Pyrolysis Properties of Lignins Extracted from Different Biorefinery Processes

Quercus mongolica was subjected to supercritical water hydrolysis and fast pyrolysis processes, to acquire the residual hydrolysis lignin (H lignin) and pyrolytic lignin (P lignin), respectively. H lignin was obtained from the solid (about 10.5 wt%) produced by a supercritical water hydrolysis pilot plant (Jeong et al., 2017; Jang et al., 2018). This solid was freeze-dried and then dissolved in a 0.5 M NaOH solution. Then, a precipitate was obtained by acidifying the solution to pH 2–3 using HCl. Then, the H lignin was collected by filtration with deionized water, after which it was freeze-dried. P lignin was prepared using a cold water separation method, as described in the literature (Scholze and Meier, 2001; Kim et al., 2016; Kim et al., 2011). Pyrolytic lignin, that is, the water-insoluble fraction of the pyrolysis oil, was obtained about 11 wt% from Quercus mongolica pyrolysis oil produced at 550 °C in a fluidized-bed pyrolysis reactor. These lignin samples were dried at 80 °C for 6 h prior to the start of the experiments. Ultimate and proximate analysis of samples of H lignin and P lignin were carried out using elemental analysis (Elementar, varo Macro cube) and thermogravimetric analysis (TGA, Pyris 8000, Perkin Elmer Co.) according to the analysis procedure described in the literature (Lee et al., 2016a). The results of the ultimate and proximate analysis are listed in Table 1. For the characterization of lignin properties such as molecular weight and functional groups, Gel-Permeation Chromatography (GPC, Viscotek, USA) and Fourier-transform Infrared spectroscopy (FT-IR) analysis were also performed. FT-IR analysis was carried out using a Nicolet iS10 (Thermo Fisher Scientific, USA) equipped with an attenuated total reflectance (ATR). A total 32 scans per sample were analyzed at 4cm^-1 spectral resolution, with in the range of 4000-600cm^-1. GPC was used for molecular mass and polydispersity analysis of lignins. Lignin samples were dissolved in tetrahydrofuran (1mg/ml) and the injection volume was 100μl.

A thermogravimetric analysis (TGA, Pyris 8000, Perkin Elmer Co.) was carried out to determine the thermal decomposition characteristics. A lignin sample (about 5 mg) was pyrolyzed using a heat profile with a range of 40–700 °C and a heating rate of 20 °C/min under a helium flow.

Two analytical pyrolysis methods, namely, singleshot (isothermal pyrolysis) and multi-shot (non-isothermal pyrolysis) Py-GC/MS, were conducted using a pyrolyzer (Py-2020iD, Frontier Laboratories Ltd.) and a gas chromatograph combined with a mass spectrometer (GC/MS, 7890A/5975C inert, Agilent Technologies). For each run, 1 mg of lignin was placed in a sample cup. For the non-isothermal pyrolysis of lignin, the sample cup was placed in the furnace and then heated to the pyrolysis end temperature at a heating rate of 20 °C/min under an He atmosphere in each pyrolysis zone, as obtained from the DTG curves. The pyrolysis vapor was transferred to the MS detector via a MicroJet Cryo-Trap (MJT-1030E, Frontier Laboratories Ltd.) and GC separation column (DB-5MS, 30 m length × 0.25 mm inner diameter × 0.25 μm film thickness) under a 50-mL/min helium flow. For the GC separation, the oven temperature was programmed to increase from 40 °C (10 min) to 310 °C (7 min) at a heating rate of 10 °C/min. The column flow was 0.7 mL/min. For the isothermal pyrolysis experiments, the sample cup was dropped into the preheated furnace at 400, 500, and 600 °C, and allowed to pyrolyze for 2 min. After the sample was pyrolyzed, the sample was analyzed under the same conditions as those of the non-isothermal pyrolysis experiments (separation column, oven temperature program). Non-isothermal and isothermal pyrolysis products were identified using the Nist 5^th library.

The GPC analysis results, mass average molecular weight (Mw), number average molecular weight (Mn) and poly dispersity (Mw/Mn), are shown in Table 2. The average molecular weight of H lignin was higher than P lignin, suggesting that higher degree of depolymerization was occurred by pyrolysis than by supercritical water hydrolysis. The molecular weight of H lignin and P lignin showed lower than that of milled wood lignin (Mw: about 6,700-12,000 Mn: about 2,000-5,500) reported previous literature because of depolymerization by pyrolysis and hydrolysis (Kim et al., 2011; Zhang et al., 2017; Wang et al., 2014). The value of poly dispersity, width of the molecular weight distribution, showed that poly dispersity of H lignin was lower than that of P lignin, indicating narrower molecular weight distribution of H lignin (Wang et al., 2014; Seo et al., 2019).

Fig. 1 showed FT-IR spectra of H lignin and P lignin. The similar FT-IR spectra presented due to same raw biomass indicating that extracted lignin structure did not change significantly depending on thermochemical process condition. The broad band at 3600-3000 cm^-1, hydroxyl groups of the aliphatic and phenolics, was presented by both lignin spectra. Phenolic hydroxyl group and C-O deformation in the methoxyl group were presented in the wavelength range 1120-1110 and 1030-1020 cm^-1, respectively. The aromatic ring band (1510-1500 cm^-1) of P lignin showed higher intensity than H lignin. The peaks of P lignin at 1700 cm^-1, unconjugated ketone and carboxyl group stretching, was higher than that of HL because pyrolytic lignin extracted from pyrolysis oil could contain some hemicellulose pyrolysis compounds (Wang et al., 2014; Zhang et al., 2017; Seo et al., 2019; Kim et al., 2011).

Fig. 1. FT-IR spectra of lignins.

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The thermogravimetry (TG) and differential thermogravimetry (DTG) curves for the H lignin and P lignin under an He atmosphere, obtained at a heating rate of 20 °C/min up to a final temperature of 700 °C, are shown in Fig. 2. Compared to the H lignin (45.5 wt.%), the P lignin produced less solid residue (31.4 wt.%) at the final temperature of 700 °C (Table 3). This result was consistent with that of a proximate analysis that found that the P lignin produced more volatile matter and a smaller amount of solids (fixed carbon + ash) than the H lignin (Table 1). Both the H and P lignins exhibited a wide decomposition temperature range because of there being many aromatic rings with various branches, the activity of the chemical bonds, and the functional groups (Wang et al., 2009). The P lignin decomposed with a weight loss at 120–600 °C, while the H lignin decomposed with a weight loss at 200–600 °C. The different thermal decomposition properties, including the decomposition temperature range and amount of solid residue at the final temperature, were derived from differences in the chemical structure and the processes. The P lignin, which has poor thermal stability, started to decompose at a lower temperature than the H lignin due to the different process conditions of fast pyrolysis and supercritical water hydrolysis. Lin et al. (Lin et al., 2015) reported on the pyrolysis of lignin obtained from different isolation processes such as milled Amur linden wood lignin (MWL), enzymatic hydrolysis corn stover lignin (EHL), wheat straw Alkali lignin (AL), and wheat straw Sulfonate lignin (SL). The four kinds of lignin produced different DTG curves including the maximum decomposition temperature and rate as a result of the different structures and compositions caused by the different isolation processes. Wang et al. (Wang et al., 2014) reported on the pyrolysis behavior of pyrolytic lignin and milled wood lignin. Compared to milled wood lignin, pyrolytic lignin decomposed at a lower temperature due to its poor thermal stability, while its char yield was lower at the final temperature. In the DTG curves for the lignins, the distinct weight loss stages obtained by dividing the decomposition temperature range of 120–600 °C are listed in Table 3. From these decomposition stages, multi-shot non-isothermal pyrolysis- GC/MS were used to understand formation of pyrolysis product in respect stages.

Fig. 2. (a) TG and (b) DTG curves of H lignin and P lignin.

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Fig. 3 shows the chromatograms of lignin pyrolysis for each temperature range, based upon a distinct weight loss in thermogravimetry analysis: 200–400 °C (Zone A), 400–500 °C (Zone B), and 500–600 °C (Zone C) for H lignin and 120–250 °C (Zone D), and 250–600 °C (Zone E) for P lignin. Fig. 3 (a), H lignin chromatograms, shows 24 representative pyrolysis peaks. The major large pyrolysis product peaks were dimethoxy phenol, hydroxy-methoxybenzoic acid, and methoxymethyl phenol in Zone A; methoxy benzenediol, methoxy phenol, and dimethoxy phenol in Zone B; and phenol, methyl phenol, and toluene in Zone C. For the P lignin, 21 predominant pyrolysis peaks are presented in Fig. 3 (b). In this case, dimethoxy propenyl phenol, methoxy prophenyl phenol, and hydroxy dimethoxy benzaldehyde were the main pyrolysis products in Zone D, while those in Zone E were hydroxy methoxy benzoic acid, dimethoxy phenol, and methoxy benzenediol. The main pyrolysis peaks of the H lignin and P lignin are listed in Table 4. The identified peaks were categorized into eight groups, namely, acids, ketones, furans, p-hydroxyphenyl type (H-unit), guaiacyl type (G-unit), syringyl type (S-unit), Catechol, and aromatic hydrocarbons (benzene, toluene, naphthalene). The peak areas and peak area % are listed in Table 5. Both the H lignin and P lignin started to decompose at 120 °C. In the case of the P lignin, the pyrolysis products were produced in the first temperature zone (120–250 °C) while for the H lignin, moisture volatilization occurred at 120–200 °C and pyrolysis products began to be produced at 200 °C. The P lignin decomposed and produced pyrolysis products at a lower temperature than the H lignin. This result was consistent with the results of the thermogravimetry analysis. Wang et al. (Wang et al., 2014) reported similar results in that the pyrolytic lignin obtained from lauan pyrolysis oil decomposed at a lower temperature than milledwood lignin isolated from the same lauan. This was caused by the pyrolytic lignin being more sensitive to temperature than the milled-wood lignin. Chang et al. (Chang et al., 2011) also compared pyrolytic lignin with Alkali lignin. Compared to Alkali lignin, pyrolytic lignin decomposed at a lower temperature due to its lower thermal stability.

Fig. 3. Chromatograms of non-isothermal pyrolysis of lignins.

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The major pyrolysis products of H lignin in Zone A were G and S units and a small amount of acids and catechol were also obtained (Table 5). In Zone B for the H lignin, relatively small amounts of acids, H unit, G unit, S unit, and catechol were produced, while a few H unit and aromatic hydrocarbons were obtained in Zone C for the H lignin. In the case of the P lignin, both Zones D and E exhibited similar pyrolysis product trends in that G and S units were mainly produced, while acids, ketones, furans, H unit, catechol, and aromatic hydrocarbons were produced in relatively small amounts. The major pyrolysis products, G and S units, were produced at 200–400 °C for H lignin, and at 120–600 °C for P lignin. Although the P lignin decomposed at a lower temperature, the pyrolysis products were obtained over a wide temperature range of 120–600 °C. In the case of the H lignin, which started to decompose at 200 °C, the decomposition reaction almost ended in Zone A (200– 400 °C). The minor pyrolysis products included ketones and furans that were obtained as a result of the non-isothermal pyrolysis of P lignin. Typically, ketones and furans were produced from the pyrolysis of carbohydrates such as xylan (Lv and Wu, 2012). This result implies that the P lignin obtained from Quercus mongolica pyrolysis bio-oil contained some hemicellulose pyrolysis compounds such as ketones and furans as shown in FT-IR result (Fig. 1). Fortin et al. (Fortin et al., 2015) investigated the structural properties of the pyrolytic lignins obtained from switchgrass fast pyrolysis oil. They reported that the hemicellulose xylan backbone remained intact and connected to the lignin through ferulate ester linkages.

The isothermal pyrolysis of lignins obtained from supercritical water hydrolysis and fast pyrolysis processes was carried out to investigate the distribution of pyrolysis products such as phenolics and aromatic hydrocarbons at different pyrolysis reaction temperatures (400, 500, and 600 °C). Table 6 lists the MS peak areas and peak area % for the isothermal pyrolysis of H and P lignin. The phenolics were divided into p-hydroxyphenyl type (H-unit), guaiacyl type (G-unit), syringyl type (S-unit), and catechol. Among them, guaiacyl and syringyl types, especially hydroxymethoxy benzoic acid and dimethoxy phenol, were the major pyrolysis products for both lignins. The guaiacyl and syringyl types were produced in large amounts at 500 °C while the guaiacyl type were the predominant compound group. As minor pyrolysis products, phydroxyphenyl type, catechol, and aromatic hydrocarbons were also obtained. The amounts of p-hydroxyphenyl type and aromatic hydrocarbons produced increased with the pyrolysis reaction temperature for both lignins. P-hydroxyphenyl type was newly detected at 500 °C in the H lignin, while the aromatic hydrocarbons were newly produced at 500 °C for both lignins. This is because the guaiacyl and syringyl types were converted to p-hydroxyphenyl type, while further reaction led to the production of aromatic hydrocarbons via cracking reactions such as demethoxylation, dealkylation, and dehydroxylation. Kim et al. (Kim et al., 2016) reported on the lignin decomposition mechanism after studying the pyrolysis of pinus radiata. They suggested reaction pathways in which p-hydroxyphenyl type were produced by the demethoxylation and dealkylation reactions of the guaiacyl and syringyl types, with aromatic hydrocarbons being produced from the dehydroxylation of p-hydroxyphenyl type. The production of the p-hydroxyphenyl type and aromatic hydrocarbons was enhanced as the reaction temperature increased. Shen et al. (Shen et al., 2015) reported similar results for the thermochemical conversion of maple, rice straw, and rice husk lignins. The pyrolysis of the three lignin samples mainly produced guaiacyl and syringyl types, while the formation of p-hydroxyphenyl type and aromatic hydrocarbons was promoted as the temperature increased, due to the enhancement of the demethoxylation and dehydroxylation reactions.