生物质油重整制氢

Energy Fuels2010,24,3251–3255:DOI:10.1021/ef1000634Published on Web04/26/2010Upgrading of Bio-oil by Catalytic Esterification and Determination of Acid Number forEvaluating Esterification DegreeJin-Jiang Wang,Jie Chang,*and Juan FanSouth China University of Technology,No.381Wushan Road,Guangzhou510641,People’s Republic of ChinaReceived January20,2010.Revised Manuscript Received April11,2010Bio-oil was upgraded by catalytic esterification over the selected catalysts of732-and NKC-9-type ion-exchange resins.The determination of the acid number by potentiometric titration was recommended bythe authors to quantify the total content of organic acids in bio-oil and also to evaluate the esterificationdegree of bio-oil in the process of upgrading.We analyzed the measurement precision and calibrated themethod of potentiometric titration.It was proven that this method is accurate for measuring the content oforganic acids in bio-oil.After bio-oil was upgraded over732and NKC-9,acid numbers of bio-oil werelowered by88.54and85.95%,respectively,which represents the conversion of organic acids to neutralesters,the heating values increased by32.26and31.64%,and the moisture contents decreased by27.74and30.87%,respectively.The accelerated aging test and aluminum strip corrosion test showed improvement ofstability and corrosion property of bio-oil after upgrading,respectively.1.IntroductionBio-oil,a liquid product from biomass fast pyrolysis,by virtue of its environmental friendliness and energy indepen-dence,is regarded as a promising energy source and receives more and more attention.1,2Nonetheless,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.3-5Consequently,upgrad-ing 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 anic acids(formic acid,acetic acid,propionic acid,etc.)in bio-oils can be converted to their corresponding esters,and the quality of bio-oil will be greatly improved.Solid acid cata-lysts,solid base catalysts,6ionic liquid catalysts,7HZSM-5, and aluminum silicate catalysts8,9were investigated for esterification of bio-oils.Not only the liquid bio-oil but also the uncondensed bio-oil vapor can be esterified,and good results can be obtained.10Esterification 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 discri-minate,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 milli-grams 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 evalua-ting the follow-up upgrading process.The stability and*To whom correspondence should be addressed.Telephone:þ86-20-87112448.Fax:þ86-20-87112448.E-mail:changjie@. (1)Czernik,S.;Bridgwater,A.V.Overview of applications of bio-mass fast pyrolysis oil.Energy Fuels2004,18,590–598.(2)Huber,G.W.;Iborra,S.;Corma,A.Synthesis of transportationfuels 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 Fuels2006,20,848–889.(5)Oasmaa,A.;Czernik,S.Fuel oil quality of biomass pyrolysisoils;State of the art for the end user.Energy Fuels1999,13,914–921.(6)Zhang,Q.;Chang,J.;Wang,T.J.;Xu,Y.Upgrading bio-oil over different solid catalysts.Energy Fuels2006,20,2717–2720.(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 Fuels2009,23,2278–2283.(8)Peng,J.;Chen,P.;Lou,H.;Zheng,X.Catalytic upgrading of bio-oil by HZSM-5in 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 Fuels2008,22, 3489–3492.(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 Fuels2010,24,673–682.(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.Pyrolysis2005,73,323–334.(13)Sipila,K.;Kuoppala,E.;Fagernas,L.;Oasmaa,A.Character-ization of biomass-based flash pyrolysis oils.Biomass Bioenergy1998, 14,103–113.corrosiveness of bio-oil before and after upgrading were also studied.2.Experimental Section2.1.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 is3000tons/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. The732resin(reference standard:Amberlite IR-120)and NKC-9resin(reference standard:Amberlyst15)were also com-mercially 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.Metrohm888Titrando 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 distribu-tion 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,and1.0g)were dissolved in methanol or ethanol and were titrated by the standard aqueous solutions ofsodium hydroxide(0.045,0.1,and0.165mol/L).Each sample was analyzed3times,and relative standard deviations of the measurements were calculated.Avoiding the potential esterifi-cation,the measurement should be carried out as soon aspossible.2.2.2.Calibration.The experiments of adding a standard sample into bio-oil were conducted to calibrate the method. Acetic acid,0.06-0.08g(precision,(0.0001g),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=(M m-M b)/M s,where M m is the mass of NaOH consumed by the mixed sample(mg),M b is the mass of NaOH consumed by bio-oil(mg),and M s is the mass of the standard sample(g).2.3.Experiment of Upgrading Bio-oil by Catalytic Esterifica-tion.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 of732resin and NKC-9resin.A total of81.5mL of methanol and58.5mL 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 of5g of resin catalyst was used.A total of0.5mL of reaction solution was sampled at a20min 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)Â100%, in which V and V0are the volumes of the standard NaOH solution consumed in neutralizing the0.5mL 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 of1:2and 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 Figure1.The experiments were carried out at the tempera-ture of50°C for5h,and the catalyst was used by10wt%of 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 every1h interval by the method previously proposed in this paper.Other physical properties were determined by the proposed methods in the round robin test.122.3.3.Aging Test.The aging properties were determined using the accelerated aging test method.12Original,diluted,and upgraded bio-oils were placed in small sealed vials and heated at80°C.Kinematic viscosity was measured at40°C at specific intervals.2.3.4.Aluminum Strip Corrosion Test.The aluminum strips were machined into3cmÂ3.5cmÂ0.1mm.The strips were cleaned and polished by silicon carbide paper,weighed,and then immersed in60mL vials containing30mL oil samples.After that,the vials were sealed and placed at50°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 Discussion3.1.Potentiometric Titration for Acid Number Determina-tion.3.1.1.Influence of Solvents on Measurement Precision. Bio-oil is composed of both water-soluble and water-inso-luble fractions and is miscible with polar solvents,such as methanol,ethanol,etc.4,13Therefore,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.Precipita-tion 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 for0.65g of bio-oil titrated by different concentrations of NaOH were shown in Figure2.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 theend Figure1.Schematic diagram of the experiment.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 0.165mol/L NaOH is larger than with 0.1mol/L NaOH because of the fact that a smaller amount of titrant was used.Because the slope in the titration curve with 0.045mol/L is smaller,the measurement precision is worse.Acid numbers of 0.400,0.650,and 1.000g of bio-oil determined with 0.1mol/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 differ-ent 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 ofpure 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 determina-tion 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 732Resin and NKC-9Resin on the Esterification of the Model Compound.The conversion of acetic acid over 732resin and NKC-9resin under 50or 70°C was shown in Figure 3.Both 732resin and NKC-9resin exhibited high activities for esterification of acetic acid.At 70°C,732resin converted 84.72%acetic acid in 120min,with no significant increase in conversion after 60min.The NKC-9resin presented slightly lower activity than 732resin,and acetic acid conversion rose with time remarkably and reached 79.74%in 120min at 70°C.Figure 2.Curves for 0.065g of bio-oil titrated with (I)0.165mol/L NaOH,(II)0.100mol/L NaOH,and (III)0.045mol/L NaOH.Table 1.Acid Numbers of Bio-oil Determined by Different Concentrations of NaOH and Relative Standard Deviationsof the Measurementsconcentration of NaOH (mol/L)acid number (mg of NaOH/g)relative standard deviation (%)0.16554.480.940.10054.160.450.04554.761.23Table 2.Acid Numbers of Bio-oil Determined with Different Sample Sizes by 0.1mol/L NaOH and Relative StandardDeviations of the Measurementsamount of bio-oil (g)acid number (mg of NaOH/g)relative standard deviation (%)0.40053.860.920.65054.160.451.00054.440.26Table 3.Measured Acid Numbers of the Standard Sample and Errorof Measurements123measured acid number (mg of NaOH/g)666.15662.89668.83relative standard deviation (%)0.45theoretical acid number (mg of NaOH/g)666.11relative error (%)0.02Figure 3.Conversion of acetic acid over 732resin and NKC-9resin under different reaction temperatures.3.2.2.Upgrading of Bio-oil.The large quantities of car-boxylic 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 H þions 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 þC n H 2n þ1OH T RCOOC n H 2n þ1þH 2O,leading to the for-mation 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 up-graded bio-oil on 732resin and NKC-9resin were lowered by 88.54and 85.95%,respectively,which represents the con-version of organic acids to neutral esters,the moisture contents decreased by 27.74and 30.87%,respectively,the heating values increased by 32.26and 31.64%,respectiv-ely,the densities were both lowered by 21.77%,and the viscosities fell by 97%approximately.In comparison to thediluted 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 H þions in a solution.Studies reveal that bio-oil is not a highly dispersed system and is a mixture of multiphase structures.14The 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.6also found that the pH value was lowered while the carboxy-lic 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 num-bers decreased dramatically in the first 3h,and about 68%of acids were converted.Acid numbers changed slightly with time in the last paratively,the acid number of the diluted bio-oil that had been stored without catalysts at room temperature for 90days decreased only from 26.28to 19.57mg 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 stabi-lity of bio-oils by measuring the variations of viscosity under accelerated aging conditions.12The kinematic viscosity of original,diluted,and upgraded bio-oils aging at 80°C wasTable 4.Characteristics of Original,Diluted,and Upgraded Bio-oilupgraded bio-oilcharacteristicsoriginal bio-oildiluted bio-oil a732NKC-9acid number (mg of NaOH/g)53.8626.28 6.177.57pH2.513.65 1.98 2.70density (kg/m 3)1.240.970.970.97H 2O content (wt %)16.628.0812.0111.49caloric value (MJ/kg)15.0119.8519.76kinematic viscosity (at 40°C)(mm 2/s)81.272.482.462.45aDiluted bio-oil refers to the mixed bio-oil with methanol but not yetreacted.Figure 4.Changes of acid numbers with time during bio-oilupgrading.Figure 5.Variations of viscosity of original and upgraded bio-oils aging at 80°C.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 ob-served when heated for 36h,and the lower layer of spongy materials gradually increased with time.The upgraded bio-oil did not show significant changes,and the viscosity varied between 1.91and 1.98mm 2/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 up-graded bio-oil needs further study.In addition to this,wealso 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 equilibri-um,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.ConclusionsAcid 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.Metha-nol is more suitable than ethanol as a titration solvent.The higher the concentration of titrant,the more obvious the tit-ration 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 re-sins.After upgrading over 732resin and NKC-9resin,the acid number of bio-oil was significantly reduced by 88.54and 85.95%,respectively,which represents the conversion of organic acids to neutral esters,the heating values increased by 32.26and 31.64%,respectively,the moisture contents were lowered by 27.74and 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 973R&D program(2010CB732205).Figure 6.Weight loss of aluminum strips corroded by original and upgraded bio-oils at 50°C.(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.。