Rhodium-mediated hydrogenolysis/hydrolysis of the aryl ether bond in supercritical carbon dioxide/water: an experimental and theoretical approach

The use of supercritical carbon dioxide (scCO(2))/water for the conversion of diphenyl ether was investigated over Rh/C catalysts to achieve the cleavage of C-O bond under mild reaction conditions. In the studied reaction system, ethereal C-O bond was cleaved by hydrogenolysis and hydrolysis, which was strongly hampered if the reaction was conducted individually in CO2 or in water. A combined effect was desirable to achieve targeted cleavage because of the beneficial effect of a mild acidic environment generated through the dissolution of CO2 in water. Optimization of hydrogen pressure suggested a competition between diphenyl ether and hydrogen as the reaction rate (TOF) was significantly decreased with increased hydrogen pressure. In addition, the role of water was also evaluated and prompted the requirement of a suitable amount of water for cleavage. Mechanistic investigations through theoretical calculations together with experimental analysis illustrated that hydrogenolysis and hydrolysis were two reaction paths responsible for the rapture of the C-O bond.