BrOnsted acid sites in zeolites

Two-Dimensional correlation analysis to study Bronsted acid sites in zeolites... 

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. 

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... 

The goal of our work was to recognise if the cyclohexanol conversion can be used as a test method in the studies of heterogeneity of Bronsted acid sites in zeolites. 

Recent results are presented illustrating principal mechanistic differences between alkane isomerization in liquid acids and over solid acids, including bifunctional catalysts. Isotopic labeling shows that butane isomerization over solid acids proceeds preferentially as a bimolecular process, i.e. via a Cg intermediate, which subsequently decomposes, preferentially into two iso-Cn structures. Bronsted acid sites in zeolites form chemical bonds with metal clusters. The resulting metal-proton adducts function as "collapsed bifunctional sites". 

Figure C2.12.2. Formation of Bronsted acid sites in zeolites. Aqueous exchange of cation M with an ammonium salt yields the ammonium form of the zeolite. Upon thermal decomposition ammonia is released and the proton remains as charge-balancing species. Direct ion-exchange of M with acidic solutions is feasible for high-silica zeolites. 

D. Freude, J. Klinowski, and H. Hamdan, Chem. Phys. Lett., 149, 355 (1988). Solid-State NMR Studies of the Geometry of Bronsted Acid Sites in Zeolitic Catalysts. 

M. Hunger, Bronsted acid sites in zeolites characterized by multinuclear solid-state NMR spectroscopy, Catal. Rev. Sci. Eng. 39 (1997) 345-393. 

Although the intramolecular cyclization of unsaturated alcohols can be catalyzed by both Lewis and Bronsted acid sites in zeolites, the reported experimental results interestingly indicated that the largest conversions to the corresponding tetrahydrofurans were achieved for zeolites showing an optimum concentration of Bronsted sites (0.15-0.20 mmol g" ). This circumstance was rationalized by computational methods as well it was proposed that the... 

M. Hunger, D. Freude, D. Fenzke, H. Pfeifer etal., H solid-state NMR studies of the geometry of Bronsted acid sites in zeolites H-ZSM-5. Chem. Phys. Lett. 191,391-395 (1992)... 

---