J. Anal. Appl. Pyrolysis 87 (2010) 65–69

Analysis of wheat straw lignin by thermogravimetry and pyrolysis–gaschromatography/mass spectrometry

Qing yang *, Shubin Wu, Rui Lou, Gaojin Lv

State Key Lab of Pulp & Paper Engineering, South China University of Technology, No. 381 Wu Shan Road, Guangzhou, Guangdong 510640, PR China

A R T I C L E I N F O

Article history:

Received 7 May 2009

Accepted 12 October 2009

Available online 24 October 2009

Keywords:

Wheat straw

Thermogravimetry

Dynamics

Lignin

Py–GC/MS

A B S T R A C T

The thermogravimetric characteristics of wheat straw enzymatic acidolysis lignin (EMAL) were

analyzed. At a temperature above 200 8C, the pyrolysis rate of wheat straw EMAL was clearly accelerated,

since all DTG curves and the maximum temperature corresponding to pyrolysis shifted to high

temperature areas, two maxima being also recorded on the DTG curves. The thermogravimetric

dynamics parameters of wheat straw EMAL were calculated by the methods of Kissinger and Ozawa,

respectively; the activation energy of wheat straw EMAL was of 103.92 and 107.69 kJ mol�1,

respectively, and the frequency factor, ln A, of 19.21 and 20.60 min�1. Pyrolysis–gas chromatography

mass spectrometry (Py–GC/MS) was applied to analyze wheat straw EMAL at 500 8C for investigating fast

thermal degradation products of lignin. The identified key marker compounds were the derivatives of

the three kinds of lignin basic units (guaiacyl, syringyl, and hydroxyphenyl units), but the guaiacyl units

dominated over the lignin-derived products.

Crown Copyright � 2009 Published by Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Journal of Analytical and Applied Pyrolysis

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1. Introduction

Fast pyrolysis, which is a process in which organic materialsare rapidly heated at 450–600 8C in absence of air [1], as one ofthermal decomposition technologies, has been progressivelydesigned and optimized for producing bio-oils. These newobjectives require lower process temperatures and efficientcooling (quenching) of the condensable vapors in order toprevent their cracking and hence maximize oils production [2].

Due to the complexity of lignin and the difficulty in extraction,the literature related to the pyrolysis behavior of lignin isscarce. Garcıa-Perez et al. [3] proposed the activation energy of30 kJ mol�1 and frequency factor of 25 min�1 for softwood barklignin with three-pseudocomponent models. Liu et al. [4] studiedthe kinetics of fir lignin pyrolysis and put forward a first-orderdecomposition model for the temperature range of 149–288 8C,in which the activation energy and frequency factor were72.9 kJ mol�1 and 1.14 � 107 s�1. Muller-Hagedorn and Bockhorn[5] obtained kinetic parameters for barley straw and Gavottlignins in a TGA based on the Levenberg–Marquardt and improvedRunge–Kutta laws. The activation energies and frequency factorfor barley straw and Gavott lignins varied from 92 to 102 kJ mol�1,from 107.3 to 107.9 min�1; and from 84 to 94 kJ mol�1 from 106.7 to

* Corresponding author. Tel.: +86 20 2223 6808.

E-mail address: qing.yb@mail.scut.edu.cn (Q. yang).

0165-2370/$ – see front matter . Crown Copyright � 2009 Published by Elsevier B.V. A

doi:10.1016/j.jaap.2009.10.006

107.1 min�1, respectively. Domınguez et al. [6] reported that theactivation energy of 17 organosolv lignins by using Borchardt–Daniels method was from 17.9 to 42.5 kJ mol�1. Wang et al. [7]estimated that the activation energy of sawdust lignin wascalculated to be 120.7–197.3 kJ mol�1 according to the distrib-uted active energy model (DAEM). Substantial differencesapparent can be observed in the kinetic parameters reported,which can be due to several factors related to the experimentalmethods, operating conditions and data analysis, but also to thechemical composition of the raw material examined in each study.

Thermoanalytical techniques, in particular thermogravimetricanalysis (TGA) and derivative thermogravimetric (DTG), allow thisinformation to be obtained in a simple and straightforwardmanner. Pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) also has been applied to examine reaction products ofthermal degradation [8] and an effective tool to investigate fastpyrolysis of biomass and on-line analysis of the pyrolysis vapors[9,10].

It has been reported that biomass pyrolysis liquids containlarge molecules derived from lignin [11], which has prejudiciousinfluences on bio-oil properties [12]. So the aim of this paperwas to study the thermal behavior of wheat straw lignin duringthe pyrolysis process, to provide data and kinetic informationnecessary for the evaluation of the process, as well as tocharacterize fast pyrolysis vapors of wheat straw lignin, in aneffort for the rational use of lignin based on thermochemicalconversion of lignin to energy and chemicals.

ll rights reserved.

Fig. 1. TG curves of wheat straw EMAL at different heating rates.

Q. yang et al. / J. Anal. Appl. Pyrolysis 87 (2010) 65–6966

2. Experimental

2.1. Preparation of EMAL

2.1.1. Cellulase enzymatic hydrolysis

The milled wheat straw powder was dewaxed with acetone in aSoxhlet apparatus for 6 h at 60 8C. The treated sample (10 g)was subjected to enzyme treatment, by using industrial cellulasewith activity of about 8000 units/ml carboxyl methyl cellulose(CMCase). The enzymatic hydrolysis was carried out at 40 8C over aperiod of 48 h at pH value 4.5 (using acetate buffer solution) and acontent of 5%. After the enzymatic hydrolysis, the impureenzymatic hydrolysis lignin was centrifuged, and washed twicewith acidic deionized water at the pH value 2.0 (HCl), freeze-dried.

2.1.2. Mild acidolysis

Impure enzymatic hydrolysis lignin (5 g) was suspended inl00 ml the dioxane/acidified deionized water solution (85:15, v/v,0.01 mol/l HCl) and extracted at 87 8C sparging nitrogen gas for 2 h.The obtained mixture solution was filtered and the lignin solutionwas collected. The solid residue was sequentially washed withfresh dioxane/deionized water solution (85:15, v/v) for 2–3 times.The total filtrates solution was neutralized with sodium bicarbo-nate and stirred for 3 h. The neutralized solution was then rotary-evaporated until a thick solution was obtained. This thick solutionwas carefully dropped into a large quantity of acidified deionizedwater (pH 2.0, HCl) and the precipitated lignin was isolated bycentrifugation, washed and freeze-dried. The obtained lignin wasfinally washed with HPLC grade hexane and dried in a vacuumdrying oven at room temperature. This material was designated asthe enzyme acid hydrolysis lignin (EMAL).

2.2. The content of carbohydrate analysis

The content of carbohydrate was analyzed on the basis ofNational Standards Methods and literature data [13]. The resultswere shown in Table 1.

2.3. Thermogravimetric analysis

Thermogravimetric experiment was performed in TG209Integrated Thermal Gravimetric Analyzer from NETZSCH Corpora-tion in Germany with high purity nitrogen as carrier gas whoseflow rate was 40 cm3/min. About 6–12 mg material was put inceramic crucible each time and heated from room temperature to800 8C with heating rates of 10, 20, 30, 40 and 50 8C/min,respectively. Calculated thermogravimetric rate data was auto-matic output through the system of TG209 Integrated ThermalGravimetric Analyzer. Thermogravimetric experiment was used toobtain the parameter of chemical dynamics, so influence of errorwas decreased as much as possible. The average size of all sampleswas less than 0.2 mm so that the mass loss of samples was underthe control of kinetics reaction.

2.4. Py–GC/MS analysis

Py–GC/MS analysis was tested on a combined system of anAmerican CDS5150 pyrolyzer and Shimazu QP2010 Plus GC-MS.Pyrolysis temperature was set up at 500 8C, heating time 1 min. GCqualitative analysis was conducted with DB-Wax fused silica

Table 1Results of the isolation of EMAL from wheat straw (wt.%).

Material Yield Purity Carbohydrate content

Wheat straw EMAL 58.7 93.4 4.3

capillary column (30 m � 0.25 mm, i.e., film thickness 0.25 mm);flow rate of He was 1.22 ml/min; column temperature, held at50 8C for 5 min, then raised to 280 8C at the rate of 10 8C/min;injector temperature 250 8C; EI-MS scan range 45–600 amu, scantime 0.5 s; EI ionization energy 70 eV. Data processing wasperformed with the PerkinElmer NIST spec version 5.

3. Results and discussion

3.1. Thermal decomposition characteristics of enzyme acid hydrolysis

lignin (EMAL)

The TG and DTG curves of wheat straw EMAL, obtained atheating rates of 10, 20, 30, 40 and 50 8C/min, separately, andplotted in Figs. 1 and 2, respectively, showed that thermaldecomposition occurred over a wide temperature range, startingat approximately 150 8C. A major loss of weight followed,accompanied by devolatilization with charring [14], which washighly exothermal [15], between 200 and 500 8C, while the charresidue at 800 8C varied between 32 and 40%, at differentheating rates. However, over 500 8C, this was followed by a slowfurther mass loss up to the final temperature, the TG curvestending to become flat. It was observed that, at a giventemperature of the same sample, the slower heating rates, thehigher the degradation degree because when heating rateincreased, the response time of sample attaining necessarytemperature of pyrolysis shortened, which was in favor ofthermal degradation [16].

Fig. 2. DTG curves of wheat straw EMAL at different heating rates.

Fig. 3. Linearization curves of Kissinger method.

Table 3Parameter values in Ozawa formula.

b (min�1) T 1/T2�103 Lnb

10 616.2 1.62 2.30

20 634.2 1.58 3.00

30 645.9 1.55 3.40

40 653.7 1.53 3.69

50 665.5 1.50 3.91

Q. yang et al. / J. Anal. Appl. Pyrolysis 87 (2010) 65–69 67

The DTG peaks differed in their position and height, because theheating rate acted as a significant factor on lignin thermaldecomposition. With increasing the heating rate from 10 to50 8C/min, the thermal decomposition rate of increased over the200–500 8C, the DTG curves being shifted towards a highertemperature zone, as well as the temperatures corresponding tothe maximum loss of mass peaks, also shifted towards highervalues. The first peak, appearing between 80 and 150 8C wasobviously responsible for the moisture retained in wheat strawEMAL, since its evolution mainly occurred around 100 8C. Atdifferent heating rates, the first maximum weight loss peak wasdue to the bulk reaction between the epoxy groups of the phenolichydroxyl groups of lignin, while the second maximum weight losspeak was subjected to heat release during the formation of charfrom lignin [17].

3.2. Kinetics of wheat straw EMAL pyrolysis

Kissinger and Ozawa methods were respectively differentialand integral thermal analysis methods which were used todetermine the apparent activation energy of degradation atdifferent heating rates. In non-isothermal kinetics, for the mostusual case of a linear heating program corresponded to a constantheating rate, the dependence of the reaction rate on thetemperature and the extent of the reaction could be describedby the following well-known differential equation (Eq. (1)):

dadt¼ k f ðaÞ (1)

where

a ¼ 1�mðtÞ �m f

m0 �m fand k ¼ A exp � E

RT

� �

m(t) was the experimental mass at each monitoring time, mf wasthe final mass, and m0 was the initial dry mass. f(a) was thedifferential form kinetic mechanism function and k was thekinetic constant which, according to the Arrhenius equation,was a function of the pre-exponential factor (A), apparentactivation energy (E), absolute temperature, and constant ofideal gas law (R).

The temperature–time relation was computed from flowingexpression (Eq. (2)):

T ¼ Toþ bt (2)

b whose unit was 8C/min was the constant heating rate, and To; theinitial temperature.

The function (Eqs. (3) and (4)) was deduced via Eqs. (1) and (2),as follows:

lnbT2

p

!¼ � E

RTPþ ln

AR

E

� �(3)

lnðbÞ ¼ lnAE

RgðaÞ � 5:331� 1:052E

RT(4)

Eqs. (3) and (4) provided the formula for calculating theapparent activation energy (E) and the constant of ideal gas law (R)

Table 2Parameter values in Kissinger formula.

b (min�1) T l/T�10�3 l/T2� l0�6 b/T2� l0�6 Ln(b/T2� l0�6)

10 616.2 1.62 2.63 26.3 �10.55

20 634.2 1.58 2.45 49.0 �9.92

30 645.9 1.55 2.40 72.0 �9.54

40 653.7 1.53 2.34 93.6 �9.28

50 665.5 1.50 2.26 113.0 �9.09

of Kissinger [18] and of Ozawa [19], respectively. g(a) was theintegral form of the kinetics mechanism function in Eq. (4).Kissinger’s method underlined the conversion of solid into solidand gas, where the temperatures of the maximum peaks of DTGcurves were monitored at different heating rates [18]. Ozawa’smethod was an integral technique which expressed straight linesat different heating rates according to relationship between thelogarithm of the heating rate and reciprocal temperature atconstant mass loss, in which the apparent activation energy ofdegradation was calculated from the slope of linear relationships[19]. The methods of Kissenger and Ozawa, as two sorts of non-isothermal kinetics which benefited from fast measurement,wide range of temperature and broad use, were applied in theexperiments.

3.2.1. Kissinger method

While temperature reached its maximum value, the effect ofthe heating rates on the peak temperature followed Eq. (3). Thepeak temperature and the heating rates plotted in Fig. 2 wereshown in Table 2. The graph of lnðb=T2

pÞ depending on 1/Tp wasplotted in Fig. 3.

Fig. 4. Linearization curves of Ozawa method.

Table 4Identification of the pyrolysis products by their mass fragments.

Peak Time

(mm)

Compound Area (%) Formula Similarity

1 1.525 Carbon dioxide 24.55 CO2 94

2 1.683 2-Butanone 4.74 C4H8O 90

3 1.842 Methanol 1.27 CH4O 86

4 1.925 Acetic acid 3.60 C2H4O2 97

5 4.400 Furfural 1.43 C5H4O2 96

6 9.250 o-Guaiacol (G) 7.31 C7H8O2 98

7 10.967 p-Creosol (G) 4.44 C8H10O2 96

8 11.350 2,3-Dihydro-benzofuran 10.80 C8H8O 91

9 12.117 p-Isopropylbenzaldehyde (H) 0.52 C10H12O 85

10 12.275 4-Ethylguaiacol (G) 1.19 C9H12O2 91

11 12.725 2-Hydroxy-5-methylacetophenone (G) 19.27 C9H10O2 88

12 13.117 Syringol (S) 8.55 C8H10O3 93

13 13.358 p-Eugenol (G) 1.14 C10H12O2 91

14 13.692 Vanillin (G) 1.20 C8H8O3 90

15 14.433 1,2,4-Trimethoxybenzene (G) 4.57 C9H12O3 83

16 14.567 (E)-Isoeugenol or cis-isoeugenol (G) 4.16 C10H12O2 93

17 16.367 Methoxyeugenol (S) 0.47 C11H14O3 84

18 25.392 Isooctyl phthalate 0.77 C24H38O4 91

G: guaiacyl lignin; S: syringyl lignin; H: hydroxyphenyl lignin.

Q. yang et al. / J. Anal. Appl. Pyrolysis 87 (2010) 65–6968

Fitting the equation in Fig. 3 was expressed as

y ¼ �12:49974� 103xþ 9:7756 (5)

The linear regression coefficient (R2) was 0.99. Slope was�12.49974 � 103, from which deduced the apparent activationenergy (E) – 103.92 kJ mol�1 – was deduced; the intercept of fitequation was 9.7756, so the pre-exponential factor (ln A) was19.21 min�1.

3.2.2. Ozawa method

ln b and 1/Tp represented the linear relation with a given valueof a in the DTG curves of different heating rates. The effect ofheating rate on the peak temperature of the DTG curves wasfollowed in Eq. (4). The peak temperature and heating rates of Fig. 2were shown in Table 3, while the graph of ln b depending on 1/Tp

was shown in Fig. 4.Fitting of the equation in Fig. 4 was expressed as

y ¼ �13:62676� 103xþ 24:46324 (6)

The linear regression coefficient (R2) was 0.98. The slope was13.62676 � 103, from which the apparent activation energy (E) – of107.69 kJ mol�1 – was deduced; the intercept of the fit equation was24.46324, so the pre-exponential factor (ln A) was 20.60 min�1.

4. Py–GC/MS of wheat straw lignin

The pyrogram of wheat straw EMAL sample was graphicallypresented in Fig. 5, and Table 4 summarized the products of lignin

Fig. 5. Py–GC/MS pyrogram of wheat straw EMAL at 500 8C.

pyrolysis by their mass fragments. The key markers of compoundsfrom the pyrogram were identified with the PerkinElmer NIST 05Library and the existing published data to obtain the highestpossible likelihood of compound identification. Jakab et al. [20]concluded that the release of monomer products was independentof the lignin type and the monomer formation was not onlydetermined by the energy required for the rupture of primarybonds (C–C or C–O–C). The most lignin-derived key markers werefrom guaiacyl (G) lignin units, next of syringyl (S) lignin units andthe least of hydroxyphenyl (H) lignin units. The yield of theguaiacyl-type was about 4.5 times higher than that of the sum ofthe syringyl-type, and the hydroxyphenyl-type and the total yieldof hydroxyphenyl-type, syringyl-type and guaiacyl-type was52.82%. Among products of lignin pyrolysis at 500 8C, the existof furfural was due to the pyrolysis of residual xylan in lignin, andthe yield of the 2,3-dihydro-benzofuran, a kind of heterocycliccarbonaceous matter, was high and its value was 10.80%. It wasreported that the carbon dioxide was released at about 100 8Chigher and coincided with the acetic acid formation [20], so thereleasing of carbon dioxide and the occurrence of acetic acid couldoriginated from hemicellulose in pyrolysis. As for methanol, Jakabet al. [21] proved that methoxyl groups were the primary source ofmethanol. The origin of isooctyl phthalate was poorly understandand would be needed further research. In accordance with thePerkinElmer NIST 05 Library, the percentage of sample notidentified in Fig. 5 was not the key markers of compounds byreason of the presence of impurity caused by ion fragments in massspectrum.

5. Conclusions

� The influence of various heating rates on wheat straw wassignificant, the pyrolysis rate of wheat straw EMAL increasingover a temperature range between 200 and 500 8C. Attemperatures above 200 8C, the pyrolysis rate of wheat strawEMAL accelerated obviously; all DTG curves and maximumpyrolysis corresponding temperature shifted towards a hightemperature area, two maximum being also recorded on the DTGcurves.� The thermogravimetric dynamics parameters of wheat straw

EMAL were calculated by the methods of Kissinger and Ozawa,respectively. Although the results of the dynamics parameterswere almost equal, the fitting degree of the Kissinger methodwas better than that of the Ozawa one.

Q. yang et al. / J. Anal. Appl. Pyrolysis 87 (2010) 65–69 69

� With Py–GC/MS analysis at 500 8C, the identified key markercompounds were the derivatives of the three kinds of lignin basicunits (guaiacyl, syringyl, and hydroxyphenyl units), but theguaiacyl units dominated over the lignin-derived products.

Acknowledgements

The work described in this paper was supported by a grant fromthe National High Technology Research and Development Programof China (863 Program) (No. 2007AA05Z456) and the NationalNatural Science Foundation of China (No. 20576043).

References

[1] A.V. Bridgewater, Chem. Eng. J. 91 (2003) 87–102.[2] R. Bayerbach, D. Meier, J. Anal. Appl. Pyrol. 85 (2009) 98–107.[3] M. Garcıa-Perez, A. Chaala, H. Pakdel, D. Kretschmer, C. Roy, J. Anal. Appl. Pyrol. 78

(2007) 117–124.

[4] Q. Liu, S.R. Wang, Y. Zheng, Z.Y. Luo, K.F. Cen, J. Anal. Appl. Pyrol. 82 (2008) 170–177.[5] M. Muller-Hagedorn, H. Bockhorn, J. Anal. Appl. Pyrol. 79 (2007) 136–146.[6] J.C. Domınguez, M. Oliet, M.V. Alonso, M.A. Gilarranz, F. Rodrıguez, Ind. Crop. Prod.

27 (2008) 150–156.[7] G. Wang, W. Li, B.Q. Li, H.K. Chen, Fuel 87 (2008) 552–558.[8] D.J. Nowakowski, M.J. Jones, J. Anal. Appl. Pyrol. 83 (2008) 12–25.[9] Q. Lu, W.Z. Li, D. Zhang, X.F. Zhu, J. Anal. Appl. Pyrol. 84 (2009) 131–138.

[10] R. Fahmi, A.V. Bridgwater, S.C. Thain, I.S. Donnison, P.M. Morris, N. Yates, J. Anal.Appl. Pyrol. 80 (2007) 16–23.

[11] J. Lede, F. Broust, F.-T. Ndiaye, M. Ferrer, Fuel 86 (2007) 1800–1810.[12] A. Oasmaa, E. Kuoppala, Energy Fuel 17 (2003) 1075–1084.[13] S.L. Shi, F.W. He, Analysis of Pulp & Papermaking, China Light Industry Press,

Beijing, 2003, pp. 311–316.[14] H.P. Yang, R. Yan, H.P. Chen, D.H. Lee, C.G. Zheng, Fuel 86 (2007) 1781–1788.[15] R. Ball, A.C. McIntosh, J. Brindley, Combust. Theory Model. 8 (2004) 281–291.[16] Y. Tang, S.B. Wu, R. Lou, Q. Yang, J South China Univ. Technol. (Nat. Sci. Ed.) 37

(2009) 22–26.[17] R. Lou, S.B. Wu, Cell. Chem. Technol. 42 (2008) 371–380.[18] H.F. Kissinger, Anal. Chem. 29 (1957) 1702–1706.[19] T. Ozawa, Bull. Chem. Soc. Jpn. 38 (1965) 1881–1886.[20] E. Jakab, O. Faix, F. Till, J. Anal. Appl. Pyrol. 40–41 (1997) 171–186.[21] E. Jakab, O. Faix, F. Till, T. Szekely, J. Anal. Appl. Pyrol. 35 (1995) 167–179.