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Los ligandos hacen la diferencia Conocimientos moleculares sobre el catalizador de Phillips durante la polimerización

Caterina Barzan¹, Alessandro Piovano¹, Luca Braglia^1,2

¹Department of Chemistry, NIS Interdepartmental Center and INSTM Reference Center, University of Turin , Via G. Quarello 15A, Turin I10135, Italy.

Journal of the American Chemical Society

|August 23, 2017

Resumen

Este estudio revela que los iones de cromo (Cr) divalentes en una coordinación de 6 veces, que interactúan con los subproductos oxigenados, son los sitios activos en los catalizadores de Phillips para la polimerización del etileno, desafiando las teorías anteriores.

Área de la Ciencia:

Catálisis heterogénea
Química de la polimerización
Ciencias de los materiales

Sus antecedentes:

El catalizador Phillips (Cr/SiO2) es crucial para la polimerización del etileno.
Comprender los sitios activos de cromo (Cr) y su coordinación es clave para optimizar el rendimiento del catalizador.
Los modelos anteriores proponían especies "desnudas" o de baja coordinación de Cr como sitios activos.

Objetivo del estudio:

Para investigar los cambios en la estructura molecular de los sitios Cr en Cr^VI/SiO₂ durante la polimerización del etileno utilizando espectroscopia sensible al operando.
Diferenciar entre la reducción de Cr por el etileno y el posterior proceso de polimerización.
Identificar las especies específicas de Cr y los entornos de coordinación responsables de la actividad catalítica.

Principales métodos:

Se utilizaron técnicas espectroscópicas sensibles al operando para controlar los sitios Cr.
Parámetros experimentales cuidadosamente controlados para separar los eventos de reducción y polimerización del cromato.
Se analizó la interacción de los sitios Cr con los subproductos de la polimerización.

Principales resultados:

Se han identificado iones Cr divalentes en una coordinación de 6 veces como los sitios activos primarios para la polimerización del etileno.
Se observó que estos sitios activos interactúan con los subproductos oxigenados, principalmente el metilformato.
Se encontraron especies Cr^VI no reducidas y Cr^II/Cr^III espectadoras presentes durante la polimerización.

Conclusiones:

Desafía la hipótesis de las especies de cromo "desnudas", proponiendo en su lugar que los sitios activos son Cr iones coordinados con ligandos oxigenados externos.
Destaca el papel importante de estos ligandos oxigenados flexibles, análogos a los ligandos auxiliares en la catálisis homogénea.
Proporciona una nueva visión mecanicista de la polimerización del etileno sobre los catalizadores Phillips.

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