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Scripta Materialia 60 (2009) 687–690

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Design of siliceous lignins – Novel organic/inorganichybrid sorbent materials

Galina Telysheva,a,* Tatiana Dizhbite,a Dmitry Evtuguin,b Nina Mironova-Ulmane,c

Galina Lebedeva,a Anna Andersone,a Oskars Bikovens,a

Jelena Chirkovaa and Lubov Belkovaa

aLatvian State Institute of Wood Chemistry, Dzerbenes Street 27, Riga LV-1006, LatviabUniversity of Aveiro, 3810-193, Aveiro, Portugal

cInstitute of Solid State Physics, University of Latvia, Riga LV-1063, Latvia

Received 19 September 2008; revised 23 December 2008; accepted 26 December 2008Available online 13 January 2009

Novel nanoporous lignin(lignocellulose)/SiO2 hybrid materials were designed using the sol–gel process. X-ray photoelectronicspectroscopy, Fourier transform infrared spectroscopy and 29Si and 13C nuclear magnetic resonance spectroscopy were appliedto characterize the hybrids’ surface and bulk structure. Nitrogen and water vapor sorption were used for assessment of porous struc-ture parameters and hydrophobization of matrices, respectively. The efficiency of the hybrid materials obtained was proved byadsorption tests with potential inorganic (Cu2+) and organic (2,4-dichlorophenoxyacetic acid) pollutants.� 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Polymers; Surface modification; Porous materials; Nanomaterials; Silica

The harmful substances that have been intro-duced into the environment in the last decades actunbalance the natural biogeocenotic processes, causingthe natural biological chains to break, with inevitabledamage and related problems, such as contaminationof objects in the ecosystem, soil exhaustion and decreas-ing biodiversity. Lignocellulose and its constituent bio-polymers are among the most abundant biological rawmaterials that are structurally based on self-assem-bling/self-healing nanosystems with a high degree ofmolecular fidelity. Lignocellulosic materials are sorptionactive towards a broad range of compounds consideredas eco-contaminants, including heavy metals, oil prod-ucts, phenols, chlorophenols and detergents. This hasbeen shown by multiple studies [1–3], and lignocellulosicmaterials originating from plant biomass are currentlybeing used to decontaminate environmental objects.

Lignins in situ are multifunctional phenolic polymerscontaining hydroxyl, carboxyl and carbonyl groups [4].Isolated from plant tissue, lignins are produced as by-products/waste in large quantities upon pulp and fuelethanol production. The three-dimensional structure

1359-6462/$ - see front matter � 2009 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2008.12.051

* Corresponding author; e-mail addresses: ligno@edi.lv; lcl@edi.lv

inherent in lignin in situ is also found for lignins isolatedfrom wood by acid hydrolysis (further hydrolysis lig-nins). The porous structure of lignins is not highly devel-oped. However, the presence of significant amounts ofdifferent oxygen-containing groups in a matrix structureof lignin provides the polymer sorption activity. Mecha-nisms of the sorption interaction include physicaladsorption, hydrogen bonding, coordination and cova-lent linking, and acidic–basic interaction. The use oftechnical lignins for the sequestration and precipitationof contaminants has been extensively studied [5–7]. Anincrease in the sorption capacity, thermostability andhydrophobicity of wood polymeric components can beachieved by the design of organic/inorganic hybrid(OIH) materials. This can be exemplified by the en-hanced efficiency of the cellulose/inorganic hybrid sor-bents for the concentration and sequestration of heavymetals and radionuclides from aqueous media [8]. Thedrastic decrease in hydrophilicity and improvement inmechanical strength of cellulose (sulphite pulp) afterits treatment with silica sol has been demonstrated [9].

Using silica as an inorganic component for synthesisof OIHs is particularly attractive [10]. Recently, cellu-lose/silica hybrids were synthesized using tetraethoxysi-lane as the silica precursors [11]. In our previous

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688 G. Telysheva et al. / Scripta Materialia 60 (2009) 687–690

study, we obtained siliceous lignins by using monomericand oligomeric organosilicon compounds as modifiers[12,13]. The products obtained could be considered asOIH materials, where the Si compound guest moleculemodulates both the microsurface and the bulk structureof the lignin host matrix. As the result of Si modifica-tion, the lignin matrix structure becomes cross-linkedby polyfunctional Si compounds. Microregions, where1–2 siloxane clusters are bonded with the lignin phenyl-propane structure unit, appear [14].

The aim of the present research was design and charac-terization of structural features of siliceous lignins andtheir sorption activity towards hazardous organics,including pesticides, and heavy metals. Taking into ac-count that, at present, the avoidance of application of or-ganic solvents in the synthesis of organic/inorganichybrids intended for broad practical application is animportant task [15], a number of siliceous lignins was pre-pared using sol–gel process in aqueous media for ligninsurface modification with Si oligomers. The similarity ofhypothetic structures of lignin and soil humic substancesand the ability of lignin to fulfil all the functions of humicsubstances in the soil allows us to consider the OIH mate-rials obtained as ‘‘Si-containing clay minerals–soil organ-ic matter” biomimetic complexes.

Two hydrolyzed lignins isolated in the laboratory con-ditions from birch and spruce wood (BHL and SHL,respectively), as well as technically hydrolyzed lignin(THL), obtained as a by-product/waste from fuel ethanolproduction from a mixture of hard- and softwood, wereused for this work as organic matrices for the synthesisof siliceous lignins via a sol–gel process (20 �C; atmo-spheric pressure). The microcrystalline cellulose (Avicell)was also used as an organic matrix for preparing OIH aslignins from fuel ethanol production usually containresidual non-hydrolyzed highly ordered cellulose. Afterdrying in vacuum at 50 �C, the powdered lignin and cellu-lose hybrid materials contained 80–90 wt.% of naturalpolymer and 10–20% of SiO2.

In order to characterize the alterations in the lignin sur-face and the bulk matrix structure resulting from interac-tion with silica precursors, the X-ray photoelectronicspectroscopy (XPS), Fourier transform infrared spectros-copy (FTIR) and nuclear magnetic resonance spectros-copy (NMR) methods were applied. In accordance withthe XPS data, lignin modification with Si-containing olig-omers is accompanied by the formation of hydrophobicsilicon-containing nanoclusters not only on the surfacebut also in the matrix bulk structure (Table 1).

The FTIR spectroscopy (using a Spectrum One Per-kin-Elmer spectrometer with a resolution of 4 cm�1, 32scans to compute one spectrum, KBr pellet technique)data indicated the presence of the silica network in lig-nin/silica hybrids. The new bands at about 465, 805,1080 cm�1, assigned to deformation and symmetric and

Table 1. Si content for the THL/silica hybrid material measured by anESCA 3MK II spectrometer (VG Scientific), taking into accountrecommendations for XPS analysis of lignocellulosic samples [16].

Silicon content (at.%)

On the sample surface 6.8 ± 0.2At a depth of 2–4 nm from the sample surface 11.5 ± 0.3

asymmetric stretching vibrations of (Si–O–Si) links,respectively [17–19], appeared in the spectra of the hy-brids. At the same time, the increasing intensity of absor-bance at 950 and 1170 cm�1 as well as at 3574–3479 cm�1

indicated the presence of Si–OH groups [20,21] in consid-erable amounts in the hybrid materials obtained.

In order to confirm the formation of the silica net-work, quantify the silica cross-link density and assessits bounding to lignin, high-resolution solid-state 29Siand 13C cross-polarization/magic angle spinning (CP/MAS) NMR spectroscopy was applied. The hybridmaterials used for this part of the study were BHL/silicaand SHL/silica hybrids, with SiO2 content of 10% and20%. For characterization of Si-modified lignin prod-ucts, the 29Si CP/MAS NMR technique was appliedfor the first time. NMR spectra were registered on aBruker Avance 400 spectrometer. Samples were packedinto a zirconium rotor sealed with Kel-FTM caps andspun at 5 kHz. The accumulation time for a signal wasat least 24 h. For processing of the spectra the Brukercomputer program XWIN-NMR-3.1 was used.

The comparison of 13C NMR data for BHL/silica hy-brids with different SiO2 contents (Fig. 1) showed that,with increasing silica content, the signals of non-pheno-lic structures (range of 54.4, 71.6 and 153 ppm) increase,as does the signal of the OCH3 group, which moves to54.4 ppm. These changes could be due to the participa-tion of a small proportion of Si atoms in the formationof covalent bonds with lignin, which changes the micro-environment of the lignin inherent groups. 29Si NMRspectra provided information about the proportions ofQn species (Qn designates a Si atom bonded to n otherSi atoms via O-bridges) in silica materials. The 29SiNMR spectra of lignin/SiO2 hybrids showed three mainresonance signals. For the SHL/silica and BHL/silica(Fig. 2) hybrids there were two major signals, at �98.6and �108.3 ppm, assigned to tertiary (Q3) and quater-nary (Q4) structures, respectively, and a small signal at�87 ppm, corresponding to secondary (Q2) structures.Q1 signal is absent in the both spectra. These data indi-cated the presence of a silica network composed essen-tially of cyclic units (Q3 structures) connected byoxygen bridges (Q4 structures) [22]. The data obtainedshow that the silica structures in lignin/silica hybrids

Figure 1. Differential solid-state 13C CP/MAS NMR spectrum ofBHL/silica with 20% silica and BHL/silica with 10% silica hybridproducts. Spectra were recorded on a Bruker Avance 400 spectrometerat 100.6 MHz (9.4 T). In all experiments the 1H and 13C 90� pulseswere �4 microseconds, contact time 2 ms, and the delay betweenpulses of 4 s.

Figure 2. 29Si CP/MAS NMR spectrum of BHL/silica hybrid productwith 20% silica. The spectrum was recorded on a Bruker Avance 400spectrometer. The acquisition parameters were as follows: 90� pulsewidth 4 microseconds, contact time 8 ms, pulse delay 60 s.

Table 2. Changes in the hydrophilic properties of the surface of THLand Avicell resulting from modification with Si oligomers.

Sample am (mM g�1) a (sites nm�2)

THL 2.70 ± 0.02 5.4 ± 0.2THL/silica 0.93 ± 0.02 1.2 ± 0.1Avicell 1.93 ± 0.01 4.7 ± 0.2Avicell/silica 1.19 ± 0.01 1.4 ± 0.1

G. Telysheva et al. / Scripta Materialia 60 (2009) 687–690 689

are highly, though not completely, condensed with theformation of Si–O–Si links, while Si–OH groups still re-main (as was also shown by FTIR), presenting anopportunity for further adsorption interactions. Theestimated degree of silica condensation was about80%, and 40–60% of the silica was incorporated intothe lignin materials.

In the micrometer range, the morphology of the sam-ples of the parent polymers and OIHs was studied byscanning electronic microscopy (SEM) using a TES-CAN TS 5136 microscope. Analysis of the micrographs(Fig. 3) showed that silica was deposited on the ligninand cellulose surface as isolated or aggregated nanopar-ticles (200–400 nm in diameter). Simultaneously, someof particles were clustered into large-scale aggregateswhich continuously merged together. The analogous sil-ica deposition was shown in Ref. [23] for cellulose/silicahybrids obtained via the sol–gel method using an organ-ic precursor of silica (tetraethyl orthosilicate).

The analysis of water vapor adsorption–desorptionisotherms (Table 2), obtained using a vacuum-staticmethod at 20 �C on the laboratory designed gravimetricline with a quartz spring, showed significant hydrophob-ization of lignin and cellulose after their transformationinto OIH materials: an approximately twofold decreasein the monolayer capacity (am) and a threefold decreasein the specific amount of water adsorption sites (a) in

Figure 3. SEM micrographs of parent Avicell (to the left) and Avicell/silica hybrid product with 10% silica (to the right).

comparison with parent wood-derived polymers wereobserved. These changes confirmed the strong organic–inorganic phase interaction in the hybrids obtained,obviously by deterioration of the initial hydrogen bondsystem in the polymeric matrices and the formation ofhydrogen bonding with Si–OH groups.

The porous structure of the samples was examined bythe nitrogen gas adsorption–desorption isotherm meth-od. N2 adsorption–desorption isotherms were measuredat�196 �C according to the recommendations of IUPAC[24] on a KELVIN 1200 analyzer. The specific surfacearea (SBET), pore volume and mean pore size of the sam-ples are summarized in Table 3. SBET was calculated byapplying the Brunauer, Emmett and Teller equation, thepore size distribution was calculated by the Barrett, Joy-ner and Halenda method and the microporosity was as-sessed using the t-plot method [25]. The results obtainedshow that the microstructures of lignin and cellulose werechanged radically as the result of silica incorporation: thevalue of SBET increased from 1.5 (cellulose) and 15 m2 g�1

(hydrolysis lignin) up to 420 and 190 m2 g�1, respectively,owing to the formation of small mesopores, with a diam-eter of 1.8–2.5 nm, and the appearance of micropores,which were absent in the parent matrices.

The change in the sorption ability of the hybrid mate-rials towards potential inorganic and organic pollutantswas studied by the sorption of Cu2+ and 2,4-dichloro-phenoxyacetic acid (2,4-D), as representatives of heavymetals and chlorophenolic pesticides, respectively. Theequilibrium data for Cu2+ sorption (batch experiments,20 �C, 0.6–317.7 mg L�1 metal concentrations, the metalcontents being determined by atomic absorption spec-trophotometry) showed that the hybrid materials sorbedmuch more Cu2+ than the parent organic polymers (2–10 times, depending on the matrix structure features).

The sorption capacity of the parent and modifiedpolymeric matrices towards 2,4-D pesticide was studiedin sorption experiments conducted by adding an aliquotof 2,4-D aqueous solution at concentrations which didnot exceed its water solubility: 13.8–221.0 mg L�1 (the2,4-D concentration in supernatant was measured usingultraviolet spectrophotometry at 283 nm). The signifi-cant increase in 2,4-D sorption was observed for sili-

Table 3. Characteristics of porous structure of lignin and cellulose-based products.

Sample Surface area,SBET (m2 g�1)

Pore volume (mm3 g�1)

Total Micropore

THL 15.0 ± 0.5 10.4 ± 0.2 <0.1THL/silica 192.5 ± 0.7 114.0 ± 0.5 13.4 ± 0.2Avicell 1.5 ± 0.2 2.5 ± 0.1 <0.1Avicell/silica 423.0 ± 0.6 196.1 ± 0.8 77.5 ± 0.5

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ceous lignins (Fig. 4). The products obtained by equilib-rium sorption of 2,4-D from its 110.5 mg L�1 aqueoussolution were used for further sorption of the bacteriumBurkholderia cepacia LMKK 491 (2-day cell culture),which is able to degrade 2,4-D. It was found that (Table4), because of the more developed nanoporosity anddiversity of their sorption centers, siliceous lignins canrealize simultaneous sorption of the organic pollutantmolecules and the degrading microorganisms, and canthus activate the process of pollutant biodegradationon interfaces.

A soil incubation experiment on the degradation of14C-ring-labeled 2,4-D in the presence of THL-basedproducts was performed using unplanted peat soil inglass vials with hermetically closed caps. Aliquots ofthe NaOH trapping solution were analyzed for 14CO2

using a Beta Multicounter System RISO GM-25-6.The results of this experiment demonstrated that sili-ceous THL provided rapid complete mineralization of2,4-D, whereas the parent THL accelerated the pesticidemineralization only a little during the first 15 days of theexperiment and did not influence the process further incomparison with the control soil sample. In the presenceof THL/silica hybrid material in the soil, practicallyexhaustive biodegradation of 2,4-D was achieved in 2weeks, and could be ascribed to the activation of theprocess of pollutant biodegradation on interfaces.

In summary, we have obtained nanoporous lignin/sil-ica and cellulose/silica hybrid materials with significantlyhigher specific surface area, pore volume and hydropho-bicity, and more uniform pore size distribution than theirparent polymeric matrices. The formation of an Si net-work composed essentially of cyclic units (Q3 structures)connected by oxygen bridges (Q4 structures) and boundedto lignin was proved by 29Si NMR spectroscopy. The dataobtained by 29Si NMR and FTIR show that the Si–O–Hgroups still remain in lignin/silica hybrids, thus presentingan opportunity for further adsorption interactions. Themodification of lignin was accompanied by the appear-

Figure 4. Isotherms of 2,4-D sorption by parent THL (1) and hybridmaterial THL/silica with 10% silica (2) (batch equilibrium experiment,20 �C).

Table 4. Sorption of 2,4-D and its bacterium degrader (B. cepacia

LMMK 626) by THL products.

Sample Presorbed2,4-D (mg g�1)

Bacteria sorbed on the samplescontaining 2,4-D (cells 106 g�1)

THL 2.8 ± 0.3 7.0 ± 0.1THL/silica 4.9 ± 0.3 14.0 ± 0.1

ance of novel sorption active sites, including hydrophobicSi-containing nanoclusters on the surface and in the bulkmatrix as well. This resulted in increased sequestration ofheavy metals and biodegradation of hazardous organiccontaminants.

This research was supported by Latvian budgetGrant 1564 and by European Commission COST Ac-tion COST-E41.

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