CO₂-Expanded Solvents:  Unique and Versatile Media for Performing Homogeneous Catalytic Oxidations

Abstract

The work summarized here demonstrates a new concept for exploiting dense phase CO(2), media considered to be "green" solvents, for homogeneous catalytic oxidation reactions. According to this concept, the conventional organic solvent medium used in catalytic chemical reactions is replaced substantially (up to 80 vol %) by CO(2), at moderate pressures (tens of bars), to create a continuum of CO(2)-expanded solvent media. A particular benefit is found for oxidation catalysis; the presence of CO(2) in the mixed medium increases the O(2) solubility by ca. 100 times compared to that in the neat organic solvent while the retained organic solvent serves an essential role by solubilizing the transition metal catalyst. We show that CO(2)-expanded solvents provide optimal properties for maximizing oxidation rates that are typically 1-2 orders of magnitude greater than those obtained with either the neat organic solvent or supercritical CO(2) as the reaction medium. These advantages are demonstrated with examples of homogeneous oxidations of a substituted phenol and of cyclohexene by molecular O(2) using transition metal catalysts, cobalt Schiff-base and iron porphyrin complexes, respectively, in CO(2)-expanded CH(3)CN.

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

“…Moreover, in scCO 2 at 80 uC, 60% phenol conversion was observed, a value comparable to that found for homogeneous Co(II) catalysts. 10 The increased conversions in scCO 2 relative to CXL is in contrast to the behavior observed using related homogeneous catalysts under similar reaction conditions. With a homogeneous [Co miscibility in the reaction medium and pore diffusion rates to the immobilized catalytic sites dictate the overall rate.…”

“…10 Volumetric expansion of acetonitrile by dense CO 2 20 and the solubility limits of [Co II (salen)] complexes in CO 2 -acetonitrile CXLs have previously been reported. 10 The expansion ratio is defined as the ratio of the volume of the equilibrated CO 2 -expanded liquid phase at pressure P and temperature T [V(P,T)] to the initial volume of the neat solvent at atmospheric pressure and temperature…”

Carbon dioxide pressure induced heterogeneous and homogeneous Heck and Sonogashira coupling reactions using fluorinated palladium complex catalysts

“…That is, CO 2 can function as a reaction promoter for liquid-phase reactions as well as gas-liquid reactions such as oxidation [20][21][22][23], hydrogenation [24][25][26][27][28][29][30], and hydroformylation [31][32][33]. For the substrates of iodobenzene and methyl acrylate, however, no positive effect of CO 2 pressurization on the rate of Heck reaction was observed [16].…”

Química verde, os desafios da química do novo milênio

“…A reatividade das moléculas usando o fluido supercrítico está baseada na alta temperatura da água ou do CO 2 , que se apresentam no estado supercrítico [45][46][47][48] . A aplicação desta técnica em processos industriais tem minimizado os problemas com os resíduos formados.…”

“…As mudanças nas propriedades físicas e químicas da água e do CO 2 no estado supercrítico têm encorajado muitos pesquisadores a se aventurarem nesta linha de pesquisa. A diminuição da permissividade da água e do gás carbônico implica no aumento da solubilidade de compostos orgânicos nestes fluidos [45][46][47][48] . O aumento das constantes dielétricas no estado supercrítico também favorece as reações catalisadas por sólidos ácidos e básicos [45][46][47][48] .…”