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Holzforschung 2015; 69(6): 769–776
*Corresponding author: Olena Sevastyanova, Department of Fibre and Polymer Technology, KTH The Royal Institute of Technology, SE-10044 Stockholm, Sweden; and Wallenberg Wood Science Center, KTH The Royal Institute of Technology, SE-10044, Stockholm, Sweden, e-mail: [email protected] Podkościelna, Magdalena Sobiesiak and Barbara Gawdzik: Faculty of Chemistry, Department of Polymer Chemistry, Maria Curie-Skłodowska University, pl. M. Curie-Skłodowskiej 5, 20-031 Lublin, PolandYadong Zhao: Department of Fibre and Polymer Technology, KTH The Royal Institute of Technology, SE-10044 Stockholm, Sweden
Beata Podkościelna, Magdalena Sobiesiak, Yadong Zhao, Barbara Gawdzik and Olena Sevastyanova*
Preparation of lignin-containing porous microspheres through the copolymerization of lignin acrylate derivatives with styrene and divinylbenzene
Abstract: A novel method for synthesizing microspheres from lignin or lignin acrylate derivatives through copo-lymerization with styrene (St) and divinylbenzene (DVB) has been developed. The copolymers were obtained by the emulsion-suspension polymerization with a con-stant molar ratio of DVB to St of 1:1 (w/w) and different amounts of lignin or its derivatives. The morphologies of the obtained materials were examined by scanning elec-tron microscopy. Two types of lignin modifications were performed to introduce vinyl groups into the lignin mol-ecules: modification with acrylic acid and modification with epichlorohydrin plus acrylic acid. The course of mod-ification was confirmed by attenuated total reflectance Fourier transform infrared spectroscopy. The thermal stability and degradation behavior of the obtained micro-spheres were investigated by thermogravimetric analysis, and the pore structure was characterized via nitrogen sorption experiments. Owing to the presence of specific functional groups and the well-developed pore struc-ture, the obtained Lignin-St-DVB microspheres may have potential application as specific sorbents for the removal of phenolic pollutants from water, as demonstrated by the solid-phase extraction technique.
Keywords: ATR-FTIR, lignin acrylation, polymeric micro-spheres, porous materials, solid-phase extraction
DOI 10.1515/hf-2014-0265Received September 26, 2014; accepted January 16, 2015; previously published online February 18, 2015
IntroductionLignin is one of the primary components of lignocellulosic materials and the second most abundant biopolymer after cellulose. Various technical lignins are currently available in large quantities as low-value by-products from the pulp and paper industry. However, the structural features of lignin have a high potential for chemical modifications, which can lead to value-added polymeric materials with specific properties (Dournel et al. 1988; Lindberg et al. 1989; Stewart 2008; Hatakeyama and Hatakeyama 2010). The modifications are typically aimed at the derivatiza-tion of phenolic and aliphatic hydroxyl groups (OHphen and OHaliph) situated at the C-α and C-γ positions of the propane side chain (Figure 2a) to obtain more reactive functional groups. The ratio of OHphen to OHaliph varies depending on the origin (hardwood or softwood) of the lignin and on the pulping process (e.g., kraft, alkali, organosolv pulping, etc.). Etherification, esterification, and reaction with isocyanates, silylation, phenolation, and oxidation/reduction are the common approaches for the OH-group modification (Laurichesse and Avérous 2014).
Esterification, as the easiest way for modification, can be performed by means of acidic compounds, acid anhydrides, and chloride acids, with the latter two being the most reactive. Often, the agents for esterification are bi-functional, which results in lignin-based polyester networks. The synthesis of polyesters, epoxy resins and elastomeric materials are performed via lignin esterifica-tion (Kondo et al. 1987; Guo and Gandini 1991; Guo et al. 1992; Hirose et al. 2005; Fang et al. 2011; Sivasankarapillai and McDonald 2011; Luong et al. 2012; Saito et al. 2012; Sivasankarapillai et al. 2012).
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770 B. Podkościelna et al.: Copolymerization of lignin acrylate derivatives
Alternatively, reactive groups can be introduced into the macromolecular structure of lignin allowing for cross-linking reactions with various polymeric systems. The introduction of acrylate functionality is one example. Naveau (1975) acrylated kraft lignin with methacrylic anhydride and methacryloyl chloride followed by copoly-merization with methyl methacrylate. Glasser and Wang (1989) used isocyanatoethyl methacrylate to modify hydroxybutyl lignin, as well as lignin and lignin-like model compounds. The copolymerization characteristics of the acrylated lignins were investigated with styrene and methyl methacrylate.
Styrene-divinylbenzene (St-DVB) is among the first developed and most popular polymeric sorbents. Because of its hydrophobic character, St-DVB interacts with ana-lytes through van der Waals forces and π-electron inter-action of the aromatic ring. To improve the sorption of polar analytes, the specific area of the adsorbent can be enlarged or polar functional groups can be introduced into the copolymer. The latter can be achieved either by copolymerization with at least of one monomer that con-tains polar functional groups or by chemical post-modi-fication of the hydrophobic St-DVB polymer to introduce polar moieties into its structure. Monomers containing various functional groups, such as methyl methacrylate, N-vinylpyrrolidone, acrylonitrile, cyanomethylstyrene, or derivatives of amines or amides, are common agents for the copolymerization with St and DVB. Chemical post-modifications of hydrophobic St-DVB polymers can be performed by introducing sulfonic, acetyl, hydroxyme-thyl, benzoyl, hydroxyl or carboxyl groups (Davankov and Tsyurupa 1990; Gawdzik and Osypiuk 2000).
In the present paper, kraft lignin was activated by modification with acrylic acid (AA) and epichlorohydrin (ECH) plus AA before copolymerization with St and DVB. A novel method for synthesizing porous microspheres via the outlined reactions was tested. The aim was to develop novel sorbents with specific chemical structures and properties of lignins of different origin. The shape, pore structure, and thermal properties of the obtained lignin-containing functionalized microspheres were investigated and the potential of the obtained microspheres were tested as sorbents for phenolic environmental pollutants by solid-phase extraction (SPE).
Materials and methodsThe schematic representation of the major experimental steps is shown in Figure 1. Kraft lignin from softwood was obtained from Sigma-Aldrich (Stockholm, Sweden). DVB, St, AA, decan-1-ol, and
Lignin
Direct modificationwith AA
Modificationwith ECH and AA
L
Synthesis of microspheres: St+DVB+L/LA/LEA
LA LEA
Solid phase extraction (SPE) experiments withphenol and its chlorinated derivatives
Figure 1: Schematic representation of the experimental steps.
L
OH
OH3C H (or lignin)
SH
OHLignin
=
a
L-(Ph-OH) +O
ClNaOH
L-OO
L-OO
+ COOH L-OOH
O
O
L-(Aliph-OH) + COOH L-O
O
LA
LE
LEA
I
II
b
CH2 CH2
CH3O
H2C
+ +
Lor
SH
St DVB LEA
LA or
c
OH
Figure 2: Schematic representation of the a) kraft lignin molecule, b) reaction of lignin directly with acrylic acid (I) and with epichloro-hydrin and acrylic acid (II), and c) copolymerization of lignin and its acryl derivatives (LA and LEA) with St and DVB.
bis(2-ethylhexyl)sulfosuccinate sodium salt (DAC, BP) were obtained from Fluka AG (Buchs, Switzerland). α,α′-Azoiso-bis-butyronitrile (AIBN) were obtained from Merck (Darmstadt, Germany). All of these chemicals were of reagent grade. ECH, sulfuric acid, propan-2-ol, benzene, NaOH, acetone, and hydroquinone were obtained from POCh (Gliwice, Poland). Triethylbenzylammonium chloride (TEBAC) was prepared in the laboratory of the Department of Polymer Chem-istry, UMCS (Lublin, Poland) by reacting benzoyl chloride with tri-ethylamine in a molar ratio of 1:3. The reaction time was 72 h and the product was filtered, washed with benzene, and dried.
Modification of lignin directly with AA (Figure 2b,I): To a 250 ml round-bottom, three-necked flask equipped with a mechanical
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B. Podkościelna et al.: Copolymerization of lignin acrylate derivatives 771
stirrer, thermometer and an azeotropic trap (Dean-Stark apparatus), 20 g of lignin, 75 ml of benzene, 75 ml of AA, 2 ml of sulfuric acid, and 2 g of hydroquinone (polymerization inhibitor) were added, and this mixture was refluxed for 5 h. The modified lignin was isolated by filtration, washed with distilled water (1 l) and acetone (100 ml), and dried.
Modification of lignin with ECH and AA (Figure 2b,II): In the first step, 15 g of lignin, 60 ml of ECH and 45 ml of propan-2-ol were added to a 250 ml round-bottom, three-necked flask equipped with a mechanical stirrer, a thermometer and a dropping funnel, and this mixture was heated at 75°C for 1 h. An aqueous solution of NaOH was added dropwise for 30 min through the dropping funnel. The reaction continued for 1 h at 75°C. The lignin (modified with epoxy groups) was isolated by filtration and then sequentially washed with MeOH, distilled water, and acetone. The product was dried at room temperature. In the second step, 15 g of lignin with epoxy groups, 50 ml of AA, 0.4 g of TEBAC (a catalyst), and 0.005 g of hydroquinone (polymerization inhibitor) were added to a 150 ml round-bottom, two-necked flask equipped with a mechanical stirrer, thermometer and condenser, and this solution was heated at 90–95°C for 5 h. The product was isolated by filtration, washed with distilled water (2 l), and dried.
Synthesis of microspheres (Figure 2c): The copolymerization of St with DVB and lignin was performed in an aqueous medium. Re-distilled water (150 ml) and 0.75 g of DAC,BP (as a surfactant) were stirred for 0.5 h at 80°C in a three-necked flask fitted with a stirrer, a water condenser, and a thermometer. Then, the solutions contain-ing DVB and St in a molar ratio of 1:1 and various amounts of lignin, unmodified or modified (Table 1), were added under stirring together with the initiator AIBN (1%) and a mixture of pore-forming diluents (10 ml of toluene and 10 ml of 1-decanol). The reaction mixture was stirred at 350 rpm for 18 h at 80°C. The microspheres were obtained by filtration, washed with hot distilled water (2 l), and then dried and extracted in a Soxhlet apparatus with boiling acetone (Podkościelna et al. 2012).
Polymer characterization: Hydroxyl and carboxyl groups in the lignin were quantified by phosphorus NMR (31P NMR) analysis with a 90° pulse angle, inverse gated proton decoupling (delay time of 10 s). Prior to analysis, lignin samples were purified by consecutive extrac-tion with toluene and pentane. A 20–25 mg lignin was functionalized
Table 1: Experimental and pore structure parameters of the St-DVB copolymers.
Copolymer
Parameter of synthesis Pore structure
St(g)
DVB(g)
La
(g)LA(g)
LEA(g)
SBET
(m2 g-1)VTOT
(cm3 g-1)W
(nm)
St-DVB 8 10 0 – – 235 0.84 15.6St-DVB-1L 8 10 1 – – 166 0.41 10.9St-DVB-3L 8 10 3 – – 154 0.36 11.0St-DVB-6L 8 10 6 – – 149 0.25 8.0St-DVB-LA 8 10 – 3 – 196 0.66 11.9St-DVB-LEA 8 10 – – 3 105 0.12 13.0
aL, the amount of unmodified lignin in grams (g).
by 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane in a 1/1.6 mixture (v/v) of CDCl3 and pyridine for 2 h at room temperature ( Granata and Argyropolous 1995).
Elemental analysis (C, H, N, and S) was performed in a Flash EA 1112 elemental analyzer (Thermo Finnigan, USA; external service pro-vided by the Elemental Analysis Unit at the Santiago de Compostela University, USC, Santiago de Compostela, Spain). Attenuated total reflectance (ATR) spectra were recorded in a Bruker TENSOR 27 Fou-rier transform infrared (FTIR) spectrophotometer (resolution 4 cm-1; 32 scans were accumulated). Prior to the field-emission SEM analy-sis (FE-SEM; Hitachi S-4800 FE-SEM), the samples were coated with a 3 nm thick gold layer (Cressington 208HR high-resolution sputter coater).
The pore structures of the copolymers were characterized by N2 adsorption at 77 K (ASAP 2405 adsorption analyzer, Micrometrics Inc., USA). Prior to the analysis, the copolymers were degassed at 140°C for 2 h. Specific surface areas were calculated by the BET method, assuming that the area of a single N2 molecule in the adsorbed state is 16.2 Å2. Pore volumes and pore size distributions were determined by the BJH method.
Thermogravimetric, derivative thermogravimetric, and differ-ential scanning calorimetric thermograms were obtained usng STA 449 F1 Jupiter thermal analyzer (Netzsch, Selb, Germany) with Al2O3 crucible with a sample weight of ∼10 mg under He atmosphere (40 ml min-1) and a heating rate 10 K min-1 between 30–800°C.
SPE experiments with phenol and its chlorinated derivatives: Aque-ous solutions of the phenolic compounds were prepared by diluting a standard MeOH solution containing 100 mg l-1 of phenol (P), 2-chlo-rophenol (ChP), 2,4-dichlorophenol (DChP), and 2,4,6-trichlorophe-nol (TChP). The final concentration in the water-diluted solution was 2 mg l-1. The phenols were pre-concentrated by laboratory cartridges filled with 100 mg of the substance. Different volumes of the solu-tions were passed through the cartridge filled with samples (adsor-bent) and connected by PTFE tubing (Chrompack, Middelburg, The Netherlands) to a water pump jet (Figure 3). The vacuum was then maintained for 5 min to dry the sorbent bed. Afterwards, the phenolic compounds were eluted with 2 ml of MeOH for each 100 ml of aque-ous solution. The quantities of the eluted phenolic compounds were determined by HPLC (three independent determinations). The recov-ery calculations were based on the assumption that MeOH completely
Syringe
Porous teflon
Porous teflon
Vacuum
Adsorbent
Figure 3: Schematic representation of the experimental set-up for the SPE method.
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772 B. Podkościelna et al.: Copolymerization of lignin acrylate derivatives
eluted the adsorbed compounds. A recovery decrement below 25% of the maximum value was assumed to be a breakthrough volume for the compound. For more details, see Gawdzik et al. (2005), Sobiesiak and Podkościelna (2010), and Sobiesiak (2011).
Results and discussion
Acrylation of lignin and analytical data
The typical functional groups (OH, OMe, and SH) are illustrated on imaginary phenylpropane units in Figure 2a, whereas the other key reactions performed in this study are illustrated in Figures 2b and 2c. The direct anchoring of the double bonds on lignin by acrylation of the ali-phatic chain is shown in Figure 2b,I as lignin reacts to LA. Because of its high acidity, the OHphen group does not par-ticipate readily in the esterification. In contrast, ECH and AA do react with phenolic groups and the formation of LE and LEA is probable (as presented in Figure 2b,II).
The ATR-FTIR (Figure 3a) and 31P NMR spectra con-firms that the kraft lignin investigated is a typical guaiacyl lignin (G lignin, softwood lignin). For example, the band at 1270 cm-1 (typical for guaiacyl units) is dominant, the band at 1505 cm-1 is larger than that at 1463 cm-1, and two separate bands at 817 and 858 cm-1 (out-of-plane C-H vibra-tion) are visible (Sarkanen and Ludwig 1971; Faix 1991; Lin and Dence 1992).
The kraft lignin in focus contained 2.5% sulfur, as indicated by the elemental analysis (Table 2). In the FTIR spectrum, a very weak band corresponding to the thiol group (-SH) was also perceptible at 2600 cm-1. Based on the 31P NMR analysis, the lignin sample contained 2.4 mmol g-1 OHaliph and 3.4 mmol g-1 OHphen groups (Table 2).
The formation of the aforementioned functional groups can also be seen on the ATR-FTIR spectra (Figure 4a). LA and LEA show strong bands at 1175 cm-1 and 1720 cm-1 (C = O stretch in ester groups), respectively,
Table 2: Amount of functional groups in lignin (based on 31P NMR) and chemical composition of unmodified lignin (L) and of lignin modified with acrylic acid (AC) and epichlorohydrin and acrylic acid (LEA) based on elemental analysis.
Functional groups (mmol g-1) Elements L LA LEA
OHaliph 2.43 C (%) 62.64 59.07 59.81OHphen cond. 1.54 H (%) 5.92 6.44 6.49OHphen guaiacyl 1.91 N (%) 0.58 0.36 0.22OHphen total 3.45 S (%) 2.53 1.17 0.63COOH 0.44 Total (%) 71.67 67.04 67.16
c
70
80
90
10070
80
90
100
St-DVB
St-DVB-LASt-DVB-LEA
60
70
80
90
100
4000 3000 2000 1000
Wavenumber (cm-1)
Tra
nsm
ittan
ce (
%)
a
Lignin (L)LALEA
1720 cm-1
1175 cm-1
4000 3000 2000 1000
Wavenumber (cm-1)
70
80
90
100
Tra
nsm
ittan
ce (
%)
b
4000 3000 2000 1000
Wavenumber (cm-1)
70
80
90
100
Tra
nsm
ittan
ce (
%)
2000 1600 1200 80088
92
96
100
St-DVBSt-DVB-3L
Tra
nsm
ittan
ce (
%)
Wavenumber (cm-1)
Figure 4: ATR-FTIR spectra of a) lignin (L) and modified lignin (LA and LEA), b) St-DVB and St-DVB copolymer with unmodified lignin (St-DVB-L), and c) St-DVB and St-DVB copolymer with modified lignins (St-DVB-LA and St-DVB-LEA).
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B. Podkościelna et al.: Copolymerization of lignin acrylate derivatives 773
whereas the intensities of the other lignin-related signals in the 1300–1000 cm-1 and 1700–1400 cm-1 regions are sig-nificantly reduced. The signal at 1175 cm-1 is weaker for LEA than for LA. Ester C-O-C stretching vibrations in acrylates result in doublets at 1180 and 1260 cm-1, which is more pro-nounced for the LEA sample. A C = C double bond typically results in a moderate band at 1680–1640 cm-1. However, this area overlaps with the strong aromatic skeletal vibra-tions of lignin. An indication of the introduced C = C bond is the increased signal at 820 cm-1, which corresponds to the bending vibrations of the = C–H group.
An intensity decrement of the band at 1086 cm-1 (C-O deformation in secondary alcohols) is visible for both the LEA and LE samples. The other typical band for second-ary alcohols at 1128 cm-1 is reduced for the former and completely disappeared for the latter. The intensity of C-O deformation band of primary alcohols is also reduced for both modified samples (1035 cm-1). The same is true for the broad O-H stretching band (3600–3050 cm-1) of the LEA sample. Expectedly, the modification with LA proceeds through the OHaliph groups, likely through both Cα (sec-ondary) and Cγ (primary) OH groups, whereas modifica-tion with ECH and AA results in the introduction of acrylic groups predominantly into the OHphen groups.
Copolymerization of lignin and its vinyl derivatives with St and DVB
When St (as a monovinyl monomer) is copolymerized with DVB (as multivinyl monomer) by suspension polymeriza-tion in the presence of a pore-forming diluent (toluene and 1-decanol in the present work), macroporous resin beads are produced that have a permanent porosity in the dry state. The commercially available St-DVB products are generally highly cross-linked polymers that possess a well-developed pore structure. In such products, the diluent controls the pore size, pore-size distribution, and total pore volume (Horák and Benés 1996). The molar ratio of St and DVB monomers was 1:1 in the present study. Lignin and its vinyl derivatives (LA, LEA) were copolymer-ized with St and DVB in varying quantities, resulting in porous lignin-containing microspheres Figure 2c.
Figure 4b presents the FTIR spectra of the St-DVB and St-DVB-L copolymers. The spectral range of 2000–800 cm-1 (inset) shows the significant differences, whereas signals resulting from St and DVB are common to both materials. Bands characteristic of aromatic systems are the 1650–1430 cm-1 regions (aromatic skeletal vibrations). Bands resulting from C-H out-of-plane vibrations in aromatic rings and vinyl compounds are between 988–830 cm-1.
Some bands with weak-to-medium intensity appear in the 1290–900 cm-1 region, which are a result of in-plane defor-mational vibrations of C-H bonds.
After introducing lignin as a component of the polymer, the bands corresponding to the hydrocarbon moiety (1600–1400, 1067, 1025, and 900 cm-1) and to the vinyl bonds (1629, 988, and 830 cm-1) of the molecule became stronger, indicating that the lignin partially pre-vented cross-linking of the polymeric network and left some of the vinyl groups unreacted, as illustrated in Figure 4b (inset) for the sample St-DVB-3L (Table 1). The most intense bands of St-DVB-L at 1030 and 1085 cm-1 are attrib-utable to C-O deformations in primary and secondary alco-hols, respectively. The broad O-H band at 3050–3600 cm-1 is no longer observed for the lignin copolymerized with St and DVB, whereas the intensity of the signal from the C-H stretch in methyl and methylene groups increased.
The spectra of the St-DVB, St-DVB-LA, and St-DVB-LEA copolymers are comparable in Figure 4c. Similar to the St-DVB-L copolymer, intensity increments of vinyl bands at 1629, 988, and 830 cm-1 can be observed for LA- and LEA-containing microspheres, indicating that the presence of lignin partially affects the cross-linking reaction between St and DVB. The spectra of St-DVB and St-DVB-LEA are quite similar, with increased signal inten-sities in the regions around 3000 (methyl and methylene groups), 1500–1700 (aromatic skeletal vibrations), and 800–900 cm-1 (C-H out-of-plane deformations). At the same time, bands of numerous functional groups intro-duced by modification make the spectrum of St-DVB-LEA quite complex (Figure 4c). The strongest signals are attrib-utable to the presence of methylene groups, and the sym-metrical and asymmetrical stretching vibrations at 2854 and 2923 cm-1, respectively. Other bands resulting from these groups can be observed at 1455(deformation vibra-tions) and 760 cm-1 (rocking vibrations). Another strong band at 1727 cm-1 is due to the C = O stretching vibrations of acrylate. The spectral range of 1300–1000 cm-1 is common for all structures containing oxygen functional groups. Asymmetrical and symmetrical stretching vibrations of C-O bonds in alkyl-aryl ethers (bond joining LEA with lignin) resulted in a band around 1270–1230 and 1050–1010 cm-1, respectively. For the St-DVB-LEA copolymer, the broad O-H band can be observed at 3600–3100 cm-1.
For chromatographic purposes, sorbent particles should possess a uniform spherical shape because this improves the efficiency of the sorption process owing to the regular flow of the mobile phase, which minimizes the diffusion effects. The actual shapes of the obtained lignin-containing microspheres were observed by FE-SEM (Figure 5), which shows particles 10–30 μm in diameter.
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774 B. Podkościelna et al.: Copolymerization of lignin acrylate derivatives
The St-DVB copolymer formed larger agglomerates. St-DVB-L and lignin acrylate formed well-separated spheri-cal granules. The St-DVB-LEA copolymer contained a significant amount of irregularly shaped particles, which likely formed by unreacted lignin derivatives, along with smaller sized microspheres.
Pore structure and thermal properties
A well-developed surface area and the presence of micro- and mesopores are essential for effective sorption processes (Sobiesiak 2011). The pore structure of St-DVB and its lignin copolymers was analyzed by the N2 adsorption-desorption method. The largest specific areas and pore volumes are observed for the copolymers obtained in synthesis No. 1 (St-DVB) without the addition of lignin (Table 1, pore structure). Compared with the parent St-DVB, copolymers with modi-fied lignins resulted into a decrement of specific surface areas and total pore volumes, most likely because some pores were blocked by lignin derivatives. This assumption is supported by the fact that the mean pore width for St-DVB-L is approximately 11 nm, whereas that for St-DVB copolymers is 15.6 nm. All of the investigated materials are mesoporous with two maximums at 40 Å (4 nm) and at 240 Å (24 nm) (Figure 6). Mesoporous materials are well suited for sorption application in liquid/water systems.
Table 3 summarizes the thermal parameters of the materials in focus. The copolymers St-DVB-LA and St-DVB-LEA began to decompose at almost the same tem-perature at 266°C, which is approximately 14°C lower than that of the unmodified St-DVB. The addition of LA or LEA
St-DVB St-DVB-L St-DVB-LA St-DVB-LEA
Figure 5: SEM images of St-DVB microspheres without lignin and with lignin (St-DVB-L) and lignin derivatives (St-DVB-LA and St-DVB-LEA).
0
0.002
0.004
0.006
0.008
0.010
10 100 1000
Por
e vo
lum
e (c
c/g*
A)
Mean pore diameter (Å)
St-DVB
St-DVB-1L
St-DVB-3L
St-DVB-6L
St-DVB-LA
St-DVB-LEA
Figure 6: Pore size distribution curves for St-DVB copolymers with unmodified lignin (St-DVB-1 L/3 L/6 L) and with acrylated lignins (St-DVB-LA and St-DVB-LEA).
Table 3: Results of thermal analysis obtained for St-DVB and St-DVB copolymers with unmodified (St-DVB-L) and acrylated (St-DVB-LA and St-DVB-LEA) lignin.
Material Tinitial (°C) Tmax. (°C) Residue at 800°C (%)
St-DVB 280 418.6 4.53St-DVB-1L 314 426.0 4.97St-DVB-3L 326 430.5 5.42St-DVB-6L 340 431.0 5.50St-DVB-LA 266 425.3 4.63St-DVB-LEA 267 419.0 10.86
to the St-DVB network caused an enrichment of oxygen-containing functional groups in the polymers, which resulted in decreased stability compared with pure cross-linked St-DVB. The temperature of the maximum rate of
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B. Podkościelna et al.: Copolymerization of lignin acrylate derivatives 775
mass loss (Tmax) and the final residue of St-DVB-LA exhib-ited values similar to those obtained for St-DVB-1L. The thermal behavior of St-DVB-LEA is different in that the Tmax is 419°C, which is very close to the Tmax of St-DVB. However, the final residue exceeded 10%, which is approximately two-fold greater than that for St-DVB.
Solid-phase extraction (SPE)
If the phenol molecule possesses any electron-withdraw-ing substituents, its affinity to the polymer in adsorp-tion experiments is stronger. Accordingly, the order of the recovery curves in Figure 6 is consistent with the amount of chlorine substituents in the test compounds, with trichlorinated phenol (TChP) possessing the highest recovery value. By introducing functional groups to the polymer, its surface becomes more hydrophilic and the role of van der Waals forces and the π-electrons of the aro-matic ring becomes larger in the sorption processes, and thus the sorption efficiency is improved.
Based on the preliminary SPE study with lignin-containing microspheres, the best results were obtained for the St-DVB-LA material, which presented the highest values of recovery (47%) and breakthrough volume (800 ml) (Figure 7c). For St-DVB-6L, the highest recovery also reached 45%, but the breakthrough had already occurred at 600 ml (Figure 7b). Considering the porous structure parameters of these materials (Table 1), the obtained SPE results are promising. For comparison, the recovery values of unmodified St-DVB are also presented (Figure 7a). The values of SBET and VTOT suggest that this material should possess superior sorption ability than its lignin-modified derivatives, but this is not the case. In the investigated range of sample volumes, the recovery values of chlo-rinated phenols (TChP and DChP) for lignin-containing St-DVB copolymers were higher than those for the pure St-DVB, even after exceeding the breakthrough volume. The difference was observed for non-chlorinated lignin, which had better affinity toward pure St-DVB. The recov-ery values obtained for the phenol with one chlorine sub-stituent (ChP) were quite similar for all of the investigated porous materials.
To explain these observations, the mean pore size (W) should be taken into account. In this trial, the St-DVB microspheres possess the widest pores of all of the pre-sented materials. The wider mesopores promote faster mass transport in the porous structure. However, the sorp-tion capacity of this material is not high enough, particu-larly for small molecules. For a more detailed investigation and comparison of the sorptive properties, additional
30a
b
c
25
20
504540353025201510
05
504540353025201510
00 200 400 600
Volume of sample solution (ml)
St-DVB-LA
St-DVB-6L
Rec
over
y (%
)
St-DVB
800 1000
5
15
10
5
0
PH ChP DChP TChP
Figure 7: Recovery curves of phenol (PH) and chlorinated phenol compounds (ChP, DChP and TChP) obtained for a) St-DVB, b) St-DVB-6 L, and c) St-DVB-LA.
optimization of the synthesis should be performed to produce materials with similar pore characteristics.
ConclusionsAcrylic derivatives of lignin were successfully prepared through two methods: reaction with AA (LA) and a two-step reaction with ECH and AA (LEA). The chemical structures of all of the new derivatives were confirmed by ATR-FTIR analysis. Polymeric mesoporous materials in the form of microspheres were prepared from St-DVB and lignin or its acrylate derivatives. The mean pore diameter was in the range of 4–24 nm, whereas the surface area and total pore volume for copolymers containing lignin were in the range of 100–200 m2 g-1 and 0.1–0.6 cm3 g-1, respectively. The addi-tion of unmodified lignin to St-DVB improved its thermal stability and as a consequence, the initial decomposition temperatures of the polymers increased from 280 to 340°C. Owing to the presence of different functional groups in the lignin macromolecule, lignin-containing porous St-DVB microspheres had better sorption properties toward
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776 B. Podkościelna et al.: Copolymerization of lignin acrylate derivatives
chlorinated phenolic compounds compared with the pure St-DVB. Thus, it can be concluded that lignin-containing microspheres could be effective sorbents for environmen-tal pollutants after a further optimization of the process.
Acknowledgments: We would like to thank the following institutions for supporting the conduct of this study: the Knut and Alice Wallenberg foundation in association with the Wallenberg Wood Science Center (WWSC), The Swed-ish Institute (Baltic Sea cooperation program, project 001-3053), and the Cost Action FP1105 WoodCellNet.
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