**5. Preparation of nanoporous materials composed of ladder-like PSQs and layered clay minerals by ion-exchange reaction**

There have been intense research activities on layered silicates pillared with inorganic or organic clusters, which are called pillared interlayer clays (PILCs) (Otsuka, 1997; Kloprogge, 1998; Ding et al., 2001). PILCs have much higher surface areas and pore volumes than the original clays. Such properties make them useful catalysts, ion-exchangers, and adsorbents. These materials are normally prepared by the intercalative ion-exchange of layered clay minerals with a variety of nano-sized pillars, such as organic ions (Ogawa et al., 1994), inorganic ions (Pichowicz & Mokaya, 2004), and sol particles (Yamanaka et al., 1992).

Even though the preparation of various polymers/clay hybrids (Pinnavaia & Beall, 2000) has already been reported, there have been a few studies on the preparation of PILCs using polymers. Because the polymers usually have a flexible structure, expansion of the interlayer space of clays by polymer incorporation is not enough for providing more free space.

The aforementioned PSQ-NH3+Cl– has motivated the author and co-workers to develop new PILCs, because it has rigidity and bulkiness to expand the interlayer of clays and ability to intercalate molecules into the interlayer of anionic clays by ion-exchange reaction due to the presence of ammonium cations in side-chains of PSQ-NH3+Cl–. In this section, therefore, the author describes the preparation of a clay pillared with rod-like cationic PSQ-NH3+Cl– (Scheme 4) (Kaneko et al., 2004b).

The synthesis was performed by pouring the PSQ-NH3+Cl– aqueous solution into an aqueous suspension of Na-saturated saponite (Na-SAP). After the mixture was vigorously stirred for 2 h at room temperature, the product was collected by filtration, washed with water, and then dried under reduced pressure at room temperature to obtain the clay pillared with PSQ-NH3+Cl– (PSQ–SAP). The resulting product was characterized by IR, CHN elemental analysis, XRD, and nitrogen adsorption–desorption isotherm measurements.

The IR spectrum of PSQ–SAP showed an absorption band at *ca*. 1515 cm–1 assigned to the ammonium ion of the PSQ component, indicating that the PSQ was inserted into the interlayer of SAP. On the basis of the CHN analysis data, the exchange amount of a repeating unit of the PSQ component in PSQ–SAP was calculated to be 126 meq/100 g SAP. This value is higher than that of the cation exchange capacity (CEC) of Na-SAP (92 meq/100 g SAP) (Bujdák et al., 2002). The distance between the charges of PSQ would be shorter than that of SAP. Therefore, excess ammonium groups and counter anions (Cl–) of PSQ-NH3+Cl– were inserted into the interlayer of SAP, which was confirmed by a Cl elemental analysis. The XRD pattern of PSQ–SAP was completely different from that of Na-SAP and PSQ-NH3+Cl–. Accordingly, PSQ–SAP was not a mixture, but an intercalated nano-order material, *i.e.*, a hybrid.

**Scheme 4.** Preparation of a clay pillared with PSQ by ion-exchange reaction of PSQ-NH3+Cl– with Nasaturated saponite.

From the nitrogen adsorption–desorption isotherms at 77K, the surface area and pore volume of PSQ–SAP derived from the *t*-plot were estimated to be 370m2/g and 0.15 cm3/g, respectively. This indicates that a porous material was prepared from the starting materials with dense structures (BET surface areas of Na-SAP and PSQ-NH3+Cl– were *ca*. 26 and 5 m2/g, respectively).

When a clay mineral with high CEC such as Li-saturated taeniolite was employed, such a porous material was not obtained by combination with PSQ-NH3+Cl– (BET surface area of the resulting product was *ca*. 53 m2/g), although a sufficient interlayer spacing existed as confirmed by the XRD measurement (*d*-value of the product was *ca*. 1.83 nm). Because the distance between the PSQs in the interlayer of taeniolite is short due to the higher CEC of the Li-saturated taeniolite (exchange amount of a repeating unit of PSQ was calculated to be 140 meq/100 g taeniolite), sufficient space was not provided. Furthermore, when polyallylamine hydrochloride (PAA-Cl) —a common cationic polymer— was used for pillaring in the SAP interlayer, a porous structure was not obtained (BET surface area of the product was *ca*. 52 m2/g). It was difficult for PAA-Cl to pillar the interlayer of SAP due to the lack of rigidity and bulkiness. From these results, it was considered that the rigidity and bulkiness of the guest polymers and a sufficient distance between charges in the host layered clay minerals are necessary for preparing clays pillared with polymers.
