Clay Mineral Catalysis of Natural Processes and Prebiotic Organic Reactions
After outlining the use of clays as solid acid catalysts in petroleum cracking, this chapter describes the role of clay minerals in the decarboxylation of fatty acids to yield short-chain alkanes. The reaction involves electron transfer from the fatty acid to structural ferric ions or edge surface aluminum ions, acting as Lewis acid sites. Clay minerals can also catalyze the transformation of organic matter in sediment (kerogen) or preserve organic matter in soil by intercalation into their layer structure. Pyrolysis studies indicate that montmorillonite can induce the decomposition and transformation of kerogen by a Brønsted acid-catalyzed process or through a synergistic interaction between Brønsted and Lewis acid sites. Pyrolysis or hydrothermal treatment of soil humic acid in the presence of clay minerals can lead to intensive aromatization. Clay minerals can also promote the formation of biomolecules and biopolymers from their simple precursors and preferentially adsorb the L-enantiomer of some amino acids over the D-optical isomer. Thus, oligopeptides can form from pristine amino acids under conditions of repeated wetting, drying, and heating, or from the corresponding amino acid adenylates. Likewise, montmorillonite can promote the self-condensation of an RNA nucleotide activated on the phosphate group with imidazole (ImpA) to yield oligomers.