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Industrial Crops and Products 41 (2013) 347– 355

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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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L.I. Grishechko et al. / Industrial Crops and Products 41 (2013) 347– 355 349

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

Author's personal copy

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.

Author's personal copy

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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