Lithium-substituted materials

The incorporation of alkali metals in the perovskite La1 jeMjeAlo.7Nio.3O3 (M = Li, Na, K) allowed to increase the catalytic performances particularly with sodium substitution x = 0.25), while the lowest amount of coke formation was found for lithium-substituted materials (x = 0.2), proving that basic promoters improved the stability of nickel-based catalysts [41],... 

Of the wide range of catalyst materials studied, lithium-substituted nickel oxides are unusual in that they form sufficiently simple structures that a relatively complete characterization is possible. In addition, the origin of the selectivity imparted in nickel oxide by lithium substitution is particularly interesting. NiO readily reacts with CH4 at temperatures above 873 K, but only CO2 and H2O are formed.i2>2 This is quite different than the case of MgO, which has little or no reactivity toward methane without the addition of alkali metal. Somehow, substitution of Li cations for Ni cations converts nickel oxide from a material which is highly active and selective for complete combustion to one that can be highly selective for hydrocarbon production. 

Kim J-H, Myung S-T, Yoon CS, Oh I-H, Sun Y-K (2004) Effect of Ti substitution of LiNio.sMni 5 xTix04 and their electrochemical properties as lithium insertion material. J Electrochem Soc 15LA1911-A1918... 

A polymer complex containing Ru(bpy)3 pendant groups was obtained by the reaction of a lithium substituted polystyrene with 2,2 -bipyridyl followed by interaction with c/>Ru(bpy)2 [131]. Another example is binding of 4,4 -dicarboxy-2,2 -bipyridyl at a copolymer of />-aminostyrene followed by reaction with c/>Ru(bpy)i (structure 19) [132]. Other copolymers with pendant Ru(bpy)3 bound via a spacer or containing additional bound 4,4 -bipyridyl are also prepared. These materials are interesting as sensitizers for visible light energy conversion. 

As demonstrated by the diversity of phosphate materials, the substitution of various anions further extends the range of possible lithium insertion materials. 

In practice, the defect structure of the materials LiJCo, M)02 and Lix(Ni, M)02 under oxidizing conditions found at cathodes, is complex. For example, oxidation of Fe3+ substituted lithium nickelate, LL(Ni, Fe)02, under cathodic conditions leads to the formation of Fe4+ and Ni4+. Conductivity can then take place by means of rapid charge hopping between Fe3+, Ni3+, Fe4+, and Ni4+, giving average charges of Fe3+S and Ni3+S. These solids are the subject of ongoing research. 

The crystalline material with the highest Li ion conductivity found to date is H-doped lithium nitride (Lapp et al., 1983), Fig. 2.11. It is essentially a vacancy conductor because the substituting hydrogen atoms in the formula Lij. H N are tightly bound as NH groups. These are located in such a way as to leave vacancies in the Li" ion conduction pathway. 

Better results were obtained for the carbamate of 163 (entry 3) [75, 80). Thus, deprotonation of the carbamate 163 with a lithium base, followed by complexation with copper iodide and treatment with one equivalent of an alkyllithium, provided exclusive y-alkylation. Double bond configuration was only partially maintained, however, giving 164 and 165 in a ratio of 89 11. The formation of both alkene isomers is explained in terms of two competing transition states 167 and 168 (Scheme 6.35). Minimization of allylic strain should to some extent favor transition state 167. Employing the enantiomerically enriched carbamate (R)-163 (82% ee) as the starting material, the proposed syn-attack of the organocopper nucleophile could then be as shown. Thus, after substitution and subsequent hydrogenation, R)-2-phenylpentane (169) was obtained in 64% ee [75]. 

The spectra of the doped materials (Cr, Ni, Zn +, Li+, Co +, AP+) are similar to those seen for the nominally stoichiometric materials, and sets of resonances between 500 and 700 ppm are seen on cation doping in addition to that of the normal spinel environment (at ca. 500 ppm). Again, these resonances are assigned to lithium ions near manganese-(IV) cations. The lower intensity of the additional resonances seen on Cr + substitution, in comparison to Zn + or Ni + substitution, is consistent with the oxidation of fewer manganese ions near the depart ions. For the Li- and Zn-doped spinels, resonances at ca. 2300 ppm were also observed, which are assigned to lithium ions in the octahedral sites of the spinel structure. In the case of Zn doping, it is clear that the preference of Zn + for the tetrahedral site of the spinel structure forces the lithium onto the octahedral site. 

Finally, Al (/= 5/2) and Co NMR spectroscopy have been used to probe AP+ in Al-doped lithium cobalt oxides and lithium nickel oxides. A Al chemical shift of 62.5 ppm was observed for the environment Al(OCo)e for an AP+ ion in the transition-metal layers, surrounded by six Co + ions. Somewhat surprisingly, this is in the typical chemical shift range expected for tetrahedral environments (ca. 60—80 ppm), but no evidence for occupancy of the tetrahedral site was obtained from X-ray diffraction and IR studies on the same materials. Substitution of the Co + by AF+ in the first cation coordination shell leads to an additive chemical shift decrease of ca. 7 ppm, and the shift of the environment A1(0A1)6 (20 ppm) seen in spectra of materials with higher A1 content is closer to that expected for octahedral Al. The spectra are consistent with a continuous solid solution involving octahedral sites randomly occupied by Al and Co. It is possible that the unusual Al shifts seen for this compound are related to the Van-Vleck susceptibility of this compound. 

A new development in silsesquioxane ehemistry is the eombination of sil-sesquioxanes with cyclopentadienyl-type ligands. Reeently, several synthetie routes leading to silsesquioxane-tethered fluorene ligands have been developed. The scenario is illustrated in Seheme 47. A straightforward aeeess to the new ligand 140 involves the 1 1 reaction of 2 with 9-triethoxysilylmethylfluorene. Alternatively, the chloromethyl-substituted c/oxo-silsesquioxane derivative 141 can be prepared first and treated subsequently with lithium fluorenide to afford 140. Compound 141 has been used as starting material for the preparation of the trimethylsilyl and tri-methylstannyl derivatives 142 and 143, respeetively, as well as the novel zirconoeene complex 144. When activated with MAO (methylalumoxane), 144 yields an active ethylene polymerization system. 

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