Study On Molecular Level Kinestics Of Complex Reaction Systems

The molecular level kinetic model is an important breakthrough in the study of kinetic model for complex reaction systems after lumped kinetic model, and already becomes an inevitable development trend of kinetic model for complex reaction system. Among those molecular level kinetic models, the single-event method not only can retain the full detail of reaction network, but also can reduce the number of model parameters to a tractable level, so it attracts wide attentions. The single-event method was built and optimized by the study of single-event kinetic model for catalytic reforming.The catalytic reforming of n-hexane, n-heptane and 2,2,4-trimethylpentane over Pt-Sn/Al22O3-Cl catalyst were studied in this paper. The results showed that isomerization is the main reaction occurred in reforming of three kinds of model compounds, cyclization and cracking reactions are much slower than isomerization. Cracking reaction of 2,2,4-trimethylpentane occurs easily due to its special molecular structure. The classical bifunctional reforming mechanism was chosen as the foundation of single-event kinetic model for catalytic reforming based on the literature survey and experimental research.The digital representation of all kinds of molecules was carried out using Boolean relation matrix. The carbon atoms in molecule were labelled stochastically. According to hybrid state of carbon atoms, the C=C double bond and charge of carbon atom were clearly represented in Boolean relation matrix by utilization of main diagonal elements, which can avoid to use additional vectors. The Golender's potential vector of molecule with C=C double bond or charge was calculated creatively based on the Boolean relation matrix and Golender's graph potential, and it can be used to confirm the unique molecule to avoid redundance of molecule storage in reaction network database. The number of non-zero elements in a row without counting main diagonal elements in matrix determines whether the corresponding carbon atom is primary secondary, tertiary or quaternary. The type of molecule can be determined by inspecting the whole matrix. The pure connection between carbon atoms drawn out form Boolean relation matrix was used to determine the relative position between carbon atoms, and the calculation formula was put forward, it is necessary for digital representations of reactionsAnd then, the digital representations of reactions occurred at metallic and acidic sites by performance of Boolean relation matrix based on reaction mechanism of catalytic reforming were carried out.In order to perform search, comparison, and storage of molecules and reactions during the reaction network generation and rate equation construction quickly, several optimization designs were executed for reaction network database:(1) the file name of molecule storage was the combination of carbon number and molecule type; (2) the file name of reaction storage was the combination of reaction type, carbon number of reactant, carbon number of first product, and second product; (3) the key and time-consuming characters of Boolean relation matrix were prestored in the database, for example, carbon number of molecule, Ring carbon atom number, Golender's potential vector, and so on.The program for reaction network generation was completed according to the constructed program flow chart based on the bifunctional reaction mechanism. The detailed reaction network necessary for single-event kinetics resolution of catalytic reforming was generated via this program. Because chain growth reactions were not considered in catalytic reforming, the reaction network is only dependent of the carbon number of starting molecule, but independent of its type, due to the reversibility between molecules with the same carbon number. The results of network generation showed that the networks generated from different molecules with the same carbon number were consistent with each other.The single-event concept was introduced based on transition state theory. The elementary-step rate coefficient is the product of single-event number and single-event rate coefficient. The number of parameters can be further reduced by thermodynamic restriction. The rate equations of single-event kinetic model were deduced based on gas-solid catalysis and bifunctional reforming mechanism.According to the definition of single-event number, the single-event number of elementary step can be obtained from global symmetry number of reactant and transition state. The automatic calculation of global symmetry number was realized by the definition of symmetry number, and some examples were given. A large number of single-event numbers of elementary steps were calculated based on the program originated from automatic calculation algorithm. It is important for study of single-event kinetic model due to its academic value.Finally, the single-event rate coefficients of C6 reforming were estimated based on the data of n-hexane reforming using the hybrid algorithm which is the combination of BFGS and simulated anneal algorithm. The results showed that obtained kinetic parameters were consistent with the bifunctional reforming mechanism; the activation energy of reactions with tertiary carbeniums is lower than that of reactions without tertiary carbeniums; the activation energy of reactions with branch degree change is higher than that of reactions without branch degree change. At the same time, the BFGS-SA hybrid algorithm appears good stability and astringency during parameter estimation.The application of single-event method in the study of molecular level kinetics for catalytic reforming was realized by above research, and this study has important theoretical significance and reference values for the research and development of single-event kinetic model for complex reaction systems.