Energy & Fuels 2006, 20, 2717-2720
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Upgrading Bio-oil over Different Solid Catalysts Qi Zhang,*,†,‡ Jie Chang,† TieJun Wang,† and Ying Xu† Guangzhou Institute of Energy ConVersion, Chinese Academy of Sciences, Guangzhou 510640, China, and Department of Thermal Science and Energy Engineering, UniVersity of Science and Technology of China, Hefei 230027, China ReceiVed May 18, 2006. ReVised Manuscript ReceiVed August 24, 2006
Solid acid 40SiO2/TiO2-SO42- and solid base 30K2CO3/Al2O3-NaOH were prepared and compared with catalytic esterification activity according to the model reaction. Upgrading bio-oil by solid acid and solid base catalysts in the conditioned experiment was investigated, in which dynamic viscosities of bio-oil was lowered markedly, although 8 months of aging did not show much viscosity to improve its fluidity and enhance its stability positively. Even the dehydration by 3A molecular sieve still kept the fluidity well. The density of upgraded bio-oil was reduced from 1.24 to 0.96 kg/m3, and the gross calorific value increased by 50.7 and 51.8%, respectively. The acidity of upgraded bio-oil was alleviated by the solid base catalyst but intensified by the solid acid catalyst for its strong acidification. The results of gas chromatography-mass spectrometry analysis showed that the ester reaction in the bio-oil was promoted by both solid acid and solid base catalysts and that the solid acid catalyst converted volatile and nonvolatile organic acids into esters and raised their amount by 20-fold. Besides the catalytic esterification, the solid acid catalyst carried out the carbonyl addition of alcohol to acetals. Some components of bio-oil undertook the isomerization over the solid base catalyst.
1. Introduction Biomass, despite containing a low carbon content, is clean, because biomass has a negligible content of sulfur, nitrogen, and ash, which gives a lower emission of SO2, NOx, and soot than that of conventional fossil fuels. Besides, zero emission of CO2 can be achieved because CO2 released from biomass will be resolved into the plants by photosynthesis quantitatively. Biomass fast pyrolysis for bio-oil production has aroused great attention and interest extensively in recent years for the excess consumption of fossil fuels and high efficiency of the biomass pyrolysis technique. Energy crisis and fuel tension make the biomass fast pyrolysis a more important area of research.1,2 Biooil from biomass fast pyrolysis is mainly produced from biomass residues in the absence of air, atmospheric pressure, a low temperature (450-550 °C), high heating rate (103-104 °C/s), and short gas residence time, to crack into short-chain molecules and be cooled to liquid rapidly. However, bio-oil has deleterious properties of high viscosity, thermal instability, corrosiveness, and chemical complexity, which set up many obstacles to their applications. The recent upgrading techniques are hydrodeoxygenation,3-53-5 catalytic cracking of pyrolysis vapors,66 * To whom correspondence should be addressed. Telephone: 086-02087057787. Fax: 086-020-87057789. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ University of Science and Technology of China. (1) Czernik, S. R.; Bridgwater, A. V. Overview of applications of biomass fast pyrolysis oil. Energy Fuels 2004, 18, 590-598. (2) Bridgwater, A. V.; Peacocke, G. V. C. Fast pyrolysis processes for biomass. Renewable Sustainable Energy ReV. 2000, 4, 1-73. (3) Pindoria, R. V.; Lim, J. Y.; Hawkes, J. E. et al. Structural characterization of biomass pyrolysis tars/oils from eucalyptus wood wastes: Effect of H2 pressure and samples configuration. Fuel 1997, 76, 1013-1023. (4) Pindoria, R. V.; Megaritis, A.; Herod, A. A. et al. A two-stage fixedbed reactor for direct hydrotreatment of volatiles from the hydropyrolysis of biomass: Effect of catalyst temperature, pressure and catalyst ageing time on product characteristics. Fuel 1998, 77, 1715-1726.
emulsification,7-97-9 steam reforming,1010 extracting chemicals from the bio-oils, etc. In this paper, solid acid and solid base catalysts were prepared and compared with upgrading the biooil properties by catalytic esterification and the mechanism involved in it was investigated. 2. Experimental Section 2.1. Catalysts Preparation. Titanium solution was prepared by stirring the mixture of Ti(OBu)4, EtOH, H2SO4, and H2O in some proportion, and TiO2 powder was obtained after aging, drying, and milling the titanium gel. Mixing SiO2 (Degussa A-380) with TiO2 powder by 40 wt % in ethanol, impregnating in H2SO4 (1 M), and being calcined in 400 °C, the solid acid catalyst 40STS were prepared. Milling the mixture of K2CO3 and Al2O3 by 30 wt % together, impregnating in NaOH (1 M) solution, and drying, the solid base catalyst 30KAN was prepared after the calcination in 500 °C under the protection of nitrogen atmosphere. 2.2. Esterification. The model reaction of esterification with ethanol and acetic acid by a molar ratio of 2.5:1 was carried out in the three-neck flask equipped with a thermometer, reflux condenser, and magnetic stirrer. The catalyst was used by 5 wt % of the reaction solution, which was sampled at a 20 min interval and (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) Nokkosmaki, M. I.; Kuoppala, E. T.; Leppamaki, E. A. et al. Catalytic conversion of biomass pyrolysis vapours with zinc oxide. J. Anal. Appl. Pyrolysis 2000, 55, 119-131. (7) Chiaramonti, D.; Bonini, M.; Fratini, E. et al. Development of emulsions from biomass pyrolysis liquid and diesel and their use in engines. Part 1: Emulsion production. Biomass Bioenergy 2003, 25, 85-99. (8) Chiaramonti, D.; Bonini, M.; Fratini, E. et al. Development of emulsions from biomass pyrolysis liquid and diesel and their use in engines. Part 2: Tests in diesel engines. Biomass Bioenergy 2003, 25, 101-111. (9) Ikura, M.; Stanciulescu, M.; Hogan, E. Emulsification of pyrolysis derived bio-oil in diesel fuel. Biomass Bioenergy 2003, 24, 221-232. (10) Takanabe, K.; Aika, K.; Seshan, K. et al. Sustainable hydrogen from bio-oil-steam reforming of acetic acid as a model oxygenate. J. Catal. 2004, 227, 101-108.
10.1021/ef060224o CCC: $33.50 © 2006 American Chemical Society Published on Web 10/14/2006
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Figure 1. Catalytic activity of solid catalysts in the model reaction. Table 1. Acetic Acid Conversion over Different Catalysts in Partial Reflux Esterification catalysts
no catalyst (%)
solid acid/ 40STS (%)
solid base/ 30KAN (%)
acetic acid conversion
15
100
23.7
measured quantitatively by NaOH standard solution titration and analysis by GC2010 (Shimadzu, FID, N2 carrier gas, column: DB1HT 30 m × 0.25 mm × 0.1 µm). By the titration method, the acetic acid conversion was calculated by applying the equation: conversion ) (1 - V/V0) × 100%, in which V and V0 are the volumes of the standard NaOH solution consumed in neutralizing 0.5 mL solution sampled in the process and at the beginning of the reaction to change the phenolphthalein indicator pink, respectively. The conversion was also measured by the internal standard method through GC2010, with isoamyl acetate as the internal standard, and the results of the measurements were kept consistent mutually. 2.3. Characterization of Bio-oil. The dynamic viscosity was measured with WFY-108D viscometers (GB/T265-88), and gross calorific value was measured using a WGR-1 calorimetric bomb (ASTM D4809). The water content was determined by the Karl Fischer titration (ASTM D1744, GB11146-89), which was performed by using Metrohm 787 KF Titrino. The acidity was evaluated by a PHC-3C precision pH-meter from the Shanghai REX Instrument Factory. Gas chromatography-mass spectrometry (GC-MS; GC, HP5890; MSD, HP5972A) was conducted to analyze the composition of biooil. The separation was realized on a column of DB-5MS, 30 m × 0.25 mm × 0.25 µm, and the oven temperature program was 40 °C (holding for 5 min) at 6 °C/min to 295 °C (holding for 10 min).
3. Results and Discussion 3.1. Catalytic Activity of the Solid Acid and Solid Base Catalysts on Model Esterification. First, in partial reflux esterification with only the reflux in 20 min, the conversion of acetic acid in 1 h was shown in Table 1. The solid acid catalyst nearly converted all of the acetic acid into esters and showed higher catalytic activity, and the solid base catalyst exhibited better activity than no catalyst, although it was not as high as the solid acid catalyst. To clarify the relation of the conversion with time, total reflux esterification was carried out. The conversion of acetic acid in the total reflux reactor of catalytic esterification over different catalysts was described in Figure 1. The solid acid catalyst 40STS presented high activity, and acetic acid conversion rose with time remarkably and reached 84% in 80 min, approaching the 88% of equilibrium conversion. The solid acid catalyst could accelerate the esterification greatly; however, the conversion over the solid base catalyst 30KAN was low.
Figure 2. Effect of catalytic esterification on bio-oil viscosity. (a) Before and (b) after dehydration by 3A molecular sieve.
3.2. Effect of Catalytic Esterification on the Properties of Bio-oil. The bio-oil studied was obtained by fast pyrolysis of biomass in the fluidized bed of 2 m × 70 mm, with two screw feeders, two cyclones, a condenser, and an electric heater, and the specific operation conditions were illustrated in the literature.11 The feedstock of pyrolysis was rice husks, and the biooil was held in a refrigerator for 1 month before its esterification. The bio-oil was a dark brown, free-flowing liquid with a distinctive smoky odor. The esterification was carried out in the flask with ethanol/bio-oil ) 2.5 by volume ratio and 5 wt % catalyst, and the reaction temperature was maintained in 50 °C for 5 h to avoid coke formation. Some properties of bio-oil were changed by catalytic esterification and detailed as follows. 3.2.1. Effect of Catalytic Esterification on Viscosity. The dynamic viscosity curves with temperature of bio-oils were illustrated in the Figure 2. At 20 °C, the dynamic viscosity of bio-oils was lowered from 48.56 to 4.81 and 6.09 mm2/s catalyzed by 40STS and 30KAN, respectively. With the dehydration of 3A molecular sieve, the dynamic viscosity of raw bio-oil increased by 2.49-fold, that catalyzed over the solid acid catalyst only increased by 7.3%, and that catalyzed over the solid base catalyst was nearly unchanged. On the other hand, the fluidity of upgraded bio-oil was kept well and the viscosity fluctuation with temperature was weakened. The dehydration by 3A molecular sieve did not make the upgraded bio-oils much more viscous. The catalytic esterification stabilized the bio-oil positively and enhanced the fluidity of bio-oil despite dehydration by 3A molecular sieve. The partially dehydrated original and upgraded bio-oils were sealed in bottles and held still in a refrigerator under about 5 °C for 8 months, and the dynamic viscosities were measured and showed in Table 2. All of the dynamic viscosities of partially dehydrated original and upgraded bio-oils increased to some extent after 8 months (11) Zheng, J.-l.; Zhu, X.-f.; Guo, Q.-x. et al. Thermal conversion of rice husks and sawdust to liquid fuel. Waste Manage. 2006, in press.
Upgrading Bio-oil oVer Different Solid Catalysts
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Figure 3. GC-MS spectroscopy of bio-oil and upgraded bio-oils. (a) Raw bio-oil, (b) upgraded bio-oil by the solid acid catalyst, and (c) upgraded bio-oil by the solid base catalyst. Table 2. Dynamic Viscosity Change of Partially Hydrated Original and Upgraded Bio-oils after 8 Months of Aging Storage
Table 4. Properties of Raw and Upgraded Bio-oils after Dehydration by 3A Molecular Sieve
dynamic viscositya (partially hydrated bio-oils) (mm2/s)
fresh aged for 8 months a
original bio-oil
upgraded bio-oil by solid acid
upgraded bio-oil by solid base
120.7 135.6
5.161 7.645
6.077 7.783
Measured at a temperature of 20 °C. Table 3. Properties of Raw and Upgraded Bio-oils
properties
original bio-oil
upgraded bio-oil by solid acid
upgraded bio-oil by solid base
pH density (kg/m3) H2O content gross calorific value (kJ/kg)
2.60 1.24 29.79 15 834.7
1.12 0.96 13.60 23 868.7
5.93 0.97 12.35 24 034.9
of aging storage. The dynamic viscosity of original bio-oil increased by 14.9 mm2/s, and those of upgraded bio-oils by solid acid and solid base catalysts increased by 2.484 and 1.706 mm2/ s, separately, whose absolute mounting magnitude were far lower than the original one, which indicated the improved stability over time. 3.2.2. Effect of Catalytic Esterification on Other Properties. The properties of bio-oils were shown in Table 3. The addition of solution diluted the bio-oil, and the water content was lowered. The esterification should follow the equation: RCOOH + R′-OH f RCOOR′ + H2O. On the basis of the ethanol addition and stoichiometric calculation, the water content in the upgraded bio-oil should be 11.45% if esterification did not occur in the reactor but, actually, water contents increased
properties
raw bio-oil
upgraded bio-oil by solid acid
upgraded bio-oil by solid base
pH density (kg/m3) H2O content gross calorific value (kJ/kg)
3.58 1.26 26.34 16 752.8
1.12 0.96 11.21 24 975.5
5.54 0.93 11.02 24 478.1
Table 5. Percent Area of Acids and Esters in Bio-oil and Upgraded Bio-oils percent area
bio-oil upgraded bio-oil by solid acid upgraded bio-oil by solid base
organic acids (nonvolatile)
esters
10.0 12.82 12.55
1.33 28.00 18.46
to 13.6 and 12.35% over 40STS and 30KAN, separately, indicating that esterification might be involved. The pH value of upgraded bio-oil catalyzed by the solid acid catalyst was lowered from 2.60 to 1.12 for the catalyst acidification, while the pH value of upgraded bio-oil catalyzed by the solid base catalyst rose to 5.93. The density decreased; dynamic viscosity was one-tenth of that of raw bio-oil; and gross calorific value increased by 50.7 and 51.8%, respectively. The dehydrated bio-oils by 3A molecular sieve were described in Table 4. The dehydration decreased the water contents of raw and upgraded bio-oils over 40STS and 30KAN by 11.6, 17.6, and 10.8%, respectively, and increased the gross calorific value by 5.8, 4.4, and 1.8%, separately. The 3A molecular sieve influenced the upgraded bio-oil over the solid acid catalyst more and did not make much difference on the density. All of the
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Table 6. Corresponding Components in Raw and Upgraded Bio-oils by the Solid Acid Catalyst components in raw bio-oil
in upgraded bio-oil by solid acid
formic acid f formic acid f formic acid anhydride f butanedioic acid f 1,2-benzenedicarboxylic acid f
ethane,1,1′,1′′-[methylidynetris(oxy)]trisacetic acid, diethoxy-,ethyl ester butanedioic acid, diethyl ester 1,2-benzenedicarboxylic acid, diethyl ester
Table 7. Corresponding Components in Raw and Upgraded Bio-oils by the Solid Base Catalyst components in raw bio-oil
in upgraded bio-oil by solid alkali
butanedioic acid f 1,2-benzenedicarboxylic acid f 3-methyl-1,2-benzenediol f 3,5-dimethylbenzoic acid f
butanedioic acid, diethyl ester 1,2-benzenedicarboxylic acid, diethyl ester 2-methyl-1,3-benzenediol 2,3-dimethylbenzoic acid
above results indicated that the direct and simple physical dehydration was not effective and ideal in upgrading the biooil, which can be taken as an assistant method. 3.3. Effect of Catalytic Esterification on the Composition of Bio-oil. Bio-oil is a complex mixture highly oxygenated with a great amount of large-size molecules, which nearly involve all species of oxygenated organics, such as esters, ethers, aldehydes, ketones, phenols, organic acids, and alcohols. The analysis on the composition of bio-oil and upgraded bio-oil was illustrated in the spectroscopy of Figure 3. The bio-oil consisted of nonvolatile components, such as furfural, benzoic acid, benzene dicarboxylic acid, and other organic acid. Most of the components identified were the phenols and the derivatives with methyl, methoxy, propenyl, ketones, and aldehydes groups attached, and nearly all of the functional groups confirmed the existence of oxygen. The comparison on the spectroscopy was shown in Table 5 and validated the remarkable variation on the composition of biooils considering the source material.11 For example, the obvious peaks at time ) 35.00 min in b and c did not appear in a, and according to analysis results, they are signals of 1,2-benzenedicarboxylic acid and bis(2-ethylhexyl)ester, which did not exist in the raw bio-oil. The area of esters was increased by 20-fold over the solid acid catalyst, including esters based on volatile acids, such as formate and acetate, and by 12.9-fold over the solid base catalyst. The evidence proved that 40STS presented the high esterification activity and both solid acid and base catalysts can convert organic acid to esters by catalytic esterification. The composition transformation of bio-oils was shown in Tables 6 and 7. The organic acid reacted with alcohol not only to esters but also to acetals by nucleophilic addition over the solid acid catalyst. Besides esterification, isomerization happened over the solid base catalyst based on the component variation.
4. Conclusions (1) Both the solid acid catalyst 40STS and the solid base catalyst 30KAN can catalyze the model esterification of ethanol and acetic acid. 40STS achieved higher catalytic activity on esterification than 30KAN. (2) The catalytic esterification stabilized the bio-oil with lowered dynamic viscosity and enhanced fluidity, which was sustained after the dehydration by 3A molecular sieve and aging for 8 months as well. (3) The upgraded bio-oils were observed with density decreased from 1.24 to 0.96 kg/m3, and the gross calorific value increased by 50.7 and 51.8% over 40STS and 30KAN, respectively. The pH value of upgraded bio-oil over the solid acid catalyst was lowered from 2.60 to 1.12 for the catalyst acidification, while the pH value of upgraded bio-oil over the solid base catalyst rose to 5.93. The water content was lowered from 29.79 to 11.21 and 11.02% by catalytic esterification over solid acid and solid base catalysts, separately, and partial dehydration by 3A molecular sieve. (4) The analysis on the composition proved that catalytic esterification occurred over both solid acid and solid base catalysts, and the proportion of esters in the upgraded bio-oils increased remarkably, by 20-fold over the solid acid catalyst. Both volatile and nonvolatile acids can be converted to esters over the 40STS catalyst, indicating its high activity. (5) Over the solid acid catalyst, the organic acid reacted with alcohol not only to esters but also to acetals by nucleophilic addition. Besides esterification, isomerization happened over the solid base catalyst based on the component variation. Acknowledgment. The authors thank the National Natural Science Foundation of China (50476090) and the Natural Science Foundation of Guangdong Province (04000378) for financial support. EF060224O
