Layered phosphates, pillaring

G. Alberti, M. Gasciola, U. Constantino, R. Vivani, Layered and pillared metal (IV) phosphates and phospho-nates. Advanced Materials 8 (1996) 291-303. 

Pillared clays have shown a diversity of behaviors indicating that they may find applications as catalysts. The theoretical pore sizes for aluminum Keggin ion- pillared clays are calculated and compared with literature data. Layered phosphates and titanates have much higher charges, which should produce smaller cavities. Methods of achieving a range of pore sizes are described and compared to preliminary experimental results on the pillaring of these classes of layered compounds. 

These layered phosphates do not swell in water. Therefore, the layers were first spread apart by intercalation of butylamine. In this condition the AI13 species was exchanged for the alkylammonium ion to form pillared products [19]. Table 12-1 gives a summary of some of the results obtained with zirconium phosphate. It is observed that the surface areas of the aluminum... 

More suitable pillars could be inorganic ions or oxides, with which it should be possible to prepare modulated pillared structures with these layered phosphates. 

The concepts developed through the pillaring of layered silicate clays and group-IV metal phosphates should be extended to other classes of layered host structures. Some work has been initiated in the area of related layered phosphates (e.g., vanadyl and uranyl phosphates), and this approach should be extended. Layered compounds with well-defined structures should receive first priority because they present the best possibilities for a molecular design approach. 

Using this approach leads to replace phosphates by other chelating agents. The first attempts concerned phosphonates and were developed by Alberti and Clearfield ([76] and Refs therein) in the early seventies. These authors showed that phosphonates could replace phosphates in a lot of syntheses, but this way was essentially devoted to layered compounds in order to increase the distance between the layers, and not with the aim of obtaining porous compounds. They mainly used monophosphonates, but some further attempts with diphosphonates or functionalized monophosphonates lead to pillared compounds which sometimes offer frameworks with an open structure. Using this way, we recently characterized [77] a porous vanadium(IV) diphosphonate (NH4)2(H20)2(V0) (V0(H20))2 (03P—CH2—P03)2 or MIL-2 (for Materials of Institut Lavoisier) in which the channels are occupied with water molecules and ammonium ions (Fig. 15 (left)). 

Microporous hosts related to zeolites, i.e., Cu(II)-exchanged alumina- and chromia-pillared layered a-Sn- and Zr-phosphates, have been studied for the polymerization of pyrrole introduced from the vapor phase. XPS data and optical spectra suggested that more than one type of polymer was formed in the pillared hosts, with mixed neutral/bipolaron states on low-oxidation level polymer. The absence of significant conductance (less than 10 S/cm) was associated with short pol5uneric units. 

Advancements in the preparation of new PLS s nearly parallels that of the zeolite and zeolite-like phases. Initially the pillared smectite clays were investigated but the quest for new materials with new properties led to e qiloring the pillaring of other layered phases. These include, most notably, the layered zirconium phosphates, double hydroxides (hydrotalcites), sihcas and metal oxides. The parallel paths of discovery in new material compositions for the layered phases and the microporous (zeoUte) phases are summarized in Table 1. A conq>arison between the pore architectures of the zeohtes and the two dimensional PLS is shown in Table 2. 

We have prepared other mixed composition pillared compounds irtiich have as their non-pillaring group the hydroxyl moiety, and thus are simply relatives of zirconium phosphate in which the layers are spread at a fixed distance apart. These products behave as expected in titration and ion-exchange experiments, and will not be further discussed here. 

Clays have also been pillared with polyoxometalate ions, such [(PW11VO40)4-].261 Other layered materials can also be pillared. Zirconium phosphate has been pillared with chromium(III) oxide.262 A layered titanate has been pillared with silica.263 The need for a separate pillaring step has been avoided in the preparation of some porous lamellar silicas which are structurally similar to pillared clays.264 Eight to twelve carbon diamines were used with tetra-ethoxysilane in ethanol with added water to make them in 18 h at room temperature. The template was recovered by solvent extraction before the silica was calcined. 

The pillaring of clays has become an established technique of preparing a new class of porous materials. While their promise as cracking catalysts has not been fulfilled, never-the-less they exhibit interesting catalytic behavior. We have extended the technique to the pillaring of many other types of non-clay layered compoimds which include phosphates, oxides and layered double hydroxides. This extension broadens the field to the point where one can choose the degree of acidity or basicity and framework metals required for specific catalytic processes. 

---