2001 59 1, 17 21 ACTA CHIMICA SINICA Vol 59, 2001 No 1, 17 21 a,d Ξ a b b a a ( a b 710069) c d ( c 100083) ( d 710072) UB3LYP/ 3-21G 3 5 298 1 223 K : 963 K, 0 = 402 27 kj/ mol ; 963 K 1 223 K, E 0 = 456 91 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, 710069) LUO Rui - Ying c YANG Yan - Qing d ( c Beijing University of Aeronautics and Astronautics, Beijing, 100083) ( d Department of Material Science, Northwestern Polytechnic University, Xi an, 710072) 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 298 1 223 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 = 402 27 kj/ mol When temperature is higher than 963 K, but below 1 223 K, the main reaction paths is the one that produces benzene radical and methyl radical The corresponding activation energy is 0 = 456 91 kj/ mol This mechanism is in accord with the experimental result Ξ E - mail : xb @pub xaonline com :2000-06 - 30, : 2000-08 - 30, (596820070) (99J K100) (Received June 30, 2000 Accepted August 30, 2000)
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, 1 073 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) ( 1 073 K), G ( ) = H - T S (3) [7 Gaussian 98 ] Dell Dimension P - 550
January 2001 : 19 2 2 [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 ; 3 1 2 ( 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 ( ) ] 3 2 1 3 2 1 3 R TS1 1, 0 = E 0 (TS1) - E 0 (R) = 402 27 1 kj/ mol 5, 1, 5, 1 1 IM TS1 TS1 1,,
20 Vol 59, 2001, [9 ] H H 400 46 370 99 459 37 593 53 661 03 659 19 3 G 409 14 363 82 454 36 595 72 653 30 654 30 Fig 3 Potential energy profiles along reaction path 500 (773 K) H 400 34 380 02 462 85 598 71 667 23 665 28 4 G 428 67 340 99 439 10 595 88 630 32 637 74 Fig 4 The main pyrolysis reaction path of toluene 800 (1 073 K) H 400 19 381 13 462 61 598 95 667 54 665 58 G 431 91 336 47 436 40 595 54 626 08 634 58 1 950 (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 TS5 0 402 27 367 72 456 91 592 13 657 85 656 24 Freq( I) 1 770 24 238 32 202 65 544 58 436 86 459 35 2 (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 400 45 377 82 462 86 597 93 666 31 664 37 G 423 10 348 46 443 80 596 37 637 77 643 09 570 (843 K) H 400 43 378 68 462 93 598 28 666 71 664 77 G 425 15 345 77 442 07 596 21 634 89 641 14 690 (963 K) H 399 87 382 47 462 08 599 07 667 73 665 77 G 436 37 330 14 433 01 595 04 620 72 630 28, G 3 S G S, G TS1 G TS1, TS2, TS3, TS4 TS5 G 1 S, 4 S S G 3 2 2 ( ) 3 3 298 1 223 K ( 2) 2, (4), 298 1 223 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
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 ; 1 073 K, k TS1 k TS2 2 (800 ), 1 ; 1 073 K ( 1 223 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 298 773 843 963 1 073 1 223 k TS1 k TS1 k TS2 k TS3 k TS4 k TS5 1 2 10-59 4 1 10-16 7 9 10-14 1 1 10-10 2 0 10-8 5 9 10-6 1 1 10-51 4 6 10-11 6 6 10-9 6 4 10-6 9 3 10-4 2 0 10-1 1 4 10-67 1 6 10-17 7 1 10-15 3 1 10-11 1 3 10-8 8 2 10-6 2 3 10-92 8 1 10-28 2 0 10-24 9 6 10-20 2 3 10-16 9 9 10-13 1 9 10-102 1 3 10-30 8 0 10-27 1 3 10-21 7 4 10-1 7 9 10-14 1 3 10-102 5 6 10-31 3 3 10-27 5 1 10-22 - 18 2 9 10 3 1 10-14 4 : (1) p 86 963 K 1, 1 R TS1 IM TS1 P R TS1, E 0 = 402 27 kj/ mol ; (2), 1 073K ( 1 223 K), k 2 k TS1 k TS2 > k TS1 1 2, 0 = 456 91 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), 1569 6 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,1998 8 Fu, X - C ; Chen, R - H Physical Chemistry ( 2), People s Education Press, Beijing, 1982, pp 235-236, pp 228-236 ( in Chinese) 9 Fu, X - C ; Chen, R - H Physical Chemistry ( 1), People s Education Press, Beijing, 1982, pp 191-198, pp 446-447 ( in Chinese) ( Ed PAN Bing - Feng)