Application of Sulfonic Acid Functionalised Hybrid Silicas Obtained by Oxidative Cleavage of Tetrasulfide Bridges as Catalysts in Esterification Reactions

Article abstract of DOI:10.1002/cctc.201200509

Hybrid materials with tetrasulfide bridges were synthesised by co-condensation between bis[3-(triethoxysilyl)propyl]tetrasulfide and either tetraethyl orthosilicate or 1,4-bis(triethoxysilyl)benzene under acidic conditions in the presence of Brij-76 as a structure-directing agent. They were post-modified by treatment with H₂O₂ for 3h or 24h. A progressive loss of the mesostructure in the resulting materials was observed upon increasing the S content in the synthesis mixtures. Thus, only those samples with the lowest tetrasulfide content could be considered as periodic mesoporous organosilicas. This effect was even more pronounced upon oxidation. The oxidised materials exhibited a considerable acidity, particularly those with the two precursors in approximately equal amounts. All sulfonic acid functionalised materials were used as catalysts in the esterification of acetic acid with ethanol, and they exhibited a comparable or very similar activity to that of Amberlyst-15. Several mesoporous and microporous hybrid materials were selected and compared to this commercial resin in the transformation of more hydrophobic substrates. Some of these hybrid materials were especially attractive for the transformation of benzyl alcohol and various secondary alcohols.

Full text of DOI:10.1002/cctc.201200509

CHEMCATCHEM
FULL PAPERS
DOI: 10.1002/cctc.201200509
Application of Sulfonic Acid Functionalised Hybrid Silicas
Obtained by Oxidative Cleavage of Tetrasulfide Bridges as
Catalysts in Esterification Reactions
Marꢀa Isabel Lꢁpez, Dolores Esquivel, Cꢂsar Jimꢂnez-Sanchidriꢃn, and
[a]
Francisco Josꢂ Romero-Salguero*
Hybrid materials with tetrasulfide bridges were synthesised by
co-condensation between bis[3-(triethoxysilyl)propyl]tetra-
sulfide and either tetraethyl orthosilicate or 1,4-bis(triethoxysi-
lyl)benzene under acidic conditions in the presence of Brij-76
as a structure-directing agent. They were post-modified by
treatment with H O for 3 h or 24 h. A progressive loss of the
terials exhibited a considerable acidity, particularly those with
the two precursors in approximately equal amounts. All sulfon-
ic acid functionalised materials were used as catalysts in the
esterification of acetic acid with ethanol, and they exhibited
a comparable or very similar activity to that of Amberlyst-15.
Several mesoporous and microporous hybrid materials were
selected and compared to this commercial resin in the trans-
formation of more hydrophobic substrates. Some of these
hybrid materials were especially attractive for the transforma-
tion of benzyl alcohol and various secondary alcohols.
2
2
mesostructure in the resulting materials was observed upon in-
creasing the S content in the synthesis mixtures. Thus, only
those samples with the lowest tetrasulfide content could be
considered as periodic mesoporous organosilicas. This effect
was even more pronounced upon oxidation. The oxidised ma-
Introduction
Many esters find important applications as flavours, fragrances,
pesticides, plasticisers and pharmaceuticals and others. Even
though the Fischer esterification has some important limita-
tions that can hamper its completion, this reaction is preferred
in the industry to avoid the preparation of acyl chlorides as in-
termediates. As the Fischer esterification is typically catalysed
by strong mineral acids (HCl, H SO , HF) or Lewis acids (AlCl ),
Sulfonic acid groups can be incorporated into hybrid materi-
als through several strategies. A common one is the co-con-
densation of a tetralkoxysilane (tetraethyl orthosilicate, TEOS,
or tetramethyl orthosilicate, TMOS) or a bridged organosilane
with 3-mercaptopropyltrimethoxysilane or 3-mercaptopropyl-
triethoxysilane with subsequent oxidation of the thiol
[2–7]
groups.
A very useful precursor also for co-condensation is
2
4
3
[8–10]
the use of heterogeneous catalysts with high activity is of
great interest for the design of greener synthetic routes. In
general, cation-exchange resins are effective catalysts. Howev-
er, they suffer from several drawbacks such as swelling and
shrinking in organic media, and also a relatively low thermal
stability in most cases, which limits their use up to approxi-
2-(4-chlorosulfonylphenyl)ethyl trimethoxysilane,
which
yields arenesulfonic acid groups just by hydrolysis under the
synthesis conditions. Another usual approach consists of graft-
ing of either silicas or organosilicas with some of the above-
[4,5,11,12]
mentioned precursors.
Furthermore, the organic moiet-
ies of periodic mesoporous organosilicas (PMOs) can be modi-
fied in several manners to incorporate sulfonic acid groups. In
general, these procedures involve the sulfonation of aromatic
[
1]
mately 1308C. Polysiloxanes incorporating arenesulfonic or al-
kylsulfonic groups can overcome all these problems because
of their higher structural stability.
[
13–15]
[16,17]
rings
or double bonds.
All these acid solids are able
[18]
to catalyse a great variety of organic reactions.
Different thioether-bridged mesoporous organosilicas con-
taining disulfide or tetrasulfide moieties have also been syn-
thesised in the last years. The oxidation of their bridges is ex-
pected to yield two sulfonic acid groups in close proximity to
each other, which can lead to an enhanced catalytic activity ac-
+
[
a] M. I. Lꢀpez, Dr. D. Esquivel, Prof. C. Jimꢁnez-Sanchidriꢂn,
Prof. F. J. Romero-Salguero
Department of Organic Chemistry
Nanochemistry and Fine Chemistry Research Institute (IUIQFN)
Faculty of Sciences
University of Cꢀrdoba
[19]
[20]
cording to some authors. Yang et al. synthesised sulfonat-
ed organosilicas by co-condensation between bis[3-(triethoxy-
silyl)propyl]disulfide and TMOS and subsequent oxidation of
the disulfide bridges with nitric acid. These materials exhibited
Marie Curie Building, Campus de Rabanales, Ctra. Nnal IV, km 396, 14014
Cꢀrdoba (Spain)
Fax: (+34)957212066
E-mail: qo2rosaf@uco.es
+
[
] Present address:
+
À1
acidities in the H content range from 0.78 to 0.89 mmolg
Department of Inorganic and Physical Chemistry
Center for Ordered Materials, Organometallics and Catalysis (COMOC)
Ghent University
and good activities for the esterification of aliphatic acids with
ethanol. Similarly, materials with tetrasulfide bridges were pre-
pared by co-condensation between bis[3-(triethoxysilyl)propyl]-
Krijgslaan 281, Building S3, 9000 Ghent (Belgium)
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 1002 – 1010 1002

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
[
21]
[22]
tetrasulfide (BTEPTS) and TMOS or TEOS. These organosili-
2
+
cas were used as adsorbents for removing Hg ions from
[
23]
waste water. Seo et al. oxidised the tetrasulfide bridges to
sulfonic acid groups with Br –HCl obtaining a surprisingly high
2
amount of acid sites and an exceptional activity in the acetyla-
tion of anisole with acetic anhydride. Clearly, this strategy de-
serves further investigations. In addition, although the result-
ing alkylsulfonic groups would be slightly weaker than the are-
nesulfonic groups, they should exhibit a higher stability be-
cause desulfonation of aromatic rings easily occurs in diluted
acidic aqueous solutions. Herein we describe the synthesis of
hybrid materials by co-condensation between the precursors
BTEPTS and either TEOS or 1,4-bis(triethoxysilyl)benzene (BTEB)
in a wide range of ratios using Brij-76 as structure-directing
agent under acidic conditions. The as-synthesised solids as
well as the resulting sulfonic acid functionalised hybrid materi-
als obtained by oxidation with H O were thoroughly charac-
2
2
Figure 2. XRD patterns of materials prepared by co-condensation of BTEB
and BTEPTS before and after the oxidation treatments.
terised and finally various esterification reactions were investi-
gated on the sulfonated samples by using Amberlyst-15 as
a reference catalyst.
a less ordered mesostructure. The mesoporous framework in
samples BS TS
was unaltered at x=90 (a =62.8 ꢅ) but it
o
x
100Àx
Results and Discussion
became affected at x=75 (a
=52.6 ꢅ). Oxidation of the mate-
o
rial S TS drastically decreased its ordering and only some
90
10
Characterization of the hybrid materials
weak shoulders were observed. Thus the only oxidised material
that exhibited a well-defined low-angle peak was BS90TS10O(3)
The XRD patterns of the as-synthesised samples S100 and BS100
(
see Experimental Section for nomenclature) revealed a hexago-
(a =60.0 ꢅ).
o
The incorporation of S into the hybrid materials was quite
nally ordered mesoporous structure with a values of 64.9 and
o
61.0 ꢅ, respectively (Figure 1 and 2), in agreement with previ-
effective in all cases (Tables 1 and 2). The larger differences be-
tween the theoretical and the measured S contents at high
x values are probably caused by the fact that the actual materi-
als were not fully condensed, thus the contribution of oxygen
Table 1. Composition and acidity for materials S TS100Àx and their corre-
x
sponding oxidised samples.
[
a]
[b]
[c]
oxidised [%] Soxidised
[d]
[e]
Material
S
[
theoretical
S
measured
S
Acidity
À1
À1
À1
À1
mmolg ] [mmolg
]
[mmolg ] [mmolg ]
S
S
S
S
S
S
S
S
S
S
90TS10
4.6
–
–
8.1
–
–
10.6
–
–
11.9
–
–
2.0
1.0
0.7
4.2
2.4
1.3
7.1
4.6
1.1
9.4
5.6
–
0
0
0.1
0.7
0.6
0.1
1.1
1.0
0.2
2.3
0.9
0.1
2.3
–
90TS10O(3)
90TS10O(24)
75TS25
66.6
64.7
0
70.9
76.2
0
67.9
77.5
0
61.1
–
0.7
0.5
0
1.7
1.0
0
3.1
0.9
0
3.4
–
75TS25O(3)
75TS25O(24)
50TS50
50TS50O(3)
50TS50O(24)
25TS75
Figure 1. XRD patterns of materials prepared by co-condensation of TEOS
and BTEPTS before and after the oxidation treatments.
2
S TS O(3)
5
75
2
S TS O(24)
5
75
S
S
S
10TS90
12.4
–
–
12.6
–
–
10.6
7.9
–
11.0
n.d.
–
0
39.8
–
0
39.9
–
0
3.1
–
0
n.d.
–
0.1
1.1
–
0.1
1.2
–
10TS90O(3)
10TS90O(24)
[
24]
ously reported data. However, only those samples S TS
x
100Àx
TS100
with x=90 and materials BS PS
with x=90 and 75 exhibit-
TS100O(3)
TS100O(24)
x
100Àx
ed a low-angle (100) peak characteristic of PMOs. This peak
was shifted to a higher angle for S TS (a =54.3 ꢅ) relative to
90
10
o
[a] Calculated from the initial composition of the starting synthesis gel;
[b] determined by elemental analysis; [c] determined by deconvolution of
the XPS spectra; [d] calculated as Smeasured ꢆSoxidised(%)/100; [e] H content
the S100 peak, which indicated a decrease in the unit cell di-
mension upon increasing the S content. Also, the peak was
wider than in the sample S100, confirming the existence of
+
determined by titration; n.d.=not determined.
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 1002 – 1010 1003

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
Table 2. Composition and acidity for materials BS TS100Àx and their corre-
x
sponding oxidised samples.
[
a]
[b]
measured
[c]
[d]
oxidised
[e]
Material
S
[
theoretical
S
S
[%]
oxidised
S
Acidity
À1
À1
À1
À1
mmolg
]
[mmolg
]
[mmolg
]
[mmolg ]
BS90TS10
2.1
–
–
4.7
–
–
8.1
–
–
10.6
–
–
1.5
0.9
0.5
3.3
2.4
1.3
6.1
3.9
2.4
8.4
6.5
1.9
10.1
7.5
–
0
0
0.2
0.5
0.5
0.2
0.8
0.9
0.1
1.4
1.1
0.1
1.2
1.1
0.1
1.2
–
BS90TS10O(3)
BS90TS10O(24)
BS75TS25
68.5
64.5
0
61.7
80.1
0
70.5
77.9
0
49.8
82.7
0
0.6
0.3
0
1.5
1.0
0
2.7
1.9
0
3.3
1.6
0
BS75TS25O(3)
BS75TS25O(24)
BS50TS50
BS50TS50O(3)
BS50TS50O(24)
BS25TS75
BS25TS75O(3)
BS25TS75O(24)
BS10TS90
11.9
–
–
BS10TS90O(3)
BS10TS90O(24)
46.6
–
3.5
–
[
[
a] Calculated from the initial composition of the starting synthesis gel;
b] determined by elemental analysis; [c] determined by deconvolution of
+
the XPS spectra; [d] calculated as Smeasured ꢆSoxidised(%)/100; [e] H content
determined by titration; n.d.=not determined.
in their compositions was higher. For the same reason, co-con-
densation with BTEB gave materials with S contents closer to
the theoretical values than preparation with TEOS. Tetrasulfide
bridges were extensively oxidised upon treatment with H O .
2
2
Complete oxidation of one tetrasulfide bridge (CÀSÀSÀSÀSÀC)
should give two sulfonic acid groups (2CÀSO H) and, as a con-
3
sequence, the S content should drastically decrease, particular-
ly after treatment for 24 h (Scheme 1). Materials S TS
O(24)
x
100Àx
Figure 3. X-ray photoelectron spectra of S 2p for representative samples. Ex-
perimental XPS curve (dotted line), deconvoluted peaks (dashed line) and
fitted curve (solid line).
with x=25 or less could not be recovered. The same occurred
for material BS TS O(24), whereas BS TS O(24) was obtained
10
90
25
75
in very low amount. In the other cases, oxidation for 24 h
made the isolation of the corresponding solids even more diffi-
cult, resulting in a lower yield. This was expected because the
probability to solubilise a fragment of the framework is directly
related to the amount of tetrasulfide bridges. The oxidation
process was further studied by X-ray photoelectron spectrosco-
py (XPS, Table 1 and 2). The as-synthesised samples only exhib-
ited one peak at approximately 164 eV, corresponding to the S
tion, which ranged from 39.8 to 82.7%, depending on the
starting co-precursor (TEOS or BTEB), the S content in the start-
ing gel and the oxidation time.
The concentration of sulfonic acid groups in the hybrid ma-
terials was determined both by elemental analysis and subse-
quent XPS measurements of the oxidised S and by titration
with NaOH (Table 1 and 2). Both procedures provided similar
concentrations of sulfonic acid groups at the highest x values.
However, the procedures gave considerably different results at
2p binding energy in the tetrasulfide bridges (Figure 3). After
treatment with H O , another peak at approximately 169 eV ap-
2
2
[
4,5]
peared.
This peak was attributed to oxidised S and its frac-
the lowest x values, particularly for materials S TS
O(3), thus
100Àx
x
indicating that these materials
were less acidic than expected.
This implies that some sulfonic
acid groups are not exchangea-
ble because of a restricted acces-
sibility and/or that some sulfonic
acid groups form pairs that after
the ion exchange treatment
+
retain
a
bridged
H
besides
+
[25]
a Na ion. In general, samples
BS TS O(3) were less acidic
x
100Àx
than S TS
O(3). Thus, maxi-
x
100Àx
+
Scheme 1. Generation of sulfonic acid groups by oxidation of tetrasulfide bridges.
mum acidities in terms of H
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 1002 – 1010 1004

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
À1
content were found to be 1.4 and 2.3 mmolg , respectively,
Catalytic activity
[18]
which are remarkably high values for this sort of materials.
Particularly, the highest acidities were obtained for those com-
positions where x=25 or 50. In most cases, a long oxidation
step decreased the amount of acid sites.
The esterification of acetic acid with ethanol is frequently used
to measure the activity of acid catalysts. In our case, most of
the oxidised materials exhibited a good catalytic performance,
in many cases similar to that of the highly active commercial
resin Amberlyst-15, whereas both the blank reaction and the
reactions performed in the presence of the non-oxidised mate-
rials revealed a low conversion (Figure 5–8). Sulfonated PMOs
and organosilicas have been found to be particularly useful for
catalysing reactions in which water is present and, in these
Materials synthesised from either TEOS or BTEB had a large
surface area and a narrow pore–size distribution in the meso-
porous range (Tables 3 and 4), which is typical of a mesoporous
silica and a PMO, respectively. Consequently, for both samples
type IV N2 adsorption–desorption isotherms were obtained.
Also for BS TS as well as the corresponding oxidised materi-
90
10
[5,13,26,27]
als type IV isotherms were obtained and thus they can be con-
sidered as PMOs. However, the isotherm of material S TS
10
cases, they can exhibit a high activity per acid site.
This
fact has been ascribed to the increased hydrophobicity around
90
[26–29]
was of type IV, whereas the isotherms of their oxidised coun-
terparts were a superposition of type I and IV. Furthermore, all
hybrid materials with x=75 and 50 gave type I isotherms,
whereas those with xꢀ25 gave type III isotherms. Accordingly,
the porosity of these materials ranged from mesoporous to mi-
croporous and finally to non-porous, following an increase in
the content of BTEPTS. As expected, the templating mecha-
nism was disrupted by the presence of the long and flexible
BTEPTS precursor at high concentrations. As an example,
the sulfonic acid groups.
A comparison of the conversion
curves revealed that the oxidised hybrids synthesised by co-
condensation with TEOS were broadly more active than those
prepared from BTEB. In addition, the oxidation treatment for
24 h did not improve the catalytic activity, which remained in
the same range. All materials were reused once. Some authors
have reported that sulfonated hybrid materials can lose a high
percentage of their catalytic activity (even up to 50%) after the
[30]
first recycle but providing a similar activity for further runs.
In other cases, a gradual loss of activity has been reported
Figure 4 depicts the isotherms for samples S TS
in the
100Àx
x
[
14,31]
whole range of compositions. For both as-synthesised and oxi-
dised samples, the surface area abruptly decreased as the con-
tent of BTEPTS increased. In general, the surface area was very
small at x<50. Microporous samples (x=75, 50) were particu-
larly affected by the oxidation treatment, especially those at
after every recycle.
oxidised materials S TS
This feature was also observed for our
and BS TS . Additionally, as also
100Àx
x
100Àx
x
[
11,32,33]
reported by other authors,
no clear relationship between
activity and acidity could be established. This relationship may
be influenced by many factors at the same time, such as the
accessibility of the acid groups, the environment around these
groups, their proximity and the diffusion of reagents and prod-
ucts into or out of the pores, among others.
24 h. In these cases, an increase in the surface area could be
observed. According to their isotherms (not shown), a longer
oxidation treatment resulted in an increase and a decrease in
the adsorption at low and high relative pressures, respectively.
Therefore, after oxidation for 24 h, these samples increased
their microporosity and, as a consequence, they exhibited
a higher surface area.
Some hybrid materials have been utilised for the esterifica-
tion of more hydrophobic substrates in stoichiometric
amounts. In these cases, all reactions were performed at 1108C
to increase the solubility of the corresponding carboxylic acid
Table 3. Textural properties of materials S
x
p p
TS100Àx and their corresponding oxidised samples. SBET =BET surface area; V =pore volume; d =pore diameter.
2
À1
3
À1
2
À1
3
À1
2
À1
3
À1
Material
S
BET [m g
]
V
p
[cm g
]
d
p
[nm] Material
S
BET [m g
]
V
p
[cm g
]
d
p
[nm] Material
S
BET [m g
]
V
p
[cm g
]
p
d [nm]
S
S
S
S
S
S
100
1113
1070
454
110
4
0.88
0.56
0.24
0.08
0.01
0.01
0.01
3.6
2.8
–
–
–
90TS10
S
S
S
S
S
90TS10O(3) 538
0.26
0.22
0.05
0.01
0.01
0.01
2.0
–
–
–
–
S
S
S
S
S
90TS10O(24) 772
75TS25O(24) 569
50TS50O(24) 483
0.38
0.28
0.25
–
–
–
2.0
–
–
–
–
75TS25
50TS50
25TS75
10TS90
75TS25O(3) 448
50TS50O(3)
25TS75O(3)
10TS90O(3)
85
4
4
25TS75O(24)
10TS90O(24)
–
–
–
–
3
–
–
TS100
TS100O(3)
4
–
TS100O(24)
–
Table 4. Textural properties of materials BS
x
p p
TS100Àx and their corresponding oxidised samples. SBET =BET surface area; V =pore volume; d =pore diameter.
2
À1
3
À1
2
À1
3
À1
2
À1
3
À1
Material
S
BET [m g
]
V
p
[cm g
]
d
p
[nm] Material
S
BET [m g
]
V
p
[cm g
]
d
p
[nm] Material
S
BET [m g
]
V
p
[cm g
]
p
d [nm]
BS100
1178
971
357
60
16
–
0.89
0.62
0.17
0.04
0.02
0.01
3.5
2.9
1.9
–
–
–
BS90TS10
BS75TS25
BS50TS50
BS25TS75
BS10TS90
BS90TS10O(3) 1088
0.67
0.19
0.15
0.01
0.01
3.2
–
–
–
–
BS90TS10O(24) 919
BS75TS25O(24) 574
BS50TS50O(24) 401
0.51
0.28
0.21
–
3.4
–
–
–
–
BS75TS25O(3)
BS50TS50O(3)
BS25TS75O(3)
BS10TS90O(3)
310
287
6
BS25TS75O(24)
BS10TS90O(24)
–
–
–
–
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 1002 – 1010 1005

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
Among both hybrid catalysts, S TS O(3) usually gave higher
90
10
conversions than BS TS O(3). Both materials were particularly
90
10
active in the transformation of palmitic acid, thus exhibiting
[34]
a great potential for biodiesel production. In general, esterifi-
cations involving secondary alcohols have lower equilibrium
constants than those with participation of primary alcohols.
This was also observed in the reactions with butan-1-ol and
butan-2-ol. Also, the reaction with benzoic acid provided a low
conversion. In this case, the highest conversion (65.1%) was
obtained for Amberlyst-15 at 24 h.
In view of the high activity of the hybrid materials with mes-
oporous structure, two more samples were also tested in some
esterification reactions, specifically, S TS O(3) and BS TS O(3)
50
50
50
50
+
exhibiting the highest acidities (H
content 2.3 and
À1
1
.4 mmolg , respectively) although still retaining non-negligi-
2
À1
ble surface areas (85 and 287 m g , respectively). Interestingly,
both materials provided a higher conversion than the mesopo-
rous hybrids and even than Amberlyst-15 in some reactions
(
Figure 10). Thus, the higher acidity of the microporous sam-
ples is clearly advantageous. Sample S TS O(3) gave rise to
50
50
the highest conversions in the esterification of octanoic acid
with secondary alcohols, that is, butan-2-ol and octan-2-ol,
with values of 46.0 and 65.4%, respectively, at 24 h.
To check the stability under these reaction conditions, both
the mesoporous and the microporous hybrid materials were
reused after the reaction of octanoic acid with butan-1-ol at
110 8C. With this aim, once used for a first reaction, they were
filtered and washed with 50 mL of acetone. After drying, they
were reused for a second reaction under the same conditions.
In Figure 11 the variation of the conversion with time on
stream for these catalysts is depicted. Similarly to the esterifica-
tion of acetic acid with ethanol, all catalysts lost some activity,
although materials BS TS O(3) and BS TS O(3) preserved
90
10
50
50
their initial activity much better.
Among the most common esters, benzyl esters are of partic-
ular interest. They serve as attractive protective groups for car-
boxylic acids in organic synthesis because they can be readily
removed by hydrogenolysis. Moreover, they are largely used in
the perfume industry. For example, benzyl acetate is found in
jasmine, gardenia and heliotrope fragrances. Thus, the esterifi-
cation of benzyl alcohol with three carboxylic acids, formic,
acetic and octanoic acids, was tested. The activities of various
sulfonated hybrid materials and Amberlyst-15 are depicted in
Figure 12.
Interestingly,
materials
SxTS
O(3)
100Àx
and
BS TS O(3) with x=90 and 50 exhibited excellent results for
x
100Àx
esterifications involving benzyl alcohol, and reached even
higher conversions with x=50 than Amberlyst-15. A clear rela-
tionship between conversion and acidity can be seen. These
results are remarkable because the overall conversions in the
esterification over Amberlyst-15 as a catalyst have rarely been
Figure 4. Nitrogen adsorption–desorption isotherms of materials S
Adsorption (*) and desorption (*) branches.
x
TS100Àx.
[17,35]
surpassed by other sulfonated materials.
Often, higher cat-
and decrease the viscosity of the reactant mixtures. Because of
their mesoporous structures, samples S TS O(3) and
alytic activities have been reported for these materials but per
acid site (turnover frequency), owing to their relatively low
population of acid sites relative to Amberlyst-15
90
10
BS TS O(3) were firstly selected to test their activity and com-
90
10
À1
pare this activity with that of Amberlyst-15. The results of
these reactions are shown in Figure 9. In general, the mesopo-
rous samples were slightly less active than Amberlyst-15.
(4.7 mmolg ). However, the catalyst reactor loadings are gen-
erally determined by mass or volume for practical applications.
In our case, all reactions were performed by using the same
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 1002 – 1010 1006

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
and prevent its use in continu-
ous flow reactors because of the
problems caused by plugging.
Thus, it was difficult to withdraw
samples for analysis if Amber-
lyst-15 was used as the catalyst.
Conclusions
Different sulfonic acid functional-
ised hybrid silicas were prepared
by oxidation of the correspond-
ing materials bearing tetrasulfide
bridges, which were previously
synthesised by co-condensation
between bis[3-(triethoxysilyl)pro-
pyl]tetrasulfide (BTEPTS) and
either tetraethyl orthosilicate
Figure 5. Overall conversion (mol%) in the esterification of acetic acid with ethanol on fresh (left) and reused
(right) sulfonic acid functionalised hybrid materials: S90TS10O(3) (*), S75TS25O(3) (*), S50TS50O(3) (&), S25TS75O(3)
&), S10TS90O(3) (!), TS100O(3) (!), Amberlyst-15 (ꢆ), blank (*).
(
(
TEOS) or bis(triethoxysilyl)ben-
zene (BTEB). As their S content
increased, the hybrid materials
underwent a change from meso-
porous to microporous structure
and finally to non-porous materi-
als. A short oxidation treatment
(
3 h) was more suitable than
a long one (24 h) to obtain
highly acidic materials without
significantly dissolving their
framework. Generally, a consider-
able amount of tetrasulfide
bridges was oxidised. The high-
est acidities were obtained for
those materials synthesised from
precursors at an approximately
Figure 6. Overall conversion (mol%) in the esterification of acetic acid with ethanol on fresh (left) and reused
right) sulfonic acid functionalised hybrid materials: S90TS10O(24) (*), S75TS25O(24) (*), S50TS50O(24) (&).
(
+
equal molar fraction. H con-
À1
tents up to 2.3 and 1.4 mmolg
were achieved for the hybrid
materials obtained from TEOS
and BTEB, respectively. All the
sulfonic
acid
functionalised
hybrid silicas were highly active
in the esterification of acetic
acid with ethanol, having con-
versions similar to that of the
commercial resin Amberlyst-15.
The samples synthesised from
Figure 7. Overall conversion (mol%) in the esterification of acetic acid with ethanol on fresh (left) and reused
right) sulfonic acid functionalised hybrid materials: BS90TS10O(3) (*), BS75TS25O(3) (*), BS50TS50O(3) (&),
BS25TS75O(3) (&), BS10TS90O(3) (!).
TEOS were more active than
(
those prepared by co-condensa-
tion with BTEB. Also, a clear rela-
tionship between activity and
weight of catalyst. An additional advantage of the sulfonated
hybrid materials is their higher structural rigidity. A significant
swelling of the resin Amberlyst-15 was observed under the re-
action conditions, particularly if formic and acetic acids were
used. This swelling would limit the applicability of the resin in
batch reactors because of its difficult filtration or decantation
acidity was observed if more hydrophobic reactants were used
in equimolar amounts for the esterification reaction. Remarka-
bly, the most acidic catalysts, that is, BS TS O(3) and particu-
50
50
larly S TS O(3), were more efficient than the resin for the
50
50
esterification of octanoic acid with secondary alcohols and for
the synthesis of various benzyl esters. Even though a decrease
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 1002 – 1010 1007

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
promising catalysts for a great
variety of acid-catalysed organic
processes.
Experimental Section
Precursors TEOS, BTEB and BTEPTS
were purchased from Aldrich. Sur-
factant Brij-76 (i.e., polyoxyethyle-
ne(10)stearyl alcohol), Amberlyst-
1
5, 1,4-dioxane (ꢁ99%), acetic
(ꢁ99.7%), octanoic (ꢁ98%) and
palmitic (90%) acids, as well as
butan-2-ol (99%) and octan-2-ol
(97%), were supplied by Aldrich.
Benzoic acid (ꢁ99.5%), H O (30%
Figure 8. Overall conversion (mol%) in the esterification of acetic acid with ethanol on fresh (left) and reused
(right) sulfonic acid functionalised hybrid materials: BS90TS10O(24) (*), BS75TS25O(24) (*), BS50TS50O(24) (&).
2
2
Figure 9. Catalytic performance of sulfonated materials with mesoporous
structures in different esterification reactions compared to the performance
of Amberlyst-15 (dashed lines) at a reaction time of 7 h. &=S90TS10O(3);
&
=BS90TS10O(3).
Figure 11. Activity of fresh (c) and recycled (a) mesoporous and micro-
porous hybrid materials in the esterification of octanoic acid with butan-1-
ol.
Figure 10. Catalytic performance of sulfonated materials with a microporous
hybrid structure in the esterification of octanoic acid with different alcohols
(bars) compared to the performance of Amberlyst-15 (dashed lines) at a reac-
tion time of 7 h.
in activity was observed upon reuse, specifically catalysts
BS TS O(3) and BS TS O(3) exhibited a considerable resist-
90
10
50
50
ance to deactivation. Owing to their high activity and their sta-
bility, unlike Amberlyst-15 that exhibits problems of swelling,
some of these sulfonic acid functionalised hybrid silicas ob-
tained by oxidative cleavage of tetrasulfide bridges are very at-
tractive catalysts for the synthesis of esters and, therefore,
Figure 12. Catalytic performance of different sulfonated materials in the
esterification of benzyl alcohol with various carboxylic acids (bars) compared
to the catalytic performance of Amberlyst-15 (dashed lines) at a reaction
time of 3 h.
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 1002 – 1010 1008

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
w/v), H SO (95–98%) and HCl (37%) were provided by Panreac.
Catalytic reaction
2
4
Also, formic acid (98%, Fluka), ethanol (99.8%, Acros), butan-1-ol
The catalytic activities of all materials were studied in different
esterification reactions and compared with the activity of Amber-
lyst-15 as a reference. The comparisons were made on a weight
basis.
(ꢁ99.5%, Fluka) and benzyl alcohol (>99%, Merck) were used. All
these chemicals were used without further purification.
The esterification of acetic acid with ethanol was conducted ac-
cording to the following procedure. The catalyst (0.150 g) was pre-
treated overnight at 1208C and then added to a mixture of acetic
acid (0.076 mol) and ethanol (0.760 mol). The resulting mixture was
Synthesis
The hybrid materials were synthesised by co-condensation of
BTEPTS and either TEOS or BTEB as the silica and organosilica pre-
cursors, respectively. The molar ratios of the precursors in the syn-
thesis mixtures (based on Si) were: 100:0, 90:10, 75:25, 50:50,
then heated to reflux under a N atmosphere. Samples collected at
2
regular intervals were analysed by gas chromatography by using
a VF-5 m 30 mꢆ0.25 mm ID capillary column. Reaction products
were identified by the use of standards and quantified with diox-
ane as the internal standard by using an FID detector.
2
5:75, 10:90 and 0:100. Briefly, Brij-76 (6 g) was added to a solution
of 37% HCl (19.6 mL) in H O (279 mL). The resulting mixture was
stirred at 508C for 24 h. Later, BTEPTS and either TEOS or BTEB
were added. The Si/Brij-76/HCl/H O molar ratio in the mixture was
2
2
The catalytic activity of some solids was further studied in esterifi-
cation reactions involving different acids (i.e., formic acid, acetic
acid, octanoic acid, benzoic acid and palmitic acid) and alcohols
1
:0.10:2.85:186.1. After stirring for 24 h at the same temperature,
the suspension was aged at 908C for 24 h under static conditions.
Finally, the solid was filtered and dried in the air.
(i.e., butan-1-ol, butan-2-ol, octan-2-ol and benzyl alcohol). The cat-
The surfactant was removed by heating the as-synthesised material
at reflux in an HCl solution (1 mL of 37% HCl and 50 mL ethanol,
per gram of solid) for 12 h. Subsequently, the solid was filtered and
washed with ethanol. This process was repeated three times. The
final material was then dried in an oven at 1008C under vacuum.
alyst (0.100 g) was pretreated overnight at 1208C and added to an
equimolar mixture of the carboxylic acid (0.0543 mol) and the alco-
hol (0.0543 mol). The reaction was performed at 1108C in a N at-
2
mosphere. Analyses were performed as previously described. The
corresponding esters were the only products detected.
The tetrasulfide bridges were transformed into sulfonic acid
groups by oxidation with H O (Scheme 1). Thus, the hybrid materi-
2
2
Acknowledgements
al (0.3 g) was stirred with 30% H O (10 mL) at 508C for either 3 or
2
2
2
4 h. The solid was then washed with 1.2m H SO (25 mL). Later, it
2 4
The authors wish to acknowledge funding of this research by
Spanish Ministerio de Ciencia e Innovaciꢀn (Project MAT2010-
was thoroughly washed with water until a neutral pH was reached.
Finally, the solid was dried at 1008C under vacuum.
1
8778), Junta de Andalucꢃa (Project P06-FQM-01741) and Fondos
The parent silica-based hybrid materials were named as S TS
x
100Àx
Feder. The technical support and facilities from Cꢀrdoba Universi-
ty’s SCAI are greatly appreciated. M.I.L. thanks Ministerio de Edu-
caciꢀn, Cultura y Deporte for a FPU teaching and research fel-
lowship.
(
with x the Si molar ratio of TEOS in the initial mixture, ranging
from 0 to 100), whereas the materials subjected to oxidation treat-
ments were designated as S TS O(3) and S TS O(24), depend-
x
100Àx
x
100Àx
ing on the duration of the treatment (3 or 24 h, respectively). Anal-
ogous organosilica-based materials were named BS TS
,
100Àx
x
BS TS
O(3) and BS TS
O(24), respectively. TS100 was the mate-
x
100Àx
x
100Àx
Keywords: acidity · heterogeneous catalysis · ion exchange ·
oxidation · silicates · sulfur
rial synthesised with BTEPTS as the sole precursor, whereas S100
and BS100 were a silica and an organosilica, respectively, obtained
exclusively from either TEOS or BTEB.
[
[
1] S. Suzuki, K. Tohmori, Y. Ono, J. Mol. Catal. 1987, 43, 41–50.
2] S. Hamoudi, S. Kaliaguine, Microporous Mesoporous Mater. 2003, 59,
1
95–204.
3] Q. Yang, J. Liu, J. Yang, M. P. Kapoor, S. Inagaki, C. Li, J. Catal. 2004, 228,
65–272.
4] B. Rꢃc, ꢇ. Molnꢃr, P. Forgo, M. Mohai, I. Bertꢁti, J. Mol. Catal. A: Chem.
006, 244, 46–57.
Characterization
[
[
[
2
X-Ray powder diffraction patterns were recorded on a Siemens d-
5
000 powder diffractometer (CuKa radiation). Elemental composi-
2
tions were determined on a Eurovector EA 3000 analyser. XPS
spectra were recorded with a SPECS Phoibos HAS 3500 150 MCD.
The residual pressure in the analysis chamber was 5ꢆ10 Pa. Ac-
5] S. Shylesh, P. P. Samuel, C. Srilakshmi, R. Parisha, A. P. Singh, J. Mol.
Catal. A: Chem. 2007, 274, 153–158.
[6] J. P. LourenÅo, M. I. Macedo, A. Fernandez, Catal. Commun. 2012, 19,
À9
curate binding energies have been determined with respect to the
position of the Si 2p peak at 103.4 eV. The peaks were decom-
posed using a least-squares fitting routine (Casa XPS software)
with a Gauss/Lorenz ratio of 70/30 and after subtraction of a linear
background. N2 isotherms were determined on a Micromeritics
ASAP 2010 analyser at À1968C. The specific surface area of each
solid was determined by using the BET method and the pore–size
distribution by the Barret–Joyner–Halenda method.
105–109.
[
[
[
7] R. Siegel, E. Domingues, R. De Sousa, F. Jꢂrꢈme, C. M. Morais, N. Bion, P.
Ferreira, L. Mafra, J. Mater. Chem. 2012, 22, 7412–7419.
8] J. A. Melero, G. D. Stucky, R. van Grieken, G. Morales, J. Mater. Chem.
2
002, 12, 1664–1670.
9] C.-C. Chen, S. Cheng, L.-Y. Jang, Microporous Mesoporous Mater. 2008,
09, 258–270.
1
[
[
[
10] J. A. Melero, G. Vicente, M. Paniagua, G. Morales, P. MuÇoz, Bioresour.
Technol. 2012, 103, 142–151.
11] P. L. Dhepe, M. Ohashi, S. Inagaki, M. Ichikawa, A. Fukuoka, Catal. Lett.
The acidity was determined by using the following procedure. Ali-
quots of 50 mg of the material were stirred with 2m NaCl (20 g) at
RT for 24 h. Then, the solid was filtered off and the resulting solu-
tion titrated with a 0.025m NaOH solution using phenolphthalein
as the indicator.
2
005, 102, 163–169.
12] G. H. Jeong, E. G. Kim, S. B. Kim, E. D. Park, S. W. Kim, Microporous Meso-
porous Mater. 2011, 144, 134–139.
[13] K. Nakajima, I. Tomita, M. Hara, S. Hayashi, K. Domen, J. N. Kondo, Catal.
Today 2006, 116, 151–156.
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 1002 – 1010 1009

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
[
14] M. P. Kapoor, Y. Kasama, M. Yanagi, T. Yokoyama, S. Inagaki, T. Shimada,
[27] K. Nakajima, I. Tomita, M. Hara, S. Hayashi, K. Domen, J. N. Kondo, Adv.
Mater. 2005, 17, 1839–1842.
H. Nanbu, L. R. Juneja, Microporous Mesoporous Mater. 2007, 101, 231–
2
39.
[28] C. M. Li, H. Yang, X. Shi, R. Liu, Q. H. Yang, Microporous Mesoporous
Mater. 2007, 98, 220–226.
[29] D. Dubꢂ, M. Rat, F. Beland, S. Kaliaguine, Microporous Mesoporous Mater.
2008, 111, 596–603.
[30] J. Liu, Q. H. Yang, M. P. Kapoor, N. Setoyama, S. Inagaki, J. Yang, L.
Zhang, J. Phys. Chem. B 2005, 109, 12250–12256.
[31] B. Karimi, H. M. Mirzaei, A. Mobaraki, Catal. Sci. Technol. 2012, 2, 828–
834.
[
[
[
[
15] A. Karam, J. C. Alonso, T. I. Gerganova, P. Ferreira, N. Bion, J. Barrault, F.
Jꢂrꢈme, Chem. Commun. 2009, 7000–7002.
16] E. De Canck, C. Vercaemst, F. Verpoort, P. Van Der Voort, Stud. Surf. Sci.
Catal. 2010, 175, 365–368.
17] D. Esquivel, E. De Canck, C. Jimꢂnez-Sanchidriꢃn, P. Van Der Voort, F. J.
Romero-Salguero, J. Mater. Chem. 2011, 21, 10990–10998.
18] J. A. Melero, R. Van Grieken, G. Morales, Chem. Rev. 2006, 106, 3790–
3
812.
[32] B. Rꢃc, P. Hegyes, P. Forgo, A. Molnar, Appl. Catal. A: Gen. 2006, 299,
193–201.
[
19] V. Dufaud, M. E. Davis, J. Am. Chem. Soc. 2003, 125, 9403–9413.
[
20] C. Li, J. Liu, L. Zhang, J. Yang, Q. Yang, Microporous Mesoporous Mater.
[33] J. Yang, H. Q. Yang, G. Wang, Z. C. Feng, J. Liu, J. Mol. Catal. A: Chem.
2006, 256, 122–129.
[34] S.-Y. Chen, T. Yokoi, C.-Y. Tang, L.-Y. Jang, T. Tatsumi, J. C. C. Chan, S.
Cheng, Green Chem. 2011, 13, 2920–2930.
[35] C. Pirez, J.-M. Caderon, J.-P. Dacquin, A. F. Lee, K. Wilson, ACS Catal.
2012, 2, 1607–1614.
2
008, 113, 333–342.
21] J. Liu, J. Yang, Q. Yang, G. Wang, Y. Li, Adv. Funct. Mater. 2005, 15, 1297–
302.
22] M. Teng, H. Wang, F. Li, B. Zhang, J. Colloid Interface Sci. 2011, 355, 23–
8.
[
[
1
2
[
[
23] O. Kwon, S. Park, G. Seo, Chem. Commun. 2007, 4113–4115.
24] M. C. Burleigh, S. Jayasundera, C. W. Thomas, M. S. Spector, M. A. Marko-
witz, B. P. Gaber, Colloid Polym. Sci. 2004, 282, 728–733.
[25] R. Mouawia, A. Mehdi, C. Reyꢂ, R. J. P. Corriu, J. Mater. Chem. 2008, 18,
Received: July 27, 2012
4193–4203.
[26] M. Rat, M. H. Zahedi-Niaki, S. Kaliaguine, T. O. Do, Microporous Mesopo-
rous Mater. 2008, 112, 26–31.
Revised: September 18, 2012
Published online on February 18, 2013
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 1002 – 1010 1010