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Oil Shale, 2008, Vol. 25, No. 3, pp. 348–358 ISSN 0208-189X
doi: 10.3176/oil.2008.3.06 © 2008 Estonian Academy Publishers
LOW-DENSITY ORGANIC AEROGELS FROM OIL
SHALE BY-PRODUCT 5-METHYLRESORCINOL
A. -L. PEIKOLAINEN
*
, F. PÉREZ-CABALLERO, M. KOEL
Institute of Chemistry, Tallinn University of Technology
Akadeemia tee 15, 12618 Tallinn, Estonia
The objective of the present work was to prepare organic aerogels using a
by-product of oil shale processing as a starting material. Low-density
organic aerogels were synthesized via sol-gel polycondensation of formal-
dehyde (FA) and either 96% 5-methylresorcinol (MR) or the technical
mixture named Honeyol™ (H) containing 59.6% of 5-methylresorcinol
among other diphenolic compounds, using supercritical CO
2
for drying the
gel obtained. Porosity and particle characteristics of MR-FA and H-FA aero-
gels can easily be controlled by varying the concentrations of precursors and
preparation conditions. Less than 4.5-hour drying resulted in MR-FA aerogel
characterized by radial shrinkage 2%, density 0.21 g/cm
3
and specific
surface area 350 m
2
/g. At the same molar ratios H-FA aerogel had 29%
shrinkage, 302 m
2
/g specific surface area and the density as low as
0.10 g/cm
3
.
The preparation techniques and morphology of MR-FA and H-FA aerogels
were compared to resorcinol-formaldehyde, phloroglucinol-formaldehyde
and phenol-formaldehyde aerogels.
Introduction
Organic aerogels are produced via polycondensation of two monomers
which form
functionalized clusters (sol-gel), and covalent cross-linking of
these clusters produces a gel. After processing these gels under supercritical
conditions an organic aerogel is obtained. Organic aerogels can further be
pyrolyzed to form highly porous carbon aerogels of low density and high
specific surface area. Carbon aerogels can be used as membranes, adsorbents
and carriers for metal catalysts; they find application in high-energy physics
and acoustic technology; low thermal conductivity allows usage as thermal
insulators [1]. Carbon aerogels are also promising for electrochemical
applications due to their electrically conductive network [2].
*
Corresponding author: e-mail annnaliisa@gmail.com
Low-Density Organic Aerogels from Oil Shale by-Product 5-Methylresorcinol
349
In this paper the preparation of 5-methylresorcinol- formaldehyde aerogel
is discussed. 5-methylresorcinol is an alternative precursor for aerogel pre-
paration because its molecular structure is similar to other precursors used
for this purpose. Moreover, quicker gelling was expected due to the addi-
tional directing methyl group in 5-methylresorcinol molecule compared to
the most studied aerogel precursor resorcinol. Methyl substitution also
increases hydrophobicity of the gel and thus smaller shrinkage while drying
was predicted. In addition, being a by-product in oil shale industry,
5-methylresorcinol is an inexpensive material in the regions where oil shale
industry is active.
Properties (density, thermal conductivity, etc.) of an aerogel are dependent
on its structure, which can be controlled by the molar ratios of reagents
(aromatic compound to formaldehyde, aromatic compound to catalyst and
solvent to aromatic compound), the choice of the catalyst and the solvent,
gelling conditions and by the way of drying the gel [3–5].
Among other gel drying techniques (conventional drying, freeze-drying),
supercritical fluid drying is preferred. At the supercritical state no meniscus
is formed between gaseous and liquid phase and capillary pressures within
the pores, causing the reduction of porosity or cracking the fragile gel
skeleton during drying, are avoided. Resulting densities, specific surface
areas and percentages of shrinkage differ accordingly to the fluid used. [6]
CO
2
as a supercritical agent is readily available, inexpensive, non-flammable
and has low critical parameters (T
c
= 31.1 °C and P
c
= 7.38 MPa) [4] that do
not decompose sol-gel polymers. Furthermore, its polarity is suitable for
removing most of the organic solvents used for such organic polymeric sol-
gel preparation [7], and recycling of CO
2
makes it environmentally friendly
processing agent.
Experimental
Materials and equipment
5-Methylresorcinol of purity
96% was obtained from Carboshale, Estonia;
Honeyol™ which is a product of Viru Keemia Grupp, Estonia, was obtained
from Department of Oil Shale Technology of Tallinn University of Technol-
ogy; the catalysts were Na
2
CO
3
(purity 99.8%) from Sigma Aldrich Labor-
chemikalien GmbH, Germany, and KOH from Chemapol Lachema Brno,
Czech Republic; formaldehyde was in the form of 35% solution in water,
obtained by dissolving paraform in distilled water, when paraform (purity
95%) was from Sigma Aldrich Laborchemikalien GmbH, Germany.
Solvents used were acetone (pure, Petrochemiczne Płock, Poland) and
methanol (HPLC reagent, Rathburn Chemicals Ltd., Scotland).
Thermostat for gelation: TECHNE Dri-Block® DB 3A, Spain; super-
critical drying was performed on a self-completed equipment consisting of
high-pressure pump HPP 4001, Czechia, thermostat: Intersmat IGC 121 C FL,
A. -L. Peikolainen et al.
350
France, and high-pressure 10-mL cell constructed in laboratory. CO
2
(99.8%)
was obtained from Eesti AGA.
Preparation of 5-methylresorcinol-formaldehyde aerogel
The preparation of organic aerogel was started from the gelation of 5-methyl-
resorcinol (MR) and formaldehyde: MR was dissolved in distilled water
(W), and then the catalyst (Cat) Na
2
CO
3
and formaldehyde solution were
added. The gelation was carried out in test tubes either at room temperature
(25 °C) or at 50 °C. After gelling, the gels were transferred from the test
tubes into the acetic acid solution of pH~4 (double-catalyzed synthesis). The
next step was solvent exchange – water in the gel was replaced with acetone,
and then the supercritical drying followed. The regime of drying the gel with
supercritical CO
2
(SCE) comprised of pressurization of CO
2
to 20 MPa at
25 °C, flowing liquid CO
2
through the gel at 12 MPa and 25 °C and super-
critical CO
2
extraction at 12 MPa and 50 °C.
In comparison to MR-FA gels, resorcinol- (R), phloroglucinol- (PG) and
phenol-formaldehyde gels and single-step acid-catalyzed MR-FA aerogel
were prepared under similar conditions.
Preparation of Honeyol™-formaldehyde aerogel
From some trials to prepare Honeyol™-formaldehyde gel (H-FA) similarly
to MR-FA gel, it became evident that H-FA needs a stronger basic catalyst
than Na
2
CO
3
and also higher temperatures for gelling. A homogenous H-FA
gel was obtained at 60 °C in the presence of KOH. In this case methanol was
used as the solvent instead of water and paraform and KOH were dissolved
in methanol. The molar ratios were calculated by the amount of 5-methyl-
resorcinol in Honeyol™.
Results and discussion
5-Methylresorcinol is a trifunctional molecule with reaction sites at the 2
nd
,
the 4
th
and the 6
th
position of the aromatic ring where the addition of
bifunctional formaldehyde results in formation of hydroxymethyl (–CH
2
OH)
groups (Fig. 1). It has been found that the reaction is fast under basic condi-
tions (slow in acidic solution) [8], and the kinetics of this reaction is
proportional to the size and the valence of the hydrated cation [9]. Na
2
CO
3
was used as a basic catalyst in MR-FA aerogels, following Pekala’s example
of R-FA aerogel preparation [1].
These intermediates further react to form methylene (–CH
2
–) and
methylene ether (–CH
2
–O–CH
2
–) bridged compounds (slow in basic, fast in
acidic solution) [8, 10].
Gel formation of MR-FA with optimal molar ratios for obtaining the
lowest shrinkage and density (MR/Cat = 60, MR/FA = 0.5 and W/MR = 45)
under alkaline conditions and at room temperature occurs within 100
minutes.
Low-Density Organic Aerogels from Oil Shale by-Product 5-Methylresorcinol
351
Fig. 1. Addition of formaldehyde to 5-methylresorcinol in the presence of basic
catalyst (Na
2
CO
3
).
Honeyol™ contains dihydroxy benzenes (Table 1) with additional sub-
stitutions at the positions where directing groups would affect FA to react
on, and with Na
2
CO
3
as the catalyst, water as the solvent and room
temperature for gelation, the gel could not be obtained.
Although H-FA gel formed after increasing the amount of catalyst, the
gel dissolved in acetone during the solvent exchange step. The gel remained
intact when methanol, which is also miscible with CO
2
, was used for the
solvent replacement instead of acetone. Further methanol was used as the
original solvent following the examples from the literature [5]. Na
2
CO
3
was
replaced with stronger catalyst KOH, which was used in smaller amounts.
The gelling of the solution with composition H/FA = 0.5, methanol/H = 45,
H/Cat = 60 takes at least 8 days at 60 °C, but as the gelling time is strongly
dependent on the catalyst amount, the time can be shortened to 1 day by
decreasing the ratio H/Cat about 10 times.
By means of IR spectra measurements the gels prepared by single- and
double-step catalyzed synthesis were compared (Fig. 2).
Table 1. Composition of Honeyol™
Component mass fraction, %
Monohydric phenols 0.8
resorcinol 5.7
4-methylresorcinol 2.8
5-methylresorcinol 59.6
2-methylresorcinol 1.7
2,5-dimethylresorcinol 8.4
5-ethylresorcinol 9.8
4,5-dimethylresorcinol 7.6
Not identified 3.6
Dihydric phenols 99.2
A. -L. Peikolainen et al.
352
0
100
200
cm
-1
T, %
single
-
step base catalyzed synthesis
single-step acid catalyzed synthesis
double-catalyzed synthesis
H-FA aerogel
MR-FA aerogel
T, %
0
100
1500-1400
-CH
2
-
1110-1000
-CH
2
-O-CH
2
-
3100-2800
-CH
2
-
Fig. 2. MR-FA and H-FA aerogels via single- or double-catalyzed synthesis.
In the spectra IR adsorption bands of –CH
2
– (2930 cm
–1
and 1450 cm
–1
)
and –CH
2
–O–CH
2
– (1100 cm
–1
) bonds are similar in single-step base
catalyzed and single-step acid catalyzed MR-FA aerogels. For the double-
catalysed aerogel, the same bonds adsorb less, referring to the effectiveness
of the single-step catalysis over the double-catalyzed synthesis. Despite that,
from further experiments the single-step acid catalysed reaction was
excluded due to unsatisfying homogeneity of the obtained material (under
the chosen conditions).
IR spectra of single-step base catalyzed and double-step base-acid
catalyzed H-FA aerogels almost overlap (Fig. 2), therefore, the use of only
basic catalyst was considered sufficient. The comparison of absorption bands
caused by methylene and ether bridges in MR-FA, H-FA, R-FA and PG-FA
aerogels is seen in Fig. 3.
Gels from both, di-substituted and tri-substituted aromatic precursors
have a similar amount of –CH
2
–O–CH
2
– bridges between the molecules,
showing the equal adsorption band at 1100 cm
–1
. Methylene bridges at the
characteristic wavenumbers (2930 cm
–1
and 1450 cm
–1
) show stronger
adsorption for R-FA aerogel than for MR-FA, H-FA and PG-FA aerogels.
The structure of resorcinol molecule has an unoccupied 5
th
position, which,
we assume, is what makes the close connection between two aromatic
molecules via –CH
2
– bridges preferable compared to tri-substituted
molecules where –OH or –CH
3
groups at the same positions can be found.
Low-Density Organic Aerogels from Oil Shale by-Product 5-Methylresorcinol
353
0
10
20
30
40
50
60
cm
-1
T, %
R-FA MR-FA H-FA PG-FA
3100-2800
-CH
2
-
1500-1400
-CH
2
-
1110-1000
-CH
2
-O-CH
2
-
Fig. 3. IR spectra of MR-FA, H-FA, R-FA and PG-FA gels with bands of methylene
and ether bridges.
The molar ratios between the precursors (MR/FA, MR/Cat, W/MR or
H/FA, H/Cat, methanol/H) were optimized, taking into account the final
density, the preparation time and the radial shrinkage during drying. The
shrinkage is calculated by the diameters of gel rods before and after drying
by the following equation (1):
[]
%%Shrinkage
before
afterbefore
radial
100⋅
∅
∅−∅
= . (1)
By excluding the step of catalyzation in acidic media according to the
results of IR spectra measurements, the preparation time can be decreased
several days.
The number of FA molecules as a cross-linking agent must exceed the
number of aromatic molecules to form three dimensional mesoporous
material [1], and from this follows that a preferred molar ratio of R/FA is
1:2. On the other hand, formaldehyde remaining in the gel after poly-
condensation could induce a collapse of mesoporous structure and decrease
the volume of mesopores in the prepared aerogel [11]. From our experiments
the MR/FA ratio 1:2 is suitable for MR-FA aerogels (Fig. 4), whose density
and shrinkage are the smallest (0.21 g/cm
3
and 2%) compared to aerogels
with MR/FA ratios 1:4 and 3:4 (MR/Cat = 60 and W/MR = 45).
A. -L. Peikolainen et al.
354
16
1:4
1:2
3:4
90
67.5
56.25
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
Molar ratios
Density, g/cm
3
0
5
10
15
20
25
30
35
40
45
50
Radial shrinkage, %
MR/Cat
MR/FA
W/MR
MR/FA=0.5
W/MR=45
t=25
o
C
MR/Cat=60
W/MR=45
t=25
o
C
MR/FA=0.5
MR/Cat=60
t=50
o
C
Fig. 4. Densities and radial shrinkages of SCE MR-FA aerogels according to molar
ratios.
To strengthen the gel structure before the supercritical drying in order to
achieve the minimal shrinkage and the lowest density of the aerogel, the gels
were aged 9 days in basic media as it has been suggested for R-FA gels [1].
Later it was found that for MR-FA aerogels with molar ratios MR/FA = 0.5,
MR/Cat = 60 and W/MR = 45, curing the gel in basic media for 2 days
showed the same results in the aspect of shrinkage and density. Optimal
duration of supercritical CO
2
drying according to our research was less than
4.5 hours, as no further decrease in aerogel density or radial shrinkage was
detected after a longer processing.
Although our experiments showed that the densities of supercritically
dried MR-FA aerogels can be decreased by raising W/MR ratio above 45
(Fig. 4), increasing W/MR to 90, the solid network of the gel becomes too
sparse for maintaining the original shape resulting in aerogel with 26%
shrinkage having the density 0.23 g/cm
3
. Also, an increased W/MR ratio,
higher temperature and longer curing time are needed for gel formation. The
compromise between the density and the gelling time was made, and the
ratio 45 was preferred in experiments.
Phenol gave no homogenous gels at these molar ratios (Phenol/FA = 0.5,
W/Phenol = 45, Phenol/Cat = 60) and temperatures. Materials prepared from
phenol and FA were either flake-like (preparation at 50 °C) or stiff and hard
bulk pieces of novolak (preparation at 90 °C). As gel-like materials were not
obtained, the experiments with phenol find no further attention.
R-FA and PG-FA solutions (molar ratios R/FA = PG/FA = 0.5, W/R =
W/PG = 45, R/Cat = PG/Cat = 60) resulted in transparent aerogels (ultrafine
pore size minimizes light scattering [1]) with densities after SCE drying
Low-Density Organic Aerogels from Oil Shale by-Product 5-Methylresorcinol
355
respectively 0.22 g/cm
3
and 0.28 g/cm
3
. Temperature 65 °C was necessary
for synthesizing PG-FA gels because of poor solubility of PG in water.
MR-FA gels with MR/Cat ratio 16 were also transparent and having smaller
pores than MR-FA gel with MR/Cat ratio 60 (opaque gel), the density and
radial shrinkage were affected by drying time at larger scale.
The effect of the supercritical drying is clearly seen in lower final
densities compared to the gels dried in ambient conditions (Fig. 5). Drying in
ambient conditions makes the gel denser due to greatly reduced porosity
(shrinkage 28–45%). PG-FA gel and all the H-FA gels cracked into pieces
while drying at room temperature and pressure.
All R-FA, MR-FA, PG-FA and H-FA aerogels consist of nanometre-
sized spherical particles and particle clusters (Fig. 6).
Comparing two H-FA aerogels (H/Cat ratios 60 and 6), the decrease of
particle and pore sizes is observable. H/Cat ratio 6 leads to transparent gels
with particle size 10 nm, while gels with H/Cat ratio 60 are opaque and
consist of more than 20 nm sized particles. The pore size distribution for
organic aerogels is wide, and no micropores were detected with nitrogen
adsorption measurements. PG-FA aerogel consists of less than 10 nm sized
particles with PG/Cat ratio 60.
It is known that an increasing amount of catalyst leads to higher density
[1], larger total pore volume and specific surface area [11–13] of the aerogel
which can be followed in Fig. 7. From the graph it is also seen that the single-
step base catalyzed synthesis has proven to be more effective than the double-
step catalysis, leading to lower density, larger specific surface area (calculated
by Brunauer-Emmett-Teller theory) and higher total pore volume.
The densities of H-FA aerogels are more affected by drying time than
MR-FA aerogels (slashes in Fig. 7 legend separate the time, in hours, of each
step of CO
2
drying regime). For H-FA density 0.10 g/cm
3
was the lowest
achieved with the single-step base catalyzed synthesis (H/Cat ratio 60) with
5 h and 35 min supercritical drying.
Fig. 5. Effect of supercritical fluid drying on the gel structure. MR-FA gel
(MR/FA = 0.75) dried a) by SCE; radial shrinkage 4% and b) in ambient conditions;
radial shrinkage 33%.
A. -L. Peikolainen et al.
356
Fig. 6. Effect of catalyst ratio on H-FA
and PG-FA aerogels. a) SCE H-FA
(H/Cat = 60); b) SCE H-FA (H/Cat = 6);
c) SCE PG-FA (PG/Cat = 60).
S
BET
=469;
(V
t
=966)
S
BET
=355;
(V
t
=624)
S
BET
=412;
(V
t
=810)
S
BET
=302;
(V
t
=581)
0.34
0.26
0.21
0.29
0.24
0.15
0.26
0.15
0.13
0.21
0.14
0.10
0
50
100
150
200
250
300
350
400
450
500
6
16
60
H/Cat
BET Surface area, m
2
/g
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Density, g/cm
3
BET surface area
base 0.33/2/2
base 0.58/2.5/2.5
base-acid 0.33/2/2
base-acid 0.58/2.5/2.5
V
t
-total pore volume, mm
3
/g
Fig. 7. Influence of catalysts and SCE regime on density, surface area and total pore
volume of H-FA aerogels.
Low-Density Organic Aerogels from Oil Shale by-Product 5-Methylresorcinol
357
The densities and the specific surface areas of aerogels prepared by the
double-catalyzed synthesis (R-FA aerogels achieve lower densities at the
double-catalyzed synthesis) from different aromatic precursors (resorcinol,
5-methylresorcinol and Honeyol™) with equal molar ratios (R/FA = MR/FA
= H/FA = 0.5; W/R = W/MR = Met/H = 45; R/Cat = MR/Cat = H/Cat = 60)
and SCE regime are compared in Fig. 8. Temperatures for nitrogen adsorp-
tion measurements were chosen based on thermogravimetric analysis
(105 °C for R-FA and MR-FA, 180 °C for H-FA).
SCE H-FA
MR-FA
SCE R-FA
SCE MR-FA
0.0
0.2
0.4
0.6
0.8
1.0
Gel
Density, g/cm
3
0
100
200
300
400
500
BET Surface area, m
2
/g
Fig. 8. Densities and BET surface areas of supercritically dried (SCE) R-FA,
MR-FA, H-FA aerogels (catalyst ratio 60) and of MR-FA aerogel dried under
ambient conditions.
Specific surface area is the largest for R-FA aerogel (455 m
2
/g) because
the gel consists of small, 7–10 nm sized particles as was discussed above,
but as was seen in Fig. 7, it was possible to make an aerogel with the similar
specific surface area – 469 m
2
/g also from Honeyol™. Specific surface area
of the gel is decreased considerably when drying is carried out at ambient
temperature and pressure.
Conclusions
For aerogel preparation, 5-methylresorcinol and its technical mixture
Honeyol™ are very competitive precursors beside well-studied resorcinol
allowing to control the gel structure easily by the same techniques, resulting
in very similar characteristics: the lowest density achieved for MR-FA
aerogel is 0.21 g/cm
3
and for H-FA 0.10 g/cm
3
. Preparation of MR-FA and
H-FA aerogels is effective via the single-step base catalyzed synthesis
requiring at least 10 times larger amount of catalyst than R-FA and PG-FA
for gelation. Acid catalyst does not contribute to strengthening the gel
structure and does not lead to desired lower densities. Supercritical condi-
A. -L. Peikolainen et al.
358
tions are necessary for drying, especially for Honeyol™-FA gels, however,
drying times for MR-FA and H-FA for obtaining aerogels with previously
mentioned densities are extremely short: 4.33 h for MR-FA and 5.58 h for
H-FA gel.
Acknowledgements
Authors express their thanks to Mai Uibu, Olga Volobujeva, Tiiu Kailas for
making necessary analysis of the materials prepared during this study.
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Presented by A. Kogerman
Received September 11, 2007
