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Fast pyrolysis of Kraft lignins fractionated by ultrafiltration
Accepted Manuscript Title: Fast pyrolysis of Kraft lignins fractionated by ultrafiltration Authors: Lupeng Shao, Xueming Zhang, Fushan Chen, Feng Xu PII: DOI: Reference:
S0165-2370(17)30555-7 https://doi.org/10.1016/j.jaap.2017.11.003 JAAP 4178
To appear in:
J. Anal. Appl. Pyrolysis
Received date: Revised date: Accepted date:
30-6-2017 15-10-2017 2-11-2017
Please cite this article as: Lupeng Shao, Xueming Zhang, Fushan Chen, Feng Xu, Fast pyrolysis of Kraft lignins fractionated by ultrafiltration, Journal of Analytical and Applied Pyrolysis https://doi.org/10.1016/j.jaap.2017.11.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fast Pyrolysis of Kraft Lignins Fractionated by Ultrafiltration Lupeng Shaoa, Xueming Zhanga, Fushan Chenb, Feng Xua,c* a
Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University,
Beijing 100083, China b
College of Marine Science and Biological Engineering, Qingdao University of
Science & Technology, Qingdao 266042, China c
Shandong Key Laboratory of Paper Science & Technology, Qilu University of
Technology, Jinan 250353, China *Corresponding author. E-mail address: [email protected] (F. Xu)
Highlights
Pyrolysis of fractionated lignins by ultrafiltration were carried out by Py-GC/MS.
Molecular weight of lignin affected the relative content of pyrolytic products.
High molecular weight lignin favored the generation of G-type compounds at 500 o
C.
Abstract The pyrolysis behavior of different lignin samples fractionated by ultrafiltration membrane technology was investigated by pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS). Results indicated that the predominant products derived from pyrolysis of lignin fractions with different molecular weights changed in the relative content, but not in the compound species. At 500 oC, high molecular weight lignin favored the generation of guaiacol-type compounds (57.33%), and low
molecular weight lignin produced more syringol-type compounds (34.33%). As temperature increased to 650 oC, methoxy groups in S and G units were easier to cleave from the benzene ring of the high molecular weight lignin leading to an increase in the relative content of phenol-type compounds. High molecular weight lignin was found to produce the highest amounts of phenol-type compounds (46.22%) and aromatic hydrocarbons (17.25%) at 800 oC, while low molecular weight lignin favored the generation of guaiacol-type and syingol-type compounds. The reveal of relevance between specific lignin fractions and pyrolysis behaviour is meaningful for the efficient transformation of lignin to specific aromatic compounds.
Keywords: Lignin; Fractionation; Molecular weight; Pyrolysis 1. Introduction Lignin, a common primary ingredient in biomass, is a natural amorphous three-dimensional polymer consisting of three primary units, cross-linked with each other by ether bonds and carbon-carbon bonds [1]. These three monolignols are known as syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) units (Fig.1), respectively. The structure of lignin varies considerably in different origins and isolation methods, particularly with regard to the type and quantity of linkages. Owing to the inherent heterogeneity and recalcitrance, lignin as a by-product of pulp and paper industry and bioethanol industry is mainly employed for power and heat purpose through combustion [2]. However, in light of the low price of natural gas, the economic benefit of burning lignin has diminished [3,4]. Therefore, new strategies need to be developed to generate value-added products from lignin [5].
The strategies to transform lignin can be broadly classified into acid/base catalyzed depolymerization, pyrolysis, liquid-phase reforming, and biodegradation. Pyrolysis is identified as one of the most promising thermochemical technologies to convert lignin into bio-oil. It represents a straightforward strategy to break down lignin into smaller fragments [6]. Furthermore, due to the significant complexity of the pyrolysis reactions, the various features and yields of the aromatic derivatives attained through lignin pyrolysis are strongly affected by the type of lignin and pyrolysis reaction conditions [7-9]. Zhang et al. investigated the effect of lignin source on pyrolysis products by Py-GC/MS, and concluded that aspen lignin produced significantly more pyrolytic products than prairie cordgrass lignin [10]. Wang et al. performed pyrolysis of four lignin polymers isolated from the same pine wood, and found that alkali lignin and milled wood lignin produced more phenols at 400 oC [11]. Jiang et al. studied the effect of temperature on the composition of lignin pyrolysis products, and found that the maximum yield of phenolic compounds was obtained at 600 oC [12]. In the attempt to improve economic benefit of lignin, challenges, including recalcitrance and heterogeneity, especially the broad distribution of the molecular weight, limit its utilization in high value-added applications. Fractionation has been proposed as an efficient approach to obtain specific lignin fractions with certain molecular weight distribution and tunable chemical functionality [13,14]. To date, three main fractionation approaches have been reported in the literature including organic solvent extraction [15,16], selective acid precipitation at reduced pH values
[17-19], and membrane ultrafiltration technology [20-22]. Tedious extraction steps and high production costs of organic solvent extraction may limit application on a large industrial scale. Colloids formation during acid precipitation may complicate the process of filtration and separation. Membrane ultrafiltration technology allows obtaining lignin fractions with specific molecular weight distribution by free-reagent treatment. The only requirement of the systems is the use of a semipermeable medium and the generation of a hydrostatic pressure difference as the driving force, which consume less energy. Jönsson evaluated the economy of isolation of kraft lignin by ultrafiltration, and found that lignin can be recovered from cooking liquor at a cost of about 60 € per tonne of lignin and from evaporated black liquor by ultrafiltration at 33 € per tonne [23]. The membrane was economic which could be recycled after treatment. Considering these, membrane ultrafiltration technology could be adopted to obtain lignin fractions for further adequate application. Thermal behaviors of lignin from different sources have been investigated widely, but study on thermal conversion of fractionated lignin samples still remain to be explored. The present study explored the pyrolysis behavior of lignin derived from ultrafiltration process. Several lignin fractions were separated from black liquor of poplar kraft pulping by using ultrafiltration membrane technology. Chemical structures of ultrafiltration lignin fractions and unfractionated lignin were explored. Then these lignin fractions were pyrolyzed at different temperatures. GC/MS coupled with the pyrolyzer was employed to characterize the condensable products. Such information is useful for the proper utilization of variant lignin fractions.
2. Materials and methods 2.1. Black liquor obtaining The black liquor of poplar kraft pulping (KP) was obtained from the Key Laboratory of Pulp and Paper Science & Technology of the Ministry of Education of Qilu University of Technology (China). Laboratory-made poplar KP was processed using followed conditions: active alkali (Na2O)-24% on a dry weight basis; sulfidity-25%; liquor to wood ratio-5:1; final temperature-165 oC; warm-up time-60 min; holding time-100 min. After reaction, the separation of liquid and solid fractions was carried out, and the liquor was further filtrated through filter paper for later use. Kraft lignin (KL) was separated from the black liquor by acid precipitation with 2M H2SO4 to pH 2. After the acid addition, the final suspensions were centrifuged to recover lignin. The isolated lignin was washed with acidified water (pH 2.0) and freeze-dried. 2.2. Lignin fractions obtained from ultrafiltration (UF) process The stirred ultrafiltration unit used in this work was supplied by Merck Millipore (Darmstadt, Germany). Three membranes (filter diameter: 76 mm) with cut-offs of 10 kDa, 5 kDa, and 1 kDa were adopted in this experiment. Lignin was sequentially fractionated according to the scheme shown in Fig.2. The filtrated solution was divided into four fractions by ultrafiltration with gradient decreasing membrane cut-off. Firstly, a membrane with 10 kDa was employed, and the retentate was collected and kept as the > 10 kDa liquid fraction. Then the filtrate was subjected to the next cut-off level (5 kDa). This process continued until 1 kDa membrane was
applied. All the retentate and filtrate liquid were collected and defined as the following names: L1 > 10 kDa, 10 kDa > L2 > 5 kDa, 5 kDa > L3 > 1 kDa, L4 < 1 kDa. Then dilute sulphuric acid (2 M) was added dropwise to the obtained liquid fractions respectively until the pH value of the mixed solution was 2.0 at ambient temperature. After the acid addition, the suspensions were centrifuged, and the precipitations were washed with acidified water (pH 2.0) and freeze-dried. The precipitated lignin fractions were defined as F1, F2, F3, and F4 corresponding to those liquid fractions mentioned above. 2.3. Lignin characterization Acid-insoluble lignin, acid-soluble lignin, carbohydrate, and ash contents in the isolated lignin fractions were determined after two-stage acid hydrolysis according to the procedure described in previous studies [15,24]. The molecular weights and polydispersity of lignin fractions were determined by Agilent 1200 gel permeation chromatography with a refraction index detector (RID) [25]. Lignin was dissolved in THF (HPLC grade), and then about 10 ul of solution was injected to the PL-gel Mix-D column (300 mm × 7.5 mm) at a flow rate of 1 ml/min. Standard calibration was performed with polystyrene standards. Before the analysis, lignin fractions were acetylated according to the previous research [26]. Fourier Transform Infrared Spectroscopy (FTIR) spectra of lignin fractions were collected in a VERTEX70 instrument (Bruker, Germany) using KBr pellet technique. The region between 4000 cm-1 and 400 cm-1 with a resolution of 4 cm-1 and 32 scans were recorded.
Thermal stability of lignin was investigated using a TA Q50 thermogravimetric analyzer (TA Instruments, USA). Approximately 10 mg of lignin was heated at a heating rate of 20 oC/min from ambient temperature to 750 oC under a nitrogen flow of 60 ml/min. 2.4. Py-GC/MS Fast pyrolysis experiments were performed by using a CDS 5200 pyrolyzer coupled with GC/MS (Agilent Technologies 7890A/5975) to obtain the distribution of pyrolysis products. Approximately 0.5 mg of lignin was loaded in a quartz tube, which was wrapped by a Pt filament. Lignin was pyrolyzed at 500, 650, and 800 oC respectively for 15s with a heating rate of 10 oC/ms. Pyrolytic volatiles were directly analysed online by GC/MS. Pure helium was used as purge gas with a constant flow rate of 1 ml/min at a 1:50 split ratio. The conditions of GC/MS were set as follows: the injector temperature was maintained at 270 oC; the chromatographic separation was carried out using a Agilent HP-5MS (30 m × 0.25 mm × 0.25 m) capillary column; the oven temperature was initially programmed from 50 oC (maintained for 2 min) to 250 oC (held for 5 min) at a heating rate of 10 oC/min. An ion trap mass spectrometer was used for the compounds detection, with a mass scan range of m/z 45-650 (EI 70 eV). Finally, the pyrolysis products were identified according to commercial NIST14 library and the previous references [27-29].
3. Results and discussion 3.1. UF process Black liquor (BL) was divided into four fractions by ultrafiltration using gradient
decreasing membrane cut-off. It can be observed from Fig. 3(a) that the color of liquor changed progressively from deep to shallow with the decreasing of membrane cut-off. The dark color of the liquor is derived from the chromophoric functional groups present in lignin, including carbonyl groups, carboxylic acids, quinones, which are soluble in alkaline medium [30,31]. As shown in Fig. 3(b), the color of corresponding lignin fractions changed obviously. It might be caused by the different amount of chromophoric functional groups in UF lignin fractions with different molecular weights. It is evident, thus, that the UF process affected the color of liquor and lignin. 3.2. Characterization of lignin fractions 3.2.1. Lignin composition In the present study, for the purpose of obtaining lignin with a more representative whole molecular weight distribution and subsequent potential application in industry, the fractionated lignins were washed three times by acidified water (pH 2.0) without additional treatment. The chemical composition of lignin fractions are shown in Table 1. The total content of AIL and ASL, ash-free lignin, varied from 81.77% to 84.71%. The amount of residual carbohydrates varied between 0 and 8.11% depending on the fractionation process. The carbohydrates might originate from polysaccharides still covalently bound to the lignin as in the Lignin–Carbohydrate Complex (LCC) [32]. The total carbohydrates content in UF lignin fractions from F1 to F4 decreased as the membrane cut-off decreased. The fact confirmed that UF was not only a fractionation process, but also a purification process.
Furthermore, xylose constituted the main hemicellulosic sugar in all obtained lignin fractions. The decrease in xylose content from 5.77% to 0 again proved that UF made the purification of lignin. As can be observed in Table 1, the ash contents were relatively low for all samples, from 0.95% to 1.84%, indicating efficient washing of the samples. 3.2.2. Molecular weights distribution Gel permeation chromatography was carried out to obtain the molecular weight distribution of the lignin fractions. The weight-average (Mw), number-average (Mn) molecular weight and polydispersity (Mw/Mn) of the lignin fractions from UF process and unfiltered lignin are shown in Table 2. As expected, the objective to obtain lignin fractions with varying molecular weight from UF process was achieved. There was a clear decreasing trend of the weight-average (Mw) and number-average (Mn) molecular weight as the membrane cut-off decreased. Lignin molecular weight is closely related to the number of α- and β-aryl ether bonds and C-C bonds among the structural units. The polydispersity of UF lignin fractions decreased when compared with KL, indicating more uniform lignin fraction were obtained by ultrafiltration. As can be seen in Table 2, the yields of F1, F2, F3, and F4 were 54.3%, 25.9%, 15.4%, and 4.4%, respectively, which decreased as the membrane cut-off decreased. The yield of F4 was only 4.4%, which was the lowest among UF lignin fractions. Considering the subsequent potential application in industry, F1, F2, and F3 were adopted for further investigation. 3.2.3. FTIR analysis
The FTIR spectra of lignin fractions are shown in Fig. 4, and the notable band assignments are listed in Table 3. As can be seen from the spectra, the four lignin fractions showed similar spectroscopic patterns [33-35]. A wide absorption band at 3420 cm-1 is assigned to O-H stretching vibration in aromatic and aliphatic OH groups. The bands at 2939 cm-1 and 2840 cm-1 represent the C-H stretching in the methyl and methylene groups, respectively. The band at 1713 cm-1 can be attributed to unconjugated carbonyl stretching vibration in ketone or aldehyde groups. The typical aromatic skeleton vibration occurs at 1605, 1510, and 1425 cm-1. The bands at 1330, 1120 and 835 cm-1 are attributed to syringyl structures in lignin, while the bands at 1265 and 1030 cm-1 are associated with guaiacyl units in lignin. The relative intensities of the peaks related to the –OH stretching vibration (3420 cm-1) in different lignin fractions were calculated considering the signal centered at 1510 cm−1 present in all FTIR spectra (pure aromatic skeletal vibrations in lignin) as invariant band in each spectrum [36]. The relative intensities were 0.76, 0.78, 0.89, and 0.89 for F1, F2, F3, and KL, respectively. Although different lignin fractions showed similar peak form, absorbance intensity varied. The reason was that UF process influenced amount of functional groups. Therefore, more analyses were carried out to identify the structural differences. 3.2.4. Thermogravimetric analysis To investigate the thermal properties of lignin fractions, TGA measurements were performed under nitrogen atmosphere. Thermogravimetric (TG) and differential thermogravimetric (DTG) curves of lignin samples are shown in Fig. 5.
The thermal degradation behavior of four lignin fractions showed the consistency at low temperature (below 170 oC). The initial weight loss stage before 170 oC mainly attributed to the release of water and some organic constituents. However, the thermal behavior of these lignin fractions was slightly different at higher temperature. Decomposition temperature corresponding to the first obvious peak decreased with the decrement of molecular weight of the lignin fractions. The first obvious mass loss peak occurred at 285, 281, and 242 oC for fraction F1, F2, and F3, respectively. The mass loss stage between 230 and 300 oC was possibly caused by the degradation of remaining hemicellulose and cleavage of weaker bonds in lignin structure [37,38]. The maximum value of mass loss rate at around 385 oC for all fractions belonged to lignin degradation. The thermal degradation of lignin occurred in a rather broad temperature range, from 200 oC to 600 oC, because of the complex structure of lignin with phenolic hydroxyl, carbonyl groups, and benzylic hydroxyl [39]. When the temperature was beyond 600 oC, the mass loss was not very evident. At this stage, decomposition and condensation reactions of aromatic rings were predominant during lignin pyrolytic process, and char was the main product [40]. DTG peaks for KL, F1 and F2, occurred at the temperature range from 600 to 700 oC, may be caused by the secondary reaction of lignin. [7, 41] The residue weights of KL, F1, F2, and F3 at 750 o
C were 35.1%, 33.8%, 25.3%, and 31.8%, respectively. The final residue weights of
F1, F2, and F3 thermal degradation were lower than KL. These phenomena may suggest that fractionation process of lignin could affect the thermal decomposition behavior.
3.3. Py-GC/MS analysis The fast pyrolysis of UF lignin fractions were conducted at various temperatures, and the pyrolysis products were identified by GC/MS. When lignin undergoes fast pyrolysis, it produces permanent gases, volatiles, and char. This study only discussed the volatile products. It is known that the chromatographic peak area of a compound is considered linear with its quantity, and peak area% (of total peak area) is linear with its relative content in the detected products. Therefore, for each product, peak area% value can be compared to reveal the changing of its relative content among the detected products [42-44]. Total ion current (TIC) chromatograms of lignin pyrolysis with different temperatures were shown in Fig. S1. The major pyrolysis products and relative contents are listed in Table 4. As can be seen from the data, products of lignin pyrolysis were mainly phenolic compounds and aromatic hydrocarbons. There were no obvious peaks corresponding to small C2-C5 oxygenated products in TIC chromatograms. This may be attributed to the low carbohydrate content. According to the typical functional groups, products were classified into five groups: aromatic hydrocarbons (AH), phenol-type compounds (H), guaiacol-type compounds (G), syringol-type compounds (S), and catechol-type compounds (C) [42]. The distributions of typical aromatic compounds are depicted in Fig. 6. As shown in Fig. 6, phenolic compounds were the dominant products from the four lignin samples at different pyrolysis temperatures. G-type compounds and S-type compounds were the primary products at 500 oC for all four lignin fractions. As reference reported, hardwood lignin has plenty of guaiacyl and syringyl units [6].
G-type and S-type phenols could be produced by the direct breaking of the β-O-4 bond which has the lowest dissociation energy in all types of linkage bonds [45]. The composition of pyrolysis products was closely correlated with the content of G and S units in different lignin fractions. Among them, guaiacol, 4-methylguaiacol, 4-ethylguaiacol, 4-vinylguaiacol, and syringol were in the majority. As temperature increased, the relative content of G-type compounds and S-type compounds decreased, while the relative content of H-type compounds increased. This can be explained by the fact that different linkages break at different temperatures. Most of the ether linkages are readily cleaved to form condensable volatile products at low temperatures. Most methoxy phenols, such as G-type and S-type phenols, are contained in the pyrolysis products due to the fact that the methoxy groups are more resistant than the alkyl-aryl ether linkages during the thermal degradation [6]. S-type phenols only derived from S units in lignin. However, S units could be degraded into G and/or H units due to the demethoxylation reaction at high temperatures [8]. Hence, the formation of syringol-type compounds was inhibited at high pyrolysis temperatures. As for the G-type phenols, they mainly derived from G units within lignin, as well as the demethoxylation of S units at relatively low temperatures. As temperatures increased, the demethoxylation reaction was enhanced, and the selectivity of H-type compounds increased. For all lignins, 3-methoxycatechol was detected in pyrolysis products. The production was enhanced with the increased temperature, probably due to the cracking of ArO-CH3 in the syringol. As shown in Table 4, the predominant pyrolysis compounds from lignin fractions
with different molecular weights changed in the relative amount, but not in the species. These differences are due to the different content ratio of the phenylpropane building blocks in different lignin fractions. The relative content of G-type compounds from F1, F2, F3, and KL pyrolysis at 500 oC were 57.33%, 54.97%, 40.16%, and 38.23%, respectively. The result seemed to follow the trend of decreasing G-type compounds relative content with decreasing lignin molecular weight (from F1 to F3). The relative content of G-type compounds from KL pyrolysis was the lowest among the four lignin fractions. Though molecular weight of KL was situated between F1 and F2, its polydispersity was the highest. This fact suggested that the molecular weight of lignin with different polydispersity could partially influence lignin pyrolysis products. The data showed that high molecular weight lignin was better for formation of G-type compounds (57.33%) at 500 oC, while low molecular weight lignin produced more S-type compounds (34.33%). In addition, syringol was produced in large quantities among the whole pyrolysis products at 500 oC. This finding indicated that syringol was easily produced from lignin regardless of the molecular weight at low temperatures. Syringol is largely employed as a flavouring and natural preservative due to its antioxidant capacity [46]. As temperature increased to 650 oC, the relative content of H-type compounds from F1, F2, F3, and KL pyrolysis were 28.9%, 27.88%, 17.2%, and 18.63%, respectively. All the H-type monomers increased obviously. The values of G-type compounds were 42.24%, 33.05%, 35.41%, and 36.76%, respectively. Among them, relative content of guaiacol and 4-methylguaiacol derived from UF lignin fractions
decreased, while values of that derived from KL varied slightly. There was no 4-ethylguaiacol generated at 650 oC. This phenomenon suggested that ethyl groups linked in benzene ring were easier cleaved with increasing temperature. The relative contents of S-type compounds were 11.3%, 17.94%, 23.32%, and 25.09%, respectively. Compared with the values obtained from 500 oC to 650 oC, the increment of H-type compounds derived from high molecular weight lignin was higher than that obtained from low molecular weight lignin. However, the decrement of S-type compounds derived from high molecular weight lignin was higher than that obtained from low molecular weight lignin. It is reasonable to suppose that the methoxy group in S units is easier cleaved from the benzene ring of the high molecular weight lignin fraction than that of the low molecular weight lignin fraction. As temperature increased, the high molecular weight lignin fraction favoured the generation of more H-type compounds. As can be seen in Fig. 6, there were almost no aromatic hydrocarbons at relatively low pyrolysis temperatures. As the pyrolysis temperature increased to 800 o
C, the aromatic hydrocarbons appeared, and the relative content from F1, F2, F3, and
KL pyrolysis were 17.25%, 9.74%, 8.96%, and 12.87%, respectively. Among them, toluene that could be produced from either block linkage cleavages of H units or aromatic demethoxylation and dehydroxylation showed the highest relative content [7]. This trend may be caused by the secondary decomposition took place at high temperature, and indicated that the elimination of phenolic hydroxyl groups from benzene ring was promoted by increased temperature. Demethoxylation reaction was
also enhanced at high pyrolysis temperatures [47]. As a result, relative contents of G-type and S-type compounds decreased, and H-type compounds increased. Especially, relative content of 4-vinylguaiacol derived from F2 and F3 were 11.47% and 10.39%. However, there was no 4-vinylguaiacol among pyrolysis products derived from F1 and KL. This suggested that the vinyl group was more cracked in the high molecular weight fraction than in the other fractions at higher temperature. Furthermore, aromatic hydrocarbons and H-type compounds from F1 pyrolysis were produced in maximum values among the four lignin fractions at 800 oC. F2 and F3 lignin fractions produced more G-type and S-type compounds. This phenomenon suggested that molecular weight of lignin had an obvious effect on the relative content of pyrolysis products at higher temperature. Results indicated that high molecular weight lignin fraction was better for formation of H-type compounds and aromatic hydrocarbons at higher pyrolysis temperature. It showed the possibility to concentrate some specific products by controlling the lignin fractionation process at different pyrolysis temperatures.
4. Conclusions The fractionation process of lignin by ultrafiltration has an effect on structural features and resulting pyrolysis behaviors. The predominant pyrolysis compounds from lignin fractions with different molecular weight changed in the relative amount, but not in the compound species. At relatively low pyrolysis temperature, high molecular weight lignin was better for formation of G-type compounds，while low molecular weight lignin produced more S-type compounds. As temperature increased,
lignin with high molecular weight produced more H-type compounds and aromatic hydrocarbons. The research gave contribution to the design of pyrolytic patterns for selective production of specific aromatic compounds. The pyrolytic utilization of lignin potentially could be a sustainable source of aromatic production resulting in their ability to be petrochemical-replaceable.
Acknowledgements The authors gratefully acknowledge the financial support from the China National Key R&D plan (2017YFD0600204).
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Fig. 1. Structures of monolignols, the primary building blocks of lignin
Fig. 2. Scheme for fractionation of lignin from black liquor by ultrafiltration technology
Fig. 3. Liquor (a) based on the same concentration and lignin fractions (b) obtained by ultrafiltration.
Fig. 4. (a) FTIR spectra of lignin fractions. (b) Magnified region of FTIR spectra of lignin fractions.
Fig. 5. TG and DTG curves of lignin fractions
Fig. 6. Pyrolysis product distributions of four lignin fractions under different temperatures in Py-GC/MS
Table 1 Composition of different lignin fractions, expressed in weight percentage and based on dry weight Lignin KL F1 F2 F3 F4 a
Composition (%) a
b
AIL
ASL
Total carbohydrates
Xylose
Glucose
Arabinose
Galactose
Ash
76.68 78.41 77.62 75.35 76.56
5.19 5.31 7.09 7.13 6.97
4.99 8.11 3.69 0.07 NDc
4.26 5.77 3.51 0.07 ND
0.42 1.47 ND ND ND
0.15 0.38 0.09 ND ND
0.16 0.49 0.09 ND ND
0.95 1.84 1.19 1.07 1.09
AIL: acid insoluble lignin. b ASL: acid soluble lignin. c ND: Not detected.
Table 2 Molecular weight distribution of different lignin fractions Sample
Mn (g/mol)
Mw (g/mol)
Mw/Mn
KL F1 F2 F3 F4
584 1141 703 484 395
1507 2640 1119 700 574
2.58 2.31 1.59 1.44 1.45
a
Mass ratio of the specific lignin fraction to the total UF lignin
Yield (%)a 54.3 25.9 15.4 4.4
Table 3 Band assignments for FTIR spectra of lignin Wavenumbers (cm-1)
Band assignment
3420 2940-2840 1713 1605 1510 1425 1330 1265 1120 1030
O-H stretching vibration in hydroxyl groups C-H stretching vibration in methyl and methylene groups C=O stretching in unconjugated ketone, carbonyl groups Aromatic skeletal vibrations Aromatic skeletal vibrations Aromatic skeletal vibrations C-O of syringyl ring C-O of guaiacyl ring Typical of S unit; C-O deformation in ester bond Aromatic C-H in-plane deformation (G>S) plus C-O deformation in primary alcohols C-H out of plane in positions 2 and 6 (S units)
835
Table 4 Pyrolysis products analysis of four lignin fractions at different temperatures Relative content (peak area%) Type
a
Compounds
RT(min)
500 °C KL
AH-type
H-type
G-type
S-type
C-type a
F1
F2
650 °C F3
Benzene
2.333
Toluene
3.381
o-Xylene
4.78
p-Xylene
4.911
Phenol
6.815
2-Methylphenol
8.026
4-Methylphenol
8.372
2,4-Dimethylphenol
9.531
Guaiacol
8.624
3-Methylguaiacol
10.016
4-Methylguaiacol
10.294
12.43
18.48
18.1
4-Ethylguaiacol
11.84
6.38
11.24
8
4-Vinylguaiacol
12.522
6.63
11.52
Syringol
13.259
27.14
26.13
4-Allylsyringol
20.448
1.57
1.34
Acetosyringone
21.156
2.25
1.46
3-Methoxycatechol
11.622
2.35
KL
F1
F2
800 °C F3
KL
F1
F2
F3
9.33
6.85
5.98
1.18
0.98
0.84
2.7
2.4
1.91
2.14
4.34 0.95
3.95
3.11
3.23
0.83
0.81
0.9 12.79
16.09
1.58
1.42
10.17
6.61
9.63
13.36
7.82
16.6
16.44
10.72
16.45
4.34
6.86
5.2
3.83
9.67
10
6.81
9.34
3.79
6
5.53
3.04
8.43
10.22
6.28
7.31
3.89
6.41
3.79
2.51
8.54
9.56
5.47
6.86
12.44
13.35
15.17
13.34
6.67
8.57
12.18
9.5
2.42
2.52
13.38
13.21
13.56
7.39
9.78
7.31
9.48
11.24
7.77
10.77
10.18
8.69
12.81
10.49
10.83
11.47
10.39
23.34
31.53
26.09
10.4
16.98
22.07
12.7
15.28
12.24
0.9
0.96
1.25
2.92
0.93
0.97
2.25
2.59
2.65
3.67
2.96
2.88
0.99 18.1
1.64
3.17
1.91
0.81 16.6
2.01
3.02
1.46
5.83
2.46
: AH: aromatic hydrocarbons; H: phenol-type compounds; G: guaiacol-type compounds; S: syringol-type compounds; C: catechol-type compounds
S0165-2370(17)30555-7 https://doi.org/10.1016/j.jaap.2017.11.003 JAAP 4178
To appear in:
J. Anal. Appl. Pyrolysis
Received date: Revised date: Accepted date:
30-6-2017 15-10-2017 2-11-2017
Please cite this article as: Lupeng Shao, Xueming Zhang, Fushan Chen, Feng Xu, Fast pyrolysis of Kraft lignins fractionated by ultrafiltration, Journal of Analytical and Applied Pyrolysis https://doi.org/10.1016/j.jaap.2017.11.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fast Pyrolysis of Kraft Lignins Fractionated by Ultrafiltration Lupeng Shaoa, Xueming Zhanga, Fushan Chenb, Feng Xua,c* a
Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University,
Beijing 100083, China b
College of Marine Science and Biological Engineering, Qingdao University of
Science & Technology, Qingdao 266042, China c
Shandong Key Laboratory of Paper Science & Technology, Qilu University of
Technology, Jinan 250353, China *Corresponding author. E-mail address: [email protected] (F. Xu)
Highlights
Pyrolysis of fractionated lignins by ultrafiltration were carried out by Py-GC/MS.
Molecular weight of lignin affected the relative content of pyrolytic products.
High molecular weight lignin favored the generation of G-type compounds at 500 o
C.
Abstract The pyrolysis behavior of different lignin samples fractionated by ultrafiltration membrane technology was investigated by pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS). Results indicated that the predominant products derived from pyrolysis of lignin fractions with different molecular weights changed in the relative content, but not in the compound species. At 500 oC, high molecular weight lignin favored the generation of guaiacol-type compounds (57.33%), and low
molecular weight lignin produced more syringol-type compounds (34.33%). As temperature increased to 650 oC, methoxy groups in S and G units were easier to cleave from the benzene ring of the high molecular weight lignin leading to an increase in the relative content of phenol-type compounds. High molecular weight lignin was found to produce the highest amounts of phenol-type compounds (46.22%) and aromatic hydrocarbons (17.25%) at 800 oC, while low molecular weight lignin favored the generation of guaiacol-type and syingol-type compounds. The reveal of relevance between specific lignin fractions and pyrolysis behaviour is meaningful for the efficient transformation of lignin to specific aromatic compounds.
Keywords: Lignin; Fractionation; Molecular weight; Pyrolysis 1. Introduction Lignin, a common primary ingredient in biomass, is a natural amorphous three-dimensional polymer consisting of three primary units, cross-linked with each other by ether bonds and carbon-carbon bonds [1]. These three monolignols are known as syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) units (Fig.1), respectively. The structure of lignin varies considerably in different origins and isolation methods, particularly with regard to the type and quantity of linkages. Owing to the inherent heterogeneity and recalcitrance, lignin as a by-product of pulp and paper industry and bioethanol industry is mainly employed for power and heat purpose through combustion [2]. However, in light of the low price of natural gas, the economic benefit of burning lignin has diminished [3,4]. Therefore, new strategies need to be developed to generate value-added products from lignin [5].
The strategies to transform lignin can be broadly classified into acid/base catalyzed depolymerization, pyrolysis, liquid-phase reforming, and biodegradation. Pyrolysis is identified as one of the most promising thermochemical technologies to convert lignin into bio-oil. It represents a straightforward strategy to break down lignin into smaller fragments [6]. Furthermore, due to the significant complexity of the pyrolysis reactions, the various features and yields of the aromatic derivatives attained through lignin pyrolysis are strongly affected by the type of lignin and pyrolysis reaction conditions [7-9]. Zhang et al. investigated the effect of lignin source on pyrolysis products by Py-GC/MS, and concluded that aspen lignin produced significantly more pyrolytic products than prairie cordgrass lignin [10]. Wang et al. performed pyrolysis of four lignin polymers isolated from the same pine wood, and found that alkali lignin and milled wood lignin produced more phenols at 400 oC [11]. Jiang et al. studied the effect of temperature on the composition of lignin pyrolysis products, and found that the maximum yield of phenolic compounds was obtained at 600 oC [12]. In the attempt to improve economic benefit of lignin, challenges, including recalcitrance and heterogeneity, especially the broad distribution of the molecular weight, limit its utilization in high value-added applications. Fractionation has been proposed as an efficient approach to obtain specific lignin fractions with certain molecular weight distribution and tunable chemical functionality [13,14]. To date, three main fractionation approaches have been reported in the literature including organic solvent extraction [15,16], selective acid precipitation at reduced pH values
[17-19], and membrane ultrafiltration technology [20-22]. Tedious extraction steps and high production costs of organic solvent extraction may limit application on a large industrial scale. Colloids formation during acid precipitation may complicate the process of filtration and separation. Membrane ultrafiltration technology allows obtaining lignin fractions with specific molecular weight distribution by free-reagent treatment. The only requirement of the systems is the use of a semipermeable medium and the generation of a hydrostatic pressure difference as the driving force, which consume less energy. Jönsson evaluated the economy of isolation of kraft lignin by ultrafiltration, and found that lignin can be recovered from cooking liquor at a cost of about 60 € per tonne of lignin and from evaporated black liquor by ultrafiltration at 33 € per tonne [23]. The membrane was economic which could be recycled after treatment. Considering these, membrane ultrafiltration technology could be adopted to obtain lignin fractions for further adequate application. Thermal behaviors of lignin from different sources have been investigated widely, but study on thermal conversion of fractionated lignin samples still remain to be explored. The present study explored the pyrolysis behavior of lignin derived from ultrafiltration process. Several lignin fractions were separated from black liquor of poplar kraft pulping by using ultrafiltration membrane technology. Chemical structures of ultrafiltration lignin fractions and unfractionated lignin were explored. Then these lignin fractions were pyrolyzed at different temperatures. GC/MS coupled with the pyrolyzer was employed to characterize the condensable products. Such information is useful for the proper utilization of variant lignin fractions.
2. Materials and methods 2.1. Black liquor obtaining The black liquor of poplar kraft pulping (KP) was obtained from the Key Laboratory of Pulp and Paper Science & Technology of the Ministry of Education of Qilu University of Technology (China). Laboratory-made poplar KP was processed using followed conditions: active alkali (Na2O)-24% on a dry weight basis; sulfidity-25%; liquor to wood ratio-5:1; final temperature-165 oC; warm-up time-60 min; holding time-100 min. After reaction, the separation of liquid and solid fractions was carried out, and the liquor was further filtrated through filter paper for later use. Kraft lignin (KL) was separated from the black liquor by acid precipitation with 2M H2SO4 to pH 2. After the acid addition, the final suspensions were centrifuged to recover lignin. The isolated lignin was washed with acidified water (pH 2.0) and freeze-dried. 2.2. Lignin fractions obtained from ultrafiltration (UF) process The stirred ultrafiltration unit used in this work was supplied by Merck Millipore (Darmstadt, Germany). Three membranes (filter diameter: 76 mm) with cut-offs of 10 kDa, 5 kDa, and 1 kDa were adopted in this experiment. Lignin was sequentially fractionated according to the scheme shown in Fig.2. The filtrated solution was divided into four fractions by ultrafiltration with gradient decreasing membrane cut-off. Firstly, a membrane with 10 kDa was employed, and the retentate was collected and kept as the > 10 kDa liquid fraction. Then the filtrate was subjected to the next cut-off level (5 kDa). This process continued until 1 kDa membrane was
applied. All the retentate and filtrate liquid were collected and defined as the following names: L1 > 10 kDa, 10 kDa > L2 > 5 kDa, 5 kDa > L3 > 1 kDa, L4 < 1 kDa. Then dilute sulphuric acid (2 M) was added dropwise to the obtained liquid fractions respectively until the pH value of the mixed solution was 2.0 at ambient temperature. After the acid addition, the suspensions were centrifuged, and the precipitations were washed with acidified water (pH 2.0) and freeze-dried. The precipitated lignin fractions were defined as F1, F2, F3, and F4 corresponding to those liquid fractions mentioned above. 2.3. Lignin characterization Acid-insoluble lignin, acid-soluble lignin, carbohydrate, and ash contents in the isolated lignin fractions were determined after two-stage acid hydrolysis according to the procedure described in previous studies [15,24]. The molecular weights and polydispersity of lignin fractions were determined by Agilent 1200 gel permeation chromatography with a refraction index detector (RID) [25]. Lignin was dissolved in THF (HPLC grade), and then about 10 ul of solution was injected to the PL-gel Mix-D column (300 mm × 7.5 mm) at a flow rate of 1 ml/min. Standard calibration was performed with polystyrene standards. Before the analysis, lignin fractions were acetylated according to the previous research [26]. Fourier Transform Infrared Spectroscopy (FTIR) spectra of lignin fractions were collected in a VERTEX70 instrument (Bruker, Germany) using KBr pellet technique. The region between 4000 cm-1 and 400 cm-1 with a resolution of 4 cm-1 and 32 scans were recorded.
Thermal stability of lignin was investigated using a TA Q50 thermogravimetric analyzer (TA Instruments, USA). Approximately 10 mg of lignin was heated at a heating rate of 20 oC/min from ambient temperature to 750 oC under a nitrogen flow of 60 ml/min. 2.4. Py-GC/MS Fast pyrolysis experiments were performed by using a CDS 5200 pyrolyzer coupled with GC/MS (Agilent Technologies 7890A/5975) to obtain the distribution of pyrolysis products. Approximately 0.5 mg of lignin was loaded in a quartz tube, which was wrapped by a Pt filament. Lignin was pyrolyzed at 500, 650, and 800 oC respectively for 15s with a heating rate of 10 oC/ms. Pyrolytic volatiles were directly analysed online by GC/MS. Pure helium was used as purge gas with a constant flow rate of 1 ml/min at a 1:50 split ratio. The conditions of GC/MS were set as follows: the injector temperature was maintained at 270 oC; the chromatographic separation was carried out using a Agilent HP-5MS (30 m × 0.25 mm × 0.25 m) capillary column; the oven temperature was initially programmed from 50 oC (maintained for 2 min) to 250 oC (held for 5 min) at a heating rate of 10 oC/min. An ion trap mass spectrometer was used for the compounds detection, with a mass scan range of m/z 45-650 (EI 70 eV). Finally, the pyrolysis products were identified according to commercial NIST14 library and the previous references [27-29].
3. Results and discussion 3.1. UF process Black liquor (BL) was divided into four fractions by ultrafiltration using gradient
decreasing membrane cut-off. It can be observed from Fig. 3(a) that the color of liquor changed progressively from deep to shallow with the decreasing of membrane cut-off. The dark color of the liquor is derived from the chromophoric functional groups present in lignin, including carbonyl groups, carboxylic acids, quinones, which are soluble in alkaline medium [30,31]. As shown in Fig. 3(b), the color of corresponding lignin fractions changed obviously. It might be caused by the different amount of chromophoric functional groups in UF lignin fractions with different molecular weights. It is evident, thus, that the UF process affected the color of liquor and lignin. 3.2. Characterization of lignin fractions 3.2.1. Lignin composition In the present study, for the purpose of obtaining lignin with a more representative whole molecular weight distribution and subsequent potential application in industry, the fractionated lignins were washed three times by acidified water (pH 2.0) without additional treatment. The chemical composition of lignin fractions are shown in Table 1. The total content of AIL and ASL, ash-free lignin, varied from 81.77% to 84.71%. The amount of residual carbohydrates varied between 0 and 8.11% depending on the fractionation process. The carbohydrates might originate from polysaccharides still covalently bound to the lignin as in the Lignin–Carbohydrate Complex (LCC) [32]. The total carbohydrates content in UF lignin fractions from F1 to F4 decreased as the membrane cut-off decreased. The fact confirmed that UF was not only a fractionation process, but also a purification process.
Furthermore, xylose constituted the main hemicellulosic sugar in all obtained lignin fractions. The decrease in xylose content from 5.77% to 0 again proved that UF made the purification of lignin. As can be observed in Table 1, the ash contents were relatively low for all samples, from 0.95% to 1.84%, indicating efficient washing of the samples. 3.2.2. Molecular weights distribution Gel permeation chromatography was carried out to obtain the molecular weight distribution of the lignin fractions. The weight-average (Mw), number-average (Mn) molecular weight and polydispersity (Mw/Mn) of the lignin fractions from UF process and unfiltered lignin are shown in Table 2. As expected, the objective to obtain lignin fractions with varying molecular weight from UF process was achieved. There was a clear decreasing trend of the weight-average (Mw) and number-average (Mn) molecular weight as the membrane cut-off decreased. Lignin molecular weight is closely related to the number of α- and β-aryl ether bonds and C-C bonds among the structural units. The polydispersity of UF lignin fractions decreased when compared with KL, indicating more uniform lignin fraction were obtained by ultrafiltration. As can be seen in Table 2, the yields of F1, F2, F3, and F4 were 54.3%, 25.9%, 15.4%, and 4.4%, respectively, which decreased as the membrane cut-off decreased. The yield of F4 was only 4.4%, which was the lowest among UF lignin fractions. Considering the subsequent potential application in industry, F1, F2, and F3 were adopted for further investigation. 3.2.3. FTIR analysis
The FTIR spectra of lignin fractions are shown in Fig. 4, and the notable band assignments are listed in Table 3. As can be seen from the spectra, the four lignin fractions showed similar spectroscopic patterns [33-35]. A wide absorption band at 3420 cm-1 is assigned to O-H stretching vibration in aromatic and aliphatic OH groups. The bands at 2939 cm-1 and 2840 cm-1 represent the C-H stretching in the methyl and methylene groups, respectively. The band at 1713 cm-1 can be attributed to unconjugated carbonyl stretching vibration in ketone or aldehyde groups. The typical aromatic skeleton vibration occurs at 1605, 1510, and 1425 cm-1. The bands at 1330, 1120 and 835 cm-1 are attributed to syringyl structures in lignin, while the bands at 1265 and 1030 cm-1 are associated with guaiacyl units in lignin. The relative intensities of the peaks related to the –OH stretching vibration (3420 cm-1) in different lignin fractions were calculated considering the signal centered at 1510 cm−1 present in all FTIR spectra (pure aromatic skeletal vibrations in lignin) as invariant band in each spectrum [36]. The relative intensities were 0.76, 0.78, 0.89, and 0.89 for F1, F2, F3, and KL, respectively. Although different lignin fractions showed similar peak form, absorbance intensity varied. The reason was that UF process influenced amount of functional groups. Therefore, more analyses were carried out to identify the structural differences. 3.2.4. Thermogravimetric analysis To investigate the thermal properties of lignin fractions, TGA measurements were performed under nitrogen atmosphere. Thermogravimetric (TG) and differential thermogravimetric (DTG) curves of lignin samples are shown in Fig. 5.
The thermal degradation behavior of four lignin fractions showed the consistency at low temperature (below 170 oC). The initial weight loss stage before 170 oC mainly attributed to the release of water and some organic constituents. However, the thermal behavior of these lignin fractions was slightly different at higher temperature. Decomposition temperature corresponding to the first obvious peak decreased with the decrement of molecular weight of the lignin fractions. The first obvious mass loss peak occurred at 285, 281, and 242 oC for fraction F1, F2, and F3, respectively. The mass loss stage between 230 and 300 oC was possibly caused by the degradation of remaining hemicellulose and cleavage of weaker bonds in lignin structure [37,38]. The maximum value of mass loss rate at around 385 oC for all fractions belonged to lignin degradation. The thermal degradation of lignin occurred in a rather broad temperature range, from 200 oC to 600 oC, because of the complex structure of lignin with phenolic hydroxyl, carbonyl groups, and benzylic hydroxyl [39]. When the temperature was beyond 600 oC, the mass loss was not very evident. At this stage, decomposition and condensation reactions of aromatic rings were predominant during lignin pyrolytic process, and char was the main product [40]. DTG peaks for KL, F1 and F2, occurred at the temperature range from 600 to 700 oC, may be caused by the secondary reaction of lignin. [7, 41] The residue weights of KL, F1, F2, and F3 at 750 o
C were 35.1%, 33.8%, 25.3%, and 31.8%, respectively. The final residue weights of
F1, F2, and F3 thermal degradation were lower than KL. These phenomena may suggest that fractionation process of lignin could affect the thermal decomposition behavior.
3.3. Py-GC/MS analysis The fast pyrolysis of UF lignin fractions were conducted at various temperatures, and the pyrolysis products were identified by GC/MS. When lignin undergoes fast pyrolysis, it produces permanent gases, volatiles, and char. This study only discussed the volatile products. It is known that the chromatographic peak area of a compound is considered linear with its quantity, and peak area% (of total peak area) is linear with its relative content in the detected products. Therefore, for each product, peak area% value can be compared to reveal the changing of its relative content among the detected products [42-44]. Total ion current (TIC) chromatograms of lignin pyrolysis with different temperatures were shown in Fig. S1. The major pyrolysis products and relative contents are listed in Table 4. As can be seen from the data, products of lignin pyrolysis were mainly phenolic compounds and aromatic hydrocarbons. There were no obvious peaks corresponding to small C2-C5 oxygenated products in TIC chromatograms. This may be attributed to the low carbohydrate content. According to the typical functional groups, products were classified into five groups: aromatic hydrocarbons (AH), phenol-type compounds (H), guaiacol-type compounds (G), syringol-type compounds (S), and catechol-type compounds (C) [42]. The distributions of typical aromatic compounds are depicted in Fig. 6. As shown in Fig. 6, phenolic compounds were the dominant products from the four lignin samples at different pyrolysis temperatures. G-type compounds and S-type compounds were the primary products at 500 oC for all four lignin fractions. As reference reported, hardwood lignin has plenty of guaiacyl and syringyl units [6].
G-type and S-type phenols could be produced by the direct breaking of the β-O-4 bond which has the lowest dissociation energy in all types of linkage bonds [45]. The composition of pyrolysis products was closely correlated with the content of G and S units in different lignin fractions. Among them, guaiacol, 4-methylguaiacol, 4-ethylguaiacol, 4-vinylguaiacol, and syringol were in the majority. As temperature increased, the relative content of G-type compounds and S-type compounds decreased, while the relative content of H-type compounds increased. This can be explained by the fact that different linkages break at different temperatures. Most of the ether linkages are readily cleaved to form condensable volatile products at low temperatures. Most methoxy phenols, such as G-type and S-type phenols, are contained in the pyrolysis products due to the fact that the methoxy groups are more resistant than the alkyl-aryl ether linkages during the thermal degradation [6]. S-type phenols only derived from S units in lignin. However, S units could be degraded into G and/or H units due to the demethoxylation reaction at high temperatures [8]. Hence, the formation of syringol-type compounds was inhibited at high pyrolysis temperatures. As for the G-type phenols, they mainly derived from G units within lignin, as well as the demethoxylation of S units at relatively low temperatures. As temperatures increased, the demethoxylation reaction was enhanced, and the selectivity of H-type compounds increased. For all lignins, 3-methoxycatechol was detected in pyrolysis products. The production was enhanced with the increased temperature, probably due to the cracking of ArO-CH3 in the syringol. As shown in Table 4, the predominant pyrolysis compounds from lignin fractions
with different molecular weights changed in the relative amount, but not in the species. These differences are due to the different content ratio of the phenylpropane building blocks in different lignin fractions. The relative content of G-type compounds from F1, F2, F3, and KL pyrolysis at 500 oC were 57.33%, 54.97%, 40.16%, and 38.23%, respectively. The result seemed to follow the trend of decreasing G-type compounds relative content with decreasing lignin molecular weight (from F1 to F3). The relative content of G-type compounds from KL pyrolysis was the lowest among the four lignin fractions. Though molecular weight of KL was situated between F1 and F2, its polydispersity was the highest. This fact suggested that the molecular weight of lignin with different polydispersity could partially influence lignin pyrolysis products. The data showed that high molecular weight lignin was better for formation of G-type compounds (57.33%) at 500 oC, while low molecular weight lignin produced more S-type compounds (34.33%). In addition, syringol was produced in large quantities among the whole pyrolysis products at 500 oC. This finding indicated that syringol was easily produced from lignin regardless of the molecular weight at low temperatures. Syringol is largely employed as a flavouring and natural preservative due to its antioxidant capacity [46]. As temperature increased to 650 oC, the relative content of H-type compounds from F1, F2, F3, and KL pyrolysis were 28.9%, 27.88%, 17.2%, and 18.63%, respectively. All the H-type monomers increased obviously. The values of G-type compounds were 42.24%, 33.05%, 35.41%, and 36.76%, respectively. Among them, relative content of guaiacol and 4-methylguaiacol derived from UF lignin fractions
decreased, while values of that derived from KL varied slightly. There was no 4-ethylguaiacol generated at 650 oC. This phenomenon suggested that ethyl groups linked in benzene ring were easier cleaved with increasing temperature. The relative contents of S-type compounds were 11.3%, 17.94%, 23.32%, and 25.09%, respectively. Compared with the values obtained from 500 oC to 650 oC, the increment of H-type compounds derived from high molecular weight lignin was higher than that obtained from low molecular weight lignin. However, the decrement of S-type compounds derived from high molecular weight lignin was higher than that obtained from low molecular weight lignin. It is reasonable to suppose that the methoxy group in S units is easier cleaved from the benzene ring of the high molecular weight lignin fraction than that of the low molecular weight lignin fraction. As temperature increased, the high molecular weight lignin fraction favoured the generation of more H-type compounds. As can be seen in Fig. 6, there were almost no aromatic hydrocarbons at relatively low pyrolysis temperatures. As the pyrolysis temperature increased to 800 o
C, the aromatic hydrocarbons appeared, and the relative content from F1, F2, F3, and
KL pyrolysis were 17.25%, 9.74%, 8.96%, and 12.87%, respectively. Among them, toluene that could be produced from either block linkage cleavages of H units or aromatic demethoxylation and dehydroxylation showed the highest relative content [7]. This trend may be caused by the secondary decomposition took place at high temperature, and indicated that the elimination of phenolic hydroxyl groups from benzene ring was promoted by increased temperature. Demethoxylation reaction was
also enhanced at high pyrolysis temperatures [47]. As a result, relative contents of G-type and S-type compounds decreased, and H-type compounds increased. Especially, relative content of 4-vinylguaiacol derived from F2 and F3 were 11.47% and 10.39%. However, there was no 4-vinylguaiacol among pyrolysis products derived from F1 and KL. This suggested that the vinyl group was more cracked in the high molecular weight fraction than in the other fractions at higher temperature. Furthermore, aromatic hydrocarbons and H-type compounds from F1 pyrolysis were produced in maximum values among the four lignin fractions at 800 oC. F2 and F3 lignin fractions produced more G-type and S-type compounds. This phenomenon suggested that molecular weight of lignin had an obvious effect on the relative content of pyrolysis products at higher temperature. Results indicated that high molecular weight lignin fraction was better for formation of H-type compounds and aromatic hydrocarbons at higher pyrolysis temperature. It showed the possibility to concentrate some specific products by controlling the lignin fractionation process at different pyrolysis temperatures.
4. Conclusions The fractionation process of lignin by ultrafiltration has an effect on structural features and resulting pyrolysis behaviors. The predominant pyrolysis compounds from lignin fractions with different molecular weight changed in the relative amount, but not in the compound species. At relatively low pyrolysis temperature, high molecular weight lignin was better for formation of G-type compounds，while low molecular weight lignin produced more S-type compounds. As temperature increased,
lignin with high molecular weight produced more H-type compounds and aromatic hydrocarbons. The research gave contribution to the design of pyrolytic patterns for selective production of specific aromatic compounds. The pyrolytic utilization of lignin potentially could be a sustainable source of aromatic production resulting in their ability to be petrochemical-replaceable.
Acknowledgements The authors gratefully acknowledge the financial support from the China National Key R&D plan (2017YFD0600204).
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Fig. 1. Structures of monolignols, the primary building blocks of lignin
Fig. 2. Scheme for fractionation of lignin from black liquor by ultrafiltration technology
Fig. 3. Liquor (a) based on the same concentration and lignin fractions (b) obtained by ultrafiltration.
Fig. 4. (a) FTIR spectra of lignin fractions. (b) Magnified region of FTIR spectra of lignin fractions.
Fig. 5. TG and DTG curves of lignin fractions
Fig. 6. Pyrolysis product distributions of four lignin fractions under different temperatures in Py-GC/MS
Table 1 Composition of different lignin fractions, expressed in weight percentage and based on dry weight Lignin KL F1 F2 F3 F4 a
Composition (%) a
b
AIL
ASL
Total carbohydrates
Xylose
Glucose
Arabinose
Galactose
Ash
76.68 78.41 77.62 75.35 76.56
5.19 5.31 7.09 7.13 6.97
4.99 8.11 3.69 0.07 NDc
4.26 5.77 3.51 0.07 ND
0.42 1.47 ND ND ND
0.15 0.38 0.09 ND ND
0.16 0.49 0.09 ND ND
0.95 1.84 1.19 1.07 1.09
AIL: acid insoluble lignin. b ASL: acid soluble lignin. c ND: Not detected.
Table 2 Molecular weight distribution of different lignin fractions Sample
Mn (g/mol)
Mw (g/mol)
Mw/Mn
KL F1 F2 F3 F4
584 1141 703 484 395
1507 2640 1119 700 574
2.58 2.31 1.59 1.44 1.45
a
Mass ratio of the specific lignin fraction to the total UF lignin
Yield (%)a 54.3 25.9 15.4 4.4
Table 3 Band assignments for FTIR spectra of lignin Wavenumbers (cm-1)
Band assignment
3420 2940-2840 1713 1605 1510 1425 1330 1265 1120 1030
O-H stretching vibration in hydroxyl groups C-H stretching vibration in methyl and methylene groups C=O stretching in unconjugated ketone, carbonyl groups Aromatic skeletal vibrations Aromatic skeletal vibrations Aromatic skeletal vibrations C-O of syringyl ring C-O of guaiacyl ring Typical of S unit; C-O deformation in ester bond Aromatic C-H in-plane deformation (G>S) plus C-O deformation in primary alcohols C-H out of plane in positions 2 and 6 (S units)
835
Table 4 Pyrolysis products analysis of four lignin fractions at different temperatures Relative content (peak area%) Type
a
Compounds
RT(min)
500 °C KL
AH-type
H-type
G-type
S-type
C-type a
F1
F2
650 °C F3
Benzene
2.333
Toluene
3.381
o-Xylene
4.78
p-Xylene
4.911
Phenol
6.815
2-Methylphenol
8.026
4-Methylphenol
8.372
2,4-Dimethylphenol
9.531
Guaiacol
8.624
3-Methylguaiacol
10.016
4-Methylguaiacol
10.294
12.43
18.48
18.1
4-Ethylguaiacol
11.84
6.38
11.24
8
4-Vinylguaiacol
12.522
6.63
11.52
Syringol
13.259
27.14
26.13
4-Allylsyringol
20.448
1.57
1.34
Acetosyringone
21.156
2.25
1.46
3-Methoxycatechol
11.622
2.35
KL
F1
F2
800 °C F3
KL
F1
F2
F3
9.33
6.85
5.98
1.18
0.98
0.84
2.7
2.4
1.91
2.14
4.34 0.95
3.95
3.11
3.23
0.83
0.81
0.9 12.79
16.09
1.58
1.42
10.17
6.61
9.63
13.36
7.82
16.6
16.44
10.72
16.45
4.34
6.86
5.2
3.83
9.67
10
6.81
9.34
3.79
6
5.53
3.04
8.43
10.22
6.28
7.31
3.89
6.41
3.79
2.51
8.54
9.56
5.47
6.86
12.44
13.35
15.17
13.34
6.67
8.57
12.18
9.5
2.42
2.52
13.38
13.21
13.56
7.39
9.78
7.31
9.48
11.24
7.77
10.77
10.18
8.69
12.81
10.49
10.83
11.47
10.39
23.34
31.53
26.09
10.4
16.98
22.07
12.7
15.28
12.24
0.9
0.96
1.25
2.92
0.93
0.97
2.25
2.59
2.65
3.67
2.96
2.88
0.99 18.1
1.64
3.17
1.91
0.81 16.6
2.01
3.02
1.46
5.83
2.46
: AH: aromatic hydrocarbons; H: phenol-type compounds; G: guaiacol-type compounds; S: syringol-type compounds; C: catechol-type compounds