Dealumination Routes and Methods for Zeolites

Zeolite dealumination is one of the most important subjects in the field of zeolite secondary synthesis and modification. For decades, zeolite scientists have been investigating dealumination routes and techniques in order to optimize the properties and functions of zeolites. Generally, there are three dealumination routes presented as follows  

Scherzer concluded that in the USY formed from NH4Y through high-temperature water-vapor treatment, the aluminum exists in three forms (1) in the USY framework, (2) outside the framework as 6-coordinated octahedral Al, (3) on the USY surface as differently coordinated Al which results in surface Al enrichment. The Al in the USY framework produces B acid centers, and although the number of the acid centers is smaller than that of the parent Y zeolite, the strength and distribution of these centers may favor cracking catalytic reactions more. 

It was also pointed out by Scherzer that for dealumination and framework stabilization 

Since the Si—O bond length (1.66 A) is shorter than that (1.75 A) of Al—O, the crystal lattice of zeolites is shrunken and the structure is stabilized after dealumination and silicon enrichment, as confirmed as follows. Of course, unavoidably there exist silicon-deficient hydroxyl nests after both dealumination and ultra-stabilization, and some mesopores will be generated in the USY framework. 

---