Olefin production, computer modeling

Ross, L. L., Shu, W. R., Computer Modeling of Hydrocarbon Pyrolysis for Olefins Production, in "Thermal Hydrocarbon Chemistry", Adv. Chem. Series,... 

Ross and Shu [38], discussing the computer modelling of hydrocarbon pyrolysis for olefin production, classify reaction models in four categories in order of increasing sophistication empirical, semi-kinetic, stoichiometric and mechanistic. Most concepts of this classification are included in Table 3 with, however, a more classical meaning of the word stoichiometry. 

Computer Modeling of Hydrocarbon Pyrolysis for Olefins Production... 

Computer modeling of hydrocarbon pyrolysis is discussed with respect to industrial applications. Pyrolysis models are classified into four groups mechanistic, stoichiometric, semi-kinetic, and empirical. Selection of modeling schemes to meet minimum development cost must be consistent with constraints imposed by factors such as data quality, kinetic knowledge, and time limitations. Stoichiometric and semi-kinetic modelings are further illustrated by two examples, one for light hydrocarbon feedstocks and the other for naphthas. The applicability of these modeling schemes to olefins production is evidenced by successful prediction of commercial plant data. 

This chapter is organized as follows. In the following section the tentative catalytic cycle proposed by Wilke and co-workers is outlined, followed, in the next section, by a short description of the computational approach employed and the catalyst model chosen. The structural and energetic aspects of all critical elementary steps of the complete catalytic cycle are presented after that. Then we propose a theoretically verified, refined catalytic reaction cycle, and follow that with the elucidation of the product distribution between linear and cyclic Cjo-olefins. Finally, the catalytic reaction courses of the [Ni°]-catalyzed co-oligomerization of 1,3-butadiene and ethylene and of the cyclooligomerization of 1,3-butadiene are compared. 

Several observations led to the proposal that some of the catalysts containing metals other than platinum do not react by the Chalk-Harrod mechanism. First, carbon-silicon bond-forming reductive elimination occurs with a sufficiently small number of complexes to suggest that formation of the C-Si bond by insertion of olefin into the metal-silicon bond could be faster than formation of the C-Si by reductive elimination. Second, the formation of vinylsilane as side products - or as the major products in some reactions of silanes with alkenes cannot be explained by the Chalk-Harrod mechanism. Instead, insertion of olefin into the M-Si bond, followed by p-hydrogen elimination from the resulting p-silylalkyl complex, would lead to vinylsilane products. This sequence is shown in Equation 16.39. Third, computational studies have indicated that the barrier for insertion of ethylene into the Rh-Si bond of the intermediate generated from a model of Wilkinson s catalyst is much lower than the barrier for reductive elimination to form a C-Si bond from the alkylrhodium-silyl complex. ... 

In short, each reaction family could be described with a maximiun of three parameters (A, Eo, a). Procurement of a rate constant from these parameters required only an estimate of the enthalpy change of reaction for each elementary step. In principle, this enthalpy change of reaction amoimted to the simple calculation of the difference between the heats of formation of the products and reactants. However, since many model species, particularly the ionic intermediates and olefins, were without experimental values, a computational chemistry package, MOPAC, ° was used to estimate the heat of formations on the fly . Ihe organization of the rate constants into quantitative structure-reactivity correlations (QSRC) reduced the number of model parameters greatly Ifom O(IO ) to 0(10). 

The reactivity of carbenes/carbenoids towards C=C bonds continues to attract much attention from theoretical and synthetic chemists alike. Calculations using different levels of theory have thus been conducted to better understand the reactivity of cyclo-propenylidene towards C=C bonds. This DFT study has suggested that the reaction involves two pathways from a common intermediate to give products featuring three-and four-membered rings, respectively. Computational methods applied for the first time to intra- and inter-molecular cyclopropanations involving oxiranyllithiums have provided mechanistic rationale for such carbenoid reactions. " While the intramolecular cyclopropanation of oxiranyllithiums equipped with an olefinic moiety (i.e., 1,2-epoxyhexene used as model substrate) proved to follow either a two-step carbolithiation pathway or a concerted methylene transfer, the latter route predominates for intermolecular cyclopropanation. 

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