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Escisión de enlaces C-C catalizados por metales en alcanos: efectos de la sustitución de metilo en las estructuras de
David W Flaherty1, David D Hibbitts, Enrique Iglesia
1Department of Chemical Engineering, University of California at Berkeley , Berkeley, California 94720, United States.
Journal of the American Chemical Society
|June 26, 2014
Resumen
Los sustitutos de metilo en los enlaces carbono-carbono impactan significativamente las tasas de hidrogenolización de los alcanos y la selectividad del producto en los catalizadores metálicos del grupo del platino. Los bonos altamente sustituidos requieren más energía para romperse, favoreciendo productos menos ramificados bajo condiciones específicas.
Área de la Ciencia:
- Catálisis Heterogénea Catálisis Heterogénea
- Química de las superficies.
- Mecanismo de reacción Mecanismo de reacción
Sus antecedentes:
- La hidrogenólisis de alcano es una reacción crucial en la refinación del petróleo y la síntesis química.
- La influencia de los patrones de sustitución de alquilo en la activación del enlace C-C sigue siendo un área activa de investigación.
- Comprender las geometrías de los estados de transición es clave para controlar los resultados de las reacciones.
Objetivo del estudio:
- Para dilucidar el efecto de los sustituyentes de metilo en las tasas de hidrogenolización de enlaces C-C y selectividades.
- Investigar las vías mecanicistas de la escisión de enlaces C-C en varios catalizadores metálicos (Ir, Rh, Ru, Pt).
- Para correlacionar los parámetros de activación con las estructuras de estado de transición para diferentes tipos de enlaces C-C.
Principales métodos:
- Estudios cinéticos experimentales de hidrogenólisis para alcanos acíclicos y cíclicos C2-C8.
- Análisis de las entalpías de activación (ΔH‡) y las entropias (ΔS‡) para la escisión de enlaces C-C.
- Estimación computacional de los valores de entropía para las geometrías de estado de transición propuestas utilizando mecánica estadística.
Principales resultados:
- Los enlaces C-C altamente sustituidos ((3)C-(x)C) exhiben mayores entalpias y entropias de activación que los menos sustituidos ((2)C-(2)C, (2)C-(1)C).
- Los estados de transición para la escisión (3)C-(x) C implican especies α,β,γ- o α,β,γ,δ-unidas, mientras que menos enlaces sustituidos forman especies α,β-unidas.
- Las bajas temperaturas y las altas presiones de H2 favorecen la escisión de enlaces C-C menos sustituidos, dando lugar a productos más ramificados.
Conclusiones:
- El grado de sustitución del enlace C-C dicta el mecanismo de hidrogenólisis y las barreras energéticas.
- Las ideas mecánicas sugieren que el control de las condiciones de reacción puede dirigir la selectividad hacia los productos ramificados deseados.
- Los hallazgos son relevantes para optimizar procesos catalíticos como la desulfurización, la desnitrogenación y la desoxigenación.
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