ZEOLITE SYNTHESIS USING A SEMICONTINUOUS REACTOR .1. CONTROLLED NUCLEATION AND GROWTH OF ZSM-5 CRYSTALS HAVING WELL-DEFINED MORPHOLOGIES

ZSM-5 has been synthesized at 90 and 130 degrees C in reactor systems in which reagents were supplied continuously, and homogeneous product slurry was withdrawn semicontinuously. In some cases, precursor crystals were nucleated in situ, whereas other preparations started from dispersions of seed crystals. Guided by a simple mathematical model, conditions were chosen to give a steady, near-linear crystal growth regime. The crystals grew from solution in a medium free of solid gel in which the predominant source of nutrient silica was the monomeric monoanion [(HO)(3)SiO](-). The products were well formed, of high crystallinity, and of narrow crystal size distribution. By providing a means for regulating solution composition and effective supersaturation, the experimental method allowed direct control over nucleation and growth processes and of the radial composition of the resulting crystals. Continuous methods of zeolite synthesis provide a versatile preparative tool and a powerful technique for studying crystallization behavior.