Pillared layered materials

More recently, crown ether pillared and functionalized a-zirconium phosphonates have been synthesized [34], Incorporation of crown ethers was achieved by first converting them to their respective phosphonic acids. X-ray diffraction studies showed that the interlamellar spacing was about 15 A when a biphospho-nic acid was used. Both a- and y-type zirconium phosphonates could be synthesized using this methodology. Preliminary results showed that these layered materials have good selectivity toward binding transition metal ions and that the interlamellar fluoride ions could be replaced by anion exchange. 

The metal phosphonate compounds usually adopt layered or pillared layered structures with the organic moieties filling in the inter-layer spaces [1-4]. The layered nature makes them interesting candidates to host intercalation reactions. Furthermore, the potential for the organic moieties to be modified by functional groups allows for the preparation of a number of new materials that have possible applications in areas such as catalysis, ion exchange, sensing and ion conduction [1,4-7]. 

Further investigations will determine how these isomorphous substitutions affect the ability of the layered materials to form stable pillared compounds. 

Other examples are pillared bentonite and other clays, which exist as thin sheets only weakly connected, if at all. These layered materials yield low levels of LCB when the catalyst activated at temperatures below its fusion temperature. However, if it is heated to 600-700 °C, so that the structure rearranges, very high LCB levels are produced in the polymer. 

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. 

Because of the structural parallelism described above, in principle the HOH layers of the heterophyllosilicates could serve like the TOT layers of the phyllosilicates to synthesize mesoporous pillared materials and, in general, layered materials, a perspective that has been recently investigated but not yet realized [33, 42]. 

Acidic clays are widely applied in the dehydration of alcohols [38]. Although similar to zeolites in their capacity to induce the formation of both alkenes and ethers, selective alkene synthesis is possible. Various layered materials (clays, ion-exchanged montmorillonite, pillared layered clays) are very active and, in general, selective in transforming primary, secondary, and tertiary aliphatic alcohols to 1-alkenes [39-43]. Al -exchanged montmorillonite, however, induces ether formation from primary alcohols and 2-propanol [41]. Substituted 1-phenyl-1-ethanols yield the corresponding styrene derivatives at high temperature (653-673 K) [44]. 

Preparation of pillared layered materials based on zeolite layers. 

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

We have shown that it is possible to obtain pillared layered materials with cavities ranging from a few angstroms to relatively large ones ( 15 A) by the choice of the layered compound and pillar. These materials can be utilized as catalysts and sorbants in both the acid forms and ion-exchanged forms, as is the case for zeolites. While they may be unsuitable for use in cracking reactions because of their lowered stability relative to zeolites, they may find use for many other types of reactions. It is now necessary to determine the physical chemical and catalytic properties of these materials, so as to find where their particular behavior can be utilized effectively. 

The successful removal of the template is proven by the very strong decrease of the CH- or CC-vibration in the Raman-spectra of the samples as compared to the iron oxide surfactant. From fig. 6 it can be infered that probably two layered materials have formed in the iron oxide surfactant. Since the layers would not be stabile without any "spacer", probably some kind of pillaring has occured during the plasma calcination. The color change from light brown to a darker brown, moreover, indicates that oxidation of the iron,... 

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