The Unique Structure–Activity Relationship of Porphyrins and Clay Mineral Systems in Modern Applications: A Comprehensive Review

J. Hassen and J. Silver

Abstract
Porphyrins are a group of organic heterocyclic macromolecules, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges. This unique and significant group of compounds has a special relationship with inorganic clay minerals, which are hydrous aluminium phyllosilicates. Clay minerals, known to be typical layered materials, act as unique hosts for porphyrin molecules or their precursor materials. When the porphyrin compound enters between the layers of a clay, it can orient horizontally, diagonally, or perpendicularly. Cation-exchanged clay can provide the acidity required for the formation of porphyrins from its precursor molecules without the need for other auxiliary factors such as heating. Clays are also capable of facilitating the incorporation of various metals into the porphyrin ring to form a metalloporphyrin. The properties of these metalloporphyrin clay complexes have been studied extensively. Modern applications of such clay-porphyrin materials include their use as heterogeneous catalysts for oxidation and polymerization reactions in the synthesis of new materials. These heterogeneous catalysts have a wide range of uses in vital sectors, such as food manufacturing, pharmaceuticals, and the chemical industry. Additionally, clay-porphyrin systems are employed in constructing various types of self-assembling artificial photosynthesis systems.

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Keywords
porphyrin-clay systems, structure-activity relationship, heterogeneous catalysts, self-assembling artificial photosynthesis systems