: (a) 1SP-4T; (b) 1SP-1T; (c) 2SP-1T; (d) 1HP-4T; (e) 1HP-1T; (f)
unfunctionalized SiO -TiO ; (g) no catalyst.
2 2 3
-TiO -SO H catalysts 0.1 g, acetic acid 17 mmol, ethanol 153
2
2
6
420 | J. Mater. Chem., 2010, 20, 6419–6421
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Table 2 Esterification yields of 1SP-1T and 1SP-4T samples with
ꢀ
lasting 6 h, while the same conversion by 1SP-4T decreased to 35%
from 100% upon using it two times, each lasting 6 h. The stability of
a
amount 0.1 g on various carboxylic acids at 80 C
1
SP-1T is comparable to that of the reported solid acid catalytic
Conversion, %
1,2
systems. It should be noted that the catalytic activity is better
retained for larger fatty acids. This aspect of the catalyst that is related
to the deactivation requires further study for definitive explanation of
the behavior.
Acid
1st use, 1 h
1st use, 6 h
2nd use, 6 h
3rd use, 6 h
Acetic
61 (97)
61 (80)
55 (77)
26 (68)
33 (66)
30 (60)
97 (100)
95 (98)
94 (95)
92 (100)
93 (100)
93 (100)
85 (35)
86 (39)
87 (39)
88 (54)
89 (55)
89 (55)
70
76
78
86
87
87
Propanoic
Butanoic
Lauric
Palmitic
Oleic
In summary, a simple one-step synthesis procedure for preparing
2 2 3
a mesoporous SiO -TiO -SO H solid acid catalyst has been pre-
sented, thereby providing a route to aromatic acid-functionalization
with multiple acid sites. The solid acids have been shown to deliver
a catalytic activity for esterification that is higher than those of the
usual aliphatic sulfonic acid-functionalized ones and commercially
available solid acid catalysts reported in the literature. This excellent
catalytic performance could be attributed to the promoted acid
strength of the sulfonic acid group in the presence of an electron
withdrawing phenyl group, and the synergically large pore size. The
a
Reaction conditions: carboxylic acid ¼ 17 mmol, EtOH ¼ 153 mmol.
1
SP-4T sample data, determined by GC, are shown in parentheses.
reveals that the catalytic activity of the developed SiO -TiO -SO H
2
2
3
was higher than that of the reported SiO -SO H prepared by two-
3
2
10
step sulfonation, 30% yield in 1 h and 90% 6 h. In the case of 1SP-4T
and 1SP-1T, TOFs were higher than those of other catalysts owing to
their relatively higher acidity and surface area. This remarkable acid
2 2 3
SiO -TiO -SO H materials are useful as eco-friendly nanocatalysts
for esterification and biofuel production.
This work was supported by the National Research Foundation of
Korea (NRF) grant funded by the Korea government (MEST)
2 2 3
catalytic activity of the newly developed solid acid, SiO -TiO -SO H,
could be attributed to the enhanced acidic strength of the catecholic
sulfonic acid due to the presence of a neighboring electron with-
drawing phenyl group as well as the increased acid density inside the
large pores by 2 acid groups in 1 molecule of catecholic acid.
The esterification of several organic acids with different chain
lengths was also carried out. As shown in Table 2, the smaller the
organic acid is, the faster it was converted because of the difference in
mass transfer. Smaller molecules diffuse readily into the pores of
mesoporous solid acids. The conversions of the larger fatty acids were
generally lower than those of the smaller organic acids. But, the larger
fatty acids also showed high conversion over 92% when the reaction
(
20100000722).
Notes and references
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Catal. Commun., 2008, 9, 769; B. R. Jermy and A. Pandurangan,
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ꢁ
2
B. R ꢁa c, P. AMoln ꢁa rForgo, M. Mohai and I. Bert oꢁ ti, J. Mol. Catal. A:
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J. A. Melero, R. V. Grieken and G. Morales, Chem. Rev., 2006, 106,
3790.
3
time was extended to 6 h. The SiO
2
2 3
-TiO -SO H solid acids still
exhibited higher catalytic activity for esterification than the other
4 C. S. Gill, B. A. Price and C. W. Jones, J. Catal., 2007, 251, 145.
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G. D. Stucky, Chem. Mater., 2000, 12, 2448.
11
3 3
solid acids reported elsewhere. SBA-15-SO H and HMS-SO H
with 2–3 nm pore showed the conversion of palmitic acid to the
ꢀ
6
B. A. Borgias, S. R. Cooper, Y. B. Koh and K. N. Raymond, Inorg.
Chem., 1984, 23, 1009; J. L. Martin and J. Takats, Can. J. Chem.,
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corresponding esters at 85% and 55%, respectively, at 85 C for 3 h.
2
Al-MCM-41 with a high surface area of 950–1023 m /g showed 58%
ꢀ
7 S. Belin, L. R. B. Santos, V. Briois, A. Lusvardi, C. V. Santilli,
S. H. Pulcinelli, T. Chartier and A. Larbot, Colloids Surf., A, 2003,
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to 90% conversion of acetic acid at 100 and 200 C in 8 h. MCM-41-
SO H prepared by a two-step conventional process showed a 63%
3
ꢀ
yield for the esterification of lauric acid at 112 C in 8 h. It is believed
2 2 3
that the large pores of this SiO -TiO -SO H catalyst provide
9
X. Mo, d. E. L oꢁ pez, K. Suwannakan, Y. Liu, E. Lotero,
J. G. Goodwin Jr and C. Lu, J. Catal., 2008, 254, 332.
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improved accessibility to large reactants, such as oleic acid and the
ester by enabling the rapid diffusion of reactant and product through
the pores, thereby minimizing consecutive reactions. Finally, it turned
out that the activity of the solid acid catalyst decreases with repeated
use. As shown in Table 2, the conversion of acetic acid by 1SP-1T
decreased from 97% to 70% upon using it three times, each time
K. Domen, J. Mol. Catal. A: Chem., 2005, 230, 85.
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1
2
2
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This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 6419–6421 | 6421