Metal oxides with Lewis acids

Polymerization of ethylene oxide can occur duriag storage, especially at elevated temperatures. Contamination with water, alkahes, acids, amines, metal oxides, or Lewis acids (such as ferric chloride and aluminum chloride) can lead to mnaway polymerization reactions with a potential for failure of the storage vessel. Therefore, prolonged storage at high temperatures or contact with these chemicals must be avoided (9). 

Various uncharged species (including metal oxides and Lewis acids) can be made organically soluble by complexation with some type of catalyst. 

Although quartz tubing is available commercially, its metal oxide content (10 to 100 ppm) is considered to be too great for use in capillary gas chromatography (10). Metal oxides are Lewis acids and can serve as adsorptive sites for electfon-donor species such as ketones and amines and as an active site for species with //-bonding... 

Catalytic activity in olefin polymerization is related to the presence of cationic metal-hydrocarbyl species [90], which can be obtained by (i) using oxide supports that have high Br0nsted and Lewis acidity, (ii) the addition of a co-catalyst to a neutral supported species or (iii) modification of the surface with Lewis acid cocatalysts prior to grafting of the metal-hydrocarbyl species (Scheme 11.8a-c) [91-97]. 

Until recently the most popular method in asymmetric catalysis was the application of metal complexes. This is not surprising, since the use of different metals, ligands and oxidation states makes it possible to tune selectivity and perform asymmetric induction very easily. Thus, the concept of asymmetric catalysis has become almost synonymous with the use of metals coordinated by chiral ligands [1,2]. In many examples the metal is a Lewis acid [3]. 

The spectral behavior of CO bonded to metal atoms (metal carbonyls) has been used to characterize the surface of solids (61). For instance, it is known that metal carbonyl interacts with surface site of metal oxides and zeolites to form a Lewis-type adduct where a CO ligand of the metal carbonyl interacts (via the oxygen atom) with surface OH groups or with co-ordinatively unsaturated metal ions (surface Lewis acid sites) (62,63). On the other hand, thermal treatment of the metal carbonyl support adducts lead to loss of CO with formation of subcarbonyls, which are anchored to the support (64,65). Papile et al. (66) reported the characterization... 

Solid superacids may be made by treating ordinary solid add catalysts with strong Br0nsted or Lewis acids. For example, if freshly precipitated titanium hydroxide or zirconium hydroxide is treated with sulfuric acid and calcined in air at 500 °C. a very active solid acid catalyst results. The solids consist mainly of the metal dioxides with sulfate ions coordinated to the metal ions on the surface. Likewise, a superacid solid catalyst can be made by treating these metal oxides with antimony penlafluonde. Both catalysts contain both Br nsted and Lewis acid sites, and they arc sufficiently active to catalyze the isomerization of n-butane at room temperature.26... 

Oxidation states of palladium-loaded Y zeolites were measured by ESR and IR spectrometry. After treatment by oxygen at 500°C the Pd is almost in the Pd(II) form, and few Pd (1%) are found in the Pd(III) form. After reduction by hydrogen at room temperature the Pd at zero oxidation state is almost atomically dispersed. The electron density of the Pd(0) is low because of its strong interaction with Lewis acid sites of the zeolite network it could even form Pd(I) (8%) (detected by ESR). This species is easily reoxidizable to Pd(II) by treatment in oxygen at 800°C. For reduction temperatures above 250°C, crystallites of metallic palladium are dispersed on the surface. 

Enhanced acidity solids including Brpnsted and Lewis acid-modified metal oxides and mixed oxides, as well as metal salts complexed with Lewis acids. 

It was recognized at an early stage in the development of silicon-transition-metal chemistry that silicon-oxygen compounds often appeared, either as by-products from preparations or as decomposition products on heating or even on storage. At first, adventitious hydrolysis or oxidation was blamed, but it soon became clear that attack on silicon by oxygen of coordinated carbonyl groups was responsible. Since metal carbonyls are known to form adducts with Lewis acids such as compound (XXXIII) (286),... 

In contrast to acidic surface metal oxides with cation oxidation states of +5 to +7, which are anchored to the support by surface hydroxyl groups, basic surface metal oxides with cation oxidation states of +1 to +3 are anchored at surface Lewis acid sites (Bredow et al., 1998 Cortez et al., 2003 Diebold, 2003 Jehng and Wachs, 1992 Vuurman et al., 1996). Raman spectra demonstrated that supported basic metal oxides are, in contrast to acidic supported metal oxides, insensitive to moisture. The Raman spectra of basic surface metal oxide species do not show the bands at about 1000 cm 1 that would indicate terminal M = 0 bonds. The spectra typically exhibit Raman bands in the wave number region of 500-700 cm-1, characteristic of M—O bonds (Chan and Wachs, 1987 Tian et al., 2006 Vuurman et al., 1996) similar behavior was observed for TiO, ZrOx, Pt02, and other oxide surface species with cations in the +4 oxidation state. 

Because of its large size, the electron cloud of FT is easily polarizable, and therefore, H is a soft base. Consequently, complexes with transition metals are usually formed in which the metals are soft Lewis acids. The metals are usually in low oxidation states, which makes them softer acids than the same metals in higher oxidation states. Typical among these compounds are Fe(CO)4H2, Re(CO)(P(C6H5)3)3H, and Mn(CO)5H. 

CO2 is a poor donor but a good electron acceptor. Owing to its acidic character, it is frequently used to probe the basic properties of solid surfaces. IR evidence concerning the formation of carbonate-like species of different configurations has been reported for metal oxides [31], which accounts for the heterogeneity of the surface revealed by micro-calorimetric measurements. The possibility that CO2 could behave as a base and interact with Lewis acid sites should also be considered. However, these sites would have to be very strong Lewis acid sites and this particular adsorption mode of the CO2 molecule should be very weak and can usually be neglected [32]. 

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