Dealuminated acid strength

The preparation methods of aluminum-deficient zeolites are reviewed. These methods are divided in three categories (a) thermal or hydrothermal dealumination (b) chemical dea-lumination and (c) combination of thermal and chemical dealumination. The preparation of aluminum-deficient Y and mordenite zeolites is discussed. The structure and physico-chemical characteristics of aluminum-deficient zeolites are reviewed. Results obtained with some of the more modern methods of investigation are presented. The structure, stability, sorption properties, infrared spectra, acid strength distribution and catalytic properties of these zeolites are discussed. 

Extensive studies of the acidity and basicity of zeolites by adsorption calorimetry have been carried out over the past decades, and many reviews have been published [62,64,103,118,120,121,145,146,153,154]. For a given zeolite, different factors can modify its acidity and acid strength the size and strength of the probe molecule, the adsorption temperature, the morphology and crystallinity, the synthesis mode, the effect of pretreatment, the effect of the proton exchange level, the Si/Al ratio and dealumination, the isomorphous substitution, chemical modifications, aging, and coke deposits. 

The initial rate of 2-MN acetylation depends on the framework Si/Al ratio of the zeolite catalyst.[27] For a series of dealuminated BEA samples (by treatment with hydrochloric acid or with ammonium hexafluorosilicate), the acetylation rate passes through a maximum for a number of framework A1 atoms per unit cell (/VA() between 1.5 and 2.0 (Si/Al ratio between 30 and 40). The activity of the protonic sites (i.e. the TOF) increases significantly with Si/Al from 420 h 1 for Si/Al = 15 to 2650 h 1 for Si/Al = 90. It should be noted that similar TOF values could be expected from the next nearest neighbour (NNN) model. Indeed all the framework A1 atoms of the zeolite (hence all the corresponding protonic acid sites) are isolated for Si/Al ratio 10.5. Therefore the acid strength of the protonic sites is then maximal as well as their activity.[57,58] This was furthermore found for m-xylcnc isomerization over the same series of BEA zeolites.1271 This increase in TOF for... 

Obviously the pore size determines which molecules can access the acidic sites inside the zeolite framework (molecular sieving effect) and is responsible for the shape selectivity observed with these materials (see later). The catalytic activity is also influenced by the acid strength of these sites which is determined by the Si/Al ratio (see above). The latter can be increased by post-synthesis removal of A1 atoms. Dealumination can be achieved by treatment with a... 

In Section V it was shown that the Si/AI ratio has a strong influence o the acidic properties of zeolites. Dealumination, as discussed previously, is a widely used means of changing the acid character of zeolite catalysts. Such changes in the acid strength distribution are manifested as changes in catalytic behavior. For example, dealumination of HY zeolites increased the catalytic activity for cumene cracking at 573 K, reaching a maximum at a... 

At the same time it has been shown that certain zeolite compositions do not exhibit uniform acid strength. Lunsford and coworkers extended the acidity-aluminum content correlation to the high Si/Al range of dealuminated H-Y zeolites and found that above approximately 35 Al/unit cell (Si/Al 4) the overall acidity of the zeolites decreased, presumably due to next nearest neighbor interactions, i.e., the intensive factor of acidity of the zeolite is diminished. It has also been established that certain dealumination procedures can affect the intensive factor of acidity. This was clearly demonstrated by Lago et al. on H-ZSM-S catalysts, in which dealumination by mild steaming resulted in a four-fold increase in the cracking activity for n-hexane. ... 

It has been shown that steam ammonia treatment of an AlaOs-SiOa hydrocracking support promotes the formation of pentacoordinated aluminum species. The mechanism is associated to a dealumination and aluminum migration from tetrahedral coordination into more distorted Al-Si environment. This generates a larger proportion of strong Lewis acid centers and a broad distribution of acid strength. 

The highly active performance of chemically AHFS-dealuminated zeolites for chlorinated VOC destruction could be accounted for by the generation of new strong acid sites, which were preferentially Bronsted sites, due to dealumination treatment. It could be concluded that a zeolite with a modest concentration of Bronsted sites, which were primarily of high acid strength, demonstrated to be effective for catalytic purposes. Likewise, it was established that chlorinated VOC oxidative decomposition was a type of reaction that required strong Bronsted acidity. 

The increase of activity observed for increasing dealuminations is not only due to the increase of the acid strength of the Bronsted acid sites due to an inductive effect of EFAL species [14,15], but also to the increase of the hydrophobicity of the zeolite s surface. 

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