Supercritical Fluids as Greener Catalytic Reaction Media part 2

 “Supercritical” carbon dioxide as “green” solvent has received significant interest in chemical reactions, in particular, in homogeneous and heterogeneous catalysis. The beneficial use of supercritical fluids with respect to rate and/or selectivity has been demonstrated for several homogeneously and heterogeneously catalyzed reactions. Among them, oxidation and hydrogenation reactions are interesting because of the inertness of carbon dioxide toward molecular oxygen and di-hydrogen 

Polymerization using supercritical CO2 and reactive oxide catalysts

The art and science of polymerization have been known for decades, and today are an essential part of many industrial processes, such as the production of plastics, paints and coatings as well as various electronic and biomedical devices. Clearly, the applications of polymers are extensive, with a number of diverse industries and important systems relying on this crucial technology. Although there are several types of polymerisation, free-radical polymerisation is one of the most commonly used processes, and is currently used extensively in a variety of systems. Traditionally, free radical polymerisaiton is initiated by the decomposition of a highly reactive and unstable species, such as an azo compound (azobisisobutyronitrile, AIBN) or a peroxide compound (di-t-butyl peroxide). These molecules undergo homolytic cleavage to produce radicals that attack the chosen monomer, thereby initiating polymerisation. Radical initiators rely on the fact that they are highly reactive, however, as they are unstable, care must be taken in their handling and storage so as to prevent them from exploding. In the present contribution, oxide-based catalysts are employed as a novel polymerisation initiator. When coupled with supercritical CO2, a relatively benign solvent, these systems represent an environmentally friendly and sustainable approach to the production of polymers. The process utilises an alternative and recyclable reaction medium, a cost effective, non-toxic and robust initiator that is highly active at relatively low concentrations, thereby eradicating the need for initiator recovery and product purification. The system is effective in the polymerisation of methyl acrylate, a commonly used monomer, and of norbornadiene, a traditionally non-reactive substrate. Analysis of the products via gel permeation chromatography has confirmed the presence of polymeric materials with molecular weights of up to 54 000 amu. Electron paramagnetic resonance has also been used to follow radical formation.  

Continuous flow reactions in supercritical CO2.

Supercritical CO2, scCO2, has been used as a solvent in many different reactions. Continuous flow fixed bed reactors have been used successfully in conjunction with scCO2 in a number of applications including hydrogenation, hydroformylation, etherification and Friedel Crafts type alkylation