Bronsted acid sites, reactions catalyzed

Possible role of the induced acidity and basicity in catalysis and environmental chemistry is discussed. The suggested mechanism explains the earlier reported promotive effect of some gases in the reactions catalyzed by Bronsted acid sites. Interaction between the weakly adsorbed air pollutants could lead to the enhancement of their uptake by aerosol particles as compared with separate adsoi ption, thus favoring air purification. 

Xylene Isomerization There are several mechanisms by which the three xylene isomers can be interconverted. The one that is of the greatest interest with respect to industrial applications is the so-called monomolecular or direct xylene isomerization route. This reaction is most commonly catalyzed by Bronsted acid sites in zeolitic catalysts. It is believed to occur as a result of individual protonation and methyl shift steps. 

Like the AVADA and the AlkyClean processes, these two processes also replace the liquid acid/base catalysts with solid acids and bases [192]. Although the reaction mechanism for the heterogeneous acid-catalyzed esterification is similar to the homogeneously catalyzed one [207,208], there is an important difference concerning the relationship between the surface hydrophobicity and the catalyst s activity. This is especially true for fatty acids, which are very lipophilic compounds. One can envisage three cases First, if there are isolated Bronsted acid sites surrounded by a... 

Dealumination processes which leave residual extraframework aluminum in a Y-type zeolite result in a decrease in the overall number of Bronsted acid sites but an increase in the strength of the remaining acid sites. The net effect is an increase in activity for acid-catalyzed reactions up to a maximum at ca. 32 framework A1 atoms per unit cell. A model for strong Bronsted acidity is proposed which includes (i) the presence of framework Al atoms that have no other A1 atoms in a 4-membered ring and (ii) complex A1 cations in the cages. The essential role of extraframework aluminum is evident from recent studies in which framework A1 has been completely removed from zeolite-Y and by experiments on the related ZSM-20 zeolite. 

Many reactions are catalyzed by acid sites on the surface of the catalyst. Isomerization, polymerization, aromatiza-tion, and cracking are catalyzed by Lewis and/or Bronsted acid sites. The precise nature of these sites is open to debate however, intuitively one can use an alkaline material to titrate acid sites and hence determine the number of such sites present. Beses, such as n-butylamine, with a series of Hammett indicators have been used for titrating acid sites. However, the system must be free from water contamination and the catalyst must be colorless to enable one to note indicator color changes. Diffusion of the indicators into the porous network can be very slow and require long equilibration times. 

Cation- and anion-exchange resins are widely applied as catalysts, when reactions can be carried out at temperatures lower than 423 K [161-171], Cation-exchange resins in acid forms have Bronsted acid sites and when exchanged with metallic cations contain Lewis acid sites, while anion exchange resins forms base species to carry out base-catalyzed reactions [169], The principles previously described in this chapter for these sites in other catalysts, can be applied for the catalytic action of these polymers. 

It is generally admitted that skeletal transformations of hydrocarbons are catalyzed by protonic sites only. Indeed good correlations were obtained between the concentration of Bronsted acid sites and the rate of various reactions, e g. cumene dealkylation, xylene isomerization, toluene and ethylbenzene disproportionation and n-hexane cracking10 12 On the other hand, it was never demonstrated that isolated Lewis acid sites could be active for these reactions. However, it is well known that Lewis acid sites located in the vicinity of protonic sites can increase the strength (hence the activity) of these latter sites, this effect being comparable to the one observed in the formation of superacid solutions. Protonic sites are also active for non skeletal transformations of hydrocarbons e g. cis trans and double bond shift isomerization of alkenes and for many transformations of functional compounds e.g. rearrangement of functionalized saturated systems, of arenes, electrophilic substitution of arenes and heteroarenes (alkylation, acylation, nitration, etc ), hydration and dehydration etc. However, many of these transformations are more complex with simultaneously reactions on the acid and on the base sites of the solid... 

In the case of an oligomerization reaction catalyzed by Bronsted acid sites, the scheme reported in Fig. 1 must be modified as represented in Fig. 2a. 

According to this scheme, the first step of the reaction is the formation of a hydrogen-bonded precursor FH B, followed by the protonation of the monomer, leading to the formation of FBH". Examples of this type are shown in Section IV.B, where the oligomerization of unsaturated molecules in protonie zeolites is discussed. It is important that this first step is common to other reactions catalyzed by Bronsted acid sites. For example, in Section IV.A, the formation of methyl-substituted benzene carbocations as intermediate species involved in the MTO process in Hp zeolite is diseussed. 

If the polymerization reaction occurs in the presence of steric constraints, the activation energy associated with the monomer insertion grows with the progress of the reaction, in the same way as discussed for the oligomerization reactions catalyzed by Bronsted acid sites (Section LB and Fig. 2a), because the available space is progressively reduced. Under such conditions, the formation of polymeric species is limited, and small oligomeric species can become observable. An example of this situation is ethene polymerization on Cr/silicalite, in which the transition metal center is grafted to the internal surface of a cavity (Section VI.C.l). 

IV. Reactions Catalyzed by Bronsted Acid Sites The MTO Process and Oligomerization of Unsaturated Hydrocarbons... 

Co-Exchanged Zeolites. Hydrothermal durability of Co-zeolites usually depends on the nature of the parent zeolite, Co exchange level, preparation method, etc. Existence of both Co and Bronsted acid sites in zeolites can play a synergistic role for catalyzing NOx reduction reaction with HCs however, the protonic sites induce catalyst deactivation by Not only can the... 

A reaction that is catalyzed by a Bronsted acid site, or H, can often be accelerated by addition of a solid acid. Materials like ion-exchange resins, zeolites, and mixed metal oxides function as solid analogues of corrosive liquid acids (e.g., H2SO4 and HF) and can be used as acidic catalysts. For example, isobutylene (IB) reacts with itself to form dimers on cross-linked polyfstyrene-sulfonic acid), a strongly acidic solid polymer catalyst ... 

The acidic 10 and 12 membered ring zeolites (H-MOR, ZSM-5, ZSM-11) can also be used to catalyze the condensation of alkenes with aldehydes to form unsaturated alcohols, acetals etc. (Prins reaction)[92]. Chang et a/. [93] showed that this reaction involves in the initial step the activation of the aldehyde by a Bronsted acid site to generate an electrophilic species. The condensation with, e.g., isobutene leads then to a primary alcohol with a positive charge at the tertiary carbon atom. Elimination of water and addition of further aldehyde molecules may lead to a broad variety of products. Some of these reactions can be effectively blocked by chosing zeolites with the appropriate pore size [94,95]. 

For example, a series of alkylaromatics afforded the corresponding aralkyl ketones in high selectivities with TBHP in the presence of CrAPO-5 (3 m %) at 80 °C (Table 5). TBHP could be replaced by Oj but this required neutralization of Bronsted acid sites on the CrAPO-5, by ion-exchange, in order to avoid acid-catalyzed decomposition of the benzylic hydroperoxide to the eorresponding phenol, which inhibits the autoxidation. The addition of a small amount of TBHP to initiate the reaction also had a beneficial effect. 

Even though the synthesis of many medium pore SAPO molecular sieves are well documented, only SAPO-11 has been studied in detail with respect to its shape selectivity and catalytic activity in acid catalyzed reactions. The reaction of m-xylene on zeolites, besides its industrial importance, is abundantly described in literature not only because it provides information on the geometry of the zeolite channels, but also because it is considered as an appropriate reaction to give information on the acidic properties of solid catalysts. Both isomerization and disproportionation are catalyzed by Bronsted acid sites , the disproportionation reactions requiring stronger acid sites than isomerization reactions. Hence SAPO molecular sieves with medium acidity should give better selectivity for m-xylene isomerization than zeolites. 

The Diels-Alder cycloaddition reaction of dihydropyran with acrolein was performed in the presence of various H-form zeolites such as H-Faujasites, H-p, H-Mordenites which differ both in their shape selective as well as their acidic properties. The activity of the different catalysts was determined and the reaction products were identified. High 3delds in cycloadduct were obtained over dealuminated HY (Si/Al=15) and Hp (Si/Al=25) compared to HM (Si/Al=10). These results were accounted for in terms of acidity, shape selectivity and microporosity vs mesoporosity properties. The activity and the regioselectivity were then discussed in terms of frontier orbital interactions on the basis of MNDO calculations for thermal and catalyzed reactions by complexing the diene and the dienophile with Bronsted and Lewis acidic sites. From these calculations, Bronsted acidic sites appeared to be more efficient than Lewis acidic sites to achieve Diels-Alder reactions. 

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