Energy Fuels 2010, 24, 3251–3255 Published on Web 04/26/2010
: DOI:10.1021/ef1000634
Upgrading of Bio-oil by Catalytic Esterification and Determination of Acid Number for Evaluating Esterification Degree Jin-Jiang Wang, Jie Chang,* and Juan Fan South China University of Technology, No. 381 Wushan Road, Guangzhou 510641, People’s Republic of China Received January 20, 2010. Revised Manuscript Received April 11, 2010
Bio-oil was upgraded by catalytic esterification over the selected catalysts of 732- and NKC-9-type ionexchange resins. The determination of the acid number by potentiometric titration was recommended by the authors to quantify the total content of organic acids in bio-oil and also to evaluate the esterification degree of bio-oil in the process of upgrading. We analyzed the measurement precision and calibrated the method of potentiometric titration. It was proven that this method is accurate for measuring the content of organic acids in bio-oil. After bio-oil was upgraded over 732 and NKC-9, acid numbers of bio-oil were lowered by 88.54 and 85.95%, respectively, which represents the conversion of organic acids to neutral esters, the heating values increased by 32.26 and 31.64%, and the moisture contents decreased by 27.74 and 30.87%, respectively. The accelerated aging test and aluminum strip corrosion test showed improvement of stability and corrosion property of bio-oil after upgrading, respectively.
the uncondensed bio-oil vapor can be esterified, and good results can be obtained.10 Esterification was proven to occur by gas chromatography-mass spectrometry (GC-MS) or Fourier transform infrared (FITR) analysis. A GC-MS chromatogram or FITR spectrum can be used for qualitative analysis of the original and upgraded bio-oils; however, there is no quantitative method proposed for evaluating the esterification degree of bio-oils. Gas chromatography can be used to quantify the organic acids in bio-oils11-13 and to evaluate the esterification degree; however, the overlapping chromatographic peaks are difficult to discriminate, and complicated pretreatment operations are often required. In this paper, we conducted the experiments of upgrading bio-oil by catalytic esterification over selected catalysts: 732- and NKC-9-type ion-exchange resins. Moreover, we developed a rapid method of acid number determination by potentiometric titration, which can be used to quantify the total amount of the organic weak acids in bio-oils and also to evaluate the esterification degree in the process of bio-oil upgrading. The acid number, which is expressed as milligrams of sodium hydroxide per gram of sample in this paper (mg of NaOH/g), refers to the quantity of base required to titrate a sample in a specified solvent to a specified end point. We investigated the precision and accuracy of the method for quantifying the organic acids in bio-oils. The acid number was used as an important index for evaluating the follow-up upgrading process. The stability and
1. Introduction Bio-oil, a liquid product from biomass fast pyrolysis, by virtue of its environmental friendliness and energy independence, is regarded as a promising energy source and receives more and more attention.1,2 Nonetheless, the drawbacks, including high acidity, low heating value, high corrosiveness, high viscosity, and poor stability of bio-oil, limit its usage as a high-grade/transportation fuel.3-5 Consequently, upgrading of bio-oil before use is desirable to give a liquid product that can be used in a wider variety of applications. Catalytic esterification is widely studied for this purpose. Organic acids (formic acid, acetic acid, propionic acid, etc.) in biooils can be converted to their corresponding esters, and the quality of bio-oil will be greatly improved. Solid acid catalysts, solid base catalysts,6 ionic liquid catalysts,7 HZSM-5, and aluminum silicate catalysts8,9 were investigated for esterification of bio-oils. Not only the liquid bio-oil but also *To whom correspondence should be addressed. Telephone: þ86-2087112448. Fax: þ86-20-87112448. E-mail:
[email protected]. (1) Czernik, S.; Bridgwater, A. V. Overview of applications of biomass fast pyrolysis oil. Energy Fuels 2004, 18, 590–598. (2) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of transportation fuels from biomass: Chemistry, catalysts, and engineering. Chem. Rev. 2006, 106, 4044–4098. (3) Bridgwater, A. V.; Peacocke, G. V. C. Fast pyrolysis processes for biomass. Renewable Sustainable Energy Rev. 2000, 4, 1–73. (4) Mohan, D.; Pittman, C. U.; Steele, P. H. Pyrolysis of wood/ biomass for bio-oil: A critical review. Energy Fuels 2006, 20, 848–889. (5) Oasmaa, A.; Czernik, S. Fuel oil quality of biomass pyrolysis oils;State of the art for the end user. Energy Fuels 1999, 13, 914–921. (6) Zhang, Q.; Chang, J.; Wang, T. J.; Xu, Y. Upgrading bio-oil over different solid catalysts. Energy Fuels 2006, 20, 2717–2720. (7) Xiong, W. M.; Zhu, M. Z.; Deng, L.; Fu, Y.; Guo, Q. X. Esterification of organic acid in bio-oil using acidic ionic liquid catalysts. Energy Fuels 2009, 23, 2278–2283. (8) Peng, J.; Chen, P.; Lou, H.; Zheng, X. Catalytic upgrading of biooil by HZSM-5 in sub- and super-critical ethanol. Bioresour. Technol. 2009, 100, 3415–3418. (9) Peng, J.; Chen, P.; Lou, H.; Zheng, X. M. Upgrading of bio-oil over aluminum silicate in supercritical ethanol. Energy Fuels 2008, 22, 3489–3492. r 2010 American Chemical Society
(10) Hilten, R. N.; Bibens, B. P.; Kastner, J. R.; Das, K. C. In-line esterification of pyrolysis vapor with ethanol improves bio-oil quality. Energy Fuels 2010, 24, 673–682. (11) Branca, C.; Giudicianni, P.; Di Blasi, C. GC/MS characterization of liquids generated from low-temperature pyrolysis of wood. Ind. Eng. Chem. Res. 2003, 42, 3190–3202. (12) Oasmaa, A.; Meier, D. Norms and standards for fast pyrolysis liquids;1. Round robin test. J. Anal. Appl. Pyrolysis 2005, 73, 323–334. (13) Sipila, K.; Kuoppala, E.; Fagernas, L.; Oasmaa, A. Characterization of biomass-based flash pyrolysis oils. Biomass Bioenergy 1998, 14, 103–113.
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Energy Fuels 2010, 24, 3251–3255
: DOI:10.1021/ef1000634
Wang et al.
corrosiveness of bio-oil before and after upgrading were also studied. 2. Experimental Section 2.1. Materials and Chemicals. The original bio-oil produced by pyrolysis of wood chips in a circulating fluidized-bed unit was provided by the Devotion Group (Guangzhou, China). The capacity is 3000 tons/year, and the yield of bio-oil is close to 70%. Methanol, ethanol, and acetic acid (AR grade) were commercially available and used without further purification. The 732 resin (reference standard: Amberlite IR-120) and NKC9 resin (reference standard: Amberlyst 15) were also commercially available and used as catalysts for the esterification of bio-oil. 2.2. Acid Number Determination. Bio-oil is usually a black or dark brown liquid, and therefore, it is difficult to choose a suitable visual indicator to signal the end point of the titration. A potentiometric method identifies the end point by monitoring the greatest slope in the titration curve at the equivalence point, which is especially suitable for the titration of turbid or colored solutions. Metrohm 888 Titrando and its internal template method of dynamic pH titration were used for the rapid determination of acid numbers of bio-oil. This template method will dynamically control the titration rate by monitoring the changes of the titration curve to optimize the density distribution of the measurements. For the optimized collection of data, the end point can be acutely observed. 2.2.1. Analysis on Measurement Precision. Different bio-oil samples (0.4, 0.65, and 1.0 g) were dissolved in methanol or ethanol and were titrated by the standard aqueous solutions of sodium hydroxide (0.045, 0.1, and 0.165 mol/L). Each sample was analyzed 3 times, and relative standard deviations of the measurements were calculated. Avoiding the potential esterification, the measurement should be carried out as soon as possible. 2.2.2. Calibration. The experiments of adding a standard sample into bio-oil were conducted to calibrate the method. Acetic acid, 0.06-0.08 g (precision, (0.0001 g), was used as a standard sample and was added into bio-oil, and the mixed sample was titrated. The measurement error of the added standard sample was employed to check the accuracy of the method. The theoretical acid number of the standard sample was calculated by the definition of the acid number, and the measured values were worked out by the formula: measured acid number = (Mm - Mb)/Ms, where Mm is the mass of NaOH consumed by the mixed sample (mg), Mb is the mass of NaOH consumed by bio-oil (mg), and Ms is the mass of the standard sample (g). 2.3. Experiment of Upgrading Bio-oil by Catalytic Esterification. 2.3.1. Activity Evaluation of Resin Catalysts. The model reaction of esterification with methanol and acetic acid was employed to evaluate the catalytic activity of 732 resin and NKC-9 resin. A total of 81.5 mL of methanol and 58.5 mL of acetic acid (molar ratio=2:1) were added into a three-neck flask equipped with a thermometer, reflux condenser, and magnetic stirrer. A total of 5 g of resin catalyst was used. A total of 0.5 mL of reaction solution was sampled at a 20 min interval and measured quantitively by NaOH standard solution titration. From the titration method, the acetic acid conversion was calculated by the equation: conversion=(1 - V/V0) 100%, in which V and V0 are the volumes of the standard NaOH solution consumed in neutralizing the 0.5 mL solution sampled in the process and at the beginning of the reaction, respectively. 2.3.2. Esterification of Bio-oil. Bio-oil and methanol were mixed in a volume ratio of 1:2 and were added to the three-neck flask equipped with a thermometer, reflux condenser, and stirrer. A schematic diagram of the reaction experiment is shown in Figure 1. The experiments were carried out at the temperature of 50 °C for 5 h, and the catalyst was used by 10 wt % of
Figure 1. Schematic diagram of the experiment.
the bio-oil. After completion of the reaction, the catalysts were filtrated and removed from the mixture. Acid numbers of bio-oil were determined at every 1 h interval by the method previously proposed in this paper. Other physical properties were determined by the proposed methods in the round robin test.12 2.3.3. Aging Test. The aging properties were determined using the accelerated aging test method.12 Original, diluted, and upgraded bio-oils were placed in small sealed vials and heated at 80 °C. Kinematic viscosity was measured at 40 °C at specific intervals. 2.3.4. Aluminum Strip Corrosion Test. The aluminum strips were machined into 3 cm 3.5 cm 0.1 mm. The strips were cleaned and polished by silicon carbide paper, weighed, and then immersed in 60 mL vials containing 30 mL oil samples. After that, the vials were sealed and placed at 50 °C. At specific intervals, the strips were taken out of the vials and washed in ethanol. The strips were weighed and then taken back to the vials until the next weight measurement time.
3. Results and Discussion 3.1. Potentiometric Titration for Acid Number Determination. 3.1.1. Influence of Solvents on Measurement Precision. Bio-oil is composed of both water-soluble and water-insoluble fractions and is miscible with polar solvents, such as methanol, ethanol, etc.4,13 Therefore, the aqueous solution of NaOH was selected as a titrant, and methanol or ethanol was used as a solvent for titration. We found that methanol was more suitable than ethanol for the increasing insolubility of bio-oil in ethanol with the addition of titrant. Precipitation of bio-oil and adherence to the pH electrode were observed, and especially, a large amount of titrant was needed. 3.1.2. Influence of the Concentration of Titrant and the Sample Size on Measurement Precision. Curves for 0.65 g of bio-oil titrated by different concentrations of NaOH were shown in Figure 2. ERC is the abbreviation for equivalence point recognition criterion, and its value is a function of the derivative of the titration curve. The value of ERC at the end 3252
Energy Fuels 2010, 24, 3251–3255
: DOI:10.1021/ef1000634
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Figure 2. Curves for 0.065 g of bio-oil titrated with (I) 0.165 mol/L NaOH, (II) 0.100 mol/L NaOH, and (III) 0.045 mol/L NaOH.
pure acetic acid was added in bio-oil, and acid numbers were measured to check if the added pure acetic acid could be accurately quantified. The measurement deviation and the error between the measured and theoretical values were listed in Table 3. From the small error of only 0.02% between theoretical and measured acid numbers, we can conclude that the potentiometric titration for acid number determination is an accurate method for measuring the content of organic acids in bio-oil, especially for acetic acid and the like. Because of the accuracy to quantify the content of organic acids, acid number determination is used for evaluating the follow-up bio-oil upgrading process. 3.2. Esterification of Bio-oil. 3.2.1. Catalytic Activity of 732 Resin and NKC-9 Resin on the Esterification of the Model Compound. The conversion of acetic acid over 732 resin and NKC-9 resin under 50 or 70 °C was shown in Figure 3. Both 732 resin and NKC-9 resin exhibited high activities for esterification of acetic acid. At 70 °C, 732 resin converted 84.72% acetic acid in 120 min, with no significant increase in conversion after 60 min. The NKC-9 resin presented slightly lower activity than 732 resin, and acetic acid conversion rose with time remarkably and reached 79.74% in 120 min at 70 °C.
Table 1. Acid Numbers of Bio-oil Determined by Different Concentrations of NaOH and Relative Standard Deviations of the Measurements concentration of NaOH (mol/L)
acid number (mg of NaOH/g)
relative standard deviation (%)
0.165 0.100 0.045
54.48 54.16 54.76
0.94 0.45 1.23
Table 2. Acid Numbers of Bio-oil Determined with Different Sample Sizes by 0.1 mol/L NaOH and Relative Standard Deviations of the Measurements amount of bio-oil (g)
acid number (mg of NaOH/g)
relative standard deviation (%)
0.400 0.650 1.000
53.86 54.16 54.44
0.92 0.45 0.26
Table 3. Measured Acid Numbers of the Standard Sample and Error of Measurements measured acid number (mg of NaOH/g) relative standard deviation (%) theoretical acid number (mg of NaOH/g) relative error (%)
1
2
3
666.15 0.45 666.11 0.02
662.89
668.83
point (EP) increases with the concentration of titrant. The higher the concentration of titrant, the more obvious the end point of titration. However, the measurement precision was not better when a higher concentration of titrant was used from the results in Table 1. The deviation of acid numbers determined with 0.165 mol/L NaOH is larger than with 0.1 mol/L NaOH because of the fact that a smaller amount of titrant was used. Because the slope in the titration curve with 0.045 mol/L is smaller, the measurement precision is worse. Acid numbers of 0.400, 0.650, and 1.000 g of bio-oil determined with 0.1 mol/L NaOH were shown in Table 2. A better precision was obtained when a larger sample size of bio-oil was used. The acid numbers determined with different masses of bio-oil are close, and the relative standard deviation is only 0.52%. 3.1.3. Accuracy. A standard sample is often used for calibration to check the accuracy of the method. Acetic acid exists in most bio-oils, and we used pure acetic acid as a standard sample for calibration. A determined amount of
Figure 3. Conversion of acetic acid over 732 resin and NKC-9 resin under different reaction temperatures.
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3.2.2. Upgrading of Bio-oil. The large quantities of carboxylic acids found in bio-oil would lead to the high acidity and corrosiveness of bio-oil. Besides, the acids would react with other components, and Hþ ions generated by ionization of acids would catalyze the potential condensation and polymerization reaction, which causes the instability of bio-oil. Fischer esterification is proposed to be the reaction pathway in conversion of carboxylic acid to esters. The esterification reaction follows the equation: RCOOH þ CnH2nþ1OH T RCOOCnH2nþ1 þ H2O, leading to the formation of water and ester. The neutral ester products are less active and corrosive than the acids, and the bio-oil is expected to be improved. The properties of original bio-oil, diluted bio-oil, and upgraded bio-oil were shown in Table 4. Diluted bio-oil refers to the mixed bio-oil with methanol before reaction. The acid number and the moisture content of the bio-oil were both lowered after bio-oil diluted with a large amount of methanol. In comparison to original bio-oil, acid numbers of upgraded bio-oil on 732 resin and NKC-9 resin were lowered by 88.54 and 85.95%, respectively, which represents the conversion of organic acids to neutral esters, the moisture contents decreased by 27.74 and 30.87%, respectively, the heating values increased by 32.26 and 31.64%, respectively, the densities were both lowered by 21.77%, and the viscosities fell by 97% approximately. In comparison to the
diluted bio-oil that was not esterified, the moisture of upgraded bio-oil was increased. This is logical because esterification would produce water. However, the pH value showed inconformity with the acid numbers and became ambiguous. The pH value corresponds to the concentration (activity, in fact) of Hþ ions in a solution. Studies reveal that bio-oil is not a highly dispersed system and is a mixture of multiphase structures.14 The pH value may not reflect the true content of acids in such a complex system, and the complexity of bio-oil will lead to uncertainties during the measurements. Zhang et al.6 also found that the pH value was lowered while the carboxylic acids in bio-oil were converted to their corresponding neutral esters. From these points of view, acid number determination is a more suitable indicator for organic acids transformation to neural esters during the bio-oil upgrading process. The changes of acid numbers with time during bio-oil upgrading were studied and shown in Figure 4. Acid numbers decreased dramatically in the first 3 h, and about 68% of acids were converted. Acid numbers changed slightly with time in the last 2 h. Comparatively, the acid number of the diluted bio-oil that had been stored without catalysts at room temperature for 90 days decreased only from 26.28 to 19.57 mg of NaOH/g. These data showed that the catalyst has a high activity for the conversion of acids. 3.2.3. Stability. It is recommended to determine the stability of bio-oils by measuring the variations of viscosity under accelerated aging conditions.12 The kinematic viscosity of original, diluted, and upgraded bio-oils aging at 80 °C was
Figure 5. Variations of viscosity of original and upgraded bio-oils aging at 80 °C.
Figure 4. Changes of acid numbers with time during bio-oil upgrading.
Table 4. Characteristics of Original, Diluted, and Upgraded Bio-oil upgraded bio-oil a
characteristics
original bio-oil
diluted bio-oil
732
NKC-9
acid number (mg of NaOH/g) pH density (kg/m3) H2O content (wt %) caloric value (MJ/kg) kinematic viscosity (at 40 °C) (mm2/s)
53.86 2.51 1.24 16.62 15.01 81.27
26.28 3.65 0.97 8.08
6.17 1.98 0.97 12.01 19.85 2.46
7.57 2.70 0.97 11.49 19.76 2.45
a
2.48
Diluted bio-oil refers to the mixed bio-oil with methanol but not yet reacted.
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described in Figure 5. Heating the original bio-oil to 80 °C totally altered its properties, and the viscosity dramatically increased with time. A serious phase separation was observed when heated for 36 h, and the lower layer of spongy materials gradually increased with time. The upgraded biooil did not show significant changes, and the viscosity varied between 1.91 and 1.98 mm2/s. The viscosity of diluted bio-oil was also steady going when aging, according to Figure 5. Dilution with small amounts of alcohol is known to stabilize bio-oil, and because of the dilution effect by double volumes of methanol, the stability did not change significantly after esterification. 3.2.4. Corrosion Property. According to the weight loss in Figure 6, upgraded bio-oil was less corrosive than the original bio-oil. We confirm that the property of uniform surface corrosion of bio-oil was improved after upgrading; however, several stains on the aluminum strip surface were also observed after the corrosion of upgraded bio-oil. Whether pitting corrosion occurred on aluminum by upgraded bio-oil needs further study. In addition to this, we
also found that the rate of weight loss of upgraded bio-oil is slightly faster than diluted bio-oil that was not esterified. This may due to an increase of the moisture content after esterification. From the point of view of chemical equilibrium, the esterification reaction will happen given enough time at room temperature even without catalysts. The catalytic process can accelerate the potential esterification and water production, and the esterified bio-oil can be further treated by dehydration and the like. 4. Conclusions Acid number determination by potentiometric titration is an accurate method for quantifying the total content of organic acids in bio-oil, which can be used as an important index for evaluating the follow-up upgrading process. Methanol is more suitable than ethanol as a titration solvent. The higher the concentration of titrant, the more obvious the titration end point, and the larger the sample size used, the better precision was obtained. The bio-oil was upgraded by catalytic esterification over the selected catalysts of 732- and NKC-9-type ion-exchange resins. After upgrading over 732 resin and NKC-9 resin, the acid number of bio-oil was significantly reduced by 88.54 and 85.95%, respectively, which represents the conversion of organic acids to neutral esters, the heating values increased by 32.26 and 31.64%, respectively, the moisture contents were lowered by 27.74 and 30.87%, respectively, the densities were both lowered by 21.77%, and the viscosities were lowered by 97% approximately. Upgraded bio-oil is more stable than the original bio-oil and as stable as the diluted bio-oil that was not esterified because of the huge dilution effect of methanol. Besides, the corrosion-proof property was improved after dilution and upgrading. The ion-exchange resins catalyze the reaction and convert most of the organic acids to their neutral esters. Catalytic esterification with methanol by resin catalysts will be a simple and effective way to improve the quality of bio-oil. Acknowledgment. We are grateful for the financial support from the National Nature Science Foundation of China (90610035) and the 973 R&D program (2010CB732205). (14) Garcia-Perez, M.; Chaala, A.; Pakdel, H.; Kretschmer, D.; Rodrigue, D.; Roy, C. Multiphase structure of bio-oils. Energy Fuels 2006, 20, 364–375.
Figure 6. Weight loss of aluminum strips corroded by original and upgraded bio-oils at 50 °C.
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