ABSTRACT
Clay mineral surfaces may be activated or modified in order to enhance catalytic efficiency as well as control sample porosity. Treatment with mineral acids is the most widely used technique for acid activation. Thermal activation is achieved by heating and calcination, sometimes in conjunction with microwave and ultrasound irradiation. Two outstanding examples of surface-modified clay catalysts are pillared interlayered clays (PILC) and porous clay heterostructures (PCH). Smectite clays can also be modified by intercalation of cationic organic species such as protonated 1,4-diazobicyclo[2,2,2] octane (Dabco), or quaternary ammonium ions such as cetyltrimethylammonium. Although useful as phase-transfer catalysts, organically modified clay minerals would require treatment with mineral acids in order to become catalytically active. Organoclays can also be obtained through the covalent grafting of silylating agents to the silicate layers. The various types of organic reactions catalyzed by acid-treated montmorillonites, notably the commercial K-series catalysts, are summarized in a series of tables. Likewise, organic conversions in the presence of PILC and acid-activated organoclays are presented in tabular form.
