The role of gel chemistry in synthesis of aluminophosphate molecular sieves

The efficiency and product purity of aluminophosphate molecular sieves by the hydrothermal synthesis are affected by the gel chemistry of a reaction mixture. A suitable gel chemistry appears to be a prerequisite for hydrothermally crystallizing the molecular sieves from hydrous gel mixtures. Factors that determine the gel chemistry are the source of aluminum and phosphorus, mixing procedure, gross chemical composition of gel including the water content, and pretreatment of gel. With all these factors specified, the pH of the reaction mixture seems to reflect the gel evolution and crystallization of molecular sieves. Besides the structure-templating effect, organic bases play an important role in determining the gel chemistry. With the addition of organic bases, the pH of the reaction mixture rises to the desirable range of 3.5-6.0. From the properly prepared gel within this pH range, the formation of aluminum phosphate dense phases is eliminated while microporous aluminophosphate is formed as a metastable phase during the hydrothermal crystallization.