ARTICLE pubs.acs.org/IECR
Classified Separation of Lignin Hydrothermal Liquefied Products Shimin Kang, Xianglan Li, Juan Fan, and Jie Chang* The Key Lab of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China ABSTRACT: Alkaline lignin was liquefied under hydrothermal conditions, and the liquefied products were effectively separated into four main types of substances: benzenediols, monophenolic hydroxyl products, weak-polar products, and water-soluble products (low-molecular-weight organic acids, alcohols, etc.). The production process and yield of each classified products are discussed. More than half of the yields of the oil products consisted of phenolics. A mechanism for phenolic production from lignin liquefaction is proposed. It suggests that the decomposition of lignin under hydrothermal conditions occurs mainly by three steps: hydrolysis and cleavage of the ether bond and the CC bond, demethoxylation, and alkylation.
’ INTRODUCTION Lignin is the only renewable polyphenolic compound and is very attractive to many industries, as it is a potential source of biophenolics as a substitute for petroleum-based phenolics. Hydrothermal upgrading is a promising liquefaction process. Recently, the formation of platform chemicals from lignin under hydrothermal conditions yielded very promising results.13 However, the chemical composition of lignin hydrothermal liquefaction oils is complicated, comprising phenolics, carboxylic acids, alcohols, substituted benzene derivatives, and so on, although phenolics are often the mainly products. Lignin-derived phenolics are valuable and useful chemicals. They have both germicidal activity and antidiarrheal properties and can be used as intermediates in the synthesis of pharmaceutical products, for the production of adhesives, and for the synthesis of specialty polymers.4 Recently, lignin liquefied phenolic oils were used as a substitute for phenol in the synthesis of resins.5 Lignin liquefied oils were found to have great potential for use as antioxidants as a result of their phenolic products.6 However, without purification or separation of the phenolic compounds in the lignin liquefied products, the nonphenolic compounds, such as carboxylic acids and alcohols, would modify the brittleness and increase the flexibility of the resins. In addition, carbohydrate admixtures or aliphatic hydroxyl groups can decrease antioxidant activity because their polar groups can hydrogen bond with lignin phenolic groups.7 Therefore, it is important to separate phenolic compounds from the organic acids, alcohols, and other components in lignin liquefied products. In addition, it is beneficial to separate benzenediols and monophenolics because the two phenolic categories also have different physical and chemical properties and applications. On the other hand, organic acids are also valuable and widely applied chemicals. Organic acids and their anions can be used as catalysts,8 corrosion inhibitors,9 and reducing agents.10 Hydrothermal liquefaction has been used to convert lignin into valuable low-molecular-weight platform chemicals. However, it is not easy to separate and identify all of the compounds formed during the thermochemical treatment processes.11 In most of the previous studies, the main object seemed to be r 2011 American Chemical Society
liquefying the lignin to liquid products (oils) rather than producing specific classified chemicals through further separation treatments. Therefore, the object of this work was to separate, classify, and identify the liquefied products by two extraction processes.
’ EXPERIMENTAL SECTION Reagent and Equipment. HCl solution (37 wt %) was obtained from Beijing Chemical Works, China. NaOH and NaHCO3 were obtained from Sinopharm Chemical Reagent Co., Ltd., China. Ethyl acetate (EA) and methylene dichloride (MD) were obtained from Guangzhou Chemical Reagent Factory, China. All of the above reagents were analytical reagents. The autoclave was obtained from Yantai Songling Chemical Equipment Co., Ltd., China. Operating Conditions. Alkaline lignin was obtained from Wuhan East China Chemical Co., Ltd., China. The elemental content (wt %) of the alkaline lignin was as follows: N, 1.35; C, 39.8; S, 2.01; H, 5.57; K, 1.77; Na, 10.8. The N, S, C, and H contents were determined with an Elementer Vario EL III instrument, whereas the K and Na contents were determined with a Hitachi Z-2010 atomic absorption spectrometer. Hydrothermal liquefaction experiments were conducted in a 250 mL stainless steel 316 autoclave. The heating power was 1.5 kW. The autoclave was loaded with 8.0 g of alkaline lignin and 100 mL of water. Then, the autoclave was purged five times with nitrogen to remove the inside air; after that, the autoclave was under an initial nitrogen pressure of 2.0 MPa. The reactants were agitated vertically using a stirrer (200 rpm). The temperature was raised to the set values, and then the autoclave was kept at the reaction temperature for 30 min. The set values in the experiments were 270, 300, and 330 °C for method 2 (see Figure 1) and 300 °C for method 1 (see Figure 1), with an error value of (1 °C. The Received: May 26, 2011 Accepted: August 20, 2011 Revised: July 15, 2011 Published: August 21, 2011 11288
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Figure 1. Separation of the lignin hydrothermal liquefied products.
Figure 2. Separation of the model product (catechol).
pressure is related to the temperature, and the pressures corresponding to temperatures of 270, 300, and 330 °C were 6.5, 8.0, and 11.8 MPa, respectively. When the reaction was finished, the heater was turned off, and the autoclave was air-cooled to quench the reaction. Separation Procedure. The separation process of the liquefied products is shown in Figure 1. The acid used in all acidification processes was 10 wt % HCl solution. Aside from the solid extracted with 100 mL of methylene dichloride (MD) by Soxhlet extraction for 24 h, all other extractions were done in a
500 mL separatory funnel. The volume of extraction agents [including ethyl acetate (EA), methylene dichloride (MD), 5% NaHCO3 solution, and 5% NaOH solution] for each extraction was 200 mL (four times). CO2 was used to control the acidity in NaOH solution phase 1 after acidification; CO2 was dissolved in excess when the pH of the solution became stable between 5 and 6. Two separation methods of MD-1 were employed: method 1 and method 2. The separation mechanism depended on the acidity value of the products and their solubilities in different solutions. The rough acidities of the lignin products are as follows: low-molecular-weight organic acid > H2CO3 > phenolics > alcohol, ester, weak-polar organics > NaHCO3. Most organic acids are water-soluble, can occur acidbase neutralization reaction with NaOH and NaHCO3, and can be recovered from organic acid salt the by acidification. The organic acids were finally obtained in EA-3 and EA-5 or EA-6. The weak-polar organics were extracted in MD-2 or MD-4. The benzenediols can be dissolved in NaHCO3 solution and were extracted in EA-6. The remaining monophenolic hydroxyl products were extracted in EA-4 or EA-7. All classified products were weighed after low-pressure rotary evaporation of the solvents. The yield of the products in the each phase was calculated as weight of evaporated products 100% 8:0 ðlignin weightÞ In Figure 2, catechol used as a model compound of benzenediols was dissolved in 500 mL of MD, and then the solution was processed by method 1. Gas Chromatography-Mass Spectrometry (GC-MS) and Mass Spectrum Analysis. The compositions of the products obtained in various phases were analyzed using a Shimadzu QP 2010 Plus system equipped with an Rxi-5 ms column (30 m 11289
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Chart 1. Chemical Structural Formulas Detected by GC-MSa
a
Values in parentheses indicate similarities (%).
0.25 mm 0.25 μm). The temperature of the injector was set at 260 °C. The compounds were identified by means of the NIST08 and NIST08s mass spectral data library. The EA-9 and EA-10 phase products were measured using electrospray ionizationmass spectrometry (ESI-MS); data processing was performed with Bruker Daltonics Data Analysis 3.3 software.
’ RESULTS AND DISCUSSION All compounds produced by lignin hydrothermal liquefaction in all phases after separation were detected by GC-MS. As shown in Chart 1, there were 100 compounds, including open-chain compounds, phenolic compounds, and other cyclic compounds. The cyclic compounds were mainly five- or six-membered, carbocyclic, or epoxy compounds. The open-chain compounds were mainly low-molecular-weight organic acids, alcohols, and esters. There were certain saccharides (cellulose and hemicellulose residues) in the raw materials. These saccharides were decomposed into short-chain alcohols and fatty acids under hydrothermal conditions; the reactions of saccharides involved isomerization, bond cleavage, and dehydration.12 In addition, some organic acids and alcohols were recovered owing perhaps to the decomposition of the propyl side chain of lignin, although it has been reported that organic acids were barely produced from the aromatic moiety in lignin.13 Some of these short-chain alcohols and fatty
acids further produced esters by a reversible esterification. There were also some cyclic esters, which might have been produced by self-esterification of hydroxy acids. The organic acids, esters, and alcohols in the water phase were collected in EA-3 as shown in Figure 3. However, compared with the compositions at 270 °C, the content of low-molecular-weight ester chain compounds decreased greatly at 300 and 330 °C, whereas the contents of organic acids and alcohols, especially 3-pentanol (5), increased. This was probably because ester decomposition played a major role at temperatures higher than 300 °C. In subcritical water, the alcohols and acids are hard to decompose, whereas the esters are readily hydrolyzed; much more alcohol and acid were obtained as a result of hydrolysis of esters at higher temperatures.14 moreover, there existed a certain amount of water-soluble epoxy compounds, including furan derivatives, in EA-3. Furan derivatives (63, 64) and 1,3-dioxolane-2-methanol (61) were found at 270 °C; however, the products did not appear at 300 and 330 °C. This phenomenon indicates that some saccharide degradation products should be secondarily completely decomposed at higher temperatures. The cyclic compounds were mainly derivatives of cyclopentanone, furan, and benzene. These derivatives of cyclopentanone and furan were produced by dehydration or dealcoholization of the saccharides. On the other hand, these products could also be cyclization products of dicarbonyl intermediates formed by aldol condensation reactions of low-molecular-mass 11290
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Figure 3. Compounds in EA-3 at various reaction temperatures (270, 300, 330 °C).
Figure 4. Compounds in EA-2 at various reaction temperatures (270, 300, 330 °C). 11291
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Figure 5. Compounds in EA-7 at various reaction temperatures (270, 300, 330 °C) and in EA-4 at 300 °C.
compounds formed from degradation products of the initial stages of liquefaction.11 Both benzene derivatives and phenolics should be produced by decomposition of lignin. As shown in Figure 4, the EA-2 phase consist of mainly three benzenediols, catechol (54) (or 1,2-benzenediol), 3-methoxy1,2-benzenediol (55), and 4-methyl-1,2-benzenediol (56). The relative content of 4-methyl-1,2-benzenediol increased at elevated temperature within the range from 270 to 330 °C. Benzenediols, especially catechol, are important chemical intermediates that are widely used in dyes, medicines, and perfumes. These separation results indicate that our approach is promising for obtaining purified benzenediols from the liquefaction of lignin. As shown in Figure 5, almost all of the compounds in EA-4 and EA-7 were monophenolic hydroxyl products. Within the range of experimental temperatures from 270 to 330 °C, the relative contents of phenol (33) and methyl- and ethyl-substituted phenols (34, 35, 3740, and 44) increased with increased temperature, whereas the relative contents of methoxyl-substituted phenols (36, 4143, 45, and 4749) were very low or even zero at 330 °C compared with those at 270 °C. Comparing the compounds in EA-7 with those in EA-4, almost all of the phenolic products were the same, aside from 20 ,60 -dihydroxy-40 -methoxyacetophenone (60), which appeared in EA-4 but not in EA-7.
However, 60 was also found in EA-6 (see Figure 6), probably because 20 ,60 -dihydroxy-40 -methoxyacetophenone can be dissolved in NaHCO3 solution and its acidity is weaker than that of H2CO3. The weak-polar products were mainly cyclopentanone and benzene derivatives, which were collected in MD-2 or MD-3 by method 1 or 2, respectively. As shown in Figure 7, the relative contents of methoxy aromatic compounds (87, 91, 95, 99, and 100) were very low or zero at 330 °C, whereas at 300 °C, the dimethoxy aromatic compounds (87 and 91) had large proportions, but the trimethoxy aromatic compounds (95, 99, and 100) were much lower compared with those at 270 °C. Some of the organic acids and benzenediols were also extracted from MD-1; these organic acids and benzenediols were soluble in NaHCO3 solution, and they were collected in EA-6 by method 1, as shown in Figure 6. Also, the organic acids and benzenediols can be separated and classified in the separation process of EA-1. Compared with EA-6 at 300 °C, it was surprising that the benzenediols were not found in EA-5. This phenomenon indicates that reactions between benzenediols and CO2 occurred; however, it was difficult to explore the reaction mechanism of these benzenediols with CO2 directly because there were many compounds in EA-5. To clarify this phenomenon, catechol 11292
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Figure 6. Compounds in EA-6 at various reaction temperatures (270, 300, 330 °C) and in EA-5 at 300 °C.
was used as a model compound of the benzenediols and was processed as shown in Figure 2. By GC-MS analysis, catechol was not detected in EA-8 and -9. However, in EA-10, highpurity catechol was found. This indicates that the catechol reacted with CO2. The EA-9 and EA-10 phase products were analyzed by mass spectrometry. As shown in Figure 8, according to the increased value of molecular weight, it can be concluded that one catechol reacted with two CO2 molecules, probably forming the dihydroxybenzenedicarboxylic acid by KobleSchmitt reaction. The possible reaction mechanism is shown in Scheme 1. The oil yield of each phase and the total oil yield separated by method 2 are shown in Figure 9. The benzenediols yield in EA-2 increased with increased temperature, reaching 2.6 wt % at 330 °C. At 300 °C, the total yield of phenolic products, including all of the monophenolic hydroxyl products and benzenediols in the EA-2, -6, and -7 phases, amounted to 14.617.1 wt %. The yield of the products at 300 °C in EA-5 was slightly higher than that in EA-6, because of the absorption of CO2 by benzenediols in forming dihydroxybenzoic acids. The yield of weakpolar products and the total oil yield reached maximum values (7.4 and 28.3 wt %, respectively) at 300 °C.
’ MECHANISM Lignins are polyphenolic compounds consisting of a variety of linkages connecting three phenylpropane units (H, S, and G); more than two-thirds of the linkages in lignin are ether linkages.15
An idealized lignin model consisting of all three basic structure units connected by α-O-4 and β-O-4 bonds was used to explain the phenolic production mechanism, as shown in Scheme 2. α-O-4 and β-O-4 bonds have relatively low bond dissociation enthalpies and should be hydrolyzed to produce the basic structure units; these basic structure units undergo further hydrolysis to their corresponding aromatic products by the cleavage of the side-chain CC bond, as shown in step 1 of Scheme 2. It was reported that cleavage of the side-chain CC bond mainly occurs between the aromatic ring and the αcarbon atom.16 The nature of the products (35, 40, 43, 45, 49, 50, 52, etc.) indicates that cleavage between the α- and βcarbons and between the β- and γ-carbons also occurred in our experiments. According the previous discussions, many methoxyl aromatic products resulted from the hydrolysis of the S and G units at 270 °C. The trimethoxyl and dimethoxyl aromatic products decreased from 270 to 300 °C, and there was a certain content of monomethoxyl aromatic products at 300 °C. In contrast, at 330 °C, the trimethoxyl and dimethoxyl phenolic products were not found, and the monomethoxyl aromatic product content was relatively low. Higher temperature was found to be conducive to more powerful conversion of the methoxyl aromatic products. It was reported that an increase in temperature under hydrothermal conditions enhances the demethoxylation of the methoxyl aromatic product guaiacol to form catechol and phenol.17 The benzene ring is relatively stable under hydrothermal conditions, so the conversion of the methoxyl aromatic products was 11293
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Figure 7. Compounds in MD-4 at various reaction temperatures (270, 300, 330 °C) and in MD-2 at 300 °C.
Figure 8. Mass spectrum analysis of EA-9 and EA-10.
probably due to the hydrolysis of the aliphatic CO bond and the aromatic CO bond of the methoxyl group. The bond energy of the aliphatic CO bond (245 kJ/mol) is smaller than that of the aromatic CO bond (356 kJ/mol),16 so the hydrolysis of the aliphatic CO bond of methoxyl group should cause major effects. The proposed mechanism of conversion of methoxyl aromatic products is shown as step 2 in Scheme 2. According to the previous discussions, the relative contents of orthoor meta-alkyl-substituted phenols (such as 34, 37, 38, 57, and 59)
Figure 9. Oil yields of products in each phase and total oil yield separated by method 2.
increase with increased temperature. These substituted phenols are hard to produce by direct pyrolysis of lignin according its unit structure, which means that these substituted phenols were formed 11294
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Scheme 1. Catechol and CO2 Reaction Mechanism
Scheme 2. Mechanism of Phenolic Production from Lignin
by further reactions of the phenolics produced in steps 1 and 2. A possible mechanism is that methanol and ethanol are produced during the hydrolysis of lignin and further react with phenolics to form substituted phenols at high temperatures, as shown in step 3 of Scheme 2. It has been reported that the high hydronium ion content of near-critical water is sufficient for synthesizing a variety of substituted phenols with various alcohols as a result of Friedel Crafts alkylation reactions.18 Moreover, elevated temperatures of 270330 °C promote the alkylation reactions.
’ CONCLUSIONS Alkaline lignin was liquefied under hydrothermal conditions, and the liquefied products were separated and classified; the total oil yield reached 28.3 wt % at a reaction temperature of 300 °C. Method 2 was successfully used to separate and classify the liquefied products, whereas method 1 was not suitable for obtaining all of the original products because benzenediols were found to react with CO2. Based on the change in the distribution 11295
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’ AUTHOR INFORMATION Corresponding Author
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(16) Demirbas, A. Mechanisms of liquefaction and pyrolysis reactions of biomass. Energy Convers. Manage. 2000, 41, 633–646. (17) Wahyudiono; Kanetake, T.; Sasaki, M.; Goto, M. Decomposition of a lignin model compound under hydrothermal conditions. Chem. Eng. Technol. 2007, 30, 1113–1122. (18) Chandler, K.; Deng, F.; Dillow, A. K.; Liotta, C. L.; Eckert, C. A. Alkylation reactions in near-critical water in the absence of acid catalysts. Ind. Eng. Chem. Res. 1997, 36, 5175–5179.
*Fax: +86 20 87112448. E-mail:
[email protected].
’ ACKNOWLEDGMENT We acknowledge financial support from the National Basic Research Program of China (973 Program) (No. 2010CB732205) and Fair of Science and Technical Achievements Resulting from Cooperation of Industry, Education and Academy (No. 2010A090200038). ’ REFERENCES (1) Lavoie, J. M.; Bare, W.; Bilodeau, M. Depolymerization of steamtreated lignin for the production of green chemicals. Bioresour. Technol. 2011, 102, 4917–4920. (2) Kruse, A. Hydrothermal biomass gasification. J. Supercrit. Fluids 2009, 47, 391–399. (3) Okuda, K.; Ohara, S.; Umetsu, M.; Takami, S.; Adschiri, T. Disassembly of lignin and chemical recovery in supercritical water and pcresol mixture—Studies on lignin model compounds. Bioresour. Technol. 2008, 99, 1846–1852. (4) Amen-Chen, C.; Pakdel, H.; Roy, C. Seperation of phenolics from eucalyptus wood tar. Biomass Bioenergy 1997, 13, 25–37. (5) Xu, J.; Jiang, J.; Lv, L.; Dai, W.; Sun, Y. Rice husk bio-oil upgrading by means of phase separation and the production of esters from the water phase, and novolac resins from the insoluble phase. Biomass Bioenergy 2010, 34, 1059–1063. (6) Kang, S.; Li, B.; Chang, J.; Fan, J. Antioxidant abilities comparison of lignins with their hydrothermal liquefaction products. BioResources 2011, 6, 243–252. (7) Ugartondo, V.; Mitjans, M.; Vinardell, M. P. Comparative antioxidant and cytotoxic effects of lignins from different sources. Bioresour. Technol. 2008, 99, 6683–6687. (8) Oledzka, E.; Narine, S. S. Organic acids catalyzed polymerization of ε-caprolactone: Synthesis and characterization. J. Appl. Polym. Sci. 2011, 119, 1873–1882. (9) Pastore, T.; Cabrini, M.; Coppola, L.; Lorenzi, S.; Marcassoli, P.; Buoso, A. Evaluation of the corrosion inhibition of salts of organic acids in alkaline solutions and chloride contaminated concrete. Mater. Corros. 2011, 62, 187–195. (10) Kim, H. S.; Kam, D. W.; Kim, W. S.; Koo, H. J. Synthesis of the LiFePO4 by a solid-state reaction using organic acids as a reducing agent. Ionics 2011, 17, 293–297. (11) Taner, F.; Eratik, A.; Ardic, I. Identification of the compounds in the aqueous phases from liquefaction of lignocellulosics. Fuel Process. Technol. 2004, 86, 407–418. (12) Kabyemela, B. M.; Adschiri, T.; Malaluan, R. M.; Arai, K. Glucose and fructose decomposition in subcritical and supercritical water: Detailed reaction pathway, mechanisms, and kinetics. Ind. Eng. Chem. Res. 1999, 38, 2888–2895. (13) Yoshida, K.; Kusaki, J.; Ehara, K.; Saka, S. Characterization of low molecular weight organic acids from beech wood treated in supercritical water. Appl. Biochem. Biotechnol. 2005, 121124, 795–805. (14) Krammer, P.; Vogel, H. Hydrolysis of esters in subcritical and supercritical water. J. Supercrit. Fluids 2000, 16, 189–206. (15) Pandey, M. P.; Kim, C. C. Lignin depolymerization and conversion: A review of thermochemical methods. Chem. Eng. Technol. 2011, 34, 29–41. 11296
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