Crystallization and crystal morphology of silicalite-1 prepared from silica gel using different amines as a base

Abstract

Silicalite-1 was synthesized by hydrothermally treating aqueous mixture suspensions of NPrn4Br (TPABr)-dispersed silica gel and different amines. Influence of the type of the amines on crystallization of silicalite-1 and the resultant crystal shapes was examined. Only when the amines with a lower pKb value than 4.50 were used, crystallization of silicalite-1 occurred, suggesting that the dissolution of the silica gels with the amines and sequential deposition of silicalite-1 from the solution play an important role in this crystallization behavior. The resultant crystal shapes of silicalite-1 were also dependent on the type of amines. The additions of NH2C2H4OH and NH2C3H6OH as an amine led to a long plate-like shape, whereas the amines with a stronger basicity, such as N(C2H5)3 and NH2C6H12NH2, gave the formation of long rod-like crystals. This difference is due to difference in crystal growth rate along the b-axis. Thus, it has some relation with the dissolution rate of the silica gel, that is, the supersaturation degree of silicate species in the reaction solution. Influences of the amount of TPABr dispersed into the silica gels and the hydrothermal period on the resultant crystal shapes were also examined. With increasing the TPABr amount, the resultant crystals became shorter in the c-axis direction. When the hydrothermal period was changed, a short crystal was formed first, and sequent growth of the crystal in the c-axis direction took place. From these observations, it follows that the increase in the amount of TPABr led to an increase in the nucleation rate of silicalite-1 crystals, giving a shorter shape. Consequently, the crystal growths of silicalite-1 along the b- and c-axis could be controlled independently.