The CAL Family of Molecular Sieves: Silicoaluminophosphates Prepared from a Layered Aluminophosphate

Among the synthesis methods to prepare zeolites, the sol-gel processes are the ones most known and used. The same may be said about the aluminophosphate-based molecular sieves. This work reviews the preparation of zeolites from layered silicates and discusses the synthesis of molecular sieves using an aluminophosphate with kanemite-type layered structure as precursor. The role of the structure directing agent and of the water content on the structure-type of each silicoaluminophosphate prepared is reviewed. In the cases focussed herein, there are two features affected by this new procedure: the acidity of the silicon substitution sites and the morphology of the particles. Concerning the acidity of these solids, a larger amount of silicon can be substituted in the framework before the onset of silicon island formation. At comparable SiO₂/Al₂O₃ molar ratios, the samples prepared by the altered procedure possess a larger number of acid sites in the borders of silicon islands than in the silicoaluminophosphates prepared by the traditional procedures(e.g. by using phosphoric acid and a source of aluminum, as precursors). In relation to the morphology, this new route leads to particles shape with vestiges of the lamellar morphology of the layered reactant, thus introducing the possibility of reduced mass transport difficulties.