Bronsted acids catalyzed isomerization

The latter isomerization via an intramolecular hydride shift is catalyzed by Lewis acids. Tin-containing P-zeoHte exhibits remarkable activity for the glucose-fructose isomerization in water. and NMR studies have elucidated this Lewis acid-catalyzed isomerization mechanism, and show a clear difference between Sn-fl (Lewis acid catalysis) and NaOH (Bronsted base catalysis). 

Current efforts are focused on the preparation of bifunctional catalysts capable of directing the Lewis acid-catalyzed isomerization of alkyl glucoside intermediates to alkyl fmctosides, and their subsequent Bronsted acid-catalyzed dehydration to... 

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. 

The Lewis acid (AICI3) and Bronsted acid (HCI) catalyzed isomerization of pentane to a more highly branched product would form 2,3-dimethylpropane ... 

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. 

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... 

Isomerization of phenols 137 over sUica gel in the solid phase furnishes the corresponding 2,3-dihydro-4-oxo-4//-l-benzopyrane derivatives 138 (equation 60) ° . The cascades of the charge-accelerated rearrangements of the ortl o-(l,l-dimethylpropenyl)phenol 139 catalyzed by Bronsted acid (e.g. trifluoroacetic acid, equation 61) as well as by Lewis acids (anhydrous AICI3 or TiCLj, equations 62 and 63) proceed via the common intermediate 140 . 

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 mechanism of the acid catalysis of a-phenylallyl chloride isomerization has not been investigated in detail. In aprotic solvents such as chlorobenzene, Bronsted acids probably catalyze the reaction by hydrogen bonding with chloride, which should stabilize the S i transition state. 

Simple use of conventional solid acids and bases affords the efficient production of furfurals. First, a soHd base catalyzes the isomerization of the aldose, such as glucose, xylose, and rhamnose, into the corresponding ketose, for example, fructose, xylulose, and rhamnulose. Second, a solid Bronsted acid dehydrates the ketose into furfurals (HMF, furfural, and MF). Such an approach is impossible for Hquid acid-base pairings because of their self neutralization. 

The disproportionation of trimethylbenzene is a possible route to produce durene (1,2,4,5-tetramethylbenzene). PILCs have been reported to be catalysts for this reaction, and the shape selective effect was discovered in this reaction from the product distribution with durene being the major product. This disproportionation reaction is catalyzed by the PILC Lewis sites, while the isomerization of trimethylbenzene (side reaction) is well correlated to the Bronsted acidity. 

Mixed WOj/Al Oj/HY catalysts prepared by calcination of physically mixed WO3, Al Oj and HY zeolite showed unique behavior in the metathesis between ethene and 2-butene to produce propene [147]. Monomeric tetrahedrally coordinated surface tungstate species responsible for the metathesis activity were formed via the interaction with Bronsted acid sites of HY zeolite. Polytungstate clusters are supposed to be less active in the metathesis reaction. The best catalyst demonstrates the 2-butene conversion close to the thermodynamic equilibrium value ( 64%) at 453 K. The catalysts are bifunctional [148] they catalyze first isomerization of 1-butene to 2-butene and then cross-metathesis between 1-butene and 2-butene to produce propene and 2-pentene. 10%W03/Al203-70%HY exhibits the highest propene yield. 

S, and S interconvert photochemically in basic emimCVAlCl3 and bpCl/AlCl, but by different mechanisms. The photoisomerization could not be studied in the Lewis acidic ionic liquids because this results in the very large Bronsted acidity of traces of HCl in the media S for instance, underwent an acid-catalyzed dimerization in acidic [emim] [Clj/AlClj. In basic [bp] [ClJ/AlClj, both and S, photoi-somerize cleanly to give a PS containing -99% S,. In addition, the electron transfers from S, and to bp+ are quite exothermic (electron transfers from the triplet states are endothermic). Thus, the isomerizations in [bp] [Cl]/AlCl3 occur by the Electron Transfer mechanism. 

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