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Full Length Article
Catalytic valorization of lignin to liquid fuels over solid acid catalyst assisted by microwave heating
Minghao Zhoua,b, Brajendra K. Sharmab, Jing Lia, Jiaping Zhaoa, Junming Xua, , Jianchun Jianga ⁎
Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry (CAF), No. 16, Suojin Five Village, Nanjing 210042, China Illinois Sustainable Technology Center, Prairie Research Institute, University of Illinois at Urbana-Champaign, One Hazelwood Dr., Champaign, IL 61820, USA
Keywords: Lignin Depolymerization Microwave Liquid fuels
In this study, three types of lignin feedstocks were introduced into the microwave assisted depolymerization process over solid acid catalysts. The effects of lignin structures, solvents and catalysts were investigated to improve bio-oil yields and properties. A common phenomenon was observed that the highest bio-oil yields were all obtained in methanol under optimal conditions with microwave heating. The highest bio-oil yield from alkaline lignin, dealkaline lignin and lignosulfonate was 57.4%, 82.9% and 70.9% respectively, all obtained in methanol. The promotional effect of methanol over HSZ-640 (or HSZ-660) in the depolymerization process was confirmed due to the higher polarity of methanol, and higher acidity of catalysts. The promotional effect on the elemental composition and HHV of bio-oil was observed, finding that HSZ-640 (or HSZ-660) exhibited to be effective to convert lignin to liquid fuels with higher HHV, and the HHV of obtained bio-oil increased to about 28–32 MJ/kg, confirming that hydrogenation/hydrodeoxygenation took place during the depolymerization.
1. Introduction Lignocellulosic biomass is one of the most promising renewable resources for the production of liquid fuels and/or fine chemicals and has attracted increasing attention on account of its global abundance and availability [1,2]. Lignin always remains after cellulose and hemicelluloses have been utilized; however, the surplus lignin resource and its main phenylpropane units make it a promising alternative to produce liquid fuels or petroleum-based aromatic chemicals. Actually, lignin has a relatively lower heating value and a much higher oxygen amount, indicating that it is urgent to develop effective technologies for the value-added utilization of the abundant lignin resources. Although an increasing number of researchers have focused on understanding the catalysts, structures, solvents and depolymerization of lignin, the depolymerization efficiency still faces great challenges due to the complex and recalcitrant structure of lignin [3–7]. The varieties of lignin sources and recalcitrant chemical bonds in lignin (C–O, C–C, etc.) make it difficult to develop a generalized depolymerization method to convert various types of lignin feedstocks to liquid fuels and/or value-added aromatic chemicals [8–10]. Therefore, it is important and crucial to investigate the specific relationship between lignin structures and depolymerization parameters (solvents, catalysts, etc.) to develop an effective and generalized valorization method for the utilization of different kinds of lignin feedstocks. ⁎
A wide range of depolymerization approaches has been proposed for the utilization of lignin, such as fast pyrolysis [11,12], oxidation , hydrothermal/reductive depolymerization [14–17], and microwave assisted depolymerization [18,19]. Previous studies have indicated that solvents and catalysts have great influence during the lignin depolymerization. Therefore, various kinds of solvents (such as methanol [20,21], ethanol [21–23], isopropanol [19,24], water [25,26], formic acid [3,27], etc.) were investigated as reaction mediums for the solvolysis and depolymerization of lignin. Both homogeneous and heterogeneous catalysts have been studied during the lignin valorization process. Precious metal based catalysts (such as Pt, Pd, and Ru, etc.) were reported to be effective during the lignin depolymerization process due to their good hydrogenation/hydrodeoxygenation performances [15,25,28]. However, the use of precious metal based catalysts will add to the cost for the large scale utilization of lignin. Instead, nonnoble metal based catalysts (Cu, Ni, and Mo, etc.) were found to be promising alternatives to convert lignin to liquid fuels and/or fine chemicals [15,29–32]. Homogeneous acidic or basic catalysts (e.g. H2SO4, NaOH) were also introduced to the lignin depolymerization processes [33,34]. Labidi et al. investigated the depolymerization of three types of organosolv lignin (acetosolv, formosolv, and acetosolv/ formosolv) in supercritical acetone, ethanol, and methanol , finding that the liquid bio-oil yield was closely dependent on the molecular weight of lignin sources. Kim studied the depolymerization of
Corresponding author. E-mail address: [email protected]
https://doi.org/10.1016/j.fuel.2018.10.144 Received 8 August 2018; Received in revised form 15 September 2018; Accepted 30 October 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
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six kinds of lignin samples from oakwood and pinwood using different delignification techniques (ethanolsolv, formasolv, Klason), confirming that the complex solvent (scEtOH-HCOOH) was effective for depolymerizing different types of lignin sources, with higher bio-oil yield (> 81 wt%) . In fact, most of those depolymerization approaches required harsh reaction conditions (either high reaction temperature or pressure), thus, it is urgent to explore mild reaction conditions and technologies to convert low HHV lignin to value-added liquid fuels and/or fine chemicals. In order to effectively convert lignin to liquid fuel with a higher heating value, microwave assisted depolymerization of three types of lignin over some solid acid catalysts was investigated. Herein, alkaline lignin (AL), dealkaline lignin (DL) and lignosulfonate (LS) were used as different lignin feedstocks to study the influence of lignin structure on the liquid bio-oil yield and its properties. The effect of catalysts, solvents and temperature was investigated in detail, in order to convert lignin to bio-oil. The synergistic effect of solvents and catalysts was found to have positive effects in converting lignin to bio-oil with higher HHV. The experimental results confirmed that lignin feedstocks, solvents, catalysts and temperature had great promotional effects on the microwave assisted depolymerization of lignin, as bio-oil yield and its properties (including elemental composition, HHV and molecular weight) improved to some extent.
where WBO, WRL and Wlignin, depict the weight of the bio-oil, residual lignin and feed lignin, respectively. The elemental composition (including C, H, O, N and S) of lignin feedstocks and obtained bio-oil was analyzed on an elemental analyzer (Thermo Fischer Flash EA 1112). GC–MS analysis was conducted on Agilent7890A to study the chemicals in bio-oil. The molecular weights (Mn and Mw) of bio-oil were calculated and analyzed according to the MALDI-TOF MS analysis results, conducted in a Bruker Autoflex Speed LRF MALDI in negative mode, with 2,5-Dihydroxybenzoic acid (DHB) as the matrix. 3. Results and discussion 3.1. Characterizations of lignin feedstocks The C, H, O, N and S content of three different lignin feedstocks used in this study were presented in Table S1, which exhibited that all three lignins contained a relatively high amount of oxygen (44.5–47.5%) and low hydrogen content (about 4–5%). Additionally, they all contained a relatively low HHV (about 16.6–18.6 MJ/kg) with a low O/C and H/C ratio (about 0.72–1.34), due to the high oxygen levels. These results indicated that the value-added utilization of lignin resources was urgently needed to produce liquid fuels or fine chemicals. The 13C NMR analysis was conducted for a detailed evaluation of present functional groups in different lignin feedstocks. The 13C NMR spectra were presented in Fig. S1, showing that different peaks were observed according to different functional groups. The peaks at 0–95.8 ppm, 95.8–166.5 ppm and 166.5–215.0 ppm were assigned to the aliphatic groups, aromatic groups and carboxyl groups respectively. As presented in Fig. S1, in the aliphatic region, peaks corresponding to the aliphatic C–C bond (0–38.4 ppm) were all not very pronounced in the three lignin feedstocks. DL exhibited a pronounced intensity at about 50 ppm in respect to that of AL and LS, assigned to C in Ar-OCH3. Furthermore, an intense peak occurred at about 73 ppm, ascribing to C-α of G type βO-4 units in lignin. In the aromatic region, three similar peaks of different intensities in the three lignin feedstocks were observed at about 106, 125 and 142 ppm. The peaks at about 106 ppm, corresponding to the aromatic C–H bond, mainly assigned to C-2/C-6, S with α-CO and/ or C-5 in G units, and peaks at 125 ppm (corresponding to aromatic C–C bond) and 142 ppm (corresponding to aromatic C–O bond) could be assigned to C-1 in S/G non-etherified units and C-3 in G units in lignin, which were all labeled in Fig. S1 in corresponding places. Peaks in the aromatic region of AL and DL exhibited a stronger intensity, which were not evident in the LS spectrum. In the carboxyl region, LS showed a more intense peak relating to AL and DL, indicating the presence of more carboxyl bonds in LS.
2. Experimental 2.1. Materials and catalysts In this study, three types of lignin (alkaline lignin, dealkaline lignin and lignosulfonate) were all purchased from TCI Chemical Reagent. Other chemical reagents (such as methanol, ethanol, etc.) were all of analytic purity grade; they were purchased from the local Sinopharm Chemical Reagent of China, and were used directly as received. HSZbased catalysts (HSZ-640, HSZ-660, HSZ-690) were purchased from Tosoh Corporation, and all catalysts were calcined at 500 °C for 4 h prior to the reaction. The characterization results of catalysts were also provided by the Tosoh Corporation. 2.2. Microwave assisted lignin depolymerization and products characterization The lignin depolymerization reaction was conducted in a Discovery SP microwave reactor (CEM Corporation, USA). Methanol, ethanol or isopropanol was chosen as both hydrogen-donor and solvent during the depolymerization process. The vessel was first filled with catalyst and lignin (catalyst/lignin mass ratio 1/2) in solvent with a solid/solvent ratio of 1 g/25 ml, and then capped. The microwave assisted depolymerization of different types of lignin was carried out under a settled reaction temperature (400 W, 140 or 160 °C 80 min in methanol; 160 or 180 °C 80 min in ethanol or isopropanol) with a stirring speed of 600 rpm. The reactor was quickly cooled down after reaction, then the liquid products and unreacted lignin were separated through centrifugation. Bio-oil (BO) was obtained after removal of solvent, and the weight of residual lignin (RL) was estimated by subtracting the weight of catalyst charged from the weight of the solid phase, as almost no char was formed during the reaction. The BO and RL yields were calculated by the weight percentage of the weights of BO and RL products in relation to the feeding lignin, respectively. The results reported in this work were all average values of two or three runs. Product yields were calculated according to the following equations:
YBO (wt%) =
WBO × 100% Wlignin
YRL (wt%) =
WRL × 100% Wlignin
3.2. Catalytic depolymerization of lignin in methanol The blank experiment carried out without catalysts showed a relatively low bio-oil yield of 40.3%. The increased bio-oil yield and decreased residual lignin yield, as shown in Table 1, confirmed the promotional effect of HSZ-based mordenite catalysts during the microwave assisted depolymerization of alkaline lignin (AL). In comparison with the blank experiment, an obvious promotion in bio-oil yield was evident during the catalytic depolymerization process, no matter what kind of catalyst was introduced in the reaction. It could be seen in Table 1 (entry 1, 3–5) that bio-oil yields increased from 40.3% to either 57.4%, 55.5% or 49.1% respectively, when HSZ-640, HSZ-660 or HSZ690 was introduced into the lignin depolymerization process. The biooil yield improved by 9–17% (see entries 1, 3–5 in Table 1), while catalysts were added in the depolymerization process, indicating that HSZ-based mordenite catalysts showed good catalytic activity during the microwave assisted lignin depolymerization in methanol. The results in Table 1 also indicated that higher reaction temperature might have positive effect on the lignin depolymerization, as the bio-oil yield 240
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Table 1 Depolymerization of alkaline lignin and product distributions under different reaction conditions in methanol. Entry
Blanka HSZ-640 HSZ-640 HSZ-660 HSZ-690 Blankb HSZ-640 HSZ-640 HSZ-660 HSZ-690 Blankc HSZ-640 HSZ-640 HSZ-660 HSZ-690
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 a b c
(Si/Al = 18)a (Si/Al = 18)a (Si/Al = 30)a (Si/Al = 240)a (Si/Al = 18)b (Si/Al = 18)b (Si/Al = 30)b (Si/Al = 240)b (Si/Al = 18)c (Si/Al = 18)c (Si/Al = 30)c (Si/Al = 240)c
160 °C, 140 °C, 160 °C, 160 °C, 160 °C, 160 °C, 140 °C, 160 °C, 160 °C, 160 °C, 160 °C, 140 °C, 160 °C, 160 °C, 160 °C,
80 min 80 min 80 min 80 min 80 min 80 min 80 min 80 min 80 min 80 min 80 min 80 min 80 min 80 min 80 min
Table 2 Product distributions during lignin depolymerization under different reaction conditions.
Product yield (wt%) BO
40.3 48.8 57.4 55.5 49.1 50.6 68.3 82.9 75.7 67.1 41.2 54.8 69.3 70.9 64.0
54.3 45.3 35.6 36.8 44.5 43.8 26.8 11.4 18.3 26.6 52.8 37.7 24.1 23.6 29.5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
AL. DL. LS.
Methanol, 140 °Ca Methanol, 160 °Ca Ethanol, 160 °Ca Ethanol, 180 °Ca Isopropanol, 160 °Ca Isopropanol, 180 °Ca Methanol, 160 °Cb Ethanol, 160 °Cb Ethanol, 180 °Cb Isopropanol, 160 °Cb Isopropanol, 180 °Cb Methanol, 160 °Cc Ethanol, 160 °Cc Ethanol, 180 °Cc Isopropanol, 160 °Cc Isopropanol, 180 °Cc
Product yield (wt%) BO
48.8 57.4 27.5 32.7 25.6 30.1 82.9 50.8 56.7 49.6 56.8 69.3 39.8 46.3 38.5 43.7
45.3 35.6 65.7 60.4 67.3 62.8 11.4 42.6 36.4 43.3 35.7 24.1 53.3 47.6 55.7 50.6
Reaction conditions: HSZ-640, 80 min. a AL. b DL. c LS.
increased from 48.8% to 57.4% (see entries 2, 3 in Table 1), when the depolymerization was conducted over HSZ-640 under 140 and 160 °C, respectively. Based on the depolymerization process, HSZ-640 could lead to a much higher bio-oil yield than HSZ-660 or HSZ-690 (see entries 3–5 in Table 1), due to the different properties of catalysts. It could be seen in Table S2 that HSZ-640 and HSZ-660 possessed much higher acidity than HSZ-690, leading to the improved liquid product yield (see entries 3–5 in Table 1). A similar promotion effect of solid acid catalysts (such as HUSY, HZSM-5, etc.) was also observed and confirmed in other reported studies [27,36,37]. Dhepe et al. reported different solid acid catalysts (HUSY (Si/Al = 15), HZSM-5 (Si/Al = 11.5), HBEA (Si/ Al = 19), etc.) could effectively convert six types of lignin feedstocks (dealkaline lignin, bagasse lignin, etc.) into value-added aromatic compounds, confirming that solid acid catalysts could be good alternatives for those precious metal based catalysts. However, the depolymerization process was carried out at relatively hash reaction conditions (250 °C, 0.7 MPa N2, 30 min), which was in need of much higher reaction temperature . Shen et al. also obtained similar experimental results, when they conducted microwave assisted lignin depolymerization over modified HUSY catalysts in formic acid . The results for the depolymerization of dealkaline lignin (entries 6–10 in Table 1) and lignosulfonate (entries 11–15 in Table 1) in methanol also confirmed that HSZ-640 and/or HSZ-660 exhibited higher bio-oil yields during the reaction. Additionally, in comparison with a blank experiment, the promotional effect of HSZ-based catalysts was also observed (entries 6–10 for dealkaline lignin, entries 11–15 for lignosulfonate), as the bio-oil yields improved when catalyst was added in the depolymerization system. Herein, HSZ-640 and HSZ-660 showed much higher catalytic activity with a higher bio-oil yield than HSZ-690 due to their higher acidity of catalysts. HSZ-640 could produce a relatively higher bio-oil yield than the other two catalysts, so in the following studies, HSZ-640 was chosen to investigate the effect of different lignin species on the depolymerization products and their properties.
depolymerization efficiency, and liquid products with different chemical compositions were obtained in different reaction conditions. However, there were very few studies focusing on the investigation of the specific relationship between lignin structures, depolymerization parameters (catalysts, solvents, and so on) and depolymerized liquid products. Thus herein, the depolymerization of alkaline lignin, dealkaline lignin and lignosulfonate was investigated to study the effect of lignin sources on the obtained liquid fuels and their properties. Apart from different lignin species, catalysts and solvents also played important roles during the microwave assisted depolymerization process. Therefore, certain microwave receptors should be introduced into the microwave assisted depolymerization system, as lignin was one kind of poor microwave receptors during the degradation . So in this study, three kinds of alcohols (methanol, ethanol and isopropanol) were chosen as both microwave receptor and hydrogen-donor solvent to study their effect during the depolymerization of alkaline lignin, dealkaline lignin and lignosulfonate. The microwave assisted depolymerization results of three types of lignin over HSZ-640 were presented in Table 2, considering the effect of solvent and reaction temperature on bio-oil yield. Herein, due to the pressure limitation of the microwave reactor (no more than 30 bar), the depolymerization in methanol was conducted at 140 °C and 160 °C respectively, and 160 °C and 180 °C were used when the depolymerization was carried out in ethanol and isopropanol. A higher reaction temperature seemed to have a positive influence on the lignin degradation, as the bio-oil yields all improved with the rise of reaction temperature during the depolymerization. Take the depolymerization results of AL as an example, the bio-oil yield from AL increased from 48.8% to 57.4% with methanol as solvent, when the temperature rose from 140 °C to 160 °C (entries 1, 2 in Table 2); and the bio-oil yields from AL improved from 27.5% and 25.6% to 32.7% and 30.1% respectively (entries 3–6 in Table 2), while ethanol and isopropanol was used as solvent. A similar promotional effect of temperature was also observed during the depolymerization of DL and LS. The results also indicated that varied solvents had an important effect on the bio-oil yield, as the microwave assisted lignin depolymerization efficiency was closely associated with the microwave absorption ability of the substrate and solvent . In comparison with the three solvents used in this study, the bio-oil yield obtained from AL, DL and LS in methanol was 57.4%, 82.9% and 69.3% respectively (entries 2, 7, 12 in
3.3. Effect of lignin feedstocks, catalysts and solvents on microwave assisted lignin depolymerization Different types of lignin feedstocks might lead to depolymerized products (bio-oil) with varied chemical compositions, thus a general summary of current studies focusing on the depolymerization of different lignin feedstocks was presented in Table S3. It could be found that lignin sources, solvents and catalysts all had a certain effect on the 241
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acidity exhibited to be much more effective for the catalytic depolymerization of three types of lignin used in this study. The highest bio-oil yields from AL and DL were all obtained over HSZ-640, and HSZ-640 and HSZ-660 produced a similar bio-oil yield from LS. The above results indicated that certain amount of acidity in catalysts was helpful for the lignin depolymerization, not the more the better, as HSZ-660 produced a little lower bio-oil yield than HSZ-640 during the depolymerization process. This phenomenon was similar to the research by Dhepe, who had tried to determine the correlation between total acid amount of different types of solid acids (HUSY (Si/Al = 15), HMOR (Si/Al = 10), HZSM-5 (Si/Al = 11.5), HBEA (Si/Al = 19) and so on) and their catalytic activity during the lignin degradation process. Unfortunately, Dhepe et al. found it was difficult to draw an exact correlation due to the complexity of the depolymerization reactions in terms of which kind of chemical bonds in lignin were interacting with active sites during the process . Apart from the effect of catalysts, the depolymerization was also greatly affected by the lignin feedstocks. As presented in Table 3, alkaline lignin seemed much more difficult to depolymerize, as the bio-oil yields from AL were much lower than those from DL and LS, no matter what kind of catalyst was used during the reaction. The depolymerization difficulty level of the three lignin sources was AL > LS > DL, taking only the bio-oil yield into consideration. Among those three lignin feedstocks, dealkaline lignin was the easiest to depolymerize, having a much higher liquid product yield, which was in accordance with Subramaniam’s research . The results in Table 3 confirmed that HSZ-based catalysts could provide possible alternatives for other catalysts, and the depolymerization efficiency and liquid product yields were highly dependent on the lignin feedstocks and catalysts used.
Table 2), and the microwave assisted depolymerization was conducted at 160 °C for 80 min over HSZ-640. It could be seen that ethanol and isopropanol gave much lower bio-oil yields under the same reaction conditions (160 °C, 80 min, see entries 3, 5 for AL, entries 8, 10 for DL, entries 13, 15 for LS in Table 2). The bio-oil yield was still much lower even the depolymerization was conducted at 180 °C in ethanol and isopropanol (entries 4, 6 for AL, entries 9, 11 for DL, entries 14, 16 for LS in Table 2). The higher bio-oil yields obtained in methanol were due to the higher polarity and better microwave absorption ability of solvent, as a good microwave receptor was necessary and helpful to improve the microwave assisted depolymerization of lignin. Herein, the polarity order of the three solvents was methanol > ethanol > isopropanol, and the bio-oil yields obtained in different solvents were in good accordance with the polarity sequence. The results in Table 2 confirmed that methanol could lead to the improvement of bio-oil yield due to the higher polarity, and it seemed that this was not dependent on the kind of lignin feedstock used during the depolymerization process. The catalytic depolymerization of dealkaline lignin and lignosulfonate with microwave heating was also investigated in methanol, ethanol and isopropanol (entries 7–11 for DL, entries 12–16 for LS in Table 2) over HSZ-640, and similar phenomena were observed that higher bio-oil yields were obtained while methanol was used as a solvent (82.9% for DL and 69.3% for LS). The results indicated that the solvent with a higher polarity coupled with microwave heating could facilitate the deconstruction of lignin, and a general rule might be concluded that polar solvents could lead to improved bio-oil yield, which was also not dependent on the lignin sources. As can be seen from Table 2, promotional effect of reaction temperature was also observed during the depolymerization of DL and LS in ethanol and isopropanol. The bio-oil yields from DL increased from 50.8% and 49.6% to 56.7% and 56.8 (entries 8–12), and bio-yields from LS increased from 39.8% and 38.5% to 46.3% and 43.7% (entries 13–16), when ethanol and isopropanol were used in the microwave assisted depolymerization process. In comparison with the bio-oil yields obtained from AL, DL and LS, it could be found that the depolymerization efficiency was greatly influenced by the varieties of lignin sources, while the bio-oil yields from DL and LS were much higher than that of AL in either methanol (entries 2, 7, 12), ethanol (entries 4, 9, 14) or isopropanol (entries 6, 11, 16) under the same reaction conditions,. These findings indicated that AL was much more difficult to depolymerize than DL and LS because of the complexity of bonds (C–C or C–O) in different lignin feedstock. In order to investigate the effect of solid acid catalysts on the depolymerization of different types of lignin, catalytic microwave assisted depolymerization of dealkaline lignin and lignosulfonate was also conducted in methanol over HSZ-640, HSZ-660 and HSZ-690, and biooil yields were presented in Table 3. It might be concluded that bio-oil yields were closely associated with catalysts and lignin sources. As can be seen in Table 3, HSZ-640 and HSZ-660 with a relatively higher
3.4. Liquefied bio-oil properties analysis In the above studies, it was confirmed that the depolymerization efficiency and bio-oil yields were highly dependent on lignin sources, solvents and catalysts during the microwave assisted depolymerization. And there was a common phenomenon that the highest bio-oil yields from AL, DL and LS were all obtained over HSZ-640 or HSZ-660 in methanol, because of the higher polarity of methanol and higher acidity of HSZ-640 and HSZ-660, which were not dependent on the lignin sources. In order to discuss the reaction conditions on the liquid product properties (such as elemental composition, O/C ratio, H/C ratio, HHV, etc.), detailed information of obtained bio-oil was presented in Table 4. After the microwave assisted depolymerization, the oxygen composition in bio-oil decreased and hydrogen composition increased, indicating the hydrogenation/hydrodeoxygenation activity of HSZbased solid acid catalysts, and those improvements in bio-oil elemental composition finally led to the increase of HHV in comparison with those original lignin feedstocks (see results in Table S1 and Table 4), which was needed for value-added utilization of lignin for liquefied fuels. The elemental compositions and HHV of bio-oil obtained from AL (entries 1–3), DL (entries 6–8) and LS (entries 11–13) in methanol confirmed the promotion effect of catalysts during the conversion of different types of lignin into liquid bio-fuels. There was an increase in hydrogen composition (about 7–9%) and a decrease in oxygen content (about 6–14%) under different reaction conditions, which was helpful to improve the heating value of those liquid fuels. As it can be seen in Table 4, the HHV improved to about 28–32 MJ/kg, in comparison with the lower HHV of different lignin feedstock (18.02 MJ/kg for AL, 18.58 MJ/kg for DL, 16.61 MJ/kg for LS in Table S1). Apart from the effect of catalysts, solvents were not only associated with the lignin depolymerization efficiency, but were also closely associated with the bio-oil properties. An obvious improvement in hydrogen content and HHV of bio-oil from different types of lignin sources was observed, no matter what kind of solvent was used during the depolymerization process (entries 1, 4, 5 for AL, 6, 9, 10 for DL, 11, 14, 15 for AL) over HSZ-640 under the optimal depolymerization conditions. In previous
Table 3 Microwave assisted depolymerization of different lignin feedstocks in methanol over different catalysts. Feedstock
Product yield (wt%) BO
1 2 3
HSZ-640 HSZ-660 HSZ-690
57.4 55.5 51.3
35.6 38.4 41.9
4 5 6
HSZ-640 HSZ-660 HSZ-690
82.9 75.7 67.1
11.4 18.3 26.6
7 8 9
HSZ-640 HSZ-660 HSZ-690
69.3 70.9 64.0
24.1 23.6 29.5
Reaction conditions: 160 °C, 80 min, in methanol. 242
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Table 4 Effect of lignin species, catalysts and solvents on elemental compositions and HHV of bio-oil from different lignin feedstocks. Entry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Elemental composition (wt%)
AL, HSZ-640a AL, HSZ-660a AL, HSZ-690a AL, HSZ-640b AL, HSZ-640c DL, HSZ-640a DL, HSZ-660a DL, HSZ-690a DL, HSZ-640b DL, HSZ-640c LS, HSZ-640a LS, HSZ-660a LS, HSZ-690a LS, HSZ-640b LS, HSZ-640c
49.83 50.46 49.95 51.41 53.04 50.49 51.38 49.69 51.35 53.98 47.84 48.11 46.96 51.22 53.57
13.16 13.54 12.96 13.61 13.93 12.82 13.16 12.58 13.45 13.88 13.02 13.31 12.86 13.46 13.17
36.82 35.77 36.81 34.7 32.75 36.49 35.19 37.46 34.94 32.52 38.94 38.38 39.91 35.12 33.03
0.05 0.11 0.13 0.15 0.16 0.06 0.10 0.14 0.11 0.15 0.07 0.08 0.11 0.06 0.08
0.14 0.12 0.15 0.13 0.12 0.14 0.17 0.13 0.15 0.17 0.13 0.12 0.16 0.14 0.15
0.55 0.53 0.55 0.51 0.46 0.54 0.51 0.56 0.51 0.44 0.61 0.60 0.63 0.51 0.55
3.17 3.22 3.11 3.17 3.15 3.04 3.07 3.04 3.14 3.08 3.26 3.32 3.28 3.15 2.95
29.72 30.51 29.52 31.03 32.17 29.55 30.41 28.89 30.78 32.53 28.66 29.16 28.07 30.73 31.38
Fig. 1. GC–MS results and product distributions of bio-oil from different lignin feedstocks (reaction conditions: over HSZ-640, in methanol, 160 °C, 80 min).
Reaction conditions: a – 160 °C, 80 min, in methanol; b – 180 °C, 80 min, in ethanol; c – 180 °C, 80 min, in isopropanol. HHV (MJ/ kg) = (34C + 124.3H + 6.3N + 19.3S − 9.8O)/100, where C, H, N, S, and O are the weight percentages of carbon, hydrogen, nitrogen, sulfur, and oxygen .
was carried out from 200 m/z to 8000 m/z, almost no macromolecules (m/z above 800) were observed from the MALDI-TOF MS results. Some phenolic dimers and trimers with molecular weight of 273 m/z, 339 m/ z, 405 m/z, 412 m/z and 456 m/z were observed main products, which were different from the oligomers observed in other studies [19,27,40,41]. As those aromatic oligomers with molecular weight of 274 m/z, 290 m/z, 316 m/z, 332 m/z, 362 m/z, 406 m/z and 454 m/z were found to be main compounds in obtained bio-oil, while the lignin depolymerization was conducted in isopropanol with microwave heating . However, the intensities of those oligomers varied while different lignin feedstocks were used in the microwave assisted depolymerization process. This variations indicated the different degradation degree of different types of lignin feedstocks, and proved that the oligomer distributions were closely associated with lignin structures. The GC–MS results of bio-oils from different types of lignin were exhibited in Fig. 1, indicating that those bio-oils had similar chemical compositions under the optimal reaction conditions. It could be obviously seen that all those three lignin feedstocks gave a wide and similar distributions of depolymerized products. However, not too many kinds of phenolic monomers were obtained from lignin feedstocks. The main monophenols in the bio-oil were p-hydroxyacetophenone, p-hydroxybenzaldehyde, p-hydroxybenzoic acid, guaiacol, vanillin, vanillyl alcohol, homovanillic acid, homovanillyl alcohol, isovanillic acid methyl ester, p-hydroxyacetovanillon, acetosyringone, sinapinic acid methyl ester, syringaldehyde, syringic acid, which could be divided into H (hydroxyphenyl), G (guaiacyl) and S (syringyl) types compounds. It could be seen in Fig. 1 that G types compounds were of relatively larger amount than H and S types compounds, and the compositions varied while different lignin source was used in the depolymerization. The presence of methyl ester (G4, S4) proved the occurrence of an esterification reaction between depolymerized phenolic acids and solvent (methanol) during the lignin depolymerization process. The different distributions in obtained bio-oil from alkaline lignin, dealkaline lignin and lignosulfonate proved that the structural characteristics were of great importance during the microwave assisted lignin depolymerization process, while varied bio-oil yields and different chemical compositions of bio-oil were observed in the reaction. Additionally, it was observed that there was relatively more kinds of G derivatives than H and S derivatives in the liquefied bio-oil. The results were in accordance with 13C NMR results, which was discussed and presented in Fig. S1 that all the three lignins used in this study were of a higher intensity of functional groups in G units.
study, it was confirmed that methanol could give higher bio-oil yields compared with ethanol and isopropanol. However, unlike the bio-oil yields, no obvious and common rule could be concluded from the elemental composition results, and only a general increase in hydrogen content and HHV was observed. The complexity of reactions that happened during the depolymerization might lead to this phenomenon. MALDI-TOF MS and GC–MS technique were employed for a detailed understanding of the molecular weight and chemical composition in the obtained bio-oil under different depolymerization conditions. The molecular weight of bio-oil was also closely associated with lignin species and solvents during the depolymerization process. The molecular weight of bio-oil from AL, DL and LS under optimal reaction conditions were presented in Table 5. Bio-oil obtained from AL, DL and LS over HSZ-640 in methanol was of a different molecular weight (entries 1, 4, 7), caused by different lignin structures. The lowest molecular weight of bio-oil was all observed in methanol, which was not dependent on the lignin sources. It could be found that bio-oil obtained in methanol (entries 1, 4, 7) exhibited to have a much lower molecular weight than those obtained from either ethanol (entries 2, 5, 8) or isopropanol (entries 3, 6, 9), which was in accordance with the depolymerization efficiency and polarity of the solvent, indicating the effect of solvent on the depolymerization and bio-oil properties. Apart from the molecular weight, those aromatic oligomers in biooil were also investigated by MALDI-TOF MS. Although the scanning Table 5 Molecular weights of bio-oil obtained from different lignin feedstocks under different reaction conditions. Entry
1 2 3 4 5 6 7 8 9
AL, methanol, 160 °C AL, ethanol, 180 °C AL, isopropanol, 180 °C DL, methanol, 160 °C DL, ethanol, 180 °C DL, isopropanol, 180 °C LS, methanol, 160 °C LS, ethanol, 180 °C LS, isopropanol, 180 °C
Molecular weight Mn
21.3 360.2 368.6 309.1 345.6 357.8 312.3 386.7 385.4
523.6 558.7 565.8 515.4 543.2 550.6 532.8 575.6 582.3
Reaction conditions: catalyzed by HSZ-640, reaction time: 80 min. 243
Fuel 239 (2019) 239–244
M. Zhou et al.
4. Conclusion 
In this study, microwave assisted depolymerization of three different types of lignin sources were investigated over HSZ-based solid acid catalysts in relatively mild reaction conditions (temperature between 140 and 180 °C). The depolymerization efficiency of lignin was closely dependent on lignin structures, catalysts and solvents. The highest bio-oil yield from alkaline lignin, dealkaline lignin and lignosulfonate was 57.4%, 82.9% and 70.9% respectively, under mild conditions. A common rule and a synergistic effect was observed, as the highest bio-oil yields were all obtained in methanol coupled with HSZ640 (or HSZ-660) and microwave heating, on account of the higher solvent polarity of methanol and higher acidity of HSZ-640 (or HSZ660). The hydrogen composition and HHV of bio-oil improved, and the oxygen content decreased, indicating that hydrogenation and/or hydrodeoxygenation happened during the depolymerization process.
     
The authors are grateful for the financial support from National Natural Science Foundation of China (31700645), and the Natural Science Foundation of Jiangsu Province (BK20170159). Minghao Zhou (201703270013) would like to acknowledge the fellowship from the China Scholarship Council (CSC).
  
Appendix A. Supplementary data
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2018.10.144.
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