Pyrolysis of Alkanes: A Computational Approach

Abstract

In this study, we investigate the kinetic and thermodynamic aspects of thermal cracking reactions of short paraffin chains by density functional theory (DFT) methods. The thermal cracking reactions have been modeled for a series of shorter unbranched alkanes at 673 K by following a free-radical mechanism. Benchmark calculations have been carried out with different functionals (B3LYP, M06-2X, PBE0, BMK, B3PW91) and basis sets (6-31G(d,p), 6-311+G(d,p)) to determine the most suitable DFT method, and the results were compared to the available experimental data. Computations were also performed at the CBS-QB3 level to evaluate the accuracy of the DFT method. The thermodynamic and kinetic properties of the initiation, hydrogen atom transfer (HAT), and decomposition (β-scission) reactions are intensely discussed to better understand the trends in product distributions at high temperatures. Evans−Polanyi (EP) relations have been used to build a linear relationship between the enthalpy of reactions and their activation energies; this process may be useful for the determination of the kinetic parameters of longer paraffin chains as well. Finally, the preexponential factors of short-chain paraffin have been calculated and classified based on the identity of the radicalic products. The latter, together with the activation energies derived from the EP relations can be used safely for the prediction of the rate constants for long paraffin chains.

A Kinetic Approach to the Pyrolysis of Naphthenic Acids

“…Furthermore, single point energy refinement on the B3LYP-optimized geometries with B3LYP/6-311++G­(2df,p) yields more reliable results (less than an average 3.7% error for each reaction type) and consistent MAE values (1.05–1.80 kcal/mol) than the other methods tested. Note that the same functional (B3LYP) has been used successfully by some of us for the pyrolysis reactions of paraffins …”

“…Note that the same functional (B3LYP) has been used successfully by some of us for the pyrolysis reactions of paraffins. 56 According to the decarboxylation and dehydration pathways displayed in Figure 1, acetic acid (AA) decarboxylates via 2 steps: the acidic H rotates around the C−O bond via TS-1a to yield trans-AA, then upon H-transfer via TS-1b decarboxylation occurs. On the other hand, proton transfer from the alkyl chain to the carbonyl oxygen via TS-2 yields water and ethenone (the simplest ketene).…”

Elucidation of Catalytic Propane Dehydrogenation Using Theoretical and Experimental Approaches: Advances and Outlook

“…C–H bond dissociation in the PDH reaction system can proceed via the homolytic or heterolytic path, depending on the nature of the reaction, i.e., whether oxidative or nonoxidative. Experimental observations have shown that homolytic dissociation occurs preferably in nonoxidative PDH due to thermodynamic factors, i.e., positive values of Δ G and Δ H of reaction. , The nonoxidative reaction follows the free radical mechanism in which the C–H bond breaks with electrons given to both C and H atoms. Both the dissociation routes occur in oxidative PDH reaction, as described by eqs – for oxidative PDH with CO 2 on the GaO catalyst .…”

“…The endothermic nature of the nonoxidative reaction also makes the process analogous to thermal cracking by increasing the concentration of unpaired electrons near the catalyst surface at high temperatures, favoring the free-radical mechanism of homolytic dissociation of the bonds. , Zhang et al simulated the PDH reaction over the ideal and oxygen-defective surface of the (1 0 1) facet of ZrO 2 , termed t -ZrO 2 , since it is the termination surface with the highest stability and is tetrahedral in shape. The density functional theory (DFT) calculations show that C–H bond dissociation occurs homolytically due to thermodynamic factors such as the exothermic nature of bond dissociation near the catalyst surface.…”

Uncertainty-calibrated deep learning for rapid identification of reaction mechanisms

“…A reliable reaction network can map out the reaction fluxes indicating how reactants convert to products, and as such, providing instructions for reaction condition optimization and process control. [1] However, chemical reaction systems encountered in various fields, such as catalysis, [2][3][4] pyrolysis, [5][6][7][8] combustion, [9,10] and biological processes, [11] involve thousands of elementary reactions and enormous intermediate molecular species or radicals. Even the seemingly simple methane combustion process is composed of around 800 chemical pathways to interconnect the reaction intermediates.…”