Reaction pathways of propene pyrolysis

Abstract

The gas-phase reaction pathways in preparing pyrolytic carbon with propene pyrolysis have been investigated in detail with a total number of 110 transition states and 50 intermediates. The structure of the species was determined with density functional theory at B3PW91/6-311G(d,p) level. The transition states and their linked intermediates were confirmed with frequency and the intrinsic reaction coordinates analyses. The elementary reactions were explored in the pathways of both direct and the radical attacking decompositions. The energy barriers and the reaction energies were determined with accurate model chemistry method at G3(MP2) level after an examination of the nondynamic electronic correlations. The heat capacities and entropies were obtained with statistical thermodynamics. The Gibbs free energies at 298.15 K for all the reaction steps were reported. Those at any temperature can be developed with classical thermodynamics by using the fitted (as a function of temperature) heat capacities. It was found that the most favorable paths are mainly in the radical attacking chain reactions. The chain was proposed with 26 reaction steps including two steps of the initialization of the chain to produce H and CH(3) radicals. For a typical temperature (1200 K) adopted in the experiments, the highest energy barriers were found in the production of C(3) to be 203.4 and 193.7 kJ/mol. The highest energy barriers for the production of C(2) and C were found 174.1 and 181.4 kJ/mol, respectively. These results are comparable with the most recent experimental observation of the apparent activation energy 201.9 +/- 0.6 or 137 +/- 25 kJ/mol.

NEW REACTION PATHWAYS OFPROPENE + BCl3DECOMPOSITION IN CHEMICAL VAPOR DEPOSITION PROCESS

“…19 and 43, the system at higher temperatures may have free radicals to conduct chain reactions. The H radical may be initially produced by firstly exciting the ground state propene (X 1 A 0 ) into its lowest triplet state (a 3 A) with an energy of 270.2 kJ/mol and then conducting a bondcleavage to produce a H atom with an energy of 148.7 kJ/mol at 298.15 K, or by direct decompositing a H atom with an energy of 226.2 kJ/mol at 1200 K. 43 The Cl radical may be produced by the decomposition of IM99 J into IM91 J þ Cl with an energy of 303.3 kJ/mol, which is believed to be moderate for initializing a reaction chain. 19 Therefore, IM97 J may be attacked by H or Cl radical at different sites.…”

Reaction rate of propene pyrolysis

“…Propene is widely used as the precursor in preparing PyC with high deposition rate 14–16. This system (propene + H 2 + Ar) has been investigated experimentally17–30 and theoretically 31–34. For example, Vaidyaraman et al28 measured the rate constant and reported an apparent activation energy of ∼21.0 kcal/mol.…”

“…Also with the similar method, Zhao et al30 observed the energy by 201.9 ± 0.6 kJ/mol for 1123–1373 K, and the deposition rate is controlled by the gas phase homogeneous reactions at these higher temperatures. In our group, in addition to a detailed theoretical study of the gas‐phase reaction thermodynamics of C3H6 pyrolysis by Xiaoping Yao et al33 Yena Qu et al34 carried out a detailed theoretical research on the gas phase reaction pathways of propene pyrolysis, which involves 110 transition states and 49 intermediates. In that work,34 the most favorable reaction pathway associated with the elementary reactions and energy barriers were presented.…”

“…This is obviously desirable and convenient in understanding the intrinsic insights of the reactions though a numerical result can be obtained with a kinetics software (e.g., COMSOL35 or KINTECUS36) to solve the coupled differential equations once the rate constants of the elementary reactions are known. Actually, the rate constants for all of the refined34 elementary reactions can be found in references 37–49. Nevertheless, some inconsistencies were found and will be discussed in more detail.…”

“…Nevertheless, some inconsistencies were found and will be discussed in more detail. For the purpose of making the known “most favorable paths”34 comparable with the known experiments,28–30 the present work would try to develop the expression of the overall (or apparent) reaction rate based on the mechanisms of Ref 34. with the steady‐state approximation and to determine the rate constants of the elementary reactions with variational transition state theory (VTST).…”