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 3,5,39,59-tetra-tert-butyl-4,49-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 O 2 as the terminal oxidant were screened in neat acetonitrile, supercritical carbon dioxide (scCO 2 ), and CO 2 -expanded acetonitrile. The highest conversions were found with the catalysts that had high affinity for dioxygen. Moreover, the greatest conversions (y60%) were obtained when reactions were done in scCO 2 , which is attributed to improved mass transfer of O 2 and substrates through the porous catalysts. Furthermore, the heterogeneous catalysts can be recycled with some loss of activity (y30%) after three cycles; nonetheless these results suggest that the polymer hosts efficiently protect the immobilized catalytic sites from destructive bimolecular routes.

Gas-Expanded Liquids: Fundamentals and Applications

“…For the oxidation of 2,6-diterf-butylphenol (DTBP) to 2,6-di-ter/-butyl-l,4-benzoquinone (DTBQ) and 3,5.3 f ,5 f -tetra-rerr-butyl-4,4'-diphenoquinone (TTBDQ), a series of porous materials with immobilized Co(II) complexes were screened as catalysts in neat acetonitrile, supercritical carbon dioxide (scC0 2 \ and C0 2 -expanded acetonitrile (88). In this case, the highest conversions were found in scC0 2 suggesting that scC0 2 , rather than CXL or liquid reaction media, provides the best mass transfer of 0 2 and of substrates through the porous catalysts.…”

Gas-Expanded Liquids

“…In contrast, CO 2 -replacement of the organic solvent had a negligible effect on conversion and selectivity values when using iodosylbenzene as oxidant. Sharma et al investigated 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). A series of porous materials with immobilized Co(II) complexes served as catalysts, and their reactivity using O 2 as the terminal oxidant were screened in neat acetonitrile, sc CO 2 , and CO 2 -expanded acetonitrile.…”

Catalytic Oxidations in Dense Carbon Dioxide

“…Subramaniam and co-workers carried out the oxidation of DTBP using molecular oxygen and immobilized Co complexes as oxidant and heterogeneous catalysts, respectively, which affords 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ) and 3,5,3′,5′-tetra-tert-butyl-4,4′-diphenoquinone (TTBDQ) as products (Scheme 14). 52 The catalysts were prepared by the template copolymerization method, in which precursor Co complexes possessing polymerizable ligand were treated with ethylene glycol dimethacrylate in the presence of azobisisobutyronitrile (AIBN) as radical initiator. Figure 20 shows the models for the immobilized metal sites which are isolated enough to prevent detrimental intermolecular interactions.…”

“…However, some ligand modification may be necessary to perform the reaction under completely homogeneous conditions (section , Scheme ). Cobalt Schiff base complexes immobilized on polymer supports can also successfully be applied for the oxidations, and the results are, in some cases, even better than those obtained in organic solvents (section , Figure ), though the catalysts are gradually deteriorated as the reaction proceeds.…”

“…CO 2 -expanded liquids (CXLs, types (v) and (vi)) have been shown as suitable media for catalytic oxidations (sections , , , , , and , ). Typically, solid as well as metal complex catalysts stay in CXLs rather than in the CO 2 -rich phase during the oxidations.…”