Supercritical Fluids as Greener Catalytic Reaction Media

Supercritical Fluids as Greener Catalytic Reaction Media

Utilization of supercritical fluids as greener reaction media for homogeneous catalysis or compressed carrier for heterogeneous catalysis has great potential in manipulating the outcomes of catalytic reactions because of high miscibility with gaseous reactants and tunable physicochemical properties as well as the favorable heat- and mass-transfer properties. We have recently demonstrated that the use of supercritical carbon dioxide (scCO2) and methanol (scCH3OH) caused a remarkable enhancement of reactivity and selectivity of homogeneous catalysts, and a significant improvement in the product selectivity of the heterogeneously catalyzed reactions over solid catalysts. Particularly, scCO2 can be used as not merely an alternative to conventional organic solvents but also an attractive carrier medium for the heterogeneously catalyzed reaction. CO2 works as an effective promoter of selective transformation. In addition, the use of scCH3OH, which acts as a methylating agent and a reaction medium, results in fine-tuning of the chemoselectivity in the methylation of organic compounds over the solid catalysts. Here, we will focus on our recent research progress in practical catalytic reactions with both high productivity and selectivity by using scCO2 and scCH3OH. The industrial outlook for the use of supercritical fluids is bright largely because of the growing awareness of the need for green chemistry.

Chemical reactions in supercritical fluids

Reactions in supercritical fluids have by now become a well-established branch of Green Chemistry.  A wide range of reactions have been carried out in both batch and continuous modes.  Reactions in supercritical CO2 (scCO2) include hydrogenation, hydroformylation, alkylation, etherification and oxidation. Recently, we have become intrigued by the possibilities of using the phase state of a reaction mixture in scCO2 to manipulate the outcome of that reaction. An example involved the etherification of symmetrical diols, where the applied pressure is used to switch the product between the diether and the desymmetrized mono-ether. This will give some recent examples from our group as well as describing some of our work on reactions in supercritical H2O.