Facile synthesis of a sulfonated carbon-silica-meso composite and mesoporous silica

Article abstract of DOI:10.1039/c1cc14673d

In this study we report a novel and simple method for preparing a sulfonated carbon-silica-meso composite showing high acidity and porosity useful for transformation of bulky molecules, where glucose was used as a carbon source as well as a non-surfactant templating precursor and the resultant composite upon calcination yielded the mesoporous silica.

Full text of DOI:10.1039/c1cc14673d

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COMMUNICATION
Facile synthesis of a sulfonated carbonÀsilica-meso composite and
mesoporous silicaw
Devaki Nandan, Peta Sreenivasulu, Sandeep K. Saxena and Nagbhatla Viswanadham*
Received 29th July 2011, Accepted 2nd September 2011
DOI: 10.1039/c1cc14673d
In this study we report a novel and simple method for preparing a
sulfonated carbon–silica-meso composite showing high acidity and
porosity useful for transformation of bulky molecules, where
glucose was used as a carbon source as well as a non-surfactant
templating precursor and the resultant composite upon calcination
yielded the mesoporous silica.
The synthesis involves drop by drop addition of sulfuric
acid to the solution of glucose and tetraethyl orthosilicate
(TEOS) with the molar ratio of TEOS : 0.385 glucose : 4.8
2 2 4
H O : 0.88 H SO , followed by carbonization of the resultant
mixture in a nitrogen atmosphere at 573 K (ESIw) to facilitate
the decomposition and transformation of the glucose moieties
into hydrophobic carbon residue bearing sulfonyl groups as
the inner core that is in close contact with the stabilizing
hydrophilic silica matrix shell designated as sulfonated carbon
silica (SCS) composite material.
a
The concentration and pK values of catalysts play a vital role
in organic transformations, where a high density of accessible
strong Brønsted acid sites possessing stability in aqueous
1
environment is desired for catalyst development. The use of
The amorphous mesoporous silica (Fig. S1, ESIw) designated
as MS is obtained by calcination of SCS at 873 K to remove the
carbon template including sulfonyl groups. The resultant MS
liquid acids such as sulfuric acid suffers from energy inefficiency
and requires separation and recycling steps of acid waste residue.
The usability of recyclable solid materials as replacement to
homogeneous acid catalysts is usually limited due to the low
8
particles have wide applications in organic mass transformations,
9
adsorption of gases and immobilization of different organic
10
2
density and strength of the acid sites on the solid surface. The
moieties and inorganic metals for various catalytic applications.
method of immobilization of homogeneous catalysts onto
solid supports such as sulfonation of activated carbon resin
and metallic oxide has come up to solve the problem of acid
density, but the procedure is time consuming and involves several
Though, there are many methods available for the preparation of
mesoporous silica in the literature, the procedures are lengthy,
time consuming and use costly surfactants as templating
1
1,12
agents.
The present work gains advantage over the existing
3,4
preparation steps. Moreover, the immobilization of acidic
functional groups is difficult and also yields low acid density.
In the preparation of an efficient alternative solid acid catalyst
for acid attributed reactions, herein we report a simple one-step
method for the synthesis of a new class of sulfonated-carbon–
silica composite catalysts. The composite material containing an
organic inner core and mesoporous silica shell with its high
surface area and acidic properties is not only suitable for further
functionalization with acid or metal ions but also provides good
mechanical and thermal stability for catalytic applications.
Herein we used glucose as a cheaper and green carbon
precursor alternative to the commonly used high cost resins, ionic
methods as it provides a simple one-step synthesis of MS using
a low cost, non-surfactant common chemical ‘‘glucose’’ as a
template precursor. To the best of our knowledge, this is the
first of its kind to synthesise mesoporous silica in a simple
method using glucose as a template precursor.
The small-angle X-ray scattering (SAXS) patterns of the
as-synthesized (SCS) and calcined (MS) samples are shown in
Fig. 1. One broad peak signifying the average pore-center-to-
1
3
pore-center correlation length is observed in both the samples.
However, the peak is highly intensified in MS and indicates the
5
surfactants and P123 block co-polymers. In order to avoid the
limitations involved in the functionalization of an organic moiety
in two step partial carbonization and sulfonation methods reported
6
,7
in known prior art, herein we adopted a novel synthesis approach
of simultaneous carbonization and sulfonation in a single step.
Catalyst and Conversion Processes Division, Indian Institute of
Petroleum, Council of Scientific and Industrial Research,
Dehradun-248005, India. E-mail: nvish@iip.res.in;
Fax: +91-135-2525702; Tel: +91-135-2525856
w Electronic supplementary information (ESI) available: Experimental
details, wide angle XRD, TPD, EDX, data on textural properties and
reaction conditions. See DOI: 10.1039/c1cc14673d
Fig. 1 Small angle XRD patterns of MS and SCS.
Chem. Commun., 2011, 47, 11537–11539 11537
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significant increase in the order of the mesostructure due to the
removal of the sulfonated-carbon moiety during calcination.
This is further reflected in nitrogen adsorption–desorption
isotherms of SCS and MS (Fig. 2A). The isotherms of both
the samples are of type IV, characteristic of mesoporous materials
according to IUPAC classification, but the hysteresis loop of the
MS sample appears to be H2 type indicating that the MS has
good pore connectivity than usually observed for large mesopores
1
4
resulting from removal of the sulfonated-carbon moiety. The
3
À1
pore volumes of SCS and MS are 0.52 cm
3
g
.87 cm g , respectively, that confirms the removal of the
and
À1
0
sulfonated-carbon moiety in SCS and is responsible for
significant increase in pore volume in MS (ESIw, Table S1).
This phenomenon is further reflected in the increase in average
pore diameter of the SCS from 2.6 nm to 5.3 nm after
calcination (MS) (Fig. 2B). The EDX elemental analysis of
samples also supported the removal of the carbon moiety after
calcination (Fig. S2, ESIw).
The SEM images of samples also support the phenomenon
of removal of the carbon moiety during calcination of SCS,
where the black spots representing the carbon moiety observed
in SCS are disappeared in MS (Fig. 3A). TPD analysis of
samples reveals that SCS has high acidity than MS (Fig. S3,
ESIw) as SCS contains the acid group bearing a carbon moiety
which disappears during calcination and is absent in the
resulting MS. Moreover, the SCS exhibited acidity similar to
Fig. 3 (A) SEM images of SCS and MS. (B) IR of SCS and MS.
stretching vibrations. The TGA analysis of SCS (Fig. S4,
ESIw) shows weight loss at two places: (1) about 8% weight
loss below 392 K and (2) about 14% weight loss between
1
5
sulfonated zirconia. The IR spectra of the samples show the
interaction between the sulfonated-carbon moiety and meso-
porous silica in SCS (Fig. 3B). It is known that sulfonated
carbon exhibits two characteristic bands representing the
5
73 K and 1023 K which can be ascribed to the removal of
water/moisture and carbon material respectively. This
envisions that the catalyst is stable at the chosen reaction
temperature i.e. 393 K under solvent free conditions.
À1
À1 16
À1
–
OSO
3
H group at 1712 cm and 1207 cm
.
In our study,
Based on the textural properties of the materials obtained
(Table S1, ESIw), we have proposed a schematic model for the
synthesis of SCS and MS (Fig. 4). Tetraethyl orthosilicate and
glucose vigorously hydrolyse under the synthetic conditions to
give –SiOH groups and sulfonyl groups bearing an aromatic
organic moiety. This supramolecular assembly of glucose
molecules helps to form the cage-like structure inside the
SiO , where the –SO H group acts as a hydrophilic agent that
the SCS also exhibited a band at 1712 cm , but the second
À1
one at 1207 cm is not distinct as it is merged with the band at
À1
À1
1
3
090 cm related to silica. The additional bands obtained at
À1
447 cm and 803 cm are due to presence of –OH and SiO
2
2
3
can facilitate the interaction between the hydrophobic carbon
moiety and the hydrophilic silica moiety for the successful
formation of the SCS composite, which upon calcinations
expels the sulfonated-carbon moiety to give MS with increased
average pore diameter and pore volume (Fig. 2B). Aggregation
2
or even close-packing of SiO can also result in the formation of
1
7
a mesoporous structure.
Thus, SiO units are self-assembled, and their structure
2
effectively sustains the local strain caused during the carbon
removal and mesopore formation. Upon calcinations, the
Fig. 2 (A) N
2
adsorption–desorption isotherms of MS and SCS.
Fig. 4 The proposed templating and sulfonation pathway for the
(B) The respective pore size distribution using the BJH method is shown.
synthesis of SCS and MS.
1
1538 Chem. Commun., 2011, 47, 11537–11539
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supramolecular assembly of sulfonated glucose molecules breaks
from the SiO structure. Contrary to the previously reported
acidic SCS and MS materials through a novel approach of
simultaneous carbonization and sulfonation of glucose in an
organic silica medium, where glucose acts as a carbon source
as well as a template precursor.
2
templating pathways using surfactants or block-co-polymers
where the interaction between the template molecules and the
18,19
silica framework is through hydrogen or ionic bonding,
in
the present study covalent bonding is observed to exist between
Authors are thankful to the director, IIP, for his encouragement.
DN and PS acknowledge CSIR, New Delhi, for awarding
fellowship. We are thankful to XRD, IR, Porosimetry, SEM,
TPD and GC groups at IIP for analysis.
the carbon moiety and SiO
2
of the SCS composite as it is
À1
confirmed by the presence of two IR bands at 1712 cm and
À1
1633 cm representing the –OSO H ester bond and aromatic
3
ring respectively. As a result, the SCS of the present study
exhibits pore expansion (Fig. 2) rather than the pore contraction
Notes and references
1
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templated materials), up on calcination.
2 M. Kitano, D. Yamaguchi, S. Satoshi, K. Nakajima, H. Kato,
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In general, the tertiary butylation of phenol is conventionally
carried out in the vapour phase reaction at higher reaction
temperatures (above 413 K) (Table S2, ESIw). However, the
recent developments in novel materials provide opportunity for
3
4
S. Suganuma, K. Nakajima, M. Kitano, D. Yamaguchi, H. Kato,
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The SCS
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material synthesized in the present study exhibiting meso-
porosity along with strong acidity is expected to be suitable for
catalytic applications involving bulky molecular transformations
and the material is tested for its performance in liquid phase
butylation of phenol. The studies indeed indicated the promising
catalytic functionality of the SCS, where the catalyst exhibited as
high as 50% conversion based on phenol (497% conversion
based on alcohol) with product selectivity of 52%, 30% and 18%
to 2-tertiary butyl phenol, 4-tertiry butyl phenol and 2,4-ditertiary
butyl phenol respectively (Table S3, ESIw). The results indicate
comparable or better performance of the SCS catalyst with the
reported results (Table S4, ESIw). The reusability of the catalyst
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(SCS) synthesized in this work was investigated by filtering the
reaction solution, washing with ethanol and drying at 393 K
between consecutive cycles.
The results confirmed that the acidity-bearing carbon com-
posite in combination with mesoporosity of the SCS catalyst
successfully provides the required activity and stability to the
catalyst. Moreover, due to its high surface area and pore volume,
the MS obtained after removal of the sulfonated-carbon moiety
is a vital host material for immobilization and functionalization
useful for various mass absorption and catalytic transformations.
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This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11537–11539 11539