Synthesis of sulfonic acid containing ionic-liquid-based periodic mesoporous organosilica and study of its catalytic performance in the esterification of carboxylic acids

Article abstract of DOI:10.1002/cplu.201402071

A new sulfonic acid containing ionic-liquid-based periodic mesoporous organosilica (PMO-IL-SO₃H) material was prepared and its catalytic application was investigated in the esterification of carboxylic acids with alcohols. The PMO-IL-SO₃H nanocatalyst was first characterized with diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and nitrogen sorption analysis. Then, the catalytic performance of this material was studied in the esterification of carboxylic acids with short- and long-chain aliphatic alcohols, cyclic alcohols, and benzylic alcohols under solvent-free conditions. The results showed that the catalyst has superior activity for the conversion of several alcohols to afford the corresponding ester products in excellent yields and high purity. Moreover, the catalyst could be recovered and reused several times without a significant decrease in activity and product selectivity. Copyright

Full text of DOI:10.1002/cplu.201402071

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DOI: 10.1002/cplu.201402071
Synthesis of a Novel Sulfonic Acid Containing Ionic-Liquid-
Based Periodic Mesoporous Organosilica and Study of Its
Catalytic Performance in the Esterification of Carboxylic
Acids
[a]
[b]
[a]
[a]
Dawood Elhamifar,* Babak Karimi,* Abbas Moradi, and Javad Rastegar
A new sulfonic acid containing ionic-liquid-based periodic mes-
rial was studied in the esterification of carboxylic acids with
short- and long-chain aliphatic alcohols, cyclic alcohols, and
benzylic alcohols under solvent-free conditions. The results
showed that the catalyst has superior activity for the conver-
sion of several alcohols to afford the corresponding ester prod-
ucts in excellent yields and high purity. Moreover, the catalyst
could be recovered and reused several times without a signifi-
cant decrease in activity and product selectivity.
oporous organosilica (PMO-IL-SO H) material was prepared and
3
its catalytic application was investigated in the esterification of
carboxylic acids with alcohols. The PMO-IL-SO H nanocatalyst
3
was first characterized with diffuse reflectance infrared Fourier
transform (DRIFT) spectroscopy, transmission electron micros-
copy (TEM), thermogravimetric analysis (TGA), and nitrogen
sorption analysis. Then, the catalytic performance of this mate-
Introduction
The esterification of carboxylic acids with alcohols is a well-
known organic transformation that is of particular interest due
to its key role in the production of valuable ester products,
such as environmentally friendly solvents, renewable fuels,
coatings, dyes, cosmetics, flavors, food oiling agents, plastic lu-
bricants, ink additives, spin finishes, textiles, and pharmaceuti-
tant challenge in this area. In this context, the application of
several solid acids, such as alumina, resins, zeolites, sulfated zir-
conia, sulfonated carbon, and heteropoly acids, have been well
[2c,4]
documented.
Alternatively, more recently, acid-functional-
ized ionic liquids have also been used as a recoverable catalyst
[5]
in the esterification of alcohols. However, the most reported
ionic-liquid-based catalytic systems were not stable and de-
[
1]
cal compounds. In particular, if fatty acids are used as a sub-
strate, the importance of this process increases owing to the
significance of fatty ester products in the oleochemical indus-
[5]
composed during the reaction process. A variety of acid-func-
tionalized mesoporous silicas have also been used as an effec-
tive nanocatalyst in this regard owing to their high surface
area, good recoverability, and reusability, as well as isolated
[
2]
try. The esterification of carboxylic acids has been conven-
tionally performed in the presence of homogeneous base cata-
lysts, such as sodium and potassium hydroxide or methoxide,
and acid catalysts, such as HCl, H SO , phosphoric acid, para-
[6]
acid sites in their channels.
Recently, we demonstrated that the confinement of a task-
specific 1-methyl-3-octylimidazolium hydrogen sulfate
2
4
[
1–3]
toluene sulfonic acid, HF, AlCl , BF , and ZnCl .
However, the
3
3
2
use of these homogeneous catalytic systems are not suitable
for practical applications because they are corrosive, toxic,
non-recoverable, and non-reusable. Other disadvantages of
these catalytic systems include difficulty in product separation,
environmental pollution, and they generally need additional
neutralization stages. Therefore, the development of a novel
heterogeneous catalyst system with appropriate catalytic per-
formance, while demonstrating good reusability, is an impor-
([OMIm]HSO ) ionic liquid inside the mesochannels of sulfonic
4
acid functionalized SBA-15 (SBA-15-Pr-SO H), which is denoted
3
as IL@SBA-15-Pr-SO H, resulted in a material that displayed
3
concomitant enhanced catalytic activity through a cooperative
mechanism; a feature that enabled the materials to effectively
promote the direct esterification of carboxylic acids with alco-
[7]
hols even at room temperature. However, owing to noncova-
lent immobilization of the employed ionic liquid, it may leach
from the surface because adsorbed [OMIm][HSO ] is partially
4
dissolved in organic phases (especially polar solvents), and
therefore, restricts long-term catalyst recovery. In a continuation
of this study, we reasoned that it might be possible to over-
come this problem by the simultaneous covalent immobiliza-
tion of sulfonic acid/ionic-liquid matrix inside the nanospaces
of a high-surface-area inorganic solid, such as mesoporous
silica. Meanwhile, we synthesized and reported a set of new
periodic mesoporous organosilica (PMO) materials that con-
tained ionic-liquid functional groups and studied their applica-
[
a] Dr. D. Elhamifar, A. Moradi, J. Rastegar
Department of Chemistry, Yasouj University
Yasouj 75918-74831 (Iran)
E-mail: d.elhamifar@yu.ac.ir
[b] Prof. Dr. B. Karimi
Department of Chemistry
Institute for Advanced Studies in Basic Sciences (IASBS)
Gava Zang, Zanjan 45137-6731 (Iran)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201402071.
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tions as efficient nanomaterials in the immobilization
and stabilization of metallic catalysts in some organic
reactions as well as in the headspace solid-phase mi-
[
8]
croextraction of polycyclic aromatic hydrocarbons.
We subsequently found that the covalent incorpora-
tion of the imidazolium functionality could not only
greatly enhance the catalytic activity of the anchored
sulfonic acid moiety in a cooperative manner, but
also provided a means of higher recyclability of the
present catalyst model than our previous system
comprising physically adsorbed ionic liquid.
Results and Discussion
First, the ionic-liquid-based periodic mesoporous or-
ganosilica (PMO-IL) was prepared according to our
[
8]
previous procedure. The PMO-IL was then treated
with a substoichiometric amount of (3-mercaptopro-
pyl)trimethoxysilane through a grafting approach to
give a mercaptopropyl-functionalized PMO nano-
3
Figure 1. TGA of PMO-IL-SO H nanocatalyst.
structure (PMO-IL-SH). Oxidation of SH groups into SO H was
elimination of the residue of the surfactant template. The third
and main weight loss from 250–6008C (22.89%) was associat-
ed with ionic liquid and propylsulfonic acid functional groups
incorporated in the material framework. These observations
strongly illustrate the successful inclusion of organic functional
groups in the material network and verify the high thermal sta-
bility of the material.
3
achieved through the treatment of the PMO-IL-SH with a solu-
tion of hydrogen peroxide at ambient temperature for 24 h,
followed by treatment with a diluted solution of sulfuric acid
[
9]
for 30 min (Scheme 1). The materials were then characterized
with several techniques, such as thermogravimetric analysis
(TGA), diffuse reflectance infrared Fourier transform (DRIFT)
spectroscopy, TEM, and nitrogen sorption analysis. TGA of
The nitrogen adsorption–desorption analysis of the prepared
materials was also performed to investigate their textural prop-
erties and pore size regularity (Figure 2). As shown, all samples
illustrate approximately the same type IV nitrogen adsorption–
desorption isotherm with an H1 hysteresis loop, according to
IUPAC classification. However, the inflection of the capillary
condensation became less sharp in the PMO-IL-SH and PMO-IL-
PMO-IL-SO H showed three weight losses at different tempera-
3
tures (Figure 1). The first weight loss observed below 1008C
(3.7%) was attributed to adsorbed water and alcoholic solvents
remaining from the extraction process, whereas the second
one (2.11%) appearing at 110 to 2908C, which corresponds to
SO H materials. Moreover, the Barrett–Joyner–Halenda (BJH)
3
pore size distribution isotherms showed a singlet sharp peak
for all materials. The BET surface areas were 655, 570, and
2
À1
5
04 m g for PMO-IL, PMO-IL-SH, and PMO-IL-SO H, respec-
3
tively (Table 1S in the Supporting Information). This study also
demonstrated that the pore sizes and pore volumes were suc-
cessively reduced through the series PMO-IL to PMO-IL-SH and
PMO-IL-SO H from 10.60 to 9.3 nm and from 1.2 to
3
3
À1
0
.93 cm g , respectively. This observation indicates successful
immobilization of propylthiol and propylsulfonic acid function-
al groups inside the mesochannels of the materials (Table 1S in
the Supporting Information). The TEM image of the PMO-IL-
SO H material was also recorded to investigate the nature and
3
stability of pores during oxidation process (Figure 3). This
image showed a hexagonally ordered mesoporous structure
with high regularity, a result that is in good agreement with ni-
trogen sorption analysis and powerfully confirms the uniform
pore size distribution and high stability of the material during
the reaction process.
The DRIFT spectrum of PMO-IL-SO H was also investigated
3
to confirm the presence of organic functional moieties in the
material network (Figure 1S in the Supporting Information).
À1
Scheme 1. Preparation of the PMO-IL-SO
3
H nanocatalyst.
The peaks cleared at n˜ =700–790 cm are attributed to CÀSi
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CÀH stretching), and a broad peak at approximately
À1
3
300 cm (stretching vibration of OÀH groups), re-
[
8]
spectively. These data prove the successful incorpo-
ration and high stability of ionic liquid and sulfonic
acid functional groups in the framework of the mate-
rial. The proton capacity of PMO-IL-SO H was also
3
measured through pH analysis of weighed samples
after ion exchange with a 1m solution of NaOH.
After characterization, the PMO-IL-SO H was ap-
3
plied as a catalyst in the esterification of carboxylic
acids with alcohols under solvent-free conditions. In
the first step, the effect of temperature and catalyst
loading was investigated in the reaction of acetic
acid with benzyl alcohol as a test model (Table 1).
The results showed that the amount of catalyst
strongly influences the reaction progress. As seen
from data summarized in Table 1, substrate conver-
sion increased with increasing amounts of catalyst
(
Table 1, entries 1–3) and the best result was ob-
tained in the presence of 5 mol% catalyst. The effi-
ciency of the catalyst was also considerably affected
by the reaction temperature. As shown, at 25 and
508C, low to moderate yields of product were ob-
tained in the presence of 5 mol% catalyst, whereas
at 708C an excellent yield was delivered (Table 1, en-
tries 3–5). To show the actual effect of sulfonic acid
groups on the catalytic cycle, in the next study the
catalytic activity of PMO-IL-SO H was compared with
3
its parents under the same reaction conditions as
those used before (Table 1, entry 3 vs. entries 6 and
7). As shown, both PMO-IL and PMO-IL-SH materials
were clearly inactive in catalyzing this reaction and
gave only a trace amount (<5%) of ester products,
whereas PMO-IL-SO H gave ester product in quantita-
3
tive yields. These results indicate that the observed
catalytic activity in PMO-IL-SO H almost exclusively
3
arises from the sulfonic acid functional groups in the
materials.
Our studies also demonstrated that the activity of
homogeneous sulfuric acid was much lower than
that of PMO-IL-SO H under the same reaction condi-
3
tions (Table 1, entry 8). The high efficiency of the
PMO-IL-SO H nanocatalyst may be attributed to iso-
3
lated acid sites incorporated in the mesochannels of
PMO-IL, where imidazolium functions are located.
This may provide a synergism cooperation between
imidazolium and sulfonic acid groups in their close
proximity and result in enhanced catalytic per-
Figure 2. Nitrogen adsorption–desorption (a) and pore size distribution (b) isotherms of
PMO-IL, PMO-IL-SH, and PMO-IL-SO H.
3
[7]
formance. In another study, the catalytic per-
formance of SBA-15 containing sulfonic acid and SiO2
stretching vibrations. The asymmetric and symmetric stretch-
with the same catalytic loading was compared with those of
ing vibrations of SiÀOÀSi bonds were observed at n˜ =1092
PMO-IL-SO H. The result significantly demonstrated that the ac-
3
À1 [8a]
and 927 cm . The other absorption peaks were observed at
tivity of both SBA-15-SO H and SiO -SO H was lower than that
3
2
3
n˜ =1443 (for CÀH deformation vibrations); 1560 (C=C stretch-
ing of imidazolium ring); 1620 (C=N stretching of imidazolium
of PMO-IL-SO H under the same reaction conditions and reac-
3
tion time (Table 1, entries 9 and 10). This observation proves
the key role of ionic-liquid units in the progress of the reac-
tion.
[
8]
ring); 1100 (S=O stretching vibration of the sulfonic acid moi-
eties); 2917, 3051 (aliphatic CÀH stretching); 3124 (unsaturated
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Table 2. The esterification of carboxylic acids with alcohols in the pres-
[
a]
3
ence of PMO-IL-SO H nanocatalyst.
[
b]
Entry
Alcohol (R’OH)
R
T [8C]
t [h]
Yield [%]
[
[
c]
c]
[d]
1
2
3
4
5
6
7
8
9
1
PhCH
4-ClC
4-MeOC
2
OH
CH
CH
CH
CH
CH
CH
3
3
3
3
3
3
70
70
60
75
75
70
75
60
70
70
60
60
60
60
75
75
18
21
17
24
24
24
24
12
24
24
15
15
16
19
20
24
95
92
96
86
85
88
86
93
89
83
93
92
92
90
90
82
6
H
4
CH
2
OH
OH
6
H
4
CH
2
[
[
[
c]
c]
c]
PhCH(OH)CH
PhCH(OH)CH
3
2
CH
3
PhCH
Ph(CH
2
CH
2
OH
2
)
3
OH
OH
OH
PhCH
CH
2
CH
2
[
c]
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
CH
CH
(CH
(CH
(CH
(CH
2
2
2
3
3
Figure 3. TEM image of PMO-IL-SO H.
PhCH
PhCH
2
2
CH
CH
2
0
2
CH OH
2
1
1
2
2
2
2
)
)
)
)
6
7
8
CH
CH
CH
2
2
2
OH
OH
OH
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
1
1
1
1
1
2
3
4
5
6
Table 1. The effect of temperature and catalyst loading on the esterifica-
[
a]
tion of acetic acid with benzyl alcohol.
[c]
2
16CH OH
[
[
c]
c]
2-octanol
cyclooctanol
[
a] Reaction conditions: alcohol (2 mmol), carboxylic acid (2 mmol), and
catalyst (5 mol%). [b] Yield of isolated product. [c] Alcohol (2 mmol) and
carboxylic acid (4 mmol). [d] Ph=C
[
b]
Entry
Catalyst
Loading
T [8C]
Yield [%]
6
5
H .
1
2
3
4
5
6
7
8
9
PMO-IL-SO
PMO-IL-SO
PMO-IL-SO
PMO-IL-SO
PMO-IL-SO
PMO-IL
3
3
3
3
3
H
H
H
H
H
2 mol%
3.5 mol%
5 mol%
5 mol%
5 mol%
0.045 g
0.045 g
5 mol%
5 mol%
5 mol%
70
70
70
50
25
70
70
70
70
70
17
57
95
43
8
<5
<5
19
58
52
It is also important to note that long-chain aliphatic esters
are very important products in the biodiesel industry. In the
present study, the results showed that long-chain aliphatic al-
cohols, such as octadeca-1-ol, also gave remarkable yields in
the reaction with acetic acid under defined reaction conditions
PMO-IL-SH
H
2
SO
SBA-15-SO
SiO -SO
4
3
H
10
2
3
H
(
Table 2, entry 14), which highlights the advantage of the cata-
[
1
a] Reaction conditions: acetic acid (4 mmol), benzyl alcohol (2 mmol),
8 h. [b] Yield determined by GC analysis.
lyst in both industrial and chemical processes. We also found
that PMO-IL-SO H was even active for the esterification of car-
3
boxylic acids with secondary aliphatic alcohols and gave a sig-
nificant yield of ester products under the desired reaction con-
ditions (Table 2, entry 15). Notably, more sterically demanding
cyclic alcohols were also smoothly converted into the corre-
sponding ester products, as exemplified by Table 2, entry 16.
These observations strongly demonstrate the remarkable abili-
ty of this nanocatalyst for the esterification of a broad range of
alcohols with carboxylic acids. The advantages of the present
catalyst system may be attributed to the hydrophobic part of
the ionic-liquid moieties and isolated sulfonic acid sites incor-
porated in the material network, as well as the high surface
area of the ordered nanostructured PMO-IL material.
With the optimized conditions, we next managed to exam-
ine the scope and limitation of this novel nanocatalyst in the
esterification of various alcohols with carboxylic acids (Table 2).
Generally, all reactions were very clean and the ester products
were obtained in good to excellent yields with no tar forma-
tion. As shown, primary benzylic alcohols, such as benzyl alco-
hol, 4-chlorobenzyl alcohol, and 4-methoxybenzyl alcohol, suc-
cessfully reacted with acetic acid in the presence of 5 mol%
catalyst and gave the corresponding ester products in excel-
lent yield (Table 2, entries 1–3). Secondary benzylic alcohols,
such as 1-phenyl ethanol and 1-phenyl propanol, which, owing
to their steric hindrance, are less reactive substrates in the
esterification reaction, also gave the ester adducts in excellent
yield at 758C (Table 2, entries 4 and 5). Other primary alcohols
with aromatic rings, such as 2-phenyl ethanol and 3-phenyl-1-
propanol, also effectively reacted with carboxylic acids and de-
livered the corresponding ester products in high yields
To investigate the role of the nanostructured PMO-IL materi-
al in the stabilization of catalytic sites, the recoverability and
reusability of the catalyst was monitored in the reaction of
acetic acid with benzyl alcohol under optimized reaction con-
ditions. To do this, after completion of the reaction, the ob-
tained mixture was filtered and the catalyst was recovered and
washed thoroughly with acetone and dried under vacuum
(Table 2, entries 6 and 7). Moreover, this new catalytic system
overnight at 708C. The recovered catalyst (RPMO-IL-SO H) was
3
was equally effective with both short- and moderate-chain pri-
mary aliphatic alcohols and the reactions proceeded smoothly
to deliver the ester products with good to excellent yields
then reused under the desired reaction conditions at least six
times and gave the corresponding ester product without any
significant decrease in selectivity and yield (Figure 4). This ob-
servation confirms the key role of the PMO-IL nanostructure in
(Table 2, entries 8–13).
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Conclusion
A new sulfonic acid immobilized
on a periodic mesoporous orga-
nosilica with an imidazolium
framework (PMO-IL-SO H) has
3
been synthesized, characterized,
and successfully applied as a cat-
alyst in the esterification of car-
boxylic acids. TGA and DRIFT
spectroscopy of the PMO-IL-
SO H material confirmed the
3
successful incorporation and
high stability of functional or-
ganic groups in the material net-
work. Nitrogen sorption analysis
and TEM indicated the high reg-
ularity and uniform pore size dis-
tribution of the mesoporous
channels with
a well-defined
hexagonal architecture. This ma-
terial was powerfully used as
3
Figure 4. Reusability of PMO-IL-SO H in the esterification of acetic acid with benzyl alcohol.
a catalyst in the esterification of
the stabilization of isolated sulfonic acid sites of the mesochan-
nels.
carboxylic acids with primary
and secondary alcohols and gave the corresponding ester
products in high yield and excellent selectivity. Some other ad-
vantages of this work include solvent-free conditions, easy
product separation and purification, non-corrosive catalyst, low
cost, and eco-friendly because the catalyst could be recovered
and reused several times. Regarding the remarkable properties
of this catalytic system, some of its applications in other chem-
ical transformations are currently underway in our laboratories.
The TEM image of the recovered catalyst after the third reac-
tion cycle showed a well-ordered hexagonal array of mesopo-
rous channels, which proved that the structure remained un-
changed after the recovery stages (Figure 5). Moreover, nitro-
gen adsorption–desorption analysis of the recovered PMO-IL-
SO H nanocatalyst illustrated a type IV isotherm and H1 hyste-
3
resis loop with a pattern the same as that of fresh catalyst. The
BET surface area, pore diameter, and pore volume of the
RPMO-IL-SO H material decreased remarkably (Table 1S in the
3
Experimental Section
Supporting Information). The BJH curve of this material also
showed a peak with high intensity, which proved that the reg-
ularity of the mesochannels remained during the applied reac-
tion conditions. Accordingly, it can be concluded that the de-
signed nanocatalyst is highly effective and stable during the
reaction process and the applied conditions do not negatively
affect the regularity of the PMO-IL nanomaterial.
Preparation of the PMO-IL-SH nanostructure
PMO-IL mesomaterial was prepared according to our previous re-
[8]
ported procedure. The prepared PMO-IL was then modified by
3-mercaptopropyl)trimethoxysilane through a grafting approach.
In a typical procedure, PMO-IL (1 g) was added to dry toluene
50 mL) at 258C with stirring for 5 min. After a clear homogeneous
solution was obtained, (3-mercaptopropyl)trimethoxysilane
0.4 mmol) was added and the obtained mixture was heated at
(
(
(
reflux under an argon atmosphere for 24 h. The resulting precipi-
tate was filtered and washed thoroughly with dry toluene to
remove unreacted (mercaptopropyl)trimethoxysilane. The final
solid was dried overnight at 708C and gave a white powder denot-
ed as PMO-IL-SH. Elemental analysis showed that the loading of
À1
thiopropyl groups on the solid surface was 0.35 mmolg .
Preparation of the PMO-IL-SO H nanocatalyst
3
The oxidation of SH groups to SO H counterparts was accom-
3
[9]
plished in the presence of hydrogen peroxide as an oxidant. Typi-
cally, PMO-IL-SH (1 g) was added to a 100 mL round-bottomed
flask containing H O (35%, 30 mL) and stirred for 24 h at room
2
2
Figure 5. TEM image of recovered PMO-IL-SO
lyst.
3
H (RPMO-IL-SO
3
H) nanocata-
temperature. Then, a dilute solution of sulfuric acid was added and
the mixture was further stirred for 30 min at the same temperature.
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due was dried at 608C for 12 h and denoted as PMO-IL-SO H. The
3
[
[
proton capacity of PMO-IL-SO H was also measured through pH
3
analysis of weighed samples after ion exchange with a 1m solution
+
of NaOH. The results showed that the loading of H on the solid
À1
surface was 2.2 mmolg .
General procedure for the esterification of carboxylic acids
with the PMO-IL-SO H nanocatalyst
3
2
2
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Alcohol (2 mmol), carboxylic acid (2–4 mmol), and catalyst
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7
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Keywords: esterification · ionic liquids · periodic mesoporous
organosilicas · carboxylic acids · sulfonic acid catalysts
5
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Received: March 18, 2014
Published online on && &&, 0000
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 0000, 00, 1 – 7
&
6
&
ÞÞ
These are not the final page numbers!

FULL PAPERS
Increased functionality: The prepara-
tion of a new sulfonic acid containing
ionic-liquid-based periodic mesoporous
D. Elhamifar,* B. Karimi,* A. Moradi,
J. Rastegar
&& – &&
organosilica (PMO-IL-SO H, see picture),
3
as well as its catalytic applications for
the esterification of carboxylic acids
with primary and secondary alcohols
under solvent-free conditions is de-
scribed.
Synthesis of a Novel Sulfonic Acid
Containing Ionic-Liquid-Based Periodic
Mesoporous Organosilica and Study of
Its Catalytic Performance in the
Esterification of Carboxylic Acids
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 0000, 00, 1 – 7
&
7
&
ÞÞ
These are not the final page numbers!