Mesoporous MCM-48 Immobilized with Aminopropyltriethoxysilane: A Potential Catalyst for Transesterification of Triacetin

Article abstract of DOI:10.1007/s10562-017-1997-5

Abstract: The ordered mesoporous silicas MCM-48, MCM-41, and SBA-15 were synthesized and functionalized with 3-aminopropyltriethoxysilane (APTES). The X-ray diffraction patterns before and after functionalization revealed no structural degradation during the process. FT-IR spectra of the materials clearly indicated the anchoring of the aminopropyl moiety with silanol groups. The amine concentrations were calculated using TG-DTA and CHN analysis. The amine-loaded materials were assessed as catalysts for the transesterification of triacetin with methanol. MCM-48-NH₂, in which the pores are interconnected in a three-dimensional manner, exhibited superior catalytic activity to one- dimensional MCM41-NH₂ and SBA-15-NH₂, even with lower concentrations of the amine group. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-017-1997-5

Catal Lett (2017) 147:1040–1050
DOI 10.1007/s10562-017-1997-5
Mesoporous MCM-48 Immobilized
with Aminopropyltriethoxysilane: A Potential Catalyst
for Transesterification of Triacetin
Mahuya Bandyopadhyay1,2 · Nao Tsunoji1 · Tsuneji Sano1
Received: 15 December 2016 / Accepted: 13 February 2017 / Published online: 28 February 2017
© Springer Science+Business Media New York 2017
Abstract The ordered mesoporous silicas MCM-48,
MCM-41, and SBA-15 were synthesized and functional-
ized with 3-aminopropyltriethoxysilane (APTES). The
X-ray diffraction patterns before and after functionaliza-
tion revealed no structural degradation during the process.
FT-IR spectra of the materials clearly indicated the anchor-
ing of the aminopropyl moiety with silanol groups. The
amine concentrations were calculated using TG-DTA and
CHN analysis. The amine-loaded materials were assessed
as catalysts for the transesterification of triacetin with
methanol. MCM-48-NH2, in which the pores are intercon-
nected in a three-dimensional manner, exhibited superior
catalytic activity to one- dimensional MCM41-NH2 and
SBA-15-NH2, even with lower concentrations of the amine
group.
Graphical Abstract
Keywords Mesoporous materials · Functionalization ·
MCM-48 · Transesterification · Triacetin
1 Introduction
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-017-1997-5) contains supplementary
material, which is available to authorized users.
The increasing global demand for energy has led to intense
research activity into alternative energy sources. Bio-based
resources are considered as a promising renewable alter-
native energy source to fossil fuels owing to the environ-
mental hazards and lack of sustainability associated with
the latter [1, 2]. However, the high viscosity of vegetable
oil, one of the most common bio-based resources, prevents
its direct use as a fuel for diesel engines. Modifications
including dilution, pyrolysis, microemulsification, and,
most successfully, transesterification, have been employed
to overcome this problem. Consequently, the production of
* Mahuya Bandyopadhyay
mouraji@hiroshima-u.ac.jp
* Nao Tsunoji
tnao7373@hiroshima-u.ac.jp
1
Department of Applied Chemistry, Graduate
School of Engineering, Hiroshima University,
Higashi-Hirosima 739-8527, Japan
2
Institute of Infrastructure, Technology, Research
and Management, IITRAM, Maninagar, Ahmedabad,
Gujarat, India
Vol:.(1234567890)
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Mesoporous MCM-48 Immobilized with Aminopropyltriethoxysilane: A Potential Catalyst for…
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biodiesel from the transesterification of vegetable and ani-
mal fats using homogeneous base catalysts has increased
drastically in recent years.
Development Company in 1992 [18]. Following appropri-
ate functionalization, these types of mesoporous materials
can serve as potential catalysts to increase the yield of, and
selectivity for, the desired products. Several approaches
have been proposed to impart specific properties to these
materials for application in areas such as catalysis, chemi-
cal sensing, and adsorption [19–23].
Natural fats and oils can be used for the production of a
variety of bio-based products because they contain multiple
reactive groups that can be modified to afford the desired
products using an appropriate choice of reaction condi-
tions and catalyst. Triglycerides, which are produced from
plants and animal fats, are regarded as a prospective source
for bio-fuel production because they have a composition
similar to that of fossil fuels [3]. Methyl esters are pro-
duced by the transesterification of triglycerides with short
chain alcohols such as methyl alcohol. However, the major
challenge in this process is to convert the material into
bio-fuels. To improve the economical aspect of these pro-
cesses, the choice of catalyst is crucial. Homogeneous cata-
lysts are ideal for liquid-phase reactions because they form
uniform mixtures with the reactants, resulting in minimum
mass-transfer limitations and high reaction rates. However,
the use of homogeneous catalysts presents certain difficul-
ties, including the disposal of toxic wastes, complicated
catalyst recovery, and time- and energy-consuming separa-
tion of the desired products. These unfavorable aspects of
the use of homogeneous catalysts have led to an increase
in the use of solid heterogeneous catalysts for large-scale
chemical reactions [4–6]. Alkali-earth-metal oxides [7],
alkali–metal-doped alumina [8, 9], metal-loaded zeolites
[10], hydrotalcites [11], enzymes [12], Amberlyst-15 [13],
the Cs salt of H3PW12O40 [13], and polyaniline sulfate [14]
have been studied as heterogeneous catalysts for the trans-
esterification of triglycerides.
The heterogenization of organic bases has been used
to prepare catalysts that exhibit excellent performance in
many reactions. Surface functionalization of mesoporous
materials allows the introduction of catalytic groups to
their internal pore surfaces through co-condensation or
post-synthesis treatment [24]. MCM-41- and MCM-48-
type materials contain a large number of –OH groups on
their internal pore surfaces that can react to form hybrid
materials. Surface modification by the formation of silanol
linkages via condensation with silane-bearing moieties,
particularly propyl chains, is the most commonly employed
methodology [25–36]. Modification with silane-terminated
aminopropyl chains, which imparts surface basicity to the
mesoporous materials, has been particularly widely stud-
ied [37, 38]. Mesoporous materials particularly MCM-41,
and SBA-15 has been widely explored in esterification
reaction so far, but almost no report has been available for
mesoporous MCM-48 materials employed in this reaction.
Guerrero et al. has reported amine functionalized MCM-
41 and SBA-15 as transesterification catalysts of glyceryl
tributyrate with methanol [39].
MCM-48, a relatively unexploited mesoporous siliceous
molecular sieve, has been investigated in the present work.
Its interconnected three-dimensional pore structure was
expected to provide a combination of catalytic activity and
product selectivity superior to that available from other cat-
alysts, including those based on more common mesoporous
molecular sieves such as MCM-41 and SBA-15, in which
the pores extend in only one direction. This limits the
accessibility to active sites in these materials, restricting
the level of functionalization, and extending diffusion dis-
tances for reactants and products. However, there remains
the question of whether it is simply the accessibility of
the active sites in functionalized MCM-48 that renders it
more active, or whether the active sites on this support have
higher catalytic activity, or indeed differ in any other way,
from those of MCM-41 and SBA-15.
The transesterification of triacetin on alkali–metal-mod-
ified nano-silicalite-1 has been recently reported [15]. In
this study, the authors doped nano-silicalite-1 with different
alkali metals, including Cs, Li, and K, and compared the
catalytic activities of the resultant materials. Zeolites attract
considerable attention as heterogeneous catalyst supports
owing to their unique pore structure, high surface area, and
high hydrothermal stability [16]. Lopez et al. [17] reported
the comparison of a variety of solid catalysts and conven-
tional liquid catalysts, such as NaOH and H2SO4, in the
transesterification of triacetin as a model reaction for that
of the larger triglycerides found in vegetable oils and fats.
However, the use of zeolite-type catalysts for liquid-phase
reactions involving biological feedstock where larger mol-
ecules are involved has met with limited success because
these types of reactions require well-defined domains to
minimize the formation of unwanted byproducts. This poor
performance has led to extensive research into increasing
the pore size of mesoporous materials while maintain-
ing their high surface area. A major breakthrough in this
research was the fabrication of MCM type mesoporous
materials by the scientists of the Mobil Research and
The objective of this study was to compare MCM-48
with MCM-41 and SBA-15 as siliceous catalyst support
materials in terms of the concentration of the supported
aminopropyl groups, and to relate these properties to the
catalytic activities of the supported basic groups. The
approach taken was to prepare the siliceous supports and
then functionalize them with 3-aminopropyltriethoxysilane
(APTES). The obtained hybrid materials were thoroughly
characterized by means of X-ray powder diffraction (XRD),
1 3

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M. Bandyopadhyay et al.
N2 adsorption, thermogravimetric and differential thermal
analysis (TG-DTA), Fourier-transform infrared (FT-IR)
spectroscopy, and solid-state MAS NMR.
at 40°C. Finally, TEOS was added, and the mixture was
stirred at room temperature for 20 h. The solution was then
poured into a Teflon-lined autoclave and aged at 90°C for
24 h. The final molar gel composition was 1.0 TEOS:0.017
Pluronic123:6.1 HCl:165 H2O. After hydrothermal treat-
ment for 24 h the product was filtered off, washed thor-
oughly with water, and dried at room temperature. The
obtained material was calcined in air at 540°C for 5 h [42].
A series of catalysts containing different concentra-
tions of amine groups were synthesized, and these amine-
functionalized mesoporous materials were then employed
as catalysts for the transesterification of triacetin with
methanol. Different reaction parameters were studied, and
the reaction conditions were optimized. The results corre-
lated well with the nature of the supports. Heterogeneity
was also tested on the most effective catalyst to confirm
the absence of any leaching of the active species from the
support surface. The excellent results obtained for amine-
functionalized MCM-48 as a catalyst for transesterification
is the key finding of this work.
2.4 Functionalization of Mesoporous Silica
In each case, 1 g of the mesoporous silica was dried under
vacuum at 70°C for 4 h. After the pre-treatment, the
mesoporous silica was dispersed in 50 mL of toluene and
an excess amount (1:1 weight ratio) of APTES (>99%,
Aldrich) was added. The mixture was vigorously stirred
under reflux for 12 h. The solid product was then collected
by filtration, washed thoroughly with toluene, and air dried
[43]. A series of mesoporous silicas with different amino-
propyl concentrations were prepared.
2 Experimental
2.1 Synthesis of MCM-48
In a typical procedure [40], cetyltrimethylammonium chlo-
ride (CTACl, >95%, TCI) was added to 1 M NaOH in a
Teflon bottle. The appropriate amount of water was then
added, and the mixture was stirred in a water bath at 50°C
until it became homogeneous, followed by the addition of
tetraethoxysilane (TEOS, Wako). The final molar gel com-
position was 1.0 TEOS:0.7 CTACl:0.5 NaOH:64 H2O. The
obtained gel was poured into a Teflon-lined autoclave and
kept at 90°C for four days. The product was filtered off,
washed thoroughly with water, and dried at room tempera-
ture. The surfactant inside the as-synthesized material was
removed by calcination at 540°C for 5 h.
2.5 Catalyst Characterization
The as-synthesized, calcined, and functionalized materials
were characterized using powder XRD using a Bruker AXS
D8 Advance diffractometer with graphite monochromatized
CuKα radiation at 40 kV and 30 mA. Scanning electron
microscopy (SEM) images were recorded using a Hitachi-
S-4800 SEM coupled with an energy-dispersive X-ray
(EDX) analyzer. 29Si magic-angle spinning (MAS) NMR
spectra were recorded at 119.18 MHz on a Varian 600PS
solid NMR spectrometer using a 6 mm diameter zirconia
rotor spinning at 4 kHz. The spectra were acquired using
6.2 µs pulses, a 100 s recycle delay, and 1000 scans. 3-(Tri-
methylsilyl) propionic-2,2,3,3-d4 acid sodium salt was used
2.2 Synthesis of MCM-41
1
as a chemical shift reference. H–13C cross-polarized (CP)
Cetyltrimethylammonium bromide (CTMABr, >98%, TCI)
was dissolved in a solution of NaOH (0.1 M) at 40°C. After
dissolution, TEOS was added to the mixture. The mixture
was stirred for 2 h and poured into a Teflon bottle. The gel
was kept at 100°C for 24 h. The final molar gel composi-
tion was 1.0 TEOS:0.12 CTABr:0.23 NaOH:130 H2O. The
product was filtered off, washed thoroughly with water, and
dried at room temperature. The surfactant template was
removed by calcination at 540°C for 5 h [41].
MAS NMR spectra were measured with a spinning fre-
quency of 4 kHz, a 90° pulse length of 5.6 µs, and a cycle
delay time of 5 s. The 13C chemical shifts were referenced
to hexamethylbenzene. Thermal analyses were performed
using an SII 7300 TG-DTA instrument (Seiko Instru-
ments). A sample of 3–4 mg was heated in a flow of air
(50 mL min−1) at a heating rate of 10°C min−1 from room
temperature to 700°C. N2 adsorption/desorption measure-
ments were carried out at −196°C using a conventional
volumetric apparatus (BELSORP-max, Bel Japan). The
calcined samples were degassed at 400°C for 16 h under
N2 flow prior to analysis. The functionalized samples were
heated at 200°C. CHN analysis was performed using a
Perkin-Elmer 2400 11 CHN analyzer at the Natural Science
Centre for Basic Research and Development (N-BARD),
Hiroshima University. IR spectra were recorded at room
2.3 Synthesis of SBA-15
The poly(ethylene glycol)-block-poly(propylene glycol)-
block-poly(ethylene glycol)-block copolymer (Pluronic
123, Aldrich) was used as a template. First, the solid copol-
ymer was mixed with 2 M HCl and stirred for 3 h until a
clear solution was obtained. The solution was then heated
1 3

Mesoporous MCM-48 Immobilized with Aminopropyltriethoxysilane: A Potential Catalyst for…
1043
temperature on an FT-IR spectrometer (NICOLET 6700) at
a resolution of 4 cm−1.
moiety. Furthermore, the intensities of the peaks between
2θ=4.7°–5.3° (MCM-48-NH2) and 2θ=3.87°–5.11°
(MCM-41-NH2) decrease significantly. In these types of
mesoporous materials, the peak intensity characteristics
depend on the scattering contrast between the pore walls
and pore channels, and usually decrease with a decrease
in scattering contrast upon anchoring of organic groups to
the pore surfaces [44]. Thus, pore occupancy by the teth-
ered organic groups is the most likely cause of the observed
decrease in peak intensity, rather than any change in the
mesostructure. Thus, the mesoporosity and homogeneity of
all the support materials appear to be maintained.
2.6 Catalytic Activity Measurement
The amine-modified mesoporous silica sample (10 mg) was
activated at 70°C for 1 h. Transesterification of triacetin
(>98%, TCI) with methanol (>99.8%, KANTO CHEMI-
CAL) was performed in a glass tube fitted with a screw cap.
For the reaction, 3 g of triglyceride triacetin was mixed
with 6.52 g methanol (i.e., at a methanol: triacetin molar
ratio of 16:1). The reaction was performed at 65°C for
270 min under stirring. After completion of the reaction,
the filtrate was separated from the catalyst and 0.0166 g
(100:1 mmol with respect to triacetin) of solid naphtha-
lene was used as an internal standard. Samples were ana-
lyzed using a Shimadzu-14B gas chromatographer (GC)
equipped with a flame ionization detector and a Stabilwax
column using Ar as a carrier gas to determine the conver-
sion of triacetin and the product yield.
The success of the post-synthetic grafting of organic
moieties is also confirmed by TG-DTA. The initial
weight loss of ~2% up to 500 K is mainly due to desorp-
tion of physically adsorbed water. The substantial weight
loss from 600 to 900 K results from the decomposition of
organic groups anchored in the mesoporous matrices. The
amount of organic materials anchored via silanol groups
was evaluated from the weight loss in this temperature
range. This result is also confirmed by the occurrence of
a sharp exothermic peak in the DTA curve at ca. 600 K for
all the materials (see Fig. S1). The amine concentrations
obtained from TG analysis of the different silica materials
are listed in Table 1. CHN analysis was also done for most
active catalysts and given in Table 1. The obtained values
are constant with that from TG analysis. To gain a better
understanding of the influence of the organic compound,
we prepared several catalysts with different amine-loading
amounts.
3 Results and Discussion
3.1 Catalyst Characterization
The XRD patterns of as-synthesized, calcined, and amine-
modified MCM-48, MCM-41, and SBA-15 are shown in
Fig. 1. The siliceous MCM-48 sample shows characteris-
tic reflections from the 211, 220, 420, and 332 planes. The
patterns imply that all the materials are well-ordered, well-
resolved, and periodic, indicating good long-range order.
For the calcined sample, a decrease in the unit cell volume
caused by the condensation of silanol groups is observed
(i.e., in the XRD pattern of MCM-48), which is indicated
by the shift of the 211 peak from 2θ=2.20° to 2θ=2.70°.
All reflections exhibit increased intensity due to increased
diffraction contrast between the channel pores and walls
upon removal of the surfactant template [40]. The peak
intensities decrease upon modification with APTES in
all cases. This decrease in peak intensity is caused by
the grafting of aminopropyl groups in the mesoporous
In order to further confirm the presence of amine groups
anchored through the grafting process, FT-IR spectra of the
materials before and after amine loading were compared, as
shown in Fig. 2. All the bare materials exhibit broad peak
at 3400 cm−1 owing to the O–H bond (data not shown)
of the silanol group, and the peaks at 1070 and 795 cm−1
correspond to Si–O–Si stretching and bending vibrations,
respectively [45]. The signal at 1516 cm−1 is attributed
to NH2 scissoring [46]. This peak is observed for all the
mesoporous silicas. The C–N stretching vibration is usually
observed in the wavelength range 1000–1200 cm−1, but this
peak cannot be resolved owing to overlap with the Si–O–Si
Fig. 1 Powder XRD patterns
of a as-synthesized, b calcined,
and c amine-loaded mesoporous
silicas A MCM-48, B MCM-41,
and C SBA-15
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M. Bandyopadhyay et al.
Table 1 Preparation and
Catalyst
Amine
Amine
BET surface
areac (m2 g−1
Pore
Pore
characteristics of various amine
modified mesoporous materials
concentrationa
concentrationb
)
volumed
diameterd
(nm)
(mmol g−1
)
(mmol g−1
)
(cc g−1
)
MCM-48
MCM-48-NH2 (1:1)
MCM-48-NH2 (1:0.5)
MCM-48-NH2 (1:0.025) 0.52
MCM-48-NH2 (1:0.01) 0.43
MCM-48-NH2 (1:0.005) 0.31
MCM-48-NH2 (1:0.001) 0.17
MCM-41
MCM-41-NH2 (1:1)
MCM-41-NH2 (1:0.025) 0.38
SBA-15
0
1.24
1.05
–
1351
400
385
498
460
900
861
1088
690
881
852
282
755
1.1
2.34
1.54
1.55
1.54
1.54
1.54
2.08
2.51
1.83
2.48
6.4
1.12
1.01
–
–
–
–
–
1.25
–
–
0.36
0.35
0.52
0.56
0.85
0.55
0.99
0.31
0.65
0.96
0.39
0.87
0
1.31
0
1.36
0.5
SBA-15-NH2 (1:1)
SBA-15-NH2 (1:0.025)
1.78
–
5.34
6.4
aDetermined from thermogravimetric analysis (TG-DTA)
bDetermined from CHN analysis
cSurface area determined by the BET method
dDetermined by the BJH method
Fig. 2 FT-IR spectra of a calcined, and b amine-loaded mesoporous silicas A MCM-48, B MCM-41, and C SBA-15
band presented at 1050–1150 cm−1. However, the peak for
the modified silicas in this range is wider, suggesting a
possible overlap of the peaks. Thus, the covalent grafting
of APTES onto the mesoporous silica materials may be
clearly established by comparing the spectra of the grafted
and unloaded materials.
materials with uniform pore-size distribution. The sur-
face area and porosity data obtained from the adsorp-
tion branch are summarized in Table 1 (see Fig. S2). The
surface area, pore volume, and pore diameter decrease
with an increase in amine concentration. For MCM-48,
the decrease is much more pronounced in the case of the
bare and 1:1 Silica:APTES (w/w) material. However, the
surface areas of the samples prepared with ratios of 1:1,
1:0.5, 1:0.025, and 1:0.01 are almost the same, and this
is reflected in the catalytic activities of these catalysts, as
discussed in Sect. 3.2. The pore diameter decreases from
2.34 to 1.54 nm, and this value is almost the same for all
the MCM-48 samples prepared with different amine con-
centrations. However, in the cases of MCM-41 and SBA-
15, there is also no significant change in pore diameter
N2 adsorption/desorption analysis of the pure and
functionalized materials are shown in Fig. 3. The adsorp-
tion/desorption isotherm data are given only for the most
active catalyst (Silica:APTES = 1:1), and the amine-
modified materials are compared with pure unloaded
mesoporous silica materials. The N2 adsorption iso-
therms of the pure samples are of type IV with a sharp
capillary condensation step between the partial pressures
0.1 and 0.8, which is characteristic of typical mesoporous
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Mesoporous MCM-48 Immobilized with Aminopropyltriethoxysilane: A Potential Catalyst for…
1045
upon decreasing the amine loading (1:0.025 silica: amine
(w/w)). Furthermore, there is no significant change in
the surface area. These two samples show significantly
lower catalytic activity, which is in perfect agreement
with the porosity data. The presence of the amine groups
reduces the effective surface area, which is indicated by
the decrease in the height of the capillary condensation
step. The pore volume and pore diameter also decrease
upon amine functionalization. This is expected, as the
amine group is tethered to both the external and internal
pore surfaces. The maximum reduction of pore diameter
is observed for the amine modified MCM-48 materials,
which is consistent with the catalytic activity results pre-
sented in Sect. 3.2.
The SEM images of the amine-loaded materials are
shown in Fig. 4. The MCM-48-NH2 particles present a
uniformly spherical morphology with a particle size of
~250 nm. The MCM-41-NH2 particles are disc shaped with
a diameter of ~2.0 µm, whereas the SBA-15-NH2 particles
appear chain-like with lengths of ~1.5 µm. It is also con-
firmed that modification with the aminopropyl group does
not alter the morphology of the samples.
Figure 5 shows TEM images of MCM-48-NH2, MCM-
41-NH2, and SBA-15-NH2. The periodic arrangement of
Fig. 3 N2 adsorption/desorption isotherms of a calcined and b amine-loaded mesoporous silicas A MCM-48, B MCM-41, and C SBA-15
Fig. 4 SEM images of A MCM-48-NH2, B MCM-41-NH2, and C SBA-15-NH2
Fig. 5 TEM images of A MCM-48-NH2, B MCM-41-NH2, and C SBA-15-NH2
1 3

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M. Bandyopadhyay et al.
walls and channels in the mesoporous materials leads to
periodic imaging contrast in the TEM observation. The
regular contrast variation in the image indicates the intact
silicate framework. There is no sign of structure distortion
upon amine grafting and subsequent prolonged washing
and drying.
The 29Si MAS NMR and 13C CP MAS NMR spectra
of the materials are given in Figs. 6 and 7, respectively. In
the 29Si MAS NMR spectra, the unloaded materials pre-
sent signals due to Si(OSi)3(OH) (Q3) and Si(OSi)4 (Q4)
at −102 and −111 ppm, respectively. After amine loading,
a broad signal at −60 to −70 ppm ascribed to C–Si(OSi)3
(T3) is clearly observed in all the mesoporous silica sam-
ples, indicating the successful silylation by amine function-
alized groups. In the case of the 13C CP MAS NMR spec-
tra, resonances around δ=10, 21, and 42 ppm, attributed
to amine propyl groups, are observed. These spectra match
well with the literature data [47], also confirming the pres-
ence of aminopropyl groups on the silica surface.
3.2 Catalytic Activity Test
3.2.1 Comparison of Various Catalysts
The catalytic activities of the materials in the transesteri-
fication of triacetin (the acetic acid triester of glycerol)
with methanol was screened. The methanolysis of tri-
acetin, which is the simplest triglyceride, has been stud-
ied here as a model reaction. As transesterification is an
Fig. 7 13C CP MAS NMR spectra of
a
MCM-41-NH2
(Silica:APTES=1:1), b MCM-48-NH2 (Silica:APTES=1:1), and c
SBA-15-NH2 (Silica:APTES=1:1)
equilibrium-controlled reaction, a higher methanol/triacetin
molar ratio is necessary to shift the equilibrium to the prod-
uct side and increase triglyceride conversion. Therefore,
the catalytic reactions were performed at 65°C for 4.5 h
with methanol/triacetin molar ratio of 16. For all reactions,
10 mg of catalyst was used. The activities are expressed
in terms of triacetion conversion, and the main desired
product, methylacetate, was identified. The catalytic activ-
ity results for different silica materials are depicted in
Fig. 8. Amine-loaded MCM-48 (Silica:APTES=1:1)
shows 78% triacetin conversion and a 52% methyl acetate
yield, whereas MCM-41-NH2 (Silica:APTES=1:1) shows
34% triacetin conversion and 6% yield, and SBA-15-NH2
(Silica:APTES=1:1) shows 37% conversion and 5% yield.
3.2.2 Effect of Reaction Time
The detailed kinetics was investigated for all the cata-
lysts, and the results are summarized in Fig. 9. The
MCM-48-NH2 (Silica:APTES = 1:1) catalyst exhibits
excellent performance in this reaction, even at shorter
reaction times. It can be concluded from the time
Fig. 6 29Si MAS NMR spectra of
a
MCM-41,
b
MCM-
41-NH2 (Silica:APTES=1:1),
c
MCM-48,
d
f
MCM-48-NH2
SBA-15-NH2
(Silica:APTES=1:1),
(Silica:APTES=1:1)
e
SBA-15, and
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Mesoporous MCM-48 Immobilized with Aminopropyltriethoxysilane: A Potential Catalyst for…
1047
MCM-41 and SBA-15 are not sufficiently active in this
reaction under the present optimized reaction conditions.
The plot of Log Co/C vs. time (see Fig. S3, Co is the ini-
tial concentration of triacetin and C is the concentration
of triacetin at different time as well as different conver-
sion level), which confirms linear relationship between
triacetin consumption and reaction time, indicating no
degradation of catalytic active site during the reaction
system.
3.2.3 Effect on Amine Loading
To gain further information concerning amine-modified
MCM-48 catalyst, MCM-48 with different amine load-
ings was prepared. For comparison, different amine-
loaded MCM-41 and SBA-15 samples were also syn-
thesized. The amine concentration was confirmed by
TG-DTA analysis. The activity of the different amine-
loaded materials is summarized in Fig. 10, and shows that
the conversion increases with an increase in amine con-
centration in all catalysts. In the case of MCM-48-NH2,
the activity increases from 12 to 78% with an increase
in amine concentration from 0.17 to 1.24 mmol g−1.
MCM-41-NH2 with an amine concentration of 0.38 mmol
g−1, and SBA-15-NH2 with an amine concentration of
0.5 mmol g−1, exhibit 4–5% triacetin conversion. Thus,
even at a lower amine concentration, MCM-48-NH2 is
a superior catalyst to MCM-41-NH2 and SBA-15-NH2.
The highest conversion is achieved with functionalized
MCM-48, although the maximum amine concentra-
tion (Silica:APTES = 1:1) is slightly lower for MCM-48
Fig. 8 Transesterification of triacetin with methyl alco-
hol over MCM-48-NH2 (Silica:APTES=1:1), MCM-41-NH2
(Silica:APTES=1:1), and SBA-15-NH2 (Silica:APTES=1:1) cata-
lysts. Black represents conversion of triacetin and grey represents
yield of methyl acetate. Reaction conditions: catalyst 0.01 g, tempera-
ture 65°C, and time 4.5 h
Fig. 9 Effect of reaction time on triacetin conversion over differ-
ent catalysts. a MCM-48-NH2 (Silica:APTES=1:1), b SBA-15-NH2
(Silica:APTES=1:1), and c MCM-41-NH2 (Silica:APTES=1:1).
Reaction conditions: catalyst 0.01 g, and temperature 65ºC
conversion plot that the catalytic performance of MCM-
48-NH2 (Silica:APTES = 1:1) is remarkably different
from that of the other two catalysts, as already indicated
by Fig. 8. A clear difference in activity is observed in
the initial stages of the reaction. The MCM-48-NH2
(Silica:APTES = 1:1) catalyst shows 22% conversion
within 30 min. The conversion increases with time and
reaches 78% after 4.5 h, whereas there is no conversion
for amine-loaded MCM-41 (Silica:APTES = 1:1) and
SBA-15 (Silica:APTES = 1:1) until 1.5 h. Thus, modified
Fig. 10 Effect of amine concentration on triacetin conversion over
different catalysts. a MCM-48-NH2, b MCM-41-NH2, and c SBA-
15-NH2. Reaction conditions: catalyst 0.01 g, temperature 65°C, and
time 4.5 h
1 3

1048
M. Bandyopadhyay et al.
(1.24 mmol g−1) than MCM-41 (1.32 mmol g−1) and the
amine concentration is found to be maximum for SBA-15
(1.36 mmol g−1).
diffusional limitation in this reaction system. The dimen-
sions of triacetin is 1.02×0.38 nm [48]. Average pore size
of MCM-48 is 2.5 nm. After amine modification the pore
size remains in the range of 1.54 nm. Hence, internal diffu-
sion of triacetin inside the pore is not a matter of concern.
The turnover frequency (TOF, see Table S1) was also cal-
culated to compare the efficiency of the catalysts based on
the estimated no. of active basic site. The TOF value was
found to be very high for MCM-48 catalyst than the other
two at any amount of catalyst mass (0.01–0.1 g). Taking
into account the fact that both MCM-48 and MCM-41 have
similar surface area and amine loaded amount, the different
framework structure plays important role in this reaction.
We can conclude that MCM-48 with three dimensional
pore structure shows high accessibility of active sites,
resulting in higher catalytic efficiency.
From the results, it seems that not only the amine con-
centration but some other factors are responsible for the
catalytic activity of the materials. The availability of the
active sites clearly plays an important role in the relative
catalytic activities. It was expected from the standpoints of
maximum amine concentration and largest pore diameter
among the mesoporous materials that SBA-15-NH2 would
be a superior catalyst to MCM-41-NH2 or MCM-48-NH2.
Therefore, these results clearly indicate that the type of
mesoporous silica is the most important factor in determin-
ing the catalytic activity of the material. The three-dimen-
sional pore structure of MCM-48 seems to be better for site
accessibility than the one-dimensional pore structures of
MCM-41 and SBA-15, although the pores in SBA-15 are
2–3 times as wide as those in MCM-48.
There are several reports of the transesterification of oils
over functionalized mesoporous MCM-41 and SBA-15. Liu
et al. [49]. reported the transesterification of dimethyl oxa-
late over a series of amino-functionalized MCM-41 mate-
rials, where APTES-modified MCM-41 promoted 55%
oil conversion. They used 1.8 g of catalyst, 10 h reaction
time, and a reaction temperature of over 150°C. 1-3 Dicy-
clohexyl-2-octylguanidine-loaded SBA-15 also showed
93% oil conversion at 65°C [50] when the reaction was
performed for 18 h and a much higher amount of catalyst
was used. Thus, it is very clear that the reaction conditions
used in the studies cited above were much more severe than
those applied in the present study.
3.2.4 Effect of Catalyst Amount
To get a deep insight into the catalytic system, the reaction
was performed by varying the catalyst amount from 0.01 to
0.1 g (Fig. 11). Conversion was found to increase for all the
catalyst with an increase in catalyst weight. When we used
0.01 g catalysts, MCM-48-NH2 gives 48% conversion after
1.5 h, whereas MCM-41-NH2 and SBA-15-NH2 shows
very less catalytic activity (6 and 5%). Even by using 0.1 g
catalyst SBA-15-NH2 and MCM-41-NH2 shows around
20% triacetin conversion, the values much smaller than that
on MCM-48-NH2 (75%). This results indicate that there is
We attempted to optimize the reaction conditions by
using the smaller amounts of catalyst and applying lower
temperatures. MCM-48-NH2 performs exceptionally well
(78% triacetin conversion) at 65°C for 4.5 h reaction time
when only 10 mg of catalyst is used. However, the pre-
sent mild reaction conditions were not adequate to activate
MCM-41 and SBA-15 in order to explore their maximum
catalytic activity. The above results indicate strongly that
amine-loaded MCM-48 performs far better than MCM-41
and SBA-15 under these reaction conditions.
3.2.5 Leaching Test
A heterogeneity test was also performed to assess the
leaching of the aminopropyl groups from the catalysts.
The catalyst was filtered off from the reaction mixture
after 1.5 h reaction time and the reaction was continued
for another 1.5 h. A control reaction was performed for 3 h
and the products of both were analyzed by GC. There is no
change in the conversion of triacetin after removal of the
catalyst when the reaction is continued for a further 1.5 h.
Furthermore, the total conversion in this reaction after 3 h
matches the conversion in the control reaction after 1.5 h.
These results strongly indicate that there is no leaching of
Fig. 11 Effect of catalyst amount on triacetin conversion over differ-
ent catalysts. a MCM-48-NH2 (Silica:APTES=1:1), b MCM-41-NH2
(Silica:APTES=1:1), and
c SBA-15-NH2 (Silica:APTES=1:1).
Reaction conditions: temperature 65°C, and time 1.5 h
1 3

Mesoporous MCM-48 Immobilized with Aminopropyltriethoxysilane: A Potential Catalyst for…
1049
the active species during the reaction, and that the catalyst
is truly heterogeneous in nature.
concentration. However, basicity is not the only factor upon
which catalytic activity depends, and the MCM-48 materi-
als with lower amine loadings still exhibited high catalytic
activity. The results suggest that functionalized MCM-48 is
a potential catalyst for this reaction and may be used for the
green synthesis of biodiesel from vegetable oil, and many
other industrially important liquid-phase reactions.
3.2.6 Reusability of the Catalysts
The reusability of the catalysts was also tested. After the
reaction, the catalyst was filtered, washed with methanol,
and dried at room temperature. After the 1st run, the cat-
alytic activity considerably decreases (from 78 to 34%).
However, for the 2nd to 4th run, the activity remains con-
stant. After the 2nd run, the activity remains constant, indi-
cating that the maximum degree of deactivation is achieved
during the 1st run, and that the remaining active sites are
available for the next cycle of reactions. The CHN analy-
sis reveals that after 4th cycle, the amine concentration of
used catalyst is 1.08 mmol g−1, which was consistent with
the initial value (1.12 mmol g−1). The XRD pattern (see
Fig. S4) after the 4th cycle reveals that there is no struc-
tural degradation or collapse during the reactions. The
FT-IR spectra (see Fig. S5) of the fresh and used catalysts
also exhibit no significant difference, which is indicative of
the good stability of the solid catalyst. From these results,
it can be concluded that the deactivation of catalysts after
1st cycle may have occurred due to the deposition of the
product or substrate within the mesopore (see TG diagram
Fig. S6 where curves are compared before and after cata-
lytic reaction).
Acknowledgements We thank Dr. M. Maeda at the Natural Science
Center for Basic Research and Development (N-BARD), Hiroshima
University for the measurement of TEM measurement.
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