- Email: [email protected]
Microwave-assisted hydrothermal carbonization of lignocellulosic materials
Materials Letters 63 (2009) 2707–2709
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Microwave-assisted hydrothermal carbonization of lignocellulosic materials M. Guiotoku a,⁎, C.R. Rambo b, F.A. Hansel a, W.L.E. Magalhães a, D. Hotza b a b
EMBRAPA—National Center for Forest Research, Colombo, PR, Brazil Group of Ceramic and Glass Materials (CERMAT), Department of chemical Engineering (EQA), Federal University of Santa Catarina (UFSC), Florianópolis, SC, Brazil
a r t i c l e
i n f o
Article history: Received 29 July 2009 Accepted 18 September 2009 Available online 27 September 2009 Keywords: Characterization methods Surfaces Microstructure Microwave Hydrothermal carbonization Cellulose
a b s t r a c t A new microwave-assisted hydrothermal carbonization (MAHC) method is reported in this work. The process uses microwave heating at 200 °C in acidic aqueous media to carbonize pine sawdust (Pinus sp.) and α-cellulose (Solucell®) at three different reaction times. Elemental analysis showed that the lignocellulosic samples subjected to MAHC yielded carbon-enriched material 50% higher than raw materials. Increase in aromaticity was confirmed by a van Krevelen diagram. SEM micrographs detected no morphological changes in pine sawdust. In contrast, SEM micrographs of carbonized α-cellulose revealed spherical-shaped particles with diameters ranging from 1 to 2 μm. These results showed that microwave-assisted hydrothermal carbonization is an innovative approach to obtain carbonized lignocellulosic materials. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Several methods to convert biomass into energy have been studied in the past decades, all of them based on thermal, biochemical and physical processes. Carbonization is one of the possible thermo chemical conversions of wood into energy, where a solid residue known as charcoal is produced through a slow process of partial thermal decomposition of wood in the presence or absence of oxygen [1]. Recently, a specific process of carbonization to produce aqueous carbon suspensions from biomass, named hydrothermal carbonization, was developed by Antonietti and his coworkers [2]. Basically, this method consists of heating biomass in the presence of a catalyst in a closed container under pressure (c.a. 100 bar), at a temperature range of 180 to 200 °C for 12 h. Hydrothermal reactions of the biomass cause the decomposition of carbohydrate structures through hydrolysis, resulting in free sugars and other byproducts (e.g. aldehydes and furfural) [3,4]. Microwave irradiation is frequently used in organic synthesis and sample digestion and has been increasingly applied to replace conventional heating methods in the processing of materials such as waste reduction [5], synthesis [6] and carbohydrate hydrolysis [7]. The advantage of microwaves in materials processing is mainly attributed to a selective, fast, and homogenous heat that significantly reduces processing time and costs [8]. Furthermore, the microwaves provide suitable conditions to obtain new products [9]. Despite the
⁎ Corresponding author. Tel.: +55 41 3675 5748; fax: +55 41 3675 5737. E-mail address: [email protected] (M. Guiotoku). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.09.049
widespread use of microwave irradiation in materials processing, its use to promote hydrothermal carbonization of lignocellulosic materials had not been explored. This work reports a new microwaveassisted hydrothermal carbonization (MAHC) of lignocellulosic materials.
2. Materials and methods Pine sawdust (PS) and α-cellulose (α-cel) (Solucell®, Bahia PulpBrazil) samples were used as raw materials. Aqueous suspensions of the lignocellulosic materials were prepared using citric acid (Vetec, Brazil) as a catalyst reagent. A 1.5 mol.L− 1 catalyst stock solution was prepared in Milli-Q water. About 500 mg of samples (a total of 10 replicates) were poured in a reaction container filled with 10 mL of stock solution. The suspensions were placed in a 100 mL PTFE sealed reactor for microwave processing. The materials were hydrothermally carbonized using a microwave labstation (Ethos Plus, Millestone, USA) with a magnetron frequency of 2.45 GHz, 1000 W at maximum power and 10 W pulse controlled power fractions. The system was heated from 20 °C to 85 °C at 22 °C min− 1, then from 85 °C to 145 °C at 7 °C min− 1, and from 145 °C to 200 °C at 14 °C min− 1, finally an isotherm was held at 200 °C for 60, 120 and 240 min. The temperature during microwave irradiation was controlled by a thermocouple installed in a reference container. After the carbonization process, the reactor was cooled at room temperature and the carbonized materials were filtered with a cellulose ester filter (0.45 μm, Millipore, USA) using a mechanical vacuum pump and subsequently washed with Milli-Q water until reaching a neutral pH. The solid products were dried (105 °C, 12 h). The yield of carbonized materials (R) was gravimetrically measured and the percentage was
2708
M. Guiotoku et al. / Materials Letters 63 (2009) 2707–2709
expressed according to: R(%) = (mcm/mrm) × 100, where mcm is the mass of carbonized materials and mrm is the mass of raw materials. The carbonized products were analyzed using an elemental CHNanalysis (Perkin Elmer, USA). Elemental analysis data from α-cellulose charcoal (α-cel charcoal) obtained by α-cellulose carbonization on a tubular furnace (900 °C, 3 h, nitrogen atmosphere) was used as reference material to complete carbonization. The morphology and microstructure of the obtained materials were evaluated using a scanning electron microscope (Philips XL-30, The Netherlands). 3. Results and discussion The influence of processing time on the carbonization yield for PS and α-cel is shown in Fig. 1. The carbonization yields of PS and α-cel presented opposite behaviors. While the first increased with time, the latter decreased. Comparatively, the amount of volatiles compounds may be higher in the PS than α-cel reaction due the presence of lignin and hemicelluloses. Therefore, the increase in carbonized PS yield may be attributed to side reactions of volatile compounds released from degradation of those extra tissues with the surface of carbonized solid material [10]. On the other hand, side reactions must be less pronounced in the α-cel because of the characteristic volatile compounds yielded in the process. This compounds could be more water soluble or degraded with time increasing, thus their incorporation in the α-cel carbonized material must have been unfavourable. Elemental analysis of the materials is shown in Table 1. No significant differences were detected in the amount of C, H and N between samples with different carbonization times. Compared with their respective raw materials, the samples subjected to the carbonization process had their carbon content increased by 40% and 57% for PS and α-cel, respectively. In contrast, the amount of O and H decreased in all carbonized materials, suggesting an aromatization process. These findings were similar to the hydrothermal carbonization of microcrystalline cellulose showed by Inoue [11], in which an increase of carbon content was verified at temperature above 300 °C. In order to qualitatively evaluate the carbonization process, H/C and O/C were plotted using the van Krevelen diagram [12], which provides information about the changes in chemical structure after carbonization. Fig. 2 shows the atomic ratios of H/C and O/C for carbonized PS, α-cel and their respective raw materials, and α-cel charcoal. In the diagram, H/C and O/C ratios of carbonized materials decrease when compared to their natural samples, suggesting that changes in the materials were taking place. The loss of H and O
Fig. 1. Influence of time in the percentage of carbonized yield of pine sawdust (—●—) and α-cellulose (—○—) samples at 200 °C and 1.5 mol L− 1 of citric acid.
Table 1 Elemental analysis for pine sawdust and α-cellulose at different times of MAHC and αcellulose charcoal. Time (min)
C (wt.%)
H (wt.%)
N (wt.%)
Pine sawdust 0 60 120 240
45.45 ± 0.06 60.01 ± 0.15 64.74 ± 0.15 63.54 ± 0.12
6.22 ± 0.09 5.51 ± 0.00 5.29 ± 0.18 5.19 ± 0.10
0.02 ± 0.01 0.02 ± 0.00 0.04 ± 0.01 0.71 ± 0.01
α-cellulose 0 60 120 240 α-cel charcoal
40.5 ± 0.15 63.11 ± 0.08 63.63 ± 0.00 63.75 ± 0.09 91.08 ± 0.12
6.43 ± 0.02 4.74 ± 0.007 4.64 ± 0.03 4.50 ± 0.04 1.33 ± 0.05
0.09 ± 0.06 0.26 ± 0.05 0.06 ± 0.02 0.46 ± 0.01 0
a
Oa (wt.%)
Atomic ratio H/C
O/C
48.31 ± 0.16 34.46 ± 0.16 29.93 ± 0.35 30.56 ± 0.20
1.16 1.10 0.98 0.98
0.79 0.43 0.35 0.36
52.98 ± 0.11 31.89 ± 0.13 31.67 ± 0.06 31.29 ± 0.10 7.59 ± 0.08
1.90 0.90 0.87 0.85 0.17
0.97 0.38 0.37 0.37 0.06
The oxygen content was determined by difference [100% − (C% + H% + N %)].
occurred by dehydrogenation, deoxygenation and dehydration processes. However, the products are not completely carbonized, since they are in the center of the diagram, amongst the raw materials and α-cel charcoal. Such partially carbonized products are commonly obtained by hydrothermal carbonization and their chemical structure can be described as an amorphous aromatic carbon OH and COOH substituted [13]. Interestingly, the H/C ratio change is less pronounced in PS material, probably due to the inability of MAHC to modify other natural biopolymers (e.g. lignin) present in PS (see Fig. 2). Fig. 3 shows SEM micrographs of PS and α-cel before and after carbonization. Carbonized PS (Fig. 3A) apparently maintained its micro-morphological features after hydrothermal carbonization (Fig. 3B). For example, carbonized PS particles exhibited round corners and original tracheid cell parts, present in the raw material. In contrast, the carbonized α-cel is characterized by a noticeable morphological change after the MAHC process (Fig. 3C and D). Initially, α-cellulose was composed of a fibrous morphology (Fig. 3C), but after carbonization the agglomerated fibers changed to spherical particles, with a size of about 1.2–2.0 μm in diameter (Fig. 3D). A mechanism that generates these spheres was proposed by Yao et al. [14] and is based on hydrolysis followed by dehydration and polymerization of polysaccharides. The spherical nuclei are formed in order to minimize the energy interface. Similar results were obtained by Titirici [15] using the conventional hydrothermal method of carbohydrates (e.g. glucose and starch). Apparently, the combination of hydrolysis with subsequent carbonization of cellulose materials to produce spherical particles is not possible by conventional hydrothermal carbonization.
Fig. 2. The van Krevelen diagram for pine sawdust (●), α-cellulose (○) and α-cellulose charcoal (⊗).
M. Guiotoku et al. / Materials Letters 63 (2009) 2707–2709
2709
Fig. 3. SEM micrographs of pine sawdust raw material (A) and hydrothermally carbonized in microwave oven material (B); α-cellulose raw material (C) and hydrothermally carbonized in microwave oven sample (D).
4. Conclusions A new method to produce hydrothermal carbonized materials using microwaves was reported. Its key feature was the occurrence of hydrolysis followed by the carbonization of lignocellulosic materials, in shorter time and milder conditions to obtain carbon-enriched materials. This is a positive outcome in terms of the green chemistry aspects of this process and is expected to be useful in development of new materials from lignocellulosics. Acknowledgements Thanks to Ludmila de Araújo Ramos (Bahia Pulp S/A) for providing α-cellulose samples. Marcela Guiotoku thanks CNPq/Brazil for the scholarship.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Bridgwater AV. Chem Eng J 2003;91:87–102. Titirici MM, Thomas A, Antonietti M. New J Chem 2007;31(6):787–9. Mochidzuki K, Sakoda A, Suzuki M. Adv Environ Res 2003;7:421–8. Fujino T, Calderón-Moreno JM, Swamy S, Hirose T, Yoshimura M. Solid State Ionics 2002;151:197–203. Menéndez JA, Domínguez A, Inguanzo M, Pis JJ. J Anal Appl Pyrol 2004;71:657–67. Corsaro A, Chiacchio U, Pistarà V, Romeo G. Curr Org Chem 2004;8:511–38. Orozco A, Ahmad M, Walker G. Trans IChemE Part B Process Safety and Environmental Protection. 2007;85(B5): 446–449. Nüchter M, Ondruschka B, Bonrath W. Gum A. Green Chem 2004;6(2):128–41. Yang KS, Yoon YJ, Lee MS, Lee WJ, Kim JH. Carbon 2002;40:897–903. Antal Jr MJ, Grnli M. Ind Eng Chem Res 2003;42:1619–40. Inoue S, Uno S. Minowa T. J Chem Eng Jpn 2008;41(3):210–5. van Krevelen DW. Fuel 1950;29:269–84. Titirici MM, Thomas A, Yu SH, Müller J, Antonietti M. Chem Mater 2007;19:4205–12. Yao C, Shin Y, Wang LQ, Windisch Jr CF, Samuels WD, Arey BW, et al. J Phys Chem C 2007;111:15141–5. Titirici MM, Antonietti M, Baccile N. Green Chem 2008;10:1204–12.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Microwave-assisted hydrothermal carbonization of lignocellulosic materials M. Guiotoku a,⁎, C.R. Rambo b, F.A. Hansel a, W.L.E. Magalhães a, D. Hotza b a b
EMBRAPA—National Center for Forest Research, Colombo, PR, Brazil Group of Ceramic and Glass Materials (CERMAT), Department of chemical Engineering (EQA), Federal University of Santa Catarina (UFSC), Florianópolis, SC, Brazil
a r t i c l e
i n f o
Article history: Received 29 July 2009 Accepted 18 September 2009 Available online 27 September 2009 Keywords: Characterization methods Surfaces Microstructure Microwave Hydrothermal carbonization Cellulose
a b s t r a c t A new microwave-assisted hydrothermal carbonization (MAHC) method is reported in this work. The process uses microwave heating at 200 °C in acidic aqueous media to carbonize pine sawdust (Pinus sp.) and α-cellulose (Solucell®) at three different reaction times. Elemental analysis showed that the lignocellulosic samples subjected to MAHC yielded carbon-enriched material 50% higher than raw materials. Increase in aromaticity was confirmed by a van Krevelen diagram. SEM micrographs detected no morphological changes in pine sawdust. In contrast, SEM micrographs of carbonized α-cellulose revealed spherical-shaped particles with diameters ranging from 1 to 2 μm. These results showed that microwave-assisted hydrothermal carbonization is an innovative approach to obtain carbonized lignocellulosic materials. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Several methods to convert biomass into energy have been studied in the past decades, all of them based on thermal, biochemical and physical processes. Carbonization is one of the possible thermo chemical conversions of wood into energy, where a solid residue known as charcoal is produced through a slow process of partial thermal decomposition of wood in the presence or absence of oxygen [1]. Recently, a specific process of carbonization to produce aqueous carbon suspensions from biomass, named hydrothermal carbonization, was developed by Antonietti and his coworkers [2]. Basically, this method consists of heating biomass in the presence of a catalyst in a closed container under pressure (c.a. 100 bar), at a temperature range of 180 to 200 °C for 12 h. Hydrothermal reactions of the biomass cause the decomposition of carbohydrate structures through hydrolysis, resulting in free sugars and other byproducts (e.g. aldehydes and furfural) [3,4]. Microwave irradiation is frequently used in organic synthesis and sample digestion and has been increasingly applied to replace conventional heating methods in the processing of materials such as waste reduction [5], synthesis [6] and carbohydrate hydrolysis [7]. The advantage of microwaves in materials processing is mainly attributed to a selective, fast, and homogenous heat that significantly reduces processing time and costs [8]. Furthermore, the microwaves provide suitable conditions to obtain new products [9]. Despite the
⁎ Corresponding author. Tel.: +55 41 3675 5748; fax: +55 41 3675 5737. E-mail address: [email protected] (M. Guiotoku). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.09.049
widespread use of microwave irradiation in materials processing, its use to promote hydrothermal carbonization of lignocellulosic materials had not been explored. This work reports a new microwaveassisted hydrothermal carbonization (MAHC) of lignocellulosic materials.
2. Materials and methods Pine sawdust (PS) and α-cellulose (α-cel) (Solucell®, Bahia PulpBrazil) samples were used as raw materials. Aqueous suspensions of the lignocellulosic materials were prepared using citric acid (Vetec, Brazil) as a catalyst reagent. A 1.5 mol.L− 1 catalyst stock solution was prepared in Milli-Q water. About 500 mg of samples (a total of 10 replicates) were poured in a reaction container filled with 10 mL of stock solution. The suspensions were placed in a 100 mL PTFE sealed reactor for microwave processing. The materials were hydrothermally carbonized using a microwave labstation (Ethos Plus, Millestone, USA) with a magnetron frequency of 2.45 GHz, 1000 W at maximum power and 10 W pulse controlled power fractions. The system was heated from 20 °C to 85 °C at 22 °C min− 1, then from 85 °C to 145 °C at 7 °C min− 1, and from 145 °C to 200 °C at 14 °C min− 1, finally an isotherm was held at 200 °C for 60, 120 and 240 min. The temperature during microwave irradiation was controlled by a thermocouple installed in a reference container. After the carbonization process, the reactor was cooled at room temperature and the carbonized materials were filtered with a cellulose ester filter (0.45 μm, Millipore, USA) using a mechanical vacuum pump and subsequently washed with Milli-Q water until reaching a neutral pH. The solid products were dried (105 °C, 12 h). The yield of carbonized materials (R) was gravimetrically measured and the percentage was
2708
M. Guiotoku et al. / Materials Letters 63 (2009) 2707–2709
expressed according to: R(%) = (mcm/mrm) × 100, where mcm is the mass of carbonized materials and mrm is the mass of raw materials. The carbonized products were analyzed using an elemental CHNanalysis (Perkin Elmer, USA). Elemental analysis data from α-cellulose charcoal (α-cel charcoal) obtained by α-cellulose carbonization on a tubular furnace (900 °C, 3 h, nitrogen atmosphere) was used as reference material to complete carbonization. The morphology and microstructure of the obtained materials were evaluated using a scanning electron microscope (Philips XL-30, The Netherlands). 3. Results and discussion The influence of processing time on the carbonization yield for PS and α-cel is shown in Fig. 1. The carbonization yields of PS and α-cel presented opposite behaviors. While the first increased with time, the latter decreased. Comparatively, the amount of volatiles compounds may be higher in the PS than α-cel reaction due the presence of lignin and hemicelluloses. Therefore, the increase in carbonized PS yield may be attributed to side reactions of volatile compounds released from degradation of those extra tissues with the surface of carbonized solid material [10]. On the other hand, side reactions must be less pronounced in the α-cel because of the characteristic volatile compounds yielded in the process. This compounds could be more water soluble or degraded with time increasing, thus their incorporation in the α-cel carbonized material must have been unfavourable. Elemental analysis of the materials is shown in Table 1. No significant differences were detected in the amount of C, H and N between samples with different carbonization times. Compared with their respective raw materials, the samples subjected to the carbonization process had their carbon content increased by 40% and 57% for PS and α-cel, respectively. In contrast, the amount of O and H decreased in all carbonized materials, suggesting an aromatization process. These findings were similar to the hydrothermal carbonization of microcrystalline cellulose showed by Inoue [11], in which an increase of carbon content was verified at temperature above 300 °C. In order to qualitatively evaluate the carbonization process, H/C and O/C were plotted using the van Krevelen diagram [12], which provides information about the changes in chemical structure after carbonization. Fig. 2 shows the atomic ratios of H/C and O/C for carbonized PS, α-cel and their respective raw materials, and α-cel charcoal. In the diagram, H/C and O/C ratios of carbonized materials decrease when compared to their natural samples, suggesting that changes in the materials were taking place. The loss of H and O
Fig. 1. Influence of time in the percentage of carbonized yield of pine sawdust (—●—) and α-cellulose (—○—) samples at 200 °C and 1.5 mol L− 1 of citric acid.
Table 1 Elemental analysis for pine sawdust and α-cellulose at different times of MAHC and αcellulose charcoal. Time (min)
C (wt.%)
H (wt.%)
N (wt.%)
Pine sawdust 0 60 120 240
45.45 ± 0.06 60.01 ± 0.15 64.74 ± 0.15 63.54 ± 0.12
6.22 ± 0.09 5.51 ± 0.00 5.29 ± 0.18 5.19 ± 0.10
0.02 ± 0.01 0.02 ± 0.00 0.04 ± 0.01 0.71 ± 0.01
α-cellulose 0 60 120 240 α-cel charcoal
40.5 ± 0.15 63.11 ± 0.08 63.63 ± 0.00 63.75 ± 0.09 91.08 ± 0.12
6.43 ± 0.02 4.74 ± 0.007 4.64 ± 0.03 4.50 ± 0.04 1.33 ± 0.05
0.09 ± 0.06 0.26 ± 0.05 0.06 ± 0.02 0.46 ± 0.01 0
a
Oa (wt.%)
Atomic ratio H/C
O/C
48.31 ± 0.16 34.46 ± 0.16 29.93 ± 0.35 30.56 ± 0.20
1.16 1.10 0.98 0.98
0.79 0.43 0.35 0.36
52.98 ± 0.11 31.89 ± 0.13 31.67 ± 0.06 31.29 ± 0.10 7.59 ± 0.08
1.90 0.90 0.87 0.85 0.17
0.97 0.38 0.37 0.37 0.06
The oxygen content was determined by difference [100% − (C% + H% + N %)].
occurred by dehydrogenation, deoxygenation and dehydration processes. However, the products are not completely carbonized, since they are in the center of the diagram, amongst the raw materials and α-cel charcoal. Such partially carbonized products are commonly obtained by hydrothermal carbonization and their chemical structure can be described as an amorphous aromatic carbon OH and COOH substituted [13]. Interestingly, the H/C ratio change is less pronounced in PS material, probably due to the inability of MAHC to modify other natural biopolymers (e.g. lignin) present in PS (see Fig. 2). Fig. 3 shows SEM micrographs of PS and α-cel before and after carbonization. Carbonized PS (Fig. 3A) apparently maintained its micro-morphological features after hydrothermal carbonization (Fig. 3B). For example, carbonized PS particles exhibited round corners and original tracheid cell parts, present in the raw material. In contrast, the carbonized α-cel is characterized by a noticeable morphological change after the MAHC process (Fig. 3C and D). Initially, α-cellulose was composed of a fibrous morphology (Fig. 3C), but after carbonization the agglomerated fibers changed to spherical particles, with a size of about 1.2–2.0 μm in diameter (Fig. 3D). A mechanism that generates these spheres was proposed by Yao et al. [14] and is based on hydrolysis followed by dehydration and polymerization of polysaccharides. The spherical nuclei are formed in order to minimize the energy interface. Similar results were obtained by Titirici [15] using the conventional hydrothermal method of carbohydrates (e.g. glucose and starch). Apparently, the combination of hydrolysis with subsequent carbonization of cellulose materials to produce spherical particles is not possible by conventional hydrothermal carbonization.
Fig. 2. The van Krevelen diagram for pine sawdust (●), α-cellulose (○) and α-cellulose charcoal (⊗).
M. Guiotoku et al. / Materials Letters 63 (2009) 2707–2709
2709
Fig. 3. SEM micrographs of pine sawdust raw material (A) and hydrothermally carbonized in microwave oven material (B); α-cellulose raw material (C) and hydrothermally carbonized in microwave oven sample (D).
4. Conclusions A new method to produce hydrothermal carbonized materials using microwaves was reported. Its key feature was the occurrence of hydrolysis followed by the carbonization of lignocellulosic materials, in shorter time and milder conditions to obtain carbon-enriched materials. This is a positive outcome in terms of the green chemistry aspects of this process and is expected to be useful in development of new materials from lignocellulosics. Acknowledgements Thanks to Ludmila de Araújo Ramos (Bahia Pulp S/A) for providing α-cellulose samples. Marcela Guiotoku thanks CNPq/Brazil for the scholarship.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Bridgwater AV. Chem Eng J 2003;91:87–102. Titirici MM, Thomas A, Antonietti M. New J Chem 2007;31(6):787–9. Mochidzuki K, Sakoda A, Suzuki M. Adv Environ Res 2003;7:421–8. Fujino T, Calderón-Moreno JM, Swamy S, Hirose T, Yoshimura M. Solid State Ionics 2002;151:197–203. Menéndez JA, Domínguez A, Inguanzo M, Pis JJ. J Anal Appl Pyrol 2004;71:657–67. Corsaro A, Chiacchio U, Pistarà V, Romeo G. Curr Org Chem 2004;8:511–38. Orozco A, Ahmad M, Walker G. Trans IChemE Part B Process Safety and Environmental Protection. 2007;85(B5): 446–449. Nüchter M, Ondruschka B, Bonrath W. Gum A. Green Chem 2004;6(2):128–41. Yang KS, Yoon YJ, Lee MS, Lee WJ, Kim JH. Carbon 2002;40:897–903. Antal Jr MJ, Grnli M. Ind Eng Chem Res 2003;42:1619–40. Inoue S, Uno S. Minowa T. J Chem Eng Jpn 2008;41(3):210–5. van Krevelen DW. Fuel 1950;29:269–84. Titirici MM, Thomas A, Yu SH, Müller J, Antonietti M. Chem Mater 2007;19:4205–12. Yao C, Shin Y, Wang LQ, Windisch Jr CF, Samuels WD, Arey BW, et al. J Phys Chem C 2007;111:15141–5. Titirici MM, Antonietti M, Baccile N. Green Chem 2008;10:1204–12.