Heterogeneous-homogeneous catalytic limiting selectivity

The introductory section (Chapter 1) presents a brief survey of the history of industrial heterogeneous and homogeneous catalysis. The survey shows that the availability of the feedstocks has had a decisive influence on the catalysts that have been developed. In some instances the chemical leads for new catalysts have lain dormant in the literature for a number years before they were developed into industrial processes. Subsequently (Chapter 2) a very limited selection of current industrial catalytic processes is described. These include the... 

Reactions that otherwise would be carried out in more than one phase (heterogeneous reactions) can be transformed to homogeneous ones, with the aid of supercritical fluids, where interphase transport limitations are eliminated. This is realized due to enhanced solubilities of the supercritical fluids. Typical examples are reactions in water (supercritical water can solubilize organic compounds), homogeneous catalytic reactions, reactions of organometallic compounds. Homogenizing one compound more than the other may also affect relative rates in complex reactions and enhance the selectivity. 

However, the use of the heterogeneous catalysts in applicative enantioselective syntheses has a limited success. Several factors contribute to this situation (1) a long time is required to achieve an effective heterogeneous enantioselective catalyst compared with the homogeneous ones, (2) a more complex structure of the heterogeneous catalyst surface on which centers coexist with different catalytic activity and selectivity, which can lead to undesired secondary reactions, and (3) an increased difficulty to create an effective asymmetric environment and to accommodate it with the multitude of reactions that are interesting to be carried out under enantioselective restrictions. 

Mother nature has resolved the various limitations involved in multi-electron processes. Unique assemblies composed of cofactors and enzymes provide the microscopic catalytic environments capable of activating the substrates, acting as multi-electron relay systems and inducing selectivity and specificity. Artificially tailored heterogeneous and homogeneous catalysts as well as biocatalysts (enzymes and cofactors) are, thus, essential ingredients of artificial photosynthetic devices. 

Homogeneous catalysts are structurally well-defined complexes and because they are soluble in the reaction mix are not subject to pore diffusion limitations as are heterogeneous catalytic materials. They are almost always highly selective towards desired products. The main consideration is that the complex be stable and reactor conditions chosen such that all the gaseous reactants are adequately dissolved and mixed in the liquid phase. Homogeneous catalysts are easily characterized by standard instrumental methods for compound identification such as XRD or spectroscopy. Deactivation is associated with attack by traces of carboxylic acidic byproducts and impurities in the feed such as 02 and chlorides that attack the ligand groups. 

---