Sulfonic Acid Functionalized Ordered Mesoporous Silica and their Application as Highly Efficient and Selective Heterogeneous Catalysts in the Formation of 1,2-Monoacetone-D-glucose

Article abstract of DOI:10.1002/cctc.201801462

A series of sulfonic acid functionalized ordered mesoporous silica (OMS), designated as RSO₃H-OMS (R=alkyl or aryl; OMS=MCM-41, IITM-56 or SBA-15), were prepared by post-synthesis grafting method. These catalysts, in general, exhibit strong acidic sites and, therefore, yield diacetone-D-glucose as main product in the D-glucose acetonation reaction. On the other hand, the functionalized catalyst can also be tuned in such a way to generate significant amount of weak-to-moderate acidic sites, which are in turn responsible for the formation of 1,2-monoacetone-D-glucose, hitherto not reported so far. These functionalized materials also show promise as they are water tolerant catalyst as well as exhibit varying acidic strengths, which allow greater flexibility for the desired product. In addition, the uniform mesopores with high surface area permit bulkier molecules to enter the active sites, thus the catalyst offers larger pliability in terms of yield and reusability. We report here, for the first time, RSO₃H-SBA-15, with sizable amount of weak-to-moderate acidic sites, as a robust heterogeneous catalyst for the formation of the targeted molecule, 1,2-monoacetone-D-glucose.

Full text of DOI:10.1002/cctc.201801462

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DOI: 10.1002/cctc.201801462
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Sulfonic Acid Functionalized Ordered Mesoporous Silica
and their Application as Highly Efficient and Selective
Heterogeneous Catalysts in the Formation of
1,2-Monoacetone-D-glucose
Nunna V. Krishna,[a] Sankaran Anuradha,[a] Reddi Ganesh,[a] Velisoju V. Kumar,[a] and
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11 Parasuraman Selvam*[a, b, c]
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Dedicate to Prof. Dr. G. Aravamudhan on the occasion of his 85th birthday
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A series of sulfonic acid functionalized ordered mesoporous
silica (OMS), designated as RSO3H-OMS (R=alkyl or aryl; OMS=
MCM-41, IITM-56 or SBA-15), were prepared by post-synthesis
grafting method. These catalysts, in general, exhibit strong
acidic sites and, therefore, yield diacetone-D-glucose as main
product in the D-glucose acetonation reaction. On the other
hand, the functionalized catalyst can also be tuned in such a
way to generate significant amount of weak-to-moderate acidic
sites, which are in turn responsible for the formation of 1,2-
monoacetone-D-glucose, hitherto not reported so far. These
functionalized materials also show promise as they are water
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tolerant catalyst as well as exhibit varying acidic strengths,
which allow greater flexibility for the desired product. In
addition, the uniform mesopores with high surface area permit
bulkier molecules to enter the active sites, thus the catalyst
offers larger pliability in terms of yield and reusability. We
report here, for the first time, RSO3H-SBA-15, with sizable
amount of weak-to-moderate acidic sites, as a robust heteroge-
neous catalyst for the formation of the targeted molecule, 1,2-
monoacetone-D-glucose.
Introduction
context, the production of 1,2-diols of D-glucose as O-
isopropylidene derivatives or acetonides (Scheme 1) is critical in
Biomass attracts much attention as a renewable feedstock as
well as for its “CO2 neutral” impact on the environment.[1]
Though, carbohydrates represent about 95% of the annually
renewable biomass, yet their unlimited potential as feedstock is
generally unexploited. In this regard, the attractiveness of
mono- and di-saccharides as organic raw materials exists in
their straightforward chemical transformation into valuable
products.[2] Further, sugars are key intermediates in biomass
conversion and, in particular, D-glucose is one of the important
sources for the production of wide-range of value added
chemicals, e.g., sorbitol, glucaric acid, etc.[3] On the other hand,
transformation of biomass into chemicals necessitates a green
process owing to environmental and ecological issues.[4] In this
Scheme 1. Acetonation reaction of D-glucose.
[a] Dr. N. V. Krishna, Dr. S. Anuradha, R. Ganesh, Dr. V. V. Kumar,
Prof. Dr. P. Selvam
National Centre for Catalysis Research and Department of Chemistry
Indian Institute of Technology-Madras
Chennai 600 036 (India)
multi-step syntheses in organic, medicinal, and carbohydrate
chemistry.[5] In addition, monoacetonide derivatives are also
well-known as anti-inflammatory and anti-pyretic with very low
toxicity, and therefore commercially demanding as a starting
material for a diverse range of bioactive molecules including
antibiotics, cytotoxic, anti-inflammatory and antipyretic
agents.[6]
In general, 1,2-monoacetone-D-glucose is produced in high
yield by hydrolysis of diacetone-D-glucose using mineral acids.[7]
The commonly used homogeneous catalysts for acetonide
(ketal formation) reaction such as organic acids, mineral acids,
E-mail: selvam@iitm.ac.in
[b] Prof. Dr. P. Selvam
School of Chemical Engineering and Analytical Science
The University of Manchester
Manchester M13 9PL (United Kingdom)
[c] Prof. Dr. P. Selvam
Department of Chemical and Process Engineering
University of Surrey
Guildford
Surrey GU2 7XH (United Kingdom)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.201801462
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Scheme 2. Generalized formation mechanism of RSO3H-OMS.
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and metal salts have several disadvantages including tedious
work-up involved in the isolation of the products.[7,8] Therefore,
the development of environmentally benign and recyclable
heterogeneous catalysts is of paramount importance. In this
regard, catalysts such as zeolites, pillared-clays, polymer-resins,
silica-sulfuric acid, etc., show promise. However, they are
sluggish in terms of reactivity and moisture sensitive under
ambient conditions.[9] In this context, sulfonic acid (RSO3H)
functionalized ordered mesoporous silicas (OMS), designated as
RSO3H-OMS; R=alkyl/aryl, are potential catalysts for D-glucose
acetonation as they are water tolerant (imparted by functional-
ized organic moiety) with high acid strengths which allows
greater selectivity for the products. In addition, large uniform
pores with high surface area permit bulky molecules to enter
the catalyst active sites. In addition, the high thermal and
Figure 1. XRD patterns and TEM images of sulfonic acid functionalized OMS:
(a,d) RSO3H-MCM-41; (b,e) RSO3H-IITM-56; (c,f) RSO3H-SBA-15.
IITM-56 and SBA-15) with different loading was prepared by
post-synthesis grafting method via oxidation of thiol-functional-
ized analogue, RSH-OMS (Scheme 2), and evaluated their
catalytic activity for the D-glucose acetonation reaction. A
preliminary account of this work was presented elsewhere.[13]
hydrothermal stability offers greater flexibility in terms of Results and Discussion
reusability of the catalyst.[10]
At this juncture, it is also noteworthy here that OMS (MCMs,
IITMs, SBAs, etc.) possess distinct types of silanol groups, viz.,
isolated, vicinal and germinal, and that the amount of each
type may vary with different synthesis procedures and
structures of OMS.[11] Furthermore, each type of these silanol
groups has varied reactivity, which in turn has influence on the
bonding characteristics of the functionalizing agent.[12] There-
fore, a knowledge of bonding of various types of silanols with
sulfonic acids is essential so as to evaluate and understand the
very different activities of the sulfonic acid functionalized silica-
based catalysts. Hence, the main objective of the present study
is to understand the nature of acidic sites in RSO3H-OMS (R=
CH2CH2CH3) and their influence on the catalytic activity towards
acetonation of D-glucose reaction. In this context, a series of
sulfonic acid functionalized OMSs (RSO3H-OMS; OMS=MCM-41,
Figure 1 depicts XRD patterns and TEM images of sulfonic acid
functionalized samples (RSO3H-OMS) with an optimized wt ratio
of OMS: 3-MPTMS=2:1. The corresponding XRD patterns of
neat (OMS, Figure S1) and thiol-functionalized (RSH-OMS, Fig-
ure S2) are also presented. All these materials, viz., RSO3H-MCM-
41, RSO3H-IITM-56 and RSO3H-SBA-15, show well-resolved
reflections typical of 2D-hexagonal structure indicating that
mesostructural order of the functionalized materials.[14] At this
juncture, it is also noteworthy that the structural integrities are
maintained even after oxidation and various other processing
treatments of the samples. Table 1 summarizes the structural
data of the various samples and, as expected, the unit cell
parameters decrease only marginally (Table S1) for the function-
alized samples.[15] These results are well-supported by the TEM
(Figure 1) wherein the highly ordered parallel arrays of 1D-pore
Table 1. Physico-chemical characteristics of OMS and RSO3H-OMS.[a]
[d]
[e]
Material
Sulfur content [wt %]
a0
SBET
DBJH
[nm]
Vp
hw
29Si MAS-NMR data [%]
Initial[b]
Final[c]
[nm]
[m2 gÀ 1
]
[cm3 gÀ 1
]
[nm]
Q4
Q3
Q2
T3
T2
T1
MCM-41
RSO3H-MCM-41
IITM-56
RSO3H-IITM-56
SBA-15
–
8.5
–
8.5
–
8.5
–
1.9
–
5.2
–
5.0
–
4.6
–
1.6
4.33
4.23
6.01
5.95
10.18
10.17
9.66
9.77
976
912
758
728
701
671
448
429
2.78
2.17
3.80
3.60
6.46
6.18
5.14
5.20
1.13
0.89
0.89
0.79
1.04
1.01
0.60
0.60
1.55
2.06
2.21
2.35
3.72
3.99
4.52
4.57
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54
62
58
52
49
38
32
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33
32
30
42
37
5
4
5
4
6
4
6
6
–
7
–
6
–
4
–
3
–
2
–
3
–
3
–
2
–
–
–
–
–
1
–
3
RSO3H-SBA-15
SBA-15(IS)
RSO3H-SBA-15(IS)f
[a] OMS-to-3-MPTMS (weight) ratio of 2:1 for RSO3H-OMS; [b] Nominal composition; [c] From XRF data; [d] Average unit cell parameter calculated using 1/d
2 =4/3 (h2 +hk+k2/a2); [e] Wall thickness, hw =a0–DBJH; [f] Silica-to-3-MPTMS (weight) ratio of 8:1.
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Figure 2. N2 sorption isotherms and PSD (inset) of various OMS and sulfonic acid functionalized OMS.
channels further substantiate the preservation of mesostructural
ordering.[16]
3700 cmÀ 1 corresponds to OÀ H stretching of water and surface
hydroxyl groups can be seen for all the samples.[20] The signals
associated with the stretching vibration of non-condensed
silanol groups are present at 960 cmÀ 1, and the strong signal
around 1630 cmÀ 1 is mainly due to the bending vibration of
adsorbed water. However, a careful look at the spectra reveal
that the water-based absorption signals show lower intensity
for the functionalized samples which could be attributed to
weaker or lower amount of water adsorption suggesting a
possible water tolerant nature as compared to their silica
analogue. On the other hand, a weak signal, assigned to SÀ H
stretching vibration, appears in the range of 2523–2579 cmÀ 1,
which is absent for RSO3H-OMS (see Figure 3) clearly demon-
strates the complete oxidation of the thiol groups.[21] Signal
related to free silanols, i.e., isolated silanols and geminal
silanols, at 3750 cmÀ 1 and vicinal silanols at 3520 cmÀ 1 appear in
FT-IR spectra only upon evacuation at higher temperatures as
these are masked by OÀ H stretching of water at room temper-
ature.[22]
Figure 4A presents the silica-subtracted FT-IR spectra of
RSO3H-OMS. Signals corresponding to the asymmetric and
symmetric S=O stretching vibrations of sulfonic acid are
typically seen in the range of 1000–1250 cmÀ 1. The Si-CH2-R
stretch appears in the 1200–1250 cmÀ 1 and asymmetric stretch-
ing of SO2 moieties appear at 1350 cmÀ 1.[23] Signal symmetries
of asymmetric S=O stretching and SiÀ CH2À R stretching vibra-
tions of RSO3H-OMS materials found to vary with the silica used
for grafting. In general, one can expect hydrophilicity around
sulfonic acid functional groups (RSO3H). However, as demon-
strated by Morales et al.[24] more hydrophobic local environ-
ments near the propyl-sulfonic acid sites of mesoporous silica
(RSO3H-SBA-15) result in reduced poisoning by water as
compared to mesoporous silica (SBA-15). NH3-TPD profiles of
the RSO3H-OMS materials are depicted in Figure 4B and their
To study the effect of sulfonic acid loading, RSO3H-MCM-41
with different OMS to 3-MPTMS ratios were prepared. Further,
the ratios were also optimized, with respect to the product
yield, and the details are given in Figure S3, Figure S4 and
Table S2. Among these, the one with 2:1 ratio of MCM-41 and
3-MPTMS showed maximum activity with selective formation of
diacetone-D-glucose. Therefore, the other two sulfonic acid
functionalized OMS materials, viz., RSO3H-IITM-56 and RSO3H-
SBA-15, were prepared with this optimum loading (Table 1).
Further, at the loading of MCM-41:3-MPTMS=2:1, the silylating
agent is grafted uniformly inside the pores with more
condensation, which generates more strong acid sites[17] that in
turn leads to high D-glucose conversion and diacetone-D-
glucose selectivity (Table S2). Upon higher loadings, the amount
of acid sites remains unchanged caused by saturation of the
sulfonic acid groups on the surface as evidenced from NH3-TPD
and CEC data (Table S2). N2 sorption isotherms of various OMS
and their corresponding RSO3H-OMS along with pore size
distributions are depicted in Figure 2. As expected, all the
samples including the optimized ones (Figure S5), show type IV
isotherm with steep H1 hysteresis loop typical characteristic of
regular mesoporous channels and narrow pore distribution.[14,18]
At this juncture, it worth mentioning here that the capillary
condensation of RSO3H-OMS materials, logically shifts to lower
relative pressure which clearly demonstrates that the sulfonic
acid groups are grafted inside the pores.[19] This is further
confirmed by the decrease in pore volume and increase in wall
thickness. The textural properties of various OMS and RSO3H-
OMS are listed in Table 1.
Figure 3 illustrates the FT-IR spectra of OMS, RSH-OMS and
RSO3H-OMS samples. The spectra of RSO3H-OMS show two
weak absorption signals at 2941 and 2886 cmÀ 1 characteristic of
asymmetric and symmetric stretching vibrations of À CH2À acidity (from both NH3-TPD and CEC) is listed in Table 2. These
groups demonstrating functionalization of OMS with 3-MPTMS
and its retention even after oxidation. A broad signal at 2950 to
materials consist of three types of acid sites, i.e., weak acid
°
°
around 157 C, moderate acid around 181 C and strong acid
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Figure 3. FT-IR spectra of: (a) silica; (b) thiol, (c) sulfonic acid-functionalized analogues of: (A) MCM-41; (B) IITM-56; (C) SBA-15.
°
around 223 C. The amount of sulfur content in RSO3H-OMS is
determined by XRF and data is presented in Table 1. The acid
amount depends directly on 3-MPTMS grafted, which in turn is
proportional to the amount of Q3 and Q2 species present in
OMS.
Figure 5 depicts the 29Si MAS-NMR spectra of OMS and
RSO3H-OMS. Table 1 presents the amount of various silica
species (silanols, siloxanes and organosiloxanes) in these
samples. Signals corresponding to different silica species (Qn =
Si(OSi)n(OH)4–n) such as Q4 due to siloxane linkages, Q3 due to
isolated and vicinal silanols and Q2 due to structural defects
(geminal silanols) appear at À 110 ppm, À 100 ppm and
À 91 ppm, respectively (Figure 5A). It can be seen for this figure
that MCM-41 has more Q3 species and less Q2 species than that
of IITM-56 and SBA-15 as the condensation of silica is more
favourable in the samples prepared under basic condition than
that prepared in acidic medium.[25] On the other hand, OMS
with thicker walls has Q3 species that are far apart to each other
and also has more structural defects (Q2 species).
Figure 4. Subtracted FT-IR spectra (A) and NH3-TPD (B) of: (a) RSO3H-MCM-
41; (b) RSO3H-IITM-56; (c) RSO3H-SBA-15.
Table 2. Acidity data and acetonation of D-glucose over RSO3H-OMS.[a]
Catalyst[b]
CEC[c]
TPD data
Total acid
content
Product selectivity [%]
[mmolgÀ 1
]
Types of acid sites
(i) (ii) (iii)
D-glucose con-
version
Diacetone-D-
1,2-Monoacetone-D-
glucose[e]
5,6-Monoacetone-D-
glucose[f]
glucose[d]
[mmolgÀ 1
]
[%]
RSO3H-SiO2
0.080
0.140
0.419
0.394
0.334
0.176
0.032 0.093 0.015 83.6
49.4
(43.1)
86.4
(84.1)
81.8
(80.5)
72.3
(72.2)
46.4
(46.2)
19.6
(20.7)
0.8
(2.6)
5.0
(6.4)
13.0
(12.3)
42.7
(42.3)
31.0
(36.2)
12.8
(13.3)
13.2
(13.1)
14.7
(15.5)
10.9
(11.5)
(80.5)
0.012 0.053 0.354 99.4
(99.0)
0.016 0.058 0.320 99.5
(99.4)
0.034 0.054 0.246 98.3
(98.2)
0.074 0.032 0.070 88.7
(88.4)
RSO3H-MCM- 0.374
41
RSO3H-IITM-
56
RSO3H-SBA-
15
0.352
0.281
0.160
RSO3H-SBA-
15(IS)g
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Figure 5. 29Si MAS-NMR of: (A) silica and (B) sulfonic acid-functionalized
analogues of: (a) MCM-41; (b) IITM-56; (c) SBA-15.
In the case of RSO3H-OMS (Figure 5B), the 29Si MAS-NMR
spectra provide information about the bonding of organo-
siloxane groups of silylating agent with silanols. Signals
corresponding to various organosilica species (Tm =RSi(OSi)m
(OH)3À m, m=1–3] such as T3, T2 and T1 appear around
À 66 ppm, À 54 ppm and À 47 ppm respectively. Presence of
more T3 species in RSO3H-MCM-41 compared to RSO3H-SBA-15
shows that silylating agent is more effectively condensed with
OMS having more number of Q3. RSO3H-SBA-15 consists of T2
and T1 species as the Q3 species essential for bonding with
organosiloxane groups of silylating agent are far apart thus
making complete condensation (T3 species generation) less
probable.[26] Based upon 29Si MAS-NMR and acidity data, we
hypothesize that strong acid sites are generated by T3 bonding
of sulfonic acid functional group with isolated and vicinal
silanols, moderate acid sites are generated by T2 bonding of
sulfonic acid functional group with isolated and geminal
silanols whereas weak acid sites are generated by T1 bonding of
sulfonic acid functional group with isolated silanols (Scheme 3).
Sulfonic acid functionalized OMS, i.e., RSO3H-OMS, are
studied for acetonation of D-glucose reaction and their catalytic
activities are shown in Figure 6 and Table 2. The D-glucose
conversion by RSO3H-OMS materials is relative to their acid
amount and their product selectivity found to vary with amount
of acid sites. For a comparison, sulfonic acid functionalized
(amorphous) silica gel, designated as RSO3H-SiO2, is also tested
for the given reaction. As expected, RSO3H-SiO2 exhibited lower
catalytic activity in comparison with the mesoporous analogues,
viz., RSO3H-OMS, as fewer amounts of sulfonic acid groups gets
grafted into the silica matrix, as seen from TPD and CEC data
given in Table 2. This could be attributed to the presence of
large amount of silanols in OMS as compared to the silica gel.
Furthermore, both RSO3H-MCM-41 and RSO3H-IITM-56 catalyst
show a similar activity in terms of 1,2-monoacetone-D-glucose
Scheme 3. Representation of different silanol groups, and the formation of
different acid sites in RSO3H-OMS by post-synthesis grafting method.
Figure 6. Variation of product selectivity over various RSO3H-OMS.
selectivity while RSO3H-SBA-15 gives slightly higher activity
owing to the presence of weaker acidic sites than the other two
catalysts (Figure 6). On the other hand, a decrease in strong
acidic sites leads to lower diacetone-D-glucose formation
whereas an increase in weak acidic sites lead to an increase in
1,2-monoacetone-D-glucose formation. Moderate acid sites do
not vary much in all the three catalysts and, therefore nearly
the same 5,6-monoacetone-D-glucose selectivity. Thus, it may
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be concluded that strong acidic sites favourably catalyse
diacetone-D-glucose, moderate acidic sites catalyse 5,6-mono-
acetone-D-glucose, an intermediate in diacetone-D-glucose
formation, while the weak acidic sites found to catalyse 1,2-
monoacetone-D-glucose (Figure 6). RSO3H-SiO2 exhibited more
selectivity towards 1,2-monoacetone-D-glucose than RSO3H-
OMS as it poses more weak acid sites, but it showed less
selectivity towards diacetone-D-glucose because of its fewer
strong acid sites (see table 2). Based on reaction results, we
have proposed reaction mechanisms for the formation of
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diacetone-D-glucose
and
1,2-monoacetone-D-glucose
(Scheme S1 and Scheme S2).[27]
Reusability of RSO3H-OMS catalysts are tested, and their
activities stay unaltered even after recycling (Table 2). However,
as expected RSO3H-SiO2 catalyst lost its activity significantly
(Figure 7). Thus, RSO3H-OMS materials are found to be efficient
Figure 8. XRD pattern (A) and TEM image (B) of: RSO3H-SBA-15(IS).
°
of generally used 100 C to increase the amount of weak acid
sites.
XRD patterns of SBA-15(IS), RSH-SBA-15(IS) and RSO3H-SBA-
15(IS) are depicted in Figure S6 and Figure 8A. All the samples
show well-resolved reflections typical of 2D-hexagonal SBA-15
type structure indicating that mesostructural order as well the
functionalized analogues is maintained even after oxidation
and various other processing treatments.[15a,c] Table 1 summa-
rizes the structural data of these various materials, and as
expected the unit cell parameters decrease only marginally for
the functionalized samples.[16] These results are well supported
by the TEM images (Figure 8B) which show highly ordered
parallel arrays of one-dimensional mesochannels which further
substantiate the preservation of mesostructural ordering in
these materials.[17] N2 sorption isotherms of SBA-15(IS) and their
corresponding functionalized analogues along with pore size
distributions is depicted in Figure 9A (see inset). It can be seen
from these figures that all the RSO3H-SBA-15(IS) materials show
type IV isotherm with steep H1 hysteresis loop characteristic of
regular mesoporous channels and narrow pore size like the
corresponding siliceous counterpart.[18] At this juncture, it is
worth mentioning here that the capillary condensation of
Figure 7. Recyclability results of sulfonic acid functionalized catalysts: (A)
RSO3H-SiO2; (B) RSO3H-MCM-41; (C) RSO3H-IITM-56; (D) RSO3H-SBA-15.
D-glucose;
Diacetone-D-glucose;
1,2-Monoacetone-D-glucose.
for acetonation of D-glucose and in particular the SBA-15
framework structure can further be tuned for selective product
formation. In addition, it can be seen from Table 2 that 1,2-
monoacetone-D-glucose selectivity is catalysed by weak acid
sites (Table 2 and Table S2) and these can be enhanced at low
sulfonic acid loading, i.e., by grafting RSO3H in SBA-15 matrix.
Slower condensation of acid synthesized ordered mesoporous
silicates permits us to tune their amount of silanols in contrast
to their base-mediated counter parts.[28] In addition, Pluronic
P123, non-ionic surfactant, used for SBA-15 synthesis has more
number of hydrophilic units per surfactant molecule which
allows mesostructure to be intact even at slower condensation
due to their more and strong interaction with silica units.
Therefore, we have tuned the framework of SBA-15 (i.e., SBA-
Figure 9. N2 sorption isotherms and PSD (inset) (A); 29Si MAS-NMR spectra (B)
of: (a) SBA-15(IS) and (b) RSO3HSBA-15(IS).
°
15(IS)) by keeping it for hydrothermal treatment at 80 C instead
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RSO3H-SBA-15(IS) materials, as expected, shifts to lower relative
pressure which clearly demonstrates that the sulfonic acid
groups are grafted inside the pores.[19] This is further affirmed
by decrease in pore volume and increase in wall thickness. The
textural properties of OMS and RSO3H-OMS materials are listed
in Table 1.
15(IS) is tested and its activity and 1,2-monoacetone-D-glucose
selectivity stay unaltered (Table 2; Figure 11).
1
2
3
4
5
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The FT-IR spectra of SBA-15(IS), and the corresponding thiol
and sulfonic acid analogues shows characteristics similar to
other RSO3H-OMS (Figure S7). Presence of more isolated silanols
signal in SBA-15(IS), in comparison to SBA-15, is clearly shown
by the presence of distinct signal around 3750 cmÀ 1.[22] Sharp
signals in subtracted FT-IR spectra of RSO3H-SBA-15(IS) materials
in the region of 1500–1000 cmÀ 1 further confirms this (Fig-
ure S8). 29Si MAS-NMR spectra of SBA-15(IS) and RSO3H-SBA-
15(IS) are depicted in Figure 9B and their amount of silica
species (silanols, siloxanes and organosiloxanes) is presented in
Table 1. Thus, by modifying SBA-15 preparation conditions, i.e.,
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°
°
hydrothermal treatment at 80 C instead of 100 C, the sample
SBA-15(IS) can have little more Q3 (À 100 ppm) and Q2
(À 90 ppm) species and are far apart than SBA-15 so that it can
graft considerable amount of functional groups.[25] Accordingly,
RSO3H-SBA-15(IS) consists more T1 species (À 47 ppm) as the Q3
species essential for bonding with organosiloxane groups of
Figure 11. Recyclability data of RSO3H-SBA-15(IS).
silylating agent are far apart (due to its thicker walls than SBA- Conclusions
15) thus making complete condensation (T3 species generation)
less probable (see Figure 9B and Table 1).
NH3-TPD profile of RSO3H-SBA-15(IS) is shown in Figure 10
and the acidity data are presented in Table 2. It is clear from
In summary, we have successfully prepared and characterized a
series of high quality sulfonic acid-functionalized ordered
mesoporous silica (RSO3H-OMS) catalysts and tested for aceto-
nation of D-glucose reaction. The type of bonding with silanols
was found to vary with wall thickness and surface roughness of
OMS used for grafting. Further, it was also found to vary with
sulfonic acid loading. Based on these data, we have deduced
different types acid sites, viz., strong, moderate and weak. The
strong acid sites are generated by sulfonic acid bonding with
isolated and vicinal silanols (T3 sites) whereas moderate acid
sites are produced by T2 type bonding of sulfonic acid moiety
with isolated and geminal silanols. On the other hand, the weak
acid sites are generated by T1 type bonding of sulfonic acid
moiety with isolated silanols.
In addition, it is also concluded that the product, i.e., 1,2-
monoacetone-D-glucose, is mainly produced by weak acid sites,
while the products, viz., 5,6-monoacetone-D-glucose and diac-
etone-D-glucose, are catalysed by the moderate and strong
acid sites, respectively. Among the different sulfonated materi-
als, the catalyst RSO3H-MCM-41 showed maximum activity with
selective formation of diacetone-D-glucose. On the other hand,
the catalysts RSO3H-SBA-15, produce considerable amount of
1,2-monoacetone-D-glucose in addition to diacetone-D-glucose.
More importantly, the SBA-15 framework structure can be
tuned in such way that it can produce weak acid sites (RSO3H-
SBA-15(IS)) that can selectively produce 1,2-monoacetone-D-
glucose in good yield hitherto not reported so far. This is
achieved by synthesizing the silica matrix at milder preparation
conditions that generate thicker walls and more isolated
silanols which are far apart leading to the formation of T1
bonding sites exclusively. Thus, the present study provides a
deeper insight into the sulfonic acid functionalized ordered
Figure 10. NH3-TPD profile of: RSO3H-SBA-15(IS).
these data that the catalysts have considerable amount of weak
acid sites. As postulated earlier, weak acid sites are generated
by T1 bonding of sulfonic acid group with isolated silanols.
Table 2 and Figure 6 give the results of acetonation of D-
glucose, and it is clear that the catalyst, RSO3H-SBA-15(IS) shows
the formation of significant amount of 1,2-monoacetone-D-
glucose in comparison with other catalysts. The results follow
the earlier postulation that weak acid sites lead to relative 1,2-
monoacetone-D-glucose formation. Reusability of RSO3H-SBA-
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source operating at 40 kV. Fourier transformed infrared spectra (FT-
mesoporous silica materials which are expected to have wider
implications in understanding their effect on various acid-
catalysed reactions.
1
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3
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5
IR) was recorded on Bruker TENSOR 27 FT-IR spectrometer with
4 cmÀ 1 resolution and 100 scans in mid IR region (4000–600 cmÀ 1
)
employing KBr pellet technique. Magic angle spinning-nuclear
magnetic resonance (29Si MAS NMR) spectra of samples were
recorded on Bruker DSX 300 spectrometer and chemical shifts were
referenced to tetramethylsilane.
Experimental Section
6
Synthesis of OMS: All the OMS, viz., MCM-41, IITM-56 and SBA-15,
were synthesized as per the procedures described elsewhere.[14]
Accordingly, MCM-41 was prepared under basic conditions using
CTAB as surfactant and fumed silica as silica source with a typical
gel composition of: 1 SiO2: 0.27 CTAB: 0.26 TMAOH: 0.13 NaOH: 68
H2O. On the other hand, IITM-56 was prepared under highly acidic
conditions using Brij-56 and TEOS with a gel composition of: 1
TEOS: 0.15 Brij-56: 11.30 HCl: 119 H2O. Likewise, SBA-15 was also
prepared under highly acidic conditions employing P123 and TEOS
with a gel composition of: 1 TEOS: 0.017 Pluronic-P123: 9.60 HCl:
108 H2O. All the materials were kept for hydrothermal treatment at
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Nitrogen adsorption-desorption isotherms were measured
at 77 K using Micromeritics ASAP 2020 surface area analyzer, in
°
which catalysts were outgassed at 180 C for 6 h before the
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measurements. Helium was used as the carrier gas. Specific
surface area was calculated using BET (Brunauer-Emmett-Teller)
equation and pore-size distribution was obtained from the
desorption branch of the isotherm by the BJH (Barrett-Joyner-
Halenda) method. Acid strengths of the samples were deter-
mined by temperature programmed desorption of ammonia
(NH3-TPD) as probe molecule which was carried out on a
Micromeritics Chemisorb 2750. Samples were activated at
°
100 C for 24 h where as a batch of SBA-15 is also kept for
°
hydrothermal treatment at 80 C for 24 h. The resulting solid
°
product was filtered, washed, dried at 60 C for 6 h and calcined at
°
200 C for 1 h in helium flow. Later, they were cooled and
°
550 C for 1 h in nitrogen atmosphere followed by 5 h in air at a
heating rate of 1 C minÀ 1. The final surfactant-free materials, viz.,
°
maintained at 120 C prior to their exposure to ammonia vapour
°
°
MCM-41, IITM-56, SBA-15 and SBA-15(IS) (SBA-15 kept at 80 C for
followed by purging with helium for 30 min. The desorption of
hydrothermal treatment) were directly used for sulfonic acid
°
ammonia was carried out by heating the reactor up to 350 C at
a uniform rate of 10 Cmin . Cation exchange capacity (CEC) of
functionalization. Amorphous silica (SRL India) with 100–120 mesh,
À 1
°
S
BET =353 m2 gÀ 1, Vp=0.46 cm3 gÀ 1 and DBJH =3.53 nm was used for
RSO3H-OMS samples was determined using an aqueous solution
of sodium chloride (2 M) as an exchange agent. About 0.05 g of
sample was added to 10 mL NaCl aqueous solution, stirred
overnight at room temperature. The resultant solution was
titrated with 0.01 M aq. NaOH using phenolphthalein as an
indicator.
comparative studies.
Preparation of RSO3H-OMS: A series of sulfonic acid functionalized
OMS, viz., RSO3H-MCM-41, RSO3H-IITM-56 and RSO3H-SBA-15 (R=
CH2CH2CH3) were prepared by post-synthesis grafting method.[29] In
°
a typical manner, OMS were evacuated in a vacuum oven at 150 C
for 5 h initially and then cooled down to room temperature; it was
then suspended in 30 mL of dry toluene and 3-MPTMS was added
with OMS : 3-MPTMS (wt ratio). The mixture was refluxed for 6 h at
Catalytic Studies: Acetonation of D-glucose was carried out in
batch reactor at atmospheric pressure consisting of a round bottom
flask reactor equipped with a magnetic stirrer. About 500 mg of D-
glucose was added to 30 mL of anhydrous acetone and 2 mL of
acetonide forming agent, viz., 2,2-dimethoxypropane (DMP), in
presence of 50 mg of RSO3H-OMS catalyst. The reaction mixture
°
110 C under N2 atmosphere. The obtained product was filtered and
washed with excess dry toluene to remove unreacted 3-MPTMS
°
present, if any, and dried at 60 C. The resulting thiol-functionalized
°
OMS, viz., RSH-OMS, was refluxed for 20 h at 60 C with 30% H2O2
(in 2 mL methanol) and then cooled down to room temperature. To
this mixture, 10 mL of 0.1 M H2SO4 was added and stirred for 4 h at
room temperature for complete oxidation. The obtained product
was then centrifuged, washed with water and ethanol. The solid
°
was refluxed at 60 C under nitrogen atmosphere for 3 h. The
mixture was cooled, and the catalyst was separated by filtration
and the resulting solution was concentrated under vacuum. The
progress of the reaction was monitored using thin layer chromatog-
raphy (TLC) and the compounds were detected by spraying the
sheets with Punkal’s reagent.
°
material was finally dried at 60 C overnight to obtain the desired
material, i.e., RSO3H-OMS.
Likewise, a series of RSO3H-MCM-41 having MCM-41: 3-MPTMS (wt
ratio) of 8:1; 4:1; 2:1, and 1:1 were also prepared. In a similar way,
sulfonic acid functionalized IITM-56 and SBA-15, i.e., RSO3H-IITM-56
and RSO3H-SBA-15, with optimized sulfonic acid loading of OMS: 3-
MPTMS (wt ratio) of 2:1 were also synthesized. In a similar manner,
RSO3H-SiO2 was synthesized for comparative studies. We have also
prepared a sample, designated as RSO3H-SBA-15(IS) with OMS:3-
MPTMS (wt ratio) of 8:1 loading, wherein the parent SBA-15 was
Products Analyses: Reaction products, viz., 5,6-O-isopropylidene-α-
D-glucofuranose (5,6-monoacetone-D-glucose), 1,2:5,6-di-O-isopro-
pylidene-α-D-glucofuranose (diacetone-D-glucose), and 1,2-O-iso-
propylidene-α-D-glucofuranose (1,2-monoacetone-D-glucose), were
analysed by high performance liquid chromatography (HPLC;
Shimadzu LC-20AT) equipped with refractive index detector. The
column used was Phenomenex column (Luna 5 μ NH2 100) and the
mobile phase was acetonitrile and water (HPLC grade) in the ratio
of 70:30. The used catalysts were regenerated by stirring it with
°
°
prepared at 80 C instead of 100 C so as to generate large amount
of weak acid sites/silanol groups.
°
methanol at 75 C for 20 min followed by cooling to room temper-
ature and by further stirring it with 10 mL of diluted acid solution
(0.1 M H2SO4) for another 30 min. The resultant mixture was filtered,
washed and dried under vacuum. This was repeated, and catalyst
reusability was tested in four consecutive runs.
Characterization: Powder X-ray diffraction (XRD) patterns of the
catalysts were recorded by a Bruker D8 Focus Advance diffractom-
eter equipped with LynxEye detector with a Ni-filtered Cu Kα
radiation source (λ=1.5406 Å) in 2θ range of 1.5 to 7.5 with a 2θ
°
step size of 0.01 and step time of 2 s. Transition electron
microscope (TEM) images were recorded using Philips CM12
transmission electron microscope operated at 120 KeV with a point-
to-point resolution of 3.4 Å. The sulfur content was measured on
Rigaku Primini X-ray fluorescence (XRF) spectrometer using Pd
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Acknowledgements
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This work is supported by a grant-in-aid under India-Australia
Fund for Scientific and Technological Cooperation Program (INT/
AUS/P-18/08). We are thankful to Prof. B. Viswanathan for the
encouragement. DST, New Delhi is gratefully acknowledged for
funding NCCR, IIT-Madras.
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Conflict of Interest
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Keywords: Sulfonic acid
Acetonation · 1,2-monoacetone-D-glucose
· mesoporous silica · D-glucose,
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Manuscript received: September 5, 2018
Version of record online: ■■■, ■■■■
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FULL PAPERS
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O.M.S! The acidity and the strength
of sulfonic acid functionalized
ordered mesoporous silica (RSO3H-
OMS) is found to be dependent on
the nature and amount of silanol
groups present on the surface of
OMS used for grafting. By tuning
their acidity, the product selectivity
of acetonation of D-glucose can be
altered.
Dr. N. V. Krishna, Dr. S. Anuradha, R.
Ganesh, Dr. V. V. Kumar, Prof. Dr. P.
Selvam*
1 – 10
Sulfonic Acid Functionalized
Ordered Mesoporous Silica and
their Application as Highly Efficient
and Selective Heterogeneous
Catalysts in the Formation of 1,2-
Monoacetone-D-glucose
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