new tannin–lignin aerogels
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Industrial Crops and Products 41 (2013) 347– 355
Contents lists available at SciVerse ScienceDirect
Industrial Crops and Products
journa l h o me page: www.elsev ier .com/ locate / indcrop
New tannin–lignin aerogels
L.I. Grishechko a,b, G. AmaralLabat a, A. Szczurek a, V. Fierro a, B.N. Kuznetsovb, A. Pizzi c, A. Celzard a,∗,1
a Institut Jean Lamour UMR Université de Lorraine – CNRS 7198, ENSTIB, 27 rue Philippe Séguin, BP 1041, 88051 Épinal Cedex 9, Franceb Institute of Chemistry and Chemical Technology, SB RAS, 42 K. Marx Street, Krasnoyarsk 660049, Russiac LERMAB ENSTIB, 27 rue Philippe Séguin, BP 1041, 88051 Épinal Cedex 9, France
a r t i c l e i n f o
Article history:Received 10 March 2012
Received in revised form 21 April 2012
Accepted 29 April 2012
Keywords:Aerogels
Tannin
Lignin
Porous structure
a b s t r a c t
Highly porous organic aerogels based on tannin and lignin have been prepared and characterized for the
first time. Hydrogels were first described, which were prepared at constant solid weight fraction and
constant pH, but with different tannin/lignin and (tannin + lignin)/formaldehyde weight ratios. A phase
diagram has been drawn, showing the range of compositions in which nice hydrogels may be obtained.
The porosity of the resultant aerogels, dried with supercritical CO2, has been systematically investigated
in terms of surface area, macro (pore width > 50 nm), meso (2–50 nm) and microporosity (<2 nm). The
impact of the composition on the porous properties was thoroughly discussed and supported by electron
microscopy studies. We show how the gradual substitution of tannin by lignin modified the pore size
distribution, although the aerogels remained almost purely mesoporous materials. Values of thermal
conductivity and mechanical resistance are also given, which are compared with those of much more
expensive, non renewable, aerogels derived from resorcinol–formaldehyde.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Organic aerogels are highly porous materials combining a
number of excellent and unique characteristics such as extreme
lightweight, high surface area and adjustable mesoporosity
(2 nm < pore width < 50 nm). Some of them are also able to swell
once immersed in a solvent. From such valuable features, organic
aerogels may find various applications. For instance, they have
been suggested as sorbents (Sengil and Ozacar, 2008, 2009), ther
mal insulators (Fischer et al., 2006; Biesmans et al., 1998; Pekala
et al., 1995; Hrubesh and Pekala, 1994) or drug delivery materials
(Ciolacu et al., 2012; GarcíaGonzález et al., 2011; Wu et al., 2005b;
Nishida et al., 2003). Besides, after pyrolysis, their carbonaceous
derivatives may be used as high surface area adsorbents (Tian et al.,
2009; Yamamoto et al., 2002; CarrascoMarin et al., 2009), catalysts
or catalyst supports (Liu et al., 2006; SanchezPolo et al., 2007; Serp
and Figueiredo, 2009), and as electrodes for doublelayer capacitor
(AmaralLabat et al., 2012c; Szczurek et al., 2010). Other applica
tions of carbon aerogels with relation to their adsorption properties
have been reviewed in a recent monograph (Celzard et al.,
in press).
In general, organic aerogels are produced by sol–gel poly
condensation reaction of monomers with aldehydes, especially
phenolic compounds such as resorcinol. The first organic aerogels
∗ Corresponding author. Tel.: +33 329 29 61 14; fax: +33 329 29 61 38.
Email address: Alain.Celzard@enstib.uhpnancy.fr (A. Celzard).1 Member of the Institut Universitaire de France, France.
were described by Pekala (1989) who produced organic aerogels
from resorcinol and formaldehyde. From that time, many devel
opments have been reported. Resorcinol–formaldehyde (RF) gels
are the best known among organic aerogels and have been widely
investigated because of their excellent and reproducible prop
erties. However, the high cost of resorcinol never allowed any
mass production of RF gels and motivated researchers to look for
new, cheaper, raw materials that might lead to similar materi
als. Nowadays, several lowcost phenolic compounds that could
be successively used as gel precursors have been reported, such as
phenol itself (Wu et al., 2005a; Mukai et al., 2005) and other kinds
of phenolic resins (AmaralLabat et al., 2012b), cresol (Zhu et al.,
2006; Li et al., 2003), tannin (Kraiwattanawong et al., 2007, 2008;
SanchezMartin et al., 2011) or lignin (Chen and Li, 2010; Chen et al.,
2011).
Tannins are phenolic compounds naturally present in a number
of plants, helping them to fight against insects and fungi. Fur
thermore, their astringent character limits the consumption of
tanninrich vegetables by herbivorous animals (Robbins, 1987).
Whereas distributed all through the cytoplasm of any vegetal cell
(Haslam, 1989), the highest concentration of such compounds is
generally found within tree barks, e.g. those of pine, oak and wattle.
In the present work, we focused our attention on tannin extracts
from Acacia mearnsii, de Wild, as precursors of organic aerogels.
Such material was indeed recently shown to replace successfully
resorcinol for preparing aerogels and cryogels, allowing a much
broader range of pore structures than those of RF gels, due to the
wider range of pH made possible by the use of tannin (Szczurek
et al., 2011a,b; AmaralLabat et al., 2012c). Wattle tannins from
09266690/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.indcrop.2012.04.052
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348 L.I. Grishechko et al. / Industrial Crops and Products 41 (2013) 347– 355
Fig. 1. Prorobinetinidin, the main flavonoid unit contained in wattle tannin.
Acacia mearnsii mainly consist of prorobinetinidin, the main
flavonoid unit presented in Fig. 1. Such units are mostly 4,6linked,
and sometimes 4,8linked, leading to oligomers whose molecular
weights range typically from 500 to 3500 g mol−1.
Though commercially available and much cheaper than resorci
nol, around 1500 US$ per metric ton, wattle tannin is produced in
limited amounts, with an average of 220,000 tons per year. Given
that such tannin represents 90% of the world production and is
traditionally applied to leatherwork and industry of adhesives, the
possibility of other applications is hindered and tends to increase
tannin cost. This is the reason why finding even cheaper raw mate
rial is required. In the present study, aerogels have been prepared
from tannin–formaldehyde formulations in which tannin has been
partly replaced by lignin.
Lignin is a phenolic, threedimensional, crosslinked polymer
occurring in plant tissues, and whose role is cementing cellulose
fibers. It is based on three phenylpropanoid monomers, see Fig. 2,
connected with each others through various interunit linkages
(Abreu et al., 1999; CalvoFlores and Dobado, 2010; Adler, 1977),
thus resulting in a complex macromolecular structure. In general,
lignin is a waste material from the pulp and paper industry, and is
most often used as fuel for the energy balance of pulping process
(Ciolacu et al., 2012 and refs. therein). Yet, considerable effort has
been made in the past for finding high valueadded applications
to lignin. For instance, it has been proved that glyoxalated lignin
can be an effective precursor of adhesive resin for formaldehyde
free particleboards (El Mansouri et al., 2007). In addition, potential
health applications of lignin have been explored, and it was shown
that lignin possesses high activity as binder of cholic acid sodium
salt, and as antitumor and antivirus (Vinardell et al., 2008).
Although it is not the first time that lignin is used as gel precur
sor, few works exist about gels based on lignin (Ciolacu et al., 2012;
Aaltonen and Jauhiainen, 2009; Raschip et al., 2011; Yamamoto
et al., 2000; Chen and Li, 2010). The present paper is the first one
dealing with tannin mixed with lignin for preparing organic aero
gels. As explained above, such precursors are incomparably cheaper
than any other phenolic compound of synthetic, non renewable,
origin. We show here how almost purely mesoporous aerogels have
(c) (b) (a)
OCH3
OH
OH
OCH3
OH
OH
OCH3
OH
OH
Fig. 2. Schematic representation of the structural units of lignin: (a) pcoumaryl
alcohol (4hydroxyl phenyl), (b) coniferyl alcohol (guaiacyl), (c) sinapyl alcohol
(syringyl).
been obtained for the first time from tannin–lignin–formaldehyde
formulations.
2. Experimental
2.1. Synthesis of tannin–lignin–formaldehyde (TLF) gels
2.1.1. MaterialsCommercial tannin powder, sold under the name Tupafin and
supplied by the company SilvaChimica (St. Michele Mondovi, Italy)
has been used. Tannin was extracted industrially in Tanzania from
the whole, dried, bark of 10years old wattle trees. Wattle barks
were leached by a solution of 1% sodium bisulphite in water at
70 ◦C. The resultant concentrated solution was then spraydried,
leading to a lightbrown powder which generally contains 80–82%
of actual phenolic flavonoid materials, 4–6% of water, 1% of amino
and imino acids, the remainder being monomeric and oligomeric
carbohydrates, in general broken pieces of hemicelluloses.
Lignin was kindly supplied by Innventia and is presently com
mercialized by the company Metso under the name LignoBoost. It is
a Kraft lignin that has been precipitated from softwood black liquor
by injection of CO2. It has been next filtered, redispersed, acidified
again, filtered once more and finally washed. As a result, a much
purer lignin as usual was obtained, having very low carbohydrate,
sulphur and ash contents, typically 0.5–1.5, 1–3, and 0.2–1 wt.% on
dry basis, respectively (Axegård, 2011). Additional details can be
found elsewhere (Tomani, 2010).
2.1.2. Preparation of TLF hydrogels50 g of lignin were first dissolved in 200 mL of distilled water
stirred magnetically at 85 ◦C during 1 h. In the meantime, 15 mL of
30 wt.% NaOH aqueous solution were added to this mixture. A dark
homogeneous solution, having a lignin concentration of 20 wt.%
and a pH of 10, was obtained. After cooling at room temperature,
this solution was used as follows in the preparation of each gel.
Various amounts of wattle tannin were dissolved in the solu
tion of lignin for obtaining different tannin/lignin weight ratios
(T/L, on dry basis): 0.11, 0.25, 0.43, 0.67, and 1.0. Next, various
amounts of 37% formaldehyde aqueous solution were added to
these tannin–lignin solutions, in order to obtain different weight
ratios (lignin + tannin)/formaldehyde ((L + T)/F, again on dry basis):
0.83, 1.0, 1.25, 1.7, and 2.5. In all the cases, the amounts were
adjusted for keeping the fraction of solid (i.e. tannin + lignin) always
equal to 26 wt.%. The different amounts of tannin and formalde
hyde slightly modified the pH of the solution before gelling, which
decreased by 1.1 pH unit when T/L increased from 0.11 to 1, and
by 0.3 pH unit when (L + T)/F decreased from 2.5 to 0.83. Thus, the
lowest pH, 8.6, was measured for T/L = 1 and (L + T)/F = 0.83. The
resultant solutions were finally introduced into glass tubes, which
were subsequently sealed and installed vertically in an oven at 85 ◦C
during 5 days.
No formulation was prepared in the absence of tannin, given
that no gelation occurred only from lignin and formaldehyde. Low
amounts of tannin allowed obtaining, in most cases, brown homo
geneous gels whose color depended on both T/L and (L + T)/F ratios.
However, all the tested formulations did not lead to nice gels. Some
of them did not gel at all, whereas others led to precipitates. Distin
guishing precipitates from gels was not always easy, especially in
the case of opaque gels. The materials were assumed to be precip
itates, despite their monolithic character, each time they shrank
significantly, leaving solvent above the solid within the tube in
which they were prepared. Moreover, precipitates were always
extremely brittle, visibly due to their particulate character. Such sit
uation is expected when the polymer nodules are very big, leading
to sedimentation instead of homogeneous gel.
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All the samples were finally soaked in absolute, dry, ethanol for
exchanging water and byproducts possibly present in the porosity
of the organic gels. Ethanol was replaced every day by fresh one,
and solvent exchange was assumed to be complete after 3 days.
2.1.3. Supercritical drying of TLF hydrogelsAfter 3 days of solvent exchange, the samples were
cut into parallelepiped pieces whose dimensions, typically
9 mm × 6 mm × 5 mm, were carefully measured with an electronic
Vernier caliper. Such measurements allowed determining the
shrinkage of the gels after drying. Supercritical drying in CO2 was
carried out in an Autosamdri – 815 automatic critical point dryer
apparatus (Tousimis, USA). For that purpose, the samples were
placed in a stainless steel sample holder and immersed in the dry
ing chamber partially filled with absolute ethanol. The chamber,
whose volume is close to 25 cm3, was next closed and cooled to
−10 ◦C before being filled with pure liquid CO2. During this step,
the pressure in the chamber increased from 0.1 to 4.83 MPa, and
ethanol was replaced by liquid CO2 (purge step) during 5 min. The
apparatus was then stopped for 60 min, in order to let the excess
CO2 diffuse inside the porosity of the gel and replace ethanol
still possibly remaining in the pores. Filling and purging steps
were repeated 3 times, again with 60 min breaks for each, to be
sure that ethanol was completely replaced by liquid CO2. After
these cycles, the temperature inside the chamber was around 0 ◦C
and was increased to 40 ◦C. The pressure consequently rose to
10.2–10.4 MPa, i.e. above the critical point of CO2, and was main
tained for 10 min. Then, the chamber was slowly depressurized at
a controlled rate of 0.45 MPa min−1, using the “bleed” meter valve
of the critical point dryer. A recent study indeed demonstrated
how important is maintaining the depressurizing rate at a given
value in order to obtain both low shrinkages and fully reproducible
aerogels (AmaralLabat et al., 2012a).
The resultant tannin–lignin–formaldehyde aerogel samples
were termed ATLFx/y, where A means aerogel, and x and y are T/L
and (L + T)/F weight ratios, respectively.
2.2. Characterization methods
2.2.1. Density, porosity and pore texture of TLF aerogelsThe bulk density, �b (g cm−3), defined as the mass of mate
rial divided by the total volume it occupies, was measured for
all aerogels using a Geopyc 1360 Envelope Density Analyser
(Micromeritics, USA). The skeletal density, �s (g cm−3), was mea
sured by helium pycnometry (Accupyc II 1340, Micromeritics, USA)
using finely crushed samples for avoiding any erroneous result
related to possibly closed porosity. From bulk and skeletal den
sities, the overall porosity, ˚ (dimensionless), and also the specific
pore volumes, Vp (cm3 g−1), of the materials could be calculated as:
˚ = 1 −�b
�s(1)
Vp =1
�b−
1
�s(2)
The pore texture of TLF aerogels was investigated by analysis
of nitrogen adsorption–desorption isotherms, obtained at −196 ◦C
with an automatic apparatus (ASAP 2020, Micromeritics, USA). The
samples were first degassed at 60 ◦C under vacuum for 24 h. Sur
face areas, SBET, were determined by the BET method (Brunauer
et al., 1938), and the micropore volume (pore size < 2 nm), Vmicro(cm3 g−1), was obtained from the tplot analysis, based on the rep
resentation of the isotherm versus the thickness of the adsorbed
layer on a nonporous reference material (Lippens and de Boer,
1965). The mesopore volume (2 nm < pore size < 50 nm), Vmeso
(cm3 g−1), was calculated as the difference V0.95− Vmicro, where
V0.95 (cm3 g−1) is the volume of liquid nitrogen corresponding to
the amount adsorbed at a relative pressure P/P0 = 0.95 (Gregg and
Sing, 1982). Finally, the poresize distributions were calculated by
application of the DFT model supplied by Micromeritics software.
2.2.2. Morphology and functional groupsSample morphology was examined using a FEI Quanta 600 FEG
scanning electron microscope (SEM). Not all samples were inves
tigated, but only a selection of aerogels having the most different
amounts of tannin and different (L + T)/F mass ratios. The average
sizes of nodules were measured with Image ProPlus 6.0 software.
Chemical differences among organic gels prepared at differ
ent mass ratios of precursors were looked for through the use of
Fouriertransform infrared (FTIR) spectroscopy. FTIR studies were
carried out with an IRAffinity1 spectrometer (Shimadzu, Japan),
using samples (1 mg) ground, dispersed in, and pressed with,
100 mg of dry KBr. The pellets were investigated in transmission
mode from 400 to 4000 cm−1 (20 scans per spectrum at a resolution
of 4 cm−1).
2.2.3. Thermal and mechanical propertiesThermal conductivity measurements were carried out at room
temperature for only two samples combining two interesting
characteristics. They were both belonging to the same series
(L + T)/F = 1.7 but with the most different lignin fractions: T/L = 0.25
and T/L = 1, and had very different BET surface areas (see next sec
tion). Thermal conductivity was measured by the transient plane
source method (Hot Disk TPS 2500, ThermoConcept, France). The
method is based on a transiently heated plane sensor, used both
as a heat source and as a dynamic temperature sensor. It consists
of an electrically conducting pattern in the shape of a double spi
ral, which has been etched out of a thin nickel foil and sandwiched
between two thin sheets of Kapton®. The plane sensor was fitted
between two well fitted cylindrical pieces of TLF sample, each one
with a smooth, flat, surface facing the sensor. The thermal conduc
tivity was then calculated with the Hot Disk 6.1 software.
Once these measurements were done, the same materials were
tested in compression, using an Instron 4206 universal testing
machine equipped with a 2 kN head. The compression was carried
out at a constant load rate of 2.0 mm min−1 during which deforma
tion and load were continuously recorded. Thermal and mechanical
tests were both carried out in the same ambient conditions, 18 ◦C
and 40–45% relative humidity.
3. Results and discussion
3.1. “Phase diagram” of TLF gels
Given that TLF formulations have been prepared with different
T/L and different (L + T)/F weight ratios, careful examination after
5 days spent in a sealed tube at 85 ◦C allowed drawing a kind of
phase diagram. Phase diagrams have been reported in the case of
polyurethane (Biesmans et al., 1998) and resorcinol–formaldehyde
(Brandt and Fricke, 2004) gels, but in a different way, including cat
alyst ratios and % of solids as the variable parameters. In the present
case, only composition parameters have been used at nearly con
stant pH and constant solid fraction, which has never been done so
far.
The phase diagram was obtained as follows. First, wet gels were
removed from their tubes and cut into thin slices (<1 mm), and the
light transmission ability of each was evaluated with naked eye.
Both T/L and (L + T)/F ratios had a strong influence on the results,
as seen in Fig. 3. The samples belonging to the red area, obtained at
low T/L ratio = 0.11, were not gelled but precipitated as hard par
ticles. However, keeping T/L = 0.11 but increasing the (L + T)/F ratio
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350 L.I. Grishechko et al. / Industrial Crops and Products 41 (2013) 347– 355
(L+T)/ F mass rati o
T/ L
ma
ss
rati
o
1,00
0,67
0,43
0,25
0,11
0,83 1 1,25 1,7 2,5
not gelled/p recipitat ion
light brown/opaqu e
Brown/opaque
dark brown/ semitransparent
dark brown or black/ not transpa ren t
black/ transpa rent
Fig. 3. Phase diagram of tannin – lignin – formaldehyde gels.
up to 2.5, opaque, homogeneous, light brown gels were obtained
(yellow area). These results clearly suggest that enough tannin is
required for the occurrence of the gelation process. Samples having
T/L ratios higher than 0.2 were all gelled, and became increasingly
darker when either T/L or (L + T)/F increased. Gels having the highest
T/L and (L + T)/F ratios were even completely black. Concurrently,
the samples were more transparent at high T/L and (L + T)/F ratios,
except for the darkest ones, for which transparency was hardly seen
and would have required extremely thin slices.
Such trends suggest that the gel backbone was more and more
tenuous and based on progressively smaller nodules when the
ratios T/L and (L + T)/F increased. Besides, there should be a slow
transition from pure precipitates, being made of dense and hard,
individual, particles, to pure gels, being lightweight and transpar
ent continuous porous polymer networks. Opaque gels observed at
intermediate values of T/L and (L + T)/F might comprise some big
ger particles embedded in a more continuous environment of less
distinct polymer nodules. These observations seem to be in con
tradiction with the former observations according to which higher
pH leads to smaller nodules (Szczurek et al., 2011b; Pekala and
Schaefer, 1993; Job et al., 2005). However, in the present case,
the range of explored pH is very narrow and beyond the usual
pH at which phenolic gels are prepared, never higher than 8. The
observed trends should thus be rather due to the amount of tannin
than to the pH value.
As developed below, the best gels, being darker and more trans
parent, are those containing more tannin, leading to higher density
but also to higher surface area and mesopore volume.
3.2. Porosity of TLF aerogels
Volume shrinkages of TLF hydrogels submitted to supercritical
drying in CO2 are first given in Table 1. These values, within the
range 26–39%, are rather low since they correspond to linear
shrinkages ranging from 9.5 to 15.2%. For the sake of comparison,
aerogels based on tannin–formaldehyde only (i.e. without lignin)
prepared in the same conditions: 26 wt.% of solid, pH 10, and
also dried with supercritical CO2, had volume shrinkage close to
60%. Given that the supercritical drying was always carried out in
strictly identical conditions, and that the solid content used in all
TLF formulations was constant, different shrinkages should be due
Table 1
Volume shrinkage (%) of tannin–lignin–formaldehyde aerogels. n.g.: not gelled.
T/L mass ratio (L + T)/F mass ratio
0.83 1 1.25 1.7 2.5
0.11 n.g. n.g. n.g. n.g. 31
0.25 27 27 26 28 28
0.43 32 37 28 26 33
0.67 36 34 35 37 39
1 37 39 31 39 37
Table 2
Bulk density (g cm−3) of tannin–lignin–formaldehyde aerogels. n.g.: not gelled.
T/L mass ratio (L + T)/F mass ratio
0.83 1 1.25 1.7 2.5
0.11 n.g. n.g. n.g. n.g. 0.24
0.25 0.19 0.21 0.22 0.25 0.25
0.43 0.21 0.26 0.26 0.26 0.33
0.67 0.29 0.28 0.29 0.33 0.41
1 0.27 0.31 0.39 0.35 0.38
to different mechanical properties of wet gels. Gels based on hard
nodules are expected to shrink a little, whereas gels based on thin
polymer chains are prone to shrink more. On average, such trend
was observed here, initially dark, transparent, gels having higher
shrinkages than light, opaque ones, as already suggested in the
previous section. Therefore, the higher was the amount of tannin in
the initial formulation, the thinner was the gel texture and hence
the higher was the shrinkage. However, volume shrinkage values
were not so different from each other, the highest difference being
less than 15% of the average value.Bulk densities of TLF aerogels
are gathered in Table 2. Their values depend on the composition,
just like for volume shrinkage. Moreover, some correlation may be
evidenced between shrinkage and density, as seen in Fig. 4. This
finding will be further supported by SEM observations (see below),
higher concentrations of tannins leading to more compact aerogels.
The amount of formaldehyde, through the ratio (L + T)/F, also had
an influence on the density, lower amounts leading to higher den
sities. However, the effect was lower than that of T/L ratio. In
0
0.1
0.2
0.3
0.4
0.5
40353025
Volume shrink age (%)
Bu
lk d
en
sity
(g
cm
-3)
Fig. 4. Correlation between volume shrinkage and bulk density of TLF aerogels.
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L.I. Grishechko et al. / Industrial Crops and Products 41 (2013) 347– 355 351
0,250,43
0,67
1
0,83
1
1,25
1,7
2,5
70
75
80
85
90
Po
ros
ity
(%
)
T/L mass ratio
(L+T)/F
mass
ratio
85-90
80-85
75-80
70-75
Fig. 5. Overall porosity of TLF aerogels as a function of their composition.
comparison, the bulk density of aerogels based on
tannin–formaldehyde also prepared with the same solid con
tent (26%), the same pH (10), and dried in supercritical CO2, was
0.44 g cm−1.The skeletal density measured by helium pycnometry
was 1.44 ± 0.03 g cm−3. This value is an average based on several
measurements made from samples belonging to different areas
of Fig. 3. No clear trend could be evidenced as a function of
composition. Overall porosity and specific pore volume were
deduced by application of Eqs. (1) and (2). The corresponding
values of porosity, ranging from 72 to 87%, are given in Fig. 5
as a function of T/L and (L + T)/F weight ratios. The specific pore
volume (not shown) presented the same variations and varied
from 1.7 to 4.6 cm3 g−1. As discussed before for bulk density, the
overall porosity decreased when the amount of tannin increased.
Adsorption and SEM studies, see below, suggest that such decrease
is related to a decrease of macroporosity due to the smaller sizes
of nodules observed at higher T/L and (L + T)/F ratios.
Nitrogen adsorption–desorption isotherms of a selection of TLF
aerogels are presented in Fig. 6. All isotherms were type IV, very
Relative pressure P/P0
Adso
rbed
N2 v
olu
me
(ST
P c
m3 g
-1)
0
100
200
300
400
500
600
700
800
0 0.2 0. 4 0.6 0. 8 1
ALTF0.25 /0.83
ALTF0.43 /0.83
ALTF0.67 /0.83
ALTF1/0.83
Fig. 6. Nitrogen adsorption (empty symbols)–desorption (full symbols) isotherms
at −196 ◦C of some TLF aerogel samples, having different T/L ratios and constant
(L + T)/F = 0.83.
typical of mesoporous solids. A broad hysteresis loop was always
present, which is associated with the secondary process of capillary
condensation and results in the complete filling of the mesopores at
high relative pressure. The hysteresis loop was H1 type, character
istic of solids crossed by channels with uniform sizes and shapes
(Chang et al., 2009). The very low amount of nitrogen adsorbed
at low relative pressure indicates the nearly absence of microp
orosity. At medium relative pressure, the adsorbed amount clearly
increased with the T/L ratio (see Fig. 6). The behavior as a function
of (L + T)/F ratio was more complex, either decreasing continuously
with at higher amounts of formaldehyde, or passing through a max
imum. A provisional explanation of this finding is suggested below.
The corresponding BET surface areas, mesopore and macropore
volumes are plotted in Fig. 7 as a function of both T/L and (L + T)/F
ratios. The macropore volumes were calculated as Vp− V0.95. Micro
pore volumes are not given, since all of them were extremely low,
0.01–0.02 cm3 g−1 on average. As expected from the former discus
sion, the BET surface changed as a function of both T/L and (L + T)/F
ratios, see Fig. 7a. Since the micropore volume is almost negligible,
the surface area is essentially due to mesoporosity, whose behavior
is thus obviously the same, see Fig. 7b. At low T/L ratios, the sur
face area decreased with the amount of formaldehyde. This finding
may be explained by tannin molecules strongly bonded into a less
porous network, or also because of the possible selfpolymerization
of the excess formaldehyde (Brown, 1967). However, at high T/L
ratios, a maximum was observed, suggesting that an optimum
amount of formaldehyde exists, the crosslinking role of formalde
hyde producing either a more tightened polymer network having
thus narrower pores (leading to a higher surface area) or a too
dense, almost nonporous, network.
The macropore volume accounts for the major part of the overall
porosity, and hence the general trends of Figs. 5 and 7c are the same.
In other words, adding more tannin and less formaldehyde to the
initial formulations produced less big pores and more narrow pores.
This feature is evidenced by the poresize distributions presented
in Fig. 8, especially for pores narrower than 30 nm. In TLF aerogels,
the porosity can thus be controlled. Pure tannin gels are expected
to present high mesopore volumes and even probably a non
negligible fraction of micropores, whereas the gradual substitution
of tannin by lignin, introducing bigsized polymer chains, leads
to higher macropore volumes and lower mesopore volumes. This
conclusion was also derived for lignin–resorcinol–formaldehyde
cryogels, in which more lignin led to wider distribution of pore
sizes (Chen and Li, 2010).
The pore texture characteristics given in Fig. 7a and b are typ
ical of those already reported for ligninresorcinolformaldehyde
aerogels (Chen and Li, 2010) and aerogels (Chen et al., 2011). BET
surface areas are, however, slightly lower on average than those
of resorcinol–formaldehyde organic aerogels, which can reach
700 m2 g−1 in suitable conditions of dilution and pH (Job et al.,
2005). Resorcinol molecule is indeed smaller than that of tannin,
the latter being even smaller than lignin. Consequently, replacing
resorcinol by tannin produced a looser packing of polymer parti
cles, leading to more big pores (macropores) and less narrow pores
(mesopores and micropores). Gradual replacement of tannin by
lignin further promoted such phenomenon.
3.3. Chemical and physical structure of TLF aerogels
Although more than 20 samples were prepared, only 5 repre
sentative materials having different T/L and (L + T)/F mass ratios
were analyzed by FTIR spectroscopy. FTIR analysis of raw tannin
and lignin has also been carried out.
First, the effect of T/L ratio is presented in Fig. 9a, using the two
extreme values 0.25 and 1 at constant (L + T)/F = 0.83. In Fig. 9a,
the spectra of pure tannin and lignin are also given for the sake
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352 L.I. Grishechko et al. / Industrial Crops and Products 41 (2013) 347– 355
(b) (a)
(c)
0,25
0,43
0,67
1
0,83
1
1,25
1,7
2,5
50
150
250
350
450
550
SB
ET (
m2/g
)
T/L mass ratio
(L+T)/F mass ratio
450- 550
350- 450
250- 350
150- 250
50-1 50
0,25
0,43
0,67
1
0,83
1
1,25
1,7
2,5
0
0,2
0,4
0,6
0,8
1
1,2
1,4
Vm
eso (
cm
3/g
)
T/L mass ratio(L+T)/F
mass
ratio
1,2-1,4
1-1,2
0,8-1
0,6-0,8
0,4-0,6
0,2-0,4
0,25
0,43
0,67
1
0,83
1
1,25
1,7
2,5
0,5
1,3
2,1
2,9
3,7
4,5
Vm
acro
(c
m3/g
)
T/L mass ratio
(L+T)/F mass ratio
3,7-4,5
2,9-3,7
2,1-2,9
1,3-2,1
0,5-1,3
Fig. 7. Pore texture characteristics of TLF aerogels: (a) BET surface area; (b) mesopore volumes; (c) macropore volumes.
of comparison. As expected, all the spectra were rather similar,
and those of TLF aerogels presented the characteristics of both tan
nin and lignin simultaneously. For instance, the following bands
or shoulders typically belong to lignins: 2920, 2850, from 1670
to 1730 cm−1, 1420 and 1220 cm−1, corresponding to asymmet
ric stretching of CH2, stretching of CH2 from aromatic methoxy
groups, stretching of carbonyls and C O from conjugated ketones,
deformation (shearing) of CH2 and CH3, and stretching of C O in
guaiacyl ring, respectively (Ciolacu et al., 2012; Pandey, 1999; El
Mansouri et al., 2011). Of course, tannin and lignin being aromatic,
phenolic, molecules, their FTIR spectra have a number of peaks in
common, such as those around 1600 and 1500 cm−1, both corre
sponding to deformation of C C in aromatic rings, respectively,
and at 1460 cm−1, originating from asymmetric C H deformation,
both characteristic of tannin and lignin (Hergert, 1971). How
ever, tannin is a much less complex molecule, so its spectra
presented less absorption bands than lignin, and only three had
higher intensity compared to those of lignin, at 1160, 1030 and
850 cm−1, corresponding to asymmetric and symmetric stretching
of C O C, and outofplane deformation of C H in aromatic rings,
respectively.
Fig. 9b presents the FTIR spectra of TLF aerogels prepared at
constant T/L ratio = 1, but different (L + T)/F weight ratios. No clear
difference could be observed, despite the obvious influence of the
amount of formaldehyde on overall porosity, surface area and pore
volumes (see again Figs. 5–7). Lower (L + T)/F ratios, corresponding
to higher amounts of formaldehyde, were expected to lead to higher
crosslinking per unit volume that might have broadened the peaks
at wavenumbers lower than 2000 cm−1. This feature would have
been consistent with the decrease of both BET surface area and
mesopore volume for higher amounts of formaldehyde, but was
not observed. In contrast, the clear increase of macroporosity with
the amount of formaldehyde (see Fig. 7c) could be explained from
SEM micrographs of aerogels having different (L + T)/F ratios.
Photos and scanning electron micrographs presented in Fig. 10
show different sample morphologies, depending on their T/L and
(L + T)/F mass ratios. The increased density of materials prepared
with higher T/L weight ratios (see again Table 2) is clearly seen
in Fig. 10a and b, more compact samples being both smaller and
darker. Besides, a number of spherical pores can be seen all over
the surface of ATLF0.25/1.7, justifying its high macropore volume
(see Fig. 7c). At low magnification, no difference between aerogels
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L.I. Grishechko et al. / Industrial Crops and Products 41 (2013) 347– 355 353
(a)
(b)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1 10 100 1000
T/L = 0.25
T/L = 0.43
T/L = 0.67
T/L = 1
(L+T)/F = 0.83
Pore width (nm)
Dif
fere
nti
al p
ore
volu
me
(cm
3 g
-1)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1 10 100 1000
(L+T)/F = 1
(L+T)/F = 1.25
(L+T)/F = 1.7
(L+T)/F = 2.5
T/L = 0.25
Pore width (nm)
Dif
fere
nti
al p
ore
volu
me
(cm
3 g
-1)
Fig. 8. Poresize distributions of some TLF aerogels having: (a) different T/L ratios
at constant (L + T)/F = 0.83; (b) different (L + T)/F ratios at constant T/L = 0.25.
could be observed by SEM, whatever the composition, so only two
examples were shown here (Fig. 10c and d). All presented flawless,
smooth surfaces, with the typical conchoidal fracture found in most
aerogels. At much higher magnification, the usual nodular struc
ture of phenolic gel was observed. Higher T/L ratios clearly led to
smaller clusters of nodules, whatever the amount of formaldehyde,
as shown in Fig. 10e–h. For example, the mean size of clusters in
ATLF0.25/0.83 (Fig. 10e) and in ATLF1/0.83 (Fig. 10f) was 65–75 nm
and less than 50 nm, respectively. More tannin also led to more
compact structure, which much less wide empty spaces within
the structure, in agreement with the results of Fig. 7c. This finding
also supports the conclusion of Chen and Li (2010) who observed
in resorcinol–lignin–formaldehyde aerogels that more lignin led
to wider pore size distribution. Comparing Fig. 10e and g on one
hand, and Fig. 10f and h on the other hand, it can be seen that
less formaldehyde (i.e. higher (L + T)/F ratios) led to smaller clus
ters: from around 75 to 45 nm for sample for ATLF0.25/0.83and
ATLF0.25/2.5, respectively, and from 50 to 35 nm for ATLF1/0.83 and
ATLF1/2.5, respectively. These conclusions are the same as those
already deduced from the phase diagram given in Fig. 3. Big clus
ters might lead to a loose packing of nodule clusters, and hence to
a higher macroporosity in aerogels characterized by low (L + T)/F
ratios. The same applies to low T/L ratios, more lignin leading to
steric hindrance due to the big size of lignin molecules, and hence
to higher macroporosity.
40080012001600200024002800320036004000
Waven umber (cm-1 )
Tra
nsm
itan
ce
Lignin Tannin ALTF 0.25/0.83 ALTF 1 .0/0.83
ALTF 1.0/0.8 3 ALTF 1.0/1.7 ALTF 1.0/2.5
Tra
nsm
itan
ce
(a)
(b)
40080012001600200024002800320036004000
Wavenumber (cm-1 )
Fig. 9. FTIR spectra of tannin, lignin and some selected TLF aerogels: (a) effect of T/L
ratio, pure tannin and lignin being given as references; (b) effect of (L + T)/F ratio.
3.4. Thermal and mechanical properties of TLF aerogels
Aerogels are known for having low thermal conductivity, and
the insulating performances are even higher for materials pre
senting simultaneously high porosity and narrow pores. These
two features are often contradictory, so two samples from the
same (L + T)/F series were chosen: ATLF0.25/1.7 and ATLF1/1.7.
These materials indeed have very different bulk densities (0.28 and
0.39 cm3 g−1, respectively) and surface areas (169 and 438 m2 g−1,
respectively). Measured thermal conductivities of ATLF0.25/1.7 and
ATLF1/1.7 were 0.041 and 0.045 W m−1 K−1, respectively. Such val
ues are extremely low, given that the corresponding porosities
are not so high for aerogels: 80 and 73%, respectively. These val
ues are indeed within the range of typical commercial insulating
materials having higher porosity, such as expanded polystyrene,
glass wool, and various kinds of insulating polymer foams (Basso
et al., 2011 and refs. therein). However, the thermal performances
of TLF aerogels remain lower than those of other organic aerogels
reported so far, for example based on cellulose (0.029 W m−1 K−1
at 0.25 g cm−3; Fischer et al., 2006), resorcinol (0.012 W m−1 K−1 at
0.157 g cm−3; Lu et al., 1992) or polyurethane (0.017 W m−1 K−1 at
0.26 g cm−3; Biesmans et al., 1998). These differences are explained
by the lower density of such gels, but mainly by their narrower pore
size distribution centered on lower pore diameters. TLF aerogels
consequently require further optimization.
Compression tests of the same samples evidenced two different
rupture behaviors. ATLF0.25/1.7 sample presented a brittle frac
ture and broke into pieces during the test. In contrast ATLF1/1.7,
of higher density, progressively collapsed but retained its mono
lithic character. However, measuring the compressive modulus
from the slope of the initial, linear, part of the compression
curves (not shown) was possible for both samples, and led to 5.89
and 22.86 MPa, respectively. Such values have the same order of
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354 L.I. Grishechko et al. / Industrial Crops and Products 41 (2013) 347– 355
Fig. 10. Pictures of TLF aerogels: (a) ATLF0.25/1.7 (0.25 g cm−3); (b) ATLF1/1.7 (0.35 g cm−3). SEM images of: (c) ATLF1.0/0.83 (50×); (d) ATLF1.0/2.5 (50×); (e) ATLF0.25/0.83
(40 000×); (f) ATLF1.0/0.83 (40 000×); (g) ATLF0.25/2.5 (40 000×); (h) ATLF1.0/2.5 (40 000×).
magnitude as those already reported for other organic aerogels,
e.g. 6–200 MPa for phenol–furfural aerogels having similar densi
ties (Pekala et al., 1995), or 4–40 MPa for resorcinol–formaldehyde
made at different pH and having a density of 0.28 g cm−3 (Pekala
et al., 1990).
4. Conclusion
The present work described organic aerogels based on tannin
and lignin as cheap, phenolic precursors. We have clearly shown
that lignin can replace part of the more expensive tannin, though
the latter itself remains incomparably cheaper than resorcinol.
More lignin produced aerogels having lower density but also lower
surface area and mesopore volume. In other words and all other
things being equal, increasing the amount of lignin increased the
volume of macropores (>50 nm) but decreased that of narrower
pores.
A compromise between density and surface area thus has to be
chosen, depending on the applications. For example, lightweight
aerogels are brittle, whereas more compact ones are much more
resistant. Such aerogels might also be used as adsorbents in the
liquid phase, those of lower density presenting probably higher
adsorption kinetics due to wide pores, but lower sorption capaci
ties due to the lower available inner surface. Such aerogels, natural
up to the 71% level, might thus find applications in biomedical and
environmental applications.
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L.I. Grishechko et al. / Industrial Crops and Products 41 (2013) 347– 355 355
Acknowledgements
The authors gratefully acknowledge the financial support of the
CPER 2007–2013 “Structuration du Pôle de Compétitivité Fibers
Grand’Est” (Competitiveness Fiber Cluster), through local (Conseil
Général des Vosges), regional (Région Lorraine), national (DRRT and
FNADT) and European (FEDER) funds. We also thank the French For
eign Ministry (MAE) and the Région Lorraine, especially one of us
(L.I.G) for her grant through the LorraineRussia ARCUS cooperation
program.
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