Comparative Research about Wheat Straw Lignin from the Black

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Comparative Research about Wheat Straw Lignin from the Black Liquor after Soda-Oxygen and Soda-AQ Pulping: Structural Changes and Pyrolysis Behavior Lilong Zhang, Keli Chen,* and Lincai Peng Faculty of Chemical Engineering, Kunming University of Science and Technology, Yunnan 650500, China ABSTRACT: Structural changes and pyrolysis behavior of lignins derived from black liquor of soda-anthraquinone (SAL) and soda-oxygen pulping (SOL) for wheat straw at similar delignification rates have been investigated. The lignin was first isolated through acid precipitation and then was characterized by various analytical techniques, including elemental analysis, gel permeation chromatography, Fourier transform infrared spectroscopy,13C−1H heteronuclear single quantum correlation spectra analysis, and thermogravimetry mass spectrometry. At similar delignification rates, the pulping process for gaining quality pulp, SOL contained more (β-O-4) structure and hydroxyl and carbonyl groups, which provided promising results for depolymerization and pyrolysis. SOL showed superior pyrolysis properties including higher combustibility gas formation and lower pyrolysis temperature. Small group organic products like acetic acid and furural could be released from SOL at 100 °C lower temperature, while more methane could be produced in a larger temperature range. short while the straw contains many parenchyma cells,9 which result in the poor performance on pulp properties. On the other hand, high content of ash and silicon10 along with parenchyma cells makes the black liquor hard to be extracted and concentrated commonly below 50%.11 Therefore, it is difficult to employ an alkali recovery process in the pulping of straw. Soda-oxygen straw pulping, as an effective and clear pulping method, has some advantages over traditional soda-AQ pulping.12−14 First, straw has a loose tissue structure suitable for cooking with oxygen. The rate of delignification during the soda-oxygen pulping for wheat straw could reach to ca. 70% before 100 °C.13 This superior property could make the cooking temperature of soda-oxygen pulping for wheat straw is usually lower than 125 °C, while the traditional soda-AQ pulping temperature usually exceeded 150 °C.15 Second, because of the synergistic action of alkali and oxygen, sodaoxygen pulping has higher lignin degradation and less carbohydrates loss, in which the brightness and yield of pulp both exceeded 50%.15 Besides, owing to that the silicon is largely kept in the fiber, BL (black liquor) with a fewer silica content displays a lower viscosity which is good for evaporation and combustion.16 However, in contrast with traditional pulp BL, there still is much confusion about the BL from sodaoxygen pulp, which restricted its industrialization process. Although our recent work preliminarily revealed the pyrolysis mechanism of the BL from soda-oxygen pulping at different pulp end points,17 further intensive works are needed including how each composition and structure changes the effect on BL’s pyrolysis behavior. Lignin, as one of the main organic compounds in the BL, accounted for 30−45% of the total solid composition, which

1. INTRODUCTION Wheat straw, as one of the agriculture residues, is abundant in many agricultural countries.1 In China, wheat is the third food crop after rice and maize.2 The annual output of wheat in China is more than 130 million tones, and wheat straw can be produced 100 million tons per year.2 As one kind of sustainable and green biomass resource, wheat straw can be used after being treated. Traditionally treatments like feeding and composting have many disadvantages, such as low conversion, low economic value, and second pollution.3 Modern biomass conversion methods like biogas, bioethanol, and biosolid fuels also can be applied to reuse this bioresource.4 However, low economic efficiency and high cost limit its popularization. Currently, in many developing countries, the waste biomass is mainly treated by land-filling or direct incineration, which could bring serious pollution of the environment,5 which results in waste of biomass resources and air pollution. Only less than 40%6 of the collected straw residues are being used as industrial raw material for producing composite products, pulp and paper, chemicals, etc. Therefore, how to efficiently make use of the wheat straw is of great significance. Pulping and paper-making, as one of the largest consumers of the biomass resources, could convert lignocellulosic biomass into fibers, paper, paper-based materials, and even cellulose, etc. Wheat straw pulp occupied over 20% of whole pulp market in China for decades.7 However, by now, only about 8 million tons of straw are being used for pulping and papermaking industry, which accounts for only 4% of the whole market.6 The inherent disadvantage of the traditional pulping method cannot be neglected. Soda-AQ (anthraquinone) cooking, as a traditional pulp method, is most widely used for pulping of straw. However, this method has been mainly faced with several problems such as low pulp yields, poor wet-end characteristics of pulp, poor paper physical properties, and troublesome environmental pollution.8 They are closely related to the properties of straw. On one hand, the single fiber of straw is © XXXX American Chemical Society

Received: June 23, 2017 Revised: September 1, 2017 Published: September 5, 2017 A

DOI: 10.1021/acs.energyfuels.7b01786 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Cooking Trials effective alkali charge % (g/g raw material)

Tmax/°C

Tmax/min

t (h/time to Tmax)

solid-to-liquor ratio

additives [% (w/w raw material)]

24 18

155 120

120 120

2 2

1:5 1:5

AQ 0.5 MgSO4 0.5

soda-AQ soda-oxygen

Table 2. Pulp Properties soda-AQ soda-oxygen

pulp yield (%/dry material)

reject yield (%/dry material)

Kappa number

viscosity (mL/g)

brightness

50.67 45.49

0.35 3.30

13.40 12.34

693 520

38.8 43.5

contained nearly 60% carbon element.18 There is no doubt that lignin is the biggest source of thermal energy in the recovery furnace. After soda-oxygen pulping, SOL obtained from BL should have different structure modification compared with SAL. The structure characters of the lignin play an important role on the behavior of pyrolysis.19 Therefore, it is of significance to make clear the association between the structure and pyrolysis properties of lignin, which could be also an essential precondition for facilitating alkali recovery of the BL from soda-oxygen pulping. Many researches about lignin structure and its pyrolysis property often focus on lignin obtained from traditional pulping. Ben20 researched the relationship between the structure of pyrolysis oil and the temperature of pyrolysis from softwood kraft lignin. Guo21 proposed the catalytic gasification mechanism of organic bound sodium groups and inorganic sodium salts in the wheat straw soda-AQ pulping BL. Liu22 investigated the relevance between lignin’s chemical structure and pyrolysis behaviors of soda alkali lignin and Alcell organosolv lignin. However, a very few attention has been paid on SOL. As a continuous effort to study the pyrolysis properties of the BL, this paper presents comparative research about wheat straw lignin from the black liquor after soda-oxygen and soda-AQ pulping. The structural characteristics of the two types of lignins, SOL and SAL, were identified by means of elemental analysis, Fourier transform infrared spectroscopy (FTIR), and heteronuclear single quantum correlation (HSQC) spectra analysis. The thermal behaviors of the lignin were examined by thermogravimetry mass spectrometry (TG-MS).

equivalent delignification for different pulping processes to investigate the effect of lignin structure changing on the thermal property at the same delignification rate. As Table 2 shows, soda-AQ pulp has a similar Kappa number with soda-oxygen pulp, which reflects that both of them have a similar delignification rate. However, a different pulping process should bring different lignin structure change, which can also bring different thermal properties, especially pyrolysis behavior. 2.3. Lignin Extracted from Black Liquor. After abundant and effective stirring with a RW 20 digital overhead type mechanical stirrer (IKA, Germany), BLs are mixed well. The BL was first acidified with 12% (w/w) sulfuric acid to pH 8.5 and centrifuged at 8000 r for 20 min to remove SiO2. The pH of the supernatant was adjusted to pH = 2.0 with the addition of 12% (w/w) sulfuric acid, and then the supernatant was centrifuged at 8000 r for 20 min to get crude lignin. The solid precipitate was sufficiently washed and subsequently freezedried as crude lignin. 2.4. Characterization of Lignin. Before characteristic analysis of lignin, the crude lignin was purified and then etherified according to the literature.24 Elements Analysis. The analysis of purified lignin elements was implemented in a Vario-I elemental analyzer and an ICP inductively coupled plasma emission spectrometer. Measurement parameters: oxidation furnace temperature 1150 °C; reduction furnace temperature: 850 °C; carrier gas flow rate of the measuring cell: 90 mL/min; carrier gas flow rate of the reference cell: 20 mL/min; oxygen flow rate: 30−80 mL/min. Molecular Weight Analysis. Gel permeation chromatography (GPC), as one method for molecular weights, was used to determined the molecular weights of the lignins, which comprised a Waters 1525 binary HPLC pump, a Waters 717 plus Autosampler, a Waters 2414 refractive index detector, and a Breeze (V3.3) GPC workstation (Waters, USA). The lignin was dissolved in the tetrahydrofuran (THF). The eluent flow was 1 mL/min, and the system was maintained at 35 °C during the analysis. Fourier-Transform Infrared Spectroscopy (FT-IR) Analysis. The FT-IR spectra of the lignin samples were recorded from a KBr disc containing about 1% finely ground samples on a Tensor 27 Bruker company in the range of 4000−400 cm−1. HSQC Analysis. 2D-NMR spectra were recorded at 25 °C in a Bruker AVANCE 800 MHz spectrometer, equipped with a cryogenically cooled z-gradient triple resonance probe. The conditions used for all acetylated lignin samples were around 60 mg in 0.5 mL of NMR solvent (CDCl3) with the central solvent peaks (δH/δC, 7.26/77.23) as internal reference. The spectral widths were 5000 and 13200 Hz for the 1H and 13C dimensions, respectively. The number of collected complex points was 1024 for the 1H dimension, with a recycle delay of 1 s. The number of transients was 64, and 256 time increments were recorded in the 13C dimension. The data matrices were zero-filled up to 1024 points in the 13C dimension. TG-MS Method. The experiments were done on the Thermo Gravimetric Analyzer (TGA, STA 449 F3 Jupiter, NETESCH) at a heating rate of 20 °C/min within the temperature range from 50 to 800 °C, and high purity nitrogen was used as carrier gas with a flow rate of 20 mL/min. About 30 mg of lignin was put in a ceramic crucible each time. The m/z was carried out from 1 to 150 amu to determine which m/z has to be specified during the TG experiments. The ion curves that were close to the noise level were omitted. Finally,

2. EXPERIMENTAL SECTION 2.1. Cooking Trials. Wheat straw was harvested from Jining, Shandong province of China in the summer of 2014. It was cut down into 3−5 cm length and air-dried for 2 days. Cooking of wheat straw was conducted in a 15 L stainless steel ZQS1-15 laboratory digester (TongDa company, Xianyang, China) with an electric heater unit. In order to meet the demand of the pulping property for papermaking mills and gaining similar Kappa number pulp, two trails’ condition was set according to previous work.23 Two trials were carried out at the same digestion reactor, and the detailed operations are given in Table 1. After cooking, the BL was extruded from the pulp with a centrifugal machine at 5000 r/min for 20 min and stored in a refrigerator. 2.2. Analysis of Pulp Properties. The resultant pulp was thoroughly washed and screened with 90 °C distilled water until more than 95% of the BL in pulp was obtained. Properties of pulp such as Kappa number (Tappi, T236 om-13), viscosity (Tappi, T230 om-13), and brightness (ISO 3688:1999, ISO 2470:1999) were measured by the referenced standard methods. The analyses were conducted with three parallels, and the relative standard deviation was below 5%. Pulp properties are displayed in Table 2. Since soda-oxygen and soda-AQ have different pulping mechanisms, it is hard to compare the delignification process even for the same raw material at the same pulp condition. Therefore, we chose the B


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Energy & Fuels only the intensities of selected ions (m/z = 16, 28, 39, 44, 45, 96, 109, 123, and 138) were monitored with the thermogravimetric parameters.

lignin units, while SOL has 13 lignin units counted as Mn. Meanwhile, as mentioned above regarding pulp property, sodaoxygen pulp has a similar Kappa number with soda-AQ pulp. Because of the synergistic effect between oxygen and alkali, soda-oxygen could delignify wheat straw to almost the same extent at 35 °C lower cooking temperature compared with traditional soda-AQ pulping. Therefore, that SOL has a lower Mw could be largely attributed to the oxidation too. The low Mw and polydispersity could suggest that SOL would has weaker thermostability than SAL. 3.3. Functional Groups of the Lignin Samples. Oxidative degradation not only makes the Mw of lignin decrease but also brings the modification of the functional group. This kind of change of functional group is inseparable with synergy between active oxygen and alkali. The infrared spectra of lignin samples are presented in Figure 1. The typical functional groups and the IR signal with the possible compounds are listed in Table 5.

3. RESULTS AND DISCUSSION 3.1. Element Analysis. Thermal property greatly depends on the composition of the organic element. Carbon element (C) is closely related to the heat value content. As shown in Table 3, C took about 50% of the lignins. Oxygen element (O) Table 3. Analysis of the Lignin Elements organic elements content, wt/% SAL SOL a

C

H

N

S

Oa

51.40 49.22

5.25 5.09

0.74 0.59

3.30 1.17

39.31 43.93

By difference.

was another dominated elemental that accounts for about 40%. As Table 3 shows, SOL has 2.2% lower C content while a 4.6% higher O content than SAL. This difference comes from oxidation that the synergistic action of alkali and oxygen could introduce more oxygenic groups into lignin during the sodaoxygen pulp process. It is interesting to note that the elemental sulfur of the SOL sample was only about 1/3 that of SAL. During the process of the alkali recovery system, sulfur containing groups in the lignin structure may react with oxygen and subsequently be eliminated or substituted during the oxidation process.25 Sulfur could transform into sulfur compounds like SO2,25 which is harmful to air environment; the less sulfur in SOL is undoubtedly helpful to reduce the emission of SO2 from burning of its BL in an alkali recovery system. 3.2. Molecular Weight of Lignin Samples. Through acid addition to adjust the BL pH = 2.0, almost lignin could be precipitated from BL, which could take apart more than 95% of the total lignin in the BL.26 It is clear that the other 5% acid dissolved lignin has weaker thermal property which can lightly affect the whole lignin property. Therefore, acid-precipitation lignin can respective whole dissolved lignin in the BL. Through GPC analysis of SAL and SOL, their weight-average molecular weight (Mw), number-average molecular weight (Mn), and polydispersity (Mw/Mn) are shown in Table 4. The

Figure 1. FTIR spectra of SAL and SOL.

Table 5. Main Functional Groups of Lignin26−29 wavenumbers (cm−1) 3800−3000 2980−2840 1690 1640, 1421

Table 4. Weight-Average (Mw) and Number-Average (Mn) Molecular Weights and Polydispersity (Mw/Mn) of the Lignin Fractions SAL SOL

Mn

Mw

Mz

D (Mw/Mn)

2861 2299

4548 3396

6859 4706

1.62 1.47

1507 1322 1000−1130 1284 1157, 998 1044 896 788−400

molecular weight of SAL (Mn 2861) was apparently higher than that of SOL (Mn 2299). Theoretically, the soda-AQ process has a higher temperature, so more high molecular weight lignin would be dissolved in the BL, which results in deeper delignification. However, like Table 1 showed, soda-oxygen pulp has less residual lignin. This may result from the degradation of lignins after dissolving in BL. Prolonged oxidation reaction of oxygen could made the high Mw molecule fraction become degraded to low molecule weight lignin groups. In comparison with SAL, SOL has lower Mn 2299 g/ mol and a narrower molecular weight distribution (D = 1.47). Since each lignin unit has a C9 formula, SAL average has 16

functional groups

compounds

O-H C-H stretch CO stretch −COO anti- and symmetric stretch aromatic skeleton vibration C-O stretch C-H stretch OH or C-O bending stretch C-O stretch C-O stretch C-C stretch

hydroxyl group alkyl lignin uronic acids lignin syringyl ring (S unit) hemicelluloses guaiacyl ring (G unit) arabinose hydroxyl group β-D-xylose the ring of the sugar and lignin

As shown in Figure 1, since both SOL and SAL belong to acid precipitated wheat straw lignin, there are many similar peaks that occurred in SAL and SOL. From the high frequency region to the low frequency region: the 3400−3500 cm−1 spectrum is attributed to the hydroxyl group (O-H stretch); the 2850−3000 cm−1 spectrum obviously represents −CH3 stretch; the signals of CO stretching vibration in acetyl are located at C


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Figure 2. HSQC NMR spectra of SAL (left) and SOL (right). The main lignin substructures are A, β-O-4′ substructures; B, phenylcoumarane substructures formed by β-5/α-O-5 linkages; C, resinol substructures formed by β-β′/α-O-γ′/γ-O-α′ linkages; G, guaiacyl units; S, syringyl units; and S′, oxidized syringyl units bearing a carbonyl group at Cα.

1732 cm−1; the 1690 cm−1 spectrum is the typical functional group of lignin; 1331 cm−1 is assigned to syringyl ring breathing with CO stretching; 1284 cm−1 is assigned to guaiacyl ring breathing with C-O stretching; and 1039 cm−1 is assigned to aromatic C-H guaiacyl type and C-H deformation of primary alcohol. To investigate the different functional group changes between SOL and SAL, concerns are focused on the changed groups. With oxygen addition in the soda-oxygen pulp process, it has more oxidative effect on the lignin structure. First, the peaks at 1507 cm−1 (aromatic skeletal vibrations) and 1625 cm−1 (guaiacyl ring breathing with C-O stretching) were overlapped by an obviously broadened band −COO anti- and symmetric stretch in the SOL. Second, 1130 cm−1 (aromatic CH guaiacyl type) was absent in the SOL cruve. Last, but not the least, SOL has a weaker peak at 1004 cm−1 (C-O stretching hydroxyl group from hydroxyl group) than SAL. It means that, after soda-oxygen pulping, oxidation could bring the oxidized functional group on the lignin structure. On the other hand, the G unit may be selectively degraded and oxidation could oxidize the hydroxyl group to carbanyl and even carboxyl groups.30 Therefore, on the basis of the above comparisons, we can infer that the soda-oxygen process has a more deconstructive

effect on lignin even under lower temperature. Taking into consideration And’s work that the lignin structure change could affect thermal property,31 we may further come up with one conjecture that those structural changes from oxidative reaction on the lignin could affect the thermal sensitivity. 3.4. Structure Analysis of the Lignin Samples. By means of FT-IR analysis, the functional groups and bonds characterization of wheat straw lignin dissolved in BL could be reflected generally. However, detailed structure information the of C and H could not be presented. 2D HSQC NMR technology, which has high resolution and sensitivity, can solve the overlap problem of one-dimensional carbon and hydrogen spectra and is widely used in analysis of polymer structures such as lignin.32 The interpretation and utilization of 2D spectra of lignin have been described in more detail previously, and the main substructures of the lignin assigned in NMR spectra are given in Figure 2. Here, 2D-NMR was used for analysis of the structures of SAL and SOL. HSQC cross-signals were assigned by comparing with the literature.33−35 The 2D HSQC NMR spectra of wheat lignins mainly showed two regions, δC/δH 50.0−95.0/2.5−5.5 ppm corresponding to the aliphatic region and δC/δH 95.0−150.0/5.5−8.0 ppm attributed to the aromatic region. It was found that all the lignin samples could contain D


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the p-coumaroylated unit in the A structure formed a conjugated structure with relative stability.42 It was, therefore, considered that the A (nonphenolic) structure was stable during the soda-oxygen cooking process. Therefore, β-O-4 (A structure) was present in higher content in SOL. However, on the other hand, SAL has a higher C content than SOL. Hence, it is considered that soda-oxygen synergy has a high reactivity in breaking the β-β structure. Through the above analysis, we found that, although some lignin structures were changed and broken in the cooking process, some parts still retained the benzene ring structures, indicating that SOL present in the BL was less seriously damaged. Moreover, the hydroxyl and carbonyl groups were still present in the lignin, revealing that the lignin obtained from the soda-oxygen was practical and suitable for use in lignin conversion. 3.5. Thermal Characteristics of SAL and SOL. On the basis of structure changes of the two lignins derived from different pulping methods at similar delignification rate, the presence of the oxygen could promote the effects of delignification and structure change. The variability in the structure characteristic indicates thermal property would bring different pyrolysis behavior. TG Analysis. Through the analysis of the TG-DTG curves shown in Figure 3, the pyrolysis characteristic of lignin could be gained. Among them, there are two obvious parts of the weight loss which represent the volatiles released from the pyrolysis process. In the initial pyrolysis stage before 250 °C, the weight loss of ca. 10% is mainly attributed to dehydration and low molecular weight organics releases.43 As shown in the DTG curves, the maximum mass loss rate (3.23 wt %/°C) of SAL was at 229 °C. The thermal decomposition rate of SOL was higher than that of SAL in this stage, and the maximum weight loss rate (3.97 wt %/°C) was attained at 233 °C. This may result from the differences in the inherent structures and chemical components. SOL contains many oxidized groups like carbonyl group as shown in Figures 4 and 5, while SAL is rich in PC ester links. As a result of it, the SOL could pyrolysize easily at the initial stage because the bond energy of the carbonyl group is lower than that of C-C.44 In the second stage, weight loss started when the temperature is raised to 300 °C. Both SAL and SOL achieve the maximum decomposition rate at the 376 °C. With the pyrolysis temperature increasing, the pyrolysis rate slowed down after 450 °C. The loss weight takes up about 30% of the whole process with the ending temperature of 500 °C. However, unlike in the initial stage, the maximum decomposition rate of SAL is lightly higher than SOL’s. This result may be caused by the different structure changes in different pulping conditions. Compared with SAL, SOL is highly enriched in S-lignin units and A (β-O-4) linkages. The content of the reactive chemical structure in the SOL which was oxidized decreased after the initial stage. In the second stage, the fragmentation of lignin and disintegration of aryl−alkyl ether bonds which are connected by C−C bonds was started.45 Therefore, SAL has more structure stable lignin to crack, and it may result in the increasing of the maximum decomposition rate. After 500 °C, the weight of the lignin could not change with the temperature increasing. The residual lignin char includes ash and some fixed carbon composition. More addition of alkali

certain carbohydrate structures due to close-knit bonds between lignins and carbohydrates.36 In the aromatic regions, the syringyl/guaiacyl (S/G) ratio of lignin samples was calculated according to C2,6-H2,6 correlations from the S units and C2-H2 plus C6-H6 correlations from the G units. The syringyl (S) units could be dissolved preferentially because of their lower degradation temperature, and S units can cleave at a much faster rate than the G unit,38 which had been confirmed in Kraft, soda, and soda-AQ pulping.39,40 Theoretically, on an equal delignification rate, the S unit could dissolve into the BL from raw material at a similar amount. However, SOL has a higher content of S units, while more G units are contained in SAL, which made the S/G ratio of SOL higher than that of SAL. It means that the S units were highly reactive in the cooking process with solid alkali and active oxygen. For soda-AQ pulping, because of the higher cooking temperature, G units dissolved into the BL has an advantage than sodaoxygen pulping. As shown in Figure 3, the main interunit linkages were observed in the SAL and SOL. In the aliphatic region, except

Figure 3. TG and DTG curves of SAL and SOL.

for the strong methoxy group signals (δC/δH 56.1/3.77 ppm), the signals of β-O-4 linkages (δC/δH 60.0/3.69 and 60.5/3.83 ppm) were the main interunit linkage, but the signals of β−β′ were weak and those of β-5′ could not even be detected. Usually, the alkyl-aryl ether (β-O-4) structure is the dominant linkage in lignin polymers. During the soda pulping, this kind of linkage is the easiest to be cleaved into soluble fractions.41 As Table 6 shows, SOL has nearly double as much A content as SAL and the former has 5 times higher B content than the latter. As Yang’s previous work reported, while the cooking with solid alkali and active oxygen could not provide a strong alkaline environment, the pH value of the BL was only 8.1 and Table 6. Percentages of the Substructures in the Lignin Samples on the Basis of Contour Integration of the HSQC Spectra37

pCA S G A B C S/G

SAL %

SOL %

5.57 62.75 37.24 27.05 0.94 44.53 1.685

6.49 67.38 32.62 55.18 5.89 30.65 2.066 E


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Figure 4. Evolution of H2O (m/z 18) and of CO2 (m/z 44) for SAL and SOL.

Figure 5. Evolution of acetic acid (m/z = 45) and furfural (m/z = 39/96) for SAL and SOL.

to a low-molecular-weight (LMW) group like some kind of organic acid and carboxyl, and the methoxyl group in the lignin are the primary target functional groups to decompose during the pyrolysis.46 Therefore, even under low temperature heating, such a structure would decarboxylate and lead to the release of volatiles at low temperature. As shown in the DTG curves in Figure 3, the thermal decomposition rate of SOL was higher than that of SAL in the first pyrolysis stage. Therefore, SOL could release more concentrated volatiles releasing from this stage. A small organic group like acetic acid and furfural released from the pyrolysis of the lignin mainly results from the cracking and reforming of the ether groups. Figure 5 shows the change current with temperature of acetic acid and furfural. The m/z = 45 and m/z = 39/96 represent the acetic acid and furfural, respectively. There are many differences between SOL and SAL in small organic production during pyrolysis. First, the half-peak width (HPW) represents the temperature span of the pyrolysis process. In the curve of m/z 45, the peak of SAL occurred at ca. 450 °C with higher HPW. At the same time, SOL’s main peak split into two peaks and one of them move to lower temperature ca. 400 °C. Second, for acetic acid, peak temperature of SOL was ca. 100 °C lower that that from SAL. It may be resulted from more oxidative organic matter which was formed during soda-oxygen pulping. Last, but not least, for m/z = 39 peaks, both SAL and SOL have shoulder peaks. However, SAL has two adjacent peaks range from 500 to 600 °C. Two peaks of SOL one step separated out into independent peaks and one of them moved to lower temperature ca. 400 °C. The result showed that the small organic groups like acetic acid and furfural were released in lower temperature. The advancement of the temperature of

in the soda-AQ pulp should make the yield of char higher, which the residual solid is ca. 43% while the AOL is ca. 38%. Although the TGA cruve could reflect the pyrolysis characteristic of the lignins obtained from different pulping conditions, the precision of the constitution produced during the gasification could not be gained. More information on this information could be obtained from MS analysis of evolved gases. 3.6. MS Analysis. Water and carbon dioxide are main degradation products of biomass,43 about 30 wt % of the mass balance for the pyrolysis of lignin, which also accounts for the most part of oxygen elimination from lignin.45 Among them, water can be released in two stages (ca. 200−300 °C and 300− 500 °C), while the carbon dioxide has one main peak (ca. 400 °C). For water curves, by contrast with SAL, SOL has taller and a more sharp peak range from 300 to 400 °C, which means that water mainly released from SOL has a higher concentration. Water could be released from SOL continuously in high density when the pyrolysis temperature reaches 550 °C, which may be resulted from the degradation of the methoxy group that had been oxidized during pulp with active oxygen. Therefore, SOL has not a small shoulder peak at 550 °C which was overlapped by high concentration water release. For the carbon dioxide peak height signal, SOL also has the higher peak than SAL, which means that SOL releases higher concentration of carbon dioxide in the similar temperature caused by the pyrolysis of the ether linkage. Another cruve m/z 16 signal stands for CH4 has two peaks. SOL has two parallel peaks at 500 and 650 °C. The first peak of SOL slightly moved left from 350 to 400 °C, while the second peak happened in high temperature ca. 650 °C. This might be due to the oxidation of the soda-oxygen pulping for the lignin and this strong oxidation degraded the lignin group F


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Figure 6. Evolution of guaiacol (m/z 109) and methylguaiacol (m/z 123, 138) for SAL and SOL.

was built which could support that soda-oxygen pulp is a potential and effective method for use in lignocellulose biorefineries.

starting thermal degradation may be attributed to its more activated structures in SOL which suffered from the alkalioxygen synergy effect during the pulping process. Through above-mentioned FTIR analysis and HSQC spectra analysis, SOL contains more carboxyl acid groups, S unit, and β-O-4 (A structure) which has higher heat sensitivity. This obvious change could well reflect that structural changes of lignin lead to different pyrolysis behavior. Volatile aromatic compounds like guaiacol (G) and methylguaiacol (MetG) represent the crack of the lignin during the taring process47 and steam gasification.48 As shown in Figure 6, corresponding mass-to-charge ratios are m/z 109 for G, 123 and 138 for MetG, respectively. A similar pattern of the maximum absorption of MS (m/z 109, 123, and 138) was obtained at the third pyrolysis stage. Among them, the curves of the m/z = 109 and m/z = 123 have obviously one main peak that occurred at ca. 600 °C. There is about 50 °C temperature gap between G and MetG, which means that MetG is harder to crack than G. For m/z = 138, the SAL has higher peak at 450 °C compared with SOL, which means that SAL could release more G units during the third pyrolysis stage. This result is well illustrated; the conclusion has been draw in the HSQC spectra analysis that soda-oxygen pulping may selectively degrade the G-unit lignin or transform it to S-unit or p-hydroxyphenyl-type lignin. Through analysis of the dehydration and the decarboxylation of the lignin, the effect of the oxygen addition pulping on the characteristic of gasification is obvious shown: (1) the fragmentation of the SAL is harder than SOL, and (2) after alkaline autoxidation of the phenolic unit with the oxidation, the gasification is easier to release.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Keli Chen: 0000-0001-9287-7632 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (21276119).

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REFERENCES

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4. CONCLUSIONS Compared with soda-AQ pulping, the soda-oxygen process provides a weakly alkaline and lower temperature environment for delignification. At a similar delignification rate, soda-oxygen pulping has a more deconstructive effect on lignin. In SOL, more selective structure changes occurred in phenolic hydroxyl and p-hydroxyphenyl, which could be oxidized into carbonyl groups. At the same time, more β-O-4 structure was retained in SOL. As the result of the structural changes, SOL presented better pyrolysis characteristic that more volatile gas was released at lower temperature than that of SAL. SOL has 100 °C lower release temperature for acetic acid and furural than SAL, and the syngas of the gasification contains a larger amount of volatile organic compounds. Therefore, the relationship between lignin structural characteristics and pyrolysis behavior G


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Energy & Fuels

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Comparative Research about Wheat Straw Lignin from the Black

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