New approach in the application of lignin for the synthesis of hybrid materials

Chemicals and eluents

Methacryloyl chloride (MCl), triethoxyvinylsilane (TEVS), decan-1-ol and bis(2-ethylhexyl)sulfosuccinate sodium salt (DAC, BP) were from Fluka AG (Buchs, Switzerland). Kraft lignin from softwood was obtained from Sigma-Aldrich (Sweden). α,α′-Azoiso-bis-butyronitrile (AIBN) and DVB (62.2% of 1,4-divinylbenzene, 0.2% of 1,2-divinylbenzene, and ethylvinylbenzene were washed with 3% aqueous sodium hydroxide solution before use) were obtained from Merck (Darmstadt, Germany). Acetone, methanol, chlorobenzene, chloroform, hexane, toluene, tetrahydrofuran (THF), acetonitrile and 1,4-dioxane were obtained from POCh (Gliwice, Poland).

Synthesis of methacrylated lignin

In a 500 cm³ round-bottomed flask equipped with a mechanical stirrer, a thermometer and a dropper, lignin (kraft, 19 g) and methylene chloride (200 mL) were placed with 20 mL of triethyleneamine in an ice bath and stirred for 0.5 h. Next, methacryloyl chloride (15 mL) was added dropwise for 1 h at a temperature range of 3–5°C. Subsequently, the flask contents were stirred for 1 h at 5°C and for 1 h at room temperature. After the reaction, the resulting precipitate was filtered off, and magnesium sulfate was added to the filtrate. The obtained lignin methacrylate derivative was extracted with methylene chloride and purified on a chromatographic column.

Copolymerization

The copolymerization of divinylbenzene with triethoxyvinylsilane and methacrylated lignin was performed in the aqueous medium. Seventy-five milliliters of redistilled water and 0.75 g of bis(2-ethylhexyl)sulfosuccinate sodium salt (surfactant) were stirred for 0.5 h at 80°C in a 250 cm³ three-necked flask fitted with a stirrer, a water condenser and a thermometer. Then, the solutions containing DVB, TEVS, different amounts of lignin (0, 1, 1.5, 2, 2.5 g; Table 1), the initiator AIBN (1 wt. %) and the mixture of pore-forming diluents was added while stirring. The reaction mixture was stirred at 350 rpm for 18 h at 80°C. The obtained microspheres were washed with distilled hot water (2 L), filtered off, dried and extracted in a Soxhlet apparatus with boiling acetone. After drying, the microspheres were fractionated with sieves. The applied polymerization conditions yielded microspheres, approximately 80% of which were in the range of 20–50 μm.

Characterization of methods

Attenuated total reflectance (ATR-FTIR) spectra were recorded on a Bruker TENSOR 27 FTIR spectrophotometer (resolution 4 cm⁻¹, 32 scans were accumulated).

¹H NMR spectra were recorded on a Brucker 300 MSL instrument (Brucker, Germany) operating at the ¹H resonance frequency of 300 MHz. Chemical shifts were referenced to deuterated chloroform (CDCl₃), which served as an internal standard. The ¹³C NMR spectrum of methacrylated lignin in chloroform was created using the same apparatus.

Prior to the field emission scanning electron microscopy analysis (FE-SEM), the samples were coated with a 3-nm thick gold layer (Cressington 208HR high-resolution sputter coater). The appearances and morphologies of the microspheres were determined using a Hitachi S-4800 FE-SEM instrument with an accelerated voltage of 1 kV.

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. The specific surface area was calculated according to the Brunauer-Emmett-Teller (BET) method with the assumption that the area occupied by a single nitrogen molecule is 16.2 Ų. The pore volumes and pore size distributions were determined by the Barrett-Joyner-Halenda (BJH) method.

Differential scanning calorimetry (DSC) thermograms were obtained with the use of a DSC Netzsch 204 calorimeter (Netzsch, Günzbung, Germany). All DSC measurements were performed in aluminum pans with pierced lids with a sample mass of ~5–10 mg under a nitrogen atmosphere (30 mL min⁻¹). Dynamic scans were performed at a heating rate of 10 K min⁻¹ in the temperature range of 20–500°C.

Thermogravimetric analysis (TG/DTG) was performed on an STA 449 Jupiter F1, Netzsch (Selb, Germany) under the following operational conditions: heating rate, 10°C min⁻¹; a dynamic atmosphere of helium (50 mL min⁻¹) in the temperature range of 25–600°C; sample mass, approximately 5 mg; sensor thermocouple type, S TG-DSC; and an alumina crucible was used as a reference empty.

The swellability coefficient B was determined by the equilibrium swelling in acetone, methanol, acetonitrile, chloroform, toluene, THF and water using the procedure from Ref. [20], [21].

B is expressed as:

where

V_s is the volume of the copolymer after swelling and

V_d is the volume of the dry copolymer.

Spectroscopic characterization of lignin methacrylic derivatives

Figure 1 presents the chemical structures of the compounds used for the synthesis of the lignin derivatives. Figure 2 illustrates the chemical structures of the monomers that were applied for the reaction of the hybrid microspheres.

Fig. 1:

Scheme of synthesis of methacrylated derivative of lignin.

Fig. 2:

Scheme of copolymerization.

The lignin methacrylic derivatives were prepared by reaction with methacryloyl chloride. The ATR-FTIR spectra of the methacrylated lignin (L-Met) and its precursor kraft lignin (L) are presented in Fig. 3. In the spectrum of the kraft lignin, C–H stretching vibrations of aromatic ring backbone methyl groups are observed at 2935 cm⁻¹. The hydroxyl group (–OH) gives a shape signal at 3350 cm⁻¹. The aromatic C=C signal is observed at 1511 cm⁻¹.

Fig. 3:

ATR-FTIR spectra of lignin and its derivative.

In the L-Met spectrum, a significant reduction of the signal intensity of the hydroxyl group is observed. This observation demonstrates the successful modification with methacrylate groups. Additionally, a new signal corresponding to the carbonyl groups at 1720 cm⁻¹ is clearly visible. The signals at approximately 1120 cm⁻¹ that are visible in both spectra correspond to the aliphatic tertiary carbon connected with hydroxyl groups (–C_tert–OH).

Figure 4a presents the ¹H-NMR spectra of the L-Met. In the spectrum, protons in the methyl groups (–CH₃) produce shape bands at δ=1.97–2.19 ppm, whereas those from the terminal methylene groups (=CH₂) produce bands at δ=5.69 and 6.25 ppm. The band at δ=7.28 ppm corresponds to the phenyl ring. The band at δ=1.14–1.16 ppm corresponds to the thiol groups from the lignin molecule, whereas the aliphatic hydroxyl groups produce a wide peak at approximately 3.90 ppm.

Fig. 4:

¹H (a) and ¹³C (b) NMR spectra of lignin and its derivative.

Figure 4b presents the ¹³C NMR spectra of methacrylated lignin. The spectrum displays a set of three peaks at 76.7, 77.1, and 77.6 ppm that come from the solvent, i.e. chloroform, and can be neglected in the spectrum interpretation. In the spectrum of L-Met, the methyl group carbon absorbs at 18.3 ppm, and the methyl group connected with oxygen (–O–CH₃) absorbs at 40.73 ppm.

The terminal carbon atom of the double bond absorbs at 127.47 ppm, and the more highly substituted one absorbs at 136.21 ppm. The signals at 123.55 and 136.21 ppm are produced by the carbon atoms of the aromatic ring. The signal for the ester carbonyl group appears at 172.77 ppm.

Characterization of the DVB-TEVS microspheres with a lignin methacrylate component

Three monomers were used for the synthesis of the polymer microspheres. Each of the monomers performed a definite function during the formation of the polymer microspheres. DVB is a 4-functional monomer that possesses two pairs of bonds that are capable of polymerization and is responsible for the crosslinking of the obtained product. Crosslinking makes microspheres suitably rigid and resistant to the action of solvents. The application of triethoxyvinylsilane enables the preparation of microspheres of appropriate porosity. This is a very important parameter when the obtained material is to be used as an effective sorbent. The last component is methacrylated lignin. The presence of a group capable of polymerization enables the chemical introduction of lignin into the structure of the polymer being formed. Moreover, the presence of the functional groups (among others, –SH, –OH, and –OCH₃) in its structure modifies the surfaces of the microspheres. Additionally, the application of the biopolymer (which is lignin) causes the prepared copolymer to be more biodegradable compared to generally applied copolymers of styrene with divinylbenzene (ST-DVB).

Morphology of DVB-TREVS-lignin microspheres

Figure 5 presents the actual shapes of the all of the obtained hybrid microspheres. One can see that the copolymers have a tendency to agglomerate. The DVB-TEVS copolymer obtained without the addition of lignin has the smallest particle size, with diameters in the range of 5–40 μm, and irregular shapes. The addition of lignin significantly improved the appearance of the microspheres. Among all of the studied copolymers, the most uniform spherical shape and a more homogeneous size distribution were obtained for the microspheres containing 1.5 g of lignin. The particles have diameters in the range of 20–45 μm.

Fig. 5:

SEM images of the obtained microspheres.

Porous structure

A well-developed surface area and the presence of micro- and meso-pores are very important for effective sorption processes [22]. For this reason, the study of the synthesis of high-porosity material is very important. The characteristic parameters of the porous structures of the studied microspheres are presented in Table 2. The specific surface areas (S_BET) and total pore volumes (V_tot) are in the ranges of 434–522 m²/g and 0.95–1.88 cm³/g, respectively. These materials are mesoporous; the average pore diameter (W) is approximately 100 Å (10 nm). The largest specific areas and pore volumes are observed for the copolymers obtained in synthesis No. 2 (DVB-TEVS-1L). With increasing lignin content, the decreases in S_BET, V_TOT as well as W are noticeable. This observation is probably connected with the blocking of the bead surface by the lignin molecule. Two maxima in the ranges of 34–37 Å and 200–360 Å can be observed for the most probable pore diameter for each material. As presented in Fig. 6, the pore size distribution for DVB-TEVS exhibits a low value for mesopores. The incorporation of lignin into the structure of the microspheres results in an increase of the size of the mesopores; their sizes are strictly connected with the quantity of lignin additives. Larger amounts of lignin cause the narrowing of the pore size distributions, and the maximum of the peaks shift toward smaller pore diameter values.

Fig. 6:

Pore size distribution curves of the synthesized copolymers.

Our previously obtained styrene-divinylbenzene microspheres with lignin and its epoxyacrylate derivatives had S_BET and V_TOT values in the range of approximately 105–166 m²/g and 0.12–0.41 cm³/g, respectively [5]. The new materials synthesized with the addition of TEVS possess almost three-fold higher porous structure parameters. The sorption efficiencies of the new materials should be much higher than those obtained previously.

Thermal properties of copolymers

Figure 7 illustrates the DSC curves for the obtained microspheres. Analysis of the DSC data leads to the conclusion that the synthesized copolymers are characterized by good thermal resistance. On the curves, one endothermic effect (in the range 430–450°C) associated with the maximum degradation of the samples is observed. On all curves, a small exothermic effect at 170°C that is connected with the crosslinking process is also visible. This effect is a quite typical phenomenon for derivatives of 4-functional divinylbenzene [23]. With increase in the amount of lignin in the structure of the microspheres, the faster decomposition takes place. The thermal decomposition for DVB-TEVS sample obtained without the addition of lignin, at the highest temperature (approximately 10°C) is observed.

Fig. 7:

DSC curves of copolymers.

In the thermogravimetric analysis (TG/DTG) of the hybrid microspheres of DVB-TEVS, one can observe a T_initialof approximately 360°C (Table 3 and Figs. 8 and 9). For the lignin-containing microspheres, all of the TG curves have almost the same course; the copolymer decomposition begins at approximately 340°C (T_initial). The thermal degradation process proceeds rapidly in one stage from 350 to 490°C with a T_max(DTG plot) in the range of 443–451°C. The final residue evaluated at 600°C is in the range of 3–16% and increases with the addition of greater amounts of lignin. The obtained microspheres have good thermal stability, and the addition of lignin causes an increase in chars formation.

Fig. 8:

TG curves of copolymers.

Fig. 9:

DTG curves of copolymers.

Swelling studies

The swellability coefficients in THF, acetone, acetonitrile, toluene, chloroform, methanol and water were studied. The results are summarized in Table 4. From these data, one can see that the DVB-TEVS microspheres do not have a tendency to swell (B=0%). In contrast, the largest swellability coefficients in all solvents are exhibited by the obtained copolymer with the greatest share of lignin in its structure (DVB-2.5L). All of the copolymers with lignin reached the greatest swellability coefficient values in tetrahydrofuran and chloroform. In water, the smallest tendencies to swell (B=0–8%) were observed. Microspheres have higher affinities for non-polar (THF, toluene, chloroform) than polar (acetone, acetonitrile, methanol, water) solvents. The swellability coefficient is closely related to the presence and amount of lignin in the structure of the microspheres.