A study on the catalytic activity and theoretical modeling of a novel dual acidic mesoporous silica

Article abstract of DOI:10.1039/c3ra47638c

A novel mesoporous silica-functionalized dual Bronsted acidic species has been introduced as an efficient catalyst for solvent-free esterification of fatty acids with ethanol. The structure of the catalyst has been characterized by FT-IR spectroscopy, thermal gravimetric analysis (TGA), TEM and N₂ adsorption-desorption. TGA of catalyst 1 showed no weight loss before 200 °C, indicating a high degree of hydrophobicity of the surface of the mesoporous silica. TEM images and nitrogen adsorption-desorption showed no noticeable changes to the structure of the catalyst before and after acid treatment. pH metric analysis was performed for the catalyst to determine the loading of the acidic sites. The structure of the catalyst was modeled by mimicking the surface of functionalized silica gel in the form of a cage-like cluster. Various conformers from the proposed structures were selected and optimized at the B3LYP/6-311++G** level of calculation. Natural bond orbital (NBO) analysis was performed to investigate the nature of hydrogen bonding of the catalyst in more detail. Based on the data gained from the optimized structures of the catalyst, a mechanism was proposed for the esterification reaction.

Full text of DOI:10.1039/c3ra47638c

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A study on the catalytic activity and theoretical
modeling of a novel dual acidic mesoporous silica†
Cite this: RSC Adv., 2014, 4, 16647
*
Majid Vafaeezadeh and Alireza Fattahi
A novel mesoporous silica-functionalized dual Brønsted acidic species has been introduced as an efficient
catalyst for solvent-free esterification of fatty acids with ethanol. The structure of the catalyst has been
characterized by FT-IR spectroscopy, thermal gravimetric analysis (TGA), TEM and N2 adsorption–
desorption. TGA of catalyst 1 showed no weight loss before 200 ꢀC, indicating a high degree of
hydrophobicity of the surface of the mesoporous silica. TEM images and nitrogen adsorption–desorption
showed no noticeable changes to the structure of the catalyst before and after acid treatment. pH
metric analysis was performed for the catalyst to determine the loading of the acidic sites. The structure
of the catalyst was modeled by mimicking the surface of functionalized silica gel in the form of a cage-
like cluster. Various conformers from the proposed structures were selected and optimized at the
B3LYP/6-311++G** level of calculation. Natural bond orbital (NBO) analysis was performed to investigate
the nature of hydrogen bonding of the catalyst in more detail. Based on the data gained from the
optimized structures of the catalyst, a mechanism was proposed for the esterification reaction.
Received 14th December 2013
Accepted 24th March 2014
DOI: 10.1039/c3ra47638c
www.rsc.org/advances
However, using homogeneous acid catalysts such as H2SO4,
H3PO4, and HCl for esterication reaction have many draw-
Introduction
backs such as releasing hazardous wastes and difficulties in
catalyst separation. It has been shown that various esterication
reactions can be efficiently performed with heterogeneous acid
catalysts as good replacements of the traditional hazardous and
corrosive homogeneous acid catalysts.4 Esterication of the
fatty acid with mesoporous silica–sulfonic acid improves reac-
tion efficiency, catalyst separation, and reusability.1a,5 In
comparison to carbon-based sulfonic acid catalyst, SiO2-based
sulfonic acids have some benets such as high loading of acidic
centers, thermal and hydrothermal stability of the silica back-
bone, and avoiding usage of concentrated sulfuric acid and
elevated temperature during the catalyst preparation.
However, the major drawback of these types of catalysts is
that the loading of organic groups are typically low in the
organically modied mesoporous silica. In the case of sup-
ported sulfonic acid this phenomena causes some technical
problems especially in the solvent-free reactions such as inef-
cient mixing of reactant(s) and catalysts. To the best of our
knowledge, only few methods are developed to design hetero-
geneous acid catalysts with enhanced loading of acidic sites.6 In
this regard, Karimi and co-worker introduced that SBA-15 pro-
pylsulfonic acid-conned Brønsted acidic ionic liquid can effi-
ciently catalyze some organic transformations at room
temperature without using special methodologies.7 The authors
proposed that the existence of a synergistic effect between the
graed sulfonic acid groups and hydrogensulfate counter anion
of the supported ionic liquid accompanied by hydrophobic
nature of the catalyst is the main reason of the extraordinary
activity of the catalyst. Other successful examples of the
Synthesis of fatty acid methyl/ethyl esters known as biodiesel
has attracted special attention in the last decade1 as they are a
good replacement of fossil fuels to diminish the global energy
crisis. The traditional route to biodiesel production is trans-
esterication of triglycerides with a short-chain alcohol (e.g.,
methanol or ethanol) using sodium hydroxide as a base cata-
lyst.2 However, sodium hydroxide is a corrosive reagent and
hence an additional neutralization step is required which leads
to production of a high volume of salty waste. Furthermore, the
residual free fatty acid (in the case of less expensive feedstock
such as waste oil) is saponied by the homogeneous alkali
catalyst, which increases the separation cost. In this regard,
several approaches based on metal oxides (such as CaO and
MgO) have been developed as heterogeneous solid catalysts.3
However, despite promising improvements, the leaching of the
active sites and low surface area are among the drawbacks of
these protocols which remain unresolved. Moreover, in some
cases to gain sufficient biodiesel products the applied metal
oxide should be mixed with strong bases such as MgO–KOH and
MgO–NaOH systems.3d
Acid catalyst esterication is a promising way to solve the
drawbacks of the mentioned traditional base catalyst reactions.
Department of Chemistry, Sharif University of Technology, Tehran, Iran. E-mail:
fattahi@sharif.edu; Fax: +98 21 66029165; Tel: +98 21 66005718
† Electronic supplementary information (ESI) available: O2,3, M2,3 and P2,3
which optimized at B3LYP/6-311++G** and their total energy values, 1H NMR
spectra of the esters and XYZ coordinates of the optimized structures. See DOI:
10.1039/c3ra47638c
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synthesis of dual Brønsted acidic catalysts were reported by Luo
and co-workers.8
To investigate the structure of the catalyst and intermedi-
ates, comparative FT-IR spectra were performed for pure silica,
They showed that both amorphous silica gel and silica material A and catalyst 1. As shown in Fig. 1, the top spectrum is
coated magnetic nanoparticles functionalized with dual for pure silica, while the next two are for intermediate A and the
Brønsted acidic ionic liquid can catalyze some multicomponent nal catalyst 1. Intermediate A and catalyst 1 show important
reactions under solvent-free conditions.8 Mesoporous silica differences relative to the pure silica.
with two Brønsted acidic sites has also been synthesized and
The bands at 2852 and 2923 cmꢁ1 in the spectra of the
applied for efficient thioacetalization of various carbonyl intermediate A belong to C–H and N–H stretching frequencies
compounds in water.9 The structure of this catalyst was theo- in the surface-bonded aminopropyl functionalities. In the case
retically studied to gain useful insight into the orientation of of catalyst 1, another band at 3041 cmꢁ1 can be interpreted as
organic groups which were attached to the surface of the cata- C–H stretching frequency of benzyl group moiety. The band at
lyst. In the current work, we wish to report detailed studies on 582 cmꢁ1 with relatively high intensity belongs to the S–O
the direct esterication of fatty acid with a novel organically frequency of –SO3H group. The double band frequency (S]O) of
modied mesoporous silica gel containing dual Brønsted acidic –SO3H group seems to be overlapped with the broad band of the
species. The method for preparation of the catalyst is simple pure silica gel around the 1100 cmꢁ1. It is worth to note that the
and can be perform with commercially available starting bands around 1470 and 1740 cmꢁ1 in both A and catalyst 1 are
materials. In this method fatty acid esterication has been attributed to the N–H bending frequencies of the surface-
performed without using any additional water-removal step bonded amine and/or ammonium functionalities.
(such as azeotropic distillation) and using molecular sieves or
Silica-based catalysts inherently tend to absorb humidity
concentrated sulfuric acid. Moreover, we have reported novel and/or water molecules which participate in the reaction media
theoretical modeling techniques to study the structure of the in the form solvent, impurity or generated as by-product. In the
catalyst for better determining and introducing the nature of case of silica supported acid catalyst this phenomenon is more
interactions of active sites with high level of calculations. In this important than others.
regards, a cage-like cluster of the silica gel which was func-
As previously studied by van Grieken et al., “acid solids are
tionalized with organic group has been selected to mimic the subjected to poisoning with water in reactions where this highly
surface of the catalyst.
polar molecule is involved”.11 Typically, the presence of water in
the silica-based catalyst is explained as a presence of a sharp
weight loss which usually occurs around 100 ꢀC in the thermal
gravimetric analysis (TGA) of the catalyst.7b However, as shown
in Fig. 2 no detectable weight loss could be assigned up to
200 ꢀC in the thermogram of the catalyst 1. This observation can
be interpreted via the fact that the surface of the mesoporous
silica modied with organic groups has high degree of
Results and discussion
As stated in the experimental section, catalyst 1 was simply
prepared by graing of the commercially available amine
precursor onto the high surface area silica gel (Scheme 1, A).
This prepared material easily reacts with benzyl bromide to
afford surface-bonded ammonium bromide specie (B). Chlor-
osulfonic acid treatment of the material10 followed by acidi-
cation and washing afforded catalyst 1 as a dry and slightly
cream powder (Scheme 1).
Scheme 1 Schematic illustration of catalyst 1 preparation.
16648 | RSC Adv., 2014, 4, 16647–16654
Fig. 1 FT-IR spectroscopy of the pure silica, material A and catalyst 1.
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Fig. 4 N2 adsorption–desorption of material B (a) and catalyst 1 (b).
Fig. 2 TGA of catalyst 1.
desorption analysis was performed for material B and catalyst 1
(Fig. 4). The results show the type IV adsorption–desorption
isotherm over the range P/P0 ¼ 0.4–0.8 with H2 hysteresis loop.
hydrophobicity. This hydrophobicity has two important effects
in the esterication reaction: (i) the functionalized silica gel The BET (Brunauer–Emmett–Teller) surface area of the catalyst
does not have tendency to absorb water and thus no signicant 1 was found to be 43 m2 gꢁ1. The value of the mean pore
deactivation of catalyst 1 occurs during the reaction (ii) it diameter of catalyst 1 was derived from the BJH (Barrett–Joyner–
facilitates mass transfer of organic starting material and Halenda) average pore diameter analysis and was calculated to
simultaneously expels out water by-product in the esterication be 6.2 nm.
reaction.7a,12
Ion exchange pH analysis was performed for catalyst 1 with 1
M solution of NaCl according to the known literature proce-
dures.7a,13 The result of experiment indicated that the loading of
TEM images of catalyst before (material B) and aer acid
treatment (catalyst 1) are shown in Fig. 3. As shown, no notice-
able changes have been made for the structure of the catalyst.
For further investigation about the structure of the catalyst
catalyst 1 is approximately 1.12 mmol H+ gꢁ1
.
The performance of catalyst 1 was initially investigated for
before and aer acid treatment, nitrogen adsorption– esterication of the 3-phenylpropionic acid with ethanol. Our
Fig. 3 TEM of the material B (a) and catalyst 1 (b).
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Table
1 The data of catalytic performance for esterification of
carboxylic acid with ethanol
Entry
Substrate
Catalyst
Solvent
Yielda (%)
1
2
3
4
5
6
7
8
3-Phenylpropionic acid
3-Phenylpropionic acid
Lauric acid
Palmitic acid
Stearic acid
3-Phenylpropionic acid
3-Phenylpropionic acid
3-Phenylpropionic acid
3-Phenylpropionic acid
3-Phenylpropionic acid
3-Phenylpropionic acid
1
1
1
1
1
A
B
1
1
1
1
—
—
—
—
—
—
—
H2O
H2O
CH3CN
Toluene
54b
91
86
85
80
Fig. 5 Schematic illustration of shifting the reaction equilibrium for
ester synthesis with catalyst 1.
c
—
38c
50d
82e
24d
19d
9
10
11
a
Yields refer to isolated pure products in the presence of 10 mol% of
proton equivalent of a weighted sample of catalyst 1 at 78 ꢀC for 12 h
and
performed at room temperature for 48 h. 150 mg of material A and
B was used as catalyst. 1.5 mL of solvent was used in each case. 20
b
2
mL ethanol, unless otherwise stated. The reaction was
c
d
e
mol% of catalyst was used.
preliminary investigation showed that an isolated yield of 54% of
the ester product was obtained when 10 mol% of the catalyst 1
was used at room temperature for 48 h (Table 1, entry 1). When
the mentioned reaction was performed at 78 ꢀC (in which
Fig. 6 Recovery experiment of catalyst 1.
3-phenylpropionic acid and the fatty acids are molten), it catalyst was recovered. The recovered material was subjected to
produced the corresponding ester in 91% yield (Table 1, entry 2). another reaction with identical conditions. The results indicate
Encouraged by these results, we then managed esterication that catalyst 1 exhibits almost similar catalytic activity for at
of some available fatty acid with ethanol. When 10 mol% of least four times without appreciable loss of efficiency.
catalyst 1 was used at 78 ꢀC (Table 1, entries 3–5), the fatty acid
Computational methods are fruitful ways to study various
ethyl ester (FAES) known as biodiesel was obtained in high behavior of the catalytic reactions such reaction mechanism,
yield. The reaction of 3-phenylpropionic acid with ethanol and intermediates and structures of catalysts in molecular dimen-
material B as catalyst at the optimized reaction condition gave sion.16 To gain more realistic insight about the structure of the
38% ester yield (Table 1, entry 7) owing to the potential catalytic catalyst we performed some computational modeling by
activity of ammonium salts for esterication reaction.12a,14
To investigate the effect of solvent, the reaction was per-
mimicking the surface of the silica.
Silica-based catalysts are common class of catalyst with broad
formed in water, toluene and acetonitrile (Table 1, entries 8–11). catalytic applications. Modeling the surface of silica gel is a
Our investigation showed that the esterication reaction can challenging step for computational study. Recently, Shanks and
proceed well in water (Table 1, entries 8 and 9). In this regard we co-worker performed interesting mechanistic study for acetic
found that high yield (82%) of ester was obtained at 78 ꢀC aer acid esterication by using simple xed-Si(OMe)3 groups to
12 h by increasing the amount of catalyst to 20 mol%. In this model the surface of silica gel.17 Using the relatively large cage-
condition, the supported organic groups provide hydrophobic like structure of silicon–oxygen sequence is a useful way to
environment in water.15 A plausible explanation for signicant perform a more accurate mimicking the surface of silica gel and
catalytic activity in water is related to the differences in polari- thus to obtain more reliable data.18 In the present study, a 6-edge
ties between the applied solvents and the surface of the catalyst. cage-like cluster9,19 of silica (Si4O6(OH)) was selected to model the
In the case of water as the highest polar solvent, it acts as a surface of silica gel. The initial structures were built by attaching
driving force to push organic substrate into the hydrophobic the organic groups of catalyst to the surface of silica gel.9 In
channels. Simultaneously, the back reaction (ester hydrolysis) preparation of catalyst 1, material B undergo one step sulfonation
could not occur due to the fact that water cannot diffuse into the of benzene ring. This electrophilic substitution reaction can
hydrophobic environment of the catalyst. The net effect is occur in the three positions (ortho, meta and para) of the benzene
shiing the reaction to completion even in the presence of ring according to values of the negative charge of carbon atoms in
water as solvent (Fig. 5).
the ring. Exploring the orientation of such electrophilic substi-
The reusability of catalyst 1 was investigated for esterica- tution can be predicted by means of computational modeling. In
tion of 3-phenylpropionic acid with ethanol at the optimized this regard, the structure of material B was optimized at B3LYP/6-
reaction condition (Fig. 6). In this regard, aer the rst run of 311++G** level of calculation.9,20 Then the values of Mulliken and
reaction, the mixture has been ltrated and the catalyst was NBO charges21 were extracted for carbon atoms of benzene ring of
recovered. Aer drying, nearly 95% of the initial weighted the optimized structures (Fig. 7).
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The values of Mulliken charge (black) show the preference of
the electrophilic substitution (sulfonation) reaction to the meta
position of benzene ring in material B.
However, the corresponding data derived from NBO charge
(red) did not show signicant difference in orientation to the
sulfonation of benzene ring in material B (Fig. 7). With these
promising results, we then tried to investigate the structure of
the catalyst 1 considering the occurrence of sulfonation reac-
tion of material B at three positions (ortho, meta and para). The
structures of ortho (O), meta (M), and para (P) substitute were
built with Spartan soware. Conformer search was performed
for each category using Merck Molecular Force Field. Then the
conformers with the lowest energy were optimized at B3LYP/6-
311++G** level. The lowest energy conformers of each geometry
of the substituted benzene ring (O1, M1 and P1) are shown in
Fig. 8.22 By comparing the energetic of the optimized structures
(Table 2), we found that in the optimized structure, a coopera-
tivity ꢁexists between the sulfonic acid head groups (–SO3H) and
HSO4 moieties. As previously proposed7a the cooperativity
factor is an effective parameter which signicantly improves the
catalytic activity.
Fig. 7 The values of Mulliken charge (black) and NBO charge (red) of
carbon atoms in benzene ring of the optimized structure of material B.
The experimental and theoretical data about the structure of
the silica-functionalized sulfonic acids and the nature of the
intra-structural interactions are rare.9 As shown in Fig. 8, the
optimized structures of catalyst 1 mainly comprises intra-
structural hydrogen bonds (H bond). The lengths of H bonds of
all optimized structures are listed in Table 2.
Table 2 The values of the total energy, significant values of E(2) (kcal
molꢁ1) and DqCT
Structure Total energya Charge transfer
rH/O E(2)
DqCT
b
To better explore the nature of H bonds, NBO analysis was
performed for the global minimum energy geometries (the
geometry with the lowest energy) in O1, M1 and P1 of the
proposed structures (Table 2). In the NBO analysis, the inter-
action between the electron pairs of the H bond acceptor atoms
and the antibonding orbitals of the H bond donor is investi-
gated as a characteristic of H bonding. The second order
perturbation stabilization energy, E(2), is related to the delo-
calization of electrons in a donor–acceptor pair and is calcu-
lated by perturbation theory.23 The higher value of E(2) indicates
O1
M1
P1
ꢁ3380.203564 lp O40 / s*H47–O46 1.438 72.09 0.777
lp O48 / s*H25–N23 1.756 23.51 0.472
lp O42 / s*H24–N23 1.666 32.93 0.588
ꢁ3380.199109 lp O42 / s*H47–O46 1.662 24.50 0.477
lp O40 / s*H25–N23 1.510 64.17 0.719
lp O13 / s*H39–O38 1.852 10.87 0.218
ꢁ3380.192690 lp O42 / s*H47–O46 1.761 16.06 0.358
lp O40 / s*H24–N23 1.800 20.69 0.511
lp O41 / s*H25–N23 1.892 13.15 0.388
a
The total energies are reported in atomic unit. b rH/O is the length of
˚
H bonds that are reported in A.
Fig. 8 Optimized structures of O1, M1 and P1.
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a higher delocalization trend of an electron from donor to participate in the reaction. In this mechanism, a non-adsorbed
acceptor orbitals, and thus a stronger interaction. The selected ethanol molecule reacts with an adsorbed carboxylic acid mol-
ꢁ
values of E(2) for donor–acceptor (lp–s*) interactions are listed ecule.4a,17,24 Despite the Brønsted acidity, the role of HSO4 is
in Table 2. It can be deduced from these data that the highest activation of ethanol via H bond assistance.
value of E(2) (72.09) is attributed to the shortest H bond (1.438 A)
˚
which exists in O1. Typically, formation of H bond leads to
charge transfer from donor sites to the acceptors. The values of
Conclusion
charge transfer (DqCT) can be calculated from the NBO analysis.
These values are known as a criterion of the H bond strength. As
shown in Table 2, shorter H bonds generally have higher DqCT
values.
In summary, we designed a novel dual Brønsted acidic meso-
porous silica with commercially available starting material for
esterication reaction. The high loading acidic sites accompa-
nied by hydrophobic nature of the catalyst facilitate esterica-
tion reaction in high yield without using special methodology.
The results showed that the reaction can also be performed in
water as an environmentally benign and cheap solvent. The
catalyst has been simply separated from the reaction mixture
and used for at least four reaction runs without noticeable loss
of activity. The structure of the catalyst was investigated using a
computational modeling technique. Mulliken and NBO charges
were calculated to explore the orientation of electrophilic
substitution sulfonation on the surface of the catalyst. For the
rst time, the presence of synergistic effect between sulfonic
Since both E(2) and DqCT values are responsible for the
strength of H bonds, therefore a relationship between these
parameters should exist (Fig. 9). According to Fig. 9, this linear
correlation between the values of E(2) and DqCT (which were
derived from the optimized structures of O1, M1 and P1) is
observed.
Based on the structural data gained from the optimized
structures of catalyst 1, a mechanism was proposed for
synthesis of FAES with ethanol (Fig. ꢁ10). The presence of
sulfonic acid moiety (–SO3H) and HSO4 in its close proximity
proposed that both of these species can simultaneously
ꢁ
acid specie (–SO3H) and HSO4 moiety on the surface of the
dual Brønsted acidic catalyst was theoretically explored and
proved. The useful structural data derived from the optimized
geometries of the catalyst 1 helped us propose a reasonable
mechanism for esterication. In this mechanism, it was
proposed that the reaction occurs on a single acidic site of thꢁe
mesoporous silica. The cooperation of –SO3H and HSO4
species of catalyst 1 was directly involved in the mechanism of
esterication. We believe that this modeling method gives more
realistic insight and useful structural information about the
catalyst in comparison with simple paper-drawing demonstra-
tions. Further computational investigations about the reaction
mechanism are being carried out in our laboratories.
Experimental
Preparation of aminopropyl silica
Fig. 9 Correlation between the values of second-order perturbation
energies E(2) (kilocalorie per mole) and charge transfer (DqCT).
Silica gel Davisil™ (CAS number 112926-00-8) grade 635
3
(average pore diameter 60 A, pore volume 0.75 cm gꢁ1, surface
˚
area 480 m2 gꢁ1) was purchased from Sigma Aldrich. First, the
silica gel was activated by reuxing in 6 M hydrochloric acid for
24 h. Then it was washed thoroughly with the deionized water to
adjust the pH of the solution to 6–7 and dried at 100 ꢀC for
overnight. The activated silica gel (5.00 g) was mixed with
10 mmol of (3-aminopropyl)triethoxysilane in dry toluene for
24 h. Then, the solid materials were ltered off and washed with
hot toluene for 24 h in a continuous extraction apparatus
ꢀ
(Soxhlet) and dried in an oven at 90 C for 12 h to give amino-
propyl silica as a white solid.
Preparation of dual Brønsted acidic mesoporous silica
(catalyst 1)
A mixture of 2.0 g aminopropyl silica and 5 mmol benzyl
bromide was reuxed in toluene for 24 h. Then, the solid was
ltrated and washed with dry toluene (2 ꢂ 50 mL) and dried at
Fig. 10 Proposed reaction mechanism for the esterification over
catalyst 1.
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90 ꢀC. Chlorosulfonic acid (10 mmol) was added to the solution
of the resulted material in 50 mL of dry chloroform and the
mixture was reuxed for 48 h.10 Then, the resulted material was
ltrated and the residual chlorosulfonic acid was carefully
washed by absolute ethanol. Finally, the material was subjected
to 2 molar solution of H2SO4 at room temperature. Aer 12 h,
the material was ltrated and washed with diethyl ether and
dichloromethane to afford catalyst 1.
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pH analysis of the catalyst25
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mL, 1 M). The resulting mixture was stirred for 2 h aer which
the pH of the solution decreased to 1.65. This is approximately
equal to a loading of 1.12 mmol H+ gꢁ1 of the catalyst 1.
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General procedure for fatty acid esterication
In a 10 mL round-bottom ask equipped with a condenser, fatty
acid (2 mmol), ethanol (2 mL) and catalyst 1 (10 mol %) were
added and heated to reux. The reaction progress was moni-
tored by TLC (thin layer chromatography) and gas chromatog-
raphy. Aer the completion of the reaction, the slurry was
ltrated and the catalyst was washed with n-hexane (50 mL) and
recovered. The residual of the remained fatty acid in organic
phase was quenched with 5% aqueous solution of NaHCO3 (2 ꢂ
3 mL). Then, the organic phase was dried over Na2SO4. The
corresponding FAES in high yield was afforded following
evaporation of the n-hexane under reduced pressure.
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16654 | RSC Adv., 2014, 4, 16647–16654
This journal is © The Royal Society of Chemistry 2014