Layered double hydroxides, pillaring

Layered double hydroxides pillared with SiW12O404-... 

The layered double hydroxides have been of interest as anion exchangers, basic catalysts, and catalyst precursors. Pillared derivatives of the type discussed earlier for the clays have been synthesized by ion exchange with polyoxometallate anions but have lower thermal stability. Several authors have reported synthesis of layered double hydroxides pillared with the Vio028 ion by direct ion exchange or by techniques that involve preswelling the layer structure with an organic anion. For example, the terephthalate anion was used in the synthesis ofVio028 and Mo7 024 intercalated phases. 

Doeuff, M., Kwon, T., and Pinnavaia, T. J. 1989. Layered double hydroxides pillared by polyoxometalate anions exafs studies and chemical synthesis. Synthetic Metals 34 609-15. 

The structure of the layered double hydroxides is the reverse of that of the clays. They are anionic materials in which the sheets are intercalated with anions instead of cations. These ions can be exchanged with several different anions. The cationic nature of the layers lends itself to pillaring by large Keg-gin anions. 

Similar methods of encapsulation are also observed in pillared clays, which were also introduced as catalysts as long ago as the early 1980s. The field has been thoroughly reviewed up to 2000 [65], Layered double hydroxide structures have also been used for the entrapment of metal coordination compounds [66],... 

It has been pointed out that these pillared intercalates are intrinsically difficult to synthesize in highly crystalline form because the layered hosts are basic, whereas most heteropolyacids are acidic and tend to decompose. Narita et al. (392) tried direct synthesis of a heteropolyanion-pillared layered double hydroxide by a coprecipitation reaction of Zn2+ and A1J+ ions in the presence of a moderately acidic lacunary Keggin anion, a-SiWn039 XRD of the product showed a basal spacing of 14.6 A, which corresponds to a gallery height of 9.9 A. The surface area was found to be 97 m2 g, which is three times that of the layered double hydroxide. 

Abbreviations AD, asymmetric dihydroxylation BPY, 2,2 -bipyridine DMTACN, 1,4-dimethyl-l,4,7-triazacyclonane EBHP, ethylbenzene hydroperoxide ee, enantiomeric excess HAP, hydroxyapatite LDH, layered double hydroxide or hydrotalcite-type structure mCPBA, meta-chloroperbenzoic acid MTO, methyltrioxorhenium NMO, A-methylmorpholine-A-oxide OMS, octahedral molecular sieve Pc, phthalocyanine phen, 1,10-phenantroline PILC, pillared clay PBI, polybenzimidazole PI, polyimide Por, porphyrin PPNO, 4-phenylpyridine-A-oxide PS, polystyrene PVP, polyvinylpyridine SLPC, supported liquid-phase catalysis f-BuOOH, tertiary butylhydroperoxide TEMPO, 2,2,6,6-tetramethyl-l-piperdinyloxy TEOS, tetraethoxysilane TS-1, titanium silicalite 1 XPS, X-ray photoelectron spectroscopy. 

In all previous cases, V was incorporated in a monomeric form. There are also methods to introduce oligomeric V into inorganic structures. Choudary et al. (48) advocated the use of a montmorillonite, pillared with V oligomers (V-PILC). V-PILC catalyzes the epoxidation of allylic alcohols with i-BuOOH. Oligomeric V is also used to pillar anionic clays such as layered double hydroxides (LDHs) with decavanadate anions (VioOjg) (49). 

Layered silicate clays intercalated by pillaring poly-oxocations are precursors to an important class of mi-croporous catalysts. Smectite clay was the only host structure known to be pillarable by purely inorganic oxo ions. Recently, layered double hydroxides (LDH) pillaring oxo ions were reported by Pinnavaia and coworkers [79, 80]. 

Chibwe, M. and Jones, W. (1990). The synthesis, chemistry and catalytic applications of layered double hydroxides in Pillared Layered Solids (I. Y. Mitchell, Ed.). Elsevier Applied Sciences, London. 

Chibwe, K. and Jones, W. (1989). Synthesis of polyoxometalate-pillared layered double hydroxides via calcined precursors. Chem. Mater. 1, 489. 

Dimotakis, E. D. and Pinnavaia, T. J. (1990). New route to layered double hydroxides intercalated by organic-anions - precursors to polyoxometalate-pillared derivatives. Inorg. Chem. 29, 2393. 

Evans, J., Pillinger, M. and Zhang, J. (1996). Structural studies of polyoxometalate-anion-pillared layered double hydroxides. J. Chem. Soc., Dalton Trans., 2963. 

Narita, E., Kaviratna, P. D. and Pinnavaia, T. J. (1993). Direct synthesis of a polyoxometalate-pillared layered double hydroxide by coprecipitation. J. Chem. Soc., Chem. Commun., 60. 

Kooli, F. and Jones, W. (1995). Direct synthesis of polyoxovanadate-pillared layered double hydroxides. Inorg. Chem. 34, 6237. 

Wang, J. D., Serrette, Y., Tian, Y. and Clearfield, A. (1995). Synthetic and catalytic studies of inorganically pillared and organically pillared layered double hydroxides. Appl. Clay Sci. 10, 103. 

The anionic polyoxometalates comprise a much larger class of metal oxide-based pillaring agents (18). It was proposed that hydrotaldtes (also referred to as layered double hydroxides, or LDHs) would be suitable host materials for pillaring with polyoxometalates. Subsequently, a route for synthesizing isopol3rmetalate-pillared hydrotalcites via organic-anion-pillared precursors was developed (19-20). 

Recent Catalytic Studies Involving Pillared Layered Double Hydroxides... 

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. 

Challier and Slade [175] reported the synthesis of nanocoinposite materials consisting of polyaniline molecules encapsulated between ultra-thin mixed metal hydroxide sheets which are propped apart by spacers of terephthalate or hexacyanoferrate(II) ions acting as pillars. The layered double hydroxides (LDHs) were prepared by the method of Drezdon [176] which were refluxed with aniline to synthesize aniline intercalated LDHs. In thermo-gravimetric studies, terephthalate/Cu/Cr LDHs as well as hexa-cyanoferrate(lI)/Cu/Al LDHs showed weight losses in two steps attributed to the removal of trapped water and thermal breakdown of the intercalated systems. The former material exhibited somewhat better thermal stability than the latter one. 

The present work summarizes our efforts to synthesize supergallery pillared derivatives of three general families of layered compounds. The host structures of interest include smectite clays, layered double hydroxides, and layered silicic acids. 

El Malki, K., de Roy, A., and Besse, J. P. 1989. New CuCr layered double hydroxide compound discussion of pillaring with intercalated tetrahedral anions. Eur. J. Solid State Inorg. Chem. 26 339-51. 

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