The one-component transient permeation of carbon dioxide through an alumina-supported silica membrane is studied for pressures up to 2.0 × 10⁷ Pa, at temperatures of 296, 313, and 358 K. The permeation is obtained for gaseous, liquid, and supercritical carbon dioxide. For all conditions high carbon dioxide fluxes were obtained. The membrane has been tested with a maximum pressure difference between the feed and the permeate side of about 2.5 × 10⁶ Pa. At a feed-side pressure of 2.0 × 10⁷ Pa and at 358 K the permeance is equal to 8.0 × 10^-8 mol m^-2 s^-1 Pa^-1. The transient transport through the microporous membrane can be described with a single mass-transfer coefficient. For a feed-side pressure between 1.0 × 10⁵ and 200 × 10⁵ Pa and a temperature of 296 K the mass-transfer coefficient increases by a factor of 80. The fact that carbon dioxide has a high flux through this type of silica membrane opens the way for regeneration of carbon dioxide at supercritical conditions.

Membrane separation technology is successfully applied for the immobilization of a homogeneous catalyst (a (1H, 1H, 2H, 2H-perfluoroalkyl)-dimethylsilyl-substituted derivative of Wilkinson's catalyst) in a continuous process that uses supercritical carbon dioxide as solvent. The catalyst is separated from the products by a microporous silica membrane (see scheme).

Perfluoro-functionalized poly(propylene imine) dendrimers have been used as reactive extractants for anionic species and as phase transfer catalysts for two types of reactions. Different generations of dendrimers have been used for applications in carbon dioxide. First, the reactive extraction of the inorganic ions permanganate and dichromate, from an aqueous phase to a supercritical carbon dioxide phase, has been studied using the functionalized dendrimers. The yield for these extractions is rather low, indicating that the perfluoro-functionalized poly(propylene imine) dendrimers are not suited for oxidation reactions with these ions in carbon dioxide. Second, the poly(propylene imine) dendrimers have been used as a phase transfer catalyst for the halogen exchange reaction of benzyl chloride into benzyl bromide and for the esterification of oxalic acid with pentafluorobenzylbromide. In contrast with the low extraction yield for the inorganic permanganate and dichromate ions, a high reaction rate was obtained for the halogen exchange reaction with the functionalized dendrimers as phase transfer catalysts. It is found that the rate of reaction for both cases depends on the generation number of the dendrimer. For the higher-generation dendrimers, it is more difficult for the substrate to migrate from the bulk carbon dioxide phase to the interior of the dendrimer. This indicates that the mass transfer of the reagents from the organic phase into the interior of the dendrimer determines the reaction rate for the fourth and higher generation dendrimers. A model is proposed to describe the extraction and the phase transfer catalysis based on the fact that the modified dendrimers combine micellar properties with those of standard phase transfer catalysts. From the different experiments it can be concluded that the perfluoro-functionalized poly(propylene imine) dendrimers have great potential as multisite phase transfer catalysts.