Journal of Porous Materials 5, 227–239 (1998) c  1998 Kluwer Academic Publishers. Manufactured in The Netherlands. Pore Structure Tailoring of Pillared Clays with Cation Doping Techniques H.Y. ZHU AND G.Q. LU Department of Chemical Engineering, The University of Queensland, St Lucia Qld 4072, Australia Abstract. Techniques and mechanism of doping controlled amounts of various cations into pillared clays without causing precipitation or damages to the pillared layered structures are reviewed and discussed. Transition metals of great interest in catalysis can be doped in the micropores of pillared clay in ionic forms by a two-step process. The micropore structures and surface nature of pillared clays are altered by the introduced cations, and this results in a significant improvement in adsorption properties of the clays. Adsorption of water, air components and organic vapors on cation-doped pillared clays were studied. The effects of the amount and species of cations on the pore structure and adsorption behavior are discussed. It is demonstrated that the presence of doped Ca 2+ ions can effectively aides the control of modification of the pillared clays of large pore openings. Controlled cation doping is a simple and powerful tool for improving the adsorption properties of pillared clay. Keywords: pillared clays, cation doping, pore structure tailoring, adsorption 1. Introduction Pillared clays (PILC) are a new class of materials, de- veloped in searching for materials with larger pore sizes than zeolites [1–5]. Preparation of PILCs is based on the phenomenon of swelling, which is a typical prop- erty of smectite clays [6]. When dispersed in water, the layered clays swell because of hydration of the inter- lamellar cations which act as counter-ions to balance the negative charges of clay layers. Therefore, polyox- ocations in aqueous solutions can be intercalated into the interlayer space by cation exchange. The aqueous solution containing polyoxocations is referred to as the pillaring solution. When heated to above 400 ◦ C, the in- tercalated polyoxocations are subjected to dehydration and dehydroxylation. As a result, they are converted to oxide pillars [7], propping the clay layers apart. A permanent micropore system is thus formed [8]. The pore openings of PILC vary from 0.4 to 2.0 nm, de- pending on the type of pillars [9]. The pore volume and specific surface area are also pillar-dependent. For instance, the predominant slit-shaped pores in alumina pillared clays (Al-PILC) has a width of about 0.7–0.8 nm, micropore volume of about 0.1 cm 3 /g and BET surface area of about 300 m 2 /g [1–5, 10] whilst for a silica-titania sol pillared clay they are about 1.2–1.5 nm, 0.15 cm 3 /g and 450 m 2 /g, respectively [11]. In the last decade, the literature on synthesis and catalytic applications of pillared clays has grown rapidly and pillared clays of various pillars have been synthesized. The pillaring of clays has also become an increasingly popular technique for synthesis of other porous-layered materials. Compared to other porous solids, pillared clays demonstrate their advantages in several aspects. First, clays are abundant material and pillared clays are generally synthesised under moderate conditions with much simpler procedures, compared to zeolite synthe- sis [4, 6]. Another feature is that some pillared clays have pore openings of about 1 nm or even larger [4], being much larger than those in zeolites (0.3 to 0.7 nm) [6]. So PILCs have a potential to serve as molecular sieves and shape-selective catalysts for a wide range of molecular sizes. Furthermore, multiple components like transition metals, rare earth elements can be read- ily introduced into PILCs by intercalating clays with mixed pillars of oxides, such as, cerium oxide-alumina [12], lanthanum oxide-alumina [13] and silica-titania [10], and/or by a subsequent cation exchange process [14–18]. PILCs have been shown to have high catalytic