Bioresource Technology xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Hydrothermal conversion of lignin to substituted phenols and aromatic ethers Rawel Singh a, Aditya Prakash a, Shashi Kumar Dhiman a, Bhavya Balagurumurthy a, Ajay K. Arora b, S.K. Puri b, Thallada Bhaskar a,⇑ a b
Bio-Fuels Division (BFD), CSIR-Indian Institute of Petroleum (IIP), Dehradun 248005, India Indian Oil Corporation, R&D Centre, Faridabad, Haryana, India
h i g h l i g h t s Hydrothermal liquefaction of lignin was performed using ethanol & methanol. Maximum liquid yield (85%) was observed at low reaction temperature of 200 °C. The major liquid products are substituted phenols and aromatic ethers. The maximum organic carbon conversion was found to be 72%.
a r t i c l e
i n f o
Article history: Received 10 January 2014 Received in revised form 19 February 2014 Accepted 21 February 2014 Available online xxxx Keywords: Lignin Bio-oil Hydrothermal liquefaction Thermo-chemical conversion Substituted phenols
a b s t r a c t Hydrothermal liquefaction of lignin was performed using methanol and ethanol at various temperatures (200, 250 and 280 °C) and residence times of 15, 30 and 45 min. Maximum liquid product yield (85%) was observed at 200 °C and 15 min residence time using methanol. Increase in temperature was seen to decrease the liquid products yield. With increase in residence time, liquid yields first increased and then decreased. FTIR and 1H NMR showed the presence of substituted phenols and aromatic ethers in liquid products and breakage of b-O-4 or/and a-O-4 ether bonds present in lignin during hydrothermal liquefaction was confirmed through FTIR of bio-residue. In comparison to the existing literature information, higher lignin conversion to liquid products and maximum carbon conversion (72%) was achieved in this study. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The growing interest in biofuels from lignocellulosic feedstock can provide a path towards replacing petroleum-based fuels with sustainable biofuels which have the potential to lower greenhouse gas (GHG) emissions. Lignocellulose consists mainly of plant cell wall materials and is a complicated natural composite with three main biopolymers, cellulose (40–50%), hemicellulose (15–25%) and lignin (15–35%). The lignocellulosics-to-ethanol process makes use of the cellulose and hemicelluloses, leaving behind lignin as waste. In addition, the pulp and paper industry also generates huge amounts of lignin. Presently, lignin is being utilised as a low-grade boiler fuel to provide heat and power to the process. However, the aromatic structure of lignin suggests that it may be a good source of valuable ⇑ Corresponding author. Tel.: +91 9410151846; fax: +91 1352660202. E-mail addresses: [email protected], [email protected] (T. Bhaskar).
bulk and specialty chemicals like aromatics (Benzene, Toluene, and Xylene, etc.), phenols, aromatic ethers, vanillin, etc., if broken into smaller molecular units by the development of appropriate thermochemical method/catalytic technology. Lignin-derived products find application as fuels, solvents, chemical reagents, and polymers. However, lignin depolymerisation with selective bond cleavage is the major challenge for converting it into valueadded chemicals. Superior to pyrolysis technology, high-pressure direct liquefaction technology has the potential to produce liquid oils that is not miscible in water, with much higher calorific values and a range of chemicals including vanillin, phenols, aldehydes, and organic acids, etc. (Huber et al., 2006). The process is best suited for wet materials as the drying of feedstock is not necessary and water is used as one of the reactants. Methanol and ethanol have lower boiling as well as critical points than water. Due to the lower dielectric constants of these solvents, high molecular weight products produced as a result of
http://dx.doi.org/10.1016/j.biortech.2014.02.076 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Singh, R., et al. Hydrothermal conversion of lignin to substituted phenols and aromatic ethers. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.02.076
2
R. Singh et al. / Bioresource Technology xxx (2014) xxx–xxx
reaction of cellulose and hemicellulose readily dissolve in them (Mazaheri et al., 2010). Liquefaction in organic solvents other than water produces a higher yield of water insoluble oily products making methanol and ethanol promising solvents for the liquefaction of biomass to produce valuable hydrocarbons (Zhou et al., 2012). Beauchet et al. used aqueous alkaline solution to depolymerise Kraft lignin, Indulin AT into gas fraction (mainly CO2), methanol, acetic acid and formic acid, aromatic monomers (19.1 wt.%) and oligomers (polyaromatic molecules) and modified lignin (45– 70 wt.%) (Beauchet et al., 2012). Gosselink et al. used supercritical fluid consisting of carbon dioxide/acetone/water (300–370 °C, 100 bar) for converting organosolv hardwood and wheat straw lignins to a phenolic oil and monomeric aromatic compounds (10– 12%) (Gosselink et al., 2012). Yuan et al. used hot-compressed water–ethanol medium for depolymerisation of alkaline lignin into oligomers using NaOH as the catalyst and phenol as the capping agent at 220–300 °C. The optimal process condition as reported in their study was 260 °C for 3 h (Yuan et al., 2010). Song et al. studied the conversion of native birch wood lignin into monomeric phenols like propylguaiacol and propylsyringol with total selectivity >90% at a lignin conversion of about 50%, over nickel-based catalysts common alcohols as solvents (Song et al., 2013). In the present investigation, lignin depolymerisation was studied in organic solvents viz. ethanol and methanol under various temperatures (200, 250 and 280 °C) and residence times of 15, 30 and 45 min. The lignin and its reaction products were analysed using various physicochemical characterisation methods such as TG, Powder X ray Diffraction (XRD), FTIR, NMR, and Total Organic Carbon (TOC) to understand and confirm the formation of monomeric lignophenols from macromolecular lignin.
2. Methods Lignin was procured from Asian Lignin Manufacturing™ (ALM) in the form of Protobind™ 1000, a renewable product obtained from agricultural fibrous feedstocks. The properties of the lignin are presented as Table S1 in Supplementary materials. Hydrothermal liquefaction of lignin was carried out in stainless steel tubular reactor (35 ml). The thermocouples were placed in the skin and heart of the reactor. In a typical hydrothermal liquefaction experiment, the reactor was loaded with lignin and ethanol/methanol (1:10 by weight). Then the reactor was purged five times with nitrogen to remove the inside air. The temperature was then raised to required point at heating rate of 10 °C min 1 and kept for a specific residence time. After the specified reaction time, the reactor was taken out of the furnace and was immediately submerged into a water bath to stop the further reaction. After cooling to room temperature, the reaction contents were filtered and washed with the respective solvent (ethanol/methanol). Upon removal of the solvent in a rotary evaporator under reduced pressure, the liquid products were obtained and weighed. The solid residue was dried in the oven and after drying weight of solid residue was noted. The liquid and solid products obtained were then analysed. Lignin and reaction products obtained after the hydrothermal upgradation (HTU) of lignin were analysed by powder XRD, and FTIR. The thermogravimetric analysis was carried out in Perkin– Elmer TG instrument. The gross calorific value of lignin has been found out using Parr 6300 Bomb Calorimeter. The bio-oil samples were analysed using FTIR and NMR. Powder XRD pattern of lignin was collected on a Bruker D8 advance X-ray diffractometer. The 1H NMR spectra of the liquid samples have been recorded in the Bruker Ultra shield 500 plus instrument and DMSO-d6 has been used as solvent. The FTIR spectra were recorded on a Nicolet 8700 FTIR
spectrometer. TOC analysis of lignin and bio-residue was performed using Shimadzu TOC-L unit with solid sample module SSM-5000A. 3. Results and discussion The presence of organic solvents is known to improve the solubility of lignin and its decomposition products (Kang et al., 2013). Moreover, methanol can be used as a hydrogen donor in lignin liquefaction, which is in favour of low oxygen content oil production (Barta et al., 2010). Maximum liquid product yield was observed at low temperature and effect of various reaction conditions is explained below (Fig. 1a and b). 3.1. Effect of temperature The reaction temperature is an important factor for the product distribution. The maximum liquid product yield was observed at 200 °C for both ethanol and methanol. Raising the temperature would promote the decomposition of lignin and repolymerisation of the intermediates simultaneously. The maximum liquid product yield was observed at 30 min residence time so this residence time has been considered to see the effect of temperature on liquid product yields. With the increase of temperature from 200 to 280 °C the yield of liquid products obtained by hydrothermal liquefaction of lignin using ethanol decreased. At 200 °C the liquid yield was maximum at 81% and with increase of temperature to 250 °C liquid product yield decreased to 75%. Further decrease of liquid product yield at 65% was observed at 280 °C. The similar trend of decrease in liquid product yield with increase in temperature was also observed at residence times of 15 and 45 min respectively for hydrothermal liquefaction of lignin using ethanol. Solid bio-residue yield increased with increase in temperature and similar result was found by Ye et al. where yield of liquid products decreased with increasing temperature from 225 to 300 °C for lignin depolymerisation in ethanol–water. This was attributed to the formation of solid residue through side reactions such as repolymerisation of lignin decomposition intermediates. This indicates that lower temperature favoured depolymerisation reaction whereas with the increase of temperature formation of higher reactive radicals promoted repolymerisation (Ye et al., 2012). Similar phenomenon is observed in coal liquefaction. Similar effect of temperature on liquid products yield was observed in case of hydrothermal liquefaction of lignin using methanol. Maximum liquid products yield (85%) was observed at 200 °C and liquid products yield gradually decreased with the increase of temperature. 3.2. Effect of residence time The maximum liquid product yield was observed at 200 °C and 30 min residence time for hydrothermal liquefaction of lignin using ethanol and the effect of residence time at different temperatures is discussed below. At 200 °C the effect of residence time on liquid products yield is negligible and liquid yield was constant at all residence times of 15, 30 and 45 min respectively. At 250 °C the liquid product yield first increased with increasing residence time from 15 to 30 min. With the further increase of residence time to 45 min the liquid product yield decreased. The liquid product yield was 60%, 75% and 55% at residence time of 15, 30 and 45 min respectively. Similar trend of decrease in liquid product yield with increase of residence time was observed at 280 °C. The liquid product yield was 42%, 65% and 41% at residence time of 15, 30 and 45 min respectively. During lignin degradation, cracking reactions and condensation occur simultaneously. After a certain reaction time when most of the easily cleaved bonds in lignin molecule
Please cite this article in press as: Singh, R., et al. Hydrothermal conversion of lignin to substituted phenols and aromatic ethers. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.02.076
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(a)
90
bio-residue yield increased beyond 30 min. Therefore, 30 min would be a desirable reaction time for hydrothermal liquefaction of lignin using methanol and ethanol. The gas yield was less in all cases and was vented after the completion of the reaction. TOC analysis of the lignin and bio-residue obtained by hydrothermal liquefaction of lignin using ethanol was studied. TOC results are in good agreement with the above results of effect of temperature and residence times on liquid product yield. Carbon conversion decreased with the increase of temperature. Maximum carbon conversion was observed at 200 °C and 30 min which is also optimum condition for production of higher liquid yields (Supplementary Table S2).
Ethanol
80
% Yield Liquid
70 30 min
60 50
15 min 45 min
40 30
3.3. Analysis of lignin
20 200
220
240
260
280
30 min
XRD spectra of lignin showed its amorphous nature. The TG–DTG of lignin feed is seen to have a single significant decomposition temperature though a slightly complex behaviour is also observed due to overlapping decomposition temperatures (Fig. S1). FTIR spectra of pure lignin shows absorbance peak at 1035 cm 1 which is due to the CAO, C@C and CACAO stretching vibrations. The peak at 1217 cm 1 is due to the CAC and CAO stretching vibrations. The peak at 1268 cm 1 corresponds to the aromatic ring vibration. The peak at 1330 cm 1 corresponds to the CAO of syringyl ring present in lignin. The peak at 1425 cm 1 is due to the CAH in plane deformation. CAH stretching vibrations in lignin are observed at 2805 and 2922 cm 1. Peak at 3427 cm 1 is due to OAH stretching vibrations and shows the presence of phenolic hydroxyl groups. The peak at 1704 shows presence of C@O groups in lignin. Aromatic ring vibrations are observed at 1514 cm 1 (Xu et al., 2013).
280
3.4. Analysis of liquid products
Temperature, ºC
(b)
90
Methanol
80
70
% Yield Liquid
3
60
50 15 min
40 45 min
30
20 200
220
240
260
Temperature, ºC
(c)
60
ETH200 ETH250 ETH280 METH200 METH250 METH280
50
% Proton
40
30
20
10
0 0.5-1.5
1.5-3.0
3.0-4.5
4.5-6.0
6.0-8.5
8.5-10.0
Chemical Shift (ppm) Fig. 1. (a) Effect of residence time and temperature on liquid products yield using ethanol. (b) Effect of residence time and temperature on liquid products yield using methanol. (c) NMR of liquid products obtained from lignin using ethanol and methanol. ETH200: 200 °C, 30 min, ethanol; ETH250: 250 °C, 30 min, ethanol; ETH280: 280 °C, 30 min, ethanol; METH200: 200 °C, 30 min, methanol; METH250: 250 °C, 30 min, methanol; METH280: 280 °C, 30 min, methanol.
such as ether bonds and carbonyl groups were cracked, lignin depolymerisation was essentially complete. Further increasing the residence time would favour the condensation of the degraded lignin intermediates, which should be the reason that solid
The FTIR spectrum of liquid products (Fig. S2) showed absorbance at 1120, 1222, 1460, 1514, 1604, 1700, and 2934 cm 1. The absorbances at 2934 and 1460 cm 1 are attributed to CAH stretching vibrations of methyl and methylene and absorbance of these peaks increased in liquid samples and indicates the presence of alkane groups in liquid products. The increased absorption peak at 1700 cm 1 suggests the presence of conjugated aromatic ketones or esters. The absorbances at 1514 and 1604 cm 1 corresponds to aryl groups. The peaks at around 1220 and 3420 cm 1 indicate the presence of phenols. The peak at 1120 cm 1 belongs to methoxy groups and is present in all the liquid products. The peak at 834 cm 1 indicates the presence of para-substituted aromatic groups (Ye et al., 2012). The FTIR spectra of liquid products obtained from the hydrothermal liquefaction of lignin using ethanol and methanol indicates the presence of phenols and aromatic ethers. NMR spectra provided complementary functional group information to FTIR spectra and the ability to quantify and compare integration areas between spectra. A summary of integrated peak area regions assigned to different functional group classes present in the liquid products obtained from hydrothermal liquefaction of lignin using methanol and ethanol at 30 min residence time are shown in Fig. 1c. The region from 0.5 to 3.0 ppm is composed of protons on aliphatic carbon. The region from 0.5 to 1.5 ppm corresponds to protons on aliphatic carbon atoms at least two bonds away from C@C or heteroatom and the region from 1.5 to 3.0 ppm corresponds to the protons on aliphatic carbon atoms that may be bonded to a C@C double bond or heteroatom. Liquid products obtained from hydrothermal liquefaction of lignin showed high proton percentage in this region. With the increase in temperature proton percentage increased in this region. The next portion of the 1H NMR spectrum at 3.0–4.5 ppm represents methoxyl
Please cite this article in press as: Singh, R., et al. Hydrothermal conversion of lignin to substituted phenols and aromatic ethers. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.02.076
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R. Singh et al. / Bioresource Technology xxx (2014) xxx–xxx
protons (Kosa et al., 2011) or methylene group that joins two aromatic rings. The latter moiety would exist in a partially decomposed lignin oligomer present in the bio-oil (Mullen et al., 2009). All the liquid products show high proton percentage in this region. The region between 4.5 and 6.6 ppm represents aromatic ether protons i.e. lignin derived methoxy phenols. The region of the spectrum between 6.0 and 8.5 ppm corresponds to the aromatic region of spectrum. The liquid products showed significant proton percentage in this region. The liquid products obtained using methanol as solvent showed higher percentage in this region. Maximum proton percentage (17%) was observed in this region using methanol at 200 °C. The downfield spectrum regions (8.5– 10 ppm) of the bio-oils arise from the aldehydes and the proton percentage was very low in this region at around 1%. The FTIR and 1H NMR of the liquid products indicate the presence of substituted phenols and aromatic ethers present in the liquid product obtained. In a study by Ye et al. liquid products obtained were mainly composed of phenolics (4-ethylphenol, 4-vinylphenol, guaiacol, 4-ethylguaiacol, and 4-vinylguaiacol) which constitute almost 85% of the identified compounds (Ye et al., 2012). The formation of substituted phenols and aromatic ethers may be due to the breakage of various CAC and CAOAC ether bonds (b-O-4 or/and a-O-4) present in lignin. The ethanol and methanol may further react with the OAH groups to give corresponding methyl and ethyl ethers. During lignin hydrothermal liquefaction, various reactions like hydrolysis and cleavage of the ether and CAC bond, demethoxylation, alkylation and condensation, etc. occur. Various linkages like b-O-4 and Ca–Cb were easier to break whereas the aromatic rings were not affected during hydrothermal liquefaction. This indicates that at milder conditions of relatively low temperature and short residence time, phenolic monomers and dimers may be formed and as temperature is increased demethoxylation and alkylation of lignin derived phenolic compounds are enhanced and various alkyl phenols may be formed (Kang et al., 2011, 2013). 3.5. Analysis of bio-residue The absorbance from the ether bond CAOAC at 1120 and 1034 cm 1 weaken in intensity for the solid residue in all samples, which is possible due to the breakage of b-O-4 or/and a-O-4 ether bonds to form hydroxyl groups and alkyl groups. The weakening of these absorbance peaks in bio-residue samples indicate that the breakage of b-O-4 or/and a-O-4 ether bonds to form hydroxyl groups and alkyl groups started at very low temperature around 200 °C. Solid residue obtained from depolymerisation of lignin using methanol and ethanol showed similar behaviour. The absorbances at 1604, 1514, and 1425 cm 1 that are attributed to aromatic nuclei decreased in the bio-residue samples. The weakened absorbance at 1700 cm 1 implies that decarbonylation occurred even at very mild condition (Ye et al., 2012). 4. Conclusion Maximum liquid products yield (85%) was observed at 200 °C with 30 min residence time using methanol. With increase in
temperature, liquid product yield decreased. FTIR and 1H NMR of liquid products confirmed the presence of substituted phenols and aromatic ethers. FTIR of bio-residue samples indicate that the breakage of b-O-4 or/and a-O-4 ether bonds to form hydroxyl groups and alkyl groups started at very low temperature (200 °C). Acknowledgements The authors thank The Director, CSIR-Indian Institute of Petroleum, Dehradun, for his constant encouragement and support. R.S. and A.P. thank Council of Scientific and Industrial Research (CSIR), New Delhi, India, for providing Senior Research Fellowship (SRF) and Junior Research Fellowship (JRF) respectively. The authors thank CSIR, Government of India for providing financial support in the form of XII Five Year Plan project (CSC0116/BioEn). The authors thank the FTIR, NMR and XRD groups at CSIR-Indian Institute of Petroleum (IIP) for providing analytical support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2014. 02.076. References Barta, K., Matson, T.D., Fetting, M.L., Scott, S.L., Iretskii, A.V., Ford, P.C., 2010. Catalytic disassembly of an organosolv lignin via hydrogen transfer from supercritical methanol. Green Chem. 12, 1640–1647. Beauchet, R., Monteil-Rivera, F., Lavoie, J.M., 2012. Conversion of lignin to aromaticbased chemicals (L-chems) and biofuels (L-fuels). Bioresour. Technol. 121, 328– 334. Gosselink, R.J.A., Teunissen, W., van Dam, J.E.G., de Jong, E., Gellerstedt, G., Scott, E.L., et al., 2012. Lignin depolymerisation in supercritical carbon dioxide/acetone/ water fluid for the production of aromatic chemicals. Bioresour. Technol. 106, 173–177. Huber, G.W., Iborra, S., Corma, A., 2006. Synthesis of transportation fuels from biomass: chemistry, catalysts and engineering. Chem. Rev. 106, 4044–4098. Kang, S., Li, X., Fan, J., Chang, J., 2011. Classified separation of lignin hydrothermal liquefied products. Ind. Eng. Chem. Res. 50, 11288–11296. Kang, S., Li, X., Fan, J., Chang, J., 2013. Hydrothermal conversion of lignin: a review. Renew. Sust. Energy Rev. 27, 546–558. Kosa, M., Ben, H., Theliander, H., Ragauskas, A.J., 2011. Pyrolysis oils from CO2 precipitated Kraft lignin. Green Chem. 13, 3196–3202. Mazaheri, H., Lee, K.T., Bhatia, S., Mohamed, A.R., 2010. Sub/supercritical liquefaction of oil palm fruit press fiber for the production of bio oil: effect of solvents. Bioresour. Technol. 101, 7641–7647. Mullen, C.A., Strahan, G.D., Boateng, A.A., 2009. Characterization of various fastpyrolysis bio-oils by NMR spectroscopy. Energy Fuels 23, 2707–2718. Song, Q., Wang, F., Cai, J., Wang, Y., Zhang, J., Yua, W., et al., 2013. Lignin depolymerization (LDP) in alcohol over nickel based catalysts via a fragmentation–hydrogenolysis process. Energy Environ. Sci. 6, 994–1007. Xu, F., Yu, J., Tesso, T., Dowell, F., Wang, D., 2013. Qualitative and quantitative analysis of lignocellulosic biomass using infrared techniques: a mini-review. Appl. Energy 104, 801–809. Ye, Y., Zhang, Y., Fan, J., Chang, J., 2012. Novel method for production of phenolics by combining lignin extraction with lignin depolymerization in aqueous ethanol. Ind. Eng. Chem. Res. 51, 103–110. Yuan, Z., Cheng, S., Leitch, M., Xu, C., 2010. Hydrolytic degradation of alkaline lignin in hot-compressed water and ethanol. Bioresour. Technol. 101, 9308–9313. Zhou, D., Zhang, S., Fu, H., Chen, J., 2012. Liquefaction of macro algae enteromorpha prolifera in sub-/supercritical alcohols: direct production of ester compounds. Energy Fuels 26, 2342–2351.
Please cite this article in press as: Singh, R., et al. Hydrothermal conversion of lignin to substituted phenols and aromatic ethers. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.02.076
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Hydrothermal conversion of lignin to substituted phenols and aromatic ethers Rawel Singh a, Aditya Prakash a, Shashi Kumar Dhiman a, Bhavya Balagurumurthy a, Ajay K. Arora b, S.K. Puri b, Thallada Bhaskar a,⇑ a b
Bio-Fuels Division (BFD), CSIR-Indian Institute of Petroleum (IIP), Dehradun 248005, India Indian Oil Corporation, R&D Centre, Faridabad, Haryana, India
h i g h l i g h t s Hydrothermal liquefaction of lignin was performed using ethanol & methanol. Maximum liquid yield (85%) was observed at low reaction temperature of 200 °C. The major liquid products are substituted phenols and aromatic ethers. The maximum organic carbon conversion was found to be 72%.
a r t i c l e
i n f o
Article history: Received 10 January 2014 Received in revised form 19 February 2014 Accepted 21 February 2014 Available online xxxx Keywords: Lignin Bio-oil Hydrothermal liquefaction Thermo-chemical conversion Substituted phenols
a b s t r a c t Hydrothermal liquefaction of lignin was performed using methanol and ethanol at various temperatures (200, 250 and 280 °C) and residence times of 15, 30 and 45 min. Maximum liquid product yield (85%) was observed at 200 °C and 15 min residence time using methanol. Increase in temperature was seen to decrease the liquid products yield. With increase in residence time, liquid yields first increased and then decreased. FTIR and 1H NMR showed the presence of substituted phenols and aromatic ethers in liquid products and breakage of b-O-4 or/and a-O-4 ether bonds present in lignin during hydrothermal liquefaction was confirmed through FTIR of bio-residue. In comparison to the existing literature information, higher lignin conversion to liquid products and maximum carbon conversion (72%) was achieved in this study. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The growing interest in biofuels from lignocellulosic feedstock can provide a path towards replacing petroleum-based fuels with sustainable biofuels which have the potential to lower greenhouse gas (GHG) emissions. Lignocellulose consists mainly of plant cell wall materials and is a complicated natural composite with three main biopolymers, cellulose (40–50%), hemicellulose (15–25%) and lignin (15–35%). The lignocellulosics-to-ethanol process makes use of the cellulose and hemicelluloses, leaving behind lignin as waste. In addition, the pulp and paper industry also generates huge amounts of lignin. Presently, lignin is being utilised as a low-grade boiler fuel to provide heat and power to the process. However, the aromatic structure of lignin suggests that it may be a good source of valuable ⇑ Corresponding author. Tel.: +91 9410151846; fax: +91 1352660202. E-mail addresses: [email protected], [email protected] (T. Bhaskar).
bulk and specialty chemicals like aromatics (Benzene, Toluene, and Xylene, etc.), phenols, aromatic ethers, vanillin, etc., if broken into smaller molecular units by the development of appropriate thermochemical method/catalytic technology. Lignin-derived products find application as fuels, solvents, chemical reagents, and polymers. However, lignin depolymerisation with selective bond cleavage is the major challenge for converting it into valueadded chemicals. Superior to pyrolysis technology, high-pressure direct liquefaction technology has the potential to produce liquid oils that is not miscible in water, with much higher calorific values and a range of chemicals including vanillin, phenols, aldehydes, and organic acids, etc. (Huber et al., 2006). The process is best suited for wet materials as the drying of feedstock is not necessary and water is used as one of the reactants. Methanol and ethanol have lower boiling as well as critical points than water. Due to the lower dielectric constants of these solvents, high molecular weight products produced as a result of
http://dx.doi.org/10.1016/j.biortech.2014.02.076 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Singh, R., et al. Hydrothermal conversion of lignin to substituted phenols and aromatic ethers. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.02.076
2
R. Singh et al. / Bioresource Technology xxx (2014) xxx–xxx
reaction of cellulose and hemicellulose readily dissolve in them (Mazaheri et al., 2010). Liquefaction in organic solvents other than water produces a higher yield of water insoluble oily products making methanol and ethanol promising solvents for the liquefaction of biomass to produce valuable hydrocarbons (Zhou et al., 2012). Beauchet et al. used aqueous alkaline solution to depolymerise Kraft lignin, Indulin AT into gas fraction (mainly CO2), methanol, acetic acid and formic acid, aromatic monomers (19.1 wt.%) and oligomers (polyaromatic molecules) and modified lignin (45– 70 wt.%) (Beauchet et al., 2012). Gosselink et al. used supercritical fluid consisting of carbon dioxide/acetone/water (300–370 °C, 100 bar) for converting organosolv hardwood and wheat straw lignins to a phenolic oil and monomeric aromatic compounds (10– 12%) (Gosselink et al., 2012). Yuan et al. used hot-compressed water–ethanol medium for depolymerisation of alkaline lignin into oligomers using NaOH as the catalyst and phenol as the capping agent at 220–300 °C. The optimal process condition as reported in their study was 260 °C for 3 h (Yuan et al., 2010). Song et al. studied the conversion of native birch wood lignin into monomeric phenols like propylguaiacol and propylsyringol with total selectivity >90% at a lignin conversion of about 50%, over nickel-based catalysts common alcohols as solvents (Song et al., 2013). In the present investigation, lignin depolymerisation was studied in organic solvents viz. ethanol and methanol under various temperatures (200, 250 and 280 °C) and residence times of 15, 30 and 45 min. The lignin and its reaction products were analysed using various physicochemical characterisation methods such as TG, Powder X ray Diffraction (XRD), FTIR, NMR, and Total Organic Carbon (TOC) to understand and confirm the formation of monomeric lignophenols from macromolecular lignin.
2. Methods Lignin was procured from Asian Lignin Manufacturing™ (ALM) in the form of Protobind™ 1000, a renewable product obtained from agricultural fibrous feedstocks. The properties of the lignin are presented as Table S1 in Supplementary materials. Hydrothermal liquefaction of lignin was carried out in stainless steel tubular reactor (35 ml). The thermocouples were placed in the skin and heart of the reactor. In a typical hydrothermal liquefaction experiment, the reactor was loaded with lignin and ethanol/methanol (1:10 by weight). Then the reactor was purged five times with nitrogen to remove the inside air. The temperature was then raised to required point at heating rate of 10 °C min 1 and kept for a specific residence time. After the specified reaction time, the reactor was taken out of the furnace and was immediately submerged into a water bath to stop the further reaction. After cooling to room temperature, the reaction contents were filtered and washed with the respective solvent (ethanol/methanol). Upon removal of the solvent in a rotary evaporator under reduced pressure, the liquid products were obtained and weighed. The solid residue was dried in the oven and after drying weight of solid residue was noted. The liquid and solid products obtained were then analysed. Lignin and reaction products obtained after the hydrothermal upgradation (HTU) of lignin were analysed by powder XRD, and FTIR. The thermogravimetric analysis was carried out in Perkin– Elmer TG instrument. The gross calorific value of lignin has been found out using Parr 6300 Bomb Calorimeter. The bio-oil samples were analysed using FTIR and NMR. Powder XRD pattern of lignin was collected on a Bruker D8 advance X-ray diffractometer. The 1H NMR spectra of the liquid samples have been recorded in the Bruker Ultra shield 500 plus instrument and DMSO-d6 has been used as solvent. The FTIR spectra were recorded on a Nicolet 8700 FTIR
spectrometer. TOC analysis of lignin and bio-residue was performed using Shimadzu TOC-L unit with solid sample module SSM-5000A. 3. Results and discussion The presence of organic solvents is known to improve the solubility of lignin and its decomposition products (Kang et al., 2013). Moreover, methanol can be used as a hydrogen donor in lignin liquefaction, which is in favour of low oxygen content oil production (Barta et al., 2010). Maximum liquid product yield was observed at low temperature and effect of various reaction conditions is explained below (Fig. 1a and b). 3.1. Effect of temperature The reaction temperature is an important factor for the product distribution. The maximum liquid product yield was observed at 200 °C for both ethanol and methanol. Raising the temperature would promote the decomposition of lignin and repolymerisation of the intermediates simultaneously. The maximum liquid product yield was observed at 30 min residence time so this residence time has been considered to see the effect of temperature on liquid product yields. With the increase of temperature from 200 to 280 °C the yield of liquid products obtained by hydrothermal liquefaction of lignin using ethanol decreased. At 200 °C the liquid yield was maximum at 81% and with increase of temperature to 250 °C liquid product yield decreased to 75%. Further decrease of liquid product yield at 65% was observed at 280 °C. The similar trend of decrease in liquid product yield with increase in temperature was also observed at residence times of 15 and 45 min respectively for hydrothermal liquefaction of lignin using ethanol. Solid bio-residue yield increased with increase in temperature and similar result was found by Ye et al. where yield of liquid products decreased with increasing temperature from 225 to 300 °C for lignin depolymerisation in ethanol–water. This was attributed to the formation of solid residue through side reactions such as repolymerisation of lignin decomposition intermediates. This indicates that lower temperature favoured depolymerisation reaction whereas with the increase of temperature formation of higher reactive radicals promoted repolymerisation (Ye et al., 2012). Similar phenomenon is observed in coal liquefaction. Similar effect of temperature on liquid products yield was observed in case of hydrothermal liquefaction of lignin using methanol. Maximum liquid products yield (85%) was observed at 200 °C and liquid products yield gradually decreased with the increase of temperature. 3.2. Effect of residence time The maximum liquid product yield was observed at 200 °C and 30 min residence time for hydrothermal liquefaction of lignin using ethanol and the effect of residence time at different temperatures is discussed below. At 200 °C the effect of residence time on liquid products yield is negligible and liquid yield was constant at all residence times of 15, 30 and 45 min respectively. At 250 °C the liquid product yield first increased with increasing residence time from 15 to 30 min. With the further increase of residence time to 45 min the liquid product yield decreased. The liquid product yield was 60%, 75% and 55% at residence time of 15, 30 and 45 min respectively. Similar trend of decrease in liquid product yield with increase of residence time was observed at 280 °C. The liquid product yield was 42%, 65% and 41% at residence time of 15, 30 and 45 min respectively. During lignin degradation, cracking reactions and condensation occur simultaneously. After a certain reaction time when most of the easily cleaved bonds in lignin molecule
Please cite this article in press as: Singh, R., et al. Hydrothermal conversion of lignin to substituted phenols and aromatic ethers. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.02.076
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(a)
90
bio-residue yield increased beyond 30 min. Therefore, 30 min would be a desirable reaction time for hydrothermal liquefaction of lignin using methanol and ethanol. The gas yield was less in all cases and was vented after the completion of the reaction. TOC analysis of the lignin and bio-residue obtained by hydrothermal liquefaction of lignin using ethanol was studied. TOC results are in good agreement with the above results of effect of temperature and residence times on liquid product yield. Carbon conversion decreased with the increase of temperature. Maximum carbon conversion was observed at 200 °C and 30 min which is also optimum condition for production of higher liquid yields (Supplementary Table S2).
Ethanol
80
% Yield Liquid
70 30 min
60 50
15 min 45 min
40 30
3.3. Analysis of lignin
20 200
220
240
260
280
30 min
XRD spectra of lignin showed its amorphous nature. The TG–DTG of lignin feed is seen to have a single significant decomposition temperature though a slightly complex behaviour is also observed due to overlapping decomposition temperatures (Fig. S1). FTIR spectra of pure lignin shows absorbance peak at 1035 cm 1 which is due to the CAO, C@C and CACAO stretching vibrations. The peak at 1217 cm 1 is due to the CAC and CAO stretching vibrations. The peak at 1268 cm 1 corresponds to the aromatic ring vibration. The peak at 1330 cm 1 corresponds to the CAO of syringyl ring present in lignin. The peak at 1425 cm 1 is due to the CAH in plane deformation. CAH stretching vibrations in lignin are observed at 2805 and 2922 cm 1. Peak at 3427 cm 1 is due to OAH stretching vibrations and shows the presence of phenolic hydroxyl groups. The peak at 1704 shows presence of C@O groups in lignin. Aromatic ring vibrations are observed at 1514 cm 1 (Xu et al., 2013).
280
3.4. Analysis of liquid products
Temperature, ºC
(b)
90
Methanol
80
70
% Yield Liquid
3
60
50 15 min
40 45 min
30
20 200
220
240
260
Temperature, ºC
(c)
60
ETH200 ETH250 ETH280 METH200 METH250 METH280
50
% Proton
40
30
20
10
0 0.5-1.5
1.5-3.0
3.0-4.5
4.5-6.0
6.0-8.5
8.5-10.0
Chemical Shift (ppm) Fig. 1. (a) Effect of residence time and temperature on liquid products yield using ethanol. (b) Effect of residence time and temperature on liquid products yield using methanol. (c) NMR of liquid products obtained from lignin using ethanol and methanol. ETH200: 200 °C, 30 min, ethanol; ETH250: 250 °C, 30 min, ethanol; ETH280: 280 °C, 30 min, ethanol; METH200: 200 °C, 30 min, methanol; METH250: 250 °C, 30 min, methanol; METH280: 280 °C, 30 min, methanol.
such as ether bonds and carbonyl groups were cracked, lignin depolymerisation was essentially complete. Further increasing the residence time would favour the condensation of the degraded lignin intermediates, which should be the reason that solid
The FTIR spectrum of liquid products (Fig. S2) showed absorbance at 1120, 1222, 1460, 1514, 1604, 1700, and 2934 cm 1. The absorbances at 2934 and 1460 cm 1 are attributed to CAH stretching vibrations of methyl and methylene and absorbance of these peaks increased in liquid samples and indicates the presence of alkane groups in liquid products. The increased absorption peak at 1700 cm 1 suggests the presence of conjugated aromatic ketones or esters. The absorbances at 1514 and 1604 cm 1 corresponds to aryl groups. The peaks at around 1220 and 3420 cm 1 indicate the presence of phenols. The peak at 1120 cm 1 belongs to methoxy groups and is present in all the liquid products. The peak at 834 cm 1 indicates the presence of para-substituted aromatic groups (Ye et al., 2012). The FTIR spectra of liquid products obtained from the hydrothermal liquefaction of lignin using ethanol and methanol indicates the presence of phenols and aromatic ethers. NMR spectra provided complementary functional group information to FTIR spectra and the ability to quantify and compare integration areas between spectra. A summary of integrated peak area regions assigned to different functional group classes present in the liquid products obtained from hydrothermal liquefaction of lignin using methanol and ethanol at 30 min residence time are shown in Fig. 1c. The region from 0.5 to 3.0 ppm is composed of protons on aliphatic carbon. The region from 0.5 to 1.5 ppm corresponds to protons on aliphatic carbon atoms at least two bonds away from C@C or heteroatom and the region from 1.5 to 3.0 ppm corresponds to the protons on aliphatic carbon atoms that may be bonded to a C@C double bond or heteroatom. Liquid products obtained from hydrothermal liquefaction of lignin showed high proton percentage in this region. With the increase in temperature proton percentage increased in this region. The next portion of the 1H NMR spectrum at 3.0–4.5 ppm represents methoxyl
Please cite this article in press as: Singh, R., et al. Hydrothermal conversion of lignin to substituted phenols and aromatic ethers. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.02.076
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protons (Kosa et al., 2011) or methylene group that joins two aromatic rings. The latter moiety would exist in a partially decomposed lignin oligomer present in the bio-oil (Mullen et al., 2009). All the liquid products show high proton percentage in this region. The region between 4.5 and 6.6 ppm represents aromatic ether protons i.e. lignin derived methoxy phenols. The region of the spectrum between 6.0 and 8.5 ppm corresponds to the aromatic region of spectrum. The liquid products showed significant proton percentage in this region. The liquid products obtained using methanol as solvent showed higher percentage in this region. Maximum proton percentage (17%) was observed in this region using methanol at 200 °C. The downfield spectrum regions (8.5– 10 ppm) of the bio-oils arise from the aldehydes and the proton percentage was very low in this region at around 1%. The FTIR and 1H NMR of the liquid products indicate the presence of substituted phenols and aromatic ethers present in the liquid product obtained. In a study by Ye et al. liquid products obtained were mainly composed of phenolics (4-ethylphenol, 4-vinylphenol, guaiacol, 4-ethylguaiacol, and 4-vinylguaiacol) which constitute almost 85% of the identified compounds (Ye et al., 2012). The formation of substituted phenols and aromatic ethers may be due to the breakage of various CAC and CAOAC ether bonds (b-O-4 or/and a-O-4) present in lignin. The ethanol and methanol may further react with the OAH groups to give corresponding methyl and ethyl ethers. During lignin hydrothermal liquefaction, various reactions like hydrolysis and cleavage of the ether and CAC bond, demethoxylation, alkylation and condensation, etc. occur. Various linkages like b-O-4 and Ca–Cb were easier to break whereas the aromatic rings were not affected during hydrothermal liquefaction. This indicates that at milder conditions of relatively low temperature and short residence time, phenolic monomers and dimers may be formed and as temperature is increased demethoxylation and alkylation of lignin derived phenolic compounds are enhanced and various alkyl phenols may be formed (Kang et al., 2011, 2013). 3.5. Analysis of bio-residue The absorbance from the ether bond CAOAC at 1120 and 1034 cm 1 weaken in intensity for the solid residue in all samples, which is possible due to the breakage of b-O-4 or/and a-O-4 ether bonds to form hydroxyl groups and alkyl groups. The weakening of these absorbance peaks in bio-residue samples indicate that the breakage of b-O-4 or/and a-O-4 ether bonds to form hydroxyl groups and alkyl groups started at very low temperature around 200 °C. Solid residue obtained from depolymerisation of lignin using methanol and ethanol showed similar behaviour. The absorbances at 1604, 1514, and 1425 cm 1 that are attributed to aromatic nuclei decreased in the bio-residue samples. The weakened absorbance at 1700 cm 1 implies that decarbonylation occurred even at very mild condition (Ye et al., 2012). 4. Conclusion Maximum liquid products yield (85%) was observed at 200 °C with 30 min residence time using methanol. With increase in
temperature, liquid product yield decreased. FTIR and 1H NMR of liquid products confirmed the presence of substituted phenols and aromatic ethers. FTIR of bio-residue samples indicate that the breakage of b-O-4 or/and a-O-4 ether bonds to form hydroxyl groups and alkyl groups started at very low temperature (200 °C). Acknowledgements The authors thank The Director, CSIR-Indian Institute of Petroleum, Dehradun, for his constant encouragement and support. R.S. and A.P. thank Council of Scientific and Industrial Research (CSIR), New Delhi, India, for providing Senior Research Fellowship (SRF) and Junior Research Fellowship (JRF) respectively. The authors thank CSIR, Government of India for providing financial support in the form of XII Five Year Plan project (CSC0116/BioEn). The authors thank the FTIR, NMR and XRD groups at CSIR-Indian Institute of Petroleum (IIP) for providing analytical support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2014. 02.076. References Barta, K., Matson, T.D., Fetting, M.L., Scott, S.L., Iretskii, A.V., Ford, P.C., 2010. Catalytic disassembly of an organosolv lignin via hydrogen transfer from supercritical methanol. Green Chem. 12, 1640–1647. Beauchet, R., Monteil-Rivera, F., Lavoie, J.M., 2012. Conversion of lignin to aromaticbased chemicals (L-chems) and biofuels (L-fuels). Bioresour. Technol. 121, 328– 334. Gosselink, R.J.A., Teunissen, W., van Dam, J.E.G., de Jong, E., Gellerstedt, G., Scott, E.L., et al., 2012. Lignin depolymerisation in supercritical carbon dioxide/acetone/ water fluid for the production of aromatic chemicals. Bioresour. Technol. 106, 173–177. Huber, G.W., Iborra, S., Corma, A., 2006. Synthesis of transportation fuels from biomass: chemistry, catalysts and engineering. Chem. Rev. 106, 4044–4098. Kang, S., Li, X., Fan, J., Chang, J., 2011. Classified separation of lignin hydrothermal liquefied products. Ind. Eng. Chem. Res. 50, 11288–11296. Kang, S., Li, X., Fan, J., Chang, J., 2013. Hydrothermal conversion of lignin: a review. Renew. Sust. Energy Rev. 27, 546–558. Kosa, M., Ben, H., Theliander, H., Ragauskas, A.J., 2011. Pyrolysis oils from CO2 precipitated Kraft lignin. Green Chem. 13, 3196–3202. Mazaheri, H., Lee, K.T., Bhatia, S., Mohamed, A.R., 2010. Sub/supercritical liquefaction of oil palm fruit press fiber for the production of bio oil: effect of solvents. Bioresour. Technol. 101, 7641–7647. Mullen, C.A., Strahan, G.D., Boateng, A.A., 2009. Characterization of various fastpyrolysis bio-oils by NMR spectroscopy. Energy Fuels 23, 2707–2718. Song, Q., Wang, F., Cai, J., Wang, Y., Zhang, J., Yua, W., et al., 2013. Lignin depolymerization (LDP) in alcohol over nickel based catalysts via a fragmentation–hydrogenolysis process. Energy Environ. Sci. 6, 994–1007. Xu, F., Yu, J., Tesso, T., Dowell, F., Wang, D., 2013. Qualitative and quantitative analysis of lignocellulosic biomass using infrared techniques: a mini-review. Appl. Energy 104, 801–809. Ye, Y., Zhang, Y., Fan, J., Chang, J., 2012. Novel method for production of phenolics by combining lignin extraction with lignin depolymerization in aqueous ethanol. Ind. Eng. Chem. Res. 51, 103–110. Yuan, Z., Cheng, S., Leitch, M., Xu, C., 2010. Hydrolytic degradation of alkaline lignin in hot-compressed water and ethanol. Bioresour. Technol. 101, 9308–9313. Zhou, D., Zhang, S., Fu, H., Chen, J., 2012. Liquefaction of macro algae enteromorpha prolifera in sub-/supercritical alcohols: direct production of ester compounds. Energy Fuels 26, 2342–2351.
Please cite this article in press as: Singh, R., et al. Hydrothermal conversion of lignin to substituted phenols and aromatic ethers. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.02.076
