Adsorption of Water Insoluble Porphyrin to Trioctahedral Layered Aluminosilicate and Its Surface Acidity

This study investigated the adsorption of a non-ionic water-insoluble organic molecule (meso-tetraphenylporphyrin, TPP) on a trioctahedral layered aluminosilicate (saponite, SP) in organic solvents in order to investigate properties of SP as an adsorbent for a wider variety of molecules, other than cationic or polar ones. The affinity of solvents for the layered aluminosilicate surface was an important factor for predicting the molecule’s adsorption proprieties. Namely, solvents with low affinity for the aluminosilicate should be selected so that molecules can approach the surface, thus prompting adsorption of the molecules. Under these conditions, TPP was adsorbed to SP due to their basicity and acidity. The acid on SP adsorbing TPP was revealed to be Lewis acid. The weaker acid strength (H0, estimated to be 0.8–1.5) compared with that of Brønsted acids appeared to be more suitable for adsorbents because molecules are recovered efficiently from SP where their intramolecular charge distributions are shifted. The acidity of SP is discussed quantitatively in this study because this has not been done thoroughly in comparison with dioctahedral aluminosilicates such as montmorillonite, which are conventionally used as catalysts.