RESUMO
Resumen En el presente estudio se realizaron cálculos con base en la Teoría del Funcional de la Densidad Electrónica (DFT) con la aproximación B3PW91/LANL2DZ para optimizar los sistemas monometálicos y bimetálicos Au9, Au8Pd, Au8Pt, AuPd8, AuPt8, Pd9 y Pt9. Los materiales fueron teóricamente evaluados como catalizadores para la oxidación de monóxido de carbono (CO) y se determinó el sistema más favorable para la adsorción de esta molécula. La sustitución de átomos de Pt y Pd por átomos de Au en los nonámeros generó un cambio en la estructura tridimensional del sistema. El análisis de reactividad global mostró que el clúster más reactivo es PÍ9, seguido por AuPt s . Los índices de Fukui identificaron los sitios más susceptibles para un ataque nucleofílico de ambos clústeres. La adsorción de CO generó una cascada de oxidación que liberó ~4,5 eV, indicando que la reacción es altamente exotérmica y exergónica. Los clústeres AuPt s y Pt 9 mostraron los valores más bajos de energía de activación de la etapa determinante del mecanismo. En general, la sustitución de un átomo de platino (o paladio) por un átomo de oro no afecta la reactividad de los nonámeros y, por tanto, se infiere que el clúster AuPt s podría ser un catalizador promisorio en la oxidación de CO.
Abstract In the current study were development calculations based on Density Functional Theory (DFT) with the B3PW91/LANL2DZ approach for optimizing both monometallic and bimetallic systems: Au9, AusPd, Au8Pt, AuPds, AuPts, Pd9 y Pt9. Such materials were theoretically tested as catalyst for the oxidation of carbon monoxide (CO) and the most favorable system for its further adsorption was determined. The substitution of Pt and Pd by Au atoms in the nonamers generated a change in the tridimensional structure of the system. The global reactivity analysis showed that the most reactive cluster is Pt9 followed by AuPts. On the other hand, the Fukui indexes identified the most susceptible sites for a nucleophilic attack of both clusters. The CO adsorption generated an oxidation cascade which liberated ∼ 4.5 eV, indicating that the reaction is highly exothermic and exergonic. Both AuPt8 and Pt9 showed the lowest values of activation energy in the determining step of the mechanism. In general, the substitution of a Pt (Pd) atom by an Au atom does not affect the reactivity of the nonamers and then it is inferred that the AuPds cluster could be a promissory catalyst in the CO oxidation.
Resumo No presente estudo, cálculos baseados na Teoria do Funcional da Densidade Eletrônica (DFT) com a abordagem B3PW91/LANL2DZ foram realizados para otimizar sistemas monometálicos e bimetálicos Au9, Au8Pd, Au8Pt, AuPd8, AuPt8, Pd9 y Pt9. Tais materiais foram teoricamente avaliados como catalisadores para a oxidação do monóxido de carbono (CO) e foi determinado o sistema mais favorável para a adsorção desta molécula. A substituição dos átomos de Pt e Pd por átomos de Au nós não-nomes gerou uma mudança na estrutura tridimensional do sistema. A análise de reatividade global mostrou que o cluster mais reativo é Pt9, seguido por AuPt8. Os índices de Fukui identificaram os sítios mais suscetíveis ao ataque nucleofílico de ambos os clusters. A adsorção de CO gerou uma cascata de oxidação que liberou ~4,5 eV, indicando que a reação é altamente exotérmica e exergônica. Os aglomerados AuPt 8 y Pt 9 apresentaram os menores valores de energia de ativação do estágio determinante do mecanismo. Em geral, a substituição de um átomo de platina (ou paládio) por um átomo de ouro não afeta a reatividade dos não-nomes e, portanto, infere-se que o aglomerado AuPt 8 pode ser um catalisador promissor na oxidação do CO.
RESUMO
In this contribution, the thermodynamic analysis of α- and ß-pinene epoxide isomerization over Fe and Cu supported on MCM-41 is presented using computational chemistry and group contribution methods (GCMs). Some physical-chemical data (T c, P c, v c, Z c, ω, T b, T fus) and thermodynamic (S°298.15, C p,298.15 °, C v,298.15 °, ΔH f,298.15 °, ΔG f,298.15 °, ΔH vb °, ΔH fus, C pL) properties obtained by different GCMs are reported for several monoterpenes and monoterpenoids, which significantly contribute to the knowledge of the properties of these compounds. Density functional theory (DFT), PBE-D3/6-311G(d,p), was employed for determining the Gibbs free energy and the heat of reaction associated with the transformation of monoterpene epoxides into aldehydes, ketones, and related oxygenated compounds in the presence of different solvents and at several temperatures. The calculations were compared with available data reported and the experimental results of the catalytic reactions. The transformation of α- and ß-pinene epoxides into aldehydes appears to be more spontaneous and favorable than their transformations into alcohols in a wide range of temperatures. These results are in agreement with the experiments over Fe/MCM-41 and Cu/MCM-41, where α-pinene epoxide isomerization yields campholenic aldehyde (50-80% selectivity) as the main product. The 1.7Fe/MCM-41 material was more active in all solvents than 1.3Cu/MCM-41 for both α- and ß-pinene epoxide isomerization. However, perillyl alcohol (20-70% selectivity) was the most favored for the isomerization reaction, except when ethyl acetate was the solvent. Enthalpy and Gibbs free energy of the studied reactions estimated by both GCMs and DFT calculations did not show large differences for most of the reactions at evaluated temperatures.
RESUMO
The reaction mechanism of the isomerization of α and ß-pinene epoxides with Fe species as catalysts was studied with density functional theory (DFT) calculations and an experimental methodology. ß-pinene epoxide can be isomerized into myrtanal and myrtenol in four steps, while in the case of perillyl alcohol, two additional steps are necessary. On the other hand, high selectivity to myrtanal obtained experimentally can be explained by the number of steps and the kind of the hydrogen transference that is easier for this compound in comparison with myrtenol and perillyl alcohol. A thermodynamic analysis showed that transformation into myrtenol, myrtanal, and perillyl alcohol is spontaneous but transformation into myrtanal is the most favorable. In the case of α-pinene epoxide rearrangement, a mechanistic study was carried out toward the optimization of the possible intermediates. Synthesis of campholenic aldehyde and carveol from α-pinene epoxide was carried out through three steps after the coordination of oxygen to iron, showing that in contrast to carveol formation, campholenic aldehyde synthesis is spontaneous. Analysis of ∇2ρ, the total energy density (H = V + G), and the |V|/G ratio evaluated at the bond critical point of the Fe-O bond showed for both epoxides that such interaction is closed shell instead of covalent. Apparently, α-pinene epoxide can be isomerized faster that ß-pinene epoxide with Fe3+, which is in agreement with previous experimental results. This is the first report where a reaction mechanism of isomerization of monoterpenes epoxides is studied based on very detailed experimental and computational methodologies.
