Mesostructure assembly in two dimensions: Synthesis, characterization and catalytic applications of acidic porous clay heterostructures

Environmental concerns have raised a great deal of interest in the substitution of traditional acid catalysts such as aluminum chloride, sulfuric and hydrofluoric acids with solid-state alternatives, particularly microporous pillared clays and mesoporous molecular sieves. The recently reported new class of solid porous materials known as porous clay heterostructures (denoted PCH) shows promising acidic properties for catalytic organic conversions. Presented here is a comprehensive study on the synthesis, characterization and catalytic activity of a new porous clay heterostructures derived from synthetic saponite.;These mesostructured intercalates have been prepared through the surfactant-directed assembly of mesoporous silica within the galleries of the layered aluminosilicate. The removal of the intragallery mixture of neutral alkyl amine and quaternary ammonium ion surfactant (Q+) by calcination afforded PCH intercalates with basal spacings of 33-35 A, BET specific surface areas of 800-920 m2 g-1, and pore volumes of 0.38-0.44 cm 3 g-1. The framework pore sizes were in the super-micropore to small mesopore region ∼15-25 A. 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 mmole CHA g-1) increased with the saponite layer charge density, indicating that the acidity is correlated with the number of protons balancing the clay layer charge after calcination. Grafting aluminum into the gallery silica by a post-synthesis treatment can significantly enhance the acidic character of the saponite derivatives. The high acidity, structural stability to 750°C, and supermicroporous to small mesoporous pore structure were verified by employing these PCH materials as catalysts for Friedel-Crafts alkylation and cumene cracking reactions.