Lewis alumina supported

Chiral Lewis acids supported on silica gel and alumina, and their use as catalysts in Diels-Alder reactions of methacrolein and bromoacrolein [103]... 

Also, manganese added to cobalt on activated carbon catalysts resulted in a decrease in bulk carbide formation during reduction and a decrease in the subsequent deactivation rate.84 Magnesium and yttrium added to the support in alumina-supported cobalt catalysts showed a lower extent of carburization. This was explained by a decrease in Lewis acidity of the alumina surface in the presence of these ions.87... 

Furthermore, the removal of these groups by thermal treatment (dehydroxyl-ation) yields coordinatively unsaturated oxygens and anions in which coordina-tively unsaturated aluminum atoms are exposed (Lewis acid sites). In general, the total concentration of OH groups on a alumina support ranges between 10 and... 

Although the mechanism of the platinum catalysis is by no means completely understood, chemists do know a lot about how it works. It is an example of a dual catalyst platinum metal on an alumina support. Platinum, a transition metal, is one of many metals known for its hydrogenation and dehydrogenation catalytic effects. Recently bimetallic platinum/rhenium catalysts are now the industry standard because they are more stable and have higher activity than platinum alone. Alumina is a good Lewis acid and as such easily isomerizes one carbocation to another through methyl shifts. 

The surface structure and acid sites of alumina-supported molybdenum nitride catalysts have been studied using temperature-programed desorption (TPD), and reduction (TPR), diffuse reflectance infrared spectroscopy, and X-ray diffraction (XRD) analysis. The nitride catalysts were prepared by the temperature-programmed reaction of alumina-supported molybdenum oxide (12.5% and 97.1%) with NH3 at temperatures of 773, 973, and 1173 K. TPR and XRD analyses showed that y-Mo2N was already formed at 973 K. On the basis of NH3-TPD measurements and IR spectroscopy, it was found that Lewis acid sites were predominant over Bronsted acid sites on the surface of Mo2N/A1203. 

However, the molybdenum-alumina and the high calcined cobalt-molybdenum-alumina samples still show an important difference. The pyridine spectra of MoCo-124 indicate a second Lewis acid site, characterized by the 1612 cm-1 band. This band differs from the weak Lewis acid sites of the alumina support (1614 cm- ) because the position is significantly different. It also appears that the strength of the bond between pyridine and the catalyst is stronger, for the 1612 cm-1 band is still present after evacuation at 250°C, while the weak Lewis band (1614 cm-1) of the alumina has disappeared at this desorption temperature. Obviously the second Lewis band for the MoCo-124 catalyst is introduced by the interaction of cobalt with the surface molybdate layer. This interaction is... 

The first successful examples of enantioselective Diels-Alder reactions catalyzed by chirally modified Lewis acids were reported by Koga [85]. The catalysts were prepared from menthol and AlEt2Cl [86]. Alumina-supported chiral menthoxy aluminum derivatives (64, 65, 66, 67) have been prepared by simple mixing of (-)-menthol, AlEt2Cl, and alumina in toluene under reflux. The reaction of methacrolein with cyclopentadiene (Eq. 20) was conducted with 67 as catalyst at -50 °C and afforded 81 % conversion with 31 % ee [87] Koga reported 57 % ee at -78 °C by use of an homogeneous catalyst [85]. Solid catalyst 69, prepared from silica gel-supported proli-nol 68 and AlEt2Cl (Eq. 21) is also an active catalyst in the same reaction, but with low enantioselectivity [87]. When the same catalyst was attached to crosslinked polystyrene (70) the ee in the reaction was lower [88]. 

Figure 12.21 shows pyridine adsorption at 25°C on an alumina pre-treated at 450 C. Only Lewis sites are visible because the Bronsted sites are too weak to be shown by pyridine. The existence of two Lewis sites L] and L2 should be noted (bands at 1 622 cm and 1617 cm ) corresponding respectively to aluminium atoms in tetrahedral and octahedral coordination sites. Lutidine adsorption on the same alumina support is used to show the Bronsted sites (Fig. 12.22 and Table 12.4). 

Another ion exchange procedure involves the interaction of a metal acetylacetonate (acac) with an oxide support. Virtually all acetylacetonate complexes, except those of rhodium and ruthenium, react with the coordinatively unsaturated surface sites of 7 alumina to produce stable catalyst precursors. On thermal treatment and reduction these give alumina supported metal catalysts having relatively high dispersions. 38 Acetylacetonate complexes which are stable in the presence of acid or base such as Pd(acac)2, Pt(acac)2 and Co(acac)3, react only with the Lewis acid, Al" 3 sites, on the alumina. Complexes which decompose in base but not in acid react not only with the Al 3 sites but also with the surface hydroxy groups. Complexes that are sensitive to acid but not to base react only slightly, if at all, with the hydroxy groups on the surface. It appears that this is the reason the rhodium and ruthenium complexes fail to adsorb on an alumina surface. 38... 

Phosphorous is generally used to enhance HDN activity by modifying Lewis acidic sites of alumina support. 

Infrared spectrum of pyridine adsorbed on undoped alumina-supported vanadium oxide catalyst, after evacuation at 150 °C, shows an absorption band at 1450 cm 1, characteristic for pyridine retained on Lewis acid sites, which has been related to V-free alumina [4,10, 11]. The intensities of the bands at 1450 cm l (related to Lewis acid sites) and 1545 cm I (related to Bronsted acid sites) have been used to determine the numer of Lewis and Bronsted acid sites on the surface of catalysts. The results are outlined in Table 1. 

The intensity of the band at 1450 crn l observed in the spectrum of the undoped alumina-supported vanadia catalyst decreases upon the addition of bismuth, potassium, phosphorous or molybdenum. These results indicate a reduction of the number of Lewis acid sites after the incorporation of metal oxides. In the case of K-doped catalysts the low intensity of the band at 1450 cm 1 clearly demonstrates that the majority of the surface acid sites disappears with the incorporation of potassium. Thus, Lewis acid sites have completely disappeared in the K(0.7) catalyst. 

In conclusion, this paper shows the effect of the addition of different metal oxides (K, Bi, P and Mo) on the catalytic behavior of an alumina-supported vanadia catalysts in the ODH of propane. In all cases, the addition of small amounts of metal oxide (MeA/ atomic ratio of 0.1) increases the selectivity to propylene, probably as a consequence of the elimination of non selective sites (Lewis acid sites) on the surface of the support. However, only in the case of K-doped catalysts the selectivity and the yield of propylene increases with the metal content. The varition of the acid-base character of catalysts and its influence on the adsorption/desorption of reactants and products could be responsible of the different performances obsen/ed. In this way. 

Figure 2. Structures of dienophiles investigated by Mayoral et al. in Diels-Alder reactions with cyclopentadiene catalyzed by Lewis acids supported on alumina or silica. Reference numbers are given in square parentheses.

Investigating the role of the surface structures (hydroxyl groups, Lewis acid sites, basic sites, and combinations thereof) and their location (faces, edges, comers, defects) in determining the dispersion and the structure of the supported species in alumina-supported catalysts. Also the nature of the anchoring bonds between surface species (e.g., metal particles) and the support should be clarified. 

In a novel approach to activate the metallocene compound with a strong Lewis acid supported on a solid support such as silica or alumina, scientists at Chevron-Phillips Chemical Company reported a series of high-activity catalysts suitable for the slurry process [5,7]. This method does not employ an alumoxane compound to activate the catalyst. 

The alumina support was also treated with a variety of Lewis acids, but the most active supports were treated with chlorinating agents such as CCl or TiCl, which were added to the alumina in a fluidized column maintained at 400-600 C. Alumina was treated with an aqueous solution of ammonium sulfate, (NHJ SO, or H SO,. 

Mote stable catalysts ate obtained by using fluorinated graphite or fluorinated alumina as backbones, and Lewis acid halides, such as SbF, TaF, and NbF, which have a relatively low vapor pressure. These Lewis acids ate attached to the fluorinated soHd supports through fluorine bridging. They show high reactivity in Friedel-Crafts type reactions including the isomerization of straight-chain alkanes such as / -hexane. 

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

Several aluminum- and titanium-based compounds have been supported on silica and alumina [53]. Although silica and alumina themselves catalyze cycloaddition reactions, their catalytic activity is greatly increased when they complex a Lewis acid. Some of these catalysts are among the most active described to date for heterogeneous catalysis of the Diels-Alder reactions of carbonyl-containing dienophiles. The Si02-Et2AlCl catalyst is the most efficient and can be... 

While our discussion will mainly focus on sifica, other oxide materials can also be used, and they need to be characterized with the same rigorous approach. For example, in the case of meso- and microporous materials such as zeolites, SBA-15, or MCM materials, the pore size, pore distribution, surface composition, and the inner and outer surface areas need to be measured since they can affect the grafting step (and the chemistry thereafter) [5-7]. Some oxides such as alumina or silica-alumina contain Lewis acid centres/sites, which can also participate in the reactivity of the support and the grafted species. These sites need to be characterized and quantified this is typically carried out by using molecular probes (Lewis bases) such as pyridine [8,9],... 

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