Bronsted acidity, origin

Bronsted acid sites in HY-zeolites mainly originate from protons that neutralize the alumina tetrahedra. When HY-zeolite (X- and Y-zeolites... 

The reachon of benzene with ethylene or propylene to form ethylbenzene or isopropylbenzene (cumene) is an industrially important transformahon, with ethylbenzene as the key building block for polystyrene and cumene as the feedstock for phenol produchon [55]. Fthylbenzene was originally produced with a Lewis acid catalyst consishng of AlCfi or a Bronsted acidic solid phosphoric acid (SPA) catalyst [56]. Both catalyst systems suffered from equipment corrosion so, in the 1980s the Mobil-Badger vapor phase alkylation process was introduced, which... 

As shown in Table 1, the amount of irreversibly adsorbed pyridine dropped to one third its original value after modification, which is caused by the methylation of surface Bronsted acidic sites through equation (1). The results were confirmed by the presence of surface methoxyl groups and the absence of Bpy peaks of adsorbed pyridine in i.r. spectra. The TPD of ammonia in Figure 3 indicates that the modification influenced mainly the number of surface acidic sites. The results in Table 1 aslo show that the drop in acidity paralleled that... 

Enthalpies of formation of the magnesium bromide salts of the Bronsted acids are calculated from the measured enthalpies of the reaction of the Bronsted acid, HB, with pentylmagnesium bromide and the known enthalpies of formation of pentyhnagnesium bromide and pentane according to equation 11. Because some of the HB enthalpies of formation have been revised and others newly measured since the original publication, the BMgBr enthalpies of formation have been recalculated and appear in Table 4. ... 

The hydrothermal stability of the SA1 sample has been compared with that of the original SMg in Figure 6. The X-Ray diffraction pattern of an A1 sepiolite before (a) and after (b) steaming at 700aC, 100% steam and a natural sepiolite (SMg) also steamed at 7000C (c). The results indicate that after this thermal treatment an important fraction of the aluminic sepiolite remains unalterated, while the natural sample is completely folded. The folding of the sepiolite is shown by the disappearance of the (1,10) reflexion at 7.2, 20, and the appearance of a new reflexion at 8.9, 20 (16). Moreover, the adsorption of pyridine shows the presence of a residual Bronsted acidity in the SA1 after the hydrothermal treatment which is approximately 30% of the untreated sample. 

The nickel and palladium compounds described above are useful in processes for polymerising various olefins, and optionally also for copolymerising olefinic esters, carboxylic acids or other functional olefins with these olefins. When (I) is used as a catalyst, a neutral Lewis acid or a cationic Lewis or Bronsted acid whose counterion is a weakly coordinating anion is also present as part of the catalyst system. The neutral Lewis acid is originally uncharged (i.e. not ionic). Suitable neutral Lewis acids include SbFs, A B and BF3. By a cationic Lewis acid is meant a cation with a positive charge such as Ag+, H+ and Na+. 

Desorption of water often converts Bronsted to Lewis acids, and readsorption of water can restore Bronsted acidity. Probe molecules, such as ammonia, pyridine, etc., are used to evaluate Bronsted and Lewis acidity. These compounds may contain water as an impurity, however. Water produced by reduction of metal oxides can also be readsorbed on acid sites. Probe molecules can in some cases react on surface acid sites, giving misleading information on the nature of the original site. Acidity, and accessibility, of hydroxyl groups or adsorbed water on zeolites and acidic oxides can vary widely. Study of adsorbed nitrogen bases is very useful in characterization of surface acid sites, but potential problems in the use of these probes should be kept in mind. 

Although no oligomerization was seen on the original Bronsted acidic ZSM-5, and oligomerization activity increased as the catalyst was progressively dried, subsequent readdition of water, which entirely removed the sites which held C02, did not eliminate, but instead somewhat increased this activity. 

Montmorillonites are more frequently used as Bronsted acid catalysts even if Lewis acidity plays a role in their catalytic activity. The origin of Bronsted acidity in metal-exchanged montmorillonites is ascribed to the polarizing influence of the cation on the water molecules in spatially restricted interlayers. The exchangeable cations are either protons or polarizing cations [e.g., aluminum, chromium(III), or iron(III)]. 

The question as to whether the acid properties of cracking catalysts are due to protons (Bronsted acids) or to electron-deficient atoms (Lewis acids) is somewhat more difficult to answer. Before considering this question, the origin of acid centers in silica-alumina compositions should be discussed. It is generally believed that acid centers, either the... 

Bronsted s original correlation was for proton-transfer reactions but many workers now use the ionisation of an acid as a general measure of polarity for reactions involving substituent groupings similar to the model acid. For example nucleophilic substitution by aryloxide ions into 2,4-dinitrochlorobenze (Eqn. 14) [9] may be... 

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