Song Liabcd,
Wenzhi Li*e,
Qi Zhang*abc,
Riyang Shuabc,
Huizhen Wange,
Haosheng Xine and
Longlong Maabc
aGuangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China. E-mail: zhangqi@ms.giec.ac.cn; Fax: +86 20 87057789; Tel: +86 20 87057789
bCAS Key Laboratory of Renewable Energy, Guangzhou 510640, PR China
cGuangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, PR China
dUniversity of Chinese Academy of Sciences, Beijing 100049, PR China
eLaboratory of Basic Research in Biomass Conversion and Utilization, Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei 230026, PR China. E-mail: liwenzhi@ustc.edu.cn
First published on 3rd January 2018
The lignin-first biorefinery method appears to be an attractive approach to produce phenolic chemicals. Herein, corn stover was employed for the production of phenolic monomers using an unsupported non-noble MoS2 catalyst. The yield of phenolic monomers was enhanced from 6.65% to 18.47% with MoS2 at 250 °C and about 75% lignin was degraded with more than 90% glucan reserved in the solid residues. The Fourier-Transform Infrared (FT-IR) and heteronuclear single quantum coherence-nuclear magnetic resonance (1H–13C HSQC-NMR) characterization suggested that the cleavage of the β-O-4, γ-ester and benzyl ether linkages were enhanced, promoting the delignification and the depolymerization of lignin. The catalyst performance was relatively effective with 14.30% phenolic monomer yield after the fifth run. The effects of the reaction temperature, the initial hydrogen pressure, the dosage of catalyst, and the reaction time were investigated. The model reactions were also proposed for the potential mechanism study. This work provides some basic information for the improvement of the graminaceous plant lignin-first process with a non-noble metal catalyst.
This is the prevalent strategy for lignin upgradation by reductive depolymerization with metal catalysts.8,9 Moreover, recent advances have been reported with different lignin sources. Noble metal catalysts, such as Ru/C, Pd/C, Pt/C, and Rh/C,10–12 and non-noble transition metal sulfide, carbide, and nitride catalysts, such as sulfided NiMo and CoMo, MoC2, and TiN,13–15 were introduced in the depolymerization of technical lignin. The active metals played an important role in the cleavage of the β-O-4 linkage through hydrogenation or hydrodeoxygenation. Moreover, technical lignin, including lignosulfonate lignin, kraft lignin, organosolv lignin, and soda lignin,16 is usually produced via pretreatments under various harsh conditions. The pretreatments dramatically lead to the structural changes by severe and irreversible condensation. In fact, the condensation, resulting from the transformation of the ether bonds into the relative stable carbon–carbon bonds during the pretreatments, makes the subsequent lignin depolymerization process harder.17 High temperature and char formation are also the common challenges in the process of lignin-to-aromatic conversion. Herein, these drawbacks have seriously restricted the depolymerization of technical lignin into value-added chemicals and biofuels under relatively mild conditions. Compared with technical lignin, native lignin has 40–60% of total intermolecular linkages in the form of β-O-4 ether bonds,16 which makes native lignin a better feedstock for lignin-to-aromatic conversion. Based on this, lignin-first as a novel concept has been put forward with native lignin to extract more valuable products from lignin.3 Lignin-first depolymerization focuses on prohibiting the lignin condensation and improving the efficiency of lignin-to-aromatic conversion.3,17 In the solvolysis process, the native lignin is extracted from biomass by organic solvents. The extracted lignin fragments are depolymerized into monomers and oligomers via a reductive pathway with a metal catalyst. In this process, the carbohydrate pulp is primarily retained in the solid residues, which can be used for producing valuable chemicals and biofuels.3,18,19 Numerous efforts have been made in the recent years. For instance, noble metal catalysts, such as Ru/C, Pd/C, Pt/C, and Rh/C, were employed for native birch.18,20,21 Furthermore, the synergistic effects of the noble metal catalysts were investigated at a relatively mild condition with an acid as a co-catalyst (such as HCl, Al(OTf)3, and metal triflates).22 To reduce the catalyst cost, the Ni-based catalysts, such as Ni/Al2O3 (ref. 23) and Ni/C,19 were introduced in the process. Based on the previous studies, the lignin-first conversion approach exhibits a high yield and selectivity for the production of phenolic monomers. The reductive metal catalyst has been proven effective for the cleavage of the lignin–carbohydrate bonds and lignin–lignin ether bonds22,23 and the acids played a significant role as the co-catalyst.24,25
Compared to lignin with a high S/G ratio from hardwoods, lignin of the graminaceous plants (e.g. corn stover) is more complex; there is a higher content of hydroxycinnamic acids, particularly p-coumaric acid (pCA) and ferulic acid (FA), which are ester-linked or ether-linked in the cell walls.26,27 However, only few studies have been conducted using lignin of graminaceous plants. For instance, Ru/C, Ni/C, and the synergistic effects with H3PO4 were studied with native corn stover as the feedstocks.24 Though the phenolic monomers were harvested, the recyclability of the catalyst was not reported. Thus, the investigation on the lignin-first process of the graminaceous plants will provide a lignin-first approach with a much wider applicability for the utilization of lignocellulose. On the whole, noble metal catalysts are high cost and the Ni-based catalysts confront with the limitation of the recyclability. Moreover, the chemocatalytic conversion of carbohydrate pulp mostly needs the involvement of acids.20,28,29 Therefore, a non-noble metal catalyst capable of hydrogenation and acid tolerance is preferred for the lignin-first process.
MoS2 is a good choice for the lignin-first reductive depolymerization. It could tolerate some common acids except strong oxidative acids and is capable of hydrogenation (HYD), hydrodesulfurization (HDS), and hydrodeoxygenation (HDO).30,31 In addition, it has been reported that Ni-modified MoS2 catalyst was effective for the cleavage of β-O-4 ether bond via a dehydroxylation–hydrogenation strategy;32 also, MoS2 has been introduced in the depolymerization of alkali lignin for bio-oil production.33 As a non-noble transition-metal sulfide catalyst, it can be easily hydrothermally synthesized by an easier method compared to the synthesis of supported NiMo or CoMo metal sulfide. The unsupported layered structure can offer more active sulphur vacancies at the edge of the slab.
In this study, the lignin-first biorefinery was further developed with the depolymerization of native corn stover lignin via a non-noble unsupported MoS2 catalyst in methanol. The effect of the reaction conditions, including the reaction temperature, the initial hydrogen pressure, the dosage of catalyst, and the reaction time, on the yield of the phenolic monomers and the liquid products were investigated in detail. In particular, FT-IR and 1H–13C HSQC-NMR techniques were utilized to perform the chemical characterization of the liquid products. The model reactions were proposed for the potential mechanism study in order to get a comprehensive understanding of the lignin-first process.
The properties of the fresh and recycled catalysts were characterized using scanning electron microscope (SEM, FEI SIRION200), X-ray diffractometer (XRD, Rigaku SmartLab), X-ray photoelectron spectrometer (XPS, Thermo-VG Scientific ESCALAB 250), Autosorb iQ Station (Quantachrome), and Autosorb-iQ-C (ASIQACIV200-2).
Gas chromatography/mass spectrometry (GC-MS, Agilent 7890/5975) and gas chromatography (GC, SHIMADZU GC-2010) with flame-ionization detection were employed for qualitative and quantitative analysis of the phenolic monomers. Acetophenone was used as an internal standard. Fourier transform infrared spectrometry (FT-IR, Nicolet 8700) and nuclear magnetic resonance spectroscopy (NMR, Bruker AVANCE AV III 400) were utilized for further analysis of the chemical composition of the liquid products.
The volatile products were confirmed by GC-MS as follows: guaiacyl compounds, including 2-methoxy-4-propenylphenol, 2-methoxy-4-ethylphenol, 2-methoxy-4-propylphenol, and methyl trans-4-hydroxy-3-methoxycinnamate (methyl ferulate, MF); p-hydroxyphenyl compounds, including 4-ethylphenol, methyl trans-p-coumarate (MpC), and methyl 3-(4-hydroxyphenyl)propionate; syringyl compounds, including 2,6-dimethoxy-4-propylphenol and 2,6-dimethoxy-4-allylphenol. In addition, 2,3-dihydrobenzofuran was detected. The yield of the products was calculated using the following equations:
YMi = Mi/ML × 100% | (1) |
YSumG = MSumG/ML × 100% | (2) |
YSumS = MSumS/ML × 100% | (3) |
YSumH = MSumH/ML × 100% | (4) |
YSum = MSum/ML × 100% | (5) |
YLP = MLP/M × 100% | (6) |
YEP = MEP/M × 100% | (7) |
YSR = MSR/M × 100% | (8) |
Fig. 1 XRD/XPS spectra of the MoS2: (a) XRD and (b) XPS, a: fresh MoS2, b: used once, c: used 4 times, and d: used 5 times. |
The effect of the reaction temperature was significant on the lignin-first process as shown in Fig. 3 and Table 1. The LP yield was stable at around 38% at over 240 °C, indicating that more volatile gas products were released. The yield of total phenolic monomers was changed from 16.69% to 19.18% as the temperature rose from 230 °C to 260 °C and slightly down to 16.79% when the temperature increased up to 270 °C. This could be attributed to the increase of the depolymerization through the cleavage of the β-O-4 linkages in supercritical methanol as the temperature increased up to 260 °C. Moreover, the repolymerization reactions might be more dominant as the reaction temperature increased from 260 to 270 °C, resulting in the decline of the phenolic monomer yield. The decline in the yield of both guaiacols and p-hydroxyphenyl phenols was in accordance with this tendency, while the yield of syringols maintained at around 4%. This change could be ascribed to the active opening at 5-position on the aromatic ring, which made the G and H-type units prone to the repolymerization reactions at a higher temperature comparing with that for the S units.36,37
Reaction temperature (°C) | Yield of phenolic monomer (%) | |||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1b | 2c | 3d | 4e | 5f | 6g | 7h | 8i | 9j | 10k | SumG | SumS | SumH | Sum | |
a Condition: 2.0 g corn stover, 40 mL methanol, 0.3 g MoS2, 3 MPa H2, and 2 h.b 1: 4-Ethylphenol.c 2:2,3-Dihydrobenzofuran. The calibration factor of an internal standard method was calculated by an effective carbon number (ECN) method explained in the ESI.d 3: 2-Methoxy-4-ethylphenol.e 4: 2-Methoxy-4-propylphenol.f 5: 2-Methoxy-4-propenylphenol.g 6: Methyl 3-(4-hydroxyphenyl)propionate. The calibration factor of an internal standard method was calculated by an effective carbon number (ECN) method explained in the ESI.h 7: 2,6-Dimethoxy-4-allylphenol.i 8: 2,6-Dimethoxy-4-propylphenol.j 9: Methyl trans-p-coumarate. The calibration factor of an internal standard method was calculated by an effective carbon number (ECN) method explained in the ESI.k 10: Methyl trans-4-hydroxy-3-methoxycinnamate. | ||||||||||||||
230 | 1.95 | 0.93 | 0.43 | 2.38 | 1.71 | 0.75 | 0.83 | 3.51 | 2.27 | 1.93 | 6.45 | 4.34 | 4.97 | 16.69 |
240 | 2.38 | 1.08 | 0.95 | 2.81 | 1.86 | 0.00 | 0.61 | 3.80 | 2.10 | 1.88 | 7.50 | 4.41 | 4.48 | 17.47 |
250 | 2.63 | 0.61 | 1.3 | 3.75 | 1.55 | 1.45 | 0.64 | 3.01 | 2.02 | 1.51 | 8.11 | 3.65 | 6.10 | 18.47 |
260 | 3.18 | 0.31 | 1.32 | 3.92 | 1.54 | 2.09 | 0.00 | 3.12 | 2.06 | 1.64 | 8.42 | 3.12 | 7.33 | 19.18 |
270 | 3.89 | 0.23 | 1.93 | 1.47 | 1.29 | 1.81 | 1.50 | 2.43 | 1.22 | 1.02 | 5.71 | 3.93 | 6.92 | 16.79 |
Therefore, the lignin fragments were constantly generated, resulting in the yield of EP to rise slowly. Moreover, the SR yield declined to 47.56%, which was less than the sum of cellulose and hemicellulose (53.04%), indicating that the degradation of the carbohydrate components occurred extensively with the increase in temperature.
Compared to that of the reaction temperature, the effect of the initial hydrogen pressure was much more significant on the lignin-first process as shown in Fig. 3 and Table 2. The liquefaction of corn stover was enhanced when the harsh reaction conditions were employed. In particular, the LP yield increased clearly and the yield of the phenolic monomers rose sharply from 9.91% to 18.47% when the initial hydrogen pressure changed from 1 MPa to 3 MPa. As the pressure was further raised to 4 or 5 MPa, the yield remained at a stable level. This suggested that a certain hydrogen pressure had a facilitation effect on the delignification and the depolymerization of lignin because in the reductive depolymerization of lignin, the hydrogenolysis and hydrodeoxygenation reactions are the hydrogen-consuming processes.8 In addition, a part of cellulose or/and hemicellulose, particularly those in the amorphous carbohydrate fractions in corn stover, might be methylated and transformed into LP or EP due to methanolysis.38 Therefore, the LP and EP yields were beyond the total lignin content in corn stover (21.10%) and the SR yield declined under the more harsh conditions.
Initial hydrogen pressure (MPa) | Yield of phenolic monomer (%) | |||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1b | 2c | 3d | 4e | 5f | 6g | 7h | 8i | 9j | 10k | SumG | SumS | SumH | Sum | |
a Condition: 2.0 g corn stover, 40 mL methanol, 0.3 g MoS2, 250 °C, and 2 h.b 1: 4-Ethylphenol.c 2:2,3-Dihydrobenzofuran. The calibration factor of an internal standard method was calculated by an effective carbon number (ECN) method explained in the ESI.d 3: 2-Methoxy-4-ethylphenol.e 4: 2-Methoxy-4-propylphenol.f 5: 2-Methoxy-4-propenylphenol.g 6: Methyl 3-(4-hydroxyphenyl)propionate. The calibration factor of an internal standard method was calculated by an effective carbon number (ECN) method explained in the ESI.h 7: 2,6-Dimethoxy-4-allylphenol.i 8: 2,6-Dimethoxy-4-propylphenol.j 9: Methyl trans-p-coumarate. The calibration factor of an internal standard method was calculated by an effective carbon number (ECN) method explained in the ESI.k 10: Methyl trans-4-hydroxy-3-methoxycinnamate. | ||||||||||||||
1 | 1.51 | 0.6 | 1.48 | 1.56 | 0.71 | 0.60 | 0.00 | 1.42 | 1.10 | 0.93 | 4.68 | 1.42 | 3.21 | 9.91 |
2 | 2.49 | 0.46 | 1.06 | 2.92 | 1.23 | 1.16 | 0.54 | 2.50 | 2.05 | 1.48 | 6.69 | 3.04 | 5.70 | 15.89 |
3 | 2.63 | 0.61 | 1.30 | 3.75 | 1.55 | 1.45 | 0.64 | 3.01 | 2.02 | 1.51 | 8.11 | 3.65 | 6.10 | 18.47 |
4 | 3.43 | 0.52 | 1.57 | 2.73 | 1.43 | 1.74 | 0.79 | 2.79 | 1.59 | 1.21 | 6.94 | 3.58 | 6.76 | 17.80 |
5 | 3.03 | 0.41 | 1.42 | 3.39 | 1.43 | 1.77 | 0.87 | 2.95 | 1.89 | 1.50 | 7.74 | 3.82 | 6.69 | 18.66 |
Together with the effects of the reaction temperature and initial hydrogen pressure, the influences of the dosage of catalyst and the reaction time were also investigated. As summarized in Table 3, the yield of the phenolic monomers was stable at around 18% with the presence of MoS2 in the reaction. The yield was three times higher than that for the blank reaction, indicating that MoS2 played a positive role in the lignin-first process. The sulfur vacancies in MoS2 could effectively adsorb oxygen in β-O-4 and other oxygen-containing linkages, thus accelerating the bond cleavage and leading to the production of monomers.32 The monomeric products were stabilized through hydrogenation,23,32 which would be proven later by NMR characterization and the analysis of the chemical components of the solid residues. The HDO might be enhanced when the reaction time was prolonged. This phenomenon could be observed from the results displayed in Table 4, with the decline of the yield of the phenolic monomers, as well as in Fig. 3, with the increasing of the EP yield. Moreover, the increase of the LP yield and the decrease of the SR yield suggested the improvement of the liquefaction and the aggravation of the degradation of the carbohydrate components.
Dosage of catalyst (wt%) | Yield of phenolic monomer (%) | |||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1b | 2c | 3d | 4e | 5f | 6g | 7h | 8i | 9j | 10k | SumG | SumS | SumH | Sum | |
a Condition: 2.0 g corn stover, 40 mL methanol, 250 °C, 3 MPa H2, and 2 h.b 1: 4-Ethylphenol.c 2: 2,3-Dihydrobenzofuran. The calibration factor of an internal standard method was calculated by an effective carbon number (ECN) method explained in the ESI.d 3: 2-Methoxy-4-ethylphenol.e 4: 2-Methoxy-4-propylphenol.f 5: 2-Methoxy-4-propenylphenol.g 6: Methyl 3-(4-hydroxyphenyl)propionate. The calibration factor of an internal standard method was calculated by an effective carbon number (ECN) method explained in the ESI.h 7: 2,6-Dimethoxy-4-allylphenol.i 8: 2,6-Dimethoxy-4-propylphenol.j 9: Methyl trans-p-coumarate. The calibration factor of an internal standard method was calculated by an effective carbon number (ECN) method explained in the ESI.k 10: Methyl trans-4-hydroxy-3-methoxycinnamate. | ||||||||||||||
0 | 0.34 | 1.13 | 0.00 | 0.26 | 0.76 | 0.00 | 0.00 | 0.00 | 2.02 | 2.14 | 3.16 | 0.00 | 2.36 | 6.65 |
5 | 1.12 | 1.72 | 1.71 | 2.40 | 1.33 | 0.00 | 0.00 | 2.87 | 4.22 | 2.55 | 7.99 | 2.87 | 5.34 | 17.92 |
15 | 2.63 | 0.61 | 1.30 | 3.75 | 1.55 | 1.45 | 0.64 | 3.01 | 2.02 | 1.51 | 8.11 | 3.65 | 6.10 | 18.47 |
25 | 2.58 | 0.36 | 2.57 | 2.52 | 1.35 | 1.18 | 0.94 | 3.48 | 1.40 | 1.22 | 7.66 | 4.42 | 5.16 | 17.60 |
35 | 2.97 | 0.00 | 2.48 | 2.40 | 1.02 | 1.79 | 1.28 | 2.82 | 0.93 | 0.73 | 6.63 | 4.10 | 5.69 | 16.42 |
45 | 2.86 | 0.00 | 2.92 | 1.83 | 0.94 | 2.52 | 2.00 | 3.14 | 0.84 | 0.48 | 6.17 | 5.14 | 6.22 | 17.53 |
Reaction time (h) | Yield of phenolic monomer (%) | |||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1b | 2c | 3d | 4e | 5f | 6g | 7h | 8i | 9j | 10k | SumG | SumS | SumH | Sum | |
a Condition: 2.0 g corn stover, 40 mL methanol, 0.3 g MoS2, 250 °C, and 3 MPa H2.b 1: 4-Ethylphenol.c 2: 2,3-Dihydrobenzofuran. The calibration factor of an internal standard method was calculated by an effective carbon number (ECN) method explained in the ESI.d 3: 2-Methoxy-4-ethylphenol.e 4: 2-Methoxy-4-propylphenol.f 5: 2-Methoxy-4-propenylphenol.g 6: Methyl 3-(4-hydroxyphenyl)propionate. The calibration factor of an internal standard method was calculated by an effective carbon number (ECN) method explained in the ESI.h 7: 2,6-Dimethoxy-4-allylphenol.i 8: 2,6-Dimethoxy-4-propylphenol.j 9: Methyl trans-p-coumarate. The calibration factor of an internal standard method was calculated by an effective carbon number (ECN) method explained in the ESI.k 10: Methyl trans-4-hydroxy-3-methoxycinnamate. | ||||||||||||||
1 | 1.48 | 1.30 | 1.74 | 2.74 | 1.87 | 0.00 | 0.00 | 4.00 | 2.33 | 2.07 | 8.42 | 4.00 | 3.81 | 17.53 |
2 | 2.63 | 0.61 | 1.30 | 3.75 | 1.55 | 1.45 | 0.64 | 3.01 | 2.02 | 1.51 | 8.11 | 3.65 | 6.10 | 18.47 |
4 | 2.25 | 0.00 | 2.52 | 2.02 | 1.40 | 0.87 | 0.69 | 2.99 | 1.57 | 1.39 | 7.33 | 3.68 | 4.69 | 15.70 |
6 | 2.45 | 0.44 | 2.82 | 1.54 | 1.19 | 0.00 | 0.00 | 2.56 | 1.24 | 1.36 | 6.91 | 2.56 | 3.69 | 13.60 |
12 | 3.21 | 0.00 | 2.29 | 0.99 | 0.57 | 0.97 | 0.98 | 1.33 | 0.36 | 0.34 | 4.19 | 2.31 | 4.54 | 11.04 |
First, the EP (bio-oil) was rich in the phenolic monomers due to the abundant related functional structure observed in the FT-IR spectra shown in Fig. 4. The peak at 3370 cm−1 was attributed to the O–H stretching vibration, while the C–H symmetric and asymmetric vibrations in methyl and methylene group appeared at 2930 and 2850 cm−1, respectively.39 The 1720 cm−1 peak was assigned to the carbonyl stretching related to methyl p-coumarate. The strong absorption peak at 1610, 1510, and 1450 cm−1 in the bio-oil products suggested the existence of the benzene structure. The peak at 1360 cm−1 indicated the presence of the alkyl groups due to the sp3 C–H bending/rocking;8 the peak at 1330 cm−1 was derived from the phenolic OH (syringyl nuclei) absorptions; the guaiacyl ring breathing with CO stretching appeared at 1270 cm−1.40 The peak at 1210 cm−1 was attributed to the asymmetric vibration of C–O–C in methylated ester. In particular, the peak at 1170 cm−1 was characterized for the ester-linked p-hydroxycinnamic acids.41 In addition, the C–O stretching in the alkoxy functional group was observed at 1110 cm−18 and the peak at 834 cm−1 was considered to be the signal of the aromatic C–H out-of-plane deformation.39 Moreover, carbohydrates were considered to be present in the ethyl acetate insoluble products because the phenolic absorptions, particularly the peaks corresponding to the benzene and alkoxyl functional structures, were weak or even faded, but O–H stretching at the β-glucosidic linkages was observed at 1195 cm−1.42 According to the FT-IR spectra, it was suggested that the liquid products of the depolymerization of native corn stover lignin were divided into two parts: one was the EP bio-oil rich of the lignin-degraded aromatic structures, including the phenolic monomers and oligomers, while the other was the ethyl acetate insoluble products mostly composed of polysaccharides. To further investigate the structure of the liquid products, 1H–13C HSQC NMR was also conducted.
The information of the chemical structural linkage of the liquid products was explained by the signals originated from the parts of the type-linkages of the lignin-carbohydrate complexes and the representative lignin monomer units in 1H–13C HSQC NMR (Fig. 5a and b for EP and Fig. 5c for the ethyl acetate-insoluble products). Four inter-linkages of lignin in the extractive-free corn stover (β-O-4 (53–60%), β-5 (27–29%), β-β (10–11%), and β-1 (3–7%)) were observed as reported earlier.43 First, the cleavage of the β-O-4 linkages occurred in the lignin-first process, which could be observed in the 1H–13C HSQC NMR characterization of liquid products with MoS2 catalysts, including ethyl acetate soluble products and ethyl acetate insoluble products as shown in Fig. 5a and c. Moreover, the yield of the phenolic monomers was much higher with the presence of MoS2 (Table 3, line 3) than that in the blank (Table 3, line 1). This suggested that the monomeric products were stabilized by MoS2 through hydrogenation, which was consistent with the previous study.23 For instance, the contours of the major linkages of β-O-4, including the signals of Cα–Hα (δC/δH, 72.9/4.30 ppm and 62.9/4.31 ppm for the γ-acylated lignin units), Cβ–Hβ (δC/δH, 83.6/4.40 ppm for G/S type and 86.3/4.10 for S/S type), and Cγ–Hγ (δC/δH, 60.5/3.50 ppm), disappeared after the catalytic process.42,44 Hence, it was concluded that the β-O-4 linkages of the lignin fragments in the reaction had been severely broken. Moreover, the cleavage of the G/S and S/S type β-O-4 linkages could be responsible for the partial generation of the phenolic monomers. In addition, it was reported that the hydroxycinnamic acid moieties of corn lignin, including about 18% p-coumaric acid (pCA) and 3–4% ferulic acid (FA), were deposited on the cell wall of corn stover and linked through the ester and ether linkages.27,45 The γ-ester of LCCs present the contours at δC/δH 62–64/4.0–4.25 ppm. The benzyl ether structures between the lignin units and carbohydrates showed the signals at δC/δH 80–81/4.4–4.6 ppm and 80–81/5.0–5.2 ppm.43 Both ester and ether linkages were not found in the liquid products. Therefore, the formation of MpC and MF monomers could be related to the cleavage of the ester and ether bonds. Moreover, the primary linkages retained in the liquid products were considered to be the β-5 linkages and the β-β linkages because Cβ–Hβ and Cγ–Hγ, correlated to the β-5 linkages, were distinguished at δC/δH 51.8/3.58 ppm and 61.6/3.43 ppm, respectively, and the signals for Cγ–Hγ in the β-β linkages were located at δC/δH 66.1/3.73 ppm.43 Moreover, there was only a trace of the β-1 linkages that was observed at δC/δH 79.1/3.97 ppm assigned to the Cβ–Hβ signals due to its relatively low content in the raw materials.43 Moreover, other types of the LCCs linkages and phenyl glycoside linkages were detected at δC/δH 100.4/4.51 ppm and 102.7/4.63 ppm, respectively,46 and β-D-xylopyranoside (βX) presented the contours at δC/δH 102.2/4.24 ppm, 73.6/2.98 ppm, 73.9/3.35 ppm, 75.5/3.74 ppm, and 61.6/3.43 ppm, which referred to the C1–H1, C2–H2, C3–H3, C4–H4, and C5–H5, respectively. The signals related to carbohydrates indicated that there were carbohydrates reserved in LP together with lignin oligomers. The results also confirmed the previous discussion that not only lignin but also the carbohydrate components were liquefied.
Fig. 5 1H–13C HSQC-NMR spectra of the liquid products: (a), (b) ethyl acetate soluble products, and (c) ethyl acetate insoluble products (the detailed typical structure refer to Fig. S1†). |
In the aromatic regions of the 1H–13C HSQC NMR spectra of EP (Fig. 5b), different lignin units (G, S, and H type) of the phenolic units are clearly shown. The spectra of the ethyl acetate-insoluble products are blank, suggesting that the phenolic components were absent. C2,6–H2,6 and C3,5–H3,5 correlated to the H units at δC/δH 128.9/6.99 ppm and 115.5/6.68 ppm, respectively. In particular, MpC showed the signals for C2,6–H2,6, Cα–Hα, and Cβ–Hβ at δC/δH 130.7/7.54 ppm, 145.4/7.56 ppm, and 114.4/6.41 ppm, respectively. C2–H2 and C6–H6 in the G units were observed at δC/δH 113.2/6.73 ppm and 119.3/6.74 ppm, respectively. C2–H2 and C6–H6 correlated to MF at δC/δH 111.8/7.23 ppm and 123.5/7.13 ppm, respectively, while the signals of Cα–Hα and Cβ–Hβ in MF were overlapped with those of MpC. The S units related to C2,6–H2,6 displayed contours at δC/δH 103.7/6.63 ppm and the condensed S units of oligomers showed contours at δC/δH 106.0/6.43 ppm. The characterization of the aromatic regions supported the FT-IR spectra results, which suggested that the EP was rich in the phenolic monomers and oligomers. It was also in good agreement with the previous reports.43,44,46
Collectively, the potential mechanism can be explained in Fig. 6. In the lignin-first process, the native corn stover lignin can be partially dissolved in supercritical methanol together with amorphous xylan and the dissolving limitation can be promoted by a MoS2 catalyst through the cleavage of the β-O-4, γ-ester, and benzyl ether linkages, resulting in the release of pCA and FA monomer. Moreover, the dissolved lignin fragments will be further subjected to a hydrocracking reaction to be depolymerized into the phenolic monomers with MoS2. In particular, the hydroxycinnamic acid moieties (mainly pCA and FA) can further transform into 4-ethylphenol, 2-methoxy-4-ethylphenol, and methyl esters. Moreover, the 2-methoxy-4-propylphenol production can be assigned to the hydrogenation of 2-methoxy-4-propenylphenol, while the other products can be released from the cleavage of the G/S and S/S type β-O-4 linkages in native lignin.24
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra11947j |
This journal is © The Royal Society of Chemistry 2018 |