The Role of Next Nearest Neighbors in Zeolite Acidity and Activity

Analysis of the Zeolite-Catalyzed H-Exchange Reaction of Methane Using Atom-In-Molecule Reactivity Indexes

“…The importance of the type of tetrahedral atom in the NNN positions of the reaction center in determining the activity is therefore convincingly demonstrated. This finding strongly supports the Al NNN theory, which states that the strength of an acid site depends on the number of Al NNN. − The success of the Al NNN theory, and the related topological density theory by Barthomeuf in predicting the ideal Si/Al ratio for maximum acidity in mordenite and faujasite, has been evidenced by experiment. , …”

Effect of NNN Site Si/Al Substitution on the Acid Strength: Mordenite

“…Provided that the possibility of every T site to accommodate an Al is equal, the probabilities of 0NNN and 1NNN are calculated as follows where R = Si/Al atomic ratio, n is the number of NNN sites, and y is the number of Al occupying the NNN sites. In case of mordenite, there are 48 T sites in a unit cell, of which 16 each are T1 and T2, while 8 each are T3 and T4, respectively.…”

Cracking and Hydrocracking

“…Even zeolite Y has problems of access by molecules in the higher boiling range [157,158], and it is for this reason that sometimes an ASA is added [159]: it is much less acidic but it does not induce any diffusion limitations, thus serving to convert the species that the zeolite cannot cope with [160] (see Figures 18.16 and 18.17). Zeolite Y can be modified by combinations of steaming and/or calcination, ion-exchange, leaching, and various chemical treatments [161,162], with the aim of controlling acid-site density, nonframework aluminum species and mesoporosity, while preserving a high crystallinity [99,163,164]. The experimental space set up by these techniques (see Figure 18.18) is so enormous that to this day new, more effective modifications of Y continue to be found.…”