Investigations of secondary nucleation by initial breeding in clear solution zeolite NaA systems

Abstract

An investigation of the effects of adding seed crystals to a clear synthesis solution, which produced molecular sieve zeolite NaA, was carried out. The initial breeding secondary nucleation mechanism, whereby microcrystalline fragments are washed off seed crystal surfaces to catalyze zeolite crystal nucleation, was studied using SEM, powder X-ray diffraction, and quasi-electric light scattering spectroscopy (QELSS). It was determined that small seed crystals (about 1–3 μm) did not promote nucleation of a new population of zeolite crystals when added to a clear solution synthesis batch. However, much larger zeolite NaA crystals (about 40 μm) were demonstrated to promote zeolite crystal nucleation by an initial breeding mechanism. Small particulates were demonstrated to exist only on the larger crystal surfaces, could be washed off, and were observed in a filtrate solution by QELSS. Lastly, it has been shown that the source of these particulates is residual aluminosilicate material in the bulk solution phase which was not incorporated into the crystal mass at the time the seed crystals were synthesized. As such, stopping the syntheses of smaller seed crystals prematurely was sufficient to cause the formation of initial-bred nuclei in those samples.