Kinetic Investigation of Thermal Decomposition Reactions of 4 -Demethypodophyllotoxin and Podophyllotoxin. PuHong Wen

Transcription

1 Advanced Materials Research nline: ISSN: , Vol. 800, pp doi: / Trans Tech Publications, Switzerland Kinetic Investigation of Thermal Decomposition Reactions of 4 -Demethypodophyllotoxin and Podophyllotoxin PuHong Wen 1 Province Key Laboratory of Phytochemistry, Baoji University of Arts and Sciences, Baoji Shaanxi , People s Republic of China 2 Department of Chemistry and Chemical Engineering, Baoji University of Arts and Science, 1 Gaoxin Road, Baoji, Shaanxi , People s Republic of China wenpuhong@hotmail.com Keywords: Thermal decomposition, 4 -Demethypodophyllotoxin, kinetic parameter. Abstract. The thermal behavior and thermal decomposition kinetic parameters of podophyllotoxin (PPT) and 4 -demethypodophyllotoxin (DMPPT) in a temperature-programmed mode have been investigated by means of DSC and TG-DTG. The kinetic model functions in differential and integral forms of the thermal decomposition reactions mentioned above for leading stage were established. The kinetic parameters of the apparent activation energy E a and per-exponential factor A were obtained from analysis of the TG-DTG curves by integral and differential methods. The most probable kinetic model function of both decomposition reactions in differential form was (1-α) 2. The values of E a indicated that the reactivity of PPT was higher than that of DMPPT in the thermal decomposition reaction. The values of the entropy of activation S, enthalpy of activation H and free energy of activation G of the reactions were estimated. Introduction In 1951, the American cancer institute first reported the structure of podophyllotoxin (See Figure1a), which is a main chemical composition of the plants, Sinopodophyllum emodi Wall etc, and a well-known natural product owing to its antimitotic activity, insecticidal activity and ability of its derivatives to inhibit DNA synthesis. Their effects on brain tumor, skin cancer, lung cancer, cervical carcinoma, penis cancer and etc have been validated [1,2]. It is one of more active fields in recent years to transform the structure of podophyllotoxin (PPT) in order to synthesize new derivatives with high function of inhibiting tumor and low toxicity, including the search of effective structure model by means of determination of insecticidal activity of the derivatives to establish ideal insecticide, and the research of their various properties and relationships between structure and function by the use of various methods such as ultraviolet, pulse radiolysis and laser photolysis and so on in order to indicate direction of such mentioned above research [3,4]. 4 -demethypodophyllotoxin (DMPPT) with 4 -H structure (See Figure1b) is one of derivatives of PPT with 4 -CH3 structure. It has higher antitumor effect than PPT substituting 4 -H for 4 -CH3 [5]. In this paper, their thermodynamic parameters of the melting process, and mechanism and kinetic parameters of the thermal decomposition reactions for leading stage were described. This is quite useful to evaluate their thermal stability under non-isothermal condition and study their thermal changes at high temperature. This work could provide help to the research and development of new antitumor agents and insecticides from PPT. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-04/03/16,23:43:49)

2 518 Recent Development on Material Science and Environmental Material H H Experimental H 3 C Fig. 1 The structures of (a) podophyllotoxin and (b) 4 -demethypodophyllotoxin. The samples of 4 -demethypodophyllotoxin (DMPPT) and podophyllotoxin (PPT) were supplied by School of Chemistry and Chemical Engineering in Lanzhou University. The structures of compounds PPT and DMPPT are shown in Figure 1. Their purity was more than 99% by HLPC determination. DSC and TG-DTG curves were obtained on a Dupont 9900 thermal analyzer and a Perkin Elmer diamond TG/DTA SII thermal analyzer. The heating rates of compounds DMPPT and PPT were 2-20 oc min 1 with the flow rate of N2 gas at 40 ml min 1 and the sample mass of mg. The heating rate was calculated according to the actual rising rate of temperature from ambient to the temperature at the end of the reaction. The temperature was calibrated with pure indium. Results and Discussion CH 3 (a) CH 3 H 3 C CH 3 Thermodynamic Data for the Melting Process of Compounds DMPPT and PPT. Typical DSC curves for compounds DMPPT and PPT were shown in Figure 2. The values of the melting point Tm, melting enthalpy Hm and melting entropy Sm obtained by such DSC curves were shown in Table 1, where it can be seen that Tm, Hm and Sm of compound DMPPT with 4 -H structure are higher than those of compound PPT with 4 -CH3. This phenomenon is accordant to their restraining tumor activities. The activities of compound DMPPT are higher than those of compound PPT [5]. H (b) Heat Flow /w g DSC (a) Endo Exo Heat Flow /w g DSC (b) Endo Exo Fig. 2 DSC curves of (a) 4 -demethypodophyllotoxin and (b) podophyllotoxin at a heating rate of 10 o C/min. Table 1 Thermodynamic data for the melting process of compounds DMPPT and PPT. Compd. Molecular formula Molecular weight T m ( ) H m (kj mol 1 ) S m (J mol 1 K 1 ) DMPPT C 21 H PPT C 22 H

3 Advanced Materials Research Vol Thermal Behavior. Typical TG-DTG curves for DMPPT and DMEP were shown in Figure 3. It is observed from DSC and DTG curves that there is a primary stage in their thermal decomposition processes. The leading mass losses and the leading decomposition occur in the stage accompanying heat effect. Weight /% TG (a) Weight /% TG (b) DTG DTG Fig. 3 TG-DTG curves of (a) 4 -demethypodophyllotoxin and (b) podophyllotoxin at a heating rate of 10 o C/min. Analysis of Kinetic Data. In order to obtain the kinetic parameters (the apparent activation energy Ea and per-exponential factor A) and the most probable kinetic model function of the leading stage decomposition reactions for compounds DMPPT and PPT, the following integral method [Eq. (1)] and differential method [Eq. (2)] are employed: The Satava-Sestak equation [6] AE E lgg( α ) = lg (1) βr RT The Ach ar-brindley-shap equation [6] 1 E ln[ dα / dt] = lna ( dα / dt = βdα / dt ) (2) f( α) RT where α is the fraction of conversion, dα / dt the rate of conversion, T the absolute temperature, R the gas constant, β the linear heating rate, while G(α) and f(α) are the integral and differential mechanism functions, respectively. The integral Eq. (1) and differential Eq. (2) were cited to obtain the values of Ea, A and the most probable kinetic model function of G(α) and f(α) from non-isothermal TG-DTG curves. The values of α, T and dα / dt obtained by the TG-DTG curves are listed in Table 2. Forty-one types of kinetic model function [6] and the data in Table 2 were put into Eq. (1) and Eq. (2) for calculation, respectively. The values of Ea, A, linear correlation coefficient r and standard mean square deviation Q were obtained by the linear least-squares and iterative methods [7]. The probable kinetic model functions of the integral and differential methods were selected by the logical choice method [8] and the corresponding kinetic parameters are listed in Table 3. Therefore, for the leading stage decomposition reactions, the second order reaction of n =2 for DMPPT and PPT are acceptable. And Eak and Ak ( kj/mol and s-1 for DMPPT, and kj/mol and s-1 for PPT) obtained by Kissinger method [6,8] are in good agreement with the calculated average values in Table 3. The values of Ea indicated that the reactivity of PPT was higher than that of DMPPT in the thermal decomposition process.

4 520 Recent Development on Material Science and Environmental Material Table 2 Data of compounds DMPPT and PPT determined by TG curves with β = 15 min 1 Compd. T 0 ( ) Data point T ( ) α dα/dt (K -1 ) Data point T ( ) α dα/dt (K -1 ) DMPPT PPT The enthalpy of activation H, free energy of activation G and entropy of activation S of the reactions at Tp were obtained by Eq. (3)-(5) [8] and the corresponding kinetic parameters are summarized in Table 3. E kbt G Aexp = exp (3) RT h TR H = E RT (4) G = H T S (5) where k B and h are the Boltzmann constant and the Planck constant, respectively. Compd. Method Table 3 Kinetic parameters obtained by the data in Table 2. Form of the most probable kinetic model function E a (kj mol 1 ) / lg(a/s 1 ) DMPPT Šatava-Šesták G(α)= (1-α) /12.22 Achar-Brindley-Shap f(α)= (1-α) / Average value /12.88 PPT Šatava-Šesták G(α)= (1-α) /11.19 Achar-Brindley-Shap f(α)= (1-α) / Average value /11.03 r/q / / / / H (kj mol 1 ) T p G (kj mol 1 ) T p =371.9 o C S (J mol 1 K 1 ) T p =378.7 o C

5 Advanced Materials Research Vol Conclusions The kinetics and mechanism of the leading stage decomposition reactions for DMPPT and PPT have been investigated. Both have same the kinetic model functions in integral and differential forms, G(α) = (1-α) 1-1 and f(α)= (1-α)2. The apparent activation energy and pre-exponential factor of these reactions have been obtained: Ea= kj mol 1, A= for DMPPT and Ea= kj mol 1, A= for PPT respectively. The values of Ea indicated that the thermal stability of skeleton structure in DMPPT was higher than that in PPT. The values of enthalpy of activation H, free energy of activation G and entropy of activation S of the reactions at Tp were , kj mol 1, J mol 1 K 1 for DMPPT and , kj mol 1, J mol 1 K 1 for PPT respectively. The differences of the thermal behavior between DMPPT and PPT are produced by substituting 4 -H for 4 -CH3 in the structure. Acknowledgments This work was supported by the Science and Technology Foundation of the Shaanxi Key Laboratory (grant no. 2003JS018), and Key Research Project (grant no. ZK1051) from Baoji University of Arts and Sciences. References [1] C. Ma and S.R. Luo: Chin Tradit Herb Drugs Vol. 23 (1992), p. 271 (in Chinese). [2] B.F. Xie, Z.Y. Cheng and Z.M. Li: Chin. J. Cancer Vol. 20 (2001), p. 368 (in Chinese). [3] S.L. Wang, M. Wang and X.Y. Sun: Chem. J. Chin. Univ. Vol. 24 (2003), p (in Chinese). [4] R. Gao, X. Tian and Y. Zhang: Chin. J. Pestic. Sci. Vol. 2 (2000), p. 1 (in Chinese). [5] Y.G. Wang, L. Tao, J.L. Pan, J.F. Shi and Y.Z. Chen: Chem. J. Chin. Univ. Vol. 18 (1997), p [6] R.Z. Hu and Q.Z. Shi: Thermal Analysis Kinetics (Science Publications, Beijing 2001) (in Chinese). [7] R.Z. Hu, Z.Q. Yang and Y.J. Liang: Thermochim. Acta Vol. 123 (1988), p [8] P.H. Wen, G.D. Feng and J.B. Zheng: Chinese Journal of Chemistry Vol. 24 (2006), p. 29.

6 Recent Development on Material Science and Environmental Material / Kinetic Investigation of Thermal Decomposition Reactions of 4'-Demethypodophyllotoxin and Podophyllotoxin / DI References [8] P.H. Wen, G.D. Feng and J.B. Zheng: Chinese Journal of Chemistry Vol. 24 (2006), p /cjoc