ARTICLE pubs.acs.org/EF
Mechanisms and Product Specialties of the Alcoholysis Processes of Poplar Components Xianwu Zou, Tefu Qin,* Yong Wang, and Luohua Huang Key Laboratory of Wood Science and Technology of State Forestry Administration (SFA), Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing 100091, People’s Republic of China ABSTRACT: To make clear the alcoholysis mechanisms of poplar components, the effects of both single and lumped components on the liquefaction rates and product distributions have been studied with acidified 1-octanol as a solvent at 130 °C in an airtight stainless-steel autoclave. The results showed that single lignin was easy to liquefy, and its liquefaction rate reached 80.70 wt %. On the contrary, the liquefaction rates of single cellulose and hemicellulose were only 25.40 and 31.20 wt %, respectively. When cellulose, hemicellulose, and lignin are lumped in poplar, the liquefaction of cellulose can be promoted. As a result, the liquefaction rate of poplar reached 83.54 wt %. Although both cellulose and hemicellulose were difficult to liquefy, the content of heavy oil from cellulose was only 2.4 wt %; on the contrary, the content of heavy oil from hemicellulose reached 22.91 wt %. Meanwhile, the content of heavy oil from lignin was up to 38.30 wt %. When cellulose, hemicellulose, and lignin coexisted in poplar, the formation of heavy oil was depressed. As a result, the content of heavy oil was only 11.02 wt %. In addition, the analysis results of gas chromatographymass spectrometry (GCMS) on the light oils suggested that cellulose degraded into glucose fractions and further cracked into acidic compounds, such as formic acid and acetic acid. Then, these acidic compounds could react with acidified 1-octanol, which led to the formation of formic acid octyl ester and acetic acid octyl ester. Meanwhile, the rearrangement and condensation took place between the fragments of cellulose degradation, which resulted in the formation of aromatic compounds. Similarly, glucosides, esters, and aromatic compounds were still present in the light oil from hemicellulose alcoholysis. Furthermore, some segments from pentose degradation, such as n-octyl-R-D-riboside and furfural, could also be found in the light oil of hemicellulose. In contrast, aromatic and phenolic compounds were the chief products from lignin degradation. The composition of light oil from poplar was not the simple summation of those from cellulose, hemicellulose, and lignin. During poplar alcoholysis, a large number of reactions occurred, including cracking, esterification, rearrangement, condensation, and dehydration. Although light oil from poplar was a very complex mixture, there were only eight main compounds, including unreactive 1-octanol, formic acid octyl ester, octyl ether, levulinic acid butyl ester, acetic acid octyl ester, 4-hydroxy-4-methyl-2-pentanone, 5-methyl-furfural, and phenol. The total content of the eight compounds reached 98.08% (area %). The simple components of light oil were beneficial to the separation of valuable chemicals and the recycle of unreactive 1-octanol.
1. INTRODUCTION Diminishing petroleum reserves and growing concerns about global climate change make it imperative to develop new processes for the production of fuel and chemicals based on renewable biomass resources.15 Previous studies have shown that there are many efficient methods for biomass transformation, including hydrolysis and fermentation,6 fast pyrolysis,7,8 liquefaction,9,10 gasification,11 etc. Among these methods, biomass liquefaction with alcoholic solvents, especially with highboiling-point alcoholic solvents, has attracted considerable attention because of their mild reaction conditions.1215 Zhang et al.14 studied the liquefaction of bagasse in ethylene glycol catalyzed by sulfuric acid at 190 °C under atmospheric pressure and found that some useful chemicals and biofuels could be obtained by this process. Yamada et al.16 investigated the product in acidcatalyzed solvolysis of cellulose using polyethylene glycol at 150 °C at atmospheric pressure and found that degradation of cellulose leads to the formation of glucosides, which then decompose, resulting in a levulinic acid structure and producing a waterinsoluble fraction. The conversion rates of both glucosides and levulinates strongly depend upon the reaction conditions of the solvolysis. Chen et al.17 assessed the liquefaction of wheat straw r 2011 American Chemical Society
in the mixture of polyethylene glycol (PEG 400) and glycerin in the presence of acid at the temperature of 130160 °C and found that the final liquefaction products could be used as the polyol component to manufacture polyurethane. Furthermore, the polyurethane foam presented better compressive strength, thermal stability and biodegradation than that manufactured from diisocyanate and polyol alone. Although biomass liquefaction with high-boiling-point alcohols has shown obvious energy efficiency in many works, it is still unclear about the effects of biomass components on the alocholysis process. To produce superior quality products from various biomass materials, it is necessary to understand the alcoholysis mechanisms of biomass components. In this paper, the effects of poplar components, including single and lumped components, on the alcoholysis processes were examined by analyzing the liquefaction rates and product distributions. In addition, the characterizations of the light oils from the alcoholysis processes of poplar components were carried out by a gas chromatograph Received: May 15, 2011 Revised: July 18, 2011 Published: July 18, 2011 3786
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Table 1. Chemical and Elemental Compositions of the Poplar Sample (wt % on a Dry Basis)
a
chemical
content
elemental
content
composition
(wt %)
composition
(wt %)
lignin hemicellulose
21.38 31.37
C H
47.02 6.01
R-cellulose
45.33
Oa
46.76
ethanolbenzene solubles
4.45
N
0.21
cold-water solubles
3.36
ash
0.54
By difference.
equipped with a mass selective detector (GCMS) to probe the interaction mechanisms of degradation fragments from poplar alcoholysis and study the feasibility of enhancement of valuable chemicals.
Figure 1. Liquefaction rates of the single and lumped components of poplar.
2. EXPERIMENTAL SECTION 2.1. Materials. The applied biomass is poplar from the suburb of Beijing, China. The preparation and characterization of the poplar sample have been fully described previously.12 Their chemical and elemental compositions are listed in Table 1. The main components of the poplar sample include cellulose, hemicellulose, and lignin. Cellulose is an extra-pure microcrystal, and lignin is alkali lignin in brown powders. Hemicellulose cannot be purchased from commercial sources, whereas xylan has been widely used as a representative component of hemicellulose.18 Here, xylan is in white powders and used as hemicellulose. The average particle size of hemicellulose (xylan) is at ∼100 μm. However, those of cellulose and lignin are all at ∼50 μm. The three main components were obtained from Sigma-Aldrich (Beijing, China). 1-Octanol (analytical reagent grade) was acidified by premixing with 3 wt % sulfuric acid (95%). 1-Octanol and sulfuric acid used were obtained from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). 2.2. Procedure of Alcoholysis and Separation. The alcoholysis experiments of cellulose, hemicellulose, lignin, and poplar were carried out in a 2 L stainless-steel autoclave (Taiatsu Techno TAS-2). In a typical run, 100 g of sample was placed in the autoclave together with 2-fold acidified alcoholic solvents. The air in the autoclave was replaced by N2. The autoclave was heated and controlled at 130 °C for 60 min. With the temperature rising from room temperature to 130 °C, the pressure P in the autoclave increased from ambient pressure to 0.150.42 MPa [P(1-octanol + poplar) = 0.42 MPa; P(1-octanol + cellulose) = 0.15 MPa; P(1-octanol + hemicellulose) = 0.18 MPa; and P(1-octanol + lignin) = 0.38 MPa]. After the reaction, the autoclave was cooled to room temperature by air and the gaseous products were vented because the yield of gaseous products was almost negligible. The solid and liquid products were poured into a beaker. In multiple-stage extraction, liquefied products were separated into three fractions: residue (insoluble in acetone), heavy oil (soluble in acetone but insoluble in 1-hexane), and light oil (soluble in 1-hexane). The procedure of separation of liquefied products has been listed in a previous work.12 During the reproducibility of the experimental runs, a (3.0% standard deviation was observed for the mass of residue, heavy oil, and light oil. 2.3. Characterization of Light Oil. The light oil was analyzed by GCMS [Shimadzu QP2010 plus; column, DB-5; cross-linked methyl siloxane, 30 m 0.32 mm 0.25 μm; temperature program, 50 °C (hold for 10 min) f 250 °C (rate of 10 °C/min) and hold for 10 min].
Figure 2. Distribution of liquefaction products derived from the alcoholysis processes of the single and lumped components of poplar. Because of the good solubility of 1-hexane, all unreactive 1-octanol solvent remained in light oil.
3. RESULTS AND DISCUSSION 3.1. Effect of Poplar Components on the Liquefaction Rates. The liquefaction rate η is given by the ratio
η¼
mass of residue in autoclave 1 100% mass of initial samples
ð1Þ
Figure 1 shows the liquefaction rates of the single and lumped components of poplar. It can be seen that the liquefaction rates of cellulose and hemicellulose were only 25.40 and 31.20 wt %, respectively. However, the liquefaction rate of lignin was up to 80.70 wt %. This result indicates that the liquefaction of lignin is easier than that of cellulose and hemicellulose. Considering that the content of cellulose in the poplar sample was 45.33 wt %, if the liquefaction rate of cellulose in the poplar sample was still 25.40 wt %, the highest liquefaction rate of poplar would be less than 66.18 wt % because the residue rate from cellulose was 45.33% (1 25.40%) = 33.82%. 3787
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Table 2. Compositions of the Light Oils from the Alcoholysis Processes of the Single and Lumped Components of Poplar concentration (area %) retention time (min)
compound name
poplar
3.075 3.129
3,3-diethoxy-1-propyne acetic acid, butyl ester
3.342
11,12-dihydroxyseychellane
0.03
3.422
furfural
0.04
3.567
2-pentanone, 4-hydroxy-4-methyl-
2.09
3.842
propanoic acid, 3-(4-benzyloxyphenyl)-
0.09
cellulose
hemicellulose
lignin
0.01 0.04 1.02 1.4
0.17
3.977
p-dimethylbenzene
0.16
4.08
ethylbenzene
0.05
4.467 5.041
2(3H)-furanone, 5-methylbenzene, 1-ethyl-2-methyl-
0.07
5.308
1,3-dioxolane-4-methanol, 2,2-dimethyl-, (S)-
0.96
5.813
diglycerol
0.11
5.825
3,4-furandiol, tetrahydro-, cis-
5.889
1-propanol, 2-(2-methoxypropoxy)-
6.058
benzene, (1-methylethenyl)-
0.12
0.05
6.436
1-octaldehyde
0.14
0.33
5.974
phenol
0.34
6.017 6.342
phenol octadecanoic acid, 8-oxo-, methyl ester
0.17 0.01
6.429
1-hexene, 3,4-dimethyl-
7.57
1-octanol
7.938
phenol, 2-methoxy-
8.112
3-cyclopentyl-1-propanol
8.562
formic acid octyl ester
7.61
8.725
2-propenal
0.02
9.157 9.348
tetralin 4-methyl-2-hexanol
0.17
9.355
propanoic acid, 2-methyl-, anhydride
0.03
9.517
2-hexenal
0.03
9.64
3-decanol
0.03
9.676
oxalic acid allyl butyl ester
0.01
9.83
acetic acid octyl ester
5.17
1.71
1.27
1.56 0.15 0.25
0.78
0.04 68.82
0.06
0.06 74.6
84.74
90.87 0.99
0.04 6.4
5.84
0.69
0.72
1.76
0.54 0.54
0.1
0.36
1.05
0.07
0.22
10.063
3-hexanol, 3,5-dimethyl-
0.07
10.403 10.603
carbonic acid cyclicvinylene ester furan, tetrahydro-2,5-dimethyl-, cis-
0.02
10.82
propane, 2-ethoxy-
0.07
10.827
octane, 1-ethoxy-
10.908
nonane, 3,7-dimethyl-
0.10
10.942
1-butanol, 2-methyl-
0.02
11.194
2,3-hexanediol
0.02
12.035
butanoic acid 1,1-dimethylethyl ester
0.02
12.043 13.343
undecane, 4,7-dimethylbutanoic acid, 2-oxo-
0.05
13.615
phenol, 4,6-di(1,1-dimethylethyl)-2-methyl-
0.08
13.616
cyclopentanone ethylene ketal
14.539
butane, 2,2-dimethyl-
0.03
15.08
levulinic acid butyl ester
6.10
3.76
0.43
0.09
15.19
octyl ether
6.84
8.08
2.62
1.52
16.375
3-ethyl-7-hydroxyphthalide
0.01
16.568 16.672
6,8-dioxahexadecane furfural, 5-methyl-
0.23 0.94
16.896
4-methyl-2-hexanol
0.13
0.1
0.11
0.06 0.12
0.14
0.09
0.06
0.29
0.5 0.49
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Table 2. Continued concentration (area %) retention time (min)
compound name
poplar
17.553 17.820
n-octyl-R-D-riboside n-octyl-β-D-glucopyrano
0.02
18.008
sulfurous acid, isobutyl pentyl ester
0.01
18.68
2-buten-1-ol, propanoate
0.04
19.096
cyclohexane, 1-methyl-2-propyl-
0.22
20.508
octane, 3,6-dimethyl-
0.16
20.523
1-octanol, 2-methyl-
cellulose
hemicellulose
0.11
0.98 0.37
0.09
lignin
0.12
Figure 3. Total ion chromatograms of the light oils from the alcoholysis processes of the single and lumped components of poplar.
However, the real liquefaction rate of poplar was 83.54 wt %. This result suggests that the liquefaction of cellulose in poplar is easier than that of pure cellulose. This phenomenon can be due to the amorphous structure of cellulose in poplar, which is relatively easy to degrade into its monomer sugars compared to crystalline cellulose.19 In addition, the free phenoxyl radicals from the decomposition of lignin can also promote the liquefaction of cellulose in biomass.20,21 3.2. Effect of Poplar Components on the Distribution of Liquefaction Products. The distribution of liquefaction products of the single and lumped components of poplar are plotted in Figure 2. Obviously, the content ψ of heavy oil shows the following increasing order:
22.91 wt %. Meanwhile, the content of heavy oil from lignin was up to 38.30 wt %. This result indicates that the heavy oil from cellulose degradation is negligible; however, hemicellulose and lignin degrade and chiefly form heavy oils. According to the content of cellulose, hemicellulose, and lignin in the poplar sample, the calculated content ψcalculated of heavy oil from the three main components should be 15.39 wt %. ψcalculated ¼ ðmass of initial biomass ðcellulose % ψcellulose þ hemicellulose % ψhemicellulose þ lignin % ψligin ÞÞ=mass of initial biomass 100% ¼ ðmass of initial biomass ð45:33% 2:4% þ 31:37% 22:91% þ 21:38% 38:30%ÞÞ=mass of initial biomass 100% ¼ 15:39%
ψcellulose < ψpoplar < ψhemicellulose < ψlignin Although it is difficult to liquefy cellulose and hemicellulose, the content of heavy oil from cellulose was only 2.4 wt %; on the contrary, the content of heavy oil from hemicellulose reached 3789
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Scheme 1
However, the real content of heavy oil from poplar sample was only 11.02 wt %. This result means that the formation of heavy oil can be depressed at the presence of lumped components.
3.3. Effect of Poplar Components on the Compositions of Light Oils. The compositions of the light oils from the alcoho-
lysis processes of the single and lumped components of poplar 3790
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Energy & Fuels are determined by GCMS analysis with the help of the mass spectra library of NIST08. The possible compounds name and content (area %) are given in Table 2. As shown in Table 2, the light oil from cellulose contains noctyl-β-D-glucopyrano, which means that glucose fractions can be combined with 1-octanol during the cellulose degradation. Meanwhile, some aromatic compounds, such as 1-ethyl-2-methyl-benzene, 1-methylethenyl-benzene, and tetralin, can be observed in the light oil from cellulose alcoholysis. Because cellulose is composed of glucose fractions, these aromatic compounds should be derived from the rearrangement and condensation between the degradation fragments during cellulose alcoholysis. Furthermore, when glucose deeply cracked into acidic compounds, such as formic acid and acetic acid, these acidic compounds could sequentially react with 1-octanol to form formic acid octyl ester and acetic acid octyl ester. The total content of the two esters reached 6.91% (area %), which was the second highest content besides unreactive 1-octanol. These results indicate that 1-octanol is not only a solvent but also a reactive reagent, which promotes cellulose to depolymerize directly into ester compounds. Similar to the cellulose alcoholysis, n-octyl-β-D-glucopyrano, aromatic compounds, and esters were still present in the light oil from hemicellulose alcoholysis. In addition, n-octyl-R-D-riboside and furfural, which derived from the degradation of pentose, can also be found in the light oil from hemicellulose alcoholysis. The result may be due to the special structure of hemicellulose, which is composed of pentose and hexose. Although the light oil from lignin contains a few formic acid octyl esters and acetic acid octyl esters, the total content of aromatic and phenolic compounds, including 2-methoxy-phenol, 1-ethyl-2-methyl-benzene, and tetralin, was 3.09% (area %), which was the second highest content besides unreactive 1-octanol. This result suggests that lignin degrades and chiefly forms aromatic and phenolic compounds. This is in agreement with the known phenolic nature of lignin.22 In addition, the real total content of formic acid octyl ester and acetic acid octyl ester in the light oil from poplar was 12.78% (area %), which was far more than the calculated total content of esters from cellulose, hemicellulose, and lignin. This result further confirms that the alcoholysis process of poplar components can be promoted at the lumped case. Moreover, when the composition of the light oil from poplar is compared to that from its main components, it can be seen that some compounds are still present but other compounds disappear. The result suggests that the composition of light oil from poplar is not the simple summation of the compositions from poplar components; degradation, rearrangement, and condensation could also take place between the degradation fragments from cellulose, hemicellulose, and lignin (Figure 3). 3.4. Liquefaction Mechanisms of Poplar Components with Alcoholic Solvent. Although the light oil from poplar liquefaction with a 1-octanol solvent is a very complex mixture, there were only eight main compounds in the light oil, including unreactive 1-octanol, formic acid octyl ester, octyl ether, levulinic acid butyl ester, acetic acid octyl ester, 4-hydroxy-4-methyl-2pentanone, 5-methyl-furfural, and phenol, with a total content reaching 98.08% (area %). The simple components of light oil were beneficial to the separation of valuable chemicals and the recycle of unreactive 1-octanol. The fundamental principle of poplar alcoholysis can also be primarily represented with the formation processes of the eight compounds.
ARTICLE
According to their chemical structure, the eight compounds can be reduced to four sources: (a) biomass cracking fragments, including 5-methyl-furfural and phenol, (b) esters derived from the esterification of acidic fragments with alcoholic solvent, including formic acid octyl ester and acetic acid octyl ester, (c) new compounds from the rearrangement and condensation of degradation fragments, including 4-hydroxy-4-methyl-2-pentanone and levulinic acid butyl ester, and (d) unreactive 1-octanol and its dehydration product, octyl ether. Therefore, the alcoholysis mechanisms of poplar components can be proposed as the following reaction pathway in Scheme 1.
4. CONCLUSION (1) With acidified 1-octanol as the solvent at 130 °C, the liquefaction rate of lignin was 80.70 wt %. On the contrary, the liquefaction rates of cellulose and hemicellulose were only 25.40 and 31.20 wt %, respectively. The liquefaction of cellulose could be promoted when cellulose, hemicellulose, and lignin lumped together in poplar. As a result, the liquefaction rate of poplar reached 83.54 wt %. (2) Although it is difficult to liquefy cellulose and hemicellulose, the content of heavy oil from cellulose was only 2.4 wt %; on the contrary, the content of heavy oil from hemicellulose reached 22.91 wt %. Meanwhile, the content of heavy oil from lignin was up to 38.30 wt %. When cellulose, hemicellulose, and lignin coexisted in biomass, the formation of heavy oil was depressed. As a result, the content of heavy oil was only 11.02 wt %. (3) When cellulose was subjected to acidified 1-octanol, it degraded into glucose fractions and further cracked into acidic compounds, such as formic acid and acetic acid. Then, these acidic compounds could be esterified into formic acid octyl ester and acetic acid octyl ester, with content being the second highest besides unreactive 1-octanol in the light oil from cellulose. Meanwhile, the rearrangement and condensation took place between the fragments of cellulose degradation, which resulted in the formation of aromatic compounds. The alcoholysis process of hemicellulose was similar to that of cellulose. In addition, noctyl-R-D-riboside and furfural, which derived from pentose degradation, could also be found in the light oil from hemicellulose alcoholysis. In contrast, aromatic and phenolic compounds were the chief products from lignin degradation. The composition of light oil from poplar was not the simple summation of those from cellulose, hemicellulose, and lignin; rearrangement and condensation could also take place between the degradation fragments from cellulose, hemicellulose, and lignin. (4) During poplar alcoholysis, a large number of reactions occurred, including cracking, esterification, rearrangement, condensation, and dehydration. Although light oil from biomass was a very complex mixture, there were only eight main compounds in the light oil, including unreactive 1-octanol, formic acid octyl ester, octyl ether, levulinic acid butyl ester, acetic acid octyl ester, 4-hydroxy-4-methyl-2-pentanone, 5-methyl-furfural, and phenol. The total content of these eight compounds reached 98.08% (area %). The simple components of light oil were beneficial to the separation of valuable chemicals and the recycle of unreactive 1-octanol. ’ AUTHOR INFORMATION Corresponding Author
*Telephone: +86-10-6288-9433. Fax: +86-10-6288-9467. E-mail:
[email protected]. 3791
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’ ACKNOWLEDGMENT The authors thank the fund for the Commonwealth Speciality Industry of SFA (200904026) for financial support of this work. ’ REFERENCES (1) Edward, L. K.; Dante, A. S.; Ryan, M. W.; Juan, C. S.-R.; Christian, A. G.; James, A. D. Science 2008, 322, 417–421. (2) Tushar, P.; Vispute, H. Z.; Aimaro, S.; Rui, X.; George, W. H. Science 2010, 330, 1222–1227. (3) Yuan, X. Z.; Li, H.; Zeng, G. M.; Tong, J. Y.; Xie, W. Energy 2007, 32, 2081–2088. (4) Yuriy, R.-L.; Christopher, J. B.; Zhen, Y. L.; James, A. D. Nature 2007, 447, 982–985. (5) Yan, N.; Zhao, C.; Luo, C.; Dyson, P. J.; Liu, H.; Kou, Y. J. Am. Chem. Soc. 2006, 128, 8714–8715. (6) Demirbas, M. F. Appl. Energy 2009, 86, S151–S161. (7) Balat, M.; Balat, M.; Kirtay, E.; Balat, H. Energy Convers. Manage. 2009, 50, 3147–3157. (8) Goyal, H. B.; Seal, D.; Saxena, R. C. Renewable Sustainable Energy Rev. 2008, 12, 504–517. (9) Yuan, X. Z.; Tong, J. Y.; Zeng, G. M.; Li, H.; Xie, W. Energy Fuels 2009, 23, 3262–3267. (10) Lu, W. P.; Wang, C.; Yang, Z. Y. Bioresour. Technol. 2009, 100, 6451–6456. (11) Yanik, J.; Ebale, S.; Kruse, A.; Saglam, M.; Yuksel, M. Int. J. Hydrogen Energy 2008, 33, 4520–4526. (12) Zou, X.; Qin, T.; Huang, L.; Zhang, X.; Yang, Z.; Wang, Y. Energy Fuels 2009, 23, 5213–5218. (13) Krzan, A.; Zagar, E. Bioresour. Technol. 2009, 100, 3143–3146. (14) Zhang, T.; Zhou, Y. J.; Liu, D. H.; Petrus, L. Bioresour. Technol. 2007, 98, 1454–1459. (15) Liu, J. J.; Chen, F. G.; Qiu, M. H. J. Biobased Mater. Bioenergy 2009, 3, 401–407. (16) Yamada, T.; Aratani, M.; Kubo, S.; Ono, H. J. Wood Sci. 2007, 53, 487–493. (17) Chen, F. G.; Lu, Z. M. J. Appl. Polym. Sci. 2009, 111, 508–516. (18) Yang, H. P.; Yan, L.; Chen, H. P.; Lee, D. H.; Zheng, C. G. Fuel 2007, 86, 1781–1788. (19) Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106, 4044–4098. (20) Demirbas, A. Energy Convers. Manage. 2000, 41 (6), 633–646. (21) Amen-Chen, C.; Pakdel, H.; Roy, C. Bioresour. Technol. 2001, 79, 277–299. (22) Dorrestijn, E.; Laarhoven, L. J. J.; Arends, I. W. C. E.; Mulder, P. J. Anal. Appl. Pyrolysis 2000, 54, 153–192.
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