Metal oxide pillars, synthesis

Protons are released upon heating which in part balance the negative charge of the host clay layers. A number of review articles have recently appeared which summarize the synthesis and physical properties of metal oxide pillared days derived fix>m the intercalation of polyoxocations of aluminum, zirconium, chromium and many other metals [10-12]. The Lewis acid sites provided by coordinatively unsaturated metal ion sites on the pillar and the Bronsted addity formed upon thermolysis imparts novel chemical catalytic properties [13,14]. Since the pores between pillars often are larger than those foimd in conventional zeolites, there is considerable interest in the use of metal oxide pillared clays for the processing of large organic molecules, espedally petroleum [14-17]. 

Recently reported meso- and macroscale self-assembly approaches conducted, respectively, in the presence of surfactant mesophases [134-136] and colloidal sphere arrays [137] are highly promising for the molecular engineering of novel catalytic mixed metal oxides. These novel methods offer the possibility to control surface and bulk chemistry (e.g. the V oxidation state and P/V ratios), wall nature (i.e. amorphous or nanocrystalline), morphology, pore structures and surface areas of mixed metal oxides. Furthermore, these novel catalysts represent well-defined model systems that are expected to lead to new insights into the nature of the active and selective surface sites and the mechanism of n-butane oxidation. In this section, we describe several promising synthesis approaches to VPO catalysts, such as the self-assembly of mesostructured VPO phases, the synthesis of macroporous VPO phases, intercalation and pillaring of layered VPO phases and other methods. 

In order to obtain a porous pillared material with improved adsorption properties, some additional modification techniques on PILCs, both during or after the synthesis, have been developed. The incorporation of metals in the pillars, when performed during the synthesis, results in the formation of mixed oxide-pillars. Heylen et al. (77) observed an enhanced adsorption at 273 K of cyclohexane, CCI4, and CO2 on Ee-/Cr-pillared montmorillonite compared to the pure Fe-pillared montmorillonite. By the synthesis of mixed oxide-pillars, specific adsorption sites are created in the PILC, exerting a positive influence on the adsorption capacity and selectivity towards gases. 

Mixed oxide pillared clays are a new form of pillared clays. They can be obtained by a combined hydrolysis of metal salts La/Al, Al/Fe, Fe/Cr, Fe/Zr, Cr/Al. Another route is the synthesis of complexes containing different metal cations GaAli2-Keggin structures or binary oxides. These mixed pillared clays will have other, more specific catalytic and adsorption properties, while the size and charge of the pillaring species can be changed. The main drawback of these pillared materials is the characterization of the pillars. 

The incorporation of metals into the pillars, when performed during the synthesis, results in the formation of mixed oxide-pillars (examples are given in Table 1). In this way specific adsorption sites are created in the PILC, exerting a positive influence on the adsorption capacity and selectivity towards gases [33]. 

Figure 3 The synthesis of pillared porous MOFs by mechanochemical LAG synthesis starting from a metal oxide (a) general schematic of the reaction and (b) examples of low-porosity pillared MOFs based on zinc fumarate, prepared by LAG...

Figure 4 The synthesis of porous MOFs directly from a metal oxide using the mechanochemical ILAG technique (a) the rapid (less than an hour) and quantitative synthesis of pillared MOFs enabled and directed by catalytic (about 100-1000 ppm) addition of nitrate or sulfate salts and (b) the quantitative formation of porous ZIFs from zinc oxide by grinding with two equivalents of 2-ethylimidazole. Analogous reactivity with 2-methylimidazole produced the popular material ZIF-8 and, with plain Him, several other open- and close-packed ZIF topologies...

Tetravalent metal phosphonates, or MELS (for Molecularly Engineered Layered Structures), provide a novel class of materials that combine many of the properties of incn-ganic metal oxides with the organic functionality more commonly found in functionalized polymeric resins. Early development work on these materials was carried out by Alberti and co-woikers [ref. 1] and Dines et al. [ref. 2]. Synthesis and characterization of related zirconium phosphates that also contain phosphonate groups as pillars have been described by Clearfield [ref 3]. There is a substantial patent estate for tetravalent metal phosphonates, and exclusive rights to this estate are owned by Catalytica [ref 4]. 

To increase die activiiy of metals or metal oxides su[q K>rted on hydrous titaninm oxide and to increase our ability to tailor-design catalysts, the synthesis of new crystalline titanates was initiated. The basis for synthesis of the new crystalline titanates was to modify the procedures developed at Sandia for preparing the hydrous metal oxides, and to utilize techniques for synthesis of zetdites and pillaring of layered materials (ref. 6-13). 

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