Stabilization and Chemical Modification of Zeolites

Dealumination processes are usually used in conjunction with production of the acid form of zeolite Y for many catalytic apphcations, and zeolites A and X are in most cases no longer used in acid catalytic applications because the high amount of aluminum in their frameworks makes them difficult to stabilize using various dealumination techniques. Successful dealumination and at least partial annealing of defects, resulting from movement of silicon cations to the aluminum vacancies, can be assessed by measurement of the reduction of the unit cell size of the zeolite. This unit cell reduction is a consequence of the relative ionic radii of AF (0.54 A) and Si + (0.40A). 

Various fluoride-containing reactants have also been used to dealuminate, dealuminate/reinsert silica and/or modify the acidity of zeolites for catalytic applications [132-137]. 

An additional way to effect the shape/size selectivity of zeolite catalysts was developed which entailed deposition of various silicon-containing deposits on the external surfaces of zeolites to create an additional diffusion barrier [131, 138]. 

Many of these treatments can be straightforwardly scaled-up to industrial quantities by adding steam to calcination furnace atmospheres and carrying out various forms of solution ion exchange or chemical treatment and/or acid extractions in suitable powder and granule contacting equipment. The products vary in stability and catalytic properties because the treated materials are no longer freshly crystal- 

United States patent Date Description Reference 

---