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Hydrothermal decomposition of alkali lignin in sub- and supercritical water
Chemical Engineering Journal 187 (2012) 410–414
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Short communication
Hydrothermal decomposition of alkali lignin in sub- and supercritical water ∗ ´ ´ Hanna Pinkowska , Paweł Wolak, Adrianna Złocinska Department of Chemical Technology, Wrocław University of Economics, ul. Komandorska 118/120, 53-345 Wrocław, Poland
a r t i c l e
i n f o
Article history: Received 16 June 2011 Received in revised form 17 January 2012 Accepted 20 January 2012 Keywords: Alkali lignin Subcritical water Supercritical water Hydrothermal decomposition Phenolic compounds Biochar
a b s t r a c t Alkali lignin as an example of waste plant biomass was subjected in a batch reactor to decomposition in sub- and supercritical water. Hydrothermolysis of alkali lignin with relatively highmolecular-weight led to the production of phenolic compounds such as guaiacol and catechol, as well as phenol, m-cresol, p-cresols and o-cresol. Both an increase growth in conversion time and an increase in reaction temperature promoted secondary reactions: degradation of alkali lignin decomposition products, leading to, their repolymerization and carbonization. The components of the liquid fractions of products were examined by GC–MS and HPLC, whereas the solid charred post-reaction residues were analyzed by elemental analysis and FTIR. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Lignin is a highly reactive amorphous polymer, comprised of cross-linked, branched aromatic monomers, with phenylpropyl units [1] and characterized by the presence of diverse functional groups (hydroxyl, methoxyl and carbonyl). Lignin is produced mainly as a byproduct of the pulp and paper industry and biomass fractionation and is treated as waste material, which is destroyed as low-grade fuel by burning or used as a low-value product (e.g. flocculating and dispersing agents). Lignin in the chemical industry is still only little recovered and used as a suitable source of chemicals, like phenolics, which possess antioxidant properties [2]. In recent years, attempts have been made to perform hydrothermal decomposition of model substances for lignin: guaiacol [3–5] and catechol [6], and of lignin [2,7–9]. The use of sub- and supercritical water (Tcr = 647.1 K, Pcr = 22.1 MPa [10]) is believed to have positive effect on the degradation of lignin material to phenolics compounds. However only a few publications deal with hydrothermolysis of raw lignin. These studies focused on “alkaline lignin” [2], completely soluble in methanol and water (C: 51.83%, H: 4.78%, N: 0.11%, and O: 43.28%) with the molecular weight estimated to be about 350 or the “organosolve” THF-soluble lignin [7–9], with the weight average molecular weight Mw = 1500 [9] or Mw = 2100 (C: 66.54%, H: 5.85%, O: 27.61%) and number average molecular weight Mn = 620 [8].
∗ Corresponding author. Tel.: +48 71 36 80 275; fax: +48 71 36 80 275. ´ E-mail address: [email protected] (H. Pinkowska). 1385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2012.01.092
The purpose of this study was to examine the trend of hydrothermal decomposition of alkali lignin- an environmentally hazardous component of pulp and paper wastewaters, in sub- and supercritical water, as a batch process, and to determine the effect of the reaction parameters (temperature and time) on the degree of raw material conversion. The alkali lignin used in the experiments was of relatively high molecular weight, with a weight average and number average molecular weight of Mw = 28,000 and Mn = 5000, respectively, insoluble in methanol and in THF. Alkali lignin was decomposed in a hydrothermal process, with water acting as the reagent, solvent and catalyst. The composition and yield of the selected elements low-molecular-weight phenolic compounds contained in the liquid product fractions was determined, and insoluble product fractions were examined. 2. Materials and methods 2.1. Material and reagents For all experiments, alkali lignin (Sigma–Aldrich) was used. All chemicals were used without further purification. Their description is given in Supplementary information. 2.2. Reactor and experimental procedure The hydrothermal decomposition of alkali lignin was performed in a 4576A-type batch reactor (Parr, USA). 5 g of alkali lignin and 95 g of water were used in each run. The hydrothermolysis was conducted at temperatures of 553, 643, 653 and 663 K. The reaction was stopped once the intended temperature was reached (zero
H. Pi´ nkowska et al. / Chemical Engineering Journal 187 (2012) 410–414
411
900–700 cm−1 were assigned as the bending vibrations of saturated aromatic hydrocarbons and aromatic C H groups.
(a) (b)
T (%)
3.2. Liquid product fractions
(c)
3800
3400
3000
2600
2200
1800
1400
1000
600
cm-1 Fig. 1. FTIR spectrum of alkali lignin – (a) and MN fractions obtained from the hydrothermolysis of alkali lignin for reaction time 240 min at temperature 553 K (b) and 663 K(c).
holding time), and after holding times of 30, 60, 90, 120 and 240 min. The liquid and solid product fractions were collected by washing the reactor vessel with methanol. Detailed description of the reactor equipment, as well the experimental procedure is given in Supplementary material. 2.3. Separation of the products of alkali lignin hydrothermolysis As a result of the hydrothermal decomposition of alkali lignin, a liquid product containing water and methanol-soluble substances (the SM fraction) and a water and methanol-insoluble solid postreaction residue (the MN fraction) were obtained. The SM fraction was separated from the MN fraction by vacuum filtration using PTFE membrane. No gaseous fraction components, formed during alkali lignin hydrothermolysis, were collected. The content of a given alkali lignin hydrothermolysis products was calculated from the relation: Yi (wt%) =
m i
mal
× 100
(1)
where mi is the mass of the i-product (g) and mal is the mass of the alkali lignin used for the reaction (g). 2.4. Analytical techniques and measurement methodology The composition of alkali lignin and MN fractions was determined by elemental analysis and Fourier transform infrared spectroscopy (FTIR). The products in liquid fractions were analyzed by gas chromatography mass spectroscopy (GC–MS) and high performance liquid chromatography (HPLC). The operating conditions of the analysis were described in Supplementary material. 3. Results and discussion 3.1. Composition of alkali lignin The composition of alkali lignin can be illustrated using the empirical formula [C10 H10.47 O3.86 N0.08 S0.08 ]n ; it contains carbon (61.31%), hydrogen (5.35%), oxygen (31.47%), nitrogen (0.49%) and sulfur (1.38%). Fig. 1(a) shows the FTIR spectrum of alkali lignin, demonstrating some typical bands [2,5]. The bands in the 3600–3200 cm−1 region were assigned to the stretching of aromatic O H groups, in the 3000–2800 cm−1 to the stretching of aromatic and aliphatic C H groups and in the 1600 and 1510 cm−1 to the stretching of aromatic C C groups. Signals in the region 1400–1300 cm−1 and
Table 1 shows the results of the qualitative examination of the SM fractions obtained from the hydrothermal decomposition of alkali lignin using GC–MS, while Fig. 1S a model chromatogram (is given in Supplementary material). In the SM product fractions obtained at 553 K and 643 K the quantitadominant substances were guaiacol, vanillin, tively 1-(4-hydroxy-3-methoxyphenyl)ethanone, and 1-(4-hydroxy3-methoxyphenyl)propanone, vanillic acid, as well as additionally at 643 K also 4-methylguaiacol, catechol and 4-methylcatechol. Except for vanillic acid at 553 K and catechol and 4-methylcatechol at 643 K, the longer was the reaction time, the smaller was the share of these components in the SM fractions. In the fractions obtained at 643 K, an increase in reaction time was accompanied by a growth in the share of, among others, phenol, catechol, 4-methylphenol and 4-methylresorcine. Dominant elements of the SM fractions after alkali lignin hydrothermolysis performed at 653 K were phenol, guaiacol, 4-methylguaiacol, catechol, 3methylcatechol, 4-methylcatechol and 4-methylresorcine. Except for phenol and catechol, the share of those elements in the SM fractions fell continuously as reaction time increased. At 663 K, as the reaction time increased, the percentage of guaiacol and catechol in the SM fractions markedly decreased, while the phenol, o-cresol and p-cresol content increased. Fig. 2 illustrates the effect of reaction temperature and time on yield of the selected SM products. Based on the experimental results, the cleavage of the C O bonds in alkali lignin took place particularly in subcritical water and also in supercritical conditions (which was promoted by the high ion product of water). Various phenols and methoxy phenols (such as guaiacol- a lignin monomer) were formed by hydrolysis as basic products. The largest guaiacol yield (11.23%) was obtained at the lowest temperature set after a holding time of 0 min, and fell gradually as both the reaction temperature and time increased. The cleavage of C C bonds in alkali lignin took place in subcritical water but at a longer reaction time, and occurred predominantly in supercritical water [11]. The main product of demethoxylation of guaiacol was catechol, formed after the homolysis of the OCH3 bond and hydrogen abstraction from water [5] and/or as a result of dealkylation of alkali lignin [2]. In subcritical conditions (at 553 K), the catechol yield increased continually with reaction time, with its highest value of 3.91% after 240 min. In subcritical water (at 643 K), as well as under supercritical conditions (at 653 K) the catechol yield initially grew in the range of reaction times from 0 to 60 min, reaching 10.11% and 9.54%, respectively. Then with an increase of reaction time (over 60 min) the yield of catechol decreased. At 663 K the highest yield of catechol was reached after a holding time of 0 min, and then decreased with the reaction time. The results in this study were similar to the work of Wahyudiono et al. [5]. In this previous report, it was found that both the conversion of guaiacol and the catechol yield were low in subcritical conditions (2.6–5.7 wt% and 1.7–3.4 wt%, respectively at 523 K and 8 MPa), and grew higher in supercritical water (58–98 wt% and 40–44 wt%, respectively at 673 K and 40 MPa). Both in sub- and supercritical water the yield of catechol rapidly increased with reaction time (till 30 min) and then decreased [2]. In the applied conditions, stable reaction products such as phenol, m,p-cresol and o-cresol formed as products of occurring secondary reactions. They were minor reaction products in subcritical water but their yield increased with reaction temperature
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H. Pi´ nkowska et al. / Chemical Engineering Journal 187 (2012) 410–414
Table 1 Composition of the SM fractions obtained from alkali lignin hydrothermolysis. RTa
9.47 9.74 10.19 10.70 10.77 11.23 11.37 11.89 11.95 12.41 12.55 13.05 13.15 13.30 13.55 13.77 13.89 14.26 14.76 15.19 15.21 15.71 15.85 15.91 16.08 16.18 16.46 16.79 16.85 16.89 17.15 17.31 18.19 a
Compound
Phenol Guaiacol o-Cresol p-Cresol 3-Methylguaiacol 4-Methylguaiacol Dimethylphenol Dimethylphenol (isomer) 4-Ethylphenol 4-Ethyl-2-methoxyphenol 1-Ethyl-4-methoxybenzene 3-Ethyl-5-methylphenol 3-Methoxycatechol 3-Methoxyphenol 2-Methoxy-methyl-ethylphenol Catechol 2,6-Dimethoxyphenol 3-Methylcatechol 4-Methylcatechol Vanillin 4-Ethoxyphenol 2-Methylresorcine 4-Methylresorcine 2-Methoxy-4-propylphenol 4,5-Dimethylresorcine 1-(4-Hydroxy-3-methoxyphenyl)ethanone 4-Hydroxybenzaldehyde 1-(4-Hydroxy-3-methoxyphenyl)propanone 4-Propylcatechol 3-Methoxy-4-hydroxybenzyl methyl ether 4-tert-Butylbenzene-1,2-diol 4-(3-Hydroxy-1-propenyl)-2-methoxyphenol Vanillic acid 8.7
Tr (K), tr (min) Share in the SM fraction (area %) 553, 0
553, 240
643, 0
643, 240
653, 0
653, 240
663, 0
663, 240
0.7 41.9 – – – 2.1 – – – 3.2 – – 0.1 – – 1.3 – – – 17.9 – – – 2.1 – 8.8 1.1 10.6 – – 0.7 1.0 15.9
2.1 28.3 – 0.5 – 6.2 – – 0.3 4.6 – – 0.7 – 0.9 10.3 0.8 – 1.6 7.0 – 0.7 0.9 0.7 – 5.3 0.7 9.6 0.7 1.2 0.7 0.5 9.8
2.0 27.3 – 1.0 0.6 10.2 – – 0.5 6.3 – – 0.5 0.5 0.8 8.5 0.7 0.7 1.7 10.7 – – 1.0 0.9 – 5.7 0.9 7.1 0.5 0.9 0.6 1.0 –
5.8 9.5 2.9 5.5 0.6 3.7 1.9 0.8 1.9 2.3 0.7 – – – 0.8 31.3 – 6.6 14.4 0.8 0.8 – 6.4 – 0.6 – – – 2.6 – – – –
3.9 24.4 1.2 3.0 1.1 5.7 1.0 0.3 1.1 3.6 – – – – – 15.7 – 6.7 18.0 0.8 1.8 – 7.5 – 1.1 0.5 – – 2.8 – – – –
8.2 8.3 4.0 4.3 3.7 2.6 2.6 1.3 3.9 1.4 0.5 0.7 0.6 – – 32.8 – 5.1 11.0 – – – 5.0 – – – – 1.9 2.0 – – – –
9.0 21.1 4.1 5.3 2.2 4.3 2.1 0.1 2.5 2.0 0.2 0.8 1.3 – – 14.2 – 6.2 13.4 0.3 1.3 – 4.2 – 0.2 0.4 – 0.3 3.8 0.3 0.1 – –
11.9 13.8 6.2 9.1 6.2 8.0 4.2 0.8 5.3 0.1 1.8 1.4 2.0 – – 12.0 – 3.6 8.7 – 0.1 – 2.1 – – 0.1 – 1.5 0.3 – 0.6 – –
Retention time (min).
and time. Phenol and cresol isomers were formed from guaiacol radical species after they underwent scission [5] or from catechol, as a result of its hydrolysis [2,6]. Phenol could be produced from catechol via cleavage of one of its C OH bonds, and formation of a phenoxy radical and H atom. The H atom reacted with catechol to yield phenol. Phenol could also be formed from cleavage of the catechol C O bond and reaction of the cyclohexyl cation with water [6]. Cresol isomers were formed from guaiacol radicals after their scission and subsequent reaction with the H atom [5]. In our study, the yields of phenol and cresol isomers grew gradually across the whole range of reaction temperatures and times. The largest yields of phenol (4.21%), m,p-cresols (3.89%) and o-cresol (3.11%) were obtained at 663 K and 240 min. The results in this study were similar to the work of Wahyudiono et al. [2]. In this previous report, it was found that both in sub- and supercritical water, the yield of phenol increased with the reaction time, achieved 11.75 wt% at 673 K and 240 min. The yield of cresol isomers increased initially with the reaction time, but was almost constant after 90 min. The amounts of m,p-cresol and ocresol approached 8.76 wt% and 3.65 wt%, respectively at 623 K and 6.98 wt% and 3.95 wt%, respectively at 673 K. In the adopted alkali lignin hydrothermal decomposition conditions, the yields of phenol and cresol isomers did not directly correspond to the changes in the guaiacol or catechol content. Although the yields of phenol and cresol isomers increased with the reaction time, the total yield of identified alkali lignin decomposition products decreased with increasing reaction temperature and time. Yields of monomeric phenols were low, probably due to their gasification and/or repolymerization.
3.3. Insoluble product fractions In experiments performed in subcritical water, the MN fractions yield was high (48.9% and 25.3% after 0 min at 553 K and 643 K, respectively), but fell as both the conversion temperature and time increased (31.1% and 13.1% after 240 min). In supercritical water the yield of MN fractions was higher than under subcritical conditions at 643 K (Fig. 3). Probably, the MN fractions obtained under subcritical conditions were composed of undissolved alkali lignin. In supercritical water the dielectric constant of water decreases with increasing temperature, promoting the dissolution of alkali lignin [4]. Under supercritical conditions, the solubility of alkali lignin in water was greater and soluble alkali lignin fragments reacted with the lower-molecular-weight soluble compounds to form polymers and cross-linked phenolic biochar. The repolymerization via radical coupling and carbonization via recondensation reactions were promoted by increasing temperature [2]. The composition of the MN fractions obtained at 553 K after duration 0 min and 30 min corresponded to the typical composition of alkali lignin. The further extension of the reaction time and the rise in temperature resulted in significant changes in the elementary composition of the solid residue. The composition of the MN fractions obtained at 553 K and 663 K and reaction time 240 min, represented by the respective formulas [C10 H7.26 O3.19 N0.07 S0.07 ] and [C10 H1.86 O0.56 N0.05 S0.07 ], significantly differed from the composition of the alkali lignin subjected to hydrothermal decomposition. Fig. 1(b) and (c) shows FTIR selected two model spectra of the MN fractions obtained from the hydrothermal decomposition of
H. Pi´ nkowska et al. / Chemical Engineering Journal 187 (2012) 410–414 50
12 guaiacol
553K
553K
phenol m,p-cresol
8
643K
653K
663K
45
catechol
10
o-cresol
6 4 2
g/100g of dry alkali lignin
g/100g of dry alkali lignin
413
40 35 30 25 20 15
0 0
30
60
90
120
150
180
210
240
10 0
30
Time (min) g/100g of dry alkali lignin
12
643K
120
150
180
210
240
Fig. 3. The effect of alkali lignin hydrothermolysis temperature and time on the yield of MN fractions.
guaiacol catechol
10
phenol m,p-cresol
8
o-cresol
6 4
0 0
30
60
90
120
150
180
210
12
4. Conclusions guaiacol
653K
catechol
10
phenol m,p-cresol
8
o-cresol
6 4 2 0 0
30
60
90
120
150
could only be observed in the 1200–650 cm−1 region, in which the disappearance of vibrations typical of ether groups and aromatic rings were observed. The spectrum 1(c) of the MN fraction obtained at 663 K with reaction time 240 min differs from the ones for the MN fractions obtained at a lower temperature. The band typical for OH group stretching vibrations is absent at 3600–3000 cm−1 . This is the result of the loss of phenolic and alcoholic groups [2], and it indicates the progressive degradation of the alkali lignin.
240
Time (min)
g/100g of dry alkali lignin
90
Time (min)
2
180
210
240
Time (min) 12
g/100g of dry alkali lignin
60
guaiacol
663K
catechol
10
phenol
The hydrothermal decomposition of alkali lignin was performed in sub- and supercritical water. Alkali lignin of a high molecular weight was successfully degraded into low-molecular-weight components. The yield (wt%) of guaiacol, catechol, phenol and cresol isomers reached the highest values of approximately 11.23% (553 K, 0 min), 11.11% (653 K, 0 min), 4.21% (663 K, 240 min) and 7.00% (663 K, 240 min) With increase in reaction temperature and time, the composition of the solid post-reaction residue differed significantly from the composition of the alkali lignin subjected to hydrothermal decomposition. This suggests that the solid residue consisted mainly of phenolic biochar. Hydrothermal decomposition of alkali lignin could become an environmentally friendly technology to produce antioxidant phenolic compounds. Hydrothermal decomposition of relatively high-molecular-weight alkali lignin is an example of an environmentally benign and efficient technique of lignin biomass treatment (e.g. from the pulp and paper industry).
m,p-cresol
8
o-cresol
Acknowledgment
6
The authors gratefully acknowledge the financial support for this work provided by the Ministry of Science and Higher Education of Poland in 2008–2010 in the form of research Project No. N N523 494134.
4 2 0 0
30
60
90
120
150
180
210
240
Time (min) Fig. 2. The effect of alkali lignin hydrothermolysis temperature and time on the yield of selected components in the SM fractions.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cej.2012.01.092. References
alkali lignin at 553 K and 663 K after reaction time of 240 min. The other FTIR spectra of MN fractions are given in Supplementary material (Fig. 2S). The FTIR spectrum (1b) and other spectra of MN products resembled one another, which indicated that their structures and functional group types were similar. Small differences
[1] Y. Yu, X. Lou, H. Wu, Some recent advances in hydrolysis of biomass in hotcompressed water and its comparisons with other hydrolysis methods, Energy Fuel 22 (2008) 46–60. [2] Wahyudiono, M. Sasaki, M. Goto, Recovery of phenolic compounds through the decomposition of lignin in near and supercritical water, Chem. Eng. Process. 47 (2008) 1609–1619.
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[3] K. Okuda, S. Ohara, M. Umetsu, S. Takami, T. Adschiri, Disassembly of lignin and chemical recovery in supercritical water and p-cresol mixture. Studies on lignin model compounds, Bioresour. Technol. 99 (2008) 1846–1852. [4] Wahyudiono, T. Kanetake, M. Sasaki, M. Goto, Decomposition of a lignin model compound under hydrothermal conditions, Chem. Eng. Technol. 8 (2007) 1113–1122. [5] Wahyudiono, M. Sasaki, M. Goto, Thermal decomposition of guaiacol in suband supercritical water and its kinetic analysis, J. Mater. Cycles Waste Manage. 13 (2011) 68–79. [6] Wahyudiono, M. Sasaki, M. Goto, Conversion of biomass model compound under hydrothermal conditions using batch reactor, Fuel 88 (2009) 1656–1664. [7] M. Saisu, T. Sato, M. Watanabe, T. Adschiri, K. Arai, Conversion of lignin with supercritical water–phenol mixtures, Energy Fuel 17 (2003) 922–928.
[8] K. Okuda, M. Umetsu, S. Takami, T. Adschiri, Disassembly of lignin and chemical recovery rapid depolymerization of lignin without char formation in water–phenol mixtures, Fuel Process. Technol. 85 (2004) 803–813. [9] Z. Fang, T. Sato, R.L. Smith, H. Inomata, K. Arai, J.A. Kozinski, Reaction chemistry and phase behavior of lignin in high-temperature and supercritical water, Bioresour. Technol. 99 (2008) 3424–3430. [10] F. Cansell, S. Rey, P. Beslin, Thermodynamic aspects of supercritical fluids processing: application to polymers and wastes treatment, Rev. I. Fr. Petrol. 53 (1998) 71–98. [11] M.P. Pandey, C.S. Kim, Lignin depolymerization and conversion: a review of thermochemical methods, Chem. Eng. Technol. 34 (2011) 29–41.
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Short communication
Hydrothermal decomposition of alkali lignin in sub- and supercritical water ∗ ´ ´ Hanna Pinkowska , Paweł Wolak, Adrianna Złocinska Department of Chemical Technology, Wrocław University of Economics, ul. Komandorska 118/120, 53-345 Wrocław, Poland
a r t i c l e
i n f o
Article history: Received 16 June 2011 Received in revised form 17 January 2012 Accepted 20 January 2012 Keywords: Alkali lignin Subcritical water Supercritical water Hydrothermal decomposition Phenolic compounds Biochar
a b s t r a c t Alkali lignin as an example of waste plant biomass was subjected in a batch reactor to decomposition in sub- and supercritical water. Hydrothermolysis of alkali lignin with relatively highmolecular-weight led to the production of phenolic compounds such as guaiacol and catechol, as well as phenol, m-cresol, p-cresols and o-cresol. Both an increase growth in conversion time and an increase in reaction temperature promoted secondary reactions: degradation of alkali lignin decomposition products, leading to, their repolymerization and carbonization. The components of the liquid fractions of products were examined by GC–MS and HPLC, whereas the solid charred post-reaction residues were analyzed by elemental analysis and FTIR. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Lignin is a highly reactive amorphous polymer, comprised of cross-linked, branched aromatic monomers, with phenylpropyl units [1] and characterized by the presence of diverse functional groups (hydroxyl, methoxyl and carbonyl). Lignin is produced mainly as a byproduct of the pulp and paper industry and biomass fractionation and is treated as waste material, which is destroyed as low-grade fuel by burning or used as a low-value product (e.g. flocculating and dispersing agents). Lignin in the chemical industry is still only little recovered and used as a suitable source of chemicals, like phenolics, which possess antioxidant properties [2]. In recent years, attempts have been made to perform hydrothermal decomposition of model substances for lignin: guaiacol [3–5] and catechol [6], and of lignin [2,7–9]. The use of sub- and supercritical water (Tcr = 647.1 K, Pcr = 22.1 MPa [10]) is believed to have positive effect on the degradation of lignin material to phenolics compounds. However only a few publications deal with hydrothermolysis of raw lignin. These studies focused on “alkaline lignin” [2], completely soluble in methanol and water (C: 51.83%, H: 4.78%, N: 0.11%, and O: 43.28%) with the molecular weight estimated to be about 350 or the “organosolve” THF-soluble lignin [7–9], with the weight average molecular weight Mw = 1500 [9] or Mw = 2100 (C: 66.54%, H: 5.85%, O: 27.61%) and number average molecular weight Mn = 620 [8].
∗ Corresponding author. Tel.: +48 71 36 80 275; fax: +48 71 36 80 275. ´ E-mail address: [email protected] (H. Pinkowska). 1385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2012.01.092
The purpose of this study was to examine the trend of hydrothermal decomposition of alkali lignin- an environmentally hazardous component of pulp and paper wastewaters, in sub- and supercritical water, as a batch process, and to determine the effect of the reaction parameters (temperature and time) on the degree of raw material conversion. The alkali lignin used in the experiments was of relatively high molecular weight, with a weight average and number average molecular weight of Mw = 28,000 and Mn = 5000, respectively, insoluble in methanol and in THF. Alkali lignin was decomposed in a hydrothermal process, with water acting as the reagent, solvent and catalyst. The composition and yield of the selected elements low-molecular-weight phenolic compounds contained in the liquid product fractions was determined, and insoluble product fractions were examined. 2. Materials and methods 2.1. Material and reagents For all experiments, alkali lignin (Sigma–Aldrich) was used. All chemicals were used without further purification. Their description is given in Supplementary information. 2.2. Reactor and experimental procedure The hydrothermal decomposition of alkali lignin was performed in a 4576A-type batch reactor (Parr, USA). 5 g of alkali lignin and 95 g of water were used in each run. The hydrothermolysis was conducted at temperatures of 553, 643, 653 and 663 K. The reaction was stopped once the intended temperature was reached (zero
H. Pi´ nkowska et al. / Chemical Engineering Journal 187 (2012) 410–414
411
900–700 cm−1 were assigned as the bending vibrations of saturated aromatic hydrocarbons and aromatic C H groups.
(a) (b)
T (%)
3.2. Liquid product fractions
(c)
3800
3400
3000
2600
2200
1800
1400
1000
600
cm-1 Fig. 1. FTIR spectrum of alkali lignin – (a) and MN fractions obtained from the hydrothermolysis of alkali lignin for reaction time 240 min at temperature 553 K (b) and 663 K(c).
holding time), and after holding times of 30, 60, 90, 120 and 240 min. The liquid and solid product fractions were collected by washing the reactor vessel with methanol. Detailed description of the reactor equipment, as well the experimental procedure is given in Supplementary material. 2.3. Separation of the products of alkali lignin hydrothermolysis As a result of the hydrothermal decomposition of alkali lignin, a liquid product containing water and methanol-soluble substances (the SM fraction) and a water and methanol-insoluble solid postreaction residue (the MN fraction) were obtained. The SM fraction was separated from the MN fraction by vacuum filtration using PTFE membrane. No gaseous fraction components, formed during alkali lignin hydrothermolysis, were collected. The content of a given alkali lignin hydrothermolysis products was calculated from the relation: Yi (wt%) =
m i
mal
× 100
(1)
where mi is the mass of the i-product (g) and mal is the mass of the alkali lignin used for the reaction (g). 2.4. Analytical techniques and measurement methodology The composition of alkali lignin and MN fractions was determined by elemental analysis and Fourier transform infrared spectroscopy (FTIR). The products in liquid fractions were analyzed by gas chromatography mass spectroscopy (GC–MS) and high performance liquid chromatography (HPLC). The operating conditions of the analysis were described in Supplementary material. 3. Results and discussion 3.1. Composition of alkali lignin The composition of alkali lignin can be illustrated using the empirical formula [C10 H10.47 O3.86 N0.08 S0.08 ]n ; it contains carbon (61.31%), hydrogen (5.35%), oxygen (31.47%), nitrogen (0.49%) and sulfur (1.38%). Fig. 1(a) shows the FTIR spectrum of alkali lignin, demonstrating some typical bands [2,5]. The bands in the 3600–3200 cm−1 region were assigned to the stretching of aromatic O H groups, in the 3000–2800 cm−1 to the stretching of aromatic and aliphatic C H groups and in the 1600 and 1510 cm−1 to the stretching of aromatic C C groups. Signals in the region 1400–1300 cm−1 and
Table 1 shows the results of the qualitative examination of the SM fractions obtained from the hydrothermal decomposition of alkali lignin using GC–MS, while Fig. 1S a model chromatogram (is given in Supplementary material). In the SM product fractions obtained at 553 K and 643 K the quantitadominant substances were guaiacol, vanillin, tively 1-(4-hydroxy-3-methoxyphenyl)ethanone, and 1-(4-hydroxy3-methoxyphenyl)propanone, vanillic acid, as well as additionally at 643 K also 4-methylguaiacol, catechol and 4-methylcatechol. Except for vanillic acid at 553 K and catechol and 4-methylcatechol at 643 K, the longer was the reaction time, the smaller was the share of these components in the SM fractions. In the fractions obtained at 643 K, an increase in reaction time was accompanied by a growth in the share of, among others, phenol, catechol, 4-methylphenol and 4-methylresorcine. Dominant elements of the SM fractions after alkali lignin hydrothermolysis performed at 653 K were phenol, guaiacol, 4-methylguaiacol, catechol, 3methylcatechol, 4-methylcatechol and 4-methylresorcine. Except for phenol and catechol, the share of those elements in the SM fractions fell continuously as reaction time increased. At 663 K, as the reaction time increased, the percentage of guaiacol and catechol in the SM fractions markedly decreased, while the phenol, o-cresol and p-cresol content increased. Fig. 2 illustrates the effect of reaction temperature and time on yield of the selected SM products. Based on the experimental results, the cleavage of the C O bonds in alkali lignin took place particularly in subcritical water and also in supercritical conditions (which was promoted by the high ion product of water). Various phenols and methoxy phenols (such as guaiacol- a lignin monomer) were formed by hydrolysis as basic products. The largest guaiacol yield (11.23%) was obtained at the lowest temperature set after a holding time of 0 min, and fell gradually as both the reaction temperature and time increased. The cleavage of C C bonds in alkali lignin took place in subcritical water but at a longer reaction time, and occurred predominantly in supercritical water [11]. The main product of demethoxylation of guaiacol was catechol, formed after the homolysis of the OCH3 bond and hydrogen abstraction from water [5] and/or as a result of dealkylation of alkali lignin [2]. In subcritical conditions (at 553 K), the catechol yield increased continually with reaction time, with its highest value of 3.91% after 240 min. In subcritical water (at 643 K), as well as under supercritical conditions (at 653 K) the catechol yield initially grew in the range of reaction times from 0 to 60 min, reaching 10.11% and 9.54%, respectively. Then with an increase of reaction time (over 60 min) the yield of catechol decreased. At 663 K the highest yield of catechol was reached after a holding time of 0 min, and then decreased with the reaction time. The results in this study were similar to the work of Wahyudiono et al. [5]. In this previous report, it was found that both the conversion of guaiacol and the catechol yield were low in subcritical conditions (2.6–5.7 wt% and 1.7–3.4 wt%, respectively at 523 K and 8 MPa), and grew higher in supercritical water (58–98 wt% and 40–44 wt%, respectively at 673 K and 40 MPa). Both in sub- and supercritical water the yield of catechol rapidly increased with reaction time (till 30 min) and then decreased [2]. In the applied conditions, stable reaction products such as phenol, m,p-cresol and o-cresol formed as products of occurring secondary reactions. They were minor reaction products in subcritical water but their yield increased with reaction temperature
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Table 1 Composition of the SM fractions obtained from alkali lignin hydrothermolysis. RTa
9.47 9.74 10.19 10.70 10.77 11.23 11.37 11.89 11.95 12.41 12.55 13.05 13.15 13.30 13.55 13.77 13.89 14.26 14.76 15.19 15.21 15.71 15.85 15.91 16.08 16.18 16.46 16.79 16.85 16.89 17.15 17.31 18.19 a
Compound
Phenol Guaiacol o-Cresol p-Cresol 3-Methylguaiacol 4-Methylguaiacol Dimethylphenol Dimethylphenol (isomer) 4-Ethylphenol 4-Ethyl-2-methoxyphenol 1-Ethyl-4-methoxybenzene 3-Ethyl-5-methylphenol 3-Methoxycatechol 3-Methoxyphenol 2-Methoxy-methyl-ethylphenol Catechol 2,6-Dimethoxyphenol 3-Methylcatechol 4-Methylcatechol Vanillin 4-Ethoxyphenol 2-Methylresorcine 4-Methylresorcine 2-Methoxy-4-propylphenol 4,5-Dimethylresorcine 1-(4-Hydroxy-3-methoxyphenyl)ethanone 4-Hydroxybenzaldehyde 1-(4-Hydroxy-3-methoxyphenyl)propanone 4-Propylcatechol 3-Methoxy-4-hydroxybenzyl methyl ether 4-tert-Butylbenzene-1,2-diol 4-(3-Hydroxy-1-propenyl)-2-methoxyphenol Vanillic acid 8.7
Tr (K), tr (min) Share in the SM fraction (area %) 553, 0
553, 240
643, 0
643, 240
653, 0
653, 240
663, 0
663, 240
0.7 41.9 – – – 2.1 – – – 3.2 – – 0.1 – – 1.3 – – – 17.9 – – – 2.1 – 8.8 1.1 10.6 – – 0.7 1.0 15.9
2.1 28.3 – 0.5 – 6.2 – – 0.3 4.6 – – 0.7 – 0.9 10.3 0.8 – 1.6 7.0 – 0.7 0.9 0.7 – 5.3 0.7 9.6 0.7 1.2 0.7 0.5 9.8
2.0 27.3 – 1.0 0.6 10.2 – – 0.5 6.3 – – 0.5 0.5 0.8 8.5 0.7 0.7 1.7 10.7 – – 1.0 0.9 – 5.7 0.9 7.1 0.5 0.9 0.6 1.0 –
5.8 9.5 2.9 5.5 0.6 3.7 1.9 0.8 1.9 2.3 0.7 – – – 0.8 31.3 – 6.6 14.4 0.8 0.8 – 6.4 – 0.6 – – – 2.6 – – – –
3.9 24.4 1.2 3.0 1.1 5.7 1.0 0.3 1.1 3.6 – – – – – 15.7 – 6.7 18.0 0.8 1.8 – 7.5 – 1.1 0.5 – – 2.8 – – – –
8.2 8.3 4.0 4.3 3.7 2.6 2.6 1.3 3.9 1.4 0.5 0.7 0.6 – – 32.8 – 5.1 11.0 – – – 5.0 – – – – 1.9 2.0 – – – –
9.0 21.1 4.1 5.3 2.2 4.3 2.1 0.1 2.5 2.0 0.2 0.8 1.3 – – 14.2 – 6.2 13.4 0.3 1.3 – 4.2 – 0.2 0.4 – 0.3 3.8 0.3 0.1 – –
11.9 13.8 6.2 9.1 6.2 8.0 4.2 0.8 5.3 0.1 1.8 1.4 2.0 – – 12.0 – 3.6 8.7 – 0.1 – 2.1 – – 0.1 – 1.5 0.3 – 0.6 – –
Retention time (min).
and time. Phenol and cresol isomers were formed from guaiacol radical species after they underwent scission [5] or from catechol, as a result of its hydrolysis [2,6]. Phenol could be produced from catechol via cleavage of one of its C OH bonds, and formation of a phenoxy radical and H atom. The H atom reacted with catechol to yield phenol. Phenol could also be formed from cleavage of the catechol C O bond and reaction of the cyclohexyl cation with water [6]. Cresol isomers were formed from guaiacol radicals after their scission and subsequent reaction with the H atom [5]. In our study, the yields of phenol and cresol isomers grew gradually across the whole range of reaction temperatures and times. The largest yields of phenol (4.21%), m,p-cresols (3.89%) and o-cresol (3.11%) were obtained at 663 K and 240 min. The results in this study were similar to the work of Wahyudiono et al. [2]. In this previous report, it was found that both in sub- and supercritical water, the yield of phenol increased with the reaction time, achieved 11.75 wt% at 673 K and 240 min. The yield of cresol isomers increased initially with the reaction time, but was almost constant after 90 min. The amounts of m,p-cresol and ocresol approached 8.76 wt% and 3.65 wt%, respectively at 623 K and 6.98 wt% and 3.95 wt%, respectively at 673 K. In the adopted alkali lignin hydrothermal decomposition conditions, the yields of phenol and cresol isomers did not directly correspond to the changes in the guaiacol or catechol content. Although the yields of phenol and cresol isomers increased with the reaction time, the total yield of identified alkali lignin decomposition products decreased with increasing reaction temperature and time. Yields of monomeric phenols were low, probably due to their gasification and/or repolymerization.
3.3. Insoluble product fractions In experiments performed in subcritical water, the MN fractions yield was high (48.9% and 25.3% after 0 min at 553 K and 643 K, respectively), but fell as both the conversion temperature and time increased (31.1% and 13.1% after 240 min). In supercritical water the yield of MN fractions was higher than under subcritical conditions at 643 K (Fig. 3). Probably, the MN fractions obtained under subcritical conditions were composed of undissolved alkali lignin. In supercritical water the dielectric constant of water decreases with increasing temperature, promoting the dissolution of alkali lignin [4]. Under supercritical conditions, the solubility of alkali lignin in water was greater and soluble alkali lignin fragments reacted with the lower-molecular-weight soluble compounds to form polymers and cross-linked phenolic biochar. The repolymerization via radical coupling and carbonization via recondensation reactions were promoted by increasing temperature [2]. The composition of the MN fractions obtained at 553 K after duration 0 min and 30 min corresponded to the typical composition of alkali lignin. The further extension of the reaction time and the rise in temperature resulted in significant changes in the elementary composition of the solid residue. The composition of the MN fractions obtained at 553 K and 663 K and reaction time 240 min, represented by the respective formulas [C10 H7.26 O3.19 N0.07 S0.07 ] and [C10 H1.86 O0.56 N0.05 S0.07 ], significantly differed from the composition of the alkali lignin subjected to hydrothermal decomposition. Fig. 1(b) and (c) shows FTIR selected two model spectra of the MN fractions obtained from the hydrothermal decomposition of
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12 guaiacol
553K
553K
phenol m,p-cresol
8
643K
653K
663K
45
catechol
10
o-cresol
6 4 2
g/100g of dry alkali lignin
g/100g of dry alkali lignin
413
40 35 30 25 20 15
0 0
30
60
90
120
150
180
210
240
10 0
30
Time (min) g/100g of dry alkali lignin
12
643K
120
150
180
210
240
Fig. 3. The effect of alkali lignin hydrothermolysis temperature and time on the yield of MN fractions.
guaiacol catechol
10
phenol m,p-cresol
8
o-cresol
6 4
0 0
30
60
90
120
150
180
210
12
4. Conclusions guaiacol
653K
catechol
10
phenol m,p-cresol
8
o-cresol
6 4 2 0 0
30
60
90
120
150
could only be observed in the 1200–650 cm−1 region, in which the disappearance of vibrations typical of ether groups and aromatic rings were observed. The spectrum 1(c) of the MN fraction obtained at 663 K with reaction time 240 min differs from the ones for the MN fractions obtained at a lower temperature. The band typical for OH group stretching vibrations is absent at 3600–3000 cm−1 . This is the result of the loss of phenolic and alcoholic groups [2], and it indicates the progressive degradation of the alkali lignin.
240
Time (min)
g/100g of dry alkali lignin
90
Time (min)
2
180
210
240
Time (min) 12
g/100g of dry alkali lignin
60
guaiacol
663K
catechol
10
phenol
The hydrothermal decomposition of alkali lignin was performed in sub- and supercritical water. Alkali lignin of a high molecular weight was successfully degraded into low-molecular-weight components. The yield (wt%) of guaiacol, catechol, phenol and cresol isomers reached the highest values of approximately 11.23% (553 K, 0 min), 11.11% (653 K, 0 min), 4.21% (663 K, 240 min) and 7.00% (663 K, 240 min) With increase in reaction temperature and time, the composition of the solid post-reaction residue differed significantly from the composition of the alkali lignin subjected to hydrothermal decomposition. This suggests that the solid residue consisted mainly of phenolic biochar. Hydrothermal decomposition of alkali lignin could become an environmentally friendly technology to produce antioxidant phenolic compounds. Hydrothermal decomposition of relatively high-molecular-weight alkali lignin is an example of an environmentally benign and efficient technique of lignin biomass treatment (e.g. from the pulp and paper industry).
m,p-cresol
8
o-cresol
Acknowledgment
6
The authors gratefully acknowledge the financial support for this work provided by the Ministry of Science and Higher Education of Poland in 2008–2010 in the form of research Project No. N N523 494134.
4 2 0 0
30
60
90
120
150
180
210
240
Time (min) Fig. 2. The effect of alkali lignin hydrothermolysis temperature and time on the yield of selected components in the SM fractions.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cej.2012.01.092. References
alkali lignin at 553 K and 663 K after reaction time of 240 min. The other FTIR spectra of MN fractions are given in Supplementary material (Fig. 2S). The FTIR spectrum (1b) and other spectra of MN products resembled one another, which indicated that their structures and functional group types were similar. Small differences
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