Oxides modified amorphous

We have explored rare earth oxide-modified amorphous silica-aluminas as "permanent" intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides "titrates" the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions. 

Systematic studies carried out at C.F.R.I. pertainining to catalytic vapour phase synthesis of pyridine bases, namely 2 4-picolines through cyclodehydrogenation reaction of acetaldehyde and anunonia have been described. Metal oxide modified amorphous silica-aliunina and crystalline alumino-silicate namely ZSM-5 zeolites were found to be active and selective catalysts towards the formation of the lower pyridines bases. The method of preparation, pretreatment vis-a-vis the acidity have been found to affect the catalytic activity and selectivity. It is interesting to observe that metal modified crystalline alumino-silicate ZSM-5 is more selective than amorphous silica-alumina for the formation of 2 4-picoline. A plausible reaction mechanism based on the findings of this study has been proposed. 

Modified Amorphous Oxide Surfaces as Metal Cation Sequestrating Agents... 

Modified Amorphous Oxide Surfaces AS Electrochemical Sensors... 

Due to their ability to coordinate metal cations, as well as interact with organic species, the amorphous oxide-modified surfaces can be successfully used to produce electrochemical sensors [10], as will be illustrated by two specific examples. 

The use of alkoxides as precursors to produce new materials [1] has increased in the last decade due to the advantages of the so-called sol-gel process [2] such as the preparation of compounds with high homogeneity, at room temperatures, in contrast with the typical high-temperature syntheses in solid-state chemistry. Such an experimental approach can lead not only to modified amorphous oxide surfaces, as discussed in Chapter 1, but also to nanostructured modified surfaces, as will be shown here. 

In an interesting interplay between GO and organosilanes (which are so extensively used to modify amorphous or nanostructured oxide surfaces), there are reports [33], of the preparation of intercalation compounds in which amino groups are inserted into the interlayer space of GO, using 3-aminopropylethoxysilanes. For such compounds, it was shown that the amino groups of... 

Moon, J., Suvaci, E., Li, T., Sostantino, S.A., Adair, I.H. Phase development of barium titanate from chemically modified amorphous titanium (hydrous) oxide precursor. J. Eur. Ceram. Soc. 22, 809-815 (2002)... 

Lampert and coworkers [36] have used a modified amorphous PEO-LiCFaSOs electrolyte for the realization of WO3 laminated windows using several types of counter-electrodes, such as niobium oxide, nickel oxide and a new class of solid redox polymerization electrodes [63]. These latter electrodes have an advantage over inorganic layers in that they can be tailored to the electrochromic material and ion specifically. Figure 8.18 illustrates the optical transmittance of a EW made of WOa/modified a-PEO/ion storage polymer [63]. 

Cobo, S., Heidkamp, J., Jacques, P. A., Fize, J., Fourmond, V., Guetaz, L., et al. (2012). A Janus cobalt-based catalytic material for electro-splitting of v/ater. Nature Materials, 11, 802—807. Consul, J. M. D., Peralta, C. A., Benvenutti, E. V., Ruiz, J. A. C., Pastore, H. O., Baibich, I. M. (2006). Direct decomposition of nitric oxide on alumina-modified amorphous and mesoporous silica-supported palladium catalysts. Journal of Molecular Catalysis A Chemical, 246, 33—38. 

In both cases, we observe an amorphous pattern no crystallites of rare earth oxide appear even at 25% wt. loading. This indicates that oxide particles remain less than 30A in diameter. The surface area, pore volume and pore size distribution of the starting Si-Al support also change on impregnation. Table 1 lists the values for yttria-modified samples of... 

Komrska Satava (1970) showed that these accounts apply only to the reaction between pure zinc oxide and phosphoric acid. They found that the setting reaction was profoundly modified by the presence of aluminium ions. Crystallite formation was inhibited and the cement set to an amorphous mass. Only later (7 to 14 days) did XRD analysis reveal that the mass had crystallized directly to hopeite. Servais Cartz (1971) and Cartz, Servais Rossi (1972) confirmed the importance of aluminium. In its absence they found that the reaction produced a mass of hopeite crystallites with little mechanical strength. In its presence an amorphous matrix was formed. The amorphous matrix was stable, it did not crystallize in the bulk and hopeite crystals only grew from its surface under moist conditions. Thus, the picture grew of a surface matrix with some tendency for surface crystallization. 

Metal oxides were also chirally modified and few of them showed a significant or at least useful e.s. Thus, while Al203/alkaloid [80] showed no enantiodifferentiation, Zn, Cu, and Cd tartrate salts were quite selective for a carbene addition (45% e.e.) [81] and for the nucleophilic ring opening of epoxides (up to 85% e.e.) [82], Recently, it was claimed that /(-zeolite, partially enriched in the chiral polymorph A, catalyzed the ring opening of an epoxide with low but significant e.s. (5% e.e.) [83], All these catalysts are notyet practically important but rather demonstrate that amorphous metal oxides can be modified successfully. 

In 1990, Choudary [139] reported that titanium-pillared montmorillonites modified with tartrates are very selective solid catalysts for the Sharpless epoxidation, as well as for the oxidation of aromatic sulfides [140], Unfortunately, this research has not been reproduced by other authors. Therefore, a more classical strategy to modify different metal oxides with histidine was used by Moriguchi et al. [141], The catalyst showed a modest e.s. for the solvolysis of activated amino acid esters. Starting from these discoveries, Morihara et al. [142] created in 1993 the so-called molecular footprints on the surface of an Al-doped silica gel using an amino acid derivative as chiral template molecule. After removal of the template, the catalyst showed low but significant e.s. for the hydrolysis of a structurally related anhydride. On the same fines, Cativiela and coworkers [143] treated silica or alumina with diethylaluminum chloride and menthol. The resulting modified material catalyzed Diels-Alder reaction between cyclopentadiene and methacrolein with modest e.s. (30% e.e.). As mentioned in the Introduction, all these catalysts are not yet practically important but rather they demonstrate that amorphous metal oxides can be modified successfully. 

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