CHEMISTRY: Guest in a Green Solvent
Julia Uppenbrink

Hydrocarbon solvents are used widely in the chemical industry for cleaning
and coating and for reaction and separation processes. Environmentally
friendly chemical synthesis uses alternative solvents such as ionic
liquids, water, and liquid and supercritical carbon dioxide (scCO2). The
last is particularly attractive because CO2 is inexpensive and
environmentally benign (except as a greenhouse gas). Furthermore, changing
the density of scCO2 alters its effectiveness as a solvent, and this can be
used to control reaction rates and yields.

Unfortunately, many reactants and catalysts do not dissolve readily in
scCO2. Several approaches have been used to overcome this problem. For
example, surfactants have been designed that enhance the solubilizing
properties of scCO2, and microemulsions of water and scCO2 have been used.
Now, Yeung et al. show that Pd nanoparticles encapsulated within
poly(propylene imine) dendrimers will catalyze chemical reactions in scCO2.
In two reactions, the catalyst produced high yield and selectivity with
little degradation. -- JU

Chem. Comm. 2001, 2290 (2001).