Brdnsted and Lewis Acids

The most common catalysts used in plastic cracking are acidic solids, mainly alumina, amorphous silica-alumina and zeolites. These materials are the catalysts typically used in the petroleum processing and petrochemical industries. They have very different textural and acid properties, which directly determine their catalytic activity and product selectivity. Thus, while the acidity of alumina is of Lewis type, both Brdnsted and Lewis acid sites may be present in amorphous silica-alumina and zeolites. This is an important factor because... 

When aromatic pinacols are reacted with an acid, products often arise from dehydration and rearrangement.5 This general conversion is known as the pinacol rearrangement. The pinacol rearrangement may be promoted by both Brdnsted and Lewis acids.6 In the procedure described here, superacidic triflic acid is reacted with an aryl pinacol and a dehydrative cyclization occurs to give the substituted phenanthrene product. Related to this conversion, the chemistry of benzopinacol in sulfuric acid and triflic acid is contrasted in Scheme 1. We have proposed that the superacidic triflic acid causes the formation of diprotonated intermediates which promote the dehydrative cyclization.4... 

Of course, this is a vastly oversimplified picture. Depending upon the extent and nature of cation exchange, and the relative order and extents of steaming and chemical treatments, the Brdnsted and Lewis acidity will change. 

The thermal stabilities of their corresponding pillar intmlayered montmorillonite clay minerals (PILCs) were characterized through the use of powder X-ray diffraction, differential thermal analyses and surface area measurements. These relative stabilities were also found to increase in the order Gaia-PILC < Alia-PILC < GaAl -PlLC. The relative Brdnsted and Lewis acidities of the montmorillonite itself, and of the clay mineral pillared with the different polyoxocations have been examined through infrared pyridine sorption studies. 

Both Brdnsted and Lewis acid catalysis have been observed and probably coordination to oxygen rather than nitrogen puts a positive charge on the middle nitrogen of the triad to give a labile species similar to that in the Schmidt reaction. 

Oiatterjee A (2006) A reactivity index study to rationalize the effect of dopants on Brdnsted and Lewis acidity occurring in MeAlPOs. J Mol Graph Model 24 262-270... 

IR pyridine adsorption studies show that the AL-PILC sample Vc calcined at 680°C (Vc-680) has a strong Lewis acidity only (Fig.4). On this sample steamed at 550°C (Fig.5) a strong Bronsted acidity is observed still detectable after evacuation at 480°C. Steaming at 650°C reduces the strength of both Brdnsted and Lewis acid sites,no more detectable after degassing at 300°C. 

Finally in Chapters 11-13, some of the more recent discoveries that have led to a renaissance in the field of organocatalysis are described. Included in this section are the development of chiral Brdnsted acids and Lewis acidic metals bearing the conjugate base of the Bronsted acids as the ligands and the chiral bifunctional acid-base catalysts. 

Flo. 8. Schematic representation of the deamination and dehydration of a zeolite with the production of Brdnsted acid (A), Bronsted base (B), and Lewis acid (C),... 

Upon adsorption of pyridine IR bands at 1540 and 1450 cm" appear characteristic for Bronsted and Lewis acid sites, respectively. The band at 1490 cm is due to the combination vibration of both types of acid sites. On pyridine adsorption the OH band typical of Bronsted acidic hydroxyl groups disappeared while those of the terminal SiOH groups only decreased in intensity (see Fig. 2). These observations reflect the different acidity of these OH groups. The parent Na-ZSM-5 sample possessed no Brdnsted acidity detectable by this method. 

These findings allowed him to ascribe the observed catalytic activity differences of many catalysts to the different acid site concentrations, not to the presence of the active centres of different acid strength. The relative concentration of Brdnsted and Lewis sites, which are able to initiate cumene cracking, present on different catalysts are shown in Table 5. 

Fig. 19 Pathway for the conversion of glucose to furyl glycolic acid (FA), via enzymatic cortalcerone synthesis, Brdnsted acid, and Lewis acid catalysis. Based on [161]...

Isopropanol was used as a probe molecule to characterize the acidity of heteropolyacid compounds because the product s distribution upon reaction depends on the nature of the surface active sites. Strong Brdnsted (H ) and Lewis acid sites catalyze the dehydration of isopropanol to propylene (di-isopropyl ether over weak Lewis acid sites), and redox/basic sites lead to the dehydrogenation of the alcohol to acetone. 

The acid properties of A1-, Ga-, and Fe-substituted MCM-41-type mesoporous silicates have been probed using ammonia adsorption at 423 K [285]. Substitution led to the formation of Bronsted and Lewis acid sites of different type and strength. Initial heats of ammonia chemisorption decreased in the same order (185 (Al) > 162 (Ga) > 144 (Fe) kJ mol ) as the degree of isomorphous substitution of framework silicon by trivalent atoms. The ratio of strong to medium-strong Brdnsted sites fell in the same order. 

The surfaces of most oxides and minerals have two different kinds of acid sites, Brdnsted and Lewis, on which hydrogen-bonding polymers can adsorb. Bronsted acids are defined as proton donors, such as the M—OH sites on oxide surfaces. The more electron withdrawing the underlying substrate, the greater is the Bronsted acidity. Lewis acid sites are defined as electron deficient or as having the ability to accept electrons. Examples of Lewis acid sites include M (OH)2 groups on oxide surfaces. 

In general, two classes of acids have to be discussed Lewis and Brdnsted acids. In macrocycles, only a few Lewis acid centres have been incorporated, e.g. tin (Newcomb et al., 1987 Newcomb and Blanda, 1988 Blanda and Newcomb, 1989 Blanda et al., 1989) and boron (Reetz et al., 1991). Here we will discuss Brpnsted acids, starting with carboxylic acids. 

Boron heterocycles bearing a hydroxy group on the boron atom are acidic. Boric, boronic and borinic acids are Lewis acids towards hydroxide ion. It was suggested that six-membered 5-hydroxy compounds behaved as Brdnsted acids. The experimental evidence for this was the similarity between the UV spectra of the hydroxy compounds in neutral and basic solution. The rationale for this fact was the maintained aromaticity which was supposed to favour the formation of, for example, ion (163) over ion (164). This suggestion was at first supported by nB NMR spectroscopy, which differentiates between anions of Lewis-acidic... 

There are thus two classes of acids on surfaces of metal oxides Lewis acids and Brdnsted acids (which are also termed proton acids). The weight of evidence (1-8) shows that strong Brpnsted acids are the primary seat of catalytic activity for skeletal transformations of hydrocarbons. In the solids under review, they consist of protons associated with surface anions. 

Goble and Lawrence attributed the high isomerization activity of chlorinated platinum-alumina catalyst to the creation of a localized dual site comprising a Lewis acid site and an adjacent platinum site. However, as has since been pointed out by Asselin et al. (88), carbonium ion intermediates over low-temperature isomerization catalysts are probably created by the same process as that observed for Friedel-CrEifts catalyst abstraction of hydride ion from the paraffin by a strong Brdnsted acid according to the equation... 

The spectra of adsorbed pyridine for MoCo-153 and MoNi-153 are compared in Figure 9. Two final calcination temperatures have been applied, 480 and 650°C. The spectra of the 480°C samples (Figure 9a and 9b) are nearly identical. The BrtSnsted acid bands are weak, while the 1612 cm l Lewis bands are strong. The intensity of the Brdnsted acid bands increases for both 650°C calcined samples (Figure 9c and 9d). The Lewis acid bands show a marked difference now. The 1612 band remains high in intensity for the MoCo-153 catalyst, but this Lewis band decreases appreciably in intensity for the MoNi-153 catalyst. 

The molybdate surface layer in the molybdenum-alumina samples is characterized by the presence of BrGnsted acid sites ( 1545 cm- ) and one type of strong Lewis acid sites (1622 cm l). Cobalt or nickel ions are brought on this surface on impregnation of the promotor. The absence of BrtSnsted acid sites is observed for both cobalt and nickel impregnated catalysts, calcined at the lower temperatures (400-500°C). Also a second Lewis band is observed at 1612 cnrl.The reflection spectra of these catalysts indicate that no cobalt or nickel aluminate phase has been formed at these temperatures. This indicates that the cobalt and nickel ions are still present on the catalyst surface and neutralize the Brdnsted acid sites of the molybdate layer. These configurations will be called "cobalt molybdate" and "nickel molybdate" and are shown schematically in Figure 11a. 

Phenol-ketone novolacs 1487, 1488 Phenol-nitrile complexes 377 Phenol radical cations 1101 fragmentation of 289-291 Phenols—see also Biphenols, Bis-phenols, Hydroxybenzenes, Polyphenols acidities of, gas-phase 310-312 acylation of 629-632, 933, 934 Lewis acid catalyzed 631 montmoriUonite-catalyzed 632 pyridine-catalyzed 631 adsorption of 944 alkylation of 606-629, 941 Brdnsted acid catalyzed 612 Lewis acid catalyzed 607-611 solid acid catalyzed 612-621 stereoselective 621-626 under supercritical conditions 621 as antioxidants 139-143, 840-901 ort/io-substituted 845 thermochemistry of 139, 140, 179 autoxidation of 1118, 1119 bromination of 649-651 jr-cation interaction of 322 chlorination of 649 comparison with isoelectronic methyl, amino and fluoro aromatic derivatives 226... 

From Fig. 1 to Fig.3, one can attribute the total acidity increase of alumina-bound ZSM-5 mainly to the increase in Lewis acidity and that of alumina-bound Y to the increase in both Lewis and Brdnsted acidity. 

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