Supramolecular heterogeneous catalysis

One may note that the analysis of solid state molecular geometries obtained from crystal structure data may reveal the changes occurring when two reagents approach each other, thus providing insight into chemical reaction paths [7.72]. 

Of special interest is the eventuality of stabilizing transition states by imprinting their features into cavities or adsorption sites using stable transition state analogs as templates. Studies towards such TSA footprint catalysis have been performed by generating TSA complementary sites as marks on the surface [7.73a] or as cavities in the bulk [7.73b] of silica gel. These imprinted catalytic sites showed pronounced substrate specificity [7.74a,b] (namely in the case of cavities [7.73 b]) and chiral selectivity [7.74c,d]. 

Similar features may be achieved with organic polymeric materials. Template imprinting of complementary cavities containing appropriate functional groups yields models of enzyme active sites [7.34, 7.35, 7.75]. They perform the synthesis of amino acids with enantioselectivity [7.76] or the esterolysis of activated esters by TSA imprinting [7.77]. 

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A researcher in the field of heterogeneous catalysis, alongside the important studies of catalysts chemical properties (i.e., properties at a molecular level), inevitably encounters problems determining the catalyst structure at a supramolecular (textural) level. A powerful combination of physical and chemical methods (numerous variants x-ray diffraction (XRD), IR, nuclear magnetic resonance (NMR), XPS, EXAFS, ESR, Raman of Moessbauer spectroscopy, etc. and achievements of modem analytical chemistry) may be used to study the catalysts chemical and phase molecular structure. At the same time, characterizations of texture as a fairytale Cinderella fulfill the routine and very frequently senseless work, usually limited (obviously in our modem transcription) with electron microscopy, formal estimation of a surface area by a BET method, and eventually with porosimetry without any thorough insight. 

Scheme 9.10 Supramolecular metal-organic polymer 17 for the heterogeneous catalysis of asymmetric hydrogenations.

The unique features of dendritic architecture and the rich chemistry of organo-transition metal complexes have been combined in metallodendrimers to create the potential for a wide range of utilitarian applications. Because dendrimers allow scientists to probe the twilight zone between homogeneous and heterogeneous catalysis as well as to apply the techniques associated with combinatorial-type chemistry, diverse new areas of the nanoworld have became accessible. Since many new avenues in supramolecular chemistry have been opened by organometalhc complexes, metallodendrimers will continue to play an important role in not only organometalhc chemistry and polymer science, but also in material science. These new interfaces will be rich areas for future science to pursue. 

The use of host-guest catalysis in industrial applications would seem further away, as even the construction of a host by supramolecular means still remains costly. In terms of space-time yields heterogeneous counterparts may be more attractive, for instance (periodic) mesoporous (organo) silicas (for PMOs [99]). Highly selective processes that do not occur otherwise may find their way to applications. The capacity of a host to distinguish between guests (substrates) is as yet not a useful property for industrial applications as mixtures are not commonly used as feedstock (apart from... 

Our research in heterogeneous liquid-phase catalysis has been supported by the Belgian federal government through an IUAP program Supramolecular Chemistry and Catalysis. ... 

Margitfalvi et al. suggested that the catalytic systems Pt- and Pd-cinchona alkaloid-keto esters, are the first examples of a new class of heterogeneous catalytic reactions of supramolecular catalysis, where the prochiral group is part of a conjugated double bond system. 

Crespilho, F.N., Silva, W.C., Zucolotto, V. Supramolecular assemblies of metallophthalocyanines physicochemical properties and applications. In Nantes, I.L. (ed.) Catalysis and Photochemistry in Heterogeneous Media, 1st edn. Research Signpost, Kerala (2007)... 

The coexistence of hydrophilic and hydrophobic nano-domains separated in space, with a local order and fluidity typical of liquids, confer to supramolecular surfactant structures remarkable properties, which are advantageous in applications involving molecular confinement within nanoscopic regions and reactivity in micro-heterogeneous media. Micelle-mediated reactions constitute the basis of the so-called micellar catalysis [62, 116], admicellar catalysis [117] or admicellar polymerisation [118] in which reaction mechanisms may be controlled at a molecular level to save energy and raw materials, as well as to avoid lengthy post-reaction purification and analytical steps. 

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