Acidic Porous Clay Heterostructures (PCH): Intragallery Assembly of Mesoporous Silica in Synthetic Saponite Clays

Mesostructured intercalates belonging to the class of solid acids known as porous clay heterostructures (PCH) have been prepared through the surfactant-directed assembly of mesoporous silica within the galleries of synthetic saponite clays with targeted layer charge densities in the range x = 1.2−1.7 e - units per Q + x [Mg 6 ](Si 8 - x Al x )O 20 (OH) 4 unit cell. The removal of the intragallery mixture of neutral alkylamine and quaternary ammonium ion surfactant (Q + ) by calcination afforded PCH intercalates with basal spacings of 33−35 Å, BET specific surface areas of 800−920 m 2 g - 1, and pore volumes of 0.38−0.44 cm 3 g - 1 . The framework pore sizes were in the supermicropore to small mesopore region ∼15−23 Å. Temperature-programmed desorption of chemisorbed cyclohexylamine (CHA) indicated the presence of both weak and strong acid sites, corresponding to desorption temperatures near 220 and 410 °C, respectively. The total acidity (0.64−0.77 mmol CHA g - 1 ) increased with the saponite layer charge density ( x ), indicating that the acidity is correlated with the number of protons balancing the clay layer charge after calcination. The high acidity, structural stability to 750 °C, and supermicroporous to small mesoporous pore structure of these intermediates make these PCH materials especially attractive candidates for acid-catalyzed conversions of organic molecules.