One Step Bio-Oil Upgrading through Hydrotreatment, Esterification

Jul 8, 2009 - Anhui Province Key Laboratory of Biomass Clean Energy, University of Science and Technology of China, Hefei 230023, P.R. China. Ind. Eng...
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Ind. Eng. Chem. Res. 2009, 48, 6923–6929

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One Step Bio-Oil Upgrading through Hydrotreatment, Esterification, and Cracking Zhe Tang, Qiang Lu, Ying Zhang,* Xifeng Zhu, and Qingxiang Guo* Anhui ProVince Key Laboratory of Biomass Clean Energy, UniVersity of Science and Technology of China, Hefei 230023, P.R. China

The crude bio-oil was upgraded in supercritical ethanol under hydrogen atmosphere by using Pd/SO42-/ZrO2/ SBA-15 catalyst. This is a novel way to upgrade bio-oil with the combination of hydrotreatment, esterification, and cracking under supercritical conditions. The results indicated that the upgrading process performed effectively and the properties of the upgraded bio-oil were improved significantly. After the upgrading process, a trace amount of tar or coke was produced and most of the organic components were kept in the upgraded bio-oil. No phase separation was observed. The amount of aldehydes and ketones decreased evidently. In particular, aldehydes were almost completely removed. Most acids were converted into corresponding esters, and at the same time many new types of esters were produced. The results of TGA and DTA indicated that macromolecular compounds were decomposed and much more volatile compounds were produced after the upgrading process. The pH value and heating value of the upgraded bio-oil increased; meanwhile, the kinematical viscosity and density decreased compared to those of the crude bio-oil. 1. Introduction The energy from biomass, which is renewable, sustainable, and environmentally benign, is widely recognized as a potential future solution to the energy problems worldwide.1,2 Different from traditional combustion or slow pyrolysis processes, the biomass fast pyrolysis technique can produce 60-75 wt % of liquid bio-oil,3 with the potential to substitute petroleum. Unfortunately due to its high instability, viscosity, corrosiveness, and polarity, bio-oil has to be upgraded before it can be used as the engine fuel. Currently the most effective techniques for bio-oil upgrading are hydrotreatment4-6 and catalytic cracking.7-11 However, the yield of upgraded oils is relatively low. The process also produces a large amount of char, coke, and tar, which may result in catalyst deactivation and reactor clogging. Therefore, it is necessary to develop more efficient processes to upgrade bio-oil. Bio-oil is a complex mixture of several hundreds of organic compounds, mainly including acids, alcohols, aldehydes, esters, ketones, sugars, phenols, phenol derivatives, components with multifunctional groups, and a large proportion (20.0-30.0 wt %) of lignin-derived oligomers.12 Some of these compounds are directly related to the undesirable properties of bio-oil. For examples, aldehydes and compounds with an unsaturated carbon bond in bio-oil are active for polymerization and condensation reactions.13 These reactions result in increasing viscosity and phase separation in the bio-oil. Carboxylic acids, such as acetic acid, make bio-oil very corrosive and extremely severe at elevated temperature,14 which imposes more requirements on construction materials of the vessels and the upgrading process before using bio-oil in transport fuels. The acids in bio-oil also promote adol reaction to accelerate bio-oil aging and the properties decline.13 Hydrotreatment is an effective way to convert aldehydes and unsaturated compounds into some more stable ones, but it requires more severe conditions such as higher temperature and hydrogen pressure to deal with acids, which is not economical and energy efficient.15 Esterification should be * To whom correspondence should be addressed. Tel.: 86-551-3603463. Fax: 86-551-360-6689. E-mail: [email protected] and [email protected].

an effective way to treat acids under milder conditions. Except for unsaturated compounds and acids, a large proportion (20.0-30.0 wt %) of lignin-derived oligomers is consisted in bio-oil, which makes bio-oil upgrading more difficult. Ligninderived oligomers cannot be vaporized. At 100 °C or more, they rapidly react and produce a tar and eventually a solid residue. However, the energy density of pure lignin-derived oligomers is much higher than that of bio-oil, so simply removing ligninderived oligomers from bio-oil will cause bio-oil heating values to significantly decrease. Shabtai et al.16 reported that supercritical ethanol can help solubilize the lignin into monomers. Thus, supercritical ethanol conditions might also be applied to upgrade lignin-derived oligomers in bio-oil. Whatever bio-oil upgrading process is used, it is imperative to find suitable catalysts. A combination of the large pore dimensions of mesoporous materials with the strong acid sites would be highly advantageous, leading to a novel and probably useful catalytic material. Bio-oil contained some macromolecular compounds such as napthalene, guaiacol, and lignin-derived oligomers.17 During upgrading, these compounds were easy to cover the active sites or block the pore of the catalyst, such as zeolites, and make the catalyst deactivate. SBA-15, a new-style mesoporous silica material, has outstanding properties compared with other silica materials, such as easy synthesis, adjustable pore size, thick pore walls, and notable hydrothermal stability.18 However, SBA-15 is a pure silica material and has limited catalytic activity.19,20 Strong acid sites are necessary for esterification of bio-oil and are expected to affect the cracking of the high molecular weight molecules of bio-oil. Incorporating superacid SO42-/ZrO2 with SBA-15 could be an effective way to generate acid sites on the surface of SAB-15.21,22 On the basis of the above considerations, it could be a promising way to upgrade bio-oil by combining hydrotreatment, esterification, and cracking under supercritical conditions. In this paper, upgrading of crude bio-oil was carried out in supercritical ethanol with hydrogen present. Ethanol acted as both a reaction medium and reactant in the supercritical upgrading process. Palladium supported on SO42-/ZrO2/SBA15 was developed for hydrotreatment, esterification, and cracking. Herein, palladium was chosen due to its high hydrotreating

10.1021/ie900108d CCC: $40.75  2009 American Chemical Society Published on Web 07/08/2009

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Table 1. Detailed Operational Conditions of Different Experiments experiment

catalyst

PH2a (MPa)

Prb (MPa)

Pfc (MPa)

1 2 3

0.2 PdSZr + 1.8 g of SZr no catalyst 2.0 g of SZr

2.0 0 0

10.5 8.5 8.5

2.0 0.5 1.0

a Initial hydrogen pressure. room temperature.

b

Pressure at 280 °C.

c

Final pressure at

capability. The textural properties of the catalysts were characterized by N2 adsorption and X-ray diffraction (XRD). The changes in organic components of bio-oil before and after upgrading were compared and analyzed in detail to elucidate the effect of this novel upgrading process. Figure 1. N2 adsorption/desorption isotherms of the samples: SBA-15, ZrO2/ SBA-15, SO42-/ZrO2/SBA-15, and Pd/SO42-/ZrO2/SBA-15.

2. Experimental Section 2.1. Materials. Palladium(II) nitrate hydrate (99.9%) was purchased from Strem Chemicals. Triblock copolymer EO20PO70EO20 (P123) was purchased from Aldrich. All other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used as received without further purification. The bio-oil investigated in this study was made in our laboratory. It was made through the flash pyrolysis of rice husk at about 550-600 °C. The details are given in ref 23. 2.2. Catalysts. SBA-15 was prepared according to the ref 24 as follows: 4.0 g of template, triblock copolymer EO20PO70EO20 (P123), was dispersed in 30.0 g of deionized water and 25.0 mL of 2.0 M HCl solution at 40 °C with stirring for 1 h, followed by addition of 18 mL of tetraethyl orthosilicate (TEOS). The mixture was stirred at 40 °C for 24 h and then crystallized in a Teflon-lined autoclave at 100 °C for 12 h. Afterward the solid product was filtered, washed with deionized water, and dried for 4 h in a desiccator at 100 °C. Finally, the sample was calcined at 550 °C in air for 5 h. ZrO2/SBA-15 and SO42-/ZrO2/SBA-15 catalysts were prepared via a two-step wetness impregnation method according to ref 25. A 16.6 g amount of zirconium nitrate pentahydrate and 5.7 g of cetyltrimethylammonium bromide (CTAB) were dispersed in anhydrous ethanol under stirring, followed by addition of 2.1 g of SBA-15. Herein, CTAB was used as a template in order to obtain materials with more ordered structure. After 3 h, the solid product was filtered and crystallized at 100 °C in a Teflon-lined autoclave under ethanol vapor atmosphere for 12 h. The obtained solid product was dried and designated as A. If A was calcined at 550 °C in air for about 3 h without other treatment, ZrO2/SBA-15 was obtained. If 3.0 g of A was further treated by 45 mL of 1 M H2SO4 solution for 3 h and then calcined at 550 °C for 3 h, SO42-/ZrO2/SBA-15 (designated as SZr) was obtained. Pd/SO42-/ZrO2/SBA-15 catalyst was prepared through the dry impregnation method. A 0.1 g amount of palladium(II) nitrate hydrate was impregnated in 1.5 g of SZr. The dried Pd(NO3)2/ SZr was reduced in hydrogen and nitrogen atmosphere at 280 °C for 3 h using a heating rate 1.0 °C/min to obtain the ultimate catalyst Pd/SO42-/ZrO2/SBA-15 (designated as PdSZr). The flow

Figure 2. X-ray diffraction patterns of SBA-15, ZrO2/SBA-15, and SO42-/ ZrO2/SBA-15; the inset shows X-ray diffraction of Pd/SO42-/ZrO2/SBA15.

rate of hydrogen and nitrogen was 10 and 100 mL/min, respectively. 2.3. Characterization of Catalysts. X-ray diffraction (XRD) analysis was conducted on a Philips X’Pert PROS X-ray diffractometer using Cu KR radiation. The data were recorded over 2θ ranges of 0.5-5° and 10-70°. Nitrogen adsorption-desorption isotherms were measured by a Micromeritics ASAP 2020 system. The Brunauer-EmmettTeller (BET) equation was used to calculate the surface area in the range of relative pressures between 0.0 and 0.2. The pore size distributions were calculated from the adsorption and desorption branches of the isotherms using the thermodynamicbased Barrett-Joyner-Halenda (BJH) method. The total pore volume was determined from the adsorption and desorption branches of the nitrogen isotherms at P/P0 ) 0.97. The chemical compositions of catalysts were measured by a Perkin-Elmer Analyst 800 atomic absorption spectrometer. 2.4. Experimental Procedure. The crude bio-oil used in this study is comprised of 33.0 wt % original bio-oil made in our lab and 67.0 wt % anhydrous ethanol. Three bio-oil upgrading experiments were carried out, and the detailed procedure is as follows. About 55.0 g of crude bio-oil and a certain amount of catalyst were filled into a 150.0 mL autoclave, and then the air in the autoclave was driven out by hydrogen. H2 (0-2.0 MPa)

Table 2. Textural Properties and Composition of Catalysts sample SBA-15 ZrO2/SBA-15 SO42-/ZrO2/SBA-15 Pd/SO42-/ZrO2/SBA-15 a

BET surface area (m2/g) micropore area (m2/g) pore volume (cm3/g) average pore diameter (nm) 828.2 217.6 218.1 218.3

100.8 23.1 17.4 15.6

1.15 0.24 0.24 0.28

6.5 4.9 5.0 5.6

The molar ratio of Zr and Si and the weight percentage of Pd were obtained from atomic absorption analysis.

compositiona pure silica Zr/Si:1.45 Zr/Si:1.45 Zr/Si:1.45, WPd ) 2.61 wt %

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Table 3. Properties of Crude Bio-Oil and Upgraded Bio-Oil bio-oils

water (wt %)

ethanol (wt %)

oila (wt %)

heating valueb (MJ/kg)

viscosity (mm2/s)

density (g/mL)

pH value

original bio-oil crude bio-oilc NoCat-oil SZr-oil PdSZr-oil

30.0 10.2 29.9 17.3 16.2

0.0 67.0 63.5 70.7 65.8

70.0 22.8 6.6 12.0 18.0

17.4 17.6 6.2 14.7 20.1

13.2 2.3 1.6 1.5 1.6

1.18 0.92 0.89 0.84 0.84

3.2 4.3 5.5 5.3 4.7

a The weight percentage of oil except water and ethanol. bio-oil and 67 wt % anhydrous ethanol.

b

Heating value without ethanol.

was injected into the autoclave at room temperature. Subsequently, it was heated to 280 °C and the reaction was carried out at this temperature for 3 h. The pressure of the system ranged from 8.5 to 10.5 MPa. After cooling to room temperature, the pressure was recorded and then the gas in the autoclave was

c

The crude bio-oil was a mixture of 33 wt % original

discharged without content analysis due to the equipment limitation. The autoclave was opened, and the upgraded biooil was taken out for property analysis without any further treatment. The respective experimental conditions are shown in Table 1.

Figure 3. Comparison of GC-MS spectra of crude bio-oil and upgraded bio-oil: (a) 2.0-13.5, (b) 13.5-30.0 min, and (c) 30.0-58.0 min.

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For convenience of description, the upgraded bio-oil obtained from the corresponding experiment was designated as NoCatoil, SZr-oil, and PdSZr-oil. 2.5. Characterization of Products. The organic components of crude bio-oil and upgraded bio-oils were analyzed by gas chromatography-mass spectrometry (GC-MS) (Thermo Trace DSQ (I)). Separation was using a 30 m × 0.25 mm × 0.25 µm TR-5MS fused-silica capillary column. The injector temperature was 280 °C in split mode. Nitrogen was the carrier gas. The GC operating conditions were as follows: oven temperature was held at 40 °C for 3.0 min and then heated to 180 °C at a rate of 4.0 °C/min; finally, it was heated to 280 °C at a rate of 10.0 °C/min and held at this temperature for 10 min. The ethanol content in the crude and upgraded bio-oils was measured by gas chromatography (GC1690, Kexiao, China) using n-propanol as an internal standard. A 30 m × 0.25 mm × 0.25 µm fused-silica capillary column (OV1701, China) was used for separation. The injector temperature was 250 °C in split mode. Nitrogen was the carrier gas. The GC operating conditions were as follows: oven temperature was held at 40 °C for 5.0 min and then heated to 250 °C at a rate of 10.0 °C/ min and held at this temperature for 5.0 min. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of the crude and the upgraded bio-oil were carried out over the range 25-600 °C at a rate of 10.0 °C/min under nitrogen or air using a DT-60H Thermogravimetric Analyzer, and the gas flow rate was 50 mL/min. The heating value, density, pH value, water content, and kinematical viscosity were measured according to ref 26. 3. Results and Discussion 3.1. Catalyst Properties. The chemical composition, BET surface area, micropore area, pore volume, and average pore diameter of SBA-15, ZrO2/SBA-15, SO42-/ZrO2/SBA-15, and Pd/SO42-/ZrO2/SBA-15 catalysts are given in Table 2. These texture parameters were calculated from nitrogen adsorption and atomic absorption measurements. From Table 2 it can be seen that after incorporation of ZrO2, the surface area, pore volume, average pore diameter, and micropore area of SBA-15 decreased evidently, which implied that some pores of SBA-15 were blocked by ZrO2 crystallites. This result was not expected since the molar ratio of Zr/Si was as high as 1.45. According to Table 2, incorporation of H2SO4 slightly affected the textural properties of ZrO2/SBA-15 support. The surface area did not change after further incorporation of 2.61 wt % Pd, but the pore volume and average pore diameter increased significantly, which could be due to reduction of part of ZrO2 crystallites during the Pd reducing process. N2 adsorption-desorption isotherms of all catalysts are shown in Figure 1. The isotherm of SBA-15 illustrated a clear H1type hysteresis loop in the relative pressure range between 0.6 and 0.8, implying this material had very regular mesoporous channels. The other three catalysts exhibited similar N2 adsorption-desorption isotherms as the SBA-15, suggesting that the ordered structure after the postincorporation processes was retained. The XRD patterns of SBA-15, ZrO2/SBA-15, and SO42-/ ZrO2/SBA-15 are shown in Figure 2. In the low-angle region, the XRD pattern of SBA-15 exhibited one very intense peak at 2θ ) 0.9° and two weak peaks at 2θ ) 1.7° and 1.8°, which could be indexed to (100), (110), and (200) diffraction peaks, representing a 2D hexagonal mesostructure with space group P6mm.27 The peak intensity of the SO42-/ZrO2/SBA-15 decreased significantly, and it was hard to define the diffraction

Figure 4. Comparison of typical compounds between crude bio-oil and upgraded bio-oil.

peaks of ZrO2/SBA-15. Since the structure of the mesoporous SBA-15 was intact after modification, the decreasing of the diffraction peaks should be caused by blocking of the mesoporous channels with ZrO2 crystallites, which is in agreement with the nitrogen adsorption results. In the high-angle region of ZrO2/SBA-15 there were four diffraction peaks at 2θ ) 30.2°, 35.2°, 50.3°, and 59.8°, indexed as the (101), (110), (200), and (211) reflections of the tetragonal ZrO2 phase, respectively. The XRD diffraction peaks of SO42-/ ZrO2/SBA-15 were almost the same as those of ZrO2/SBA-15, indicating that H2SO4 incorporation did not affect the shape and size of ZrO2 crystallites. With regard to the high-angle XRD patterns of Pd/SO42-/ZrO2/SBA-15, except for the four peeks attributed to the ZrO2 crystallite, one low and broad peak at 2θ ) 40.2° could be attributed to the Pd crystallite. The crystallite size of Pd could be calculated according to the Scherrer formula, which was about 3.7 nm. 3.2. Comparison of Crude and Upgraded Bio-Oils. 3.2.1. General Properties of Bio-Oils. General properties of crude bio-oil and upgraded bio-oils are listed in Table 3. It is necessary to mention that the heating values of the crude and upgraded bio-oils were derived from the measured data by deducting the contribution of ethanol. After upgrading, the pH value of bio-oils was increased while the kinematical viscosity and density were decreased. 3.2.1.1. NoCat-Oil. The NoCat-oil was obtained by upgrading the crude bio-oil under supercritical conditions without hydrogen and any catalyst presence. The final pressure in the autoclave was built up to 0.5 MPa, suggesting that a certain amount of gas was produced. Phase separation was observed, with bio-oil on the top and a large amount of tar and coke on the bottom. The bio-oil contained 29.9 wt % water and 6.6 wt % oil content, indicating that many components in the crude bio-oil were polymerized and carbonized or cracked to gas-phase compounds during the upgrading process. This may explain why the heating value of the obtained bio-oil was as low as 6.2 MJ/kg. Therefore, supercritical ethanol itself could not upgrade crude bio-oil effectively. 3.2.1.2. SZr-Oil. The SZr-oil was obtained by adding SZr as catalyst under same conditions as the NoCat-oil. The final pressure in the autoclave was built up to 1.0 MPa, which was higher than that of the NoCat process. No phase separation was observed in the SZr-oil. Small but significant amount of tar and coke was formed. These results indicated that the superacidic SZr catalyst could promote the deep cracking and somehow inhibit the polymerization and condensation reactions. The heating value of the SZr-oil was still lower than that of the crude bio-oil. It demonstrated that SZr was not an effective catalyst for bio-oil upgrading under supercritical conditions.

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Table 4. Major Organic Component and Relative Content of Crude Bio-Oil and Upgraded Bio-Oil with Catalyst PdSZr crude bio-oil

area %

PdSZr-oil

area %

acids acetic acid palmitic acid octadec-9-enoic acid stearic acid

13.7 0.5 0.5 0.1

acetic acid

1.5

esters methyl 2-oxopropanoate 2-oxopropyl acetate

0.3 1.0

ethyl acetate ethyl propionate ethyl 2-hydroxyacetate ethyl butyrate ethyl 2-hydroxypropanoate ethyl but-2-enoate ethyl 2-ethoxyacetate ethyl 4-oxopentanoate ethyl 2-methyl-4-oxopentanoate diethyl succinate diethyl 2-methylsuccinate ethyl palmitate ethyl octadec-9-enoate ethyl furan-2-carboxylate

10.5 2.7 0.9 1.4 1.6 0.8 1.2 2.4 1.1 1.8 1.1 0.7 0.8 0.4

1.8 3.8 3.4 2.9 1.9 1.0 1.4 0.4 1.0 1.0

phenol 2-methoxyphenol 4-ethylphenol 2-methoxy-4-methylphenol 4-ethyl-2-methoxyphenol 2,6-dimethoxyphenol 4-ethylbenzene-1,2-diol

1.4 2.5 3.8 2.0 1.7 0.9 0.6

4.1 2.5 1.9 1.5

5-ethylfuran-2-carbaldehyde

0.2

2.8 0.7 1.1 0.6 0.6 1.20 0.35 0.8 3.38 0.92 0.34

cyclopent-2-enone 5-methyldihydrofuran-2(3H)-one 2-methylcyclopentanone 2-methylcyclopent-2-enone 2,3-dimethylcyclopent-2-enone 1-(furan-2-yl)ethanone hexane-2,5-dione heptane-2,5-dione

0.3 0.4 0.3 1.7 0.5 0.3 0.2 0.2

2.06 0.13

2-ethoxyethanol 4-ethoxybutan-2-ol pentan-2-ol 1,1-diethoxyethane 1,1-diethoxypropane 2-methoxy-1,3-dioxolane 2-methoxy-1,3-dioxolane

0.2 1.0 0.6 1.2 5.5 0.4 1.9

phenols phenol 2-methoxyphenol 4-ethylphenol 2-methoxy-4-methylphenol 4-ethyl-2-methoxyphenol 2-methoxy-4-vinylphenol 2-methoxy-4-(prop-1-enyl) phenol 4-allyl-2,6-dimethoxyphenol 4-methylbenzene-1,2-diol 4-ethylbenzene-1,2-diol aldehydes furfural 4-methylbenzaldehyde 4-hydroxy-3-methoxybenzaldehyde 3-(4-hydroxy-3,5-dimethoxyphenyl) acrylaldehyde ketones 1-hydroxypropan-2-one 1-hydroxybutan-2-one butan-2-one 3-methylfuran-2(3H)-one 4-methylfuran-2(5H)-one furan-2(5H)-one 1-(furan-2-yl)ethanone 2-methylcyclopent-2-enone 2-hydroxy-3-methylcyclopent-2-enone 1-(4-hydroxy-3-methoxyphenyl) propan-2-one 4-(4-hydroxy-3-methoxyphenyl) butan-2-one others propanol 7,8-dimethoxy-1,2,3,9b-tetrahydrodibenzo[b,d] furan-4(4aH)-one

3.2.1.3. PdSZr-Oil. The PdSZr-oil was obtained by using our designed Pd/SO42-/ZrO2/SBA-15 catalyst in supercritical ethanol under hydrogen atmosphere. The final pressure in the autoclave was same as the initial one. However we could not exclude the possibility that the production of carbonaceous gas was same as the consumption of H2, since the gas components were not analyzed. After upgrading process, trace amount of tar or coke was formed. It indicated that introducing hydrogen and hydrotreating catalyst to the bio-oil upgrading process could effectively inhibit the polymerization and condensation reactions. The heating value was significantly higher in the PdSZr-oil than

that in the crude bio-oil. These results suggested that the PdSZr process was an effective way for bio-oil upgrading. 3.2.2. Comparison of GC-MS Chromatogram of Crude Bio-Oil and PdSZr-Oil. To reveal the PdSZr-oil quality improvement from the molecular level, detailed component changes of the bio-oils were investigated by GC-MS. The comparison of the GC-MS spectra of crude and upgraded bio-oil is presented in Figure 3. The total analytical time of GC-MS was 58.0 min. For easy comparison and to display all peaks clearly, the whole chromatograms are shown separately

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Figure 5. TG/DTA plots of crude bio-oil and upgraded bio-oil: (a) TG in air and nitrogen and (b) DTA in air.

in Figure 3a (2.0-13.5 min), 3b (13.5-30.0 min), and 3c (30.0-58.0 min). It can be seen in Figure 3a that the components in the crude bio-oil were different from the ones in the PdSZr-oil. Small molecular weight components of acids, aldehydes, ketones, and furans were the major components of the crude bio-oil. After the upgrading process, acids, aldehydes, furans, and some of the ketones disappeared. Esters and a small amount of ethers and ketones were produced and became the dominant components in the upgraded bio-oil. Evidently, ethyl acetate was converted from acetic acid through esterification in the existence of ethanol with the PdSZr catalyst. Other esters may be produced from some other compounds through quite complicated pathways. The mechanism of the reaction is still under investigation. From Figure 3b it can be seen that the types of phenols decreased after upgrading. Furthermore, the unsaturated double bonds at the substituted groups of phenols, such as 4-allyl-2methoxyphenol and 2-methoxy-4-(prop-1-enyl) phenol in the crude bio-oil, disappeared in the upgraded bio-oil. Simultaneously, the amount of phenols with corresponding saturated substituted groups, such as 2-methoxy-4-propyl-phenol, increased. Apparently, double bonds were reduced through hydrotreating bio-oil in a hydrogen atmosphere with catalyst PdSZr. At the same time, some new esters were produced after upgrading. Figure 3c shows similar results as Figure 3a and 3b. After the upgrading process, the acids in crude bio-oil were converted into esters and the double bonds at the substituted groups of phenols were saturated.

It is worth mentioning that when another GC-MS instrument (Thermo Trace DSQ (II)) equipped with a TR-35MS fusedsilica capillary column was used to compare the bio-oils, it was observed that sugars which could be detected in the crude biooil almost completely disappeared after upgrading. The results are not shown here. It is known that the GC/MS technique could not give the quantitative analysis of the compounds. However, the chromatographic peak area of a compound is considered linear with its quantity, and the peak area % is linear with its concentration. Therefore, the corresponding chromatographic peak area % can be compared to show the changing of its relative content in bio-oils. All of the organic components of crude bio-oil and PdSZr-oil obtained from GC-MS analysis were classified, and their relative contents were added together and are shown in Figure 4. Major organic components and relative contents are given in Table 4. As can be seen from Figure 4 and Table 4, crude bio-oil was mainly comprised of acids, phenols, esters, aldehydes, and ketones. After the upgrading process, most of the acids were converted into esters. The percentage of acids peak areas decreased from 14.8% to 1.5%. Meanwhile, the percentages of esters peak areas increased from 1.5% to 30.4%. The top three esters in upgraded bio-oil were ethyl acetate (10.5%), ethyl propionate (2.7%), and ethyl 4-oxopentanoate (2.4%). Furthermore, the type of ester increased from 2.0 to 25.0. The percentages of peak areas of phenols, aldehydes, and ketones decreased after the upgrading process. In particular, the percentages of peak areas of aldehydes decreased from 10.0% to 0.2%, indicating that most of the aldehydes were reduced or removed. The ketones changed from 13.3% to 5.0%. This result indicated ketones were more difficult to be reduced or removed compared with aldehydes. 3.2.3. TGA/DTA of Crude Bio-Oil and PdSZr-Oil. The experimental data of TGA and DTA are presented in Figure 5a and 5b. It can be seen in Figure 5a that the thermogravimetric behavior of the bio-oils was similar in nitrogen as in air below 400 °C. The change of the TG curve centralized below 100 °C, and in this range, the weight loss rate of the upgraded bio-oil was faster than that of the crude bio-oil. It should be due to the volatilization of ethanol, water, and other light components. At 100 °C in a nitrogen atmosphere, the residue of crude bio-oil and upgraded bio-oil was about 19.7 and 4.8 wt %, respectively. According to Table 3, except water and ethanol, the weight percentage of crude bio-oil and PdSZr-oil are 22.8 and 18.0 wt %, indicating that much more components in upgraded bio-oil were evaporated under 100 °C than those in crude bio-oil under the TG measured conditions. Therefore, after the upgrading process, much more volatile light components were produced, which is consistent with the results obtained from GC-MS analysis. More importantly it can be seen that there is a clear exothermic peak at about 500 °C in the DTA curve of the crude bio-oil in Figure 5b, which does not exist in that of the upgraded bio-oil. The exothermic peak is characteristic of the burning of char residues. Therefore, no char residue was produced from the upgraded bio-oil during analysis. Since the char residue was mainly formed from the large and nonvolatile compounds and trace amount of tar and coke were produced during the upgrading process, it is believed that the large and nonvolatile compounds were converted to smaller and more volatile ones during the upgrading process and were retained in the upgraded bio-oil.

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4. Conclusions In this study, bio-oil was upgraded by one-step hydrotreatment, esterification, and cracking in supercritical ethanol using PdSZr catalyst. A trace amount of tar or coke was produced during the upgrading process, and most of the organic components were kept in the upgraded bio-oil. The quality of the biooil was improved evidently. The pH value and heating value of the upgraded bio-oil increased; meanwhile, the kinematical viscosity and density decreased. Aldehydes and ketones were reduced to stable compounds by hydrotreating. A large amount of esters was converted from acids and some other compounds in crude bio-oil through esterification and became dominant components of the upgraded bio-oil. Larger molecular weight compounds were decomposed and retained in the upgraded biooil. Hydrogen atmosphere and hydrotreating catalyst supported on materials with acidic sites and large pore size were imperative to get satisfactory results for bio-oil upgrading under supercritical conditions. This bio-oil upgrading system can effectively inhibit polymerization and condensation reactions and thus inhibit the formation of tar or coke. Currently, more experiments are underway to explore different multifunctional catalysts. A detailed bio-oil upgrading mechanism is also under investigation. In general, very complicated reactions happened during bio-oil upgrading under selected conditions. More work needs to be done to optimize the bio-oil upgrading process. Acknowledgment We gratefully acknowledge the financial support provided by the National Nature Science Foundation of China 200806077, Knowledge Innovation Program of Chinese Academy of Science (KGCX2-YW-3306), and National Basic Research Program of China (2007CB210205). Literature Cited (1) Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek, G.; Cairney, J.; Eckert, C. A.; Frederick, J. W. J.; Hallett, J. P.; Leak, D. J.; Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T. The Path Forwards for Biofuels and Biomaterials. Science 2006, 311, 484– 489. (2) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of Transportation Fuels from Biomass: Chemistry, catalysts, and engineering. Chem. ReV. 2006, 106 (9), 4044–4098. (3) Mohan, D.; Pittman, C. U.; Steele, P. H. Pyrolysis of Wood/Biomass for Bio-oil: A Critical Review. Energy Fuels 2006, 20, 848–889. (4) Love, G. D.; Snape, C. E.; Carr, A. D.; Houghton, R. C. Release of Covalently-bound Alkane Biomarkers in High Yields from Kerogen via Catalytic Hydropyrolysis. Org. Geochem. 1995, 23, 981–986. (5) Senol, O. I.; Viljava, T. R.; Krause, A. O. I. Hydrodeoxygenation of Methyl Esters on Sulphided NiMo/γ-Al2O3 and CoMo/γ-Al2O3 Catalysts. Catal. Today 2005, 100, 331–335. (6) Zhang, S. P.; Yan, Y. J.; Li, T. C.; Ren, Z. W. Upgrading of Liquid Fuel from the Pyrolysis of Biomass. Bioresour. Technol. 2005, 96, 545– 550. (7) Adjaye, J. D.; Bakhshi, N. N. Production of Hydrocarbons by Catalytic Upgrading of a Fast Pyrolysis Bio-oil. Part l: Conversion over Various Catalyst. Fuel Process. Technol. 1995, 45, 161–183.

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ReceiVed for reView January 21, 2009 ReVised manuscript receiVed June 22, 2009 Accepted June 23, 2009 IE900108D