Fuel Processing Technology 90 (2009) 657–663

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Fuel Processing Technology

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Liquefaction of rice straw in sub- and supercritical 1,4-dioxane–water mixture

Hui Li a,1, Xingzhong Yuan a,⁎, Guangming Zeng a, Jingyi Tong a,1, Yan Yan b,2, Hongtao Cao a,1, Lihua Wang a,1,Mingyang Cheng a,1, Jiachao Zhang a,1, Dan Yang a,1

a College of Environmental Science and Engineering, Hunan University, Changsha 410082, Chinab School of Environment, Beijing Normal University, Beijing 100875, China

⁎ Corresponding author. Tel.: +86 731 8821413; fax:E-mail addresses: lihuiluoyang@126.com (H. Li), yxz

zgming@hnu.cn (G. Zeng), tongjingyi.hnue@yahoo.comyanhudielan@126.com (Y. Yan), caohtr@yahoo.com.cn (H(L. Wang), chengmingyang123@163.com (M. Cheng), jiayd20030310224@sina.com (D. Yang).

1 Tel.: +86 731 8821413; fax: +86 731 8823701.2 Tel.: +86 10 58803693; fax: +86 731 8823701.

0378-3820/$ – see front matter © 2008 Elsevier B.V. Adoi:10.1016/j.fuproc.2008.12.003

a b s t r a c t

a r t i c l e i n f o

Article history:

The critical liquefaction of ri Received 14 May 2008Received in revised form 24 November 2008Accepted 2 December 2008

Keywords:SubcriticalSupercriticalSynergistic1,4-DioxaneGC–MSFTIR

ce straw in sub- and supercritical 1,4-dioxane–watermixturewas investigated in a500 mL autoclave at temperature of 260–340 °C, resistance time of 0–20 min, and volume ratios 0–100 vol.%(1,4-dioxane:mixture). The yields of oil and PA+A (preasphaltene and asphaltene) were in the range of29.64–57.30 wt.% and 6.42–22.68 wt.%, depending on the temperature, resistance time and volume ratio.The synergistic capability of 1,4-dioxane–water mixture could allow the great decomposition of the tubularstructure of lignocelluloses. It was shown by the results that the “oxygen-transfer” reaction, deoxygenationand decarboxylation may occur in the liquefaction of rice straw with 1,4-dioxane–water mixture, whiledeoxygenation and decarboxylation may be the main reaction. The oil and PA+A fractions obtained atdifferent volume ratios were analyzed by FTIR and GC–MS to investigate the effect of the ratios on the type ofthe compounds in the liquid products. It is shown that the nucleophilic and hydrolytic functions of watermight be weaken at the higher ratio of 1,4-dioxane runs, resulting the lower amount of phenolic, acidic,hydrocarbon and ester derivatives in the oil and PA+A fractions.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Recently, thermo-chemical conversion of biomass feedstock intoliquid products such as bio-oils in hot compressed water has attractedintensive interest due to the advantage of low environmental impactover fossil fuels [1–9].

Water is a cheap and common solvent as well as an effectiveliquefaction agent in the liquefaction process, the high critical valuefor water (374.3 °C, 22.1 MPa) means that the sub- and supercriticalwater liquefaction process requires challenging operative conditions.In addition, another drawback of utilizing water as the solvent forliquefaction of biomass is the higher value of oxygen concentration inthe liquefied products, resulting low-heating values for the liquidproducts. Hence, many attempts were made to investigate theliquefaction conversion of biomass to liquid products with loweroxygen values in the low-critical-value organic solvents, such as

+86 731 8823701.@hnu.cn (X. Yuan),.cn (J. Tong),. Cao), wlh0603100@163.com

chao.1984@163.com (J. Zhang),

ll rights reserved.

ethylene glycol [10], ethyl acetate [11], 2-propanol [12], ethylenecarbonate [13], glycerine [14] and ethanol [15].

It was reported by Yao [16] that somemixed solvents had a synergisticcapability to inhibit the formation of the solid residue, and enhance theliquefaction conversion of biomass. These mixtures with synergisticcapability could be composed of two solvents with different polarity:(1) an electron donor solvent with middle intensity; (2) an electronacceptor solvent with high polarity, containing the hydroxyl group.

In this study,1,4-dioxaneweremixedwith deionizedwater, preparedas synergistic reaction medium. The objectives of the current researchare to clarify the distributions of oil, PA+A (preasphaltene andasphaltene), and residue according to the origins of liquefactiontemperatures, resistance time and volume ratios, and to investigate thecharacteristics of the products obtained with different ratios.

2. Experiment

2.1. Materials

The dried rice straw was mainly obtained from a farm in Hunan,southern China. First, the samplewas dried in an oven at 105 °C for 24 h,ground in a rotary cutting mill, and then screened into fractions ofparticle diameter 30–120mesh.The results of component, elemental andthermal analysis are given in Table 1.

The critical values of the 1,4-dioxane–water mixture calculated bythe CHEMCAD program are shown in Table 2.

Table 1Component, elemental and thermal analysis of rice straw.

Analysis

Chemical composition Wt.%

Lignin 9.22Cellulose 41.33Semi-cellulose 24.60

Elemental composition

C 36.81H 5.025Oa 56.69HHV 16.74 MJ/kg

a By differences.

Table 2The critical values (Tc, Pc) of the used solvents (°C, MPa).

1,4-dioxane 1,4-dioxane:water(20 vol.%)

1,4-dioxane:water(50 vol.%)

1,4-dioxane:water(80 vol.%)

Water

Tc Pc Tc Pc Tc Pc Tc Pc Tc Pc

314.0 5.21 345.0 14.97 326.6 10.82 317.6 7.73 374.3 22.10

658 H. Li et al. / Fuel Processing Technology 90 (2009) 657–663

2.2. Experimental procedure and separation

Liquefaction experiments were conducted in a 500 mL GSH-0.5type autoclave at the reaction temperature. The reactor was loadedwith 10 g (dry basis) of rice straw and 200 mL mixture. Thereactants were agitated vertically using stirrer (95 rpm). Thetemperature was then raised up to the reaction temperature.Afterwards, it was cooled down to the room temperature by fanand cool water.

The procedure for separating liquefaction products is shown inFig. 1. After completing the cooling period, the gas product wasvented without being further analyzed. The autoclave contentswere poured into a beaker. The liquefied products were removedfrom the autoclave by washing with 150 mL deionized water threetimes, and then they were filtered. After removal of the mixtureunder reduced pressure at 70 °C in a rotary evaporator, aqueousphase product was designated as Oil. The dark brown solid waswashed with tetrahydrofuran (THF) and filtrated under vacuum for15 min. The THF insoluble fraction was called as Residue, while theTHF soluble fraction was designated as Preasphaltene and Asphal-tene (PA+A). The product losing in the evaporation was called aslight fraction (LF).

2.3. Analysis

2.3.1. HHV analysisThe higher heating value (HHV) of rice straw was determined

using a calorimeter (KS Auto Calculation Bomb Calorimeter KLSR-4000).

2.3.2. Elemental analysisThe element composition of oil, PA+A and rice straw was

analyzed by CHNOS Elemental Analyzer Vario EL III (ElementarAnalysen-systeme GmbH, Germany).

2.3.3. FTIRThe spectra were obtained from a Spectrum. FTIR (Perkin Elmer,

Germany) spectrophotometer by the potassium bromide disctechnique.

2.3.4. Gas chromatography–mass spectrometry (GC–MS)GC–MS analyses were conducted on a Trace GC, Palaris Q GC–

MS spectrometer (Thermo-Finnigan, USA) using carbon capillarycolumn, DB-1 (film thickness, 0.25 mm; column dimensions,30 m×0.25 mm), with He as the carrier gas. The columntemperature of GC used in this study was programmed from 130to 230 °C with an increasing rate of 8 °C/min. The temperature ofthe injection chamber was 250 °C, and the temperature of transferline was 230 °C. Mass range was 40–450 m/z.

3. Results and discussion

The yield of each product is calculated as follows:

Yield of oil = mass aqueous phase productð Þ� = mass biomass added × 100kð Þ

ð1Þ

Yield of PA + A = mass THF solubleð Þ = biomass added × 100kð Þ ð2Þ

Yield of residue = mass THF insolubleð Þ= mass biomass added × 100kð Þð3Þ

Liquefaction conversion = 100k − yield of residue ð4Þ

3.1. Effect of reaction temperature

The yields of products for various temperatures (260–340 °C) in1,4-dioxane–water mixture were presented in Fig. 2 with the ratio of50 vol.% (1,4-dioxane:mixture), resistance time of 5 min, finalpressure of 5.6–22.3 MPa, and the results in Table 2 showed thatthe liquefaction operations in this study were essentially at sub- andsupercritical conditions.

As reported in Fig. 2, the yield of residue decreased from 12.18 to7.56 wt.% with the temperature increased from 260 to 340 °C, whilethe yield of oil decreased from 49.00 to 33.01 wt.% with thetemperature increased from 260 to 320 °C. In addition, an increase ofreaction temperature from 260 to 280 °C and from 280 to 320 °Cresulted in an increment in yield of PA+A from 9.08 to 19.70 wt.%,and a decrease from 19.70 to 12.56 wt.%. These results are consistentwith the previous studies. It was shown by several investigations[15,17] that hydrolysis and repolymerization were involved in theliquefaction. During the liquefaction, biomass was decomposed anddepolymerized to fragments of lighter molecules, and then theseunstable fragments rearranged through condensation, cyclization,and polymerization to form new compounds. However, the yields ofoil and PA+A obtained at 340 °C were higher than those obtained at320 °C. It could be due to that the liquefaction operation at 340 °C(pressure: 22.3 MPa) with 1,4-dioxane–water mixture (50 vol.%)were at the supercritical conditions (Table 2). The chemical reactions,which would otherwise occur in a multiphase system at conventionalconditions, could occur in a single fluid phase at the supercriticalcondition [18]. Therefore, the formation of residue during repoly-merization may be reduced at supercritical condition and, conse-quently, the formation of oil and PA+A was promoted.

3.2. Effect of resistance time

Experiments of rice straw liquefied in mixture at differentresistance time (0, 5, 10, 15 and 20 min) have been carried out at300 °C, with 10 g of rice straw, and 200 mL mixture (50 vol.%). Theresults are presented in Fig. 3.

As the data shown in Fig. 3, the yield of oil decreased with theresistance time increasing in the range of 0–20 min. The oil formedwas less than 44.00 wt.% at the resistance time of 20 min, while the

Fig. 1. Procedure for separation of critical liquefaction products.

659H. Li et al. / Fuel Processing Technology 90 (2009) 657–663

maximum yield of oil (57.30 wt.%) was obtained at the resistancetime of 0 min. In addition, an increase of resistance time in therange of 0–10 min and 10–20 min resulted in an increment in yield ofPA+A from 14.00 to 20.12 wt.%, and a decrease in that from 20.12 to6.42 wt.%, while the yield of residue was opposite. The minimumyield of residue (7.05 wt.%) was obtained at 300 °C, resistance time10 min. In other words, the maximum liquefaction conversion of ricestraw was 92.95 wt.% at that condition. The yield of oil and PA+Adecreased as the resistance time was prolonged in the range of 10

Fig. 2. Yield of products as a function of liquefaction temperature in sub- andsupercritical liquefaction of rice straw in 1,4-dioxane–water mixture with a ratio of50 vol.%, resistance time 5 min.

and 20 min, owing to the formation of residue by repolymerization,condensation, cyclization and polymerization [17].

3.3. Effect of solvent ratios

Experiments of rice straw liquefied in solvent with different 1,4-dioxane:mixture volume ratios (0, 20, 50, 80 and 100 vol.%) have beencarried out at 300 °C, 10 g of rice straw, resistance time 5 min. The Oiland PA+A obtained at those conditions were analyzed by CHNOSElemental Analyzer. The results were presented in Fig. 4, and Table 3.

Fig. 3. Effect of resistance time on the yield of products (Oil, PA+A and Residue: wt.%)in 1,4-dioxane–water mixture with a ratio of 50 vol.%, liquefaction temperature 300 °C.

Fig. 4. Effect of volume ratio on the yield of products (Oil, PA+A and Residue: wt.%)with 1,4-dioxane–water mixture at 300 °C.

Table 3Effect of volume ratio on the element composition (wt.%) and HHV (MJ/kg) of the liquidproducts with 1,4-dioxane–water mixture at 300 °C.

Ratio Oil PA+A

C H O HHV C H O HHV

0 69.25 4.91 25.84 25.85 72.57 6.88 20.55 30.7720 66.23 6.90 26.87 27.51 79.25 8.69 12.06 37.1750 56.19 6.43 37.38 21.54 75.48 7.32 17.20 32.9980 60.69 5.42 33.89 22.24 74.74 7.12 18.14 32.28100 62.40 6.54 31.06 24.94 77.66 9.14 13.20 37.07

HHV (MJ/kg)=0.3383C+1.442(H–O/8).

660 H. Li et al. / Fuel Processing Technology 90 (2009) 657–663

As the data shown in Table 3, the carbon composition and HHV ofoil and PA+A are higher than that of rice straw, while the oxygencomposition of oil and PA+A are lower than that of rice straw. Theseresults are consistent with previous study. It was reported byDemirbas that deoxygenation and decarboxylation were involvedduring the liquefaction process, resulting in low-oxygen-value ofproducts comparing that of the feedstock [19].

Moreover, the oxygen composition of oil were higher than those ofPA+A, while the HHVs were opposite. The lowest oxygen composi-tion of oil and the highest oxygen composition of PA+A were bothobtained at the pure water run. Those results suggest that 1,4-dioxanemay promote the transfer of oxygen from PA+A to oil fraction duringliquefaction, leading to slightly increment of the oxygen compositionin oil fraction. In other words, the “oxygen-transfer” reaction,deoxygenation and decarboxylation may occur in the liquefaction ofrice straw with 1,4-dioxane–water mixture, while deoxygenation anddecarboxylation may be the main reaction.

In Fig. 4, the yields of oil obtained at the ratios of 50, 80 and 100 vol.%were higher than that obtained inpurewater run. It can be probably dueto the relation between the value of oxygen concentration and polarityofwater soluble fraction (oil) in biomass liquefactionproducts [20]. Highoxygen composition increases the polarity of the liquid componentsand makes them more water-soluble. In this study, “oxygen-transfer”capability of 1,4-dioxanemaygive a contribution to the increment of thepolarity of oil fractions (Table 3). However, the yield of oil (PA+A) is notincreasing (decreasing) with the volume ratio increasing in the range of0 and 100 vol.%, and the maximum yield of PA+A (22.68 wt.%) wasobtained at the ratio of 20 vol.%. This difference may be considered dueto that the yield of residue decreasing with the increasing ratio in therange of 0–50 vol.%, and increased with the ratio from 50–100 vol.%. Inother words, as the ratio was higher or lower than 50 vol.%, rice strawmay be not completely decomposed.

Among the three components of biomass, ligninwas themost difficultone to decompose, and consequently, the amount of solid residueincreased in proportion to the lignin content in the liquefaction process.In biomass, lignin occurs mostly as lignocellulose in complex associationwith cellulose [19]. The distinctive structural characteristics of lignocellu-lose make them resistant to attack by sole solvent during the liquefactionprocess. When using 1,4-dioxane–water mixture as reaction solvent, thewater acts as a nucleophile and reacts with some active centers in theprotolignin [21,22]. The 1,4-dioxane solvent solubilizes the cellulose andsemi-cellulose, and impregnates the plant tissue, carrying the reagents tothe protolignin and the resulting lignin fragments from the inner part ofthe cell to the solution [21,22]. Therefore, the synergistic capability of 1,4-

dioxane–watermixture (especially with ratio of 50 vol.%) could allow thegreat decomposition of the tubular structure of lignocellulose, and thenfree phenoxyl radicals were formed. These free phenoxyl radicals had arandom tendency to form products through condensation or repolymer-ization, and consequently the production of residue was reduced.

3.4. Characteristics of the oil and PA+A

The oil and PA+A fractions obtained at 300 °C, resistance time5 min, different 1,4-diaxane:mixture volume ratios (0, 20, 50, 80 and100 vol.%) were analyzed by FTIR (Fig. 5) and GC–MS (Tables 4 and 5)in order to investigate the type of the organic compounds in the bio-oil.

3.4.1. FTIRComparing with the spectra, the greater intensity and higher

resolution of the 3000–2800 cm−1 bands suggest the oil and PA+A tobe more highly aliphatic in character. The intense band centered at1725 cm−1, attributed to hemicelluloses in the original rice straw,appears to increase in intensity in the oil and PA+A fractions. In thisregion contributions are to be expected from a conjugated carbonyl orcarboxyl structure (1710 cm−1) and from unconjugated carbonyl,carboxyl structures and/or esters of conjugated acids (1715 cm−1),which arise from the breakdown of the original lignin [23]. Thecellulose band at 898 cm−1, which was clearly discernible in the ricestraw, was found to be absent in the spectra of the oil and PA+A,indicating the expected total breakdown of cellulose during liquefac-tion. The appearance of bands below 900 cm−1 in the oil and PA+A(bands between 900 and 675 cm−1) may be attributed to C–H out-of-plane deformation bands on aromatic rings, indicating a high degreeof aromatic ring substitution [24]. Bands between 1170 and 970 cm−1

(C–O bonds in polysaccharides, lignin alcohols and ether bands)appeared less intense in the oil and PA+A spectra, especially the1050 cm−1 band. However, aromatic ether bands (up to 1330 cm−1)and phenolic stretching bands (near 1270 and 1235 cm−1) appearedas more intense lignin ethers aromatic in the oil and PA+A spectra.

FTIR spectra of the oil and PA+A fractions presented many similarfeatures on chemical structures detectable by FT-infrared spectro-scopy. However, some significant differences could be observedwhen comparing spectra from oil and PA+A. Carbonyl stretching(1725 cm−1), OH in-plane stretching (1070 cm−1) and C–O stretching(1050 cm−1) bands were found to be more intense in the spectra ofthe oil, attributed to a high degree of aliphatic alcohols and esters inthe oil.

In addition, the influences of solvent ratio on the characterizationof oils and PA+A fractions were investigated in this study. The greaterintensity and higher resolution of the 3000–2800 cm−1 and 900–675 cm−1 bands suggest the oil and PA+A from mixture and pure1,4-dioxane runs to bemore highly aliphatic and aromatic in character,indicating a great decomposition of cellulose and lignin. Moreover, theintensity of absorbance at 1725 cm−1 (carbonyl stretching) washigher in case of oil fractions obtained with mixture and pure 1,4-dioxane runs, which suggested for a greater decomposition of semi-

Fig. 5. FTIR spectra of rice straw (1), oil (2) and PA+A (3). (a) Ratio of 0, (b) 20 vol.%,(c) 50 vol.%, (d) 80 vol.% and (e) 100 vol.%.

661H. Li et al. / Fuel Processing Technology 90 (2009) 657–663

cellulose and original lignin in those conditions than that in purewater run. However, the absorbance intensity of carbonyl stretching(1725 cm−1) from PA+A fractions in the runs of 1,4-dioxane–watermixture (80vol.%) andpure 1,4-dioxanewas lower than that in theotherruns. It might be due to that “oxygen-transfer” capability of 1,4-dioxanecontaining abundant oxygen had given a significant contribution to theincrement of the polarity of water soluble with carbonylic groups.Hence, the mighty absorptions caused by carbonylic groups weredisplayed in water-solubles (oil fractions).

3.4.2. GC–MSCombining the information of FTIR mentioned above, the oil and

PA+A fractions obtained at 300 °C,with different 1,4-diaxane:mixturevolume ratios (0, 20, 50, 80 and 100 vol.%) were analyzed by GC–MS inorder to investigate the type of the organic compounds in those twofractions. Clearly, the liquefaction products were such unknown andcomplex mixtures of organic compounds that no calibration of the MSdetector was set, mainly due to the lack of an appropriate standard

mixture for calibration. Tables 4 and 5 list the tentative compounds ofthe oil and PA+A fractions, which are the most probable compoundsidentified by the MS search file (NIST library).

From the results of GC–MS analyses, the chemical compositions ofoil were the inclusion of a lot of complex compounds such as phenols,aromatics, esters (straight chain and aromatic esters), alkanes,alcohols, etc, while the compositions in the PA+Awere the inclusionof a lot of complex compounds such as phenols, aromatics, aromaticesters, aldehydes, alkanes, ketones, aethers, etc.

In Tables 4 and 5, it is clearly seen that phenolic compounds are themajor compounds identified in all the liquefied products, followed byester derivatives, hydrocarbon, organic acids and alcohols. Theinfluence of solvent ratio has been investigated for the subcriticalliquefaction (Table 2) of rice straw with ratios of 0, 20, 50, 80 and100 vol.%, respectively. As can be seen from Tables 4 and 5, the higherthe ratio of water in the reaction solvent, the higher the amounts ofphenolic and acidic compounds obtained in the oil and PA+Afractions. In other words, the amounts of phenolic and acidic

Table 5GC–MS analysis of the PA+A fractions of liquefaction products from rice strawwith 1,4-dioxane–water mixture (vol.%) at 300 °C.

Peak Compound 0 20% 50% 80% 100%

11.10 3,5-bis(1,1-dimethylethyl)-phenol ○ Δ Δ ● ●12.89 1,1-dimethoxy-(Z)-9-octadecene Δ Δ ● Δ ○15.06 Docosane ○ ○ ○ ○ ○15.36 2-methyl-1-hexadecanol – – ● ● ○15.55 Oleic acid, 3-(octadecyloxy)propyl ester – ● ● ● –

15.74 1-(4-hydroxy-3,5-dimethoxyphenyl)-ethanone ● ○ ● ● ●16.25 2-[4-methyl-6-(2,6,6-trimethylcyclohex-1-enyl)

hexa-1,3,5-trienyl]cyclohex-1-en-1-carboxaldehyde

● ● ● – ●

17.09 1,2-benzenedicarboxylic acid,butyl 2-ethylhexyl ester

Δ Δ Δ Δ Δ

17.75 Dotriacontane ○ ○ ○ – –

18.11 Picrotoxinin ● ● ● ● ●18.20 3-ethyl-5-(2-ethylbutyl)-octadecane ● ○ ● ● ●18.37 1,2-benzenedicarboxylic acid, butyl octyl ester Δ Δ Δ ○ –

19.07 hexadecanoic acid, 1-(hydroxymethyl)-1,2-ethanediyl ester

Δ ○ Δ ● –

19.89 C32H48O6 – ● ● ● ●20.07 4-methylandrostane 2,3-diol-1,17-dione – – ● – ●20.26 Phorbol – ● ● ● ●20.83 Nonacosane ○ ○ ○ ● ○21.37 Octadecanoic acid ○ ● ● ● ●23.94 2-methyl-1-(3,4-dimethoxyphenyl)-1-(2,4-

diisopropoxy-phenyl)propane● ● ● ● ●

27.57 Tetratetracontane – – ● ● ●28.26 Bis(2-ethylhexyl) phthalate ● ● ● ● ●29.31 4,4-methylenebis(1,1-dimethylethyl)-phenol ○ ● ● ● ●

● The results of chromatographic area (% of total area) belonging to the identifiedcompounds N1.0%.○ The results of chromatographic area (% of total area) belonging to the identifiedcompounds N3.0%.Δ The results of chromatographic area (% of total area) belonging to the identifiedcompounds N5.0%.

662 H. Li et al. / Fuel Processing Technology 90 (2009) 657–663

compounds of the oil and PA+A fractions in the solvent with ratio inthe range of 0–50 vol.% could be higher than those in the ratio of 80and 100 vol.%. The similar explanation in the previous sections for thefunction of 1,4-dioxane and water during the liquefaction process maybe adopted here. During the liquefaction with the ratio of 80 and100 vol.%, the hydrolysis of cellulose was inhibited as the lack of water.Meanwhile, the function of water as a nucleophile, which could reactwith some active centers in the protolignin, was weaken as the lack ofwater [21,22].

There are two formation mechanisms for acidic derivatives. First,during the liquefaction, the cellulose was hydrolyzed to produce furanderivatives including 2-furancarboxaldehyde and 5-methyl-2-furan-carboxaldehyde, and the further decomposition of these products ledto form acetic fragment, which is a rich source of acidic compounds.Second, the small functional groups cracked from the lignin mono-mers could also be the source of acidic compounds. These results arein good agreement with the previous study [25]. Meanwhile, it isshown by wood chemistry that lignin has phenylpropane units, whichare a highly rich source of phenolic compounds. During theliquefaction process, breaking of β-aryl and benzoylether bonds inlignin produced phenolic compounds and further decomposition ofphenolic compounds produced benzendiol derivatives [26]. Therefore,formation of phenolic compounds is considered as decomposition oflignin [27,28].

The amount of hydrocarbon and ester derivatives obtained in thepure 1,4-dioxane run could be slightly lower than that in the otherruns. During the liquefaction of biomass, the further decomposition offuran derivatives, which is obtained from the hydrolysis of cellulose,might contribute to the formation of esters [25]. In addition, the smallfragments such as alcohols, acids, aldehydes, and ketones wereobtainedwithout loss of water via the retro-aldol condensation duringthe liquefaction of cellulose [29]. Finally, the small pieces having C–Oor C–O bonds may be connected by the dissociation of O–H bonds andthe recombination of C–H bonds to form hydrocarbons. However, the

Table 4GC–MS analysis of the oil fractions of liquefaction products from rice straw with 1,4-dioxane–water mixture (vol.%) at 300 °C.

Peak Compound 0 20% 50% 80% 100%

11.10 3,5-bis(1,1-dimethylethyl)-phenol Δ ○ ● ● Δ13.11 [3.3.1.1(3,7)]decane-2,6-diol,2,6-bis

(aminomethyl)-tricyclol– – ○ ○ ○

15.06 Docosane ○ ● Δ ● –

15.74 1-(4-hydroxy-3,5-dimethoxyphenyl)-ethanone ○ ○ Δ ○ ●17.09 1,2-benzenedicarboxylic acid,

butyl 2-ethylhexyl ester○ – Δ – –

17.75 Dotriacontane ● – – – –

18.11 Picrotoxinin ● – – – ●18.20 3-ethyl-5-(2-ethylbutyl)-octadecane ○ ○ ○ ● ○18.37 1,2-benzenedicarboxylic acid, butyl octyl ester Δ Δ Δ Δ ○18.78 Docosanoic acid, 1,2,3-propanetriyl ester ○ ○ ● ● ○19.07 Hexadecanoic acid, 1-(hydroxymethyl)-1,2-

ethanediyl esterΔ Δ ○ Δ ○

19.39 Octadecanoic acid, 2-hydroxy-1,3-propanediyl ester

– – Δ – –

20.26 Phorbol – – – ● ●20.83 Nonacosane Δ Δ ● ○ ○21.37 Octadecanoic acid ● ○ ● ● ●21.64 15-methyl-hexadecanoic acid, methyl ester – – Δ – –

23.94 2-methyl-1-(3,4-dimethoxyphenyl)-1-(2,4-diisopropoxy-phenyl)propane

○ ○ ● ○ ●

28.26 Bis(2-ethylhexyl) phthalate – ● – ● –

29.31 4,4-methylenebis(1,1-dimethylethyl)-phenol ● ○ ● ● ●

● The results of chromatographic area (% of total area) belonging to the identifiedcompounds N1.0%.○ The results of chromatographic area (% of total area) belonging to the identifiedcompounds N3.0%.Δ The results of chromatographic area (% of total area) belonging to the identifiedcompounds N5.0%.

hydrolysis of cellulose could be reduced, and the formation ofhydrocarbon and ester derivatives could be inhibited in the pure1,4-dioxane run.

4. Conclusion

The formation of residue during repolymerization may be reducedand the formation of oil and PA+A was promoted at supercriticalcondition, comparing those obtained at subcritical condition.

The maximum yield of oil (57.30 wt.%) was obtained at theresistance time of 0 min, while the maximum liquefaction conversionof rice straw was 92.95 wt.% at the resistance time of 10 min at 300 °C,10 g of rice straw, and 200 mL 1,4-dioxane–water mixture (50 vol.%).

The “oxygen-transfer” reaction, deoxygenation and decarboxyla-tion may occur in the liquefaction of rice straw with 1,4-dioxane–water mixture, while deoxygenation and decarboxylation may be themain reaction.

Phenolic compounds are the major compounds identified in all theliquefied products, followed by ester derivatives, hydrocarbon, organicacids and alcohols. The nucleophilic and hydrolytic functions of waterwere weaken at the high ratio of 1,4-dioxane runs, resulting the loweramount of phenolic, acidic, hydrocarbon and ester derivativescompounds in the oil and PA+A fractions.

Acknowledgements

This research was financially supported by the National NaturalScience Foundation of China (No. 50678062), the National BasicResearch Program of China (2005CB724203), Chinese NationalNatural Foundation for Distinguished Young Scholars Project(No.50225926) and Key Project for Science and Technology Research,Ministry of Education (No. 108100).

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