5735r 2010 American Chemical Society pubs.acs.org/EF

Energy Fuels 2010, 24, 5735–5740 : DOI:10.1021/ef100896qPublished on Web 09/29/2010

Aromatics Production via Catalytic Pyrolysis of Pyrolytic Lignins from Bio-Oil

Yan Zhao,† Li Deng,† Bin Liao,‡ Yao Fu,*,† and Qing-Xiang Guo*,†,‡

†Anhui Province Key Laboratory of Biomass Clean Energy, Department of Chemistry,University of Science and Technology of China, Hefei 230026, China, and, and ‡Key Laboratory of Cellulose and

Lignocellulosics Chemistry, Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou 510650, China

Received July 14, 2010. Revised Manuscript Received August 28, 2010

Herein we reported a promising method for the production of aromatics from pyrolytic lignin (PL).Compared to the lignins derived from pulping process, the PLs obtained from bio-oil give more aromatics(40% in carbon yield) and do not generate eff luvial gas containing sulfur. More importantly, phenols arethe main products with selectivity over 90% at 600 �Cwithout catalyst. In the presence of ZSM-5, the cokedeposition was not obvious and the selectivity for aromatic hydrocarbons is more than 87%, indicatingrobustness of ZSM-5 for deoxygenating and high tolerance to PLs. Therefore we have demonstrated thecatalytic pyrolysis of PLs is an alternative way to produce fuel additives and useful chemicals.

1. Introduction

Traditional chemical industry has been dependent on petro-leum for more than a half century, which results in rapiddepletion of fossil resource and the problem of greenhouse gasemissions. It requires strategies for alternative fuels and chemi-cals production from biomass especially from nonediblebiomass.1-3 In this respects, fast pyrolysis is a promisingtechnology for biomass utilization and has many advantages.The product of this technology, knownas bio-oil, has lower costthan other biofuels, a wide range of feedstock, and more than60% energy of the feedstock.2 However, it is of poor qualities:

low heating value, low thermal stability, high viscosity, andpoor volatility, which limited its usage in internal combustionengines.4 To upgrade the bio-oil, researchers developed severalmethods including esterification,5-7 catalytic reforming,8-11

hydrogenation,12,13 and ketonization.14,15 Because of the differ-ent reactivity and interactions of more than 300 components inthe oil, it is rather difficult to improve its qualities to meet therequirementof transportation fuels bya single treatment.Hencethe combination of different methods and tools is necessary.Herein, we propose a strategy containing separation and cata-lytic conversion steps for bio-oil upgrading (Scheme 1). As wehave previously shown, bio-oil can be separated effectively intotwo fractions: the distillate and the nonvolatile PL by thedistillation in glycerol.16 Moreover, in a recent publicationby Huber et al., alkanes, hydrogen, and polyols can be obtainedfrom the water-soluble bio-oil via aqueous phase reforming(APR).17 Considering the similarity of the organic compositionof the distillate and the water-soluble bio-oil, it is believed thatthe distillate can also be converted to alkanes by the APRtechnique.With the aim of converting the other fraction of bio-oil and finishing theupgrading strategy for thewhole bio-oil, thepyrolysis of PL has been carried out in this work.

It is well-known that lignin is the second most abundantcomponent of biomass, which accounts for about 30% ofplant biomass. Indeed lignin can be regarded as the majoraromatic source of a biobased economy. The selective trans-formation of lignin to aromatics with high yield and less coke

*To whom correspondence should be addressed. Fax: þ86-551-3606689. E-mail: fuyao@ustc.edu.cn (Y.F.); qxguo@ustc.edu.cn(Q.X.G.).(1) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of transportation

fuels from biomass: Chemistry, catalysts, and engineering. Chem. Rev.2006, 106 (9), 4044–4098.(2) Corma, A.; Iborra, S.; Velty, A. Chemical routes for the transfor-

mation of biomass into chemicals.Chem. Rev. 2007, 107 (6), 2411–2502.(3) Zakzeski, J.; Bruijnincx, P. C. A.; Jongerius, A. L.; Weckhuysen,

B. M. The catalytic valorization of lignin for the production of renew-able chemicals. Chem. Rev. 2010, 110 (6), 3552–3599.(4) Czernik, S.; Bridgwater, A. V. Overview of applications of bio-

mass fast pyrolysis oil. Energy Fuels 2004, 18 (2), 590–598.(5) Zhang, Q.; Chang, J.; Wang, T. J.; Xu, Y. Upgrading bio-oil over

different solid catalysts. Energy Fuels 2006, 20 (6), 2717–2720.(6) Peng, J.; Chen, P.; Lou, H.; Zheng, X. M. Upgrading of bio-oil

over aluminumsilicate in supercritical ethanol.EnergyFuels 2008, 22 (5),3489–3492.(7) Xiong, W. M.; Zhu, M. Z.; Deng, L.; Fu, Y.; Guo, Q. X.

Esterification of Organic Acid in Bio-Oil using Acidic Ionic LiquidCatalysts. Energy Fuels 2009, 23, 2278–2283.(8) Sharma, R. K.; Bakhshi, N. N. Catalytic Upgrading of Pyrolysis

Oil. Energy Fuels 1993, 7 (2), 306–314.(9) Gayubo, A. G.; Aguayo, A. T.; Atutxa, A.; Aguado, R.; Olazar,

M.; Bilbao, J. Transformation of oxygenate components of biomasspyrolysis oil on a HZSM-5 zeolite. H. Aldehydes, ketones, and acids.Ind. Eng. Chem. Res. 2004, 43 (11), 2619–2626.(10) Gayubo, A. G.; Aguayo, A. T.; Atutxa, A.; Aguado, R.; Bilbao,

J. Transformation of oxygenate components of biomass pyrolysis oil ona HZSM-5 zeolite. I. Alcohols and phenols. Ind. Eng. Chem. Res. 2004,43 (11), 2610–2618.(11) Nilsen,M.H.; Antonakou, E.; Bouzga, A.; Lappas, A.;Mathisen,

K.; Stocker,M. Investigation of the effect ofmetal sites inMe-Al-MCM-41(Me = Fe, Cu or Zn) on the catalytic behavior during the pyrolysis ofwooden based biomass.Microporous Mesoporous Mater. 2007, 105 (1-2),189–203.

(12) Elliott, D. C. Historical developments in hydroprocessing bio-oils. Energy Fuels 2007, 21 (3), 1792–1815.

(13) Tang, Z.; Lu, Q.; Zhang, Y.; Zhu, X. F.; Guo, Q. X. One StepBio-Oil Upgrading throughHydrotreatment, Esterification, and Crack-ing. Ind. Eng. Chem. Res. 2009, 48 (15), 6923–6929.

(14) Gartner, C.A.; Serrano-Ruiz, J. C.; Braden,D. J.; Dumesic, J. A.Catalytic Upgrading of Bio-Oils by Ketonization. Chemsuschem 2009,2 (12), 1121–1124.

(15) Deng, L.; Fu, Y.; Guo, Q. X. Upgraded Acidic Components ofBio-oil through Catalytic Ketonic Condensation. Energy Fuels 2009,23 (1), 564–568.

(16) Deng, L.; Yan, Z.; Fu, Y.; Guo, Q. X. Green Solvent for FlashPyrolysis Oil Separation. Energy Fuels 2009, 23, 3337–3338.

(17) Vispute, T. P.; Huber, G. W. Production of hydrogen, alkanesand polyols by aqueous phase processing of wood-derived pyrolysis oils.Green Chem. 2009, 11 (9), 1433–1445.

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Energy Fuels 2010, 24, 5735–5740 : DOI:10.1021/ef100896q Zhao et al.

formation was described as “new technology” by Zakzeskiet al.3 In terms of heating value and elemental composition, itis more adaptable to produce fuels than cellulose and hemi-cellulose and drawing more and more attention. Kou et al.carried out a two stephydroprocess over noble-metal catalystsconverting it to gasoline and diesel selectively.18,19 The Acelllignin obtained fromorganosolv pulping processwas catalyticpyrolyzed with acetone for the production of gasoline rangehydrocarbons.20 Under pyrolysis conditions, natural lignin isbroken down into volatile aromatics and nonvolatile oligo-mers, the so-called PL, which will polymerize with aldehydesand phenols forming coke in bio-oil causing poor stability ofthe oil. Nevertheless the PL contributes a large part of theheating valueofbio-oil for its lowoxygen content. Zhang et al.transformed it to stable organics by employing rutheniumcatalysts combining the supercritical ethanol technique.21

Bakhshi et al upgraded PL over HZSM-5 using tetralin ashydrogen-donating dilute.8 Regarding the economic feasibil-ity of the fuels and chemicals, zeolite catalysts are superior forindustrial scale process because they do not need noblemetalsandexternalhydrogenforcatalytic cracking.Theyplay importantroles not only in the petroleum industry but also in biomassconversion. For instance, carbohydrates, more than a half ofwhich is oxygen, can be converted to aromatics over zeoliteswith a maximum carbon yield of 30%.22,23 In this study,employing the powerful tools, an initial approach to the above-mentioned “new technology” has been carried out. The PLsproduce near 40% aromatics under pyrolysis condition.

2. Experimental Section

2.1. Materials. The bio-oil used in this work was producedthrough the fast pyrolysis of rice husk in the fluidized reactor atabout 550 �C. The PLs were obtained from bio-oil by twomethods. The PL precipitated from water (WPL) was prepared

according to the method of Scholze,24,25 and the PL separatedfrom the mixture of glycerol and bio-oil (GPL) was obtainedas described elsewhere.16 The other two lignins: alkali lignin(AL) and kraft lignin (KL) were purchased from Sigma-AldrichCo. Ltd.

The catalysts ZSM-5, HZSM-5, and β-zeolite were providedby the catalyst plant of Nankai University, and MCM-41 wasobtained from Fuxu Zeolite Co. Ltd. SBA-15 was synthesizedaccording to the method described elsewhere.26 The typicalproperties of them are listed in Table 1.

2.2. Experimental Procedure.The pyrolysis of PLswas carriedout in a tubular reactor (1 cm i.d.) made of quartz glass. Asdescribed in Figure 1, before the reaction, 0.5 g of lignin and0.5 g of zeolite were loaded in the branched tube and the heatingzone with quartz wool, respectively. During the pyrolysis, ligninwas fed into the heating zone by a piston continuously within3 min, and the temperature was kept for another 2 min withnitrogen at the rate of 50 mL/min. Then liquid product wascondensed and collected in a liquid nitrogen trap. For catalystregeneration, air (50mL/min) was employed to remove the cokeat 600 �C.

Scheme 1. Strategy for Bio-Oil Upgrading with Combined Treatments

Table 1. Typical Properties of the Catalysts

entry catalystsBET surfacearea/m2/g

average porediameter/nm Si/Al ratio

1 ZSM-5 420 0.5a 50a

2 HZSM-5 350 0.5a 50a

3 β-zeolite 63 0.7a 50a

4 MCM-41 1000a 3.8a 50a

5 SBA-15 807 6.9 pure silica

aProvided by the manufacturers.

Figure 1. Schematic diagram of experimental apparatus for catalyticpyrolysis of PLs.

(18) Yan, N.; Zhao, C.; Dyson, P. J.; Wang, C.; Liu, L. T.; Kou, Y.Selective Degradation of Wood Lignin over Noble-Metal Catalysts in aTwo-Step Process. Chemsuschem 2008, 1 (7), 626–629.(19) Zhao, C.; Kou, Y.; Lemonidou, A. A.; Li, X. B.; Lercher, J. A.

Highly Selective Catalytic Conversion of Phenolic Bio-Oil to Alkanes.Angew. Chem., Int. Ed. 2009, 48 (22), 3987–3990.(20) Thring, R. W.; Katikaneni, S. P. R.; Bakhshi, N. N. The

production of gasoline range hydrocarbons from Alcell lignin usingHZSM-5 catalyst. Fuel Process. Technol. 2000, 62 (1), 17–30.(21) Tang, Z.; Zhang, Y.; Guo, Q. X. Catalytic Hydrocracking of

Pyrolytic Lignin to Liquid Fuel in Supercritical Ethanol. Ind. Eng.Chem. Res. 2010, 49 (5), 2040–2046.(22) Carlson, T. R.; Vispute, T. R.; Huber, G. W. Green gasoline by

catalytic fast pyrolysis of solid biomass derived compounds. Chem-suschem 2008, 1 (5), 397–400.(23) Carlson, T. R.; Tompsett, G. A.; Conner, W. C.; Huber, G. W.

Aromatic Production fromCatalytic Fast Pyrolysis of Biomass-DerivedFeedstocks. Top. Catal. 2009, 52 (3), 241–252.(24) Scholze, B.; Meier, D. Characterization of the water-insoluble

fraction from pyrolysis oil (pyrolytic lignin). Part I. PY-GC/MS, FTIR,and functional groups. J. Anal. Appl. Pyrol. 2001, 60 (1), 41–54.

(25) Scholze, B.; Hanser, C.; Meier, D. Characterization of the water-insoluble fraction from fast pyrolysis liquids (pyrolytic lignin) Part II.GPC, carbonyl goups, and C-13-NMR. J. Anal. Appl. Pyrol. 2001, 58,387–400.

(26) Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson,G. H.; Chmelka, B. F.; Stucky, G. D. Triblock copolymer syntheses ofmesoporous silica with periodic 50 to 300 angstrom pores. Science 1998,279 (5350), 548–552.

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Energy Fuels 2010, 24, 5735–5740 : DOI:10.1021/ef100896q Zhao et al.

2.3. Analytical Determination. Nitrogen adsorption/desorp-tion isotherms was measured by a Micromeritics ASAP 2020analyzer. The surface area was determined using the Barrett-Emmet-Taller (BET) method, and the average pore size ofSBA-15 was determined by the Barret-Joyner-Halenda (BJH)method.

Liquid samples were analyzed by a gas chromatograph-massspectrometer (GC-MS; Thermal Trace GCUltra with a Polar-isQ ion trap mass spectrometer) equipped with a TR-35MScapillary column (30 m� 0.25 mm� 25 μm). Split injection wasperformed at a split ratio of 50 using helium (99.999%) as thecarrier gas. The water content in the liquid samples was deter-mined by Karl Fischer titration.

The gas was collected by a gasbag and sampled for analysisusing aGCwith thermal conductivity detector (TCD). The yieldof gaseous products were measure by weight difference (weightof gas=weight of feed-weight of liquid-weight of coke). Thecomponents were determined by the external standard methodusing calibration gas (a mixture of H2, CO, CO2, CH4, C2H6,C2H4, C3H8, and C3H6). The elemental analysis of lignins andcoke was performed employing a Vario EL III analyzer. Thethermal gravity analysis (TGA) of ligninswas carried out using aShimazu DTG-60H thermogravimetric analyzer. The measure-ments were carried out at a rate 30 �C/min to 1000 �C undernitrogen atmosphere.

3. Results and Discussion

3.1. Lignins Properties. To compare the difference inpyrolysis behavior of PLs and other lignin compounds, fourkinds of lignins, WPL, GPL, AL, andKL, were used as feed.The later two lignins are the byproduct of paper industry.Because of the different treatments, the four lignins havedifferent properties.According to elemental analysis (Table 2),the chemical formulas of WPL, GPL, AL, and KL areCO0.34H1.17N0.02, CO0.36H1.17N0.02, CO0.59H1.25S0.03Na0.03,and CO0.29H1.45S0.10Na0.10, respectively. The C, O, and Hcontents of lignins are similar to coal. Assuming the productsof the cracking reaction are toluene, CO, and H2O, thetheoretical molar carbon yield of toluene from PLs is about79% (eq 1) while the value for glucose is 63%.23 It suggestsPLs will produce more aromatics than carbohydrates. Thehydrogen-to-carbon effective ratio defined as eq 2 is anotherimportant factor to compare the relative amount of hydro-gen and predict the hydrocarbons yield. For example, thevalue of glucose, PLs, benzene, and alkene are 0, 0.5, 1.0, and 2,respectively. It means the feed with the higher ratio mayproduce more hydrocarbons using less hydrogen.

C100O35H117 f 247=22C7H8 ð79% carbon yieldÞþ 471=22COþ 523=22 H2O ð1Þ

Heff=C ¼ ðH- 2OÞ=C ð2ÞFor PLs, the sulfur contents of PLs are trace (<0.1%) andomitted in the formulas,while thevaluesofALandKLareratherhigh because of the sulfur compounds added in pulping process.

The molecular weight and the degree of polymerization ofthe lignins are given in Table 3. It is clear that the two PLs areoligomers with a degree of polymerization (DP) less than8 while the other two are polymers with the weight averagemolecular weight more than 50 000.

The results of TGA clearly show that WPL leaves only34 wt % of coke (dry basis), and GPL leaves 37 wt % cokeat 800 �C. For differential thermal gravimetry (DTG), theweight loss peak of GPL appears at higher temperature,which is probably caused by the polymerization when heatedin glycerol. Although the peak temperature of AL is close toWPL, the coke yields of it aremuchhigher (see the SupportingInformation for TGA and DTG curves). It seems that highdegree of polymerization and salt composition increase thecoke yield.

3.2. Pyrolysis of Lignin without Catalyst. The lignins arethe solid oligomers and polymers which cannot contact theinternal surface of the zeolites directly. When heated in thepresence of catalysts, they are broken down forming pyro-lytic vapors first and then the vapors enter the pores ofzeolites and undergo catalytic reactions. In order to find theproduct distributions of the vapors formed in the firststep, the lignins were pyrolyzed without catalyst. As shownin Figure 2, the distribution was examined at differenttemperature. Generally, at 600 �C, lignins yield the mostorganic liquid products and moderate coke and gas. PLsexhibit higher selectivity for the aromatics and the carbonyield are 40% and 37% for WPL and GPL. Regarding thestructural properties of GPL and WPL, the difference inyield of aromatics may be attributed to the molecular weightand condensation with some aldehydes.16 Higher tempera-ture will increase the yield of gas while reduce the liquids andcoke formation.

Table 2. Elemental Composition of the Lignins

entry lignin C/wt% H/wt% O/wt%a N/wt% S/wt% Na/wt%c

1 WPL 63.7 6.26 28.86 1.18 trace trace2 GPL 62.54 6.12 30.04 1.30 trace trace3 AL 49.01 5.15 38.77 0.19 4.00b 2.884 KL 41.68 5.08 33.97 0.14 11.13b 8.00

aThe oxygen contentwas estimated by difference. bThe sulfur contentwas provided by theAldrichCo., Ltd. cThe sodiumcontentwas estimatedaccording to the sulfur content.

Table 3. Molecular Weight and TGA of the Lignins

entry lignin Mn Mw DPbweight

loss/wt%DTG peak

temperature/�C

1 WPL 644a 1 047a 6-7 66.31 3132 GPL 757a 1 331a 8-9 62.57 3573 AL 10 000c 60 000c ∼400 42.72 3414 KL 7 000c 52 000c ∼350 54.69 293

aAdapted from ref 16. bEstimated according to ref 25. cThe values areprovided by Aldrich Co. Ltd.

Figure 2. Pyrolysis product distributions of the lignins withoutcatalyst: aromatics (green), gas (cyan), and coke (gray).

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Energy Fuels 2010, 24, 5735–5740 : DOI:10.1021/ef100896q Zhao et al.

For AL and KL, the liquid products are at least only6% and 7% and more coke and gas are generated at 600 �C.What’s worse is that the gas is of effluvial smell which iscaused by the high sulfur content. Because of the poorqualities, AL and KL were not used as feed in the followingstudy.

Figure 3 exhibits the aromatic selectivity of liquid productsfrom PLs. The products were classified into three parts:oxygenates (e.g., phenols and substituted benzaldehydes),arenes (excluding polycyclic aromatic hydrocarbons (PAHs)),and PAHs. It is evident that the selectivity for oxygenatesdeclines and that for arenes increases gradually as the tempera-ture rises. The PAHs appear at 700 �Cand increase to 32%and45% for WPL and GPL at 800 �C. Compared to pyrolyzinglignocellulosic biomass, pyrolysis of PL without catalysts willnot produce furans, acids, and levoglucan derived from cellu-lose and hemicellulose. Thus it is an effective transformation ofPL to aromatic oxygenates, mainly phenols (93%), with highselectivity. This process may find an application for the pro-duction of phenols, which are important chemicals especiallyfor resin industry.

The gas composition fromPLswas shown in Figure 4. TheCH4 and CO are the predominating products accounting forover 30% and 60%. According to the composition, thehigher heating value of the gas is about 4900 kcal/N m3.Although the temperature does notmake the obvious changein gas composition, it does increase the yield of gas (Figure 2).Besides, at high temperature (800 �C), water-gas shiftplays an important role in gas generation which is inferredby the observation of the decrease in yield of both coke andwater.

3.3. Catalytic Effects on Products Distribution. Five zeo-lites, ZSM-5, HZSM-5, β-zeolite, MCM-41 and SBA-15,were used to catalyze the pyrolysis of PL. The reactiontemperature of 600 �C was chosen, for more aromatics invapors are generated from PL at this temperature. Figure 5shows the products distribution. In the presence of ZSM-5,PL produce more aromatics and the carbon yield of aro-matics is not reduced evidently compared to the pyrolysiswithout catalyst (39% versus 40% for WPL). Under cata-lytic pyrolysis condition, coke formation comes from twoparts: thermal decomposition of lignins irrelative to catalystand from the coke deposition on zeolites. Therefore ZSM-5causes the least coke deposition on the surface for the carbon

yield of coke is 42% and identical to that without catalyst.The catalytic performance of HZSM-5 is inferior to ZSM-5.It decreases the aromatic yield to 30% and increases thecoke yield to about 50%due to the coke deposition caused bythe stronger acidity. Unfortunately, the other three zeolitesgive a much lower yield of aromatics (16-21%) and increasethe carbon yield of coke (55-60%) obviously. AlthoughSBA-15 is neutral, the great internal surface is not completelyinert and the mesoporous channels enable the entrance andadsorption of larger intermediates, which may result inserious coke formation. These results indicate that pore sizeis another important factor regarding the shape selectivecatalysis.

As shown inFigure 6, the aromatic selectivity was changeddramatically by microporous zeolites, especially by ZSM-5.In the presence of ZSM-5, the selectivity for deoxygenatedproducts are 88% (57% arenes þ 31% PAHs) and 87%(52% arenesþ 35% PAHs) forWPL andGPL, respectively.These aromatics can be used directly as high octane fuel addi-tives in gasoline or hydrogenated to cyclic alkanes in a second-ary process.22 Compared to the results of pyrolysis withoutcatalyst, most of oxygenates are converted to arenes and PAHsover microporous zeolites. However, the mesoporous catalysts

Figure 3. Aromatic selectivity of the PLs at different temperature:oxygenates (blue), arenes (yellow), and PAHs (red).

Figure 4.Gas selectivity of the PLs at different temperature:methane(red), CO (yellow), CO2 (green), C2-4 alkanes and alkenes (blue),and H2 (gray).

Figure 5. Pyrolysis products distribution of the PLs with zeolites:aromatics (green), gas (cyan), and coke (gray).

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are incapable of deoxygenating for the main products are stilloxygenates similar to the noncatalytic thermal conversion,indicating the catalytic conversion and noncatalytic thermalconversion of the primary intermediate occur simultaneously.The typical results of aromatic selectivitywithorwithoutZSM-5are listed in Table 4. It demonstrates the ZSM-5 deoxygenatedphenols remarkably. For instance, the selectivity for phenol and4-ethylphenolweredeclined from12%and17%to7%and2%,respectively. Moreover 4-methylbenzaldehyde was eliminatedcompletely in the presence of ZSM-5. As a result, toluenebecomes the dominant product with a selectivity of 32%. Also,the second most abundant product is naphthalene. The selec-tivity for it is raised from 0 to 13% by ZSM-5 (see the Support-ing Information for more details). Under catalytic pyrolysiscondition, the selectivity for gas products is similar to thatwithout catalyst. The results of gas composition with zeoliteswere given in Figure S5 in the Supporting Information.

3.4. Catalyst Regeneration. Considering the catalyst cost,the catalytic pyrolysis does not only require high activity andselectivity but also demands a long catalyst life. With thisrespect, ZSM-5 regeneration test was carried out at 600 �Cusing GPL as feed. Although the coke deposition on catalystis not severe, the yield of coke is rather high. However, thecoke can be burned to supply heat to keep the temperature ofthe pyrolysis reactor23 and the process of purging air at high

temperature to also regenerate the catalyst. As shown inFigure 7, the yield of aromatics decreased slightly by 2%after one catalytic run and the catalytic activity was keptin the following runs. The obvious changes in the yield ofcoke and gas were not found either. Figure 8 shows theselectivity for arenes ranges from 52% to 57% in five runs.Only the selectivity for PAHs declines sharply from 35% to25% after one run. These results suggest the ZSM-5 is stableunder the pyrolysis condition and tolerant to phenols fromthe PLs.

4. Conclusions

In summary, we have demonstrated that the PLs separatedfrom bio-oil can be transformed into aromatic hydrocarbonsand phenols, which are important bulk chemicals. Underpyrolysis conditions, the PLs generate 40% aromatics basedon carbon yield at 600 �Candmore than90%of the aromaticsare phenols which are chemicals of many applications. Fromthe catalytic tests, it is clear that ZSM-5 is more effective, interms of the reactivity, selectivity, and coke deposition. Theyield of aromatic is up to 39%, and the selectivity for aromatic

Figure 6. Aromatic selectivity of the PLs with zeolites: oxygenates(blue), arenes (yellow), and PAHs (red).

Table 4. Typical Results of Aromatic Selectivity with ZSM-5 Using

GPL as Feeda

selectivity/%

compounds blank ZSM-5

benzene trace 9.20toluene 3.23 31.57ethylbenzene 2.43 7.212,3-dimethylphenol 12.94 tracephenol 11.95 6.68cresol 16.11 5.074-ethylphenol 16.76 1.914-methylbenzaldehyde 13.48 tracenaphthalene trace 13.041-methylnaphthalene trace 8.391H-indene trace 6.14

aReaction conditions: 0.5 g of lignin with or without 0.5 g of ZSM-5,at 600 �C with nitrogen at the rate of 50 mL/min.

Figure 7. Pyrolysis products distribution of theGPL with ZSM-5 at600 �C in recycled catalytic runs: aromatics (green), gas (cyan), andcoke (gray).

Figure 8. Aromatic selectivity of the GPL with ZSM-5 in recyclecatalytic runs: oxygenates (blue), arenes (yellow), and PAHs (red).

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Energy Fuels 2010, 24, 5735–5740 : DOI:10.1021/ef100896q Zhao et al.

hydrocarbons is more than 85%. In general, the catalyticpyrolysis of PL does not only accomplish a route for the bio-oil upgrading but also extends the ranges of the application ofbio-oil.

Improvements should be carried out with the aim ofmaximizing the liquid products and minimizing the cokeformation. Although coke formation caused by thermaldecomposition can be mitigated by rising temperaturesolely, the yield of aromatics will also be reduced signif-icantly. In our opinion, the problem may be solved bychanging the pressure inside the tubular reactor (> or< 1 atm), together with robust catalysts. We believe thatfurther optimization will eventually make the process

viable for the industrial production of fuel additives andchemicals.

Acknowledgment. This work was supported by NationalBasic Research Program of China (Grant 2007CB210205),Knowledge Innovation Program of Chinese Academy of Science(GrantKGCX2-YW-3306), theNSFC-GuangdongProvince JointFund (Grant U0834005), and NCET (Grant 080519).

Supporting Information Available: Thermal gravity analysis(TGA) curves of the lignins, comparison of aromatic selectivitywith and without ZSM-5 using GPL as feed, and gas selectivityof the PLs with zeolites. This material is available free of chargevia the Internet at http://pubs.acs.org.