Indian Journal of Chemistry
Vol. 47, June 2008, pp. 815-820
Formation of mesopores in resorcinol-formaldehyde composite resin
Mahasweta Nandi, Krishanu Sarkar & Asim Bhaumik*
Department of Materials Science and Centre for Advanced Materials
Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
Email: msab@iacs.res.in
Received 10 January 2008; revised 12 May 2008
A new nanostructured resorcinol-formaldehyde material has been synthesized by hydrothermal condensation of
resorcinol and formaldehyde at 363 K under mild alkaline condition in the presence of supramolecular assembly of cationic
surfactant, cetyltrimethylammonium bromide as structure directing agent. The material has been characterized by powder
X-ray diffraction, N2 sorption, transmission and scanning electron microscopy , thermogravimetric and differential thermal
analysis and UV-vis spectroscopy. The X-ray and TEM image analyses reveal disordered wormhole-like mesostructure with
pores of ca. 2.5 nm. These composite materials exhibit photoluminescence property at room temperature, which may be
utilized for the fabrication of novel organic optical devices.
Synthesis of mesoporous materials usually follows
templating route, involving 2D or 3D supramolecular
assembly (true liquid crystalline phases)1 formed by
surfactant molecules as the core, around which the
desired solid matrix is formed. Subsequent removal of
the templating molecules from this composite can
generate mesoporosity, and the size of the pores
generated depends on the dimension of the selfassemblies of the surfactant molecules. Since the first
report on the synthesis of ordered mesoporous silica,
MCM-41/48, by Mobil researchers2 in 1992, the
synthesis of ordered nanostructured materials by soft
templating approach has been widely practiced.
Mesoporous materials synthesized so far usually have
very high surface area and tunable pore diameters visà-vis related microporous materials3, and hence have
attracted widespread attention for adsorption,
exchanger, catalysis and application as hosts for the
synthesis of nanomaterials. Functionalized mesoporous molecular sieves have also found enormous
potential applications in the field of catalysis4-7,
adsorption8, electronics9 and sensors10. In recent times
mesoporous carbons11 synthesized by pyrolyzing the
mesophase pitch and resins have attracted significant
interest in this context, whereas very little work has
been done on purely organic porous nanostructured
materials12. On the other hand considerable attention
has been given to the synthesis of mixed
cresol-formaldehyde13-15 or resorcinol-formaldehyde
resins16-18, organic aerogels and their corresponding
pyrolyzed carbon aerogels. Owing to their
controllable pore size distribution, high surface area,
low electrical resistivity and low thermal
conductivity, these are efficient materials as
electrodes for super capacitors and fuel cells19,
catalyst supports and high temperature insulation
materials20. Aerogels derived from sol-gel
polycondensation of cresol or rescorcinol with
formaldehyde under basic condition have been
studied in detail16. In general, preparation of carbon
aerogel proceeds through three steps: sol-gel
polycondensation of an aqueous solution of cresol or
rescorcinol with formaldehyde to form wet organic
aerogel, supercritical or ambient pressure drying of
the wet organic aerogel and finally pyrolysis of the
organic aerogel to the carbon aerogel. Carbon
aerogels prepared from resorcinol-formaldehyde
organic aerogels synthesized in the presence of a
neutral block copolymeric template, (Pluronic F127)
has been reported21. The various carbon aerogels
obtained have been found to have appreciable surface
areas, which can be varied by changing the
conditions. Also, the variation of the pore size was
large, from 2-100 nm. In the present work, we have
not attempted to prepare the carbon aerogel of the
resorcinol-formaldehyde composite by pressure
drying and pyrolysis. Rather, we have prepared the
mesostructured
resol
material
using
the
supramolecular assembly of cationic surfactant in situ
in the synthesis of gel and thereafter hydrothermal
treatment of the same. This was followed by the
removal of the structure directing agent (SDA) by
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INDIAN J CHEM, SEC A, JUNE 2008
solvent extraction to make the pores free from the
surfactants and obtain the corresponding all-organic
porous framework material. Recently, we have
synthesized mesoporous organic frameworks using
ionic surfactant and mixture of non-ionic and ionic
surfactants22. The resorcinol-formaldehyde nanostructured materials thus synthesized were found to have
photoluminescence property, and hence, can be
applied to design novel optical devices, which could
open up new potential applications for these allorganic porous materials.
Materials and Methods
Cationic surfactant, cetyltrimethylammonium
bromide, (CTAB, Loba Chemie) was used as the
SDA. Organic precursor gels were synthesized via
polycondensation of resorcinol (Loba Chemie) with
formaldehyde, HCHO (E-Merck, 37%) in an aqueous
alkaline solution of NaOH (E-Merck). In a typical
synthesis, 3.64 g of CTAB (0.01 mole) was first
dissolved in 20 ml of water. To it 1.6 g of sodium
hydroxide (0.04 mole) dissolved in 10 ml of water
was added under stirring condition. When the mixture
became homogeneous, 3.24 g of HCHO (0.04 mole)
was added to this solution. Finally, 2.2 g of resorcinol
(0.02 mole) dissolved in 15 ml of water was added
dropwise to it under constant vigorous stirring. After
stirring for 2 h at room temperature the gel was
transferred to an airtight polypropylene bottle and
kept at 363 K for 3 days. The solid product was then
obtained through filtration, repeated washing with
water and drying under vacuum at ambient
temperature. Surfactant was removed from the
as-synthesized sample by extracting twice with
HCl/water solution for 4 h at room temperature. When
the as-synthesized sample was extracted it showed a
considerable amount of weight loss, (ca. 41%), which
could be attributed to the loss of the surfactant
molecules from the pores of the nanostructured
as-synthesized sample. In order to study the effect of
time and temperature of the reaction as well as the
concentration of the surfactants, a few more samples
were synthesized. The detail has been discussed later
in this article.
For characterization, X-ray diffraction patterns of
the as-synthesized and the acid-extracted samples
were recorded on a Bruker-AXS D8 Advance
diffractometer using Cu-K (=1.5406Å) radiation.
TEM images were recorded in a JEOL JEM 2010
transmission electron microscope and SEM images in
a JEOL JEM 6700F field emission scanning electron
microscope. Thermogravimetry (TG) and differential
thermal analysis (DTA) of the samples were carried
out on a TA instrument Q600 DSC/TGA thermal
analyzer. N2 adsorption/desorption isotherms of the
sample was obtained using a Quantachrome Autosorb
1-C instrument, at 77 K. Prior to the measurement, the
sample was degassed at 363 K for 8 h under high
vacuum. UV-visible diffuse reflectance spectra were
obtained on a Shimadzu UV 2401PC spectrophotometer with an integrating sphere attachment.
BaSO4 pellet was used as background standard. The
excitation and emission spectra were recorded on a
Fluoromax-P Horiba Jobin-Yvon luminescence
spectrometer, using a solid sample holder at room
temperature. The powder samples were pressed to
form a smooth, opaque flat disk for the optical study.
The band pass for the excitation and emission
monochromators was set at 2.5 nm.
Results and Discussion
The low angle XRD patterns of the as-synthesized
(a) and extracted (b) samples are shown in Fig. 1. A
single low angle peak was observed for both the
samples with no detectable peak at high angle, which
indicates that the samples are nanostructured
containing disordered mesophases, with no short or
long range ordering. The extracted sample showed
relatively weaker intensity and broader peak width
(Fig. 1b) over the as-synthesized sample (Fig. 1a).
This result suggests that the nanostructure has been
restored after the re moval of the surfactant. However,
the arrangement of the pores became more disordered
after the removal of SDA. In order to study the effect
of surfactant concentration, time and temperature
dependence of the reaction, a series of samples were
synthesized which varied in their surfactant
concentration, time and temperature of reaction. The
results have been given in Fig. 1. The sample
synthesized with the same composition and at same
temperature but hydrothermally treated only for 1 day
(c) showed much lower intensity of the peak. No
improvement in XRD was obtained for the samples
synthesized at higher and lower temperatures, viz.,
393 K (d) and 348 K (e) for 3 days. Two other batches
were also prepared by varying the concentration of
surfactant in the reaction mixtures; one with twice the
amount of CTAB (f) and other containing half the
amount of CTAB with respect to sample (a). Here
also no marked improvement was noticed as far as
NANDI et al.: FORMATION OF MESOPORES IN RESORCINOL-FORMALDEHYDE COMPOSITE RESIN
sample (b) was concerned. On the other hand, for
sample (f) there was no XRD peak at all in the
mesoporous region. Thus, the conditions chosen for
the reaction are by far the most optimized conditions.
To study the nature of the surfactant molecules, it is
important to have an insight into the reaction
mechanism. The –OH groups of resorcinol moiety in
basic aqueous condition remain as Oˉ and this can
817
Fig. 1 –Low angle XRD pattern of resorcinol-formaldehyde
samples. [As-synthesized samples with (a) rescorcinol:CTAB=2:1
at 363 K for 3 days; (b) extracted sample of (a);
(c) rescorcinol:CTAB=2:1 at 363 K for 1 day;
(d) rescorcinol:CTAB=2:1 at 393 K for 3 days;
(e) rescorcinol:CTAB=2:1 at 348 K for 3 days;
(f) rescorcinol:CTAB=1:1 at 363 K for 3 days;
(g) rescorcinol:CTAB=4:1 at 363 K for 3 days].
interact with the cation derived from CTAB, i.e.,
CTA+. Thus, the ionic interaction directs the growth
of the mesostructure. If instead an anionic template
had been used there would be no ionic interactions
between two negatively charged species. As far as the
use of non-ionic templates is concerned, examples
where non-ionic template Pluronic F127 has been
used as a template for the synthesis of mesostucture
are known21. Thus, we see that this kind of framework
can be derived very easily from cationic and nonionic templates.
The TEM image of the as-synthesized sample is
shown in Fig. 2. The image confirms the formation of
low electron density spherical spots of 2.0-2.5 nm
diameter, corresponding to the small to medium size
mesopores and their disordered arrangements. Thus,
from the XRD pattern and TEM image analysis it may
be concluded that these nanostructured resorcinolformaldehyde composites have disordered wormholelike mesostructure. Supramolecular-templated mesoporous silicas, e.g., MSU-123 or KIT-124, also exhibit
similar common pore center-to-center correlation
length, which is characteristic of a wormhole-like
structure. The SEM image of the as-synthesized
sample exhibits granular/spherical morphology
(Fig. 3). In these samples very tiny spheres of
dimension 30-40 nm were found, which assembled
together to form large spherical aggregates.
The quantitative determination of the content of
resorcinol-formaldehyde in the surfactant-free sample
was estimated by using TG and DTA in the presence
of N2 flow. The TG and DTA plots of the resorcinolformaldehyde sample show the first weight loss up to
373 K due to desorption of physisorbed water (about
3.3 wt %; Fig. 4). This is followed by a sharp
decrease in the weight at temperatures between 473
Fig. 2 –TEM image of as-synthesized sample.
Fig. 3 –SEM image of as-synthesized sample.
2  (deg.)
INDIAN J CHEM, SEC A, JUNE 2008
818
and 573 K, which may be attributed to the loss of
resorcinol-formaldehyde fragments present in the
material. A considerable endothermic peak in the
DTA plot centered at 518 K suggests that most of the
resorcinol-formaldehyde fragment was decomposed at
this point. The total weight loss for the resorcinolformaldehyde fragment was ca. 29.9 wt % in the
temperature range 473-573 K. However, from 573873 K a gradual weight loss of ca. 25.2 wt % occurred
with an exothermic peak at ca. 723 K, which could be
due to the complete conversion of the material into
carbon.
We have estimated the surface area from the N2
adsorption-desorption isotherms of the extracted
sample (not shown here). No sharp increase for N2
uptake corresponding to the capillary condensation
was observed in the mesopore region. The BET
surface area for this sample was 4.9 m2g-1. Pore size
distribution of this sample estimated by employing the
BJH model suggests a broad distribution with maxima
at 2.3 nm. Since the framework of this porous resin
composite is completely organic containing aromatic
fragments, bond twisting or bond distortion may have
occurred during degassing leading to lower surface
area in this material. Moreover, since the resin surface
is highly hydrophilic, removal of water molecules
during degassing may also result in collapse of the
mesostructure. Porous carbon synthesized from
phenol and formaldehyde also exhibits similar low
surface area25 (ca. 5 m2g-1). For purely organic
mesoporous polyaniline22a samples a similar low
surface area was observed for most of the batches and
Temp. (K)
Fig. 4 – TG and DTA plot for the extracted sample.
the highest value of surface area exhibited was
ca. 45 m2g-1. It is relevant to mention here that this is
a preliminary report on purely organic mesoporous
material and further study to improve adsorption
properties is in progress.
In the UV-visible spectra (Fig. 5) strong absorption
band at ca. 278 nm with a weaker one at 225 nm and
two shoulders at 325 and 490 nm for resorcinol were
observed (Fig. 5c), attributed to the various
chromophoric functionalities in the molecule,
whereas, the as-synthesized (Fig. 5a) and extracted
samples (Fig. 5b), showed no absorption near 278 nm;
rather a broad band was observed at higher
wavelengths centered around 325 and 375 nm
respectively. The hump at 490 nm that can be
observed for resorcinol is somewhat shifted to higher
wavelength, at 527 and 525 nm respectively for the
as-synthesized and extracted samples. Absence of any
absorption band from 220-290 nm thus suggests that
all the resorcinol moieties have been fully polycondensed with formaldehyde in the material.
The photoluminescence spectra (both excitation
and emission) of the as-synthesized sample have been
given in Fig. 6. The emission spectrum was obtained
by excitation of the polymer at the maximum
absorption wavelength (λex= 349 nm). The maxima in
the emission spectra were observed at 468 nm (weak
intensity) and 565 nm (strong intensity),
corresponding to deep blue and greenish light,
Fig. 5 – UV-visible diffuse
(a) as-synthesized sample, (b)
(c) resorcinol.
reflectance spectra of
extracted sample, and,
NANDI et al.: FORMATION OF MESOPORES IN RESORCINOL-FORMALDEHYDE COMPOSITE RESIN
Fig. 6 – Photoluminescence spectra of as-synthesized sample.
[(a), excitation, and, (b) emission].
819
respectively. The existence of some small peaks
around 470 nm peak region may be attributed to the
nano-scale particle size distribution26 of the sample.
Thus, upon irradiation with light of 349 nm, the material excites an electron from the HOMO to LUMO to
generate the singlet excited state and subsequently the
excited polymer resin relaxes to the ground state with
the emission of blue and greenish light. The formation
of the stable resin framework may tighten the entire
skeleton, resulting in much weaker vibrations and
more relaxation27, which may be responsible for its
photoluminescence at room temperature.
Based on the above, we propose a reaction scheme
(Scheme 1) for the formation of mesoporous
resorcinol-formaldehyde framework. This scheme is
constructed on the basis of sol-gel framework
structures formed when resorcinol condenses with
formaldehyde in the presence of a base. Here the
Reaction scheme and proposed model (partial) for the formation of resorcinol-formaldehyde matrix in presence of template.
Scheme 1
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INDIAN J CHEM, SEC A, JUNE 2008
condensation can take place randomly in all direction
of the activated aromatic ring, giving rise to a stable
cross-linked resin framework structure. The polar
head groups of the supramolecular assembly of the
cationic surfactant can interact with phenoxide anions
under alkaline pH conditions leading to this novel
nanostructured material.
Conclusions
A new nanostructured resorcinol-formaldehyde
composite has been synthesized by using supramolecular assembly of a cationic surfactant CTAB through
in situ aqueous polycondensation of resorcinol with
formaldehyde under alkaline condition. Powder XRD
and TEM studies suggests no long range ordering in
these samples and wormhole-like disordered
mesopores of dimension ca. 2.3 nm extending in all
direction. UV-visible absorption data suggests the
formation of resorcinol-formaldehyde polymeric
network in these samples. This composite material
exhibits photoluminescence at room temperature. We
expect this novel, all organic porous resin to find
potential applications in photoresponsive materials
and in fabrication of organic optical devices etc.
Acknowledgement
MN and KS thank CSIR for Senior Research
Fellowships. This work was partly funded by the
Ramanna Fellowship and by the NanoScience and
Technology Initiative project grants of Department of
Science and Technology, New Delhi.
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