Immobilized metal complexes in porous hosts: catalytic oxidation of substituted phenols in CO2 media

Abstract

Development of processes that utilize heterogeneous catalysis in environmentally beneficial media is of fundamental and practical importance. The oxidation of 2,6-di-tert-butylphenol (DTBP) to 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ) and 3,5,3′,5′-tetra-tert-butyl-4,4′-diphenoquinone (TTBDQ) has been investigated to evaluate the factors necessary to achieve high product conversion and selectivity in various media. A series of porous materials with immobilized Co(II) complexes served as catalysts and their reactivities using O₂ as the terminal oxidant were screened in neat acetonitrile, supercritical carbon dioxide (scCO₂), and CO₂-expanded acetonitrile. The highest conversions were found with the catalysts that had high affinity for dioxygen. Moreover, the greatest conversions (∼60%) were obtained when reactions were done in scCO₂, which is attributed to improved mass transfer of O₂ and substrates through the porous catalysts. Furthermore, the heterogeneous catalysts can be recycled with some loss of activity (∼30%) after three cycles; nonetheless these results suggest that the polymer hosts efficiently protect the immobilized catalytic sites from destructive bimolecular routes.