Fuel 217 (2018) 202–210
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Full Length Article
Understanding the relationship between the structure and depolymerization behavior of lignin Jaeyong Parka, Asim Riaza, Rizki Insyanib, Jaehoon Kima,b, a b
T
⁎
School of Mechanical Engineering, Sungkyunkwan University, 2066 Seobu-Ro, Jangan-Gu, Suwon, Gyeong Gi-Do 16419, Republic of Korea SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, 2066 Seobu-Ro, Jangan-Gu, Suwon, Gyeong Gi-Do 16419, Republic of Korea
G RA P H I C A L AB S T R A C T
A R T I C L E I N F O
A B S T R A C T
Keywords: Lignin Lignin structure Depolymerization Supercritical ethanol Formic acid
Various lignin depolymerization methods have been proposed. Nevertheless, the relationship between the structure of lignin and its depolymerization behavior has not been widely investigated. Herein, six types of lignin samples were produced from oakwood (OW, hardwood) and pinewood (PW, softwood) using three different delignification techniques (ethanolsolv, formasolv, and Klason). The content of ether linkages in the OW-derived lignins was approximately three times higher than that in the PW-derived lignins because of the presence of the sinapyl alcohol unit in the former. The contents of ether linkages in the lignin isolated via the different methods followed the order: formasolv > ethanolsolv > Klason. The lignin samples were depolymerized in a mixture of supercritical ethanol (scEtOH) and formic acid at temperatures of 250–350 °C. At 350 °C, regardless of the lignin type, high conversion (> 95%) and a high bio-oil yield (> 81 wt%) could be achieved, demonstrating that the combined use of scEtOH-HCOOH was very effective for the depolymerization of various types of lignin. At the low temperatures of 250–300 °C, the lignin conversion and bio-oil yield were highly dependent on the amount of ether linkages; for example, at 300 °C, the use of OW-derived formasolv lignin resulted in a high bio-oil yield (86.2 wt%), whereas the use of OW-derived Klason lignin resulted in a very low bio-oil yield (27.9 wt%). The properties of the bio-oils produced from the different types of lignin were discussed.
⁎ Corresponding author at: School of Mechanical Engineering and SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, 2066 Seobu-Ro, Jangan-Gu, Suwon, Gyeong Gi-Do 16419, Republic of Korea. E-mail address:
[email protected] (J. Kim).
https://doi.org/10.1016/j.fuel.2017.12.079 Received 15 October 2017; Received in revised form 8 December 2017; Accepted 19 December 2017 Available online 02 January 2018 0016-2361/ © 2017 Elsevier Ltd. All rights reserved.
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1. Introduction
for example, Xabier et al. compared the depolymerization behavior of three types of organosolv lignin (acetosolv, formosolv, and acetosolv/ formosolv) produced from a single wood resource using supercritical acetone, ethanol, and methanol [27]. In supercritical acetone at 300 °C and 6.6–6.7 MPa, the use of an acetosolv/formosolv lignin for 40 min resulted in a higher oil yield (38 wt%) than obtained with individual acetosolv and formosolv lignin (32–34 wt%). One of the key parameters determining the oil yield is the molecular weight of lignin; the oil yield was found to be lower when the molecular weight of lignin was lower. Jun et al. studied the catalytic solvolysis of organosolv lignins produced from Chinese fir and maple in a mixture of supercritical ethanol and 1butanol in the presence of a Ru/C catalyst at 300 °C under a pressure of 40 MPa for a reaction time of 40 min [28]; the lower oil yield from Chinese fir lignin (32 wt%) than that from maple lignin (41 wt%) was attributed to the different lignin origins according to the wood types. Although the previous studies illustrated a plausible relationship between the lignin sources and bio-oil yields, there is still a lack of understanding of the fundamental properties of lignins and their depolymerization behaviors. Herein, we investigate the chemical and physical properties of lignin produced by using two different types of lignocellulosic biomass (oak (hardwood) and pine (softwood)) and three different lignin separation methods (ethanolsolv, formasolv, Klason). The produced lignin was depolymerized in a supercritical mixture of ethanol (scEtOH) and formic acid (HCOOH) to gain fundamental understanding of the properties of lignin and its depolymerization behavior. The yields and properties of the lignin-derived oils from the depolymerization of oakwood-derived and pinewood-derived lignins are discussed in detail.
Lignocellulosic biomass is considered one of the most promising non-edible renewable resources for producing bio-derived fuels and chemicals because of its natural abundance and global availability [1–4]. In a typical sugar platform pathway for producing biofuels and biochemicals (e.g., cellulosic bioethanol and biobutanol), it is necessary to separate lignin from cellulose using an appropriate pretreatment method [5–7]. When the current, active commercialization status of lignocellulosic biofuels worldwide is taken into account, a huge amount of lignin (which corresponds to 25–35 wt% of lignocellulosic biomass [8]) is generated from biorefinery plants as a byproduct [3,9]. Approximately 40% of the lignin produced from cellulosic biofuel plants would be required as a source of internal energy from combustion [10]; thus, the development of effective techniques for lignin valorization to produce liquid fuels and value-added chemicals is highly desired. The production of surplus lignin in the near future and its rich aromatic structure make it a promising alternative to petroleum-based aromatic chemicals. Nowadays, however, only a very small fraction (1–2%) of the lignin produced from pulping liquors is used to produce specialty chemicals [11]. One reason for the limited use of lignin as a feedstock for producing liquid fuels and chemicals is the highly recalcitrant and complex nature of lignin, which makes depolymerization very difficult. Moreover, the presence of various types of chemical bonds in the lignin derived from different wood sources and the unpredictable changes in the chemical bonding during delignification make it difficult to develop a “generalized” depolymerization technique; a depolymerization method that is effective for a certain type of lignin is not necessarily effective for other types of lignin. Therefore, it is crucial to understand the relationship between the lignin structure and its depolymerization behavior to develop an efficient valorization technique for ultimate utilization of various types of lignin. Various methods of separating lignin from cellulose have been developed, including the steam explosion, Kraft, alkali, concentrated strong acid hydrolysis, and organosolv approaches [7]. During the pretreatment of lignocellulosic biomass, the structure of “native” lignin is ultimately changed; the degree and extent of the structural change is highly dependent on the pretreatment method selected for lignin separation. Most of the pretreatment methods proceed at elevated temperatures of 100–200 °C in the presence of acid or base catalysts. Under these conditions, CeC coupling reactions tend to occur, making the isolated lignin more recalcitrant to depolymerization [12,13]. Therefore, the chemical nature of isolated lignin (phenyl propanol monomers, ether and condensed linkages, bonding energies, substituent groups, etc.) is not only dependent on the lignin source (e.g., hardwood, softwood), but also on the separation technique. This makes it very difficult to develop a generalized depolymerization method for the effective production of value-added aromatic chemicals and fuel additives from various types of lignin. Various lignin depolymerization approaches have been proposed, including fast pyrolysis [14], the ionic liquid-assisted method [15], biological degradation [16], and hydrothermal/solvothermal reactions with or without a catalyst [17,18]. Among these approaches, the hydrothermal/solvothermal technique is considered very promising because of its high liquid yield (up to 95 wt%) with low char formation (5–20 wt%) [19]. To enhance the conversion of lignin under hydrothermal/solvothermal conditions, various types of solvents (e.g., water, methanol, ethanol, isopropyl alcohol (IPA), acetone [20–22]) and catalysts (e.g., NaOH [23], KOH [24], H-USY (ultra-stable zeolite Y, Si/ Al = 15) [25], CuMgAlOx [22], Pt/Al2O3 [26]) have been evaluated. To develop a versatile method for the depolymerization of lignin produced from various lignocellulose sources and using different pretreatment methods, it is necessary to gain comprehensive understanding of the relationship between the lignin structure and its depolymerization behavior. However, only a few studies have been dedicated to understanding the depolymerization behavior using different lignin sources;
2. Materials and methods 2.1. Materials Oakwood (Ouercus, OW, hardwood) and pinewood (Pinus, PW, softwood) were purchased from a local market in South Korea. The cellulose, hemicellulose, and lignin content were analyzed by using the Van Soest method [29], as listed in Table S1. HPLC grade ethanol, acetone, and dichloromethane (DCM) were purchased from Burdick & Jackson (USA). Aqueous sulfuric acid (H2SO4, ≥99.5%, ACS reagent grade) was purchased from Sigma-Aldrich (USA). Formic acid (reagent grade) and HCl (35%, extra pure grade) were purchased from Daejung Chemical & Metal (South Korea). Distilled-de-ionized (DDI) water was prepared by using an AQUAMax™-Basic 360 water purification system (Younglin Instrument Co., Ltd., South Korea). High-purity N2 (99.999%) for purging the reactor was purchased from JC Gas Company (South Korea). 2.2. Lignin separation methods To produce the ethanolsolv lignin, a batch reactor with an inner volume of 140 mL was filled with 10 g of wood, 80 mL of ethanol/water mixture (50:30 v/v), and 1 g of sulfuric acid as a catalyst. The reactor was heated to 190 °C by using cartridge heaters and a heating furnace and kept for 1 h. After delignification, the solid and black liquid products were collected from the reactor and separated by centrifugation at 4000 rpm for 10 min. The solid products contained cellulose and a small fraction of unreacted lignin. A 240-mL aliquot of water was then added to the recovered filtrate to precipitate the fragmented lignin in the black liquid. This precipitated lignin was then dried in a drying oven overnight at 80 °C. The ethanolsolv lignin samples produced using oakwood and pinewood are designated as OW-E and PW-E, respectively. For the production of formasolv lignin, the batch reactor was filled with 10 g of wood, 80 mL of a formic acid/water mixture (80:20 w/w), and 0.2 g of HCl as a catalyst. The reactor was then heated to 120 °C and kept for 1.5 h. After the reaction, the same separation protocol that was employed to recover the ethanolsolv lignin was used to obtain the 203
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2.4. Analysis and characterization
formasolv lignin. The formasolv lignin samples produced using oakwood and pinewood are designated as OW-F and PW-F, respectively. Klason lignin was prepared by a two-step acid hydrolysis reaction; first, the wood sample was hydrolyzed with a concentrated strong acid (72% H2SO4) for 2 h at room temperature. The reaction mixture was then diluted to give a 3% H2SO4 aqueous solution by adding DDI water, and the mixture was reacted at 110 °C under reflux conditions for 4 h for the subsequent weak acid hydrolysis reaction. After the two-step acid hydrolysis reaction, the holocellulose was dissolved in aqueous sulfuric acid and the lignin precipitated at the bottom of the flask. The precipitated lignin was separated from the dissolved holocellulose by centrifugation. The recovered lignin was washed with DDI water several times and then dried in a drying oven at 80 °C for 12 h. The Klason lignin samples produced using oakwood and pinewood are designated as OW-K and PW-K, respectively. The yields of lignin recovered from each delignification method followed the order: Klason (100 wt %) > ethanolsolv (63–71 wt%) > formasolv (53–54 wt%), as listed in Table S2.
To quantify the ether linkages in the lignin samples (e.g., β-O-4, αO-4, 4-O-5), the vanillin and syringaldehyde contents were analyzed using nitrobenzene oxidation, as proposed by Chen [30]. Prior to nitrobenzene oxidation, the wood samples were ground and extracted using a Soxhlet apparatus with a toluene/ethanol mixture (2:1) at 120 °C for 6 h. Fifty milligrams of dried lignin or 200 mg of dried wood, 7 mL of 2 M NaOH, and 0.48 g of nitrobenzene were placed into a stainless-steel bomb reactor with an inner volume of 11 mL. The reactor was then heated to 170 °C and the oxidation was allowed to proceed for 2.5 h. After the reaction, the liquid mixture was collected from the reactor and water and DCM were added to the reaction mixture. The whole solution was kept in a separating funnel for 4 h to separate the oxidized products in the aqueous phase from nitrobenzene in the DCM phase. The aqueous phase was collected from the funnel and then acidified to pH 2–3 using aqueous HCl. The acidified species in the aqueous phase were then extracted using DCM in three liquid-liquid extraction cycles. The DCM phase was evaporated using a rotary evaporator at 40 °C and 0.04 MPa to recover the products for further analysis. Thermogravimetric analysis (TGA) was conducted over the temperature range of 30–800 °C using a TA Instruments Q50 TGA instrument at a heating rate of 10 °C min−1. The gas flow rate was fixed at 60 mL min−1. Elemental analysis (EA) of the samples was conducted with a Vario EL cube elemental analyzer equipped with a TCD detector (Elementar Analysensysteme GmbH, Germany). The combustion tube and the reduction tube temperature were maintained at 1150 °C and 850 °C, respectively. The oxygen content was analyzed using a TCD detector in O-mode with a pyrolysis tube at 1170 °C. The higher heating value (HHV) of the raw lignin samples and bio-oil was calculated using the DIN 51900 standard:
2.3. Lignin depolymerization reaction The depolymerization experiments were performed by using a custom-built, batch reactor made of SUS 316 having an inner volume of 140 mL, equipped with a magnetically driven stirrer. The reactor was rated to rated 500 °C at 50 MPa. In each lignin depolymerization reaction, 3 g of lignin, 60 g of ethanol and 12 g of formic acid were placed into the reactor. The reactor was closed and purged with N2 gas three times through a purge line dipped into the solution to remove dissolved oxygen in the liquid phase and the oxygen in the reactor head, and then pressurized at 1 MPa with N2. The reactor was heated to the experimentally desired temperatures of 250–350 °C at a heating rate of approximately 20 °C min−1. After reaching the desired reaction temperature, the reaction was allowed to proceed for 1 h. After the reaction, the reactor was quickly quenched to 100 °C using a water basket, in order to prevent further possible reaction during the cooling process, and was then further cooled to room temperature using an electric fan. The reactor pressure was recorded after complete cooling to calculate amount of gas produced. The produced gas was collected in a 0.5-L Tedlar bag for compositional analysis. The reaction mixture in the reactor was then collected in a beaker by washing with acetone. The liquid and solid products were separated using a Whatman filter paper. The solid residue was dried in a drying oven at 70 °C for 6 h. The liquid product, which contained solvent, bio-oil, and byproduct, was evaporated at 50 °C and 0.02 MPa using a rotary evaporator. After the evaporation process, the bio-oil was further dried in a vacuum oven at 60 °C at ∼0 MPa for 6 h to remove the residual solvent. All the experiments were performed in triplicate and the average values are reported. The conversion and the yields of bio-oil, solid residue, and gas product were calculated using the following equations:
HHV (MJ. kg−1) = (34 C+ 124.3 H+ 6.3 N+ 19.3S−9.8O)/100
where C, H, N, S, and O are the weight percentages of carbon, hydrogen, nitrogen, sulfur, and oxygen, which were obtained by EA. The molecular weight distribution of bio-oil was characterized by using a Bruker UltrafleXtreme™ matrix-assisted laser desorption ionization time-of-flight mass spectrometer (MALDI-TOF/MS, USA). The composition of the produced gases was analyzed by using a PerkinElmer Clarus 600 GC-Model Arnel 1115PPC Refinery Gas Analyzer (RGA, PerkinElmer, CT, USA). The detailed specifications of the RGA-GC have been reported previously [31]. The bio-oil was characterized using an Agilent Technologies 7890A GC with a time-offlight mass spectrometer (TOF/MS) detector for both qualitative and quantitative analysis. A detailed description of the GC-TOF/MS equipment is given elsewhere [32]. The functional groups on the sample surface were characterized by using a NICOLET iS10 Fourier-transform infrared (FT-IR) spectrometer (Thermo Electron Co., USA). 3. Results and discussion 3.1. Characterization of lignin
Conversion Weight of dry ash free lignin-Weight of organics in solid residue = Weight of dry ash free lignin
Yield of bio oil (wt%) =
Weight of bio oil × 100 Weight of dry ash free lignin
Yield of solid residue (wt%) =
Yield of gas (wt%) =
The lignin samples were characterized by proximate and ultimate analyses, as listed in Table 1. All the lignin samples produced from the different types of wood and by the different delignification methods had similar contents of volatile matter (49–55 wt%) and fixed carbon (45–49 wt%). The ash content of the Klason lignin was slightly higher than that of the ethanolsolv and formasolv lignin samples. During the ethanolsolv and formasolv processes, the lignin was dissolved in the organic solvents and recovered by precipitation in water. On the other hand, during the Klason process, the lignin remained in the solid form and holocellulose was dissolved in the acidic aqueous solvent, and thus some fraction of ash present in the wood sample could be trapped in the solid lignin phase. The sulfur contents of the ethanolsolv and formasolv lignin samples were under the detection limit of EA (< 0.01 wt%), but
(1)
× 100
Weight of solid residue × 100 Weight of dry lignin
Weight of produced gas × 100 Weight of dry ash free lignin
(5)
(2)
(3)
(4) 204
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Table 1 Proximate and ultimate analysis of the lignin feedstocks used in this study. Proximate analysis (wt%, d.b.)a
OW-E OW-F OW-K PW-E PW-F PW-K c,d
Ultimate analysis (wt%, d.a.f)b
VMc
FCd
Ashe
Moisture
C
H
O
N
S
O/C
H/C
50.6 54.4 54.2 52.5 53.5 49.0
48.2 44.5 44.0 45.7 46.2 49.1
0.1 N.D. 0.6 N.D. 0.2 0.6
1.1 1.1 1.2 1.8 0.3 1.3
69.2 63.6 60.9 68.0 66.4 60.6
5.13 5.15 4.84 5.02 5.08 4.57
23.9 28.2 30.1 24.7 26.4 26.3
N.D.f N.D. N.D. N.D. N.D. N.D.
N.D. N.D. 5.85 N.D. N.D. 5.92
0.26 0.33 0.37 0.27 0.30 0.33
0.92 0.97 0.95 0.89 0.92 0.90
VM and FC were determined using TGA in N2. a On a dry basis. b On a dry, ash-free basis. c VM: volatile matter. d FC: fixed carbon. e Ash content was determined using TGA in air. f Not detectable: under the detection limit of EA (0.01 wt%).
formasolv method was used as compared to the ethanolsolv and Klason methods. During the lignin separation reaction, the α-aryl ether and βaryl ether bonds of lignin in the wood undergo hydrolysis to produce small soluble fractions, which are detached from the cellulose. Protonation of the hydroxyl group in the α or β position with acid catalysts can convert the α or β carbon into a carbocation, which can break the ether bond. On the other hand, the formed carbocation may be recondensed in an intramolecular manner to produce cyclic species or may react with other fragmented lignins [35,36]. The extent of the recondensation reaction may depend on the relative amount of guaiacyl alcohol and sinapyl alcohol units in a given lignin species because the sinapyl alcohol, in which the ortho position of the aromatic ring is occupied by methoxy groups, can effectively suppress self-recondensation [36]. Therefore, the OW-derived lignin samples gave rise to higher V + S yields than the PW-derived lignin samples. In addition to the wood source, the degree of recondensation is also known to be dependent on the delignification method and conditions [12]. As listed in Table 2, the formasolv method resulted in consistently higher V + S yields than achieved with the ethanolsolv method, regardless of the wood type. The lower reaction temperature (120 °C) of the formasolv method than that of the ethanolsolv method (190 °C) may account for the lower degree of condensation in the former case. In addition, when the Klason method was used, much lower V + S yields were obtained than achieved with the formasolv and ethanolsolv methods because of the use of a large quantity of strong acid [35]. Because the CeC bond dissociation energy (BDE) (234.9–483.4 kJ mol−1) is larger than the CeO BDE (182.7–311.6 kJ mol−1) [37], the high degree of recondensation can make lignin depolymerization more difficult. The functional groups in the lignin samples were analyzed using FTIR, and the results are shown in Fig. 1. Signals of hydroxyl (νeCeOH at ∼3430 cm−1, including vibrations from COOH and H2O), carbonyl (νC]O at 1710 cm−1), skeletal aromatic ring vibrations (νeC]C at 1605 and 1510 cm−1), and CeH (νeCeH at 1460 and 1365 cm−1) groups were clearly detectable in the profile of the lignin samples, which agrees well with previous observations [28,38]. The signals of the CeO groups at 1320 and 1120 cm−1 only appeared in the profile of the OWderived lignin samples, indicating that the corresponding ether bonds originated from the syringol group. On the other hand, the signals of the CeO groups at 1270 and 1030 cm−1 (which correspond to the guaiacol group) in the profile of the PW-derived lignin samples had a stronger transmittance than observed for the OW-derived counterparts. Compared to the ethanolsolv and formasolv lignin samples, the Klason lignin exhibited weak peaks at 1320 and 1270 cm−1, indicating that fewer CeO bonds of the guaiacol and syringol groups were present. This agrees well with the nitrobenzene oxidation results. The lignin samples were not completely soluble in acetone or formic acid, and thus only the molecular weights of the soluble fraction were
the Klason lignin contained approximately 5 wt% sulfur, which may have originated from the sulfuric acid treatment. To quantify the ether linkages (e.g., β-O-4, α-O-4, 4-O-5) in the lignin samples, nitrobenzene oxidation was employed; during the nitrobenzene oxidation, vanillin (V) and syringaldehyde (S) were formed by cleavage of the ether linkages [30], which are the major chemical bonds in lignin. As listed in Table 2, the amount of V and S produced during the nitrobenzene oxidation was highly dependent on the wood source as well as the type of lignin produced using the various isolation methods. When the wood samples were oxidized, the total V + S yield from OW was approximately three times higher than that produced from PW because of the presence of syringaldehyde in the oxidized mixture. Lignin from hardwood contains both guaiacyl alcohol (G) and sinapyl alcohol (S) units with G/S ratios in the range of 1–4, which leads to a high ether linkage content in the range of 64–86%. On the other hand, softwood lignin consists of almost 95% guaiacyl alcohol units, and thus the amount of ether linkages (59–74%) in softwood is lower than that in hardwood [33,34]. The V + S content of the lignin samples was an order of magnitude smaller than that of the “intact” lignin present in the corresponding woods. This indicates that the structure of lignin changed significantly to form CeC condensed linkages at the expense of the ether linkages during the lignin separation process. Notably, the V + S yields from the lignin samples were highly dependent on the delignification methods. For example, when OW was used, the V+S yield followed the order: OW-F (129.3 µmol g−1) > OW-E > (116.4 µmol g−1) > OW-K (83.4 µmol g−1). A similar trend in the V yield was observed for the PWderived lignin samples. This indicates that less condensation to form condensed linkages at the expense of ether linkages occurred when the
Table 2 Nitrobenzene oxidation data. Feedstock
Vanillin (V) (µmol g−1)
Syringaldehyde (S) (µmol g−1)
Total (V + S) (µmol g−1)
OWa OW-Eb OW-Fc OW-Kd PWe PW-E PW-F PW-K
520.1 49.8 35.6 40.6 459.3 35.4 68.9 23.0
788.2 66.6 93.7 42.8 0 0 0 0
1308.3 116.4 129.3 83.4 459.3 35.4 68.9 23.0
a b c d e
Oakwood. Ethanolsolv lignin from oakwood. Formasolv lignin from oakwood. Klason lignin from oakwood. Pinewood.
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Fig. 2. (a) Yields of bio-oil and solid reside produced from the depolymerization of OW-E, OW-F, and OW-K at temperatures of 250–350 °C. Reaction conditions: 9–10 MPa (250 °C), 19–23 MPa (300 °C), 30–46 MPa (350 °C), 3 g lignin, 60 g ethanol, 12 g formic acid, 60 min. (b) Correlation between the amount of ether linkages and bio-oil yield.
Fig. 1. FT-IR spectra of the lignin samples (a) wavenumber range from 4000 to 800 cm−1, (b) wavenumber range from 1400 to 1100 cm−1.
analyzed using MALDI-TOF/MS, as shown in Fig. S1. The Klason lignin was not soluble any solvents, and thus its molecular weight could not be analyzed. The most intense peak of the lignin samples produced from the ethanolsolv method was observed at 900 m/z, whereas that produced from the formasolv method had a maximum peak at 1200 m/z. In addition, the formasolv lignin samples contained a large amount of high-molecular-weight species with signals in the range of 2000–3500 m/z. This indicates that the lignin structure underwent less fragmentation during the formasolv process than in the ethanolsolv process. The low delignification temperature of the formasolv method may be responsible for the high-molecular-weight fraction in the formasolv lignin samples.
state can donate hydrogen [39–44] and the addition of formic acid to the reaction mixture enhanced the hydrogen generation [45,46] which effectively quenches the radicals and suppresses repolymerization during the cracking reaction [47–51]. In addition, formation of ether bonding followed by elimination-hydrogenolysis could help depolymerize lignin [52–54]. As shown in Fig. S2, H2 comprised approximately 30 mol% of the gases produced during the depolymerization. In addition to its role as a hydrogen donor, supercritical ethanol can participate in the reaction by ring alkylation [55–57] and esterification [58,59]. Self-decomposition of ethanol to produce low-molecularweight oxygenated hydrocarbons [39] may also occur during the depolymerization in scEtOH. These factors may result in the yield of biooil produced from OW-F being over 100%. Although the yields of bio-oil produced from the different types of OW-derived lignins were quite similar at the high reaction temperature of 350 °C, the conversion and bio-oil yields were significantly dependent on the types of lignin at lower reaction temperatures of 250–300 °C, as shown in Fig. 2a. When the reaction temperature decreased from 350 to 250 °C, the conversion of OW-K and the bio-oil yield decreased significantly from 97.7% to 19.3% and 89.9 to 18.7 wt %, respectively. In contrast, when OW-E and OW-F were used, the conversion and bio-oil yield decreased to a lower extent as compared to OW-K; for example, at the low reaction temperature of 250 °C, the yields of bio-oil from OW-E and OW-F were still as high as 68.0 wt% and 77.5 wt%, respectively. In the case of OW-K, the significant drop in the conversion and bio-oil yield at low temperature may be attributable to the high content of recalcitrant CeC linkages formed during the
3.2. Depolymerization of lignin produced from OW To investigate the lignin depolymerization behavior and the properties of the bio-oils from the lignin samples produced by the different separation methods, OW-E, OW-F, and OW-K were reacted with formic acid (5:1 w/w ratio) in scEtOH at different reaction temperatures of 250–350 °C. At the end of the 1 h reaction, the pressure increased to ∼10 MPa at 250 °C and to ∼46 MPa at 350 °C. As listed in Table S3, almost complete conversion of lignin with a high bio-oil yield of > 90 wt% and a low solid-residue yield (< 2.5 wt%) could be achieved at the high reaction temperature of 350 °C, regardless of the lignin samples used. This indicates that the combined use of scEtOH and formic acid at 350 °C is very effective for the conversion of various types of lignin into bio-oils. This may be because ethanol in its supercritical 206
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Fig. 3. GC-TOF/MS chromatograms of bio-oils produced from OW-derived lignin. Reaction conditions: 350 °C, 30–46 MPa, 3 g lignin, 60 g ethanol, 12 g formic acid, 60 min.
achieved with the scEtOH + HCOOH mixture above 350 °C, regardless of the type of lignin used. The molecular weights of the bio-oil produced from the OW-derived lignin samples are shown in Fig. S3a. After depolymerization, the peak
Klason process. Overall, as shown in Fig. 2b, there is a strong correlation between the amount of ether linkages (determined by nitrobenzene oxidation) and the bio-oil yield at the low-to-medium temperature range of 250–300 °C, whereas a high-yield bio-oil (> 81 wt%) was 207
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comparable to that observed in the depolymerization of OW-F and OWE. Therefore, despite the low content of labile ether linkages in the PWderived lignins produced by the formasolv and ethanolsolv methods, the high depolymerization activity associated with scEtOH + formic acid is not significantly affected by the structure of lignin derived from different wood sources. The lowest conversion (95.1%) and bio-oil yield (81.7 wt%) were achieved with PW-K because of the high content of CeC linkages. However, even when the highly recalcitrant PW-K was used as the feedstock, the bio-oil yield was higher than that achieved via the acid-catalyzed depolymerization of dealkaline lignin at 250 °C for 30 min (60 wt% THF soluble) [25], base-catalyzed depolymerization of organosolv lignin at 300 °C (52 wt%) for 40 min [23], and metalcatalyzed depolymerization of alkali lignin at 265 °C (45.8 wt%) for 1 h [61] (see Table S6 for a comparison of the bio-oil yield). This indicates that scEtOH + formic acid is a highly versatile and effective medium for the depolymerization of various types of lignin. The molecular weight distributions of the bio-oils produced from the PW-derived lignins are shown in Fig. S3b. As in the case of the OWderived lignins and their corresponding bio-oils, the peaks were shifted to the low-molecular-weight direction for the bio-oils produced from PW-derived lignin. The bio-oils produced from the different lignin samples had similar molecular weight distributions. Again, this clearly indicates the effective depolymerization of the various types of lignin in the scEtOH + formic acid medium. The GC-TOF/MS chromatograms of the bio-oils produced from the PW-derived lignins are shown in Fig. 4. Again, the main chemicals were mono-aromatic species in the middle retention time and short-chain oxygenated species were also present in the short reaction time zone, as in the case of the bio-oils produced from the OW-derived lignins. The absence of syringol in the bio-oil produced from the PW-derived lignin samples is quite straightforward because the lignins were derived from softwood. However, the bio-oil produced from PW-derived lignin did not contain long-chain fatty acid esters and acid. This could be because of the absence of fatty acid species in the PW.
of the bio-oil produced from OW-E shifted from 900 m/z (Fig. S1) to 500 m/z. A similar peak shift to the low-molecular-weight range was observed for the bio-oil produced from OW-F. In addition, the signals of the high-molecular-weight species (> 2000 m/z) observed in the profile of the OW-F sample (Fig. S1) disappeared in the profile of the bio-oil. The molecular weight of the bio-oil produced from OW-K was similar to that from OW-E. This indicates that scEtOH + HCOOH is a highly versatile medium for the effective depolymerization of various types of lignins with different chemical bonds. The chemical species in the bio-oils produced from OW-E, OW-F, and OW-K at 350 °C were analyzed using GC-TOF/MS and the chromatograms are shown in Fig. 3. The chemical species in the bio-oils were categorized into three groups: linear and branched short-chain oxygenated species in the short retention times (< 10 min), monoaromatic species in the middle retention time (10–20 min), and longchain fatty acid alkyl esters in the long retention time (> 20 min) zone. The linear and branched short-chain oxygenated species (see Table S4 for detailed information) may have originated from the decomposition of ethanol and formic acid at 350 °C [39]. To confirm the decomposition of ethanol and formic acid, a mixture of 60 g of ethanol and 12 g of formic acid (without lignin) was treated at 350 °C and 29–35 MPa for 30 min and the product was analyzed using GC-TOF/MS, as shown in Fig. S4. Similar types of oxygenated species were formed during the blank reaction. At 250–300 °C, relatively less of the short-chain oxygenated species was observed (Fig. S5), indicating less ethanol decomposition at the low liquefaction temperatures. Among the monoaromatic species, the most abundant compounds were guaiacol, syringol, and their alkylation compounds (e.g., creosol and 3,5-dimethoxy4-hydroxy toluene). The long-chain fatty acid ethyl esters (e.g., octadecanedioic acid and E-11-hexadecenoic acid ethyl ester) in the bio-oil produced from the OW-derived lignins may have originated from the esterification of woody fat with ethanol [60]. To confirm this, Soxhlet extraction of OW in hexane for 6 h was conducted and the recovered liquid extract is shown in Fig. S6; a substantial amount of woody fat could be extracted. Similarly, long-chain fatty acids and their ester species were detected in the OW-derived lignins (Fig. S7). As shown in Fig. S5, the low-temperature liquefaction resulted in a relatively large amount of carbonyl-containing or double bond-containing branched mono-aromatics such as 1-(4-hydroxy-3-methoxyphenyl)-2-propanone (8, Fig. S5a), 2,6-dimethoxy-4-(2-propenyl)phenol (10, Fig. S5a), and 3,5-dimethoxy-4-hydroxyphenylacetic acid (15, Fig. S5a). This suggests that the deoxygenation and hydrogenation ability associated with scEtOH may be lower at the lower reaction temperatures. This agrees well with the EA results, as listed in Table S5. As the reaction temperature increased, the O/C ratios of the bio-oil produced from the OW-derived lignins decreased and the HHV of the bio-oil increased. For example, when the reaction temperature increased from 250 to 350 °C, the O/C ratio of the bio-oil produced from OW-K decreased from 0.31 to 0.25 and the HHV increased from 25.8 to 30.4 MJ kg−1.
4. Conclusion Six different types of lignin samples were depolymerized in a mixture of supercritical ethanol (scEtOH) and formic acid at temperatures of 250–350 °C. The content of ether linkages in the samples was highly dependent on the wood source (i.e., whether it was derived from oakwood, OW or pinewood, PW) and the lignin isolation method (ethanolsolv, E; formasolv, F; Klason, K). This may be because the formation of condensed linkages at the expense of ether linkages is dependent on the presence of the sinapyl alcohol unit and the isolation conditions. The amount of ether linkages in the lignin samples played an important role in determining the bio-oil yield in the low-to-medium temperature range; for example, the content of ether linkages followed the order: OW-F (129.3 µmol g−1) > OW-E (116.4 µmol g−1) > OW-K −1 (83.4 µmol g ), and the bio-oil yield followed the order: OW-F (77.5 wt%) > OW-E (68.0 wt%) > OW-K (18.7 wt%) at 250 °C. On the other hand, at the high temperature of 350 °C, all the lignin samples, even PW-K with an extremely low amount of ether linkages (23.0 µmol g−1), could be converted effectively into high-yield bio-oil (> 81 wt%). The compositional analysis suggested that during the depolymerization, hydrodeoxygenation and hydrogenation occurred to some degree.
3.3. Comparison of the reactivity of the lignin samples derived from OW and PW As discussed in Section 3.1, the lignin samples produced from PW had lower V + S contents than those produced from OW, suggesting that the PW-derived lignin samples may have lower depolymerization activity. To examine the difference in the depolymerization behavior depending on the wood sources, the PW-derived lignin samples were depolymerized in a mixture of scEtOH and formic acid at 350 °C for 1 h. During the course of liquefaction, the pressure in the reactor increased from 30 to 46 MPa. As listed in Table S3, when PW-F was used, a high conversion of 99.4% with a high bio-oil yield of 90.7 wt% and a low solid residue yield of 0.6 wt% were achieved. The use of PW-E resulted in a slightly lower conversion (97.1%) and a lower bio-oil yield (88.1 wt %) as compared to those of PW-F. This depolymerization behavior is
Acknowledgements This work was supported by the New and Renewable Energy Core Technology Program (No. 20143030090940) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) financed by the Ministry of Trade, Industry and Energy, Republic of Korea. Additional support from the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the 208
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Fig. 4. GC-TOF/MS chromatograms of bio-oils produced from PW-derived lignin. Reaction conditions: 350 °C, 30–46 MPa, 3 g lignin, 60 g ethanol, 12 g formic acid, 60 min.
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Ministry of Science, ICT & Future Planning (No. 2017M1A2A2087635) was appreciated.
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