Acid centres

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. 

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],... 

The main characteristics of FAB-MS are indicated in Table 6.15. FAB ionisation is relatively simple to perform. However, parameter optimisation and data interpretation of the resulting FAB spectra can be complex. Matrix selection for additive analysis is crucial. Solubility of the additives in the matrix is essential for production of viable spectra. FAB/FIB is well suited to organic compounds which exhibit some polarity, and contain either acidic and/or basic functional groups. Compounds with basic groups run well in positive ionisation mode, and those with acidic centres run well in the negative ionisation... 

Abramova, A. V., Slivinsky, Ye. V., Goldfarb, Y.Y., Kitaev, L. Ye., Kubasov A. A., Modeling of Nature and Strength of Acid Centres in Ultrastable Zeolites as a Component of Hydrocracker Catalysts, in Hydrotreatment and Hydrocracking of Oil Fractions. 1999, Elsevier Science B. V New York. pp. 377-380. 

Fejes, P. Kiricsi, I. Hannus, I. Tihanyi, T. Kiss, A., Poisoning of acidic centres in zeolites with sodium azide, Imelik, B. Naccache, C. Taarit, Y. Ben Vedrine, J.C. Coudrier, G Praliaud, H., Eds., Catalysis by Zeolites, Studies in Surface Science and Catalysis 5, Elsevier Sci. Publ. Co. Amsterdam, 1980, pp. 135-140. 

Lewis bound form with a strong band at 1440 cm together with a weak 1490cm band. This implies that the Bronsted acidity is associated with the strongly bound water and as this water is removed the pyridine becomes coordinated to a Lewis bound site either nearby or at the undercoordinated A1 site produced by the removal of surface bound water. This transformation of Bronsted to Lewis acid centres is well established in catalyst chemistry as the sample... 

Pd/Cu-zeolites are also catalysts for the oxidative acetoxylation of propylene to allylacetate [32-39]. The best results are obtained on a catalyst which is pretreated with an alkali solution to neutralize the acidic centres and containing Pd and Cu in an atomic ratio of 1.1 [37]. The alkali treatment suppresses the acid catalyzed addition of acetic acid to propylene, resulting in the formation of isopropyl acetate, which is observed over non-neutralized Na- and H-Y, as well as over unreduced and reduced Pd/Cu-NaY. Experiments with... 

The catalytic activity Z -5 -Jype. zeolitesmodified by polyvalent cations (Ca, Mg, x, In, Dy, So, Ga, A1, Be ), were investigated in reactions of toluene al lation by ethylene and transalkylation of ethylbenzene. The presence in these samples of aprotic acid centres of different strength and absence of prot-ic centres were established by IR spectroscopy technique of adsorbed CO. The strength of aprotic centres was characterized by the heat of CO adsorption and was shown to be a main factor determining the selectivity of catalytic action of the systems studied. 

Both the bis-alkyl derivatives, [(Me3Si)2CH]2M (M = Ge, Sn, Pb) and the metallocenes, (CsH5)2M (M = Sn, Pb), function as two-electron donors towards both transition metal and main group metal Lewis acid centres forming complexes of the types [(Me3Si)2CH]2M —>... 

The experimental evidence for the second hypothesis was the observed increase of the cracking rate of alkanes after addition of small amounts of alkenes to the feed. Both early theories assumed the continuation of the cracking reaction by intermolecular transfer of the charge from the products to fresh starting molecules, that is like an ionic chain mechanism with the catalyst acting only as an initiator. The problem was further clouded by the fact that two types of acid centres exist on the surface, the Br0n-... 

A complete separation of a carbonium ion from the hydride ion is very probably not necessary. It has been shown [73] by MO calculations that any attack by a charged species on an atom bonded to a carbon atom causes activation of the bonds from a /3-carbon atom to the substituents. In this way, the splitting of the Cp—Cy bond can be induced by adsorption of the alkane on a strongly acidic site. The preferential cracking of a saturated hydrocarbon chain in /3-positions to the position where a carbonium ion might be formed was observed early and named the /3-rule by Thomas [2], The question remains open as to which type of acidic centre is able to activate an alkane molecule. The fact that an aluminosilicate catalyst is poisoned for the cracking of alkanes by irreversibly adsorbed ammonia suggests a Lewis site [240], viz. 

It may be concluded from all these results that the presence of acid centres is unavoidable for a catalyst to be active in olefin hydration. The possible role of basic centres is less clear they might participate in a fast step which follows the rate-determining step. 

Specific rate coefficients (related to unit amount of acid centres) were approximately the same for solid catalysts as well as for HC1 [474]. However, when a montmorillonite clay activated by adsorption of protons on its surface was used as the catalyst in ethyl acetate hydrolysis [475], a higher specific rate coefficient (about 1.8 times at 25°C) was found for the reaction catalysed by adsorbed protons than by dissolved acid, this result being explained by the authors by an increase of activation entropy in the former case. 

The yield of the reaction (Table 1) clearly depends or the nature of the solid and on the experimental conditions (temperature, time). Thus, with silica and alumina, amorphous solids with a relatively low acidity, the acetophenone oxime molecule reacts with a very low yield, being the only reaction product the hydrolysis one, acetophenone. With the synthetic mixed oxide silica-alumina, that possesses simultaneously BrOnsted and Lewis acidic centres, the conversion is quantitative, being also the major product the hydrolysis one (3), when the reaction is carried out at 160°C. 

Activity The mechanistic and kinetic study has shown that odCB isomerisation is catalyzed by the acid centres of the solid. Furthermore,... 

Lateral macrobicycles are dissymmetric by design thus, monoelectronic reduction of the Cu(ll) ion bound to the [12]-N2S2 macrocyclic subunit in the bis-Cu(ll) cryptate 45, gives a mixed valence Cu(i)-Cu(ll) complex [4.6]. Macrotricycle 46 forms a dinuclear Cu(ll) cryptate that acts as a dielectronic receptor and exchanges two electrons in a single electrochemical wave [4.7]. Complexes of type 47 combine a redox centre and a Lewis acid centre for the potential activation of a bound substrate [4.8]. 

In Charton s work pKa data for 4-X-substituted-bicyclo[2.2.2.]octane-l-carboxylic acids in 50% w/w EtOH-H20 are the basis for primary molecular skeleton, the substituent is somewhat remote from the acidic centre and the geometry for 4-X and 1-COOH closely resembles that for groups in a 1,4-disubstituted benzene)81. However, the pKa value for the vinyl-substituted acid in this solvent was not available, so the 07 value for the vinyl group was calculated by substituting the pKa value for vinylacetic acid in water at 25 °C (4.352) in the regression equation 10 ... 

The titanium atom in all oxidation states appears to be an acidic centre. 

We have already seen a number of models for the zinc(II) containing enzymes such as carbonic anhydrase in Section 11.3.2. Zinc is an essential component in biochemistry, and forms part of the active site of more then 100 enzymes, of which hydrolases (such as alkaline phosphatase and carboxypeptidase A), transferases (e.g. DNA and RNA polymerase), oxidoreductases (e.g. alcohol dehydrogenase and superoxide dismutase) and lysases (carbonic anhydrase) are the most common. In addition, the non-enzyme zinc finger proteins have an important regulatory function. In many of these systems, the non-redox-active Zn2+ ion is present as a Fewis acidic centre at which substrates are coordinated, polarised and hence activated. Other roles of zinc include acting as a template and playing a structural or regulatory role. 

As AI2O3 contains considerable amounts of acidic centres on the surface, it was suggested that the syndiospecific polymerisation of styrene takes place via polyinsertion involving Ti C cationic species. Non-cationic species with the Ti C bond on the alumina surface are believed to promote isospecific polymerisation. 

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