Abstract
Ethanol organosolv lignin separated from bamboo was depolymerized by low-power microwave radiation (~80 W) using ethanol as a swelling agent and formic acid as a hydrogen donor solvent. After increasing the temperature from 100 to 200 °C, the total amount of phenolic compounds in the products increased from 8.1% to 40.8%, and both the weight average molecular weight (Mw) and number average molecular weight (Mn) of the products from the lignin depolymerization decreased. With extended reaction time from 20 to 60 min, the total amount of phenolic compounds and molecular weight did not remarkably change. In addition, Fourier transform infrared (FT-IR) spectroscopy showed that oxidative fracture was the primary way that lignin depolymerized. The severity factor played an important role in converting lignin into small molecular substances, and the evaluation showed that the microwave temperature was more influential on the lignin depolymerization than the reaction time. Because depolymerization and repolymerization of fragments both occurred during the microwave radiation process, it is critical to inhibit repolymerization of degraded fragments for the efficient degradation of lignin. This study not only provides a theoretical basis for studying the mechanism of microwave-assisted lignin degradation but is also important for the determination of a cost-effective lignin depolymerization method.
Download PDF
Full Article
Low-Power Microwave Radiation-assisted Depolymerization of Ethanol Organosolv Lignin in Ethanol/Formic Acid Mixtures
Dengle Duan,a,b Yunfeng Zhao,a,b Liangliang Fan,a,b Leilei Dai,a,b Jiaqian Lv,a Roger Ruan,a,b,c Yunpu Wang,a,b,* and Yuhuan Liu a,b,*
Ethanol organosolv lignin separated from bamboo was depolymerized by low-power microwave radiation (~80 W) using ethanol as a swelling agent and formic acid as a hydrogen donor solvent. After increasing the temperature from 100 to 200 °C, the total amount of phenolic compounds in the products increased from 8.1% to 40.8%, and both the weight average molecular weight (Mw) and number average molecular weight (Mn) of the products from the lignin depolymerization decreased. With extended reaction time from 20 to 60 min, the total amount of phenolic compounds and molecular weight did not remarkably change. In addition, Fourier transform infrared (FT-IR) spectroscopy showed that oxidative fracture was the primary way that lignin depolymerized. The severity factor played an important role in converting lignin into small molecular substances, and the evaluation showed that the microwave temperature was more influential on the lignin depolymerization than the reaction time. Because depolymerization and repolymerization of fragments both occurred during the microwave radiation process, it is critical to inhibit repolymerization of degraded fragments for the efficient degradation of lignin. This study not only provides a theoretical basis for studying the mechanism of microwave-assisted lignin degradation but is also important for the determination of a cost-effective lignin depolymerization method.
Keywords: Lignin; Low microwave radiation; Depolymerization; Ethanol; Formic acid
Contact information: a: Nanchang University, State Key Laboratory of Food Science and Technology, Nanchang 330047, China; b: Nanchang University, Engineering Research Center for Biomass Conversion, Ministry of Education, Nanchang 330047, China; c: Center for Biorefining and Department of Bioproducts and Biosystems Engineering University of Minnesota, 1390 Eckles Ave., St. Paul, MN 55108, USA;
* Corresponding authors: wangyunpu@ncu.edu.cn; liuyuhuan@ncu.edu.cn
INTRODUCTION
After cellulose, lignin is the second most abundant natural polymer on earth (Li et al. 2012). Lignin is widely distributed in plant cell walls and possesses a complex structure, which consists of coniferyl alcohol, sinapyl alcohol, and coumaryl alcohol polymers. Lignin functions as a biological barrier against pathogens (Toledano et al. 2014b). At present, valuable applications for this underutilized resource have attracted considerable attention. With the development of ethanol biorefineries, lignin has become a noteworthy potential feedstock for biofuels and aromatic chemicals (Jiang et al. 2010). For example, as the primary plant biomass component, lignin contains numerous oxygen functional groups, such as hydroxyl, carbonyl, and carboxyl groups. Therefore, phenolic chemicals can be obtained from lignin by chemical degradation processes (Wahyudiono et al. 2008). In addition, by breaking down the lignin into small molecules, several compounds, such as vanillin, can be commercially produced (Hocking 1997). However, lignin degradation remains a challenge because of the highly complex structure of lignin, technological barriers, and adverse economic considerations (Badamali et al. 2008; Jiang et al. 2010).
Several studies have investigated the mechanism of lignin degradation, including fast pyrolysis, hydrogenolysis, oxidation, lignin depolymerization in supercritical solvents, ionic liquid pretreatment, and catalysts in lignin conversion (Pandey and Kim 2011). Under rapid pyrolysis conditions, lignin is converted directly into a liquid product, which is referred to as “bio-crude” or “bio-oil.” The resulting “bio-crude” or “bio-oil” can be upgraded through various processes, such as those used in traditional petroleum refineries (Patwardhan et al. 2011). Wang et al. (2014) reported that after catalytic pyrolysis by the Zn/ZSM-5 catalyst coupled with a microwave pyrolysis reactor, lignin was primarily degraded and dehydrated to produce propenyl–guaiacols. Badamali et al. (2009) reported the complete degradation of the lignin model phenolic monomer, which was catalyzed by Co(salen)/SBA-15 under microwave radiation for 40 min. Wen et al. (2014) pretreated poplar alkaline lignin with ionic liquids and found that after pretreatment, the β-O-4′ bond of lignin was cleaved, resulting in an increase in phenolic –OH. However, a dehydration reaction occurred that resulted in a decrease in aliphatic –OH. In addition, the β-β′ and β-5′ linkages were depolymerized, resulting in a decrease in the molecular weights (Wen et al. 2014). One of the disadvantages of these methods is the requirement for extreme reaction conditions, such as high pressure or temperature, to accomplish the depolymerization of lignin (Nanayakkara et al. 2014).
At present, environmental interest is the key to lignin degradation. Environmentally friendly processes have attracted increasing attention (Gu et al. 2012). Liquefaction is a good method to degrade lignin, and solvent selection is crucial to improve the economics of the process (Xiao et al. 2011; Xie et al. 2015). Ethanol as a swelling agent and formic acid as a hydrogen donor solvent have been used to establish a mild and environmentally friendly system to convert lignin to small molecular products (Pandey and Kim 2011; Gasson et al. 2012). Ethanol is attractive for lignin degradation because of its high solvency for biomass and low critical temperature (Gasson et al. 2012). In addition, using ethanol as a solvent results in a less oxygenated oil fraction and virtually no solid residue (Gellerstedt et al. 2008; Gasson et al. 2012). Formic acid decomposes into CO2 and active hydrogen under heating conditions, which results in lignin hydrogenation (Pandey and Kim 2011).
Bamboo is a highly abundant resource in China with a large plantation area and annual yield. Bamboo is composed of hemicellulose, cellulose, and lignin (Dong and Xiong 2014). Bamboo lignin depolymerization leads to the industrial production of valuable industrial and food products, such as vanillin, vinylguaiacol, optically active lignans, ferulic acid, and monolignol dimers (Lou et al. 2010).
Compared with traditional heating, microwave radiation has many advantages, including low reaction temperatures, short reaction times, and high yields of the target products (Ouyang et al. 2015). However, the majority of studies concerning microwave radiation have focused on depolymerizing lignin by high microwave power, i.e., in general, greater than 400 W (Dong et al. 2014; Toledano et al. 2014b). Studies on low microwave power-assisted depolymerization of bamboo lignin are few, and this lack of research on this topic is not beneficial to the study of the lignin depolymerization mechanism under microwave heating. Research on low microwave radiation assistance in lignin depolymerization not only provides a theoretical basis for studying the mechanism of microwave-assisted lignin degradation, but is also important for determining a cost-effective method for the depolymerization of lignin.
In the present study, low microwave radiation (~ 80 W) was applied in ethanol organosolv lignin depolymerization when using ethanol as a swelling agent and formic acid as a hydrogen donor solvent. Ethanol organosolv lignin was isolated from bamboo and subjected to mild microwave radiation at temperatures ranging from 100 to 200 °C and reaction times of 20 to 60 min. The product characteristics from the lignin depolymerization were evaluated.
EXPERIMENTAL
Materials and Reagents
Bamboo culms (natural material) were obtained from Nanchang, Jiangxi Province, China. Ethanol (analytical grade) was from Xi Long Scientific Co. (Guangdong, China), formic acid (analytical grade) was obtained from Da Mao Reagent Co. (Tianjin, China), and tetrahydrofuran (THF, HPLC grade) was purchased from Solarbio (Beijing, China).
Methods
Extraction of ethanol organosolv lignin from bamboo
The bamboo lignin was extracted using the method of Fan et al. (2015). In brief, dried bamboo culms were cut into small pieces, sieved, extracted with methylbenzene/ ethanol (2:1, v/v), and finally air-dried to obtain a dewaxed bamboo powder. The resulting dried bamboo powder (150 g) and 50% ethanol aqueous solution (8:1, v/v) were cultivated in an autoclave at 180 °C for 3 h with a 10 °C/min heating rate, kept within a ± 2 °C range. After cultivation, the mixture was vacuum-filtered and dried as described by Fan et al. (2015) to obtain the bamboo lignin powder.
Microwave depolymerization
Lignin was depolymerized by ethanol and formic acid with microwave heating. All experiments were conducted using a microwave reaction system (CEM Corporation, USA), and the conditions of each experiment are shown in Table 1. Lignin (0.5 g) was mixed with 8 mL of formic acid and 4 mL of ethanol in each tube, and the tube was covered with a rubber cap with a small hole. The experiments were conducted at various times (20 to 60 min) and temperatures (100 to 200 °C). The power of the microwave used in this work was less than 80 W. Before the experiment, the sample was placed in a microwave digestion instrument and stirred with a magnetic rotor for 2 min. The temperature was raised to the designated temperature at low power (80 W, ~2 min), followed by a steady step, during which the power varied to maintain the temperature at the target value. The temperature in the microwave reactor system was monitored by infrared sensing. To prevent hotspot problems, a magnetic rotor was added in the tube before the reaction was initiated. After the reaction, the samples were cooled to room temperature (~20 min). Then, the formic acid and ethanol were evaporated under vacuum, and the resulting mixture was stored in darkness for subsequent experiments. Each experiment was conducted in duplicate to ensure consistent results.
The severity factor, R0, combines the temperature and time into a single factor, the effective reaction conditions, and is defined as follows (Kim et al. 2014),
dt (1)
(2)
where Ti is the reaction temperature (°C), Tb is the base temperature (100 °C), w is the fitting parameter (14.75), and t is the reaction time (min).
Table 1. List of Reaction Conditions and Severity Factors
Product characterization
The samples were dissolved in methyl alcohol prior to analysis. The composition of the phenolic compounds of the samples was identified using a gas chromatograph–mass spectrometer (GC–MS) (Agilent Technologies, California, USA) coupled with a thermal conductivity detector (TCD) and an Agilent capillary column (DB-35 ms, California, USA; 0.25 mm × 30 m × 0.25 μm). The column temperature was kept at 60 °C for 2 min and then raised to 300 °C at a rate of 10 °C/min. The injection sample size was 2 μL, with a split ratio of 20:1. The ion source temperature was 230 °C for the mass selective detector. Helium was selected as the carrier gas, and the flow rate was 1.0 mL/min. The compounds were identified by comparing the spectrograms with those in the NIST data library. The relative content of each compound in the products was determined by semi-quantitative methods according to the chromatographic percentage.
Molecular weight determination
The molecular weight of the product was determined by a gel permeation chromatography–high-performance liquid chromatography (GPC–HPLC) instrument (G1312B binary pump, Agilent Technologies, State of California, USA, UV detector at 280 nm; GPC column at a column temperature of 30 °C). The samples were dissolved in THF prior to analysis. The samples were then filtered through a 0.45-μm syringe filter. THF, at a flow rate of 0.50 mL/min, was used as the mobile phase. A polystyrene standard calibration curve was used to calibrate the number average molecular weight (Mn) and weight average molecular weight (Mw) of the samples. This method is also useful for evaluating the degree of lignin depolymerization.
Characterization of functional groups
Characterization of the functional groups of the raw material and depolymerization products was accomplished by Fourier transform infrared (FT–IR) spectroscopy (Nicolet iS5, Thermo, USA). The samples were ground with KBr, and the spectra were recorded in the range of 4000 to 700 cm−1 with 16 scans and at a 4 cm−1 spectral resolution per sample.
RESULTS AND DISCUSSION
Compounds of Depolymerization Products Identified by GC–MS
The experiments were conducted to study the influence of microwave temperature and reaction time on the products obtained under mild conditions. After treatment under various microwave conditions, the lignin was converted into small molecular chemical compounds. This research primarily exhibited the value-added phenolic compounds obtained from lignin depolymerization. Figures 1 and 2 show that seven compounds were obtained. Among these compounds, the G-type ones included guaiacol, vanillin, and homovanillyl alcohol, and the S-type ones included syringol, syringaldehyde, acetosyringone, and desaspidinol.
Fig. 1. The components and their relative area percentage of phenolic compounds in the products identified by GC-MS at different microwave temperatures (30 min), where (1) is guaiacol, (2) is vanillin, (3) is homovanillyl alcohol, (4) is syringol, (5) is syringaldehyde, (6) is acetosyringone, and (7) is desaspidinol
Figure 1 shows that changing the microwave temperature exerted a considerable influence on the product distribution of the phenolic compounds. Guaiacol was not detected by GC–MS when the microwave temperature was lower than 160 °C. Homovanillyl alcohol was not detected by GC–MS when the microwave temperature was lower than 140 °C. At a temperature of 100 °C, no desaspidinol was observed in the products from lignin depolymerization, possibly because of several relatively stable chemical bonds (Hu et al. 2013). In general, as the microwave temperature increased, the yield of vanillin gradually increased, which indicated that the rate of vanillin formation reaction was higher than that of the polycondensation reaction, except at 180 °C (Dong et al. 2014). The syringol yield exhibited the same tendency as that of vanillin; therefore, it is suggested that the rate of the syringol formation reaction was higher than that of the polycondensation reaction, except at 160 °C. The desaspidinol yield gradually increased as the microwave temperature increased from 100 to 180 °C and then remained constant until 200 °C. The change in yield of syringaldehyde and acetosyringone as the microwave temperature increased from 100 to 200 °C may have been due to the bond dissociation energy from the C-C and C-O bonds, which indicates a competitive kinetic relationship (Hu et al. 2013; Dong et al. 2014). In general, acetosyringone was not remarkably changed by the microwave temperature, i.e., it changed from 1.28% to 1.32% as the microwave temperature increased from 100 to 200 °C; therefore, there was a possible dynamic balancing state between the rate of the acetosyringone formation and polycondensation reactions.
Fig. 2. The components and their relative area percentage of phenolic compounds in the products identified by GC-MS at different microwave times (160 °C), where (1) is guaiacol, (2) is vanillin, (3) is homovanillyl alcohol, (4) is syringol, (5) is syringaldehyde, (6) is acetosyringone, and (7) is desaspidinol
As can be seen in Fig. 2, the guaiacol yield at 160 °C was not noticeably influenced by the microwave time. When the microwave time was increased from 20 to 60 min, the guaiacol yield changed from 0.72% to 0.35%. For homovanillyl alcohol, the yield generally decreased from 2.47% to 0.80% as the reaction time increased from 20 to 60 min. The yields of acetosyringone and desaspidinol were not remarkably changed by the reaction time, which suggests there was a possible dynamic balancing state between the rate of the formation reaction and polycondensation reaction at 160 °C, regardless of the reaction time. With an increase in the reaction time from 20 to 60 min, the vanillin and syringaldehyde yields increased from 0.76% to 1.55% and 1.01% to 2.09%, respectively. During the reaction at 160 °C and 50 min, the syringol yield reached its highest value (3.27%). The phenolic compounds from the lignin depolymerization reaction are mostly derived from the cleavage of bonds among three phenylpropane units, namely, syringylpropane (3,5-diamethoxy-4-hydroxyphenylpropane) (S), guaiacylpropane (4-hydroxy-3-methoxy-phenylpropane) (G), and 4-hydroxyphenylpropane (H) groups (Monteil-Rivera et al. 2013; Toledano et al. 2014b). Figures 1 and 2 show that the total amount of S-type compounds was higher than that of the G-type compounds. In addition, the total amount of phenolic compounds increased from 8.1% to 40.8% with increasing microwave temperature. However, no major change was observed for the total amount of phenolic compounds with the extension of the reaction time from 20 to 60 min. The variation in the quantities of phenolic compounds from the lignin depolymerization reaction may have been due to the hydrogenolyzation of the three phenylpropane units under the selected conditions, which may have been remarkably affected by the use of the severity factors (Table 1) (Kim and Park 2013; Toledano et al. 2014b). When the reaction time was set at 30 min, the value of R0 noticeably increased, from 1.48 to 4.47, with an increase in the microwave temperature. However, when the microwave temperature was set to 160 °C and the reaction time increased, no evident change was observed in the R0 value. Therefore, the influence of the microwave temperature on lignin depolymerization at a set time was larger than that of the reaction time at a set microwave temperature.
Fig. 3. The possible mechanism of phenolic compound generation from lignin depolymerization (Shen et al. 2015)
The possible generation mechanism of phenolic compounds from the lignin depolymerization reaction is shown in Fig. 3. The presence of desaspidinol is not clearly understood. Toledano et al. (2014b) reported that for lignin depolymerization to aromatics via mild microwave-assisted hydrogen-free hydrogenolysis and when tetralin was used as a solvent, no desaspidinol was found in the products. In contrast, when formic acid was used as a solvent, desaspidinol was found in the products from the lignin depolymerization. Toledano et al. (2014b) concluded that the formation of desaspidinol depends on the solvent utilized in the system. Jiang et al. (2010) studied the composition of the lignin pyrolysis products at various temperatures (from 400 to 800 °C). Desaspidinol was detected in both Alcell lignin and Asian lignin pyrolysis products by pyrolysis GC–MS. As the temperature increased from 400 to 800 °C, the proportion of desaspidinol decreased from 0.555 wt.% to 0.275 wt.% in the Alcell lignin pyrolysis vapor, and from 0.134 wt.% to 0.038 wt.% in the Asian lignin pyrolysis vapor (Jiang et al. 2010).
Molecular Weight Determination
Table 2 presents the Mw and Mn values, as well as the molecular weight distribution (Mw/Mn), of the depolymerization products. For a reaction time of 30 min, Mw and Mn underwent a dynamic change process with increasing microwave temperature. The Mw and Mn values at 120 °C (1121 and 660, respectively) were lower than those values at 100 °C (1335 and 754, respectively), which suggests there was a depolymerization phenomenon for lignin under low-power microwave heating as the temperature increased from 100 to 120 °C. However, as the temperature further increased to 140 °C, the values of Mw and Mn were 1304 and 801, respectively, which were higher than at 120 °C. This suggested that there was a repolymerization phenomenon for the molecules produced under low-power microwave heating as the temperature increased from 120 to 140 °C. The repolymerization reaction is primarily caused by the reaction between the unstable oligomer or intermediate products and original lignin, which results in unconverted lignin with a higher molecular weight than the initial lignin (Li et al. 2007; Toledano et al. 2014a). As the microwave temperature increased from 140 to 180 °C, the values of Mw and Mn decreased from 1304 and 801 to 1026 and 348, respectively. This finding suggested that lignin was depolymerized into several small molecules. In addition, as the temperature further increased to 200 °C, the values of Mw and Mn slightly increased to 1055 and 358, respectively; therefore, the lignin molecule may have repeated the depolymerization and repolymerization reactions as the temperature increased from 100 to 200 °C under low-power microwave conditions. However, the Mw and Mn values decreased in general, which indicated that the rate of the repolymerization reaction was lower than that of the depolymerization reaction (Roberts et al. 2011; Toledano et al. 2014a). When the temperature was 180 °C, the lowest values were obtained for the Mw and Mn, which were 1026 and 348, respectively. It was concluded that microwaves can assist the lignin depolymerization reaction within a certain temperature range. This outcome was slightly different from that reported by Dong et al. (2014), who studied lignin depolymerized by formic acid with a fixed microwave power mode (600 W) and found that by increasing the microwave temperature, both Mn and Mw decreased at first, and then increased, with their lowest values obtained at 160 °C (4529 and 1886, respectively). When the microwave temperature exceeded 160 °C, the repolymerization reaction played the dominant role in the lignin degradation process, which improved the Mw and Mn to a certain extent (Dong et al. 2014).
Table 2. Molecular Weight-Average (Mw), Number-Average (Mn), and Distributions (Mw/Mn) of the Depolymerization Products
The effect of the microwave time on the molecular weight of the depolymerization products was similar to the effect of the microwave temperature. At 160 °C, Mw and Mn also showed a dynamic change process. After microwave heating for 30 min at 160 °C, the Mw and Mn values (1173 and 532, respectively) were lower than those obtained at 20 (1284 and 551, respectively) and 40 min (1361 and 663, respectively). Further increasing the microwave heating time to 50 min resulted in the Mw and Mn decreasing to 1187 and 507, respectively. Then, at a reaction time of 60 min, the Mw and Mn increased to 1256 and 544, respectively. With the extension of the microwave time from 20 to 60 min at 160 °C, the lignin molecule repeatedly underwent the depolymerization and repolymerization reactions. However, no remarkable differences in the values of Mw and Mn were observed when the microwave time increased from 20 to 60 min at 160 °C, as there was only a slight decrease for Mw and Mn between 20 (1284 and 551, respectively) and 60 min (1256 and 544, respectively). Dong et al. (2014) reported that longer microwave heating times inhibited the apparent depolymerization of lignin. Ouyang et al. (2015) showed that, compared with traditional heating, microwave heating promoted the alkali lignin depolymerization of wheat straw and prolonged microwave heating times accelerated the recondensation of depolymerized lignin fragments.
FT-IR Spectra
The FT-IR spectra can be used to determine the physicochemical properties and chemical functional groups of lignin (Wen et al. 2013). The FT-IR spectra of the lignin before and after depolymerization under various microwave conditions were characterized in the 700 to 4000 cm−1 range, and the results are shown in Figs. 4 and 5.
Fig. 4. FT-IR spectra of the lignin before and after depolymerization at various microwave temperatures and 30 min
The main peaks of the chemical functional groups present in the lignin before and after depolymerization were observed at 3423, 2937, 2840, 1720, 1654, 1603, 1514, 1462, 1425, 1328, 1268, 1217, 1118, 1032, 913, and 833 cm−1. Among these peaks, the broad peak at approximately 3423 cm−1 was related to the –OH stretching absorption of the aliphatic hydroxyl and phenolic hydroxyl groups.
Fig. 5. FT-IR spectra of the lignin before and after depolymerization at 160 °C and various microwave times
Figures 4 and 5 show that the peak around 3423 cm−1 of the degradation products was evidently weakened by the microwave radiation, which suggests that the amount of –OH groups in the lignin depolymerization products decreased after the microwave treatment. The signal of the peak at 1720 cm−1 assigned to the stretching C=O groups was sharper and clearer in the degraded products than in the raw lignin, which indicates a higher carbonyl content in the depolymerization products. This was explained by the fact that the oxidation of hydroxyl groups into carbonyl groups was promoted by the microwave radiation (Xiao et al. 2011).
The band at 2937 cm−1, which was assigned to the C-H vibration in methyl, was weaker in the lignin depolymerization products compared with the raw lignin; therefore, a decrease in the methyl groups is indicated. The decrease in methyl meant the abscission of the C3 structure and stood for the cleavage of the β-O-4 bond, which suggests the generation of monophenolic compounds and the breakage of long carbon chains (Xiao et al. 2011; Ouyang et al. 2014). This was in accordance with the aforementioned results of the GC-MS and molecular weight analyses. In addition, the signals at 1268 and 1217 cm−1 originated from C-O groups in the guaiacol and syringol rings, respectively, and were weakened after the lignin depolymerization, which indicates the cleavage of C-O bonds and a decrease of methoxyl groups in the lignin structure after treatment by microwave radiation.
The signal at 1514 cm−1 was assigned to the aromatic skeletal vibrations and was present in all of the spectra of the lignin before and after depolymerization, which demonstrates that the aromatic rings still existed after depolymerization. However, the absorption peak at 1514 cm−1 was weakened in the lignin depolymerization products compared with the raw lignin, which suggests that the aromatic rings were partially opened. Figures 4 and 5 show that the spectra patterns of the lignin before and after depolymerization had a slight difference, which demonstrates that the main chemical functional groups of the lignin before and after degradation under microwave radiation were slightly different. In addition, the depolymerization products contained incompletely degraded lignin.
CONCLUSIONS
- As the temperature increased from 100 to 200 °C, the total amount of phenolic compounds in the products increased. Among the products, the total amount of S-type compounds in the products was higher than that of the G-type compounds. Additionally, the Mw and Mn of the products from lignin depolymerization decreased. However, when the reaction time was increased from 20 to 60 min at 160 °C, both the total amount of phenolic compounds and molecular weight did not remarkably change.
- Increasing the microwave temperature within a certain range effectively promoted lignin degradation, whereas lengthening the reaction time of microwave radiation at a certain microwave temperature had a minimal effect on the lignin depolymerization.
- After lignin depolymerization, the products contained relatively smaller fragments and a portion of incompletely degraded lignin, and the aromatic rings were still present in the products and partially opened. Under low microwave radiation, lignin depolymerization occurred mainly through oxidation cleavage. During microwave treatment, the lignin primarily underwent depolymerization and repolymerization reactions. Our future work will be focused on the separation and extraction of aromatic compounds from lignin depolymerization product, and then further apply to manufacture high-value bulk and fine chemicals.
ACKNOWLEDGMENTS
This work was supported financially by the International Science and Technology Cooperation Program of China (2014DFA61040) and the Program of the Natural Science Foundation of China (21466022).
REFERENCES CITED
Badamali, S. K., Clark, J. H., and Breeden, S. W. (2008). “Microwave assisted selective oxidation of lignin model phenolic monomer over SBA-15,” Catalysis Communications 9(13), 2168-2170. DOI: 10.1016/j.catcom.2008.04.012
Badamali, S. K., Luque, R., Clark, J. H., and Breeden, S. W. (2009). “Microwave assisted oxidation of a lignin model phenolic monomer using Co(salen)/SBA-15,” Catalysis Communications 10(6), 1010-1013. DOI: 10.1016/j.catcom.2008.12.051
Dong, Q., and Xiong, Y. (2014). “Kinetics study on conventional and microwave pyrolysis of moso bamboo,” Bioresource Technology 171, 127-131. DOI: 10.1016/j.biortech.2014.08.063
Dong, C., Feng, C., Liu, Q., Shen, D., and Xiao, R. (2014). “Mechanism on microwave-assisted acidic solvolysis of black-liquor lignin,” Bioresource Technology 162, 136-141. DOI: 10.1016/j.biortech.2014.03.060
Fan, L., Ruan, R., Liu, Y., Wang, Y., and Tu, C. (2015). “Effects of extraction conditions on the characteristics of ethanol organosolv lignin from bamboo (Phyllostachys pubescens Mazel),” BioResources 10(4), 7998-8013. DOI: 10.15376/biores.10.4.7998-8013
Gasson, J. R., Forchheim, D., Sutter, T., Hornung, U., Kruse, A., and Barth, T. (2012). “Modeling the lignin degradation kinetics in an ethanol/formic acid solvolysis approach. Part 1. Kinetic model development,” Industrial and Engineering Chemistry Research 51(32), 10595-10606. DOI: 10.1021/ie301487v
Gellerstedt, G., Li, J., Eide, I., Kleinert, M., and Barth, T. (2008). “Chemical structures present in biofuel obtained from lignin,” Energy & Fuels 22(6), 4240-4244. DOI: 10.1021/ef800402f
Gu, X., Kanghua, C., Ming, H., Shi, Y., and Li, Z. (2012). “La-modified SBA-15/H2O2 systems for the microwave assisted oxidation of organosolv beech wood lignin,” Maderas. Ciencia y tecnologia 14(1), 31-41. DOI: 10.4067/S0718-221X2012000100003
Hocking, M. B. (1997). “Vanillin: Synthetic flavoring from spent sulfite liquor,” Journal of Chemical Education 74(9), 1055. DOI: 10.1021/ed074p1055
Hu, J., Shen, D., Xiao, R., Wu, S., and Zhang, H. (2013). “Free-radical analysis on thermochemical transformation of lignin to phenolic compounds,” Energy & Fuels 27(1), 285-293. DOI: 10.1021/ef3016602
Jiang, G., Nowakowski, D. J., and Bridgwater, A. V. (2010). “Effect of the temperature on the composition of lignin pyrolysis products,” Energy & Fuels 24(8), 4470-4475. DOI: 10.1021/ef100363c
Kim, H. G., and Park, Y. (2013). “Manageable conversion of lignin to phenolic chemicals using a microwave reactor in the presence of potassium hydroxide,” Industrial and Engineering Chemistry Research 52(30), 10059-10062. DOI: 10.1021/ie400719v
Kim, Y., Kreke, T., Mosier, N. S., and Ladisch, M. R. (2014). “Severity factor coefficients for subcritical liquid hot water pretreatment of hardwood chips,” Biotechnology and Bioengeering 111(2), 254-263. DOI: 10.1002/bit.25009
Li, J., Henriksson, G., and Gellerstedt, G. (2007). “Lignin depolymerization/ repolymerization and its critical role for delignification of aspen wood by steam explosion,” Bioresource Technology 98(16), 3061-3068. DOI: 10.1016/j.biortech.2006.10.018
Li, M.-F., Sun, S.-N., Xu, F., and Sun, R.-C. (2012). “Microwave-assisted organic acid extraction of lignin from bamboo: Structure and antioxidant activity investigation,” Food Chemistry 134(3), 1392-1398. DOI: 10.1016/j.foodchem.2012.03.037
Lou, R., Wu, S.-b., and Lv, G.-j. (2010). “Effect of conditions on fast pyrolysis of bamboo lignin,” Journal of Analytical and Applied Pyrolysis 89(2), 191-196. DOI: 10.1016/j.jaap.2010.08.007
Monteil-Rivera, F., Phuong, M., Ye, M., Halasz, A., and Hawari, J. (2013). “Isolation and characterization of herbaceous lignins for applications in biomaterials,” Industrial Crops and Products 41, 356-364. DOI: 10.1016/j.indcrop.2012.04.049
Nanayakkara, S., Patti, A. F., and Saito, K. (2014). “Chemical depolymerization of lignin involving the redistribution mechanism with phenols and repolymerization of depolymerized products,” Green Chemistry 16(4), 1897-1903. DOI: 10.1039/C3GC41708E
Ouyang, X., Tan, Y., and Qiu, X. (2014). “Oxidative degradation of lignin for producing monophenolic compounds,” Journal of Fuel Chemistry and Technology 42(6), 677-682. DOI: 10.1016/S1872-5813(14)60030-X
Ouyang, X., Zhu, G., Huang, X., and Qiu, X. (2015). “Microwave assisted liquefaction of wheat straw alkali lignin for the production of monophenolic compounds,” Journal of Energy Chemistry 24(1), 72-76. DOI: 10.1016/S2095-4956(15)60286-8
Pandey, M. P., and Kim, C. S. (2011). “Lignin depolymerization and conversion: A review of thermochemical methods,” Chemical Engineering and Technology 34(1), 29-41. DOI: 10.1002/ceat.201000270
Patwardhan, P. R., Brown, R. C., and Shanks, B. H. (2011). “Understanding the fast pyrolysis of lignin,” ChemSusChem 4(11), 1629-1636. DOI: 10.1002/cssc.201100133
Roberts, V., Stein, V., Reiner, T., Lemonidou, A., Li, X., and Lercher, J. A. (2011). “Towards quantitative catalytic lignin depolymerization,” Chemistry-A European Journal 17(21), 5939-5948. DOI: 10.1002/chem.201002438
Shen, D., Liu, N., Dong, C., Xiao, R., and Gu, S. (2015). “Catalytic solvolysis of lignin with the modified HUSYs in formic acid assisted by microwave heating,” Chemical Engineering Journal 270, 641-647. DOI: 10.1016/j.cej.2015.02.003
Toledano, A., Serrano, L., and Labidi, J. (2014a). “Improving base catalyzed lignin depolymerization by avoiding lignin repolymerization,” Fuel 116, 617-624. DOI: 10.1016/j.fuel.2013.08.071
Toledano, A., Serrano, L., Pineda, A., Romero, A. A., Luque, R., and Labidi, J. (2014b). “Microwave-assisted depolymerisation of organosolv lignin via mild hydrogen-free hydrogenolysis: Catalyst screening,” Applied Catalysis B-Environmental 145, 43-55. DOI: 10.1016/j.apcatb.2012.10.015
Wahyudiono, Sasaki, M., and Goto, M. (2008). “Recovery of phenolic compounds through the decomposition of lignin in near and supercritical water,” Chemical Engineering and Processing: Process Intensification 47(9-10), 1609-1619. DOI: 10.1016/j.cep.2007.09.001
Wang, L., Lei, H., Bu, Q., Ren, S., Wei, Y., Zhu, L., Zhang, X., Liu, Y., Yadavalli, G., Lee, J., et al. (2014). “Aromatic hydrocarbons production from ex situ catalysis of pyrolysis vapor over zinc modified ZSM-5 in a packed-bed catalysis coupled with microwave pyrolysis reactor,” Fuel 129, 78-85. DOI: 10.1016/j.fuel.2014.03.052
Wen, J.-L., Xue, B.-L., Xu, F., Sun, R.-C., and Pinkert, A. (2013). “Unmasking the structural features and property of lignin from bamboo,” Industrial Crops and Products 42, 332-343. DOI: 10.1016/j.indcrop.2012.05.041
Wen, J.-L., Yuan, T.-Q., Sun, S.-L., Xu, F., and Sun, R.-C. (2014). “Understanding the chemical transformations of lignin during ionic liquid pretreatment,” Green Chemistry 16(1), 181-190. DOI: 10.1039/C3GC41752B
Xiao, W., Han, L., and Zhao, Y. (2011). “Comparative study of conventional and microwave-assisted liquefaction of corn stover in ethylene glycol,” Industrial Crops and Products 34(3), 1602-1606. DOI: 10.1016/j.indcrop.2011.05.024
Xie, J., Qi, J., Hse, C., and Shupe, T. F. (2015). “Optimization for microwave-assisted direct liquefaction of bamboo residue in glycerol/methanol mixtures,” Journal of Forestry Research 26(1), 261-265. DOI: 10.1007/s11676-015-0032-1
Article submitted: March 2, 2017; Peer review completed: May 15, 2017; Revised version received and accepted: May 23, 2017; Published: June 7, 2017.
DOI: 10.15376/biores.12.3.5308-5320