ARTICLE pubs.acs.org/IECR
Enhanced Bio-oil Yield from Liquefaction of Cornstalk in Sub- and Supercritical Ethanol by AcidChlorite Pretreatment Hua-Min Liu,† Bing Feng,‡ and Run-Cang Sun*,†,§ †
State Key Laboratory of Pulp and Paper Engineering and ‡School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China § The Institute of Biomass Chemistry and Technology, Beijing Forestry University, Beijing 100083, China ABSTRACT: Acidchlorite pretreatment was developed to enhance bio-oil yield and decrease the optimum reaction temperature of cornstalk liquefaction in a stainless steel reactor (0.5 L) at temperatures of 200340 °C. The effects of liquefaction temperature and ethanol amount on the bio-oil yield from liquefaction of cornstalk before and after pretreatment were investigated. The results showed that the pretreatment could markedly enhance the bio-oil yield and decrease the optimum temperature. The highest bio-oil yield (31.4%) was obtained from the liquefaction at the reaction temperature of 260 °C from the 1.5 h pretreated cornstalk. Unpretreated and pretreated cornstalks differed in chemical components, in which the acidchlorite pretreatment increased carbohydrate content and removed some amount of lignin. Scanning electron microscopy showed that the pretreated cornstalks had extensive anomalous porosity and lamellar structures. X-ray analysis showed that this pretreatment process was unable to break apart inter- and intrachain hydrogen bonding in cellulose fibrils. GC/MS analysis showed that the pretreatment had an important effect on the formation of various compounds in the bio-oil.
1. INTRODUCTION Recently there has been more and more interest in the conversion of biomass to liquid fuels because of the limited amount of fossil fuels available and their increasing price, the need for national energy independence and safety, and the reduction in greenhouse gas (GHG) emissions caused by the use of fossil oil.1 At present, thermochemical conversion of biomass is one of the most common and convenient routes for conversion into liquid including pyrolysis and liquefaction processes. During the pyrolysis process, the biomass is performed under high temperatures (7001000 °C) and at a high heating rate (104 °C/s) in the absence of air.2 Another important method of converting biomass into liquid fuel is liquefaction in solvents (such as water, ethanol, and acetone) by heat, in which biomass could be decomposed into liquid at a mild temperature and a low pressure compared to the pyrolysis process. The relative amounts of gaseous, liquid (bio-oil), and solid (residue) products were dependent on the operating conditions, such as biomass and solvent type, temperature, residence time, and heating rate, among which the liquefaction temperature is the most significant.35 Lignocellulosic biomass, mostly from agricultural and forestry sources, is mainly composed of cellulose, hemicelluloses, and lignin. Carbohydrates (cellulose and hemicelluloses) account for 5575% of lignocellulosic biomass by dry weight.6 The carbohydrate polymers are tightly bound to the lignin, mainly by hydrogen bonds but also by some covalent bonds. Lignin is built up by oxidative coupling of three major C6C3 (phenylpropanoid) monomers, namely, sinapyl alcohol (S), coniferyl alcohol (G), and p-coumaryl alcohol (H), which form an irregular structure in a tridimensional network inside the cell walls. The major interunit linkage is β-O-aryl ether type. Lignincarbohydrate complexes (LCC), which are formed by lignin covalently linked r 2011 American Chemical Society
with carbohydrates through bonds such as ester and ether, prevent plant biomass degradation.7,8 Therefore, due to their structural features, biomass has limited accessibility to solvents in the process of liquefaction, and delignification would help to break down LCC and separate carbohydrates and lignin for improving liquefaction. Many researchers have used pretreatment methods to increase the accessible surface area of lignocellulose and improve biomass digestibility at the enzymatic hydrolysis stage, which is the key to biomass conversion.9,10 Pretreatment methods are generally categorized into physical, chemical, physicochemical, and biological processes.11 The physical pretreatment process normally uses mechanical comminution and pyrolysis, where the biomass is exposed to high temperatures.12,13 Although this procedure is quite simple, the high energy consumption associated with it makes it not preferable to be implemented in a commercial scale production. Biological pretreatment involves the utilization of microbes and enzymes to degrade the biomass, and the mechanism has been reported to have the potential to be applied in the pretreatment of biomass.14 The chemicals commonly applied in the pretreatment process are either acid or alkaline, which are everyday industrial chemicals carrying minimal toxicity in their applied concentrations.15 The more popular and established laboratory method for the pretreatment is acidchlorite delignification utilizing an aqueous solution of acetic acid and sodium chlorite. This method effectively bleaches and then solubilizes lignin at moderate temperatures.16 Received: May 2, 2011 Accepted: August 17, 2011 Revised: August 15, 2011 Published: August 17, 2011 10928
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deionized water until a neutral pH was obtained, then dried at 105 °C for 24 h, and kept in a desiccator at room temperature before use. The pretreatment yields were defined as YPre ¼
Figure 1. Procedure for separation of products.
Liquefaction of biomass has been carried out in different solvents with various catalysts,17,18 but there is little literature on liquefaction of biomass by using pretreatment for enhancing bio-oil yield and decreasing the reaction temperature. In this study, the liquefaction of cornstalk to bio-oil consisted of two main steps: first, the cornstalk was pretreated using the method of acidchlorite delignification; second, the liquefaction of the pretreated cornstalk was performed in a stainless steel reactor (0.5 L) in sub- and supercritical ethanol. The objective of this study was to confirm the feasibility of biomass liquefaction by acidchlorite pretreatment for designing a cost-effective (less energy consumption) biomass liquefaction system.
2. MATERIALS AND METHODS 2.1. Materials and Solvents. The cornstalk sample was collected from the city of Guangzhou, which is located in the south of China. The raw material was first ground using a highspeed rotary cutting mill and sieved though 40 mesh. The ground cornstalk was extracted with distilled water and ethanol; then it was dried at 105 °C for 24 h and kept in desiccators at room temperature. The cornstalk used in this study contained about 49.6% carbon, 5.9% hydrogen, 43.5% oxygen, 1.0% nitrogen, and 5.5% ash based on the weight of dry sample. The solvents used were analytical grade ethanol and acetone. 2.2. Pretreatment. Acidchlorite pretreatment of cornstalk was performed in a triangle flask using sodium chlorite and acetic acid at 75 °C. Each 100 g of cornstalk was placed in a triangle flask (2 L) with deionized water (6 mL/g biomass), sodium chlorite (0.2 mg/g biomass), and glacial acetic acid (0.2 mL/g biomass). Each triangle flask was then sealed using a triangle flask (0.1 L) and placed in a reciprocating water bath at 75 °C. Every dose of sodium chlorite and glacial acetic acid was added and the flask was resealed and placed back in the reciprocating water bath at 75 °C after each 0.5 h. The pretreated cornstalk residues at different pretreatment times (1.5 and 3 h) were washed with
WSR 100% W
ð1Þ
where YPre is the pretreatment yield (wt %), W is the unpretreated cornstalk (g), and WSR is the pretreated cornstalk (g). 2.3. Liquefaction and Separation. For each run, cornstalk and ethanol were fed into a 0.5 L magnetically driven stirred autoclave. Then the reactor was purged four times with nitrogen to remove the air/oxygen in the reactor airspace. Agitation was set at 300 rpm and maintained for all experiments. The reactor was heated and the temperature was maintained at the set temperature for the desired holding time. After the reaction was completed, the autoclave was cooled to room temperature by cool water. The gas was then released from the autoclave, reducing the pressure to atmospheric pressure. The solid and liquid mixture was removed from the autoclave for separation. The procedure for the separation is shown in Figure 1. After that, water and acetone extracts were removed in a rotary evaporator and the corresponding fraction was weighed and designated as water-soluble oil (WSO) or heavy oil (HO). Solid residue was defined as the acetone insoluble fraction. The results obtained in this study are reported using the parameters defined as YWSO ¼
WWSO 100% WDry ð1 YA Þ
ð2Þ
YHO ¼
WHO 100% WDry ð1 YA Þ
ð3Þ
YRE ¼
WR 100% WDry ð1 YA Þ
ð4Þ
YB ¼ YWSO þ YHO
ð5Þ
where YWSO is the water soluble oil yield (wt %), YHO is the heavy oil yield (wt %), YB is the bio-oil yield (wt %), YRE is the residue yield (wt %), YA is the ash yield (wt %), WDry is the cornstalk flour (g), WWSO is the water-soluble oil (g), WHO is the heavy oil (g), and WR is the residue (g). Each experiment was repeated twice and the relative errors were within 8%. 2.4. Analytical Methods. Cornstalk derived oil (WSO and HO) was analyzed by a gas chromatograph equipped with a mass selective detector (GC/MS; Agilent 7890A/5975, USA). Both the injector and the detector were kept at 280 °C, and the carrier gas He velocity was 1 mL/min. An HP-5 column (5% phenyl methyl siloxane, 30 mm 0.25 mm) was used. In order to give a good product separation, the oven program was 1 min isothermal at 40 °C, followed by a heating rate of 10 °C/min to a final 280 °C and a hold for 2 min at the final temperature. Cornstalk was dried and analyzed using a Vario EL CHNO elemental analyzer. The ash content of the cornstalk was determined by burning at 650 °C. A field emission scanning electron microscope (S-3700N, Hitachi Limited, Japan) was used to image the dried fiber fraction of unpretreated and pretreated cornstalks. The samples were sputtered with a thick layer of gold spread uniformly from all sides at two different angles. 10929
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Figure 2. Chemical components and pretreatment yields of the unpretreated and pretreated cornstalks.
The chemical components of raw and pretreated cornstalks were determined, in which the cellulose was determined by the HNO3ethanol method.19 For the determination of lignin and hemicelluloses, hemicelluloses were determined according to Chinese standard methods.20 The contents of lignin and hemicelluloses were calculated as biomassholocelluloseinsoluble ash and holocellulosecellulose, respectively. The crystallinities of unpretreated and pretreated cornstalks were measured using an XRD-6000 instrument (Shimadzu, Japan) with Cu Kα radiation source (λ = 0.154 nm) at 40 kV and 30 mA. Samples were scanned at a speed of 2°/min, ranging over 2θ = 540°, and a step size of 0.02° at room temperature. The crystallinity index (CrI) was calculated from X-ray diffraction (XRD) data and determined based on the formula by Segal et al.21 as follows: CrI ¼
I002 Iam 100 I002
ð6Þ
in which I002 is the intensity for the crystalline portion of biomass (i.e., cellulose) at about 2θ = 22.5°, and Iam is the peak for the amorphous portion (i.e., cellulose, hemicelluloses, and lignin) at about 2θ = 18.7° in most of the literature. It should be noted in this study that the second highest peak after 2θ = 22.5° was at 2θ = 16.8°, and this was assumed to correspond to the amorphous region.22,23
3. RESULTS AND DISCUSSION 3.1. Structural Constituents and Pretreatment Yields of Cornstalk. The optimum temperature for bio-oil yield may
depend on the relative abundance of cellulose, hemicelluloses, and lignin in a biomass. Therefore, the composition of the biomass is a key factor affecting the efficiency of bio-oil production during conversion processes. To increase the bio-oil yield from liquefaction of cornstalk and decrease the reaction temperature, the pretreatment focused on removing compositional impediments and modifying physicochemical structural barriers. Therefore, differences in the chemical components of the unpretreated and pretreated cornstalks were studied. Figure 2 shows the differences in the main chemical composition of unpretreated cornstalk and the cornstalks pretreated with delignification times of 1.5 and 3 h by the acidchlorite method. As shown from Figure 2, the unpretreated and pretreated cornstalks
Figure 3. SEM images of unpretreated and pretreated cornstalks. (a) Unpretreated cornstalk. (b) Cornstalk pretreated at the time of 1.5 h. (c) Cornstalk pretreated at the time of 3 h.
were different in cellulose, hemicellulose, and lignin contents. Hemicellulose and lignin contents of unpretreated cornstalk and the cornstalk pretreated at the time of 1.5 h were 20.0 and 28.2% and 15.6 and 28.9%, respectively, suggesting that the delignification for 1.5 h did not affect lignin significantly. However, when the pretreatment time increased to 3 h, the lignin content of the cornstalk decreased from 28.2 to 12.2% and the hemicellulose content increased from 20.0 to 29.0%. Delignification pretreatment led to increasing the remaining hemicelluloses compared to the unpretreated cornstalk. The cellulose content of the cornstalk pretreated for 1.5 and 3 h was 50.1 and 53.3%, respectively, higher than the content of 46.4% in the unpretreated cornstalk. The delignification process was intended to disrupt the hemicelluloselignin complex. The development of pretreatment processes strong enough to separate the cell wall arrangement and mild enough to avoid a significant chemical degradation of biomass components was a challenge for today’s 10930
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Figure 4. XRD patterns of unpretreated and pretreated cornstalks. (A) Untreated; (B) 1.5 h; (C) 3 h.
chemical industry.24 For the novel pretreatment methods it is advisable to use cheap and easily recoverable chemicals and lowcost equipment. A high pretreatment yield and low energy intensive approaches are highly desired. The effect of pretreatment on the residue yield of cornstalk is given in Figure 2. These results show that the pretreatment with acidchlorite gave relatively higher yields of residue. The yields of cornstalk residue obtained by treatment at the times of 1.5 and 3 h accounted for 93.2 and 77.2%, respectively. 3.2. Scanning Electron Microscopic Observations of Cornstalk Structure. Scanning electron microscopy (SEM) was used to compare morphological changes in the cornstalk before and after pretreatment. Figure 3 shows the SEM images of the unpretreated and pretreated cornstalks. Before delignification pretreatment, the cornstalk surface was tight and smooth. However, after pretreatment it showed extensive anomalous porosity and lamellar structures, and fibers became relatively fluffy. The fibrils were separated from the initial connected structure and exposed. Thus acidchlorite pretreatment breaks down the physical structural barriers of the cornstalk and separates its chemical components with greater surface area and roughness, resulting in an increase of the accessibility of the cornstalk to ethanol and accelerating the reaction rate in the liquefaction process. 3.3. Biomass Crystallinity. Various pretreatments can change cellulose crystal structures by disrupting inter- and intrachain hydrogen bonding of cellulose fibrils.25 For biomass, it is difficult to determine the true cellulose crystallinity of the entire material including the hemicelluloses and lignin in addition to amorphous cellulose.22,26 However, X-ray measurements of the crystallinity index are still the best option to estimate their impact on biomass crystallinity. The features of cornstalk after acidchlorite pretreatment were examined using powder X-ray diffraction (XRD) and were also compared to those of the unpretreated sample. The crystallinity index (CrI) for all samples is calculated from the XRD data, and the results and the XRD spectra are shown in Figure 4. As can be seen, the unpretreated cornstalk is lowly crystalline (41.8), and after acidchlorite pretreatment there is an increase in the CrI (42.8 and 48.5). This increase in CrI after acidchlorite pretreatment suggested that the amorphous cellulose broke down significantly under the acidchlorite condition. However, this pretreatment process was unable to break apart the inter- and intrachain hydrogen bonding in cellulose fibrils.
Figure 5. Bio-oil yield obtained from liquefaction of unpretreated and pretreated cornstalks at different temperatures. (a) Residue yield at different temperatures. (b) WSO yield at different temperatures. (c) HO yield at different temperatures. (d) Bio-oil yield at different temperatures. Conditions: residence time of 0 min; 10 g of cornstalk; 100 mL of ethanol. 10931
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3.4. Effect of Pretreatment on Bio-oil Yield at Different Liquefaction Temperatures. The temperature plays an im-
portant role which influences the yield of bio-oil in the liquefaction process. To investigate the effect of acidchlorite pretreatment on bio-oil yields obtained from liquefaction of the cornstalk, the liquefaction of the three samples (unpretreated cornstalk and cornstalks pretreated at the times of 1.5 and 3 h) were performed under the same conditions such as the ratio of biomass to ethanol, heating rate, reaction temperature, and time. The experiments in pure ethanol runs were performed at temperatures and pressures of 200340 °C and 17 MPa, respectively. The maximum level is higher than the critical point of ethanol (243 °C, 6.4 MPa). Figure 5 shows the bio-oil and residue yields from liquefaction of the three samples at different temperatures. Obviously, regardless of the before and after pretreated cornstalk liquefaction runs, the yield of residue decreased continually with the temperature increased from 200 to 340 °C. Compared to the liquefaction of the unpretreated cornstalk, all pretreatments decreased the residue yield over the whole range of the temperatures tested, suggesting that the pretreatment enhanced the conversion rates (100% YRE) of cornstalk liquefaction. This might be due to the break of intra- and intermolecular chemical bonds, resulting in enhancing the surface area and in better ethanol accessibility, which increased the reaction rates. Preliminary experiments showed that the acidchlorite pretreatment led to improvement in the efficiency of cornstalk liquefaction and increased the conversion yield of bio-oil in the reaction temperatures from 200 to 320 °C. The maximum biooil yield from liquefaction of unpretreated cornstalk was 23.4% (340 °C). Comparatively, the pretreated cornstalk conversions showed a higher bio-oil yield compared to that of unpretreated cornstalk liquefaction. For example, for 1.5 and 3 h of the acidchlorite pretreatment, the maximum bio-oil yields were 31.4 and 29.4% when the reaction temperature reached 260 °C. This is because the pretreatment enhances the accessibility of ethanol and the susceptibility of cornstalk. Furthermore, due to different bio-oil formation temperatures for cellulose, hemicelluloses, and lignin, the optimum reaction temperature for bio-oil yield may depend on the relative abundances of cellulose, hemicelluloses, and lignin in the cornstalk.27 However, further increasing the reaction temperature led to a decrease in the bio-oil yield. The decrease in the bio-oil yield was mainly attributed to the conversion of bio-oil to gas and residue products by isomerization, dehydration, and fragmentation at the higher reaction temperature in the presence of ethanol.28,29 Since the higher temperature and longer pretreatment time resulted in a lower bio-oil yield and pretreatment yield, lower reaction temperature and shorter pretreatment time were more favorable. The targeted products in biomass liquefaction were composed of WSO and HO. WSO mainly consisted of simple organic acids, alcohols, furfural, sugars, etc., which are primarily formed from holocellulose via depolymerization and hydrolysis reactions.30,31 In contrast, the HO primarily composed of phenols, phenolic compounds, long-chain carboxylic acids, etc., results from the pyrolysis/hydrolysis/degradation of lignin or from the dehydration of intermediate products derived from cellulose and hemicelluloses.30,31 As shown in Figure 5, the maximum WSO yields from the liquefaction of the cornstalks pretreated at the times of 1.5 and 3 h were 19.6 (260 °C) and 22.2% (260 °C), respectively. It can be concluded that the
Figure 6. Effect of ethanol amount on bio-oil and residue yield obtained from liquefaction of unpretreated and pretreated cornstalks. (a) Effect of ethanol amount on WSO yield. (b) Effect of ethanol amount on HO yield. (c) Effect of ethanol amount on bio-oil yield. (d) Effect of ethanol amount on residue yield. Conditions: residence time of 0 min; reaction temperature of 300 °C; 100 mL of ethanol. 10932
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Table 1. GC/MS Analysis Results for the WSOs Obtained from Liquefaction of the Three Samples at 300 °C with 100 mL of Ethanol content (%) no.
a
a
RT (min)
compound
untreated
1.5 h
3h
0.2
0.2
1
4.11
toluene
1.1
2
5.34
2-pentanone, 4-hydroxy-4-methyl-
2.1
3 4
5.90 6.63
p-xylene dimethyl sulfone
2.1 0.4
0.3
1.6
1H-pyrrole-2-carboxaldehyde
formula
Mw
C7H8
92
C6H12O2
116
C8H10 C2H6O2S
106 94
C5H5NO
95
0.9
C5H5NO
95
2.5
C7H6O2
122
5
8.37
6
10.24
3-pyridinol
2.2
0.1
7
11.52
benzoic acid
4.3
8
11.57
ethyl hydrogen succinate
9
11.60
acetamide, N-butyl-
10
14.72
2-methoxy-4-vinylphenol
1.2
11 12
15.32 15.55
benzoic acid, 3-chlorophenol, 2,6-dimethoxy-
1.4 2.4
13
15.64
ethyl-α-D-glucopyranoside
1.5
18.5
14
16.73
vanillin
6.6
1.3
15
17.03
ethyl-β-D-riboside
16
17.32
D-arabinose
17
17.40
methyl-α-D-ribofuranoside
18
17.90
2-pyrrolidinecarboxylic acid-5-oxo-, ethyl ester
19 20
18.74 19.10
2-naphthalenamine, 6-nitroacexamic acid
21
19.56
3-isopropoxyalanine
5.0
C6H13O3N
147
22
20.17
3,4-altrosan
2.9
C6H10O5
162
C6H10O4
146
C4H9NO
87
0.3
C9H10O2
150
0.5 0.9
C7H5O2Cl C8H10O3
156 154
1.0 1.1
11.6
22.1
C8H16O6
208
C8H8O3
152
14.7
C7O5H14
178
3.7
C5H10O5
150
C6H12O5
164
2.3 21.6
3.9
3.3
C7H11O3N
157
5.0
2.3 6.8
C10H8O2N2 C8H15O3N
188 173
RT, residence time.
presence of high hemicelluloses and cellulose (see Figure 2) improved the formation of WSO. Lignin is one of the major constituents of biomass. It is an aromatic polymer in which the monomeric guaiacylpropane units are connected by both etheric and carboncarbon linkages. Demirbas32 studied the effect of lignin content on the liquefaction products of biomass and showed that the absolute values of the correlation coefficients between the lignin content and the HO yield indicated a strong positive correlation. The maximum HO yield of 14.4% for all the liquefaction runs was obtained when the cornstalk pretreated for 1.5 h was liquefied at 280 °C. It should be noted that longer pretreatment time resulted in lower yields of HO because of less lignin content in the pretreated cornstalk (see Figure 2). 3.5. Effect of Ethanol Amount on the Liquefaction of Unpretreated and Pretreated Cornstalks. In biomass, lignin carbohydrate complexes (LCC), which are formed by lignin covalently linked with carbohydrates through bonds such as ester and ether, prevent plant biomass degradation. Therefore, the purpose of the pretreatment is to break down LCC and increase the accessible surface area for solvent attack in the liquefaction process. In section 3.4, the promising results of increased bio-oil and decreased optimum reaction temperature were found by acidchlorite pretreatment. Solvent plays an important role in biomass liquefaction, with or without catalyst.33 To further investigate the behavior of liquefaction of the pretreated cornstalk in the presence of ethanol, the experiments of liquefaction of the unpretreated and pretreated cornstalks at different ethanol amounts have been carried out, and the results are shown in
Figure 6. The results reflected that a high amount of ethanol had a negative impact on the residue yield obtained from liquefaction of the unpretreated cornstalk, suggesting that the low amount of ethanol led to high conversion rates (100% YRE). The major roles of solvent during the liquefaction of biomass were to decompose the biomass and provide free radicals at higher temperature. The presence of free radicals could stabilize liquefaction intermediates and prevent them from forming the compounds, which are more difficult to decompose. This would enhance the bio-oil yield and inhibit the forming of residue.34 However, when the cornstalk was pretreated with acidchlorite at two different times, the yield of residue obtained at the ethanol amount of 40 mL was higher than those obtained at ethanol amounts of 20, 60, 80, and 100 mL. These different trends of residue yield between liquefactions of the unpretreated and pretreated cornstalks might be due to the surface area and the chemical components of the cornstalk (section 3.1), which resulted in the changing of liquefaction reaction types and reaction orders. The yields of both WSO and bio-oil from liquefaction of the three samples increased when ethanol was constantly added to the system. It should be noted that the addition of ethanol reduced the rate of formation of HO first, and then with the sequential increment of ethanol led to the increment of HO. The possible reason might be that the increment of ethanol contributed to the increment of reaction pressure and the density of supercritical ethanol; therefore, the compounds in the sludge and free radicals obtained from the decomposing process were extracted into ethanol and then reacted with 10933
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Table 2. GC/MS Analysis Results for the HOs Obtained from Liquefaction of the Three Samples at 300 °C with 100 mL of Ethanol content (%) no.
RT (min)
1 2
4.12 4.61
compound toluene 3-penten-2-one, 4-methyl-
Mw
untreated
1.5 h
3h
formula
14.9 3.5
12.5 3.1
17.3 3.3
C7H8 C6H10O
92 98
3
5.35
2-pentanone, 4-hydroxy-4-methyl-
14.8
17.1
16.9
C12H12O2
116
4
5.92
p-xylene
1.1
23.9
0.9
C8H10
106
5
6.32
oxime, methoxyphenyl-
C8H9O2N
151
6
6.35
cyclohexanone
7
7.68
benzene, 1,3,5-trimethyl-
8
7.77
phenol
9 10
8.19 8.94
decane cyclohexanone, 3,3,5-trimethyl-
11
9.86
undecane
1.2
12
14.55
1-hexadecene
13
16.78
hexadecanoic acid, ethyl ester
supercritical ethanol, resulting in the increment of HO.34,35 Meanwhile, ethanol as hydrogen donor, and hydrocracking of heavier molecules to light ones, was due to its penetration and hydrogen-supply abilities.36 3.6. Characteristics of the Liquid Products. In order to investigate the effect of the pretreatment on the type of organic compounds formed in the bio-oil products, the WSO and HO fractions obtained from liquefaction of the three samples at 300 °C with 100 mL of ethanol were analyzed by GC/MS. Clearly, the liquefaction products were unknown and complex mixtures of organic compounds, so no calibration of the MS detector was set, mainly due to the lack of an appropriate standard mixture for calibration. Therefore, the percentage values for individual compounds in the bio-oil do not represent the actual concentrations of these compounds. Tables 1 and 2 list the tentatively assigned compounds of the WSO and HO products, which are the most probable compounds identified by the MS search file (NIST library). The GC/MS spectrum demonstrates that the WSOs obtained from cornstalk liquefaction were composed of a variety of acids, esters, and phenol (Table 2), such as benzoic acid, 3-chlorobenzoic acid, 2,6-dimethoxyphenol, 2-pyrrolidinecarboxylic acid-5oxoethyl ester, acexamic acid, etc. By comparing the products obtained from liquefaction of various samples, it is observed that the WSO fractions strongly depend on the pretreatment by acidchlorite. As shown from Table 2, the compositional differences were relatively small among the HOs originating from various cornstalks. Phenol (e.g., toluene, 4-hydroxy-4methyl-2-pentanone, phenol, 1,3,5-trimethylbenzene) and cyclohexanone were the major components of all three HOs. In addition, there were also various amounts of p-xylene, hexadecanoic acid ethyl ester, 1-hexadecene, and decane. As a result of the disintegration of the cellulose, hemicelluloses, and lignin, the bio-oil from liquefaction is composed of different molecular structures. For example, the phenolic compounds were primarily originated from degradation of lignin, although they might also result from cellulose via hydrolysis to sugars followed by dehydration and ring closure reactions.37 The esters, alcohols, ketones, and aldehydes are probably formed from decomposition of the cellulose and hemicelluloses biomass fraction.38
3.6 32.1
C6H10O
98
0.4
C9H12
120
4.2
1.7
C6H6O
94
1.7
1.1 0.4
C10H22 C9H16O
142 140
0.9
C11H24
156
C16H32
224
C18H36O2
284
26.9 0.7
3.2 0.5
0.9
0.4
4. CONCLUSIONS The cornstalk was subjected to acidchlorite pretreatments before liquefaction in ethanol at sub- and supercritical conditions. Compared to the liquefaction of unpretreated cornstalk under the same condition, the acidchlorite pretreatment was effective for enhancing the bio-oil yield and decreasing the optimum reaction temperature, and the pretreatment time of 1.5 h produced the highest bio-oil yield at the reaction temperature of 260 °C. A high ethanol amount was demonstrated to be favorable for a high yield of bio-oil. The differences of the main chemical components between the unpretreated and pretreated cornstalks were observed; i.e., the pretreatment resulted in an increase in carbohydrate content due to the preferable removal of lignin. SEM observations showed an extensive anomalous porosity and lamellar structures in the pretreated cornstalks, thus increasing the accessible surface area for ethanol attack and resulting in a high bio-oil yield. X-ray analysis showed that acidchlorite pretreatment process did not change inter- and intrachain hydrogen bonding in cellulose fibrils. Finally, GC/MS analysis showed that the pretreatment had an important effect on the formation of various compounds in the bio-oil. ’ AUTHOR INFORMATION Corresponding Author
*E-mail: e-mail:
[email protected]. Tel.: +86-10-62336972. Fax: ++86-10-62336972.
’ ACKNOWLEDGMENT We sincerely acknowledge financial support from the State Forestry Administration (200804015), the Major State Basic Research Projects of China (973-2010CB732204), the National Natural Science Foundation of China (30930073), and the China Ministry of Education (111). ’ REFERENCES (1) Zhang, B.; Keitz, M. V.; Valentas, K. Thermochemical liquefaction of high-diversity grassland perennials. J. Anal. Appl. Pyrolysis 2009, 84, 18. 10934
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