The Kinetics of Cyclization Reactions on Polyaromatics from First Principles

Abstract

Ab initio density functional theory calculations are presented on cyclization reactions of polyaromatics involved in coke formation during the thermal cracking of hydrocarbons. During coke formation, cyclization can take place at various sites, differing from each other by the local polyaromatic structure. This local structure also determines the minimum number of carbon atoms that must be added to allow the formation of a new ring. Kinetic parameters are calculated for the various ring-closure reactions by means of transition state theory. The activation energy is largely affected by the local structure of the polycyclic aromatic hydrocarbon, whereas the frequency factor varies significantly in terms of the length of the attached alkyl chain. The calculations, as presented, give a microscopic insight into the mechanisms that contribute to barrier formation and to the value of the frequency factor. The relative importance of cyclization at different sites, under conditions typical for an industrial cracking unit, is studied on the basis of the calculated rate constants at various temperatures. The results suggest that the nature of coke formation is autocatalytic: the larger the macroradicals, the faster the subsequent reactions that lead to further growth of the polyaromatic surface. This type of calculation is the first step towards the development of structural relations for the kinetic parameters in terms of the local structure of the coke matrix.

Hydrogen Abstraction Acetylene Addition and Diels−Alder Mechanisms of PAH Formation: A Detailed Study Using First Principles Calculations

“…B3LYP/6-311G** calculations were also applied earlier to investigate the mechanism of cyclization reactions for the alkyl chain radicals bonded to various PAHs including benzene, naphthalene, anthracene, and so forth, containing 1−4 aromatic rings. − The calculated barriers were found to be significantly higher as compared to those of our predictions for the cyclization involving vinyl-type radical side chains and varied from 8.0 to 12.0 kcal/mol depending on the PAH. For instance, for the C 6 H 5 −C 4 H 8 (1-phenylbutyl) radical, the barrier for the ring closure was calculated to be 11.8 kcal/mol, whereas our predictions for the cyclization barriers of the A4b radical are 3.6 and 4.6 kcal/mol at the G3 and B3LYP levels, respectively.…”

Hydrogen Abstraction Acetylene Addition and Diels−Alder Mechanisms of PAH Formation: A Detailed Study Using First Principles Calculations

“…B3LYP/6-311G** calculations were also applied earlier to investigate the mechanism of cyclization reactions for the alkyl chain radicals bonded to various PAHs including benzene, naphthalene, anthracene, and so forth, containing 1−4 aromatic rings. − The calculated barriers were found to be significantly higher as compared to those of our predictions for the cyclization involving vinyl-type radical side chains and varied from 8.0 to 12.0 kcal/mol depending on the PAH. For instance, for the C 6 H 5 −C 4 H 8 (1-phenylbutyl) radical, the barrier for the ring closure was calculated to be 11.8 kcal/mol, whereas our predictions for the cyclization barriers of the A4b radical are 3.6 and 4.6 kcal/mol at the G3 and B3LYP levels, respectively.…”

Reactivity Indices for Radical Reactions Involving Polyaromatics

“…Within the field of polyaromatics it may be convenient to introduce a specific nomenclature to identify the various positions. 15,71 One distinguishes between benzene-, naphthalene-, anthracene-, phenanthrene-, benzophenanthrene-and dibenzophenanthrene-like sites. This nomenclature is clarified in Figure 1, where all hydrogens are assigned to a specific site.…”

“…We only report reaction barriers at 0 K, in analogy with discussions in previous sections. The results are reported in Table and are taken from ref . To compare various reactions, a suitable normalization is required for the magnetic susceptibility anisotropy since this property is size-dependent.…”

“…For example if one looks at naphthalene, there are two possibilities: abstraction of hydrogen number 1 (and equally 4, 5, and 8) or abstraction of hydrogen number 2 (and equally 3, 6, and 7). Within the field of polyaromatics it may be convenient to introduce a specific nomenclature to identify the various positions. , One distinguishes between benzene-, naphthalene-, anthracene-, phenanthrene-, benzophenanthrene- and dibenzophenanthrene-like sites. This nomenclature is clarified in Figure , where all hydrogens are assigned to a specific site.…”

Reactivity and aromaticity of polyaromatics in radical cyclization reactions

“…A detailed discussion of the geometries and frequencies was given in Ref. 13. We only give a brief overview of the results for the understanding of this article.…”

Hydrogen Abstraction Acetylene Addition and Diels−Alder Mechanisms of PAH Formation: A Detailed Study Using First Principles Calculations

“…B3LYP/6-311G** calculations were also applied earlier to investigate the mechanism of cyclization reactions for the alkyl chain radicals bonded to various PAHs including benzene, naphthalene, anthracene, and so forth, containing 1−4 aromatic rings. − The calculated barriers were found to be significantly higher as compared to those of our predictions for the cyclization involving vinyl-type radical side chains and varied from 8.0 to 12.0 kcal/mol depending on the PAH. For instance, for the C 6 H 5 −C 4 H 8 (1-phenylbutyl) radical, the barrier for the ring closure was calculated to be 11.8 kcal/mol, whereas our predictions for the cyclization barriers of the A4b radical are 3.6 and 4.6 kcal/mol at the G3 and B3LYP levels, respectively.…”

Reactivity Indices for Radical Reactions Involving Polyaromatics

“…Within the field of polyaromatics it may be convenient to introduce a specific nomenclature to identify the various positions. 15,71 One distinguishes between benzene-, naphthalene-, anthracene-, phenanthrene-, benzophenanthrene-and dibenzophenanthrene-like sites. This nomenclature is clarified in Figure 1, where all hydrogens are assigned to a specific site.…”

“…We only report reaction barriers at 0 K, in analogy with discussions in previous sections. The results are reported in Table and are taken from ref . To compare various reactions, a suitable normalization is required for the magnetic susceptibility anisotropy since this property is size-dependent.…”

“…For example if one looks at naphthalene, there are two possibilities: abstraction of hydrogen number 1 (and equally 4, 5, and 8) or abstraction of hydrogen number 2 (and equally 3, 6, and 7). Within the field of polyaromatics it may be convenient to introduce a specific nomenclature to identify the various positions. , One distinguishes between benzene-, naphthalene-, anthracene-, phenanthrene-, benzophenanthrene- and dibenzophenanthrene-like sites. This nomenclature is clarified in Figure , where all hydrogens are assigned to a specific site.…”

Reactivity and aromaticity of polyaromatics in radical cyclization reactions

“…A detailed discussion of the geometries and frequencies was given in Ref. 13. We only give a brief overview of the results for the understanding of this article.…”

Hydrogen Abstraction Acetylene Addition and Diels−Alder Mechanisms of PAH Formation: A Detailed Study Using First Principles Calculations

“…B3LYP/6-311G** calculations were also applied earlier to investigate the mechanism of cyclization reactions for the alkyl chain radicals bonded to various PAHs including benzene, naphthalene, anthracene, and so forth, containing 1−4 aromatic rings. − The calculated barriers were found to be significantly higher as compared to those of our predictions for the cyclization involving vinyl-type radical side chains and varied from 8.0 to 12.0 kcal/mol depending on the PAH. For instance, for the C 6 H 5 −C 4 H 8 (1-phenylbutyl) radical, the barrier for the ring closure was calculated to be 11.8 kcal/mol, whereas our predictions for the cyclization barriers of the A4b radical are 3.6 and 4.6 kcal/mol at the G3 and B3LYP levels, respectively.…”

Reactivity Indices for Radical Reactions Involving Polyaromatics

“…Within the field of polyaromatics it may be convenient to introduce a specific nomenclature to identify the various positions. 15,71 One distinguishes between benzene-, naphthalene-, anthracene-, phenanthrene-, benzophenanthrene-and dibenzophenanthrene-like sites. This nomenclature is clarified in Figure 1, where all hydrogens are assigned to a specific site.…”

“…We only report reaction barriers at 0 K, in analogy with discussions in previous sections. The results are reported in Table and are taken from ref . To compare various reactions, a suitable normalization is required for the magnetic susceptibility anisotropy since this property is size-dependent.…”

“…For example if one looks at naphthalene, there are two possibilities: abstraction of hydrogen number 1 (and equally 4, 5, and 8) or abstraction of hydrogen number 2 (and equally 3, 6, and 7). Within the field of polyaromatics it may be convenient to introduce a specific nomenclature to identify the various positions. , One distinguishes between benzene-, naphthalene-, anthracene-, phenanthrene-, benzophenanthrene- and dibenzophenanthrene-like sites. This nomenclature is clarified in Figure , where all hydrogens are assigned to a specific site.…”

Reactivity and aromaticity of polyaromatics in radical cyclization reactions

“…A detailed discussion of the geometries and frequencies was given in Ref. 13. We only give a brief overview of the results for the understanding of this article.…”