Hydrogen miscibility with SCFs

SCFs, like gases, have no surface tension they diffuse rapidly to occupy the entire volume of a system. This also means that if other gases are introduced, they will also diffuse to fill the entire volume and mix perfectly. Unlike the solubility of gases in liquid solvents, which is relatively low and decreases as temperature increases, gases are totally miscible with SCFs and can be said to have perfect solubility. The concentration of hydrogen in a supercritical mixture of hydrogen... 

Solubihty of H2 in most organic solvents is very low. As hydrogenation is conducted in traditional solvents, diffusion of H2 from bulk gas phase to the H2/liquid interface, absorption at the H 2/liquid interface, and diffusion into Hquid bulk phase are necessary, and this influences the progress of the reaction. H2 is completely miscible with SCFs, and its concentration at the catalyst surface can be greatly increased, so high reaction rates can be achieved [21]. 

A further useful feature of SCFs is their total miscibility with gases. This circumvents the problem of low gas solubility in organic solvents for reactions such as hydrogenation, carbonylation or hydroformylation, and obviates the need for stirring a single homogeneous phase is always present. 

In a first approximation, the new methods correspond to the conventional solvent techniques of supported catalysts (cf Section 3.1.1.3), liquid biphasic catalysis (cf Section 3.1.1.1), and thermomorphic ( smart ) catalysts. One major difference relates to the number of reaction phases and the mass transfer between them. Owing to their miscibility with reaction gases, the use of an SCF will reduce the number of phases and potential mass transfer barriers in processes such as hydrogenation, carbonylations, oxidation, etc. For example, hydroformylation in a conventional liquid biphasic system is in fact a three-phase reaction (g/1/1), whereas it is a two-phase process (sc/1) if an SCF is used. The resulting elimination of mass transfer limitations can lead to increased reaction rates and selectiv-ities and can also facilitate continuous flow processes. Most importantly, however, the techniques summarized in Table 2 can provide entirely new solutions to catalyst immobilization which are not available with the established set of liquid solvents. 

The high miscibility with H2, the smaller number of phase boundaries (SCF/catalyst vs gas/liquid/catalyst), and the high diffusion rates were recognized relatively early as potentially useful properties of SCFs in hydrogenation reactions using heterogeneous noble metal catalysts. A substantial amount of research in this field has been and still is being carried out in industrial laboratories or in cooperation with the chemical industry. Therefore, the information in the published literature reflects only a limited fraction of the most recent developments. 

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