Enhanced Acidity Solids

The acidic sites of solid acids may be of either the Brpnsted (proton donor, often OH group) or Lewis type (electron acceptor). Both types have been identified by IR studies of solid surfaces using the pyridine adsorption method. The absorption band at 1460 cm 1 is assigned to pyridine coordinated with the Lewis acid site, and another absorption at 1540 cm 1 is attributed to the pyridinium ion resulting from the protonation of pyridine by the Brpnsted acid sites. Various solids displaying acidic properties, whose acidities can be enhanced to the superacidity range, are listed in Table 2.6. 

The natural clay minerals are hydrous aluminum silicates with iron or magnesium replacing aluminum wholly or in part, and with alkali or alkaline earth metals present as essential constituents in some others. Their acidic properties and natural abundance have favored their use as catalysts for cracking of heavy petroleum fractions. With the exception of zeolites and some specially treated mixed oxides for which superacid properties have been claimed, the acidity as measured by the color changes of absorbed Hammett bases is generally far below the superacidity range. They are inactive for alkane isomerization and cracking below 100 °C and need co-acids to reach superacidity. 

Subsequently, the same authors138 described the preparation of a solid superacid catalyst with acid strength of H0 = —16 with a sulfuric acid-treated zirconium oxide. They exposed Zr(OH)4 to 1A sulfuric acid and calcined it in air at approximately 600°C. The obtained catalyst was able to isomerize (and crack) butane at room temperature. The acidity was examined by the color change method using Hammett indicators added to a powdered sample placed in sulfuryl chloride. The 

Cation exchange resins, polymeric perfluorinated resinsulfonic acids 

The X-ray photoelectron and IR spectra showed that the catalyst possessed bidentate sulfate ion coordinated to the metal. The specific surface areas were much larger than those of the zirconium oxides, which had not undergone the sulfate treatment. The interesting feature of these catalysts is the high temperature at which they are prepared, which means that they maintain their acidity at temperatures as high as 500°C and should thus be easy to regenerate and reuse. 

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

Reaction conditions markedly affect the Diels-Alder reaction.521,522 First observed by Yates and Eaton,526 A1C13 accelerates cycloadditions. Other Lewis acids as well as acidic solids (clays, zeolites) were later shown to exhibit similar effects attributed to the coordination of the catalyst with the dienophile. High pressure521,522 and water521,522,527 were also found to show beneficial effects. Reaction conditions also enhance selectivities of the Diels-Alder reaction (see discussion below). 

Solid superacids can be further divided into various groups depending on the nature of the acid sites. The acidity may be a property of the solid as part of its chemical structure (possessing Lewis or Brpnsted sites the acidity of the latter can be further enhanced by complexing with Lewis acids). Solid superacids can also be obtained by deposition on or intercalation of strong acids into an otherwise inert or low-acidity support. 

Sulfated zirconia is a good example of a structural Lewis acid which has been chemically treated to enhance acidity. It has been extensively studied as a solid acid catalyst for vapour phase reactions and we1112 and others14 have found that a mesoporous version of this material is a particularly effective catalyst for liquid phase Friedel-Crafts alkylation reactions and to a lesser extent Friedel-Crafts benzoylations. The commercial (MEL Chemicals Ltd) material SZ999/1 shows a nitrogen isotherm characteristic of a mesoporous solid (surface area 162 m2g, pore volume 0.22 cm3g )- Whereas microporous and mesoporous materials are capable of rapidly catalysing the alkylation of benzene with various alkenes (Table 1), on reuse only the mesoporous... 

At present, the Z(2-C1) group is predominately used for N -protection of lysine residues in the context of the Boc/Bzl chemistry in solution and on solid supports it is up to 60 times more stable than the related Ai -Z derivatives. Similarly, the Ai -Z(2-Br) derivatives show approximately a 60-fold enhanced acid stability versus the Z protection.b l... 

E25.13 (a) Enhanced acidity. When Ar replaces on lattice site, the charge is balanced by H 0 increasing the acidity of the solid catalyst. 

The addition of further o-methoxy groups to the p-alkoxybenzyl alcohol framework naturally results in linkers with enhanced acid sensitivities. One of the first linkers of this type was 2-(4-hydroxy-3-methoxyphenoxy)acetic acid (11) developed by Sheppard and colleagues for the Fmoc solid phase synthesis of protected peptide acids for fragment condensation [41]. Treatment of peptidyl resins with... 

Skeletal isomerization of n-paraffin is a crucial reaction to enhance the octane number and produce clean fuels. Liquid acids such as HF and H2SO4 catalyze this reaction, but they lead to problems of pollution, corrosion, and toxicity. A solution is thus to replace these catalysts by acidic solids, which are more respectful towards the environment and limit corrosion problems. Among various acidic solids sulfated zirconia attracts the attention, because it exhibits an exceptional acidity and a high catalytic activity in many reactions such as the isomerization of n-paraffin. In 1962 V, C. F. Holm and G. C. Bailey described at first the catalytic properties of sulfated zirconia [1], then in 1979 Hino and... 

In selecting a solid acid to catalyse any given reaction, zeolites are not the only candidates. Many solids fulfil the general requirement for solid acidity and some possess favourable attributes of enhanced acid strength and resistance to deactivation. Some of the more important varieties are discussed below. 

These views naturally spill over into the discussion of solid acids. Acidity of Broensted sites in zeolites can be enhanced by modification of the local chemical environment of the framework and, since much evidence reveals the presence of both Broensted and Lewis sites, it is not unreasonable to suggest that enhanced acidity can arise fiom a synergistic interaction between bofli types of site (Scheme 3)... 

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Another example of reagent-induced asymmetric synthesis is the enantioselective preparation of phosphoramides 6 by addition of dialkylzine reagents to A-diphenylphosphinoylimincs 4 in the presence of the enantiomerically pure 1,2-amino alcohols 5a or 5 b (diethylzinc does not add to A-silyl- or A-phenylimines)12. Phosphoramides 6 (crystalline solids) are obtained in moderate to good yield and good enantioselectivity. The latter can be enhanced by recrystallization. Acidic hydrolysis with dilute 3 M hydrochloric acid/tetrahydrofuran provides the corresponding amines 7 without any racemization. 

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