Attempts of asymmetric adsorption on clays

Chapter 1 considers the possible relationships of earthly clays and other minerals to the origin of chirality in organic molecules. Attempts to establish experimental evidence of asymmetric adsorption on clays were unsuccessfiil, but die search for chirality did find naturally occurring enantiomorphic crystals like quartz. Asymmetric adsorption of organic molecules on quartz crystals such as separation of racemic mixtures, like Co or Cr complexes, alcohols and other compounds, allowed for the conclusion that quartz crystals can serve as possible sources of chirality but not of homochirality. This latter conclusion results fi om the finding that all studied locations of quartz crystals contain equal amounts of d- and /-forms. The preparations of synthetic adsorbents such as imprinting silica gels are also considered. More than 130 references are analyzed. 

There are many natural minerals and salts that posses optical activity in their crystalline state owing to their chiral lattices, sueh as quartz, einnabar, mica, chlorates, bromates, and iodates. Crystal chirality of other minerals, like aluminosilicates, such as zeolites, were not investigated, but these minerals are considered by many investigators as possible sourees of chirality and the origin of homochirality in our biosphere The optieal activities of clays have not revealed reliable evidence of chirality and therefore they do not play any positive role in our understanding of the origin or of the amplification of homochirality in nature. 

On the other hand, if aluminosilicates are used as supports for chiral metal complex catalysts, they might reveal a contribution to asymmetric action as was found in the case of the hydrogenation catalyst [CoSalen] complex supported on hectorite 

Even though clays have no chirality associated with their crystal structure, Julg and other authors claim asymmetric effects. As stated 

Measurement of the (-) rotation (a 90) in (/-quartz (a 0) has definite difficulties because of double refraction. That is why the (+)-rotation only is usually accepted for (/-quartz. 

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