Basic sites of alumina

Influence of the Acid Function.- It has been shown that most of the coke deposition on the bifunctional naphtha reforming catalyst occurs on the acid function. Barbier et al. showed the importance of the acid sites of the support by performing TPO studies of coke from cyclopentane reaction on three catalysts Pt(0.59)/AI2O3 and the same catalyst modified by the addition of H BO and by the addition of KOH. The neutralization of basic sites of alumina by H3BO3 modified the quantity and localization of coke very little, whereas the neutralization of acid sites by KOH produced a decrease of 90% of the quantity of coke deposited on the support. Thus it is apparent that the polymerization leading to coke is essentially of an acid nature. A similar effect was observed in the coking produced... 

The trans elimination can take place if the basic sites of the alumina attack the hydrogen from one side of the plane and the hydroxyl group is removed from the opposite side of the plane by the acidic sites of the alumina. This may be possible if the reaction occurs within the pores of molecular dimensions (46) or within the crevices of the aluminas. Crevice sites on silica-alumina catalyst have been proposed by Burwell and co-workers (57) on the basis of racemization and exchange reactions of hydrocarbons. 

The basic and acid sites on alumina surfaces have been represented graphically (43, 72). In order for the acid and the basic sites of the alumina to attack tra s-l,4-cyclohexanediol from different planes of the catalyst surface it is necessary for the dehydration to be restricted to submicroscopical holes or crevices or to occur between channels of those particles. Since the basic and acid sites of the alumina have to surround the cyclohexanediol, as in the solvolytic reaction, the alumina therefore can be considered as a pseudosolvent for such dehydration reactions. 

DFT calculations were performed on the interaction of [Mo(CO)6] with acidic and basic surface sites of alumina, by using tetrahedral and octahedral clusters as a model [20]. Two kinds of interactions were detected a weak one between an AP acidic Lewis site and the oxygen atom of a CO ligand, and a true chemical bond between the oxygen atom of an AlO basic site and the metal centre. 

The Peri model of alumina also demonstrates that basic sites of various strength, consisting of oxygen atoms in various arrangements (isolated... 

The adsorption of sulfur dioxide of the catalysts rapidly decreased with the vanadium content of the samples. As sulfur dioxide is not chemisorbed on bulk vanadium pentoxide and also appears not to chemisorb on vanadate species, on V20s/Y-Al203 catalysts this probe can be considered to be selective for the titration of the basic sites of vanadium-free alumina. Thus, by selectively probing uncovered alumina, SO2 further allows the distinction between the acid-base features of the vanadium oxide layer and those of uncovered alumina. 

FIGURE 7.10 Surface concentrations and their intensity distributions for (A) the acidic and (B) the basic sites of adsorbents used in frying oil. A, activated carbon B, alumina (basic) C, alumina (neutral) D, alumina (acidic) E, bleaching earth F, diatomaceous earth G, silica and H, magnesium silicate. 

Ring openings of epoxides with various nucleophiles are catalyzed by acid or base and are accompanied by configurational inversion on the substituted carbon (23). Posner found that y-alumina facilitated nucleophilic ring openings of epoxides with amines, alcohols, and carboxylates to give P-functionalized alcohols stereospecihcally (trans) in good yields under mild reaction conditions (24). This catalytic behavior of alumina was assumed to be due to the cooperative function of acidic and basic sites on alumina. 

To further study the basicity, benzoic acid measurements were performed on the four alumina samples. The four alumina samples showed colour change with the addition of only two indicators namely phenolphthalein (pKa = 9.3) and 4-chloro2-nitroaniline (pKa=17.2). The amount of benzoic acid adsorbed is given in Table 5. Addition of alkali/acid to AD-101 which has originally a strong basic site redistributes the acid and basic sites on alumina. 

A facile explanation of the failure of acidity to enhance activity of any of the catalysts while base enhances the activity of a poor catalyst without affecting the active ones is that the Claus active catalysts owe their activity to basic sites. In general, alumina shows strong surface acidity when activated at 450 °C in vacuum, and intensive investigation has shown this to happen in Lewis acid sites (8). There is also evidence that basic sites occur on alumina. Schwab and Krai (9) obtained evidence for basic sites by boron trifluoride adsorption on alumina, and Yamadaya et al. (10) reported up to 0.4 mmole/g of basic sites on alumina from benzoic acid titrations. Pines and Manassen (11) obtained indirect evidence of such sites from studies of the dehydration of primary alcohols. 

The nature of acidic and basic sites on alumina can be represented as follows (14) ... 

Both models describe the transient stop-effect behaviour and can be distinguished through periodic experimentation. The former is very similar to the reaction mechanism claimed by Koubek et al.[l], who associates S with an acid site on which the alcohol or the amine is strongly adsorbed and S2 with a basic site on which the reactant is more weekly adsorbed. Koubek takes advantage on the stop-effect to measure the acidic and basic character of aluminas. 

The surface of alumina in aqueous environment is well characterized [7-10). It has amphoteric sites, that is, acidic sites of aluminum and basic sites of hy-... 

To explain the predominance of anti stereospecificity in dehydration over alumina, participation by both acidic and basic sites of the catalyst is invoked. Furthermore, as the alumina must surround the substrate, it has been regarded as a solvating agent and an analogy to solvolytic elimination has been drawn ... 

Hoekman et al. [40] studied CO2 methanation reaction over Haldor Topspe commercially available methanation catalysts consisting of Ni and NiO on an alumina substrate with total nickel loading of 20-25% and an operating temperature range of 190-450 C in an extruded ring-shaped catalyst. Approximately 60% conversion of CO2 was observed at r= 300-350°C and stoichiometric CO2/H2 ratio. Aldana et al. [41] found that Ni over ceria-zirconia (prepared by sol-gel synthesis) shows an initial COj activity of almost 80%, with a CH4 selectivity of 97.3%, decreasing down to 84.7% after 90 hours of reaction. By IR operando analysis, they found that for Ni-ceria-zirconia catalysts the main mechanism for CO2 methanation does not require CO as reaction intermediate and the mechanism is based on CO2 adsorption on weak basic sites of the support. 

On alumina the metal ac l acetonates are partly decomposed by catalytic action of the acidic and basic sites of the support surface. The sui e reaction involves the split of acetyl acetonate ligands which may adsorb on Al -sites in thdr enolate form (see Figure 4), and possibfy the further convosion to acetone and acetic add strongly adsorbed on Al >sites. In consecutive reaction stq>s carbon oxides originating from decompodtion form surface carbonates. V(acac)3 adsorbed on alumina is, in contrast to Pt(acac)2, and Cr(acac)3, nearly wholly decompo to acetyl acetone and consecutive products this is attributed to the low stability of this metal ace acetonate. 

For adsorption, active aluminas can be considered as possessing both Lewis and Brensted acidic and basic sites of various strengths and concentrations. Acidity is contributed by lattice AF+ ions, protonated hydroxyls, and some acidic hydroxyls. Basicity is a result of O - anion vacancies and basic hydroxyls. A useful schematic representing the various surface functionalities is shown in Figure 3 [15]. 

Various hydrated aluminum hydroxides serve as starting material for TLC alumina. By a series of nonuniform thermal dehydration processes, a variety of aluminas are obtained, and the ones most suitable for TLC are the crystalline modifications of x-ALOs and y-ALO (Rossler, 1969 Snyder, 1975). The physicochemical properties or the exact nature of adsorption sites of alumina are not well understood. Snyder (1975) has suggested that exposed A1 atoms, strained Al-O bonds, and perhaps other cationic sites serve as adsorption sites, whereas, unlike silica, surface hydroxyl groups are probably not important. Acids are probably retained by interaction with basic sites such as surface oxide ions. Gasparic and Churacek (1978) report that every A1 atom is surrounded by six atoms of... 

Basic refractory materials include lime, magnesia, various materials composed chiefly of alumina (bauxite, diaspore, laterite, gibb-site, etc.), dolomite and most of the rarer refractory oxides, particularly zirconia. 

Many solids have foreign atoms or molecular groupings on their surfaces that are so tightly held that they do not really enter into adsorption-desorption equilibrium and so can be regarded as part of the surface structure. The partial surface oxidation of carbon blacks has been mentioned as having an important influence on their adsorptive behavior (Section X-3A) depending on conditions, the oxidized surface may be acidic or basic (see Ref. 61), and the surface pattern of the carbon rings may be affected [62]. As one other example, the chemical nature of the acidic sites of silica-alumina catalysts has been a subject of much discussion. The main question has been whether the sites represented Brpnsted (proton donor) or Lewis (electron-acceptor) acids. Hall... 

Due to the formation of Ca/Al mixed oxide on the surface, the Ca -modified alumina has a completely different structure compared to the spinel one This leads to a different type of surface Lewis acid/basic sites, rendering the catalyst 30 times less active. 

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. 

The transformation of a cyclopentanol/cyclohexanone mixture allows us to estimate simultaneously the acidity and the basicity of catalysts. Two reactions take place the hydrogen transfer (HT) on basic sites and the alcohol dehydration (DEH) on acid sites. This reaction was carried out at two temperatures over four aluminas. Theta alumina seems to be the most basic of the aluminas tested. Correlation between model reaction and IR study were also discussed. 

In catalyst preparation, one can use this knowledge to determine the relative contributions of various hydroxyl groups before and after application of the active phase onto the support. In this way Sibeijn etal. [31] established that rhenium oxide attached to acidic sites of the alumina support exhibits higher activity for the metathesis of olefins than rhenium oxide on neutral or basic sites. As, however, rhenium species preferentially exchange with basic hydroxyls, one needs to increase the loading above a certain value (6 wt% for an alumina of 200 m2/gram) before the catalyst exhibits appreciable activity [31]. 

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