DFT Kinetic Study of the Pyrolysis Mechanism of Toluene Used for Carbon Matrix

Transcript

1 , ACTA CHIMICA SINICA Vol 59, 2001 No 1, a,d Ξ a b b a a ( a b ) c d ( c ) ( d ) UB3LYP/ 3-21G K : 963 K, 0 = kj/ mol ; 963 K K, E 0 = kj/ mol /,,,, UB3LYP/ 3-21G 3 DFT Kinetic Study of the Pyrolysis Mechanism of Toluene Used for Carbon Matrix WANG Hui a,d 3 YANG Hai - Feng a ZHAI Gao - Hong b WEN Zhen - Yi b RAN Xin - Quan a SHI Qi - Zhen a ( a Chemistry Department, b Modern Physics Institute, Northwest University, Xi an, ) LUO Rui - Ying c YANG Yan - Qing d ( c Beijing University of Aeronautics and Astronautics, Beijing, ) ( d Department of Material Science, Northwestern Polytechnic University, Xi an, ) Abstract Based on the thermodynamic data, the pyrolysis mechanism of the carbon matrix toluene used for carbon material was studied by dynamic computation using the UB3LYP/ 3-21G 3 method in the Gaussian 98 program package The transition states were found by the QST2 method and were proved by IRC calculations The activation energies of the five reaction paths were calculated, and the rate constants from K were obtained by the transition states theory The dynamic calculation results show, when the pyrolysis temperature of toluene is lower than 963 K,the main reaction path is the breaking of the C H bonds of the methyl on the benzene ring The reaction is completed via the process : reactant intermediate product, with reactant intermediate as rate - control step and activation energy 0 = kj/ mol When temperature is higher than 963 K, but below K, the main reaction paths is the one that produces benzene radical and methyl radical The corresponding activation energy is 0 = kj/ mol This mechanism is in accord with the experimental result Ξ E - mail : xaonline com : , : , ( ) (99J K100) (Received June 30, 2000 Accepted August 30, 2000)

2 18 Vol 59, 2001 Keywords carbon/ carbon composites material, carbon matrix toluene, pyrolysis mechanism, dynamics research, UB3LYP/ 3-21G 3 method /,, ( UAM1 UHF/ 3-21G 3 UB3LYP/ 3-21G 3 ), /,, / 1, [1 3 ] ( 1),,, /, UB3LYP/ 3-21G 3 1 Fig 1 Design of toluene pyrolysis reaction path,, ( E 0 ) 2 1, UB3LYP/ 3-21G 3 QST2,,, ( IRC) readisotopes 1 2 (298 K,773 K,843 K,963 K, K,1 223 K) 0 H G S, S Deutsch [4,5 ],773 K [6 C H, : ] : 0 ( ) = E elec ( ) + ZPE (1) H ( ) = 0 + vib ( ) + rot ( ) + transs ( ) + nrt (2) ( K), G ( ) = H - T S (3) [7 Gaussian 98 ] Dell Dimension P - 550

3 January 2001 : [8 ] : k = k T/ h exp ( - G / RT), = k T/ h exp ( S / R - H / RT) (4) ; k Boltzmann ; h Planck K 0 E : ; K, 3 ; ( IM) (TS) (1), 2 3 [8 ] ( k) 2 [ :nm : ( ) ] Fig 2 The optimized geometries and structure parameters of transition state and intermediate at UB3LYP/ 3-21G 3 level [ bond lengths (nm), bond angles and dihedral ( ) ] R TS1 1, 0 = E 0 (TS1) - E 0 (R) = kj/ mol 5, 1, 5, 1 1 IM TS1 TS1 1,,

4 20 Vol 59, 2001, [9 ] H H G Fig 3 Potential energy profiles along reaction path 500 (773 K) H G Fig 4 The main pyrolysis reaction path of toluene 800 (1 073 K) H G (1 223 K) Table 1 The calculated activation energies of pyrolysis reaction for 5 path and the sole imaginary frequency of TS ( I/ cm - 1 ) TS1 TS1 TS2 TS3 TS4 TS Freq( I) (kj/ mol) (kj/ mol) Table 2 Calculated activation enthalpy (kj/ mol), activation free energy (kj/ mol) of the thermolysis process of five types at different temperature TS1 TS1 TS2 TS3 TS4 TS5 25 (298 K) H G (843 K) H G (963 K) H G , G 3 S G S, G TS1 G TS1, TS2, TS3, TS4 TS5 G 1 S, 4 S S G ( ) K ( 2) 2, (4), K 5 ( 3) [9 ], H, 1 1 R TS1 IM TS1 P, 2 ( E vib ( ) + R TS1, rot ( ) + transs ( ) + nrt) TS1 k TS1 TS1 k TS1

5 January 2001 : 21,, 1 2 ;, 1 R TS1, 1 3,4 5 k TS1 k TS2, k TS3, k TS4 k TS5, : 963 K, 1,2 3,4 5 k k TS1 > k TS2 > k TS3 > k TS4 > k TS5, 1 ; K, k TS1 k TS2 2 (800 ), 1 ; K ( K), k 2 k TS1 k TS2 > k TS1,, 3 ( k/ s - 1 ) Table 3 The calculated rate constant ( k/ s - 1 ) at different temperature for reaction 1, 2, 3, 4 and 5 T/ K k TS1 k TS1 k TS2 k TS3 k TS4 k TS : (1) p K 1, 1 R TS1 IM TS1 P R TS1, E 0 = kj/ mol ; (2), 1 073K ( K), k 2 k TS1 k TS2 > k TS1 1 2, 0 = kj/ mol References 1 Wang, H ; Luo, R - Y ; Yang, Y - Q ; Ran, X - Q ; Wen, Z - Y Chin J Mater Res 2001, 15 (2), 151 (in Chinese) 2 Zhai, G - H ; Wang, H ; RAN, X - Q ; Wen, Z - Y ; Luo, R - Y ; Yang Y - Q Mater Sci Eng 2000, 18 (4), 10 (in Chinese) 3 Wang, H ; Zhai, G - H ; Ran, X - Q ; Shi, Q - Z ; Wen, Z - Y ; Luo, R - Y ; Yang Y - Q Chin J Inorg Chem 2000, 16 (6), 879 (in Chinese) 4 Ooya, S ; Kobayashi, H Carbon 1967, 51, 29 5 Deutsch, S ; Keieger, K A J Phy Chem 1962, 66 (19), Frisch, E ; Frisch, M J Gaussian 98 User s Reference, 2nd Edn, 7 Frisch, M J ; Trucks, G W ; Schlegel, H B ; Scuseria, G E ; Robb, M A ; Cheeseman, J R ; Zakrzewski, V G ; Montgomery, J A ; Stratmann, Jr R E ; Burant, J C ; Dapprich, S ; Millam, J M ; Daniels, A D ; Kudin, K N ; Strain, M C ; Farkas, O ; Tomasi, J ; Barone, V ; Cossi, M ; Cammi, R ; Mennucci, B ; Pomelli, C ; Adamo, C ; Clifford, S ; Ochterski, J ; Petersson, G A ; Ayala, P Y ; Cui, Q ; Morokuma, K ; Malick, D K ; Rabuck, A D ; Raghavachari, K ; Foresman, J B ; Cioslowski, J ; Ortiz, J V ; Stefanov, B B ; Liu, G ; Liashenko, A ; Piskorz, P ; Komaromi, I ; Gomperts, R ; Martin, R L ; Fox, D J ; Keith, T ; Al - Laham, M A ; Peng, C Y ; Nanayakkara, A ; Gonzalez, C ; Challacombe, M ; Gill, P M W ; Johnson, B ; Chen, W ; Wong, M W ; Andres, J L ; Gonzalez, C ; Head - Gordon, M ; Replogle, E S ; Pople, J A Gaussian 98, Revision A 6, Gaussian, Inc, Pittsburgh PA, Fu, X - C ; Chen, R - H Physical Chemistry ( 2), People s Education Press, Beijing, 1982, pp , pp ( in Chinese) 9 Fu, X - C ; Chen, R - H Physical Chemistry ( 1), People s Education Press, Beijing, 1982, pp , pp ( in Chinese) ( Ed PAN Bing - Feng)